observation of a threshold enhancement in the p invariant mass spectrum
DESCRIPTION
Phys. Rev. Lett. 93 , 112002 (2004) Cited 28 times. Observation of a threshold enhancement in the p invariant mass spectrum. H.X. Yang ( BES Collaboration ) IHEP [email protected] Oct.28 - Nov.01, 2006, Guilin. Outline. Introduction Features of signal and event selection - PowerPoint PPT PresentationTRANSCRIPT
Observation of a threshold enhancement
in the p invariant mass spectrum
H.X. Yang(BES Collaboration)
Oct.28 - Nov.01, 2006, Guilin
Phys. Rev. Lett. 93, 112002 (2004) Cited 28 times
Outline
Introduction
Features of signal and event selection
Reliability of the result
Summary
IntroductionIntroduction
L ~ ~51030 /cm2s at J/ peak
Ecm~2-5 GeV
The Beijing Electron Positron Collider
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=1.78%(1+p2) z = 3.1 cm
BESII Detector
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
Motivation
The enhancement near the ppbar mass threshold observed by BESII in could be interpreted as a ppbar bound state (multi-quark meson).
The new observation of BESII as well as new resonances observed by other experiments makes the searches for multiquark states become a very hot topic of high energy physics field.
It is of special importance to search for possible baryon-antibaryon resonances or bound states.
,
,,, ppnppp
ppJ /
..ccpM ..ccpM
Data MC(Phase Space)
Data Data
pMpM
11
Strong enhancement near the threshold of ..ccpM
S-wave BW fit results
• M = (2075 +/- 12) MeV
Γ = (90 +/- 35) MeV
BR = (1.18+/- 0.28)*10**(-4)
• About 7σ statistical significance
Features of signal and event selection
Features of signal (I)
Signal means
• Λ signal (very strong constraint)
After selection, the clear Λ signal in data shows high purity of signal.
../ ccpKJ
Data MC
Features of signal (II)
• Four charged tracks, no photon, no pi0
• Three heavy particles: p, pbar, Kaon
• So kinematics (with correct mass assignment) provides very strong constraint in the event selection, and in the kinematic fit can reject most background events.
/JppK MM
0missP
Key points in event selection
Keep • High purity of signal ( Λ mass as a control)• Efficiency as high as possible• Systematic uncertainties from selection criteria
as small as possible
Low momentum feature should be considered
Event Selection
• Four good charged tracks ( Pxy>50 MeV ) • PID: Kaon-ID and at least one proton-ID• 4-C kinematic fit PID information is used in the mass assignment
• Remove events with combined information of Emiss and Mpk_miss
no requirement on Λ vertex reconstruction
..ccpK
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
Strong enhancement near the threshold of pm
Phase Space
Observed in both of and p
mpm
pM pM
pK Kp
S-wave BW fit results
•M = (2075 12 5) MeV
Γ = (90 35 9) MeV
BR = (1.18 0.28 0.4) 10-4
• 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 ~ 7σ statistical significance high L hypotheses fail
Reliability of the result
Extensive and careful checks!
Similar enhancement also observed in pK'
Fix the parameters, 4 away from phase space.
Is the enhancement from N* or Λ*?
• Hard to understand why N* or Λ* only contribute to very small phase space
pKM KM
Other checks
• Can the enhancement be interpreted as known PDG particles? NO
• MC Input/Output test:
check systematic uncertainties of fitting bias – is small.
• Other fits (L=1, 2, 3): P-wave fine, other failed
• 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
• 这一研究成果与新发现的质子-反质子奇异结构具有同样重要意义:有可能是一个多夸克态粒子(六夸克态)。
• 自然界已发现的由夸克组成的粒子中的夸克数均为两个或三个,因此发现(多于三个夸克组成的)多夸克态粒子对检验和发展量子色动力学 QCD 理论具有非常重要的意义。也将是新的物质存在形式。
• 两个 PRL Referee 均对该文章给予好评。(文章被直接接收)
• 被高能所作为重要科研成果对外宣传
重要科学意义
PRL Referee 给予好评• Referee A: …
由于 BES 等实验的多个新发现的共振峰,以及关于五夸克态粒子的争论,使得存在重子-反重子束缚态及多夸克态粒子的问题再次引起极大关注。…
• Referee B:
这篇文章研究了关于重子-反重子质量阈值共振结构的重要课题…。 BES 是第一个报道质子-反质子阈值增长结构的实验。而这篇文章观测到类似的 质量阈值增长结构,支持了前面的发现结果。…
p
Summary• We observed an enhancement at 2.08 GeV in the invariant
mass spectrum of
• Various checks show that the result is robust.
• It has been fitted with S-wave B-W function successfully:
mass = 2075+/-12+/-5 MeV
width=90+/35+/-9 MeV
Br=(1.18+/0.28+/-0.40)x10E-4
• It is very unlikely that the enhancement is from pure N* and Λ* interference, although they contribute to Dalitz plot.
• A possible multi-quark state candidate
p
THANK YOU!
• 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
• W/o any constrains, PWA with N* and Λ* can fit data:
(S= -997)
• However, it needs many unexpected big BRs and many large destructive interferences to cancel these big BRs.
• Keep in mind: Ndata=5433
Mathematical fit
Fraction of Ndata Nevent
N*(1900) 3/2+ 108% 5900
N*(2050) 3/2+ 33% 1800
Λ*(1890) 3/2+ 21% 1100
Λ*(1810) 1/2+ 9% 500
Λ*(1800) 1/2- 34% 1900
(1/2- is P-wave decay, which should be suppressed )
Big BRs
• Typically, BR(J/psibaryon anti-baryon) ~ 2x10-3
e.g., BR(J/psi p pbar) =2.2x10-3
• So, if there is no suppression, the typical number of observed events for one decay mode is:
BR(J/psi Λ Λ *)BR(Λ *pK) BR(Λpπ) ε N J/psi
= 2x10-3 x0.1 x0.1 x0.6 x5.8x107
~ 700 events
Physics constrains (I)
• Phase space ~ p
• Angular momentum barrier
~ (p / M J/psi) 2L+1
• If the width is large, some fraction of N* or Λ* cannot be produced.
• Global efficiency may be lower.
Physics constrains (II) – near threshold suppression
• E.g., Since Λ* decays both to pK and Σπ,
and most BRs of them are of the same order, so this channel can provide constrains on Λ*(1520), Λ*(1600), Λ*(1690) production rate.
• No clear evidence for strong production of Λ*(1800), Λ*(1810), Λ*(1890).
Near threshold suppression is a reasonable estimation.
Physics constrains (III) – from other channels
Nevent/2 (each decay mode, not include c.c.)
N*(1900) 3/2+ ~ 300 -400 N*(2050) 3/2+ ~ 100 -150 Λ*(1890) 3/2+ ~ 100 -150 Λ*(1810) 1/2+ ~ 150 -200 Λ*(1800) 1/2- ~ 20 - 30 (1/2- is P-wave decay, which should be suppressed near threshold)
Estimated Nevent in Ndata
• Constrain the Nevent of near threshold states ~ 100-200
• The PWA fit cannot reproduce the enhancement near pΛ threshold (S=-900).
PWA with constrains and w/o X(2075)
• Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold.
• Significance of X(2075) >> 5 sigma. (S=-952)
PWA with constrains and with X(2075)
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.
• Fit the similar enhancement in with the same
parameters of X(2075) as in J/psi.
• The significance of X(2075) in psi’ data is ~4.0 σ.
• The similar enhancements in the psi’ data and in the Belle data ( in BpΛπ process ) CANNOT be from N*(2050), Λ*(1890) … interference.
Fit the enhancement in pK'
pK'
Summary of PWA studies• Reproducing the pΛ threshold enhancement with pure N* and
Λ* interferences needs many unexpected big BRs and large destructive interferences.
• PWA fits with pure N* and Λ* and with constrains can hardly reproduce the enhancement.
• PWA fit with X(2075) can easily reproduce the enhancement (independent of constrains) with high significance.
• The similar enhancements in the psi’ data and in the Belle data ( in BpΛπ process ) CANNOT be from N*(2050), Λ*(1890) … interference.
Systematic uncertainties
被高能所作为重要科研成果对外宣传