Measurement of the production cross section of prompt J/ψ mesons in association with a W ± boson in pp collisions at $ \sqrt{s} $ = 7 TeV with the ATLAS detector

Abstract

Measurement of the production cross section of prompt J/ψ mesons in association with a W ± boson in pp collisions at √ s = 7 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.ch Abstract: The process pp → W ± J/ψ provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb −1 of √ s = 7 TeV pp collisions at the LHC, the first observation is made of the production of W ± + prompt J/ψ events in hadronic collisions, using W ± → µν µ and J/ψ → µ + µ − . A yield of 27.4 +7.5 −6.5 W ± + prompt J/ψ events is observed, with a statistical significance of 5.1σ. The production rate as a ratio to the inclusive W ± boson production rate is measured, and the double parton scattering contribution to the cross section is estimated. The ATLAS collaboration 20 IntroductionStudy of the production of a W boson in association with a prompt J/ψ meson offers new tests of Quantum Chromodynamics (QCD) at the perturbative/non-perturbative boundary as well as developing the framework for future probes of the Higgs sector and beyond-thestandard-model searches in such final states. Perturbative calculations of heavy quarkonium production in hadronic collisions distinguish between terms that produce a heavy quark system (QQ) in a colour-singlet (CS) or a colour-octet (CO) state. The relative importance of these terms for inclusive J/ψ production is a subject of debate [1][2][3][4][5][6][7]. In the case of prompt J/ψ production in association with a W ± boson, the relative contributions of CS and CO processes differ from the inclusive process. Some theoretical studies [8,9] suggest W ± + prompt J/ψ production should be dominated by colour-octet processes, and thus be a distinctive test of the non-relativistic QCD (NRQCD) framework [10,11]. In contrast, recent work [12] suggests that in 7 TeV pp collisions, CO and CS (in particular, electromagnetic W ± γ * →W ± J/ψ) contributions to the W ± + prompt J/ψ cross section are comparable. Measurements of the production cross sections can help distinguish between these models. A search for the related processes W ± +Υ(1S) and Z + Υ(1S) performed by the CDF experiment saw no excess of events above the expected background and set upper limits on the production rate [13].Observation and measurement of W ± + prompt J/ψ production for the first time represents a step in our understanding toward measurements of the Higgs boson in rare quarkonia and associated vector boson decay modes, first proposed in ref. [14]. Recent -1 - JHEP04(2014)172phenomenological studies [15] have emphasised the value of these rare decay modes to provide a unique probe of the Higgs boson charm couplings. Such final states can also be sensitive probes of beyond-the-standard-model (BSM) frameworks. The presence of an anomalous rate of W ± /Z + prompt J/ψ/Υ associated production over standard model predictions can, for example, be an indication of a sig...

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