Flavour independent neutral Higgs boson

searches at LEP

Ivo van VulpenNIKHEF

On behalf of the LEP collaborations

EPS conference 2005

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 2

The LEP collider:

e+e- collider at CERN operated 1989-2000 (4 experiments: ALEPH, DELPHI, L3, OPAL)

s mZ (LEP1) and s = 130-209 GeV (LEP2)

Most results (95% CL limits) based on LEP2: Int. Lum. 2.6 fb-1

LEP and the LEP data-set

http://lephiggs.web.cern.ch/LEPHIGGS/www/Welcome.html

LEP combined results on Higgs searches:

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 3

SM Higgs boson

Weak boson fusion

In the SM we know everything about the Higgs boson

mh > 115.3 GeV (expected)

mh > 114.4 GeV (observed)

(except its mass)

B-tagging

udsc-jets

b-jets

Higgs-strahlungh bb (>80%)

qq (70%) (20%)l+l- (10%)

Production: hz(mh,√s) Decay: h(mh)

2 b-jets + a “Z”. Excluded SM mh:

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 4

Simplest SM extension: 2 complex Higgs Doublets 5 Higgs bosons (3 neutral)

Neutral Higgs boson production Higgs boson decay

Depends on model parameters

Neutral Higgs boson production at LEP

Models: 2HDM(I): One doublet for all fermions

2HDM(II): One up-type doublet One down-type doublet

mainly hbb

Associated productionHiggs strahlungM

SSM

hZ )(sin2 SM

hZ

SM)bb(hbbh

)(cos)(sin

2

2

hA )(cos2 SM

hZ

hbb decays suppressed (light quarks and gluons)

No sensitivity in standard analyses

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 5

What ifa neutral Higgs boson is produced, but

… does not couple (strongly) to b-quarks

… decays into light quarks or gluon pairs

… has a cross section X times smaller or larger than SM prediction

In general: Test predictions of any (new) model against LEP data

Experimentalist: Exclude neutral Higgs boson production, with (H hadrons) over mH range

YES NO Cancel

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 6

Neutral Higgs boson production at LEP

Details Higgs sectorSU(2) x U(1)

Consider Higgs boson decays into all quark pairs and gluons

Exclude and

Topology based (no b-tag information)

Flavour independent (hadronic) Higgs boson search:

Worst case scenario

2ffffhA

EWhAffffhA σ σ C

2)ffZ(h

EWhZ)ffZ(h σ σ C

)(mh2hZC )m,(m Ah

2hAC

Assumption: h small (w.r.t detector resolution)

e+e- hZ

e+e- hA

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 7

hZ production

qq/qql+l-: Z-like object + recoil mass

Gluon pairs versus light-quark pairs: higher selection efficiency worse di-jet mass resolution

Experimental issues:

4-jets: topology+ masses (3 pairings)

Weak boson fusion

Higgs-strahlungh qq/gg (100%)

qq (70%) (20%)l+l- (10%)

3 final states

SM Backgrounds:

qq(), W+W- , ZZ

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 8

hZ production: single experiment

DELPHI (all channels)

Topologies: qqqq, qq, qql+l-

Mh range: 4 < Mh <115 GeV

Mh (GeV)Mqq (GeV)

C2

Z(h

h

ad

ron

s)

Single channel (qq+-)

only 3% of all decays,but very clean

SM hZ cross section

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 9

hZ production: LEP combined Flavour Independent

Even without b-tag information, still a strong limit

Expected Observed (GeV/c2) (GeV/c2)

LEP results on hZ production:

SM 115.3 114.4

Flav.Ind. 113.0 112.9

Mh (GeV)

C

2Z

(h

had

ron

s)

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 10

hA production4-jet event at LEP

h

A

SM Backgrounds:

qq() ~ 8 pb W+W- ~ 8 pb ZZ ~ 0.5 pb

Analyses depend on h and A masses Topology (small mh or mA: 4 jets 3 jets)

Gluon pairs versus light-quark pairs: higher selection efficiency worse di-jet mass resolution

4-jet final state

Experimental issues:

Pairing ambiguities: 4 jets (3 pairings)

qq/gg (100%)

qq/gg (100%)

Associated production

OPAL: also consider hAA

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 11

Interpretation in 2HDM models (parameter scan): see talk by P. Akesson

Mh (GeV) Mh (GeV)

MA (

GeV

)

MA (

GeV

)

LEP Flavour Independent results on hA production

For C2 =1: mh+mA < 140 GeV

hAA taken into account(6 parton final state)

Exclude hA(hadronic) cross-sections relative to SM:

DELPHIhAhadrons

)m,(m Ah2hAC

Ivo van Vulpen (NIKHEF) EPS conference (Lissabon, July 2005) slide 12

Conclusions

Limits on hZ and hA production (h,Agluons/quarks)LEP combined results on hZ (No signal observed)Cross section exclusion as function of mh (mA)

Important experimental LEP legacy:

More model independent summary of LEP data

Important test for (future) model builders… SUSY, 2-3-4-5HDM, CP (non-)conserving, etc.

LHC cannot easily cover low mass regions(allows check for any LHC prediction)