Perspectives in forward physics

1
Perspectives in forward physics

• Mario Campanelli
• James Monk
• University College London

2
Introduction

• LHC has been often presented as a discovery
  machine, thinking of BSM spectacular signatures
• In the beginning, the conventional wisdom is that
  we should re-discover the SM.
• On the other hand
• Almost everything is a discovery when you run at
  5 times more energy
• Many issues are still pending perhaps not big
  surprises expected, but worth looking at there
  are still things to discover in the SM

3
Forward physics

• The only happy guys when luminosity is low
• Not too much pileup to dirty rapidity gaps
• Most of the cross sections are high enough anyway
• Two main lines
• Diffraction
• QCD evolution

4
Diffraction

• Processes with Q2ltlt s can proceed through the
  exchange of color-singlets (pomerons) and produce
  events with large rapidity gaps.
• Extensively studied at Hera, and at the Tevatron

5
Soft Survival Factor

• The gap in radiation in a diffractive event will
  be filled in if another interaction happens on
  top of it.
• Even without pile-up there would be multiple
  interactions from the same protons that take part
  in the diffractive scattering.
• The probability that there is no such secondary
  scatter is the soft survival factor
• Necessary to connect the predicted cross-sections
  with the measured (lower) cross-sections.
• Observed to be 0.1 at the Tevatron.
• Different models predict values 0.03 at the LHC
• Model dependent variation with t2, ??

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6
pp?jet-gap-jet
Exchanged colourless object either Pomeron (spin
0) or Reggeon (spin?0). -t2 must be sufficiently
large for jet to hit calorimeter

• Modified Herwig Iproc 2400
• Modified because at leading order as does not run
  at the Reggeon-proton vertex (justified by HERA
  and Tevatron data)

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7
Compare modified Herwig within Athena (at
1800GeV) to Tevatron data
Low N excess in signal
DØ data (hep-ex/9809016)
Modified Herwig signal Vs. Pythia 2?2 background
No. calorimeter towers ? lt 1 2 jets ? gt 1.9
and gap ?? gt 4 2 jets with ET gt 30 GeV.
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ET of the second jet

• Choose signal (Herwig) events where the central
  (?lt1) multiplicity 2
• Can determine what fraction of events are colour
  singlet exchange as the ET of the second jet
  increases
• This fraction should increase with the jet ET

9
Gap Fraction

• Rises with ET as does data
• Soft survival not included here
• Gap fraction broadly compatible with data
  assuming soft survival of 0.1

10
QCD parton evolution

• Matrix element calculation only available to a
  fixed order, higher order contributions estimated
  with evolution equations that resum part of the
  perturbative expansion.
• Main evolution equations
• Dokschitzer Gribov Lipatov Altarelli Parisi
  (DGLAP)
• Balitski Fadin Kripov Lipatov (BFKL)
• Ciafaloni Catani Fiorani Marchesini (CCFM)

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DGLAP evolution

• Used by Pythia, Rapgap etc.
• Sums over ln(Q2) terms
• Ordering on virtuality of propagators, ie kt and
  x of emitted partons

Kt ordering
12
BFKL evolution

• No exact Monte Carlo implementation, similar to
  CDM (in 2 slides)
• Sums over ln(1/x) terms (instead of ln Q2)
• Strong ordering in x, random walk in kt
• More energetic forward jets (low-x region)
• Less energy-angle correlation

13
CCFM evolution

• Implemented in CASCADE
• Based on kt factorization
• Angular (instead of kt) ordering
• Uses unintegrated PDFs
• Similar to DGLAP at high Q2, BFKL at low-x (ideal
  case?)

14
Color Dipole Model (CDM)

• Implemented in ARIADNE (presently not available
  in Atlas)
• Color dipoles created between colored objects
  decay into gluons, that create more diples etc.

Random walk in kt
MonteCarlo implementation of BFKL
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Observables

• Differences are mainly visible in forward jets,
  the most far away from the (common) matrix
  element.
• Most sensitive region low-x and low-Q2
• Ex. HERA

x
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Observables at LHC (from CMS)
Very sensitive to 1/x -gt E much more
discriminating than Et
Et
Knutteson, Hera4LHC workshop
Health warning Ariadne contains no q?gg
splitting. Questionable suitability for LHC
studies!
E