past and future experiments Spin physics in Drell-Yan processes: past and future experiments Michela Chiosso University of Torino – Dip. Fisica Generale I.N.F.N - Torino IWHSS 2008 Torino – April 2nd 2008

Drell-Yan dilepton production Target p This process is electromagnetic and exactly calculable Propagator of the virtual photon in the amplitude  factor M-2 in the cross section

Past Drell-Yan experiments This process was first described by Sydney Drell and Tung-Mow Yan in 1970. First quantitative Drell-Yan experiments: late 1970s  waiting for spectrometers able to measure pbarn cross-sections in the presence of much larger background. 1970s - 1980s - Omega, CERN (Corden at al, 1978, 1980) - CIP, FERMILAB (Anderson et al 1976, Hogan et al 1979) NA3, CERN (Badier et al 1979, 1989) CFS, FERMILAB (Lederman et al, 1982) CHFMNP, CERN (Antreasyan et al) E605, FERMILAB (Moreno et al, 1989) … 1980s – 1990s - E615, FERMILAB (Conway et al, 1989) - NA10, CERN (Anderson et al, 1984) E772, FERMILAB (McGaughey et al, 1994) NA51, CERN (Abreu et al, 1998) E866 (E.A. Hawker et al, 1998) …

Present and future Drell-Yan facilities RHIC collider S=(200)2 GeV2 J-PARC fixed target S= 60-100 GeV2 FAIR collider S=200 GeV2 fixed target S=30-80 GeV2 COMPASS fixed target S=100-400 GeV2 (CERN-SPS) NICA S=400 GeV2 (JINR) E906 fixed target S=230 GeV2 (FMI) CMS collider S=196 TeV2 (CERN-LHC) Les Bland Yuji Goto Klaus Peters This talk Alexander Sorin

Investigating PDFs with Drell-Yan processes Since many years Drell-Yan process has been playing a key role in the study of parton distribution functions (PDFs) Flavor asymmetry of nucleon parton distribution functions E772, E866, NA51, E906 p, k mesons parton distribution functions NA10, E615, NA3 Boer-Mulders transverse momentum dependent parton distribution Function (TMD PDFs) E615, NA10, E866 Recently it is drawing back attention as a unique tool to directly access spin dependent parton distribution functions Transversity Sivers function

Flavor asymmetry in the nucleon sea E772, E866, NA51, E906 Light sea-quarks flavor asymmetry

Pion quarks distribution function NA10, -----E615 P.J. Sutton, A.D. Martin, Phys.Rev.D, 45 (7) 1992 No available experimental data at small x (x≤0.2) to determine sea-quark distr. unambiguously Performed various fits with sea carrying an increasing fraction of pion momentum p+p- valence distr. are the same, but different contr. to DY through quark charge squared gluon distribution: Prompt photon production valence distribution: Fit to Drell-Yan data sea-quark distribution: Fit to Drell-Yan data

p, k mesons parton distribution functions Light mesons parton distribution functions p, k mesons parton distribution functions NA10, E615, NA3 pion structure function NA3 :

Angular distribution & Boer-Mulders PDF Unpolarized angular distribution in Collin-Soper frame λ=1,μ=ν=0 at LO, in collinear approximation  transversely polarized g, no transverse momenta: Lam-Tung relation: 1-l-n=0 NLO prediction: small deviation from for Pt<3GeV/c n ≠ 0: Boer-Mulders effect if PT2«M2 NLO corrections, at large qt

Angular distribution & Boer-Mulders PDF 1.There is a sizable cos2f asymmetry (n up to 0.3) in the unpolarized pion-induced Drell-Yan: the Lam-Tung sum rule is violated beyond the QCD-improved parton model. NA10 , E615 2. l = -1 at large xf E615 3. No azimuthal asymmetry in proton-induced Drell-Yan E866

Angular distribution & Boer-Mulders PDF Violation of Lam-Tung sum rule Boer-Mulders function can lead to azimuthal dependence

Angular distribution & Boer-Mulders PDF E866 at Fermilab 800 GeV/c p+d 800 GeV/c p+p No noticeable flavour asymmetry between Sea-quark Boer-Mulders function is relatively small

Spin dependent parton distribution functions Polarized Drell-Yan Transversity  direct access to without convolution with fragmentation function , like in SIDIS Sivers function Boer-Mulders function

Spin dependent parton distribution functions Double polarization ( ) transversity Single polarization ( ) Sivers Boer-Mulders transversity No polarization ( ) Boer-Mulders

Spin dependent PDFs with Drell-Yan processes What do we need to access spin dependent PDFs through DY? polarized beam + polarized target (beam) unpolarized beam + polarized target or Valence Larger asymmetries (SSA and DSA) in region with large valence quark content. High luminosity: very small DY cross section

Spin physics with Drell-Yan processes in COMPASS Measures of single-spin DY asymmetries: Polarised DY SSA ~ sin(f-fs) + sin(f+fs) Beam: p - p S=100÷400 GeV2 Polarized target: NH3 / 6LiD Polarization: >80% / >40% Dilution factor: 0.15 / 0.35 Luminosity Unpolarised DY ds ~ cos(2f)  · Using p- beam it is necessary to make an assumption connecting pion and proton PDFs p - p ~1031 cm-2 s-1

COMPASS spectrometer layout MW1 Beam PT ECAL1 HCAL1 Ibeam ~ 1108 p/s 25% of current PT Polarized target 180 mrad acceptance 2 cells, 15 cm each

Kinematic range M<J/y  dominated by semileptonic decay of charmed hadrons M>J/y Drell-Yan dilepton production major contribution M must be large enough to apply pQCD But production rate falls off rapidly with M Safe region: Out from resonances regions dominated by strong production mechanism

Kinematic range x1, x2 Sizeable single and double spin asymmetries in valence quark region COMPASS s=100-400 GeV2 M=4-9 GeV/c2 t=0.16-0.8 @ s=100 GeV2 t=0.08-0.4 @ s=200 GeV2 t=0.05-0.3 @ s=300 GeV2 t=0.04-0.2 @ s=400 GeV2

Kinematic range COMPASS S=100GeV2 S=300GeV2 S=200GeV2 x2 x2 x2 x1 x1 Q2 Q2

Kinematic range: COMPASS acceptance in “valence” region: 0.1  x1/2  0.5 x1vs x2 t Sensitive to Sivers effect at low PT: PT << Q PT

Kinematic range: COMPASS vs other experiments x2 vs x1 COMPASS E866 E615

Spin physics in Drell-Yan processes at COMPASS SUMMARY What… Spin dependent PDFs transversity, Sivers function, Boer-Mulders function How… 1·108 p- beam on NH3/6LiD polarized target expected luminosity: 1031 cm-2 sec-1 expected events rate: ~ 34000 in 150 days of run S=300 GeV2 , M(m+m-): 4-9 GeV/c When Beyond 2010 November 11-12, 2007 Test beam at CERN SPS: verified radiation conditions, PT and spectrometer peformances with high intensity hadron beam EOI in preparation