Cosmological Relaxation of the Electroweak Scale (1504.07551v2)

Abstract: A new class of solutions to the electroweak hierarchy problem is presented that does not require either weak scale dynamics or anthropics. Dynamical evolution during the early universe drives the Higgs mass to a value much smaller than the cutoff. The simplest model has the particle content of the standard model plus a QCD axion and an inflation sector. The highest cutoff achieved in any technically natural model is 10^8 GeV.

• The paper proposes a relaxion mechanism that dynamically tunes the Higgs mass during inflation, offering an alternative solution to the hierarchy problem.
• It compares two models: one using a QCD axion framework and another with a new strong interaction, achieving energy cutoffs up to 10⁸ GeV.
• The study highlights that future experiments targeting axion-like particles and cosmological signatures could test this naturalness framework.

An Examination of the Cosmological Relaxation of the Electroweak Scale

The paper, "Cosmological Relaxation of the Electroweak Scale" by Graham, Kaplan, and Rajendran, presents a novel approach to addressing the electroweak hierarchy problem. The authors propose a mechanism leveraging cosmological dynamics to account for the small mass of the Higgs boson relative to the high energy cutoff scale of the Standard Model.

Overview and Core Proposal

Traditional solutions to the hierarchy problem typically invoke new physics at the electroweak scale, such as supersymmetry or compositeness, or dichotomously, rely on anthropic principles within a multiverse context. These methods often entail fine-tuning of model parameters and face significant constraints from observational and collider experiments. In contrast, this paper outlines a self-consistent mechanism driven by early-universe dynamics to naturally set the electroweak scale without necessitating additional weak-scale physics or anthropic selection.

Central to their solution is the concept of a "relaxion," akin to a QCD axion but extended to influence the Higgs mass during cosmological inflation. Through a rolling mechanism across its potential landscape and soft symmetry-breaking interactions with the Higgs field, the relaxion dynamically adjusts the Higgs mass during inflation. When the Higgs field's vacuum expectation value (vev) reaches the Standard Model value, new strong interaction dynamics generate barriers in the relaxion’s potential, effectively halting its evolution.

Theoretical Implications and Model Details

The paper introduces two primary relaxion models. The first model incorporates the standard model supplemented by a QCD axion and inflation sector, achieving a cutoff of around $10^5$ TeV. However, this version encounters issues with the strong CP problem. To mitigate this, the authors introduce a second model where the relaxion's potential barriers arise from a new strong interaction, decoupled from QCD, facilitating a higher cutoff up to $10^8$ GeV.

A notable aspect of these models is their reliance on technically natural parameters, emphasizing a potential field behavior unaffected by perturbative quantum corrections. The framework demands a long field excursion and small parameter values, deemed stable under radiative corrections. Additionally, the models exploit the inflationary epoch, wherein Hubble friction allows the relaxion to roll across its landscape, ensuring the Higgs mass is tuned within a technically natural range.

Practical and Theoretical Implications

Graham et al.'s proposal invites significant implications for both the academic pursuit of naturalness and the practical search for physics beyond the Standard Model. Observational tests of this model would differ substantially from conventional avenues. Instead of collider signals or supersymmetric partners, experiments should focus on detecting subtle effects induced by light scalar fields and axion-like particles, potentially modulating fundamental constants.

Furthermore, these models touch on broader theoretical inquiries, such as the viability of axion monodromy and questions surrounding the ultraviolet completion of such dynamics. They also hint at possible applications to other scalar field-driven naturalness issues, including the enigmatic cosmological constant problem.

Future Directions

The conceptual innovation of cosmological relaxation offers a tantalizing reorientation of naturalness paradigms in particle physics. Continued exploration could refine the inflation models to avoid speculative territory, better understand possible UV completions, and seek tangible observational proxies. Confirmation of these ideas would demand novel experimental designs, such as ultra-sensitive axion searches or probing new cosmological relics, potentially reshaping the approach to naturalness in the coming decade.

The paper thus presents a compelling argument for re-evaluating the means of tackling the electroweak hierarchy problem, extending the landscape of theoretical physics aimed at understanding the fundamental constants of nature.