How Does Short Distance Behavior Affect the Nucleus

How Does Short Distance BehaviorAffect the Nucleus

Don Geesaman12 January 2007DNP QCD Town Meeting

2Don Geesaman Short Distance Behavior in Nuclei 12 January 2007

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?


We want to describe a nucleus

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

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?


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


Our visual images

OR

“nucleons” held apart by short range repulsionbut even in 208Pb, half the nucleons are in the surface

average spacing at ρnm ~ 1.8 fmRadius of a nucleon ~ 0.8 fmaverage spacing at 3ρnm ~ 1.3 fm


What do we know about short distance behavior in nuclei?

Strong N-N potential does have impacts

NN Interaction NN Correlation Functions


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


Direct measurement

JLab E97-006Rohe et al.PRL 93, 182501 (04)

0.61+/- .06 protonsin pm>240 MeV/c and Em> 40 MeV


Spectroscopic factors


Distribution of spectroscopic strength (from Dickhoff)

Note ab initio calculations do very good job in, for example 7Li – SRC+LRC.


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

]/)exp[(11)(

aRrr

This is only illustrative. There are a number of other mechanisms that also lead to changes in the shell structure as N/Z varies.


Does the impact of correlations change dramatically away from valley of stability?

S = Sn-Sp for neutron knockout and

Sp-Sn for proton knockout

History1960’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.

22O34Ar

From Gade and Tostevin, NSCL

Note Rs is ratio to shell modelnot spectroscopic factor


Another way to look at high momentum components: x>1 data

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!

CLAS Egiyan et al.


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 resultS.R. Beane et al, PRL 97 (2006)


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


Alexa et al PRL 82, 1374 (99)

e-d elastic scattering

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.

Schiavilla et al PRL 94, 072303 (05)

)',(4 peeHe Perhaps the smoking gun?


Quark Meson Coupling predictions

)',(4 peeHe


Parton Distributions in Nuclei

1984 – Parton distributions are differentEMC 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

Alde et al (Ferm

ilab E772) P

hys. Rev. Lett. 64 2479 (1990)


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


Advantages of 120 GeV Main InjectorThe (very successful) past:

Fermilab E866/NuSeaFermilab E866/NuSea Data in 1996-1997 1H, 2H, and nuclear targets 800 GeV proton beam

The future:

Fermilab E906Fermilab E906 Data in 2009 1H, 2H, and nuclear targets 120 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 x Luminosity for same detector

rate as 800 GeV beam

50 50 x statistics!! statistics!!

Fixed Target

Beam lines

Tevatron 800 GeV

Main Injector

120 GeV


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


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


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 massneutron stars2.1 ± 0.2 M Nice et al. astro-ph/05080502.1 ± 0.28 M, R=13.8 ± 1.8 kmOzel, Nature 441, 04858 (2006)

Correlation between neutron skin thickness in finite nucleiand pressure of β-equilibrated matter in neutron stars


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)


What really happens at high density?

Stone, Guichon, Matevosyan and Thomas


Summary Success

– Two body correlations mapped outDickhoff: “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


xtarget xbeam

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.

Drell-Yan scattering: A laboratory for sea quarks

E906 S

pect.

Monte

Carlo


Separating structure and dynamics

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How Does Short Distance Behavior Affect the Nucleus