More on the stop story

Last Tuesday a number of new exclusion limits for 3rd generation superpartners by ATLAS collaboration was made public. To a casual observer, the seminar presenting the results may have resembled a reenactment of the Saint Valentine's day massacre. Nevertheless I will argue here -- more out of contrariness than conviction -- that SUSY may be battered, bruised, covered in slime, and badly bleeding, but formally she is not yet dead. For most particle theorists, the death will be pronounced scalar partners of the top and bottom quarks are excluded up to ~500 GeV, the precise threshold depending on taste, education, and diet. That's because excluding light stops and sbottoms is going to squash any remaining hopes that supersymmetry can address the naturalness problem of electroweak symmetry breaking, for which purpose it was originally invented. However, the LHC collaborations have yet not presented any robust limits on the stop masses. Instead, here's what the LHC have taught us about SUSY so far:

• Since summer 2011 we know that in a generic case when all colored superpartners including stops have comparable masses and decay to the lightest supersymmetric particle (LSP) producing a considerable amount of missing energy then stops have to be heavier than about 1 TeV.
• Since last Tuesday we know if the only squarks below TeV are those of the 3rd generation then gluinos have to be heavier than about 600-900 GeV, depending on the details of the supersymmetric spectrum.

An example of the recent ATLAS bounds is shown on the right. The assumption here is that the gluino decays via an intermediate virtual stop into 2 top quarks + LSP. In that case, combination of searches in the 2 same-sign lepton channels, and the 1 lepton + 1 b jet channel excludes gluino masses all the way up to 700 GeV, a bit more if the LSP is nearly massless. Even stronger bounds are reported assuming gluinos decaying via a virtual sbottom squark into 2 bottom quarks + LSP. In that case gluino masses up to 900 GeV are excluded, see below.


However there still exists one important loophole. One can imagine that gluinos are very heavy, and only the 3rd generation squarks have masses below 1 TeV. In that case, the 3rd generation squarks have to be produced directly in gluon-gluon collision without a help from intermediate gluinos, in which case the production cross section is suppressed. Strangely enough, limits on direct stop production are currently almost non-existing. As explained in my previous post, there exist some theorist-level limits derived by reinterpreting other LHC searches which claim that stops need to be heavier than 200-300 GeV, depending on how they decay. Thus, for all we know, the stop might have a similar mass to the top quark without having been noticed at the Tevatron or LHC! Plugging this loophole is now the most important issue in SUSY phenomenology. For the direct production of sbottoms, who should come in package with stops due to the SU(2) symmetry of weak interactions, the current experimental situation is a bit better. An ATLAS search published last December excludes sbottoms below ~400 GeV for a very light LSP, although this search completely loses steam when LSP is heavier than ~100 GeV . Hopefully, we will learn much more at the Moriond conference 2 weeks from now.

To summarize, the year 2012 will surely go down in history as the Higgs year. But among more studious future historians of science it may also be remembered as the year when SUSY, at least in its natural form, was finally laid to grave...

See the new ATLAS notes here and here for more details on the searches for gluino mediated stop production. See also Peter's blog.