588 research outputs found
Theoretical Expectations For High Mass Photon Pairs in L+ L- Gamma Gamma Events at LEP/SLC
Recently, the L3 collaboration has reported the observation of four events in
the reactions e+ e- --> L+ L- + (2 Photons), L = e, mu, tau, with the invariant
photon pair mass near 60 GeV in a data sample collected in the L3 detector
corresponding to 950,000 produced Z0's. More recently, more data from the other
LEP collaborations have become available. In this paper, we use the Monte Carlo
genrator YFS3 and our recent exact results on e+ e- --> L+ L- + (2 Photons) to
assess the QED expectations for such L3-type high mass photon pair events in e+
e- --> L+ L- + (n Photons) near the Z0 resonance.Comment: 9 pages (LaTeX + 6 uu-encoded figures), UTHEP-93-1002 (version with
corrected preprint number
Multiple photon effects in fermion-(anti)fermion scattering at SSC energies
We use the theory of Yennie, Frautschi and Suura to realize, via Monte Carlo
methods, the process f\,\bbbarf\to f'\,\bbbarf'+n\gamma at SSC and LHC
energies, where and are quarks or leptons. QED infrared divergences
are canceled to all orders in perturbation theory. The resulting Monte Carlo
event generator, SSC-YFS2, is used to study the effects of initial-state photon
radiation on these processes in the SSC environment. Sample Monte Carlo data
are presented and discussed. We find that the respective multiple-photon
effects must be taken into account in discussing precise predictions for SSC
physics processes.Comment: 14 pages + 8 figures + 1 tabl
Precision Predictions for (Un)Stable W+W- Pair Production At and Beyond LEP2 Energies Beyond LEP2 Energies
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 KORALW1.42 in an appropriate
combination, we achieve a physical precision on the signal process, as isolated
with LEP2 MC Workshop cuts, below 0.5 per cent. 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
Dittmaier {\it et al.}. In this way, we show that the total (physical plus
technical) precision tag for the WW signal process cross section is 0.4 per
cent for 200 GeV, for example. Results are also given for 500 GeV with an eye
toward the LC.Comment: 19 pages, 2 figs;corrected Tab. 3;improved refs.,figs.,text;improved
refs.,text;improved tex
CLEO Contributions to Tau Physics
We review many of the contributions of the CLEO experiment to tau physics.
Topics discussed are: branching fractions for major decay modes and tests of
lepton universality; rare decays; forbidden decays; Michel parameters and spin
physics; hadronic sub-structure and resonance parameters; the tau mass, tau
lifetime, and tau neutrino mass; searches for CP violation in tau decay; tau
pair production, dipole moments, and CP violating EDM; and tau physics at
CLEO-III and at CLEO-c.Comment: Invited talk at the Seventh International Workshop on Tau Lepton
Physics (TAU02), Santa Cruz, Ca, USA, Sept 2002, 14 pages, LaTeX, 21 eps
figure
Final State Radiative Effects for the Exact O(alpha) YFS Exponentiated (Un)Stable W+W- Production At and Beyond LEP2 Energies
We present the LL final state radiative effects for the exact O(alpha) YFS
exponentiated (un)stable WW pair production at LEP2/NLC energies using Monte
Carlo event generator methods. The respective event generator, version 1.12 of
the program YFSWW3, wherein both Standard Model and anomalous triple gauge
boson couplings are allowed, generates n(\gamma) radiation both from the
initial state and from the intermediate W+ W- and generates the LL final state
W decay radiative effects. Sample Monte Carlo data are illustrated.Comment: 16 pages, 8 figures, 2 table
Polarized Moller Scattering Asymmetries
The utility of polarized electron beams for precision electroweak studies is
described. Parity violating Moller scattering asymmetries in e-e- --> e-e- are
discussed. Effects of electroweak radiative corrections and the running
sin^2(theta_W(Q^2)) are reviewed. The sensitivity of E158 (a fixed target e-e-
experiment at SLAC) and future e-e- collider studies to ``new physics'' is
briefly outlined.Comment: Updated Fig.
CLEO Results on Tau Michel Parameters
We present measurements of the tau Michel Parameters made by the CLEO
experiment. Three different analyses are performed: a spin-independent lepton
spectrum analysis and a second spin-dependent analysis using \vs\
events, and a third spin-dependent analysis using \vs\
events. the results are used to derive limits on the general
four-fermion couplings, the mass of the charged Higgs in the MSSM, and a
right-handed in left-right models. Many of these measurements are more
precise than the PDG world averages.Comment: Invited talk at the TAU'98 Workshop, 14-17 September 1998, Santander,
Spain. 7 pages, 7 figures, LaTeX -> postscrip
Radiative Tau Lepton Pair Production as a Probe of Anomalous Electromagnetic Couplings of the Tau
We calculate the squared matrix element for the process e+ e- --> tau+ tau-
gamma allowing for anomalous magnetic and electric dipole moments at the tau
tau gamma vertex. No interferences are neglected and no approximations of light
fermion masses are made. We show that anomalous moments affect not only the
cross section, but also the shape of the photon energy and angular
distributions. We also demonstrate that in the case of the anomalous magnetic
dipole moment, the contribution from interference involving Standard Model and
anomalous amplitudes is significant compared to the contribution from anomalous
amplitudes alone. A program to perform the calculation is available and it may
be employed as a Monte Carlo generator.Comment: 14 pages, 8 figures submitted to Nuclear Physics
Gauge Invariant YFS Exponentiation of (Un)stable Production At and Beyond LEP2 Energies
We present the theoretical basis and sample Monte Carlo data for the YFS
exponentiated calculation of at and beyond LEP2 energies, where the left-handed parts
of and are the respective upper and lower components of an
doublet, . The problem of gauge invariance of the radiation
from the unstable charged spin~1 is solved in an entirely physical
manner. Our formulas are illustrated in a proto-typical YFS Monte Carlo event
generator YFSWW2, wherein both Standard Model and anomalous triple gauge boson
couplings are allowed.Comment: Latex replaces postscript, 17 page
Measuring the FSR--inclusive pi+pi- cross section
Final state radiation (FSR) in pion--pair production cannot be calculated
reliably because of the composite structure of the pions. However, FSR
corrections have to be taken into account for a precise evaluation of the
hadronic contribution to g-2 of the muon. The role of FSR in both energy scan
and radiative return experiments is discussed. It is shown how FSR influences
the pion form factor extraction from experimental data and, as a consequence,
the evaluation of a_mu^had. In fact the O(alpha) FSR corrections should be
included to reach the precision we are aiming at. We argue that for an
extraction of the desired FSR--inclusive cross section sigma^(gamma)_had a
photon--inclusive scan measurement of the ``e+e- to pi+pi- + photons'' cross
section is needed. For exclusive scan and radiative return measurements in
contrast we have to rely on ad hoc FSR models if we want to obtain either
sigma^(gamma)_had or the FSR--exclusive cross section sigma^(0)_had. We thus
advocate to consider seriously precise photon--inclusive energy scan
measurements at present and future low energy e+e- facilities. Then together
with radiative return measurements from DAFNE and BABAR and forthcoming scan
measurements at VEPP-2000 we have a good chance to substantially improve the
evaluation of a_mu^had in the future.Comment: 18 pages, 13 Figure
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