1

Siena 7Siena 7thth Topical Seminar: Topical Seminar: The Higgs in two jet modesThe Higgs in two jet modes

W. J. Murray

RALStandard Model

results

2

LEP AcceleratorLEP Accelerator Very clean environment Excellent performance!

year ’95 ’96 ’97 ’98 ’99 2000

s 130-136 161-172 183 189 192 196 200 202 204 205 207 208

Lum/expt (pb–1) 3 3 10 10 60 170 30 80 80 40 10 70 130 10

2.5 fb-1 @ Ecm>189 GeV

Lum. (pb–1) ALEPH DELPHI L3 OPAL LEP

s > 189 GeV/c2 629 610 627 599 2465

s > 206 Ge/c2 130 142 139 130 542

Thanks to Steve Myers and friends

3

Higgs Searches at LEP 1Higgs Searches at LEP 1

0.0 0.0 m mHH 65 GeV/c 65 GeV/c22

Excluded at 95% C.L.Excluded at 95% C.L.

EventsEventsexpectedexpectedat LEP1at LEP1

•These were typically two-jet modes

•Many modes:

Stable,,ee,,,,bb

•Clean Z decays (ll, ) used

•Prior to LEP only some patchy constraints

The mass range to 0 now excluded, no holes.

4

Closing in on the Higgs!Closing in on the Higgs!

0100200300400500600700800900

1000

Excluded by EWfitsAllowed

Excluded byDirect Search

EW fits assume a Higgs

Search looks for one

After A. Wagner, ICHEP 2000 Frequentist

Bayesian

5

Total LEP Higgs Total LEP Higgs Predicted LEP events,

4 experiments

17 @

115Ge

V

6

LEP Higgs production: HZLEP Higgs production: HZ

Make a Higgs and a Z together

So need Energy greater than Higgs mass plus Z mass

‘Higgstrahlung’ process

7

tau

b quarks

gluons

WW

other

Higgs and Z decay channelsHiggs and Z decay channels

Higgs decay modes

taus

quarks

neutrinos

electrons

muons

Z decay modes

B quarksAny quark

WW

Assuming: Both Z & H are made!

Marumi described green x green – I get the odds and ends!

8

Fusion: WW and ZZFusion: WW and ZZ

Any quark

WW

•These diagrams allow non-resonant Higgs production

•WW always much larger than ZZ

•Has been suggested as way to beat kinematic threshold

(Ebeam-Mz)

H

e+

e-

W

9

HHiiggs ggs cross-section cross-section

Any quark

WW

•Cross-section ~ few fb at 115:

Not accessible with ½ fb-1 per experiment

s=192GeVfb

MH

10

LEP Higgs channelsLEP Higgs channelsDecaymode

Br

bb 73.6% 7.2%Gluons 6.6%

ww 8.1%

Higgs decay

Z decay Fraction

bb qq 51.5%

bb 14.7% Any ll 6.7%

bb 2.5% qq 5.0%

Total 80.9%

Higgs decays, 115GeV

Search channels

This talk

11

How is the Higgs search done?How is the Higgs search done?•Classify events into one category, pre-select

•Assign measured particles to four initial fermions: quark, e/

•tag b quark jets, vtx+leptons

•Decide which fermions are from Z, which from H

Not obvious in cases…

•Require compatibility with Z mass for Z pair

•Fit for MH, assuming E, p conserved, Mz at least for H channel

12

Higgs backgroundsHiggs backgroundsDecay Mode Higgs Z

Background Comment

bb ZZ, Z(double ISR), Z

Large missing E, but smaller missing ET

Any ll ZZ, anything Low rate, good mass resolution

bb

qq

ZZ, WW, Z/ Hard..included for SUSY

13

Channel comparison at 115GeVChannel comparison at 115GeVDecay Mode

Signal Events with s/b:

Higgs Z >0.3 > 0.5 >1.0 >3.0

bb 1.7 1.1 0.4 0.06

Any ll 1.1 0.9 0.7 0.24

bb

qq

0.4 0.3 0.2 0.10

Two jets 3.2 2.3 1.3 0.39

bb qq 4.1 3.0 1.6 0.36

14

Channel sensitivity (3Channel sensitivity (3 evidence) evidence)

Probability of 3 assuming

MH

2 jets and 4 jets comparable

Need to combine all

45%

20%

15

B-tagging – crucial stepB-tagging – crucial step

FACTOR >500 IN WW REJECTION REACHED

LOG SCALE

bcuds

•Tag of b’s reduces many backgrounds

•Semi-leptonic W’s dangerous

•Mis-tag is well controlled

16

ZZ cross-checksZZ cross-checks• B-tag important for

ZZ selection

• Proof that this very similar channel can be measured

• A major background is under control

17

Significant high mass eventsSignificant high mass events

0.400.5112.6quarks205.4OPAL10

0.400.5114.0quarks206.5ALEPH9

0.410.5115.4208.1ALEPH8

0.470.6118.1ee205.0ALEPH7

0.490.6114.3quarks206.7DELPH6

0.530.7110.7quarks206.6OPAL5

0.530.7115.0206.4L34

0.640.9110.0quarks206.5ALEPH3

1.212.4112.9quarks206.7ALEPH2

1.734.6114.3quarks206.7ALEPH1

ws/bMZ decayEnergyEXP

18

Top three candidates:Top three candidates:

L3 H candidate, s/b=0.7– 114GeV Higgs mass– But very collinear – too much?

ALEPH Hee candidate, s/b=0.6– 118GeV mass– Evidence for brehmstrahlung…Mass 99?

ALEPH H candidate, s/b=0.5– 115GeV mass– Kinematic fit poor

All well b-tagged

All have problems not reflected in their

s/b estimates

19

L3 HL3 H candidate: candidate:Two clear b-quarks

Lots of energy missing

Could be HZ, with Z decaying to invisible neutrinos

Mass is 114.5GeV

Or maybe it is just a pair of quarks.

Importance dropped in publication

20

L3 HL3 H characteristics characteristics

Mass and neural network output both

at signal peak

Used for discriminato

r

21

Four fermion backgrounds

acollinear

Two fermion collinear

Total well modelled

Collinearity of HCollinearity of H

We cannot add criteria in the light

of the data.

Otherwise classical statistical

analysis is impossible.

L3 candidate

22

How is significance assessed?How is significance assessed?•Maximum likelihood fit to observed distribution

Done in 1D or 2D (e.g. Mass,Neural-Net)

•Each bin needs signal and background estimates, from simulation, dependent upon Ecms, channel etc.

i

ii

i b

bsSL ln

•This is a weighted sum of events.

•L is compared with distributions expected for background and signal to quantify probabilities

`Qi’

23

•Positive –

background like

•Negative –

signal like

•Height gives power

•Green – 1

•Yellow – 2

Log-likelihood for two jetsLog-likelihood for two jets

Most powerful

24

Log-likelihood for two jet channelsLog-likelihood for two jet channels

Good agreement

Tiny excess at 117

25

No single leptonic channel has much discrimination

Results in each compatible with either hypothesis.

PDF’s for channelsPDF’s for channels

26

Combined leptonic PDFCombined leptonic PDF

Disfavours signal

Excludes nothing

27

Comparison with Hqq PDFComparison with Hqq PDF

Rather similar distributions (sensitivity) – results somewhat different

28

Combined two-jet limitCombined two-jet limit

MH>114.2GeV/c2

Exactly what would be expected

29

CLCLbb for two jets combined for two jets combined

Good agreement

with background expectation

30

Marumi showed you this allready

Significance reduced to

2.1

Hqq alone 1-CLHqq alone 1-CLbbLEP 1-CLLEP 1-CLbb

31

Conclusions from Two Jet search:Conclusions from Two Jet search:

MMHH > 114.2GeV/c > 114.2GeV/c22, nothing unusual seen, nothing unusual seen

Statistical analysis has its limitationsStatistical analysis has its limitations

Just as powerful as four jet searchJust as powerful as four jet search

Results should only be used combined with four jets:Results should only be used combined with four jets:

MMHH>114.1GeV/c>114.1GeV/c22

32

My own conclusions:My own conclusions:

• Precision electroweak measurements Precision electroweak measurements DEMANDDEMAND a a Higgs:Higgs:

•MMWW agrees with Higgs predictions to 1 per mille, agrees with Higgs predictions to 1 per mille, •MMtoptop agreement to 10% agreement to 10%mmHH = 88 GeV/c = 88 GeV/c22

- 35- 35

+ 53+ 53

mmHH = 115.6 GeV/c = 115.6 GeV/c22- 0.8- 0.8

+ 0.8+ 0.8

Just a statistical fluke?Just a statistical fluke?

In about 4 years, In about 4 years, Tevatron+LHC should decideTevatron+LHC should decide

More Precision Measurements with

Lepton Colliders should follow after 2012We need to build

TESLA/NLC

Lancon will

explain this

• Direct Searches (~2.1Direct Searches (~2.1 effect) effect)

33

Higgstrahlung rather collinear

Total much less so

Acollinearity of HAcollinearity of HHiggstrahlung

WW fusion

total

L3 candidate