D0 Search for the Higgs Boson in Multijet Events

0.03 MB·English

by  Alex Melnitchouk

DØ Search for the Higgs Boson in Multijet
Events
Alex Melnitchouk for the DØ Collaboration
DepartmentofPhysicsandAstronomy,
6 TheUniversityofMississippi,
0 108LewisHall,P.O.Box1848
0 University,MS38677-1848,USA.
2

n Abstract. WepresenttwosearchesfortheHiggsbosonin√s=1.96TeVpp¯ collisionsusingdata
a
taken with the DØ detector during Run II of the Fermilab Tevatron collider. The first study is a
J
searchforneutralHiggsbosonsproducedinassociationwithbottomquarks.Thecrosssectionsfor
4
theseprocessesareenhancedinmanyextensionsofthestandardmodel(SM),suchasinitsminimal
2
supersymmetricextensionat large tanb . The results of our analysisagree with expectationsfrom

theSM,andweuseourmeasurementstosetupperlimitsontheproductionofneutralHiggsbosons
1
v inthemassrangeof90to150GeV.
1 ThesecondstudyisasearchforthestandardmodelHiggsbosonproducedinassociationwiththe
4 Zboson.Westudythepp¯ ZH n n¯bb¯ channel,whichisoneofthemostsensitivewaystosearch
→ →
0 forlightHiggsbosonsattheTevatron.Weselectmultijeteventswithlargeimbalanceintransverse
1 momentum and two b-tagged jets. Then we search for a peak in invariant mass distribution of
0 two b-tagged jets. After subtracting the backgrounds, we set the 95% C.L. upper limits on the
6 s (pp¯ ZH n n¯) BR(H bb¯)forHiggsmassesbetween105and135GeV.
0 → → × →
/ Keywords: Higgsboson,StandardModel,Supersymmetry
x
PACS: 14.80.Bn,14.80.Cp
e
-
p
e INTRODUCTION
h
:
v
The Higgs boson, which is hypothesized to be responsible for electroweak symmetry
i
X
breaking,istheonlyparticleintheSMthathasnotbeendirectlyobserved.Higgsboson
r alsoappearsintheextensionsoftheSM,suchassupersymmetry(SUSY).Intheminimal
a
supersymmetric extension of the SM (MSSM) [1], there are two Higgs fields and five
physicalHiggsbosons,threeofwhichareneutral.Tevatronexperimentscanbesensitive
tobothSM HiggsbosonandMSSM neutral Higgsbosons.
In both searches presented in this paper [2, 3], final states contain b-jets. Higgs
signal would appear as a "bump" in the di-b-jet invariant distribution. Therefore, good
understandingofthecalorimeterresponseandb-taggingarethemainingredientsofboth
analyses.
STANDARD MODEL HIGGS SEARCH IN MULTIJET EVENTS
We use data collected by the DØ detector [4] between March 2003 and June 2004,
corresponding to an integrated luminosity of about 261 pb 1. We select events with
−
two acoplanar b-jets with transverse energy above 20 GeV. We require that missing
transverseenergy in the event is above25 GeV. In addition, we use kinematicvariables
basedon vectorsumoftransversemomentaofjetsand tracks.
We distinguish between two types of background, such as physics background and
instrumental background. Physics backgrounds are mainly due to misidentified b-jets
in Z+jets and W+jets processes with final state neutrinos that escape the detector.
Instrumental backgrounds are multijet events in which energy of jets is mismeasured.
Physics backgrounds are estimated using using PYTHIA, COMPHEP, and ALPGEN
Monte Carlo event generators. Monte Carlo samples were processed through the DØ
detector simulation and reconstruction software. Instrumental background is estimated
fromdata.
No excess of events is observed. We derive upper limits on s (pp¯ ZH) BR(H
bb¯) for four Higgs mass points in the range between 105 and 135 G→eV. The×results a→re
plottedin Figure 1.
SUPERSYMMETRIC HIGGS SEARCH IN MULTIJET EVENTS
Werequirethreejetsabove15GeV atthehighesttriggerlevel.Intheofflinewerequire
at least three b-tagged jets. Both the p thresholds and pseudorapidity range selections
T
are optimized for Higgs mass and number of required jets. Signal rates and kinematics
arenormalizedto NLOcalculations.
MajorsourceofbackgroundisSMmultijetproduction.Therearetwomaincategories
ofmultijetbackground.Onecontainsgenuineheavy-flavorjets,whiletheotherhasonly
light-quark or gluon jets that are mistakenly tagged as b-quark jets, or correspond to
gluonsthat branch intonearly collinearbb¯ pairs. Background shape is determined from
double b-tagged data by applying the tag rate function to non-b-tagged jets. Then the
backgroundisnormalizedto thedataoutsidethesignalregion.
No excess of events is observed and upper limits on neutral Higgs production cross-
section are derived. The limit is shown as a function of Higgs mass and tanb in Figure
2.
ACKNOWLEDGMENTS
WethankthestaffsatFermilabandcollaboratinginstitutions,andacknowledgesupport
from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom and
RFBR (Russia); CAPES, CNPq, FAPERJ, FAPESP and FUNDUNESP (Brazil); DAE
andDST(India);Colciencias(Colombia);CONACyT(Mexico);KRFandKOSEF(Ko-
rea); CONICET and UBACyT (Argentina); FOM (The Netherlands); PPARC (United
Kingdom); MSMT (Czech Republic); CRC Program, CFI, NSERC and WestGrid
Project (Canada); BMBF and DFG (Germany); SFI (Ireland); Research Corporation,
AlexandervonHumboldtFoundation,and theMarieCurieProgram.
REFERENCES
1. H.P.Niles,Phys.Rept.110,1(1984);H.E.HaberandG.L.Kane,Phys.Rept.117,75(1985).
103
b) DØ Run II Preliminary
p
( ZH fi n n bb, L =261pb-1
) b 102 int
b
95% C.L. upper limit
fiH 10 Measured
(
B
Expected

·
H) 1
Z
Standard Model
fi p10-1
p
(

s10-2
110 120 130 140
Higgs Mass (GeV)
FIGURE1. 95%C.L.upperlimitsonthecrosssectionforZHproductiontimesthebranchingfraction
forH bb¯
→

110000 MSSM Higgs bosons DØ

P bbf (fi bb), f = h, H, A

8800 E

L

t
a
6600
d
bbnn e
aa d
tt4400 u No mixing
l
c
x Max. mixing
2200 E
8800 110000 112200 114400
mm ((GGeeVV))
AA
FIGURE 2. The 95%C.L. upperlimit on tanb as a functionof m for two scenariosof MSSM, “no
A
mixing"and"maximalmixing".AlsoshownarethelimitsobtainedbytheLEPexperimentsforthesame
twoscenariosoftheMSSM[5]
2. http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/HIGGS/H10/H10.pdf
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etal.,Phys.Rev.Lett.95,151801(2005);hep-ex/0504018,Fermilab-Pub-05/058-E.
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5. TheLEPWorkingGroupforHiggsBosonSearches,LHWG-Note2004-01.