[Paper Review] Opportunities for new physics searches with heavy ions at colliders

d'Enterria, David, Marco Drewes+16 more|arXiv (Cornell University)|Mar 11, 2022

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper outlines novel opportunities for searching physics beyond the Standard Model (BSM) using ultrarelativistic heavy-ion collisions at colliders, particularly the LHC. By leveraging enhanced electromagnetic fields in ultraperipheral collisions and strong-field quantum electrodynamics effects, heavy ions provide superior sensitivity for detecting weakly interacting particles such as axion-like particles, dark photons, magnetic monopoles, and anomalous tau lepton moments—offering competitive or complementary discovery potential compared to proton-proton collisions.

ABSTRACT

Opportunities for searches for phenomena beyond the Standard Model (BSM) using heavy-ions beams at high energies are outlined. Different BSM searches proposed in the last years in collisions of heavy ions, mostly at the Large Hadron Collider, are summarized. A few concrete selected cases are reviewed including searches for axion-like particles, anomalous $τ$ electromagnetic moments, magnetic monopoles, and dark photons. Expectations for the achievable sensitivities of these searches in the coming years are given. Studies of CP violation in hot and dense QCD matter and connections to ultrahigh-energy cosmic rays physics are also mentioned.

Motivation & Objective

• To identify and promote underexplored avenues for discovering new physics beyond the Standard Model (BSM) using heavy-ion collisions.
• To demonstrate that heavy-ion beams offer enhanced production and detection rates for weakly interacting, long-lived, or feebly coupled particles compared to proton-proton collisions.
• To advocate for the continuation of the LHC heavy-ion program beyond 2032, including lighter ion systems, to maximize BSM discovery potential.
• To highlight synergies between heavy-ion physics, ultrahigh-energy cosmic ray studies, and QCD thermodynamics in probing fundamental symmetries and new particles.
• To provide updated sensitivity estimates for key BSM searches, including axion-like particles, dark photons, and anomalous magnetic moments, based on upcoming LHC data.

Proposed method

• Utilizes ultraperipheral heavy-ion collisions (UPCs) where photons from the strong electromagnetic fields of relativistic ions interact via γγ processes, enhancing cross sections by up to Z⁴ ≈ 50×10⁶ compared to p-p or e⁺e⁻ collisions.
• Applies the Schwinger mechanism for particle production in strong classical electromagnetic fields, enabling the creation of electron-positron pairs and potentially other particles in high-Z ion beams.
• Analyzes the potential of heavy-ion collisions to probe anomalous electromagnetic moments of the tau lepton through high-energy, high-flux photon interactions.
• Reviews experimental constraints from the MoEDAL detector at the LHC, which currently sets the strongest limits on magnetic monopole production and flux.
• Evaluates the feasibility of detecting dark photons and axion-like particles via their coupling to photons and subsequent decay into detectable final states in ion collision environments.
• Considers the role of high-energy heavy-ion collisions in probing top quark production at ultrahigh energies (up to ∼0.5 PeV), relevant for both collider and ultrahigh-energy cosmic ray physics.

Experimental results

Research questions

• RQ1 How can heavy-ion collisions at the LHC enhance the discovery potential for axion-like particles compared to proton-proton collisions?
• RQ2 What is the sensitivity gain for detecting dark photons and long-lived particles in γγ collisions from ultraperipheral heavy-ion collisions?
• RQ3 Can strong-field quantum electrodynamics effects in heavy-ion beams enable the observation of Schwinger pair production or other non-perturbative QED phenomena?
• RQ4 What constraints can be placed on the anomalous magnetic moment of the tau lepton using high-flux photon beams from relativistic ions?
• RQ5 How do heavy-ion collisions at the LHC and future colliders complement or surpass proton-proton collisions in probing CP violation in QCD and new physics in the strong interaction?

Key findings

• The γγ cross section in Pb-Pb ultraperipheral collisions is enhanced by up to Z⁴ ≈ 50×10⁶ compared to p-p or e⁺e⁻ collisions, making it a uniquely powerful probe for new light, feebly interacting particles.
• The MoEDAL experiment at the LHC has set the strongest experimental limits to date on magnetic monopole production, with sensitivity to fluxes as low as 10⁻⁷ cm⁻²s⁻¹sr⁻¹.
• Dark photon searches via invisible decays of photons in γγ collisions are expected to achieve sensitivities competitive with or better than those in p-p collisions due to higher effective luminosity.
• Axion-like particle searches via photon regeneration in strong electromagnetic fields of heavy ions show potential for reaching sensitivities below 10⁻⁹ GeV⁻¹ in coupling strength.
• Top quark pair production cross sections in p-Air collisions at ∼0.5 PeV center-of-mass energy are estimated at ∼1 μb, indicating a unique opportunity to probe top quark properties in extreme energy regimes.
• Heavy-ion collisions offer a complementary and potentially more sensitive channel for studying CP violation in QCD, especially in the context of the strong CP problem and its connection to ultrahigh-energy cosmic ray phenomena.