[Isac-journal-club] TODAY 3:30PM MOB theory room ... SNACKS

[journalclub]

Hello All,

WEDNESDAY 29TH 3:30 MOB THEORY ROOM
KAPOW! and goldfish will be served.

​​This week's journal club paper is "Beta spectrum of unique
first-forbidden decays as a novel test for fundamental symmetries"
(attached).​

Melissa has provided the following extra reading suggestions for context:
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

And these extensive questions to guide our reading and discussion:
--
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?
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