Measurement of the nuclear polarization in€¦ · Contribute to independent check on the value of...

Measurement of the nuclear polarization inoptically-pumped 37K:

Progress towards a measurement of theβ-asymmetry parameter

Benjamin Fenker

Texas A&M University Cyclotron InstituteTRIUMF Neutral Atom Trap

Symmetries in Subatomic PhysicsVictoria, BC

June 8 2015

1 / 28

Acknowledgments

The TRINAT Collaboration

TRIUMF - John Behr, Alexandre Gorelov, Konstantin Olchanski,Ioana Craiciu, Claire Warner, Claire Preston

Texas A & M - Spencer Behling, Michael Mehlman,Dan Melconian, Praveen Shidling, Eames Bennett

U of Manitoba - Melissa Anholm, Gerald Gwinner

Tel Aviv - Daniel Ashery, Iuliana Cohen

TRIUMF & ISAC Target & Beam Delivery GroupFunding Agencies

USA: DOE DE-FG02-93ER40773 & Early Career ER41747

Canada: NSERC, NRC through TRIUMF, WestGrid

Israel: Israel Science Foundation

2 / 28

Outline

Motivation - Testing the SM with nuclearphysics

TRINAT - TRIUMF’s Neutral Atom Trap

Polarization through optical pumping

Systematics in the polarizationmeasurement

Outlook and future plansshakeoff

Ion MCP

uniform E fieldB Coil Electrodes for

beta

electron MCP

3 / 28

Motivation: Fundamental Symmetries

Search for possible right-handed currents SU(2)L ⊗U(1)Y

?→ SU(2)R ⊗SU(2)L ⊗U(1)Y

Contribute to independent check on the value of Vud

Energy dependence tests recoil-order corrections, weakmagnetism, second-class currents

Angular correlations in β-decay are sensitive to new physics 10−3 precision constrains SM extensions, while 10−4 has

discovery potentiald5W

dEdΩedΩν∼ 1+aβν

pepν cos(θeν)

EeEν+b

me

Ee+

P

(

Aβpe

Eecos(θe)+Bν

pνEν

cos(θν)

)

+ . . .

θe

~pe ~pν

θνdud

duu

We

ν4 / 28

Overview

Magneto-Optical Trap (MOT) Provides a cold (∼ 1mK), localized (∼Ø1mm) source of atoms Shallow trap so products emerge unperturbed

Polarization Axis

Trap Light

OP Light

Nuclear DetectorsBC 408

Plastic Scintillator

Si−strip detector

Mirror

MCP

~E

5 / 28

Overview

Magneto-Optical Trap (MOT) Optical Pumping Polarizes the Atoms

σ± lasers drive biased random walk towards Pnucl =±1

Polarization Axis

Trap Light

OP Light



Si−strip detector

Mirror

MCP

~E

5 / 28

Overview

Magneto-Optical Trap (MOT)

Optical Pumping Polarizes the Atoms Nuclear Detectors

β-telescopes measure position, energy along polarization axis

Polarization Axis

Trap Light

OP Light



Si−strip detector

Mirror

MCP

~E

5 / 28

Overview

Magneto-Optical Trap (MOT)

Optical Pumping Polarizes the Atoms Nuclear Detectors

β-telescopes measure position, energy along polarization axis

BC 408Plastic Scintillator

Si−strip detector

Mirror

MCP

~E

5 / 28

β-detection

Scintillators record full energy; backgrounds from untrappedatoms, annihilation

Shake-off electron MCP tags events that decay from the trap Silicon ∆E detectors suppress background from γs Collected statistics for 0.2% measurement of Aobs

Energy [keV]0 1000 2000 3000 4000 5000 6000

Cou

nts

1

10

210

310

410

510 All Events-

Shakeoff eSi-strip detectorTriple coincidence

BC 408Plastic Scintillator

Si−strip detector

MCP

~E

6 / 28

Optical Pumping

Stretched state has F = 2, MF = 2 or equivalently Iz =32 , Jz =

12

Zeeman sublevels feel Bz = 2G along quantization axis Stretched state corresponds to atomic and nuclear polarization Photoionization is a monitor of excited state population Use this to monitor trap size, position, temperature, polarization

Ene

rgy

355 nm light

F = 1

F = 2

F = 1

F = 2S1/2

P1/2

P3/2

Angular MomentumFz

Fz =−2 Fz =−1 Fz = 0 Fz = 1 Fz = 2

E−field

MC

P

K+

Note:~F =~I +~J

7 / 28

Optical Pumping


12


Ene

rgy

Optical PumpingLight

Repump

F = 1

F = 2

F = 1

F = 2S1/2

P1/2

P3/2

Angular MomentumFz

Fz =−2 Fz =−1 Fz = 0 Fz = 1 Fz = 2

∆F z

=+

1

∆Fz =±1,0

E−field

MC

P

K+

Note:~F =~I +~J

7 / 28

Optical Pumping


12


Ene

rgy

355 nm light

F = 1

F = 2

F = 1

F = 2S1/2

P1/2

P3/2

Angular MomentumFz

Fz =−2 Fz =−1 Fz = 0 Fz = 1 Fz = 2

E−field

MC

P

K+

Note:~F =~I +~J

7 / 28

Photoions monitor trap parameters

Polarized measurements must be done with MOT off

With MOT off, cloud expands; alternate counting/trappingP

hoto

ion

Eve

nts

1

10

210

310

410

BQuench MOT Collects, Cools

Optical Pumping

Time since MOT off / microseconds0 500 1000 1500 2000 2500 3000 3500 4000

z po

s / m

m∆

-10

-5

0

-5

8 / 28

Photoionization signal

With time-of-flight and position cuts, this signal is very clean

∆z position / mm

-20 -15 -10 -5 0 5 100

50

100

150

200

250

300

350

400

450

MOT / 1000.0FWHM = 3.2 mm

Optical PumpingFWHM = 2.3 mm

Photoion time-of-flight [ns]

1220 1260 1300 1340 1380 14200

50

100

150

200

250

300

MOT / 1000.0FWHM = 19 ns

Optical PumpingFWHM = 16 ns

9 / 28

Polarization Signal

This strong signal allows clean measurement of polarization

Initial peak proportional to number of atoms, laser power,provides normalization

Tail region provides information about the degree of polarization

Time [µs]-200 0 200 400 600 800 1000 1200 1400

Polarized

Pho

toio

nE

vent

s

0

10

20

30

40

50

60

70

Directly measurenon-stretchedpopulation, but

Polarization dependson how this smallpopulation isdistributed amongstsublevels

Small tail measuresdeviation from unity

10 / 28

Polarization Signal

This strong signal allows clean measurement of polarization

Initial peak proportional to number of atoms, laser power,provides normalization

Tail region provides information about the degree of polarization

Time [µs]-200 0 200 400 600 800 1000 1200 1400

Polarized

Pho

toio

nE

vent

s

0

10

20

30

40

50

60

70

Directly measurenon-stretchedpopulation, but

Polarization dependson how this smallpopulation isdistributed amongstsublevels

Small tail measuresdeviation from unity

10 / 28

Polarization Model

H = H0︸︷︷︸

Coulomb

+ Hso︸︷︷︸

Spin-Orbit

+ Hhf︸︷︷︸

Hyperfine

+ HB︸︷︷︸

Magnetic Field︸︷︷︸

Atomic Hamiltonian

−e~d ·~E(t)︸︷︷︸

Laser Term

HSO =~L ·~SHhf =~I · (~L+~S)~E(t) = E0 cos(kz −ωLt)εq

HB =−~µ·~B=−gF µB~B · (~I +~L+~S)

〈P〉= Tr(ρP) = Tr(ρIz) 〈T 〉 ∼ Tr(ρI2z )

Density Matrix:ρii - population of state iρij - correlation between i , j

Time evolution:d ρdt = 1

i~ [H (t), ρ]+R(t)

Tremblay, P. and Jacques C. PRA 41(9), 4989 (1990)Renzoni, F. et al. PRA 63(6), 065401 (2001)

Systematics in the polarization measurement

Photoionization signal is an indirect measure of the polarization

Light ellipticity and a transverse magnetic field affect thephotoionization curve similarly but result in different polarization

-Steady state P1/2 population

0 0.02 0.04 0.06 0.08 0.1 0.12×10−3

Nuc

lear

Pol

ariz

atio

n

0.994

0.995

0.996

0.997

0.998

0.999

1 Bx = 0.0, Vary s3

s3 = 1.0, Vary Bx

s3 = 1.0

s3 = 0.993Bx = 0.0G

Bx = 0.15 G

Off-line studies:Bx ≤ 66mG

Stokesparameter:

〈s3〉=I+− I−I++ I−

≥+0.9893

≤−0.9983

CPT “dark” statesare minimized

11 / 28

Depolarizing mechanisms - Stokes Parameter s3

s3 characterizes the degree of circular polarization

s0 is equivalent to the total power contained in the beams3

s0=

I+− I−I++ I−

If |s3|/s0 < 1.0, atoms can be pumped out of the stretched state

Ene

rgy

F = 1

F = 2

F = 1

F = 2S1/2

P1/2

P3/2

Angular MomentumFz

Fz =−2 Fz =−1 Fz = 0 Fz = 1 Fz = 2

Equilibrium is reached withnot all atoms in the fullystretched state

12 / 28

Depolarizing mechanisms - Stokes Parameter s3

s3 characterizes the degree of circular polarization

s0 is equivalent to the total power contained in the beams3

s0=

I+− I−I++ I−

If |s3|/s0 < 1.0, atoms can be pumped out of the stretched state

Ene

rgy

F = 1

F = 2

F = 1

F = 2S1/2

P1/2

P3/2

Angular MomentumFz

Fz =−2 Fz =−1 Fz = 0 Fz = 1 Fz = 2

Equilibrium is reached withnot all atoms in the fullystretched state

12 / 28

UHV Viewport

SiC Mirror

s3 =−0.9950

s3 =−0.9980

s′3 =−0.9959 s′3 =−0.9938

Warneret al. RSI 85, 113106 (2014).

Depolarizing mechanisms - Transverse magnetic field

Magnetic field perpendicular to polarization axis causesprecession

Atoms in the stretched state precess toother ground states

~B = Bx x +Bz zH~B =−~µ·~BHBx = gF µBBxFx = gF µBBx

F++F−2

13 / 28

Depolarizing mechanisms - Transverse magnetic field

Magnetic field perpendicular to polarization axis causesprecession

Atoms in the stretched state precess toother ground states

~B = Bx x +Bz zH~B =−~µ·~BHBx = gF µBBxFx = gF µBBx

F++F−2

13 / 28

Trim coils minimize transverse magnetic fieldScan current and minimize optical pumping tail to find Iideal

Compare Iideal and Iactual , find Bx = 33mG.

Conservatively assign 100% uncertainty → Bx ≤ 66 mG

Preliminary Results

Depolarizing mechanisms are almost 100% correlated

Perform separate fits with either s3 or B⊥ fixed

Cou

nts

/mic

rose

cond

0

1

2

3

4

5

6

7

8

σ− −2log(L)comb/ν = 757/1168

Time / microsecond200 400 600 800 1000 1200 1400 1600 1800

Polarized

Polarized

Cou

nts

/mic

rose

cond

0

2

4

6

8

10

12 σ+

Cou

nts

/mic

rose

cond

0

1

2

3

4

5

6

7

8

σ− −2log(L)comb/ν = 771/1169

Time / microsecond200 400 600 800 1000 1200 1400 1600 1800

Polarized

Polarized

Cou

nts

/mic

rose

cond

0

2

4

6

8

10

12 σ+

Bx = 66mG

I(σ−) = 2.2(3)Wm−2

s3(σ−) =−0.9967(9)

P =−0.994(1)stat

I(σ+) = 2.1(2)Wm−2

s3(σ+) = +0.9915(16)

P =+0.990(2)stat

Bx = 4(36)mG

I(σ−) = 2.0(2)Wm−2

s3(σ−) =−0.9938

P =−0.9916(4)stat

I(σ+) = 2.0(2)Wm−2

s3(σ+) = +0.9893

P =+0.9890(3)stat

14 / 28

Preliminary Results - Global Fit

Vary delay-time after AC-MOTFit with common s3 and Bx

σ− σ+

Cou

nts

/ 2.0

mic

rose

cond

s

0

0.51

1.52

2.5

3

3.54

E = 395 V/cm8.4 hours−2logL/d.o.f.=5617/8028

Cou

nts

/ 1.0

mic

rose

cond

s

0

1

2

3

4

5

6

E = 535 V/cm15.2 hours

Cou

nts

/ 10.

0 m

icro

seco

nds

0

1

2

3

4

5

6

7


Cou

nts

/ 1.0

mic

rose

cond

s

0

1

2

3

4

5

67

8


Time [microseconds]200 400 600 800 1000 1200 1400 1600 1800

Cou

nts

/ 4.0

mic

rose

cond

s

0

1

2

3

4

5E = 415 V/cm

1.6 hours

0

0.51

1.52

2.5

3

3.54

0

1

2

3

4

5

6

0

1

2

3

4

5

6

7

0

1

2

3

4

5

67

8

Time [microseconds]200 400 600 800 1000 1200 1400 1600 1800

0

1

2

3

4

5

15 / 28

Preliminary Results

Uncertainties / 10−4

σ+ σ−

Polarization Alignment Polarization AlignmentDepolarizing Mechanism 8 15 4 5

Global Fit vs. Average 1 3 1 2Fit ∆ vs. Fit I 3 6 3 6

Uncertainty in Bz 1 3 1 1Binning 2 3 2 5

Initial Alignment (T0 =−1) 18 42 15 36Hyperfine Pumping 1 2 1 3Sum Systematics 20 45 16 37

Statistics 6 16 8 19Uncertainty 22 48 18 42

Central Value 0.9898 -0.9761 -0.9920 -0.9808

P = 〈MI〉I = 0.991(2) T =

I(I+1)−3〈M2I 〉

I(2I−1) =−0.978(5)

16 / 28

Conclusions

Nuclear polarization gives access to more β-decay observables Optical pumping achieves high polarization in an open geometry

→ P = 0.991±0.002 Will not dominate the uncertainty for present data set We expect

dAβAβ

≤ 0.5% (ASMβ =−0.5706(7))

Future plans include modeling our MOT to reduce uncertaintyfrom initial sublevel distribution

Polarization measurement at 10−4 precision requires moresystematic studies

Uncertainty / 10−4

2012 2014

Asymmetry (Stat.) 62 20Polarization (Stat.) 60 7Polarization (Systematics) 56 18Detector Response 64Asymmetric Number of Trapped Atoms 25Trap Movement 18Timing Errors 9Mirror Thickness 2

S. Behling “Measurement Of The Standard Model Beta Asymmetry Parameter, Aβ, in 37K” Ph. D Thesis. Texas A & M University, 2015. 17 / 28

THANK YOU

Backup slides . . .18 / 28

Acknowledgments

The TRINAT Collaboration

TRIUMF - John Behr, Alexandre Gorelov, Konstantin Olchanski,Ioana Craiciu, Claire Warner, Claire Preston

Texas A & M - Spencer Behling, Michael Mehlman,Dan Melconian, Praveen Shidling, Eames Bennett

U of Manitoba - Melissa Anholm, Gerald Gwinner

Tel Aviv - Daniel Ashery, Iuliana Cohen

TRIUMF & ISAC Target & Beam Delivery GroupFunding Agencies

USA: DOE DE-FG02-93ER40773 & Early Career ER41747

Canada: NSERC, NRC through TRIUMF, WestGrid

Israel: Israel Science Foundation

19 / 28

Comparison with Geant4

Cou

nts

0

200

400

600

800

1000Data

Geant4 SimulationPRELIMINARY

χ2/ν = 1.6

Energy [keV]1000 2000 3000 4000 5000

y dat

a−

y mc

√y m

c

-4

-2

0

2

4

20 / 28

Preliminary Results


σ+ σ−






Central Value 0.9898 -0.9761 -0.9920 -0.9808

P = 〈MI〉I = 0.991(2) T =

I(I+1)−3〈M2I 〉

I(2I−1) =−0.978(5)

21 / 28

Preliminary Results


σ+ σ−






Central Value 0.9898 -0.9761 -0.9920 -0.9808

P = 〈MI〉I = 0.991(2) T =

I(I+1)−3〈M2I 〉

I(2I−1) =−0.978(5)

21 / 28

Future work: Initial sublevel distribution?

Has little effect on equilibrium state but can affect theshape of the initial peak

Polarization is limited by “unpolarized” β-asymmetry

Alignment (T ) unconstrained (for now)

Initi

al P

opul

atio

n F

ract

ion

0

0.1

0.2

0.3

0.4

0.5

T = 0.0

Initi

al P

opul

atio

n F

ract

ion

0

0.1

0.2

0.3

0.4

0.5

T = -0.34

|1, -1> |1, 0> |1, 1> |2, -2> |2, -1> |2, 0> |2, 1> |2, 2>

Initi

al P

opul

atio

n F

ract

ion

0

0.1

0.2

0.3

0.4

0.5

T = -1.0

Preliminary Results


σ+ σ−






Central Value 0.9898 -0.9761 -0.9920 -0.9808

P = 〈MI〉I = 0.991(2) T =

I(I+1)−3〈M2I 〉

I(2I−1) =−0.978(5)

21 / 28





0 20 40 60 80 100 120 140 160 180 2000

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045 =00T=-0.340T=-10T

ES Population

Preliminary Results


σ+ σ−






Central Value 0.9898 -0.9761 -0.9920 -0.9808

P = 〈MI〉I = 0.991(2) T =

I(I+1)−3〈M2I 〉

I(2I−1) =−0.978(5)

21 / 28





Model MOT dynamics to limit initial alignment

Work in progress

Optics Layout

30 mW, 770 nm Laser

AOM for ON/OFF

1/2-wave plate

Polarizing beam-splitter



Polarization Maintaining Fiber

Deflects poorly polarized light

ON OFF

High quality polarizer

High quality polarizer LCVR - 1/2-wave plate or zero

High quality1/4-wave plate

High quality1/4-wave plate

UHV Viewport w/PCTFE Gasket

UHV Viewport w/PCTFE Gasket

SiC Mirror

SiC Mirror

TRAP

Control Computer

Controls Optical Pumping ON/OFF

LCVR - 1/2-wave plate or zero

Controls Polarization State

22 / 28

Why 37K?

Atomic structure allows for laser-trapping AND optical pumping

Isobaric analogue decay simplifies nuclear structure corrections

Strong branch to ground state is a very clean decay

Iπ = 32+ → 3

2+

is a mixed Fermi-Gamow Teller decay

∆t1/2 = 0.08%(Shidling et al. 2014)∆BR = 0.14%∆QEC = 0.003%

3/2+

3/2+

5/2+

MIRROR

MIRROR

β+

3718Ar19

3719K18

2.07% QEC = 3.4 MeV

97.89% QEC = 6.1 MeV

23 / 28

Why 37K?




Iπ = 32+ → 3

2+



∆F t = 0.18%∆ρ = 0.4%

3/2+

3/2+

5/2+

MIRROR

MIRROR

β+

3718Ar19

3719K18

2.07% QEC = 3.4 MeV

97.89% QEC = 6.1 MeV

23 / 28

Why 37K?




Iπ = 32+ → 3

2+



∆F t = 0.18%∆ρ = 0.4%

Aβ(0) =−0.5706(7)→∆Aβ = 0.12%

3/2+

3/2+

5/2+

MIRROR

MIRROR

β+

3718Ar19

3719K18

2.07% QEC = 3.4 MeV

97.89% QEC = 6.1 MeV

23 / 28

Photoionization Events

BC408 Plastic Scintillator

Silicon Strip Detector

rMCPeMCP

Nuclear polarization

355 nm light ~E

~B

Photoelectrone−

K+ Monitor of trapposition, size,temperature

Ultra-clean measure ofnuclear polarization P

24 / 28

Measure Vud with mirror nuclei

Nuclearcorrection

RadiativecorrectionExperimental

unce

rtain

ty

0+−→ 0+ neutron

mirrorpionnuclei

Vud

25 / 28

TRINAT’s x2-MOT System

Collection trap is coupled to TRIUMF-ISAC beam line

Transfer atoms to second trap for precision measurement

MCP

MCP

Push

beam 14"

Collection chamber Detection chamber

Ion

Neutralizer

beam

Electrostatichoops

26 / 28

Initial Polarization

Atoms can be polarized in a MOT if beams are unbalanced. Weavoid this

Use β-asymmetry of trapped (unpolarized) atoms to constraininitial polarization

S. Behling “Measurement Of The Standard Model Beta Asymmetry Parameter, Aβ, in 37K” Ph. D Thesis. Texas A & M University, 2015.

27 / 28

Correlation measurements with polarized nuclei

LTNO

Brute-forcealignment ofnuclear spin

P calculatedknowing thetemperature

Backscatteringfrom sourceholder

Stern-Gerlach

Physicallyseparatepolarized atoms

Very highpolarization, butinefficient

Optical Pumping

MI

γ

State selection;very high P

Open geometryminimizesbackscattering

Must measurepolarization

28 / 28

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