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flav-io / flavio

A Python package for flavour physics phenomenology in the Standard model and beyond
http://flav-io.github.io/
MIT License
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add observables and measurement for combined (l1+ l2-, l2+ l1-) LFV BR(B->Ml1l2,l2l1) #57

Closed peterstangl closed 6 years ago

peterstangl commented 6 years ago

@DavidMStraub, this PR defines the Observable instance for the combined emu+mue LFV BR(B->pi(emu+mue)) as used in https://arxiv.org/pdf/hep-ex/0703018.pdf. To this end, I have put everything used to define the LFV M->Pll Observable instances into the function _define_obs_B_Mll and removed the currently unnecessary loop for br in ['BR',]:. I hope this fine.

Please double-check that my assumption BR(B->pi(emu+mue)) = BR(B+->pi+ emu) + BR(B+->pi+ mue) is correct.

Note that like in the last two commits in master, the test introduced in 4056db5df4387a3aea32adbe833a369714ca2604 is failing (KeyError: 'test_obs 1').

DavidMStraub commented 6 years ago

Thanks! I added a few comments.

DavidMStraub commented 6 years ago

By the way, the failing test is my fault. Will fix it soon, you can ignore it in the meantime.

peterstangl commented 6 years ago

@DavidMStraub I have added the B0 decay and implemented both the B+ and the B0 decays in such a way that it would now be very easy to also add the combined l1 l2+l2 l1 LFV decays with combinations of leptons in the final state different from e,mu as well as decays to kaons instead of pions. Would there be some use of already adding these other decays?

DavidMStraub commented 6 years ago

Good question. They are measured separately so it's not really needed, on the other hand since it's trivial to add them I guess it doesn't hurt.

DavidMStraub commented 6 years ago

You could also add the measurement from https://arxiv.org/pdf/hep-ex/0703018.pdf while you're at it :)

peterstangl commented 6 years ago

@DavidMStraub concerning the measurements, did you read my comment https://github.com/flav-io/flavio/pull/57#discussion_r210670958? Before adding them, it would be nice to understand what's actually going on there. Probably I'm just missing something.

DavidMStraub commented 6 years ago

No sorry, I had missed that comment.

The way they write it it's indeed confusing. I would understand it like this:

To quote a combined limit for the isospin modes, they assume there is no isospin asymmetry, which implies BR(B+ -> pi+ l+ l-) = 2 tau_B+/tau_B0 BR(B0 -> pi0 l+ l-) and they call this BR(B -> p l+ l-). Note that the absence of isospin asymmetry is an assumption for the non-LFV modes which is violated at some level in the SM, but its certainly a valid assumption for the LFV modes.

The table of limits now gives three numbers for each decay mode: the charged decay, the neutral decay, and the limit assuming isospin symmetry to hold. The above relation of course does not apply to these limits. Let's take a simple counterexample: if your detector cannot see pi0s at all, the limit on the B0 mode will be completely gone, and the limit on the BR assuming isospin symmetry will be given directly by the limit on the B+ mode.

By the way, I fear the factor of 2 between the neutral and charged mode is missing in flavio :(

DavidMStraub commented 6 years ago

Missing factor of 1/2 added in 0d95eec5a72c2b19a7ce0a1c29e7d4fce0580397

DavidMStraub commented 6 years ago

The failing test should now also be fixed in 1dec447aa5748bb14bdc080854760ce7fc0523c0.

peterstangl commented 6 years ago

Thanks a lot for the explanation, this makes sense!

I have added the remaining decays and rebased on master.

coveralls commented 6 years ago

Coverage Status

Coverage decreased (-0.05%) to 90.704% when pulling 25c55f5c00a762d9c6c7030fa16e214b50eab6f5 on peterstangl:B_pi_emu_mue into 1dec447aa5748bb14bdc080854760ce7fc0523c0 on flav-io:master.

peterstangl commented 6 years ago

I will also add the measurement from https://arxiv.org/pdf/hep-ex/0703018.pdf to this PR

peterstangl commented 6 years ago

@DavidMStraub I have also added the two measurements from https://arxiv.org/pdf/hep-ex/0703018.pdf. I have only included the LFV measurements since the non-LFV BR(B->pill) is not included as observable at the moment (and the PDG would provide lower limits for these channels than hep-ex/0703018).

DavidMStraub commented 6 years ago

Very good!

Yes in fact B->pimumu has been observed and measured by LHCb in the meantime. It is not yet in flavio because the hadronic contributions are really tricky due to |Vud Vus| ~ |Vtd Vts| (but they don't enter the LFV decays fortunately).