M. Contalbrigo (on behalf of HERMES collaboration) Measurement of Transverse Asymmetries from Interference Fragmentation at HERMES M. Contalbrigo (on behalf of HERMES collaboration) INFN and Ferrara University Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Leading Twist Distribution Functions proton proton’ quark quark’ Probabilistic interpretation in helicity base: 1/2 R 1/2 L f1(x) + q(x) spin averaged (well known) 1/2 R 1/2 L g1(x) - Dq(x) helicity diff. (known) -   dq(x) helicity flip (unknown) h1(x) No probabilistic interpretation in the helicity base (off diagonal) 1/2 -1/2 R L Transversity base u = 1/2(uR + uL) u = 1/2(uR - uL) Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Probing transversity Chiral-odd: requires another chiral-odd partner Impossible in inclusive DIS Direct measurement only with Drell-Yan Indirect Measurement: convolution with novel fragmentation functions ppe+e-X epe’p(p)X epe’X h1 x h1 h1 x Collins function h1 must couple to another chiral-odd function h1 x IFF Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

SIDIS cross section Trans. component of relative momentum survives integration over the Ph┴ of pair Collinear kinematics (factorization) Simple product of unknown functions Explicit dependence on transverse momentum of hadron Ph┴ Convolution of two unknown functions Competing mechanism (Sivers) Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Two-pion asymmetry Interference from different partial waves of p+p- system Indipendent probe of h1 Theoretically clean channel Limited statistic power Needs 2pion fragmentation function from e+e- machines RT Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Event selection Standard DIS cuts Pions identified with RICH All possible combination of detected p+p- were included for each events 0.60 < mpp < 0.95 GeV DE (GeV) The asymmetry is evaluated as a function of x,z,Mpp and the angles phi_Rperp, phi_S and theta. N_up( down) is the number of semi-inclusive pi+-pi- pairs detected while the target is in up (down) spin-state. These numbers are normalized to the corresponding numbers of DIS events (Ni_up, Ni_down) Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Azimuthal asymmetry Linear fit function The asymmetry is evaluated as a function of x,z,Mpp and the angles phi_Rperp, phi_S and theta. N_up( down) is the number of semi-inclusive pi+-pi- pairs detected while the target is in up (down) spin-state. These numbers are normalized to the corresponding numbers of DIS events (Ni_up, Ni_down) Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Partial wave expansion The contribution to the asymmetry is due to interference of different partial waves of the final state p+p – (partial wave expansion truncated at wave p) Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

The azimuthal moments was extracted from AUT using a 2D linear c2 fit The extracted values of the moments are not influenced by the presence of this term in the fit function consistent with zero 1 Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Azimuthal moments The azimuthal moments are being extracted from AUT using a 2D non-linear c2 fit Poorly constrained Preliminary studies show minor influence on the present result The extracted values of the moments were not influenced by extra terms due to partial integration of Ph Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

T-odd and chiral-odd dihadron fragmentation function Azimuthal moment First evidence of a T-odd and chiral-odd dihadron fragmentation function Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

POSITIVE ASYMMETRY in the whole range of Mpp-mass Azimuthal moment POSITIVE ASYMMETRY in the whole range of Mpp-mass Jaffe et al. Phys.Rev.Lett.80,1166(1998) No evidence of the sign-change at the r0 mass Vedere articolo di Jaffe per capire se e’ un modello o no Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

POSITIVE ASYMMETRY in the whole range of Mpp-mass Azimuthal moment Radici et al. Phys.Rev.D65,074031 (2002) POSITIVE ASYMMETRY in the whole range of Mpp-mass Before Trento conventions Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Azimuthal moment POSITIVE ASYMMETRY in the whole range of Mpp-mass The non linear behaviour in Mpp requires carefully study of acceptance related sys POSITIVE ASYMMETRY in the whole range of Mpp-mass Capire perche’ possono ridurre l’interferenza M(GeV) Bacchetta and Radici hep-ph/0608037 Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Conclusions The non-zero Single Spin Asymmetry (SSA) measured implies 2-pion SIDIS can be used to probe transversity First evidence for a non-zero interference fragmentation function; No evidence of a sign change of SSA at r0 mass Preliminary results on extra-terms in the fit do not show significant contribution Double statistics is expected with 2005 data Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Azimuthal moments If 2-p pair are isotropically Integration on q can not be complete If 2-p pair are isotropically distributed in the phase-space q-distribution is proportional to sinq: A CUT ON THE MOMENTA BIASES SIGNIFICANTLY THE q DISTRIBUTION!! Contalbrigo Marco 2h transverse asymmetry Kyoto (Japan), 06-10-2006

Bacchetta et al. [hep-ph/0212300]

ppe+e-X epe’h(h)X epe’X h1 x h1 h1 x Collins function Impossible in inclusive DIS Direct measurement only with Drell-Yan Indirect Measurement: convolution with novel fragmentation functions h1 x h1 h1 x Collins function h1 must couple to another chiral-odd function h1 x IFF The extracted values of the moments were not influenced by the addition of subleading terms like: