21,110,305 research outputs found
Spin Polarizabilities of the Nucleon from Polarized Low Energy Compton Scattering
As guideline for forthcoming experiments, we present predictions from Chiral
Effective Field Theory for polarized cross sections in low energy Compton
scattering for photon energies below 170 MeV, both on the proton and on the
neutron. Special interest is put on the role of the nucleon spin
polarizabilities which can be examined especially well in polarized Compton
scattering. We present a model-independent way to extract their energy
dependence and static values from experiment, interpreting our findings also in
terms of the low energy effective degrees of freedom inside the nucleon: The
polarizabilities are dominated by chiral dynamics from the pion cloud, except
for resonant multipoles, where contributions of the Delta(1232) resonance turn
out to be crucial. We therefore include it as an explicit degree of freedom. We
also identify some experimental settings which are particularly sensitive to
the spin polarizabilities.Comment: 30 pages, 19 figure
Many-body effects in nuclear structure
We calculate, for the first time, the state-dependent pairing gap of a finite
nucleus (120Sn) diagonalizing the bare nucleon-nucleon potential (Argonne v14)
in a Hartree-Fock basis (with effective k-mass m_k eqult to 0.7 m), within the
framework of the BCS approximation including scattering states up to 800 MeV
above the Fermi energy to achieve convergence. The resulting gap accounts for
about half of the experimental gap. We find that a consistent description of
the low-energy nuclear spectrum requires, aside from the bare nucleon-nucleon
interaction, not only the dressing of single-particle motion through the
coupling to the nuclear surface, to give the right density of levels close to
the Fermi energy (and thus an effective mass m* approximately equal to m), but
also the renormalization of collective vibrational modes through vertex and
self-energy processes, processes which are also found to play an essential role
in the pairing channel, leading to a long range, state dependent component of
the pairing interaction. The combined effect of the bare nucleon-nucleon
potential and of the induced pairing interaction arising from the exchange of
low-lying surface vibrations between nucleons moving in time reversal states
close to the Fermi energy accounts for the experimental gap.Comment: 5 pages, 4 figures; author list correcte
Leading neutron spectra
It is shown that the observation of the spectra of leading neutrons from
proton beams can be a good probe of absorptive and migration effects. We
quantify how these effects modify the Reggeized pion-exchange description of
the measurements of leading neutrons at HERA. We are able to obtain a
satisfactory description of all the features of these data. We also briefly
discuss the corresponding data for leading baryons produced in hadron-hadron
collisions.Comment: 17 pages, 8 figures; sentence and reference added, reference
corrected, to be published in EPJ
Quantum-hydrodynamical picture of the massive Higgs boson
The phenomenon of spontaneous symmetry breaking admits a physical
interpretation in terms of the Bose-condensation process of elementary spinless
quanta. In this picture, the broken-symmetry phase emerges as a real physical
medium, endowed with a hierarchical pattern of scales, supporting two types of
elementary excitations for k \to 0: a massive energy branch E_a(k) \to M_H,
corresponding to the usual Higgs boson field, and a collective gap-less branch
E_b(k) \to 0. This is similar to the coexistence of phonons and rotons in
superfluid He-4 that, in fact, is usually considered the condensed-matter
analog of the Higgs condensate.
After previous work dedicated to the properties of the gap-less, phonon
branch, in this paper we use quantum hydrodynamics to propose a physical
interpretation of the massive branch. On the base of our results, M_H coincides
with the energy-gap for vortex formation and a massive Higgs boson is like a
roton in superfluid He-4. Within this interpretation of the Higgs particle,
there is no "naturalness" problem since M_H remains a naturally intermediate,
fixed energy scale, even for an ultimate ultraviolet cutoff Lambda \to \infty.Comment: Latex file, 20 pages, no figure
Measurement of the LT-asymmetry in \pi^0 electroproduction at the energy of the \Delta (1232) resonance
The reaction p(e,e'p)pi^0 has been studied at Q^2=0.2 (GeV/c)^2 in the region
of W=1232 MeV. From measurements left and right of q, cross section asymmetries
\rho_LT have been obtained in forward kinematics \rho_LT(\theta_\pi^0=20deg) =
(-11.68 +/- 2.36_stat +/- 2.36_sys)$ and backward kinematics
\rho_LT(\theta_\pi^0=160deg) =(12.18 +/- 0.27_stat +/- 0.82_sys). Multipole
ratios \Re(S_1+^* M_1+)/|M_1+|^2 and \Re(S_0+^* M_1+)/|M_1+|^2 were determined
in the framework of the MAID2003 model. The results are in agreement with older
data. The unusally strong negative \Re(S_0+^* M_1+)/|M_1+|^2 required to bring
also the result of Kalleicher et al. in accordance with the rest of the data is
almost excluded.Comment: 7 pages, 7 figures, 4 tables. Changed content. Accepted for
publication in EPJ
S_3 Flavor Symmetry and Leptogenesis
We consider leptogenesis in a minimal
S_3 extension of the standard model with an additional Z_2 symmetry in the
leptonic sector. It is found that the CP phase appearing in the neutrino mixing
is the same as that for the CP asymmetries responsible for leptogenesis.
Because of the discrete S_3 x Z_2 flavor symmetries, the CP asymmetries are
strongly suppressed. We therefore assume that the resonant enhancement of the
CP asymmetries takes place to obtain a realistic size of baryon number
asymmetry in theuniverse. Three degenerate right-handed neutrino masses of
O(10) TeV are theoretically expected in this model.Comment: 25 pages, 3 figure
Valence transition in the periodic Anderson model
A very rich phase diagram has recently been found in CeCuSi from
high pressure experiments where, in particular, a transition between an
intermediate valence configuration and an integral valent heavy fermion state
has been observed. We show that such a valence transition can be understood in
the framework of the periodic Anderson model. In particular, our results show a
breakdown of a mixed-valence state which is accompanied by a drastic change in
the \textit{f} occupation in agreement with experiment. This valence transition
can possibly be interpreted as a collapse of the large Fermi surface of the
heavy fermion state which incorporates not only the conduction electrons but
also the localized \textit{f} electrons. The theoretical approach used in this
paper is based on the novel projector-based renormalization method (PRM). With
respect to the periodic Anderson model, the method was before only employed in
combination with the basic approximations of the well-known slave-boson
mean-field theory. In this paper, the PRM treatment is performed in a more
sophisticated manner where both mixed as well as integral valent solutions have
been obtained. Furthermore, we argue that the presented PRM approach might be a
promising starting point to study the competing interactions in
CeCuSi and related compounds.Comment: 9 pages, 3 figures included; v2: completely revised and extended
versio
Inclusive Photoproduction of D* Mesons with Massive Charm Quarks
We have calculated the next-to-leading order cross sections for the inclusive
production of D* mesons in gamma-p collisions at HERA in two approaches using
massive or massless charm quarks. The usual massive theory for the direct cross
section with charm quarks only in the final state was transformed into a
massive theory with MS-bar subtraction by subtracting the mass divergent and
additional finite terms calculated earlier in connection with the process
gamma+gamma -> D*+X. This theory approaches the massless theory with increasing
transverse momentum. The difference between the massive and the massless
approach with MS-bar subtraction is studied in detail in those kinematic
regions relevant for comparison with experimental data. With these results and
including the resolved cross section which is dominated by the part originating
from the charm in the photon, we compute the fully inclusive D* cross section
and compare it with preliminary data from the ZEUS collaboration at HERA. We
find on average good agreement.Comment: 18 pages, 9 figures, figures modified to include statistical and
systematic experimental error
Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at TeV
Inclusive transverse momentum spectra of primary charged particles in Pb-Pb
collisions at = 2.76 TeV have been measured by the ALICE
Collaboration at the LHC. The data are presented for central and peripheral
collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross
section. The measured charged particle spectra in and GeV/ are compared to the expectation in pp collisions at the same
, scaled by the number of underlying nucleon-nucleon
collisions. The comparison is expressed in terms of the nuclear modification
factor . The result indicates only weak medium effects ( 0.7) in peripheral collisions. In central collisions,
reaches a minimum of about 0.14 at -7GeV/ and increases
significantly at larger . The measured suppression of high- particles is stronger than that observed at lower collision energies,
indicating that a very dense medium is formed in central Pb-Pb collisions at
the LHC.Comment: 15 pages, 5 captioned figures, 3 tables, authors from page 10,
published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/98
Well-posedness for the motion of an incompressible liquid with free surface boundary
We study the motion of an incompressible perfect liquid body in vacuum. This
can be thought of as a model for the motion of the ocean or a star. The free
surface moves with the velocity of the liquid and the pressure vanishes on the
free surface. This leads to a free boundary problem for Euler's equations,
where the regularity of the boundary enters to highest order. We prove local
existence in Sobolev spaces assuming a "physical condition", related to the
fact that the pressure of a fluid has to be positive.Comment: To appear in the Annals of Mat
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