The β\beta-function for Yukawa theory at large NfN_f

We compute the $\beta$-function for a massless Yukawa theory in a closed form at the order $\mathcal{O}(1/N_f)$ in the spirit of the expansion in a large number of flavours $N_f$. We find an analytic expression with a finite radius of convergence, and the first singularity occurs at the coupling value $K=5$.Comment: 15 pages, 2 figures. v3 error in the computation of the n-loop scalar self-energy corrected. The first singularity occurs at $K=5$, qualitatively the analysis is unchange

The abelian gauge-Yukawa β\beta-functions at large NfN_f

We study the impact of the Yukawa interaction in the large-$N_f$ limit to the abelian gauge theory. We compute the coupled $\beta$-functions for the system in a closed form at $\mathcal{O}(1/N_f)$.Comment: 13 pages, 6 figures. v3 matches the published versio

Axiflavon-Higgs Unification

In this talk, a unified model of scalar particles that addresses the flavour hierarchies, solves the strong CP problem, delivers a dark matter candidate, and radiatively triggers electroweak symmetry breaking is discussed. The recently proposed axiflavon is embedded together with an (elementary) Goldstone Higgs-sector in a single multiplet (and thereby also a model of flavour and strong CP conservation for the latter is provided). Bounds on the axion decay constant follow from requiring a SM-like Higgs potential at low energies and are confronted with constraints from flavour physics and astrophysics. In the minimal implementation, the axion decay constant is restricted to $f_a \approx (10^{11}-10^{12})$ GeV, while adding right-handed neutrinos allows for a heavy-axion model at lower energies, down to $f_a \sim 10$ TeV.Comment: Contribution to the 2019 EW session of the 54th Rencontres de Moriond, 6 pages, 2 figure

Bubble-resummation and critical-point methods for β\beta-functions at large NN

We investigate the connection between the bubble-resummation and critical-point methods for computing the $\beta$-functions in the limit of large number of flavours, $N$, and show that these can provide complementary information. While the methods are equivalent for single-coupling theories, for multi-coupling case the standard critical exponents are only sensitive to a combination of the independent pieces entering the $\beta$-functions, so that additional input or direct computation are needed to decipher this missing information. In particular, we evaluate the $\beta$-function for the quartic coupling in the Gross-Neveu-Yukawa model, thereby completing the full system at $\mathcal{O}(1/N)$. The corresponding critical exponents would imply a shrinking radius of convergence when $\mathcal{O}(1/N^2)$ terms are included, but our present result shows that the new singularity is actually present already at $\mathcal{O}(1/N)$, when the full system of $\beta$-functions is known.Comment: 11 pages, 7 figures; v2: references added, matches the published versio

Extracting the mass scale of a second Higgs boson from a deviation in h(125)h(125) couplings

We investigate the correlation between a possible deviation in the discovered Higgs boson $h(125)$ couplings from the Standard Model prediction and the mass scale ($M_{\text{2nd}}$) of the next-to-lightest Higgs boson in models with non-minimal Higgs sectors. In particular, we comprehensively study a class of next-to-minimal Higgs sectors which satisfy the electroweak $\rho$ parameter to be one at tree level. We derive an upper limit on $M_{\text{2nd}}$ by imposing bounds from perturbative unitarity, vacuum stability, triviality and electroweak precision data as functions of the deviation in the $hVV$ ($V=W,Z$) couplings. Furthermore, we discuss the complementarity between these bounds and the current LHC data, e.g., by considering direct searches for additional Higgs bosons and indirect constraints arising from the measured $h(125)$ signal strengths.Comment: 37 pages, 36 figure

Hierarchies and new symmetries: from flavored axions to composite Higgs

We investigate possible extensions of the Standard Model according to either elementary or composite nature of the Higgs boson. In the former scenario, we present a model of flavored axion Higgs unification in which electroweak symmetry breaking occurs dynamically and selects a small region for the axion decay constant that will be fully tested at forthcoming dark matter and flavor experiments. Moreover, if the Higgs is elementary, yukawa couplings are regarded as fundamental interactions: this relates to possible UV completions based on asymptotic safety for which we provide new insights by studying models containing a large multiplicity of fermion fields. Finally, we introduce a new realization of a composite Higgs that resolves the tension with null-signals of new physics at the LHC. It is based on a new symmetry structure that we call soft breaking, whose realization in the context of a warped extra dimension is particularly simple. When combined with other possible symmetries this allows for a completely natural model to emerge predicting composite resonances above 2 TeV with fine-tuning at the minimum 10% already implied by LEP and thus restoring naturalness in the light of the LHC data

Has NANOGrav found first evidence for cosmic strings?

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has recently reported strong evidence for a stochastic common-spectrum process affecting the pulsar timing residuals in its 12.5-year data set. We demonstrate that this process admits an interpretation in terms of a stochastic gravitational-wave background emitted by a cosmic-string network in the early Universe. We study stable Nambu-Goto strings in dependence of their tension $G\mu$ and loop size $\alpha$ and show that the entire viable parameter space will be probed by an array of future experiments.Comment: 7 pages, 3 figures. v2: revised treatment of the higher cosmic-string modes [see Eq. (6)], resulting in a few numerical but no qualitative changes. v3: matches version published in PR

A Natural Composite Higgs via Universal Boundary Conditions

We present a novel realization of a composite Higgs, which can naturally produce top partners above the current LHC bounds without increasing the tuning above 10%. The essential ingredients are softened breaking of the Higgs shift symmetry as well as maximal symmetry, which turn out to perfectly complement each other. The 5D realization of this model is particularly simple: universal UV and IR boundary conditions for the bulk fermions containing the SM fields will cure the problems of existing holographic composite Higgs models and provide a complete viable model for a naturally light Higgs without much tuning.Comment: 25 pages, 3 figures; v2: matches version submitted to journal; v3: revised version, results unchanged; v4: reference adde