Measurement of Collins asymmetries in inclusive production of charged pion pairs ine+e−annihilation atBABAR

Abstract

We present measurements of Collins asymmetries in the inclusive process e þ e − → ππX, where π stands for charged pions, at a center-of-mass energy of 10.6 GeV. We use a data sample of 468 fb −1 collected by the BABAR experiment at the PEP-II B factory at SLAC, and consider pairs of charged pions produced in opposite hemispheres of hadronic events. We observe clear asymmetries in the distributions of the azimuthal angles in two distinct reference frames. We study the dependence of the asymmetry on several kinematic variables, finding that it increases with increasing pion momentum and momentum transverse to the analysis axis, and with increasing angle between the thrust and beam axis.

Measurement of Collins asymmetries for kaons and pions at BABAR

“…It is consistent with the expected symmetry between z 1 and z 2 . The results for ππ are in agreement with previous results from Belle [4] and BABAR [2]. Results for Kπ and KK pairs are obtained for the first time.…”

“…These subsamples have indeed a different mix of favoured and disfavoured terms, where favoured means that the fragmenting quark matches one of the valence quark of the produced hadron, for example a u quark fragmenting into a π + . The BABAR collaboration has extended its analysis published in 2014 measuring Collins asymmetries for pion pairs [2] with a new analysis published at the end of 2015, where the hadrons can be either pions or kaons [3]. It is the first such study involving kaons, thus giving information on the fragmentation of the strange quark.…”

Imaging the proton

“…For e + e − annihilation processes, no useful data on multiplicities and/or unpolarized cross section have been released so far to constrain the unpolarized TMD-FF, only predictions on effects from TMD evolution at various scales of current experiments have been recently published [24]. Some preliminary data with large error bars on azimuthal asymmetries, originating from a specific polarized TMD-FF named Collins function, have recently become public [25,26]. Hence, the uncertainties on the nonperturbative part of TMDs is still large and the discussion on how to properly implement evolution equations in the TMD framework is still open (for a recent review, see Ref.…”

Measurements of fragmentation functions and implications for semi-inclusive deep-inelastic scattering

“…In particular, when studying the transversemomentum structure of the nucleon through semi-inclusive DIS it will be unavoidable to also know the transverse-momentum structure of hadronization as the transverse momentum of the hadrons in semi-inclusive DIS arises through convolutions of the transverse-momentum dependence of both the parton distribution and fragmentation functions. Fragmentation functions are mainly constrained through measurements of semi-inclusive DIS [2,3,4,5] and of hadron production in electron-positron annihilation, especially the very precise data from the B factories [6,7,8,9,10,11,12]. [In case of transverse-momentum integrated fragmentation functions, data on hadron production in proton-proton collision is also used in global analyses.]…”