Physics of the Nucleon Sea Quark Distributions

Abstract

Sea quark distributions in the nucleon have naively been expected to be generated perturbatively by gluon splitting. In this case, there is no reason for the light quark and anti-quark sea distributions to be different. No asymmetries in the strange or heavy quark sea distributions are predicted in the improved parton model. However, recent experiments have called these naive expectations into question. A violation of the Gottfried sum rule has been measured in several experiments, suggesting that u < d in the proton. Additionally, other measurements, while not definitive, show that there may be an asymmetry in the strange and anti-strange quark sea distributions. These effects may require nonperturbative explanations. In this review we first discuss the perturbative aspects of the sea quark distributions. We then describe the experiments that could point to nonperturbative contributions to the nucleon sea. Current phenomenological models that could explain some of these effects are reviewed.

Flavor structure of the nucleon sea

“…Details of these models have been reviewed in several articles [27,28,29,30]. For example, the meson-cloud model [31,32] treats the proton as a linear combination of a bare proton plus pion-nucleon and pion-delta states:…”

“…The charm structure functions F γp 2c in the relatively large-x region measured by the EMC collaboration [106,107,108,109,110] were inconsistent with the sole production of charm by photongluon fusion [29]. This brought the speculation about the charm of nonperturbative origin, so-called "intrinsic charm" (IC).…”

Charm antiquark and charm quark in the nucleon

“…Therefore what is the effect of charm in the structure of the proton is still an open and interesting question. A comprehensive review on the nucleon sea, which includes light quark, intrinsic strange and charm, can be found in [21]. The chiral quark model or more precisely the quark meson fluctuation model (in SU(2) version) was first suggested in [22] to explain the sea flavor asymmetry, d −ū > 0.…”

DISTORTIONS IN THE NEGATIVE ENERGY DIRAC SEA: VIOLATION OF THE GOTTFRIED SUM RULE AND $\Delta \bar u$ IN THE PROTON

“…At present, there are several possible explanations for it, including mesonic (mainly pionic 3 ) contributions and the Pauli exclusion principle at the quark level 4 -for recent reviews see Refs. 5,6,7,8 . Here we report our recent work 9 , on this problem, aimed at putting the phenomenon into a more general context.…”