Impersonating the Standard Model Higgs Boson: Alignment without Decoupling (1310.2248v1)

Abstract: In models with an extended Higgs sector there exists an alignment limit, in which the lightest CP-even Higgs boson mimics the Standard Model Higgs. The alignment limit is commonly associated with the decoupling limit, where all non-standard scalars are significantly heavier than the $Z$ boson. However, alignment can occur irrespective of the mass scale of the rest of the Higgs sector. In this work we discuss the general conditions that lead to "alignment without decoupling", therefore allowing for the existence of additional non-standard Higgs bosons at the weak scale. The values of $\tan\beta$ for which this happens are derived in terms of the effective Higgs quartic couplings in general two-Higgs-doublet models as well as in supersymmetric theories, including the MSSM and the NMSSM. Moreover, we study the information encoded in the variations of the SM Higgs-fermion couplings to explore regions in the $m_A - \tan\beta$ parameter space.

• The paper demonstrates that alignment without decoupling enables SM-like Higgs behavior in 2HDMs without requiring heavy extra scalars.
• It establishes explicit conditions through cubic equations in tanβ and effective quartic couplings to achieve natural alignment.
• The findings challenge conventional decoupling assumptions and highlight potential light-scalar signatures for future collider searches.

Analysis of "Impersonating the Standard Model Higgs Boson: Alignment without Decoupling"

The paper "Impersonating the Standard Model Higgs Boson: Alignment without Decoupling," authored by Marcela Carena, Ian Low, Nausheen R. Shah, and Carlos E. M. Wagner, provides an in-depth paper of the conditions under which the Higgs sector of two-Higgs-doublet models (2HDMs) can mimic the behavior of the Standard Model (SM) Higgs boson without necessarily demanding the heavy scalar states to decouple. This phenomenon is referred to as "alignment without decoupling."

Extended Higgs Sectors and the Alignment Limit

In the context of the SM, the Higgs boson is responsible for electroweak symmetry breaking, and its properties are fixed by its mass and vacuum expectation value (VEV). When extending the Higgs sector, such as in 2HDMs, new scalar particles arise, potentially altering the Higgs couplings observed in the SM. A crucial regime in such models is the "alignment limit," where the lightest CP-even Higgs boson behaves like the SM Higgs boson. Traditionally, this limit is tied closely to the decoupling limit, where additional Higgs scalars are much heavier than the $Z$ boson, ensuring that their effects on low-energy observables are negligible.

Decoupling vs. Alignment without Decoupling

The paper highlights the theoretical possibility of alignment without decoupling, a regime where the SM-like Higgs does not require other scalar particles to be heavy. This condition decouples the concept of alignment from the decoupling limit in mass parameters, allowing for potentially light additional Higgs bosons at the weak scale. The authors delve into the mathematical conditions necessary for alignment without decoupling, focusing on the interplay between different parameters such as $\tan\beta$ and the effective quartic couplings in both general 2HDMs and specific supersymmetric models like the MSSM and NMSSM.

Key Findings and Numerical Results

The paper derives explicit conditions crucial for achieving alignment without heavy decoupling, expressed through cubic equations involving $\tan\beta$. The paper reveals that alignment can be achieved naturally in certain parameter spaces without excessive fine-tuning, particularly when radiative corrections introduce small or zero $\lambda_6$ and $\lambda_7$ in the scalar potential.

Numerical analyses expose possible parameter spaces within the MSSM and NMSSM where alignment without decoupling might occur. The authors explain that these scenarios might manifest through novel solutions to the alignment conditions which effectively disentangles the SM-like Higgs behavior from the heavy state mass scale.

Implications and Future Directions

The ramifications of this paper are particularly profound for the ongoing search for physics beyond the Standard Model. Realizing alignment without decoupling could imply the existence of additional light scalars that current searches might not exclude. This phenomenon challenges typical expectations and necessitates precision measurements of Higgs couplings, especially if future experiments observe properties consistent with the SM even without the detection of new particles.

Furthermore, the paper opens new theoretical perspectives in constructing viable 2HDMs and interpreting data from high-energy experiments like the LHC. It calls for innovative strategies in both model-building and experimental searches to address regions in parameter space that might previously have been dismissed under conventional assumptions of decoupling.

In conclusion, this paper offers a comprehensive exploration of a nuanced area in Higgs phenomenology, expanding our understanding of possible manifestations of extended Higgs sectors while providing theoretical support for experimental pursuits in identifying new scalar states at the weak scale.