Breaking up the Proton: An Affair with Dark Forces

Area of Science:

• Particle Physics
• High-Energy Physics
• Cosmology

Background:

• Deep inelastic scattering (DIS) of electrons/positrons off protons probes the internal structure of protons.
• The Standard Model of particle physics can be extended with hypothetical particles like the dark photon.
• Existing electroweak precision observables offer constraints on dark photon properties.

Purpose of the Study:

• To derive model-independent bounds on the kinetic mixing parameter (ε) of a dark photon using HERA DIS data.
• To investigate the impact of dark photon exchange on parton distribution functions (PDFs) and scaling violations.
• To assess the potential of future colliders, such as the Large Hadron Electron Collider (LHC-e), for probing dark photons.

Main Methods:

• Analysis of existing HERA data for deep inelastic scattering of e^{±} off protons.
• Fitting HERA data to determine parton distribution functions (PDFs) and derive bounds on the dark photon kinetic mixing parameter (ε).
• Utilizing PDF sum rules to analyze the effects of dark photon exchange on scaling violations.

Main Results:

• A model-independent bound of ε ≲ 0.02 was obtained for dark photon masses ≲ 10 GeV, slightly improving on bounds from electroweak precision observables.
• For higher dark photon masses (up to 5 TeV), the limit on ε weakens monotonically (ε ≲ 1).
• Dark photon exchange leads to non-Bjorken-x-dependent scaling violations, serving as a potential signature in experimental data.

Conclusions:

• Deep inelastic scattering provides competitive and complementary constraints on dark photons compared to electroweak precision measurements.
• The effects of dark photons cannot be trivially absorbed into refitted PDFs and manifest as distinct scaling violations.
• Future colliders like the Large Hadron Electron Collider (LHC-e) will significantly enhance sensitivity to dark photons across a wide mass range.