PowerPoint-Pr

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6th International Workshop on Hyperons, Charm and
Beauty Hadrons Chicago, June 27-July 3, 2004
Searches for Exotic Baryons at HERA
Karin Daum Wuppertal
on behalf of the H1, HERMES and ZEUS
collaborations

• Outline
• HERA kinematics at HERA
• Strange Pentaquarks
• q (Hermes, Zeus) X-- (Zeus)
• Exotic Anticharmed Baryon (H1, Zeus)
• Conclusions

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The Hera accelerator
DESY Hamburg Germany
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Hera kinematics in ep collisions
vs 300-318 GeV (energy c.m.) Wm(gp) (hadronic
energy) Q2-q2 (photon virtuality)
?
2 kinematic regimes Q ? 0 GeV
Photoproduction Q gt 1 GeV Electroproduction
(DIS)
e
p
?s
K
Dp event in DIS
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Fixed target kinematics at Hera
²H
?
vs 10 GeV (energy c.m.) Wm(gp) (hadronic
energy) Q2-q2 (photon virtuality)
²H
2 kinematic regimes Q ? 0 GeV
Photoproduction Q gt 1 GeV Electroproduction
(DIS)
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The ? search at HERMES

Aerogel
Kinematic Range 0.02 lt x lt 0.8 at Q2 gt 1 GeV2
and W gt 2 GeV
Very good proton ID
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The ? search at HERMES

e ²H ? ? X ? K0s p X

• Hadron identification using RICH
• Suppress contamination from ?? p?
• Define appropriate event topology

Unambiguous invariant mass reconstruction due to
PID
Peak at 1528?2.6 MeV ?8?2 MeV 3.7 ?
Peak at 1527?2.3 MeV ?9.2?2 MeV 4.3 ?
78?18 events
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The ? search in ZEUS

e p ? ? X ? K0s p X

• DIS (Q2gt1 GeV2)96-00 data 121 pb-1
• Identify K0s
• Remove ?? p? and converted ?s
• Limited particle identification via
• dE/dx
• select proton candidates
• p(p)lt1.5 GeV, dE/dxgt1.15 mips

• DIS (Q2gt1 GeV2)96-00 data 121 pb-1
• Identify K0s ,
• Remove ?? p? and converted ?s

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The ? search in ZEUS

Clear signal visible only at Q²gt 20 GeV²
MC normalised to background, No ?-bumps included
Peak at 1521.5?1.5 MeV ?6.1?1.5 MeV 4.6 ?
221 ?48 events
Fit a 2nd Gaussian to account for some of the
differences
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Is there a Q at HERA
HERMES
ZEUS

• Clear Signal for L(1520) ? pK-

• No Signal for Q ? pK Q isoscalar

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The ? search in eN collisions at HERA

Experiment Kinematics K0s p/ K0s
Factor 35 !
Effect of phase space/acceptance ?
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Search for ? at ZEUS
--
All charge combinations
preliminary
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Evidence for anti-charmed baryon by H1
If a strange pentaquark Why not a charmed one also
96-00 data 75 pb-1 DIS Q²gt1 GeV²
Golden channel D? D0 ? s ?(K?)?s
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Particle identification

• Reasonable PID via dE/dx
• all data calibrated
• parameterisation
• accurate to 3-5
• MIP resolution 8

Most probable dE/dx
Use dE/dx for background suppression
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Opposite sign Dp mass distribution
Apply mass difference technique M(Dp)m(K??
p)-m(K??)MPDG(D)
Background well described by D MC and wrong
charge D from data
narrow resonance at M3099? 3(stat.) ? 5 (syst.)
MeV
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Typical Dp candidates
HERA-I
All events scanned No anomalies observed e.g.
split tracks, wrong reconstruction

p
s
-
K
p
0
D

p
D
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Does the resonance come from Ds?
Look at the correlation of ?M(D) vs. M(Dp)
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Physics changes on-resonance ?
Look at momentum distribution of proton
candidates w/o dE/dx
No dE/dx cuts !
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Dp in photo-production

• total 4900 D to start
• Dp peak at the same mass in ?p
• no enhancement in non-charm bg
• 95 bg due to non-charm

Background well described by wrong charge D from
data
Photo-production more difficult due to large
non-charm background
but
independent confirmation of the signal
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Significance estimate
5.4 s
Background only hypothesis Nb51.72.7
Background fluctuation probability 4x10-8
(Poisson)5.4 s (Gauss)
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Search for charmed PQ, qc? Dp, in ZEUS
Selection differs from H1 analysis
D.I.S.
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Search for charmed PQ, qc? Dp, in ZEUS
Photoproduction DIS
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Conclusions

• Positive evidence for the production of q at
  HERA
• No doubly charged partner ? q is an isoscalar
• large differences in the q/ K0s yields from
  HERMES
• and ZEUS
• Negative results on the search for X-- by ZEUS
• Evidence for a neutral anti-charmed baryon state
• decaying to Dp from H1
• - not confirmed by ZEUS

prelim.
prelim.
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Backup Slides
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Like-sign Dp mass distribution
Background well described by D MC and wrong
charge D from data
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Does the resonance come from protons ?
ltL(p)gt 0.92
Use this region with L(p)gt0.5
Signal is there for well identified protons
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Is the physics different in the signal region?

• ? Single particle momentum spectra
• are steeply falling
• This feature is preserved in the
• combinatorial background of
• invariant mass analyses

• In decays particles are also
• emitted in the direction of flight
• ? Particles from a decay should
• have a harder spectrum than the
• combinatorial background

H1
Check the proton momentum !
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Is the physics different in the 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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Basics of kinematic tests
2-Body Decay
Dp rest frame
wrong mass assignment
CPQ MC
independent of decay angle cos? only for
correct mass assignment
correct mass assignment
M(Dp) GeV
CPQ MC
CPQ MC
M(Dp) GeV
M(Dp) GeV
M(Dp) GeV
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Basics of kinematic tests
2-Body Decay
Dp rest frame
wrong mass assignment
CPQ MC
Integrated in cos?
correct mass assignment
M(Dp) GeV
CPQ MC
CPQ MC
M(Dp) GeV
Do we see a band like structure in the
M(Dp)-M(Dx) plane in data? ? Lets have a look
M(Dp) GeV
M(Dp) GeV
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Signal due to D? ?
NO!
Go to the Dp signal region
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Lots of further kinematic test

• Reflections from a possible signal in DK mass
  distribution ruled out
• Possible contributions from D ?D ? with
  ?-conversion ruled out
• Possible contributions from D /D ?D K
  ruled out
• Possible peak structures in all possible mass
  correlations with all possible mass hypotheses of
  the particles making the D and the Dp system to
  search for real or fake resonances, e.g ?, ? , ?
  , K ,?, f no enhancements found
• Possible peak structures in all possible mass
  correlations among the proton candidate the
  remaining charged particles of the event with all
  possible mass assignments to search for real or
  fake peaks, no enhancements found

0
0
0
S1 S2
0
0 S
2
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Remarks on Dp searches at LEP
D signals for different xE
Rb ?22, Rc ?17
D _at_ LEP are produced predominantly by beauty
D acceptance vs. xE?
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Remarks on Dp searches at LEP
Rb ?22, Rc ?17
D _at_ LEP are produced predominantly by beauty
ltxEgtcc ?0.488
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Remarks on Dp searches at LEP
beauty
charm
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D from Dp and direct D at HERA
MC
MC
ltxEgtcc ?0.56
ltxEgtcc ?0.43
Ds from Dp significantly softer than normal
Ds
Should also hold for LEP !