Twohadron asymmetries

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Two-hadron asymmetries _at_

• Adam Mielech
• INFN Trieste
• on behalf of COMPASS collaboration

7-10th. September 2005
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Study of single spin asymmetries for 2hadron
production in SIDIS

• is a way to probe transverse polarization
  distributions of the nucleon coupled to
  interference fragmentation functions
• so, it is complementary to the 1hadron case,
  where transversity is probed by
  the Collins effect.

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Definitions
Angles defined in Breit system
azimuthal angle of the spin of the fragmenting
and initial quark
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Asymmetry
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• 200 physicists from 12 countries
• longitudinally polarised muon beam
• longitudinally or transversely polarised
  deuteron (6LiD) target
• momentum and calorimetry measurement
• particle identification

Localisation

LHC
SPS
N
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Collected statistics with transverse target
polarization

• 2002
• 127 days of data taking, 1.6106 SI DIS events
• 2003
• 14 days of data taking, 42002
• 2004
• 14 days of data taking, 2003 data doubled

Today data 20022003 are presented
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Event selection

• Selection of events
• Q2 gt1 (GeV/c)2
• 0.9gtygt0.1
• At least one /- hadron pair

1
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Hadron selection

• Selection of hadrons
• Tracks which survived muon rejection criteria
• zh1,h2gt0.1 xfh1,h2gt0.1 (current fragmentation
  region)
• (zh1 zh2)lt0.9 (remove exclusive mesons)

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Additional cut

• RTgt0.05 GeV/c

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Collected statistics multiplicities
h-
h
Final sample of hadron pairs after cuts -
20022003 2.8 M - 0.22 pair / DIS event
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Asymmetry
... for the process
assumming hp
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Asymmetry
Spectator model for FF
Radici, Jakob, Bianconi, PRD 65, 074031
Jaffe, Jin, Tang Phys.Rev. Lett. 80,1166(1998)
d0 d1 strong interaction pp phase shift (data)
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Asymmetry
Spectator model prediction
Radici, Jakob, Bianconi, PRD 65, 074031
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Asymmetry
Radici, Jakob, Bianconi, PRD 65, 074031
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Systematic checks

• In order to estimate systematics effects false
  asymmetries were investigated so far
• asymmetry calculated for the unpolarized sample
  created by scrambling the data of the real
  (polarized) sub periods
• asymmetry calculated for slices of one target
  cell
• Width of fRS bins were changed

Results shows that false asymmetries, are not
larger than the statistical uncertenties of
measured asymmetries.
Results remains stable.
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Nearest future plans

• Finalize analysis of 2004 data and systematic
  studies
• Look at the other ways to define oppositely
  charged hadron combinations e.g. h1 -gt
  leading hadron or, h2 -gt second leading
  hadron leading in terms of pT, z, ...
• Use of RICH to extract asymmetries for
  identified hadron pairs (so far we assuming
  that all hadrons are pions)

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Compass RICH

• Fully operated for transverse data since 2003
• Separation between p, K and p in the momentum
  range 3-50 GeV/c

beam
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RICH response
Hadron masses calculated from the measured
Cherenkov angle
p
K
p

• Threshold momenta
• pp2 GeV/c
• pK9 GeV/c
• pP17 GeV/c

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Fractions of hadrons recognizedby RICH
For single leading (z) hadrons, with additional
cut z gt 0.25
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2hadron case using RICH
Pairs of hadrons to be analyzed pp, pK, KK
K0
r0
K
f
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2hadron case using RICH
counts
counts
counts
Identification for standard cuts (zh1gt0.1
zh2gt0.1)
counts
Cleaner identification for more energetic
particles (zh1gt0.25 zh2gt0.25)
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Summary and outlook

• 2hadron data collected in years 2002 and 2003
  with transversely polarized deuteron target were
  analyzed
• All measured asymmetries are small and compatible
  with 0.
• Performed checks indicate that systematic effects
  are not larger than real asymmetries and their
  statistical uncertainties.
• RICH studies were done in order to extract
  asymmetries for identified hadrons and hadron
  pairs pp, pK and KK in near future.
• Different 2hadron combination choice methods are
  investigated, some results are expected in
  nearest future.
• Data collected in year 2004 are analyzed, results
  expected soon.