About LHC Progress

There are rumours appearing here and there about further imminent delays of the LHC start-up. As for the facts, two weeks ago Lyn Evans, who is the LHC project leader, gave a colloquium about the current status of the LHC. He did not mention any delays, but he described in great detail the efforts they are currently undertaking and the problems that have emerged. The video recording of this talk is available here. In particular, at 52:35 Lyn devotes some time to the plug-in modules between magnet interconnects, whose faults spawned the recent rumours. While I personally don't understand why can't they tie up the magnets with strings (or superstrings in the case of superconducting magnets), i guess those more experimentally oriented may get some insight. Anyway, experts advise not to get excited with this particular problem; there will be many others. As Lyn himself put it, at this point we are far less deterministic.

Deep Throats and Phase Transitions

Both of my readers have expressed their concern about my lack of activity in the last weeks. OK, let's say i wasn't in mood and go back to business.

Last week John March-Russell gave a seminar entitled Throats with Faster Holographic Phase Transitions. This sounds very encouraging to stay for another coffee in the cafeteria. This time, however, the first intuition would be wrong, as behind this awkward title hides an interesting and less studied piece of physics.

The story is about the Randall-Sundrum model (RS1, to be precise): five-dimensional theory in approximately AdS5 space cut off by the Planck and the TeV branes. The question is what happens with this set-up at high temperatures. There is a point of view from which the high-temperature phase can be simply understood. Here at CERN the local folks believe that the Randall-Sundrum set-up is a dual description of a normal (though strongly coupled) gauge theory in four dimensions. Therefore at high temperatures such phenomena as deconfinement or the emergence of a gluon plasma should be expected. How this phase transition manifests itself in the 5D description?

This last question was studied several years ago in a paper by Creminelli et al based on earlier results by Witten. It turns out that one can write down another solution of the Einstein equations that describes a black hole in the Ads5 space. The black hole solution is a dual description of the high-temperature deconfined phase: the TeV brane (whose presence implies the existence of a mass gap in the low-temperature phase) is hidden behind the black hole horizon.

Which of the two solutions dominates, that is to say, which one gives the dominant contribution to the path integral depends on the free energy F = E- T S. One can calculate that at zero temperature the RS1 solution has lower free energy. But the black hole solution has entropy associated with the black hole horizon and its free energy ends up being lower at high enough temperature. This black hole solution effectively describes a high-temperature expanding universe filled with a hot gluon plasma. As the temperature goes down to the critical value, the RS1 solution with a TeV brane becomes energetically more favorable and a first order phase transition occurs.

Creminelli et al computed the critical temperature at which free energies of the two phases are equal. They also estimated the rate of phase transition between the black hole and the RS1 phases. It turns out that, with the assumption they made about the mechanism stabilizing the fifth dimension, the rate is too low so that the phase transition could never be completed. The universe expands too fast and, although bubbles of the RS1 phase do form, they do not collide. One ends up with an empty ever-inflating universe. From this analysis it seems that, if RS1 is to describe the real world, the temperature of the universe should never exceed the critical one. Although this assumption does not contradict any observations, it makes life more problematic (how to incorporate inflation, bariogenesis...)

According to John, the problem with too slow phase transitions is not general but specific to the stabilization mechanism assumed by Creminelli et al. In his recent paper, John studied a modified version of RS1 - a string-inspired set-up called the Klebanov-Tseytlin throat. From the picture it is obvious that the Klebanov-Tseytlin throat is dual to a punctured condom. John found that in this modifed set-up the phase transition is fast enough to complete. The key to the success seems to be the fact that the different stabilization mechanism results
in a strong breaking of conformal symmetry in IR.

So much for now, more details in the paper. I think this subject is worth knowing about. It connects various areas of physics and cosmology and does not seem to be fully explored yet. First order phase transitions, like the one in RS1, may also leave observable imprints in the gravity waves spectrum, as discussed here.

Slides available.

Drowning the Hierarchy Problem

For a change, the third week of the New Physics workshop turned out to be very interesting. In this post I tell you about Gia Dvali and his brand new idea of solving the hierarchy problem. Several other talks last week deserves attention and I hope to find more time to write about it.

Gia first argued for the following result. Suppose there exists N particle species whose mass is of order M. Further suppose that these species transform under exact gauged discrete symmetries. Then there is a lower bound on the Planck scale:

$M_p > N^{1/2} M$

The proof goes via black holes. As argued in the old paper by Krauss and Wilczek, gauged discrete symmetries should be respected by quantum gravity. Therefore, if we make a black hole out of particles charged under a gauged discrete symmetry, the total charge will be conserved. For example, take a very large number N of particles, each carrying a separate Z2 charge. Form a black hole using an odd number of particles from each species, so that the black hole carries a Z2^N charge. Then wait and see what happens. According to Hawking, the black hole should evaporate. But it cannot emit the charged particles and reduce its charge before its temperature becomes of order M. The relation between the black hole temperature and mass goes like $T \sim M_p^2/M_{BH}$. Thus, by the time the charge starts to be emitted, the black hole mass is reduced to $M_{BH} = M_p^2/M$. To get rid of all its charge the black hole must emit at least N particles of mass M, so its mass at this point must satisfy $M_{BH} > N M$. From this you easily obtain Gia's bound.

The bound has several interesting consequences. One is that it can be used to drown the hierarchy problem in the multitude of new particles. Just assume there exists something like 10^32 new charged particle species at the TeV scale. If that is the case, the Planck scale cannot help being 16 orders of magnitude higher than the TeV scale. For consistency, gravity must somehow become strongly interacting at the TeV scale, much as in the ADD or RS model, so that the perturbative contributions to the Higgs mass are cut off at the TeV scale. Thus, in Gia's scenario the LHC should also observe the signatures of strongly interacting gravity.

You might say this sounds crazy...and certainly it does. But, in fact, the idea is not more crazy than the large extra dimensions of the ADD model. The latter is also an example of many-species solution to the hierarchy problem. In that case there are also 10^32 degrees of freedom - the Kaluza-Klein modes of the graviton, which make gravity strongly interacting at TeV. The difference is that most of the new particles is much lighter than TeV, which creates all sorts of cosmological and astrophysical problems. In the present case these problems can be more readily circumvented.

Transparencies available on the workshop page.

LHC: The First Year

The New Physics workshop is at its peak. The TH seminar room is cracking in its seams and there are at least two talks every day. Unfortunately, the theory talks last week were ranging from not-so-exctiting to pathetic. Therefore I clench my teeth and report on a talk by an experimentalist. Fabiola Gianotti was talking about the ATLAS status and plans for physics with first data.

Experimentalists (much as happy families) are all alike. They can never resist showing us a hundred spectacular pictures of their cherished detectors and another hundred showing the tunnel at sunrise and sunset. They feel obliged to inform how many kilometers of cable was wasted for each particular part of a detector. They stuff each cm^2 of the transparancies with equally indispensable pieces of information. Usually, this leaves little space for interesting physics. This time, however, was slightly different. Having finished with the pictures, Fabiola told us several interesting things about early physics with the ATLAS detector.

ATLAS is already alive, kicking and collecting data from cosmic-ray muons. The LHC will go online in Spring 2008. That is to say, if all goes smoothly, if nothing else explodes and if Holger Nielsen refrains from pulling the cards. In the unlikely case of everything going as planned, the first collisions at 14 TeV will take place in July. The plan for ATLAS is to collect a hundred inverse picobarn of data by the end of the year, and a few inverse femtobarn by the end of 2009.

The main task in 2008 will be to discover the Standard Model. 100/pb translates roughly to 10^6 W bosons and to 10^5 Z boson. The decays of W and Z have been precisely measured before, so this sample can be used for callibrating the detectors. We will also see some 10^4 top-antitop pairs with fully- or semi-leptonic decays. Thus, the top will be detected on european soil and we will know for sure that the Tevatron didnt make it up. In the first year, however, the top samples will serve no purpose other than callibration. For example, the top mass resolution will be of order 10 GeV (the ultimate precision is 1 GeV but this is a song of the future), far worse than the current Tevatron sensitivity. Apart from that, the QCD jets background at high pT will be measured, something notoriously difficult to estimate theoretically. In short, 2008 studies will be boring but necessary to prepare the stage for future discoveries.

Is there any hope for a discovery in 2008? Fabiola pointed out the case of a 1 TeV vector resonance decaying to an electron pair. This would stand out like a lamppost over the small and well-understood Drell-Yann background and could be discovered with as little as 70/pb of data. The problem is that LEP and LEP2 put an indirect constraint on the mass of such a resonance to be larger than a few TeV. So not much hope for that.

Another hope for an early discovery is supersymmetry with its multitude of coloured particles. From the plot we can see that a gluino lighter than 1.5 TeV could be discovered with 100/pb of data. A quick discovery would be important, as it would set the green light for the ILC project. The problem in this case is that a discovery requires a good understanding of the missing energy spectra. Most likely, we will have to wait till 2009.

The higgs boson is a more difficult case. From the plot below you can see that there is no way to see anything at 100/pb. However, with a few inverse picobarns the whole interesting range of higgs masses will be covered. Thus, according to Fabiola, the higgs puzzle should be resolved by the end of 2009.

Last thing worth mentioning is the ongoing effort to visualize the huge kinetic energy that will be stored in the LHC beam. This time the energy was compared to that of a British aircraft carrier running at 12 knots. The bottom line is the following. If you spot an aircraft carrier on Lac Leman, don't panic, it's just the LHC that lost the beam.

The slides are available via the workshop page here.

Restoration of the Fourth Generation

I'm not as successful as the others in spreading rumours. So i'm sadly returning to writing about things i've seen with my very eyes. Last week there has been several talks that would deserve a post. I pick up perhaps not the most interesting but the most comprehensible one: Graham Kribs talking about Four Generations and Higgs Physics.
There are three generations of quarks and leptons. Everyone knows that the fourth one does not exist. The Bible says it is excluded at 99.999% confidence level. Graham is spreading the heresy of claiming otherwise. He adds the 4th generation of chiral matter to the Standard Model and reanalyses the constraints on the parameters of such a model.

First of all, there are limits from direct searches. The LEPII experiment set the lower limit of roughly 100 GeV on the masses of the 4th electron and neutrino. The bounds on the 4th generation quarks from the Tevatron are more stringent. CDF excludes the 4th up-type quark lighter than 256 GeV, and Graham argues that a similar bound should hold for a down-type quark.

Indirect limits from flavour physics can be taken care of by asuming that the mixing between the three and the fourth generations is small enough. More troubling are the contraints from the electroweak precision tests. For example, the new quark doublet contributes to the S paramater:
$\Delta S = \frac{N_c}{6 \pi} \left ( 1 - 2 Y \log \frac{m_u^2}{m_d^2} \right )$
If the 4th generation quarks are degenarate in mass, the S parameter comes out too large. Splitting the masses could help keeping the S parameter under control, though it generates new contributions to the T parameter. Nevertheless, Graham argues that there is a large enough window, with the up-type quark some 50 GeV heavier than the down-type quark, where all the precision constraints are satisfied.

The fourth generation may be discovered by direct searches at the Tevatron or at the LHC. But its most dramatic effect would be a profound modification of the higgs physics. Within the Standard Model, the higgs is produced in a hadron collider dominantly via the gluon fusion:
The particle in the loop is the top quark - the only coloured particle in the Standard Model with a sizable coupling to the higgs boson. With the 4th generation at hand, we have two more quarks strongly coupled to the higgs boson. As a result, the higgs production cross section dramatically increases, roughly by the factor of 9. The first hint of the 4th generation could come from the Tevatron who would see the higgs with an abnormally large cross section. In fact, the Tevatron has already excluded the 4th generation scenario for a range of the higgs boson mass (see the latest higgs exclusion plot here).

The slides from this and other talks can be found here. If interested in more details, look at the recent paper of Graham. While reading, try not to forget that the 4th generation does not exist.

Model Building Month

Today the TH Institute "New Physics and the LHC" kicks off here at CERN. As the organizers put it, it will be mostly targeted to model-builders, with the participation of string theorists and collider phenomenologists. Regardless of that looming participation, the program looks quite promising. After the dog days of summer you can count on more activity in this blog in the coming weeks.

Also today, on the other end of the world began Lepton-Photon'07. I wouldn't bother to mention if not for certain rumours. Rumours of rumours, so to say. I mean the rumours of the alleged non-standard higgs signal at the Tevatron whispered from blog to blog since May. The new rumour is that the relevant multi-b channel analysis will be finally presented at that conference. In spite of the fact that there are neither photons nor leptons in the signal :-)



Update: well....nothing like that seems to have happened. My deep throat turned out shallow. So we are left to rumours and gossips for some more time....
Update #2: Finally, only CDF presented a new analysis of that channel. Their excess is 1.5 only sigma. Details in this public note, a wrap-up by Tommaso here. Now we're waiting for D0. Come on guys, dont be ashamed...

Entropic Principle

I'm back. Just in time to report on the curious TH seminar last Wednesday. Jim Cline was talking about The entropic approach to understanding the cosmological constant. I am lucky to live in a special moment of history and to observe fascinating sociological processes going on in particle physics. I'm dying to see where this will lead us...

Everyone knows about the anthropic principle. In short, certain seemingly fundamental parameters are assumed to be enviromental variables that take random values in different parts of the universe. We can live only in those corners where the laws of physics support more or less intelligent life. This could explain why the values of certain physical parameters appear fine-tuned.

One undeniable success of this approach is Weinberg's prediction concerning the cosmological constant. Weinberg assumed that intelligent life needs galaxies. Keeping all other cosmological parameters fixed, galaxies would not form if the energy density in the cosmological constant exceeded that in matter today by roughly a factor of 5000. If all values of the cosmological constant are equally probable we should observe a value close to the upper limit. Weinberg's explanation points to a value some 1oo0 times larger than the observed one. However it remains attractive, lacking any fundamental understanding of the smallness of the cosmological constant.

Weinberg's paper appeared in 1987, well before the first indications for accelerated expansion from supernovae started to show up. That was a prediction in the common-sense use of this word. Since then, the anthropic principle was mostly employed to predict things we already know. The successful applications include the electroweak breaking scale, the dark matter abundance and the CMB temperature. As for the things we don't know, predictions based on the anthropic principle turn out to be less sharp. For example, the supersymmetry breaking scale is predicted by anthropic reasoning to be at a TeV, or at the Planck scale, or somewhere in between.

The entropic principle, proposed earlier this year by Bousso et al, is a new twist in this merry game. The idea is to replace the selection criterion based on the existence of intelligent life with something more objective. Bousso et al argue that the right criterion is maximizing the entropy production in a causal diamond. The argument is that life of any sort cannot exist in a thermal equilibrium but requires free energy. They argue that free energy divided by the temperature at which it is radiated (that is the entropy increase) is a good measure of a complexity that may arise in a given spacetime region. They proceed to calculating the entropy production (dominated by infrared radiation of dust heated by starlight) for various values of the cosmological constant and find that it is peaked close to the observed value. This allows them to conclude that ...this result improves our confidence in the entropic principle and the underlying landscape.... More details on this computation can be found in the paper or in this post on Reference Frame.

Jim in his talk reviewed all that minus the sneer. He also played his own part in the game. He predicted the primordial density contrast. His conclusion is that the observed value 10^(-5) is not unlikely, which further improves our confidence in the entropic principle and the underlying landscape. Pushing the idea to the edge of absurd, he also made an attempt to predict the dark matter abundance. He took an obscure model of gravitino dark matter with non-conserved R-parity. In this model, the dark matter particles decay, thus producing entropy. He argued that the entropy production is maximized close to the observed value of the dark matter abundance, which further improves our confidence in the entropic principle and the underlying landscape. I guess i missed something here. The observed universe is as we see it because the entropy production from the synchrotron radiation of the dark matter decay products may support some odd forms od life? At this point, pulling cards does not seem such a weird idea anymore...