observables with the Nab...

Stefan Baeβler

The measurement of neutron beta decay

observables with the Nab spectrometer

1 Inst. Nucl. Part. Phys.

The neutrino electron correlation coefficient 𝒂

𝑑Γ ∝ 1 + 𝑎𝑝𝑒𝐸𝑒

cos 𝜃𝑒𝜈 + 𝑏𝑚𝑒

𝐸𝑒

𝜈𝑒

p 𝜃𝑒𝜈

Novel approach to determine cos 𝜃𝑒𝜐:

Kinematics in Infinite Nuclear Mass Approximation:

1. Energy Conservation: 𝐸𝜐 = 𝐸𝑒,𝑚𝑎𝑥 − 𝐸𝑒,𝑘𝑖𝑛

2. Momentum Conservation:

𝑝𝑝2 = 𝑝𝑒

2 + 𝑝𝜐2 + 2𝑝𝑒𝑝𝜐 cos 𝜃𝑒𝜐

𝐸𝑒

𝜈𝑒

p 𝜃𝑒𝜈

𝐸𝑒,𝑘𝑖𝑛 = 450 keV

1 + 𝑎𝑝𝑒𝐸𝑒

cos 𝜃𝑒𝜈 𝑝𝑝2

pp2 [MeV2/c2]

0.0 0.5 1.0 1.5

𝐸𝑒

𝜈𝑒

p 𝜃𝑒𝜈

pp2 [MeV2/c2]

0.0 0.5 1.0 1.5

J.D. Bowman, Journ. Res. NIST 110, 40 (2005)

𝐸𝑒

𝜈𝑒

p 𝜃𝑒𝜈

Properties of 𝑝𝑝2 distribution for fixed 𝐸𝑒:

Edges 𝑝𝑝2

𝑚𝑖𝑛,𝑚𝑎𝑥= 𝑝𝑒 ± 𝑝𝜐

Slope ∝ 1 + 𝑎𝑝𝑒

𝐸𝑒cos 𝜃𝑒𝜈 𝑝𝑝

cos 𝜃𝑒𝜈 𝑝𝑝2 = +1

cos 𝜃𝑒𝜈 𝑝𝑝2 = −1

Electron spectrum:

0 2 0 0 4 0 0 6 0 0 8 0 0

𝐸𝑒,𝑘𝑖𝑛 (keV)

b = +0.1

The Fierz Interference Term 𝒃

𝑑Γ ∝ 𝜚 𝐸𝑒 1 + 𝑎𝑝𝑒𝐸𝑒

𝐸𝑒

𝜈𝑒

p 𝜃𝑒𝜈

n = (ddu)

p = (udu)

νe gV , gA

Coupling Constants in Neutron Decay

𝑛 → 𝑝 + 𝑒− + 𝜈 𝑒

Coupling Constants of the Weak Interaction

gV = GF·Vud·1

gA = GF·Vud·λ

n = (ddu)

p = (udu)

νe gV , gA

𝑛 → 𝑝 + 𝑒− + 𝜈 𝑒

gV = GF·Vud·1

gA = GF·Vud·λ 𝜏𝑛−1 ∝ 𝑔𝑉

2 + 3𝑔𝐴2

n = (ddu)

p = (udu)

νe gV , gA

𝑛 → 𝑝 + 𝑒− + 𝜈 𝑒

gV = GF·Vud·1

2 + 3𝑔𝐴2

𝜆 =𝑔𝐴𝑔𝑉

n = (ddu)

p = (udu)

νe gV , gA

n + νe ↔ p + e-

Primordial Nucleosynthesis Solar cycle

p + p → 2H+ + e+ + νe

𝑛 → 𝑝 + 𝑒− + 𝜈 𝑒

Start of Big Bang Nucleosynthesis,

Primordial 4He abundance Start of Solar Cycle, determines amount of

Solar Neutrinos

gV = GF·Vud·1

2 + 3𝑔𝐴2

𝜆 =𝑔𝐴𝑔𝑉

Neutron Lifetime Measurements

Beam: Decay rate: 𝑑𝑁

𝑑𝑡=

𝜏𝑛

UCN from source

Storage

bottle(s)

Shutter UCN

detector

𝑑𝑡=

𝜏𝑛

Bottle: Neutron counts : 𝑁 = 𝑁0𝑒−

𝜏𝑒𝑓𝑓

with 1

𝜏𝑒𝑓𝑓

𝜏𝑛+

𝜏𝑤𝑎𝑙𝑙

1985 1990 1995 2000 2005 2010 2015 2020

Experiment publication

material bottle not used beam

UCN from source

Storage

bottle(s)

Shutter UCN

detector

𝑑𝑡=

𝜏𝑛

𝜏𝑒𝑓𝑓

with 1

𝜏𝑒𝑓𝑓

𝜏𝑛+

1985 1990 1995 2000 2005 2010 2015 2020

material bottle not used beam

UCN from source

Storage

bottle(s)

Shutter UCN

detector

𝑑𝑡=

𝜏𝑛

𝜏𝑒𝑓𝑓

with 1

𝜏𝑒𝑓𝑓

𝜏𝑛+

1985 1990 1995 2000 2005 2010 2015 2020

material bottle not used magnetic bottle beamUCNtau

Motivation for Nab

Determination of ratio 𝜆 = 𝑔𝐴 𝑔𝑉 from

𝐴 = −2 Re 𝜆 + 𝜆 2 1 + 3 𝜆 2 or

𝑎 = 1 − 𝜆 2 1 + 3 𝜆 2 :

−1.30 −1.28 −1.26 −1.24

UCNA (2010) ( )

PERKEO II (1997) ( )

Stratowa (1978)

PERKEO I (1986)

Liaud (1997)

( ) PERKEO II (2002)

PERKEO II (2013)

UCNA (2013) ( )

Mostovoi (2001)

Yerozolimskii (1997)

Byrne (2002)

My average:

𝜆 = −1.2756(11)

Δ𝜆 𝜆 = 0.03%

(Nab goal)

𝜆 = 𝑔𝐴/𝑔𝑉

aCORN (2017)

UCNA (2017)

Motivation for Nab

Most recent 2+1+1 flavor Lattice-QCD result from

PNDME: T. Bhattacharya et al., PRD 94, 054508 (2016)

Note: 𝜆 should be fixed by standard model.

However, precision of its calculation from

first principles is insufficiently precise:

PNDME’16

LHPC’12

LHPC’10

RBC/UKQCD’08

Lin/Orginos’07

RQCD’14

QCDSF/UKQCD’13

ETMC’15

CLS’12

RBC’08

1.00 1.25 1.50 1.75

𝑁𝑓=2

𝑁𝑓=2+1

𝑁f=

2+1+1 𝜆 = 𝑔𝐴/𝑔𝑉

𝐴 = −2 Re 𝜆 + 𝜆 2 1 + 3 𝜆 2 or

𝑎 = 1 − 𝜆 2 1 + 3 𝜆 2 :

−1.30 −1.28 −1.26 −1.24

UCNA (2010) ( )

Stratowa (1978)

PERKEO I (1986)

Liaud (1997)

PERKEO II (2013)

UCNA (2013) ( )

Mostovoi (2001)

Byrne (2002)

My average:

𝜆 = −1.2756(11)

Δ𝜆 𝜆 = 0.03%

(Nab goal)

aCORN (2017)

UCNA (2017)

Motivation for Nab

Most recent 2+1+1 flavor Lattice-QCD result from

PNDME: T. Bhattacharya et al., PRD 94, 054508 (2016)

Note: 𝜆 should be fixed by standard model.

However, precision of its calculation from

first principles is insufficiently precise:

PNDME’16

LHPC’12

LHPC’10

RBC/UKQCD’08

Lin/Orginos’07

RQCD’14

QCDSF/UKQCD’13

ETMC’15

CLS’12

RBC’08

1.00 1.25 1.50 1.75

𝑁𝑓=2

𝑁𝑓=2+1

𝑁f=

2+1+1 𝜆 = 𝑔𝐴/𝑔𝑉

𝐴 = −2 Re 𝜆 + 𝜆 2 1 + 3 𝜆 2 or

𝑎 = 1 − 𝜆 2 1 + 3 𝜆 2 :

−1.30 −1.28 −1.26 −1.24

UCNA (2010) ( )

Stratowa (1978)

PERKEO I (1986)

Liaud (1997)

PERKEO II (2013)

UCNA (2013) ( )

Mostovoi (2001)

Byrne (2002)

My average:

𝜆 = −1.2756(11)

Δ𝜆 𝜆 = 0.03%

(Nab goal)

aCORN (2017)

UCNA (2017)

2. Goal: Test of unitarity of Cabibbo-

Kobayashi-Maskawa (CKM) matrix from

𝑉𝑢𝑑2𝜏𝑛 1 + 3𝜆2 = 4908.7 19 s and

𝑉𝑢𝑑2 + 𝑉𝑢𝑠

2 + 𝑉𝑢𝑏2 = 1

For neutron data to be competitive, want:

Δ𝜏𝑛 𝜏𝑛 ~0.3 s Δ𝜆 𝜆 ~0.03%

0+→0+

neutron (λ, τStorage)

neutron (λ, τBeam)

Kl3 (Nf=2+1+1)

tau →hadrons

mirror nuclei

What if the test of CKM unitarity fails?

𝑑′𝑠′𝑏′𝐷′

𝑉𝑢𝑑 𝑉𝑢𝑠 𝑉𝑢𝑏 𝑉𝑢𝐷𝑉𝑐𝑑 𝑉𝑐𝑠 𝑉𝑐𝑏 𝑉𝑐𝐷𝑉𝑡𝑑 𝑉𝑡𝑠 𝑉𝑡𝑏 𝑉𝑡𝐷𝑉𝐸𝑑 𝑉𝐸𝑠 𝑉𝐸𝑏 𝑉𝐸𝐷

𝑑𝑠𝑏𝐷

Like all precision measurements, a failure of the unitarity test would not point to a single cause: Various possibilities exist, among those are: 1. Heavy quarks:

2. Exotic muon decays: All determinations of 𝑉𝑢𝑑 use 𝐺𝐹 from muon lifetime. If the muon had additional decay modes (𝜇 → 𝑋 + 𝑌 +⋯), 𝐺𝐹 (and 𝑉𝑢𝑑) would be determined wrong. E.g., 𝜇+ → 𝑒+ + 𝜈 𝑒 +𝜈𝜇 (wrong

neutrinos) would be very relevant for neutrino factories.

3. (Semi-)leptonic decays of nuclei through something other than exchange of 𝑊± bosons:

𝑉𝑢𝐷2 = 1 − 𝑉𝑢𝑏

2 − 𝑉𝑢𝑠2 − 𝑉𝑢𝑑

W. Marciano, A. Sirlin, PRL 56, 22 (1986) P. Langacker, D. London, PRD 38, 886 (1988)

K.S. Babu and S. Pakvasa, hep-ph/0204236

Specific models: E.g. W. Marciano, A. Sirlin, PRD 35, 1672 (1987). R. Barbieri et al., PLB 156, 348 (1985) K. Hagiwara et al., PRL 75, 3605 (1995) A. Kurylov, M. Ramsey-Musolf, PRL 88, 071804 (2000) 12

Energy scale of new physics: Λ ≥ 11 TeV V. Cirigliano et al., NPB 830, 95 (2010)

Other searches for Beyond Standard Model Physics: S,T interactions (fermions with “wrong”

helicity), e.g. through W’ bosons; weak magnetism; second class currents; …

LHC-Search for 𝑝𝑝 → 𝑒 + 𝜈 + other stuff and 𝑝𝑝 → 𝑒 + 𝑒 + other stuff

Scalar(S) and tensor(T) interactions in beta decay

8 TeV, 20 fb-1

Low-energy experiments (mostly 0+ → 0+,

𝜋 → 𝑒𝜈𝛾); 𝑔𝑆,𝑇 from quark model

Low-energy experiments,

𝑔𝑆,𝑇 from Lattice QCD

Low-energy experiments (add 𝑏 < 10−3

in n, 6He); 𝑔𝑆,𝑇 from quark model

Low-energy experiments,

𝑔𝑆,𝑇 from Lattice QCD

14 TeV, 10,300 fb-1

Nab spectrometer @ FNPB @ SNS

Fundamental Neutron Physics Beamline (FNPB)

@ Spallation Neutron Source (SNS)

Cold Neutron Beam

decay volume

0-1 kV

- 30 kV

magnetic filter

region (field

maximum)

Neutron

TOF region

(low field)

4 m flight path skipped

Segmented

Si detector Ø 81 mm

decay volume

0-1 kV

- 30 kV

magnetic filter

region (field

maximum)

Neutron

TOF region

(low field)

Nab spectrometer operation

Segmented

Si detector Ø 81 mm

decay volume

0-1 kV

- 30 kV

magnetic filter

region (field

maximum)

Neutron

TOF region

(low field)

Original configuration: D. Počanić, S. Baeßler, D. Bowman, V. Cianciolo, G. Greene, S. Penttilä et al., NIM A 611, 211 (2009)

Asymmetric configuration: S. Baeßler, D. Počanić, D. Bowman, S. Penttilä et al., arXiv:1209.4663

Nab spectrometer operation

• Measurement of 𝐸𝑒,𝑘𝑖𝑛 and tp for each event; protons

only in upper detector

→ 1 tp2 gives an estimate for 𝑝𝑝

2, magnetic field

shape gives a narrow detector response function

• Long TOF region improves proton TOF resolution

• Two detector geometry allows to suppress electron

backscattering

Nab spectrometer principle: measurement of Ee and tp

decay volume

0-1 kV

- 30 kV

magnetic filter

region (field

maximum)

Neutron

TOF region

(low field)

2, magnetic field

backscattering

Proton Trajectory

Magnetic Field

conver

𝑝∥

𝑝⊥

𝑝∥

𝑝⊥

Electron energy measurement with backscattering

suppression

detected Ee [keV]

0 50 100 150 200 250 300

detected Ee for e- in lower detector

detected Ee with only lower detector

Detector response for

incoming Ee = 300 keV

2, magnetic field

backscattering

decay volume

0-1 kV

- 30 kV

magnetic filter

region (field

maximum)

Neutron

TOF region

(low field)

Nab data analysis

pp2 [MeV2/c2]

0.0 0.5 1.0 1.5

Full GEANT4 spectrometer simulation:

Nab data analysis

𝑡𝑝 =𝑚𝑝 ∙ spectrometer length

𝑝𝑝−component along 𝐵

pp2 [MeV2/c2]

0.0 0.5 1.0 1.5

0 0.002 0.004 0.006

Inverse squared proton TOF 1 𝑡𝑝2 [1/μs2]

Data analysis: Use edge to determine or verify the spectrometer TOF response function.

Then, use central part to determine slope and

correlation coefficient a.

Full GEANT4 spectrometer simulation:

Nab data analysis

𝑡𝑝 =𝑚𝑝 ∙ spectrometer length

𝑝𝑝−component along 𝐵

pp2 [MeV2/c2]

0.0 0.5 1.0 1.5

0 0.002 0.004 0.006

Inverse squared proton TOF 1 𝑡𝑝2 [1/μs2]

Statistical uncertainty budget for 𝒂 coefficient

Planned statistical uncertainty budget:

3.6×108 events can be detected in 6 weeks (Decay volume V = 246 cm3, 12.7% of decay

protons go to upper detector, 50% efficiency in use of beam time, 1600 decays/s),

corresponding to Δ𝑎

𝑎 𝑠𝑡𝑎𝑡∼ 2.4 ⋅ 10−3 for this period.

→ 𝚫𝒂

𝒂 𝒔𝒕𝒂𝒕∼ 𝟕 ⋅ 𝟏𝟎−𝟒 can be reached, but it requires 70 weeks of data taking.

lower 𝑬𝐞,𝒌𝒊𝒏 cutoff none 100 keV 100 keV 100 keV

upper 𝒕𝒑 cutoff none none 40 μs 30 μs

𝚫𝐚 (𝑵, 𝒂, 𝒃 variable) 2.4/√N 2.5/√N 2.7/√N 3.0/√N

𝚫𝐚 (𝑵, 𝒂, 𝒃, 𝑬𝒄𝒂𝒍, 𝑳 variable) 2.6/√N 2.7/√N 2.9/√N 3.2/√N

𝚫𝐚 (𝑵, 𝒂, 𝒃, 𝑬𝒄𝒂𝒍, 𝑳 variable, inner 75% of data) 3.4/√N 3.5/√N 3.8/√N 4.3/√N

As above, 10% bg 4.2/√N 4.4/√N 4.6/√N 5.0/√N

Compare to Δ𝑎 𝑎 = 4 % of best existing experimental results (ACORN 2017),

and ∼ 1 % planned for ACORN and aSPECT

Experimental parameter Main specification Systematic

uncertainty Δa/a

Magnetic field

... curvature at pinch Δ𝛾/𝛾 = 2% with 𝛾 = 𝑑2 𝐵𝑧(𝑧)/𝑑𝑧2/𝐵𝑧(0) 5.3·10-4

… ratio rB = BTOF/B0 (Δ𝑟𝐵)/𝑟𝐵 = 1% 2.2·10-4

… ratio rB,DV = BDV/B0 (Δ𝑟𝐵,𝐷𝑉)/𝑟𝐵,𝐷𝑉 = 1% 1.8·10-4

Length of the TOF region none

Electrical potential inhomogeneity:

… in decay volume / filter region 𝑈𝐹 − 𝑈𝐷𝑉 < 10 mV 5·10-4

… in TOF region 𝑈𝐹 − 𝑈𝑇𝑂𝐹 < 200 mV 2.2·10-4

Neutron Beam:

… position Δ𝑧𝐷𝑉 < 2 mm 1.7·10-4

… profile (including edge effect) Slope at edges < 10%/cm 2.5·10-4

… Doppler effect small

… Unwanted beam polarization 𝑃𝑛 ≪ 10−4 can be small

Adiabaticity of proton motion 1·10-4

Detector effects:

… Electron energy calibration Δ𝐸𝑒,𝑘𝑖𝑛 < 0.2 keV 2·10-4

… Shape of electron energy response fraction of events in tail to 1% 5.7·10-4

… Proton trigger efficiency 𝜖𝑝 < 100 ppm/keV 3.4·10-4

… TOF shift due to detector/electronics Δ𝑡𝑝 < 0.3 ns 3·10-4

Residual gas 𝑝 < 2 ⋅ 10−9 torr 3.8·10-4

Background / Accidental coincidences small

Sum 1.2·10-3

Systematic uncertainty budget for 𝒂 coefficient

Goal: 𝚫𝐛 ≤ 𝟑 ⋅ 𝟏𝟎−𝟑

Systematic uncertainties:

1. Electron energy determination

2. Most stringent requirement: Non-linearity of 0.01%

3. Background

2% of events in tail

(deadlayer, bremsstrahlung) Y

i e l d

detected Ee [keV]

0 50 100 150 200 250 300

Detector response to decay

electron with 𝐸𝑒 = 300 keV

decay volume

- 30 kV

magnetic filter

region (field

maximum)

Neutron

TOF region

(low field)

-30 kV

Electron spectrum:

0 2 0 0 4 0 0 6 0 0 8 0 0

b = +0.1

The measurement of the Fierz Interference Term 𝒃

𝐸𝑒

𝜈𝑒

p 𝜃𝑒𝜈

Goal: 𝚫𝐛 ≤ 𝟑 ⋅ 𝟏𝟎−𝟑

2. Most stringent requirement: Non-linearity of 0.01%

3. Background

i e l d

detected Ee [keV]

0 50 100 150 200 250 300

Electron spectrum:

0 2 0 0 4 0 0 6 0 0 8 0 0

b = +0.1

The measurement of the Fierz Interference Term 𝒃

𝐸𝑒

𝜈𝑒

p 𝜃𝑒𝜈

Status of Nab experiment

Fundamental Neutron Physics Beamline

@ Spallation Neutron Source

Magnet system tested successfully at manufacturer, is

expected to arrive at ORNL this Friday.

Magnet system tested successfully at manufacturer, is

expected to arrive at ORNL this Friday.

𝐵 measured 𝐵 calculated

-200 -100 0 100 300 400 500 600

𝑧/cm 200

Nab (and UCNB) detector properties

• DAQ system (A. Sprow, C. Crawford et al.): All

waveforms recorded. Risetime ∼ 40 ns, fall time

∼ 4 µs.

• Energy resolution a few keV, threshold ≤ 10 keV

• Noise low enough to detect protons (average

deposited energy: 18 keV)

• Bad pixels need investigation

Ø 81 mm

Back side

LabVIEW controller C++ coincidence logic

LabVIEW FPGA low-threshold trapezoid trigger on ADC Optical fiber bus, clock & sync 23

∼ 4 µs.

LANL test run with Ce-139 source, Analysis H. Li

Pulse height [ADC reading]

500 1000 1500 2000 2500 3000 3500

γ: 33 keV

Ce-139 source

β: 27 keV

β: 127 keV

β: 160 keV

Ø 81 mm

Back side

∼ 4 µs.

LANL test run with Ce-139 source, Analysis H. Li

Pulse height [ADC reading]

500 1000 1500 2000 2500 3000 3500

γ: 33 keV

Ce-139 source

β: 27 keV

β: 127 keV

β: 160 keV

Ø 81 mm

Back side

Electron, ~ 100 keV, followed by

Proton, ~18 keV, in neighboring pixel

Main electrode system: Simulation of the effect of openings

Symmetry axis Electric potential [V]

Assumption for simulation: Potential of bore tube different by 1 V from electrodes

Top and bottom part of electrode system, before coating:

Parts are in hand, and are awaiting cleaning and coating.

Main electrode system: Fabrication

Symmetry axis

Electrode surface coating: What is the issue?

Most undergraduate textbooks: No electric field in empty hole inside conductor…

...and I have no objection if the conductor is homogenous. However, if not:

E field

Material 1 Material 2

EF,2 Φ1 Φ2

EF,1 EF,2

E field

EF,2 Φ1 Φ2

EF,1 EF,2

E field

Consequence: Need to establish that electric field is known, or known to be absent, in flight path.

0 50 100 150 200 250 300 350 0

Angle [°]

Work Function [meV]

Work function measurements

In collaboration with Prof. I. Baikie, KP Technologies 27

Main electrode system

We previously identified two coating methods that provide good homogeneity of the work function, and are compatible with UHV conditions. A copper/silver spray (which was good always when the coating looked good eye) and a graphite spray (which was mostly good when the coating looked good by eye). We decided to go with copper/silver.

But: Manufacturer changed recipe:

OLD AND GOOD NEW

Present status of testing: • New spray seems to be equally good in terms of work function

uniformity (𝜎 < 10 meV) • Vacuum test so far inconclusive • Cryogenic testing not yet done Next step: Align main electrode substrates, do coating, characterize

WF / meV

0.0 0.5 1.0 1.5 2.0 2.5 0.0

X [cm]

a Department of Physics, Arizona State University, Tempe, AZ 85287-1504 b Department of Physics, University of Virginia, Charlottesville, VA 22904-

4714 c Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 d Department of Physics and Astronomy, University of Sussex, Brighton

BN19RH, UK e Department of Chemistry and Physics, University of Tennessee at

Chattanooga, Chattanooga, TN 37403 f Department of Physics and Astronomy, University of Kentucky,

Lexington, KY 40506 g Department of Physics, University of Manitoba, Winnipeg, Manitoba, R3T

2N2, Canada

h KIT, Universität Karlsruhe (TH), Kaiserstraße 12, 76131 Karlsruhe,

Germany i Department of Physics and Astronomy, University of Tennessee,

Knoxville, TN 37996 j Department of Physics and Astronomy, University of South Carolina,

Columbia, SC 29208 k Los Alamos National Laboratory, Los Alamos, NM 87545 l Department of Physics, University of Winnipeg, Winnipeg, Manitoba

R3B2E9, Canada m Department of Physics, North Carolina State University, Raleigh, NC

27695-8202 n Universidad Nacional Autónoma de México, México, D.F. 04510, México o University of Michigan, Ann Arbor, MI 48109

The Nab collaboration

R. Alarcona, S.B.b,c (Project Manager), S. Balascutaa, L. Barrón Palosn, K. Bassi, N. Birgei, A. Blosef, D.

Borissenkob, J.D. Bowmanc (Co-Spokesperson), L. Broussardc, A.T. Bryantb, J. Byrned, J.R. Calarcoc,i, T.

Chuppo, T.V. Ciancioloc, J.N. Clementb, C. Crawfordf, W. Fanb, W. Farrarb, N. Fomini, E. Frležb, J. Fryb,

M.T. Gerickeg, M. Gervaisf, F. Glückh, G.L. Greenec,i, R.K. Grzywaczi, V. Gudkovj, J. Hamblene, C.

Hayesm, C. Hendruso, T. Itok, H. Lib, C.C. Lub, M. Makelak, R. Mammeig, J. Martinl, M. Martineza, D.G.

Matthewsf, P. McGaugheyk, C.D. McLaughlinb, P. Muellerc, D. van Pettenb, S.I. Penttiläc (On-site

Manager), D. Počanićc (Co-Spokesperson), G. Randalla, N. Roaneb, C.A. Roysem, K.P. Rykaczewskic, A.

Salas-Baccib, E.M. Scotti, S.K. Sjuek, A. Smithb, E. Smithk, A. Sprowf, E. Stevensb, J. Wexlerm, R.

Whiteheadi, W.S. Wilburnk, A.Youngm, B.Zeckm

Main project funding:

Active and recent collaborators:

Goal: 𝚫𝐛 < 𝟑 ⋅ 𝟏𝟎−𝟑

2. Background

i e l d

detected Ee [keV]

0 50 100 150 200 250 300

decay volume

- 30 kV

magnetic filter

region (field

maximum)

Neutron

TOF region

(low field)

-30 kV

Electron spectrum:

0 2 0 0 4 0 0 6 0 0 8 0 0

E e , k i n ( k e V )

b = +0.1

The determination of the Fierz Interference term 𝒃

𝐸𝑒

𝜈𝑒

p 𝜃𝑒𝜈

Goal: 𝚫𝐛 < 𝟑 ⋅ 𝟏𝟎−𝟑

2. Background

i e l d

detected Ee [keV]

0 50 100 150 200 250 300

Electron spectrum:

0 2 0 0 4 0 0 6 0 0 8 0 0

E e , k i n ( k e V )

b = +0.1

The determination of the Fierz Interference term 𝒃

𝐸𝑒

𝜈𝑒

p 𝜃𝑒𝜈

• Main uncertainties in previous best experiment (PERKEO II): statistics, detector, background, polarization

• Superior detector energy resolution, good enough time resolution

• Keep coincidences to improve background

• Statistics @ SNS or NIST is an issue for 𝐴

• Polarization measurement seems manageable (XSM or He-3)

Segmented

Si detector

magnetic filter

region (field

maximum)

TOF region

(field r B · B 0 )

Polarizer:

Supermirror

or Helium-3

Flipper

• Beta asymmetry (to extract 𝐴) :

reflect all protons to bottom detector,

use top detector for electrons

• Proton asymmetry (to extract 𝐵):

detect protons at top

Planned continuation (SNS or NIST): Polarized neutrons

Polarimetry with

Helium-3

• The Nab collaboration is setting up the Nab spectrometer at the Spallation Neutron Source. Start of commissioning planned in summer 2018. Until tomorrow, the big reason for the delay has been the magnet.

• Goal: Δa/a ≤ 10-3 and Δb ≤ 3·10-3. This is in line with the requirements for standard model tests:

1) Compare renormalization constant 𝜆 = 𝑔𝐴/𝑔𝑉 to direct determination on lattice.

2) Unitarity test of Cabbibo-Kobayashi-Maskawa matrix in first row.

3) Search for new physics at above the TeV scale that manifests itself as S,T interaction.

Summary

33 Thank you for the attention!

observables with the Nab...

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Transcript of observables with the Nab...

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