Corrected mass range in Pythia. Fixes LHCBGAUSS-1628

Gen/LbPythia8/src/Lib/Pythia8Production.cpp

@@ -443,8 +443,13 @@ void Pythia8Production::updateParticleProperties(const LHCb::ParticleProperty*
   double lifetime = thePP->lifetime()*Gaudi::Units::c_light;
   if (lifetime >= 1.e16 * Gaudi::Units::mm) lifetime = 0;
   double width = lifetime == 0 ? 0 : Gaudi::Units::hbarc / lifetime;
-  pd.mMin(id, (thePP->mass() - (thePP->maxWidth() ? thePP->maxWidth() : 
-                15*width)) / Gaudi::Units::GeV); 
+  double min_mass = (thePP->mass() - (thePP->maxWidth() ? thePP->maxWidth() : 
+                15*width)) / Gaudi::Units::GeV;
+  // Ensure that minimal mass is not negative
+  if(min_mass < 0){
+    min_mass = 0;
+  }
+  pd.mMin(id, min_mass); 
   pd.mMax(id, (thePP->mass() + 15*width) / Gaudi::Units::GeV);
   pd.mWidth(id, width / Gaudi::Units::GeV);
   pd.tau0(id, lifetime / Gaudi::Units::mm);