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Register a new userConstraints on Unparticle Interactions from Invisible Decays of Z, Quarkonia and Neutrinos
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Unparticles ($U$) interact weakly with particles. The direct signature of unparticles will be in the form of missing energy. We study constraints on unparticle interactions using totally invisible decay modes of $Z$, vector quarkonia $V$ and neutrinos. The constraints on the unparticle interaction scale $Lambda_U$ are very sensitive to the dimension $d_U$ of the unparticles. From invisible $Z$ and $V$ decays, we find that with $d_U$ close to 1 for vector $U$, the unparticle scale $Lambda_U$ can be more than $10^4$ TeV, and for $d_U$ around 2, the scale can be lower than one TeV. From invisible neutrino decays, we find that if $d_U$ is close to 3/2, the scale can be more than the Planck mass, but with $d_U$ around 2 the scale can be as low as a few hundred GeV. We also study the possibility of using $V (Z)to gamma + U$ to constrain unparticle interactions, and find that present data give weak constraints.
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We study unparticle effects on particle and antiparticle osillations in meson-antimeson, and muonium-antimuonium systems. Unlike usual tree level contributions to meson oscillations from heavy particle exchange with small $Gamma_{12}$, the unparticle may have sizeable contributions to both $M_{12}$ and $Gamma_{12}$ due to fractional dimension $d_U$ of the unparticle. We find that very stringent constraints on the unparticle and particle interactions can be obtained. If unparticle effect dominates the contributions (which may happen in $D^0-bar D^0$ mixing) to meson mixing parameters $x$ and $y$, we find that $x/y =cot(pi d_U)$. Interesting constraints on unparticle and particle interactions can also be obtained using muonion and antimuonion oscillation data. We also comment on unparticle effects on CP violation in meson oscillations.
We study the LHC sensitivity to probe a long-lived heavy neutrino $N$ in the context of $Z$ models. We focus on displaced vertex signatures of $N$ when pair produced via a $Z$, decaying to leptons and jets inside the inner trackers of the LHC experiments. We explore the LHC reach with current long-lived particle search strategies for either one or two displaced vertices in association with hadronic tracks or jets. We focus on two well-motivated models, namely, the minimal $U(1)_{B-L}$ scenario and its $U(1)_{X}$ extension. We find that searches for at least one displaced vertex can cover a significant portion of the parameter space, with light-heavy neutrino mixings as low as $|V_{lN}|^2approx 10^{-17}$, and $l=e,mu$ accessible across GeV scale heavy neutrino masses.
Invisible neutrino decay modes are difficult to target at laboratory experiments, and current bounds on such decays from solar neutrino and neutrino oscillation experiments are somewhat weak. It has been known for some time that Cosmology can serve as a powerful probe of invisible neutrino decays. In this work, we show that in order for Big Bang Nucleosynthesis to be successful, the invisible neutrino decay lifetime is bounded to be $tau_ u > 10^{-3},text{s}$ at 95% CL. We revisit Cosmic Microwave Background constraints on invisible neutrino decays, and by using Planck2018 observations we find the following bound on the neutrino lifetime: $tau_ u > (1.3-0.3)times 10^{9},text{s} , left({m_ u}/{ 0.05,text{eV} }right)^3$ at $95%$ CL. We show that this bound is robust to modifications of the cosmological model, in particular that it is independent of the presence of dark radiation. We find that lifetimes relevant for Supernova observations ($tau_ u sim 10^{5},text{s}, left({m_ u}/{ 0.05,text{eV} }right)^3$) are disfavoured at more than $5,sigma$ with respect to $Lambda$CDM given the latest Planck CMB observations. Finally, we show that when including high-$ell$ Planck polarization data, neutrino lifetimes $tau_ u = (2-16)times 10^{9},text{s} , left({m_ u}/{ 0.05,text{eV} }right)^3$ are mildly preferred -- with a 1-2 $sigma$ significance -- over neutrinos being stable.
Gamma-ray bursts (GRBs) are expected to provide a source of ultra high energy cosmic rays, accompanied with potentially detectable neutrinos at neutrino telescopes. Recently, IceCube has set an upper bound on this neutrino flux well below theoretical expectation. We investigate whether this mismatch between expectation and observation can be due to neutrino decay. We demosntrate the phenomenological consistency and theoretical plausibility of the neutrino decay hypothesis. A potential implication is the observability of majoron-emitting neutrinoless double beta decay.
Unparticles as suggested by Georgi are identities that are not constrained by dispersion relations but are governed by their scaling dimension, d. Their coupling to particles can result in macroscopic interactions between matter, that are generally an inverse nonintegral power of distance. This is totally different from known macroscopic forces. We use the precisely measured long-ranged spin-spin interaction of electrons to constrain unparticle couplings to the electron. For 1<d<1.5 the axial vector unparticle coupling is excluded; and for 1<d<1.3 the pseudoscalar and vector couplings are also ruled out. These bounds and the ones for other ranges of d exceed or are complementary to those obtained previously from exotic positronium decays.
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