14 Search Results

We explore generalized global symmetries in theories of physics beyond the standard model. Theories of $Z^{\prime }$ bosons generically contain “noninvertible” chiral symmetries, whose presence indicates a natural paradigm to break this symmetry by an exponentially small amount in an ultraviolet completion. For example, in models of gauged lepton family difference such as the phenomenologically well motivated $\mathrm{U}(1{)}_{L_{\mu }-L_{\tau }}$ , there is a noninvertible lepton number symmetry which protects neutrino masses. We embed these theories in gauged non-Abelian horizontal lepton symmetries, e.g., $\mathrm{U}(1{)}_{L_{\mu }-L_{\tau }}\subset \mathrm{SU}(3{)}_H$ , where the generalized symmetries are broken nonperturbatively by the existence of lepton family magnetic monopoles. In such theories, either Majorana or Dirac neutrino masses may be generated through quantum gauge theory effects from the charged lepton Yukawas, e.g., $y_{\nu }\sim y_{\tau }\exp (-S_{\mathrm{inst}})$ . These theories require no bevy of new fields nor additional global symmetries but are instead simple, natural, and predictive: The discovery of a lepton family $Z^{\prime }$ at low energies will reveal the scale at which $L_{\mu }-L_{\tau }$ emerges from a larger gauge symmetry. Published by the American Physical Society 2024

Large classes of dark sector models feature mass scales and couplings very different from the ones we observe in the Standard Model (SM). Moreover, in the freeze-in mechanism, often employed by the dark sector models, it is also required that the dark sector cannot be populated during the reheating process like the SM. This is the so called asymmetric reheating. Such disparities in sizes and scales often call for dynamical explanations. In this paper, we explore a scenario in which slow evolving conformal field theories (CFTs) offer such an explanation. Building on the recent work on conformal freeze-in (COFI), we focus on a coupling between the Standard Model Hypercharge gauge boson and an anti-symmetric tensor operator in the dark CFT. We present a scenario which dynamically realizes the asymmetric reheating and COFI production. With a detailed study of dark matter production, and taking into account limits on the dark matter (DM) self-interaction, warm DM bound, and constraints from the stellar evolution, we demonstrate that the correct relic abundance can be obtained with reasonable choices of parameters. The model predicts the existence of a dark photon as an emergent composite particle, with a small kinetic mixing also determined by the CFT dynamics, which correlates it with the generation of the mass scale of the dark sector. At the same time, COFI production of dark matter is very different from those freeze-in mediated by the dark photon. This is an example of the physics in which a realistic dark sector model can often be much richer and with unexpected features.

We consider theories in which a dark sector is described by a Conformal Field Theory (CFT) over a broad range of energy scales. A coupling of the dark sector to the Standard Model breaks conformal invariance. While weak at high energies, the breaking grows in the infrared, and at a certain energy scale the theory enters a confined (hadronic) phase. One of the hadronic excitations can play the role of dark matter. We study a “Conformal Freeze-In” cosmological scenario, in which the dark sector is populated through its interactions with the SM at temperatures when it is conformal. In this scenario, the dark matter relic density is determined by the CFT data, such as the dimension of the CFT operator coupled to the Standard Model. We show that this simple and highly predictive model of dark matter is phenomenologically viable. The observed relic density is reproduced for a variety of SM operators (“portals”) coupled to the CFT, and the resulting models are consistent with observational constraints. The mass of the COFI dark matter candidate is predicted to be in the keV-MeV range.

We extend the definition of asymptotic multiparticle states of the S-matrix beyond the tensor products of one-particle states. We identify new quantum numbers called pairwise helicities, or q_ij, associated with asymptotically separated pairs of particles. We first treat all single particles and particle pairs independently, allowing us to generalize the Wigner construction, and ultimately projecting onto the physical states. Our states reduce to tensor product states for vanishing q_ij, while for vanishing spins they reproduce Zwanziger’s scalar dyon states. This construction yields the correct asymptotic states for the scattering of electric and magnetic charges, with pairwise helicity identified as q_ij = e_ig_j – e_jg_i.

On-shell methods are particularly suited for exploring the scattering of electrically and magnetically charged objects, for which there is no local and Lorentz invariant Lagrangian description. In this paper we show how to construct a Lorentz-invariant S-matrix for the scattering of electrically and magnetically charged particles, without ever having to refer to a Dirac string. A key ingredient is a revision of our fundamental understanding of multi-particle representations of the Poincaré group. Surprisingly, the asymptotic states for electric-magnetic scattering transform with an additional little group phase, associated with pairs of electrically and magnetically charged particles. The corresponding “pairwise helicity” is identified with the quantized “cross product” of charges, e₁g₂- e₂g₁, for every charge-monopole pair, and represents the extra angular momentum stored in the asymptotic electromagnetic field. We define a new kind of pairwise spinor-helicity variable, which serves as an additional building block for electric-magnetic scattering amplitudes. We then construct the most general 3-point S-matrix elements, as well as the full partial wave decomposition for the 2 → 2 fermion-monopole S-matrix. In particular, we derive the famous helicity flip in the lowest partial wave as a simple consequence of a generalized spin-helicity selection rule, as well as the full angular dependence for the higher partial waves. Our construction provides a significant new achievement for the on-shell program, succeeding where the Lagrangian description has so far failed.

New light scalar particles in the mass range of hundreds of GeV, decaying into a pair of W/Z bosons can appear in several extensions of the SM. The focus of collider studies for such a scalar is often on its direct production, where the scalar is typically only mildly boosted. The observed W/Z are therefore well-separated, allowing analyses for the scalar resonance in a standard fashion as a low-mass diboson resonance. In this work we instead focus on the scenario where the direct production of the scalar is suppressed, and it is rather produced via the decay of a significantly heavier (a few TeV mass) new particle, in conjunction with SM particles. Such a process results in the scalar being highly boosted, rendering the W/Z’s from its decay merged. The final state in such a decay is a “fat” jet, which can be either four pronged (for fully hadronic W/Z decays), or may be like a W/Z jet, but with leptons buried inside (if one of the W/Z decays leptonically). In addition, this fat jet has a jet mass that can be quite different from that of the W/Z/Higgs/top quark-induced jet, and may be missed by existing searches. In this work, we develop dedicated algorithms for tagging such multi-layered “boosted dibosons” at the LHC. As a concrete application, we discuss an extension of the standard warped extra dimensional framework where such a light scalar can arise. We demonstrate that the use of these algorithms gives sensitivity in mass ranges that are otherwise poorly constrained.

Here, we have recently established a new method for measuring the mass of unstable particles produced at hadron colliders based on the analysis of the energy distribution of a massless product from their two-body decays. The central ingredient of our proposal is the remarkable result that, for an unpolarized decaying particle, the location of the peak in the energy distribution of the observed decay product is identical to the (fixed) value of the energy that this particle would have in the rest-frame of the decaying particle, which, in turn, is a simple function of the involved masses. In addition, we utilized the property that this energy distribution is symmetric around the location of peak when energy is plotted on a logarithmic scale. The general strategy was demonstrated in several specific cases, including both beyond the standard model particles, as well as for the top quark. In the present work, we generalize this method to the case of a massive decay product from a two-body decay; this procedure is far from trivial because (in general) both the above-mentioned properties are no longer valid. Nonetheless, we propose a suitably modified parametrization of the energy distribution that was used successfully for the massless case, which can deal with the massive case as well. We test this parametrization on concrete examples of energy spectra of Z bosons from the decay of a heavier supersymmetric partner of top quark (stop) into a Z boson and a lighter stop. After establishing the accuracy of this parametrization, we study a realistic application for the same process, but now including dominant backgrounds and using foreseeable statistics at LHC14, in order to determine the performance of this method for an actual mass measurement. The upshot of our present and previous work is that, in spite of energy being a Lorentz-variant quantity, its distribution emerges as a powerful tool for mass measurement at hadron colliders.

Recently (arXiv:1608.00526), a new framework for warped higher-dimensional compactifications with “bulk” standard model (SM) was proposed: in addition to the UV (Planck scale) and IR (a couple of TeV) branes, there is an intermediate brane, taken to be around 10TeV. The SM matter and Higgs fields propagate from the UV brane down to this intermediate brane only, while gauge and gravity fields propagate in the entire bulk. Such a configuration renders the lightest gauge Kaluza-Klein (KK) states within LHC reach, simultaneously satisfying flavor and CP constraints. In addition, the usual leading decay modes of the lightest KK gauge bosons into top and Higgs bosons are suppressed. This effect permits erstwhile subdominant channels to become significant. These include flavor-universal decays to SM fermions and Higgs bosons, and a novel channel — decay to a radion and a SM gauge boson, followed by radion decay to a pair of SM gauge bosons. In this work, we first delineate the parameter space where the above mentioned cascade decay of gauge KK particles dominates, and thereby can be the discovery mode at the LHC. We then perform a detailed analysis of the LHC signals from this model, finding that 300/fb suffices for evidence of KK-gluon in tri-jet, jet + di-photon and jet + di-boson channels. However, KK photon in photon + di-jet, and KK-W in leptonic W + di-jet require 3000/fb. The crucial feature of this decay chain is a “double” resonance, i.e. 3-particle and 2-particle invariant mass peaks, corresponding to the KK gauge boson and the radion respectively.

The standard paradigm of collisionless cold dark matter is in tension with measurements on large scales. In particular, the best fit values of the Hubble rate H₀ and the matter density perturbation σ₈ inferred from the cosmic microwave background seem inconsistent with the results from direct measurements. We show that both problems can be solved in a framework in which dark matter consists of two distinct components, a dominant component and a subdominant component. The primary component is cold and collisionless. The secondary component is also cold, but interacts strongly with dark radiation, which itself forms a tightly coupled fluid. The growth of density perturbations in the subdominant component is inhibited by dark acoustic oscillations due to its coupling to the dark radiation, solving the σ₈ problem, while the presence of tightly coupled dark radiation ameliorates the H₀ problem. The subdominant component of dark matter and dark radiation continue to remain in thermal equilibrium until late times, inhibiting the formation of a dark disk. We present an example of a simple model that naturally realizes this scenario in which both constituents of dark matter are thermal WIMPs. Our scenario can be tested by future stage-IV experiments designed to probe the CMB and large scale structure.