4,299 research outputs found
How to use SANC to improve the PHOTOS Monte Carlo simulation of bremsstrahlung in leptonic W-Boson decays
Using the SANC system we study the one-loop electroweak standard model
predictions, including virtual and real photon emission, for the decays of the
on-shell vector boson, W --> L ANTI-NU (GAMMA). The complete one-loop
corrections and exact photon emission matrix element are taken into account.
For the phase-space integration, the Monte Carlo technique is used. This
provides a useful element, first for the evaluation of the theoretical
uncertainty of PHOTOS. Later we analyse the source of the differences between
SANC and PHOTOS and we calculate the additional weight, which once installed,
improves predictions of PHOTOS simulations. We can conclude that, after the
correction of the weight is implemented, the theoretical uncertainty of PHOTOS
simulations due to an incomplete first-order matrix element is reduced to below
alpha/pi, for observables not tagging the photon in a direct way, and to 10%
otherwise. This is interesting for applications in the phenomenology of the
ongoing LEP2 and future LC and LHC experimental studies.Comment: Submitted to Acta Physica Polonica. 8 pages, 5 figure
PHOTOS Monte Carlo for precision simulation of QED in decays - History and properties of the project
Because of properties of QED, the bremsstrahlung corrections to decays of
particles or resonances can be calculated, with a good precision, separately
from other effects. Thanks to the widespread use of event records such
calculations can be embodied into a separate module of Monte Carlo simulation
chains, as used in High Energy Experiments of today. The PHOTOS Monte Carlo
program is used for this purpose since nearly 20 years now. In the following
talk let us review the main ideas and constraints which shaped the program
version of today and enabled it widespread use. We will concentrate specially
on conflicting requirements originating from the properties of QED matrix
elements on one side and degrading (evolving) with time standards of event
record(s). These issues, quite common in other modular software applications,
become more and more difficult to handle as precision requirements become
higher.Comment: Prepared for XI International Workshop on Advanced Computing and
Analysis Techniques in Physics Research, Amsterdam, the Netherlands, April 23
200
Comparison of SANC with KORALZ and PHOTOS
Using the SANC system we study the one-loop electroweak standard model
prediction, including virtual and real photon emissions, for the decays of
on-shell vector and scalar bosons B --> f anti-f (gamma), where B is a vector
boson, Z or W, or a Standard Model Higgs. The complete one-loop corrections and
exact photon emission matrix element are taken into account. For the
phase-space integration, the Monte Carlo technique is used. For Z decay the QED
part of the calculation is first cross-checked with the exact one-loop QED
prediction of KORALZ. For Higgs boson and W decays, a comparison is made with
the approximate QED calculation of PHOTOS Monte Carlo. This provides a useful
element for the evaluation of the theoretical uncertainty of PHOTOS, very
interesting for its application in ongoing LEP2 and future LC and LHC
phenomenology.Comment: Submitted to Acta Physica Polonica. 9 pages, 6 figure
Scalar QED, NLO and PHOTOS Monte Carlo
Recently, due to improvement at experiments, QED bremsstrahlung in B meson decays into pair of scalars (\pi's and/or K's) is of phenomenological interest. In practical application where experimental acceptance must be taken into account, PHOTOS Monte Carlo is often used for simulation of these QED effects. Phenomenologically sound predictions, valid over all phase space can not be obtained for complex objects, with the scalar QED alone. We will nonetheless use scalar QED to test the performance of PHOTOS. We present the analytical form of the kernel used in the older versions of PHOTOS, and the exact one with respect to first order scalar QED. Matrix element and phase space jacobians are factorized in the final weight. In this paper we also present aspects of program design, that are related to phase space generation, especially when mass terms become significant. The discussed effects are way beyond the direct phenomenological interest of today. We use this opportunity to present some foundations of the program organization that assure its precision, which may be useful for future extensions. An agreement of better than 0.01% with independent calculations of scalar QED is demonstrated
PHOTOS Monte Carlo for precision simulation of QED in decays
Because of properties of QED, the bremsstrahlung corrections to decays of particles or resonances can be calculated, with a good precision, separately from other effects. Thanks to the widespread use of event records such calculations can be embodied into a separate module of Monte Carlo simulation chains, as used in High Energy Experiments of today. The PHOTOS Monte Carlo program is used for this purpose since nearly 20 years now. In the following talk let us review the main ideas and constraints which shaped the program version of today and enabled it widespread use. We will concentrate specially on conflicting requirements originating from the properties of QED matrix elements on one side and degrading (evolving) with time standards of event record(s). These issues, quite common in other modular software applications, become more and more difficult to handle as precision requirements become higher
Universal Interface of TAUOLA Technical and Physics Documentation
Because of their narrow width, tau decays can be well separated from their
production process. Only spin degrees of freedom connect these two parts of the
physics process of interest for high energy collision experiments. In the
following, we present a Monte Carlo algorithm which is based on that property.
The interface supplements events generated by other programs, with tau decays.
Effects of spin, genuine weak corrections or of new physics may be taken into
account at the time when a tau decay is generated and written into an event
record.Comment: 1+44 pages, 17 eps figure
Constrained invariant mass distributions in cascade decays. The shape of the "-threshold" and similar distributions
Considering the cascade decay in which
are massive particles and are massless particles, we
determine for the first time the shape of the distribution of the invariant
mass of the three massless particles for the sub-set of decays in
which the invariant mass of the last two particles in the chain is
(optionally) constrained to lie inside an arbitrary interval, . An example of an experimentally
important distribution of this kind is the `` threshold'' -- which is
the distribution of the combined invariant mass of the visible standard model
particles radiated from the hypothesised decay of a squark to the lightest
neutralino via successive two body decay,: \squark \to q \ntlinoTwo \to q l
\slepton \to q l l \ntlinoOne , in which the experimenter requires
additionally that be greater than . The
location of the ``foot'' of this distribution is often used to constrain
sparticle mass scales. The new results presented here permit the location of
this foot to be better understood as the shape of the distribution is derived.
The effects of varying the position of the cut(s) may now be seen more
easily.Comment: 12 pages, 3 figure
Invariant mass distributions in cascade decays
We derive analytical expressions for the shape of the invariant mass
distributions of massless Standard Model endproducts in cascade decays
involving massive New Physics (NP) particles, D -> Cc -> Bbc -> Aabc, where the
final NP particle A in the cascade is unobserved and where two of the particles
a, b, c may be indistinguishable. Knowledge of these expressions can improve
the determination of NP parameters at the LHC. The shape formulas are
composite, but contain nothing more complicated than logarithms of simple
expressions. We study the effects of cuts, final state radiation and detector
effects on the distributions through Monte Carlo simulations, using a
supersymmetric model as an example. We also consider how one can deal with the
width of NP particles and with combinatorics from the misidentification of
final state particles. The possible mismeasurements of NP masses through `feet'
in the distributions are discussed. Finally, we demonstrate how the effects of
different spin configurations can be included in the distributions.Comment: 39 pages, 14 figures (colour), JHEP clas
Matching NLO parton shower matrix element with exact phase space: case of W -> l nu (gamma) and gamma^* -> pi^+pi^-(gamma)
The PHOTOS Monte Carlo is often used for simulation of QED effects in decay
of intermediate particles and resonances. Momenta are generated in such a way
that samples of events cover the whole bremsstrahlung phase space. With the
help of selection cuts, experimental acceptance can be then taken into account.
The program is based on an exact multiphoton phase space. Crude matrix element
is obtained by iteration of a universal multidimensional kernel. It ensures
exact distribution in the soft photon region. Algorithm is compatible with
exclusive exponentiation. To evaluate the program's precision, it is necessary
to control the kernel with the help of perturbative results. If available,
kernel is constructed from the exact first order matrix element. This ensures
that all terms necessary for non-leading logarithms are taken into account. In
the present paper we will focus on the W -> l nu and gamma^* -> pi^+ pi^-
decays. The Born level cross sections for both processes approach zero in some
points of the phase space. A process dependent compensating weight is
constructed to incorporate the exact matrix element, but is recommended for use
in tests only. In the hard photon region, where scalar QED is not expected to
be reliable, the compensating weight for gamma^* decay can be large. With
respect to the total rate, the effect remains at the permille level. It is
nonetheless of interest. The terms leading to the effect are analogous to some
terms appearing in QCD. The present paper can be understood either as a
contribution to discussion on how to match two collinear emission chains
resulting from charged sources in a way compatible with the exact and complete
phase space, exclusive exponentiation and the first order matrix element of QED
(scalar QED), or as the practical study of predictions for accelerator
experiments.Comment: 24 page
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