120 research outputs found
Positive pion absorption on 3He using modern trinucleon wave functions
We study pion absorption on 3He employing trinucleon wave functions
calculated from modern realistic NN interactions (Paris, CD Bonn). Even though
the use of the new wave functions leads to a significant improvement over older
calculations with regard to both cross section and polarization data, there are
hints that polarization data with quasifree kinematics cannot be described by
just two-nucleon absorption mechanisms.Comment: 14 pages, 6 figure
Large Extra Dimensions and Decaying KK Recurrences
We suggest the possibility that in ADD type brane-world scenarios, the higher
KK excitations of the graviton may decay to lower ones owing to a breakdown of
the conservation of extra dimensional ``momenta'' and study its implications
for astrophysics and cosmology. We give an explicit realization of this idea
with a bulk scalar field , whose nonzero KK modes acquire vacuum
expectation values. This scenario helps to avoid constraints on large extra
dimensions that come from gamma ray flux bounds in the direction of nearby
supernovae as well as those coming from diffuse cosmological gamma ray
background. It also relaxes the very stringent limits on reheat temperature of
the universe in ADD models.Comment: 16 pages, late
Scalar mesons moving in a finite volume and the role of partial wave mixing
Phase shifts and resonance parameters can be obtained from finite-volume
lattice spectra for interacting pairs of particles, moving with nonzero total
momentum. We present a simple derivation of the method that is subsequently
applied to obtain the pi pi and pi K phase shifts in the sectors with total
isospin I=0 and I=1/2, respectively. Considering different total momenta, one
obtains extra data points for a given volume that allow for a very efficient
extraction of the resonance parameters in the infinite-volume limit.
Corrections due to the mixing of partial waves are provided. We expect that our
results will help to optimize the strategies in lattice simulations, which aim
at an accurate determination of the scattering and resonance properties.Comment: 19 pages, 12 figure
Supersymmetric Extra Dimensions: Gravitino Effects in Selectron Pair Production
We examine the phenomenological consequences of a supersymmetric bulk in the
scenario of large extra dimensions. We assume supersymmetry is realized in the
bulk and study the interactions of the resulting bulk gravitino Kaluza-Klein
(KK) tower of states, with supersymmetry breaking on the brane inducing a light
mass for the zero-mode gravitino. We derive the 4-d effective theory, including
the couplings of the bulk gravitino KK states to fermions and their scalar
superpartners. The virtual exchange of the gravitino KK states in selectron
pair production in polarized \epem collisions is then examined. We find that
the leading order operator for this exchange is dimension six, in contrast to
that of bulk graviton KK exchange which induces a dimension eight operator at
lowest order. The resulting kinematic distributions for selectron production
are dramatically altered from those in D=4 supersymmetric scenarios, and can
lead to a enormous sensitivity to the fundamental higher dimensional Planck
scale, of order .Comment: 48 pg
Kaluza-Klein gravitino production with a single photon at e^+ e^- colliders
In a supersymmetric large extra dimension scenario, the production of
Kaluza-Klein gravitinos accompanied by a photino at e^+ e^- colliders is
studied. We assume that a bulk supersymmetry is softly broken on our brane such
that the low-energy theory resembles the MSSM. Low energy supersymmetry
breaking is further assumed as in GMSB, leading to sub-eV mass shift in each KK
mode of the gravitino from the corresponding graviton KK mode. Since the
photino decays within a detector due to its sufficiently large inclusive decay
rate into a photon and a gravitino, the process e^+ e^- -> photino + gravitino
yields single photon events with missing energy. Even if the total cross
section can be substantial at sqrt(s)=500 GeV, the KK graviton background of
e^+ e^- -> photon + graviton is kinematically advantageous and thus much
larger. It is shown that the observable, sigma(e^-_L)-sigma(e^-_R), can
completely eliminate the KK graviton background but retain most of the KK
gravitino signal, which provides a unique and robust method to probe the
supersymmetric bulk.Comment: Reference added and typos correcte
What is the structure of the Roper resonance?
We investigate the structure of the nucleon resonance N^*(1440) (Roper)
within a coupled-channel meson exchange model for pion-nucleon scattering. The
coupling to pipiN states is realized effectively by the coupling to the sigmaN,
piDelta and rhoN channels. The interaction within and between these channels is
derived from an effective Lagrangian based on a chirally symmetric Lagrangian,
which is supplemented by well known terms for the coupling of the Delta isobar,
the omega meson and the 'sigma', which is the name given here to the strong
correlation of two pions in the scalar-isoscalar channel. In this model the
Roper resonance can be described by meson-baryon dynamics alone; no genuine
N^*(1440) (3 quark) resonance is needed in order to fit piN phase shifts and
inelasticities.Comment: 55 pages, 14 figure
Three-Nucleon Forces from Chiral Effective Field Theory
We perform the first complete analysis of nd scattering at
next-to-next-to-leading order in chiral effective field theory including the
corresponding three-nucleon force and extending our previous work, where only
the two-nucleon interaction has been taken into account. The three-nucleon
force appears first at this order in the chiral expansion and depends on two
unknown parameters. These two parameters are determined from the triton binding
energy and the nd doublet scattering length. We find an improved description of
various scattering observables in relation to the next-to-leading order results
especially at moderate energies (E_lab = 65 MeV). It is demonstrated that the
long-standing A_y-problem in nd elastic scattering is still not solved by the
leading 3NF, although some visible improvement is observed. We discuss
possibilities of solving this puzzle. The predicted binding energy for the
alpha-particle agrees with the empirical value.Comment: 36 pp, 20 figure
Couplings of light I=0 scalar mesons to simple operators in the complex plane
The flavour and glue structure of the light scalar mesons in QCD are probed
by studying the couplings of the I=0 mesons and to the
operators , and to two photons. The Roy dispersive
representation for the amplitude is used to determine the
pole positions as well as the residues in the complex plane. On the real axis,
is constrained to solve the Roy equation together with elastic
unitarity up to the K\Kbar threshold leading to an improved description of
the . The problem of using a two-particle threshold as a matching
point is discussed. A simple relation is established between the coupling of a
scalar meson to an operator and the value of the related pion form-factor
computed at the resonance pole. Pion scalar form-factors as well as two-photon
partial-wave amplitudes are expressed as coupled-channel Omn\`es dispersive
representations. Subtraction constants are constrained by chiral symmetry and
experimental data. Comparison of our results for the couplings with
earlier determinations of the analogous couplings of the lightest I=1 and
scalar mesons are compatible with an assignment of the ,
, , into a nonet. Concerning the gluonic operator
we find a significant coupling to both the and the
.Comment: 31 pages, 5 figure
Vector meson production and nucleon resonance analysis in a coupled-channel approach for energies m_N < sqrt(s) < 2 GeV I: pion-induced results and hadronic parameters
We present a nucleon resonance analysis by simultaneously considering all
pion- and photon-induced experimental data on the final states gamma N, pi N, 2
pi N, eta N, K Lambda, K Sigma, and omega N for energies from the nucleon mass
up to sqrt(s) = 2 GeV. In this analysis we find strong evidence for the
resonances P_{31}(1750), P_{13}(1900), P_{33}(1920), and D_{13}(1950). The
omega N production mechanism is dominated by large P_{11}(1710) and
P_{13}(1900) contributions. In this first part, we present the results of the
pion-induced reactions and the extracted resonance and background properties
with emphasis on the difference between global and purely hadronic fits.Comment: 54 pages, 26 figures, discussion extended, typos corrected,
references updated, to appear in Phys. Rev.
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