Searching for Dark Photons at the LHeC & FCC-he
Zeren Simon Wang
1909.02312work with M. D’Onofrio and O. Fischer
November 27, 2019Sixth LHC LLP Community Workshop
Ghent University, Ghent, Belgium
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 1 / 11
Motivation
Dark Matter
Hidden sector theories predict new particles to interact with theStandard Model (SM) field content via feebly coupled mediatorparticles
Non-gravitational portals between the dark sector and the SM:scalar: dark Higgs
pseudoscalar: axion-like particles
vector: dark photons
neutrino: heavy neutral leptons
Studied at the HL-LHC, CEPC, FCC-ee/hh, beam-dump, andfixed-target experiments
Sensitivity of electron-proton colliders may be unique:LHeC & FCC-he
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 2 / 11
The dark photon model
Extend the SM gauge group by an additional (broken) gauge group:U(1)X
L ⊃ −1
4FµνF
µν − 1
4XµνX
µν − ε
2XµνF
µν
Applying a field re-definition to get rid of the kinetic mixing termXµνF
µν
L ⊃ −∑f
f̄ ε e qf /A′f
Two parameters: mass mγ′ and mixing ε
Production and decay controlled by one parameter ε
Focus on MeV-GeV mass range in this work
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 3 / 11
LHeC and FCC-he, and dark photon production
Ee (GeV) Ep (TeV)√s (TeV) Lint (ab−1)
LHeC 60 7 1.3 1FCC-he 60 50 3.5 3
p
e−
X
γ ′
e−
γ
p X
e−
e−
γ ′
LHeC, Pt(X)>10 GeV
LHeC, Pt(X)>5 GeV
FCC-he, Pt(X)>10 GeV
FCC-he, Pt(X)>5 GeV
0 2 4 6 8 10
0.05
0.10
0.50
1
5
10
mγ' [GeV]
σ(e
-p→e-pγ')/ϵ2[nb]
DIS: Q2 � m2p ' 1 GeV2
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 4 / 11
Signal estimation
mγ' [MeV]100
300
500
-4 -2 0 2 4
η(γ')
arbitraryunits
mγ' [MeV]100
300
500
0 10 20 30 40 50 60
P(γ')
arbitraryunits
Ndv(√s,L,mX , ε)
= σ(M, ε)L ×∫
D(ϑ, γ)Pdv(xmin(ϑ), xmax(ϑ),∆xlab(τ, γ)) dϑdγ
Pdv = Exp
(−xmin
∆xlab
)− Exp
(−xmax
∆xlab
)xmin = 200µm, xmax =∞,∆xlab = τlab|~v | = βγ′γγ′ τc
η(e/µ) η(jets)
LHeC (-4.3,4.9) (-5,5)FCC-he (-5.0,5.2) (-5.5,5.5)
In practice, apply |η| < 4.7
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 5 / 11
Dark photon decays
[Raggi, Kozhuharov,’15]
Γ(γ′ → l+l−) =1
3αQED mγ′ ε2
√1− 4m2
l
m2γ′
(1 +
2m2l
m2γ′
)Γtotal(γ
′) =Γ(γ′ → e−e+)
BR(γ′ → e−e+)
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 6 / 11
Background
Real low-energy photons interacting with the detector material or thebeam pipe → e−e+ pairs
location of the secondary vertex coincides with the detector material orthe beam pipeeasily rejected
Long-lived hadrons such as KS , KL, and Λ
lifetime far away from IP (3cm, 15m, and 8cm)hadronic activity is aligned with the proton beam and propagatesmostly into the forward hemisphere of the detectortheir primary decay channels are only marginally consistent with oursignal signaturetheir masses are well known
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 7 / 11
Results I: 90% C.L. sensitivity reaches
N=1
N=10
N=10010 nm
1 μm
100 μm10 mm
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81.×10-5
5.×10-5
1.×10-4
5.×10-4
0.001
0.005
mγ' [GeV]
ϵ
LHeC, Pt (X)=5 GeV
N=1
N=10
N=10010 nm
1 μm
100 μm10 mm
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81.×10-5
5.×10-5
1.×10-4
5.×10-4
0.001
0.005
mγ' [GeV]
ϵ
LHeC, Pt (X)=10 GeV
N=1
N=10
N=10010 nm
1 μm
100 μm10 mm
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81.×10-5
5.×10-5
1.×10-4
5.×10-4
0.001
0.005
mγ' [GeV]
ϵ
FCC-he, Pt (X)=5 GeV
N=1
N=10
N=10010 nm
1 μm
100 μm10 mm
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81.×10-5
5.×10-5
1.×10-4
5.×10-4
0.001
0.005
mγ' [GeV]
ϵ
FCC-he, Pt (X)=10 GeV
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 8 / 11
Results II: 90% C.L. sensitivity reaches
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81.×10-5
5.×10-5
1.×10-4
5.×10-4
10-3
mγ' [GeV]
ϵ
Present exclusion
LHeC, Pt(X) = 5 GeV
LHCb
Nbkg=0, signal efficiency = 100%
Nbkg=0, signal efficiency = 20%
Nbkg=100, signal efficiency = 100%
Nbkg=100, signal efficiency = 20%
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81.×10-5
5.×10-5
1.×10-4
5.×10-4
10-3
mγ' [GeV]
ϵ
Present exclusion
FCC-he, Pt(X) = 5 GeV
LHCb
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 9 / 11
Results III: Comparison
[Physics Briefing Book: Input for the European Strategy for Particle Physics Update 2020 [1910.11775 ]]
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 10 / 11
Summary
Portals to the dark sector
Vector portal and dark photon model
Electron-proton colliders may explore a unique territory in theparameter space for MeV-GeV dark photons
Thank You!
Z.S. Wang Searching for Dark Photons at the LHeC & FCC-he 11 / 11