121260316
Proton-proton bremsstrahlung: Consequences of different on-shell-point conditions
Background: Proton-proton bremsstrahlung (pp{gamma}) is investigated both theoretically and experimentally. The pp{gamma} amplitudes can be classified according to the number and location of on-shell points at which they are evaluated. Quantitative understanding of the effect on the pp{gamma} cross section of using different on-shell-point conditions is lacking, but it is essential to identifying the photon emission mechanism governing the pp{gamma} process. Method: Four different pp{gamma} amplitudes, which include four-on-shell-point amplitudes and one-on-shell-point amplitude, are generated from a realistic one-boson-exchange (ROBE) model for pp scattering. These ROBE amplitudes are used to investigate the consequence of using different on-shell-point conditions in calculating the pp{gamma} cross sections. Purpose: We verify the validity of the ROBE pp{gamma} amplitudes. We explore similarities between the four-on-shell-point ROBE and two-u-two-t special (TuTts) soft-photon pp{gamma} amplitudes and important differences with the one-on-shell-point ROBE and Low pp{gamma} amplitudes. We demonstrate that the precision Kernfysisch Versneller Instituut (KVI) data can only be described by the four-on-shell-point (ROBE or TuTts) amplitude. We use the ROBE four-on-shell-point amplitude and one-on-shell-point amplitude to investigate systematically the effect of using different on-shell-point conditions to calculate the pp{gamma} cross section. Furthermore, we identify a general principle that governs the process. This general principle is alsomore » applicable to other bremsstrahlung processes involving the scattering of two identical nucleons. Results: (i) The four-on-shell-point ROBE (or TuTts) amplitude describes the high-precision KVI data much better than does the one-on-shell-point ROBE (or Low) amplitude. Although the contribution from the anomalous magnetic moment of the proton is very significant, it does not completely dominate the KVI pp{gamma} cross sections. (ii) The four-on-shell-point ROBE (or TuTts) amplitude describes the TRIUMF data better than does the one-on-shell-point ROBE (or Low) amplitude. (iii) The effect on the pp{gamma} cross section of using different on-shell-point conditions is significant in the hard-photon region, i.e., for small proton scattering angles {theta} (={theta}{sub 3}={theta}{sub 4}, symmetric scattering angles) far from the elastic limit ({theta}{yields}45 deg.); in contrast, the effect becomes insignificant in the vicinity of the elastic limit. Near the limit as {theta} tends to 45 deg. (or as the photon energy K approaches zero), the four-on-shell-point and one-on-shell-point amplitudes approach one another, a general principle applicable to all bremsstrahlung processes because only kinematics is involved. Conclusion: The four-on-shell-point ROBE amplitude provides a quantitative description of pp{gamma} cross sections. The anomalous magnetic moment is an important component of the photon emission mechanism. The four-on-shell-point property of the full ROBE amplitude is essential to properly describing the complete range of the precision KVI data and the TRIUMF data, although the one-on-shell-point ROBE amplitude is adequate in the region near the elastic limit.« less
13345134
D-branes in topological minimal models: the Landau–Ginzburg approach
We study D-branes in topologically twisted N = 2 minimal models using the Landau-Ginzburg realization. In the cases of A and D-type minimal models we provide what we believe is an exhaustive list of topological branes and compute the corresponding boundary OPE algebras as well as all disk correlators. We also construct examples of topological branes in E-type minimal models. We compare our results with the boundary state formalism, where possible, and find agreement.
121496385
Meson exchange currents in neutron-proton bremsstrahlung
Background: The meson exchange current (MEC) contribution is important in the neutron-proton bremsstrahlung process (np gamma) when the two nucleon-scattering angles are small. However, our understanding of such effects is limited, and the reason why meson exchange current effects dominate the np gamma cross section has not been thoroughly investigated. Purpose: The primary focus of this investigation is to understand the origin of the MEC contribution, to identify the leading MEC amplitudes, and to comprehend why these MEC amplitudes dominate the np gamma cross sections. Method: We used a new method that combines the one-boson-exchange (OBE) approach with the soft-photon approach to define 10 different np gamma amplitudes. These amplitudes are used to calculate np gamma cross sections at 225 MeV for nucleon laboratory scattering angles lying between 12(degrees) and 43(degrees). The results of these calculations are then compared to investigate the meson exchange current effect in np gamma. Results: (i) The OBE amplitude M(np gamma,mu)(PS) and the two-u-two-t special (TuTts) soft-photon amplitude M(np gamma,mu)(TuTts) predict quantitatively similar np gamma cross sections. (ii) The MEC effect is found to be significant when the two nucleon-scattering angles are far from the elastic limit (45(degrees)), but the effect is insignificant when the nucleon angles approach the elastic limit. (iii) The origin of the MEC effect and the leading MEC amplitudes have been identified in this investigation. Furthermore, the reason is now clear why the leading MEC amplitudes dominate the np gamma cross section when the nucleon-scattering angles are small. (iv) The contribution from the anomalous magnetic moments of the proton and the neutron is confirmed to be negligibly small. (v) In general, the theoretical cross sections using the amplitude M(np gamma,mu)(PS), or the amplitude M(np gamma,mu)(TuTts), are consistent with the triple differential cross sections recently measured at the Los Alamos National Laboratory. However, there exists an unexplained discrepancy between theory and experiment in some cases. Conclusions: The findings of this investigation have enhanced our understanding of the meson exchange current effect in np gamma. The comparative amplitude method introduced can be used for other bremsstrahlung investigations.
18559665
Topological sigma-models with H-flux and twisted generalized complex manifolds
We study the topological sector of N = 2 sigma-models with HH-flux. It has been known for a long time that the target-space geometry of these theories is not Kahler and can be described in terms of a pair of complex structures, which do not commute, in general, and are parallel with respect to two different connections with torsion. Recently an alternative description of this geometry was found, which involves a pair of commuting twisted generalized complex structures on the target space. In this paper, we define and study the analogs of A and B-models for N = 2 sigma-models with HH-flux and show that the results are naturally expressed in the language of twisted generalized complex geometry. For example, the space of topological observables is given by the cohomology of a Lie algebroid associated to one of the two twisted generalized complex structures. We determine the topological scalar product, which endows the algebra of observables with the structure of a Frobenius algebra. We also discuss mirror symmetry for twisted generalized Calabi-Yau manifolds.
120831466
Noncoplanarity effects in proton-proton bremsstrahlung
Noncoplanarity in proton-proton bremsstrahlung is investigated. Significant effects are observed for certain photon polar angles, psi(gamma). Such noncoplanarity effects, not of dynamical origin, are possibly responsible for past disagreements between theory and experiment. The Harvard noncoplanar coordinate system, which avoids kinematic singularities in the cross section, is used in our calculations and is recommended for use in the analysis of experimental data. Alternative methods of presenting cross sections are discussed. [S0556-2813(98)50110-0].
123491963
Noncoplanarity in proton-proton bremsstrahlung
Using the soft-photon approximation, we address the issue of the importance of noncoplanarity effects in proton-proton bremsstrahlung, We investigate the noncoplanar cross section as a function of the noncoplanarity angle (φ) over bar for the entire range of the photon polar angle psi(gamma). The (φ) over bar dependence is shown to provide a significant variation in the cross section, for a given psi(gamma). Thus, there can be some uncertainty in determining experimental coplanar cross sections. To avoid the phase-space singularities of spherical geometry. we utilize the Harvard noncoplanar geometry. A detailed explication of the Harvard geometry is provided, Comparison of our calculations with experimental data is included.
120312696
A study of the CKM angle $\gamma$ using the LHCb experiment and a distributed workload management system
The LHCb experiment, at the Large Hadron Collider (LHC), will make precision CP violation measurements and search for new physics beyond the Standard Model of particle physics. The 14 TeV proton-proton collisions at the LHC will produce the full range of B-hadrons, at a data rate of peta-bytes per year. In order to process this unprecedented amount of data, distributed computing resources (the Grid) located across four continents are coordinated by a series of workload management systems (Grid middlewares). DIRAC, the LHCb distributed workload management system for event simulation, reconstruction and user analysis is extended from the Linux operating platform to the Windows platform, allowing a transparent integration of Windows resources to the existing Linux system. The B±→ D/D (KS π π−) K± decay, a key channel for the precision measurement of the CKM angle γ, is studied using the multiplatform DIRAC system. The expected annual yield from this channel for 2 fb−1 of data (a full data taking year) is ∼4200 events, with a total physics background to signal ratio (B/S) of < 0.54 at the 90% confidence level. The dominant background is expected to arise from a real D/D candidate reconstructed with a fake kaon from the underlying event, with a B/S = 0.35± 0.03 where the error is statistical. The potential background arising from a real D/D candidate reconstructed with a mis-identified pion is greatly reduced, with a B/S < 0.095 at the 90% confidence level, by the use of particle identification information from the RICH sub-detector. The estimated LHCb statistical sensitivity to γ is ∼12° for 2 fb−1 of data, using a model dependent Dalitz analysis with a model error of ∼9°.
4995821
Drell-Yan lepton pair production at NNLO QCD with parton showers
We present a simple approach to combine NNLO QCD calculations and parton showers, based on the unlops technique. We apply the method to the computation of Drell-Yan lepton-pair production at the Large Hadron Collider. We comment on possible improvements and intrinsic uncertainties.
118708445
Jet vetoes for Higgs boson production at future hadron colliders
We study Higgs boson production in exclusive jet bins at possible future 33 and 100 TeV proton-proton colliders. We compare the cross sections obtained using fixed-order perturbation theory with those obtained by also resuming large logarithms induced by the jet-binning in the gluon-fusion and associated production channels. The central values obtained by the best-available fixed-order predictions differ by $10-20\%$ from those obtained after including resummation over the majority of phase-space regions considered. Additionally, including the resummation dramatically reduces the residual scale variation in these regions, often by a factor of two or more. We further show that in several new kinematic regimes that can be explored at these high-energy machines, the inclusion of resummation improvement is mandatory.
18039096
X-ray Shots of Cyg X-1 1
X-ray shots of Cyg X-1 in different energy bands and spectral states have been studied with PCA/RXTE observations. The detailed shot structure is obtained by superposing many shots with one millisecond time bin through aligning their peaks with an improved algorithm. In general, the shots are composed of a slow rise and fast decay. The shot structures in the different states are different. The duration of shot in the high state is shorter than that in the low and transition states. The shot profile in the high energy band is more asymmetric and narrower than that in the low energy band. The average hardness of shot is lower than that of steady emission in the transition and low states but higher than that in the high state. The time lags between the shots in higher and lower energy bands have been found in the different states. In transition states, the time lag is the largest among the different states of Cyg X-1, and it is the smallest in the low state. The implications of the observed shot features for shot models are discussed. Subject headings: accretion: accretion disk stars: individual (Cygnus X-1) X-rays: stars
119342728
Deuteron Compton scattering in effective field theory : Spin-dependent cross sections and asymmetries
Polarized Compton scattering on the deuteron is studied in nuclear effective field theory. A set of tensor structures is introduced to define 12 independent Compton amplitudes. The scalar and vector amplitudes are calculated up to next-to-next-to-leading order in low-energy power counting. Significant contribution to the vector amplitudes is found to come from the spin-orbit type of relativistic corrections. A double-helicity-dependent cross section {delta}{sub 1}d{sigma}/d{omega}=(d{sigma}{sub +1-1}-d{sigma}{sub +1+1})/2d{omega} is calculated to the same order, and the effect of the nucleon isoscalar spin-dependent polarizabilities is found to be smaller than the effect of isoscalar spin-independent ones. Contributions of spin-independent polarizabilities are investigated in various asymmetries, one of which has an effect as large as 12% (26%) at the center-of-mass photon energy 30 (50) MeV.