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Measurement of F /F , D/U Ratios and A=3 EMC Effect in Deep Inelastic

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Measurement of F /F , D/U Ratios and A=3 EMC Effect in Deep Inelastic n p MeAsurement of F2 /F2 , d/u RAtios and A=3 EMC Effect in Deep Inelastic Electron Scattering Off the Tritium and Helium MirrOr Nuclei (JLab Conditionally Approved Experiment C12-10-103) Makis Petratos Kent State University For the JLab MARATHON Collaboration PAC37 – January 2011 Jefferson Lab PAC30: Physics goals of experiment: “Highlights of 12 GeV Program” Conditional approval based on on “review of safety aspects of 3H target” PAC36: Physics again very highly rated. Conditional approval based on detailed SBS detector design 3H target conditional approval removed Electron-Nucleon Deep Inelastic Scattering (DIS): Interaction is mediated via exchange of a virtual photon absorbed by one of the nucleon’s quarks. Nucleon breaks up into several X hadrons. X e’ γ★ Q2 e N DIS and Quark Parton Model • Cross Section – Nucleon structure functions F1 and F2: d 2 F ( ,Q2 ) 2F ( ,Q2 ) 2 cos2 1 sin 2 2 4 2 2 ddE' 4E sin 2 M F M v2 E E' R L 2 1 1 F Q2 2 2 T 1 Q 4EEsin ( / 2) • Quark-Parton Model (QPM) interpretation in terms of 2 quark probability distributions qi(x) (large Q and ν): 1 2 F (x) x e2q (x) F1(x) ei qi (x) 2 i i 2 i i • Bjorken x: fraction of nucleon momentum carried by struck quark: x Q2 / 2M n p F2 /F2 in Quark Parton Model • Assume isospin symmetry: u p (x) d n (x) u(x) u p (x) d n (x) u(x) d p (x) un (x) d(x) d p (x) u n (x) d (x) p n p n s (x) s (x) s(x) s (x) s (x) s(x) • Proton and neutron structure functions: p 4 1 1 F2 x (u u) (d d ) (s s) 9 9 9 n 4 1 1 F x (d d ) (u u) (s s) 2 9 9 9 • Nachtmann inequality: n p 1/ 4 F2 / F2 4 SLAC Measurements End Station A 1968-1972 Friedman, Kendal, Taylor Nobel 1991 n p F2 /F2 extracted from proton and deuterium deep inelastic data using Hamada-Johnston potential in a Fermi-smearing model. Data in disagreement with SU(6) prediction: 2/3=0.67! SLAC/MIT and (later) CERN Data • Nachtmann inequality satisfied: n p • For x → 0 : F2 /F2 → 1: Sea quarks dominate with: u u d d s s n p • For x → 1 : F2 /F2 → 1/4: High momentum partons in proton (neutron) are up (down) quarks, and: s s 0 • For medium and high x, safe to assume that (with d and u denoting now quark plus antiquark distributions): n F2 [1 4(d / un)] p 1/ 4 F2 / F2 4 F p [4 (d / u)] 2 Theory Overview (I) • SU(6)-breaking Diquark Model (Close/Feynman, 1970’s) n p • Phenomenological explanation: fall off of F2 /F2 and its x=1 limit easily explainable using SU(6) nucleon wave function (Close, 1973) or Regge theory (Feynman, 1975) and assuming that the spectator diquark configuration with total spin s=1 is suppressed relative to that with s=0. • Limit at x=1 in agreement with the SLAC/CERN data. • Constituent Quark Model (Isgur and Carl, 1980-2000) • Suppression fully understood in the hyperfine-perturbed quark model: color-magnetic hyperfine interaction generated by gluon exchange is proportional to quark spin dot products si·sj. Force is repulsive(attractive) for parallel(antiparallel) spins and allows the u(x) and d(x) quark momentum distributions to be different. • Limit for x=1 in agreement with the diquark model and data. Theory Overview (II) • Perturbative QCD (Farrar and Jackson, 1975) • Simple consideration of isospin and helicity structure of nucleon’s quark wave function within perturbative QCD dictates that for diquark spin alignment (s=1) only exchange of longitudinal gluons is permitted, resulting in suppression of n p Compton scattering amplitude, which causes the F2 /F2 fall off. • Problem: pQCD x=1 limit different than that of quark/diquark model! • QCD Counting Rules (Brodsky et al., 1995) • Early pQCD findings substantiated within first-principles calcu- lations of Counting Rules of QCD predicting the high-x behavior of quark distributions: q(x)~(1-x)2n-1+2Δh, where n is the minimum number of not interacting (spectator) quarks and Δh is the difference in helicity between the struck quark and the nucleon. • Same x=1 limit as pQCD. n n F2 / F2 , d/u Ratios and A1 Limits for x→1 n p n p F2 /F2 d/u A1 A1 SU(6) 2/3 1/2 0 5/9 Diquark Model/Feynman 1/4 0 1 1 Quark Model/Isgur 1/4 0 1 1 Perturbative QCD 3/7 1/5 1 1 QCD Counting Rules 3/7 1/5 1 1 A1: Asymmetry measured with polarized electrons and nucleons. Equal in QPM to probability that the quark spins are aligned with the nucleon spin. Extensive experimental programs at CERN, SLAC, DESY and JLab. Extensive recent review on the valence/high-x structure of the nucleon: R. J. Holt and C. D. Roberts, Rev. Mod. Phys. 82, 2991 (2010). EMC Effect • Nuclear F2 structure function per nucleon is different than that of deuterium: large x and A dependence. • Quark distribution functions modified in the nuclear medium. • Possible explanations include: • Binding effects beyond nucleon Fermi motion • Enhancement of pion field with increasing A • Influence of possible multi-quark clusters • Change in the quark confinement scale in nuclei • No universally accepted theory for the effect explanation. • A=3 data will be pivotal for understanding the EMC effect. • Theorists: Ratio of EMC effect for 3H and 3He is the best quantity for quantitative check of the theory, free of most uncertainties. A Dependence EMC Effect SLAC E139, 1984 J. Gomez et al. Nucleon momentum probability distributions in nuclei different than those in deuterium. Effect increases with mass number A. Binding/EMC Effect in Deuteron • Deuteron structure function convolution: Calculated in a covariant frame-work in terms of free nucleon structure functions (Melnitchouk and Thomas 1996) : F d (x,Q2 ) f (y) F p (x,Q2 ) F n (x,Q2 ) dy 2 2 2 Spectral function f(y) accounts for Fermi Motion AND Binding/Off-Shell Effects, where y is the fraction of the nucleus momentum carried by the struck nucleon. • Density Model: EMC effect for deuteron scales with nuclear charge density as for heavy nuclei (Frankfurt and Strikman 1988, applied by Whitlow et al. 1992) : F d F A 2 1 d 2 1 p p d F2 F2 A d F2 SLAC DIS Data Whitlow: Density Model M&T: Convolution Model Bodek: Fermi-Smearing Paris N-N potential n/ p Standard analysis of F2 F2 from proton and deuteron data shows large uncertainty at high-x values depending on nucleon-nucleon potential used. The three analysis methods indicate tremendous uncertainties in high-x n p behavior of F2 /F2 and d/u ratios … d/u essentially unknown at large x! Most recent CTEQ6X analysis shows similar large uncertainties at high x: A. Accardi et al., Phys. Rev D81, 034016 (2010). 3 3 Nucleon F2 Ratio Extraction from He/ H • Form the “SuperRatio” of EMC ratios for A=3 mirror nuclei: 3 3 He H 3 3 F2 3 F2 R( He) R( He) p n R( H) p n R 3 2F2 F2 F2 2F2 R( H ) 3 3 • If R=σL/ σT is the same for He and H, measured DIS cross section ratio must be equal to the structure function ratio as calculated from above equations: 3 He 3 F He 2F p F n 2 R 2 2 3 H 3 H F p 2F n F2 2 2 • Solve for the nucleon F2 ratio and calculate R* (expected to be very close to unity) using a theory model: n 3He 3H F2 2R F2 / F2 p 3 3 He H F2 2F2 / F2 R 3He and 3H Structure Functions • Nucleon structure function in nucleus A in the Impulse Approximation: extension of deuteron model (Melnitchouk and Thomas): F NA (x) dy [ f (y)] F N (x / y) f F N 2 N / A 2 N / A 2 • For 3He: 3He p n F2 2 f 3 F2 f 3 F2 p/ He n/ He • With isospin symmetry: f 3 f 3 f n/ H p/ He p f 3 f 3 f p/ H n/ He n 3 3H p n • Then for H: F2 fn F2 2 f p F2 • Charge symmetry breaking effects are small! SuperRatio R* = R(3He)/R(3H) has been calculated by three expert groups to deviate from 1 only up to ~1.5% taking into account all possible effects: * I. Afnan et al., Phys. Lett. B493, 36 (2000); Phys. Rev. C68, 035201 (2003) * E. Pace, G Salme, S. Scopetta, A. Kievsky, Phys. Rev. C64, 055203 (2001) * M. Sargsian, S. Simula, M. Strikman, Phys. Rev. C66, 024001 (2002) The Experiment • 3He/3H DIS measurements can be best run in Hall A: • Beam Energy: 11.0 GeV – Beam Current: 25 µA • Small angles (15o - 25o): use Left HRS system • Large angles (42o - 57o): use BigBite system • Standard electron detection packages for both systems • ~700 hours for d/u measurement (@ 100% efficiency) • ~300 hours for R=σL/σT measurement (@ 100% efficiency) • Desirable to check that the ratio R is the same for 3He and 3H with Rosenbluth separation of cross section versus ε (degree of γ* longitudinal polarization) using the Left HRS system. Need dedicated 3.3, 5.5, 7.7 GeV energies. • Need target system with helium/tritium/deuterium cells.
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