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)