Evidence for a Narrow S = +1 Baryon Resonance in Photoproduction from the Neutron [Contents] 1. Introduction 2. Principle of experiment 3. Experiment at LEPS 4. Results 5. Summary Physics Colloquium Ⅰ July 7, 2004 Shibata lab. 01_1010_2 Taisuke Koroku T. Nakano et al. Phys. Rev. Lett. 91 (2003)

1. Introduction Hadron is formed from quarks. Quark has “color” (r, g, b and their anticolors). Observed particles must be “colorless”. q qq Baryon － Meson qq For example, quark configuration qqqqq is also possible. five-quark ⇒ It’s exotic five-quark state ! It is valuable to study QCD. －

I3I3 S  + (1530)  (2070) N (1710)  (1890) S = +1 S = 0 S = -1 S = -2 I = 0 I = 1/2 I = 1 I = 3/2 Five-quark state ・ Five-quark state is first predicted by A. Manohar in ・ Ten types five-quark states were theoretically predicted by Diakonov, Petrov and Polyakov ( 1997 ). a molecular meson-baryon ・ They were considered to be a molecular meson-baryon penta-quark resonance or penta-quark. uuddsS = +1 ・ Θ + has a quark configuration uudds and S = MeV/c 2 ・ The region of its mass ( 1530 MeV/c 2 ) had never been searched before. － Prediction of five-quark states

K+ nK+ n Θ + γ + n → K - Θ + n (udd) γ K + (us) K - (us) n (udd) l l Θ + (uudds) l K + (us) l 2. Principle of experiment γ N reaction

3. Experiment at LEPS The experiment was carried out at LEPS (Laser-Electron Photon facility at SPring-8) in Hyogo, Japan. Compton backscattering Beam photons are produced by Compton backscattering of laser light. Recoil photons are measured. 8 GeV electron in SPring-8 storage ring e-e- 1.5~2.4 GeV γ recoil e - Ar laser 351 nm Ar laser

Spectrometer Two targets ： Plastic scintillator ・ Plastic scintillator composed of Hydrogen and Carbon nuclei Liquid-hydrogen ・ Liquid-hydrogen (LH 2 ) K-K- K+K+ n Three drift chambers Dipole magnet TOF wall LH 2 Plastic scintillator Two sets of events from two targets are clearly distinguished. Beam Beam

Analysis method K +, K - were identified. P K + P K - ⇒ P K +, P K - were measured. Θ + mass was calculated using missing-mass MM γK -. n γ Θ+Θ+ K+K+ K-K- n’ K+K+ MM γK - distribution at 1.4 ~ 1.8 GeV was studied.

Cuts ・ Vertex cut (select the events from plastic scintillator) ・ Photoproduction of φ meson excluded ・ etc. Event selection After these cut, 109 events satisfied all the criteria. n γ Θ+Θ+ K+K+ K-K- n K+K+ Θ + event n γ K+K+ n K-K- Φ production event φ

The effect of Fermi motion is removed using equation below. Two missing mass MM γK - and MM γK + K - have the same Fermi motion ⇒ Its effect is canceled. Correction for the Fermi motion ・ Nucleons in nuclei are affected by the Fermi motion. n in 12 C γ K+K+ K-K- n Θ+Θ+ K+K+ affected by the Fermi motion

4. Result A peak was observed. Mass ： 1.54 ± 0.01 GeV/c 2 Width ： ≦ 25 MeV/c 2 Width ： ≦ 25 MeV/c 2 Gaussian significance ： 4.6σ The MM γK - spectrum c solid histogram: events from plastic scintillator dotted histogram: background events from the LH 2 normalized by a fit in the region above 1.59 GeV/c 2 After the correction for the Fermi motion

5. Summary ・ Five-quark states had been theoretically predicted. Θ + (uudds)1530 MeV ・ Θ + (uudds) was predicted at the mass of 1530 MeV. ・ The experiment was carried out at the LEPS of Spring-8. Beam: High energy photon 1.5~2.4 GeV Target: n in 12 C ・ The events were selected with several cuts. ・ The Fermi motion was corrected. ・ A sharp baryon resonance peak was found. Mass: 1.54±0.01 GeV/c 2 Width: ≦ 25 MeV/c 2 Gaussian significance: 4.6σ It’s a molecular meson-baryon resonance or a penta-quark baryon Θ +. -

Two missingmass have the same Fermi motion. ⇒ The effect of Fermi motion is removed using equation below. n γ Θ+Θ+ K+K+ K-K- n K+K+

groupreactionmass [MeV]width LEPSγn → K + K - (n)1540 ± 10 ＜ 25 DIANAK + Xe → K 0 pXe’1539 ± 2 ＜ 9＜ 9 CLASγd → K + K - p(n)1542 ± 5 ＜ 21 SAPHIRγp → K + K 0 (n)1540 ± 6 ＜ 25 ITEPνA → K 0 pX1533 ± 5 ＜ 20 CLASγp → π + K - K + (n)1555 ± 10 ＜ 26 HERMESe + (27.6GeV) d → K 0 pX1528 ± 313 ± 9 SVDp(70GeV) A → K 0 pX1526 ± 3 ＜ 24 COSYpp → K 0 pΣ ± 5 ＜ 18 JINRp(10GeV) A → K 0 pX1545 ± 1216 ± 4 ZEUSe + p(cm300GeV) → e’K 0 pX1522 ± 38 ± 4 Summary of the experiments of penta-quark