216 research outputs found
Epistemic and Ontic Quantum Realities
Quantum theory has provoked intense discussions about its interpretation since its pioneer days. One of the few scientists who have been continuously engaged in this development from both physical and philosophical perspectives is Carl Friedrich von Weizsaecker. The questions he posed were and are inspiring for many, including the authors of this contribution. Weizsaecker developed Bohr's view of quantum theory as a theory of knowledge. We show that such an epistemic perspective can be consistently complemented by Einstein's ontically oriented position
The pion structure function and jet production in
Despite its theoretical and practical importance, the pion structure is still
badly constrained, particularly at low and in the sea-quark and gluon
sectors. Recently ZEUS have presented data on dijet photoproduction with a
leading neutron, which is dominated by slightly off-shell pion exchange and can
be used to constrain the pion densities down to . We
compare a recent NLO calculation to the ZEUS data and find that the lower gluon
densities of SMRS seem to be preferred by the data. Theoretical uncertainties,
in particular from the pion flux, are discussed in some detail.Comment: Talk presented at the Ringberg Workshop on ``New Trends in HERA
Physics 2001''. 12 pages, 10 postscript figure
SUSY signals at HERA in the no-scale flipped SU(5) supergravity model
Sparticle production and detection at HERA are studied within the recently
proposed no-scale flipped supergravity model. Among the various
reaction channels that could lead to sparticle production at HERA, only the
following are within its limit of sensitivity in this model: , where are the
two lightest neutralinos and is the lightest chargino. We study the
elastic and deep-inelastic contributions to the cross sections using the
Weizs\"acker-Williams approximation. We find that the most promising
supersymmetric production channel is right-handed selectron ()
plus first neutralino (), with one hard electron and missing energy
signature. The channel leads to comparable rates but also
allows jet final states. A right-handedly polarized electron beam at HERA would
shut off the latter channel and allow preferentially the former one. With an
integrated luminosity of {\cal L}=100\ipb, HERA can extend the present LEPI
lower bounds on by
\approx25\GeV, while {\cal L}=1000\ipb will make HERA competitive with
LEPII. We also show that the Leading Proton Spectrometer (LPS) at HERA is an
excellent supersymmetry detector which can provide indirect information about
the sparticle masses by measuring the leading proton longitudinal momentum
distribution.Comment: 11 pages, 8 figures (available upon request as uuencoded file or
separate ps files), tex (harvmac) CTP-TAMU-15/93, CERN/LAA/93-1
Stochastic motion of test particle implies that G varies with time
The aim of this letter is to propose a new description to the time varying
gravitational constant problem, which naturally implements the Dirac's large
numbers hypothesis in a new proposed holographic scenario for the origin of
gravity as an entropic force. We survey the effect of the Stochastic motion of
the test particle in Verlinde's scenario for gravity\cite{Verlinde}. Firstly we
show that we must get the equipartition values for which
leads to the usual Newtonian gravitational constant. Secondly,the stochastic
(Brownian) essence of the motion of the test particle, modifies the Newton's
2'nd law. The direct result is that the Newtonian constant has been time
dependence in resemblance as \cite{Running}.Comment: Accepted in International Journal of Theoretical Physic
Aspects of Two-Photon Physics at Linear e+e- Colliders
We discuss various reactions at future e+e- and gamma-gamma colliders
involving real (beamstrahlung or backscattered laser) or quasi--real
(bremsstrahlung) photons in the initial state and hadrons in the final state.
The production of two central jets with large pT is described in some detail;
we give distributions for the rapidity and pT of the jets as well as the
di--jet invariant mass, and discuss the relative importance of various initial
state configurations and the uncertainties in our predictions. We also present
results for `mono--jet' production where one jet goes down a beam pipe, for the
production of charm, bottom and top quarks, and for single production of W and
Z bosons. Where appropriate, the two--photon processes are compared with
annihilation reactions leading to similar final states. We also argue that the
behaviour of the total inelastic gamma-gamma cross section at high energies
will probably have little impact on the severity of background problems caused
by soft and semi--hard (`minijet') two--photon reactions. We find very large
differences in cross sections for all two--photon processes between existing
designs for future e+e- colliders, due to the different beamstrahlung spectra;
in particular, both designs with >1 events per bunch crossing exist.Comment: 51 pages, 13 figures(not included
Massive stars as thermonuclear reactors and their explosions following core collapse
Nuclear reactions transform atomic nuclei inside stars. This is the process
of stellar nucleosynthesis. The basic concepts of determining nuclear reaction
rates inside stars are reviewed. How stars manage to burn their fuel so slowly
most of the time are also considered. Stellar thermonuclear reactions involving
protons in hydrostatic burning are discussed first. Then I discuss triple alpha
reactions in the helium burning stage. Carbon and oxygen survive in red giant
stars because of the nuclear structure of oxygen and neon. Further nuclear
burning of carbon, neon, oxygen and silicon in quiescent conditions are
discussed next. In the subsequent core-collapse phase, neutronization due to
electron capture from the top of the Fermi sea in a degenerate core takes
place. The expected signal of neutrinos from a nearby supernova is calculated.
The supernova often explodes inside a dense circumstellar medium, which is
established due to the progenitor star losing its outermost envelope in a
stellar wind or mass transfer in a binary system. The nature of the
circumstellar medium and the ejecta of the supernova and their dynamics are
revealed by observations in the optical, IR, radio, and X-ray bands, and I
discuss some of these observations and their interpretations.Comment: To be published in " Principles and Perspectives in Cosmochemistry"
Lecture Notes on Kodai School on Synthesis of Elements in Stars; ed. by Aruna
Goswami & Eswar Reddy, Springer Verlag, 2009. Contains 21 figure
Resolved Photon Processes
We review the present level of knowledge of the hadronic structure of the
photon, as revealed in interactions involving quarks and gluons ``in" the
photon. The concept of photon structure functions is introduced in the
description of deep--inelastic scattering, and existing
parametrizations of the parton densities in the photon are reviewed. We then
turn to hard \gamp\ and \gaga\ collisions, where we treat the production of
jets, heavy quarks, hard (direct) photons, \jpsi\ mesons, and lepton pairs. We
also comment on issues that go beyond perturbation theory, including recent
attempts at a comprehensive description of both hard and soft \gamp\ and \gaga\
interactions. We conclude with a list of open problems.Comment: LaTeX with equation.sty, 85 pages, 29 figures (not included). A
complete PS file of the paper, including figures, can be obtained via
anonymous ftp from
ftp://phenom.physics.wisc.edu/pub/preprints/1995/madph-95-898.ps.
Measurement of D* Meson Cross Sections at HERA and Determination of the Gluon Density in the Proton using NLO QCD
With the H1 detector at the ep collider HERA, D* meson production cross
sections have been measured in deep inelastic scattering with four-momentum
transfers Q^2>2 GeV2 and in photoproduction at energies around W(gamma p)~ 88
GeV and 194 GeV. Next-to-Leading Order QCD calculations are found to describe
the differential cross sections within theoretical and experimental
uncertainties. Using these calculations, the NLO gluon momentum distribution in
the proton, x_g g(x_g), has been extracted in the momentum fraction range
7.5x10^{-4}< x_g <4x10^{-2} at average scales mu^2 =25 to 50 GeV2. The gluon
momentum fraction x_g has been obtained from the measured kinematics of the
scattered electron and the D* meson in the final state. The results compare
well with the gluon distribution obtained from the analysis of scaling
violations of the proton structure function F_2.Comment: 27 pages, 9 figures, 2 tables, submitted to Nucl. Phys.
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
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