[Isac-journal-club] Journal Club Paper and Discussion

[Journal Club]

Hi everyone,

Next week's journal club will be featuring the paper "Beta spectrum of
unique first-forbidden decays as a novel test for fundamental
symmetries" (attached).

If you'd like some additional reading material to put this paper into
context, I have a couple recommendations.
For background on beta decay kinematics including exotic couplings in
the allowed limit:
https://journals-aps-org.ezproxy.library.ubc.ca/pr/abstract/10.1103/PhysRev.106.517
For present limits on tensor
couplings:http://journals.aps.org.ezproxy.library.ubc.ca/prc/abstract/10.1103/PhysRevC.89.025501

I've also generated a rather large set of questions for you all to
think about while reading the paper (below).


--
Reading/Discussion Questions:
* What is the difference between allowed, superallowed, and forbidden
transitions?
* Under our present understanding of the standard model, which
couplings (eg vector, axial-vector, scalar, pseudoscalar, tensor, ...)
are involved in each of these transition types?
* What is a unique first-forbidden transition?  What property makes it
"unique"?

* Eq. (9) characterizes the probability distribution for the kinematic
behavior of an unpolarized Gamow-Teller decay in terms of the
parameters "a_betanu" and "b".  (A Fermi decay can also be
characterized by the same dependence on these two parameters, though
the numerical values of the parameters depend on different physical
couplings)  Regarding "a_betanu" and "b":
  - * What would physically change about the decay process's
kinematics if you change their numerical values?
  - * What quantities would you want to measure in a decay in order to
extract numerical values for them?

* Eq. (11) characterizes the probability distribution for the
kinematic behavior of an unpolarized unique first-forbidden decay.
Regarding Eq. (11):
  - * Which term in this equation describes a new set of dependencies
that weren't present in the allowed limit?
  - * What effect does this "extra" term have on decay kinematics?
ie, given standard model physics (no tensor couplings), what behavior
is different in a unique first-forbidden transition than in the
allowed limit?
  - * What quantities would one need to measure in order to ascertain
that a particular decay behaves according to Eq. (11), rather than Eq.
(9)?
  - * What quantities would one need to measure in order to extract
numerical values for possible tensor couplings?
* Why do the authors believe that current experiments "cannot fit
separately both a_betanu and the Fierz term"?
* All else being equal, is it really easier to observe/constrain the
presence of tensor currents by observing a decay that behaves
according to Eq. (11), rather than according to Eq. (9)?  In
particular:
  - * Is it easier if you only look at the beta energy spectrum?
  - * Is it easier if you have access to other information about the
decay kinematics?

* When an expression is accurate "up to recoil-order corrections,"
what approximation has been made?
* Recoil-order and relativistic/radiative corrections have small,
annoying-to-calculate effects on the spectra involved in a decay.  In
extracting BSM physics from an interaction such as the one described
here, how much do we need to worry about these corrections?
* What is the difference (both physically and mathematically) between
'right-handed' and 'left-handed' tensor couplings?
* Why are the limits for right-handed tensor interactions so much less
constraining than those for left-handed tensor interactions?

* Is it really necessary to introduce a 20 keV detector resolution
effect into the simulation?
* Are the limits extracted from these proof-of-concept simulations
competetive with the true limits that have been measured
experimentally?
* Is the authors' suggestion that someone should really do an
experiment like this on unique first-forbidden decays a good idea
overall?  Are there any downsides?

Bonus:
* The authors neglect any dependence on nuclear spin-polarization in
this discussion.  Under what conditions is this acceptable?  Is it
ever okay for a spin-1 parent nucleus?
--


Enjoy!

- Melissa
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