1. Beyond-standard model physics searches in neutron and nuclear beta decay
Stefan Baeβler
Inst. Nucl. Part. Phys.

2. Table of contents
Beyond-standard model physics searches in neutron and nuclear beta decay:
Is the Cabbibo Kobayashi Maskawa (CKM) matrix unitary?
𝑑 ′ 𝑠 ′ 𝑏 ′ = 𝑉 𝑢𝑑 𝑉 𝑢𝑠 𝑉 𝑢𝑏 𝑉 𝑐𝑑 𝑉 𝑐𝑠 𝑉 𝑐𝑏 𝑉 𝑡𝑑 𝑉 𝑡𝑠 𝑉 𝑡𝑏 ∙ 𝑑 𝑠 𝑏
Various unitarity tests possible; the most precise one is the one in the first row:
𝑉 𝑢𝑑 𝑉 𝑢𝑠 𝑉 𝑢𝑏 2 =1
V-A structure of weak interaction: Scalar- and tensor (S,T) interactions, which could be mediated by non-standard intermediate bosons, causes beta decays with one of the leptons having the opposite helicity. e-
𝜎 𝑛
𝜎 𝑒 e-
𝜎 𝑒 p n
𝜎 𝜈
𝜎 𝜈
𝜈 𝑒
𝜈 𝑒

3. Established determination of 𝑽 𝒖𝒅
0 + ,1
Use beta decay. Most precise in superallowed beta decays:
(Partial) lifetime:
𝑡 1/2 , 𝐵𝑅
1 𝑓 𝐸 𝑡 = 2𝜋 ℏ <𝑓𝑒𝜈 𝐻 𝑖> 2
Integration:
1 𝑓𝑡 = 𝐺 𝐹 2 𝑉 𝑢𝑑 2 𝐾 <𝑓 𝑂 𝑖> 2
Mass difference 𝑄
0 + ,1
Can be computed with precision only in some cases,
In which the nuclear wave function overlap is well-known.
For superallowed decays, 𝑂=1 and <𝑓 𝑂 𝑖> 2 ≈2.
→ Good to a few percent, see left
3080
𝑓𝑡 (𝑠)
3060
3040 10 20 30 40
𝑍 of daughter
(from Hardy, Towner, PRC 91, (2015))

4. Established determination of 𝑽 𝒖𝒅
0 + ,1
Use beta decay. Most precise in superallowed beta decays:
(Partial) lifetime:
𝑡 1/2 , 𝐵𝑅
1 𝑓 𝐸 𝑡 = 2𝜋 ℏ <𝑓𝑒𝜈 𝐻 𝑖> 2
Integration:
1 𝑓𝑡 = 𝐺 𝐹 2 𝑉 𝑢𝑑 2 𝐾 <𝑓 𝑂 𝑖> 2
Mass difference 𝑄
0 + ,1
If there is an issue, it is believed to be the isospin-breaking correction that is nuclear structure dependent.
Can be computed with precision only in some cases,
In which the nuclear wave function overlap is well-known.
Improvement: Apply corrections:
ℱ𝑡=𝑓𝑡 1+ 𝛿 𝑅 ′ 1+ 𝛿 𝑁𝑆 − 𝛿 𝐶 10 C 22 Mg 38 Ca 46 V 14 O 26 m Al 34 Cl 38 m K 50 Mn 62 Ga 74 Rb 34
3090 Ar 42 Sc 54 Co
Energy-dependent rad. corr.
Isospin-breaking correction (to matrix element)
ℱ𝑡 (𝑠)
3080
Nucleus-dependent rad. corr.
This quantity contributes with largest uncertainty to 𝑉 𝑢𝑑
3070
Inner rad. corr.
3060 10 20 30 40
1 ℱ𝑡 = 2𝐺 𝐹 2 𝑉 𝑢𝑑 2 𝐾 1+ Δ 𝑉 𝑅
𝑍 of daughter
(from J. Hardy, I. Towner, PRC 91, (2015))

5. Determination of 𝑽 𝒖𝒔
Use Kl3 (Ke3 or Kμ3) semileptonic decays, e.g. 𝐾 𝐿 → 𝜋 + + 𝑒 − + 𝜈 𝑒 :
Γ 𝐾𝑙3 =(𝑘𝑛𝑜𝑤𝑛)⋅ 𝑉 𝑢𝑠 𝑓 + 𝐾𝜋 𝐼 𝐾 𝑙 1+ 𝛿 𝐸𝑀 𝐾𝑙 + 𝛿 𝑆𝑈 2 𝐾𝑙
Correction to phase space integral for charged Kaons
Lepton formfactor
phase space integral
Radiative correction
Current issue: 𝑓 + 𝐾𝜋 0 needs to come from theory (Lattice-QCD). Present averages are:
𝑁 𝑓
𝑓 + 𝐾𝜋 0
𝑉 𝑢𝑠
PDG16 average, based on…
2+1
0.9677(37)
𝑒𝑥𝑝+𝑅𝐶 9 𝐿𝑎𝑡
PRD87, (2013), JHEP 06, 164 (2015)
2+1+1
0.9704(24)(22)
𝑡ℎ exp
PRL112, (2014)
Use ratio of leptonic decays: 𝐾 ± → 𝜇 ± + 𝜈 𝜇 / 𝜈 𝜇 +𝛾 to 𝜋 ± → 𝜇 ± + 𝜈 𝜇 / 𝜈 𝜇 +𝛾
Γ 𝐾𝜇2 Γ 𝜋𝜇2 = 𝑉 𝑢𝑠 𝑓 + 𝐾 𝑉 𝑢𝑑 𝑓 + 𝜋 corrections
Result is 𝑉 𝑢𝑠 = 𝑒𝑥𝑝 𝑅𝐶 𝐿𝑎𝑡
Use hadronic tau decays: E.g., 𝜏→𝐾+ 𝜈 𝜏
Γ∝| 𝑉 𝑢𝑠 ​ 2 (with theory input)
Result is 𝑉 𝑢𝑠 =0.2202(15)
NB: I haven’t talked about 𝑉 𝑢𝑏 . 𝑉 𝑢𝑏 is determined from decays of B/D mesons. Its value, 𝑉 𝑢𝑏 = ⋅ 10 −3 is controversial, but small enough to be neglected here.

6. Test of CKM unitarity in the first row.
𝑉 𝑢𝑑 2 =1− 𝑉 𝑢𝑏 2 − 𝑉 𝑢𝑠 2
CKM Unitarity test ( 𝑉 𝑢𝑑 𝑉 𝑢𝑠 𝑉 𝑢𝑏 2 =1 ) was perfect for a decade, after being off for even longer (Kaon experiment has moved). Now, updates of lattice calculations of the Kaon form factors put agreement in question.

7. Observables in Neutron Beta Decay
Observables in neutron beta decay, as a function of generally possible coupling constants (assuming only Lorentz-Invariance):
Jackson et al., PR 106, 517 (1957), C. F. v.Weizsäcker, Z. f. Phys. 102,572 (1936), M. Fierz, Z. f. Phys. 105, 553 (1937) e-
𝜎 𝑛 p n
𝜈 𝑒
Fierz interference term
Beta-Asymmetry
Neutrino-Electron-Correlation
Neutron lifetime 8

8. Coupling Constants of the Weak Interaction
gA = GF·Vud·λ
𝜏 𝑛 −1 ∝ 𝑔 𝑉 2 +3 𝑔 𝐴 2
Coupling Constants in Neutron Decay
𝐴=𝐴 𝜆= 𝑔 𝐴 𝑔 𝑉
p = (udu) u e- W± νe
gV , gA
gV = GF·Vud·1 d
n = (ddu)
n → p + e- + νe
Primordial Nucleosynthesis
Solar cycle W± e- νe n p
n + νe ↔ p + e- W± e+ νe
p + p
2H+
p + p → 2H+ + e+ + νe
Start of Big Bang Nucleosynthesis, Primordial 4He abundance
Start of Solar Cycle, determines amount of Solar Neutrinos 9

9. Neutron Lifetime Measurements
Beam: Decay rate: 𝑑𝑁 𝑑𝑡 = 𝑁 𝜏 𝑛
Bottle: Neutron counts : 𝑁= 𝑁 0 𝑒 − 𝑡 𝜏𝑒𝑓𝑓
with 1 𝜏𝑒𝑓𝑓 = 1 𝜏 𝑛 𝜏 𝑤𝑎𝑙𝑙
UCN from source
UCN Storage bottle(s)
Shutter
UCN
detector
Many new experiments:
Somehow improved material bottles (e.g. Serebrov et al.)
Magnetic bottles (e.g. UCNτ, C.-Y. Liu et al., LANL; τSPECT, W. Heil, M. Beck et al., TRIGA Mainz; HOPE, O. Zimmer et al., ILL Grenoble, PENELOPE, S. Paul et al., TU München )
Beam Lifetime (only at NIST)

10. The Beta Asymmetry 𝜎 𝑛 e- p n 𝜈 𝑒 PERKEO II
𝜎 𝑛 e- p n
𝜈 𝑒
Electron Detector (Plastic Scintillator)
Decay Electrons
𝜆=−1.2755(30)
from M.P. Mendenhall et al., PRC 87, (2013)
Polarized Neutrons
Note: The PERKEO III collaboration plans to release their beta asymmetry result, with Δ𝐴 𝐴 ∼2⋅ 10 −3 , corresponding to 𝛿𝜆 𝜆 ∼7⋅ 10 −4 , any day now.
Split Pair Magnet
PERKEO II
Magnetic Field
𝜆=−1.2748(+13/−14) from D. Mund et al., PRL 110, (2013)

11. Determination of coupling constants
Determination of ratio 𝜆= 𝑔 𝐴 𝑔 𝑉 from 𝐴= −2 Re 𝜆 + 𝜆 𝜆 or 𝑎= 1− 𝜆 𝜆 2 :
Note: 𝜆 should be fixed by standard model. However, precision of its calculation from first principles is insufficient:
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
𝑁 𝑓 =2
𝑁 𝑓 =2+1
2+1+1
𝑁 f =
𝜆 = 𝑔 𝐴 / 𝑔 𝑉
Δλ/λ = 0.03%
(Nab goal)
PERKEO II (2013)
UCNA (2013)
UCNA (2010) ( )
Byrne (2002)
Average:
( )
PERKEO II (2002)
(21)
Mostovoi (2001)
Yerozolimskii (1997)
PERKEO II (1997) ( )
Liaud (1997)
PERKEO I (1986)
Stratowa (1978)
-1.28
-1.26
-1.24
𝜆=𝑔𝐴/𝑔𝑉
Most recent flavor Lattice-QCD result from PNDME: T. Bhattacharya et al., PRD 94, (2016)

12. Test of CKM unitarity in the first row.
For neutron data to be competitive with superallowed decays, one wants:
Δ 𝜏 𝑛 𝜏 𝑛 ∼0.3 s
(Neutron lifetime experiment underway were discussed earlier)
Δ𝜆 𝜆 ∼3⋅ 10 −4

13. The PERC facility @ FRM II
Electron or proton detector
Scintillator (e-),
Silicon, …
PERC facility:
Active decay volume in a 8 m long neutron guide
e,p selector is magnetic Filter: phase space, systematics
PERC can be coupled with various spectrometers
PERC Magnet delivery planned for fall 2017
Goal for beta asymmetry 𝐴:
Δ𝐴 𝐴 ≤5⋅ 10 −4 ⇔ Δ𝜆 𝜆 ≤2⋅ 10 −4
(Requires polarization control)!
D. Dubbers et al., Nucl. Instr. Meth. A 596 (2008) 238 and arXiv:
MAC-E filter (“aSPECT”)
R×B spectrometer …
G. Konrad, SMI
Cryostat
𝐵1 = 6 T
𝐵0 = 1.5 T
User
System

14. ( 𝑝 𝑝 is inferred from proton time-of-flight)
Idea of SNS n e-
𝜈 𝑒 p
Fundamental Neutron Physics Beamline Spallation Neutron Source (SNS)
𝜃 𝑒𝜈
𝑑Γ∝𝜚 𝐸 𝑒 1+𝑎 𝑝 𝑒 𝐸 𝑒 cos 𝜃 𝑒𝜈 +𝑏 𝑚 𝑒 𝐸 𝑒
8 meters
Kinematics in Infinite Nuclear Mass Approximation:
Energy Conservation:
𝐸 𝜈 = 𝐸 𝑒,𝑚𝑎𝑥 − 𝐸 𝑒
Momentum Conservation:
𝑝 𝑝 2 = 𝑝 𝑒 2 + 𝑝 𝜈 2 +2 𝑝 𝑒 𝑝 𝜈 cos 𝜃 𝑒𝜈
Goal: Determine 𝑝 𝑝 2 , 𝐸 𝑒 spectrum
Δ𝑎 𝑎 ≤ 10 −3 ⇔ Δ𝜆 𝜆 ≤3⋅ 10 −4
( 𝑝 𝑝 is inferred from proton time-of-flight)
Cold Neutron Beam from left
General Idea: J.D. Bowman, Journ. Res. NIST 110, 40 (2005)
Original configuration: D. Počanić et al., NIM A 611, 211 (2009)
Asymmetric configuration: S. Baeßler et al., J. Phys. G 41, (2014)

15. What if the test of CKM unitarity fails?
Like all precision measurements, a failure of the unitarity test would not point to a single cause:
Various possibilities exist, among those are:
Heavy quarks:
Exotic muon decays:
All direct determinations of 𝑉 𝑢𝑑 use 𝐺 𝐹 from muon lifetime. If the muon had additional decay modes (𝜇→𝑋+𝑌+…), 𝐺 𝐹 (and 𝑉 𝑢𝑑 ) would be determined wrong. E.g., 𝜇 + → 𝑒 + + 𝜈 𝑒 +𝜈 𝜇 (wrong neutrinos) would be very relevant for neutrino factories.
(Semi-)leptonic decays of nuclei through something other than exchange of 𝑊 ± bosons:
𝑑′ 𝑠′ 𝑏′ 𝐷′ = 𝑉 𝑢𝑑 𝑉 𝑢𝑠 𝑉 𝑢𝑏 𝑉 𝑢𝐷 𝑉 𝑐𝑑 𝑉 𝑐𝑠 𝑉 𝑐𝑏 𝑉 𝑐𝐷 𝑉 𝑡𝑑 𝑉 𝑡𝑠 𝑉 𝑡𝑏 𝑉 𝑡𝐷 𝑉 𝐸𝑑 𝑉 𝐸𝑠 𝑉 𝐸𝑏 𝑉 𝐸𝐷 ∙ 𝑑 𝑠 𝑏 𝐷
𝑉 𝑢𝐷 2 =1− 𝑉 𝑢𝑏 2 − 𝑉 𝑢𝑠 2 − 𝑉 𝑢𝑑 2
W. Marciano, A. Sirlin, PRL 56, 22 (1986)
P. Langacker, D. London, PRD 38, 886 (1988)
K.S. Babu and S. Pakvasa, hep-ph/
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, (2000)
Energy scale of new physics: Λ≥11 TeV V. Cirigliano et al., NPB 830, 95 (2010)

16. Search for effective scalar (S) and tensor (T) interaction
𝜎 𝑛
𝜎 𝑒 e-
𝜎 𝑒 p n
𝜎 𝜈
𝜎 𝜈
𝜈 𝑒
𝜈 𝑒
Traditional low energy effective Lagrangian:
ℒ 𝑒𝑓𝑓 =− 𝐶 𝑉 𝑒 𝛾 𝜇 𝜈 𝑒 ⋅ 𝑝 𝛾 𝜇 𝑛− 𝐶 𝑉 ′ 𝑒 𝛾 𝜇 𝛾 5 𝜈 𝑒 ⋅ 𝑝 𝛾 𝜇 𝑛
− 𝐶 𝐴 𝑒 𝛾 𝜇 𝛾 5 𝜈 𝑒 ⋅ 𝑝 𝛾 𝜇 𝛾 5 𝑛− 𝐶 𝐴 ′ 𝑒 𝛾 𝜇 𝜈 𝑒 ⋅ 𝑝 𝛾 𝜇 𝛾 5 𝑛
+ 𝐶 𝑆 𝑒 𝜈 𝑒 ⋅ 𝑝 𝑛− 𝐶 𝑆 ′ 𝑒 𝛾 5 𝜈 𝑒 ⋅ 𝑝 𝑛
+ 𝐶 𝑇 𝑒 𝜎 𝜇𝜈 𝜈 𝑒 ⋅ 𝑝 𝜎 𝜇𝜈 𝑛− 𝐶 𝑇 ′ 𝑒 𝜎 𝜇𝜈 𝛾 5 𝜈 𝑒 ⋅ 𝑝 𝜎 𝜇𝜈 𝑛
+ 𝐶 𝑃 ⋅ 𝑒 𝛾 5 𝜈 𝑒 ⋅ 𝑝 𝛾 5 𝑛− 𝐶 𝑃 ′ ⋅ 𝑒 𝜈 𝑒 ⋅ 𝑝 𝛾 5 𝑛
+ h.c.
In SM, 𝐶 𝑉 = 𝐶 𝑉 ′ =1, 𝐶 𝐴 = 𝐶 𝐴 ′ =𝜆, all others zero.
The Fierz term is unique in that it is the only term that is first-order sensitive to S,T
Decay rate ∝𝜚 𝐸 1+𝑏 𝑚 𝑒 𝐸

17. Search for effective scalar (S) and tensor (T) interaction
Low-Energy search for scalar current: Determine Fierz term 𝑏 𝐹 =−( 𝐶 𝑆 + 𝐶 𝑆 ′ )/ 𝐶 𝑉 in superallowed Fermi decays: J. Hardy, I. Towner find 𝐶 𝑆 + 𝐶 𝑆 ′ 2 𝐶 𝑉 = (90% CL)
(from J. Hardy, I. Towner, PRC 91, (2015))
Improvements possible with measurements using light nuclei
Using this, and combination of ℱ 𝑡 −1 ∝ 1+ 𝑏 𝐹 𝛾 𝑚 𝑒 𝐸 , 𝜏 𝑛 −1 ∝(1+3 𝜆 2 ) 1+ 𝑏 𝑛 𝑚 𝑒 𝐸 with
𝑏 𝑛 ∝ 𝐶 𝑆 + 𝐶 𝑆 ′ 𝐶 𝑉 +3 𝐶 𝑇 + 𝐶 𝑇 ′ 𝐶 𝐴 , and beta symmetry 𝐴= 𝐴 0 / 1+ 𝑏 𝑛 𝑚 𝑒 𝐸 (for 𝜆):
R.W. Pattie et al. get 𝐶 𝑇 + 𝐶 𝑇 ′ 2 𝐶 𝐴 =−0.0007(27) (90% CL)
from R.W. Pattie Jr., PRC 88, (2013) & PRC 92, (E) (2015)
SB 2016 update: 𝐶 𝑇 + 𝐶 𝑇 ′ 2 𝐶 𝐴 =− (90% CL) after G. Konrad, ArXiV:1007:3027
Resolution of neutron lifetime issue would give large improvement.

18. Effective field theory for beta decay at quark level
Effective low-energy Lagrangian:
ℒ 𝑒𝑓𝑓 =− 𝐺 𝐹 𝑉 𝑢𝑑 𝜖 𝐿 ⋅ 𝑒 𝛾 𝜇 1− 𝛾 5 𝜈 𝑒 ⋅ 𝑢 𝛾 𝜇 1− 𝛾 5 𝑑
+ 𝜖 𝑅 ⋅ 𝑒 𝛾 𝜇 1− 𝛾 5 𝜈 𝑒 ⋅ 𝑢 𝛾 𝜇 1+ 𝛾 5 𝑑
+ 𝜖 𝑆 ⋅ 𝑒 1− 𝛾 5 𝜈 𝑒 ⋅ 𝑢 𝑑
+ 𝜖 𝑇 ⋅ 𝑒 𝜎 𝜇𝜈 1− 𝛾 5 𝜈 𝑒 ⋅ 𝑢 𝜎 𝜇𝜈 1− 𝛾 5 𝑑
− 𝜖 𝑃 ⋅ 𝑒 𝛾 𝜇 1− 𝛾 5 𝜈 𝑒 ⋅ 𝑢 𝛾 5 𝑑
+ terms with 𝜖 that involve righthanded 𝜈 𝑒
+h.c.
Unobservable, suppressed with 𝑞/ 𝑀 𝑁
Unobservable, all corr. coeff. only sensitive in second order
Connection with traditional coupling constants from nuclear and neutron physics:
𝐶 𝑉 + 𝐶 𝑉 ′ 2 = 𝐺 𝐹 𝑉 𝑢𝑑 𝑔 𝑉 1+ 𝜖 𝐿 + 𝜖 𝑅
𝐶 𝐴 + 𝐶 𝐴 ′ 2 =− 𝐺 𝐹 𝑉 𝑢𝑑 𝑔 𝐴 1+ 𝜖 𝐿 − 𝜖 𝑅
𝐶 𝑆 + 𝐶 𝑆 ′ 2 = 𝐺 𝐹 𝑉 𝑢𝑑 𝑔 𝑆 𝜖 𝑆
𝐶 𝑇 + 𝐶 𝑇 ′ 2 = 𝐺 𝐹 𝑉 𝑢𝑑 𝑔 𝑇 𝜖 𝑇
𝑢 𝑑= 𝑔 𝑆 𝑝 𝑛
𝑢 𝜎 𝜇𝜈 𝑑= 𝑔 𝑇 𝑝 𝜎 𝜇𝜈 𝑛 …

19. Scalar(S) and tensor(T) interactions in beta decay
Other searches for Beyond Standard Model Physics: S,T interactions (fermions with “wrong” helicity), e.g. through W’ bosons; weak magnetism; second class currents; …
Low-energy experiments (mostly 0 + → 0 + , 𝜋→𝑒𝜈𝛾); 𝑔 𝑆,𝑇 from quark model
Low-energy experiments (add 𝑏 < 10 −3 in n, 6He); 𝑔 𝑆,𝑇 from quark model
Low-energy experiments,
𝑔 𝑆,𝑇 from Lattice QCD
Low-energy experiments,
𝑔 𝑆,𝑇 from Lattice QCD
LHC:
8 TeV, 20 fb-1
LHC:
14 TeV, 10,300 fb-1
LHC-Search for 𝑝𝑝→𝑒+𝜈+ other stuff and 𝑝𝑝→𝑒+𝑒+ other stuff

20. region (field maximum)
The determination of the Fierz Interference term 𝒃 in neutron decay with Nab n e-
𝜈 𝑒 p
𝜃 𝑒𝜈
𝑑Γ∝𝜚 𝐸 𝑒 1+𝑎 𝑝 𝑒 𝐸 𝑒 cos 𝜃 𝑒𝜈 +𝑏 𝑚 𝑒 𝐸 𝑒
decay volume
0 kV
- 30 kV
+1 kV
magnetic filter
region (field maximum)
Neutron
beam
TOF region
(low field)
4 m flight path skipped
1 m flight path skipped
0 V
-30 kV 2 4 6 8 E e , k i n ( V ) Y l d a r b . u t s
= +0.1 SM
Electron spectrum:
Goal: 𝑏 𝑛 ≤3⋅ 10 −3

21. The determination of the Fierz Interference term 𝒃 𝝂 in neutron decay with UCNB @ LANL
Key component
Diagram courtesy of: Albert Young
𝐵 ex𝑝 = 𝑁 −− − 𝑁 ++ 𝑁 −− + 𝑁 ++ ∝ 𝐵+ 𝑏 𝜈 𝑚 𝑒 /𝐸 1+𝑏 𝑚 𝑒 /𝐸
Goal: 𝑏 𝜈 ≤ 2..3 ⋅ 10 −3

22. 6He little-b measurement at UW
Fierz interference term 𝒃 from He-6 beta decay
6He little-b measurement at UW
M. Fertl1, A. Garcia1, M. Guigue4, P. Kammel1, A. Leredde2, P. Mueller2, R.G.H. Robertson1, G. Rybka1, G. Savard2, D. Stancil3, M. Sternberg1, H.E. Swanson1, B.A. Vandeevender4, A. Young3
1University of Washington, 2Argonne National Lab, 3North Carolina State University, 4Pacific Northwest National Laboratory
Use cyclotron radiation spectroscopy. Similar to Project 8 setup for tritium decay.
M. Fertl et al., PRL 114, (2015)
Goal: measure “little b” to 10-3 or better in 6He Statistics not a problem.
Determine e’s energy at birth

23. Thank you for your attention!
Summary and outlook
Measurement of coupling constants in weak interaction allows test of the unitarity of the CKM matrix is most precisely done in the first row; some tension (again). Substantial improvement of precision not in sight.
Measurement of Fierz term in beta decay allows the search for new physics that manifests itself at low energies as scalar and/or tensor interaction. This is competitive to searches at LHC, and experimental improvements are likely in the near future.
The latter needs form factors ( 𝑔 𝑆 and 𝑔 𝑇 ) to connect nuclear and quark-level description. Recent work in theory (Lattice QCD) determined them at the 10% level. Experimental verification desirable.
Thank you for your attention!
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