Measurement of the lepton charge asymmetry in inclusive W production in pp collisions at √s = 7 TeV

CERN-PH-EP-2011-024 ; CMS-EWK-10-006-003. - 2011. Preprint in CERN Document Server


A measurement of the lepton charge asymmetry in inclusive pp → WX production at √s = 7 TeV presented based on data recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of 36 inverse-picobarns. This high precision measurement of the lepton charge asymmetry, performed in both the W → eν and W → μν channels, provides new insights into parton distribution functions.

Approved plots

(click on plots for PDF versions)


Fig. 1: Example of signal extraction in the W → eν channel (top two plots) and the W → μν channel (bottom two plots) for the bin in lepton pseudorapidity 0<|η|<0.4. The left plots corresponds to positive leptons; the right plots, to negative leptons. In the W → eν channel, the signal yield is extracted from a templated fit to the transverse missing energy distribution; in the W → μν channel, the signal is extracted from an unbinned maximum likelihood fit to the distribution of the modified isolation variable ξ.


Fig. 2 : Lepton charge asymmetry in six bins of absolute lepton pseudorapidity |η|, in the W → eν channel (light blue points) and the W → μν channel (light pink points). The size in pseudorapidity of the six bins are different for electrons and muons; they are indicated as strips in the lower part of the bottom panel. The electron channel covers pseudorapidities in the range 0<|η|<2.4 with a gap at the barrel/endcap transition of the electromagnetic calorimeter ECAL; the muon channel covers pseudorapidities in the range 0<|η|<2.1.The top panel is obtained with a threshold on the lepton transverse momentum of 25 GeV/c; the bottom panel, with a threshold of 30 GeV/c. No threshold on the transverse missing energy (related to the transverse energy of the undetected neutrino) is applied. The error bars correspond to the sum in quadrature of statistical and systematic uncertainties. The CMS data are compared to two theory predictions at the NLO in QCD obtained with the MCFM program and two different sets of Parton Density Functions, CT10W and MSTW2008NLO. The indicated 90% C.I. bands are obtained following the prescriptions of the PDF4LHC working group.

Additional plots


Theory curves courtesy Juan Rojo (NNPDF).


Theory curves courtesey Katerina Lipka (HERAPDF).


The CMS lepton charge asymmetry, for muons, with a pT threshold at 20 GeV, is shown together with the LHCb data points.
The data points are compared to a NLO prediction with MCFM + MSTW08NLO.

This plot, produced for the first LPCC Electroweak Working Group Meeting, is very preliminary

  • CMS charge asymmetry with a threshold at 20 GeV is not available in the electron channel (due to trigger requirements)
  • The systematic uncertainties on the CMS points are not finalized:
    the 20 GeV CMS points should not be used by PDF fitters
  • The binning of the CMS point is not that of the publication. The first three bins have been divided in two. This is a first attempt to define a binning more consistent between ATLAS and CMS in the central region
  • The LHCb data points are not completely corrected, and cannot in principle be compared to CMS points
  • LHCb are updating their lepton charge asymmetry results to their full 2010 dataset
  • ATLAS are working at correcting their data points so that they can be included in such representation
  • Eventually the ATLAS, CMS and LHCb asymmetry will be presented for lepton pT threshold at 25 GeV and 30 GeV, with no cut on the missing transverse energy (neutrino pT) and the transverse mass, and combined

-- GautierHamelDeMonchenault - 18-Mar-2011

Topic attachments
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PDFpdf Fig2_25_HERA10.pdf r1 manage 16.2 K 2011-03-19 - 22:57 GautierHamelDeMonchenault  
PNGpng Fig2_25_HERA10.png r1 manage 41.2 K 2011-03-19 - 22:55 GautierHamelDeMonchenault  
PDFpdf Fig2_25_HERA15.pdf r1 manage 16.2 K 2011-03-19 - 22:58 GautierHamelDeMonchenault  
PNGpng Fig2_25_HERA15.png r1 manage 41.4 K 2011-03-19 - 22:56 GautierHamelDeMonchenault  
PDFpdf Fig2_25_NNPDF.pdf r1 manage 16.5 K 2011-03-19 - 22:58 GautierHamelDeMonchenault  
PNGpng Fig2_25_NNPDF.png r1 manage 44.3 K 2011-03-19 - 22:54 GautierHamelDeMonchenault  
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PNGpng Fig2_30_NNPDF.png r1 manage 43.9 K 2011-03-19 - 22:55 GautierHamelDeMonchenault  
PDFpdf WAsymm_20_ACL.pdf r1 manage 15.2 K 2011-04-17 - 08:38 GautierHamelDeMonchenault  
PNGpng WAsymm_20_ACL.png r1 manage 32.4 K 2011-04-17 - 08:39 GautierHamelDeMonchenault  
PDFpdf fig1.pdf r1 manage 36.5 K 2011-03-18 - 09:53 GautierHamelDeMonchenault  
PNGpng fig1.png r1 manage 44.2 K 2011-03-18 - 09:54 GautierHamelDeMonchenault  
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