Disentangling running coupling and conformal effects in QCD

We investigate the relation between a postulated skeleton expansion and the conformal limit of QCD. We begin by developing some consequences of an Abelian-like skeleton expansion, which allows one to disentangle running-coupling effects from the remaining skeleton coefficients. The latter are by construction renormalon-free, and hence hopefully better behaved. We consider a simple ansatz for the expansion, where an observable is written as a sum of integrals over the running-coupling. We show that in this framework one can set a unique Brodsky-Lepage-Mackenzie (BLM) scale-setting procedure as an approximation to the running-coupling integrals, where the BLM coefficients coincide with the skeleton ones. Alternatively, the running-coupling integrals can be approximated using the effective charge method. We discuss the limitations in disentangling running coupling effects in the absence of a diagrammatic construction of the skeleton expansion. Independently of the assumed skeleton structure we show that BLM coefficients coincide with the conformal coefficients defined in the small $\beta_0$ (Banks-Zaks) limit where a perturbative infrared fixed-point is present. This interpretation of the BLM coefficients should explain their previously observed simplicity and smallness. Numerical examples are critically discussed.Comment: 38 pages; Revised version (to appear in PRD); includes modifications of section 3 and an added section 4. Sections 6 and 7 of the original version have been extracted and will be published separatel

Massive quark scattering at strong coupling from AdS/CFT

We extend the analysis of Alday and Maldacena for obtaining gluon scattering amplitudes at strong coupling to include external massive quark states. Our quarks are actually the N=2 hypermultiplets which arise when D7-brane probes are included in the AdS_5 x S^5 geometry. We work in the quenched approximation, treating the N=2 matter multiplets as external sources coupled to the N=4 SYM fields. We first derive appropriate massive-particle boundary conditions for the string scattering worldsheets. We then find an exact worldsheet which corresponds to the scattering of two massive quarks and two massless gluons and extract from this the associated scattering amplitude. We also find the worldsheet and amplitude for the scattering of four massive quarks. Our worldsheet solutions reduce to the four massless gluon solution of Alday and Maldacena in the limit of zero quark mass. The amplitudes we compute can also be interpreted in terms of 2-2 scattering involving gluons and massive W-bosons.Comment: 46 pages, 11 figures, v4: additional comments added to intr

Fixing the conformal window in QCD

A physical characterization of Landau singularities is emphasized, which should trace the lower boundary N_f^* of the conformal window in QCD and supersymmetric QCD. A natural way to disentangle ``perturbative'' from ``non-perturbative'' contributions to amplitudes below N_f^* is suggested. Assuming an infrared fixed point persists in the perturbative part of the QCD coupling even below N_f^* leads to the condition \gamma(N_f^*)=1, where \gamma is the critical exponent. Using the Banks-Zaks expansion, one gets 4<N_f^*<6. This result is incompatible with the existence of an analogue of Seiberg duality in QCD. The presence of a negative ultraviolet fixed point is required both in QCD and in supersymmetric QCD to preserve causality within the conformal window. Evidence for the existence of such a fixed point in QCD is provided.Comment: 10 pages, 1 figure, extended version of a talk given at the QCDNET2000 meeting, Paris, September 11-14 2000; main new material added is evidence for negative ultraviolet fixed point in QC

Opportunities offered by naturally occurring materials in lightweight aerostructures design

The raising concerns towards the environmental impact of the air transport sector have led the international regulation authorities to put pressure on the aeronautical industry in order to achieve ambitious goals both in the short and medium-term. The expected growth in the aviation sector for the upcoming years poses a greater challenge to both airplane manufacturers and operators who need to adopt revolutionary solutions to provide an effective response towards a more sustainable energy supply, including steep reductions in pollutant emissions and a significant improvement in recyclability. Natural materials are a viable path to address this challenge as they rely upon sustainable production processes and offer considerable advantages in terms of renewability, recyclability and biodegradability. Furthermore, evidences from recent research suggest that some natural materials offer mechanical properties equivalent or even better than conventional materials used for engineering applications in the aerospace sector. This paper presents a review of the progresses achieved in the use of natural based materials in aeronautical structures. Most of these materials are in the form of polymer based composites with either the inclusion of natural reinforcing fibres and resins or other types of natural materials used as core constituents in sandwich components, such as cork based composites. The presented data were collected from different research works and cover a broad range of loading conditions as experienced by aircraft under normal operational scenarios and taking into consideration relevant airworthiness requirements, such as fire resistance and damage tolerance. Results are very encouraging and support the use of natural based materials in aircraft structural components, opening a window of tangible opportunities for the design of more fuel efficient and environmentally sustainable aircraft in the near future

On the renormalization of multiparton webs

We consider the recently developed diagrammatic approach to soft-gluon exponentiation in multiparton scattering amplitudes, where the exponent is written as a sum of webs - closed sets of diagrams whose colour and kinematic parts are entangled via mixing matrices. A complementary approach to exponentiation is based on the multiplicative renormalizability of intersecting Wilson lines, and their subsequent finite anomalous dimension. Relating this framework to that of webs, we derive renormalization constraints expressing all multiple poles of any given web in terms of lower-order webs. We examine these constraints explicitly up to four loops, and find that they are realised through the action of the web mixing matrices in conjunction with the fact that multiple pole terms in each diagram reduce to sums of products of lower-loop integrals. Relevant singularities of multi-eikonal amplitudes up to three loops are calculated in dimensional regularization using an exponential infrared regulator. Finally, we formulate a new conjecture for web mixing matrices, involving a weighted sum over column entries. Our results form an important step in understanding non-Abelian exponentiation in multiparton amplitudes, and pave the way for higher-loop computations of the soft anomalous dimension.Comment: 60 pages, 15 figure

A laser obstacle detection and avoidance system for manned and unmanned aircraft applications

This paper presents the key design features, the numerical simulations and the experimental ground/flight test activities performed to verify the functionalities of an obstacle detection and avoidance system suitable for various classes of manned and unmanned aircraft. The Laser Obstacle Avoidance and Monitoring (LOAM) system is proposed as one of the key non-cooperative sensors adopted for avoiding obstacles/intruders in the context of a future Sense-and-Avoid (SAA) capability. After a brief description of the system architecture and of the main data processing algorithms, avoidance trajectory generation and performance estimation models are described. A simulation of the avoidance trajectory generation algorithm is performed in a realistic scenario. Additionally, a brief overview of ground and flight test activities performed on various platforms and their main results is also presented. Some of the key aspects of the LOAM Human Machine Interface and Interaction (HMI2) design are also outlined. The demonstrated detection and avoidance performances and the robust trajectory generation algorithm ensure a safe avoidance of all classes of obstacles (i.e. ground and aerial) in all weather conditions and flight phases

LIDAR obstacle warning and avoidance system for unmanned aerial vehicle sense-and-avoid

The demand for reliable obstacle warning and avoidance capabilities to ensure safe low-level flight operations has led to the development of various practical systems suitable for fixed and rotary wing aircraft. State-of-the-art Light Detection and Ranging (LIDAR) technology employing eye-safe laser sources, advanced electro-optics and mechanical beam-steering components delivers the highest angular resolution and accuracy performances in a wide range of operational conditions. LIDAR Obstacle Warning and Avoidance System (LOWAS) is thus becoming a mature technology with several potential applications to manned and unmanned aircraft. This paper addresses specifically its employment in Unmanned Aircraft Systems (UAS) Sense-and-Avoid (SAA). Small-to-medium size Unmanned Aerial Vehicles (UAVs) are particularly targeted since they are very frequently operated in proximity of the ground and the possibility of a collision is further aggravated by the very limited see-and-avoid capabilities of the remote pilot. After a brief description of the system architecture, mathematical models and algorithms for avoidance trajectory generation are provided. Key aspects of the Human Machine Interface and Interaction (HMI2) design for the UAS obstacle avoidance system are also addressed. Additionally, a comprehensive simulation case study of the avoidance trajectory generation algorithms is presented. It is concluded that LOWAS obstacle detection and trajectory optimisation algorithms can ensure a safe avoidance of all classes of obstacles (i.e., wire, extended and point objects) in a wide range of weather and geometric conditions, providing a pathway for possible integration of this technology into future UAS SAA architectures

Next-to-next-to-leading soft-gluon corrections for the top quark cross section and transverse momentum distribution

I present results for top quark production in hadronic collisions at LHC and Tevatron energies. The soft-gluon corrections to the differential cross section are resummed at next-to-next-to-leading-logarithm (NNLL) accuracy via the two-loop soft anomalous dimension matrices. Approximate next-to-next-to-leading-order (NNLO) differential and total cross sections are calculated. Detailed theoretical predictions are shown for the t tbar cross section and the top quark p_T distribution at the Tevatron and the LHC.Comment: 23 pages, 14 figures; additional results and figure

Next-to-eikonal corrections to soft gluon radiation: a diagrammatic approach

We consider the problem of soft gluon resummation for gauge theory amplitudes and cross sections, at next-to-eikonal order, using a Feynman diagram approach. At the amplitude level, we prove exponentiation for the set of factorizable contributions, and construct effective Feynman rules which can be used to compute next-to-eikonal emissions directly in the logarithm of the amplitude, finding agreement with earlier results obtained using path-integral methods. For cross sections, we also consider sub-eikonal corrections to the phase space for multiple soft-gluon emissions, which contribute to next-to-eikonal logarithms. To clarify the discussion, we examine a class of log(1 - x) terms in the Drell-Yan cross-section up to two loops. Our results are the first steps towards a systematic generalization of threshold resummations to next-to-leading power in the threshold expansion.Comment: 66 pages, 19 figure