Recent Spectroscopy Results from the H1 and ZEUS Experiments

1
Recent Spectroscopy Results from the
(H1 and) ZEUS Experiments
Achim Geiser, DESY Hamburg
for the ZEUS
collaboration XXXIXth
Rencontres de Moriond QCD and
Hadronic Interactions at High Energy
La Thuile, Italy, 29. March 04

• Selected main topic
• evidence/search for pentaquarks
• Introduction
• Evidence for a resonance consistent with q -gt
  K0p
• and its antiparticle (ZEUS)
• ( Evidence/search for a charmed pentaquark
  (H1/ZEUS)
• -gt see talk by K. Lipka tomorrow)

hep-ex/04040xx (soon)
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What are pentaquarks?

• QCD only colourless states can exist as free
  particles (confinement)
• conventional wisdom
• bound multiquark states (hadrons)
• qq (mesons)
• qqq (baryons)
• but other colourless combinations not
  forbidden
• qqqq (tetraquarks)
• qqqqq (pentaquarks) ...
• can they be bound tightly enough to have narrow
  width?

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Evidence for pentaquarks

• some QCD models yes!
• e.g. Diakonov et al. 1997 uudds q m1530
  MeV, G lt 15 MeV
• recently observed by several
• fixed target experiments
• q n K, LEPS, CLAS, SAPHIR
• (establishes exotic quantum numbers)
• m1540-5 MeV, width resolution
• q p K0s, DIANA, ITEP, HERMES,
• SVD, COSY-TOF
• (more precise measurements)
• m1527-3 MeV, width resolution
• evidence for uussd, ddssu by NA49
• (not seen by HERA-B)

fixed target gt pentaquark uses valence quarks
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Pentaquarks in high energy collisions?

• uudds q established, I0 no q0, q
  (?)
• what about uudds q- ? (this talk)
• (can only be produced in fragmentation)
• what about uuddc/uuddc qc ? -gt talk K. Lipka
  tomorrow
• (need high CMS energy)
• look in high energy collider experiments,
• e.g. deep inelastic ep scattering (DIS) at
  HERA

?s 300 (318) GeV ep CM energy before (after)
1998 W m(gp) gp CM energy Q2
-q2 photon virtuality, squared momentum
transfer Q2 gt 1 GeV2 DIS Q2 lt 1 GeV2
photoproduction
q2
p
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Search for q p K0s / q- p K0s in ZEUS

• inclusive DIS event sample Q2 gt 1 GeV2
• data 1996-2000, 121 pb-1
• K0 selection
• pT(K0) gt 0.3 GeV
• eliminate L and
• photon conversion
• candidates
• 866800 K0s

6
Intermezzo look for K0s K0s resonances

• several resonances
• observed
• produced in
• gluon-rich
• environment
• f0(1710)
• glueball
• candidate??

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Search for q p K0s / q- p K0s in ZEUS

• select protons by defining ionization band in
  dE/dx
• dE/dx gt 1.15 mips, p(p)lt1.5 GeV gt 60
  proton purity

p
p
K
K-
p
p
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Check look for L0(1520) p K c.c.

• well established narrow d-wave uds state, isospin
  0, charge 0
• isospin 1 partner is expected to have higher
  mass
• gt do NOT expect S or X in same place
• select p,K using dE/dx
• resonance seen
• in p K- c.c.,
• not seen in p K c.c.
• mass and natural width
• consistent with PDG
• M 1519.5 - 1.0 MeV
• G 15.6 - 1.0 MeV
• no evidence for
• hypothetical q(1530)
• (I1 pentaquark)

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Search for q p K0s / q- p K0s in ZEUS

• select p/p and K0s
• in inclusive DIS
• (Q2gt1 GeV2)
• plot m(pK0s) m(pK0s)
• structure emerges
• for increasing Q2
• or low W
• around m1525 MeV

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Search for q p K0s / q- p K0s in ZEUS

• fit bg single Gaussian
• peak at
• 1522-2 MeV
• width consistent
• with resolution
• statistical
• significance
• 3.9 s
• still structure left of
• peak?
• also try different
• background shape

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Search for q p K0s / q- p K0s in ZEUS

• fit bg two Gaussians
• (2nd Gaussian parametrizes potential
• resonance or empirical shoulder in
• background shape)
• m 1521.5 - 1.5 (stat)
• 2.8-1.7 (sys) MeV
• Gaussian width 6.1 - 1.5 MeV
• still compatible with
• experimental resolution of 2 MeV
• (Breit-Wigner fit G 8-4 MeV)
• significance 4.6 s
• signal seen in both charges (inset)
• if interpreted as q q-
• antipentaquark!

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Search for q p K0s / q- p K0s in ZEUS

• summary of
• fit results
• p(fluctuation)
• m 1500-1560 MeV
• s 1.5-12 MeV 6 x 10-5
• other systematic checks
• (not part of the default selection)
• remove proton candidates consistent with
  misidentified p from K -gt K0 p
• momentum imbalance require E(p) gt E(K0)
• gt peak significance increases further (including
  Q2gt1 GeV2),
• but complicated background shape -gt do not
  use
• check that reflections from Ds- -gt K0 K- or
  other charm resonances
• can not produce such a peak
• assign proton mass to pion enriched sample using
  dE/dx
• mix K0 and p from different events
• -gt no peaks

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Summary and Conclusions

• pentaquarks and other exotic multiquark states
  are allowed in principle in QCD, but are they
  narrow enough to be seen?
• recent evidence from fixed target
  experiments, suggested by some QCD models yes!
  (q uudds, m 1530 MeV)
• ZEUS finds evidence for a narrow resonance in p
  K0s and p K0s
• in DIS events at HERA, consistent with q
  q-
• m 1521.5 - 1.5(stat) 2.8-1.7(sys) MeV
• width still consistent with exp. resolution
  of 2 MeV (G 8-4 MeV)
• if pentaquark interpretation confirmed
• first observation of the q-
  state (antipentaquark)
• new era in spectroscopy this is just the
  beginning!