ZZ production in pPb collisions at LHCb
Hengne Li a , , ∗ a Guangdong Provincial Key Laboratory of Nuclear Science, Institute of Quantum Matter, South ChinaNormal University, Guangzhou 510006, China
E-mail: [email protected]
This article presents results of the Z boson production in the proton-lead collisions at √ s NN = .
02 TeV and √ s NN = .
16 TeV collected in 2013 and 2016, respectively, by the LHCb detectorat the LHC in forward and backward rapidity. The great precision of the 2016 data are foundcompatible with nPDFs theoretical predictions within large theoretical uncertainties, and can beuseful to constraint new predictions.
HardProbes20201-6 June 2020Austin, Texas On behalf of the LHCb collaboration. ∗ Speaker © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] S e p production in pPb collisions at LHCb Hengne LiThe properties of W/Z bosons have been extensively studied at electron-positron and hadroncolliders. The production cross-sections of W/Z bosons at hadron colliders can be well describedby perturbative Quantum Chromodynamics (pQCD) at next-to-next-to-leading order (NNLO), andthe radiative corrections and the input electroweak parameters are also precisely known. Themeasurements of their production cross-sections in proton-proton (pp) collisions can be used toconstrain the initial conditions such as the Parton Distribution Functions (PDFs) of the proton [1, 2].In the same respect, the production of the W/Z bosons can also precisely probe of the nuclearPDFs, especially the that of heavy quarks and gluon, which are currently less precisely con-strained [3]. In proton-ion and ion-ion collisions, the PDFs of nucleons confined in nuclei are foundto be different with respect to those of free nucleons, which are called nuclear PDFs (nPDFs) [4–8].The differences between nPDFs and PDFs are often referred as nuclear modifications, which areunderstood as a reflection of the various initial state nuclear matter effects on the free nucleons.These effects include the nuclear shadowing [9] appearing as a suppression for Bjorken- x ( x in thefollowing, the fraction of a nucleon momentum carried by a parton) below 0.05 , the anti-shadowingeffect [10, 11] raising the PDFs for x around 0.1 , the EMC effect [12] suppressing the PDFs around0 . < x < . x around 1. The nPDFsare crucial for the studies of the Quark Gluon Plasma (QGP) in the ion-ion collisions, in order todisentangle cold and hot nuclear matter effects.Moreover, since the W/Z bosons and their leptonic decay products do not participate stronginteractions, they inherit perfectively the initial conditions without being modified by the hadronicmedium in the intermediate and final states. Therefore, they can be used to better differentiatebetween properties of the initial- and final-state effects, and the proton-ion collisions provide anideal environment to study the initial-state nuclear matter effects, hence to constrain the nPDFs.In this article, we present results of the Z boson production in the proton-lead collisions [13,14] at the LHC using data collected during 2013 and 2016 by the LHCb detector. The LHCbdetector [15, 16] is a fully instrumented single-arm spectrometer in the forward region covering apseudorapidity acceptance of 2 < η <
5, providing a high tracking momentum resolution down tovery low transverse momentum ( p T ) and precise vertex reconstruction capability. The proton-leaddatasets with their recorded integrated luminosities are given in Table 1.2013 2016 √ s NN L − − − − Table 1:
Summary of the LHCb pPb datasets and the recorded integrated luminosities.
The Z boson production cross-sections in the dimuon decay channel are measured in the fiducialvolume in both the forward (pPb) and backward (Pbp) collision configurations [13, 14] based onthe following equation: σ Z → µ + µ − = [ N cand · ρ ] /[L · (cid:15) ] , where σ Z → µ + µ − is the production cross-section to be measured, N cand is the number of Z → µ + µ − candidates passing signal selection, ρ is the signal purity of the selected Z candidates, L is the integrated luminosity, and (cid:15) is thetotal efficiency including trigger, reconstruction and selection efficiencies. The fiducial volume isdefined as 60 < m µ + µ − <
120 GeV, 2 . < η µ ± < .
5, and p µ ± T >
20 GeV. The purity is measured2 production in pPb collisions at LHCb
Hengne Liusing data-driven methods, and the efficiencies are estimated using Monte-Carlo (MC) samplestogether with tag-and-probe data driven corrections.The invariant mass distributions of selected signal candidates are shown in Fig. 1 for datasetstaken in 2016 at √ s NN = .
16 TeV.
60 80 100 120 [GeV] - m + m m C a nd i d a t e s / ( G e V ) - m + m fi Z Pb data) p ( - m + m fi Z(MC PYTHIA8)
LHCb Preliminary = 8.16 TeV NN sPb p Forward (a)
60 80 100 120 [GeV] - m + m m C a nd i d a t e s / ( G e V ) - m + m fi Z data) p (Pb - m + m fi Z(MC PYTHIA8)
LHCb Preliminary = 8.16 TeV NN s p PbBackward (b)
Figure 1: (color online) The dimuon invariant mass distributions after the offline selection for pPb (a) andPbp (b) configurations, using datasets taken in 2016 at √ s NN = .
16 TeV. The red line shows the distributionsfrom simulation generated using PYTHIA 8 [17] with CTEQ6L1 [18] PDF set, normalised to the number ofobserved candidates.
JHEP09(2014)030 [ nb ] − µ + µ → Z σ syst. stat. ⊕ syst. FEWZ NNLO + MSTW08FEWZ NNLO + MSTW08 + EPS09 (NLO) = 5 TeV NN s Pb p LHCb
Figure 3 . Experimental results and the theoretical predictions for the Z → µ + µ − production cross-section. The inner error bars of the experimental results show the systematic uncertainties. Theuncertainties on the theoretical predictions are negligible compared to those on the experimentalresults. where β is the correction factor for the difference in the detector acceptance of the muonsbetween the forward and backward directions. It is evaluated using NNLO Fewz calcula-tions to be β = 2 . +0 . − . (theo.) ± . +0 . − . (PDF), where the first uncertainty isfrom the variation of the renormalisation and factorisation scale, the second the numericaland the last the uncertainty from the PDF uncertainties. The scale variation always leadsto an enhancement of β .The numbers of candidates in the common y range are 2 in the forward and 4 in thebackward samples. The measured value for R FB is R FB (2 . < | y | < .
0) = 0 . +0 . − . (stat.) +0 . − . (syst.) , where the first uncertainty is statistical, defined as the 68% confidence interval with sym-metric coverage. The 99.7% (i.e. 3 σ ) confidence interval with symmetric coverage is[0 . , . β . The systematic uncertaintiesbetween the forward and the backward directions on the purity and the reconstruction, se-lection, trigger and muon-identification efficiency are assumed to be fully correlated. Theprobability to observe a value of R FB no larger than that measured, assuming no nuclearmodifications (i.e. the true value is R FB = 1), is 1.2 %. This corresponds to a deviationwith a 2 . σ significance. The probability is estimated with a toy Monte Carlo assumingPoissonian distributions for the number of candidates.– 8 – (a) pPb Pbp [ nb ] mmfi Z s FEWZ NNPDF31FEWZ NNPDF31 + EPPS16FEWZ NNPDF31 + nCTEQ15Data
LHCb Preliminary = 8.16 TeV NN s (b) Figure 2: (color online) Measured fiducial cross-sections of Z production and theoretical calculationsusing various PDF sets with or without nuclear modification. Figure (a) is for dataset taken in 2013 at √ s NN = .
02 TeV and figure (b) is for dataset taken in 2016 at √ s NN = .
16 TeV.
The resulting fiducial cross-sections for centre-of-mass energies at √ s NN = R FB ), is particularly sensitive to cold nuclear effects. R FB is measured in the common rapid-ity region (2 . < | y ∗ | < .
0) in the centre-of-mass frame of the produced Z boson using 20163 production in pPb collisions at LHCb
Hengne Li - - * y D a t a / T h e o r y pQCQ + NNPDF31 + nCTEQ15 <4.5) m h >20 GeV, 2< m T LHCb 8.16 TeV (p <4.5) m h >20 GeV, 2< m T LHCb 5.02 TeV (p <4) m h >20 GeV, 2.5< m T ALICE 5.02 TeV (p |<2.4) l h >20 GeV, | lT CMS 5.02 TeV (pATLAS 5.02 TeV (full lepton phase space)
LHCb Preliminary) - l + l fi (Z s pPb, Figure 3: (color online) Comparison of LHCb 8.16 TeV results with previous 5.02 TeV results from ATLAS,CMS, ALICE, and LHCb. The uncertainties on the data over theory ratios include only the experimentalstatistical and systematic uncertainties; the PDF uncertainties are shown separately on the line at one by thegrey band. The central values of the LHCb and ALICE results at 5.02 TeV are shifted to left and right by 0.1units in rapidity, respectively, for better visibility. dataset [14] as R . < | y ∗ | < . = . ± . ( stat ) ± . ( syst ) ± . ( lumi ) , which is compati-ble with theoretical calculations using FEWZ with the following nPDFs: R . < | y ∗ | < . , NNPDF3 . + EPPS16 = . ± . ( theo . ) ± . ( num . ) ± . ( nPDF ) , and R . < | y ∗ | < . , NNPDF3 . + nCTEQ15 = . ± . ( theo . ) ± . ( num . ) ± . ( nPDF ) , where, the uncertainty “num.” is from the numerical precision.In summary, LHCb provides an excellent opportunity to probe the cold nuclear matter effectsin the very forward region using Z boson production. Results of pPb collisions at 5.02 TeV and8.16 TeV results are presented, which are compatible with theoretical predictions involving nPDFs,where the 8.16 TeV results give the highest precision in the forward region at LHC, thus can beuseful in constraining the current nPDFs. Acknowledgments
We acknowledge the support from Science and Technology Program of Guangzhou (No. 2019050001).
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