Measurement of the double-differential high-mass Drell-Yan cross section in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

This paper presents a measurement of the double-differential cross section for the Drell-Yan Z/γ * → + − and photon-induced γγ → + − processes where is an electron or muon. The measurement is performed for invariant masses of the lepton pairs, m , between 116 GeV and 1500 GeV using a sample of 20.3 fb −1 of pp collisions data at centre-of-mass energy of √ s = 8 TeV collected by the ATLAS detector at the LHC in 2012. The data are presented double differentially in invariant mass and absolute dilepton rapidity as well as in invariant mass and absolute pseudorapidity separation of the lepton pair. The single-differential cross section as a function of m is also reported. The electron and muon channel measurements are combined and a total experimental precision of better than 1% is achieved at low m . A comparison to next-to-next-to-leading order perturbative QCD predictions using several recent parton distribution functions and including next-to-leading order electroweak effects indicates the potential of the data to constrain parton distribution functions. In particular, a large impact of the data on the photon PDF is demonstrated. JHEP08(2016)009The ATLAS collaboration 44 IntroductionThe Drell-Yan (DY) process [1] of lepton pair production in hadronic interactions, pp → Z/γ * + X with Z/γ * → + − , is a powerful tool in understanding the nature of partonic interactions and of hadronic structure in detail. The study of this process has been fundamental in developing theoretical perturbative calculations of quantum chromodynamics (QCD) which are now performed at next-to-next-to-leading-order (NNLO) accuracy [2][3][4][5]. Measurements from the Large Hadron Collider (LHC) of neutral-and charged-current Drell-Yan processes mediated by Z/γ * and W exchange respectively at centre-of-mass energies of √ s = 7 TeV and 8 TeV have been recently published by the ATLAS [6-8], CMS [9][10][11][12] and LHCb [13][14][15][16][17] collaborations. These data provide new constraints on the parton distribution functions (PDFs) of the proton, some of which have been used in recent global PDF fits [18][19][20].Although on-shell Z and W boson measurements provide the greatest experimental precision, they are restricted in the kinematic range of partonic momentum fraction x, and four-momentum transfer Q = m , the invariant mass of the dilepton pair. Offshell measurements provide complementary constraints in a wider range of x and Q. In the neutral-current case, the off-shell measurements are dominated by the electromagnetic quark couplings to the virtual photon γ * , whereas the on-shell measurements are dominated by the weak axial and vector couplings of the quarks to the Z boson. Therefore, the measurements have different sensitivity to the up-type and down-type quarks. At large m the measurements offer constraints on the large-x antiquark PDFs which are poorly known. In addition, off-shell measurements may also be sensitive to the largely unconstrained photon PDF [7, 8, 21, 22] through the photon-induced (PI) process γγ → + ...

Photon-initiated processes at high mass

“…13 of Ref. [19]. 6 While a full refit would be required to make a completely firm conclusion, this nonetheless provides a strong indication that the larger photon-initiated cross section predictions allowed by the higher-x NNPDF photon uncertainty band are already disfavored by this ATLAS data, while our results, which predict lower photon-initiated cross sections, are qualitatively consistent.…”

“…Within this approach, we will find that the decrease in the γγ luminosity is qualitatively similar to the quark and gluon cases. We will also discuss how the recent ATLAS measurement of high-mass lepton pair production [19] and the corresponding extraction of the photon PDF presented in this analysis lends qualitative support to our results.…”

“…Indeed, this expectation is supported by the recent ATLAS measurement [19] of high-mass lepton pair production in the 116 < M ll < 1500 GeV region at ffiffi ffi s p ¼ 8 TeV, corresponding to 0.015 ≲ x ≲ 0.2. We show the predicted cross section for this mass region in Fig.…”

“…However, in Ref. [19] a Bayesian reweighting exercise is performed, and it is found that a photon PDF which lies systematically on the lower end of the NNPDF2.3 uncertainty band is preferred, with greatly reduced uncertainties; see in particular Fig. 13 of Ref.…”

The photon content of the proton

“…(5.3). The polarisation asymmetry to order ασ 0 is 5) where the 1/ term is an infrared divergence, and we have used the 't Hooft-Veltman scheme for γ 5 . Combining with eqs.…”