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Physics with W+jets at CDF towards better understanding of W+2jet process, with regard to the dijet resonant peak. Koji Sato on behalf of CDF Collaboration KEK Theory Meeting on Particle Physics Phenomenology February 29, 2012. Contents. Introduction

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Physics with W+jets at CDFtowards better understanding of W+2jet process,with regard to the dijet resonant peak

Koji Sato

on behalf of CDF Collaboration

KEK Theory Meeting on Particle Physics Phenomenology

February 29, 2012


  • Introduction

    • Tevatron Accelerator and the CDF Experiment

  • How The “Anomaly” Started

    • Dijet mass spectrum in WW/WZ→lnjjanalysis

  • 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”


Tevatron Accelerator and the CDF Experiment

Tevatron run ii
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 Termination

  • Tevatron’slast beam was terminated on Sep 30, 2011.

Tevatron run ii luminosity history
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!!


C ollider d etector at f ermilab
Collider Detector at Fermilab

Multi-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
The CDF Collaboration

~600 physicists from

14 nations and 60 institutions




McGill Univ.

Univ. of Toronto

Argonne National Lab.

Baylor Univ.

Brandeis Univ.

UC Davis

UC Los Angeles

UC San Diego

UC Santa Barbara

Carnegie Mellon Univ.

Univ. of Chicago

Duke Univ.


Univ. of Florida

Harvard Univ.

Univ. of Illinois

The Johns Hopkins Univ.



Michigan State Univ.

Univ. of Michigan

Univ. of New Mexico

Northwestern Univ.

The Ohio State Univ.

Univ. of Pennsylvania

Univ. of Pittsburgh

Purdue Univ.

Univ. of Rochester

Rockefeller Univ.

Rutgers Univ.

Texas A&M Univ.

Tufts Univ.

Wayne State Univ.

Univ. of Wisconsin

Yale Univ.



IFAE, Barcelona

CIEMAT, Madrid

Univ. of Cantabria

JINR, Dubna

ITEP, Moscow



LPNHE, Paris

Univ. Karlsruhe



Univ. of Geneva

Univ. of Athens



Glasgow Univ.

Univ. of Liverpool

Univ. of Oxford

Univ. College London




Okayama Univ.

Osaka City Univ.

Univ. of Tsukuba

Waseda Univ.


Univ. of Bologna, INFN

Frascati, INFN

Univ. di Padova, INFN

Pisa, INFN

Univ. di Roma, INFN


Univ. di Udine


Academia Sinica

How the anomaly started
How the “anomaly” started

Dijet mass spectrum in WW/WZ→lnjjanalysis

Diboson production cross section
Diboson Production Cross Section

  • 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 l n jj decay mode
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)


Dijet mass spectrum in w 2jets
Dijet mass spectrum in W+2jets

Overview of the analysis and 4.3 fb-1 results

Jet definition
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
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
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
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 l n jj final state
Dijet Mass Spectrum in lnjjFinal 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
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 l n jj final state1
Dijet Mass Spectrum in lnjjFinal 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 cuts
4.3 fb-1 with Different Cuts

Jet Et>65 GeV

Pt(jj)>40 GeV

Jet Et>30 GeV

Pt(jj)>40 GeV

(nominal selection)

Jet Et>30 GeV

Pt(jj)>60 GeV

A web post at cms
A Web Post at CMS

  • 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 properties
Studies on W+2jets properties

Summer 2011 update with 7.3 fb-1

Summer 2011 update
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.

M jj distribution by lepton type
MjjDistribution by Lepton Type



Some kinematics 1
Some Kinematics 1

Crosscheck jet energy scale
Crosscheck - Jet Energy Scale

  • Largest systematic source, considered in the fit.

  • Shifted JES by +2s, 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
Crosscheck – Alternative Generator

  • With alternative W+jets modeling with Sherpa 1.2.2.

Nlo effect 4 3 fb 1 analysis
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 3.4s (3.2s with nominal analysis).

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

  • CDF excludes 156.5 < mH < 173.7 GeV/c2at 95% C.L.

  • Tevatron excludes 156 < mH < 177 GeV/c2at 95% C.L.

Sm higgs properties at tevatron
SM Higgs Properties at Tevatron

  • 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
CDF Analyses

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

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

Event selection and reconstruction
Event Selection and Reconstruction

Reconstructed Higgs Mass

  • 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 |Pzν |.

    • Take the real part if imaginary solution.



Analysis scheme
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+jetsubprocess.

      • #signal events is also fluctuated. The fit returned zero-consistent signal contribution (for all mass points) this time.

  • Input variables to likelihood discriminant
    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.

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

    Likelihood discriminant
    Likelihood Discriminant

    Likelihood Discriminant

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

    Higgs cross section limit with channel
    Higgs Cross Section Limit with Channel

    4.6 fb-1

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


    • 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
    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.

    Top mass measurement in l jets events



















    Validity of jet energy scale

    Top Mass Measurement in L+jets Events

    • Event reconstruction with kinematic fit.

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

    In-situ JES calibration

    8.7 fb-1

    Inclusive jet selection

    Dijet mass in L+2jets

    Inclusive jet selection

    Removesystematicsassociatedwith 3rd jet veto

    Top componentdoubles, similarexcessfeature

    Nlo effect 4 3 fb 1 analysis1

    Dijet mass in L+2jets

    NLO Effect (4.3 fb-1 analysis)

    Large version

    CDF Higgs Searches

    Large version

    Likelihood discriminant1


    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.

    ith variable for S/B separation





    • For an event with a value for ithvariable 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)

    value for the event being analyzed

    Likelihood templates for cem


    Likelihood Templates for CEM

    • Signal and background templates for central electron ().

    • For mH=180 GeV/c2.

    Likelihood templates for cmup


    Likelihood Templates for CMUP

    • Signal and background templates for central muon().

    • For mH=180 GeV/c2.

    Likelihood templates for cmx


    Likelihood Templates for CMX

    • Signal and background templates for intermediate muon(0.6<).

    • For mH=180 GeV/c2.

    Systematics table


    Systematics Table

    Systematics that also affect the background shape

    Background summary


    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:

    Higgs cross section limit with channel with table


    Higgs Cross Section Limit with Channel (with Table)

    4.6 fb-1