Electromagnetic Nuclear Physics Overview Seamus Riordan Stony Brook - PowerPoint PPT Presentation

Electromagnetic Nuclear Physics Overview Seamus Riordan Stony Brook University seamus.riordan@stonybrook.edu June 17, 2015 Seamus Riordan — Cornell IEB 2015 Nucl. EM 1/30

EM Nuclear Physics Overview ... summarize the current experimental situation, and highlight opportunities for progress with high-current electron beams in the 10-500 MeV energy range. 10-500 MeV range covers: E ∼ Few 100 MeV - nucleon properties, lowest resonances E > π - π at threshold E ∼ Few - 10s MeV - Nuclear excitations Both real and virtual γ interactions have been critical in our understanding of the strong nuclear force Broadly FF, neutron, isovector, and polarization observables are popular experimental areas Seamus Riordan — Cornell IEB 2015 Nucl. EM 2/30

Nucleon Structure Protons and neutrons are the “ground state” of QCD E < 500 MeV probes non-perturbative structures Important to consider elastic processes (static structure), polarizabilities, and intermediate state properties Seamus Riordan — Cornell IEB 2015 Nucl. EM 3/30

Form Factors for Nucleons Scattering matrix element, M ∼ j µ J µ Q 2 Generalizing to spin 1/2 with arbitrary structure, one-photon exchange, using parity conservation, current conservation the current parameterized by two form factors j µ J µ = e ¯ F 1 ( q 2 ) γ ν + i κ u ( p ′ ) 2 M q ν σ µν F 2 ( q 2 ) � � u ( p ) µ q Form Factors Dirac - F 1 , chirality non-flip µ p’ p µ Pauli - F 2 , chirality flip µ J Seamus Riordan — Cornell IEB 2015 Nucl. EM 4/30

Sachs Form Factors Replace with Sachs Form Factors G E = F 1 − κτ F 2 G M = F 1 + κ F 2 Limit as Q 2 → 0 E ( Q 2 = 0) = 1 , M ( Q 2 = 0) = µ p = G p G p 2 . 79 E ( Q 2 = 0) = 0 , M ( Q 2 = 0) = µ n = G n G n − 1 . 91 − 6 dG EM � Q 2 → 0 = � r 2 EM � � dQ 2 � Rosenbluth Formula � � � E ′ G 2 E + τ G 2 , τ = Q 2 d Ω = d σ d σ M tan 2 θ � + 2 τ G 2 M � 4 M 2 d Ω E 1 + τ 2 � � Mott Seamus Riordan — Cornell IEB 2015 Nucl. EM 5/30

G E / G M through Spin Observables Akhiezer and Rekalo (1968) - Polarization offers access to G E / G M Typically have fewer systematic contributions from nuclear structure and radiative effects eN , e ′ � N ′ Polarization Transfer, � ( E e + E e ′ ) tan θ e / 2 G E = − P t 2 M G M P l polarization axis e � N , e ′ N ′ Polarized Beam/Target � e’ φ∗ θ e momentum transfer θ∗ ω, q � 2 τ ( τ + 1) tan( θ/ 2) G E / G M e A ⊥ = − ( G E / G M ) 2 + ( τ + 2 τ (1 + τ ) tan 2 ( θ/ 2)) Seamus Riordan — Cornell IEB 2015 Nucl. EM 6/30

Proton Results G p 1 M generally follow dipole - G D = � 2 exponential distribution 1 + Q 2 / (0 . 71 GeV 2 ) � 1.1 1.5 1.0 M D p G /G 1.0 p E µ p G / M p p Janssens G Borkowski µ Bartel 0.9 Sill Litt 0.5 Berger Bosted Walker Walker Andivahis Andivahis Christy 0.0 10 -2 10 -1 10 -1 1 10 1 10 2 2 2 2 Q [GeV ] Q [GeV ] JLab, Jones et al. , G p E different from G n M using polarization Neglect of hard two-photon exchange can cause systematic errors in extraction Results testing this are now being produced Seamus Riordan — Cornell IEB 2015 Nucl. EM 7/30

Proton Results G p 1 M generally follow dipole - G D = � 2 exponential distribution 1 + Q 2 / (0 . 71 GeV 2 ) � 1.1 1.0 RCQM - G. Miller (2005) π Diquark - Cloet (2012) 1.0 D M p 0.5 G /G p µ p E G / M p p G Borkowski µ 0.9 Sill Punjabi 0.0 Bosted Gayou Walker Puckett Reanalysis Andivahis Puckett 0 2 4 6 8 10 10 -2 10 -1 1 10 2 2 2 2 Q [GeV ] Q [GeV ] JLab, Jones et al. , G p E different from G n M using polarization Neglect of hard two-photon exchange can cause systematic errors in extraction Results testing this are now being produced Seamus Riordan — Cornell IEB 2015 Nucl. EM 7/30

Two-photon Exchange Results - CLAS, VEPP-III 1.04 Results from CLAS and 1.03 VEPP-III with e + / e − available R 2 γ 1.02 LNP Kinematic coverage over broad 1.01 ǫ and Q 2 up to ∼ 1 . 5 GeV 2 1.00 Both show definite effects of 0.99 ε 0.0 0.2 0.4 0.6 0.8 1.0 exchange and agreement with 2 1.5 1 0.5 0 Q 2 (GeV 2 ) reconciliation 1.04 1.04 World data Zhou and Yang (N only) 1.03 1.03 Blunden et al. (N only) Zhou and Yang (N+ � ) R 2 γ 1.02 1.02 p) p) LNP + - CLAS TPE (e (e � � 1.01 1.01 R’ = 1 1.00 Point-like proton 0.99 0.99 0.0 0.2 0.4 ε 0.6 0.8 1.0 0.98 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2 Q (GeV/c) 1.2 1 0.8 0.6 0.4 0.2 0 Q 2 (GeV 2 ) D. Adikaram et al I.A. Rachek et al Phys. Rev. Lett. 114, 062003 Phys. Rev. Lett. 114, 062005 Seamus Riordan — Cornell IEB 2015 Nucl. EM 8/30

Two-photon Exchange Results - OLYMPUS OLYMPUS at DESY - Milner et al e + / e − ratio Will provide data up to Q 2 = 2 . 2 GeV 2 at 1% level Higher Q 2 in addition with other data will provide stronger constraints Ended running in 2013 - Under analysis with hope for results at the end of 2015 Seamus Riordan — Cornell IEB 2015 Nucl. EM 9/30

Discrepancy with Muonic Hydrogen Lamb Shift Lamb shift breaks degeneracy in 2 S 1 / 2 and 2 P 1 / 2 - Hyperfine � � r 2 splitting, is sensitive to p � µ -hydrogen more sensitive due to smaller Bohr radius, increases as m 3 , m µ / m e ∼ 200 e ( p , e ′ ) and spectroscopy agree µ − H 2 off by more than 6 σ ! Missing QED effects? Proton distorting? New coupling to just µ − ? Tie to g µ − 2 problem? Theory and experiment review: Pohl et al. Annu. Rev. Nucl. Part. Sci 2013. 63: 175-204 H.S. Margolis, S cience 339 , 405 (2013) Seamus Riordan — Cornell IEB 2015 Nucl. EM 10/30

Mainz low Q 2 G p E results J.C. Bernauer et al. PRL 105, 242001 (2010) Rosen. Sep. 1.1 [13] Hanson et al. 1.08 [2] Borkowski et al. [15] Christy et al. Janssens et al. 1.06 G M /( µ p G std. dipole ) Price et al. Bosted et al. Rosenbluth separation of over 1400 Berger et al. Bartel et al. 1.04 1.02 cross sections from Mainz, Q 2 up 1 0.98 to 1 GeV 2 0.96 0.94 0 0.2 0.4 0.6 0.8 1 Results have some systematic 1.1 discrepancies with previous 1.05 1 experiments - normalization errors µ p G E /G M 0.95 0.9 Includes two photon effects, proton 0.85 [13] w/o TPE Gayou et al. [13] w/ TPE Milbrath et al. Pospischil et al. radiative effects not large 0.8 [2] Punjabi et al. Dieterich et al. Crawford et al. Jones et al. Ron et al. [17] 0.75 0 0.2 0.4 0.6 0.8 1 Q 2 / (GeV/c) 2 E � 1 / 2 = 0 . 879 ± 0 . 008 fm , consistent � r 2 M � 1 / 2 = 0 . 777 ± 0 . 016 fm , smaller by about 0 . 1 fm ! � r 2 M � 1 / 2 = 0 . 85 ± 0 . 03 fm from other global fit (Zhan et al.) � r 2 Seamus Riordan — Cornell IEB 2015 Nucl. EM 11/30

Latest low Q 2 G p E results X. Zhan et al. Phys. Lett. B 705, 59 (2011) Pol. Trans. 1.1 [13] Hanson et al. 1.08 [2] Borkowski et al. [15] Christy et al. Janssens et al. G M /( µ p G std. dipole ) 1.06 Price et al. Bosted et al. Berger et al. Bartel et al. 1.04 1.02 1 0.98 0.96 0.94 0 0.2 0.4 0.6 0.8 1 1.1 1.05 1 µ p G E /G M 0.95 0.9 0.85 [13] w/o TPE Gayou et al. [13] w/ TPE Milbrath et al. Pospischil et al. 0.8 [2] Punjabi et al. Dieterich et al. Crawford et al. Jones et al. Ron et al. [17] 0.75 0 0.2 0.4 0.6 0.8 1 Q 2 / (GeV/c) 2 Discrepancy with other data, but G p E slope values are in agreement with Bernauer Bernauer magnetic radius from new unseen “wiggle” JLab data from 0 . 01 − 0 . 08 GeV 2 with polarized target under analysis. Seamus Riordan — Cornell IEB 2015 Nucl. EM 12/30

Latest low Q 2 G p E results X. Zhan et al. Phys. Lett. B 705, 59 (2011) Pol. Trans. 1.1 [13] Hanson et al. 1.08 [2] Borkowski et al. [15] Christy et al. Janssens et al. G M /( µ p G std. dipole ) 1.06 Price et al. Bosted et al. Berger et al. Bartel et al. 1.04 1.02 1 0.98 0.96 0.94 0 0.2 0.4 0.6 0.8 1 1.1 1.05 1 µ p G E /G M 0.95 0.9 0.85 [13] w/o TPE Gayou et al. [13] w/ TPE Milbrath et al. Pospischil et al. 0.8 [2] Punjabi et al. Dieterich et al. Crawford et al. Jones et al. Ron et al. [17] 0.75 0 0.2 0.4 0.6 0.8 1 Q 2 / (GeV/c) 2 Discrepancy with other data, but G p E slope values are in agreement with Bernauer Bernauer magnetic radius from new unseen “wiggle” JLab data from 0 . 01 − 0 . 08 GeV 2 with polarized target under analysis. Seamus Riordan — Cornell IEB 2015 Nucl. EM 12/30

New Charge Radius Measurements MUSE at PSI PRad Gasparian et al. Very low Q 2 e − Q 2 = 2 × 10 − 4 − 0 . 14 GeV 2 Gilman et al. Elastic µ − and µ + No magnetic elements - high precision calorimeter Q 2 = 0 . 002 − 0 . 07 GeV 2 Seamus Riordan — Cornell IEB 2015 Nucl. EM 13/30

Precision Radius Measurements - Under Analysis Data taken at Mainz will use initial state radiation reaches to effectively low Q 2 Will extend to Q 2 ∼ 10 − 4 GeV 2 Under analysis - preliminary results in weeks? Seamus Riordan — Cornell IEB 2015 Nucl. EM 14/30