1. The EMC effect – history and future
K. Rith,
LNF Frascati
Quark- and gluon-distributions are different for free nucleons and for bound nucleons inside nuclei

2. Open question: Do quarks and gluons play any role for the understanding of nuclear forces?
Specifically:
Can at least the short-range part be directly described by the exchange of quarks, gluons or multigluon states? (Analogue: Van der Waals force)
Can the model of nuclear forces mediated by meson exchange currents be replaced by a fundamental theory based on the strong interaction between quarks and gluons?
Is confinement influenced by the nuclear medium?
Do nucleons swell due to the neighbourhood of other nucleons?
Do they form multiquark clusters or even one big bag?

3. Deep-inelastic Lepton-Nucleon-Scattering
hadrons
k‘= (E‘,k‘)
Q2 = -(k-k‘)2 = 2EE‘(1-cos)
 = E - E‘, y = /E
x = Q2/(2M) = fraction of nucleon‘s momentum P, carried by struck quark 
1/ Q2 xP *
nucleon P
k= (E, k)
From angular and momentum distribution of scattered leptons
Internal structure of the nucleon
Structure functions F1(x,Q2), F2(x,Q2), g(x,Q2)

4. d21/dxdQ2 = 42/Q4 F2(x,Q2)/x
[1 –y –Q2/4E2 + (1 -2m2/Q2)(y2 + Q2/E2)/(2[1 + R(x,Q2)])]
F2(x,Q2) = x  zq2 [ q(x,Q2) + q(x,Q2) ]
q = u,d,s,..
R(x,Q2) = [ F2(x,Q2) ( 1 + Q2/2 ) – 2xF1(x,Q2) ] / 2xF1(x,Q2)
If RA1(x,Q2) = RA2(x,Q2) :
(d21/dxdQ2)A1 / (d21/dxdQ2)A2 = F2A1(x,Q2)/ F2A2(x,Q2)

5. End of the 1970‘s:
Second generation of DIS experiments: CDHS, CHARM, CCFRR, CHIO, EMC, BCDMS majority used nuclear targets (Fe, CaCO3, C,.. )
Main aim: study scale breaking of structure functions predicted by QCD, determine QCD, gluon distribution g(x,Q2) via Altarelli-Parisi equations
Underlying assumption: Quark and gluon distributions obtained from nuclear targets are identical to those from free nucleons

6. Apart from Fermi-motion
Assumption: Nucleons do not change their internal properties (mass, radius, spin…) when being embedded in nuclei
Apart from Fermi-motion
Bodek, Ritchie
Berlad et al.
…………….. Frankfurt,
Strikman
qN(x) is convolution of
quark momentum distribution in free nucleon and
nucleon momentum distribution in nucleus

7. The EMC experiment at CERN
H2, D2
Fe calorimeter target

8. EMC data for F2N(Fe) and F2N(D)
Fit to Fe-data
Expectation for D-data including Fermi-motion

9. The EMC effect statistical errors
J.J. Aubert et al., Phys. Lett. 123B (1983) 275
statistical errors
Published: March 31, 1983 25th anniversary
A lot of excitement: up to now 814 citations

10. !
Consequence: Quark (and gluon) distributions are modified by the nuclear environment
Big surprise for high-energy physicists, but in principle expected by nuclear physicists and possible effects discussed in the 70th at several conferences about ‚Quarks in nuclei‘
First review:
Proceedings of the 18th Rencontre de Moriond, March 13-19, 1983, pp

11. Fe,Al Data from SLAC - 1 e‘ e H D N1 = NWalls + NH,D N2 = NWalls
empty
N1 = NWalls + NH,D
N2 = NWalls
NH,D = N1-N2
H,D
Archeology
1983
Fe,Al

12. Data from SLAC-1, archeology
A. Bodek et al., PRL 50 (1983) 1431; PRL 51 (1983) 543

13. Data from SLAC-2, dedicated experiment
R.G. Arnold et al., PRL 52 (1984) 727 ; J. Gomez et al., PRD 49 (1994) 4348 ?

14. Data from SLAC-2, A-dependence

15. Data from neutrino experiments

16. EMC Spectrometer – phase 3
Problem with old H and D data at low x due to correlated inefficiencies of drift chambers W4/5, cured by additional proportional chambers P4/5

17. Data from EMC – phase 3 No enhancement at very low x,
J. Ashman et al., PL B202 (1988) 603
No enhancement at very low x,
Some enhancement at 0.1 < x < 0.3

18. Shadowing data from EMC – phase 3
M. Arneodo et al., PL B211 (1988) 493

19. Multiquarkclusters – Short Range Correlations?
Large-x behaviour
Multiquarkclusters – Short Range Correlations?
SLAC
Origin: superfast nucleons and/or superfast quarks

20. Multiquarkclusters – Short Range Correlations?
Large-x behaviour
Multiquarkclusters – Short Range Correlations?
CLAS, K.S. Egiyan et al., P.R.L. 96 (2006)082501
To be studied in detail at JLAB12 – Hall C (E )

21. Overall picture of nuclear effects

22. Interpretation Reviews: e.g.: M. Arneodo, Phys. Rep. 240 (1994) 301
D.F. Geesaman et al., Ann. Rev. Nucl. Part. Sci. 45 (1995) 337
Several hundred publications with different approaches
No unique model for the whole x-range
Complications:
‚Any configuration of quarks, antiquarks and gluons
coupled to overall color-singlet can be expanded
in a basis of mesons, baryons and antibaryons‘
‚Nobody knows how to boost the wavefunction of a
bound system into the infinite momentum frame‘

23. Some approches Convolution F2A(x,Q2) =   dy fcA(y) F2c(x/y, Q2)
c = ‚cluster‘: N, , , 6q, ………
fcA(y): probability of finding ‚cluster‘ of
momentum y in nucleus A
F2c(x/y, Q2): quark distribution in c A
c x
Badly known, a lot of freedom

24. Change of confinement scale, swelling of nucleons, i.e., Q2 rescaling
Idea: relevant quantity is (QR)
Data should be identical for (QDRD)2 = (QARA)2
small x F2
large x
nucleus Q2
Require increase of about 15%
But: from quasielastic scattering (y-scaling): radius increase is at most ~3% (Sick et al.)

25. Change of nucleon mass, x-rescaling
pi = (M + Ei, pi ) N
Ei = removal energy A
xi‘ = Q2/2piq = Q2/[2(M+Ei) - 2 pi q]
x‘  x / (1 + <Ei>/M) > x, <Ei>  - 25 MeV
Contains both ‚binding correction‘ and ‚Fermi-motion‘

26. Conventional nuclear physics with improved nucleon wavefuctions, removal energies and correlated many body approach (applicable for 0.3 < x < 0.9 ?)
Example:
C. Ciofi degli Atti and S. Liuti, PL B225 (1988) 215
Reasonable agreement for 0.3 < x < room for additional contributions 2

27. Shadowing at high Q2
Generalized vector-meson dominance model in lab frame (property of photon)
mean free path: L = 1/( VN) 2.5 fm L
fluctuation length:d = 2/(mv2 + Q2)
 = 15 GeV d  10 fm d
d >> L
Absorption on surface
A/AN ~ A-1/3
d  1/Mx 1/(1 + mv2/Q2)
Effect dies out for x ~ 0.1

28. D ~ 1/Q2: transv. resolution
Parton-parton fusion: ‚overcrowding‘ of low-x partons in infinite momentum frame (property of nucleus)
d‘  d M/p
Lorentz contracted nucleon
D ~ 1/Q2: transv. resolution D
z ~ 1/xp: longt. size of gluon
z > d‘, i.e., x < 1/Md  0.1
DA‘ z
Low x gluons (and seaquarks) of different nucleons overlap and interact
Modified gluon and quark distributions

29. The NMC experiment at CERN
Main aims:
Precision measurement of F2p, F2D, F2n/F2p, F2p-F2n
Precision measurement of F2A1/F2A2 (x,Q2) and (RA1-RA2)(x,Q2) for several nuclei; dependence on nuclear density and radius

30. Collected statistics: ~2 108 DIS events
Helium
Lithium
Collected statistics: ~2 108 DIS events

31. Relevant publications from NMC:
P. Amaudruz et al., Z. Phys. C 51 (1991) 387
Z. Phys. C 53 (1992) 73
Phys. Lett. B 294 (1992) 120
Nucl. Phys. B 371 (1992) 553
M. Arneodo et al., Phys. Lett. B 332 (1994) 3
Nucl. Phys. B 441 (1995) 3
Nucl. Phys. B 441 (1995) 12
Nucl. Phys. B. 481 (1996) 3-22
Nucl. Phys. B 481 (1996) 23-39

32. (H)/(D) = (N11 N22)/(N12 N21) A1 A2 A3 A4 A5 A6 I1
Complementary target setup: Minimize systematic errors due to incident flux I and acceptance A A1 A2 I1 H D I2 D H
(H)/(D) = (N11 N22)/(N12 N21) A1 A2 A3 A4 A5 A6 I1 I2 I3 6
 / = ………………..

33. NMC – Example of target arrangement

36. Detailed study of shadowing region
E665: M.R. Adams et al., Phys. Rev. Lett. 68 (1992) 3266;
Z. Phys. C 67 (1995), 403

37. Dependence on nuclear mass A and density 

38. Dependence on nuclear radius A1/3
a + b A-1/3 + c A-2/3
a + b A-1/3

39. Ultimate experiment: Polarised 67Ho98 (J = 7/2)
Dependence on A1/3 or  ?
Ultimate experiment: Polarised 67Ho98 (J = 7/2)
4He(=0.089)/3He(=0.051)
JLAB-proposal E ,…
4/3 =1.75
But:
precise knowledge of F2n/F2p at large x required
R4/R3  (4/3)1/3 =1.10 ??

40. Q2-dependence

41. Q2-dependence
Sn/C
F2A1/F2A2 = a + b ln Q2

45. Gluon ‚overcrowding‘ in infinite momentum frame (property of nucleus)
d‘  d M/p
Lorentz contracted nucleon
D ~ 1/Q2: transv. resolution
z ~ 1/xp: longt. size of gluon
z > d‘, i.e., x < 1/Md  0.1
DA‘ z
Low x gluons (and seaquarks) of different nucleons overlap and interact
Modified gluon and quark distributions

46. Modification of gluon distribution
QCD: If quark distributions are modified by the nuclear environment, then also the gluon distribution must change
Is enhancement at 0.1 < x < 0.3 due to ‚merged‘ gluons?
Experimental tool: Inelastic J/-production
(Hard scale: mass of c-quark)
e+, +
e-, -
J/ * c pt c g c

47. Modification of gluon distribution
P. Amaudruz et al., Nucl. Phys. B 371 (1992) 553
Inelastic J/ production: GSN(x)/GC(x) = 1.13  0.08

48. Modification of gluon distribution
T. Gousset, H.J. Pirner, PLB 375 (1996) 349
f1(x) = F2Sn(x)/F2C(x);
r(x) = GSn(x)/GC(x) from Q2-dependence

49. Additional information from Drell-Yan
proton }X - +
xtarget
xbeam

50. Additional information from Drell-Yan (E772)
Selection: x1 – x2 > 0.3
Ratio ~ qA1 / qA2
No indication of enhancement of sea-quarks , Valence-only effect?

51. Additional information from Drell-Yan
George Bertsch in Science: Where are the nuclear pions? Answers in terms of Gluonic fields
Berger and Coester: Standard nuclear convolution:
F_2^A = \int dy f_N^A(y) F_2^N(x/y) + \int dy f_\pi^A(y) F_2^\pi(x/y)
where the F_2’s are the DIS structure functions of the nucleus, nucleon and pion and f_N/\pi^A are the nucleon/pion distribution functions. Other baryons are added as well and all is subject to conservation rules on baryon number and small f’s have total momentum conservation rules
Jung and Miller: Start with Berger and Coester, but uses different pionic distribution function in nucleus from Ericson and Thomas—points out that Berger and Coester should have used a light cone rather that instant-(time) form nuclear wave function.
Brown et al.: Brown, Rho rescaling—inside a nuclear environment masses of hadron composed of light quarks all decrease with density (at the same rate—see eqn 2.23). The rescaling is based on a partial restoration of Chiral Symmetry. This the also effects the coupling strengths. Based on this, one obtains a modified pion density distribution and follows the convolution model of Berger and Coester.
Dieperink and Korpa: First, range of curves is for various Migdal parameters. Looks at real and imaginary parts of particle-hole (nucleon-hole?) and \Delta-hole contributions. Also look at the self-energy of the pion propagator. The dressed pion propagator included nucleon and \Delta terms. These affect the pion distribution function (f_pi)
Very precise data expected from FNAL Main Injector DY

52. Additional information from neutrinos - MINERA

53. Additional information from neutrinos - MINERA
Also H,D
Linear combinations of () and ():
Separate valence (xF3) and sea (q)

54. Overall picture of nuclear effects

55. Outlook - Polarized EMC effect
I.C. Cloet, W. Bentz, A.W. Thomas, PLB 642 (2006) 210

57. Detailed study of shadowing region - 2

58. Dependence on nuclear radius A1/3 and scaling parameter n(x,Q2,A)
a + b A-1/3 + c A-2/3
a + b A-1/3
n  number of gluons probed by hadronic fluctuations of photon
B. Kopeliovich and B. Povh, PL B367 (1996) 329

59. Nuclear dependence of R = L/T
(d21/dxdQ2)A1 / (d21/dxdQ2)A2 = F2A1(x,Q2)/ F2A2(x,Q2)
requires RA1(x,Q2) = RA2(x,Q2)
Dependence of R on A could indicate nuclear effects on g(x) or different higher twist contributions to RA1 and RA2
Method: use different beam energies Ei
A1/A2(Ei) = (F2A1/F2A2)[(1+RA2)(1+RA1)] [(1+ziRA1)(1+ziRA2)]
 (F2A1/F2A2){1 – R (1-zi)/[(1+R)(1+ziR)]}
with
R = RA1 - RA2, R = ½(RA1 + RA2)
zi = [1 + ½ (yi2 + Q2/Ei2)/(1 – yi – Q2/4Ei2)] -1

60. Nuclear dependence of R = L/T
3 beam energies Ei: 120 GeV, 200 GeV, 280 GeV
R = RSn – RC =  (stat.)  (syst.)