Measurement of the Polarised Drell-Yan process at COMPASS M arcia - PowerPoint PPT Presentation

COMPASS Measurement of the Polarised Drell-Yan process at COMPASS M´ arcia Quaresma, LIP - Lisbon on behalf of the COMPASS Collaboration 2 nd June 2014, MESON 2014 - Krakow Co-financed by: 2nd June 2014, MESON 2014 - Krakow 1 / 16

COMPASS Transverse Momentum Dependent Parton Distribution Functions - TMD PDFs The nucleon structure in leading order QCD, taking into account k T , is described by 8 PDFs for each quark flavour. Sivers, Boer-Mulders, transversity and pretzelosity are accessible via either the single polarised Drell-Yan measurement or the transversely polarised SIDIS. quark NUCLEON nucleon unpolarized longitudinally pol. transversely pol. f f unpolarized 1 1T k T number density Sivers longitudinally pol. g g 1L 1T QUARK k T helicity transversely pol. h h 1 1 transversity k T k T h h k T 1T 1L Boer−Mulders pretzelosity 2nd June 2014, MESON 2014 - Krakow 2 / 16

COMPASS Polarised Drell-Yan process P a ( b ) , beam (target) hadron momentum s = ( P a + P b ) 2 , centre of mass energy squared Quark-antiquark annihilation, with x a ( b ) = q 2 / (2 P a ( b ) · q ), momentum fraction carried dilepton production H a ( P a ) by the quark from H a ( b ) u ( k a ) ¯ x F = x a − x b , Feynman x l − ( l ) X Q 2 = q 2 = M µµ 2 = sx a x b , dimuon invariant mass γ ∗ ( q ) l + ( l ′ ) u ( k b ) squared H b ( P b , S ) k T a ( b ) , quark intrinsic transverse momentum This process is an excellent tool to access TMD PDFs: No fragmentation functions involved, but the convolution of two PDFs. The use of a negative pion beam allows the annihilation between the valence quark ¯ u from π − with a valence quark u from proton to be dominant. ֒ → All the TMD PDFs are expected to be sizeable in the valence quark region The QCD TMD approach is valid in the region Q ( M µµ > 4 GeV / c 2 ) ≫ � p T � ∼ 1 GeV / c A drawback of this process is its very low cross-section (fraction of nb for M µµ > 4 GeV / c 2 ) ֒ → Imposing an experiment with high luminosity 2nd June 2014, MESON 2014 - Krakow 3 / 16

COMPASS Azimuthal Asymmetries Considering an unpolarised π − beam and a transversely polarised proton target the σ DY at LO can be written as: d 4 qd Ω = α 2 d σ cos 2 φ ) + |− → σ U { (1 + D [sin 2 θ ] A cos 2 φ S T | [ A sin φ S em Fq 2 ˆ sin φ S U T + D [ sin 2 θ ] ( A sin (2 φ + φ S ) sin (2 φ + φ S ) + A sin (2 φ − φ S ) sin (2 φ − φ S ))] } T T Each angular modulation present in the DY cross-section has an amplitude that contains the convolution of two TMD PDFs. These amplitudes are accessed via the measurement of the angular azimuthal asymmetries between the two oppositely transversely polarised target cells. Each asymmetry relates to: A cos 2 φ Boer-Mulders h ⊥ 1 ( π ) ⊗ Boer-Mulders h ⊥ 1 ( p ) U A sin φ S unpolarised PDF f 1 ( π ) ⊗ Sivers f ⊥ 1 T ( p ) T A sin (2 φ + φ S ) Boer-Mulders h ⊥ 1 ( π ) ⊗ pretzelosity h ⊥ 1 T ( p ) T A sin (2 φ − φ S ) Boer-Mulders h ⊥ 1 ( π ) ⊗ transversity h 1 ( p ) T 2nd June 2014, MESON 2014 - Krakow 4 / 16

COMPASS COMPASS @ CERN COmmon Muon Proton Apparatus for Structure and Spectroscopy Fixed target experiment at the end of M2 SPS beam line Around 240 collaborators from 13 countries and 23 institutes 2nd June 2014, MESON 2014 - Krakow 5 / 16

COMPASS Experimental setup Polarised target, NH 3 dilution factor 22% polarisation up to 90% ) S A S ( r e t e m o r t c e p S e l g n A l l a m S ) S A L ( r e t e Large angular acceptance ( ± 180 mrad) m o r t c e Two target cells ( NH 3 ) with opposite p S e polarisations transverse to the beam l g n A e g r a L N N d u Target spin reversal Beam π every few days π − @ 190 GeV / c N u N d 2nd June 2014, MESON 2014 - Krakow 6 / 16

COMPASS Experimental setup - Hadron absorber and beam plug A hadron absorber made of alumina will be placed downstream of the target to stop the hadrons and with a beam plug in the centre to stop the non-interacting beam. The hadron absorber will introduce multiple scattering on muons and there will be a degradation of the resolutions. To partially solve this problem a vertex detector is introduced in the first part of the absorber. In parallel to the polarised DY measurements, unpolarised DY measurements will also be performed using nuclear targets, W and some thin lighter materials: 2nd June 2014, MESON 2014 - Krakow 7 / 16

COMPASS TMDs (Non-)Universality – DY <> SIDIS There is a theoretical prediction that Sivers ( f ⊥ 1 T ) and Boer-Mulders ( h ⊥ 1 ) functions must change sign when accessed from DY or SIDIS due to the fact that these functions are time-reversal odd functions. SIDIS DY µ µ γ * X p h f ⊥ 1 T ( x , k T ) | DY = − f ⊥ 1 T ( x , k T ) | SIDIS h ⊥ 1 ( x , k T ) | DY = − h ⊥ 1 ( x , k T ) | SIDIS The experimental confirmation of this sign change is considered a crucial test of the QCD TMD approach. 2nd June 2014, MESON 2014 - Krakow 8 / 16

COMPASS Sivers asymmetry - COMPASS vs HERMES COMPASS 2010 proton data Siv COMPASS positive hadrons x <0.032 p 0.1 0.1 0.1 COMPASS positive hadrons x >0.032 A π + HERMES PRL 103 (2009) 0.05 0.05 0.05 0 0 0 −0.05 −0.05 −0.05 0.5 1 0.5 1 1.5 Siv COMPASS negative hadrons x <0.032 p 0.1 0.1 0.1 A COMPASS negative hadrons x >0.032 π - HERMES PRL 103 (2009) 0.05 0.05 0.05 0 0 0 −0.05 −0.05 −0.05 0.5 1 0.5 1 1.5 −2 −1 10 10 h p (GeV/ c ) x z The Q 2 coverage between the 2 experiments is different: T COMPASS: x > 0 . 032, ❁ Q 2 ❃ = 8.7 GeV / c 2 (PLB 717 2012) x > 0 . 032, ❁ Q 2 ❃ = 2.4 GeV / c 2 (PRL 103 2009) HERMES: For h − the asymmetry is zero, for h + the asymmetry is positive and slightly different between the two experiments, being the difference assigned to the Q 2 coverage. 2nd June 2014, MESON 2014 - Krakow 9 / 16

COMPASS Q 2 vs x domain in COMPASS In COMPASS we have the opportunity to access the TMD PDFs from both DY and SIDIS processes. Sivers asymmetry from SIDIS - h + and h − 2 (GeV/c) 1 Q 2 vs. x at COMPASS 2 10 0.9 2 Drell-Yan (MC) Q 0.8 0.7 0.6 0.5 SIDIS (2010 proton data) 10 0.4 0.3 0.2 0.1 1 0 -3 -2 -1 10 10 10 x There is a phase space overlap between the two measurements. However to properly compare the extracted TMDs, their Q 2 evolution must be taken into account. Recently the SIDIS analysis was performed in 4 Q 2 bins, one of the bins being Q 2 > 16 (GeV / c ) 2 , the so-called DY range. δ A sin( φ h − φ S ) ≈ 0 . 01 for both h + and h − in SIDIS for Q 2 > 16 (GeV / c ) 2 , same statistical UT error as expected for Sivers from DY. 2nd June 2014, MESON 2014 - Krakow 10 / 16

COMPASS Feasibility of the experiment 5 10 ) 2 # J/ = 6787 109 ψ ± Events/(20 MeV/c COMPASS DY 2009 M = 3.042 0.004 GeV/c 2 ± Total In 2009 a 3 days data taking beam test was 4 10 2 = 0.217 0.003 GeV/c σ ± J/ ψ done using a hadron absorber prototype, two M preliminary Continuum 3 polyethylene target cells and a negative pion 10 ψ ’ beam at 190 GeV / c with an intensity of 2 10 1 . 5 × 10 7 π/ s . A double trigger based on calorimeter signals was also used. 10 The analysis confirmed the expectations. 1 The J /ψ yields were confirmed considering −1 10 0 1 2 3 4 5 6 7 8 the low efficiencies involved. The mass and 2 M (GeV/c ) µ µ the mass resolution were in agreement with 700 Events/(1 cm) COMPASS DY 2009 the MC simulations. 600 2 M >2.5 GeV/c In the future the trigger will be based on µ µ preliminary 500 hodoscopes with a high efficiency, purity and target pointing capability. 400 300 The two target cells and the beam plug are distinguishable even if the absorber was not 200 ideal. For the future the Z vtx resolution will 100 be better because of the better absorber and of the inclusion of the vertex detector. -250 -200 -150 -100 -50 0 50 100 z (cm) 2nd June 2014, MESON 2014 - Krakow 11 / 16

COMPASS 2014/2015 Geometrical acceptance The dimuons geometrical acceptance in the HMR ( M µµ > 4 GeV / c 2 ) is 39%. Dimuon p acceptance x acceptance Dimuon mass acceptance F T 0.6 0.6 1 0.9 0.55 0.55 0.8 0.5 0.5 0.7 0.45 0.45 0.6 0.4 0.4 0.5 0.4 0.35 0.35 0.3 0.3 0.3 0.2 0.25 0.25 0.1 0.2 0.2 0 4 5 6 7 8 9 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 -1 -0.5 0 0.5 1 2 p (GeV/c) x M (GeV/c ) F T acceptance acceptance cos θ acceptance φ φ CS CS S 1 0.6 0.6 0.9 0.55 0.55 0.8 0.5 0.5 0.7 0.45 0.45 0.6 0.5 0.4 0.4 0.4 0.35 0.35 0.3 0.3 0.3 0.2 0.25 0.25 0.1 0 0.2 0.2 -1 -0.5 0 0.5 1 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 cos φ φ θ CS CS S For the extraction of the asymmetries the differential acceptance must be taken into account and to be well known. 2nd June 2014, MESON 2014 - Krakow 12 / 16