A project to investigate possibly interesting ttbar studies in heavy-ion collisions.

• establish if Pythia8 provides decay times

  - Liliana: Pythia8 does not provide decay/production times/distances.
        Check in LilianasCode/results/pt0_En5TeV.root the decay/prod_time/position histograms (all entries are set to zero)
  

• understand the question of transverse v. longitudinal boost. Which is more relevant? What about the fact that the medium is moving v. top/W velocities (which are not speed of light)?

  - Liliana: To be entirely correct, we could separate the 3 velocity components and apply the boost indepedently (?). The results that are shown so far have a minimum value (taken with only transverse boost) and a maximum value (energy boost). The correct value will be inside this interval. As for the medium, a Bjorken expansion model is usually considered to describe the hydrodynamical evolution. This is the most simple model. As such, in a first approximation the average quantities depend only on the proper time (altough, if one takens into account flucuations, then a dependency with rapidity have to be introduced) 
        A very brief summary of these models can be found here: https://www.physi.uni-heidelberg.de/~fschney/2008SS-Preseminar/Model.pdf
  

• implement a decoherence time that uses either

  cos theta = (1-2m^2/E^2)

  or

  cos theta = (1-2m^2/(pt^2+m^2))

  or even the actual theta between the W decay products (requires a bit more work).

  • Liliana: Done! (with actual theta between the W decay products)

• establish how well we can correlate kinematics with decay+decoherence time. This can be done just as a function of top and W pt. E.g. inclusive, pt>200, pt>300, pt>500. The question is going to be one of compromise between getting some spread in lifetimes and still having enough events for a measurement to be possible. It'll be fairly borderline for LHC, and probably OK for FCC.

  • Liliana: Check in LilianasCode/Images the Comparison files for LHC and FCC. It is shown the results for decay distance and decay distance + coherence time with different cuts for the minimum pt of the W boson. The upper plots represent the results using a transverse boost and the lower plots using a transverse + longitudinal boost. By increasing the pt cut, we are able to probe latter stages of the medium, altough the statistics for the LHC case, when pt > 500 GeV, is very low: from a total of 1Million events, only 400 survive... see LilianasCode/Images/Decay_ptW500_En100TeV.eps file Imposing a cut in the top will reduce even more this statistics.

• then move towards actual schematic analysis? And try to see how robust it is with background subtraction.