Table Of ContentSearch for supersymmetry in hadronic final states with M
T2
Hannsjo¨rgWeber1,aonbehalfoftheCMScollaboration
1InstituteforParticlePhysics,ETHZu¨rich,8093Zurich,Switzerland
Abstract. We present the results of a search for physics beyond the Standard Model (BSM) using data of
1.1 fb−1 integrated luminosity collected by the CMS experiment at the LHC. Fully hadronic final states were
selected based on the “stransverse” mass variable M and interpreted in various models of supersymmetry
T2
(SUSY).TwocomplementaryanalyseswereperformedtargetingdifferentareasoftheSUSYphasespace.All
2
backgrounds were estimated using both simulation and data-driven methods. As no excess of events over the
1
expectedbackgroundwasobservedexclusionlimitswerederived.
0
2
n
1 Introduction described in [5], Sect. 13.4. As seeds (inital axes) the di-
a
rection of the two (massless) jets are chosen which have
J
Wedescribeasearch[1]forphysicsbeyondtheStandard thelargestinvariantmass.Wethenassociateajetk tothe
3
Model in pp collisions collected by the Compact Muon pseudojetirather jiftheLunddistanceisminimal:
2
Solenoid(CMS)detector[2]attheLargeHadronCollider
] (LHC)atacentre-of-massenergyof7TeV.Theresultsare (E −pcosθ ) Ei ≤(E −p cosθ ) Ej .
x basedonadatasampleof1.1fb−1ofintegratedluminosity i i ik (E +E )2 j j jk (E +E )2
i k j k
e
collectedin2011.Weusethe“stransversemass”variable
-
p M [3] to select new physics candidates out of fully ha-
T2
e dronicevents.Wedivideoursearchintotwochannels:one 2.1 Advantagesof M
T2
h
targetshighsquarkandgluinomasseswithahighM cut,
[ T2
the other heavy squarks and light gluinos with a medium Inordertogainabetterunderstandingofthebehaviourof
1 MT2cutbutincludingab-tagandhighjetmultiplicities. MT2 wetakethecasewherewesetallmassestozeroand
v Inthefollowingwedescribethepropertiesof M insec. assume no initial-state radiation (ISR) or upstream trans-
T2
9 2, our analyis strategy and event selection in sec. 3, and versemomentum1. InthissimplecaseMT2becomes
5 thebackgroundestimationinsec.4.Insec.5westatethe
46 resultsofoursearch,anddrawaconclusioninsec.6. (MT2)2 =2pvTis(1)pvTis(2)(1+cosφ12), (2)
.
1 where pvis(i) is the transverse momentum of pseudojet i,
T
20 2 The search variable MT2 andφ12 theanglebetweenthetwopseudojetsinthetrans-
verse plane. It can be observed that for symmetric events
1
: The variable MT2 was introduced to measure the mass of (pvTis(1) = pvTis(2))withlargeacoplanarity MT2 behaveslike
v primarypair-producedparticleswherebothparticlesdecay themissingtransversemomentum(MET).ThusSUSYwith
Xi intodetectedandundetectedparticles(e.g.thelightestsu- expected large MET will accumulate in the high MT2 re-
persymmetricparticle(LSP)).Itisageneralizationofthe gion. However, back-to-back systems or balanced events
r
a transverse mass mT in case of two identical decay chains willpopulatethe regionwithsmall MT2.Thus MT2 isro-
eachcontainingunobservedparticles.Thevariable M is bustagainstQCDjetmismeasurements:Mismeasurements
T2
definedas alongoneofthepseudojetsresultsinMT2 ≈0GeV,while
forasymmetricmismeasurementsstillM <MET.
(cid:104) (cid:16) (cid:17)(cid:105) T2
M (m )= min max m(1),m(2) , (1)
T2 χ T T
pχ(1)+pχ(2)=pmiss
T T T
3 Analysis Strategy and Event Selection
wherem isthetransversemassofthevisiblesystemand
T
thecorrespondingLSPχofthedecayingsparticle:
Inthisanalysiswehaveestablishedtwosearchchannelsin
(cid:113) (cid:16) (cid:17) ordertobesensitivetodifferentregionsintheSUSYphase
m(Ti) = (mvis(i))2+m2χ+2 ETvis(i)ETχ(i)−pTvis(i)·pTχ(i) . space.Oneapproach,theHighMT2analysis,targetsevents
resultingfromheavysparticleproductionwhichischarac-
In this analysis the stransverse mass MT2 is not used for terizedbylargeMETand MT2.Thesecondapproach,the
massmeasurementsbutratherasadiscoveryvariable[4]. LowM analysis,isdesignedtobesensitivetotheregion
T2
In order to associate all visible decay products to the wheresquarksareheavyandgluinosrelativelylight.Here
decaychainsofthetwosparticlesweclusterthejetsofan
eventintotwo“pseudojets”usingahemispherealgorithm 1 Upstreamtransversemomentumisthetransversemomentum
whichisnotclusteredtothepseudojets(e.g.jetsoutsideofac-
a e-mail:[email protected] ceptance).
EPJWebofConferences
gluino-gluino production is dominant, the gluinos giving 4.2 Z →ννbackgroundestimation
rise to three-body decays with small MET. Also, as stops
andsbottomsareexpectedtoberelativelylight,theseevents Z →ννisanirreduciblebackgroundbecausetheproduced
canbeenrichedwithb-quarks.Thusthetwostrategiesre- neutrinosleavethedetectorunmeasuredandthusgenerate
quiretwodifferentsetsofselectioncutsstatedintable1. realMET.ThenumberofZ → νν+Jetseventspassingthe
eventselectioncanbeestimatedfromW →µν+Jetsvia
Table1.Eventselectioncutswhicharespecifictotheirstrategies. 1
N (est)=W(µν)· ·R (4)
Zνν (cid:15) (cid:15) ZW
HighM LowM acc reco/iso
T2 T2
atleast3jets2 atleast4jets
whereW(µν)isthenumberofW →µνeventspassingthe
atleast1b-tag
eventselectionwithadditionallyrequiringonemuon,R
H >600GeV3 H >650GeV ZW
T T istheratioofZ →ννeventstoW →µνevents,(cid:15) isthe
M >400GeV M >150GeV acc
T2 T2 acceptance,and(cid:15) isthecombinedreconstructionand
reco/iso
isolation efficiency. In order to reduce the tt¯background
in the W → µν selection a b-tag veto has been applied,
Besidesthischannelspecificselectionwealsorequire: whiletheresidualtt¯backgroundhasbeenestimatedfrom
– Lepton(e, µ)vetotoreduceW+Jetsandtt¯background. the b-tagged region. Furthermore (cid:15)reco/iso has been calcu-
– min∆φ(MET, anyjet)>0.3toreducefurtherQCD. latedfromTag&ProbestudiesonZ →lleventswhilethe
– |MHT−MET|<70GeVtominimizetheinfluenceof acceptanceandtheratioRZW aretakenfromMC.
unclusteredenergy(e.g.ISR)totheM shape4.
T2
– MET tail cleaning cuts (e.g. noise filters) to filter out
eventswithunphysicalMET. 4.3 W andTopbackgroundestimation
For the selection of data we require the data to pass H
T The background due to W and Top has two sources: Ei-
triggerpaths.
ther a lepton (e, µ) from a W has been unobserved due
to acceptance cuts, or has been “lost” due to failing ei-
4 Background Estimation Strategy theridentification,isolation,orreconstructioncriteria.The
other source are W decays into neutrinos and taus which
Foreachtypeofbackgrounddata-drivenestimationmeth- decayhadronically.
ods have been designed: QCD is estimated from the bulk
of the M distribution as described in sec. 4.1. In order The number of events with a “lost” lepton has been
T2
toreducetheeffectofsignalcontaminationandstatistical estimatedfromthenumberofeventswithoneleptonfound
fluctuations the electroweak and top background is esti- in data. This number is then corrected for the probability
mated from an adjacent control region in M . The pre- toloosealeptonviatheformula
T2
diction is taken from data in the control region scaled by
1−ε
Monte-Carlo (MC) ratio of the event yield in the signal Npassveto =(Nreco−Nbg) e,µ, (5)
region over the yield in the control region. Similarly the e,µ e,µ e,µ εe,µ
uncertaintiesarescaledbyaMCratio.Thecontrolregion
forHigh MT2 isdefinedas200GeV < MT2 < 400GeV, where Ner,eµco is the number of events containing a lepton,
forLowM itisdefinedas100GeV< M <150GeV. Nbg istheexpectedbackgroundfromprocessesotherthan
T2 T2 e,µ
W orTop,andε istheprobabilityforaW →lν(l=e,µ,
e,µ
orτ→e,µ)passingallselectionandreconstructioncuts.
4.1 QCDbackgroundestimation
The number of events with hadronic tau decays are
takenfromsimulationandvalidatedbydata:TheW → lν
TheQCDestimationmethodisbasedonthetwovariables
M and∆φ = min∆φ(MET, anyjet).Thesetwovari- (l=e,µ,τ)kinematicsinsimulationhasbeenvalidatedin
T2 min data with one muon. Furthermore, it has been shown that
ables are strongly correlated, but a factorization method
canbeappliedifthefunctionalformfortheratior(M )= taudecaysarewellmodelledinthesimulation[6]justify-
N(∆φ ≥ 0.3)/N(∆φ ≤ 0.2) is known. From simT2ula- ingtheuseofsimulation.
min min
tionstudiesithasbeenfoundthatfor M > 50GeVthe
T2
ratiofallsexponantially:
5 Results
N(∆φ ≥0.3)
r(M )= min =exp(a−b·M )+c (3)
T2 N(∆φ ≤0.2) T2 The M distributionsfortheHigh M analysisandLow
min T2 T2
M analysis are shown in Figs. 1 and 2, table 2 summa-
This behaviour was confirmed in data. The estimate has T2
rizestheresultsofthetwoanalysisstrategies.Asnoexcess
been performed by a fit from data in the QCD dominated
overbackgroundhasbeenfoundlimitshavebeenset.
regionof50GeV< M <80GeVtoextracttheparame-
T2
tersaandb.Inordertogetalsoparametercfromdataits
valuewasfixedtothevalueoftheratioatM =200GeV
T2
5.1 ExclusionLimits
wheretheratiostillfallsexponentially.
2 Thejetselectionrequiresp >20GeV, |η|<2.4 First,modelindependentlimitsonσ×BRwithinourac-
T
3 H isthescalarsumofalljet-p . ceptancehasbeenderivedbycomputinga95%upperlimit
T T
4 MHTisthenegativevectorialsumofalljet-p . onthenumberofeventsusingaCL formulation[7].These
T s
HadronColliderPhysicsSymposium2011
Events 11111000004523High MT2 Analysis CMS Preliminary, s = 7 TQWZTLdeVo+MaC+,pt jL6eDaje t=ts s1.1 fb 1 2) (GeV/cm1/2456700000000C~Mt = LSPSq~ (P75r0e)tGlaieMMMmnVbTTTi n=222 aOEE10rxxbqy,~pp(s ,1 ...A 0– 0 lli 00imsm) qs~G=( ei(i1 t VtN2s=0s5 , 0L( (gg~~)N7 ONGm((eL11 VL)>T02OO 05e0)00)V))GG,ee LVV =q~(1 5010)G.eV1 CDLLfEEbD0PP -F 1 22~g ~g, ~~q,~cl –~q,–1 t,a tnabn=b3=,5 m, <m0<0
1 300 g~(750)GeV
10 1 200 q~(500)GeV
g~(500)GeV
0 200 400 600 0 500 1000 1500 2000
M m (GeV/c2)
T2 0
Fig. 3. Combined exclusion limit in the mSUGRA/CMSSM
Fig.1.M distributionfortheHighM analysis.TheMCback-
T2 T2 (m , m )planewithtanβ=10.
groundisnormalizedto1.1fb−1.ApossibleSUSYsignal(LM6) 0 1/2
isoverlayed.Dataareshownasdotsontopofthebackground.
Events 111000423Low MT2 Analysis CMS Preliminary, s = 7 TQWZTLdeVo+MaC+,pt jL9eDaje t=ts s1.1 fb 1 (GeV)mLSP114680200000pp00000 → ~g l~gCos,wM ~g=MS →7 TP T2 2reebVl i +mL L=inS1a.P1ry; fmb-1( ~q)>>m(~g) 00000000........122334455555ε× A (GeV)mLSP114680200000pp00000 → ~g l~gCos,wM ~g=σσσMS →7ppp TrrrP oooT2 ddd2re eb===Vl i +σm31L /N×L=3iLn S 1Oσ×a.-P Q1Nrσy;LC fNODmbL-Q-O1(C ~-qQD)C>D>m(~g) 11100-σ1) (pb) (CLper limit on s
10 200 00..015 200 CL up
1 400 600 800 1000 1200 0 400 600 800 1000 1200 10-25%
10 1 m~g (GeV) m~g (GeV) 9
Fig.4.Modelpp→g˜g˜, g˜ →bbχ˜0fortheLowM selectionasa
0 100 200 300 400 T2
MT2 functionofthemassparametersmLSPandmg˜:Signalefficiencies
(left),95%CLupperlimitoncrosssectionofthemodel(right).
Fig.2.M distributionfortheLowM selection.TheMCback-
T2 T2
groundisnormalizedto1.1fb−1.ApossibleSUSYsignal(LM9)
isoverlayed.Dataareshownasdotsontopofthebackground.
6 Conclusion
Weconductedasearchforsupersymmetryinhadronicfi-
Table2.ExpectedyieldinthesignalregionfromStandardModel
nalstatesusingthe M variablecalculatedfrommassless
(SM)backgroundinsimulationandfromdata-drivenbackground T2
pseudojets.Adatasetcontaining1.1fb−1 ofintegratedlu-
predictions,aswellasdatayieldforbothanalysisstrategies. √
minosity in s = 7 TeV pp collisions recorded by the
SM-MC Backgroundprediction Data CMSdetectorduringthe2011LHCrunwasanalyzed.Two
HighMT2 10.6 12.6±1.3(stat)±3.5(syst) 12 complementaryanalyseswereperformedtoprobealarger
LowMT2 14.3 10.6±1.9(stat)±4.8(syst) 19 SUSYphasespace.Inbothanalysesthetailofthe MT2 is
sensitivetoapossibleSUSYsignal.Asnoevidencefora
signalwasfound,wesetupperlimitsonthecrosssection
Table3.Observedandexpectedlimitsonσ×BRwithintheac- times branching ratio within our acceptance. Exclusions
ceptanceotthetwoanalysisstrategies. limitswereestablishedinthemSUGRA/CMSSMparame-
terspace,aswellasinaSimplifiedModeltopology.
σ×BR(pb)
observedlimit expectedlimit
HighM 0.010 0.011
T2 References
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T2
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T2
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efficienciesareshowninfig.4.
