Search for Charged Higgs Bosons from Top Quark Decays in <em>pp</em> Collisions at s√=1.96  TeV

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Reference

Search for Charged Higgs Bosons from Top Quark Decays in pp Collisions at s√=1.96  TeV

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al.

Abstract

We report the results of a search for a charged Higgs boson in the decays of top quarks produced in pp collisions at a center-of-mass energy of 1.96 TeV. We use a data sample corresponding to an integrated luminosity of 193  pb−1 collected by the upgraded Collider Detector at Fermilab. No evidence for charged Higgs production is found, allowing 95% C.L.

upper limits to be placed on BR(t→H+b) for different charged Higgs decay scenarios. In addition, we present in the minimal supersymmetric standard model (mH±,tanβ) plane the first exclusion regions with radiative and Yukawa coupling corrections.

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Search for Charged Higgs Bosons from Top Quark Decays in pp Collisions at s√=1.96  TeV. Physical Review Letters , 2006, vol.

96, no. 04, p. 042003

DOI : 10.1103/PhysRevLett.96.042003

Available at:

http://archive-ouverte.unige.ch/unige:38326

Disclaimer: layout of this document may differ from the published version.

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Search for Charged Higgs Bosons from Top Quark Decays in p p Collisions at p s

1:96 TeV

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T. Yoshida,⁴⁰I. Yu,²⁷S. S. Yu,⁴³J. C. Yun,¹⁶L. Zanello,⁴⁹A. Zanetti,⁵²I. Zaw,²¹F. Zetti,⁴⁴X. Zhang,²³ J. Zhou,⁵⁰and S. Zucchelli⁵

(CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, 11529 Taiwan, 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, Pennsylvania 15213, USA

13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

15Duke University, Durham, North Carolina 27708, USA

16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

17University of Florida, Gainesville, Florida 32611, USA

18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

19University of Geneva, CH-1211 Geneva 4, Switzerland

20Glasgow University, Glasgow G12 8QQ, United Kingdom

21Harvard University, Cambridge, Massachusetts 02138, USA

042003-2

22Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

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;

Seoul National University, Seoul 151-742;

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 Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 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 of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy

50Rutgers University, Piscataway, New Jersey 08855, USA

51Texas A&M University, College Station, Texas 77843, USA

52Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy

53University of Tsukuba, Tsukuba, Ibaraki 305, Japan

54Tufts University, Medford, Massachusetts 02155, USA

55Waseda University, Tokyo 169, Japan

56Wayne State University, Detroit, Michigan 48201, USA

57University of Wisconsin, Madison, Wisconsin 53706, USA

58Yale University, New Haven, Connecticut 06520, USA (Received 24 October 2005; published 3 February 2006)

We report the results of a search for a charged Higgs boson in the decays of top quarks produced inpp collisions at a center-of-mass energy of 1.96 TeV. We use a data sample corresponding to an integrated luminosity of193 pb¹collected by the upgraded Collider Detector at Fermilab. No evidence for charged Higgs production is found, allowing 95% C.L. upper limits to be placed onBRt!Hbfor different charged Higgs decay scenarios. In addition, we present in the minimal supersymmetric standard model (m_H;tan) plane the first exclusion regions with radiative and Yukawa coupling corrections.

DOI:10.1103/PhysRevLett.96.042003 PACS numbers: 14.65.Ha, 14.80.Cp, 13.85.Qk

One of the open questions in the standard model (SM) of particle physics involves the mechanism of electroweak symmetry breaking (EWSB). Within the SM, it is postu- lated that a single scalar doublet field breaks the symmetry, resulting in a single observable particle of unknown mass called the Higgs boson [1]. To date, the SM Higgs boson has not been observed, and extensions of the SM have been proposed with different Higgs phenomenologies. The sim-

plest extension of the SM Higgs sector introduces another Higgs doublet. In two-Higgs doublet models, EWSB re- sults in five Higgs bosons, three of which are neutral (h⁰; H⁰; A⁰) and two of which are charged (H). The minimal supersymmetric extension of the SM (MSSM) includes a two-Higgs doublet sector, in which one doublet couples to the up-type quarks and neutrinos and the other to the down-type quarks and charged leptons. The obser-

vation of a charged Higgs boson would provide unambig- uous evidence of a Higgs sector richer than that predicted by the SM.

At the Tevatron, direct production of a single charged Higgs is expected to be negligible, and the direct produc- tion ofHHvia the weak interaction is expected to have a relatively small cross section on the order of 0.1 pb [2].

The production oftt pairs, with a theoretical production cross section of 6:7^0:7_0:9 pb [3] for m_t175 GeV=c², may offer another source of charged Higgs production. If kinematically allowed, the top quark can decay toHb, competing with the SM decayt!Wb. This mechanism can provide a larger production rate of charged Higgs and offers a much cleaner signature than that of direct production.

Previous searches for the charged Higgs boson have been performed atps

1:8 TeVin the_h 6E_T jets

‘ channels, where the missing energy 6E_T is defined in Ref. [4],_hdenotes a tau lepton which decays to hadrons, and where ‘eor in Ref. [5] and‘e,or _h in Ref. [6]. In the framework of the tauonic Higgs model, in which the charged Higgs decays exclusively to , these searches set limits directly onBRt!Hbbased on the measured production rate. These results are then translated into limits ontan, the ratio of vacuum expectation values of the two-Higgs doublets. Other searches obtained limits in the (m_H;tan) plane, assuming that the charged Higgs decays only to in the6E_Tjets_hchannel in Ref. [7]

and assuming that the charged Higgs decays to, cs, and tb!Wbb in the6E_Tjets‘channel with‘eor in Ref. [8], wheret is a virtual top quark.

These limits utilize tree-level MSSM predictions of the t!Hband charged Higgs branching fraction as a func- tion of tan. It is now known that higher-order radiative corrections significantly modify these predictions. The corrections strongly depend on the parameters of the model and are particularly large at high values of tan [9]. In addition, it is also predicted that, in the lowtan region, the charged Higgs has a sizable branching fraction to Wh⁰.

CDF has recently reported measurements of thettpro- duction cross section in the ‘ 6E_T jetsX channels, where ‘e; and where X ‘ (the ‘‘dilepton’’ chan- nel), X_h (‘‘leptontau’’), Xone or more tagged jets [10] (‘‘leptonjets;1tag’’), andXtwo or more tagged jets (‘‘leptonjets;2 tags’’). These measure-

ments are carried out under the assumption BRt! Hb 0and use data samples corresponding to an inte- grated luminosity of up to193 pb¹[11–13]. In this analy- sis, we consider the possibility oft!Hband recast the cross section results to set limits on charged Higgs pro- duction. Depending on the top and Higgs branching ratios, the number of expected events in these decay channels can show an excess or deficit with respect to SM expectations.

As published, these measurements allow the categoriza- tion of a single event in multiple channels. In this analysis, extra requirements are applied to each channel in order to force the association of every event to a single channel. The ttsignal acceptance and non-ttSM background contribu- tion to each of these exclusive channels are recalculated, and the changes from the original cross section analyses are found to be mostly negligible. The only exception to this is the 1tag and 2tags leptonjets channels, where the latter is a proper subset of the former. Removal of this 100% overlap changes the 1tag channel to exactly one tag, ‘‘1tag.’’ The results for these new exclusive channels in terms of background, number of observed events, and number of SM-expected events are shown in Table I.

We assume that the charged Higgs boson can decay only to , cs, tb, or Wh⁰, leading to five possible decay modes for a single top quark: (i)t!Wb, (ii)t!Hb, H!, (iii) t!Hb,H!cs, (iv) t!Hb,H! tb, and (v) t!Hb, H!Wh⁰, h⁰ !bb. Charge conjugated decays are implied. Allowing for a nonzero BRt!Hb, the acceptance of the detector for a given ttchannelkis

Ak X⁵

i;j1

B_iB_jij;kt;_H; m_H; m_h⁰; (1) whereB_i(B_j) represent the branching fractions of the top quark (antiquark) to decay via modei(j) as listed above, and _ij;k is the efficiency to detect a ttevent whose top quarks decay via modesiandjin channelk.

The efficiencies _ij;k are obtained from Monte Carlo (MC) simulation of tt events generated with different masses of the top, H, and h⁰. The MC generator

PYTHIA [14] is modified to include the decay H!tb and is used for the generation of thettevents. The detector simulation and reconstruction algorithms for muons, elec- trons, and jets are identical to those used in the SMttcross TABLE I. Number of events in each exclusive channel from non-ttSM background sources, observed in data, and total expected assuming^prod_tt 6:7 pbandBRt!Hb 0.

Channel Background events Data events SM-expected events

Dilepton 2:70:7 13 10:91:4

Leptonjets;1tag 21:83:0 49 54:04:3

Leptonjets;2tags 1:30:3 8 101

Leptontau 1:30:2 2 2:30:3

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section measurements for the four channels. MC efficien- cies are scaled for known differences between MC simu- lation of the detector response and that observed in data.

The dependence of the efficiencies on the width of the top quark (_t) and the width of the charged Higgs (_H) is taken into account using the simulated tt events. The systematic uncertainties on _ij;k for the process tt! WbWb are listed in Refs. [11–13] and do not differ much for the other possible decay modes.

The expected number of events in channelkis

_k^prod_tt AkLkn^back_k ; (2) where^prod_tt is thettproduction cross section andrepre- sents a generic model from which the nine quantities (five BR’s, _t, _H, m_H, and m_h⁰) needed to calculate the acceptanceAk, can be derived.Lk is the integrated lumi- nosity, and n^back_k is the number of expected background events in channel k (shown in Table I). We assume the inclusion of the Higgs sector does not modify the value of thettproduction cross section and set it to^prod_tt 6:7 0:9 pb.

For each channel, a likelihood is constructed based on the Poisson probability to observeN_kevents when a given model predicts _k events. Since the four channels were constructed to be mutually exclusive, the product of their likelihoods is taken to form a final likelihood. The corre- lations of the efficiencies, backgrounds, and systematic uncertainties between channels are taken into account.

The posterior probability distribution of the parameter of interest is constructed from the likelihood and a prior probability density.

In the MSSM, the nine quantities needed to calculate the acceptance are predicted from a specific set of MSSM parameters, includingm_Handtan. We use the computa- tional package CPSUPERH [15] to compute all the Higgs masses and branching ratios. This program includes QCD, supersymmetric QCD, and supersymmetric electroweak radiative corrections up to the two-loop leading logarithms and applies these corrections to the top and bottom Yukawa couplings. The top branching ratio to charged Higgs is computed with the same level of accuracy from custom code developed in collaboration with the authors of Ref. [9]. In the context of the MSSM with m_A⁰< m_H,

CPSUPERH predicts that the H decay to WA⁰ is non- negligible for masses of H below 100 GeV=c². In this case, CPSUPERH also predicts the mass of the A⁰ to be similar to that of the h⁰, and we assume the kinematics of the decay toWA⁰to be identical to that ofWh⁰when the h⁰ andA⁰ masses are equal. Thus, we assign to the decay H!Wh⁰ a branching ratio of BRH ! Wh⁰ BRH!WA⁰, effectively considering both decays.

As an example of how a charged Higgs alters the balance between the top decay channels, Fig. 1(a) shows the ex- pected number of events in each of the exclusive channels

as a function of tanfor m_H120 GeV=c². The other relevant MSSM parameters are detailed in the caption. The figure demonstrates the excess expected in the leptontau channel and the deficit expected in the other channels for large tanvalues. For values of tan around 7, BRt! Hbgoes to zero and the SM expectation for the different channels is recovered. The relationship between the chan- nels changes with charged Higgs mass. Values of tan which result in a non-self-consistent Higgs sector are reported by CPSUPERH and are shown as the theoretically inaccessible regions in Fig. 1.

Figure 1(b) shows the posterior probability obtained for the four channels when the number of observed events is equal to that expected from the SM. The posterior is obtained by means of a flat prior inlog₁₀tan. This prior allows for a smooth variation of the top and charged Higgs branching ratios as a function oflog₁₀tan. The proba- bility is integrated over its maximum density region to obtain expected upper and lower limits on tan at the 95% confidence level (C.L.).

Using the number of events observed in the data and repeating this procedure for different Higgs masses results

0 2 4 6 8 10 12

Expected SM

0 20 40

2

0 2 4 6 8

0 0.5 1

P(tan β)

95 % Area 4

6 60

Number of Events 0

Lepton+Jets, =1 tag Dilepton

Lepton+Tau

Lepton+Jets, ≥ 2 tags Expected MSSM

tan β (a)

(b)

x10-2 10

10^-1 _{1 10 10}2

tan β

10^-1 _{1 10 10}2

Theoretically Inaccessible

FIG. 1 (color online). Predictions form_H 120 GeV=c²and mt175 GeV=c² as a function of tan for 193 pb¹. The MSSM parameters are defined in Ref. [18] and are set to M_SUSY1000 GeV=c², 500 GeV=c², A_tA_b 2000 GeV=c², A500 GeV=c², M₂M₃M_QM_U M_DM_EM_LM_SUSY, and M₁0:498M₂. (a) Expected number of events in each of the channels. (b) Posterior proba- bility density obtained when the number of observed events is equal to that expected from the SM.

in the exclusion region shown in Fig. 2. We determine this exclusion region for several sets of benchmark parameters, including the maximal and minimal light Higgs mass scenarios described in Ref. [16]. The complete character- ization of these scenarios and their results are described in Ref. [17]. In all the benchmarks used, the lowtanregion is excluded in a similar region as shown in Fig. 2. The high tan exclusion region, however, can be significantly re- duced and even vanishes, due to parameters of particular benchmarks that suppress BRt!Hb. The obtained exclusion limits strongly depend on the prior probability used. Using a flat prior intan, which is characterized by sudden changes in the top and charged Higgs branching ratios, yields significantly different exclusion regions. It is important to note that, even if all the corrections were turned off and tree-level calculations were used, the results would be significantly stronger than those obtained in Ref. [8] under the same conditions.

In the hightanregion, the decayH! is expected to dominate in a large fraction of the MSSM parameter space. In this region, the tauonic Higgs model is a good approximation, and we explicitly setBRH! 1 and evaluate the posterior probability as a function of BRt!Hb. The value of _H has little effect on the results as width corrections to the efficiency are small; we set_H1:4 GeV=c². The width of the top is set to_t 1:4 GeV=c²= 1BRt!Hb, and the value ofm_h⁰ is irrelevant in this model. We perform the scan inBRt! Hb from 0 to 1. A posterior probability density of BRt!Hbis obtained using a flat prior that is constant between 0 and 1 and null elsewhere. The 95% C.L. is obtained by integrating the posterior over the maximum density region. This procedure is repeated for different

charged Higgs masses. In the region80 GeV=c²m_H 160 GeV=c², we exclude BRt!Hb>0:4 at 95%

C.L.

Finally, in order to reduce the model dependence, we place limits onBRt!Hbthat hold for any combina- tion of charged Higgs branching ratios. For a specific charged Higgs mass, we divide each of the five charged Higgs branching ratios into 21 bins. This results in 1771 possible branching ratio combinations subject to the relation P

iBRH!X_i 1. For each combination, we obtain a limit onBRt!HbassumingBRh⁰ !bb 0:9 and m_h⁰ 70 GeV=c². The least restrictive limit is quoted, and the analysis is repeated for different charged Higgs masses. The results are shown in Fig. 3. ForBRt! Hb>0:9(hatched region), the width of the top is larger than14 GeV=c² and the analytical corrections to the effi- ciencies start losing accuracy.

In summary, we have performed a search for a charged Higgs boson in top quark decays using measurements of the top pair production cross section in four different final states, and we find no evidence of signal in the region 80 GeV=c² m_H 160 GeV=c². In the context of the MSSM with full radiative corrections, we exclude the low tanregion for all benchmarks in Ref. [17]. The hightan region cannot be excluded independent of the MSSM parameters. In the tauonic Higgs model, in which the

)

±b

→ H t BR(

0 0.2 0.4 0.6 0.8 1

)2c/VeG( ±Hm

80 90 100 110 120 130 140 150

160 SM Expected

Expected σ

± 1 SM

CDF Run II Excluded

c2

=70 GeV/

h0

m

2c/VeG 41 > tΓ

FIG. 3 (color online). Results for the charged Higgs branching ratio independent analysis with m_t175 GeV=c². The dark solid region represents the CDF run II excluded region in the [m_H;BRt!Hb] plane. The expected exclusion limits are indicated by a black solid line and the 1 confidence band around it is obtained by generating pseudoexperiments. The hatched region of BRt!Hb>0:9indicates that the width of the top is larger than14 GeV=c².

β

10^-1 1 tan 10 10²

)2c (GeV/±Hm

60 80 100 120 140 160

60 80 100 120 140 160

LEP (ALEPH, DELPHI, L3 and OPAL) only s

→ c or H±

ν τ

±→ Assuming H

Theoretically inaccessible Theoretically inaccessible

SM Expected Expected σ

± 1 SM

CDF Run II Excluded LEP Excluded SM Expected

Expected σ

± 1 SM

CDF Run II Excluded LEP Excluded

FIG. 2 (color online). The MSSM results obtained with 193 pb¹ at CDF. The SM-expected exclusion limits are indi- cated by black solid lines and the1confidence band around it is obtained by generating pseudoexperiments. The darkest solid region represents the area excluded at 95% C.L. The solid lower region is the LEP combined results from direct searches [19].

Other relevant MSSM parameters are detailed in the caption of Fig. 1.

042003-6

charged Higgs decays exclusively to, the BRt!Hb is constrained to be less than 0.4 at 95% C.L. If no assumption is made on the charged Higgs decay, the BRt!Hb is constrained to be less than 0.91 at 95%

C.L.

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 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 Particle Physics and Astronomy Research Council and the Royal Society, UK;

the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; in part by the European Community’s Human Potential Programme under Contract No. HPRN-CT-2002-00292; and the Academy of Finland.

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