Tuesday 25 November 2008


Several interesting facts concerning the LHC are available on the slides from a recent talk of Jörg Wenninger. The talk describes in some detail the events between the glorious first beam on September 10 and the fatal accident on September 19. It explains what caused the accident, describes the steps already made to avoid similar problems in the future, and presents options for the repair schedule.

For those less interested in technical details, this page is the most relevant one:

In short, the operation will be restarted in late summer 2009 if they decide not to implement the full safety upgrade program. In that case, probably, both the energy and the luminosity will be smaller than previously assumed. On the other hand, if they decide for a full upgrade of the pressure relief system (which implies heating up all sectors) there will probably be no beam in 2009.

Thursday 20 November 2008


Edward Witten appeared here at CERN about the time of the first LHC beam. The coincidence suggests that he might have been created in a particle collision. That is however unlikely, since the entropy of one Witten is huge, even larger than that of a dragon (not to mention the fact that there were never any collisions at the LHC). In view of that, a more plausible explanation is that Edward is spending his sabbatical at CERN. My allusion to dragons was not entirely off-topic though, because Edward's presence seems to provoke awe and fear among the local string folk. I haven't yet quite discoverd why, but the story goes that around lunchtime each day they lock their offices, close the shutters, and hide in fireproof drawers.

Anyway, people out there don't want to know how Witten is doing, but what he is doing, so back to work... Last Tuesday Edward gave a talk entitled M2-Branes With Half Supersymmetry. The topic is far beyond my expertise and you should not expect any insight from me. I will try to summarize the main points, although it feels like reciting Bhagavad Gita in original.

Edward considers the 11-dimensional M-theory in the background of AdS4 x S7/$Z_n x Z_m$, which can be obtained as the near horizon geometry of a stack of M2 branes. This background preserves half of the original supersymmetry which corresponds to N=4 supersymmetry in 4D. The two discrete orbifold symmetries act on separate SO(4) components of the SO(8) symmetry group of S7. There are two orbifold fixed points who are $A_{n-1}$ and $A_{m-1}$ singularities on which $SU(n)$ and $SU(m)$ gauge theories live. Sitting on the Zn fixed point we see the SU(n) gauge theory in AdS4 x S3/$Z_m$ and, analogously, in the Zm fixed point we see the SU(m) gauge theory in AdS4 x S3/$Z_n$.

Edward argues that there are several interesting facts about this set-up:

  • The theory has a huge landscape of vacua that can be parametrized by elements x of SU(n) satisfying the condition $x^m = 1$ (because of the orbifolding) and elements y of SU(m) satisfying $y^n = 1$. There are $(\stackrel{n+m-1}{n}) \cdot (\stackrel{n+m-1}{m})$ such elements, so that the number of vacua grows factorially with n and m. It is surprising that so many vacua are encountered in a set-up with such a large amount of supersymmetry.
  • One can view the M2 branes as SU(m) instantons on $R_4/Z_n$ or, equivalently, as SU(n) instantons on $R_4/Z_m$. For some reason, the former point of view is called the Higgs branch, while the latter is called the Coulomb branch.
  • String theorists have their ways to count the number of instantons via D-brane configurations sitting at an orbifold point and the effective description in terms of quiver theories. Here, the quiver diagram for the Higgs branch contains the chain $SU(m) -> SU(m_0) x ... x SU(m_{n-1})$ and $U(p) -> U(p_0) x ... x U(p_{n-1})$ linked by bi-fundamental matter, where $p$ is the number of SU(m) instantons. Similarly, the Coulomb branch has the quiver with $SU(n) -> SU(n_0) x ... x SU(n_{m-1})$ and $U(\bar p) -> U(\bar p_0) x ... x U(\bar p_{n-1})$.
  • The integers m and n have a clear M-theory interpretation but the numbers of instantons p and $\bar p$ do not. But Gaiotto and Witten recently demonstrated the existence of a mirror symmetry that relates n and p, and also m and $\bar p$. This mirror symmetry allows one to describe both Higgs and Coulomb branches of M-theory.

This is it. I did not attempt to explain the physics but just to give a flavor of what Edward is brooding on these days. And don't ask me about the applications. God knows.

Sunday 16 November 2008

Hitchhiker's Guide to Ghosts and Spooks in Particle Physics

On Halloween this year the CDF collaboration at Fermilab's Tevatron announced the presence of ghosts in their detector. And not just one meager Poltergeist rattling his chain, but a whole hundred-thousand army. As for today, the ghosts could not be exorcised by systematical effects. While waiting for theorists to incorporate the ghosts into their favorite models of new physics it is good to know that the CDF anomaly is by no means the only puzzling experimental result in our field. There are other ghosts at large: I guess most of them are due to unknown systematical errors, but some may well be due to new physics. Below I pick up a few anomalous results in subjective order of relevance. The list is not exhaustive - you are welcome to complain about any missing item.

So, off we go. In this post I restrict to collider experiments, leaving astrophysics for the subsequent post.

Muon Anomalous Magnetic Moment

This experimental result is very often presented as a hint to physics beyond the Standard Model. For less oriented: there is nothing anomalous in the anomalous magnetic moment itself - it is a well-understood quantum effect that is attributed to virtual particles. But in the muon case, theoretical predictions slightly disagree with experiment. The E821 experiment in Brookhaven measured $a_\mu = (11 659 208 \pm 6)\cdot 10^{-10}$. The Standard Model accounts for all but $28\cdot 10^{-10}$ of the above, which represents a 3.4 sigma discrepancy.

The discrepancy can be readily explained by new physics, for example by low-energy supersymmetry or by new light gauge bosons mixing with the photon. But there is one tiny little subtlety. The Standard Model prediction depends on low-energy QCD contributions to the photon propagator that cannot be calculated from first principles. Instead, one has to use some experimental input that can be related to the photon propagator using black magic and dispersion relations. Now, the discrepancy between theory and experiment depends on whether one use the low-energy e+e- annihilation or the tau decays as the experimental input. The quoted 3.4 sigma arises when the electron data are used, whereas the discrepancy practically disappears when the tau data are used. It means that some experimental data are wrong, or some theoretical methods employed are wrong, or both.

In near future, a certain measurement may help to resolve the puzzle. The troublesome QCD contribution can be extracted from a process studied in BaBar, in which a photon decays into two pions (+ initial state radiation). There are rumors that the preliminary BaBar results point to a larger QCD contribution (consistent with the tau data). This would eradicate the long-standing discrepancy of the muon anomalous magnetic moment. But, at the same time, it would imply that there is a flaw in the e+e- annihilation data, which would affect other measurements too. Most notably, the electron data are used as an input in determining the hadronic contribution to the electromagnetic coupling, which is one of the key inputs in fitting the Standard Model parameters from electroweak observables. As pointed out in this paper, if the low-energy QCD contribution where larger than implied by the electron data, the central value of the fitted Higgs boson mass would decrease. Currently, the electroweak fit determines the Higgs boson mass as $77^{+28}{}_{-22}$, which is already uncomortable with the 114 GeV direct search limit. Larger QCD contributions consisent with the tau data would increase this tension. Interesting times ahead.

Forward-Backward Asymmetry

CERN's LEP experiment has been desperately successful: it beautifully confirmed all theoretical predictions of the Standard Model. The mote in the eye is called $A_{fb}^b$: the forward-backward asymmetry in decays of the Z-boson into the b-quarks. This observable measures the asymmetry in the Z boson interactions with left-handed b-quarks and right-handed ones. The results from LEP and SLD led to a determination of $A_{FB}^b$ that deviates 3 sigma from the Standard Model prediction. On the other hand, the total decay width of the Z-boson into the b-quarks (summarized in the so-called Rb) seems to be in a good agreement with theoretical predictions.

One possible interpretation of these two facts is that the coupling of the Z-boson to the right-handed b-quarks deviates from the Standard Model, while the left-handed coupling (who dominates the measurement of Rb) agrees with the Standard Model. At first sight this smells like tasty new physics - the Zbb coupling is modified in many extensions of the Standard Model. In practice, it is not straightforward (though not impossible) to find a well-motivated model that fits the data. For example, typical Higgsless or Randall-Sundrum models predict large corrections to the left-handed b-quark couplings, and smaller corrections to the right-handed b-quark couplings, contrary to what is suggested by the electroweak observables.

Maybe this discrepancy is just a fluke, or maybe this particular measurement suffers from some systematic error that was not taken into account by experimentalists. But the funny thing is that this measurement is usually included in the fit of the Standard Model parameters to the electroweak observables because...it saves the Standard Model. If $A_{FB}^b$ was removed from the electroweak fit, the central value of the Higgs boson would go down, leading to a large tension with the 114 GeV direct search limit.

Bs Meson Mixing Phase

The results from BaBar and Belle led to one Nobel prize and zero surprises. This was disappointing, because flavor-changing processes studied in these B-factories are, in principle, very sensitive to new physics. New physics in sd transitions (kaon mixing) and bd transitions is now tightly constrained. On the other hand, bs transitions are less constrained, basically because the B-factories were not producing Bs mesons. This gap is being filled by the Tevatron who has enough energy to produce Bs mesons and study its decays to J/psi. In particular, the mass difference of the two Bs eigenstates was measured and a constraint on the phase of the mixing could be obtained. The latter measurement showed some deviation from the Standard Model prediction, but by itself it was not statistically significant.

Later in the day, the UTfit collaboration combined the Bs meson data with all other flavor data. Their claim is the Bs mixing phase deviates from the Standard Model prediction at the 3 sigma level. This could be a manifestation of new physics, though it is not straightforward to find a well-motivated model where the new physics shows up in bs transitions, but not in bd or sd transitions.

NuTeV Anomaly

Nu-TeV was an experiment at Fermilab whose goal was a precise determination of the ratio of neutral current to charged current reactions in neutrino-nucleon scattering. Within the Standard Model, this ratio depends on the Weinberg angle $\sin \theta$. It turned out that the magnitude of the Weinberg angle extracted from the NuTeV measurement deviates at the 3 sigma level from other measurements.

It is difficult to interpret this anomaly in terms of any new physics scenario. A mundane explanation, e.g. incomplete understanding of the structure of the nucleons, seems much more likely. The dominant approach is to ignore the Nu-TeV measurement.

HyperCP Anomaly

This measurement was sometimes mentioned in the context of the CDF anomaly, because the scales involved are somewhat similar. Fermilab's HyperCP experiment found evidence for decays of the hyperon (a kind of proton with one s quark) into one proton and two muons. This by itself is not inconsistent with the Standard Model. However, the signal was due to three events where the invariant mass of the muon pair was very close to 214 MeV in each case, and this clustering appears very puzzling.

The HyperCP collaboration proposed that this clustering is due the fact that the hyperon first decays into a proton and some new particle with the mass 214 MeV, and the latter particle subsequently decays into a muon pair. It is very hard (though, again, not impossible) to fit this new particle into a bigger picture. Besides, who would ever care for 3 events?

GSI Anomaly

For dessert, something completely crazy. The accelerator GSI Darmstadt can produce beams of highly ionized heavy atoms. These ions can be stored for a long time and decays of individual ions can be observed. A really weird thing was noticed in a study of hydrogen-like ions of praseodymium 140 and promethium 142. The time-dependent decay probability, on top of the usual exponential time-dependence, shows an oscillation with a 7s period.

So far the oscillation remains unexplained. There were attempts to connect it to neutrino oscillations, but this has failed. Another crazy possibility is that the ions in question have internal excitations with a small $10^{-15}$ eV mass splitting.

Monday 10 November 2008

Hidden Valley Revealed?

As for today, the CDF anomaly has no convincing explanation. Strangely enough, HEP-ph is not flooded by new physics models (yet?), maybe because a down-to-earth explanation appears far more likely to most of us. The situation will clarify when D0 presents their own analysis of the analogous multi-muon events. I'm confident that D0 is working hard now, since the prospect of kicking the CDF butt (if the anomaly is a detector effect) must be tempting for them. While waiting for D0, one may wonder if there is a new physics scenario that could address the signatures reported by CDF. The so-called hidden valley scenario was pointed here and there, so I thought it would useful to explain the idea behind that romantic name.

Hidden valley refers to a large class of theories which, apart from the Standard Model, contain a hidden sector with a low mass scale. Here, low means no more than 100 GeV; could be 10 GeV, could be 1 GeV... In order to explain why the new particles were not copiously produced at LEP,
the hidden sector must be very weakly coupled to the SM. For example, the interactions between the Standard Model and the hidden valley might be mediated via a new U(1) gauge symmetry under which both sectors are charged. The Higgs boson could also be a mediator between the two sectors. If the mass of the mediator is large (more than, say, 100 GeV), then the interactions between the two sectors is very weak at low energies. This is illustrated in the picture on the right. While LEP has not enough energy to climb over the potential barrier produced by the large mass of the mediator, the LHC is powerful enough to overcome the barrier and explore the new sector. The Tevatron is not in this picture because everybody thought it was already passè.

What makes the hidden sector? Basically, the sky and your imagination is the limit. Below, I will talk about one particular scenario that is especially interesting from the point of view of collider studies. It might be that the hidden sector is described by a strongly interacting theory somewhat resembling our QCD. That is to say, there are hidden quarks confined by hidden strong forces that binds them into hidden mesons, hidden pions and similar stuff. A particle collision in our collider, after crossing the potential barrier, would produce a pair of hidden quarks who subsequently hadronize and cascade-decay to lighter hidden hadrons. But one crucial difference between our QCD and the hidden QCD is that the latter does not have a stable particle at the end of the decay chain (or at least, not all the decay chains end in a stable hidden particle) so that the hidden stuff eventually decays back into the Standard Model particles. Because of the small interaction strength between the two sectors, some hidden hadrons may have a relatively long life-time, which leads to highly displaced vertices in our detector. Often, with large multiplicities of soft particles in jets. Sounds familiar?

The hidden-valley scenario was proposed more than 2 years ago by Matt Strassler and Kathryn Zurek. They didn't have a particular motivation in mind other than exploring exotic collider signatures (although strongly interacting hidden sectors are common in supersymmetric model-building, and no commandment forbids the mass scale in those sectors be GeV-ish, they could also host the dark matter particle). This approach represents the recent change of season in particle theory. In the old days, the particle folk touched only to those models that were "strongly motivated" or "leading candidates". The outcome may prove valuable to posterity, especially to anthropologists. With the LHC approaching, the emphasis has shifted to interesting collider signatures, and fundamental motivations are no longer mandatory. It may well be that shooting at random will prove more successful than following our theoretical prejudices. In this respect hidden valleys have much in common with unparticles, who are similarly unmotivated. The reason why unparticles received much more attention is that they are quite sharply defined, and for this reason they are more comfortable as a bandwagon. On the other hand, hidden valley is a very wide concept, and by the time the model B-71 variant 69 is discussed, the audience is switching to online newspapers. But things may change soon, if the CDF anomaly won't go way...

It should be clear, however, that for the moment there is no hidden-valley model that would explain the CDF anomaly. The biggest problem is the large number of anomalous events reported by CDF. Given that CDF sees some $10^5$ anomalous events, the cross section for the production of the hidden valley particles should be larger than 100 pb. That's already a lot - much more than the standard Higgs production cross section at the Tevatron, and of the similar of magnitude as the production b-quark pairs. Moreover, the required cross section may be even more ridiculous if not all decays of the unknown particles go into muons. For example, in this attempt to fit the signal with 8-tau decays the estimate for the cross-section is 100nb. This seems to be at odds with the assumption that the hidden sector is very weakly coupled to the Standard Model. Furthermore, CDF sees no sign of resonant production, which would be expected if the mediator between the two sectors is not too heavy. Clearly, there's some work to do, for experimenters and theorists alike.

Update: As if a blog post were not enough ;-), here is Matt's brand new paper discussing possible connections of the hidden-valley scenario to the CDF anomaly.