Title: Observation%20of%20a%20threshold%20enhancement%20in%20the%20p?%20invariant%20mass%20spectrum
1Observation of a threshold enhancement in the p?
invariant mass spectrum
- H.X. Yang
- (BES Collaboration)
- IHEP
- yanghx_at_mail.ihep.ac.cn
- Oct.28 - Nov.01, 2006, Guilin
2Outline
- Introduction
- Features of signal and event selection
- Reliability of the result
- Summary
3Introduction
4The Beijing Electron Positron Collider
L 5??1030 /cm2?s at J/? peak Ecm2-5 GeV
5BESII Detector
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/p1.78?(1p2) ?z 3.1 cm
6World J/? and ?(2S) Samples (106)
Largest from BES
J/?
?(2S)
2002
2001
7Motivation
- 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.
8Strong enhancement near the threshold of
Data
MC (Phase Space)
11
Data
Data
9S-wave BW fit results
- M (2075 /- 12) MeV
- G (90 /- 35) MeV
- BR (1.18/- 0.28)10(-4)
- About 7s statistical significance
10Features of signal and event selection
11Features of signal (I)
- Signal means
- ? signal (very strong constraint)
-
- After selection, the clear ? signal in data
shows high purity of signal.
Data
MC
12Features 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.
13Key points in event selection
- Keep
- High purity of signal ( ? mass as a control)
- Efficiency as high as possible
- Systematic uncertainties from selection criteria
as small as possible -
- Low momentum feature should be
considered
14Event Selection
- Four good charged tracks ( Pxygt50 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 ? vertex reconstruction
15High Purity of Signal after Selection
- It can be shown by the clean ? signal
- MC background study only 12
- Dominantly from
?
Data
Data/MC
16Strong enhancement near the threshold of
Phase Space
17Observed in both of and
18S-wave BW fit results
P-wave BW fit results
- M (2075 ?12 ? 5) MeV
- G (90 ? 35 ? 9) MeV
- BR (1.18 ? 0.28 ? 0.4) ?10-4
- M (2044 ?17) MeV
- G (20 ? 45) MeV
- ?2/d.o.f 32.5/26
- ?2/d.o.f 31.1/26
- 7s statistical significance
high L hypotheses fail
The systematic errors are carefully studied in
S-wave case.
19Reliability of the result
- Extensive and careful checks!
20 Similar enhancement also observed in
Fix the parameters, 4? away from phase space.
21Is the enhancement from N or ??
- Hard to understand why N or ? only contribute
to very small phase space
22Other 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 (L1, 2, 3) P-wave fine, other failed
23The distribution is consistent with
S-Wave
- MC (phase space) also show non-uniform and
asymmetric distribution of . - The enhancement is consistent with S wave.
Err Data His MC
24??????
- ?????????????-???????????????????????????(????)?
- ???????????????????????????,????(?????????)???????
???????????QCD ???????????????????????? - ?? PRL Referee ??????????(???????)
- ????????????????
25PRL Referee ????
- Referee A
-
- ?? BES ?????????????,?????????????,??????-?????
???????????????????? - Referee B
- ???????????-?????????????????BES
????????-????????????????????????
????????,???????????
26Summary
- 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
- width90/35/-9 MeV
- Br(1.18/0.28/-0.40)x10E-4
- It is very unlikely that the enhancement is from
pure N and ? interference, although they
contribute to Dalitz plot. - A possible multi-quark state candidate
27THANK YOU!
28Why Dalitz plot not uniform for
events
MC
DATA
- This is due to acceptance
- It can be shown in
- distribution, where is the decay
angle of p in
29Mathematical fit
- W/o any constrains, PWA with N and ? 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 Ndata5433
30Big BRs
- Fraction of Ndata
Nevent - N(1900) 3/2 108
5900 - N(2050) 3/2 33
1800 - ?(1890) 3/2 21
1100 - ?(1810) 1/2 9
500 - ?(1800) 1/2- 34
1900 - (1/2- is P-wave decay, which should be
suppressed )
31Physics constrains (I)
- Typically, BR(J/psi?baryon 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? ? ? )BR(? ?pK) BR(??pp) e N
J/psi - 2x10-3 x0.1 x0.1 x0.6 x5.8x107
- 700 events
32Physics constrains (II) near threshold
suppression
- Phase space p
- Angular momentum barrier
- (p / M J/psi) 2L1
- If the width is large, some fraction of N or ?
cannot be produced. - Global efficiency may be lower.
33Physics constrains (III) from other channels
- E.g.,
- Since ? decays both to pK and Sp, and most
BRs of them are of the same order, so this
channel can provide constrains on ?(1520),
?(1600), ?(1690) production rate. - No clear evidence for strong production of
?(1800), ?(1810), ?(1890). - Near threshold suppression is a reasonable
estimation.
34Estimated Nevent in Ndata
- Nevent/2 (each
decay mode, not include c.c.) - N(1900) 3/2 300 -400
- N(2050) 3/2 100 -150
- ?(1890) 3/2 100 -150
- ?(1810) 1/2 150 -200
- ?(1800) 1/2- 20 - 30
- (1/2- is P-wave decay, which should be
- suppressed near threshold)
35PWA with constrains and w/o X(2075)
- Constrain the Nevent of near threshold states
100-200 - The PWA fit cannot reproduce the enhancement near
p? threshold (S-900).
36PWA with constrains and with X(2075)
- Adding X(2075), PWA fit can reproduce the
enhancement near p? threshold. - Significance of X(2075) gtgt 5 sigma. (S-952)
37Interference of excited baryons?
- PWA fits with pure N and ? 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.
38Fit the enhancement in
- 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
s. - The similar enhancements in the psi data and in
the Belle data ( in B?p?p process ) CANNOT be
from N(2050), ?(1890) interference.
39Summary of PWA studies
- Reproducing the p? threshold enhancement with
pure N and ? interferences needs many
unexpected big BRs and large destructive
interferences. - PWA fits with pure N and ? 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 B?p?p process ) CANNOT be
from N(2050), ?(1890) interference.
40Systematic uncertainties
41????????????????