Parity Violating Electron Scattering at Jefferson Lab Prof. Kent - - PowerPoint PPT Presentation

Parity Violating Electron Scattering at Jefferson Lab

• Prof. Kent Paschke

Intro to PVeS

Parity Symmetry

• p

p,

• L
• L,
• S
• S

x,y,z x,y,z

Parity transformation

Parity transformation is analogous to reflection in a mirror: . . . reverses momentum but preserves angular momentum . . .takes right-handed (helicity = +1) to left-handed (helicity = -1).

Right handed Left handed

Parity symmetry: interaction must be the same after parity transformation

Helicity: spin in direction of motion

1 ± =

• =

p S h

• Weak decay of 60Co Nucleus

60Co 60Ni

1957 – Parity Violation observed

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

Weak charge determines strength of weak force

Left-handed particles (Right-handed antiparticles) have weak charge Right-handed particles (left-handed antiparticles) are “weak charge neutral”

• bserved

not observed

• bserved

60Co 60Ni

L R

right-handed anti-neutrino

L R

not observed

60Co 60Ni

left-handed anti-neutrino

Mirror Asymmetry

• Incident beam is longitudinally polarized
• Change sign of longitudinal polarization
• Measure fractional rate difference

APV is of the

• rder of 10-6
• r 1 ppm
• M

MZ

• γ

Z0 γ

2

~

L R L R PV

A

• +
• =

2 Z

M M + =

• Weak and EM amplitudes interfere:

Elastic Electron-Nucleon Scattering

If proton is not point-like: electric and magnetic form factors GE and GM

parameterize the effect of proton structure.

{ }

) 2 / ( tan 2 1

2

• +
• =
• d

d d d

Mott Dirac

• +

+ +

• =
• )

2 / ( tan 2 1 ) (

2 2 2 2

• M

M E Mott Rosenbluth

G G G d d d d

e e p p

If proton is point-like: The differential cross-section

(scattering probability) is given by simple scattering theory

e e p p p p If the proton were like the electron: GE = 1 (proton charge) GM = 1 (and the magnetic moment would be 1 Bohr magneton).

τ=Q2/4M2 is a

convenient kinematic factor

Function of (E,θ).

Cross-section for infinitely heavy, fundamental target

Charge & Current Distributions

Q2 (GeV/c)2

GE

n

GE for the neutron r2 ρ(r)

r [fm]

charge distribution for the neutron At Q2 = 0, the form factor represent an integral over the nucleon

• charge and magnetic moment

At small Q2, GE

n measures the “charge radius”

Vector form factors GE, GM are functions of Q2

• > they measure scattering probability as a function of “wavelength”

Fourier transform of the charge and magnetic current distributions

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)

Increasing mass

Quarks are to the particle zoo what valence electrons are to the periodic table 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

But this is a “deep” probe… Do the strange quark affect the static properties of the nucleon? 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 Measuring all three enables separation of up, down and strange contributions

Low-Q2 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

The weak form factor of a nucleus

PREx: Pb Radius Experiment

Nuclear theory predicts a neutron “skin” on heavy nuclei Direct measurements involve messy QCD, but the neutral weak current can do this job!

208Pb

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?

1 0.08 Weak charge 1 Electric charge neutron proton

Rn calibrates the equation of state

• f neutron rich matter

P()

pressure density

From 208Pb to a Neutron Star

• Crust Thickness
• Explain Glitches in Pulsar Frequency ?

Combine PREX Rn with

• bserved neutron star radii
• Strange star ? Quark Star ?
• Phase Transition to “Exotic” Core ?

Crab Pulsar

This EOS is needed to understand the biggest of nuclei

New Physics

The Annoying Standard Model

• Rare or Forbidden Processes
• Symmetry Violations
• Electroweak One-Loop Effects
• Double beta decay..
• neutrinos, EDMs..
• Muon g-2, beta decay..

Nuclear Physics Long Range Plan: What is the new standard model?

• Precise predictions at level of 0.1%
• Indirect access to TeV scale physics

Fundamental Symmetries Initiative in nuclear physics

Low Q2 offers unique and complementary probes of new physics

( i t j u s t w

• n

’ t b r e a k ! )

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 APV

Goal: part per million asymmetry measurement at the few percent level How do you pick a tiny signal out of a noisy environment?

Lockin Amplifier

• utput

apparatus

modulator lockin input

injector accelerator target spectrometer detector

! ! ! 10 ~

14

N

% 5 2 1 1 = = N A A

A

• 6

10

• +
• +
• +
• =
• PV

A

 Beam helicity is chosen pseudo-randomly at 30 Hz

• Helicity state, followed by its complement
• Data analyzed as “pulse-pairs”

calculated at 15Hz Measure the asymmetry with 0.06% precision, millions of times

Huge detected rate… requires analog integration (not individual counting) 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

Polarized e- Source Hall A

1500 MHz RF, with 3 interleaved 500 MHz beams Up to 5 passes, up to 1.2 GeV per pass. Independent extraction and separation to 3 experimental halls

A

B

C

Superconducting, continuous wave, recirculating linac

CEBAF at JLab

C

B

Continuous Electron Beam Accelerator Facility

Linac tunnel Bending magnets in arc Accelerator requires 20 MW power

Hall A

• Bending (dipole) magnet – 450 tons
• 1.6 T magnetic field
• 450 bend angle
• 3,500,000 J stored energy
• Resolution (momentum) – 0.01%
• Total spectrometer – 1000 tons

Hall A Spectrometers

12 m dispersion sweeps away inelastic events

Clean separation of elastic events by magnetic optics

Overlap the elastic line and integrate the flux

Large bend and heavy shielding reduce backgrounds at the focal plane

Focal plane dispersive axis (mm)

Cherenkov cones PMT PMT

e-

Integrating Cerenkov Shower Calorimeter

• Electromagnetic shower through brass radiator
• Cerenkov light from shower in quartz layers
• Analog integration of PMT signal

Spectrometer and Detector

Polarized Source and False Asymmetries

Polarized Electrons for Measuring APV

 Beam helicity is chosen

pseudo-randomly at 30 Hz

• Helicity state, followed by

its complement

• Data analyzed as “pulse-

pairs” Strain gives high polarization (~85%) but also introduces anisotropy

Beam must look the same for the two polarization states Helicity-correlated asymmetries in the electron beam create FALSE ASYMMETRY

Causes of beam asymmetries

This is called the Δ phase

(one could imagine an α phase, too, but it’s not important)

Perfect ±λ/4 retardation leads to perfect ±circular polarization in each state A com

• mmon
• n reta

tardati tion of

• n offset
• ve
• ver-p
• phaseshifts

ts one ne s sta tate te, und nder-p

• phaseshifts

ts the oth

• ther

Anisotropy couples to residual “Δ” linear polarization to produce an intensity asymmetry AQ. Gradient in charge asymmetry creates a helicity-dependent beam profile centroid.

Big asymmetry Small asymmetry

Goal: <1% linear polarization gradients across beamspot

Controlling Beam Asymmetries

Energy: -0.25 ppb X Target: 1 nanometers X Angle: <1 nanoradian Y Target : 1 nm Y Angle: <1 nrad

HAPPEX-II (Hydrogen)

Carefu ful c l config figuratio tion o

• f p

f pola lariz ized s source surpassed B Beam A Asymmetr try G Goals ls

X BPM

micron

• Δx’ (HWP OUT)
• Δx’ (HWP IN)

Major achievement from close Major achievement from close collaboration between nuclear and collaboration between nuclear and accelerator scientists accelerator scientists

Average position the Average position the same to same to ~1 nm ~1 nm

Polarized-Electron Source

I will lead the effort to control helicity-correlated beam asymmetries for the QWeak and PREx experiments.

• a new optics test stand will be set up here at UVa:

laser, optics, electro-optics, slow controls, photodetectors, DAQ.

• tests will be performed at JLab

Polarimetry

Compton Polarimeter

Compton Int. Point γ detector e- detector Hall A magnets Precise measure of beam polarization is needed

Resonant cavity “photon target”, up to 2kW intensity

> <

• =

=

• +
• +

+

• th

e n n n n

A P P A

• exp

Polarimetry

Hall A Upgrade (PREx, HAPPEX-III, PVDIS)

• New green Fabry-Perot cavity

Hall C “Upgrade” (QWeak)

• Build a Compton polarimeter

from scratch My plan: help build the Hall C Compton, and a UVa student should be the new polarimetry analysis blackbelt

1% polarimetry required at low energy in both halls

Hall A Upgrade: laser cavity

How do you build a green Fabry-Perot cavity?

S = 0.30699952 r = 0.99975200

Norminal IR power (mW)

65.0 185.0 305.0 425.0 545.0 665.0 785.0 0.03 8.11 16.20 24.28 32.37 40.45 48.54

Conversion Efficiency: 3.1 % / (W cm)

Photo detector Beam Splitter Cavity Oscillator Phase Shifter Mixer Low Pass Filter

Tunable Laser

PID- Regulator

Error signal

Pound-Drever-Hall (PDH) locking

Where do you get a high-power, tunable laser? PPLN Second Harmonic Generation double 1064 nm to 532 nm major R&D at the cutting edge of laser technology

Results from UVa Grad Student Tharanga Jinasundera

Hall C photon-electron collider

Electron beam pulse 2 ps long every 2 nsec. (499 MHz) A 499 MHz laser gets a huge boost in luminosity

Oscillator

1.064um 30ps/1W 75/750Mhz

Amplifier 3

LBO

Iso Shutter Wp Wp Iso P CL CL HR CL CL CL CL L L CL CL C0 Amplifier 2 Amplifier 1 Pre-amplifier AC

Ast.C.

PPSLT

C1 C2 PM C3 C4

PD

PM

PD

WP BPP

VND BE Is Is Is Shutters PicoM A EO2 EO1 MS

Goal: Develop a 25W average power 499 MHz pulsed laser at 532 nm, by doubling 1064 nm from fiber amplifier with gain-switched seed SINGLE-SHOT RF pulsed laser can get the job done Interaction Region Design, Data Acquisition, Slow Controls, Analysis design, Analysis Software… Huge amount of work to do!

Summary

HAPPEX-4He: HAPPEX-H: δAPV = ± 120 (stat) ± 50 (syst) ppb δAPV /APV = ± 3.6% (stat) ± 1.9% (syst) 12GeV PVDIS: δAPV /APV = ± 0.5% (stat) ± 0.5% (syst) 12GeV Moller: δAPV = ± 0.5 (stat) ± 0.3 (syst) ppb HAPPEX-III: δAPV /APV = ± 2.5% (stat) ± 1.5% (syst) PREx: δAPV = ± 15 (stat) ± 5 (syst) ppb QWeak: δAPV = ± 7 (stat) ± 5 (syst) ppb PV-DIS: δAPV /APV = ± 2.0% (stat) ± 1.3% (syst)

The far future (past your time frame here):

State-of-the-art precision

Recent PVeS Students

• T. Brian Humensky (Princeton)

SLAC E158 and HAPPEX Polarized Source Expert Choose to switch to astrophysics Associate Fellow at the Kavli Institute for Cosmological Physics at the University of Chicago Bryan Moffit (W&M) HAPPEX-II DAQ/Software/Analysis/Simulation MIT postdoc at JLab Lisa Kaufman (UMass) HAPPEX-II Polarized Source Expert Choose to switch to neutrino physics: UMd postdoc at SLAC Pictured in Milos, Greece, for the 2006 PAVI conference

The PVeS program at JLab

• The experiments are important, the techniques are interesting, and the

results matter to researchers in many fields

• Small collaboration, big experiment: you become THE expert on many

systems, but still have a broad exposure

• Established program, new opportunities: We have a record of success in

these measurements, but a whole new set of challenges to face!

• Excellent training for all manner of sub-atomic physics, with a mix of

detector hardware, electronics, optics, analysis, design, and simulation

• UVa will play a BIG role in PVeS at JLab, with myself, X. Zheng and G. Cates

as leaders in the program

• The future for this field is very bright.
• Timing! Starting in mid-2009 (HAPPEX-III and PV-DIS, followed by PREx

in early 2010. Typical PVeS analyses are much shorter (~1 year) than typical nuclear physics experiments, so late 2011 is a realistic schedule!