Cherenkov radiation by particles traversing the background radiatio n

High energy particles traversing the Universe through the cosmic microwave backgroung radiation can, in principle, emit Cherenkov radiation. It is shown that the energy threshold for this radiation is extremely high and its intensity would be too low due to the low density of the "relic photons gas" and very weak interaction of two photons.Comment: 6 pages, LATEX, no Figs.; to be published in JETP Lett. 75 (N4) (2002

Regulatory Control of Vehicle and Power Plant Emissions: How Effective and at What Cost?

Passenger vehicles and power plants are major sources of greenhouse gas emissions. While economic analyses generally indicate that a broader market-based approach to greenhouse gas reduction would be less costly and more effective, regulatory approaches have found greater political success. Vehicle efficiency standards have a long history in the U.S and elsewhere, and the recent success of shale gas in the U.S. leads to a focus on coal–gas fuel switching as a way to reduce power sector emissions. We evaluate a global regulatory regime that replaces coal with natural gas in the electricity sector and imposes technically achievable improvements in the efficiency of personal transport vehicles. Its performance and cost are compared with other scenarios of future policy development including a no policy world, achievements under the Copenhagen accord, and a price-based policy to reduce global emissions by 50% by 2050. The assumed regulations applied globally achieve a global emissions reduction larger than projected for the Copenhagen agreements, but they do not prevent global GHG emissions from continuing to grow, and the reduction in emissions is achieved at a high cost compared to a price-based policy. Diagnosis of the reasons for the limited yet high-cost performance reveals influences including the partial coverage of emitting sectors, small or no influence on the demand for emissions-intensive products, leakage when a reduction in fossil use in the covered sectors lowers the price to others, and the partial coverage of GHGs.We thank BP for their support of this study. The MIT Integrated Global System Model (IGSM) and its economic component used in the analysis, the Emissions Prediction and Policy Analysis (EPPA model, are supported by a consortium of government, industry and foundation sponsors of the MIT Joint Program on the Science and Policy of Global Change, including U.S. Department of Energy, Office of Science (DE-FG02-94ER61937). For a complete list of sponsors see http://globalchange.mit.edu/sponsors/current.html

Sequencing of folding events in Go-like proteins

We have studied folding mechanisms of three small globular proteins: crambin (CRN), chymotrypsin inhibitor 2 (CI2) and the fyn Src Homology 3 domain (SH3) which are modelled by a Go-like Hamiltonian with the Lennard-Jones interactions. It is shown that folding is dominated by a well-defined sequencing of events as determined by establishment of particular contacts. The order of events depends primarily on the geometry of the native state. Variations in temperature, coupling strengths and viscosity affect the sequencing scenarios to a rather small extent. The sequencing is strongly correlated with the distance of the contacting aminoacids along the sequence. Thus $\alpha$-helices get established first. Crambin is found to behave like a single-route folder, whereas in CI2 and SH3 the folding trajectories are more diversified. The folding scenarios for CI2 and SH3 are consistent with experimental studies of their transition states.Comment: REVTeX, 12 pages, 11 EPS figures, J. Chem. Phys (in press

Breit interaction correction to the hyperfine constant of an external s-electron in many-electron atom

Correction to the hyperfine constant $A$ of an external s-electron in many-electron atom caused by the Breit interaction is calculated analytically: $\delta A/A =0.68 Z\alpha^2$. Physical mechanism for this correction is polarization of the internal electronic shells (mainly $1s^2$ shell) by the magnetic field of the external electron. This mechanism is similar to the polarization of vacuum considered by Karplus and Klein long time ago. The similarity is the reason why in both cases (Dirac sea polarization and internal atomic shells polarization) the corrections have the same dependence on the nuclear charge and fine structure constant. In conclusion we also discuss $Z\alpha^2$ corrections to the parity violation effects in atoms.Comment: 8 pages, 2 figure

One Loop Multiphoton Helicity Amplitudes

We use the solutions to the recursion relations for double-off-shell fermion currents to compute helicity amplitudes for $n$-photon scattering and electron-positron annihilation to photons in the massless limit of QED. The form of these solutions is simple enough to allow {\it all}\ of the integrations to be performed explicitly. For $n$-photon scattering, we find that unless $n=4$, the amplitudes for the helicity configurations (+++...+) and (-++...+) vanish to one-loop order.Comment: 27 pages + 4 uuencoded figures (included), Fermilab-Pub-93/327-T, RevTe

Collapse of Randomly Self-Interacting Polymers

We use complete enumeration and Monte Carlo techniques to study self--avoiding walks with random nearest--neighbor interactions described by $v_0q_iq_j$, where $q_i=\pm1$ is a quenched sequence of ``charges'' on the chain. For equal numbers of positive and negative charges ($N_+=N_-$), the polymer with $v_0>0$ undergoes a transition from self--avoiding behavior to a compact state at a temperature $\theta\approx1.2v_0$. The collapse temperature $\theta(x)$ decreases with the asymmetry $x=|N_+-N_-|/(N_++N_-)$Comment: 8 pages, TeX, 4 uuencoded postscript figures, MIT-CMT-

Thermodynamically Important Contacts in Folding of Model Proteins

We introduce a quantity, the entropic susceptibility, that measures the thermodynamic importance-for the folding transition-of the contacts between amino acids in model proteins. Using this quantity, we find that only one equilibrium run of a computer simulation of a model protein is sufficient to select a subset of contacts that give rise to the peak in the specific heat observed at the folding transition. To illustrate the method, we identify thermodynamically important contacts in a model 46-mer. We show that only about 50% of all contacts present in the protein native state are responsible for the sharp peak in the specific heat at the folding transition temperature, while the remaining 50% of contacts do not affect the specific heat.Comment: 5 pages, 5 figures; to be published in PR

Design Equation: A Novel Approach to Heteropolymer Design

A novel approach to heteropolymer design is proposed. It is based on the criterion by Kurosky and Deutsch, with which the probability of a target conformation in a conformation space is maximized at low but finite temperature. The key feature of the proposed approach is the use of soft spins (fuzzy monomers) that leads to a design equation, which is an analog of the Boltzmann machine learning equation in the design problem. We implement an algorithm based on the design equation for the generalized HP model on the 3x3x3 cubic lattice and check its performance.Comment: 7 pages, 3 tables, 1 figures, uses jpsj.sty, jpsjbs1.sty, epsf.sty, Submitted to J. Phys. Soc. Jp

Radiative Corrections to the Muonium Hyperfine Structure. I. The α2(Zα)\alpha^2 (Z\alpha) Correction

This is the first of a series of papers on a systematic application of the NRQED bound state theory of Caswell and Lepage to higher-order radiative corrections to the hyperfine structure of the muonium ground state. This paper describes the calculation of the $\alpha^2 (Z\alpha)$ radiative correction. Our result for the complete $\alpha^2 (Z\alpha)$ correction is 0.424(4) kHz, which reduces the theoretical uncertainty significantly. The remaining uncertainty is dominated by that of the numerical evaluation of the nonlogarithmic part of the $\alpha (Z\alpha )^2$ term and logarithmic terms of order $\alpha^4$.Comment: 56 pages, Rev.tex V3.0 and epsf.tex. 12 postscript files are called in the text. Version accepted by Phys. Rev. D. A new table is adde