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

[Melissa Anholm]

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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