Transcript CMS Ecal Laser Monitoring System

W+jets and Z+jets studies at CMS
“Candles” and “Ladders”
Analysis Overview:
• Data-driven strategy to study properties of W/Z + jets production in
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final states with electrons and muons
Focus on LHC startup: order
Using different jet definitions - in some cases, detector-wise orthogonal
Two inter-related analyses:
Z+jets candle analysis
• Test of “Berends-Giele” (BG) scaling through the measurement of the
W/Z+jets @ LHC Overview
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• W/Z+jets have a large cross section at LHC
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with final states including
• leptons
• jets
• missing transverse energy (neutrinos)
Dominant background for SM measurements
(ex. and Higgs production) and new physics
searches involving jets/leptons/MET final
states
Analysis Strategy
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W specific requirements:
– >= 1 lepton
orthogonal selection
– Z mass veto
– extra muon veto (e)
– MET > 15 GeV (QCD rejection)
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W/Z+jets ratio analysis
• Test of BG scaling through the measurement of the W+n jets / W+(n+1)
jets ratio as a function of n, along with the double ratio
• High-efficiency event selection to increase S/B
Common selection requirements:
– Single non-isolated HLT lepton trigger
– Electron/muon reconstruction
– Lepton identification
– Lepton isolation
– Lepton - primary vertex compatibility
– Jet clustering
– Electron(s) from W(Z) cleaning from jet
collections
– Jet counting
ratio to suitable level for ML fit to extract yields
• Z candle: only minimal isolation and vertex
requirements necessary due to discriminating
power of di-lepton invariant mass
• W/Z+jets ratio: synchronized event selection
for W and Z events => maximal cancellation of
systematic uncertainties in the double ratio.
Z specific requirements:
– >= 2 leptons
– Z mass window
Event Reconstruction
and Cut-Based W+jets,
Z+jets selection
• Rather than a “cutting-and-counting”, we use
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maximum likelihood fits to extract event
yields, as a function of the number of jets
Additional orthogonal discriminating variable
used to validate PDF parameterizations
Control samples from data are used to
determine distribution shapes and
efficiencies required by the fits, minimizing
the reliance on Monte Carlo simulation
Z+n jets / Z+(n+1) jets ratio as a function of n
High efficiency signal selection provides “candle” dataset for
• detector and physics object commissioning studies
• normalization of irreducible Z()+jets background in MET+jets new
physics searches
• New physics searches with SM Z bosons in the final state, observed as
a deviation from BG scaling
Maximum
Likelihood Fits
Data control
samples
• Predict W+ >= 3,4 jet yields from lower jet multiplicities, revealing
excesses from new physics processes with leptons, jets and MET
Jet Clustering
• In these analyses, each yield measurement is done as a function of
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inclusive jet multiplicity
We consider several types of jet with different experimental constituents
(all clustered with the SISCone algorithm and a cone size of 0.5)
 calo-jets: jets clustered from the calorimeter (ECAL+HCAL) cells reprojected w.r.t. the primary vertex (standard)
pT > 30 GeV/c, |h| < 3.0
track-jets: jets clustered from tracks consistent with the event primary
vertex (lower noise levels relative to calorimeters)
pT > 15 GeV/c, |h| < 2.4
JES corrected calo-jets: synchronized with the above calo-jets definition
(mature detector understanding)
pT > 60 GeV/c, |h| < 3.0
Particle Flow jets: synchronized with the above calo-jets definition
pT > 60 GeV/c, |h| < 3.0
These types of jets
have orthogonal detector systematics: calorimeter vs tracker
probe different regions of phase space: different cuts in pT, 3.0 vs 2.4 in 
Maximum Likelihood Fits
• Maximal likelihood fits are performed for each jet multiplicity in order to
extract signal and background yields.
• For Z+jets events, the fit is based
Z “candle” dataset
on the di-lepton invariant mass
• For W+jets events, the fit is based
• The yields from the maximum likelihood fits are used to calculate the
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ratios related to BG scaling and between the W/Z+jets yields - most
systematic uncertainties (PDF’s, jet energy scale, lepton isolation,
etc.) cancel in these ratios
Additionally, sPlots statistical background subtraction, in conjunction
with the maximal likelihood fit, can be used to produce a “pure”, high
efficiency, Z(ll)+jets sample - which can be used for a number of
applications related to detector and physics object commissioning
and background normalization
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on the W transverse mass
In the W fit, two different ‘categories’
are defined: a ‘heavy flavor’ (hf)
enriched (
) and depleted region
(
), dominated by signal and
single top + , respectively.
• The hf enriched region is defined
by a cut on variables related to the
impact parameters of tracks
matched to jets in the event, Dxyevt
and Dszevt.
BG scaling in W/Z+jets events
BG scaling for W/Z+jets
Deviations from BG scaling
Background control samples for m(ll) and mTW shapes
W/Z+jets ratio
hf variable control samples for signal and background
• Low jet multiplicities can be
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used to predict the high
multiplicity yields.
Comparison with
measurements of the higher
multiplicities can quantify
deviations from BG scaling
due to new physics in Z and
MET+jets+lepton final states
• Z+jets candle sample:
• High signal efficiency
with sPlots statistical
background subtraction
Z()+jets background normalization
MET correction for W()+jets events
deriived from Z()+jets events
Christopher S. Rogan, California Institute of Technology - HCP2009 - Evian-les-Bains