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Sensitivity of the DarkQuest Experiment to Muon-Philic Scalars Decaying to Photons


Core Concepts
The DarkQuest experiment at Fermilab has the potential to discover new light muon-philic scalar particles that decay into photons, which could explain the observed muon anomalous magnetic moment and provide insights into dark matter.
Abstract
  • Bibliographic Information: Blinov, N., Gori, S., & Hamer, N. (2024). Diphoton Signals of Muon-philic Scalars at DarkQuest. arXiv preprint arXiv:2405.17651v2.
  • Research Objective: This paper investigates the sensitivity of the proposed DarkQuest experiment at Fermilab to discover new light muon-philic scalar particles that decay into photons. This study focuses on a minimal scalar model with a primary interaction with muons, which could explain the observed muon anomalous magnetic moment ((g −2)µ anomaly).
  • Methodology: The authors utilize Monte Carlo simulations to model various signal production channels, including muon bremsstrahlung, meson decays (π± →µ±νS and K± →µ±νS), and photon-induced processes. They also carefully analyze potential background processes, such as SM long-lived particle decays, neutral meson production from the attenuated proton beam, and muon deep-inelastic scattering. The study considers different event selection strategies to mitigate these backgrounds and enhance signal sensitivity.
  • Key Findings: The study reveals that muon bremsstrahlung and meson decays are the dominant production mechanisms for the muon-philic scalar in the relevant mass range. The compact geometry of the DarkQuest experiment, while advantageous for detecting short-lived particles, introduces significant background challenges. However, the authors demonstrate that strategic event selections, such as requiring two well-separated photons in the ECal and a muon hit, can effectively suppress these backgrounds.
  • Main Conclusions: The research concludes that the DarkQuest experiment possesses the sensitivity to probe a significant portion of the currently allowed parameter space for muon-philic scalars that could explain the (g −2)µ anomaly, particularly in the mass range below twice the muon mass.
  • Significance: This study highlights the potential of the DarkQuest experiment to make groundbreaking discoveries in particle physics, particularly in the search for new light particles and dark matter candidates. The findings contribute valuable insights for optimizing the experiment's design and analysis strategies to maximize its sensitivity to muon-philic scalars and other potential new physics signatures.
  • Limitations and Future Research: The background estimates, while comprehensive, rely on certain simplifying assumptions and could be systematically underestimated. Further refinements to the background modeling, incorporating more detailed simulations of electromagnetic and hadronic showers, would enhance the accuracy of the sensitivity projections. Additionally, exploring alternative signal regions and event selection criteria could further improve the experiment's reach and potentially uncover other interesting new physics scenarios.
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Stats
The difference between the experimental measurement of the muon anomalous magnetic moment and the Standard Model prediction is (251 ± 59) × 10−11. DarkQuest will collect ~10^18 protons on target (POT) in a two-year run. The muon-philic scalar model requires a coupling constant gS ∼4×10−4 for mS ≲2mµ to explain the (g −2)µ anomaly. The DarkQuest experiment's compact geometry allows for the detection of particles with short decay lengths, but also introduces background challenges. An additional 2 m of iron shielding could reduce the background from SM long-lived particles to below 10 events for 10^18 POT. Event selections requiring two photon hits in the ECal and a muon hit can help mitigate backgrounds. The study found that muon bremsstrahlung and meson decays are the dominant production mechanisms for the muon-philic scalar. Backgrounds from muon deep-inelastic scattering can be reduced through selections on photon energy and angle distributions.
Quotes
"These scalars can be copiously produced by meson decays and muon bremsstrahlung." "We point out that thanks to DarkQuest’s compact geometry, muons can propagate through the dump and efficiently produce dark scalars near the end of the dump." "We find that the backgrounds can be sufficiently reduced for DarkQuest to test currently-viable (g −2)µ parameter space."

Key Insights Distilled From

by Nikita Blino... at arxiv.org 10-07-2024

https://arxiv.org/pdf/2405.17651.pdf
Diphoton Signals of Muon-philic Scalars at DarkQuest

Deeper Inquiries

How does the sensitivity of DarkQuest to muon-philic scalars compare to other proposed experiments searching for similar particles, such as SHiP or NA64?

DarkQuest exhibits a unique sensitivity to muon-philic scalars, particularly in the low mass regime (mS ≲ 2mµ), which complements the reach of other proposed experiments like SHiP and NA64. Here's a comparative breakdown: DarkQuest: This experiment leverages its compact geometry, allowing for the detection of particles with shorter decay lengths. This is particularly advantageous for scenarios where the muon-philic scalar is produced near the end of the beam dump, a feature not readily accessible to experiments with larger dump sizes. DarkQuest's sensitivity is primarily driven by: Muon Bremsstrahlung: Muons traversing the dump can radiate scalars, with the signal yield dominated by production in the last few decay lengths of the shield. Meson Decays: Rare three-body decays of mesons like π± and K± provide a significant contribution to scalar production. SHiP (Search for Hidden Particles): Characterized by a much larger decay volume compared to DarkQuest, SHiP excels in probing long-lived particles. Its sensitivity to muon-philic scalars would primarily stem from: Meson Decays: Similar to DarkQuest, rare meson decays play a crucial role. However, due to the longer decay volume, SHiP is more sensitive to smaller couplings, leading to longer scalar lifetimes. Proton Bremsstrahlung: While subdominant, protons interacting within the target can also radiate scalars. NA64: Operating in a different energy regime, NA64 utilizes a high-energy electron beam. Its sensitivity to muon-philic scalars is primarily through: Missing Momentum Signatures: Scalars produced via electron-nucleus interactions could be detected by reconstructing the missing momentum in the event. In essence, DarkQuest's strength lies in probing shorter lifetimes, thanks to its compact design and the ability to detect scalars produced near the end of the dump. SHiP, with its larger decay volume, excels in probing longer lifetimes. NA64, using an electron beam, provides a complementary approach through missing momentum techniques. The three experiments together offer a comprehensive coverage of the muon-philic scalar parameter space.

Could the observed (g −2)µ anomaly be explained by other new physics models beyond muon-philic scalars, and how would DarkQuest's sensitivity to these alternative models compare?

The observed (g −2)µ anomaly indeed can be explained by a variety of new physics models beyond muon-philic scalars. Some prominent alternatives include: Dark Photons: These hypothetical particles, weakly coupled to the electromagnetic current, can contribute to (g −2)µ through loop diagrams. DarkQuest could be sensitive to dark photons if they decay into detectable particles, such as electron-positron pairs, within the detector volume. Axion-Like Particles (ALPs): These light, weakly interacting particles are often invoked in solutions to the strong CP problem. ALPs can couple to photons and muons, potentially explaining the anomaly. DarkQuest's sensitivity to ALPs would be similar to its sensitivity to muon-philic scalars, relying on their decay into photons. Leptoquarks: These hypothetical particles carry both lepton and quark quantum numbers, mediating interactions between these sectors. Certain leptoquark models can contribute to (g −2)µ. DarkQuest could potentially probe leptoquarks if they decay into final states involving muons or photons. New Heavy Particles in Loops: Models with heavier particles, such as additional Higgs bosons or supersymmetric partners, can also explain the anomaly through their contributions to loop diagrams. While DarkQuest might not directly produce these heavy particles, their presence could be inferred indirectly if they modify the production or decay rates of lighter states accessible to the experiment. DarkQuest's sensitivity to these alternative models would depend on their specific couplings and decay channels. Generally, the experiment is well-suited for probing models with light, weakly interacting particles that decay into photons, electrons, or muons.

If DarkQuest successfully discovers a muon-philic scalar, what are the broader implications for our understanding of dark matter and the early universe?

The discovery of a muon-philic scalar at DarkQuest would be a groundbreaking event with profound implications for our understanding of dark matter, the early universe, and fundamental physics: Confirmation of New Physics Explaining (g −2)µ: It would solidify the existence of physics beyond the Standard Model, specifically addressing the long-standing muon anomalous magnetic moment anomaly. This would mark a significant leap in our understanding of fundamental particle physics. Insights into Dark Matter: Muon-philic scalars could potentially interact with dark matter particles, serving as a portal between the visible and dark sectors. This connection could provide crucial clues about the nature of dark matter, its production in the early universe, and its interactions with ordinary matter. Constraints on Early Universe Cosmology: The properties of the discovered scalar, such as its mass, lifetime, and couplings, would offer valuable insights into the conditions of the early universe. These parameters could constrain cosmological models, shedding light on processes like dark matter freeze-out and Big Bang nucleosynthesis. New Forces and Interactions: The existence of a muon-philic scalar implies a new fundamental force mediated by this particle. This would revolutionize our understanding of fundamental interactions, potentially leading to the development of new theoretical frameworks beyond the Standard Model. Flavor Physics Implications: The muon-specific nature of the scalar raises intriguing questions about the flavor structure of the universe. It could hint at new flavor symmetries or mechanisms that distinguish between different generations of leptons. In summary, the discovery of a muon-philic scalar at DarkQuest would not only resolve a long-standing puzzle in particle physics but also open up new avenues for exploring the mysteries of dark matter, the early universe, and the fundamental building blocks of nature.
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