2,915 research outputs found

### Space-Time Noncommutativity, Discreteness of Time and Unitarity

Violation of unitarity for noncommutative field theory on compact space-times
is considered. Although such theories are free of ultraviolet divergences, they
still violate unitarity while in a usual field theory such a violation occurs
when the theory is nonrenormalizable. The compactness of space-like coordinates
implies discreteness of the time variable which leads to appearance of
unphysical modes and violation of unitarity even in the absence of a
star-product in the interaction terms. Thus, this conclusion holds also for
other quantum field theories with discrete time. Violation of causality, among
others, occurs also as the nonvanishing of the commutation relations between
observables at space-like distances with a typical scale of noncommutativity.
While this feature allows for a possible violation of the spin-statistics
theorem, such a violation does not rescue the situation but makes the scale of
causality violation as the inverse of the mass appearing in the considered
model, i.e., even more severe. We also stress the role of smearing over the
noncommutative coordinates entering the field operator symbols.Comment: 10 pages, plain LaTe

### Searching for intrinsic charm in the proton at the LHC

Despite rather long-term theoretical and experimental studies, the hypothesis
of the non-zero intrinsic (or valence-like) heavy quark component of the proton
distribution functions has not yet been confirmed or rejected. The LHC with
$pp$-collisions at $\sqrt{s}=$ 7--14 TeV will obviously supply extra unique
information concerning the above-mentioned component of the proton. To use the
LHC potential, first of all, one should select the parton-level (sub)processes
(and final-state signatures) that are most sensitive to the intrinsic heavy
quark contributions. To this end inclusive production of $c(b)$-jets
accompanied by photons is considered. On the basis of the performed theoretical
study it is demonstrated that the investigation of the intrinsic heavy quark
contributions looks very promising at the LHC in processes such as
$pp\rightarrow \gamma+ c(b)+X$.Comment: 13 pages, 6 figure

### OGSA/Globus Evaluation for Data Intensive Applications

We present an architecture of Globus Toolkit 3 based testbed intended for
evaluation of applicability of the Open Grid Service Architecture (OGSA) for
Data Intensive Applications.Comment: To be published in the proceedings of the XIX International Symposium
on Nuclear Electronics and Computing (NEC'2003), Bulgaria, Varna, 15-20
September, 200

### Aharonov-Bohm Effect in Noncommutative Spaces

The Aharonov-Bohm effect on the noncommutative plane is considered.
Developing the path integral formulation of quantum mechanics, we find the
propagation amplitude for a particle in a noncommutative space. We show that
the corresponding shift in the phase of the particle propagator due to the
magnetic field of a thin solenoid receives certain gauge invariant corrections
because of the noncommutativity. Evaluating the numerical value for this
correction, an upper bound for the noncommutativity parameter is obtained.Comment: 2nd version, some clarifications added, the results compared with the
existing experimental dat

### Quantum Field Theory on the Noncommutative Plane with $E_q(2)$ Symmetry

We study properties of a scalar quantum field theory on the two-dimensional
noncommutative plane with $E_q(2)$ quantum symmetry. We start from the
consideration of a firstly quantized quantum particle on the noncommutative
plane. Then we define quantum fields depending on noncommutative coordinates
and construct a field theoretical action using the $E_q(2)$-invariant measure
on the noncommutative plane. With the help of the partial wave decomposition we
show that this quantum field theory can be considered as a second quantization
of the particle theory on the noncommutative plane and that this field theory
has (contrary to the common belief) even more severe ultraviolet divergences
than its counterpart on the usual commutative plane. Finally, we introduce the
symmetry transformations of physical states on noncommutative spaces and
discuss them in detail for the case of the $E_q(2)$ quantum group.Comment: LaTeX, 26 page

### Measurement of the energy resolution and calibration of hybrid pixel detectors with GaAs:Cr sensor and Timepix readout chip

This paper describes an iterative method of per-pixel energy calibration of
hybrid pixel detectors with GaAs:Cr sensor and Timepix readout chip. A
convolution of precisely measured spectra of characteristic X-rays of different
metals with the resolution and the efficiency of the pixel detector is used for
the calibration. The energy resolution of the detector is also measured during
the calibration. The use of per-pixel calibration allows to achieve a good
energy resolution of the Timepix detector with GaAs:Cr sensor: 8% and 13% at 60
keV and 20 keV, respectively

### Two particle Quantummechanics in 2+1 Gravity using Non Commuting Coordinates

We find that the momentum conjugate to the relative distance between two
gravitating particles in their center of mass frame is a hyperbolic angle. This
fact strongly suggests that momentum space should be taken to be a hyperboloid.
We investigate the effect of quantization on this curved momentum space. The
coordinates are represented by non commuting, Hermitian operators on this
hyperboloid. We also find that there is a smallest distance between the two
particles of one half times the Planck length.Comment: 18 pages Latex, 2 eps figure

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