Evidence for a Narrow S = +1Evidence for a Narrow S = +1Baryon Resonance in PhotoproductionBaryon Resonance in Photoproduction

from the Neutronfrom the Neutron

[Contents]1. Introduction2. Principle of experiment3. Experiment at LEPS4. Results5. Summary

Physics ColloquiumⅠ July 7, 2004

Shibata lab.01_1010_2 　 Taisuke Koroku

T. Nakano et al.Phys. Rev. Lett. 91 (2003) 012002

1. IntroductionHadron is formed from quarks.

Quark has “color” (r, g, b and their anticolors).Observed particles must be “colorless”.

qq qBaryon

－

Meson

q q

For example, quark configuration qqqqq is also possible.

⇒ It’s exotic five-quarkfive-quark state !It is valuable to study QCD.

－

I3

S

++ (1530)(1530)

(2070)

N (1710)

(1890)

S = +1S = +1

S = 0S = 0

S = -1S = -1

S = -2S = -2

I = 0I = 0

I = 1/2I = 1/2

I = 1I = 1

I = 3/2I = 3/2

・ Five-quark stateFive-quark state is first predicted by A. Manohar in 1984.・ Ten types five-quark states were theoretically predicted by Diakonov, Petrov and Polyakov (1997).

・ They were considered to be a molecular meson-baryona molecular meson-baryon resonance or penta-quarkpenta-quark.

・ Θ+ has a quark configuration uuddsuudds and S = +1S = +1.

・ The region of its mass (15301530 MeV/c MeV/c22) had never been searched before.

－

Prediction of five-quark states

K+ 　n

γ 　 + 　 n 　→　K-　 ΘΘ++

n (udd)

γ

K+

(us)

K- (us)

n (udd)

l

lΘ+ (uudds)

l

K+

(us)

l

2. Principle of experiment

γ N reaction

3. Experiment at LEPSThe experiment was carried out at LEPS (Laser-Electron Photon facility at SPring-8) in Hyogo, Japan.

Beam photons are produced by Compton backscatteringCompton backscatteringof laser light. 　 Recoil photons are measured.

8 GeV electronin SPring-8 storage ring

e-1.5~2.4 GeV γ1.5~2.4 GeV γ

recoil e-

351 nm Ar laserAr laser

SpectrometerTwo targets ：・ Plastic scintillatorPlastic scintillator composed of Hydrogen and Carbon nuclei・ Liquid-hydrogenLiquid-hydrogen (LH2)

K-K+

n

Three drift chambers

Dipole magnet

TOF wall

LH2

Plasticscintillator

Two sets of events from two targetsare clearly distinguished.

BeamBeam

BeamBeam

Analysis method

K+, K- were identified. ⇒ PPKK++, PPKK-- were measured.

Θ+ mass was calculated using missing-mass MMγK-.

n

γ

Θ+

K+

K-

n’

K+

MMγK- distribution at 1.4 ~ 1.8 GeV was studied.

22 )()( KnKPPPMM

22')(

MPP nK

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+

K+

K-

n

K+

Θ+ eventn

γ

K+

n

K-

Φ production event

φφ

The effect of Fermi motion is removed using equation below.

Two missing mass MMγK- and MMγK+K- have the sameFermi motion ⇒ Its effect is canceled.

Correction for the Fermi motion・ Nucleons in nuclei are affected by the Fermi motion.

n in 12C

γ

K+

K-

nΘ+

K+

affected by the Fermi motionaffected by the Fermi motion

NKKK

c

KMMMMMMMM

4. Result

A peak was observed.　 MassMass ：： 1.54 ± 0.01 GeV/c1.54 ± 0.01 GeV/c22

WidthWidth ：≦ ：≦ 25 MeV/c25 MeV/c22

　 Gaussian significanceGaussian significance ： ： 4.6σ4.6σ

The MMγK- spectrumc

solid histogram: events from plastic scintillatordotted histogram: background events from the LH2 normalized by a fit in the region above 1.59 GeV/c2

After the correction for the Fermi motion

5. Summary・ Five-quark states had been theoretically predicted.・ ΘΘ++ (uudds) (uudds) was predicted at the mass of 1530 MeV1530 MeV.・ The experiment was carried out at the LEPS of Spring-8.

Beam: High energy photon 1.5~2.4 GeVTarget: n in 12C

・ The events were selected with several cuts.・ The Fermi motion was corrected.・・ A sharp baryon resonance peak was found.A sharp baryon resonance peak was found.

Mass: 1.54±0.01 GeV/cMass: 1.54±0.01 GeV/c22

Width: ≦ 25 MeV/cWidth: ≦ 25 MeV/c22

Gaussian significance: 4.6σGaussian significance: 4.6σ

It’s a molecular meson-baryon resonance

or a penta-quark baryon Θ+.

--

NKKK

c

KMMMMMMM

22 )( K

realnK

PPPMM 2}){( fermi

nKn PPPP

)( ferminn

realn PPP

ferminKn PPPPM ・)(22

22 )( KK

realnKK

PPPPMM

ferminKKnKKPPMMMMMM

・

2)( 2222

Two missingmass have the same Fermi motion.⇒ The effect of Fermi motion is removed using equation below.

n

γ

Θ+

K+

K-

n

K+

2}){( ferminKKn PPPPP

ferminKKnn PPPPPM ・)(22

group reaction mass [MeV] width

LEPS γn → K+K-(n) 1540 ± 10 ＜ 25

DIANA K+Xe → K0pXe’ 1539 ± 2 ＜ 9

CLAS γd → K+K-p(n) 1542 ± 5 ＜ 21

SAPHIR γp → K+K0(n) 1540 ± 6 ＜ 25

ITEP νA → K0pX 1533 ± 5 ＜ 20

CLAS γp → π+K-K+(n) 1555 ± 10 ＜ 26

HERMES e+(27.6GeV) d → K0pX 1528 ± 3 13 ± 9

SVD p(70GeV) A → K0pX 1526 ± 3 ＜ 24

COSY pp → K0pΣ+ 1530 ± 5 ＜ 18

JINR p(10GeV) A → K0pX 1545 ± 12 16 ± 4

ZEUS e+p(cm300GeV) → e’K0pX 1522 ± 3 8 ± 4

Summary of the experiments of penta-quark