Experimental Searches for Flux Tube Exotic Mesons. J. Napolitano. Rensselaer Polytechnic Institute, Troy NY Abstract

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1 Experimental Searches for Flux Tube Exotic Mesons J. Napolitano Rensselaer Polytechnic Institute, Troy NY Abstract We summarize the experimental signatures and searches for exotic mesons, based on theoretical prejudices of the ux tube model. Specic examples are given for the f 1 (1285) and b 1 (1235) systems. 1 Introduction The naive quark model provides an excellent description of hadronic structure at low energies. We readily observe just about all the states that it predicts, and there is at best spotty evidence for states that it says should be absent. Any extension of the quark model, which attempts to incorporate some of the additional degrees of freedom suggested by QCD, must explain why these additional degrees of freedom are so dicult to observe. One would also hope that such extensions would suggest fruitful new avenues to explore, in our quest for so-called \exotic" hadrons. The ux tube model [1] incorporates gluonic degrees of freedom into the quark model, by seizing on the idea that the vacuum expels glue similar to the way a superconductor expels magnetic ux. The force between two quarks, therefore, appears as a \ux tube" of glue which connects them. This ux tube can also be excited, providing a new degree of freedom in the meson excitation spectrum. Furthermore, this model helps explain why gluonic excitations of mesons have not been observed, and where they might be found [2]. First, it naturally shows that the energy needed to excite the ux tube itself is quite large, 1 GeV/c 2 or more, so these mesons would begin to appear near 2 GeV/c 2. Secondly, however, it strongly suggests an unexpected decay mode for gluonic excitations with exotic quantum numbers. Such excitations arise from a q q pair with combined spin S = 1, and where the ux tube is excited by one unit of internal angular momentum. Since this angular momentum is quantized along the axis between the two quarks, when the exotic meson decays and the ux tube breaks into two tubes with a new qq pair on the ends, it is not possible to absorb this angular momentum into the relative motion of the two new mesons. Instead, it must be absorbed into the internal spin of one of the

2 mesons. That is, one expects decays to an S-wave and P -wave pair of mesons, such as b 1 (1235) and f 1 (1285). The b 1 (1235) and f 1 (1285) have rather complicated decay modes themselves, rendering the subsequent data analysis quite onerous. This explains why these states have been largely undetected. One would not have attempted such a measurement (or set a graduate student to task on it) without a good reason! The ux tube model also suggests a good place to look for these objects. Since one starts with an S = 1 meson, such as the (770), and plucks the ux tube to create an exotic, then it is prudent to begin with a \beam" of mesons. Photon beams are a copious source of 's, and in fact the process p! +? p is entirely dominated by 0 production at high energies. Consequently, one is tempted to look at the reactions p! b p, p! b 1 p, and p! f 1 0 p. Of course, given the availability and higher cross sections aorded by pion beams, it is worthwhile to search in the analogous reactions as well. 2 Searches Soon after the original suggestion [2] that photoproduction of S + P ststems might be a good place to look for gluonic mesons, the CERN -photon collaboration [3] reanalyzed some of their earlier data. A weak signal was identied, although with very few events it was clearly inconclusive and no amplitude analysis could be attempted. Since then, several new searches have been carried out or are in progress:? p! f 1 (1285)? p with f 1 (1285)! K + K S? (BNL E818) [4] Be! f 1 (1285) X with f 1 (1285)! K K S (FNAL E687) [5] p! b 1 (1235) p with b 1 (1235)!! (SLAC BC) [6] BNL Experiment E852: 18 GeV/c? on a hydrogen target {? p! f 1 (1285)? p with f 1 (1285)! +? {? p! b 0 1(1235)? p with b 0 1(1235)!! 0 {? p! b 1 (1235) n with b 1 (1235)!! 2.1 CERN -Photon Experiment Data was acquired at CERN using a high energy tagged photon beam [7] on a hydrogen target, analyzing downtream particles using the spectrometer. The

3 group published several results from this experiment, including the reaction! p! +? p [8]. They noted that the signal b 1 (1235)!! was enhanced for 1:6 M(! +? ) 2:0 GeV/c 2. After the suggestion [2] that this would be a good place to search for new states decaying to b 1, the collaboration reanalyzed their data, looking carefully at the b 1 mass distribution. [3] Although statistics are poor, the mass distribution suggests two new states, one consistent with the! 3 (1670), and another new state at 1890 MeV/c 2 and a width near 200 MeV/c 2. The production cross section was 10 nb. No amplitude analysis was possible, given the poor statistics and signicant background. 2.2 BNL Experiment E818 The reaction? p! f 1 (1285)? p, with f 1 (1285)! K + K S? and K S! +?, has the advantage of all charged particles in the nal state. The made it possible to carry out a search for ux tube exotics in a dedicated experiment using an existing facility, the Multiparticle Spectrometer Facility (MPS) at BNL, without the need for ne-grained photon calorimetry. Enough events were accumulated so that a partial wave amplitude analysis could be carried out. Indeed, the published result [4] shows that besides the dominant J P C = 1 ++ f 1 (1285)? partial wave, there was also a signicant exotic J P C = 1?+ f 1 (1285)? wave at nearly the same level. Furthermore, the relative phase between these two waves is suggestive of a single broad resonance in each. The peak of the exotic wave gives an exotic meson mass near 2 GeV/c 2. Statistics are still rather poor, however, and the signal needs to be conrmed. 2.3 FNAL Experiment E687 As part of Fermilab experiment E687, which is designed primarily for the photoproduction of heavy avors, the Tennessee group investigated light quark spectroscopy in high energy photoproduction. The mean, tagged photon beam energy was 100 GeV. An analysis [5] of f 1 (1285) nal states does indeed suggest a peak in the mass spectrum near 1.8 GeV/c 2. Furthermore, the angular distribution in the helicity frame is consistent with J P C = 1?+. Once again, however, statistics are rather poor and a full amplitude analysis was not attempted.

4 2.4 SLAC Bubble Chamber A recent analysis [6] of the reaction p! +? +? 0 p with 19.3 GeV photon beams has aimed at isolating the b 1 (1235)!! signal and correlating it with the remaining. This data was taken with a Compton backscatter photon beam at SLAC in the large hydrogen-lled bubble chamber. The most striking feature of this data is the large production of ++! p + with a cross section of 0:6 b. There is no obvious structure that remains after events consistent with ++ production are removed. 2.5 BNL Experiment E852 In order to carry out a systematic and complete search for mesons with \unusual" quantum numbers, the MPS facility at BNL was extended with a negrained lead glass calorimeter [9] for downstream photons, and also with more precise target recoil charged [10] and neutral [11] detectors. Data was taken with a wide range of trigger topologies in 1994 and 1995, and that analysis is well underway. The facility was further upgraded with a Cerenkov detector for K= identication and more data was acquired in Nearly all data were taken with an 18 GeV/c? beam on a liquid hydrogen target. Present ux tube exotic analyses include? p! f 1 (1285)? p with f 1 (1285)! +? and! ;? p! b 0 1 (1235)? p with b 0 1 (1235)!!0 and!! +? 0 where 0! ; and? p! b 1 (1235) n with b 1 (1235)!!. These reactions are all complementary in terms of production mechanism (i.e. neutral or charged particle exchange) and isospin selectivity of the nal excited meson state. Figure 1 shows preliminary data on the reaction? p! +? +? 0 n. We plot the mass distribution for! +? as well as for the three two-particle mass combinations +?,! + and!?. The two-particle masses are plotted with the restriction that 2:1 M(! +? ) 2:3 GeV/c 2. There is a strong signal in 0 (770)! +? indicating that states decaying to! 0 will be important in this mass range. There is also a strong signal for b? 1 (1235)!!?, while b + 1 (1235)!! + may be absent. Therefore, interfering waves with different isospin will contribute to the b 1 decays. A full partial wave analysis of this data is in progress.

5 Figure 2.1 Preliminary data from BNL experiment E852 for? p!! +? n at 18 GeV/c. References [1] Nathan Isgur and Jack Paton. Flux tube model for hadrons in QCD. Physical Review D, 31:2910, [2] Nathan Isgur, Richard Kokoski, and Jack Paton. Gluonic excitations of mesons: Why they are missing and where to nd them. Physical Review Letters, 54:869, [3] M. Atkinson et al. Diractive photoproduction of a b 1 system. Zeitschrift Physik C, 34:147, 1987.

6 [4] J. H. Lee et al. Spin-parity analysis of the f 1 (1285)? system in the reaction? p! f 1 (1285)? p at 18 GeV/c. Physics Letters B, 323:227, [5] Kathleen Danyo Blackett. The photoproduction of possible hybrid mesons at he i = 110 GeV. PhD thesis, University of Tennessee, Knoxville, TN, August [6] G. R. Blackett, K. Danyo, T. Handler, M. Pisharody, and G. T. Condo. The photoproduction of the b 1 (1235) system. LANL preprint server, hep-ex, , [7] D. Aston et al. The GeV tagged photon facility at CERN. Nuclear Instruments and Methods, 197:287, [8] M. Atkinson et al. The reaction p! p! +? for photon energies of GeV. Nuclear Physics B, 229:269, [9] R. R. Crittenden et al. A 3000 element lead-glass electromagnetic calorimeter. Nuclear Instruments and Methods A, 387:377, [10] Z. Bar-Yam et al. A cylindrical drift chamber with azimuthal and axial position readout. Nuclear Instruments and Methods A, 386:235, [11] T. Adams et al. Design and performance of a cesium iodide detector. Nuclear Instruments and Methods A, 368:617, 1996.