Hasil untuk "Physics"

W. Heiss

A short résumé is given about the nature of exceptional points (EPs) followed by discussions about their ubiquitous occurrence in a great variety of physical problems. EPs feature in classical as well as in quantum mechanical problems. They are associated with symmetry breaking for -symmetric Hamiltonians, where a great number of experiments has been performed, in particular in optics, and to an increasing extent in atomic and molecular physics. EPs are involved in quantum phase transition and quantum chaos; they produce dramatic effects in multichannel scattering, specific time dependence and more. In nuclear physics, they are associated with instabilities and continuum problems. Being spectral singularities they also affect approximation schemes. This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to ‘Quantum physics with non-Hermitian operators’.

Jonathan L. Feng

The identity of dark matter is a question of central importance in both astrophysics and particle physics. In the past, the leading particle candidates were cold and collisionless, and typically predicted missing energy signals at particle colliders. However, recent progress has greatly expanded the list of well-motivated candidates and the possible signatures of dark matter. This review begins with a brief summary of the standard model of particle physics and its outstanding problems. I then discuss several dark matter candidates motivated by these problems, including weakly interacting massive particles (WIMPs), superWIMPs, light gravitinos, hidden dark matter, sterile neutrinos, and axions. For each of these, I critically examine the particle physics motivations and present their expected production mechanisms, basic properties, and implications for direct and indirect detection, particle colliders, and astrophysical observations. Upcoming experiments will discover or exclude many of these candidates, and progress may open up an era of unprecedented synergy between studies of the largest and smallest observable length scales.

Ryan C. Rhodus

T. Guhr, A. Mueller-Groeling, H. Weidenmueller

We review the development of random-matrix theory (RMT) during the last fifteen years. We emphasize both the theoretical aspects, and the application of the theory to a number of fields. These comprise chaotic and disordered systems, the localization problem, many-body quantum systems, the Calogero-Sutherland model, chiral symmetry breaking in QCD, and quantum gravity in two dimensions. The review is preceded by a brief historical survey of the developments of RMT and of localization theory since their inception. We emphasize the concepts common to the above-mentioned fields as well as the great diversity of RMT. In view of the universality of RMT, we suggest that the current development signals the emergence of a new “statistical mechanics”: Stochasticity and general symmetry requirements lead to universal laws not based on dynamical principles.

T. Piran

Gamma-ray bursts (GRB's), short and intense pulses of low-energy $\ensuremath{\gamma}$ rays, have fascinated astronomers and astrophysicists since their unexpected discovery in the late sixties. During the last decade, several space missions\char22{}BATSE (Burst and Transient Source Experiment) on the Compton Gamma-Ray Observatory, BeppoSAX and now HETE II (High-Energy Transient Explorer)\char22{}together with ground-based optical, infrared, and radio observatories have revolutionized our understanding of GRB's, showing that they are cosmological, that they are accompanied by long-lasting afterglows, and that they are associated with core-collapse supernovae. At the same time a theoretical understanding has emerged in the form of the fireball internal-external shocks model. According to this model GRB's are produced when the kinetic energy of an ultrarelativistic flow is dissipated in internal collisions. The afterglow arises when the flow is slowed down by shocks with the surrounding circumburst matter. This model has had numerous successful predictions, like the predictions of the afterglow itself, of jet breaks in the afterglow light curve, and of the optical flash that accompanies the GRB's. This review focuses on the current theoretical understanding of the physical processes believed to take place in GRB's.

S. M. Sze

S. Carroll, V. Duvvuri, M. Trodden et al.

We show that cosmic acceleration can arise due to very tiny corrections to the usual gravitational action of general relativity, of the form ${R}^{\ensuremath{-}n}$ with $ng0.$ This model eliminates the need for a nonzero cosmological constant or any other form of dark energy, attributing a purely gravitational origin to the acceleration of the universe.

N. Engheta, R. Ziolkowski

R. Kubo

J. Als-Nielsen, D. McMorrow

D. Lyth, A. Riotto

This is a review of particle-theory models of inflation, and of their predictions for the primordial density perturbation that is thought to be the origin of structure in the Universe. It contains mini-reviews of the relevant observational cosmology, of elementary field theory and of supersymmetry, that may be of interest in their own right. The spectral index n(k), specifying the scale dependence of the spectrum of the curvature perturbation, will be a powerful discriminator between models, when it is measured by Planck with accuracy Δn∼0.01. The usual formula for n is derived, as well as its less familiar extension to the case of a multi-component inflaton; in both cases the key ingredient is the separate evolution of causally disconnected regions of the Universe. Primordial gravitational waves will be an even more powerful discriminator if they are observed, since most models of inflation predict that they are completely negligible. We treat in detail the new wave of models, which are firmly rooted in modern particle theory and have supersymmetry as a crucial ingredient. The review is addressed to both astrophysicists and particle physicists, and each section is fairly homogeneous regarding the assumed background knowledge.

A. S. Grove

D. Bouwmeester, A. Ekert, A. Zeilinger

T. Sjostrand, P. Ed'en, C. Friberg et al.

Pythia version 6 represents a merger of the Pythia 5, Jetset 7 and SPythia programs, with many improvements. It can be used to generate high-energy-physics ‘events’, i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions. The underlying physics is not understood well enough to give an exact description; the programs therefore contain a combination of analytical results and various models. The emphasis in this article is on new aspects, but a few words of general introduction are included. Further documentation is available on the web.

N. Davidson, T. Przedzinski, Z. Was

Abstract For five years now, PHOTOS Monte Carlo for bremsstrahlung in the decay of particles and resonances has been available with an interface to the C++ HepMC event record. The main purpose of the present paper is to document the technical aspects of the PHOTOS Monte Carlo installation and present version use. A multitude of test results and examples are distributed together with the program code. The PHOTOS C++ physics precision is better than its FORTRAN predecessor and more convenient steering options are also available. An algorithm for the event record interface necessary for process dependent photon emission kernel is implemented. It is used in Z and W decays for kernels of complete first order matrix elements of the decays. Additional emission of final state lepton pairs is also available. Physics assumptions used in the program and properties of the solution are reviewed. In particular, it is explained how the second order matrix elements were used in design and validation of the program iteration procedure. Also, it is explained that the phase space parameterization used in the program is exact. Program summary Program title: Photos++ Catalogue identifier: AEYF_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEYF_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 53720 No. of bytes in distributed program, including test data, etc.: 1297747 Distribution format: tar.gz Programming language: C++. Computer: PC. Operating system: Linux, MacOS. RAM: Bytes. Libraries take less than 2 MB. Memory complexity is O ( n ) with around 2–4 MB for events with 10 k particles. Classification: 11.1, 11.2. Nature of problem: Algorithm described in this paper can be used to add final state radiation to the event generated by external software using selected event record format. It can also be used on a sample of events loaded from data file. User can define parts of the decay tree on which algorithm can be invoked. The influence of the next-to-leading-order corrections, along with other options regarding electron–positron pair, muon pair and photon emission, can be studied. Solution method: The event record is traversed and a list of all decaying particles is created. Decays where program is not supposed to act and decays excluded by the user are removed from the list. The photon and pair adding algorithm is invoked separately on each remaining decay. If one or more particle is added to the decay, the kinematic of the whole decay tree is updated. Restrictions: Application of the algorithm strongly depends on the content on the event record. Insufficient precision or missing information may deteriorate quality of the results of the algorithm or prevent algorithm from working on the event or its parts. See Section  2 for more details. Running time: 10-30 s per 100k events for small events (less than 1k particles). The complexity strongly depends on the event content and user selection of excluded decays. The theoretical pessimistic complexity of the algorithm is O ( n 2 ) . However, such cases are highly unrealistic. In our tests, the average complexity is around O ( n 1.2 ) .

T. Schmitz

Yunping LIAO, Lixin WU, Guoji ZENG et al.

To address the challenge of achieving low cost, simplicity, and reliability in geological disaster monitoring and early warning, this study proposes a micro-vibration monitoring and pre-warning technology based on micro-electro-mechanical system (MEMS) sensors. Focusing on the common “dynamic” precursors of geological disasters, such as displacement, tilt, and vibration, and adhering to the comprehensive sensing principle of “hazard source–transmission path–affected body”, a pre-warning algorithm based on multi-point anomaly intensity factors was developed. Additionally, a micro-vibration monitoring and warning device with edge computing capabilities was designed. The effectiveness of the technology was systematically validated through in-situ tests on unstable rock masses and large-scale physical model experiments of landslides. Experimental results demonstrate that the monitoring device can effectively detect weak precursor signals prior to rockfall and landslide instability. Leveraging its edge computing capability, the warning device achieves pre-warning judgments within seconds, significantly enhancing the timeliness and accuracy of warnings. The micro-vibration monitoring and warning device is suitable for monitoring and pre-warning slopes and embankments in mountainous areas, particularly around residential areas. When integrated with traditional monitoring and warning technologies, it enables refined risk prevention and control of geological disasters.

E. Mörtsell, S. Dhawan

The Λ Cold Dark Matter model (ΛCDM) represents the current standard model in cosmology. Within this, there is a tension between the value of the Hubble constant, H0, inferred from local distance indicators and the angular scale of fluctuations in the Cosmic Microwave Background (CMB). In terms of Bayseian evidence, we investigate whether the tension is significant enough to warrant new physics in the form of modifying or adding energy components to the standard cosmological model. We find that late time dark energy explanations are not favoured by data whereas a pre-CMB decoupling extra dark energy component has a positive, although not substantial, Bayesian evidence. A constant equation of state of the additional early energy density is constrained to 0.086+0.04−0.03. Although this value deviates significantly from 1/3, valid for dark radiation, the latter is favoured based on the Bayesian evidence. If the tension persists, future estimates of H0 at the 1% level will be able to decisively determine which of the proposed explanations is favoured.

Sankar Das Sarma, D. Deng, L. Duan

The marriage of the two fields may give birth to a new research frontier that could transform them both.

C. Weber, David Zwicker, F. Jülicher et al.

Phase separating systems that are maintained away from thermodynamic equilibrium via molecular processes represent a class of active systems, which we call active emulsions. These systems are driven by external energy input, for example provided by an external fuel reservoir. The external energy input gives rise to novel phenomena that are not present in passive systems. For instance, concentration gradients can spatially organise emulsions and cause novel droplet size distributions. Another example are active droplets that are subject to chemical reactions such that their nucleation and size can be controlled, and they can divide spontaneously. In this review, we discuss the physics of phase separation and emulsions and show how the concepts that govern such phenomena can be extended to capture the physics of active emulsions. This physics is relevant to the spatial organisation of the biochemistry in living cells, for the development of novel applications in chemical engineering and models for the origin of life.