Searches for Higgs Bosons at CDF

Thomas WrightUniversity of Michigan

SLAC Experimental SeminarFebruary 13, 2007

2

Run 2 at the Tevatron

• World’s highest-energy collider

• Record luminosity 278E30 cm-

2s-1

• Will have 2 fb-1 on tape in ~1 week!

• Integrating 40-45 pb-1/week

• Results shown here use data samples of ~1 fb-1

• Look for updates on expanded samples this summer

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The CDF II Detector

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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

5

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)

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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

2.5 MHz crossing rate (396 ns)

Output 20-30 kHz

Synchronous

Latency 25-30 s

Output ~700 Hz

Output 70-90 Hz

Readout latency ~650 s

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The Higgs Boson of the Standard Model

• Electroweak symmetry can be broken using the “Higgs mechanism”– 4 new scalar fields– 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 exclude

mH < 114.4 GeV/c2 @ 95% CL

– SM fit requires mH < 166 GeV/c2 @ 95% CL (199 if including LEP2 direct searches) as of last year

• New CDF W mass (most precise single measurement) moves best fit to 80+36-26 GeV/c2 (was 85+39-28)

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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

(pb-1)

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The H WW* l l Channel

• Largest BR for mH > 135 GeV/c2

• Uses gg H production– Now including VBF

(6-10% extra cross section)

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

W l BR not included

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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 closer in

• Treat each bin of as a separate counting experiment

W-

W+

e+

e-

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H WW* Cross Section Limits

• Sensitivity about 6x SM level at 160 GeV

• For 6 fb-1 and combined with DØ, within factor 2 of SM

• Get more events

• WZ search almost doubled acceptance by adding new lepton types

• Get more out of events

• More than just

• Multivariate (NN)

• Matrix elements

exc

lud

ed

at

95%

C.L

.

12

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

(pb-1)

13

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)

Tag this jet

b-fraction ~80%

measure tag efficiency in data and MC

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Fake B-Tags (mistags)

• Fake tags are (almost) symmetric in displacement 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

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The WH lbb 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 1.8-2.1%

• 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)

• This channel uses a neural net filter on the b-tags to reject half of the background (~10% signal loss)

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WH Backgrounds

Use W+1-jet bin to test W+HF bkgd

Top pair cross sectionMeasured from theW+3,4-jets events

SM Higgs would be about two events

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WH Dijet Mass

At least one jet b-tagged with NN Both jets b-tagged

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WH Cross Section Limits

exc

lud

ed

at

95%

C.L

.

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The ZH bb Channel

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

• Event selection– Missing-ET > 75 GeV– Lepton veto– Exactly two jets with ET >

60 and 20 GeV– Missing-ET not aligned with

either jet• Acceptance is 0.5-0.8% (for ZH)

– About half the events would be WH with lost lepton

• Backgrounds include– QCD with fake missing-ET

– QCD bb production– W/Z + jets– Top production– Diboson production

Missing ETb-jet

b-jet

y

x

2nd jet

Fake Missing ET

1st jet

180o

A di-jet QCD event:

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ZH Dijet Mass

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ZH Cross Section Limits

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The ZH llbb Channel

• Look for ZH also in Z decays to charged leptons (e or )

• Lose in BR, gain in background

• Z+jets and top pairs

• Instead of dijet mass, enhance S/B using neural networks

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ZH Cross Section Limits

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Combined Limits (Relative to SM)

• Not including new WW*

• Factor of ~8 away from SM prediction at 115 GeV

• Expect factor 2-3 from more luminosity

• Another factor of √2 from combination with DØ

• As with WW*, need to improve the analyses in order to reach SM sensitivity

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Multivariate B-Tagger

• Multivariate b-tag algorithm in the pipeline

• Much like the one used by SLD

• Plots are for simulation only, performance characterization on data is in progress

• Up to 30% higher b-tag efficiency compared to the NN tagger already used in the WH search channel

• More double-tagged events

• Better S/B

• Better dijet mass resolution

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Double-Tagging in WH

Similar limits for exclusive 1-tag and 2-tag samples

20% improvement over inclusive ≥1-tag result

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The W/Z+H qqbb Channel

• Was competitive with the leptonic W channel in Run I (but a little lucky)

• 70% of W/Z decays are into hadrons

• Base selection is four jets with two b-tagged– Search in tagged dijet mass– Can use the four-jet trigger

designed for all-hadronic top pairs – also working on a new dedicated trigger

• Background dominated by QCD multijet production (with real tags)– Data-driven estimates in

progress

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Higgs in the MSSM

• Minimum of five scalars: h, H, A, H+, H-

– Separate couplings for up-type and down-type fermions

• Properties of the Higgs sector largely determined by mZ and by two other parameters:

– mA : mass of pseudoscalar– tan : ratio of down-type

to up-type couplings

• If tan is small, then h looks a lot like a standard model Higgs

• If tan is large– Production via b quarks

can be greatly enhanced (factor ~tan2)

– Decays to bb (~90%) and (~10%) dominate

• LEP-II searches have excluded mA<93 GeV/c2 and tan < 5-10

b

0

b

b

0

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MSSM Higgs Masses

• At high tan, the A becomes nearly degenerate with h or H– Can search for single resonance, double the cross section

• The other neutral Higgs is like SM Higgs (no production enhancement)

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Scenario Dependence

• Higgs properties are largely, but not completely, determined by mA and tan

• Loop corrections introduce dependence on other SUSY parameters– M. Carena et al., Eur.Phys.J. C45

(2006) 797-814 (hep-ph/0511023)

• b is a function of the other SUSY parameters and depends on the “benchmark” scenario

• b ~ tan (sign of critical)

• For tan = 50, = -200– mh

max: b = -0.21

– no-mixing: b = -0.11

• Need both channels to get the full story

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The gg+bb Channel

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

• Best signature is one decay into e or and the other hadronically (46% BR)– Now also e+ channel (+6%)

• Event selection– One e or with pT > 10

GeV/c– One hadronic with pT > 15

GeV/c, mass < 1.8 GeV/c2 (including 0’s)

– Or, e+ with pT > 6 GeV/c– Opposite charge– Missing-ET not recoiling

against leptons (rejects W l)

• Acceptance is 1.2-2.1%

• Backgrounds include– Z – W l +jet fake had– QCD multijet (both fake)

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Higgs Discriminant

• Because of multiple neutrinos cannot reconstruct invariant mass• In cases where taus are not back-to-back can use missing-ET projections

– Low efficiency• Instead, use “visible mass”

– Mass of the visible parts of the two taus and the missing-ET

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Sample Fit

• Signal is normalized to 95% CL exclusion limit

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MSSM Di-Tau Limits

• Background-only pseudoexperiments indicate <2 significance when considering the entire mvis range

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MSSM Interpretation

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The gg bb bbbb Channel

• bb final state not unique enough• Require one of the additional b’s

to have high pT and b-tag it• Results are for a different cross section

than the case (factor ~4)

• Basic event selection is three b-tagged jets• Search in dijet mass of two leading jets

• Background expected to be ~100% QCD multijet production

• Almost all bbq, bbc, bbb• Data-driven estimates exist

• At high tan the Higgs can developsignificant width – fit templates arefunction of cross section

• We expect CDF result on 1 fb-1 outwithin a few weeks

*editorial comment belongs to me, not DØ

Dawson, Jackson, Reina, Wackeroth hep-ph/0603112

?*

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SM Higgs at the LHC

• Discovery prospects are excellent

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

• Experience gained at the Tevatron will be very useful– tt event reconstruction– dijet mass reconstruction– W/Z + jets background

estimation techniques– b-tagging and ID at

hadron colliders

• Starting from a bump at the Tevatron gets us there that much faster!

38

Tevatron Prospects

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Summary

• CDF is searching for the Higgs in a variety of production and decay scenarios– Tools are in place to combine results from different channels– Lots of effort going into adding new channels and improving

the existing ones

• MSSM Higgs searches looking quite interesting

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Backup Material

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The Standard Model

• Matter is made out of fermions: – quarks and leptons– 3 generations

• Forces are carried by Bosons:– Electroweak: ,W,Z– Strong: gluons

• Higgs boson:– Gives mass to particles

• Much is still unknown– Is the EW symmetry really

broken by a Higgs? What kind of Higgs(es)?

– Are there any other particles? New gauge bosons (Z’, W’)? Extra generations of quarks? Extra dimensions? Superpartners?

• Let’s have a look!

HH

43

What about Neutrinos?

• Not nearly enough material to stop them in the detector

• Instead, infer their presence by energy imbalance

• i.e. add up everything you see, then reverse it

• Commonly called “missing-ET”

• Only get net neutrino momentum if >1

• Only works in transverse plane

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PDFs

45

46

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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)

Signal region D predicted by

Model event kinematics from sideband

48

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

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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

50

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

51

Untagged “Control Sample”

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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

53

The CDF II Detector

SOLENOID

= 2.0

END WALL HAD CAL.

CENTRAL OUTER TRACKER

CLC

CENTRAL HAD CALORIMETER

PLUG HAD CAL.

= 1.0

= 3.0

PLU

G E

M C

AL.

MUON CHAMBERS

Silicon Vertex Detecto

r

CENTRAL EM CALORIMETER

54

Electrons and Photons

Electron ID :

•Covers ||<3.6

•||<2 (w/ trk)

•ID eff. ~ 80-90%

Photon ID :

•Coverage : ||<2.8

•ID eff. ~ 80%

55

Muons

Muon ID :

•Coverage : ||<1

•ID eff. ~ 90-100%

56

Taus

h ID

cone

isolation

Tau ID :•Narrow iso. cluster

•Low # tracks

• 0 identification

•Coverage : ||<1

•ID eff. ~ 46%

Neutrinos inferred from energy imbalance

57

Jets and b-Tagging

Jet ID :

•Cluster of CAL towers

•Coverage : ||<3.6

Heavy Flavor Jet Tagging :

• Id HF jets via finding displaced vertex

•Coverage : ||<1.5

•Efficiency : 40-45%

x

y

z

Lxy

bdo