Seminar on the LHC Today

Today I’m at SUNY Stony Brook (SUNY = State University of New York) out on Long Island, to give a talk to the particle physics theorists there (and whoever else shows up.) One topic I’ll cover involves the still very mysterious measurements made by the CDF and DZero experiments (at Fermilab’s Tevatron collider) that suggest an asymmetry in the production of top quarks. You can read about that mystery here. Obviously one would want to make a similar measurement at the LHC (though you can’t quite make the same one) but there are other measurements that would give insights, and this is something I’ve been thinking about. First, models of new physics that attempt to create such an asymmetry at the Tevatron tend to produce various other types of asymmetries at the Large Hadron Collider. [I wrote a paper a few months ago on one example with Rutgers postdocs Nathaniel Craig and Can Kilic (Kilic is now a professor at U. Texas), and am working on another with some Rutgers students.]  Second, if we think there might be a large asymmetry involving top quarks, we would clearly like to know if there are similar asymmetries for other quarks. This isn’t that easy to find out, however. The top quark is special, because its decays are rather spectacular, and it is relatively easy to identify events in which a top quark and a top anti-quark have been produced. But all the other quarks just appear in the detector as a single jet (a spray of hadrons), and look very similar to one another and to gluons. Fortunately, there are some tricks that work, to some degree, for identifying charm and bottom quarks some of the time. And it turns out that the Tevatron has enough data (or so I estimated in a paper back in February; there’s also a paper mentioning this for bottom quarks by Bai, Hewett, Kaplan and Rizzo) that even though the observable signals would be small, it should probably be possible for CDF or DZero to measure similar forward-backward asymmetries both for charm quarks and for bottom quarks. At the LHC it’s tougher, but it should be possible to look an asymmetry in bottom quarks by sometime next year [as Harvard postdoc David Krohn, Harvard student Dilani Kahawala and I argued couple of months ago.]  Well, “should be”; any theorist’s estimate of whether something is possible, in an environment as complicated as a hadron collider, should be understood as suggestive, not definitive. Until the experimenters look into this in greater detail, we won’t know for sure if these measurements are really possible. But I’m optimistic.

The second topic I’ll talk about, time permitting, is how to fill one of the largest gaps still remaining in the search strategies for supersymmetry, and for other speculative theories that lead to similar phenomena. There was a lot of silly noise in the blogs and in the press that supersymmetry was basically ruled out by this summer’s results… But after numerous level-headed theorists have weighed in disagreeing with this statement, the LHC community now seems to be more aware that there are several very natural ways that supersymmetry could easily have evaded the searches carried out so far, which were by no means comprehensive. Some colleagues and I wrote a paper this spring pointing out a particularly large gap, and suggesting a set of search strategies that would fill it; you can read a little about that here. The experimenters are making plans to fill in these gaps (perhaps independently of our suggestions, but we’d like to think we had a little positive influence), and I expect we’ll see significant results in the next few months. Certainly by next spring I imagine that the noose on supersymmetry and the like will be much, much tighter than it is right now.

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