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New results on the Q+ at LEPS
will appear on arXiv:0812.1035
Takashi NAKANO
(RCNP, Osaka University)
Outline
• Introduction
• Data analysis and results
• Summary and Prospects
New Hadron WS@Nagoya Univ., December 6th, 2008.
What are pentaquarks?
Baryon.
Minimum quark content is 5 quarks. qqqqQ
“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
Baryon masses in constituent quark
model
mu ~ md = 300 ~ 350 MeV, ms=mu(d)+130~180 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)
1700~1900 MeV for Q+
Fall-apart decay problem
•DPP predicted the Q+ with M=1530MeV, G<15MeV, and Jp=1/2+.
•Naïve QM (and many Lattice calc.) gives M=1700~1900MeV with Jp=1/2-.
•But the negative parity state must have very wide width (~1 GeV) due to
“fall apart” decay.
Ordinary baryons
Positive Parity?
qq creation
•Positive parity requires P-
state excitation.
•Expect state to get heavier.
•Need counter mechanism.
For pentaquark
diquark-diquark, diquark-
triquark, or strong Fall apart
interaction with “pion”
cloud?
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.
Experimental status
•Not seen in the most of the high energy experiments: The
production rate of Q+/L(1520) is less than 1%.
•Production rate depends on reaction mechanism.
•No signal observation in CLAS gp, KEK-PS (p-,K-), (K+,p+)
experiments.
•K* coupling should be VERY small.
•The width must be less than 1 MeV. (DIANA and KEK-B)
reverse reaction of the Q+ decay: Q+ n K+
•K coupling should be small.
•LEPS could be inconsistent with CLAS gd experiment
(CLAS-g10).
•Strong angle or energy dependence.
Slope for mesons
Slope for baryons
Slope for pentaquarks??
S. Nam et al. hep-ph/05005134
dominant if possible without K*
exchnge
n
n
p p
Super Photon ring-8 GeV SPring-8
• Third-generation synchrotron radiation facility
• Circumference: 1436 m
• 8 GeV
• 100 mA
• 62 beamlines
LEPS beamline
in operation since 2000
g
LEPS spectrometer
Charged particle spectrometer with forward acceptance
PID from momentum and time-of-flight measurements
SVTX DC1 TOF
AC(n=1.03)
Photons
Target
Dipole Magnet
Start Counter 0.7 Tesla DC2 DC3
Quasi-free production of Q+ and L(1520)
detected
K- qLab < 20 degrees K+
Eg=1.5~2.4 GeV
K+ K-
γ γ
Q+ L1520
n p
p n n p
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.
Possible minimum momentum of the
spectator
K-
tagged detected Spectator
γ K+ nucleon pCM
vpn
d
at rest
pn - pCM
Nucleon from
We know 4 momentum of pn system decay or scattering
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.
2-fold roles of pmin
quasi-free
coherent
inelastic
Clean-up Estimation of pF
Missing masses before & after pmin cut
MM (g , K + K ) MM d (g , K + K )
Inelastic and coherent events are removed.
LEPS and CLAS f exclusion cut condition
CLAS
LEPS
Signal acceptance of f exclusion cut
LEPS
MC
default
M ( K + K ) 1.04 GeV/c 2
M2(pK-) vs M2(K+K-)
L(1520)
f contribution
M2(pK-) vs M2(K+K-) after f exclusion cut
L(1520)
L(1520) events are not concentrated near
the cut boundary.
What characterize the signal and
background?
pmin for background events are almost determined
by Fermi motion (deuteron wave function).
Approximated M(NK) calculation
M vs. pmin Fermi-motion effect
corrected
Q+ MC
corrected with
M’
uncorrected
M ( NK ) MM (g , K ) + M '( pmin )
MM (g , K ) only depends on Eg and p K .
Randomized Minimum Momentum Method
Mean and s of pmin depends on MM(g,K), but the
dependence is week.
Statistical improvement with the RMM
104 times
Fit to a single RMM specrum
(dashed line) and 3 RMM
spectra (solid line).
How to estimate the significance?
2. The significance is estimated from the difference in log
3. Fit M(nK) distribution to mass distributions with signal
1. Fit M(nK) distribution to mass distributions generated
contributions (L(1520)the Q+) represented by a of
likelihood (-2lnL) with or change in the number
by the RMM with MM(g,K) and randomized pmin.
Gaussian function with a fixed width (s).
degrees of freedom taken into account (ndf=2).
Results of L(1520) analysis
Structure with a width less
than 30 MeV/c2 requires a
physics process or fluctuation.
(-2lnL) =55.1 for ndf=2 7.1s
Prob(7.1s ) 1.2 1010
Results of Q+ analysis
(-2lnL) =31.1 for ndf=2 5.2s
Prob(5.2s ) 2 107
M2(nK+) vs. M2(pK-)
L(1520)
Q+
a proton is a spectator for M(nK+)
We assume
a neutron is a spectator for M(pK-)
Results of Q+ analysis after L(1520) exclusion
(-2lnL) =30.4 for ndf=2 5.2s
Various BG models: minimum significance = 5.1s
•For the K+K- mode, the analysis was improved recently by optimizing φ
exclusion cut and updating tagger reconstruction routine.
• The signal yield of γ p→K+Λ(1520) →K+K-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 Θ+ peak
Λ(1520) peak for LD2 data
New data Previous data
Height=137.3±8.0 Height=47.5±4.6
S/N =1.65±0.14 S/N =1.71±0.22
Fitting was carried out with fixed width(16MeV/c2)
Ratio of height = 2.89±0.32
Difference between LEPS and CLAS
for gn K-Q+ study
LEPS CLAS
Good forward angle coverage Poor forward angle coverage
Poor wide angle coverage Good wide angle coverage
Low energy Medium energy
Symmetric acceptance for K+ and K- Asymmetric acceptance
>1.04 GeV/c2
MKK~ MKK > 1.07 GeV/c2
Select quasi-free process Require re-scattering or large
Fermi momentum of a spectator
LEPS: qLAB < 20 degree |t| < 0.6 GeV2
CLAS: qLAB > 20 degree Q+ might be a soft object.
Setup of TPC experiment
Test experiment with a new TPC and a new LH2 target was
started in January, 2008.
Schematic view of the LEPS2 facility
大強度化：二連レーザー入射
逆コンプトン散乱
8 GeV electron 長距離非回折ビーム
円形電子ビーム
Recoil electron ~10 7 光子/秒（現LEPS ~10 6 ）
(Tagging) 高エネルギー化：アンジュレータ
からの放射光Ｘ線の
Laser or
反射再入射（東北大）
反射Ｘ線
Eg < 7.5GeV(現LEPS < 3GeV)
a) SPring-8 SR ring
GeV g-ray
屋内
屋外
b) Laser hutch
5m
4pガンマ検出器（東北大）
崩壊解析用スペクトロメータ
反応同定用スペクトロメータ
米国ブルックヘブン国立研究所
より、E949検出器を移設予定 高速データ収集システム
c) Experimental hutch
Q+ search experiment at J-PARC
Reverse reaction of the Q+ decay using a low
energy K+ beam gives an unambiguous answer.
K＋n → Q+ → KS0p
Cross-section depends on only the spin and the
decay width.
for J = ½
p G 2
s ( 2 J + 1) dE 26.4 G mb/MeV
8k 2 (E M ) + G / 4
2 2
CEX (K＋n→KS0p) ~7 mb
Inside 1 Tesla solenoid
p+
TPC
Forward DCs
LD2 target
K+
~800 MeV/c ~420 MeV/c
proton
BeO degrader ~40 cm
p-
Prospects
1.Improved analysis with improved f cut was finished. The positive
results will be open soon (arXiv:0812.1035 ).
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.
