Long-lived Stau Research Articles

Detecting hidden sector dark matter at HL-LHC and HE-LHC via long-lived stau decays

We investigate a class of models where the supergravity model with the standard model gauge group is extended by a hidden sector $U(1)_X$ gauge group and where the lightest supersymmetric particle is the neutralino in the hidden sector. We investigate this possibility in a class of models where the stau is the lightest supersymmetric particle in the MSSM sector and the next-to-lightest supersymmetric particle of the $U(1)_X$-extended SUGRA model. In this case the stau will decay into the neutralino of the hidden sector. For the case when the mass gap between the stau and the hidden sector neutralino is small and the mixing between the $U(1)_Y$ and $U(1)_X$ is also small, the stau can decay into the hidden sector neutralino and a tau which may be reconstructed as a displaced track coming from a high $p_T$ track of the charged stau. Simulations for this possibility are carried out for HL-LHC and HE-LHC. The discovery of such a displaced track from a stau will indicate the presence of hidden sector dark matter.

Long-lived stau, sneutrino dark matter and right-slepton spectrum

The minimal supersymmetric (SUSY) standard model (MSSM) augmented by right chiral sneutrinos may lead to one such sneutrino serving as the lightest supersymmetric particle and a non-thermal dark matter candidate, especially if neutrinos have Dirac masses only. In such cases, if the lightest MSSM particle is a stau, the signal of SUSY at the LHC consists in stable charged tracks which are distinguishable from backgrounds through their time delay between the inner tracker and the muon chamber. We show how to determine in such scenarios the mass hierarchy between the lightest neutralino and right sleptons of the first two families. The techniques of neutralino reconstruction, developed in earlier works, are combined with the endpoint of the variable MT 2 in smuon (selectron) decays for this purpose. We show that one can thus determine the mass hierarchy for smuons (selectrons) and neutralinos up to 1 TeV, to the level of 5-10%.

A solution to lithium problem by long-lived stau

We review a nonstandard Big-Bang Nucleosynthesis (BBN) scenario within the minimal supersymmetric standard model, and propose an idea to solve both the [Formula: see text] and [Formula: see text] problems. Each problem is a discrepancy between the predicted abundance in the standard BBN and observed one. We focus on the stau, a supersymmetric partner of tau lepton, which is a long-lived charged particle when it is the next lightest supersymmetric particle and is degenerate in mass with the lightest supersymmetric particle. The long-lived stau forms a bound state with a nucleus, and provides nonstandard nuclear reactions. One of those, the internal conversion process, accelerates the destruction of [Formula: see text] and [Formula: see text], and leads to a solution to the [Formula: see text] problem. On the other hand, the bound state of the stau and [Formula: see text] enhances productions of n, D, T and [Formula: see text]. The over-production of [Formula: see text] could solve the [Formula: see text] problem; while the over-productions of D and T could conflict with observations, and hence the relevant parameter space of the stau is strictly constrained. We therefore need to carefully investigate the stau–[Formula: see text] bound state to find a condition of solving the [Formula: see text] problem. The scenario of the long-lived stau simultaneously and successfully fits the abundances of light elements (D, T, [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]) and the neutralino dark matter to the observed ones. Consequently, the parameter space both of the stau and the neutralino is determined with excellent accuracy.

Displaced vertices and long-lived charged particles in the NMSSM with right-handed sneutrinos

We study LHC signatures of displaced vertices and long-lived charged particles within the context of the Next-to-Minimal Supersymmetric Standard Model with right-handed (RH) sneutrinos. In this construction the RH neutrino can be produced directly from Higgs decays or in association with a RH sneutrino when the latter is the lightest supersymmetric particle. The RH neutrino is generally long-lived, since its decay width is proportional to the neutrino Yukawa, a parameter which is predicted to be small. The RH neutrino late decay can therefore give rise to displaced vertices at the LHC, which can be identified through the decay products, which involve two leptons (2ℓ + T ) or a lepton with two jets (ℓjj). We simulate this signal for the current LHC configuration (a centre of mass of 8 TeV and an integrated luminosity of $ \mathcal{L} $ = 20 fb−1), and a future one (13 TeV and $ \mathcal{L} $ = 100 fb−1). We show that a region of the parameter space of this model can be probed and that the RH neutrino mass can be reconstructed from the end-point of the two-lepton invariant mass distribution or the central value of the mass distribution for two jets plus one lepton. Another exotic signature of this construction is the production of a long-lived stau. If the stau is the next-to-lightest supersymmetric particle, it can decay through diagrams involving the small neutrino Yukawa, and would escape the detector leaving a characteristic trail. We also simulate this signal for various benchmark points and show that the model can be within the reach of the future run of the LHC.

Multi-lepton signatures at LHC from sneutrino dark matter

We investigate multi-lepton LHC signals arising from an extension at the grand unification scale of the standard minimal supersymmetric model (MSSM) involving right-handed neutrino superfields. In this framework neutrinos have Dirac masses and the mixed sneutrinos are the lightest supersymmetric particles and hence the dark matter candidates. We analyze the model parameter space in which the sneutrino is a good dark matter particle and has a direct detection cross-section compatible with the LUX bound. Studying the supersymmetric mass spectrum of this region, we find several signatures relevant for LHC, which are distinct from the predictions of the MSSM with neutralino dark matter. For instance two opposite sign and different flavor leptons, three uncorrelated leptons and long-lived staus are the most representative. Simulating both the signal and expected background, we find that the multi-lepton signatures and the long-lived stau are in the reach of the future run of LHC with a luminosity of 100/fb. We point out that if one of these signatures is detected, it might be an indication of sneutrino dark matter.

A 119-125 GeV Higgs from a string derived slice of the CMSSM

The recent experimental hints for a relatively heavy Higgs with a mass in the range 119-125 GeV favour supersymmetric scenarios with a large mixing in the stop mass matrix. It has been shown that this is possible in the constrained Minimal Super-symmetric Standard Model (CMSSM), but only for a very specific relation between the trilinear parameter and the soft scalar mass, favouring A ≈ −2m for a relatively light spectrum, and sizable values of tan β. We describe here a string-derived scheme in which the first condition is automatic and the second arises as a consequence of imposing radiative EW symmetry breaking and viable neutralino dark matter in agreement with WMAP constraints. More specifically, we consider modulus dominated SUSY-breaking in Type II string compactifications and show that it leads to a very predictive CMSSM-like scheme, with small departures due to background fluxes. Imposing the above constraints leaves only one free parameter, which corresponds to an overall scale. We show that in this construction $ A = - 3/\sqrt {2} m \simeq - 2m $ and in the allowed parameter space tan β ≃ 38 − 41, leading to 119 GeV < m h < 125 GeV. The recent LHCb results on BR(B s → μ + μ −) further constrain this range, leaving only the region with m h ~ 125. GeV. We determine the detectability of this model and show that it could start being probed by the LHC at 7(8) TeV with a luminosity of 5(2) fb−1, and the whole parameter space would be accessible for 14 TeV and 25 fb−1. Furthermore, this scenario can host a long-lived stau with the right properties to lead to catalyzed BBN. We finally argue that anthropic arguments could favour the highest value for the Higgs mass that is compatible with neutralino dark matter, i.e., m h -125 GeV.

Big-bang nucleosynthesis with a long-lived charged massive particle includingHe4spallation processes

We propose helium-4 spallation processes induced by long-lived stau in supersymmetric standard models, and investigate an impact of the processes on light elements abundances. We show that, as long as the phase space of helium-4 spallation processes is open, they are more important than stau-catalyzed fusion and hence constrain the stau property.

Squark mass measurement in the long-lived stau scenario at the LHC

The long-lived stau scenario is an interesting possibility at the LHC. We study squark mass measurements in this scenario; in particular, we show that left- and right-handed squark masses are both measurable. In SUSY events, multiple jets are expected, which become a source of combinatorial backgrounds. In order to reduce such backgrounds a hemisphere analysis is applied, and we discuss mass measurements of squarks in decay modes of q˜L into qW˜0 or q′W˜± and also q˜R into qB˜.

Spin and chirality determination of superparticles with long-lived stau at the LHC

Long-lived stau shows up in various supersymmetric models, like gauge-mediated supersymmetry (SUSY) breaking model. At the LHC experiment, long-lived stau is useful not only for the discovery of SUSY signals but also for the study of the detailed properties of superparticles. We discuss a method to obtain information on spins and chiralities of superparticles in the framework with long-lived stau. We also show that such a study can be used to distinguish SUSY model from other models of new physics, like the universal extra dimension model.

Tau polarization measurements at the LHC in supersymmetric models with a long-lived stau

Supersymmetry (SUSY) with a long-lived stau is an attractive scenario in the LHC experiments because one can directly observe stau tracks in each SUSY event, and thus precise measurements of SUSY particle masses are possible. In this scenario, we discuss the possibility to observe/measure parity violation in interactions among SUSY particles. Such a measurement will be important in determining the spins and chiralities of SUSY particles. We use the last step of the cascade-decay chain: ${\ensuremath{\chi}}^{0}\ensuremath{\rightarrow}\stackrel{\texttildelow{}}{\ensuremath{\tau}}\ensuremath{\tau}\ensuremath{\rightarrow}\stackrel{\texttildelow{}}{\ensuremath{\tau}}(l\ensuremath{\nu}\overline{\ensuremath{\nu}})$, where the polarization of the tau lepton can be determined statistically by looking at the energy distribution of the final state lepton. Comparing with the theoretical formula of the neutralino differential decay width, one can extract the size of parity violation in the interaction vertices among the stau, the tau lepton, and the neutralino. We perform a Monte Carlo simulation to see if the effect is visible at the LHC experiments.

Measurement of the superparticle mass spectrum in the long-lived stau scenario at the LHC

In supersymmetric scenarios with a long-lived stau, the LHC experiments provide us with a great environment for precise mass measurements of superparticles. We study a case in which the mass differences between the lightest stau and other sleptons are ∼ 10 GeV or larger, so that the decay products of heavier sleptons are hard enough to be detected. We demonstrate that the masses of neutralinos, sleptons, and squarks can be measured with a good accuracy.

Thermal relic abundances of long-lived staus

In supersymmetric models with a long-lived stau being the lightest Standard Model superpartner, the stau abundance during primordial nucleosynthesis is tightly constrained. Considering the complete set of stau annihilation channels in the minimal supersymmetric Standard Model (MSSM) with real parameters for scenarios in which sparticle coannihilations are negligible, we calculate the decoupling of the lighter stau from the primordial plasma and identify processes which are capable to deplete the resulting stau abundance significantly. We find particularly efficient stau annihilation at the resonance of the heavy CP-even Higgs boson and for a lighter stau with a sizeable left–right mixing due to enhanced stau–Higgs couplings. Even within the constrained MSSM, we encounter both effects leading to exceptionally small values of the resulting stau abundance. Prospects for collider phenomenology are discussed and possible implications of our findings are addressed with emphasis on gravitino dark matter scenarios.

Big-bang nucleosynthesis and gravitinos

We derive big-bang nucleosynthesis (BBN) constraints on both unstable and stable gravitino taking account of recent progress in theoretical study of the BBN processes as well as observations of primordial light-element abundances. In the case of unstable gravitino, we set the upper limit on the reheating temperature assuming that the primordial gravitinos are mainly produced by the scattering processes of thermal particles. For stable gravitino, we consider B-ino, stau, and sneutrino as the next-to-the-lightest supersymmetric particle and obtain constraints on their properties. Compared with the previous works, we improved the following points: (i) we use the most recent observational data, (ii) for gravitino production, we include contribution of the longitudinal component, and (iii) for the case with unstable long-lived stau, we estimate the bound-state effect of stau accurately by solving the Boltzmann equation.

SUSY at the Pole

We study the role neutrino telescopes could play in discovering supersymmetric extensions of the Standard Model with a long-lived stau next-to-lightest superparticle. In such a setup, pairs of staus are produced by cosmic neutrino interactions in the Earth matter. In optimistic scenarios, one can expect several pair events per year in a cubic kilometre detector such as IceCube. We also show that no significant event rate can be expected for decays of staus stopped in the detector.

Lepton flavour violation in future linear colliders in the long-lived stau NLSP scenario

We analyze the prospects of observing lepton flavour violation in future e-e- and e+e- linear colliders in scenarios where the gravitino is the lightest supersymmetric particle, and the stau is the next-to-lightest supersymmetric particle. The signals consist of multilepton final states with two heavily ionizing charged tracks produced by the long-lived staus. The Standard Model backgrounds are very small and the supersymmetric backgrounds can be kept well under control by the use of suitable kinematical cuts. We discuss in particular the potential of the projected International Linear Collider to discover lepton flavour violation in this class of scenarios, and we compare the estimated sensitivity with the constraints stemming from the non-observation of rare decays.

A running spectral index in supersymmetric dark-matter models with quasistable charged particles

We show that charged particles decaying in the early Universe can induce a scale-dependent or running spectral index in the small-scale linear and nonlinear matter power spectrum and discuss examples of this effect in minimal supersymmetric models in which the lightest neutralino is a viable cold-dark-matter candidate. We find configurations in which the neutralino relic density is set by coannihilations with a long-lived stau and the late decay of staus partially suppresses the linear matter power spectrum. Nonlinear evolution on small scales then causes the modified linear power spectrum to evolve to a nonlinear power spectrum similar to (but different in detail) models parametrized by a constant running alphas = dns/dlnk by redshifts of 2 to 4. Thus, Lyman-alpha forest observations, which probe the matter power spectrum at these redshifts, might not discriminate between the two effects. However, a measurement of the angular power spectrum of primordial 21-cm radiation from redshift z[approximate]30–200 might distinguish between this charged-decay model and a primordial running spectral index. The direct production of a long-lived charged particle at future colliders is a dramatic prediction of this model.