Drell-Yan studies in ppbar reactions at FAIR
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rell–Yan Studies in ppbar Reactions at FAIR1, 2 M. Destefanis for the PANDA Collaboration Universitá degli Studi di Torino and INFN Torino Abstract—The nucleonic structure is far to be completely understood. A transverse momentum dependent description of the nucleon structure is a crucial milestone for several forthcoming studies in a wide range of experimental scenarios. By mean of antiproton beams, eventually polarized, that will be available at the future FAIR facility with a beam momentum up to 15 GeV/c, the nonperturbative region of the QCD could be accessed. One of the main goal of the forthcoming experiments at FAIR is the investigation of those Drell– Yan lepton pairs produced in protonantiproton annihilations, taking advantage of the high expected lumi nosities. Drell–Yan studies are a unique tool to access the spin depending properties of the nucleon, and namely its transverse degrees of freedom. Transverse Momentum Dependent (TMD) Parton Distribution Functions (PDFs), in particular the Boer–Mulders, the Sivers and the Transversity distribution functions, could be widely investigated by mean of the corresponding experimental azimuthal asymmetries. In later stages of FAIR, single and doublespin asymmetries could be investigated as well. The Drell–Yan physics program which could be accessed at FAIR will be discussed in details, with a particular focus on the PANDA experimental scenario. DOI: 10.1134/S1063779613060063 21
1. INTRODUCTION
The Parton Distribution Functions (PDF’s) and the Fragmentation Functions (FF’s) are both needed to completely describe the nucleonic structure. At leading twist, three PDF’s are needed to describe the cross section of the unpolarized data: f1(x), g1(x), and h1(x). The f1(x) function describes the probability of finding a quark with a fraction x of the longitudinal momentum of the parent hadron, regardless on the quarks spin orientation. The g1(x) function is the lon gitudinal polarization (helicity) distribution of the quarks. The h1(x), or Transversity, describes the quarks transverse spin distribution inside a transversely polar ized hadron. While the first two PDF’s have a probabi listic interpretation in the helicity base and many experimental data, characterized by a high accuracy, can be found in the literature, the Transversity is not diagonal in the helicity base, and it is still subject of deep investigations. Moreover, the h1(x) could not be extracted from the historical deep inelastic scattering (DIS) data [1] since it is a chirally odd function. The three mentioned distribution functions are connected by the Soffer inequality 1 h1 ( x ) ≤ f1 ( x ) + g1 ( x ) , 2
To describe in a consistent way the polarized cross sections in high energy hadronhadron scattering [2], a Transverse Momentum Dependent (TMD) approach, which takes into account the transverse momentum of the partons (kT), has to be introduced. In this scenario, the three PDF’s above described are no more enough, and eight independent PDFs, func tions of x and kT, are needed. Fig. 1 shows suc
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