Showing papers in "Physical Review in 2000"

Precision predictions for (un)stable W+ W- pair production at and beyond LEP-2 energies

Abstract: We present precision calculations of the processes e+e− → 4 fermions, in which the double resonant W+W− intermediate state occurs. Referring to this latter intermediate state as the signal process, we show that, by using the YFS Monte Carlo event generators YFSWW3-1.14 and KORALW 1.42 in an appropriate combination, we achieve a physical precision on the signal process, as isolated with LEP2 MC Workshop cuts, below 0.5%. We stress the full gauge invariance of our calculations and we compare our results with those of other authors where appropriate. In particular, sample Monte Carlo data are explicitly illustrated and compared with the results of the program RacoonWW of Denner et al. In this way, we show that the total (physical and technical) precision tag for the WW signal process cross section is 0.4% for 200 GeV, for example. Results are also given for 500 GeV with an eye toward future linear colliders. Submitted to Phys. Lett. B † Work partly supported by the Maria Sk lodowska-Curie Joint Fund II PAA/DOE-97-316, the European Commission Fifth Framework contract HPRN-CT-2000-00149, and the US Department of Energy Contracts DE-FG05-91ER40627 and DE-AC03-76ER00515. CERN-TH/2000-337 DESY-00-179 UTHEP-00-0101 November, 2000 The award of the 1999 Nobel Prize for physics to G. ’t Hooft and M. Veltman, and the success of the predictions of their formulation [1] of the renormalized non-Abelian quantum loop corrections for the Standard Model [2] of the electroweak interactions in confrontation with data of LEP experiments, underscores the need to continue to test this theory at the quantum loop level in the gauge boson sector itself. This emphasizes the importance of the on-going precision studies of the processes e+e− → W+W− + n(γ) → 4f + n(γ) at LEP2 energies [3–5], as well as the importance of the planned future higher energy studies of such processes in LC physics programs [6–9]. We need to stress that hadron colliders also have considerable reach into this physics and we hope to come back to their roles elsewhere [10]. In what follows, we present precision predictions for the event selections (ES) of the LEP2 MC Workshop [11], for the processes e+e− → W+W− + n(γ) → 4f + n(γ), based on our new exact O(α)prod YFS-exponentiated LL O(α2) FSR leading-pole approximation (LPA) formulation, as it is realized in the MC program YFSWW3-1.14 [12, 13], in combination with all four-fermion processes MC event generator KoralW-1.42 [14, 15] so that the respective four-fermion background processes are taken into account in a gaugeinvariant way. Indeed, gauge invariance is a crucial aspect of our work and we stress that we maintain it throughout our calculations. Here, FSR denotes final-state radiation and LL denotes leading-log as usual. Recently, the authors in Refs. [16] have also presented MC program results for the processes e+e− → W+W− + n(γ) → 4f + n(γ), n = 0, 1, in combination with the complete background processes that feature the exact LPA O(α) correction. Thus, we will compare our results, where possible, with those in Refs. [16] in an effort to check the over-all precision of our work. As we argue below, the two sets of results should agree at a level below 0.5% on observables such as the total cross section. More specifically, in YFSWW3-1.13 [13], the leading-pole approximation (LPA) is used to develop a fully gauge-invariant YFS-exponentiated calculation of the signal process e+e− → W+W−+n(γ) → 4f +n(γ), which features the exact O(α) electroweak correction to the production process and the O(α2) LL corrections to the final-state decay processes. The issue is how to combine this calculation with that of KoralW-1.42 in Ref. [14,15] for the corresponding complete Born-level cross section with YFS-exponentiated initial-state O(α3) LL corrections. In this connection, we point out that the LPA enjoys some freedom in its actual realization, just as does the LL approximation in the precise definition of the big log L, without spoiling its gauge invariance. This can already be seen from the book of Eden et al. [17], wherein it is stressed that the analyticity of the S-matrix applies to the scalar form factors themselves in an invariant Feynman amplitude, without any reference to the respective external wave functions and kinematical (spinor) covariants. The classic example illustrated in Ref. [17] is that of pion–nucleon scattering, with the amplitude M = ū(p2)[A(s, t) + B(s, t)(6q1+ 6q2)]u(p1), (1) where the pi are the nucleon 4-momenta, the qi are the pion 4-momenta, u(p) is the usual Dirac wave function of the nucleon, and the invariant scalar functions A(s, t) and B(s, t) of the Mandelstam invariants s = (p1 + q1) , t = (q2− q1) realize the analytic properties

Measurements of B ---> D(*)(S)+ D+(*) branching fractions

Abstract: This article describes improved measurements by CLEO of the $B^0 \\to D_s^+ D^{*-}$ and $B^0 \\to D_s^{*+} D^{*-}$ branching fractions, and first evidence for the decay $B^+ \\to D_s^{(*)+} \\bar{D}^{**0}$, where $\\bar{D}^{**0}$ represents the sum of the $\\bar{D}_1(2420)^0$, $\\bar{D}_2^*(2460)^0$, and $\\bar{D}_1(j=1/2)^0$ L=1 charm meson states. Also reported is the first measurement of the $D_s^{*+}$ polarization in the decay $B^0 \\to D_s^{*+} D^{*-}$. A partial reconstruction technique, employing only the fully reconstructed $D_s^+$ and slow pion $\\pi_s^-$ from the $D^{*-} \\to \\bar{D}^0 \\pi^-_s$ decay, enhances sensitivity. The observed branching fractions are ${\\mathcal B} (B^0 \\to D_s^+ D^{*-}) = (1.10 \\pm 0.18 \\pm 0.10 \\pm 0.28)%$, ${\\mathcal B} (B^0 \\to D_s^{*+} D^{*-}) = (1.82 \\pm 0.37 \\pm 0.24 \\pm 0.46)%$, and ${\\mathcal B} (B^+ \\to D_s^{(*)+} \\bar{D}^{**0}) = (2.73 \\pm 0.78 \\pm 0.48 \\pm 0.68)%$, where the first error is statistical, the second systematic, and the third is due to the uncertainty in the $D_s^+ \\to \\phi \\pi^+$ branching fraction. The measured $D_s^{*+}$ longitudinal polarization, $\\Gamma_L/\\Gamma = (50.6 \\pm 13.9 \\pm 3.6)%$, is consistent with the factorization prediction of 54%.

Formation and evolution of the transverse anisotropy with nanocrystallization in amorphous Fe73.5CuNb3Si13.5B9 ribbons

Abstract: The evolution of the magnetic domain patterns has been studied in amorphous Fe73.5CuNb3Si13.5B9 ribbons by suitable "long duration" thermal treatments. It is shown that by annealing at 550 degrees C for annealing time t(a) from 1 to 150 h. very fine nanocrystalline bcc-FeSi rains are homogeneously formed in the amorphous matrix. Although with increasing t(a) the grain size remains very fine (less than or equal to 11 nm), the coercivity H-c increases rapidly from 0.14 A/m for t(a) = 3 h to 133 A/m for t(a) = 150 h. For the nanocrystalline ribbons with t(a) = 3 h, the domain structure is characterized by a few broad longitudinal together with some broad transverse domain patterns, connected to the minimum coercive field. With increasing annealing time t(a) greater than or equal to 10 h, only transverse domain patterns are observed and the transverse domain width gradually becomes narrow. By applying an external magnetic field. the magnetization processes indicate that the easy magnetization may be between the longitudinal and transverse directions for the samples annealed for t(a) = 1, 3, and 10 h, while in the sample of t(a) = 30 h. the easy magnetization in domains is transverse to the ribbon direction. The induced transverse anisotropy of 380 J/m(3) in the sample of t(a) = 30 h is determined from the domain width. The estimated H-c for the coherent rotation process in the sample of t(a) = 30 h is compatible with the experimentally observed value.

Spatial and energy distribution of muons in gamma induced airshowers

Abstract: The FLUKA Monte Carlo program is used to calculate the effects of hadroproduction by primary gamma rays incident upon the Earth's atmosphere; for the results presented in this paper, only primary angles at 0{sup o} from zenith are considered. The FLUKA code is believed to be quite accurate in reproducing experimental photon hadroproduction data in the 1 GeV to 10 TeV energy range studied. The charged pions which are so produced can decay to muons with sufficient energy to reach ground level. The number of these muons and their radial and energy distribution are studied for incident gamma ray energies from 1 GeV to 10 TeV. The number of these muons is not negligible; they can, in certain circumstances, be used to study potential sources of gamma rays such as gamma ray bursts. It is found, for example, that a 10 TeV incident primary gamma ray produces, on average, 3.4 muons which reach ground level; the gamma ray energy which produces the maximum number of muons at ground level depends on the spectral index of the primary gamma spectrum, a constant which describes how the primary gamma flux rises with decreasing primary energy. For example, for a differential spectral indexmore » of 2.7, there is a broad maximum number of muons coming from {approx}30 GeV primary gamma ray energy.« less

A Measurement of the mass and full width of the eta(c) meson

Abstract: In a sample of 7.8 million J/{psi} decays collected in the Beijing Spectrometer, the process J/{psi}{yields}{gamma}{eta}{sub c} is observed for five different {eta}{sub c} decay channels: K{sup +}K{sup -}{pi}{sup +}{pi}{sup -}, {pi}{sup +}{pi}{sup -}{pi}{sup +}{pi}{sup -}, K{sup {+-}}K{sub S}{sup 0}{pi}{sup {+-}} (with K{sub S}{sup 0}{yields}{pi}{sup +}{pi}{sup -}), {phi}{phi} (with {phi}{yields}K{sup +}K{sup -}) and K{sup +}K{sup -}{pi}{sup 0}. From these signals, we determine the mass of {eta}{sub c} to be 2976.6{+-}2.9{+-}1.3 MeV. Combining this result with a previously reported result from a similar study using {psi}(2S){yields}{gamma}{eta}{sub c} detected in the same spectrometer gives m{sub {eta}{sub c}}=2976.3{+-}2.3{+-}1.2 MeV. For the combined samples, we obtain {gamma}{sub {eta}{sub c}}=11.0{+-}8.1{+-}4.1 MeV. (c) 2000 The American Physical Society.