Slide 1

12009/07/071 Improved Measurement of d / u Asymmetry in the Nucleon Sea Phys. Rev. D 64, 052002 (2001). R. S. Towell et al. FNAL E866 / NuSea Collaboration Jinnouchi / Shibata Lab. 06_16830 Ryo Nagai sea quark d / u Contents 1. Introduction 2. Experiment 3. Analysis 4. Results 5. Summary Slide 2 22 1. Introduction valence quark gluon splitting d d d u u d d u u sea quarks gluon proton This experiment is effective to verify this assumption. This experiment is effective to verify this assumption. This experiment measured dimuon momentum from Drell-Yan process (see next page). The goal This paper presents improved analysis results by using the previous data. The usual assumption Sea quark-antiquark pair is produced perturbatively from gluon splitting. The numbers of uu and dd pairs should be approximately equal, because the mass difference of u and d is small. determine s pd /2s pp over a wide kinematic range determine d / u in the proton sea quarks Slide 3 33 Drell-Yan process Fig. shows Drell-Yan process in the actual experiment. This experiment used proton and deuteron as targets. 4-momentum of the muon pair was measured. x 1, x 2 : Bjorken- x of the quarks from the beam and target Drell-Yan process (Feynman diagram) proton-beam fixed target (proton or deuteron) At nucleon level X : hadrons, p : proton, q : quark g * : virtual photon, m: muon At quark level (1- x 2 ) p 2 (1- x 1 ) p 1 Slide 4 44 Measuredmuon 4-momentum p 3 =(E 3, p 3 ), p 4 =(E 4, p 4 ) The Feynman- x ( x F ) and dimuon mass ( M ) are defined as: s=(p 1 +p 2 ) 2 ~2E 1 m p : the total 4-momentum squared of the initial nucleons p g || : the center-of-mass longitudinal momentum of the virtual photon Thus, x 1 and x 2 are determined by the measurement. The p-d Drell-Yan cross section can be expressed as s pd ~s pp +s pn. For x 1 >>x 2, Drell-Yan cross section ratio is expressed The right side was simplified further assuming d(x) u for x < 0.25. d/ud/u x2x2 Slide 9 99 5. Summary This paper reports improved analysis of the measurement of the Drell-Yan cross section ratio per nucleon of p+d to p+p. The experiment used 800 GeV proton beam and measured 4- momentum of dimuon. From this measurement the large asymmetry of the uu and dd pairs in the nucleon sea is extracted as a function of x. This result is different from the expectation, the same numbers of uu and dd pairs are produced perturbatively. The result suggests possibility that there is a non-perturbative origin for the production of the uu and dd pairs. Slide 10 Slide 11 11 Passive origin of d / u asymmetry Perturbative origin Pauli blocking less than 1% Non-perturbative origin Meson cloud model express the physical proton as the combination of a proton with a symmetric sea and a series of virtual meson-baryon state Chiral quark model Similar to the meson model Except the virtual meson couples directly to a quark (not to the nucleon) Slide 12 12 Meson Cloud Model u u d u proton d d u d u p- meson valence quark sea quarks Light gray region p-meson cloud ud ud u d u d u d d d d u d d Slide 13 13 Feynman- x (x F ) Feynman- x (x F ) is defined as : The center-of-mass 4-momentum The center-of-mass longitudinal momentum is Invariant mass (center-of-mass) is Using these expressions, Slide 14 14 Dimuon mass (M) Dimuon mass (M) is defined as: The law of conservation of energy and momentum is Using the law, M is calculated as Invariant mass Using the expression of the invariant mass, p 2 = 0 (fixed target) m 1, m 2 ~ m p m p