Measurement of light-by-light scattering in ATLAS · 2017. 4. 21. · Positron Production in...
Transcript of Measurement of light-by-light scattering in ATLAS · 2017. 4. 21. · Positron Production in...
Measurement of light-by-light scattering inATLAS
Marcin Guzik
AGH University of Science and Technology, Cracow
Seminarium środowiskowe Fizyki Cząstek Białasówka21 April 2017
1/29
Electromagnetic interactions in Pb+Pb collisions
2RPb < b
[Fermi, Nuovo Cim. 2 (1925) 143][Weizsacker, Z. Phys. 88 (1934) 612][Williams, Phys. Rev. 45 (10 1934) 729]
Equivalent Photon Approximation (EPA)
σEPAA1A2(γγ)→A1A2X =
∫∫dω1dω2 n1 (ω1)n2 (ω2)σγγ→X (Wγγ)
with n (b, ω) = Z2αemπω
∣∣∣∣∫ dq⊥q2⊥F(Q2)Q2 J1 (bq⊥)
∣∣∣∣2Q2 < 1
R2 and ωmax ≈ γR
2/29
LHC as a photon-photon colliderpp collisions Pb+Pb collisions
Pros
• harder EPA γ spectrum(ωmax ∼ TeV)
• more data available(∼ 35 fb−1
)Cons
• large pile-up (multipleinteractions per bunch crossing)
• problems with triggering on lowpT objects
Pros
• AA (γγ) x-sec ∝ Z4
• gluonic x-sec ∝ A2
⇒ lower QCD bkg.
• low pile-up (< 1%)
Cons
• softer EPA γ spectrum(ωmax ∼ 0.1TeV)
• relatively small datasample
[PR D94 (2016) 3, 032011]
[ALICE Collaboration,EPJC 73 (2013) 2617]
3/29
Search for light-by-light scattering[arXiv:1702.01625]
Motivation
• first direct observation ofγγ → γγ scattering
• previous indirect measurements used:a) multi-photon Breit-Wheeler
reaction(ω + nω0 → e+e−) [PRL 79 (1997) 1626]
b) photon splittingc) Delbruck scattering
Pb Pb
γ γ
γγ
Pb Pb
4/29
Positron Production in Multiphoton Light-by-Light Scattering[Physical Review Letters 79, 1626 (1997)]
95% CL on the residualbackground from showersof lost beam particles af-ter subtracting the laser-offpositron rate
simulation of trident processω + nω0 → e′e+e−
power law fittedto the data
Abstract
5/29
Scattering of 1.3 Mev Gamma-Rays by an Electric Field[Phys. Rev. 90, 720 (1953)]
6/29
Experimental investigation of high-energy photon splitting in atomic fields[Phys. Rev. Lett. 89 (2002) 061802], [arXiv:hep-ex/0111084]
• source: Compton scatteredlaser light
• Compton scattered electron Emeasured
• both final γ detected
7/29
Standard Model Predictions
Recent SM Predictions for the LHC[A. Szczurek et al. PRC 93 (2016) 4, 044907], [D. d’Enterria et al. PRL 111 (2013) 080405]
8/29
The LHC and the ATLAS detector
2015 Pb+Pb dataL ≈ 0.5 nb−1
√sNN = 5.02 TeV
9/29
The ATLAS detector components
ηTracking in 2T Solenoid
MuonsPhotons
2015 Pb+Pb dataL ≈ 0.5 nb−1
√sNN = 5.02 TeV
10/29
LbyL - Photon Identification
γ cuts: ET > 3 GeV, |η| < 2.37
Shower shape variables used to γ PID
• Eratio ≡ ratio of the energy difference associated with thelargest and second largest energy deposits to the sum of thesedeposits in the first layer of EM calo
• f1 ≡ fraction of energy reconstructed in the first layer withrespect to the total energy of the cluster
• Weta2 ≡ lateral width of the shower in the middle layer
γπ0
11/29
Search for light-by-light scattering[arXiv:1702.01625]
Trigger
• total ET in calorimeterbetween 5 and 200 GeV
• no more than one hit ininner MBTS
• less than 10 hits in the pixeldetector
Main sources of bkg.
• Central ExclusiveProduction (CEP) gg→ γγ
• misidentification of electronsfrom γγ → ee
Event Selection
• two photons withET > 3 GeV, |η| < 2.37
• no tracks from IP
• mγγ > 6 GeV, pγγT < 2 GeV
• Aco =(
1− ∆φπ
)< 0.01
acoplanarity cutγγ0 0.01 0.02 0.03 0.04 0.05 0.06
γγC
EP
+
Nsi
gN
/ si
gN
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
ATLAS
= 5.02 TeVNNsPb+Pb
< 2 GeVγγT
p
background MCγγsignal + CEP
Pb Pb
γ
γ
Pb Pb
12/29
More about background
acoplanarityγγ0.02 0.04 0.06 0.08 0.1 0.12
Events/0.01
0
1
2
3
4
5
6
7
8
9
10ATLAS
= 5.02 TeVNNsPb+Pb⩾1
ZDCData, n
MCγγCEP
CEP of γγ
• normalisation from fitto the data in regionof Aco > 0.02
• ±0.01 as a systematic
• ZDC based studies asa cross check
13/29
More about background γγ → e+e−
[GeV]-e+em0 5 10 15 20 25 30 35 40 45 50
Eve
nts
/ GeV
0
100
200
300
400
500
600
700
800ATLAS
=5.02 TeVNNsPb+Pb
| < 2.47eη > 2.5 GeV, |eTE
-1bµData, 480
MC -e+e→γγ
[GeV]-e+e
Tp
0 1 2 3 4 5
Eve
nts
/ 0.2
GeV
0
100
200
300
400
500
600
700
800
900ATLAS
=5.02 TeVNNsPb+Pb
| < 2.47eη > 2.5 GeV, |eTE
-1bµData, 480
MC -e+e→γγ
eη3− 2− 1− 0 1 2 3
Ele
ctro
ns /
0.1
0
100
200
300
400
500
600ATLAS
=5.02 TeVNNsPb+Pb
| < 2.47eη > 2.5 GeV, |eTE
-1bµData, 480
MC -e+e→γγ
[GeV]eTE
0 5 10 15 20 25
Ele
ctro
ns /
0.5
GeV
0
200
400
600
800
1000
1200 ATLAS
=5.02 TeVNNsPb+Pb
| < 2.47eη > 2.5 GeV, |eTE
-1bµData, 480
MC -e+e→γγ
14/29
More about background γγ → e+e−
acoplanarityγγ0 0.1 0.2 0.3 0.4 0.5 0.6
Eve
nts
/ 0.0
2
0
5
10
15
20
25
30ATLAS
=5.02 TeVNNsPb+Pb
= 1 control regiontrkN
-1bµData, 480 MC-e+e→γγ
10× MC γγ→γγ
acoplanarityγγ0 0.1 0.2 0.3 0.4 0.5 0.6
Eve
nts
/ 0.0
2
0
5
10
15
20
25
30ATLAS
=5.02 TeVNNsPb+Pb
= 2 control regiontrkN
-1bµData, 480 MC-e+e→γγ
10× MC γγ→γγ
• precision limited by the data statistic
• conservative uncertainty of 25% assigned to MC
15/29
Trigger and photon PID efficiency studies
Trigger efficiency (γγ → e+e−)
• independent trigger: coincidence of signals in both ZDC sides anda requirement on the total ET in the calorimeter below 50 GeV.
• events with only two reconstructed tracks and two EM energyclusters (with cl Aco < 0.2)
• about 70% at(Ecl1T + Ecl2T
)= 6 GeV to 100% above 9 GeV
• error function parametrisation used to reweight the MC• MBTS veto studied using supporting trigger (98± 2)%
γ PID (γγ → e+e−)
• one photonand two tracks
• FSR selection:pttγT < 1 GeV
[GeV]TE
0 2 4 6 8 10 12 14 16 18
Pho
ton
PID
effi
cien
cy
0
0.2
0.4
0.6
0.8
1
1.2
1.4ATLAS
=5.02 TeVNNsPb+Pb
-1bµData, 480
MC
17/29
Photon reconstruction efficiency studies
[GeV]γTE
0 2 4 6 8 10 12 14 16 18 20
Eve
nts
/ GeV
0
20
40
60
80
100
120ATLAS
=5.02 TeVNNsPb+Pb
=1γ=1, NeN
< 2 GeVtrk2
T> 5 GeV, pe
TE
-1bµData, 480
MC -e+e→γγ
[GeV]γ-e+e
Tp
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Eve
nts
/ 0.2
GeV
0
20
40
60
80
100
120ATLAS
=5.02 TeVNNsPb+Pb
=1γ=1, NeN
< 2 GeVtrk2
T> 5 GeV, pe
TE
-1bµData, 480
MC -e+e→γγ
γ reco (γγ → e+e−) withhard bremsstrahlung
• one electron withET > 5 GeV andtwo good qualitytracks
• unmatched track:ptrk2T < 2 GeV
[GeV]trk2T
- peTE
0 2 4 6 8 10 12 14 16 18
Pho
ton
reco
nstr
uctio
n ef
ficie
ncy
0
0.2
0.4
0.6
0.8
1
1.2
1.4ATLAS
=5.02 TeVNNsPb+Pb
-1bµData, 480
MC
18/29
Search for light-by-light scattering[arXiv:1702.01625]
Photon Performance Studies(done with γγ → l+l− events)
• trigger efficiency studies
• γ reconstruction with hardbremsstrahlung
• γ PID with FSR radiation
• γ energy scale andresolution
Systematic Uncertainty
dominated by:
• γ reco
• γ PID
[GeV]cluster2T + Ecluster1
TE0 5 10 15 20 25 30 35 40 45 50
Eve
nts
/ 0.5
GeV
0
50
100
150
200
250
300ATLAS
=5.02 TeVNNsPb+Pb
| < 2.47eη > 2.5 GeV, |eTE
-1bµData, 480
/ndf=150/89)2χdefault MC (
/ndf=210/89)2χ scale +5% (TE
/ndf=285/89)2χ scale -5% (TE
Source of uncertainty Relative uncertainty
Trigger 5%Photon reco efficiency 12%Photon PID efficiency 16%Photon energy scale 7%Photon energy resolution 11%
Total 24%19/29
Search for light-by-light scattering[arXiv:1702.01625]
Results: data - 13 event, expected - 7.3 signal and 2.6 bkg. eventsSelection γγ → e+e− CEP gg → γγ Hadronic Other Total Signal Data
fakes fakes background
Preselection 74 4.7 6 19 104 9.1 105Ntrk = 0 4.0 4.5 6 19 33 8.7 39pγγT < 2 GeV 3.5 4.4 3 1.3 12.2 8.5 21Aco < 0.01 1.3 0.9 0.3 0.1 2.6 7.3 13
Uncertainty 0.3 0.5 0.3 0.1 0.7 1.5
acoplanarityγγ0 0.01 0.02 0.03 0.04 0.05 0.06
Eve
nts
/ 0.0
05
0
2
4
6
8
10
12
14 ATLAS
= 5.02 TeVNNsPb+Pb
Signal selectionno Aco requirement
-1bµData, 480 MC γγ→γγ
MC -e+e→γγ MC γγCEP
|γγ
|y0 0.5 1 1.5 2 2.5 3
Eve
nts
/ 0.5
0
2
4
6
8
10
12
14 ATLAS
=5.02 TeVNNsPb+Pb
Signal selectionwith Aco < 0.01
-1bµData, 480 MC γγ→γγ
MC -e+e→γγ MC γγCEP
20/29
Search for light-by-light scattering[arXiv:1702.01625]
Results:• significance of 4.4σ estimated using profile likelihood
method (expected significance of 3.8σ)• x-sec measured in fiducial region of pγT > 3 GeV, |ηγ | < 2.4,
mγγ > 6 GeV, pγγT < 2 GeV, Aco < 0.01σ = 70± 20 (stat.)± 17 (syst.)nbSM predictions: 45± 9 nb ([PRL 111 (2013) 080405]), 49± 10 nb ([PRC 93 (2016) no.4, 044907])
γγ→γγµ
0 0.5 1 1.5 2 2.5 3
ln L
∆-
2
0
5
10
15
20
25
Observed
SM expected
ATLAS
=5.02 TeVNNsPb+Pb
-1bµL dt = 480 ∫
σ1
σ2
σ3
σ4
σ5
[GeV]γγT
p0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Eve
nts
/ GeV
0
2
4
6
8
10
12
14
16
18
20ATLAS
=5.02 TeVNNsPb+Pb
Signal selectionwith Aco < 0.01
-1bµData, 480 MC γγ→γγ
MC -e+e→γγ MC γγCEP
21/29
BSM Physics in HI - Axion Search With UPC[arXiv:1607.06083]
La = 12 (∂a)2 − 12m
2aa2 − 14
aΛFF
Pb Pb
γ γaγγ
Pb Pb
expected axion searches sensitivity
22/29
BSM Physics in HI - Axion Search With UPC[arXiv:1607.06083]
La = 12 (∂a)2 − 12m
2aa2 − 1
4 cos2 θWaΛBB
Pb Pb
γ γaγγ
Pb Pb
expected axion searches sensitivity
23/29
BSM Physics in HI - Axion Search With UPC[arXiv:1607.06083]
La = 12 (∂a)2 − 12m
2aa2 − 14
aΛFF
Pb Pb
γ γaγγ
Pb Pb
σALP = 5 nb, ma = 15 GeV and 40 GeVassumed E resolution of 0.5 GeV
24/29
BSM Physics in HI - Axion Search With UPC
[GeV]γγm0 5 10 15 20 25 30
Eve
nts
/ 0.5
GeV
0
2
4
6
8
10
12 ATLAS
=5.02 TeVNNsPb+Pb
Signal selectionAco < 0.01
-1bµData, 480 MC γγ→γγ
MC -e+e→γγ MC γγCEP
25/29
LbyL Scattering Constraint on Born-Infeld Theory[arXiv:1703.08450]
LQED = −14FµνFµν → LBI = β2
(1−
√1 + 1
2β2FµνFµν − 16β4
(FµνFµν
)2)
26/29
LbyL Scattering Constraint on Born-Infeld Theory[arXiv:1703.08450]
• limit on Born-Infeld scale M =√β & 100 GeV 5 orders of
magnitude grater than the previous one from PVLAS• in case of Born-Infeld SM extention with U (1)Y realized
nonlineary MY = cos θWM & 90 GeV• such theory has finite-energy electroweak monopole solution
less constrained by higgs than in other extensions of SMwhich because of MY → Mmonopole & 11 TeV is out of reachat the LHC
27/29
Summary
• The first direct evidence for γγ → γγ scattering withsignificance of 4.4σ has been reported.• improvements in the precision expected with more Pb+Pb
data to be collected in 2018
• This result has already been used by J. Ellis etal. [arXiv:1703.08450] to derive the limits on Born-Infeld theory
• According to S. Knapen et al. [arXiv:1607.06083] UPC data can beused in BSM searches eg. for ALP
28/29
Thank You for Your Attention!
29/29