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Explore 2002-2003 data on transverse spin effects, azimuthal asymmetries, and event selection in the study of Collins and Sivers effects on the deuteron by COMPASS Collaboration. Investigate the small asymmetries in hadron fragmentation and quark transverse momentum. Interpret the findings with explanations from BELLE measurements and theoretical predictions.
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Collins and Sivers asymmetries on the deuteron from COMPASS Andrea Bressan (University and I.N.F.N. Trieste) on behalf of the COMPASS Collaboration PANIC05 Particles and Nuclei International Conference Santa Fe, NM - October 24-28, 2005
LHC COMPASS SPS Andrea Bressan University -INFN Trieste
COMPASS data sample RICH PID • 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 E-Calorimetry Andrea Bressan University -INFN Trieste
Target 3He – 4He Dilution refrigerator (T~50mK) superconductive Solenoid (2.5 T) Dipole (0.5 T) For transversity: 1 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 > 2000 hrs Andrea Bressan University -INFN Trieste
COMPASS acceptance Q21(GeVc)2 Excellent for non-perturbative & perturbative physics - small xBj & very small Q2 Q2 > 100 (GeV/c)2 Andrea Bressan University -INFN Trieste
Transverse Spin Effects in SIDIS • 3 possible polarimeters to measure ΔTq 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 Λ hyperon • First results on 02-03 data Andrea Bressan University -INFN Trieste
Azimuthal asymmetries of single hadrons • Collinseffect predicts an azimuthal asymmetry infragmentation of transversely polarized quarks with finite transverse momentum to unpolarized hadrons 2) Siverseffect predicts an azimuthal asymmetry from the un-polarized quarks, i.e. an azimuthal modulation of initial quark transverse momentum for a transversely polarized nucleon Andrea Bressan University -INFN Trieste
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 Andrea Bressan University -INFN Trieste
Event Selection • Leading Hadron • 0.25<z<1 • Pt>0.1 GeV/c • z-missing cut • All Hadrons • z>0.2 • Pt>0.1 GeV/c • DIS cuts: • Q2>1 (GeV/c)2 • 0.1<y<0.9 • W>5 GeV/c2 • Hadrons • Track Length<10 X0 • Energy Deposit in HCALs > 5(8) GeV Q2=2.4 (GeV/c)2 pT0.51 GeV/c z0.45 xBj0.035 Andrea Bressan University -INFN Trieste
Collins and Sivers effects Only statistical errors shown Phys. Rev. Lett. 94, 202002 (2005) Systematic errors are smaller than the quoted statistical errors. Andrea Bressan University -INFN Trieste
Collins and Sivers effects Only statistical errors shown Phys. Rev. Lett. 94, 202002 (2005) Systematic errors are smaller than the quoted statistical errors. Andrea Bressan University -INFN Trieste
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.35:S139-S142,2004) and Anselmino et al. (hep-ph/0507181) Andrea Bressan University -INFN Trieste
Just a flash: fits and calculations • M. Anselmino et al. • Sivers on Deuterium • W. Vogelsang and F. Yuan Deuterium • Set I for CFF Andrea Bressan University -INFN Trieste
Expected accuracy for transversity Andrea Bressan University -INFN Trieste
RICH response Hadron masses calculated from the measured Cherenkov angle Threshold momenta: • pp=2 GeV/c • pK=9 GeV/c • pP=17 GeV/c For single leading (z) hadrons, with additional cut: z > 0.25 Andrea Bressan University -INFN Trieste
ΔTq(x) from Λ polarization Informations on ΔTq(x) can be access also in μN μ’ΛX • Alternative approch to Collins single hadron asymmetries • The chiral-odd partner of ΔTq(x) is the fragmentation function Andrea Bressan University -INFN Trieste
ΔTq(x) from Λ 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. Andrea Bressan University -INFN Trieste
2006: proton run for transverse PT • New magnet: larger acceptance • Higher statistics at large x Andrea Bressan University -INFN Trieste
Expected accuracy for transversity • 30 days of running For Sivers expected errors are almost the same • M. Anselmino et al. Protons Andrea Bressan University -INFN Trieste
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. Andrea Bressan University -INFN Trieste
Save slides Andrea Bressan University -INFN Trieste
Flavor decomposition To measure the shape of Andrea Bressan University -INFN Trieste
Estimators Weaker assumptions on acceptance effects less sensitive to distorsions set by Cahn effects Reconstruction time independent in the solid angle Andrea Bressan University -INFN Trieste
Extraction of Collins (Sivers) asym’s • Evaluating the population as a function of Φ, independently in both target cells; • Extracting 2 asymmetries and calculating the weighted mean; j = C, S, (Φj calculated always with spin = ↑)and F is the muon flux, n the number of target particles, σ the spin averaged cross-section, and aj the product of angular acceptance and efficiency of the spectrometer; Andrea Bressan University -INFN Trieste
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 Veto straws,MWPC,Gems,SciFi Gems,SciFi,DCs,straws Silicon SciFi Micromegas,DC,SciFi BMS Andrea Bressan University -INFN Trieste
Measuring ΔTq(x) Tq(x) is chiral odd: decouples from leading twist DIS because helicity of quark must flip. It doesn’t play any role in inclusive DIS; How to access it? Andrea Bressan University -INFN Trieste
Measuring ΔTq(x) Convolution with fragmentation functions (measurements of ff ongoing at BELLE) Direct Measurement Chiral-odd: requires another chiral-odd partner ppl+l-X epe’hX Andrea Bressan University -INFN Trieste
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) y x h z q y x q kT z Andrea Bressan University -INFN Trieste
Collins single-spin asymmetries on deuterons • W. Vogelsang and F. Yuan hep-ph/0507266 • Fit Parameters: Set II for CFF Andrea Bressan University -INFN Trieste
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. Andrea Bressan University -INFN Trieste
2hadron case using RICH Pairs of hadrons to be analyzed: pp, pK, KK K0 r0 K* f Andrea Bressan University -INFN Trieste
2hadron case using RICH Identification for “standard” cuts (zh1>0.1 & zh2>0.1) Better SNR for zh1>0.25 and zh2>0.25 Andrea Bressan University -INFN Trieste