Higgs Searches at CDF

Higgs Searches at CDF

Thomas WrightUniversity of Michigan

SLAC Experimental SeminarFebruary 21, 2006

T. Wright SLAC Experimental Seminar 2

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 exclude

mH > 114.4 GeV/c2 @ 95% CL

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


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


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!


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


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 ~400 Hz

Output 70-90 Hz

Readout latency ~650 s


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)


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

Efficiency data/MC scale factor

SF = 0.91 0.06


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


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 1.5-1.7%• 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)


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


Untagged “Control Sample”


Tagged Background Summary

Use W+1-jet bin to fix W+HF bkgd(scale up by 20%)

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


Tagged Dijet Mass


WH Cross Section Limits


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

Missing ETb-jet

b-jet

y

x

2nd jet

Fake Missing ET

1st jet

180o

A di-jet QCD event:


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


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


ZH Cross Section Limits


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


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

W-

W+

e+

e-


H WW* Cross Section Limits


SM Higgs Limits Summary


Limits Scaled by SM Cross Sections


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


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


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

b

0 b

b

0

TeV4LHC working group


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)


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


MSSM Interpretation

• final state less sensitive to mixing effects than bbbb

• As bb cross section decreases, BR increases to compensate

D0 search in the bbbb final state


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)


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) m SVT beam

= 48m


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


Summary

• 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!

Backup Material


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


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


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

Documents

Higgs Searches at CDF