Search for Light Gravitinos in Events with Photons and Missing Transverse Momentum at HERA

0.16 MB·English

by  H1 Collaboration

DESY 04-227 ISSN 0418-9833
November2004
Search for Light Gravitinos in Events with Photons and
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0 Missing Transverse Momentum at HERA
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H1 Collaboration

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Abstract
e
-
p
e A search for gravitinos produced in e±p collisions is performed using the H1detector
h atHERA.Thedataweretakenatacentre-of-massenergyof319GeVandcorrespondtoan
:
v integrated luminosity of64.3pb−1 for e+pcollisions and 13.5pb−1 fore−pcollisions. If
i
X R-parity isnot conserved, thet-channel exchange ofaselectron can produce aneutralino,
which, inmodels wherethegravitino isthelightest supersymmetric particle, subsequently
r
a
decaysintoaphotonandalightgravitino. Theresultingeventsignature,whichinvolvesan
isolatedphoton,ajetandmissingtransverseenergy, isanalysedforthefirsttimeatHERA.
Nodeviation from the Standard Model isfound. Exclusion limitson the cross section and
on R-parity-violating Yukawacouplings arederived inaGauge Mediated Supersymmetry
Breakingscenario. Theresultsareindependentofthesquarksector. Neutralinosandsuper-
symmetric partners of the left-handed electron with masses up to 112GeV and 164GeV,
respectively, canberuledoutatthe95%confidence levelforR-parity-violating couplings
λ′ equalto1.
To besubmittedtoPhys. Lett. B
A.Aktas10, V. Andreev26, T. Anthonis4, A. Asmone33,A. Babaev25, S. Backovic37, J.Ba¨hr37,
P. Baranov26, E. Barrelet30,W. Bartel10, S. Baumgartner38,J. Becker39, M.Beckingham21,
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R.C.W. Henderson17, H. Henschel37, O. Henshaw3, G. Herrera24, I. Herynek31, R.-D. Heuer11,
M.Hildebrandt34,K.H. Hiller37, P. Ho¨ting2, D. Hoffmann22,R. Horisberger34,
A.Hovhannisyan36, M.Ibbotson21,M. Ismail21,M. Jacquet28, L. Janauschek27, X. Janssen10,
V.Jemanov11, L.Jo¨nsson20,D.P. Johnson4, H. Jung20,10, D. Kant19, M.Kapichine8,
M.Karlsson20, J.Katzy10, N.Keller39,I.R. Kenyon3,C. Kiesling27, M.Klein37,
C. Kleinwort10,T. Klimkovich10, T.Kluge1, G.Knies10,A. Knutsson20, B. Koblitz27,
V.Korbel10, P. Kostka37, R. Koutouev12,A. Kropivnitskaya25,J. Kroseberg39, K.Kru¨ger14,
J.Ku¨ckens10, M.P.J. Landon19,W. Lange37, T.Lasˇtovicˇka37,32, P. Laycock18, A. Lebedev26,
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V.Nagovizin25, K.Nankov10, B. Naroska11,J. Naumann7, Th.Naumann37, P.R. Newman3,
C. Niebuhr10, A. Nikiforov27, D.Nikitin8, G. Nowak6, M.Nozicka32, R. Oganezov36,
B. Olivier3, J.E.Olsson10,D. Ozerov25,A. Paramonov25, C. Pascaud28, G.D. Patel18,
M.Peez29, E.Perez9, A. Perieanu10, A.Petrukhin25,D. Pitzl10,R. Placˇakyte˙27, R. Po¨schl10,
B. Portheault28, B. Povh12, N. Raicevic37,P. Reimer31,B. Reisert27,A. Rimmer18, C. Risler27,
E.Rizvi19, P. Robmann39, B. Roland4, R. Roosen4, A.Rostovtsev25, Z.Rurikova27,
S. Rusakov26,K. Rybicki6,†, D.P.C. Sankey5,E. Sauvan22,S. Scha¨tzel13, J.Scheins10,
F.-P. Schilling10, P. Schleper11, S. Schmidt27, S. Schmitt39, M.Schneider22, L. Schoeffel9,
A.Scho¨ning38, V. Schro¨der10, H.-C. Schultz-Coulon14, C. Schwanenberger10,K. Sedla´k31,
F. Sefkow10, I. Sheviakov26,L.N. Shtarkov26, Y. Sirois29, T.Sloan17,P. Smirnov26,
Y.Soloviev26, D. South10, V. Spaskov8,A. Specka29,H. Spitzer11, R. Stamen10, B. Stella33,
J.Stiewe14,I. Strauch10, U. Straumann39, V. Tchoulakov8, G.Thompson19, P.D. Thompson3,
F. Tomasz14, D. Traynor19,P. Truo¨l39, G. Tsipolitis10,40, I. Tsurin37, J.Turnau6,
E.Tzamariudaki27, A. Uraev25, M. Urban39, A. Usik26, D.Utkin25, S. Valka´r32,
1
A.Valka´rova´32, C. Valle´e22, P. Van Mechelen4, N. Van Remortel4, A.Vargas Trevino7,
Y.Vazdik26, C. Veelken18,A. Vest1, S. Vinokurova10, V.Volchinski36,K. Wacker7,
J.Wagner10, G. Weber11, R. Weber38, D. Wegener7, C. Werner13, N.Werner39, M.Wessels1,
B. Wessling11, G.-G. Winter10, Ch. Wissing7, E.-E. Woehrling3, R. Wolf13, E.Wu¨nsch10,
S. Xella39, W. Yan10,V. Yeganov36, J. Zˇa´cˇek32,J. Za´lesˇa´k31,Z. Zhang28, A. Zhelezov25,
A.Zhokin25, H.Zohrabyan36, andF. Zomer28
1 I. PhysikalischesInstitutder RWTH, Aachen,Germanya
2 III. PhysikalischesInstitutder RWTH, Aachen,Germanya
3 Schoolof Physicsand Astronomy,UniversityofBirmingham,Birmingham,UKb
4 Inter-UniversityInstituteforHighEnergies ULB-VUB, Brussels;UniversiteitAntwerpen,
Antwerpen;Belgiumc
5 RutherfordAppletonLaboratory,Chilton,Didcot,UKb
6 Institutefor NuclearPhysics, Cracow,Polandd
7 Institutfu¨rPhysik, Universita¨tDortmund,Dortmund,Germanya
8 JointInstituteforNuclear Research,Dubna,Russia
9 CEA, DSM/DAPNIA,CE-Saclay,Gif-sur-Yvette, France
10 DESY, Hamburg,Germany
11 Institutfu¨r Experimentalphysik,Universita¨tHamburg,Hamburg,Germanya
12 Max-Planck-Institutfu¨r Kernphysik,Heidelberg,Germany
13 PhysikalischesInstitut,Universita¨tHeidelberg,Heidelberg,Germanya
14 Kirchhoff-Institutfu¨rPhysik,Universita¨tHeidelberg, Heidelberg,Germanya
15 Institutfu¨r experimentelleundAngewandtePhysik, Universita¨tKiel, Kiel, Germany
16 InstituteofExperimentalPhysics,SlovakAcademyof Sciences, Kosˇice, SlovakRepublicf
17 DepartmentofPhysics, UniversityofLancaster,Lancaster,UKb
18 DepartmentofPhysics, UniversityofLiverpool,Liverpool,UKb
19 Queen MaryandWestfieldCollege, London,UKb
20 PhysicsDepartment,Universityof Lund,Lund, Swedeng
21 PhysicsDepartment,Universityof Manchester,Manchester,UKb
22 CPPM, CNRS/IN2P3 - UnivMediterranee,Marseille- France
23 DepartamentodeFisicaAplicada,CINVESTAV, Me´rida,Yucata´n,Me´xicok
24 DepartamentodeFisica,CINVESTAV, Me´xicok
25 InstituteforTheoreticaland ExperimentalPhysics, Moscow,Russial
26 Lebedev PhysicalInstitute,Moscow,Russiae
27 Max-Planck-Institutfu¨r Physik,Mu¨nchen,Germany
28 LAL, Universite´ deParis-Sud,IN2P3-CNRS, Orsay,France
29 LLR, EcolePolytechnique,IN2P3-CNRS, Palaiseau,France
30 LPNHE, Universite´sParisVIandVII, IN2P3-CNRS, Paris,France
31 InstituteofPhysics,Academyof Sciences oftheCzech Republic,Praha,Czech Republice,i
32 FacultyofMathematicsandPhysics,Charles University,Praha,Czech Republice,i
33 DipartimentodiFisicaUniversita` di RomaTreand INFNRoma 3,Roma, Italy
34 PaulScherrer Institut,Villigen,Switzerland
35 FachbereichC, Universita¨tWuppertal,Wuppertal,Germany
36 Yerevan PhysicsInstitute,Yerevan, Armenia
37 DESY, Zeuthen, Germany
38 Institutfu¨r Teilchenphysik,ETH, Zu¨rich,Switzerlandj
2
39 Physik-Institutder Universita¨tZu¨rich,Zu¨rich,Switzerlandj
40 Alsoat PhysicsDepartment,NationalTechnicalUniversity,ZografouCampus, GR-15773
Athens,Greece
41 Alsoat RechenzentrumUniversita¨tWuppertal,Wuppertal,Germany
43 Alsoat UniversityofP.J.Sˇafa´rik,Kosˇice, SlovakRepublic
44 Alsoat CERN, Geneva, Switzerland
† Deceased
a SupportedbytheBundesministeriumfu¨r BildungundForschung,FRG,under contract
numbers05 H11GUA /1,05H1 1PAA/1,05 H11PAB /9,05H1 1PEA/6,05 H11VHA/7 and
05H1 1VHB/5
b SupportedbytheUKParticlePhysicsand AstronomyResearchCouncil, andformerlybythe
UKScienceandEngineeringResearchCouncil
c SupportedbyFNRS-FWO-Vlaanderen,IISN-IIKW andIWT andbyInteruniversityAttraction
PolesProgramme,BelgianScience Policy
d PartiallySupportedbythePolishStateCommitteeforScientificResearch,
SPUB/DESY/P003/DZ118/2003/2005
e SupportedbytheDeutscheForschungsgemeinschaft
f SupportedbyVEGA SRgrant2/4067/24
g SupportedbytheSwedishNaturalScienceResearchCouncil
i SupportedbytheMinistryof Educationof theCzech Republicundertheprojects
INGO-LA116/2000andLN00A006, byGAUKgrantno 173/2000
j SupportedbytheSwissNationalScienceFoundation
k SupportedbyCONACYT, Me´xico, grant400073-F
l PartiallySupportedbyRussianFoundationforBasicResearch, grantno. 00-15-96584
3
1 Introduction
Supersymmetry (SUSY) [1] is an attractive concept which remedies some shortcomings of the
StandardModel(SM).Thisfermion-bosonsymmetryleadstoanextensionoftheparticlespec-
trum by associating to each SM particle a supersymmetric partner, differing in its spin by half
a unit. The masses of the new particles are related to the symmetry breaking mechanism. In
GaugeMediated SupersymmetryBreaking (GMSB) models,new “messenger”fields are intro-
ducedwhichcoupletothesourceofsupersymmetrybreaking. Thebreakingisthentransmitted
to the SM fields and their superpartners by gauge interactions [2]. The gravitino, G˜, is the
lightestsupersymmetricparticle(LSP) and can beas lightas 10−3 eV.
The next-to-lightest supersymmetric particle (NLSP) is generally either the lightest neu-
tralino χ˜0 or a slepton ℓ˜, which decays to the stable gravitino via χ˜0 γG˜ or ℓ˜ ℓG˜. The
1 1 → →
distinguishing event topology involves a high energy photon or lepton and significant missing
energy due to the undetected gravitino. Such topologies have been studied at LEP [3] and the
Tevatron [4,5]. No significant deviation from the SM was found. In these studies R-parity
(R ) was assumed to be conserved. An investigation of R -violating (R/ ) SUSY in a GMSB
p p p
scenario is performed in this analysis. A search for R/ resonant single neutralino production
p
χ˜0 via t-channel selectron exchange, e±q χ˜0q′, is performed in e+p and e−p collisions. It
1 → 1
is assumed that the χ˜0 is the NLSP and that the decay χ˜0 γG˜ occurs with an unobservably
1 1 →
small lifetime and dominates over R/ neutralino decays. Feynman diagrams of the analysed
p
processesaredepictedinFig.1. Theresultingexperimentalsignatureisaphoton,ajetoriginat-
ing from the scattered quark and missing transverse momentum due to the escaping gravitino.
The main SM background arises from radiative charged current (CC) deep inelastic scattering
(DIS)withajet, aphotonanda neutrinointhefinal state.
γ γ
χ˜0 χ˜0
1 1
e+ e−
e˜ e˜
L L
˜ ˜
d uj G u dk G
λ′ λ′
1j1 11k
Figure 1: Dominant diagrams for neutralino production via R/ selectron exchange in e+p and
p
e−p scattering,withsubsequentneutralinodecay intoagravitinoand aphoton.
ResonantsquarkproductioninR/ SUSYhasbeeninvestigatedpreviouslyatHERAinmod-
p
elsinwhichtheLSPiseitheragaugino[6]oralightsquark[7]. Squark massdependentlimits
on various R/ Yukawa couplingshave been derived. In contrast, the process considered in this
p
analysisis completelyindependentofthesquarksector.
Thedatacorrespond toan integratedluminosityof64.3pb−1 fore+p collisionsrecorded in
1999 and 2000 and 13.5pb−1 for e−p collisionsrecorded in 1998 and 1999. The energy of the
4
incomingelectron1 isE = 27.6GeVandtheenergyoftheincomingprotonisE = 920GeV.
e p
Thustheelectron-protoncentre-of-mass energy is319GeV.
2 The Supersymmetric Model
This analysis considers a supersymmetric model where the gravitino is the LSP and in which
R isnotconserved–ascenariowhichhasbeenusede.g. in[8]andhasbeenconsideredbefore
p
in the context of dark matter [9]. R-parity is a discrete multiplicative symmetry which can
be written as R = ( 1)3B+L+2S, where B denotes the baryon number, L the lepton number
p
−
and S the spin of a particle. The most general supersymmetric theory that is renormalisable
and gauge invariant with respect to the SM gauge group [10] contains R/ Yukawa couplings
p
betweenthesupersymmetricpartneroftheleft-handedelectrone˜ ,aleft-handedup-typequark
L
uj and a right-handed down-typeanti-quark d¯k, where j and k denotegeneration indices. The
L R
correspondingpart oftheLagrangianreads
= λ′ e˜ ujd¯k +h.c. (1)
LR/p − 1jk L L R
At HERA, the presence of couplings λ′ and λ′ could lead to neutralino production in e+p
1j1 11k
and e−p collisions, respectively, via t-channel selectron exchange (see Fig. 1). The search
presented here is performed under the simplifying assumption that one of the couplings λ′
1j1
(j = 1,2) or λ′ (k = 1,2,3) dominates2. If the initial state lepton is a positron the dominant
11k
hard scattering process at the large Bjorken x values relevant here involves a down quark from
theproton(seeFig.1,left). Iftheinitialstateleptonisan electronmainlyupquarksareprobed
(seeFig.1,right). ForagivenR/ coupling,theχ˜0productioncrosssectionforaninitialelectron
p 1
is roughly a factor of two larger than that for an initial positron, reflecting the different parton
densities for valence up and down quarks in the proton. Due to the contribution of diagrams
involvingantiquarksintheinitialstate(notshowninFig.1),thecrosssectionforχ˜0 production
1
in e+p (e−p) collisions via a λ′ coupling is larger than that for production via a λ′ (λ′ )
111 121 112
coupling of the same strength. The relative difference amounts to at most 15% (8%) for e+p
(e−p) processes, for low masses of the produced neutralino. The cross sections of the e−p
processesinduced byλ′ and λ′ are thesamewithinafew percent.
112 113
The GMSB model used here is inspired by [11]. While the gaugino mass spectrum and
gauge couplings are derived from this minimal model, the slepton masses are treated as free
parameters. The supersymmetric partner of the left-handed electron can be much lighter than
the supersymmetricpartner of the right-handed one as, for example, in the Hybrid Multi-Scale
Supersymmetric Model HMSSM-I [12]. This allows small mass differences ∆m = m(e˜ )
L
−
m(χ˜0).
1
TheGMSBmodelischaracterised bysixnew parameters inadditiontothoseoftheSM:
Λ, M, N, tanβ, sign(µ) and √F . (2)
1Inthefollowingelectronwillbeusedtorefertobothelectronandpositronunlessexplicitlyotherwisestated.
2The coupling λ′ is not studied here because the production of a top quark together with a neutralino is
131
suppressedduetothehightopquarkmass.
5
The parameter Λ sets the overall mass scale for the SUSY particles, M is the mass of the
messenger particles, N is the number of sets of messenger particles, tanβ is the ratio of the
Higgsvacuumexpectation valuesand sign(µ) is thesign oftheHiggs sectormixingparameter
µ. The intrinsic SUSY breaking scale is √F, which also determines the G˜ mass according to
m 2.5 F/(100 TeV)2eV. Furthermore, √F affects the neutralino decay rate according
toG˜Γ(≃χ˜0 ·γG˜) 1/F2; low values of √F thus suppress the R/ decays of the neutralino. In
the SU1SY→param∼eter space considered here, the branching ratio Bp R(χ˜0 γG˜) exceeds 95%
1 →
if √F lies in the range between the present experimental limit of 221GeV [13] and 1TeV.
Thus,√F isnotvariedbutitisassumedthattheR/ decays oftheneutralinodonotcontribute.
p
In the range considered for √F the neutralino lifetime is short enough to have no effect on
the detection efficiency. The contributions of the heavier neutralinos χ˜0 (i = 2,3,4) to the
i
consideredsignalaresmalland thusneglected.
3 The H1 Detector
In the following the detector components most relevant for this analysis are briefly described.
The main components of the tracking system are the central drift and proportional chambers
whichcoverthepolarangle3range20◦ < θ < 160◦andaforwardtrackdetector(7◦ < θ < 25◦).
The tracking system is surrounded by a finely segmented liquid argon (LAr) calorimeter [14]
which covers the range 4◦ < θ < 154◦ and which has an energy resolution of σ /E
E
≃
12%/pE(GeV) 1% for electrons and σ /E 50%/pE(GeV) 2% for hadrons, as
E
⊕ ≃ ⊕
obtainedintestbeammeasurements[15]. Thetrackingsystemandcalorimetersaresurrounded
by a superconducting solenoid and its iron yoke instrumented with streamer tubes. The lat-
ter are used to detect hadronic showers which extend beyond the LAr and to identify muons.
The luminosity is determined from the rate of Bethe-Heitler events (ep epγ) measured in a
→
luminositymonitor. Adetailed descriptionoftheH1 experimentcan befound in[16].
4 Event Simulation
InordertoestimatetheexpectedSMcontributionstothesignatureunderstudyandtodetermine
thedetection efficiencies for a possibleSUSY signal, completeMonteCarlo (MC) simulations
oftheH1detectorresponseareperformed. ForeachpossibleSMsourceasampleofMCevents
isusedcorrespondingtoaluminosityofmorethan10timesthatofthedata. Forthesimulation
of the charged and neutral current (CC and NC) DIS backgrounds, the DJANGO [17] event
generator is used which includes first order QED radiation as modelled by HERACLES [18].
The parton densities in the proton are taken from the CTEQ5L [19] parameterisation. The
direct and resolved photoproduction of light and heavy quark flavours is generated using the
PYTHIA [20] program. The SM predictions for ep eZX and ep eW±X are calculated
→ →
usingthe leading order generator EPVEC [21] with the next-to-leadingorder QCD corrections
implementedusinga reweightingmethod[22].
3Thepolarangleθismeasuredwithrespecttotheprotonbeamdirection.
6
Thesignaltopologyis simulatedusingtheSUSYGEN generator [23]. The partondensities
are evaluated at the scale of the Mandelstam variable t. Efficiencies are determined by inter-
−
polation between calculations at different points in the parameter space, where the neutralino
mass m(χ˜0) is varied from 50GeV to 140GeV and the selectron mass m(e˜ ) from m(χ˜0) to
1 L 1
200GeV,bothinstepsoftypically15GeV.
˜
± → ˜0 ′ → ′
5 Search for the Process e q χ q γGq
1
5.1 Event Preselection
The process e±q χ˜0q′ γG˜q′ is characterised by missing transverse energy, a jet and an
→ 1 →
electromagneticclusterin the calorimeter. The eventsused in thisanalysis are triggered by the
LAr system with an efficiency of typically 95% for the chosen kinematic region. Background
events not related to ep collisions are suppressed by requiring a primary interaction vertex re-
constructed within 35cm in z of the nominal vertex position, by using topological filters
±
against cosmic and proton-beam related background and by requiring an event time which is
consistentwiththebunchcrossingtime.
Events are selected if the missing transverse momentum determined from the energy de-
posits in the calorimeter is greater than 25GeV. The events are required to contain at least
one hadronic jet in the range 10◦ < θ < 145◦ and an identified photon with a polar angle
jet
θ greater than 10◦, both with transverse momenta greater than 5GeV. Hadronic jets are re-
γ
constructed from energy deposits in the calorimeter using a cone algorithm in the laboratory
frame with a radius p(∆η)2 +(∆φ)2 = 1, where η = lntanθ/2 is the pseudorapidity and
−
φdenotestheazimuthalangle.
PhotonsareidentifiedusingashowershapeanalysisofenergydepositsintheLArcalorime-
ter. For θ > 20◦ an electromagnetic cluster is only accepted as a photon candidate if it is not
γ
associatedwitha charged track inthecentral tracking system. In addition,thephotonmustnot
liewithintheconeofanyreconstructed jet withp > 5GeV.
T,jet
5.2 Systematic Uncertainties
The systematic errors on the SM background expectation are evaluated by considering the fol-
lowinguncertainties.
Theuncertaintyontheelectromagneticenergy scaleofthecalorimetervariesfrom 0.7%
•
to 3%dependingon thecalorimeterregion[24].
For the jet transverse momenta selected in this analysis (typically above 20GeV) the
•
uncertainty onthehadronicenergy scaleis 2%[25].
Theuncertaintyon thetrack reconstructionefficiency is 2%.
•
7
An uncertainty of 10% is attributed to the SM cross sections for CC and NC DIS as
•
implemented in the MC simulation which arises mainly from the parton densities of the
protonat highx.
Themeasurementoftheintegratedluminosityhas aprecisionof1.5%.
•
Furthermore, the followinguncertainties related to the modellingof theSUSY signal are taken
intoaccount.
Thetheoreticaluncertaintyofthesignalcrosssectionduetotheuncertaintyoftheparton
•
densities, which is typically a few percent and does not exceed 7% for e−p scattering or
17%fore+p scatteringanywhereintheparameterspacestudied.
Choosingeithertheinvariantmassofthefinalstateparticlesorthetransversemomentum
•
of the final state quark instead of the square root of the Mandelstam variable t as the
−
hard scale at which the parton distributionsare evaluated yields an additional theoretical
uncertainty ofup to10%at largeselectronand neutralinomasses.
A relative uncertainty of 10% is attributed to the signal detection efficiencies, resulting
•
mainlyfrom theinterpolationbetween theneutralinoand selectronmasses.
All systematic errors are added in quadrature separately for the signal and the background.
Theresultinguncertaintiesare between11%and22%inbothcases.
5.3 Final Selection and Results
Afterthepreselection,describedinSection5.1,12candidateeventsareselectedinthecomplete
e±p data sampleand 11.5 1.5 events are expected from SM background processes, predomi-
±
nantlyfrom radiativeCC DIS(95%). Thedistributionsofthepolarangleθ andthetransverse
γ
momentum p of the photon candidates are shown in Fig. 2 (a) and (b), respectively. Fig. 2
T,γ
(c) shows the transverse momentum p calculated from the hadronic energy deposits in the
T,h
calorimeter. ThesumoftheE p ofallmeasuredparticlesispresentedinFig.2(d). Thedis-
z
−
tributions illustrate the good understanding of the SM processes. For comparison, a simulated
SUSY signalforaχ˜0 massof125GeVisalso shown.
1
To reduce the CC DIS background, p > 15GeV and E p > 15GeV are required for
T,γ z
−
thefinalselection. ThesecutsarealsodepictedinFig.2. Nocandidateeventisfoundinthee+p
data set, to be compared with 1.8 0.2 expected from SM processes. In the e−p data sample,
±
1 candidate event is found while the SM prediction is 1.1 0.2. The SM expectation arises
±
predominantlyfrom CC DIS (90%) with small contributionsfrom NC DIS and the production
of W and Z bosons where the final state electron is misidentified as a photon. With all cuts
applied, the final selection efficiency for the signal ranges between 10% for low and 35% for
high neutralino masses. The largest contribution to the inefficiency arises from the missing
transverseenergy requirement.
Assuming that the massless gravitino is the only non-interacting particle in the event, its
kinematics are reconstructed by exploiting the conservation of transverse momentum and the
8
7 9
s s
t t
n H1 data (a) n 8 (b)
6 l
e e
ev 5 SM MC ev 7 GMSB:mm((χχ~~0))== 112255 GGeeVV
6 1
(arb. norm.)
4 5
3 4
3
2
2
1
1
0 0
0 20 40 60 80 100 120 140 160 0 20 40 60 80 100
θ ( o ) p (GeV)
γ T,γ
7
s s 10
t t
n (c) n (d)
6
e e
v v 8
e 5 e
4 6
3
4
2
2
1
0 0
0 20 40 60 80 100 120 0 10 20 30 40 50 60 70
p (GeV) E-p (GeV)
T,h z
Figure 2: Distributions of the polar angle (a) and transverse momentum (b) of photon candi-
dates, hadronic transverse momentum (c) and the sum of the E p of all measured particles
z
−
(d) after preselection. The complete e±p data set is compared with the SM prediction. The
signal expected for a neutralino with a mass of 125GeV is shown with arbitrary normalisation
(dashedhistogram). Thearrows indicateadditionalcutsappliedonp and E p inthefinal
T,γ z
−
selection.
constraint(E p )+(E p ) = 2E . Thefour-vectorofthisparticleisthenaddedtothatof
− z G˜− z,G˜ e
thephotontoreconstructtheinvariantmassmofthedecayingneutralino. ThedataandtheSM
expectation for this distribution are shown in Fig. 3. From the simulation of the SUSY signal,
also shown in Fig. 3, the mass resolution is determined to be around 10GeV. The candidate
eventhas areconstructed invariantneutralinomassof36 4GeV.
±
6 GMSB Model Interpretations
As no significant deviationfrom theSM is observed, constraintson GMSB modelsat the 95%
confidence level (CL) are derived using a modified frequentist approach based on likelihood
ratios, which takes statistical and systematic uncertainties into account [26]. For a given neu-
tralino mass m(χ˜0), the limits are obtained by counting the number of observed and expected
1
events in a certain mass interval. In the investigated range of 50GeV < m(χ˜0) < 140GeV,
1
9