Hunting for Exotic Particles at the LHC

Author: Denis Avetisyan

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The MoEDAL-MAPP experiment offers a unique approach to searching for charged, long-lived particles that could reveal physics beyond our current understanding.

This review assesses the potential of MoEDAL-MAPP to detect beyond-the-Standard-Model particles with intermediate and high electric charges, complementing searches at ATLAS and CMS.

Despite the Standard Model’s successes, compelling evidence suggests the existence of physics beyond its current framework. This mini-review, ‘Prospects for detecting charged long-lived BSM particles at MoEDAL-MAPP experiment: A mini-review’, assesses the potential of the MoEDAL-MAPP experiment to uniquely probe these new particles, specifically those carrying electric charge and exhibiting extended lifetimes. Our synthesis of recent studies demonstrates that MoEDAL-MAPP offers complementary sensitivity to ATLAS and CMS, particularly for scenarios involving slow-moving, multiply-charged states. Given the anticipated increase in luminosity at the High Luminosity LHC, how will dedicated experiments like MoEDAL-MAPP refine our understanding of these elusive beyond-the-Standard-Model signatures?

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The Illusion of Completeness

Despite its remarkable predictive power and consistent validation through experiments like those at the Large Hadron Collider, the Standard Model of particle physics remains incomplete. This foundational theory, describing the fundamental forces and particles of the universe, fails to account for observed phenomena like dark matter, dark energy, and the mass of neutrinos. Furthermore, it offers no explanation for the matter-antimatter asymmetry in the universe, nor does it incorporate gravity in a consistent manner. These unresolved puzzles strongly suggest the existence of physics beyond the Standard Model, driving ongoing research into theoretical frameworks – such as supersymmetry, extra dimensions, and string theory – and motivating the search for new particles and interactions that could provide a more complete understanding of the cosmos.

Theoretical frameworks extending the Standard Model of particle physics frequently posit the existence of long-lived particles (LLPs), differing dramatically from the fleeting particles typically observed in high-energy collisions. These LLPs, unlike those decaying almost instantly, would travel a measurable distance before disintegrating, potentially appearing as displaced vertices or exhibiting unusual signatures within detectors. The motivation for these particles arises from attempts to address shortcomings in the Standard Model – such as explaining dark matter, neutrino masses, or the matter-antimatter asymmetry – and their predicted lifetimes vary widely, ranging from millimeters to kilometers. Consequently, the Large Hadron Collider offers a unique opportunity to produce and detect these elusive entities, demanding innovative experimental techniques designed to capture their extended decay paths and differentiate them from background noise.

The pursuit of long-lived particles (LLPs) necessitates a departure from conventional detection methods at high-energy colliders. Traditional detectors, designed to capture the immediate aftermath of particle collisions, often fail to register LLPs that travel a measurable distance before decaying. Consequently, physicists are developing innovative search strategies focused on identifying the decay products of these escaped particles – often photons, leptons, or displaced vertices – far from the interaction point. These techniques include utilizing specialized detector volumes positioned at larger radii, employing timing detectors to resolve the flight path of LLPs, and leveraging machine learning algorithms to distinguish subtle LLP signatures from background noise. The challenge lies in reconstructing the original particle’s properties from these delayed and often fragmented remnants, demanding creative experimental designs and sophisticated data analysis pipelines to fully explore this potentially groundbreaking area of physics.

Hunting Ghosts: A Dedicated Search

MoEDAL (Monopole and Other Exotic Particle Search) is a dedicated experiment installed at the Large Hadron Collider (LHC) specifically designed to search for particles beyond those predicted by the Standard Model. Its primary targets include highly ionizing particles such as magnetic monopoles, as well as long-lived particles (LLPs) which may decay after traversing a significant distance. Unlike conventional LHC detectors focused on immediate interaction signatures, MoEDAL is designed to detect these exotic particles even if they do not interact strongly within the primary interaction point, offering a complementary search strategy to existing experiments. The detector’s unique design allows it to operate passively, recording the tracks of potentially interacting particles without requiring active electronic components in the immediate vicinity of the LHC beam pipe.

MoEDAL’s passive operation distinguishes it from typical particle detectors which rely on active components like electronics and magnetic fields for particle tracking and identification. Instead, MoEDAL employs a modular detector consisting of alternating layers of aluminum and plastic scintillator. Interacting particles traverse these layers, and the aluminum layers induce ionization, while the plastic layers record the passage of charged particles via scintillation light. This configuration allows MoEDAL to continuously record data without requiring external power or triggering systems, enabling it to detect rare, highly ionizing particles that might otherwise be missed by active detectors.

Nuclear Track Detectors (NTDs) employed within the MoEDAL experiment utilize a chemical etching process to create a lasting record of charged particle trajectories. These detectors, composed of layers of polycarbonate plastic, undergo a controlled etching procedure where the passage of a charged particle creates latent damage tracks. Subsequent chemical etching preferentially removes material along these damaged regions, forming microscopic, cone-shaped pits aligned with the particle’s path. The opening angle of the cone is inversely proportional to the particle’s velocity, and the pit length correlates to the particle’s energy loss. This process effectively ‘freezes’ the particle’s trajectory, allowing for offline reconstruction and analysis even after the initial interaction event has passed, which is crucial for detecting slowly moving or non-interactive particles.

Simulating the Unseen: Validating the Hunt

A fast simulation framework has been developed to model the response of the MoEDAL detector to charged Long-Lived Particles (LLPs). This framework utilizes a parameterized detector model to efficiently generate simulated events, allowing for rapid assessment of detector sensitivity and optimization of analysis strategies. The simulation incorporates key aspects of the MoEDAL detector, including the arrangement of silicon microstrip detectors and the magnetic field, to accurately represent the expected signal and background contributions. The computational efficiency of this framework enables the generation of large datasets necessary for evaluating the discovery potential of MoEDAL across a range of LLP scenarios and parameter spaces.

The MoEDAL fast simulation framework enables the rapid generation and analysis of large datasets of simulated events, crucial for optimizing the detector’s search strategies. This is achieved through parameterized modeling of detector response, allowing physicists to efficiently explore a wide range of long-lived particle (LLP) scenarios and refine analysis techniques. By simulating events with an integrated luminosity of 300 \text{ fb}^{-1}, the framework facilitates estimation of MoEDAL’s discovery potential, quantified by signal significance criteria of Nsig = 1, 2, and 3, and allows for systematic variation of analysis parameters to maximize sensitivity.

Projections based on simulations utilizing an integrated luminosity of 300 fb⁻¹ demonstrate MoEDAL’s sensitivity to a diverse range of Long-Lived Particle (LLP) scenarios, encompassing those predicted by models beyond the Standard Model, including supersymmetry (SUSY). Discovery potential was assessed using criteria defined by expected signal significance, specifically evaluating scenarios with Nsig values of 1, 2, and 3, representing different levels of statistical evidence for a potential signal. This methodology allows for a quantitative assessment of MoEDAL’s capacity to identify or constrain parameters within these theoretical frameworks, providing crucial insight into potential new physics.

The MoEDAL experiment exhibits comparable or improved sensitivity to the ATLAS and CMS experiments in identifying long-lived particles (LLPs) with electric charges between 3e and 6e, where ‘e’ represents the elementary charge. This enhanced sensitivity stems from MoEDAL’s unique detector design, specifically its use of nuclear track detectors arranged around the beam interaction point. These detectors are highly effective at identifying highly ionizing particles, characteristic of LLPs within this charge range, and offer a complementary search strategy to the all-purpose detectors ATLAS and CMS. The sensitivity comparison is based on projections from simulated data and analysis of expected signal yields.

With the Run 2 detector geometry, MoEDAL’s sensitivity to staus, a potential supersymmetric particle, extends to a mass of 1.7 TeV. This mass reach is determined through detailed simulations incorporating an integrated luminosity of 300 fb⁻¹ and is based on search strategies optimized for stau pair production and decay. Future projections indicate an improved mass reach with an idealized detector geometry, suggesting potential for discovery beyond the current limitations of the Run 2 configuration. This improvement is predicated on optimizing detector acceptance and resolution for enhanced signal identification and background rejection.

A Chorus of Detectors: Broadening the Search

Beyond the specialized MoEDAL experiment, the ATLAS and CMS detectors at the Large Hadron Collider employ a variety of techniques to seek long-lived particles (LLPs). These general-purpose experiments aren’t limited to a single search strategy; instead, they utilize displaced vertex searches, which identify particles decaying some distance from the primary collision point, and actively hunt for heavy stable charged particles that would leave unusual ionization signatures within the detector. By analyzing the products of processes like the Drell-Yan interaction, researchers can identify potential deviations from established Standard Model predictions, effectively broadening the scope of the search and increasing the likelihood of discovering these elusive particles. This multifaceted approach, combining distinct methodologies, offers a robust pathway to exploring the full potential of the LHC in the quest for new physics beyond our current understanding.

ATLAS and CMS experiments at the Large Hadron Collider leverage the well-understood Drell-Yan process – a fundamental interaction producing lepton pairs – as a pathway to create long-lived particles (LLPs). By meticulously analyzing the decay products of these LLPs, researchers specifically hunt for instances where two photons (γ) emerge as a resonance – an excess of diphoton events at a particular energy. This signal would deviate from predictions based on the Standard Model of particle physics, hinting at the production and subsequent decay of a new, exotic particle. The precision of these searches, combined with the unique sensitivity of the MoEDAL experiment, offers a comprehensive approach to uncover potential new physics beyond the established framework.

The Large Hadron Collider’s potential for discovering new physics is maximized not through a single experiment, but through a synergistic approach. MoEDAL, with its dedicated search for long-lived particles escaping the main detectors, offers a unique sensitivity that complements the broad reach of ATLAS and CMS. While ATLAS and CMS excel at identifying particles produced directly or decaying rapidly within their detectors – often leveraging processes like the Drell-Yan interaction to create potential new resonances – MoEDAL targets particles that travel further before decaying. This combined strategy ensures a comprehensive exploration of possible new phenomena, covering a wider range of particle lifetimes and decay modes than any single experiment could achieve. The coordinated efforts allow physicists to build a more complete picture, increasing the likelihood of uncovering deviations from the Standard Model and expanding the frontiers of particle physics.

The pursuit of particles beyond the Standard Model, as detailed in this review of the MoEDAL-MAPP experiment, reveals a persistent tension between theoretical expectation and observational constraint. Each attempt to detect these long-lived, highly charged particles feels akin to charting a course through an opaque darkness, hoping for a signal amidst considerable uncertainty. As John Dewey observed, “Every great advance in science has issued from a new audacity of imagination.” This audacity drives physicists to construct increasingly complex models, yet the universe consistently reminds one that each measurement is a compromise between the desire to understand and the reality that refuses to be understood. The MoEDAL-MAPP experiment, with its unique approach to detecting multiply charged particles, embodies this courageous attempt to glimpse beyond the event horizon of current knowledge.

Where Do the Signals Go?

The pursuit of particles beyond the Standard Model is, at its core, an exercise in constructing elaborate illusions. This review of MoEDAL-MAPP’s potential merely delineates a more nuanced space for those illusions to dissolve. The experiment’s sensitivity to highly charged, long-lived particles offers a specific window, a fleeting glimpse, before the inevitable confrontation with silence. It is a testament to ingenuity, but also a reminder that even exquisitely designed detectors cannot compel nature to reveal its secrets.

The competitive edge highlighted-particularly for intermediate and high charges-is not a triumph, but a refinement of the search parameters. One adjusts the lens, hoping for a clearer image, knowing full well that the fundamental limitations remain. Each null result isn’t a failure of instrumentation, but an expansion of the darkness, a wider horizon beyond which any model, however elegant, may simply vanish.

Future iterations will undoubtedly push the boundaries of sensitivity, increasing the volume of the search. But one should remember that the universe isn’t obligated to fill the space. It is a humbling prospect. Every theory is just light that hasn’t yet vanished, and the event horizon, whether literal or conceptual, always awaits.

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Original article: https://arxiv.org/pdf/2512.23387.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

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2025-12-31 20:16