Parity-violating electron scattering JLAB Juliette Mammei

Outline June 1-19, 2015HUGS I.Introduction II.Theory A.Standard Model B.Madam Wu C.Z’eldovich D.Emmy Noether III.Experimental Considerations A.Beam quality B.Target stability C.Backgrounds D.Apparatus symmetry E.Detector linearity F.Collimator precision G.Magnet stability H.Raster synchronization IV.Past Experiments A.SLAC E158 B.SAMPLE C.Mainz A4 D.G0 E.HappEx I-IV F.Qweak G.PREX H.PVDIS V.Future Experiments A.PREX II B.CREX C.MOLLER D.Qweak (Mainz) E.PVDIS F.SOLID VI.Summary 2

Introduction June 1-19, 2015HUGS 3

PVES Historical view

Why Parity-Violating Electron Scattering (PVES)? June 1-19, 2015HUGS o Search for physics Beyond the Standard Model (BSM) with low energy (Q 2 <<M 2 ) precision tests complementary to high energy measurements Neutrino mass and their role in the early universe 0νββ decay, θ 13, β decay,… Matter-antimatter asymmetry in the present universe EDM, DM, LFV, 0νββ, θ 13 Unseen Forces of the Early Universe Weak decays, PVES, g μ -2,… LHC new physics signals likely will need additional indirect evidence Neutrons: Lifetime, P- & T-Violating Asymmetries (LANSCE, NIST, SNS...) Muons: Lifetime, Michel parameters, g-2, Mu2e (PSI, TRIUMF, FNAL, J-PARC...) PVES: Low energy weak neutral current couplings, precision weak mixing angle (SLAC, Jefferson Lab, Mainz) o Study nuclear and nucleon properties Strange quark content of nucleon Neutron radii of heavy nuclei 5

Standard Model of Particles and Interactions June 1-19, 2015HUGS 6

Interactions Particles June 1-19, 2015HUGS 7

June 1-19, 2015HUGS 8

Symmetries HUGS Emmy Noether (18??-19??) June 1-19, 2015 Conservation laws imply symmetries Conservation of: Linear momentum Angular momentum Energy Implies: Rotational invariance Translational invariance Time invariance 9

What is unitarity and why is it required? June 1-19, 2015HUGS CKM matrix 10

Parity June 1-19, 2015HUGS quantum mechanical operator that reverses the spatial sign ( P: x -> -x ) We describe physical processes as interacting currents by constructing the most general form which is consistent with Lorentz invariance Note: P (V*V) = +1 P (A*A) = + 1 P (A*V) = -1 Parity Time Reversal Charge Conjugation 11

A brief history of parity violation June 1-19, 2015HUGS R R L L 1930s – weak interaction needed to explain nuclear β decay 1950s – parity violation in weak interaction; V-A theory to describe 60 Co decay 1970s – neutral weak current events at Gargamelle late 1970s – parity violation observed in electron scattering - SLAC E122 12

Nuclear beta decay June 1-19, 2015HUGS "Beta spectrum of RaE" by HPaul - Own work. Licensed under CC BY-SA 4.0 via Wikimedia Commons e- pn 13

EM and Weak Interactions : Historical View June 1-19, 2015HUGS EM: e + p  e + p elastic scattering V x V Parity Violation (1956, Lee, Yang; 1957, Wu) required modification to form of current - need axial vector as well as vector to get a parity-violating interaction (V - A) x (V - A) Note: weak interaction process here is charged current (CC) e- p p p n 14

Parity-violation in charge current maximal June 1-19, 2015HUGS Madam Wu Bleckneuhaus, with English language captions by Stigmatella aurantiaca electrons favored the direction opposite to that of the nuclear spin 15

What about a neutral weak current? June 1-19, 2015HUGS e- p n p p ? 16

Neutral weak currents observed June 1-19, 2015HUGS The prediction of the Z 0 implied the existence of previously unobserved neutral current processes like: These processes were first discovered in 1973: Z0 Z0   e-e- e-e-  + e -   + e - What about parity violation? 17

The neutral weak current, Zel’Dovich June 1-19, 2015HUGS parity non-conservation via weak – EM interference parity-violating asymmetry e- longitudinally polarized e- Four drops of ink in a 55-gallon barrel of water would produce an "ink concentration" of 1 ppm!!! 18

Magnetic spectrometer Background and kinematic separation SLAC Experiment E122 June 1-19, 2015HUGS Polarimetry Integrating detectors High luminosity from photoemission from NEA GaAs cathode Rapid helicity-flip (sign of e- polarization) Huge achievement! Highest P 2 I ever, by far. Developed for this experiment at SLAC and used ever since 19

SLAC Experiment E122 June 1-19, 2015HUGS sin 2 θ W =0.20±0.03 GWS --‐ Nobel Prize 1979 Parity Non-Conservation in Inelastic Electron Scattering, C.Y. Prescott et. al, 1978 Deep inelastic scattering: Y dependence reflects quark axial/electron vector coupling strength At high x LeftRight γ Charge0, ±1, ±1/3, ±2/3 W ChargeT= ±1/2 0 Z ChargeT-qsin 2 θ W -qsin 2 θ W A PV ~ 100 ± 10 ppm 20

Standard Model of Electroweak Interactions June 1-19, 2015HUGS Glashow-Weinberg-Salam Model (1967): unified EM and weak forces as an electroweak force  SU(2) L x U(1) gauge theory with spontaneous symmetry breaking fermions: Interaction of fermions with gauge bosons:  W +,- Z0Z0 sin 2  W – “weak mixing angle”, parameterizes the mixing between the two neutral currents 21

Running of coupling constants June 1-19, 2015HUGS What about sin 2 θ W ? 22

Running of sin 2 θ W June 1-19, 2015HUGS Present: “d-quark dominated” : Cesium APV (Q A W ): SM running verified at ~ 4  level “pure lepton”: SLAC E158 (Q e W ): SM running verified at ~ 6  level Future: “u-quark dominated” : Q weak (Q p W ): projected to test SM running at ~ 10  level “pure lepton”: MOLLER (Q e W ): projected to test SM running at ~ 25  level + +  23

Width of the Z 0 HUGSJune 1-19, 2015 Measure a variety of electroweak processes with couplings to all possible fermions Extract values of (sin 2  W ) eff in a consistent renormalization scheme from all processes Production of real Z 0 bosons in e + e - annihilation 24

Spontaneous symmetry breaking HUGSJune 1-19, 2015 Why do the weak bosons have mass? Higgs mechanism Higgs field – scalar (not a vector) field that permeates all of space As universe cooled, symmetry was broken and 3 of the electroweak bosons absorbed 3 of the Higgs bosons, gaining mass but leaving the photon massless and one Higgs boson to be discovered at CERN 25

Low Energy Weak Neutral Current Standard Model Tests These three types of experiments are a complementary set for exploring new physics possibilities well below the Z pole Low energy weak charge “triad” (M. Ramsey-Musolf) probed in weak neutral current experiments parity-violating Moller scattering e + e  e + e Cesium Atomic Parity Violation primarily sensitive to neutron weak charge JLAB Q p weak : parity-violating e-p elastic scattering e + p  e + p June 1-19, 2015HUGS 26

Some PVES Experiments June 1-19, 2015HUGS 27

PVES Experiments at JLAB June 1-19, 2015HUGS JLAB has a rich program of PVES experiments to measure nuclear and nucleon properties and to perform precision tests of the Standard Model to search for new physics SM Tests Nuclear properties Nucleon properties QweakMOLLER PREX CREX G0 Happex PVDIS SOLID 28

Feynman Diagrams June 1-19, 2015HUGS Further reading: Looking for consistency in the construction and use of Feynman diagrams Peter Dunne, Phys. Educ. 36 No 5 (September 2001)

The Dirac Equation June 1-19, 2015HUGS Dirac equation for free electron: where: with: leads to electron four-vector current density: where the adjoint is: satisfies the continuity equation: 30

Cross section June 1-19, 2015HUGS The incident flux times the differential cross section is proportional to the product of the square of the matrix element and the Lorentz invariant phase space All the physics is in the matrix element 31

the matrix element June 1-19, 2015HUGS external lines vertex factors propagator 32

external lines the matrix element June 1-19, 2015HUGS vertex factors propagator 33

How do we measure ? June 1-19, 2015HUGS 2 + =

Hand-waving derivation of the parity-violating asymmetry in electron-proton scattering June 1-19, 2015HUGS 35

Asymmetry June 1-19, 2015HUGS 36