High energy photon interactions @ LHCKrzysztof Piotrzkowski

Université Catholique de Louvain

• LHC as a high energy and p collider

• Tagging photoproduction at LHC

• Benchmark processes in and p

• Summary/Outlook

On behalf of the

Louvain Photon Group J. de Favereau, V. Lemaître, Y. Liu, S. Ovyn, T. Pierzchała, KP and X. Rouby

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

LHC as a High Energy Collider

pp

Phys. Rev. D63 (2001) 071502(R)hep-ex/0201027

DISCLAIMER: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.

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How measure these events?

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

pp

RP

p beam

scattered p

..i.e. ‘Roman pot’ detectorsput as far (> 100 m) from the IPand as close to the beam (2 mm)as possible

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Forward proton trajectories (for high luminosity optics)

J.de Favereau & X.Rouby

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Beampipes

~ 7 m

Proton Detectors at 220m from IP5: TotemSee talk by J.Whitmore

See talk by B.Cox

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

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p 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 events

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

S(W)=f(x1)fp(x2)

W2=4Epx1x2

dpp/dW = S g/q

pp= dW S g/q

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

Ex:Photon-gluon luminosity

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

For example, forward energyflows (into 5>||>3) for twohemispheres (red-photon and blue-proton) is compared forassociated WH photo-production

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

Selecting photon-proton events at low luminosity

S.Ovyn

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Exclusive lepton pairs

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

known

DY:qq

Key signature:Acoplanarity angle for dileptons

Done at HERA, being done at Tevatron!

Y.Liu

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T. Pierzchała(using Calchep)

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T. Pierzchała(using Calchep)

Minimal

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Single W photoproduction: Studied at HERA

cf. PLB 471 (2000) 411Diener et al.

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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 (q t) could be used to study anomalous qt coupling; level of Wq background (~ 100 fb/GeV) indicates possibility of significant improvement wrt HERA

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Inclusive (pp) vs. photo-production: Examples

Process/[pb] pp or p

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)

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

Needs large suppression, more difficult at high luminosities

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Anomalous quartic gauge couplings

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ZZ

Two-photon production of W and Z boson pairs at LHC is ideal to study quartic gauge couplings a0

W, acW, a0

Z, 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

assuming no background

(suppressed by 10-3)

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T. Pierzchała (using Calchep)

Exclusive two-photon pair production

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

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

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~

~

Simple production mechanisms and decay modes:

~ ~ 0

Charged Leptons

and

Missing energy

WW is background

SUSY: chargino case

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• 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!

Summary/Outlook