Is Dark Light or Dark Photons Possible in Theoretical Physics?

Dark photons, also known as “dark” force carriers, are hypothetical particles that are predicted to exist in certain theories of physics beyond the Standard Model. These particles are thought to carry forces associated with dark matter, which is a mysterious substance that is believed to make up around 85% of the universe’s mass. Dark matter is difficult to detect in the laboratory, as it does not absorb, reflect, or emit electromagnetic radiation, making it challenging to observe using traditional detection methods.

The Dark Photon Hypothesis

One theory that has been proposed to explain the nature of dark matter is the dark photon hypothesis. According to this hypothesis, dark photons could be responsible for the interactions between dark matter and the ordinary sector of the universe. Dark photons are thought to be able to mix with ordinary neutral gauge bosons, such as photons, W and Z bosons, which carry the electromagnetic and weak forces. This mixing could allow dark photons to couple in a relevant way to dark matter and, potentially, to other hypothetical dark sector particles.

The mixing between dark photons and ordinary photons can be described by the following Lagrangian term:

L_mix = -(ε/2) F_μν F'_μν

where F_μν and F'_μν are the field strength tensors of the ordinary photon and the dark photon, respectively, and ε is the kinetic mixing parameter, which determines the strength of the mixing.

The kinetic mixing between dark photons and ordinary photons can lead to a variety of observable effects, such as:

1. Invisible Decays of Ordinary Particles: If the dark photon is lighter than the particles in the Standard Model, it can lead to invisible decays of these particles, such as the decay of a neutral pion into a dark photon and a neutrino.

2. Anomalous Magnetic Moments: The kinetic mixing between dark photons and ordinary photons can also affect the anomalous magnetic moments of charged particles, such as the electron and the muon.

3. Beam Dump Experiments: Dark photons can be produced in beam dump experiments, where a high-energy beam of particles is directed onto a dense target. The dark photons can then decay into visible particles, which can be detected by the experiment.

4. Cosmological and Astrophysical Signatures: The presence of dark photons can also have cosmological and astrophysical signatures, such as affecting the cosmic microwave background radiation or the dynamics of dark matter halos in galaxies.

Recent Experimental Developments

There have been several recent studies that have suggested that dark photons could explain certain data from high-energy scattering experiments. For example, a new analysis conducted by an international team of physicists suggests that dark photons could explain certain data from deep inelastic scattering (DIS) experiments.

The analysis, which was led by Nicholas Hunt-Smith and colleagues at the University of Adelaide, Australia, focused on the discrepancy between the measured and predicted values of the proton’s structure function, F_2(x,Q^2), in DIS experiments. The researchers found that the inclusion of a dark photon contribution to the proton’s structure function could potentially resolve this discrepancy.

The dark photon contribution to the proton’s structure function can be described by the following expression:

F_2^{dark}(x,Q^2) = (ε^2 α'/α) F_2^{em}(x,Q^2) (m_γ'/Q)^2 / (1 + (m_γ'/Q)^2)

where ε is the kinetic mixing parameter, α' and α are the fine-structure constants of the dark and ordinary photons, respectively, m_γ' is the mass of the dark photon, and F_2^{em}(x,Q^2) is the electromagnetic structure function of the proton.

The analysis by Hunt-Smith and colleagues suggests that a dark photon with a mass of around 200 MeV and a kinetic mixing parameter of ε ≈ 10^-3 could potentially explain the observed discrepancy in the DIS data.

Another study, led by researchers at the Jefferson Lab in Virginia, US, looked at how dark photons could possibly affect collisions between particles at incredibly high energies by kinetically mixing with standard photons. Byproducts created by these collisions can give scientists a good picture of the universe at ultrasmall scales, as well as what physical laws are at play on these levels.

The researchers found that the presence of a dark photon could lead to deviations from the Standard Model predictions in the production of certain particles, such as dileptons (pairs of leptons, such as electrons or muons) or hadrons (particles made up of quarks and gluons).

These studies do not prove the existence of dark photons, but they do provide evidence that supports the dark photon hypothesis. Further research is needed to confirm the existence of dark photons and to understand their properties and interactions with other particles.

Challenges and Future Directions

Despite the promising results from these studies, there are still several challenges and open questions in the field of dark photon research:

1. Detection Challenges: Dark photons are extremely difficult to detect directly, as they do not interact strongly with ordinary matter. This makes it challenging to design experiments that can unambiguously identify the presence of dark photons.

2. Theoretical Uncertainties: The properties of dark photons, such as their mass and coupling strength, are not well-constrained by theory. This makes it difficult to make precise predictions about the observable effects of dark photons.

3. Competing Explanations: The discrepancies in high-energy scattering data that have been attributed to dark photons could also have other explanations, such as unknown systematic effects or the presence of other new physics.

4. Cosmological Constraints: The existence of dark photons can also have implications for the early universe and the formation of large-scale structures. These cosmological constraints need to be taken into account when considering the viability of the dark photon hypothesis.

Despite these challenges, the search for dark photons remains an active area of research in theoretical and experimental particle physics. Future experiments, such as the proposed SHiP (Search for Hidden Particles) experiment at CERN, are designed to specifically search for dark photons and other hypothetical dark sector particles. The continued development of new detection techniques and the refinement of theoretical models will be crucial in advancing our understanding of the nature of dark matter and the possible role of dark photons in the universe.

References:

1. Dark photons could explain high-energy scattering data
2. Searching for Dark Photons at the LHC
3. Probing the Dark Sector with Beam Dump Experiments