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

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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

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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!

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