Search

We study how light scalar fields can change the stellar landscape by triggering a new phase of nuclear matter. Scalars coupled to nucleons can develop a non-trivial expectation value at finite baryon density. This sourcing of a scalar reduces the nucleon mass and provides an additional energy density and pressure source. Under generic conditions, a new ground state of nuclear matter emerges, with striking implications for the configuration of stellar remnants. Notably, neutron stars in the new ground state can be significantly heavier than QCD equations of state currently predict. We also find hybrid stellar compositions and stable self-bound objects with sizes as small as the Compton wavelength of the scalar. We discuss several specific realizations of this scenario: the QCD axion and lighter generalizations thereof and linearly or quadratically coupled scalar fields effectively equivalent to a class of scalar-tensor modification of gravity. Lastly, we explore phenomenological signatures relevant to electromagnetic and gravitational wave observations of neutron stars, such as atypical compactness and instability gaps in radii., Comment: 56 pages, 11 figures

We study the effects of exceptionally light QCD axions on the stellar configuration of white dwarfs. At finite baryon density, the non-derivative coupling of the axion to nucleons displaces the axion from its in-vacuum minimum which implies a reduction of the nucleon mass. This dramatically alters the composition of stellar remnants. In particular, the modifications of the mass-radius relationship of white dwarfs allow us to probe large regions of unexplored parameter space without requiring that axions are dark matter., Comment: 13 pages, 12 figures, added appendices, published version

We derive causality constraints on the simplest scalar-tensor theories in which black holes differ from what General Relativity predicts, a scalar coupled to the Gauss-Bonnet or the Chern-Simons terms. Demanding that time advances are unobservable within the regime of validity of these effective field theories, we find their cutoff must be parametrically of the same size as the inverse Schwarzschild radius of the black holes for which the non-standard effects are of order one. For astrophysical black holes within the range of current gravitational wave detectors, this means a cutoff length at the km. We further explore the leading additional higher-dimensional operators potentially associated with the scale of UV completion and discuss their phenomenological implications for gravitational wave science., Comment: 28 pages, 1 figure; added references

We study the renormalization group of generic effective field theories that include gravity. We follow the on-shell amplitude approach, which provides a simple and efficient method to extract anomalous dimensions avoiding complications from gauge redundancies. As an invaluable tool we introduce a modified helicity $\tilde{h}$ under which gravitons carry one unit instead of two. With this modified helicity we easily explain old and uncover new non-renormalization theorems for theories including gravitons. We provide complete results for the one-loop gravitational renormalization of a generic minimally coupled gauge theory with scalars and fermions and all orders in $M_{Pl}$, as well as for the renormalization of dimension-six operators including at least one graviton, all up to four external particles., Comment: 52 pages, 9 figures, 5 appendices

Finite density effects can destabilize the metastable vacua in relaxion models. Focusing on stars as nucleation seeds, we derive the conditions that lead to the formation and runaway of a relaxion bubble of a lower energy minimum than in vacuum. The resulting late-time phase transition in the universe allows us to set new constraints on the parameter space of relaxion models. We also find that similar instabilities can be triggered by the large electromagnetic fields around rotating neutron stars., Comment: 30 pages, 5 figures and several appendices

We consider matter density effects in theories with a false ground state. Large and dense systems, such as stars, can destabilize a metastable minimum and allow for the formation of bubbles of the true minimum. We derive the conditions under which these bubbles form, as well as the conditions under which they either remain confined to the dense region or escape to infinity. The latter case leads to a phase transition in the universe at star formation. We explore the phenomenological consequences of such seeded phase transitions., Comment: 33 pages, 6 figures and several appendices

We study the phenomenology of a strongly-interacting top quark at future hadron and lepton colliders, showing that the characteristic four-top contact operators give rise to the most significant effects. We demonstrate the extraordinary potential of a 100 TeV proton-proton collider to directly test such non-standard interactions in four-top production, a process that we thoroughly analyze in the same-sign dilepton and trilepton channels, and explore in the fully hadronic channel. Furthermore, high-energy electron-positron colliders, such as CLIC or the ILC, are shown to exhibit an indirect yet remarkable sensitivity to four-top operators, since these constitute, via renormalization group evolution, the leading new-physics deformations in top-quark pair production. We investigate the impact of our results on the parameter space of composite Higgs models with a strongly-coupled (right-handed) top quark, finding that four-top probes provide the best sensitivity on the compositeness scale at the future energy frontier. In addition, we investigate mild yet persisting LHC excesses in multilepton plus jets final states, showing that they can be consistently described in the effective field theory of such a new-physics scenario., Comment: 27 pages + references, 11 figures, 5 tables. v2: slight modification of title, minor additions to discussion (including HL-LHC projection), references added, results unchanged; version published in JHEP

We show how the properties of the QCD axion change in systems at finite baryonic density, such as neutron stars. At nuclear saturation densities, where corrections can be reliably computed, we find a mild reduction of the axion mass and up to an order of magnitude enhancement in the model-independent axion coupling to neutrons. At moderately higher densities, if realized, meson (kaon) condensation can trigger axion condensation. We also study the axion potential at asymptotically large densities, where the color-superconducting phase of QCD potentially leads to axion condensation, and the mass of the axion is generically several orders of magnitude smaller than in vacuum due to the suppressed instantons. Several phenomenological consequences of the axion being sourced by neutron stars are discussed, such as its contribution to their total mass, the presence of an axionic brane, or axion-photon conversion in the magnetosphere., Comment: 39 pages + appendices and updated references, 6 figures

We construct the general effective field theory of gravity coupled to the Standard Model of particle physics, which we name GRSMEFT. Our method allows the systematic derivation of a non-redundant set of operators of arbitrary dimension with generic field content and gravity. We explicitly determine the pure gravity EFT up to dimension ten, the EFT of a shift-symmetric scalar coupled to gravity up to dimension eight, and the operator basis for the GRSMEFT up to dimension eight. Extensions to all orders are straightforward., Comment: 30 pages + appendices and references. v2: typos fixed, references added, new appendix on gravity in d > 4 spacetime dimensions; matches version to be published in JHEP

We present an exceptional twin-Higgs model with the minimal symmetry structure for an exact implementation of twin parity along with custodial symmetry. Twin particles are mirrors of the Standard Model yet they carry hypercharge, while the photon is identified with its twin. We thoroughly explore the phenomenological signatures of hypercharged naturalness: long-lived charged particles, a colorless twin top with electric charge $2/3$ that once pair-produced, bounds via twin-color interactions and can annihilate to dileptons or a Higgs plus a photon or a $Z$, and glueballs produced from Higgs decays and twin-quarkonium annihilation that either decay displaced, or are stable on collider scales and eventually decay to diphotons. Prospects for detection of these signatures are also discussed., Comment: 37 pages, 7 figures

We construct the effective field theory for a single massive higher-spin particle in flat spacetime. Positivity bounds of the S-matrix force the cutoff of the theory to be well below the naive strong-coupling scale, forbid any potential and make therefore higher-derivative operators important even at low energy. As interesting application, we discuss in detail the massive spin-3 theory and show that an extended Galileon-like symmetry of the longitudinal modes, even with spin, emerges at high energy., Comment: 22 pages, 2 figures and 2 appendices; v2 improved discussion, extra appendix and references added

We derive new positivity bounds for scattering amplitudes in theories with a massless graviton in the spectrum in four spacetime dimensions, of relevance for the weak gravity conjecture and modified gravity theories. The bounds imply that extremal black holes are self-repulsive, $M/|Q|<1$ in suitable units, and that they are unstable to decay to smaller extremal black holes, providing an S-matrix proof of the weak gravity conjecture. We also present other applications of our bounds to the effective field theory of axions, $P(X)$ theories, weakly broken galileons, and curved spacetimes., Comment: v4: two more appendices, discussion of Froissart bound extended

Partial compositeness as a theory of flavor in the lepton sector is assessed. We begin presenting the first systematic analysis of neutrino mass generation in this context, and identifying the distinctive mass textures. We then update the bounds from charged lepton flavor and CP violating observables. We put forward a $U(1)^3\times CP$ symmetry of the composite sector, in order to allow the new physics to be not far above the TeV scale. This hypothesis effectively suppresses the new contributions to the electron EDM and $\mu\to e\gamma$, by far the most constraining observables, and results in a novel pattern of flavor violation and neutrino masses. The CP violation in the elementary-composite mixing is shown to induce a CKM phase of the correct size, as well as order-one phases in the PMNS matrix. We compare with the alternative possibility of introducing multiple scales of compositeness for leptons, that also allow to evade flavor and CP constraints. Finally, we examine violations of lepton flavor universality in $B$-meson semi-leptonic decays. The neutral-current anomalies can be accommodated, predicting strong correlations among different lepton flavors, with a few channels close to the experimental sensitivity., Comment: Reference from ACME collaboration added and bounds from electron EDM updated. Typos and minor errors corrected

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., Comment: 10 pages + 3 appendices and references, 4 figures; v2: bounds optimized, improved discussion, extended outlook, conclusion unchanged. Extra appendix and references added

We present a version of the twin Higgs mechanism with minimal symmetry structure and particle content. The model is built upon a composite Higgs theory with global $SO(6)/SO(5)$ symmetry breaking. The leading contribution to the Higgs potential, from the top sector, is solely cancelled via the introduction of a Standard Model neutral top partner. We show that the inherent $Z_2$ breaking of this construction is under control and of the right size to achieve electroweak symmetry breaking, with a fine-tuning at the level of $5-10\%$, and compatibly with the observed Higgs mass. We briefly discuss the particular phenomenological features of this scenario., Comment: 12 pages, 2 figures, 3 appendices

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., Comment: 32 pages on how Thomson might have discovered supersymmetry; v2: few typos fixed and comments added

We study the effective theory of a generic class of hidden sectors where supersymmetry is broken together with an approximate R-symmetry at low energy. The light spectrum contains the gravitino and the pseudo-Nambu-Goldstone boson of the R-symmetry, the R-axion. We derive new model-independent constraints on the R-axion decay constant for R-axion masses ranging from GeV to TeV, which are of relevance for hadron colliders, lepton colliders and B-factories. The current bounds allow for the exciting possibility that the first sign of SUSY will be the R-axion. We point out its most distinctive signals, providing a new experimental handle on the properties of the hidden sector and a solid motivation for searches of axion-like particles., Comment: 7 pages, 2 figures. v2: typos fixed, appendix added. v3: R-axion as motivation for ALP phenomenological studies made explicit, minor clarifications. Accepted for publication on PRL

We study the conditions for a new scalar resonance to be observed first in diphotons at the LHC Run-2. We focus on scenarios where the scalar arises either from an internal or spacetime symmetry broken spontaneously, for which the mass is naturally below the cutoff and the low-energy interactions are fixed by the couplings to the broken currents, UV anomalies, and selection rules. We discuss the recent excess in diphoton resonance searches observed by ATLAS and CMS at 750 GeV, and explore its compatibility with other searches at Run-1 and its interpretation as Goldstone bosons in supersymmetry and composite Higgs models. We show that two candidates naturally emerge: a Goldstone boson from an internal symmetry with electromagnetic anomalies, and the scalar partner of the Goldstone of supersymmetry breaking: the sgoldstino. The dilaton from conformal symmetry breaking is instead disfavoured by present data, in its minimal natural realization., Comment: 18 pages + refs, 2 figures. v2: typos corrected, references added, discussions extended and three new plots. Conclusion unchanged. v3: published version

The appearance of the light Higgs boson at the LHC is difficult to explain, particularly in light of naturalness arguments in quantum field theory. However light scalars can appear in condensed matter systems when parameters (like the amount of doping) are tuned to a critical point. At zero temperature these quantum critical points are directly analogous to the finely tuned standard model. In this paper we explore a class of models with a Higgs near a quantum critical point that exhibits non-mean-field behavior. We discuss the parametrization of the effects of a Higgs emerging from such a critical point in terms of form factors, and present two simple realistic scenarios based on either generalized free fields or a 5D dual in AdS space. For both of these models we consider the processes $gg\to ZZ$ and $gg\to hh$, which can be used to gain information about the Higgs scaling dimension and IR transition scale from the experimental data., Comment: 25p+ appendix, 3 figures; v2: references added, minor improvements, match published version

Deviations of the top electroweak couplings from their Standard Model values imply that certain amplitudes for the scattering of third generation fermions and longitudinally polarized vector bosons or Higgses diverge quadratically with momenta. This high-energy growth is a genuine signal of models where the top quark is strongly coupled to the sector responsible for electroweak symmetry breaking. We propose to profit from the high energies accessible at the LHC to enhance the sensitivity to non-standard top-$Z$ couplings, which are currently very weakly constrained. To demonstrate the effectiveness of the approach, we perform a detailed analysis of $tW \to tW$ scattering, which can be probed at the LHC via $pp\to t\bar{t}Wj$. By recasting a CMS analysis at 8 TeV, we derive the strongest direct bounds to date on the $Ztt$ couplings. We also design a dedicated search at 13 TeV that exploits the distinctive features of the $t\bar{t}Wj$ signal. Finally, we present other scattering processes in the same class that could provide further tests of the top-Higgs sector., Comment: 37 pages, 10 figures; v2: minor improvements in the discussion, references added. Matches version published in JHEP

Light top partners are the prime sign of naturalness in composite Higgs models. We explore here the possibility of non-standard top partner phenomenology. We show that even in the simplest extension of the minimal composite Higgs model, featuring an extra singlet pseudo Nambu-Goldstone boson, the branching ratios of the top partners into standard channels can be significantly altered, with no substantial change in the generated Higgs potential. Together with the variety of possible final states from the decay of the pseudo-scalar singlet, this motivates more extensive analyses in the search for the top partners., Comment: 30 pages, 6 figures, JHEP version

Vacuum energy changes during cosmological phase transitions and becomes relatively important at epochs just before phase transitions. For a viable cosmology the vacuum energy just after a phase transition must be set by the critical temperature of the next phase transition, which exposes the cosmological constant problem from a different angle. Here we propose to experimentally test the properties of vacuum energy under circumstances different from our current vacuum. One promising avenue is to consider the effect of high density phases of QCD in neutron stars. Such phases have different vacuum expectation values and a different vacuum energy from the normal phase, which can contribute an order one fraction to the mass of neutron stars. Precise observations of the mass of neutron stars can potentially yield information about the gravitational properties of vacuum energy, which can significantly affect their mass-radius relation. A more direct test of cosmic evolution of vacuum energy could be inferred from a precise observation of the primordial gravitational wave spectrum at frequencies corresponding to phase transitions. While traditional cosmology predicts steps in the spectrum determined by the number of degrees of freedom both for the QCD and electroweak phase transitions, an adjustment mechanism for vacuum energy could significantly change this. In addition, there might be other phase transitions where the effect of vacuum energy could show up as a peak in the spectrum., Comment: 28 pages, LaTeX, 7 figures

We construct a model of inflation based on a low-energy effective theory of spontaneously broken global scale invariance. This provides a shift symmetry that protects the inflaton potential from quantum corrections. Since the underlying scale invariance is non-compact, arbitrarily large inflaton field displacements are readily allowed in the low-energy effective theory. A weak breaking of scale invariance by almost marginal operators provides a non-trivial inflaton minimum, which sets and stabilizes the final low-energy value of the Planck scale. The underlying scale invariance ensures that the slow-roll approximation remains valid over large inflaton displacements, and yields a scale invariant spectrum of perturbations as required by the CMB observations., Comment: 18 pages, 3 figures

We present an overview of composite Higgs models in light of the discovery of the Higgs boson. The small value of the physical Higgs mass suggests that the Higgs quartic is likely loop generated, thus models with tree-level quartics will generically be more tuned. We classify the various models (including bona fide composite Higgs, little Higgs, holographic composite Higgs, twin Higgs and dilatonic Higgs) based on their predictions for the Higgs potential, review the basic ingredients of each of them, and quantify the amount of tuning needed, which is not negligible in any model. We explain the main ideas for generating flavor structure and the main mechanisms for protecting against large flavor violating effects, and present a summary of the various coset models that can result in realistic pseudo-Goldstone Higgses. We review the current experimental status of such models by discussing the electroweak precision, flavor and direct search bounds, and comment on UV completions and on ways to incorporate dark matter., Comment: 58 Pages, 5 Figures, 5 Tables. Invited review article accepted for publication in The European Physical Journal C

We re-examine the feasibility of the higgs-like particle discovered at the LHC being a dilaton: the Goldstone boson of spontaneous breaking of scale invariance. We review the expected phenomenological deviations from the SM higgs and compare with other Goldstone higgs scenarios, with particular emphasis on double higgs production., Comment: 4 pages, 2 figures, 3 tables. Proceedings for LHCP 2013: First Large Hadron Collider Physics Conference, Barcelona, May 13-18, 2013

We examine the possibility that the recently discovered 125 GeV higgs like resonance actually corresponds to a dilaton: the Goldstone boson of scale invariance spontaneously broken at a scale f. Comparing to LHC data we find that a dilaton can reproduce the observed couplings of the new resonance as long as f ~ v, the weak scale. This corresponds to the dynamical assumption that only operators charged under the electroweak gauge group obtain VEVs. The more difficult task is to keep the mass of the dilaton light compared to the dynamical scale, Lambda ~ 4 pi f, of the theory. In generic, non-supersymmetric theories one would expect the dilaton mass to be similar to Lambda. The mass of the dilaton can only be lowered at the price of some percent level (or worse) tuning and/or additional dynamical assumptions: one needs to suppress the contribution of the condensate to the vacuum energy (which would lead to a large dilaton quartic coupling), and to allow only almost marginal deformations of the CFT., Comment: 30 pages, 4 figures; v2: references added, typos fixed, discussions clarified, accepted for publication in EPJC

The coupling of a composite Higgs to the standard model fields can deviate substantially from the standard model values. In this case perturbative unitarity might break down before the scale of compositeness is reached, which would suggest that additional composites should lie well below this scale. In this paper we account for the presence of an additional spin 1 custodial triplet of rhos. We examine the implications of requiring perturbative unitarity up to the compositeness scale and find that one has to be close to saturating certain unitarity sum rules involving the Higgs and the rho couplings. Given these restrictions on the parameter space we investigate the main phenomenological consequences of the spin 1 triplet. We find that they can substantially enhance the Higgs di-photon rate at the LHC even with a reduced Higgs coupling to gauge bosons. The main existing LHC bounds arise from di-boson searches, especially in the experimentally clean channel where the charged rhos decay to a W-boson and a Z, which then decay leptonically. We find that a large range of interesting parameter space with 700 GeV < m(rho) < 2 TeV is currently experimentally viable., Comment: 37 pages, 12 figures; v4: sum rule corrected, conclusions unchanged

The LHC has started to explore the TeV energy regime, probing the SM beyond LEP and Tevatron. We show how the dijet measurements at the LHC are able to test certain sectors of the SM at an unprecedented level. We provide the best bounds on all possible four-quark interactions and translate them into limits on the compositeness scale of the quarks and gluons. We also provide constraints on extra gauge bosons, Z', W' and G', and on new interactions proposed to explain the present measurement of the forward-backward asymmetry of the top., Comment: 22 pages, 6 figures. v2: New bounds derived from the 2011 CMS dijet data set

We characterize models where electroweak symmetry breaking is driven by two light Higgs doublets arising as pseudo-Nambu-Goldstone bosons of new dynamics above the weak scale. They represent the simplest natural two Higgs doublet alternative to supersymmetry. We construct their low-energy effective Lagrangian making only few specific assumptions about the strong sector. These concern their global symmetries, their patterns of spontaneous breaking and the sources of explicit breaking. In particular we assume that all the explicit breaking is associated with the couplings of the strong sector to the Standard Model fields, that is gauge and (proto)-Yukawa interactions. Under those assumptions the scalar potential is determined at lowest order by very few free parameters associated to the top sector. Another crucial property of our scenarios is the presence of a discrete symmetry, in addition to custodial SO(4), that controls the $T$-parameter. That can either be simple CP or a $Z_2$ that distinguishes the two Higgs doublets. Among various possibilities we study in detail models based on SO(6)/SO(4)$\times$ SO(2), focussing on their predictions for the structure of the scalar spectrum and the deviations of their couplings from those of a generic renormalizable two Higgs doublet model., Comment: 54 pages

We investigate grand unified theories (GUTs) in scenarios where electroweak (EW) symmetry breaking is triggered by a light composite Higgs, arising as a Nambu-Goldstone boson from a strongly interacting sector. The evolution of the standard model (SM) gauge couplings can be predicted at leading order, if the global symmetry of the composite sector is a simple group G that contains the SM gauge group. It was noticed that, if the right-handed top quark is also composite, precision gauge unification can be achieved. We build minimal consistent models for a composite sector with these properties, thus demonstrating how composite GUTs may represent an alternative to supersymmetric GUTs. Taking into account the new contributions to the EW precision parameters, we compute the Higgs effective potential and prove that it realizes consistently EW symmetry breaking with little fine-tuning. The G group structure and the requirement of proton stability determine the nature of the light composite states accompanying the Higgs and the top quark: a coloured triplet scalar and several vector-like fermions with exotic quantum numbers. We analyse the signatures of these composite partners at hadron colliders: distinctive final states contain multiple top and bottom quarks, either alone or accompanied by a heavy stable charged particle, or by missing transverse energy., Comment: 55 pages, 13 figures, final version to be published in JHEP

The Higgs spectrum of the minimal composite Higgs model, based on the SO(5)/SO(4) coset, consists of a unique Higgs doublet whose phenomenology does not differ greatly from the Standard Model (SM). Nevertheless, extensions beyond this minimal coset structure exhibit a richer Higgs spectrum and therefore very different Higgs physics. We explore one of these extensions, the SO(6)/SO(5) model, whose Higgs spectrum contains a CP-odd singlet scalar, eta, in addition to the Higgs doublet. Due to the pseudo-Nambu-Goldstone nature of these Higgs bosons, their physical properties can be derived from symmetry considerations alone. We find that the mass of eta can be naturally light, opening up the possibility that the SM Higgs decays predominantly to the singlet, and therefore lowering the LEP bound on its mass to 86 GeV. We also show that eta can have interesting consequences in flavour-violating processes, as well as induce spontaneous CP-violation in the Higgs sector. The model can also have anomalies, giving rise to interactions between the SM gauge bosons and eta which, if measured at the LHC, would give quantitative information about the structure of the high energy theory., Comment: 13pp

In models of electroweak symmetry breaking in which the SM fermions get their masses by mixing with composite states, it is natural to expect the top quark to show properties of compositeness. We study the phenomenological viability of having a mostly composite top. The strongest constraints are shown to mainly come from one-loop contributions to the T-parameter. Nevertheless, the presence of light custodial partners weakens these bounds, allowing in certain cases for a high degree of top compositeness. We find regions in the parameter space in which the T-parameter receives moderate positive contributions, favoring the electroweak fit of this type of models. We also study the implications of having a composite top at the LHC, focusing on the process pp-> t\bar t t\bar t (b\bar b) whose cross-section is enhanced at high-energies., Comment: 26 pages, 11 figures