Hi everyone,

I have a question that maybe is stupid, but I’m a beginner, so have pity!

I have some data: for each event I have four particles, and for each particle I have their values of pt, eta, phi and m.

For each particles I collected that values in a TLorentzVector (for example i call it “elec1”) in such ways that:

elec1[0]=pt

elec1[1]=eta

elec1[2]=phi

elec1[3]=m

*That’s the question:*

*What can I do if I want to calculate the invariant mass of two of these four-vectors (for example elec1 and elec2)?*

If I try using something like this:

```
TLorentzVector sum;
float m;
sum=elec1+elec2;
m=sum.M();
```

I obtain something that haven’t any physic meaning.

*That’s probably because I have to use four-momentum of each particle instead of four vector like elec. But I can’t found a way to trasform a vector in a system of variables like elec in the corresponding four momentum of the same particle, thus in the system EPxPyPz.*

Can you help me?

Thank you!