flav-io/flavio v0.25

This release brings some major changes to the way new physics Wilson coefficients are specified, including some backwards-incompatible ones and changes in dependencies. Please read on as you might have to update your code.

New Wilson coefficient exchange format interface

The Wilson coefficient exchange format (WCxf) is an initiative to define a data exchange format for new physics Wilson coefficients (see also the paper). It allows interfacing codes using different bases and even different EFTs, like the Standard Model EFT (SMEFT) vs. the weak effective theory (WET) used by flavio. The new WCxf Python package takes care of the basis translation and matching. The initial values of a flavio.WilsonCoefficient() instance can now be set, alternatively to the existing set_initial method, by passing in a wcxf.WC() instance to the new set_initial_wcxf method.

Thanks to Jason Aebischer @jasonaebischerGIT, Jacky Kumar @jackypheno, and Xuanyou Pan @pxylisa for contributions to this.

New feature: semi-leptonic decays with wrong-flavour neutrinos

In semi-leptonic charged-current decays like B→Dlν, in the SM the neutrino always has the same flavour as the charged lepton. Beyond the SM, there can be additional contributions with "wrong-flavour" neutrinos that do not interfere with the SM contribution but have the same experimental signature. Based on an implementation by Michael A. Schmidt @micha-a-schmidt, such contributions have now been consistently added in all semi-leptonic and leptonic B, D, and K decays. For the naming of the Wilson coefficients, see the next section.

Backwards-incompatible changes in Wilson coefficients

Introducing the WCxf interface and the new effects in semi-leptonic decays requried some changes in names or definitions of Wilson coefficients:

• New physics in four-quark operators (like QCD and electroweak penguins) so far has only been used in semi-leptonic FCNC decays, but the implementation was not complete. Support for four-quark operators has now been deprecated, i.e. a warning will be issued when you use it and you should not expect it to be consistent. The feature will be reimplemented in a more complete way in a future version.
• For the dipole operators, so far the "effective" coefficients like C7eff_bs had been used. These have now been removed in favour of the "plain" coefficients like C7_bs. Note that with no new physics in four-quark operators, this is actually equivalent for the new physics contribution, but you must use the new notation or an error will be raised.
• The semi-leptonic charged-current operators have been renamed and rescaled to be more consistent with the litature:
  • CV_ becomes CVL_
  • CVp_ becomes CVR_
  • CS_ becomes CSR_ and the factor of m_b in the definition of the operator has been removed (so the new Wilson coefficient is dimensionless)
  • CSp_ becomes CSL_ and the factor of m_b in the definition of the operator has been removed (so the new Wilson coefficient is dimensionless)
• The "right-flavour" semi-leptonic charged-current operators have been renamed to be distinguished from the new "wrong-flavour" ones:
  • _bcenu becomes _bcenue,
  • _bcmunu becomes _bcmunumu, etc.
  • new: _bcenumu, _bcenutau, _bcmunue, etc.

The complete list of allowed Wilson coefficients can now also be found as PDF on the WCxf web site.

Additional dependencies

Most of the functions pertaining to CKM matrix parametrizations have been moved to the separate ckmutil package. This, in addition to the wcxf package, is now an installation dependency. If you upgrade flavio using pip3 install --upgrade flavio, those two packages will be installed automatically. If you are instead using a local development install, you might have to install them manually using pip3 install wcxf cmutil.

End of support for Python 3.4

As announced in the previous release, this release does not support Python 3.4 anymore. Please read the previous announcement for upgrade options if you still use this old Python version.

Other changes

A regression introduced in v0.24 that broke the likelihood profiler in some cases was fixed.