Tuesday 21 February 2012

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.

11 comments:

Anonymous said...

Over Motl's dead body?

Anonymous said...

Will Gordon Kane attend the funeral?

Rastus Odinga Odinga said...

It's hilarious, Motl was on the edge of his seat on Feb 14, only to hear.... nothing. He updated his blog entry, but took care not to let it rise again to the top [as he usually does for updates] and then posted a lot of crap so as to push it down as far as possible. He did add something, however: he tried to blame the theorists at CERN for all this embarrassment. It's all a conspiracy, no doubt!

Jester said...

Rastus, it's more complicated than that. The stop rumor was indeed circulating among theorists, although it concerned an excess in one CMS search that was being interpreted as gluino mediated stop production. Anyway, after the latest ATLAS update the stop interpretation of that excess is unlikely.

Anonymous said...

A Higgs of 125 GeV is good news for the MSSM since is within the range expected. Such sort of intermediate value kills several BSM options like Technicolor, Slit SUSY, Higgssless models,.. So the MSSM is in better shape compared to many other options. On the other hand a value of 125 GeV within the MSSM requires a relatively heavy spectrum It is unlikely that the 8 TeV run will be able to discover SUSY, we know that from the start. This is no excuse: a 125 GeV Higgs requires a heavy SUSY spectrum. Indeed this means that the MSSM requires a 1-0.1 percent fine-tuning and you may argue it should be abandoned on naturality grounds. However SUSY still solves the BIG hierarchy problem of 16 orders of magnitude, which is the tough problem. Aparent fine-tunings at 1-0.1 percent level are abundant in physics and insisting that in physics all parameters should be of order one is missleading. An example of fine-tuning of this size is the masses of the u,d quarks which have to be fine-tuned so that both the proton and deuteron are stable and heavy elements can be produced. Nobody abandons the SM because of that. So, yes, many of us think that we will have to wait for a high luminosity 14 TeV LHC to discover SUSY. And if nothing but a 125 Higgs is found in the 8 TeV run, that possibility will become stronger and stronger.

Anonymous said...

Amen to what Nima said.

Anonymous said...

No, it is not Nima thinking. Nima would have tried to convince you that a fine-tuned MSSM is nothing but Split SUSY with Squarks a bit light.....

Anonymous said...

I'm sure that Gordon Kane is happy that that light stop rumor was a dud as it would have surely killed his favorite pet model with 10-100 TeV squarks, which he's been promoting over the past five years.

Luboš Motl said...

I just want to say an obvious thing, namely that nothing is changing about the outstanding rumor that the LHC has detected stop squarks.

All the new ATLAS papers only use 2.05/fb at most while the folks have had access to 5/fb for quite some time.

Jester will ultimately pay those $10,000 because SUSY will prove him wrong. However, whether or not some sfermions may be light remains to be seen.

Otherwise, I am amazed what kind of anonymous junk you allow to be posted on your blog. I would trace them and make sure that the authors are moved to a dumping ground for toxic waste.

Rastus Odinga Odinga said...

Hi Jester, sure I understand that. Never mind. What a pity that you didn't state your bet with LM with a sub-clause stating that he has to pay you $100 each time he wrongly predicts that a discovery of supersymmetry is about to be announced. That would give you a nice little steady flow of income right through to retirement. Yours I mean, not of course LM's....

Anonymous said...

So no conclusive proof of the Higgs particle and none of the stop either. Anyone working on Higgsless theories?