Transverse Spin Structure with muon beam and Drell-Yan measurements at COMPASS

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1 Transverse Spin Structure with muon beam and Drell-Yan measurements at COMPASS Anna Martin Trieste University and INFN on behalf of the COMPASS Collaboration SPIN-Praha Praha-009 ADVANCED STUDIES INSTITUTE SYMMETRIES AND SPIN

2 Transverse Spin Structure of the Nucleon international theoretical and experimental effort SIDIS: HERMES at DESY COMPASS at CERN spin experiments at JLab and future projects erhic, ELIC, ENC at FAIR N l γ l' X h hard pp scattering: spin experiments at RHIC / BNL and several future projects for Drell-Yan: COMPASS experiments at JParc / KEK Panda and PAX at FAIR / GSI Nica at JINR SPASCHARM at IHEP SPIN-Praha-009

3 OUTLINE the COMPASS experiment the spin structure of the nucleon transverse spin effects in SIDIS transversity and Collins asymmetry Sivers function and asymmetry future measurements at COMPASS transverse spin effects in Drell-Yan spin asymmetries COMPASS plans SPIN-Praha-009

4 Common Muon and Proton Apparatus for Structure and Spectroscopy fixed target experiment at the CERN SPS approved in 1997 with a broad physics programme hadron beams: hadron spectroscopy search for exotics pion/kaon polarizabilities... muon beam: nucleon spin structure ΔG/G helicity distributions transverse spin effects... SPS SPIN-Praha-009

5 Common Muon and Proton Apparatus for Structure and Spectroscopy fixed target experiment at the CERN SPS approved in 1997 with a broad physics programme data taking since 00: L: E. Zemlianishkina B. Pawlukiewicz today muon beam deuteron ( 6 LiD) polarised target proton (NH 3 ) polarised target hadron beam LH target L/T target polarisation 4:1 T: G. Pesaro, tomorrow 006 L target polarisation 007 L /T target polarisation 1:1 S. Grabmueller J. Bernhard muon beam: 160 GeV/c longitudinal polarisation -80% intensity 10 8 µ + /spill (4.8s/16.s) P. Jasinski T. Schlueter today SPIN-Praha-009

6 COMPASS spectrometer muon beam high energy beam large angular acceptance broad kinematical range two stages spectrometer Large Angle Spectrometer (SM1) Small Angle Spectrometer (SM) variety of tracking detectors to cope with different particle flux from θ = 0 to θ 00 mrad SciFi Straws Silicon DC Micromegas MWPC GEMs W45 E/HCAL SM E/HCAL MuonWall Polarised Target SM1 MuonWall ~ 50 m μ beam SPIN-Praha-009 ECT*, July 3, 009 RICH calorimetry, PID RICH detector F. Bradamante

7 COMPASS spectrometer muon beam radiator C 4 F 10 threshold: π ~ GeV/c K ~ 10 GeV/c Polarised Target μ beam SPIN-Praha-009 ECT*, July 3, 009 RICH RICH detector F. Bradamante

8 The Target System solid state target operated in frozen spin mode during data taking with transverse polarization, polarization reversal after ~ 4-5 days : 6 LiD (polarised deuteron) dilution factor f = 0.38 polarization P T = 50% two 60 cm long cells with opposite polarization 006: PTM replaced with the large acceptance COMPASS magnet (180 mrad) target cells 3 target cells 007: NH 3 (polarised protons) dilution factor f = 0.14 polarization P T = 90% SPIN-Praha-009

9 OUTLINE the COMPASS experiment the spin structure of the nucleon transverse spin effects in SIDIS transversity and Collins asymmetry Sivers function and asymmetry future measurements at COMPASS transverse spin effects in Drell-Yan spin asymmetries COMPASS plans SPIN-Praha-009

10 Structure of the Nucleon key process: Deep Inelastic Scattering E N l Semi-Inclusive DIS: the incident and scattered leptons, and at least one final state hadron are measured SPIN-Praha-009 γ q l' E target fragmentation current fragmentation σ h ln lhx h q σ the lepton interacts with a nucleon s parton exchanging a virtual photon q virtual photon four-momentum x lq lq = q /Mν q(x) D h q (z) ν = E E' y = ν/ E Inclusive DIS: only the incident and scattered leptons are measured Q = Q > 0 γ = Q z= E /ν Q >> M W =(P+q) >> M h /ν

11 COMPASS kinematics 160 GeV/c muon beam DIS event selection: Q >1 (GeV/c) 0.1<y<0.9 W>5 GeV/c dn/dq COMPASS 007 transverse proton data preliminary Q (GeV/c) SPIN-Praha-009 dn/dw wide kinematical range 10 COMPASS 007 transverse proton data preliminary W (GeV/c )

12 COMPASS kinematics 160 GeV/c muon beam DIS event selection: Q >1 (GeV/c) 0.1<y<0.9 W>5 GeV/c Hadron selection p T >0.1 GeV/c z>0. SPIN-Praha-009 h T dn/dz dn/dp COMPASS 007 transverse proton data preliminary p h T COMPASS 007 transverse proton data preliminary (GeV/c) z

13 Structure of the Nucleon three distribution functions are necessary to describe the quark structure of the nucleon at LO: q(x) : number density or unpolarised distribution f 1 probability density of finding a quark with a fraction x of the longitudinal momentum of the parent nucleon Δq(x) = q unpolarised -q : longitudinal polarization or helicity distribution DIS (CERN, HERA, SLAC) g 1 and hard nucleon scattering measurements in a longitudinally polarised nucleon, probability density of finding a quark with a momentum fraction x and spin parallel to that of the parent nucleon very well known well known Δ T polarised q(x) = q -q DIS/SIDIS : transverse measurements polarization at or transversity distribution SLAC, EMC, SMC, HERMES, COMPASS h 1 in a transversely polarised nucleon, probability density of finding a quark with a momentum fraction x and spin parallel to that of the parent nucleon new poorly known SPIN-Praha-009

14 OUTLINE the COMPASS experiment the spin structure of the nucleon transverse spin effects in SIDIS transversity and Collins asymmetry Sivers function and asymmetry future measurements at COMPASS transverse spin effects in Drell-Yan spin asymmetries COMPASS plans SPIN-Praha-009

15 Transversity distribution proposed in 79 (Ralston & Soper), reappraised in 90 properties Δ T q(x) Δq(x) probes the relativistic nature of quark dynamics no contribution from the gluons simple Q evolution positivity (Soffer) bound first moment: tensor charge sum rule for transverse spin in Parton Model framework it is related to GPD s is chiral-odd more difficult to measure ΔT q q + Δq Δ 1 T q dx Δ 1 = Δ T T q(x) q + L q + L g Bakker, Leader, Trueman, PRD 70 (04) SPIN-Praha-009

16 How to measure Δ T q(x) Δ T q(x) is chiral-odd cannot be measured in inclusive DIS it can be measured in SIDIS on transversely polarized targets: the observable is the convolution of Δ T q(x) with another chiral-odd quantity several channels are being investigated G. Pesaro talk in ln l hx the chiral-odd partner is the Collins fragmentation function, which describes a possible left-right asymmetry of the hadrons in the hadronization process of a transversely polarized quark SPIN-Praha-009

17 Collins asymmetry ln l hx the observable is the so-called Collins asymmetry, an azimuthal modulation in the cross-section Φ C of the type sinφ C Φ C = φ h + φ S - π φ h azimuthal angle of the hadron, φ S azimuhtal angle of the spin of the nucleon A Coll transversity eq q Δ T q eq q recently measured by HERMES (proton target) and COMPASS (deuteron and proton targets) convincing evidence that it is non zero from the proton data q Δ D 0 T h q D h q Collins FF SPIN-Praha-009

18 Collins asymmetry SIDIS results clear non-zero effects first seen by HERMES on p proton target SPIN-Praha-009

19 Collins asymmetry proton target SIDIS results clear non-zero effects first seen by HERMES on p ~ zero asymmetries measured by COMPASS on d over the whole x-range understood as u d cancellation COMPASS final deuteron target SPIN-Praha-009

20 Collins asymmetry SIDIS results clear non-zero effects first seen by HERMES on p ~ zero asymmetries measured by COMPASS on d over the whole x-range understood as u d cancellation independent measurement of Collins effect using e + e π X BELLE data (first measurements from LEP data) + π proton target COMPASS final deuteron target SPIN-Praha-009

21 Collins asymmetry SIDIS results clear non-zero effects first seen by HERMES on p independent measurement of Collins effect using e + e π X BELLE data (first measurements from LEP data) + π proton target COMPASS final ~ zero asymmetries measured by COMPASS on d over the whole x-range understood as u d cancellation deuteron target first extraction of the Collins FF and of transversity using HERMES e - p COMPASS μ + d BELLE e + e - data M. Anselmino et al., arxiv : v 1[hep -ph] 3 Dec 008 SPIN-Praha-009

22 Collins asymmetry SIDIS results clear non-zero effects first seen by HERMES on p independent measurement of Collins effect using e + e π X BELLE data (first measurements from LEP data) + π proton target COMPASS final ~ zero asymmetries measured by COMPASS on d over the whole x-range understood as u d cancellation deuteron target first extraction of the Collins FF and of transversity using HERMES e - p COMPASS μ + d BELLE e + e - a major result! data M. Anselmino et al., arxiv : v 1[hep -ph] 3 Dec 008 SPIN-Praha-009

23 Collins asymmetry new preliminary results from COMPASS proton target run in 007 (much interest in the international community) DIS 009 COMPASS 007 preliminary large signal in the valence region SPIN-Praha-009

24 Collins asymmetry new preliminary results from COMPASS proton target run in 007 good agreement with HERMES in the x overlap region an important and not obvious result, given the different energies of HERMES and COMPASS 160 GeV/c 8 GeV/c W>1.8 GeV/c SPIN-Praha-009

25 Collins asymmetry conclusion: transversity is different from zero and can be measured in SIDIS thanks to the Collins effect by A. Bacchetta the work has just started SPIN-Praha-009

26 Collins asymmetry conclusion: transversity is different from zero and can be measured in SIDIS thanks to the Collins effect by A. Bacchetta the work has just started more data are needed to map the Q, x, z and p dependence SPIN-Praha-009

27 Structure of the Nucleon taking into account the quark intrinsic transverse momentum k T, at leading order 8 PDFs are needed for a full description nucleon polarization U L T quark polarization U f 1 number density f 1T L g 1 helicity g 1T TMDs interesting properties T h 1 h 1L h 1 transversity h 1T SPIN-Praha-009

28 SIDIS cross-section section leading order Collins asymmetry f 1 h 1 h 1L q h 1 f 1T h 1T g 1 g 1T d 6 σ 4πα 4 Q sx {[1+(1 y) + ] e f q (x)d q (z, p S ph (1 y) 4z M M N h sin(φ h ) + S p ) λ 1 ph + λ S y(1 y) T cos( φ φ ) h S zm q q S y(1 y) e g (x)d (z,p ) e L q q 1 1 h q q 1 1 h ph (1)q q (1 y) cos(φ h ) e h (x)h (z,p ) q q 1 1 h 4z M M N h (1)q q L e h (x)h (z,p ) q q 1L 1 h ) h q q (1 y) T sin( φ + φ e h (x)h (z,p ) h S q q 1 1 h zm h 1 ph (1)q q (1 y+ y ) T sin( φ φ e f (x)d (z,p ) h S q q 1T 1 h zm 3 N ph ()q q (1 y) T 3 sin( 3φ φ e h (x)h (z,p ) h S q q 1T 1 h 6z M M N h + S ) + S ) e g (x)d (z,p e q q 1T 1 h N (1)q q ) } unpol L pol. target T pol. target pol. beam & target SPIN-Praha-009

29 SIDIS cross-section section presently, the most famous TMD PDFs are: the Boer-Mulders function the Sivers function correlates the nucleon spin and the quark k t (tr. pol. N) and h 1T h 1 correlates the quark transverse spin and the quark k t (unpol. N) f 1T which correlates the quark transverse spin and the quark k t (tr. pol. N) d 6 σ 4πα 4 Q sx {[1+(1 y) + ] e f q (x)d q (z, p S ph (1 y) 4z M M N h sin(φ h ) + S p ) λ 1 ph + λ S y(1 y) T cos( φ φ ) h S zm q q S y(1 y) e g (x)d (z,p ) e L q q 1 1 h q q 1 1 h ph (1)q q (1 y) cos(φ h ) e h (x)h (z,p ) q q 1 1 h 4z M M N h (1)q q L e h (x)h (z,p ) q q 1L 1 h ) h q q (1 y) T sin( φ + φ e h (x)h (z,p ) h S q q 1 1 h zm h 1 ph (1)q q (1 y+ y ) T sin( φ φ e f (x)d (z,p ) h S q q 1T 1 h zm 3 N ph ()q q (1 y) T 3 sin( 3φ φ e h (x)h (z,p ) h S q q 1T 1 h 6z M M N h + S ) + S ) e g (x)d (z,p e q q 1T 1 h N all important for assessing the orbital angular momentum of the quarks (1)q q ) } SPIN-Praha-009

30 Sivers function f 1T correlates the nucleon spin and the quark k t (transversely polarised nucleon) proposed in 1990 initially thought to be zero (Collins, 1993) resurrected in 00 (Brodsky, Hwang, Schmitt) FSI, gauge link if different from zero, it should be responsible of a modulation in Φ S = φ h - φ S of the hadron produced inclusively on a transversely polarized target Φ S A Siv q q e q e q q f 1T f 1 D D q 1 q 1 SPIN-Praha-009

31 SIDIS cross-section section the Collins and the Sivers terms depend on different angles and both asymmetries can be extracted from the same data (as done by COMPASS and HERMES) all the structure functions can be extracted simultaneously from the different azimuthal modulations SPIN-Praha-009 d 6 σ 4πα 4 Q sx {[1+(1 y) + ] e f q (x)d q (z, p S ph (1 y) 4z M M N h sin(φ h ) + S p ) λ 1 ph + λ S y(1 y) T cos( φ φ ) h S zm q q S y(1 y) e g (x)d (z,p ) e L q q 1 1 h q q 1 1 h ph (1)q q (1 y) cos(φ h ) e h (x)h (z,p ) q q 1 1 h 4z M M N h (1)q q L e h (x)h (z,p ) q q 1L 1 h ) h q q (1 y) T sin( φ + φ e h (x)h (z,p ) h S q q 1 1 h zm h 1 ph (1)q q (1 y+ y ) T sin( φ φ e f (x)d (z,p ) h S q q 1T 1 h zm 3 N ph ()q q (1 y) T 3 sin( 3φ φ e h (x)h (z,p ) h S q q 1T 1 h 6z M M N h + S ) + S ) Collins asymmetry e g (x)d (z,p e q q 1T 1 h N (1)q Sivers asymmetry q ) }

32 Sivers asymmetry SIDIS results strong signal seen by HERMES in π + production on transversely polarized protons protontarget no signal seen by COMPASS on transversely polarized deuterons, interpreted as u- and d-quark cancellation (as for the Collins asymmetry) PLB 673 (009) 17 π ± K ± first extractions of the Sivers function K 0 from pion SIDIS data good fits to the existing data SPIN-Praha-009

33 Sivers asymmetry COMPASS preliminary results from 007 proton data Transversity 008 no signal over the whole x range marginal compatibility with HERMES π + data COMPASS 007 preliminary an intriguing result SPIN-Praha-009

34 comparison with recent predictions based on HERMES p data from Anselmino et al. from Goeke et al. SPIN-Praha-009

35 Sivers asymmetry COMPASS preliminary results from 007 proton data Transversity 008 no signal over the whole x range marginal compatibility with HERMES π + data COMPASS 007 preliminary an intriguing result new high energy data are necessary to clarify the energy dependence suggested by the COMPASS result new data will also allow to perform precise measurements of the K Sivers asymmetry SPIN-Praha-009

36 Transverse Spin Effects in SIDIS one year (150 days) of data taking at COMPASS with the transversely polarised proton target, and the present spectrometer and muon beam, will allow to improve the knowledge of transversity and will clarify the Sivers issue projected statistical errors for the Sivers asymmetry HERMES π +, DIS007 COMPASS h +, Transversity 008 COMPASS h +, projection 160 Gev proton target SPIN-Praha-009

37 Transverse Spin Effects in SIDIS one year (150 days) of data taking at COMPASS with the transversely polarised proton target, and the present spectrometer and muon beam, will allow to improve the knowledge of transversity and will clarify the Sivers issue REQUEST TO CERN SPSC: (CERN-SPSC SPSC-I-38, 1 January 009; CERN-SPSC /M-769, Addendum to the COMPASS Proposal, June 009) run one full year with transversely polarised proton target in 010, to perform new precise measurements of transverse spin effects in SIDIS and one full year with longitudinally polarised proton target this new measurement will mostly conclude the exploratory phase of transverse spin effects in SIDIS more systematic measurements in SIDIS will be needed in future, and CERN can have an important role F. Kunne talk agreed! SPIN-Praha-009

38 OUTLINE the COMPASS experiment the spin structure of the nucleon transverse spin effects in SIDIS transversity and Collins asymmetry Sivers function and asymmetry future measurements at COMPASS transverse spin effects in Drell-Yan spin asymmetries COMPASS plans SPIN-Praha-009

39 The Drell-Yan process in hadron-hadron scattering a quark and an antiquark annihilate in a timelike virtual photon which decays in a lepton pair the phase-space is defined by the variables x 1 and x s p L M total energy longitudinal momentum of the lepton pair invariant mass of the lepton pair SPIN-Praha-009

40 The Drell-Yan process DY cross section ~ convolution of PDFs l l' complementary information N γ h h SIDIS cross section ~ convolution of PDF and FF SPIN-Praha-009

41 The Drell-Yan process to access TMDs measurement of azimuthal distributions of the lepton plane Collins-Soper frame θ, φ angular distribution (unpolarized) at LO and in the collinear approximation one gets QCD corrections: Lam-Tung sum rule SPIN-Praha-009

42 The Drell-Yan process Lam-Tung sum rule: large violations of the sum rule seen in experiments at CERN (NA10) and FNAL (E615) cos φ modulation, up to 30% such a modulation could arise from the product of Boer-Mulders functions: (beam PDF target PDF) E615, Conway et al (1989) SPIN-Praha-009

43 The Drell-Yan process a long history of successful DY experiments up to now, none with beam and/or polarized target to access the spin dependent PDFs COMPASS can do it SPIN-Praha-009

44 The Drell-Yan process on a transversely polarized target Collins-Soper frame θ, φ φ S target transverse spin vector S T wrt lepton plane access spin dependent TMDs SPIN-Praha-009 sum over q (qbar) flavour

45 The Drell-Yan process in π - p in the valence region, u quark-dominance σ DY f u π f u p where extraction of the u-quark Sivers function model dependent extraction of transversity and Boer-Mulders functions Testing non-perturbative QCD confronting Drell-Yan and SIDIS results provides a crucial test of non-perturbative QCD check the predictions: due to the T-odd character of the Sivers and Boer- Mulders functions STRONG PHYSICS CASE SPIN-Praha-009

46 Why Drell-Yan at COMPASS? COMPASS is a multi-purpose spectrometer: availability of both muon and pion beams unique polarized target, well suitable for transversity studies a muon detection system spectrometer with wide angular acceptance COMPASS plans measurement of DY with high energy π beam and transversely polarised NH3 target soon (LoI, CERN-SPSC SPSC-I-38, 1 January 009, Proposal in preparation) not an easy experiment in the COMPASS environment many MC simulations test beam in 007 test beam in 008 test beam in 009 SPIN-Praha-009

47 Drell-Yan at COMPASS signal and background the dimuon mass spectrum is known from past DY experiments (spectrum from NA50) SPIN-Praha-009 M > 4 GeV/c : safe region to study Drell-Yan

48 Drell-Yan at COMPASS beam tests 007: 160 GeV/c π - beam, NH 3 target, no absorber to test the feasibility of the measurement ~ dimuon events in <1 hours of data taking SPIN-Praha-009

49 Drell-Yan at COMPASS beam tests 008: 160 GeV/c π - beam, at higher beam intensity too high detector occupancy: absorber 009: an prototype absorber will be placed after the (dummy) target to check detectors occupancy, extent of the combinatorial background, mass and vertex resolution, and radiation issues SPIN-Praha-009

50 Drell-Yan at COMPASS COMPASS is a radiation supervised area the dose limits in the control room must stay < 3 μsv/h all the region around target and absorber must be shielded SPIN-Praha-009

51 Drell-Yan at COMPASS experimental apparatus ~ the existing one with the existing PT, plus an absorber just after the target an improved muon trigger in the LAS in ~50% of the events, both the muons are detected in the LAS new hodoscopes already being built and to be used in the new SIDIS measurements SPIN-Praha-009

52 Drell-Yan at COMPASS expected precision π - beam of 190 GeV/c the acceptance is higher in the valence quarks region, where SSA are expected to be larger preliminary preliminary preliminary preliminary SPIN-Praha-009

53 Drell-Yan at COMPASS expected precision π - beam of 190 GeV/c with the transversely polarized NH 3 target (10 cm long) and a beam intensity of particles/second, a luminosity of cm - s -1 can be obtained in two years of data taking one can collect more than DY events in the region 4 < Mμμ < 9 GeV/c the statistical error in the measured asymmetries is expected to be sin( φ δa S φ) 1 % SPIN-Praha-009

54 Drell-Yan at COMPASS expected precision π - beam of 190 GeV/c two years of data taking predictions for the Sivers asymmetry (based on HERMES Sivers asymmetry in SIDIS) in the COMPASS phase-space 4 < M < 9 GeV/c SPIN-Praha-009

55 further Drell-Yan measurements what if COMPASS could dispose of a RF separated beam? ( p, p ) p / K would be possible in the future in this case thus extraction of the Sivers function ( K -, p ) extraction of valence Sivers, transversity and Boer-Mulders functions access to unpolarized kaon distribution functions (poorly known) SPIN-Praha-009

56 Drell-Yan at COMPASS - conclusion DY is a well understood process it provides unique information of the hadron structure and dynamics, and of TMD PDFs, complementary to SIDIS COMPASS allows to probe the valence quark region, where the TMD effects are expected to be larger the π - p part of the program can start soon: COMPASS can provide the first ever DY data on a polarized target and test the prediction on the sign of the Sivers function Proposal in preparation 1 st phase: π - p collisions using the polarized NH 3 target and possibly using a long liquid H target p p K - nd phase: collisions and collisions if RF separated beam will be available p SPIN-Praha-009

57 COMPASS plans short term Starting in 010: SIDIS measurements with transversely pol protons (1 year) SIDIS measurements with longitudinally pol protons (1year) Proposal in preparation DY on transversely polarised p target DVCS with LH target and polarised p target Hadron program: not over further measurements mainly depending on the results from the data taking SPIN-Praha-009