differential z cross section in the electron channel

V.Rekovic, Differential xsec Z->ee, EWK Preapproval 1

Differential Z Cross Section in the ElectronChannel

Bryan Dahmes, Giovanni Franzoni, Jason Haupt, Kevin Klapoetke,

Jeremy Mans, Vladimir Rekovic

8/2/2011


Outline

• Theory and motivation for the analysis• Measurement

– Strategy– Efficiencies and acceptance– Bin migration and unsmearing– Errors– Sensitivity to PDF’s

• The result– With 32 pb-1 in frozen AN-10-367 and AN-11-029

• Updated results– With 36pb-1

8/2/2011

Motivation for Z shape studies

• Parton density functions are critical for all processes at a hadron collider. PDF’s need to be measured from LHC data.

• We want to measure them with Z differential cross sections, Y and qT.

Z/Drell Yan Production

8/2/2011 3V.Rekovic, Differential xsec Z->ee, EWK Preapproval

Changes due primarily to inclusion of ds/dY fromTevatron.


Pythia Tunes

8/2/2011

€

1σdσ (Z →e+e−)

dqT

Measurement Strategy

• Analysis equation:

• We conduct two separate analyses: X is either rapidity (Y) or transverse momentum (qT) of Z boson.

• This is a Shape measurement. We are not measuring cross section.• Main components of the analysis:

– Z • Fast MC• Bin Migration and Unfolding

– Error estimation


€

1σdσ (Z →e+e−)

dXi

=(ε × A)

N obs −N bkg

N iobs −N i

bkg

Δ i(ε × A)i

Measurement Strategy II

In the qT analysis consider only ECAL electrons

with|η|< 2.1 to match muon analysis.

In the Y analysisconsider ECAL electrons within

tracking acceptance |η|< 2.5.use HF electrons to significantly

extend the accessible rapidity range; HF electron ID based on longitudinal and transverse shower shape variables.

Not currently using electrons in ECAL outside the tracker acceptance.

HFECAL



Data for the Analyses

8/2/2011

• Dataset/EG/Run2010A-Dec22ReReco v1/RECO 2.9 pb-1

/Electron/Run2010B-Dec22ReReco v1/ 29.1 pb-1

An error in a GoodLumi file was found immediately before the pre-approval freeze. As a result, only 32 pb−1, not 36 pb−1, is used in the frozen ANs.

• HLT

Z Definitions/Electron SelectionZ definitionin dσ/dY

Electron 1 (HLT matched) Electron 2

ECAL-ECAL ECAL + track + HLT ECAL + trackECAL-HF ECAL + track + HLT HF

ECAL Electron1 (2) Definition

GSF track matchedEWK Electron WP80 (WP95)PT > 20 GeV

HLT

HF Electron Definition

HF EM ClusterHF Electron IDPT > 20 GeV


• Single electron efficiencies are measured with the tag & probe technique and framework• Tag = ECAL electron, that passed WP80 and matches to HLT path• Probe = With invariant mass (60-120 GeV) SCluster → GsfElectron → WP80(95)→ HLT

Z definition in dσ/dqT

Electron 1 (HLT matched) Electron 2

ECAL 2.1-ECAL 2.1 ECAL with |η| <2.1 + track + HLT ECAL with |η| <2.1 + track


Factorization of Single Electron Efficiencies• Offline electron efficiency can be factorized due to several

contributions:

• HLT efficiency is measured w.r.t. offline:

• For HF there is no trigger nor track requirement:

€

εoffline =N(Superclusters)N(Electrons)

⎡ ⎣ ⎢

⎤ ⎦ ⎥MC

×N(TrackMatched)N(Superclusters) ⎡ ⎣ ⎢

⎤ ⎦ ⎥data

×N(WP80)

N(TrackMatched) ⎡ ⎣ ⎢

⎤ ⎦ ⎥data

€

ε full = ε offline ×N(L1+HLT)N(offline)

⎡ ⎣ ⎢

⎤ ⎦ ⎥data

€

εhf =N(HFClusters)N(Electrons)

⎡ ⎣ ⎢

⎤ ⎦ ⎥MC

×N(HLTElectronID)N(HFClusters)

⎡ ⎣ ⎢

⎤ ⎦ ⎥data

8/2/2011


Single Electron Efficiencies

[GeV/c]TP20 40 60 80 100 120

Single

Elect

ron Ef

ficien

cy

0.8

0.85

0.9

0.95

1

-2.50 to -1.57 dη

1.57 to 2.50 dη

-1.44 to 0.00 dη

0.00 to 1.44 dη

PRELIMINARYSun Jan 30 02:08:30 2011

GSF Track MatchingZ-shape measurement is differential in Y, qT. Therefore integral efficiencies don’t suffice. In view of the convolution step they need to be extracted as a function of:

pT, ηdet In Tag and Probe, side band background subtraction for single electron efficiency

8/2/2011

Efficiency * Acceptance

• To extract the efficiency of measured Z as a function of the Z rapidity or Z transverse momentum we start with Z->ee events from “fast” Monte Carlo and convolve single electron efficiencies. You may want to think about it as MC evaluation of :

where X is standing for either Y or qT.

.

– “Fast” Monte Carlo uses smearing functions on gen level particles (with FSR in a cone) to shift their pT , positions in HF, and to simulate ECAL energy resolution.

€

(ε × A)Zmeas X( ) = P(ηd +, pT +,ηd −, pT −;X meas)ε e+(ηd +, pT +) × ε e−(ηd −, pT −)∫ dηd +dη d −dpT +dpT −


,


• Xeemeas is not necessarily equal to Xee

true , due to physics and detector effects. • FSR photon can fall outside its cluster so Xmeas can be altered.• emission of bremsstrahlung photons, energy loss in the tracker, intrinsic

resolution of calorimeter energy and position measurements.• If these effects are uneven across measurement range, the measured

spectrum X can be different from the true spectrum, due to events migrating across the bins. We can correct the measurement by unsmearing it using either of the two recipes:1. by average response for each bin, measured by ratio

which in analysis equation replaces by2. by migration matrix that accounts for all possible migrations properly weighted.

Inverted migration matrix can then used to unfold the final measurement. In case of low statistics this artificially introduces large errors.

Bin Migration

8/2/2011

€

ρi =N i

true

N imeas

€

M(i, j) = P(X jZ ,meas | X i

Z ,true)€

(ε × A)imeas

€

(ε × A)itrue

Fast MC: Data-driven smearing for ECAL/HF

• Model energy resolution for MC smearing:

€

σHF

E= a

E⊕c

• Derive smearing parameters by comparing invariant mass of smeared MC (eg colored histograms) to DATA, to Minimize χ2 to obtain function terms.

€

σEB

E= c + a

E⋅ f (η d )

€

f (η d ) = 1−b1η d + b2η d2( )

€

σEE

E= aET

⋅ f (η d )


For HF σ is of a Gaussian, for EE, EB σ is of a Crystal Ball

Fast MC reproduces single electron and di-electron variables that compare well to data

Leading Electron

PT

ECAL-HF dielectron

mass

Type 1 ECAL-ECAL Z


Type 2 ECAL-HF Z

HF Electron PT

ηe


Eff x Acc of Measured Z

8/2/2011

qT[GeV]

€

ε full =N(Superclusters)N(Electrons)

⎡ ⎣ ⎢

⎤ ⎦ ⎥MC

×N(TrackMatched)N(Superclusters) ⎡ ⎣ ⎢

⎤ ⎦ ⎥data

×N(WP80)

N(TrackMatched) ⎡ ⎣ ⎢

⎤ ⎦ ⎥data

×N(L1+HLT)N(WP80)

⎡ ⎣ ⎢

⎤ ⎦ ⎥data


Unsmearing due to Average Bin Migration

Unsmearing for ds/dY Unsmearing for ds/dqT

8/2/2011

€

ρi =N i

true

N imeas

Systematic Uncertainties

• Different sources of systematic errors are considered:– From electron efficiencies– Energy scale– Background subtraction

• Uncertainties in the PDF’s used to compute efficiencies give rise to systematics to the measurement

8/2/2011 V.Rekovic, Differential xsec Z->ee, EWK Preapproval 17

small

significant


Error from Energy Scale

8/2/2011

Two sources of systematatic:• Vary energy scale: +/- 1% EB, +/- 3% EE• Vary local energy scale to account for uncertainty in transparency corrections:

+/- 0.13% |eta| for EB +/-2 +/- 1.5% |eta| for EE

Y qT[GeV]


Background (QCD)

Extract BG contribution from DATA = SIGNAL + BG– SIGNAL is described with POWHEG

smeared Fast MC.– BG sample is the QCD enriched sample

obtained by inverting ID cuts:candidates that fail ECAL WP95 (ID or isolation) or HF ID.

– For each bin derive nominal line shape of BG, described as

where

For each bin, fit Mee to SIGNAL + BG shapes.

• Uncertainty in BG is dominated by statistics. Will decrease with future increased data sample.

8/2/2011

Eg: 0.2 < YZ < 0.3

PDF systematic


• POWHEG + FastMC is used to determine EffxAcc for the measurement.• How much is the uncertainty on EffxAcc coming from used PDF model affecting the

uncertainty of the measurement? Is it compromising the sensitivity to PDF constraints?Ansewer: NO.

• impact 0.1% in the central Y region, and below 0.5% in Y measurement range• Impact at most 0.6% in qT measurement range

SAMPLE: 40 M events in POWHEG passedthrough FastMC, reweighted for 52 PDF CT10w vectors.

SAMPLE: 200 M events in POWHEG with |ηe| < 2.5 through FastMC, reweighted for 52 PDF CT10w vectors.

|ηgen,e| < 2.5


PDF Sensitivities – Can we constrain PDFs?

Largest sensitivity in Y – vector 23 Largest sensitivity in qT – vector 5

8/2/2011

CT10w vector 23 CT10w vector 5

Y qT[GeV]

CT10w consist of 26 vectors, each with +ive and –ive variationY and qT analysis suggest largest sensitivity to different PDF vectors of CT10w.


Sensitivities of Y and qT Analyses to CT10w

8/2/2011

As expected:Y and qT analyseshave differentsensitivity to PDF models in CT10w.

Maximum sensitivity isabout 3%.


All Errors for Y Analysis

8/2/2011

Statistics dominated.Largest systematic from BG (stat), which will decrease with more acquired integrated luminosity.

Negligible PDF errors& unsmearing


All Errors for qT Analysis

8/2/2011

Largest systematic from Energy Scale and BG (stat). The later will decrease with more acquired integrated luminosity.

Negligible PDF & unsmearing errors


Result for Y

8/2/2011

€

dσ (Z →e+e−)dY

i

=N i

obs −N ibkg

Δ i(ε × A)i


The Final Result for |Y|

8/2/2011


The Final Result for qT (linear)

8/2/2011

smeared


The Final Result for qT (log)

8/2/2011

smeared

V.Rekovic, Differential xsec Z->ee, EWK Preapproval 298/2/2011

Updates:Results with 36 pb-1


Result for Y with 36 pb-1

8/2/2011

€

dσ (Z →e+e−)dY

i

=N i

obs −N ibkg

Δ i(ε × A)i


The Final Result for |Y|with 36 pb-1

8/2/2011


The Final Result for qT with 36 pb-1 (linear)

8/2/2011


The Final Result for qT with 36 pb-1

8/2/2011


Conclusions• We performed a measurement of differential cross section in Y and qT

of the Z boson in electron channel with 32 pb-1 of 2010 data– Analyses are statistically dominated– Important systematic is on BG estimation which will be reduced with

increased data sample in 2011.• Notes AN-10-367 and AN-11-029 are frozen, but updates are

included in this presentation.– We add 4 pb-1 of data with new JSON file released few days before freeze.– Final plots of POHEG prediction in frozen qT AN-11-029 were not unfolded for

smearing. The updates with 36 pb-1 presented today include unsmearing.– As a cross check, we measured inclusive cross section, and observed

agreement with the result from VBTF– Comparison of data will be discussed in the following talk.

8/2/2011


BACK-UP

8/2/2011


Single Electron Efficiencies (T&P)

This is probably for BACK-UP8/2/2011


Eff x Acc vs. qT wrt Mesarued and wrt True Z

8/2/2011

Bin migration and unfolding in Y


• Migration matrix from the FSR and smearing implemented in fast Monte Carlo• Unfolding matrix obtained by inversion• Systematics from unfolding, less or much less than 1%:

• Base unfolding matrix based on smearing parameters• Compare it with results from varying ±1σ the smearing in fast Monte Carlo• Systematic defined as quadrature sum of variations in each bin


Bin migration and unfolding in qT

8/2/2011

• Migration matrix from the FSR and smearing implemented in fast Monte Carlo• Unfolding matrix obtained by inversion• Systematics from unfolding, less or much less than 1%:

• Base unfolding matrix based on smearing parameters• Compare it with results from varying ±1σ the smearing in fast Monte Carlo• Systematic defined as quadrature sum of variations in each bin


Systematic Uncertainty on Bin Migration for Y

8/2/2011

for average bin unfoldingCumulative syst error due to Fast MC is 0.2%.

For matrix unfoldingcumulative syst error due to Fast MC is 0.2%.


Systematics from electron efficiencies:

Z0Y-3 -2 -1 0 1 2 3F

racti

on

al E

rro

r F

rom

Bin

Co

rrela

tio

ns

-410

-310

-210

Error FromGSF Track MatchHF Electron IDHLTSuperclusterWP80WP95

CMS 2010 PRELIMINARY

Z0Y-3 -2 -1 0 1 2 3F

ract

ion

al E

rro

r F

rom

Eff

icie

ncy

Sta

tistic

s

-310

-210

-110 Error FromGSF Track MatchHF Electron IDHLTSuperclusterWP80WP95Sum in Quad

CMS 2010 PRELIMINARY

statistical bin correlated

[GeV]T

q1 10 210

Fra

ctio

na

l Err

or

Fro

m B

in C

orr

ela

tion

s

-410

-310

-210

-110 Error FromGSF Track MatchHLTSuperclusterWP80WP95Sum in Quad

1 10 210

Fra

ctio

nal E

rror

Fro

m B

in C

orre

latio

ns

-410

-310

-210

-110

Error FromGSF Track MatchHLTSupercluster

WP80WP95

qT

8/2/2011

Slide for BACK UP


PDF errors to Eff x Acc vs qT

8/2/2011

Den = No cut on Y of gen Z Den =|Y (genZ) < 2Den = ECAL-ECAL

CT10w40 M POWHEG events

CT10w400 M POWHEG events


Inclusive Cross Section

8/2/2011

From ds/dY analysis

From ds/dqT analysis

differential z cross section in the electron channel

Documents

differential xsec zee

z rapidity

zee events

z differential cross

z transverse momentum

efficiency of measured

z fast mcbin migration

ewk preapprovalspent