[Halld-physics] JEF comments

[Michael Williams]

Hi,

I read your JEF proposal and I had a few comments.  It is a very nice proposal.  


B boson:

* This is a very good idea as it is complementary to the many searches for charged final states.   Is it worth saying something to the effect that the limits for X->ee, mumu will continue to improve in the coming years since there are many LHC searches (via rare B-meson decays, etc) and also purpose-built experiments planned, but to my knowledge there aren't any planned precision measurements of the neutral decay modes in the near future.  Are there?  If I'm right then it could be worth stressing more that this experiment falls into a niche that is important and woefully unexplored or even planned to be explored.   Also, if an experiment did find X -> ee then you'd be in a great position to search for that X -> pi0 gamma (you'd know the mass) and such measurements would be required to reveal the nature of this new X.   Seeing X -> ee doesn't mean you've seen a dark photon so more measurements would be needed and yours would be a good one.  

* You reference one paper for the B -> pi0 gamma decay and it's a good paper but perhaps you can stress that you are not doing a model-dependent search.   You may need to set model-dependent limits, but if you see a narrow spike on the SM background then you've got a new particle and it may or may not be due to the physics you discuss.   I.e., the importance of this search goes beyond the specific dark matter, U(1)', motivations mentioned.  

* It seems odd that there is no mention of eta -> gamma A' (dark photon).  I suspect that we are not competitive but given that we can do charged tracking and you're building a great calorimeter the reader should be thinking what about A'->ee?  A brief mention of this would be welcome.  


Quark mass ratio:

I was curious here if you could exclude m_u = 0 with your measurement.   I don't think that is above 5sigma yet (I could be wrong).  One solution to the strong CP problem is m_u = 0 which is strongly disfavored by experiment but if it's not 5 sigma yet, and you could push it over 5 sigma, that would be interesting since it would rule out the massless quark solution.


CPV in eta Dalitz plots:

I wrote a paper a few years ago on a very powerful model-independent way to search for CPV in multivariate spaces:

 M. Williams, Observing CP violation in many-body decays, Phys.Rev.D 84, 054015 (2011). [arXiv:1105.5338]

For a single eta decay my method would work well.   The difference w.r.t. the example I used in the paper is that the two samples would be the two halves of a single Dalitz plot rather than two Dalitz plots from the decay of a particle and its anti-particle.   I could investigate what kind of limits one could set using my method in this context if there's some theory input of where in the model to insert the CPV. 


Very minor:

* You should define JEF in the text.   As of now I think it's only defined in the title. 


Congrats on a very nice proposal.

Cheers,

Mike