Drell-Yan pairs from pion scattering on longitudinally polarized nucleons COMPASS DY meeting, Torino, March Oleg Teryaev BLTP, JINR, Dubna

Outline Single Spin Asymmetries on QCD - Sources of Imaginary Phases Perturbative phases and helicities High twist in pion DY and pion light-cone wave function (distribution amplitude) Comparison with (nucleon)spin-averaged angular distributions Conclusions

Single Spin Asymmetries Simpler experimentally – more difficult theoretically. Main properties: – Parity: transverse polarization – Imaginary phase – can be seen from the imaginary i in the (quark) density matrix : T-odd effects in CP conserving theory; CP violation – complex couplings Various mechanisms – various sources of phases

Pion beam + polarized target Allows to study various ingredients of pion structure – rather different from nucleon Most fundamental one – pion-light cone distribution amplitude – similar to Generalized Distribution amplitudes – complementarity to GDA studies at COMPASS - manifested in SSA in DY at large x: Brandenburg, Muller, O.T. (95) Where to measure ? COMPASS – unique opportunity!

Pion Light-cone Distribution in pion-(q)proton scattering

Longitudinal polarization- “partonometer” Two extra terms in angular distribution, proportional to longitudinal polarization

Models for light-cone distributions and angular-weighted x-sections

Size of coefficients in angular distributions

New development (Bakulev, Mikhailov, Stefanis Bunch of DA’s

CLEO data on pion transition FF New ( BMS) parametrizations

Data vs parametrizations Bounds for DA

Role of COMPASS data Sensitive to INTERNAL points rather than end-poind behaviour –COMPLEMENTARITY Detailed prediction accounting for evolutions required (Bakulev, Stefanis, OT)

New preliminary results (BST) 3D plots for asymmetry parameter – different pion light-cone distributions Asymptotic BMS CZ

Slices in transverse momentum Asymptotic BMS CZ

Inverse moments Important ingredients

Perturbative LSSA –sensitive to helicity distributions +QCD COUPLING Perturbative phases – but no mass suppression (Ralston, Pire; Carlitz, Willey)

Azimuthal Asymmetries in unpolarized DY Sensitive test of QCD – reflect the existence of natural scattering PLANE Besides T-odd (as before) are T-EVEN

Unpolarized DY asymmetries Angular distribution (leptons c.m. frame): Lam –Tung relation (sum rule)

Status of Lam-Tung relation Holds at LO and (approximately) at NLO QCD violated by higher twists (Brodsky et al.), correlations of T-odd distributions(Boer), entanglement of quarks in QCD vacuum (Nachtmann) (and EXPERIMENTALLY). Physical origin?!

Kinematic azimuthal asymmetry from polar one Only polar z asymmetry with respect to m! - azimuthal angle appears with new

Generalized Lam-Tung relation Relation between coefficients (high school math sufficient!) Reduced to standard LT relation for transverse polarization ( =1) LT - contains two very different inputs: kinematical asymmetry+transverse polarization

How realistic is “m-model”?- averaging procedure Theorem for averages Convenient tool for asymmetries analysis (e.g. Collins suppression with respect to Sivers) For N-dimesinsional integration – N-1- dimensional “orbits”

Application for average in semi-inclusive Drell-Yan Choose Average : Representitive of “orbit” crossing scattering plane – cannot depend on n

GLT relation -applicability Appears for KINEMATICAL asymmetry in semi-inclusive process (only one physical plane exists) Violated if there is azimuthal asymmetry already in the subprocess (with respect to m) – NLOQCD, HT, Boer-Mulders functions.

Polarized vs Unpolarized Look for Lam-Tung and SSA Violation - Signal for new mechanisms: HT for longitudinal polarization BM for transverse polarization Sivers function at large PT (P.G. Ratcliffe, OT, in progress) – special role of pion beam in the observation of sign difference between SIDIS and DY

CONCLUSIONS Pion beam scattering on polarized target – access to pion structure Longitudinal polarization at large x – sensitive to pion distribution ampitude Perturbative SSA – test of QCD mechanisms, measurement of QCD coupling Comparative study of longitudinal, transverse and unpolarized data