TRANSVERSE SPIN EFFECTS IN COMPASS Andrea Bressan University

TRANSVERSE SPIN EFFECTS IN COMPASS Andrea Bressan (University and INFN – Trieste) On behalf the COMPASS collaboration

COmmon Muon and Proton Apparatus for Structure and Spectroscopy NA 58 Czech Republic, Finland, France, Germany, India, Israel, Italy, Japan, Poland, Portugal, Russia Bielefeld, Bochum, Bonn, Burdwan, Calcutta, CERN, Dubna, Erlangen, Freiburg, Heidelberg, Helsinki, Lisbon, Mainz, Miyazaky, Moscow, Munich, Nagoya, Prague, Protvino, Saclay, Tel Aviv, Torino, Trieste, Warsaw 28 Institutes, ~230 physicists 10/25/202 Andrea Bressan 2

longitudinally polarised muon beam longitudinally or transversely polarised target calorimetry particle identification luminosity: ~5. 1032 cm-2 s-1 beam intensity: 2. 108 m+/spill (4. 8 s/16. 2 s) beam momentum: 160 Ge. V/c LHC SPS N 10/25/202 Andrea Bressan 3

The Spectrometer for the Muon Programme Trigger-hodoscopes μ Filter ECal & HCal 50 SM 2 TWO STAGE SPECTROMETER: RICH SM 1 6 Li. D MWPC Straws Target μ V Ge 0 16 10/25/202 m 0. 003 < x < 0. 5 10 -3 < Q 2 < 10 (Ge. V/c Gems Drift chambers Micromegas Sci. Fi Silicon Andrea Bressan LAT, PID 4

Polarized Target New COMPASS target magnet: 180 mrad geometrical acceptance excellent field homogeneity To match larger acceptance: new microwave cavity 3 target cells: reduction of false asymmetries Target material: NH 3 high polarisation very long relaxation time (∼ 4000 h) magnetic field rotation without polarisation loss Polarisation of NH 3 in 2007: -92%, +88%, -83% 10/25/202 Target Andrea Bressan Polarization reversed every week 5

2007 Transverse data taking statistics 2007 Compass Data taking Begin of run: 18 May 2007 End of run: 11 November 2007 Split between transverse and longitudinal target polarization: - m on tape for transverse (40. 0 x 1012) - m on tape for longitudinal (41. 5 x 1012) For the extraction of the asymmetries (this analysis) only a fraction of the full statistic available has been used : nearly 20% of the whole data sample available. 10/25/202 Andrea Bressan 6

DIS Event Selection DIS cuts: 10/25/202 Q 2>1 (Ge. V/c)2 0. 1<y<0. 9 W>5 Ge. V/c 2 Andrea Bressan 7

Hadron Selection All hadrons Track Length<10 X 0 Energy Deposit in HCALs>Thr. ( 4 Ge. V HCal 1 and 5 Ge. V Hcal 2 ) Only 1 HCAL fired p. T>0. 1 Ge. V/c z>0. 2 Total statistic for this analysis 10/25/202 Positive hadrons Negative hadrons 5703231 4505088 Andrea Bressan 8

Data quality checks Data taking stability is needed: A dedicated set of quality checks have been developed and applied to fulfill this condition Different estimators have been considered: the detector profiles stability the number of primary vertices per event the number of tracks per primary vertex beam particles per primary vertex the number of K 0 per primary vertex the reconstructed mass of the K 0 meson stability of many kinematical variables: ( zvtx, Em’, fm’, x. Bj Q 2, y, W, Ehad , fhad. Lab , qhad. Lab , fhad. GNS , qhad. GNS , pt) 10/25/202 Andrea Bressan 9

Mean of kinematical quantities 10/25/202 Andrea Bressan 10

Azimuthal modulations Collins and Sivers angles C = h - S’ S = h - S S ’ azimuthal angle of spin vector of fragmenting quark ( S’ = p - S) h azimuthal angle of hadron momentum 10/25/202 Andrea Bressan 11

SIDIS azimuthal asymmetries All possible 8 azimuthal asymmetries extracted at once. . Sivers Collins 6 further modulations M. Diehl, S. Sapeta, Eur. Phys. J C 41 (2005) 515 -533 hep-ph/0503023 10/25/202 Andrea Bressan 12

Asymmetry Extraction Splitting middle cell into two parts two couples of cells with opposite polarization two independent values for the asymmetries period Extraction: 2 D Binned Maximum Log-Likelihood Fit: eight by eight grid in fh and f. S; in each bin of the matrix one expects Nj counts : 10/25/202 Andrea Bressan 13

Asymmetry Extraction - II Separation of acceptance and spin dependent modulations: Coupling of two cells (u, d) with opposite polarization ( ) and two periods (p 1, p 2) with opposite target polarization: Reasonable assumption: 4 · 64 = 256 nonlinear equations 1 + 8 + 3 · 64 = 201 fit parameter, Poisson distribution to account for low statistics: Tests for systematic errors: • For false asymmetries: combination of cells with same polarization • Comparison of 5 estimators for asymmetry extraction included the one used in previous analysis (deuteron data) For this analysis: overall systematic error is 30% and 50% of the statistical error for Collins and Sivers respectively 10/25/202 Andrea Bressan 14

Collins asymmetry – proton data systematic errors ~ 0. 3 sstat at small x, the asymmetries are compatible with zero in the valence region the asymmetries are different from zero, of opposite sign for positive and negative hadrons, and have the same strength and sign as HERMES 10/25/202 Andrea Bressan 15

Compass proton data comparison with M. Anselmino et al. predictions 10/25/202 Andrea Bressan 16

Collins Final on Deuteron 10/25/202 Andrea Bressan 17

Sivers asymmetry – proton data systematic errors ~ 0. 5 sstat the measured symmetries are small, compatible with zero 10/25/202 Andrea Bressan 18

Sivers Final on Deuteron from COMPASS 10/25/202 Andrea Bressan 19

Sivers asymmetry– proton data comparison with the most recent predictions from M. Anselmino et al. ar. Xiv: 0805. 2677 10/25/202 Andrea Bressan 20

Results: Sivers asymmetry comparison with predictions from S. Arnold, A. V. Efremov, K. Goeke, M. Schlegel and P. Schweitzer, ar. Xiv: 0805. 2137 10/25/202 Andrea Bressan 21

Transverse L polarization 10/25/202 Andrea Bressan 22

Data Selection Secondary vertex downstream of primary vertex. PT > 23 Me. V/c to exclude e+e− pairs Proton and pion momenta > 1 Ge. V/c Q 2 > 1 (Ge. V/c)2 0. 1 < y < 0. 9 Use of RICH (2007 data) Λ decay distance DΛ > 7 σD Collinearity < 10 mrad 10/25/202 Andrea Bressan 23

Results with proton target ~60% higher statistics with respect deuteron data (after) Systematic errors have been estimated to be smaller than statistical errors from false polarization. No dependence on x. 10/25/202 Andrea Bressan 24

(Old) Results on Deuteron L L § Analysis for 2002 -2004 deuteron data (and no RICH ID) § Only statistical errors are shown (systematic effects not larger than the statistical errors). § Small tendency for Λ, but not significant for deuteron target. 10/25/202 Andrea Bressan 25

Summary First preliminary results of COMPASS 2007 proton transverse run: Collins Asymmetry: different from zero, comparable to HERMES agreement with predictions of Anselmino et al Sivers Asymmetry: small and compatible with zero within present statistical errors Accessing Collins from L: 10/25/202 small and compatible with zero within present statistical errors Andrea Bressan 26

Thank You 10/25/202 Andrea Bressan 27

The 3 rd Twist-2 structure function three quark distribution functions (DF) are necessary to describe the structure of the nucleon at LO q(x) f 1 q (x) unpolarised DF quark with momentum x. P in a nucleon well known – unpolarised DIS vector charge Dq(x) g 1 q(x) helicity DF quark with spin parallel to the nucleon sp in a longitudinally polarised nucleon known – polarised DIS axial charge DTq(x) = q↑↑(x) - q↑↓(x) h 1 q(x), tensor charge 10/25/202 transversity DF quark with spin parallel to the nucleon spin in a transversely polarised nucleon largely unknown ALL 3 OF EQUAL IMPORTANCE Andrea Bressan 28

Misura di ΔTq(x) Chiral-odd: requires another chiral-odd partner Inclusive DIS p p l+l-X lp l’h 1 h 2 X lp l’ΛX impossible direct measurement ΣΔTq(x) ·ΔTq(x) convolution with spin dependent fragment. func. SIDIS (e. g. COMPASS and HERMES ) Hard scattering (e. g. RHIC) • Drell-Yan • Single Spin Asym (e. g. p↑p→πX) 10/25/202 Andrea Bressan Hard scattering • Drell-Yan (e. g. GSI) 29

Transversity DF DTq(x) = q↑↑(x) - q↑↓(x) h 1 q(x), d. Tq(x) q=uv, dv, qsea quark with spin parallel to the nucleon spin in a transversely polarised nucleon Properties: • probes the relativistic nature of quark dynamics • no contribution from the gluons simple Q 2 evolution • Positivity: Soffer bound……………. . Soffer, PRL 74 (1995) • first moments: tensor charge………. • sum rule for transverse spin in Parton Model framework………… Bakker, Leader, Trueman, PRD 70 (04) • it is related to GPD’s • is chiral-odd: decouples from inclusive DIS 10/25/202 Andrea Bressan 30

SIVERS Mechanism The Sivers DF is probably the most famous between TMDs… gives a measure of the correlation between the transverse momentum and the transverse spin Requires final/initial state interactions of the struck quark with the spectator system and the interference between different helicity Fock states to survive time-reversal invariance Time-reversal invariance implies: …to be checked In SIDIS: 10/25/202 Andrea Bressan 31

Global Fits 10/25/202 Andrea Bressan 32

Global Fit 10/25/202 Andrea Bressan 33

First Extraction of DTq HERMES, COMPASS BELLE 10/25/202 Andrea Bressan 34

Global Analysis Stefano Melis DIS 10/25/202 Andrea Bressan 35

Sivers PDF 10/25/202 Andrea Bressan 36

2007 Transverse data taking statistics 2007 Compass Data taking Begin of run: 18 May 2007 End of run: 11 November 2007 Split between transverse and longitudinal target polarization: - m on tape for transverse (40. 0 x 1012) - m on tape for longitudinal (41. 5 x 1012) Data taking Period Target Polarization Week 26 whole + - + data sample Week 31 -+of the available. Week 27 - + Week 39 +-+ Week 42 a +-+ Week 28 Week 43 -+- For the extraction of the asymmetries (this analysis) only a Week 25 of - +the - full statistic Week 30 + - +has been Week 41 : nearly - + - 20% fraction available used 10/25/202 +-+ Week 40 Andrea Bressan -+- 37