Article
Reference
Search for Doubly Charged Higgs Bosons with Lepton-Flavor-Violating Decays Involving τ Leptons
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We search for pair production of doubly charged Higgs particles (H±±) followed by decays into electron-tau (eτ) and muon-tau (μτ) pairs using data (350 pb−1) collected from pp collisions at s√=1.96 TeV by the CDF II experiment. We search separately for cases where three or four final-state leptons are detected, and combine results for exclusive decays to left-handed eτ (μτ) pairs. We set an H±± lower mass limit of 114(112) GeV/c2 at the 95% confidence level.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for Doubly Charged Higgs Bosons with Lepton-Flavor-Violating Decays Involving τ Leptons. Physical Review Letters , 2008, vol. 101, no. 12, p. 121801
DOI : 10.1103/PhysRevLett.101.121801
Available at:
http://archive-ouverte.unige.ch/unige:38525
Disclaimer: layout of this document may differ from the published version.
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Search for Doubly Charged Higgs Bosons with Lepton-Flavor-Violating Decays Involving Leptons
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Y. Zheng,8,band S. Zucchelli5 (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
4Baylor University, Waco, Texas 76798, USA
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254, USA
7University of California, Davis, Davis, California 95616, USA
8University of California, Los Angeles, Los Angeles, California 90024, USA
9University of California, San Diego, La Jolla, California 92093, USA
10University of California, Santa Barbara, Santa Barbara, California 93106, USA
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, PA 15213, USA
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637
14Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
16Duke University, Durham, North Carolina 27708, USA
17Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
18University of Florida, Gainesville, Florida 32611, USA
19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
20University of Geneva, CH-1211 Geneva 4, Switzerland
21Glasgow University, Glasgow G12 8QQ, United Kingdom
22Harvard University, Cambridge, Massachusetts 02138, USA
121801-2
23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
24University of Illinois, Urbana, Illinois 61801, USA
25The Johns Hopkins University, Baltimore, Maryland 21218, USA
26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
27Center for High Energy Physics: Kyungpook National University, Daegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea;
Sungkyunkwan University, Suwon 440-746, Korea;
Korea Institute of Science and Technology Information, Daejeon, 305-806, Korea;
Chonnam National University, Gwangju, 500-757, 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
31Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
32Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
33Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
34University of Michigan, Ann Arbor, Michigan 48109, USA
35Michigan State University, East Lansing, Michigan 48824, USA
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
42University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
43LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
44University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
45Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
46University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
47Purdue University, West Lafayette, Indiana 47907, USA
48University of Rochester, Rochester, New York 14627, USA
49The Rockefeller University, New York, New York 10021, USA
50Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
51Rutgers University, Piscataway, New Jersey 08855, USA
52Texas A&M University, College Station, Texas 77843, 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 7 December 2007; published 16 September 2008)
We search for pair production of doubly charged Higgs particles (H) followed by decays into electron-tau (e) and muon-tau () pairs using data (350 pb1) collected from pp collisions at ffiffiffi
ps
¼ 1:96 TeVby the CDF II experiment. We search separately for cases where three or four final-state leptons are detected, and combine results for exclusive decays to left-handede() pairs. We set anHlower mass limit of114ð112ÞGeV=c2at the 95% confidence level.
DOI:10.1103/PhysRevLett.101.121801 PACS numbers: 14.80.Cp, 13.85.Rm
The standard model (SM) Higgs mechanism provides a framework in which particles can acquire mass while preserving local gauge invariance. The complex scalar Higgs doublet of the SM is just one of many viable implementations, and many extensions to the SM contain Higgs triplets [1–3]. For example the left-right symmetric ðSUð2ÞLSUð2ÞRUð1ÞBLÞ extension of the electro-
weak force [2] casts parity violation as a low-energy phenomenon by invoking a right-handed weak interaction broken above the electroweak scale. This model predicts small but nonzero neutrino masses (consistent with recent experiments [4,5]) related to the suppression of the right- handed weak current [2]. Another model with an extended Higgs sector is the Higgs triplet model [3], which predicts a
massive left-handed Majorana neutrino without requiring a right-handed neutrino. An important phenomenological feature of the above models is the prediction of doubly charged Higgs bosons (H) as part of a Higgs triplet.
Doubly charged Higgs bosons couple to Higgs and elec- troweak gauge bosons and either left-handed or right- handed charged leptons (‘), and are, respectively, denoted HL orHR [6].
The only significant production mode at the Fermilab Tevatron is predicted to beqq!=Z!HþþH, and the leptonic decay modes dominate forH in the mass range mðHÞ<ðmðWÞ þmðHÞÞ [7]. Lepton-flavor- violating (LFV) decay modes are allowed, and may be particularly large (e.g., the branching fraction for the mode may be near 1=3) in the Higgs triplet model if the mass hierarchy of the quarks and charged leptons also holds for the neutrino sector [8].
The HL (HR ) is excluded below 99 GeV=c2 (97 GeV=c2) at the 95% C.L. by previous searches at LEP [9], assuming production cross sections according to the left-right symmetric models [2] and 100% branching ratio to any one dilepton decay channel. Recent searches from the Fermilab Tevatron have resulted in 95% C.L.
lower mass limits of 136, 133, and115 GeV=c2 forHL in the,ee, andechannels, respectively, and a lower mass limit of113 GeV=c2for theHRin thechannel [10].
We present the first results from hadron colliders on HþþL HL pair production and subsequent decay through LFV channels involving taus. We use data correspond- ing to an integrated proton-antiproton luminosity of 350 pb1 [11] collected at ffiffiffi
ps
¼1:96 TeV by the CDF II experiment at the Fermilab Tevatron, and set mass limits in the left-right symmetric model [2,7] for exclusive decays in the eand channels. We present limits on the cross section times branching ratio squared, B2, which can be interpreted in the context of various models [7].
CDF II [12,13], a cylindrical detector with concentric layers, has inner silicon strip detectors (SVX) and a wire drift chamber (COT) for tracking inside a solenoidal coil.
The COT provides tracking in the pseudorapidity region
jj&1:3, while the SVX covers the regionjj&1:9. At
radii outside the solenoid coil, sampling electromagnetic and hadronic calorimeters cover the regionjj<3:6with a projective tower geometry. In the central region (jj 1:0), the electromagnetic calorimeter (CEM) has an em- bedded multiwire proportional chamber (CES), with anode wires parallel to the beam direction, and orthogonal cath- ode strips. The CES has 2 cm strip or wire spacing and provides 2 mm spatial resolution of electromagnetic showers. The region 1:1 jj 3:6 is covered by the
‘‘plug’’ electromagnetic (PEM) and hadronic calorimeters.
At the largest radii there are scintillator and drift tube muon detectors in the regionjj<1:5.
We use several sets of selection criteria to characterize lepton candidates. All ‘‘tight’’ leptons must be in the central region, while ‘‘loose’’ leptons satisfy jj<1:3. Tight electrons [14] have tracks in the COT matched to energy clusters in both the CEM and CES. They pass requirements on the electromagnetic to hadronic calorime- ter deposition ratio, the CEM energy to COT track mo- mentum ratio, and a tower-to-tower energy sharing variable. Loose electrons only have tracks matched to CEM or PEM clusters with electromagnetic to hadronic calorimeter deposition ratios consistent with the electron hypothesis. Tight muons [10] are minimally ionizing in the calorimeters and have tracks in the COT that extrapolate to hits in the outer muon detectors. Loose muons are simply isolated tracks, as described below. In order to suppress background from jets misidentified as leptons, an electron or muon is selected to be isolated by requiring that the sum of the transverse momenta of all other tracks in a cone of angle 0.4 rad with respect to the lepton’s direction be less than2 GeV=c.
Identification of hadronically decaying taus (h) is fully described elsewhere [14]. In tau reconstruction, all tracks are assumed to correspond to charged pions, and all track- less CES and CEM clusters are assumed to correspond to 0 mesons. A tighth must have 1 or 3 localized tracks, and can have additional localized 0 candidates. The localization is defined by a variable size ‘‘signal cone’’
(between 3and 10, depending on the tau’s momentum) around the highest pT track associated with the h. The region between the signal cone and a larger 30 cone serves as an isolation annulus in which the summed pT of all tracks must be less than2 GeV=cand the summedET
of all 0 mesons must be less than 0.5 GeV. The 4-momentum of ah is taken to be the vector sum of the four-momenta of the tau’s tracks and0 candidates in the signal cone. The charge of ahis the sum of the charges of its tracks, and must equal1. A loosehis the same as a tighth in the regionjj<1:0, but has additional accep- tance for 1:0<jj<1:3. Since the CES does not cover the latter region, 0 related cuts are dropped, and the energy of a loose h is estimated from the plug calorimeters.
To increase signal acceptance, systems of one or three isolated, localized tracks in the region jj<1:3 are also considered as loose lepton candidates. For such candidates, the signal and isolation cone sizes are 10 and 30, re- spectively. These ‘‘isolated track systems’’ (ITSs) have acceptance for e, , and leptons. The efficiencies of lepton reconstruction, identification, and isolation require- ments are measured in data using electrons from decays of mesons, electrons and muons from decays ofZbosons, and taus fromW bosons.
We require at least three reconstructed isolated charged leptons to suppress large cross-section backgrounds such as dijets,þjets, andWð!‘‘Þ þjets. Events are clas- 121801-4
sified according to the number of isolated highpT leptons detected, and separate selections are used for the3-‘and 4-‘signatures. The data are collected by lepton plus iso- lated track triggers [15]. These triggers require one central lepton (e or ) and a second central isolated track. The integrated luminosities of the e and samples are 350 pb1 and322 pb1, respectively. Trigger efficiencies for electrons (muons) are estimated from events with pho- ton conversions and Z!ee (J= ! and Z!) decays. The efficiency for the isolated track is measured from a jet sample. The overall trigger efficiencies are 95% for H masses in the range 80–130 GeV=c2. The specific lepton requirements for the e and searches are summarized in TableI.
We use CTEQ5L parton density functions (PDFs) in the
PYTHIA generator [16] and a GEANT-based [17] detector simulation, scaled to next-to-leading order (NLO) cross sections [7], to estimate the signal and background pro- cesses. Our signal MC samples scan theH mass range 80–130 GeV=c2at10 GeV=c2intervals. The potential SM backgrounds for both the 3-‘ and 4-‘ searches are:
Z=!leptons produced in association with1hadronic jet(s) or photon(s);ZZandWZwith both bosons decaying leptonically; ttwith leptonically decaying W bosons; W bosons decaying leptonically produced in association with 2 hadronic jets; and ‘‘QCD’’ events (no leptons, 3 hadronic jets). For the e signature, þ hadronic jets events are also a potential background, while cosmic ray muons are a potential background in thechannel. The backgrounds with the larger production cross sections (e.g., QCD,W) are suppressed by multiple powers of the lepton misidentification rates (102 for jet!, and 104 for jet!e,).
Event selection for the 3-‘ events begins with the re- moval of events that are consistent with cosmic ray muons [18] or low-mass Drell-Yan lepton pairs (MðeþeÞ<
30 GeV=c2; MðþÞ<30 GeV=c2). Also, events con- sistent with Zþ production with the photon misidenti- fied as an electron are efficiently removed by requiring at least 20 GeVof missing transverse energy (E6 T) [13]. Signal events with at least onedecaying to an electron typically have E6 T>20 GeV, due to the significant fraction of the ’s energy carried off by the two neutrinos, while Zþ events are typically well measured, and thus have smallE6 T.
Similarly, in the4-‘search, events consistent with having four final-state electrons must have at least 20 eVofE6 T. No attempt is made to reconstruct the fullH mass, but we do require the presence of a like-signeorpair with an invariant mass in the range30–125 GeV=c2. This selection is nearly 100% efficient for signal but reduces diboson and top backgrounds.
To further reduce backgrounds, in particular Zþjets, we impose a requirement on the scalar sum of the lepton transverse energies and E6 TðYTÞ. The YT requirement de- pends on whether an event is tagged as aZboson decay. It is more efficient to remove events consistent withZboson decays byYTthan by a direct mass cut, because some of the signal has oppositely charged leptons in theZmass range, but large YT values compared toZþjets events. TheYT
cut values for tagged and untagged events, as well as the mass window used in Zboson tagging, are optimized by running pseudoexperiments and choosing the sets of cut values that result in the best expected limits onHþþ. The esearch usesYTcuts of 190 GeV for untagged events and 300 GeV for events tagged asZboson candidates, defined as an eþe pair in the mass range 71–111 GeV=c2. The
\Mu{\rm T} search usescuts of 190 GeV for untagged events, and 350 GeV for events tagged as Zboson candi- dates, defined as a þ pair in the mass range 76–116 GeV=c2. In the analysis, a muon with a se- verely mismeasuredpTmay lead to spuriously highYT. We minimize the mismeasurement risk by imposing additional cuts on the highestpT tracks in the events.
Events with four isolated leptons have less background than trilepton events, so less restrictive cuts are applied. We first requireYT>120 GeV. Events tagged asZbosons are required to haveE6 T>20 GeVin theesearch andYT>
150 GeVin thesearch. As withYT andZ-veto for the 3-‘ channels, pseudoexperiments were conducted with various values of both cuts, and the cuts that resulted in the best expected limits were chosen for each analysis. The acceptances for the3-‘and4-‘channels are roughly equal, and the combined acceptance grows approximately line- arly with H mass from 8% at 85 GeV=c2 to 14% at 135 GeV=c2. Observed and expected event yields for sig- nal and background for the 3-‘ and 4-‘ searches are shown in Table II. The signal event yields assume B2¼89:4 fb, corresponding to exclusive decays of 110 GeV=c2 H to e () pairs in models [2,3]. The Zþjets process is the most significant single background, with 0:15þ0:110:07ðstatÞ expected events for each of the com- bined (3-‘þ4-‘) and e searches. The combined background from WZ and ZZ production amounts to 0:120:02(0:200:02) events for the e () search.
ttbackground is0:01þ0:020:01(0:06þ0:020:01) events in thee() search. Cosmic ray, þjets, and QCD backgrounds are negligible and determined from data.
Systematic uncertainties on backgrounds from NLO cross section uncertainties are 4% for Z and W boson TABLE I. Kinematic and geometric lepton requirements (cut
values) for the e search. For the search, the first lepton changes frometo, and the third lepton changes fromhoreto isolated track.
Signature Lepton Flavor ET(PT) jj
1st (tight) e >20 GeV <1:0 3-‘ 2nd (tight) h ore >15 GeV <1:0 3rd (loose) h ore >10 GeV <1:3 4-‘ 4th (loose) Isolated Track >10 GeV=c <1:3
production processes and 8% for diboson and top quark production processes [19]. A 6% uncertainty applies to the integrated luminosity of our data set. A 28% (21%) system- atic uncertainty is used for theW!‘‘ (Z!‘‘) back- ground predictions to account for imperfect knowledge of the jet!hmisidentification rate. Imperfect simulation of the track curvature resolution is accounted for by a 0.1 event systematic uncertainty on the combined backgrounds for the search. The combined systematic uncertainty for all backgrounds amounts to 0.04 (0.11) events for thee () search. The total uncertainties on backgrounds, shown in TableII, are statistically dominated. Systematic uncertainties on the signal cross section include NLO cross section uncertainties (7.5%) [7], luminosity (6%) [11], and parton density function (PDF) uncertainty (5%) [20]. The uncertainty on signal acceptance (6.1%) is driven by un- certainties on track isolation efficiency (4.5% and 6% for 3-‘ and 4-‘ channels, respectively), and 0 isolation efficiencies (1.5% and 2% for 3-‘ and 4-‘ channels, respectively).
We find that the background predictions agree with data in all control samples, including samples in the kinematic regionYT<150 GeVenriched with QCD,Zboson, andW boson events. To check our predictions in the high-YT
regime while keeping the analysis ‘‘blind,’’ we check the number of events that pass all analysis selections except track isolation for the second tight lepton (TableI). After finalizing all selection requirements and our limit setting procedure, we search the signal regions in both the3-‘and 4-‘channels. We observe no events in either the3-‘or4-‘ channels for both the andesearches, which is con- sistent with the SM backgrounds of0:24þ0:270:24eevents and
0:390:23 events. Limits are set using a Bayesian method based on a Poisson likelihood, with a flat prior for signal cross section and Gaussian priors for uncertain- ties on signal, background acceptance, and integrated lu- minosity. The3-‘and4-‘channels are treated as separate measurements, taking into account correlated systematic uncertainties [14]. We set an upperB2 limit for the process pp !HþþL HL!eþþe of 74 fb at the 95% C.L., which corresponds in models [2,3] to a mass limit of 114 GeV=c2. The process pp !HþþL HL! þþis excluded above a cross section of 78 fb at the 95% C.L., corresponding to a mass limit of 112 GeV=c2 in the same models. The exclusion curves are shown in Fig. 1.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
This CDF 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 Foun- dation; the A.P. Sloan Foundation; the Bundes- ministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Science and Tech- nology Facilities Council and the Royal Society, UK; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; the European Community’s Human Potential Programme under Contract No. HPRN-CT- 2002-00292; and the Academy of Finland.
BR2(H++ )=100%
BR2(H++ e )=100%
95% C.L.upper limit:
(ppH++H--)xBR2 (H++/--) (fb)
mH++ (GeV/c2)
_
FIG. 1 (color online). Theoretical production cross sections for the pair production of left-handed H, and 95% C.L. limit curves forðpp!HþþH!Þ B2ð‘þþ‘Þ, for I¼e (solid), (dashed). The vertical dashed line corresponds to limits from experiments at LEP2 for exclusiveHL decays to any one dilepton channel [9].
signal (110 GeV=c2,B2¼89:4 fb) and background in the 3-‘and4-‘ searches.MLS (MOS) represent the invariant mass requirements on the like (opposite) sign leptons. The Z veto refers to the additionalYTrequirement onZboson tagged events.
The uncertainties are combined statistical and systematic.
e Selection Exp. Signal Background Data
3-‘ Lepton ID 2:940:11 37:81:3 34
MLS,MOS 2:890:11 35:41:2 29 YT=Zveto 2:40:09 9:650:66 8 YT 1:970:08 0:24þ0:270:24 0
4-‘ Lepton ID 1:610:07 0:180:06 0
YT=Zveto 1:600:07 0:04þ0:050:04 0
Selection Exp. Signal Background Data
3-‘ Lepton ID 3:060:04 30:01:4 28
MLS,MOS 2:990:04 24:61:26 20 YT=Zveto 2:350:04 6:60:86 7 YT 1:800:03 0:270:22 0
4-‘ Lepton ID 1:650:03 0:250:08 0
YT=Zveto 1:640:03 0:140:05 0
121801-6
aVisiting from University of Athens, 15784 Athens, Greece.
bVisiting from Chinese Academy of Sciences, Beijing 100864, China.
cVisiting from University of Bristol, Bristol BS8 1TL, United Kingdom.
dVisiting from University Libre de Bruxelles, B-1050 Brussels, Belgium.
eVisiting from University of California Irvine, Irvine, CA 92697, USA.
fVisiting from University of California Santa Cruz, Santa Cruz, CA 95064, USA.
gVisiting from Cornell University, Ithaca, NY 14853, USA.
hVisiting from University of Cyprus, Nicosia CY-1678, Cyprus.
iVisiting from University College Dublin, Dublin 4, Ireland.
jVisiting from University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.
kVisiting from University of Heidelberg, D-69120 Heidelberg, Germany.
lVisiting from Universidad Iberoamericana, Mexico D.F., Mexico.
mVisiting from University of Manchester, Manchester M13 9PL, United Kingdom.
nVisiting from Nagasaki Institute of Applied Science, Nagasaki, Japan.
oVisiting from University de Oviedo, E-33007 Oviedo, Spain.
pVisiting from Queen Mary, University of London, London, E1 4NS, United Kingdom.
qVisiting from Texas Tech University, Lubbock, TX 79409, USA.
rVisiting from IFIC(CSIC-Universitat de Valencia), 46071 Valencia, Spain.
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