Monday, 3 December 2012
The LHC routinely measures cross sections of processes predicted by the standard model. Unlike the Higgs or new physics searches, these analyses are not in the spotlight, are completed at a more leisurely pace, and are forgotten minutes after publication. One such observable is the WW pair production cross section. Both CMS and ATLAS measured that cross section in the 7 TeV data using the dilepton decay channel, both obtaining the result slightly above the standard model prediction. The situation got more interesting last summer after CMS put out a measurement based on a small chunk of 8 TeV data. The CMS result stands out more significantly, 2 sigma above the standard model, and the rumor is that in 8 TeV ATLAS it is also too high.
It is conceivable that new physics leads to an increase of the WW cross section at the LHC. This paper proposes SUSY chargino pair production as an explanation. If chargino decays dominantly to a W boson and an invisible particle - neutralino or gravitino, the final state is almost the same as the one searched by the LHC. Moreover, if charginos are light the additional missing energy from the invisible SUSY particles is small, and would not significantly distort the WW cross section measurement. A ~110 GeV wino would be pair-produced at the LHC with the cross section of a few pb - in the right ballpark to explain the excess.
Such light charginos are still marginally allowed. In the old days, the LEP experiments excluded new charged particles only up to ~100 GeV, LEP's kinematic reach for pair production. At the LHC, the kinematic reach is higher, however small production cross section of uncolored particles compared to the QCD junk the makes chargino searches challenging. In some cases, charginos and neutralinos have been recently excluded up to several hundred GeV (see e.g. here), but these strong limits are not bullet proof as they rely on trilepton signatures. If one can fiddle with the SUSY spectrum so as to avoid decays leading to trilepton signatures (in particular, the decay χ1→ LSP Z* must be avoided in the 2nd diagram) then 100 GeV charginos can be safe.
Of course, the odds for the WW excess not being new physics are much higher. The excess at the LHC could simply be an upward fluctuation of the signal, or higher-order corrections to the WW cross section in the standard model may have been underestimated. Still, it will be interesting to observe where the cross section will end up after the full 8 TeV dataset is analyzed. So, if you have a cool model that overproduces WW (but not WZ) pairs, now may be the right moment to step out.