Search for <em>B</em>_s⁰→<em>μ⁺μ⁻</em> and <em>B</em>_d⁰→<em>μ⁺μ⁻</em> Decays in <em>pp</em> Collisions with CDF II

Article

Reference

Search for B

→ μ

μ

and B

→ μ

μ

Decays in pp Collisions with CDF II

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al.

Abstract

We report on a search for B0s→μ+μ− and B0d→μ+μ− decays in pp collisions at s√=1.96  TeV using 364   pb−1 of data collected by the CDF II detector at the Fermilab Tevatron Collider.

After applying all selection requirements, we observe no candidates inside the B0s or B0d mass windows. The resulting upper limits on the branching fractions are B(B0s→μ+μ−)

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Search for B

→ μ

μ

and B

→ μ

μ

Decays in pp Collisions with CDF II. Physical Review Letters , 2005, vol. 95, no. 22, p. 221805

DOI : 10.1103/PhysRevLett.95.221805

Available at:

http://archive-ouverte.unige.ch/unige:38290

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

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Search for B

!

and B

!

Decays in p p Collisions with CDF II

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0031-9007=05=95(22)=221805(7)$23.00 221805-1 © 2005 The American Physical Society

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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, 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, 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

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

221805-2

23University of Illinois, Urbana, Illinois 61801, USA

24The Johns Hopkins University, Baltimore, Maryland 21218, USA

25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan

27Center for High Energy Physics: Kyungpook National University, Taegu 702-701; 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

42University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, 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 15 August 2005; published 23 November 2005; corrected 2 December 2005) We report on a search forB⁰_s!andB⁰_d! decays inpp collisions atps

1:96 TeV using364 pb¹of data collected by the CDF II detector at the Fermilab Tevatron Collider. After applying all selection requirements, we observe no candidates inside theB⁰_s orB⁰_d mass windows. The resulting upper limits on the branching fractions areBB⁰_s !<1:510⁷andBB⁰_d!<3:9 10⁸at 90% confidence level.

DOI:10.1103/PhysRevLett.95.221805 PACS numbers: 13.20.He, 12.15.Mm, 12.60.Jv, 13.85.Qk

In the standard model (SM), flavor changing neutral current (FCNC) decays are highly suppressed and can occur only through higher order diagrams. The decay rate for the FCNC decayB⁰_s ! [1] is proportional to the Cabibbo-Kobayashi-Maskawa matrix elementjV_tsj². The rate ofB⁰_d! decays is further suppressed by the ratio of jV_td=V_tsj². The SM expectations for these branching fractions are BB⁰s! 3:42 0:54 10⁹ and BB⁰_d ! 1:000:14 10¹⁰[2], which are about 2 orders of magnitude smaller than the current experimental sensitivity. However, new physics contributions can significantly enhance these branching fractions. An observation of these decays at the Tevatron would be unambiguous evidence for physics

beyond the SM. The best existing experimental bound on B is <4:110⁷ [3] (<8:310⁸ [4]) for B⁰_sB⁰_d ! at the 90% confidence level (C.L.).

In minimal supersymmetric (SUSY) extensions of the SM, additional diagrams involving SUSY particles also contribute to FCNC decay rates and the branching fraction BB⁰_s;d! / tan⁶, where tan is the ratio of vacuum expectation values of the two neutral CP-even Higgs fields. Large values oftanenhance the decay rate to a level observable by the Tevatron experiments [5]. For example, increases of 1–3 orders of magnitude are ob- tained in the minimal SO(10) models [6], which favor large values of tan. For the minimal flavor violating (MFV) models, B⁰_d! remains suppressed relative to

B⁰_s ! due to jV_td=V_tsj². This may not be true for non-MFV models such as R-parity violating SUSY [7], which can produce large enhancements, even for low val- ues oftan, in either or both of theB_sandB_dFCNC decay rates. Thus, a simultaneous observation ofB⁰_s;d! decays can be important in determining the flavor structure of the new physics. In the absence of an observation, any improvements to the limits can be used to constrain sig- nificantly many SUSY models [5–8].

In this Letter, we report on a search forB⁰_s !and B⁰_d!decays using364 pb¹ of data collected by the upgraded Collider Detector at Fermilab (CDF II). This data set includes 171 pb¹ of data from our previous measurement [9]. We significantly improve the sensitivity of the search over our former analysis by doubling our data sample, extending the muon acceptance, and using a like- lihood ratio technique for signal and background separa- tion. The limits we present here are the most stringent to date and supersede our previous results.

The CDF II detector is described in detail in Ref. [10].

The inner tracking system is composed of a silicon micro- strip detector (SVX II) [11] surrounded by an open-cell wire drift chamber (COT) [12]. These tracking detectors are immersed in a 1.4 T magnetic field and measurep_T, charged particle momentum in the plane transverse to the beam line. Four layers of planar drift chambers (CMU) [13] detect muon candidates with p_T>1:4 GeV=c and provide coverage in the pseudorapidity range jj<0:6, where lntan₂andis the angle of the track with respect to the beam line. The central muon extension (CMX) consists of conical sections of drift tubes and extends the coverage to0:6<jj<1:0 for muon candi- dates withp_T>2:0 GeV=c.

The data used in this analysis are selected by two classes of dimuon triggers: for the CMU-CMU (U-U) triggers, both muon candidates are reconstructed in the CMU cham- bers, while for the CMU-CMX (U-X) triggers, one of the muon candidates is reconstructed in the CMX chambers.

The inclusion of the U-X trigger increases the signal acceptance by about 50%. The details of the trigger system and selection requirements can be found in Refs. [9,10].

Since they have different sensitivities, we treat U-U and U-Xchannels separately, combining the results at the end.

The offline reconstruction begins by identifying two muon candidates of opposite charge which satisfy the on- line dimuon trigger requirements. To avoid regions of rap- idly changing trigger efficiency, we omit CMU (CMX) muon candidates with p_T <22:2 GeV=c. The random combinatoric backgrounds are suppressed by requiring the vector sum of the muon transverse momenta to bejp~_T j>

4 GeV=c. The remaining pairs of muon tracks are then refit, under the constraint that they come from the same three-dimensional (3D) space point, and are required to satisfy vertex fit quality criteria. The 3D decay length is given byL_3D L~p~=jp~j, where L~ is the displace- ment vector from the primary to the dimuon vertex. The

primary vertex is determined using a constrained vertex fit of all tracks in the event, excluding the pair and other secondary decay tracks. For each B candidate, we estimate the proper decay time ML_3D=jp~j, where Mis the invariant mass andp~is the momen- tum vector of the dimuon system. Additional background is reduced by demandingL_3D<1:0 cm(to remove poorly reconstructed tracks), the uncertainty onL_3Dto be less than 150m, and2< <0:3cm, wherecand is the total uncertainty on. There are 22 459 (14 305) di- muon candidates that fulfill all the above trigger and offline reconstruction requirements in theU-U(U-X) channel. At this stage, the data sample is dominated by random com- binatoric background.

We model the signal B⁰_s;d! decays using the

PYTHIA Monte Carlo (MC) program [14]. The PYTHIA

events are passed through a full detector simulation and satisfy the same requirements as data. Thep_T spectrum of the Bhadron in the MC sample is weighted to match the measurement from Ref. [10]. The signal MC samples are used primarily for analysis optimization and for estimating the efficiency of selection requirements.

For the final event selection, we use the following four discriminating variables: M, , the 3D opening angle between vectorsp~andL, and the~ B-candidate track isolation (I) [15]. We exploit the long lifetime ofBmesons to reject prompt combinatoric background with the decay length () requirement. Additional combinatoric back- ground and partially reconstructedBhadrons are removed with the pointing angle requirement. Since b-quark frag- mentation is hard,Bhadrons carry most of the transverse momentum of thebquark and, thus, are isolated. We use the variable I to enhance the heavy flavor content of the sample and also to reject partially reconstructedBhadrons, which are less isolated. Figure 1 compares the distributions of these variables for data (which is background- dominated) to MC signal events. Based on the observed distributions, we apply two additional loose requirements, I >0:5 and <0:7 rad, to further reject background while maintaining 92% of signal efficiency. We collec- tively refer to all the selection requirements applied up to this point as the ‘‘baseline’’ requirements. In the data, 6242 (4908) events survive the baseline requirements in the U-UU-Xsample.

To enhance signal and background separation, we con- struct a multivariate likelihood ratio based on the input variables: I, , and probability P e^=cBsd, where _Bsd is the world average Bsd lifetime. We use thePvariable instead ofin constructing the likelihood ratio, because the P distribution is nearly flat and, therefore, better behaved in the likelihood. We define the likelihood ratio to be

L_R

Q

i

P_sx_i Q

i

P_sx_i Q

i

P_bx_i; (1) 221805-4

where x₁ I, x₂ , x₃P, and Psbx_i is the probability that a signal (background) event has an ob- served x_i. The probability distributions for the signal events are obtained from the signal MC samples and the background distributions are taken from the data sidebands (defined below). The resulting L_R distributions for the signal and background events are shown in Fig. 2.

Although a subset of the data was used previously [9], we adopt an optimization strategy that uses only events in

the data sidebands in order to avoid biases in our choice of final selection criteria. The search window is defined by 5:169< M<5:469 GeV=c², corresponding to approxi- mately 6times the two-track invariant mass resolution, which is estimated to be_M 24 MeV=c². The sideband regions 4:669< M<5:169 GeV=c² and 5:469<

M<5:969 GeV=c² are used to estimate the back- ground in the signal region and for extensive analysis cross-checks.

TheB⁰_s!branching fraction is obtained by nor- malizing to the number of B!J= K!K decays collected by the same trigger. The B !J= K mode is reconstructed using the same baseline require- ments as the signal mode [16] but including an additional p_T>1 GeV=c requirement on the kaon candidate. The upper limit on the branching fraction is given by

BB⁰s!^90%C:L:N^90%

B⁰s

N_{B B B}⁰_s

base B base B⁰s

1

LR

B⁰_s

f_u

f_sBB !J= K; (2) where N_B^90%0

s is the number of B⁰_s! decays at the 90% C.L. for N observed and N_b expected background events. The value ofN^90%

B⁰s is estimated using the Bayesian approach of Ref. [17], assuming a flat prior for theBB⁰_s! , and incorporating Gaussian uncertainties into the limit. The number of reconstructed B!J= K candi- dates,N_B, determined from the data using sideband sub- traction is 178560 for the U-U and 69639for the U-X channel. The parameter _B0

s (_B) is the trigger acceptance and ^base

B⁰s ( ^base_B ) is the efficiency of the baseline requirements for the signal (normalization) mode. We apply the likelihood ratio requirement only to the signal mode, and, therefore, the efficiency of the likelihood ratio ^LR

B⁰s appears only in the denominator of Eq. (2). We use the fragmentation ratiof_u=f_s3:710:41[18]. The branching fraction BB!J= K !K 5:880:26 10⁵ is obtained from Ref. [17]. The ex- pression for BB⁰_d! is derived by replacing B⁰_s withB⁰_d and the fragmentation ratio withf_u=f_d 1.

The expected number of background events is estimated by extrapolating the number of sideband events passing the baseline requirements to the signal window and then scal- ing by the expected rejection factor for a given L_R requirement. The parameter is determined from the background L_R distribution, which we generate by ran- domly sampling the , , and I distributions from the data sidebands to improve statistical precision on . The relative uncertainty onN_bis15%(19%) for theU-U (U-X) channel. The dominant contribution comes from the limited statistics of the input distributions used to generate the backgroundL_R distribution.

We have cross-checked our background estimate proce- dure using control samples from the data: like sign Likelihood Ratio

0 0.2 0.4 0.6 0.8 1

Ld/Nd N/1R

10^-3 10^-2 10^-1

Background

Signal

FIG. 2 (color online). The likelihood ratio distribution for signal (dashed line) fromPYTHIAMC samples and background (solid line) from data sidebands in the U-U channel. Similar shapes are observed in theU-X channel.

(GeV/c )2 µ

Mµ

4.8 5 5.2 5.4 5.6 5.8

1/N dN/dM

0 0.05 0.1 0.15 0.2

(cm) λ 0 0.05 0.1 0.15 0.2 0.25 0.3

λ1/N dN/d

10^-4 10^-3 10^-2 10^-1

(rad) Θ

∆

0 0.5 1 1.5

Θ∆1/N dN/d

0 0.05 0.1 0.15 0.2 0.25 0.3

Isolation 0 0.2 0.4 0.6 0.8 1

1/N dN/dIsolation

0 0.05 0.1 0.15 0.2 0.25 0.3

FIG. 1 (color online). Distributions of the discriminating var- iables for data (dominated by background) shown in the solid histogram and signalB⁰s ! events shown in the dashed histogram.

events, events with <0, and a fake-muon en- hanced sample in which we demand at least one muon candidate fail the muon quality requirements. We compare our background predictions to the number of events observed in the search window for a wide range of L_Rrequirements. No statistically significant discrepancies are observed. For example, requiringL_R>0:99and sum- ming over all control samples, we predict 3.8 (4.0) back- ground events and observe 4 (3) in theU-U(U-X) channel.

In addition, we have considered background contributions fromB⁰_s;d!hh, whereh orK, in the signal window and determined that the contribution from those decays is negligible.

The acceptance ratio_B=_B⁰

s;dobtained from the signal MC sample is0:2970:020(0:1910:013) for theU-U (U-X) channel. The uncertainty includes contributions from systematic variations of the modeling of the B-hadronp_T spectrum and longitudinal beam profile and from the statistics of the MC sample.

The quantity ^baseincludes the trigger and offline recon- struction efficiencies. The trigger efficiency is determined from inclusive data samples unbiased with respect to the triggers used here. The ratio ofBtoB⁰_strigger efficiencies is measured to be less than 0.1% away from unity. We evaluate the single track COT, SVX II, and muon efficien- cies using a data sample of inclusiveJ= !decays collected on single-muon triggers. The relevant double- track efficiencies are computed by convolution withB⁰_s ! andB!J= KMC events surviving the trigger requirements. The offline reconstruction efficiency be- tween signal and normalization mode also largely cancels in the ratio with the exception of the kaon efficiency from theBdecay. Last, we obtain the efficiency of the remain- ing baseline requirements from the signal MC sample and cross-check the results by comparing B data and MC samples. Combining all effects, we find ^base_B = ^base_B0

s;d

0:9200:034(0:9150:034) for the U-U (U-X) chan- nel. The uncertainty is dominated by systematic uncertain- ties accounting for kinematic differences between J= ! andB⁰_s;d! decays.

The efficiency of the likelihood requirement ^L_B^R0 s;d

is estimated from the signal MC sample. The efficiency varies from about 70% for L_R>0:90 to 35% for L_R>

0:99. We assign a relative systematic uncertainty of 5%

to bothU-U andU-X channels based on comparisons of B!J= K MC and data samples.

We optimize the analysis based on thea prioriexpected 90% C.L. upper limit onBB⁰_s;d!. The expected limit for a given set of optimization requirements is com- puted by summing the 90% C.L. limits over all possible observations N, weighted by the corresponding Poisson probability when expecting N_b. We scan over a range of L_R requirements and determine the optimal value to be L_R>0:99. With the optimized selection requirements, the expected number of background events is the same in the

B⁰_s and B⁰_d search windows. Within the signal region of 60 MeV=c²2:5_Mabout the world averageB⁰_s orB⁰_d mass [17], N_b is 0:810:12 [0:660:13], and the B⁰_s single-event sensitivity is 1:00:2 10⁷ [1:5 0:3 10⁷] for the U-U [U-X] channel. Using these criteria, we observe no events inside either the B⁰_s or B⁰_d signal box as shown in Fig. 3. The one candidate event that survived in the previous analysis [9] has anL_Rvalue of 0:8and fails the selection requirements here. Using Eq. (2) and combining the U-U and U-X channels, taking into account the correlated uncertainties, we derive 90% (95%) C.L. limits of BB⁰_s !<1:510⁷ (2:0 10⁷) and BB⁰_d !<3:910⁸ (5:110⁸).

The new limits improve the previous limits [3,4] by a factor of 2 and can be used to reduce the allowed parameter space of a broad spectrum of SUSY models [6 –8].

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 Foundat- ion; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foun- dation; the Particle Physics and Astronomy Research Council and the Royal Society, U.K.; the Russian Foun- dation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; in part by the European

Likelihood Ratio

0.8 0.9 1

2 c/VeG /µµM

4.8 5 5.2 5.4 5.6 5.8

Bd

Bs

FIG. 3 (color online). The invariant mass distribution versus likelihood ratio for events satisfying baseline require- ments forU-U(solid triangles) andU-X(open circles) channels.

TheB⁰_s (solid box) andB⁰_d (dashed box) signal regions are also shown. The one candidate event observed in the previous analy- sis is highlighted with a star symbol (atL_R 0:8).

221805-6

Community’s Human Potential Programme under Con- tract No. HPRN-CT-2002-00292; and the Academy of Finland.

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[15] The B-candidate isolation is defined as I jp~_T j=

P

ipⁱ_T jp~_T j, where the sum is over all tracks with ²²

p 1; and are, respectively, the

azimuthal angle and pseudorapidity of trackiwith respect top~. Also see V. Krutelyov, Ph.D. thesis, Texas A&M University, 2005 (unpublished).

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