[L'angolo del PhD] Alessandro Palma - XXII Ciclo - 2009
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Studies on the dielectron spectrum with the first data of the CMS experiment at the LHC
Dottorato di Ricerca in Fisica XXII ciclo – Seminario Finale
Alessandro Palma Roma, 23 Ottobre 2009
Supervisors: Prof. Egidio Longo Dr. Riccardo Paramatti Dr. Paolo Meridiani
23 Ottobre 2009 Alessandro Palma 2
Outlook of the work
LHC experiments will start data taking before 2010
Precise tests of Standard Model (SM) and search for new TeV-physics
Focus on first months of CMS data (integrated luminosity 100 pb-1)
Important to calibrate detectors with physics events and “re-discover” SM
1. Calibration studies: use Z ee events to calibrate the CMS e.m. calorimeter
• Absolute energy scale
• Intercalibration of different calorimeter regions
2. Measurement of electron charge misidentification
• Assess and monitor quality of electron reconstruction algorithms
• Crucial ingredient in both SM and beyond-SM physics channels
• Early SM application: measurement of W+/W- cross section ratio
Physics studies developed in the framework of CMS Electroweak Group @CERN
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The Large Hadron Collider (LHC) at CERN
First collisions at √s = 7 TeV before 2010, then centre-of-mass energy will be stepped up
Results shown here are for √s = 10 TeV (intermediate step before 14 TeV)
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The Compact Muon Solenoid (CMS) experiment
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The CMS electromagnetic calorimeter (ECAL)
75,848 PbWO4 scintillating crystals
Barrel (0 < |η| < 1.479): 61,200 crystals
Endcaps (1.48 < |η| < 2.7): 2 x 7,324 crystals
Energy resolution and design performance
Stochastic
[a = 2.7%]
Noise
[b ≈ 200 MeV]
Constant
[c = 0.5%]
Endcap
Endcap
Barrel
Test beam results
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ECAL calibration with Z ee events
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Simulated Z ee event in CMS e+e- invariant mass
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Precalibration of the ECAL before startup
Barrel 1. Test beam electrons (on 10/36 of Barrel)
• Intercalibration at 0.4% level
2. Cosmic ray flux (on all the Barrel)
• Provides intercalibration of 1-2%
depending on pseudorapidity
3. Light Yield (LY) lab measures (on all the Barrel)
• Provides intercalibration of 4-5%
Endcaps
Expected intercalibration at startup: 7-10%
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ECAL calibration with Z ee events Description of the method
In each event “i”, the quadratic mass ratio
folds a weighted average of the
miscalibrations of the regions ”j” of the
calorimeter hit by the Z-electrons
The weights are given by the energy
fraction carried by region “j”
Event after event, for each region “j” a
histogram is filled with the quadratic mass
ratio with its weight
After a number of events, the histogram is
fitted and its peak gives an estimate of the
region miscalibration
The procedure is repeated iteratively
23 Ottobre 2009 Alessandro Palma 9
ECAL calibration with Z ee events MonteCarlo validation of the method
The ECAL regions to study can be defined in different ways, according to:
same η-ring
same manifacturer
1. Introduce ad-hoc miscalibration
2. Compare miscalib and 1/recalib constants at convergence
η-rings
Spread around y=x
gives recalibration precision
(improving w/ statistics)
miscalib
1/recalib
How to validate the method?
23 Ottobre 2009 Alessandro Palma 10
ECAL calibration with Z ee events Applications of the method in Barrel
Find corrections to electron energy in bins of (η, ET)
Increasing Bremsstrahlung
Brings worse energy reconstruction
Material budget in front of ECAL
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ECAL calibration with Z ee events Applications of the method in Endcaps
Intercalibration of η-rings of crystals
Expected miscalibration
at LHC startup
ECAL Endcaps
f(x) = p0 + p1/√x
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Measurement of electron charge misidentification rate from data
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Electron with wrong reconstructed charge
Simulated Z ee event in CMS
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Measurement of electron charge misID Origin of charge misID [1/2]
Bremsstrahlung emission followed by conversion “confuses” reco algorithms
pT resolution Brem yield
e- e+
e+
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Measurement of electron charge misID Origin of charge misID [2/2]
If the explanation is correct, for wrongly-
reconstructed electrons:
o Transverse Impact Parameter (TIP) is
expected to be larger
o Azimuthal angle φ has worse resolution
and biased determination
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Measurement of electron charge misID Description of the method [1/2]
Tag & Probe (TP) method applied to Zee events:
one electron (Tag) must pass a stringent track quality selection in order to ensure
that its charge is correctly reconstructed
the other electron (Probe) usually passes looser selection
Tag-Probe invariant mass in the range [85,95] GeV/c2 to reduce background
the method measures the charge misID rate on the Probe
Probe misID rate =
(# of TP events where Tag and Probe have same charge) / (# of TP events)
A typical selection for Tag (with efficiency ~10%) is:
• ECAL Barrel only
• ET > 20 GeV
• num. of track-hits > 10
• χ2 of track < 1.2
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Measurement of electron charge misID Results: charge misID rate vs reconstructed electron quantities
Probe here is a track-isolated electron with ET>20 GeV
MisID rate behaves as expected
Good agreement with MonteCarlo check on the Probe charge (reco vs gen-level electron)
Integrated misID value (1.52 ±0.09)% Statistical error w/ 100 pb-1 data
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Measurement of electron charge misID Systematic uncertainties [1/2]
• Invariant mass window
Agreement with MonteCarlo truth
checked with various invariant mass
windows
• MisID on the Tag electron
If misID for Tag is >0 (Ptag), the
method measures the sum of Tag
misID + Probe misID
Ptag can be extracted from Tag-Tag
events and subtracted
• Probe definition
Stability of the method checked with
various “Probe” definitions
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Measurement of electron charge misID Systematic uncertainties [2/2]
• Charge symmetry
No significant differences found in
misID results when requesting positive
and negative Tags
• Background level
Background found not significant
(S/B ~ 100)
Systematics below 0.1% even when
background enhanced by 3 (to account
for uncertainty in QCD yield)
Overall systematics ~0.1%
(comparable with stat. @ 100 pb-1)
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Application of electron charge misID to W+/W- cross section ratio
High-pT
isolated
electron
Missing transverse
energy (neutrino)
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Application to W+/W- cross section ratio Positive and negative W bosons at CMS
LHC initial p-p state favours W+
production
o integrated W+/W- ratio is > 1
u-type quarks carry more of the proton
momentum than d-type
o boosted W’s are more often positive
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Application to W+/W- cross section ratio Relevance of the measurement: constraining PDFs
PDF investigation: LHC will be able to explore a new region in (x, Q2) plane
Different PDF models give different W+/W- predictions
Average ratio W+/W- ≈ 1.4
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Application to W+/W- cross section ratio W eν selection
One electron with ET > 15 GeV at online reconstruction (to filter events live to a
sustainable rate)
One electron with ET > 30 GeV at offline reconstruction
No 2nd electron with ET > 20 GeV (to reduce Z ee background)
Track, ECAL, HCAL isolation and tight eleID requirements (to reduce QCD jets)
Missing Transverse Energy (MET) distribution
of selected events (signal & background):
• gives a flavour of the S/B ratio
• shows how distinctive MET is in W eν
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Application to W+/W- cross section ratio Number of selected events with 100 pb-1 data
S/B = 2.3
A robust strategy of background subtraction is under study in CMS
Background will not be considered in the following
CHAPTER 4. CHARGE MISIDENTIFICATION CORRECTION TO THE W+/W−
CROSS SECTION RATIO
Physics channel Selected events
W → eν 371937Z → ττ 1589W → τν 5018Z → ee 23650γ + jets 53138QCD 76877
tt 1875
Table 4.5: Number of signal and background events passing the selection for a statistics of100 pb−1
asymmetrically, and the final state electron has the same charge of the τ because it1119
comes from the τ → eνeντ decay.1120
Fig. ?? and ?? show the pseudorapidity and transverse energy distribution of the recon-1121
structed positive/negative electrons coming from the decay of positive/negative W bosons1122
respectively (except for the misID phenomenon that will be covered later on in this chap-1123
ter): the transverse energy distributions are very similar, while the pseudorapidity plot shows1124
how negative electrons populate less than positrons the high-pseudorapidity regions of the1125
detector.1126
The experimental method to measure the integrated W+/W− ratio in the electron channel1127
consists of measuring the integrated e+/e− ratio: once the background has been properly1128
subtracted, in the absence of electron charge misID the e+/e− ratio indeed corresponds to the1129
W+/W− ratio.1130
In a physics analysis perspective, both the integrated and the differential e+/e− ratio1131
(as a function of the electron pseudorapidity or transverse energy) provide a useful tool to1132
constraint the Parton Distribution Functions (PDF), as highlighted in Section ??.1133
96
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Application to W+/W- cross section ratio W+/W- ratio with misID correction
Correcting for electron charge misID
becomes important when:
1. W+/W- ratio becomes large
2. misID rate becomes large
i.e. at high values of electron |η|
N+,-:observed
W+/W-
T+,-:true W+/W- misID rate
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Application to W+/W- cross section ratio Charge misID rate for W electrons
Apply Tag&Probe method to electrons passing the selection requested for W
Check charge symmetry so that the same misID values can be applied to W+ and W-
events
Integrated misID rate: (1.27±0.08)%
23 Ottobre 2009 Alessandro Palma !Electron -2 -1 0 1 2
R
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
W data - misID corr.
CTEQ6L1 (MonteCarlo truth)
26
Application to W+/W- cross section ratio Constraining PDFs without misID correction
W “data” was generated starting from CTEQ6L1 (LO) PDF library
If no misID correction is inserted, at high |η| agreement with MonteCarlo gets weak
23 Ottobre 2009 Alessandro Palma
!Electron -2 -1 0 1 2
R
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
W data - misID corr.
CTEQ6L1 (MonteCarlo truth)
27
Application to W+/W- cross section ratio Constraining PDFs with misID correction
When misID correction is inserted, at high |η| data fit MonteCarlo better
Integrated W+/W-
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Conclusions
An iterative method has been elaborated that allows calibration of
regions of the CMS em calorimeter, that with 100 pb-1 data allows:
o Barrel: tuning of (η, ET)-dependent correctiosn to the electron energy
o Endcaps: intercalibration of η-rings at permille level
A “Tag&Probe” method to extract the electron charge misID rate
from data has been developed
o Good stability and agreement with MonteCarlo
o Important in Standard Model analyses and beyond
In measuring W+/W- ratio, inserting misID correction is relevant to
constraint proton PDFs