Measurement of the EMC Effect in Light Nuclei

Measurement of the EMC Effect in Light Nuclei and at Large x Dave Gaskell Jefferson Lab Nu. INT 07 May 31 2007 1. Motivation – why more studies of the EMC Effect? 2. JLab Experiment E 03 -103 3. Some preliminary results

EMC Effect and Quark Distributions in Nuclei • Measurements of (EMC, SLAC, BCDMS) have demonstrated modification of quark distributions in nuclei • x-dependence is the same for all nuclei – Shadowing: x<0. 1 – Anti-shadowing: 0. 1<x<0. 3 – EMC effect: x>0. 3 • Size of the effect depends on A (i. e. , minimum at x=0. 7)

Explaining the EMC Effect • Models of the EMC Effect fall into two broad categories – “Conventional” nuclear physics approaches – these can include • Fermi motion alone or with binding effects • More realistic nuclear structure (spectral functions) • Exchange mesons (nuclear pions) – “Exotic” models – some examples are • Multiquark clusters • Dynamical rescaling • All models can (approximately) describe the EMC effect region – Some inconsistent with other data (Drell-Yan) – Others fail to describe all x – Some describe the EMC effect ignoring binding/conventional nuclear effects

EMC Effect Calculations Benhar, Pandharipande, and Sick Phys. Lett. B 410, 79 (1997) Conventional Models • Some combination of Fermi motion and binding • Fermi motion + binding + nuclear pions Exotic Models • Dynamical rescaling • Multiquark clusters K. E. Lassila and U. P. Sakhatme Phys. Lett. B 209, 343 (1988)

Existing EMC Data • SLAC E 139 most extensive and precise data set for x>0. 2 • Measured s. A/s. D for A=4 to 197 – 4 He, 9 Be, C, 27 Al, 40 Ca, 56 Fe, 108 Ag, and 197 Au – Size at fixed x varies with A, but shape is nearly constant • Data set could be improved with – Higher precision data for 4 He – Addition of 3 He data – Precision data at large x SLAC E 139

JLab Experiment E 03 -103 • Measurement of the EMC Effect in light nuclei (3 He and 4 He) and at large x spokespersons DG and J. Arrington, graduate students J. Seely and A. Daniel – 3 He, 4 He amenable to calculations using “exact” nuclear wave functions – Large x dominated by binding, conventional nuclear effects • Ran in Hall C at JLab summer and fall 2004 (w/E 02 -109 – see Donal Day’s talk) • A(e, e’) at 5. 77 Ge. V – Targets: H, 2 H, 3 He, 4 He, Be, C, Al, Cu, and Au – Six angles to measure Q 2 dependence

EMC Effect at Low W 2 • At large x (>0. 6), E 03 -103 not in canonical DIS regime W 2>4 Ge. V 2 • Nuclear structure functions show scaling at relatively low Q 2 in the resonance region (duality) – J. Arrington , et al. , PRC 64: 014602 (2001) • EMC ratio extracted using resonance region data from JLab E 89 -008 – 1. 2<W 2<3. 0 Ge. V 2 at Q 2≈4 Ge. V 2 • Where there is overlap, JLab resonance data agrees well with SLAC (DIS) results J. Arrington, et al. , PRC 73: 035205 (2006)

E 03 -103 (and E 02 -019) Analysis • Analysis approaching final stages • “Data processing” pretty much final – Track reconstruction – Efficiencies – Charge symmetric backgrounds • 1 st pass cross sections have been used to iterate: – Radiative corrections – Bin-centering corrections • To do: – Radiative corrections for heavy (thick) targets – Coulomb corrections

Preliminary E 03 -103 Results: Carbon • E 03 -103 C/D ratios show good agreement with SLAC/EMC data • Serves as good check of analysis procedure (charge symmetric background, radiative corrections)

Preliminary E 03 -103 Results: Carbon • C/D ratio used to perform detailed check of Q 2 dependence • Data taken at 10 Q 2 settings • No Q 2 dependence seen for W 2>1. 5 Ge. V 2 and Q 2>2. 5 Ge. V 2

Preliminary E 03 -103 Results: 4 He • 4 He/D ratio consistent with SLAC result • Consistent with SLAC parameterization with A=12

Carbon to 4 He Comparison Preliminary 4 He/D, C/D ratios C/4 He: nuclear dependence of cross section nearly identical

Preliminary E 03 -103 Results: 3 He/D ratio shows significant EMC effect larger than most calculations shape at large x differs from heavier nuclei Note large correction for proton excess SLAC fit vs. NMC fit for n/p ~ few % effect on ratio Most calculations of n/p that include off-shell effects are fit to SLAC data – we can check consistency

Heavy Targets - Coulomb Corrections • Initial (scattered) electrons are accelerated (decelerated) in Coulomb field of nucleus with Z protons – Not accounted for in typical radiative corrections – Usually, not a large effect at high energy machines – not true at JLab (6 Ge. V!) • E 03 -103 uses modified Effective Momentum Approximation (EMA), Aste and Trautmann, Eur, Phys. J. A 26, 167 -178(2005) – E E+D, E’ E’+D – D = -¾ V 0, V 0 = 3 a(Z-1)/(2 rc)

Preliminary E 03 -103 Results: Gold • E 03 -103 data w/Coulomb corrections (5 -10%) • Coulomb corrections for SLAC data (not applied) would be 1% • E 03 -103 results still pre-preliminary • Radiative corrections are large target RL ~ 6% • Some residual Q 2 dependence not yet understood Coulomb corrections prescription?

EMC Effect at Large x • JLab resonance region data suggested that shape at large x changes with A – Large x “crossover” moves to larger x for heavier nuclei • Preliminary E 03 -103 results see no significant A dependence to large x shape • Large x behavior provides stringent test of EMC effect models – shape dominated by binding and Fermi motion

Summary • JLab E 03 -103 has measured – Precision nuclear structure ratios for light nuclei – Structure function ratios at large x for a large range of nuclei (3 He Au) • Some preliminary conclusions – Structure function ratios appear to scale for W 2<4 Ge. V 2 – EMC effect in 4 He about as large as for Carbon – EMC effect in 3 He not small – Shape of structure ratio at large x similar for nuclei with A>3 • With E 02 -019, will provide a large body of cross section data on nuclear targets over a large Q 2 range

A-dependence of EMC Effect • EMC Effect models must predict correct Adependence – Density dependent vs. A dependent? • Carbon and 4 He have similar density • Preliminary C/D, 4 He/D results favor denisty dependent EMC Effect 4 He 12 C

Isoscalar Corrections 1. SLAC param. (1 -0. 8 x) 2. CTEQ 3. NMC fit Isoscalar correction applied to data 3 He Au