Day 3 of the SEARCH workshop (see here for an introduction and overviews of Day 1 and Day 2) opened with my own talk, entitled “On The Frontier: Where New Physics May Be Hiding”. The issue I was addressing is this:
Even though dozens of different strategies have been used by the experimenters at ATLAS and CMS (the two general purpose experiments at the Large Hadron Collider [LHC]) to look for various types of new particles, there are still many questions that haven’t been asked and many aspects of the data that haven’t been studied. My goal was to point out a few of these unasked or incompletely asked questions, ones that I think are very important for ATLAS and CMS experts to investigate… both in the existing data and also in the data that the LHC will start producing, with a higher energy per proton-proton collision, in 2015.
I covered four topics — I’ll be a bit long-winded here, so just skip over this part if it bores you.
1. Non-Standard-Model (or “exotic”) Higgs Decays: a lightweight Higgs particle, such as the one we’ve recently discovered, is very sensitive to novel effects, and can reveal them by decaying in unexpected ways. One class of possibilities, studied by a very wide range of theorists over the past decade, is that the Higgs might decay to unknown lightweight particles (possibly related in some way to dark matter). I’ve written about these possible Higgs decays a lot (here, here, here, here, here, here and here). This was a big topic of mine at the last SEARCH workshop, and is related to the issue of data parking/delaying. In recent months, a bunch of young theorists (with some limited help and advice from me) have been working to write an overview article, going systematically through the most promising non-Standard-Model decay modes of the Higgs, and studying how easy or difficult it will be to measure them. Discoveries using the 2011-2012 data are certainly possible! and at least at CMS, the parked data is going to play an important role.
2. What Variants of “Natural” Supersymmetry (And Related Models) Are Still Allowed By ATLAS and CMS Searches? A natural variant of supersymmetry (see my discussion of “naturalness”=genericity here) is one in which the Higgs particle’s mass and the Higgs field’s value (and therefore the W and Z particles’ masses) wouldn’t change drastically if you were somehow to vary the masses of superpartner particles by small amounts. Such variants tend to have the superpartner particle of the Higgs (called the “Higgsino”) relatively light (a few hundred GeV/c² or below), the superpartner of the top (the “top squark”, with which the Higgs interacts very strongly) also relatively light, and the superpartner of the gluino up in the 1-2 TeV range. If the gluino is heavier than 1.4 TeV or so, then it is too heavy to have been produced during the 2011-2012 LHC run; for variants with such a heavy gluino, we may have to wait until 2015 and beyond to discover or rule them out. But it turns out that if the gluino is light enough (generally a bit above 1 TeV/c²) it is possible to make very general arguments, without resort to the three assumptions that go into the most classic searches for supersymmetry, that almost all such natural and currently accessible variants are now ruled out. I say “almost” because there is at least one class of important exceptions where the case is clearly not yet closed, and for which the gluino mass could be well below 1 TeV/c². [Research to completely characterize the situation is still in progress; I'm working on it with Rutgers faculty member David Shih and postdocs Yevgeny Kats and Jared Evans.] What we’ve learned is applicable beyond supersymmetry to certain other classes of speculative ideas.
3. Long-Lived Particles: In most LHC studies, it is assumed that any currently unknown particles that are produced in LHC collisions will decay in microscopic times to particles we know about. But it is also possible that one or more new type of particle will decay only after traveling a measurable distance (about 1 millimeter or greater) from the collision point. Searching for such “long-lived” particles (with lifetimes longer than a trillionth of a second!) is complicated; there are many cases to consider, a non-standard search strategy is almost always required, and sometimes specialized trigger strategies are needed. Until recently, only a few studies had been carried out, many with only 2011 data. A very important advance occurred very recently, however, when CMS produced a study, using the full 2011-2012 data set, looking for a long-lived particle that decays to two jets (or to anything that looks to the detector like two jets, which is a bit more general) after traveling up to a large fraction of a meter. The specialized trigger that was used requires about 300 GeV of energy or more to be produced in the proton-proton collision in the form of jets (or things that look like jets to the triggering system.) This is too much for the search to detect a Higgs particle decaying to one or two long-lived particles, because a Higgs particle’s mass-energy [E=mc2 energy] is only 125 GeV, and it is rather rare therefore for 300 GeV of energy in jets-et-al to be observed when a Higgs is produced. But in many speculative theories with long-lived particles, this amount of energy is easily obtained. As a result, this new CMS search clearly wipes out, at one stroke, many variants of a number of speculative models. It will take theorists a little while to fully understand the impact of this new search, but it will be big. Still, it’s by no means the final word. We need to push harder, improving and broadening the use of these methods, in order that decays of the Higgs itself to long-lived particles can be searched for. This has been done already in a handful of cases (for example if the long-lived particle decays not to jets but to a muon/anti-muon pair or an electron/positron pair, or if the long-lived particle travels several meters before it decays) and in some cases it is already possible to show that at most 1 in 100 to 1000 Higgs particles produce long-lived particles of this type. For some other cases, the triggers developed for the parked data may be crucial.
4. “Soft” Signals: A frontier that has never been explored, but which theorists have been talking about for some years, is one in which a high-energy process associated with a new particle is typically accompanied by an unusually large number of very low-energy particles (typically photons or hadrons with energy below a few GeV). The high-energy process is mimicked by certain common processes that occur in the Standard Model, and consequently the signal is drowned out, like a child’s voice in a crowded room. But the haze of a large number of low-energy particles that accompanies the signal is rare in the mimicking processes, so by keeping only those collisions that show something like this haze, it becomes possible to throw out the mimicking process most of the time, making the signal stand out — as though, in trying to find the child, one could identify a way to get most of the people to leave the room, reducing the noise enough for the child’s voice to be heard. [For experts: The most classic example of this situation arises in certain types of objects called "quirks", though perhaps there are other examples. For non-experts: I'll explain what quirks are some other time; it's a sophisticated story.]
I was pleased that there was lively discussion on all of these four points; that’s essential for a good workshop.
After me there were talks by ATLAS expert Erez Etzion and CMS’s Steve Wurm, surveying a large number of searches for new particles and other phenomena by the two experiments. One new result that particularly caught my eye was a set of CMS searches for new very heavy particles that decay to pairs of W and/or Z particles. The W and Z particles go flying outwards with tremendous energy, and form the kind of jet-like objects I mentioned yesterday in the context of Jesse Thaler’s talk on “jet substructure”. This and a couple of other related measurements are reflective of our moving into a new era, in which detection of jet-like W and Z particles and jet-like top quarks has become part of the standard toolbox of a particle physicist.
The workshop concluded with three hour-long panel discussions:
- on the possible interplay between dark matter and LHC research (for instance: how production of “friends” of dark matter [i.e., particles that are somehow related to dark matter particles] may be easier to detect at the LHC than production of dark matter itself)
- on the highest priorities for the 2013-2014 shutdown period before the LHC restarts (for instance, conversations between theorists and experimentalists about the trigger strategies that should be used in the next LHC run)
- on what the opportunities of the 2015-2020 run of the LHC are likely to be, and what their implications may be (for instance, the ability to finally reach the 3 TeV/c2 mass range for the types of particles one would expect in the so-called “Randall-Sundrum” class of extra-dimensions models; the opportunities to look for very rare Higgs, top and W decays; and the potential to complete the program I outlined above of ruling out all but a very small class of natural variants of supersymmetry.)
All in all, a useful workshop — but its true value will depend on how much we all follow up on what we discussed.