Evidence for the direct decay of the 125 GeV Higgs boson to fermions

0.34 MB·

by  CMS Collaboration

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP/2013-037
2014/11/04
CMS-HIG-13-033
Evidence for the direct decay of the 125GeV Higgs boson to
fermions
4
∗
1 The CMS Collaboration
0
2

v
o
N
Abstract
2

]
x
The discovery of a new boson with a mass of approximately 125GeV in 2012 at the
e
- LHC [1–3] has heralded a new era in understanding the nature of electroweak sym-
p
metrybreakingandpossiblycompletingthestandardmodelofparticlephysics[4–9].
e
h Since the first observation in decays to γγ, WW, and ZZ boson pairs, an extensive
[
set of measurements of the mass [10, 11] and couplings to W and Z bosons [11–13],

3 as well as multiple tests of the spin-parity quantum numbers [10, 11, 13, 14], have
v
revealed that the properties of the new boson are consistent with those of the long-
7
2 sought agent responsible for electroweak symmetry breaking. An important open
5 question is whether the new particle also couples to fermions, and in particular to
6
down-typefermions,sincethecurrentmeasurementsmainlyconstrainthecouplings
.
1 tothe up-typetop quark. Determinationof thecouplings todown-type fermionsre-
0
quires direct measurement of the corresponding Higgs boson decays, as recently re-
4
1 portedbytheCMSexperimentinthestudyofHiggsdecaystobottomquarks[15]and
:
v τ leptons [16]. In this letter we report the combination of these two channels which
i results, for the first time, in strong evidence for the direct coupling of the 125GeV
X
Higgs boson to down-type fermions, with an observed significance of 3.8 standard
r
a deviations,when4.4areexpected.
PublishedinNaturePhysicsasdoi:10.1038/nphys3005.
(cid:13)c 2014CERNforthebenefitoftheCMSCollaboration. CC-BY-3.0license
∗SeeAppendixAforthelistofcollaborationmembers

1
The CMS and ATLAS experiments at the Large Hadron Collider (LHC) have reported the
discovery of a new boson [1–3] with a mass near 125GeV and with production rates, decay
rates, and spin-parity quantum numbers [10–14] compatible with those expected for the stan-
dard model Higgs boson [4–9]. In the standard model, the Higgs boson is a spin zero particle
predicted to arise from the Higgs field which is responsible for electroweak symmetry break-
ing[17,18]. Assuch,thestandardmodelHiggsbosoncouplesdirectlytotheWandZbosons,
andindirectlytophotons. Todate,significantsignalshavebeenreportedinchannelswherethe
bosondecaystoeitherγγ,WW,orZZbosonpairs[11–13]aspredictedbythetheory. Overall,
theseresultsdirectlydemonstratethatthenewparticleisintimatelyrelatedtothemechanism
of spontaneous electroweak symmetry breaking, whereby the W and Z bosons become mas-
sive,andthusithasbeenidentifiedasaHiggsboson.
The standard model also predicts that the Higgs field couples to fermions through a Yukawa
interaction, giving rise to the masses of quarks and leptons. The structure of the Yukawa in-
teraction is such that the coupling strength between the standard model Higgs boson and a
fermion is proportional to the mass of that fermion. As the masses of many quarks and lep-
tons are sufficiently well known from experiment, it is possible within the standard model
to accurately predict the Higgs boson decay rates to these fermions. The existence of such
decays and the corresponding rates remain to be established and measured by experiment.
Indirect evidence for the Higgs coupling to the top quark, an up-type quark and heaviest ele-
mentaryparticleknowntodate,isimpliedbyanoverallagreementofthegluon-gluonfusion
production channel cross section with the standard model prediction. However, the masses
ofdown-typefermionsmaycomeaboutthroughdifferentmechanismsintheoriesbeyondthe
standard model [19]. Therefore, it is imperative to observe the direct decay of this new par-
ticle to down-type fermions to firmly establish its nature. As a consequence of the Yukawa
interactiondiscussedabove,themostabundantfermionicHiggsbosondecayswillbetothird-
generation quarks and leptons, namely the bottom quark and the τ lepton, since the decay of
aHiggsbosonwithamassaround125GeVtotopquarksiskinematicallynotallowed. There-
fore, the most promising experimental avenue to explore the direct coupling of the standard
model Higgs boson to fermions is in the study of the decay to bottom quark-antiquark pairs
(denotedasbb)aswellastotaulepton-antileptonpairs(denotedasττ).
Recently, the CMS Collaboration reported on a search for the decays of the new boson to bb
quark pairs [15] as well as to ττ lepton pairs [16] based on data collected in 2011 and 2012. In
this letter, we report on the combination of the results from the study of these two decays to
down-typefermion-antifermionpairs,performedforthefirsttimeattheLHC.
The CMS apparatus comprises several detectors specialised in identifying different types of
particles. These detectors are arranged inside and outside a superconducting solenoid of 6m
internal diameter, that provides a magnetic field of 3.8T. The detector electronics process col-
lisioninformationatarateofupto40MHz, anddecidewhetherornotthecrossingofproton
beam bunches that took place at the centre of the detector produced proton-proton collisions
of sufficient interest. Through a layered decision system, from the 20MHz of proton bunch
crossings, fewer than 1kHz are saved for further analysis. A detailed description of the full
CMS apparatus can be found in Ref. [20]. Data were collected at the LHC which delivered
proton-proton collisions at centre-of-mass energies of 7TeV (2011) and 8TeV (2012). A total
integrated luminosity of 5.1 (4.9)fb−1 and 18.9 (19.7)fb−1, for the H → bb (H → ττ) decay,
hasbeenanalyzedat7and8TeV,respectively. Eachprotonbunchcrossinggaverisetoalarge
number of simultaneous proton-proton collisions, on average 9 in 2011 and 21 in 2012. Such
a large number of overlapping collisions presented exceptional challenges in reconstructing
the individual particles produced in the collision where an interesting interaction took place.
2
Those challenges were successfully met thanks to the CMS tracking system, able to separate
collisionverticesascloseas0.5mmalongthebeamdirection.
TheHiggsbosonlifetimeis ∼10−22 seconds. Asaconsequence, thedetectorsattheLHConly
record the interactions of its decay products. For a Higgs boson mass of 125GeV, the chan-
nels expected to be experimentally accessible include the decays to two photons, two W or Z
bosons,abbquarkpair,andaττ leptonpair. Thelasttwoareespeciallychallengingsincethe
backgrounds due to standard model processes other than Higgs boson production are over-
whelming and the invariant mass distributions of the reconstructed decay products do not
produce sharp peaks. Techniques to estimate the background contributions directly from the
collision data have been developed to face this challenge. The results are typically expressed
intermsofupperlimitsontheproductoftheproductionanddecayrates,significancesforthe
background-only hypothesis to describe excesses as large, or larger, than those present in the
data,andsignalstrengthsrelativetotheexpectationfortheproductionanddecayofthestan-
dard model Higgs boson. Both the observations in data and the expectations for the standard
model Higgs boson are quoted. The expected quantities are important measures of the sen-
sitivity of a particular search which do not depend on statistical fluctuations affecting actual
observations in the data. Thus, they can also be used to compare the sensitivity of different
experiments.
The CMS search for a Higgs boson decaying into a bb quark pair is reported in Ref. [15]. The
inclusive production of Higgs bosons in this decay channel is extremely hard to exploit due
to the overwhelming direct production of bb quark pairs, as predicted by quantum chromo-
dynamics(QCD).Forthisreason,presentsearchestargettheso-called“VHproduction”mode
in which the Higgs boson is produced in association with a heavy vector boson, V, namely a
W or a Z boson, identified via its leptonic decay. Requiring the presence of additional leptons
withhightransversemomentumprovidesaneffectivehandletoreducetheQCDbackground.
TheHiggsbosoncandidateisreconstructedfromtwojetsofparticles, eachofwhichhasbeen
identified as having a high probability of arising from the hadronisation of a bottom quark.
The identification is based on properties of hadrons containing bottom quarks, such as their
long lifetimes which lead to displaced vertices, or their decay to muons, both producing dis-
tinctive signatures that the CMS detector is sensitive to. Owing to the 10% mass resolution
of the identified pair of b-quark jets [15], the standard model Higgs boson signal is expected
to produce a broad enhancement over the background in the bb invariant mass distribution.
The result of this analysis shows an excess over the background expectation for Higgs boson
masshypotheses,m ,intherangeof120–135GeV,withanobserved(expectedforthestandard
H
model Higgs boson) significance of 2.1 (2.1) standard deviations, σ, at a Higgs boson mass of
125GeV. Correspondingly, the strength of this excess relative to the expectation for the stan-
dardmodelHiggsbosonis1.0±0.5,wheretheuncertaintyincludestheeffectofbothstatistical
and systematic sources of uncertainty. A result obtained by the CDF and D0 experiments at
theTevatronproton-antiprotoncollider,whichoperatedatacentre-of-masscollisionenergyof
1.96TeV, initially revealed an excess of bb events consistent with the mass of the Higgs boson
observed at the LHC, yielding an observed significance of 2.8σ for m = 125GeV [21]. In a
H
subsequent analysis the observed significance decreased, with the CDF and D0 experiments
reporting an excess 1.59+0.69 times the expectation for the standard model Higgs boson with
−0.72
m = 125GeV [22]. At the LHC, in previous studies based solely on the 2011 data, the CMS
H
and ATLAS Collaborations have reported 95% confidence level (CL) upper limits of 2.5 to 5.5
timesthestandardmodelexpectationforaHiggsbosoninthe110–130GeVmassrange[23,24].
EvidenceforaHiggsbosondecayingtoaττ leptonpairisreportedbytheCMSCollaboration
in Ref. [16]. The lifetime of the τ lepton is 2.9×10-13s, short enough that τ leptons decay be-
3
fore reaching any of the CMS detector systems. The predominant decay modes of τ leptons
are to a lighter lepton, i.e. an electron or a muon, and two neutrinos, or to one or more pions
and a neutrino [25]. In all cases, among the decay products there are neutrinos which cannot
bedirectlydetected. However,thepresenceofneutrinosamongtheproductsofaninteraction
can be inferred from an overall imbalance in the transverse momentum, measured using the
visibleproductsoftheinteraction. The τ-leptondecaysinvolvingpionsaredominatedbythe
decaystoasinglechargedpion,achargedandaneutralpion,orthreechargedpionsobserved
asacollimatedjetofparticles. AllofthesedecaymodescanbeidentifiedbytheCMSdetector,
providing an important handle to suppressthe QCD background. The main irreducible back-
ground is due to the copiously produced Z bosons which, like the Higgs boson, can decay to
a ττ lepton pair. The mass difference between the Z boson (91.2GeV) and the region where
a Higgs boson was observed (∼125GeV), together with a dedicated ττ mass reconstruction
technique [16], allow for a good separation of the two contributions. The gluon-gluon fusion
production, the associated VH production, and the vector-boson fusion production processes
areprobedthroughtheclassificationoftheeventsaccordingtokinematicandtopologicalprop-
ertiesofthereconstructedparticles. Theresultoftheanalysisshowsabroadexcessoverback-
groundintheHiggsbosonmassrangeof115–130GeVwithanobserved(expected)significance
of 3.2σ (3.7σ) at a Higgs boson mass of 125GeV. Correspondingly, the signal strength relative
to the expectation for the standard model Higgs boson is found to be 0.78±0.27 at the Higgs
boson mass of 125GeV. Earlier, the CDF and D0 experiments at the Tevatron reported, for
m = 125GeV, an observed (expected) 95% CL upper limit of 7.0 (5.7) times the standard
H
model expectation [22]. Using the LHC data taken at 7TeV, the ATLAS and CMS Collabora-
tions reported, for m = 125GeV, observed (expected) 95% CL upper limits of 3.7 (3.5) [26]
H
and4.2(3.1)[27]timesthestandardmodelexpectation,respectively.
The CMS measurements in the VH → bb and H → ττ searches [15, 16] are both consistent,
within the precision of the present data, with each other and with the expectation for the pro-
duction and decay of the standard model Higgs boson. Hence, as a next step, a combination
of these two results, requiring the simultaneous analysis of the data selected by the two indi-
vidual measurements, is performed. Such a simultaneous fit to both datasets accounts for all
statisticalandsystematicuncertaintiesandtheircorrelations. Thestatisticalmethodsusedfor
the combination were developed by the ATLAS and CMS Collaborations in the context of the
LHC Higgs Combination Group. The presence of the signal is assessed by performing a fit to
the observed data and using a test statistic based on the profile likelihood ratio. Systematic
uncertainties are incorporated through nuisance parameters and are treated according to the
frequentistparadigm. AdetaileddescriptionofthemethodologycanbefoundinRefs.[28,29]
andresultspresentedhereareobtainedusingasymptoticformulae[30]. Toquantifyanexcess
ofeventsoverthebackgroundexpectation,theteststatisticisdefinedas
L(obs.|b, θˆ )
q = −2 ln 0 , (1)
0 L(obs.|µˆ ·s+b, θˆ)
wherethelikelihood,L,appearinginthenumeratoristhatofthebackground-onlyhypothesis,
sstandsforthesignalexpectedunderthestandardmodelHiggsbosonhypothesis,µisasignal
strength modifier introduced to accommodate deviations from standard model Higgs boson
predictions, b stands for background contributions, and θ are nuisance parameters describing
thesystematicuncertainties. Thevalueθˆ maximisesthelikelihoodinthenumeratorunderthe
0
background-only (µ = 0) hypothesis, while µˆ and θˆ define the point at which the likelihood
reaches its global maximum in the signal-plus-background fit. The post-fit model, obtained
after the signal-plus-background fit to the data, is used when deriving expected quantities.
4
Thepost-fitmodelcorrespondstotheparametricbootstrapdescribedinthestatisticsliterature,
and includes information gained in the fit regarding the values of all parameters [31, 32]. The
p-value for the background-only hypothesis is calculated as the tail probability p = P(q ≥
0 0
qobs.|b),whereqobs. isthevalueoftheteststatisticobservedindata.
0 0
WhiletheexpectedHiggsbosonsignalintheVH → bbanalysisconsistsexclusivelyofHiggs
bosons decaying to down-type fermions, that is not the case in some event categories in the
H → ττ analysis where a sizable contribution from H → WW decays is expected. The con-
tribution from H → WW decays has kinematic properties sufficiently different from those of
H → ττ decays such that the two contributions do not overlap. In order to exclusively assess
the fermionic decays of a Higgs boson, the expected contribution from H → WW decays is
consideredasabackgroundprocess. GiventhediscoveryofaHiggsbosonwithpropertiesas
expectedinthestandardmodelandamasscloseto125GeV,thepp → H → WWbackground
contributionistakenfromtheexpectationforthestandardmodelHiggsbosonwithamassof
125GeV,includingallassociateduncertainties.
Figure1shows,asafunctionofthemasshypothesisfortheHiggsbosondecayingtofermions,
the median expected and the observed p-value for the background-only hypothesis. The ex-
pectation is calculated after having performed the signal-plus-background fit to the observed
data. Thebackground-onlyhypothesisincludesthepp → H(125GeV) → WWprocessforev-
eryvalueofm . The p-valuecanbeexpressedintermsofGaussiantailprobabilitiesandgiven
H
in units of standard deviation (σ), shown in the right vertical axis of Fig. 1. For all m values
H
tested,theevidenceagainstthebackground-onlyhypothesisisfoundtobe3σ ormore,witha
maximumof3.8σform = 125GeV,correspondingtoa p-valueof7.3×10-5.
H
Thestudyofthefermionic-decaycontributionunderthe m = 125GeVhypothesisisparticu-
H
larlyrelevantbecauseofthediscoveryofaHiggsbosonwithamassnear125GeVintheanaly-
sisofnon-fermionicdecays. SincethemassresolutionintheH → bbandH → ττ channelsis,
atbest,around10%,andtheexpectedyieldsdonotchangesignificantlyaroundm = 125GeV,
H
the exact choice of m does not affect the conclusions. A scan of the profile likelihood is per-
H
formedtoestimatethebest-fitsignalstrengthrelativetothestandardmodelexpectationaswell
asconfidenceintervals. Asdescribedearlier,theH → WWcontributionisnottreatedaspartof
thesignal. TheresultisshowninFig.2, fromwhichacombinedsignalstrengthof0.83±0.24
withrespecttothestandardmodelexpectationisinferredfortheproductionofaHiggsboson
decayingtodown-typefermions. Thestatisticalcomponentoftheuncertaintyrepresents80%
of the total uncertainty. This result is compatible with the expectation for the standard model
Higgsboson,assummarisedinTable1.
In conclusion, the existing CMS searches for a Higgs boson decaying into bottom quarks and
τ leptons are consistent with the standard model prediction of a Yukawa structure, where the
fermioniccouplingsareproportionaltothefermionmasses. CombiningtheresultsoftheCMS
measurements in the H → bb and H → ττ decay channels and assuming that the Higgs
bosonisproducedasexpectedinthestandardmodel,reveals,forthefirsttime,strongevidence
for the direct coupling of the 125GeV Higgs boson to down-type fermions, with an observed
(expected)significanceof3.8(4.4)standarddeviations.
5
CMS s = 7 TeV, L = 5 fb-1; s = 8 TeV, L = 19- 20 fb-1
e
u102 VH fi bb (exp.) (obs.)
al VH fi t t (exp.) (obs.)
v
p- 10 H fi t t (non-VH) (exp.) (obs.)

al Combined (exp.) (obs.)
c
o
L 1 0s
1s
10-1
2s
10-2
10-3 3s
10-4
4s
10-5
10-6
110 115 120 125 130 135
m (GeV)
H
Figure 1: The p-value for the background-only hypothesis as a function of the Higgs boson
masshypothesisforboththeobserveddata(obs.) andpseudo-dataconstructedusingtheme-
dian expectation for the standard model Higgs boson after the signal-plus-background fit to
thedata(exp.). Tohelpvisualizetheresults,smoothlinesconnectthepointsatwhichtheprob-
abilitiesareevaluated. TheH → ττ analysisresultsarebrokendownintoacontributionfrom
VH → ττ categories,whichsharetheproductionprocesswiththeVH → bbanalysis,andan-
other contribution with all other H → ττ event categories. For every mass hypothesis tested,
thenon-fermionicdecaycontributionsaretakentobethoseofthestandardmodelHiggsboson
with a mass of 125GeV. Non-fermionic decays are considered as part of the background and
consequentlydonotcontributetothemeasurement.
Table1: SummaryofresultsfortheHiggsbosonmasshypothesisof125GeV. The p-valuesfor
thebackground-onlyhypothesisareexpressedintermsofone-sidedGaussiantailsignificances
andareprovidedinunitsofstandarddeviation(σ). Theexpectedsignificanceisthatobtained
after the fit of the signal-plus-background hypothesis to the data. Note that the expected sig-
nificance of 2.1σ quoted in Ref. [15] for the VH → bb channel was obtained before the fit of
thesignal-plus-backgroundhypothesistothedata. Thebest-fitvalueofthesignalstrengthrel-
ativetotheexpectationfromthestandardmodel, µ, summarisestheprofilelikelihoodscanof
Fig. 2. For simplicity, uncertainties have been symmetrised. The statistical component repre-
sentsmorethan80%oftheuncertainties.
Channel Significance(σ) Best-fit
(m = 125GeV) Expected Observed µ
H
VH → bb 2.3 2.1 1.0±0.5
H → ττ 3.7 3.2 0.78±0.27
Combined 4.4 3.8 0.83±0.24
6
5s
CMS s = 7 TeV, L = 5 fb-1; s = 8 TeV, L = 19- 20 fb-1
20
L

n
l 18 m = 125 GeV
H
D
2 fi
VH bb
16 4s
- 3.8s H fi t t
14 Combined
12
3.2s
10
3s
8
6
2.1s
4 2s
standard
model
2
1s
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
m
Figure2: Scanoftheprofilelikelihoodasafunctionofthesignalstrengthrelativetotheexpec-
tation for the production and decay of a standard model Higgs boson, µ, for m = 125GeV.
H
Thestatisticalsignificanceagainstthebackground-only(µ = 0)hypothesisisshownforthetwo
channels and their combination. By definition, the expectation for the standard model Higgs
bosonwithamassof125GeVisµ = 1. Thenon-fermionicdecaycontributionsexpectedforthe
standardmodelHiggsbosonwithamassof125GeVareconsideredaspartofthebackground
andthereforenotscaledwithµ.
References 7
Acknowledgements
This letter is dedicated to our dear colleague and friend Professor Lorenzo Foa` who passed
away on January 13, 2014. Lorenzo’s contribution to CMS was unique and left an indelible
impactontheexperimentinallitsphases: LorenzowastheCMSprincipalrefereeintheCERN
LHC Committee during the approval process, initiated the participation of many groups dur-
ing the formation of the CMS Collaboration, provided prescient advice and guidance during
hisperiodasCERNDirectorofResearch,wasChairoftheCMSCollaborationBoardduringthe
criticalphaseofconstruction,andwasthefirstChairoftheThesisAwardCommitteeenabling
himtostayclosetoyoungphysicists.
We congratulate our colleagues in the CERN accelerator departments for the excellent perfor-
manceoftheLHCandthankthetechnicalandadministrativestaffsatCERNandatotherCMS
institutesfortheircontributionstothesuccessoftheCMSeffort. Inaddition,wegratefullyac-
knowledge the computing centres and personnel of the Worldwide LHC Computing Grid for
deliveringsoeffectivelythecomputinginfrastructureessentialtoouranalyses. Finally,weac-
knowledge the enduring support for the construction and operation of the LHC and the CMS
detector provided by the following funding agencies: BMWF and FWF (Austria); FNRS and
FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS,
MoST,andNSFC(China);COLCIENCIAS(Colombia);MSESandCSF(Croatia);RPF(Cyprus);
MoER,SF0690030s09andERDF(Estonia);AcademyofFinland,MEC,andHIP(Finland);CEA
andCNRS/IN2P3(France);BMBF,DFG,andHGF(Germany);GSRT(Greece);OTKAandNIH
(Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU (Re-
public of Korea); LAS (Lithuania); MOE and UM (Malaysia); CINVESTAV, CONACYT, SEP,
and UASLP-FAI (Mexico); MBIE (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland);
FCT (Portugal); JINR (Dubna); MON, RosAtom, RAS and RFBR (Russia); MESTD (Serbia);
SEIDI and CPAN (Spain); Swiss Funding Agencies (Switzerland); NSC (Taipei); ThEPCenter,
IPST, STAR and NSTDA (Thailand); TUBITAK and TAEK (Turkey); NASU (Ukraine); STFC
(UnitedKingdom);DOEandNSF(USA).
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