We review the theoretical motivations and experimental status of searches for stable massive particles (SMPs) which could be sufficiently long-lived as to be directly detected at collider ...experiments. The discovery of such particles would address a number of important questions in modern physics including the origin and composition of dark matter in the universe and the unification of the fundamental forces. This review describes the techniques used in SMP-searches at collider experiments and the limits so far obtained on the production of SMPs which possess various colour, electric and magnetic charge quantum numbers. We also describe theoretical scenarios which predict SMPs along with the phenomenology needed to model their production at colliders and interactions with matter. In addition, the interplay between collider searches and open questions in cosmology such as dark matter composition is addressed.
The spectral functions of the one-band half-filled one-dimensional Hubbard chain are calculated using the exchange-correlation potential formalism developed recently. The exchange-correlation ...potential is adopted from the exact potential derived from the Hubbard dimer. Within an approximation in which the full Green's function is replaced by a noninteracting one, the spectral functions can be calculated analytically. Despite the simplicity of the approximation, the resulting spectra are in favorable agreement with the more accurate results obtained from the dynamic density-matrix renormalization group method. In particular, the analytically calculated band gap as a function of U is in close agreement with the exact gap obtained from the Bethe ansatz. In addition, the formal general solution to the equation of motion of the Green's function is presented, and the difference between the traditional self-energy approach and the exchange-correlation potential formalism is also discussed and elaborated. A simplified Holstein Hamiltonian is considered to further illustrate the general form of the exchange-correlation potential.
The concept of multiple partonic interactions in hadronic events is vital for the understanding of both minimum-bias and underlying-event physics. The area is rather little studied, however, and ...current models offer a far from complete coverage, even of the effects we know ought to be there. In this article we address one such topic, namely that of rescattering, where an already scattered parton is allowed to take part in another subsequent scattering. A framework for rescattering is introduced for the P
ythia
8 event generator and fully integrated with normal multiparton interactions and initial- and final-state radiation. Using this model, the effects on event structure are studied, and distributions are shown both for minimum-bias and jet events.
At the LHC all processes are QCD ones, whether 'signal' or 'background'. In this review the frontiers of current QCD research are addressed, towards increased understanding, improved calculational ...precision, and role in potential future discoveries. Issues raised include: the limits of perturbative QCD calculations and parton distribution usage, the nature of multiparton interactions, the impact of colour reconnection on physical observables, the need for progress on hadronization modelling, the improvements of parton showers and their combination with the matrix-element description, the use of QCD concepts in Beyond-the-Standard-Model scenarios and the key position of event generators and other software in the successful exploration of LHC physics. On the way, several questions are posed, where further studies are needed.
The magnon-phonon interaction is receiving growing attention due to its key role in spin caloritronics and the emerging field of acoustic spintronics. At resonance, this magnetoelastic interaction ...forms magnon polarons, which underpin exotic phenomena such as magnonic heat currents and phononic spin, but is mostly investigated using mesoscopic spin-lattice models. Motivated to integrate the magnon-phonon interaction into first-principles many-body electronic structure theory, we set out to derive the exchange contribution, which is subtler than the spin-orbit contribution, using Schwinger functional derivatives. To avoid having to solve the famous Hedin-Baym equations self-consistently, the phonons are treated as perturbations to the electronic structure. A formalism based on imposing a crossing-symmetric electron-electron interaction is developed in order to treat charge and spin on equal footing to respect the Pauli exclusion principle. Due to spin conservation, the magnon-phonon interaction first enters to second order through the magnon-magnon interaction, which renormalizes the magnons. We show by iteration that the magnon-magnon interaction contains a "screened T matrix"term and an arguably more important term which, in the local-spin limit, enables first-principles phonon emission and absorption amplitudes, predicted by phenomenological magnetoelastic models. These terms are, respectively, of first and second order in the screened collective four-point interaction W-a crossing-symmetric analog of Hedin's W. Proof-of-principle results are presented at varying temperatures for an isotropic magnon spectrum in three dimensions in the presence of a flat optical phonon branch.
Production and hadronization of heavy quarks Norrbin, E.; Sjöstrand, T.
The European physical journal. C, Particles and fields,
10/2000, Letnik:
17, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Heavy long-lived quarks, i.e. charm and bottom, are frequently studied both as tests of QCD and as probes for other physics aspects within and beyond the standard model. The long lifetime implies ...that charm and bottom hadrons are formed and observed. This hadronization process cannot be studied in isolation, but depends on the production environment. Within the framework of the string model, a major effect is the drag from the other end of the string that the c/b quark belongs to. In extreme cases, a small-mass string can collapse to a single hadron, thereby giving a non-universal flavor composition to the produced hadrons. We here develop and present a detailed model for the charm/bottom hadronization process, involving the various aspects of string fragmentation and collapse, and put it in the context of several heavy-flavor production sources. Applications are presented from fixed-target to LHC energies.
A selection of the latest and most frequently used parton distribution functions (PDFs) is incorporated in
Pythia8
, including the Monte Carlo-adapted PDFs from the MSTW and CTEQ collaborations. This ...article examines the differences in PDFs as well as the effect they have on results of simulations and compare with data collected by the CDF experiment. Monte Carlo-adapted PDFs do a better job than leading- and next-to-leading order PDFs for many observables, but there is room for further improvements.
Orbital magnetization is known empirically to play an important role in several magnetic phenomena, such as permanent magnetism and ferromagnetic superconductivity. Within the recently developed ..."modern theory of orbital magnetization," theoretical insight has been gained into the nature of this often neglected contribution to magnetism but is based on an underlying mean-field approximation. From this theory, a few treatments have emerged which also take into account correlations beyond the mean-field approximation. Here, we apply the scheme developed in a previous work F. Aryasetiawan et al., Phys. Rev. B 93, 161104(R) (2016) to the spin-12 Haldane-Hubbard model to investigate the effect of charge fluctuations on the orbital magnetization within the GW approximation. Qualitatively, we are led to distinguish between two quite different situations: (i) When the lattice potential is larger than the nearest-neighbor hopping, the correlations are found to boost the orbital magnetization. (ii) If the nearest-neighbor hopping is instead larger than the lattice potential, the correlations reduce the magnetization. The boost and reduction are identified to stem from interband and intraband correlations, respectively, and the relative importance of the two varies with the strength of the lattice potential. We finally study graphene with parameters obtained from first principles.
An essential ingredient in many model Hamiltonians, such as the Hubbard model, is the effective electron-electron interaction U, which enters as matrix elements in some localized basis. These matrix ...elements provide the necessary information in the model, but the localized basis is incomplete for describing U. We present a systematic scheme for computing the manifestly basis-independent dynamical interaction in position representation, U(r,r′;ω), and its Fourier transform to time domain, U(r,r′;τ). These functions can serve as an unbiased tool for the construction of model Hamiltonians. For illustration we apply the scheme within the constrained random-phase approximation to the cuprate parent compounds La2CuO4 and HgBa2CuO4 within the commonly used one- and three-band models, and to nonsuperconducting SrVO3 within the t2g model. Our method is used to investigate the shape and strength of screening channels in the compounds. We show that the O2px,y−Cu3dx2−y2 screening gives rise to regions with strong attractive static interaction in the minimal (one-band) model in both cuprates. On the other hand, in the minimal (t2g) model of SrVO3 only regions with a minute attractive interaction are found. The temporal interaction exhibits generic damped oscillations in all compounds, and its time integral is shown to be the potential caused by inserting a frozen point charge at τ=0. When studying the latter within the three-band model for the cuprates, short time intervals are found to produce a negative potential.
Orbital magnetization is known empirically to play an important role in several magnetic phenomena, suchas permanent magnetism and ferromagnetic superconductivity. Within the recently developed ...“modern theoryof orbital magnetization,” theoretical insight has been gained into the nature of this often neglected contributionto magnetism but is based on an underlying mean-field approximation. From this theory, a few treatments haveemerged which also take into account correlations beyond the mean-field approximation. Here, we apply thes cheme developed in a previous work F. Aryasetiawan et al., Phys. Rev. B 93, 161104(R) (2016) to thespin- 1/2 Haldane-Hubbard model to investigate the effect of charge fluctuations on the orbital magnetizationwithin the GW approximation. Qualitatively, we are led to distinguish between two quite different situations:(i) When the lattice potential is larger than the nearest-neighbor hopping, the correlations are found to boostthe orbital magnetization. (ii) If the nearest-neighbor hopping is instead larger than the lattice potential, thecorrelations reduce the magnetization. The boost and reduction are identified to stem from interband andintraband correlations, respectively, and the relative importance of the two varies with the strength of the latticepotential. We finally study graphene with parameters obtained from first principles.
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