2,525 research outputs found
Stability of solitons in time-modulated two-dimensional lattices
We develop stability analysis for matter-wave solitons in a two-dimensional
(2D) Bose-Einstein condensate loaded in an optical lattice (OL), to which
periodic time modulation is applied, in different forms. The stability is
studied by dint of the variational approximation and systematic simulations.
For solitons in the semi-infinite gap, well-defined stability patterns are
produced under the action of the attractive nonlinearity, clearly exhibiting
the presence of resonance frequencies. The analysis is reported for several
time-modulation formats, including the case of in-phase modulations of both
quasi-1D sublattices, which build the 2D square-shaped OL, and setups with
asynchronous modulation of the sublattices. In particular, when the modulations
of two sublattices are phase-shifted by {\delta}={\pi}/2, the stability map is
not improved, as the originally well-structured stability pattern becomes fuzzy
and the stability at high modulation frequencies is considerably reduced. Mixed
results are obtained for anti-phase modulations of the sublattices
({\delta}={\pi}), where extended stability regions are found for low modulation
frequencies, but for high frequencies the stability is weakened. The analysis
is also performed in the case of the repulsive nonlinearity, for solitons in
the first finite bandgap. It is concluded that, even though stability regions
may be found, distinct stability boundaries for the gap solitons cannot be
identified clearly. Finally, the stability is also explored for vortex solitons
of both the "square-shaped" and "rhombic" types (i.e., off- and
on-site-centered ones).Comment: Nonlinear Dynamics, to be publishe
New constraints on light vectors coupled to anomalous currents
We derive new constraints on light vectors coupled to Standard Model (SM)
fermions, when the corresponding SM current is broken by the chiral anomaly.
Cancellation of the anomaly by heavy fermions results, in the low-energy
theory, in Wess-Zumino type interactions between the new vector and the SM
gauge bosons. These interactions are determined by the requirement that the
heavy sector preserves the SM gauge groups, and lead to (energy / vector
mass)^2 enhanced rates for processes involving the longitudinal mode of the new
vector. Taking the example of a vector coupled to baryon number, Z decays and
flavour changing neutral current meson decays via the new vector can occur with
(weak scale / vector mass)^2 enhanced rates. These processes place
significantly stronger coupling bounds than others considered in the
literature, over a wide range of vector masses.Comment: 6 pages, 1 figure; v2: corrected small numerical errors in Fig.
Parametric Resonance Production of Ultralight Vector Dark Matter
Vector bosons heavier than eV can be viable dark matter candidates
with distinctive experimental signatures. Ultralight dark matter generally
requires a non-thermal origin to achieve the observed density, while still
behaving like a pressureless fluid at late times. We show that such a
production mechanism naturally occurs for vectors whose mass originates from a
dark Higgs. If the dark Higgs has a large field value after inflation, the
energy in the Higgs field can be efficiently transferred to vectors through
parametric resonance. Computing the resulting abundance and spectra requires
careful treatment of the transverse and longitudinal components, whose dynamics
are governed by distinct differential equations. We study these equations in
detail and find that the mass of the vector may be as low as eV,
while making up the dominant dark matter abundance. This opens up a wide mass
range of vector dark matter as cosmologically viable, further motivating their
experimental search.Comment: discussion clarified, matches publication in PR
Light vectors coupled to bosonic currents
New spin-1 particles with masses below the weak scale are present in many
theories of beyond Standard Model (SM) physics. In this work, we extend
previous analyses by systematically considering the couplings of such a vector
to the bosonic sector of the SM, focusing on models that lead to mass-mixing
with the Z boson. These couplings generically lead to enhanced emission of the
vector's longitudinal mode, both in Higgs decays and in flavor changing meson
decays. We present bounds in the SM+X effective theory and investigate their
model-dependence. For the case of Higgs decays, we point out that tree-level
vector emission is, depending on the model, not always enhanced, affecting the
constraints. For meson decays, which are the dominant constraints at small
vector masses, we find that while decay constraints can be weakened by
fine-tuning UV parameters, it is generically difficult to suppress the
stringent constraints from kaon decays.Comment: 11 pages, 5 figure
Spontaneous symmetry breaking in a split potential box
We report results of the analysis of the spontaneous symmetry breaking (SSB)
in the basic (actually, simplest) model which is capable to produce the SSB
phenomenology in the one-dimensional setting. It is based on the
Gross-Pitaevskii - nonlinear Schroedinger equation with the cubic
self-attractive term and a double-well-potential built as an infinitely deep
potential box split by a narrow (delta-functional) barrier. The barrier's
strength, epsilon, is the single free parameter of the scaled form of the
model. It may be implemented in atomic Bose-Einstein condensates and nonlinear
optics. The SSB bifurcation of the symmetric ground state (GS) is predicted
analytically in two limit cases, viz., for deep or weak splitting of the
potential box by the barrier. For the generic case, a variational approximation
(VA) is elaborated. The analytical findings are presented along with systematic
numerical results. Stability of stationary states is studied through the
calculation of eigenvalues for small perturbations, and by means of direct
simulations. The GS always undergoes the SSB bifurcation of the supercritical
type, as predicted by the VA at moderate values of epsilon, although the VA
fails at small epsilon, due to inapplicability of the underlying ansatz in that
case. However, the latter case is correctly treated by the approximation based
on a soliton ansatz. On top of the GS, the first and second excited states are
studied too. The antisymmetric mode (the first excited state) is destabilized
at a critical value of its norm. The second excited state undergoes the SSB
bifurcation, like the GS, but, unlike it, the bifurcation produces an unstable
asymmetric mode. All unstable modes tend to spontaneously reshape into the
asymmetric GS.Comment: Physical Review E, in pres
Direct Detection Signals from Absorption of Fermionic Dark Matter
We present a new class of direct detection signals; absorption of fermionic
dark matter. We enumerate the operators through dimension six which lead to
fermionic absorption, study their direct detection prospects, and summarize
additional constraints on their suppression scale. Such dark matter is
inherently unstable as there is no symmetry which prevents dark matter decays.
Nevertheless, we show that fermionic dark matter absorption can be observed in
direct detection and neutrino experiments while ensuring consistency with the
observed dark matter abundance and required lifetime. For dark matter masses
well below the GeV scale, dedicated searches for these signals at current and
future experiments can probe orders of magnitude of unexplored parameter space.Comment: 7 pages, 2 figures. v2: published in PRL with minor revisions and
changes to Fig 2 (no change to results
Dark forces coupled to non-conserved currents
New light vectors with dimension-4 couplings to Standard Model states have
(energy / vector mass)^2 enhanced production rates unless the current they
couple to is conserved. These processes allow us to derive new constraints on
the couplings of such vectors, that are significantly stronger than the
previous literature for a wide variety of models. Examples include vectors with
axial couplings to quarks and vectors coupled to currents (such as baryon
number) that are only broken by the chiral anomaly. Our new limits arise from a
range of processes, including rare Z decays and flavor changing meson decays,
and rule out a number of phenomenologically-motivated proposals.Comment: 20 pages, 3 figures; v2: fixed small numerical errors in Fig 1 and
Fig
Holding spatial solitons in a pumped cavity with the help of nonlinear potentials
We introduce a one-dimensional model of a cavity with the Kerr nonlinearity
and saturated gain, designed so as to keep solitons in the state of shuttle
motion. The solitons are always unstable in the cavity bounded by the usual
potential barriers, due to accumulation of noise generated by the linear gain.
Complete stabilization of the shuttling soliton is achieved if the linear
barrier potentials are replaced by nonlinear ones, which trap the soliton,
being transparent to the radiation. The removal of the noise from the cavity is
additionally facilitated by an external ramp potential. The stable dynamical
regimes are found numerically, and their basic properties are explained
analytically.Comment: Optics Letters, to be publishe
Running Measurement Protocol for the Quantum First-detection problem
The problem of the detection statistics of a quantum walker has received
increasing interest, connected as it is to the problem of quantum search. We
investigate the effect of employing a moving detector, using a projective
measurement approach with fixed sampling time , with the detector moving
right before every detection attempt. For a tight-binding quantum walk on the
line, the moving detector allows one to target a specific range of group
velocities of the walker, qualitatively modifying the behavior of the quantum
first-detection probabilities. We map the problem to that of a stationary
detector with a modified unitary evolution operator and use established methods
for the solution of that problem to study the first-detection statistics for a
moving detector on a finite ring and on an infinite 1D lattice. On the line,
the system exhibits a dynamical phase transition at a critical value of ,
from a state where detection decreases exponentially in time and the total
detection is very small, to a state with power-law decay and a significantly
higher probability to detect the particle. The exponent describing the
power-law decay of the detection probability at this critical is 10/3,
as opposed to 3 for every larger . In addition, the moving detector
strongly modifies the Zeno effect
Spontaneous symmetry breaking of self-trapped and leaky modes in quasi-double-well potentials
We investigate competition between two phase transitions of the second kind
induced by the self-attractive nonlinearity, viz., self-trapping of the leaky
modes, and spontaneous symmetry breaking (SSB) of both fully trapped and leaky
states. We use a one-dimensional mean-field model, which combines the cubic
nonlinearity and a double-well-potential (DWP) structure with an elevated
floor, which supports leaky modes (quasi-bound states) in the linear limit. The
setting can be implemented in nonlinear optics and BEC. The order in which the
SSB and self-trapping transitions take place with the growth of the
nonlinearity strength depends on the height of the central barrier of the DWP:
the SSB happens first if the barrier is relatively high, while self-trapping
comes first if the barrier is lower. The SSB of the leaky modes is
characterized by specific asymmetry of their radiation tails, which, in
addition, feature a resonant dependence on the relation between the total size
of the system and radiation wavelength. As a result of the SSB, the instability
of symmetric modes initiates spontaneous Josephson oscillations. Collisions of
freely moving solitons with the DWP structure admit trapping of an incident
soliton into a state of persistent shuttle motion, due to emission of
radiation. The study is carried out numerically, and basic results are
explained by means of analytical considerations.Comment: Physical Review A, in pres
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