Collins and Sivers asymmetries on the deuteron from COMPASS data The International Workshop on Transverse Polarisation Phenomena in Hard Processes Collins and Sivers asymmetries on the deuteron from COMPASS data Igor Horn, University of Bonn on behalf of the COMPASS Collaboration Villa Olmo (Como), 7−10th. September 2005

Outline Motivation Experimental configuration Data selection Results from 2002 beamtime Phenomenological models Statistical accuracy with 2003/4 data Summary and outlook

Physics at COMPASS With the muon beam With hadron beams Gluon Polarization G/G transverse spin distribution functions Tq (x) Flavor dependent polarized quark helicity densities q(x)  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

Spin structure functions 3 distribution functions are necessary to describe the spin structure of the nucleon at LO (twist-2), all of equal importance! Tq(x) q(x) q(x) decouples from leading twist DIS because helicity of quark must flip; not observable in inclusive DIS;

Convolution with fragmentation function How to measure ΔTq(x)? epe’X ppl+l-X epe’hX Impossible in DIS Direct Measurement Convolution with fragmentation function Semi-inclusive DIS (COMPASS and HERMES) 2) Scattering of nucleons: Drell-Yan (RHIC) 3) Scattering of nucleon and antinucleon: Drell-Yan (GSI)

Developments in transversity In the last ten years: Measurements at DESY, CERN, Jlab and RHIC Great development in the theory of transversity: transversity is not suppressed at high energies; Remarkable role of ΔTq(x), complementary to Δq(x). In the last couple of years: Role of the kT dependent distribution functions clarified (Cahn and Sivers effects, …). Key features of transversity: Probes relativistic nature of quarks No gluon analog for spin-1/2 nucleon Different Q2 evolution and sum rule than Δq(x)

Transverse spin physics at COMPASS Possible quark polarimeters: Azimuthal distribution of single hadrons Azimuthal dependence of the plane containing hadron pairs (see A.Mielech ’s talk) Measurement of transverse polarization of spin ½ baryons, e.g.  hyperon (see A. Ferrero’s talk)

Azimuthal asymmetries of single hadrons Collins effect predicts an azimuthal asymmetry in fragmentation of transversely polarized quarks with finite transverse momentum to un-polarized hadrons 2) Sivers effect predicts an azimuthal asymmetry from the un-polarized quarks, -- an azimuthal modulation of quark transverse momentum for a transversely polarized nucleon.

Collins and Sivers angles Collins: AColl ~ sin C C = h - S’ = h + S –p Sivers: ASiv ~ sin S  S = h - S S -- azimuthal angle of spin vector of initial-state quark/nucleon S’ -- azimuthal angle of spin vector of fragmenting quark with S’’ = p -S (spin flip) h -- azimuthal angle of hadron momentum

Collins and Sivers effects Collins and Sivers effect distinguishable AUTsin( - s‘ ) AUTsin( - s ) 

Experimental configuration trigger-hodoscopes DW45 SM2 dipole Polarized Target straws Muon-filter2,MW2 RICH_1 HCAL1 Gem_11 SM1 dipole ECAL2,HCAL2 MWPC Gems Scifi Muon-filter1,MW1 Veto straws,MWPC,Gems,SciFi Gems,SciFi,DCs,straws Silicon SciFi Micromegas,DC,SciFi Beam: 2 . 108 µ+/ spill (4.8s / 16.2s) Beam momentum: 160 GeV/c Luminosity: ~5 . 1032 cm-2 s-1 Beam polarisation: ~76%

The frozen spin Target 1 2 two 60 cm long Target Cells with opposite polarization Relaxation time > 2000 hrs superconductive Solenoid (2.5 T) 3He – 4He Dilution refrigerator (T~50mK) Dipole (0.5 T) For transversity measurements the polarization was reversed once a week 1 to 2 1 2 2002,2003,2004: 6LiD Achieved polarization: ~50% Dilution factor: ~0.38

Kinematical variables and cuts Primary vertex: m, m’ + hadron To ensure that we have DIS events and avoid large radiative corrections 0.1 Y 0.9 Q2 1 (GeV/c)2 Y=0.33 Q2=2.4 (GeV/c)2 Q2 > 1 (GeVc)2 W 5 GeV/c2 W=9.4 GeV/c2 xBj0.035 2002 data

Event selection: further cuts Hadron selection: energy deposit in hadron calorimeters (if present) > 5 GeV(HCAL1) or 8 GeV (HCAL2) penetration length < 10 X0 no π / K / p separation by RICH for the 2002 data Kinematic cuts on hadrons: pt > 0.1 GeV/c; z > 0.25; z > 1- ∑ zi (leading hadron analysis) (zi = fraction of available energy carried by hadron) pt > 0.1 GeV/c; z > 0.20 (analysis with all hadrons)

Event selection 2002 statistics: Positive hadrons ~ 8.7 ·105 2002 data z 0.25 pT0.1 GeV/c pT0.51 GeV/c z 0.45 2002 statistics: Positive hadrons ~ 8.7 ·105 Negative hadrons ~7.0 ·105 Total ~ 1.6 ·106

Monte Carlo studies Good agreement between MC and real data! MC events generated with Lepto 6.5.1 Data MC Taken into account: Trigger geometry Tracking efficiencies rms: 0.068 rad Good agreement between MC and real data! all reconstructed hadrons correctly reconstructed leading hadrons correctly reconstructed leading hadron, but leading hadron is not charged π (~20% of the final sample, mainly K and p (RICH not used in analysis) ) z=0.25

Extraction of Collins and Sivers asymmetries Rate asymmetries: j = C, S; F muon flux n target density σ cross-section aj acceptance efficiency f dilution factor PT target polarization DNN depolarization factor Asymmetries for two target cells are ( separately ) extracted, using periods of opposite polarization, then combined:

Systematics Ratio of the acceptances and efficiencies for both target cells vs. Coll, Siv does NOT change between two spin orientations Results STABLE under the following tests: ● Splitting the target cells in two parts ● Splitting the data in high and low hadron momenta ● Using a different method to extract the raw asymmetry ● Different binning Systematic errors are smaller than statistical errors

Results: x z p [ Gev/c ] A A Statistical Errors Collins Sivers T Phys. Rev. Lett. 94, 202002 (2005) Statistical Errors A Collins A Sivers x z p [ Gev/c ] T

Results: x z p [ Gev/c ] A A Statistical Errors Collins Sivers T Phys. Rev. Lett. 94, 202002 (2005) Statistical Errors A Collins A Sivers x z p [ Gev/c ] T

Interpretations Small asymmetries, some explanations: Cancellation between proton and neutron; Too small Collins mechanism. If and large as from preliminary measurement by BELLE (hep-ex/0507063), this is 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.35:S139-S142,2004) and Anselmino et al. (hep-ph/057181)

Sivers single-spin asymmetries M. Anselmino et al. hep-ph/057181

Prediction of Sivers asymmetries on a proton target M. Anselmino et al. hep-ph/057181 Asymmetry predicted to be larger with z>0.4, 0.2<pT<1 GeV/c and x>0.02

Sivers single-spin asymmetries on a deuteron target W. Vogelsang and F. Yuan hep-ph/0507266

Collins single-spin asymmetries on a deuteron target W. Vogelsang and F. Yuan hep-ph/0507266 Fit Parameters: Set I for CFF

Collins single-spin asymmetries on a deuteron target W. Vogelsang and F. Yuan hep-ph/0507266 Fit Parameters: Set II for CFF

Collected data Data sample with transverse spin: π Year Number of SPS spills Number of good DIS events ~45300 ~1.6 ∙ 106 ~42900 ~4 ∙ (2002) ~94000 ~2 ∙ (2003) π Data sample increased in years 2003/4: trigger system enlarged; 2004 longer run. RICH particle identification is available for 2003-2004 data! E-calorimeter information is available for 2004 data K p

Expected statistical accuracy for Collins asymmetries 30 days

Summary and Outlook Collins and Sivers single-spin asymmetries shown from 2002 data published by PRL; The 2002 asymmetries are small and compatible with the non zero proton asymmetries measured by HERMES; 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.

Thank you for your attention!