Tuesday 25 March 2008

UT fit longo magis quam acri bello

Spring is in the air: birds are singing, trees are blossoming, new physics is popping out from beneath the snow. During this month three different deviations from the Standard Model have been reported, all three in flavour physics:
- 3.8 sigma deviation in leptonic decays of Ds mesons.
- 5.3 sigma deviation of the CP asymmetry in B -> K \pi decays.
- 3 sigma deviation in the phase of B_s mesons mixing.
Sigma's come and go. Yet I suspect the last one on the list may stay longer. Not that I have serious arguments for that - just a hunch slash prejudice. Last Wednesday there was a seminar here at CERN about it: Maurizio Pierini was speaking in the name of the UTfit collaboration
who recently reported the deviation. Maurizio made a very good job explaining what's behind that analysis.

The story relates to the transitions between $B_s$ and $\bar B_s$ meson. This is a $\Delta F = 2$ flavour violating process, since a b-quark turns into an anti-b-quark. In the Standard Model, all flavour violation originates from a 3x3 unitary matrix called the CKM matrix. Numerous measurements of the angles and the one physical phase of this matrix resulted in the now-famous LSD plots, considered an important contribution to modern art. A load of results from the B factories led to overconstraining the CKM matrix, and thus to constraining contributions from beyond the Standard Model. New physics in sd transitions (kaon mixing) and bd transitions is tightly constrained. On the other hand, bs transitions are less constrained, basically because B factories were not producing $B_s$ mesons. This gap has been recently filled by the Tevatron who is able to produce $B_s$ mesons and study the $B_s \to J/\Psi \phi$ decay channel. In particular, the mass difference $\Delta m_s$ of the two $B_s$ eigenstates was measured. Furthermore, a constraint on the phase of the mixing $\phi_s$ vs. $\Delta \Gamma$ could be obtained. The latter measurement showed some deviation from the Standard Model prediction, but by itself it was not statistically significant.

New physics at the TeV or higher scale generically contributes to quark transitions between the generations. In the effective theory, where the new physics states are integrated out, we model the effect of the heavy particles by non-renormalizable four-fermion terms in the effective hamiltonian, for example $H_{NP} \sim (\bar b s)^2$. The coefficient of this operator is complex in general. One defines
$C_{B_s} e^{-2 i \phi_{B_s}} = <\bar B_s|H_{SM} + H_{NP}|B_s>/ <\bar B_s|H_{NP}|B_s>$
Analogous parameters are defined for $B_d$ mesons and for kaons. The new parameters affect the same observables as the CKM matrix elements and the new phases are an additional source of CP violation.
What the UTfit collaboration did is to add these new parameters to the unitarity triangle fit and to constrain them using all available data, including the Tevatron data on $B_s$ mixing. One complication is that D0 in their analysis employed some assumptions about the strong phases obtained from the $B_d$ sector and UTfit needed to apply some wizardry to disentangle that. The impact of the Tevatron data on the fit is shown on the plot above. The right panel shows the constraints in the $\Delta\Gamma - \phi_s$ plane before taking into account the input from the Tevatron. Both CDF and D0 pick up the region that is away from $\phi_s = 0$. The final result is the constraint on $C_{B_s}$ and $\phi_{B_s}$ shown below. While the amplitude is consistent with the SM value $C_{B_s} = 1$, the phase deviates from 0 at the 3 sigma level.
A few points stressed by Maurizio:
- The quoted 3-sigma is not just combining the two 2-sigma deviations reported by CDF and D0, but it's a result of combining their measurements with all other flavour constraints.
- It is highly non-trivial that CDF and D0 results point to the same region of the $\Delta Gamma - \phi_{s}$ plane.
- The determination of the phase is not affected by lattice uncertainties, who only affect $C_{B_s}$.

The situation should clarify in the near future. First of all, D0 should debug their analysis from the strong phases assumptions. The statistics will improve and maybe BELLE will also join the game. If the effect is real it's a fantastic news for LHCb - a Cinderella among the LHC detectors - who will be able to study the $B_s$ sector with much better precision. It'd be funny if flavour were the only place where new physics showed up at the LHC. Such a next-to-nightmare scenario is not so unlikely.

Slides of Maurizio's talk available here. For a thoroughly unenthusiastic discussion of the same results, see another blog.

Friday 14 March 2008

Warped Passages

The past week was quite packed with events. CERN TH hosted a workshop on Monte Carlo tools but, since I'm opposed to gambling, I'm not going to cover it here. The cherry on the top was the series of lectures on warped extra dimensions delivered by the better half of Randall-Sundrum. Undeniably, Lisa Randall is one of a handful of particle physicists enjoying a celebrity status. She is of course famous for giving us RS-1, the second best cited paper in the history of particle theory. On the popular science front, she has her own book that proves her good taste for music and bad taste for poetry. She even made it into the shiny world of American TV shows. Back to our small world, her seminars invariably create a lot of stir-up. Her previous performance at CERN sent shudders through the blogosphere, leading to several broken friendships and one auto-da-fe. Personally, I don't like being burnt alive, so I'd better be ending this general introduction and jump into the safety of physics blogging.

The RS model is the last truely original and noteworthy idea spawned by particle theory so far. Introducing a warped 5th dimension allows us to accommodate, in a consistent and natural way, vastly different scales in one theory. By AdS/CFT, the fifth dimension can be viewed as an effective weakly coupled description of some strongly interacting hidden sector. Although possible uses of the RS framework are much wider, most of the current work is focused on applications to the TeV-scale physics. In this way, the large hierarchy between the Planck scale and the electroweak scale can be understood as a manifestation of the warped fifth dimension.

The industry has produced many constructions based on the RS paradigm. Currently, the most interesting version seems to be the one with the higgs field and the third generation fermions localized close the IR brane, while the light SM fermions are localized close to the UV brane (SM gauge fields are evenly smeared along the fifth dimension). If the higgs field lives close to the IR brane, it feels the effective cut-off scale of order TeV, so that the quantum corrections to its mass are not sensitive to the Planck scale. Furthermore, one can implement dynamical electroweak symmetry breaking by making the higgs field a part of a 5D gauge field. Finally, by localizing the light fermions away from the IR brane, one can explain the fermion mass hierarchies and the CKM mixing angles. Thus, this version of RS models can address both electroweak symmetry breaking and flavour physics.

In her lectures, Lisa concentrated on phenomenological aspects of such a modern RS-type set-up. She reviewed possible collider signals and search strategies. There are several things we should look at:

1. The KK graviton, a spin-2 particle with large couplings (TeV suppressed, instead of Planck suppressed as for the massless graviton) is often consider the hallmark of the RS scenario. However, the prospects of seeing it at the LHC are not that bright. One reason is that the KK graviton is sharply localized close to the IR brane and it has suppressed couplings to the light quarks localized in UV. All in all, the discovery reach for the KK graviton extends only up to the mass of 2 TeV. Such a small mass is rather unlikely; the KK graviton is by a factor 1.5 heavier than the lightest KK modes of the SM gauge bosons, and typical constraints on the masses of the latter are around 3 TeV.
2. Black holes are even more spectacular, but the chances of spotting one at the LHC are inversely proportional to their spectacularity. See this old Tommaso's post for a wrap-up.
3. KK gluons provide a much more promising opportunity for a discovery. The LHC production cross-section of these heavy partners of the QCD gluon turns out to be larger than that of the KK gravition. The KK gluon interacts most strongly with fermions localized in IR, thus it will decay dominantly into a pair of top quarks. This should be visible as a peak over the SM top pair production, see the left figure. The discovery reach, however, crucially depends on how well we can identify top quark jets at the LHC. If the efficiency is not good enough, the KK gluon signal can be easily swamped by the light quark jet background, see the right figure.
4. Flavour physics appears interesting too. The RS framework leads to the scenario called next-to-minimal flavour violation (one of these names invented in a stroke of imagination) in which flavour violating interactions occur via mixing of the light quarks with the third generation. Although the RS set-up contains severak new TeV particles, their flavour non-universal couplings are aligned with the CKM matrix structure, and the corrections to most low energy flavour observables are under control. Yet some observables, like $\epsilon_K$ in the kaon mixing, generically come out too large, which suggests that some flavour symmetries should be implemented in the RS model. No compelling model has emerged so far, yet the generic feature that we expect are large flavour-changing neutral current in the third generation showing up, for example, in the top quark decays into the charm quark.

All three lectures are available for everyone to admire.

Sunday 9 March 2008

Moriond Summary

-Weather conditions: Mostly sunny
-Snow conditions: Bad at the beginnig, perfect in the middle, getting worse towards the end of the week
-Broken bones: at least two
-Minor casualties: at least one blogger
-Mont Blanc: still there

From that you can infer that the conference was quite a success. As for the side activities, you may have a look at the theory and experiment summaries. Although there has not been major developments in particle theory in the last year, some decent work was done nevertheless. I most enjoyed the talk of Chris Hill on anomaly mediated interactions in the Standard Model that had been previously missed (sic). I also liked the talks of Elias Kiritsis on AdS/QCD, Fedor Bezrukov on inflation with the SM Higgs, and Jose Ramon Espinosa on unparticles mixing with the Higgs boson. See also the wrap-up of my fellow-blogger, though at the moment it stops at day 1.

Wednesday 5 March 2008

Porno at CERN

Although I'm on holidays, I have a report due on a CERN seminar last week. Last Thursday, Irina Arefeva gave a talk about wormhole production at the LHC. The claim is that, in the context of models with the TeV quantum gravity scale, the LHC might produce not only black holes but also another class of objects called wormholes. There has been quite a lot of noise around this work, mostly in tabloids like the Sun and New Scientist, and the echoes resounded in the blogosphere too. Peter linked Irina's paper to an article that coined the term science pornography. Lubos, on the other hand, tried to point out interesting aspect of this direction of research. In this post I'll brood on the question if that paper is pornography or not.

Wormholes are solutions of the Einstein equations with equal time slices being not simply connected. The spacetime containing wormholes allows for closed time-like curves, which conflicts with our notion of causality. There exists a vast literature aiming to understand these weird objects. The questions is if they are physical or they are merely a pathology of Einstein gravity that should be discarded. One conclusion I would draw from this debate is that we are lacking tools to address some major questions in quantum gravity. In particular, the limitations of the sum-over-histories approach to quantum gravity become rather clear.

Pornography is notoriously difficult to define. The typical attidude is "I recognize it when I see it" (but, once you're watching, you might get accused of being too interested). It is difficult to draw a line beyond which science pornography begins. I can perfectly imagine social environments in which extra dimensions or supersymmetry are considered hard porno. Similarly, wormholes, just like nudity, do not equal pornography; it all depends on the wider context.

Still one can attempt a more formal characterization of pornography. In my opinion, one defining feature is certain blunt simplicity, going straight to the point and avoiding any philosophical divagations. The work presented by Irina matches that description quite accurately. The bulk of the talk was a review of wormhole solutions in general relativity. Solutions appropriate for ADD large extra dimensions were not mentioned. At the end of the day the claim was made that, since black holes are produced with geometrical cross sections, wormhole production should behave similarly. Why? The question of wormhole decays was not addressed at all. How would they differ from black hole decays?(the latter are expected to produce Hawking radiation, but wormholes do not have a horizon). In short, there was nothing that would improve our understanding of the subject. Yet, at this point, this could be just a bad talk based on a bad paper. What ultimately makes it into a vulgar porno is calling the paper Time Machine at the LHC. A cheap trick that is supposed to arouse the reader with the prospects of time travel. I'm afraid that this kind of presentation gives scientific research a bad name.

It's not that the whole subject of wormholes should never be addressed. There are certainly many interesting questions to be asked, that could at least help us understanding the limitations of our understanding of gravity. It doesn't take much to turn pornography into art. Try it. You may become Tinto Brass of particle physics :-)

Sunday 2 March 2008


I'm spending this week in La Thuile - a ski resort in the Italian Alps. This is where the electroweak session of Rencontres de Moriond takes place. In Moriond, the ethical dilemma Tommaso once wrote about is most accute. The conference venue is just 50 meters from the ski lift and theres is a discount on skipass for the participants. This year, however, the snow conditions are pretty bad, so I'm actually attending to some of the talks.

The useful thing about going 100 kilometers south and one kilometer up from Geneva is that I could learn something about the LHC. Lyn Evans, who is the alpha and the omega of the LHC project, gave a talk this morning about the construction progress. He showed the new cooldown schedule which looks like this

It seems that the delay is not as bad as some of us feared. Sector 45 will be the last to be completed because it needs to be warmed up for the installation of the inner triplet magnet. Still, it is expected to be cold again in the middle of June. This is the earliest when beam commissioning may start. The current plan is to get first collisions by the end of August. This, of course, depends on a number of unpredictable factors, yet it is reassuring that so far no major problems have arisen. Another interesting thing Lyn said is that the preferred option at the moment is to run the machine at 5-6 TeV (10-12 TeV in the center of mass frame, instead of 14 TeV) in the first year.

The new schedule should soon appear on the LHC homepage. The link to Lyn's talk is here.