Detector-size upper bounds on dark hadron lifetime from cosmology

Lingfeng Li,Yuhsin Tsai

doi:10.1007/jhep05(2019)072

Abstract

We show that in a confining hidden valley model where the lightest hidden particles are dark hadrons that have mass splittings larger than mathcal{O}(0.1) GeV, if the lightest dark hadron is either stable or decays into Standard Model (SM) hadrons/charged leptons during the big-bang nucleosynthesis (BBN), at least one of the heavier dark hadrons needs to decay into SM particles within mathcal{O}(10) nanosec. Once being produced at collider experiments, this heavier dark hadron is likely to decay within mathcal{O}(10) meter distance, which strengthens the motivation of searching for long-lived particles with sub-meter scale decay lengths at colliders. To illustrate the idea, we study the lifetime constraint in scenarios where the lightest dark particle is a pseudo-scalar meson, and dark hadrons couple to SM particles either through kinetic mixing between the SM and dark photons or by mixing between the SM and dark Higgs. We study the annihilation and decay of dark hadrons in a thermal bath and calculate upper bounds on the lightest vector meson (scalar hadron) lifetime in the kinetic mixing (Higgs portal) scenario. We discuss the application of these lifetime constraints in long-lived particle searches that use the LHCb VELO or the AT-LAS/CMS inner detectors.

Highlights

In this work, we focus on dark sectors that have O(1−10) GeV dark hadrons made of rather heavy fermionic dark quarks, and the dark confinement scale (Λd) is lower than the dark quark mass but higher than 15% of the light dark meson mass
We show that in a confining hidden valley model where the lightest hidden particles are dark hadrons that have mass splittings larger than O(0.1) GeV, if the lightest dark hadron is either stable or decays into Standard Model (SM) hadrons/charged leptons during the big-bang nucleosynthesis (BBN), at least one of the heavier dark hadrons needs to decay into SM particles within O(10) nanosec
We study the thermal history of O(1 − 10) GeV scale dark hadrons in a confining hidden valley model, in which the lightest dark meson has a slow decay that can violate the BBN or dark matter (DM) density constraints

Summary

Evolution of the hidden hadron density

−1 ds 3H(x) dx σ−2hv Yh2 − σ+2hv Yl2 + σ−hv YhYl − σ+hv Yl2. Here Yl,h = nl,h/s is the comoving number of dark hadrons, and s is the entropy density with its value determined by the SM temperature T. Since eq (2.9) depends on the assumptions of dark yukawa coupling, and the same Boltzmann suppression makes the Tdec from eq (2.9) to be similar to eq (2.8), we will only include eq (2.8) from the φh decay-inverse decay when solving the temperature evolution in eq (2.5). Since the relic φl abundance is mainly determined by (∆m/mh) and Tdec, and Tdec is insensitive to the hadronic cross sections, the final Yl is quite insensitive to the σ±hv value This is not true, if the inverse decay of φh is highly efficient to keep the SM-dark sector thermal equilibrium down to a very low temperature. The thermal equilibrium makes dark sector warmer and allows φl to keep converting into φh until its number density times σ±(2)hv is smaller than Hubble In this case Yl is more sensitive to σ±hv. For the lifetime bound we are interested in, cτφh 1 cm, and the result is insensitive to σ±hv Λ2d

Upper bounds on the dark hadron lifetime

Photon portal scenario

10 GeV 3 mηd mZd 4 100 GeV

Higgs portal scenario

GeV Λd mχd 2Λd

Application to the long-lived particle searches

Vector meson decay in the photon mixing scenario

Scalar hadron decay in the Higgs portal scenario

Dark shower signals

Findings

Conclusion