arXiv subjects

Z. Chacko

Publications and source records attributed to Z. Chacko.

At least 19 recordsLinked to original sources

arXiv2025

United States Muon Collider Community White Paper for the European Strategy for Particle Physics Update

This document is being submitted to the 2024-2026 European Strategy for Particle Physics Update (ESPPU) process on behalf of the US Muon Collider community, with its preparation coordinated by the interim US Muon Collider Coordination Group. The US Muon Collider Community comprises a few hundred American scientists. The purpose of the document is to inform ESPPU about the US plans for Muon Collider research and development (R&D), explain how these efforts align with the broader international R&D initiatives, and present the US community vision for the future realization of this transformative project.

hep-ex↗

arXiv2022

Muon Collider Forum Report

A multi-TeV muon collider offers a spectacular opportunity in the direct exploration of the energy frontier. Offering a combination of unprecedented energy collisions in a comparatively clean leptonic environment, a high energy muon collider has the unique potential to provide both precision measurements and the highest energy reach in one machine that cannot be paralleled by any currently available technology. The topic generated a lot of excitement in Snowmass meetings and continues to attract a large number of supporters, including many from the early career community. In light of this very strong interest within the US particle physics community, Snowmass Energy, Theory and Accelerator Frontiers created a cross-frontier Muon Collider Forum in November of 2020. The Forum has been meeting on a monthly basis and organized several topical workshops dedicated to physics, accelerator technology, and detector R&D. Findings of the Forum are summarized in this report.

hep-ex↗

arXiv2019

The CLIC Potential for New Physics

The Compact Linear Collider (CLIC) is a mature option for the future of high energy physics. It combines the benefits of the clean environment of $e^+e^-$ colliders with operation at high centre-of-mass energies, allowing to probe scales beyond the reach of the Large Hadron Collider (LHC) for many scenarios of new physics. This places the CLIC project at a privileged spot in between the precision and energy frontiers, with capabilities that will significantly extend knowledge on both fronts at the end of the LHC era. In this report we review and revisit the potential of CLIC to search, directly and indirectly, for physics beyond the Standard Model.

hep-ph↗

arXiv2015

The Twin Higgs: Natural Electroweak Breaking from Mirror Symmetry

We present `twin Higgs models', simple realizations of the Higgs as a pseudo-Goldstone boson that protect the weak scale from radiative corrections up to scales of order 5 - 10 TeV. In the ultra-violet these theories have a discrete symmetry which interchanges each Standard Model particle with a corresponding particle which transforms under a twin or mirror Standard Model gauge group. In addition, the Higgs sector respects an approximate global SU(4) symmetry. When this global symmetry is broken, the discrete symmetry tightly constrains the form of corrections to the pseudo-Goldstone Higgs potential, allowing natural electroweak symmetry breaking. Precision electroweak constraints are satisfied by construction. These models demonstrate that, contrary to the conventional wisdom, stabilizing the weak scale does not require new light particles charged under the Standard Model gauge groups.

hep-ph↗

arXiv2010

Probing Resonant Leptogenesis at the LHC

We explore direct collider probes of the resonant leptogenesis mechanism for the origin of matter. We work in the context of theories where the Standard Model is extended to include an additional gauged U(1) symmetry broken at the TeV scale, and where the light neutrinos obtain mass through a Type I seesaw at this scale. The CP asymmetry that generates the observed matter-antimatter asymmetry manifests itself in a difference between the number of positive and negative like-sign dileptons N(\ell^+\ell^+)-N(\ell^-\ell^-) that arise in the decay of the new Z' gauge boson to two right-handed neutrinos N, and their subsequent decay to leptons. The relatively low efficiency of resonant leptogenesis in this class of models implies that the CP asymmetry, ε, is required to be sizable, i.e. of order one. In particular, from the sign of the baryon asymmetry of the Universe, \emph{an excess of antileptons is predicted}. We identify the domains in M_{Z'}--M_N space where such a direct test is possible and find that with 300~fb^{-1} of data and no excess found, the LHC can set the $2σ$ exclusion limit ε\lesssim 0.22.

hep-ph↗

arXiv2009

Neutrino Mass Seesaw at the Weak Scale, the Baryon Asymmetry, and the LHC

We consider theories where the Standard Model (SM) neutrinos acquire masses through the seesaw mechanism at the weak scale. We show that in such a scenario, the requirement that any pre-existing baryon asymmetry, regardless of its origin, not be washed out leads to correlations between the pattern of SM neutrino masses and the spectrum of new particles at the weak scale, leading to definite predictions for the LHC. For type I seesaw models with a TeV scale Z' coupled to SM neutrinos, we find that for a normal neutrino mass hierarchy, at least one of the right-handed neutrinos must be `electrophobic', decaying with a strong preference into final states with muons and tauons rather than electrons. For inverted or quasi-degenerate mass patterns, on the other hand, we find upper bounds on the mass of at least one right-handed neutrino. In particular, for an inverted mass hierarchy, this bound is 1 TeV, while the corresponding upper limit in the quasi-degenerate case is 300 GeV. Similar results hold in type III seesaw models, albeit with somewhat more stringent bounds. For the Type II seesaw case with a weak scale SU(2) triplet Higgs, we again find that an interesting range of Higgs triplet masses is disallowed by these considerations.

hep-ph↗

arXiv2009

Supersymmetry in Slow Motion

We construct new theories of electroweak symmetry breaking that employ a combination of supersymmetry and discrete symmetries to stabilize the weak scale up to and beyond the energies probed by the LHC. These models exhibit conventional supersymmetric spectra but the fermion-sfermion-gaugino vertices are absent. This closes many conventional decay channels, thereby allowing several superpartners to be stable on collider time scales. This opens the door to the possibility of directly observing R-hadrons and three flavors of sleptons inside the LHC detectors.

hep-ph↗

arXiv2008

A Composite Twin Higgs Model

Twin Higgs models are economical extensions of the Standard Model that stabilize the electroweak scale. In these theories the Higgs field is a pseudo Nambu-Goldstone boson that is protected against radiative corrections up to scales of order 5 TeV by a discrete parity symmetry. We construct, for the first time, a class of composite twin Higgs models based on confining QCD-like dynamics. These theories naturally incoporate a custodial isospin symmetry and predict a rich spectrum of particles with masses of order a TeV that will be accessible at the LHC.

hep-ph↗

arXiv2008

The Quirky Collider Signals of Folded Supersymmetry

We investigate the collider signals associated with scalar quirks ("squirks") in folded supersymmetric models. As opposed to regular superpartners in supersymmetric models these particles are uncolored, but are instead charged under a new confining group, leading to radically different collider signals. Due to the new strong dynamics, squirks that are pair produced do not hadronize separately, but rather form a highly excited bound state. The excited ``squirkonium'' loses energy to radiation before annihilating back into Standard Model particles. We calculate the branching fractions into various channels for this process, which is prompt on collider time-scales. The most promising annihilation channel for discovery is W+photon which dominates for squirkonium near its ground state. We demonstrate the feasibility of the LHC search, showing that the mass peak is visible above the SM continuum background and estimate the discovery reach.

hep-ph↗

arXiv2007

Symmetry Breaking Patterns for the Little Higgs from Strong Dynamics

We show how the symmetry breaking pattern of the simplest little Higgs model, and that of the smallest moose model that incorporates an approximate custodial SU(2), can be realized through the condensation of strongly coupled fermions. In each case a custodial SU(2) symmetry of the new strong dynamics limits the sizes of corrections to precision electroweak observables. In the case of the simplest little Higgs, there are no new light states beyond those present in the original model. However, our realization of the symmetry breaking pattern of the moose model predicts an additional scalar field with mass of order a TeV or higher that has exactly the same quantum numbers as the Standard Model Higgs and which decays primarily to third generation quarks.

hep-ph↗

arXiv2007

Direct Detection of Supersymmetric Particles in Neutrino Telescopes

In supersymmetric theories where the lightest supersymmetric particle is the gravitino the next to lightest supersymmetric particle is typically a long lived charged slepton. In this paper, following our earlier proposal, we perform a detailed study of the production of pairs of these particles induced by the interactions of high energy cosmic neutrinos with nucleons in the earth, their propagation through the earth and finally their detection in neutrino telescopes. We investigate the charged slepton energy loss in detail and establish that the relatively small cross-section for the production of supersymmetric particles is partially compensated for by the very long range of these heavy particles. The signal, consisting of two parallel charged tracks emerging from the earth, is characterized by a track separation of a few hundred meters. We perform a careful analysis of the main background, coming from direct di-muon production, and show that it can be separated from the signal due to its characteristically smaller track separation. We conclude that neutrino telescopes will complement collider searches in the determination of the supersymmetry breaking scale, and may even provide the first evidence for supersymmetry at the weak scale.

hep-ph↗

arXiv2006

Folded Supersymmetry and the LEP Paradox

We present a new class of models that stabilize the weak scale against radiative corrections up to scales of order 5 TeV without large corrections to precision electroweak observables. In these `folded supersymmetric' theories the one loop quadratic divergences of the Standard Model Higgs field are cancelled by opposite spin partners, but the gauge quantum numbers of these new particles are in general different from those of the conventional superpartners. This class of models is built around the correspondence that exists in the large N limit between the correlation functions of supersymmetric theories and those of their non-supersymmetric orbifold daughters. By identifying the mechanism which underlies the cancellation of one loop quadratic divergences in these theories, we are able to construct simple extensions of the Standard Model which are radiatively stable at one loop. Ultraviolet completions of these theories can be obtained by imposing suitable boundary conditions on an appropriate supersymmetric higher dimensional theory compactified down to four dimensions. We construct a specific model based on these ideas which stabilizes the weak scale up to about 20 TeV and where the states which cancel the top loop are scalars not charged under Standard Model color. Its collider signatures are distinct from conventional supersymmetric theories and include characteristic events with hard leptons and missing energy.

hep-ph↗

arXiv2005

A Twin Higgs Model from Left-Right Symmetry

We present twin Higgs models based on the extension of the Standard Model to left-right symmetry that protect the weak scale against radiative corrections up to scales of order 5 TeV. In the ultra-violet the Higgs sector of these theories respects an approximate global symmetry, in addition to the discrete parity symmetry characteristic of left-right symmetric models. The Standard Model Higgs field emerges as the pseudo-Goldstone boson associated with the breaking of the global symmetry. The parity symmetry tightly constrains the form of radiative corrections to the Higgs potential, allowing natural electroweak breaking. The minimal model predicts a rich spectrum of exotic particles that will be accessible to upcoming experiments, and which are necessary for the cancellation of one-loop quadratic divergences. These include right-handed gauge bosons with masses not to exceed a few TeV and a pair of vector-like quarks with masses of order several hundred GeV.

hep-ph↗

arXiv2005

Natural Little Hierarchy from a Partially Goldstone Twin Higgs

We construct a simple theory in which the fine-tuning of the standard model is significantly reduced. Radiative corrections to the quadratic part of the scalar potential are constrained to be symmetric under a global U(4) x U(4)' symmetry due to a discrete Z_2 "twin" parity, while the quartic part does not possess this symmetry. As a consequence, when the global symmetry is broken the Higgs fields emerge as light pseudo-Goldstone bosons, but with sizable quartic self-interactions. This structure allows the cutoff scale, Λ, to be raised to the multi-TeV region without significant fine-tuning. In the minimal version of the theory, the amount of fine-tuning is about 15% for Λ= 5 TeV, while it is about 30% in an extended model. This provides a solution to the little hierarchy problem. In the minimal model, the "visible" particle content is exactly that of the two Higgs doublet standard model, while the extended model also contains extra vector-like fermions with masses ~(1-2)TeV. At the LHC, our minimal model may appear exactly as the two Higgs doublet standard model, and new physics responsible for cutting off the divergences of the Higgs mass-squared parameter may not be discovered. Several possible processes that may be used to discriminate our model from the simple two Higgs doublet model are discussed for the LHC and for a linear collider.

hep-ph↗

arXiv2005

A Minimally Fine-Tuned Supersymmetric Standard Model

We construct supersymmetric theories in which the correct scale for electroweak symmetry breaking is obtained without significant fine-tuning. We calculate the fine-tuning parameter for these theories to be at the 20% level, which is significantly better than in conventional supersymmetry breaking scenarios. Supersymmetry breaking occurs at a low scale of order 100 TeV, and is transmitted to the supersymmetric standard-model sector through standard-model gauge interactions. The Higgs sector contains two Higgs doublets and a singlet field, with a superpotential that takes the most general form allowed by gauge invariance. An explicit model is constructed in 5D warped space with supersymmetry broken on the infrared brane. We perform a detailed analysis of electroweak symmetry breaking for this model, and demonstrate that the fine-tuning is in fact reduced. A new candidate for dark matter is also proposed, which arises from the extended Higgs sector of the model. Finally, we discuss a purely 4D theory which may also significantly reduce fine-tuning.

hep-ph↗

arXiv2005

Massive Gravity on a Brane

At present no theory of a massive graviton is known that is consistent with experiments at both long and short distances. The problem is that consistency with long distance experiments requires the graviton mass to be very small. Such a small graviton mass however implies an ultraviolet cutoff for the theory at length scales far larger than the millimeter scale at which gravity has already been measured. In this paper we attempt to construct a model which avoids this problem. We consider a brane world setup in warped AdS spacetime and we investigate the consequences of writing a mass term for the graviton on a the infrared brane where the local cutoff is of order a large (galactic) distance scale. The advantage of this setup is that the low cutoff for physics on the infrared brane does not significantly affect the predictivity of the theory for observers localized on the ultraviolet brane. For such observers the predictions of this theory agree with general relativity at distances smaller than the infrared scale but go over to those of a theory of massive gravity at longer distances. A careful analysis of the graviton two-point function, however, reveals the presence of a ghost in the low energy spectrum. A mode decomposition of the higher dimensional theory reveals that the ghost corresponds to the radion field. We also investigate the theory with a brane localized mass for the graviton on the ultraviolet brane, and show that the physics of this case is similar to that of a conventional four dimensional theory with a massive graviton, but with one important difference: when the infrared brane decouples and the would-be massive graviton gets heavier than the regular Kaluza--Klein modes, it becomes unstable and it has a finite width to decay off the brane into the continuum of Kaluza-Klein states.

hep-th↗

arXiv2005

Relaxing the Upper Bound on the Mass of the Lightest Supersymmetric Higgs Boson

We present a class of supersymmetric models in which the lightest Higgs-boson mass can be as large as a few hundred GeV (200 - 300 GeV) while the successful MSSM prediction for gauge coupling unification is preserved. The theories are formulated on a 5D warped space truncated by two branes, and a part of the Higgs sector is localized on the infrared brane. The structure of the Higgs sector in the four dimensional effective theory below the Kaluza-Klein mass scale is essentially that of the next-to-minimal supersymmetric standard model (NMSSM), or related theories. However, large values of the NMSSM couplings at the weak scale are now possible as these couplings are required to be perturbative only up to the infrared cutoff scale, which can in general be much lower than the unification scale. This allows the possibility of generating a large quartic coupling in the Higgs potential, and thereby significantly raising the Higgs-boson mass bound. We present two particularly simple models. In the first model, the quark and lepton fields are localized on the ultraviolet brane, where the grand unified symmetry is broken. In the second model, the quark and lepton fields are localized on the infrared brane, and the unified symmetry is broken both on the ultraviolet and infrared branes. Our theories potentially allow the possibility of a significant reduction in the fine-tuning needed for correct electroweak symmetry breaking, although this is somewhat model dependent.

hep-ph↗

arXiv2004

Acceleressence: Dark Energy from a Phase Transition at the Seesaw Scale

Simple models are constructed for "acceleressence" dark energy: the latent heat of a phase transition occurring in a hidden sector governed by the seesaw mass scale v^2/M_Pl, where v is the electroweak scale and M_Pl the gravitational mass scale. In our models, the seesaw scale is stabilized by supersymmetry, implying that the LHC must discover superpartners with a spectrum that reflects a low scale of fundamental supersymmetry breaking. Newtonian gravity may be modified by effects arising from the exchange of fields in the acceleressence sector whose Compton wavelengths are typically of order the millimeter scale. There are two classes of models. In the first class the universe is presently in a metastable vacuum and will continue to inflate until tunneling processes eventually induce a first order transition. In the simplest such model, the range of the new force is bounded to be larger than 25 microns in the absence of fine-tuning of parameters, and for couplings of order unity it is expected to be \approx 100 microns. In the second class of models thermal effects maintain the present vacuum energy of the universe, but on further cooling, the universe will "soon" smoothly relax to a matter dominated era. In this case, the range of the new force is also expected to be of order the millimeter scale or larger, although its strength is uncertain. A firm prediction of this class of models is the existence of additional energy density in radiation at the eV era, which can potentially be probed in precision measurements of the cosmic microwave background. An interesting possibility is that the transition towards a matter dominated era has occurred in the very recent past, with the consequence that the universe is currently decelerating.

astro-ph↗