共查询到20条相似文献,搜索用时 31 毫秒
1.
G.J. Gounaris J. Layssac P.I. Porfyriadis F.M. Renard 《The European Physical Journal C - Particles and Fields》1999,10(3):499-513
We study the helicity amplitudes and the observables of the process at high energy. As in the case of the process studied before, the relevant diagrams in the standard model (SM) involve W, charged-quark, and lepton loops, while in SUSY we also have contributions from charginos and charged-sfermion or Higgs-loop
diagrams. Above 250 GeV, the dominant SM amplitudes are themselves dominated by the W loop, and as for , they are helicity conserving and almost purely imaginary. We discuss the complementary information provided by for the identification of possible nonstandard effects. This process, together with , should provide very useful information on the nature of possible new physics particles, above the threshold of their direct
production.
Received: 26 April 1999 / Published online: 8 September 1999 相似文献
2.
G.J. Gounaris P.I. Porfyriadis F.M. Renard 《The European Physical Journal C - Particles and Fields》1999,9(4):673-686
We study the helicity amplitudes of the process at high energy, which in the standard and SUSY models first arise at the one-loop order. In the standard model (SM), the
diagrams involve , charged-quark, and lepton loops, while in SUSY, we also have contributions from chargino-, charged-sfermion, and Higgs-loop
diagrams. The SUSY contributions are most important in the region above the threshold for producing the supersymmetric partners;
there, they interfere most effectively with the primarily imaginary SM amplitudes. Simple expressions for the relevant one-loop
functions are given which provide a direct overview of the behaviour of the helicity amplitudes in the whole parameter space
at high energies. The various characteristics of a large set of observables are studied in detail.
Received: 3 February 1999 / Pulished online: 30 June 1999 相似文献
3.
We present an updated next-to-leading order analysis of the branching ratio and photon spectrum, including consistently the effects of Fermi motion in the heavy-quark expansion. For
the Standard Model, we obtain for the integral over the high-energy part of the photon spectrum with GeV, where the first error reflects the uncertainty in the input parameters, and the second one the uncertainty in the calculation
of Fermi motion. This prediction agrees with the CLEO measurement of the same quantity within one standard deviation. From
a reanalysis of the CLEO data, we obtain for the total branching ratio using the measured rate above 2.2 GeV, and using a fit to the photon energy spectrum. Both values are consistent with the Standard Model prediction of . Our analysis contains an improved discussion of renormalization-scale dependence and QED corrections. We also study the
sensitivity of the branching ratio and photon spectrum to hadronic parameters such as the b-quark mass, and to possible contributions from New Physics beyond the Standard Model.
Received: 13 May 1998 / Published online: 3 September 1998 相似文献
4.
We present a general purpose Monte Carlo program for the calculation of any infrared safe observable in and production at hadron colliders at next-to-leading order in . We treat the leptonic decays of the W and Z-boson in the narrow-width approximation, but retain all spin information via decay angle correlations. The effect of anomalous
triple gauge boson couplings is investigated and we give the analytical expressions for the corresponding amplitudes. Furthermore,
we propose a way to study the effect of anomalous couplings without introducing the ambiguity of form factors.
Received: 14 February 2000 / Published online: 18 May 2000 相似文献
5.
G.J. Gounaris P.I. Porfyriadis F.M. Renard 《The European Physical Journal C - Particles and Fields》2001,19(1):57-76
If the standard model (SM) Higgs particle is sufficiently heavy, then its contribution to should be largely imaginary, interfering with the also predominantly imaginary SM “background” generated by the W loop. For standard model Higgs masses in the region GeV, this interference is found to be constructive and increasing the Higgs signal. In the minimal SUSY case an interference
effect should also appear for the contribution of the heavier CP-even neutral Higgs boson , provided it is sufficiently heavy. The effect is somewhat reduced, though, by the smallness of the width and the and ZZ branching ratios. The interference is again found to be constructive for part of the parameter space corresponding to sfermion
masses at the TeV scale and maximal stop mixing. For both the SM and the SUSY case, regions of the parameter space exist,
though, where the interference may be destructive. It is therefore essential to take these effects into account when searching
for possible scalar Higgs-like candidates. To this aim, we present the complete analytic expressions for both resonance and
background amplitudes.
Received: 20 October 2000 / Revised version: 5 January 2001 / Published online: 23 February 2001 相似文献
6.
Z. Was 《The European Physical Journal C - Particles and Fields》2005,44(4):489-503
One of the necessary steps in constructing a high-precision option of KKMC, a Monte Carlo program for the high-precision simulation
of fermion pair production at LEP and Linear Collider energies, was to make a careful study of the appropriate matrix elements
calculated from QED and the complete standard model. In particular, the installation of the double bremsstrahlung matrix element
for the process
into the scheme of coherent exclusive exponentiation (CEEX) was necessary. In the CEEX scheme one has to define an extrapolation
and/or reduction procedure to enable the use of the matrix elements for kinematical configurations with a large number of
outgoing particles. The process under study is particularly interesting because of the gauge cancellation of contributions
for photon emission from incoming fermion lines and t-channel W. The QED U(1) gauge properties require terms of the triple and quartic gauge couplings to be taken into consideration as
well. A natural separation of the complete amplitude into gauge invariant parts was found and is among the main results of
the paper. Each part has a well defined physical interpretation, which after partial integration over phase space provides
infrared singular, leading-log, next-to-leading-log, and other terms. Contributions related to the triple and quartic gauge
coupling of W (extracted with the help of an expansion around the contact W-interaction) have been ordered as well. The separation can be of broader interest; it originates from the rigorous calculation
of matrix elements; it visualizes, in the language of spin amplitudes, the properties of factorization necessary for the common
multi-process picture. For example, the multiple photon algorithm of PHOTOS, based on the parton shower-like approach, profits
from similar considerations. These somewhat speculative aspects of the calculation will be mentioned in the paper as well.
Received: 24 April 2005, Revised: 22 July 2005, Published online: 11 October 2005
Supported in part by the EU grant MTKD-CT-2004-510126, in patnership with the CERN Physics department, and Polish State Committee
for Scientific Research (KBN) grant 2 P03B 091 27 for years 2004-2006. 相似文献
7.
The forward elastic scattering of light on light, i.e., the reaction in the forward direction, is analyzed utilizing real analytic amplitudes. We calculate , the ratio of the real to the imaginary portion of the forward scattering amplitude, by fitting the total cross section data in the high energy region 5 GeV GeV, assuming a cross section that rises asymptotically as ln2 s. We then compare , the ratio of the even portions of the pp and forward scattering amplitudes, as well as to [1], the value for Compton scattering. Within errors, we find that the three -values in the c.m.s. energy region 5 GeV 130 GeV are the same, as predicted by a factorization theorem of Block and Kaidalov [2].
Received: 18 June 2002 / Published online: 9 August 2002 相似文献
8.
G.J. Gounaris P.I. Porfyriadis F.M. Renard 《The European Physical Journal C - Particles and Fields》2001,20(4):659-675
The process first arises at the one loop level, and as such it provides us with remarkable tests of the structure of the electroweak
Higgs sector. These tests are complementary to those in the gauge sector involving . We show that in the standard model (SM) where , as well as in the supersymmetric case where or , observables exist (like e.g. the energy dependence, angular distribution, photon polarization dependence or final Z polarization) which present rather spectacular properties. Such properties involve strong threshold effects with steps, bumps
or peaks, reflecting the type of Higgs and heavy quarks and chargino masses and couplings predicted by the SM and supersymmetric
models.
Received: 13 March 2001 / Published online: 13 June 2001 相似文献
9.
S. Fajfer P. Singer J. Zupan 《The European Physical Journal C - Particles and Fields》2003,27(2):201-218
We present a calculation of the rare decay modes and in the framework of the standard model. For the short distance part we have derived QCD corrections to the Wilson coefficients
involved, including C9. The latter is found to be strongly suppressed by the corrections, leading to diminished values for the branching ratios in the 10-10 range. Within SM the exclusive decays are dominated by long distance effects. Non-resonant contributions are estimated using
heavy quark and chiral symmetries to be at the level of , compared to the contributions arising from , with . The total SM branching ratio is predicted to be in the range (1-. We also consider contributions coming from MSSM with and without R parity conservation. The effects from MSSM are significant only for the R parity violating case. Such contributions enhance the branching ratio to , based on appropriately allowed values for C9 and C10. This selects as a possible probe of new physics.
Received: 25 October 2002 / Published online: 24 January 2003 相似文献
10.
L. T. Hue A. B. Arbuzov T. T. Hong T. Phong Nguyen D. T. Si H. N. Long 《The European Physical Journal C - Particles and Fields》2018,78(11):885
Radiative corrections to the decay \(h\rightarrow Z\gamma \) are evaluated in the one-loop approximation. The unitary gauge is used. The analytic result is expressed in terms of the Passarino-Veltman functions. The calculations are applicable for the Standard Model as well for a wide class of its gauge extensions. In particular, the decay width of a charged Higgs boson \(H^{\pm }\rightarrow W^{\pm }\gamma \) can be derived. The consistence of our formulas and several specific earlier results is shown. 相似文献
11.
We give explicit expressions for the amplitudes associated with the supersymmetric (SUSY) contributions to the process in the context of SUSY extensions of the standard model (SM) with non-universal soft SUSY breaking terms. From experimental
data we deduce limits on the squark mass insertions obtained from different contributions (gluinos, neutralinos and charginos).
Received: 20 April 2001 / Revised version: 14 December 2001 / Published online: 5 April 2002 相似文献
12.
Multi-ring signatures of
appearance via the oscillation
are formulated for a water Cherenkov detector. These signatures are appropriate for long-baseline neutrino experiments operating at relatively high neutrino energies
GeV that emphasize the matter effect. The NC background is less for selected multi-ring events than for 1e-like events, and may be directly estimated from the data. Our results suggest that best sensitivity to
and to the sign of
can be reached with baselines over some 6000 km.Received: 14 April 2003, Revised: 19 June 2003, Published online: 26 September 2003M.A. Kubantsev: Now at Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA 相似文献
13.
C.A. Nelson 《The European Physical Journal C - Particles and Fields》2001,19(2):323-337
Due to the absence of hadronization effects and the large mass, top quark decay will be uniquely sensitive to fundamental electroweak physics at the Tevatron, at the LHC, and at a
future linear collider. A “complete measurement” of the four helicity amplitudes in decay is possible by the combined use of andW polarimetry in stage-two spin-correlation functions (S2SC). In this paper, the most general Lorentz-invariant decay density
matrix is obtained for the decay sequence where and [or ], and likewise for . These density matrices are expressed in terms of b-polarimetry helicity parameters which enable a unique determination of the relative phases among the amplitudes. Thereby, S2SC distributions and single-sided b-W-interference distributions are expressed in terms of these parameters. The four b-polarimetry helicity parameters involving the amplitude are considered in detail. polarimetry signatures will not be suppressed in top quark analyses when final angles-and-energy variables are used for .
Received: 26 January 2001 / Published online: 15 March 2001 相似文献
14.
Dang Van Soa Takeo Inami Hoang Ngoc Long 《The European Physical Journal C - Particles and Fields》2004,34(3):285-289
We study the production of bileptons, new gauge bosons of lepton number two, in the minimal 3-3-1 model in high energy
collisions. If the bilepton masses are in the range of 300 GeV the reaction will give observable cross-sections in future colliders.Received: 5 August 2003, Published online: 23 March 2004Dang Van Soa: On leave of absence from Department of Physics, Hanoi University of Education, Hanoi, VietnamHoang Ngoc Long: On leave of absence from Institute of Physics, NCST, P.O. Box 429, Bo Ho, Hanoi 10000, Vietnam 相似文献
15.
Power-suppressed corrections coming from the end-point integration regions in the amplitude of the process
at large Q
2 and small squared center-of-mass energy W
2 are calculated in the QCD hard-scattering approach where the amplitudes factorize in a hard perturbatively calculable part and a generalized distribution amplitude. The running coupling method and the technique of infrared renormalon calculus are applied to obtain Borel resummed expressions for the two main components of the process amplitude. Numerical estimates for these power corrections are presented. They are sizeable when Q
2 < 10 GeV2.Received: 29 March 2004, Revised: 11 May 2004, Published online: 1 October 2004 相似文献
16.
The three-vertex loop amplitude in a strong magnetic field are analyzed in a general form by using the asymptotic behavior of the electron propagator in an external field. The process \(\gamma \gamma \to \nu \bar \nu\) is studied in terms of the scalar-vector-vector (SVV), pseudoscalar-vector-vector (PVV), vector-vector-vector (VVV), and axial-vector-vector-vector (AVV) combinations of couplings. It is shown that only in the case of the SVV combination does the amplitude grow linearly with increasing magnetic-field strength, the amplitudes evaluated with the other combinations of couplings (PVV, VVV, and AVV) featuring no linearly increasing terms. The process \(\gamma \gamma \to \nu \bar \nu\) is also studied within the left-right model, which is an extension of the Standard Model of electroweak interactions and which may involve an effective scalar ννee coupling. Possible astrophysical manifestations of this process are discussed. 相似文献
17.
We calculate the branching ratios and CP-asymmetries for , and decays, in the perturbative QCD approach. In this approach, we calculate non-factorizable and annihilation type contributions,
in addition to the usual factorizable contributions. Our result is in agreement with the branching ratio of , measured by the CLEO and BABAR collaborations. We also predict large CP-asymmetries in these decays. These channels are useful to determine the CKM angle .
Received: 5 November 2001 / Published online: 8 February 2002 相似文献
18.
N.G. Stefanis W. Schroers H.-Ch. Kim 《The European Physical Journal C - Particles and Fields》2000,18(1):137-156
We develop and discuss in technical detail an infrared-finite factorization and optimized renormalization scheme for calculating
exclusive processes, which enables the inclusion of transverse degrees of freedom without entailing suppression of calculated
observables, like form factors. This is achieved by employing an analytic, i.e., infrared stable, running strong-coupling
which removes the Landau singularity at by a minimum power-behaved correction. The ensuing contributions to the cusp anomalous dimension – related to the Sudakov
form factor – and to the quark anomalous dimension – which controls evolution – lead to an enhancement at high of the hard part of exclusive amplitudes, calculated in perturbative QCD, while simultaneously improving its scaling behavior.
The phenomenological implications of this framework are analyzed by applying it to the pion's electromagnetic form factor,
including the NLO contribution to the hard-scattering amplitude, and also to the pion–photon transition at LO. For the pion
wave function, an improved ansatz of the Brodsky–Huang–Lepage type is employed, which includes an effective (constituent-like)
quark mass, GeV. Predictions for both form factors are presented and compared to the experimental data, applying Brodsky–Lepage–Mackenzie
commensurate scale setting. We find that the perturbative hard part prevails at momentum transfers above about 20 GeV, while at lower values the pion form factor is dominated by Feynman-type contributions. The theoretical prediction for the form factor indicates that the true pion distribution amplitude may be somewhat broader than the asymptotic one.
Received: 19 May 2000 / Revised version: 16 August 2000 / Published online: 13 November 2000 相似文献
19.
Eri Asakawa Jun-ichi Kamoshita Akio Sugamoto Isamu Watanabe 《The European Physical Journal C - Particles and Fields》2000,14(2):335-345
We present the effects of heavy CP-even (H) and CP-odd (A) Higgs bosons on the production cross section of the process
at the energy around the mass poles of the Higgs bosons. It is found that interference between H and A with small mass gap, as well as the ones between Higgs bosons and continuum, contributes to the cross section, if the photon
beams are polarized and if we observe the helicity of the top quarks. It is demonstrated in the framework of the minimal supersymmetric
extension of the standard model that the H and A contributions can be sizable at future colliders for small values of . The methods to measure the CP-parity of the Higgs boson are also presented. The statistical significance of detecting the Higgs signals and measuring the
Higgs CP-parity is evaluated.
Received: 16 December 1999 / Revised version: 30 January 2000 / Published online: 6 April 2000 相似文献
20.
A. Bredenstein S. Dittmaier M. Roth 《The European Physical Journal C - Particles and Fields》2004,36(3):341-363
We have constructed a Monte Carlo generator (the corresponding FORTRAN code can be obtained from the authors upon request) for lowest-order predictions for the processes
and
in the standard model and extensions thereof by an effective
coupling as well as anomalous triple and quartic gauge-boson couplings. Polarization is fully supported, and a realistic photon beam spectrum can be taken into account. For the processes
all helicity amplitudes are explicitly given in a compact form. The presented numerical results contain, in particular, a survey of cross sections for representative final states and their comparison to results obtained with the program package Whizard/Madgraph. The impact of a realistic beam spectrum on cross sections and distributions is illustrated. Moreover, the size of various contributions to cross sections, such as from weak charged- or neutral-current, or from strong interactions, is analyzed. Particular attention is paid to W-pair production channels
where we investigate the impact of background diagrams, possible definitions of the W-pair signal, and the issue of gauge-invariance violation caused by finite gauge-boson widths. Finally, the effects of triple and quartic anomalous gauge-boson couplings on cross sections as well as the possibility to constrain these anomalous couplings at future
colliders are discussed.Received: 19 May 2004, Published online: 16 July 2004 相似文献