Resolve "Add helicity probability"

CHANGELOG.rst

 Unreleased changes
 ------------------
+* Add field for right handed helicity probability for outgoing lepton in GiBUUOutput
 
 v1.6.1
 ----------------------------

km3buu/output.py

@@ -464,6 +464,30 @@ class GiBUUOutput:
         y = pq / pk
         return y
 
+    @staticmethod
+    def right_helicity_probability(roottuple_data):
+        """
+        Calculate the probability for the outgoing lepton from the neutrino vertex
+        to have a right handed helicity
+
+        Definition: p = ( 1-|p|/(E+m) )/2
+
+        Parameters
+        ----------
+            roottuple_data: awkward.highlevel.Array
+        """
+        sec_lepton_pdgid = np.sign(
+            roottuple_data.process_ID) * (7 + 2 * roottuple_data.flavor_ID +
+                                          np.abs(roottuple_data.process_ID))
+        sec_lepton_mass = np.array(
+            [Particle.from_pdgid(p).mass * 1e-3 for p in sec_lepton_pdgid])
+        total_lepton_P = np.sqrt(roottuple_data.lepOut_Px**2 +
+                                 roottuple_data.lepOut_Py**2 +
+                                 roottuple_data.lepOut_Pz**2)
+        rh_probability = 0.5 * (1 - total_lepton_P /
+                                (roottuple_data.lepOut_E + sec_lepton_mass))
+        return rh_probability
+
     @property
     def A(self):
         grp = self.jobcard["target"]
@@ -539,6 +563,7 @@ class GiBUUOutput:
         visEfrac = visible_energy_fraction(ak.flatten(retval.E),
                                            ak.flatten(retval.barcode))
         retval["visEfrac"] = ak.unflatten(visEfrac, counts)
+        retval["rh_prob"] = GiBUUOutput.right_helicity_probability(retval)
         return retval
 
     @property

km3buu/tests/test_output.py

@@ -106,6 +106,11 @@ class TestGiBUUOutput(unittest.TestCase):
         mask = self.output.free_particle_mask
         np.testing.assert_array_equal(mask[0], [True, False, True, True, True])
 
+    def test_right_handed_helicity(self):
+        arr = self.output.arrays
+        np.testing.assert_array_almost_equal(
+            arr.rh_prob[:3], [4.518065e-05, 9.428467e-06, 3.488926e-05])
+
 
 @pytest.mark.skipif(not KM3NET_LIB_AVAILABLE,
                     reason="KM3NeT dataformat required")