Measurement of σ_{Λ_b⁰}/σ_B⁰×B(Λ_b^o -&gt; Λ_c⁺π⁻)/B(B⁰ -&gt; D⁺π⁻) in <em>pp</em> Collisions at s√=1.96  TeV

Article

Reference

Measurement of σ

b

0

/σ

×B(Λ

-> Λ

π

)/B(B

-> D

π

) in pp Collisions at s√=1.96  TeV

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al.

Abstract

We present the first observation of the baryon decay Λ0b→Λ+cπ− followed by Λ+c→pK−π+ in 106  pb−1 pp collisions at s√=1.96  TeV in the CDF experiment. In order to reduce systematic error, the measured rate for Λ0b decay is normalized to the kinematically similar meson decay B0→D+π− followed by D+→π+K−π+. We report the ratio of production cross sections (σ) times the ratio of branching fractions (B) for the momentum region integrated above pT>6  GeV/c and pseudorapidity range |η|

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of σ

b

0

/σ

×B(Λ

-> Λ

π

−

)/B(B

-> D

π

) in pp Collisions at s√=1.96  TeV. Physical Review Letters , 2007, vol. 98, no.

12, p. 122002

DOI : 10.1103/PhysRevLett.98.122002

Available at:

http://archive-ouverte.unige.ch/unige:38377

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

1 / 1

Measurement of

b

=

B

!

= B B

! D

in p p Collisions at p s

1:96 TeV

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J. C. Yun,¹⁶L. Zanello,⁵¹A. Zanetti,⁵⁴I. Zaw,²¹X. Zhang,²³J. Zhou,⁵²and 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

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

23University of Illinois, Urbana, Illinois 61801, USA

122002-2

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, Korea;

Seoul National University, Seoul 151-742, Korea;

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

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

36Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

37University of New Mexico, Albuquerque, New Mexico 87131, USA

38Northwestern University, Evanston, Illinois 60208, USA

39The Ohio State University, Columbus, Ohio 43210, USA

40Okayama University, Okayama 700-8530, Japan

41Osaka City University, Osaka 588, Japan

42University of Oxford, Oxford OX1 3RH, United Kingdom

43University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

44LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

45University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

46Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, I-56127 Pisa, Italy

47University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

48Purdue University, West Lafayette, Indiana 47907, USA

49University of Rochester, Rochester, New York 14627, USA

50The Rockefeller University, New York, New York 10021, USA

51Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy

52Rutgers University, Piscataway, New Jersey 08855, USA

53Texas A&M University, College Station, Texas 77843, USA

54Istituto Nazionale di Fisica Nucleare, University of Trieste/ Udine, Italy

55University of Tsukuba, Tsukuba, Ibaraki 305, Japan

56Tufts University, Medford, Massachusetts 02155, USA

57Waseda University, Tokyo 169, Japan

58Wayne State University, Detroit, Michigan 48201, USA

59University of Wisconsin, Madison, Wisconsin 53706, USA

60Yale University, New Haven, Connecticut 06520, USA (Received 28 December 2005; published 19 March 2007)

We present the first observation of the baryon decay ⁰_b !_c followed by _c !pK in 106 pb¹ppcollisions at

ps

1:96 TeVin the CDF experiment. In order to reduce systematic error, the measured rate for ⁰_b decay is normalized to the kinematically similar meson decay B⁰!D followed by D!K. We report the ratio of production cross sections () times the ratio of branching fractions (B) for the momentum region integrated abovep_T>6 GeV=cand pseudorapidity range jj<1:3: pp!⁰_bX=pp!B⁰X B⁰_b!c=BB⁰!D 0:82 0:08stat 0:11syst 0:22B_c !pK .

DOI:10.1103/PhysRevLett.98.122002 PACS numbers: 14.20.Mr, 13.30.Eg, 14.65.Fy

Weak decays of baryons containingbquarks are a good laboratory for testing the Heavy Quark Effective Theory (HQET) [1]. The⁰_bbaryon is the ground state of theudb quark system, and, in the heavy quark limit, the light degrees of freedom are in the state of zero total angular momentum [2]. Fully hadronic b!cud transitions are

more complicated in baryons than in mesons because there are diagrams which are not present in the decays of the latter. Various extensions of HQET have been used to evaluate the ⁰_b!_c decay rate [3], but the predic- tions vary over a large range. However, in Soft Collinear Effective Theory (SCET) [4], all tree-level amplitudes

can be properly evaluated, resulting in an explicit predic- tion for the ratio of branching fractions B⁰_b ! _c=BB⁰ !D 1:7 [5]. The decays of ⁰_b are also interesting because they may provide a means to determine Cabibbo-Kobayashi-Maskawa (CKM) matrix elements with different systematic uncertainties than the determinations from the decays ofBmesons [6].

This is the first reconstruction of a hadronic decay of ab baryon at a hadron collider that does not use aJ= in the final state. In addition, our sample has more than an order of magnitude more events than any previous sample of fully reconstructed⁰_b decays, and, for the same luminos- ity, is about 5 times larger than a sample of_b !J=

decays. Sincebbaryons are not produced at theBfactories operating at the4Sresonance, studying them comprises a unique facet of the B physics program at Collider Detector at Fermilab (CDF II) [7]. In particular, a large sample of fully reconstructed⁰_bdecays would allow CDF to study other properties ofbbaryons, e.g., to measure the lifetime of⁰_b, and also to search for decays of heavierb baryons such as_b !⁰_b.

This Letter presents a measurement of a ratio of⁰_band B⁰ branching fractions multiplied by the ratio of produc- tion cross-sections,

Rpp !⁰_bX pp !B⁰X

B⁰_b!_c

BB⁰!D; (1) where thequantities are the cross-sections for⁰_bandB⁰ production in the pseudorapidity range jj<1:3 with momentum in the transverse plane, p_T, above 6 GeV=c [8]. This ratio compares the branching fractions of the topologically similar, fully reconstructed decays ⁰_b ! _c and B⁰!D, where the charmed hadrons decay via similar three-body channels _c !pK andD!K[9]. The quantityRis obtained from

RN⁰

b

NB⁰

BD !K B_c !pK

B⁰

⁰_b; (2) where the first factor is the ratio of observed signal yields, the second factor is the ratio of the (external) daughter branching fractions [10], and the third factor is the ratio of reconstruction efficiencies calculated from the Monte Carlo simulations. To obtain R, the ratio of the reconstruction efficiencies of two topologically and kine- matically similar decay modes is evaluated, reducing the systematic errors on the measured quantity.

The upgraded CDF II detector is well-suited for the detailed study of weak decays of heavy baryons. In par- ticular, the advent of the Silicon Vertex Trigger (SVT) [11], which uses precise position measurements to select events containing weakly decaying heavy hadrons, allows CDF II to collect many hadronic decay modes of heavy baryons for the first time. This measurement is performed using a 106 pb¹ sample of pp collisions collected by CDF II between February 2002 and June 2003. This data sample

corresponds to 10¹⁰ b hadron decays produced in the central detector region. A full description of the CDF II detector can be found elsewhere [7]. The detector compo- nents pertinent to this analysis are the silicon microstrip vertex detector (SVX II) [12], the drift-chamber central tracker (COT) [13] and a three-tiered trigger system (Levels 1, 2, and 3). The five double-sided layers of the SVX II used in this analysis provide up to 10 position measurements. Of these, up to five are in ther-[8] plane (each precise to about15m), three are longitudinal, and two are small-angle-stereo. Thestrips are parallel to the z-axis, longitudinal strips are inclined at 90, and the small-angle-stereo strips are inclined at 1.2. The SVX II spans the radii between 2.5 and 10.6 cm and covers the pseudorapidity range jj<2. The COT has 96 measure- ment layers between the radii of 40 and 137 cm. These are organized into alternating axial and small-angle-stereo (2) superlayers. The COT has a smaller pseudorapidity coverage (jj<1:3) than the SVX II. Both tracking de- tectors are immersed in a 1.41 T magnetic field parallel to thezaxis.

This analysis, in particular, relies on the SVT, which operates as a part of the Level 2 trigger system. The trigger makes it possible to select events at a rate of100 Hzfrom the1 MHzinteraction rate. The components of the three level trigger system pertinent to this measurement are the Extremely Fast Tracker (XFT) at Level 1 and the SVT at Level 2. The XFT uses four axial superlayers of the COT to find tracks withp_T>1:5 GeV=c. The SVT combines the XFT measurement withr-hits from the SVX II detector.

The track finding is performed using a large lookup table of hit patterns. The found track candidates are fitted for curvature, angle projected onto the transverse plane and impact-parameter [14]. The impact-parameter measure- ment allows the selection of long-lived particles in the trigger decision.

The signal (⁰_b!_c) and normalization (B⁰! D) events are collected using the same trigger. At Level 1, two tracks must satisfyp_T>2:0 GeV=c, a scalar sum of transverse momenta p_T1p_T2>5:5 GeV=c, and an angular separation projected onto the transverse plane of <135. At Level 2, the transverse momentum cuts are repeated, and it is required that each track has impact parameter d₀>120m, with an angular separation be- tween the tracks projected onto the transverse plane of (2< <90). Finally, the distance evaluated in the transverse plane from the primary vertex to the two-track intersection point must be greater than200m.

Additional criteria are imposed on the triggered sample in order to reject as many background events as possible while keeping most of the signal. To reconstruct ⁰_b! _c, minimum acceptable COT and SVX hit require- ments are imposed, and all combinations of four tracks that pass such requirements are considered. Particle identifica- tion at CDF is possible only on a statistical basis and is not 122002-4

used in this analysis. Thep_T of the proton candidate from the_c and thecandidate from the⁰_bmust be greater than2:0 GeV=c, which strongly favors these particles to be the two which caused the event to pass the trigger. Thep_T of the proton candidate must be larger than thep_T of the candidate from the _c. The p_T of the ⁰_b and _c candidates must be greater than 7:5 GeV=c and 4:5 GeV=c, respectively. In Eq. (1), ⁰

b and B⁰ are

defined forp_T >6 GeV=c. The events in the data sample must satisfy p_T >7:5 GeV=c, and the difference is ac- counted for by using the Monte Carlo simulation based on thep_Tdistributions of both⁰_bandB⁰measured in data [15].

Each of the unstable particles (⁰_b,B⁰,_c, andD) is reconstructed by considering all valid combinations of tracks and requiring them to satisfy the decay hypothesis.

The charmed hadrons_c andD are reconstructed first:

each triplet of tracks that satisfies the selection criteria (detailed below) is constrained to pass through the same point, called the decay vertex. The decay vertex is deter- mined by varying the track parameters of the stable daugh- ters within their uncertainties to minimize the ². The _cD candidate is then combined with a fourth track to form a⁰_bB⁰candidate. The full topology of the decay is then imposed in another kinematic fit, resulting in a simultaneous measurement of the ⁰_b (B⁰) and_c (D) vertices.

Using these measurements, the reconstructed invariant mass of the _c must be between 2:269 GeV=c² and 2:301 GeV=c². Other selection criteria rely on L_xy, the projection onto the x-y (transverse) plane of the decay length measured from the production vertex to the decay point; the production vertex is estimated by the position of the beam line averaged over each run calculated for thez coordinate of the secondary vertex. A product of the proper decay time and the speed of light, ct, is also used. It is derived from L_xy: ct L~_xyp^_Tmc=p_T, where L~_xy is the decay vector of⁰_b (B⁰) projected onto thex-yplane, p_Tis the transverse momentum,p^_Tis the unit vector in the direction of the transverse momentum, andmis the world average mass of the ⁰_b (B⁰). In order to suppress the combinatorial background from the interaction point, we imposect⁰_b>225m(compared to thebbaryon mean decay length of36824m[10]). Calculated relative to the ⁰_b decay point, ct_c must bect_c>65m.

For a true _c, a small negative ct may arise due to resolution effects; on the other hand, thect_c of com- binatorial background candidates may have large negative values. The distance of closest approach in the transverse plane of the trajectory of the⁰_bcandidate to the primary vertex must be less than85m.

The normalization mode (B⁰!D) is reconstructed using selection criteria identical to those of the signal mode, except for a different invariant mass requirement for the D candidate and no analogy to the p_Tp>

p_T cut. The D candidate invariant mass must be between 1:848 GeV=c² and1:888 GeV=c². The distribu- tions of the invariant mass of ⁰_b!_c and B⁰! Dcandidates are shown in Figs.1and2, respectively.

Figure1shows the binned likelihood fit to the invariant mass distribution of ⁰_b candidates. The large Gaussian peak at5:6 GeV=c² is the⁰_b !_csignal. The dash- dotted curve corresponds to the exponential combinatorial background. This component is constrained by the data in the invariant mass region above the ⁰_b mass. The small asymmetric peak at5:5 GeV=c²(solid line) corresponds to contributions from fully reconstructed B-meson decays resulting in a final state with four tracks, where at least

2) Mass (GeV/c π-

c

Λ+

5 5.5 6 6.5 7

)2 Events / ( 0.02 GeV/c

0 50 100 150 200

2) Mass (GeV/c π-

c

Λ+

5 5.5 6 6.5 7

)2 Events / ( 0.02 GeV/c

0 50 100 150 200

Other B-meson decays decays Λb

Other

Combinatorial background Four particle B-meson decays

FIG. 1. ⁰_b!_c yield with binned likelihood mass fit.

The background shapes are defined in the text.

2) Mass (GeV/c π-

D+

4.6 4.8 5 5.2 5.4 5.6 5.8 6

)2 Events / ( 0.01 GeV/c

0 20 40 60 80 100 120 140 160 180

2) Mass (GeV/c π-

D+

4.6 4.8 5 5.2 5.4 5.6 5.8 6

)2 Events / ( 0.01 GeV/c

0 20 40 60 80 100 120 140 160

180 Physics backgrounds Combinatoric background

FIG. 2. B⁰!D yield with binned likelihood fit. The background shapes are defined in the text.

one track is misidentified. This shape is obtained using a full detector simulation of these modes. It is consistent with the shape ofB⁰!Dcandidates found in the⁰_b sample. The dotted and dashed curves correspond to all the other B-meson and ⁰_b backgrounds, respectively. These shapes are determined from a large parametric Monte Carlo sample which includes all known decays of B, B⁰, and B_s and⁰_b hadrons. Finally, there is a very small Gaussian distribution (not shown) from the Cabibbo- suppressed mode⁰_b!_cK(fixed to an expected 8% of the signal yield [16]). The total distribution is F_tot G_signalE_combF_four-trackF_other-BF_other-⁰_bG_cK, whereGindicates a Gaussian distribution,Eindicates an exponential, and F indicates a more complicated func- tional form.

In the fit, the width of⁰_bsignal is fixed to26:4 MeV=c², obtained by scaling the B⁰ width in data by the ratio of widths from the Monte Carlo simulation. The relative contribution of each background type in the fit is guided by two constraints: the first describes the normalization of F_four-track relative to F_other-B (i.e., N_four-track=N_other-B), where N is number of events; the second describes the normalization ofF_other-⁰_b relative to (F_other-BF_four-track) (i.e., N_other⁰

b=N_four-trackN_other-B). The value of each constraint is inferred from the relative abundance of the background types in the large parametric Monte Carlo sample. The value of the (N_four-track=N_other-B) constraint is checked by reconstructing theB⁰!D mode among _c candidates from the region of the invariant mass corresponding to the⁰_b signal. The total ² of this fit is 80.6 for 88 degrees of freedom, corresponding to the fit probability of 70%.

Figure2shows the mass fit for the D candidates.

The large Gaussian peak at5:27 GeV=c² is theB⁰ signal.

The dashed curve corresponds to the exponential combi- natorial background. The dotted curve corresponds to all physics backgrounds, both from B⁰ decays (including B⁰ !DK) as well as decays of other bhadrons. The total ² of this fit is 70.9 for 94 degrees of freedom, corresponding to the fit probability of 96%.

The quantityRdefined in Eq. (1) is calculated from the signal yields according to (2). The signal yields areN⁰

b

21419andNB⁰ 79032, respectively. Each recon- struction efficiency is defined for the ⁰_b(B⁰) with p_T >

6 GeV=c and jj<1:3. The exact configuration of the CDF II detector varied over the course of collecting the data used in the analysis, with the average efficiency of 0:5%. However, the ratio of reconstruction efficiencies is stable within statistical errors across the different periods of running, with the average valueB⁰=⁰_b 1:65 0:03stat. From Eq. (2) we thus obtain R0:82 0:08stat.

The systematic uncertainty on the measurement ofRis dominated by the error onB_c !pK, yielding a

relative error of 27% [10]. Since this uncertainty is inde- pendent of our measurement, it is quoted separately. The bulk of the remaining systematic uncertainty (12.1%) comes from B⁰=⁰_b. It arises from the imperfect knowledge of the ⁰_b lifetime (⁵₄%), the production p_T spectra of both⁰_b(7.6%) andB⁰(4%), the⁰_bpolarization (7%), and the_c resonant substructure (1%). The uncer- tainty due to the finite size of the Monte Carlo samples is 1.9%. The uncertainty due to the difference between the proton and trigger efficiency is 0.6%. The systematic uncertainty onN⁰

b=NB⁰ (5.7%) is largely due to a lack of detailed knowledge of a variety of branching fractions contributing to background shapes that are obtained from the Monte Carlo simulation and fixed in the fit. To evaluate the uncertainty due to these shapes, the branching fractions of the largest decay modes contributing to each of the shapes were varied simultaneously in the simulation, and the shapes were reevaluated. This procedure yields uncer- tainties of 4.3% and 0.9% for the⁰_band theB⁰mass fits, respectively. Uncertainties on the mass resolutions of both ⁰_bandB⁰, which are also fixed in the mass fits, contribute 2.8% and 1.8%, respectively. Finally, the contribution of the⁰_b!_cKshape is varied by a factor of 2, contrib- uting 1.6% to the systematic error. The total systematic error excluding the uncertainty on B_c !pK is 13.5%.

A direct comparison with a theoretical prediction of R_BRB⁰_b!_c=BB⁰!D 1:7 [5] can be performed if one assumes that _bandB⁰ have the same dependence onp_T, and then usef_baryon=f_d from high-p_T measurements. From Ref. [10], we obtain f_baryon=f_d0:250:04, yielding R_BR3:31:2, which compares well with [5].

In summary, we have observed the decay⁰_b!_c for the first time, and measured R0:820:08stat 0:11syst 0:22Bc. The overall error is dominated by the large uncertainty on B_c !pK. The ⁰_b! _csample is the largestb-baryon sample in existence, and, once augmented by new data, can be used for a variety of other_b measurements, including its lifetime and pro- duction properties.

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 Bundesminis- terium 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;

122002-6

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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[7] D. Acostaet al., Phys. Rev. D71, 032001 (2005) [8] CDF II uses a cylindrical coordinate system with the

z-axis along the nominal beam line. The transverse plane

(x; y) is perpendicular to thez-axis. Azimuthal angle,, is measured from the x-axis. Polar angle, , is measured from the z-axis. Pseudorapidity is defined as tanh¹cos. Transverse momentum, p_T, is the compo- nent of the particle’s momentum projected onto the trans- verse plane.

[9] Inclusion of the respective charge conjugate modes is assumed throughout this Letter. Specifically, ⁰_b ! _c, B⁰!D, _c !pK , and D! K.

[10] S. Eidelmanet al., Phys. Lett. B592, 1 (2004).

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[14] The curvature of a particle track is inversely proportional to the transverse momentum (p_T). The impact-parameter of a particle track is defined as the distance of closest approach of the particle track to the primary vertex in the transverse plane.

[15] S. Yu, Ph.D. thesis, University of Pennsylvania, 2005. hep- ex/0504059.

[16] This choice is motivated by K. Abeet al., Phys. Rev. Lett.

87, 111801 (2001); however, we assign a large systematic error to it.