UCN (Ultracold Neutrons) Jeff Martin Outline What is a UCN? Interactions of UCN How to make UCN Fun things to do with UCN

Ultracold Neutrons UCN are neutrons that are moving so slowly that they are totally reflected from a variety of materials. So, they can be confined in material bottles for long periods of time. Typical parameters: –velocity < 8 m/s –temperature < 4 mK –kinetic energy < 300 neV Interactions: –gravity: V=mgh –weak interaction (allows UCN to decay) –magnetic fields: V=-  B –strong interaction

Gravity V = mgh For a neutron on the planet Earth: m = 1 GeV/c 2, g = 10 m/s 2, h = 3 m  V = 300 neV Recall, T UCN < 300 neV Uses: –UCN spectrometer –gravitational levels experiment y x 3 m UCN no UCN

Weak Interaction n  p + e keV neutrons live for about 15 minutes dW/dT e TeTe 782 keV Causes free neutrons to decay proton electron electron anti-neutrino

Magnetic Interaction The neutron has a magnetic moment:  = -1.9  N = - 60 neV/T The potential energy of a magnetic moment in a magnetic field is: V = -   B x spin aligned V = |  B| 7 T

Magnetic Interaction The neutron has a magnetic moment:  = -1.9  N = - 60 neV/T The potential energy of a magnetic moment in a magnetic field is: V = -   B x spin anti-aligned V = -|  B| 7 T

Strong Interaction: QM in 3D Central Potential V(r) Solve by separation of variables: Answer:

QM in 3D Central Potential V(r) Some interesting consequences: QM in 3D is much like QM in 1D, but with an infinite wall at the origin.

Strong Interaction Attractive Nuclear Force

Scattering Length Weak potential Strong potential Many different potentials can give rise to the same value for “a” Odds are, a > 0

Fermi Potential For many nuclei in a solid, For a all the same, and small lattice spacing cf. neutron, Let’s replace V(r) by an effective potential with the same a:

Fermi Potential Even attractive potential can lead to repulsive effective potential! (the “Fermi Potential”) Just as long as a > 0 Largest Fermi potential is for Nickel-58 ( 58 Ni) V 0 = 335 neV

Absorption of UCN Loss/bounce: f ~ d/d loss ~ For a vessel of typical size L ~ 10 cm, the neutrons will bounce around for a time t = L/fv ~ s before being absorbed

How to make UCN Conventional Method: –Take neutrons from a reactor core –E n = 5-10 MeV –bring into thermal equilibrium with nuclei –Energy distribution of “cooled” neutrons follows Maxwell-Boltzmann distribution:

Low efficiency Fraction of neutrons below 8 m/s is only: at 300 K at 30 K Use a few tricks to boost the UCN yield: 1. vertical extraction 2. turbine

ILL Neutron Source Institut Laue-Langevin Grenoble, France Turbine operation: neutron neutron hits co-rotating blade and stops highest UCN density achieved: ~ 41 UCN/cm 3

Superthermal Source Consider a moderator with two energy levels: E = 0,  Neutrons coming into contact with the moderator can lose energy by exciting transitions in the moderator. n mod  UCN mod* +  down-scattering up-scattering In thermal equilibrium, these rates are equal. The trick is to not allow the UCN to come into thermal equilibrium the moderator.

Superfluid 4 He Superthermal Source When energy of neutron is equal to energy of phonon, down-scattering can occur. Another advantage: neutron- 4 He cross section is negligible! under development at NIST for neutron lifetime measurement

Solid Deuterium Cold n UCN Phonon Cooling removes phonons

“Superthermal” Sources What makes a good superthermal UCN source? –Low neutron absorption –High single phonon energy Light atoms Weak crystal –Long elastic interaction length Solid deuterium has these properties!

UCN Losses in SD 2 Nuclear absorption on deuterium –  ~ 150 ms Phonon up-scattering –  ~ 150 T SD2 = 5 K Nuclear absorption on hydrogen cont. –  ~ % H Conversion of para-deuterium –  ~ 150 1% para-D 2

D 2 molecule has two molecular states: –Ortho (symmetric spin) + L=0,2,… –Para (antisymmetric spin) + L=1,3,… Energy difference between ground state (ortho) and excited state (para) is about 5 meV (80 K). At T=300K, D 2 gas is 33% para and 67% ortho. Ortho and Para-D 2

Scattering from para-D 2 UCN para ortho CN At T=300K, 33% of D 2 gas is para At low T, conversion of para to ortho takes months Use magnetic substance to speed conversion measure para-fraction using Raman scattering

Pulsed Neutrons 1 GeV proton beam on Tungsten target produces ~ 18 neutrons/proton Neutrons are produced via proton- induced “spallation” Can produce large bursts of neutrons, allowing SD 2 to cool between pulses

Los Alamos Neutron Science Center LANSCE UCN Source Proton Linac

First SD 2 UCN detection with prototype source Total flight path ~ 2 m 50 ml SD 2 0 ml SD 2 Proton pulse at t = 0 detector W SD 2 p beam

Bottling Mode

UCN lifetime in SD 2 Measured for the first time Critical parameter in determining maximum  UCN Strong dependence on T and ortho/para ratio in SD 2 C. L. Morris et al. Phys. Rev. Lett. 89, (2002).

World Record density achieved ILL (1975) Previous record for bottled UCN = 41 UCN/cm 3 (at ILL) A. Saunders et al, nucl-ex/

Physics with UCN Precise measurements of neutron interactions offer window into fundamental physics: Neutron Beta Decay –Electroweak interaction tests –Probe for physics beyond the standard model, e.g. SUSY Neutron Electric Dipole Moment, Neutron-Antineutron oscillations –should not exist in standard model –Probe for physics beyond the standard model, e.g. SUSY Neutron Quantum States in Gravitational Field –Particle-in-well energy levels –Potential for precise tests of quantum mechanics, equivalence principle, modifications of gravity.

The Standard Model six quarks six leptons four gauge bosons 17 parameters + Force carriers W, Z, γ, g In the standard model, these are all the particles that exist

Shortcomings of the Standard Model Why so many parameters to fit? Why is mass range so vast? Why is the calc. Higgs mass unstable against corrections? How to incorporate gravity? Where’s the dark matter? How to generate matter-antimatter asymmetry? Belief: this is an effective theory below 100 GeV These drawbacks motivate theoretical extensions to the standard model (SUSY, string theory), and motivate searches for cracks in the standard model.

Cabibbo, Kobayashi, Maskawa (CKM) Matrix                                                               b s d b s d b s d VVV VVV VVV b s d w w w tbtstd cbcscd ubusud w w w Note: this matrix must be unitary! Weak processes allow transitions between generations Weak eigenstates of quarks are different from mass eigenstates Weak eigenstatesMass eigenstates Particle Data Group 2001 Central Values Otherwise, something is missing from the theory.

A Precise Test of Unitarity From data, we find: V ud 2 =0.9487± ( nuclear decays) V us 2 =0.0482± ( from e.g. K +  π 0 e + ν e ) V ub 2 = ± ( B meson decays) World Data 2002 In the standard model, we expect: off by 2.2 sigma

Neutron  -decay in the quark model… u d d u d u e e-e- n p W-W-

Why measure G A and G V ? G A related to strong interaction modifications (QCD) to quark axial-vector electroweak interaction G V is related to fundamental quark electroweak coupling (conserved vector current, CVC) W-W- e-e- G V =G F V ud --  W-W- e-e- GFGF d u e e u quark couples to d w Universality (almost)

Status of  -decay Neutron lifetime A-Correlation

Supersymmetry --  W-W- e-e- e  ~ ~ 00 ~ d W-W- e-e- e d ~ u ~ 00 ~u Sensitive to loop corrections  -decay sensitive to differences in squark/slepton couplings

The UCNA Experiment T. J. Bowles 4 (co-PI), R. Carr 1, B. W. Filippone 1, A. Garcia 9, P. Geltenbort 3, R. E. Hill 4, S. A. Hoedl 9, G. E. Hogan 4, T. M. Ito 6, S. K. Lamoreaux 4, C.-Y. Liu 4, M. Makela 8, R. Mammei 8, J. W. Martin 10, R. D. McKeown 1, F. Merrill 4, C. L. Morris 4, M. Pitt 8, B. Plaster 1, K. Sabourov 5, A. Sallaska 9, A. Saunders 4 (co-PI), A. Serebrov 7, S. Sjue 9, E. Tatar 2, R. B. Vogelaar 8, Y.-P. Xu 5, A. R. Young 5 (co-PI), and J. Yuan 1 1 W. K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA Idaho State University, Pocatello, ID Institut Laue-Langevin, BP 156, F Grenoble Cedex 9, France 4 Los Alamos National Laboratory, Los Alamos, NM North Carolina State University, Raleigh, NC University of Tennessee, Knoxville, TN St.-Petersburg Nuclear Physics Institute, Russian Academy of Sciences, Gatchina, Leningrad District, Russia 8 Virginia Polytechnic Institute and State University, Blacksburg, VA Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, WA University of Winnipeg, Winnipeg, MB R3B 2E9, Canada

Experimental Method to Measure A Endpoint energy 782 keV Focus electrons onto detectors using a strong (1 T) magnetic field

A(E)  N + - N - N + + N - How to Measure a Beta-Asymmetry Field defines n-polarization direction, “focuses” electrons onto detectors.

Layout in Area B 800 MeV protons UCN Source beta- spectrometer polarizer magnet

proton beam direction shield package UCN port remote extraction

UCN to expt “prepolarizer” magnet polarizer magnet beta-spectrometer magnet UCN guide insertion future UCN guide path Status

UCNA Experiment

Experimental Parameters Goal precision:  A/A = 0.2% –collection of 2  10 8 decays Decay rate 100 Hz, 21 days data-taking UCN polarization > 99.9% Systematics –Total systematic corrections 0.17% –Total systematic uncertainty 0.04%

An Important Systematic Uncertainty: Backscattering UCNA Experimental Goal: Asymmetry to 0.2% Residual correction due to backscattering 0.1% Calibration of low-energy electron backscattering in energy range of neutron beta-decay barely sufficient, will ultimately limit precision in future experiments.

New Measurements of Backscattering Electron gun Beam diagnostics Backscattering chamber Electron Beam See: JWM et al, Phys. Rev. C 68, (2003) and M. J. Betancourt et al, in preparation.

UCNA Schedule UCN source installed – April 2004 Shutdown over summer was extended UCNA commissioning – February 2005 Production running – summer 2005 More experiments to follow Silicon detectors, proton detectors

Future of UCNA Silicon Detectors –A, b, a wm Proton Detectors –B, a

Conclusions UCN have many fascinating properties. Recent advances in UCN production will allow us to use these properties to make new precision measurements of the fundamental interactions of the neutron.

Possibilities for B and a measurement of proton emission asymmetry, proton spectrum gives sensitivity to B and a, respectively accelerate protons into secondary electron emitter detect secondaries in conventional detectors

Neutron Electric Dipole Moment (EDM) Existence of EDM implies violation of Time Reversal Invariance CPT Theorem then implies violation of CP conservation Observed in mixing, but not enough to explain matter/antimatter asymmetry of universe

Sources of EDM Present Exp. Limit < e-cm Standard Model value: e-cm Supersymmetry or Multi-Higgs models can give 10 5 xSM Significant discovery potential with new high sensitivity n EDM experiment (also atomic EDM’s - 199Hg)

Basic Technique B E For B ~ 1mG = 3 Hz For E = 50kV/cm and d n = 4x e·cm  = 0.2  Hz J

New EDM Experiment Superfluid LHe UCN converter with high E-field 2-3 orders-of-magnitude Improvement possible

Nature: 1/17/02 Nesvishevsky, et al ILL Grenoble

Quantum States in Gravity Field 1-d Schrodinger potential problem V z mgz

Height Selects Vertical Velocity Quantized energy levels!

Classical expectation Energy levels are observed at expected absorber heights.