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Register a new userShining light through the Higgs portal with $gammagamma$ colliders
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High-energy $gammagamma$ colliders constitute a potential running mode of future $e^+ e^-$ colliders such as the ILC and CLIC. We study the sensitivity of a high-energy $gammagamma$ collider to the Higgs portal scenario to a hidden sector above the invisible Higgs decay threshold. We show that such $gammagamma$ collisions could allow to probe the existence of dark sectors through the Higgs portal comparatively more precisely than any other planned collider facility, from the unique combination of sizable cross-section with clean final state and collider environment. In addition, this search could cover the singlet Higgs portal parameter space yielding a first-order electroweak phase transition in the early Universe.
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We review scenarios in which the particles that account for the Dark Matter (DM) in the Universe interact only through their couplings with the Higgs sector of the theory, the so-called Higgs-portal models. In a first step, we use a general and model-independent approach in which the DM particles are singlets with spin $0,frac12$ or $1$, and assume a minimal Higgs sector with the presence of only the Standard Model (SM) Higgs particle observed at the LHC. In a second step, we discuss non-minimal scenarios in which the spin-$frac12$ DM particle is accompanied by additional lepton partners and consider several possibilities like sequential, singlet-doublet and vector-like leptons. In a third step, we examine the case in which it is the Higgs sector of the theory which is enlarged either by a singlet scalar or pseudoscalar field, an additional two Higgs doublet field or by both; in this case, the matter content is also extended in several ways. Finally, we investigate the case of supersymmetric extensions of the SM with neutralino DM, focusing on the possibility that the latter couples mainly to the neutral Higgs particles of the model which then serve as the main portals for DM phenomenology. In all these scenarios, we summarize and update the present constraints and future prospects from the collider physics perspective, namely from the determination of the SM Higgs properties at the LHC and the search for its invisible decays into DM, and the search for heavier Higgs bosons and the DM companion particles at high-energy colliders. We then compare these results with the constraints and prospects obtained from the cosmological relic abundance as well as from direct and indirect DM searches in astroparticle physics experiments. The complementarity of collider and astroparticle DM searches is investigated in all the considered models.
Future $gammagamma$ colliders allow the production of the heavy neutral MSSM Higgs bosons $H$ and $A$ as single resonances. The prospects of finding these particles in the $bbar{b}$ and the neutralino-pair final states have been analysed. The $H,A$ bosons can be discovered for medium values of $tanbeta$ with masses up to 70--80% of the initial $e^pm e^-$ c.m. energy. This production mode thus covers parts of the supersymmetric parameter space that are not accessible at other colliders.
Confining hidden sectors are an attractive possibility for physics beyond the Standard Model (SM). They are especially motivated by neutral naturalness theories, which reconcile the lightness of the Higgs with the strong constraints on colored top partners. We study hidden QCD with one light quark flavor, coupled to the SM via effective operators suppressed by the mass $M$ of new electroweak-charged particles. This effective field theory is inspired by a new tripled top model of supersymmetric neutral naturalness. The hidden sector is accessed primarily via the $Z$ and Higgs portals, which also mediate the decays of the hidden mesons back to SM particles. We find that exotic $Z$ decays at the LHC and future $Z$ factories provide the strongest sensitivity to this scenario, and we outline a wide array of searches. For a larger hidden confinement scale $Lambdasim O(10);mathrm{GeV}$, the exotic $Z$ decays dominantly produce final states with two hidden mesons. ATLAS and CMS can probe their prompt decays up to $Msim 3;mathrm{TeV}$ at the high luminosity phase, while a TeraZ factory would extend the reach up to $Msim 20;mathrm{TeV}$ through a combination of searches for prompt and displaced signals. For smaller $Lambda sim O(1);mathrm{GeV}$, the $Z$ decays to the hidden sector produce jets of hidden mesons, which are long-lived. LHCb will be a powerful probe of these emerging jets. Furthermore, the light hidden vector meson could be detected by proposed dark photon searches.
Pairs of Standard Model fermions form dimension-3 singlet operators that can couple to new dark sector states. This fermion portal is to be contrasted with the lower-dimensional Higgs, vector and neutrino singlet portals. We characterise its distinct phenomenology and place effective field theory bounds on this framework, focusing on the case of fermion portals to a pair of light dark sector fermions. We obtain current and projected limits on the dimension-6 effective operator scale from a variety of meson decay experiments, missing energy and long-lived particle searches at colliders, as well as astrophysical and cosmological bounds. The DarkEFT public code is made available for recasting these limits, which we illustrate with various examples including an integrated-out heavy dark photon.
We present a new model of Stealth Dark Matter: a composite baryonic scalar of an $SU(N_D)$ strongly-coupled theory with even $N_D geq 4$. All mass scales are technically natural, and dark matter stability is automatic without imposing an additional discrete or global symmetry. Constituent fermions transform in vector-like representations of the electroweak group that permit both electroweak-breaking and electroweak-preserving mass terms. This gives a tunable coupling of stealth dark matter to the Higgs boson independent of the dark matter mass itself. We specialize to $SU(4)$, and investigate the constraints on the model from dark meson decay, electroweak precision measurements, basic collider limits, and spin-independent direct detection scattering through Higgs exchange. We exploit our earlier lattice simulations that determined the composite spectrum as well as the effective Higgs coupling of stealth dark matter in order to place bounds from direct detection, excluding constituent fermions with dominantly electroweak-breaking masses. A lower bound on the dark baryon mass $m_B gtrsim 300$ GeV is obtained from the indirect requirement that the lightest dark meson not be observable at LEP II. We briefly survey some intriguing properties of stealth dark matter that are worthy of future study, including: collider studies of dark meson production and decay; indirect detection signals from annihilation; relic abundance estimates for both symmetric and asymmetric mechanisms; and direct detection through electromagnetic polarizability, a detailed study of which will appear in a companion paper.
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