Explanation of theWWexcess at the LHC by jet-veto resummation

Abstract

The W + W − 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-p T jets with p T > p veto T that is imposed in the experimental analyses to reduce backgrounds. Jet veto introduces a second mass scale p veto T to the problem in addition to the invariant mass of the W + W − 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 O(π 2 ) terms when the standard, positive sign is chosen for the squared renormalization scale, µ 2 . We analytically resum the large logarithms including the π 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 π 2 terms are both comparably important.

Reemergence of rapidity-scale uncertainty in soft-collinear effective theory

“…[22]. Note that the central values of the predictions, defined by r = 1 and µ = p veto T , are clearly unaffected.…”

“…We are now ready to solve the rapidity RGEs (20) and (22) for an arbitrary but fixed µ. Solving them is trivial as the right-hand sides of all the ν-RGEs are independent of ν andν.…”

“…Even though the conceptual points we will be making are quite general, for definiteness we discuss a SCET formalism that is applicable to the resummation of jetveto logarithms for the production of a color-neutral object [23], such as a higgs boson [17,23,24] [21] [25][26][27], W + W − [22,28] or other di-bosons [29], at a hadron collider. Jet veto rejects an event if it contains a jet whose transverse momentum is larger than a prescribed jet-veto scale p veto T .…”

“…As an application of our factorization formula and method of estimating rapidity scale uncertainty, we will update our earlier work of the W W production with jetveto at the LHC [22], which used the formulation of Ref. [17] and thereby lacked scale uncertainties from ν dependence as well as suffered from another accidental cancellation that led to an underestimation of the total scale uncertainties.…”

“…For the NLL, h 2 was discarded andB and B are set to the corresponding PDFs. The 'π 2 resummation' [40][41][42] is included in all the cases as in our earlier work [22]. 8 The plot on the right is based on the naive factorization formula as used in Ref.…”