hidden valley (particle physics)

hidden valley (particle physics)
Name	Hidden valley (particle physics)
Caption	Schematic of a hidden sector coupled to the Standard Model via a portal
Field	Particle physics
First proposed	Early 2000s
Founders	M. J. Strassler, K. M. Zurek

hidden valley (particle physics) Hidden valley models posit a new, secluded sector with its own gauge group, matter content, and dynamics that mixes weakly with the Standard Model through portal interactions. These frameworks were developed to address anomalies and gaps in searches at colliders such as the Large Hadron Collider and to provide novel dark matter candidates relevant to observations by experiments like Planck and WMAP. Hidden valleys can produce long-lived particles, displaced vertices, and unconventional signatures that motivate specialized searches at facilities including ATLAS, CMS, and proposed detectors like FASER and MATHUSLA.

Introduction

Hidden valley scenarios extend the Standard Model by introducing a secluded sector governed by a new gauge symmetry (for example, a confining SU(N) gauge group) with its own matter fields and bound states. Communication between the visible and hidden sectors occurs through portals mediated by particles associated with well-known frameworks such as the Higgs boson, a dark photon often called the U(1)' gauge boson, or heavy connector states from theories like supersymmetry and Grand Unified Theory. Motivations derive from puzzles in particle physics and astrophysics including the hierarchy problem, anomalies in flavor physics seen by experiments like LHCb, and the relic abundance measured by Planck.

Theoretical Framework

Model building for hidden valleys employs ingredients from established paradigms: confining dynamics reminiscent of Quantum Chromodynamics with a dark analogue of hadrons and glueballs; weakly coupled Abelian sectors related to kinetic mixing with the hypercharge gauge boson; and mediator states inspired by supersymmetry or extra dimensions such as Kaluza–Klein modes in Randall–Sundrum setups. Portals commonly considered include the Higgs boson portal coupling to scalar operators, the kinetic mixing portal between U(1) gauge bosons and hypercharge, and higher-dimension operators suppressed by heavy scales from Grand Unified Theory or String Theory constructions. Techniques from lattice Quantum Chromodynamics and effective field theory developed for Chiral Perturbation Theory are adapted to compute spectra, decay constants, and lifetime estimates for hidden-sector bound states.

Phenomenology and Signatures

Hidden valley phenomenology produces diverse collider signatures: displaced vertices, emerging jets, soft-unclustered energy patterns, and invisible decays that challenge triggers and reconstruction in detectors like CMS and ATLAS. Long-lived particles stemming from suppressed portal couplings can lead to macroscopic decay lengths probed by dedicated experiments such as MATHUSLA, FASER, CODEX-b, and upgrades at the LHCb detector. Cosmological and astrophysical probes use observations from Planck, WMAP, indirect detection experiments like Fermi Gamma-ray Space Telescope, and direct searches such as XENON1T and LUX-ZEPLIN to constrain relic densities and interaction strengths. Precision observables from LEP, flavor measurements at BaBar and Belle II, and neutrino experiments including IceCube also provide complementary constraints.

Experimental Searches and Constraints

Search strategies have been developed by collaborations at major facilities: ATLAS and CMS have published searches for displaced vertices, displaced lepton jets, and emerging jets; LHCb has leveraged low-mass, displaced-track capabilities; fixed-target and beam-dump experiments such as NA62, SHiP, and SeaQuest probe light mediators and dark photons. Limits on kinetic mixing parameters and branching fractions derive from reinterpretations of results from LEP, measurements at BaBar and Belle II, and precision electroweak fits involving SLC and Tevatron data. Proposed far detectors like MATHUSLA and FASER are designed to extend sensitivity to extremely long lifetimes beyond current collider reach.

Model Variants and Examples

Representative models include confining hidden valleys with dark pions and dark rho resonances analogous to Quantum Chromodynamics, Abelian dark photon models motivated by kinetic mixing first discussed in contexts like Holdom's work, supersymmetric hidden sectors coupled via gauge mediation as in models from Giudice and Rattazzi, and warped-extra-dimension realizations using Randall–Sundrum geometries. Specific constructions tie to broader frameworks: hidden sectors in String Theory compactifications, dark sectors related to asymmetric dark matter proposals inspired by baryogenesis mechanisms explored by Sakharov principles, or dark glueball scenarios informed by lattice studies from collaborations such as the RBC and UKQCD groups.

Implications for Cosmology and Dark Matter

Hidden valleys provide viable dark matter candidates including stable dark hadrons, asymmetric dark matter linked to baryon asymmetry from mechanisms probed by Sakharov-condition studies, and metastable relics relevant to indirect searches by Fermi Gamma-ray Space Telescope and AMS-02. Cosmological constraints arise from measurements of the cosmic microwave background by Planck and WMAP, big bang nucleosynthesis bounds from light-element observations, and structure formation probes such as those by the Sloan Digital Sky Survey. Thermal histories depend on portal strengths and mediator masses, affecting freeze-out, freeze-in, and reannihilation scenarios explored in the literature by authors including Kolb and Turner.

Category:Particle physics