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Higgs searches at L3
D. Teyssier
To cite this version:
DANIELTEYSSIER
onbehalfofthe L3Collaboration
IPNLyon,4rue EnricoFermi,Campusde laDoua,
69622VilleurbanneCedex,FRANCE
E-mail: [email protected]
Standardandnon-minimalHiggssearcheswereperformedwithdatacollectedby theL3detectorine
+
e collisionsat189 202GeVcenter-of-massenergies. The standardsearchmethodologyisdescribed. Anexampleofnon-standardsearchis reviewedaswellandresultsattheabovementionned center-of-massenergiesare presented.
1 Standard Model Higgssearches.
1.1 Introduction.
The Standard Model (SM) provides fermions and gauge bosons masses
throughelectroweaksymmetrybreaking,theso-calledHiggsmecanism
1 .
In-troducingonecomplexscalarHiggsdoublet,thethreegaugebosonsW
and
Z 0
gettheirlongitudinalpolarisation,andbecomemassive 2
. Itremainsone
degree of freedom, corresponding to a new fundamental scalar particle, the
Higgsboson.
1.2 Productionmecanism.
ThemainprocesstoproducetheHiggsbosonatLEP2istheHiggs-Strahlung.
ThecorrespondingFeynmandiagramispresentedbelow(Fig.1). Ano -shell
Z boson isproduced, andis decayinginto an on-shellZ boson and aHiggs
boson.
e
-e
+
Z
*
Z
h
0
e
-e
+
Z
*
Z
h
0
e
-e
+
Z
*
Z
h
0
e
-e
+
Z
*
Z
h
0
e
-e
+
Z
*
Z
h
0
e
-e
+
W,Z
W,Z
ν
e
,e
+
ν
e
,e
-h
beproducedbyWorZfusion(Fig.1). Butthecrosssectionissmallcompared
toHiggs-Strahlungprocess(Fig.2).
10
-3
10
-2
10
-1
1
90
100
110
120
130
M
H
[GeV]
Cross section
[pb
]
Total cross section
Higgsstrahlung
Fusion + interference
√s = 206.6 GeV
Figure2. CrosssectionforHiggsbosonproduction.
1.3 Higgs bosondecay.
The couplingbetweenthe Higgs boson andthe charged fermions is
propor-tionnaltotheirmasses. Intherangeofmassofinterest,theHiggsbosonwill
decaypreferentiallyintobquarks(Fig.3). TheZbosonbranchingratioswill
determinethedi erenttopologies.
1.4 Fourtopologies.
The Z boson candecay into apair ofquarks(four-jet topology,65% ofthe
totalnumberofevents),orapairof neutrinos(missinsenergychannel, 19%
ofevents),orapairofelectronsormuons (leptonicchannels,6% ofevents).
Finally,theZ ortheHiggsbosoncandecayintoapairoftaus,andtheother
particleinto apairofquarks(in thiscase,wegettwojetsand twomini-jets
fromthetaus,9%ofevents).
1.5 Analysisprocedure.
First ofall,loosecutsareusedto reducethemostcopiousbackgrounds,like
thetwo-photonbackground(e
+
e !e
+
e qq). Afterwards,weapply tighter
10
-2
10
-1
1
100
105
110
115
120
125
130
M
H
[GeV]
Branching ratio
H
→ bb
−
H
→ cc
_
H
→ ZZ
*
H
→ WW
*
H
→ τ
+
τ
−
H
→ gg
Figure3. Higgsbosonbranchingratios.
alltopologiesas theHiggs boson will decayinto b
b. Soweareableto build
a global variable, which can either be the output of a neural network or a
likelihood function. Combining this information with the Higgs mass
mea-surement,weobtainthe naldiscriminantvariable(Fig.4),usedtocalculate
thecon dencelevel(CL)foreachmasshypothesis.
0
50
100
150
200
40
60
80
100
M
5C
eq
Events / 2 GeV
1
10
10
2
10
3
0
0.2
0.4
0.6
0.8
1
L
HZ
Events / 0.05
Weconstruct astatistical estimator(designedhereby 2ln(Q)), depending
ontheHiggsmasshypothesis,soastotestthecompatibilityofthedatawith
thesignalplusbackgroundhypothesis,orthebackgroundonlyhypothesis. This
estimatoristakento be: 2ln(Q(m H ))=2s tot 2 X N i ln[1+s i (m H )=b i ] (1) where m H
is the Higgs mass hypothesis, s i
and b
i
the expected signal and
backgroundnumbersofeventsrespectively. N i
takesthevaluesofd i (data),b i ors i +b i
followingthehypothesis. Byintegratingthe 2ln(Q)distributions,
wecanobtain:
the(1 CL
b
)(Fig.5)representingthecompatibilitybetweenthedataand
theSMbackgroundexpectations.
theCL
s
(Fig.5)whichallowsus tosetalimit.
10
-3
10
-2
10
-1
1
105
110
115
120
m
H
[GeV]
1
−
CL
b
Observed
Background
±1σ band
±2σ band
L3
a)
10
-3
10
-2
10
-1
1
105
110
115
120
m
H
[GeV]
CL
s
112.4
112.0
Observed
Background
±1σ band
±2σ band
L3
b)
Figure5. Con dencelevels.
Theobservedlowerlimitonm
H
is112.0GeVat the95%CL,whilethe
expected lower limit is 112.4 GeV
3
. Form
H
= 115 GeV, the background
probability(1 CL
b
)is 32%. Preliminary combinedsearches from thefour
LEPexperiments,Aleph,Delphi,OpalandL3giveanobservedlowerbound
onitsmass ata95%CL
4
: m
H
>114:1 GeV.Ontheother hand,precision
measurementsofelectroweakobservables,which involvevirtual Higgsboson
exchanges,giveanupperboundof
5
: m
H
2.1 Two Higgs doubletmodels oftype II
In general two Higgs doublet models of type II
2
, ve scalar Higgs bosons
appearafter symmetry breaking. Both searchesfor the productionof the h
and AHiggs bosonsin theprocessese
+
e !hZ and e
+
e !hA areused.
Wedidnotuseanyb-tagginginordertoperforma avour-independentsearch,
sincethecouplingoftheHiggsbosontothebquarkscouldbesuppressed.
Thedominantproductionmechanismsarethefollowing:
e
-e
+
Z
*
Z
h
0
e
-e
+
Z
*
Z
h
0
e
-e
+
Z
*
Z
h
0
e
-e
+
Z
*
Z
h
0
e
-e
+
Z
*
Z
h
0
e
-e
+
Z
*
A
0
h
0
e
-e
+
Z
*
A
0
h
0
Figure6. hZandhAprocesses
The cross sections can be parametrized as a function of the Standard
Model(SM)crosssectionsas:
hZ =: SM hZ (2) hA =:: SM hZ (3) where SM hZ
is the cross sectionfor e +
e !hZ in the SM, and arethe
couplings,totheZnormalizedto theSMones
= g 2 hZZ (g SM hZZ ) 2 (4) = g 2 hAZ (g SM hZZ ) 2 (5)
andisaphasespacefactor.
2.2 Oneexample : the search for h;A!qq +
Wegivetheresultforasearchoftheprocessh;A!qq +
, withanupper
limitonthe parameter(Fig.7). Theanalysisprocedureisthesameasthat
describedin Ref. 6
50
60
70
80
90
100
50
60
70
80
90
100
η
/
= 0.15
η
/
= 0.3
η
/
= 1.0
M
(GeV)
M
ττ
(GeV)
L3
Figure7. Upperlimitontheparameter.
3 Conclusion.
There was an excellent LEP performance in 2000 :
p
s up to 209
GeV, and more than 200 pb
1
of integrated luminosity collected per
ex-periment. The observed lower bound on SM Higgs boson found by
L3 is 112.0 GeV. Other results concerning avour-independent analyses
and Minimal Supersymetric Standard Model searches can be found in :
http://l3www.cern.ch/conferences/contributions.html
References
1. F. Englert et R. Brout, Phys:R ev:Lett:13(1964)321, P.W. Higgs,
Phys:R ev:Lett:12(1964)132,T.Kibble,Phys:R ev:155(1967)1554.
2. J.Gunion, H.Haber,G.Kaneet S.Dawson,TheHiggs Hunter'sGuide,
FrontiersinPhysicsSeries(vol. 80),ed. Addison-Wesley (1990).
3. L3Collaboration,P.Achardetal.,StandardmodelHiggs bosonwith the
L3experiment atLEP,Phys. Lett. B517(2001)319-331.
4. ALEPH,DELPHI,L3etOPALCollaborations,TheLEPworkinggroup
forHiggs bosonsearches,Search for the StandardModel Higgs boson at
LEP,preliminaryCERN-EP/2001.
5. TheLEPCollaborationsALEPH,DELPHI,L3etOPAL,theLEP
Elec-troweak Working Group and the SLD Heavy Flavour and Electroweak
Groups, A Combination of Preliminary Electroweak Measurementsand
Constraintson theStandardModel, CERN-EP/2001-021.
