Flavio

This is a bug fix release correcting a bug affecting the branching ratio of leptonic B_c decays, B_c→lν.

Running masses and couplings from CRunDec

This release replaces the old implementation of running quark masses and couplings by making use of the [CRunDec package](https://arxiv.org/abs/1703.03751) by F. Herren and M. Steinhauser. This was made possible by directly wrapping the C code in a new Python package [rundec-python](https://github.com/DavidMStraub/rundec-python) imported by flavio. By providing binary wheels for Windows, Linux, and Mac OS on PyPI, the package can be easily installed without compilation. The new implementation speeds up the calculation of the running α_s and quark masses by more than two orders of magnitude!

Running Wilson coefficients from WETrunner

This release replaces the old implementation of running Wilson coefficients, based on numerical integration of the RGEs, by the "exact" leading order evolution matrices as computed in [arXiv:1704.06639](https://arxiv.org/abs/1704.06639). This speeds up the calculation of running Wilson coefficients by a factor of 5, with more potential for improvement in the future.

New dependencies

Due to the above improvements, there are a number of new dependencies: `wetrunner` and `rundec`, as well as the most recent version of `wcxf`. These will be automatically installed when you run `pip3 install --upgrade flavio`. If you are using a development installation, you can install them manually:

bash
pip3 install --upgrade rundec wetrunner wcxf

V_cb and new physics in b→clν

This release features a host of improvements facilitating state-of-the-art analyses of the CKM element V_cb in the Standard Model and of new physics in b→clν transitions. The improvements have been developed for the paper [arXiv:1801.01112](https://arxiv.org/abs/1801.01112) by Martin Jung and David Straub.

New observables

- Angular observables in B→D*lν binned in the three kinematical angles
- Longitudinal and transverse differential branching ratios in B→D*lν

New measurements

- All relevant measurements of B→Dlν and B→D*lν branching ratios (differential as well as total) and angular observables, split by lepton flavour where available

Improved theory predictions

- The CLN form factors have been completely rewritten and now properly take into account all relevant theoretical uncertainties.
- The default setting is now to consistently use CLN form factors for B→D and B→D*, with the parameters including information from a fit to lattice QCD and light-cone sum rules results. See [arXiv:1801.01112](https://arxiv.org/abs/1801.01112) for details.

Breaking changes

Due to the introduction of angular observables that are differential in angles (rather than q²), some of the plotting functions in `flavio.plots` have been renamed:

- `q2_plot_th_diff` → `diff_plot_th`
- `q2_plot_th_bin` → `bin_plot_th`
- `q2_plot_exp` → `bin_plot_exp`

`bin_plot_exp` now return a list of bins, which can be conveniently used as an input to `bin_plot_th` when comparing experiment and theory.

Other improvements

- In functions requiring numerical optimization, in particular the likelihood profiler in `flavio.statistics.fitters.profiler`, the MIGRAD algorithm provided by the `iminuit` package has been added as an additional option. By default, this algorithm is now used as fallback if the preferred algorithm SLSQP (provided by `scipy`) fails. The functionality requires the `iminuit` package to be installed. Thanks to jackypheno (Jacky Kumar) for help.

Bug fixes

- The FLHA interface to SARAH was broken due to changes in the SARAH output (41)
- There was a sign error in the renormalization group mixing between the ΔF=2 scalar and tensor operators (SLL-TLL and SRR-TRR)

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)](https://wcxf.github.io/) is an initiative to define a data exchange format for new physics Wilson coefficients (see also [the paper](https://arxiv.org/abs/1712.05298)). 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](https://github.com/wcxf/wcxf-python) 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](https://wcxf.github.io/assets/pdf/WET.flavio.pdf).

Additional dependencies

Most of the functions pertaining to CKM matrix parametrizations have been moved to the separate [ckmutil](https://github.com/DavidMStraub/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](https://github.com/flav-io/flavio/releases/tag/v0.24), 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.

This is a relatively minor release, but major changes are in the pipeline which also require some future changes in dependencies, please read on below.

New processes

- The longitudinal polarization fraction in B→K*νν, `'FL(B0->K*nunu)'`
- The branching ratio of the inclusive semi-tauonic decay `'BR(B->Xctaunu)'` as well as lepton flavour ratios like `'Rtaul(B->Xclnu)'`

Other improvements

- The default values for masses and lifetimes have been updated to PDG 2017 (from PDG 2016).

Bugs fixed

- Several typos in the QCD factorization contributions to B→Vll decays spotted by SatoshiMishima. The numerical impact is small.

Dropping support for Python 3.4

**This will be the last version of flavio working with Python 3.4.** The next version will require at least Python 3.5.
Python 3.5 was released in September 2015; the current version is 3.6. If you have a reasonably recent Linux distribution (e.g. Ubuntu 16.04 LTS or newer), you should be fine (you can check your Python version by running `python3 --version`). If you are on an outdated system and cannot update Python, have a look at [conda](https://conda.io/docs/). It allows you to install the newest version of Python in a virtual environment without root privileges. Conda is also the best choice if you are on Windows.

What's new in this release:

Parallel likelihood profiling

This release features major improvements to the likelihood profiler introduced in v0.22. Most notably, it can now make use of parallel processing, by simply passing the argument `threads=N` to the `run` method (see the [API docs](https://flav-io.github.io/apidoc/flavio/statistics/fitters/profiler.m.html) for details).

Other improvements

- The B->D and B->π form factors have been updated to use the [FLAG 2017](http://itpwiki.unibe.ch/flag/images/e/e1/FLAG_webupdate.pdf) lattice average by default
- The `best_fit` method to determine the best-fit point of a `FastFit` is now more robust (i.e., faster and less likely to fail)

Breaking changes

- The BCL parametrization of B->P form factors has been changed such that the kinematic relation f₊(0)=f₀(0) is automatically fulfilled, by fixing the highest z-expansion parameter of f₀

Bug fixes

- The Isgur-Wise relation for the B->P tensor form factor has been corrected
- Avoid Interpolation artifacts in contour plots (37), thanks to Peter Stangl (peterstangl)
- Fix to effective Wilson coefficient C₈ (38), thanks to haolinli1991