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Search for X - - Pentaquark in COMPASS
Presented by G.Brona (gbrona_at_cern.ch) On behalf
of the COMPASS collaboration

• Outline
• NA49 signal
• COMPASS experiment
• Reconstruction of ?0 and ?-
• Observation of ?(1530)0
• ?-- pentaquark search results
• Summary

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NA49 signal
C. Alt et al., Phys. Rev. Lett. 92, 042003 (2004)
NA49 (CERN)

• proton beam, liquid hydrogen target,
• peak 1860MeV, FWHMlt20MeV, S36, ?4.0

1862 2 MeV FWHM 17 MeV S 67.5 ? 5.6
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COMPASS Collaboration
COmmon Muon and Proton Apparatus for
Structure and Spectroscopy
Muon and hadron programmes

• Gluon polarization
• Inclusive DIS asymmetry
• Flavour separation
• Transversity
• Diffractive vector mesons production
• ? polarization
• Primakoff effect
• Hadron spectroscopy

250 physicists from 28 institutes, 11 countries
SPS (Super Proton Synchrotron) Beryllium
target ?/K beam 160 GeV/c polarized muon beam
The beam intensity is 2108 m per spill (4.8 s)
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COMPASS Detector
m
SAS (lt30 mrad, gt4GeV/c)
LAS (gt30 mrad, gt0.4GeV/c)
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X- - search
?
polarisationgt50
?
? -
?
p
? -
?0
? -
? -

• Good event
• incoming and outgoing muons identified
• a primary vertex inside geometrical limits of a
  6LiD target

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?0 and ?0 selection

• Topology
• Secondary vertex with 2 outgoing oppositely
  charged particles
• downstream from the primary one
• and located outside the target
• ?2 of a vertex lt 4

• Kinematical cuts
• Momentum of ?0 candidate lt 140 GeV/c
• cos?lt0.9, where ? is the angle between the ?0
  momentum
• vector in the LAB system and the momentum of
  the ? in
• the ?0 CMS

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?0 and ?0 signals
?0 1 245 800 2400 events
?0 638 000 2200 events
?0 (?0 ) in 7 MeV window around the peak are used
in the analysis
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?- and ? selection

• Topology
• All particles not used in ?0 reconstruction...
• ... and which are not muons...
• ... and do not point into primary vertex are
  assumed to be pions
• The closest distance of approach between ? and
  ?0 lt 0.8 cm
• ... and is between primary vertex and ?0 vertex

• Kinematical cuts
• Momentum of a pion candidate lt 140 GeV/c

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?- and ? signals
? - 17 930 210 events (1640 for NA49)
? 10 620 180 events (551 for NA49)
?- (?) in 10 MeV window around the peak are used
in the analysis
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Reconstruction of ?? mass spectra

• Topology
• All particles not used in ? - reconstruction...
• ... and which are not muons...
• ... and point into primary vertex are assumed
  to be pions

• Kinematical cuts
• Momentum of a pion candidate lt 140 GeV/c

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?(1530)0 and ?(1530)0 signals
?(1530)0 1090 90 events
?(1530)0 630 70 events
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?? mass spectra
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Results
Assuming


(COMPASS)
(NA49)
we would thus expect 400 ? - - pentaquarks in
COMPASS
Upper limit 70 events (99 CL)

• Negative results were also reported by
• WA89_at_340GeV/c
• HERA-B_at_920GeV/c
• ZEUS_at_27-920GeV/c
• BaBar_at_9-3.1GeV/c
• ...

Presently NA49 seems to be the only one seeing
this state
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Summary

• The analysis was triggered by NA49 observation
• (C. Alt et al., Phys. Rev. Lett. 92, 042003
  (2004))
• Analysis was performed on the 2002-2003 COMPASS
  data statistics (? -) 10 times larger than in
  NA49 experiment 400 ? -- events expected
• Signal larger than 70 events excluded (99 CL)
• COMPASS 2002-2003 data do not confirm the
  existence of ? -- pentaquark

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COMPASS Detector
m filter
Trackers Si,SciFi,Micromegas, GEM,DC,Straw,MWPC
Magnets
Polarised target
Trigger hodoscopes
HE Calorimeters
beam m
RICH
SAS (lt30 mrad, gt4GeV/c)
LAS (gt30 mrad, gt0.4GeV/c)
20022003 - 3109 interactions were recorded