Photon-initiated processes at ATLAS

Transcription

1 Photon-initiated processes at ATLAS Stewart Martin-Haugh Rutherford Appleton Laboratory 4 April 2018 Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 1 / 38

2 Introduction LHC is a hadron collider Protons are also electrically charged photon-related effects Clean environments to probe soft QCD, QED Also sensitive to electroweak physics (not covered today) Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 2 / 38

3 Light-by-light scattering: theory Maxwell equations are linear in sources and fields: superposition principle Purely quantum effect: self-interaction of photons requires loops First calculation: Euler-Heisenberg Lagrangian, 1935 (1st order) L = 1 ( E 2 B 2) [ + 2α2 (E m 4 B 2) ] (E B) Full calculation 1951 Karplus and Neuman Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 3 / 38

4 Muon g 2: theory = ge e S Electron magnetic moment (from Dirac equation): M me Parameterise deviation ae = (ge 2)/2 = α/2π + O(α2 ) Classic quantum electrodynamics prediction ae agrees with QED calculation to aµ affected by QCD including light-by-light scattering Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 4 / 38

5 Muon g 2: experiment BNL E761 experiment a exp µ = ± 5.4 (stat.) ± 3.3 (syst.) a SM µ = ± 1 ± 34 ± σ deviation from SM! New measurement in progress at Fermilab Large contribution from light-by-light scattering Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 5 / 38

6 Light-by-light scattering at ATLAS (Nature Physics) First evidence for light-by-light scattering Possible at very intense lasers: not yet seen Very clean events: only see two photons, lead ions remain intact Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 6 / 38

7 Heavy ion collisions Low instantaneous luminosity (µ < 0.005) Select ultra-peripheral collisions 480 µb 1, s NN = 5.02 TeV Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 7 / 38

8 Light-by-light scattering in heavy ion collisions Electric field of colliding nucleus depends on Z, γ, impact parameter b Z = 82: prefer lead to protons E max Zeγ b 2 up to E max = V m 1 in LHC heavy ion environment cf Schwinger limit (point for quantum effects to become important: E S = m2 e c3 eħ V m 1 Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 8 / 38

9 Equivalent photon approximation (EPA) Fermi, Weiszsäcker and Williams EM field of highly relativistic charged particle can be approximated by photons Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 9 / 38

10 Equivalent photon approximation (EPA) Cross section is convolution of γγ γγ with equivalent photon fluxes σ A1 A 2 A 1 A 2 X ( sa1 A 2 ) b 1/2 : effective impact parameter from each nucleus ω 1/2 : photon energy N((ω 1, b 1 ): equivalent photon flux, depends on nucleus form factors S abs (b): survival probability for each nucleus W γγ, Y γγ : diphoton invariant mass and mean rapidity = See Kusek-Gawenda, Lebiedowicz, Szczurek σ γγ γγ ( sa1 A 2 ) N (ω 1, b 1 ) N (ω 2, b 2 ) S 2 abs (b) 2πbdb db x db y W γγ 2 dw γγ dy γγ Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 10 / 38

11 Event display Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 11 / 38

12 Event selection Require 5 GeV <= Level 1 trigger EM energy <= 200 GeV Veto events with > 10 hits in Pixel detector or > 1 hit in Minimum Bias Trigger Scintillators, or tracks with p T > 100 MeV Exactly two unconverted photons, m γγ > 6 GeV, p γγ T < 2 GeV Acoplanarity requirement on photon clusters: 1 ϕ(cl 1, cl 2 )/π < 0.01 Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 12 / 38

13 Photon reconstruction Require unconverted photons (suppress electron background) E T > 3 GeV, η γ < 2.4 PID eff. from FSR Reco. eff. from γγ e + e + hard brem Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 13 / 38

14 Backgrounds Background gg γγ CEP γγ e + e production γγ qq including π 0 from dijets UPC with 2 brem γ Mitigation track/mbts hit veto & data-driven normalisation track veto, unconverted photons only Negligible Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 14 / 38

15 Monte Carlo samples Process γγ γγ CEP gg γγ γγ e + e γγ qq Generator Dedicated calculation EPA spectrum from SuperChic2 SuperChic2 STARlight Herwig++ (normalised to Pb-Pb yields) Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 15 / 38

16 γγ e + e background estimation MC prediction validated in regions with 2 photons and 1 or 2 additional tracks Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 16 / 38

17 Results σ fid = N data N bkg C Ldt σ fid = 70 ± 24 (stat.) ± 17 (syst.) nb Theory predictions: 45 ± 9 nb d Enterria & Silveira 49 ± 10 nb Kusek-Gawenda, Lebiedowicz, Szczurek Slight excess, well within uncertainties Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 17 / 38

18 Uncertainties Statistics-limited: Factor of 2 more Pb-Pb data in 2018 Expect more signal at lower photon E T : reconstruction challenge Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 18 / 38

19 Statistical interpretation 4.4 σ observed, 3.8 σ expected Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 19 / 38

20 Excluding Born-Infeld models Born-Infeld (1934) theory: early alternative to QED, influential in QFT formulation, of interest to string theory Predicts enhanced light-by-light scattering Significant exclusion (Ellis, Mavromatos, You) from ATLAS result β = mass 2 free parameter L SM = 1 4 F µνf µν L BI = β 2 (1 ) β 2 F µνf µν 1 16β F 4 µν F µν Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 20 / 38

21 Exclusive di-muon production Photon-initiated, protons stay intact Main backgrounds: single and double-dissociative production σpp(γγ) l EPA + l pp = P(x 1 )P(x 2 )σ γγ l+ l (m2 l + l )dx 1dx 2 x 1, x 2 : fractions of proton momentum carried by photons P(x 1 ), P(x 2 ): equivalent photon spectra for each proton m l+ l : lepton pair invariant mass (note m l + l /s = x 1x 2 ) Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 21 / 38

22 Absorptive effects Proton is not a point particle: QCD effects Can use measurement to probe structure of proton Model in two ways 1. Finite-size parameterisation: assume r p = 0.64 fm, simple exponential form for survival factor 2. SuperChic2: full amplitude calculation Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 22 / 38

23 Event selection Search for dimuon pairs associated to a 2-track (p T > 400 MeV) vertex Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 23 / 38

24 Event display Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 24 / 38

25 Pileup Require 2-track vertex > 1 mm from any other vertex, decreases signal efficiency with increasing pileup Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 25 / 38

26 Trigger strategy Low mass trigger(10 < m µµ < 30 GeV): require p µµ t < 2 GeV, di-muon vertex z 10 mm from event primary vertex OR with looser (Drell-Yan oriented) triggers to enhance statistics Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 26 / 38

27 Monte Carlo samples Process Generator Exclusive γγ µµ Herwig 7 Single-dissociative γγ µµ LPAIR JETSET Double-dissociative γγ µµ PYTHIA 8 + NNPDF2.3QED Z /γ µµ, Z /γ ττ PowhegBox v2 + CT14 + PYTHIA 8 + CTEQ6L1 Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 27 / 38

28 Backgrounds Require 12 < m µµ < 70 GeV: remove on-peak Z production Reweight DY charged-particle spectrum to match data Single-dissociative production dominant Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 28 / 38

29 Results Final cross section using binned maximum likelihood fit to dimuon acoplanarity Signal, SD and DD contributions, DY allowed to float Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 29 / 38

30 Results Measured Herwig (bare EPA) Finite-size parameterisation SuperChic2 σexcl. fid. γγ µµ = 3.12 ± 0.07 (stat.) ± 0.14 (syst.) pb σγγ µµ EPA = 3.56 ± 0.05 pb EPA, corr. σγγ µµ = 3.06 ± 0.05 pb σγγ µµ SC2 = 3.45 ± 0.05 pb Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 30 / 38

31 Results Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 31 / 38

32 Photon PDFs Photon PDFs key for LHC physics including e.g. WH production Recent progress from LUXQED collaboration: start with cross section for heavy neutral lepton production σ lp L+X (f γ ) = σ lp L+X (F 2, F L ) Write f γ in terms of structure functions (well-measured from ep scattering), obtain f γ to O(α 2 ) Beyond current γγ ll precision LUXQED uncertainties Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 32 / 38

33 Other measurements γγ ll in heavy ions: observed by ATLAS, ALICE γγ WW in pp: evidence from ATLAS and CMS γγ H: ATLAS and CMS searches [fb] σ tot [fb/gev] or dσ/dw γγ s = 13 TeV - γγ µ + µ + γγ W W γγ γγ γγ H (total) [GeV] Dyndal, Schoeffel W γγ Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 33 / 38

34 The detector that never was: FELIX Unsuccessful proposal: Forward ELastic and Inelastic experiment at the LHC Interesting idea: bend slightly scattered protons out of beam for measurement, re-insert Webpage archived Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 34 / 38

35 Exclusive physics with proton taggers TOTEM + CMS, ATLAS Forward Proton Tagger (AFP) HL-LHC: ATLAS High Granularity Timing Detector Associate transverse physics with scattered protons in forward region Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 35 / 38

36 Conclusions First evidence for light-by-light scattering Precise measurement of exclusive dimuon production Scope for complementary future measurements of Higgs and W production Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 36 / 38

37 Backup: acoplanarity tuning for light-by-light scattering Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 37 / 38

38 Backup: dilepton triggers Use dedicated low-mass trigger HLT 2mu6 10invm30 pt2 z10 with cuts tuned for elastic dimuon production, OR with low-mass Drell-Yan triggers to increase statistics mµµ < 30 GeV: HLT 2mu6 10invm30 pt2 z10 HLT mu6 iloose mu6 11invm24 noos HLT mu6 iloose mu6 24invm60 noos mµµ > 30 GeV: HLT 2mu10 Stewart Martin-Haugh (RAL) Photon-initiated processes at ATLAS 38 / 38