Pentaquark%20states%20in%20High%20Energy%20ep%20Collisions

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Pentaquark states in High Energy ep Collisions
Tim Greenshaw (given by Stephen
Maxfield) University of Liverpool IoP HEPP Half
Day Meeting, Durham, October 27th 2004
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The pentaquark dead or alive?

• The strange pentaquark
• Prejudices and predictions.
• Strange pentaquark seen by some...
• ...but not by others
• The charmed pentaquark
• A close look at the positive evidence.
• The negative evidence.
• Some things that can go wrong when looking for
  pentaquarks.
• Further pentaquark results.
• What have we learned?

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Prejudices and predictions

• Particle data book in 1984

• In 1997, using chiral soliton model, Dyakanov,
  Petrov and Polyakov predicted existence of
  resonance at 1530 MeV with width lt 15
  MeV.
• Triggered new wave of experimental activity.

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The strange pentaquark first observation

• In the process gn ? nKK-, LEPS observed peak in
  m(nK) spectrum.
• m 1540 10 5 MeV.
• G lt 25 MeV at 90 CL.
• No. of events N 43.
• Significance 4.6 s (S/vB).
• Minimum quark content
• Mass and width consistent with chiral soliton
  model prediction.

• LEPS 2000...2001 data

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More observations

• CLAS (1999) gd ? npKK-
• m 1542 5 MeV.
• G lt 21 MeV.
• N 43, significance 5.2 s.

• CLAS, gp ? npKK-n
• m 1555 10 MeV.
• G lt 26 MeV.
• N 41, significance 7.8 s.

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More observations

• SAPHIR (1997...1998) gp ? nKKS0
• m 1540 4 2 MeV.
• G lt 21 MeV
• N 63 13, significance 4.8 s.

• DIANA (1986) KXe ? pKS0Xe
• m 1539 4 2 MeV
• G lt 9 MeV
• N 29, significance 4.4 s.
• pKS0 so not necessarily exotic state!

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More observations

• HERMES, ed ? eKS0pX
• m 1528 3 2 MeV.
• G 17 9 3 MeV.
• Significance
• 4...6 s (S/vB)
• 3...4 s (N/dN from fit).

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More observations

• ZEUS, ep ? eKS0pX

• G 8 4 MeV.
• N 221 48.
• Significance 3.9...4.6 s (fit).
• Peak seen in both

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More observations

• Compendium of neutrino interactions in neon and
  deuterium (Asratyan et al)
• m 1533 5 MeV
• G lt 20 MeV
• N 27, significance 6.7 s.

• SVD-2, pA ? pKS0X
• m 1526 3 3 MeV
• G lt 24 MeV
• N 50, significance 5.6 s.

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More observations

• COSY-ToF, pp ? SpKS0

• m 1530 5 MeV
• G lt 18 MeV
• N 120, significance 3.7...5.9 s.

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Strange pentaquark summary take one

• There is apparently overwhelming evidence for a
  Q state with mass about 1540 MeV.
• In units of s, significances given as 4.6, 5.2,
  7.8, 4.8, 4.4, 3...4, 3.9...4.6, 6.7, 5.6 and
  3.7...5.9.
• According to the PDG 2004 the width of this
  state is 1 MeV (based on re-analysis of DIANA
  data).

• The strange pentaquark seems to be alive and well!

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Negative searches for the strange pentaquark

• ALEPH, ee-
• CDF, proton antiproton

• L3, gg

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Negative searches for the strange pentaquark

• HERAb
• Belle

• BaBar

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Negative searches for the strange pentaquark

• BES
• BR lt 0.84 x 10-5 at 90 CL.

• BES
• BR lt 1.1 x 10-5 at 90 CL.

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Revisiting old data

• Dalitz plots for KN ? KNp studied in hydrogen
  and deuterium bubble chambers.
• No cuts to enhance possible Q signal.
• Dominant features due to decays K(892) ? Kp and
  D(1232) ? Np.
• Lines show expected positions of Q resonance.

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Strange pentaquark summary take two

• Many high statistics experiments fail to see the
  Q.
• Are experiments consistent?
• Look at R N(Q)/N(L(1520))
• Positive experiments
• SAPHIR, R 0.3
• HERMES, R 1.6...3.5
• ZEUS, R 0.2 (estimate!)
• SVD-2, R gt 0.2
• Negative experiments
• ALEPH, R lt 0.1
• BaBar, R lt 0.01
• Belle, R lt 0.02

• Is the pattern of the masses seen by the
  positive experiments peculiar?
• Maybe the strange pentaquark isnt so healthy
  after all?

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The charmed pentaquark

• If why not
• A few predictions
• m(Qc0) 2710 MeV (Jaffe, Wilczek,
  hep-ph/0307341).
• m(Qc0) 2704 MeV (Wu, Ma hep-ph/0402244).
• Such a Qc0 would be too light to decay to D
  mesons, but could decay weakly to Tsp-.
• m(Qc0) 2985 50 MeV, G(Qc0) 21MeV,
  Karliner, Lipkin (hep-ph/0307343).
• m(Qc0) 2938...2997 MeV, (Cheung,
  hep-ph/0308176).

• Such a Qc0 could decay to D-p.
• If m(Qc0) gt m(D) m(p) 2948 MeV, Qc0 can
  decay to Dp.
• This decay mode can be dominant, (Karliner,
  Lipkin, hep-ph/0401072).

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Charmed pentaquark search experimental
considerations

• D pseudoscalar meson,without... and with
  lifetime tag
• Have either huge background or low yield.

• D much easier to find due to low Q value of
  decay D ? D0pS.
• Look at mass difference Dm m(K-ppS)
  m(K-p)in chain D ? D0pS ? K-ppS

Background described by wrong charge D0 sample,
Dm m(K-p-pS) m(K-p-).
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Charmed pentaquark observation

• Combine D with particles that have reasonable
  likelihood of being anti-protons from dE/dx
  measurements

• Resulting mass spectrum

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Charmed pentaquark observation

• Significance assessment
• NS NB 95 (within 2s of peak).
• NB 51.7 (Bg. only fit).
• Prob. signal produced by fluctuation (Poisson
  statistics) is 4x10-8.
• Equivalent to 5.4s.

• Signal also observed in independent
  photoproduction sample
• Summary take one charmed pentaquark looks
  healthy!

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Negative searches for the charmed pentaquark

• CDF, proton antiproton

• ZEUS, ep

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Negative searches for the charmed pentaquark

• ALEPH, ee-

• Are experiments consistent?
• Look at R N(Qc)/N(D)
• Positive experiment
• H1, R 0.01
• Negative experiment
• ZEUS, R lt 0.0035 (not same phase space as H1
  result?)
• Summary take two the charmed pentaquark also
  looks sick.

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Some things that can go wrong and the charmed
pentaquark statistics and fake peaks

• How significant is a 5s signal?
• Generate 40 random histograms with 600 events
  each from the parent distribution
• Three of these histograms shown on right...

• ...together with the CLAS Q signal, S/vB 5.2
  s.
• But in case of Qc, BG seems to be well described
  and shows no evidence of hump.

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More observations

• JINR, np ? npKK.

• Profusion of states seen in m(nK) spectrum

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Reflections

• E.g. of reflection
• Momentum of decay products in D1 rest frame
• Reconstruct mass misidentifying p as p, i.e.
  using expression
• Result is mass that is too large
• mrec 3.04 GeV.
• mD1 2.42 GeV.
• Boost to Lab. smears rec. mass.
• Max. and min. rec. mass given by extremes of
  cosq.

• Mass independent of q only for correct particle
  assignments

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Reflections

• Integrating over cosq

• Expected band seen in m(Dp) at const. m(Dp), no
  evidence for band in m(Dp) at const. m(Dp) as
  would be case if p assignment incorrect

D2 ? Dp
D1 ? Dp
m(qc)
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Reflections

• Estimate expected contribution of D1 and D2
  reflections from data.

• Now label p as p and recalculate mass.
• Hence obtain N(D1) N(D2) in signal region.
• Compatible with MC expectation 3.5

Loose D cutsand p selection
D cuts as for Qcand p selection
D cuts as for Qcand p selection
M(Dp) m(Kppp) - m(Kpp) m(D)PDG
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Split tracks and the Qc

• A single track may be found twice, e.g. due to
  multiple scattering in a tracking chamber that
  causes kink in track not recognised by pattern
  recognition software.
• Perhaps seen opposite in candidate D event?
• If take K from D decay and re-use track,
  identifying it as p second time round, obtain
  m(Dp) 3.1 GeV.

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Split tracks and the Qc

• Monte Carlo of D ? Kpp events where K split and
  identified as p.
• Resulting m(Dp) spectrum in DIS

• Mass peak is at m(Dp) 3.1 GeV, but broader
  than observations.
• However, after applying selection cuts...

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Split tracks and the Qc

• No evidence for such effects seen in H1 data
  scan all events in signal region.

• Check difference of transverse momenta of K and p
  tracks does not peak at zero.

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Split tracks and the Q

• Monte Carlo of K0 ? pp- events where p split
  and identified as p second time round.
• Resulting m(K0p) spectrum in DIS

• Mass peak is at m(K0p) 1.54 GeV, but broader
  than observations.
• After applying H1 selection cuts still broader
  than Q observations?

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More observations

• NA49, pp collisions
• Evidence for X- -(1862)
• m 1862 2 MeV.
• G lt 18 MeV.
• N 69, significance 5.8 s.
• Also evidence for X0(1862).

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More contradictions

• ALEPH, ee-, no evidence for X(1862) states

• No evidence for X(1862) states at CDF, proton
  anti-proton
• Also null result from HERAb and BaBar (see
  earlier).

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Coincidences and creativity

• Chiral Soliton Model approx. to QCD in limit NC ?
  8 for NC 3 many exotic states, discarded as
  artefacts.
• Couplings used in Dyakanov, Petrov and Polyakov
  prediction out of date, error of 200 MeV in
  m(X- -).
• With modern values, difficult to get G(Q) lt 10
  MeV.

• The observation of the Q has resulted in new
  ideas in spectroscopy.
• Diquark-triquark picture (Karliner Lipkin)

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Coincidences and creativity

• Diquark-diquark-antiquark picture (Jaffe and
  Wilczek)

• Tetrahedron picture (Yu-xin Liu, Jing-sheng Li,
  Cheng-guang Bao)

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Coincidences and creativity

• Lattice should tell us what QCD is really doing
  in this non-perturbative region, but so far...

PQ exists? Parity of lowest state? Anti-charmed PQ exists? Operator
Csikor et al Yes Negative Dont know Colour variant of KN
Sasaki Yes Negative No Diquark-diquark-antiquark
Kentucky No Not positive Dont know Simple KN
Chiu-Hsieh Yes Positive Yes Diquark-diquark-antiquark
MIT Yes Negative Dont know Diquark-diquark-antiquark
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What have we learned are pentaquarks dead
or alive?

• Whether pentaquarks are dead or alive, there is
  much within QCD that remains to be understood!
• Where experiments are poorly understood it is
  hard to do good theory and where theory is poorly
  understood it is hard to do good experiments.
• The evidence for the existence of pentaquarks is
  conflicting.
• If they exist, it would appear that the
  production mechanism is exotic experiments must
  measure cross-sections and identify kinematic
  regions in which pentaquarks are observed
  further analysis and data needed.

• If pentaquarks are real, explaining their width
  is difficult.
• Pentaquarks dead or alive?
• The box has yet to be properly opened!