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The EMC Effect Exploring the Structure of Nucleons in Nuclei

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The EMC Effect Exploring the Structure of Nucleons in Nuclei The EMC Effect Exploring the Structure of Nucleons in Nuclei Dave Gaskell Thomas Jefferson National Accelerator Facility April 15, 2014 1 Outline • Nucleons in the nucleus • The EMC Effect – quarks in the nucleus – Early measurements – x, Q2, nuclear dependence, universality • Recent results and implications – EMC effect and local density – EMC-SRC (Short Range Correlations) connection – Flavor dependence • Summary 2 CEBAF’s Original Mission Statement • Key Mission and Principal Focus (1987): – The study of the largely unexplored transition between the nucleon-meson and the quark- gluon descriptions of nuclear matter. The Role of Quarks in Nuclear Physics • Related Areas of Study: – Do individual nucleons change their size, shape, and quark structure in the nuclear medium? – How do nucleons cluster in the nuclear medium? – What are the properties of the force which binds quarks into nucleons and nuclei at distances where this force is strong and the quark confinement mechanism is important? 3 Models of the Nucleus Mean field picture Nucleons move independently within an average potential (ex: Fermi gas) à No need to build up nucleus from all possible pairwise interactions à Very successful for describing nuclear shell structure, other nuclear properties à No mechanism for high momentum components in nuclear wave function, or clustering effects Nuclei from NN interactions Start with realistic model of NN interaction, build nucleus from pairwise interactions (Argonne v18 + Green’s Function Monte Carlo calculations) à Requires significant computing power à Excellent description of nucleon momentum distribution over full range (short and long distances) 4 Nuclei from NN Interactions • Starting from “effective” models of interactions between protons and neutrons – we can build up any nucleus we want à (only limited by computing power) 5 Nucleons in the Nucleus • In either picture (mean field or NN interaction), nucleons are the fundamental constituents of the nucleus* • Nucleon sub-structure not relevant in these models – Energy scales very different: Fermi momenta ~ hundreds of MeV, quark substructure relevant at GeV scales • We now know that quark distributions are modified in the nucleus à Is this important for our understanding of the nucleus? – Conversely, what are the origins of this modification? • Deep Inelastic Scattering provides an excellent probe for exploring modifications to nucleon structure in the nucleus 6 Deep Inelastic Scattering Cross section for inclusive lepton (electron) scattering: dσ α 2 E = L W µν dΩdE / Q4 E / µν 2 / 2 dσ 4α (E ) ⎡ 2 2 θ 2 2 θ ⎤ / = 4 W2 (ν,Q )cos + 2W1(ν,Q )sin dΩdE Q ⎣⎢ 2 2⎦⎥ 2 In the limit of large Q , 2 2 structure functions scale MW1(ν ,Q ) → F1(x) Q 2 x = νW1(ν ,Q ) → F2 (x) 2Mν 7 F2 and Parton Distributions • F2 interpreted in the quark-parton model as the charge-weighted sum over quark distributions F (x) e2 xq (x) 2 = ∑ i i i 2 2 • At finite Q , F2 not Q independent à scaling violations can be predicted in pQCD • At fixed x, scaling can be tested via logarithmic derivative of F2 w.r.t. 2 to Q d ln(F2 ) 2 = constant d ln(Q ) • In addition, corrections due to the finite mass of the nucleon lead to further scaling violations à these can be partially accounted for by examining data in terms of Nachtmann variable, x 2x ξ = 4M 2 x2 1+ 1+ Q2 8 Quarks in the Nucleus Fe p n Typical nuclear binding energies F2 /(ZF2 + (A− Z)F2 ) à MeV while DIS scales à GeV (super) Naïve expectation: A p n F2 (x) = ZF2 (x) + (A− Z)F2 (x) More sophisticated approach includes effects from Fermi motion M / m A A N N F2 (x) = dyfi ( y)F2 (x / y) ∑∫x i Quark distributions in nuclei were not expected to be significantly Figure from Bickerstaff and Thomas, different (below x=0.6) J. Phys. G 15, 1523 (1989) Calculation: Bodek and Ritchie PRD 23, 1070 (1981) 9 Discovery of the EMC Effect • First published measurement of nuclear dependence of F2 by the European Muon Collaboration in 1983 • Observed 2 mysterious effects – Significant enhancement at small x à Nuclear Pions! (see my thesis) – Depletion at large x à the “EMC Effect” Aubert et al, Phys. Lett. B123, 275 (1983) • Enhancement at x<0.1 later went away 10 Discovery of the EMC Effect • First published measurement of nuclear dependence of F2 by the 1.2 European Muon (D) Collaboration in 1983 2 • Observed 2 mysterious 1 (Fe) / F effects 2 F – Significant 0.8 enhancement at small x à Nuclear Pions! (see 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 my thesis) x – Depletion at large x à the “EMC Effect” Aubert et al, Nucl. Phys. B293, 740 (1987) • Enhancement at x<0.1 later went away 11 Confirmation of the Effect VOLUME 51, NUMBER 7 PHYSICAL REVIEW LETTERS 15 AUGUST 1~)8$ I I Rochester- SLAG- MIT(E87) R ochester- SLAC- Ml T(E49B) SLAC re-analysis of old (g &Q & 20 (GeV/'c) ) (2&Q'&20 ( solid target data used 0 FMC (9&Q &l70(GeV/c) ) l —o SLAC (0.9& g SLAG (Cu/D) Q'& for measurements of (0.~ 9& l.~ 6 ( Bodek- Ritchi cryotarget wall l.2- Sme Bodek- Ritchie Smeori backgrounds ~Fe Op lI II ll ll il à Effect for x>0.3 0.9— confirmed 0.8 '& Photo production à No large excess at (b) (Q =0, &=l5 GeV) &&~ Photoproduction very low x (Q =O, v= l5GeV), I I 0 0.2 0 4 0.6 0.8 l.0 0 0.2 0.4 0.6 0.8 l.o FIG. 1. (a) oA~/O'D and (b) or~/aD vs x. Only random errors are shown. Point-to-point systematic errors have been added linearly (outer bars) where applicable. The normalization errors of + 2.3% and + 1.1% for cr A~/oD (E498) and oF~/aD (E87), respectively,Bodek etare al,not PRLincluded. 50, All1431data (1983)for W» 1.and8 GeV PRLare 51,included. 534 (1983)The data have been cor- rected for the small neutron excess and have not been corrected for Fermi-motion effects. The curve indicates the expected ratio if Fermi-motion effects were the only effects present (Ref. 11). High-Q2az, /oD data from EMO (Ref. 2), Iow-g o'Ay/ao and ac„/o'D data from Ref. 9, and photoproduction o'A~/oD and oz, /oD data from Ref. 13 are = shown for comparison. The systematic error in the EMC data is + 1.5% at x 0.35 and increases to + G%%uo for the points at x= 0.05 and x= 0.65. 12 sumably higher-twist effects in the language of intercept at x=0 of 1.15+0.04+0.011 and a slope QCD, may be important. of -0.45~0.08. Our slope for steel is consistent Figure 1(b) shows our recent measurements' with the slope of —0.52 + 0.04+ 0.21 reported by of oz,/crD in a similar Q' range, and the EMC da- the EMC collaboration. ' The fitted slopes, which ta' at much higher Q'. Also shown a,re values' axe not affected by overall normalization uncex for oc„/oD for (Q') = l.2 (GeV/c)' as well as oF, / tainties, indicate that the nuclear distortions in gD from photoproduction data. " These data from aluminum and steel exhibit a simila, r trend. heavier targets taken together also indicate that The understanding of the mechanisms responsi- at low Q' shadowing effects may cancel some of ble for the distortion of the structure functions of the nuclear enhancement at low x. These addi- nucleons bound in a large nucleus has been the tional Q'-dependent nuclear higher-twist effects, subject of several recent theoretical papers. like higher-twist effects in the nucleon, are ex- These include ideas such as six-quark bags, " pected to be small at large values of Q'. There- pions and quasipions in nuclei, "delta resonances fore, the extraction of AQcD from structure-func- in nuclei, "diquark states, "a.nd percolation of tion data taken with nuclear targets at high values quarks from nucleon to nucleon in a large nucle- of Q' may not be affected by these terms. us. " The data indicate that there are three inter- We have performed a linear fit to the a„,/cD esting regions: (a) the low-x region where shad- ratios for our data in the range 0.2 & x & 0.6 [(Q') owing may be important at low Q', (b) the inter- = 5.35 (GeV/c)'] and obtain an intercept at x =0 of mediate-x region where quark distributions in nu- 1.11+0.02+ 0.023 (where the second error is sys- clei become distorted, and (c) the high-x region tematic) and a slope of —0.30+ 0.06. A similar where Fermi motion is important. The theoreti- fit to our crF, /crD results' [see Fig. 1(b)] over the cal understanding of these effects is still in a range 0.2 & x & 0.6 [(Q') =6.55 (GeV/c)'] yields an very qualitative state and new experiments de- 536 Subsequent Measurements A program of dedicated BCDMS (Fe) measurements quickly SLAC E139 (Fe) followed 1.2 EMC (Cu) D The resulting data is remarkably consistent over / A 1 a large range of beam energies and measurement techniques 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x 13 EMC Effect Measurements Laboratory/ Beam Energy Target Year collaboration (GeV) SLAC E87/E49B e 8.7-20 D, Al, Fe 1983 SLAC E139 e 8-24.5 D,4He, Be, C, Ca, Fe, Ag, Au 1994,1984 SLAC E140 e 3.75-19.5 D, Fe, Au 1992,1990 CERN NMC µ 90 6Li, 12C, 40Ca 1992 µ 200 D, 4He, C, Ca 1991, 1995 µ 200 Be, C, Al, Ca, Fe, Sn, Pb 1996 CERN BCDMS µ 200 D, Fe 1987 µ 280 D, N, Fe 1985 CERN EMC µ 100-280 D, Cu 1993 µ 280 D, C, Ca 1988 µ 100-280 D, C, Cu, Sn 1988 µ 280 H, D, Fe 1987 µ 100-280 D, Fe 1983 FNAL E665 µ 490 D, Xe 1992 µ 490 D, Xe 1992 DESY HERMES e 27 D, 3He, N, Kr 2000, 2003 Jefferson Lab e 6 D, 3He, 4He, Be, C, Cu, Au 2009 e 6 D, C, Cu, Au 2004 (thesis) Geesaman, Saito, and Thomas, Ann.
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