d¯/ūasymmetry and the origin of the nucleon sea

Abstract

The Drell-Yan cross section ratios, (pϩd)/(pϩp), measured in Fermilab E866, have led to the first determination of d (x)/ū (x), d (x)Ϫū (x), and the integral of d (x)Ϫū (x) for the proton over the range 0.02 рxр0.345. The E866 results are compared with predictions based on parton distribution functions and various theoretical models. The relationship between the E866 results and the NMC measurement of the Gottfried integral is discussed. The agreement between the E866 results and models employing virtual mesons indicates that these non-perturbative processes play an important role in the origin of the d ,ū asymmetry in the nucleon sea. ͓S0556-2821͑98͒02821-5͔ PACS number͑s͒: 13.85. Qk, 11.55.Hx, 12.39.Ki, 24.85.ϩp Recent measurements ͓1-3͔ have revealed a marked asymmetry in the distributions of up and down quarks in the nucleon sea. While no known symmetry requires ū to equal d , a large d /ū asymmetry was not anticipated. The principal reason for expecting symmetry between up and down quarks in the sea is an assumption that the sea originates primarily from q-q pairs produced from gluons. As the masses of the up and down quarks are small compared to the confinement scale, nearly equal numbers of up and down pairs should result. Indeed, a theoretical investigation ͓4͔ of the lightquark asymmetry in the nucleon concluded that perturbative processes do not give rise to asymmetries in the up, down sea exceeding 1%. Thus a large d /ū asymmetry requires a non-perturbative origin for an appreciable fraction of these light antiquarks. This paper draws together several implications arising from this observed d /ū asymmetry-the effect of these measurements on existing parton distributions, an examination of the compatibility of the measurements of this asymmetry, and the origin of the effect.The issue of the equality of ū and d was first encountered in measurements of the Gottfried integral ͓5͔, defined as

Flavor asymmetry of sea quarks in the unquenched quark model

“…The Gottfried integral was first determined in 1991 by the New Muon Collaboration (NMC) in muon-induced deep-inelastic scattering experiments as S G = 0.240 ± 0.016 [1], which was later re-evaluated as 0.235 ± 0.026 [2]. The measurement of the flavor asymmetry of the nucleon sea via the violation of the Gottfried sum rule by NMC was later confirmed in Drell-Yan experiments [4][5][6][7] and SIDIS experiments [8].…”

“…The quark-parton model predicts a flavor symmetric sea that leads to the Gottfried sum rule S G = 1/3 [13], whereas any deviation from this value is an indication of thed/ū asymmetry of the nucleon sea, thus providing a clean evidence of the existence of nonperturbative higher Fock components (such as qqq-qq configurations) in the proton wave function. The first clear evidence of a flavor asymmetric sea and a related violation of the Gottfried sum rule came in 1991 from the New Muon Collaboration (NMC) [1], which was later confirmed in Drell-Yan experiments [4][5][6][7] that probe the ratioū/d, as well as in semi-inclusive deep-inelastic scattering (SIDIS) experiments [8]. All these experiments show evidence that there are morē d quarks in the proton than there areū quarks [10].…”

“…Introduction. From an experimental point of view, the flavor asymmetry betweenū andd quarks in the proton is now well established [1][2][3][4][5][6][7][8]; for review, see also Refs. [9,10].…”

Chiral quark model analysis of nucleon quark sea isospin asymmetry and spin polarization

“…However, recently there has been a remeasurement of both theū/d asymmetry as well as of theū −d asymmetry by the NuSea Collaboration 1 [19,20]. Their data differ substantially from the earlier measurements.…”

Quantifying finite-momentum effects in the quark quasidistribution functions of mesons

“…The fact that the pion is responsible for important contributions to the structure of the nucleon has been known for some time, and the recognition of this fact has led to the introduction of pionic modes into many models [2] of proton structure. At the same time, ambiguities remain concerning the structure of the pion itself (as well as of other light mesons) and its role in the nucleon's quark substructure and flavor asymmetries [3]. This fact has inspired a number of experimental measurements of the pion structure function or parton distribution functions (PDFs) over the years, including an approved JLab measurement of the π structure function via spectator tagging in the Sullivan process [4].…”