New measurements of the EMC effect in few-body nuclei

John Arrington, Physics Division, Argonne

Second meeting of the APS Topical Group on Hadron Physics

GHP06, Nashville, TN

23 Oct 2006

2

Overview

Nuclear structure functions and EMC effects– Review of measurements – Limitations in the data

JLab E03-103– Status of the analysis– Preliminary results and future plans

3

Nuclear structure functions and the EMC effect

Structure functions related to nuclear quark distributions

– Non-trivial nuclear dependence (“EMC effect”) observed by EMC, BCDMS, SLAC experiments

– Much larger than effect expected from Fermi motion

– Significant (20+%) deviations between heavy nuclei and deuterium

Universal dependence on xBj

– Shadowing : x<0.1

– Anti-shadowing: 0.1<x<0.3

– EMC effect x>0.3

Size of the effect varies with AE139 (Fe) EMC (Cu) BCDMS (Fe)

4

EMC effect: Large x region

SLAC E139

– Most precise data for large x region

– Nuclei from A=4 to 197

Conclusions

– Independent of Q2

– Universal x-dependence (shape) for all A

– Magnitude varies with A• Scales with ave. density• Scales with A

Limitations

– Poor precision at large x

– Limited data for low A

5

EMC effect: Easy to explain (too easy?)

Many “exotic” explanations

– Fermi motion insufficient

– Dynamical rescaling, nuclear pions, multi-quark clusters, medium modification to nucleon structure, etc…

More careful treatment of “conventional” effects

– Reexamination of binding effects

– Effects of nucleon correlations

Unique explanation?

– Most models could provide an EMC-like effect• Some could explain only part of the observed effect• Many appeared inconsistent with other measurements

such as Drell-Yan or limits on medium modification• Others fully explained the data without any contribution

from binding.

6

EMC effect: Conventional vs. exotic

More systematic understanding necessary

– Fermi motion and binding are clearly important, and must be included in any evaluation of “exotic” effects

– A reliable binding calculation will tell us if we’re need a new explanation for THIS or THIS

– Binding calculations must be evaluated against high-x data, which is dominated by Fermi motion and binding

– Current data is limited at large x, where one can evaluate binding models, and low-A, where uncertainties due to nuclear structure are smallest

Benhar, Pandharipande, and Sick, Phys. Lett. B469, 19 (1999)

7

EMC effect: Importance of few-body nuclei

Calculations predict different x-dependence for 3He and 4He

– Different models predict different x-dependence

– Some models predict different shapes for 3He and 4He

– Smaller nuclear structure uncertainties

Sensitive to A-dependence

– Scaling EMC effect with <> very different from A

Data lower quality

– Lower precision for 4He

– No data at large x for 3He

8

EMC effect: JLab E03-103

3He and 4He calculations by Pandharipande and Benhar

SLAC fit to heavy nuclei (scaled to 3He)

HERMES data E03103 projected uncertainty

– Lower Q2 allows access to large-x region, but only for W2<4 GeV2

JLab E03-103: JA and D. Gaskell, spokespersons, Jason Seely and Aji Daniel – thesis students

– Measured structure function for 1H, 2H, 3He, 4He, Be, C, Al, Cu, and Au

– High density 3,4He cryotargets allow improved statistics and systematics compared to SLAC E139

9

EMC effect: Scaling at lower Q2, W2

Measurement is not entirely in usual DIS region (W2>4 GeV2)

– DIS-scaling behavior has been observed to extend to lower Q2, W2 in nuclei

– Yields resonance region EMC ratios identical to DIS results

– Can be understood as consequence of quark-hadron duality

– E03-103 data at higher Q2, with precise measurement of Q2 dependence

JA, et al., PRC73:035205 (2006)

B.W.Filippone, et al., PRC45:1582 (1992) JA, et al., PRC64:014602 (2001)

W.Melnitchouk, R. Ent, and C. Keppel, Phys. Rept. 406:127 (2005)

Q2 ¼ 4 GeV2

1.3 < W2 < 2.8 GeV2

10

E03-103: Experimental details

Source of uncertainty

Statistics

*Density fluctuations

Absolute density

* - size of correction is 8% at 4 uA vs.

4% at 80 uA

SLAC E139

1.0-1.2%

1.4%

2.1%

JLab E03-103

0.5-0.7%

0.4%

1.0%

Main improvement over SLAC due to improved He targets:

Requires larger scattering angle to reach same Q2

Larger contamination Large charge-symmetric

background Larger Coulomb

distortion corrections

Main drawback is lower beam energy

Ratio of e+ to e- production

11

Ran in Hall C at Jefferson Lab, summer/fall of 2004 (EMC and x>1) Cross section extraction

– Calibrations, efficiency corrections, background subtraction completed

– Finalizing model-dependence in radiative corrections, bin centering, etc…

– Investigating Coulomb distortion corrections (heavy nuclei) EMC Ratios

– Isoscalar EMC correction (requires n/p)

E03-103: Analysis status

12

E03-103: Carbon EMC ratio

Preliminary results, 12C/2H ratio compared to SLAC and EMC

13

E03-103: Carbon EMC ratio

Data taken for 12C and 2H at ten Q2 setting

– Measure Q2 dependence of the nuclear structure functions

– Verify Q2 independence of the EMC ratios Extracted EMC ratio for Carbon for the five highest Q2 settings

Data show no indication of Q2 dependence, even at the lowest values of Q2, W2 shown

21435

14

E03-103: Helium-4 EMC ratio

Preliminary results, 4He/2H ratio compared to SLAC

Results consistent with SLAC fit to A=12 (using A-dependent fit)

Consistent with density-dependent fit to A=4

15

E03-103: Comparison of Carbon and Helium-4

Preliminary results, 4He/2H and 12C/2H ratios

12C/4He ratio compared to previous 12C EMC ratios

16

E03-103: Comparison of Carbon and Helium-4

12C much heavier than 4He, but has similar average density

Preliminary results for 4He/2H and 12C/2H ratios favor density-dependent EMC effect

4He 12C

17

E03-103: Helium-3 EMC ratio

Preliminary results, 3He EMC ratio compared to HERMES

Blue is uncorrected 3He/2H ratio

Green includes correction for proton excess in 3He (isoscalar EMC ratio)

Larger EMC effect than most models

Different x-dependence than calculations, heavier nuclei

Might suggest larger EMC effect than expected for deuterium

However, result is very sensitive to isoscalar correction

18

E03-103: Helium-3 EMC ratio

Good news:– 197Au correction is

roughly equal but has opposite sign; constrain combination of n/p and A-dependence of EMC [Working on Coulomb corrections, and checks on radiative corrections for heavy nuclei]

– 3He/(2H+1H) ratio less sensitive to neutron cross section

Bad news:

– Large correction (5-20%) to extract isoscalar ratio

– SLAC, NMC fits to n/p yield corrections that differ by 2-4% for x<0.6; difference changes sign at large x

19

Summary, and more to come…

Preliminary results from E03-103

– EMC effect nearly identical for 4He and 12C

– Indications of a large EMC effect in 3He

– Ratios independent of Q2 down to quite low Q2 values

Results on the way

– More reliable 3He isoscalar ratios, ratios to proton+deuteron

– EMC ratios for Be, C, Al, Cu, Au – emphasis on large x region

– Absolute cross sections for 1H, 2H, 3He, 4He

• Test models of n/p + nuclear effects in few-body nuclei

• E02-019: absolute cross sections for x>1 for 2H, 3He, 4He

– More detailed, quantitative studies of the Q2 dependence• Structure functions• Ratios

20

Scaling of the nuclear structure functions

F2(x,Q2) consistent with QCD evolution in Q2 for low x values (x<0.5)

Huge scaling violations at large x (especially for x>1)

F2(,Q2) consistent with QCD evolution in Q2 to much larger values

Scaling violations are mostly the “target-mass” corrections (plus a clear contribution from the QE peak)

Nearly independent of A

21

Scaling of the nuclear structure functions

Low Q2 JLab data (from E89-008, 4 GeV) are consistent with extrapolated structure function from high Q2 SLAC data [ fixed dln(F2)/dln(Q2) ]

Above =0.65, there is a large gap between JLab, SLAC data, but there are indications of scaling up to =0.75

Our data fill in the gap up to ¼0.75, show indications of scaling up to ¼0.9

22

EMC ratios as a function of x and

EMC ratios vs x and – Small difference for x<.4

– SLAC E139 and JLab E03-103 have similar range in Q2, nearly identical difference between x,

SLAC E139

JLab E03-103

23

Isoscalar corrections: 3He and 197Au

F2n/F2p from SLAC, CTEQ, NMC (bottom)

Correction to 3He (top right) and 197Au (bottom right)

24

25

Nuclear Structure: High Momentum Nucleons

Short-range structure in nuclei [E02-019 (Arrington)]

– Inclusive quasielastic scattering from nuclei at x>1

– Study distribution of extremely high momentum nucleons (>1 GeV/c) in 2H, 3He, 4He, and heavy nuclei

– Goal is to understand high-momentum components and map out strength of Short Range Correlations (SRCs) in nuclei• Important part of nuclear structure, relevant to topics in nuclear and

particle physics: neutron star structure, medium modification in sub-threshold hadron production, and -A interactions in supernovae, in neutrino oscillation experiments, and in 13 measurements

– Ran in 2004, data analysis in progress

26

Properties of Bound Nucleons: EMC effect

Medium modification: The EMC effect

– Clear observation of nuclear (density) dependence in parton distributions

– Still no consensus on the cause of the effect• Binding is important at all x values, but may also require modification to

internal structure of the nucleon

EMC effect in 3He and 4He[E03-103 (Arrington)]

– Allows reliable few-body calculations of binding

– Test A dependence and x dependence for very light nuclei

– First 3He data above x=0.4, factor of two improvement for 4He

Projected 3He uncertainties

HERMES

27

12C

Properties of Bound Nucleons: EMC effect

Preliminary results: A- and x-dependence

– SLAC E139: Can scale with ln(A) or <>

– New data suggest it scales with density

– Also indicates identical x-dependence for 4He and 12C

PRELIMINARY(statistical errors)

PRELIMINARY(statistical errors)

x=0.6

x=0.6

4He

4He

28

Properties of Bound Nucleons: JLab @ 12 GeV

EMC effect

– Map out A-dependence at large x (test binding calculations)

– Precise measurements in anti-shadowing region

DIS from 3He and 3H

– Ratio of 3H/3He provides measurements of n/p and d(x)/u(x), without uncertainty associated with nuclear effects

– Combine with precise 3He/2H ratio to extract isoscalar EMC effect for A=3

Inclusive scattering from nuclei at x>1

– At 6 GeV, dominated by QE scattering, map out distribution of high-p nucleons

– At 12 GeV, begin to probe quark distributions at x>1

Three proposals for PAC30