1

Tomoaki Hotta (RCNP, Osaka Univ.)

for

The LEPS Collaboration

Cracow Epiphany Conference, Jan 6, 2005

• Introduction• LEPS experiment• Results from new LD2 data• Summary and outlook

LEPS Results on +

2

Theoretical Prediction of Baryon

M1890-180*Y] MeV

D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305

(Chiral Soliton Model)

• Exotic: S = +1– cannot be (qqq) state

• Low mass: 1530 MeV• Narrow width: ~ 15 MeV (30 MeV)*• J=1/2+

*The width corrected by R. Jaffe (hep-ph/0401187)

3

First evidence of from LEPS

n K KK n

1.540.01 GeV< 25 MeVGaussian significance 4.6Target: neutron in Carbon nucleus

Background level is estimated by a fit in a mass region above 1.59 GeV.

Assumption:• Background is from non-resonant K+K- production off the neutron/nucleus• … is nearly identical to non-resonant K+K- production off the proton

+

Phys.Rev.Lett. 91 (2003) 012002

hep-ex/0301020

4

Evidence for Pentaquark StatesSpring8 ELSA

JLab-p

HERMES

ITEP

pp ++.

COSY-TOFDIANA

SVD/IHEP

JLab-d

ZEUSCERN/NA49

H1

Nomad

a lot of evidence

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Questions:

• “Existence of the + ” is the most important issue

– some inconsistencies in the measured mass & width.

– negative results (mainly) from high energy experiments.

→ Can we see the peak again in the new LEPS data ?

• True mass, width• Spin & Parity

• Production mechanism, cross sections…

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• Data taken from Oct. 2002 to Jun. 2003.

• ~2×1012 photons on a 15cm-long LD2 target.

• Less Fermi motion effect.

• LH2 data were taken in the same period with ~ 1.4×1012 photons on the target.

# of photons: LH2:LD2 ≈ 2:3

we expect

# of events from protons: LH2:LD2≈ 2:3

# of events: LH2:LD2≈ 1:3

LEPS New LD2 and LH2 runs

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Super Photon ring-8 GeV SPring-8

• Third-generation synchrotron radiation facility

• Circumference: 1436 m

• 8 GeV

• 100 mA

• 62 beamlines

8

Laser Electron Photon facility at SPring-8

in operation since 2000

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LEPS detector

1m

10

Charged particle identification

Mass(GeV)

Mo

men

tum

(G

eV

)

K/ separation (positive charge)

K++

Mass/Charge (GeV)

Eve

nts

Reconstructed mass

d

p

K+

K-

+-

(mass) = 30 MeV(typ.) for 1 GeV/c Kaon

11

Reaction diagrams

pK

nKnp

nKpKpn

)1520(

)()1520()(

)()(

*

*

n

K─

K+

n+

p p

p

K+

K─

p

n n

N (1020) NK+K- N

S=+1

S=-1

“Exotic”

“Standard” baryon

Meson resonance

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background

N/

N

rati

o

Invariant mass (K+K-) (GeV)

Eve

nts

Invariant mass (K+K-) (GeV)

Real data

MC()

N : Real data – MC()N : MC()

ratio was almost energy independent.

13E dependent cut point : “N/N ratio × Relative acceptance = R”

exclusion cut

Invariant mass (K+K-) (GeV)

MM

( ,

K- )

(G

eV

)

1.8<E<2.0 GeV 2.0<E<2.2 GeV 2.2<E<2.4 GeV

Monte Carlo simulation (K+K-n 3 body phase space)

M=1.019

Expected signal region

“Rerative acceptance” = N(1.50<MM(,K-)<1.55)/N(all)

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Energy dependent exclusion cut

E(GeV)

R=0.01

R=0.05

R=0.20K

K in

v. M

ass

(GeV

)

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Cut dependence of K+ missing mass for the LH2 data

MM (GeV)

MM (GeV)

MM (GeV)

R=0.01 0.02 0.03

0.05 0.07 0.10

0.20 0.50 1.00

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Cut dependence of K+ missing mass for LD2 data

MM (GeV)

MM (GeV)

MM (GeV)

0.02 0.030.01

0.05 0.07 0.10

0.20 0.50 1.00

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Comparison of MM(,K+) for

the LH2 and LD2 data

MM (GeV) MM (GeV)

LH2:LD2 ratio of events is ~2:3

consistent with the expectation.

LH2LD2

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Comparison of MMd(,K+K-)

MM (GeV) MM (GeV)

Remove dKKd contributions by requiring MMd(,KK)>1.89 GeV

LH2 LD2

Mising mass for dKKX

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After removing deuterium elastic scattering

contributions

MM (GeV) MM (GeV)

Further require 0.89<MM(,KK)>0.99 GeV

S/N at (1520) peak is better than 1.

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Cut dependence of K+ missing mass for LD2 data after MMd cut

MM (GeV)

MM (GeV)

0.20

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Check if the cuts generate the “+” peak artificially by

analyzing KKN phase space MC

sample MC sampleLH2 data

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KKN phase-space MC data after applying the same

selection cuts.

MM (GeV)

MM

(G

eV)

MM (GeV)

No narrow peak in MM(,K-).

Symmetric between K+ and K-.

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MC data after the cuts.

MM (GeV) MM (GeV)

MM

(G

eV)

No narrow peak in MM(,K-).

contribution is estimated to be less than 15% of the final sample.

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MM(,K-) for the LH2 data

MM (GeV)

MM(GeV)

0.20

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+ search in MM(,K-) of the LD2 data

• Apply the same selection cuts.• See if the peak is reproduced.• See if the peak has reasonable

dependence on the exclusion cut variation.

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MM(,K-) for the LD2 data

0.20

MM(GeV)

MM(GeV)

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Summary of LD2 data analysis

MM (GeV)

• K+K- from LD2 target

• MMd(,K+K-)>1.89 GeV

• 0.89< MM Ｎ (,K+K-)<0.99 GeV

• exclusion cut at R=0.2

• Fermi motion correction

Reliable background estimation is essential to confirm the existence of the peak.Statistics of LH2 data is small.

increase statistics by mixed event technique

(K. Hicks, Ohio univ.)

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Mixed event analysis with

KKN phase space MC data

• Mix K+, K-, from different events.

• Apply the same selection cuts again on the mixed events.

• Check if the shape of the original distribution is reproduced by the mixed events.

MM (GeV)

Mixed event analysis seems to work fine for the exclusive reaction!

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Mixed event analysis

MM (GeV) MM (GeV)

• LH2 mixed events are obtained by removing L(1520) contributions.

• The mixed event spectra are compared with the LD2 missing mass spectra.

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After removing (1520)

MM (GeV)

• Background level around 1.53 GeV in 4 bins is ~220 events IF we take the mixed event BG method.

• The excess above the BG level is ~90 events.

•The peak position, width, significance strongly depends on the BG shape.

• The mixed event BG method may not work if the major BG is due to narrow resonances in K-p or K+K- channels.

•We need further BG study and it is in progress.

32

Summary and outlook

• Evidence for an S=+1 baryon (+) around 1.54 GeV with a narrow width has been observed.

• LEPS higher statistics experiment has re-observed the peak. – unlikely to be due to statistical fluctuations.

• Need to understand the “background” shape.• Further analysis in progress.

• Experiment with larger acceptance detector (TPC) is planned in near future.