We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of standard model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the µm scale up to the Big Bang Nucleosynthesis limit of ∼10 7 m. Neutral LLPs with lifetimes above ∼ 100 m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. We study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC, assuming backgrounds can be rejected as expected. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at ATLAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underlines the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector.

We propose a mechanism called axiogenesis where the cosmological excess of baryons over antibaryons is generated from the rotation of the QCD axion. The Peccei-Quinn (PQ) symmetry may be explicitly broken in the early universe, inducing the rotation of a PQ charged scalar field. The rotation corresponds to the asymmetry of the PQ charge, which is converted into the baryon asymmetry via QCD and electroweak sphaleron transitions. In the concrete model we explore, interesting phenomenology arises due to the prediction of a small decay constant and the connections with new physics at the LHC and future colliders and with axion dark matter.Introduction.-One of the goals of fundamental physics is to understand the origin of the Universe. For this purpose, the Standard Model (SM) of particle physics needs an extension to explain the cosmological excess of matter over antimatter. Mechanisms to generate the baryon asymmetry have been intensively studied in the literature under the name of baryogenesis [1][2][3][4][5][6][7][8].The SM also needs an extension to explain the smallness of CP violation in QCD [9] which on theoretical grounds is expected to be large [10]. This is known as the strong CP problem and can be elegantly solved by the Peccei-Quinn (PQ) mechanism [11,12]. The so-called the PQ symmetry is spontaneously broken to yield a pseudo Nambu-Goldstone boson, the axion [13,14]. The PQ symmetry is explicitly broken by the quantum effects of QCD of the Adler-Bell-Jeckiw type [15,16]. The quantum effects give a potential to the axion and drive the axion field value to the point where CP symmetry is restored, solving the strong CP problem. The axion is also a dark matter candidate [17][18][19], which makes the PQ mechanism even more attractive.We discover that when the PQ mechanism is introduced into the SM, the baryon (B) and lepton (L) asymmetries are generated in a wide class of models. We call the following baryogenesis scheme as axiogenesis, which in general includes two main ingredients: 1) an asymmetry of the PQ charge is generated in the early universe as a coherent rotation in the axion direction and 2) the PQ asymmetry is later transferred to the B + L asymmetry via the QCD and electroweak sphaleron transitions. We contrast axiogenesis with other existing baryogenesis models after we introduce a concrete example. (We may convert the B + L asymmetry into the B − L asymmetry by some B − L breaking interaction. The investigation of such a scenario will be considered in a future work [20].)The PQ symmetry is an approximate global symmetry which is explicitly broken by the QCD anomaly. Given that the symmetry is not exact, it is conceivable that the PQ symmetry is significantly broken in the early universe, and the rotation of the axion is induced. In fact, it is expected that quantum gravity does not allow for a global symmetry [21][22][23][24][25] and the PQ symmetry is at best understood as an accidental symmetry explicitly broken by higher dimensional operators [26][27][28][29]. Even when one requires that t...

In the conventional misalignment mechanism, the axion field has a constant initial field value in the early Universe and later begins to oscillate. We present an alternative scenario where the axion field has a nonzero initial velocity, allowing an axion decay constant much below the conventional prediction from axion dark matter. This axion velocity can be generated from explicit breaking of the axion shift symmetry in the early Universe, which may occur as this symmetry is approximate.

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 matter, X, may be generated by new physics at the TeV scale during an early matterdominated (MD) era that ends at temperature T R TeV. Compared to the conventional radiation-dominated (RD) results, yields from both Freeze-Out and Freeze-In processes are greatly suppressed by dilution from entropy production, making Freeze-Out less plausible while allowing successful Freeze-In with a much larger coupling strength. Freeze-In is typically dominated by the decay of a particle B of the thermal bath, B → X. For a large fraction of the relevant cosmological parameter space, the decay rate required to produce the observed dark matter abundance leads to displaced signals at LHC and future colliders, for any m X in the range keV < m X < m B and for values of m B accessible to these colliders. This result applies whether the early MD era arises after conventional inflation, when T R is the usual reheat temperature, or is a generic MD era with an alternative origin. In the former case, if m X is sufficiently large to be measured from kinematics, the reheat temperature T R can be extracted. Our result is independent of the particular particle physics implementation of B → X, and can occur via any operator of dimension less than 8 (4) for a post-inflation (general MD) cosmology. An interesting example is provided by DFS axion theories with TeV-scale supersymmetry and axino dark matter of mass GeV to TeV, which is typically overproduced in a conventional RD cosmology. If B is the higgsino, h, Higgs, W and Z particles appear at the displaced decays,h → hã, Zã andh ± → W ±ã . The scale of axion physics, f , is predicted to be in the range (3 × 10 8 − 10 12 ) GeV and, over much of this range, can be extracted from the decay length.