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March 19th 2018 - PDGG MC EvGen 2: Hadronization models
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Second session of MC Generator PDG review, Greg chairs, Ben moderates (thanks Ben!). Section: 41.2 in 2016 PDG.

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Details:

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Q U E S T I O N S
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- Q: what does color connected mean? e.g. red-anti-red: are the color-flow lines unbroken? Then it is color-connected.
- What would ‘intermediate gluons’ look like in this case?
- String breaks are generated from the leading, or outermost, hadrons? fig. 41.1 in 2016 review helps answer this.
- One can split off a single on-shell hadron in each step, making it straightforward to ensure that only states constant with known hadrons are produced? Basically this is just saying that conservation laws can be applied as each hadron is created.
- So I guess they sample this probability distribution when making the hadrons?(eqn. 41.10) Ben confirms.
- The requirement that the fragmentation is time-independent imposes a left-right symmetry - why?? To ensure things like charge are conserved? They just mean the function has to be symmetric, nothing to do with any physical symmetries (e.g. eta symmetry) which is how I interpreted it.
- What really defines the cluster bounds? (solution in the instructions at top)
- uds question: what?? is it just saying isospin is conversed? Yes.

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A G E N D A    I T E M S
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- Next week Patrick and Rebecca at Connecting the Dots. The week after is Spring Break, I would prefer for the club not to take a 2-week break so recommend that it continue in my absence during CTD and resume after Spring Break.
- Next topic Strong force coupling constant:  http://dx.doi.org/10.1140/epjc/s10052-015-3376-y ? All agreed. And chair? tbd. Will ask people after next week.
- I spoke to Elisabetta who is a division fellow working partly with the PDG, and mentioned we could give her some feedback on the section we were reading. I propose people do this anonymously via google-doc over the break. Some early comments:
    - I think a list of the steps followed by a breakdown would improve this section, for example.
    - And tone between clustering and string descriptions is different

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G E N E R A L
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Greg: I will assume everyone focused on PDG as it was more in-depth. Hadronization is fundamentally non-pertubativ and so very little QCD, which is imortnant because of the transitions between the modes.

- This means much of the focus is on the energy-scales of the interactions.
    - String section: suggested thing is that it’s defined on behalf of linear confinement. This models that force as a string, when that string breaks. Fundamentally Lorentz invariant. String coefficient sets he scale: 1 GeV/fm.
    - Where does this scale come from? From phenomoneolgy, in nuclear that’s the definition. Hard scatter and soft scatter.
    - Ben: everything above the hadron mass. The string instant has dimensions, in turning parsons into hadrons, strange is surprised with respect to other particles, this comes naturally
    - Greg: fundamentally this is a 3D object. We often think of Feynman diagrams being 2D so I found it harder to
    - cyclical gluons? How do gluons decay on their own when not associated
    - Ben: alice, strangeness suppression goes away in really high multiplicity p-p events. They say, this is because you have overlapping strings so you have more energy to make strange quarks. Rope model.
    - 90% are pions, 10% ar kaons, 1% are protons
- Can you use a different string model? JetSet came before
- Based on a different observation
- Shower evolution scale, in e+e- Q is the energy imparted to scattering process, e.g. pT of parton,
- We also talked about each step in both the cluster and string model.

A broad outline:

String:
Steps, evolve the string till it reaches breaking point
On breaking, decide what particles are produced.
Then assign the produced quarks to Hadronic multiplets with fixed pT^2 and m^2
Final step: select the fraction, z, of the fragmenting end-point quark’s longitudinal momentum is carried by the created hadron.

Cluster:
First: forces g->qqbar and then assigns flavours and momenta to produced pairs
If a cluster formed has an invariant mass > 3-4 GeV, it is made to break up into 2 clusters, along an axis defined from the original cluster
This happens until all clusters fall below the mass
If a cluster has low enough mass it is allowed to deny directly to a hadron with nearby clusters absorbing excess momenta; the probability of this happening is a parameter
the remaining clusters (that are less than 3-4 GeV and more than the cutoff for decaying to a single hadron) are now considered to be a spectrum of excited mesons.
Each of these decays isotropically to 2 hadrons with various probabilities: parameters include: flavours and spin

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Next time: Cheyenne will chair the final PDG MC EvGen
session on 42.1.8, 42.1.9, and 42.1.10 in the 2017 review.
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