Charmonium — In and out of the nucleus

NUCLEAR PHYSICS A Nuclear Physics A629 (1998) 358c-365c

ELSEVIER

Charmonium In and Out of the Nucleus -

Kamal K. Seth a aNorthwestern University, Evanston, IL 60208, USA

Abstract The bound states of charm and anticharm quarks, charmonium, offer a unique opportunity to explore the relationship between the traditional way of understanding hadronic interactions in nuclear physics, i.e., in terms of meson exchanges, and the QCD based description in terms of gluon exchanges. Inserting charmonium, for example J / ~ , into the nucleus leads to many interesting possibilities. Several of these are mentioned. One, namely J/~b attenuation in nuclei, is discussed in some detail because of its importance as the possible signature of quarkgluon plasma(QGP) formation in relativistic heavy ion collisions. A technique for the direct measurement of J/~b -nucleon cross section is proposed. 1. I N T R O D U C T I O N The subject of my talk is charmonium. Charmonium is the collective name for the bound states of a charm quark and a charm antiquark (c~). The discovery of the J/~b, the 1 - - state of charmonium in 1974, essentially launched the present era of QCD. Since the qq interaction is flavor invariant, in principle it can be studied in systems of any of the six quark flavors we now have - u,d,s,c,b,t. However, a reasonable compromise between the desirability of avoiding relativistic complications (which dicates preference for the heaviest quarks) and the need to have healthy cross sections in order to have the ability to make precision measurements (which dictates preference for the light quarks) leads immediately to charmonium. The fact that charmonium has a rich spectrum of bound states (see Fig. 1) provides additional justification for concentrating on charmonium. 2. CHARMONIUM IN FREE SPACE Essentially all the pre-1985 knowledge of charmonium spectroscopy came from e + e - annihilation experiments done at e + e- colliders. The knowledge was extensive, but its precision was limited, primarily for the reason that e + e - annihilation (which proceeds through the intermediate state of one photon [ 1 - - ]) can directly access only the 1 - - vector states, J/zb, ~', ~b" ..... This limitation was successfully overcome by the recent studies of charmonium states formed in pp annihilations, which proceed by 2 or 3 gluon intermediate states, and can directly access states of all 3"Pc . The pioneering experiment was R704 at CERN, but the technique realized its full potential only in Fermilab experiment E760 and its successor experiment E835, which is currently running. I need not go into a detailed description of E760 because most of 0375-9474/98/$19 © 1998 Elsevier Science B.V. All rights reserved. PII S0375-9474(97)00711-2

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its results have already been published. I will give a brief overview, and refer you to a detailed list o f references. [1] The principle o f the Fermilab experiment E760 is quite simple. A variable energy circulating b e a m o f ,~ 4 × 1011 stochastically cooled antiprotons in the Antiproton Accumulator traverses an internal hydrogen gas cluster jet target of density 5 x 1014 a t o m s / c m 2 to yield a luminosity £ --~ 10a%m-2s -1 (now raised to < 3 x 10alcm-2s -1 with a cryogenically cooled target). The detector was optimized for the detection and identification of electrons and photons. It was able to successfully reject hadronic background which was as m u c h as 1011 times larger than the desired e + e - signals. Unparalleled mass resolution (e.g., F W H M ~ 500-650 k e V at 3.1 GeV) was achieved (see Fig. 2) because o f the quality of the stochastically cooled antiproton b e a m and the extremely thin gas target. 2.1. The New Results from E760

Fermilab experiment E760 has m a d e a number of improtant contributions to c h a r m o n i u m spectroscopy. 1. The width of J A b

was measured directly, and found to be ,-~ 50% larger than what the

K.K. Seth~Nuclear Physics A629 (1998) 358c-365c

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indirect e + e - measurements had claimed and what was accepted for the last 20 years. This has large potential impact on gluon condensate estimates. 2. The widths of X1 (3P1) and X2 (aP2) states were accurately measured for the first time. The earlier e + e - experiments were essentially able to obtain crude upper limits only. Our measurements for the first time enable estimates of the strong coupling constant to be made for P-states. 3. Multipole analysis of the radiative decay of X2 provides an estimate of (1+1%)/rn~, where 1% is the anomalous magnetic moment of the c-quark, and mc is its mass. A finite value of 1% may be indicative of compositeness of quarks. The present results are inconclusive because of large statistical errors, but much better results are expected from E835. 4. For the first time the 1Px (1 - + ) state of a particle-antiparticle system was unambiguously identified and a precision determination of its mass was made. This is of crucial importance in determining the spin-spin interaction in the confining part of the qq potential. . The e + e - experiments have not been very successful in identifying singlet states of -onia. Only the r/c (1S0) is known. E760 has confirmed its existence but finds that its parameters, mass and width, are quite different. Even more precise results are expected from E835. In addition to its new results for charmonium resonances, E760 has obtained many extremely interesting results in light quark physics. Q It has discovered new mesons: X(1520) and X(2000) in 7r° 7r° decay, and X(1488), X(1748) and X(2100) in r/~ decay.

K.K. Seth~Nuclear Physics A629 (1998) 358c-365c

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• It has measured pp forward scattering parameters, particularly the p parameter, in the 3-6 GeV/c region with unprecedented precision. • It has measured form factors of protons for timelike momentum transfers in the Q2 = 9 15 GeV 2 region and found precocious saturation, much like that observed for spacelike momentum transfers in electron elastic scattering. 3. CHARMONIUM IN THE NUCLEUS It has become a well-known clich6 that nuclear physics has to come to terms with QCD. This, of course, does not mean that 157Gd has to be understood in terms of 471 quarks. What it does mean, however, is that the forces and symmetries of nuclear physics have to be understood in terms of those of QCD. In particular, it means that the phenomenological description of nucleon-nucleon forces, so successfully developed in nuclear physics, has to find its underpinnings in QCD. The complex relationship between meson exchange models and quark and gluon exchanges of QCD have to be understood. The best way to understand these relationships is to inject into the nuclear medium a hadron which can only interact with the nucleon via gluon exchange. Such a hadron is one which has no quarks in common with nucleons; charmonium is the ideal choice. (See Fig. 3) S

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Introducing charmonium into the nuclear environment creates some very unusual opportunities. Several of these have been discussed elsewhere. [2, 3] Among the most exciting proposals are: • Charmonium ( J / ~ , ~c ) bound in nuclei • Short range NN correlations in nuclei by sub-threshold production of JAb • Charmed nuclei • Charmonium mass renormalization in nuclei

• JAb -nucleon cross section and QGP signature.

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K.K. Seth~Nuclear Physics A629 (1998) 358c-365c

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L(fm) Figure 4. Compilation of J / ~ attenuation data from several experiments. The straight line shows the Gerschel-Htifner fit with Cr~bs -----6.3 + 1.2 mb. The inset shows the latest prediction of Armesto and Capella [10] without invoking QGP formation.

4.4. The See-saw Early photoproduction experiments (E 7 ----- 75 - 200 GeV), interpreted via VMD, gave a(J/%b -N) = 1 - 2 mb. The J / ~ suppression observed by NA38 experiments at CERN in O-Cu, O-U and S-U collisions, [4] could not be explained with ~r~bs = I - 2 mb. Therefore, Satz [5] concluded that "theoretically conceivable.., quark matter formation" Brodsky & Mueller [6], on the other hand, pointed out that because of the color transparency effect, "cross section for J/~b scattering on nucleons cannot be extracted from high energy photoproduction and hadroproduction reactions" and therefore cr~bs( J / ~ b ) ¢ 1 - 2 mb. Indeed, in 1992 Gerschel & Hufner [7] showed that with an e m p i r i c a l a ~ b ~ ( J / ¢ ) = 6.3 4- 1.3 mb all J/~b suppression data on 7-A, h-A and A - A could be consistently explained. (See Fig. 4) They posed the question "Where is QGP?" in the title of their paper. The success of Gerschel & Hufner raised questions about the meaning of cr~bs ( J / ~ b ) = 6.3

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mb. Is it a ( J / ¢ -N)? Is it a ( J / ¢ - co-movers)? Is it rr((c~ g) - N)? What happened to color trasparency? What about feed-down from other resonances? An infinite number of conjectures ( = number of papers ) were offered, and no apparent consensus was reached. In the meanwhile, NA50 dropped a bomb! [8] In Pb-Pb collisions at (158 GeV/c)/A, 50% larger J / ¢ suppression was observed than could be explained with a~bs = 6.3 mb. Once again, the shout went up: "Is it QGP, yet?" Once again, the spoilers (Gavin et al. [9], Capella et al. [10]) claim that NA50 results too can be explained in the model of a~bs(J/¢ -N) + cr(J/¢ + co-movers). Perhaps it is not QGP, yet!

5. A DIRECT MEASUREMENT OF a ( J / ¢

- N)

In order to avoid the problems inherent in high energy experiments on J / ¢ production and attenuation in nuclei it is necessary to: 1. Produce J / ¢ exclusively, i.e., resonant production on a nuclear proton. This eliminates feed downs and comovers. 2. Produce c~ pair in pure color singlet state. This avoids the color octet problem. 3. Produce c5 pair with such low momentum that it will hadronize into J / ¢ before meeting the next nucleon. This avoids the color transparency problem. The obvious answer to the above requirements is to produce JAb at resonance by antiproton annihilation on a nuclear proton.

p + A-+ J / ¢ + (A - 1)-4 l+l - + (A - 1),

(4)

and to scan the Fermi-broadened J / ¢ resonance as a function of the antiproton momentum. J / ¢ can then be identified by its decay into leptons e + e - and/or # + # - .

5.1. The Proposed Experiment In the reaction p + p--+ J / ¢ -+ l + l- J / ¢ is produced resonantly at P ~ ) = 4.07 GeV/c with F ( J / ¢ ) = 0.1 MeV, rrp~k(J/¢ ) ,~ 360 nb. In the reaction p + A--+ J / ¢ + (A - 1)--+ e + e - + (A - 1)

(5)

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The physics background under the J / ¢ peak is due to the timelike form factor of the proton iO + p--+ e + + e -

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which varies from ,-~ 40 to 10 pb in the region of interest. (See Fig. 5) Thus, in a measurement made, for example, at the Fermilab/7 accumulator, with the present E760/E835 detector, we have the following parameters for the experiment:

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70 m A antiprotons circulating at ~ 0.65 MHz, 1012 atoms/cm a Xe atoms in the gas jet target, £ = 2.5 × 10 ~1 cm -2 s -1, a(peak) = 185 pb = 185 x l O -36 cm 2, acceptance times efficiency ~e --~ 0.5, Count rate (peak) = 200 / day Thus an 11 point scan across the Fermi-broadened J / ¢ (3.60 - 4.60 GeV/c) will require 300 hours, to give cr(J/~b - N) to 4-5%. In order to test the correctness of the assumptions made, and the method of data analysis, several targets, from C to Pb will need to be measured. This would indeed be a very rewarding measurement, and I hope that we can someday actually make it. The author wishes to thank the organizers of QULEN '97 for inviting him. He also wishes to thank T. Pedlar for his help in the preparation of this paper. This work was done with the support of the U.S. Department of Energy.

REFERENCES 1.

2.

The E760 Collaboration, T.A. Armstrong, etal., Phys. Rev. Lett. 68 (1992) 1468,Nucl. Phys. B373 (1992) 35,Phys. Rev. Lett. 69 (1992) 2337, Phys. Rev. D47 (1993) 772,Phys. Rev. Lett. 70 (1993) 1212, Phys. Rev. Lett. 70 (1993) 2988, Phys. Lett. B308 (1993) 394, Phys. Lett. B307 (1993) 399, Phys. Rev. D48 (1993) 3037, Phys. Rev. D52 (1995) 4839, Phys. Lett. B385 (1996) 479. Kamal K. Seth, Proc. Int. Workshop on Flavor and Spin in Hadronic and Electromagnetic Interactions, Torino 1992, ed. by E Balestra etal., (Editrice Compositori, Bologna, 1993), pp. 179-200.

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3. Various authors, Proc. SuperLEAR Workshop, Zurich 1991, ed. by C. Amsler and D. Umer (Institute of Physics, Bristol, 1992), pp. 229-286. 4. C. Baglin etal., Phys. Lett. B220 (1989) 471; Phys. Lett. B255 (1991) 459. 5. H. Satz, Proc. Int. Lepton-Photon Symposium, Geneva 1991, ed. by S. Hagerty etal., (World Scientific, Singapore, 1992), pp. 273-300. 6. S.J. Brodsky and A. H. Mneller, Phys. Lett. B206 (1988) 685. 7. C. Gerschel and J. Htifner, Phys. Lett. B207 (1988) 253; Zeit. Phys. C 56 (1992) 171. 8. NA50 Collaboration: M. Gonin etaL, Nucl. Phys. A610 (1996) 404c. 9. S. Gavin and R. Vogt, Nucl. Phys. A610 (1996) 442c;Phys. Rev. Lett. 78 (1997) 1006. 10. A. Capella etal., Phys. Lett. B393 (1997) 43; also preprint hep-ph/9706452/(1997).