========================================
October 9th 2017 - Drell-Yan xsection
========================================

First meeting with this paper. Nikola chairs.
We got up to Section 5 in this session.

------------
Topics:
    - What is the Drell-Yan process?
	- Why is the Z not just produced at rest? There are 3 mechanisms to consider.
	- What is resummation? 
	- In what way does ISR/FSR differ from Bremmstrahlung?
	- The definitions of Bare/Born/Dressed leptons
	
-----------
Details:
    - The cross-section of the Drell-Yan process was the significant variable here. There was some discussion as to 
	  why this measurment was being made, for the most part to tune generators.
	- Whilst the acop-phi was clear, there was some discussion as to the nature of teh theta-eta-star variable. 
	  It seemed clear that: tanh( eta ) = pz/|p|, where p here is the 3-momentum, but we decided to think more about
	  what the significance of that might be. Laura suggested that they might have just tried to find a kinematic 
	  variable, that didn't rely too heavily on pT resolution, that was in a range of 0-1. 
	- If Z is not produced at rest from the hard scatter (leading to back-to-back leptons), then there may have been some ISR. 
	  Either that or the Z was produced from quarks with small, but non-negligable pT. 
	- Bremmstrahlung requires a medium to occur within, this is because energy and momentum must be conserved. e.g. if the
	  process was taken in the centre-of-mass frame of the electron , it would appear to just produce a photon from rest, violating 
	  kinematic conservation laws. Whereas, if it scatters off of an atom, the atom can absorb the minute recoil. In ISR/FSR
	  no medium is required. ISR/FSR can be a gluon or photon. No medium is required because in this instance kinematic
	  conservation laws do not need to be satisfied, so before the ISR/FSR occurs, the lepton is v i r t u a l. This is an important point
	  and explains why we never distinguish the contributions of the two in a single diagram.
	- Resummation is the sophisticated treatment of Feynman diagrams, that would normally lead to divergences, so they don't diverge. 
	- Bare, dressed, born leptons are nicely defined here: http://cds.cern.ch/record/2022743/files/ATL-PHYS-PUB-2015-013.pdf on pg.3-4
	- The bare leptons are leptons “after QED FSR”. The bare e or µ are the stable particles that appear in the
	  generator record, with QED FSR corrections enabled in the generation.
	- For dressed leptons, a cone or a jet algorithm can be used to cluster all photons around the direction of
	  the bare lepton, forming a lepton after partial QED radiation recovery.
	- Born leptons are fully comparable particle-level objects and their measurements can be directly combined,
	  provided interference effects between initial and final state QED radiation can be neglected.
    - Z/gamma -> tau tau is not considered because this is a precision measurement and it decays primarily to jets with much lower resojution than
	  the lighter two leptons. 

******************************
 Next time: section 5 onwards
******************************