Photon-photon collisions at the LHC Lucian Harland-Lang, University - PowerPoint PPT Presentation

Photon-photon collisions at the LHC Lucian Harland-Lang, University College London UK HEP forum, Cosener’s House, 3 Nov 2016 1

Outline • Motivation: why study collisions at the LHC? γγ • Exclusive production: ‣ How do we model it? ‣ How do we measure it? ‣ Example processes: lepton pairs, anomalous couplings, light-by-light scattering, axion-like particles and massive resonances. ‣ Outlook - tagged protons at the LHC. • Inclusive production: ‣ How well do we understand it? ‣ Connection to exclusive case- precise determination. ‣ Predictions for LHC/FCC. 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 is it interesting? • 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 processes ( ). W , Z , WH , ZH , WW , tt , jets... • For consistent treatment of these, must incorporate QED in initial state: photon- X R initiated production. 4

Why is interesting? • 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 . . . . . . . . . . . . . . . . . . . .

Production mechanisms Exclusive final state can be produced via three different mechanisms, depending on kinematics and quantum numbers of state: C-even, couples to gluons Gluon-induced f g ( x 1 , · · · ) p 1 x 1 Q ⊥ X S eik S enh Couples to photons x 2 p 2 f g ( x 2 , · · · ) C-odd, couples to photons + gluons V ( z, k ? ) ( z, ~ k ? ) Q Photon-induced V M = J/ , 0 , Υ , Υ 0 , . . . production via QCD (left) and photon � ¯ Q (1 � z, � ~ k ? ) � ~  ~  W 2 Photoproduction p p 8 F ( x,  ) = @ G ( x,  ) / @ log  2

Selecting exclusive events 1) Gap-based selection: no extra activity in large enough rapidity region. ‣ No guarantee of pure exclusivity - BG with proton breakup outside veto region. Large enough gap BG small and can be subtracted. ⇒ ‣ Pile-up contaminating gap? Either: low pile-up running (dedicated runs/ LHCb defocussed beams) or can veto on additional charged tracks only l + l − , W + W − (already used to select charged - -by ATLAS/CMS/LHCb). VETO VETO . . . . . . . . . . . . . . . . 3 9 . . . . . . . . . . . . . . . . . . . . . . . . . .

Selecting exclusive events pp → p + X + p 2) Proton tagging: • Defining feature of exclusive events: protons intact after collision, → If we can measure the outgoing protons, possible to select purely exclusive event sample. • Basic principle: use LHC beam magnet as a spectrometer. After interaction protons have and will gradually bend out of E < √ s/ 2 beam line. • Insert ‘roman pot’ detectors at from beam line and O (100 m) O (mm) from IP. Reconstruct momenta and measure arrival time of protons. 10

Proton tagging at the LHC • These detectors are installed: ‣ CMS-TOTEM Precision Proton Spectrometer - CT-PPS. ‣ ATLAS Forward Proton - AFP. • • In both cases ‘roman pot’ detectors installed at from IPs. ∼ 200 m • Measure position ( proton momentum loss) and arrival time ∼ • ( pile-up rejection) of protons. → � � • In early stages of data taking. In 2017 will both be fully ready to • take data during normal LHC running. � � � � 220m 215m 204m ! IP5 • 2 new horizontal 2 horizontal box-shaped RPs cylindrical RPs (1 in LS1) 11

Timing and pile-up rejection ~5 cm N. Cartaglia, INFN, April 2015 • Pile-up! At LHC expect interactions per bunch crossing: ∼ 50 ‣ If we measure two intact protons, which of these central interactions is the right one?? ‣ Probability for two protons from independent single-diffractive interactions ( ) is high. What about this BG? pp → p + X • Solution: fast timing detectors measure arrival time of protons → convert to expected position of central vertex. For ∼ 10 ps z precision can control pile-up BG. Achieved in current detectors with further improvements foreseen. 12

Mass acceptance • Momentum loss of protons related to mass of central system: ξ M 2 X = ξ 1 ξ 2 s • The acceptance is directly related to distance of the RPs from ξ d the IP: for have . ξ ↓ d ↑ → Decreasing leads to acceptance at larger . Turns out d M X • Detectors at d ∼ 400 m that for this gives . M X & 500 GeV d ∼ 200 m under discussion ⇒ pp→p+X+p, X→γγ covers . M X ∼ M h pp p JJ p, z=204m 215m → ⊕ ⊕ ⊕ 100 acceptance(%) simulation d=15 σ 90 CMS-TOTEM d=20 σ 80 70 14 TeV, β *=55 cm 60 50 40 30 20 10 0 500 1000 1500 2000 13 M (GeV) X

Production mechanisms Recall three production mechanisms: C-even, couples to gluons Gluon-induced f g ( x 1 , · · · ) p 1 x 1 Q ⊥ X S eik S enh Couples to photons x 2 p 2 f g ( x 2 , · · · ) C-odd, couples to photons + gluons V ( z, k ? ) ( z, ~ k ? ) Q Photon-induced V M = J/ , 0 , Υ , Υ 0 , . . . production via QCD (left) and photon � ¯ Q (1 � z, � ~ k ? ) � ~  ~  W 2 Photoproduction p p 14 F ( x,  ) = @ G ( x,  ) / @ log  2

f g ( x 1 , · · · ) p 1 vs. x 1 Q ⊥ X S eik S enh x 2 p 2 f g ( x 2 , · · · ) production via QCD (left) and photon • Naively expect strong interaction to dominate- . α S � α • However QCD enhancement can also be a weakness: exclusive event requires no extra gluon radiation into final state. Requires introduction of Sudakov suppressing factor: � µ 2 � 1 − ∆ d k 2 α s ( k 2 � ⊥ ) � � � � T g ( Q 2 ⊥ , µ 2 ) = exp ⊥ zP gg ( z ) + P qg ( z ) dz − k 2 2 π Q 2 0 ⊥ q ⊥ • Increasing larger phase space for extra gluon emission M X ⇒ stronger suppression in exclusive QCD cross section. Gluons like to radiate! 15

vs. gg γγ • Situation summarised in ‘effective’ exclusive and . gg γγ luminosities. This Sudakov suppression in QCD cross section leads to enhancement in already* for - well before M X & 200 GeV γγ CT-PPS/AFP mass acceptance region. → Can study collisions at the LHC with unprecedented . γγ s γγ pp p JJ p, z=204m 215m → ⊕ ⊕ ⊕ 100 acceptance(%) simulation d=15 σ 90 CMS-TOTEM p d=20 σ KMR-2001 80 p 70 60 50 40 30 20 10 0 500 1000 1500 2000 M (GeV) X *Caveat - this is enhancement in initial state only. 23 Of course depends on coupling to produced state. 16

Heavy ions • LHC is not just a proton-proton collider- in addition have heavy ions ( ) collisions. AA, Ap • On the face of it strange thing to consider for exclusive production… • However for heavy ion physics it is quite natural… 17

Heavy ions - ultra-peripheral collisions • Ions do not necessarily collide ‘head-on’ - for ‘ultra-peripheral’ collisions, with the ions can interact purely via EM and b > R 1 + R 2 remain intact exclusive -initiated production. γγ ⇒ • Ions interact via coherent photon exchange- feels whole charge ∝ Z 4 Z 4 ∼ 5 × 10 7 of ion cross section . For e.g. Pb-Pb have ⇒ enhancement! • Photon flux in ion tends to be cutoff at high , but potentially M X very sensitive to lower mass objects with EW quantum numbers. 18

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) � � � 19

collisions - theory γγ 20