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 [email protected] Univ., December 6th, 2008. What are pentaquarks?. Baryon.
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New results on the Q+ at LEPS
will appear on arXiv:0812.1035
Takashi NAKANO
(RCNP, Osaka University)
New Hadron [email protected] Univ., December 6th, 2008.
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
M ~ 3mq + (strangeness)+(symmetry)
NambuGoldstone bosons of spontaneously brokenchiral symmetry.
M ~ 5mq + (strangeness) +(symmetry)
1700~1900 MeV for Q+
mu ~ md = 300 ~ 350 MeV, ms=mu(d)+130~180 MeV
Fall apart
Fallapart decay problemOrdinary baryons
Positive Parity?
For pentaquark
…would be a breakthrough in hadron physics.
SVTX
DC1
AC(n=1.03)
Target
Dipole Magnet
0.7Tesla
DC2
DC3
Start Counter
LEPS spectrometerCharged particle spectrometer with forward acceptance
PID from momentum and timeofflight measurements
Photons
detected
K
qLab < 20 degrees
K+
Eg=1.5~2.4 GeV
K+
K
γ
γ
Q+
L(1520)
n
p
n
p
p
n
p
n
Data was taken in 20022003.
spectator
Pmin
K
detected
tagged
Spectator
nucleon
K+
pCM
γ
vpn
d
at rest
 pCM
p n
Nucleon from
decay or scattering
We know
4 momentum of pn system
Mpn and ptot
pCMand vpn
Direction of pCM is assumed so that the spectator can have the minimum momentum for given pCM and vCM.
Inelastic and coherent events are removed.
M2(pK) vs M2(K+K) after fexclusion 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
DM vs. pmin
Fermimotion effect
corrected
Q+ MC
corrected with
DM’
uncorrected
Randomized Minimum Momentum Method
Mean and s of pmin depends on MM(g,K), but the dependence is week.
Statistical improvement with the RMM
Fit to a single RMM specrum (dashed line) and 3 RMM spectra (solid line).
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 (Dndf=2).
1. Fit M(nK) distribution to mass distributions generated by the RMM with MM(g,K) and randomized pmin.
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 Dndf=2
7.1s
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
Results of Q+ analysis after L(1520) exclusion
D(2lnL) =30.4 for Dndf=2
5.2s
Various BG models: minimum significance = 5.1s
The next step is...
The remaining thing to check is possible bias in the analysis.
3times statistics of LD2 data was collected from 20062007 with the same experimental setup.
(almost the same statistics for LH2 data)
Blind analysis will be carried out to check the Θ+ peak
New data
Previous data
Height=137.3±8.0
S/N =1.65±0.14
Height=47.5±4.6
S/N =1.71±0.22
Fitting was carried outwith fixed width(16MeV/c2)
Ratio of height = 2.89±0.32
LEPS
Good forward angle coverage
Poor wide angle coverage
Low energy
Symmetric acceptance for K+ and K
MKK>1.04 GeV/c2
Select quasifree process
CLAS
Poor forward angle coverage
Good wide angle coverage
Medium energy
Asymmetric acceptance
MKK > 1.07 GeV/c2
Require rescattering 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.
Test experiment with a new TPC and a new LH2 target was started in January, 2008.
大強度化：二連レーザー入射
長距離非回折ビーム
円形電子ビーム
~10 7 光子/秒（現LEPS ~10 6 ）
高エネルギー化：アンジュレータ
からの放射光Ｘ線の
反射再入射（東北大）
Eg < 7.5GeV(現LEPS < 3GeV)
逆コンプトン散乱
8 GeV electron
Recoil electron
(Tagging)
30m長直線部
（現LEPS 7.8m)
Laser or
反射Ｘ線
a) SPring8 SR ring
GeV gray
屋内
屋外
b) Laser hutch
5 m
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
高速データ収集システム
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
c) Experimental hutch
K＋n → Q+ → KS0p
for J = ½
CEX (K＋n→KS0p) ~7 mb
Inside 1 Tesla solenoid
+
TPC
Forward DCs
LD2 target
K+
~800 MeV/c
~420 MeV/c
proton
BeO degrader ~40 cm

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 JPARC.