[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

description

The first part of this work describes how to use Z -> e+e- events in order to calibrate the CMS electromagnetic calorimeter, which makes use of scintillating crystals in order to precisely measure the energy of electrons and photons coming from the proton-proton interactions. Using the very precise knowledge of the Z mass coming from LEP experiments, it is possible to set the absolute scale of the calorimeter as well as calibrating regions of the calorimeter with various topologies, and finely correct the calorimeter response to electrons. Focus is put on the first weeks of data taking. The second part of this work concentrates on the misidentification of the electric charge of electrons/positrons in CMS. It will be shown how it is possible to extract the charge misidenti fication rate from the fi rst CMS data, this time relying on the fact that electrons coming from the Z decay are always oppositely-charged. Measuring this charge misidenti fication rate not only allows to perform a real-time check of the reconstruction quality during data taking, but also has an important role in the study of some physics channels. One of the studies where the charge misidentification has an important in influence is the W+/W- cross section ratio, that represent a test of the Standard Model which does not need a precise knowledge of the machine luminosity, that will be difficult to achieve with the first data.

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

Page 1: [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 Roma, 23 Ottobre 2009

Supervisors: Prof. Egidio Longo Dr. Riccardo Paramatti Dr. Paolo Meridiani

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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

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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?

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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)%

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

R

1.1

1.2

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1.4

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2.1

W data - misID corr.

CTEQ6L1 (MonteCarlo truth)

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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

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!Electron -2 -1 0 1 2

R

1.1

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W data - misID corr.

CTEQ6L1 (MonteCarlo truth)

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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