observation of near-threshold enhancement at bes hongxun yang representing bes collaboration ihep...
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Observation of near-threshold enhancement at BES
HongXun YangRepresenting BES Collaboration
September 26- 30, 2004
Kppp ,,
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Outline
• Introduction
• threshold enhancement in
• threshold enhancement in
• threshold enhancement in
• Summary
pp
p
K
ppJ /
pKJ /
pKJ /
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Introduction
BESII Detector
Data Sample
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BESII
VC: xy = 100 m TOF: T = 180 ps counter: r= 3 cm MDC: xy = 220 m BSC: E/E= 22 % z = 5.5 cm dE/dx= 8.5 % = 7.9 mr B field: 0.4 T p/p=1.7%(1+p2) z = 2.3 cm
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World J/ and (2S) Samples (106)
J/ (2S)
0
10
20
30
40
50
60
MarkI I I DM2 BES I BES I I
0
2
4
6
8
10
12
14
MKI MKII MKIII CBAL BESI BESII
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Observation of threshold enhancement in
pp bound state (baryonium)?
ppJ /
Phys. Rev. Lett., 91 (2003) 022001
pp
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Near pp threshold enhancement in
enhancement
c
ppJ /
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Fit Result
M=1859 MeV/c2
< 30 MeV/c2 (90% CL)
J/pp
M(pp)-2mp (GeV)
0 0.1 0.2 0.3
3-body phase spaceacceptance
2/dof=56/56
fitted peak location
acceptance weighted BW +3 +5
10 25
BESII
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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
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• 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 dynamical threshold enhancement in cross sections (at LEAR)
pp
pp
pmppM 2)(
|M|2 |M|2BES BES
elasticelasticM ~|| 2 annlabann PM ~|| 2
Both arbitrary normalization Both arbitrary normalization
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Any inconsistency? NO!
• For example: with Mres = 1859 MeV, Γ = 30 MeV, J=0, BR(ppbar) ~ 10%, an estimation based on:
At Ecm = 2mp + 6 MeV ( i.e., pLab = 150 MeV ), in elastic process, the resonant cross section is ~ 0.6 mb : much smaller than the continuum cross section ~ 94 20 mb .
Difficult to observe it in cross sections.
4/)(4
)(4
)12)(12(
)12(22
2
22
2
21
rescm
outin
pcmres mE
BB
mE
c
SS
J
pp
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Why can it be seen in J/ decays, but not in cross sections?
• Reason is simple:
J/ decays do not suffer large t-channel “background”. It is an s-channel effect !
pp
>>
p p p p
p
p p p p
p
/J
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Final State Interaction ? —— Not favored
1. Theoretical calculation (Zou and Chiang, PRD69 034004 (2003)) shows: The enhancement caused by one-pion-exchange (OPE) FSI is too small to explain the BES structure.
2. The enhancement caused by Coulomb interaction is even smaller than one-pion-exchange FSI !
BES
one-pion-exchange FSI
|M|2 |M|2
Both arbitrary normalization
BES
pmppM 2)(
Both arbitrary normalization
Coulomb interaction
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Final State Interaction ? —— Not favored
Theoretical calculation might be unreliable, however, according to Watson’s theorem, we can use elastic scattering experiments to check the FSI effect, i.e.,if the BES structure were from FSI, it should be the same as in elastic scattering: But it is NOT ! FSI cannot explainthe BES structure.
/J
p
p
p p
p pelastic scattering
|M|2 BES
Both arbitrary normalization
elasticelasticM ~|| 2
pmppM 2)(
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Threshold Enhancements in J/ decays and B decays
• They may come from different mechanism:
There is “fragmentation” mechanism in B decays but NOT in J/ decays.
KppB
Belle
BES II
ppJ /
“Threshold” enhancement in B decays is much wider and is not really at threshold. It can be explained by fragmentation mechanism.
Threshold enhancement in J/ decays is obviously much more narrow and just at threshold, and it cannot be explained by fragmentation mechanism.
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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 other decay modes are desirable.
E. Fermi, C.N. Yang, Phys. Rev. 76, 1739 (1949)
…I.S. Sharpiro, Phys. Rept. 35, 129 (1978)C.B. Dover, M. Goldhaber, PRD 15, 1997 (1977)…A. Datta, P.J. O’Donnell, PLB 567, 273 (2003)]M.L. Yang et al., hep-ph/0405087
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Observation of threshold enhancement in
p
pKJ /
Phys. Rev. Lett. 93, 112002 (2004)
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High Purity of Signal after Selection
• It can be shown by the clean Λ signal
• MC background study: only 1~2%
Dominantly from
Data
..ccpK
Data/MC
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Strong enhancement near the threshold of pm
Phase Space
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Observed in both of and pmpm
pM pM
pK Kp
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S-wave BW fit results
• M = (2075 12 5) MeV
Γ = (90 35 9) MeV
BR = (5.9 1.4 2.0) 10-5
2/d.o.f = 31.1/26 ~ 7σstatistical significance
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Similar enhancement also observed in pK'
Fix the parameters, 4 away from phase space.
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• MC (phase space) also show non-uniform and asymmetric distribution of .
• The enhancement is consistent with S wave.
The distribution is consistent with S-Wave
pcos
)15.2( .. GeVM ccp pcos
Err: DataHis: MC
pcos
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Observation of threshold enhancement in
K
pKJ /
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)(GeV/c2KΛM
Eve
nts
/10
MeV
Nx
Nx
Nx
)(GeV/c2 MMM KKΛ
PS, eff. corrected
(Arbitrary normalization)
Near-threshold enhancement in MK
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We perform PWA studies on the KΛ mass threshold structure:
The most important we want to study is its production BR
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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
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• 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)
Ln
L
N(1720), N(1900), (1520), (1690) …. included in the PWA fit
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Dalitz plot (data)
Dalitz plot (PWA)
Eve
nts
/10M
eV Crosses: data
Hist.: PWA fit projection
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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
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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
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• A strong enhancement is observed near the mass threshold of MK at BES II.
• Preliminary PWA with various combinations of possible N* and Λ* in the fits —— The structure Nx*has:
Mass 1500~1650MeV
Width 70~110MeV
JP favors 1/2-
consistent with N*(1535)
The most important is:
It has large BR(J/ψ pNX*) BR(NX* KΛ) 2 X 10-4 ,
suggesting NX*has strong coupling to KΛ.
indicating it could be a KΛ molecular state
(5 - quark system).
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• A unique very narrow threshold enhancement is observed in decays at BES II:– It is not observed in elastic cross section
it cannot be explained by FSI.
– It is obviously different from the structure observed in B decays and it cannot be explained by fragmentation.
• We need to understand the nature of the strong anomalous threshold enhancements in J/ decays: multiquark states or other dynamical mechanism ? (keeping in mind that there are no strong threshold enhancements in many cases)
pp
ppppJ /
Kppp ,,
Summary
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THANK YOU!
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BES-I Resultpp/J
Threshold enhancement
But NOT claimed in Phys. Rev. Lett. 76(1996) 3502
Too small statistics to draw any conclusion on the threshold enhancement
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• 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
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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
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• This is due to acceptance• It can be shown in distribution, where is
the decay angle of p in
Why Dalitz plot not uniform for events GeVM ccp 15.2..
pcos
p
..ccp
MCDATA
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Interference of excited baryons?
• PWA fits with pure N* and Λ* can hardly reproduce the enhancement. (with reasonable constrains production rate for excited baryons)
• PWA fit with X(2075) can easily reproduce the enhancement with high significance. (independent of constrains)
It is unlikely that the enhancement is purely from * and N* interference.
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Systematic uncertainties
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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(2075)
N(1535 ) =NX
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Is the STRONG threshold enhancement universal in J/ decays ? —— NO !
• Actually in many other cases we do NOT see STRONG threshold enhancements !
• For example: In J/ decays at BES II
np J /
)( pM
)(KKMpmppM 2)(
KKJ /
)( pKM
pKJ /
0/ ppJ
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Conclusion
Mass(GeV) Width(MeV) Fraction(%) Nevent Likelyhood
a 1.52~1.62 110 22.3(39.5) 1210(2146) -940(-924)
b 1.56 80~110 44.4(48.5) 2412(2635) -845(-839)
c 1.62 70 14.7(26.0) 799(1413) -952(-932)
d 1.6~1.64 70 17.1(23.2) 929(1260) -880(-873)
e 1.57 90 20.6(31.3) 1119(1701) -957(-946)
f 1.62 70~90 19.9(19.3) 1081(1049) -970(-961)
g 1.58 80 15.6(16.6) 845(902) -954(-951)
h(free) 107.7 5851 -963
h(fixed) 51.5 2798 -945
i(free) 68.6 3727 -901
i(fixed) 39.8 2162 -898
j 72.0 3912 -1028
Total 1.5~1.65 70~110 >14.7 >800