arXiv subjects

D. M. Ghilencea

Publications and source records attributed to D. M. Ghilencea.

At least 37 records · Page 2Linked to original sources

arXiv2019

Spontaneous breaking of Weyl quadratic gravity to Einstein action and Higgs potential

We consider the (gauged) Weyl gravity action, quadratic in the scalar curvature ($\tilde R$) and in the Weyl tensor ($\tilde C_{μνρσ}$) of the Weyl conformal geometry. In the absence of matter fields, this action has spontaneous breaking in which the Weyl gauge field $ω_μ$ becomes massive (mass $m_ω\sim$ Planck scale) after "eating" the dilaton in the $\tilde R^2$ term, in a Stueckelberg mechanism. As a result, one recovers the Einstein-Hilbert action with a positive cosmological constant and the Proca action for the massive Weyl gauge field $ω_μ$. Below $m_ω$ this field decouples and Weyl geometry becomes Riemannian. The Einstein-Hilbert action is then just a "low-energy" limit of Weyl quadratic gravity which thus avoids its previous, long-held criticisms. In the presence of matter scalar field $ϕ_1$ (Higgs-like), with couplings allowed by Weyl gauge symmetry, after its spontaneous breaking one obtains in addition, at low scales, a Higgs potential with spontaneous electroweak symmetry breaking. This is induced by the non-minimal coupling $ξ_1ϕ_1^2 \tilde R$ to Weyl geometry, with Higgs mass $\proptoξ_1/ξ_0$ ($ξ_0$ is the coefficient of the $\tilde R^2$ term). In realistic models $ξ_1$ must be classically tuned $ξ_1\ll ξ_0$. We comment on the quantum stability of this value.

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arXiv2018

Two-loop corrections to Starobinsky-Higgs inflation

Higgs inflation and $R^2$-inflation (Starobinsky model) are two limits of the same quantum model, hereafter called Starobinsky-Higgs. We analyse the two-loop action of the Higgs-like scalar $ϕ$ in the presence of: 1) non-minimal coupling ($ξ$) and 2) quadratic curvature terms. The latter are generated at the quantum level with $ϕ$-dependent couplings ($\tildeα$) even if their tree-level couplings ($α$) are tuned to zero. Therefore, the potential always depends on both Higgs field $ϕ$ and scalaron $ρ$, hence multi-field inflation is a quantum consequence. The effects of the quantum (one- and two-loop) corrections on the potential $\hat W(ϕ,ρ)$ and on the spectral index are discussed, showing that the Starobinsky-Higgs model is in general stable in their presence. Two special cases are also considered: first, for a large $ξ$ in the quantum action one can integrate $ϕ$ and generate a "refined" Starobinsky model which contains additional terms $ξ^2 R^2\ln^p (ξ\vert R\vert/μ^2)$, $p=1,2$ ($μ$ is the subtraction scale). These generate corrections linear in the scalaron to the "usual" Starobinsky potential and a "running" scalaron mass. Second, for a small fixed Higgs field $ϕ^2 \ll M_p^2/ξ$ and a vanishing classical coefficient of the $R^2$-term, we show that the "usual" Starobinsky inflation is generated by the quantum corrections alone, for a suitable non-minimal coupling ($ξ$).

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arXiv2018

Quantum implications of a scale invariant regularisation

We study scale invariance at the quantum level (three loops) in a perturbative approach. For a scale-invariant classical theory the scalar potential is computed at three-loop level while keeping manifest this symmetry. Spontaneous scale symmetry breaking is transmitted at quantum level to the visible sector (of $ϕ$) by the associated Goldstone mode (dilaton $σ$) which enables a scale-invariant regularisation and whose vev $\langleσ\rangle$ generates the subtraction scale ($μ$). While the hidden ($σ$) and visible sector ($ϕ$) are classically decoupled in $d=4$ due to an enhanced Poincaré symmetry, they interact through (a series of) evanescent couplings $\proptoε^k$, ($k\geq 1$), dictated by the scale invariance of the action in $d=4-2ε$. At the quantum level these couplings generate new corrections to the potential, such as scale-invariant non-polynomial effective operators $ϕ^{2n+4}/σ^{2n}$ and also log-like terms ($\propto \ln^k σ$) restoring the scale-invariance of known quantum corrections. The former are comparable in size to "standard" loop corrections and important for values of $ϕ$ close to $\langleσ\rangle$. For $n=1,2$ the beta functions of their coefficient are computed at three-loops. In the infrared (IR) limit the dilaton fluctuations decouple, the effective operators are suppressed by large $\langleσ\rangle$ and the effective potential becomes that of a renormalizable theory with explicit scale symmetry breaking by the "usual" DR scheme (of $μ=$constant).

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arXiv2017

Standard Model with spontaneously broken quantum scale invariance

We explore the possibility that scale symmetry is a quantum symmetry that is broken only spontaneously and apply this idea to the Standard Model (SM). We compute the quantum corrections to the potential of the higgs field ($ϕ$) in the classically scale invariant version of the SM ($m_ϕ=0$ at tree level) extended by the dilaton ($σ$). The tree-level potential of $ϕ$ and $σ$, dictated by scale invariance, may contain non-polynomial effective operators, e.g. $ϕ^6/σ^2$, $ϕ^8/σ^4$, $ϕ^{10}/σ^6$, etc. The one-loop scalar potential is scale invariant, since the loop calculations manifestly preserve the scale symmetry, with the DR subtraction scale $μ$ generated spontaneously by the dilaton vev $μ\sim\langleσ\rangle$. The Callan-Symanzik equation of the potential is verified in the presence of the gauge, Yukawa and the non-polynomial operators. The couplings of the non-polynomial operators have non-zero beta functions that we can actually compute from the quantum potential. At the quantum level the higgs mass is protected by spontaneously broken scale symmetry, even though the theory is non-renormalizable. We compare the one-loop potential to its counterpart computed in the "traditional" DR scheme that breaks scale symmetry explicitly ($μ=$constant) in the presence at the tree level of the non-polynomial operators.

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arXiv2017

Wilson lines and UV sensitivity in magnetic compactifications

We investigate the ultraviolet (UV) behaviour of 6D N=1 supersymmetric effective (Abelian) gauge theories compactified on a two-torus ($T_2$) with magnetic flux. To this purpose we compute offshell the one-loop correction to the Wilson line state self-energy. The offshell calculation is actually necessary to capture the usual effective field theory expansion in powers of $(\partial/Λ)$. Particular care is paid to the regularization of the (divergent) momentum integrals, which is relevant for identifying the corresponding counterterm(s). We find a counterterm which is a new higher dimensional effective operator of dimension d=6, that is enhanced for a larger compactification area (where the effective theory applies) and is consistent with the symmetries of the theory. Its consequences are briefly discussed and comparison is made with orbifold compactifications without flux.

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arXiv2016

Bounds on supersymmetric effective operators from heavy diphoton searches

We identify the bounds on supersymmetric effective operators beyond MSSM, from heavy diphoton resonance ($X$) negative searches at the LHC, where $X$ is identified with the neutral CP-even (odd) $H$ ($A$) or both (mass degenerate). While minimal supersymmetric models (MSSM, etc) comply with the data, a leading effective operator of $d=6$ can contribute significantly to diphoton production $σ\sim 1$ fb, well above its MSSM value and in conflict with recent data. Both the $b\bar b$ and $gg$ production mechanisms of $H$ and $A$ can contribute comparably to this. We examine the dependence of the diphoton cross section $σ$ on the values of $m_X$, $Λ$ and $\tanβ$, under the experimental constraints from the SM-like higgs couplings $hgg$ and $hγγ$ (due to mixing) and from the $b\bar b$ and $t\bar t$ searches. These give $Λ$ larger than $\sim 5$ TeV for $m_X$ in the range $0.5-1$ TeV. We show how to generate the $d=6$ effective operator from microscopic (renormalizable) models. This demands the presence of vector-like states beyond the MSSM spectrum (and eventually but not necessarily a gauge singlet), of mass near $Λ$ and thus outside the LHC reach.

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arXiv2016

Two-loop scale-invariant scalar potential and quantum effective operators

Spontaneous breaking of quantum scale invariance may provide a solution to the hierarchy and cosmological constant problems. In a scale-invariant regularization, we compute the two-loop potential of a higgs-like scalar $ϕ$ in theories in which scale symmetry is broken only spontaneously by the dilaton ($σ$). Its vev $\langleσ\rangle$ generates the DR subtraction scale ($μ\sim\langleσ\rangle$), which avoids the explicit scale symmetry breaking by traditional regularizations (where $μ$=fixed scale). The two-loop potential contains effective operators of non-polynomial nature as well as new corrections, beyond those obtained with explicit breaking ($μ$=fixed scale). These operators have the form: $ϕ^6/σ^2$, $ϕ^8/σ^4$, etc, which generate an infinite series of higher dimensional polynomial operators upon expansion about $\langleσ\rangle\gg \langleϕ\rangle$, where such hierarchy is arranged by {\it one} initial, classical tuning. These operators emerge at the quantum level from evanescent interactions ($\proptoε$) between $σ$ and $ϕ$ that vanish in $d=4$ but are demanded by classical scale invariance in $d=4-2ε$. The Callan-Symanzik equation of the two-loop potential is respected and the two-loop beta functions of the couplings differ from those of the same theory regularized with $μ=$fixed scale. Therefore the running of the couplings enables one to distinguish between spontaneous and explicit scale symmetry breaking.

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arXiv2016

Manifestly scale-invariant regularization and quantum effective operators

Scale invariant theories are often used to address the hierarchy problem, however the regularization of their quantum corrections introduces a dimensionful coupling (dimensional regularization) or scale (Pauli-Villars, etc) which break this symmetry explicitly. We show how to avoid this problem and study the implications of a manifestly scale invariant regularization in (classical) scale invariant theories. We use a dilaton-dependent subtraction function $μ(σ)$ which after spontaneous breaking of scale symmetry generates the usual DR subtraction scale $μ(\langleσ\rangle)$. One consequence is that "evanescent" interactions generated by scale invariance of the action in $d=4-2ε$ (but vanishing in $d=4$), give rise to new, finite quantum corrections. We find a (finite) correction $ΔU(ϕ,σ)$ to the one-loop scalar potential for $ϕ$ and $σ$, beyond the Coleman-Weinberg term. $ΔU$ is due to an evanescent correction ($\proptoε$) to the field-dependent masses (of the states in the loop) which multiplies the pole ($\propto 1/ε$) of the momentum integral, to give a finite quantum result. $ΔU$ contains a non-polynomial operator $\sim ϕ^6/σ^2$ of known coefficient and is independent of the subtraction dimensionless parameter. A more general $μ(ϕ,σ)$ is ruled out since, in their classical decoupling limit, the visible sector (of the higgs $ϕ$) and hidden sector (dilaton $σ$) still interact at the quantum level, thus the subtraction function must depend on the dilaton only. The method is useful in models where preserving scale symmetry at quantum level is important.

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arXiv2016

One-loop potential with scale invariance and effective operators

We study quantum corrections to the scalar potential in classically scale invariant theories, using a manifestly scale invariant regularization. To this purpose, the subtraction scale $μ$ of the dimensional regularization is generated after spontaneous scale symmetry breaking, from a subtraction function of the fields, $μ(ϕ,σ)$. This function is then uniquely determined from general principles showing that it depends on the dilaton only, with $μ(σ)\sim σ$. The result is a scale invariant one-loop potential $U$ for a higgs field $ϕ$ and dilaton $σ$ that contains an additional {\it finite} quantum correction $ΔU(ϕ,σ)$, beyond the Coleman Weinberg term. $ΔU$ contains new, non-polynomial effective operators like $ϕ^6/σ^2$ whose quantum origin is explained. A flat direction is maintained at the quantum level, the model has vanishing vacuum energy and the one-loop correction to the mass of $ϕ$ remains small without tuning (of its self-coupling, etc) beyond the initial, classical tuning (of the dilaton coupling) that enforces a hierarchy $\langleσ\rangle\gg \langleϕ\rangle$. The approach is useful to models that investigate scale symmetry at the quantum level.

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arXiv2015

Comments on the nilpotent constraint of the goldstino superfield

Superfield constraints were often used in the past, in particular to describe the Akulov-Volkov action of the goldstino by a superfield formulation with $L=(Φ^\dagger Φ)_D + [(fΦ)_F + h.c.]$ endowed with the nilpotent constraint $Φ^2=0$ for the goldstino superfield ($Φ$). Inspired by this, such constraint is often used to define the goldstino superfield even in the presence of additional superfields, for example in models of "nilpotent inflation". In this review we show that the nilpotent property is not valid in general, under the assumption of a microscopic (ultraviolet) description of the theory with linear supermultiplets. Sometimes only weaker versions of the nilpotent relation are true such as $Φ^3=0$ or $Φ^4=0$ ($Φ^2\not=0$) in the infrared (far below the UV scale) under the further requirement of decoupling all additional scalars (coupling to sgoldstino), something not always possible (e.g. if light scalars exist). In such cases the weaker nilpotent property is not specific to the goldstino superfield anymore. We review the restrictions for the Kahler curvature tensor and superpotential $W$ under which $Φ^2=0$ remains true in infrared, assuming linear supermultiplets in the microscopic description. One can reverse the arguments to demand that the nilpotent condition, initially an infrared property, be extended even in the presence of additional superfields, but this may question the nature of supersymmetry breaking or the existence of a perturbative ultraviolet completion with linear supermultiplets.

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arXiv2015

Effective operators in SUSY, superfield constraints and searches for a UV completion

We discuss the role of a class of higher dimensional operators in 4D N=1 supersymmetric effective theories. The Lagrangian in such theories is an expansion in momenta below the scale of "new physics" ($Λ$) and contains the effective operators generated by integrating out the "heavy states" above $Λ$ present in the UV complete theory. We go beyond the "traditional" leading order in this momentum expansion (in $\partial/Λ$). Keeping manifest supersymmetry and using superfield {\it constraints} we show that the corresponding higher dimensional (derivative) operators in the sectors of chiral, linear and vector superfields of a Lagrangian can be "unfolded" into second-order operators. The "unfolded" formulation has only polynomial interactions and additional massive superfields, some of which are ghost-like if the effective operators were {\it quadratic} in fields. Using this formulation, the UV theory emerges naturally and fixes the (otherwise unknown) coefficient and sign of the initial (higher derivative) operators. Integrating the massive fields of the "unfolded" formulation generates an effective theory with only polynomial effective interactions relevant for phenomenology. We also provide several examples of "unfolding" of theories with higher derivative {\it interactions} in the gauge or matter sectors that are actually ghost-free. We then illustrate how our method can be applied even when including {\it all orders} in the momentum expansion, by using an infinite set of superfield constraints and an iterative procedure, with similar results.

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arXiv2014

Gauged R-symmetry and its anomalies in 4D N=1 supergravity and phenomenological implications

We consider a class of models with gauged U(1)_R symmetry in 4D N=1 supergravity that have, at the classical level, a metastable ground state, an infinitesimally small (tunable) positive cosmological constant and a TeV gravitino mass. We analyse if these properties are maintained under the addition of visible sector (MSSM-like) and hidden sector state(s), where the latter may be needed for quantum consistency. We then discuss the anomaly cancellation conditions in supergravity as derived by Freedman, Elvang and Körs and apply their results to the special case of a U(1)_R symmetry, in the presence of the Fayet-Iliopoulos term ($ξ$) and Green-Schwarz mechanism(s). We investigate the relation of these anomaly cancellation conditions to the "naive" field theory approach in global SUSY, in which case U(1)_R cannot even be gauged. We show the two approaches give similar conditions. Their induced constraints at the phenomenological level, on the above models, remain strong even if one lifted the GUT-like conditions for the MSSM gauge couplings. In an anomaly-free model, a tunable, TeV-scale gravitino mass may remain possible provided that the U(1)_R charges of additional hidden sector fermions (constrained by the cubic anomaly alone) do not conflict with the related values of U(1)_R charges of their scalar superpartners, constrained by existence of a stable ground state. This issue may be bypassed by tuning instead the coefficients of the Kahler connection anomalies (b_K, b_{CK}).

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arXiv2014

Naturalness in low-scale SUSY models and "non-linear" MSSM

In MSSM models with various boundary conditions for the soft breaking terms (m_{soft}) and for a higgs mass of 126 GeV, there is a (minimal) electroweak fine-tuning Delta\approx 800 to 1000 for the constrained MSSM and Delta\approx 500 for non-universal gaugino masses. These values, often regarded as unacceptably large, may indicate a problem of supersymmetry (SUSY) breaking, rather than of SUSY itself. A minimal modification of these models is to lower the SUSY breaking scale in the hidden sector (\sqrt f) to few TeV, which we show to restore naturalness to more acceptable levels Delta\approx 80 for the most conservative case of low tan_beta and ultraviolet boundary conditions as in the constrained MSSM. This is done without introducing additional fields in the visible sector, unlike other models that attempt to reduce Delta. In the present case Delta is reduced due to additional (effective) quartic higgs couplings proportional to the ratio m_{soft}/(\sqrt f) of the visible to the hidden sector SUSY breaking scales. These couplings are generated by the auxiliary component of the goldstino superfield. The model is discussed in the limit its sgoldstino component is integrated out so this superfield is realized non-linearly (hence the name of the model) while the other MSSM superfields are in their linear realization. By increasing the hidden sector scale \sqrt f one obtains a continuous transition for fine-tuning values, from this model to the usual (gravity mediated) MSSM-like models.

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arXiv2014

SUSY naturalness without prejudice

Unlike the Standard Model (SM), supersymmetric models stabilize the electroweak (EW) scale $v$ at the quantum level and {\it predict} that $v$ is a function of the TeV-valued SUSY parameters ($γ_α$) of the UV Lagrangian. We show that the (inverse of the) covariance matrix of the model in the basis of these parameters and the usual deviation $δχ^2$ (from $χ^2_{min}$ of a model) automatically encode information about the "traditional" EW fine-tuning measuring this stability, {\it provided that} the EW scale $v\sim m_Z$ is indeed regarded as a function $v=v(γ)$. It is known that large EW fine-tuning may signal an incomplete theory of soft terms and can be reduced when relations among $γ_α$ exist (due to GUT symmetries, etc). The global correlation coefficient of this matrix can help one investigate if such relations are present. An upper bound on the usual EW fine-tuning measure ("in quadrature") emerges from the analysis of the $δχ^2$ and the s-standard deviation confidence interval by using $v=v(γ)$ and the theoretical approximation (loop order) considered for the calculation of the observables. This upper bound avoids subjective criteria for the "acceptable" level of EW fine-tuning for which the model is still "natural".

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arXiv2013

Fixing the EW scale in supersymmetric models after the Higgs discovery

TeV-scale supersymmetry was originally introduced to solve the hierarchy problem and therefore fix the electroweak (EW) scale in the presence of quantum corrections. Numerical methods testing the SUSY models often report a good likelihood L (or chi^2=-2ln L) to fit the data {\it including} the EW scale itself (m_Z^0) with a {\it simultaneously} large fine-tuning i.e. a large variation of this scale under a small variation of the SUSY parameters. We argue that this is inconsistent and we identify the origin of this problem. Our claim is that the likelihood (or chi^2) to fit the data that is usually reported in such models does not account for the chi^2 cost of fixing the EW scale. When this constraint is implemented, the likelihood (or chi^2) receives a significant correction (delta_chi^2) that worsens the current data fits of SUSY models. We estimate this correction for the models: constrained MSSM (CMSSM), models with non-universal gaugino masses (NUGM) or higgs soft masses (NUHM1, NUHM2), the NMSSM and the general NMSSM (GNMSSM). For a higgs mass m_h\approx 126 GeV, one finds that in these models (delta_chi^2)/ndf> 1.5 (approx 1 for GNMSSM), which violates the usual condition of a good fit (total chi^2/ndf approx 1) already before fitting observables other than the EW scale itself (ndf=number of degrees of freedom). This has (negative) implications for SUSY models and it is suggested that future data fits properly account for this effect, if one remains true to the original goal of SUSY. Since the expression of delta_chi^2 that emerges from our calculation depends on a familiar measure of fine-tuning, one concludes that EW fine-tuning is an intrinsic part of the likelihood to fit the data that includes the EW scale (m_Z^0).

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arXiv2013

A new approach to Naturalness in SUSY models

We review recent results that provide a new approach to the old problem of naturalness in supersymmetric models, without relying on subjective definitions for the fine-tuning associated with {\it fixing} the EW scale (to its measured value) in the presence of quantum corrections. The approach can address in a model-independent way many questions related to this problem. The results show that naturalness and its measure (fine-tuning) are an intrinsic part of the likelihood to fit the data that {\it includes} the EW scale. One important consequence is that the additional {\it constraint} of fixing the EW scale, usually not imposed in the data fits of the models, impacts on their overall likelihood to fit the data (or chi^2/ndf, ndf: number of degrees of freedom). This has negative implications for the viability of currently popular supersymmetric extensions of the Standard Model.

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arXiv2013

Low-scale SUSY breaking and the (s)goldstino physics

For a 4D N=1 supersymmetric model with a low SUSY breaking scale (f) and general Kahler potential K(Phi^i,Phi_j^*) and superpotential W(Phi^i) we study, in an effective theory approach, the relation of the goldstino superfield to the (Ferrara-Zumino) superconformal symmetry breaking chiral superfield X. In the presence of more sources of supersymmetry breaking, we verify the conjecture that the goldstino superfield is the (infrared) limit of X for zero-momentum and Lambda->\infty. (Lambda is the effective cut-off scale). We then study the constraint X^2=0, which in the one-field case is known to decouple a massive sgoldstino and thus provide an effective superfield description of the Akulov-Volkov action for the goldstino. In the presence of additional fields that contribute to SUSY breaking we identify conditions for which X^2=0 remains valid, in the effective theory below a large but finite sgoldstino mass. The conditions ensure that the effective expansion (in 1/Lambda) of the initial Lagrangian is not in conflict with the decoupling limit of the sgoldstino (1/m_sgoldstino\sim Lambda/f, f<Lambda^2).

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arXiv2012

The fine-tuning cost of the likelihood in SUSY models

In SUSY models, the fine tuning of the electroweak (EW) scale with respect to their parameters gamma_i={m_0, m_{1/2}, mu_0, A_0, B_0,...} and the maximal likelihood L to fit the experimental data are usually regarded as two different problems. We show that, if one regards the EW minimum conditions as constraints that fix the EW scale, this commonly held view is not correct and that the likelihood contains all the information about fine-tuning. In this case we show that the corrected likelihood is equal to the ratio L/Delta of the usual likelihood L and the traditional fine tuning measure Delta of the EW scale. A similar result is obtained for the integrated likelihood over the set {gamma_i}, that can be written as a surface integral of the ratio L/Delta, with the surface in gamma_i space determined by the EW minimum constraints. As a result, a large likelihood actually demands a large ratio L/Delta or equivalently, a small chi^2_{new}=chi^2_{old}+2*ln(Delta). This shows the fine-tuning cost to the likelihood (chi^2_{new}) of the EW scale stability enforced by SUSY, that is ignored in data fits. A good chi^2_{new}/d.o.f.\approx 1 thus demands SUSY models have a fine tuning amount Delta<<exp(d.o.f./2), which provides a model-independent criterion for acceptable fine-tuning. If this criterion is not met, one can thus rule out SUSY models without a further chi^2/d.o.f. analysis. Numerical methods to fit the data can easily be adapted to account for this effect.

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