Search for Heavy Stable Charged Particles in pp collisions at sqrt(s)=7 TeV

0.35 MB·English

by  CMS Collaboration

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP/2010-067
2011/01/11
CMS-EXO-10-011
Search for Heavy Stable Charged Particles in pp Collisions
√
at s = 7 TeV
1 The CMS Collaboration∗
1
0
2

n
a
J

9

]
x Abstract
e
-
p
e The result of√a search at the LHC for heavy stable charged particles produced in pp
h collisions at s = 7 TeV is described. The data sample was collected with the CMS
[ detector and corresponds to an integrated luminosity of 3.1 pb−1. Momentum and

1 ionization-energy-loss measurements in the inner tracker detector are used to iden-
v
tifytrackscompatiblewithheavyslow-movingparticles. Additionally,trackspassing
5
4 muon identification requirements are also analyzed for the same signature. In each
6 case, no candidate passes the selection, with an expected background of less than
1
0.1 events. A lower limit at the 95% confidence level on the mass of a stable gluino
.
1 is set at 398 GeV/c2, using a conventional model of nuclear interactions that allows
0
1 charged hadrons containing this particle to reach the muon detectors. A lower limit
1 of 311 GeV/c2 is also set for a stable gluino in a conservative scenario of complete
:
v charge suppression, where any hadron containing this particle becomes neutral be-
i forereachingthemuondetectors.
X
r
a
SubmittedtotheJournalofHighEnergyPhysics
∗SeeAppendixAforthelistofcollaborationmembers

1
1 Introduction
Heavy stable (or quasi-stable) charged particles (HSCPs) appear in various extensions of the
standardmodel(SM)[1–8]. IfthelifetimeofanHSCPproducedattheLargeHadronCollider
(LHC)islongerthanafewnanoseconds,theparticlewilltraveloverdistancesthatarecompa-
(cid:38)
rableorlargerthanthesizeofatypicalparticledetector. Inaddition,iftheHSCPmassis 100
GeV/c2, asignificantfractionoftheseparticleswillhaveavelocity, β ≡ v/c, smallerthan0.9.
TheseHSCPswillbedirectlyobservablethroughthedistinctivesignatureofahighmomentum
(p)particlewithananomalouslylargerateofenergylossthroughionization(dE/dx).
Previous collider searches for HSCPs have often been performed under the assumption that
theseparticlesloseenergyprimarilythroughlow-momentum-transferinteractions,evenifthey
arestronglyinteracting,andarethereforelikelytoreachtheoutermuonsystemsofthedetec-
torsandbeidentifiedasmuons[9–12]. Theinteractionswithmatterexperiencedbyastrongly-
interacting HSCP, which is expected to form a bound state (R-hadron) [13] in the process of
hadronization, can lead to it flipping the sign of its electric charge or becoming neutral. A
recent study [14] on the modeling of nuclear interactions of HSCPs traveling through matter,
favours a scenario of charge suppression. In this model the probability is close to unity for
an R-hadroncontainingagluino,g(thesupersymmetricpartnerofthegluon),orasupersym-
(cid:101)
metric bottom squark, to emerge as a neutral particle after traversing an amount of material
typicalofthedetectorsoperatingatLEP,theTevatron,orLHC.Ifthismodeliscorrect,thema-
jorityoftheseHSCPswouldnotbeobservedinthemuonsystemofatypicalcolliderdetector.
Experimental strategies that do not rely on the muon-like behavior for the HSCPs are there-
(cid:46)
fore of great importance. For instance, searches have been performed for very slow (β 0.4)
R-hadronscontainingagluinobroughttorestinthedetector[15,16].
√
In this article we present a search for HSCPs produced in pp collisions at s = 7 TeV at the
LHC with the Compact Muon Solenoid (CMS) detector [17]. The search is based on the data
sample collected between April and August 2010 corresponding to an integrated luminosity
of 3.06 pb−1. We use triggers requiring: a high-transverse-momentum muon (p > 9 GeV/c);
T
or a dimuon pair (p > 3 GeV/c for each muon); or calorimeter-based missing transverse
T
energy(Emiss > 100GeV),tosearchforHSCPsfailingmuonidentificationoremergingmainly
T
asneutralparticlesaftertraversingthecalorimeters;orahigh-transverse-energyjet(E > 100
T
GeV) to search for HSCPs accompanied by substantial hadronic activity. The analysis makes
useoftwoapproaches. Inafirstselection,referredtoas“tracker-only”,theHSCPcandidatesare
searched for as individual tracks reconstructed in the inner tracker detector with large dE/dx
and p . A second selection, referred to as “tracker-plus-muon”, additionally requires that the
T
track is identified as a muon in the outer muon detector. For both selections, the mass of the
candidateiscalculatedfromthemeasured panddE/dx.
2 The CMS Detector
ThecentralfeatureoftheCMSdetectorisa3.8Tsuperconductingsolenoidof6minternaldi-
ametersurroundingasiliconpixelandstriptracker,acrystalelectromagneticcalorimeter,and
abrass-scintillatorhadroniccalorimeter. Muonsaremeasuredingaseousdetectorsembedded
in the iron return yoke. Centrally produced charged particles are measured in the tracker by
three layers of silicon pixel detectors, followed by ten microstrip layers. At pseudorapidities
(η ≡ −lntan(θ/2), where θ is the polar angle measured with respect to the beam direction)
above ≈ 1.5, particles are tracked in two pixel and twelve strip layers arranged in disks per-
pendicular to the beam axis. In this analysis, the dE/dx measurement is based only on the
2 3 CandidateSelectionandBackgroundEstimation
information from the silicon strip detectors. The dE/dx measurement precision is limited by
thesiliconstripanalogue-to-digitalconverter(ADC)modulesthatarecharacterizedbyamax-
imum number of counts per channel corresponding to about three times the average charge
released by a minimum–ionizing particle (MIP) in 300 µm of silicon. This is the thickness of
themodulesmountedintheinnermostsiliconstripcentrallayers. The p resolutionfortracks
T
measured in the central (forward) region of the silicon tracker is 1% (2%) for p values up to
T
50 GeV/c and degrades to 10% (20%) at p values of 1 TeV/c. The trigger and reconstruction
T
efficienciesforHSCPsinthemuondetectorsarelimitedbytherequirementsonthearrivaltime
oftheparticlesatthemuonsystem. Theserequirementsaffecttheefficiencyfordetectingslow
HSCPs. The dependence of the muon trigger efficiency on the particle velocity (β) is studied
using data and Monte Carlo (MC) simulations and found to decrease, below β = 0.7. The
muon trigger becomes completely inefficient at β = 0.5. A much more detailed description of
theCMSapparatuscanbefoundelsewhere[17].
3 Candidate Selection and Background Estimation
CandidateHSCPsarepre-selectedbyrequiringatrackwith|η| < 2.5, p > 15GeV/c,relative
T
uncertainty on the p less than 15%, and transverse (longitudinal) impact parameter with re-
T
specttothereconstructedprimarycollisionvertexlessthan0.25(2.0)cm. Candidatetracksare
alsorequiredtohaveatleastthreemeasurementsinthesilicon-stripdetector. Forthetracker-
plus-muonselection,weadditionallyrequirethetracktobecompatiblewithtracksegmentsre-
constructedinthemuonsystem. Asanestimatorofthedegreeofcompatibilityoftheobserved
charge measurements with the MIP hypothesis, a modified version of the Smirnov-Cramer-
von Mises [18, 19] discriminant is used (the modification applied to the original form of the
discriminant eliminates the sensitivity to incompatibility with the MIP hypothesis due to low
ionization):
(cid:32) (cid:34) (cid:35)(cid:33)
3 1 ∑N (cid:18) 2i−1(cid:19)2
I = × + P × P − , (1)
as i i
N 12N 2N
i=1
whereNisthenumberofchargemeasurementsinthesilicon-stripdetectors,P istheprobabil-
i
ity for a MIP to produce a charge smaller or equal to the i–th charge measurement for the ob-
servedpathlengthinthedetector,andthesumisoverthetrackmeasurementsorderedinterms
ofincreasing P. Thechargeprobabilitydensityfunctionusedtocalculate P isobtainedusing
i i
tracks with p > 5 GeV/c in events collected with a minimum bias trigger. Non-relativistic
HSCPcandidateswillhavethevalueofthediscriminant I approachingunity. Figure1shows
as
normalized distributions of p and I in data and two MC samples, for candidates passing
T as
the tracker-only pre-selection. The first MC sample contains events from QCD processes. The
second MC sample contains signal events from pair-production of stable g with a mass of 200
(cid:101)
GeV/c2. BothsamplesaregeneratedwiththePYTHIAv6.422[20]MCpackage. Moredetailson
the simulation of the signal sample will be given below. The MC QCD simulations are found
to reproduce the data, and the simulated signal is clearly separated. Because of the limited
numberofavailablesimulatedeventswithlowtransverse-momentumtransfers,theMCQCD
distributionsdisplaybin-to-binvariationsinthesizeofthestatisticalerrors.
Themostprobablevalueoftheparticle dE/dx isdeterminedusingaharmonicestimator I of
h
gradek = −2:
(cid:18) 1 (cid:19)1/k
I = ∑ck , (2)
h N i
i
where c is the charge per unit path length in the detector of the i-th measurement for a given
i
3
CMS s = 7 TeV 3.06 pb -1 CMS s = 7 TeV 3.06 pb -1
c 2
0 GeV/10-1 TracMkeCr -- ~gO n20ly0 cks/0.0 1 TracMkeCr -- ~gO n20ly0
2 MC - QCD a MC - QCD
cks/ Data of tr10-1 Data
action of tra1100--32 fraction 1100--32
fr
10-4 10-4
10-5 10-5
10-6
10-6
10-7
0 500 1000 0 0.2 0.4 0.6 0.8 1
p (GeV/c) I
T as
Figure1: Normalizeddistributionsof p (left)and I (right)indataandtwoMCsamples,for
T as
candidates passing the tracker-only pre-selection. The two MC samples contain events from
QCDprocessesandfrompair-productionofgwithamassof200GeV/c2,respectively.
(cid:101)
track. Inordertoestimatethemass(m)ofhighlyionizingparticles,thefollowingrelationship
between I , p,andmisassumed:
h
m2
I = K +C. (3)
h p2
Equation 3 reproduces the Bethe-Bloch formula [21] with an accuracy of better than 1% in the
range 0.4 < β < 0.9, which corresponds to 1.1 < (dE/dx)/(dE/dx) < 4.0. The empirical
MIP
parameters K and C aredeterminedfromdatausingasampleoflow-momentumprotons, for
whichthefittedvaluesare K = 2.579±0.001MeVcm−1 c2 and C = 2.557±0.001MeVcm−1,
andthemassresolutionis7%. Thereconstructedmassdistributionforkaonsandprotonsisin
verygoodagreementwiththeoneobtainedfromMCfollowingthisprocedure[22]. Formasses
above100GeV/c2,themassresolutionisexpectedtoworsenbecauseofthedeteriorationofthe
momentumresolutionandbecauseofthelimitonthemaximumchargethatcanbemeasured
by the silicon strip tracker ADCs, which also affects the mass scale. For a 300 GeV/c2 HSCP,
themassresolutionis12%andthereconstructedpeakpositionisat265GeV/c2.
The search is performed as a counting experiment. Signal candidates are required to have I
as
and p greater than threshold values and the mass to be in the range of 75 to 2000 GeV/c2,
T
allowing sensitivity to HSCP masses as low as 100 GeV/c2. The I distribution for the pre-
as
selected tracks, and in particular its tail, depends strongly on the number of charge measure-
ments on the track. Thus, to increase the sensitivity of the search, pre-selected tracks are di-
vided into subsamples according to the number of silicon strip measurements. The I (p )
as T
threshold in each subsample is determined by requiring a constant efficiency on data for all
subsamples, when the threshold is applied separately. A method that exploits the absence
of correlation between the p and dE/dx measurements in data is used to estimate the back-
T
groundfromMIPs. Inagivensubsample j,thenumberoftracksthatareexpectedtopassboth
thefinal p and I thresholdssetforthesubsampleisestimatedas D = B C /A ,where A is
T as j j j j j
thenumberoftracksthatfailboththe I and p selectionsand B (C )isthenumberoftracks
as T j j
thatpassonlythe I (p )selection. The B andC tracksarethenusedtoformabinnedproba-
as T j j
bilitydensityfunctionin I (p)forthe D tracks. Finally,usingthemassdetermination(Eq.3),
h j
thefullmassspectrumofthebackgroundinthesignalregion Dispredicted.
4 4 Results
Bycomparingthepredictedandobservednumberoftracksforseveralverylooseselectionsin
a control region of the mass spectrum, corresponding to masses below 75 GeV/c2, the predic-
tionisfoundtounderestimatesystematicallytheobservationby12%(5%)forthetracker-only
(tracker-plus-muon) selection. After correcting the predicted background by this amount, the
remaining background systematic uncertainty is conservatively estimated as twice the r.m.s.
of the prediction-to-observation ratio distribution The resulting uncertainty on the predicted
backgroundis14%(17%).
As significant background rejection can be obtained without a sizable effect on the signal effi-
ciency,thefinalselectionisoptimizedbyrequiringthetotalexpectedbackgroundinthesearch
regiontobe∼ 0.05events. Thislow-backgroundchoiceoptimizesthediscoverypotentialeven
if just a handful of events are observed, and at the same time maintains significant exclusion
sensitivityinthecasethatnoeventsareobserved.
4 Results
In addition to the final “tight” selection, the result of a “loose” selection is reported in Table 1.
The loose selection retains a relatively large number of background candidates and allows us
tocomparethebackgroundpredictionwiththeobserveddata. Figure2showsgoodagreement
between the observed and predicted mass spectrum obtained using the loose selection for the
tracker-plus-muonandtracker-onlycandidates.
CMS s = 7 TeV 3.06 pb -1 CMS s = 7 TeV 3.06 pb -1
2V/c103 Tracker + Muon 2V/c Tracker - Only
e e
G Data-based prediction G Data-based prediction
0 0
ks / 5102 DMaCt a- g~ 400 ks / 5102 DMaCt a- g~ 300
c c
a a
Tr Tr
10
10
1 1
10-1 10-1
0 500 1000 0 500 1000
Mass (GeV/c2) Mass (GeV/c2)
Figure 2: Mass spectrum for the loose selection defined in Table 1 for the tracker-plus-muon
(left) and tracker-only (right) candidates. Shown are: observed spectrum (black dots with
theerrorbars),data-basedpredictedbackgroundspectrum(redtriangles)withitsuncertainty
(green band) and the spectrum predicted by MC for a signal of pair-produced stable g with a
(cid:101)
massof400(left)and300(right)GeV/c2 (bluehistogram).
The results of the search with the final selection are also presented in Table 1. No candidate
HSCPtrackisobservedineitherthetracker-onlyortracker-plus-muonanalysis.
Giventhenullresult,crosssectionupperlimitsatthe95%C.L.aresetontheHSCPproduction
for two benchmark scenarios: direct production of g pairs and ˜t pairs. For a given mass, the
(cid:101) 1
crosssectionforgproductionisexpectedtobemuchlargerthanthatfor˜t productionatboth
(cid:101) 1
theTevatronandtheLHC.Thushighermasslimitscanbesetfortheformeratbothmachines.
5
Table 1: Selections used in the analysis and results of the search. The tracker-plus-muon and
tracker-only selections are labeled as “Mu” and “Tk”, respectively. As explained in the text,
theactual I (p )thresholdsaredeterminedinthevarioussubsamplesbytherequirementofa
as T
constant efficiency for candidate selection, (cid:101) ((cid:101) ). These thresholds, indicated by Imin (pmin),
I pT as T
are therefore reported as a range of values. Expected and observed number of candidates in
thesignalregionarereportedinthe“Expected”and“Observed”rows,respectively. Top: loose
selection. Bottom: tightselection.
LOOSE Mu Tk
(cid:101) 3.2×10−2 1.0×10−2
I
Imin 0.049-0.162 0.007-0.278
as
(cid:101) 1.0×10−1 3.2×10−2
pT
pmin (GeV/c) 34-36 59-62
T
Expected 281±2(stat.)±49(syst.) 426±1(stat.)±62(syst.)
Observed 307 452
TIGHT Mu Tk
(cid:101) 1.0×10−4 1.0×10−4
I
Imin 0.184-0.782 0.186-0.784
as
(cid:101) 1.0×10−3 3.2×10−4
pT
pmin (GeV/c) 115-118 154-210
T
Expected 0.025±0.002(stat.)±0.004(syst.) 0.074±0.002(stat.)±0.011(syst.)
Observed 0 0
However,asthemassofaproducedparticleincreases,theratiooftheproductioncrosssection
at the LHC to that at the Tevatron increases. For g masses in the region of 350 GeV/c2, the
(cid:101)
increaseinrelativecrosssectionoutweighsthedifferenceinintegratedluminositybetweenthe
current Tevatron and LHC data sets, enabling the LHC to set the most sensitive limits on the
searchforg.
(cid:101)
Eventswithpairproductionofgand˜t ,withmassvaluesintherange130-900GeV/c2,aregen-
(cid:101) 1
eratedwith PYTHIA inordertocomputetheefficiencyofourselectiononthesesignals. The˜t1
andg(cid:101)aretreatedasstableinallthesesamplesandtheirhadronizationisperformedbyPYTHIA.
Aparameterrelevanttothegpairproduction,andnottothe˜t pairproduction,isthefraction,
(cid:101) 1
f, of produced g hadronizing into a g-gluon state (R-gluonball). This fraction is an unknown
(cid:101) (cid:101)
parameterofthehadronizationmodelandaffectsthefractionof R-hadronsthatareneutralat
production,whichinturnaffectsthedetectionefficiency. Inthisstudy,resultsareobtainedfor
twodifferentvaluesof f,0.1and0.5,toshowtheeffectofthehadronizationmodeluncertainty
onthesensitivityofthesearch. TheinteractionsoftheHSCPswiththeCMSapparatusandthe
detector response are simulated in detail with the GEANT4 v9.2 [23, 24] toolkit. The R-hadron
strong interactions with matter are modeled as in Ref. [25, 26]. This model, like a number of
others[14,27–29],assumesthattheprobabilityofaninteractionbetweentheheavypartonand
aquarkinthetargetnucleonislowsincethecrosssectionvarieswiththeinversesquareofthe
partonmassaccordingtoperturbativeQCD.Theadoptedmodelchoosesapragmaticapproach
basedonanalogywithobservedlowenergyhadronscattering. However,giventheverylarge
uncertaintiesonthedynamicsunderlyingR-hadroninteractions,anextremelypessimisticsce-
narioofcompletechargesuppression,whereeachnuclearinteractionsufferedbytheR-hadron
causes it to become neutral, is also considered. The tracker-only selection is expected to have
sensitivity even in such a scenario. The total signal efficiency is reported in Table 2 for some
combinations of models and selections. Relatively small differences are found between the
6 4 Results
Table 2: Total signal selection efficiency and cross section upper limits for different combina-
tions of models and selections: pair production of supersymmetric stop and gluinos; tracker-
plus-muon (Mu) and tracker-only (Tk) selections; different fractions, f, of R-gluonball states
producedafterhadronizationandchargesuppression(ch. suppr.) scenario.
gluinomass(GeV/c2) 200 300 400 500 600 900
Theoreticalcrosssection(pb) 606 57.2 8.98 1.87 0.470 0.0130
Mu;f=0.1
Totalefficiency(%) 7.17 10.4 13.1 15.1 14.5 9.18
Expected95%C.L.limit(pb) 15.1 10.4 8.25 7.16 7.47 11.8
Observed95%C.L.limit(pb) 14.5 9.98 7.92 6.88 7.17 11.3
Mu;f=0.5;
Totalefficiency(%) 3.84 5.46 7.03 8.23 8.10 4.98
Expected95%C.L.limit(pb) 28.2 19.8 15.4 13.1 13.3 21.7
Observed95%C.L.limit(pb) 27.1 19.0 14.8 12.6 12.8 20.9
Tk;f=0.1;ch. suppr.
Totalefficiency(%) 0.59 2.44 4.16 6.39 8.60 7.66
Expected95%C.L.limit(pb) 188 45.5 26.7 17.4 12.9 14.5
Observed95%C.L.limit(pb) 176 42.6 25.0 16.2 12.1 13.6
stopmass(GeV/c2) 130 200 300 500 800
Theoreticalcrosssection(pb) 120 13.0 1.31 0.0480 0.00110
Mu;
Totalefficiency(%) 2.99 9.50 14.7 19.6 14.0
Expected95%C.L.limit(pb) 36.1 11.4 7.35 5.52 7.71
Observed95%C.L.limit(pb) 34.7 10.9 7.06 5.30 7.39
Tk;ch. suppr.
Totalefficiency(%) 0.02 1.19 3.55 7.27 7.68
Expected95%C.L.limit(pb) 5540 93.2 31.3 15.3 14.5
Observed95%C.L.limit(pb) 5180 87.2 29.2 14.3 13.5
7
Table3: Sourcesofsystematicerrorsandcorrespondingrelativeuncertainties.
SourceofSystematicError RelativeUncertainty(%)
Theoreticalcrosssection 10-25
Integratedluminosity 11
Triggerefficiency 12
Muonreconstructionefficiency 5
Trackreconstructionefficiency < 5
Momentumscale < 5
Ionizationenergylossscale < 3
Totaluncertaintyonsignalacceptance 15
tracker-plus-muonandtracker-onlyselectionexceptinthechargesuppressionscenario,where
thetracker-plus-muonselectioniscompletelyinefficient.
Thisanalysisisfoundtobecomplementarytothesearchforlong-livedstoppedparticlespre-
sented in [16]. Indeed, for the case of g with f = 0.1 and mass values below 500 GeV/c2, the
(cid:101)
fractionofHSCPsthathave β < 0.4andpassthefinalselectionislessthan0.5%. Thereforethe
twoanalysesexploredifferentrangesofproducedparticlevelocitieswithnooverlap.
The main sources of systematic uncertainty affecting the results presented in the following
are summarized in Table 3. The uncertainty on the signal selection efficiency is estimated to
be 15% for all considered combinations of models and selections. The main source of this
uncertainty is an assumed 10% uncertainty on the jet energy scale [30], which affects both the
jet and Emiss trigger efficiency by about 10%. In a more recent study [31], the estimate of the
T
uncertainty on the jet energy scale has been reduced by a factor of two. However, in this
analysis we have conservatively chosen to retain the earlier estimate of 10%. The uncertainty
onthemuontriggerefficiencyandtheimpactofanimperfectsimulationofthesynchronization
of the muon trigger and readout electronics are studied with data and MC. They result in an
overall uncertainty on the signal selection efficiency of less than 5%. The uncertainty on the
offlinetrackreconstructionefficiency[32,33],trackmomentumscale[34]andionizationenergy
lossscale[22]isalsofoundtoyieldnomorethan5%uncertaintyontheoverallsignalselection
efficiency. Theuncertaintyontheabsolutevalueoftheintegratedluminosityisestimatedtobe
11% [35].
The upper limit on the cross section is computed at 95% C.L. using a Bayesian method with a
flatsignalpriorandalog-normalpriorusedforintegrationoverthenuisanceparameters[18,
19, 21]. In order to obtain a conservative upper limit we set the expected background to zero.
The tracker-plus-muon selection provides better limits than the tracker-only for all scenarios
buttheonewithcompletechargesuppression. Foreachconsideredscenario,thecrosssection
upper limit obtained with the most sensitive selection is reported in Table 2 and Fig. 3, along
withthetheoreticalpredictionsforgand˜t pairproductioncomputedatnext-to-leadingorder
(cid:101) 1
(NLO) + next-to-leading log (NLL) [36–39] using the PROSPINO v2 program [40]. The g(cid:101) theo-
retical predictions refer to the case where the squarks and gluino are degenerate in mass. In
the heavy squark limit these cross sections are about 10% higher. For the case of ˜t , beyond
1
LO, the cross section does not only depend on the ˜t mass, but also, though to a much lesser
1
extent[41],onthegmass,theaveragemassofthefirstandsecondgenerationsquarksandthe
(cid:101)
stop mixingangle. For thisreason, the˜t theoretical predictions reported inTable 2and Fig. 3
1
refer to the SPS1a’ benchmark scenario [42]. All systematic uncertainties discussed above are
includedinthecrosssectionupperlimitsreportedinTable2andFig.3. Fromtheintersection
8 6 Acknowledgements
of the cross section limit curve and the lower edge of the theoretical cross section band we set
a 95% C.L. lower limit of 398 (357) GeV/c2 on the mass of pair-produced g with f = 0.1(0.5),
(cid:101)
using the tracker-plus-muon selection. The analogous limit on the ˜t mass is 202 GeV/c2. In
1
thechargesuppressionscenarioweset,withthetracker-onlyselection,a95%C.L.gmasslimit
(cid:101)
of311GeV/c2 for f = 0.1.
5 Conclusions
In summary, the CMS detector has been used to identify highly ionizing, high-p particles
T
and measure their masses. Two searches have been conducted: a very inclusive and model
independent one that uses highly-ionizing tracks reconstructed in the inner tracker detector,
andanotherrequiringalsothatthesetracksbeidentifiedintheCMSmuonsystem. Ineachcase,
theobserveddistributionofthecandidatemassesisconsistentwiththeexpectedbackground.
Wehavesetlowerlimitsonmassesofstablestronglyinteractingsupersymmetricparticles. For
the case of g with f = 0.1 and ˜t , a lower mass limit of 398 and 202 GeV/c2, respectively, is
(cid:101) 1
setatthe95%C.L.withtheanalysisthatusesmuonidentification. Inapessimisticscenarioof
completechargesuppressiontheabovegmasslimitisreducedto311GeV/c2 andisobtained
(cid:101)
withthetracker-onlyselection. Thelimitspresentedhereonstablegarethemostrestrictiveto
(cid:101)
date.
6 Acknowledgements
We are grateful to Anna Kulesza and Michael Krammer for providing the theoretical produc-
tioncrosssectionsandassociateduncertaintiesatnext-to-leadingorderforpairproductionof
gand˜t . WewishtocongratulateourcolleaguesintheCERNacceleratordepartmentsforthe
(cid:101) 1
excellent performance of the LHC machine. We thank the technical and administrative staff
at CERN and other CMS institutes, and acknowledge support from: FMSR (Austria); FNRS
and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN;
CAS, MoST, and NSFC (China); COLCIENCIAS (Colombia); MSES (Croatia); RPF (Cyprus);
AcademyofSciencesandNICPB(Estonia);AcademyofFinland,ME,andHIP(Finland);CEA
and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and
NKTH(Hungary);DAEandDST(India);IPM(Iran);SFI(Ireland);INFN(Italy);NRFandWCU
(Korea);LAS(Lithuania);CINVESTAV,CONACYT,SEP,andUASLP-FAI(Mexico);PAEC(Pak-
istan);SCSR(Poland);FCT(Portugal);JINR(Armenia,Belarus,Georgia,Ukraine,Uzbekistan);
MSTandMAE(Russia);MSTD(Serbia);MICINNandCPAN(Spain);SwissFundingAgencies
(Switzerland);NSC(Taipei);TUBITAKandTAEK(Turkey);STFC(UnitedKingdom);DOEand
NSF(USA).
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