COMPASS results: TRANSVERSITY

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COMPASS results TRANSVERSITY
Paolo Pagano On behalf of the COMPASS
collaboration
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Talk outline

• The physics case of transverse spin effects
• Probing transversity in SIDIS
• The COMPASS experiment
• The results on the deuteron
• for 1 hadron SSA and interpretation
• for hadron pairs SSA and interpretation
• 5. Conclusions and Outlook

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Spin structure functions

3 distribution functions are necessary to
describe the spin structure of the nucleon at
LO
All of equal importance !
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Measuring ?Tq(x)
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Transversity

• In the last ten years
• Great development in the theory of transversity
• Remarkable role of ?Tq(x), notably complementary
  to ?q(x).
• In the last couple of years
• Role of the kT structure functions clarified
  (Cahn and Sivers effects, ).

• Tensor charge (1991 1992)
• in analogy with
• Soffer inequality (1995)
• Leader sum rule (2004)
• in analogy with

• Key features of transversity
• Probes relativistic nature of quarks
• No gluon analog for spin-1/2 nucleon
• Different Q2 evolution and sum rule than ?q(x)
• Sensitive to valence quark polarization

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Transverse Spin Physics at COMPASS

• )

• Possible quark polarimeters suggested using
  SIDIS
• Azimuthal distribution of single
  (leading)hadrons
• Azimuthal dependence of the plane containing
  hadron pairs
• Measurement of transverse polarization of spin ½
  baryons (e.g. L hyperon)

Published RESULTS 2005
Preliminary RESULTS 2005
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Azimuthal asyms of single hadrons

• Collins effect predicts an azimuthal asymmetry
  in the quark fragmentation
• Look at leading hadron (the most sensitive to the
  parent struck quark)
• The larger z Eh/n, the stronger the signal
• 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)

Left
Right
The quark prefers to fragment in one direction
Non vanish Lq
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Collins and Sivers asyms
Collins angle
Sivers angle
Asyms in FC and FS extracted independently Asym
s calculated as function of x, z and pt and for
Leading Hadrons and for All Hadrons
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Azimuthal dependence of the plane containing
hadron pairs

• Another quark polarimeter comes from interference
  in fragmentation
• The struck quark fragments in two hadrons. The
  fragmentation function has a component dependent
  on the transverse spin momentum (as for the
  Collins function)

Left
Right
A
The hadron pair prefers to fragment in one
direction
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Hadron pair azimuthal asymmetry
Azimuthal angle
A
Calculated as function of x, z and Mh for all
pairs of positive and negative hadrons.
A. Bacchetta and M. Radici, Proceedings of DIS
2004, hep-ph/0407345
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Physics Goals of COMPASS

• With the muon beam
• Gluon Polarization ?G/G
• transverse spin distribution functions ?Tq (x)
• Flavor dependent polarized
• quark helicity densities ?q(x)
• L physics
• Diffractive VM-Production

• With hadron beams
• Primakoff-Reactions
• polarizability of ? and K
• glueballs and hybrids
• charmed mesons and baryons
• semi-leptonic decays
• double-charmed baryons

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Spectrometer 2002 ? 2004
Beam 160 GeV µ 2 . 108 µ/spill (4.8s/16.2s)
Pm 76
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
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The polarized 6LiD-Target
For transversity
3He 4He Dilution refrigerator (T50mK)
superconductive
Solenoid (2.5 T)
Dipole (0.5 T)



reversed once a week
two 60 cm long Target-Containers with opposite
polarization During data taking for
transversity dipole field always ? Relaxing
time gt 2000 hrs
Polarization 50 Dilution factor 0.38
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COMPASS data sample

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

• Data sample increased in years 2003/4
• trigger system upgraded
• DAQ upgraded
• 2004 longer run.
• 2003 and 2004 data production over(analysis in
  progress)

RICH
E-Calo
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Kinematical distributions and general cuts

To access DIS region and get rid of radiative
corrections
Use of the calorimeters
Q2 ?1 (GeV/c)2
0.1 ?Y? 0.9
?Q2?2.4 (GeV/c)2
?Y?0.33
W? 5 GeV/c2
?xBj??0.035
?W?9.4 GeV/c2
2002 data
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Cuts for Collins and Sivers SSA
2002 data

pT?0.1 GeV/c
z? 0.25 (0.2 ah)
?pT??0.51 GeV/c
?z? ?0.45
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Extraction of Collins and Sivers asymmetries

• 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 ?) F
is the muon flux n the number of target
particles s the spin averaged cross-section aj
the product of angular acceptance and
efficiency of the spectrometer
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Evaluation of systematics

• Stability of the asymmetries
• i) in time
• ii) in two halves of the target cells
• iii) according to the hadron momentum.
• Evaluated 3 different estimators and
  compatibility in the results checked.
• Look at the paper
• First Measurement of the Transverse Spin
  Asymmetries of
• the Deuteron in Semi-Inclusive Deep Inelastic
  Scattering
• (The COMPASS Collaboration)

Phys. Rev. Lett. 94, 202002 (2005)
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Collins and Sivers effects (1)

• Asymmetries as a function of x, z, pt
• 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 (2)

• Asymmetries as a function of x, z, pt
• Only statistical errors shown

Phys. Rev. Lett. 94, 202002 (2005)
Systematic errors are smaller than the quoted
statistical errors.
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Interpretation of Collins SSA

• Small asymmetries. Two explanations
• Cancellation between proton and neutron
• Too small Collins mechanism.
• If and large as from preliminary
  measurement by BELLE (see hep-ex/0507063), this
  is evidence for cancellation in iso-scalar
  target
• Also a new phenomenological fit of the data by
• Vogelsang and Yuan (see hep-ph/0507266).

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COMPASS (deut.) vs. HERMES (protons) Positive
hadrons

• HERMES
• Negative Collins asymmetries
• Positive Sivers asymmetries.
• HERMES data points for Collins effect flipped by
  a phase of p to match COMPASS notation !

HERMES data points from A. Airapetian et al,
Phys. Rev. Lett. 94 (2005) 012002DC53
(hep-ex/0408013)
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COMPASS (deut.) vs. HERMES (protons) Negative
hadrons

• HERMES
• Large Positive Collins asymmetries
• No Sivers effect.
• HERMES data points for Collins effect flipped by
  a phase of p to match COMPASS notation !

HERMES data points fromA. Airapetian et al,
Phys. Rev. Lett. 94 (2005) 012002DC53
(hep-ex/0408013)
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Theoretical work on Sivers SSA
M. Anselmino et al. hep-ph/057181 (including
HERMES Prel. Results shown at DIS05)

• Extracting the Sivers function from polarized
  SIDIS data and making predictions
• Phenomelogical model whose parameters are
  constrained by HERMES proton measurements
• COMPASS 2002 preliminary results for Sivers
  effect are in agreement with the model.

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Prediciton of Sivers asymmetries on a proton
target
M. Anselmino et al. hep-ph/057181
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Expected accuracy for Collins asymmetries

• Accuracy from total statistics accumulated with
  LiD (deuteron) target
• Projections from 30 days of data taking (included
  in COMPASS schedule for 2006) with NH3 (proton)
  target
• Taking into account
• Variation of statistical errors
• taking into account the variation of

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Kinematics from the analysis on hadron pairs

2002-2003 2.8 106 combinations
Select all combinations of positive (h1) and
negative (h2)
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Extraction of asymmetries for hadron pairs

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

FRS calculated always with spin ? F is the
muon flux n the number of target particles s the
spin averaged cross-section a the product of
angular acceptance and efficiency of the
spectrometer
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Effects on hadron pairs

• Asymmetries as a function of x, z, Mh
• Only statistical errors shown

Systematic errors are smaller than the quoted
statistical errors.
Presented for the first time _at_ DIS2005
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Theoretical work on hadron pair (asym vs.
invariant mass)
R. L. Jaffe, X. Jin and J. Tang, Phys. Rev.
Lett. 80, 1166 (1998)
Radici, Jakob, Bianconi, PRD 65, 074031
H? (z,M2pp-)sind0 sind1 sin(d0-d1)
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Theoretical work on hadron pairs (asym vs. xbj
and z)
Radici, Jakob, Bianconi, PRD 65, 074031
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Summary and Outlook

• Collins and Sivers SSA shown from 2002 data
  published by PRL
• The asymmetries are small and compatible with
  zero
• Making use of all the deuteron data, accuracy
  will improve by a factor 3.
• SSA from hadron pairs calculated from 2002 2003
  statistics
• The asymmetries are small and compatible with
  zero
• Using 2004 data, accuracy improves by a factor
  1.4.
• Stay tuned because
• New channels in hadron pair production are under
  study.
• PID is being included in all the analyses
• New results on Lambda transverse polarization are
  coming.
• Data (of comparable statistics) will be collected
  on a transversely polarized proton target (NH3)
  in 2006.

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Thank you!
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Estimators
Weaker assumptions on acceptance effects less
sensitive to distorsions set by Cahn effect
Reconstruction time independent in the solid angle
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Why a proton run?
To measure the shape of