184 research outputs found
Radio emission from cosmic ray air showers: simulation results and parametrization
We have developed a sophisticated model of the radio emission from extensive
air showers in the scheme of coherent geosynchrotron radiation, providing a
theoretical foundation for the interpretation of experimental data from current
and future experiments. Having verified the model through comparison of
analytic calculations, Monte Carlo simulations and historical experimental
data, we now present the results of extensive simulations performed with our
Monte Carlo code. Important results are the absence of significant asymmetries
in the total field strength emission pattern, the spectral dependence of the
radiation, the polarization characteristics of the emission (allowing an
unambiguous test of the geomagnetic emission mechanism), and the dependence of
the radio emission on important air shower and observer parameters such as the
shower zenith angle, the primary particle energy, the depth of the shower
maximum and the observer position. An analytic parametrization incorporating
the aforementioned dependences summarizes our results in a particularly useful
way.Comment: 33 pages, 24 figures, final version as accepted for publication by
Astropart. Physics, only minor updates since V
Modeling the Pion and Kaon Form Factors in the Timelike Region
New, accurate measurements of the pion and kaon electromagnetic form factors
are expected in the near future from experiments at electron-positron
colliders,using the radiative return method. We construct a model for the
timelike pion electromagnetic form factor, that is valid also at momentum
transfers far above the resonance. The ansatz is based on vector
dominance and includes a pattern of radial excitations expected from dual
resonance models.The form factor is fitted to the existing data in the timelike
region, continued to the spacelike region and compared with the measurements
there and with the QCD predictions. Furthermore, the model is extended to the
kaon electromagnetic form factor. Using isospin and SU(3)-flavour symmetry
relations we extract the isospin-one contribution and predict the kaon weak
form factor accessible in semileptonic decays.Comment: 31 pages, 7 figures,latex, one reference changed, version to appear
in Eur.Phys.J
Timelike form factors at high energy
The difference between the timelike and spacelike meson form factors is
analysed in the framework of perturbative QCD with Sudakov effects included. It
is found that integrable singularities appear but that the asymptotic behavior
is the same in the timelike and spacelike regions. The approach to asymptotia
is quite slow and a rather constant enhancement of the timelike value is
expected at measurable large . This is in agreement with the trend
shown by experimental data.Comment: 17 pages, report DAPNIA/SPhN 94 0
Gluon Shadowing in DIS off Nuclei
Within a light-cone quantum-chromodynamics dipole formalism based on the
Green function technique, we study nuclear shadowing in deep-inelastic
scattering at small Bjorken xB < 0.01. Such a formalism incorporates naturally
color transparency and coherence length effects. Calculations of the nuclear
shadowing for the \bar{q}q Fock component of the photon are based on an exact
numerical solution of the evolution equation for the Green function, using a
realistic form of the dipole cross section and nuclear density function. Such
an exact numerical solution is unavoidable for xB > 0.0001, when a variation of
the transverse size of the \bar{q}q Fock component must be taken into account.
The eikonal approximation, used so far in most other models, can be applied
only at high energies, when xB < 0.0001 and the transverse size of the \bar{q}q
Fock component is "frozen" during propagation through the nuclear matter. At xB
< 0.01 we find quite a large contribution of gluon suppression to nuclear
shadowing, as a shadowing correction for the higher Fock states containing
gluons. Numerical results for nuclear shadowing are compared with the available
data from the E665 and NMC collaborations. Nuclear shadowing is also predicted
at very small xB corresponding to LHC kinematical range. Finally the model
predictions are compared and discussed with the results obtained from other
models.Comment: 29 pages including 7 figures; Fig.7 modified, some references and
corresponding discussion adde
Nonperturbative \gamma^* p Interaction in the Diffractive Regime
One of the challenging aspects of electroproduction at high-energy is the
understanding of the transition from real photons to virtual photons in the
GeV^2 region. We study inclusive electroproduction on the proton at small x_B
using a nonperturbative dipole-proton cross section calculated from the gauge
invariant gluon field correlators as input. By quark-hadron duality, we
construct a photon light cone wave function which links the ``hadronic''
behavior at small Q^2 to the ``perturbative'' behavior at large Q^2. It
contains quark masses which implement the transition from constituent quarks at
low Q^2 to current quarks at high Q^2. Our calculation gives a good description
of the structure function at fixed energy for Q^2<=10 GeV^2. Indications for a
chiral transition may already have been seen in the photon-proton cross
section.Comment: 28 pages, LaTeX, eps
QCD Form Factors and Hadron Helicity Non-Conservation
Recent data for the ratio shocked the
community by disobeying expectations held for 50 years. We examine the status
of perturbative QCD predictions for helicity-flip form factors. Contrary to
common belief, we find there is no rule of hadron helicity conservation for
form factors. Instead the analysis yields an inequality that the leading power
of helicity-flip processes may equal or exceed the power of helicity conserving
processes. Numerical calculations support the rule, and extend the result to
the regime of laboratory momentum transfer . Quark orbital angular
momentum, an important feature of the helicity flip processes, may play a role
in all form factors at large , depending on the quark wave functions.Comment: 25 pages, 5 figure
Radio emission of highly inclined cosmic ray air showers measured with LOPES
LOPES (LOFAR Prototype Station) is an array of dipole antennas used for
detection of radio emission from air showers. It is co-located and triggered by
the KASCADE (Karlsruhe Shower Core and Array Detector) experiment, which also
provides informations about air shower properties. Even though neither LOPES
nor KASCADE are completely optimized for the detection of highly inclined
events, a significant number of showers with zenith angle larger than 50
have been detected in the radio domain, and many with very high field
strengths. Investigation of inclined showers can give deeper insight into the
nature of primary particles that initiate showers and also into the possibility
that some of detected showers are triggered by neutrinos. In this paper, we
show the example of such an event and present some of the characteristics of
highly inclined showers detected by LOPES
Radio emission of extensive air shower at CODALEMA: Polarization of the radio emission along the v*B vector
Cosmic rays extensive air showers (EAS) are associated with transient radio
emission, which could provide an efficient new detection method of high energy
cosmic rays, combining a calorimetric measurement with a high duty cycle. The
CODALEMA experiment, installed at the Radio Observatory in Nancay, France, is
investigating this phenomenon in the 10^17 eV region. One challenging point is
the understanding of the radio emission mechanism. A first observation
indicating a linear relation between the electric field produced and the cross
product of the shower axis with the geomagnetic field direction has been
presented (B. Revenu, this conference). We will present here other strong
evidences for this linear relationship, and some hints on its physical origin.Comment: Contribution to the 31st International Cosmic Ray Conference, Lodz,
Poland, July 2009. 4 pages, 8 figures. v2: Typo fixed, arxiv references adde
Hadron Helicity Violation in Exclusive Processes: Quantitative Calculations in Leading Order QCD
We study a new mechanism for hadronic helicity flip in high energy hard
exclusive reactions. The mechanism proceeds in the limit of perfect chiral
symmetry, namely without any need to flip a quark helicity. The fundamental
feature of the new mechanism is the breaking of rotational symmetry of the hard
collision by a scattering plane in processes involving independent quark
scattering. We show that in the impulse approximation there is no evidence for
of the helicity violating process as the energy or momentum transfer is
increased over the region 1 GeV^2 < Q^2 < 100 GeV^2. In the asymptotic region
Q^2> 1000 GeV^2, a saddle point approximation with doubly logarithmic accuracy
yields suppression by a fraction of power of Q^2. ``Chirally--odd" exclusive
wave functions which carry non--zero orbital angular momentum and yet are
leading order in the high energy limit, play an important role.Comment: uuencoded LaTeX file (21 pages) and PostScript figure
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