sparsh navin – university of birmingham mcnet studentship at university of lund
DESCRIPTION
Low x meeting, Ischia 2009. Diffraction in PYTHIA. Sparsh Navin – University of Birmingham MCnet studentship at University of Lund Supervisor - Torbjörn Sjöstrand . Outline: Diffraction in PYTHIA – old description PYTHIA vs PHOJET - PowerPoint PPT PresentationTRANSCRIPT
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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
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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
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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
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PYTHIA “old” vs PHOJET - pT
Inelastic = SD + DD + ND
SD
Pythia
Phojet
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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
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• Energy dependent
Pomeron Flux factor
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PYTHIA “old” vs “new” - pT
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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