Structure and Dynamics of the Quasi-Liquid Layer at the Surface of Ice from Molecular Simulations

We characterized the structural and dynamical properties of the quasi-liquid layer (QLL) at the surface of ice by molecular dynamics simulations with a thermodynamically consistent water model. Our simulations show that for three low-index ice surfaces only the outermost molecular layer presents short-range and mid-range disorder and is diffusive. The onset temperature for normal diffusion is much higher than the glass temperature of supercooled water, although the diffusivity of the QLL is higher than that of bulk water at the corresponding temperature. The underlying subsurface layers impose an ordered template, which produces a regular patterning of the ice/water interface at any temperature, and is responsible for the major differences between QLL and bulk water, especially for what concern the dynamics and the mid-range structure of the hydrogen-bonded network. Our work highlights the need of a holistic approach to the characterization of QLL, as a single experimental technique may probe only one specific feature, missing part of the complexity of this fascinating system.Comment: 6 Figure

Inelastic Dark Matter at the LHC Lifetime Frontier: ATLAS, CMS, LHCb, CODEX-b, FASER, and MATHUSLA

Visible signals from the decays of light long-lived hidden sector particles have been extensively searched for at beam dump, fixed-target, and collider experiments. If such hidden sectors couple to the Standard Model through mediators heavier than $\sim 10$ GeV, their production at low-energy accelerators is kinematically suppressed, leaving open significant pockets of viable parameter space. We investigate this scenario in models of inelastic dark matter, which give rise to visible signals at various existing and proposed LHC experiments, such as ATLAS, CMS, LHCb, CODEX-b, FASER, and MATHUSLA. These experiments can leverage the large center of mass energy of the LHC to produce GeV-scale dark matter from the decays of dark photons in the cosmologically motivated mass range of $\sim 1-100$ GeV. We also provide a detailed calculation of the radiative dark matter-nucleon/electron elastic scattering cross section, which is relevant for estimating rates at direct detection experiments.Comment: 21 pages, 9 figure

Exotic Decays Of A Heavy Neutral Higgs Through HZ/AZ Channel

Models of electroweak symmetry breaking with extended Higgs sectors are theoretically well motivated. In this study, we focus on the Two Higgs Doublet Model with a low energy spectrum containing scalars $H$ and a pseudoscalar $A$. We study the decays $A\rightarrow HZ$ or $H\rightarrow AZ$, which could reach sizable branching fractions in certain parameter regions. With detailed collider analysis, we obtain model independent exclusion bounds as well as discovery reach at the 14 TeV LHC for the process: $gg\rightarrow A/H\rightarrow HZ/AZ$, looking at final states bbll, \tau\tau ll and ZZZ(4l2j) for l =e,\mu. We further interpret these bounds in the context of the Type II Two Higgs Doublet Model, considering three different classes of processes: $A\rightarrow h^0Z$, $A\rightarrow H^0Z$, and $H^0 \rightarrow AZ$, in which $h^0$ and $H^0$ are the light and heavy CP-even Higgses respectively. For 100 fb$^{-1}$ integrated luminosity at the 14 TeV LHC, we find that for parent particle mass around 300-400 GeV, $A\rightarrow h^0Z$ has the greatest reach when $H^0$ is interpreted as the 126 GeV Higgs: most regions in the tan\beta-sin(\beta-\alpha) parameter space can be covered by exclusion and discovery. For 126 GeV $h^0$, only relatively small tan\beta<10 (5) can be reached by exclusion (discovery). For $A\rightarrow H^0Z$, the reach is typically restricted to sin$(\beta-\alpha)\sim\pm 1$ with tan\beta <10 in bbll and \tau\tau ll channels. The ZZZ (4l2j) channel, on the other hand, covers a wide range of 0.3<|sin(\beta-\alpha)|<1 for tan\beta<4. $H^0\rightarrow AZ$ typically favors negative values of sin(\beta-\alpha), with exclusion/discovery reach possibly extending to all values of tan\beta. A study of exotic decays of extra Higgses would extend the reach at the LHC and provides nice complementarity to conventional Higgs search channels.Comment: 32 pages, 21 figure

Light Charged Higgs Bosons to AW/HW via Top Decay

While current ATLAS and CMS measurements exclude a light charged Higgs ($m_{H^\pm}<160$ GeV) for most of the parameter region in the context of the MSSM scenarios, these bounds are significantly weakened in the Type II 2HDM once the exotic decay channel into a lighter neutral Higgs, $H^\pm \to AW/HW$, is open. In this study, we examine the possibility of a light charged Higgs produced in top decay via single top or top pair production, with the subsequent decay $H^\pm \rightarrow AW/HW$, which can reach a sizable branching fraction at low $\tan\beta$ once it is kinematically permitted. With a detailed collider analysis, we obtain exclusion and discovery bounds for the 14 TeV LHC assuming the existence of a 70 GeV neutral scalar. Assuming ${\rm BR}(H^\pm \rightarrow AW/HW)=100\%$ and ${\rm BR}(A/H \rightarrow \tau\tau)=8.6\%$, the 95% exclusion limits on ${\rm BR}(t \rightarrow H^+ b)$ are about 0.2% and 0.03% for single top and top pair production respectively, with an integrated luminosity of 300 ${\rm fb}^{-1}$. The discovery reaches are about 3 times higher. In the context of the Type II 2HDM, discovery is possible at both large $\tan\beta > 17$ for 155 GeV $< m_{H^\pm} <$ 165 GeV, and small $\tan\beta < 6$ over the entire mass range. Exclusion is possible in the entire $\tan\beta$ versus $m_{H^\pm}$ plane except for charged Higgs masses close to the top threshold. The exotic decay channel $H^\pm \to AW/HW$ is therefore complementary to the conventional $H^\pm \rightarrow \tau\nu$ channel.Comment: 21 pages. arXiv admin note: text overlap with arXiv:1408.411

Tagging single Tops

Top taggers which identify and reconstruct boosted top quarks have been established as novel tools for a multitude of LHC analyses. We show how single top production in association with a light-flavor or bottom jet can be observed in the boosted phase space regime. The full top reconstruction as part of the tagging algorithm allows us to define a distinctive kinematic angle which clearly separates different single top production processes.Comment: 13 pages, 9 figure