arXiv subjects

Maurizio Piai

Publications and source records attributed to Maurizio Piai.

At least 109 records · Page 6Linked to original sources

arXiv2012

A composite light scalar, electro-weak symmetry breaking and the recent LHC searches

We construct a model in which electro-weak symmetry breaking is induced by a strongly coupled sector, which is described in terms of a five-dimensional model in the spirit of the bottom-up approach to holography. We compute the precision electro-weak parameters, and identify regions of parameter space allowed by indirect tests. We compute the spectrum of scalar and vector resonances, which contains a set of parametrically light states that can be identified with the electroweak gauge bosons and a light dilaton. There is then a little desert, up to 2-3 TeV, where towers of resonances of the vector, axial-vector and scalar particles appear.

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arXiv2012

Towards the string dual of tumbling and cascading gauge theories

We build type IIB backgrounds that can be interpreted as the dual description of field theories in which the dynamics shows many non-trivial phenomena, generalizing the baryonic branch of the Klebanov-Strassler system. We illustrate the steps of the explicit construction with a particularly interesting example. The dual field theory exhibits the expected behavior of an N=1 supersymmetric gauge theory which, over different ranges of the radial direction, is undergoing a cascade of Seiberg dualities, a period of running, a cascade of Higgsings (tumbling) and finally gaugino condensation.

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arXiv2012

A tale of two cascades: Higgsing and Seiberg-duality cascades from type IIB string theory

We construct explicitly new solutions of type IIB supergravity with brane sources, the duals of which are N = 1 supersymmetric field theories exhibiting two very interesting phenomena. The far UV dynamics is controlled by a cascade of Seiberg dualities analogous to the Klebanov- Strassler backgrounds. At intermediate scales a cascade of Higgsing appears, in the sense that the gauge group undergoes a sequence of spontaneous symmetry breaking steps which reduces its rank. Deep in the IR, the theory confines, and the gravity background has a non-singular end of space. We explain in detail how to generate such solutions, discuss some of the Physics associated with them and briefly comment on the possible applications.

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arXiv2011

Towards multi-scale dynamics on the baryonic branch of Klebanov-Strassler

We construct explicitly a new class of backgrounds in type-IIB supergravity which generalize the baryonic branch of Klebanov-Strassler. We apply a solution-generating technique that, starting from a large class of solutions of the wrapped-D5 system, yields the new solutions, and then proceed to study in detail their properties, both in the IR and in the UV. We propose a simple intuitive field theory interpretation of the rotation procedure and of the meaning of our new solutions within the Papadopoulos-Tseytlin ansatz, in particular in relation to the duality cascade in the Klebanov-Strassler solution. The presence in the field theory of different VEVs for operators of dimensions 2, 3 and 6 suggests that this is an important step towards the construction of the string dual of a genuinely multi-scale (strongly coupled) dynamical model.

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arXiv2011

Light scalars from a compact fifth dimension

We consider a general five-dimensional sigma-model coupled to gravity, with any number of scalars and general sigma-model metric and potential. We discuss in detail the problem of the boundary conditions for the scalar fluctuations, in the case where the fifth dimension is compact, and provide a simple (and very general) algorithmic procedure for computing the spectrum of physical scalar fluctuations of the fully back-reacted system. Focusing in particular on the conditions under which the spectrum of scalar excitations (glueballs) contains parametrically light states, we apply the formalism to some especially simple toy models, which can be thought of as the gauge/gravity duals of strongly-coupled, non-conformal four-dimensional gauge theories. Our examples are chosen both within the context of phenomenological effective field theory constructions (bottom-up approach), and within the context of consistent truncations of ten-dimensional string theories in the supergravity limit (top-down approach). In one of the examples, a light dilaton is present in the spectrum in spite of the presence of a bad naked singularity in the deep IR, near which the RG flow of the dual theory is certainly very far away from any fixed point. If this feature were to persist in a complete model in which the singularity is resolved, this would prove that a light dilaton is to be expected in at least certain walking technicolor theories. We provide here all the technical details for testing this statement, once such a complete model is identified.

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arXiv2010

Towards holographic walking from N=4 super Yang-Mills

We propose that a holographic description of `walking' behaviour, namely quasi-conformal dynamics relevant for technicolor models, can be obtained from relevant deformations of N=4 super Yang-Mills. We consider deformations which drive the theory close to the N=1 Leigh-Strassler fixed point, eventually deviating from it in the deep IR. We use the Pilch-Warner dual supergravity description of the flow between the N=4 and the N=1 fixed points to focus on observables that only require knowledge of the walking region. These include large anomalous dimensions of quark bilinear operators, which we study via probe D7-branes. We also make a first attempt at describing the theory beyond the walking region by introducing an infrared cut-off, in the spirit of hard-wall models. In this case we find a light, dilaton-like scalar state, but whether this mode persists in the exact theory remains an open question.

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arXiv2010

Mass-degenerate Heavy Vector Mesons at Hadron Colliders

We study the LHC phenomenology of a couple of mass-degenerate heavy new gauge bosons with the quantum numbers of the Z and photon. We give a leading-order estimate of the number of events expected in Drell-Yan processes in terms of the parameters of the model (mass and coupling) and of the LHC machine specifications (luminosity and energy). We consider the feasibility of measuring a forward-backward asymmetry for various choices of the parameters and estimate the potential reach. We comment on how the results may affect future collider design and the results for a specific model of electro-weak symmetry breaking by way of example.

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arXiv2010

Wilson Loops in string duals of Walking and Flavored Systems

We consider the VEV of Wilson loop operators by studying the behavior of string probes in solutions of Type IIB string theory generated by Nc D5 branes wrapped on an internal manifold. In particular, we focus on solutions to the background equations that are dual to field theories with a walking gauge coupling as well as for flavored systems. We present in detail our walking solution and emphasize various general aspects of the procedure to study Wilson loops using string duals. We discuss the special features that the strings show when probing the region associated with the walking of the field theory coupling.

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arXiv2010

Lectures on walking technicolor, holography and gauge/gravity dualities

Dynamical electro-weak symmetry breaking is an appealing, strongly-coupled alternative to the weakly-coupled models based on an elementary scalar field developing a vacuum expectation value. In the first two sections of this set of lectures, I summarize the arguments, based on low-energy phenomenology, supporting walking technicolor as a realistic realization of this idea. This pedagogical introduction to walking technicolor, and more generally to the physics of extensions of the standard model, makes extensive use of effective field theory arguments, symmetries and counting rules. The strongly-coupled nature of the underlying interactions, and the peculiar quasi-conformal behavior of the theory, require to use non-perturbative methods in order to address many fundamental questions within this framework. The recent development of gauge/gravity dualities provides an ideal set of such non-perturbative instruments. The remaining two sections illustrate the potential of these techniques with two technical examples, one within the bottom-up phenomenological approach to holography in five-dimensions, the other within a more systematic top-down construction derived from ten-dimensional type-IIB supergravity.

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arXiv2010

A light scalar from walking solutions in gauge-string duality

We consider the type-IIB background generated by the strong-coupling limit of Nc D5 branes wrapped on S2, and focus our attention on a special class of solutions that exhibit walking behavior. We compute numerically the spectrum of scalar fluctuations around vacua of this class. Besides two cuts, and sequences of single poles converging on one of the branch points, the spectrum contains one isolated scalar, the mass of which is suppressed by the length of the walking region. Approximate scale-invariance symmetry in the walking region suggests that this be interpreted as a light dilaton, the pseudo-Goldstone boson of dilatations.

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arXiv2009

Protecting unparticles from the MSSM Higgs sector

We construct a model of an unparticle sector consisting of a supersymmetric SU(N) gauge theory with the number of flavors in the Seiberg conformal window. We couple this sector to the MSSM via heavy messengers. The resulting low energy theory has a Higgs coupling to unparticles. The Higgs vev drives the hidden Seiberg sector to a new conformal fixed point. The coupling to the Higgs mediates supersymmetry breaking to the Seiberg sector, and breaks conformal invariance at a lower scale. The low energy theory contains light stable and metastable mesons. Higgs decay into this sector gives signatures which are similar to those of "hidden valley" models. Decays of the lightest superpartner of standard model particles into the hidden sector reveal potentially observable unparticle kinematics.

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arXiv2009

Walking Dynamics from String Duals

Within the context of a String Theory dual to N=1 gauge theories with gauge group SU(Nc) and large Nc, we identify a class of solutions of the background equations for which a suitably defined dual of the gauge coupling exhibits the features of a walking theory. We find evidence for three distinct, dynamically generated scales, characterizing walking, symmetry breaking and confinement, and we put them in correspondence with field theory by an analysis of the operators driving the flow.

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arXiv2008

Dynamical electro-weak symmetry breaking from deformed AdS: vector mesons and effective couplings

We study a modification of the five-dimensional description of dynamical electro-weak symmetry breaking inspired by the AdS/CFT correspondence. Conformal symmetry is broken in the low-energy region near the IR brane by a power-law departure from the pure AdS background. Such a modification--while not spoiling the identification of the IR brane with the scale of confinement-- has a dramatic effect on both the coupling of the first composite states to the standard model currents and their self-couplings. Chiral symmetry breaking can take place at a scale larger than the IR cut-off. This study shows that observables, such as the precision parameter $\hat{S}$, which depend on the couplings of the lightest composite states to the currents are very sensitive to the details of the dynamics in the low energy region where conformal symmetry is lost and electro-weak symmetry is broken just above the scale of confinement. Therefore results of calculations of these observables in AdS/CFT inspired scenarios should be interpreted conservatively. The most important phenomenological consequence for physics at the LHC is that the bound on the mass scale of the heavy excitations (technirho mesons) in a realistic model is in general lower than in the pure AdS background with a simple hard-wall cut off in the IR.

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arXiv2007

Vector mesons from AdS/TC to the LHC

With the use of the AdS/CFT dictionary, a five-dimensional effective description of dynamical electro-weak symmetry breaking with walking behavior is constructed. The minimal model contains only two new parameters, the confinement scale and the effective coupling of the new strong sector. This parameter space is restricted by the precision electro-weak constraints and by the requirement that the five-dimensional coupling be perturbative (corresponding to the large-N regime in four-dimensional language). The lightest observable new states are a set of four nearly degenerate spin-1 states with the same quantum numbers as the standard-model electro-weak gauge bosons, and masses in the few TeV range. Their decay rate is dominated by two-fermion final states. The number of pp -> mu mu and pp -> mu nu events is studied as a function of the LHC integrated luminosity and of the two free parameters. Discovery at the LHC is possible over a significant part of the allowed parameter space up to masses of 4 TeV already with 10/fm of integrated luminosity.

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arXiv2007

SU(3) Family Gauge Symmetry and the Axion

We analyze the structure of a recently proposed effective field theory (EFT) for the generation of quark and lepton mass ratios and mixing angles, based on the spontaneous breaking of an SU(3) family gauge symmetry at a high scale F. We classify the Yukawa operators necessary to seed the masses, making use of the continuous global symmetries that they preserve. One global U(1), in addition to baryon number and electroweak hypercharge, remains unbroken after the inclusion of all operators required by standard-model-fermion phenomenology. An associated vacuum symmetry insures the vanishing of the first-family quark and charged-lepton masses in the absence of the family gauge interaction. If this U(1) symmetry is taken to be exact in the EFT, broken explicitly by only the QCD-induced anomaly, and if the breaking scale F is taken to lie in the range 10 to 9 - 10 to 12 GeV, then the associated Nambu-Goldstone boson is a potential QCD axion.

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arXiv2006

Neutrinos and SU(3) Family Gauge Symmetry

We include the standard-model (SM) leptons in a recently proposed framework for the generation of quark mass ratios and Cabibbo-Kobayashi-Maskawa (CKM) mixing angles from an SU(3) family gauge interaction. The set of SM-singlet scalar fields describing the spontaneous breaking is the same as employed for the quark sector. The imposition at tree-level of the experimentally correct Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix, in the form of a tri-bi maximal structure, fixes several of the otherwise free parameters and renders the model predictive. The normal hierarchy among the neutrino masses emerges from this scheme.

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arXiv2006

Walking in the third millennium

Based on recent progress in the study of strong dynamics in conformal field theories, I construct a simple, one family model of dynamical electro-weak symmetry breaking. Non-perturbative effects can be computed systematically. The model does not suffer from the well known, parametrically big, phenomenological difficulties of traditional technicolor models, is compatible with all present experimental data and is testable at the LHC.

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arXiv2006

Precision electro-weak parameters from AdS5, localized kinetic terms and anomalous dimensions

I compare the tree level estimate of the electro-weak precision parameters in two (exactly solvable) toy models of dynamical symmetry breaking in which the strong dynamics is assumed to be described by a five-dimensional (weakly coupled) gravity dual. I discuss the effect of brane-localized kinetic terms, their use as regulators for the couplings of otherwise non-normalizable modes, and the impact of a large deviation from its natural value for the scaling dimension of the background field responsible for spontaneous symmetry breaking. The latter is assumed to model the effects of walking dynamics, i.e. of a large anomalous dimension of the chiral condensate, it has a strong impact of the spectrum of spin-1 fields and, as a consequence, on the electro-weak precision parameters. The main conclusion is that models of dynamical symmetry breaking based on a large-Nc strongly interacting SU(Nc) gauge theory are compatible with precision electro-weak constraints, and produce a very distinctive signature testable at the LHC. Some of the considerations discussed are directly relevant for analogous models in the context of AdS-QCD.

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