Over the weekend I wrote about the SEARCH workshop’s first day; today I’ll describe its final two days. First I’ll give you a broad overview, and then, for more expert readers, a couple of especially interesting developments that caught my attention.
The vast amount of information pouring out of the Large Hadron Collider [LHC] is simply overwhelming. Sunday and Monday we heard 16 talks by LHC experimenters, evenly split between ATLAS and CMS, the two general purpose detectors at the LHC. Each of these talks described several complex measurements aimed at looking for a wide variety of hypothetical phenomena — for any sign of speculative things that theorists have proposed (conceptual ideas such as supersymmetry and extra dimensions, and more generally, new types of particles including heavy“partners” of the top quark, undetectable particles [such as those that may make up dark matter], new particles that can decay to quark-antiquark pairs or lepton-antilepton pairs or pairs of photons, and so on.) So far none of these measurements has turned up anything unexpected, and it is appearing very unlikely that data from 2011, the first year of full-fledged LHC operation, will lead to an easy, quick and surprising discovery. But it is still very early in the LHC’s decade-long program, so collectively we just have to buckle down, take more data and work harder.
There were a number of very interesting discussion sessions, during which many useful (and mostly friendly and constructive) exchanges occurred between theorists and experimentalists and between members of ATLAS and CMS. Almost all of this was quite technical so I won’t give many details, except to say that I learned a lot, and also saw lots of places where I thought the experimentalists could extract more information from their data.
The workshop concluded with a panel discussion — the only point during the entire workshop when theorists were formally asked to say something.
The panel consisted of Michael Peskin (senior statesman [and my Ph.D. advisor] famous for many reasons, including fundamental work on the implications of highly precise measurements ), Nima Arkani-Hamed (junior statesman, and famous for helping develop several revolutionary new ways of approaching the hierarchy problem), Riccardo Rattazzi (also famous for conceptual advances in dealing with the hierarchy problem), Gavin Salam (famous for his work advancing the applications of the theory of quarks and gluons, including revolutionary methods for dealing with jets), and myself (famous for talking too much… though come to think of it, that was true of the whole panel, except Gavin.) And Raman Sundrum, one of the organizers (and famous for his collaboration with Lisa Randall in introducing “warped” extra dimensions, and also anomaly-mediated supersymmetry breaking [which was competitive with a paper by Rattazzi and his colleagues]) informally participated too. The discussion was recorded, and I assume they will post it. If I’m not too embarrassed by it I’ll provide the link.
Meanwhile I mentioned in my last post that I think there are big issues surrounding whether the LHC can effectively trigger on exotic decays of the Higgs particle (if there are any), under current operating conditions. Informally, a bunch of theorists interested in this question met yesterday afternoon, with a couple of experimental spectators. We tried to build a list of exotic Higgs decays for which (a) one can hope to make a measurement with 2012 data, and (b) it isn’t obvious that current triggering strategies will work very well, and (c) additional triggering strategies might conceivably improve the situation. Then I tried to encourage individual theorists to do pick one of these decays and do a quick study to see whether an interesting search really could be made with this year’s data, assuming triggering on these events were carried out. The experimenters interested in this issue have indicated to us that they need our work to be done within about a month — a very short time as far as even preliminary studies are concerned. If any of my colleagues are reading this, please consider volunteering to help out.
Ok, now a few specifics from the workshop. Oh, first a bon mot from Daniel Whiteson, member of ATLAS and professor at the University of California at Irvine. “With the current data, a Standard Model Higgs at 125 GeV is like Mitt Romney: the most likely option, but the least exciting.”
One important thing I learned from John Butterworth of ATLAS (who writes regularly [and very well indeed] for the Guardian newspaper, and is currently in charge of coordinating ATLAS’s precise measurements of many properties of the Standard Model) was about ATLAS’s recent demonstration that they can measure tau lepton “polarization” [polarization = whether the spin of the tau is aligned with or counter to its direction of motion]. It is very interesting to know whether taus produced in a particular process are preferentially polarized; in many Standard Model processes they are polarized in one way, but in many new phenomena they might end up polarized the other way or on average unpolarized. The ability to measure this so well reflects how extraordinary ATLAS is as a detector compared to the previous generation of detectors. Though they’re not ready to say so publicly, I am sure CMS will also be able to do this too, as its capabilities are generally quite comparable to those of ATLAS. Anyway, this is a new experimental tool that may be very important in future measurements, and theorists like me should get to work and give the experimenters new suggestions on how to use it!
Another new tool that has been coming for a while involves the study of jet substructure. We heard about this as applied to “top-tagging” — identifying top quarks when they are moving so fast that the particles to which they decay create jets that kind of overlap. The new jet technology that was developed in the last decade, by many people, has allowed a number of new techniques to be suggested, and Petar Maksimovic (from Johns Hopkins University) of CMS gave a beautiful talk, in which he showed that a particular variant of these techniques (developed by his theory colleagues at Johns Hopkins [Kaplan, Reherman, Schwartz and Tweedie, who were inspired by work of Butterworth, Davidson, Rubin and Salam]) seem to work when applied in real data. It’s too early to be sure just how well things work, but it looks very promising, as you can see in the photo below, where Maksimovich shows that W particles decaying to a nearly-overlapping quark and antiquark can be identified as an obvious and rather narrow peak above more random sources of similar looking jets.
Finally, a very interesting comment was made by theorist George Sterman, one of the world’s greatest experts on the theory of quarks and gluons. When I visited SUNY Stony Brook last fall, I had a conversation with him in which he suggested a particular reason why the unexpectedly large asymmetry in the production of top quarks observed by the Tevatron experiments CDF and DZero might really be due just to a subtle mismatch between theoretical calculation and the experimental methodology. It was the most compelling idea I’d heard so far, but it wasn’t very precise yet. Apparently in just the last few days, Sterman managed to flesh this idea into something concrete.
First, George referred to new updated results presented by CDF (see figure above) at the Moriond conference, which show that although there is an excess, it has the same shape [as a function of top quark-antiquark invariant mass and rapidity] as the asymmetry predicted in the Standard Model. As he said, “it looks just like the Standard Model, only more so.” And then he argued that the complicated formula for the top quark asymmetry from quark-antiquark collisions (within the Standard Model) could be written, with certain well-defined approximations, in a simple form that
- clarifies the reason for the shape of the Standard Model prediction
- shows how any accidental or intentional removal of top quark-antiquark events that have an extra jet would increase the asymmetry as the logarithm of M2/Delta2, where M is the invariant mass of the top quark/antiquark pair, and Delta2 = S2-M2, where S is the invariant mass of the quark and antiquark that initiated the collision
- suggests how this idea could potentially be tested [by changing the cuts on the extra jet and thus changing the average M2/Delta2]
As he said himself, the idea is still a very new one and might be in contradiction with what the experimenters at CDF actually have done. But in any case it was classic theoretical physics; taking very complicated formulas, using a deep understanding of the problem to make clever simplifying approximations, and extracting a physically relevant, conceptually clear lesson. Kudos to Professor Sterman; I generally hope he’s right, because every other explanation that any of us have proposed using new particles looks awful, and we’d really like to put this mystery behind us soon.