Physics with Low Energy (<100 MeV) Gamma-Rays Blaine Norum University of Virginia 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays Blaine Norum, Univ of Virginia
Outline: Range: few MeV ≤ Eγ ≤ ~ 100 MeV Nucleonic degrees of freedom Focus on few-body photo-disintegration deuteron d(γ,n) Bethe-Heitler asymmetries Compton scattering 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 2
Gerasimov-Drell-Hearn (GDH) Sum Rule Original motivation for pursuing deuteron photo-disintegration was problem with GDH Sum Rule for neutron: GDH for proton satisfied GDH for neutron apparently violated 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 3
Gerasimov-Drell-Hearn (GDH) Sum Rule 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 4
Gerasimov-Drell-Hearn (GDH) Sum Rule 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 5
Gerasimov-Drell-Hearn (GDH) Sum Rule Test by measuring GDH integrand for deuterium at low energies: 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 6
Blowfish Neutron Detector Array 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 7
HIFROST Frozen Spin Target 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 8
Preliminary Measurements 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 9 Differential cross section at 90 o in CoM
Problems in Paradise! 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 10 Maximum discrepancy at Eγ ~ 12 MeV, E np ~ 9 MeV
Other hints? 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 11 Discrepancy in σ LT shows same behavior at all energies Discrepancy in σ L+T changes sign at E np = 9 MeV
Induced Neutron Polarization 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 12 Unitary cusp at E np = 9 MeV suggests new channel?
Neutron Polarimeter Array 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 13
Takeaway 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 14 Problems with deuteron – problems with other, perhaps all other light nuclei?
Bethe-Heitler Asymmetries 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 15 The interference of the two-photon and one-photon exchange amplitudes is the primary effect which causes the electron-positron asymmetry in BH pair production. The two-photon BH amplitude is closely related to the two-photon exchange amplitude in which is believed to cause the breakdown of the Rosenbluth separation.
Experimental Equipment 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 16 All equipment taken from previous experiments at Saskatchewan or Jefferson Lab
Compton Scattering 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 17 o Measurement of polarizabilities via Compton Scattering provides stringent test of calculations that link the effective low-energy description of nucleons to QCD, and Lattice QCD predictions o In order to understand the subtle differences of the pion clouds around the proton and neutron induced by explicit chiral symmetry breaking in QCD, we need the neutron polarisabilities with uncertainties comparable to those of the proton [High-Accuracy Analysis of Compton Scattering in Chiral Effective Field Theory: Status and Future, HG, DP, JMG, arXiv: v3 [nucl-th] 19 Aug 2013].
Role of polarizabilities in the mass problem 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 18 o The net effect of u-d quark mass difference and electromagnetic contributions is kn own ( to % precision) ; o How to disentangle the two contributions? (LQCD), EM is the issue! o The EM self-energy of the nucleon can be related to the measured elastic/inelastic cross sections; o Largest source of error is from β− β n (where error from neutron dominates) Cottingham : Ann Phys (NY) 25, 424 (1963), AW-L, CEC, GAM, PRL, 108, (2012)
Taking the next steps, 4 He, D, and H 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 19 Cryocooler K Base T = 3.5 K L = 20 cm V = 0.24 Lit D/H/cm 2 = 10 24
The Target and the HINDA for Compton Scattering Experiments 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 20
Newest Data Set and State of the Affair (Deuteron) 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 21 L. S. Myers, PRL 113, (2014) HWG, JAM, DRP, GF, PPNP, 67 (2012) , HG, Private Communication, 2015 JAM, DRP, HWG, EJP, A 49 (2013) 12
Compton Scattering, Making use of the Polarization The T-matrix for the Compton scattering of incoming photon of energy with a spin ( ) ½ target is described by six structure functions. = photon polarization, k is the momentum Maximum sensitivity Maximum sensitivity 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 22
HIGS: Linearly polarized gamma ray measurement EE B PT with 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 23
Evangeline Downie GWU; Sokhoyan, Vahe, Mainz, private communication, 2015 Latest results from Mainz HIGS Measurement will take place in late Spring 2016 at 85 MeV. ~ 2.5% and < 10% with sum- rule free fits. 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 24
Spin Polarizabilities Require polarized beam and polarized target 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 25
Conclusions and Acknowledgements 10/16/15 FACET II Science Workshop Physics with Low Energy (<100 MeV) Gamma Rays 26 I barely touched on what is possible. Questions range from very fundamental nucleon physics to basic nuclear structure questions. To be continued this afternoon … Acknowledgements:Special thanks to: Brad Sawatzky Mohammad Ahmed for Rob Pywellthe Compton scattering Don Crabbmaterial Henry Weller