Fraction of DP events

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CP

³

– Origins SDU, Odense 24 November 2009

QCD and Event Generators

Torbj ¨orn Sj ¨ostrand

Lund University

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The structure of an event

Warning: schematic only, everything simplified, nothing to scale, . . .

p

p/p

Incoming beams: parton densities

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p

p/p

u g

W

⁺

d

Hard subprocess: described by matrix elements

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p

p/p

u g

W

⁺

d

c s

Resonance decays: correlated with hard subprocess

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p

p/p

u g

W

⁺

d

c s

Initial-state radiation: spacelike parton showers

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p

p/p

u g

W

⁺

d

c s

Final-state radiation: timelike parton showers

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p

p/p

u g

W

⁺

d

c s

Multiple parton–parton interactions . . .

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p

p/p

u g

W

⁺

d

c s

. . . with its initial- and final-state radiation

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Beam remnants and other outgoing partons

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Everything is connected by colour confinement strings

Recall! Not to scale: strings are of hadronic widths

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The strings fragment to produce primary hadrons

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Many hadrons are unstable and decay further

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These are the particles that hit the detector

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A tour to Monte Carlo

. . . because Einstein was wrong: God does throw dice!

Quantum mechanics: amplitudes =⇒ probabilities

Anything that possibly can happen, will! (but more or less often)

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The Monte Carlo method

Want to generate events in as much detail as Mother Nature

=⇒ get average and fluctutations right

=⇒ make random choices, ∼ as in nature

σ

final state

= σ

hard process

P

tot,hard process→final state

(appropriately summed & integrated over non-distinguished final states) where P

_tot

= P

res

P

_ISR

P

_FSR

P

_MI

P

_remnants

P

hadronization

P

_decays

with P

_i

=

^Q_j

P

_ij

=

^Q_j ^Q_k

P

_ijk

= . . . in its turn

=⇒ divide and conquer

an event with n particles involves O(10n) random choices, (flavour, mass, momentum, spin, production vertex, lifetime, . . . ) LHC: ∼ 100 charged and ∼ 200 neutral (+ intermediate stages)

=⇒ several thousand choices

(of O(100) different kinds)

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The Big Picture: Putting It Together

Process Selection Resonance Decays

Parton Showers Multiple Interactions

Beam Remnants

Hadronization Ordinary Decays

Detector Simulation ME Generator

ME Expression

SUSY/. . . spectrum calculation

Phase Space Generation

PDF Library

τ Decays

B Decays

need standardized interfaces (LHA/LHEF, LHAPDF, SUSY LHA, HepMC, . . . )

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The workhorses: what are the differences?

HERWIG, PYTHIA and SHERPA intend to offer a convenient framework for LHC physics studies, but with slightly different emphasis:

PYTHIA (successor to JETSET, begun in 1978):

• originated in hadronization studies: the Lund string

• leading in development of multiple parton interactions

• pragmatic attitude to showers & matching

• the first multipurpose generator: machines & processes HERWIG (successor to EARWIG, begun in 1984):

• originated in coherent-shower studies (angular ordering)

• cluster hadronization & underlying event pragmatic add-on

• large process library with spin correlations in decays

SHERPA (APACIC++/AMEGIC++, begun in 2000):

• own matrix-element calculator/generator

• extensive machinery for CKKW matching to showers

• leans on PYTHIA for MPI and hadronization

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MCnet: Competitors and Collaborators

EU Marie Curie training network funded 2007 – 2010:

HERWIG, PYTHIA SHERPA, ThePEG, ARIADNE, VINCIA, . . . Transition Fortran → C++

and LHC preparations.

Also generator validation (RIVET) and tuning (PROFESSOR).

4 postdocs & 2 graduate students.

Annual Monte Carlo school;

even years with CTEQ:

2009: Lund;

2010: Karlsruhe.

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Multiparton Interactions

Z. Physik C34 (1987) 163 – 174:

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Signal and Background

Double Parton Scattering

1 2

3

4 | p

_⊥1

+ p

_⊥2

| ≈ 0

| p

_⊥3

+ p

_⊥4

| ≈ 0 dσ/dϕ flat

Double BremsStrahlung

1 2

3 4

| p

_⊥1

+ p

_⊥2

| ≫ 0

| p

_⊥3

+ p

_⊥4

| ≫ 0

dσ/dϕ peaked at ϕ ≈ 0/π for AFS/CDF

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Preliminary D0 results (2009):

(GeV)

jet2

p

T

10 12 14 16 18 20 22 24 26 28 30

Fraction of DP events

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

tune A, Pythia 6.420 tune S0, Pythia 6.420 data

+ 3 jets + X γ

agreement and precision “too good to be true”;

tunes 7 and 3 years old, respectively, and not to this kind of data

S0: Peter Skands (Copenhagen → Lund → Fermilab → CERN)

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Collective Effects of Multiparton Interactions

QCD: linear confinement model confirmed by lattice QCD.

Extended to hadronization:

r r

... ... ...

⇓ r r

... ... ...

r r

⇓ r r

. ...

... ... ... ...

r r

... ... ...

Gluon in N

_C

→ ∞ limit:

r r b b

hp

_⊥

i(n

_ch

) is sensitive to colour flow

p p

long strings to remnants ⇒ much n

_ch

/interaction ⇒ hp

_⊥

i(n

_ch

) ∼ flat

p p

short strings (more central) ⇒ less

n

_ch

/interaction ⇒ hp

_⊥

i(n

_ch

) rising

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0.6 0.8 1 1.2 1.4 1.6

0 10 20 30 40 50

0.6 0.8 1 1.2 1.4 1.6

0 10 20 30 40 50

N_ch (|η|<1.0, p_⊥>0.4GeV)

<p T>[GeV]

Average Charged Particle p_T (|η|<1.0, p_⊥>0.4GeV)

1960 GeV p+pbar

Inelastic, Non-Diffractive

Pythia 6.421

Data from CDF Collaboration, Phys. Rev. D79(2009)112005

CDF data Perugia 0 Perugia NOCR A-Pro

A_CR-Pro

Atlas-DC2-Pro

(P.Skands)

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