Collins and Sivers asymmetries on the deuteron from COMPASS

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Collins and Sivers asymmetries on the deuteron
from COMPASS
PANIC05 Particles and Nuclei International
Conference Santa Fe, NM - October 24-28, 2005

• Andrea Bressan
• (University and I.N.F.N. Trieste)
• on behalf of the COMPASS Collaboration

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LHC

COMPASS
SPS

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COMPASS data sample

• In so far ( 3 years )
• only DIS off 6LiD
• 20 of the time with transverse target
  polarization
• only runs with transverse polarized target being
  analyzed

runs good runs used events in the analysis
2002 475 453 (100 SPS spills) 1.6 106
2003 479 429 (100 SPS spills) 4(2002)
2004 496 470 (200 SPS spills) 2(2003)
E-Calorimetry
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Target
For transversity
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2
reversed once a week
Polarization 50 Dilution factor 0.38
two 60 cm long Target-Containers with opposite
polarization During data taking for
transversity dipole field always ? Relaxation
time gt 2000 hrs
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COMPASS acceptance


Excellent for non-perturbative perturbative
physics - small xBj very small Q2 ?
Q2 gt 100 (GeV/c)2
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Transverse Spin Effects in SIDIS

• 3 possible polarimeters to measure ?Tq are
  explored in COMPASS
• Azimuthal dependence of the plane containing
  hadrons pairs
• Preliminary results on 02-03 data (Rainer
  Joosten)
• Collins effects of (leading) h
• Published results on 2002 data
• (PRL 94 (2005) 202002 Collins and Sivers)
• Transverse polarization of ? hyperon
• First results on 02-03 data

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Azimuthal asymmetries of single hadrons

1. Collins effect predicts an azimuthal asymmetry in
   fragmentation of transversely polarized quarks
   with finite transverse momentum to unpolarized
   hadrons

2) Sivers effect predicts an azimuthal asymmetry
from the un-polarized quarks, i.e. an azimuthal
modulation of initial quark transverse momentum
for a transversely polarized nucleon
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Collins and Sivers Asymmetries
?S azimuthal angle of spin vector of incident
quark (nucleon) ?S azimuthal angle of spin
vector of fragmenting quark (after
scattering) ?h azimuthal angle of hadron
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Event Selection

• DIS cuts
• Q2gt1 (GeV/c)2
• 0.1ltylt0.9
• Wgt5 GeV/c2
• Hadrons
• Track Lengthlt10 X0
• Energy Deposit in HCALs gt 5(8) GeV

• Leading Hadron
• 0.25ltzlt1
• Ptgt0.1 GeV/c
• z-missing cut
• All Hadrons
• zgt0.2
• Ptgt0.1 GeV/c

?pT??0.51 GeV/c
?Q2?2.4 (GeV/c)2
?z? ?0.45
?xBj??0.035
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Collins and Sivers effects
Only statistical errors shown
Phys. Rev. Lett. 94, 202002 (2005)
Systematic errors are smaller than the quoted
statistical errors.
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Collins and Sivers effects
Only statistical errors shown
Phys. Rev. Lett. 94, 202002 (2005)
Systematic errors are smaller than the quoted
statistical errors.
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Interpretations

• Small asymmetries, some explanations
• Cancellation between proton and neutron
• Too small Collins mechanism.

• and large (as from preliminary
  measurement by BELLE, hep-ex/0507063) means
  evidence for cancellation in isoscalar target

• Phenomenological fits of the data from HERMES and
  prediction for COMPASS by Vogelsang and Yuan
  (hep-ph/0507266), Efremov et al. (hep-ph/0412353,
  Phys.Part.Nucl.35S139-S142,2004) and Anselmino
  et al. (hep-ph/0507181)

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Just a flash fits and calculations
M. Anselmino et al. Sivers on Deuterium
W. Vogelsang and F. Yuan Deuterium
Set I for CFF
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Expected accuracy for transversity

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RICH response
Hadron masses calculated from the measured
Cherenkov angle

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

For single leading (z) hadrons, with additional
cut z gt 0.25
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?Tq(x) from ? polarization
Informations on ?Tq(x) can be access also in
µN??? µ??X

• Alternative approch to Collins single hadron
  asymmetries

• The chiral-odd partner of ?Tq(x) is the
  fragmentation function

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?Tq(x) from ? polarization

• Negative trend but, not very significand
  deviation from zero yet.
• Systematic effects not larger than statistical
  errors.
• Factor 2 in statistics expected from 2004 data.

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2006 proton run for transverse PT

• New magnet larger acceptance
• Higher statistics at large x

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Expected accuracy for transversity
30 days of running
For Sivers expected errors are almost the same
M. Anselmino et al. Protons
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Summary and Outlook

• The 2002 single hadron asymmetries (Collins and
  Sivers) are small and compatible with the zero
• Both COMPASS (deuteron) and HERMES (protons) can
  be described by the same model, implying for
  deuteron a cancellation between protons and
  neutrons. Existing phenomenological models are in
  a good agreement with COMPASS and HERMES data
• Making use of existing deuteron data (2002-2004),
  accuracy will improve by a factor 3, results
  are coming soon
• Data (of comparable statistics) will be collected
  on a transversely polarized proton target (NH3)
  in 2006 and will allow a flavor separation.

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THANK YOU
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Save slides
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Flavor decomposition
To measure the shape of
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Estimators
Weaker assumptions on acceptance effects less
sensitive to distorsions set by Cahn effects
Reconstruction time independent in the solid angle
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Extraction of Collins (Sivers) asyms

• Evaluating the population as a function of F,
  independently in both target cells
• Extracting 2 asymmetries and calculating the
  weighted mean

j C, S, (Fj calculated always with spin ?)
and F is the muon flux, n the number of target
particles, s the spin averaged cross-section, and
aj the product of angular acceptance and
efficiency of the spectrometer
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Spectrometer 2002 ? 2004
Beam 160 GeV µ 2 . 108 µ/spill (4.8s/16.2s)
Pm 80
trigger-hodoscopes
DW45
straws
SM2 dipole
Muon-filter2,MW2
HCAL1
RICH_1
Gem_11
ECAL2,HCAL2
SM1 dipole
MWPC Gems Scifi
Polarised Target
Muon-filter1,MW1
straws,MWPC,Gems,SciFi
Veto
Gems,SciFi,DCs,straws
Silicon
Micromegas,DC,SciFi
SciFi
BMS
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Measuring ?Tq(x)

• ?Tq(x) is chiral odd decouples from leading
  twist DIS because helicity of quark must flip. It
  doesnt play any role in inclusive DIS

How to access it???
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Measuring ?Tq(x)
Chiral-odd requires another chiral-odd partner
p?p??ll-X
ep??eh?X
How to access it How to access it
SIDIS(e.g. COMPASS and HERMES ) SIDIS(e.g. COMPASS and HERMES )
Hard scattering (e.g. RHIC) Drell-Yan Single Spin Asym (e.g. p?p?pX) Hard scattering (e.g. GSI) Drell-Yan
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SIDIS off transversely polarized target

• Collins effect predicts an azimuthal asymmetry in
  the quark fragmentation
• The quark prefers to fragment in one direction
• Look at leading hadron (it comes from the struck
  quark)
• The larger is z (fraction of the available
  energy carried by the hadron), the stronger is
  the signal
• But an azimuthal asymmetry can also come from the
  un-polarised quarks namely from an azimuthal
  modulation of quark transverse momentum for a
  transversely polarized nucleon (Sivers effect)

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Collins single-spin asymmetries on deuterons
W. Vogelsang and F. Yuan hep-ph/0507266
Fit Parameters Set II for CFF
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Theoretical work on the subject

• The role of Cahn and Sivers effects in
  DIS
• M. Anselmino et al. (hep-ph/0501196)
• Phenomelogical model whose parameters are
  constrained by HERMES proton measurements
• COMPASS preliminary results for Sivers effect
  (shown at SPIN04) are in agreement with the
  model.
• Interpretation of Collins effect in progress.

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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
Identification for standard cuts (zh1gt0.1
zh2gt0.1)
Better SNR for zh1gt0.25 and zh2gt0.25