Rapidity etc.

Say we have a track-jet consisting of two tracks, both at eta = 2. One has phi = 0.4 the other phi = -0.4
These tracks hit the detector elements at eta=2.
Now we build a jet as usual, a few lines of ROOT code:
root [0] TLorentzVector p1, p2, j;
root [1] p1.SetPtEtaPhiM(10,2,-0.4,0)
root [2] p2.SetPtEtaPhiM(10,2,+0.4,0)
root [3] j = p1 + p2;
root [4] j.Eta()
(double) 2.0794904
root [5] j.Rapidity()
(double) 2.0000000
On line 3 we build the jet as usual as the 4-vector sum of its constituents.
Note that the jet eta is plain wrong!
It claims the jet is at eta = 2.08. This is biased away from the correct geometrical value which is 2.0 (where the detector hits are, and all jet constituents).
It is hence not ideal to cut on jet |eta|. We should make our acceptance selection based on |y|.
The issue here is that we use 4-vector addition to build the jet, and then we use eta which is not Lorentz invariant (differences in y are invariant under boost along z-axis, certainly not for eta). One crude fix would be not to use 4-vector addition to calculate eta. For topoclusters, a weighted sum is used. A better fix is to use "proper physics" (4 vector addition) but move away from quantities that are not Lorentz invariant (eta and ET).
Also note that eta vs y is a quite small effect. The example above is pretty extreme.


  • arXiv:1801.09721 (hep-ph) Pileup mitigation at the LHC: a theorist's view (252 pages, 101 figures. Report originally presented as a habilitation thesis)
  • arXiv:1810.07988 (hep-ph) Pileup mitigation at the Large Hadron Collider with Graph Neural Networks.

Min-bias & underline events

Machine Learning


  • arXiv:1903.05825 (hep-ph) Timing information at HL-LHC: Complete determination of masses of Dark Matter and Long lived particle.


  • Slac Summer Institute 2016 ”New Horizons on the Energy Frontier”, focusing on the Run 2 LHC, its upgrades and possible future e+e- and hadron colliders.

CERN Yellow Report

ATLAS 25-year book

Physics Beyond Colliders study

-- KenjiHamano - 2016-09-21

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