6 research outputs found
Minimal Anomalous U(1)' Extension of the MSSM
We study an extension of the MSSM by an anomalous abelian vector multiplet
and a St\"uckelberg multiplet. The anomalies are cancelled by the Green-Schwarz
mechanism and the addition of Chern-Simons terms. The advantage of this choice
over the standard one is that it allows for arbitrary values of the quantum
numbers of the extra U(1). As a first step towards the study of hadron
annihilations producing four leptons in the final state (a clean signal which
might be studied at LHC) we then compute the decays Z'\to Z_0 \g and . We find that the largest values of the decay rate is
GeV, while the expected number of events per year at LHC is at most of the
order of 10.Comment: 45 pages, 8 eps figures, feynmf. Phenomenological section expanded. 2
plots and references adde
Neutrino Induced Coherent Pion Production off Nuclei and PCAC
We review the Rein--Sehgal model and criticize its use for low energy
neutrino induced coherent pion production. We have studied the validity of the
main approximations implicit in that model, trying to compare with physical
observables when that is possible and with microscopical calculations. Next, we
have tried to elaborate a new improved model by removing the more problematic
approximations, while keeping the model still reasonably simple. Last, we have
discussed the limitations intrinsic to any approach based on the partial
conservation of the axial current hypothesis. In particular, we have shown the
inability of such models to determine the angular distribution of the outgoing
pion with respect to the direction of the incoming neutrino, except for the
kinematical point.Comment: 19 latex pages, 7 figures, 1 table. Version accepted for publication
in Physical Review
Breakdown of PCAC in diffractive neutrino interactions
We test the hypothesis of partially conserved axial current (PCAC) in high
energy diffractive neutrino production of pions. Since the pion pole
contribution to the Adler relation (AR) is forbidden by conservation of the
lepton current, the heavier states, like the a_1 pole, \rho-\pi-cut, etc.,
control the lifetime of the hadronic fluctuations of the neutrino. We evaluate
the deviation from the AR in diffractive neutrino-production of pions on proton
and nuclear targets. At high energies, when all the relevant time scales
considerably exceed the size of the target, the AR explicitly breaks down on an
absorptive target, such as a heavy nucleus. In this regime, close to the black
disc limit, the off-diagonal diffractive amplitudes vanish, while the diagonal
one, \pi->\pi, which enters the AR, maximizes and saturates the unitarity
bound. At lower energies, in the regime of short lifetime of heavy hadronic
fluctuations the AR is restored, i.e. it is not altered by the nuclear effects.Comment: 10 pages, 5 figure
Theoretical study of neutrino-induced coherent pion production off nuclei at T2K and MiniBooNE energies
We have developed a model for neutrino-induced coherent pion production off
nuclei in the energy regime of interest for present and forthcoming neutrino
oscillation experiments. It is based on a microscopic model for pion production
off the nucleon that, besides the dominant Delta pole contribution, takes into
account the effect of background terms required by chiral symmetry. Moreover,
the model uses a reduced nucleon-to-Delta resonance axial coupling, which leads
to coherent pion production cross sections around a factor two smaller than
most of the previous theoretical estimates. In the coherent production, the
main nuclear effects, namely medium corrections on the Delta propagator and the
final pion distortion, are included. We have improved on previous similar
models by taking into account the nucleon motion and employing a more
sophisticated optical potential. As found in previous calculations the
modification of the Delta self-energy inside the nuclear medium strongly
reduces the cross section, while the final pion distortion mainly shifts the
peak position to lower pion energies. The angular distribution profiles are not
much affected by nuclear effects. Nucleon motion increases the cross section by
15% at neutrino energies of 650 MeV, while Coulomb effects on charged pions are
estimated to be small. Finally, we discuss at length the deficiencies of the
Rein-Sehgal pion coherent production model for neutrino energies below 2 GeV,
and in particular for the MiniBooNE and T2K experiments. We also predict flux
averaged cross sections for these two latter experiments and K2K.Comment: 19 latex pages, 10 figures, 2 tables. Minor changes. Version accepted
for publication in Physical Review
Diffractive neutrino-production of pions on nuclei: Adler relation within the color-dipole description
Effects of coherence in neutrino-production of pions off nuclei are studied
employing the color dipole representation and path integral technique. If the
nucleus remains intact, the process is controlled by the interplay of two
length scales. One is related to the pion mass and is quite long (at low Q^2),
while the other, associated with heavy axial-vector states, is much shorter.
The Adler relation is found to be broken at all energies, but especially
strongly at \nu > 10 GeV, where the cross section is suppressed by a factor
~A^{-1/3}. On the contrary, in a process where the recoil nucleus breaks up
into fragments, the Adler relation turns out to be strongly broken at low
energies, where the cross section is enhanced by a factor ~A^{1/3}, but has a
reasonable accuracy at higher energies, where all the coherence length scales
become long.Comment: 9 pages, 5 figure