498 research outputs found
How to Generate Four-Fermion Phase Space
We present a scheme for integrating the matrix element of an arbitrary
e^+e^-\to f_1f_2\bar f_3\bar f_4 process over the complete four-fermion phase
space, or its any part, by means of the Monte Carlo technique. The presented
algorithm has been successfully implemented in the KORALW Monte Carlo code.Comment: 16 page
Four-quark final state in W-pair production: Case of signal and background
We discuss theoretical predictions for W-pair production and decay at LEP2
and higher energies in a form suitable for comparison with raw data. We present
a practical framework for calculating uncertainties of predictions given by the
KORALW and grc4f Monte Carlo programs. As an example we use observables in the
decay channel: the total four-quark (four-jet) cross section
and two-quark/jet invariant-mass distribution and cross section, in the case
when the other two may escape detection. Effects of QED bremsstrahlung,
effective couplings, running W and Z widths, Coulomb interaction and the
complete tree level set of diagrams are discussed. We also revisit the question
of technical precision of the new version 1.21 of the KORALW Monte Carlo code
as well as of version 1.2(26) of the grc4f one.
Finally we find predictions of the two programs to have an overall physical
uncertainty of 2%.
As a side result we show, on the example of an invariant mass
distribution, the strong interplay of spin correlations and detector cut-offs
in the case of four-fermion final states.Comment: 26 pages, LaTe
Coherent Exclusive Exponentiation for Precision Monte Carlo Calculations of Fermion Pair Production / Precision Predictions for (Un)stable W+W- Pairs
We present the new Coherent Exclusive Exponentiation (CEEX), in comparison to
the older Exclusive Exponentiation (EEX) and the semi-analytical Inclusive
Exponentiation (IEX), for the process e+e- -> f-bar f + n(gamma),
f=mu,tau,d,u,s,c,b, with validity for centre of mass energies from tau lepton
threshold to 1 TeV. We analyse 2f numerical results at the Z-peak, 189 GeV and
500 GeV. We also present precision calculations of the signal processes e+e- ->
4f in which the double resonant W+W- intermediate state occurs using our
YFSWW3-1.14 MC. Sample 4f Monte Carlo data are explicitly illustrated in
comparison to the literature at LEP2 energies. These comparisons show that a TU
for the signal process cross section of 0.4 percent is valid for the LEP2 200
GeV energy. LC energy results are also shown.Comment: 5 pages, 4 figures, Presented at ICHEP200
W-Pair Production with YFSWW/KoralW
A theoretical description of W-pair production in terms of two complementary
Monte Carlo event generators YFSWWand KoralW is presented. The way to combine
the results of these two programs in order to get precise predictions for WW
physics at LEP2 and LC energies is discussed.Comment: LateX file, 6 pages, conference contributio
Precision W-pair physics with the YFSWW3 and KoralW Monte Carlos
We present the recent developments in the precision studies of W-pair and
single-W processes in e+e- collisions achieved with the help of the KoralW and
YFSWW3 Monte Carlo generators. We focus on the theoretical precision of the
measurements of M_W and anomalous couplings on the example of lambda coupling.
We present the mechanism of running these two independent codes in the form of
one Concurrent Monte Carlo code. We describe also the extensions of KoralW
necessary to emulate the kinematical region of single-W process.Comment: Talk given by M. Skrzypek at 6th International Symposium on Radiative
Corrections, 8-13 September 2002, Kloster Banz, German
Precision Predictions for (Un)Stable WW/4f Production in e +e- Annihilation: YFSWW3/KoralW-1.42/YFSZZ
We present precision calculations of the processes e+ e- -> 4-fermions in
which the double resonant W+ W- and ZZ intermediate states occur. Referring to
these latter intermediate states as the 'signal processes', 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 WW 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 Denner et al. In this way, we cross check that the total (physical
oplus 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. For the analogous ZZ case, we cross check that our YFSZZ
calculation yields a total precision tag of 2 per cent, when it is compared to
the results of ZZTO and GENTLE of Passarino and Bardin et al., respectively.Comment: 14 pages, 1 figure, 4 tables, presented at RADCOR2000 by B.F.L. War
Electric Charge Screening Effect in Single-W Production with the KoralW Monte Carlo
Any Monte Carlo event generator in which only initial state radiation (ISR)
is implemented, or ISR is simulated independently of the final state radiation
(FSR), may feature too many photons with large transverse momenta, which deform
the topology of events and result in too strong an overall energy loss due to
ISR. This overproduction of ISR photons happens in the presence of the final
state particle close to the beam particle of the same electric charge. It is
often said that the lack of the electric charge screening effect between ISR
and FSR is responsible for the above pathology in ISR. We present an elegant
approximate method of curing the above problem, without actually reinstalling
FSR. The method provides theoretical predictions of modest precision: < 2%. It
is, however, sufficient for the current 1W data analysis at the LEP2 collider.
Contrary to alternative methods implemented in other MC programs, our method
works for the ISR multiphotons with finite p_T. Although this method is not an
exact implementation of the complete/exact ISR, FSR and their interference, it
is very closely modelled on it. We present a variety of numerical results
obtained with the newest version of the KoralW Monte Carlo, in which this
method is already implemented
Exact Gauge Invariant YFS Exponentiated Monte Carlo for (Un)Stable for (Un)Stable Production At and Beyond LEP2 Energies
We realize, by Monte Carlo event generator methods, the exact O}(\alpha)e^+e^- \to W^+ W^- (\to f_1\bar f'_1 + \bar f_2
f'_2)f_if'_iSU_{2L}i=1,2n(\gamma)W^+ W^-$. Sample Monte Carlo data are illustrated.Comment: 12 pages, 4 figures, 1 Latex file which includes the figure
The Monte Carlo Program KoralW version 1.51 and The Concurrent Monte Carlo KoralW&YFSWW3 with All Background Graphs and First Order Corrections to W-Pair Production
The version 1.51 of the Monte Carlo (MC) program KoralW for all processes is presented. The most important change
since the previous version 1.42 is the facility for writing MC events on the
mass storage device and re-processing them later on. In the re-processing one
may modify parameters of the Standard Model in order to fit them to
experimental data. Another important new feature is a possibility of including
complete corrections to double-resonant W-pair
component-processes in addition to all background (non-WW) graphs. The
inclusion is done with the help of the YFSWW3 MC event generator for fully
exclusive differential distributions (event-per-event). Technically, it is done
in such a way that YFSWW3 runs concurrently with KoralW as a separate slave
process, reading momenta of the MC event generated by KoralW and returning the
correction weight to KoralW. KoralW introduces the
correction using this weight, and finishes processing the event (rejection due
to total MC weight, hadronization, etc.). The communication between KoralW and
YFSWW3 is done with the help of the FIFO facility of the UNIX/Linux operating
system. This does not require any modifications of the FORTRAN source codes.
The resulting Concurrent MC event generator KoralW&YFSWW3 looks from the user's
point of view as a regular single MC event generator with all the standard
features.Comment: 8 figures, 5 tables, submitted to Comput. Phys. Commu
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