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)