[L'angolo del PhD] Alessandro Palma - XXII Ciclo - 2009

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 Supervisors: Prof. Egidio Longo
Roma, 23 Ottobre 2009 Dr. Riccardo Paramatti
Dr. Paolo Meridiani

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

23 Ottobre 2009 Alessandro Palma 2

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)


The Compact Muon Solenoid (CMS) experiment


The CMS electromagnetic calorimeter (ECAL)
Energy resolution and design performance
  75,848 PbWO4 scintillating crystals
Constant
  Barrel (0 < |η| < 1.479): 61,200 crystals
[c = 0.5%]
  Endcaps (1.48 < |η| < 2.7): 2 x 7,324 crystals

Stochastic Noise
[a = 2.7%] [b ≈ 200 MeV]

Endcap

Test beam results

Endcap
Barrel


ECAL calibration with Z  ee events

Simulated Z  ee event in CMS e+e- invariant mass


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%


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


MonteCarlo validation of the method

The ECAL regions to study can be defined in different ways, according to:
 same η-ring
 same manifacturer How to validate the method?

1.  Introduce ad-hoc miscalibration
η-rings 2.  Compare miscalib and 1/recalib
constants at convergence
1/recalib

Spread around y=x
gives recalibration precision miscalib
(improving w/ statistics)


Applications of the method in Barrel

Find corrections to electron energy in bins of (η, ET)
Material budget in front of ECAL

Increasing Bremsstrahlung
Brings worse energy reconstruction

Applications of the method in Endcaps

Intercalibration of η-rings of crystals

f(x) = p0 + p1/√x

ECAL Endcaps

Expected miscalibration
at LHC startup


Measurement of electron charge misidentification
rate from data

Electron with wrong reconstructed charge

Simulated Z  ee event in CMS


Measurement of electron charge misID
Origin of charge misID [1/2]

Bremsstrahlung emission followed by conversion “confuses” reco algorithms

e+
e+
e-

Brem yield pT resolution


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


Description of the method [1/2]

Tag & Probe (TP) method applied to Zee 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


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


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

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)


Application of electron charge misID
to W+/W- cross section ratio

High-pT
isolated Missing transverse

electron energy (neutrino)


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


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


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


CHAPTER 4. CHARGE MISIDENTIFICATION CORRECTION TO THE W + /W −
Number of selected events with 100 pb-1 data
CROSS SECTION RATIO

Physics channel Selected events
W → eν 371937
Z → ττ 1589
W → τν 5018
Z → ee 23650
γ + jets 53138
QCD 76877
tt 1875

Table 4.5: Number of signal and background events passing the selection for a statistics of
100 pb−1 S/B = 2.3

 A robust strategy of background subtraction is under study in CMS
asymmetrically, and the ﬁnal state electron has the same charge of the τ because it
 
comes from the Background decay. be considered in the following
τ → eνe ντ will not

Fig. ?? and ?? show the pseudorapidity and transverse energy distribution of the recon-
structed positive/negative electrons coming from the decay of positive/negative 23 bosons
23 Ottobre 2009 Alessandro Palma W
respectively (except for the misID phenomenon that will be covered later on in this chap-
ter): the transverse energy distributions are very similar, while the pseudorapidity plot shows

W+/W- ratio with misID correction

N+,-:observed misID rate T+,-:true W+/W-
W+/W-

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|


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)%


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
2.1
R

2 W data - misID corr.

1.9
CTEQ6L1 (MonteCarlo truth)
1.8

1.7

1.6

1.5

1.4

1.3

1.2

1.1
-2 -1 0 1 2
23 Ottobre 2009 Alessandro Palma
Electron ! 26

Constraining PDFs with misID correction

  When misID correction is inserted, at high |η| data fit MonteCarlo better

2.1
R

2
W data - misID corr.
1.9

1.8 CTEQ6L1 (MonteCarlo truth)

1.7

1.6

1.5 Integrated W+/W-

1.4

1.3

1.2

1.1
-2 -1 0 1 2
Electron !

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


[L'angolo del PhD] Alessandro Palma - XXII Ciclo - 2009

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