430 research outputs found
Density Matrix in Quantum Mechanics and Distinctness of Ensembles Having the Same Compressed Density Matrix
We clarify different definitions of the density matrix by proposing the use
of different names, the full density matrix for a single-closed quantum system,
the compressed density matrix for the averaged single molecule state from an
ensemble of molecules, and the reduced density matrix for a part of an
entangled quantum system, respectively. We show that ensembles with the same
compressed density matrix can be physically distinguished by observing
fluctuations of various observables. This is in contrast to a general belief
that ensembles with the same compressed density matrix are identical. Explicit
expression for the fluctuation of an observable in a specified ensemble is
given. We have discussed the nature of nuclear magnetic resonance quantum
computing. We show that the conclusion that there is no quantum entanglement in
the current nuclear magnetic resonance quantum computing experiment is based on
the unjustified belief that ensembles having the same compressed density matrix
are identical physically. Related issues in quantum communication are also
discussed.Comment: 26 pages. To appear in Foundations of Physics, 36 (8), 200
Pseudo-supersymmetry, Consistent Sphere Reduction and Killing Spinors for the Bosonic String
Certain supergravity theories admit a remarkable consistent dimensional
reduction in which the internal space is a sphere. Examples include type IIB
supergravity reduced on S^5, and eleven-dimensional supergravity reduced on S^4
or S^7. Consistency means that any solution of the dimensionally-reduced theory
lifts to give a solution in the higher dimension. Although supersymmetry seems
to play a role in the consistency of these reductions, it cannot be the whole
story since consistent sphere reductions of non-supersymmetric theories are
also known, such as the reduction of the effective action of the bosonic string
in any dimension D on either a 3-sphere or a (D-3)-sphere, retaining the gauge
bosons of SO(4) or SO(D-2) respectively. We show that although there is no
supersymmetry, there is nevertheless a natural Killing spinor equation for the
D-dimensional bosonic string. A projection of the full integrability condition
for these Killing spinors gives rise to the bosonic equations of motion (just
as happens in the supergravity examples). Thus it appears that by extending the
notion of supersymmetry to "pseudo-supersymmetry" in this way, one may be able
to obtain a broader understanding of a relation between Killing spinors and
consistent sphere reductions.Comment: Latex, 15 page
The design and performance of an improved target for MICE
The linear motor driving the target for the Muon Ionisation Cooling Experiment has been redesigned to improve its reliability and performance. A new coil-winding technique is described which produces better magnetic alignment and improves heat transport out of the windings. Improved field-mapping has allowed the more precise construction to be demonstrated, and an enhanced controller exploits the full features of the hardware, enabling increased acceleration and precision. The new user interface is described and analysis of performance data to monitor friction is shown to allow quality control of bearings and a measure of the ageing of targets during use
Pseudo-Supergravity Extension of the Bosonic String
We construct a ``pseudo-supersymmetric" fermionic extension of the effective
action of the bosonic string in arbitrary spacetime dimension D. The theory is
invariant under pseudo-supersymmetry transformations up to the quadratic
fermion order, which is sufficient in order to be able to derive Killing spinor
equations in bosonic backgrounds, and hence to define BPS type solutions
determined by a system of first-order equations. The pseudo-supersymmetric
theory can be extended by coupling it to a Yang-Mills pseudo-supermultiplet.
This also allows us to construct ``\alpha' corrections'' involving quadratic
curvature terms. An exponential dilaton potential term, associated with the
conformal anomaly for a bosonic string outside its critical dimension, can also
be pseudo-supersymmetrised.Comment: Latex, 16 page
Simulating (electro)hydrodynamic effects in colloidal dispersions: smoothed profile method
Previously, we have proposed a direct simulation scheme for colloidal
dispersions in a Newtonian solvent [Phys.Rev.E 71,036707 (2005)]. An improved
formulation called the ``Smoothed Profile (SP) method'' is presented here in
which simultaneous time-marching is used for the host fluid and colloids. The
SP method is a direct numerical simulation of particulate flows and provides a
coupling scheme between the continuum fluid dynamics and rigid-body dynamics
through utilization of a smoothed profile for the colloidal particles.
Moreover, the improved formulation includes an extension to incorporate
multi-component fluids, allowing systems such as charged colloids in
electrolyte solutions to be studied. The dynamics of the colloidal dispersions
are solved with the same computational cost as required for solving
non-particulate flows. Numerical results which assess the hydrodynamic
interactions of colloidal dispersions are presented to validate the SP method.
The SP method is not restricted to particular constitutive models of the host
fluids and can hence be applied to colloidal dispersions in complex fluids
The design, construction and performance of the MICE target
The pion-production target that serves the MICE Muon Beam consists of a titanium cylinder that is dipped into the halo of the ISIS proton beam. The design and construction of the MICE target system are described along with the quality-assurance procedures, electromagnetic drive and control systems, the readout electronics, and the data-acquisition system. The performance of the target is presented together with the particle rates delivered to the MICE Muon Beam. Finally, the beam loss in ISIS generated by the operation of the target is evaluated as a function of the particle rate, and the operating parameters of the target are derived
Two-body decays in the minimal 331 model
The two-body decays of the extra neutral boson Z_2 predicted by the minimal
331 model are analyzed. At the three-level it can decay into standard model
particles as well as exotic quarks and the new gauge bosons predicted by the
model. The decays into a lepton pair are strongly suppressed, with and . In the bosonic
sector, Z_2 would decay mainly into a pair of bilepton gauge bosons, with a
branching ratio below the 0.1 level. The Z_2 boson has thus a leptophobic and
bileptophobic nature and it would decay dominantly into quark pairs. The
anomaly-induced decays and , which occurs
at the one-loop level are studied. It is found that and at most. As for the and decays, with H a relatively light Higgs boson, they
are induced via Z'-Z mixing. It is obtained that
and . We also examine the flavor changing neutral
current decays and , which may have branching
fractions as large as and , respectively, and thus may be of
phenomenological interest.Comment: 14 pages, 3 figures, submitted to Physical Review
Characterisation of the muon beams for the Muon Ionisation Cooling Experiment
A novel single-particle technique to measure emittance has been developed and used to characterise seventeen different muon beams for the Muon Ionisation Cooling Experiment (MICE). The muon beams, whose mean momenta vary from 171 to 281 MeV/c, have emittances of approximately 1.2–2.3 π mm-rad horizontally and 0.6–1.0 π mm-rad vertically, a horizontal dispersion of 90–190 mm and momentum spreads of about 25 MeV/c. There is reasonable agreement between the measured parameters of the beams and the results of simulations. The beams are found to meet the requirements of MICE
Search for lepton-flavor violation at HERA
A search for lepton-flavor-violating interactions and has been performed with the ZEUS detector using the entire HERA I
data sample, corresponding to an integrated luminosity of 130 pb^{-1}. The data
were taken at center-of-mass energies, , of 300 and 318 GeV. No
evidence of lepton-flavor violation was found, and constraints were derived on
leptoquarks (LQs) that could mediate such interactions. For LQ masses below
, limits were set on , where
is the coupling of the LQ to an electron and a
first-generation quark , and is the branching ratio of
the LQ to the final-state lepton ( or ) and a quark . For
LQ masses much larger than , limits were set on the four-fermion
interaction term for LQs that couple to an electron and a quark
and to a lepton and a quark , where and are
quark generation indices. Some of the limits are also applicable to
lepton-flavor-violating processes mediated by squarks in -Parity-violating
supersymmetric models. In some cases, especially when a higher-generation quark
is involved and for the process , the ZEUS limits are the most
stringent to date.Comment: 37 pages, 10 figures, Accepted by EPJC. References and 1 figure (Fig.
6) adde
Multijet production in neutral current deep inelastic scattering at HERA and determination of alpha_s
Multijet production rates in neutral current deep inelastic scattering have
been measured in the range of exchanged boson virtualities 10 < Q2 < 5000 GeV2.
The data were taken at the ep collider HERA with centre-of-mass energy sqrt(s)
= 318 GeV using the ZEUS detector and correspond to an integrated luminosity of
82.2 pb-1. Jets were identified in the Breit frame using the k_T cluster
algorithm in the longitudinally invariant inclusive mode. Measurements of
differential dijet and trijet cross sections are presented as functions of jet
transverse energy E_{T,B}{jet}, pseudorapidity eta_{LAB}{jet} and Q2 with
E_{T,B}{jet} > 5 GeV and -1 < eta_{LAB}{jet} < 2.5. Next-to-leading-order QCD
calculations describe the data well. The value of the strong coupling constant
alpha_s(M_Z), determined from the ratio of the trijet to dijet cross sections,
is alpha_s(M_Z) = 0.1179 pm 0.0013(stat.) {+0.0028}_{-0.0046}(exp.)
{+0.0064}_{-0.0046}(th.)Comment: 22 pages, 5 figure
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