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Higgs Searches at CDF. Thomas Wright University of Michigan SLAC Experimental Seminar February 21, 2006. The Higgs Boson of the Standard Model. Electroweak symmetry can be broken using the “Higgs mechanism” One complex doublet of fields – 4 degrees of freedom

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Higgs searches at cdf

Higgs Searches at CDF

Thomas Wright

University of Michigan

SLAC Experimental Seminar

February 21, 2006

The higgs boson of the standard model
The Higgs Boson of the Standard Model

  • Electroweak symmetry can be broken using the “Higgs mechanism”

    • One complex doublet of fields – 4 degrees of freedom

    • Three give mass to the W’s and Z

    • Other manifested as a single scalar – the “Higgs boson”

  • If there is such a particle, precision electroweak measurements favor a low mass

    • LEP2 searches excludemH > 114.4 GeV/c2 @ 95% CL

    • SM fit requires mH < 186 GeV/c2 @ 95% CL (219 if including LEP2 direct searches)

SLAC Experimental Seminar

Higgs production and decay
Higgs Production and Decay

Ideally, use gg  H  bb, WW

But, QCD bb background too high

For low mH, use WH+ZH, H  bb (associated production)

At high mH the WW decay mode opens up – can use gg  H production

SLAC Experimental Seminar

Run 2 at the tevatron
Run 2 at the Tevatron

Have reached 1.8E32 cm-2s-1 twice now in the past few weeks

Recording 15-20 pb-1 per week

However, 3-month shutdown starts next week

Results shown here use data up to August 2004 (~300 pb-1)

Updates with more data coming soon!

SLAC Experimental Seminar

The cdf ii detector
The CDF II Detector

Azimuthally symmetric “barrel” geometry

Central detectors cover ||<1

Plug calorimeter extends to ||<3.6

Silicon extends to z =  50 cm (luminous region z ~ 25 cm)

Tracking out to ||<2

SLAC Experimental Seminar

The cdf ii trigger system

2.5 MHz crossing rate (396 ns)

Output 20-30 kHz


Latency 25-30 s

Output ~400 Hz

Readout latency ~650 s

Output 70-90 Hz

The CDF II Trigger System

  • Interaction rate very high, but most not “interesting”

  • Limited bandwidth to mass storage – must be choosy

  • Level1 system

    • Synchronous – no deadtime

    • Single CAL towers (photons and jets), COT tracks (with pair correlations), track-tower matches (electrons and taus), muons, missing energy

  • Level2 system

    • Asynchronous - ~5% deadtime

    • All Level1 objects, plus CAL clusters (jets) and silicon tracking

  • Level2 accept triggers full detector readout (few % deadtime)

  • Level3 runs a version of the offline reconstruction – final rate reduction before writing to tape

  • Always tuning the system to accommodate higher luminosity

SLAC Experimental Seminar

Particle identification
Particle Identification

  • Charged leptons identified by characteristic energy deposition patterns

  • Presence of neutrinos is inferred from energy imbalance – “missing energy”

  • Because net pz of the scattering partons is not known, mostly work in the transverse plane (i.e pT, ET, missing-ET)

  • B-jet identification uses the silicon tracker

    • 8 layers, 704 ladders, 722432 channels

    • Total sensor area = 6 m2

    • SVX II – 5 double-sided layers (r + rz)

    • L00 r only – mounted directly to beampipe (R = 1.4 cm)

SLAC Experimental Seminar

B jet identification b tagging
B-Jet Identification (b-tagging)

  • B-hadrons are long-lived – search for displaced vertices

  • Construct event-by-event primary within beamspot (10-32 m)

  • Fit displaced tracks and cut on Lxy significance ( ~ 200 m)

  • Calibrate performance from data (low-pT lepton samples)

b-fraction ~80%

measure tag efficiency in data and MC

Tag this jet

Efficiency data/MC scale factor

SF = 0.91  0.06

SLAC Experimental Seminar

Fake b tags mistags
Fake B-Tags (mistags)

  • Fake tags are (mostly) symmetric in Lxy

  • Rate of tags with Lxy<0 is a good estimate for the mistag rate

  • Parametrize mistag rate which can be applied to any sample

  • ~30% correction for tags from /KS and interactions with detector material

Lxy > 0

Lxy < 0

SLAC Experimental Seminar

The wh l bb channel
The WH  lbb Channel

  • Event selection

    • Isolated e or  with pT>20 GeV/c

    • Missing-ET > 20 GeV

    • Exactly two jets with ET>15 GeV

    • At least one b-tagged jet

  • Acceptance is 1.5-1.7%

  • Backgrounds include

    • Non-W events (fake lepton, fake missing-ET, b decays)

    • W + mistagged jets

    • W + heavy flavor jets

    • Diboson production (WW, WZ, ZZ)

    • Z

    • Top quark production (including single top)

SLAC Experimental Seminar

W jets simulation
W + jets Simulation

  • Lots of activity in recent years

  • We use the ALPGEN generator

    • Tree-level W + N partons

    • Also W+c+Np, W+cc+Np, W+bb+Np

  • HERWIG parton shower adds soft gluon radiation

  • Monte Carlo prediction normalized to observed number of W+jets

  • Fraction of events containing heavy quarks calibrated from data

    • b-tag rates in data and ALPGEN multijet samples

    • Scale ALPGEN prediction by 1.5  0.4

SLAC Experimental Seminar

Untagged control sample
Untagged “Control Sample”

SLAC Experimental Seminar

Tagged background summary
Tagged Background Summary

Use W+1-jet bin to

fix W+HF bkgd

(scale up by 20%)

Top pair cross section

Measured from the

W+3,4-jets events

SLAC Experimental Seminar

Tagged dijet mass
Tagged Dijet Mass

SLAC Experimental Seminar

Wh cross section limits
WH Cross Section Limits

SLAC Experimental Seminar

The zh bb channel


Missing ET

A di-jet QCD event:

2nd jet



Fake Missing ET


1st jet


The ZH bb Channel

  • Distinctive final state of b-jets recoiling against missing-ET

  • Event selection

    • Missing-ET > 70 GeV

    • Lepton veto

    • Exactly two jets with ET > 60 and 25 GeV

    • Missing-ET not aligned with either jet

  • Acceptance is 0.5-0.8%

  • Backgrounds include

    • QCD with fake missing-ET

    • QCD bb production

    • W/Z + jets

    • Top production

    • Diboson production

SLAC Experimental Seminar

Zh bb backgrounds
ZH bb Backgrounds

QCD bb background normalization fixed in Control Region 1 – extrapolate into others

Other backgrounds checked in Control Region 2

Now search in the signal region

SLAC Experimental Seminar

Zh dijet mass cut
ZH Dijet Mass Cut

  • Final selection is a dijet mass window cut

    • Require mean  20 GeV/c2

    • Straight counting experiment

  • Expect 4.4  0.9  0.5 background events, observe 6 (for mH = 120)

  • Future iterations will bin the dijet mass and count within each bin as in the WH search

SLAC Experimental Seminar

Zh cross section limits
ZH Cross Section Limits

SLAC Experimental Seminar

The h ww l l channel
The H  WW* l l Channel

  • Largest BR for mH > 135 GeV/c2

  • Uses gg  H production

    • Larger cross section than associated production

    • Suffer from W  l BR

  • Event selection

    • Two isolated leptons with pT > 20 and 10 GeV/c

    • Opposite charge

    • Missing-ET > mH/4

    • If missing-ET aligned with lepton, > 50 GeV

    • mll > mH/2-5 GeV/c2

    • pT,1+pT,2+missing-ET < mH

    • Jet veto

  • Including BR’s, acceptance is 0.3-0.7% depending on mH

W  l BR not included

SLAC Experimental Seminar

H ww backgrounds





H  WW* Backgrounds

  • Predominantly WW

  • Also Drell-Yan and other diboson channels, and from fake leptons

  • Not possible to reconstruct Higgs mass due to multiple neutrinos

  • Can exploit scalar nature of Higgs

    • Leptons from H  WW* are close in 

  • Treat each bin of  as a separate counting experiment, analogous to dijet mass in WH search

SLAC Experimental Seminar

H ww cross section limits
H  WW* Cross Section Limits

SLAC Experimental Seminar

Sm higgs limits summary
SM Higgs Limits Summary

SLAC Experimental Seminar

Limits scaled by sm cross sections
Limits Scaled by SM Cross Sections

SLAC Experimental Seminar

Closing the gap
Closing the Gap

  • Scale all channels to 300 pb-1 and combine sensitivities relative to SM

  • Would need ~50 fb-1 to exclude mH = 115 GeV/c2 (!)

  • Still much that can be done (improvements in (S/B)2)

    • Improve dijet mass resolution (goal is 10%, factor 1.7)

    • Better b-tag/mistag separation (factor 1.5)

    • Extend lepton acceptance (factor 1.8)

    • Multivariate separation of signal/bkgd (factor 1.75)

    • Include WH signal in ZH search (factor ~2)

    • CDF/D0 combination (factor 2)

  • Prospects are good to probe the 115 GeV/c2 region with a few fb-1

SLAC Experimental Seminar

Example the zh llbb channel
Example – The ZH  llbb Channel

Still in development – no results yet

Very clean channel – bkgd almost all Z+bb and Z+mistag

Comparison with Run I result indicates ~25% better limit from neural network over dijet mass alone

SLAC Experimental Seminar

Higgs in the mssm






Higgs in the MSSM

  • Two complex doublets lead to five scalars: h, H, A, H+, H-

  • Properties of the Higgs sector can be predicted from only a few parameters

    • Most interesting: mA and tan

  • bb vertex ~ tan2

    • Production via b quarks can be greatly enhanced

    • Decays to bb (~90%) and  (~10%) dominate

    • W and Z couplings NOT enhanced – BR’s low even for high m

  • In many “benchmark” scenarios, the A is degenerate with either h or H at high tan

TeV4LHC working group

SLAC Experimental Seminar

The bb channel
The bb  Channel

  • High cross section and unique final state (not QCD)

  • Best signature is one  decay into e or  and the other hadronically

  • Event selection

    • One e or  with pT > 10 GeV/c

    • One hadronic  with pT > 15 GeV/c, mass < 1.8 GeV/c2

    • Opposite charge

    • Missing-ET not recoiling against leptons (rejects W  l)

  • Acceptance is 1-2%

  • Backgrounds include

    • Z 

    • W  l +jet  fake had

    • QCD multijet (both  fake)

SLAC Experimental Seminar

Cross section limits
Cross Section Limits

  • Use the “visible mass” to further separate signal/background

    • Mass of the lepton, had, and missing-ET

Got a little bit unlucky above 120 GeV/c2

SLAC Experimental Seminar

Mssm interpretation
MSSM Interpretation

D0 search in the bbbb final state

 final state less sensitive to mixing effects than bbbb

As bb  cross section decreases,  BR increases to compensate

SLAC Experimental Seminar

Future prospects in the bb channel
Future Prospects in the bb  Channel

Combine CDF and D0 (with similar sensitivity)

Acceptance improves by 30% (lepton coverage, more decay modes)

Assumed no improvement in systematic uncertainties (unlikely)

SLAC Experimental Seminar

The gg bb bbbb channel
The gg  bb bbbb Channel

  • Use events with three b-tagged jets, search for a dijet mass peak

  • Backgrounds are QCD production of bbbb or bb+mistagged jet

  • Trigger is a major issue

    • Even the 70 GeV jet trigger is prescaled by 8

  • Solution is to move part of the b-tagging into the trigger

    • Use the silicon vertex tracker (SVT) in Level 2

    • Three central jets, two matched to SVT tracks with high impact parameter

  • Redefine b-tag to include SVT requirement, measure data/MC scale factor using same methods

  • Interpretation in MSSM complicated by Higgs width (can be 20-30% of mA at high tan)

(35  33) mm SVT  beam

 s = 48mm

SLAC Experimental Seminar

Sm higgs at the lhc
SM Higgs at the LHC

  • Pretty much a “sure thing” (if it exists, and Tevatron doesn’t get there first)

  • Strategies for low-mass region are different – more focused on backgrounds than cross section

  • So, is what we are learning at the Tevatron useful? Of course!

    • tt event reconstruction

    • dijet mass reconstruction

    • W/Z + jets background estimation techniques

    • b-tagging and  ID at hadron colliders

  • Year-long series of “TeV4LHC” workshops explored all of these topics and more

SLAC Experimental Seminar


  • CDF is searching for the Standard Model Higgs in a variety of production and decay scenarios

  • Tools are in place to combine results from different channels

  • Existing analyses not sensitive to SM Higgs even with full anticipated Run 2 data sample

    • First iterations – focus is on correctness

    • Many improvements being pursued to improve sensitivity

  • Data samples growing quickly

  • MSSM Higgs searches starting to look pretty exciting

Should be an interesting next few years!

SLAC Experimental Seminar

Non w background to wh channel

Signal region D predicted by

Model event kinematics from sideband

Non-W Background to WH Channel

  • Use missing-ET and isolation ratio (assumed uncorrelated) in sidebands to extrapolate into signal region

  • Isolation ratio = (lepton pT)/(non-lepton energy in cone with - radius 0.4 around the lepton)

SLAC Experimental Seminar

B tag efficiency measurement
B-Tag Efficiency Measurement

  • Large b-hadron mass gives a wide pT,rel distribution relative to non-b contributions

  • Fit untagged and tagged jets with b and one of four non-b templates to get b-tag efficiency

  • Spread of results using the four non-b used as a systematic error

SLAC Experimental Seminar

Dijet mass resolution
Dijet Mass Resolution

Raw: what we use now

H1: track + CAL energy flow

MTL: correct for soft leptons

Hyperball: multivariate nearest-neighbor algorithm, pick the most likely “true” dijet mass

SLAC Experimental Seminar