LOW X MEETING, Paphos, Cyprus, 17 June -1 July 2012

Predictions of Soft Processes at the LHC
Implemented in PYTHIA8
Konstantin Goulianos*
The Rockefeller University
* with:
Robert Ciesielski
1
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CONTENTS
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INTRODUCTION
This is an adaptation of the MBR (Minimum Bias Rockefeller) simulation
RENORM to PYTHIA8.
‣ successfully tested at Fermilab fixed target and collider experiments.
RENORM predictions are based on a parton model approach, in which
diffraction is derived from inclusive PDFs and color factors.
Gap Cross Sections:
‣ Gap cross sections vs gap width:
Absolute normalization!
‣ Hadronization of dissociated proton:
‣ Implemented by introducing a (non-perturbative) “quark string” and
tuning it to reproduce the MBR multiplicity and p T distributions.
‣ dN/dh, p T , and particle ID (new in this implementation; the original
MBR produced only p ± and p 0 )
‣ Unique unitarization procedure based on a saturated exchange.
Total Cross section: based on a saturated Froissart bound leading to a ln 2 s
dependence.
‣ Immune to eikonaalization models that plague the field!
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STUDIES OF DIFFRACTION IN QCD
Non-diffractive
color-exchange gaps
exponentially suppressed
Diffractive
Colorless vacuum exchange
large-gap signature
Incident hadrons
acquire color
and break upart
rapidity gap
p
Incident hadrons retain
their quantum numbers
remaining colorless
CONFINEMENT
Goal: probe the QCD nature of the diffractive exchange
P
O
M
E
R
O
N
p p p p
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DEFINITIONS
SINGLE DIFFRACTION
x,t
M X
p p’
xp
p
1- x L
ξ
CAL
ξ
Σ
all
i1
M 2 X
s
E
i-tower
T
since no radiation
no price paid for increasing
diffractive gap size
s
e
η i
dσ
d Δη
t0
p’
Rap-gap
Dh=-lnx
constant
dσ
dξ
0
dN/dh
ln ln s
dσ 1
2 2
1
ξ
ln Mx 2
low-x 2012 Cyprus Soft Processes at LHC in PYTHIA8 K. Goulianos
dM
M X
M
5

DIFFRACTION AT CDF
Elastic scattering s T =Im f el (t=0) Total cross section
f
gap
h
OPTICAL
THEOREM
f
h
SD DD DPE /CD SDD
Single Diffraction or
Single Dissociation
Double Diffraction or
Double Dissociation
Double Pom. Exchange or
Central Dissociation
Single + Double
Diffraction (SDD)
JJ, b, J/y, W
p
exclusive
JJ…ee…mm...gg
p
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540 GeV
1800 GeV
FACTORIZATION BREAKING IN
SOFT DIFFRACTION
diffractive x-section suppressed relative
to Regge prediction as √s increases
p
p
x,t
p’
M
Factor of ~8 (~5)
suppression at
√s = 1800 (540) GeV
RENORMALIZATION
√s=22 GeV
C
D
F
see KG, PLB 358, 379 (1995)
Question:
does factorization breaking
affect t-distributions
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DD at CDF
gap probability
x-section
renormalized
EDS-2011 Vietnam CDF Diffraction and Exclusive Production 8

SDD at CDF
• Excellent agreement
between data and MBR
(MinBiasRockefeller) MC
EDS-2011 Vietnam CDF Diffraction and Exclusive Production 9

CD (DPE) at CDF
• Excellent agreement
between data and MBR
low and high masses are
correctly implemented
EDS-2011 Vietnam CDF Diffraction and Exclusive Production 10

Cross Sections
tot
el
SD SD DD CD
SD single diffraction (single dissociation)
DD double dissociation (double diffraction)
CD central dissociation (double pomeron exchange)
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Total, elastic, and inelastic x-sections
GeV 2
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Total, elastic, and inelastic x-sections
versus √s
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Difractive x-sections
a 1 =0.9, a 2 =0.1, b 1 =4.6 GeV -2 , b 2 =0.6 GeV -2 , s′=s e -Dy , k=0.17,
kb 2 (0)=s 0 , s 0 =1 GeV 2 , s 0 =2.82 mb or 7.25 GeV -2
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Difractive x-sections of √s
Supress x-sections at small gaps by a factor S using the error function with
Dy S =2 for SD and DD, and Dy=Dy 1 +Dy 2 =2 for CD (DPE).
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SD and DD at 7 TeV
MBR vs PYTHIA8-4C
The differences between the PYTHIA8(4C) and MBR predictions are mainly
due to the (1/M2) 1+e behavior, with e=1.104 in MBR vs 1,08 in PYTHIA8(4C).
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CD (DPE) x-sections at 7 TeV versus
(a) Dy=Dy1+Dy2 and (b) Dy1
Both figures are MBR predictions with a Dy=2 cut-off in the error function.
The normalization is absolute with no model uncertainty other than that due to the
determination of the parameters in the formulas as determined from data
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Thank you for your
attention
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References in arXiv:1205.1446v1 [hep-ph]
multiplicities
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