How to calculate the invariant mass of a pair of particles

We will use the TLorentzVector class from ROOT, which has useful functions for converting coordinates, adding together four-momenta, and calculating the invariant mass.



In [1]:

    
from ROOT import TLorentzVector









    














    














    



Welcome to ROOTaaS 6.06/06

Some arbitrary numbers for illustration



In [2]:

    
pt1 = 25.0
eta1 = 1.0
phi1 = 0.25
energy1 = 50.0

pt2 = 29.0
eta2 = 1.5
phi2 = -0.35
energy2 = 62.0

Put these numbers into two TLorentzVector objects representing the four-momenta of the two particles.



In [3]:

    
p1 = TLorentzVector()
p1.SetPtEtaPhiE(pt1, eta1, phi1, energy1)

p2 = TLorentzVector()
p2.SetPtEtaPhiE(pt2, eta2, phi2, energy2)

Add together the four-momenta of the two particles, and calculate the invariant mass of the combined system:



In [4]:

    
total = p1 + p2
m = total.M()
print(m)









    



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