The EMC Effect Gerald A. Miller, Univ. of Washington

Transcription

1 I NTERNATIONAL JOURNAL OF HIGH-ENERGY PHYSICS CERNCOURIER The EMC Effect Gerald A. Miller, Univ. of Washington V OLUME 53 NUMBER 4 M AY 2013 Deep in the nucleus: a puzzle revisited HEAVY IONS The key to fi nding out if a collision is head on p31 ASTROWATCH Planck reveals an almost perfect universe p12 IT S A HIGGS BOSON The new particle is identifi ed p21 Higinbotham, Miller, Hen CERN Courier 53N4( 13)24 What is the EMC effect? What does it mean? 1 What are the consequences?

2 EMC Effect -Outline Basic issues -what is the EMC effect, why important? Simple ideas What is Drell-Yan pair production, why relevant? Why nucleon structure must be modified for nucleons in nuclei Simple models and examples of tests Two-nucleon effects are important Necessities for future progress 2

3 Fundamental questions of Nuclear Physics Is the nucleus made only of nucleons and mesons? How does the nucleus emerge from QCD, a theory of quarks and gluons? Why is the nucleon-meson picture of nuclei so accurate? Is the gluonic content of the nucleus the same as for nucleons in free space? Is there a non-perturbative sea in the nucleon? No one asked such questions before the EMC effect 3 was discovered

4 Primer- Deep Inelastic P e Q x P + Scattering- large Electron loses energy and is scattered by some angle: two variables:,q 2 Large,Q 2 scattering on elementary quark. Four-momentum conservation! dynamics depend on x = Q2 2M x ratio of quark p + to proton momentum P + (P ± P 0 ± P 3 ) The EMC effect involves deep inelastic scattering from nuclei EMC= European Muon Collaboration ,Q 2 Nucleon from PDG NNPDF2.3 (NNLO) g/10 s c 4 2 xf(x,µ 2 =10 GeV ) x u d v d u v

5 The EMC EFFECT Fig. 2: The image shows the ratio of deep-inelastic cross sections of Ca to D from NMC (solid circles) and SLAC (open circles). The downward slope from 0.3 < x < 0.7 and subsequent rise from x B > 0.7 is a universal characteristic of EMC data and has became known as the EMC effect. The reduction of the ratio at lower values of x B, where valence Fig. 1: Image of the EMC data as it appeared in the November 1982 issue of the CERN Courier. This image nearly derailed the highly cited refereed publication (Aubert et al., 1983), as the editor argued that the data had already been published. Valence quark momentum is reduced Nucleon structure is modified: valence quark momentum depleted. How do quarks work in a nucleus? BUT EFFECTS ARE SMALL ~15% 5

6 The EMC EFFECT Fig. 2: The image shows the ratio of deep-inelastic cross sections of Ca to D from NMC (solid circles) and SLAC (open circles). The downward slope from 0.3 < x < 0.7 and subsequent rise from x B > 0.7 is a universal characteristic of EMC data and has became known as the EMC effect. The reduction of the ratio at lower values of x B, where valence Fig. 1: Image of the EMC data as it appeared in the November 1982 issue of the CERN Courier. This image nearly derailed the highly cited refereed publication (Aubert et al., 1983), as the editor argued that the data had already been published. Valence quark momentum is reduced Nucleon structure is modified: valence quark momentum depleted. How do quarks work in a nucleus? BUT EFFECTS ARE SMALL ~15% 5

7 Simple models of reduction of valence quark momentum quarks in nuclei move through a larger confinement volume (uncertainty principle) bound nucleons are larger than free ones nucleons in nuclei move in 6, 9 or 3A quark bags nuclear binding, Fermi motion, pionic effects 6

8 Simple models of reduction of valence quark momentum quarks in nuclei move through a larger confinement volume (uncertainty principle) bound nucleons are larger than free ones nucleons in nuclei move in 6, 9 or 3A quark bags nuclear binding, Fermi motion, pionic effects EMC Everyone s Model is Cool (1985) 6

9 One thing I learned since 85

10 One thing I learned since 85 One model is not cool- unmodified nucleon -free structure function

11 Deep Inelastic scattering from nucleinucleons only free structure function Hugenholz van Hove theorem nuclear stability implies (in rest frame) P + =P - =M A Smith Miller 02 Binding causes no EMC effect SLAC-E139 Pb P + =A(M N - 8 MeV) average nucleon k + k + =M N -8 MeV, Not much spread F 2A /A~F 2N no EMC effect Smith & Miller Phys.Rev. C65 (2002) , Phys.Rev. C66 (2002) Phys.Rev. C65 (2002)

12 Nucleons and pions P A + = P N + + P π + =M A P π + /M A =.04, explain EMC try Drell-Yan, Bickerstaff, Birse, Miller 84 proton(x 1 ) nucleus(x 2 ) x 1 x 2 Phys.Rev.D33:3228,1986 Phys.Rev.Lett.53:2532,1984.

13 Nucleons and pions P A + = P N + + P π + =M A P π + /M A =.04, explain EMC Drell-Yan, E772 PRL 69,1726 (92) π fails No one s model is cool

14 Nucleons and pions P A + = P N + + P π + =M A P π + /M A =.04, explain EMC Drell-Yan, E772 PRL 69,1726 (92) π fails No one s model is cool Bertsch, Frankfurt, Strikman crisis in nuclear theory conventional physics does not work

15 Nucleons, pions, shadowing, universal nucleon modification A model containing all of these aspects does account for DIS and DY-Kulagin Petti No underlying dynamics-can t predict other phenomena 11

16 Single nucleon modification by nuclei Does it make sense? It is inevitable. Pion cloud modified by nucleus, W cloud too Neutron in nucleus is modified, lifetime changed from 15 minutes to forever Binding changes energy denominator, suppresses component peν Change energy denominator change wave fun Also Strong fields polarize nucleons- analog of Stark effect p n

17 Inevitability of medium modifications-(e,e p) (, ~q) (E f,~p+ ~q ) M A p Form factor modified because p 2 6= M 2 Problem: conventional nuclear wave function involves on-mass shell nucleons (E s, ~p ) Nucleus A-1 13

18 Single nucleon in a nucleus 1947 Shell Model 1968 SLAC-MIT nucleon not a point:! quarks don t interact R R 1982 EMC or 14

19 Nucl.Phys. B250 (1985) PLC Suppression Frankfurt Strikman 1995 BLC Frank Jennings Miller Phys.Rev. C54 (1996) Nucleon has large blob-like configuration BLC and small point like configuration PLC! BLC is affected by nucleus PLC, not affected. Result PLC suppressed/blc enhanced 15

20 Nucleon in medium- 5 models Nuclear matter External fields 1. QMC- quarks in nucleons (MIT bag) exchange mesons with nuclear medium, quark mass 2. Use NJL instead of bag Cloét 3. CQSM- quarks in nucleons (soliton) exchange infinite pairs of pions, vector mesons with nuclear medium, m q 4. Suppression of point-likeconfigurations, 5. Enhancement of blob-like configurations polarization

21 Spin experiments-njl in medium g 1n, g 1p in nuclei! other way to enhance EMC? ratio of g 1 medium to free Bentz, Cloet, Thomas Spin Kuhn talk

22 Chiral Quark Soliton Model Diakonov, Petrov, Polykov, quarks couple to vacuum instantons Vacuum dominated by Negele et al hep-lat/ instantons topological charge density quarks with spontaneously generated masses interact with pions!! Nucleon is soliton in pion field M=420 MeV good nucleon properties, DIS and magnetic moments

23 Chiral Quark Soliton Model of Nucleus- Smith, Miller 2 π exchange attraction ω (vector meson) exchange - repulsion Phys.Rev. C72 (2005) Phys.Rev. C70 (2004) Phys.Rev.Lett. 91 (2003) , Phys.Rev.Lett. 98 (2007) Double self consistency profile function and k f

24 Results Smith & Miller 03,04,05 g 1 ratio EMC ratio full Nuclear Drell Yan valence only sea is not much modified About same as DIS, not larger contrasts QMC Kuhn talk

25 Another medium effect: form factors CQS QMC Changes in the internal structure of bound nucleons result also in bound nucleon form factors. Observable effects predicted: = = = CQS: J.R. Smith and G.A. Miller, Phys. Rev. C 70, (2004) QMC: D.H. Lu et al., Phys. Lett. B 417, 217 (1998) NJL: I.C. Cloet, W. Bentz, and A.W. Thomas (to be published) S Strauch slide Chiral Quark Soliton (CQS), Quark Meson Coupling (QMC), Skyrme, Nambu-Jona-Lasinio (NJL), GPD Models. Model predictions: are density and Q 2 dependent, show similar behavior, consistent with experimental data (within large uncertainties). 21 7

26 1 S. Strauch ratio of G E /G M 2 H and 1 H polarization-transfer data are similar low pm data 10% Effect 4 He data are significantly different than 2 H, 1 H data 2 H: B. Hu et al., PRC 73, (2006). 4 He: S. Dieterich et al., PLB 500, 47 (2001); S. S., et al., PRL 91, (2003); M. Paolone, et al., PRL 105, (2010); S. Malace et al., PRL 106, (2011) 22

27 Enhancement of Blob-like Configurations- Frank,Jennings, Miller PRC 54, 920 free place in medium: normal size components attracted energy goes down PLC does not interact- color screening-fs BLC is enhanced quarks lose momentum in medium

28 1995 Frank, Jennings, Miller

29 Enhancement of Blob-Like Configurations energy denominator increased EMC ratio Frank,Jennings Miller 95 FS-PLC has NO int. with medium evaluated as QCD Stark, not modified energy denominator

30 More data [4-1] J. Seely et al., Phys. Rev. Lett. 103, (2009) [4-2] N. Fomin, et al., Phys. Rev. Lett. 108, (2012) (a) (b) (c) Figure 4.1: The electron scattering cross section ratios (per-nucleon) of various nuclei to deuterium. (a) Ratios for 0.2 < x B < 0.9 [4-1]; (b) Ratios for 12 C for 0.3 < x B < 2 [4-1, 4-2]; (c) Ratios for 0.8 < x B < 1.8 [4-2]. [4-4] L.B. Weinstein et al., Phys. Rev. Lett. 106, (2011) Figure 4.2: (left) The strength of the EMC effect (the EMC slope) plotted versus the average nuclear density [4-1]. (right) The strength of the EMC effect plotted versus the SRC scale factors [4-4]. The drawing in the upper left shows deep inelastic electron scattering from a quark in a nucleon. The drawing in the lower right shows 26

31 More data [4-1] J. Seely et al., Phys. Rev. Lett. 103, (2009) [4-2] N. Fomin, et al., Phys. Rev. Lett. 108, (2012) (a) (b) (c) Figure 4.1: The electron scattering cross section ratios (per-nucleon) of various nuclei to deuterium. (a) Ratios for 0.2 < x B < 0.9 [4-1]; (b) Ratios for 12 C for 0.3 < x B < 2 [4-1, 4-2]; (c) Ratios for 0.8 < x B < 1.8 [4-2]. [4-4] L.B. Weinstein et al., Phys. Rev. Lett. 106, (2011) Coincidence? Figure 4.2: (left) The strength of the EMC effect (the EMC slope) plotted versus the average nuclear density [4-1]. (right) The strength of the EMC effect plotted versus the SRC scale factors [4-4]. The drawing in the upper left shows deep inelastic electron scattering from a quark in a nucleon. The drawing in the lower right shows 26

32 Inclusive A(e,e ) measurements! At high nucleon momentum, distributions are similar in shape for light and heavy nuclei: SCALING. n A Adapted from Ciofi degli Atti ( k) = C n ( k) A D high k! Short distance two-nucleon relative wave function same in all nuclei.! One can get the probability of 2N-SRC in any nucleus, from the scaling factor. Piasetzky talk 27

33 Implications of data Previous models need to be extended to a larger range of x Detailed nuclear dependence must be explained (Dutta talk) Challenge: is EMC effect best described as being on single nucleon or a correlated pair of nucleons? 28

34 Are nucleons in the SRC modified? Need to start with a nuclear model of SRC and compute resulting EMC effect caused by modified structure function Medium modification due to mean field (previously discussed models) alternate hypothesis 29

35 Phenomenology Hen et al Int. J. Mod. Phys.E22, ( 13) Ciofi degli Atti & Simula, PRC53,1689 (96)-nuclear model- Spectral function has two terms - low momentum, mean-field MF part 80%, high momentum short-ranged correlations SRC 20% Strikman & Frankurt: relate nuclear DIS to nucleon structure function using spectral function PLB 183, 254 (87) Modification of nucleon structure function is associated with MF or with SRC- two models 30

36 niversal modification to nucleon in SRC Universal modification to nucleon in MF Which gives a better fit? 31 Int. J. Mod. Phys.E22, ( 13)

37 ([1]* (x-[0])+[2]*(x -[0])^2)/((1./56)*(5./9)*1.094*s qrt (x )*(2 6* 2* (1-x )^ *(9. / 8)* (1-x )^4 )+ (11. / 9)* *(1-x)^7) ([1]* (x-[0])+[2]*(x- [0])^2)/((1./56)*(5./9)*1.094*s qrt (x )*(2 6* 2* (1-x )^ *(9. / 8)*(1 -x )^4 )+ (11. / 9)* *(1-x)^7) M o Modified Structure Functions Modified Structure Function (SRC) Modified Structure Function MF N / F 2 N F N / F 2 N F x x Is the EMC effect due to nucleons in SRC pairs or to nucleons moving in the mean field? 32

38 ([1]* (x-[0])+[2]*(x -[0])^2)/((1./56)*(5./9)*1.094*s qrt (x )*(2 6* 2* (1-x )^ *(9. / 8)* (1-x )^4 )+ (11. / 9)* *(1-x)^7) ([1]* (x-[0])+[2]*(x- [0])^2)/((1./56)*(5./9)*1.094*s qrt (x )*(2 6* 2* (1-x )^ *(9. / 8)*(1 -x )^4 )+ (11. / 9)* *(1-x)^7) M o Modified Structure Functions Modified Structure Function (SRC) Modified Structure Function MF N / F 2 N F N / F 2 N F x x Is the EMC effect due to nucleons in SRC pairs or to nucleons moving in the mean field? YES 32

39 Requirements & Goals Model the free distributions Good support Consistency with nuclear properties Describe deep inelastic and di-muon production data- valence plus sea describe detailed A (N,Z) dependence Predict new phenomena

40 Ways to search for medium modification of nucleon structure Quasi-elastic scattering, Coulomb sum rule form factors of bound nucleons Quasi-elastic, recoil polarization- DIS on deuteron, detect spectator A dependence Parity violating DIS G E /G M 34

41 Summary nucleon structure is modified by nucleus minimum model requirements- EMC, DY, nuclear saturation, A-dependence predict new phenomena needed better evaluations of models experimental tests form factors in medium, ( ea! e 0 XN ) spectator tag, PVDIS nuclear gluon distribution?? new experiments Jlab and others to find out how quarks work in a nucleus

42 Where do we stand after 33 years? 36

43 Where do we stand after 33 years? 36