281 research outputs found
Mapping the phase diagram of strongly interacting matter
We employ a conformal mapping to explore the thermodynamics of strongly
interacting matter at finite values of the baryon chemical potential .
This method allows us to identify the singularity corresponding to the critical
point of a second-order phase transition at finite , given information
only at . The scheme is potentially useful for computing thermodynamic
properties of strongly interacting hot and dense matter in lattice gauge
theory. The technique is illustrated by an application to a chiral effective
model.Comment: 5 pages, 3 figures; published versio
Net-proton probability distribution in heavy ion collisions
We compute net-proton probability distributions in heavy ion collisions
within the hadron resonance gas model. The model results are compared with data
taken by the STAR Collaboration in Au-Au collisions at sqrt(s_{NN})= 200 GeV
for different centralities. We show that in peripheral Au-Au collisions the
measured distributions, and the resulting first four moments of net-proton
fluctuations, are consistent with results obtained from the hadron resonance
gas model. However, data taken in central Au-Au collisions differ from the
predictions of the model. The observed deviations can not be attributed to
uncertainties in model parameters. We discuss possible interpretations of the
observed deviations.Comment: 4 pages, 2 figure
Coevolution with bacteriophages drives genome-wide host evolution and constrains the acquisition of abiotic-beneficial mutations
This is the author accepted manuscript. The final version is available from OUP via the DOI in this record.Studies of antagonistic coevolution between hosts and parasites typically focus on resistance and infectivity traits. However, coevolution could also have genome-wide effects on the hosts due to pleiotropy, epistasis, or selection for evolvability. Here, we investigate these effects in the bacterium Pseudomonas fluorescens SBW25 during approximately 400 generations of evolution in the presence or absence of bacteriophage (coevolution or evolution treatments, respectively). Coevolution resulted in variable phage resistance, lower competitive fitness in the absence of phages, and greater genome-wide divergence both from the ancestor and between replicates, in part due to the evolution of increased mutation rates. Hosts from coevolution and evolution treatments had different suites of mutations. A high proportion of mutations observed in coevolved hosts were associated with a known phage target binding site, the lipopolysaccharide (LPS), and correlated with altered LPS length and phage resistance. Mutations in evolved bacteria were correlated with higher fitness in the absence of phages. However, the benefits of these growth-promoting mutations were completely lost when these bacteria were subsequently coevolved with phages, indicating that they were not beneficial in the presence of resistance mutations (consistent with negative epistasis). Our results show that in addition to affecting genome-wide evolution in loci not obviously linked to parasite resistance, coevolution can also constrain the acquisition of mutations beneficial for growth in the abiotic environment.This work was funded by European Research Council and NERC (UK)
High density QCD on a Lefschetz thimble?
It is sometimes speculated that the sign problem that afflicts many quantum
field theories might be reduced or even eliminated by choosing an alternative
domain of integration within a complexified extension of the path integral (in
the spirit of the stationary phase integration method). In this paper we start
to explore this possibility somewhat systematically. A first inspection reveals
the presence of many difficulties but - quite surprisingly - most of them have
an interesting solution. In particular, it is possible to regularize the
lattice theory on a Lefschetz thimble, where the imaginary part of the action
is constant and disappears from all observables. This regularization can be
justified in terms of symmetries and perturbation theory. Moreover, it is
possible to design a Monte Carlo algorithm that samples the configurations in
the thimble. This is done by simulating, effectively, a five dimensional
system. We describe the algorithm in detail and analyze its expected cost and
stability. Unfortunately, the measure term also produces a phase which is not
constant and it is currently very expensive to compute. This residual sign
problem is expected to be much milder, as the dominant part of the integral is
not affected, but we have still no convincing evidence of this. However, the
main goal of this paper is to introduce a new approach to the sign problem,
that seems to offer much room for improvements. An appealing feature of this
approach is its generality. It is illustrated first in the simple case of a
scalar field theory with chemical potential, and then extended to the more
challenging case of QCD at finite baryonic density.Comment: Misleading footnote 1 corrected: locality deserves better
investigations. Formula (31) corrected (we thank Giovanni Eruzzi for this
observation). Note different title in journal versio
The impact of intra-specific diversity in the rhizobia-legume symbiosis
Rhizobia - nitrogen-fixing, root-nodulating bacteria - play a critical role in both plant ecosystems and sustainable agriculture. Rhizobia form intracellular infections within legumes roots where they produce plant accessible nitrogen from atmospheric nitrogen and thus reduce the reliance on industrial inputs. The rhizobia-legume symbiosis is often treated as a pairwise relationship between single genotypes, both in research and in the production of rhizobial inoculants. However in nature individual plants are infected by a high diversity of rhizobia symbionts. How this diversity affects productivity within the symbiosis is unclear. Here, we use a powerful statistical approach to assess the impact of diversity within the Rhizobium leguminosarum - clover symbiosis using a biodiversity-ecosystem function framework. Statistically, we found no significant impact of rhizobium diversity. However this relationship was weakly positive - rather than negative - indicating that there is no significant cost to increasing inoculant diversity. Productivity was influenced by the identity of the strains within an inoculant; strains with the highest individual performance showed a significant positive contribution within mixed inoculants. Overall, inoculant effectiveness was best predicted by the individual performance of the best inoculant member, and only weakly predicted by the worst performing member. Collectively, our data suggest that the Rhizobium leguminosarum - clover symbiosis displays a weak diversity-function relationship, but that inoculant performance can be improved through the inclusion of high performing strains. Given the wide environmental dependence of rhizobial inoculant quality, multi-strain inoculants could be highly successful as they increase the likelihood of including a strain well adapted to local conditions across different environments
A mechanism for the Double-Spin Asymmetry in Electromagnetic Production at HERMES
We calculate the contribution of meson and pomeron exchanges to the
double-spin asymmetry in -meson electromagnetic production at HERMES
energies. We show that the observed double-spin asymmetries, which are large,
can be explained by the interference between the natural parity -secondary
Reggeon and the unnatural parity anomalous exchanges.Comment: 7 pages, 3 figures, Late
Net-charge probability distributions in heavy ion collisions at chemical freeze-out
We explore net charge probability distributions in heavy ion collisions
within the hadron resonance gas model. The distributions for strangeness,
electric charge and baryon number are derived. We show that, within this model,
net charge probability distributions and the resulting fluctuations can be
computed directly from the measured yields of charged and multi-charged
hadrons. The influence of multi-charged particles and quantum statistics on the
shape of the distribution is examined. We discuss the properties of the net
proton distribution along the chemical freeze-out line. The model results
presented here can be compared with data at RHIC energies and at the LHC to
possibly search for the relation between chemical freeze-out and QCD cross-over
lines in heavy ion collisions.Comment: 21 pages, 6 figure
The spectral function of the omega meson in nuclear matter from a coupled-channel resonance model
We calculate the spectral function of the omega meson in nuclear matter at
zero temperature by means of the low-density theorem. The omega N forward
scattering amplitude is calculated within a unitary coupled-channel effective
Lagrangian model that has been applied successfully to the combined analysis of
pion- and photon-induced reactions. While the peak of the omega spectral
distribution is shifted only slightly, we find a considerable broadening of the
omega meson due to resonance-hole excitations. For omega mesons at rest with
respect to the surrounding nuclear medium, we find an additional width of about
60 MeV at saturation density.Comment: 26 pages, 10 figures, added short discussio
Hydrodynamics near the QCD Phase Transition: Looking for the Longest-Lived Fireball
We propose a new strategy for the experimental search of the QCD phase
transition in heavy ion collisions: One may tune collision energy around the
point where the lifetime of the fireball is expected to be longest. We
demonstrate that the hydrodynamic evolution of excited nuclear matter does
change dramatically as the initial energy density goes through the "softest
point" (where the pressure to energy density ratio reaches its minimum). For
our choice of equation of state, this corresponds to epsilon_i approx. = 1.5
GeV/fm^3 and collision energy E_lab/A approx. = 30 GeV (for Au+Au). Various
observables seem to show distinct changes near the softest point.Comment: 7 pages, 3 Postscript figures (tar compressed and uuencoded)
submitte
Pion Cloud Contribution to K+ Nucleus Scattering
A careful reanalysis is done of the contribution to nucleus
scattering from the interaction of the kaon with the virtual pion cloud. The
usual approximations made in the evaluation of the related kaon selfenergy are
shown to fail badly. We also find new interaction mechanisms which provide
appreciable corrections to the kaon selfenergy. Some of these contribute to the
imaginary part below pion creation threshold. The inclusion of these new
mechanisms in the inelastic part of the optical potential produces a
significant improvement in the differential and total nuclear cross
sections. Uncertainties remain in the dispersive part of the optical potential.Comment: 27 pages, 17 figures (not all of them included, please request them),
report UG-DFM-2/9
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