PPT - BNL

Document Sample
PPT - BNL Powered By Docstoc
					       Measurement of the Cross Section
          for W Boson Production at
             s = 500 GeV at STAR
                                  Justin Stevens
                            for the STAR Collaboration

                                    APS Meeting
                                  February 16, 2010



Justin Stevens - APS 2010
 Barrel EM
Calorimeter:
                                   Detector
Lepton Energy
  Veto jets




                                                    Endcap EM
                                                   Calorimeter:
                                                     Veto jets

                                           Time Projection Chamber (TPC):
                                                       Vertex
                                                  Charge Separation
                                                      Veto jets
  Pythia+Geant p+p→W→e+ν event @ 500 GeV
 Justin Stevens - APS 2010                                            2
            500 GeV Data Set from Run 9
     W Trigger:
     High Tower Hardware L0 Trigger
     (ET > 7.3 GeV)
     High ET 2x2 Cluster Software L2
     (ET > 13 GeV)




                                       Integrated Luminosity @ 500 GeV:
                                       •Vernier Scan technique used to measure cross
                                       section for high tower trigger
                                             BHT 3  481 nb  10 (stat.)  110 (syst.)
                                       •Scaling the number of background-subtracted
                                       high tower triggers by 1/σBHT3 yields the
                                       integrated luminosity of L=13.7 pb-1
                                             (See talk by Ross Corliss: Session D7)

Justin Stevens - APS 2010                                                        3
          W Algorithm: Lepton Isolation
                                        Lepton Isolation Cuts:
                                        •Require TPC track with pT > 10 GeV
                                        •Extrapolate track to Barrel Calorimeter
                                        •Require highest 2x2 cluster around
                                        pointed tower sum ET > 15 GeV
                            4x4         •Require excess ET in 4x4 cluster < 5%
                                        •Match track to 2x2 cluster position
                                  2x2



                                                                            Signal
                                                                            region




                                                      2x2 cluster ET/4x4 cluster ET
Justin Stevens - APS 2010                                                       4
W Algorithm: Suppress QCD Background
                                           Suppress jets with leading hadron
                                              •Near side jet-cone veto
                                           Suppress di-jets and multi-jet events
                                              •Away side pT sum veto
                                              •Require an imbalance in pT of the
                                              lepton cluster and any jets reconstructed
                                              outside the near side jet cone

                                            Signal
                                                               Signal
                                            region
                                                               region




                            EMC Cluster ET / 0.7 Cone ET                  Away side pT sum (GeV)

Justin Stevens - APS 2010                                                                5
    e+/e- Charge Separation at High PT
                                                     BEMC
                               TPC
                             TPC
                               TPC

vertex                                                            positron PT = 5 GeV

                                                                  electron PT = 5 GeV

                                                                   +/- distance D ~ 1/PT
                                                                     PT=5 GeV : D~15 cm
                      200 cm of tracking
                                                                     PT=40 GeV : D ~2 cm


                                                   positrons




                                                   electrons




 Justin Stevens - APS 2010                 EMC Cluster ET (GeV)                         6
                      Background Subtraction
50
         (W+ shown)
           W    
             e    e
           MC Normalized
           to L=13.7 pb-1


                                           70                Region of interest
50                                                              ET > 25 GeV
            Run 9 Data              Total
           Missing Endcap
            Vetoed QCD
                                 Background
            Background


                                                   EMC Cluster ET (GeV)
50                                                                 Background Systematic
              Normalized at                                        •Calculate different data driven
                                                  Data             QCD background shapes by
               ET<19 GeV              Ws         Driven            varying pT Balance and away
               Data Driven
                                                QCD Bkgd.          side pT cuts
               QCD Bkgd.
                                                                   •Vary normalization region
                                                                   (ET < 17 and ET < 21 GeV)
                                                                   •The largest deviation in each
          EMC Cluster ET (GeV)                                     bin gives an estimate of the
 Justin Stevens - APS 2010                                         systematic uncertainty
                            STAR Ws from Run 9




Justin Stevens - APS 2010                        8
              Data/MC Shape Comparison




        Monte-Carlo is full PYTHIA+GEANT simulation of W→e+ν events at 500 GeV


Justin Stevens - APS 2010                                                        9
                            Cross Section

         σ W   dEe  dηe e e 
                   T
                          d2σ Weν 1 1 1 1 obs
                                                      
                          dη dE T L εtrig εvertex εreco
                                                        NW  Nback   
              Kinematic acceptance : ηe  1 and Ee  25 GeV
                                                 T


    Efficiencies Calculated from full PYTHIA + GEANT simulations




Justin Stevens - APS 2010                                                10
               Cross Section Uncertainties
 • W Reconstruction Systematic
        – Track Reconstruction: 15-20%
        – Vertex Reconstruction: 4%
        – Energy Scale: < 1%
 • Normalization/Luminosity Systematic
        – Vernier scan absolute cross section: 23%
 • Background Systematic
        – Vary data driven QCD background shape and
          normalization region

Justin Stevens - APS 2010                             11
 W Production Cross Section at STAR



                            Run 9 STAR Preliminary (p+p 500 GeV)
                            W        
                                     e 
                                              61 3 (stat.) 13 (syst.) 14 (lumi.) pb
                                                             
                                                              10



                            W        
                                     e 
                                              17  2 (stat.) 3 (syst.  4 (lumi.) pb
                                                              4      )


                                     There is reasonable agreement
                                      between the measured and
                                        expected cross sections.




Justin Stevens - APS 2010                                                        12
                            Conclusions
• STAR has measured the production cross section
  for W → e+ν in p+p collisions at s = 500 GeV
• Charge separation provided by the TPC
  demonstrated for lepton ET up to 50 GeV
• Large QCD background reduced to 9% and 16% of
  the W + and W- signal, respectively at E e > 25 GeV
                                           T

• The theoretically expected and experimentally
  measured cross sections are in agreement to
  within uncertainties

Justin Stevens - APS 2010                           13

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:0
posted:3/27/2013
language:English
pages:13