Observation of the Narrow State <em>X</em>(3872)→<em>J/ψπ⁺π⁻</em> in <em>pp</em> Collisions at s√=1.96  TeV

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Reference

Observation of the Narrow State X (3872)→ J/ψπ

π

in pp Collisions at s√=1.96  TeV

CDF II Collaboration

CAMPANELLI, Mario (Collab.), et al.

Abstract

We report the observation of a narrow state decaying into J/ψπ+π− and produced in 220   pb−1 of p̄ p collisions at s√=1.96   TeV in the CDF II experiment. We observe 730±90 decays. The mass is measured to be 3871.3±0.7(stat)±0.4(syst)   MeV/c2, with an observed width consistent with the detector resolution. This is in agreement with the recent observation by the Belle Collaboration of the X(3872) meson.

CDF II Collaboration, CAMPANELLI, Mario (Collab.), et al . Observation of the Narrow State X (3872)→ J/ψπ

π

in pp Collisions at s√=1.96  TeV. Physical Review Letters , 2004, vol. 93, no.

07, p. 072001

DOI : 10.1103/PhysRevLett.93.072001

Available at:

http://archive-ouverte.unige.ch/unige:38061

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

1 / 1

Observation of the Narrow State X3872 ! J=

in pp Collisions at p s

1:96 TeV

D. Acosta,¹⁴T. Affolder,⁷M. H. Ahn,²⁵T. Akimoto,⁵²M. G. Albrow,¹³D. Ambrose,⁴¹S. Amerio,⁴⁰D. Amidei,³¹ A. Anastassov,⁴⁸K. Anikeev,²⁹A. Annovi,⁴²J. Antos,¹M. Aoki,⁵²G. Apollinari,¹³J-F. Arguin,³⁰T. Arisawa,⁵⁴

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072001-1 0031-9007=04=93(7)=072001(6)$22.50  2004 The American Physical Society 072001-1

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L. Zanello,⁴⁷A. Zanetti,⁵¹I. Zaw,¹⁸F. Zetti,⁴²J. Zhou,⁴⁸A. Zsenei,¹⁶and S. Zucchelli³ (CDF II Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy

4Brandeis University, Waltham, Massachusetts 02254, USA

5University of California at Davis, Davis, California 95616, USA

6University of California at Los Angeles, Los Angeles, California 90024, USA

7University of California at Santa Barbara, Santa Barbara, California 93106, USA

8Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

9Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

10Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

11Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

12Duke University, Durham, North Carolina 27708, USA

13Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

14University of Florida, Gainesville, Florida 32611, USA

15Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy

16University of Geneva, CH-1211 Geneva 4, Switzerland

17Glasgow University, Glasgow G12 8QQ, United Kingdom

18Harvard University, Cambridge, Massachusetts 02138, USA

19The Helsinki Group, Helsinki Institute of Physics, and Division of High Energy Physics, Department of Physical Sciences, University of Helsinki, FIN-00014 Helsinki, Finland

20Hiroshima University, Higashi-Hiroshima 724, Japan

21University of Illinois, Urbana, Illinois 61801, USA

22The Johns Hopkins University, Baltimore, Maryland 21218, USA

23Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

24High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan

25Center for High Energy Physics, Kyungpook National University, Taegu 702-701, Seoul National University, Seoul 151-742, and SungKyunKwan University, Suwon 440-746, Korea

072001-2 072001-2

26Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

27University of Liverpool, Liverpool L69 7ZE, United Kingdom

28University College London, London WC1E 6BT, United Kingdom

29Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

30Institute of Particle Physics, McGill University, Montre´al, Canada H3A 2T8, and University of Toronto, Toronto, Canada M5S 1A7

31University of Michigan, Ann Arbor, Michigan 48109, USA

32Michigan State University, East Lansing, Michigan 48824, USA

33Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

34University of New Mexico, Albuquerque, New Mexico 87131, USA

35Northwestern University, Evanston, Illinois 60208, USA

36The Ohio State University, Columbus, Ohio 43210, USA

37Okayama University, Okayama 700-8530, Japan

38Osaka City University, Osaka 588, Japan

39University of Oxford, Oxford OX1 3RH, United Kingdom

40Universita´ di Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

41University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

42Istituto Nazionale di Fisica Nucleare, University and Scuola Normale Superiore of Pisa, I-56100 Pisa, Italy

43University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

44Purdue University, West Lafayette, Indiana 47907, USA

45University of Rochester, Rochester, New York 14627, USA

46The Rockefeller University, New York, New York 10021, USA

47Instituto Nazionale de Fisica Nucleare, Sezione di Roma, University di Roma I, ‘‘La Sapienza,’’ I-00185 Roma, Italy

48Rutgers University, Piscataway, New Jersey 08855, USA

49Texas A&M University, College Station, Texas 77843, USA

50Texas Tech University, Lubbock, Texas 79409, USA

51Istituto Nazionale di Fisica Nucleare, Universities of Trieste and Udine, Italy

52University of Tsukuba, Tsukuba, Ibaraki 305, Japan

53Tufts University, Medford, Massachusetts 02155, USA

54Waseda University, Tokyo 169, Japan

55Wayne State University, Detroit, Michigan 48201, USA

56University of Wisconsin, Madison, Wisconsin 53706, USA

57Yale University, New Haven, Connecticut 06520, USA (Received 4 December 2003; published 9 August 2004)

We report the observation of a narrow state decaying intoJ= and produced in220 pb¹ofpp collisions at ps

1:96 TeV in the CDF II experiment. We observe 73090 decays. The mass is measured to be 3871:30:7stat 0:4systMeV=c², with an observed width consistent with the detector resolution. This is in agreement with the recent observation by the Belle Collaboration of the X3872meson.

DOI: 10.1103/PhysRevLett.93.072001 PACS numbers: 14.40.Gx, 12.39.Mk

The study of bound states of charm-anticharm quarks revolutionized our understanding of hadrons beginning with the discovery of the J= meson in 1974 [1]. Al- though numerous charmonium (cc) states are now known, others should be observable. Recently, the Belle Collaboration reported a new particleX3872observed in exclusive decays ofBmesons produced ineecolli- sions [2]. This particle has a mass of3872 MeV=c² and decays into J= . A natural interpretation of this particle would be a previously unobserved charmonium state, but there are no such states predicted to lie at or near the observed mass with the right quantum numbers to decay intoJ= [3,4]. Within the framework of QCD, mesons may also arise from more complex systems than the conventional quark-antiquark bound state [5].

The proximity of the X3872 mass to the sum of the D⁰ and D⁰ masses suggests that X3872 may be a weakly bound deuteronlike ‘‘molecule’’ composed of a

D and D. Another possibility is that X3872 is a ccg hybrid meson — a cc system possessing a valence gluon.

These novel interpretations have excited great interest in X3872[6]. Whether it is a new form of hadronic matter or a conventional cc state in conflict with theoretical models, X3872 is an important object of study. Here we report the observation of aJ= resonance pro- duced inclusively inpp collisions and which is consistent withX3872.

The analysis uses a data sample of pp collisions at s

p 1:96 TeV with an integrated luminosity of 220 pb¹ collected with the upgraded collider detector (CDF II) at the Fermilab Tevatron between February 2002 and August 2003. The important components of the CDF II detector for this analysis include a tracking system composed of a silicon-strip vertex detector (SVX II) [7] surrounded by an open-cell drift chamber system called the central outer tracker (COT) [8]. The

SVX II detector comprises five concentric layers of double-sided sensors located at radii between 2.5 and 10.6 cm. On one side of the sensors, axial strips measure positions in the plane transverse to the beam line. Strips on the other side are used for stereo measurements. The latter strips are tilted with respect to the axial strips: on one layer by1:2 , another by1:2 , and three by90 . The active volume of the COT is a 3.1 m long cylinder covering radii from 43 to 132 cm with 8 superlayers of 12 wires each. In order to provide three-dimensional tracking, superlayers of axial wires alternate with super- layers of2 stereo angle wires and superlayers of2 stereo angle wires. The central tracking system is im- mersed in a 1.4 T solenoidal magnetic field for the mea- surement of charged particle momenta transverse to the beam line,p_T. The outermost detection system consists of planes of multilayer drift chambers for detecting muons [9]. The central muon system (CMU) coversjj 0:6, where lntan=2 and is the angle of the particle with respect to the direction of the proton beam.

Additional muon chambers (CMX) extend the rapidity coverage tojj 1:0.

In this analysis, J= ! decays are recorded using a dimuon trigger. The CDF II detector has a three-level trigger system. The level-1 trigger uses tracks in the muon chambers with a clear separation in azimuth from neighboring tracks. The extremely fast tracker (XFT) [10] uses information from the COT to select tracks based on p_T. XFT tracks with p_T 1:5 GeV=c (p_T 2:0 GeV=c) are extrapolated into the CMU (CMX) muon chambers and compared with the positions of muon tracks. If there are two or more XFT tracks with matches to muon tracks, the event passes the level-1 trigger.

Dimuon triggers have no requirements at level 2. At level 3, the full tracking information from the COT is used to reconstruct a pair of opposite-sign muon candidates in the mass range from 2:7 to 4:0 GeV=c². Events passing the level-3 trigger are recorded for further analysis.

The offline analysis makes use of the best available calibrations of the tracking system for reconstructing events. Well-reconstructed tracks are selected by accept- ing only those with3axial SVX II hits and>20axial and >16 stereo COT hits. Tracks are refit to take into account the ionization energy loss appropriate for the particle hypotheses under consideration [11]. Dimuon candidates are selected in the mass range from 2:8 to 3:2 GeV=c² after being constrained to originate from a common point in a three-dimensional vertex fit. The resulting signal-to-background ratio for J= candidates is about 5 to 1 [12]. Pairs of charged tracks, both having p_T 0:35 GeV=cand assumed to be pions, are then fit with the dimuon candidates to a common vertex. In this three-dimensional vertex fit, the dimuon mass is con- strained to be the world averageJ= mass [13]. We require that the²for theJ= vertex fit must be less than40 for 6 degrees of freedom.

The number of J= candidates per event passing the above preselection requirements can be quite large for events with a high multiplicity of charged tracks. These events contribute a large amount of combinatorial back- ground relative to a small potential signal. We reject events that have more than 12 preselection candidates with masses below 4:5 GeV=c². A large number of can- didates are accepted at this stage. However, after the final selection the average number of J= candidates within the mass window of 3:65–4:0 GeV=c² is less than 1:2 per event for events with at least one accepted candidate. The specific number of preselection candidates allowed per event is determined by the optimization procedure described below.

In order to suppress J= backgrounds, we tighten the selection criteria to ²<15for the 1 degree of freedom dimuon vertex fit, dimuon invariant mass within60 MeV=c²(4standard deviations) of the world averageJ= mass,p_TJ= 4 GeV=c,²<25for the J= vertex fit,p_T 0:4 GeV=c, andR0:7for both pions. HereRis defined as

^{2 2}

p , where

is the difference in azimuthal angle between the pion and the J= candidate and is the difference in pseudorapidity.

The values used in the above selection criteria are determined by an iterative optimization procedure in which the significance S=

SB

p is maximized. The quantitiesSandBrespectively represent the numbers of signal and background candidates obtained as a function of the values of the selection parameters. B is available from background fits of the data in a window around 3872 MeV=c². We use 2S !J= decays to model the X3872 yield S as the selection is varied.

The 2S signal is much larger than that of the X3872and must therefore be scaled down for the sig- nificance calculation. The scale factor is determined such that S matches the observed X yield from a reference selection. Because the X3872 signal is considerably smaller than the background, the denominator of the significance ratio is dominated byB, and the optimization is not sensitive to the precise value of the scaling.

The J= mass distribution of the selected can- didates is displayed in Fig. 1. Besides the large peak showing the 2S, a small peak is observed at a J= mass around 3872 MeV=c². To fit the mass distribution, we model each peak by a single Gaussian and use a quadratic polynomial to describe the back- ground. A binned maximum likelihood fit of the mass spectrum between3:65and4:0 GeV=c² is also shown in Fig. 1. The fit yields signals of 5790140 2Scandi- dates and580100X3872candidates.

The ‘‘wrong-sign’’J= mass distribution is also shown in Fig. 1, and no significant structures are appar- ent. We examine the hypothesis that the 3872 peak may originate from another state by incorrect assignment of the pion mass. The masses ofJ= candidates in a window around the 3872 peak are recomputed for the

072001-4 072001-4

alternate hypothesesJ= h₁h₂, whereh₁h₂ areK, KK,p,pK, andpp(and charge conjugates). This results in broad mass distributions with no peaklike structures. Thus, the 3872 peak is not an artifact of some other state, known or unknown, decaying into a J= and a pair of hadrons in which one or both hadrons are misassigned as pions.

TheX3872signal reported by the Belle Collaboration favors largemasses. Our data support this conclu- sion as well. We divide the data into two subsamples:

candidates with dipion masses greater, or less, than 0:5 GeV=c². From the Belle results, this is a large enough value to probe the high-mass behavior of the X3872 candidates and yet not eliminate all the 2Sreference signal from the high-mass subsample. Figure 2 shows the resultingJ= mass distributions. The prominence of theX3872peak is enhanced over the background in the high-mass sample, and no peak is apparent for low masses. Fitting the high-mass spectrum between3:65and 4:0 GeV=c² gives 3530100 2S candidates and 73090X3872candidates. The fitted mass and width of the 2Sare3685:650:09statMeV=c²and3:44 0:09statMeV=c², respectively. ForX3872we obtain a mass of3871:30:7statMeV=c² and a width of4:9 0:7 MeV=c². The latter value is consistent with detector resolution. Our mass is in good agreement with the Belle result of3872:00:6stat 0:5systMeV=c² [2].

RequiringM>0:5 MeV=c² reduces the back- ground by almost a factor of 2 and apparently increases the amount of fittedX3872signal. A significant part of the additional signal is attributable to an increase in the fitted width. The original fit over all dipion masses re- turns a smaller but consistent width of4:20:8 MeV=c². We conclude that theX3872signal yield after the dipion requirement is unchanged within statistics, and thus there is little signal with dipion masses below0:5 GeV=c². The same conclusion is reached by direct examination of the low dipion-mass distribution shown in Fig. 2. We use the high-mass sample for measuring the X3872 mass as the improved signal-to-noise ratio reduces the statisti- cal uncertainty.

The fit displayed in Fig. 2 has a ² of 74.9 for 61 de- grees of freedom, which corresponds to a probability of 10:9%. To estimate the significance of the signal, we first count the number of candidates in the three bins centered on the peak, i.e., 3893. The three-bin background is estimated from the fit to be 3234 candidates, leaving a signal of 659 candidates. In a Gaussian approach, this corresponds to a significance of659=

p3234

11:6stan- dard deviations. The Poisson probability for 3234 to fluc- tuate up to or above 3893 is in good agreement with the Gaussian estimate, considering the approximations of each method.

The systematic uncertainty on the mass scale is related to the momentum scale calibration, the various tracking systematics, and the vertex fitting. These effects

2) Mass (GeV/c π-

π+

ψ J/

3.65 3.70 3.75 3.80 3.85 3.90 3.95 4.00

2 Candidates/ 5 MeV/c

0 500 1000 1500 2000 2500 3000

) > 0.5 GeV/c2

π-

π+

M(

) < 0.5 GeV/c2

π-

π+

M(

3.80 3.85 3.90 3.95 900

1000 1100 1200 1300

CDF II1400

FIG. 2 (color online). The mass distributions of J=

candidates with m>0:5 GeV=c² (points) and m<0:5 GeV=c² (open circles). The curve is a fit with two Gaussians and a quadratic background. The inset shows an enlargement of the high dipion-mass data and fit.

2) Mass (GeV/c π

π ψ J/

3.65 3.70 3.75 3.80 3.85 3.90 3.95 4.00

2 Candidates/ 5 MeV/c

1000 2000 3000 4000 5000 6000

π-

π+

ψ J/

π±

π±

ψ J/

3.80 3.85 3.90 3.95 1700

1800 1900 2000 2100

CDF II2200

FIG. 1 (color online). The mass distributions of J=

and J= candidates passing the selection described in the text. A large peak for the 2Sis seen in theJ=

distribution as well as a small signal near a mass of 3872 MeV=c². The curve is a fit using two Gaussians and a quadratic background to describe the data. The inset shows an enlargement of the J= data and fit around 3872 MeV=c².

were studied in detail for our measurement of the mass difference mD_s mD [11], where the systematic uncertainty was0:21 MeV=c². A larger systematic un- certainty arises for ourX3872 mass determination be- cause it is an absolute measurement. We use the 2S mass to gauge our systematic uncertainty. With the dipion mass requirement, the 2S mass is measured to be 0:3 MeV=c² below the world average mass of3685:96 0:09 [13], a difference substantially larger than the statistical uncertainty of 0:1 MeV=c². However, studies of the stability of the 2S mass for different selection requirements indicate that an uncertainty of0:4 MeV=c² should be assigned. Variations of the fit model and fit range have negligible effect on the mass.

In summary, we report the observation of a state con- sistent with X3872 decaying into J= . From a sample of73090candidates we measure theX3872 mass to be 3871:30:7stat 0:4systMeV=c² and find that the observed width is consistent with the detec- tor resolution. This is in agreement with the measurement by the Belle Collaboration using B decays [2]. The average mass from the two experiments, assuming uncorrelated systematic uncertainties, is 3871:7 0:6 MeV=c². Our large sample of this new particle opens up avenues for future investigations, such as production mechanisms, the dipion mass distribution, and spin- parity analysis.

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 Foundation; the A. P. Sloan Foundation; the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foun- dation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and

the Royal Society, United Kingdom; the Russian Foun- dation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; the European Com- munity’s Human Potential Programme under Contract No. HPRN-CT-20002, Probe for New Physics; and the Research Fund of Istanbul University Project No. 1755/

21122001.

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