Boosting by sum

[beojan]

EDIT: For anyone trying to do this themselves, a boost to the rest frame should actually be p4.boost(-p4.sum().boostp3).

I have a JaggedArray of TLorentzVectors. I want to boost the vectors in each row to the rest frame of that row.

I'm using p4 = p4.boost(p4.sum().boostp3) (well the equivalent, since hh = p4.sum()), but I get the error:

Traceback (most recent call last):
  File "./train.py", line 44, in <module>
    p4 = p4.boost(hh.boostp3)
  File "/usr/lib/python3.7/site-packages/uproot_methods/classes/TLorentzVector.py", line 236, in boost
    v = self.p3 + gamma2*bp*p3 + gamma*p3*self.t
  File "/usr/lib/python3.7/site-packages/numpy/lib/mixins.py", line 25, in func
    return ufunc(self, other)
  File "/usr/lib/python3.7/site-packages/awkward/array/jagged.py", line 924, in __array_ufunc__
    result = getattr(ufunc, method)(*inputs, **kwargs)
  File "/usr/lib/python3.7/site-packages/uproot_methods/classes/TLorentzVector.py", line 301, in __array_ufunc__
    raise TypeError("(arrays of) TLorentzVector can only be added to/subtracted from other (arrays of) TLorentzVector")
TypeError: (arrays of) TLorentzVector can only be added to/subtracted from other (arrays of) TLorentzVector

[jpivarski]

This is actually an issue in uproot-methods, since it deals with physics calculations. I've opened a PR on that repo: scikit-hep/uproot3-methods#43, but I'm not entirely sure if this fixes the problem because I don't have good test cases. (When I make up values of 4-vectors, I get NaN because they weren't realistic 4-vectors that are within light cones.)

Could you give that a test? Thanks!

[beojan]

You could use setptetaphim to generate four vectors

[jpivarski]

I used TLorentzVectorArray.from_ptetaphim, but I didn't think too hard about the numbers I put in it.

If I don't hear from you today about the correctness of the calculation, I'll take that pull request and leave everything else the same.

(The pull request itself fixes an issue unrelated to the calculation of boost: it's that TLorentzVectors, projected to TVector3s, incorrectly kept their identities as TLorentzVectors and then there was an attempt to add some TVector3s and some supposed TLorentzVectors, which would be wrong if that's what they actually were.)