Observation%20of%20a%20threshold%20enhancement%20in%20the%20p?%20invariant%20mass%20spectrum

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Observation 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

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Outline

• Introduction
• Features of signal and event selection
• Reliability of the result
• Summary

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Introduction
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The Beijing Electron Positron Collider
L 5??1030 /cm2?s at J/? peak Ecm2-5 GeV
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BESII 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
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World J/? and ?(2S) Samples (106)
Largest from BES
J/?
?(2S)
2002
2001
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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.

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Strong enhancement near the threshold of
Data
MC (Phase Space)
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Data
Data
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S-wave BW fit results

• M (2075 /- 12) MeV
• G (90 /- 35) MeV
• BR (1.18/- 0.28)10(-4)
• About 7s statistical significance

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Features of signal and event selection
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Features of signal (I)

• Signal means
• ? signal (very strong constraint)
• After selection, the clear ? signal in data
  shows high purity of signal.

Data
MC
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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.

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Key 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

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Event 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

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High Purity of Signal after Selection

• It can be shown by the clean ? signal
• MC background study only 12
• Dominantly from

?
Data
Data/MC
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Strong enhancement near the threshold of
Phase Space
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Observed in both of and
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S-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.
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Reliability of the result

• Extensive and careful checks!

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Similar enhancement also observed in
Fix the parameters, 4? away from phase space.
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Is the enhancement from N or ??

• Hard to understand why N or ? only contribute
  to very small phase space

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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 (L1, 2, 3) P-wave fine, other failed

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The 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
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??????

• ?????????????-???????????????????????????(????)?
• ???????????????????????????,????(?????????)???????
  ???????????QCD ????????????????????????
• ?? PRL Referee ??????????(???????)
• ????????????????

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PRL Referee ????

• Referee A
• ?? BES ?????????????,?????????????,??????-?????
  ????????????????????
• Referee B
• ???????????-?????????????????BES
  ????????-????????????????????????
  ????????,???????????

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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
• 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

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THANK YOU!
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Why 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

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Mathematical 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

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Big 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 )

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Physics 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

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Physics 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.

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Physics 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.

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Estimated 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)

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PWA 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).

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PWA 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)

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Interference 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.

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Fit 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.
  

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Summary 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.

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Systematic uncertainties
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