Hadron Physics with Real and Virtual Photons at JLab

Transcription

1 Hadron Physics with Real and Virtual Photons at JLab Elton S. Smith Jefferson Lab Virtual photons shape of the nucleon Elastic scattering (form factors) Inelastic scattering (uark distributions) Exclusive scattering (Generalized Parton Distributions) Real photons production of new particles Gluonic excitations Elton S. Smith APSE 2010, Osaka Japan June 14-18,

2 Why use electron and photon probes? Elton S. Smith APSE 2010, Osaka Japan June 14-18,

3 Nucleons are hydrogen atoms of QCD >1 fm nucleons fm bound constituent uarks < 0.1 fm bare uarks and glue S=1/2 S=1/2 S=1/2 Q< 1 GeV Q ~ 1 GeV Q> 1 GeV Elton S. Smith APSE 2010, Osaka Japan June 14-18,

4 Relevant degrees of freedom LQCD Bowman Phys.Rev.D (2005) Dynamic Quark Mass Constituent uark mass Constituent uark model with fixed uark masses only justified at photon point and low Q 2. Elton S. Smith APSE 2010, Osaka Japan June 14-18,

5 Real and virtual photon beams radiator Real Photon Energy = ν = E E Mass = 0 E target E θ Spatial resolution ~1/Q γ Virtual Photon Quantum fluctuation Energy = ν = E E Mass = -Q 2 = -4EE sin 2 θ/2 Elton S. Smith APSE 2010, Osaka Japan June 14-18, 2010 p 5

6 Pictures of the proton n π + m n = 940 MeV m π = 140 MeV d u u m u ~ 4 MeV m d ~ 8 MeV m p = 938 MeV Quark contribution to mass < 0.2% Quark contribution to spin < 25% Elton S. Smith APSE 2010, Osaka Japan June 14-18,

7 Neutron charge distribution data taken with deuterium target (must subtract proton contribution) neutron charge radius determined by neutron-electron scattering: <r E2 >= fm 2 why is neutron r.m.s. charge radius negative? p π π - ρ(r) <n> = + n p n 0 r 8 Q = 4πρ(r)r 2 dr = 0 <r 2 > = 4π ρ(r)r 4 dr 0 Elton S. Smith APSE 2010, Osaka Japan June 14-18,

8 Charge distributions and Form Factors Non-relativistic picture Elton S. Smith APSE 2010, Osaka Japan June 14-18,

9 First hints of proton substructure Otto Stern (1932) measured the proton magnetic moment µ p ~ 2.5 µ dirac (first indication that the proton was not a point-like particle) Robert Hofstadter used the Stanford University Mark III electron accelerator (energies from 100 to 236 MeV) for the first measurement of the proton s radius (~10-13 cm) via elastic electron scattering 1961 Nobel Prize in Physics Elton S. Smith APSE 2010, Osaka Japan June 14-18,

10 JLab accelerator CEBAF Continuous Electron Beam 1499 MHz operation Energy GeV 200 µa, polarization 75% Luminosity up to cm 2 s -1 de/e ~ 10-4 Simultaneous delivery 3 halls A B C Elton S. Smith APSE 2010, Osaka Japan June 14-18,

11 126 GeV CEBAF add Hall D (and beam line) Upgrade magnets and power supplies CHL-2 Enhance euipment in existing halls Elton S. Smith APSE 2010, Osaka Japan June 14-18,

12 Nucleon Form Factors: Last Ten Years Proton Neutron µ p G E p /GM p unexpected result! J. Arrington PANIC08 Elton S. Smith APSE 2010, Osaka Japan June 14-18,

13 Elton S. Smith APSE 2010, Osaka Japan June 14-18,

14 Gep: Charge and currents in the proton Naïve expectation is that the charge and currents are determined from the spatial distribution of uark charges and spins Constant ratio result obtained by traditional Rosenbluth measurements Asymptotic predictions using perturbative QCD (Brodsky 1975) anticipated constant ratio. Elton S. Smith APSE 2010, Osaka Japan June 14-18,

15 What do the polarization methods imply? Proton charge and current densities are different Neutron shows negative charge at large radius Elton S. Smith APSE 2010, Osaka Japan June 14-18,

16 Elastic and inclusive measurements inclusive measurements Excited State mass = W elastic scattering W 2 = M p 2 electron electron P Proton P photon electron P Proton electron photon xp Proton x is the longitudinal fraction of the proton s momenta carried by the struck uark Elton S. Smith APSE 2010, Osaka Japan June 14-18,

17 Modern Rutherford experiment Inclusive measurements Large-angle scattering implies atomic charge concentrated in small volume (nucleus) ~ sin 2 (θ/2) Nucleon charge concentrated in small volume (uarks) Elton S. Smith APSE 2010, Osaka Japan June 14-18,

18 1-dim view from inclusive measurements What have we learned? nucleon has a substructure made of uarks uarks are spin ½ objects Longitudinal momentum distribution of uarks (x) and (x) 50% of nucleon momentum is carried by uarks, the remainder by gluons Less than 25% of the nucleon spin is carried by uark helicity. Inclusive Scattering Compton Scattering Elton S. Smith APSE 2010, Osaka Japan June 14-18,

19 Deep Exclusive Scattering W 2 = (γ+p ) 2 electron γ P proton γ electron photon P Proton Deeply Virtual Compton Scattering (DVCS) Probes the nucleon uark structure and correlations at the amplitude level Elton S. Smith APSE 2010, Osaka Japan June 14-18,

20 DVCS is golden channel DVCS Bethe-Heitler e e γ + γ e e e e γ 2 p GPD s p p p p p dσ dx B dydtdφ 3 α = x ( ) 2 2 B y * * π e Q 1 + ε T BH DVCS Beam Spin Asymmetry ~ Im ( T ) T ~ H ( ξ, ξ, t)... T DVCS T BH BH T DVCS T DVCS T BH Elton S. Smith APSE 2010, Osaka Japan June 14-18,

21 Beyond form factors and uark distributions Generalized Parton Distributios X. Ji, D. Mueller, A. Radyushkin ( ) Proton form factors, transverse charge & current densities Correlated uark momentum and helicity distributions in transverse space - GPDs Structure functions, uark longitudinal momentum & helicity distributions Elton S. Smith APSE 2010, Osaka Japan June 14-18,

22 r F( ) = H ( x, d ) = Interpretation of the GPD s Analogy with form factors r i r 3 ρ re d r ( ) r measured relative to r R cm = m r M 2 i b b e f ( x, b ξ i i Charge Form Factor Parton Distribution GPD s = 0 b measured relative to R CM = x r i i where f ( x, b ) is a parton density of uarks with momentum fraction x at a distance b from R CM Ref. Burkardt Elton S. Smith APSE 2010, Osaka Japan June 14-18,

23 Insight from New Measurements New information on proton structure G E (Q 2 ) G M (Q 2 ) different charge, magnetization distributions Connection to GPDs: spin-space-momentum correlations Model-dependent extraction of charge, magnetization distribution of proton: J. Kelly, Phys. Rev. C 66, (2002) u-uark phase-space distributions at fixed x A.Belitsky, X.Ji, F.Yuan, PRD69: (2004) z(fm) x=0.1 x=0.4 1 fm x=0.7 Quark and nucleon spins aligned anti-aligned J. Arrington PANIC08 G.Miller, PRC 68: (2003) Elton S. Smith APSE 2010, Osaka Japan June 14-18,

24 Real Photons for production of Gluonic Excitations Elton S. Smith APSE 2010, Osaka Japan June 14-18,

25 Electromagnetic and color forces O(α) ~ 0.01 γ 1 r 2 ±1 charge O(α s ) ~ 1 g ±3 color charges Elton S. Smith APSE 2010, Osaka Japan June 14-18,

26 Nucleon resonances Atomic spectra show excited states with narrow line widths γ Emission spectrum from Iron Hadronic spectra show excited states with large line widths γ W=Massofresonance Elton S. Smith APSE 2010, Osaka Japan June 14-18,

27 Quarks are confined inside colorless hadrons Quarks combine to neutralize color force mesons baryons Configurations outside the standard uark model molecules pentauark glueball meson hybrid meson Elton S. Smith APSE 2010, Osaka Japan June 14-18,

28 Quark binding and configuration of gluons Flux tube JKM, Nucl. Phys. B (Proc. Suppl.) 63A-C (1998) 326 forms between From G. Bali Confinement arises from flux tubes andtheir excitation leads to a new spectrum of mesons Elton S. Smith APSE 2010, Osaka Japan June 14-18,

29 Normal Mesons color singlet bound states Spin/angular momentum configurations & radial excitations generate the known spectrum of light uark mesons. Starting with u - d - s we expect to find mesons grouped in nonets - each characterized by a given J, P and C. Spin 0 Spin 1 K 0 K + K* π π 0 η η π + ρ ω φ K K 0 K* J PC = Not-allowed: exotic J PC = Allowed combinations Elton S. Smith APSE 2010, Osaka Japan June 14-18,

30 Quantum Numbers of Hybrid Mesons Quarks Excited Flux Tube Hybrid Meson S = 0 L = 0 J PC = J PC = 0 + like π, K J PC = S = 1 L = 0 J PC = J PC = 1 like γ,ρ Exotic J PC = Flux tube excitation (and parallel uark spins) lead to exotic J PC Elton S. Smith APSE 2010, Osaka Japan June 14-18,

31 Mass Predictions Lowest mass expected to be π 1 (1 + ) at 1.9±0.2 GeV Lattice GeV GeV GeV Elton S. Smith APSE 2010, Osaka Japan June 14-18,

32 Photon beam and experimental area North linac Tagger area Hall D East arc Electron Beam dump Counting House Top View 75 m Photon Beam dump Radiator Electron beam Tagger Area Coherent Bremsstrahlung photon beam Collimator Solenoid- Based detector Experimental Hall D Elton S. Smith APSE 2010, Osaka Japan June 14-18, 2010 Page 32

33 Linearly Polarized Photon Beam Rates based on: 12 GeV endpoint 20 µm diamond crystal 300 na electron beam diamond collimator: 76m collimator diameter: 3.5 mm Leads to 10 7 γ/s on target (after the collimator) 4 Photon Beam Intensity Spectrum nominal tagging interval Design goal is to build an experiment with ultimate rate capability of 10 8 tagged γ/s on target. tagged δe/e = 0.1% Pol = 40% photon energy (GeV) Elton S. Smith APSE 2010, Osaka Japan June 14-18, 2010 Page 33

34 Finding an Exotic Wave An exotic wave (J PC = 1 -+ ) was generated at level of 2.5 % with 7 other waves. Events were smeared, accepted, passed to PWA fitter. X(exotic) ρπ 3π 500 events/20 MeV generated Mass Input: 1600 MeV Output: /- 3 MeV PWA fit Width Input: 170 MeV Output: 173 +/- 11 MeV Statistics shown here correspond to a few days of running. Double-blind M. C. exercise Mass (3 pions) (GeV) 1.8 Elton S. Smith APSE 2010, Osaka Japan June 14-18,

35 Summary Over the past 10 years, Jefferson Lab 6 GeV program has enriched our understanding of the current and charge distributions inside the proton and neutron. Measurements of exclusive reactions at 12 GeV will be possible due to thehigh luminosityofthe CEBAF 100% duty-cycle electron accelerator at Jefferson Lab. The Q 2 range will double in the region of valence uarks over that accessible today. Electron beams promise to deepen our understanding of the nucleon with studies of the Generalized Parton Distributions, which provide information about the transverse position distribution of uarks within the nucleon. The polarized photon beam will be used to map the spectrum of hybrid mesons with masses up to 2.5 GeV starting with those with the uniue signature of exotic J PC uantum numbers. Elton S. Smith APSE 2010, Osaka Japan June 14-18,