1,058 research outputs found
The mass insertion approximation without squark degeneracy
We study the applicability of the mass insertion approximation (MIA) for
calculations of neutral meson mixing when squark masses are not degenerate and,
in particular, in models of alignment. We show that the MIA can give results
that are much better than an order of magnitude estimate as long as the masses
are not strongly hierarchical. We argue that, in an effective two-squark
framework, m_q=(m_1+m_2)/2 is the best choice for the MIA expansion point,
rather than, for example, m_q^2=(m_1^2+m_2^2)/2.Comment: 7 pages, revtex
A Perturbative Calculation of the Electromagnetic Form Factors of the Deuteron
Making use of the effective field theory expansion recently developed by the
authors, we compute the electromagnetic form factors of the deuteron
analytically to next-to-leading order (NLO). The computation is rather simple,
and involves calculating several Feynman diagrams, using dimensional
regularization. The results agree well with data and indicate that the
expansion is converging. They do not suffer from any ambiguities arising from
off-shell versus on-shell amplitudes.Comment: 22 pages, 8 figures. Discussion of effective range theory added,
typos correcte
Invariant tensors for simple groups
The forms of the invariant primitive tensors for the simple Lie algebras A_l,
B_l, C_l and D_l are investigated. A new family of symmetric invariant tensors
is introduced using the non-trivial cocycles for the Lie algebra cohomology.
For the A_l algebra it is explicitly shown that the generic forms of these
tensors become zero except for the l primitive ones and that they give rise to
the l primitive Casimir operators. Some recurrence and duality relations are
given for the Lie algebra cocycles. Tables for the 3- and 5-cocycles for su(3)
and su(4) are also provided. Finally, new relations involving the d and f su(n)
tensors are given.Comment: Latex file. 34 pages. (Trivial) misprints corrected. To appear in
Nucl. Phys.
Polaronic Signatures in Mid-Infrared Spectra: Prediction for LaMnO3 and CaMnO3
Hole-doped LaMnO3 and electron-doped CaMnO3 form self-trapped electronic
states. The spectra of these states have been calculated using a two orbital
(Mn eg Jahn-Teller) model, from which the non-adiabatic optical conductivity
spectra are obtained. In both cases the optical spectrum contains weight in the
gap region, whose observation will indicate the self-trapped nature of the
carrier states. The predicted spectra are proportional to the concentration of
the doped carriers in the dilute regime, with coefficients calculated with no
further model parameters.Comment: 6 pages with 3 figures imbedde
Conformality or confinement: (IR)relevance of topological excitations
We study aspects of the conformality to confinement transition for
non-supersymmetric Yang-Mills theories with fermions in arbitrary chiral or
vectorlike representations. We use the presence or absence of mass gap for
gauge fluctuations as an identifier of the infrared behavior. Present-day
understanding does not allow the mass gap for gauge fluctuations to be computed
on R*4. However, recent progress allows its non-perturbative computation on
R*3xS*1 by using either the twisted partition function or deformation theory,
for a range of S*1 sizes depending on the theory. For small number of fermions,
Nf, we show that the mass gap increases with increasing radius, due to the
non-dilution of monopoles and bions, the topological excitations relevant for
confinement on R*3xS*1. For sufficiently large Nf, we show that the mass gap
decreases with increasing radius. In a class of theories, we claim that the
decompactification limit can be taken while remaining within the region of
validity of semi-classical techniques, giving the first examples of
semiclassically solvable Yang-Mills theories at any size S*1. For general
non-supersymmetric vectorlike or chiral theories, we conjecture that the change
in the behavior of the mass gap on R*3xS*1 as a function of the radius occurs
near the lower boundary of the conformal window and give non-perturbative
estimates of its value. For vectorlike theories, we compare our estimates of
the conformal window with existing lattice results, truncations of the
Schwinger-Dyson equations, NSVZ beta function-inspired estimates, and degree of
freedom counting criteria. For multi-generation chiral gauge theories, to the
best of our knowledge, our estimates of the conformal window are the only known
ones.Comment: 40 pages, 3 figures; modified various comments, reference adde
Excited Baryons in Large N_c QCD Revisited: The Resonance Picture Versus Single-Quark Excitations
We analyze excited baryon properties via a 1/N_c expansion from two
perspectives: as resonances in meson-nucleon scattering, and as single-quark
excitations in the context of a simple quark model. For both types of analysis
one can derive novel patterns of degeneracy that emerge as N_c --> \infty, and
that are shown to be compatible with one another. This helps justify the
single-quark excitation picture and may give some insight into its successes.
We also find that in the large N_c limit one of the S_{11} baryons does not
couple to the pi-N channel but couples to the eta-N channel. This is
empirically observed in the N(1535), which couples very weakly to the pi-N
channel and quite strongly to the eta-N channel. The comparatively strong
coupling of the N(1650) to the pi-N channel and weak coupling to eta-N channel
is also predicted. In the context of the simple quark model picture we
reproduce expressions for mixing angles that are accurate up to O(1/N_c)
corrections and are in good agreement with mixing angles extracted
phenomenologically.Comment: 13 pages, ReVTeX
Electronic Raman scattering and photoluminescence from LaSrMnO exhibiting giant magnetoresistance
Raman and Photoluminescence (PL) experiments on correlated metallic
LaSrMnO have been carried out using different excitation
wavelengths as a function of temperature from 15 K to 300 K. Our data suggest a
Raman mode centered at 1800 cm and a PL band at 2.2 eV. The intensities
of the two peaks decrease with increasing temperature. The Raman mode can be
attributed to a plasmon excitation whose frequency and linewidths are
consistent with the measured resistivities. The PL involves intersite
electronic transitions of the manganese ions.Comment: 10 pages + 4 eps figures, Revtex 3.0, figures available on reques
Scherk-Schwarz Supersymmetry Breaking for Quasi-localized Matter Fields and Supersymmetry Flavor Violation
We examine the soft supersymmetry breaking parameters induced by the
Scherk-Schwarz (SS) boundary condition in 5-dimensional orbifold field theory
in which the quark and lepton zero modes are quasi-localized at the orbifold
fixed points to generate the hierarchical Yukawa couplings. In such theories,
the radion corresponds to a flavon to generate the flavor hierarchy and at the
same time plays the role of the messenger of supersymmetry breaking. As a
consequence, the resulting soft scalar masses and trilinear -parameters of
matter zero modes at the compactification scale are highly flavor-dependent,
thereby can lead to dangerous flavor violations at low energy scales. We
analyze in detail the low energy flavor violations in SS-dominated
supersymmetry breaking scenario under the assumption that the compactification
scale is close to the grand unification scale and the 4-dimensional effective
theory below the compactification scale is given by the minimal supersymmetric
standard model. Our analysis can be applied to any supersymmetry breaking
mechanism giving a sizable -component of the radion superfield, e.g. the
hidden gaugino condensation model.Comment: revtex4, 22 pages, some numerical errors are corrected in
phenomenological analysis, main conclusion does not chang
Interactions between brown-dwarf binaries and Sun-like stars
Several mechanisms have been proposed for the formation of brown dwarfs, but
there is as yet no consensus as to which -- if any -- are operative in nature.
Any theory of brown dwarf formation must explain the observed statistics of
brown dwarfs. These statistics are limited by selection effects, but they are
becoming increasingly discriminating. In particular, it appears (a) that brown
dwarfs that are secondaries to Sun-like stars tend to be on wide orbits, a\ga
100\,{\rm AU} (the Brown Dwarf Desert), and (b) that these brown dwarfs have a
significantly higher chance of being in a close (a\la 10\,{\rm AU}) binary
system with another brown dwarf than do brown dwarfs in the field. This then
raises the issue of whether these brown dwarfs have formed {\it in situ}, i.e.
by fragmentation of a circumstellar disc; or have formed elsewhere and
subsequently been captured. We present numerical simulations of the purely
gravitational interaction between a close brown-dwarf binary and a Sun-like
star. These simulations demonstrate that such interactions have a negligible
chance () of leading to the close brown-dwarf binary being captured by
the Sun-like star. Making the interactions dissipative by invoking the
hydrodynamic effects of attendant discs might alter this conclusion. However,
in order to explain the above statistics, this dissipation would have to favour
the capture of brown-dwarf binaries over single brown-dwarfs, and we present
arguments why this is unlikely. The simplest inference is that most brown-dwarf
binaries -- and therefore possibly also most single brown dwarfs -- form by
fragmentation of circumstellar discs around Sun-like protostars, with some of
them subsequently being ejected into the field.Comment: 10 pages, 8 figures, Accepted for publication in Astrophysics and
Space Scienc
Compton scattering beyond the impulse approximation
We treat the non-relativistic Compton scattering process in which an incoming
photon scatters from an N-electron many-body state to yield an outgoing photon
and a recoil electron, without invoking the commonly used frameworks of either
the impulse approximation (IA) or the independent particle model (IPM). An
expression for the associated triple differential scattering cross section is
obtained in terms of Dyson orbitals, which give the overlap amplitudes between
the N-electron initial state and the (N-1) electron singly ionized quantum
states of the target. We show how in the high energy transfer regime, one can
recover from our general formalism the standard IA based formula for the cross
section which involves the ground state electron momentum density (EMD) of the
initial state. Our formalism will permit the analysis and interpretation of
electronic transitions in correlated electron systems via inelastic x-ray
scattering (IXS) spectroscopy beyond the constraints of the IA and the IPM.Comment: 7 pages, 1 figur
- …