Notes June 25, 2019


Today: sections 6, 7, and 8


Section 6
-About PDFs; kind of short, and not all that helpful perhaps.  Jennet added “an excellent reference for PDFs” in the google doc: QCD and collider physics


-Jennet: “Why does the parton shower need to know about the pdf?”
---Marjorie: backward evolution (she takes that back)
---Marjorie: combination of MPI and soft stuff
---There are always tradeoffs when it comes to pdfs
---For tuning, LEP has very good quality data for quark stuff
---We’re taking a single QCD process and breaking it into chunks based on what time-scales they correspond to.  This is an approximation of course
---We also are stitching together calculations at different orders.  E.g the 1 jet cross section is inclusive in 1 or more jets, but some region of the phase space there corresponds to the 2+ jet calculation.  One must remove that phase space from the 1 jet cross section to avoid double counting.  Scheme is known as e.g. AMC@NLO
---(We found a paper ATL-PHYS-PUB-2014-021 which describes pdfs for ATLAS from the physics modelling group: see table 4)
---(Also, see “A useful talk with ttbar as an example process” from the google doc)




-MPI and primordial kT
---Jennet’s main takeaway from the MPI section: “we don’t care about the extra jets, we just care about the color flow”.  Figure 8 shows with and without MPI distributions
---Marjorie: Pythia sometimes messes up scattering of colorless objects (eg WW scattering), because it doesn’t treat the objects as colorless


-Jennet’s heavy ion question (see google doc)
---Marjorie says there isn’t too much relation between their modeling and ours


-Jennet: “Main takeaway from hadronization section is that there are two ways to do it: string and cluster (angular-ordered) models.  We get uncertainties by basically running twice and seeing the difference”
---Both have some mechanism for giving you antenna patterns that would come from actual calculations (which come from field interferences, same as you would get in E+M)
---Jennet wishes there was a video to show how these things happen


-Jennet: How do you get baryons from a string model?
---Marjorie: di-quarks, which are not mesons.  They are the 8 from the 3x3 of qcd, as opposed to the 1.  That is then combined with another 3 of course.  They are not stable objects.
---Marjorie: This is one reason why LEP data is very important.  Lots of info about e.g. how often you make baryons, kaons, lamda’s etc.  “important engineering numbers”
---Cesar asks about future colliders.  Marjorie: You hope that when you extrapolate to higher energy things will be fine.  But you really hope that you have something to tune on.  The predictions for the LHC were really not that bad.


-Jennet’s lack of charm and bottom at string break question
---Marjorie: charm and bottom is created in the actual parton shower, as you’re coming down from high energy.  But if you produce things in a color flux tube, you need to make sure that momentum and energy are conserved.  You can basically say the string as an energy per unit length.  The probability to get a charm quark out of the string is like 10^-7.  You get about ⅓ as many strange as up or down quarks


-Jennet: What does EvtGen do?
---Marjorie: It re-decays.  Take existing event record.  Find things you want to re-decay.  Remove the previous decay products.  Then evtgen redecays things.  You only use it to decay color singlet objects (e.g. mesons).  Need to have full pdg decay tables


-Jennet made a slideshow of various knobs that can be adjusted for doing MC.  See the google doc