QCD Multi-jet background in W+jets and Rjets

Alessandro Tricoli (CERN)

on behalf of

W+jets and Rjets groups

25th February 2013 SM W/Z-Physics Group Meeting

A. Tricoli 225th February 2013

Overview

QCD multi-jet background to Z+jets is small and under control Not discussed today

QCD background to W+jets has shown some unexpected features in both electron and muon channels

Far larger than in 2010 data analysis Muon channel has as much or higher background than the electron channel Poor fit quality and some model dependence in electron channel Discrepancy wrt W inclusive analysis results

A lot of effort in W+jets and Rjets groups to understand it Some questions answered, some to be further investigated

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Relevance of QCD background QCD background is an important source of uncertainty for both W+jets and Rjets

(no cancellation as QCD is very small in Z+jets) While Z+jets events are very pure, W+jets events are not, does not cancel in Rjets QCD is dominant background in 0,1,2,3 jet bins, up to ˜15% of data sample In 2010 QCD multijet uncertainty was 2nd or 3rd of all syst’s for W+ ≥4 jets (11-20%) How sure are we that we are measuring features of W+jets in all kinematic regions and

not its background? With no bias? Need to have a precise and robust estimation of the background level to reduce

uncertainty and produce accurate cross-section measurements In 2011 JES has been reduced so must QCD uncertainty

o JES 4-5% in W+1 jet, mostly cancels in Rjets (≤1% for 1 jet) Need precision ≤30% (W+1) and ≤10% (R+1) not to be dominated by QCD

2011 W+jets has had some issues in both electron and muon channels Why multi-jet background ˜3-5x larger in 2011 than 2010? Why multi-jet background level similar in electron and muon channels?

W->e 2011n Njets=0 Njets=1 Njets=2 Njets=3 Njets=4 Njets=5

Nominal (%) 4.00 12.0 11.8 10.8 7.4 4.2

W-> 2011mn Njets=0 Njets=1 Njets=2 Njets=3 Njets=4 Njets>=5

Nominal (%) 5.19 14.5 13.8 12.06 8.8 8.3

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2010 vs 2011 - electrons

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Large increase in the QCD fraction between 2010 and 2011Same template definition in both years (and same lepton thresholds in both years for this comparison)Different electron identification tight (2010) -> tight++ (2011)Different primary and supporting triggers in 2010 and 2011Is factor 3-5 difference due to pileup?

N Jet QCD frac 2010 QCD frac 2011

>= 0 0.014 0.07

>= 1 0.035 0.15

>= 2 0.041 0.13

>= 3 0.039 0.10

>= 4 0.05 0.06

>= 5 0.05 0.04

Same kinematric selection in both years

MET cut is applied to both fractions

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2010 vs 2011 - muons

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Large increase in the QCD fraction between 2010 and 2011The same isolation cut is used in both yearsIs difference of 3x-5x between 2010 and 2011 due to the broadening of the MET distribution with more pile-up?

2010muon pT > 20 GeV

2011muon pT > 20 GeV

Njet=0 0.015 0.0681

Njet=1 0.050 0.190

Njet=2 0.066 0.174

Njet=3 0.069 0.167

Njet=4 0.0079 0.126

Njet=5 0.080 0.117

MET cut is applied to both fractions

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Technique – electrons (I) Wen+jets method (similar to W inclusive analysis):

QCD template from data: o use OR of three prescaled loose(1) triggers:

• EF_e20(22,22vh)_loose, EF_e20(22,22vh)_looseTrk EF_e20(22,22vh)_loose1

o select events with loose electrons passing track quality cuts and failing tighto anti-isolation applied

QCD normalization:o fitting METo QCD template from data + Signal and other BKGs from MC

Fit done in exclusive jet bins up to last bin which is inclusive Fit done in two different data-periods: D-K, L-M

o Templates extracted for D-K and L-M periods in data and MC

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Technique – electrons (II) QCD Template definition

Electron identification cut reversal is the same as W inclusive analysiso Pass loose and track quality cuts, o Fail tight, ignoring Conversion cut, Cluster-Track Matching cut in Phi and Tight in Eta

Anti-isolation (to reduce signal contamination) is differento W+jets uses reversal of isolation cut used in signal selection:

• not isEiso98Etcone20 || not isEiso97Ptcone40 o W inclusive uses EtCone30/Et > 0.2

const unsigned int template_mask_pass = (0              | 0x1 << egammaPID::TrackPixel_Electron              | 0x1 << egammaPID::TrackSi_Electron              | 0x1 << egammaPID::TrackMatchEta_Electron);                      const unsigned int template_mask_fail = (egammaPID::ElectronTight              & ~(egammaPID::ElectronLoose)              & ~(0x1 << egammaPID::ConversionMatch_Electron)              & ~(0x1 << egammaPID::TrackMatchPhi_Electron)              & ~(0x1 << egammaPID::TrackMatchEtaTight_Electron));

((el_isEM->at(qcd_i) & egammaPID::ElectronLoose) == 0) ((el_isEM->at(qcd_i) & template_mask_pass) == 0) && ((el_isEM->at(qcd_i) & template_mask_fail) != 0))

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Technique – Comparison with W inc. - electrons W+jets fit results different from W inclusive:

larger background and worse fit chi2 A lot of effort put in comparing our results

o Cutflow, distributions, MC’s samples, binning, fit range code checks etc. Coarser binning choice in W inclusive (2 GeV vs 5 GeV) improves chi2 Reduced fit range as in W inclusive improves fit quality Missing supporting trigger reduced QCD template statistics in W+jets analysis Differences in QCD template definition and W MC dataset (small effect)

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With same binning, MC DS, fit range and triggers we find consistent background fractions for ≥ 0 jets, but chi2/dof is still different (31 vs 3) differences may be due to

different calibration? W incl. cuts out a small

region 1.6 < |η| < 1.7

W+jets group W inclusive group

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Addendum

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Source of disagreement on χ2 between between W inclusive and W+jets TFractionFitter does not look in the bin error, instead looks at bin content assumes error

is √(N) Should scale N and error such that the fractional error stays the same while the absolute

error changes to be equal to √ (N) This is what Inclusive group does => by applying this method we get lower χ2

Remaining sources of (small) disagrement (<4%) - do we need to pursue this further? Should we make these changes in W+jets,Rjets analyses?

1) rescaling W pT2) cutting out 1.7>|eta|>1.63) different calibration package4) different background MCs

Robert King

W incl. W+jets

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Technique – Fit stability - electrons Fit Stability and Optimisation:

QCD fit results have been checked with different o Anti-isolation definitions: etcone20,30,40, ptcone40 / ET >0.20

1) W contamination in control sample reduced by 3x or more with anti-isolation After anti-isolation ≤1% W contamination with any anti-isolation choice (Alpgen Wenu+jets through QCD selection) Etcone has lower W contamination

2) QCD fraction with and without anti-isolation changes by 1.5% at most Both QCD fraction, EW SF and Chi2 are little affected by different

isolations Etcone has EW SF and chi2 closer to 1

o Identification cut reversal Different combinations of isEM bits to fail or pass

Small effects on QCD background fractions

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Background composition - electrons

Since backgrounds are of the same level in e and m channels, has background composition in e-channel has changed in 2011? Fakes -> H.F.?

Check assumption of multi-jet background dominated by fakes in electron channel

Separate electron fakes from H.F. real electrons in W(ev)+jets

Use 3 components to fit to the data: EW+Top and 2 QCD templates (cut reversal+anti-isolation) with addition of

1) impact param significance >102) Impact param significance <10

 Fit quality improves Fake electrons are dominating contribution (still to assess contributions from conversions)

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Alternative fitting variables - electrons

Tried Df between electron and vector sum of jets Df template gives very different results from MET template leading to much

worse agreement with data Df fit gives suspiciously larger EW SF (>7%) Method abandoned

Exploring now fit of Isolation, extracting W template from Z sample

Work in progress, promising preliminary results

W+1Period L-M

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Technique - muons Wmn+jets method:

QCD template from data: with reverted impact parameter cuto d0 significance > 3o differently from W inclusive analysis that uses anti-isolation, due to bias

in jet kinematics QCD normalization by fitting MET on data with QCD template + Signal and

other BKGs (from MC) Fit done in exclusive jet bins up to last bin which is inclusive No splitting in data-periods Consistent results by Monica and Andrew

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Reversed D0 Reversed Iso

Using isolation leads to a bias in the leading jet pT distribution Shown here for QCD b-bbar MC

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Technique – Comparison with W inc. - muons W+jets fit results different from W inclusive:

larger background and worse chi2 Trying to compare our results

o Inclusive group uses range of 0-40 GeV to avoid high-MET region that suffers from large shape differences

o Tried Reverse Isolation consistently with W inclusive analysis 1.5% difference in fraction, no big difference in Chi2

o Using Sherpa: 1.5% difference in fraction, no big difference in Chi2 Chi2 still very different between W+jets and W inclusive analyses

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Inclusive fit Njet>=0 QCD Fraction Chi2/dof (dof)

Rev D0 Rev Iso Rev D0 Rev Iso

Range 10-100 GeV 0.0794 +/- 0.00040.0625 +/-

0.000225.7 (44)

35 (44)

Range 0-40 GeV 0.0969 +/- 0.00080.0803 +/-

0.000512.6 (19)

3.9 (19)

Range 10-100 GeV, Sherpa 0.0756 +/- 0.00040.0588 +/-

0.0003100 (44)

134 (44)

Range 0-40 GeV, Sherpa 0.0788 +/- 0.00080.0654 +/-

0.000515.9 (19)

28 (19)

No MET cut applied here

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Fitting Tools - muons

TFraction RooFit

Njet=0 0.0607 +/- 0.0006

0.0594 +/- 0.0004

Njet=1 0.2074 +/- 0.0012

0.2068 +/- 0.0008

Njet=2 0.1903 +/- 0.0026

0.1896 +/- 0.0018

Njet=3 0.1722 +/- 0.0054

0.1708 +/- 0.0037

Njet=4 0.1252 +/- 0.0107

0.1253 +/- 0.0076

Errors are about ~1.5x smaller with RooFit than with TFRactionFitter. Central values are comparable No MET cut applied here

Similar effect seen in Top background fit is TFractionFitter uncertainty realistic? Aren’t they overestimated? Problem of fit convergence in Top background fit with TFractionFitter

while none with RooFit

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Fitting Isolation - muons

Binning was too coarse, so perhaps the fit is not the best.

Use Isolation as alternative fitting variable to cross check results with MET

Black = WjetsYellow = QCDBlue = Fit result

MET Iso Iso chi2/dof (dof)

Njet=0 0.0519 +/- 0.0006

0.0476 +/- 0.0005 486 (9)

Njet=1 0.145 +/- 0.0012

0.1185+/- 0.0008 64 (9)

Njet=2 0.138 +/- 0.0026

0.1109+/- 0.0015 11 (9)

Njet=3 0.126 +/- 0.0054

0.1084+/- 0.0030 2.6 (9)

Njet=4 0.088 +/- 0.0107

0.0851 +/- 0.0049 0.74 (9)

Chi2 is not better than with MET Fit results vary quite a bit from MET fits

up to 2.7% Further investigation needed

MET cut is applied for both

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Pileup – electrons (I) Big jump in pileup in Period L and M wrt previous periods

<m>~5, 10 in Periods D-K and L-M Primary and Supporting (background) electron triggers changed during the

year 2011 and not always coherently in terms of selection and rates data period statistical weight is different in signal and background samples: D-K weigh more than L-M in background sample than in signal sample Pileup effect in whole 2011 data sets not reflected in QCD background sample MET distribution is pileup sensitive Period dependent fits in electron channel: D-K, L-M (fit quality improves)

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Pileup – electrons (II)

EW Scale Factors

Chi2/dof

QCD Fraction

Splitting Data and MC sets in D-K and L-M templates Fit quality improves (chi2/dof and EW SF closer to 1) QCD fraction increase by 1%-2% overall Low QCD fraction in D-K, large in L-M => Pileup Effect !?

Results irreproducible on MC due to small statistics Used in both W+jets and Rjets as default

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EW S.F. are at times still large 1%-12% indication EW process

mis-modeling?Chi2/dof not yet 1, especially with 0,1,2 jets QCD Template mis-

modeling?

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MC study – electron channel

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MC does not show same data-period dependence as in data, probably due to poor statistics

However well reproduces pileup Cannot use MC to understand data-

period dependence

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Pileup – muons (I)

No need of period dependence fit in Muon Channel since same trigger used for signal and background samples pileup effects (e.g. broadening of MET

distribution) in signal sample is well reproduced by background sample

Oppositely to electron channel, QCD fraction seems to go down in L-M, by a factor ˜3 wrt D-K Is this because isolation becomes tighter with more pileup? Still to be explained larger QCD fraction in 2011 than in 2010

QCD control sample

Signal sample

Clear period-dependence of isolation distribution(D-K,L-M)

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Pileup – muons (II)

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Period Inclusive Njet>=0

All 0.0795 +/- 0.0004

D-F 0.1236 +/- 0.0011

G-H 0.1393 +/- 0.0009

I 0.1217 +/- 0.0011

J 0.1091 +/- 0.0013

K 0.1053 +/- 0.0009

L 0.0518 +/- 0.0006

M 0.0399 +/- 0.0006

No MET cut applied here

Factor up to 3.4x in difference between L-M and D-K Results to be cross checked

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Uncertainties

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Wen =0 jets =1 jet =2 jets =3 jets =4 jets =5 jets

Rel uncertainty (LM, DK)

15-25% 9-10% 7-11% 10-11% 17-28%

44-69%

Electron channel Preliminary uncertainties, including:

o W/Z model dependence (Sherpa vs Alpgen)o Change of fit rangeo Change of anti-isolation - dominating sourceo Statistical uncertainty

Muon channel Preliminary uncertainties, including:

o Template shape (anti-isolation and changes of cuts)o Change of fit rangeo Alternative fit variable [Df(l,sumed-jets)]

background dominant systematics on Rjets:˜3% on 0,1,2,3 jets

Wmn =0 jets =1 jet =2 jets =3 jets =4 jets =5 jets

Rel uncertainty (%) 50% 14% 17% 26% 46% 53%

Uncertainties do not cover difference between data-periods

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On-going Discussion

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Ongoing discussion between W+jets and Rjets groups on should we worry/understand disagreement between 2010 and 2011? should we worry/understand levels and data-period dependence of background

in electron and muon channels? should we worry/understand remaining disagreement wrt W inclusive analysis?

No evidence of mistakes in 2011 analysis that could lead to such a difference wrt 2010 Detector levels are in reasonable agreement “things” have changed between 2010 and 2011 (what?) Data-period dependence not confirmed by MC but not refuted either Or maybe 2010 analysis was wrong?

Large chi2/dof and other unknowns will/can eventually translate in larger uncertainties on background estimation

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Conclusions and Outlook

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Great progress in the last few week in understanding QCD background in W+jets events – fast pace! Better understanding now of pileup contribution in the electron channel Improved agreement with W inclusive analysis results in electron channel Systematic studies of stability of results undertaken

o e.g. Model dependence, change of control sample definition, fitting tools A few systematics checks remain to finish in electron channel

Trigger bias, optimal control sample definition (e.g. isEM tight++/medium++) Cross check results with alternative fitting variable: isolation Reassess W model dependence in fit results

Still a few open questions in muon channel Source of increase in background fraction in 2011 wrt 2010 Remaining discrepancies with W inclusive analysis

Debate within W+jets and Rjets group on priorities

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Backup

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W+jets and R+jets Selection (II)object selection

R+jets uses same selections as in W+jets and Z+jets

Preselection: -trigger

- primary vertex (at least one good vertex with >=3 track ) -MET cleaning and LAr Hole Veto

Lepton selection:

Z selection: 2 OS leptons, 66 GeV< mll<116 GeV

W selection: MET > 25 GeV, mT(W) > 40 GeV

Jets selection:

Electrons: Author 1or 3, Quality flag (OQ&1446 !=0) pT>25 GeV (W),20 GeV (Z) |n|<2.47 (excluding 1.37-1.52)

Muons:STACO Combined with cleaning cuts(MCP recommendations), pT>25 GeV (W),20 GeV(Z) |n|<2.4

Electrons: 2 OS medium electrons Muons: 2 OS muons, at least the 1st with d0sign (<3), z0(<10mm), isolation requ. (ptcone20 / pt < 0.1)

Electrons: 1 tight++ electron

Muons: 1 muon with d0sign,z0, isolation requ.

Electron TriggerW: e20_medium(D-I), e22_medium (K), e22vh_medium1 (L-M) Z : 2e12_medium(D-I), 2e12T_medium (K), 2e12Tvh_medium (L-M)

MuonTrigger:mu18_MG (D-I), mu18_MG_medium (J,M)

MET: MET_RefFinal, JES and lepton scale prop with MissingETUtility

Antikt4TopoEM at EM*JES scale pT > 30 GeV, |y| < 4.4 |JVF| > 0.75 ( for jets with |η| < 2.4) ΔR(jet-lepton) > 0.5

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Fit StabilityDifferent anti-isolations

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