Measurement of the low-mass Drell-Yan differential cross section at s $$ \sqrt{s} $$ = 7 TeV using the ATLAS detector

Abstract

The differential cross section for the process Z/γ * → ( = e, µ) as a function of dilepton invariant mass is measured in pp collisions at √ s = 7 TeV at the LHC using the ATLAS detector. The measurement is performed in the e and µ channels for invariant masses between 26 GeV and 66 GeV using an integrated luminosity of 1.6 fb −1 collected in 2011 and these measurements are combined. The analysis is extended to invariant masses as low as 12 GeV in the muon channel using 35 pb −1 of data collected in 2010. The cross sections are determined within fiducial acceptance regions and corrections to extrapolate the measurements to the full kinematic range are provided. Next-to-next-to-leading-order QCD predictions provide a significantly better description of the results than next-to-leadingorder QCD calculations, unless the latter are matched to a parton shower calculation. Keywords: Hadron-Hadron Scattering Conclusion 24The ATLAS collaboration 29 IntroductionThe Drell-Yan (DY) process of dilepton production in hadronic interactions [1] provides important information on the partonic structure of hadrons which is distinct from that obtained in deep inelastic scattering (DIS) measurements (for a recent review see ref.[2] and the references therein). Recent measurements from ATLAS [3, 4], CMS [5][6][7] and LHCb [8, 9] provide further information in a new kinematic domain. Measurements reported here are made below the mass of the Z resonance and extend to a lower invariant mass than previous ATLAS measurements. In addition the cross sections are normalized by luminosity rather than to the Z mass peak cross section. The data are compared to theoretical calculations of the DY process, which can now reliably be performed at nextto-next-to-leading-order (NNLO) precision [10][11][12][13]. Calculations at next-to-leading-order (NLO) accuracy are also available matched to resummations at leading-logarithm (LL) or next-to-leading logarithm (NLL) precision [14, 15] to accommodate soft collinear partonic emission in the initial state. A quantitative comparison of the data to the calculations is presented including a QCD fit to the parton distribution functions, and a detailed discussion of theoretical uncertainties is given.-1 - JHEP06(2014)112Measurements in the region of low dilepton invariant mass, m < 66 GeV, provide complementary constraints on the parton distribution functions (PDFs) to measurements near to the mass of the Z resonance. At low m , the cross section is dominated by the electromagnetic coupling of qq pairs to the virtual photon (γ * ), whereas at masses near the Z pole the axial and vector weak couplings of the qq pair to the Z boson dominate. Therefore the observations reported here have a different sensitivity to up-type and downtype quarks and anti-quarks compared to measurements near the Z resonance.The new kinematic region accessible at the LHC operating at a centre-of-mass energy of √ s = 7 TeV and the rapidity coverage of the ATLAS detector allow low partonic momentum fractions, x ∼ 3 × 10 −4 ...

Parton distributions from high-precision collider data

“…Aside from the updated legacy datsets, in NNPDF3.1 a large number of recent measurements from ATLAS, CMS and LHCb are included. For ATLAS, we now include the Z boson (p Z T , y Z ) and (p Z T , M ll ) double differential distributions measured at 8 TeV [71]; the inclusive W + , W − and Z rapidity distributions at 7 TeV from the 2011 dataset [72], the top-quark pair production normalized y t distribution at 8 TeV [73]; total cross-sections for top quark pair production at 7, 8 and 13 TeV [74,75]; inclusive jet cross-sections at 7 TeV from the 2011 dataset [76]; and finally low mass Drell-Yan M ll distributions at 7 TeV from the 2010 run [77]. The transverse momentum spectrum at 7 TeV (2011 dataset) [78] will be studied in Sec.…”

“…Also new to NNPDF3.1 is the ATLAS low-mass Drell-Yan data from Ref. [77]. We use only the low-mass DY cross-sections in the muon channel measured from 35 pb −1 2010 dataset, which extends down to M ll = 12 GeV.…”

Parton distributions with small-x resummation: evidence for BFKL dynamics in HERA data

“…The settings of the fits described in this work follow closely those of the recent NNPDF3.1 global analysis [79]. In particular, the same input dataset is used, which includes fixed-target [125][126][127][128][129][130][131][132] and HERA [68] DIS inclusive structure functions; charm and bottom cross-sections from HERA [133]; fixed-target Drell-Yan (DY) production [134][135][136][137]; gauge boson and inclusive jet production from the Tevatron [138][139][140][141][142]; and electroweak boson production, inclusive jet, Z p T distributions, and tt total and differential cross-sections from ATLAS [143][144][145][146][147][148][149][150][151][152][153][154][155][156][157], CMS [158][159][160][161][162][163][164][165][166][167][168][169] and LHCb …”

“…Specifically, we show in Fig. 6.4 the predictions for the low-mass DY cross-sections from ATLAS at 7 TeV [156], the lowest invariant mass bin for the CMS Drell-Yan cross-sections double-differential in y and M ll at 8 TeV [195], as well as for forward W + and Z production at be smaller than the corresponding PDF errors (and in some cases also smaller than the shift between the NNLO and NNLO+NLLx curves), we can conclude from this exercise that the inclusion of these data into a fully consistent small-x resummed global PDF fit might provide further evidence for BFKL dynamics, this time from high-precision electroweak LHC crosssections as opposed to from lepton-proton deep-inelastic scattering.…”

Drell-Yan plus jet production and BFKL evolution

“…Despite these facts, many theoretical and phenomenological analyses of the DY process were made [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. The theoretical improvement was accompanied by many experimental results from fixed-target [32][33][34][35] and collider [36][37][38][39][40] experiments.…”