Visible Sector Research Articles

Light hidden photon production in high energy collisions

The visible and dark sectors of particle physics can be connected via the kinetic mixing between the ordinary (γ) and hidden photon (γ′). If the latter is light its production in high energy collisions of ordinary particles occurs via the γ − γ′ oscillations similarly to the neutrino processes. Generically, the experiments are insensitive to mass of the hidden photon, if it is lighter than 1 MeV, and it does not decay into e+e−-pair. Still, one can use the missing energy and scattering off the detector as signatures to search for light hidden photon. Presence of media suppresses production of the light vectors making the experiments insensitive to the entire model. In media the light hidden photon production is typically suppressed due to the dump of the γ − γ′ oscillations making the experiments insensitive to the entire model. We present analytic formulas for the light hidden photon production, propagation and detection valid for searches at colliders and beam-target experiments and apply them to estimate the impact on the sensitivities of NA64, FASER, MATHUSLA, SHiP, T2K, DUNE and NA62 for the background-free case.

Non-comoving cosmology

One of the fundamental assumptions of the standard ΛCDM cosmology is that, on large scales, all the matter-energy components of the Universe share a common rest frame. This seems natural for the visible sector, that has been in thermal contact and tightly coupled in the primeval Universe. The dark sector, on the other hand, does not have any non-gravitational interaction known to date and therefore, there is no a priori reason to impose that it is comoving with ordinary matter. In this work we explore the consequences of relaxing this assumption and study the cosmology of non-comoving fluids. We show that it is possible to construct a homogeneous and isotropic cosmology with a collection of fluids moving with non-relativistic velocities. Our model extends ΛCDM with the addition of a single free parameter β0, the initial velocity of the visible sector with respect to the frame that observes a homogeneous and isotropic universe. This modification gives rise to a rich phenomenology, while being consistent with current observations for β0<1.6× 10−3 (95% CL). This work establishes the general framework to describe a non-comoving cosmology and extracts its first observational consequences for large-scale structure. Among the observable effects, we find sizeable modifications in the density-velocity and density-lensing potential cross-correlation spectra. These corrections give rise to deviations from statistical isotropy with a dipolar structure. The relative motion between the different fluids also couples the vector and scalar modes, the latter acting as sources for metric vector modes and vorticity for all the species.

Gaugino portal baryogenesis

We study baryogenesis via a gaugino portal, the supersymmetric counterpart to the widely studied kinetic mixing portal, to a hidden sector. CP and baryon number violating decays of a hidden sector gaugino into the visible sector can produce the observed baryon asymmetry of the Universe. The tiny portal coupling is crucial in producing late out-of-equilibrium decays, after washout processes that can erase the asymmetry have gone out of equilibrium. We study this mechanism within various scenarios, including freeze-in or freeze-out of the hidden gaugino, as well as extended frameworks where the hidden sector contains a weakly interacting massive particle (WIMP) dark matter candidate. This mechanism can produce the desired asymmetry over a wide range of mass scales, including for hidden gaugino masses as low as 10 GeV. We also discuss possible related signals with direct collider searches, at low energy experiments, and in dark matter direct and indirect detection.

Vortices in a Generalized Maxwell-Higgs Model with Visible and Hidden Sectors

We investigate the presence of vortices in generalized Maxwell-Higgs models with a hidden sector. The model engenders U(1)×U(1) symmetry, in a manner that the sectors are coupled via the visible magnetic permeability depending only on the hidden scalar field. We develop a first-order framework in which the hidden sector decouples from the visible one. We illustrate the results with two specific examples that give rise to the presence of vortices with internal structure.

Interplay of scalar and fermionic components in a multi-component dark matter scenario

We explore the multi-component dark matter (DM) scenario considered in a simple extension of the standard model with an inert scalar doublet and a singlet fermionic field providing the two DM candidates. The DM states are made stable under the unbroken Z_2times Z_2' discrete symmetry. An additional gauge singlet scalar field is introduced to facilitate the interaction of the dark fermion with the visible sector. Presence of a charged fermionic field having the same Z_2 charge as that of the inert scalar field allows exploring the dark matter mass regions otherwise disallowed, like in the standard Inert Doublet Model (IDM) scenarios. With these arrangements, it is shown that the light DM scenario and the desert region in the intermediate mass range of DM in the standard IDM case can be made compatible with the relic density bounds and direct detection limits. Further, detailed parameter space study is carried out keeping the coexistence of both the scalar and fermionic components in focus, showing that sizable parameter space regions are available for the entire mass range of 10 mathrm GeV le M_{DM}le 2000 GeV.

Singleton portals to the twin sector

The mirror twin Higgs framework allows for a natural Higgs mass while being consistent with collider bounds on colored symmetry partners to standard model quarks. This mechanism relies crucially on a discrete symmetry which relates each standard model field to a mirror partner. These partners are charged under gauge groups identical to, but distinct from, those in the standard model. The minimal twin Higgs scenario provides only one low-energy connection between the visible and twin sectors, the light Higgs boson. We present a new class of portals connecting the two sectors, using fields that have no twin partner under the discrete symmetry. Scalar, fermion, and vector states may provide such singleton portals, each with unique features and experimental signatures. The vector portal, in particular, provides a variety of renormalizable interactions relevant for the LHC. We provide concrete constructions of these portals and determine their phenomenology and opportunities to probe the twin sector at the LHC. We also sketch a scenario in which the structure of the twin sector itself can be tested.

Starobinsky-like inflation and soft-SUSY breaking

We study a version of Starobinsky-like inflation in no-scale supergravity (SUGRA) where a Polonyi term in the hidden sector breaks supersymmetry (SUSY) after inflation, providing a link between the gravitino mass and inflation. We extend the theory to the visible sector and calculate the soft-SUSY breaking parameters depending on the modular weights in the superpotential and choice of Kähler potential. We are led to either no-scale SUGRA or pure gravity mediated SUSY breaking patterns, but with inflationary constraints on the Polonyi term setting a strict upper bound on the gravitino mass m3/2< 103 TeV. Since gaugino masses are significantly lighter than m3/2, this suggests that SUSY may be discovered at the LHC or FCC.

Dark photon dark matter produced by axion oscillations

Despite growing interest and extensive effort to search for ultralight dark matter in the form of a hypothetical dark photon, how it fits into a consistent cosmology is unclear. Several dark photon dark matter production mechanisms proposed previously are known to have limitations, at least in certain mass regimes of experimental interest. In this letter, we explore a novel mechanism, where a coherently oscillating axion-like field can efficiently transfer its energy density to a dark photon field via a tachyonic instability. The residual axion relic is subsequently depleted via couplings to the visible sector, leaving only the dark photon as dark matter. We ensure that the cosmologies of both the axion and dark photon are consistent with existing constraints. We find that the mechanism works for a broad range of dark photon masses, including those of interest for ongoing experiments and proposed detection techniques.

Dark quark nuggets

"Dark quark nuggets", a lump of dark quark matter, can be produced in the early universe for a wide range of confining gauge theories and serve as a macroscopic dark matter candidate. The two necessary conditions, a nonzero dark baryon number asymmetry and a first-order phase transition, can be easily satisfied for many asymmetric dark matter models and QCD-like gauge theories with a few massless flavors. For confinement scales from 10 keV to 100 TeV, these dark quark nuggets with a huge dark baryon number have their masses vary from $10^{23}~\mathrm{g}$ to $10^{-7}~\mathrm{g}$ and their radii from $10^{8}~\mathrm{cm}$ to $10^{-15}~\mathrm{cm}$. Such macroscopic dark matter candidates can be searched for by a broad scope of experiments and even new detection strategies. Specifically, we have found that the gravitational microlensing experiments can probe heavier dark quark nuggets or smaller confinement scales around 10 keV; collision of dark quark nuggets can generate detectable and transient electromagnetic radiation signals; the stochastic gravitational wave signals from the first order phase transition can be probed by the pulsar timing array observations and other space-based interferometry experiments; the approximately massless dark mesons can behave as dark radiation to be tested by the next-generation CMB experiments; the free dark baryons, as a subcomponent of dark matter, can have direct detection signals for a sufficiently strong interaction strength with the visible sector.

Hidden sectors in string theory: kinetic mixings, fifth forces and quintessence

Light moduli fields in string compactifications can have interesting implications for particle physics and cosmology. Fifth force bounds impose stringent constraints on the interactions of such moduli with the visible sector. To be consistent with the bounds, they need to be part of hidden sectors which interact with the Standard Model with weaker-than-Planck suppressed interactions. We consider scenarios in which the visible sector degrees of freedom are localised in the compactification and light moduli arise as closed string degrees of freedom associated with hidden sectors which are geometrically separated (in the extra-dimensions) from the Standard Model. Kinetic mixings lead to interactions between the moduli and the visible sector — we compute these using Kähler potentials of string/M-theory compactifications. We argue that in general these interactions provide a lower bound on the strength of the interactions between the moduli and the visible sector. The interactions scale with inverse powers of the volume of the compactification, thus fifth force bounds can be translated to lower bounds on the volume of the extra-dimensions. We find that compactification volumes have to be large to evade the bounds. This imposes interesting constraints on quintessence model building in string theory. Our results for the strength of the interactions can also be used to quantify the fine-tuning necessary for the stability of the potential of a light modulus against quantum corrections involving visible sector loops.

Structure formation and exotic compact objects in a dissipative dark sector

We present the complete history of structure formation in a simple dissipative dark-sector model. The model has only two particles: a dark electron, which is a subdominant component of dark matter, and a dark photon. Dark-electron perturbations grow from primordial overdensities, become non-linear, and form dense dark galaxies. Bremsstrahlung cooling leads to fragmentation of the dark-electron halos into clumps that vary in size from a few to millions of solar masses, depending on the particle model parameters. In particular, we show that asymmetric dark stars and black holes form within the Milky Way from the collapse of dark electrons. These exotic compact objects may be detected and their properties measured at new high-precision astronomical observatories, giving insight into the particle nature of the dark sector without the requirement of non-gravitational interactions with the visible sector. Our analysis demonstrates that a full study of the structure-formation history is needed to assess correctly the viability of exotic compact objects in dissipative dark-sector models.

Ultraviolet completion of a composite asymmetric dark matter model with a dark photon portal

Composite asymmetric dark matter scenarios naturally explain why the dark matter mass density is comparable with the visible matter mass density. Such scenarios generically require some entropy transfer mechanism below the composite scale; otherwise, their late-time cosmology is incompatible with observations. A tiny kinetic mixing between a dark photon and the visible photon is a promising example of the low-energy portal. In this paper, we demonstrate that grand unifications in the dark and the visible sectors explain the origin of the tiny kinetic mixing. We particularly consider an ultraviolet completion of a simple composite asymmetric dark matter model, where asymmetric dark matter carries a B − L charge. In this setup, the longevity of asymmetric dark matter is explained by the B − L symmetry, while the dark matter asymmetry originates from the B−L asymmetry generated by thermal leptogenesis. In our minimal setup, the Standard Model sector and the dark sector are unified into SU(5)GUT × SU(4)DGUT gauge theories, respectively. This model generates required B − L portal operators while suppressing unwanted higher-dimensional operators that could wash out the generated B − L asymmetry.

Baryogenesis and dark matter from B mesons

We present a new mechanism of Baryogenesis and dark matter production in which both the dark matter relic abundance and the baryon asymmetry arise from neutral $B$ meson oscillations and subsequent decays. This set-up is testable at hadron colliders and $B$-factories. In the early Universe, decays of a long lived particle produce $B$ mesons and anti-mesons out of thermal equilibrium. These mesons/anti-mesons then undergo CP violating oscillations before quickly decaying into visible and dark sector particles. Dark matter will be charged under Baryon number so that the visible sector baryon asymmetry is produced without violating the total baryon number of the Universe. The produced baryon asymmetry will be directly related to the leptonic charge asymmetry in neutral $B$ decays; an experimental observable. Dark matter is stabilized by an unbroken discrete symmetry, and proton decay is simply evaded by kinematics. We will illustrate this mechanism with a model that is unconstrained by di-nucleon decay, does not require a high reheat temperature, and would have unique experimental signals -- a positive leptonic asymmetry in $B$ meson decays, a new decay of $B$ mesons into a baryon and missing energy, and a new decay of $b$-flavored baryons into mesons and missing energy. These three observables are testable at current and upcoming collider experiments, allowing for a distinct probe of this mechanism.

Sub-GeV-scale signatures of hidden braneworlds up to the Planck scale in a SO(3,1)-broken bulk

Many-brane Universes are at the heart of several cosmological scenarios related to physics beyond the Standard Model. It is then a major concern to constrain these approaches. Two-brane Universes involving [Formula: see text]-broken 5D bulks are among the cosmological models of interest. They also allow considering matter exchange between branes, a possible way to test these scenarios. Neutron disappearance (reappearance) toward (from) the hidden brane is currently tested with high-precision experiments to constrain the coupling constant [Formula: see text] between the visible and hidden neutron sectors. When dealing with the sub-GeV-scale quantum dynamics of fermions, any pair of braneworlds can be described by a noncommutative two-sheeted space–time [Formula: see text] from which [Formula: see text] emerges. Nevertheless, the calculation of the formal link between [Formula: see text] for a neutron and [Formula: see text]-broken 5D bulks remains an open problem until now although necessary to constrain these braneworld scenarios. Thanks to a phenomenological model, we derive [Formula: see text] — for a neutron — between the two braneworlds endowed with their own copy of the Standard Model in an [Formula: see text]-broken 5D bulk. Constraints on interbrane distance and brane energy scale (or brane thickness) are discussed. While brane energy scale below the GUT scale is excluded, energy scale up to the Planck limit allows neutron swapping detection in forthcoming experiments.

Reheating and dark radiation after fibre inflation

We study perturbative reheating at the end of fibre inflation where the inflaton is a closed string modulus with a Starobinsky-like potential. We first derive the spectral index ns and the tensor-to-scalar ratio r as a function of the number of efoldings and the parameter R which controls slow-roll breaking corrections. We then compute the inflaton couplings and decay rates into ultra-light bulk axions and visible sector fields on D7-branes wrapping the inflaton divisor. This leads to a reheating temperature of order 1010 GeV which requires 52 efoldings. Ultra-light axions contribute to dark radiation even if Δ Neff is almost negligible in the generic case where the visible sector D7-stack supports a non-zero gauge flux. If the parameter R is chosen to be small enough, ns≃ 0.965 is then in perfect agreement with current observations while r turns out to be of order r≃ 0.007. If instead the flux on the inflaton divisor is turned off, Δ Neff≲ 0.6 which, when used as a prior for Planck data, requires ns≃ 0.99. After R is fixed to obtain such a value of ns, primordial gravity waves are larger since r≃ 0.01.

Multipartite dark matter with scalars, fermions and signatures at LHC

Basic idea of this analysis is to achieve a two-component dark matter (DM) framework composed of a scalar and a fermion, with non-negligible DM-DM interaction contributing to thermal freeze out (hence relic density), but hiding them from direct detection bounds. We therefore augment the Standard Model (SM) with a scalar singlet (S) and three vectorlike fermions: two singlets (χ1, χ2) and a doublet (N). Stability of the two DM components is achieved by a discrete {mathcal{Z}}_2 times {mathcal{Z}}_2^{prime } symmetry, under which the additional fields transform suitably. Fermion fields having same {mathcal{Z}}_2 times {mathcal{Z}}_2^{prime } charge (N, χ1 in the model) mix after electroweak symmetry breaking (EWSB) and the lightest component becomes one of the DM candidates, while scalar singlet S is the other DM component connected to visible sector by Higgs portal coupling. The heavy fermion (χ2) plays the role of mediator to connect the two DM candidates through Yukawa interaction. This opens up a large parameter space for the heavier DM component through DM-DM conversion. Hadronically quiet dilepton signature, arising from the fermion dark sector, can be observed at Large Hadron Collider (LHC) aided by the presence of a lighter scalar DM component, satisfying relic density and direct search bounds through DM-DM conversion.

Low energy supergravity revisited (I)

General forms of the Kähler and superpotenials that lead to consistent low energy broken Supersymmetry originating from N[Formula: see text]=[Formula: see text]1 Supergravity have been classified and used for model building since more than three decades. We point out the incompleteness of this classification when hidden sector vacuum expectation values are of the order of the Planck mass. Focusing in this paper mainly on the case of minimal Kähler potential, we adopt a rigorous approach that retrieves on the one hand the known forms, and demonstrate on the other hand the existence of a whole set of new forms for the superpotential of which we give a complete classification. The latter forms involve a new type of chiral superfields having the unusual property of belonging neither to the hidden sector nor to the conventional observable sector. Comparing the obtained forms with the conventional ones, we argue how new possibilities for model building can arise, and discuss the gravity mediation of soft as well as additional hard (but parametrically small) Supersymmetry breaking, in the presence of the new type of chiral superfields. In the simplest case, we study the vacuum structure, characterize the masses and couplings of the scalar components to the hidden and observable sectors and discuss briefly the physical role they could play. In the generic case, we estimate the magnitude and possible consequences of the hard breaking of Supersymmetry in terms of the interplay between hidden and visible sector mass scales.

Dark sector portal with vectorlike leptons and flavor sequestering

We consider models with fermionic dark matter that transforms under a non-Abelian dark gauge group. Exotic, vectorlike leptons that also transform under the dark gauge group can mix with standard model leptons after spontaneous symmetry breaking and serve as a portal between the dark and visible sectors. We show in an explicit, renormalizable model based on a dark SU(2) gauge group how this can lead to adequate dark matter annihilation to a standard model lepton flavor so that the correct relic density is obtained. We identify a discrete symmetry that allows mass mixing between the vectorlike fermions and a single standard model lepton flavor, while preventing mixing between these fields and the remaining standard model leptons. This flavor sequestering avoids unwanted lepton-flavor-violating effects, substantially relaxing constraints on the mass scale of the vectorlike states. We discuss aspects of the phenomenology of the model, including direct detection of the dark matter.

Recent results of the NA64 experiment at the CERN SPS

We report on the results of the search for a new sub-GeV vector boson (A’) mediated production of Dark Matter (χ) in the fixed-target experiment, NA64, at the CERN SPS. The A’, called dark photon, could be generated in the reaction e-Z → e-ZA’ of 100 GeV electrons dumped against an active target which is followed by the prompt invisible decay A’ $ \to \chi \bar{\chi } $. The experimental signature of this process would be an event with an isolated electron and large missing energy in the detector. From the analysis of the data sample collected in 2016 corresponding to 4.3 × 1010 electrons on target no evidence of such a process has been found. The constraints on the A’ mixing strength with photons, 10−5 ≤ ɛ ≤ 10−2, for the A’ mass range mA’ ≤ 1GeV are derived. Significantly more data were taken in 2017 and 2018. The analysis of these data is not yet finished, although the sensitivity is already estimated as about 3 times better than in the previously published result on the 2016 data. For models considering scalar and fermionic thermal Dark Matter interacting with the visible sector through the vector portal the experiment starts to be sensitive to the regions of the dark-matter parameter $ y = \in^{2} \alpha_{D} (\frac{{m_{\chi } }}{{m_{{A^{\prime}}} }})^{4} $ predicted from the cosmological observations of Dark Matter. We also report the improved results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of e+e- pairs observed in the excited 8Be* nucleus decays. Due to its coupling to electrons, the X could be produced in the bremsstrahlung reaction e-Z → e-ZX by a beam of electrons incident on an active target in the NA64 experiment at the CERN SPS and observed through the subsequent decay into a e+e- pair. With 8.4 × 1010 electrons on target no evidence for such decays was found, allowing to set limits on the X - e- coupling in the range 1 × 10−4 ≤ ɛe ≤ 6 × 10−4 excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A’) from non-observation of the decay A’ → e+e- of the bremsstrahlung A’ with a mass ≤ 23MeV. Future plans of the NA64 experiment are presented.

Strongly interacting massive particles through the axion portal

Dark matter could be a thermal relic comprised of strongly interacting massive particles (SIMPs), where $3 \rightarrow 2$ interactions set the relic abundance. Such interactions generically arise in theories of chiral symmetry breaking via the Wess-Zumino-Witten term. In this work, we show that an axion-like particle can successfully maintain kinetic equilibrium between the dark matter and the visible sector, allowing the requisite entropy transfer that is crucial for SIMPs to be a cold dark matter candidate. Constraints on this scenario arise from beam dump and collider experiments, from the cosmic microwave background, and from supernovae. We find a viable parameter space when the axion-like particle is close in mass to the SIMP dark matter, with strong-scale masses of order a few hundred MeV. Many planned experiments are set to probe the parameter space in the near future.