655 research outputs found
Spatial Constraint Corrections to the Elasticity of dsDNA Measured with Magnetic Tweezers
In this paper, we have studied, within a discrete WLC model, the spatial
constraints in magnetic tweezers used in single molecule experiments. Two
elements are involved: first, the fixed plastic slab on which is stuck the
initial strand, second, the magnetic bead which pulls (or twists) the attached
molecule free end. We have shown that the bead surface can be replaced by its
tangent plane at the anchoring point, when it is close to the bead south pole
relative to the force. We are led to a model with two parallel repulsive
plates: the fixed anchoring plate and a fluctuating plate, simulating the bead,
in thermal equilibrium with the system. The bead effect is a slight upper shift
of the elongation, about four times smaller than the similar effect induced by
the fixed plate. This rather unexpected result, has been qualitatively
confirmed within the soluble Gaussian model. A study of the molecule elongation
versus the countour length exhibits a significant non-extensive behaviour. The
curve for short molecules (with less than 2 kbp) is well fitted by a straight
line, with a slope given by the WLC model, but it does not go through the
origin. The non-extensive offset gives a 15% upward shift to the elongation of
a 2 kbp molecule stretched by a 0.3 pN force.Comment: 28 pages, 6 figures An explanatory figure has been added. The
physical interpretation of the results has been made somewhat more
transparen
Measurement of the parity violating 6S-7S transition amplitude in cesium achieved within 2 \times 10^{-13} atomic-unit accuracy by stimulated-emission detection
We exploit the process of asymmetry amplification by stimulated emission
which provides an original method for parity violation (PV) measurements in a
highly forbidden atomic transition. The method involves measurements of a
chiral, transient, optical gain of a cesium vapor on the 7S-6P_{3/2}
transition, probed after it is excited by an intense, linearly polarized,
collinear laser, tuned to resonance for one hyperfine line of the forbidden
6S-7S transition in a longitudinal electric field. We report here a 3.5 fold
increase, of the one-second-measurement sensitivity, and subsequent reduction
by a factor of 3.5 of the statistical accuracy compared with our previous
result [J. Gu\'ena et al., Phys. Rev. Lett. 90, 143001 (2003)]. Decisive
improvements to the set-up include an increased repetition rate, better
extinction of the probe beam at the end of the probe pulse and, for the first
time to our knowledge, the following: a polarization-tilt magnifier,
quasi-suppression of beam reflections at the cell windows, and a Cs cell with
electrically conductive windows. We also present real-time tests of systematic
effects, consistency checks on the data, as well as a 1% accurate measurement
of the electric field seen by the atoms, from atomic signals. PV measurements
performed in seven different vapor cells agree within the statistical error.
Our present result is compatible with the more precise Boulder result within
our present relative statistical accuracy of 2.6%, corresponding to a 2 \times
10^{-13} atomic-unit uncertainty in E_1^{pv}. Theoretical motivations for
further measurements are emphasized and we give a brief overview of a recent
proposal that would allow the uncertainty to be reduced to the 0.1% level by
creating conditions where asymmetry amplification is much greater.Comment: Article 21 pages, 6 figures, 3 tables Typos, addition of few comments
and little more data (1 week) leading to a slight reduction of the error bar
Accepted for publication in Phys.Rev.
Atomic interferometer measurements of Berry's and Aharonov-Anandan's phases for isolated spins S > 1/2 non-linearly coupled to external fields
The aim of the present paper is to propose experiments for observing the
significant features of Berry's phases for S>1, generated by spin-Hamiltonians
endowed with two couplings, a magnetic dipole and an electric quadrupole one
with external B and E fields, as theoretically studied in our previous work.
The fields are assumed orthogonal, this mild restriction leading to geometric
and algebraic simplifications. Alkali atoms appear as good candidates for
interferometric measurements but there are challenges to be overcome. The only
practical way to generate a suitable E-field is to use the ac Stark effect
which induces an instability of the dressed atom. Besides atom loss, this might
invalidate Berry's phase derivation but this latter problem can be solved by an
appropriate detuning. The former puts an upper limit to the cycle duration,
which is bounded below by the adiabatic condition. By relying upon our previous
analysis of the non-adiabatic corrections, we have been able to reach a
compromise for the Rb hf level F=2, m=0 state, which is our candidate
for an interferometric measurement of the exotic Berry's phase generated by a
rotation of the E-field around the fixed B-field. By a numerical simulation we
have shown that the non-adiabatic corrections can be kept below the 0.1% level.
As an alternative candidate, we discuss the chromium ground state J=S=3, where
the instability problem is easily solved. We make a proposal to extend the
measurement of Aharonov-Anandan's phase beyond S=1/2 to the Rb hf level
F=m=1, by constructing, with the help of light-shifts, a Hamiltonian able to
perform a parallel transport along a closed circuit upon the density matrix
space, without any adiabatic constraint. In Appendix A, Berry's phase
difference for S=3/2 and 1/2, m=1/2 states is used to perform an entanglement
of 3 Qbits.Comment: 23 pages, 6 figures, modifications in the introduction, two
paragraphs adde
A new Manifestation of Atomic Parity Violation in Cesium: a Chiral Optical Gain induced by linearly polarized 6S-7S Excitation
We have detected, by using stimulated emission, an Atomic Parity Violation
(APV) in the form of a chiral optical gain of a cesium vapor on the 7S -
6P transition,consecutive to linearly polarized 6S-7S excitation. We
demonstrate the validity of this detection method of APV, by presenting a 9%
accurate measurement of expected sign and magnitude. We underline several
advantages of this entirely new approach in which the cylindrical symmetry of
the set-up can be fully exploited. Future measurements at the percent level
will provide an important cross-check of an existing more precise result
obtained by a different method.Comment: 4 pages, 2 figure
Geometric Phases generated by the non-trivial spatial topology of static vector fields coupled to a neutral spin-endowed particle. Application to 171Yb atoms trapped in a 2D optical lattice
We have constructed the geometric phases emerging from the non-trivial
topology of a space-dependent magnetic field, interacting with the spin
magnetic moment of a neutral particle. Our basic tool is the local unitary
transformation which recasts the magnetic spin interaction under a diagonal
form. Rewriting the kinetic term in the "rotated" frame requires the
introduction of non-Abelian covariant derivatives, involving the gradients of
the Euler angles which define the orientation of the local field. Within the
rotated frame, we have built a perturbation scheme,assuming that the
longitudinal non-Abelian field component dominates the transverse ones, to be
evaluated to second-order. The geometry embedded in the longitudinal gauge
vector field and its curl, the geometric magnetic field, is described by the
associated Aharonov-Bohm phase. As an illustration, we study the physics of
cold 171Yb atoms dressed by two sets of circularly polarized beams, forming
square or triangular 2D optical lattices. The geometric field is computed
explicitly from the Euler angles. The magnitude of 2nd-order corrections due to
transverse fields can be reduced to the percent level by a choice of light
intensity which keeps the dressed atom loss rate below 5 s^{-1}. An auxiliary
optical lattice confines the atoms within 2D domains where the geometric field
is pointing upward.Comment: 12 pages, 4 figures. Comments and one figure added about the effect
of the additional scalar potential (sec. V.B). To be published in J. Phys.
A:Math. Theo
Calculating the hadronic vacuum polarization and leading hadronic contribution to the muon anomalous magnetic moment with improved staggered quarks
We present a lattice calculation of the hadronic vacuum polarization and the
lowest-order hadronic contribution to the muon anomalous magnetic moment, a_\mu
= (g-2)/2, using 2+1 flavors of improved staggered fermions. A precise fit to
the low-q^2 region of the vacuum polarization is necessary to accurately
extract the muon g-2. To obtain this fit, we use staggered chiral perturbation
theory, including the vector particles as resonances, and compare these to
polynomial fits to the lattice data. We discuss the fit results and associated
systematic uncertainties, paying particular attention to the relative
contributions of the pions and vector mesons. Using a single lattice spacing
ensemble (a=0.086 fm), light quark masses as small as roughly one-tenth the
strange quark mass, and volumes as large as (3.4 fm)^3, we find a_\mu^{HLO} =
(713 \pm 15) \times 10^{-10} and (748 \pm 21) \times 10^{-10} where the error
is statistical only and the two values correspond to linear and quadratic
extrapolations in the light quark mass, respectively. Considering systematic
uncertainties not eliminated in this study, we view this as agreement with the
current best calculations using the experimental cross section for e^+e^-
annihilation to hadrons, 692.4 (5.9) (2.4)\times 10^{-10}, and including the
experimental decay rate of the tau lepton to hadrons, 711.0 (5.0)
(0.8)(2.8)\times 10^{-10}. We discuss several ways to improve the current
lattice calculation.Comment: 44 pages, 4 tables, 17 figures, more discussion on matching the chpt
calculation to lattice calculation, typos corrected, refs added, version to
appear in PR
Constraints on the parity-violating couplings of a new gauge boson
High-energy particle physics experiments allow for the possible existence of
a new light, very weakly coupled, neutral gauge boson (the U boson). This one
permits for light (spin-1/2 or spin-0) particles to be acceptable Dark Matter
candidates, by inducing sufficient (stronger than weak) annihilation cross
sections into e+e-. They could be responsible for the bright 511 keV gamma ray
line observed by INTEGRAL from the galactic bulge.
Such a new interaction may have important consequences, especially at lower
energies. Parity-violation atomic-physics experiments provide strong
constraints on such a U boson, if its couplings to quarks and electrons violate
parity. With the constraints coming from an unobserved axionlike behaviour of
this particle, they privilegiate a pure vector coupling of the U boson to
quarks and leptons, unless the corresponding symmetry is broken sufficiently
above the electroweak scale.Comment: 6 page
Entropic Elasticity of Double-Strand DNA Subject to Simple Spatial Constraints
The aim of the present paper is the study of the entropic elasticity of the
dsDNA molecule, having a cristallographic length L of the order of 10 to 30
persistence lengths A, when it is subject to spatial obstructions. We have not
tried to obtain the single molecule partition function by solving a
Schodringer-like equation. We prefer to stay within a discretized version of
the WLC model with an added one-monomer potential, simulating the spatial
constraints. We derived directly from the discretized Boltzmann formula the
transfer matrix connecting the partition functions relative to adjacent
"effective monomers". We have plugged adequate Dirac delta-functions in the
functional integral to ensure that the monomer coordinate and the tangent
vector are independent variables. The partition function is, then, given by an
iterative process which is both numerically efficient and physically
transparent. As a test of our discretized approach, we have studied two
configurations involving a dsDNA molecule confined between a pair of parallel
plates.Comment: The most formal developments of Section I have been moved into an
appendix and replaced by a direct derivation of the transfer matrix used in
the applications. of Section II. Two paragraphs and two figures have been
added to clarify the physical interpretation of the result
- …