Overview of the SRC/EMC experiments

Transcription

1 Overview of the SRC/EMC experiments Donal Day Hall C Winter Meeting 2016 E E

2 PAC Report E (x > 1): Inclusive Scattering from Nuclei at x > 1 in the Quasi-elastic and Deep-inelastic Regimes E (EMC high-x): Detailed Studies of the Nuclear Dependence of F2 in the Light Nuclei The PAC considers this physics compelling. Although it is not yet clear that theory can directly connect these studies to the short-range part of the N-N interaction, the two are obviously related, and studies such as these may offer one of the few remaining routes to improving our knowledge of the N-N interaction in this corner of abiding uncertainty. The PAC had a very similar impression of these two experiments: both are top-quality experiments whose design reflects impressive insight into the SRC area. Both experiments involve compelling observables that will provide qualitative new information on short-range correlations in nuclei. For example, the x > 1 experiment (E ) will seek to establish or refute the existence of a second plateau at x > 2.2 in the heavy to deuteron ratio, hints of which have been observed in previous JLab data. Such a plateau would provide a dramatic signature of 3-body correlations at work and valuable information on their strength and isospin dependence. The committee was equally fascinated by EMC high-x (E ) and the ingenious suggestion of a connection between the EMC effect and the local density of nuclear matter, supported by similarities in their A-dependence and isospin-dependence. The PAC enthusiastically supports the full beam requests of both experiments: 32 and 23 days for E and E respectively, both in Hall C.

3 Structure of the nucleus ω,ρ π σ ~ 1 fm 0 =5 0 2N-SRC 1fm 3N-SRC =0.16 GeV/fm fm Determined by N-N potential nucleons are bound energy (E) distribution shell structure nucleons are not static momentum (k) distribution on average: binding energy: ~ 8 MeV distance: 1.7 fm r r d Densely packed at small distances multiples of NM Gold nucleus - 60% of the volume is occupied - very closely packed Medium modifications

4 Inclusive Electron Scattering from Nuclei Two distinct processes Quasielastic from the nucleons in the nucleus e k e 0 k + q, W 2 = M 2 e k e 0 W 2 (M n + m ) 2 M A M A 1, k M A M A 1, k x > 1 x < 1 Inelastic and DIS from the quark constituents of the nucleon. Inclusive final state means no separation of two dominant processes x = Q 2 /(2mυ) υ,ω=energy loss

5 k 1 k 2 q dσ 2 dω e de e = α2 Q 4 E e E e L µν W µν p p X The two processes share the same initial state P A P A - 1 QES in IA DIS d 2 d d / d 2 d d / Z Z d~k d~k Z Z de ei S i (k, E) Å } () Spectral function de W (p,n) 1,2 S i (k, E) Å } Spectral function The limits on the integrals are determined by the kinematics. Specific (x, Q 2 ) select specific pieces of the spectral function. n(k) = Z de S(k, E) However they have very different Q 2 dependencies σ ei elastic (form factor) 2 1/Q 4 W 1,2 scale with ln Q 2 dependence Exploit this dissimilar Q 2 dependence

6 Spectral Function, S(k,E) 3 He Strength is spread out in E, all of which must be integrated over to get n(k) n(k) = Z de S(k, E) A ridge at approx E = k 2 /2/m reflects the correlation in the gs

7 Q 2 = 1.5 QES Integration limits over spectral function The limits on the integrals are determined by the kinematics. Specific (x, Q 2 ) select specific pieces of the spectral function. Å } e e W 2 = M 2 n k M A M A 1, k DIS Q 2 = 1.5 e e k M A M A 1, k W 2 (M n + m ) 2 Å }

8 EMC (Before JLAB) F A 2 (x) 6= ZF p 2 (x)+nfn 2(x) J J Aubert et al Phys. Lett. B Followed by electron- and muon-scattering data from SLAC, Fermilab, and the New Muon collaboration (NMC) at CERN Accepted behavior the effect had a universal shape was independent of Q 2 increased with nuclear mass number A scaled with the average nuclear density Oct CERN Courier Everyone s Model is Cool bound nucleons are larger than free ones quarks in nuclei move in quark bags with 6, 9 and even up to 3A quarks, influence of nuclear binding, enhancement of pion-cloud effects and a nuclear pionic field No complete picture consistent with other data, i.e DY Shadowing: Attributed to the hadronic structure of the photon: Interference between single and multiple interactions caused the reduction in the cross-section known as shadowing. EMC: quarks in nuclei move throughout a larger confinement volume and, as the uncertainty principle implies, they carry less momentum than quarks in free nucleons.

9 Nuclear Dependence of the EMC Effect SLAC E139 studied the nuclear dependence of the EMC effect at fixed x Results consistent with Simple logarithmic A dependence Average nuclear density 4 He 4 He much lighter than 12 C, but has similar average 9 Be much lower density than 12C, but similar mass 3 He has low A and low density x=0.6 Many models of the EMC effect assume the size of the EMC effect scales with average nuclear density Constraining form of nuclear dependence can confirm or rule out this assumption Light nuclei help test scaling with mass vs. density Enter JLab E03-103

10 JLab E03-103: Light nuclei 12 C Consistent shape for all nuclei (curves show shape from SLAC fit) If shape (x-dependence) is same for all nuclei: the slope (0.35<x<0.7) can be used to study dependence on A EMC Effect and Local Nuclear Density 9 Be 4 He 8 J. Seely, et al., PRL103, (2009) Credit: P. Mueller, via John Arrington

11 Independent Particle Shell Model NN potential - AV18 Independent particle states of a uniform potential - a mean field. Wood-Saxon V (MeV) r(fm) Long mean free paths No two-body interactions The single-particle energies ξα and wave function Φα are the basic quantities - can be accessed in knockout reactions The spectral function should exhibit a structure at fixed energies with momentum distributions characteristic of the shell (orbit). Enormous strong force acting So many nucleons to collide with How can nucleons possibly complete whole orbits (10 21 /s) without interacting? Correlations must exist - and they do S( p, E) = X (p) 2 (E + )

12 How do we know short range correlations exist? 0.08 (e,e p) at NIKHEF FIG. 11. I r(urn) of Central density is saturated - nucleons can be packed only so close together: p ch * (A/Z) = constant J.W. Negele RMP 54 (913) 1982 relative spectroscopic factor (%) He 4 He 6 Li 7 Li 12 C 16 O 30 Si 31P32S 40 Ca 48 Ca 51 V 208 Pb 90 Zr Occupation numbers scaled down by a factor mass number A

13 Theory suggests that SRCs are a common feature for all nuclei What many calculations indicate is that the tail of n(k) for different nuclei has a similar shape - reflecting that the NN interaction, common to all nuclei, is the source of these dynamical correlations. n(k) = Z de S(k, E) n(k) Ciofi/Simula Benhar k (MeV/c) k < 250 MeV/c 85% of nucleons 40% of KE k > 250 MeV/c 15% of nucleons 60% of KE pmin (GeV/c) Selection by kinematics x

14 SRC in (e,e ) at JLab Simple SRC Model: 1N, 2N, 3N contributions dominate at x 1, 2, 3 2N, 3N configurations at rest (total p pair = 0) Isospin independent K. Egiyan et al, PRL96, (2006) CLAS - <Q 2 > 1.5 GeV 2 E Q 2 =2.7 GeV 2 N. Fomin, et al., PRL 108 (2012) Experimental observations: Clear evidence for 2N-SRC at x>1.5 Suggestion of 3N-SRC plateau(?) Isospin dependence? 22

15 Connection between SRCs and EMC effect: Importance of two-body correlations? J. Seely, et al., PRL103, (2009) N. Fomin, et al., PRL 108, (2012) J. Arrington, A. Daniel, D. Day, N. Fomin, D. Gaskell, P. Solvignon, PRC 86 (2012) Given the fact that the inclusive data integrate over very different parts of the spectral function this probably deserves more study. L. Weinstein, et al., PRL 106, (2011) O. Hen, et al, PRC 85, (2012)

16 Nuclear Dependence of EMC and SRCs Arrington et al, PRC 86, (2012) High virtuality Local density a 2 number of high momentum nucleons (pn) pairs N tot /N iso* R 2N accounts for difference in pair counting for EMC/SRC; nucleons close together Detailed study of nuclear dependence of EMC effect and SRCs does not favor either picture Can we distinguish between these two pictures via some new observable? Flavor dependence of the EMC effect

17 Flavor dependence and SRCs 4 He 2-body density from np/4 High momentum nucleons from SRCs emerge from tensor part of NN interaction np pairs dominate à Probability to find 2 nucleons close together nearly the same for np, nn, pp pp For r 12 < 1.7 fm: S.C. Pieper and R.B. Wiringa, Ann. Rev. Nucl. Part. Sci 51, 53 (2001) If EMC effect due to high virtuality, flavor dependence of EMC effect emerges naturally à If EMC effect from local density, np/pp/nn pairs all contribute (roughly) equally

18 Flavor dependence and SRCs High momentum nucleons in the nucleus come primarily from np pairs à The relative probability to find a high momentum proton is larger than for neutron for N>Z nuclei - protons get paired often more. Probability to find SRC Under the assumption the EMC effect comes from high virtuality (high momentum nucleons), effect driven by protons (u-quark dominates) > similar flavor dependence is seen in some meanfield approaches M. Sargsian, arxiv: [nucl-th] and arxiv: [nucl-th]

19 Flavor Dependence of the EMC Effect Mean-field calculations predict a flavor dependent EMC effect for N Z nuclei Medium modified quark distributions Free nucleon quark distributions Cloët, Bentz, and Thomas, PRL 102, (2009) Isovector-vector mean field (ρ) causes u (d) quark to feel additional vector attraction (repulsion) in N Z nuclei Experimentally, this flavor dependence has not been observed directly

20 Flavor dependence from 40 Ca and 48 Ca CBT model predicts a ~3% effect for 48 Ca at x=0.6 à N/Z = 1.4 Assuming no flavor dependence, difference between 40 Ca and 48 Ca should be less than < 1% Will be measured at 12 GeV E Spokespersons: Arrington, Gaskell, Daniel σ A /σ D x

21 σ vs energy loss F 2 versus x F 2 versus ξ Heavier nuclei QEP is averaged out - duality quark pdfs at x > 1 =2x/(1 + p 1+4M 2 x 2 /Q 2 ) In OPE F 2 0 (not F 2 ) should be independent Q 2 in absence of QCD evolution and higher twists. F 2 (x, Q 2 )= x2 2 r 3 F0 2(, Q 2 ) + 6M2 x 3 Q 2 r 4 h 2(, Q 2 )+ 12M4 x 4 Q 4 r 5 g 2 (, Q 2 ) PDFs at x > 1 sensitive to SRC. The bulk of the strength for x come from the high momentum nucleons generated by short range correlations in nuclei. Large Q 2, large x additionally sensitive to small admixtures of exotic components - e.g. 5% 6q cluster in D leads to dramatic effect on large x pdfs: Mulders and Thomas F 2 (0) (,Q 2 ) E BCDMS SLAC = 1.25 = Q 2 (GeV) 2 Fomin et. al., Phys.Rev.Lett. 105 (2010)

22 Q 2 2 ut as source of the EMC effect, because they would require very large non-hadronic componen hich were often excluded by other measurements. Figure 12 provides a simple example: It show e nuclear structure function for deuterium, as calculated from a convolution of neutron and proto ructure functions (red), and compares it to the structure function Q 2 obtained by assuming that 5 > 17 for x > 1 f the deuteron wave function is described by a 6-quark bag, using the model of Mulders an homas [34] for the quark distribution for the 6-q bag. The difference is at most 2% througho e region of large EMC effect on the (0.3 short < x range < 0.8), correlations and so that one provide would need the high-momentum an extremelypart large of exot the s omponent Red - QES, in nuclei Blue to - explain RR, Green the EMC - DIS, effect line in terms total of this (convolution kind of non-hadronic model) allow us to separate the contribution of superfast quarks that come contribution from high uclei. and those that come from other configurations in nuclei. Sensitivity to non-hadronic components 5% 6-quark bag x<1 x>1 5% 6-quark bag Mulders & Thomas

23 .few remaining routes to improving our knowledge of the N-N interaction in this corner of abiding uncertainty. N-N interaction L S J = 1 L T(L+S+T odd) 2S+1 L J S S P P P P D D D D 3 Possible Two-nucleon Nucleon states The SR NN attraction dominated tensor interaction, which yields high momentum isosinglet (np) pairs. Absent in the isotriplet channel (pp, nn, np). The two-body distribution should be identical to the deuteron distribution, n 2 (k) = n D (k), and the ratio of scattering cross sections between a heavy nucleus A and the deuteron to yield a 2 (A, Z ) Explains the SRC ratios, isospin asymmetry

24 Tensor force responsible for dominant part of SRC and correlations are largely of pn pairs ρ NN (q,q=0) (fm 6 ) He 4 He Schiavilla et al. PRL 98, Li (2007), VMC and AV18/UIX 8 Be q (fm -1 ) JLAB A(e,e NN) data from Hall A R. Subedi et al. Science 320, 1476 (2008)

25 EMC Å} HMS Å} super-fast quarks quark distribution functions medium modifications What s planned? XEMC part 2 SHMS HMS Å} SHMS Å} SRC, n(k), σ FSI

26 E Overview in greater detail Request 23 days of beam time in Hall C Higher Q 2, expanded range in x Lower-x coverage improves shape comparison Large expansion in high-x DIS region Angle scan to test Q 2 -independence More complete set of light nuclei 40 Ca, 48 Ca comparison Improved 3 He extraction θ=35 o for high-x coverage: W 2 >2 0.3<x<0.92; improved high-x coverage, mainly needed for isoscalar ratios, e.g. 3 He/(D+p) 6 GeV DIS range 11 GeV DIS range θ=20 o for most nuclei: W 2 >2 0.1<x<0.88 with high cross section, small pion and charge-symmetric backgrounds, reasonable radiative corrections Main 6 GeV setting

27 neutrons # of Targets Isotopes with measured charge radii Isotopes with measured charge radii 20 proposed included N = Z protons # of We like to to add additional heavier nuclei Vary N/Z for approximately fixed mass Vary mass for approximately fixed N/Z EMC/SRC target set Start trying to disentangle A dependence and N/Z dependence 1 H 2 H 3 He 4 He 6,7 Li 9 Be 10,11 B 12 C 40 Ca 48 Ca Seek out ranges of n/p ratios for isospin dependence studies Cu Au Z N Sym A Ti Ni Ni Ru Pd Ag U Th 228 n/p n/p world set proposed included generated on by Donal Day Avg density Average Density

28 E and E Hall C experiments will provide more inclusive data à E x>1 à E EMC Effect Will provide additional data on light and medium-heavy targets à 2 H, 3 He, 4 He à 6 Li, 7 Li, Be, 10 B, 11 B, C à Al, 40 Ca, 48 Ca, Cu First running in Hall C after completion of 12 GeV Upgrade will include a few days for EMC/x>1 measurements on 10 B, 11 B, and Al (parasitic) - begins???? Beam Time request for 2018?