1. Spin-averaged Nucleon Structure at “Large” x
John Arrington
Physics Division
Argonne National Laboratory
HiX2014: The 4th International Workshop on Nucleon Structure at Large Bjorken x
Laboratori Nazionali di Frascati, Nov 17, 2014
Odd talk to be giving, given that this is my “low-x” program: x>1 (SRCs), x=1,3 (FFs, TPE), x~1 (CT), x<1 (EMC).

2. High-x experiments are providing benchmark data for hadron structure
Why high-x?
Valence structure of the hadron
Key aspect of structure in Constituent Quark Models
QCD and QCD-inspired predictions of x1 behavior
Combination of proton and neutron provides unique sensitivity to models/symmetries of nucleon distributions
Study quark – hadron transition, quark-hadron duality
Resonance structure of the hadron
Combination of proton and neutron provide sensitivity to isospin structure
High-x experiments are providing benchmark data for hadron structure

3. Why high-x now? Jefferson Lab is the “valence” machine
Dramatic progress in 6 GeV Era; remarkable coverage of resonance, duality physics
12 GeV provides huge opportunities in the duality/QCD region
Other experiments (Drell-Yan, n-A scattering) providing complementary new information
Future EIC provides some additional opportunities to explore high-x region
Physics plan still being fully developed
High-x structure at low Q2 provides key input (via evolution) to very high Q2

4. Large x is essential for particle physics
Large Hadron Collider
Parton distributions at large x are important input into simulations of hadronic background at colliders, e.g. the LHC.
High x at low Q2 evolves into low x at high Q2
Small uncertainties at high x are amplified
HERA anomaly: (1996): excess of neutral and charged current events at Q2 > 10,000 GeV2
Leptoquarks????
~0.5% added to u(x) at x > 0.75
S. Kuhlmann et al, PLB 409 (1997)
[Owens, Accardi, other PDF talks?]
Lake of Geneva
CMS
LHCb
Airport
ALICE
ATLAS

5. Outline Proton Neutron F2n/F2p, d(x)/u(x) Nuclei and nuclear effects
Duality
pdfs as x1, hadron structure
Neutron
Resonance structure, Duality
Recent extractions, new experimental techniques
F2n/F2p, d(x)/u(x)
Recent deuteron/proton, model-dependent extractions
Model-independent and model-less-dependent measurements
Flavor dependence: Drell-Yan, n-A scattering, form factors
Nuclei and nuclear effects
New information/ideas on A dependence, isospin dependence

6. Impact of proton structure function data
Data at very high x sensitive to:
Target mass corrections
High twist effects
Soft gluon resummation [Corcella]
Quark-hadron duality [Keppel]
Constrain u(x) with minimal sensitivity to nuclear effects [Owens, Accardi]
Test hadronic models
F2 ~ (1-x)a as x1
Much better with n/p ratio
Data taken in DIS at JLab, Drell-Yan at FNAL, with more on the way
FNAL E906/SeaQuest [Reimer]
JLab 12 [Malace]
CTEQ6X, A. Accardi et al, PRD 81, (2010)

7. Quark-Hadron Duality
Proton structure function in resonance region follows, on average, the DIS curve

8. Duality: there and back again
1970: Bloom-Gilman Duality [532 citations] 1996: “parasitic” measurements of duality in unpolarized e-p scattering I.Niculescu, et al., PRL 85 (2000) 1186; ibid, 85 (2000) 1182
Deuteron (neutron) duality
Nuclear duality (scaling)
Separated structure functions
Semi-inclusive duality
Parity-violating duality
Spin duality
Neutron structure
EMC effect
12 GeV SIDIS
Flavor dependence, nuclear effects in light nuclei
Both duality and duality-enabled physics having large impact
EMC/SRC correlation

9. Large-x parton distributions in Drell-Yan
xtarget
xbeam

10. Large-x parton distributions
Proton beam, proton target: 4u(x)+d(x) with no nuclear corrections
Sensitive to u(x)
Some sensitivity to d(x), combined with DIS and proton-deuteron Drell-Yan
Fermilab E906 will add much more precise high-x data.
JLab inclusive scattering from H, 2H at large x, Q2
Extend duality studies, increase reach/precision for DIS at large x
Planned for first running period in Hall C
JLab E : S. Malace, M. Christy, C. Keppel, M. Niculescu

11. Impact of neutron structure function data
Data at very high x sensitive to:
Resonance structure [Niculescu, Malace]
Quark-hadron duality [Keppel]
Neutron + proton data:
Separate u(x) and d(x) [Owens, Accardi]
Test hadronic models
F2n/F2p or d(x)/u(x) as x1
Approaches to extracting F2n/F2p
Model-dependent extraction from deuteron
Model-less-dependent extractions
3H/3He ratios
“Tagged” measurements on A=2
Model-independent extractions
PVDIS on proton
S. Malace, PRL 104 (2010)
CJ11, A. Accardi et al, PRD 84, (2011)

12. Extracting neutron resonance structure
First approach: extract neutron structure function from deuteron, proton data
S. Malace, PRL 104 (2010)
Left: proton, neutron, deuteron structure functions
Right: Ratio of F2(neutron) to theory calculation over 1st, 2nd, 3rd resonances and full resonance region

13. Spectator Tagging: Deuteron as an effective neutron target
“Tag” scattering from slow (nearly on-shell) nucleon in 2H
Proton and neutron have equal and opposite momenta
Use low-momentum spectator, coming opposite to virtual photon direction, to tag interaction with low-momentum nucleon (and correct for its momentum)
BONUS: Neutron structure function in resonance region, DIS for lower x
BONUS12: Extend DIS from tagged neutron to large x
L.L. Frankfurt and M.I. Strikman, Phys. Rep. 76, 217 (1981)
C. Ciofi degli Atti and S. Simula, Phys. Lett. B319, 23 (1993); Few-Body Systems 18, 55 (1995)
S. Simula, Phys. Lett. B387, 245 (1996); Few-Body Systems Suppl. 9, 466 (1995)
W. Melnitchouk, M. Sargsian and M.I. Strikman, Z. Phys. A359, 99 (1997)

14. Neutron structure function from tagged neutrons
F2n from BONUS, two beam energies Curve from fit to previous extraction from deuteron Duality tests for F2n [Niculescu]
S. Tkachenko, et al., PRC 89 (2014)

15. Focus on F2n/F2p, ratio of d(x)/u(x) as x  1
Several predictions based on simple assumptions about symmetries in proton
SU(6)-symmetric wave function of the proton in the quark model (spin up):
u and d quarks identical
N and D degenerate in mass
d/u = 1/2, F2n/F2p = 2/3
SU(6) symmetry is broken: N-D Mass Splitting
Mechanism produces mass splitting between S=1 and S=0 diquark spectator.
symmetric states are raised, antisymmetric states are lowered (~300 MeV).
S=1 suppressed => d/u = 0, F2n/F2p = 1/4, for x -> 1
pQCD: helicity conservation (qp) => d/u =2/(9+1) = 1/5, F2n/F2p = 3/7 for x -> 1
Dyson-Schwinger Eq.: Contains finite size S=0 and S=1 diquarks
d/u = 0.28, F2n/F2p = 0.49

16. PDF predictions at large-x
Nucleon Model
F2n/F2p
d/u
Du/u
Dd/d
A1n
A1p
SU(6)
2/3
1/2
-1/3
5/9
Valence Quark
1/4 1
DSE contact interaction
0.41
0.18
0.88
0.38
0.83
DSE realistic interaction
0.49
0.28
0.65
-0.26
0.17
0.59
pQCD
3/7
1/5

17. Neutron structure function as x  1
Attempts to extract F2n(x) from deuteron and proton data yield large range of results, depending on the model of the deuteron [green and red points]
Treat data as fixed Q2: neglects Q2 dependence of smearing function
Result can vary strongly with assumed Q2
PDF analyses used to assume d/u0
“Scaled EMC effect”: Use F2A/F2D data as measure of nuclear effects; scale to density of the deuteron
Neglects several effects:
Q2 dependence of smearing
A-dependence of Fermi motion
Difference in F2p and F2n

18. Neutron Structure Function
Two recent analysis addressed these issues
Structure function analysis: interpolate data to fixed Q (J. Arrington, et al., JPG 36 (2009) )
PDF analysis (CJ6.X): calculate smearing, TMC, for each x, Q2 point (A. Accardi, et al., PRD 81 (2010) )
Both avoid high-Q2 approximations in convolution
Assume identical p, n higher twist
Results shown for initial F2n/F2p analysis
Consistent with CTEQ6x analysis, which included off-shell effects similar to Melnitchouk and Thomas
JA, F.Coester, R.J.Holt, T.S.-H.Lee, J.Phys.G36, (2009)

19. Detailed investigations of model-dependence
Extracted n/p ratio including estimates of all uncertainties:
Vary N-N potential, convolution, off-shell effects
JA, W. Melnitchouk, J. Rubin, arXiv:

20. Detailed investigations of model-dependence
Extracted n/p ratio including estimates of all uncertainties:
Vary N-N potential, convolution, off-shell effects
NOT the best way to extract d/u
Similar analysis done for CJ12 pdfs
pdf style analysis can include more data (less sensitive to deuteron model), apply constraints (d(x)>0), go beyond LO,...
*A. Accardi, et al., PRD84 (2011)
Our analysis isolates model dependence, provides ‘baseline’ for comparison to model-independent n/p extractions
JA, W. Melnitchouk, J. Rubin, arXiv:

21. Detailed investigations of model-dependence
Separation of pdfs from TMC, HT correction, impose constraint d(x)>0 Include additional data, look at strangeness, antiquarks, etc…
Uncertainty dominated by data rather than model dependence: New high-x DIS data helps
Compared to F2n analysis:
Positivity constraint pushes results up
Bands shown here are full range of (many) models; F2n analysis shows estimated 1-sigma range
While this analysis appears to suggest larger d/u, results are in good agreement; main difference is the way they are presented
GEn ?
CJ12 J. Owens et al, PRD 87 (2013)

22. GEn as constraint on d(x)/u(x) as x  1?
Neutron transverse charge density
Neutron IMF transverse charge density has negative core
Distribution ‘squeezed’ to smaller impact parameter for high-x
Negative core comes from d-quark charge dominance at large x, i.e. u/d < 0.5 for neutron
 d/u < 0.5 for proton at some large x
Question: can this limit be made tighter?
Up Quark contribution
Down Quark
G. Miller and JA, PRC 78 (2008) (R)

23. Conclusions 3 approaches for reducing model dependence experimentally
F2n has “relatively” small uncertainty, including model dependence
F2n/F2p fairly well known to x=0.6, barring significant “EMC effect” in 2H
Additional high-x, high-Q2 data can further constrain F2n
E : Malace, Christy, Keppel, Niculescu
Model-independent constraints on F2n/F2p at large x still vital
Improved precision in extraction of F2n
Sensitive to nuclear effects in the deuteron through comparison to extraction from inclusive deuteron data
3 approaches for reducing model dependence experimentally
A=3
A=2
A=1

24. A=3 Solution: Add a nucleon
Problem: Extraction from A=2 introduces model-dependence
A=3 solution:
3H and 3He have larger nuclear corrections
n/p can be extracted from 3H/3He ratio (many systematics cancel)
n/p extraction sensitive only to difference between 3H, 3He corrections
No DIS data on the triton!
I. Afnan et al., Phys. Lett. B493, 36 (2000); Phys. Rev. C68, (2003)
E. Pace, G Salme, S. Scopetta, A. Kievsky, Phys. Rev. C64, (2001)
M. Sargsian, S. Simula, M. Strikman, Phys. Rev. C66, (2002)

25. A=3 Solution: Add a nucleon
Mirror symmetry of A=3 nuclei
Extract F2n/F2p from ratio of measured 3He/3H structure functions
R = ratio of ”EMC ratios” for 3He and 3H
Relies only on difference in nuclear effects
Calculated to within ~1% up to x  0.8

26. A=3 Solution: Add a nucleon
Thermo-mechanical design,
B. Brajuskovic et al, NIM A, arXiv:
E MARATHON: G. Petratos, J. Gomez, R.J.Holt, R. Ransome
Scheduled for 2016 at JLab
Hall A BigBite Spectrometer

27. A=2 Solution: “remove” a nucleon
“Tag” scattering from slow (nearly on-shell) nucleon in 2H
Proton and neutron have equal and opposite momenta
Use low-momentum spectator, coming opposite to virtual photon direction, to tag interaction with low-momentum nucleon (and correct for its momentum)
BONUS12: DIS from tagged neutron at large x
EIC: Forward spectator tagging will allow for broad set of measurements from tagged nearly on-shell (or far off-shell) nucleon

28. A=2 Solution: “remove” a nucleon
Tag low momentum, backward angle
spectator proton in deuteron: yields electron scattering from a free neutron target
GEM-based radial TPC in JLab CLAS spectrometer
Measure neutron structure function F2 to study quark structure of the nucleon at large x
Slide credit: C. Keppel

29. F2n/F2p from BONUS at 6 GeV Pspectator< 100 MeV/c
N.K. Baillie, et al, PRL 108, (2012)
Pspectator< 100 MeV/c
spectator < 90o
Textbook physics :)
Plot is in the new edition of Gauge Theories of the Strong, Weak, and Electromagnetic Interactions (Chris Quigg)
xBj coverage in ~DIS region not quite enough to get to most interesting region for d(x)/u(x)

30. Tagged Neutron in the Deuteron – BONUS + CLAS12
JLab E , S. Bueltmann, H. Fenker, M. Christy, C. Keppel, et al

31. Spectator tagging at an EIC [Nadel-Turonski]
Ee=8 GeV, EN=30 GeV
Luminosity=3.5E33
26 weeks at 50% efficiency
Detect ~30 GeV proton – “super BoNuS”
EIC in China or USA or …
EIC has > 100 x luminosity of HERA, polarized e and light ions, nuclear beams
Thanks to A. Accardi, R. Ent, C. Keppel 31

32. A=1 Solution: actually remove a nucleon
Use proton ‘target’, but modify sensitivity to u(x) vs d(x)
Drell-Yan with proton beam (high-x quarks), p/D target (low-x antiquarks)
Neutrino-scattering: requires proton target, limited x resolution
PVDIS: SOLID at JLab12 [Zheng]
MINOS with 2 years data using high energy tune, LH2 target nm m- d W+ m+ u W-

33. Slide credit: S. Riordan

34. Slide credit: S. Riordan

35. Nuclei and nuclear effects
EMC effect: Modification of high-x pdfs in nuclei
Light nuclei [Gaskell] – Unpolarized EMC effect
Antiquark EMC effect [Reimer, Kovarik, Kumano] – Drell-Yan on nuclei
Isospin dependence [Cloet] – Parity-violating electron scattering

36. EMC effect: A-dependence
SLAC E139
Most precise large-x data
Nuclei from A=4 to 197
Conclusions
Universal x-dependence
Magnitude varies
Scales with A (~A-1/3)
Scales with density
J. Gomez, et al., PRD49, 4349 (1994)

37. Importance of light nuclei
1) Mass vs. density dependence
4He is low mass, higher density
9Be is higher mass, low density
3He is low mass, low density (no data)
Calculations almost exclusively use nuclear matter, extrapolate to finite nuclei by scaling with density, A,…
2) Constrain 2H – free nucleon difference
JLab E03-013: JA and D.Gaskell
USING EXTRAPOLATION TO FREE NUCLEON HAS LIMITATIONS, BUT CLEARLY IMPROVED BY ADDING LIGHT NUCLEI

38. EMC effect in light nuclei
J.Seely, et al., PRL103, (2009)
Credit: P. Mueller
N. Fomin, et al., PRL108 (2012)
Short-range correlations in light nuclei
NOTHING SURPRISING WITHOUT 9Be; BOTH EFFECTS HAD ALWAYS BEEN ASSUMED TO SCALE APPROX WITH DENSITY.

39. Recent question: Isospin dependence of EMC effect
Typically assumed that EMC effect is identical for u(x) and d(x)
Becoming hard to believe, at least for non-isoscalar nuclei
EMC/SRC connection + SRC n-p dominance suggests enhanced EMC effect in minority nucleons [Gaskell]
48Ca, 208Pb expected to have significant neutron skin: neutrons preferentially sit near the surface, in low density regions
Recent calculations show difference for u-, d-quark, as result of scalar and vector mean-field potentials in asymmetric nuclear matter [I. Cloet, et al, PRL 109, (2012); PRL 102, (2009)] [Cloet]
In all 3 cases, neutron rich nuclei have enhanced EMC effect for up quarks, suppressed effect for down quarks --> cancellation in unpolarized EMC effect
Measurements:
EMC effect on 40Ca, 48Ca [Gaskell]
parity-violating DIS from 48Ca (SoLID spectrometer at JLab) [Zheng]

40. Summary
The large-x structure of hadrons is essential for a complete picture
Valence region defines the hadron
Test models of the nucleon
Large-x studies on nuclei probe the impact of colored medium
Large-x data important for understanding high-energy QCD studies
New 21st century tools have positioned us well for the next decade
JLab 6 GeV provided wealth of data for valence structure of hadrons
FNAL E906/SeaQuest, MINERnA, and other stand-alone experiments
JLab 12 GeV promises dramatic improvements and will address several key questions
A future EIC can provide unique high-x capabilities