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Nuclear/Particle/Astrophysics with Slow Neutrons. W. M. Snow Physics Department Indiana University Center for the Exploration of Energy and Matter [email protected]. 1. Neutron technology and slow neutrons 2. Neutron lifetime 3. NN weak interaction: parity violation - PowerPoint PPT Presentation
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W. M. SnowPhysics DepartmentIndiana University
Center for the Exploration of Energy and [email protected]
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