Search for WH associated production at Dø Tevatron

0.91 MB·English

by  Florian Miconi

Search for WH associated production at Dø Tevatron
FlorianMiconi1,a
Universite´ deStrasbourg,IPHC,23rueduLoess67037Strasbourg,France
CNRS,UMR7178,67037Strasbourg,France
Abstract. TheHiggsmechanismintroducedin1964givesasatisfactorysolutiontoamajorproblemofthe
standardmodelofelementaryparticles:theoriginofthemass.ItpredictstheexistenceoftheHiggsscalarboson,
2 whichhasnotbeendiscoveredexperimentallyyet.TheTevatron,ahadronacceleratorbasedatFermiNational
1 AcceleratorLaboratorynearChicago,hasdelivereddatatoitstwomulti-purposedetectorsCDFandDZERO
0 since 1983 up to september 2011. Leaving us about 11 fb−1 of data per experiment to analyze. Associated
2 productionofaHiggsbosonandavectorgaugebosonWorZisthemainsearchchannelforalightstandard
Higgsboson(i.e.below135GeV/c2).UsingdatacollectedbyDZERO,wearelookingforthisproductionmode
n
taking advantage of sophisticated techniques to improve the signal sensitivity such as b-jet identification and
a
J multivariatediscriminants.Intheend,astatisticalapproachallowsustosetanupperlimitontheratiobetween
theobserved(resp.expected)Higgsproductioncrosssectionanditstheoreticalcrosssection.Thelatestresult
9
obtainedintheWHchannelusing8.5 fb−1atDZEROis4.6(resp.3.5)fora115GeV/c2Higgsboson.
2

]
x 1 Introduction in two b quarks, making its signal impractical to extract
e
withintheoverwhelmingQCDbackgroundgeneratedata
-
p ThemassoftheHiggsbosonisunpredictedbytheHiggs hadroniccollider.Thus,themostsensitivechannelarethe
e mechanismofthestandardmodel.Nevertheless,theoreti- WHandZHassociatedproduction.Thedecayproductsof
h
calconstraintsexistbutareveryloose(consideringtrivial- theWortheZallowingtodrasticallyreducethehadronic
[
ity and vacuum stability). Combination of direct searches background.

1 at colliders, and precision measurements of electroweak WepresentherethesearchforWHassociatedproduc-
v observables depending on the Higgs mass allows a more tionatDøTevatron.DøisonethetwoTevatron’smultipur-
6 strictconstraintoverthemassrange.Weshowonthefig- posedetector,designedtostudyhighmassstatesandlarge
2 ure1theLEPexclusionupto114.4GeV/c2 andthelatest transverse momentum phenomena, design that suits very
0
Tevatronexclusion(156 < m < 177GeV/c2)alongwith wellthesearchforastandardHiggsboson.Wewillpresent
6 H
the standard fit on the electroweak parameters measure- theWHanalysisstrategy,startinginthesection2withits
.
1 ments[2].Theseresultsclearlyshowapreferenceforthe selection, we will then in the sections 3 and 5 look at the
0 lowmassregion. techniquesusedtoimprovethesignalefficiencybyselect-
2
ingthefinalstatewithb-jetsidentificationalgorithmsand
1
combiningdiscriminantvariableswithmultivariateanaly-
:
v sis (MVA). We will also present in the section 4 how we
i partly recover radiation jets to improve the di-jet mass in
X
the3jetschannelusingaradiationrecoverymethod.
r
a
2 The WH analysis selection
Thesearchisbasedoneventswiththefollowingkinemat-
icalandtopologicalcharacteristics:
– Thereconstructedprimaryvertexmustlayinthecen-
tralregion60cmaroundthecenterofthedetectoralong
Fig.1.DirectandindirectexperimentalconstraintsontheHiggs thebeamaxis.
bosonmass. – One central (0 ≤ η ≤ 2.5) charged lepton (electron
ormuon)withatransversemomentumhigherthan15
GeV mustbepresent.
Productiondecaymodesarewellestablishedinthestan- – Weselecteventswithamissingtransverseenergyabove
dard model as a function of the mass of the Higgs bo- 15GeV,signatureofaneutrinocomingfromthedecay
son. Gluon fusion has the greatest cross section over all oftheWboson.
themassregion.Butatlowmass,theHiggsbosondecays – We consider only the events with 2 or 3 central (0 ≤
η ≤ 2.5)jetswithatransversemomentumhigherthan
a e-mail:[email protected] 20GeV.
EPJWebofConferences
– Wealsorequireaminimalvalueonamultivariatedis-
criminant referred as MVA QCD aiming at reducing
theQCDbackground.
– Other leptons are vetoed in order to be orthogonal to
theZH →llbbanalysis.
Theanalysisisthenseparatedintwoorthogonalchan-
nels with exactly two jets and three jets. These channels
arefurtherdividedinorthogonalsub-channelswithexactly
oneb-taggedjetand2b-taggedjets[3].
Fig. 2. Diagram of a b-jet. IP corresponds to the track impact
parameter
3 b-jet identification
Being able to discriminate b-jets from lighter flavor jets
is crucial for the low mass Higgs search. This tool relies
on the characteristics of b quarks and B mesons. Indeed
b-quarkistheheaviestquarkthatleadtohadronisationbe-
fore its decay and the resulting B-hadron has a very long
lifetime (about 1.5 ps). These illustrated by figure 2 at-
tributesleadto:
– A secondary vertex, about 3 mm apart from the pri-
maryvertex.
– HighimpactparametertracksproducedbytheB-hadron
decay.
Fig.3.Higgsinvariantmassforthe2jetschannelwith2b-tagged
– A higher secondary vertex mass and a distinctive jet
jets
angularopening.
– Thepossiblepresenceofamuoninthejetreconstruc-
tioncone.
a b-jet (FSR) to reconstruct the Higgs invariant mass but
SeveralalgorithmshavebeendevelopedatDøtotakead- we have to be careful not to use an initial state radiation
vantageofthesefeatures[4]. (ISR)jettodoso.Figure4showsMonteCarlosimulations
ofa115GeVHiggs,about25%improvementisachieved
– SVT(SecondaryVertexTagger):basedontherecon-
onthedijet-massresolutionifwerecovereveryFSRjets.
structionofasecondaryvertexusingaKalmanfilter.
But in the detector, there’s no simple way to discriminate
– JLIP(JetLIfetimeProbabilitytagger):takesadvantage
aFSRjetfromanISRjet.Weshowthattheminimalangle
of the impact parameter significance of tracks associ-
between the third and first or second jet (referred to al-
ated to a jet, to construct the probability that this jet
phamin)isefficienttodiscriminatethesetwocases.Inthe
is coming from the primary vertex and therefor more
end,theprobleminathreejetsselectionistofindtheright
likelytobealightjet.Heavyflavorjetsaredistributed
jet combination to reconstruct the Higgs invariant mass.
atlowvaluesofthisvariable.
TheFSRrecoverymethodconsistsintwosteps:
– CSIP(CountingSignedImpactParametertagger):counts
thenumberoftrackswithhighimpactparameter. – First,jetsareorderedbydecreasingb-jetidentification
variable value. This way, the two leading jets are the
Each of these algorithms show good performances in
mostlikelytobetheb-jetscomingfromtheHiggs.
b-jetsidentification.Asithasbeenobservedthatinforma-
– Knowingthattheremainingjetisverylikelytobethe
tiongivenbyeachalgorithmisnot100%correlatedsince
FSRorISRjet,weaddittotheinvariantmasscalcula-
they don’t exploit exactly the same b-jet features, it has
tionifthevalueofalphaminisbelowanoptimumcut
beendecidedallthesevariablesintoamorepowerfulone
thatwefoundtobe1.
usingmultivariateanalysis.Contrarilytosequentialcuts,a
multivariateanalysisallowstotakeintoaccountthecorre- The method is currently implemented in most of the
lations between input variables. On figure 3 is shown the low mass analyses at Dø and shows up to 20% improve-
Higgs invariant mass reconstructed in the 2 jets channel mentonthelimitobtainedinthe3jetschannel.
with2jetsb-taggedusingtheb-taggingmultivariateanal-
ysis.
5 Optimization using a multivariate
analysis
4 Three jets channel special treatment:
radiation recovery Thesamewaymultivariateanalysishelpedtoachievebet-
terperformancesoftheb-taggingprocedure,itcanbevery
Higgs invariant mass is the most powerful single variable helpfultooptimizethesensitivitytoaHiggssignalbytak-
to search for a Higgs signal. In the 3 jets channel, its re- ing advantage of the discriminating power of less impor-
constructionislessstraightforwardthaninthe2jetschan- tantvariablesandnotonlytheinvariantmass.Themethod
nel. We want to take into account radiation coming from chosen for the WH analysis is the Boosted Decision Tree
HadronColliderPhysicssymposium2011
^2 figure6.ResultscombiningallTevatronanalysesarevery
ntries/47GeV/c22050000 pdthraeordmfigmisuiorndege7lsi[on7vc]ee.rthalelothbeselrovwedmliamssitriasnbgeeloawsctawnicbeethseeesntaonn-
E
1500
1000
500
00 50 100 150 200 250 300 350 400
Higgs invariant mass (GeV/c^2)
Fig.4.Higgsinvariantmasswith(redorlightgrey)andwithout
(black)theradiationrecoverymethod.
(BDT) developed with the TMVA package [5]. A deci-
sion tree splits each event in order to optimize the sig-
nal/backgroundseparation.Theresultingnodes(leaves)con- Fig.6.LimitontheHiggsproductionbasedontheWHanalysis
tinuetobesplituntilwereachtherequiredsignal/background presentedinthisreport.
purity.Toselectinputvariablesweensurethateachiswell-
modeledandshowgooddiscriminatingpowerbetweensig-
nalandbackground.AdedicatedMVAistrainedforeach
Higgsmasshypothesiscoveringthelowmassregionfrom
100GeVto150GeVin5GeVsteps.Figure5showsthe
resulting output of the BDT in the 2 jets channel with 2
b-tagged jets for a 115 GeV Higgs, with the background
events distributed towards 0 and well separated from the
signalevents.
Fig.7.LimitontheHiggsproductionbasedonthecombination
ofCDFandDøresults.
References
Fig.5.Finaldiscriminantvariableusedinthe2jetschannelwith
2b-taggedjets
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