low-mass higgs searches at the tevatron

21 Jul. 2006; p.1ICHEP 2006: Tev Low-mass Higgs Ben Kilminster

Low-mass Higgs Searches at the

Tevatron

Ben Kilminster

Ohio State University/CDF

for CDF/D0

4 New results in H->bb channels :ZH-> bb

D0 - 0.3 fb-1

CDF - Update from 0.3 to 1 fb-1

WH-> lbbD0 - 0.4 fb-1

CDF - Update from 0.8 to 1 fb-1

ZH->llbbD0 - New Channel ! 0.4 fb-1

CDF - New Channel ! 1 fb-1

L=1 fb-1

L=1 fb-1

L=1 fb-1


Mass ~ Inertia : how hard it is to move free quark or lepton Mass caused by transition between left-handed fermion to

right-handed particle via Higgs field, H0

For instance, top quark mass, Mt :

Standard Model mass generation via Higgs

Mt = tR <H0> tL

tR tL tR

<H0> <H0>


What we know about Higgs

Required Higgs boson not yet discovered !!

“Standard Model” (SM)• Simplest Higgs mechanism

possible• Higgs is 1 particle

H spin 0 electrically neutral interacts with all SM

particles couples more strongly with

higher mass particles

SM not wrong yet !

Expected Higgs Mass

LEP Direct : MH > 114 GeV @ 95%

New CDF/D0 top mass (174.1 2.1 GeV) & new LEP W mass (80.392 0.029 GeV)

MH = 85 +39 -28 GeV MH < 166 GeV @ 95 % CLLEP EWWG

Low mass Higgs Favored !!


What we know about Higgs

Production (pp @ 1.96 TeV c.o.m.)

Low mass region: MH < 135 GeVH bb dominatesWH & ZH - easier to identify than gg -> H

Decay by mass [GeV]

Decay

Excluded

68% CL 95% CL Most

likelyMH


Fermilab’s Tevatron World’s highest-energy particle

collisions ~4 miles circumference protons-

antiprotons 2 multi-purpose detectors: D and CDF

Run I (1992-1996) s = 1.8 TeV Integrated luminosity 120 pb-1

Run II (2001-present) s = 1.96 TeV Integrated luminosity by July ‘06:

Delivered > 1.6 fb-1

Higgs analyses use up to 1 fb-1

Design goal of 8 fb-1 by 2008

1 fb-1 delivered May 2005

Good slope after

shutdown!

July, 2006


Review of low mass Higgs channels

ZH l+l- bb

2 b jets ~ 1/2 MH each2 leptons ~ 50 GeV eachZ mass constraintCleanest signal

WH lbb

2 b jets ~ 1/2 MH each1 lepton ~ 50 GeV eachMissing ET ~ 50 GeVHighest production X-sec

ZH bb

2 b jets ~ 1/2 MH each0 leptonsMissing ET ~ 100 GeVLargest expected signal

Negative tag (wrong side)

B-Tagging Techniques

Positive tag (right side) Interaction pointprimary vertex

Lxy > 0

All channels have 2 jets originating from b quarks Require one or both to be “b-tagged”

2nd vertexLxy < 0

2nd vertex Interaction point

Charm-jets and mistagged jets can be controlled by strictness of cut on LXY / XY

“Mistags” of tagged light-quark jets can be understood from “negative tags”

Algorithm exploits long b lifetime and large mass to look for displaced vertices or tracks with impact parameter


B-Tagging Techniques at CDF

Can improve purity with a Neural Network trained to discriminate b from c and light jets

B-Tag Efficiency Light quark mistag rate(Positive Tag) (Negative Tag)


Identifying bb resonances : D0

Z-> bb H->bb benchmark Can be used to determine b-jet energy scale New D0 analysis finds evidence for Z->bb in dijet data

Background derived from data 1168 Events in peak (300 pb-1) MZ = 81.0 2.2 GeV measured

83 2 GeV expected (from MC)


ZH -> bb

ZH bb

2 b jets ~ 50 GeV each0 leptonsMissing ET ~ 90 GeVMost expected signal

2nd jet

Fake Missing ET

1st jet

180o

Di-jet QCD

Most difficult background:

Tev’s most sensitive Channel

ZH MET+bb at CDF

MET in QCD control region:• no leptons• met close to second jet

Mjj in EWK control region:• one lepton• met away from second jet

Mjj Signal region:• no leptons• met away from second jet

Improvements : (S/√B)2=6.3 gain in Lum.• Includes WH -> lbb ( lepton not detected)• Improved EWK lepton veto• Dijet mass fit separately 1-tag, 2-tags

ZH / SM = 14 for MH : 115 GeV

#Leptons

(MET, J2)

L=1 fb-1


ZH-> bb D0

Instrumental background (from energy mismeasurement) in signal region understood by parameterization of Met

Result: Dijet mass fit in 1 b-tag & 2 b-tags L = 261 pb-1 ZH < 3.4 pb for MH : 115 GeV


WH -> l bb

WH lbb

2 b jets ~ 50 GeV each

1 lepton ~ 40 GeV eachMissing ET ~ 40 GeVWH Highest production X-sec

Most difficult background:W+bb jet production


WH->l bb CDF

Variety of b-jet identification scenarios optimized to find the best a priori limit• BEST : Separate 1-tag + NN-tag and 2-tag scenario

Result: Dijet mass fit WH < 3.4 pb for MH : 115 GeV

L=1 fb-1


WH->lbb D0

Result: Dijet mass fit in 1 b-tag & 2 b-tags L = 378 pb-1

ZH < 2.4 pb for MH : 115 GeV


ZH -> l+l- bb

ZH l+l- bb

2 b jets ~ 50 GeV each

2 leptons ~ 40 GeV eachZ mass constraintCleanest signal


ZH->llbb D0 New analysis with 389 pb-1 (Z->ee), 320 pb-1 (Z->+-)

Dijet mass before b-tagging Dijet mass after 2 b-tags

Process before b-tag after 2 tagsZ+bb 17 3Z+jj 937 5ttbar 11 4ZZ+ZW 26 0.6QCD 44 0.6

ZHM=115

= 0.1 evts

Total BKG : 13 evts Result: Dijet mass fit ZH < 7.9 pb (Z->ee) ZH < 11 pb (Z-> ) for MH = 115 GeV


2D Neural Network trained to separate Signal from Background Z+jets vs. ZH “x” axis (85% BKG) ZH vs. ttbar “y” -axis (8% BKG)

Optimized design with 9 inputs

ZH->llbb CDF Method

(1,0)

(1,1)

(0,0)

ZH

TT

Z+jets

Training on : TT,ZH,Z+jetsAllow other shapes to fall in place:

Fakes, ZZ, ZW

ZZ, ZW

Fakes

2D Neural Net Discriminant


ZH->llbb CDF results

(1,0)

(1,1)

(0,0)ZH

TT

Z+j ZZ, ZW

Fakes

ZH v

s TTB

AR a

xis

Results in Data : ee, combined

Expected : 103 +- 17 Observed : 104 events

Result: Entire 2D distribution fit Brand new result : 1 fb-1

ZH < 2.2 pb @ 95% CLfor MH : 115 GeV L=1 fb-1

Note: ZH * 5 !


Summary of Observed limits

Analysis CDF limit @ MH = 115 GeV(factor above SM)

D0 limit (factor above SM)

ZH -> bb Includes WH (miss lep)(14)

3.4 pb(41)

WH -> lbb 3.4 pb(23)

2.4 pb(16)

ZH -> llbb 2.2 pb(27)

6.1 pb(75)

Summary CDF/D0 fully exploring all Low Mass Higgs

ZH -> l+ l- bb channel added by both CDF and D0 CDF has updated WH-> l bb, ZH -> llbb with 1 fb-1

Experimental techniques providing factors of equivalent luminosity

Accelerator Division, CDF, and D0 working together against the clock !

4

8

fb-10

Now

Projected Luminosity

2009 400

Limits will improve with luminosity and smarts !

4 - 8 fb-1 can find us a light Higgs


BACKUPS


Higgs: ZHllbb

SummaryCDF & D0 Preliminary


SM / MSSM CompatibilityIf MSSM is theory, is it worth looking for SM Higgs ? For MA > 200 GeV

light MSSM Higgs h behaves like SM HiggsWh and Zh couplings same as WH and ZHH branching ratios same as h

SM searches valid If only one Higgs accessible at Tevatron/LHC, LC may be required

to distinguish SM from MSSM (Carena, Haber, Logan, Mrenna Phys.Rev.D65 055005, 2002)

MA < 200 GeV For large tan (> 3), SM-like Higgs is suppressed

Discovery potential mainly in MSSM Small tan , SM searches valid


CDF sees Zbb decays in Run 2

Double b-tagged events with no extra jets and a back-to-back topology are the signal-enriched sample: Et

3<10 GeV, 12>3

Among 85,784 selected events CDF finds 3400±500 Zbb decays

- signal size ok- resolution as expected- jet energy scale ok!

This is a proof that we are in business with small S/N jet resonances!

CDF expects to stringently constrain the b-jet energy scale with this dataset

low-mass higgs searches at the tevatron

Documents