Beyond the Standard Model: Hunting Higgs Decays for New Physics

Author: Denis Avetisyan

New calculations reveal how the dimuon decay rates of multiple Higgs bosons can vary within an extended theoretical framework, offering potential avenues for discovery at the Large Hadron Collider.

This review details branching ratio predictions for $H_{1,2,3}
ightarrow μ^{+}μ^{-}$ within the Next-to-Two-Higgs-Doublet Model (N2HDM), highlighting the impact of Yukawa coupling types.

Despite the Standard Model’s successes, the Higgs sector remains a prime target for searches beyond established physics. This work, ‘Branching Ratios of $H_{1,2,3} \rightarrow μ^{+}μ^{-}$ in the Broken-Phase N2HDM’, investigates the dimuon decay channels of the three CP-even Higgs bosons within a Next-to-Two-Higgs-Doublet Model, revealing branching ratios significantly altered by modified Yukawa couplings and scalar mixing. Incorporating one-loop radiative corrections, we demonstrate how precision measurements of these decay rates can constrain viable regions of the N2HDM parameter space and potentially explain observed enhancements in the dimuon signal. Could detailed analyses of $H \rightarrow μ^{+}μ^{-}$ decays at current and future colliders ultimately reveal the existence of this extended Higgs sector?

The Fragile Foundations of Observation

The discovery of the Higgs boson, confirmed through observations of its decay into a pair of muons – a process denoted H→μμ – solidified a cornerstone of the Standard Model of particle physics. This fundamental particle, responsible for conferring mass upon other particles, continues to be intensely scrutinized. Precise measurements of the Higgs boson’s mass, spin, and interactions are paramount, as any deviation from Standard Model predictions could signal the existence of new, undiscovered particles or forces. The H→μμ decay channel, while relatively rare, provides a clean signature for identifying the Higgs boson and allows physicists to probe its properties with high accuracy, making it an indispensable tool in the ongoing quest to understand the universe at its most fundamental level. Continued analysis of data from the Large Hadron Collider, focusing on this decay, remains crucial for validating the Standard Model and potentially unveiling physics beyond it.

The Higgs boson, while confirming a key prediction of the Standard Model, doesn’t offer a complete picture of reality. Consequently, physicists meticulously measure its properties, particularly branching ratios – the probability of the Higgs decaying into different particles. Deviations from predicted branching ratios would signal the presence of new, undiscovered particles or interactions beyond the Standard Model. For example, if the Higgs decays into a pair of photons more, or less, frequently than expected, it could indicate interactions with particles contributing to dark matter. These precise measurements, conducted at the Large Hadron Collider and future colliders, effectively function as a sensitive probe for new physics, potentially unveiling phenomena that currently lie beyond the reach of direct observation and pushing the boundaries of $particle physics$ .

The pursuit of physics beyond the Standard Model increasingly focuses on theoretical frameworks capable of addressing unresolved mysteries, such as the nature of dark matter. One prominent avenue of investigation involves the Two-Higgs-Doublet Model (2HDM), which proposes the existence of additional Higgs bosons beyond the one discovered in 2012. This model doesn’t simply add particles; it alters the mechanism of electroweak symmetry breaking and modifies the strengths with which fundamental particles interact – known as Yukawa couplings. By precisely measuring the properties of the known Higgs boson and searching for evidence of these additional, heavier Higgs bosons, physicists aim to indirectly detect dark matter candidates that may interact with these new particles. The 2HDM offers a compelling explanation for dark matter while simultaneously providing a potential solution to inconsistencies within the Standard Model itself, making it a key focus for current experimental searches at the Large Hadron Collider and beyond.

Beyond the Standard Model, theoretical frameworks like the Two-Higgs-Doublet Model propose alterations to fundamental interactions, specifically impacting Yukawa couplings – the values dictating particle mass – and the mechanism of electroweak symmetry breaking, which explains the origin of mass itself. These extended models aren’t simply modifications; they predict measurable shifts in how the Higgs boson interacts with other particles, and changes to the way fundamental forces behave at high energies. Consequently, physicists require extraordinarily precise data analysis from experiments like the Large Hadron Collider to discern subtle deviations from Standard Model predictions. This high-precision approach seeks to map the Higgs boson’s interactions with unprecedented detail, effectively testing the validity of these extended models and potentially revealing pathways to understanding phenomena like dark matter and the matter-antimatter asymmetry in the universe. The pursuit involves meticulously examining decay rates and interaction strengths, searching for any statistical anomaly that could signal the presence of new, undiscovered particles or forces.

Data Streams and the Illusion of Certainty

The ATLAS experiment at the Large Hadron Collider (LHC) collects data from proton-proton collisions during distinct operational periods, referred to as “runs”. Run 2, comprising data collected from 2016 to 2018, delivered an integrated luminosity of approximately 150 fb^-1 at a center-of-mass energy of 13 TeV. The currently ongoing Run 3, initiated in 2022, is characterized by significantly increased luminosity and is projected to provide an integrated luminosity exceeding 300 fb^-1 by its conclusion. This progression to higher integrated luminosities directly translates to improved statistical precision in measurements of known processes and enhanced sensitivity to rare phenomena, allowing for increasingly stringent tests of the Standard Model and searches for physics beyond it. The increased collision rate during Run 3 also facilitates studies of heavy-ion collisions and provides a larger dataset for detector performance calibration and validation.

The High-Luminosity LHC, comprising Run 3 data collection, significantly increases the rate of proton-proton collisions compared to previous runs. This higher luminosity directly translates to a larger integrated dataset, improving the statistical power of analyses focused on rare Standard Model processes and searches for physics beyond the Standard Model. Specifically, the increased event rate enhances the ability to observe decay channels with small branching ratios, and to resolve subtle deviations from theoretical predictions that might otherwise be obscured by statistical uncertainties. The improvement in sensitivity is proportional to the square root of the integrated luminosity, meaning even modest increases in dataset size yield substantial gains in statistical reach.

The decay of the Higgs boson into two muons, denoted as H→μμ, provides a crucial channel for investigating Higgs boson properties due to the relatively clean signature and well-understood muon reconstruction. The branching ratio for this decay is small – approximately 0.02% – but it is precisely predicted within the Standard Model and highly sensitive to new physics. Measurements of the H→μμ signal rate allow for precise determination of the Higgs boson coupling strength to muons, providing a direct test of the Standard Model’s flavor predictions and a potential window for observing deviations indicative of beyond-Standard-Model physics, such as contributions from new particles or modified Higgs couplings. The sensitivity is enhanced by the large datasets collected by the ATLAS experiment, enabling precise measurements of both the signal yield and background contributions.

The extraction of statistically significant results from data acquired by the ATLAS experiment necessitates the application of advanced statistical methods, most notably global fit techniques. These methods combine information from multiple independent measurements and decay channels, accounting for systematic uncertainties and correlations to optimize parameter estimation. Global fits utilize likelihood functions, often incorporating $\chi^2$ minimization or profile likelihood ratios, to determine best-fit values and assess the statistical significance of observed signals. The process involves constructing a comprehensive statistical model that encapsulates all relevant theoretical predictions, experimental acceptances, and background contributions, allowing for a simultaneous determination of multiple parameters and a robust assessment of potential new physics effects. Sophisticated algorithms and substantial computational resources are required to handle the complexity of these analyses and ensure reliable results.

Extending the Framework: A Cascade of Possibilities

The Next-to-Two-Higgs-Doublet Model (N2HDM) builds upon the Two-Higgs-Doublet Model (2HDM) by incorporating a real scalar singlet field. This addition expands the parameter space available for describing electroweak symmetry breaking and Higgs boson interactions, moving beyond the constraints of the 2HDM. The increased complexity allows for potentially addressing limitations within the Standard Model, such as the origin of neutrino masses or the nature of dark matter, through modifications to the Higgs potential and couplings. The singlet field introduces additional degrees of freedom and mixing effects, which can significantly alter the predicted mass spectra and decay patterns of the physical Higgs bosons compared to the 2HDM.

The Two-Higgs-Doublet Model (2HDM) encompasses several variations – Type I, Type II, Type X, and Type Y – each distinguished by its unique coupling scheme between the two Higgs doublets and fermions. In Type I, both Higgs doublets couple to all fermions, while Type II features one doublet coupling to up-type fermions and the other to down-type fermions. Type X and Type Y restrict these couplings further; Type X couples one doublet to all fermions while the other does not couple to any, and Type Y couples one doublet to up-type fermions and the other to charged leptons, but neither to down-type quarks. These differing coupling arrangements directly impact the decay rates and production cross-sections of the Higgs bosons, $H_1$ and $H_2$ , providing a means to differentiate between the models through experimental observation. Specifically, the branching ratios for decays into fermions and gauge bosons are sensitive to these coupling structures.

Accurate determination of Higgs boson branching ratios is essential for differentiating between various two-Higgs-doublet models (2HDMs) and extensions like the Next-to-Two-Higgs-Doublet Model (N2HDM). Calculations within the N2HDM predict a dimuon branching ratio for the H1 boson-across Type I, Type X, Type II, and Type Y models-of 2.17 x 10^-4. This value is currently consistent with Standard Model predictions, posing a challenge for direct discrimination. However, precise measurements at experiments like the LHC, coupled with global fit analyses, are necessary to constrain N2HDM parameter space and potentially reveal deviations from the Standard Model that could indicate the presence of additional Higgs bosons.

Accurate interpretation of experimental data from the Large Hadron Collider requires sophisticated statistical tools for analyzing Higgs boson properties within the Next-to-Two-Higgs-Doublet Model (N2HDM). Programs such as HiggsSignals and EVADE facilitate this analysis, allowing researchers to perform global fits and assess the compatibility of N2HDM parameters with observed data. Recent calculations focusing on the Type X N2HDM reveal that the dimuon branching ratio for the heavier Higgs boson, H2, can reach a maximum value of 4 x 10^-6. This predicted branching ratio is of particular interest as it approaches, and in some parameter spaces may exceed, the current sensitivity limits of LHC detectors, potentially allowing for direct observation of this decay channel and providing strong evidence for physics beyond the Standard Model.

The Shadow of the Unknown: Implications and Limits

The pursuit of physics beyond the Standard Model frequently centers on scenarios that subtly modify established predictions, and the Two-Higgs-Doublet Model (N2HDM) offers a compelling framework for this exploration. This model posits the existence of an additional Higgs boson alongside the one already discovered, potentially altering the decay patterns and interaction strengths of all Higgs particles. By meticulously comparing experimental observations – such as decay rates and production cross-sections – with the precise predictions of the Standard Model and the N2HDM, physicists can search for deviations indicative of new physics. These discrepancies, even if exceedingly small, could unveil the presence of additional particles or interactions not currently accounted for, offering critical insights into fundamental questions concerning dark matter, matter-antimatter asymmetry, and the ultimate nature of reality. The N2HDM, therefore, serves as a vital testing ground for extending the Standard Model and pushing the boundaries of particle physics.

The meticulous scrutiny of the Higgs boson’s characteristics, achieved through high-precision measurements at facilities like the Large Hadron Collider, offers a powerful pathway to explore physics beyond the Standard Model. These investigations aren’t simply about confirming known properties; they focus on detecting minute deviations from predicted behavior. Sophisticated data analysis techniques, including advanced statistical modeling and machine learning algorithms, are crucial for isolating these subtle signals from the background noise inherent in particle collisions. By precisely mapping the Higgs boson’s interactions – its decay modes, production rates, and couplings to other particles – physicists can search for evidence of new particles or forces that might be influencing its behavior. Even seemingly insignificant discrepancies could hint at the existence of previously unknown phenomena, potentially unlocking answers to some of the most fundamental questions in particle physics, such as the nature of dark matter and the origin of mass.

The Higgs boson’s interactions, or coupling structure, offer a unique window into fundamental mysteries beyond the Standard Model, notably the composition of dark matter. Current theories suggest dark matter particles may interact with Standard Model particles via channels mediated by the Higgs boson, implying a connection between the Higgs’s properties and the elusive dark sector. Precise measurements of how the Higgs boson couples to various particles – including itself – can therefore constrain models of dark matter, potentially revealing its mass, interaction strength, and even its composition. A deviation from the Standard Model’s predicted coupling strengths could signal the existence of new particles influencing these interactions, effectively providing indirect evidence for dark matter and opening pathways to understand its nature. Investigations into these coupling structures are thus paramount in the ongoing quest to unveil the universe’s hidden components and complete the Standard Model.

The continued operation and data collection capabilities of the Large Hadron Collider remain paramount to extending the frontiers of particle physics. Current theoretical calculations, specifically within the framework of the N2HDM, predict significantly suppressed dimuon branching ratios for the neutral Higgs boson H2 in Type I and Type Y scenarios, falling below $10^{-7}$ . However, H2 in the Type II scenario demonstrates a moderately enhanced branching ratio, reaching approximately $1 \times 10^{-6}$ . These subtle yet potentially observable differences in decay pathways underscore the importance of high-precision measurements and comprehensive data analysis, offering a crucial avenue for identifying deviations from Standard Model predictions and potentially unveiling new physics beyond current understanding.

The calculation of branching ratios within the N2HDM isn’t simply a matter of predicting decay channels; it’s an exercise in mapping the contours of potential failure. Each predicted ratio, each deviation from Standard Model expectations, represents a point where current understanding might fracture. This work acknowledges that certainty is an illusion, and true resilience lies in embracing the inevitable revelation of the unexpected. As John Dewey observed, “Education is not preparation for life; education is life itself.” Similarly, this research isn’t about preparing for discovery, but actively participating in the ongoing evolution of particle physics, where every observation, even those initially deemed anomalous, provides valuable insight into the underlying reality.

Future Prospects

The calculation of branching ratios, even within a comparatively constrained space like the N2HDM, doesn’t yield prediction so much as maps of plausible failure modes. The model’s freedom in Yukawa couplings isn’t a strength, but an acknowledgment that any attempt at precise prediction is, at best, a temporary reprieve from the underlying chaos. The LHC, as an instrument, merely amplifies certain of these failures, making them visible. A signal observed in the dimuon channel isn’t confirmation of a specific parameter set, but rather the transient alignment of probabilities.

The true challenge lies not in refining these branching ratio calculations, but in accepting the inevitability of model inadequacy. Future work should prioritize the development of robust statistical frameworks capable of extracting information from incomplete and contradictory data. A guarantee of discovery is a fallacy; instead, the field must embrace a methodology that quantifies uncertainty and acknowledges the inherent limitations of any theoretical construct. Stability, after all, is merely an illusion that caches well.

The exploration of alternative decay channels, and the systematic investigation of higher-order corrections, will undoubtedly refine the signal landscape. However, these refinements are merely tactical adjustments. The strategic imperative remains the same: to move beyond the pursuit of “correctness” and towards a more nuanced understanding of how complex systems – like the Higgs sector – inevitably evolve towards states of unpredictable complexity. Chaos isn’t failure – it’s nature’s syntax.

Original article: https://arxiv.org/pdf/2601.15328.pdf

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

The Fragile Foundations of Observation

Data Streams and the Illusion of Certainty

Extending the Framework: A Cascade of Possibilities

The Shadow of the Unknown: Implications and Limits

Future Prospects

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