221 research outputs found
Self-diffusion of reversibly aggregating spheres
Reversible diffusion limited cluster aggregation of hard spheres with rigid
bonds was simulated and the self diffusion coefficient was determined for
equilibrated systems. The effect of increasing attraction strength was
determined for systems at different volume fractions and different interaction
ranges. It was found that the slowing down of the diffusion coefficient due to
crowding is decoupled from that due to cluster formation. The diffusion
coefficient could be calculated from the cluster size distribution and became
zero only at infinite attraction strength when permanent gels are formed. It is
concluded that so-called attractive glasses are not formed at finite
interaction strength.Comment: 10 figure
Diffusion limited cluster aggregation with irreversible slippery bonds
Irreversible diffusion limited cluster aggregation (DLCA) of hard spheres was simulated using Brownian cluster dynamics. Bound spheres were allowed to move freely within a specified range, but no bond breaking was allowed. The structure and size distribution of the clusters was investigated before gelation. The pair correlation function and the static structure factor of the gels were determined as a function of the volume fraction and time. Slippery bonds led to local densification of the clusters and the gels, with a certain degree of order. At low volume fractions densification of the clusters occurred during their growth, but at higher volume fractions it occurred mainly after gelation. At very low volume fractions, the large-scale structure (fractal dimension), size distribution and growth kinetics of the clusters was found to be close to that known for DLCA with rigid bonds. Restructuring of the gels continued for long times, indicating that aging processes in systems with strong attraction do not necessarily involve bond breaking. The mean-square displacement of particles in the gels was determined. It is shown to be highly heterogeneous and to increase with decreasing volume fraction
Crystallization and dynamical arrest of attractive hard spheres
International audienceCrystallization of hard spheres interacting with a square well potential was investigated by numerical simulations using so-called Brownian cluster dynamics. The phase diagram was determined over a broad range of volume fractions. The crystallization rate was studied as a function of the interaction strength expressed in terms of the second virial coefficient. For volume fractions below about 0.3 the rate was found to increase abruptly with increasing attraction at the binodal of the metastable liquid-liquid phase separation. The rate increased until a maximum was reached after which it decreased with a power law dependence on the second virial coefficient. Above a critical percolation concentration, a transient system spanning network of connected particles was formed. Crystals were formed initially as part of the network, but eventually crystallization led to the breakup of the network. The lifetime of the transient gels increased very rapidly over a small range of interaction energies. Weak attraction destabilized the so-called repulsive crystals formed in pure hard sphere systems and shifted the coexistence line to higher volume fractions. Stronger attraction led to the formation of a denser, so-called attractive, crystalline phase. Nucleation of attractive crystals in the repulsive crystalline phase was observed close to the transition
An Automatic Invisible Axion In The SUSY Preon Model
It is shown that the recently proposed preon model which provides a unified
origin of the diverse mass scale and an explanation of family replication as
well as of inter-family mass-hierarchy, possesses a Peccei-Quinn symmetry whose
spontaneous breaking leads to an automatic invisible axion. Existence of the
PQ-symmetry is simply a consequence of supersymmetry and requirement of
minimality in the field-content and interactions, which propose that the
lagrangian should possess only those terms which are dictated by the gauge
principle and no others. In addition to the axion, the model also generates two
superlight Goldstone bosons and their superpartners which are cosmologically
safe.Comment: (TeX file) 16 Page
Signatures of Nucleon Disappearance in Large Underground Detectors
For neutrons bound inside nuclei, baryon instability can manifest itself as a
decay into undetectable particles (e.g., ), i.e.,
as a disappearance of a neutron from its nuclear state. If electric charge is
conserved, a similar disappearance is impossible for a proton. The existing
experimental lifetime limit for neutron disappearance is 4-7 orders of
magnitude lower than the lifetime limits with detectable nucleon decay products
in the final state [PDG2000]. In this paper we calculated the spectrum of
nuclear de-excitations that would result from the disappearance of a neutron or
two neutrons from C. We found that some de-excitation modes have
signatures that are advantageous for detection in the modern high-mass,
low-background, and low-threshold underground detectors, where neutron
disappearance would result in a characteristic sequence of time- and
space-correlated events. Thus, in the KamLAND detector [Kamland], a
time-correlated triple coincidence of a prompt signal, a captured neutron, and
a decay of the residual nucleus, all originating from the same
point in the detector, will be a unique signal of neutron disappearance
allowing searches for baryon instability with sensitivity 3-4 orders of
magnitude beyond the present experimental limits.Comment: 13 pages including 6 figures, revised version, to be published in
Phys.Rev.
Observing Nucleon Decay in Lead Perchlorate
Lead perchlorate, part of the OMNIS supernova neutrino detector, contains two
nuclei, 208Pb and 35Cl, that might be used to study nucleon decay. Both would
produce signatures that will make them especially useful for studying
less-well-studied neutron decay modes, e.g., those in which only neutrinos are
emitted.Comment: 6 pages, 2 figure
Phase separation and percolation of reversibly aggregating spheres with a square-well attraction potential
International audienceReversible aggregation of spheres is simulated using a novel method in which clusters of bound spheres diffuse collectively with a diffusion coefficient proportional to their radius. It is shown that the equilibrium state is the same as with other simulation techniques, but with the present method more realistic kinetics are obtained. The behaviour as a function of volume fraction and interaction strength was tested for two different attraction ranges. The binodal and the percolation threshold were determined. The cluster structure and size distribution close to the percolation threshold were found to be consistent with the percolation model. Close to the binodal phase separation occurred through the growth of spherical dense domains, while for deep quenches a system spanning network is formed that coarsens with a rate that decreases with increasing attraction. We found no indication for arrest of the coarsening
Radiative processes (tau -> mu gamma, mu -> e gamma and muon g-2) as probes of ESSM/SO(10)
The Extended Supersymmetric Standard Model (ESSM), motivated on several
grounds, introduces two vectorlike families (16 + 16-bar) of SO(10)) with
masses of order one TeV. It is noted that the successful predictions of prior
work on fermion masses and mixings, based on MSSM embedded in SO(10), can be
retained rather simply within the ESSM extension. These include an
understanding of the smallness of V_{cb} ~ 0.04 and the largeness of nu_mu -
nu_tau oscillation angle, sin^2 2 theta_{nu_mu nu_tau}^{osc} ~ 1. We analyze
the new contributions arising through the exchange of the vectorlike families
of ESSM to radiative processes including tau -> mu gamma, mu -> e gamma, b -> s
gamma, EDM of the muon and the muon (g-2). We show that ESSM makes significant
contributions especially to the decays tau -> mu gamma and mu -> e gamma and
simultaneously to muon (g-2). For a large and plausible range of relevant
parameters, we obtain: a_mu^{ESSM} ~ +(10-40) times 10^{-10}, with a correlated
prediction that tau -> mu gamma should be discovered with an improvement in its
current limit by a factor of 3-20. The implications for mu -> e gamma are very
similar. The muon EDM is within reach of the next generation experiments. Thus,
ESSM with heavy leptons being lighter than about 700 GeV (say) can be probed
effectively by radiative processes before a direct search for these vectorlike
leptons and quarks is feasible at the LHC.Comment: 27 pages LaTex, 2 figure
Unifying flipped SU(5) in five dimensions
It is shown that embedding a four-dimensional flipped SU(5) model in a
five-dimensional SO(10) model, preserves the best features of both flipped
SU(5) and SO(10). The missing partner mechanism, which naturally achieves both
doublet-triplet splitting and suppression of d=5 proton decay operators, is
realized as in flipped SU(5), while the gauge couplings are unified as in
SO(10). The masses of down quarks and charged leptons, which are independent in
flipped SU(5), are related by the SO(10). Distinctive patterns of quark and
lepton masses can result. The gaugino mass M_1 is independent of M_3 and M_2,
which are predicted to be equal.Comment: revised version-to appear in PRD, 23 pages, 3 figures, ReVTeX
Gauging U(1) symmetries and the number of right-handed neutrinos
In this letter we consider that assuming: a) that the only left-handed
neutral fermions are the active neutrinos, b) that is a gauge symmetry,
and c) that the assignment is restricted to the integer numbers, the
anomaly cancellation imply that at least three right-handed neutrinos must be
added to the minimal representation content of the electroweak standard model.
However, two types of models arise: i) the usual one where each of the three
identical right-handed neutrinos has total lepton number L=1; ii) and the other
one in which two of them carry L=4 while the third one carries .Comment: Published version in PLB forma
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