Nucleon Valence Quark Structure

Transcription

1 Nucleon Valence Quark Structure Z.-E. Meziani, S. Kuhn, O. Rondon, W. Melnitchouk Physics Motivation Nucleon spin and flavor structure High-x quark distributions Spin-flavor separation Moments of structure functions Spin-averaged moments Color field polarizabilities Conclusion January 17,

2 Motivation Understand the nucleon structure in the valence quark region What is required? Complete knowledge of parton distribution functions (PDFs). Unpolarized, helicity dependent and transversity distribution functions Theoretically large x exposes valence quarks - free of sea effects x->1 behavior sensitive test of spin-flavor symmetry breaking important for higher moments of PDFs - compare with lattice QCD intimately related with resonances, quark-hadron duality January 17,

3 Experimentally Precision data are scarce even after 30 years of experiments A special combination of beam energy and polarized luminosity makes the 12 GeV upgrade figure of merit unique Jlab at 12 GeV is the only place where this physics can be done How to proceed? Perform measurements of spin-independent and spin dependent structure functions through inclusive and semi-inclusive deep inelastic scattering (DIS) Targets are unpolarized and polarized hydrogen, deuterium and helium-3 Make polarized target and detectors standard equipment in Halls January 17,

4 Unpolarized structure functions F 1 (x,q 2 ) and F 2 (x,q2 ) Proton & neutron measurements provide d/u distributions ratio Inclusive DIS U Polarized structure functions g 1 (x,q 2 ) and g 2 (x,q 2 ) Proton & neutron measurements combined with d/u provide the spinflavor distributions u/u & d/d Q 2 :Four-momentum transfer x : Bjorken variable ν : Energy transfer M : Nucleon mass W : Final state hadrons mass L T January 17,

5 Examples of existing data January 17,

6 Virtual photon-nucleon asymmetries In the quark-parton model: F 1 (x, Q 2 ) = 1 2 f e 2 f q f(x, Q 2 ) g 1 (x, Q 2 ) = 1 2 f e 2 f q f(x, Q 2 ) January 17,

7 World data for A 1 Proton Neutron January 17,

8 Polarized quarks as x--> 1 SU(6) symmetry: A 1 p = 5/9 A 1 n = 0 d/u=1/2 u/u = 2/3 d/d = -1/3, Broken SU(6) via scalar diquark dominance A 1 p 1 A 1 n 1 d/u 0 u/u 1 d/d -1/3 Broken SU(6) via helicity conservation A 1 p 1 A 1 n 1 d/u 1/5 u/u 1 d/d 1 Note that q/q as x--> 1 is more sensitive to spin-flavor symmetry breaking effects than A 1 January 17,

9 A 1 n at 11 GeV with MAD W>1.2 Examples from different halls A 1 p at 11 GeV with CLAS++ January 17,

10 A n 1 and Helicity-Flavor Decomposition A 1 n in Hall A d d = 4 15 u u = 4 15 g1 n F1 n g1 p F1 p (4+ u d ) 1 15 (4+ d u ) 1 15 g1 p F1 p g1 n F1 n (1+4 u d ) (1+4 d u ), January 17,

11 Unpolarized Neutron to Proton ratio In the large x region (x>0.5) the ratio F 2 n /F2 p is not well determined due to the lack of free neutron targets Impact: determine valence d quark momentum distribution extract helicity dependent quark distributions through inclusive DIS high x and Q 2 background in high energy particle searches. construct moments of structure functions January 17,

12 Unpolarized Neutron to Proton ratio Spectator tagging Nearly free neutron target by tagging low-momentum proton from deuteron at backward angles DIS from A=3 nuclei Mirror symmetry of A=3 nuclei Extract F 2 n /F2 p from ratio of 3 He/ 3 H structure functions Small p ( MeV/c) Minimize on-shell extrapolation (neutron only 7 MeV off-shell) Backward angles (θ pq > 110 o ) Minimize final state interactions Super ratio R= ratio of EMC ratios for 3 He and 3 H Calculated to within 1% Most systematic and theoretical uncertainties cancel January 17,

13 Unpolarized Neutron to Proton Ratio Hall A 11 GeV with MAD HallB 11 GeV with CLAS++ January 17,

14 Semi-inclusive DIS Spin-flavor decomposition of valence and sea quarks by tagging hadron (e.g. π, K) in current fragmentation region dσ = f e 2 f q f (x) D h f(z) z = E h /ν (z) quark--> D hadron fragmentation function unpolarized or polarized beam and target mass of unobserved X system, W x > 2 GeV January 17,

15 Semi-inclusive DIS Factorization of current and target fragmentation Berger criterion η>2 to avoid contamination from target fragmentation (z > 0.4) needs to be established experimentally at 11 GeV current jet π K N Λ η= η CM W = 5 GeV target jet π K z π zd val(z) x F π N ( η) January 17,

16 Helicity-Flavor Decomposition Hall A with MAD Hall B with CLAS++ January 17,

17 Flavor decomposition of the sea January 17,

18 Moments of structure functions Define moments of F 2 Hall C with SHMS Note: Integral includes elastic Contribution Relate moments to matrix elements of (quark & gluon) operators of given twist via OPE Calculate directly in lattice QCD January 17,

19 Moments of Structure Functions January 17,

20 Moments of Structure Functions January 17,

21 g 2 at JLab with GeV Proton, Hall B with CLAS++ Neutron, Hall A with MAD January 17,

22 January 17,

23 Color Field Polarizabilities January 17,

24 Conclcusion JLab at 12 GeV is a unique facility to determine large x quark distributions Semi-inclusive DIS will allow unprecedented flavor decomposition for valence and sea quarks First quantitative insights into the effect of color E and B fields on the nucleon structure January 17,