ЯДЕРНАЯ ФИЗИКА, 2014, том 77, № 1, с. 130-134
= ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ
NEW RESEARCH OF CHARMONIUM OVER DID THRESHOLD USING THE ANTIPROTON BEAM WITH MOMENTUM RANGING FROM 1 TO 15 GeV/c
© 2014 M. Yu. Barabanov1)*, A. S. Vodopyanov1), S. L. Olsen2)
Received January 14, 2013
The spectroscopy of charmonium cc is discussed. It is a good testing tool for the theories of strong interactions, including: QCD in both the perturbative and non-perturbative regimes, LQCD, potential models and phenomenological models. For this purpose an elaborated analysis of the charmonium spectrum is given, and attempts to interpret recent experimental data in the above DD threshold region are considered. Experiments using antiproton beams take advantage of the intensive production of particle— antiparticle pairs in antiproton—proton annihilations. Experimental data from different collaborations are analyzed with special attention given to new states with hidden charm that were discovered recently. Some of these states can be interpreted as higher-laying S, P, and D wave charmonium states. But much more data on different decay modes are needed before firmer conclusions can be made. These data can be derived directly from the experiments using a high quality antiproton beam with momenta ranging from 1 to 15 GeV/c.
DOI: 10.7868/S0044002714010048
1. INTRODUCTION
The study of the strong interactions and properties of hadronic matter produced in the process of antiproton—proton annihilation is a subject of substantial current interest. Studies of charmonium as well as other, new forms of matter, such as gluonic excitations, using an antiproton beam with momentum ranging from 1 to 15 GeV/c is a promising technique for addressing these issues [1,2]. Gluonic excitations imply the existence of glueballs and hybrids. Char-monium and charmed hybrids with different quantum numbers would be copiously produced in antiproton— proton annihilation processes. The accuracy of mass and width measurements depends only on the quality of antiproton beam (high luminosity, minimal beam momentum spread, small lateral beam dimensions). Such a facility would make it possible to extract the information about excited states of charmonium and charmed hybrids which can be extremely useful for understanding the nature of strong interactions. Detailed analysis of the charmonium system is a promising way to understand the dynamics of quark interactions at small distances. The size of charmonium is of an order of less than 1 fm, so that one of the
1)1 Joint Institute for Nuclear Research, Dubna, Russia.
2)Department of Physics and Astronomy, Seoul National
University, Korea.
E-mail: barabanov@jinr.ru
main doctrines of QCD — asymptotic freedom is accessible. Charmonium spectroscopy is a good testing tool for the theories of strong interactions including: QCD in both the perturbative and non-perturbative regimes; QCD-inspired purely phenomenological potential models; non-relativistic QCD and LQCD.
An analysis of charmonium spectrum is carried out, and attempts to interpret a great quantity of experimental data above DD threshold of 3.73 GeV/c2 are undertaken. But much more data on different decay modes are needed for a more compelling analysis. These data could be derived directly from the PANDA experiment that will use a high quality antiproton beam with momentum ranging from 1 to 15 GeV/c. The advantage of antiproton beam consists in the copious production of particle—antiparticle pairs which is observed to occur in antiproton—proton annihilation. This allows one to carry out spectroscopic research with good statistics and high accuracy; including the possibility of measuring the masses, widths, and branching ratios of different charmonium states with high precision.
The charmonium system has been investigated in detail, initially in e+e- reactions, and subsequently — on a restricted scale but with high precision — in 'p'p annihilations (e.g., experiments R704 at CERN and E760/E835 at Fermilab). Despite these efforts a number of unsolved questions dealing with charmonium remain: the singlet lD2 and triplet 3DJ char-
monium states are yet to be seen; the higher laying singlet 15o, 1P1 and triplet 3S\, 3PJ charmonium states are poorly investigated; only few partial widths of the 3PJ states are known; some of the measured decay widths do not fit theoretical schemes and additional experimental checks and/or reconsideration of the corresponding theoretical models is needed; more data on different decay modes are needed to clarify the situation; almost nothing is known about the partial widthsof the 1D2 and 3DJ states; the domain above the DD threshold is poorly studied.
During the last several years about twenty new states (the so-called XYZ particles) with hidden charm were discovered by different experimental groups like Belle, BaBar, BES, CLEO, CDF, D0. Most of these states were observed in the above DD threshold region in some specific channel (beside X(3872) and Y(4260) states). These new particles are produced from B-meson decays and in electron-positron or two-photon collisions. Many recently discovered states above the DD threshold still require verification and explanation. As of now, their interpretation is far from obvious [2-6].
In general, one can identify four main classes of charmonium decays that are especially interesting and promising from a scientific point of view [1, 7]: decays into particle—antiparticle or DD pair: pp — — cc — baryon—antibaryon EE, AA, or DD pair; decays into light hadrons: pp — cc — pn, pp —
cc — wi0, pp — cc — , pp — cc
n+n ;
radiative decays: pp — cc — YVc, YXcJ; decays with J/^, and hc in the final state: pp — cc —
— J/^ + X ^ pp — cc — J/^n+n-,
— J/^n°n°; pp — cc — + X
and pp
ф'п+n , pp
cc
+ X ^ pp — cc — hcn+n , pp — cc
pp — cc pp — cc cc — hc
• hcn0n0.
+
2. DISCUSSION OF THE RESULTS OF CHARMONIUM SPECTRUM CALCULATIONS
For this purpose an analysis of the spectrum of the singlet 1So, 1Pi, D and triplet 3Si, 3Pj, 3 D j charmonium states in the mass region above DD threshold has been carried out. Different decay modes of charmonium such as decays into particle— antiparticle or DD pair, decays into light hadrons and decays with J/^, , and hc in the final state are investigated. These modes have small total widths and significant branching ratios, conditions that facilitate their experimental detection.
A combined approach based on the quarkonium potential model [8, 9] and a confinement model for decay products that involves a three-dimensional sphere
M, MeV/c2 5000
4600
4200
3800
3400
3000
1 1-1 у (5060)
Г y (4660)
,-, ш (4540)
(4415)
y (4360)
—=X (4160) —= y (4260)
Ш (4160)
- —= X (3940) ^^^ Ш (4040)
- _Ш (3770)
- П (2S) ш (2S)
- _n (1S) i
0-
1-
J
pc
Fig. 1. The mass spectrum of singlet 1S0 and triplet 3Si states of charmonium. The horizontal dotted line denotes DD threshold.
M, MeV/c2 4100
3900 3700 3500 3300
_ X(3915) _ —= Xc0 (2P) y(3940) —= xc1 (2P) Z(3930) —= xc2 (2P)
-
- — xc1 (1P)
_ — xc0 (1P)
0++ 1++ 2++ JPC
Fig. 2. The same as Fig. 1, but for triplet 3Pj states of charmonium.
embedded into the four-dimensional Euclidean space [10—13], predicts that more than twenty states of charmonium exist in the mass region above the DD threshold. We find that this approach not only predicts new states, but describes the existing experimental data with high accuracy. Special attention is given to the recently discovered new states containing hidden charm (i.e., the XYZ particles). A study of experimental data from Belle, BaBar, BES, CLEO, CDF, and D0 Collaborations concludes that eleven of new recently discovered states can be interpreted as S, P, D-wave charmonium states (two singlet 1S0, two singlet 1D2, three triplet 3S1, three triplet 3PJ, and one triplet 3D1) charmonium states. This assumption can be verified by the PANDA experiment with its high-quality antiproton beam.
Figures 1, 2 illustrate the spectrum of singlet 1S0 and triplet 3S1, 3PJ states of charmonium. The black boxes correspond to the established charmo-
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M, MeV/c2 4800 г
4600
4400 4200 4000
3800
7(4630)
■ 7(4274)
■ 7(4140)
2-
1-
2-
3- J
pc
Fig. 3. The same as Fig. 1, but for singlet 1D2 and triplet
3DJ charmonium states.
nium states, the black—white boxes indicate recently experimentally revealed XYZ particles that can be interpreted as higher-lying charmonium states. Additional charmonium states indicated by black-white boxes are predicted by our calculations [12— 14]. In this scheme, the X(3940) and X(4160) are interpreted as radially excited singlet 1S0 states of charmonium; the Y(4260), Y(4360), and Y(4660) as radially excited triplet 3S1 states of charmonium, and the X(3915), Y(3940), and Z(3930) as radial excited triplet 3PJ states of charmonium. Finally, the white boxes correspond to states that have not yet been found; these are interpreted as higher-lying radially excited states of charmonium.
Figure 3 illustrates the spectrum of singlet 1D2 and triplet 3DJ states of charmonium. The black— white boxes correspond to XYZ states that may be interpreted as D-wave charmonium states; the Y(4140) and Y(4274) are interpreted as singlet 1D2 states of charmonium. The Y(4630), with quantum numbers JPC equal to 1 , was observed by Belle in e+e- into AcAc; it is interpreted as a radially excited triplet 3D1 charmonium state. This assumption could be confirmed by PANDA in the channels considered above. The white boxes correspond to the charmonium states that have not yet been found, but predicted by this approach. They may be also interpreted as higher-lying 1D2 and 3DJ states of charmonium.
3. CALCULATION OF THE WIDTHS OF CHARMONIUM STATES
To confirm that the predicted S, P, D-wave charmonium states actually exist and can be found experimentally, their widths and branching ratios are calculated. The feature of all charmonium states
is their narrownes
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