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Evidence for a Narrow Exotic Anti-Charmed Baryon
State )
Jan Hladký, Institute of Physics AS CR Prague, H1
Collaboration

• ) published in Phys. Lett. B588 (2004) 17-28

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Outlook
Introduction
D and Dp analysis
Conclusions

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Motivation Pentaquark problem HERA is also
charm factory H1 detector has Si vertex
detectors D golden channel Charm baryons
searches
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Pentaquarks proposals
Gell-Mann 1964
Several model concepts
Lot of experiments
- several successfull ?
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Pentaquarks from meson and baryon multiplets
Mesons qq from SU(3)f
Baryons qqq from SU(3)f
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H1 experiment at HERA
27.6 GeV
820, 920 GeV
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D meson selection

• 1996 2000 Data
• Lint 75 pb-1
• 1 GeV2 lt Q2 lt 100 GeV2 0.05 lt y lt
  0.7
• pt (D) gt 1. 5 GeV
• -1. 5lt ?(D) lt 1.
• pt (K) pt (p) gt 2 GeV
• Elasticity z(D) gt 0.2

Good Signal/Background 3400 Ds to start with
D signal region subsequently used
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Proton selection

• dE/dx calibrated for 1996-2000 data
• parameterization accurate to 3-5
• 8 average resolution

Most probable dE/dx
Normalized likelihood based on measured dE/dx
expectations for p, K, p and resolution
L(p)L(K)L(p) 1
Final proton selection ( L(p)gt0.1 p(p)gt2 )
.or. L(p)gt0.3
dE/dx used for background suppression
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The D- p analysis

• Looking for a narrow state near threshold
• Expected 4-particle mass resolution about 35 MeV
  not favourable for a narrow state ? use mass
  difference technique m(Dp)-m(D)
• Cut on the normalized proton likelihood L(p) for
  pion suppression
• Take a D candidate add a track consistent with a
  proton and opposite charge of the D using mp
  for its mass

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Pentaquark in H1 setup
e 27,6 GeV
p
820/920 GeV
K p
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D-p cc in DIS for 1996 - 2000
M(Dp) m(Kppp)-m(Kpp)m(D)PDG
Background well described by D MC and wrong
charge D from data
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Signal in both D- p and in D p
M(Dp) m(Kppp)-m(Kpp)m(D)PDG
M(Dp)3.096 0.006 GeV
M(Dp)3.102 0.003 GeV
Signal of similar strength observed for both
charge combinations at compatible M (Dp)
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Signal region in D- p richer in D-
Dp Side bands


DM(D) GeV
M(Dp) GeV
Dp signal region
Normalization to the width of the windows in
M(Dp)
M(Dp) m(K??p)-m(K??)m(D)PDF
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Is the D-p signal due to protons?
M(Dp)3.104 0.003 GeV
Use this region with L(p)gt0.5
M(Dp) m(K??p)-m(K???)m(D)PDF
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Is physics different in signal region?
Fit slope with ??exp -?p(p)
DM(D) GeV
M(Dp) GeV
The momentum spectrum of the particles in the
signal region is harder than in the M(Dp) side
bands
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Lot of different tests have been made
signal is not affected !!!
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Significance estimation
NS Nb 95 Dp candidates (within 2s)
Nb 45.0?2.8 from background signal hypothesis
(fit)
5.4 s

• ? Significance estimate based on the background
  only hypothesis Nb 51.7?2.7
• Use of different background functions as well as
  the background model from data and MC
• Significance determined in a binning free method
• Background fluctuation probability 4 x 10-8
  (Poisson) ? 5.4 s (Gauss)
• Change in likelihood of fits 6.2 s

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CONCLUSIONS
H1 has observed a narrow resonance in both D-
p and D p M(Dp) 3099? 3 (stat.) ? 5
(syst.) MeV , Gaussian width ? 12 ? 3(stat.)
MeV. Significance is 5.4 sigma.
Background fluctuation probability is smaller
than 4?10-8. The signal is also observed in
an independent photoproduction sample. It is
interpreted as an anti-charmed baryon decaying
into Dp and its charge conjugate. The
minimal quark content is uudd . It is a
candidate for a charmed pentaquark state. Up
to now is not confirmed by other experiments.

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HAVE A GOOD TIME IN THE GOLDEN CITY
PRAGUE !
OF
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CONCLUSIONS