Current Status of JPC-exotic Mesons

UCLEAR PHYSIC~

ELSEVIER

Current Status of

Nuclear Physics B (Proc. Suppl.) 56A (1997) 234--239

PROCEEDINGS SUPPLEMENTS

Jet-exotic Mesons

S. U. Chung * ~Brookhaven National Laboratory, Upton, N Y 11973 A n exotic meson with j P c = 1-+ has been studied by three groups, in the vicinity of the a2(1320) meson in the

decay channel ~ . The current status and future pro6pects of this exotic meson, especially on the new result coming from the data of BNL-E852 Collaboration, are given.

1. I n t r o d u e t l o n

2. O b s e r v a t i o n o f t h e ~h(1405) m e s o n

Although the quark model has been successful in describing and classifying hadrons, there is mounting evidence that the simple picture of a meson consisting of a quark and an anti-quark must be extended to include multi-quark states and those containing valence gluons. JPC-exotic mesons, i.e. the states belonging to j e t = 0 + - , 1 - + 2 + - and 0 - - , cannot be quarkonia. The current status and future prospects of a ~r~7 state with jPc = 1-+ are given.

The ~h(1405) state, called the M(1405) meson, was originally reported by the GAMS group [2], seen in the reaction 7r-p -~ ~r°~n at 100 GeV/c. The forward-backward asymmetry, obtained from the cosine of the Jackson angle in the ~r°~ system, shows a substantial non-sero swing in the a2(1320) region. This can happen only if there is a strong interference effect between even-odd waves in the a2(1320) region. After a partial-wave analysis of the ~r°z/ system, the GAMS group reported that both the Do and the P0 are resonant in the vicinity of the a2(1320). Because the a2(1320) and the M are seen to be produced via unnatural-parity exchange, it could be concluded that the b1(1235) exchange is the dominant one. This also indicates that the M should decay into the bx(1235)~r final state. There exist two additional pieces of evidence regarding the M/~h(1405) meson. The VES experiment [3] studied the reaction ~r-N -~ ~r-,/N at 37 GeV/c. The M/~h(1405) signal is present but small with large errors in the 7rT/channel, as seen in fig. 1. The E179 collaboration [4] at KEK studied the 7r-W system in the reaction ~r-p --~ ~r-17p at 6.3 GeV/c. As seen in fig. 2, both D+ and P+ waves peak at the a2(1320) mass region; therefore, p- or f2-exchange processes seem to play a central role in the production of these waves. Since the phase variation between these waves seems constant throughout the a2(1320) mass region, one must conclude that both are resonant. They have determined that m(a~) : 1325.1-4-5.1 MeV and r(.,) : 126.2 ~ 14.2 MeV for the a2(1320), and m(~ra) = 1323.1:k4.6 MeV and r(Th) -- 143.2+12.5 MeV for the M/~h(1405).

A short comment is in order on notations for the partial waves commonly encountered in the literature. A pure wave coupling to a system of two pseudoscalars can be completely specified by the orbital angular m o m e n t u m (L), its s-component (M) and the reflectivity quantum number (e). The latter is usually chosen such that it coincides with the naturality of the exchanged particles (Reggeons) in lr N or K N production processes. The spin J is simply L and the parity P is given by ( - ) ~ . The usual notation is IJPCM') or le LM} = LM,. Here C is the charge conjugation. If M = 0, there is only one ~, i . e . ~ = - 1 . In addition, one makes the usual assumption that the partial waves for M > 1 are negligible, so that M can be dropped from the subscript above. One then merely deals with the following three partial waves, without ambiguity: L0, L+ and L_, where the notations S, P, Ds F are used for L : 0, i, 2, 3, etc. In this note, an JPC-exotlc meson is denoted following the standard convention in use, i.e. the name is determined by the combination {P, C} of the meson. Hence, an isovector jPc = i - + state is denoted ~rl, while its isosca]ar partners are given the names ~a and ~ . It should be noted that the Particle Data Group [1] suggests a different convention for exotic mesons. 0920-5632/97/$17.00 © 1997 Elsevier Science B.V, All rights reserved. PII: S0920-5632(97)00281-8

S.U. Chung/Nuclear Physics B (Proc. Suppl.) 56A (1997) 234 239

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S. U Chung/Nuclear Physics B (Proc. SuppL) 5614 (1997) 234-239 3. N e w r e s u l t on t h e .]PC : 1-+ xT] s t a t e The BNL-E852 Collaboration [5] is in the process of carrying out a study of the reactions x - p -+ ~Tx-p and x - p --+ Wx°n at 18 GeV/c, where the decay is observed in the channels T7 or x + x - x °. The data presented in this note are based on 45,000 events fitting the reaction x - p -+ T/x-p with ~ --~ 3'7- The mass spectrum of r/x- is shown in Fig. 3, and the dotted histogram corresponds to nonr/ background in the data. It is seen that the a2(1320) peak is most prominent, with relatively pure r/events in this region. The partial-wave analysis has been done on the data, but the results are highly preliminary and are not presented here; however, one can glean a lot from a study of the structures in the unnormalised moments,

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with a shorthand notation in which A 2 stands for IAI2 and AB for 2Re{AB°}. In Fig. 3 are shown the experimental moments H(30), H(32), H(40) and H(42). As expected, one sees that the aa(1320) peak is dominated by H(40) and H(42) moments; as they are both large and negative, one concludes that the D+ wave must be dominant in the a= region. The crucial observation is that the experimental moments H(30) and H(32) are both large in the a= region as well and have the same sign (negative). From (2), one finds that the only possible solution requires presence of

237

a substantial P+ wave in the a2 region. A study of the finite acceptance of the apparatus reveals that it cannot account for the structures seen in the H(30) and H(32) m o m e n t s - - a more quantitative measure is given in the next paragraph. The next crucial feature of the moments is that the mass and the width of the peaks in all the moments H(30), H(32), H(40) and H(42) are very much like those of the (=2(1320) itself. From this one must conclude that the P+ wave is resonant, as is the a2 of course, and that the two phases track with each other throughout the a= region. A full likelihood analysis of the data has been performed, taking into account the finite acceptance of the apparatus in the fits. One finds that S-, P- and D-waves are sufficient to give acceptable fits to the data below 1.8 G~V. The required waves are So, P0, P - , P+, Do, D_ and D+. Of these, only the P+ and D+ waves are significant, as expected, in the a= region. It can be shown that the eightfold ambiguities endemic for a twopseudoscalar system are confined to the waves of unnatural-parity exchange (e = - 1 ) and there are no ambiguities involving the two waves, D+ and P+, with natural-parity exchange. The results of a fit----still preliminary--are given in Fig. 4; it is seen that the fit requires a substantial D-wave as well as a P-wave in the a= region. All the moments of fig. 3 are well described by the fit (not shown). Note that the phase difference (AO) between P+ and D+ is very well measured throughout the a= region and is a slowly decreasing function of the mass, similar to that seen in the VES data (see fig. 1). The mass dependence can be fit with two Breit-Wigner forms for D+ and P+ plus an additional phase which has a slowly-varying linear dependence in mass in the a2 region. Thus the observed structures in D+ and P+ could be interpreted as being both resonant. 4. Discussions and Future P r o s p e c t s

The KEK group sees a state at mass 1323 MeV and width 143 MeV in the xr/ channel, produced via natural-parity exchange. This result is in conflict with an earlier GAMS result which showed a 1 - + state with unnatural-parity exchange. (Sadovsky [6] emphasised that the analysis technique, originally employed by the GAMS group, was unreliable.) The VES group also studled the xl? channel but they report no significant 1- + state in the 1.3-1.4 GeV region. Now the new results from the BNL-E852 Collaboration tend to confirm those of the KEK data. It should be noted

S.U Chung/Nuclear Physics B (Proc. Suppl.) 56A (1997)234-239

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S. U Chung/Nuclear Physics B (Proc. Suppl.) 56A (1997) 23~239 that the BNL data at 18 GeV/c is three times the KEK data ion the same reaction but at a much lower beam energy of 6.3 GeV/c). A definitive statement from the BNL Collaboration is expected soon, since their current analysis is based on 1/4 of the total data sample they have taken in 1994 and 1995. In addition to the ~rx(1320) covered in this note, the VES group[3] claims a 1 - + candidate at 1620 MeV in the decay channels ~rp and ~rTf, produced via natural-parity exchange. The BNLE818 Collaboration[7] reported observation of a 1 -+ fl(1285)~r state in the 1.9-2.0 GeV mass region. All of these are in need of confirmation. It is hoped that the situation will soon be very much clarified by the impending new results from partial-wave analyses on the BNL-E852 data, which consist of some 10s triggers collected over two years. In addition to ~pr, the decay channels under study include T/'~, 1717, ~wTr, (a'lr~r)-, A(1285)~, w~r, bl(1235)Tr and w~/. The E852 Collaboration is planning to take more data in 1997 with a segmented Cerenkov Counter for ~r/K separation. Because gluons couple equally to all flavors, a comprehensive study of gluonic hadrons necessarily involves ai final states. The decay channels of interest include KlClr, KtC~rTr, KK~I, KtCw, etc. The relevant experiments, both on-going and planned throughout the world, should shed light on the missing and uncertain members of the quarkonium family and those that lie beyond, in the near future. New results are expected from the analyses being carried out on the IHEP data taken by the VES and the GAMS group. Further new information should be forthcoming from the massive statistics being accumulated by the Crystal Barrel and Obelix collaborations at LEAR, on the I~P and/~n annihilations at rest and in flight. The WA76 collaboration with the Omega Spectrometer at CERN, and its extension WA91, have been collecting data since 1991, for a study of exotic mesons in the central production at the SPS. Towards the end of this decade and at the beginning of the next millennium, one can look forward to new experiments such as COMPASS planned at CERN SPS, Japan Hadron Facility with 50-GeV protons at KEK, Beijing c-charm factory, and upgraded AGS and RHIC at BNL.

239

Acknowledgments The author wishes to thank the organizers for inviting him to speak in LEAP96, an excellent international conference devoted to non-perturbative QCD. This research was supporte~"by the U.S. Department of Energy under contract number DEAC02-76CH00016. REFERENCES

I.

Review of Particle Properties, Particle Data Group, Phys. Rev. D50, Part I (1994). 2. D. Aide, et ~., Phys. Lett. B205, 397 (1988). 3. G . M . Beladidze, et ~., Phys. Lett. B313, 276

(1093). 4. H. Aoyagi, et ~., Phys. Lett. B314, 246 (1993). 5. The BNL-E852 collaboration consists of BNL, IHEP (Protvino/Russia), Indiana, U. Mass.(Dartmouth), Moscow State, Northwestern (Evanston), Notre Dame, RPI (Troy, NY). The group is currently carrying out a systematic study of the exotic mesons produced in ~r-p interactions at 18 GeV/c at the existing BNL Multi-Particle Spectrometer (MPS), augmented by charged-particle and photon detectors around the target and at the downstream end of the MPS magnet. 6. S.A. Sadovsky, 'On the Choice of Minimization functional in the partial-wave analysis,' IHEP preprint 91-15, IHEP/Protvlno, 1991. 7. J . H . Lee, et ~., Phys. Lett. B323, 227 (1994).