Add a Decfile for Xicc2LcPi decay, with updated Xicc lifetime

We would like to add a new DecFile for the Xicc++ -> Lc+ pi+ decay, which sets the Xicc++ lifetime to its up-to-date value, 256 fs:

• 26264055 : Xicc++_Lcpi,pKpi-res=GenXicc,DecProdCut,t=256fs,WithMinPT,3GeV.dec

We use "Xicc++_Lcpi,pKpi-res=GenXicc,DecProdCut,WithMinPT,3GeV.dec (26264052)" as the template for writting the new Decay File. The differences between the old and the new Decay Files are:

1. Add the script for ParticleValue settings. Set tau = 256 fs.
2. Change the description of the Lambda_c decay. Use the description with the Lambda(1520) resonance taken into account.

The current extra flag is 5, which is the same as that of the original Decay File (26264052). According to the LHCb Event Type Definition Rules, it seems that the correct value should be 6 because the Xicc++ is in the decay chain. We tried to change the event type to be 26264060, but we cannot generate the .xgen file successfully with "26264060". The generated option file (26264060.py) and Gauss log file (26264060.log) for the event type 26264060 can be found in the attachment. The problem might be related to the line in the option file:

Generation().Special.GenXiccProduction.Commands = ["mixevnt imix 1", "loggrade ivegasopen 0", "loggrade igrade 1", "vegasbin nvbin 300", "counter xmaxwgt 5000000", "confine pscutmin 0.0", "confine pscutmax 7.0"]

After removing it, we can generate the .xgen file with the event type 26264060.

The Decfile submitted has been tested locally. 400 events are generated, and we checked the mass and the proper-time distributions of the Xicc++ candidates. The results look nice, and they can be found in the (Xicc2LcPi_mass_tau_check.pdf).

Thanks!

Mengzhen