Observation of Z production in proton-lead collisions at LHCb

The paper is available here.

arXiv: 1406.2885 LHCB-PAPER-2014-022

JHEP 1407 (2014) 094

More detailed information ANA note 2014-025

Abstract

The first observation of Z boson production in proton-lead collisions at a centre-of-mass energy per proton-nucleon pair of √sN N = 5 TeV is presented. The data sample corresponds to an integrated luminosity of 1.6 nb−1 collected with the LHCb detector. The Z candidates are reconstructed from pairs of oppositely charged muons with pseudorapidities between 2.0 and 4.5 and transverse momenta above 20 GeV/c. The invariant dimuon mass is restricted to the range 60 − 120 GeV/c2. The Z production cross-section is measured to be σZ→μ+ μ− (fwd) = 13.5+5.4 −4.0(stat.) 1.2(syst.) nb in the direction of the proton beam and σZ→μ+μ− (bwd) = 10.7+8.4−5.1(stat.) 1.0(syst.) nb in the direction of the lead beam, where the first uncertainty is statistical and the second systematic.

Figures

(pdf and eps versions are available under attachments).

Caption Figure
Figure 1: Invariant dimuon mass distribution of selected Z candidates in (top) the backward and (bottom) the forward sample are shown by the black data points with error bars. The red line shows the distribution obtained from simulation using Pythia8 with the MSTW08 PDF set normalised to the number of observed candidates. Fig1a.pngFig1b.png
Figure 2: Rapidity distribution of selected Z candidates in (top) the backward and in (bottom) the forward sample are shown by the black data points. The red line shows the distribution obtained from simulation using Pythia8 with the MSTW08 PDF set normalised to the number of observed candidates. The top x-axis shows the absolute rapidity, y Lab , in the laboratory frame, the bottom one the absolute rapidity, y, in the centre-of-mass frame. Fig2a.pngFig2b.png
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. The uncertainties on the theoretical predictions are negligible compared to those on the experimental results. Fig3.png
Supplementary Figure 1_S: Regions in xA -Q2 that LHC, RHIC (purple area) or fixed target experiments (DIS +DY; yellow area) are sensitive to. The centre-of-mass energy per proton-nucleon pair for LHC is √spN = 5 TeV. The red line shows the kinematic boundary for this particular centre-of-mass energy. The blue area shows the sensitive region for LHCb for energy scales Q2 > 10 GeV2. The black horizontal line corresponds to the mass of the Z boson indicating that LHCb is sensitive down to xA 2 10−4 for measurements of the Z production. The green area shows the phase-space corresponding approximately to the ATLAS and CMS detector and the central part of the ALICE detector. Process dependent phase spaces of these experiments can be seen in Ref. [1]. The regions for RHIC and fixed target experiments have been also taken from Ref. [1]. The corresponding rapidity in the laboratory frame is shown by the dashed lines and indicated by the numbers in the plot. For the rapidity, y, in the centre-of-mass frame there is a shift of y Lab − y= ∆ y = +0.47 with respect to the rapidity, y Lab, in the laboratory frame. Fig1-S.png
Supplementary Figure 2-S: The track multiplicity distributions of events containing J/ψ candidates in (top) the backward and in (bottom) the forward direction are shown by the red data points. For comparison the distributions for events with J/ψ (black solid line) as well as Z (black dots) candidates in pp collisions at √s = 8TeV recorded in 2012 are displayed. The similarity of the distributions in the forward sample between pp and p Pb collisions is rather accidental as the two effects of a smaller pile-up in p Pb collisions and of collisions of the proton with a lead nucleus cancel each other out. The higher track multiplicity in the backward sample is caused by the lead nucleus propagating toward the detector. Fig2a-S.pngFig2b-S.png
Supplementary Figure S3: Distribution of R FB(2.5 < abs(y) < 4.0) for a true value of 1.0 from Toy Monte Carlo. The solid red line shows the central value and the dashed blue one R FB(2.5 < abs(y) < 4.0) = 1.0 The filled part of the histogram shows the region below the observed value, it corresponds to a p-value of 0.012. The discrete behaviour of the plot originates from the fact that the ratio is built from integers. Fig3-S.png

-- KatharinaMueller - 11 Jun 2014

Topic attachments
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PDFpdf Fig1-S.pdf r1 manage 420.0 K 2014-06-11 - 09:30 KatharinaMueller  
PNGpng Fig1-S.png r1 manage 411.7 K 2014-06-11 - 09:30 KatharinaMueller  
Unknown file formateps Fig1a.eps r1 manage 49.8 K 2014-06-11 - 09:22 KatharinaMueller  
PDFpdf Fig1a.pdf r1 manage 14.0 K 2014-06-11 - 09:22 KatharinaMueller  
PNGpng Fig1a.png r1 manage 13.6 K 2014-06-11 - 09:22 KatharinaMueller  
Unknown file formateps Fig1b.eps r1 manage 50.7 K 2014-06-11 - 09:22 KatharinaMueller  
PDFpdf Fig1b.pdf r1 manage 14.2 K 2014-06-11 - 09:22 KatharinaMueller  
PNGpng Fig1b.png r1 manage 14.1 K 2014-06-11 - 09:22 KatharinaMueller  
PDFpdf Fig2a-S.pdf r1 manage 15.0 K 2014-06-11 - 09:30 KatharinaMueller  
PNGpng Fig2a-S.png r1 manage 15.9 K 2014-06-11 - 09:30 KatharinaMueller  
PDFpdf Fig2a.pdf r1 manage 14.2 K 2014-06-11 - 09:22 KatharinaMueller  
PNGpng Fig2a.png r1 manage 12.5 K 2014-06-11 - 09:22 KatharinaMueller  
Unknown file formateps Fig2b-S.eps r1 manage 52.8 K 2014-06-11 - 09:30 KatharinaMueller  
PDFpdf Fig2b-S.pdf r1 manage 14.6 K 2014-06-11 - 09:30 KatharinaMueller  
PNGpng Fig2b-S.png r1 manage 15.5 K 2014-06-11 - 09:30 KatharinaMueller  
Unknown file formateps Fig2b.eps r1 manage 52.1 K 2014-06-11 - 09:23 KatharinaMueller  
PDFpdf Fig2b.pdf r1 manage 14.5 K 2014-06-11 - 09:23 KatharinaMueller  
PNGpng Fig2b.png r1 manage 12.9 K 2014-06-11 - 09:23 KatharinaMueller  
Unknown file formateps Fig3-S.eps r1 manage 54.4 K 2014-06-11 - 09:30 KatharinaMueller  
PDFpdf Fig3-S.pdf r1 manage 13.9 K 2014-06-11 - 09:30 KatharinaMueller  
PNGpng Fig3-S.png r1 manage 12.6 K 2014-06-11 - 09:30 KatharinaMueller  
Unknown file formateps Fig3.eps r1 manage 52.6 K 2014-06-11 - 09:23 KatharinaMueller  
PDFpdf Fig3.pdf r1 manage 13.8 K 2014-06-11 - 09:23 KatharinaMueller  
PNGpng Fig3.png r1 manage 15.3 K 2014-06-11 - 09:23 KatharinaMueller  
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