Investigating the Production of W and Z Bosons at LHCb

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Investigating the Production of W and Z Bosons at
LHCb

• Stephanie Donleavy

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Investigations

• Will look at
• Numbers of bosons produced
• W and Z production and decay mechanisms
• Comparison between theories

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Parton Distribution Functions (PDFs)

• A PDF describes probabilities of finding a quark
  or gluon with particular values of x and Q2
• PDFs extracted from fits to experimental data
• Theoretical prediction for W and Z production at
  LHC
• NLO electroweak PDF
• Theory error NLO error PDF error so precise
  Standard Model test can be made where theory and
  PDF are well known
• Where PDF errors are large, experimental data
  can be used to constrain them

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At LHCb

• Mainly B physics but many Ws and Zs produced as
  well.
• At an integrated luminosity of 500 pb-1
• 2.44 x 106 W bosons
• 1.2 x 105 Z bosons

(In the LHCb acceptance)
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W and Z Production

• Bosons produced at different angles depending on
  method of production
• E.g.
• qval qsea -gt forward
• qsea qsea -gt transverse

In LHCb boson seen is mostly produced by qval
qsea interactions
In this plot, x is the fraction of proton
momentum and Q is momentum transfer
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Decay channels
10.57 branching ratio
3.37 branching ratio

• I will investigate muon channels

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Kinematic Parameters

• Pseudorapidity
• Rapidity
• Angular Distributions
• Transverse mass
• Invariant mass

Use mcfm-5.1 to generate data A Monte Carlo for
FeMto barn processes (Only to generator level
no detector simulation is carried out)
Theory paper used Parton Distributions and the
LHC W and Z production. The European Physical
Journal C, 14, 133-145 (2000), A.D.Martin,
R.G.Roberts, W.J.Stirling, R.S.Thorne
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Theoretical Rapidity Distributions
Rapidity Distributions for Ws
(Rapidity)
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Rapidity Distributions
(Using CTEQ6L)
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Comparing PDF Sets

• Different PDF sets have different errors for
  different regions of rapidity
• Due to different theoretical assumptions
• Contributions also from data measurements
• Plots studied
• pseudorapidity of lepton
• rapidity of boson or lepton

PDF sets used CTEQ6M (Coordinated
Theoretical-Experimental Project on QCD) and
MRST2001 (Martin, Roberts, Sterling and Thorne)
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Systematic PDF Errors in Rapidity
Errors are from a standard error set devised by
the authors of the PDF set
Errors greater at central rapidities
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Percentage Errors

• Percentage errors are much lower in central
  regions of rapidity
• Ratios mean that some uncertainties cancel out so
  overall error is much less

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Conclusions and Further Plans

• W and Z production and decay theory
  investigated.
• PDF Sets found to be mostly within 8 of each
  other's predictions.

• For continuation to thesis,
• 2.5 fb-1 of data expected (i.e. 11 x 106 Ws and
  6 x 105 Zs)
• Use LHCb code
• Look at selection of W/Z candidates
• Understand trigger and background

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EXTRA PLOTS
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• Angular distribution

Theta is the angle of the lepton's direction with
respect to the boost direction in the rest frame
of the W.
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Acceptances
LHCb acceptance is 1.8 - 4.9 in Pseudorapidity
Acceptance ()
Cross-section (nb)
Acceptance cuts are made on lepton
From generator using cteq6m
108.75 2.61 / -2.94
79.98 2.06 / -2.38
57.78 1.32 / -1.59
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(No Transcript)
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Principle Decay Modes
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Comparing MRST and CTEQ
Plots were made of pull using the following
definition
Overall Plot shown below

• Width is not equal to 1 so separate plots were
  made for each process to analyse problem.
• Errors were found not to be independent.

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Conclusions from PDF Sets
Plots made of Pull vs Pseudorapidity for each
process
PDF Sets found to be mostly within 8 of each
other's predictions. Main differences occurred
in ratio plots at high values of rapidity /
pseudorapidity - where there is a lack of data
and theoretical assumptions are made. CTEQ
errors generally larger than MRST assumption of
u-d sea ratio
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