arXiv subjects

Maurizio Piai

Publications and source records attributed to Maurizio Piai.

At least 91 records · Page 5Linked to original sources

arXiv2017

Dilaton EFT Framework For Lattice Data

We develop an effective-field-theory (EFT) framework to analyze the spectra emerging from lattice simulations of a large class of confining gauge theories. Simulations of these theories, for which the light-fermion count is not far below the critical value for transition to infrared conformal behavior, have indicated the presence of a remarkably light singlet scalar particle. We incorporate this particle by including a scalar field in the EFT along with the Nambu-Goldstone bosons (NGB's), and discuss the application of this EFT to lattice data. We highlight the feature that data on the NGB's alone can tightly restrict the form of the scalar interactions. As an example, we apply the framework to lattice data for an SU(3) gauge theory with eight fermion flavors, concluding that the EFT can describe the data well.

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arXiv2017

Calculable mass hierarchies and a light dilaton from gravity duals

In the context of gauge/gravity dualities, we calculate the scalar and tensor mass spectrum of the boundary theory defined by a special 8-scalar sigma-model in five dimensions, the background solutions of which include the 1-parameter family dual to the baryonic branch of the Klebanov-Strassler field theory. This provides an example of a strongly-coupled, multi-scale system that yields a parametrically light mass for one of the composite scalar particles: the dilaton. We briefly discuss the implications of these findings towards identifying a satisfactory solution to both the big and little hierarchy problems of the electro-weak theory.

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arXiv2017

Glueballs on the Baryonic Branch of Klebanov-Strassler: dimensional deconstruction and a light scalar particle

Within gauge/gravity duality, we compute the scalar and tensor mass spectrum in the boundary theory defined by the five-dimensional sigma-model coupled to gravity obtained by constraining to eight scalars the truncation on T$^{1,1}$ that corresponds to the Papadopoulos-Tseytlin (PT) ansatz. We study fluctuations around the 1-parameter family of backgrounds that lift to the baryonic branch of the Klebanov-Strassler (KS) system, and interpolates between the KS background and the Maldacena-Nunez one (CVMN). We adopt a gauge invariant formalism in the treatment of the fluctuations that we interpret as states of the dual theory. The tensor spectrum interpolates between the discrete spectrum of the KS background and the continuum spectrum of the CVMN background, in particular showing the emergence of a finite energy range containing a dense set of states, as expected from dimensional deconstruction. The scalar spectrum shows analogous features, and in addition it contains one state that becomes parametrically light far from the origin along the baryonic branch.

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arXiv2017

Symmetry restoration at high-temperature in two-color and two-flavor lattice gauge theories

We consider the $SU(2)$ gauge theory with $N_f=2$ flavors of Dirac fundamental fermions. We study the high-temperature behavior of the spectra of mesons, discretizing the theory on anisotropic lattices, and measuring the two-point correlation functions in the temporal direction as well as screening masses in various channels. We identify the (pseudo-)critical temperature as the temperature at which the susceptibility associated with the Polyakov loop has a maximum. At high temperature both the spin-1 and spin-0 sectors of the light meson spectra exhibit enhanced symmetry properties, indicating the restoration of both the global $SU(4)$ and the axial $U(1)_A$ symmetries of the model.

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arXiv2017

Parity doubling in two-flavor SU(2) at high temperature

We study the mass spectrum of mesons at high temperature in $SU(2)$ gauge theory with two flavors of Dirac fundamental fermions. Numerical simulations are carried out on anisotropic lattices using Wilson fermions, with lattice parameters tuned so that Euclidean symmetry is restored at low energy. We determine the pseudo-critical temperature $T_c$ using renormalized Polyakov loops. We calculate temporal and spatial meson correlation functions across $T_c$, and observe a clear sign of parity doublings above $T_c$ in both vector and scalar channels. The degeneracy between parity partners in the spectrum indicates that the enhanced $SU(4) \times U(1)_A$ global symmetry of the model is restored at high temperature.

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arXiv2017

Large mass hierarchies from strongly-coupled dynamics

Motivated by the absence of signals of new physics at the LHC, which seems to imply the presence of large mass hierarchies, we investigate the theoretical possibility that these could arise dynamically in new strongly-coupled gauge theories extending the standard model of particle physics. To this purpose, we study lattice data on non-Abelian gauge theories in the (near-)conformal regime---specifically, $\mathrm{SU}(2)$ with $N_{\mathrm{f}}=1$ and $2$ dynamical fermion flavours in the adjoint representation. We focus our attention on the ratio $R$ between the masses of the lightest spin-2 and spin-0 resonances, and draw comparisons with a simple toy model in the context of gauge/gravity dualities. For models in which large anomalous dimensions arise dynamically, we show indications that this mass ratio can be large, with $R > 5$. Moreover, our results suggest that $R$ might be related to universal properties of the IR fixed point. Our findings provide an interesting step towards understanding large mass ratios in the non-perturbative regime of quantum field theories with (near) IR conformal behaviour.

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arXiv2016

A spartan model for the LHC diphoton excess

We propose a simple model accommodating the reported 750 GeV diphoton excess seen in the first 13-TeV run of the LHC. It leads to testable predictions, in particular for di-lepton production, at higher integrated luminosity. We append to the minimal standard model a new gauge sector with its own SU(2) symmetry group. A new complex doublet scalar field provides mass for the new vectors and describes the 750-GeV resonance. An adequate rate for the diphoton signals, with resonant production via photon fusion, requires the VEV of the new scalar field to be somewhat less than the electroweak scale. This in turn requires the new heavy vectors to have sub-TeV masses and be relatively strongly coupled. A new global U(1) symmetry plays a key role. Current precision-electroweak constraints are respected.

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arXiv2016

Large mass hierarchies from strongly-coupled dynamics

Besides the Higgs particle discovered in 2012, with mass 125 GeV, recent LHC data show tentative signals for new resonances in diboson as well as diphoton searches at high center-of-mass energies (2 TeV and 750 GeV, respectively). If these signals are confirmed (or other new resonances are discovered at the TeV scale), the large hierarchies between masses of new bosons require a dynamical explanation. Motivated by these tentative signals of new physics, we investigate the theoretical possibility that large hierarchies in the masses of glueballs could arise dynamically in new strongly-coupled gauge theories extending the standard model of particle physics. We study lattice data on non-Abelian gauge theories in the (near-)conformal regime as well as a simple toy model in the context of gauge/gravity dualities. We focus our attention on the ratio $R$ between the mass of the lightest spin-2 and spin-0 resonances, that for technical reasons is a particularly convenient and clean observable to study. For models in which (non-perturbative) large anomalous dimensions arise dynamically, we show indications that this mass ratio can be large, with $R>5$. Moreover, our results suggest that $R$ might be related to universal properties of the IR fixed point. Our findings provide an interesting step towards understanding large mass ratios in the non-perturbative regime of quantum field theories with (near) IR conformal behaviour.

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arXiv2016

Spectrum-doubled Heavy Vector Bosons at the LHC

We study a simple effective field theory incorporating six heavy vector bosons together with the standard-model field content. The new particles preserve custodial symmetry as well as an approximate left-right parity symmetry. The enhanced symmetry of the model allows it to satisfy precision electroweak constraints and bounds from Higgs physics in a regime where all the couplings are perturbative and where the amount of fine-tuning is comparable to that in the standard model itself. We find that the model could explain the recently observed excesses in di-boson processes at invariant mass close to 2 TeV from LHC Run 1 for a range of allowed parameter space. The masses of all the particles differ by no more than roughly 10%. In a portion of the allowed parameter space only one of the new particles has a production cross section large enough to be detectable with the energy and luminosity of Run 1, both via its decay to WZ and to Wh, while the others have suppressed production rates. The model can be tested at the higher-energy and higher-luminosity run of the LHC even for an overall scale of the new particles higher than 3 TeV.

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arXiv2015

On the glueball spectrum of walking backgrounds from wrapped-D5 gravity duals

We compute the mass spectrum of glueball excitations of a special class of strongly-coupled field theories via their type-IIB supergravity dual. We focus on two subclasses of backgrounds, which have different UV-asymptotics, but both of which exhibit walking behavior, in the weak sense that the gauge coupling of the dual field theory exhibits a quasi-constant behavior at strong coupling over a range of energies, before diverging in the deep IR. We improve on earlier calculations, by making use of the fully rigorous treatment of the 5-dimensional consistent truncation, including the rigorous form of the boundary conditions. In both cases there is a parametrically light scalar glueball. In the first case, this is a physical state, while in the second case this result is unphysical, since the presence of higher-order operators in the dual field theory makes the whole (physical) spectrum depend explicitly on a (unphysical) UV-cutoff scale.

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arXiv2015

Hyperscaling violation and Electroweak Symmetry Breaking

We consider a class of simplified models of dynamical electroweak symmetry breaking built in terms of their five-dimensional weakly-coupled gravity duals, in the spirit of bottom-up holography. The sigma-model consists of two abelian gauge bosons and one real, non-charged scalar field coupled to gravity in five dimensions. The scalar potential is a simple exponential function of the scalar field. The background metric resulting from solving the classical equations of motion exhibits hyperscaling violation, at least at asymptotically large values of the radial direction. We study the spectrum of scalar composite states of the putative dual field theory by fluctuating the sigma-model scalars and gravity, and discuss in which cases we find a parametrically light scalar state in the spectrum. We model the spontaneous breaking of the (weakly coupled) gauge symmetry to the diagonal subgroup by the choice of IR boundary conditions. We compute the mass spectrum of spin-1 states, and the precision electroweak parameter S as a function of the hyperscaling coefficient. We find a general bound on the mass of the lightest spin-1 resonance, by requiring that the indirect bounds on the precision parameters be satisfied, that implies that precision electroweak physics excludes the possibility of a techni-rho meson with mass lighter than several TeV.

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arXiv2015

Constrained Flavor Breaking

We explore predictive flavor models based on subgroups of the standard-model $SU(3)^5$ flavor symmetry. Restricting to products of $SU(3)$, we find that a global $SU(3)^3$ flavor symmetry, broken only by two Yukawa spurions, leads to a relation among down-type quark, up-type quark and charged-lepton Yukawa matrices: $Y_d \propto Y_u Y_e^\dagger$. As a result, the charged-lepton mass ratios are expressed in terms of quark mass ratios and mixing angles. Large leptonic mixing angles appear to be natural and lead to contributions to flavor-changing neutral currents in the charged-lepton sector, which can be tested in future precision experiments.

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arXiv2015

Multiscale confining dynamics from holographic RG flows

We consider renormalization group flows between conformal field theories in five (six) dimensions with a string (M-theory) dual. By compactifying on a circle (torus) with appropriate boundary conditions, we obtain continuous families of confining four-dimensional theories parametrized by the ratio $Λ_{\rm {\tiny flow}}/Λ_{\rm \tiny{QCD}}$, with $Λ_{\rm \tiny{flow}}$ the scale at which the flow between fixed points takes place and $Λ_{\rm \tiny{QCD}}$ the confinement scale. We construct the dual geometries explicitly and compute the spectrum of scalar bound states (glueballs). We find a `universal' subset of states common to all the models. We comment on the modifications of these models, and the corresponding fine-tuning, required for a parametrically light `dilaton' state to be present. We also comment on some aspects of these theories as probed by extended objects such as strings and branes.

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arXiv2014

Gauge/gravity dualities and bulk phase transitions

We consider D7-branes probing several classes of Type IIB supergravity backgrounds, and study the classical problem of finding equilibrium configurations for the embedding functions. This is a method employed to model chiral symmetry breaking in the gravity dual of a strongly-coupled, confining gauge theory. We unveil and discuss a new type of phase transition appearing in the gravity systems, which is similar in nature and meaning to bulk phase transitions on the lattice. The existence of this genre of phase transition puts a new, intrinsic limit on the region of parameter space which can be used to study the physics of the dual field theory. We complete the analysis of D7 embeddings in wrapped-D5 supergravity backgrounds, and explain in what cases chiral-symmetry breaking is sensibly modelled by the gravity construction.

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arXiv2014

On the stability of multi-scale models of dynamical symmetry breaking from holography

We consider two classes of backgrounds of Type IIB supergravity obtained by wrapping D5-branes on a two-cycle inside the conifold. The field theory dual exhibits confinement and, in addition, a region in which the dynamics is walking, at least in the weak sense that the running of the coupling is anomalously slow. We introduce quenched matter in the fundamental, modelled by probe D7-branes which wrap an internal three-dimensional manifold and lie at the equator of the transverse two-sphere. In the space spanned by the remaining internal angle and the radial coordinate the branes admit two embeddings. The first one is U-shaped: the branes merge at some finite value of the radius. The second one is disconnected and extends along the entire radial direction at fixed angular separation. We interpret these two configurations as corresponding to chiral-symmetry breaking and preserving phases, respectively. We present a simple diagnostic tool to examine the classical stability of the embedding, based on the concavity/convexity conditions for the relevant thermodynamic potentials. We use this criterion to show that U-shaped probes that explore the walking region are unstable, hence providing a dynamical origin for the tachyonic mode found in the literature. Whenever this occurs, the disconnected solution becomes favored energetically. We find that in one of the two classes of backgrounds the U-shaped embedding is always unstable, and thus never realised dynamically. Consequently, these models cannot be used to describe chiral-symmetry breaking. In the second category of solutions, our analysis reveals the presence of a first-order phase transition between chiral-symmetry broken and restored phases. Interestingly, this is in the same class that contains a parametrically light scalar in the spectrum of glueballs of the dual field theory.

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arXiv2013

Correlators of Circular Wilson Loops from Holography

We study the correlators of two circular Wilson loops of different radii at strong coupling. In our setup one Wilson loop is located inside the other. We use holography to calculate the connected two-point function. Both an AdS background and a confining background are considered. As the computation for the confining case cannot be carried out analytically we solve the problem numerically. In the AdS case our results agree with similar holographic calculations. In the case of a confining background we find an asymptotic area law, in agreement with the result of the lattice strong coupling expansion. We also elaborate on the subtle issue of the interplay between connected and disconnected string worldsheets.

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arXiv2012

The decay constant of the holographic techni-dilaton and the 125 GeV boson

We critically discuss the possibility that the 125 GeV boson recently discovered at the LHC is the holographic techni-dilaton, a composite state emerging from a strongly-coupled model of electroweak symmetry breaking. This composite state differs from the SM for three main reasons. Its decay constant is in general larger than the electroweak scale, hence suppressing all the couplings to standard model particles with respect to an elementary Higgs boson, with the exception of the coupling to photons and gluons, which is expected to be larger than the standard-model equivalent. We discuss three classes of questions. Is it possible to lower the decay constant, by changing the geometry of the holographic model? Is it possible to lower the overall scale of the strong dynamics, by modifying the way in which electroweak symmetry breaking is implemented in the holographic model? Is there a clear indication in the data that production mechanisms other than gluon-gluon fusion have been observed, disfavoring models in which the holographic techni-dilaton has a large decay constant? We show that all of these questions are still open, given the present status of theoretical as well as phenomenological studies, and that at present the techni-dilaton hypothesis yields a fit to the data which is either as good as the elementary Higgs hypothesis, or marginally better, depending on what sets of data are used in the fit. We identify clear strategies for future work aimed at addressing these three classes of open questions. In the process, we also compute the complete scalar spectrum of the two-scalar truncation describing the GPPZ model, as well as the decay constant of the holographic techni-dilaton in this model.

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arXiv2012

Holographic Technidilaton and LHC searches

We analyze in detail the phenomenology of a model of dynamical electroweak symmetry breaking inspired by walking technicolor, by using the techniques of the bottom-up approach to holography. The model admits a light composite scalar state, the dilaton, in the spectrum. We focus on regions of parameter space for which the mass of such dilaton is 125 GeV, and for which the bounds on the precision electroweak parameter S are satisfied. This requires that the next-to-lightest composite state is the techni-rho meson, with a mass larger than 2.3 TeV. We compute the couplings controlling the decay rates of the dilaton to two photons and to two (real or virtual) Z and W bosons. For generic choices of the parameters, we find a suppression of the decay into heavy gauge bosons, in respect to the analog decay of the standard-model Higgs. We find a dramatic effect on the decay into photons, which can be both strongly suppressed or strongly enhanced, the latter case corresponding to the large-N regime of the dual theory. There is a correlation between this decay rate of the dilaton into photons and the mass splitting between the techni-rho meson and its axial-vector partner: if the decay is enhanced in respect to the standard-model case, then the heavy spin-1 resonances are nearly degenerate in mass, otherwise their separation in mass is comparable to the mass scale itself.

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