Outline

1
New results on the Q at LEPS
will appear on arXiv0812.1035
Takashi NAKANO (RCNP, Osaka University)

• Outline
• Introduction
• Data analysis and results
• Summary and Prospects

New Hadron WS_at_Nagoya Univ., December 6th, 2008.
2
What are pentaquarks?

• Baryon.
• Minimum quark content is 5 quarks.
• Exotic penta-quarks are those where the
  antiquark has a different flavor than the other 4
  quarks
• Quantum numbers cannot be defined by 3 quarks
  alone.

Q uudds
Baryon number 1/3 1/3 1/3 1/3 1/3 1
Strangeness 0 0 0 0 1 1
e.g. uuddc, uussd
c.f. L(1405) uudsu or uds
3
Baryon masses in constituent quark model
mu md 300 350 MeV, msmu(d)130180 MeV

• Mainly 3 quark baryons
• M 3mq (strangeness)(symmetry)
• p, K, and h are light
• Nambu-Goldstone bosons of spontaneously broken
  chiral symmetry.
• 5-quark baryons, naively
• M 5mq (strangeness) (symmetry)
• 17001900 MeV for Q

4
Fall-apart decay problem

• DPP predicted the Q with M1530MeV, Glt15MeV,
  and Jp1/2.
• Naïve QM (and many Lattice calc.) gives
  M17001900MeV with Jp1/2-.
• But the negative parity state must have very wide
  width (1 GeV) due to fall apart decay.

Ordinary baryons
Positive Parity?

• Positive parity requires P-state excitation.
• Expect state to get heavier.
• Need counter mechanism.
• diquark-diquark, diquark-triquark, or strong
  interaction with pion cloud?

For pentaquark
5
What are the fundamental building blocks for Q

• (3 quarks) p(K) cloud?
• N p K bound state?
• di-quark di-quark anti-quark?
• 5-quark?
• ..

would be a breakthrough in hadron physics.
6
Experimental status

• Not seen in the most of the high energy
  experiments The production rate of Q/L(1520)
  is less than 1.
• No signal observation in CLAS gp, KEK-PS (p-,K-),
  (K,p) experiments.
• The width must be less than 1 MeV. (DIANA and
  KEK-B) reverse reaction of the Q decay Q ? n
  K
• LEPS could be inconsistent with CLAS gd
  experiment (CLAS-g10).

• Production rate depends on reaction mechanism.

• K coupling should be VERY small.

• K coupling should be small.

• Strong angle or energy dependence.

7
Slope for mesons
Slope for baryons
Slope for pentaquarks??
8
S. Nam et al. hep-ph/05005134
without K exchnge
dominant if possible
n
n
p
p
9
Super Photon ring-8 GeV SPring-8

• Third-generation synchrotron radiation facility
• Circumference 1436 m
• 8 GeV
• 100 mA
• 62 beamlines

10
LEPS beamline
in operation since 2000
g
11
LEPS spectrometer
Charged particle spectrometer with forward
acceptance PID from momentum and time-of-flight
measurements
Photons
12
Quasi-free production of Q and L(1520)
detected
K-
K
qLab lt 20 degrees
Eg1.52.4 GeV
K
K-
?
?
Q
L(1520)
n
p
n
p
p
n
p
n
Data was taken in 2002-2003.
spectator
Pmin

• Both reactions are quasi-free processes.
• The major BG is f productions.
• Fermi-motion should be corrected.
• Existence of a spectator nucleon characterize
  both reactions.

13
Possible minimum momentum of the spectator
K-
detected
tagged
Spectator nucleon
K
pCM
?
vpn
d
at rest
- pCM
p n
Nucleon from decay or scattering
We know
4 momentum of pn system
Mpn and ptot
pCM and vpn
Direction of pCM is assumed so that the spectator
can have the minimum momentum for given pCM and
vCM.
14
2-fold roles of pmin
quasi-free
coherent
inelastic
Clean-up
Estimation of pF
15
Missing masses before after pmin cut
Inelastic and coherent events are removed.
16
LEPS and CLAS f exclusion cut condition
CLAS
LEPS
17
Signal acceptance of f exclusion cut
MC
LEPS default
18
M2(pK-) vs M2(KK-)
L(1520)
f contribution
19
M2(pK-) vs M2(KK-) after f exclusion cut
L(1520)
L(1520) events are not concentrated near the cut
boundary.
20
What characterize the signal and background?
pmin for background events are almost determined
by Fermi motion (deuteron wave function).
21
Approximated M(NK) calculation
Fermi-motion effect
DM vs. pmin
corrected
Q MC
corrected with DM
uncorrected
22
Randomized Minimum Momentum Method
Mean and s of pmin depends on MM(g,K), but the
dependence is week.
23
Statistical improvement with the RMM
Fit to a single RMM specrum (dashed line) and 3
RMM spectra (solid line).
24
How to estimate the significance?
2. Fit M(nK) distribution to mass distributions
with signal contributions (L(1520) or Q)
represented by a Gaussian function with a fixed
width (s).
3. The significance is estimated from the
difference in log likelihood (-2lnL) with the
change in the number of degrees of freedom taken
into account (Dndf2).
1. Fit M(nK) distribution to mass distributions
generated by the RMM with MM(g,K) and randomized
pmin.
25
Results of L(1520) analysis
Structure with a width less than 30 MeV/c2
requires a physics process or fluctuation.
D(-2lnL) 55.1 for Dndf2
7.1s
26
Results of Q analysis
D(-2lnL) 31.1 for Dndf2
5.2s
27
M2(nK) vs. M2(pK-)
L(1520)
Q
a proton is a spectator for M(nK) a neutron is a
spectator for M(pK-)
We assume
28
Results of Q analysis after L(1520) exclusion
D(-2lnL) 30.4 for Dndf2
5.2s
Various BG models minimum significance 5.1s
29

• For the KK- mode, the analysis was improved
  recently by optimizing f exclusion cut and
  updating tagger reconstruction routine.
• The signal yield of ? p?K?(1520) ?KK-p
  increased 60.
• Solid method to estimate the background shape
  and signal significance is developed.
• The results will be published soon.

The next step is...
The remaining thing to check is possible bias in
the analysis. 3times statistics of LD2 data was
collected from 2006-2007 with the same
experimental setup. (almost the same statistics
for LH2 data) Blind analysis will be carried out
to check the T peak
30
?(1520) peak for LD2 data
New data
Previous data
Height137.38.0 S/N 1.650.14
Height47.54.6 S/N 1.710.22
Fitting was carried out with fixed
width(16MeV/c2) Ratio of height 2.890.32
31
Difference between LEPS and CLASfor gn ? K-Q
study
LEPS Good forward angle coverage Poor wide angle
coverage Low energy Symmetric acceptance for K
and K- MKKgt1.04 GeV/c2 Select quasi-free process
CLAS Poor forward angle coverage Good wide angle
coverage Medium energy Asymmetric acceptance MKK
gt 1.07 GeV/c2 Require re-scattering or large
Fermi momentum of a spectator

LEPS qLAB lt 20 degree t lt 0.6 GeV2 CLAS
qLAB gt 20 degree
Q might be a soft object.
32
Setup of TPC experiment
Test experiment with a new TPC and a new LH2
target was started in January, 2008.
33
Schematic view of the LEPS2 facility
???????????? ?????????
??????? 10 7 ??/?(?LEPS 10 6
) ?????????????? ??????X??
?????(???) Eg lt 7.5GeV(?LEPS lt 3GeV)
????????
8 GeV electron
Recoil electron (Tagging)
30m ???? (?LEPS 7.8m)
Laser or ??X?
a) SPring-8 SR ring
GeV g-ray
??
??
b) Laser hutch
5 m
4p??????(???) ????????????? ????????????? ????????
???
?????????????? ???E949????????
c) Experimental hutch
34
Q search experiment at J-PARC

• Reverse reaction of the Q decay using a low
  energy K beam gives an unambiguous answer.
• Kn ? Q ? KS0p
• Cross-section depends on only the spin and the
  decay width.

• for J ½

• 26.4 ? mb/MeV

CEX (Kn?KS0p) 7 mb
Inside 1 Tesla solenoid
?
TPC
Forward DCs
LD2 target
K
800 MeV/c
420 MeV/c
proton
BeO degrader 40 cm
?-
35
Prospects
1.Improved analysis with improved f cut was
finished. The positive results will be open soon
(arXiv0812.1035 ). 2.New data set with 3 times
more statistics has been already taken. 3. Blind
analysis will be carried out to check the peak
(in this year). 4. If the peak is confirmed, a
new experiment with a Time Projection Chamber has
been carried out since Jan 2008. ? wider angle
coverage and Q reconstruction in pKs decay
mode. 5. If the peak is confirmed, the study will
be expanded at LEPS2. We will also submit a
proposal to do a complete search for Q by using
a low energy K beam at J-PARC.