## Monday, December 20, 2010

Today in arxiv has appeared something that I had never seen before, a whole collection of review articles devoted to a single topic, the ADS/CFT. There is an "overview" article that serves as reference of the others: Review of AdS/CFT Integrability: An Overview.

If I haven't counted bad there are 23 articles. And this is a review!. Clearly that is an indication of the fact that AdS/CfT is a very active line of research with many branches.

## Thursday, December 02, 2010

### Behind the super event horizont

This is a very brief note to tell that I have not forgotten about the blog.

There are a couple of combined reason for my long absence. Possible one of the most important ones is that my favourite (and expensive) wireless keyboard is out of work because it's USB connector seems to be slightly broken. Now I must return to writing with a wired keyboard, which is uncomfortable and it results in laziness for writing.

I also have had a few math exams, which take time out, I have been busy with activities out of physics (doing an small software develpment, have returned to get piano lesson because of a very good economic offer, deciding which martial art I want learn now that there is no instructors here in the one I did any more, etc).

In the physics side I have been refreshing my knowledge in some topics (for example renormalization) and studying new ones. Specially I am very involved in black hole physic. Both in classical semiclassical gravity (dynamic horizons, vadya metrics, etc) with the idea of see if it is possible to realize some idea I have about "entropic force" effects in black hole physics. Also I am haveing a close attention to a somewhat technical points of black holes in supergravity and string theory, far beyond the basic (although very important) results of the nineties that appear in most books. I hope that these two areas could result in something useful, but who know?

Anyway, I am recovering from the trauma that has supposed the dead of my favourite keyboard and hopefully I'll return to write more often here.

## Tuesday, October 05, 2010

### Where to hide a vacuum?

This last mounth happened many interesting things and there were a few good papers. Possibly I´l mention some of them in the future, but today just want to write a brief entry about a pre-idea.

The energy of vacuum is an ugly problem for QFT and string theory. Both in the theoretical side, the vacuum energy would be a lot greater than 0, and in the experimental side, the universe seems to be accelerating according to the famous last nineties observations. The observational side is less worrying: It is due to an imbalance among redshifth and brightness of supernovae. Well, one could try to imaginate an missing light theory (MLT) where some photons are lost in the travel from the distant supernovae to us. This would result in a lost of intensity that could explain the observations. The way to "loose photons" would be by means that some (very rare, of course) interaction with K gravitons in a warped scenario, or in F-theory like scenaries where unification of gauge bossons is in upper dimensional branes one could try to search ways to loose some photon in that branes. Ok, this an idea of myself, and I have not purchased it too much, there are good reasons to think it is not an excellent/important idea.

Or it could be that the accelerated expansios is real but not due to a cosmological constant. One way would be the entropic force scenarie of Gerge Smooth where the acceleration is due to a entropic force caused by the entropy of the cosmological horizon. Or, maybe some "dark energy" theorie.

All of those phenomemenological problems and solutions don't address the fundamental problem that there is a vacuum energy in QFT that should be great. The most extended way is a"landscape + antropic" solution based on the Weimberg ideas and developed by Polchinsky, Bousso, etc and has derived in the multiverse scenario.

Reading the articles in the CC problem (for example, this of WItten: The Cosmological Constant From The Viewpoint Of String Theory or this other of Polchinsky: The Cosmological Constant and the String Landscape ) one can learn that the main objective is to get an small vacuum energy. But I thin that there is another alternative. To put the vacuum energy out of the reach of gravity. That sound like a hard problem because gravity couples to everything, that seems to be one of the most imprtant insights of general relativity. Well, of course I have not a mechanism to hide the vacuum energy for gravity or I would be trying to publish the paper where I develop it to some important review (and it would be in the arxiv just now), butt he key point is that if there is a way to do so it would give an elegant solution to a long standing problem and it could be, possibly, consistent with all the cosmological constraints, including accelerated expansion, inflation, CMB, etc. Well, maybe one could try to relate the mechanism to the observation that some people has made among the CC (presumably) observed value and the neutrino masses. Or, better, to try to relate it to the (presumably) recent observation of different masses among a neutrino and an antineutrino.Or one could forget neutrinos at all, of course.

Well, it is a pre-idea, as I said. For the most conservative readers my recommendation is to look at the two papers I linked (if they haven't already made it) and forget my comments ;-).

## Monday, August 30, 2010

### Verlinde videos on emergent gravity

Reading the kea's new blog I advertised that there were two videos of Verlinde talking about emergent gravity:

1)KIPT video

2) Perimeter video

At the beginning of this summer Verlinde had announced it's presence in a string theory conference in Madrid but, unfortunately, at last he didn't assist. Because of that I was curious to see in video what I missed in direct and looked at the first of the videos. The realization is somewhat poor in various moments and you can't see Verlinde typing equations in the blackboard or you can't see complete the slides. Still you can follow the conference so it is ok.

In the conference he argues some thing that dn't appear in his paper On the Origin of Gravity and the Laws of Newton. Searching on verlinde's papers n arxiv one can find that some of the things he says on the conference about Berry phase or Born-Oppenheimer/adiabatic could be somewhat rooted on this other paper: Black Hole Berry Phase

While watching the video I have got the impression that it could be interesting to see it one could get an alternative construction of general relativity using entropy instead of geometry as the way to go from special relativity, that is, constant speed reference systems, to accelerated systems. In particular I thought about it when Verlinde does sme considerations about Rindler space.

If that would be possible one would have a different way to consider the questions of general relativity. Also if that entropic viewpoint of general relativity makes sense one could use as a framework to formulate actual theories, including string theory, and, maybe, it could seed some light on it. In that sense it would be more an entropic formulation of general relativity that anything else.

Of course he last two paragraphs are just my considerations, and wouldn't be confused with what Verlinde says.

To end this entry just to mention that a few days ago Peter G.O. Freund posted the paper Emergent Gauge Fields where he argues that the entropic force scenario for gravity could mean, because of ADS/CFT, that all other forces are emergent. The paper is discussed in Peter WOits blog Everything is Emergent. Verlinde mentions similar things at the end of the talk.

Well, as I said in my previous post about entropic gravity I agree with the common view that this idea is very vague. Still I liked the talk over the paper. Even if the idea of gravity through entropy fails the talk is entertaining and worth to watch.

## Monday, July 26, 2010

### Higgs search at ICHEP + four interesting articles in arxiv.

The great conference of the moment is ICHEP. It has a dedicated blog where very well known people as Tommao Dorigo (quantum diaries survivor), Jester (resonances) and many others are posting about the conferences.

Today is a great day in the conference. In one hand the French president Nicolas Sarkozy will give a talk (not sure what he will speak about, possibly about the financiation of CERN) and, just after that, the Tevatron people will expose the combined D0 and CDF results concerning the Higgs search.where it will be settled the question or wetter the gossip Tomasso spread a few days go had some foundation or he will, definitively, be charged with the nickname of "the Paris Hilton of Physics" as he was referred in a previous talk of the tevatron fellowship a few days ago. I don't really like that descalifications, but maybe it was just a joke.

Anyway, you can follow those two conferences live here. The tevatron talk begins at 4 30(spanish hour).

Out of ICHEP there are interesting things also.

These weekend at least two interesting paper appeared in arxiv. On one of them Topological Influence between Monopoles and Vortices: a Possible Resolution of the Monopole Problem the authors, Shingo Kobayashi. Michikazu Kobayashi, Yuki Kawaguchi, Muneto Nitta, Masahito Ueda propose a new way to eliminate monopoles from the spectrum of grand unification theories such as SU(5) or SU(10).

That monopoles are considered an inevitable aspect of that theories, appearing in the spontaneous symmetry breaking hat leads from the unified theories to the standard model. Like monopoles are not observed some mechanism is required to explain their absence (assuming the unification of forces is a reality). The usual mechanism is thought to be inflation that dilutes their density to one in a cosmological horizon. Mathematically the presence, of that monopoles is linked to the second homotopy group, $$\pi_2(G/H)$$, of the quotient group G/H where G is the grand unification group and H is the resulting group after symmetry breaking, being non trivial. If the subgroup H contains an U(1) them it can be shown that $$\pi_2(G/H)\cong \pi_1(H)\cong \mathbb{Z}$$.

The author claim that using the, so called, Alice strings that can possibly also appear in the GUT. They have found a loophole in the argument that leads to the monopoles non-existence (or existence of at most one monopole) Monopoles are rotated into antimonopoles if rotated along Alice Strings. Because of that in certain cases monopoles can annihilate in pairs in the presence of that strings. To analyse the question they describe a combined monopole + Alice string by means of a different type of homotopy groups, the abe group. Remember that the usual n-homotopy groups are, loosely speaing, inequivalent ways to embed an n-sphere in the target space. Abe homotopy groups are constructed in a similar way using instead of spheres pinched torus.

I don't know too much of Alice strings,beyond what is exposed in the article so while I don't read more about them I can't take a persona opinion about how reliable that loophole is. In any case the article is very well written and very elegant mathematically so it deserves to be truth more than many others simply because of that ;).

Another very interesting article of last weekend was SU(3) × SU(2) × U(1) Vacua in F-Theory by Kang-Sin Choi. The title says what it offers. The abstract clarifies it:

The Standard Model group and matter spectrum is obtained in vacua of F-theory, without resorting to an intermediate unification group. The group SU(3) x SU(2) x U(1)_Y is the commutant to SU(5)_t \times U(1)_Y structure group of a Higgs bundle in E_8 and is geometrically realized as a deformation of I_5 singularity. Lying along the unification groups of E_n, our vacua naturally inherit their unification structure. By modding SU(5)_t out by Z_4 monodromy group, we can distinguish Higgses from lepton doublets by matter parity. Turning on universal G-flux on this part, the spectrum contains three generations of quarks and leptons, as well as vectorlike pairs of electroweak and colored Higgses. Minimal Yukawa couplings is obtained at the renormalizable level.

I will add that not depending on an unification group the monopoles existences is also ruled out. In fact in F-theory GUTs the gauge groups live in different number of dimensions that matter fermions. I never have stopped to think if that has any influence in the monopoles prediction, but I guess not so this kind of scenario, without unification, solves that problem. In fact semirealistic standard model can be obtained in string theory in heterotic or branneworld construction, so it is nt exclusive of F-theory. That spoils the former idea of the eighties of string theory as a generalization of the grand unification schemes but it doesn't mean string theory fails, of course.

Today also have appeared at least two very interesting papers. In this case both share an author, Tom Banks. One of them is TASI Lectures on Holographic Space-Time, SUSY and Gravitational Effective Field Theory and it's abstract says:

I argue that the conventional field theoretic notion of vacuum state is not valid in quantum gravity. The arguments use gravitational effective field theory, as well as results from string theory, particularly the AdS/CFT correspondence. Different solutions of the same low energy gravitational field equations correspond to different quantum systems, rather than different states in the same system. I then introduce {\it holographic space-time} a quasi-local quantum mechanical construction based on the holographic principle. I argue that models of quantum gravity in asymptotically flat space-time will be exactly super-Poincare invariant, because the natural variables of holographic space-time for such a system, are the degrees of freedom of massless superparticles. The formalism leads to a non-singular quantum Big Bang cosmology, in which the asymptotic future is required to be a de Sitter space, with cosmological constant (c.c.) determined by cosmological initial conditions. It is also approximately SUSic in the future, with the gravitino mass \$K \Lambda^{1/4}

The other is Pedagogical notes on black holes, de Sitter space, and bifurcated horizons and this is its abstract:

I discuss black hole evaporation in two different coordinate systems and argue that the results of the two are compatible once one takes the holographic principle into account. de Sitter space is then discussed along similar lines. Finally I make some remarks about smooth initial conditions in GR, which evolve to space-times with bifurcate horizons, and emphasize the care one must take in identifying spaces of solutions of General Relativity which belong to the same quantum theory of gravity. No really new material is presented, but the point of view I take on all 3 subjects is not widely appreciated.

I am just reading the first, so I can't say too much about them just now. moreover, if my memory is right banks was the thesis director of Lubos Motl so one would expect that he will write about these articles if the climate wars, or maybe the Higgs search announcement of today, allow him to do so. If not maybe I will edit this entry to say something about the first of the bank's articles that looks really intriguing.

P.S. IMPORTANT NOTE: I have just realized that there seems to be a problem with the template of the blog and that latex is not working any more in these, and all the other, entries. I'll try to see if the probem can be fixed.

## Tuesday, July 13, 2010

### Brief note: a warpdrive to vixra and some recent tantalizing experimental resuts

WARPDRIVES

Reading this post at the vixra bog I got aware of this paper:

Warp Drive Basic Science Written For ”Aficionados”.Chapter II - Jose Natario.

Warp drives are a nice idea. They are inspired by the TV and cinema serial Star Trek of which I am not a particular fan, although I have seen a few chapters and most of the movies.

The key concept is that when you want to to go from a point A to a point B you contract the space in front of you and expand the one behind you. in that way it is possible to go from A to B in an arbitrary small amount of time as far as one can contract/expand the space arbitrarily.

This wordy idea was first formalized by Alcubierre in pure GR framework. The construction of the warp-drive solution requires the use NEC (null energy condition) violating matter as much as wormholes or other exotic solution in GR require.

The last I read (sorry, I don't remember the actual link) of the alcubierre solution was that the back reaction of quantum effects ruined it (or at least it's use as a time machine).

The article I link here focuses in a different kind of warp drive, the Natario warp drive. I didn't previously know of this. I have nether read the full article. What I read looks coherent (as any article in arxiv on the same subject I mean) and I don't see any particular sign of crackpottery. But it is a vixra article, that is, ii is not intended (as far as I know) to any peer to peer review so, well, you know, read it at your own risk. B.T.W. despite the name it is not written for "aficionados": At least not for the kind of aficcionados that ignore genera relativity.

EXPERIMENTAL RESULTS

Htese recent days there were an unsuall amount of experiments with surprasing results.

On of them was a result on the possible violation of the CPT symmetry by neutrinos. This was infered by the observed fact that neutrinos and antineutrinos were found t behave differently and have different mass. You can read about in, for example: MINOS: hints of CPT-violation in the neutrino sector

Another surprising result was the observed fact that in muonic atoms the proton radio decreased, contradicting QED (quantum electro dynamics).The most recent blog entry in the subject is written by "mr TGD" (no offense intended) Matti Pitkannen: The incredibly shrinking proton

And last, but not least, the rumor of a 3 sigma detection of ligh (around 115 Gev) higgs bosson in the tevatron. The last entry on the subject is own to Lubos Motl: What a light Higgs would mean for particle physics

A lot of things, and a lot of papers in arxiv today. And I have many task to do tday so I will end the entry here.

## Wednesday, June 16, 2010

### Quick note: Two proposals in breaking of the equivalence principle

Until Friday it is very unlikely that I have time to properly read the two articles that I am going to link here. But I find that they are challenging enough to leve a note abut them.

The first article was cited yesterday in the arxiv blog: New Quantum Theory Separates Gravitational and Inertial Mass (if you are Spanish and prefer reading it in that language there is a translated version by kanijo: Nueva teoría cuántica separa las masas gravitatoria e inercial.

Those entries make reference to the following arxiv paper: Inertial And Gravitational Mass In Quantum Mechanics

I haven't read the actual paper, as I said I have not too much free time this week and to be honest reading the Tech Review entry I was not too inclined to do so. But somewhat amazingly (how many probabilities are there of having such a provocative proposal two consecutive days from independent sources) the first article in arxiv hep-th of today makes a similar claim based, this time, in string theory:The influence of D-branes' backreaction upon gravitational interactions between open strings

This is the abstract:

We argue that gravitational interactions between open strings ending on D3-branes are largely shaped by the D3-branes' backreaction. To this end we consider classical open strings coupled to general relativity in Poincare AdS5 backgrounds. We compute the linear gravitational backreaction of a static string extending up to the Poincare horizon, and deduce the potential energy between two such strings. If spacetime is non-compact, we find that the gravitational potential energy between parallel open strings is independent of the strings' inertial masses and goes like 1/r at large distance r. If the space transverse to the D3-branes is suitably compactified, a collective mode of the graviton propagates usual four-dimensional gravity. In that case the backreaction of the D3-branes induces a correction to the Newtonian potential energy that violates the equivalence principle. The observed enhancement of the gravitational attraction is specific to string theory; there is no similar effect for point-particles.

It is curious how physicist use the same majestatic plural as the pope in their papers because the "we" correspond to a single author, Raphael Benichou. He has seven published articles in arxiv -being this the first one in solitary-, in different topics of string theory.

As one can suspect the violation of the principle of equivalence (PE) is only for the string living in the brane (yes, the article treats about a braneworld scenario) and it is conserved in the bulk. Well, I must say that now -reading the discussion of the article- I have just learned that this is not the first stringy paper that raises the question of the violation of the PE. Apparently the novelty is that in the previous results the rupture was based in generic effects and it applied to point particles. The result of the article is purely stringy, it resorts in features of open strings in the brane. Well, fine, but a braneworld scenario is itself very stringy in its nature and I barely can guess a good reason to have such a world if it is not in string theory. Still this effect would be doubly stringy motivated.

The set up of the paper is a toy model using D-brane models that are far away from realistic D-brane phenomenology. I didn't read the article in detail so I don't now how much the separation of the conventional gravitational potential depends on the details. Also I must say that I haven't seen an explicit mention on how weak the effect is and if there is some possibility of seeing it in an actual experiment. The fact is that what is predicted is a slower decay of the gravitational attraction at large distances (in that respect it would be something like a MOND, modified Newtonian dynamics, of the kind used to explain rotations of galaxies without the need of dark matter. I am not sure why the author doesn't say anything about this point in the discussion *). This is different from the usual result in braneword gravitational phenomenology where one looks for an increase of gravity at very small distances due to the fact that the graviton propagates in the bulk a certain very small distance.

Well, my provisional conclusion is that the first article, discussed in arxiv blog, seems to be far from being the first proposal based on "quantum gravity" of a violation of the equivalence principle. It's distinction would be that it makes not use at al of string theory, which depending on tastes could be a pus or a minus. Obviously both approaches are totally different the one of the other. I have just made a very quick view of the non stringy article and I can't comment anything about it. Probably it is easies to read for people with few or null basic in string theory. Inparticular I guess that everyone with a solid basic in QFT and GR could read it safely and take his own conclusions.

I thin that maybe Lubos will say something more about the paper later today. But there are another interesting papers in arxiv so maybe not (I seriously doubt that Distler would say anything). It is a pity that there are not actually more very active blogs by string theorist out there. I believe hat my understanding of string theory has grown enough so that what I say could have some credibility, but still it is sure that there is a lot people out there that could make a better job than me and everyone in the field would beneficiate if some of them would blog in a regular basic.

*UPDATE: Ok, seemingly my knowledge of MONDS was very outdated. Consulting wikipedia entry on MOND I see that the modern form doesn't relay on modifications of newtonian potential. In the same page it is cited an arxiv paper by bekenstein reviewing MOND that could be an interesting reading for anyone wh is curious abut MOND theories. Advise: There are many good reasons to doubt on the validity of MONDs, of course. But at least it would be good t know with certainty what it says and so avoid trivial mistakes ;).

## Monday, June 07, 2010

### In Arxiv today: Supersymmetry breaking

Today there are some interesting papers in arxiv, although not so many as the previous Monday. Some of them are purely mathematical, or concern formal aspects.

I like formal aspects, and mathematics, but because I had some leaks on recent phenomenology I am provisionally interested in more mundane topics. One key aspect in phenomenology is supersymmetry breaking. To begin with the MSSM (minimal supersymmetricla standard model) only can get supersymmetry breaking by means of soft terms which are outside the model itself. These terms must arise by some method that generates them.

The older studied method is gravity mediates supersymmetry breaking. There the breaking os supersymmetry happens at high energies,because of gravity related mechanisms and gets communicate to the MSSM by means of moduli (that is, scalar neutral fields).

Another mechanism is gauge mediate supersymmetry breaking. There supersymmetry breaking happens in a hidden sector (in heterotic E8xE8 string theory typically the "other" E8) and gets transmitted to the visible sector by some gauge field.

Still another method is AMSB (anomaly mediatesd smmetry breaking). And there are a few more. All of theme have their own advantages and problems.

Instead of trying to give details here I will refer the readers to this paper A Comparison of Supersymmetry Breaking and Mediation Mechanisms by S.P. de Alwis where a review of the topic is made. It also includes many links to the key papers in the subject so it is a perfect way to get a proper guide to the subject if because of some reason you can´t or want to read a whole book about the subject. The review also gives references of string theoretical realizations of the mechanism, which is a great plus.

## Tuesday, June 01, 2010

### Heterotic phenomenology

I have talked quite often in this blog about F-theory. This is partially due to "historical" reasons, that is, the F-theory GUT revolution happened recently, while this blog growth. Also the influence of a friend of mine to let learn algebraic geometry was a plus because F-theory relies a lot in that area of maths.
Of course another reason is that they are very good developed framework.

But that doesn't mean that there is not development in other areas of string theory. In particular from the eighties heterotic strings where the best candidate for a phenomenological model. Even today most books in string theory (such as the Becker- Becker-Swhartz one) use the heterotic to teach the math of compactification.

Today it has appeared an interesting paper in heterotic phenomenology so I will say a few things about the subject. Heterotic string/m-Theory models are mainly build by the compactification mechanism. In that aspect they differ from many advances in string theory phenomenology. The fact that a group of parallel N D-branes automatically give an U(n) gauge theory was an star point for building local models in which gravitational degrees of freedom can be ignored for many purposes. That derived in a lot of development of D-brane models in Type II A and type II B string models, and, later, the non-perturbative counterpart of type II-B, F theory, where in addition to D-branes one has (P,q) branes. In fact something is done about non local F-Theory models and some is made for M-theory from the duality among some F-theory and M-theory set ups. But here I am going to talk about the most conventional approach, full compactifications. I am not sure about it, but I think that the reason why not too much development in local M-theory models is because not too much is known for certain (despite the Bagger-Lambert minirevolution of two years ago) about the M-theory branes, although possibly that would apply better to type II A M-theory that to heterotic M-theory.

I am going now with some references. The article of today roots in his model of the year 2006: The Exact MSSM Spectrum from String Theory. I let here the abstract of the paper:

We show the existence of realistic vacua in string theory whose observable sector has exactly the matter content of the MSSM. This is achieved by compactifying the E_8 x E_8 heterotic superstring on a smooth Calabi-Yau threefold with an SU(4) gauge instanton and a Z_3 x Z_3 Wilson line. Specifically, the observable sector is N=1 supersymmetric with gauge group SU(3)_C x SU(2)_L x U(1)_Y x U(1)_{B-L}, three families of quarks and leptons, each family with a right-handed neutrino, and one Higgs-Higgs conjugate pair. Importantly, there are no extra vector-like pairs and no exotic matter in the zero mode spectrum. There are, in addition, 6 geometric moduli and 13 gauge instanton moduli in the observable sector. The holomorphic SU(4) vector bundle of the observable sector is slope-stable.

The observable sector of the theory has an SU(3)C × SU(2)L × U(1)Y × U(1)B−L gauge group. The B-L additional group is beyond the MSSM, but that is not as bad as it seems as they discuss in the paper of today. Additionally they have:

Matter spectrum:
– 3 families of quarks and leptons, each with a right-handed neutrino
– 1 Higgs–Higgs conjugate pair
– No exotic matter fields
– No vector-like pairs (apart from the one Higgs pair)

3 complex structure, 3 K¨ahler, and 13 vector bundle moduli

This sector is obtained by by two steps. First a Spin(10) group can arise from the
spontaneous breaking of the observable sector E8 group by an SU(4) gauge instanton
on an internal Calabi-Yau threefold. Later The Spin(10) group is then broken by discrete Wilson lines to a gauge group containing SU(3)C × SU(2)L × U(1)Y as a factor.

The structure of the hidden sector depends on the choice of a stable, holomorphic
vector bundle V ′. The topology of V ′, that is, its second Chern class, is constrained by two conditions: first, the anomaly cancellation equation:

$$c2(V´) = c2(TX) - c2(V) - [W]$$

Here c2 means the second Chern class of the vector bundle V and [W] is a possible effective five-brane class. Ok, 'll stop writing the details that can be read in the paper. The important part is that they don't obtain in detail the aspects of the hidden sector (the sector of the other E8 group of the E8xE8 heterotic string). They simply assume it's existence.

Since 2006 that model has been further developed and has lead to this paper today: The Mass Spectra, Hierarchy and Cosmology of B-L MSSM Heterotic Compactifications

The two papers even share one co-author, Burt A. Ovrut. The abstract reads:

The matter spectrum of the MSSM, including three right-handed neutrino supermultiplets and one pair of Higgs-Higgs conjugate superfields, can be obtained by compactifying the E_{8} x E_{8} heterotic string and M-theory on Calabi-Yau manifolds with specific SU(4) vector bundles. These theories have the standard model gauge group augmented by an additional gauged U(1)_{B-L}. Their minimal content requires that the B-L gauge symmetry be spontaneously broken by a vacuum expectation value of at least one right-handed sneutrino. In previous papers, we presented the results of a quasi-analytic renormalization group analysis showing that B-L gauge symmetry is indeed radiatively broken with an appropriate B-L/electroweak hierarchy. In this paper, we extend these results by 1) enlarging the initial parameter space and 2) explicitly calculating all renormalization group equations numerically, without approximation. The regions of the initial parameter space leading to realistic vacua are presented and the B-L/electroweak hierarchy computed over these regimes. At representative points, the mass spectrum for all sparticles and Higgs fields is calculated and shown to be consistent with present experimental bounds. Some fundamental phenomenological signatures of a non-zero right-handed sneutrino expectation value are discussed, particularly the cosmology and proton lifetime arising from induced lepton and baryon number violating interactions.

Since the 2006 paper math sophistication has grown and in the way of the theory the have used things such as monads, spectral covers or cohomological methods to calculate the texture of Yukawa couplings and other parameters. The key ingredient is still a Calabi-Yau manifolds with Z3xZ3 homotopy and a vector bundle with SU(4) structure group. The observable matter spectrum is basically the same of the previous paper. As I said before they state that: The existence of the extra U(1)B􀀀L gauge factor, far from being being extraneous or problematical, is precisely what is required to make a heterotic vacuum with SU(4) structure group phenomenologically viable. The reason is the following. As is well-known, four-dimensional N = 1 supersymmetric theories generically contain two lepton number violating and one baryon number violating dimension four operators in the superpotential. The former,
if too large, can create serious cosmological di culties, such as in baryogenesis
and primordial nucleosynthesis , as well as coming into conflict with direct measurements of lepton violating decays.

Well, the details can be read in the paper. The important thing is that the model is mature enough to allow explicit and accurate renormalization group analysis of the effective field theory and do precise predictions of some aspects.

This is not the only line of investigation in heterotic string theory. As I have stated this gives mainly an MSSM but no unification group scheme is followed. But there are such kind of constructions. As early as in 2005 there is a paper doing such a thing from heterotic M-theory: An SU(5) Heterotic Standard Model-

The authors are Vincent Bouchard, Ron Donagi. and the abstract says:

We introduce a new heterotic Standard Model which has precisely the spectrum of the Minimal Supersymmetric Standard Model (MSSM), with no exotic matter. The observable sector has gauge group SU(3) x SU(2) x U(1). Our model is obtained from a compactification of heterotic strings on a Calabi-Yau threefold with Z_2 fundamental group, coupled with an invariant SU(5) bundle. Depending on the region of moduli space in which the model lies, we obtain a spectrum consisting of the three generations of the Standard Model, augmented by 0, 1 or 2 Higgs doublet conjugate pairs. In particular, we get the first compactification involving a heterotic string vacuum (i.e. a {\it stable} bundle) yielding precisely the MSSM with a single pair of Higgs.

If one reads the paper one can see that it cites the papers in heterotic string that are the basic of the other models. This can look a bit surprising since one article is about heterotic string (which has 10 dimensions)and the other about heterotic M-theory (which has 11 dimensions). Nut they actually work with compactifications in a calaby-Yau threfold. That is because the eleventh dimension of the heterotic M-theory has an special character and no compactification of is is done. On the contrary the type II A M-theory has a more conventional eleventh dimensions and it requires compactification on G2 holonomy bundles, which are harder to work. Well, I am far from being an expert in the heterotic phenomenology, but I thought that the today paper was a good occasion to say some things about it.

Besides this paper today there has been many other interesting papers. Fortunately Lubos has written an entry doing a brief comments on them and I prefer link you to that entry to get the info: A generous hep-th Tuesday

Update: In the Lubos entry (where very gently this post is linked, thanks Lubos ;)) the papers about heterotic phenomenology have been discussed and someone posted two papers about G2 heterotic compactifications, concretelly : http://arxiv.org/abs/0810.3285 and http://arxiv.org/abs/0905.1968.

It has also been discussed an issue about the prediction of the paper considered here saying that the mass of the Higgs boson was around 101 - 106 GeV. while the LEP had excluded with a 95% confidence level a Higgs mass minor than 114 GeV. Lubos argues that 95% is not enought to exclude the value f there are goo theoretical reasons to do s. I would add that Jester (resonance) wrote a post stating that the LEP exclusion ny worked for conventional Higgs. I don't remember the details so I can't say if that is relevant, but I recommend the readers of this blog to make a search in resonances.

## Monday, May 10, 2010

### Uber-naturalness

The most interesting paper today n arxiv hep_th is very probably this: Uber-naturalness: unexpectedly light scalars from supersymmetric extra dimensions

This is the abstract:

Standard lore asserts that quantum effects generically forbid the occurrence of light (non-pseudo-Goldstone) scalars having masses smaller than the Kaluza Klein scale, M_KK, in extra-dimensional models, or the gravitino mass, M_3/2, in supersymmetric situations. We argue that a hidden assumption underlies this lore: that the scale of gravitational physics, M_g, (e.g. the string scale, M_s, in string theory) is of order the Planck mass, M_p = 10^18 GeV. We explore sensitivity to this assumption using the spectrum of masses arising within the specific framework of large-volume string compactifications, for which the ultraviolet completion at the gravity scale is explicitly known to be a Type IIB string theory. In such models the separation between M_g and M_p is parameterized by the (large) size of the extra dimensional volume, V (in string units), according to M_p: M_g: M_KK: M_3/2 = 1: V^{-1/2}: V^{-2/3}: V^{-1}. We find that the generic size of quantum corrections to masses is of the order of M_KK M_3/2 / M_p ~ M_p / V^{5/3}. The mass of the lighest modulus (corresponding to the extra-dimensional volume) which at the classical level is M_V ~ M_p/V^{3/2} << M_3/2 << M_KK is thus stable against quantum corrections. This is possible because the couplings of this modulus to other forms of matter in the low-energy theory are generically weaker than gravitational strength (something that is also usually thought not to occur according to standard lore). We discuss some phenomenological and cosmological implications of this observation

One of the good things of this kind of theories is that a combination of supersymmetry and large extra dimensions allow that the gravitino wouldn't be the LSP in the kind of theories where it would usually be it. That is good if the controversial results claiming the existence of dark matter of 10 GeV are confirmed. As far as I know the gavitino couldn't be the particle responsible for that kind of DM. That's bad because the gravitino is the LSP in the Vafa F-theory GUTs. But if the idea of this paper can be translated to F-theory (which after all shares some of the characteristics of the LV theories disused in this paper) it could give them the flexibility to not be immediately ruled out if the DM experiments confirm the discovering.

Obviously the interest of the paper goes beyond that particular purpose of my own concern. It is a theoretic paper of wide interest and not a phenomenologistic paper devoted to a narrow particular subject. Anyway, it is clear that if someone wants to properly appreciate this paper beyond string theory he needs a good familiarity with supersymmetry, as explained in, for example, the book of the previous entry and cosmological issues of dark matter and inflation.

Update: A litle of meditation shows an obvious obstacle to use plainly this constructions inf F-theory, The idea of this paper relies in a variant of gravity mediated supersymmetry breaking. In F-theory supersymmetry is broken bye a variant of the guidice-massiero mechanism that belongs to gauge mediated supersymmetry breaking. Worse, the very idea of F-theory GUTS relies in the gravity decoupling so it seems hard to incorporate this idea in an straightforward way. Ok, I never have claimed to be at all an expert in F-theory or phenomenology. But I fell s if my understanding of this topics is growing fast, maybe in a few centuries I could publish something worthfull ;).

## Friday, April 30, 2010

### Sparticles

Supersymmetry is easy, isn't it?. I have posted (in spanish) a brief introduction to the subject. And I'll make more posts abut it. I am almost sure that everyone who has passed a course in QFT can understand the basics of supersymmetry.

After all, you have a superpartner for every particle, that has opposite statistic. You have funny names for the superpartnerts. The partner of the electron is the slectron, the quark has the squark, etc. And for bosonoics fields the thing is easy also. To the photon corresponds the photino, to the gluons the gluino, to the higgs the higssino, etc.

Well, them if you remember that for every broken global symmetry there is an associated particle, the goldstone boson them you know what a goldstino is. Isn't it?

Well, if you read the book that gives title to the entry(the full title is theory and phenomenology of sparticles) you will realize that maybe the goldstino could not be exactly what you think.

And, of course, you will learn a lot more about phenomenology of N=1 supersymmetry in 4 dimensiones. That includes the MSSM, cosmology of supersymmetry models, etc, etc. The books is from 2004 (reprinted in 2007) so it is, I guess, terribly outdated. I find it a lot better option that the,somewhat similar in it's purpose, book of Michel Dine (supersymmetry and superstrings). Ok, this book doesn't cover superstring, and it has many more pages. But with the actual trend of growing interest in phenomenology, motivated by the dark matter experiments, and the beginning of operation of the LHC it looks as a good reference book to get good info about the characteristics of neutralino, methods of supersymmetry breaking or whatever.

P.S. I'll try o update the previous posts, as promised, as soon as possible.

## Wednesday, April 28, 2010

### Enrique Álvarez on vacuum energy decay

When I studied theoretical physics in the UAM the everybody (well, at least everybody studying he specialty of theoretical physics) favorite teacher was Enrique Álvarez. Possibly, I don't know because I never coincided with him, today Ibañez could discuss the crown, but in that times Ibañez was in CERN and didn't teach.

Particularly great was his course in an asignature devoted to classical mechanics where he explained constrained lagrangian system, and the Dirac way to deal with them. That supposed that I had not any problem in the future understanding quantization of gauge theories, string theories, BRST operators and all that which is a great advantage. Also his course in general relativity was great.

Well, I am happy to see today in arxiv an article of him totally worth to post about: http://arxiv.org/abs/1004.4867

The article makes comments, and insights, on a previous work of Polchinsky the one of the Polchinsky action)in 2007: http://arxiv.org/abs/0709.2899 (I think he talked about that topic in the strings 2008 conferences celebrated in CERN)

The idea is that in de Sitter space the vacuum is not stable in the presence of interactions among particles. I am still reading both articles so I will not comment on them now, but I will edit this post (yes, I promise xD) as soon as I read them and explain what's going on.

But I am sure that some readers of this blog, who also had Álvarez as a teacher, will not wait me and read the article themselves. After all the requisites for reading are mainly some familiarity with basic QFT and quantization on curved spaces. In fact one of the points of this post is to advise them of the existence of the article and so avoid the possibility that they would miss it.

## Friday, April 09, 2010

### Á guide for anonymous LHC backholics

A few days ago it was published on arxiv the following paper: Black hole/string ball production, possibly at LHC

I am busy reading any, many things nowadays and I couldn't read it immediately - despite of it's few number of pages - but today, at last, I read it.

The main interest of the papers is that it makes a quick review of the ideas that lead to the proposal of black hole creation on the LHC and to it's characteristics. Most interestingly still, it gives references to the relevant literature.

One of the focus +of the paper is the way on which the Hawking radiation is expected to behave. It is said that the first expectation was that most of the radiation would be send t Kaluza-Klein models of the gaviton that scape to the bulk. In particular a paper stating that was: A Model for High Energy Scattering in Quantum Gravity (note that the article treats other aspects as well).

Later it was argued that instead there would be a lot of hawking emission into brane particles: Black Holes Radiate Mainly on the Brane . In this paper, like in the others, the treatment is semiclassical. No construction of black out of D-branes is used. Neither Hawking radiation is described by string theory objects. Such descriptions exist, to a certain extent, but are not developed enough to use them in this contexts. In fat an analytic description of a black hole in braneworlds is inexistent even in classical gravity and some reasonable approximations have to be made. This approximations are valid for small black holes whose size is much smaller than the length of the extra dimension.

The amount of radiation send to the bulk or to the brane is important. If all the radiation is send to the brane no Hawking radiation would be observed in the LHC.

Well. I think that this article is citing mainly orthodox string theory friendly papers. Be aware that other people argue that a microcanonical statistical ensemble would be necessary. In that assumptions the lifetime of the back holes could be large enough to leave the detectors (but still there woulld, of course, no danger of earth swallowing black holes).

The theme of black hole emission has behind it a lot of publications and many, many, possibilities have been conjectured. One of that is the possibility of formation of a "cromosphere" or an "electrosphere". By that it is understood that the hawking radiation reorganizes into a permanent orbiting matter that obstruct the rest of the outgoing radiation to some extent. A recent paper in arxiv (sorry, I have no time to search the reference now) argues against it.

Also there have been more exotic ideas. For example Louis Crane abrogate for the possibility of creating black holes by collapsing large clouds of dust, of a few tons, by means of implosions. He argues that the resulting black hole would emit in supersymmetric modes and that the radiation could be modulated to use the black hole as a device to impulsate spaceships able to go into other solar systems. I have discussed this, and the one of microcanonical ensembles, possibility in my other blog. You can search there for them (if you speak Spanish).

As the readers could guess some of the suggestions in the literature have more wide acceptation, possible with very good reasons, that others. And, except for the Louis Crane idea, all depends on braneworld sceneries. Recently there was an article in arxiv that gives a wide sight into realizations of that models into full string theory. When I would have read it I'll try to summarize some of the proposals here.

UPDATE: Today Lubos has commented about an article treating black holes: Kerr black hole: the CFT entropy works for all M,J. Of course it is worth reading.

## Tuesday, March 30, 2010

### LHC live webcast

Today at 8 30 it was expected that the LHC would make the first collisions at 7 TeV. There has been some problem and one of the beans has been lost. It is expected that around 11 a second try would be made.

You can follow the events in real time in the live webcast

## Thursday, March 18, 2010

### Strings 2010

I have been reading in the internet about the strings 2010, which this year has place in Texas, for a while. But I had not paid too much attention because I thought that, as usually, the event would be in summer.

Today I have been shocked when I saw a quick note in U duality blog saying that the conferences have been going this week.

This wee I was not particularly busy and if I just would have read carefully the dates I could have seen the livecast (available here).

Well, I still can see the slides of the talks of the past three days and the remaining ones (if the livecast works properly).

Well, at least I can see that still are very important people pending so it still there is some fun.

The most amazing of this is the total absence of mentions to the conferences in the everybody's favorite string theory superhero. Seemingly some global warming supervillian has taken him out from his usual missions ;).

Other than this in the meantime from the last post there has been an announcement about possible dark matter detection (you can read it in resonances, shores of the Dirac sea and , the older entry, in Lubos blog. I am planing to write my own entry, in Spanish, in my other blog).

Also Smoot et all published their second paper applying, as announced, their idea of an entropic force due to CEH(cosmic event horizons)to inflation. I'll post about it sometime in the near future. In particular I'll see if I can say something on how that idea could work together with dynamical horizons formalism for black holes and if it could somewhat influence the picture of black hole evaporation.

Of course there have been other interesting papers. For example a few authors took the F-theory GUT's strategy (that is, phenomenology using local models) to construct an MSSM from F-theory. Certainly it doesn't look that great compared with GUT, but if the dark matter detection is true, and the detected particle is something similar to a neutralino that MSSM F-theoretic scenery could be most easily compatible with that discovery than the GUT. As far as I have been able to think about it one possibility- taken account that in a certain sense F-theory GUTS are a universal extra dimension for the gauge bosons, that is, they live in a 7-brane- would be to seek Kaluza-Klein dark matter. The problem is that the involved gauge bosons are the unification ones and not the electroweaks,as it would be required for viable light dark matter. Well, maybe there are workarounds.

Also I wonder how the Smoot proposal, if admitted truth,could relax the conditions of the Vafa GUT (some of them are related to cosmology) to allow a better fit with a possible neutralino. In fact I would like to be able to analyze that kind of questions myself, but I still need to learn more in detail these questions.A good thing is that since I have the tablet PC I am advancing a lot faster. The point is that these, and for sure others, questions are worth of investigation (they are very close to measurable phenomena). As fr as I see they are much more realistic lines of research that the Verlinde proposal, which still has a good echo in arxiv. Possibly the ultimate reason behind it is not the quality and viability of the ideas but simple the difficulty of them. Easier research have more followers, simply because they offer more published papers, and them academic positions.

That is not exclusive of physic. A friend of me told me that the same thing happens in maths. Well, I understand that this thing happen. But maybe it would be good for science if the academy wouldn't be the only way to go. Or at least if it wouldn't be so burocrathic and there would be intermediate ways in which people could do some paid work in high end physics, at least sometimes, proved that the work is reasonably good. Well, I know it is not going to happen so it doesn't worth to loose time with the idea.

P.S. I'll edit the entry to ad the lacking links when I have a while for it.

## Wednesday, February 24, 2010

### Entropic force goes nobel

Yeahm I know it is a somewhat misleading title ;).What I am meaning is that a nobel laureate -and a very famouse one, the cosmologist Geogre F. Smoot- wrote a paper (in collaboration with other people) in the subject of entropic force.

It is this: Entropic Accelerating Universe

This is the abstract:

To accommodate the observed accelerated expansion of the universe, one popu-
lar idea is to invoke a driving term in the Friedmann-Lema^ tre equation of dark
energy which must then comprise 70% of the present cosmological energy density.
We propose an alternative interpretation which takes into account the temperature
intrinsic to the information holographically stored on the screen which is the surface
of the universe. Dark energy is thereby obviated and the acceleration is due to an
entropic force naturally arising from the information storage on a surface screen.
We consider an additional quantitative approach based upon the entropy and sur-
face terms usually neglected in General Relativity and show that this leads to the
entropic accelerating universe

Certainly the claim is very ambitious. The authors pretend to explain the observed accelerated universe in terms of the entropic force. If truth it could perfectly be a noble prize winning article.

Certainly it is surprising to see such a respected physicist going into the risk of using a so criticized idea (the entropic force -as an origin of gravity- has received a lot of critics in the blogosphere and also some arxiv papers argumenting against it). Even thought reading the aper one sees that there are a few differences between the assumptions of Verlinde and the ones in this paper.

This authors accept the validity of general relativity. They don't say that is is a consequence of an entropic force in a holographic screen. What they do, instead, is to modify slightly the deduction of the Einstein equation from a conventional variational principle. Their only modifications goes into the usually discarded boundary surface term. It is there where they insert an "holographic screen entropic force" (I guess that this is the kind of terminology that a marketing department would suggest for a mass media successful theory).

It is the boundary term which is responsible for the accelerating universe. The content of the holographic entropic term is, according to the prescriptions of Verlinde, proportional to an Unrhu temperature to an horizon. In that point the authors make a discussion of possibilities of choice (the other alternatives being a Hawking or a de Sitter temperature).

The next section uses a purely entropic force argument to calculate the pressure due to the changes of entropy and the result agree with that of the dark energy.

The concluding section stablish definitively that the entropic force fits the data and wonders about further compatibilities, to be analysed in the future, of the idea related to big bang nucleosinthesis, cyclic models and inflation.

Well, certainly this paper relax the assumptions of Verlinde- which totally break with Einstein gravity- and with that relatively weak change they explain one of the mos puzzling issues of the actual cosmology. I think that still the introduction of the unruh temperature in the surface term is somewhat ad hoc (or at least that it would need further clarification). But the payback of it is very impressive.

Anyway, I am almost sure that this paper is going to be heavily discussed by the usual suspects so it is a good idea for the readers of this entry to keep an eye on them.

B.T.W. for today I had the idea to discus a line of papers which discuss how to extend the Higgs mechanism to gravity. he foundational paper of that topic seems to go back to t' Hooft and this month there have been two papers on the subject. Previously to find that recent papers I had wondered myself about that possibility (considering it with a mood of "what if?"), but clearly I arrived too late and another people have worked in that idea already. Still I think is a nice topic to read about and I will tell something about it in the future.

P.S. If I have to make a guess and it is accepted that the main content of this explanation for dark energy would be the anthropic arguments based on the landscape of string theory the idea of the paper discussed here looks better. As a plus, accepting general relativity it is not obvious that it is contrary to string theory. In fact possibly this article would rise a motivation to discuss "microscopic" implementations of the entropic force idea, as presented in this paper. Or I can be totally wrong. Certainly I had not thought too much about this topic before this paper, maybe it is time to consider it serioulsly.

P.S. 2 A second good point that I see is that the cosmological constant (or any variant explanation for dark energy) is a priori as ad hoc as an "entropic force", s introduced by these authors. And certainly it looks a more elegant solution. Sill I am reticent in some aspects,some kind of circularity "de sitter=>horizont=> temperature=>entropic force=>de sitter", but time, and further thinking, will say.

## Monday, February 15, 2010

### Let the entropy be with you

This new year "sensation" has been a paper by the well known string theorist Erick Verlinde about a possible entropic force like origin of gravity. The actual paper is this.

The article has been extensively discussed in the blogosphere and I will not add too much about it. The main reason of publishing about the topic is that I commented to a friend it's existence and he asked me for the link. A secondary reason is the possibility to paraphrase Obi Wan Kennoby (or possibly it's master, whose role was played by an actor who actually had a degree in Physics, which of course was the reason because he was a much better jedi that Obi one ; ).

About the paper itself all I can say is that the first paper about Physics was one by the two Verinde Brothers in the topic of Planckian scatering. This about entropic force is the first one I readed in the archos tabletPC that I comment in here. T wirte it I am using a B-move trackball remote mouse + keyboard which is wonderful way to get total control of your PC from a comfortable distance.

By the way, anyone with an undergraduate knowledge of physics which includes statistical mechanics and general relativity and who hs read about string theory in divulgaive books such as the elegant universe can read and understand the Verlinde paper by himself and make his own judgement of it's merits so n advise s needed.

## Tuesday, January 19, 2010

### Physics everywhere

In science fiction novels of not too long ago it was presented the idea of porting a whole library of textbooks in a pocket computer.

Well, that is possible now. One option are the smartphones, specially the incoming new generation with screens of up to 4 inches or even greater. In today phones it is also possible, but possible the screens are too small to do a decent reading and they are useful more like a way to consult a reference in a given moment that to do regular lectures. There are some workarounds that work for plain text, but are not too good for physics/math pdfs. Also you will need to be sure to know what software to get. For example for android operative systems the only decent way seems to be to go to the document to go suit. For windows mobile, Symbian, or even the Ipne OS there are wider variety of options.

But even with the incoming generation of mobiles (which are lovely anyway because of the many, many features they have)it is better to search for more comfortable options.

In this sense there are to main ways, the internet tablets and the tablet PCs. In fact the different among them is not too clear (possible the internet tablets are smaller). This year, possible inspired by the rumour of an "apple tablet" there are many, many new tablet PCs in the way, covering different ranges of prices and features.

I have decided not to wait and I have purchased one of the first available ones, an archos 9

The name makes reference to the 9 inches of the screen size. The OS is windows 7. There in the market products with 7 inches screens, but I think that for a comfortable reading 9 inches is the minimum size. Also influenced me that the archos 7 (or archos 5 for what matters) used android, which is a baby OS.

So now I have in a device of a size and weight of an slim pocket book all the reference material I could wise (in pdf or djvu format), and I even can run mathematica (I prefer it over matlab). And of course I can do all the other things that one can do with a normal computer. Ok, the performance is worst that in a desktop computer) and a touch screen is not as good (for my taste) as a regular mouse and keyboard. But still it is lovely the possibility of doing physics everywhere. And, anyway, it is not a good idea to charge that computers with too many programs if one wants a minimally decent perfomance.

Well, for sure that has been already possible since a few year with regular notebooks and ultra small notebooks/netbooks, but I think that the tablet PCs make an actual difference. It is definitively fine to keep reading the same arxiv pdf you were reading in your home while you are in the bus or the underground (without the need to fill your house with too many printed papers).

By the way, an important tip. Most pdfs have wide margins that make their reading in a not too big screen awkward (the zoom options of the reading programs not always solve that problem in a the satisfactory way one would expect). It is a very good idea to get a program to crop the pdfs such as Nitro pdf or pdffills. Of course these programs have many other possibilities that just cropping pdfs, but possible that will be the one you will use the most (the second one could, maybe, create pdfs scanning your favourite books).

Well, the technology gives us great possibilities. Lets hope this decade would be better for physics that the past one that, I coincide with Jester (resonances), is possibly the worst in hundreds of years.