News from Pythia Neutrino grassroots discussion @ Fermilab (March 15 - - PowerPoint PPT Presentation

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News from Pythia

Neutrino grassroots discussion @ Fermilab (March 15 2016)

Stefan Prestel, remotely :(

SLIDE 2

Pros and cons of PYTHIA 8 Sophisticated showers for DIS available. p under investigation. This is a plus! Only for pp .

• No ep, γp or γγ incoming beams.
• Fewer in-built processes.

+ Evolved MPI model, sophisticated diffractive machinery. + polarisation in production and decay. + More perturbative physics: Matching and Merging! + Simple card files: Should be better match for software

• frameworks. Compatible with modern in- and outputs

+ Simple and extensive online documentation

http://home.thep.lu.se/ torbjorn/pythia82html/Welcome.html

PYTHIA6 development has stopped. PYTHIA6 support is not

high-priority for developers any longer.

2 / 8

SLIDE 3

Pros and cons of PYTHIA 8 Sophisticated showers for DIS available. p under investigation. This is a plus! Only for pp .

• No ep, γp or γγ incoming beams.
• Fewer in-built processes.

+ Evolved MPI model, sophisticated diffractive machinery. + τ polarisation in production and decay. + More perturbative physics: Matching and Merging! + Simple card files: Should be better match for software

• frameworks. Compatible with modern in- and outputs

+ Simple and extensive online documentation

http://home.thep.lu.se/∼torbjorn/pythia82html/Welcome.html

PYTHIA6 development has stopped. PYTHIA6 support is not

high-priority for developers any longer.

2 / 8

SLIDE 4

Pros and cons of PYTHIA 8 Sophisticated showers for DIS available. γp/γγ under investigation. This is a plus! Only for pp .

• No ep, γp or γγ incoming beams.
• Fewer in-built processes.

+ Evolved MPI model, sophisticated diffractive machinery. + τ polarisation in production and decay. + More perturbative physics: Matching and Merging! + Simple card files: Should be better match for software

• frameworks. Compatible with modern in- and outputs

+ Simple and extensive online documentation

http://home.thep.lu.se/∼torbjorn/pythia82html/Welcome.html

PYTHIA6 development has stopped. PYTHIA6 support is not

high-priority for developers any longer.

2 / 8

SLIDE 5

LHC lessons . . LHC is a jet machine need to get jet production right need to get jet evolution right Before After NLO merging

3 / 8

SLIDE 6

LHC lessons . . LHC is a jet machine need to get jet production right need to get jet evolution right Before After NLO merging .

LHC physics requires accurate & precise QCD calculations:

Next-to-leading order, next-to-next-to-leading order, “not-so-approximate” all-order resummation… Data can only be described if we Combine multiple accurate fixed-order calculations with each

• ther, and with all-order resummation, into a single precise

prediction ( = matrix element merging) Done in PYTHIA 8

3 / 8

SLIDE 7

In retrospect, also important for HERA data . .

SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA

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parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level

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SHERPA predictions for the jet cross sections in H1 (in Q2 and E2

T,B/Q2).

Plot taken arXiv:0912.3715 Exact psp factorisation enables ME corrections. Good agreement after combining many multi-jet matrix elements w/ each other and w/ shower.

SLIDE 8

In retrospect, also important for HERA data . .

SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA SHERPA

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parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level parton level

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2

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SHERPA predictions for the jet cross sections in H1 (in Q2 and E2

T,B/Q2).

Plot taken arXiv:0912.3715 Exact psp factorisation enables ME corrections. Good agreement after combining many multi-jet matrix elements w/ each other and w/ shower.

. No such improvements possible in PYTHIA 6, which also has other issues for DIS.

SLIDE 9

The DIS issue: Parton showers and cross section

Naive shower

Full final state changed

Pythia 6 Pythia 8

Lepton intact Lepton intact

Hard scattering

Does not reduce to lowest-order Reduces to modified hard process Reduces to correct hard process Now has virtuality (mass) t

Initial state radiation in a traditional PS proceeds by

• Take massless incoming line, shift to accommodate virtuality t.
• Split the massive incoming line to produce the emission.

Naive introduction of a virtuality t means xAfter ̸= xBjorken Before . . ⇒ Shower changes momentum fraction of the “core” process. ⇒ Must recalculate scattering cross section dσ = f(x, Q2)dˆ σ ⇒ after each emission! ⇒ Not possible / practical. Fix in pp: “Backward evolution”. Electrons take recoil. Bad for DIS.

5 / 8

SLIDE 10

The new Pythia 8 model Pythia 6 model

• Redefine hard scattering

⋄ Energy sharing very messy ⋄ Not coherent ⋄ Holes in phase space ⋄ Jet rates technically depend on custom structure functions. − Cannot easily improved with full MEs → uncertain for large W2 + GVMD model for W2 ≲ 1GeV

a Pythia 8 model (DIRE)

• Exact
• n-shell

phase space factorization ⋄ Straight-forward energy sharing ⋄ Coherence built in ⋄ Full phase space coverage ⋄ Depends

• nly
• n

standard structure functions(∗) + Exact phsp factorization allows merging with exact MEs − No diffraction yet for W2 ≲ 1GeV

(∗) up to power corrections from difference of kernels to DGLAP.

6 / 8

SLIDE 11

Relevance for neutrinos? . . For PYTHIA 8, neutrino scattering is deep inelastic scattering. Still new in PYTHIA 8. We know that LHC improvements may fix issues with HERA (high-W2) data (work in progress) How high W2 for current experiments? Varied Pythia 6 parameters?

7 / 8

SLIDE 12

Summary and Outlook . . Things to do in PYTHIA 8: Diffractive model, Low-multiplicity hadronization, general- ized proton structure. . . Opportunities:

PYTHIA 8 is maintained and developed (in C++). Hadroniza-

tion tunes more up-to-date. Well-defined showers may allow better interface to non-perturbative physics. High-W2 physics can be made more reliable. Questions Is neutrino phenomenology sensitive to improvements in PYTHIA 8? What about nuclear effects, also for eA and pA?

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