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Laser Electron Photon Facility

In the laser-electron-photon group of the experimental nuclear physics section, the quark-nuclear physics is studied aiming at the experimental elucidation of the character of quantum systems composed of quarks and gluons, which are called "hadrons", using a photon beam with very high energy and good quality. The experiments are carried out at SPring-8 in Nishiharima, that is a synchrotron radiation ring with the world highest energy (8 billion electron volts: 8 GeV). Hereafter, let's briefly introduce in question form.

(1) What is a laser-electron photon?

Since the photon does not have charge, it is not possible to get energy with a usual accelerator. However, if laser light comes into head-on collision with the accelerated high energy electron by Compton scattering (elastic scattering of an electron and a photon), a scattered photon will get energy and come out almost backward. We call this "laser-electron photon (LEP)". Actually, when the laser of 350-nm wavelength is applied to the 8-GeV electrons of SPring-8, a polarized photon beam with a maximum energy of 2.4 GeV is obtained.

(2) Why can "hadron" be studied when the GeV-photon is used ?

In general, if light is applied to a material, it may be examined up to the size of the wavelength. Using the famous expression, E = h, the above-mentioned 2.4-GeV photon gives 0.5-fm wavelength which is shorter than the size of a hadron, typically of a proton (~1 fm) (Here, a femto-meter (fm) is a super-tiny size with as many as six digits smaller than a nano-meter, which we hear often recently.) Thus the research is attained to its substructure, the world of quarks and gluons. Especially, it is only our group that has LEP with the energy above the production threshold of a -meson in the world which consists of a strange quark and an anti-strange quark.

(3) What and how to measure ?

First, the high energy photon beam is hit to a hydrogen or a nuclear target. The particles emitted after a photo-nuclear reaction are measured with some detectors and the information on the production particles and the reaction mechanism is derived.
Since many of particles produced are unstable and will immediately decay to other two or more particles, it is important to measure them simultaneously. We have a high resolution spectrometer consisting of a large dipole magnet and several position detectors for the forward outgoing particles, and a three-dimensional tracking detector called time-projection-chamber (figure below) for the detection of charged particles emitted sideway which has been put in practical use recently. Moreover an electromagnetic calorimeter covering a large solid angle is also prepared for the particles decaying to two -rays like a neutral -meson.

(4) What is a "pentaquark", the signal of which was reported by newspapers?

Up to now, only two groups of hadrons have been found, namely, the particle which consists of three quarks (baryon) and the particle which consists of a quark and an anti-quark pair (meson), although the particle consisting of four or more quarks are not theoretically prohibited. Our group has observed a signal of an exotic particle (named +) that seemed to be a baryon with an anti-strange quark for the first time in the world among the particles produced by the reaction of a GeV-photon with a neutron in the nucleus (figure below). Since the particle which consists of two quarks and one anti-quark is not allowed from the constraint of the theory, this particle should have at least four quarks and one anti-quark, and thus is considered to be a "pentaquark". The proof of its existence and the clarification of its physical character are just under advance all over the world. Our group also makes this the top priority and is tackling its research.

(5) What kind of other themes are being studied ?

Search for the effect of glueball (a particle made only of gluons) through the phi-meson photoproduction near threshold, clarification of the structure of the excited state of hyperon (a baryon with strange quark(s)), especially,(1405), extraction of the modification of mesons in the nuclear medium, search for the undiscovered baryon resonance, etc, are other challanged themes. The experimental study is being done with state-of-the-art technologies in order to understand the matter from the quark-gluon level, also expecting the encounter with unknown phenomena. An understanding of the structure of the hadron containing strangeness is in particular of much interest, because it is connected also with the huge macroscopic system such as a neutron star.

Time Projection Chamber

Missing mass spectrum obtained from the measurement
of K- mesons from the neutron target.
(The shaded region is that from the proton target.)




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