Measurement of the exclusive Υ production cross-section in pp collisions at s = 7 $$ \sqrt{s}=7 $$ TeV and 8 TeV

Abstract

A study is presented of central exclusive production of Υ(nS) states, where the Υ(nS) resonances decay to the µ + µ − final state, using pp collision data recorded by the LHCb experiment. The cross-section is measured in the rapidity range 2 < y(Υ) < 4.5 where the muons are reconstructed in the pseudorapidity range 2 < η(µ ± ) < 4.5. The data sample corresponds to an integrated luminosity of 2.9 fb −1 and was collected at centreof-mass energies of 7 TeV and 8 TeV. The measured Υ(1S) and Υ(2S) production crosssections are σ(pp → pΥ(1S)p) = 9.0 ± 2.1 ± 1.7 pb and σ(pp → pΥ(2S)p) = 1.3 ± 0.8 ± 0.3 pb, where the first uncertainties are statistical and the second are systematic. The Υ(1S) crosssection is also measured as a function of rapidity and is found to be in good agreement with Standard Model predictions. An upper limit is set at 3.4 pb at the 95% confidence level for the exclusive Υ(3S) production cross-section, including possible contamination from χ b (3P ) → Υ(3S)γ decays. The LHCb collaboration 15 IntroductionCentral exclusive production (CEP) of Υ(nS) (n = 1, 2, 3) resonances in pp collisions is thought to occur by photoproduction through the exchange of a photon and a pomeron (a colour-singlet system) between two protons, as illustrated in figure 1. Since the protons do not dissociate, typically only a small component of momentum transverse to the beam direction (p T ) is exchanged in the interaction. The photoproduction of Υ resonances at LHCb can be computed using perturbative quantum chromodynamics (QCD), given the high photon-proton centre-of-mass energy, W , and the cross-section depends on the square of the gluon parton-density function, g(x), where Bjorken-x is the fraction of the proton's momentum carried by the gluon [1]. Measurements of the production cross-sections for the Υ(nS) resonances in the forward region covered by the LHCb detector are sensitive to g(x) in the region of small x down to approximately 1.5 × 10 −5 , where the knowledge of g(x) is limited. Furthermore, predictions for the Υ(nS) cross-sections at leading order (LO) and next-to-leading order (NLO) in the strong-interaction coupling differ greatly for the values of W probed in Υ(nS) resonance production, and there are significant variations depending on the models used to describe the Υ wave function and the t-channel exchange [1][2][3]. Quarkonia photoproduction has been studied in exclusive production at HERA [4-9], the Tevatron [10] and the LHC [11][12][13]. At LHCb, exclusive production is associated with the absence of significant detector activity apart from that associated with the exclusive candidate. The background from proton dissociation occurring outside the detector acceptance is characterised as having a value of Υ candidate p T which is larger than that for exclusive production.-1 -

Solution to the Balitsky-Kovchegov equation with the collinearly improved kernel including impact-parameter dependence

“…The prediction is compared with experimental data obtained at HERA by H1 [73] and ZEUS [74] experiments. It is also compared with the two latest measurements -in proton-proton collisions at √ s = 7 TeV and √ s = 8 TeV by LHCb [75], and in proton-lead collisions at √ s = 5.02 TeV by the CMS experiment [76]. The description of the data is good, although the large uncertainties prevent us from making any strong conclusions regarding the agreement of the predictions with the data.…”

“…Comparison of the predictions of the model (solid lines) with the HERA data from H1[73] and ZEUS[74], and LHC data from LHCb[75] and CMS[76] for the W γp dependence ofthe exclusive photoproduction cross section of the Υ(1S) meson. of double and single collinear logarithms.…”

Dissociative production of vector mesons at electron-ion colliders

“…The exclusive photoproduction cross section is compared with ZEUS [40] and H1 [41] data from HERA. It is also compared with LHCb data taken in proton-proton collisions at √ s = 7 TeV and √ s = 8 TeV at the LHC [42].…”

Central Exclusive Production at LHCb

“…LHCb studied CEP of ϒ(nS) states, where the resonances decay to the µ + µ − final state [11]. For the µ + µ − final state both muons lie in the pseudorapidity range 2 < η(µ ± ) < 4.5 and the ϒ(nS) candidate is reconstructed in the rapidity range 2 < y(ϒ(nS)) < 4.5.…”