Overview of the Search for Dark Matter inside Accelerators

Introduction

What follows is an overview of the search for dark matter in accelerators based on talk on cosmology & astroparticle physics. We cover in brief, the ongoing efforts to produce and identify dark matter within high-energy accelerators, such as the Large Hadron Collider (LHC). The writeup highlights the core ideas in the search for dark matter in accelerators, focusing on collider searches rather than detection methods. I provide an overview of how accelerators work and the strategies employed to detect dark matter within these high-energy environments. I describe the core methods and techniques used in collider searches to identify potential dark matter signatures. Furthermore, I discuss the key challenges and limitations that scientists face in these efforts. Lastly, I explore the theoretical frameworks guiding dark matter production then offer a glimpse into the expected future progress in this exciting field of research.

Overview of Dark Matter

Dark matter constitutes a substantial portion of the universe's total mass, forming the backbone of the cosmic web and playing a crucial role in the formation of galaxies and clusters. Despite its significance, dark matter remains undetected by direct electromagnetic observations, characterized by its unique properties. Dark matter is non-luminous, meaning it does not emit, absorb, or reflect light, rendering it invisible to traditional astronomical instruments. It is also massive, contributing about 85% of the total mass in the universe, and it interacts very weakly with ordinary matter, primarily through gravitational forces.

Moreover, dark matter is cold, moving at non-relativistic speeds, which allows it to clump together and form large-scale structures observed in the universe. Its stability over the age of the universe is another critical property, ensuring that dark matter particles have remained consistent throughout cosmic history to account for the observed gravitational effects. These characteristics make dark matter a challenging target for detection, necessitating innovative approaches and advanced technologies to study its elusive nature.

Detection Methods

Efforts to detect dark matter are broadly categorized into direct detection, indirect detection, and collider searches. Direct detection experiments aim to observe dark matter particles interacting with ordinary matter in highly sensitive underground detectors. These detectors seek to identify rare collisions between dark matter particles and atomic nuclei. For instance, experiments like LUX, XENON1T, and PandaX focus on detecting Weakly Interacting Massive Particles (WIMPs) through nuclear recoils. Axions, another dark matter candidate, are searched for via axion-photon conversion in magnetic fields, as seen in the ADMX experiment.

Indirect detection methods, on the other hand, look for the byproducts of dark matter annihilation or decay, such as gamma rays, neutrinos, or positrons. Telescopes and detectors on Earth and in space, like Fermi-LAT or H.E.S.S., observe these byproducts. Additionally, the CAST experiment searches for axions by looking for their conversion to photons in the magnetic fields of the Sun or galaxies. X-ray telescopes like Chandra are used to detect sterile neutrinos by observing their decay products.

In principle, interaction with ambient dark matter which forms the galactic dark matter halo may be possible since the earth plows through this dark matter as we trek across space in the great march about our galaxy’s center. A key challenge in detection however is the sparsity of dark matter and its general tendency to ignore our humble baryonic matter. An alternative to these detection methods which could offer greater experimental control in the study of dark matter would be to produce dark matter directly, in house, so to speak. This might be accomplished by creating dark matter inside of particle colliders, circumventing certain problems with detector apparatuses but presenting challenges of its own.

Collider searches, the primary focus of this paper, involve producing dark matter particles in high-energy collisions at particle accelerators, such as the Large Hadron Collider (LHC). The presence of dark matter is inferred from missing energy and momentum in the detector, as these particles do not interact with the detector material directly, leading to characteristic missing transverse energy (MET) signatures.

Introduction to Collider Searches

Particle accelerators use electromagnetic fields to propel charged particles, such as protons or electrons, to high speeds and contain them in well-defined beams. The high-energy collisions of these particles allow us to probe the fundamental structure of matter and the forces governing the universe. Instruments placed at collision points observe and record the results of these collisions, with detectors like ATLAS and CMS analyzing the events.

Accelerators offer a unique and powerful approach to searching for dark matter. There are several compelling reasons to use accelerators for this purpose. Firstly, accelerators can produce dark matter particles in high quantities, overcoming the low density of dark matter in space. For instance, while the local dark matter density is about 0.3 GeV/cm³, colliders might produce far higher densities, making detection more feasible and the subsequent study of dark matter more feasible.

Secondly, accelerators can accelerate particles to very high speeds and energies, enabling the study of a wide range of dark matter candidates by overcoming the "low mass threshold" problem. This is crucial because in space, dark matter particles might move slowly, but in a collider, they can be accelerated to near-light speeds, allowing detailed study of their properties. Thirdly, collider experiments can be controlled precisely, and run at a time of our choosing. This contrasts with detector type experiments which enable observations only at the rate which nature gives them to us.

Finally, many dark matter models predict particles as thermal relics from the Big Bang. These models give a specific target for searches in colliders. Thermal relic dark matter particles should have specific properties, such as mass and interaction cross-section, that can be tested in collider experiments.

Colliders can broadly be classified into lepton colliders or hadron colliders, both of which can be employed for the study of dark matter. Lepton colliders, which involve point-like elementary particles, allow for precise knowledge of the total energy and momentum involved in collisions, making them precision machines. Hadron colliders, on the other hand, involve composite particles like protons, which means the precise momentum and energy of the constituent particles cannot be determined exactly. However, the complex structure of hadronic collisions can offer richer opportunities for discovery.

Key Facilities

At this time, there are several key facilities that play a significant role in dark matter searches through collider experiments. Lepton accelerators, such as the Large Electron-Positron Collider (LEP) at CERN, are valuable due to their well-collimated beams and known total momentum. These characteristics make them ideal for detailed studies. Hadron accelerators, such as the Tevatron at Fermilab, have been crucial in past dark matter searches. The Tevatron operated from 1983 to 2011 and was a proton-antiproton collider. However, the difficulty and cost of antimatter creation limit the practical advantages of using such colliders for dark matter searches. Other notable facilities include SuperKEKB and Belle II in Japan, which focus on B-meson physics and searches for dark sector particles through high-luminosity electron-positron collisions. The SLAC National Accelerator Laboratory has also explored high-energy collisions and provided data for dark matter searches through experiments like the Stanford Linear Collider (SLC) and the PEP-II collider.

The Large Hadron Collider (LHC) stands out as the most prominent facility for dark matter searches. The LHC has a circumference of 27 kilometers and is situated about 100 meters below ground level on the Franco-Swiss border near Geneva, Switzerland. It uses superconducting magnets to steer and focus particle beams, which operate at a temperature of 1.9 Kelvin, colder than outer space. The LHC provides the necessary energy to potentially produce WIMPs, expected to have masses in the GeV-TeV range. High energy and luminosity are crucial for increasing the chances of detecting rare events, such as WIMP production. From here, I continue with descriptions applying to the high energy proton-proton collisions that occur in the LHC in order to present a clear idea of how the search for dark matter can be carried out inside a collider. Comments about alternative scenarios, such as in lepton colliders will also be made at times but it should be kept in mind that a variety of important facilities are engaged in the search. With this key background in mind then let’s explore the main ideas relevant to identifying dark matter inside a collider.

MET: Basic Explanation

Missing transverse energy (MET) signatures are a crucial tool in collider searches for dark matter. In a hadron collider like the LHC, the precise energy along the beam axis is unknown, but the transverse momentum before collision is zero. The resulting collision fragments become jets registered by the detector. By conservation of momentum, the net sum of their transverse momentum should be zero. If non-zero transverse momentum is found, it could indicate that the missing momentum went into dark matter, escaping detection.

In lepton accelerators, the total momentum before collision is known precisely, as they involve point-like particles. This simplifies MET identification but the principles remain the same and MET signatures in general are a key way in which dark matter could be identified. The name MET comes from the notation, wherein transverse energy is represented by ET, and the missing energy transverse becomes missing ET, However, it's worth noting that the signature MET may not correctly apply to dark matter. In the case of particles like neutrinos their momentum and energy are in essence the same but this need not hold true for dark matter. We are looking for missing transverse momentum more accurately, but the search strategy nonetheless is generally called MET.

Challenges of MET: Accelerator Challenges

Subatomic particles are incredibly small, and in the case of proton-proton collisions in the LHC, the difficulty can be likened to throwing needles and trying to get them to collide head-on in mid-air. The LHC accomplishes this by firing large quantities of protons very accurately. For example, experiments in Run 2 used 2,808 bunches of 100 billion protons each, focused down to fit within the diameter of a human hair, racing around the LHC for 11,245 turns per second to achieve 1 billion proton-proton collisions per second.

After successfully colliding protons, the problem shifts to analyzing the resulting complex cascade of particle interactions. Protons, composed of quarks and gluons, enter the collision as color singlets. Inside the protons, energetic partons (quarks and gluons) engage in hard scattering, governed by parton distribution functions (PDFs). The hard scattering between partons results in the production of high-energy particles, which then fragment and hadronize into jets. The proton remnants leave behind hadronic debris, consisting of low-energy particles.

The analysis of jets in detectors like CMS and ATLAS involves multiple layers of detection. The inner layer of silicon detectors tracks particles with high precision and identifies displaced vertices. The electromagnetic tracker follows the paths of charged particles, allowing scientists to reconstruct their trajectories and determine their properties. Electromagnetic and hadronic calorimeters are designed to absorb the energy of incoming electrons, photons, and hadrons, stopping them and measuring the energy they deposit. Muon detectors in the outer radius of the detector measure the momentum of muons, which can penetrate more material than other particles. Strong magnetic fields within the detector bend the paths of charged particles, and by observing the curvature of these paths, we can calculate the momentum of the particles. Jets, streams, and sprays of particles are then analyzed looking for MET. However, accurately accounting for all that happened in the collision is crucial to say that a MET signal might be dark matter.

Several factors limit the effectiveness of accelerator-based dark matter searches. One significant limitation is the parton distribution functions (PDFs), which represent the probability density for finding a particle with a specific longitudinal momentum fraction. The variability in PDFs across different parton types leads to uncertainties in the initial state, making it harder to identify MET signatures.

Another limitation is the energy of particle colliders, such as the LHC. The energy of these colliders is much lower than the energy scales of the early universe. For example, the LHC has a nominal collision energy of 13 TeV, while the early universe had energies around 10E16 TeV. This disparity limits the ability to probe dark matter particles with masses higher than what the collider can produce.

The lifetime of dark matter particles is another challenge. Dark matter particles are expected to be very long-lived, but in accelerators, we can only observe particles over very short timescales. Extrapolating from short-lived particles to make claims about dark matter's long lifetime is uncertain.

Additionally, detecting a particle in a collider and proving it's dark matter involves complementary experiments. Just detecting a particle doesn't confirm it's dark matter; it must be shown to have the right properties and interactions. Moreover, the identification can be difficult as dark matter may be hidden by background noise and errors in measurement can present a false signal for MET.

Background Noise

Background noise presents a significant challenge in identifying true dark matter signals. Mismeasurement of visible matter can appear as a false signal of MET. Various sources of background noise include neutrinos, which are weakly interacting particles that can mimic the signals of dark matter.

For instance, the Z boson, a neutral particle mediating the weak nuclear force, can decay into a neutrino-antineutrino pair about 20% of the time. The W boson, another mediator of the weak force, can decay into a charged lepton (such as an electron or muon) and a neutrino. Tau leptons, heavier cousins of electrons and muons, decay into lighter leptons (electrons or muons) and neutrinos. Accurately detecting and measuring all visible particles is crucial to ensuring that any observed MET is due to invisible particles like dark matter and not undetected visible particles.

Mitigation strategies include enhancing detector capabilities to improve the identification and measurement of all particles, including those at the edges of detection thresholds. Developing sophisticated data analysis methods to differentiate between MET caused by neutrinos and that caused by potential dark matter particles is also critical.

Theoretical Framework

Dark matter needs to be integrated into both Standard Model (SM) extensions, referred to as Beyond the Standard Model (BSM) theories, and cosmological models. This ensures that new theories fit with our existing understanding of the universe, represented by models like the SM and Lambda Cold Dark Matter (Lambda CDM).

The essential role of theory could be described thusly: we have learned a good deal about the universe already and can explain much of what occurs in nature from the scale of subatomic particles to the scale of the cosmos. Theories for dark matter candidates then must fit with the existing pieces of the puzzle, which is to say they must connect with and logically extend our existing understanding or show superior fit to experimental data (no mean feat) if they are to overturn reigning theories.

Theories for dark matter can be broadly classified along a spectrum from simple to complete. Simple models assume just a particular portal of interaction, while complete theories propose a complex picture of dark matter in relation to the standard model with specific decay chains. Complete theories, such as Supersymmetry (SUSY), propose a comprehensive framework that extends the Standard Model by adding a large number of new particles and interactions. These models aim to solve multiple problems in particle physics, including the hierarchy problem and dark matter. Simplified models focus on minimal extensions to the Standard Model, introducing only the necessary components to explain dark matter. These often involve fewer new particles and simpler interactions, such as those described by portal theories.

One approach in theory is the direct production of dark matter particles in association with visible Standard Model particles in simplified models. The presence of dark matter is inferred from events where a high-energy particle is produced along with significant missing transverse energy (MET). Key examples include monophoton events (a single high-energy photon plus MET), monojet events (a single jet of hadrons plus MET), and events involving a W or Z boson, or a Higgs boson, decaying into visible particles plus MET.

In more complex models like Supersymmetry (SUSY), dark matter particles are produced via known decay chains. SUSY models predict a superpartner for each particle, often involving specific decay chains that produce dark matter particles. For instance, heavier particles like squarks can decay into quarks and neutralinos, the latter being a stable, neutral dark matter candidate. These decay chains result in missing energy because neutralinos do not interact with the detector.

Displaced Vertices

In addition to missing transverse energy (MET) signatures, the search for dark matter at the LHC has expanded to include another key signature known as displaced vertices. This concept broadens the potential for detecting dark matter by considering scenarios where DM particles do not completely escape the detector before interacting. Instead, they may travel a certain distance and then interact, producing detectable particles with a delay.

A displaced vertex occurs when a dark particle is produced at the initial interaction vertex—the point where the particles collide. Rather than interacting immediately, as most Standard Model particles do, this dark particle travels some distance before it interacts again, producing visible particles. This second interaction happens away from the original collision point, thus creating a "displaced vertex."

The presence of displaced vertices is a unique and significant indicator because it suggests interactions involving new physics, such as dark sector interactions or portals. Standard Model particles typically interact immediately at the collision point, so the observation of a displaced vertex strongly implies the existence of particles with different interaction properties. These could include dark sector portals or short-lived particles like Feebly Interacting Particles (FIPs).

Displaced vertices provide a distinct and powerful signal in the search for dark matter, complementing the MET approach by potentially revealing interactions that would otherwise go unnoticed. By expanding the detection strategies to include these signatures, researchers at the LHC are better equipped to uncover the elusive nature of dark matter and its interactions.

Recent Progress and Future Prospects

Recent experiments have significantly advanced our understanding and constrained the properties of dark matter. Experiments like Belle-II, NA62, and FASER have presented new bounds for dark photons, especially in low coupling scenarios. ATLAS and CMS, two of the general-purpose detectors at the LHC, have significantly constrained the bounds of SUSY dark matter during Run 2, with analysis continuing into 2023.

Looking to the future, the LHC began Run 3 between July and November 2022 and will continue through 2025. This run operates at an increased energy of 13.6 TeV, up from the previous 13 TeV in earlier runs, with upgrades to increase the luminosity by a factor of 10. These enhancements will significantly improve the collider's ability to detect rare events, including potential dark matter production.

Interest in this area of research is significant and other future prospects are also on the horizon. A proposed next-generation collider, the Future Circular Collider (FCC), aims to further push the boundaries of our understanding. The FCC if built would have a circumference of 100 km, much larger than the current 27 km LHC, and aims to reach collision energies up to 100 TeV. This increase in energy and scale would provide unprecedented opportunities to enhance the search for dark matter, supersymmetry, and other BSM theories.

Conclusion

To conclude, let’s summarize this look at the intricate and ambitious search for dark matter within high-energy accelerators, particularly the LHC. Dark matter, which constitutes approximately 85% of the universe's mass, forms the backbone of the cosmic web and is pivotal for understanding the structure and evolution of the cosmos. Despite its significant role, dark matter remains invisible to electromagnetic observations, necessitating advanced and innovative detection methods.

Dark matter searches can be broadly categorized into two main strategies: detection and production. Detection efforts focus on observing ambient dark matter from the galactic halo, using direct and indirect detection methods.

Production efforts involve attempting to create and identify dark matter within particle accelerators. These collider searches rely on strategies such as missing transverse energy (MET) signatures and displaced vertices. To do so however, we face a tremendous needle in the haystack problem, searching for nearly undetectable signals in exceedingly rare events. Colliders also have energy limitations, and must filter for a complicated background of possible interactions to find statistically significant results. Despite these major challenges, the potential for discovery remains significant.

The theoretical framework for dark matter spans a spectrum from simplified models to complete theories. Simplified models, such as Effective Field Theories (EFTs), describe minimalistic hypothetical dark matter interactions. In contrast, complete theories like Supersymmetry (SUSY) propose detailed hypothetical decay chains based on an extended particle framework. These theories provide specific targets for experimental searches, narrowing the field and refining our understanding of dark matter. Experiments in turn provide limits on the ranges for dark matter under a given theory, further narrowing the search and refining theoretical models.

Recent progress in dark matter research has been marked by notable findings from experiments. These experiments set new bounds on dark matter properties, with ongoing data analysis continuously enhancing our understanding. The future prospects are promising, with the LHC's Run 3 aiming to operate at increased energy and luminosity, significantly boosting the potential for rare event detection. Additionally, the proposed Future Circular Collider (FCC) promises unprecedented opportunities for advancing our search for dark matter, supersymmetry, and other Beyond Standard Model (BSM) theories.

As a parting note I’d like to highlight that the search for dark matter in accelerators encapsulates a profound scientific endeavor that combines the very large with the very small. On the cosmic scale, humans are like minute ants on a tiny dust mote, yet on the scale of particles, we ourselves are akin to vast universes. This quest to uncover the nature of dark matter has necessitated a massive collaboration from thousands of scientists and engineers, pushing the boundaries of human knowledge and technological capability. I find it beautiful and humbling that as we continue to explore this frontier, each discovery brings us closer to understanding the fundamental nature of the universe, where we came from, and where we could go.

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