Article
Reference
Measurement of Bottom-Quark Hadron Masses in Exclusive J/ψ Decays with the CDF Detector
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We measure the masses of b hadrons in exclusively reconstructed final states containing a J/ψ→μ−μ+ decay using 220 pb−1 of data collected by the CDF II experiment. We find:
m(B+)=5279.10±0.41(stat.)±0.36(sys.) MeV/c2, m(B0)=5279.63±0.53(stat.)±0.33(sys.) MeV/c2, m(B0s)=5366.01±0.73(stat.)±0.33(sys.) MeV/c2, m(Λ0b)=5619.7±1.2(stat.)±1.2(sys.) MeV/c2. m(B+)−m(B0)=−0.53±0.67(stat.)±0.14(sys.) MeV/c2, m(B0s)−m(B0)=86.38±0.90(stat.)±0.06(sys.) MeV/c2,
m(Λ0b)−m(B0)=339.2±1.4(stat.)±0.1(sys.) MeV/c2. The measurements of the B0s, Λ0b mass, m(B0s)−m(B0) and m(Λ0b)−m(B0) mass difference are of better precision than the current world averages
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of Bottom-Quark Hadron Masses in Exclusive J/ψ Decays with the CDF Detector. Physical Review Letters , 2006, vol. 96, no. 20, p. 202001
DOI : 10.1103/PhysRevLett.96.202001
Available at:
http://archive-ouverte.unige.ch/unige:38342
Disclaimer: layout of this document may differ from the published version.
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Measurement of Bottom-Quark Hadron Masses in Exclusive J= Decays with the CDF Detector
D. Acosta,16J. Adelman,12T. Affolder,9T. Akimoto,54M. G. Albrow,15D. Ambrose,15S. Amerio,42D. Amidei,33 A. Anastassov,50K. Anikeev,15A. Annovi,44J. Antos,1M. Aoki,54G. Apollinari,15T. Arisawa,56J-F. Arguin,32 A. Artikov,13W. Ashmanskas,15A. Attal,7F. Azfar,41P. Azzi-Bacchetta,42N. Bacchetta,42H. Bachocou,28W. Badgett,15 A. Barbaro-Galtieri,28G. J. Barker,25V. E. Barnes,46B. A. Barnett,24S. Baroiant,6G. Bauer,31F. Bedeschi,44S. Behari,24
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J. Zhou,50and S. Zucchelli4
(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
4Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
5Brandeis University, Waltham, Massachusetts 02254, USA
6University of California, Davis, Davis, California 95616, USA
7University of California, Los Angeles, Los Angeles, California 90024, USA
8University of California, San Diego, La Jolla, California 92093, USA
9University of California, Santa Barbara, Santa Barbara, California 93106, USA
10Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
11Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
12Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
13Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
14Duke University, Durham, North Carolina 27708
15Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
16University of Florida, Gainesville, Florida 32611, USA
17Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
18University of Geneva, CH-1211 Geneva 4, Switzerland
19Glasgow University, Glasgow G12 8QQ, United Kingdom
20Harvard University, Cambridge, Massachusetts 02138, USA
21Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
22Hiroshima University, Higashi-Hiroshima 724, Japan
23University of Illinois, Urbana, Illinois 61801, USA
202001-2
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; 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 di Roma ‘‘La Sapienza’’, I-00185 Roma, Italy
50Rutgers University, Piscataway, New Jersey 08855, USA
51Texas A&M University, College Station, Texas 77843, USA
52Texas Tech University, Lubbock, Texas 79409, 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 July 2005; published 22 May 2006)
We measure the masses of bhadrons in exclusively reconstructed final states containing a J= ! decay using220 pb1of data collected by the CDF II experiment. We find:mB 5279:10 0:41stat:0:36sys:MeV=c2, mB0 5279:630:53stat:0:33sys:MeV=c2, mB0s 5366:01 0:73stat:0:33sys:MeV=c2, m0b 5619:71:2stat: 1:2sys: MeV=c2. mB mB0 0:530:67stat: 0:14sys: MeV=c2, mB0s mB0 86:380:90stat: 0:06sys: MeV=c2, m0b mB0 339:21:4stat:0:1sys: MeV=c2. The measurements of theB0s,0bmass,mB0s mB0andm0b mB0mass difference are of better precision than the current world averages.
DOI:10.1103/PhysRevLett.96.202001 PACS numbers: 14.40.Nd, 13.25.Hw, 13.30.Eg, 14.20.Mr
In the standard model of particle physics, hadrons are composite, colorless particles made up of partons (quarks and gluons) which interact via the strong or color force.
The theory that describes these interactions is quantum chromodynamics (QCD) [1]. The masses of hadrons are fundamental physical observables and their study forms the spectroscopy of quark systems bound by QCD. At the low energy scale of hadron masses, QCD observables cannot be evaluated using perturbation theory. Lattice QCD calculations have to be used to evaluate mass spectra
from first principles. Of particular interest is the study of the heaviest known hadrons, those containing a bottom orb quark [2]. The techniques of lattice QCD are the main tools to compute the properties of hadrons and play a crucial role in the computation of parameters used to extract informa- tion on CP violation and possible new physics from b-hadron measurements. A recent breakthrough in lattice QCD [3], using unquenched methods, allowed a calcula- tion of the Bc meson mass to a precision of 0.3% [4], an improvement by a factor of 3 over previous calculations.
Current theoretical uncertainties on the masses of heavy- light hadrons, which contain aband a light quark, are of the order of 25 MeV=c2, where the most precise predic- tions are those of b hadron mass differences. The new method will reduce uncertainties further, close to those achieved by experiment. The uncertainty is dominated by the light quark chiral extrapolation; other uncertainties are expected to be reduced below 1% [5,6]. Precise experi- mental measurements to compare with lattice results are of interest as an essential test of QCD and to provide con- fidence in other applications of lattice calculations. We present here the most precise individual measurements to date for the masses of theB,B0,B0s, and0bparticles.
The data used in this analysis were obtained with the Collider Detector at Fermilab (CDF II) operating at the ps
1:96 TeVTevatron pp collider. The data were col- lected between February 2002 and August 2003 and corre- spond to an integrated luminosity of220 pb1. The CDF II detector is described in detail elsewhere [7]. This analysis relies on the tracking system and the muon detectors. The tracking system is comprised of a silicon microstrip vertex detector (SVX II) [8] and a drift chamber operating in a 1.4 T solenoidal magnetic field. The SVX II system con- sists of 5 concentric silicon layers made of double-sided silicon covering the radii from 2.5 cm to 10.6 cm. The impact parameter resolution is about40m, including a 30mcontribution from the beam spot. The Central Outer Tracker (COT) [9] is an open cell drift chamber measuring 310 cm in length, with an inner radius of 41 cm extending to a radius of 138 cm and provides a large lever arm for curvature measurements. Each cell contains a plane of 12 sense wires tilted by 35with respect to the radial direction to compensate for the drift Lorentz angle. The COT is segmented radially into eight superlayers. For superlayers 1, 3, 5, and 7 wires form a2stereo angle with respect to the beam direction, while for superlayers 2, 4, 6, and 8 wires are oriented along the beam direction. The measured momentum resolution ispT=pT0:15%pT=GeV=c.
Muon detectors consist of multilayer drift chambers lo- cated around the outside of the calorimeters [10]. The central muon system covers a range in pseudorapidity of jj<0:6. The central muon extension extends the pseu- dorapidity range to0:6<jj<1:0.
Data are selected with a three-level trigger system. The Level 1 portion of the dimuon trigger uses the extremely fast tracker [11], providing a coarse track reconstruction based on fast digitization of drift chamber signals. Only tracks with a measured transverse momentum larger than 1:5 GeV=c are considered further. Two such tracks, matched to distinct hits in the muon systems, are required to pass the Level 1 dimuon trigger. No additional require- ments are made at Level 2. At Level 3, a detailed recon- struction is performed and opposite sign dimuon events with an invariant mass in the range 2:7–4:0 GeV=c2 are accepted and written to tape. Stored events are recon- structed using the full set of calibrations.
The followingbhadron decay modes are reconstructed:
B!J= K, B0 !J= K 0, B0s !J= , B0! J= KS0, and 0b !J= 0. The daughter particles are reconstructed in the decay modes K 0 !K, ! KK,KS0!, and0 !p. Charge conjugate modes are included implicitly. To reconstruct a given final state we try all possible combinations of particle hypoth- eses, since hadronic particle identification capabilities are limited. For a given particle hypothesis tracks are corrected for energy loss with the corresponding mass assigned to the track [12]. The correction procedure [13] makes use of the material information in a GEANT [14] description of the CDF detector. Material is integrated only at radii larger than that of the reconstructed decay vertex of long-livedKS0 and0particles. High track quality is ensured by requiring at least 20 axial and 16 stereo hits in the COT. To ensure a precise measurement of the bhadron decay vertex, only tracks with at least 3 axial SVX hits are considered. The SVX hit requirement is not applied to daughter tracks from K0Sand0. We find that about 70% ofKs0that have tracks in the COT decay outside the silicon tracker. A muon is reconstructed from tracks matched to track stubs in the muon chambers.
The mass reconstruction begins by constraining two selected muons of opposite charge to a common 3D ver- tex. Candidates with a resulting dimuon mass within 80 MeV=c2 of the world average J= mass [15] are se- lected. A pT threshold of 400 MeV=cis required on all tracks, except 0 daughters, for which all available tracks are used. A uniform threshold of 2 GeV=c is im- posed on the momentum transverse to the beam direction of K, K 0, and candidates. Mass windows of 80 MeV=c2, 10 MeV=c2, and 40 MeV=c2 around the world average masses [15] are required to select K 0,, and K0S, respectively. Combinations with a p mass be- tween 1:10 GeV=c2 and1:13 GeV=c2 are selected as 0 candidates. The KS0 and 0 flight directions are recon- structed as the vector connecting the J= and the KS0 or 0vertices. We require theKS0and0momentum vectors to be within 0.25 and 0.57 of their flight direction, respectively. In the final track fit J= ,K0S, and0 candi- dates are constrained to their world average masses. A 3D pointing constraint back to theb-decay vertex is applied to K0Sand0 candidates.
A cut on the b hadron transverse momentum pT 6:5 GeV=cis applied. For b hadrons,c is in the range 400–500m, whereis the proper lifetime. In order to reduce background, the two-dimensional decay length of the b hadron, Lxy, defined as LxyX~j~p~pT
Tj , is required to exceed 100m, where X~ is the vector between the pro- duction vertex and the decay vertex of thebhadron.
To calibrate the momentum scale of the CDF tracking system, three values must be determined: the energy lost by a track when passing through the material in the inner detector, the radius of the tracker (for the track curvature
202001-4
measurement), and the strength of the magnetic field (for the track curvature-to-momentum conversion). The effect of the tracker radius is indistinguishable from the magnetic field strength in the calibration, so we neglect the tracker radius and describe the procedure in terms of a magnetic
field calibration. We use a sample of over 1106, inclu- siveJ= !decays to calibrate the track energy loss and magnetic field. An underestimate of the material re- sults in undercorrected energy loss and introduces a de- pendence of the reconstructed dimuon mass onpT. ThepT dependence is a signature for inadequate material assess- ment, as an incorrect value of the magnetic field produces a shift in invariant mass independent of thepT of the recon- structed particle. The first calibration step tunes the amount of material to remove the momentum dependence. Next the magnetic field is scaled so that the reconstructed J= ! mass agrees with the world average. The effect of the calibration steps on the momentum dependence of the dimuon mass is illustrated in Fig. 1. Final state radiation in the decay of the J= leads to an asymmetry in the otherwise Gaussian distribution of the measured dimuon invariant mass. We use a Monte Carlo simulation to correct the resulting bias in bins ofJ= pT during calibration.
After reconstruction of candidates the mass is extracted using an unbinned log-likelihood fit, with the signal distri- bution modeled as a Gaussian. The shape of the back- ground is investigated using an inclusive Monte Carlo sample ofbhadron decays. A detailed detector simulation based onGEANTis used. TheB !J= K sample con- tains significant contributions from partially reconstructed B0!J= K 0 !K and misreconstructed (GeV/c)
ψ of J/
pT
0 5 10
)2 ) (MeV/c- µ+ µm(
3075 3080 3085 3090 3095 3100 3105
slope )/(GeV/c) (MeV/c2
0.058
± with B-field corr. -0.026
0.043
± with tuned material 0.049
0.043
± no corrections 0.904
FIG. 1 (color online). The reconstructed mass of dimuons from J= decays, as a function of transverse momentum. The three sets represent various stages of corrections: the solid circles indicate no correction, the triangles add the material tuning and the squares show all corrections including the magnetic field scale.
2) ) (GeV/c K+
ψ J/
M(
5.00 5.05 5.10 5.15 5.20 5.25 5.30 5.35 5.40 5.45 5.50 2 Events/(5 MeV/c )
0 50 100 150 200 250 300
350 B+→ J/ψ K+ N(B )=2264Fit Prob: 52.0% + ±53 a)
2) ) (GeV/c K0*
ψ J/
M(
5.00 5.05 5.10 5.15 5.20 5.25 5.30 5.35 5.40 5.45 5.50 2 Events/(5 MeV/c )
0 50 100 150 200 250
±42 N(B )=12790
K0*
ψ
→ J/
B0 Fit Prob: 14.0%
b)
2) ) (GeV/c φ
ψ J/
M(
5.10 5.15 5.20 5.25 5.30 5.35 5.40 5.45 5.50 5.55 5.60 2 Events/(5 MeV/c )
0 10 20 30 40 50 60
±13
s
N(B )=1850
φ ψ
→ J/
0
Bs
Fit Prob: 75.0%
c)
2) ) (GeV/c Λ0
ψ J/
M(
5.3 5.4 5.5 5.6 5.7 5.8 5.9
2 Events/(6 MeV/c )
0 5 10 15 20 25 30 35
±10
b)=89 Λ0 0 N(
Λ ψ
→ J/
0
Λb Fit Prob: 23.3%
d)
FIG. 2 (color online). The invariant mass distribution forJ= K,J= K0 ,J= , andJ= 0candidates. The results of the log- likelihood fits are superimposed. The fit probability obtained from a2test is shown. The left shoulder in Fig. 2(a) originates from partially reconstructed decays, as explained in the text.
B!J= decays, which are modeled in the back- ground probability distribution function. Partially recon- structed B0!J= K 0 !K decays populate the left shoulder in Fig. 2(a). Events fromB !J=
decays appear on the right side of the signal peak. The misreconstruction ofK 0 !Kdue to swapped tracks assignments of K and is taken into account for B0 ! J= K 0decays. No significant contributions are found for the other decay modes. Comparisons between data and fits are shown in Fig. 2.
The systematic uncertainties are summarized in Tables I and II. The largest systematic uncertainties originate from the momentum scale and tracker alignment. Deviations from the well-measured world averages in the 0 ! , 0!, and!high statis- tics samples are used to determine the uncertainty of the momentum scale. The observed deviations, scaled by theQ value of the respective decay, provide an estimate of the systematic uncertainty. The second uncertainty of impor- tance originates from the relative alignment of SVX and COT. It is evaluated by comparing mass measurements using the combined tracker information to those using the COT information alone. Certain tracker misalignments cause a straight track to be reconstructed with some,
‘‘false,’’ curvature. The net observed effect is an increase in momentum for negatively charged tracks and a decrease for positively charged tracks. False curvature effects can- cel, to first order, in charge symmetric samples. We derive
a parametric correction to remove the charge dependence and use the mass shift due to this correction as a measure of the systematic error. Uncertainties due to the vertex fit are evaluated using different mass and pointing constraints in the fit. The uncertainty labeled ‘‘resolution bias’’ is due to a correlation between the reconstructed decay vertex posi- tion and the measured opening angle and curvature of the daughter particles. Background systematics are determined by varying the background description. In theBcase, the mass shift due to inclusion and exclusion of the misrecon- structed B!J= is assigned as systematic uncer- tainty. In the B0 case, the systematic uncertainty is derived by varying the amount of the reflection contribu- tion within1of expectation.
We obtain the following results:
mB 5279:100:41stat0:36sys MeV=c2; mB0 5279:630:53stat0:33sys MeV=c2; mB0s 5366:010:73stat0:33sys MeV=c2; m0b 5619:71:2stat1:2sys MeV=c2:
These results are in agreement with the current world av- erages:mB 5279:00:5 MeV=c2,mB0 5279:4 0:5 MeV=c2, and mB0s 5369:62:4 MeV=c2 [16].
Our new 0b mass measurement agrees with two of the three previous measurements and is in excellent agreement
Source B0!J= K 0 B!J= K B0s !J=
Tracking
Momentum scale 0.20 0.22 0.20
Alignment 0.18 0.18a 0.18a
False curvature 0.02b 0.02 0.02b
Vertex fitting 0.10 0.10a 0.10a
Resolution bias 0.13 0.13 0.13
Background systematics
K-swap inK 0 0.06 — —
J= contamination — 0.13 —
Total uncertainty 0.33 0.36 0.33
aFromB0.
bFromB.
TABLE II. Summary of systematic uncertainties for the0bmass measurement inMeV=c2. The high statisticsB0values have been used for the0bsystematics.
Source B0!J= KS0 0b!J= 0
Tracking
Momentum scale 0.2 0.2
Alignment 1.0 1.0a
Vertex fitting 0.7 0.7a
Total uncertainty 1.2 1.2
aFromB0!J= K0S.
202001-6
with CDF’s Run I measurement [17–19]. The achieved precision is better than the current world average of m0b 56249 MeV=c2 [16].
For the mass differences, most systematic uncertainties cancel. The momentum scale uncertainty is scaled down to the size of the mass difference. For the mass constrained 0 and K0S decay modes a contribution arises from the uncertainty of the input masses. The remaining systematic uncertainty originates from differences in the fit models.
We minimize systematic effects in m0b mB0 by using theB0!J= KS0decay mode, which is topologically similar to0b!J= 0, and where we measuremB0 5280:460:63stat, in agreement with the more precise mass determination from the B0 !J= K 0 decay mode.
The uncertainties are summarized in Table III. We obtained the following results for the mass differences:
mB mB0 0:530:670:14 MeV=c2; mB0s mB0 86:380:900:06 MeV=c2; m0b mB0 339:21:40:1 MeV=c2: These are the most precise measurements of mB0s mB0andm0b mB0to date. We look forward to a rigorous comparison of these measurements with precision calculations from lattice QCD.
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.
This work was supported by the US 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 Particle Physics and Astronomy Research Council and the Royal Society, UK; the Russian Foundation for Basic Research; the Comision Interministerial 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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TABLE III. Summary of systematic uncertainties for thebhadron mass differences inMeV=c2.
Mass difference Momentum scale Fit model Inputs Total uncertainty
mB mB0 0.00 0.14 — 0.14
mB0s mB0 0.01 0.06 — 0.06
mB0s mB 0.01 0.13 — 0.13
m0b mB0 0.05 — 0.03 0.06
