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 H  1, new measurements required: (1) simple systems with relatively “clean” theoretical interpretation (2) excellent control of systematic errors An Area of Vigorous Activity H  1  2.4f  H  0  -1.4h  0

n,p system: n + p→ d +  n, 4 He 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)

n + p→ d +  Pulsed neutron beams will be used to identify velocity dependent systematic errors 3 He polarizers provide additional control of polarization systematic errors Projected systematic errors in A   1  level, statistical errors  5  10 -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 V ud 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) Semi-leptonic decay E. Fermi, Z. Phys 88, 161 (1934) 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 3 He polarizers/analyzers can now provide absolute polarimetry at the 0.1 percent level for CN experiments Development of SD 2 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/cm 3 (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: 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! 3 He polarizers Pulsed neutron beams Hadronic weak interactions Neutron beta-decay Superthermal He and SD 2 sources Neutron beta-decay T non-invariance (EDMs)