the lep higgs saga - welcome to the infn roma home page · 2008. 5. 26. · the lep collider a good...

The LEP Higgs Saga

Pippa WellsCERN

2 August 2003

LEP Higgs Saga Pippa Wells 1

Search for the Higgs Boson at LEP

• Quick reminder

– The LEP machine and detectors

– Standard Model processes

• Constraints on the Higgs mass

• Higgs production and decay

• How to tell if candidates are significant?

• What happened in 2000...?


The LEP Collider

A good fill lasted around

• 10 h at LEP1 (√

s ≈ MZ)

• 3 h at LEP2

0

20

40

60

80

100

120

-25 -22.5 -20 -17.5 -15 -12.5 -10 -7.5 -5 -2.5 0Time (hours)

E (

GeV

) o

r L

(1031

cm-2

s-1)

A day at LEP (example from 1999)EnergyLuminosity

LEP Accelerator and four detectors operated 24h a day, from Spring to

Autumn, 1989 to 2000


LEP2 Machine Performance

• Impressive machine performance - increase beam energy and

maintain high luminosity.

• Superconducting RF acceleration system pushed beyond design

gradient.

• Max. integrated luminosity per year: 65pb−1 (LEP1), 254pb−1 (LEP2)16

1

172

183

189

192

196

200

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205

207

209

0

50

100

150

Ecm (GeV)

Inte

gra

ted

Lu

mi (

pb

-1)

1996

1997

1998

1999

2000


LEP: e+e− collisions

ALEPH, DELPHI, L3, OPAL

each recorded:

• 4.5M Z at LEP1

including off-peak data.

• 10k W-pair at LEP2

LEP1 dominated by Z

resonance

Several processes are

important at LEP2

Any sign of e+e− → HZ?

Pick out H and Z decay

products against big

backgrounds.

√s⎯

(GeV)

Cro

ss s

ectio

n (p

b)

e+e−→e+e–qq−

e+e−→qq−(γ)

e+e−→µ+µ−(γ)

e+e−→W+W−

e+e−→ZZ

e+e−→γγe+e−→W+W−γ

e+e−→HZmH = 115 GeV

L3preliminary

10-1

1

10

10 2

10 3

10 4

80 100 120 140 160 180 200LEP Higgs Saga Pippa Wells 6

Cut-away view of OPAL

θ ϕ

x

y

z

Hadron calorimetersand return yoke

Electromagneticcalorimeters Muon

detectors

Jetchamber

Vertexchamber

Microvertexdetector

Z chambers

Solenoid andpressure vessel

Time of flightdetector

Presampler

Silicon tungstenluminometer

ForwarddetectorOverall size 12×12×12 m


Z to hadrons event in ALEPH

Made on 9-Sep-1993 11:43:03 by D

RE

VE

RM

AN

N w

ith DA

LI_D

1.DALI

Run=9063 Evt=7848 ALEPH

• This example has 3 jets e+e− → qqg

• Jets include many tracks (curved in B=1.5 T field) and energy clustersLEP Higgs Saga Pippa Wells 8

e+e− → Z → e+e− event in OPAL

• Lepton pair events have

low multiplicity

• Electrons are identified

by a track in the central

detector, and a large

energy deposit in the

electromagnetic

calorimeter, E /p = 1.

Run : even t 4093 : 1150 Da t e 930527 T i me 20751

Ebeam 45 . 658 Ev i s 94 . 4 Em i s s - 3 . 1 V t x ( - 0 . 05 , 0 . 08 , 0 . 36 )

Bz=4 . 350 Th r us t =0 . 9979 Ap l an=0 . 0000 Ob l a t =0 . 0039 Sphe r =0 . 0001

C t r k ( N= 2 Sump= 92 . 4 ) Eca l ( N= 9 SumE= 90 . 5 ) Hca l ( N= 0 SumE= 0 . 0 )

Muon ( N= 0 ) Sec V t x ( N= 0 ) Fde t ( N= 1 SumE= 0 . 0 )

Y

XZ

200 . cm.

Cen t r e o f s c r een i s ( 0 . 0000 , 0 . 0000 , 0 . 0000 )

50 GeV2010 5


e+e− → Z → µ+µ− event in L3

• Muons penetrate entire

detector. Little energy in

the calorimeters.

• L3 emphasizes lepton

and photon id with a

precise BGO crystal

ECAL, and large muon

spectrometer.

• Tracking volume

relatively small (r=1m)

• ALL detectors inside

r=6m solenoid, B=0.5T.

HCAL

BGO

Tracking

MuonChambers


e+e− → Z → τ+τ− event in DELPHI

• Tau lepton decays

dominated by 1 and 3

charged tracks, with or

without neutrals, missing

neutrino(s), back-to-back

very narrow “jets”.

• DELPHI has extra

particle ID detectors,

RICH.

DELPHI Interactive AnalysisRun: 23438Evt: 581

Beam: 45.6 GeV

Proc: 8-Mar-1992

DAS : 18-Jun-199103:22:19

Scan: 29-Apr-1992

TD TE TS TK TV ST PA

Act

Deact

44

( 44)

0

( 0)

45

( 48)

0

( 4)

0

( 0)

0

( 0)

8

( 9)

0

( 6)

0

( 9)

0

( 5)

0

( 0)

0

( 0)

0

( 0)

0

( 0)

X

Y

Z

Barrel RICH


Tagging heavy quarks - leptons

Heavy hadrons have weak decays, sometimes final state leptons, long

lifetimes, characteristic masses and event shapes.

b and c hadrons have ≈ 20% of

decays to leptons with high p and

for b hadrons with high pT

Electrons: ionisation in tracking

chambers dE /dx , E /p, shower

shape

Muons: Match between central track

and muon chambers

Leptons also give charge of the

decaying hadron

Example shows e+e− → Z → bbevent in L3

L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3

µ+p = 17 GeVp = 1.7 GeV

⊥

e+ E = 14 GeVp = 2.7 GeV

⊥


Tagging heavy quarks - lifetimes

Heavy hadrons have long

lifetime and large boost

d0 and L are signed

quantities. A badly

measured track may

intercept the “wrong-side” of

the beam spot.

Rely on silicon microvertex

detectors for resolution.

Use several variables

together.

Impact parameter, d0

Decay length, L

Primary vertex

Secondary vertex

B

B

Fragmentationtrack


W pair events at LEP2

WW → qqqq (four jets) WW → qq�ν (two jets, lepton and

missing energy)


WW and ZZ production at LEP23 diagrams “CC03” 2 diagrams “NC02”

e-

e+

νe

f

ff

f

W

W

e-

e+

γ

f

ff

f

W

W

e-

e+

Z

f

ff

f

W

W

e-

e+

e

f

ff

f

Z

Z

e-

e+

e

f

ff

f

Z

Z

√s (GeV)

σ WW

(pb

)

YFSWW and RacoonWW

LEP PRELIMINARY

11/07/2003

0

10

20

160 180 200

16

17

18

190 195 200 205 0

0.5

1

180 190 200

√s (GeV)

σ ZZ (

pb)

ZZTO and YFSZZLEP PRELIMINARY

11/07/2003

ZZ production is an irreducible background to ZH production.LEP Higgs Saga Pippa Wells 15

Global electroweak fits and Higgs Mass

Fit to data from LEP,

SLD, Tevatron...

Electroweak variables

depend on m2t and

log mH through radiative

corrections

Consistency between

predicted top and W

mass (Z pole) and direct

measurements

Preference for low Higgs

mass.80.2

80.3

80.4

80.5

80.6

130 150 170 190 210

mH [GeV]114 300 1000

mt [GeV]

mW

[G

eV]

Preliminary

68% CL

∆α

LEP1, SLD Data

LEP2, pp− Data


Constraints on Higgs mass

Electroweak fits

mH < 237 GeV (95% CL)

Theory: self consistency

of SM to GUT scale

≈ 1016 GeV

130 < mH < 190 GeV.

mH higher - theory

non-perturbative,

mH lower - vacuum

unstable. 0

1

2

3

4

5

6

10020 400

mH [GeV]

∆χ2

Excluded Preliminary

∆αhad =∆α(5)

0.02761±0.00036

0.02747±0.00012

incl. low Q2 data

Theory uncertainty


Higgs production cross-section

e-

e+

Zf

f

H

Z*

10-3

10-2

10-1

1

10

10 2

10 3

10 4

60 80 100 120 140 160 180 200

√s– (GeV)

σ (p

b)

10-3

10-2

10-1

1

10

10 2

10 3

10 4

60 80 100 120 140 160 180 200

e+e- → hadrons

e+e- → HZ

ZZ

WW

60

mh = 70 GeV 90 110 115

e-

e+

Z*

f

f

H

Z

With a luminosity of about 100pb−1 and reasonable detection efficiency,

sensitive to a cross section of O(0.1) pb.

Need LEP2 to produce mH >∼ 65 GeV. Reach mH <∼√

s − MZ

Must take into account many background processes


Higgs decay branching ratios

“Higgs couples to mass”

τ+τ-

cc_

gg

W*W*Z*Z*

mH

[GeV]

10-2

10-1

1

60 70 80 90 100 110 120

bb_

Hig

gs B

ranc

hing

Rat

io

BR(%) Higgs Z boson115 GeV

qq 70

bb 74 15cc 4 12gg 6 0�+�− 10τ+τ− 7 3νν 20W∗W∗ 8Z∗Z∗ 1


HZ decays

q

q

b

b

−

−ν

b

b

ν

−

−

e+ or µ+

b

b

e− or µ−

−

τ+

b

b

τ−

−

Four jets, 60%

H → bb , Z → qq

Missing energy, 18%

H → bb , Z → νν

Leptonic, 6%

H → bb , Z → �+�−

Tau channels, 9%

H → bb(τ+τ−) , Z → τ+τ−(qq)LEP Higgs Saga Pippa Wells 20

Got some candidates? Are they significant?

bi expected number of background

si(mH) expected signal, function of “test

mass” mH

Count these in bins of reconstructed Higgs

mass mrecH and a global discriminating vari-

able G

si (MH),bi

MHrec

G

Discriminant G takes into account b-tagging, τ-id, kinematic variables that

distinguish signal and background.

“How Higgs-like is the event?”

Expectations account for luminosity, Ecm, resolution, efficiency...

Compare likelihoods of “s + b” and “b only”.


Likelihood comparison

Likelihoods of s + b and b from Poisson probabilities of observing

ni data events in each bin:

Q(mH) =Ls+b

Lb=

∏i

(si + bi)ni e−(si +bi )/ni

bnii e−bi /ni

More convenient to work with log likelihood:

−2 ln Q(mH) = 2stot − 2∑

i

ni ln(

1 +si (mH)

bi

)

Sum is over all bins, channels (four jet, missing energy...), and experiments.


Likelihood ratio, −2 ln Q−2 ln Q vs. test mass mH.

Find expected (median)

curves and statistical spread

from a set of ficticious MC

samples of the same

luminosity/Ecm mix as the

data.

Compare with the

observation - here’s an

example of what you might

hope to see in the real data!

Taking slices at different test

masses - separation of b and

s+b decreases as mass

increases.

-10

-7.5

-5

-2.5

0

2.5

5

7.5

10

105 110 115

"b"

"s+b"

"Observed"

MH (GeV)

-2ln

(Q)

1σ

2σ

} Expected


Confidence levels

This is a slice through the previous

plot at one test mass, showing the

distributions for MC experiments,

and the data observation.

Separation of b and s+b curves

indicates sensitivity of analysis.

For EACH test mass, mH, define

confidence levels

b1−CLCL

−20 −15

s+b

−10

Pro

b. D

ensi

ty

−5 0 5 10 15

−2 ln Q

"s+b"

"b"

observed

1 − CLb Measure of inconsistency with “b”

CLs+b Measure of inconsistency with “s + b”

CLs = CLs+b/CLb Lower bound on Higgs mass


5 September 2000 LEPC

LEPC - The CERN Committee in charge of the LEP physics programme

One of a planned series of presentations of results from the four

experiments during 2000 in case something new came up during the last

year of LEP running at higher energy than ever before...

150pb−1 per experiment with Ecm > 200 GeV

of which 75pb−1 per experiment with Ecm > 206 GeV

Here are some slides shown in that meeting...


��

��

��

-10

-7.5

-5

-2.5

0

2.5

5

7.5

10

100 105 110 115

Background Only

Background+Signal

Observed

MH (GeV)

-2ln

(Q)

L3

-10

-7.5

-5

-2.5

0

2.5

5

7.5

10

100 105 110 115

Background Only

Background+Signal

Observed

MH (GeV)

-2ln

(Q)

ALEPH

-10

-7.5

-5

-2.5

0

2.5

5

7.5

10

100 105 110 115

Background Only

Background+Signal

Observed

MH (GeV)

-2ln

(Q)

DELPHI

-10

-7.5

-5

-2.5

0

2.5

5

7.5

10

100 105 110 115

Background Only

Background+Signal

Observed

MH (GeV)

-2ln

(Q)

OPAL

��


��

��

��

-10

-7.5

-5

-2.5

0

2.5

5

7.5

10

105 110 115

Background Only

Background+Signal

Observed

MH (GeV)

-2ln

(Q)

ADLO

10-5

10-4

10-3

10-2

10-1

105 110 115MH (GeV)

1-C

Lb

3σ

ADLO

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��


ALEPH Events - four jets with b tags

Zoom right inside

the beam pipe:

Made on 4-O

ct-2000 09:39:52 by DR

EV

ER

MA

NN

with D

AL

I_F1.Filenam

e: DC

054698_004881_001004_0937.PS_H1_X

Y_X

Y_T

U

0 −0.3cm 0.2cmX’

0 −1cm

1cm

Y’

µ

Made on 15-Sep-2000 13:12:13 by D

RE

VE

RM

AN

N w

ith DA

LI_F1.

Filename: D

C054698_004881_000915_1311.PS_H

1_XY

_WW

DALI

Run=54698 Evt=4881 ALEPH

End of detectorEnd of tracks

ECM = 206.7


5 September Decision

Approve 1 month extension of LEP running from scheduled stop on

1 October to 2 November 2000.

Hope that this will allow time to double the luminosity above 206 GeV (add

75pb−1 per experiment)

(Big end-of-LEP celebration on 11 October had to go ahead!)

Slides from the 3 November meeting...


�

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Data Sets

• REFERENCE data set ... where it all begun ...

data set combined for the Sept 5 LEP seminar ...

Revisited ... changes within the experiments⇒ Recalibration of data

⇒ Revision of procedures (corrections)

⇒ Improvements ... better sensitivity

• DELTA set ... data collected since “REF”

(... until the “cutoff date” ... Oct 18-25)

• TOTAL = REF + DELTA

Integrated luminosities ... A+D+L+O = “ADLO”(contributions from single experiments ... within ±5%)

Not included ... latest data ... ≈30 pb−1

L (pb−1) REF DELTA TOTAL

Ecm > 200 GeV 596.6 213.7 810.3

Ecm > 206 GeV 303.5 184.5 488.0

P. Igo-Kemenes - LEP Seminar - Nov. 3, 2000 Page 5


�

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�

−2 ln(Q) ... REF, DELTA, TOTAL

-10

-5

0

5

10

15

20

25

100 102 104 106 108 110 112 114 116 118 120

mH(GeV/c2)

-2 ln

(Q)

Observed

Expected background

Expected signal + background

LEP REF

-10

-5

0

5

10

15

20

25

100 102 104 106 108 110 112 114 116 118 120

mH(GeV/c2)

-2 ln

(Q)

Observed

Expected background


LEP DELTA

-10

-5

0

5

10

15

20

25

100 102 104 106 108 110 112 114 116 118 120

mH(GeV/c2)

-2 ln

(Q)

Observed

Expected background


LEP TOTAL⇐Minimum @mH ≈ 115 GeV

Agreement with SM Higgs cross-sect. for

mH = 115.0+1.3−0.9 GeV

P. Igo-Kemenes - LEP Seminar - Nov. 3, 2000 Page 8LEP Higgs Saga Pippa Wells 31

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Distributions of Reconstructed Mass

Sequence: “Loose”, “Medium” and “Tight” selection (∗)

0

10

20

30

40

Even

ts /

3 G

eV/c

2

√s– = 200-210 GeV

LEP S/B=0.3backgroundhZ Signal(mh=115 GeV)

allcnd= 200bgd= 201.75sgl= 10.26

> 109 GeV2720.416.11

0

5

10

15

Even

ts /

3 G

eV/c

2

√s– = 200-210 GeV


allcnd= 59bgd= 55.26sgl= 4.66

> 109 GeV63.562.94

0

2

4

6

8

0 20 40 60 80 100 120Reconstructed Mass mH [GeV/c2]

Even

ts /

3 G

eV/c

2

√s– = 200-210 GeV


allcnd= 24bgd= 22.79sgl= 2.74

> 109 GeV41.131.78

(∗) Special selection ... not biasing the mass distribution



�

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Results by Experiments

for TOTAL data set, @ mH = 115 GeV

-20 -10 0 -20 -10 0

-20 -10 0 -20 -10 0

ALEPH DELPHI

L3

-2 ln Q

Prob

. Den

sity

OPAL

ObservedBackgroundSignal



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SUMMARY

REFERENCE ⇒ TOTAL

2.2σ ⇒ 2.9σ

One expt “s+b”-like ⇒ Three expt “s+b”-like

4-jet “s+b”-like ⇒ 4-jet, E-miss “s+b”-like

Perfect compatibility with SM Higgs cross section

for

mH = 115.0+1.3−0.9 GeV

! ALL THIS IS VERY EXCITING !

Current bound on Higgs boson mass

mH > 113.5 GeV @95% c.l.

for 115.3 GeV expected



Run LEP in 2001?

Evidence was consistent with a hint of Higgs production at 115 GeV

• 3/4 experiments somewhat more “s+b” than “b”

• Two channels more “s+b” than “b”

• Spread of s/b and mrecH for significant candidates consistent with Higgs

BUT

• Evidence still weak (< 3σ - a “discovery” is usually considered to be

5σ . Fluctuations happen.)

• No guarantee that extra running would confirm a discovery

• Big impact on LHC schedule and resources (civil engineering directly

delayed by LEP extension)

• LHC could see this Higgs boson, and if it’s a light SUSY Higgs could

simultaneously investigate other SUSY particles...

LEP SHUTDOWN DEFINITIVELY AT THE END OF 2000


The final word on the SM Higgs (April 2003)

Full dataset, calibration updates, some improvements to analyses.

-30

-20

-10

0

10

20

30

40

50

106 108 110 112 114 116 118 120

mH(GeV/c2)

-2 ln

(Q)

ObservedExpected for backgroundExpected for signal plus background

LEP


The final word on the SM Higgs (April 2003)

0

0.02

0.04

0.06

0.08

0.1

0.12

-15 -10 -5 0 5 10 15

-2 ln(Q)

Pro

babi

lity

dens

ity

ObservedExpected for backgroundExpected for signalplus background

LEPmH = 115 GeV/c2

(a)

Higgs boson excluded up to 114.4 GeV at 95% CL


The End

The LEP experiements have published more than 1000 papers on many

topics.

High precision tests of the Standard Model have been made, and are

sensitive to radiative corrections.

The electroweak data prefer a light Higgs boson.

The SM Higgs boson search gives a limit at 114.4 GeV, with an

inconclusive hint of a signal at around 115 GeV.

Pass the baton to the Tevatron (Run II - CDF, D0 in progress) and the LHC

(ATLAS, CMS first data in 2007).


the lep higgs saga - welcome to the infn roma home page · 2008. 5. 26. · the lep collider a good...

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