1

Sparsh Navin – University of Birmingham

MCnet studentship at University of Lund

Supervisor - Torbjörn Sjöstrand

Diffraction in PYTHIA

Low x meeting, Ischia 2009

Outline:• Diffraction in PYTHIA – old description• PYTHIA vs PHOJET• Diffraction in PYTHIA – new description and distributions• Outlook

2

Process Typesinelasticelastictotal

...SD DD NDσ +σ +σ

Single Diffraction - SD

Double Diffraction - DD

Non Diffractive - ND

3

Diffraction in PYTHIA – “old”

Event Generation: • Diffractive cross sections given by model by Schuler and Sjöstrand (Phys.

Rev. D 49, 2257 (1994))• Diffractive mass ( ) and momentum transfer (t) generated according to:

2

2

1~ b|t|2

X X

d s edtdM M

Particle Production:

• above mass of incoming particles => isotropical decay into 2-body state

• More massive system treated as a string with quantum numbers of the original hadron

21 XM GeVc

XM

4

Pomeron couples to valence quarksDominates at small

Diffraction in “old” PYTHIA – stretching the string

Version 6.214 (Fortran)

• q and g contributions are set by a user-defined fixed ratio

Version 8.1 (C++)

• Slopes (value of p) in q and g case are different

• mass ( ) dependence

Has a gluon (g) and quark (q) contribution

XM

P( )P( ) p

X

q Ng M

XM

XM

Pomeron couples to gluonDominates at large

5

PYTHIA “old” vs PHOJET - pT

Inelastic = SD + DD + ND

SD

Pythia

Phojet

6

PYTHIA “old” vs PHOJET - Nch

Inelastic = SD + DD + ND

SD

Pythia

Phojet

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Diffraction in PYTHIA – “new”Event Generation:• Cross sections - same way as before• Diffractive mass ( ) and momentum transfer (t) picked by Pomeron flux model

Particle Production: • Pomeron-p collisions• Pomeron PDF with - dependence from H1 data - H1 2007 DPDF Fit Jets and H1 2006 Fits A and B - Pion PDF also available • Standard PYTHIA machinery for multiple interactions, parton showers, hadronization

Mass separation:• For non-perturbative description (as before) - longitudinally stretched strings• For perturbative

XM

2Q

1.2 < M 10 XGeV GeV

M 10 X GeV

8

• Energy dependent

Pomeron Flux factor

9

PYTHIA “old” vs “new” - pT

10

PYTHIA “old” vs “new” - Nch

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Comments

PP

proton

ExE

gg

P

Ex

E

2X PM x s

Diffractive hard scattering cross section:

2/ ( )/ ( )2 2

Pomeron flux Pomeron PDF

ˆ( ) ( , ) P proton P q g P q g

d df x f x QdQ dQ

In the massless limit1p1p

2p

Px

gx

X

Not known from first principles Multiple interactions => screening of diffractive rates

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• Does cut-off depend on the diffractive mass?• Introduce a screening factor to go from ep to pp collisions• Momentum sum of PDFs

Future Plans

0Tp

• Include Central Diffraction 1p

1p

2p2p

X

LRG

LRG

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Summary• Earlier versions of PYTHIA had a primitive description of diffraction• No hard diffraction – caused the difference in pT and multiplicity tails

compared to PHOJET• New version has Pomeron description of diffraction• Hard collisions can be simulated• Better agreement with PHOJET

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• Trigger efficiencies – ALICE• SD, DD, ND events in ALICE detectors• Diffraction in Phojet• Extraction of fractions from data - ALICE

Back up slides

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Trigger Efficiencies and corrections

p ro cessp ro cessto ta ltrig g ered efNN /Efficiency

• Need to know the fraction (f) and the efficiency (e) for each process. • Efficiency is process, trigger and generator dependent

MB1 = SPD or V0A or V0C

Process SD DD ND

Fraction (f) 0.187 0.127 0.686

Efficiency (e) 0.714 0.864 0.999

NDNDDDDDSDSD efefef

Process SD DD ND

Fraction (f) 0.134 0.063 0.803

Efficiency (e) 0.767 0.938 0.999

MB1 efficiencies:

Pythia: 92.9%

Phojet: 96.4%

Reason for difference: f – uncertainty in

cross sectionse – uncertainty in

kinematics

Major difference is in diffractive events

~2-4% effect each

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Single Diffraction (SD) in ALICE

V0A V0C

Outgoing undiffracted p beam

Diffractive system

0

Pseudorapidity gap

ITS

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Double Diffraction (DD) in ALICE

V0A V0C

Diffractive system (2)Diffractive

system (1)

0

Pseudorapidity gap

ITS

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Non Diffractive (ND) events

V0C

No pseudorapidity gap

V0A

ITS

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Central elements of Phojet- R. Engel workshop on soft diffraction at LHC 26/6/09

Two component PomeronOnly one pomeron with soft and hard contributionsTopological identification of different terms (Dual parton model)Soft and hard partons differ in impact parameter distributionApplication of existing parton density parametrisationInitial and final state radiation (leading logQ^2 parton showers)

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Extraction of fractions• Trigger on bunch crossing• Define 8 uncorrelated trigger types using SPD, V0A and V0C• Meausre

NININDNDDDDDSDSDgent ri g

gen

NIgen

NIgen

NIt ri g

gen

NDgen

NDgen

NDt ri g

gen

DDgen

DDgen

DDt ri g

gen

SDgen

SDgen

SDt ri g

gent ri g

NIt ri g

NDt ri g

DDt ri g

SDt ri gt ri g

efefefefNN

NN

NN

NN

NN

NN

NN

NN

NN

NN

NNNNN

calcmeasured

2

2

))((Error)()(

tr ig meas uredtr ig

meas uredtr igcalctr ig

iNiNiN

• Program works out combinations of fractions to generate so as to minimise 2 calctrigN

trigN

- Z.Matthews 20/03/09, ALICE first physics meeting