First Flavor-Tagged Determination of Bounds on Mixing-Induced <em>CP</em> Violation in <em>B_s⁰ -&gt; J/ψϕ</em> Decays

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Reference

First Flavor-Tagged Determination of Bounds on Mixing-Induced CP Violation in B

-> J/ψϕ Decays

CDF Collaboration

CLARK, Allan Geoffrey (Collab.), et al.

Abstract

This Letter describes the first determination of bounds on the CP-violation parameter 2βs using B0s decays in which the flavor of the bottom meson at production is identified. The result is based on approximately 2000 B0s→J/ψϕ decays reconstructed in a 1.35  fb−1 data sample collected with the CDF II detector using pp¯ collisions produced at the Fermilab Tevatron. We report confidence regions in the two-dimensional space of 2βs and the decay-width difference ΔΓ. Assuming the standard model predictions of 2βs and ΔΓ, the probability of a deviation as large as the level of the observed data is 15%, corresponding to 1.5 Gaussian standard deviations.

CDF Collaboration, CLARK, Allan Geoffrey (Collab.), et al . First Flavor-Tagged Determination of Bounds on Mixing-Induced CP Violation in B

-> J/ψϕ Decays. Physical Review Letters , 2008, vol. 100, no. 16, p. 161802

DOI : 10.1103/PhysRevLett.100.161802

Available at:

http://archive-ouverte.unige.ch/unige:38506

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

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First Flavor-Tagged Determination of Bounds on Mixing-Induced CP Violation in B

! J= Decays

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J. C. Yun,¹⁷L. Zanello,⁵⁰A. Zanetti,⁵³I. Zaw,²²X. Zhang,²⁴Y. Zheng,^8,dand 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, Pennsylvania 15213, USA

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

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

161802-2

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 14 December 2007; published 22 April 2008)

This Letter describes the first determination of bounds on theCP-violation parameter 2s usingB⁰s

decays in which the flavor of the bottom meson at production is identified. The result is based on approximately 2000B⁰s!J= decays reconstructed in a 1:35 fb¹ data sample collected with the CDF II detector usingpp collisions produced at the Fermilab Tevatron. We report confidence regions in the two-dimensional space of2_s and the decay-width difference . Assuming the standard model predictions of2_sand, the probability of a deviation as large as the level of the observed data is 15%, corresponding to 1.5 Gaussian standard deviations.

DOI:10.1103/PhysRevLett.100.161802 PACS numbers: 13.25.Hw, 11.30.Er

The accurate determination of charge-conjugation- parity (CP) violation in meson systems has been one of the goals of particle physics since the effect was first discovered in neutral kaon decays in 1964 [1]. Standard model CP-violating effects are described through the

Cabibbo-Kobayashi-Maskawa (CKM) mechanism [2], which successfully describes the phenomenology of CP violation inB⁰ andBdecays with a single phase respon- sible for allCPviolation effects [3]. However, comparable experimental knowledge fromB⁰_sdecays has been lacking.

In theB_ssystem, the mass eigenstatesB_sL andB_sH are admixtures of the flavor eigenstatesB⁰_sandB⁰_s. This causes oscillations between theB⁰_sandB⁰_sstates with a frequency proportional to the mass difference of the mass eigenstates, m_sm_Hm_L. In the standard model this effect is explained in terms of second-order weak processes involv- ing virtual massive particles that provide a transition am- plitude between the B⁰_s andB⁰_s states. The magnitude of this mixing amplitude is proportional to the oscillation frequency, while its phase, responsible for CP violation inB⁰_s!J= decays, is 2^SM_s 2 arg_V^VtsVtb

csV_cb[4], where V_ij are the elements of the CKM quark mixing matrix. Global fits of experimental data tightly constrain theCPphase to small values in the context of the standard model,2^SM_s 0:04[5]. The presence of physics beyond the standard model could contribute additional processes and modify the magnitude or the phase of the mixing amplitude. The recent precise determination of the oscil- lation frequency [6] indicates that contributions of new physics to the magnitude are unlikely [7]. However, new physics may contribute significantly to the observed CP phase2_s2^SM_s ^NP_s [7–9], where^NP_s is due to the additional processes. The decay-width difference between the mass eigenstates, _LH, is also sensitive to the same new physics phase. If^NP_s 2^SM_s we expect 2j₁₂jcos2_s[9], wherej₁₂jis the off-diagonal element of theB⁰_s-B⁰_s decay matrix from the Schro¨dinger equation describing the time evolution ofB⁰_smesons [10].

In this Letter we present the first study of the B⁰_s ! J= decay [11] in which the initial state is identified as B⁰_s or its antiparticle B⁰_s in a process known as ‘‘flavor tagging.’’ Such information is necessary to separate the time evolution of mesons produced asB⁰_sorB⁰_s. By relating this time development with the CP eigenvalue of the final state that is accessible through the angular distribu- tions of the J= and mesons, we obtain direct sensitivity to the CP-violating phase. This phase enters the time development with terms proportional to both jcos2_sjandsin2_s. Analyses ofB⁰_s!J= decays that do not use flavor tagging provide information on, and are primarily sensitive to jcos2_sj and jsin2_sj, leading to a fourfold ambiguity in the determination of2_s [10,12].

This measurement uses 1:35 fb¹ of data collected by the CDF experiment at the Fermilab Tevatron using a dimuon trigger which preferentially selects events contain- ing J= ! decays [13]. The CDF II detector is described in detail in Ref. [13] with the detector subsys- tems relevant for this analysis discussed in Ref. [14].

We reconstruct theB⁰_s !J= decay from the decays J= ! and !KK and require these final state particles to originate from a common point. We use an artificial neural network (ANN) [15] to separateB⁰_s ! J= signal from background. In the ANN training, we consider the following variables: particle identification of

kaons using the time-of-flight detector and the specific ionization energy loss (dE=dx) in the multiwire drift cham- ber, the momentum components of the B⁰_s and candi- dates transverse to the proton beam direction, the invariant mass of thecandidate, and the quality of a kinematic fit to the trajectories of the final state particles. We have trained the ANN with signal events from simulated data that are passed through the standard GEANT-based [16]

simulation of the CDF II detector [17] and are recon- structed as in real data. We useB⁰_s!J= mass sideband candidates, defined as those having mJ= 2 5:2861;5:3131 [ 5:4211;5:4481GeV=c², as the back- ground sample in the ANN training. Applying the selection on the output variable of the ANN, we observe 2,01973 B⁰_s!J= signal events with a signal to background ratio of approximately one. The invariant J= mass distribution is shown in Fig. 1. An event-specific primary interaction point is used in the calculation of the proper decay time, tmB⁰_sL_xyB⁰_s=p_TB⁰_s, where L_xyB⁰_s is the distance from the primary vertex to the B⁰_s!J=

decay vertex projected onto the momentum of theB⁰_sin the plane transverse to the proton beam direction,mB⁰_sis the mass of the B⁰_s meson [3], and p_TB⁰_s is its measured transverse momentum.

The orbital angular momenta of the vector (spin 1) mesons,J= and, produced in the decay of the pseudo- scalar (spin 0) B⁰_s meson, are used to distinguish the CP-even S- and D-wave final states from the CP-odd P-wave final state. We measure the decay angles_T,_T, and _T, defined in Ref. [10], in the transversity basis [18].

2) ) (GeV/c φ

ψ Mass(J/

5.3 5.35 5.4 5.45

2 Candidates per 2.0 MeV/c

0 100

200 data

fit

2) ) (GeV/

φ ψ Mass(J/

5.3 5.35 5.4 5.45

FIG. 1. Invariant KK mass distribution with the fit projection overlaid. The vertical lines indicate the mass sideband regions.

161802-4

The transverse linear polarization amplitudes A_k and A_? correspond to CP even andCP odd final states at t0, respectively. The longitudinal polarization amplitude A₀ corresponds to a CP even final state. The polarization amplitudes are required to satisfy the condition jA₀j² jA_kj² jA_?j²1.

In order to separate the time development of the B⁰_s meson from that of theB⁰_s meson, we identify the flavor of theB⁰_s orB⁰_smeson at the time of production by means of flavor tagging. Two independent types of flavor tags are used, each exploiting specific features of the production of bquarks at the Tevatron, which are primarily produced in bbpairs. The first type of flavor tag infers the production flavor of theB⁰_sorB⁰_smeson from the decay products of the otherbquark in the event. This is known as an opposite- side flavor tag (OST). The OST decisions are based on the charge of muons or electrons from semileptonicBdecays [14] or the net charge of the opposite-side jet [14]. If multiple tags are available for an event, the decision from the highest dilution flavor tag is chosen [14]. The tag dilution D, defined by the probability to correctly tag a candidate P_tag 1D=2, is estimated for each event.

The calibration of the OST dilution is determined from B!J= KandB⁰ !J= K⁰decays. The second type of flavor tag identifies the flavor of the reconstructedB⁰_s or B⁰_smeson at production by correlating it with the charge of an associated kaon arising from fragmentation processes [19], referred to as a same-side kaon tag (SSKT). The SSKT algorithm and its dilution calibration on simulated data are described in Ref. [6]. The average dilution is11 2%for the OST and274%for the SSKT, where the uncertainties contain both statistical and systematic effects.

The measured efficiencies for a candidate to be tagged are 961%for the OST and501%for the SSKT.

An unbinned maximum likelihood fit is performed to extract the parameters of interest,2_sand, plus addi- tional parameters (referred to as ‘‘nuisance parameters’’) that include the signal fractionf_s, the meanB⁰_s width _LH=2, the mixing frequencym_s, the magnitudes of the polarization amplitudesjA₀j²,jA_kj², andjA_?j², and the strong phases kargA_kA₀ and ? argA_?A₀. The fit uses information on the reconstructedB⁰_scandidate mass m and its uncertainty _m, the B⁰_s candidate proper decay timetand its uncertainty_t, the transversity angles

~

fcos_T; _T;cos _Tg, and tag information D and , where D is the event-specific dilution and f1;0;1gis the tag decision, in which 1corresponds to a candidate tagged as B⁰_s, 1 to a B⁰_s, and 0 to an untagged candidate. The single-event likelihood is de- scribed in terms of signal (P_s) and background (P_b) proba- bility distribution functions (PDFs) as

f_sP_smj_mP_st; ~; jD; tP_stP_sD

1f_sP_bmP_btj_tP_bP~ _btP_bD: (1)

The signal mass PDFP_smj_mis parametrized as a single Gaussian with a standard deviation determined separately for each candidate, while the background mass PDF, P_bm, is parametrized as a first order polynomial. The distributions of the decay time uncertainty and the event- specific dilution are observed to be different in signal and background, so we include their PDFs explicitly in the likelihood. The signal PDFs P_standP_sDare deter- mined from sideband-subtracted data distributions, while the background PDFs P_bt andP_bD are determined from theJ= invariant mass sidebands. The PDFs of the decay time uncertainties,P_standP_bt, are described with a sum of gamma function distributions, while the dilution PDFs P_sD and P_bD are included as histo- grams that have been extracted from data.

The time and angular dependence of the signal PDF P_st; ~; ;jD; _tfor a single flavor tag can be written in terms of two PDFs,PforB⁰_sandP forB⁰_s, as

P_st; ~; jD; t

1D

2 Pt; ~j_t ~ 1D

2 Pt; ~ j_t ;~ (2) which is trivially extended in the case of two independent flavor tags (OST and SSKT). The detector acceptance effects on the transversity angle distributions, , are~ modeled with B⁰_s !J= simulated data. Three- dimensional joint distributions of the transversity angles are used to determine ~ in order to correctly account for any dependencies among the angles. The time and angular probabilities forB⁰_s can be expressed as

Pt; ~ / jA₀j²Tf₁ jA~ _kj²Tf₂~ jA_?j²Tf₃ jA~ _kjjA_?jUf₄~ jA₀jjAkjcoskTf₅~

jA₀jjA_?jVf₆;~ (3) where the functionsf₁~ . . .f₆~ are defined in Ref. [10].

The probabilityPforB⁰_sis obtained by substitutingU! U and V!V. The time-dependent term T is defined as

T e^t cosht=2 cos2_ssinht=2 sin2_ssinm_st;

where 1 for P and 1 for P. The other time- dependent terms are defined as

U e^t sin?kcosm_st cos?k cos2_ssinm_st cos_?ksin2_s sinht=2;

V e^t sin_?cosm_st cos_?cos2_s sinm_st cos_?sin2_ssinht=2:

These relations assume that there is no directCPviolation in the system. The time dependence is convolved with a Gaussian proper time resolution function with standard deviation _t, which is adjusted by an overall calibration factor determined from the fit using promptly decaying background candidates. The average of the resolution function is 0.08 ps, with a root-mean-square deviation of 0.04 ps.

We model the background lifetime PDFP_btj_twith a delta function at t0, one and two exponentials with negative slope for t <0 and t >0, respectively, all of which are convolved with the Gaussian resolution function.

The background angular PDFs are factorized, P_b~ P_bcos_TP_b’_TP_bcos _T, and are obtained using B⁰_s mass sidebands events.

Possible asymmetries between the tagging rate and di- lution ofB⁰_sandB⁰_smesons have been studied with control samples and found to be statistically insignificant. We allow important sources of systematic uncertainty, such as the determination of overall calibration factors associ- ated with the proper decay time resolution and the dilu- tions, to float in the fit. The mixing frequency m_s17:770:12 ps¹ is constrained in the fit within the experimental uncertainties [6]. Systematic uncertain- ties coming from alignment, detector sculpting, back- ground angular distributions, decays from other B mesons, the modeling of signal and background are found to have a negligible effect on the determination of both and_srelative to statistical uncertainties.

The signal probability distribution is invariant under the simultaneous transformation (2_s!2_s, ! , k!2k, and ? !?), causing the likelihood function to have two minima. This symmetry can be removed by restricting any of the above parameters within appropriate ranges. However, even after removal of the exact symmetry, approximate symmetries remain, pro- ducing local minima. Since the log-likelihood function is nonparabolic, we cannot meaningfully quote point esti- mates. Instead we choose to construct a confidence region in the2_splane.

We use the Feldman-Cousins likelihood ratio ordering [20] to determine the confidence level (CL) for a2040 grid evenly spaced in 2_s2 =2;3=2 and 2 0:7;0:7. The other parameters in the fit are treated as nuisance parameters (e.g., B⁰_s mean width, transversity amplitudes, strong phases) [21]. To ensure that the ob- tained confidence regions provide the quoted coverage against deviations of the nuisance parameters from their values measured in our fit to data, we perform pseudoex- periments by randomly sampling the nuisance parameter space within5of the fit values and confirm coverage of the 68% and 95% confidence regions shown in Fig.2. The solution centered in 02_s=2 and >0 corre- sponds to cos_?<0 and cos_?k>0, while the opposite is true for the solution centered in=2_s

and<0. Assuming the standard model predicted val- ues of 2_s0:04 and 0:096 ps¹ [9], the proba- bility to observe a likelihood ratio equal to or higher than what is observed in data is 15%. Additionally, we present a Feldman-Cousins confidence interval of2_s, whereis treated as a nuisance parameter, and find that 2_s2 0:32;2:82 at the 68% confidence level. The CP phase 2_s, , , and the linear polarization amplitudes are consistent with those measured in Ref. [10]. We also ex- ploit current experimental and theoretical information to extract tighter bounds on theCP-violating phase. Applying the constraint j₁₂j 0:0480:018 [9] in the relation 2j₁₂jcos2_s, we obtain 2_s2 0:24;1:36 [ 1:78;2:90at the 68% C.L.

In summary we present confidence bounds on the CP-violation parameter 2_s and the width difference from the first study of B⁰_s !J= decays using flavor tagging. Assuming the standard model predicted values of 2_s0:04 and 0:096 ps¹, the probability of a deviation as large as the level of the observed data is 15%, which corresponds to 1.5 Gaussian standard devia- tions. Treating instead as a nuisance parameter and fitting only for2_s, we find that2_s2 0:32;2:82at the 68% confidence level. The presented experimental bounds restrict the knowledge of2_sto two of the four solutions allowed in measurements that do not use flavor tagging [10,12] and improve the overall knowledge of this parameter.

(rad) βs

2

0 2 4

) -1 (psΓ∆

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

95% C.L.

68% C.L.

SM prediction

FIG. 2. Feldman-Cousins confidence region in the2_s plane, where the standard model favored point is shown with error bars [9]. The intersection of the horizontal and vertical dotted lines indicates the reflection symmetry in the2s plane.

161802-6

We would like to thank U. Nierste for several useful suggestions. We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contri- butions. 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 Science and Technology Facilities Council and the Royal Society, U.K.; the Institut National de Physique Nucleaire et Physique des Particules/CNRS, France; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; the European Community’s Human Potential Programme;

the Slovak R&D Agency; and the Academy of Finland.

aDeceased.

bVisitor from University of Athens, 15784 Athens, Greece.

cVisitor from Universidad Auto´noma of Madrid, E-28049 Madrid, Spain.

dVisitor from Chinese Academy of Sciences, Beijing 100864, China.

eVisitor from University of Bristol, Bristol BS8 1TL, United Kingdom.

fVisitor from University Libre de Bruxelles, B-1050 Brussels, Belgium.

gVisitor from University of California Irvine, Irvine, CA 92697, USA.

hVisitor from University of California Santa Cruz, Santa Cruz, CA 95064, USA.

iVisitor from Cornell University, Ithaca, NY 14853, USA.

jVisitor from University of Cyprus, Nicosia CY-1678, Cyprus.

kVisitor from University College Dublin, Dublin 4, Ireland.

lVisitor from University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.

mVisitor from University of Heidelberg, D-69120 Heidelberg, Germany.

nVisitor from Universidad Iberoamericana, Mexico D.F., Mexico.

oVisitor from University of Manchester, Manchester M13 9PL, England, United Kingdom.

pVisitor from Nagasaki Institute of Applied Science, Nagasaki, Japan.

qVisitor from University de Oviedo, E-33007 Oviedo, Spain.

rVisitor from Queen Mary, University of London, London, E1 4NS, England, United Kingdom.

sVisitor from Texas Tech University, Lubbock, TX 79409, USA.

tVisitor from IFIC (CSIC-Universitat de Valencia), 46071 Valencia, Spain.

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