Monday 9 August 2010

His First Inverse Picobarn

The LHC, more precisely ATLAS, just passed the 1pb-1 milestone:

One inverse picobarn of integrated luminosity is 1/1000 of what is planned for LHC Run I. At the 7 TeV center-of-mass energy this luminosity translates to:
  • 200 000 W bosons,
  • 60 000 Z bosons,
  • 200 top quark pairs,
  • 10-20 Higgs bosons, if bastard's mass is around 120 GeV,
  • A couple of gluino pairs (Poisson permitting) in a parallel universe where gluinos exist and weigh 500 GeV.

Thursday 5 August 2010


A new paper entitled It's on is now out on hep-ph. When particle theorists refer to "it" they don't mean sex, unlike ordinary people. Here "it" stands for the LHC who is not only "on" but already produces interesting constraints on new physics. In particular, the latest jets + missing energy search performed by ATLAS excludes a new region of the susy parameter space with a light, 150-300 GeV gluino. One can learn two interesting things from the paper:
  1. 1) that with just 70 nb-1 of LHC data one can obtain non-trivial constraints on vanilla susy models that in some cases are more stringent than the existing Tevatron constraints,
  2. 2) and that the susy combat group in ATLAS missed the point 1.
A gluino is the fermionic partner of the QCD gluon, as predicted by supersymmetry. A pair of gluinos can be produced for example by colliding 2 gluons. Since the protons circulating in the LHC ring are filled to the brim with gluons, gluinos would pop out as pop corn if only they existed and were light enough. For example, a 200 GeV gluino would be produced at the LHC7 with the stunning cross section of 0.6 nb. Thus, even the small amount of LHC data collected so far could contain a few tens of gluino events. Once produced, gluinos immediately decay to standard model and other susy particles (if they don't then the whole story is completely different, and is not covered by the latest ATLAS search). When the gluino is the next-to-lightest susy particle apart from a neutralino then it decays via an off-shell squark into 2 quarks and the neutralino. The signature at the LHC is thus a number of high-pT QCD jets (from the quarks) and large missing energy (from the neutralinos who escape the detector).

There is a lot of jet events at the LHC, but fortunately only a small fraction of them is accompanied by large missing energy. In the 70nb-1 of data, after requiring 40 GeV of missing pT, and with some additional cuts on the jets one finds only four such dijet events, zero 3-jet events, and one 4-jet event. Thus, even a small number of gluinos would have stood out in this sample. The resulting constraints on the gluino vs. neutralino masses are plotted below (the solid black line)
In the region where the mass difference between the gluino and the neutralino is not too large, the LHC constraints beat those from the Tevatron, even though the latter are based on 100000 more luminosity! Obviously, the constraints will get much better soon, as the LHC has already collected almost 10 times more luminosity and doubles the data sample every week.

These interesting constraints were not derived in the original experimental note from ATLAS. Paradoxically, many experimentalists are not enthusiastic about the idea of interpreting the results of collider searches in terms of directly observable parameters such as masses and cross sections. Instead, they prefer dealing with abstract parameters of poorly motivated theoretical constructions such as mSUGRA. In mSUGRA one makes a guess about the masses of supersymmetric particle at the scale $10^{14}$ times higher then the scale at which the experiment is performed, and from that input one computes the masses at low energies. The particular mSUGRA assumptions imply a large mass difference between the gluino and the lightest neutralino at the weak scale. In this narrow strip of parameter space the existing Tevatron searches happen to be more sensitive for the time being.