10,124 research outputs found
The Exchange Gate in Solid State Spin Quantum Computation: The Applicability of the Heisenberg Model
Solid state quantum computing proposals rely on adiabatic operations of the
exchange gate among localized spins in nanostructures. We study corrections to
the Heisenberg interaction between lateral semiconductor quantum dots in an
external magnetic field. Using exact diagonalization we obtain the regime of
validity of the adiabatic approximation. We also find qualitative corrections
to the Heisenberg model at high magnetic fields and in looped arrays of spins.
Looped geometries of localized spins generate flux dependent, multi-spin terms
which go beyond the basic Heisenberg model.Comment: 13 pages, 8 figure
Anomalous Multiplicity Fluctuations from Phase Transitions in Heavy Ion Collisions
Event-by-event fluctuations and correlations between particles produced in
relativistic nuclear collisions are studied. The fluctuations in positive,
negative, total and net charge are closely related through correlations. In the
event of a phase transitions to a quark-gluon plasma, fluctuations in total and
net charge can be enhanced and reduced respectively which, however, is very
sensitive to the acceptance and centrality. If the colliding system experiences
strong density fluctuations due, e.g., to droplet formation in a first-order
phase transition, all fluctuations can be enhanced substantially. The
importance of fluctuations and correlations is exemplified by event-by-event
measurement of the multiplicities of 's and charged particles since
these observables should anti-correlate in the presence of co-mover or
anomalous absorption.Comment: revised version to appear in Phys. Rev. C, 5 page
Radiative and Collisional Jet Energy Loss in a Quark-Gluon Plasma
We calculate radiative and collisional energy loss of hard partons traversing
the quark-gluon plasma created at RHIC and compare the respective size of these
contributions. We employ the AMY formalism for radiative energy loss and
include additionally energy loss by elastic collisions. Our treatment of both
processes is complete at leading order in the coupling, and accounts for the
probabilistic nature of jet energy loss. We find that a solution of the
Fokker-Planck equation for the probability density distributions of partons is
necessary for a complete calculation of the nuclear modification factor
for pion production in heavy ion collisions. It is found that the
magnitude of is sensitive to the inclusion of both collisional and
radiative energy loss, while the average energy is less affected by the
addition of collisional contributions. We present a calculation of for
at RHIC, combining our energy loss formalism with a relativistic
(3+1)-dimensional hydrodynamic description of the thermalized medium.Comment: 4 pages, 4 figures, contributed to Quark Matter 2008, Jaipur, Indi
Induced photon emission from quark jets in ultrarelativistic heavy-ion collisions
We study the induced photon bremsstrahlung from a fast quark produced in
AA-collisions due to multiple scattering in quark-gluon plasma. For RHIC and
LHC conditions the induced photon spectrum is sharply peaked at photon energy
close to the initial quark energy. In this region the contribution of the
induced radiation to the photon fragmentation function exceeds the ordinary
vacuum radiation. Contrary to previous analyses our results show that at RHIC
and LHC energies the final-state interaction effects in quark-gluon plasma do
not suppress the direct photon production, and even may enhance it at p_{T}
about 5-15 GeV.Comment: 11 pages, 4 figure
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