Tuesday, November 30, 2010

HERA excluded R-parity violating squarks up to 275-290 GeV

There are many experimental particle physics preprints on the arXiv today.

The CMS experiment at the LHC studies die photon Bremsstrahlung which may be useful for various future calibration purposes etc.

HERA tunnel

BABAR has looked at two-photon physics and complicated decays of charmonium states.

In a widely publicized result, ATLAS at the LHC publishes its results about the dijet asymmetry in lead-lead collisions. Some jets - fraternities of hadrons arising from a single quark or gluon - apparently lose lots of energy by going through a hot, dense medium. This QCD effect can't be seen in the proton-proton collisions.

A London physicist defends the "simple" measurement of all resonances between 1.91 GeV and 2.41 GeV.

D0 at the Tevatron measured the WW and WZ production. It all agrees with the Standard Model and excludes new W-prime bosons and Randall-Sundrum gravitons up to 700 GeV or so.

The Tevatron teams including both D0 and CDF publish their extended Higgs boson search. Taking 12/fb of combined p-pbar data into account and looking at many channels, they exclude the modest interval 158-175 GeV we've heard about previously.

Another paper linked to D0 at the Tevatron talks about soft QCD results.

But I personally find the search for squarks at HERA to be the most interesting paper. The LHC and the Tevatron are not the only two colliders whose data are still being evaluated. HERA at DESY, Germany - closed in 2007 - collided positrons or electrons with protons at the 319 GeV center-of-mass energy. Taking about 0.2/fb of data of each type, they excluded upper-type squarks below 275 GeV and lower-type squarks below 290 GeV at the 95% confidence level.

It makes some sense to call the squarks below this threshold or 300 GeV "light". So HERA has ruled out light squarks. Many SUGRA models predicts squarks below 1 TeV. With this HERA insight and related exclusions at the Tevatron, it is somewhat unlikely that squarks are already seen at the available LHC data.

However, as onymous has emphasized and I previously missed, the exclusion only applies to R-parity violating theories - so the result is far less interesting than argued above. Of course, I believe that R-parity is conserved, because the LSP should be the dark matter candidate etc. Even more seriously, they need to assume some strange lepton-violating couplings. You should ignore the last sentence of the previous paragraph. If I appreciated these "details", the HERA paper would probably not become the top hep-ex paper for me.

The 50/pb of proton-proton collisions per detector that the LHC has acquired so far is probably marginally exceeding the reach of the whole Tevatron in the search for new physics at this stage. It's still plausible that new physics is already seen in the data and will be presented during the winter conferences.

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