Add BC_VPPHAD model
Adding new decay model created by @aluchins for the Bc+ -> Jpsi p pbar pi+ process. The work under the model is ongoing, soon we expect some code modifications with description of one more Bc+ decay with the protons in the final state. This MR is for the Sim09, but will certainly need also to be ported to Sim10. Probability distributions are attached. probs.pdf
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added Decays EvtGen HepForge Generators enhancement labels
Ping @jback
It looks like the form factors and amplitudes for this new
BC_VPPHADmodel are essentially the same as those already implemented for BC_VHAD with just an extra p-pbar amplitude added.To avoid duplicating code, we should add this new mode to the already existing BC_VHAD model by checking if an additional p-pbar pair is in the defined decay daughter list and add the extra vertex amplitude terms automatically.
An internal integer flag can be set (to 1) if the p-pbar state is found (otherwise it is 0), which can then be used in conditional if statements to add the extra amplitude terms and set the max probability appropriately.
The
EvtWPPHad::WCurrent_ppPifunction can be moved to EvtWHad.We would also need to see some example kinematic variable distributions (momentum, angle, probability etc.) for the new mode so that we can implement a set of test histograms for the HepForge (Sim10) version.
Dear @jbackThank you very much for your comment. I think in a long run we'll followow this approach.
However now the model for ppbar pairs in the final state is not mature enough.
We expect some modifications:- addtion of more final states with larger numeber of light mesons
- modification of amplitude itself, both in the W-current and the resonance conntent - based on our current analysis, the amplitude most likely to be corrected to achive better matching to experimental data. Here one can expect both a small tuning and large redesign, depending on the resut of comparison of the model with data.
It is not excluded that relatively large update of the model will be required for (1) and/or (2) on a itmesscale of months. Keeping this in mind we woudl prefer to keep this new model separated from the standard "mesons-only" BC_VHAD model, that are well tested and are known to provide the excellent description of data. After the detailed tests of the model against data (probably it also requires publishig of data) we can revisit the code and merge two models together. For a time being we prefere to keep the models separate.
We'll provide the distributions asap, most likely before the end of ths week.
Deal @jback
It seems to me, that it could be better to create a separate model for this decay. There are several reasons for it:
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For Bc-meson form-factor calculation, the new model uses the same function, as an old one, this function is stored in a separate file (EvtBCVFF2.cpp) and is not duplicated.
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The amplitude calculation in the case of Dirac particles in the final state (like in Bc -> psi p ~p pi) is different from the case when only scalars are present (Bc -> psi pi pi pi). You have to write two extra loops over the proton's and anti-proton's polarizations.
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The old model seems to be too overloaded and hard to be debugged. One of the benefits of creating a new model is that we can collect such decays as Bc -> psi p ~p pi, Bc -> psi p ~p +3pi, etc in it.
Best, Alexey
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