Physics with W+jets at CDFtowards better understanding of W+2jet process,

with regard to the dijet resonant peak

Koji Satoon behalf of CDF Collaboration

KEK Theory Meeting on Particle Physics Phenomenology February 29, 2012

Contents• Introduction

– Tevatron Accelerator and the CDF Experiment• How The “Anomaly” Started

– Dijet mass spectrum in WW/WZ→lnjj analysis• Dijet Mass Spectrum in W+2jets

– Overview of the analysis and 4.3 fb-1 results• Studies of the W+2jets Properties

– Summer 2011 update with 7.3 fb-1

• Search for – First pretag W+2jets analysis after the “anomaly”

INTRODUCTIONTevatron Accelerator and the CDF Experiment

Tevatron Run II collisions at s = 1.96

TeV (1.8 TeV in Run I). Run II from Summer

2001 through Autumn 2011.

Collisions at world highest energy until Nov 2009.

Energy frontier for ～25 years!!

Two multi-purpose detectors for wide range of physics studies.

Tevatron Termination• Tevatron’s last beam was terminated on Sep 30, 2011.

Tevatron Run II — Luminosity History

• Typical Peak Luminosity : ～ 4 1032 cm2 s-1.• ～ 8 pb-1/week.• Total Integrated Luminosity:

– Delivered: 11.8 fb-1.– Recorded by CDF: 9.98 fb-1.– 8.86 fb-1 with good silicon.

• Typical data taking efficiency of CDF:

~ 85% to the end. No significant drop after 10 years of running!!

(CDF)

Collider Detector at FermilabMulti-purpose detector

Tracking in 1.4 T magnetic field. Coverage |h|<~1.

Precision tracking with silicon. 7 layers of silicon detectors.

EM and Hadron Calorimeters. sE/E ~ 14%/E (EM). sE/E ~ 84%/E (HAD).

Muon chambers.

The CDF Collaboration~600 physicists from

14 nations and 60 institutionsMcGill Univ.Univ. of Toronto

Argonne National Lab.Baylor Univ.Brandeis Univ.UC DavisUC Los AngelesUC San DiegoUC Santa BarbaraCarnegie Mellon Univ.Univ. of ChicagoDuke Univ.FermilabUniv. of FloridaHarvard Univ.Univ. of IllinoisThe Johns Hopkins Univ.LBNLMITMichigan State Univ.Univ. of MichiganUniv. of New MexicoNorthwestern Univ.The Ohio State Univ.Univ. of PennsylvaniaUniv. of PittsburghPurdue Univ.Univ. of RochesterRockefeller Univ.Rutgers Univ.Texas A&M Univ.Tufts Univ.Wayne State Univ.Univ. of WisconsinYale Univ.

JINR, DubnaITEP, Moscow

Univ. Karlsruhe

Univ. of Geneva

Glasgow Univ.Univ. of LiverpoolUniv. of OxfordUniv. College London

Univ. of Bologna, INFNFrascati, INFNUniv. di Padova, INFNPisa, INFNUniv. di Roma, INFNINFN-TriesteUniv. di Udine

IFAE, BarcelonaCIEMAT, MadridUniv. of Cantabria

LPNHE, Paris

KHCL

KEKOkayama Univ.Osaka City Univ.Univ. of TsukubaWaseda Univ.

Academia Sinica

USA Canada

Russia

Germany

Switzerland

UK

Italy

Spain

France

Korea

Japan

Taiwan

Slovakia

Univ. of Athens

Greece

HOW THE “ANOMALY” STARTEDDijet mass spectrum in WW/WZ→lnjj analysis

Diboson Production Cross Section

Process SM cross section (pb)

Measured Cross section (pb)

Luminosity (fb-1) year

WW (→2l) 12.4 ± 0.8 13.6 ±3.0 0.8 2006

WZ (→3l) 3.7 ± 0.3 3.9 ± 0.8 7.1 2011

ZZ (→4l) 1.4 ± 0.1 2.18 ± 0.69 6.1 2011

ZZ (→ll )nn 1.4 ± 0.1 1.45+0.60-0.51 5.9 2010

• Detailed study of diboson production processes provides stringent test of TGC.

• New physics can affect the production cross sections.

Diboson production cross section measurements at CDF:

WW/WZ Production in lnjj Decay Mode• Autumn 2009 analysis using 4.3 fb-

1.– e/m with Pt>20 GeV , |h|<1.– MET>25 GeV.– ≥2 jets with Et>20GeV , |h|<2.4.– Mt(e,MET)>30 GeV.– Df(MET,j1)>0.4.– Pt(jj)>40 GeV.

• Measured cross section: s(WW+WZ)= 18.1±3.3(stat.) ±2.5(syst)

～ 1.1s

DIJET MASS SPECTRUM IN W+2JETSOverview of the analysis and 4.3 fb-1 results

Jet Definition• Jets are clustered using JETCLU

algorithm with DR<0.4.• Electrons and jets are

distinguished according to their lateral and longitudinal shower shape.

• Jet energies are corrected for calorimeter response and non-linearity.

• Correction for out-of-cone energy and spectator interactions, which is physics process dependent, is not done in this analysis.

Jet Energy Measurement• Uncertainty on Jet energy

measurement is ≤3% in the relevant Et and h region.– h-dependent correction by

dijet balancing in dijet events.

– Energy from different interactions is parameterized as a function of the number of reconstructed primary-vertexes.

– Absolute scale tuned to the scale of better calibrated EM calorimeter by g+jet balancing in g+jet event.

Modeling of Physics Processes• Considered backgrounds contributing to W+(≥)2jets eventes:

• Pythia6.126, Alpgen 2.10_prime, MadEvent4.• Event kinematics of QCD multijet (non-W) is modeled by:

– “AntiElectron” events: events with electron candidates which fail two of non-kinematic (shower shape) cuts.

– Non-isolated Muon events.

Normalization of Background• We scale diboson, single top, and Z+jets backgrounds

according to their theoretical cross sections.• Normalization of W+jets and QCD multijet processes are

obtained by fitting the MET distribution.– Fit to data passing through event selection, but before the

MET cut.

Dijet Mass Spectrum in lnjj Final State• Spring 2011 analysis using 4.3 fb-1.

– e/m with Pt>20 GeV , |h|<1.– MET>25 GeV.– 2 jets with Et>30 GeV , |h|<2.4.– Mt(e,MET)>30 GeV.– Df(MET,j1)>0.4.– Pt(jj)>40 GeV.

• An excess is seen.• The W mass peak around 80 GeV does NOT

look to be described very well.

CDF’s usual cut: Etjet>20 GeV

Systematic Uncertainties• The following systematic samples are considered.

– Jet Energy Scale: ±1s.– Renormalization and factorization scale for W+jets:

• Nominal value: .• Fluctuate between half and double the nominal value.

– Modeling of QCD background:• Use Non-isolated Muon for both e/m channels.• Isolation cut is fluctuated between >0.3, >0.2(nominal),

and >0.15.• These systematic sources are considered as source to affect

both rate and shape of the background in the final fit.

Dijet Mass Spectrum in lnjj Final State• Spring 2011 analysis using 4.3 fb-1.

– e/m with Pt>20 GeV , |h|<1.– MET>25 GeV.– 2 jets with Et>30GeV , |h|<2.4.– Mt(e,MET)>30 GeV.– Df(MET,j1)>0.4.– Pt(jj)>40 GeV.

• An excess is seen.

• 3.2s deviation from estimated background, after considering the systematic uncertainties on background modeling.

4.3 fb-1 with Different CutsJet Et>65 GeVPt(jj)>40 GeV

Jet Et>30 GeVPt(jj)>40 GeV(nominal selection)

Jet Et>30 GeVPt(jj)>60 GeV

A Web Post at CMS

http://cmsdoc.cern.ch/~ttf/CDFDiJetScale/AnimatedDijet.gif

• Scale the background Mjj distribution by up to 7%.• The excess goes away if a ≥5% scale is assumed.• In CDF analysis, JES shape uncertainty is considered in the final

fit.• However, this method will end up with ～ 10% discrepancy in

overall normalization.

• Author not known.• I didn’t find a description

about this plot, either.

We will revisit this plot later.

STUDIES ON W+2JETS PROPERTIESSummer 2011 update with 7.3 fb-1

Summer 2011 update• Summer 2011 update using 7.3 fb-1.• Excess corresponding to ～ 4 pb.• Statistically 4.7s deviation from

estimated background. – 4.1s even when we consider

systematic effect.– D0 did not see such an excess.

Mjj Distribution by Lepton Type

Electron: Muon:

Some Kinematics 1

Some Kinematics 2

Some Kinematics 3

Some Kinematics 4

Crosscheck - Jet Energy Scale• Largest systematic source, considered in the fit.• Shifted JES by +2 , s which corresponds to a shift by ～ 7% for

the left plot.• We still see a notable excess ～ 4.1s.• Plus, JES won’t explain the discrepancy in angular distributions.

Crosscheck – Alternative Generator• With alternative W+jets modeling with Sherpa 1.2.2.

NLO Effect (4.3 fb-1 analysis)• Ratio between MCFM and ALPGEN+PYTHIA was calculated as

a function of Mjj.• Reweight ALPGEN sample by the obtained ratio.• This procedure returned a statistical significance of 3.4 (s 3.2 s

with nominal analysis).

SEARCH FOR First pretag W+2jets analysis after the “anomaly”

SM Higgs Search Status at CDF/Tevatron(these results will be updated very very soon)

• CDF excludes 156.5 < mH < 173.7 GeV/c2 at 95% C.L.

• Tevatron excludes 156 < mH < 177 GeV/c2 at 95% C.L.

SM Higgs Properties at Tevatron

bb WW• mH<135 GeV (low mass):

– gg→H→bb is difficult to see.– Look for WH/ZH with leptonic vector boson decays.

• mH>135 GeV (high mass):– Easiest to look for H→WW with one or two W

decaying to lepton.

CDF Analyses

• Sensitivity for mH>135 GeV/c2 is dominated by H→WW*→lnln mode!

• We hope to improve the sensitivity of the experiment by adding a new analysis channel: .

𝑯→𝑾𝑾 (∗)→ 𝒍𝝂 𝒍𝝂

𝑯→𝒁 𝒁(∗)→𝟒 𝒍

𝑪𝒐𝒎𝒃𝒊𝒏𝒆𝒅

Event Selection and Reconstruction• Event selection:

– e/m with Pt>20 GeV , |h|<1.– MET>20 GeV.– 2 jets with Et>20GeV , |h|<2.0.– 60<Mt(e,MET)<100 GeV.

• Pzν Reconstruction:

– Solve equation: m(e,ν) = 80.419 GeV.– Pick up the solution with smaller absolute value |Pz

ν |.– Take the real part if imaginary solution.

Reconstructed Higgs Mass

GeV/c2

Arbi

trar

y

Analysis Scheme• We unify 6 kinematic variables into a likelihood discriminant.

– j1: the jet closer by to the lepton.• Background Estimation:

– MET fit to obtain crude W+jets/QCD normalization.– Fit the likelihood discriminant to data with each background

fluctuated within the stat./syst. uncertainties.• Break down W+jets into W+qq/W+qg/W+gg, and each

subprocess is floated independently.• Systematic uncertainties are taken into account in the fit,

including JES and Q2 of W+jet subprocess.• # signal events is also fluctuated. The fit returned zero-

consistent signal contribution (for all mass points) this time.

Input Variables to Likelihood Discriminant• Data-MC agreement with this background estimation is good

for these kinematic variables!– Some other unused variables (jet Et, jet h, Mt(l,MET)) still suffer

discrepancy, though improved by this procedure.

𝑴 𝒍𝝂 𝒋𝒋❑ 𝑴 𝒋𝒋

❑ 𝚫𝐑 (𝐥 , 𝐣𝟏)

𝚫𝐑 ( 𝐣 , 𝐣) 𝚫𝝓(𝐥 , 𝐣𝐣) 𝑷𝑻𝒍𝒆𝒑

Open red histograms show 100xsignal for mH=180 GeV/c2.

Likelihood Discriminant

Open red histogram shows 100xsignal for mH=180 GeV/c2.Likelihood Discriminant

Higgs Cross Section Limit with Channel

• Excludes ()>s 5.7×sSM at 95% C.L for mH=180 GeV/c2.

4.6 fb-1

Summary• Overviewed recent two W+2jet analyses:

– Dijet mass spectrum analysis – analysis

• We are having difficulty in modeling the W+2jet background at CDF. Large discrepancy between data and MC is seen:– in dijet mass analysis (harder cuts).– , , in Higgs analysis.

• Situation for W+2jet analyses applying b-tagging, such as ,are better, but probably due to lower statistics per analysis channel.

Summary 2• Several studies have been done to improve the data-MC agreement:

– In context with Dijet mass analysis (harder cuts):• Shift jet energy scale to an extreme.• Alternative W+jets generator (Sherpa).• Study with NLO description (MCFM).

– In context with Higgs analysis:• Event Reweighting of ALPGEN so that a particular kinematic

distribution has perfect data-MC agreement. We tried reweighting with , , but none of these improved wide range of distributions.

• Breaking down W+2jet into W+qq/W+qg/W+gg led to the first Higgs search result in pretag W+2jet topology, but the improvement is not enough for some kinematics.

• We haven’t found a definitive prescription.

BACKUP

Top Mass Measurement in L+jets Events

t

tq

q

g

g

b

b

W+

W-

l+

n

q’

q15% 85%

100%

100%

In-situ JES calibration

• Event reconstruction with kinematic fit.

• 2D likelihood fit with mtop and DJES as free parameters.

8.7 fb-1

Validity of jet energy scale

Inclusive jet selection

45

Top component doubles, similar excess feature

Remove systematics associated with 3rd jet veto

Dijet mass in L+2jets

NLO Effect (4.3 fb-1 analysis)Dijet mass in L+2jets

Larg

e ve

rsio

n

CDF Higgs Searches

Likelihood Discriminant• analysis composes a likelihood discriminant from 6 kinematic

variables in order to improve Signal/Background separation.• Signal template is modeled by PYTHIA Higgs sample, and

background is modeled by ALPGEN W+2jet sample.

value for the event being analyzed

Si

Bi

ith variable for S/B separation

signalbkgd.

• For an event with a value for ith variable as shown in right plot, the likelihood for this single variable is defined as:

• The likelihood discriminant is defined as:

(i runs through all the variables considered in the likelihood composition)

analysis

Likelihood Templates for CEM• Signal and background templates for central electron ().• For mH=180 GeV/c2.

𝑴 𝒍𝝂 𝒋𝒋❑ 𝑴 𝒋𝒋

❑ 𝚫𝐑 (𝐥 , 𝐣𝟏)

𝚫𝐑 ( 𝐣 , 𝐣) 𝚫𝝓(𝐥 , 𝐣𝐣)𝑷𝑻

𝒍𝒆𝒑

analysis

Likelihood Templates for CMUP• Signal and background templates for central muon ().• For mH=180 GeV/c2.

𝑴 𝒍𝝂 𝒋𝒋❑ 𝑴 𝒋𝒋

❑ 𝚫𝐑 (𝐥 , 𝐣𝟏)

𝚫𝐑 ( 𝐣 , 𝐣) 𝚫𝝓(𝐥 , 𝐣𝐣)𝑷𝑻

𝒍𝒆𝒑

analysis

Likelihood Templates for CMX• Signal and background templates for intermediate muon

(0.6<).

• For mH=180 GeV/c2.

𝑴 𝒍𝝂 𝒋𝒋❑ 𝑴 𝒋𝒋

❑ 𝚫𝐑 (𝐥 , 𝐣𝟏)

𝚫𝐑 ( 𝐣 , 𝐣) 𝚫𝝓(𝐥 , 𝐣𝐣)𝑷𝑻

𝒍𝒆𝒑

analysis

Systematics Table analysis

Syst

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cs th

at a

lso

affec

t th

e ba

ckgr

ound

sha

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Background Summary• Construction of the likelihood discriminant depends on the Higgs signal

MC, so we perform the fit for each analyzed Higgs mass point.

Estimated number of background:

Expected signal yield:

analysis

Higgs Cross Section Limit with Channel (with Table)

4.6 fb-1

analysis