Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)1 COMPASS results: TRANSVERSITY Paolo Pagano On behalf of the COMPASS collaboration

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)2 Talk outline 1.The physics case of transverse spin effectsThe physics case of transverse spin effects 2.Probing transversity in SIDISProbing transversity in SIDIS 3.The COMPASS experimentThe COMPASS experiment 4.The results on the deuteron for 1 hadron SSA for 1 hadron SSA and interpretationinterpretation for hadron pairs SSAfor hadron pairs SSA and interpretationinterpretation 5.Conclusions and OutlookConclusions and Outlook

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)3 Spin structure functions 3 distribution functions are necessary to describe the spin structure of the nucleon at LO: All of equal importance !  T q(x)  q(x) q(x)

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)4 Measuring Δ T q(x) How to access it: SIDIS (e.g. COMPASS and HERMES ) Hard scattering (e.g. RHIC) –Drell-Yan –Single Spin Asym (e.g. p ↑ p→πX) Hard scattering (e.g. GSI) –Drell-Yan Chiral-odd: requires another chiral-odd partner Impossible in DISDirect Measurement ppl+l-Xppl+l-Xep   e’h  X Convolution with fragmentation function ep   e’X

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)5 Transversity In the last ten years: Great development in the theory of transversity; Remarkable role of Δ T q(x), notably complementary to Δq(x). In the last couple of years: Role of the k T 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 Q 2 evolution and sum rule than Δq(x) -Sensitive to valence quark polarization

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)6 ) 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.  hyperon) Published RESULTS 2005 Preliminary RESULTS 2005 Not covered in this talk

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)7 Azimuthal asym’s 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 = E h /, 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) LeftRight Non vanish L q The quark prefers to fragment in one direction

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)8 Collins and Sivers asym’s Asym’s in  C and  S extracted independently; Asym’s calculated as function of x, z and p t and for “Leading Hadrons” and for “All Hadrons” Collins angle: Sivers angle:

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)9 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): The hadron pair prefers to fragment in one direction A LeftRight A

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)10 Hadron pair azimuthal asymmetry Calculated as function of x, z and M h for all pairs of positive and negative hadrons. Azimuthal angle: A A. Bacchetta and M. Radici, Proceedings of DIS 2004, hep-ph/

COMPASSFixed target experiment at CERN SPS 240 physicists from 12 countries 28 institutes 2000 technical run 2001 commissioning run physics runs (muon program) 2005 SPS stop 2006-… resume physics

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)12 With the muon beam Gluon Polarization  G/G transverse spin distribution functions  T q (x) Flavor dependent polarized quark helicity densities  q(x)  physics Diffractive VM-Production Physics Goals of COMPASS With hadron beams Primakoff-Reactions -polarizability of  and K glueballs and hybrids charmed mesons and baryons -semi-leptonic decays -double-charmed baryons

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)13 SM1 dipole SM2 dipole Polarised Target HCAL1 Muon-filter1,MW1 Micromegas,DC,SciFi Gems,SciFi,DCs,straws MWPC Gems Scifi trigger-hodoscopes Silicon Spectrometer 2002  2004 RICH_1Gem_11 ECAL2,HCAL2 straws,MWPC,Gems,SciFi straws Muon-filter2,MW2 DW45 SciFi Veto Beam: 160 GeV µ µ/spill (4.8s/16.2s) P  ~ 76%

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)14 The polarized 6 LiD-Target two 60 cm long Target-Containers with opposite polarization During data taking for transversity dipole field always  Relaxing time > 2000 hrs superconductive Solenoid (2.5 T ) 3 He – 4 He Dilution refrigerator (T~50mK) Dipole (0.5 T) reversed once a week 1 2 For transversity: Polarization: 50% Dilution factor: 0.38

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)15 COMPASS data sample In so far ( 3 years ) –only DIS off 6 LiD –only runs with transverse polarized target being analyzed runsgood runs used events in the analysis (1h) SPS spills 1.6 · SPS spills ~4(2002) SPS spills ~2(2003) RICH E-Calo Data sample increased in years 2003/4: –trigger system upgraded; –DAQ upgraded; –2004 longer run and 2004 data production over (analysis in progress)

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)16 Kinematical distributions and general cuts  Q 2  =2.4 (GeV/c) 2  Y  =0.33  W  =9.4 GeV/c 2  x Bj  hadrons p E muons Use of the calorimeters To access DIS region and get rid of radiative corrections 2002 data Q 2  1 (GeV/c)  Y  0.9 W  5 GeV/c 2

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)17 Cuts for Collins and Sivers SSA  p T  0.51 GeV/c  z   useful statisticsLead had’s Positive hadrons8.7 ·10 5 Negative hadrons7.0 ·10 5 Total1.6 · data p T  0.1 GeV/c z  0.25 (0.2 ah)

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)18 Extraction of Collins and Sivers asymmetries Evaluating the population as a function of Φ, independently in both target cells; Extracting 2 asymmetries and calculating the weighted mean. j = C, S (Φ j calculated always with spin = ↑) F is the muon flux n the number of target particles σ the spin averaged cross-section a j the product of angular acceptance and efficiency of the spectrometer;

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)19 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, (2005)

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)20 Collins and Sivers effects (1) Systematic errors are smaller than the quoted statistical errors. Asymmetries as a function of x, z, p t Only statistical errors shown Phys. Rev. Lett. 94, (2005)

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)21 Collins and Sivers effects (2) Systematic errors are smaller than the quoted statistical errors. Asymmetries as a function of x, z, p t Only statistical errors shown Phys. Rev. Lett. 94, (2005)

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)22 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/ ), this is evidence for cancellation in iso-scalar target; Also a new phenomenological fit of the data by Vogelsang and Yuan (see hep-ph/ ).

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)23 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 π to match COMPASS notation ! HERMES data points from: A. Airapetian et al, Phys. Rev. Lett. 94 (2005) [DC53] (hep-ex/ )

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)24 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 π to match COMPASS notation ! HERMES data points from: A. Airapetian et al, Phys. Rev. Lett. 94 (2005) [DC53] (hep-ex/ )

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)25 Theoretical work on Sivers SSA “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. M. Anselmino et al. hep-ph/ (including HERMES Prel. Results shown at DIS05)

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)26 Prediciton of Sivers asymmetries on a proton target M. Anselmino et al. hep-ph/057181

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)27 Expected accuracy for Collins asymmetries A Coll statistical errrors: Positive hadrons (to the left): Red; 2002 – 2004 Blue; 2006 Blue. Negative hadrons (to the right): Red; 2002 – 2004 Blue; 2006 Blue. Accuracy from total statistics accumulated with LiD (deuteron) target; Projections from 30 days of data taking (included in COMPASS schedule for 2006) with NH 3 (proton) target : Taking into account: –Variation of statistical errors: –taking into account the variation of:

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)28 Kinematics from the analysis on hadron pairs : combinations Select all combinations of positive (h 1 ) and negative (h 2 )

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)29 Extraction of asymmetries for hadron pairs Evaluating the population as a function of Φ RS, independently in both target cells; Extracting 2 asymmetries and calculating the weighted mean. Φ RS calculated always with spin = ↑; F is the muon flux n the number of target particles σ the spin averaged cross-section a the product of angular acceptance and efficiency of the spectrometer;

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)30 Effects on hadron pairs Asymmetries as a function of x, z, M h Only statistical errors shown Systematic errors are smaller than the quoted statistical errors. Presented for the first DIS2005

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)31 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, H ∢ (z,M 2  +  - ) ∼ sin  0 sin  1 sin(  0 -  1 ) A Mh sin 

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)32 Theoretical work on hadron pairs (asym vs. x bj and z) AxAx sin   (z) ∢ Radici, Jakob, Bianconi, PRD 65,

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)33 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 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 (NH 3 ) in 2006.

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)34 Thank you!

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)35 Save slides

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)36 Estimators Weaker assumptions on acceptance effects less sensitive to distorsions set by Cahn effect Reconstruction time independent in the solid angle

Prague (CZ), August 1stPaolo Pagano (INFN - Trieste)37 Why a proton run? To measure the shape of