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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 (n, n decay
asymmetry)
– Neutron Electric Dipole Moment Fermilab7/13/06
The Caltech UCN group
Nick HutzlerGary ChengJenny HsiaoRiccardo SchmidKevin HickersonJunhua YuanBrad Plaster Bob CarrBF
Ultra-Cold Neutrons (UCN)
(Fermi/Zeldovich)• What are UCN ?
– Very slow neutrons (v < 8 m/s ’ > 500 Å )that cannot penetrate into
certain materials
Neutrons can be trapped in bottles
or by magnetic field
Recall (for short-range potential): At low energies (kr0<<1; eg s-wave) elastic scattering determined solely by scattering length a
For k ’0elasa2
Neutron “repulsion” comes from coherent scattering from nuclei in solids.
Material Bottles
V(r)
r
m
n a 2V 0
2
F
Solid
The coherent nuclear potential can lead to repulsive pseudopotential (Fermi potential)
for a > 0
Attractive potential can also lead to neutron absorption but often Lmfp >> n (~10-5 probability per bounce)
For EUCN < VF, UCN are trapped
EUCN
Fermi Pseudo-potential
Typical Fermi Potentials
Material VF (neV)
Al 54
58Ni 350
Ti - 48
Graphite 180
Stainless Steel 188
Diamond-like Carbon
282
neutron velocityvn ~ 8 m/s
• B-field and neutron magnet moment produces a potential
• Thus can produce a 3D potential well at a field minimum (traps one spin state)
Magnetic Bottles also possible
BV
Ioffe Trap
For vFor vnn<8 m/s need B<6T<8 m/s need B<6T
UCN Properties
m 3 ~:gravity to dueheight Maximum
mK 4"T"
nm) (50 A 500 λ
mi/hr) 18( m/s 8v
UCN
o
UCN
UCN
g3m
UCN
How to make UCN?
• Conventional Approach:– Start with neutrons from nuclear
reactor core
– Use collisions with nuclei to slow down neutrons
m/s10 4 vMeV 105E
7n
n
Some of neutron’s energy lostto nuclear recoil in each collision
Gives a Maxwell-Boltzmann Distribution
But…– Only small
fraction of neutron distribution is UCN
)H (liquid K 20Tfor 10~K 300Tfor 10~
26
-8
– Can improve some via gravity and moving turbines
– Record density at Institut Laue-Langevin (ILL) reactor in Grenoble
bottle in stored UCN/cm 40 3(1971)
Best vacuum on earth~104 atoms/cm3
Can we make more?
Higher Density UCN Sources
• Use non-equilibrium system (aka Superthermal)– Superfluid 4He
(T<1K)
Very few 11K phonons if T<1K
minimal upscattering
11K (9Å ) incident nproduces phonon &becomes UCN
NIST NIST nn Experiment Experiment
(neutron)
– Solid deuterium (SD2) Gollub &
Boning(83)– Small absorption probability– Faster UCN production– Small Upscattering if T < 6K
UCN
Phonon
Cold Neutron
LANSCE
UCN Source
Proton Linac (½ mile long)
(Los Alamos Neutron Science CEnter)
Schematic of prototype SD2 source
Liquid N2
Be reflector
Solid D2
77 K polyethylene
Tungsten Target
58Ni coated stainless guide
UCN Detector
Flapper valve
LHe
Caltech, LANL, NCState, VaTech, Princeton, Russia, France, JapanCollaboration
First UCN detection
Total flight path ~ 2 m sec 0.33 m/s 6
1 m 2
50 ml SD2
0 ml SD2
Proton pulse at t = 0
New World Record UCN Densit
y
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)]
Previous record for bottled UCN = 41
UCN/cm3 (at ILL)
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)
Macroscopic Quantum States in a Gravity
Field1-d Schrödinger potential problem
neutron in ground state“bounces” ~ 15 m high
V
z
mgz
Height Selects Vertical Velocity
UCN @ ILL
Neutron Energy Levels in Gravity
Classical expectation
May allow improved tests of Gravity at short distances
(need more UCN!)
Nesvizhevsky, et al, Nature 2002
Neutron Beta Decayeνepn
MeV 0.78K.E.νe,p,
min. 10t min., 15τ2
1n
or in quark picture…
udd
udu
e
e-
n
p
W-
Precision neutron decay measurements
• Neutron lifetime essential in Big-Bang Nucleosynthesis Calculations• Can provide most precise measurement
of Vud
• < 0.3% measurements can be sensitive to new physics (from loops in electroweak field theory)
bsd
VVVVVVVVV
bsd
tbtstd
cbcscd
ubusud
w
w
w
Weak eigenstates Mass eigenstates
Single complex phaseis possible
(gives “CP” Violation-more later)
a.k.a. Radiative Corrections
Particle Data Group
Primordial He Abundance
Serebrov et al.,
Phys. Lett. B 605, 72 (2005)
(878.5 ± 0.7 ± 0.3) seconds
Neutron Lifetime versus Year
Data points used by Particle Data Group
(PDG) 2004 for averaging
Big-Bang Nucleosynthesis ConstraintsWMAP
New neutron lifetime
New Lifetime Experiment using
our Solid Deuterium
Sourceis under
development
Neutron Decay in the Standard Model
)Δf(1GG31VG
1
cm
2πτ
RVA ud2F
45e
73
n
1122
GA = Axial vector weak coupling constantGV = Vector weak coupling constant
V
A2 G
Gλ ,
λ 31
λ)12λA
(GA/GV from parity
violating decay asymmetry in n decay
22
1
VA ud2F
45e
73
nGG31VG
1
cm
2πτ
2
ud2F
45e
73
nVG
1
cm
2πτ
45e
73
n cm
2πτ
GF = Fermi Constant (known from decay)Vud = up-down quark weak coupling (more later)
f = phase space integralR = Electroweak radiative correction
Note: Z0 Boson (M=91 GeV) gives 2% correction!
• Vud in Standard Model
(from vs. -decay)
Sensitivity to New Physics?Kurylov&Ramsey-Musolf
Phys. Rev. Lett. 88, 071804 (2002)
W- e-
GFVud-
W- e-
GF d u
e e
-
W- e-
e
~ ~0~ d
W- e-
e
d~ u~
0~ u
q
μM
Mlooploop
SMud
superud lnΔ :)Δ(1VV ~
~
• Supersymmetric particles produce loop corrections
Asymmetry Measurement with UCN
-
-exp NN
NNA
NN
)cos1 θβA(NN 0e e-
n
sCorrection Radiative kElectrowea RC
RCτA,V nud
),(f
UCNA
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 > 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
Experiment Layout
Superconducting Spectrometer
Electron Detectors
UCN Guides
Neutron Polarizing Magnets
UCN Source
Liquid N2
Be reflector
Solid D2
77 K poly
Tungsten Target
LHe
UCNA experimentExperiment commissioning underwayInitial goal is 0.2% measurement of A-correlation (present measurement ~ 1%)
UCNA
Most Recent Collaborator
Neutron Electric Dipole Moment (EDM)
• Why Look for EDMs?– Existence of EDM implies violation of
Time Reversal Invariance
+-
J
+-
J-
d dJ Jt t
• Time Reversal Violation seen in K0–K0 system• May also be seen in early Universe
- Matter-Antimatter asymmetrybut the Standard Model effect is too small !
Cartoon
Quantum Picture – Discrete Symmetries
-+-B-++J
+--E-+-d-+-TPC
EdBH
Non-Relativistic Hamiltonian
C-evenP-evenT-even
C-evenP-oddT-odd
(-t) (t) T :Reversal Time(-x,-y,-z) z)y,(x, P :Parity
C :nConjugatio Charge nn
J
J and
J
J Assume
dd
• 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
Possible Mechanisms for Matter/Antimatter Asymmetry in
the Universe
• Standard Model EDMs are due to observed CP violation in the K0/B0-system but…– e- and quark EDM’s are zero at first order– Need at least two “loops” to get EDM’s
• Thus EDM’s are VERY small in standard model
Origin of EDMs
Neutron EDM in Standard model is~ 10-32 e-cm (=10-19 e-fm)
Electron EDM in Standard Model is< 10-40 e-cm
Physics Beyond the Standard Model
• New physics (e.g. SuperSymmety = SUSY) has additional CP violating phases in added couplings – New phases: (CP) should be ~ 1 (why not?)(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/M2
SUSY
2SUSY
CPn M
sind
Note: exp. limit: dn < 0.3 x 10-25 e-cm
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 Pospelov, et al, 2004
A/
MSUSY=750GeV
/
e-
199Hg
n
Chang, et al, hep-ph/0503055
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
n-EDM vs Time
Simplified Measurement of EDM
B-field
E-field1. Inject polarized particle2. Rotate spin by /23. Flip E-field direction4. Measure frequency shift
h
EdB
22
Must know B very well
ILL-Grenoble neutron EDM Experiment
Trapped Ultra-Cold Neutrons (UCN) withNUCN = 0.5 UCN/cc
|E| = 5 kV/cm
100 sec storage time
d = 3 x 10-26 e-cm
Harris et al. Phys. Rev. Lett. 82, 904 (1999)
Careful magnetometry is essential !
199199Hg MagnetometerHg Magnetometer
New EDM Experiment
Superfluid He UCN converter with high E-field
>2 orders-of-magnitude improvement possible
(ASU/Berkeley/Caltech/Duke/Harvard/Indiana/Kentucky/LANL/MIT/NIST/NCSU/ORNL/HMI/SFU/Tennessee/UIUC/Yale)(AMO/HEP/NP/Low Temp expertise)
New Technique for n-EDM
B-field
E-field1. Inject polarized neutron & polarized 3He
2. Rotate both spins by 90o
4. Flip E-field direction
3. Measure n+3He capturevs. time
(note: >>)
3He functions as “co-magnetometer”
EDM SensitivityEDM @
ILLEDM @
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
d (e-cm)/day 3 x 10-25 3 x 10-27
SNR (signal noise ratio) 1 1
UCNm
dmNT|E|2
σ SNR/11
Spallation Neutron Source (SNS)
@ Oak Ridge National Lab
1 GeV proton beam 1.4 MW on spallation target
Fundamental Fundamental Physics Physics BeamlineBeamline
EDM Experiment at SNS
Isolated floor
HeLiquifier
New n-EDM Sensitivity
2000 2010
EDM @ SNS
dn < 1x10-28 e-cm
Future Outlook
– Improvements in UCN densities > 102 possible in next generation sources
– First superthermal UCN source experiments are underway
– Substantial promise for future high precision neutron experiments
The End
Additional Slides…
• Hadronic (strongly interacting particles) EDMs are from – QCD (a special parameter in Quantum
Chromodynamics – QCD)
– or from the quarks themselves
Origin of Hadronic EDMs
EDM from QCD
• This is the Strong CP problem in QCD
• Small QCD 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 QCD
(Harnik et al arXiv:hep-ph/0411132)
– Remains an unsolved theoretical “problem”
Hadronic EDM from Quarks
• Quark EDM contributes via
q q
q q
g
qd qd~
Quark EDMQuark ChromoEDM e.g.
cmed recall cm-e dbut
n
Model Standardn
25exp
31
1010
Atomic EDMs • Schiff Theorem
– Neutral atomic system of point particles in Electric field readjusts itself to give zero E field at all charges
-Q
+QWith E-field
E
Determination of Vud
New n !!
UC
NA
EDM Measurementsparticle Present Limit
(90% CL)(e-cm)
Laboratory Possible Sensitivity
(e-cm)
Standard Model(e–cm)
e- (Tl)e- (PbO) e- (YbF)e- (GGG)
1.6 x 10-27 BerkeleyYaleSussex LANL/Indiana
10-29
10-29
10-30
<10-40
9.3 x 10-19 CERNBNL <10-24
<10-36
n nnn
6.3 x 10-26 ILLILLPSISNS
1.5 x 10-26
~ 2 x 10-28
~ 7 x 10-28
< 1 x 10-28
~10-32
199Hg129Xe225Ra223Rn d
1.9 x 10-27 SeattlePrincetonArgonneTRIUMFCOSY/JPARC?
5 x 10-28 10-31
10-28
1 x 10-28
<10-27
~10-33
~10-34
Status of new EDM experiment
• Estimated cost ~ 18 M$
• Approved with highest priority for SNS fundamental physics beamline
• Recent R&D indicates no “showstoppers”
• DOE recently (12/05) granted first stage funding approval (Critical Design 0). Funding from NSF also being sought
Status of Electroweak Baryogenesis
• Appeared to be “ruled out” several years ago– First order phase transition doesn’t work for
MHiggs > 120 GeV– MSSM parameters ineffective (CP<<1)
• 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: arXiv:hep-ph/0412354
Note: Leptogenesis is also possibleNote: Leptogenesis is also possible
How to measure an EDM?
BS if ; BSμ
2 Bμdt
Sdτ
Sμ
2μ ;BμH
N
N
Classical Picture:• If the spin is not aligned with B there will be a precession due to the torque • Precession frequency given by
Recall magnetic moment in B field:
fS
iS
Sd
d
Ein dfor E2d
B2μ
dt
dS
S
1
dt
dω
NNN
ω
Cabibbo-Kobayashi-Maskawa (CKM) Matrix
Unitarity, , (or lack thereof) of CKM matrix tests existence of possible new physics
(eg. Supersymmetry – “beyond the Standard Model”)
u,c,t quarks couple weakly to superposition of other quarks
eg. |Vud|2 + |Vus|2 + |Vub|2 = 1
b
s
d
VVV
VVV
VVV
b
s
d
tbtstd
cbcscd
ubusud
w
w
w
Weak eigenstates Mass eigenstates
Single complex phaseis possible
(gives CP Violation-more later)