Tuesday 22 March 2011

The World after Moriond Electroweak

Rencontres de Moriond is a conference series taking place high in the Italian Alps where particle physics experiments like to present their latest analyses. This year the quality of snow was not quite satisfactory, but the quality of physics results somewhat made up for it. Some highlights have already been discussed on this and other blogs, but I think it's worth giving a short recap anyway.
  • New SUSY searches from ATLAS and CMS.
    Searches in several new channels have been presented: jets+MET+b-tags (relevant for sbottom production), trileptons (relevant for gauge mediated models), e-mu resonance (relevant for certain R-parity violating scenarios), and so on. No excess has been seen, and the parameter space of SUSY has been further constrained. Later I may write something more about the significance of these results, but actually the most striking observation here is how both experiments illustrated their analyses. CMS showed a series of cartoons that pretty accurately summarizes the evolution of SUSY from the early 90s till today:
    ATLAS, on the other hand, for the illustration picked a screen from Impossible Mission, a computer game from the 80s. Those who grew up on ZX Spectrum remember well that the game was long, frustrating, and actually impossible to complete ;-)
  • New Higgs combination from the Tevatron.
    In the last couple of years we got used to Tevatron shrinking the available range for the Higgs mass. This time the 95 percent exclusion range is actually slightly worse than in summer 2010 due to an upward background fluctuation. This may be a sign that the Tevatron Higgs searches are approaching the end of the line, and the full data set that will be analyzed next year may not bring significant improvements. See Tommaso's blog for more comments.
  • First exciting Higgs results from the LHC.
    The ATLAS and CMS searches for Higgs-like scalars in the tau-tau final state chops off a new portion the MSSM Higgs parameter space. See this post for more details.
  • LHC measurements of top quark properties.
    Top is at the moment the hottest issue in particle theory due to the anomaly in the forward-backward asymmetry of the top quark production measured by Tevatron's CDF. Most new physics explanations predict new phenomena that should affect various top quark properties measured at the LHC. CMS flashed some plots the very important distribution of the differential top pair production cross section as a function of the pair invariant mass.Nothing unusual can be seen there, except for a small glitch near 700 GeV. This measurement should severely constrain some explanations of the CDF anomaly, for example those involving the heavy gluon partner.
Two additional results are worth pointing out. Both are rather a show-off at this point, but they demonstrate the amazing potential of the LHC experiment and promise interesting physics in the coming year.
  • CMS observation of single top.
    It took 8 years of the Tevatron Run-2 to pinpoint the single top production, and even now it cannot be extracted from the background without some neural network hocus pocus. On the other hand, CMS was able to observe that process with just 35 fb-1 of data. As new physics often modifies the coupling of the top quark to W and b, which in turn affects the cross section for the single top production, single top may provide important constraints or discoveries in the near future. See also this post on Symmetry Breaking.
  • LHCb limits Bs → μμ.
    This is supposed to be the flag measurement of the LHCb experiment. The importance of this rare decay process stems from the fact that the branching fraction is very suppressed in the Standard Model whereas it can easily be enhanced in many theories beyond the Standard Model, in particular in the MSSM. The first LHCb limit is already close to that from the Tevatron, so that LHCb should take over already this summer. See Collider Blog for more details on the measurement.
This week is taking place the second part of Moriond'11 oriented more on QCD, so we are guaranteed another load of interesting results. As for me, I'm dying to see more of dijets and top quarks results.


Anonymous said...

Some post-SM theories predict that the cross section of the W and the Z, at a given energy, depend on their polarization. Longitudinal and transversal polarization should have different cross sections.

Is there a way to measure such an effect at the Tevatron or the LHC?

Anonymous said...

Thank for the nice summary. Poor susy :)

Kea said...

Cool, thanks. Gotta love Susy's vanity and gruesome demise.

piscator said...

CMS really needed only 35 fb-1 of data to observe single top production - well impressive, obviously far better than the Tevatron ;)

Jester said...

Anon#1, yes one can measure W polarization by looking at the angular distribution of its decay products. CMS has already done this measurement, see http://indico.in2p3.fr/getFile.py/access?contribId=92&sessionId=2&resId=0&materialId=slides&confId=4403

wolfgang said...


according to Wikipedia the bug on the ZX version

"only sometimes made the game unwinnable; not always"

so this indicates that SUSY could still be the winner 8-)

Jester said...

Oh, I didn't know there was a bug...I've always thought it was impossible on purpose. I take your point: there may be another computer, I mean another universe, where finding SUSY is possible ;-)

Luboš Motl said...

Dear Jester, see Impossible Mission. The Atari 7800 version had a bug that always made it unwinnable; the ZX Spectrum had the same bug but it only made it unwinnable sometimes.

Most importantly, let me mention that as a kid, I spend years with Commodore 64. C64 had the original version of the game and it didn't have any bug. ;-) So let's hope that if and then the exciting thing happens, you won't disappear into another Universe with my $10,000. ;-)

wolfgang said...

... the real question is what $10,000 will buy in a few years 8-(

Unknown said...

Wolfgang: maybe a subscription to Penthouse magazine?