4,941 research outputs found
Lattice study of the Coleman--Weinberg mass in the SU(2)-Higgs model
Radiative symmetry breaking is a well known phenomenon in perturbation
theory. We study the problem in a non-perturbative framework, i.e. lattice
simulations. The example of the bosonic sector of the SU(2)-Higgs model is
considered. We determine the minimal scalar mass which turns out to be higher
than the mass value given by 1-loop continuum perturbation theory.Comment: Contribution to ICHEP-02, Amsterdam, 24-31 July 2002, 2 pages, 1
figur
Lattice QCD at finite temperature and density
QCD at finite temperature and density is becoming increasingly important for
various experimental programmes, ranging from heavy ion physics to
astro-particle physics. The non-perturbative nature of non-abelian quantum
field theories at finite temperature leaves lattice QCD as the only tool by
which we may hope to come to reliable predictions from first principles. This
requires careful extrapolations to the thermodynamic, chiral and continuum
limits in order to eliminate systematic effects introduced by the
discretization procedure. After an introduction to lattice QCD at finite
temperature and density, its possibilities and current systematic limitations,
a review of present numerical results is given. In particular, plasma
properties such as the equation of state, screening masses, static quark free
energies and spectral functions are discussed, as well as the critical
temperature and the QCD phase structure at zero and finite density.Comment: 32 pages, typos corrected, reference added. Lectures given at 45.
Internationale Universitatswochen fur Theoretische Physik: (Schladming Winter
School on Theoretical Physics): Conceptual and Numerical Challenges in
Femto-Scale and Peta-Scale Physics, Schladming, Styria, Austria, 24 Feb - 3
Mar 200
Excited hadrons as a signal for quark-gluon plasma formation
At the quark-hadron transition, when quarks get confined to hadrons, certain
orbitally excited states, namely those which have excitation energies above the
respective states of the same order as the transition temperature
, may form easily because of thermal velocities of quarks at the
transition temperature. We propose that the ratio of multiplicities of such
excited states to the respective states can serve as an almost model
independent signal for the quark-gluon plasma formation in relativistic
heavy-ion collisions. For example, the ratio of multiplicities of
and when plotted
with respect to the center of mass energy of the collision (or vs.
centrality/number of participants), should show a jump at the value of
beyond which the QGP formation occurs. This should happen
irrespective of the shape of the overall plot of vs. . Recent
data from RHIC on vs. N for large values of
N may be indicative of such a behavior, though there are large error
bars. We give a list of several other such candidate hadronic states.Comment: 19 pages, RevTex, no figures, minor change
On World Problems. Facts and Implications - A Hungarian View
This volume of Hungarian studies was compiled as part of the UNESCO Major Programme I (Reflection on World Problems and Future-oriented Studies) within the framework of the contract concluded between UNESCO and IIASA. The subject of the contract was the preparation of a study entitled World Problems and their Perceptions. The study was compiled in coordination with the Hungarian National Member Organization of IIASA. The Report, in addition to the State of the Art part, contains information about the Task Force Meeting which UNESCO and IIASA organized in Budapest, February 1985, on a theme in accordance with the title of the report and the contributions of which will appear later in book form.
This Collaborative Paper is part of the background material of the Report. It shows a possible perception and interpretation of a group of problems, and the implications arising therefrom.
The Report sets out to show what concealed cultural or professional presuppositions there are behind the weight of the different conceptions of so-called global or world problems and how they differ in relation to each other. In this connection it reflects on the UNESCO Medium-Term Plan: "Whatever the approaches adopted, studies on world problems are inevitably linked, by the very nature of the field they cover -- which is none other than the present and future of societies -- to choices based on presuppositions regarding standards or on systems of values or even on ideological choices. They seem to require, as their necessary complement, an open-minded examination carried out in a context which promotes intellectual cooperation and within which all currents of thought can be freely expressed; and no organization seems to offer such a context more successfully than UNESCO."
The Report tries to justify the hypothesis formulated in UNESCO's Medium-Term Plan and make clear the necessary existence of differing cultural and professional world views in this respect. It shows that there is no one correct point of view, that many rational conceptions of equal value to each other exist, and that in relation to these problems only the acceptance of plural rationality can lead to common solutions. While this conception seems trivial, practice shows that in seeking solutions this approach is not unique and not at all self-explanatory. The Report and the Collaborative Paper show how the economic, social and cultural conditions, as well as practice, determine the world view, how wide the differences are, and that they necessarily lead to various problem formulations and action plans. In actual fact these outlined differences and knowledge of their explanation can lead the international organizations to the formulation of fuller and, probably, more successful action programmes than previously.
In the Collaborative Paper we offer a selection of Hungarian studies prepared recently in various specialized fields, we give an insight into the workshop debates and results of the Hungarian research teams, and last but not least, we try to show the options formed an different topics by Hungarian researchers.
The volume is not a homogeneous-topic selection, but at the same time we endeavored, by examining the facts, to concentrate on different features of the unequal development taking place globally and to indicate the interconnection between the studies. The authors of the studies examine, in a complex way, the world economic. and world political changes of the past years, the present state and tendencies of world factors and draw conclusions as to the expected developments.
Naturally, as with all selections, the content of our volume cannot cover the wide range of problems -- also formulated by UNESCO's programmes, even though we plan several selections during the programme period. With this initiative, while wishing to give a sample of the work and opinions of Hungarian researchers, we would also like to encourage other national research teams to make a similar statement of opinion, so that we can contribute directly to the dialogue in the range of topics of the world problems programme being discussed also in international organizations.
The range of studies closes with a summary, in which in addition to a summing up of the contents and conclusions of the studies, a sharper formulation is made of those questions to which we want particularly to draw attention at the present stage of the World Problems Programme, covering an extremely wide range of topics
Towards a controlled study of the QCD critical point
The phase diagram of QCD, as a function of temperature T and quark chemical
potential mu, may contain a critical point (mu_E,T_E) whose non-perturbative
nature makes it a natural object of lattice studies. However, the sign problem
prevents the application of standard Monte Carlo techniques at non-zero baryon
density. We have been pursuing an approach free of the sign problem, where the
chemical potential is taken as imaginary and the results are Taylor-expanded in
mu/T about mu=0, then analytically continued to real mu.
Within this approach we have determined the sensitivity of the critical
chemical potential mu_E to the quark mass, d(\mu_E)^2/dm_q|_{\mu_E=0}. Our
study indicates that the critical point moves to {\em smaller} chemical
potential as the quark mass {\em increases}. This finding, contrary to common
wisdom, implies that the deconfinement crossover, which takes place in QCD at
mu=0 when the temperature is raised, will remain a crossover in the mu-region
where our Taylor expansion can be trusted. If this result, obtained on a coarse
lattice, is confirmed by simulations on finer lattices now in progress, then we
predict that no {\em chiral} critical point will be found for mu_B \lesssim 500
MeV, unless the phase diagram contains additional transitions.Comment: 4 pages, 6 figures, proceedings of Quark Matter 2008, Jaipur (India),
Feb. 2008, to appear in J. Phys.
Variable - temperature scanning optical and force microscope
The implementation of a scanning microscope capable of working in confocal,
atomic force and apertureless near field configurations is presented. The
microscope is designed to operate in the temperature range 4 - 300 K, using
conventional helium flow cryostats. In AFM mode, the distance between the
sample and an etched tungsten tip is controlled by a self - sensing
piezoelectric tuning fork. The vertical position of both the AFM head and
microscope objective can be accurately controlled using piezoelectric coarse
approach motors. The scanning is performed using a compact XYZ stage, while the
AFM and optical head are kept fixed, allowing scanning probe and optical
measurements to be acquired simultaneously and in concert. The free optical
axis of the microscope enables both reflection and transmission experiments to
be performed.Comment: 24 pages, 9 figures, submitted to the journal "Review of Scientific
Instruments
Four-dimensional lattice results on the MSSM electroweak phase transition
We present the results of our large scale 4-dimensional (4d) lattice
simulations for the MSSM electroweak phase transition (EWPT). We carried out
infinite volume and continuum limit extrapolations and found a transition whose
strength agrees well with perturbation theory. We determined the properties of
the bubble wall that are important for a successful baryogenesis.Comment: 5 pages, 3figures. Talk presented at Johns Hopkins Workshop on
Nonperturbative Quantum Field Theory Methods and their Applications (19-21
August 2000.
EoS of finite density QCD with Wilson fermions by Multi-Parameter Reweighting and Taylor expansion
The equation of state (EoS), quark number density and susceptibility at
nonzero quark chemical potential are studied in lattice QCD simulations
with a clover-improved Wilson fermion of 2-flavors and RG-improved gauge
action. To access nonzero , we employ two methods : a multi-parameter
reweighting (MPR) in and and Taylor expansion in . The use
of a reduction formula for the Wilson fermion determinant enables to study the
reweighting factor in MPR explicitly and heigher-order coefficients in Taylor
expansion free from errors of noise method, although calculations are limited
to small lattice size. As a consequence, we can study the reliability of the
thermodynamical quantities through the consistency of the two methods, each of
which has different origin of the application limit.
The thermodynamical quantities are obtained from simulations on a lattice with an intermediate quark mass(. The MPR
and Taylor expansion are consistent for the EoS and number density up to
and for the number susceptibility up to . This
implies within a given statistics that the overlap problem for the MPR and
truncation error for the Taylor expansion method are negligible in these
regions.
In order to make MPR methods work, the fluctuation of the reweighting factor
should be small. We derive the equation of the reweighting line where the
fluctuation is small, and show that the equation of the reweighting line is
consistent with the fluctuation minimum condition.Comment: 20 pages, 11 figures. Accepted to JHEP. Discussions are added.
Figures for Taylor coefficients (Fig. 7) are modifie
Modeling the dynamics of a tracer particle in an elastic active gel
The internal dynamics of active gels, both in artificial (in-vitro) model
systems and inside the cytoskeleton of living cells, has been extensively
studied by experiments of recent years. These dynamics are probed using tracer
particles embedded in the network of biopolymers together with molecular
motors, and distinct non-thermal behavior is observed. We present a theoretical
model of the dynamics of a trapped active particle, which allows us to quantify
the deviations from equilibrium behavior, using both analytic and numerical
calculations. We map the different regimes of dynamics in this system, and
highlight the different manifestations of activity: breakdown of the virial
theorem and equipartition, different elasticity-dependent "effective
temperatures" and distinct non-Gaussian distributions. Our results shed light
on puzzling observations in active gel experiments, and provide physical
interpretation of existing observations, as well as predictions for future
studies.Comment: 11 pages, 6 figure
Electroweak Phase Transition in the MSSM: 4-Dimensional Lattice Simulations
Recent lattice results have shown that there is no Standard Model (SM)
electroweak phase transition (EWPT) for Higgs boson masses above \approx 72
GeV, which is below the present experimental limit. According to perturbation
theory and 3-dimensional (3d) lattice simulations there could be an EWPT in the
Minimal Supersymmetric Standard Model (MSSM) that is strong enough for
baryogenesis up to m_h \approx 105 GeV. In this letter we present the results
of our large scale 4-dimensional (4d) lattice simulations for the MSSM EWPT. We
carried out infinite volume and continuum limits and found a transition whose
strength agrees well with perturbation theory, allowing MSSM electroweak
baryogenesis at least up to m_h = 103 \pm 4 GeV. We determined the properties
of the bubble wall that are important for a successful baryogenesis.Comment: 4 pages, 4 figures included; lightest Higgs mass bound relaxed
(abstract, fig. 3 changed), version to appear in Phys. Rev. Letter
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