Article
Reference
Search for Doubly Charged Higgs Bosons Decaying to Dileptons in pp Collisions at s√=1.96 TeV
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
e present the results of a search for doubly charged Higgs bosons (H±±) decaying to dileptons (ll′) using ≈240 pb−1 of pp¯ collision data collected by the CDF II experiment at the Fermilab Tevatron. In our search region, given by same-sign ll′ mass mll′>80 GeV/c2 (100 GeV/c2 for ee channel), we observe no evidence for H±± production. We set limits on σ(pp¯→H++H−−→l+l'+l−l'−) as a function of the mass of the H±± and the chirality of its couplings. Assuming exclusive same-sign dilepton decays, we derive lower mass limits on H±±L of 133, 136, and 115 GeV/c2 in the ee, μμ, and eμ channels, respectively, and a lower mass limit of 113 GeV/c2 on H±±R in the μμ channel, all at the 95% confidence level.
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Search for Doubly Charged Higgs Bosons Decaying to Dileptons in pp Collisions at s√=1.96 TeV. Physical Review Letters , 2004, vol. 93, no. 22, p. 221802
DOI : 10.1103/PhysRevLett.93.221802
Available at:
http://archive-ouverte.unige.ch/unige:38091
Disclaimer: layout of this document may differ from the published version.
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Search for Doubly Charged Higgs Bosons Decaying to Dileptons in p p Collisions at
p s
1:96 TeV
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0031-9007=04=93(22)=221802(7)$22.50 221802-1 2004 The American Physical Society
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(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
5Brandeis University, Waltham, Massachusetts 02254, USA
6University of California–Davis, Davis, California 95616, USA
7University of California–Los Angeles, Los Angeles, California 90024, USA
8University of California–San Diego, La Jolla, California 92093, USA
9University of California–Santa Barbara, Santa Barbara, California 93106, USA
10Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
11Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
12Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 , USA
13Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
14Duke University, Durham, North Carolina 27708
15Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
16University of Florida, Gainesville, Florida 32611, USA
17Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
18University of Geneva, CH-1211 Geneva 4, Switzerland
19Glasgow University, Glasgow G12 8QQ, United Kingdom
20Harvard University, Cambridge, Massachusetts 02138, USA
21The Helsinki Group, Helsinki Institute of Physics; and Division of High Energy Physics, Department of Physical Sciences, University of Helsinki, FIN-00044, Helsinki, Finland
22Hiroshima University, Higashi-Hiroshima 724, Japan 221802-2
23University of Illinois, Urbana, Illinois 61801, USA
24The Johns Hopkins University, Baltimore, Maryland 21218, USA
25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
27Center for High Energy Physics, Kyungpook National University, Taegu 702-701 Korea;
Seoul National University, Seoul 151-742 Korea;
and SungKyunKwan University, Suwon 440-746 Korea
28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
29University of Liverpool, Liverpool L69 7ZE, United Kingdom
30University College London, London WC1E 6BT, United Kingdom
31Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
32Institute of Particle Physics, McGill University, Montre´al, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
33University of Michigan, Ann Arbor, Michigan 48109, USA
34Michigan State University, East Lansing, Michigan 48824, USA
35Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
36University of New Mexico, Albuquerque, New Mexico 87131 , USA
37Northwestern University, Evanston, Illinois 60208, USA
38The Ohio State University, Columbus, Ohio 43210, USA
39Okayama University, Okayama 700-8530, Japan
40Osaka City University, Osaka 588, Japan
41University of Oxford, Oxford OX1 3RH, United Kingdom
42Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, University of Padova, I-35131 Padova, Italy
43University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
44Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy
45University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
46Purdue University, West Lafayette, Indiana 47907, USA
47University of Rochester, Rochester, New York 14627, USA
48The Rockefeller University, New York, New York 10021, USA
49Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University di Roma ‘‘La Sapienza,’’ I-00185 Roma, Italy
50Rutgers University, Piscataway, New Jersey 08855, USA
51Texas A&M University, College Station, Texas 77843, USA
52Texas Tech University, Lubbock, Texas 79409, USA
53Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
54University of Tsukuba, Tsukuba, Ibaraki 305, Japan
55Tufts University, Medford, Massachusetts 02155, USA
56Waseda University, Tokyo 169, Japan
57Wayne State University, Detroit, Michigan 48201, USA
58University of Wisconsin, Madison, Wisconsin 53706, USA
59Yale University, New Haven, Connecticut 06520, USA (Received 25 June 2004; published 23 November 2004)
We present the results of a search for doubly charged Higgs bosons (H) decaying to dileptons (ll0) using240 pb1ofppcollision data collected by the CDF II experiment at the Fermilab Tevatron. In our search region, given by same-sign ll0 massmll0>80 GeV=c2 (100 GeV=c2 foreechannel), we observe no evidence for H production. We set limits on pp!H H!l l0 ll0 as a function of the mass of theH and the chirality of its couplings. Assuming exclusive same-sign dilepton decays, we derive lower mass limits onHL of 133, 136, and115 GeV=c2in theee,, and echannels, respectively, and a lower mass limit of113 GeV=c2onHR in thechannel, all at the 95% confidence level.
DOI: 10.1103/PhysRevLett.93.221802 PACS numbers: 14.80.Cp, 12.60.Cn, 12.60.Fr, 13.85.Rm
The standard model (SM) gives a good description of the known fundamental particles, using the SU3C SU2LU1Y gauge group to describe their nongravi- tational interactions. The SU2LU1Y electroweak gauge symmetry is broken toU1EMby the Higgs mecha- nism, but a Higgs boson has yet to be observed. In addition to the SM SU2L Higgs doublet, a number of models [1– 3] predict new Higgs doublets or triplets con- taining doubly charged Higgs bosons (H). For ex-
ample, the left-right symmetric model [2], predicated on a right-handed version of the weak force SU2R, requires a Higgs triplet. The model predicts light neutrino masses by the seesaw mechanism [4], consistent with recent data on neutrino oscillations [5]. Furthermore, the left-right symmetric model suggests light [O100 GeV=c2] doubly charged Higgs particles if su- persymmetry is a property of nature [3] and is therefore of interest for direct searches at high-energy colliders.
H bosons couple directly to leptons, photons,Wand Z bosons, and singly charged Higgs bosons (H). The HL and HR bosons, respectively, couple to left- and right-handed particles and may have different fermionic couplings. Their coupling to a pair of W bosons is ex- perimentally constrained to be small due to the small observed value ofjEW1j[6], resulting in a negligible cross section for the processpp !W !WH.H production would be dominated by the reaction pp ! Z=!H H, whose cross section is independent of theH fermionic couplings at tree level.
TheH decays predominantly to charged leptons if mH<2mHandmHmH< mW[7]. The leptonic decays conserve the quantum numberBL, whereBis the baryon number andLis the lepton number. TheH couplingshll0 to electrons and muons are experimentally constrained by the absence of H production ine e collisions (hee<0:05) [8], and the nonobservation of the decays !3e (heehe<3:2107) and !e (hhe<2106) [9]. The experimental constraints on the couplings (quoted here formH 100 GeV=c2) weaken with increasingH mass. The h coupling is probed by measurements of the anomalous magnetic mo- ment of the muong2; the previous limith<0:25 [9] has not been reanalyzed using the most recent g 2 measurement [10].
Direct searches by the OPAL, L3, and DELPHI Collaborations in e e collisions [11] have excluded H bosons below masses of about100 GeV=c2, assum- ing exclusiveH decay to a given dilepton channel. A recent search by the D0 Collaboration in thechannel [12] has excludedHL below a mass of118 GeV=c2. In this Letter, we describe a search for doubly charged resonances in the same-sign ee, e, and channels, using 240 pb1 [13] of data collected at
ps 1:96 TeVby the CDF (Collider Detector at Fermilab) II experiment at the Fermilab Tevatron. We present our results using theH production model [4] and set the world’s highest mass limits for a range of couplings toe and. We probe the range 105< hll0<0:5, which cor- responds to narrow resonances that decay promptly (c <
10m, whereis the lifetime).
The CDF II detector [14] consists of an inner tracking detector, a lead (iron) scintillator sampling calorimeter for measuring electromagnetic (hadronic) showers, and outer drift chambers for muon identification. The inner detector includes a high-resolution wire chamber [the central outer tracker (COT) [15] ] which, along with the central calorimeter and muon system, covers the pseudor- apidity intervaljj<1[16].
Our strategy is to search for one of the pair-produced H bosons to maximize the sensitivity and to permit detection of any singly produced doubly charged reso- nance. The event triggers can be classified by the require- ments of (i) two energy clusters withET>18 GeVin the electromagnetic calorimeter (2EM), (ii) a central electro-
magnetic cluster with ET>18 GeVand matching track pT>9 GeV=c (1EM), or (iii) a COT track with pT>
18 GeV=cwith an associated track segment (‘‘stub’’) in the muon detectors.
The same-sign eesample is selected primarily using the 2EM trigger. In the off-line analysis, we require two same-sign central electrons with calorimeter ET>
30 GeV and COT track pT>10 GeV=c. Electrons are identified using the ratio of calorimeter energy (E) to track momentum (p) pcE <4, longitudinal and lateral shower profiles, track-cluster matching, calorimeter iso- lation energy in a surrounding cone, and photon- conversion identification using the tracker. The same- signee sample corresponds to an integrated luminosity of 23513 pb1. The luminosity is determined by measuring the rate of inelastic collisions, and the uncer- tainty has equal contributions from the uncertainties on the inelastic cross section and on the acceptance of the luminosity counters.
The same-signsample is selected using the single- muon trigger, with a consistent off-line requirement of a matching stub. We select tracks withpT>25 GeV=cthat are minimum ionizing, i.e., have small electromagnetic and hadronic energy depositions in the calorimeters. The cosmic-ray muon background is suppressed by requiring the muons to originate from the beam line, to be coinci- dent in time with each other and with app collision, and to be consistent with a pair of outgoing particles [17].
Track-quality requirements and calorimeter isolation suppress hadronic-jet backgrounds. The integrated lumi- nosity of the same-signsample is24214pb1.
The same-signesample is selected mainly using the 1EM trigger. We require a central electron and a track matched to a muon stub. The stub requirement signifi- cantly reduces background, but also reduces the fiducial acceptance of H!e relative to the and ee samples. The integrated luminosity of the same-signe sample is24014 pb1. All electron and muon tracks are constrained to the transverse position of the beam to improve their momentum resolution.
We calculate trigger efficiencies using separate un- biased triggers, and the tracking and lepton-identification efficiencies usingZ!ee=events. We obtain96:6 0:4% and100:00 0:000:02% as the efficiencies of the 1EM and 2EM triggers, respectively. The muon trigger effi- ciencies, including the matching-stub requirements, are 77:11:3% and 93:90:8% for jj<0:6 and 0:6<jj<1, respectively, each corresponding to a separate detector subsystem. The tracking efficiency is high (>99%) for isolated particles within the COT fidu- cial volume. The lepton-identification efficiencies are 92:70:3% and 90:80:2% for electrons and muons, respectively. The corresponding efficiencies measured in simulated [18] Z events are 89:30:1%
and 91:30:1%. The simulated H detection effi- ciency is corrected by the ratio of data to simulated Z boson efficiencies.
221802-4
The potential backgrounds from SM processes are (i) hadrons that decay to leptons or are misidentified as such, (ii) leptonic decays of W bosons, produced in association with hadronic jet(s) (W jet), (iii)Z= de- cays (Drell-Yan), where the same-sign track comes from a photon conversion, (iv)WZproduction, where both theW andZdecay leptonically, and (v) cosmic rays.
The hadronic background is estimated using lepton- triggered events with two same-sign lepton candidates [21], each failing the identification requirements (‘‘fail- ing lepton candidate’’). The ratio of the number of lepton candidates passing to the number failing the requirements (the ‘‘pass-fail ratio’’) is measured using jet data samples.
These samples are selected either usingET>100 GeVor ET>20 GeVjet triggers, or using single-lepton triggers and excluding leptonic W and Z decays. The pass-fail ratio isO0:05, with a systematic uncertainty of80%
arising from its sample dependence. It is used to apply a weight to each candidate lepton (as a function ofET) in events with two failing lepton candidates to obtain the dilepton mass distribution.
TheW jet background is determined by applying the pass-fail ratio as a weight toW data events which have a second failing lepton and 25< E6 T<60 GeV. The ex- pected misidentified-W contribution (from jets) is sub- tracted to prevent double counting. We use simulated [18]
W jet events to correct for the acceptance of the E6 T requirement. Background fromWproduction, where the photon converts to ane epair, is implicitly included in this estimate. It is studied explicitly using the simulation and found to be negligible.
Background fromZ=!e eoccurs when one elec- tron radiates a photon which subsequently converts to an e e pair. When a same-sign conversion electron has higher momentum than the prompt electron and is asso- ciated with the cluster, the event is reconstructed with two same-sign electrons. The mass dependence is obtained from simulated [18] Drell-Yan events. The simulated sample is normalized using the number of same-sign candidates in the Z mass region (80 GeV=c2< mee<
100 GeV=c2), after subtracting jet and W jet contributions.
Background from WZ!lll production is estimated using simulation [18]. We use the production cross section of 4.0 pb [22] and apply the trigger, tracking, and lepton- identification efficiencies to the events that pass the kine- matic and geometric selection.
The cosmic-ray background is estimated using COT timing information. We use an independently identified sample of cosmic rays to estimate the residual contribu- tion surviving the timing requirements made in the analysis. The expected cosmic-ray background is found to be0:020:02events, which we take to be negligible.
Figure 1 shows the total background and the data as a function ofmll0for each sample. The predominantly back- to-back lepton topologies, the kinematic thresholds, and
0 50 100 150 200 250
10-1
1 10
0 50 100 150 200 250
10-1
1 10
Same-sign ee
2) Mass (GeV/c )2 Number of Events/(10 GeV/c
Data Drell-Yan Jets W+Jets WZ
0 50 100 150 200 250
10-2
10-1
0 50 100 150 200 250
10-2
10-1
µ µ Same-sign
2) Mass (GeV/c )2 Number of Events/(10 GeV/c
Jets W+Jets WZ
0 50 100 150 200 250
10-2
10-1
0 50 100 150 200 250
10-2
10-1
µ Same-sign e
2) Mass (GeV/c )2 Number of Events/(10 GeV/c
Jets W+Jets WZ
FIG. 1. The same-sign dilepton mass distributions of the ee data and the cumulative SM contributions to theee(top), (middle), ande(bottom) samples. The solid line is the overall sum of the indicated areas. No same-signoreevents are observed.
TABLE I. Integrated backgrounds in the low-mass (<80 GeV=c2) and high-mass (100–300 GeV=c2 for ee, 80–300 GeV=c2forande) regions.
Background Low-mass region High-mass region Z=!ee 0:460:13 0:370:11 Jets!ee 0:47 0:230:19 0:62 0:710:44 W jet!ee 0:140:08 0:360:21 WZ!ee 0:070:02 0:110:03
Total ee 1:10:4 1:5 0:90:6
Jets! 0:30 0:240:16 0:19 0:350:17 W jet! 0:320:22 0:400:27
WZ! 0:210:04 0:190:03
Total 0:80:4 0:8 0:50:4
Jets!e 0:090:05 0:060:05
W jet!e 0:22 0:240:15 0:250:17 WZ!e 0:120:02 0:120:03
Total e 0:40:2 0:40:2
the typical lepton pT from W or Z decays lead to the observed peaked shapes of the background distributions.
The search is performed in the region of mll0 >
80 GeV=c2 for the and e samples, and in the re- gion of mee>100 GeV=c2 for theeesample. The low- mass regions (mll0<80 GeV=c2) are used to check our background predictions. Table I summarizes the total background predictions. We estimate1:10:4(ee),0:8 0:4 (), and 0:40:2 (e) events in the low-mass regions and observe one ee event (mee70 GeV=c2) and no or e events. As an additional check, we compare the predicted and observed backgrounds for same-sign dilepton events with one failing lepton candi- date andE6 T <15 GeV. The expectations of5421(ee), 7:63:1(), and2:40:8(e) events are consistent with the observed numbers of 63 (ee), 8 (), and 2 (e) events.
The same-sign dilepton mass resolution is3:5% of the mass. The intrinsic H width is equal to P
l;l0h2ll0mH=8 [6] and contributes negligibly to the reconstructed mass if P
l;l0h2ll0<0:5. We define search windows of10%of a givenH mass, corresponding to a3 window. We predict the acceptances as a func- tion ofHmass using the simulation [18], including the efficiency scale factors. The acceptance systematic uncer- tainty is dominated by the parton distribution function uncertainty, which we estimate to be 4% using the Martin-Roberts-Stirling-Thorne prescription [23]. In the mass range of interest, the acceptances are34%for the eeandchannels and18%for theechannel.
No events are found in the high-mass regions. This null result yields a 95% C.L. upper limit on the cross section as a function ofH mass (Fig. 2). We calculate the limit using a Bayesian method [24] with a flat prior for the signal and Gaussian priors for background and acceptance uncertainties. Through comparison with the theoretical cross sections [25], we obtain mass limits of 133, 136, and 115 GeV=c2, for exclusiveHL decays toee,, ande, respectively, and113 GeV=c2 for exclusiveHR
decays to . Figure 3 shows these results in the mass- coupling plane, along with the current world limits.
In summary, we have performed an inclusive search for doubly charged resonances in same-signeedata with mee>100 GeV=c2, and same-signandedata with mll0>80 GeV=c2. We have found no evidence for new doubly charged resonances and have significantly ex- tended the existing mass limits on doubly charged Higgs bosons decaying exclusively to ee (mH
L >
133 GeV=c2), (mH
L >136 GeV=c2 and mH
R >
113 GeV=c2), ore(mH
L >115 GeV=c2) final states.
We thank M. Mu¨hlleitner and M. Spira for calculat- ing the next-to-leading order H production cross section. We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions. This work was supported by the U.S.
Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, U.K.; the Russian Foundation for Basic Research; the Comision Interministerial de Ciencia y Tecnologia, Spain; this work was supported in part by the European Community’s Human Potential Programme under Contract No. HPRN-CT-20002, Probe for New Physics;
and this work was supported by Research Fund of Istanbul University Project No. 1755/21122001.
80 90 100 110 120 130 140 150
0 0.02 0.04 0.06 0.08 0.1 0.12
80 90 100 110 120 130 140 150
0 0.02 0.04 0.06 0.08 0.1
0.12 ee
Theory (R) Theory (L) µ
e
µ µ
2) Mass (GeV/c
±
H±
BR (pb)×Cross section
FIG. 2 (color). Experimental limits on cross section branching ratio (BR) at 95% C.L. as a function ofH mass (solid curves). Dotted curves show the theoretical next-to- leading order total cross sections [25] forHLandHR.
90 100 110 120 130 140 150 160 170 10-5
10-4
10-3
10-2
10-1
90 100 110 120 130 140 150 160 170 10-5
10-4
10-3
10-2
10-1
CDF:
µ µ
L→
±
H±
→ ee
L±
H±
µ
→ e
L±
H±
µ µ
R±→ H±
2) Mass (GeV/c
±
H±
OPAL Exclusion Single
Production
→ ee
±
H±
Coupling (h L3, OPAL, DELPHI ll’→±± H µµ→L±± HOD
FIG. 3 (color). The doubly charged Higgs lower mass limits versus lepton coupling (hll0) from this analysis, assuming ex- clusive decay to a given dilepton pair. Our limits are valid for hll0>105. Previous limits [8,11,12] are also shown.
221802-6
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