SPIN @ Prague 2007 Collins and Sivers asymmetries Girisan Venugopal University of Bonn (on behalf of the COMPASS collaboration) Collins and Sivers asymmetries on the deutron target

COMPASS experiment @ CERN LHC SPS N fixed target experiment at the CERN SPS 240 physicists from 28 Institutes, 11 Countries 11/12/2018 Girisan Venugopal University of Bonn

Girisan Venugopal University of Bonn COMPASS spectrometer high energy beam large angular acceptance broad kinematical range longitudinally polarised muon beam longitudinally or transversely polarised target MuonWall SM2 E/HCAL E/HCAL Straws DC MWPC SciFi Silicon Micromegas GEMs SM1 Polarised Target Muon Wall RICH m beam 11/12/2018 Girisan Venugopal University of Bonn

The COMPASS polarized target 11/12/2018 Girisan Venugopal University of Bonn

Girisan Venugopal University of Bonn Data taking 2002-2004 transversely polarized deuteron target (6LiD) ~ 20% of the running time 2002 11 days 2003 9 days 2004 14 days trigger (large x, Q2) DAQ, on line filter Reconstructed DIS events (106) 11/12/2018 Girisan Venugopal University of Bonn

Girisan Venugopal University of Bonn Event selection DIS cuts: Q2 > 1 0.1 < y < 0.9 W > 5 GeV/c Hadron selection: z > 0.2 pt > 0.1 GeV/c Q2 > 1 Statistics 2002 - 2004: 8.5 * 106 positive hadrons 7.0 * 106 negative hadrons 11/12/2018 Girisan Venugopal University of Bonn

Hadron identification Cherenkov thresholds: π ~ 3 GeV/c K ~ 9 GeV/c p ~ 17 GeV/c 5 m 6 m 3 m C4F10 photon detectors 2 σ π/K separation at 43 GeV/c Statistics 2003-2004 positive negative  3.4 * 106 2.8 * 106 K 0.7 * 106 0.4 * 106 11/12/2018 Girisan Venugopal University of Bonn

Transverse Spin Physics At leading order, the inner structure of the nucleon can be described with three Parton Distribution Function (PDF) q(x) Momentum distribution Well known – Unpolarized DIS Δq(x) Helicity distribution known – Polarized DIS ΔTq(x) Transversity distribution Largely unknown 11/12/2018 Girisan Venugopal University of Bonn

Measuring Transversity Inclusive DIS 11/12/2018 Girisan Venugopal University of Bonn

Measuring Transversity Drell-Yan process SIDIS FF DTq(x) Measured in COMPASS 11/12/2018 Girisan Venugopal University of Bonn

Measuring Transversity Properties of Transversity distribution fuction is chiral-odd: decouples from inclusive DIS Observable effects are given only by the product of Tq (x) with an other chiral-odd function Can be measured in SIDIS on a transversely polarised target via “quark polarimetry” “Collins” asymmetry “Collins” Fragmentation Function two-hadron asymmetry ( presented by Frank Massmann) “Interference” Fragmentation Function Λ polarisation Fragmentation Function of q↑Λ 11/12/2018 Girisan Venugopal University of Bonn

Collins effect in SIDIS The “quark polarimetry” relies on the Collins effect The fragmentation function of a transversely polarised quark has a spin dependent part “Collins” FF: 11/12/2018 Girisan Venugopal University of Bonn

Collins effect in SIDIS Fragmentation of transversely polarized quarks to unpolarized hadrons The “Collins angle” is C = h + S - p h,,s azimuthal angles of the hadron, transverse spin of the initial quark Transversity distribution function Collins fragmentation Function (is being measured in Belle Expt) 11/12/2018 Girisan Venugopal University of Bonn

Girisan Venugopal University of Bonn Collins asymmetries π K only statistical errors shown (~1%), systematic errors considerably smaller small asymmetries compatible with 0 for both +π, K and -π, K (ΔTu, ΔTd cancel in deuterium ) 11/12/2018 Girisan Venugopal University of Bonn

Collins: Interpretation Global fit performed by M. Anselmino et.al 2006: final results from 2002-2004 data NPB765 (2007)31 Talk given by A. Prokudin (International Workshop on Structure and Spectroscopy, Freiburg) 11/12/2018 Girisan Venugopal University of Bonn

Girisan Venugopal University of Bonn Sivers effect The intrinsic transverse momentum of unpolarized quarks in a transversely polarized nucleon can introduce a different azimuthal asymmetry It is related to the parton orbital angular momentum in a transversely polarized nucleon The “Sivers angle” is S = h - S h,,s azimuthal angles of the hadron, transverse spin of the initial quark Sivers Function 11/12/2018 Girisan Venugopal University of Bonn

Girisan Venugopal University of Bonn Sivers asymmetries π K only statistical errors shown (~1%), systematic errors considerably smaller small asymmetries compatible with 0 for both +π, K and -π, K 11/12/2018 Girisan Venugopal University of Bonn

Sivers: Interpretation Global fit performed by M. Anselmino et.al 2006: final results from 2002-2004 data NPB765 (2007)31 Talk given by A. Prokudin (International Workshop on Structure and Spectroscopy, Freiburg) 11/12/2018 Girisan Venugopal University of Bonn

Sivers: Interpretation The measured asymmetry on deuteron compatible with zero has been interpreted as Evidence for the Absence of Gluon Orbital Angular Momentum in the Nucleon S.J. Brodsky and S. Gardner, PLB643 (2006) 22 11/12/2018 Girisan Venugopal University of Bonn

Girisan Venugopal University of Bonn Summary & Outlook The measured asymmetries on a Deuteron target are very small and compatible with zero Theoretical work aimed at a first extraction of transversity and Sivers distribution function is ongoing by different groups COMPASS measures this year with proton target 11/12/2018 Girisan Venugopal University of Bonn

Girisan Venugopal University of Bonn Thank you all !!! 11/12/2018 Girisan Venugopal University of Bonn

Collins : Interpretation A.V. Efremov, K. Goeke and P. Schweitzer, PRD73 (2006) 094025 (ΔTq from Chiral Quark Soliton Model ) COMPASS 2002-2004 11/12/2018 Girisan Venugopal University of Bonn

Sivers: Interpretation M. Anselmino et al., PRD72 (2005) 094007 13 parameter fit to HERMES and COMPASS 2002 data to extract the Sivers functions comparison with COMPASS 2002-2004 data ok COMPASS 2002-2004 11/12/2018 Girisan Venugopal University of Bonn