motivation indirect searches

Higgs Searches at LEP2

E. KneringerUniversity of Innsbruck / Austria

Collaboration

LAKE LOUISE WINTER INSTITUTEElectroweak Physics14 - 20 February 1999

E. KNERINGER LLWI - 1999 2

Motivationindirect searches

results from

direct searches with

Outline The Standard Model Higgs

Boson SUSY Higgses

The Neutral Higgs Bosons of the MSSM

Charged Higgses in General Two Doublet Models

Invisible Higgs

LEP EWWG summer ‘98


The Standard Model Higgs Boson

Situation at LEP2

Production and decay

Decaymode

BRmH=90 GeV/c2

bb 84 % 8 %cc 4 %gg 4 %


Search in Four CharacteristicEvent Topologies

missing energy 4 jets

general requirements: compatibility with mZ

b-jets

general requirements: compatibility with mZ

b-jets

Z

2 isolatedhigh energeticleptons

l = e,

Z

candidates


Background Processes

tools to distinguish signal from background processes

tagging of b-quark jets lepton identification (e, ) kinematic mass fits

bkg (pb)189 GeV

qq() ~100

WW ~16

ZZ ~1


Analyses extensively using Monte Carlo simulations

for neural net training for setting cuts

optimizing the analyses for the individual channels to get the best combined analysis

highest efficiency with lowest background best Higgs mass exclusion limit if there is no hint for a signal

Selection N95exp Nbkg

exp NZZexp Nobs

bbqq 39.3 % 7.7 18.5 10.1 20 bb 24.3 % 1.6 3.7 3.1 7 bbll 72.9 % 1.5 13.2 11.8 13 bb 20.1 % 0.2 0.8 0.6 2 qq 18.9 % 0.3 1.8 1.2 0

mH

entr

ies

38.0 42total 11.3

ZZ background subtraction at 189 GeV

189 GeV data


Results for the SM Higgs all data observed limit:

mH > 90.4 GeV/c2 at 95 % C.L.*)

from gedanken experiments: expected limit: mH > 93.4 GeV/c2

bad luck

other LEP experiments have typically mH > 94 GeV/c2

(expected and observed) current best limit from L3:

mH > 95.5 GeV/c2

already above Z mass!*) C.L.: frequentist; P (signal > data ) > 95 %

ALEPH preliminary


Neutral Higgs Bosons of the MSSM Reminder

2 parameters to parameterize model at tree-level: mh, tan (mA, ) Mass relations at tree-level

mh < mZ |cos 2| (+ radiative corrections !!!) mh < mA < mH

mH > mZ (mH > max{mZ,mA}) mH± > mW (mH±

2 = mW

2 + mA2 )

mA large decoupling of heavy Higgses

2 complementary production processes:

mh s mZ

hZHiggs strahlung

sin2()

mh ,mA s/2

hAassociated

pair-production cos2()

Decay mode BR

bbbb 85 %bbbb 15 %

2 final states


Analyses

candidate: 4 b-jet event

Selection N85exp Nbkg

exp Nobs

bbbb 58.3 % 3.4 9.0 13

bb 24.0 % 0.2 0.6 0

mA + mhen

trie

s

189 GeV data


Results for the MSSM Higgses all ALEPH data

combined hA selection at large

tan (eq. cos2() ~ 1):

mA > 82.2 GeV/c2

mh > 82.2 GeV/c2

at 95% C.L. lucky

reinterpretation ofhZ + hA searchesat smaller valuesof cos2()

...

expected and observed C.L.for equal h and A massesand cos2() =1

130 - 189 GeV data


sin2() - mh plane

130 - 183 GeV data

hA

hZ


mh - tan plane

results can also be expressed in the mh - tan plane

for the benchmark sets of MSSM parameters(mtop, MSUSY, ...)

and for two extreme configurations of stop mixing(controlled by At, )

no mixing maximal mixing

theoretically

excluded

theoretically

excluded

excluded

130 - 183 GeV data


Charged Higgs search for pair-produced H+ H

in the framework of generaltwo-doublet models

mW = mZ cosW at tree level, i.e. = 1 absence of flavor changing neutral currents

decays into heaviest energetically accessible fermion pairs

three analyses for thethree channels

leptonic hadronic cs sc mixed cs

cross section of the same size at LEP2 energies

(for the mass limits reachable)

mH+ = 59 GeV/c2


final state final state topology:

two acoplanar taus missing energy carried away by neutrinos

(at least four neutrinos!)

charged Higgscharged Higgs


cs final state

final state topology: two hadronic jets thin -jet + missing energy

two complementary approaches:

global analysis acoplanarity missing momentum thrust

topological analysis -jet reconstruction

sensitivitywell below mW


reconstructed jet-jet mass


Performance of analyses 183 GeV data Expected number of background events from SM processes as

a function of mH± , compared to the number of events observed(mixed + four jet channel: sliding mass window):

60 events found by the three selections 57.9 events expected from SM processes consistency + no accumulation in the cssc mass distribution

combine analyses

put limits



Results

only up to 183 GeV data NO evidence for a signal

mass limits as a function of ()

Limit at 95% C.L. on the mass of charged Higgs bosons as a function of (); the hatched region is excluded by the combination of all three channels.

mH± > 59 GeV/c2

indep. of () [95% C.L.]



() for type II doublet models

predictionprediction

H+ decay widths at tree level:

For mc,ms << mH+ we havetree(H+ cs) ~ Nc g

2 mH+ / (32 mW2) [mc

2 cot2ß + ms2 tan2ß] |Vcs|

2

tree(H+ cb) ~ Nc g2 mH+ / (32 mW

2) [mc2 cot2ß + mb

2 tan2ß] |Vcb|2

tree(H+ ) ~ g2 mH+ / (32 mW2) [ m

2 tan2ß]

0.972

0.042


Invisible Higgs decays

SMHZZinvh 2.

hZee

Acoplanar leptons with mass ~ MZ

Acoplanar jets with mass ~ MZ

Hll Hqq

Expected backgr. 3.8 8.8

Observed events 5 8


Summary SM Higgs

with 250 pb-1 collected with ALEPH at s = 161, 172, 183 and 189 GeV 95% C.L. lower limit is m H > 90.4 GeV/c2

very preliminary, very conservative

MSSM Higgs also lower energy data at 130 GeV included m h , mA > 82.2 GeV/c2 for large tan

Charged Higgs data at 189 GeV not yet included m H+ > 59 GeV/c2 independent of ()

Invisible Higgs h neutralinos & GUT mass relation (M1 ~ 0.5 M2) relaxed for 22 = 1: m h > 88 GeV/c2 @ 95% C.L.


Confidence Level define estimator as a measure for the experiment in

question to contain signal events: n = # of observed events s = expected # of signal events Pi

n = prob. that i signal events areless signal like than observed

small background like large signal like

define observed confidence level c where data is the estimator of the actual experiment s is the p.d.f. of the estimator for signal events

expected confidence level <c >average of c where the # of observed events is

distributed according to the expected background

(i.e. in the absence of signal)

no event observed = es , data = min , c = es

n

i

ni

is Pi

se

0 !

data

dmc hs

min

)ˆ,(


interpretation of C.L.

m H > 90.4 GeV/c2 at 95% C.L. means: given the hypothesis that the Higgs mass is

and < 90.4 GeV/c2

be data the estimator as observed in the data

and signal the estimator for a gedanken experiment where the above hypothesis is true (i.e. distributed according to s )

then P (signal > data ) > 95 % the quoted limit is the largest mass hypothesis for which this

is true

it does NOT mean: P(m H > 90.4) > 95% or P(m H < 90.4) < 5%

hm̂hm̂

motivation indirect searches

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