5,858 research outputs found
The Formation of Population III Stars in Gas Accretion Stage: Effects of Magnetic Fields
The formation of Population III stars is investigated using resistive
magnetohydrodynamic simulations. Starting from a magnetized primordial
prestellar cloud, we calculate the cloud evolution several hundreds of years
after first protostar formation, resolving the protostellar radius. When the
natal minihalo field strength is weaker than B \lesssim 10^-13 (n/1 cm^-3)^-2/3
G (n is the hydrogen number density), magnetic effects can be ignored. In this
case, fragmentation occurs frequently and a stellar cluster forms, in which
stellar mergers and mass exchange between protostars contribute to the mass
growth of these protostars. During the early gas accretion phase, the most
massive protostar remains near the cloud centre, whereas some of the less
massive protostars are ejected. The magnetic field significantly affects
Population III star formation when B_amb \gtrsim 10^-12 (n/1 cm^-3)^-2/3 G. In
this case, because the angular momentum around the protostar is effectively
transferred by both magnetic braking and protostellar jets, the gas falls
directly onto the protostar without forming a disk, and only a single massive
star forms. In addition, a massive binary stellar system appears when B_amb
\sim 10^-12 (n/1cm^-3)^-2/3 G. Therefore, the magnetic field determines the end
result of the formation process (cluster, binary or single star) for Population
III stars. Moreover, no persistent circumstellar disk appears around the
protostar regardless of the magnetic field strength, which may influence the
further evolution of Population III stars.Comment: 59 pages, 21 figures, Accepted for publication in MNRAS. For high
resolution figures see
http://jupiter.geo.kyushu-u.ac.jp/machida/arxiv/PopIII
The lattice QCD simulation of the quark-gluon mixed condensate g<\bar{q} \sigma G q> at finite temperature and the phase transition of QCD
The thermal effects on the quark-gluon mixed condensate g<\bar{q} \sigma G
q>, which is another chiral order parameter, are studied using the SU(3)c
lattice QCD with the Kogut-Susskind fermion at the quenched level. We perform
the accurate measurement of the mixed condensate as well as the quark
condensate for 0MeV<=T<=500MeV. We observe the sharp decrease of both the
condensates around T_c \simeq 280MeV, while the thermal effects below T_c are
found to be weak. We also find that the ratio m_0^2 = g<\bar{q} \sigma G
q>/ is almost independent of the temperature even in the very
vicinity of T_c, which indicates that the two condensates have nontrivial
similarity in the chiral behaviors. We also present the correlation between the
condensates and the Polyakov loop to understand the vacuum structure of QCD.Comment: Talk given at the XXII International Symposium on Lattice Field
Theory (LATTICE 2004), Fermilab, Batavia, Illinois, USA, 21-26 June 2004,
Lattice2004(non-zero), 3 pages, 3 figure
CP Violations in Lepton Number Violation Processes and Neutrino Oscillations
We examine the constraints on the MNS lepton mixing matrix from the present
and future experimental data of the neutrino oscillation and lepton number
violation processes. We introduce a graphical representation of the CP
violation phases which appear in the lepton number violation processes such as
neutrinoless double beta decay, the conversion, and the K decay,
Using this graphical representation, we derive the
constraints on the CP violation phases in the lepton sector.Comment: 21pp, REVTeX, 9 Figure
Spin 3/2 Penta-quarks in anisotropic lattice QCD
A high-precision mass measurement for the pentaquark (5Q) Theta^+ in
J^P=3/2^{\pm} channel is performed in anisotropic quenched lattice QCD using a
large number of gauge configurations as N_{conf}=1000. We employ the standard
Wilson gauge action at beta=5.75 and the O(a) improved Wilson (clover) quark
action with kappa=0.1210(0.0010)0.1240 on a 12^3 \times 96 lattice with the
renormalized anisotropy as a_s/a_t = 4. The Rarita-Schwinger formalism is
adopted for the interpolating fields. Several types of the interpolating fields
with isospin I=0 are examined such as (a) the NK^*-type, (b) the
(color-)twisted NK^*-type, (c) a diquark-type. The chiral extrapolation leads
to only massive states, i.e., m_{5Q} \simeq 2.1-2.2 GeV in J^P=3/2^- channel,
and m_{5Q} = 2.4-2.6 GeV in J^P=3/2^+ channel. The analysis with the hybrid
boundary condition(HBC) is performed to investigate whether these states are
compact 5Q resonances or not. No low-lying compact 5Q resonance states are
found below 2.1GeV.Comment: 15 pages, 6 figures, 4 table
Survival of charmonia above Tc in anisotropic lattice QCD
We find a strong evidence for the survival of and as
spatially-localized (quasi-)bound states above the QCD critical
temperature , by investigating the boundary-condition dependence of their
energies and spectral functions. In a finite-volume box, there arises a
boundary-condition dependence for spatially spread states, while no such
dependence appears for spatially compact states. In lattice QCD, we find almost
{\it no} spatial boundary-condition dependence for the energy of the
system in and channels for . We also
investigate the spectral function of charmonia above in lattice QCD using
the maximum entropy method (MEM) in terms of the boundary-condition dependence.
There is {\it no} spatial boundary-condition dependence for the low-lying peaks
corresponding to and around 3GeV at . These facts
indicate the survival of and as compact
(quasi-)bound states for .Comment: 4 pages, 1 figur
Anisotropic Lattice QCD Studies of Penta-quark Anti-decuplet
Anti-decuplet penta-quark baryon is studied with the quenched anisotropic
lattice QCD for accurate measurement of the correlator. Both the positive and
negative parity states are studied using a non-NK type interpolating field with
I=0 and J=1/2. After the chiral extrapolation, the lowest positive parity state
is found at m_{Theta} \simeq 2.25 GeV, which is too massive to be identified
with the experimentally observed Theta^+(1540). The lowest negative parity
state is found at m_{Theta}\simeq 1.75 GeV, which is rather close to the
empirical value. To confirm that this state is a compact 5Q resonance, a new
method with ``hybrid boundary condition (HBC)'' is proposed. The HBC analysis
shows that the observed state in the negative parity channel is an NK
scattering state.Comment: A talk given at International Workshop PENTAQUARK04, July 20-23, 2004
at SPring-8, Japan, 8 pages, 7 figures, 2 table
Two Langevin equations in the Doi-Peliti formalism
A system-size expansion method is incorporated into the Doi-Peliti formalism
for stochastic chemical kinetics. The basic idea of the incorporation is to
introduce a new decomposition of unity associated with a so-called Cole-Hopf
transformation. This approach elucidates a relationship between two different
Langevin equations; one is associated with a coherent-state path-integral
expression and the other describes density fluctuations. A simple reaction
scheme is investigated as an illustrative example.Comment: 14page
Kinetically driven helix formation during the homopolymer collapse process
Using Langevin simulations, we find that simple 'generic' bead-and-spring
homopolymer chains in a sufficiently bad solvent spontaneously develop helical
order during the process of collapsing from an initially stretched
conformation. The helix formation is initiated by the unstable modes of the
straight chain, which drive the system towards a long-lived metastable
transient state. The effect is most pronounced if hydrodynamic interactions are
screened.Comment: 4 pages, 4 figure
On U_q(SU(2))-symmetric Driven Diffusion
We study analytically a model where particles with a hard-core repulsion
diffuse on a finite one-dimensional lattice with space-dependent, asymmetric
hopping rates. The system dynamics are given by the
\mbox{U[SU(2)]}-symmetric Hamiltonian of a generalized anisotropic
Heisenberg antiferromagnet. Exploiting this symmetry we derive exact
expressions for various correlation functions. We discuss the density profile
and the two-point function and compute the correlation length as well
as the correlation time . The dynamics of the density and the
correlations are shown to be governed by the energy gaps of a one-particle
system. For large systems and depend only on the asymmetry. For
small asymmetry one finds indicating a dynamical exponent
as for symmetric diffusion.Comment: 10 pages, LATE
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