SoftQCD:all = on              soft hadronic interactions
HardQCD:all = on              hard QCD scatterings
PartonLevel:MPI = on          multiple parton interactions
PartonLevel:ISR = on          initial-state showers
PartonLevel:FSR = on          final-state showers
PhaseSpace:pTHatMin = 10.0    only hard subprocesses above 10 GeV


*** Details are as follows ***

SoftQCD:all = on
Enables all soft QCD processes.
Soft QCD refers to low-momentum-transfer interactions — the “messy” non-perturbative part of hadron collisions, such as:
- elastic scattering
- diffractive events
- minimum bias interactions
- soft inelastic processes
These are important when you want realistic full proton-proton background events.

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HardQCD:all = on
Enables all hard QCD processes.
Hard QCD means interactions with large momentum transfer, where perturbative QCD applies. This includes parton-parton scatterings like:
qq->qq
qg->qg
gg->gg
heavy quark production (c-cbar, b-bbar)

These are typically the main source of jets and energetic secondary particles.
So this line tells PYTHIA to simulate all hard strong-interaction subprocesses.

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PartonLevel:MPI = on
Turns on Multiple Parton Interactions (MPI).
In a proton-proton collision, one proton contains many quarks/gluons.
More than one pair can interact in the same event.
So instead of just one hard scattering, PYTHIA can simulate several 
simultaneous partonic scatterings. This is very important for:

- underlying event
- event multiplicity
- realistic detector occupancy

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PartonLevel:ISR = on
Turns on Initial State Radiation (ISR).
Before the hard collision, incoming partons may radiate gluons:
q->qg, g->gg
That changes the incoming energy and momentum of the colliding partons.

ISR affects:
- transverse momentum distributions
- event shapes
- jet structure

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PartonLevel:FSR = on
Turns on Final State Radiation (FSR).
After the hard interaction, outgoing partons radiate additional gluons.
This produces:
- parton showers
- jet development
- more secondary particles

For example: q->qg after the scattering.
Without FSR, jets would be unrealistically simple.

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PhaseSpace:pTHatMin = 10.0
Sets the minimum transverse momentum scale for hard subprocesses:
pTmin=10 GeV
This means PYTHIA will only generate hard interactions whose partonic transverse momentum exceeds 10 GeV.

Why use this?
Because QCD cross sections become enormous at low pT. Setting a cutoff:

- avoids too many soft hard-QCD events
- speeds up simulation
- focuses on energetic collisions