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The EMC effect – history and future

The EMC effect – history and future. K. Rith, LNF Frascati 26.5.2008. Quark- and gluon -distributions are different for free nucleons and for bound nucleons inside nuclei. Open question : Do quarks and gluons play any role for the understanding of nuclear forces ?. Specifically :.

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The EMC effect – history and future

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  1. The EMC effect – history and future K. Rith, LNF Frascati 26.5.2008 Quark-andgluon-distributions are differentfor 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 ofnuclear 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 nucleonsswell 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 ofnucleon‘s momentumP, 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/Q4F2(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. 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 J.J. Aubert et al., Phys. Lett. 123B (1983) 275 statistical errors Published: March 31, 1983 25th anniversary A lot of excitement: up to now814 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. 207-222

  11. Data from SLAC - 1 e‘ e H D empty N1 = NWalls + NH,D N2 = NWalls NH,D = N1-N2 H,D 1970-72 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 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. Large-x behaviour Multiquarkclusters – Short Range Correlations? SLAC Origin: superfast nucleons and/or superfast quarks

  20. 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 (E12-06-105)

  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 innucleus A F2c(x/y, Q2): quark distribution inc A c x Badly known, a lot of freedom

  24. Change of confinement scale, swelling of nucleons, i.e.,Q2rescaling 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 2 Reasonable agreement for 0.3 < x < 0.7 room for additional contributions

  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. Parton-partonfusion: ‚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. Helium-4 4.00 Lithium-6 6.05 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. 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

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

  35. Dependence on nuclear mass A and density 

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

  37. Dependence on A1/3 or  ? Ultimate experiment: Polarised 67Ho98 (J = 7/2) 4He(=0.089)/3He(=0.051) JLAB-proposal E-03-103,… 4/3 =1.75 But: precise knowledge of F2n/F2p at large x required R4/R3 (4/3)1/3 =1.10 ??

  38. Q2-dependence

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

  40. 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

  41. 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

  42. 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

  43. 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

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

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

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