[PWGDQ] use correct momentum scaling when computing invariant mass#16365
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The mu+mu- invariant mass from global forward tracks in now computed by extrapolating the MFT tracks with the momentum rescaled to the MCH one (momentum scaling method), while previously it was computed by directly extrapolating the global refitted tracks.
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The mu+mu- invariant mass from global forward tracks in now computed by extrapolating the MFT tracks with the momentum rescaled to the MCH one (momentum scaling method), while previously it was computed by directly extrapolating the global refitted tracks.
The PR also fixes a small bug in the code that applies the momentum rescaling, since it was previously scaling the momentum of the global refitted track instead of the MFT standalone track.
On MC data, the new code provides a slight improvement of the mass resolution for J/psi and psi(2S) (checked with few AO2Ds from LHC25i4), as seen below (red: new code with correct momentum scaling, blue: old code with global refitted track). The reason why the global refitted track gives a slightly worse mass resolution in MC data should be investigated, but the question is independent from the changes in this PR.