4,147 research outputs found
A quantum jump description for the non-Markovian dynamics of the spin-boson model
We derive a time-convolutionless master equation for the spin-boson model in
the weak coupling limit. The temporarily negative decay rates in the master
equation indicate short time memory effects in the dynamics which is explicitly
revealed when the dynamics is studied using the non-Markovian jump description.
The approach gives new insight into the memory effects influencing the spin
dynamics and demonstrates, how for the spin-boson model the the co-operative
action of different channels complicates the detection of memory effects in the
dynamics.Comment: 9 pages, 6 figures, submitted to Proceedings of CEWQO200
A remark on higher dimension induced domain wall defects in our world
There has been recent interest in new types of topological defects arising in
models with compact extra dimensions. We discuss in this context the old
statement that if only SU(N) gauge fields and adjoint matter live in the bulk,
and the coupling is weak, then the theory possesses a spontaneously broken
global Z(N) symmetry, with associated domain wall defects in four dimensions.
We discuss the behaviour of this symmetry at high temperatures. We argue that
the symmetry gets restored, so that cosmological domain wall production could
be used to constrain such models.Comment: 12 pages. Presentation clarified, references added; to appear in
Phys.Lett.
Finite-Size Scaling of Vector and Axial Current Correlators
Using quenched chiral perturbation theory, we compute the long-distance
behaviour of two-point functions of flavour non-singlet axial and vector
currents in a finite volume, for small quark masses, and at a fixed gauge-field
topology. We also present the corresponding predictions for the unquenched
theory at fixed topology. These results can in principle be used to measure the
low-energy constants of the chiral Lagrangian, from lattice simulations in
volumes much smaller than one pion Compton wavelength. We show that quenching
has a dramatic effect on the vector correlator, which is argued to vanish to
all orders, while the axial correlator appears to be a robust observable only
moderately sensitive to quenching.Comment: version to appear in NP
Finite temperature Z(N) phase transition with Kaluza-Klein gauge fields
If SU(N) gauge fields live in a world with a circular extra dimension,
coupling there only to adjointly charged matter, the system possesses a global
Z(N) symmetry. If the radius is small enough such that dimensional reduction
takes place, this symmetry is spontaneously broken. It turns out that its fate
at high temperatures is not easily decided with straightforward perturbation
theory. Utilising non-perturbative lattice simulations, we demonstrate here
that the symmetry does get restored at a certain temperature T_c, both for a
3+1 and a 4+1 dimensional world (the latter with a finite cutoff). To avoid a
cosmological domain wall problem, such models would thus be allowed only if the
reheating temperature after inflation is below T_c. We also comment on the
robustness of this phenomenon with respect to small modifications of the model.Comment: 18 pages. Revised version, to appear in Nucl.Phys.
The ultraviolet limit and sum rule for the shear correlator in hot Yang-Mills theory
We determine a next-to-leading order result for the correlator of the shear
stress operator in high-temperature Yang-Mills theory. The computation is
performed via an ultraviolet expansion, valid in the limit of small distances
or large momenta, and the result is used for writing operator product
expansions for the Euclidean momentum and coordinate space correlators as well
as for the Minkowskian spectral density. In addition, our results enable us to
confirm and refine a shear sum rule originally derived by Romatschke, Son and
Meyer.Comment: 16 pages, 2 figures. v2: small clarifications, one reference added,
published versio
Heavy quark medium polarization at next-to-leading order
We compute the imaginary part of the heavy quark contribution to the photon
polarization tensor, i.e. the quarkonium spectral function in the vector
channel, at next-to-leading order in thermal QCD. Matching our result, which is
valid sufficiently far away from the two-quark threshold, with a previously
determined resummed expression, which is valid close to the threshold, we
obtain a phenomenological estimate for the spectral function valid for all
non-zero energies. In particular, the new expression allows to fix the overall
normalization of the previous resummed one. Our result may be helpful for
lattice reconstructions of the spectral function (near the continuum limit),
which necessitate its high energy behaviour as input, and can in principle also
be compared with the dilepton production rate measured in heavy ion collision
experiments. In an appendix analogous results are given for the scalar channel.Comment: 43 pages. v2: a figure and other clarifications added, published
versio
States with a Democratic governor and women-headed public health agencies were more likely to implement Covid-19 stay-at-home orders earlier
Much of the US response to the COVID-19 pandemic has been the responsibility of state governments, with some issuing stay-at-home orders earlier than others, and some not issuing them at all. In new research which analyses the timing of state stay-at-home orders, Laine P. Shay finds that Democratic states were over 400 percent more likely to implement early stay-at-home orders, and almost 250 percent more likely if their state public health agency was headed by a woman
Term limits in state legislatures are linked to more powerful speakers
Fifteen US states have introduced some form of term limit on state legislators which prevents them from running for office for more than set number of times. While some past research has suggested that term limits can lead to weaker state legislatures, new research from Laine P. Shay suggests that they can empower the party in power’s Speaker. He finds that compared to states without term limits, a state with the most stringent term limit law is associated with an 11 percent increase in tools delegated to the Speaker’s position
Determination of the weak Hamiltonian in the SU(4) chiral limit through topological zero-mode wave functions
A new method to determine the low-energy couplings of the weak
Hamiltonian is presented. It relies on a matching of the topological poles in
of three-point correlators of two pseudoscalar densities and a
four-fermion operator, measured in lattice QCD, to the same observables
computed in the -regime of chiral perturbation theory. We test this
method in a theory with a light charm quark, i.e. with an SU(4) flavour
symmetry. Quenched numerical measurements are performed in a 2 fm box, and
chiral perturbation theory predictions are worked out up to next-to-leading
order. The matching of the two sides allows to determine the weak low-energy
couplings in the SU(4) limit. We compare the results with a previous
determination, based on three-point correlators containing two left-handed
currents, and discuss the merits and drawbacks of the two procedures.Comment: 38 pages, 9 figure
Weak low-energy couplings from topological zero-mode wavefunctions
We discuss a new method to determine the low-energy couplings of the weak Hamiltonian in the -regime. It relies on a matching of the
topological poles in of three-point functions of two pseudoscalar
densities and a four-fermion operator computed in lattice QCD, to the same
observables in the Chiral Effective Theory. We present the results of a NLO
computation in chiral perturbation theory of these correlation functions
together with some preliminary numerical results.Comment: 7 pages. Contribution to Lattice 200
- …