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Simone Blasi

Publications and source records attributed to Simone Blasi.

35 records · Page 2Linked to original sources
arXiv2022

Holographic Composite Higgs Model Building: Soft Breaking, Maximal Symmetry, and the Higgs Mass

We study the emergence and phenomenological consequences of recently proposed new structures, namely soft breaking of the Higgs shift symmetry and `maximal symmetry' of the composite sector, in holographic realizations of composite Higgs models. For the former, we show that soft breaking can also successfully be implemented in a full 5D warped model, where symmetry-restoring universal boundary conditions for the fermion fields allow to break the problematic connection between a realistically light Higgs and anomalously light top partners. For the latter, we demonstrate that the minimal incarnation of maximal symmetry in the holographic dual leads to a sharp prediction of $m_h\approx 197$ GeV for $f=800$ GeV. We find that a viable implementation is possible with sizable negative gauge brane kinetic terms, allowing for $m_h=125$ GeV. Overall, both approaches offer promising directions to improve the naturalness also of holographic realizations of composite Higgs models.

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arXiv2022

Minimal SU(6) Gauge-Higgs Grand Unification

We present a minimal viable Gauge-Higgs Grand Unification scenario in warped space based on a $SU(6)$ bulk symmetry - unifying the gauge symmetries of the SM and their breaking sector. We show how the issue of light exotic new states is eliminated by appropriately breaking the gauge symmetry on the UV and IR boundaries by either brane scalars or gauge boundary conditions. The SM fermion spectrum is naturally reproduced including Dirac neutrinos and we compute the Higgs potential at one-loop, finding easily solutions with a realistic $m_h \sim 125$ GeV. The problem of proton decay is addressed by showing that baryon number is a hidden symmetry of the model. Among the phenomenological consequences, we highlight the presence of a scalar leptoquark and a scalar singlet. The usual $X,Y$ gauge bosons from $SU(5)$ GUTs are found at collider accessible masses.

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arXiv2022

Domain walls seeding the electroweak phase transition

Topological defects can act as local impurities that seed cosmological phase transitions. In this paper we study the case of domain walls, and how they can affect the electroweak phase transition in the Standard Model extended with a $Z_2$-odd scalar singlet. When the transition is two-step, the early breaking of the $Z_2$ symmetry implies the formation of domain walls which can then act as nucleation sites for the second step. We develop a method based on dimensional reduction to calculate the rate of the catalyzed phase transition within the 3d theory on the domain wall surface. By comparison with the standard homogeneous rate, we conclude that the seeded phase transition is generically faster and it ultimately determines the way the phase transition is completed. We comment on the phenomenological implications for gravitational waves and baryogenesis.

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arXiv2021

Baryogenesis via relativistic bubble expansion

We present a novel baryogenesis mechanism in which the asymmetry is sourced from heavy particles which either gain their mass or are created during bubble expansion in a strong first order phase transition. These particles then decay in a CP and baryon number violating way inside the bubble. The particles are inherently out-of-equilibrium and sufficiently dilute after wall crossing so the third Sakharov condition is easily met. Washout is avoided provided the reheat temperature is sufficiently below the scale of the heavy particles. The mechanism relies on moderate supercooling and relativistic walls which -- in contrast to electroweak baryogenesis -- generically leads to a sizable gravitational wave signal, although in the simplest realisations at frequencies beyond upcoming detectors. We present a simple example model and discuss the restrictions on the parameter space for the mechanism to be successful. We find that high reheat temperatures $T_{\rm RH} \gtrsim 10^{10}$ GeV are generally preferred, whereas stronger supercooling allows for temperatures as low as $T_{\rm RH} \sim 10^{6}$ GeV, provided the vacuum energy density is sufficiently suppressed. We briefly comment on using resonantly enhanced CP violation to achieve even lower scales.

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arXiv2021

Unification of Gauge Symmetries... including their breaking

In this talk, we present a minimal viable scenario that unifies the gauge symmetries of the Standard Model (SM) and their breaking sector. Our Gauge-Higgs Grand Unification setup employs 5D warped space with a $SU(6)$ bulk gauge field that includes both a $SU(5)$ grand unified theory (GUT) and a Higgs sector as a scalar component of the 5D vector field, solving the hierarchy problem. By appropriately breaking the gauge symmetry on the boundaries of the extra dimension the issue of light exotic new states, appearing generically in such models, is eliminated and the SM fermion spectrum is naturally reproduced. The Higgs potential is computed at one-loop, finding straightforward solutions with a realistic $m_h = 125$ GeV. The problem of proton decay is addressed by showing that baryon number is a hidden symmetry of the model. The presence of a scalar leptoquark and a scalar singlet is highlighted, which might play a role in solving further problems of the SM, allowing for example for electroweak baryogenesis. Finally, the $X$ and $Y$ gauge bosons from $SU(5)$ GUTs are found at collider accessible masses, opening a window to the unification structure at low energies.

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arXiv2021

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.

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arXiv2021

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 $Gμ$ and loop size $α$ and show that the entire viable parameter space will be probed by an array of future experiments.

astro-ph.CO↗
arXiv2020

Fingerprint of Low-Scale Leptogenesis in the Primordial Gravitational-Wave Spectrum

The dynamical generation of right-handed-neutrino (RHN) masses in the early Universe naturally entails the formation of cosmic strings that give rise to an observable signal in gravitational waves (GWs). Here, we show that a characteristic break in the GW spectrum would provide evidence for a new stage in the cosmological expansion history and a suppression of the RHN mass scale compared to the scale of spontaneous symmetry breaking. The detection of such a spectral feature would thus represent a novel and unique possibility to probe the physics of RHN mass generation in regions of parameter space that allow for low-scale leptogenesis in accord with electroweak naturalness.

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arXiv2020

A critical look at $β$-function singularities at large $N$

We propose a self-consistency equation for the $β$-function for theories with a large number of flavours, $N$, that exploits all the available information in the Wilson-Fisher critical exponent, $ω$, truncated at a fixed order in $1/N$. We show that singularities appearing in critical exponents do not necessarily imply singularities in the $β$-function. We apply our method to (non-)abelian gauge theory, where $ω$ features a negative singularity. The singularities in the $β$-function and in the fermion mass anomalous dimension are simultaneously removed providing no hint for a UV fixed point in the large-$N$ limit.

hep-th↗
arXiv2019

Bubble-resummation and critical-point methods for $β$-functions at large $N$

We investigate the connection between the bubble-resummation and critical-point methods for computing the $β$-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 $β$-functions, so that additional input or direct computation are needed to decipher this missing information. In particular, we evaluate the $β$-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 $β$-functions is known.

hep-th↗
arXiv2019

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.

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arXiv2019

Softened Goldstone-Symmetry Breaking

We propose a new way of breaking the Goldstone symmetry in composite Higgs models, restoring the global symmetry in the mixings between the elementary and composite fermions by completing the former to full representations of this symmetry. The Goldstone symmetry is in turn broken softly by vector-like mass terms in the {\it elementary} sector only. The resulting softened explicit breaking allows for a light Higgs boson, as found at the LHC, and a heavy top quark, without the need of light top partners around the Goldstone scale $f \sim {\rm TeV} \ll m_{\rm comp.}$, which remain elusive at the LHC, while we recover the standard scenario in the limit of infinite vector-like masses.

hep-ph↗
arXiv2018

The $β$-function for Yukawa theory at large $N_f$

We compute the $β$-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 $K=5$.

hep-ph↗
arXiv2018

A Common Source for Scalars: Axiflavon-Higgs Unification

We propose a unified model of scalar particles that addresses the flavour hierarchies, solves the strong CP problem, delivers a dark matter candidate, and provides the trigger for electroweak symmetry breaking. Besides furnishing a unification of the recently proposed axiflavon with a Goldstone-Higgs sector, the scenario can also be seen as adding a model of flavour (and strong CP conservation along with axion dark matter) to elementary Goldstone-Higgs setups. In particular, we derive bounds on the axion decay constant from the need to generate a SM-like Higgs potential at low energies, which we confront with constraints from flavour physics and cosmology. In the minimal implementation, we find that the axion decay constant is restricted to a thin stripe of $f_a \approx (10^{11}-10^{12})$ GeV, while adding right-handed neutrinos allows to realize a heavy-axion model at lower energies, down to $f_a \sim 10$ TeV.

hep-ph↗
arXiv2017

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

hep-ph↗
arXiv2017

Effects of custodial symmetry breaking in the Georgi-Machacek model at high energies

The model proposed by Georgi and Machacek enables the Higgs sector to involve isospin triplet scalar fields while retaining a custodial $SU(2)_V$ symmetry in the potential and thus ensuring the electroweak $ρ$ parameter to be one at tree level. This custodial symmetry, however, is explicitly broken by loop effects of the $U(1)_Y$ hypercharge gauge interaction. In order to make the model consistent at high energies, we construct the most general form of the Higgs potential without the custodial symmetry, and then we derive the one-loop $β$-functions for all the model parameters. Assuming the $δ_i$ quantities describing the custodial symmetry breaking to be zero at low energy, we find that $|δ_i|$ are typically smaller than the magnitude of the $U(1)_Y$ gauge coupling and the other running parameters in the potential also at high energy without spoiling perturbativity and vacuum stability. We also clarify that the mass degeneracy among the $SU(2)_V$ 5-plet and 3-plet Higgs bosons is smoothly broken by $\sim 0.1\%$ corrections. These results show that the amount of the custodial symmetry breaking is well kept under control up to energies close to the theory cutoff.

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