6,531 research outputs found
Scale Invariance vs. Conformal Invariance: Holographic Two-Point Functions in Horndeski Gravity
We consider Einstein-Horndeski gravity with a negative bare constant as a
holographic model to investigate whether a scale invariant quantum field theory
can exist without the full conformal invariance. Einstein-Horndeski gravity can
admit two different AdS vacua. One is conformal, and the holographic two-point
functions of the boundary energy-momentum tensor are the same as the ones
obtained in Einstein gravity. The other AdS vacuum, which arises at some
critical point of the coupling constants, preserves the scale invariance but
not the special conformal invariance due to the logarithmic radial dependence
of the Horndeski scalar. In addition to the transverse and traceless graviton
modes, the theory admits an additional trace/scalar mode in the scale invariant
vacuum. We obtain the two-point functions of the corresponding boundary
operators. We find that the trace/scalar mode gives rise to an non-vanishing
two-point function, which distinguishes the scale invariant theory from the
conformal theory. The two-point function vanishes in , where the full
conformal symmetry is restored. Our results indicate the strongly coupled scale
invariant unitary quantum field theory may exist in without the full
conformal symmetry. The operator that is dual to the bulk trace/scalar mode
however violates the dominant energy condition.Comment: Latex, 28 pages, comments and references adde
Design of a Wideband Antenna With Stable Omnidirectional Radiation Pattern Using the Theory of Characteristic Modes
Enhancement of singly and multiply strangeness in p-Pb and Pb-Pb collisions at 158A GeV/c
The idea that the reduction of the strange quark suppression in string
fragmentation leads to the enhancement of strange particle yield in
nucleus-nucleus collisions is applied to study the singly and multiply strange
particle production in p-Pb and Pb-Pb collisions at 158A GeV/c. In this
mechanism the strange quark suppression factor is related to the effective
string tension, which increases in turn with the increase of the energy, of the
centrality and of the mass of colliding system. The WA97 observation that the
strange particle enhancement increases with the increasing of centrality and of
strange quark content in multiply strange particles in Pb-Pb collisions with
respect to p-Pb collisions was accounted reasonably.Comment: 8 pages, 3 PostScript figures, in Latex form. submitted to PR
Recommended from our members
A review of net zero energy buildings in hot and humid climates: Experience learned from 34 case study buildings
Sustainable development in the building sector requires the integration of energy efficiency and renewable energy utilization in buildings. In recent years, the concept of net zero energy buildings (NZEBs) has become a potential plausible solution to improve efficiency and reduce energy consumption in buildings. To achieve an NZEB goal, building systems and design strategies must be integrated and optimized based on local climatic conditions. This paper provides a comprehensive review of NZEBs and their current development in hot and humid regions. Through investigating 34 NZEB cases around the world, this study summarized NZEB key design strategies, technology choices and energy performance. The study found that passive design and technologies such as daylighting and natural ventilation are often adopted for NZEBs in hot and humid climates, together with other energy efficient and renewable energy technologies. Most NZEB cases demonstrated site annual energy consumption intensity less than 100 kW-hours (kWh) per square meter of floor space, and some buildings even achieved “net-positive energy” (that is, they generate more energy locally than they consume). However, the analysis also shows that not all NZEBs are energy efficient buildings, and buildings with ample renewable energy adoption can still achieve NZEB status even with high energy use intensity. This paper provides in-depth case-study-driven analysis to evaluate NZEB energy performance and summarize best practices for high performance NZEBs. This review provides critical technical information as well as policy recommendations for net zero energy building development in hot and humid climates
A Survey on Model-based, Heuristic, and Machine Learning Optimization Approaches in RIS-aided Wireless Networks
Reconfigurable intelligent surfaces (RISs) have received considerable
attention as a key enabler for envisioned 6G networks, for the purpose of
improving the network capacity, coverage, efficiency, and security with low
energy consumption and low hardware cost. However, integrating RISs into the
existing infrastructure greatly increases the network management complexity,
especially for controlling a significant number of RIS elements. To unleash the
full potential of RISs, efficient optimization approaches are of great
importance. This work provides a comprehensive survey on optimization
techniques for RIS-aided wireless communications, including model-based,
heuristic, and machine learning (ML) algorithms. In particular, we first
summarize the problem formulations in the literature with diverse objectives
and constraints, e.g., sum-rate maximization, power minimization, and imperfect
channel state information constraints. Then, we introduce model-based
algorithms that have been used in the literature, such as alternating
optimization, the majorization-minimization method, and successive convex
approximation. Next, heuristic optimization is discussed, which applies
heuristic rules for obtaining low-complexity solutions. Moreover, we present
state-of-the-art ML algorithms and applications towards RISs, i.e., supervised
and unsupervised learning, reinforcement learning, federated learning, graph
learning, transfer learning, and hierarchical learning-based approaches.
Model-based, heuristic, and ML approaches are compared in terms of stability,
robustness, optimality and so on, providing a systematic understanding of these
techniques. Finally, we highlight RIS-aided applications towards 6G networks
and identify future challenges.Comment: This paper has been accepted by IEEE Communications Surveys and
Tutorial
Characterisation of the interface adhesion of elastic-plastic thin film/rigid substrate systems using a pressurized blister test numerical model
Heuristic Algorithms for RIS-assisted Wireless Networks: Exploring Heuristic-aided Machine Learning
Reconfigurable intelligent surfaces (RISs) are a promising technology to
enable smart radio environments. However, integrating RISs into wireless
networks also leads to substantial complexity for network management. This work
investigates heuristic algorithms and applications for optimizing RIS-aided
wireless networks, including greedy algorithms, meta-heuristic algorithms, and
matching theory. Moreover, we combine heuristic algorithms with machine
learning (ML), and propose three heuristic-aided ML algorithms, namely
heuristic deep reinforcement learning (DRL), heuristic-aided supervised
learning, and heuristic hierarchical learning. Finally, a case study shows that
heuristic DRL can achieve higher data rates and faster convergence than
conventional DRL. This work aims to provide a new perspective for optimizing
RIS-aided wireless networks by taking advantage of heuristic algorithms and ML
- …