Article
Reference
Search for a Higgs Boson Decaying to Two W Bosons at CDF
CDF Collaboration
CLARK, Allan Geoffrey (Collab.), et al.
Abstract
We present a search for a Higgs boson decaying to two W bosons in pp collisions at s√=1.96 TeV center-of-mass energy. The data sample corresponds to an integrated luminosity of 3.0 fb−1 collected with the CDF II detector. We find no evidence for production of a Higgs boson with mass between 110 and 200 GeV/c2, and determine upper limits on the production cross section. For the mass of 160 GeV/c2, where the analysis is most sensitive, the observed (expected) limit is 0.7 pb (0.9 pb) at 95% Bayesian credibility level which is 1.7 (2.2) times the standard model cross section.
CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . Search for a Higgs Boson Decaying to Two W Bosons at CDF. Physical Review Letters , 2009, vol. 102, no. 02, p. 021802
DOI : 10.1103/PhysRevLett.102.021802
Available at:
http://archive-ouverte.unige.ch/unige:38543
Disclaimer: layout of this document may differ from the published version.
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Search for a Higgs Boson Decaying to Two W Bosons at CDF
T. Aaltonen,24J. Adelman,14T. Akimoto,56M. G. Albrow,18B. A´ lvarez Gonza´lez,12S. Amerio,44,yD. Amidei,35 A. Anastassov,39A. Annovi,20J. Antos,15G. Apollinari,18A. Apresyan,49T. Arisawa,58A. Artikov,16W. Ashmanskas,18 A. Attal,4A. Aurisano,54F. Azfar,43P. Azzurri,47,bbW. Badgett,18A. Barbaro-Galtieri,29V. E. Barnes,49B. A. Barnett,26 V. Bartsch,31G. Bauer,33P.-H. Beauchemin,34F. Bedeschi,47D. Beecher,31S. Behari,26G. Bellettini,47,zJ. Bellinger,60
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(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3University of Athens, 157 71 Athens, Greece
4Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
5Baylor University, Waco, Texas 76798, USA
6Istituto Nazionale di Fisica Nucleare Bologna, University of Bologna, I-40127 Bologna, Italy
7Brandeis University, Waltham, Massachusetts 02254, USA
8University of California, Davis, Davis, California 95616, USA
9University of California, Los Angeles, Los Angeles, California 90024, USA
10University of California, San Diego, La Jolla, California 92093, USA
11University of California, Santa Barbara, Santa Barbara, California 93106, USA
12Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
13Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
14Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
15Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia
16Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
17Duke University, Durham, North Carolina 27708, USA
18Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
19University of Florida, Gainesville, Florida 32611, USA
20Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
21University of Geneva, CH-1211 Geneva 4, Switzerland
22Glasgow University, Glasgow G12 8QQ, United Kingdom
23Harvard University, Cambridge, Massachusetts 02138, USA
24Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
25University of Illinois, Urbana, Illinois 61801, USA
021802-2
26The Johns Hopkins University, Baltimore, Maryland 21218, USA
27Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
28Center 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
29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
30University of Liverpool, Liverpool L69 7ZE, United Kingdom
31University College London, London WC1E 6BT, United Kingdom
32Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain
33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
34Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8;
and University of Toronto, Toronto, Canada M5S 1A7
35University of Michigan, Ann Arbor, Michigan 48109, USA
36Michigan State University, East Lansing, Michigan 48824, USA
37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
38University of New Mexico, Albuquerque, New Mexico 87131, USA
39Northwestern University, Evanston, Illinois 60208, USA
40The Ohio State University, Columbus, Ohio 43210, USA
41Okayama University, Okayama 700-8530, Japan
42Osaka City University, Osaka 588, Japan
43University of Oxford, Oxford OX1 3RH, United Kingdom
44Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, University of Padova, I-35131 Padova, Italy
45LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France
46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
47Istituto Nazionale di Fisica Nucleare Pisa, University of Pisa, University of Siena and Scuola Normale Superiore, I-56127 Pisa, Italy
48University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
49Purdue University, West Lafayette, Indiana 47907, USA
50University of Rochester, Rochester, New York 14627, USA
51The Rockefeller University, New York, New York 10021, USA
52Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, Sapienza Universita` di Roma, I-00185 Roma, Italy
53Rutgers University, Piscataway, New Jersey 08855, USA
54Texas A&M University, College Station, Texas 77843, USA
55Istituto Nazionale di Fisica Nucleare Trieste/Udine, University of Trieste/Udine, Italy
56University of Tsukuba, Tsukuba, Ibaraki 305, Japan
57Tufts University, Medford, Massachusetts 02155, USA
58Waseda University, Tokyo 169, Japan
59Wayne State University, Detroit, Michigan 48201, USA
60University of Wisconsin, Madison, Wisconsin 53706, USA
61Yale University, New Haven, Connecticut 06520, USA (Received 24 September 2008; published 15 January 2009)
We present a search for a Higgs boson decaying to twoWbosons inppcollisions at ffiffiffi ps
¼1:96 TeV center-of-mass energy. The data sample corresponds to an integrated luminosity of3:0 fb1collected with the CDF II detector. We find no evidence for production of a Higgs boson with mass between 110 and 200 GeV=c2, and determine upper limits on the production cross section. For the mass of160 GeV=c2, where the analysis is most sensitive, the observed (expected) limit is 0.7 pb (0.9 pb) at 95% Bayesian credibility level which is 1.7 (2.2) times the standard model cross section.
DOI:10.1103/PhysRevLett.102.021802 PACS numbers: 14.80.Bn, 13.85.Rm
The Higgs boson in the standard model (SM) breaks the electroweak SUð2ÞLUð1ÞY symmetry. While this sym- metry is now well established, the mechanism of the symmetry breaking has not yet been identified. Direct searches at the LEP experiments have set a lower limit on the Higgs boson massmHof114:4 GeV=c2at 95% C.L.
in the context of the SM [1]. Precision measurements provide the indirect upper limit mH<144 GeV=c2 at
95% C.L. through radiative corrections to the SM predic- tions of the particle masses and couplings [2]. However, these indirect limits assume no significant contributions to the radiative corrections due to as yet unobserved processes.
In this Letter, we report a search for the process gg! H!WWðÞin a3:0 fb1integrated luminosity sample of pp collisions at ffiffiffi
ps
¼1:96 TeVproduced by the Fermilab
Tevatron and collected by the CDF II detector. For a SM Higgs boson with a mass that is not directly excluded by the LEP experiments, the dominant production mechanism at the Tevatron is gluon-gluon fusion which proceeds via a virtual top quark loop [3]. FormH>135 GeV=c2, the SM Higgs boson decays primarily to theWW [3], where one of the final stateW bosons is virtual formHbelow 2 times theW mass.
The events are reconstructed in the ll final state, whose branching fraction is 6.0% of the WW decays, where l is either an electron e or a muon , including those fromleptons produced in the W decays. The SM Higgs boson branching fraction toWWvaries from 7.5%
at115 GeV=c2 to 73.5% at200 GeV=c2with a maximum of 96.5% at 170 GeV=c2 [3]. Previous searches set limits ranging from 10 to 40 times the predicted SM rate, depending on the value ofmH [4].
The CDF II detector is a multipurpose solenoidal spec- trometer surrounded by calorimeters and muon detectors [5]. The geometry is described using the azimuthal angle and the pseudorapidity ln½tanð=2Þ, whereis the polar angle with respect to the proton beam axis (positivez axis). The transverse energyET is Esin, where E is the energy associated with a calorimeter element or energy cluster. Similarly, pT is the track momentum component transverse to the beam line.
The events we consider must pass one of four online selections, triggers, before being recorded. One electron trigger requires an electromagnetic (EM) energy cluster in the central (jj<1:1) calorimeter with ET >18 GeV matched to a track found in the drift chamber with pT >
8 GeV=c. A second electron trigger requires an EM energy cluster withET>20 GeVin the forward (1:2<jj<2:0) calorimeter and the missing transverse energy E6 T >
15 GeV. The variable E6~T, used to infer the presence of neutrinos, is defined asP
iET;in^T;i and E6 T jE6~Tj, where
^
nT;iis the transverse component of the unit vector pointing from the interaction point to calorimeter elementi. Muon triggers are based on track segments in the muon chambers matched to a drift-chamber track with pT>18 GeV=c.
Trigger efficiencies are measured using leptonicW andZ data samples [6].
To improve the signal acceptance while maintaining acceptable background rejection for the Wþjets and Wprocesses where a jet oris misidentified as a lepton, we use a modified version of the lepton identification strategy developed for the WZ observation analysis [7].
Candidate leptons are separated into six mutually exclusive categories: two for electrons; three for muons; and one for tracks that extrapolate outward to detector regions with insufficient calorimeter coverage for energy measurement.
The electron categories are central (jj<1:1) using a drift-chamber-based tracking algorithm and forward (1:2<jj<2:0) using a silicon-detector-based tracking algorithm. One of the muon categories uses the muon
chambers and the other two use tracks matched with en- ergy deposits consistent with minimum ionization in the central or forward calorimeters.
All lepton candidates are required to be isolated such that the sum of the ET for the calorimeter elements in a cone ofR¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðÞ2þ ðÞ2
p <0:4around the lepton is less than 10% of the ET (for electrons) orpT (for muons and track lepton candidates). For lepton types in the central region where the track finding efficiency is sufficient, we also apply a track-based isolation criterion which requires there is no more than 10% of the electronETor muonpTin other tracks within a cone of R <0:4around the lepton track.
The Higgs boson candidates are selected from events with exactly two lepton candidates. At least one lepton is required to match a trigger lepton candidate and haveET>
20 GeV (pT>20 GeV=c) for electrons (muons). We loosen this requirement to 10 GeV (GeV=c) for the other lepton to increase the kinematic acceptance, particularly for lowermHwhere oneWis off shell below theW boson mass and produces a lower pT lepton. We also require a dilepton invariant massm‘‘>16 GeV=c2to suppress mis- identified multijet events.
Aside fromH!WWðÞproduction, other SM processes that can lead to two high-pT leptons include Drell-Yan (DY),tt,WW,WZ, andZZproduction, andW production in association with a photon (W) or a jet (Wþjets) misidentified as a lepton. Thettcontribution is suppressed by requiring fewer than two reconstructed jets withET>
15 GeVandjj<2:5in the event. The DY background is suppressed by requiring sufficiently largeE6 Tin the event to remove contributions from mismeasured leptons or jets.
This is achieved by requiringE6 T;rel>25 GeV, whereE6 T;rel
is E6 TsinE6 T;ð‘;jetÞ if E6 T;ð‘;jetÞ<2 and E6 T otherwise, andE6 T;ð‘;jetÞis the angle between theE6~T direction and the nearest lepton or jet. The observedE6 T is corrected for muons and track-only lepton candidates, because they do not deposit all of their energy in the calorimeter. Except for Wþjets, acceptances are determined using data simulated with the MC@NLOprogram forWW [8],PYTHIAforH! WWðÞ, DY, WZ, ZZ, and tt [9], and the generator de- scribed in Ref. [10] forW.
The response of the CDF II detector is then estimated with a GEANT-4-based simulation [11] to which an effi- ciency correction of up to 10% per lepton is applied based on measurements of the lepton reconstruction and identi- fication efficiencies using observedZ!‘þ‘events. An additional correction is applied to the W background estimate based on a measurement in data of the photon- conversion veto efficiency. The Wþjets contribution is estimated from data by extrapolating from a sample of events that contain an identified lepton and a jet jl con- taining a track or EM energy cluster similar to those required in the lepton identification. The contribution of each event to the total yield is scaled by the probability that
021802-4
thejlis identified as a lepton. This probabilitypðpT;jlÞis determined for each lepton type as a function of thejlpT using multijet events collected with jet-based triggers. A correction topðpT;jlÞis applied for the small real lepton contribution using Monte Carlo simulation of singleWand Zboson production.
Based on the procedure above, we expect 76891 background events and observe 779 in the selected region.
The expected composition of the background is35649 WW,24:93:9WZ,21:83:5ZZ,25:55:0tt,138 31DY,90:524:1W, and11127Wþjets, where the indicated uncertainties include the systematic uncertainties described below. As a cross-check of the background model, we measure the fraction of qq !WW events in the sample with a similar method to the signal extraction described below and find it to be consistent with the expectation. Table I shows the dependence of expected yield on mH. Since the level of non-WWðÞ background depends on the lepton identification categories, this infor- mation is used to divide the sample into high and low signal-to-background (S=B) classes. The lowS=Bcategory largely consists of candidates with a forward lepton.
After selection, the dominant background is theqq ! WWprocess which differs from the signal process only by spin, production mechanism, and resonant structure.
Because of the two neutrinos in the final state, a simple mass peak cannot be used to isolate the resonant gg! H!WWðÞ process from the backgrounds. Instead, we combine two different multivariate techniques to differ- entiate signal and background. One is a matrix element (ME) technique, which uses an event-by-event calculation of the probability density for each contributing process to produce the observed event. If all details of the collision properties and the detector response are modeled in the ME calculation, this method provides the optimal sensitivity to the signal. However, there are several approximations used in the calculations: theoretical differential cross sections are only implemented to leading order, a simple parame- terization of the detector response is used, and for some small (WZandtt) or difficult to model (DY) backgrounds, we do not calculate a probability density. In order to improve these approximations, we extend the ME calcu- lation with a neural network (NN) which exploits the more complete model implemented in the simulated data and Wþjets model.
The event probability density for the ME method is
Pðx~obsÞ ¼ 1 h i
Z d LOðy~Þ
d ~y ðy~ÞGðx~obs; ~yÞd ~y;
where the elements of y~ (x~obs) are the true (observed) values of the lepton momenta and E6 T, d LO=d ~y is the parton level differential cross section [12], ðy~Þ is a pa- rameterization of detector acceptance and efficiency func- tion, andGðx~obs; ~yÞis the transfer function representing the detector resolution and aPYTHIA-based estimate of trans- verse momentum of the‘‘E6 Tsystem due to the initial state radiation. The constant h i normalizes the total event probability to unity. This calculation integrates the theo- retical differential cross section over the missing informa- tion due to two unobserved neutrinos in the final state. We form a likelihood ratio discriminant which is the signal probability divided by the sum of signal and background probabilities LRH!WWðÞðx~obsÞ P PHðx~obsÞ
Hð~xobsÞþP
ikiPið~xobsÞ where ki are the expected background fractions of WW, ZZ, W, and Wþjets. The LR distributions are shown in Fig. 1(a). Additional ME likelihood ratios LRWW,LRZZ, LRW, and LRWþjets are defined analogously to LRH!WWðÞ.
For the final results, a NN discriminant is used to extend the ME calculation using as input the ME likelihood ratios in addition to various kinematic variables. For each of the Higgs boson masses investigated, a NN is trained on signal events and an appropriately weighted composition of back- ground events. The NN classifies events as signal or back- ground based on the inputs LRH!WWðÞ, LRWW, LRZZ, LRW, LRWþjets, ‘‘, R‘‘, m‘‘, E6 T, E6 T;ð‘;jetÞ, and E6 T;rel, where ‘‘ andR‘‘ are the separation between the two leptons inandR, respectively. We find that the most discriminating input variables areLRH!WWðÞ,R‘‘, and E6 T;rel. An example of the NN output is shown in Fig.1(b). The presented results use theNEUROBAYES[13]
program. Comparable results are also obtained using the
TMVA-MLP [14] program, demonstrating the robustness of the technique.
For the signal and backgrounds modeled by Monte Carlo simulation, the same procedures as Ref. [7] are used to assess the systematic uncertainties on the lepton selection efficiency, trigger efficiency, parton-distribution function, and luminosity to be 1.4% to 2.0%, 2.1% to 7.1%, 1.9%–
4.1%, and 6%, respectively, depending on mode. The cross section uncertainties are 10% forWW [12],WZ[12],ZZ [12], and W [15], and 15% for tt [16]. Based on a comparison of simulated WW events generated with
MC@NLOandPYTHIA, we assign a systematic uncertainty on the acceptance due to higher order QCD effects of 5.5%
for WW events and 10% for the other modes which are only simulated at leading order.
TABLE I. Expected Higgs boson events as a function ofmH.
mH (GeV=c2) 110 120 130 140 150 160 170 180 190 200
Expected yield 0.5 1.9 4.3 7.0 9.3 11.6 11.0 9.0 6.4 5.1
The systematic uncertainty on theWþjets background is estimated to be 24% from differences in the observed probability that a jet is identified as a lepton for jets collected using different jet ET trigger thresholds. These variations correspond to changing the parton composition of the jets and the relative amount of contamination from real leptons. Because only the E6 T;rel requirement sup- presses the DY background, there is an uncertainty due to the E6 T resolution modeling, which is estimated to be
20% based on comparisons of the data and Monte Carlo simulation in a sample of dilepton events. For the W background contribution, there is an additional uncertainty of 20% from the detector material description and photon- conversion veto efficiency.
A Bayesian credibility level (C.L.) is calculated for each mHhypothesis based on the combined binned likelihood of the discriminant distributions for the high and low S=B samples. A posterior density is obtained by multiplying this likelihood by Gaussian prior densities for the back- ground normalizations and systematic uncertainties leav- ing ðgg!HÞ BðH!WWðÞÞ with a uniform prior density. A 95% C.L. limit is then determined such that 95% of the posterior density for B falls below the limit. Limits as a fraction of the SM cross section
SMðgg!HÞare calculated by including SMðgg!HÞ as a parameter whose prior density is determined by the 10% theoretical uncertainty of its next-to-next-to-leading- log prediction [17]. The resulting cross section limits for both discriminants are shown in Table II and Fig. 2.
Because the NN uses the ME calculation as input, it is the full result; the ME results are given only for compara- tive purposes.
In conclusion, we have presented limits on the produc- tion of a Higgs boson through gluon fusion followed by its decay to a pair ofW bosons. A combination of matrix ele- ment and neural network techniques is used to discriminate signal from background. Studies using the two techniques independently achieve consistent results with a sensitivity approximately 0:1 SM worse than the combination at mH¼160 GeV=c2. The consistency of results obtained with different algorithms provides evidence of the robust- ness of the multivariate techniques. At the most sensitive value of mH ¼160 GeV=c2, the observed limit is 1.7 times the SM prediction where the median expected limit is 2.2, corresponding to a downward fluctuation slightly larger than 1 standard deviation. Compared to an optimized selection and a likelihood based on the‘‘variable, the multivariate discriminators gain a factor of 1.7 to 2.5 in effective integrated luminosity depending on mH. This measurement also constrains alternative models in which
TABLE II. Expected and observed upper limits on ðgg!HÞ BðH!WWðÞÞandð BÞ=ð BÞSM for variousmH.
mH(GeV=c2) 110 120 130 140 150 160 170 180 190 200
Using Matrix Element Only
Expected (pb) 3.6 2.6 2.2 1.9 1.5 0.9 0.9 1.1 1.2 1.3
Observed (pb) 2.8 1.5 1.1 0.9 0.8 0.7 0.6 0.7 1.0 1.5
Expected/SM 63.7 19.6 9.4 6.0 4.3 2.4 2.6 3.8 6.0 8.2
Observed/SM 50.3 10.9 4.7 3.0 2.3 1.7 1.8 2.6 5.0 10.3
Using Neural Net Discriminator
Expected (pb) 3.0 2.3 1.9 1.7 1.4 0.9 0.8 1.0 1.1 1.2
Observed (pb) 2.5 1.7 1.2 1.1 0.9 0.7 0.7 0.7 1.0 1.6
Expected/SM 54.0 17.1 8.4 5.4 3.9 2.2 2.4 3.5 5.6 7.7
Observed/SM 44.6 13.2 5.3 3.5 2.6 1.7 2.2 2.7 5.5 10.6
Matrix Element Likelihood Ratio
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Events / 0.04 1
10
102 Other
→ WW H Data WW
Neural Net Score
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Events / 0.05 1
10
Other
→ WW H Data WW
(a)
(b)
FIG. 1 (color online). The likelihood ratio distribution from (a) the ME discriminate alone and (b) the full NN score for mH¼160 GeV=c2. The Higgs boson distribution is normalized to the SM expectation. The distributions are shown only for the highS=Bclass, which provides the majority of the sensitivity.
021802-6
thegg!Hcoupling is enhanced by additional particles in the virtual loops of the production amplitude [18].
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 En- ergy 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 Science and Technology 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 Ministerio de Ciencia e Innovacio´n, Spain; the Slovak R&D Agency; and the Academy of Finland.
aDeceased
bVisitor from University of Massachusetts Amherst, Amherst, MA 01003, USA.
cVisitor from Universiteit Antwerpen, B-2610 Antwerp, Belgium.
dVisitor from University of Bristol, Bristol BS8 1TL, United Kingdom.
eVisitor from Chinese Academy of Sciences, Beijing 100864, China.
fVisitor from Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.
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jVisitor from University of Cyprus, Nicosia CY-1678, Cyprus.
kVisitor from University College Dublin, Dublin 4, Ireland.
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oVisitor from Queen Mary, University of London, London, E1 4NS, United Kingdom.
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eeOn leave from J. Stefan Institute, Ljubljana, Slovenia.
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