15,725 research outputs found
Spin effects in the fragmentation of transversely polarized and unpolarized quarks
We study the fragmentation of a transversely polarized quark into a non
collinear (k_T not= 0) spinless hadron and the fragmentation of an unpolarized
quark into a non collinear transversely polarized spin 1/2 baryon. These
nonperturbative properties are described by spin and k_T dependent
fragmentation functions and are revealed in the observation of single spin
asymmetries. Recent data on the production of pions in polarized semi-inclusive
DIS and long known data on Lambda polarization in unpolarized p-N processes are
considered: these new fragmentation functions can describe the experimental
results and the single spin effects in the quark fragmentation turn out to be
surprisingly large.Comment: 5 pages + 2 eps figures, uses aipproc.sty and epsfig.sty. Talk
delivered by M. Anselmino at the "14th International Spin Physics Symposium",
SPIN2000, October 16-21, 2000, Osaka, Japa
Transverse Lambda polarization in unpolarized semi-inclusive DIS
The long-standing problem of transverse Lambda polarization in high-energy
collisions of unpolarized hadrons can be tackled by considering new, spin and
k_T-dependent quark fragmentation functions for an unpolarized quark into a
polarized, spin-1/2 hadron. Simple phenomenological parameterizations of these
new "polarizing fragmentation functions", which describe quite well the
experimental data on Lambda and Lambda-bar hyperons produced in p-A processes,
are utilized and extended here to give predictions for transverse Lambda
polarization in semi-inclusive DISComment: LaTeX, 4 pages, 2 ps figures, uses sprocl.sty and epsfig.sty; Talk
delivered by F. Murgia at the IX International Workshop on Deep Inelastic
Scattering (DIS2001), Bologna, 27 April - 1 May 200
Double transverse spin asymmetries in vector boson production
We investigate a helicity non-flip double transverse spin asymmetry in vector
boson production in hadron-hadron scattering, which was first considered by
Ralston and Soper at the tree level. It does not involve transversity functions
and in principle also arises in W-boson production for which we present the
expressions. The asymmetry requires observing the transverse momentum of the
vector boson, but it is not suppressed by explicit inverse powers of a large
energy scale. However, as we will show, inclusion of Sudakov factors causes
suppression of the asymmetry, which increases with energy. Moreover, the
asymmetry is shown to be approximately proportional to x_1 g_1(x_1) x_2 \bar
g_1(x_2), which gives rise to additional suppression at small values of the
light cone momentum fractions. This implies that it is negligible for Z or W
production and is mainly of interest for \gamma^* at low energies. We also
compare the asymmetry with other types of double transverse spin asymmetries
and discuss how to disentangle them.Comment: 12 pages, Revtex, 2 Postscript figures, uses aps.sty, epsf.sty;
figures replaced, a few minor other correction
The elliptic genus from split flows and Donaldson-Thomas invariants
We analyze a mixed ensemble of low charge D4-D2-D0 brane states on the
quintic and show that these can be successfully enumerated using attractor flow
tree techniques and Donaldson-Thomas invariants. In this low charge regime one
needs to take into account worldsheet instanton corrections to the central
charges, which is accomplished by making use of mirror symmetry. All the
charges considered can be realized as fluxed D6-D2-D0 and anti-D6-D2-D0 pairs
which we enumerate using DT invariants. Our procedure uses the low charge
counterpart of the picture developed Denef and Moore. By establishing the
existence of flow trees numerically and refining the index factorization
scheme, we reproduce and improve some results obtained by Gaiotto, Strominger
and Yin. Our results provide appealing evidence that the strong split flow tree
conjecture holds and allows to compute exact results for an important sector of
the theory. Our refined scheme for computing indices might shed some light on
how to improve index computations for systems with larger charges.Comment: 37 pages, 12 figure
Universality of T-odd effects in single spin and azimuthal asymmetries
We analyze the transverse momentum dependent distribution and fragmentation
functions in space-like and time-like hard processes involving at least two
hadrons, in particular 1-particle inclusive leptoproduction, the Drell-Yan
process and two-particle inclusive hadron production in electron-positron
annihilation. As is well-known, transverse momentum dependence allows for the
appearance of unsuppressed single spin azimuthal asymmetries, such as Sivers
and Collins asymmetries. Recently, Belitsky, Ji and Yuan obtained fully color
gauge invariant expressions for the relevant matrix elements appearing in these
asymmetries at leading order in an expansion in the inverse hard scale. We
rederive these results and extend them to observables at the next order in this
expansion. We observe that at leading order one retains a probability
interpretation, contrary to a claim in the literature and show the direct
relation between the Sivers effect in single spin asymmetries and the
Qiu-Sterman mechanism. We also study fragmentation functions, where the process
dependent gauge link structure of the correlators is not the only source of
T-odd observables and discuss the implications for universality.Comment: 29 pages, Revtex, 26 Postscript figures; abstract, introduction and
section VIIC significantly modified and appendix B replace
Black Hole Meiosis
The enumeration of BPS bound states in string theory needs refinement.
Studying partition functions of particles made from D-branes wrapped on
algebraic Calabi-Yau 3-folds, and classifying states using split attractor flow
trees, we extend the method for computing a refined BPS index, arXiv:0810.4301.
For certain D-particles, a finite number of microstates, namely polar states,
exclusively realized as bound states, determine an entire partition function
(elliptic genus). This underlines their crucial importance: one might call them
the `chromosomes' of a D-particle or a black hole. As polar states also can be
affected by our refinement, previous predictions on elliptic genera are
modified. This can be metaphorically interpreted as `crossing-over in the
meiosis of a D-particle'. Our results improve on hep-th/0702012, provide
non-trivial evidence for a strong split attractor flow tree conjecture, and
thus suggest that we indeed exhaust the BPS spectrum. In the D-brane
description of a bound state, the necessity for refinement results from the
fact that tachyonic strings split up constituent states into `generic' and
`special' states. These are enumerated separately by topological invariants,
which turn out to be partitions of Donaldson-Thomas invariants. As modular
predictions provide a check on many of our results, we have compelling evidence
that our computations are correct.Comment: 46 pages, 8 figures. v2: minor changes. v3: minor changes and
reference adde
U(N|M) quantum mechanics on Kaehler manifolds
We study the extended supersymmetric quantum mechanics, with supercharges
transforming in the fundamental representation of U(N|M), as realized in
certain one-dimensional nonlinear sigma models with Kaehler manifolds as target
space. We discuss the symmetry algebra characterizing these models and, using
operatorial methods, compute the heat kernel in the limit of short propagation
time. These models are relevant for studying the quantum properties of a
certain class of higher spin field equations in first quantization.Comment: 21 pages, a reference adde
Quantum theories of (p,q)-forms
We describe quantum theories for massless (p,q)-forms living on Kaehler
spaces. In particular we consider four different types of quantum theories: two
types involve gauge symmetries and two types are simpler theories without gauge
invariances. The latter can be seen as building blocks of the former. Their
equations of motion can be obtained in a natural way by first-quantizing a
spinning particle with a U(2)-extended supersymmetry on the worldline. The
particle system contains four supersymmetric charges, represented quantum
mechanically by the Dolbeault operators and their hermitian conjugates. After
studying how the (p,q)-form field theories emerge from the particle system, we
investigate their one loop effective actions, identify corresponding heat
kernel coefficients, and derive exact duality relations. The dualities are seen
to include mismatches related to topological indices and analytic torsions,
which are computed as Tr(-1)^F and Tr[(-1)^F F] in the first quantized
supersymmetric nonlinear sigma model for a suitable fermion number operator F.Comment: 44 pages, 2 figures, a reference adde
First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment
In this paper we investigate by means of first-principles density functional
theory calculations the (111) surface of the Ag-Cu alloy under varying
conditions of pressure of the surrounding oxygen atmosphere and temperature.
This alloy has been recently proposed as a catalyst with improved selectivity
for ethylene epoxidation with respect to pure silver, the catalyst commonly
used in industrial applications. Here we show that the presence of oxygen leads
to copper segregation to the surface. Considering the surface free energy as a
function of the surface composition, we construct the convex hull to
investigate the stability of various surface structures. By including the
dependence of the free surface energy on the oxygen chemical potential, we are
able compute the phase diagram of the alloy as a function of temperature,
pressure and surface composition. We find that, at temperature and pressure
typically used in ethylene epoxidation, a number of structures can be present
on the surface of the alloy, including clean Ag(111), thin layers of copper
oxide and thick oxide-like structures. These results are consistent with, and
help explain, recent experimental results.Comment: 10 pages, 6 figure
Predictions from a stochastic polymer model for the MinDE dynamics in E.coli
The spatiotemporal oscillations of the Min proteins in the bacterium
Escherichia coli play an important role in cell division. A number of different
models have been proposed to explain the dynamics from the underlying
biochemistry. Here, we extend a previously described discrete polymer model
from a deterministic to a stochastic formulation. We express the stochastic
evolution of the oscillatory system as a map from the probability distribution
of maximum polymer length in one period of the oscillation to the probability
distribution of maximum polymer length half a period later and solve for the
fixed point of the map with a combined analytical and numerical technique. This
solution gives a theoretical prediction of the distributions of both lengths of
the polar MinD zones and periods of oscillations -- both of which are
experimentally measurable. The model provides an interesting example of a
stochastic hybrid system that is, in some limits, analytically tractable.Comment: 16 page
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