ultra-cold neutrons: from nev to tev what are ucn? (how cold?) how to produce ucn? – conventional...
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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
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The Caltech UCN group
Nick HutzlerGary ChengJenny HsiaoRiccardo SchmidKevin HickersonJunhua YuanBrad Plaster Bob CarrBF
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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
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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
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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
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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 velocityvn ~ 8 m/s
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• 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
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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
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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
m/s10 4 vMeV 105E
7n
n
Some of neutron’s energy lostto nuclear recoil in each collision
Gives a Maxwell-Boltzmann Distribution
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But…– Only small
fraction of neutron distribution is UCN
)H (liquid K 20Tfor 10~K 300Tfor 10~
26
-8
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– 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?
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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)
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– Solid deuterium (SD2) Gollub &
Boning(83)– Small absorption probability– Faster UCN production– Small Upscattering if T < 6K
UCN
Phonon
Cold Neutron
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LANSCE
UCN Source
Proton Linac (½ mile long)
(Los Alamos Neutron Science CEnter)
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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
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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
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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)
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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
Field1-d Schrödinger potential problem
neutron in ground state“bounces” ~ 15 m high
V
z
mgz
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Height Selects Vertical Velocity
UCN @ ILL
Neutron Energy Levels in Gravity
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Classical expectation
May allow improved tests of Gravity at short distances
(need more UCN!)
Nesvizhevsky, et al, Nature 2002
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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-
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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
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Particle Data Group
Primordial He Abundance
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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
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Big-Bang Nucleosynthesis ConstraintsWMAP
New neutron lifetime
New Lifetime Experiment using
our Solid Deuterium
Sourceis under
development
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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!
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• 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
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Asymmetry Measurement with UCN
-
-exp NN
NNA
NN
)cos1 θβA(NN 0e e-
n
sCorrection Radiative kElectrowea RC
RCτA,V nud
),(f
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 > 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
Superconducting Spectrometer
Electron Detectors
UCN Guides
Neutron Polarizing Magnets
UCN Source
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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
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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
+-
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
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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
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• 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
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• 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
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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: (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
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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 Pospelov, et al, 2004
A/
MSUSY=750GeV
/
e-
199Hg
n
Chang, et al, hep-ph/0503055
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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
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
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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)
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Careful magnetometry is essential !
199199Hg MagnetometerHg Magnetometer
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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)
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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”
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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
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Spallation Neutron Source (SNS)
@ Oak Ridge National Lab
1 GeV proton beam 1.4 MW on spallation target
Fundamental Fundamental Physics Physics BeamlineBeamline
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EDM Experiment at SNS
Isolated floor
HeLiquifier
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New n-EDM Sensitivity
2000 2010
EDM @ SNS
dn < 1x10-28 e-cm
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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
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The End
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Additional Slides…
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• Hadronic (strongly interacting particles) EDMs are from – QCD (a special parameter in Quantum
Chromodynamics – QCD)
– or from the quarks themselves
Origin of Hadronic EDMs
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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”
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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
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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
-Q
+QWith E-field
E
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Determination of Vud
New n !!
UC
NA
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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
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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
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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
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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
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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)