Ultra-Cold Neutrons: From neV to TeV

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Ultra-Cold Neutrons From neV to TeV

• What are UCN? (how cold?)
• How to produce UCN?
• Conventional reactor sources
• New superthermal sources
• Experiments with UCN
• Neutron beta-decay (tn, n decay asymmetry)
• Neutron Electric Dipole Moment

Fermilab 7/13/06
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(No Transcript)
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The Caltech UCN group
Nick Hutzler Gary Cheng Jenny Hsiao Riccardo
Schmid Kevin Hickerson Junhua Yuan Brad Plaster
Bob Carr BF
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Ultra-Cold Neutrons (UCN) (Fermi/Zeldovich)

• What are UCN ?
• Very slow neutrons
• (v lt 8 m/s l gt 500 Å )
• that cannot penetrate into
• certain materials
• Neutrons can be
• trapped in bottles
• or by magnetic
• field

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Material Bottles
Neutron repulsion comes from coherent
scattering from nuclei in solids.
Recall (for short-range potential) At low
energies (kr0ltlt1 eg s-wave) elastic
scattering determined solely by scattering
length a
For k 0 selas 4pa2
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Fermi Pseudo-potential
EUCN
The coherent nuclear potential can lead to
repulsive pseudopotential (Fermi potential) for a
gt 0
For EUCN lt VF, UCN are trapped
Attractive potential can also lead to neutron
absorption but often Lmfp gtgt ln (10-5
probability per bounce)
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Typical Fermi Potentials
Material VF (neV)
Al 54
58Ni 350
Ti - 48
Graphite 180
Stainless Steel 188
Diamond-like Carbon 282
neutron velocity vn 8 m/s
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Magnetic Bottles also possible

• B-field and neutron magnet moment
• produces a potential
• Thus can produce a 3D potential well
• at a field minimum (traps one spin state)

Ioffe Trap
For vnlt8 m/s need Blt6T
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UCN Properties
g
3m
UCN
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How to make UCN?

• Conventional Approach
• Start with neutrons from nuclear
• reactor core
• Use collisions with nuclei to slow down neutrons

Some of neutrons energy lost to nuclear recoil
in each collision
Gives a Maxwell-Boltzmann Distribution
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But

• Only small fraction of neutron distribution is
  UCN

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• Can improve some via gravity and moving turbines
• Record density at
• Institut Laue-Langevin
• (ILL) reactor in
• Grenoble

(1971)
Best vacuum on earth 104 atoms/cm3
Can we make more?
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Higher Density UCN Sources

• Use non-equilibrium system
• (aka Superthermal)
• Superfluid 4He
• (Tlt1K)

(neutron)
11K (9Å ) incident n produces phonon becomes UCN
NIST tn Experiment
Very few 11K phonons if Tlt1K \ minimal
upscattering
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• Solid deuterium (SD2) Gollub Boning(83)
• Small absorption probability
• Faster UCN production
• Small Upscattering if T lt 6K

UCN
Cold Neutron
Phonon
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LANSCE
(Los Alamos Neutron Science CEnter)
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Schematic of prototype SD2 source
Flapper valve
58Ni coated stainless guide
Liquid N2
Be reflector
LHe
Solid D2
77 K polyethylene
UCN Detector
Tungsten Target
Caltech, LANL, NCState, VaTech, Princeton,
Russia, France, Japan Collaboration
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First UCN detection
Total flight path 2 m
50 ml SD2
0 ml SD2
Proton pulse at t 0
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New World Record UCN Density



Previous record for bottled UCN 41 UCN/cm3 (at
ILL)
Measurements of Ultra Cold Neutron Lifetimes in
Solid Deuterium PRL 89,272501 (2002)
Demonstration of a solid deuterium source of
ultra-cold neutrons Phys. Lett. B 593, 55 (2004)
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Physics with higher density UCN Sources

• Macroscopic Quantum States
• Neutron decay (lifetime correlations)
• Solid Deuterium Source
• Neutron Electric Dipole Moment (EDM)
• Superfluid He Source
• Also possible
• Characterize structure of large ( 500Å )
  systems
• (Polymers, Biological molecules)

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Macroscopic Quantum States in a Gravity Field
1-d Schrödinger potential problem
neutron in ground state bounces 15 mm high
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Height Selects Vertical Velocity
Neutron Energy Levels in Gravity
UCN _at_ ILL
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Nesvizhevsky, et al, Nature 2002
May allow improved tests of Gravity at short
distances (need more UCN!)
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Neutron Beta Decay
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Precision neutron decay measurements

• Neutron lifetime essential in Big-Bang
• Nucleosynthesis Calculations
• Can provide most precise measurement of Vud
• lt 0.3 measurements can be sensitive to new
  physics (from loops in electroweak field theory)

Mass eigenstates
Weak eigenstates
ö
æ
ö
æ
ö
æ
d
V
V
V
d

ç

ç

ç
ub
us
ud
w
Single complex phase is possible (gives CP
Violation -more later)

s
V
V
V
s

ç

ç

ç
cb
cs
cd
w

ç

ç

ç
b
V
V
V
b
ø
è
ø
è
ø
è
tb
ts
td
w
a.k.a. Radiative Corrections
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Particle Data Group
Primordial He Abundance
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Neutron Lifetime versus Year
Data points used by Particle Data Group (PDG)
2004 for averaging

Serebrov et al., Phys. Lett. B 605, 72
(2005) (878.5 0.7 0.3) seconds
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Big-Bang Nucleosynthesis Constraints
WMAP
New neutron lifetime
New Lifetime Experiment using our Solid
Deuterium Source is under development
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Neutron Decay in the Standard Model
GF Fermi Constant (known from m decay) Vud
up-down quark weak coupling (more later)
GA Axial vector weak coupling constant GV
Vector weak coupling constant
f phase space integral DR Electroweak
radiative correction Note Z0 Boson (M91
GeV) gives 2 correction!
GA/GV from parity violating decay asymmetry in n
decay
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Sensitivity to New Physics?
KurylovRamsey-Musolf Phys. Rev. Lett. 88, 071804
(2002)

• Vud in Standard Model
• (from m vs. b-decay)

• Supersymmetric particles produce loop corrections

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Asymmetry Measurement with UCN
UCNA
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Asymmetry measurement with UCN

• All previous measurements of GA/GV used cold
  neutrons from a reactor
• Gives continuous, large, background from reactor
  and polarizer (magnetized solid)
• Gives gt 1 systematic error due to neutron
  polarization (95-98)
• New experiment uses pulsed UCN source
• Neutron decays counted with the source off
• Can produce high neutron polarization (99.9)
  using 6T magnetic field

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Experiment Layout
Neutron Polarizing Magnets
UCN Guides

UCN Source
Superconducting Spectrometer

Electron Detectors
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UCNA experiment
Experiment commissioning underway Initial goal is
0.2 measurement of A-correlation
(present measurement 1)
UCNA
Liquid N2
Be reflector
LHe
Solid D2
77 K poly
Tungsten Target
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Most Recent Collaborator
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Neutron Electric Dipole Moment (EDM)

• Why Look for EDMs?
• Existence of EDM implies violation of Time
  Reversal Invariance

Cartoon

-

• Time Reversal Violation seen in K0K0 system
• May also be seen in early Universe
• - Matter-Antimatter asymmetry
• but the Standard Model effect is too small !

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Quantum Picture Discrete Symmetries
Non-Relativistic Hamiltonian
123
123
C-even P-even T-even
C-even P-odd T-odd
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Possible Mechanisms for Matter/Antimatter
Asymmetry in the Universe

• Sakharov Criteria
• Baryon Number Violation
• Departure from Thermal Equilibrium
• CP C violation
• Standard Model CP violation is insufficient
• Must search for new sources of CP
• B-factories, Neutrinos, EDMs

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Origin of EDMs

• Standard Model EDMs are due to observed CP
  violation in the K0/B0-system but
• e- and quark EDMs are zero at first order
• Need at least two loops to get EDMs
• Thus EDMs are VERY small in standard model

Neutron EDM in Standard model is 10-32 e-cm
(10-19 e-fm)
Electron EDM in Standard Model is lt 10-40 e-cm
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Physics Beyond the Standard Model

• New physics (e.g. SuperSymmety SUSY) has
  additional CP violating phases in added couplings
  
• New phases (fCP) should be 1 (why not?)
• Contributions to EDMs depends on masses of new
  particles
• In Minimal Supersymmetric Standard Model
• dn 10-25 (e-cm)x (200 GeV)2/M2SUSY

Note exp. limit dn lt 0.3 x 10-25 e-cm
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Possible impacts of non-zero EDMs

• Must be new Physics
• Sharply constrains models
• beyond the Standard Model
• (especially with LHC data)
• May account for matter-
• antimatter asymmetry of
• the universe

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Experimental EDMs

• Present best limits come from atomic systems and
  the free neutron
• Electron EDM - 205Tl
• Quark ColorEDM - 199Hg
• Quark EDM - free neutron
• Future best limits (x10 1000) may come from
• Molecules (PbO, YbF e-)
• Liquids (129Xe nuclear)
• Solid State systems (Gadolinium-Gallium-Garnet
  e-)
• Storage Rings (Muons, Deuteron)
• Radioactive Atoms (225Ra, 223Rn)
• New Technologies for Free Neutrons
• Switzerland PSI
• France ILL
• US Spallation Neutron Source SNS

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n-EDM vs Time











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Simplified Measurement of EDM
E-field
1. Inject polarized particle
2. Rotate spin by p/2
3. Flip E-field direction
4. Measure frequency shift
B-field
Must know B very well
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ILL-Grenoble neutron EDM Experiment
Trapped Ultra-Cold Neutrons (UCN) with NUCN
0.5 UCN/cc E 5 kV/cm 100 sec storage
time sd 3 x 10-26 e-cm
Harris et al. Phys. Rev. Lett. 82, 904 (1999)
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Careful magnetometry is essential !
199Hg Magnetometer
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New EDM Experiment
(ASU/Berkeley/Caltech/Duke/Harvard/Indiana/Kentuck
y/LANL/ MIT/NIST/NCSU/ORNL/HMI/SFU/Tennessee/UIUC/
Yale)
(AMO/HEP/NP/Low Temp expertise)
Superfluid He UCN converter with high E-field
gt2 orders-of-magnitude improvement possible
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New Technique for n-EDM
E-field

• Inject polarized neutron
• polarized 3He

2. Rotate both spins by 90o
3. Measure n3He capture vs. time (note
sihgtgtshh)
4. Flip E-field direction
B-field
3He functions as co-magnetometer
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EDM Sensitivity
EDM _at_ ILL EDM _at_ SNS
NUCN 1.3 x 104 2 x 106
E 10 kV/cm 50 kV/cm
Tm 130 s 500 s
m (cycles/day) 270 50
sd (e-cm)/day 3 x 10-25 3 x 10-27
SNR (signal noise ratio) 1 1
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Spallation Neutron Source (SNS) _at_ Oak Ridge
National Lab
1 GeV proton beam 1.4 MW on spallation target
Fundamental Physics Beamline
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EDM Experiment at SNS
He Liquifier
Isolated floor
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New n-EDM Sensitivity








EDM _at_ SNS

dn lt 1x10-28 e-cm
2000
2010


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Future Outlook

• Improvements in UCN densities gt 102 possible in
  next generation sources
• First superthermal UCN source experiments are
  underway
• Substantial promise for future high precision
  neutron experiments

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The End
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Additional Slides
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Origin of Hadronic EDMs

• Hadronic (strongly interacting particles) EDMs
  are from
• qQCD (a special parameter in Quantum
  Chromodynamics QCD)
• or from the quarks themselves

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EDM from qQCD

• This is the Strong CP problem in QCD
• Small qQCD does not provide any new symmetry for
  LQCD
• Popular solution is axions (Peccei-Quinn
  symmetry) new term in LQCD
• No Axions observed yet
• Extra dimensions might suppress qQCD
• (Harnik et al
  arXivhep-ph/0411132)
• Remains an unsolved theoretical problem

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Hadronic EDM from Quarks

• Quark EDM contributes via

g
g
q
q
q
q
Quark ChromoEDM e.g.
Quark EDM
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Atomic EDMs

• Schiff Theorem
• Neutral atomic system of point particles in
  Electric field readjusts itself to give zero E
  field at all charges

With E-field
Q
-Q
E
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Determination of Vud
New tn !!
UCNA
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EDM Measurements
particle Present Limit (90 CL) (e-cm) Laboratory Possible Sensitivity (e-cm) Standard Model (ecm)
e- (Tl) e- (PbO) e- (YbF) e- (GGG) 1.6 x 10-27 Berkeley Yale Sussex LANL/Indiana 10-29 10-29 10-30 lt10-40
m m 9.3 x 10-19 CERN BNL lt10-24 lt10-36
n n n n 6.3 x 10-26 ILL ILL PSI SNS 1.5 x 10-26 2 x 10-28 7 x 10-28 lt 1 x 10-28 10-32
199Hg 129Xe 225Ra 223Rn d 1.9 x 10-27 Seattle Princeton Argonne TRIUMF COSY/JPARC? 5 x 10-28 10-31 10-28 1 x 10-28 lt10-27 10-33 10-34
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Status of new EDM experiment

• Estimated cost 18 M
• Approved with highest priority for SNS
  fundamental physics beamline
• Recent RD indicates no showstoppers
• DOE recently (12/05) granted first stage funding
  approval (Critical Design 0). Funding from NSF
  also being sought

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Status of Electroweak Baryogenesis

• Appeared to be ruled out several years ago
• First order phase transition doesnt work for
  MHiggs gt 120 GeV
• MSSM parameters ineffective (fCPltlt1)
• Recent work has revived EW baryogenesis
• First order phase transition still viable
• (with new gauge degrees of freedom)
• Resonance in MSSM during phase transition

Lee, Cirigliano, and Ramsey-Musolf
arXivhep-ph/0412354
a Note Leptogenesis is also possible
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How to measure an EDM?
Recall magnetic moment in B field

• Classical Picture
• If the spin is not aligned with B there will be
  a precession
• due to the torque
• Precession frequency given by

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Cabibbo-Kobayashi-Maskawa (CKM) Matrix
u,c,t quarks couple weakly to superposition of
other quarks
Unitarity,
, (or lack thereof) of CKM matrix tests
existence of possible new physics (eg.
Supersymmetry beyond the Standard Model)
eg. Vud2 Vus2 Vub2 1