Investigating the Production of
W and Z bosons at LHCb
Stephanie Donleavy
Will look at:
• Numbers of bosons produced
• W and Z production and decay
mechanisms
• Compare to theory
At LHCb
•
Mainly b physics but many Ws and Zs
produced as well.
•
At an integrated luminosity of 500 pb-1:
– 1.5 x 106 W+s, 9.4 x 105 W-s
– 1.2 x 105 Z0s
(In the LHCb acceptance)
Bosons produced at different angles
depending on method of production:
E.g.
➢ q + q -> forward
➢ q + q -> transverse
In LHCb boson seen is
mostly produced by q + q
interactions
• Principle decay modes:
I will investigate muon channels
Histogrammed parameters:
–
–
–
–
–
Pseudorapidity
Rapidity
Angular Distributions
Transverse mass
Invariant mass
Plotted for each
boson and compared
to theory.
Use mcfm-5.1 to generate data – A Monte Carlo for FeMto
barn processes
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
Rapidity Theory
Rapidity Distributions for Ws
  E  pz  
0.5

ln
Rapidity:


(
E

p
)
z 

It is a measure of a particle's
angle wrt the beam axis
Rapidity Results
(Y-axes are just number of particles)
PDF sets - motivations
• In regions where the PDF is most accurate
the most precise test of the standard
model can be made
• In regions without accurate PDF sets, data
input to theory to improve knowledge and
prediction capability
PDF sets used : cteq6m and mrst2001
Comparing different sets
Different PDF sets have different errors for different
regions of rapidity
•Due to different theoretical assumptions
Program made to plot:
• pseudorapidity of lepton and / or neutrino
• rapidity of boson or lepton
The following graph was produced of rapidity to look at
PDF errors in different regions
Errors greater at central rapidities
Plots of Percentage error
•Percentage errors are much lower in central regions
of rapidity
•Ratios mean that some unknowns cancel out so
overall error is much less
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.
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
Further Plans:
For continuation to thesis,
•~2.5 fb-1 of data expected
•Use LHCb code
•Look at selection of W/Z candidates
•Understand trigger and background
EXTRA PLOTS
• Angular distribution:
Theta is the angle of the lepton's direction with respect to the
boost direction in the rest frame of the W.
Acceptances
LHCb acceptance is 1.8 - 4.9 in Pseudorapidity
Boson
Acceptance
(%)
Crosssection (nb)
Acceptance cuts are
made on lepton
From generator using
cteq6m
W+
27.6
108.75 +2.61 / -2.94
W-
24.1
79.98 +2.06 / -2.38
Z0
13.9
57.78 +1.32 / -1.59