Polarised Drell-Yan Process in the COMPASS Experiment

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1 Polarised Drell-Yan Process in the COMPASS Experiment Ma rcia Quaresma, LIP - Lisbon on behalf of the COMPASS collaboration 1th April 16, DIS 16 Hamburg COMPASS Ma rcia Quaresma (LIP) CERN/FIS-NUC/17/15 DIS 16 1th April 16 1 / 14

2 COMPASS experiment at CERN COmmon Muon Proton Apparatus for Structure and Spectroscopy Fixed target experiment at the end of M SPS beam line Around 4 collaborators from 13 countries and institutes Data taking since and approved up to 18 Márcia Quaresma (LIP) DIS 16 1 th April 16 / 14

3 Nucleon structure - TMD PDFs The nucleon structure in leading order QCD, taking into account k T, is described by 8 TMD PDFs for each quark flavour. Quark longitudinally polarised unpolarised transversely polarised f 1 h 1 unpolarised unpolarised PDF g 1 h 1L Nucleon longitudinally polarised helicity f 1T g 1T h 1 transversely polarised worm-gear T h 1T Sivers transversity Boer-Mulders worm-gear L pretzelosity k T COMPASS contribution: Studying TMD PDFs dependence on several kinematic variables Accessing them through two different processes, SIDIS and DY Theoretical prediction of the Sivers and Boer-Mulders sign change when extracted from SIDIS or from DY Crucial test of the QCD TMD approach DY is an excellent tool to access TMD PDFs at COMPASS: All the TMD PDFs are expected to be sizeable in the valence quark region - dominant region when π is used as beam QCD TMD approach valid for Q (M µµ > 4 GeV/c ) p T 1 GeV/c Márcia Quaresma (LIP) DIS 16 1 th April 16 3 / 14

4 DY and SIDIS processes DY and SIDIS cross-sections are written in terms of angular modulations. The amplitude of each angular modulation contains: l lʹ π PDF X γ * l γ * p PDF X DY: PDF PDF l + p PDF FF h X SIDIS: PDF FF Two transversely and oppositely polarised target cells: π N u N u Experimental extraction of the amplitudes: N d Target spin reversal every few days polarisation reversal each weeks to cancel possible systematics N d Measurement of the azimuthal asymmetries: N u (φ S, φ h ) N d (φ S, φ h ) N d (φ S, φ h ) N u (φ S, φ h ) Márcia Quaresma (LIP) DIS 16 1 th April 16 4 / 14

5 All the asymmetries are extracted together using the Unbinned Maximum Likelihood Method. Márcia Quaresma (LIP) DIS 16 1 th April 16 5 / 14 DY and SIDIS cross sections in terms of LO asymmetries DY: dσ d 4 qdω = α Φq ˆσ U {(1 + cos (θ) + sin (θ)a cos(φ) UU +S T [(1 + cos(θ))a sin(φ S ) sin(φ S ) cos(φ)) + sin (θ) (A sin(φ+φ S ) sin(φ + φ S ) + A sin(φ φ S ) sin(φ φ S ))]} SIDIS: dσ dxdydzdψdφ h dp ht Φ = 4 (P a P b ) Ma M b, ˆσ U = F 1 UU = α y γ (1 + xyq (1 ε) x ) σ U {1 + ε cos(φ h )A cos(φ h ) UU +S T [sin(φ h φ S )A sin(φ h φ S ) + ε sin(φ h + φ S )A sin(φ h +φ S ) +ε sin(3φ h φ S )A sin(3φ h φ S ) ] +S T P l [ 1 ε cos(φ h φ S )A cos(φ h φ S ) LT ]} ε = (1 y 1 4 γ y ) / (1 y + 1 y γ y ), γ = Mx Q, σ U = F UU,T l lʹ P ht H A H B S T hadron plane l φ S P h y y S T φ S θ l + H A φ h q T lepton plane x q z φ z x y x z

6 LO asymmetries in DY and SIDIS and TMD PDFs A cos(φ CS ) UU A sin(φ S ) A sin(φ CS φ S ) A sin(φ CS +φ S ) DY: h q 1,π hq 1,p Boer-Mulders f q 1,π f q 1T,p Sivers h q 1,π hq 1,p Transversity h q 1,π hq 1T,p pretzelosity A cos(φ h) UU A sin(φ h φ S ) A sin(φ h+φ S ) A sin(3φ h φ S ) SIDIS: h q 1 H1q h f q 1T Dh 1q h q 1 Hh 1q h q 1T Hh 1q A cos(φ h φ S ) g q LT 1T Dh 1q Nucleon Quark transversely longitudinally polarised polarised unpolarised longitudinally transversely unpolarised polarised polarised f 1 f 1T k T unpolarised PDF Sivers g 1 g 1T helicity worm-gear T transversity h 1 h 1L h 1 h 1T Boer-Mulders worm-gear L pretzelosity Márcia Quaresma (LIP) DIS 16 1 th April 16 6 / 14

7 COMPASS results on Sivers asymmetry COMPASS SIDIS proton data from 7 and 1 (PLB 744 (15) 5) p A Siv COMPASS π + x<.3.1 COMPASS π + x> HERMES π + PRL 13 (9) p A Siv COMPASS K x<.3 + COMPASS K x>.3 + HERMES K PRL 13 (9) 1 1 (GeV/c) Q DY and SIDIS phase-space overlap at large Q 1 Q vs. x at COMPASS Drell-Yan (MC) SIDIS (1 proton data) x z COMPASS: Q = 8.7 GeV/c HERMES: Q =.4 GeV/c p h (GeV/c) T x.3..1 The Q evolution has to be studied Márcia Quaresma (LIP) DIS 16 1 th April 16 7 / 14

8 SIDIS Data MultiDimensional Analysis Sivers asymmetry ) S - ϕ h sin(ϕ A D analysis: 4 Q, each divided in bins of x, z, p Th, W (also a 3D analysis was performed) + h 1<Q /(GeV/c) <4 h - z >. COMPASS preliminary Proton 1 data )4<Q sin(ϕ - ϕ A h S /(GeV/c) <6.5 )6.5<Q sin(ϕ - ϕ A h S.5 /(GeV/c) < ) S - ϕ h sin(ϕ A.5 Q /(GeV/c) > x z p (GeV/c) T W (GeV/c ) Márcia Quaresma (LIP) DIS 16 1 th April 16 8 / 14

9 COMPASS general purpose spectrometer Two stages spectrometer, wide angular acceptance, ±18 mrad Muon and hadron beams Polarised target (longitudinally and transversely polarised NH 3 and 6 LiD) About 35 detector planes Particles identification: calorimeters, RICH and µ Filters Márcia Quaresma (LIP) DIS 16 1 th April 16 9 / 14

10 Polarised DY programme Hadron absorber Polarised target Two cells of NH 3 Polarisation 8% Dilution factor % Hadron beam 19 GeV/c π beam (small contamination of K and p) Due to small cross-section, measurement requires high luminosity Hadron absorber downstream of target Stops hadrons and non interacting beam Degrade resolutions, two target cells, vertex detector Nuclear targets: Al and W unpolarised DY studies Márcia Quaresma (LIP) DIS 16 1 th April 16 1 / 14

11 Drell-Yan measurement - 14 pilot run NH 3 target not polarised, also Al and W targets I beam π/s 17 days of stable data taking used for the analysis First COMPASS unpolarised high mass DY data Events ongoing analysis M µµ >4 GeV/c COMPASS DY 14 Clear signal from all targets: NH 3, Al, W Reasonable Z vtx resolution Z vtx (cm) Márcia Quaresma (LIP) DIS 16 1 th April / 14

12 DY 14 pilot run - Results Events 6 1 COMPASS DY Total J/ψ Continuum ψ' Combinatorial Bkg ongoing analysis x ongoing analysis COMPASS DY 14 M µµ > 4 GeV/c M µµ (GeV/c ) Almost background free high mass DY events Unpolarised DY analysis is ongoing: x 1 Valence quarks region coverage Studies of the flavour dependent EMC effect (modification of quark distributions in nuclei) available data from past experiments not enough accurate Dedicated studies on the Lam Tung sum rule violation precision improvement w.r.t. past experiments Márcia Quaresma (LIP) DIS 16 1 th April 16 1 / 14

13 Polarised Drell-Yan measurement - 15 run 1 st World Experiment NH 3 polarised target (plus Al and W targets) I beam 1 8 π/s - very high intensity Four months of stable data taking are being analysed One period already produced M µµ > 4 GeV/c 8 µ + µ pairs expected from polarised target δa sin(φ S ).8% Models predict A sin(φ S ) from 5% to 1% Sivers asym: A sin(φ S ) = A (φ γ φ S ) = A N, the same pion PDF is used (PRD 45 (199) 349) P. Sun and F. Yuan, PRD 88 (13) 1141, x F = x π x p, p T < GeV/c Echevarria et al, PRD 89 (14) 7413, x F = x p x π, p T < 1 GeV/c Márcia Quaresma (LIP) DIS 16 1 th April / 14

14 Final remarks The first ever polarised DY measurement using π beam and proton polarised target was done by COMPASS in 15. These data are being analysed. Unique opportunity to extract the TMD PDFs from both SIDIS and DY in the same experiment. DY results will have a key contribution in the test of the Sivers sign change theoretical prediction. Unpolarised DY data from nuclear targets is being analysed, statistical improvement w.r.t. past experiments. The possibility to have a second year of polarised DY data taking in 18 is under consideration. First COMPASS DY polarised data preliminary results should be ready soon Márcia Quaresma (LIP) DIS 16 1 th April / 14