24,060 research outputs found
Universality in snowflake aggregation
Aggregation of ice crystals is a key process governing precipitation. Individual ice crystals exhibit considerable diversity of shape, and a wide range of physical processes could influence their aggregation; despite this we show that a simple computer model captures key features of aggregate shape and size distribution reported recently from cirrus clouds. The results prompt a new way to plot the experimental size distributions leading to remarkably good dynamical scaling. That scaling independently confirms that there is a single dominant aggregation mechanism at play, albeit our model (based on undeflected trajectories to contact) does not capture its form exactly
Production of large transverse momentum dileptons and photons in , and collisions by photoproduction processes
The production of large dileptons and photons originating from
photoproduction processes in , and collisions is calculated. We
find that the contribution of dileptons and photons produced by photoproduction
processes is not prominent at RHIC energies. However, the numerical results
indicate that the modification of photoproduction processes becomes evident in
the large region for , and collisions at LHC energies.Comment: 10 figure
On the Application of Gluon to Heavy Quarkonium Fragmentation Functions
We analyze the uncertainties induced by different definitions of the momentum
fraction in the application of gluon to heavy quarkonium fragmentation
function. We numerically calculate the initial fragmentation
functions by using the non-covariant definitions of with finite gluon
momentum and find that these fragmentation functions have strong dependence on
the gluon momentum . As , these fragmentation
functions approach to the fragmentation function in the light-cone definition.
Our numerical results show that large uncertainties remains while the
non-covariant definitions of are employed in the application of the
fragmentation functions. We present for the first time the polarized gluon to
fragmentation functions, which are fitted by the scheme exploited in
this work.Comment: 11 pages, 7 figures;added reference for sec.
Derivation of the Lorentz Force Law, the Magnetic Field Concept and the Faraday-Lenz Law using an Invariant Formulation of the Lorentz Transformation
It is demonstrated how the right hand sides of the Lorentz Transformation
equations may be written, in a Lorentz invariant manner, as 4--vector scalar
products. This implies the existence of invariant length intervals analogous to
invariant proper time intervals. This formalism, making essential use of the
4-vector electromagnetic potential concept, provides a short derivation of the
Lorentz force law of classical electrodynamics, the conventional definition of
the magnetic field, in terms of spatial derivatives of the 4--vector potential
and the Faraday-Lenz Law. An important distinction between the physical
meanings of the space-time and energy-momentum 4--vectors is pointed out.Comment: 15 pages, no tables 1 figure. Revised and extended version of
physics/0307133 Some typos removed and minor text improvements in this
versio
Tuning Monte Carlo Generators: The Perugia Tunes
We present 9 new tunes of the pT-ordered shower and underlying-event model in
PYTHIA 6.4. These "Perugia" tunes update and supersede the older "S0" family.
The data sets used to constrain the models include hadronic Z0 decays at LEP,
Tevatron minimum-bias data at 630, 1800, and 1960 GeV, Tevatron Drell-Yan data
at 1800 and 1960 GeV, and SPS min-bias data at 200, 546, and 900 GeV. In
addition to the central parameter set, called "Perugia 0", we introduce a set
of 8 related "Perugia Variations" that attempt to systematically explore soft,
hard, parton density, and colour structure variations in the theoretical
parameters. Based on these variations, a best-guess prediction of the charged
track multiplicity in inelastic, nondiffractive minimum-bias events at the LHC
is made. Note that these tunes can only be used with PYTHIA 6, not with PYTHIA
8. Note: this report was updated in March 2011 with a new set of variations,
collectively labeled "Perugia 2011", that are optimized for matching
applications and which also take into account some lessons from the early LHC
data. In order not to break the original text, these are described separately
in Appendix B. Note 2: a subsequent "Perugia 2012" update is described in
Appendix C.Comment: 46 page
Jets associated with Z^0 boson production in heavy-ion collisions at the LHC
The heavy ion program at the LHC will present unprecedented opportunities to
probe hot QCD matter, that is, the quark gluon plasma (QGP). Among these
exciting new probes are high energy partons associated with the production of a
Z^0 boson, or Z^0 tagged jets. Once produced, Z^0 bosons are essentially
unaffected by the strongly interacting medium produced in heavy-ion collisions,
and therefore provide a powerful signal of the initial partonic energy and
subsequent medium induced partonic energy loss. When compared with theory,
experimental measurements of Z^0 tagged jets will help quantify the jet
quenching properties of the QGP and discriminate between different partonic
energy loss formalisms. In what follows, I discuss the advantages of tagged
jets over leading particles, and present preliminary results of the production
and suppression of Z^0 tagged jets in relativistic heavy-ion collisions at LHC
energies using the Guylassy-Levai-Vitev (GLV) partonic energy loss formalism.Comment: To appear in the proceedings of the 2010 Winter Workshop on Nuclear
Dynamics, which was held in Ocho Rios, Jamaica, mon
Enhancement of prompt photons in ultrarelativistic proton-proton collisions from nonlinear gluon evolution at small-
In this paper we estimate the influence of nonlinear gluon evolution in the
production of prompt photons at the LHC pp collider. We assume the validity of
collinear factorization and consider the EHKQS parton distributions, which are
solutions of the GLR-MQ evolution equations and describe quite well the DESY
HERA data, as input in our calculations. We find that both single and
double photon production are enhanced for low- photons and central
rapidities, while this effect is absent for the high- photons. The
implications of this effect for the Quark-Gluon Plasma searches and for the QCD
background to Higgs are also discussed.Comment: 4 pages, 4 figures. Version to be published in Physical Review
Detection of Coulomb Charging around an Antidot in the Quantum Hall Regime
We have detected oscillations of the charge around a potential hill (antidot)
in a two-dimensional electron gas as a function of a large magnetic field B.
The field confines electrons around the antidot in closed orbits, the areas of
which are quantised through the Aharonov-Bohm effect. Increasing B reduces each
state's area, pushing electrons closer to the centre, until enough charge
builds up for an electron to tunnel out. This is a new form of the Coulomb
blockade seen in electrostatically confined dots. Addition and excitation
spectra in DC bias confirm the Coulomb blockade of tunnelling.Comment: 4 pages, 4 Postscript figure
Shadowing Effects on the Nuclear Suppression Factor, R_dAu, in d+Au Interactions
We explore how nuclear modifications to the nucleon parton distributions
affect production of high transverse momentum hadrons in deuteron-nucleus
collisions. We calculate the charged hadron spectra to leading order using
standard fragmentation functions and shadowing parameterizations. We obtain the
d+Au to pp ratio both in minimum bias collisions and as a function of
centrality. The minimum bias results agree reasonably well with the BRAHMS data
while the calculated centrality dependence underestimates the data and is a
stronger function of p_T than the data indicate.Comment: 18 pages, 3 figures, final version, Phys. Rev. C in pres
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