Parity Violating Electron Scattering at Jefferson Lab Prof. Kent Paschke
Intro to PVeS
Parity Symmetry Parity transformation x , y , z � � x , � y , � z � � � � Right handed Left handed p � � � � L � S � Parity transformation is analogous p , L , S to reflection in a mirror: . . . reverses momentum but preserves angular momentum Helicity: spin in direction of motion � � = � = ± . . .takes right-handed (helicity = +1) h S p 1 to left-handed (helicity = -1). Parity symmetry : interaction must be the same after parity transformation 60 Ni 1957 – Parity Violation observed 60 Co Weak decay of 60 Co Nucleus
Charge and Handedness Electric charge determines strength of electric force Electrons and protons have same charge magnitude: same strength Neutrinos are “charge neutral”: do not feel the electric force not observed observed Weak charge determines strength of weak force Right-handed particles Left-handed particles (left-handed antiparticles) (Right-handed antiparticles) are “weak charge neutral” have weak charge 60 Co 60 Ni 60 Ni 60 Co right-handed observed R L anti-neutrino not observed left-handed R L anti-neutrino
Mirror Asymmetry • Incident beam is longitudinally polarized • Change sign of longitudinal polarization • Measure fractional rate difference 2 � = M + Weak and EM amplitudes interfere: M � Z γ Z 0 A PV is of the M Z � � � = � A R L ~ order of 10 -6 γ 2 PV � + � M R L or 1 ppm �
Elastic Electron-Nucleon Scattering If proton is point-like: The differential cross-section (scattering probability) is given by simple scattering theory e e τ =Q 2 /4M 2 is a Function of (E, θ ). convenient Cross-section for infinitely kinematic factor heavy, fundamental target � { } d 2 � � d = + � 1 2 tan ( / 2 ) p p � p p � d d Mott Dirac If proton is not point-like: electric and magnetic form factors G E and G M parameterize the effect of proton structure. � � � + � 2 2 d ( G G ) � e e d = + � � 2 2 E M 2 G tan ( / 2 ) � � � M � d + � d 1 � � Mott Rosenbluth If the proton were like the electron: G E = 1 (proton charge) p p G M = 1 (and the magnetic moment would be 1 Bohr magneton).
Charge & Current Distributions Vector form factors G E , G M are functions of Q 2 -> they measure scattering probability as a function of “wavelength” Fourier transform of the charge and magnetic current distributions G E for the neutron charge distribution for the neutron n r 2 ρ (r) G E Q 2 (GeV/c) 2 r [fm] At Q 2 = 0, the form factor represent an integral over the nucleon • charge and magnetic moment At small Q 2 , G E n measures the “charge radius”
The weak form factor of a nucleon
The Simple Nucleon The nucleon is composed of three quarks (up and down flavors) interacting via the Strong force (Quantum Chromodynamics) Quarks are to the particle zoo what valence electrons are to the periodic table Increasing mass It’s simple: the nucleon is three marbles in a bag! Not so fast. The strong force is weird! It grows with distance, and is huge at “large” distances (10 -15 m). Gluons (strong carriers) interact with themselves. Strong glue is sticky. Does this mess play a role in the long-distance interaction of the proton? How well can the quark model really predict static properties?
Strangeness in the Sea The sea contains all flavors, but • the u and d sea can’t be distinguished from the valance • the heavier quarks (c,b,t) are too heavy to contribute much • strange – different flavor, same mass scale! From hard-scattering, we know that the strange sea exists. ~4% of the momentum of the nucleon is carried by strange quarks But this is a “deep” probe… Do the strange quark affect the static properties of the nucleon? Low-Q 2 Elastic electron scattering from the nucleus measures charge radius and magnetic moment A strange contribution would be the first unambiguous low-energy failure of the naïve quark model Measuring all three enables separation of up, down and strange contributions
The weak form factor of a nucleus
PREx: Pb Radius Experiment Nuclear theory predicts a Direct measurements involve neutron “skin” on heavy nuclei messy QCD, but the neutral weak current can do this job! 208 Pb proton neutron Electric 1 0 charge Weak charge 0.08 1 P ( � ) density pressure R n calibrates the equation of state of neutron rich matter Why do we care? • Key prediction of nuclear theory, so this tests understanding of nuclear structure • This parameter important in atomic physics, heavy ion physics, radioactive beams, etc. • hmm… what other neutron-rich matter is interesting?
From 208 Pb to a Neutron Star This EOS is needed to understand Crab Pulsar the biggest of nuclei • Crust Thickness • Explain Glitches in Pulsar Frequency ? Combine PREX R n with observed neutron star radii • Phase Transition to “Exotic” Core ? • Strange star ? Quark Star ?
New Physics
( i The Annoying Standard Model t j u s t w o n ’ t b r e a Nuclear Physics Long Range Plan: k ! ) What is the new standard model? Low Q 2 offers unique and complementary probes of new physics • Rare or Forbidden Processes - Double beta decay.. • Symmetry Violations - neutrinos, EDMs.. • Electroweak One-Loop Effects - Muon g-2, beta decay.. • Precise predictions at level of 0.1% • Indirect access to TeV scale physics Fundamental Symmetries Initiative in nuclear physics
Direct vs Indirect Searches (according to Hans Christian Andersen)
Electroweak Physics Away from Z pole • Low energy observables probe interference between SM and NP • Current “low energy” experiments are accessing scales beyond 10 TeV
How the measurement is done
Measuring A PV Goal: part per million asymmetry measurement at the few percent level How do you pick a tiny signal out of a noisy environment? output Lockin Amplifier injector accelerator apparatus target spectrometer detector lockin input modulator � 1 1 � + � � � = = 14 A 5 % = � 10 � N ~ 10 ! ! ! 6 A PV A A � + + � � 2 N Beam helicity is chosen pseudo-randomly at 30 Hz • Helicity state, followed by its complement Huge detected rate… requires analog integration (not individual counting) • Data analyzed as “pulse-pairs” High luminosity and polarization: state-of-the-art electron source High precision requires low noise electronics, precision beam monitors Tiny asymmetry requires careful control of false asymmetries calculated at 15Hz Measure the asymmetry with 0.06% precision, millions of times
CEBAF at JLab Superconducting, continuous wave, recirculating linac A B 1500 MHz RF, with C 3 interleaved 500 MHz beams Up to 5 passes, up to 1.2 GeV per pass. Polarized e - Source Independent extraction C Hall A and separation to B 3 experimental halls
Continuous Electron Beam Accelerator Facility Accelerator requires 20 MW power Bending magnets in arc Linac tunnel
Hall A
Hall A Spectrometers • Bending (dipole) magnet – 450 tons • 1.6 T magnetic field • 45 0 bend angle • 3,500,000 J stored energy •Resolution (momentum) – 0.01% •Total spectrometer – 1000 tons
Spectrometer and Detector Clean separation of elastic events by magnetic optics Large bend and heavy shielding Integrating Cerenkov Shower Calorimeter reduce backgrounds at the • Electromagnetic shower through brass radiator focal plane • Cerenkov light from shower in quartz layers • Analog integration of PMT signal Focal plane dispersive axis (mm) 12 m dispersion sweeps away PMT inelastic events Cherenkov cones Overlap the e - elastic line and PMT integrate the flux
Polarized Source and False Asymmetries
Polarized Electrons for Measuring A PV Strain gives high polarization (~85%) but also introduces anisotropy Beam helicity is chosen pseudo-randomly at 30 Hz • Helicity state, followed by its complement • Data analyzed as “pulse- pairs” Beam must look the same for the two polarization states Helicity-correlated asymmetries in the electron beam create FALSE ASYMMETRY
Causes of beam asymmetries Perfect ± λ /4 retardation leads to perfect ±circular polarization in each state Anisotropy couples to residual “ Δ ” A com ommon on reta tardati tion of on offset linear polarization to produce an ove over-p -phaseshifts ts one ne s sta tate te, intensity asymmetry A Q . und nder-p -phaseshifts ts the oth other Big Gradient in charge asymmetry asymmetry creates a helicity-dependent beam profile centroid. This is called the Δ phase Small asymmetry (one could imagine an α phase, too, but it’s not important) Goal: <1% linear polarization gradients across beamspot
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