hard diffraction in pythia 8...pythia 8 cdf cuts (0.37 0.02)% (0.28 0.01)% pythia 8 too suppressed....

$: Hard diffraction in PYTHIA 8...PYTHIA 8 CDF cuts (0.37 0.02)% (0.28 0.01)% Pythia 8 too suppressed. Fractions are sensitive to variations of model parameters: P PDF and ux, free parameters$
lund un i v er s i ty dept . o f a stronomy and theoret i ca l phy s i c s

Hard diffraction in PYTHIA 8

Christine O. Rasmussen

• PYTHIA 8

• Soft diffraction

• Hard diffraction

• Preliminary results

• Conclusion and outlookOct. 5 2015

Slide 1/18


PYTHIA 8 is a general purpose event generator forhigh-energy collisions.

It attempts to describe all parts. [Figure: T. Sjostrand]

Christine O. Rasmussen — Hard diffraction in PYTHIA 8 — Oct. 5 2015

Slide 2/18


PYTHIA 8Currently 8 authors (incl. 3 post-docs and 2 PhD students).

New main features as of version 8.2:• New models of colour reconnections

(S. Argyropoulos, J. Christiansen, P. Skands + T. Sjostrand)

• Variety of matching and merging schemes(S. Prestel + L. Lonnblad)

• Weak showers, matching and merging with weak showers(J. Christiansen, S. Prestel + T. Sjostrand)

• Many new tunes - default Monash 2013 tune(P. Skands et. al)

Ongoing work:• Model for hard γγ events

(I. Helenius + T. Sjostrand)

• New model for hard diffraction(C. Rasmussen + T. Sjostrand)

• Exclusive processes(R. Zlebic + L. Lonnblad)

An Introduction to PYTHIA 8.2 [Comput.Phys.Commun. 191 (2015) 159]


Slide 3/18


Soft diffractionσtot calculated using Donnachie-Landshoff parametrisation.

Diffractive and elastic topologies calculated withPomeron-based parametrisation of Schuler-Sjostrand.

p p

p X

P

SD

Xp

p Y

P

DD

p p

p p

XP

P

CD

Nondiffractive (ND) cross section inferred from the above,

σND = σtot − σel −∑

X=S,C,D

σXD


Slide 4/18


Soft diffraction

Low-mass region:MX ≤ 10 GeV.

• Represent MX aslongitudinal string

• Quark = 1 string,gluon = 2 strings

• Probability to kick outa gluon or quark fromproton: P(q)

P(g) = NMp

• No ISR, FSR, MPI

• Fragment with LundString fragmentationmodel

High mass region:MX > 10 GeV.

• Based onIngelman-Schleinapproach

• Set up Pp system

• MPI machinery decideinteractions

• Includes interleavedMPI, ISR, FSRevolution in Pp system

• Now includes 7 modelsfor Pomeron flux and5 for Pomeron PDF


Slide 5/18


Soft diffraction

[S. Navin: arXiv:1005.3894[hep-ph]]

MPI gives a smooth mergingof hard jets and soft events.

Gap always survives, as MPInot allowed in hadron-hadronsystem.

MPI activity in SD tuned to give approximatelysame amount as in ND:

〈nMPI〉(ND) ∼σhard

pp

σND⇒

〈nMPI〉(SD) ∼σhardPp (Nogap survival)

σPp(Nogap survival)=σhardPp

σeffPp

with σeffPp = 10 mb, tunable.


Slide 6/18


Hard diffractionObjective: Allow for truly hard diffractive processes,e.g. high-p⊥ QCD, electroweak etc.Question: Given a hard scattering, what is the probabilityfor this to have been created in a diffractive process?Available: Parton id, x and Q2.Method: Evaluate the diffractive PDFand use dynamical gap survival.Assumption 1: The hadronic PDFs can be split intonondiffractive and diffractive,

fi (x ,Q2) = f ND

i (x ,Q2) + f Di (x ,Q2)

Assumption 2: The diffractive PDF factorises,

f Di (x ,Q2) =

∫ 1

x

dxPxP

∫ tmax

tmin

dt fP/p(xP, t) fi/P(x/xP,Q2)


Slide 7/18


Hard diffractionThe probabilities for either sides to be diffractive are

PB = f Di (xa,Q

2)/fi (xa,Q2)

PA = f Di (xb,Q

2)/fi (xb,Q2)

Dynamical gap survival:

X

p

p p’

a

b

P

7 MPI

3 MPI

SD ab → X process with beam remnants from both proton andPomeron.


Slide 8/18


Hard diffraction

Dynamical gap survival introduces additional suppression.

pp → Gap + W pp → Gap + Z

D0 [Phys.Lett.B574(2003)169] (0.89+0.19−0.17) % (1.44+0.61

−0.52) %

(pp → p′ + W ) x 2 (pp → p′ + Z) x 2CDF [Phys.Rev.D82(2010)112004] (1.0±0.11) % (0.88±0.22) %

(pp → p′ + W ) x 2 (pp → p′ + Z) x 2PYTHIA 8 CDF cuts (0.37 ± 0.02) % (0.28 ± 0.01) %

Pythia 8 too suppressed.Fractions are sensitive to variations of model parameters:

P PDF and flux, free parameters of MPI model.


Slide 9/18


Hard diffraction

How are these fractions and the particle distributions affectedwhen we change

• the Pomeron parametrisations,

• the regulator pref⊥0,

• the impact-parameter dependence of the MPI model?

Will affect the chosen value of xP and connected variablessuch as the mass of the diffractive system, the size of therapidity gap, the squared momentum transfer and the angleat which the proton is bent.

Will affect the rapidity gap survival rate, the charged particlespectrum and the underlying event.


Slide 10/18


Hard diffractionQCD 2→ 2 processes with p⊥ > 20 GeVat√s = 8 TeV pp collider

10−4

10−2

100

1/N

dN/dxP

0.06

0.12

0.18

0.24

0.30

1/N

dN/dM

Sch-SjoMBR

0.04

0.08

0.12

0.16

0.20

1/N

dN/d

∆η

10−4 10−2 100

xP

0.60.81.01.21.4

Rat

ioto

Sch-

Sjo

0 2000 4000 6000 8000M

0.60.81.01.21.4

Rat

ioto

Sch-

Sjo

0 2 4 6 8 10∆η

0.60.81.01.21.4

Rat

ioto

Sch-

Sjo

Normalized to unit area


Slide 11/18


Hard diffractionQCD 2→ 2 processes with p⊥ > 20 GeVat√s = 8 TeV pp collider

0 50 100 150 200 250 300 350 400

nch

10−4

10−3

10−2

10−1

1001/N

dN/dn

ch

Pythia 8 defaultspref⊥0 = 2.78

pref⊥0 = 1.78

Normalized to unit area


Slide 12/18


Hard diffraction

Changing the pref⊥0 increases the diffractive fraction, as a

higher value gives less MPIs.

pp → Gap + W pp → Gap + Z

D0 [Phys.Lett.B574(2003)169] (0.89+0.19−0.17) % (1.44+0.61

−0.52) %

(pp → p′ + W ) x 2 (pp → p′ + Z) x 2CDF [Phys.Rev.D82(2010)112004] (1.0±0.11) % (0.88±0.22) %

(pp → p′ + W ) x 2 (pp → p′ + Z) x 2PYTHIA 8 CDF cuts pref⊥0 = 2.28 (0.37 ± 0.02) % (0.28 ± 0.01) %PYTHIA 8 CDF cuts pref⊥0 = 2.78 (0.61 ± 0.02) % (0.48 ± 0.01) %


Slide 13/18


Diffractive dijet production at the Tevatron.

SD dijets: pp→ X p, [X → X ′ + jet + jet]

E jetT > 7 GeV, |η|jet < 4.2

jet

jetp

p p′

c

d

a

b

P

[Phys.Rev.Lett.84.(2000) 5043]

HERA parametrisations does not describe CDF data


Slide 14/18


Preliminary results

Kinematical distributions of SD jets

NDSD PDF selectionSD MPI selection

10−5

10−4

10−3

10−2

10−1

Mean ET of jets with surviving antiproton

1/N

dN/

dE∗ T

0 10 20 30 40 50

0.6

0.8

1

1.2

1.4

E∗T

MC

/Dat

a

NDSD PDF selectionSD MPI selection

0

0.05

0.1

0.15

0.2

0.25

0.3

Mean η of jets with surviving antiproton

1/N

dN/

dη∗

-4 -2 0 2 4

0.6

0.8

1

1.2

1.4

η∗

MC

/Dat

a

E ∗T not steep enoughNotation:Setup Pp system after probabilistic criterion = PDF selectionSetup Pp system after MPI criterion = MPI selection


Slide 15/18


Preliminary results

10−3 10−2 10−1

xp

10−5

10−4

10−3

10−2

10−1

100

Rat

ioSD

/ND

PDF selectionMPI selection

CDF data

[Phys.Rev.D86.(2012) 032009]

Pythia 8 not as steep as data.


Slide 16/18


Preliminary results

10−3 10−2 10−1

xp

10−5

10−4

10−3

10−2

10−1

100

Rat

ioSD

/ND

PDF selectionMPI selection

MPI selection, pref⊥0 = 2.78

CDF data

[Phys.Rev.D86.(2012) 032009]

Tweaking pref⊥0 helps, but still too flat


Slide 17/18


Conclusion and outlook

• We have developed a new model for hard diffractionwith dynamical gap survival

• Model is implemented in PYTHIA 8, publicly available

• Some kinematical distributions disagree with CDF data -we obtain too hard events

• Tuning MPI parameters could improve distributions

• Development of new Pomeron flux (and PDF?)

• Retuning of the soft diffractive machinery


Slide 18/18

hard diffraction in pythia 8...pythia 8 cdf cuts (0.37 0.02)% (0.28 0.01)% pythia 8 too suppressed....

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