NCNR and NIST Neutron research and probing matter with cold neutrons OR Neutrons ignored no longer!
Fundamental Physics With Cold and Ultra-cold Neutrons Albert Young North Carolina State University.
Transcript of Fundamental Physics With Cold and Ultra-cold Neutrons Albert Young North Carolina State University.
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]
Decay of the neutron, the neutron’s static moments, fundamental physical constants, as well as tests of basic theories (such as quantum mechanics), etc…
Measurements, utilizing low energy neutrons, of… 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 anti-matter?
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
Hadronic Weak Interactions
Neutron Beta-Decay
Neutron EDM
Some examples where new opportunities are being pursued:
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
Example: p + n → p + n
(treated in depth in David Bowman’s talk this afternoon)
Currently parameterized in terms of a meson exchange
model:
Effects are small!
Theory suggests a range of acceptible values for couplings
To resolve experimentally correct value for H1, new measurements required:
(1) simple systems with relatively “clean” theoretical interpretation
(2) excellent control of systematic errors
An Area of Vigorous Activity
H12.4f
H0-1.4h
0
• n,p system: n + p→ d +
• n,4He system: Rotation of the transverse polarization of neutron
after transmission through liquid helium
(spin rotation)
Cold Neutron Beam Experiments(neutrons with energies of a few meV)
8105A
rad/m104 7
H045.0
rad/m 23.025.023.031.1 02 HHHH
n + p→ d +
• Pulsed neutron beams will be used to identify velocity dependent systematic errors
• 3He polarizers provide additional
control of polarization systematic
errors
Projected systematic errors in A 110-9 level, statistical errors 510-9
Detector array used in recent test of npdg experiment
Allowed DDH
Projected Limits from Cold Neutron Beams Measurements
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
•Low Z ensures radiative corrections small
•Spin ½+ ½+ decay restricts the number of contributing form factors, ensures angular correlations have simple form
Simplicity
-decay of quarks (no strong interaction)
J u c t U
d
s
b
h cquarks ( ) ( , , ) . .
1 5
J leptonse h c
( )
. . 1 5
Semi-leptonic decay
E. Fermi, Z. Phys 88, 161 (1934)
)()(
2leptonsquarksF
effJJ
GH
CKM matrix
Electroweak Standard Model
Quark current
Lepton current
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)
• Angular correlations measurements have been performed using cold neutron beams: dominant systematic errors have involved neutron polarization & backgrounds.
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
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
• A great many extenstions to the standard model, including supersymmetry, left-right symmetric models, expanded Higgs sectors, etc…introduce unconstrained T non-invariant 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
T non-invariance in beta-decay
Neutron static electric dipole moment (EDM)
For neutrons (briefly)
(see Norval Fortson’s talk later today)
Limit sought in LANSCE EDM expt
(roughly)
Limits from EDM have provided critical
guidance for theory
New opportunity to measure the neutron EDM using a superthermal He UCN source
•Source of UCNs
•Insulator for required large electric fields
•Detector for UCN spin state
Measurement performed in liquid He (similar to n lifetime), with the liquid He serving a variety of functions:
1
nd
NEP
Improvement factors
Density: x 8
Electric Field: x 5
Coherence time: x 5
(see Martin Cooper’s talk in the working group session)
x 200
Measure torque that an electric field exerts on the precessing
neutron spin
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:
• New opportunities to utilize UCNs in the US:
• Involves a close collaboration between many strong university groups and US national laboratories…we’re excited to seize these opportunities!
3He polarizers
Pulsed neutron beams
Hadronic weak interactions
Neutron beta-decay
Superthermal He and SD2 sources
Neutron beta-decay
T non-invariance (EDMs)