Cell Cycle Control by Xenopus p28^(Kix1) a Developmentally Regulated Inhibitor of Cyclin-dependent Kinases

We have isolated Xenopus p28^(Kix1), a member of the p21^(CIP1)/p27^(KIP1)/p57^(KIP2) family of cyclin-dependent kinase (Cdk) inhibitors. Members of this family negatively regulate cell cycle progression in mammalian cells by inhibiting the activities of Cdks. p28 shows significant sequence homology with p21, p27, and p57 in its N-terminal region, where the Cdk inhibition domain is known to reside. In contrast, the C-terminal domain of p28 is distinct from that of p21, p27, and p57. In co-immunoprecipitation experiments, p28 was found to be associated with Cdk2, cyclin E, and cyclin A, but not the Cdc2/cyclin B complex in Xenopus egg extracts. Xenopus p28 associates with the proliferating cell nuclear antigen, but with a substantially lower affinity than human p21. In kinase assays with recombinant Cdks, p28 inhibits pre-activated Cdk2/cyclin E and Cdk2/cyclin A, but not Cdc2/cyclin B. However, at high concentrations, p28 does prevent the activation of Cdc2/cyclin B by the Cdk-activating kinase. Consistent with the role of p28 as a Cdk inhibitor, recombinant p28 elicits an inhibition of both DNA replication and mitosis upon addition to egg extracts, indicating that it can regulate multiple cell cycle transitions. The level of p28 protein shows a dramatic developmental profile: it is low in Xenopus oocytes, eggs, and embryos up to stage 11, but increases approximately ~ 100-fold between stages 12 and 13, and remains high thereafter. The induction of p28 expression temporally coincides with late gastrulation. Thus, although p28 may play only a limited role during the early embryonic cleavages, it may function later in development to establish a somatic type of cell cycle. Taken together, our results indicate that Xenopus p28 is a new member of the p21/p27/p57 class of Cdk inhibitors, and that it may play a role in developmental processes

Identification of Colour Reconnection using Factorial Correlator

A new signal is proposed for the colour reconnection in the hadronic decay of W+ W- in e+e- collisions. Using Pythia Monte Carlo it is shown that this signal, being based on the factorial correlator, is more sensitive than the ones using only averaged quantities.Comment: 6 pages 1 postscript figur

One-side forward-backward asymmetry at the LHC

Forward-backward asymmetry $A_{\rm FB}$ is an essential observable to study the nature of coupling in the standard model and physics beyond the standard model, as shown at LEP and Tevatron. As a proton-proton collider, the LHC does not have the preferred direction contrary to her counterpart, namely, LEP and Tevatron. Therefore $A_{\rm FB}$ is not applicable at the LHC. However for the proton the momentum of valence quark is usually larger than that of the sea quark. Utilizing this feature we have defined a so-called one-side forward-backward asymmetry $A_{\rm OFB}$ for the top quark pair production at LHC in the previous work. In this paper we extend our studies to the charged leptons and bottom quarks as the final states. Our numerical results show that at the LHC $A_{\rm OFB}$ can be utilized to study the nature of the couplings once enough events are collected.Comment: 19 pages, 7 figures, 1 table, published versio

Strongly Correlated Topological Superconductors and Topological Phase Transitions via Green's Function

We propose several topological order parameters expressed in terms of Green's function at zero frequency for topological superconductors, which generalizes the previous work for interacting insulators. The coefficient in topological field theory is expressed in terms of zero frequency Green's function. We also study topological phase transition beyond noninteracting limit in this zero frequency Green's function approach.Comment: 10 pages. Published versio

Interacting topological phases and modular invariance

We discuss a (2+1) dimensional topological superconductor with $N_f$ left- and right-moving Majorana edge modes and a $\mathbb{Z}_2\times \mathbb{Z}_2$ symmetry. In the absence of interactions, these phases are distinguished by an integral topological invariant $N_f$. With interactions, the edge state in the case $N_f=8$ is unstable against interactions, and a $\mathbb{Z}_2\times \mathbb{Z}_2$ invariant mass gap can be generated dynamically. We show that this phenomenon is closely related to the modular invariance of type II superstring theory. More generally, we show that the global gravitational anomaly of the non-chiral Majorana edge states is the physical manifestation of the bulk topological superconductors classified by $\mathbb{Z}_8$.Comment: 11 page

Detecting the Majorana fermion surface state of 3^3He-B through spin relaxation

The concept of the Majorana fermion has been postulated more than eighty years ago; however, this elusive particle has never been observed in nature. The non-local character of the Majorana fermion can be useful for topological quantum computation. Recently, it has been shown that the 3He-B phase is a time-reversal invariant topological superfluid, with a single component of gapless Majorana fermion state localized on the surface. Such a Majorana surface state contains half the degrees of freedom of the single Dirac surface state recently observed in topological insulators. We show here that the Majorana surface state can be detected through an electron spin relaxation experiment. The Majorana nature of the surface state can be revealed though the striking angular dependence of the relaxation time on the magnetic field direction, $1/T_1 \propto sin^2 \theta$ where $\theta$ is the angle between the magnetic field and the surface normal. The temperature dependence of the spin relaxation rate can reveal the gapless linear dispersion of the Majorana surface state. We propose a spin relaxation experiment setup where we inject an electron inside a nano-sized bubble below the helium liquid surface.Comment: 6 pages, 2 figures; reformatted with reference adde

Topological superconductivity and Majorana fermions in half-metal / superconductor heterostructure

As a half-metal is spin-polarized at its Fermi level by definition, it was conventionally thought to have little proximity effect to an s-wave superconductor. Here we show that, with interface spin-orbit coupling, $p_x +ip_y$ superconductivity without spin degeneracy is induced on the half-metal, and we give an estimate of its bulk energy gap. Therefore a single-band half-metal can give us a topological superconductor with a single chiral Majorana edge state. Our band calculation shows that two atomic layers of VTe or CrO$_2$ is a single-band half-metal for a wide range ($\sim$0.1eV) of Fermi energy and thus is a suitable candidate material.Comment: 7 pages, 4 figure

Fluctuation-induced Topological Quantum Phase Transitions in Quantum Spin Hall and Quantum Anomalous Hall Insulators

We investigate the role of quantum fluctuations in topological quantum phase transitions of quantum spin Hall insulators and quantum anomalous Hall insulators. Employing the variational cluster approximation to obtain the single-particle Green's function of the interacting many-body system, we characterize different phases by direct calculation of the recently proposed topological order parameter for interacting systems. We pinpoint the influence of quantum fluctuations on the quantum spin Hall to Mott insulator transition in several models. Furthermore, we propose a general mechanism by which a topological quantum phase transition can be driven by the divergence of the self energy induced by interactions

A General Theorem Relating the Bulk Topological Number to Edge States in Two-dimensional Insulators

We prove a general theorem on the relation between the bulk topological quantum number and the edge states in two dimensional insulators. It is shown that whenever there is a topological order in bulk, characterized by a non-vanishing Chern number, even if it is defined for a non-conserved quantity such as spin in the case of the spin Hall effect, one can always infer the existence of gapless edge states under certain twisted boundary conditions that allow tunneling between edges. This relation is robust against disorder and interactions, and it provides a unified topological classification of both the quantum (charge) Hall effect and the quantum spin Hall effect. In addition, it reconciles the apparent conflict between the stability of bulk topological order and the instability of gapless edge states in systems with open boundaries (as known happening in the spin Hall case). The consequences of time reversal invariance for bulk topological order and edge state dynamics are further studied in the present framework.Comment: A mistake corrected in reference

Quantitative Test of SO(5) Symmetry in the Vortex State of Nd1.85Ce0.15CuO4Nd_{1.85}Ce_{0.15}CuO_4

By numerically solving models with competing superconducting and antiferromagnetic orders, we study the magnetic field dependence of the antiferromagnetic moment in both the weak and strong field regimes. Through a omparison with the neutron scattering results of Kang et al and Matsuura et al.on $Nd_{1.85}Ce_{0.15}CuO_4$, we conclude that this system is close to a SO(5) symmetric critical point. We also make a quantitative prediction on increasing the upper critical field $B_{c2}$ and the superconducting transition temperature $T_c$ by applying an in-plane magnetic field.Comment: 4 pages, 3 figures v3: final version PRL 92, 107002 (2004