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Krzysztof Piotrzkowski Université Catholique de Louvain. High energy photon interactions @ LHC. LHC as a high energy gg and g p collider Tagging photoproduction at LHC Benchmark processes in gg and g p Summary/Outlook.

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high energy photon interactions @ lhc

Krzysztof Piotrzkowski

Université Catholique de Louvain

High energy photon interactions @ LHC

LHC as a high energy gg and gp collider

Tagging photoproduction at LHC

Benchmark processes in gg and gp


On behalf of the Louvain Photon GroupJ. de Favereau, V. Lemaître, Y. Liu, S. Ovyn, T. Pierzchała, KP and X. Rouby


LHC as a High Energy gg Collider



Phys. Rev. D63 (2001) 071502(R)


  • Highlights:
  • gg CM energy W up to/beyond 1 TeV (and under control)
  • Large photon flux F therefore significant gg luminosity
  • Complementary (and clean) physics to ppinteractions, eg studies of exclusive production of heavy particles might be possible opens new field of studying very high energy gg (and gp) physics


This is NOT meant for studying all photon interactions at the LHC but those for which the QCD background is strongly suppressed, as for example in the exclusive production of leptons or gauge bosons.

K. Piotrzkowski - UCLouvain


How measure these events?



scattered p


p beam

Measure (gg) X in the CMS or ATLAS detector and the scattered protons using very forward detectors..

..i.e. ‘Roman pot’ detectors

put as far (> 100 m) from the IP

and as close to the beam (2 mm)

as possible

K. Piotrzkowski - UCLouvain


Forward proton trajectories (for high luminosity optics)

J.de Favereau & X.Rouby

K. Piotrzkowski - UCLouvain

proton detectors at 220m from ip5 totem
Proton Detectors at 220m from IP5: Totem

…or, Proton Detectors at 420m from IP1/5: FP420

See talk by B.Cox


~ 7 m

See talk by J.Whitmore

K. Piotrzkowski - UCLouvain


gp interactions @ LHC – super HERA @ CERN

Photon-proton interactions can also be tagged at the LHC; and have significantly higher energy reach and luminosity yield than for gg events

  • Example assumptions:
  • 0.01 < x1 < 0.1, photon tagging range
  • 0.005 < x2 < 0.3, Bjorken-x range for partons

+ use MRST2001 (at Q2=104 GeV2) for proton pdf

Ex:Photon-gluon luminosity



dspp/dW = Ssgg/q

spp= dWSsgg/q

K. Piotrzkowski - UCLouvain


Selecting photon-proton events at low luminosity

At LHC luminosity up to 1033 (small event pileup) rapgap signature can be used:

For example, forward energy

flows (into 5>|h|>3) for two

hemispheres (red-photon and

blue-proton) is compared for

associated WH photo-


Finally, exclusivity cuts can be applied, by requiring of just two opposite tracks, for example…


K. Piotrzkowski - UCLouvain


Exclusive lepton pairs


Done at HERA, being done at Tevatron!

Key signature:

Acoplanarity angle for dileptons

Calibration process both for luminosity and energy scales, has striking signatures and can be well triggered and reconstructed by central detectors alone

gg  mm


DY:qq mm

K. Piotrzkowski - UCLouvain


T. Pierzchała

(using Calchep)

K. Piotrzkowski - UCLouvain


T. Pierzchała

(using Calchep)


K. Piotrzkowski - UCLouvain

single w photoproduction studied at hera
Single W photoproduction: Studied at HERA

Diener et al.

cf. PLB 471 (2000) 411

K. Piotrzkowski - UCLouvain


Higgs and top photoproduction

Associated photoproduction of WH has significant cross-section at LHC

and much better signal-to-background ratio; low mass region should be accesible.

Important backgrounds to WH deserve their own studies, as photo-production of top pairs, WZ, or single top (as at HERA):

Single top production (gq t) could be used to study anomalous gqt coupling; level of Wq background (~ 100 fb/GeV) indicates possibility of significant improvement wrt HERA

K. Piotrzkowski - UCLouvain

inclusive pp vs photo production examples

Needs large suppression, more difficult at high luminosities

Q1: Can one select photon-induced events among very many pp interactions?

Inclusive (pp) vs. photo-production: Examples

Process/s[pb] pp gg or gp

WW ~ 70 0.2

tt ~ 600 1.5

WH ~ 1.2 0.03

Q2: What about diffractive irreducible backgrounds?

A: For above, only important for top photo-production (similar size)

K. Piotrzkowski - UCLouvain

anomalous quartic gauge couplings
Anomalous quartic gauge couplings

K. Piotrzkowski - UCLouvain


(suppressed by 10-3)

assuming no background

gg ZZ

Two-photon production of W and Z boson pairs at LHC is ideal to study quartic gauge couplings

a0W, acW, a0Z, acZ

(LEP limits are poor due to limited phase space)

  • Should be possible to detect these events (esp. fully leptonic decays) even at highest pp luminosities

K. Piotrzkowski - UCLouvain


Exclusive two-photon pair production

Large cross-section for WW pairs extending to very high invariant masses – possibility of searches for physics BSM

T. Pierzchała (using Calchep)

Muon pairs produced with invariant masses beyond 100 GeV, hence pairs of massive charged particles could be searched for

K. Piotrzkowski - UCLouvain


SUSY: chargino case

Simple production mechanisms and decay modes:

Charged Leptons


Missing energy

WW is background

K. Piotrzkowski - UCLouvain



  • High-energy (at electroweak scale and beyond) photon interactions have significant cross-sections at the LHC!
  • Tagging high energy photon (and diffractive) interactions at LHC can be done by supplementing central detectors with very forward spectrometers.
  • This offers new, exciting and complementary physics studies in parallel to ‘nominal’ ones
  • We should make sure that from Day 1 of LHC running triggers and selection algorithms for exclusive events in pp are in place…
  • We must make the best of the LHC!

K. Piotrzkowski - UCLouvain