An Explanation of the WW Excess at the LHC by Jet-Veto Resummation

The WW production cross section measured at the LHC has been consistently exhibiting a mild excess beyond the SM prediction, in both ATLAS and CMS at both 7-TeV and 8-TeV runs. We provide an explanation of the excess in terms of resummation of large logarithms that arise from a jet-veto condition, i.e., the rejection of high-pT jets with pT > pT(veto) that is imposed in the experimental analyses to reduce backgrounds. Jet veto introduces a second mass scale pT(veto) to the problem in addition to the invariant mass of the WW pair. This gives rise to large logarithms of the ratio of the two scales that need to be resummed. Such resummation may not be properly accounted for by the Monte Carlo simulations used in the ATLAS and CMS studies. Those logarithms are also accompanied by large pi^2 terms when the standard, positive sign is chosen for the squared renormalization scale. We analytically resum the large logarithms including the pi^2 terms in the framework of soft collinear effective theory (SCET), and demonstrate that the SCET calculation not only reduces the scale uncertainties of the SM prediction significantly but also renders the theory prediction well compatible with the experiment. We find that resummation of the large logarithms and that of the pi^2 terms are both comparably important.