Evidence for the Exclusive Decay <em>B_c^±</em> -&gt; <em>J/ψπ^±</em> and Measurement of the Mass of the <em>B_c^±</em> Meson

Article

Reference

Evidence for the Exclusive Decay B

-> J/ψπ

and Measurement of the Mass of the B

Meson

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al .

Abstract

We report the first evidence for a fully reconstructed decay mode of the B±c meson in the channel B±c→J/ψπ±, with J/ψ→μ+μ−. The analysis is based on an integrated luminosity of 360   pb−1 in pp collisions at 1.96 TeV center of mass energy collected by the Collider Detector at Fermilab. We observe 14.6±4.6 signal events with a background of 7.1±0.9 events, and a fit to the J/ψπ± mass spectrum yields a B±c mass of 6285.7±5.3(stat)±1.2(syst)  MeV/c2. The probability of a peak of this magnitude occurring by random fluctuation in the search region is estimated as 0.012%.

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Evidence for the Exclusive Decay B

-> J/ψπ

and Measurement of the Mass of the B

Meson. Physical Review Letters, 2006, vol.

96, no. 08, p. 082002

DOI : 10.1103/PhysRevLett.96.082002

Available at:

http://archive-ouverte.unige.ch/unige:38324

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

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Evidence for the Exclusive Decay B

! J=

and Measurement of the Mass of the B

Meson

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(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

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

082002-2

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, University of Padova, 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 23 May 2005; revised manuscript received 14 November 2005; published 28 February 2006) We report the first evidence for a fully reconstructed decay mode of the Bc meson in the channel B_c !J= , withJ= !. The analysis is based on an integrated luminosity of360 pb¹inpp collisions at 1.96 TeV center of mass energy collected by the Collider Detector at Fermilab. We observe 14:64:6signal events with a background of7:10:9events, and a fit to theJ= mass spectrum yields aB_c mass of6285:75:3stat 1:2systMeV=c². The probability of a peak of this magnitude occurring by random fluctuation in the search region is estimated as 0.012%.

DOI:10.1103/PhysRevLett.96.082002 PACS numbers: 13.25.Hw, 14.40.Lb, 14.40.Nd

Within the standard model of elementary particles, five of the six different kinds of quarks combine in quark- antiquark pairs to make mesons. TheB_c meson combines the two heaviest of these quarks as a bottom-charm quark- antiquark pair. Although it has been observed in semilep- tonic decay modes [1,2], up to now no evidence for theB_c has been found in fully reconstructed decay modes [3– 6].

Consequently, its massMB_chas not been measured with good precision.

Nonrelativistic potential models predict the b and c quarks to be tightly bound with a ground state mass in the approximate range6200–6300 MeV=c²[7– 9]. Recent QCD-based perturbative computations up toO⁴_spredict MB_cto be630717 MeV=c²[10,11]. Most recently, a three-flavor lattice QCD calculation obtains MB_c 630412statsyst¹⁸₀ cutoff effectsMeV=c² [12].

Several of the predictedB_c decay modes contain aJ=

meson [13]. These are among the most easily reconstruc-

tible B_c decays at CDF, owing to an efficient dimuon trigger giving high purity J= ! reconstruction.

The CDF Collaboration made the first observation of the B_c meson in the semileptonic decay channels B_c ! J= l_lX, in a sample of 110 pb¹ of data at ps

1:8 TeVin run I at the Tevatron [1]. The symbolXdenotes possible undetected decay particles. With a signal of 20:4^6:2_5:5 events, theB_c mass was measured to be6:40 0:39stat 0:13systGeV=c². Recently, the D0 Collab- oration reported a preliminary observation of aB_c signal in the decay channel B_c !J= X in a sample of 210 pb¹ of run II data [2].

In this Letter we report first evidence for theB_c meson in the fully reconstructed decay channel B_c !J= , withJ= !. The analysis is based on a data set of 360 pb¹ inpp collisions collected at

ps

1:96 TeVby CDF at the Tevatron during run II.

The CDF II detector consists of a magnetic spectrometer surrounded by calorimeters and muon chambers and is described in detail elsewhere [14]. The components most relevant to this analysis are briefly described here. The tracking system is in a 1.4 T axial magnetic field and consists of a silicon microstrip detector (L00, SVX, ISL, in increasing order of radius) [15–17] surrounded by an open-cell wire drift chamber (COT) [18]. The muon de- tectors used for this analysis are the central muon drift chambers (CMU), covering the pseudorapidity rangejj<

0:6 [19,20], and the extension muon drift chambers (CMX), covering0:6<jj<1:0. Cylindrical coordinates are used with thezaxis in the proton beam direction.

This measurement uses events containing pairs of muons, each with jj<1:0, selected with a three-level trigger. At the first trigger level, muon-candidate track segments in CMU and CMX are matched to COT tracks obtained with a hardware processor [21]. Dimuon triggers use combinations of CMU-CMU and CMU-CMX muons with p_T>1:52:0 GeV=c for CMU (CMX) muons, where p_T is the momentum transverse to the beam line.

At the second level, opening angle and opposite-charge cuts are imposed on the muon pairs. At the third level, three-dimensional (3D) tracking is performed to select muon pairs with invariant mass, M, between 2700 and4000 MeV=c².

To reconstruct the B_c !J= decay offline, we make several requirements on the quality of the tracks and the J= candidate. To ensure good primary and sec- ondary vertex resolution, each track must have an r position measurement on at least three of five SVX layers.

ForJ= identification, we require matching between the COT muon tracks and the muon chamber track seg- ments. In addition, we require that 3042< M<

3152 MeV=c², the average J= mass resolution in our sample being 14 MeV=c². Each other charged particle track withp_T >400 MeV=cis treated as a pion candidate to be combined with theJ= . The pion candidate and the

two muons are then fitted to a common 3D vertex, with M constrained to the world average J= mass value [22]. All combinations for which the vertex fit con- verged are retained. The primary vertex position is calcu- lated from the other tracks in each event.

The B_c search was performed using the following analysis method. The mass values of the J= combi- nations in the search window 5600< MJ= <

7200 MeV=c², referred to asB_c candidates, were tempo- rarily hidden. The search window was chosen to corre- spond to the2standard deviation region around the CDF run I measurement of theB_c mass [1]; it is approximately 100 times wider than the expectedB_c mass resolution.

In order to optimize the significance of a possible B_c signal, we varied the selection criteria to maximize the function QS_F=1:5

B_av

p [23]. Here,S_F is the ac- cepted fraction of signal events, in this case taken from a Monte Carlo sample, and the backgroundB_avis the number of selectedB_c candidates within the search window, scaled to correspond to a mass range of63 MeV=c², based on the average mass resolution of a B_c candidate within the search window. The term 1.5 is appropriate for optimizing a search for a signal at least3above background fluctua- tions. The distributions of the selection variables for the signal events were evaluated using samples of simulated B_c !J= decays. These were generated with a B_c mass of6400 MeV=c², a lifetime of 0.46 ps [1], andp_Tand rapidity distributions according to a leading order pertur- bative QCD calculation [24]. A harder p_T spectrum [25]

was used as an alternative to check the stability of the optimal selection criteria; these were not very sensitive to variations of the p_T spectrum or the assumed lifetime within its experimental uncertainty. The Monte Carlo B_c decays were processed with full detector simulation and the same trigger and reconstruction criteria as the data. The distributions of the selection variables for the background were taken from the data in the search window, in which the contribution from a signal is expected to be small.

Optimized cuts were determined for the following se- lection variables: the J= three-track 3D vertex fit (²<9for 4 degrees of freedom), the pion track contri- bution to the vertex fit (²<2:6), the impact parameter in r of the B_c candidate with respect to the primary vertex (<65m), the maximumctwheretis the proper decay time of theB_c candidate (<750m), the transverse momentum of the pion (>1:8 GeV=c), the 3D angle be- tween the momentum of theB_c candidate and the vector joining the primary to the secondary vertex ( <0:4 rad), and the significance of the projected decay length of the B_c candidate onto its transverse momentum direction [L_xy=L_xy>4:4]. After these selections, 390 candidates remain in the search window, with no two candidates from the same beam crossing.

A sample of B mesons, reconstructed in the decay mode B !J= K, was analyzed as a control sample 082002-4

in order to check our understanding of the reconstruction of the relevant variables in the simulation. TheB!J= K decay topology is the same as that ofB_c !J= , apart from the different masses and lifetimes. The B mass distribution, shown in Fig. 1, was obtained using the same selection requirements as optimized for theB_c can- didates, but without the cut on maximum ct. A total of 237857 B!J= K signal events is found, with a fitted mass of 5279:00:3 MeV=c². The fit takes into account a small contribution from the Cabibbo-suppressed decay B!J= . The average mass resolution is 11:50:3 MeV=c², in agreement with the simulation, which can thus be used with confidence to evaluate the expected mass resolution forB_c decays. TheByield is used to calculate the expected B_c yield. The relative trigger and reconstruction efficiency, _Bc=_B, is in the range 35%–85%, with uncertainties arising from theB_c p_T spectrum and theB_c lifetime. On the basis of theB yield, previous CDF cross section measurements [1], and theoretical calculations [13,26 –31] of the branching frac- tions of the B_c !J= and B_c !J= l decay modes, a B_c yield in the range of 10 to 50 events is expected.

A search procedure was then defined to identify any possible signal in the data and to estimate its significance.

This was based on a scan of the search region in 10 MeV=c²intervals, with a sliding fit window extending from100to200 MeV=c²in mass around each nominal peak position, m. This window was chosen to minimize possible contributions from partially reconstructedB_c de- cays below the peak position (e.g., intoJ= and more than one additional particle). For each value ofm, a fit function was defined as a Gaussian signal with meanm, combined with a linear background term. The Gaussian resolution was fixed as a linear function ofmbased on Monte Carlo simulation, and varied from 13 to 19 MeV=c² over the search region. The three fit parameters were the number of signal (S) and background (B) events and the linear back-

ground slope. The output of a scan was defined to be the largest value of S=1:5

pB

, _max, obtained from the 131 fits performed in the mass interval 5700 MJ= 7000 MeV=c².

The distribution of _max for the null hypothesis was obtained from Monte Carlo experiments [32], in which the mass spectra were derived from a smooth background model. This model was necessarily approximate owing to the initially hidden mass distribution. The model consisted of a linear background, to describe combinatoric events, and a ‘‘physical’’ background to describe partially recon- structedB_c decays in the mass range below6400 MeV=c². Studies showed that the main source of combinatoric back- ground are events in which a genuineJ= is paired with an uncorrelated track. The shape of the physical background was based on Monte Carlo simulations of inclusiveB_c ! J= Xdecays, with branching ratios taken from Ref. [13].

Applied to the 390 event data sample, the scan procedure found a _max near m6290 MeV=c², which is compat- ible with aB_c signal of196events. Using a large set of Monte Carlo simulations, we modeled the shape of the observed background, and, analyzing it in the same way as the data, evaluated the probability that a random enhance- ment has a _max value exceeding that of the data. This probability was found to be 0.17%.

After the above steps had been performed, further checks on the previously hidden events revealed that the existing pion selection allowed two classes of fitted tracks that were unsuitable for theB_c search. The first class had insufficient number of COT hits to give good mass resolu- tion and so was not compatible with a search for a narrow Gaussian signal; the second class had poor SVX resolution in the z direction and was dominated by combinatorial background. The above two classes of events contributed 10% to theBsignal; they would be expected to contribute fewer than two events to the B_c signal, but they increase the combinatorial background by about 40% over the J= mass range. After removal of both classes of poor quality tracks in addition to the original optimized cut selection, 220 candidates remained. These were re- quired to have good SVX z resolution on both the pion track and at least one of the muon tracks. This final track selection, which maximizesQ, is therefore not fully blind;

it is based also on the observed properties of theBsignal and the overall properties of theB_c candidate sample.

Figure 2 shows the mass spectrum for the 220 event sample. The main features are the B_c !J= signal peak near 6290 MeV=c², a linear combinatorial back- ground above this peak, and a broad enhancement below the peak which can be attributed to the physical back- ground from partially reconstructed B_c decays. We per- form a global unbinned likelihood fit over the entire mass range to obtain the mass and yield for theB_c signal. The fit included a Gaussian signal with a variable mass but with a resolution whose mass-dependent value was determined by

2) K mass (MeV/c ψ

J/

5200 5300 5400 5500 5600

2 Entries in 5 MeV/c

0 50 100 150 200 250 300 350

400 B^±→ J/ψ K^±

FIG. 1 (color online). The invariant mass distribution of the B!J= K candidates. The curve is a fit to the data.

the Monte Carlo simulation, together with background modeled as a linear combinatorial term and a broad low- mass Gaussian contribution for the physical background. A signal of14:64:6events is obtained centered at a mass of 6285:75:3 MeV=c². The standard deviation of the Gaussian at the central value of the signal mass is 15:5 MeV=c². The background within a region of 2 standard deviations from this mass value is 7:10:9 events. The statistical significance of the signal is dis- cussed below. Within the signal region, the distributions of the selection variables agree within statistics with those of the Monte Carlo simulation.

Systematic uncertainties on theB_c mass determination due to measurement uncertainties on the track parameters (0:3 MeV=c²) and the momentum scale (0:6 MeV=c²) are evaluated from the corresponding uncertainties on the B mass analysis [33]. Further uncertainties are due to the possible differences in thep_T spectra of theBand B_c mesons (0:5 MeV=c²) and our limited knowledge of the background shape used in the final mass fit as well as uncertainty in the signal width (0:9 MeV=c²) [34].

The total systematic uncertainty is evaluated to be 1:2 MeV=c².

The signal peak is robust under variations of the pion track quality selection. We have investigated several meth- ods for determining the best figure of significance for such a peak over a broad mass range. The method that gives the best sensitivity to a real signal is based on the standard significance measureS=

pB

. We repeated the Monte Carlo scans for the new track selection to determine the null hypothesis distribution for S=

pB

. Applying to the Monte Carlo simulations the same global fit method as to the data, we find that the probability that a random en- hancement anywhere in the range 5800–7000 MeV=c² exceeds the value of S=

pB

for the experimental peak is 0.012%.

In view of the limited statistics of the observed mass peak, an independent consistency check was performed. If the mass peak is due to fully reconstructed B_c !J=

decays, partially reconstructed B_c !J= trackX decays should be detectable in the mass region below the peak but not in the region above. The pion candidate in partially reconstructed decays should have a small impact parameter d_xy relative to the J= vertex, consistent with being physically associated with it, whereas the pion can- didate in combinatorial background events should have a broad d_xy distribution reflecting random association with theJ= vertex.

To investigate this, we relax the cuts on , the im- pact parameter of theB_c candidate, and the² of the 3D vertex fit, so as to make a signal in the d_xy distribu- tion visible over the broader combinatorial background.

We compare the distribution ofd_xy of the pion candidate in the region 5600< MB_c<6190 MeV=c² (lower side band) to that in the region6390< MB_c<7200 MeV=c² (upper side band), where the main contribution should be combinatorial.

Figure 3 (top panel) shows the difference between the lower (4900–5100 MeV=c²) and upper (5400–

5700 MeV=c²) sidebands for the d_xy distribution in the B data sample, with a large excess of events visible at small d_xy values. Figure 3 (bottom panel) shows the cor- responding plot obtained using theB_c candidate sample.

An enhancement is visible with a shape compatible with that seen in the B sample. TheB curve, rescaled to fit the B_c data, provides a good description of this distribu- tion. The excess of low d_xy events in the B_c sample is evaluated to be 24459, where the uncertainty is statis- tical only. This result is consistent with Monte Carlo esti-

mµEntries / 10

0 50 100 150 200 250 300

350 J/ψ K^±

µm)

xy ( Impact Parameter d 0 50 100 150 200 250 300 350 400 450 500 -20

0 20 40

60 J/ψπ^±

FIG. 3 (color online). Impact parameter of the third track relative to the J= vertex for the lower sideband region, after subtraction of the same distribution for the upper side band: (top panel) the curve is the sum of two Gaussians, fitted to the B data points; (bottom panel) theB_c data points, overlaid with the above curve, rescaled. In both cases the selection criteria were relaxed.

2) invariant mass (MeV/c π

ψ J/

5600 5800 6000 6200 6400 6600 6800 7000 7200

2 Entries / 10 MeV/c

0 1 2 3 4 5 6

6100 6200 6300 6400 6500 0

2 4 6

FIG. 2. The invariant mass distribution of theJ= candi- dates and results of an unbinned likelihood fit in the search window. The inset shows the peak section of the distribution.

The broad enhancement below 6:2 GeV=c² is attributable to partially reconstructedBc mesons.

082002-6

mates based on the calculations of [13]. This supports the hypothesis that the broad physical background below the signal peak, evident in Fig. 2, is in fact associated with partially reconstructedB_c decays.

In conclusion, we observe a peak in the J=

mass spectrum at a mass of 6285:75:3stat 1:2syst MeV=c². This peak is consistent with a narrow, weakly decaying particle state and is interpreted as the first evidence for fully reconstructed decays of theB_c meson.

The mass value has much improved precision over the results obtained in B_c semileptonic decays [1,2]. There is also good agreement with recent theoretical predictions for theB_c mass around6300 MeV=c²[10 –12].

We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.

We also thank A. V. Berezhnoy, C. H. Chang, and X. G. Wu for making available their calculations of B_c production spectra. This work was supported by the U.S. Department of Energy and the 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 Particle Physics and Astronomy Re- search Council and the Royal Society, U.K., the Russian Foundation for Basic Research, the Comision Inter- ministerial de Ciencia y Tecnologia, 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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