116,722 research outputs found
Nuclear matter incompressibility coefficient in relativistic and nonrelativistic microscopic models
We systematically analyze the recent claim that nonrelativistic and
relativistic mean field (RMF) based random phase approximation (RPA)
calculations for the centroid energy E_0 of the isoscalar giant monopole
resonance yield for the nuclear matter incompressibility coefficient, K_{nm},
values which differ by about 20%. For an appropriate comparison with the RMF
based RPA calculations, we obtain the parameters for the Skyrme force used in
the nonrelativistic model by adopting the same procedure as employed in the
determination of the NL3 parameter set of an effective Lagrangian used in the
RMF model. Our investigation suggest that the discrepancy between the values of
K_{nm} predicted by the relativistic and nonrelativistic models is
significantly less than 20%.Comment: Revtex file (13 pages), appearing in PRC-Rapid Com
Hadron production in heavy ion collisions: Fragmentation and recombination from a dense parton phase
We discuss hadron production in heavy ion collisions at RHIC. We argue that
hadrons at transverse momenta P_T < 5 GeV are formed by recombination of
partons from the dense parton phase created in central collisions at RHIC. We
provide a theoretical description of the recombination process for P_T > 2 GeV.
Below P_T = 2 GeV our results smoothly match a purely statistical description.
At high transverse momentum hadron production is well described in the language
of perturbative QCD by the fragmentation of partons. We give numerical results
for a variety of hadron spectra, ratios and nuclear suppression factors. We
also discuss the anisotropic flow v_2 and give results based on a flow in the
parton phase. Our results are consistent with the existence of a parton phase
at RHIC hadronizing at a temperature of 175 MeV and a radial flow velocity of
0.55c.Comment: 25 pages LaTeX, 18 figures; v2: some references updated; v3: some
typos fixe
Bohr's 1913 molecular model revisited
It is generally believed that the old quantum theory, as presented by Niels
Bohr in 1913, fails when applied to few electron systems, such as the H2
molecule. Here we find new solutions within the Bohr theory that describe the
potential energy curve for the lowest singlet and triplet states of H2 about as
well as the early wave mechanical treatment of Heitler and London. We also
develop a new interpolation scheme which substantially improves the agreement
with the exact ground state potential curve of H2 and provides a good
description of more complicated molecules such as LiH, Li2, BeH and He2.Comment: 6 pages, 9 figure
Ambiguities in statistical calculations of nuclear fragmentation
The concept of freeze out volume used in many statistical approaches for
disassembly of hot nuclei leads to ambiguities. The fragmentation pattern and
the momentum distribution (temperature) of the emanated fragments are
determined by the phase space at the freeze-out volume where the interaction
among the fragments is supposedly frozen out. However, to get coherence with
the experimental momentum distribution of the charged particles, one introduces
Coulomb acceleration beyond this freeze-out. To be consistent, we investigate
the effect of the attractive nuclear force beyond this volume and find that the
possible recombination of the fragments alters the physical observables
significantly casting doubt on the consistency of the statistical model.Comment: 11 pages+3 figure
Vacuum Squeezing in Atomic Media via Self-Rotation
When linearly polarized light propagates through a medium in which
elliptically polarized light would undergo self-rotation, squeezed vacuum can
appear in the orthogonal polarization. A simple relationship between
self-rotation and the degree of vacuum squeezing is developed. Taking into
account absorption, we find the optimum conditions for squeezing in any medium
that can produce self-rotation. We then find analytic expressions for the
amount of vacuum squeezing produced by an atomic vapor when light is
near-resonant with a transition between various low-angular-momentum states.
Finally, we consider a gas of multi-level Rb atoms, and analyze squeezing for
light tuned near the D-lines under realistic conditions.Comment: 10 pages, 6 figures; Submitted to PR
Direct Observation of Interband Spin-Orbit Coupling in a Two-Dimensional Electron System
We report the direct observation of interband spin-orbit (SO) coupling in a
two-dimensional (2D) surface electron system, in addition to the anticipated
Rashba spin splitting. Using angle-resolved photoemission experiments and
first-principles calculations on Bi/Ag/Au heterostructures we show that the
effect strongly modifies the dispersion as well as the orbital and spin
character of the 2D electronic states, thus giving rise to considerable
deviations from the Rashba model. The strength of the interband SO coupling is
tuned by the thickness of the thin film structures
Flipping SU(5) out of Trouble
Minimal supersymmetric SU(5) GUTs are being squeezed by the recent values of
alpha_s, sin^2 theta_W, the lower limit on the lifetime for p to nubar K decay,
and other experimental data. We show how the minimal flipped SU(5) GUT survives
these perils, accommodating the experimental values of alpha_s and sin^2
theta_W and other constraints, while yielding a p to e/mu+ pi0 lifetime beyond
the present experimental limit but potentially accessible to a further round of
experiments. We exemplify our analysis using a set of benchmark supersymmetric
scenarios proposed recently in a constrained MSSM framework.Comment: 12 pages LaTex, 3 eps figure
Detection of SUSY in the Stau-Neutralino Coannihilation Region at the LHC
We study the feasibility of detecting the stau neutralino
(stau_1-neutralino_1)coannihilation region at the LHC using tau leptons. The
signal is characterized by multiple low energy tau leptons from
neutralino_2-->tau stau_1-->tau tau neutralino_1 decays, where the stau_1 and
neutralino_1 mass difference (Delta M) is constrained to be 5-15 GeV by current
experimental bounds including the bound on the amount of neutralino cold dark
matter. Within the framework of minimal supergravity models, we show that if
hadronically decaying tau's can be identified with 50% efficiency for visible
pt >20 GeV the observation of such signals is possible in the final state of
two tau leptons plus large missing energy and two jets. With a gluino mass of
830 GeV the signal can be observed with as few as 3-10 fb^-1 of data (depending
on the size of Delta M). Using a mass measurement of the tau pairs with 10
fb^-1 we can determine dM with a statistical uncertainty of 12% for Delta M =
10 GeV and an additional systematic uncertainty of 14% if the gluino mass has
an uncertainty of 5%.Comment: 15 pages. 9 Figures, Latex, Typing error in the title as it appeared
in the web listing is corrected, paper is unchange
A new scenario for string unification
We present a new scenario for gauge coupling unification in flipped SU(5)
string models, which identifies the scale of SU(3) and SU(2)
unification with the empirical ~GeV scale, and the
scale of SU(5) and U(1) unification with the theoretical ~GeV string unification scale. The vacuum shift
necessary for the cancellation of the anomalous and an SU(4)
hidden sector with fractionally-charged particles, play a crucial role in the
dynamical determination of all intermediate mass scales in this scenario.Comment: 8 pages, LaTeX, 2 figures (uuencoded
Gravitational GUT Breaking and the GUT-Planck Hierarchy
It is shown that non-renormalizable gravitational interactions in the Higgs
sector of supersymmetric grand unified theories (GUT's) can produce the
breaking of the unifying gauge group at the GUT scale ~GeV. Such a breaking offers an attractive alternative to the
traditional method where the superheavy GUT scale mass parameters are added ad
hoc into the theory. The mechanism also offers a natural explanation for the
closeness of the GUT breaking scale to the Planck scale. A study of the minimal
SU(5) model endowed with this mechanism is presented and shown to be
phenomenologically viable. A second model is examined where the Higgs doublets
are kept naturally light as Goldstone modes. This latter model also achieves
breaking of at but cannot easily satisfy the current
experimental proton decay bound.Comment: 11 pages, REVTeX, 1 figure included as an uuencoded Z-compressed
PostScript file. Our Web page at
http://physics.tamu.edu/~urano/research/gutplanck.html contains ready to
print PostScript version (with figures) as well as color version of plot
- …