“Almost direct” photons

Quantum chromodynamics has intensified the interest and enhanced the importance of measuring the production of direct photons in hadron collisions. We point out that lepton pairs of low invariant mass (m² < m²_ψ) provide an equally good probe for testing Q.C.D. Their experimental observation is not only easier, the relevant data may already exist.     

“Loop the loop”

The applicability of the collective coordinate method (saddle-point approximation) for large-N planar models is discussed. Some unstated assumptions are clarified. Statements that Wilson loops form a complete set of gauge invariant operators are also examined and a set of generalized algebraic Mandelstam relations among Wilson loops is presented. The inclusion of loops that wind around themselves and cross many times, as independent variables, is stressed.     

On “diagonal” Bianchi cosmologies

A list of the possible “diagonal” Bianchi cosmologies is given, with a proof that it is complete.     

Schlieren “PIV” for turbulent flows

The possibility of using commercial PIV equipment combined with schlieren optics to measure the velocity fields of turbulent flows is explored. Given a sufficiently high Reynolds number and adequate refractive flow differences, turbulent eddies can serve as the PIV “particles” in a schlieren image or shadowgram. The PIV software analyzes motion between consecutive schlieren or shadowgraph frames to obtain velocity fields. Velocimetry examples of an axisymmetric sonic helium jet in air and a 2D turbulent boundary layer at Mach 3 are shown. Due to optical path integration, axisymmetric flows require the inverse Abel transform to extract center-plane velocity data. Conditions for optimum schlieren sensitivity are examined. In its present embodiment, “schlieren PIV” is not useful for laminar flows nor for fully 3D flows. Otherwise it functions much like standard PIV under conditions where individual particles are not resolved and velocimetry is instead based on correlation of the motion of turbulent structures. “Schlieren PIV” shows significant promise for general refractive turbulent flow velocimetry if its integrative nature can be overcome through sharp-focusing optics.     

Designing the “perfect” projection screen

A perfect diffuser would place 100% of the light leaving the projector in that small region of space where there will be audience eyes to observe it. It would not allow light from sources other than the projector to reach the eyes from the screen. The screen should be affordably priced and cosmetically unremarkable, e.g. seamless. The image seen by any observer should be equally bright over the whole screen. I discuss a way to approximate the perfect projection screen using kinoform diffusers, a Fresnel lens and a mirrored surface.     

Novel comprehension of “common path” principle

The idea of “common path” has been widely applied in optical instrument design for 30 years and even today. But the meaning of “common path” has not yet been explained clearly and sometimes confusion has been created. In this paper an “adaptive principle” is proposed and recommended on optical instrument system. It suggests that the designer not only arranges the measurement system to obtain measurement signal but also sets a channel to give prediction of noise or disturbance in real time or short term. Such a recommendation is based on the recent studies on nonlinear dynamics and atmospheric disturbance by means of experiments as well as theoretical analysis.     

“Proper Time” Method Calculation of the “One Loop” Thermodynamical Potential for QED in External Electromagnetic Field

The extension of the “proper time” method to the statistical field theory is obtained. The procedure is used to calculate the “one loop” contribution to the thermodynamical potential in electrodynamics, when the electron positron gas is located in a combination of an inhomogeneous electrostatic field and an uniform magnetic one. Also, a “proper time” representation of the temperature Green function for this problem is given.     

Feasibility of “intermittent” active control of combustion instabilities in liquid fueled combustors using a “smart” fuel injector

This paper describes an experimental investigation of the feasibility of an “intermittent” active control approach for suppressing combustion instabilities in liquid fueled combustors. The developed controller employs a “smart” fuel injector that can modify the spray properties in response to changes in combustor operating conditions. This action weakens or breaks up the coupling between the combustion process and combustor acoustic modes oscillations, thus preventing the excitation of large amplitude instabilities. This approach differs significantly from previously proposed active control methods, both in concept and implementation, as it requires only “intermittent” modification of the combustion process by a single control action as opposed to the continuous action required by most other active control methods. The “smart” fuel injector used in this study consisted of a double-staged, air-assisted atomizer in which counter swirling, primary (inner stage) and secondary (outer stage) air streams were supplied to the injector through separate sets of tangentially oriented orifices. Control of the ratio of air mass flow rates supplied to these two stages, by use of a diverter valve, resulted in significant changes in the spray shape and its axial, tangential, and radial velocity components. This variation in spray properties of the “smart” injector was characterized for different values of the inner to outer air flow rate ratio in cold flow tests with a PDPA system. These results were then correlated with the characteristics of the “intermittently” controlled combustor. Measured quantities included the instability amplitudes, axial dependence of the mean and oscillatory heat release amplitudes, and the characteristics of the recirculation zones, which were all shown to depend on the fuel spray properties. The results of this study demonstrate the feasibility of using “smart” fuel injectors with capabilities for varying the combustion process characteristics to reduce the amplitudes of detrimental combustion instabilities in real engines to acceptable levels.     

Photon contribution to the “super-hard” pomeron

It is shown that direct photons provide a leading twist mechanism for diffractive jet production in which the jets carry away all of the momentum lost by the proton. Two-photon processes are thus expected to asymptotically dominate “super-hard” pomeron events in ep collisions. We report the expected rates from these events for recent ZEUS and H1 data cuts. We also estimate the direct photon contribution to the “super-hard” pomeron events observed by the CERN UA8 group for collisions. It is again argued that direct photons are the leading mechanism for these events. We find that direct photons are an appreciable fraction of the events seen by UA8.     

A comment on “ghosts and geometry”

Recent attempts at geometric interpretations of Feynman-De Witt-Faddeev-Popov ghosts and Becchi-Rouet-Stora symmetries of gauge theories are reviewed critically, and an interpretation in terms of the infinite-dimensional group of gauge transformations is restated. This interpretation seems adequate both in the path-integral approach and in canonical quantization with indefinite metric.     

Discovering and “rediscovering” the W boson

One of the first publications by the ATLAS collaboration using data from the Large Hadron Collider at CERN dealt with the measurement of the production cross section of the W boson. The collaboration “rediscovered” the W in order to, among other things, check whether the detector and analysis methods were working well. Originally, the discovery of the W had been announced in 1983 by the CERN management, referring mainly to work done by its UA1 collaboration. In both the discovery and the “rediscovery”, the convergence of two distinct sets of criteria of data selection was an important concern of the researchers. In 1983, this concern figured prominently in the published paper whereas in 2010 it was mainly dealt with inside the collaboration.     

Optimum excitation of “enhanced” central transition populations

Central transition (CT) sensitivity enhancement schemes that transfer polarization from satellites to the CT through selective saturation or inversion of neighboring satellite transitions have provided a welcome improvement for magic-angle spinning spectra of half-integer quadrupole nuclei. While many researchers have investigated and developed different methods of creating enhanced CT populations, here we investigate the conversion of these enhanced CT populations into observable CT coherence. We show a somewhat unexpected result that a conversion pulse length optimized for maximum sensitivity on equilibrium populations may not be optimum for an enhanced (non-equilibrium) polarization. Furthermore, CT enhancements can be lost if excessive rf field strength is used to convert this enhanced polarization into CT coherence. While a maximally enhanced CT signal is expected when using a perfectly selective CT conversion pulse, we have found that significant sensitivity loss can occur when using surprisingly low rf field strengths, even for sites with relatively large quadrupole coupling constants. We have systematically investigated these issues, and present some general guidelines and expectations when optimizing the conversion of enhanced (non-equilibrium) CT populations into observable CT coherence.