Monday, 13 February 2012

How to find a stop

Lately there's been a surge of interest in hypothetical scalar partners of the top quark, the stops in short. So it may be a good moment to sell a few technical details to a larger audience. For theorists, a stop is easy to distinguish experimentally: it looks like a top but with a twiddle on top. However experimentalists are not as smart, and they have to invest much more time and effort in order to identify stops at the LHC.

What it looks like depends first of all on how it decays. Even the minimal SUSY model offers countless possibilities. Leaving out the case of stable stops, in the MSSM stops ultimately decays to a number of known particles from the Standard Model plus the lightest supersymmetric partner (LSP) who is assumed to be a very weakly interacting particle showing up as missing momentum in a detector. Some possible decay chains are:
Stop → top + LSP, Stop → W + sbottom → W + b + LSP, Stop → bottom + chargino → bottom + W + LSP, etc.
The bottom line is that the MSSM stop should manifest itself at the LHC as an excess of events with:
  • top and/or bottom quarks,
  • significant missing energy due to the LSP.
Now, how to produce it. Being top partners, stops carry a color charge, hence they can be produced in gluon collisions which are easy to come by at the LHC. However, on the plot you see that production of stop pairs is far less frequent than that of gluinos and 1st generation squarks of similar mass. In physics jargon, s-channel production of scalar particles is velocity suppressed as a consequence of angular momentum conservation. This is the main reason why, as you'll see below, the LHC limits on stop pair production are so much weaker than those on gluinos. However, there is a trick to boost the stop production rate by producing them indirectly in gluino decays: gluino → stop + top, as long as the gluino is not much heavier than the stops. As a bonus, this production mode generates more junk in the detector that could be helpful in discriminating signal from background. For example, one can imagine the sequence:
pp → 2 gluinos → 2 stops + 2 tops → 4 tops + 2 LSPs
which leaves us with 4 top quarks in the final state. The 4-top production rate in the Standard Model is very small, therefore observation of such a final state at this point would be a clear sign of new physics. Another place where this sort of cascades could show up are the searches for same-sign top quarks.

What is the experimental situation so far? As far as I know, the LHC collaborations have not yet published any limits on direct stop production. On the other hand, gluino mediated stop production was targeted in this note based on 1 fb-1 of ATLAS data. The plot shows that gluinos decaying in the sequence:
gluino → top + stop → 2 tops + bottom + chargino→ 2 tops + bottom + W + LSP
cannot be lighter than 500 GeV. During the next 30 days leading to the Moriond conference many more searches based on larger data samples will be released, starting with the Valentine Day ATLAS talk.

For today, we can get some idea of the current LHC sensitivity from this paper, which compiles a large number of SUSY searches and recasts the results in terms of limits on stops. The LHC reach for direct stop production (right plots) is poor, corresponding to stop mass of only 200-300 GeV. For gluino mediated stop production (plots below) the limits are much better and extend to approximately 700 GeV gluino masses (though the precise limits may depend on details of the SUSY spectrum; it is probably possible to design spectra for which these limits are somewhat weaker). Amusingly, the dedicated ATLAS search does not seem to be the most sensitive probe of gluino mediated stop production. Instead, more stringent limits come from vanilla SUSY searches (decaying top quarks produce jets, b-jets, and/or leptons that can be picked up by these searches). We'll see very soon whether the coming experimental analyses will significantly improve these limits.
Update: see the slides of the Feb 14 ATLAS talk for more limits on stops.


Anonymous said...

The "Atlas preliminary" plot has a feature I've seen before, namely that the excluded range is far less than expected but not in a peak, but across the entire range. (CLs to CLs expected) Is this to be expected if something's really there? I assume it's a result of what exactly is plotted, but what exactly do we expect to see here?

Jester said...

Yeah, that ATLAS search has a 1 sigma excess. However, stop interpretation of that excess is unlikely, due to constraints from other SUSY searches.

Chris Austin said...

"production of stop pairs is far less frequent than that of gluinos and 1st generation squarks of similar mass. In physics jargon, s-channel production of scalar particles is velocity suppressed as a consequence of angular momentum conservation."

How can angular momentum conservation suppress production of stop pairs relative to production of 1st or 2nd generation squark pairs of similar mass?

Jester said...

1st generation squarks can be produced in quark-quark collisions via a t-channel gluino exchange

Chris Austin said...

Thanks. In other words, if I understand it right, the 1st generation quark emits or absorbs a gluino, and simultaneously transforms into a 1st generation squark.

Anonymous said...

1 sigma *excess* ? One should rather say "1 sigma agreement"

muon said...

It is worth noting that the new ATLAS result, based on more than 1 fb-1, is weaker than the old ATLAS result, based only 0.035 fb-1. Such is the "mystery" of 1-sigma fluctuations... ;)