Experiment at SPring-8

ELSEVIER

Nuclear Physics A670 (2000) 332c-339~ www.elsevier.nl/locatelnpe

Experiment T. T. T. Y. A. H.

at Spring-8

Nakano, J.K. Ahn, H. Ejiri, M. Pujiwara, T. Hotta, N. Matsuoka, T. Matsumura, Mibe, M. Nomachi, H. Toki, a C.W. Wang, S.C. Wang, b H. Kawai, T. Ooba, ’ Iwata, Y. Miyachi, T. Toyama, A. Wakai, d K. Hicks, e H. Akimune, Y. Asano, Sugaya, f S. Date, N. Kumagai, Y. Ohashi, H. Toyokawa, s K. Imai, M. Yosoi, Ichikawa, h T. Kishimoto, A. Sakaguchi, M. Sumihama, i S. M&no, j and Shimizu k

“RCNP, Osaka University, 10-l Mihogaoka, Ibaraki, Osaka 567-0047, JAPAN bInstitute of Physics, Academia Sinica Nankang, Taipei 11529, Taiwan CDepartment of Physics, Chiba University, l-33 Yayoi-cho, Inage, Chiba 263-8522, JAPAN dDepartment of Physiscs, Nagoya University, Chikusa, Nagoya 4648602 eDepartment of Physiscs, Ohio University, Athens, Ohio, 45701, USA ‘JAERI, Spring-8, 323-3, Mihara, Mikazuki, Sayou, Hyogo 6795143, JAPAN sJASR1, Spring-8, 323-3, Mihara, Mikazuki, Sayou, Hyogo 6795143, JAPAN hDepartment of Physics, Kyoto University, Kitashirakawa Oiwake-cho, Sakyoku, Kyoto 6068502, JAPAN ‘Department of Physics, Osaka University, l-l Machikaneyama, Toyonaka, Osaka 56&0043, JAPAN jwakayama Medical College, Kimiidera 811-1, Wskayama, 641-0012, JAPAN kDepartment of Physics, Yamagata University, Kojirakawa 1-412, Yamagata 990-8560, JAPAN The GeV photon beam at Spring-8 is produced by backward-Compton scattering of laser photons from 8 GeV electrons. The maximum energy of the photon will be above 3 0375-9474/00/$ - see front matter 8 2000 Elsevier Science B.V PI1 SO375-9474(00)00124-X

All rights reserved.

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T. Nakano et al./Nuclear Physics A670 (2000) 332c-339c

GeV, and the beam intensity will be l0 T photons/see. Polarization of the photon beam will be 100 % at the maximum energy with fully polarized laser photons. We report the outline of the quark nuclear physics project with this high-quality high-intensity beam. 1. L E P S Facility

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Figure 1. Plan view of the Laser-Electron Photon facility at SPring-8 (LEPS).

The Spring-8 facility is the most powerful third-generation synchrotron radiation facility with 61 beamlines. We use a beamline, BL33LEP (Fig. 1), for the quark nuclear physics studies exclusively. The beamline has a 7.8-m long straight section between two bending magnets. Polarized laser photons are injected from a laser hutch toward the straight section where Backward-Compton scattering (BCS) [1] of the laser photons from the 8 GeV electron beam takes place (Fig. 2). The BCS photon beam is transfered to the experimental hutch, 60 m downstream of the straight section. Figure 3 shows the maximum energy of the BCS photon as a function of a laser-photon energy for the cases of 8 GeV and 6 GeV electron beams. The maximum energy depends more strongly on the electron-beam energy than on the laser-photon energy. SPring-8 is the only facility where a high-intensity BCS photon beam above 2 GeV is obtainable. If laser lights are 100 % circular polarized, a backward-Compton-scattered photon is also polarized. The polarization drops as the photon energy decreases as shown in Figure 4. However, an energy of laser photons is easily changed so that the polarization remains reasonably high in the energy region of interest. The intensity, position, and polarization

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T. Nakano et al./Nuclear Physics A670 (2000) 332c-339c

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of the laser lights which do not interact with the electron beam are monitored at the end of the beamline. Circular Polarization i.o

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Figure 5. Scattered angle of BCS photons at SPring-8.

Figure 5 shows the relation between the energy of BCS photons and the scattered angle. The scattered angles of BCS photons above 1.5 GeV are less than 0.15 mrad which corresponds to a beam size of 1 cm at the distance of 60 m from the collision point. This directivity gives a great advantage in using a compact detector and target system. The beam energy is determined by measuring the energy of a recoil electron with a tagging counter. The tagging counter is located at the exit of the bending magnet after

T. Nakano et al./Nuclear Physics A670 (2000) 332c-339c

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the straight section. It consists of multi-layers of a 0.1 mm pitch silicon strip detector (SSD) and plastic scintillator hodoscopes. Electrons in the energy region of 4.5 - 6.5 GeV are detected by the counter. The corresponding photon energy is 1.5 - 3.5 GeV. The position resolution of the system is much better than a required resolution. The energy resolution (RMS) of 15 MeV for the photon beam is limited by the energy spread of the electron beam and an uncertainty of a photon-electron interaction point. In the first stage, we use a conventional Ar laser. A 25 W A r laser has a 2 W output at 351 mn (3.5 eV). The maximum energy of the BCS photon with this laser light is 2.4 GeV. If we use a multi-line mode around 351 nm, the laser output power is about 5 W. The corresponding intensity of the BCS photon beam amounts to l0 T photons/sec, which is the maximum intensity approved for the project.

2. Physics P r o g r a m s In this section, a few physics programs which can be studied with a charged-particle spectrometer which is under construction. It is well known that all the hadron-hadron total cross sections (including a 7P total cross section) in a wide energy range are reproduced very well in terms of two s-dependent terms with s -°5 and s °'°s dependences, where s is the Mandelstam s variable [2]. The Reggeon exchange model clearly suggests that the s -°'5 term originates from the p meson (T = 1, J " -- 1-. ) trajectory. On the other hand, the s °'°s term requires the introduction of an unobserved Regge trajectory, whose a(t --- 0) has to be 1.08. The trajectory is called the pomeron because of the great contribution by Pomeranchuk [3]. One can identify the Pomeron trajectory with a gluon trajectory since the first particle state on the trajectory appears at m 2 ~ 4 GeV 2 with J = 2 (2 ++ glueball). At high energies, diffractive photo-production of a vector meson from a proton target is well described as a pomeron-exchange process in the framework of the Regge theory and of the Vector Dominance Model (VDM) [4]; a high energy photon converts into a vector meson and then it is scattered from a proton by an exctmnge of the pomeron [5]. Within QCD, interactions between hadrons can be due to quark-exchange and ghion-exchange. Phenomenologically, we know that the quark-exchange processes can be simulated b y meson-exchange which can be calculated from effective Lagrangians based on QCD. This has been very successful in understanding various photoproduction data at low energies. On the other hand, the gluon-exchange mechanism is poorly understood. For the ¢ production, the meson-exchange contribution is small because the couplings of the ¢ meson to other non-strange mesons are weak due to the OZI suppression. Precise measurements of da/dt and spin observables in the low energy will provide an important information on the Pomeron exchange dynamics through interferences with the meson exchange amplitudes [6]. Moreover, the ¢ production measurements near the threshold may reveal the existence of another gluon-exchange trajectory (a 0 ++ glueball trajectory) whose contribution falls off rapidly at high energies. In the measurement of the ¢ production, one can also address the question concerning the s$ components in nucleon. Again, polarization data obtained by using polarized photons axe crucial. Spin observables magnify small amplitudes hidden in a dominant amplitude by interference effects. The interference between a pomeron-exchange amplitude and a small amplitude due to

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T Nakano et al./Nuclear Physics A670 (2000) 332c-339c

a direct knockout of sg in the nucleon may cause a large asymmetry of the production cross sections between the spin parallel and anti-parallel configurations of the polarized photon and polarized nucleon [7]. A photo-production of A(1405) from a proton is also of great interest. A recent theoretica] work within a framework of a chiral unitary model treats the A(1405) as a dynamically generated KN resonance. The detection of K + from the 7P --* K+A(1405) is sufficient to determine the shape and strength of the A(1405), which are predicted by the theory [8]. The final topic is a search for a a meson via a Primakov process [9]. In this study, we search for enhancement of two-pion production from a heavy target in a very forward angle, with a small invariant mass, and in a small momentum transfer region. The enhancement is expected if there is a state with positive charge parity and with a wide width, which couples both to two gammas and two pions. 3. D e t e c t o r

Figure 6. The LEPS detector setup.

The LEPS detector (Fig. 6) consists of charged-particle tracking counters, a dipole magnet, and a time-of-flight wall. The design of the detector is optimized for a ¢ photoproduction in forward angles. It has a large forward acceptance and good coverage of momentum transfer t for the photoproduction as shown in Figure 7.

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The opening of the dipole magnet is 135-cm wide and 55-cm height. The length of the pole is 60 cm, and the field strength at the center is 1 T. The vertex detector consists of 2 planes (x- and y-) of single-sided SSDs (SVTX) and 5 planes (x,x',y,y',u) multi-wire drift chamber (DC1), which are located upstream of the magnet. The stereo ambiguity (pairing ambiguity) for two-track events are solved with DC1. The thickness of each SSD is 300#m and it results in angle spread (a) of ,-, 1 mrad. The corresponding deterioration in 2-kaon invariaat mass and momentum transfer t measurements are 300 KeV and 2 x 10-3GeV 2 respectively. Two sets of MWDCs (DC2 and DC3) are located downstream of the magnet. The active area size of DC2 and DC3 is 200cm(W) × 80cm(H). Each set has 5 planes: x,x',y,y', and u(v) in order to solve both left-right ambiguity and stereo ambiguity locally. Figure 8 shows the momentum resolution as a function of a momentum. The identification of momentum analyzed particles is performed by measuring a time of flight from a target to the TOF wall. The start signal for the TOF measurement is provided by a RF signal from the 8-GeV ring, where electrons are bunched at every 2 nsec (508MHz) with a width (a) of 12 psec. Since the speeds of the both electron beam and a laser-electron photon are same, the arrival time of the laser-electron photon at the target is synchronized with the RF signal. A stop signal is provided by the TOF wan consisting of 40 2m-long plastic scintillation bar with a cross section of 4cm (t) x 12 cm (w). The resolution of the T O F counter is less than 100 psec. When the TOF wall is located at 3 m from the magnet, a flight-length of a charged particle is about 4 m. In this case, the detector has a capability of 4-a K/~r separation up to 2 GeV/c.

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7:. Nakano et al./Nuclear Physics A670 (2000) 332c-339c

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A ¢ meson is identified through the reconstruction of the K K invariant mass. The typical mass resolution is 600 keV, which is much smaller than the ¢ width. The measurement error of a momentum transfer is about 0.01 GeV 2, and it mainly comes from the measurement error of the incident photon energy. The elastic ¢ photoproduction from a proton and a quasi-free ¢ photoproduction from a nucleon are separated by reconstructing a recoiled target mass from the incident photon energy and the energy-momentum of a ¢. Figure 9 shows the reconstructed mass distribution by a simulation. The narrow peak at the proton mass corresponds to the elastic production, and the broad tail corresponds to the quasi-free production. The broadening is due to a Fermi momentum of a nucleon in a nucleus. The beam intensity and profile during experiments are monitored by a beam monitor consisting of a pre-radiator to convert a photon into an electron-positron pair, a thin plastic scintillation counter, and SSD counters. For future upgrade, developments of an aerogel cherenkov counter, a gamma counter, and a polarized target are in progress. 3.1. S t a t u s and Schedule The construction of the beamline and the laser hutch were completed in September of 1998. The laser system was installed in the winter shutdown period of 1998-1999. The construction of the experimental hutch and the detector system will be completed by the end of March, 1999. The commissioning of the Laser-Electron Photon beam will be carried out from May to July in 1999. Then, the first test experiment will start in September, 1999.

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REFERENCES

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