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

Mateusz Dyndal (CERN) on behalf of ATLAS and CMS collaborations

Moriond EW 16-23 Mar 2019

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

M. Dyndal17 Mar 2019 Light-by-light scattering in ATLAS and CMS in Run2

Nucleus intact No neutrons

Nucleus breaks up Multiple neutrons

Rapidity gap

No rapidity gap

UPC MEASUREMENTS

PHOTONUCLEAR DIJETS

�13Ax

3−10 2−10 1−10 1

b / G

eV ]

µ [ Ax

d THd

σ∼2 d

12−10

10−10

8−10

6−10

4−10

2−10

1

210

410

610

< 50 GeVTH42 <

) -1 10× < 59 GeV ( TH50 <

) -2 10× < 70 GeV ( TH59 <

) -3 10× < 84 GeV ( TH70 <

) -4 10× < 100 GeV ( TH84 <

) -5 10× < 119 GeV ( TH100 <

) -6 10× < 141 GeV ( TH119 <

) -7 10× < 168 GeV ( TH141 <

) -8 10× < 200 GeV ( TH168 <

PreliminaryATLAS-12015 Pb+Pb data, 0.38 nb

= 5.02 TeVNNs

=0.4 jetsR tkanti- > 20 GeVlead

Tp

> 35 GeVjetsm

Not unfolded for detector response

DataPythia+STARlightscaled to data

φΔ0 1 2 3

φΔ

/dN

dev

tN

1/

4−10

3−10

2−10

1−10

1

10

210

= 2jetN = 3 (/ 10)jetN > 3 (/ 100)jetN

PreliminaryATLAS-1Pb+Pb 2015, 0.38 nb

= 5.02 TeV, 0nXnNNs = 0.4 jetsR tanti-k

Not unfolded fordetector response

have zero neutrons in one direction and one or more neutrons in the opposite direction, referred to as the“0nXn” event topology. The photon-going direction is defined to be the direction in which zero neutronsare observed. Background events are removed by requiring a minimum rapidity gap in this directionand requiring that there is no large gap in the opposite direction. Corrections are applied to accountfor signal events removed by these requirements, and thus they are not part of the fiducial definitionof the measurement. Event-level observables are constructed from all jets having transverse momentapT > 15 GeV and pseudo-rapidities |⌘ | < 4.4. Events are required to have two or more such jets and atleast one jet with pT > 20 GeV. The jets are used to define the event-level variables:

HT ⌘X

i

pT i , mjets ⌘2666664*,X

i

Ei+-

2

��X

i

~pi

��23777775

1/2

, yjets ⌘12

ln P

i Ei +P

i pz iPi Ei �

Pi pz i

!, (1)

where i runs over the measured jets in an event, E and ~p represent jet energies and momentum vectors,respectively, and pz represents the longitudinal component of the jet momenta. The signs of pz are chosento be positive in the photon-going direction. A further requirement is imposed that the jet-system mass,mjets, satisfies mjets > 35 GeV.

The di�erential cross-sections are measured as a function of HT and

z� ⌘mjetsp

se+yjets , xA ⌘

mjetsps

e�yjets . (2)

In the limit of 2! 2 scattering kinematics, xA corresponds to the ratio of the energy of the struck partonin the nucleus to the (per nucleon) beam energy. z� = x� y, where y is the energy fraction carried by thephoton. For direct processes, x� is unity, while for resolved events, it is the fraction of the photon’s energycarried by the resolved parton entering the hard scattering.

The remainder of this note is structured as follows: Section 2 describes the ATLAS detector and thetriggers used for the measurements in this analysis. Section 3 describes the data and Monte Carlo (MC)samples used in the analysis and provides information on how the MC sample obtained from P��is re-weighted for use in Pb+Pb collisions. Section 5 describes all aspects of the data analysis and themeasurement of the photo-nuclear dijet production cross-sections. Section 6 discusses the evaluation ofthe systematic uncertainties, and Section 7 discusses possible backgrounds to the measurement. Section 8presents the final results figures with comparison to Monte Carlo and theory. Section 9 summarizes thisnote and provides conclusions.

2 ATLAS detector

The measurements described in this note are performed using the ATLAS detector [18] in the Run 2configuration. They rely on the calorimeter system, the inner detector, the zero degree calorimeters,and the trigger system. The calorimeters, which cover the pseudo-rapidity range |⌘ | < 4.91, are usedfor measuring the jets and for the rapidity gap analysis. The inner detector is used to measure chargedparticle tracks over |⌘ | < 2.5. The zero degree calorimeters (ZDCs), which measure neutrons emitted at1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector

and the z-axis along the beam pipe. The x-axis points from the IP to the center of the LHC ring, and the y-axis pointsupward. Cylindrical coordinates (r, �) are used in the transverse plane, � being the azimuthal angle around the z-axis. Thepseudorapidity is defined in terms of the polar angle ✓ as ⌘ = � ln tan(✓/2).

4

have zero neutrons in one direction and one or more neutrons in the opposite direction, referred to as the“0nXn” event topology. The photon-going direction is defined to be the direction in which zero neutronsare observed. Background events are removed by requiring a minimum rapidity gap in this directionand requiring that there is no large gap in the opposite direction. Corrections are applied to accountfor signal events removed by these requirements, and thus they are not part of the fiducial definitionof the measurement. Event-level observables are constructed from all jets having transverse momentapT > 15 GeV and pseudo-rapidities |⌘ | < 4.4. Events are required to have two or more such jets and atleast one jet with pT > 20 GeV. The jets are used to define the event-level variables:

HT ⌘X

i

pT i , mjets ⌘2666664*,X

i

Ei+-

2

��X

i

~pi

��23777775

1/2

, yjets ⌘12

ln P

i Ei +P

i pz iPi Ei �

Pi pz i

!, (1)

where i runs over the measured jets in an event, E and ~p represent jet energies and momentum vectors,respectively, and pz represents the longitudinal component of the jet momenta. The signs of pz are chosento be positive in the photon-going direction. A further requirement is imposed that the jet-system mass,mjets, satisfies mjets > 35 GeV.

The di�erential cross-sections are measured as a function of HT and

z� ⌘mjetsp

se+yjets , xA ⌘

mjetsps

e�yjets . (2)

In the limit of 2! 2 scattering kinematics, xA corresponds to the ratio of the energy of the struck partonin the nucleus to the (per nucleon) beam energy. z� = x� y, where y is the energy fraction carried by thephoton. For direct processes, x� is unity, while for resolved events, it is the fraction of the photon’s energycarried by the resolved parton entering the hard scattering.

The remainder of this note is structured as follows: Section 2 describes the ATLAS detector and thetriggers used for the measurements in this analysis. Section 3 describes the data and Monte Carlo (MC)samples used in the analysis and provides information on how the MC sample obtained from P��is re-weighted for use in Pb+Pb collisions. Section 5 describes all aspects of the data analysis and themeasurement of the photo-nuclear dijet production cross-sections. Section 6 discusses the evaluation ofthe systematic uncertainties, and Section 7 discusses possible backgrounds to the measurement. Section 8presents the final results figures with comparison to Monte Carlo and theory. Section 9 summarizes thisnote and provides conclusions.

2 ATLAS detector

The measurements described in this note are performed using the ATLAS detector [18] in the Run 2configuration. They rely on the calorimeter system, the inner detector, the zero degree calorimeters,and the trigger system. The calorimeters, which cover the pseudo-rapidity range |⌘ | < 4.91, are usedfor measuring the jets and for the rapidity gap analysis. The inner detector is used to measure chargedparticle tracks over |⌘ | < 2.5. The zero degree calorimeters (ZDCs), which measure neutrons emitted at1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector

and the z-axis along the beam pipe. The x-axis points from the IP to the center of the LHC ring, and the y-axis pointsupward. Cylindrical coordinates (r, �) are used in the transverse plane, � being the azimuthal angle around the z-axis. Thepseudorapidity is defined in terms of the polar angle ✓ as ⌘ = � ln tan(✓/2).

4

Excellent agreement with PYTHIA6 reweighed to STARLIGHT

jet variables:

ATLAS-CON

F-2017-011

Quasi-real photons from Pb ions @LHC

2

▪ Boosted nuclei are intense source of (quasi-real) photons ▪ Equivalent photon flux

▪ Q ~ 1/R ~ 0.06 GeV ▪ √sNN = 5.02 TeV

—> Lorentz factor γ ~2700 ▪ Emax ≾ γ/R ~ 80 GeV ▪ Each flux scales with Z2

▪ Various types of interactions possible:

Photon-pomeron(e.g. exclusive J/Psi)

Photo-nuclear(e.g. photoproduction of jets)

Photon-photon(e.g. dilepton production)

[Fermi, Nuovo Cim. 2 (1925) 143]

(μ+)

(μ-)


CRACOVIANTHEORETICAL

RESULTS FOR UPC

EPA

�� SCATTERING

FOUR-LEPTONPRODUCTION

ELECTRONS

MUONS

PROTON-ANTIPROTONPRODUCTION

CONCLUSION

�� SCATTERING

BOXESΓ

Γ

Γ

ΓW

W

W

Γ

Γ

Γ

Γ

W

W W

Γ

Γ

Γ

Γ

W

W

W

Γ

Γ

Γ

ΓW

W

W

Γ

Γ

Γ

ΓW

W WΓ

Γ

Γ

Γ

W

W

W

Γ

Γ

Γ

Γ

F

F

F

F

Γ

Γ

Γ

Γ

W

W

W

W

Γ

Γ

Γ

Γ

F

F

F

F

Γ

Γ

Γ

Γ

W

W

W

W

Γ

Γ

Γ

Γ

F

FF

F

Γ

Γ

Γ

Γ

W

WW

W

Γ

Γ

Γ

Γ

W

WΓ

Γ

Γ

Γ

W W

Γ

Γ

Γ

ΓW W

Γ Γ " Γ Γ

(GeV)s

-310 -210 -110 1 10 210 310

(p

b)

σ

-310

-210

-110

1

10

210

310

410

510

610

710

γγ → γγ

totalleptonsquarksW-bosons

Fermionic box LO QED - FormCalc.

The one-loop W box diagram - LoopTools.

We have compared our results with:I Jikia et al. (1993),I Bern et al. (2001),I Bardin et al. (2009).

Bern et al. consider QCD and QED corrections

(two-loop Feynman diagrams) to the one-loop

fermionic contributions in the ultrarelativistic limit

(s, |t|, |u| � m2f

). The corrections are quite small

numerically.

CRACOVIAN THEORETICAL RESULTS FOR UPC KRAKÓW, DECEMBER 1, 2017 9 / 40

▪ Light-by-light (γγ → γγ) scattering ▪ Forbidden at tree-level ▪ Tested indirectly in electron/muon g-2 measurements

▪ Another examples: Delbruck scattering and photon splitting processes

▪ This reaction is accessible in Pb+Pb collisions at the LHC

▪ Cross-section scales ~with Z4 ▪ Initial photon-photon system has very soft pT (< 0.1 GeV)

▪ At high energies, proposed as a clean channel to study:

▪ Anomalous gauge couplings ▪ Contributions from BSM particles

Motivation

3

elementary cross section

d’Enterria et al. PRL 111 (2013) 080405 Klusek-Gawenda et al. PRC 93 (2016) 044907

region probed with LHC measurements


The ATLAS and CMS detectors

TAS

R

50 1 2 43 6 7 8

Barrel

FCALLUCID

Tracking

EndCap

ZDC/TAN

|η|109

µ-chambers

MBTS

4


▪ 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. 212 (2017) 258]

γγ → l+l- measurements

5

arXiv:1810.04602ATLAS-CONF-2016-025

pTe > 2 GeV

|ηe| < 2.4

(μ+)

(μ-)

http://arxiv.org/abs/1810.04602

https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2016-025/


▪ Photons ▪ ET > 3 GeV (ATLAS), ET > 2 GeV (CMS)

▪ Standard photon reconstruction/ identification schemes re-optimized for low-ET case

▪ Veto extra particle activity ▪ Requiring no tracks(pT > 100 MeV, |η| < 2.5)

▪ CMS: no activity in calorimeters, above noise thresholds

▪ Selecting back-to-back topology ▪ pT

γγ < 2 GeV (1 GeV CMS) ▪ Acoplanarity < 0.01

Event and object selections

6

- Different sets of cuts are used to deliver a very good separation between e/γ and fake signature of QCD

- 3 (2) main operating point with increasing background rejection power have been defined for electrons (photons)- e: loose, medium, tight- γ: loose, tight

Electron/Photon Identification

11

An example of γ/π0: cut on strip variable

reject the π0

stri

p m

iddle

bac

k

γ π0

γγ → ee(γγ) background event candidate

EM shower for EM shower for


▪ γγ → ee events are used for: ▪ Trigger efficiency studies ▪ 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)

Photon performance studies

7

ee selection: 2 electrons, 2 tracks, Aco < 0.01


▪ 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 noiseor cosmic-ray muons (<0.1 event)

▪ Fake photons from hadrons (e.g. γγ → qq) ▪ Exclusive di-meson (e.g. π0π0) production

- strongly suppressed for m > 5 GeV ▪ Bottomonia (γγ → ηb → γγ: σ ~ 1 pb

or γPb → Υ → γηb → 3γ)

Background processes

8

1. Introduction 1

1 IntroductionElastic light-by-light (LbyL) scattering, gg ! gg, is a pure quantum mechanical process thatproceeds at leading order in the quantum electrodynamics (QED) coupling a, via virtual boxdiagrams containing charged particles (Fig. 1, left). In the standard model (SM), the box dia-gram involves charged fermions (leptons and quarks) and boson (W±) contributions at highenergies. Although LbyL scattering via an electron loop has been indirectly tested throughthe high-precision measurements of the anomalous magnetic moment of the electron [1] andmuon [2], its direct observation in the laboratory remains elusive still today due to its verysmall cross section sgg µ a4 ⇡ 3 ⇥ 10�9. Out of the two closely-related processes—photonscattering in the Coulomb field of a nucleus (Delbruck scattering) [3] and photon splitting ina strong magnetic field (“vacuum birefringence”) [4, 5]—only the former has been clearly ob-served [6]. However, as demonstrated in [7], the LbyL process can be experimentally observedin ultraperipheral interactions of ions, with impact parameters larger than twice the radius ofthe nuclei, exploiting the very large fluxes of quasi-real photons emitted by the nuclei acceler-ated at TeV energies [8]. Ions accelerated at high energies generate strong electromagnetic fieldswhich, in the equivalent photon approximation (EPA) [9–11], can be considered as g beams ofvirtuality Q

2 < 1/R2, where R is the effective radius of the charge distribution. For lead (Pb)

nuclei with radius R ⇡ 7 fm, the quasi-real photon beams have virtualities �Q2 < 10�3 GeV2.

Since each photon flux scales as the square of the ion charge Z2, gg scattering cross sections in

PbPb collisions are enhanced by a factor Z4 ' 5 ⇥ 107 compared to similar proton-proton or

electron-positron interactions.

Figure 1: Schematic diagrams of light-by-light scattering (gg ! gg, left), QED dielectron pro-duction (gg ! e+e�, centre), and central exclusive diphoton production (gg ! gg, right) inultraperipheral PbPb collisions (where the (⇤) superindex indicates a potential electromagneticexcitation of the outgoing ions).

Many final states have been measured in photon-photon interactions in ultraperipheral col-lisions (UPCs) of proton and/or lead beams at the Large Hadron Collider (LHC), includinggg ! `+`� [12–20], gg ! W

+W

� [21–23], as well as a first evidence of gg ! gg reported bythe ATLAS experiment [24]. The final state signature of interest in this analysis is the exclusiveproduction of two photons, PbPb ! gg ! Pb(⇤)ggPb(⇤), where the diphoton final state ismeasured in the otherwise empty central part of the detector, and the outgoing Pb ions survivethe interaction and escape undetected (with a potential electromagnetic excitation denoted bythe (⇤) superindex) at very low angles with respect to the beam (Fig. 1, left). The dominant back-grounds are the QED production of an exclusive electron-positron pair (gg ! e+e�) where thee± are misidentified as photons (Fig. 1, centre), and gluon-induced central exclusive production

provided as a function of the sum of cluster transverse energies (Ecl1T +Ecl2

T ). The e�ciency grows fromabout 70% at (Ecl1

T + Ecl2T ) = 6 GeV to 100% at (Ecl1

T + Ecl2T ) > 9 GeV. The e�ciency is parameterised

using an error function fit which is then used to reweight the simulation. Due to the extremely low noise,very high hit reconstruction e�ciency and low conversion probability of signal photons in the pixel de-tector (around 10%), the uncertainty due to the requirement for minimal activity in the ID is negligible.The MBTS veto e�ciency was studied using �� ! `+`� events (` = e, µ) passing a supporting triggerand it is estimated to be (98 ± 2)%.

Photons are reconstructed from EM clusters in the calorimeter and tracking information provided bythe ID, which allows the identification of photon conversions. Selection requirements are applied toremove EM clusters with a large amount of energy from poorly functioning calorimeter cells, and atiming requirement is made to reject out-of-time candidates. An energy calibration specifically optimisedfor photons [38] is applied to the candidates to account for upstream energy loss and both lateral andlongitudinal shower leakage. A dedicated correction [39] is applied for photons in MC samples to correctfor potential mismodelling of quantities which describe the properties (“shapes”) of the associated EMshowers.

The photon particle-identification (PID) in this analysis is based on three shower-shape variables: thelateral width of the shower in the middle layer of the EM calorimeter, the ratio of the energy di↵erenceassociated with the largest and second largest energy deposits to the sum of these energies in the firstlayer, and the fraction of energy reconstructed in the first layer relative to the total energy of the cluster.Only photons with ET > 3 GeV and |⌘| < 2.37, excluding the calorimeter transition region 1.37 < |⌘| <1.52, are considered. The pseudorapidity requirement ensures that the photon candidates pass throughregions of the EM calorimeter where the first layer is segmented into narrow strips, allowing for goodseparation between genuine prompt photons and photons coming from the decay of neutral hadrons. Aconstant photon PID e�ciency of 95% as a function of ⌘ with respect to reconstructed photon candidatesis maintained. This is optimised using multivariate analysis techniques [40], such that EM energy clustersinduced by cosmic-ray muons are rejected with 95% e�ciency.

Preselected events are required to have exactly two photons satisfying the above selection criteria, witha diphoton invariant mass greater than 6 GeV. In order to reduce the dielectron background, a vetoon the presence of any charged-particle tracks (with pT > 100 MeV, |⌘| < 2.5 and at least one hit inthe pixel detector) is imposed. This requirement further reduces the fake-photon background from thedielectron final state by a factor of 25, according to simulation. It has almost no impact on �� ! ��signal events, since the probability of photon conversion in the pixel detector is relatively small andconverted photons are suppressed at low ET (3–6 GeV) by the photon selection requirements. Accordingto MC studies, the photon selection requirements remove about 10% of low-ET photons. To reduce otherfake-photon backgrounds (for example, cosmic-ray muons), the transverse momentum of the diphotonsystem (p��T ) is required to be below 2 GeV. To reduce background from CEP gg ! �� reactions, anadditional requirement on diphoton acoplanarity, Aco = 1��/⇡ < 0.01, is imposed. This requirementis optimised to retain a high signal e�ciency and reduce the CEP background significantly, since thetransverse momentum transferred by the photon exchange is usually much smaller than that due to thecolour-singlet-state gluons [41].

5


▪ ATLAS ▪ 13 events observed in data ▪ 7.3 signal events and 2.6 background events expected

▪ CMS ▪ 14 events observed in data ▪ 11.1 signal events and 4.0 background events expected

Results

9


▪ Observed (expected) significance: ▪ ATLAS: 4.4σ (3.8σ) ▪ CMS: 4.1σ (4.4σ)

▪ σATLAS = 70 ±20 (stat) ±17 (sys) nb ▪ pT

γ > 3 GeV, mγγ > 6 GeV, |ηγ| < 2.4 ▪ SM predictions: 49 ±5 nb

▪ σCMS = 120 ±46 (stat) ±28 (sys) ±4 (th) nb

▪ pTγ > 2 GeV, mγγ > 5 GeV, |ηγ| < 2.4

▪ SM predictions: 138 ±14 nb ▪ Ratio to e+e- cross section is also

measured: R = (25 ± 10 (stat) ± 6 (sys)) × 10−6

Results

10

Nature Phys. 13 (2017) 852

arXiv:1810.04602

https://doi.org/10.1038/nphys4208


M. Dyndal 17 Mar 2019 Light-by-light scattering in ATLAS and CMS in Run2

5.1 Diphoton analysis efficiencies 9

0 1 2 3 4 5 6 7 8 9 10 (GeV)TPhoton E

0

5

10

15

20

25

30

Phot

ons

/ (1

GeV

)Data

(MC)γ γ → γ γLbL ) + other bkgγ γ →CEP (gg (MC)-e+ e→ γ γQED

(5.02 TeV)-1bµPbPb 390

CMS

2− 1.5− 1− 0.5− 0 0.5 1 1.5 2ηPhoton

0

5

10

15

20

25

30

Phot

ons

/ (0.

8) Data (MC)γ γ → γ γLbL ) + other bkgγ γ →CEP (gg (MC)-e+ e→ γ γQED


CMS

3− 2− 1− 0 1 2 3φPhoton

0

5

10

15

20

25

Phot

ons

/ (1.

05)

Data (MC)γ γ → γ γLbL ) + other bkgγ γ →CEP (gg (MC)-e+ e→ γ γQED


CMS

0 2 4 6 8 10 12 14 16 18 20 (GeV)γγDiphoton m

0

5

10

15

20

25

Even

ts /

(2.5

GeV

)



CMS

2− 1.5− 1− 0.5− 0 0.5 1 1.5 2Diphoton y

0

2

4

6

8

10

12

14

Even

ts /

(0.8

)



CMS

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 (GeV)

TDiphoton p

0

2

4

6

8

10

12

14

Even

ts /

(0.2

GeV

)



CMS

Figure 5: Distributions of the single photon ET, h, and f, as well as diphoton pT, rapidity,and invariant mass measured for the fourteen exclusive events passing all selection criteria(squares), compared to the expectations of LbL scattering signal (orange histogram), QED e+e�MC predictions (yellow histogram), and the CEP plus other backgrounds (light blue histogram,scaled to match the data in the Af > 0.02 region). Signal and QED e+e� MC samples are scaledaccording to their theoretical cross sections and integrated luminosity. The error bars aroundthe data points indicate statistical uncertainties. The horizontal bars around the data symbolsindicate the bin size.

acoplanarity. It is found to be #gg = (20.7 ± 0.4)%, mostly driven by the inefficiencies of thesingle photon reconstruction and identification, and of the trigger (#g,reco+ID, #gg,trig. ⇡ 70%).The quoted uncertainty here is statistical only, reflecting the finite size of the LbL scattering

15

Figure 6: Observed (full line) and expected (dotted line) 95% CL limits on the production crosssection s(gg ! a ! gg) as a function of the ALP mass ma in ultraperipheral PbPb collisionsat

psNN = 5.02 TeV. The inner (green )and outer (yellow) bands indicate the regions containing

68 and 95%, respectively, of the distribution of limits expected under the background-onlyhypothesis.

2�10 1�10 1

4�10

3�10

F~,a

F⇥ag

⇤) -1(G

eV⌅

1/

log |⇧

(OPAL)⇥2⌃-e+e

dumpsBeam

CMS

10 20 30 40 50 60 70 80 90 100(GeV)am

, observed⇥⇥⌃PbPb (5.02 TeV) , expected⇥⇥⌃PbPb (5.02 TeV)

(CMS)⇥2⌃pp

(ATLAS)⇥3⌃pp

(ATLAS)⇥2⌃pp(OPAL)⇥3⌃-e+e

⌃| linear scale

Figure 7: Exclusion limits at 95% CL in the ALP-photon coupling gag versus ALP massma plane, for the operators aFeF/4L (left, assuming ALP coupling to photons only) andaBeB/4L cos2 qW (right, including also the hypercharge coupling, thus processes involving theZ boson) derived in Refs. [30, 55] from measurements at beam dumps [59], in e+e� collisionsat LEP-I [55] and LEP-II [56], and in ppcollisions at the LHC [13, 57, 58], and compared to thepresent PbPb limits.

BSM interpretations

11

▪ Measurements can be interpreted in terms of limits on specific BSM models

a

Pb

Pb

Pb

Pb

�

�

Ze

Ze

Fig. 1: Exclusive ALP production in ultra-peripheral Pb-Pb collisions.

LEP and LHC [13–15]. In Fig. 2, we show the expected sensitivity from performing a bump hunt inm�� for UPCs, assuming a luminosity for the current (1 nb�1) and the high luminosity (10 nb�1) Pb-Pbruns.1 For each mass point we computed the expected Poisson limit [16]. The dominant backgroundsare estimated to be light-by-light scattering [3] and fake photons from electrons, and become negligiblefor m�� & 20 GeV. In the region which there is background, we assume the entire signal falls into abin of width 1 GeV. The signal selection criteria in this case are ET > 2 GeV and |⌘| < 2.5 for thetwo photons and |�� ⇡| < 0.04. The analogous limit from the exclusive p-p analysis performed byCMS [17] is also shown, which is very weak due to low photon luminosities. For the FF operator theheavy-ion limits are significantly stronger, whereas for the BB operator, traditional p-p collider limitsare enhanced due to additional production channels through the Z coupling.

Light-by-light scattering has been measured by the ATLAS collaboration [2], and the results wereconsistent with our estimates and those in earlier computations [18–20]. Using the observed m�� spec-trum, we then derive an observed limit on ALPs for F eF and B eB couplings, which are shown in blackin Fig. 2. In detail, we generated Monte Carlo samples for the ALP signal using a modified version ofthe STARlight code [21],2 which assigns a small virtuality to the photons and as such leads to a typicalp��T . 100 MeV for the recoil of the ��-system. We then follow the ATLAS analysis and apply thefollowing selection cuts on the signal:

1. Require exactly two photons with ET > 3 GeV and |⌘| < 2.4

2. Demand |�� ⇡| < 0.03, where �� is the azimuthal angle between the two photons

The signal efficiency is ⇠70% near threshold and becomes fully efficient if the sum of the photon ener-gies exceeds 9 GeV. The selection criteria are slightly different from our previous theoretical analysis,however we note that only the larger ET cut leads to noticeable changes for the efficiencies. Giventhat we do not model photon identification at the detector level, we apply an extra total reconstructionefficiency of 90%, which roughly takes into account the per-photon ID efficiency of 95% measured byATLAS.

The m�� spectrum measured by ATLAS is plotted in bin-widths of 3 GeV, starting at m�� = 6GeV. For our exclusion, we generated samples with m�� = 7, 10, 13, 16, ... GeV, and assume that all theevents are contained in their respective bins after final selection. We further assume that ATLAS did notobserve any events with m�� & 30 GeV. The 95% exclusion limits on the coupling 1/⇤ are obtainedassuming only statistical uncertainties. A more detailed CLs analysis that includes a proper treatment ofsystematics would yield slightly more conservative limits, and we encourage the experimental commu-nity to include such an analysis as it is beyond the scope of our simulation framework.

In summary, we have found that heavy-ion collisions at the LHC can provide the best limits onALP-photon couplings for 7 GeV < ma < 100 GeV, confirming our previous estimates. The very

1Limits from the p-Pb runs are not competitive despite their higher luminosity, because of the less advantageous Z2 scalingof the production rate. Collisions with lighter elements, e.g. Ar-Ar, may set relevant limits if the luminosity could be enhancedby two to three orders of magnitude, as compared to current Pb-Pb run.

2Our patch for ALP production is now included in the latest STARlight release.

2

Ellis et al., PRL 118 (2017) 261802

Light-by-Light Scattering Constraint on Born-Infeld Theory

John EllisTheoretical Particle Physics and Cosmology Group,

Physics Department, King’s College London, London WC2R 2LS, UK ;

Theoretical Physics Department, CERN, CH-1211 Geneva 23, Switzerland

Nick E. MavromatosTheoretical Particle Physics and Cosmology Group,

Physics Department, King’s College London, London WC2R 2LS, UK

Tevong YouDAMTP, University of Cambridge, Wilberforce Road, Cambridge, CB3 0WA, UK;

Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge, CB3 0HE, UK

The recent measurement by ATLAS of light-by-light scattering in LHC Pb-Pb collisions is thefirst direct evidence for this basic process. We find that it excludes a range of the mass scale of anonlinear Born-Infeld extension of QED that is . 100 GeV, a much stronger constraint than thosederived previously. In the case of a Born-Infeld extension of the Standard Model in which the U(1)Yhypercharge gauge symmetry is realized nonlinearly, the limit on the corresponding mass reach is⇠ 90 GeV, which in turn imposes a lower limit of & 11 TeV on the magnetic monopole mass in sucha U(1)Y Born-Infeld theory.

Over 80 years ago, soon after Dirac proposed his rel-ativistic theory of the electron [1] and his interpreta-tion of ‘hole’ states as positrons [2], Halpern [3] in 1933and Heisenberg [4] in 1934 realized that quantum ef-fects would induce light-by-light scattering, which wasfirst calculated in the low-frequency limit by Euler andKockel [5] in 1935. Subsequently, Heisenberg and Eu-ler [6] derived in 1936 a more general expression forthe quantum nonlinearities in the Lagrangian of Quan-tum Electrodynamics (QED), and a complete calcula-tion of light-by-light scattering in QED was publishedby Karplus and Neuman [7] in 1951. However, measure-ment of light-by-light scattering has remained elusive un-til very recently. In 2013 d’Enterria and Silveira [8] pro-posed looking for light-by-light scattering in ultraperiph-eral heavy-ion collisions at the LHC, and evidence for thisprocess was recently presented by the ATLAS Collabo-ration [9], at a level consistent with the QED predictionsin [8] and [10].

In parallel with the early work on light-by-light scat-tering in QED, and motivated by a ‘unitarian’ idea thatthere should be an upper limit on the strength of the elec-tromagnetic field, Born and Infeld [11] proposed in 1934a conceptually distinct nonlinear modification of the La-grangian of QED:

LQED = �1

4Fµ⌫F

µ⌫!

LBI = �2⇣1�

r1 +

1

2�2Fµ⌫Fµ⌫ �

1

16�4(Fµ⌫ Fµ⌫)2

⌘,

(1)

where � is an a priori unknown parameter with the di-mension of [Mass]2 that we write as � ⌘ M2, and Fµ⌫

is the dual of the field strength tensor Fµ⌫ . Interest

in Born-Infeld theory was revived in 1985 when Frad-kin and Tseytlin [12] discovered that it appears whenan Abelian vector field in four dimensions is coupled toan open string, as occurs in models inspired by M the-ory in which particles are localized on lower-dimensional‘branes’ separated by a distance ' 1/

p� = 1/M in some

extra dimension 1. Depending on the specific brane sce-nario considered, M might have any value between afew hundred GeV and the Planck scale ⇠ 1019 GeV. Forthe purposes of this paper, we consider only the relevantterms of fourth order in the gauge field strengths in (1).

Until now, there has been no strong lower limit onthe Born-Infeld scale � or, equivalently, the brane massscale M and the brane separation 1/M . A constraintcorresponding to M & 100 MeV was derived in [14] fromelectronic and muonic atom spectra, though the deriva-tion has been questioned in [15]. Measurements of pho-ton splitting in atomic fields [16] were considered in [17],where it was concluded that they provided no limit onthe Born-Infeld scale and it was suggested that measure-ments of the surface magnetic field of neutron stars [18]might be sensitive to M =

p� ⇠ 1.4⇥ 10�5 GeV. More

recently, measurements of nonlinearities in light by thePVLAS Collaboration [19] are somewhat more sensitiveto the individual nonlinear terms in (1), but are insensi-tive to the particular combination appearing in the Born-Infeld theory, as discussed in [20] where more referencescan be found.

1Remarkably, the maximum field strength is related to the fact

that the brane velocity is limited by the velocity of light [13],

confirming the insight of Born and Infeld [11].

arX

iv:1

703.

0845

0v2

[hep

-ph]

24

May

201

7

Axion-like particles

Born-Infeld extension of QED

arXiv:1810.04602

gγγa < 2x10-4 GeV-1 for 6<ma<30 GeV

(see e.g. Knapen et al., PRL 118 (2017) 171801)

http://arxiv.org/abs/arXiv:1810.04602


▪ New measurement performed using 1.73 nb-1 of data collected in November 2018

▪ 3.5x more statistics (cf. 2015 dataset)

▪ Several analysis improvements (wrt previous measurement) ▪ Better trigger (higher efficiency at lowest photon ET’s) ▪ Photon PID based on Neural Network discriminator ▪ …

New ATLAS measurement

12

ATLAS-CONF-2019-002

[GeV]cluster2TE+cluster1

TE4 6 8 10 12 14

Leve

l-1 tr

igge

r effi

cien

cy

0

0.2

0.4

0.6

0.8

1ATLAS Preliminary

=5.02 TeVNNsPb+Pb -1Data 2018, 1.7 nb

Fit to dataStat

syst⊕Stat

[GeV]TPhoton E0 5 10 15 20 25

Phot

on P

ID e

ffici

ency

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05ATLAS Preliminary

=5.02 TeVNNsPb+Pb FSR photons-1Data 2018, 1.7 nb

MCγγ → γγ


Aco0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Even

ts /

0.00

5

0

5

10

15

20

25

30

35

40

45

50 PreliminaryATLAS

= 5.02 TeVNNsPb+Pb

-1Data 2018, 1.7 nb)γγ → γγSignal (

γγ →CEP gg ee→ γγ

Sys. unc.

▪ Signal region (Aco < 0.01) ▪ 59 events observed in 2018 data (12 ±3 background events expected)

▪ Aco < 0.005 region is ued to extract significance ▪ 42 events observed (6 ±2 background events expected) ▪ 8.2σ (6.2σ) observed (expected)

▪ Updated cross-section: σATLAS = 78 ±13 (stat) ±8 (sys) nb ▪ SM predictions: 49 ±5 nb


13

ATLAS-CONF-2019-002

[GeV]γγm5 10 15 20 25 30

Even

ts /

GeV

0

2

4

6

8

10

12

14

16

18 PreliminaryATLAS

= 5.02 TeVNNsPb+Pb



Sys. unc.


γγ → γγ event candidate from Nov 2018 Pb+Pb data

14

mγγ = 29 GeV

ATLAS-CONF-2019-002


Summary▪ Using LHC as a photon-photon collider works very well

▪ First high-energy evidence for LbyL scattering by ATLAS & CMS ▪ Based on 2015 Pb+Pb dataset @ 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 2018 @ 5.02 TeV ▪ 8.2σ (6.2σ) observed (expected) significance ▪ Fiducial cross-section measured with 20% precision

15


References

16

▪ ATLAS Collaboration, Observation of light-by-light scattering in ultraperipheral Pb+Pb collisions with the ATLAS detector, ATLAS-CONF-2019-002

▪ CMS Collaboration, Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at √sNN = 5.02 TeV arXiv:1810.04602 [hep-ex]

▪ ATLAS Collaboration, Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC, Nature Phys. 13 (2017) 852

▪ ATLAS Collaboration, Measurement of high-mass dimuon pairs in ultra-peripheral lead-lead collisions at √sNN = 5.02 TeV with the ATLAS detector at the LHC, ATLAS-CONF-2016-025


https://doi.org/10.1038/nphys4208

https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2016-025/


Backup

17



18

3− 2− 1− 0 1 2 3γγ

y0

5

10

15

20

25

Even

ts /

0.6

PreliminaryATLAS

= 5.02 TeVNNsPb+Pb



Sys. unc.

[GeV]γγ

Tp

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Even

ts /

0.2

GeV

0

5

10

15

20

25 PreliminaryATLAS

= 5.02 TeVNNsPb+Pb



Sys. unc.



19

0 10 20 30 40 50 60 70 80 [GeV]eem

1

10

210

310

410

510

610

Even

ts /

2 G

eV Preliminary ATLAS

= 5.02 TeVNNsPb+Pb

ee selection

-1Data 2018, 1.7 nbee MC→γγ

Sys. unc.

0 10 20 30 40 50 60 70 80 [GeV]eem

0.60.8

11.21.4

Dat

a / M

C 3− 2− 1− 0 1 2 3eey

500

1000

1500

2000

2500

Even

ts /

0.1

Preliminary ATLAS

= 5.02 TeVNNsPb+Pb

ee selection


Sys. unc.

3− 2− 1− 0 1 2 3eey

0.60.8

11.21.4

Dat

a / M

C



20

ratioE0 0.2 0.4 0.6 0.8 1

Even

ts /

0.01

2

0

0.01

0.02

0.03

0.04

0.05

0.06

Signal

Background

ATLAS Preliminary=5.02 TeVNNSPb+Pb

[GeV]trk2T

- peT,1E

0 2 4 6 8 10 12 14 16 18 20Ph

oton

reco

nstru

ctio

n ef

ficie

ncy

0.4

0.5

0.6

0.7

0.8

0.9

1 ATLAS Preliminary

=5.02 TeVNNsPb+Pb (hard-brem) selectionγee


M. Dyndal 17 Mar 2019 Light-by-light scattering in ATLAS and CMS in Run2

▪ First EFT constraints on nonlinear Lorentz-violating operators in QED

▪ Based on the ATLAS measurement, constraints are put on 126 nonlinear operators with d = 8

BSM interpretations

21

[arXiv:1812.11672]

(…)


PbPb (γγ) → PbPb X process calculations

[Fermi, Nuovo Cim. 2 (1925) 143]

[Weizsacker, Z. Phys. 88 (1934) 612] [Williams, Phys. Rev. 45 (10 1934) 729]

The cross section for AA (γγ) → AA X process can be calculated using:

(1) Number of equivalent photons (EPA) by integration of relevant EM form factors:

(2) EW γγ → X (elementary) cross section

Impact parameter > 2R

22

Strong fields, up to 1025 Vm−1 at the LHC


Theoretical uncertainties & higher-order corrections

23

▪ Theory uncertainties dominated by modeling of nuclear form factors (~10%)

▪ Higher-order corrections relatively small(<3%) for |eta|<2.5

arXiv:1902.00268

arXiv:hep-ph/0109079

arXiv:1606.01058


LHC as a photon-photon collider

▪ pp collisions + harder EPA spectrum (ωmax ~ TeV)

- large pile-up (multiple interactions per bunch crossing) + large datasets available, O(10 fb-1)

- hard to trigger on low-pT objects

▪ Pb+Pb collisions - softer EPA spectrum (ωmax~100 GeV) + AA (γγ) cross-sections scale as Z4 + gluonic cross-sections scale as ~A2 * (lower QCD bkg expected wrt pp) + low pile-up (<1%)* - Short LHC Pb+Pb campaigns (cf. pp)

24


(GeV)s0 0.5 1 1.5 2 2.5 3

) (pb

)γγ

→ γγ (σ

-310

-210

-110

1

10

210

310tensor contributions

total(1275)2f(1320)2a’(1525)2f(1565)2f(1700)2a

θcos-1 -0.5 0 0.5 1

(pb)

θ)/d

cos

γγ → γγ(

σd

-310

-210

-110

1

10

210

310

410 = 1.3 GeVs

(1320), total2

(1270) & a2f, total2f, helicity-22f, helicity-02f, total2a

FIG. 4: The energy dependence of the s-channel tensor-meson resonances (left) and the angulardistributions at

√s = 1.3 GeV (right).

(GeV)s0 1 2 3 4

(pb)

σ

-310

-110

10

310

510

710γγ → γγ

fermionic contributionsleptonsquarksmesonic contributionsscalarspsudoscalarstensors

(2050)4f

(GeV)s0 1 2 3 4

(pb)

σ

-310

-110

10

310

510

710| < 0.6θ, |cosγγ → γγ

fermionic contributionsleptonsquarksmesonic contributionsscalarspsudoscalarstensors

FIG. 5: The energy dependence of the meson exchange contributions compared with the fermion-box ones. Results integrated over full z-range (left) and for |z| < 0.6 (right) are plotted. Thef4(2050) meson contribution is calculated from (2.19).

the order of 0.5 GeV while in low-energy e+e− collisions because of limited phase spaceand the presence of two-photon bremsstrahlung background. The region of f2(1270)seems quite interesting as here some enhancement could be potentially identified by theBelle II at SuperKEKB for instance. Imposing a cut |z| < 0.6 (see the right panel of Fig. 5)improves the signal (meson exchanges) to background (boxes) ratio.

The meson exchange contributions are limited only to√

s < 4 GeV, and should not

9

pseudoscalars

Role of QCD meson exchanges

Lebiedowicz et al.

Phys. Lett. B 772 (2017) 330-335

γ(p1)

γ(p2)

γ(p3)

γ(p4)ps

γ(p1)

γ(p2)

γ(p3)

γ(p4)

pt

γ(p1)

γ(p2)

γ(p4)

γ(p3)

pu

FIG. 1: Diagrams for light-by-light scattering via a time-like (s-channel) and a space-like (t-channeland u-channel) meson exchanges.

variables used in the present paper are

s = (p1 + p2)2 = (p3 + p4)

2 ,

t = (p1 − p3)2 = (p2 − p4)

2 ,

u = (p2 − p3)2 = (p1 − p4)

2 ,

ps = p1 + p2 = p3 + p4 ,

pt = p2 − p4 = p3 − p1 ,

pu = p1 − p4 = p3 − p2 ,

p2s = s , p2

t = t , p2u = u . (2.3)

The amplitude for the reaction (2.1) with the meson exchanges is written as

Mλ1λ2→λ3λ4= ∑

MPS=π0,η,η′(958),ηc(1S),ηc(2S)

M(MPS)λ1λ2→λ3λ4

+ ∑MS= f0(500), f0(980),a0(980), f0(1370),χc0(1P)

M(MS)λ1λ2→λ3λ4

+ ∑MT= f2(1270),a0(1320), f ′2(1525)

M(MT)λ1λ2→λ3λ4

. (2.4)

In Table I we have collected possible potential resonances that may contribute to theprocess (2.1). The contribution of axial-vector mesons vanishes for on-shell photons dueto the Landau-Yang theorem [13]. The two-photon branching fractions for the resonancesare relatively well known and were measured in recent years by the Belle and BaBarcollaborations.

A. Pseudoscalar meson exchanges

The amplitude for the pseudoscalar meson exchange is written as

iM(MPS)λ1λ2→λ3λ4

= (ϵµ33 )∗ iΓ

(MPSγγ)µ3µ4

(p3, p4) (ϵµ44 )∗ i∆(MPS)(ps) ϵ

µ11 iΓ

(MPSγγ)µ1µ2

(p1, p2) ϵµ22

+(ϵµ33 )∗ iΓ

(MPSγγ)µ3µ1

(−p3, p1) ϵµ11 i∆(MPS)(pt) (ϵ

µ44 )∗ iΓ

(MPSγγ)µ4µ2

(p4, p2) ϵµ22

+(ϵµ44 )∗ iΓ

(MPSγγ)µ4µ1

(p4, p1) ϵµ11 i∆(MPS)(pu) (ϵ

µ33 )∗ iΓ

(MPSγγ)µ3µ2

(−p3, p2) ϵµ22 ,

(2.5)

where ϵµii are the polarisation vectors of the photons with the helicities λi.

3

25


▪ Installed at ±140 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)

Zero Degree Calorimeters

26

3

At a center of mass energy of√

sNN = 200 GeV per nu-cleon pair, the production cross section is expected to be33,000 b, or 4,400 times the hadronic cross section [1, 2].

The electromagnetic fields are strong enough, with cou-pling Zα ≈ 0.6, (Z is the nuclear charge and α ≈ 1/137the fine-structure constant), that conventional perturba-tive calculations of the process are questionable. Manygroups have studied higher-order calculations of pair pro-duction. Some early coupled-channel calculations pre-dicted huge (order-of-magnitude) enhancements in thecross section [3] compared to lowest-order perturbativecalculations.

Ivanov, Schiller and Serbo [4] followed the Bethe-Maximon approach [5], and found that at RHIC,Coulomb corrections to account for pair production in theelectromagnetic potential of the ions reduce the cross sec-tion 25% below the lowest-order result. For high-energyreal photons incident on a heavy atom, these Coulombcorrections are independent of the photon energy anddepend only weakly on the pair mass [5]. However, forintermediate-energy photons, there is a pair-mass depen-dence, and also a difference between the e+ and e− spec-tra due to interference between different order terms [6].

In contrast, initial all-orders calculations based on solv-ing the Dirac equation exactly in the ultra-relativisticlimit [7] found results that match the lowest-order per-turbative result [8]. However, improved all-orders calcu-lations have agreed with the Coulomb corrected calcula-tion [9]. These all-orders calculations do not predict thekinematic distributions of the produced pairs.

Any higher-order corrections should be the largestclose to the nuclei, where the photon densities are largest.These high-density regions have the largest overlap atsmall ion-ion impact parameters, b. Small-b collisions canbe selected by choosing events where the nuclei undergoCoulomb excitation, followed by dissociation. The disso-ciation also provides a convenient experimental trigger.Pair production accompanied by mutual Coulomb exci-tation should occur at smaller b, and have larger higher-order corrections than for unaccompanied pairs.

Previous measurements of e+e− pair production wereat much lower energies [10, 11]. The cross sections, pairmasses, angular and pT distributions generally agreedwith the leading-order QED perturbative calculations.These studies did not require that the nuclei break up,and so covered a wide range of impact parameters.

This letter reports on electromagnetic production ofe+e− pairs accompanied by Coulomb nuclear breakupin

√sNN = 200 GeV per nucleon pair Au-Au collisions

[12], as is shown in Fig. 1. An e+e− pair is producedfrom two photons, while the nuclei exchange additional,independent photons, which break up the nuclei. Werequire that there be no hadronic interactions, which isroughly equivalent to setting the minimum impact pa-rameter bmin at twice the nuclear radius, RA, i.e. about13 fm. The Coulomb nuclear breakup requirement selects

Au

e

Au*Au

e

+

Au*

−

FIG. 1: Schematic QED lowest-order diagram for e+e− pro-duction accompanied by mutual Coulomb excitation. Thedashed line shows the factorization into mutual Coulomb ex-citation and e+e− production.

moderate impact parameter collisions (2RA < b <≈ 30fm) [13, 14]. Except for the common impact parameter,the mutual Coulomb dissociation is independent of thee+e− production [15, 16]. The cross section is

σ(AuAu → Au∗Au∗e+e−) =

!d2bPee(b)P2EXC(b) (1)

where Pee(b) and P2EXC(b) are the probabilities of e+e−

production and mutual excitation, respectively at im-pact parameter b. The decay of the excited nucleus usu-ally involves neutron emission. P2EXC(b) is based onexperimental studies of neutron emission in photodisso-ciation [17]. For small b, a leading-order calculation ofP2EXC(b) may exceed 1. A unitarization procedure isused to correct P2EXC(b) to account for multiple inter-actions [14, 17].

The most common excitation is a giant dipole reso-nance (GDR). GDRs usually decay by single neutronemission. Other resonances decay to final states withhigher neutron multiplicities. In mutual Coulomb disso-ciation, each nucleus emits a photon which dissociates theother nucleus. The neutrons are a distinctive signaturefor nuclear breakup.

We consider two different pair production calculationsfor Pee(b). The first uses the equivalent photon approach(EPA) [1], which is commonly used to study photopro-duction. The photon flux from each nucleus is calculatedusing the Weizsacker-Williams method. The photons aretreated as if they were real [2]. The e+e− pair produc-tion is then calculated using the lowest-order diagram[18]. The photon pT spectrum for a photon with energyk is given by [19, 20]

dN

dpT≈

F 2(k2/γ2 + p2T )p2

T

π2(k2/γ2 + p2T )2

(2)

where F is the nuclear form factor and γ is the Lorentzboost of a nucleus in the laboratory frame. This calcula-tion uses a Woods-Saxon distribution with a gold radius

Pb

Pb

Pb*

Pb*


▪ CEP gg → γγ background ▪MC simulation (with data-driven normalization) is cross-checked in the analysis of ZDC activity

▪ Aco > 0.01 used as a control region ▪ Energy deposits corresponding to at least 1 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: ▪ What we see in the detector (Aco > 0.01) is consistent with the incoherent CEP background + some ee events with Coulomb breakup (not included in the plot)(signal region: 11/13 events have no ZDC activity)

ZDC check (ATLAS)

27

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

Documents

Transcript of Light-by-light scattering in ATLAS and CMS in...