add rapidity getter

src/Constants.h

 #ifndef BASE_CONSTANTS_H
 #define BASE_CONSTANTS_H
 
+#include <map>
 #include <Rtypes.h>
 
 namespace AnalysisTree {
@@ -17,6 +18,7 @@ constexpr ShortInt_t UndefValueShort = -999;
 constexpr Integer_t UndefValueInt = -999;
 constexpr bool Unused{false};
 
+const std::map<int, float> MassMap = { {211, 0.14}, {-211, 0.14}, {2212, 0.938} };
 
 namespace Exyz{
   enum Exyz : ShortInt_t {

src/Track.h

@@ -105,6 +105,11 @@ class Track : public Container {
   Floating_t GetPhi() const  { return atan2(py_, px_); }
   Floating_t GetEta() const  { return 0.5 * log ( (GetP() + pz_) / (GetP()-pz_) ); }
   Floating_t GetP() const  { return sqrt(px_*px_ + py_*py_ + pz_*pz_); }
+  Floating_t GetRapidity(int pid) const  {
+    const float mass = MassMap.find(pid)->second;
+    const float e = sqrt( mass*mass + GetP()*GetP() );
+    return 0.5*log( (e+GetPz()) / (e-GetPz()) );
+  }
 
   void MemoryTest()
   {