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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