5386232
B- -> pi- pi0/rho- rho0 to NNLO in QCD factorization
The approximate tree decays B - → π - π 0 / ρ - ρ 0 may serve as benchmark channels for testing the various theoretical descriptions of the strong interaction dynamics in hadronic B meson decays. The ratios of hadronic and differential semileptonic B→ πlv/ ρlv decay rates at maximum recoil provide particularly clean probes of the QCD dynamics. We confront the recent next-to-next-to-leading order calculation in the QCD factorization framework with experimental data and find support for the factorization assumption. A detailed analysis of all tree-dominated B → ππ/πρ/ρρ decay modes seems to favor somewhat enhanced color-suppressed amplitudes, which may be accommodated in QCD factorization by a small value of the first inverse moment of the B meson light-cone distribution amplitude, λ B ≃ 250 MeV. Precise measurements of the semileptonic B → ρlv spectrum could help to clarify this point.
118509342
Master integrals for the two-loop penguin contribution in non-leptonic B-decays
A bstractWe compute the master integrals that arise in the calculation of the leading penguin amplitudes in non-leptonic B-decays at two-loop order. The application of differential equations in a canonical basis enables us to give analytic results for all master integrals in terms of iterated integrals with rational weight functions. It is the first application of this method to the case of two different internal masses.
119143806
Higher order QCD corrections in exclusive charmless B decays
We discuss exclusive charmless B decays within the Standard Model of particle physics. In particular we compute part of the NNLO corrections for hadronic two-body decays in the framework of QCD Factorization. We also present a conceptual analysis of heavy-to-light form factors at large recoil.
14116539
NNLO corrections to inclusive semileptonic B decays in the shape-function region
Abstract We compute 2-loop QCD corrections to the hard coefficient functions which arise in the factorization formula for B → X u l ν decays in the shape-function region. Our calculation provides the last missing piece required for a NNLO analysis of inclusive semileptonic B decays, which may significantly reduce the theoretical uncertainty in the extraction of the CKM matrix element | V u b | . Among the technical aspects, we find that the 2-loop hard coefficient functions are free of infrared singularities as predicted by the factorization framework. We perform a brief numerical analysis of the NNLO corrections and include a discussion on charm mass effects.
43936826
The transverse-momentum spectrum of Higgs bosons near threshold at NNLO
A bstractWe give next-to-next-to-leading order (NNLO) predictions for the Higgs production cross section at large transverse momentum in the threshold limit. Near the partonic threshold, all radiation is either soft or collinear to the final state jet which recoils against the Higgs boson. We find that the real emission corrections are of moderate size, but that the virtual corrections are large. We discuss the origin of these corrections and give numerical predictions for the transverse-momentum spectrum. The threshold result is matched to the known NLO result and implemented in the public code PeTeR.
56450914
NNLO corrections to semileptonic and hadronic B decays
We review recent progress in the perturbative calculations for semileptonic and hadronic B meson decays with an emphasis on the phenomenological implications of the next-to-next-to-leading order (NNLO) corrections. Specifically, we consider CP-averaged branching ratios of tree-dominated B -> pi pi/pi rho/rho rho decays and the inclusive determination of |V_ub| from B -> X_u l nu.
9002585
Progress with FEL-based coherent electron cooling
Cooling intense high-energy hadron beams remains a major challenge for accelerator physics. Synchrotron radiation is too feeble, while efficiency of two other cooling methods falls rapidly either at high bunch intensities (i.e. stochastic cooling of protons) or at high energies (i.e. e-cooling). The possibility of coherent electron cooling, based on high-gain FEL and ERL, was presented at last FEL conference [1]. This scheme promises significant increases in luminosities of modern high-energy hadron and electron-hadron colliders, such as LHC and eRHIC. In this paper we report progress made in the past year on the development of this scheme of coherent electron cooling (CeC), results of analytical and numerical evaluation of the concept as well our prediction for LHC and RHIC. We also present layout for proof-of-principle experiment at RHIC using our R&D ERL which is under construction.
118415064
Fringe field simulations of a non-scaling FFAG accelerator
Fixed-field Alternating Gradient (FFAG) accelerators offer the potential of high-quality, moderate energy ion beams at low cost. Modeling of these structures is challenging with conventional beam tracking codes because of the large radial excursions of the beam and the significance of fringe field effects. Numerous tune resonances are crossed during the acceleration, which would lead to beam instability and loss in a storage ring. In a non-scaling FFAG, the hope is that these resonances can be crossed sufficiently rapidly to prevent beam loss. Simulations are required to see if this is indeed the case. Here we simulate a non-scaling FFAG which accelerates protons from 31 to 250 MeV. We assume only that the bending magnets have mid-plane symmetry, with specified vertical bending field in the mid-plane (y=0). The magnetic field can be obtained everywhere using a power series expansion, and we develop mathematical tools for calculating this expansion to arbitrary order when the longitudinal field profile is given by an Enge function. We compare the use of a conventional hard-edge fringe with a more accurate, soft-edge fringe field model. The tune 1/3 resonance is the strongest, and crossing it in the hard-edge fringe model results in a 21% loss of the beam. Using the soft-edge fringe model the beam loss is less than 6%.
37838867
Numerical algorithms for modeling electron cooling in the presence of external fields
The design of the high-energy cooler for the Relativistic Heavy Ion Collider (RHIC) recently adopted a nonmagnetized approach. To prevent recombination between the fully stripped gold ions and co-propagating electrons, a helical undulator magnet has been proposed. In addition, to counteract space-charge defocusing, weak solenoids are proposed every 10 m. To understand the effect of these magnets on the cooling rate, numerical models of cooling in the presence of external fields are needed. We present an approach from first principles using the VORPAL parallel simulation code. We solve the n-body problem by exact calculation of pair-wise collisions. Simulations of the proposed RHIC cooler are discussed, including fringe field and finite interaction time effects.
119104094
Modulator simulations for coherent electron cooling using a variable density electron beam
Increasing the luminosity of relativistic hadron beams is critical for the advancement of nuclear physics. Coherent electron cooling (CEC) promises to cool such beams significantly faster than alternative methods. We present simulations of 40 GeV/nucleon Au+79 ions through the first (modulator) section of a coherent electron cooler. In the modulator, the electron beam copropagates with the ion beam, which perturbs the electron beam density and velocity via anisotropic Debye shielding. In contrast to previous simulations, where the electron density was constant in time and space, here the electron beam has a finite transverse extent, and undergoes focusing by quadrupoles as it passes through the modulator. The peak density in the modulator increases by a factor of 3, as specified by the beam Twiss parameters. The inherently 3D particle and field dynamics is modeled with the parallel VSim framework using a $\delta$f PIC algorithm. Physical parameters are taken from the CEC proof-of-principle experiment under development at Brookhaven National Lab.
55044187
VLASOV AND PIC SIMULATIONS OF A MODULATOR SECTION FOR COHERENT ELECTRON COOLING
Next generation ion colliders will require effective cooling of high-energy hadron beams. Coherent electron cooling (CEC) can in principle cool relativistic hadron beams on orders-of-magnitude shorter time scales than other techniques [1]. We present Vlasov-Poisson and delta-f particlein-cell (PIC) simulations of a CEC modulator section. These simulations correctly capture the subtle time and space evolution of the density and velocity wake imprinted on the electron distribution via anisotropic Debye shielding of a drifting ion. We consider 1D and 2D reduced versions of the problem, and compare the exact solutions of Wang and Blaskiewicz [2] with Vlasov-Poisson and delta-f PIC simulations. We also consider interactions under non-ideal conditions where there is a density gradient in the electron distribution, and present simulations of the ion wake. COHERENT ELECTRON COOLING Coherent electron cooling (CEC) is an exciting new technique for rapidly cooling high-energy hadron beams [1]. CEC begins in the same way as conventional electron cooling, with hadrons (in this case ions) and electrons co-propagating at matched mean velocities. The proposed Brookhaven CEC consists of three sections: a modulator, where the ion imprints a density wake on the electron distribution, an FEL, where the density wake is amplified by an FEL, and a kicker, where the amplified wake interacts with the ion, resulting in dynamical friction for the ion. In this paper we consider only the modulator section. We calculate the wakes in the electron distribution due to the presence of the ions. In the modulator section the ions drift in a nearly uniform distribution of electrons. Although these beams are highly relativistic in the laboratory frame, all velocities are non-relativistic in the “beam frame” drifting with the mean speed of the particles. The response of the electrons is classical Debye shielding, with a few complications. Accelerated electrons have a non-isotropic velocity distribution, and the electron density may also be non-uniform.