38,524 research outputs found
The Fine-Tuning Problem of the Electroweak Symmetry Breaking Mechanism in Minimal SUSY Models
We calculate the region of the MSSM parameter space (i.e. , ,
, \ldots) compatible with a correct electroweak breaking and a realistic
top-quark mass. To do so we have included {\em all} the one-loop corrections to
the effective potential and checked their importance in order to obtain
consistent results. We also consider the fine-tuning problem due to the
enormous dependence of on (the top Yukawa coupling), which is
substantially reduced when the one-loop effects are taken into account. We also
explore the reliability of the so-called "standard" criterion to estimate the
degree of fine-tuning. As a consequence, we obtain a new set of upper bounds on
the MSSM parameters or, equivalently, on the supersymmetric masses perfectly
consistent with the present experimental bounds.Comment: talk given at the XVI Kazimierz Meeting on Elementary Particle
Physics, Kazimierz (Poland) 24-28 May 1993, 4 pages in standard LATEX + 2
figures (not included but available upon request), CERN-TH.7024/9
Quasi-cycles in a spatial predator-prey model
We show that spatial models of simple predator-prey interactions predict that
predator and prey numbers oscillate in time and space. These oscillations are
not seen in the deterministic versions of the models, but are due to stochastic
fluctuations about the time-independent solutions of the deterministic
equations which are amplified due to the existence of a resonance. We calculate
the power spectra of the fluctuations analytically and show that they agree
well with results obtained from stochastic simulations. This work extends the
analysis of these quasi-cycles from that previously developed for well-mixed
systems to spatial systems, and shows that the ideas and methods used for
non-spatial models naturally generalize to the spatial case.Comment: 18 pages, 4 figure
Geometrical approach to tumor growth
Tumor growth has a number of features in common with a physical process known
as molecular beam epitaxy. Both growth processes are characterized by the
constraint of growth development to the body border, and surface diffusion of
cells/particles at the growing edge. However, tumor growth implies an
approximate spherical symmetry that makes necessary a geometrical treatment of
the growth equations. The basic model was introduced in a former article [C.
Escudero, Phys. Rev. E 73, 020902(R) (2006)], and in the present work we extend
our analysis and try to shed light on the possible geometrical principles that
drive tumor growth. We present two-dimensional models that reproduce the
experimental observations, and analyse the unexplored three-dimensional case,
for which new conclusions on tumor growth are derived
SUSY Soft Breaking Terms from String Scenarios
The general SUSY soft breaking terms for a large class of phenomenologically
relevant string scenarios (symmetric orbifolds) are given. They show a certain
lack of universality, but not dangerous for flavor changing neutral currents.
To get more quantitative results a specific SUSY breaking mechanism has to be
considered, namely gaugino condensation in the hidden sector. Then, it turns
out that squark and slepton masses tend to be much larger than scalar masses
(), which probably is a quite general fact. Experimental
bounds and the requirement of a successful electroweak breaking without fine
tuning impose further restrictions on the soft breaking terms. As a consequence
the gluino and chargino masses should be quite close to their present
experimental limits, whereas squark and slepton masses should be much higher (>
1 TeV).Comment: (Talk presented at the SUSY-93 Conference, Boston, March 29 - April
2, 1993), 11 pages, CERN--TH.6922/9
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