High Energy Physics - Phenomenology (hep-ph)

Abstract: We continue our examination of prospects for discovery of heavy Higgs bosons of natural SUSY (natSUSY) models at the high luminosity LHC (HL-LHC), this time focussing on charged Higgs bosons. In natSUSY, higgsinos are expected at the few hundred GeV scale whilst electroweak gauginos inhabit the TeV scale and the heavy Higgs bosons, H, A and H^\pm could range up tens of TeV without jeopardizing naturalness. For TeV-scale heavy SUSY Higgs bosons H, A and H^\pm, as currently required by LHC searches, SUSY decays into gaugino plus higgsino can dominate H^\pm decays provided these decays are kinematically accessible. The visible decay products of higgsinos are soft making them largely invisible, whilst the gauginos decay to W, Z or h plus missing transverse energy (MET). Charged Higgs bosons are dominantly produced at LHC14 via the parton subprocess, gb-> H^\pm t. In this paper, we examine the viability of observing signtures from H^\pm -> \tau\nu, H^\pm -> tb and H^\pm -> W, Z, h + MET events produced in association with a top quark at the HL-LHC over large Standard Model (SM) backgrounds from (mainly) t\bar{t}, t\bar{t}V and t\bar{t}h production (where V=W, Z). We find that the greatest reach is found via the SM H^\pm(-> \tau\nu) +t channel with a subdominant contribution from the H^\pm(-> tb) +t channel. Unlike for neutral Higgs searches, the SUSY decay modes appear to be unimportant for H^\pm searches at the HL-LHC. We delineate regions of the m_A vs. \tan\beta plane, mostly around m_A \sim 1-2 TeV, where signals from charged Higgs bosons would serve to confirm signals of a heavy, neutral Higgs boson at the 5\sigma level or, alternatively, to exclude heavy Higgs bosons at the 95% confidence level at the high luminosity LHC.

Abstract: The latest results from the long baseline neutrino experiments show a hint of non-zero CP violation in the neutrino sector. In this article, we study the CP violation effects in the upcoming long-baseline neutrino experiments DUNE and T2HK. Non-standard interactions can affect the cleaner determination of CP violation parameter. It has been argued that the NSI can help alleviate the tension between the recent $\delta_{CP}$ measurements of NO$\nu$A and T2K experiments. We consider here the dual NSI due to $\epsilon_{e\mu}$ and $\epsilon_{e\tau}$, arising simultaneously to see the effects in neutrino oscillation probabilities. Moreover, the CP asymmetry parameter $A_{CP}$ exhibits a clear distinction between normal and inverted mass orderings in the DUNE experiment.

Abstract: Pseudoscalar meson dominance has implications for nucleon structure which follow from an Extended Partial Conservation of the Axial Current (EPCAC). The minimal resonance saturation of the nucleon pseudoscalar form factor of the lowest pseudoscalar and isovector mesons compatible with pQCD short distance constraints and chiral symmetry. Using PDG tabulated pseudoscalarisovector masses and widths we obtain $g_{\pi NN} = 13.21(^{+0.11}_{-0.06})$, to be compared with the most precise determinations from $np, pp$ scattering, $g_{\pi^+ np} = 13.25(5)$ from the Granada-2013 database. Equivalently a Goldberger-Treiman discrepancy $\Delta_{\rm GT} = 1.8^{+0.9}_{-0.4}\%$ is found. Our results are consistent with almost flat strong pion-nucleon-nucleon vertices.

Abstract: Precision measurements of the possible coupling of spin to other scalars, vectors and pseudovectors has proven to be a sensitive way to search for new particle physics beyond the standard model. Indeed, in addition to searching for exotic spin-spin interactions, studies have been undertaken to look for couplings of spin to gravity, the relative velocity between particles, and preferred directions. Several laboratory experiments have established upper bounds on the energy associated with various fermion spin-orientations relative to Earth. Here, we combine these results with a model of Earth in order to investigate the possible long-range spin-velocity interactions associated with the exchange of ultralight ($m_{z'}<1$ neV) or massless scalar or vector bosons. We establish stringent bounds on the strength of these couplings between electrons, neutrons, protons and nucleons.

Abstract: The flavor-changing neutral current (FCNC) decays of charmed hadrons with missing energy ($\slashed E$) can serve as potentially promising hunting grounds for hints of new physics, as the standard-model backgrounds are very suppressed. A few of such processes have been searched for in recent experiments, particularly $D^0\to\slashed E$ by Belle and $D^0\to\pi^0\slashed E$ and $\Lambda_c^+\to p\slashed E$ by BESIII, resulting in upper bounds on their branching fractions. We consider them to illuminate the possible contributions of the quark transition $c\to u\slashed E$ with a couple of invisible spinless bosons carrying away the missing energy, assuming that they are not charge conjugates of each other and hence can have unequal masses. We find that these data are complementary in that they constrain different sets of the underlying operators and do not cover the same ranges of the bosons' masses, but there are regions not yet accessible. From the allowed parameter space, we show that other $D$-meson decays, such as $D\to\rho\slashed E$, and the charmed-baryon ones $\Xi_c\to(\Sigma,\Lambda)\slashed E$ can have sizable branching fractions and therefore may offer further probes of the new-physics interactions. We point out the importance of $D^0\to\gamma\slashed E$ which are not yet searched for but could access parts of the parameter space beyond the reach of the other modes. In addition, we look at a scenario where the invisibles are instead fermionic, namely sterile neutrinos, and a scalar leptoquark mediates $c\to u\slashed E$. We discuss the implications of the aforesaid bounds for this model. The predictions we make for the various charmed-hadron decays in the different scenarios may be testable in the near future by BESIII and Belle II.

Abstract: We propose to distinguish the nature of neutrino masses, Dirac vs Majorana, from the spectrum of gravitational waves generated. We study two simple models of Majorana and Dirac mass genesis motivated by generating small neutrino masses without assuming tiny Yukawa couplings. For Majorana neutrinos, spontaneous breaking of the gauged $B-L$ symmetry gives a cosmic string induced gravitational wave signal flat over a large range of frequencies, whereas for Dirac neutrinos, spontaneous and soft-breaking of a $Z_2$ symmetry generate a peaked gravitational wave spectrum from annihilation of domain walls. The striking difference between the shape of the spectra in the two cases can help differentiate between Dirac vs Majorana neutrino masses in the two class of models considered, complementing results of neutrinoless double beta decay experiments.