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How Does Short Distance Behavior Affect the Nucleus

How Does Short Distance Behavior Affect the Nucleus. Don Geesaman 12 January 2007 DNP QCD Town Meeting . Why. We built JLab and did experiments at SLAC, FNAL, DESY... because the short-distance behavior of nuclei was not understood.

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How Does Short Distance Behavior Affect the Nucleus

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  1. How Does Short Distance BehaviorAffect the Nucleus Don Geesaman 12 January 2007 DNP QCD Town Meeting

  2. Why • We built JLab and did experiments at SLAC, FNAL, DESY... because the short-distance behavior of nuclei was not understood. • the nucleus is more than mean-field and long-range correlations. • High momentum transfer = short distances • short range components of N-N interaction • High momentum transfer = resolve the QCD structure • where are the QCD effects in nuclei? • We know at high temperature or density things must change. • how high is high • is transition continuous or abrupt? • where do neutron stars lie?

  3. We want to describe a nucleus • Pure QCD Description • what are the clusters of quarks in a nucleus? • know the parton distributions change • EMC effect • shadowing • x>1 • The problem is always whether our description of a bare proton is good enough and then how to actually calculate many body effects? • Hadronic Description • exemplified by ab initio calculations with potentials • NN • NNN + NNNN + • Bare form factors • Meson exchange currents • Past two decades have shown this is remarkably successful

  4. Issues in Proton Structure – new data has been critical! • Nucleon form factors • spin carried by the quarks and gluons and angular momentum • nature of the sea

  5. average spacing at ρnm ~ 1.8 fm Radius of a nucleon ~ 0.8 fm average spacing at 3ρnm ~ 1.3 fm Our visual images OR “nucleons” held apart by short range repulsion but even in 208Pb, half the nucleons are in the surface

  6. What do we know about short distance behavior in nuclei? • Strong N-N potential does have impacts NN Interaction NN Correlation Functions

  7. What do we know about short distance behavior in nuclei? • Impact of correlations on high momentum structure of wave functions • direct observation • high momentum components in (e,e’p) • x>1 correlations • indirect (quenching) effects • reduction of single particle strength – Spectroscopic factors • apparent changes in bare form factors – quenching of GA

  8. Direct measurement JLab E97-006 Rohe et al. PRL 93, 182501 (04) 0.61+/- .06 protons in pm>240 MeV/c and Em> 40 MeV

  9. Spectroscopic factors

  10. Distribution of spectroscopic strength (from Dickhoff) Note ab initio calculations do very good job in, for example 7Li – SRC+LRC.

  11. Basic “facts” of nuclear physics that may be wrong in neutron-rich nuclei The radius and diffuseness of the neutron and proton distributions are similar R=1.2 A1/3, a~ 0.55 fm The magic numbers of the shell model are fixed. The deformations of the neutrons and protons are similar The valence quasi-particles are renormalized by about 0.6 by short-range correlations. The charge-independence of the strong interaction makes isospin a good quantum number This is only illustrative. There are a number of other mechanisms that also lead to changes in the shell structure as N/Z varies.

  12. Does the impact of correlations change dramatically away from valley of stability? From Gade and Tostevin, NSCL History 1960’s: Shell Model and transfer reactions assumed pure single particle states. 1970’s: electron scattering showed only 60% occupancy in valence single particle states. 1980’s: Understood based on correlations. 1990’s: Correlations viewed as universal, approximately nucleus independent. 2000’s: In nuclei far from stability, observed large changes in correlation effects. 22O 34Ar S = Sn-Sp for neutron knockout and Sp-Sn for proton knockout Note Rs is ratio to shell model not spectroscopic factor

  13. Another way to look at high momentum components: x>1 data CLAS Egiyan et al. • 2 and 3 nucleon correlations It appears that correlations dominate the deep inelastic structure functions at high x. Not likely to tell us about quark substructure!

  14. Towards better understanding of short range behaviour • NN, NNN Data • a number of puzzles • what is the key experiment? • Lattice –very long way to go • Effective field theory • Need at least N3LO – Chi2 of order 1 Still a fit to data, but about ½ the free parameters! • 3NF – still working on N3LO • Other baryon-nucleon interactions: Modern lattice QCD result S.R. Beane et al, PRL 97 (2006)

  15. Nucleon-Baryon Interactions • Λ-N No one pion exchange • small spin-orbit interaction • perhaps more direct window on short range behavior • Will low energy data (scattering length, hypernuclear spectroscopy) provide enough constraints? • Σ – N and Ξ – N Important for neutron star matter. How to probe? • P P Interactions • G parity says short range part changes • problem is absorption is so strong that little information seems to be obtainable

  16. e-d elastic scattering Alexa et al PRL 82, 1374 (99) Does structure of baryons change in nuclei? So far JLAB has taught us that hadron structure/interactions do not change much (to the precision we can determine today) at normal matter densities. Perhaps the smoking gun? Schiavilla et al PRL 94, 072303 (05)

  17. Quark Meson Coupling predictions

  18. Alde et al (Fermilab E772) Phys. Rev. Lett. 64 2479 (1990) Parton Distributions in Nuclei • 1984 – Parton distributions are different EMC effect – nucleon carries smaller fraction of momentum or changes structure Shadowing • 1990 – little change in sea quarks for x>0,1 • 2007 • x >1 data dominated by correlations • still need flavor separation and larger x range for antiquarks. • Will we finally be able to tag parton distributions vs the momentum and binding energy of spectator particles? • predicted large effects in spin structure

  19. Still only one high precision measurement of antiquarks: Where are the nuclear pions? • The binding of nucleons in a nucleus modifies the x dependence. • Most contemporary models still predict large effects to antiquark distributions as x increases. • Models must explain both DIS-EMC effect and Drell-Yan • Sufficient uncertainly that CTEQ is worried about using neutrino data on Fe to establish nucleon antiquark distributions. • MINERva – neutrino A dependence Smith and Miller

  20. Main Injector 120 GeV Tevatron 800 GeV Advantages of 120 GeV Main Injector The future: Fermilab E906 • Data in 2009 • 1H, 2H, and nuclear targets • 120 GeV proton Beam The (very successful) past: Fermilab E866/NuSea • Data in 1996-1997 • 1H, 2H, and nuclear targets • 800 GeV proton beam • Cross section scales as 1/s • 7 x that of 800 GeV beam • Backgrounds, primarily from J/ decays scale as s • 7 xLuminosity for same detector rate as 800 GeV beam 50 x statistics!! Fixed Target Beam lines

  21. Can we measure binding energy and spectator momentum dependence? • Test technical issue of how to include binding in calculation • Do we see nuclear dependence change for high momentum spectators which involve short distance interactions- Spectator tagging? SLAC fit to heavy nuclei (scaled to 3He) Calculations by Pandharipande and Benhar for 3He and 4He Approximate uncertainties for 12 GeV coverage

  22. Nuclear Effects in Spin Dependence • Why its big? • Quark-Meson Coupling model: • Lower Dirac component of confined light quark modified most by the scalar field

  23. Correlation between neutron skin thickness in finite nuclei and pressure of β-equilibrated matter in neutron stars Neutron Stars • probe densities to 6 ρNM • Is neutron matter superfluid? • low density – yes • higher density ??? • Do we see transition to kaon-condensed, hyperson, or quark matter ? • Nuclear Observables: • neutron skins • N/Z dependence of giant resonances • nuclear equation of state studies • Astronomical observations • What are the limits on mass and radii? • cooling? Recent observation of high mass neutron stars 2.1 ± 0.2 M Nice et al. astro-ph/0508050 2.1 ± 0.28 M, R=13.8 ± 1.8 km Ozel, Nature 441, 04858 (2006)

  24. Constraints on neutron star equations of state Mass-Radius constraints from observations and model predictions for the mass-radius of nucleonic stars, hybrid stars and strange quark stars. (From Jaikumar, Page and Reddy)

  25. What really happens at high density? Stone, Guichon, Matevosyan and Thomas

  26. Summary • Success • Two body correlations mapped out Dickhoff: “unique for a correlated many body system” • beginning to get information on three body correlations • correlations may be quite different in nuclei far from stability • Still to do – and a lot harder than we had hoped • QCD description of short range N-N behavior • definitive evidence for changes in proton structure in nuclei beyond easily understood (if hard to calculate) mean-field effects. • Spin and binding/spectator momentum effects • flavor dependence - extend nuclear anti-quark measurements to regions where effects may be much larger. • a long way to go to be confident about what happens in neutron stars

  27. xtarget xbeam Drell-Yan scattering: A laboratory for sea quarks Detector acceptance chooses xtarget and xbeam. • Fixed target  high xF = xbeam – xtarget • Valence Beam quarks at high-x. • Sea Target quarks at low/intermediate-x. E906 Spect. Monte Carlo

  28. Separating structure and dynamics

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