W. M. SnowPhysics DepartmentIndiana University

Center for the Exploration of Energy and Matterwsnow@indiana.edu

Nuclear/Particle/Astrophysics with Slow Neutrons

1. Neutron technology and slow neutrons2. Neutron lifetime 3. NN weak interaction: parity violation4. Search for neutron electric dipole moment: time reversal violation5. Neutron gravitational bound states and search for extra dimensions

For more: J. Nico and W. M. Snow, Annual Reviews of Nuclear and Particle Science 55, 27-69 (2005)

Thanks for slides to: Geoff Greene, Chen-Yu Liu, Jen-chieh Peng,Philip Harris, Hartmut Abele, Hiro Shimizu, T. Soldner,…

Nuclear/Particle/Astrophysics with Slow Neutrons...

is Particle Physics: but at an energy of 10-20 TeV, using a low energy decelerator

employs a particle which, according to Big Bang Cosmology, is lucky to be alive

is “Nuclear” physics, but with an “isotope of nothing”

relies on experimental techniques and ideas from nuclear, particle, atomic, and condensed matter physics

is pursued at facilities built mainly for chemistry, materials science, and biology

Neutron Properties

Mass: mn=939.56536(8) MeV [mn> mp+ me, neutrons can decay]

Size: rn~10-5Angstrom=1 Fermi [area~ 10-25 cm2=0.1 “barn”]

Internal Structure: quarks [ddu, md~ mu~few MeV ] + gluons

Electric charge: qn=0, electrically neutral [qn<10-21e]

Spin: sn= 1/2 [Fermi statistics]

Magnetic Dipole Moment: n/ p = -0.68497935(17)

Electric Dipole Moment: zero[dn < 3x10-26 e-cm]

Lifetime: n=885.7 +/- 0.8 seconds

Why is it such hard work to get slow neutrons?

E

E=0

p n

VNN

Neutrons are bound in nuclei, needseveral MeV for liberation. We want E~kT~25 meV (room temperature) or less

How to slow down a heavy neutralparticle with Mn= Mp ? Lots of collisions…

E 0 E/2

[1/2]N=(1 MeV)/(25 meV) for N collisions

Neutrons are unstable when free->they can’t be accumulated easily

The ILL reactor

Cold Source

Inside the ILL Reactor

1 m

H11

thermal

cold

hot

The Spallation Neutron Source

$1.4B--1GeV protons at 2MW, started in 2007. Short (~1 usec) pulse– mainly for high TOF resolution

Target

Moderators

“Slow” Neutrons: MeV to neV

235U

n 2MeV

ν β

γ

γβ

ν

235Un 0.1eV

235Un

n

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10010-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

W(En)

En [eV]

T30K T293K

UCN Very cold Cold ThermEpitherm

30K

300KNuclear reactor/Spallation source

Neutron Moderator(LH2, LD2)

meVneV

Neutron Energy, Momentum, and Wavelength

Neutrons in Condensed Matter

|k> |p>

for a “thermal” neutron (EK=mv2/2=3/2 kBT, T=300K-> EK=25 meV)the de Broglie wavelength of the neutron is ≈ 2 Angstroms

Thermal neutrons have the right energies and momenta to match excitations (phonons, spin waves, molecular rotations…) and static

structures (crystals, molecular shapes,…) in condensed media

phonon

|k>|p>

diffraction

Many Condensed Matter Phenomena lie within the Ranges of Length & Time Seen by Neutron Scattering

0.001 0.01 0.1 1 10 100

Q (Å-1)

Energy Transfer (meV)

Neutrons in Condensed Matter Research

Spallation - Chopper ILL - without spin-echo ILL - with spin-echo

Crystal Fields

Spin Waves

Neutron Induced

ExcitationsHydrogen Modes

Molecular Vibrations

Lattice Vibrations

and Anharmonicity

Aggregate Motion

Polymers and

Biological Systems

Critical Scattering

Diffusive Modes

Slower Motions

Resolved

Molecular Reorientation

Tunneling Spectroscopy

Surface Effects?

[Larger Objects Resolved]

Proteins in Solution Viruses

Micelles PolymersMetallurgical

SystemsColloids

Membranes Proteins

Crystal and Magnetic

Structures

Amorphous Systems

Accurate Liquid Structures

Precision Crystallography Anharmonicity

1

100

10000

0.01

10-4

10-6

Mome ntum DistributionsElastic Scatte ring

Elec tron- phonon

Interactio ns

Itine rant Mag nets

Mole cular Motions

Cohe rent Modes in Glass es

and Liquids

SPSS - Cho ppe r Spe c tro me te r

Potential step -> neutron index of refraction

€

with

n = 1−V0

E n Neutron kinetic energy

€

V0 =2π ah2n0

mn

If a > 0, total external reflection

θ

1nout =

E

V1n 0

in −=

<Vstrong>=2h2bs/m, ~+/- 100 neV<Vmag>=B, ~+/- 60 neV/Tesla<Vgrav>=mgz~100 neV/m<Vweak>=[2h2bw/m]s·k/|k|~10-7<Vstrong>

All forces contribute tothe neutron opticalpotential:

Neutron optical guides at ILL/Grenoble (top view)

Cold Neutron Guide Hall at NIST

n

What methods are used to polarize neutrons?

B

B gradients (Stern-Gerlach,sextupole magnets)electromagneticF=()B

Reflection from magneticmirror: electromagnetic+strongf=a(strong) +/- a(EM) with | a(strong)|=| a(EM)|f+=2a, f-=0

B

Transmission throughpolarized nuclei: strong≠ - T ≠ TSpin Filter:T=exp[-L]

L

Ultra-Cold Neutrons (UCN) (Fermi/Zeldovich)

What are UCN ?– Very slow neutrons

(v < 8 m/s, > 500 Å, E<Voptical )

that cannot penetrate into

certain materials

Neutrons can be trapped

in material bottles or by

magnetic fields

The weak interaction: just like EM, but not really:

3 “weak photons” [W+, W-, Z0], can change quark type

e- e-

e-e-

one EM photon

e- e-

e-

Z0

e-

u d

u

W+-

d

V(r)=e2/r, m ‘V’(r)≈[e2/r]exp(-Mr) , MZ,W≈ 80-90 GeV

“Empty” space (vacuum) is a weak interaction superconductor |B|

vacuum superconductor

penetration depth

r

weak field

our “vacuum”

1/ MZ,W

r

u u

u

Z0

u

The weak interaction violates mirror symmetry and changes quark type

u e

W+-

d

Only the weak interaction breaks mirror symmetry: not understood

weakinteraction = eigenstates

[CKM] quark mass eigenstates

Vud in n decayMatrix must be unitary

r->-r in mirror, but s->+sDiscovered by C.S.Wu

Neutron -decay

Input for theory of Big Bang element creation

Clean extraction of fundamental parameters of the electroweak theory.

Neutron LifetimeEffect on

Primordial 4HeAbundance in

Universe

Influence of : Shift by 9 (1%) = 885.7(8)s Yp = 0.2479(6) = 878.5(8)s Yp = 0.2463(6)

Astrophysical observations: large systematic uncertainties,

Yp = 0.238(2)(5), Yp = 0.232 … 0.258, …

878.5(8)s

885.7(8)s

Neutron Lifetime Measurement with aProton Trap and Flux Monitor (Dewey et al)

dN(t)/dt=-N(t)/, measure decay rate and total # of neutrons in a known beam volume

Protons from neutron decay trapped in a Penning trap and counted

Neutron # in trap inferred from flux monitor

In-Beam Lifetime Apparatus @ NIST

Neutron Lifetime Using UCN Bottle

0 10 20 30 401.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Sto

r-1 *

100

0 [

s-1]

[s-1]

Measure storage time, vary S/V

Lifetime extrapolated toS/V->0 limit

“History” of Neutron Lifetime Measurements

World average [PDG]

Last (single)measurement

N-N Weak Interaction: Size and Mechanism

Relative strength of weak / strong amplitudes:

NN weak amplitudes first-order sensitive to qq correlations

Weak interaction violates parity. Use parity violation to isolate the weak contribution to the NN interaction.

NN repulsive core → 1 fm range for NN strong force~1 fm

= valence + sea quarks + gluons + …

NN strong force at low energy mediated by mesons

QCD possesses only vector quark-gluon couplings → conserves parity

Both W and Z exchange possessmuch smaller range [~1/100 fm]

weak

PV Gamma Asymmetry in Polarized Neutron Capture

Asymmetry A of gamma angular distribution upon polarized neutron capture due to weak NN interaction [from snp]

Goal: 1x10-8 at SNS/7000 MW-hours

Asymmetry depends mainly on the weak pion coupling f for n-p

PV gamma asymmetry in n+D->3H+ also possible (SNS letter of intent)

n p

d

NPDGamma Experimental Setup at Los Alamos (LANSCE)

B0=10 gauss

€

dωdΩ

=14π

(1+Aγcos(Θσn⋅kn))

Parity Violating Asymmetry in n+p->D+gamma at LANSCE

Analogous to optical rotation in an “handed” medium.

Transversely-polarized neutrons corkscrew due to the NN weak interaction

PV Spin Angle is independent of incident neutron energy in cold neutron regime,

In combination with n-p experiment, determine weak interactions between nucleons and use it to calculate parity violation in atoms

( ) ( )kfff PVPC

vv⋅+= σ0

Another Parity-Violating Observable: Neutron Spin Rotation

( )−++=↑2

1i

( )zeze PVPCPVPC ii −+ −−+− )()(

2

1 φφφφ

PVPVPV flϕφφϕ 42 ==−= −+

Refractive index dependenton neutron helicity

Cross section of Spin Rotation Apparatusn

Side View

Liquid Helium Motion Control System

Nonmagnetic cryostat: target feedthroughs and liquid motion control system

Distribution of Raw Asymmetries

Data underanalysis

Result consistentwith zero at1E-6 rad/mlevel

Future experiments inH,D, and 4Hepossible

Where’s the antimatter in the universe?Diffuse -ray flux

expected from annihilation

Cohen, De Rujula, Glashow; astro-ph/9707087

In the lab we make equal amounts of matter and antimatterSo why is the universe lopsided? Is it just an accident?

Sakharov Criteria to generate matter/antimatter asymmetry from the laws of physics– Baryon Number Violation (not yet seen)– C and CP Violation (seen but too small by ~1010)– Departure from Thermal Equilibrium (no problem?)

A.D. Sakharov, JETP Lett. 5, 24-27, 1967

Matter/Antimatter Asymmetry in the Universe in Big Bang, starting from zero

Relevant neutron experimental effortsNeutron-antineutron oscillations (B)Electric Dipole Moment searches (T=CP)

Neutron Electric Dipole Moment

sdxdxxd nn ˆ)( 3 ==∫ rr

Non-zero dn violates both P and T

Under a parity operation: Under a time-reversal operation:

EEssrr

−→→ ,ˆˆ EEssrr

→−→ ,ˆˆ

EdEd nn

rrrr⋅−→⋅ EdEd nn

rrrr⋅−→⋅

Factor ~10 per 8 years

EDM limits: the first 50 yearsE

xper

imen

tal L

imit

on

d (

e cm

)

1960 1970 1980 1990

ϕ

Left-Right

10-32

10-20

10-22

10-24

10-30

MultiHiggs SUSY

ϕ

Standard Model

Electro-magneticneutron:

electron:

10-34

10-36

10-38

2000

10-20

10-30

Barr: Int. J. Mod Phys. A8 208 (1993)

Reality check

If neutron were the size of the Earth...

x-e

+e

... current EDM limit on neutron would correspond to charge separation of

x 10

EDM Measurement Principle

() – () = – 4 E d/ h

assuming B unchanged when E is reversed.

B0 E<Sz> = + h/2

<Sz> = - h/2

h(0) h() h()

B0 B0 E

Present EDM limit corresponds to energy difference of 1E-22 eV!

nEDM apparatus

at ILLusingUCN

N S

Four-layer mu-metal shield High voltage lead

Quartz insulating cylinder

Coil for 10 mG magnetic field

Upper electrodeMain storage cell

Hg u.v. lamp

PMT to detect Hg u.v. lightVacuum wall

Mercury prepolarising

cell

Hg u.v. lampRF coil to flip spins

Magnet

UCN polarising foil

UCN guide changeover

Ultracold neutrons

(UCN)

UCN detector

• Use 199Hg co-magnetometer to sample the variation of B-field in the UCN storage cell

• Limited by low UCN flux of ~ 5 UCN/cm3

• Figure–of-merit ~ E(NΤ)1/2

UCN production in liquid helium

1.03 meV (11 K) neutrons downscatter by emission of phonon in liquid helium at 0.5 K

Upscattering suppressed: Boltzmann factor e-E/kT is small if T<<11K

n = 8.9 Å; E = 1.03 meV

Landau-Feynman dispersion curve for 4He excitations

Dispersion curve for free neutrons

R. Golub and J.M. Pendlebury Phys. Lett. 53A (1975), Phys. Lett. 62A (1977)

nEDM experiment at SNS, ~2014

GOAL: ~1E-28e-cm

Figure–of-merit ~ E(NΤ)1/2

New approach aims at N 100 N, T 5 T, E 5 E

Classical/QM Bouncing Neutrons

Neutron Probability Distributions Above the Mirror

Vhorizont~4-15 m/s Vvertic~2 cm/s

Δ≈Δ

hE

Selection of vertical and horizontal velocity components

General scheme of the experiment (flow-through integral mode)

Experimental Apparatus

the Experiment

Observation/Comparison to Theory

Large Extra Dimensions of Spacetime?New Forces of nature? Why not?

Large Extra Dimensions of Spacetime?New Forces of nature? Why not?

Infinite dimension

compactdimension

R

Maybe gravity is so weak because some of its fluxflows into extra “compact”dimensions of size

If so, V is not 1/r if r~

Limits for alpha and lambda

Green: Neutron Limits

)1()( /21 rermm

GrV −⋅+⋅

= )1()( /21 rermm

GrV −⋅+⋅

=

V.V.Nesvizhevsky

- Search for extra fundamental forces at short distances of 1 nm - 10 m

-Verification of electrical neutrality of neutrons

Perturbation frequency, Hz

Transition probability

induced by

vibrating mass

eVE 18min 10−≈

6

12

min 10−≈−EE

E

ijji wEE ⋅=− h

Hz26021 ≈

Quantum trap

Resonance transitions

Potential Applications in fundamental physics

Nuclear/Particle/Astrophysics with Slow Neutrons

Uses techniques and concepts from atomic physics,condensed matter physics,low temperature physics, optics

Neutron physics measurements test fundamental laws of weak interaction and can be used to search for new phenomena

New facilities are on the way (NIST, SNS, JSNS, UCN facilities).

for more: http://www.bnl.gov/NPSSNico/Snow, Annual Reviews Nuclear Particle Science 55, 27-69 (2005)

Neutron EDM Experiment with Ultra Cold Neutrons

• Use 199Hg co-magnetometer to sample the variation of B-field in the UCN storage cell

• Limited by low UCN flux of ~ 5 UCN/cm3 • Figure–of-merit ~ E(NΤ)1/2

A new approach aims at N 100 N, T 5 T, E 5 E

Most Recent ILL Measurement

P, CP, T, and CPT

Parity violation (1956)– only in weak interaction

CP violation (1964)– parametrised but not understood– only seen so far in K0 & B0 systems– Doesn’t seem to be responsible for baryon

asymmetry of universe

T violation (1999)– CPT is good symmetry: T CP

KS

KL

e-60Co

Detector

Big Bang Nucleosynthesis (BBN)kT

[M

eV]

t [s]

d destroyed by (kT < 2.2MeV,

but N / NB 109)

en p e −→ + +

n

p

1

7

N

N≈

decay rate

1 200

en e p ++ ↔ +en p e −+ ↔ +

2n

p

expN m c

N kT

⎛ ⎞Δ= −⎜ ⎟

⎝ ⎠

reaction rate

1

0.1

Freeze-out if Hubble

expansion rate greater than reaction rate

n

p

1

6

N

N≈

Tf

All rates depend on baryon density NB/N = 101010

3

3 4

DD He

He He

n ppn

+ → ++ →+ →

He

tot p n

n2

4 NM

M N N≈

+

4He formation

Vud from neutron decay needs the neutron lifetime

Classical Theory of Weak Decay

( ) ( )p nF5 5 e

p

1 122

ud

GH p q n e

mV ν μ

μ μν

μ μγ γ σ γ γ νλ⎧ ⎫−⎪ ⎪

= + + −⎨ ⎬⎪ ⎪⎩ ⎭

Standard Model for neutron decay:

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛=

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

′′′

b

s

d

U

b

s

d

CKM⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛=

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

′′′

b

s

d

U

b

s

d

CKM

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

=

VVVVVVVVV

tbtstd

cbcscd

ubusud

U CKM ⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

=

VVVVVVVVV

tbtstd

cbcscd

ubusud

U CKM

|Vud|2 + |Vus|

2 + |Vub|2 = 1-Δ |Vud|

2 + |Vus|2 + |Vub|

2 = 1-Δ

5 41 2 2 2

3 7(1 3 )2ud

Re

F

fV

mG

ch

− = +

Unitarity of CKM matrix

Expression for neutron lifetime in Standard Model