7,359 research outputs found

### CMS experiment at the LHC: Commissioning and early physics

The CMS collaboration used the past year to greatly improve the level of
detector readiness for the first collisions data. The acquired operational
experience over this year, large gains in understanding the detector and
improved preparedness for early physics will be instrumental in minimizing the
time from the first collisions to first LHC physics. The following describes
the status of the CMS experiment and outlines early physics plans with the
first LHC data.Comment: To appear in the Proceedings of the 21st Rencontres de Blois: Windows
on the Universe, Blois, France, 21-27 Jun 200

### Comment on "Clock Shift in High Field Magnetic Resonance of Atomic Hydrogen"

In this Comment, we reanalyze the experiments on the collision frequency
shift of the b-c and a-d hyperfine transitions in three-dimensional atomic
hydrogen in the presence of, respectively, a and b-state atoms. Accurate
consideration of the symmetry of the spatial and spin part of the diatomic
wavefunction yields the difference a_T-a_S=0.30(5) \AA between the triplet and
singlet s-wave scattering lengths of hydrogen atoms. This corrects the
factor-of two error of the commented work [Phys. Rev. Lett. 101, 263003
(2008)].Comment: 1 pag

### Possibility of Geometric Description of Quasiparticles in Solids

New phenomenological approach for the description of elementary collective
excitations is proposed. The crystal is considered to be an anisotropic
space-time vacuum with a prescribed metric tensor in which the information on
electromagnetic crystalline fields is included. The quasiparticles in this
space are supposed to be described by the equations structurally similar to the
relativistic wave equations for particles in empty space. The generalized
Klein-Gordon-Fock equation and the generalized Dirac equation in external
electromagnetic field are considered. The applicability of the proposed
approach to the case of conduction electron in a crystal is discussed.Comment: 17 pages, latex; to appear in Int. Jnl. Mod. Phy

### Fragmentation of CDF jets: perturbative or non-perturbative?

Presented are the most recent jet fragmentation results from CDF: inclusive distributions of charged particle momenta and their kT in jets; average track multiplicities, as well as angular distributions of multiplicity flow, for a wide range of jet energies with ET from 40 to 300 GeV. The results are compared with Monte-Carlo and, when possible, analytical calculations performed in resummed perturbative QCD approximations (MLLA)

### Hyperfine frequency shift in two-dimensional atomic hydrogen

We propose the explanation of a surprisingly small hyperfine frequency shift
in the two-dimensional (2D) atomic hydrogen bound to the surface of superfluid
helium below 0.1 K. Owing to the symmetry considerations, the microwave-induced
triplet-singlet transitions of atomic pairs in the fully spin-polarized sample
are forbidden. The apparent nonzero shift is associated with the
density-dependent wall shift of the hyperfine constant and the pressure shift
due to the presence of H atoms in the hyperfine state $a$ not involved in the
observed $b\to c$ transition. The interaction of adsorbed atoms with one
another effectively decreases the binding energy and, consequently, the wall
shift by the amount proportional to their density. The pressure shift of the
$b\to c$ resonance comes from the fact that the impurity $a$-state atoms
interact differently with the initial $b$-state and final $c$-state atoms and
is also linear in density. The net effect of the two contributions, both
specific for 2D hydrogen, is comparable with the experimental observation. To
our knowledge, this is the first mentioning of the density-dependent wall
shift. We also show that the difference between the triplet and singlet
scattering lengths of H atoms, $a_t-a_s=30(5)$ pm, is exactly twice smaller
than the value reported by Ahokas {\it et al.}, Phys. Rev. Lett. {\bf101},
263003 (2008).Comment: 4 pages, no figure

### Thermal compression of two-dimensional atomic hydrogen to quantum degeneracy

We describe experiments where 2D atomic hydrogen gas is compressed thermally
at a small "cold spot" on the surface of superfluid helium and detected
directly with electron-spin resonance. We reach surface densities up to 5e12
1/cm^2 at temperatures of approximately 100 mK corresponding to the maximum 2D
phase-space density of about 1.5. By independent measurements of the surface
density and its decay rate we make the first direct determination of the
three-body recombination rate constant and get the value of 2e-25 cm^4/s for
its upper bound, which is an order of magnitude smaller than previously
reported experimental results.Comment: 4 pages, 4 postscript figures, bibliography (.bbl) file, submitted to
PR

### Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain

The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes

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