Update src/Particle.h

src/Particle.h

@@ -22,9 +22,9 @@ class Particle : public Track {
   PdgCode_t GetPid() const { return pid_; }
   Floating_t GetMass() const {return mass_;}
 
-  void SetPid(PdgCode_t pid) {
+  void SetPid(PdgCode_t pid, Floating_t mass=-1000.f) {
     pid_ = pid;
-    mass_ = GetMassByPdgId(pid);
+    mass_ = mass != -1000.f ? GetMassByPdgId(pid) : mass;
   }
 
   template<typename T>