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Krzysztof Piotrzkowski. High Energy Photon Interactions at the LHC. Introduction: LHC as a high energy gg and g p collider and tagging with forward protons Gauge boson photoproduction Luminosity measurement with lepton pairs Associated Higgs photoproduction Comments & Outlook.

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high energy photon interactions at the lhc

Krzysztof Piotrzkowski

High Energy Photon Interactions at the LHC

Introduction: LHC as a high energy gg and gp collider and tagging with forward protons

Gauge boson photoproduction

Luminosity measurement with lepton pairs

Associated Higgs photoproduction

Comments & Outlook

Team: J. de Favereau, O. Militaru, S.Ovyn, T. Pierzchala, X. Rouby and M. Vander Donckt


LHC as a High Energy gg Collider



  • 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

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



Provided efficient measurement of forward-scattered protons one can study high-energy gg collisions at the LHC

K.Piotrzkowski - Louvain University



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.

This IS meant for studying production of selected

final states in photon interactions at the LHC.

K.Piotrzkowski - Louvain University


Kinematics/gg Luminosity

VirtualityQ2 of colliding photons vary between kinematical minimum = Mp2x2/(1-x) where x is

fraction of proton momentum carried by a photon,

and Q2max ~ 1/proton radius2

W2 = s x1 x2

Photon flux 1/Q2

Q2 - Q2min sq2/4

for x>0.0007, Q2<2GeV2

protons scattered at

`zero-degree’ angle

Use EPA à la Budnev et al.*

* error found in the elastic (Q2 integrated) g flux for protons!

dWSgg =‘gg : pp luminosity’

Note: it’s few times larger if one of protons is allowed to break up

K.Piotrzkowski - Louvain University


Problem: Same signature (one or two very forward protons) has

also central diffraction (i.e. pomeron-pomeron scattering) in strong


Two processes might interfere, however the transverse momentum of the scattered protons are in average much softer in

two-photon case

Q2 < 0.01 GeV2

a) `true’ distributions; b) distributions smeared due to beam intrinsic pT; all plots normalized for pT2 < 2 GeV2

Assuming ultimate pT

resolution  100 MeV; i.e.

neglecting detector effects

pT gives powerful separation handle provided

that size of gg and pomeron-pomeron cross-sections are not too different

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gg Physics Menu - Highlights


s ~ 80 pb


s ~ 10 pb (at W=MH=200 GeV)

All ?


s ~ 1 pb

s ~ 100-500 nb

+ SUSY processes

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Significant production rates and very clean signatures available - both transverse and longitudinal missing


for single tags

  • H/A bosons -ggbbH/A?
  • Surprises???

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photia development

J. de Favereau

  • Use Pythia6.152 to simulate ggandgpinteractions
  • Introduce photon spectra for proton beams (forggand gpcase) with proper normalization (pp cross-section calculations)
  • Elastic and inelastic production possible (but no p dissociation simulation yet)
  • Direct photo-production
  • Run Photia with Oscar/Orca

K.Piotrzkowski - Louvain University

cms efficiency
CMS efficiency

J. de Favereau

First look at leptonic Final states : p -> XW -> l

Efficiency at L1 and HLT (ORCA 8_1_2)

L1 : 48 %

HLT : 29 %

! No endcap muons in orca : HLT efficiency is underestimated !


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anomalous ww production ii



J. de Favereau & T. Pierzchala

ds/dpT [pb/GeV]

Ex.: gg at 1 TeV


pT [GeV]

Anomalous WW production (II)

At a generator level so far:

  • k changes mostly normalization, l also affects the pT shape
  • From statistical point of view better than LEP2 limits seems feasible
  • Looking also into a single W photoproduction

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= x

K.Piotrzkowski - Louvain University



= x

K.Piotrzkowski - Louvain University



ds/dpT at W=300 and 800 GeV

SM ac=10-6GeV-2ac=4.10-6GeV-2

ac=10-6GeV-2 ac=4.10-6GeV-2

K.Piotrzkowski - Louvain University

gauge boson photoproduction
Gauge boson photoproduction
  • Hope for large sensitivity in QGC, will study WW anomalous production for LED and strong W sector

gg  gg (also not possible at tree level), eg. sensitivity to massive monopole contributions (large pT physics)

gg  ZZ suppressed in SM (~10-3), good place to look for BSM

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= x

K.Piotrzkowski - Louvain University


Precise luminosity measurements at hadron

colliders: QED at work

Old and successful idea – use

a process with the precisely

known cross-section and measure

its rate – in 1973 Budnev et al.

proposed two-photon lepton pair

production for pp collisions

At the QED only level (protons are point-like charges) the calculations are as precise as for Bhabha scattering, for example, and if the pair pT is small the hadronic corrections are negligible, at 10-4 level – Khoze et al. EPJ C19 (2001) 313.

In the gg l+l- case the precision is limited by the

experimental uncertainties!

K.Piotrzkowski - Louvain University

large cross section clear signatures

Main features of the two-photonprocess

Large cross-section + clear signatures

Example forelectron energy: i=15GeV

photon energy: 1, 2me2/ (5GeV, 50eV)

forward production dominates

pair mass: W2me (< 20MeV)

pair pT: PTme (< 20MeV)

single electron pT: pT(1),pT(2)me

acoplanarity angle:   PT/pT

leptons are back-to-back

And nothing else produced!

K.Piotrzkowski - Louvain University

forward e e pairs in pp
Forward e+e-pairs in pp

Most important distributions for the signal and the backgrounds

KP & A. Shamov ‘99:

K.Piotrzkowski - Louvain University

luminosity with very forward pairs in castor
Luminosity with very forward pairs in CASTOR

QED process for luminosity monitoring

(KP & D.Bocian)

pp  pp e+e-

Electrons are

in forward range

5 <  < 7

Cross section ~barns

EM calorimetry essential

Tracking important (T2?)

Can get the luminosity to 1-2%?

 small angles of few mrad

 high rates of few kHz

s varies strongly with pair energy


K.Piotrzkowski - Louvain University


Forward e+e- pairs in CASTOR in ion collisions at the LHC

Recent studies: published in Acta Phys. Pol. B.

Use modified LPAIR (ME generator of

lepton pairs by Vermaseren), i.e.

Introduce new type of projectile,

so far only elastic (form-factor)

High rates for e+e- pairs within the CASTOR acceptance

Excellent candidate for the luminosity measurement for ion collisions

K.Piotrzkowski - Louvain University

pp luminosity
pp luminosity

Try to get first estimate on the systematics: sensitivity to energy scale

+ resolutions

Energy resolution seems to be not too critical (D. Bocian)

K.Piotrzkowski - Louvain University


Another possibility


Measure centrally produced pairs in a given energy range

Needs central triggering, hence large lepton pT

Cross-section is small – could be used for making final Luminosity calibration


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Calibration&monitoring tool:

two-photon ee/mm production

Setting W scale + background control + elastic/inelastic separation, and RP luminosity measurement

Elastic di-muons

Single p tags

(ie. x>0.01) +

pT(m)>5 GeV


m-pair invariant mass

Note: It does not assume proton trigger!

K.Piotrzkowski - Louvain University


Tagging gp interactions - super HERA at CERN

Given tagging capability an exciting possibility of measuring photon-proton interactions at the LHC opens up:

- significantly higher energy reach and luminosity yield than for gg events is expected

  • Initial assumptions:
  • 0.01 < x < 0.1, photon tagging range
  • 0.005 < x < 0.3, Bjorken-x range for quarks and gluons
          • (arbitrary for the moment, could be extended)

+ use MRST2001 (at Q2=104 GeV2) for partons

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Photon-gluon luminosity spectra

Note: at Wgg > 400 GeV photon-gluon luminosity is about 10% of the

nominal pp

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Photon-quark luminosity spectra

Note: at Wgq > 300 GeV photon-quark luminosity is about one third of the

nominal pp (and still significant beyond 1 TeV)

K.Piotrzkowski - Louvain University


gp Physics Menu - Highlights

s ~ 25 pb at W=400 GeV


s ~ 40 pb for gq Wq at W > 200 GeV


  • • anomalous W and Z production at Wgq  1 TeV
  • top pair production - top charge + mass determination?
  • single top production and anomalous Wtb vertex
  • SM BEH - for example,g b  H b, g q H W q
  • SUSY studies (complementary to the nominal ones) -
  • H+ t production (and H++), b and t spairs, t~c pair, ...
  • Exotics: compositness, excited quarks, ...

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already studied at hera

Diener et al.

Already studied at HERA

cf. PLB 471 (2000) 411

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comments outlook

Working position


Detectors a few m apart


Exciting physics prospects for g interactions @ LHC; RP detectors

Essential – full FS reconstruction, background control, trigger, etc.

420m important too, mostly to increase acceptance/event statistics

Note: HERA tagging method ‘without’ RP! Routinely used at high L…

Parking position

RF screen

Note: This is an efficient method for non-elastic protons (f!)

K.Piotrzkowski - Louvain University