127 research outputs found
Polaron Effective Mass, Band Distortion, and Self-Trapping in the Holstein Molecular Crystal Model
We present polaron effective masses and selected polaron band structures of
the Holstein molecular crystal model in 1-D as computed by the Global-Local
variational method over a wide range of parameters. These results are augmented
and supported by leading orders of both weak- and strong-coupling perturbation
theory. The description of the polaron effective mass and polaron band
distortion that emerges from this work is comprehensive, spanning weak,
intermediate, and strong electron-phonon coupling, and non-adiabatic, weakly
adiabatic, and strongly adiabatic regimes. Using the effective mass as the
primary criterion, the self-trapping transition is precisely defined and
located. Using related band-shape criteria at the Brillouin zone edge, the
onset of band narrowing is also precisely defined and located. These two lines
divide the polaron parameter space into three regimes of distinct polaron
structure, essentially constituting a polaron phase diagram. Though the
self-trapping transition is thusly shown to be a broad and smooth phenomenon at
finite parameter values, consistency with notion of self-trapping as a critical
phenomenon in the adiabatic limit is demonstrated. Generalizations to higher
dimensions are considered, and resolutions of apparent conflicts with
well-known expectations of adiabatic theory are suggested.Comment: 28 pages, 15 figure
Lattice dynamics effects on small polaron properties
This study details the conditions under which strong-coupling perturbation
theory can be applied to the molecular crystal model, a fundamental theoretical
tool for analysis of the polaron properties. I show that lattice dimensionality
and intermolecular forces play a key role in imposing constraints on the
applicability of the perturbative approach. The polaron effective mass has been
computed in different regimes ranging from the fully antiadiabatic to the fully
adiabatic. The polaron masses become essentially dimension independent for
sufficiently strong intermolecular coupling strengths and converge to much
lower values than those tradition-ally obtained in small-polaron theory. I find
evidence for a self-trapping transition in a moderately adiabatic regime at an
electron-phonon coupling value of .3. Our results point to a substantial
independence of the self-trapping event on dimensionality.Comment: 8 pages, 5 figure
Effects of dimensionality and anisotropy on the Holstein polaron
We apply weak-coupling perturbation theory and strong-coupling perturbation
theory to the Holstein molecular crystal model in order to elucidate the
effects of anisotropy on polaron properties in D dimensions. The ground state
energy is considered as a primary criterion through which to study the effects
of anisotropy on the self-trapping transition, the self-trapping line
associated with this transition, and the adiabatic critical point. The effects
of dimensionality and anisotropy on electron-phonon correlations and polaronic
mass enhancement are studied, with particular attention given to the polaron
radius and the characteristics of quasi-1D and quasi-2D structures.
Perturbative results are confirmed by selected comparisons with variational
calculations and quantum Monte Carlo data
Search for supersymmetry with a dominant R-parity violating LQDbar couplings in e+e- collisions at centre-of-mass energies of 130GeV to 172 GeV
A search for pair-production of supersymmetric particles under the assumption
that R-parity is violated via a dominant LQDbar coupling has been performed
using the data collected by ALEPH at centre-of-mass energies of 130-172 GeV.
The observed candidate events in the data are in agreement with the Standard
Model expectation. This result is translated into lower limits on the masses of
charginos, neutralinos, sleptons, sneutrinos and squarks. For instance, for
m_0=500 GeV/c^2 and tan(beta)=sqrt(2) charginos with masses smaller than 81
GeV/c^2 and neutralinos with masses smaller than 29 GeV/c^2 are excluded at the
95% confidence level for any generation structure of the LQDbar coupling.Comment: 32 pages, 30 figure
Search for oscillations using inclusive lepton events
A search for Bs oscillations is performed using a sample of semileptonic b-hadron decays collected by the ALEPH experiment during 1991-1995. Compared to previous inclusive lepton analyses, the prop er time resolution and b-flavour mistag rate are significantly improved. Additional sensitivity to Bs mixing is obtained by identifying subsamples of events having a Bs purity which is higher than the average for the whole data sample. Unbinned maximum likelihood amplitude fits are performed to derive a lower limit of Dms>9.5 ps-1 at 95% CL. Combining with the ALEPH Ds based analyses yields Dms>9.6 ps-1 at 95% CL.A search for B0s oscillations is performed using a sample of semileptonic b-hadron decays collected by the ALEPH experiment during 1991-1995. Compared to previous inclusive lepton analyses, the proper time resolution and b-flavour mistag rate are significantly improved. Additional sensitivity to B0s mixing is obtained by identifying subsamples of events having a B0s purity which is higher than the average for the whole data sample. Unbinned maximum likelihood amplitude fits are performed to derive a lower limit of Deltam_s>9.5ps^-1 at 95% CL. Combining with the ALEPH D-s based analyses yields Deltam_s>9.6ps^-1 at 95% CL
Limit on oscillation using a jet charge method
A lower limit is set on the B_{s}^{0} meson oscillation parameter \Delta m_{s} using data collected from 1991 to 1994 by the ALEPH detector. Events with a high transverse momentum lepton and a reconstructed secondary vertex are used. The high transverse momentum leptons are produced mainly by b hadron decays, and the sign of the lepton indicates the particle/antiparticle final state in decays of neutral B mesons. The initial state is determined by a jet charge technique using both sides of the event. A maximum likelihood method is used to set a lower limit of \, \Delta m_{s}. The 95\% confidence level lower limit on \Delta m_s ranges between 5.2 and 6.5(\hbar/c^{2})~ps^{-1} when the fraction of b quarks from Z^0 decays that form B_{s}^{0} mesons is varied from 8\% to 16\%. Assuming that the B_{s}^{0} fraction is 12\%, the lower limit would be \Delta m_{s} 6.1(\hbar/c^{2})~ps^{-1} at 95\% confidence level. For x_s = \Delta m_s \, \tau_{B_s}, this limit also gives x_s 8.8 using the B_{s}^{0} lifetime of \tau_{B_s} = 1.55 \pm 0.11~ps and shifting the central value of \tau_{B_s} down by 1\sigma
Measurement of the B lifetime and production rate with D combinations in Z decays
The lifetime of the \bs meson is measured in approximately 3 million hadronic Z decays accumulated using the ALEPH detector at LEP from 1991 to 1994. Seven different \ds decay modes were reconstructed and combined with an opposite sign lepton as evidence of semileptonic \bs decays. Two hundred and eight \dsl candidates satisfy selection criteria designed to ensure precise proper time reconstruction and yield a measured \bs lifetime of \mbox{\result .} Using a larger, less constrained sample of events, the product branching ratio is measured to be \mbox{\pbrresult
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