Flavor Separation in Λ - Baryon Fragmentation

It is shown that neutrino and antineutrino deep inelastic scattering of unpolarized and polarized Λ and Λ productions, can provide a clean separation of unpolarized and polarized fragmentation functions of a quark into a Λ, for both light-flavor quarks and antiquarks and also for strange quarks. Combining with Λ and Λ productions in polarized electron deep inelastic scattering, one can systematically measure or check the various flavor and spin dependent fragmentation functions. Such measurements can provide crucial tests of different predictions concerning the spin structure of hadrons and the quark-antiquark asymmetry of the nucleon sea. PACS numbers: 13.87.Fh, 13.15.+g, 13.60.Hb, 14.20.Jn Accepted for publication in Physics Review Letters Mailing address Spin physics is currently one of the most active research directions of hadron physics due to the rich phenomena, which are different from the naive theoretical expectations and the large number of experimental facilities which can provide precision measurements of many physical quantities related to the underlying spin structure of hadrons. Among the various topics the strange content of the proton is one of the most attractive due to its close connections to the proton spin problem [1] and to the quark-antiquark asymmetry of the nucleon sea [2]. Though there have been many progresses and achievements, both theoretically and experimentally, our current knowledge of the strange quark content of the proton is still very poor, since one is still unclear, whether or not [1, 2], strange quarks are highly polarized inside the proton, and it is even more obscure, whether or not [2, 3], the strange quark-antiquark distributions are symmetric. Thus a precision measurement of the strange-antistrange polarizations of the proton is one of the most challenging and significant tasks for hadron quark structure. From another point of view, the quark distribution of a quark inside a hadron is related by crossing to the fragmentation function of the same flavor quark to the same hadron, by a simple reciprocity relation [4] qh(x) ∝ D h q (z), (1) where z = 2p · q/Q is the momentum fraction of the produced hadron from the quark jet in the fragmentation process, and x = Q/2p · q is the Bjorken scaling variable corresponding to the momentum fraction of the quark from the hadron in the deep inelastic scattering (DIS) process. Although such an approximate relation may be only valid at a specific scale Q, which deserves further studies, it could provide reasonable connection between different physical quantities and lead to different predictions about the fragmentations based on our understanding of the quark structure of a hadron [3]. Thus measurements of quark fragmentations into a hadron can also provide some new insights into the quark structure of the hadron. Among the various produced hadrons, Λ hyperon is most suitable to study the polarized fragmentation due to its self-analyzing property owing to the characteristic decay mode Λ → pπ with a large branching ratio of 64%. The technology to disentangle between Λ and