pentaquark search and other multiquark candidates at bes
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Pentaquark search and other multiquark candidates at BES
Xiaoyan SHEN
Representing BES Collaboration
Institute of High Energy Physics, CASshenxy@ihep.ac.cn
ICHEP 2004August 16 - 22, 2004, Beijing, P. R. China
Outline Introduction
Pentaquark searches at BES
Other multiquark candidates at BES
* pp threshold enhancement in
* p threshold enhancement in
* K threshold enhancement in
Summary
ΛpKJ/ψ ΛpKJ/ψ
pγpJ/ψ
0
10
20
30
40
50
60
CBAL MK II MK III DM2 BES I BES II
BESII Detector
VC: xy = 100 m TOF: T = 180 psMDC: xy = 220 m BSC: E/E= 21 % dE/dx= 8.5 % = 7.9 mr p/p=1.78(1+p2) z = 2.3 cm counter: r= 3 cm B field: 0.4 T z = 5.5 cm
Introduction
0
2
4
6
8
10
12
14
MKI MKII MKIII CBAL BESI BESII CLEOc
J/
(2S)
World J/ and (2S) Samples (×106) BESII
58M J/
BESII 14M (2S)
Pentaquark searches at BES
not forbidden by QCD:
definite evidence of pentaquark states would be an important addition to our understanding of QCD.
a baryon with S=+1 is a natural candidate
+(1540)
experimental status of +(1540)
(LEPS) nK
MΘ
0SpK
0SpKnK
“Positive” experiments: many
“Negative” experiments: many
BES: ALEPH: RΛ*(1520) < 0.1 DELPHI: L3: HERA-B: RΛ*(1520) < 0.02 SPHINX: RΛ*(1520) < 0.02 CDF:
Need more experimental facts (through different processes)
BES: e+ e- collision; has relatively clean data samples with less backgrounds
investigate the pentaquark state in the hadronic decays of charmonium
pKnK
nKpK
nKpKnKpKJS
s
s
ss
or
or
))(( & ))((/ & )2(
Multiquark candidates at BES
1. Near pp threshold enhancement in
enhancement
c
Phys. Rev. Lett., 91 (2003) 022001
ppJ /
Fit results
Mass: M=1859 MeV/c2
Width: < 30 MeV/c2 (90% CL)
J/pp
M(pp)-2mp (GeV)0 0.1 0.2 0.3
BG curve Eff. curve
2/dof=56/56
Fitted peak
Fitted curve +3 +510 25
BES II
MARK-III & DM2 Results
Threshold enhancement
pp/J
Claimed inPhys. Rep. 174(1989) 67-227
Too small statistics to draw any conclusion on the threshold enhancement,
e.g., cannot exclude known particles such as (1760)
MARK-III
DM2
This enhancement is important:
FSI not favored mass≤2mp , width is narrow Hard to be explained as a conventional qq meson
pp bound state (baryonium)?
+ n +
deuteron:
loosely bound 3-q 3-
q color singlets with Md = 2mp-
baryonium:
loosely bound
3-q 3-q color singlets with Mb = 2mp-
?
attractive nuclear force attractive force?
There is lots & lots of literature about this possibility
Observations of this structure in otherdecay modes are desirable.
2. Observation of an enhancement near mass threshold in process
p pKJ /
The clear Λ signal in data shows high purity of signal.
Data/MC
S-wave BW fit results
M = (2075 12 5) MeV
Γ = (90 35 9) MeV
BR = (5.9 1.4 2.0) 10-5
• M = (2044 17) MeV Γ = (20 45) MeV• 2/d.o.f = 32.5/26
P-wave BW fit results
The systematic errors arecarefully studied in S-wave case.
2/d.o.f = 31.1/26 About 7σ statistical significance high L hypotheses fail
Accepted by P. R. L.
Hep-ex/0405050
Near threshold enhancement is also observed inp pKS)2(
Fix the parameters, 4 away from phase space.
Accepted by P. R. L. Hep-ex/0405050
3. Near threshold enhancement in..ccK
../ ccpKJ
)(GeV/c2KΛM
Eve
nts
/10
MeV
Nx
Nx
Nx
)(GeV/c2 MMM KKΛ
PS, eff. corrected
(Arbitrary normalization)
PWA is performed to pKJ /
possible N* and *states listed in PDG are fitted N(1720), N(1900), (1520), (1690), …
many different combinations are tried
different form factors are used
different JP of Nx is tried
also tried N(1535) to fit Nx
Mass and Width scan
M 1520 – 1620 MeV
110 MeV
JP = 1/2-
Total fit (S=-952)
Nevent:
Fraction Nevent
NX 22% 1210
Mass scan(GeV/c2)
An example:
Width scan(GeV/c2)L
n
L
N(1720), N(1900), (1520), (1690) …. included in the PWA fit
JP check with various combinations
JP ½- ½+ 3/2- 3/2+ non A -940 -848 -848 -930 -813 B -845 -783 -806 -833 -752 C -952 -841 -844 -916 -768 D -880 -768 -752 -822 -650 E -957 -889 -893 -944 -875 F -970 -920 -925 -963 -919 G -954 -925 -919 -944 -909
fit results
Cases Mass(GeV) Width(MeV) Fraction(%) Nevent Log Likelyhood
a 1.52 ~ 1.62 110 22.3 1210 -940
b 1.56 80 ~110 44.4 2412 -845
c 1.62 70 14.7 799 -952
d 1.6 ~1.64 70 17.1 929 -880
e 1.57 90 20.6 1119 -957
f 1.62 70 ~ 90 19.9 1081 -970
g 1.58 80 15.6 845 -954
1.50 ~1.65 70 ~110 >14.7 >800
An abnormal enhancement is observed in the mass threshold of MK
Its mass and width:
(large uncertainty near threshold, high statistics is crucial!)Mass 1500~1650MeV
Width 70~110MeV
JP favors 1/2-
large BR(J/pNx)Br(NxK) (2*10-4). What is it??
Possibly N*(1535) with large coupling to K K molecular state ??
no (1540) seen in both J/ and (2S) hadronic decays.
a near pp threshold enhancement observed in J/pp decay. Baryonium??
a near p threshold enhancement observed in both
J/ pK and (2S) pK decays.
a near K threshold enhancement observed in
J/pK. large coupling to K. K molecular state??
Summary
FSI ? —— Not favored1. Theoretical calculation (Zou and Chiang, PRD69 034004 (2003)) sh
ows that the enhancement caused by one-pion-exchange FSI is too small to explain the BES structure.
2. For FSI, the BES structure should be the same as in elastic scattering (Bugg, hep-ph/0406293): But it is NOT !
BES
elasticelasticM ~|| 2
BES
one-pion-exchange FSI
pmppM 2)(
|M|2 |M|2
Both arbitrary normalization Both arbitrary normalization
The enhancement caused by Coulomb interaction is even smaller than one-pion-exchange FSI !
With threshold kinematic contributions removed, there are very smooth threshold enhancements in elastic “matrix element” and very small enhancement in annihilation “matrix element”:
much weaker than what BES observed !
NO strong threshold enhancement in collision (at LEAR)
pp
pp
pmppM 2)(
|M|2 |M|2BES BES
elasticelasticM ~|| 2 annlabann PM ~|| 2
Both arbitrary normalization Both arbitrary normalization
Any inconsistency? NO!
For example: with Mres = 1859 MeV, Γ = 30 MeV, J=0, BR(ppbar) ~ 10%, based on:
At Ecm = 2mp + 6 MeV ( i.e., pLab = 150 MeV ), in elastic process, the resonant cross section is only ~ 0.6 mb : much smaller than the continuum cross section
~ 94 +/- 20 mb
Very difficult to observe it in collisions.
4/)(4
)(4
)12)(12(
)12(22
2
22
2
21
rescm
outin
pcmres mE
BB
mE
c
SS
J
pp
PWA of the near-threshold enhancement(NX) in mK
PWA with: a: NX,N(1720),N(1900), (1520), (1570), (1690), (1810), X(2075) b: NX,N(1720),N(1900) , (1520), (1690), (1810), X(2075) c: NX,N(1720),N(1900), (1520), (1570), (1690), (1890),X(2075) d: NX,N(1720),N(1900), (1520), (1690), (1890),X(2075) e: NX,N(1720),N(1900), (1520), (1570), (1690), (1810), (1890),X(2075) f: NX,N(1720),N(1900),N(2050), (1520), (1570), (1690), (1810), (1890),X(2075) g: NX,N(1720),N(1900),N(2050), (1520), (1570), (1690), (1810), (1890)J(p) 1/2(-) 3/2(+) 3/2(+) 3/2(+) 3/2(-) 1/2(-) 3/2(-) 1/2(+) 3/2(+) 1(-)m(GeV)1.535,1.720, 1.900, 2.050, 1.5195, 1.570, 1.690, 1.810, 1.890, 2.080
(GeV) 0.150, 0.150 0.300, 0.200, 0.0156, 0.070, 0.060, 0.150, 0.100, 0.080 h:N(1535),N(1650),N(1720),N(1900), (1520), (1570), (1690), (1810), (1890),X(2075) i: N(1535),N(1650),N(1720),N(1900), (1520), (1690), (1810), (1890),X(2075)
J(p) 1/2(-) 1/2(-), mN(1650)=1.650, N(1650)=0.150 j: 18Res All possible N* and *states listed in PDG and N(1900)(3/2-),N(2050)(1/2+,3/2+), (1570), X(2
075)
N(1535 ) =NX
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