Light-by-light scattering in ATLAS and CMS in Run2

Transcription

1 Light-by-light scattering in ATLAS and CMS in Run2 Mateusz Dyndal (CERN) on behalf of ATLAS and CMS collaborations Moriond E 6-23 Mar 209

2 Quasi-real photons from Pb N/dΔφ 7 Mar 209 M. Dyndal Light-by-light -scattering in ATLAS and CMS in Run2 2 ATLAS Preliminary Pb+Pb 205, 0.38 nb for signal events removed by the of the measurement. Event-level p T > 5 GeV and pseudo-rapiditi least one jet with p T > 20 GeV. T Boosted nuclei are intense source of (quasi-real) photons Equivalent photon flux Q ~ /R ~ 0.06 GeV snn = 5.02 TeV > Lorentz factor γ ~2700 Emax γ/r ~ 80 GeV Each flux scales with Z 2 Various types of interactions possible: Photon-pomeron (e.g. exclusive J/Psi) Photo-nuclear (e.g. photoproduction of jets) X H T p T i, m jets have i 0nX are o where i runs over the measured je and r respectively, and p z representsfor thes to be positive in the photon-going of th m jets, satisfies m jets > 35 GeV. p T > least The di erential cross-sections are [Fermi, Nuovo Cim. 2 (925) 43] jet variables: z In the limit of 2! 2 scattering wher kin in the nucleus to the (per nucleon) respe photon. For direct processes, (μ+) xto be is carried by the resolved partonmente jets, (μ-) The d The remainder of this note is stru triggers used for the measurement samples used in the analysis and is re-weighted Photon-photon for use in Pb+Pb c In th measurement of the photo-nuclear in the the systematic uncertainties, and photo S (e.g. dilepton production) presents the final results figures carri w 2

3 Motivation Light-by-light (γγ γγ) scattering Forbidden at tree-level Tested indirectly in electron/muon g-2 measurements Klusek-Gawenda et al. Another examples: SCATTERING PRC 93 (206) Delbruck scattering and photon splitting processes B OXES d Enterria et al. PRL (203) " This reaction is accessible in Pb+Pb collisions at the LHC Cross-section scales ~with Initial photon-photon system has very soft pt (< 0. GeV) At high energies, proposed as a clean channel to study: 7 Mar total leptons quarks -bosons γγ γγ 4 2 Anomalous gauge couplings Contributions from BSM particles 5 elementary cross section Z4 σ (pb) M. Dyndal -3-3 region probed with LHC measurements -2 - escattering have compared our results with: Light-by-light in ATLAS and CMS in Run2 2 3 s (GeV) 3

4 The ATLAS and CMS detectors R µ-chambers Barrel EndCap Tracking MBTS FCAL LUCID TAS ZDC/TAN η 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 4

5 γγ l+l- measurements Good cross-check for: Detector performance in UPC events Testing the validity of calculations (photon fluxes etc.) Measurements in agreement with theory predictions [STARlight MC, Comp.Phys.Comm. 22 (207) 258] (μ+) (μ-) ATLAS-CONF arxiv: p T e > 2 GeV η e < Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 5

6 Event and object selections Photons E T > 3 GeV (ATLAS), E T > 2 GeV (CMS) strip middle back Standard photon reconstruction/ identification schemes re-optimized for low-e T case EM shower for γ EM shower for π 0 Veto extra particle activity Requiring no tracks (p T > 0 MeV, η < 2.5) CMS: no activity in calorimeters, above noise thresholds γγ ee(γγ) background event candidate Selecting back-to-back topology p T γγ < 2 GeV ( GeV CMS) Acoplanarity < Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 6

7 Photon performance studies γγ ee events are used for: Trigger efficiency studies ee selection: 2 electrons, 2 tracks, Aco < 0.0 Photon reconstruction/identification efficiencies (using e.g. hard-bremsstrahlung photons due to interaction with the material of the tracker) Photon energy scale/resolution (EM cluster properties from γγ ee) 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 7

8 Background processes Main background Central Exclusive Production gg γγ Misidentified electrons from γγ ee Dedicated control regions are used Other background processes (found to be negligible) Fake photons induced by calo noise or cosmic-ray muons (<0. event) Fake photons from hadrons (e.g. γγ qq) Exclusive di-meson (e.g. π 0 π 0 ) production - strongly suppressed for m > 5 GeV Bottomonia (γγ η b γγ: σ ~ pb or γpb Υ γη b 3γ) o reduce backgroun Aco = / and reduce the CEP 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 8

9 Results ATLAS CMS 3 events observed in data 7.3 signal events and 2.6 background events expected 4 events observed in data. signal events and 4.0 background events expected 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 9

10 Results Observed (expected) significance: ATLAS: 4.4σ (3.8σ) CMS: 4.σ (4.4σ) Nature Phys. 3 (207) 852 σ ATLAS = 70 ±20 (stat) ±7 (sys) nb p T γ > 3 GeV, mγγ > 6 GeV, η γ < 2.4 SM predictions: 49 ±5 nb σ CMS = 20 ±46 (stat) ±28 (sys) ±4 (th) nb p T γ > 2 GeV, mγγ > 5 GeV, η γ < 2.4 SM predictions: 38 ±4 nb Ratio to e+e- cross section is also measured: R = (25 ± (stat) ± 6 (sys)) 6 arxiv: Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2

11 CMS LbL γ γ γ γ (MC) BSM interpretations CEP (gg γ γ ) + other bkg QED γ γ e+e- (MC) CEP (gg γ γ ) + other bkg QED γ γ e+e- (MC) arxiv: v2 [hep-ph] 24 Photons / ( Photons / ( G CMS LbL γ γ γ γ (MC) 25 a U()Y Born-Infeld theory. Over 80 years ago, soon after Dirac proposed his relativistic theory of the electron [] and his interpretation of hole states as positrons [2], Halpern [3] in 933 and Heisenberg [4] in 934 realized that quantum effects would induce light-by-light scattering, which was first calculated in the low-frequency limit by Euler and Kockel [5] in 935. Subsequently, Heisenberg and Euler [6] derived in 936 a more general expression for the quantum nonlinearities in the Lagrangian of Quantum Electrodynamics (QED), and a complete calculation of light-by-light scattering in QED was published by Karplus and Neuman [7] in 95. However, measurement of light-by-light scattering has remained elusive until very recently. In 203 d Enterria and Silveira [8] proposed looking for light-by-light scattering in ultraperipheral heavy-ion collisions at the LHC, and evidence for this process was recently presented by the ATLAS Collaboration [9], at a level consistent with the QED predictions in [8] and []. In parallel with the early work on light-by-light scattering in QED, and motivated by a unitarian idea that there should be an upper limit on the strength of the electromagnetic field, Born and Infeld [] proposed in 934 a conceptually distinct nonlinear modification of the Lagrangian of QED: Measurements can be interpreted in terms of limits on specific BSM models Figure 6: Observed (full line) and expected (dotted line) 95% CL limits on the production cross Photon ET (GeV) Photon η section! gg) as a function of the ALP mass ma in ultraperipheral PbPb collisions p s(gg! a PbPb 390 µb (5.02 TeV) PbPb 390 µb (5.02 TeV) 25 )and outer (yellow) bands indicate the regions containing at25 saxion-like = 5.02 TeV. The inner (green NN Data Data particles CMS CMS γ γ γ γrespectively, (MC) LbL γ γ γof γ (MC) 68 and LbL 95%, of the distribution limits expected under the background-only 20 e.g. (gg Knapen γ γ ) + other bkg et al., CEP (gg γ γ ) + other bkg 20 (seecep hypothesis. γ γ e e (MC) QED γ γ e e (MC) PRL QED 8 (207) 780) - Events / (2.5 GeV) Photons / (.05) Ze Pb Pb a Pb 0 LQED = Pb Ze : Exclusive ALP production in0ultra-peripheral Pb-Pb3collisions. 0 CMS log linear scale Photon φ γγ Diphoton m (GeV) arxiv: In Fig. 2, we show the expected sensitivity from performing a bump hunt in Events / (0.8) / (GeV-) g ~ PbPb µb (5.02 TeV) pp 2390 (CMS) PbPb 390 µb- (5.02 TeV) Data e e CMS 3 (OPAL) pp 2 (ATLAS) ) and g a luminosity for the high luminosity ( nblbl )γ Pb-Pb LbL the γ γ current γ γ (MC)( nb γ γ γ (MC) CEP (gg γ γ ) + other bkg CEP (gg γ γ ) + other bkg pp 3 (ATLAS) oint we computed the expected Poisson limit [6]. The dominant backgrounds 2 2 QED γ[3] γ and e+e- (MC) γ γ e+e- (MC) -by-light scattering fake photons from electrons, and become QED negligible the region which there is background, we assume the entire signal falls into a 8 selection criteria in this case are ET > 2 GeV and 8 < 2.5 for the e signal < The analogous limit from the exclusive p-p analysis performed by 6, which is very weak due to low photon luminosities. For the F F operator the 4 stronger, whereas for the B B operator, traditional 4 p-p collider -4 - limits nificantly γγa a itional2production channels through the Z coupling a,aff Data - Events / (0.2 GeV) - e+e 2 (OPAL) g 20 CMS 7 Mar 209 Figure 7: Fµ F µ! 4 r Fµ F µ 2 (Fµ F µ )2 6 4 ( where is an a priori unknown parameter with the dimension of [Mass]2 that we write as M 2, and F µ is the dual of the field strength tensor Fµ. Interest < 2x GeV for 6<m <30 GeV Ellis PbPb (5.02 TeV) and, observed ttering0 hasdumps been measured by the ATLAS collaboration [2], 0 the results were PbPb2(5.02 TeV) 0, expected mates and those in earlier computations [8 20]. Using the observed m specdiphoton p (GeV) Diphoton y 2 T e e40couplings, for F F 20and30B B which 70 are 80 shown 90 0 observed limit on ALPs in black m (GeV) a f,version generated Monte Carlo samples the ALP signal using a modified of as diphoton pt, rapidity, re 5: Distributions of thefor single photon ET, h, and as well 2 which assigns a small virtuality to the photons and as such leads to a typical ], invariant mass measured for the fourteen exclusive events passing all selection criteria Beam LBI = Born-Infeld extension of QED et al., PRL 8 (207) Light-by-light scattering ATLAS and CMS in Run2 M. Dyndal Exclusion limits at 95% CL in the ALP-photon coupling gag inversus ALP mass

12 New ATLAS measurement New measurement performed using.73 nb - of data collected in November x more statistics (cf. 205 dataset) Several analysis improvements (wrt previous measurement) Better trigger (higher efficiency at lowest photon ET s) Photon PID based on Neural Network discriminator Level- trigger efficiency ATLAS Preliminary Pb+Pb s NN =5.02 TeV - Data 208,.7 nb Fit to data Stat Stat syst cluster cluster2 E T +E T [GeV] Photon PID efficiency ATLAS Preliminary ATLAS-CONF Pb+Pb s NN =5.02 TeV - Data 208,.7 nb FSR photons γγ γγ MC Photon E T [GeV] 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 2

13 New ATLAS measurement Signal region (Aco < 0.0) Events / events observed in 208 data (2 ±3 background events expected) Aco < region is ued to extract significance 42 events observed (6 ±2 background events expected) 8.2σ (6.2σ) observed (expected) Updated cross-section: σ ATLAS = 78 ±3 (stat) ±8 (sys) nb SM predictions: 49 ±5 nb ATLAS Preliminary Pb+Pb s NN = 5.02 TeV Data 208,.7 nb Signal (γγ γγ) CEP gg γγ γγ ee Sys. unc Aco - Events / GeV ATLAS-CONF ATLAS Preliminary Pb+Pb s NN = 5.02 TeV Data 208,.7 nb Signal (γγ γγ) CEP gg γγ γγ ee Sys. unc [GeV] - m γγ 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 3

14 γγ γγ event candidate from Nov 208 Pb+Pb data mγγ = 29 GeV ATLAS-CONF Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 4

15 Summary Using LHC as a photon-photon collider works very well First high-energy evidence for LbyL scattering by ATLAS & CMS Based on 205 Pb+Pb 5.02 TeV Good sensitivity for specific BSM models Axion-like particles Higher-dimension operators, New: observation of this process by ATLAS Based on recent Pb+Pb LHC runs from November 5.02 TeV 8.2σ (6.2σ) observed (expected) significance Fiducial cross-section measured with 20% precision 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 5

16 References ATLAS Collaboration, Observation of light-by-light scattering in ultraperipheral Pb+Pb collisions with the ATLAS detector, ATLAS-CONF CMS Collaboration, Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at snn = 5.02 TeV arxiv: [hep-ex] ATLAS Collaboration, Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC, Nature Phys. 3 (207) 852 ATLAS Collaboration, Measurement of high-mass dimuon pairs in ultraperipheral lead-lead collisions at snn = 5.02 TeV with the ATLAS detector at the LHC, ATLAS-CONF Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 6

17 Backup 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 7

18 New ATLAS measurement Events / 0.2 GeV 25 ATLAS Preliminary 20 5 Pb+Pb s NN = 5.02 TeV Data 208,.7 nb Signal (γγ γγ) CEP gg γγ γγ ee Sys. unc. - Events / Data 208,.7 nb Signal (γγ γγ) CEP gg γγ γγ ee Sys. unc. ATLAS Pb+Pb Preliminary s NN = 5.02 TeV γγ p [GeV] T y γγ 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 8

19 New ATLAS measurement Events / 2 GeV Data 208,.7 nb γγ ee MC Sys. unc. ATLAS Preliminary Pb+Pb s NN ee selection = 5.02 TeV Events / Data 208,.7 nb γγ ee MC Sys. unc. ATLAS Preliminary Pb+Pb s NN ee selection = 5.02 TeV Data / MC m ee [GeV] m ee [GeV] Data / MC y ee y ee 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 9

20 New ATLAS measurement Events / ATLAS Preliminary Pb+Pb S NN =5.02 TeV Signal Background E ratio Photon reconstruction efficiency ATLAS Preliminary Pb+Pb s NN =5.02 TeV eeγ (hard-brem) selection - Data 208,.7 nb γγ ee MC e trk2 E T, - p [GeV] T 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 20

21 BSM interpretations First EFT constraints on nonlinear Lorentz-violating operators in QED Based on the ATLAS measurement, constraints are put on 26 nonlinear operators with d = 8 [arxiv:82.672] ( ) 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 2

22 PbPb (γγ) PbPb X process calculations Strong fields, up to 25 Vm at the LHC [Fermi, Nuovo Cim. 2 (925) 43] Impact parameter > 2R [eizsacker, Z. Phys. 88 (934) 62] [illiams, Phys. Rev. 45 ( 934) 729] The cross section for AA (γγ) AA X process can be calculated using: () Number of equivalent photons (EPA) by integration of relevant EM form factors: (2) E γγ X (elementary) cross section 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 22

23 Theoretical uncertainties & higher-order corrections Theory uncertainties dominated by modeling of nuclear form factors (~%) Higher-order corrections relatively small (<3%) for eta <2.5 arxiv: arxiv:hep-ph/09079 arxiv: Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 23

24 LHC as a photon-photon collider pp collisions + harder EPA spectrum (ω max ~ TeV) - large pile-up (multiple interactions per bunch crossing) + large datasets available, O( fb - ) - hard to trigger on low-p T objects Pb+Pb collisions - softer EPA spectrum (ω max ~0 GeV) + AA (γγ) cross-sections scale as Z 4 + gluonic cross-sections scale as ~A 2 * (lower QCD bkg expected wrt pp) + low pile-up (<%)* - Short LHC Pb+Pb campaigns (cf. pp) 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 24

25 Role of QCD meson exchanges Lebiedowicz et al. Phys. Lett. B 772 (207) σ (pb) γ γ γ γ, cosθ < 0.6 fermionic contributions leptons quarks mesonic contributions scalars psudoscalars pseudoscalars tensors - γ(p ) γ(p 3 ) γ(p ) -3 γ(p 3 ) γ(p ) p t p u γ(p 4 ) s (GeV) γ(p 2 ) p s γ(p 4 ) γ(p 2 ) γ(p 4 ) γ(p 2 ) γ(p 3 ) FIG.: Diagrams forlight-by-lightscattering via a time-like (s-channel) and a space-like (t-channel 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 25

26 Zero Degree Calorimeters Installed at ±40 m from the ATLAS IP (where the beam pipe splits) Detect very forward (8.3 < η < +inf) neutral particles (incl. neutrons) Usually used in HI collisions to provide a measurement of the centrality (correlated to the number of forward neutrons) Very useful to tag the ultra-peripheral events (e.g. 0nXn or XnXn topologies) Pb Au e + Pb* Au* Pb Au* e Pb* Au 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 26

27 ZDC check (ATLAS) CEP gg γγ background MC simulation (with data-driven normalization) is cross-checked in the analysis of ZDC activity Aco > 0.0 used as a control region Energy deposits corresponding to at least forward neutron emission Expectations: Pb+Pb CEP occurs at relatively small impact parameters (b~2r) -> large probability for nuclear break-up Moreover: the probability for extra Coulomb break-up is ~80% for b=2r (from STARlight) Conclusions: hat we see in the detector (Aco > 0.0) is consistent with the incoherent CEP background + some ee events with Coulomb breakup (not included in the plot) (signal region: /3 events have no ZDC activity) 7 Mar 209 M. Dyndal Light-by-light scattering in ATLAS and CMS in Run2 27