Photon-photon collisions at the LHC Lucian Harland-Lang, University College London IPPP seminar, Durham, 6 Oct 2016 In collaboration with Valery Khoze and Misha Ryskin 1
Outline • Motivation: why study collisions at the LHC? γγ • Exclusive production: ‣ How do we model it? ‣ Example processes: lepton pairs, anomalous couplings, light-by-light scattering, axion-like particles. ‣ Outlook. • Inclusive production: ‣ How well do we understand it? ‣ Connection to exclusive case- precise determination. ‣ Predictions for LHC/FCC. ‣ Comparison to LUXqed. 2
The proton and the photon • The proton is an electrically charged object- it can radiate photons. p p p → As well as talking about quarks/gluons in the initial state, we should consider the photon. • How large an effect is this? Where is it significant? Can it be a background to other processes? How can we exploit this QED production mode? 3
Why bother? • In era of high precision phenomenology at the LHC: NNLO calculations rapidly becoming the ‘standard’. However: S ( M Z ) ∼ 0 . 118 2 ∼ 1 1 α 2 α QED ( M Z ) ∼ 70 130 → EW and NNLO QCD corrections can be comparable in size. • Thus at this level of accuracy, must consider a proper account of EW corrections. At LHC these can be relevant for a range of W , Z , WH , ZH , WW , tt , jets... processes ( ). • For consistent treatment of these, must incorporate QED in initial state: photon- X R initiated production. 4
Why bother? • Unlike the quarks/gluons, photon is colour-singlet object: can naturally lead to exclusive final state, with intact outgoing protons. • Exclusive photon-initiated processes of great interest. Potential for clean, almost purely QED environment to test electroweak sector and probe possible BSM signals. • Protons can be measured by tagging detectors installed at ATLAS/ CMS. Handle to select events and provides additional information. X R . . . . . . . . . . . . . . . . 3 5 . . . . . . . . . . . . . . . . . . . .
Exclusive production 6
Central Exclusive Production Central Exclusive Production (CEP) is the interaction: pp → p + X + p • Diffractive: colour singlet exchange between colliding protons, with large rapidity gaps (‘+’) in the final state. • Exclusive: hadron lose energy, but remain intact after the collision. • Central: a system of mass is produced at the collision point and M X only its decay products are present in the central detector. . . . . . . . . . . . . . . . . . . . . . . 7 . . . . . . . . . . . . . . . . . . . .
Selecting exclusive events • Exclusive final states can be selected in two ways: ‣ Measuring intact protons with purpose-built detectors purely ⇒ exclusive signal. ‣ Demanding no additional hadronic activity in large enough rapidity region. Some BG from events where proton breakup occurs outside veto region, but generally under control and can subtract. • Latter possible at all LHC experiments. Common method - charged final state ( ) and veto on extra tracks. l + l − , W + W − ... • Former also possible at LHC - proton tagging detectors installed at from O (100 m) ATLAS/CMS interaction points (AFP, CT-PPS). 8
Production mechanisms Exclusive final state can be produced via three different mechanisms, depending on quantum numbers of state: C-even, couples to gluons Gluon-induced f g ( x 1 , · · · ) p 1 (double pomeron exchange): x 1 Q ⊥ X S eik S enh Couples to photons x 2 p 2 f g ( x 2 , · · · ) Photon-induced C-odd, couples to photons + gluons V ( z, k ? ) ( z, ~ k ? ) Q V M = J/ , 0 , Υ , Υ 0 , . . . production via QCD (left) and photon � ¯ Q (1 � z, � ~ k ? ) Photoproduction � ~ ~ W 2 p p 9 F ( x, ) = @ G ( x, ) / @ log 2
SuperChic • Have developed a MC for a range of CEP processes, widely used for LHC analyses. Available on Hepforge: 22 /50 c) p p � p+ � � + p 20 � Events per 18 Data SuperCHIC MC 16 (Normalized to data) 14 12 10 8 6 4 2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 | - | (rad) � � � 10
Modelling exclusive collisions γγ • In exclusive photon-mediated interactions, the colliding protons must both coherently emit a photon, and remain intact after the interaction. How do we model this? • Answer is well known- the ‘equivalent photon approximation’ (EPA): cross section described in terms of a flux of quasi-real photons radiated from the proton, and the subprocess cross section. γγ → X PHYSICS REPORTS (Section C of Physics Letters) 15, no. 4 (1975) 181—282. NORTH-HOLLAND PUBLISHING COMPANY THE TWO-PHOTON PARTICLE PRODUCTION MECHANISM. PHYSICAL PROBLEMS. APPLICATIONS. EQUIVALENT PHOTON APPROXIMATION V.M. BUDNEV, I.F. GINZBURG, G.V. MELEDIN and V.G. SERBO USSR Academy of Science, Siberian Division, Institute for Mathematics, Novosibirsk, USSR Received 25 April 1974 Revised version received 5 July 1974 .4 bstract: This review deals with the physics of two-photon particle production and its applications. Two main problems are discussed first, what can one find out from the investigation of the two-photon production of hadrons and how, and second, how can the two-photon production of leptons be used? The basic method for extracting information on the -y-y h (hadrons) transition the ee eeh reaction is discussed in detail. 11
Equivalent photon approximation • Initial-state emission can be to very good approximation p → p γ factorized from the process in terms of a flux: γγ → X q 2 d 2 q i ⊥ i ) + x 2 ✓ ◆ n ( x i ) = 1 Z α i ⊥ (1 − x i ) F E ( Q 2 2 F M ( Q 2 i i ) q 2 i ⊥ + x 2 q 2 i ⊥ + x 2 π 2 i m 2 i m 2 x i p p • Cross section then given in terms of `luminosity’: γγ d L EPA = 1 γγ s n ( x 1 ) n ( x 2 ) d M 2 X d y X with d L EPA d σ pp → pXp γγ σ ( γγ → X ) ˆ X R ∼ d M 2 d M 2 X d y X X d y X Not exact equality: see later 12
Proton form factors • Where does photon flux come from? Consider e.g. elastic scattering: ep M ∼ l µ H µ proton rest frame • Most general form for hadronic current is � γ µ F 1 ( Q 2 ) + i σ µ ν q ν H µ = eP ( p 0 ) F 2 ( Q 2 ) P ( p ) 2 m p F 1 ( Q 2 ): ‘Dirac’ form factor, proton spin preserved F 2 ( Q 2 ): ‘Pauli’ form factor, proton spin flipped 13
Proton form factors G E = F 1 − Q 2 • Defining: G M = F 1 + F 2 F 2 4 m p get well known ‘Rosenbluth’ formula: d σ ep → ep 2 + Q 2 ✓ ◆ F E ( Q 2 ) cos 2 θ F M ( Q 2 ) sin 2 θ ∝ dcos θ 2 m 2 2 p F E ( Q 2 ) = 4 m 2 p G 2 E ( Q 2 ) + Q 2 G 2 M ( Q 2 ) F M ( Q 2 ) = G 2 M ( Q 2 ) where 4 m 2 p + Q 2 • Here are the proton electric/magnetic form factors the G E /G M ∼ Fourier transform of the charge/magnetic moment distribution within proton. 14
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