共查询到20条相似文献,搜索用时 203 毫秒
1.
Di-jet production is studied in collisions of quasi-real photons at e + e- centre-of-mass energies
from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive
-clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy
of the two leading jets, and as a function of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process,
, for different regions of
. Angular distributions in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of
and
is presented, where
is the jet pseudo-rapidity. Different regions of the
-
-space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.Received: 20 December 2002, Published online: 17 October 2003 相似文献
2.
For the first time at LEP the production of prompt photons is studied in the collisions of quasi-real photons using the OPAL data taken at
centre-of-mass energies between 183 GeV and 209 GeV. The total inclusive production cross-section for isolated prompt photons in the kinematic range of transverse momentum
GeV and pseudorapidity
is determined to be
. Differential cross-sections are compared to the predictions of a next-to-leading-order (NLO) calculation.Received: 6 May 2003, Published online: 4 November 2003 相似文献
3.
production in
interactions has been detected via its decays into
,K
+
K
-
K
+
K
- and
in the data taken with the DELPHI detector at LEP1 and LEP2 energies. The two-photon radiative width averaged over all observed decay channels is
= 13.9
2.0 (stat.)
1.4(syst.)
2.7 (BR) keV. No direct decay channel
has been observed. An upper limit
< 5.5 keV at 95% confidence level has been evaluated for this decay mode.Received: 3 July 2003, Published online: 7 November 2003 相似文献
4.
M. Boglione M. R. Pennington 《The European Physical Journal C - Particles and Fields》2003,30(4):503-512
A guide to the composition of the enigmatic f
0(980) and a
0(980) states is their formation in
-radiative decays. Precision data are becoming available from the KLOE experiment at the DA
NE machine at Frascati, as well as results from SND and CMD-2 at VEPP-2M at Novosibirsk. We show how the coupling of the f
0(980) to this channel can be extracted from these, independently of the background provided by
production. To do this we use the fact that the behaviour of both the f
0(980) and
cannot be determined by these data alone, but is strongly constrained by experimental results from other hadronic processes as required by unitarity. We find that the resulting coupling for the
is
GeV with a background that is quite unlike that assumed if unitarity is neglected. This provides an object lesson in how unitarity teaches us to add resonances. Not surprisingly the result is crucially dependent on the pole position of the f
0(980), for which there are still sizeable uncertainties. At present this leads to an uncertainty in the
branching ratio which can only be fixed by further precision data on the f
0(980). Nevertheless, the
is now the same order of magnitude as the experimental
.Received: 25 March 2003, Published online: 5 September 2003 相似文献
5.
A model describing the leptoproduction of
mesons is considered. For the amplitude of the photon dissociation into a
pair, the light-cone wave function is used. Scattering of the colorless
pair off the nucleon is computed within reggeon exchange phenomenology. The transition of the scattered quark and antiquark into the final
meson is treated with the aid of the parton-hadron duality concept. Numerical calculations of
and r0400 describe the world data at Q2 < 4 GeV2 rather reasonably. The calculations of the
meson spin density matrix show that the computed matrix elements except r0400 are in good agreement with the available experimental data at Q2 up to 8 GeV2 and the total mass of the
system greater than 4 GeV. The Regge phenomenology predictions at the highest experimentally available energies agree with both the HERA data and calculations performed within the perturbative QCD approach even for very high Q2 (
-20 GeV2). The predicted scale of the S-channel helicity non-conservation is in reasonable agreement with the experimental data.Received: 18 September 2003, Revised: 17 December 2003, Published online: 13 February 2004 相似文献
6.
We present a complete one-loop study of the process
and the predicted cross section in a
Linear Collider. A suitable numerical code, PLATONlc, valid for any set of real MSSM parameters, is released. This study and code are complementary to those suitable for dark matter detection through the inverse process
describing neutralino-neutralino annihilation at rest, which were presented previously. If SUSY is realized in nature, both codes should be very helpful in future astrophysical and collider studies of the neutralino sector.Received: 6 November 2003, Published online: 9 January 2004Programme dActions Intégrées Franco-Hellenique, Platon 04100 UM. 相似文献
7.
Cirigliano V. Ecker G. Neufeld H. Pich A. 《The European Physical Journal C - Particles and Fields》2004,33(3):369-396
We perform a complete analysis of isospin breaking in
amplitudes in chiral perturbation theory, including both strong isospin violation (
) and electromagnetic corrections to next-to-leading order in the low-energy expansion. The unknown chiral couplings are estimated at leading order in the 1/Nc expansion. We study the impact of isospin breaking on CP conserving amplitudes and rescattering phases. In particular, we extract the effective couplings g8 and g27 from a fit to
branching ratios, finding small deviations from the isospin-limit case. The ratio
measuring the
enhancement is found to decrease from
in the isospin limit to
in the presence of isospin breaking. We also analyze the effect of isospin violation on the CP violation parameter
, finding a destructive interference between three different sources of isospin violation. Within the uncertainties of large-Nc estimates for the low-energy constants, the isospin violating correction for
is below 15%.Received: 21 November 2003, Published online: 4 February 2004Work supported in part by IHP-RTN, Contract No. HPRN-CT2002-00311 (EURIDICE) and by Acciones Integradas, Project No. 19/2003 (Austria), HU2002-0044 (MCYT, Spain) 相似文献
8.
We compute the cross section for exclusive double-diffractive
production at the Tevatron,
, and the LHC. We evaluate both the gg and
-channel exchange contributions to the process. The observation of exclusive
production at the Tevatron will provide a check on the model predictions and offer an opportunity to confirm the expectations for exclusive double-diffractive Higgs production at the LHC.Received: 10 September 2004, Revised: 8 October 2004, Published online: 1 December 2004 相似文献
9.
M. Benayoun P. David L. DelBuono Ph Leruste H. B. O’Connell 《The European Physical Journal C - Particles and Fields》2003,31(4):525-547
We examine the decay modes
within the context of the hidden local symmetry (HLS) model. Using numerical information derived in previous fits to the
and Ve
+
e
- decay modes in isolation and the
lineshape determined in a previous fit to the pion form factor, we show that all aspects of these decays can be predicted with fair accuracy. Freeing some parameters does not improve the picture. This is interpreted as strong evidence in favor of the box anomaly in the
decays, which occurs at precisely the level expected. We also construct the set of equations defining the amplitudes for
and
at the chiral limit, as predicted from the anomalous HLS Lagrangian appropriately broken. This provides a set of four equations depending on only one parameter, instead of three for the traditional set. This is also shown to match the (two-angle, two-decay-constant)
-
mixing scheme recently proposed and is also fairly well fulfilled by the data. The information returned from the fits also matches expectations from previously published fits to the
decay modes in isolation.Received: 29 June 2003, Revised: 17 September 2003, Published online: 4 November 2003 相似文献
10.
The anomalous weak dipole moments of the
lepton are measured in a data sample collected by ALEPH from 1990 to 1995 corresponding to an integrated luminosity of 155 pb-1. Tau leptons produced in the reaction
at energies close to the
mass are studied using their semileptonic decays to
,
,
or
. The real and imaginary components of both the anomalous weak magnetic dipole moment and the CP-violating anomalous weak electric dipole moment,
,
,
and
, are measured simultaneously by means of a likelihood fit built from the full differential cross section. No evidence of new physics is found. The following bounds are obtained (95% CL):
,
,
, and
.Received: 25 September 2002, Published online: 29 August 2003 相似文献
11.
M. Dubinin H. J. Schreiber A. Vologdin 《The European Physical Journal C - Particles and Fields》2003,30(3):337-344
We examine the prospects for measuring the
coupling of a standard model-like Higgs boson with a mass between 120 and 160 GeV at the future TESLA linear e
+
e
- collider, assuming an integrated luminosity of 1 ab-1 and a center-of-mass energy of 500 GeV. We consider the Higgs boson produced in association with
via the W W fusion reaction
, followed by the rare decay into a Z boson and a photon,
. Accounting for all main background contributions, a precision of 27% can be achieved in unpolarized e
+
e
- collisions for M
H
= 140 GeV. With appropriate initial state polarisations
BF(
)/BF(
), or the precisions on the
partial width, can be improved to 17% and provide valuable information on the
coupling. For M
H
= 120 and 160 GeV, the small significance of the signals in unpolarized collisions sets upper limits of 79% respectively 72% at 90% confidence level on the
branching fraction.Received: 5 March 2003, Revised: 14 July 2003, Published online: 5 September 2003 相似文献
12.
The hadronic photon structure function
is measured from data taken with the ALEPH detector at LEP. At centre-of-mass energies between
and
an integrated luminosity of
is analyzed in two ranges of Q2 with
GeV2 and 67.2 GeV2. Detector effects and acceptance are corrected for with a Tikhonov unfolding procedure. The results are compared to theoretical predictions and measurements from other experiments.Received: 12 June 2003, Published online: 11 August 2003 相似文献
13.
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
14.
Measurements of the time-dependent CP-asymmetry in the decay
and its charge conjugate by the BELLE and BABAR collaborations currently yield
and
, characterizing the direct and mixing-induced CP-asymmetries, respectively. We study the implication of these measurements on the CKM phenomenology taking into account the available information in the quark mixing sector. Our analysis leads to the results that the ratio |P
c
/T
c
| involving the QCD-penguin and tree amplitudes and the related strong phase difference
in the
decays are quite substantial. Using the isospin symmetry to constrain |P
c
/T
c
| and
, where
parameterizes the penguin-induced contribution, we present a fit of the current data including the measurements of
and
. Our best-fits yield
,
,
,
, and
. At 68% C.L., the ranges are
,
,
,
and
. Currently en vogue dynamical approaches to estimate the hadronic matrix elements in
decays do not provide a good fit of the current data.Received: 5 April 2004, Published online: 14 July 2004A.Ya. Parkhomenko: On leave of absence from Department of Theoretical Physics, Yaroslavl State University, Sovietskaya 14, 150000 Yaroslavl, Russia. 相似文献
15.
A. J. Buras R. Fleischer S. Recksiegel F. Schwab 《The European Physical Journal C - Particles and Fields》2003,32(1):45-54
The standard-model interpretation of the ratios of charged and neutral
rates, R
c and R
n, respectively, points towards a puzzling picture. Since these observables are affected significantly by colour-allowed electroweak (EW) penguins, this
puzzle could be a manifestation of new physics in the EW penguin sector. Performing the analysis in the R
n- R
c plane, which is very suitable for monitoring various effects, we demonstrate that we may, in fact, move straightforwardly to the experimental region in this plane through an enhancement of the relevant EW penguin parameter q. We derive analytical bounds for q in terms of a quantity L, which measures the violation of the Lipkin sum rule, and point out that strong phases around
are favoured by the data, in contrast to QCD factorisation. The
modes imply a correlation between q and the angle
that, in the limit of negligible rescattering effects and colour-suppressed EW penguins, depends only on the value of L. Concentrating on a minimal flavour-violating new-physics scenario with enhanced Z
0 penguins, we find that the current experimental values on
require roughly
. As the
data give
, L has either to move to smaller values once the
data improve or new sources of flavour and CP violation are needed. In turn, the enhanced values of L seen in the
data could be accompanied by enhanced branching ratios for the rare decays
,
,
and
. Most interesting turns out to be the correlation between the
modes and
, with the latter depending approximately on a single scaling variable
.Received: 8 September 2003, Published online: 12 November 2003 相似文献
16.
J. G. Messchendorp 《The European Physical Journal A - Hadrons and Nuclei》2003,18(2-3):417-419
Preliminary differential cross-sections of the reactions
and
with A = 1
H, 12
C, and
nat
Pb are presented. A significant nuclear-mass dependence of the
invariant-mass distribution is found in the
channel. The dependence is not observed in the
channel. The in-medium observation in the
channel is consistent with an in-medium modification of the
interaction in the I = J = 0 channel, changing width and pole position of a
resonant state.Received: 30 September 2002, Published online: 22 October 2003PACS:
13.60.Le Meson production - 21.65. + f Nuclear matterJ.G. Messchendorp: For the TAPS and A2 Collaborations 相似文献
17.
We discuss the time-dependent CP asymmetry of the decay
in an extension of the standard model with both a two Higgs doublet and additional fourth-generation quarks. We show that, although the standard model with a two Higgs doublet and the standard model with fourth-generation quarks alone are not likely to largely change the effective
from the decay
, the model with both an additional Higgs doublet and fourth-generation quarks can easily account for the possible large negative value of
without conflicting with other experimental constraints. In this model, additional large CP violating effects may arise from the flavor-changing Yukawa interactions between neutral Higgs bosons and the heavy fourth-generation down type quark, which can modify the QCD penguin contributions. With the constraints obtained from
processes such as
and
, this model can lead to an effective
as large as - 0.4 in the CP asymmetry of
.Received: 25 March 2004, Revised: 20 April 2004, Published online: 18 June 2004 相似文献
18.
P. H. Chankowski Ł Sławianowska 《The European Physical Journal C - Particles and Fields》2004,33(1):123-136
We investigate the potential effects of the scalar flavor changing neutral currents that are generated e.g. in supersymmetry with
in the
transitions. Using the experimental upper limit on
we place stringent model independent constraints on the impact these currents may have on the rates
and
. We find that in the first case, contrary to the claim made recently in the literature, the maximal potential effects are always smaller than the uncertainty of the standard model NNLO prediction, that is of order 5-15%. In the second case, the effects can be large, but the experimental errors combined with the unsettled problems associated with the relevant form factors do not allow for any firm conclusion about the detectability of a new physics signal in this process. In supersymmetry the effects of the scalar flavor changing neutral currents are further constrained by the experimental lower limit on the B0s-
mass difference, so that most likely no detectable signal of the supersymmetry generated scalar flavor changing neutral currents in the processes
and
is possible.Received: 9 September 2003, Revised: 8 October 2003, Published online: 15 January 2004 相似文献
19.
A resonance search has been made in the
invariant-mass spectrum with the ZEUS detector at HERA using an integrated luminosity of
. The decay channels
and
(and the corresponding antiparticle decays) were used to identify
mesons. No resonance structure was observed in the
mass spectrum from more than 60 000 reconstructed
mesons. The results are not compatible with a report of the H1 Collaboration of a charmed pentaquark,
.Received: 14 September 2004, Revised: 29 September 2004, Published online: 9 November 2004 相似文献
20.
We compute the one-loop
-functions describing the renormalisation of the coupling constant
and the frequency parameter
for the real four-dimensional duality-covariant non-commutative
-model, which is renormalisable to all orders. The contribution from the one-loop four-point function is reduced by the one-loop wavefunction renormalisation, but the
-function remains non-negative. Both
and
vanish at the one-loop level for the duality-invariant model characterised by
. Moreover,
also vanishes in the limit
, which defines the standard non-commutative
-quantum field theory. Thus, the limit
exists at least at the one-loop level.Received: 19 March 2004, Published online: 5 May 2004 相似文献