Download Fundamental Physics With Cold and Ultra-cold

Fundamental Physics With
Cold and Ultra-cold Neutrons
Albert Young
North Carolina State University
Fundamental neutron physics
[Fr. Physique Fondamental, c. 1975, first used to describe a
variety of interdisciplinary research activities carried out
at the high flux reactor of the Institut Laue Langevin,
Grenoble]
Measurements, utilizing low
energy neutrons, of…
Decay of the neutron, the
neutron’s static moments,
fundamental physical
constants, as well as tests
of basic theories (such as
quantum mechanics), etc…
Why is the universe “Left-Handed?”
How much matter is in the universe? How much is
“Dark Matter”
Why does the universe have matter and no antimatter?
What is the origin of the Time Reversal Asymmetry?
Where is the “Physics Beyond the Standard Model?”
• A number of breakthroughs in the past five
years (both CN and UCN regimes)
→Opportunities for new experiments…
• Very strong involvement by university groups
and labs in the U.S. and abroad
Some examples where new opportunities
are being pursued:
Hadronic Weak Interactions
Neutron Beta-Decay
Neutron EDM
Some Examples I Won’t Have the
Opportunity to Discuss:
• N-N Oscillations, or searches for B-L violating
interactions: see Frank Plasil’s talk in the working group session
• Neutron interferometry and tests of quantum mechanics
• Low energy neutron studies of reactions relevant for
nuclear astrophysics
the list goes on…
The Hadronic Weak Interaction
(treated in depth in David Bowman’s talk this afternoon)
Example: p + n → p + n
Currently parameterized in
terms of a meson exchange
model:
Theory suggests a range of
acceptible values for couplings
Effects are small!
An Area of Vigorous Activity
H12.4f
H0-1.4h0
To resolve experimentally correct value for H1, new measurements required:
(1) simple systems with relatively “clean” theoretical interpretation
(2) excellent control of systematic errors
Cold Neutron Beam Experiments
(neutrons with energies of a few meV)
• n,p system:
n + p→ d + 
A  5 108
 0.045H 
• n,4He system:
Rotation of the transverse polarization of neutron
after transmission through liquid helium
(spin rotation)
  4 107 rad/m
 1.31H  0.23H   0.25H 2  0.23H0  rad/m
n + p→ d + 
Projected systematic errors in A  110-9 level, statistical errors  510-9
• Pulsed neutron beams will be used to identify velocity dependent systematic errors
• 3He polarizers provide additional
control of polarization systematic
errors
Detector array used in recent test of npdg experiment
Projected Limits from Cold Neutron Beams
Measurements
Allowed DDH
Neutron Beta-Decay
• Measurements provide fundamental data on the
electroweak interaction, for example the CKM matrix
element Vud and the weak axial form factor of the nucleon.
• Lifetime measurements provide essential input data to high
precision models of big bang nucleosynthesis.
• Extreme simplicity of this system permits a high precision
confrontation between the electroweak standard model
and experiment (we can probe for new physics).
•Semi-leptonic decay
• Single nucleon system
Simplicity
•Low Z ensures radiative corrections small
•Spin ½+  ½+ decay restricts the number of
contributing form factors, ensures angular
correlations have simple form
b-decay of quarks (no strong interaction)
H 
b eff
GF ( quarks)  (leptons)

J
J
2
Semi-leptonic decay
E. Fermi, Z. Phys 88, 161 (1934)
Electroweak Standard Model
Quark current


J ( quarks)  ( u, c, t )    1   5 
CKM matrix
Lepton current


d 
 
U  s   h. c.
 
b 
J  ( leptons )   e   1   5   
 h. c.
Breakthroughs
• Most precise lifetime measurements have been performed
with ultra-cold neutrons for past 10 years (neutrons with
energies below about 350 neV, which can be stored in
material and magnetic bottles)
Advent of superfluid He superthermal UCN source provides
higher UCN densities and, when coupled to a magnetic trap,
reduce systematic corrections to lifetime by 1 to 2 orders of
magnitude
• Angular correlations measurements have been performed
using cold neutron beams: dominant systematic errors have
involved neutron polarization & backgrounds.
3He polarizers/analyzers can now provide absolute
polarimetry at the 0.1 percent level for CN experiments
Development of SD2 superthermal source provides copius
extracted UCNs for angular correlations measurements
A Superthermal Solid Deuterium UCN Source
at LANSCE
World record densities
achieved this June
Compare to previous record of 41 UCN/cm3
(at ILL). Note: over two orders of magnitude
improvement may ultimately be possible!
Time Reversal Non-invariance
(see Norval Fortson’s talk later today)
• A great many extenstions to the standard model, including
supersymmetry, left-right symmetric models, expanded
Higgs sectors, etc…introduce unconstrained T noninvariant phases: experiments are required to determine at
what level these phases actually appear.
• Cosmological models of the matter-antimatter asymmetry
require T non-invariance to be present in the early universe
at some level
For neutrons
T non-invariance in beta-decay
(briefly)
Neutron static electric dipole moment (EDM)
Limits from EDM have
provided critical
guidance for theory
Limit sought in
LANSCE EDM expt
(roughly)
New opportunity to measure the neutron EDM
using a superthermal He UCN source
(see Martin Cooper’s talk in the working group session)
Measurement performed in liquid He (similar to n lifetime),
with the liquid He serving a variety of functions:
•Source of UCNs
•Insulator for required large electric fields
•Detector for UCN spin state
Measure torque that an electric
field exerts on the precessing
neutron spin

d n  EP N

1
Improvement factors
Density:
x8
Electric Field:
x5
Coherence time:
x5
x  200
Summary
• There are numerous, ongoing projects to probe
fundamental physics with neutrons which show steady
progress…the physics motivation is compelling
• New opportunities generated for CN beam research:
3He
polarizers
Hadronic weak interactions
Pulsed neutron beams
Neutron beta-decay
• New opportunities to utilize UCNs in the US:
Superthermal He and SD2
sources
Neutron beta-decay
T non-invariance (EDMs)
• Involves a close collaboration between many strong
university groups and US national laboratories…we’re
excited to seize these opportunities!