21,504 research outputs found
Bounds on Cubic Lorentz-Violating Terms in the Fermionic Dispersion Relation
We study the recently proposed Lorentz-violating dispersion relation for
fermions and show that it leads to two distinct cubic operators in the
momentum. We compute the leading order terms that modify the non-relativistic
equations of motion and use experimental results for the hyperfine transition
in the ground state of the ion to bound the values of the
Lorentz-violating parameters and for neutrons. The resulting
bounds depend on the value of the Lorenz-violating background four-vector in
the laboratory frame.Comment: Revtex 4, four pages. Version to match the one to appear in Physical
Review
Axial and Vector Correlator Mixing in Hot and Dense Hadronic Matter
We study the manifestations of chiral symmetry restoration which have a
significance for the parity mixing. Restricting to pions and nucleons we
establish a formalism for the expression of the vector correlator, which
displays the mixing of the axial correlator into the vector one and unifies the
cases of the heat bath and the dense medium. We give examples of mixing
cross-sections. We also establish a link between the energy integrated mixing
cross-sections and the pion scalar density which governs the quenching factors
of coupling constants, such as the pion decay one, as well as the quark
condensate evolution.Comment: 12 pages, Latex, 4 PostScript Figure
Electrical Characterization of a Thin Edgeless N-on-p Planar Pixel Sensors For ATLAS Upgrades
In view of the LHC upgrade phases towards the High Luminosity LHC (HL-LHC),
the ATLAS experiment plans to upgrade the Inner Detector with an all-silicon
system. Because of its radiation hardness and cost effectiveness, the n-on-p
silicon technology is a promising candidate for a large area pixel detector.
The paper reports on the joint development, by LPNHE and FBK of novel n-on-p
edgeless planar pixel sensors, making use of the active trench concept for the
reduction of the dead area at the periphery of the device. After discussing the
sensor technology, and presenting some sensors' simulation results, a complete
overview of the electrical characterization of the produced devices will be
given.Comment: 9 pages, 9 figures, to appear in the proceedings of the 15th
International Workshops on Radiation Imaging Detector
A nature-inspired feature selection approach based on hypercomplex information
Feature selection for a given model can be transformed into an optimization task. The essential idea behind it is to find the most suitable subset of features according to some criterion. Nature-inspired optimization can mitigate this problem by producing compelling yet straightforward solutions when dealing with complicated fitness functions. Additionally, new mathematical representations, such as quaternions and octonions, are being used to handle higher-dimensional spaces. In this context, we are introducing a meta-heuristic optimization framework in a hypercomplex-based feature selection, where hypercomplex numbers are mapped to real-valued solutions and then transferred onto a boolean hypercube by a sigmoid function. The intended hypercomplex feature selection is tested for several meta-heuristic algorithms and hypercomplex representations, achieving results comparable to some state-of-the-art approaches. The good results achieved by the proposed approach make it a promising tool amongst feature selection research
Handling dropout probability estimation in convolution neural networks using meta-heuristics
Deep learning-based approaches have been paramount in recent years, mainly due to their outstanding results in several application domains, ranging from face and object recognition to handwritten digit identification. Convolutional Neural Networks (CNN) have attracted a considerable attention since they model the intrinsic and complex brain working mechanisms. However, one main shortcoming of such models concerns their overfitting problem, which prevents the network from predicting unseen data effectively. In this paper, we address this problem by means of properly selecting a regularization parameter known as Dropout in the context of CNNs using meta-heuristic-driven techniques. As far as we know, this is the first attempt to tackle this issue using this methodology. Additionally, we also take into account a default dropout parameter and a dropout-less CNN for comparison purposes. The results revealed that optimizing Dropout-based CNNs is worthwhile, mainly due to the easiness in finding suitable dropout probability values, without needing to set new parameters empirically
Performance of Irradiated Thin Edgeless N-on-P Planar Pixel Sensors for ATLAS Upgrades
In view of the LHC upgrade phases towards the High Luminosity LHC (HL-LHC),
the ATLAS experiment plans to upgrade the Inner Detector with an all-silicon
system. Because of its radiation hardness and cost effectiveness, the n-on-p
silicon technology is a promising candidate for a large area pixel detector.
The paper reports on the joint development, by LPNHE and FBK of novel n-on-p
edgeless planar pixel sensors, making use of the active trench concept for the
reduction of the dead area at the periphery of the device. After discussing the
sensor technology, a complete overview of the electrical characterization of
several irradiated samples will be discussed. Some comments about detector
modules being assembled will be made and eventually some plans will be
outlined.Comment: 6 pages, 13 figures, to appear in the proceedings of the 2013 Nuclear
Science Symposium and Medical Imaging Conference. arXiv admin note: text
overlap with arXiv:1311.162
Development of Edgeless n-on-p Planar Pixel Sensors for future ATLAS Upgrades
The development of n-on-p "edgeless" planar pixel sensors being fabricated at
FBK (Trento, Italy), aimed at the upgrade of the ATLAS Inner Detector for the
High Luminosity phase of the Large Hadron Collider (HL-LHC), is reported. A
characterizing feature of the devices is the reduced dead area at the edge,
achieved by adopting the "active edge" technology, based on a deep etched
trench, suitably doped to make an ohmic contact to the substrate. The project
is presented, along with the active edge process, the sensor design for this
first n-on-p production and a selection of simulation results, including the
expected charge collection efficiency after radiation fluence of comparable to those expected at HL-LHC (about
ten years of running, with an integrated luminosity of 3000 fb) for the
outer pixel layers. We show that, after irradiation and at a bias voltage of
500 V, more than 50% of the signal should be collected in the edge region; this
confirms the validity of the active edge approach.Comment: 20 pages, 9 figures, submitted to Nucl. Instr. and Meth.
Novel Silicon n-on-p Edgeless Planar Pixel Sensors for the ATLAS upgrade
In view of the LHC upgrade phases towards HL-LHC, the ATLAS experiment plans
to upgrade the Inner Detector with an all-silicon system. The n-on-p silicon
technology is a promising candidate for the pixel upgrade thanks to its
radiation hardness and cost effectiveness, that allow for enlarging the area
instrumented with pixel detectors. We report on the development of novel n-in-p
edgeless planar pixel sensors fabricated at FBK (Trento, Italy), making use of
the "active edge" concept for the reduction of the dead area at the periphery
of the device. After discussing the sensor technology and fabrication process,
we present device simulations (pre- and post-irradiation) performed for
different sensor configurations. First preliminary results obtained with the
test-structures of the production are shown.Comment: 6 pages, 5 figures, to appear in the proceedings of the 9th
International Conference on Radiation Effects on Semiconductor Materials
Detectors and Device
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