18828707
Production of Large Transverse Momentum W Bosons at the Tevatron
We discuss the production of W bosons at large transverse momentum in p pbar collisions at the Tevatron Run I and II. W boson hadroproduction is a process of relevance in testing the Standard Model and in estimates of backgrounds to new physics. The next-to-leading order cross section in the region of large transverse momentum is dominated by threshold soft-gluon corrections which arise from incomplete cancellations near partonic threshold between graphs with real emission and virtual graphs due to the limited phase space available for real gluon emission. In this contribution it is shown how, by including next-to-next-to-leading-order (NNLO) soft-gluon corrections, the transverse momentum distribution of the W at the Tevatron is modestly enhanced and the dependence on the factorization and renormalization scales is significantly reduced.
119065474
Small-x phenomenology – Summary of the 3rd Lund small-x workshop in 2004
A third workshop on small-x physics, within the Small-x Collaboration, was held in Hamburg in May 2004 with the aim of overviewing recent theoretical progress in this area and summarizing the experimental status.
111837585
Conformal Signatures in Mueller-Navelet and Forward Jets in DIS
5866670
The Gluon Green's Function at Small x
In this contribution a recently proposed iterative procedure is used to study the BFKL gluon Green's function at next-to-leading order. This is done in QCD and in N=4 supersymmetric Yang-Mills theory. The study includes an analysis of the evolution with energy and of angular dependences. A discussion of a novel resummation of running coupling terms in the QCD case is included.
18570212
The High Energy Limit of QCD: BFKL Cross Sections
In this contribution we describe in some detail different aspects of the construction of BFKL cross sections. We focus on several effects which are relevant at next-to-leading order. In particular, we describe QCD coherence in DIS final states, improvements of the collinear region in multi-Regge kinematics, inclusive jet production at next-to-leading order, and azimuthal angle decorrelations of Mueller-Navelet jets at hadron colliders.
1754173
What HERA May Provide
More than 100 people participated in a discussion session at the DIS08 workshop on the topic What HERA may provide. A summary of the discussion with a structured outlook and list of desirable measurements and theory calculations is given.
2038251
Time-dependent Ionization in Radiatively Cooling Gas
We present new computations of the equilibrium and nonequilibrium cooling efficiencies and ionization states for low-density radiatively cooling gas containing the elements H, He, C, N, O, Ne, Mg, Si, S, and Fe. We present results for gas temperatures between 10^4 and 10^8 K, assuming dust-free and optically thin conditions, and no external radiation. For nonequilibrium cooling we solve the coupled time-dependent ionization and energy loss equations for a radiating gas cooling from an initially hot, ≳5 × 10^6 K, equilibrium state, down to 10^4 K. We present results for heavy element compositions ranging from 10^(-3) to 2 times the elemental abundances in the Sun. We consider gas cooling at constant density (isochoric) and at constant pressure (isobaric). We calculate the critical column densities and temperatures at which radiatively cooling clouds make the dynamical transition from isobaric to isochoric evolution. We construct ion ratio diagnostics for the temperature and metallicity in radiatively cooling gas. We provide numerical estimates for the maximal cloud column densities for which the gas remains optically thin to the cooling radiation. We present our computational results in convenient online figures and tables.
11565526
Gravitational lensing of the x-ray background by clusters of galaxies
Gravitational lensing by clusters of galaxies affects the cosmic X-ray background (XRB) by altering the observed density and flux distribution of background X-ray sources. At faint detection flux thresholds, the resolved X-ray sources appear brighter and diluted, while the unresolved component of the XRB appears dimmer and more anisotropic, because of lensing. The diffuse X-ray intensity in the outer halos of clusters might be lower than the sky-averaged XRB, after the subtraction of resolved sources. Detection of the lensing signal with a wide-field X-ray telescope could probe the mass distribution of a cluster out to its virialization boundary. In particular, we show that the lensing signature imprinted on the resolved component of the XRB by the cluster A1689 should be difficult, but possible, to detect out to 8' at the 2-4 σ level, after 106 s of observation with the forthcoming Advanced X-Ray Astrophysics Facility (AXAF) satellite. The lensing signal is fairly insensitive to the lens redshift in the range 0.1 zl 0.6. The amplitude of the lensing signal is, however, sensitive to the faint-end slope of the number-flux relation for unresolved X-ray sources, and it can thus help constrain models of the XRB. A search for X-ray arcs or arclets could identify the fraction of all faint sources that originate from extended emission of distant galaxies. The probability for a 3 σ detection of an arclet that is stretched by a factor of approximately 3 after a 106 s observation of A1689 with AXAF is roughly comparable to the fraction of all background X-ray sources that have an intrinsic size of approximately 1''.
1239184
General Relativistic Effects in the Core Collapse Supernova Mechanism
We apply our recently developed code for spherically symmetric, fully general relativistic (GR) Lagrangian hydrodynamics and multigroup flux-limited diffusion (MGFLD) neutrino transport to examine the effects of GR on the hydrodynamics and transport during collapse, bounce, and the critical shock reheating phase of core collapse supernovae. GR effects were examined by performing core collapse simulations from several precollapse models in the Newtonian limit, in a hybrid limit consisting of GR hydrodynamics and Newtonian transport, and in the fully GR limit. Comparisons of models computed with GR versus Newtonian hydrodynamics show that collapse to bounce takes slightly less time in the GR limit and that the shock propagates slightly farther out in radius before receding. After a secondary quasi-static rise in the shock radius, the shock radius declines considerably more rapidly in the GR simulations than in the corresponding Newtonian simulations. During the shock reheating phase, core collapse computed with GR hydrodynamics results in a substantially more compact structure from the center out to the stagnated shock, the shock radius being reduced by a factor of 2 after 300 ms for a 25 M model and 600 ms for a 15 M model, times being measured from bounce. The inflow speedmore » of material behind the shock is also increased by about a factor of 2 throughout most of the evolution as a consequence of GR hydrodynamics. Regarding neutrino transport, comparisons show that the luminosity and rms energy of any neutrino flavor during the shock reheating phase increases when switching from Newtonian to GR hydrodynamics. This arises from the close coupling of the hydrodynamics and transport and the effect of GR hydrodynamics to produce more compact core structures, hotter neutrinospheres at smaller radii. On additionally switching from Newtonian to GR transport, gravitational time dilation and redshift effects decrease the luminosities and rms energies of all neutrino flavors. This decrease is less in magnitude than the increase in neutrino luminosities and rms energies that arise when switching from Newtonian to GR hydrodynamics, with the result that a fully GR simulation gives higher neutrino luminosities and harder neutrino spectra than a fully Newtonian simulation of the same precollapse model.« less
118707424
A new rare type of supernovae: hypervelocity stellar collisions at galactic centers
When a binary star system is tidally disrupted by a supermassive black hole at a galactic nucleus, one star is ejected at a high speed while the other remains in a tightly bound orbit around the black hole. The cluster of tightly bound stars builds over time, eventually creating a steady state in which the rate of collisions between these stars is similar to the rate of capturing new stars. A large fraction of the collisions occur near the periapsis of the orbits around the black hole, where the kinetic energies are sufficient to generate an explosive disruption of the two stars involved. The typical flare brightens for several days, with a peak luminosity that is comparable to the lower-luminosity end of known supernovae. The explosion lightcurve is followed by a longer flare due to accretion of ejected matter onto the black hole. Dedicated searches in the near universe could observe several such "collisional-supernovae" per year.
119095544
Final state interactions and new physics in B ---> pi K decays
Within the standard model, and if one assumes that soft rescattering effects are negligible, the $\mathrm{CP}$ asymmetry ${\mathcal{A}}_{\mathrm{CP}}^{\mathrm{dir}}{(B}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}K)$ is predicted to be very small and the ratio ${R=B(B}_{d}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{\mp}}{K}^{\ifmmode\pm\else\textpm\fi{}}{)/B(B}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}K)$ provides a bound on the angle $\ensuremath{\gamma}$ of the unitarity triangle, ${\mathrm{sin}}^{2}\ensuremath{\gamma}l~R.$ We estimate the corrections from soft rescattering effects using an approach based on Regge phenomenology, and find effects of order $10%$ with large uncertainties. In particular, we conclude that ${\mathcal{A}}_{\mathrm{CP}}^{\mathrm{dir}}\ensuremath{\sim}0.2$ and ${\mathrm{sin}}^{2}\ensuremath{\gamma}\ensuremath{\sim}1.2R$ could not be taken unambiguously to signal new physics. Using SU(3) relations, we suggest experimental tests that could constrain the size of the soft rescattering effects thus reducing the related uncertainty. Finally, we study the effect of various models of new physics on ${\mathcal{A}}_{\mathrm{CP}}^{\mathrm{dir}}$ and on $R.$