Measurement of the forward-backward asymmetry of lepton pairs and the weak-mixing angle at CMS

Physics Analysis Summary in CERN Document Server: CMS-PAS-EWK-10-011

Abstract

We present the measurement of the forward-backward asymmetry AFB for opposite sign lepton pairs (e+e- and μ+μ-) produced via an inter mediate Z/γ* in LHC proton-proton collisions at √s = 7 TeV. We also present a measurement of the effective weak-mixing angle (θeff) via a multivariate likelihood fit of the rapidity, di-lepton invariant mass, and decay angle distributions. Our results are based on an integrated luminosity of approximately 40 inverse-picobarns. We present AFB as a function of mass in 40<M(ll)<600 GeV range. From the dominant u-ubar and d-dbar → Z/γ* → μ+μ- processes, we measure a sine square θeff of 0.22870.00770.0036. We find both the AFB and the sine square θeff measurements to be consistent with the Standard Model predictions within uncertainties.

Approved plots (click on plots to get PDF versions)

CosThetaCS_e_ratio.png CosThetaCS_mu_ratio.png

Distribution of the cosine of the helicity angle (polar angle of the lepton momentum) in the Collins-Soper frame. On the left, in the electron channel; on the right, in the muon channel. The data are compared to a full CMS simulation of events generated with POWHEG.


AFB_uncor_e.png AFB_uncor_mu.png

Uncorrected forward-backward asymmetry AFB in eleven bins of the di-lepton invariant mass. On the left, in the electron channel; on the right, in the muon channel. The forward-backward asymmetry is constructed based on the sign of the cosine of the helicity angle expressed in the Collins-Soper frame. The data are compared to a full CMS simulation of events generated with POWHEG.


ZDYPLot_CmsMCG-v2_60_120_PAS.png

High statistics simulation test of the unbinned maximum likelihood fit used to extract the value of the weak mixing angle. The three observables used in the fit are: the rapidity of the di-muon pair (top left), the cosine of the helicity angle in the Collins-Soper frame (top right) and the invariant mass of the di-muon pair (bottom left in linear scale, bottom right in logarithmic scale). The high statistics simulated data is represented by black points, the projections of the fit results are represented by solid blue lines.


ZDYPLot_CmsDataG_v3_60_120_PAS.png

Result of the unbinned maximum likelihood fit to the data, corresponding to approximately 40 inverse-picobarns of integrated luminoisty. The three observables used in the fit are: the rapidity of the di-muon pair (top left), the cosine of the helicity angle in the Collins-Soper frame (top right) and the invariant mass of the di-muon pair (bottom left in linear scale, bottom right in logarithmic scale). The CMS data is represented by black points, the projections of the fit results are represented by solid blue lines. The projection of the background-only model is represented by a dashed blue line, to illustrate the negligible contribution of backgrounds in this analysis.

-- GautierHamelDeMonchenault - 18-Mar-2011

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf AFB_uncor_e.pdf r1 manage 14.5 K 2011-03-18 - 11:20 GautierHamelDeMonchenault  
PNGpng AFB_uncor_e.png r1 manage 26.0 K 2011-03-18 - 11:20 GautierHamelDeMonchenault  
PDFpdf AFB_uncor_mu.pdf r1 manage 14.5 K 2011-03-18 - 11:21 GautierHamelDeMonchenault  
PNGpng AFB_uncor_mu.png r1 manage 25.8 K 2011-03-18 - 11:21 GautierHamelDeMonchenault  
PDFpdf CosThetaCS_e_ratio.pdf r1 manage 17.7 K 2011-03-18 - 11:21 GautierHamelDeMonchenault  
PNGpng CosThetaCS_e_ratio.png r1 manage 30.6 K 2011-03-18 - 11:21 GautierHamelDeMonchenault  
PDFpdf CosThetaCS_mu_ratio.pdf r1 manage 17.5 K 2011-03-18 - 11:21 GautierHamelDeMonchenault  
PNGpng CosThetaCS_mu_ratio.png r1 manage 31.1 K 2011-03-18 - 11:21 GautierHamelDeMonchenault  
PDFpdf ZDYPLot_CmsDataG_v3_60_120_PAS.pdf r1 manage 47.3 K 2011-03-18 - 11:21 GautierHamelDeMonchenault  
PNGpng ZDYPLot_CmsDataG_v3_60_120_PAS.png r1 manage 44.2 K 2011-03-18 - 11:21 GautierHamelDeMonchenault  
PDFpdf ZDYPLot_CmsMCG-v2_60_120_PAS.pdf r1 manage 95.5 K 2011-03-18 - 11:21 GautierHamelDeMonchenault  
PNGpng ZDYPLot_CmsMCG-v2_60_120_PAS.png r1 manage 38.5 K 2011-03-18 - 11:22 GautierHamelDeMonchenault  
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Topic revision: r3 - 2011-03-25 - GautierHamelDeMonchenault
 
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