1,721,036 research outputs found
Space-time fluctuations and ultra-high energy cosmic ray interactions
No full textThe intimate geometry of space–time is expected to suffer stochastic fluctuations as a result of quantum gravitational
effects. These fluctuations may induce observable consequences on the propagation of high energy particles over large
distances, so that the strength and the characteristics of these fluctuations may be constrained, mainly in the range of
energies of interest for cosmic ray physics. While invoked as a possible explanation for the detection of the puzzling
cosmic rays with energies in excess of the threshold for photopion production (the so-called super-Greisen–Zatsepin–
Kuzmin particles), we demonstrate here that lower energy observations may provide strong constraints on the role of a
fluctuating space–time structure
A fluctuating energy-momentum may produce an unstable world
No full textQuantum gravitational effects may induce stochastic fluctuations in the structure of space–time, to produce a
characteristic foamy structure. It has been known for some time now that these fluctuations may have observable
consequencies for the propagation of cosmic ray particles over cosmological distances. We note here that the same
fluctuations, if they exist, imply that some decay reactions normally forbidden by elementary conservation laws, become
kinematically allowed, inducing the decay of particles that are seen to be stable in our universe. Due to the strength of
the prediction, we are led to consider this finding as the most severe constraint on the classes of models that may
describe the effects of gravity on the structure of space–time. We also propose and discuss several potential loopholes of
our approach, that may affect our conclusions. In particular, we try to identify the situations in which despite a
fluctuating energy–momentum of the particles, the reactions mentioned above may not take place
MICROCANONICAL FERMIONIC AVERAGE METHOD IN THE SCHWINGER MODEL - A REALISTIC COMPUTATION OF THE CHIRAL CONDENSATE
No full textThe microcanonical fermionic average method has been used so far in the context of lattice models
with phase transitions at finite coupling. To test its applicability to asymptotically free theories, we have implemented it in two-dimensional QED, i.e., the Schwinger model. We exploit the possibility,
intrinsic to this method, of studying the whole plane without extra computer cost, to follow
constant physics trajectories and measure the limit of the chiral condensate. We recover the
continuum result within three decimal places. Moreover, the possibility, intrinsic to the method, of
performing simulations directly in the chiral limit allows us to compute the average plaquette energy
at m = 0, the result being in perfect agreement with the expected value
Approaching space-time through velocity in doubly special relativity
No full textWe discuss the definition of velocity as dE=d|p|, where E, p are the energy and momentum of a particle, in doubly special relativity (DSR). If this definition matches dx/dt appropriate for the space-time
sector, then space-time can in principle be built consistently with the existence of an invariant length scale.
We show that, within different possible velocity definitions, a space-time compatible with momentum-space
DSR principles cannot be derived
THE MICROCANONICAL FERMIONIC AVERAGE METHOD FOR ASYMPTOTICALLY FREE THEORIES - A TEST IN THE SCHWINGER MODEL
No full textWe have applied the Microcanonical Fermionic Average method to QED_2, i.e. the Schwinger Model, to test its applicability to Asymptotically Free Theories. We present here the results of the simulations, compared to the continuum results. Since the M.F.A. method allows the study of the whole plane at very small computer cost, we are able to verify the scaling of the chiral condensate at high degree, and obtain the continuum result within 3 decimal places. We present also results for the plaquette energy
Three and two colours finite density QCD at strong coupling: A new look
No full textSimulations in finite density, lattice QCD by means of the Monomer–Dimer–Polymer
algorithm show a signal of first-order transition at finite temporal size. This behaviour agrees with
predictions of the mean field approximation, but is difficult to reconcile with infinite mass
analytical solution. The MDP simulations are considered in detail and severe convergence
problems are found for the SU(3) gauge group, in a wide region of chemical potential. Simulations of SU(2) model show discrepancies with MDP results as well
FERMIONIC EFFECTIVE ACTION AND THE PHASE-STRUCTURE OF NONCOMPACT QUANTUM ELECTRODYNAMICS IN 2+1 DIMENSIONS
No full textWe study the phase diagram of noncompact QED_3 using the microcanonical fermionic average method described elsewhere. We present evidence for a continuous phase transition line in the plane, extending down to an arbitrarily small flavour number N
Continuous chiral transition in strongly coupled compact QED with the standard torus topology
No full textWe analyze the phase diagram of compact QED on the torus with a chirally symmetric four fermion interaction. Inside a
mean field approximation for the four fermion term, a line of first order phase transitions and another one of second order
are found in the plane. Approaching the second order line a continuum limit can be defined. Critical exponents vary
along this line in a similar way as in the non-compact model, suggesting that a non-trivial interacting continuum theory can
be constructed
Chiral condensate, susceptibilities, critical coupling and indices in QED(4).
No full textWe measure chiral susceptibilities in the Coulomb phase of noncompact QED_4 in 8^4, 10^4 and 12^4 lattices. The MFA approach allows simulations in the chiral limit which are therefore free from arbitrary mass extrapolations. Using the critical couplings extracted from these calculations, we study the critical behaviour of the chiral condensate, which we find in disagreement with the predictions of logaritmically improved scalar Mean Field theory
Finite density fat QCD
No full textLattice formulation of finite baryon density QCD is problematic from the computer simulation point of view;
it is well known that for light quark masses the reconstructed partition function fails to be positive in a wide
region of parameter space. For a large bare quark mass, problems related to the phase of the determinant are
still present but restricted to a small region in the chemical potential . We present evidence for a transition
line that, starting from the temperature critical point at , moves towards a smaller with increasing
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