arXiv subjects

Francesco Riva

Publications and source records attributed to Francesco Riva.

At least 37 records · Page 2Linked to original sources

arXiv2018

ZZ and Zgamma still haven't found what they are looking for

Neutral diboson processes are precise probes of the Standard Model (SM) of particle physics, which entail high sensitivity to new physics effects. We identify in terms of dimension-8 effective operators the leading departures from the SM that survive in neutral diboson processes at high-energy, and which interfere with the unsuppressed SM helicity contributions. We describe symmetries and selections rules that single out those operators, both for weakly and strongly coupled physics beyond the SM. Finally, we show that unitarity and causality enforce, via dispersion relations, positivity constraints on the coefficients of these effective operators, reducing the parameter space which is theoretically allowed.

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arXiv2018

Electroweak Precision Tests in High-Energy Diboson Processes

A promising avenue to perform precision tests of the SM at the LHC is to measure differential cross-sections at high invariant mass, exploiting in this way the growth with the energy of the corrections induced by heavy new physics. We classify the leading growing-with-energy effects in longitudinal diboson and in associated Higgs production processes, showing that they can be encapsulated in four real "high-energy primary" parameters. We assess the reach on these parameters at the LHC and at future hadronic colliders, focusing in particular on the fully leptonic $WZ$ channel that appears particularly promising. The reach is found to be superior to existing constraints by one order of magnitude, providing a test of the SM electroweak sector at the per-mille level, in competition with LEP bounds. Unlike LHC Run-1 bounds, which only apply to new physics effects that are much larger than the SM in the high-energy tail of the distributions, the probe we study applies to a wider class of new physics scenarios where such large departures are not expected.

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arXiv2017

The Other Fermion Compositeness

We discuss the only two viable realizations of fermion compositeness described by a calculable relativistic effective field theory consistent with unitarity, crossing symmetry and analyticity: chiral-compositeness vs goldstino-compositeness. We construct the effective theory of $\mathcal{N}$ Goldstini and show how the Standard Model can emerge from this dynamics. We present new bounds on either type of compositeness, for quarks and leptons, using dilepton searches at LEP, dijets at the LHC, as well as low-energy observables and precision measurements. Remarkably, a scale of compositeness for Goldstino-like electrons in the 2 TeV range is compatible with present data, and so are Goldstino-like first generation quarks with a compositeness scale in the 10 TeV range. Moreover, assuming maximal $R$-symmetry, goldstino-compositeness of both right- and left-handed quarks predicts exotic spin-1/2 colored sextet particles that are potentially within the reach of the LHC.

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arXiv2017

Beyond Amplitudes' Positivity and the Fate of Massive Gravity

We constrain effective field theories by going beyond the familiar positivity bounds that follow from unitarity, analyticity, and crossing symmetry of the scattering amplitudes. As interesting examples, we discuss the implications of the bounds for the Galileon and ghost-free massive gravity. The combination of our theoretical bounds with the experimental constraints on the graviton mass implies that the latter is either ruled out or unable to describe gravitational phenomena, let alone to consistently implement the Vainshtein mechanism, down to the relevant scales of fifth-force experiments, where general relativity has been successfully tested. We also show that the Galileon theory must contain symmetry-breaking terms that are at most one-loop suppressed compared to the symmetry-preserving ones. We comment as well on other interesting applications of our bounds.

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arXiv2017

Strongly Interacting Light Dark Matter

In the presence of approximate global symmetries that forbid relevant interactions, strongly coupled light Dark Matter (DM) can appear weakly coupled at small energy and generate a sizable relic abundance. Fundamental principles like unitarity restrict these symmetries to a small class, where the leading interactions are captured by effective operators up to dimension-8. Chiral symmetry, spontaneously broken global symmetries and non-linearly realized supersymmetry are examples of this. Their DM candidates (composite fermions, pseudo Nambu-Goldstone Bosons and Goldstini) are interesting targets for LHC missing-energy searches.

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arXiv2016

On the Validity of the Effective Field Theory Approach to SM Precision Tests

We discuss the conditions for an effective field theory (EFT) to give an adequate low-energy description of an underlying physics beyond the Standard Model (SM). Starting from the EFT where the SM is extended by dimension-6 operators, experimental data can be used without further assumptions to measure (or set limits on) the EFT parameters. The interpretation of these results requires instead a set of broad assumptions (e.g. power counting rules) on the UV dynamics. This allows one to establish, in a bottom-up approach, the validity range of the EFT description, and to assess the error associated with the truncation of the EFT series. We give a practical prescription on how experimental results could be reported, so that they admit a maximally broad range of theoretical interpretations. Namely, the experimental constraints on dimension-6 operators should be reported as functions of the kinematic variables that set the relevant energy scale of the studied process. This is especially important for hadron collider experiments where collisions probe a wide range of energy scales.

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arXiv2016

Helicity Selection Rules and Non-Interference for BSM Amplitudes

Precision studies of scattering processes at colliders provide powerful indirect constraints on new physics. We study the helicity structure of scattering amplitudes in the SM and in the context of an effective Lagrangian description of BSM dynamics. Our analysis reveals a novel set of helicity selection rules according to which, in the majority of 2 to 2 scattering processes at high energy, the SM and the leading BSM effects do not interfere. In such situations, the naive expectation that dimension-6 operators represent the leading BSM contribution is compromised, as corrections from dimension-8 operators can become equally (if not more) important well within the validity of the effective field theory approach.

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arXiv2016

The Last Gasp of Dark Matter Effective Theory

We discuss an interesting class of models, based on strongly coupled Dark Matter (DM), where sizable effects can be expected in LHC missing energy (MET) searches, compatibly with a large separation of scales. In this case, an effective field theory (EFT) is appropriate (and sometimes necessary) to describe the most relevant interactions at the LHC. The selection rules implied by the structure of the new strong dynamics shape the EFT in an unusual way, revealing the importance of higher-derivative interactions previously ignored. We compare indications from relic density and direct detection experiments with consistent LHC constraints, and asses the relative importance of the latter. Our analysis provides an interesting and well-motivated scenario to model MET at the LHC in terms of a handful of parameters.

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arXiv2016

Digamma, what next?

If the 750 GeV resonance in the diphoton channel is confirmed, what are the measurements necessary to infer the properties of the new particle and understand its nature? We address this question in the framework of a single new scalar particle, called digamma ($\digamma$). We describe it by an effective field theory, which allows us to obtain general and model-independent results, and to identify the most useful observables, whose relevance will remain also in model-by-model analyses. We derive full expressions for the leading-order processes and compute rates for higher-order decays, digamma production in association with jets, gauge or Higgs bosons, and digamma pair production. We illustrate how measurements of these higher-order processes can be used to extract couplings, quantum numbers, and properties of the new particle.

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arXiv2016

Patterns of Strong Coupling for LHC Searches

Even though the Standard Model (SM) is weakly coupled at the Fermi scale, a new strong dynamics involving its degrees of freedom may conceivably lurk at slightly higher energies, in the multi TeV range. Approximate symmetries provide a structurally robust context where, within the low energy description, the dimensionless SM couplings are weak, while the new strong dynamics manifests itself exclusively through higher-derivative interactions. We present an exhaustive classification of such scenarios in the form of effective field theories, paying special attention to new classes of models where the strong dynamics involves, along with the Higgs boson, the SM gauge bosons and/or the fermions. The IR softness of the new dynamics suppresses its effects at LEP energies, but deviations are in principle detectable at the LHC, even at energies below the threshold for production of new states. Our construction provides the so far unique structurally robust context where to motivate several searches in Higgs physics, diboson production, or WW scattering, which were so far poorly justified. Perhaps surprisingly, the interplay between weak coupling, strong coupling and derivatives, which is controlled by symmetries, can override the naive expansion in operator dimension, providing instances where dimension-8 dominates dimension-6, well within the domain of validity of the low energy effective theory. This result reveals the limitations of an analysis that is both ambitiously general and restricted to dimension-6 operators.

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arXiv2016

What is the gamma gamma resonance at 750 GeV?

Run 2 LHC data show hints of a new resonance in the diphoton distribution at an invariant mass of 750 GeV. We analyse the data in terms of a new boson, extracting information on its properties and exploring theoretical interpretations. Scenarios covered include a narrow resonance and, as preliminary indications suggest, a wider resonance. If the width indications persist, the new particle is likely to belong to a strongly-interacting sector. We also show how compatibility between Run 1 and Run 2 data is improved by postulating the existence of an additional heavy particle, whose decays are possibly related to dark matter.

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arXiv2015

Rosetta: an operator basis translator for Standard Model effective field theory

We introduce Rosetta, a program allowing for the translation between different bases of effective field theory operators. We present the main functions of the program and provide an example of usage. One of the Lagrangians which Rosetta can translate into has been implemented into FeynRules, which allows Rosetta to be interfaced into various high-energy physics programs such as Monte Carlo event generators. In addition to popular bases choices, such as the Warsaw and Strongly Interacting Light Higgs bases already implemented in the program, we also detail how to add new operator bases into the Rosetta package. In this way, phenomenological studies using an effective field theory framework can be straightforwardly performed.

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arXiv2015

Model-independent precision constraints on dimension-6 operators

We discuss electroweak precision constraints on dimension-6 operators in the effective theory beyond the standard model. We identify the combinations of these operators that are constrained by the pole observables (the W and Z masses and on-shell decays) and by the W boson pair production. To this end, we define a set of effective couplings of W and Z bosons to fermions and to itself, which capture the effects of new physics corrections. This formalism clarifies which operators are constrained by which observable, independently of the adopted basis of operators. We obtain numerical constraints on the coefficients of dimension-6 operator in a form that can be easily adapted to any particular basis of operators, or any particular model with new heavy particles.

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arXiv2014

(Re-)Inventing the Relativistic Wheel: Gravity, Cosets, and Spinning Objects

Space-time symmetries are a crucial ingredient of any theoretical model in physics. Unlike internal symmetries, which may or may not be gauged and/or spontaneously broken, space-time symmetries do not admit any ambiguity: they are gauged by gravity, and any conceivable physical system (other than the vacuum) is bound to break at least some of them. Motivated by this observation, we study how to couple gravity with the Goldstone fields that non-linearly realize spontaneously broken space-time symmetries. This can be done in complete generality by weakly gauging the Poincare symmetry group in the context of the coset construction. To illustrate the power of this method, we consider three kinds of physical systems coupled to gravity: superfluids, relativistic membranes embedded in a higher dimensional space, and rotating point-like objects. This last system is of particular importance as it can be used to model spinning astrophysical objects like neutron stars and black holes. Our approach provides a systematic and unambiguous parametrization of the degrees of freedom of these systems.

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arXiv2014

Composite Charge 8/3 Resonances at the LHC

In composite Higgs models with partial compositeness, the small value of the observed Higgs mass implies the existence of light fermionic resonances, the top partners, whose quantum numbers are determined by the symmetry (and symmetry breaking) structure of the theory. Here we study light top partners with electric charge 8/3, which are predicted, for instance, in some of the most natural composite Higgs realizations. We recast data from two same sign lepton searches and from searches for microscopic blackholes into a bound on its mass, M8/3 > 940 GeV. Furthermore, we compare potential reach of these searches with a specifically designed search for three same-sign lepton, both at 8 and 14 TeV. We provide a simplified model, suitable for collider analysis.

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arXiv2014

Vices and Virtues of Higgs EFTs at Large Energy

We study constraints on new physics from Higgs production at the LHC in the context of an effective field theory (EFT), focusing on Higgs searches in $HV$ ($V=W,Z$) associated production which are particularly sensitive to the high-energy behavior of certain dimension-6 operators. We show that analyses of these searches are generally dominated by a kinematic region where the generic EFT expansion breaks down, and establish under which conditions they can nevertheless be meaningful. For example, constraints from these searches on the Wilson coefficients of operators whose effects grow with energy can be established in scenarios where a particular combination of fermions and the Higgs are composite and strongly coupled: then, bounds from Higgs physics at high energy are complementary to LEP1 and competitive with LEP2.

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arXiv2014

BSM Primary Effects

Using the predictive power of the effective field theory approach, we present a physical parametrization of the leading effects beyond the SM (BSM), that give us at present the best way to constrain heavy new-physics at low-energies. We show that other BSM effects are not independent from these ones, and we provide the explicit correlations. This information is useful to know where to primarily look for new physics in future experiments, and to know how this new physics is related to previous measurements, most importantly in electroweak-symmetry breaking processes or Higgs physics.

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arXiv2014

Towards the Ultimate SM Fit to Close in on Higgs Physics

With the discovery of the Higgs at the LHC, experiments have finally addressed all aspects of the Standard Model (SM). At this stage, it is important to understand which windows for beyond the SM (BSM) physics are still open, and which are instead tightly closed. We address this question by parametrizing BSM effects with dimension-six operators and performing a global fit to the SM. We separate operators into different groups constrained at different levels, and provide independent bounds on their Wilson coefficients taking into account only the relevant experiments. Our analysis allows to assert in a model-independent way where BSM effects can appear in Higgs physics. In particular, we show that deviations from the SM in the differential distributions of h->Vff are related to other observables, such as triple gauge-boson couplings, and are then already constrained by present data. On the contrary, BR(h-> Z+gamma) can still hide large deviations from the SM.

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