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 study a simplified model of the SM Higgs boson decaying to a degenerate pair of scalars which travel a macroscopic distance before decaying to SM particles. This is the leading signal for many well-motivated solutions to the hierarchy problem that do not propose additional light colored particles. Bounds for displaced Higgs decays below 10 cm are found by recasting existing tracker searches from Run I. New tracker search strategies, sensitive to the characteristics of these models and similar decays, are proposed with sensitivities projected for Run II at √ s = 13 TeV. With 20 fb −1 of data, we find that Higgs branching ratios down to 2 × 10 −4 can be probed for centimeter decay lengths.

The production mechanism of a 750 GeV diphoton resonance, either via gluon or photon fusion, can be probed by studying kinematic observables in the diphoton events. We perform a detector study of the two production modes of a hypothetical scalar or tensor diphoton resonance in order to characterize the features of the two scenarios. The nature of the resonance production can be determined from the jet multiplicity, the jet and diphoton rapidities, the rate of central pseudorapidity gaps, or the possible detection of forward protons from elastic photoproduction for events in the signal region. Kinematic distributions for both signals and expected irreducible diphoton background events are provided for comparison along with a study of observables useful for distinguishing the two scenarios at an integrated luminosity of 20 fb −1 . We find that decay photons from a 750 GeV scalar resonance have a preference for acceptance in the central detector barrel, while background events are more likely to give accepted photons in the detector end caps. This disfavors the interpretation of the large number of excess events found by the the Run-2 CMS diphoton search with one photon detected in the end cap as a wide spin-0 resonance signal. However, one expects more end cap photons in the case of spin-2 resonance.

We present the leading experimental constraints on supersymmetric models with R-parity violation (RPV) and a long-lived lightest superpartner (LSP). We consider both the well-motivated dynamical RPV scenario as well as the conventional holomorphic RPV operators. Guided by naturalness, we study the cases of stop, gluino, and higgsino LSPs with several possible leading decay channels in each case. The CMS displaced dijet and the ATLAS multitrack displaced vertex searches have been fully recast, with all cuts and vertex reconstruction algorithms applied. Heavy charged stable particle searches by CMS are also applied. In addition, we consider representative bounds for prompt LSP decays that are directly applicable. Our main results are exclusion plots in the m LSP −τ LSP plane for the various scenarios. We find that the natural parameter space (mt < 800 GeV, mg < 1500 GeV, mH < 800 GeV) is excluded for a long-lived LSP (τ LSP 1 mm).

Twin Higgs models solve the little hierarchy problem without introducing new colored particles, however they are often in tension with measurements of the radiation density at late times. Here we explore viable cosmological histories for Twin Higgs models. In particular, we show that mixing between the SM and twin neutrinos can thermalize the two sectors below the twin QCD phase transition, significantly reducing the twin sector's contribution to the radiation density. The requisite twin neutrino masses of O(1 − 20) GeV and mixing angle with SM neutrinos of 10 −3 − 10 −5 can be probed in a variety of current and planned experiments. We further find that these parameters can be naturally accessed in a warped UV completion, where the neutrino sector can also generate the Z2-breaking Higgs mass term needed to produce the hierarchy between the symmetry breaking scales f and v. INTRODUCTIONTwin Higgs (TH) models provide an elegant solution to the hierarchy problem without introducing new states that are charged under the SM gauge symmetries [1]. Instead, a mirror sector with its own SU(3)×SU(2)×U(1) gauge symmetry is assumed. The Z 2 symmetry relating the SM and mirror sectors protects the Higgs mass from large radiative corrections, with the twin partners cancelling the SM quadratic divergences at one loop. Other variations of neutral naturalness include [2][3][4][5][6][7][8]. While this idea is very efficient at hiding new physics from the LHC and future colliders, it often leads to tension with cosmological observations due to the appearance of new light relativistic degrees of freedom (DOF), namely the twin photon and twin neutrinos.The standard assumption of TH models is that only the Higgs portal connects the SM and the mirror sectors. This maintains thermal equilibrium between the two sectors down to temperatures of a few GeV, below which the twin sector decouples [9]. At this point the twin and SM sectors have similar energy densities, and the twin photon and neutrinos contribute significantly to the radiation density at late times. In particular, the Mirror Twin Higgs (MTH) model-the scenario where the mirror sector is a full copy of the SM-predicts an exceedingly large contribution to the correction of the total radiation density (usually expressed in terms of ∆N eff , as measured from Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB)).Recently, several solutions have been proposed for the cosmological problems of the Twin Higgs, including the Fraternal Twin Higgs (FTH) [10], hard Z 2 -breaking in the Yukawa couplings [11], and SM reheating from a light right-handed neutrino [12]. Further cosmological aspects of Twin Higgs models, including dark matter, have been studied in [11][12][13][14][15][16][17][18][19].In this paper we propose that the neutrino portal can also naturally be used to connect the twin and the SM sectors. Mixing between the SM and twin neutrinos appears in many simple implementations of the twin neutrino sector. We show that such mixing can lower the decoupling tem...

The exclusive reaction γp → pK + K − was studied in the photon energy range 3.0 − 3.8 GeV and momentum transfer range 0.6 < −t < 1.3 GeV 2 . Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. In this kinematic range the integrated luminosity was approximately 20 pb −1 . The reaction was isolated by detecting the K + and the proton in CLAS, and reconstructing the K − via the missing-mass technique. Moments of the di-kaon decay angular distributions were extracted from the experimental data. Besides the dominant contribution of the φ meson in the P -wave, evidence for S −P interference was found. The differential production cross sections dσ/dt for individual waves in the mass range of the φ resonance were extracted and compared to predictions of a Regge-inspired model. This is the first time the t-dependent cross section of the S-wave contribution to the elastic K + K − photoproduction has been measured.

We present a novel class of composite Higgs models in which the top and gauge partners responsible for cutting off the Higgs quadratic divergences form a continuum. The continuum states are characterized by their spectral densities, which should have a finite gap for realistic models. We present a concrete example based on a warped extra dimension with a linear dilaton, where this finite gap appears naturally. We derive the spectral densities in this model and calculate the full Higgs potential for a phenomenologically viable benchmark point, with percent level tuning. The continuum top and gauge partners in this model evade all resonance searches at the LHC and yield qualitatively different collider signals.

We show that the relatively small but coincident excesses observed around 2 TeV in the ATLAS Run 1 and Run 2 hadronic diboson searches --- when a cut on the number of tracks in the fat jets is not applied --- and the null results of all remaining high-mass diboson searches are compatible with the decay of a triboson resonance $R$ into $WZ$ plus an extra particle $X$. These decays can take place via new neutral ($Y^0$) or charged ($Y^\pm$) particles, namely $R \to Y^0 \, W$, with $Y^0 \to Z X$, or $R \to Y^\pm Z$, with $Y^\pm \to W X$. An obvious candidate for such intermediate particle is a neutral one $Y^0$, given a $3.9\sigma$ excess found at 650 GeV by the CMS Collaboration in searches for intermediate mass diboson resonances decaying to $ZV$, with $V=W,Z$. We discuss discovery strategies for triboson resonances with small modifications of existing hadronic searches.Comment: LaTeX 32 pages, 38 figures. v2: Figs. 6, 9 and 11 modified. v3: added VH search in hadronic final state. v4: final published versio