Among the physics goals of LHC experiments, precision tests of the Standard Model in the Strong and Electroweak sectors play an important role. Because of nature of the proton-proton processes, ...observables based on the measurement of the direction and energy of leptons provide the most precise signatures. In the present paper, we concentrate on the angular distribution of Drell–Yan process leptons, in the lepton-pair rest-frame. The vector nature of the intermediate state imposes that distributions are to a good precision described by spherical polynomials of at most second order. We show that with the proper choice of the coordinate frames, only one coefficient in this polynomial decomposition remains sizable, even in the presence of one or two high
p
T
jets. The necessary stochastic choice of the frames relies on probabilities independent from any coupling constants. This remains true when one or two partons accompany the lepton pairs. In this way electroweak effects can be better separated from strong interaction ones for the benefit of the interpretation of the measurements. Our study exploits properties of single gluon emission matrix elements which are clearly visible if a conveniently chosen form of their representation is used. We rely also on distributions obtained from matrix element based Monte Carlo generated samples of events with two leptons and up to two additional partons in test samples. Incoming colliding protons’ partons are distributed accordingly to PDFs and are strictly collinear to the corresponding beams.
Precision tests of the Standard Model in the Strong and Electroweak sectors play a crucial role, among the physics program of LHC experiments. Because of the nature of proton–proton processes, ...observables based on the measurement of the direction and energy of final state leptons provide the most precise probes of such processes. In the present paper, we concentrate on the angular distribution of leptons from
W
→
ℓ
ν
decays in the lepton-pair rest-frame. The vector nature of the intermediate state imposes that distributions are to a good precision described by spherical harmonics of at most second order. We argue, that contrary to general belief often expressed in the literature, the full set of angular coefficients can be measured experimentally, despite the presence of escaping detection neutrino in the final state. There is thus no principle difference with respect to the phenomenology of the
Z
/
γ
→
ℓ
+
ℓ
-
Drell–Yan process. We show also, that with the proper choice of the reference frames, only one coefficient in this polynomial decomposition remains sizable, even in the presence of one or more high
p
T
jets. The necessary stochastic choice of the frames relies on probabilities independent from any coupling constants. In this way, electroweak effects (dominated by the
V
-
A
nature of
W
couplings to fermions), can be better separated from the ones of strong interactions. The separation is convenient for the measurements interpretation.
Matching and comparing the measurements of past and future experiments call for consistency checks of electroweak (EW) calculations used for their interpretation. On the other hand, new calculation ...schemes of the field theory can be beneficial for precision, even if they may obscure comparisons with earlier results. Over the years, concepts of
Improved Born
,
Effective Born
, as well as of effective couplings, in particular of
sin
2
θ
W
eff
mixing angle for EW interactions, have evolved. In our discussion, we use four versions of DIZET EW library for phenomenology of practically all HEP accelerator experiments over the last 30 years. We rely on the codes published and archived with the KKMC Monte Carlo program for
e
+
e
-
→
f
f
¯
n
(
γ
)
and available for the TauSpinner as well. TauSpinner re-weighs generated events for introduction of EW effects. To this end, DIZET is first invoked, and its results are stored in data file and later used. Documentation of TauSpinner upgrade, to version 2.1.0, and that of its new arrangement for semi-automated benchmark plots are provided. In our paper, focus is placed on the numerical results, on the different approximations introduced in Improved Born to obtain Effective Born, which is simpler for applications of strong or QED processes in
pp
or
e
+
e
-
colliders. The
τ
lepton polarization
P
τ
, forward–backward asymmetry
A
FB
and parton-level total cross section
σ
tot
are used to monitor the size of EW effects and effective
sin
2
θ
W
eff
picture limitations for precision physics. Collected results include: (i)
Effective Born
approximations and
sin
2
θ
W
eff
, (ii) differences between versions of EW libraries and (iii) parametric uncertainties due to, for example,
m
t
or
Δ
α
h
(
5
)
(
s
)
. These results can be considered as benchmarks and also allow to evaluate the adequacy of
Effective Born
with respect to
Improved Born
. Definitions are addressed too.
Formula omitted-leptons produced in pp collisions allow to measure Standard Model parameters and offer probes for New Physics. The TauSpinner program can be used to modify spin (or production matrix ...elements) effects in any Formula omitted sample. It relies on the kinematics of outgoing particles: Formula omitted lepton(s) (also Formula omitted in case of W-mediated processes, optionally also four-moments of accompanying hard jets) and Formula omitted decay products. No other information is required from the event record. With calculated spin (or production/decay matrix element) weights, attributed on the event-by-event basis, modifications to the spin/decay/production features, is possible without the need for regenerating events. With TauSpinner algorithms, the experimental techniques developed over years since LEP 1 times are already used and extended for LHC applications. The purpose of the present publication is to systematically document physics basis of the program, and to overview its application domain and systematic errors.
Because of their narrow width,
τ 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
τ decays. Effects of spin, including transverse degrees of freedom, genuine weak corrections or of new physics may be taken into account at the time when a
τ decay is generated and written into an event record. The physics content of the C++ interface is already now richer than its FORTRAN predecessor.
Program title: TAUOLA++, versions 1.0.2, 1.0.3, 1.0.4, 1.0.5, 1.0.6
Catalogue identifier: AELH_v1_0
Program summary URL:
http://cpc.cs.qub.ac.uk/summaries/AELH_v1_0.html
Program obtainable from: CPC Program Library, Queenʼs University, Belfast, N. Ireland
Licensing provisions: Standard CPC licence,
http://cpc.cs.qub.ac.uk/licence/licence.html
No. of lines in distributed program, including test data, etc.: 649 068
No. of bytes in distributed program, including test data, etc.: 6 544 479
Distribution format: tar.gz
Programming language: C++, FORTRAN77
Computer: PCs, workstations
Operating system: Linux, MacOS
RAM:
<
10
MB
Classification: 11.2
External routines: HepMC (
http://lcgapp.cern.ch/project/simu/HepMC/), optional; PYTHIA8 (
http://home.thep.lu.se/~torbjorn/Pythia.html)
Subprograms used:
Cat Id
Title
Reference
ADSM_v2_0
MC-TESTER
Comput. Phys. Commun. 182 (2011) 779
Nature of problem: The code of Monte Carlo generators often has to be tuned to the needs of large HEP Collaborations and experiments. In particular
τ lepton decays need to be added (or modified) to the previously generated (or measured) events encapsulated in an event record.
Solution method: The new algorithm, the universal interface of TAUOLA which works with the HepMC event record of C++ applications is documented. It uses the
τ decay generator as described in 2 with the updates explained in 1. For the new interface spin treatment was improved. For example it features complete spin effects in processes mediated by
Z
/
γ
⁎
interactions. The effects of electroweak corrections can be taken into account in this case as well. In general, the program superseeds its FORTRAN predecessor 1. The event record analysis as well as initialization is also modernized.
Restrictions: The input event record must meet the requirements described in Section 2.3.1 of the documentation.
Unusual features: Two sets of installation scripts; an additional tool for calculating tables for electroweak corrections.
Running time: Depends on the size and complexity of the events. Small events (<50 particles), require 1 to 7 minutes for processing 1 M events on PC/Linux with a 2.4 GHz processor.
References:
1
P. Golonka, B. Kersevan, T. Pierzchala, E. Richter-Was, Z. Was, M. Worek, Comput. Phys. Commun. 174 (2006) 818.
2
S. Jadach, Z. Was, R. Decker, J.H. Kühn, Comput. Phys. Commun. 76 (1993) 361.
Abstract $$\uptau $$ τ -leptons produced in pp collisions allow to measure Standard Model parameters and offer probes for New Physics. The TauSpinner program can be used to modify spin (or production ...matrix elements) effects in any $$\uptau $$ τ sample. It relies on the kinematics of outgoing particles: $$\uptau $$ τ lepton(s) (also $$\upnu _\uptau $$ ντ in case of W-mediated processes, optionally also four-moments of accompanying hard jets) and $$\uptau $$ τ decay products. No other information is required from the event record. With calculated spin (or production/decay matrix element) weights, attributed on the event-by-event basis, modifications to the spin/decay/production features, is possible without the need for regenerating events. With TauSpinner algorithms, the experimental techniques developed over years since LEP 1 times are already used and extended for LHC applications. The purpose of the present publication is to systematically document physics basis of the program, and to overview its application domain and systematic errors.
We present the system for maintaining the versions of two packages: the
TAUOLA of
τ-lepton decay and
PHOTOS for radiative corrections in decays. The following features can be chosen in an automatic ...or semi-automatic way: (1) format of the common block
HEPEVT; (2) version of the physics input (for
TAUOLA): as published, as initialized by the CLEO collaboration, as initialized by the ALEPH collaboration (it is suggested to use this version only with the help of the collaboration advice), new optional parametrization of matrix elements in 4
π decay channels; (3) type of application: stand-alone, universal interface based on the information stored in the
HEPEVT common block including longitudinal spin effects in the elementary
Z
/
γ
∗
→
τ
+
τ
−
process, extended version of the standard universal interface including full spin effects in the
H
/
A
→
τ
+
τ
−
decay, interface for
KKMC Monte Carlo, (4) random number generators; (5) compiler options. The last section of the paper contains documentation of the
programs updates introduced over the last two years.
Title of program:
tauola-photos-F, release II
Catalogue identifier:ADXO_v1_0
Program summary URL:
http://cpc.cs.qub.ac.uk/summaries/ADXO_v1_0
Programs obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland
Computer: PC running GNU/Linux operating system
Programming languages and tools used:
CPP: standard C-language preprocessor,
GNU Make builder tool, also FORTRAN compiler
No. of lines in distributed program, including test data, etc.: 194 118
No. of bytes in distributed program, including test data, etc.:2 481 234
Distribution format: tar.gz
Catalogue identifier:ADXO_v2_0
Program summary URL:
http://cpc.cs.qub.ac.uk/summaries/ADXO_v2_0
No. of lines in distributed program, including test data, etc.:308 235
No. of bytes in distributed program, including test data, etc.:2 988 363
Distribution format:tar.gz
Does the new version supersede the previous version:Yes
Nature of the physical problem: The code of Monte Carlo generators often has to be tuned to the needs of large HEP Collaborations and experiments. Usually, these modifications do not introduce important changes in the algorithm, but rather modify the initialization and form of the hadronic current in
τ decays. The format of the event record (
HEPEVT common block) used to exchange information between building blocks of Monte Carlo systems often needs modification. Thus, there is a need to maintain various, slightly modified versions of the same code. The package presented here allows the production of ready-to-compile versions of
TAUOLA S. Jadach, Z. Wa̧s, R. Decker, J.H. Kühn, Comput. Phys. Comm. 76 (1993) 361; A.E. Bondar, et al., Comput. Phys. Comm. 146 (2002) 139 and
PHOTOS E. Barberio, Z. Wa̧s, Comput. Phys. Comm. 79 (1994) 291 Monte Carlo generators with appropriate demonstration programs. The new algorithm, universal interface of
TAUOLA to work with the
HEPEVT common block, is also documented here. Finally, minor technical improvements of
TAUOLA and
PHOTOS are also listed.
Method of solution: The standard
UNIX tool: the C-language preprocessor is used to produce a ready-to-distribute version of
TAUOLA and
PHOTOS code. The final FORTRAN code is produced from the library of ‘pre-code’ that is included in the package.
Reasons for new version: The functionality of the version of TAUOLA and PHOTOS changed over the last two years. The changes, and their reasons, are documented in Section 9, and our new papers cited in this section.
Additional comments: The updated version includes new features described in Section 9 of the paper. PHOTOS and TAUOLA were first submitted to the library as separate programs. Summary details of these previous programs are obtainable from the CPC Program Library.
Typical running time: Depends on the speed of the computer used and the demonstration program chosen. Typically a few seconds.
The TauSpinner algorithm is a tool that allows one to modify the physics model of the Monte Carlo generated samples due to the changed assumptions of event production dynamics, but without the need ...of re-generating events. With the help of weights τ-lepton production or decay processes can be modified accordingly to a new physics model. In a recent paper a new version TauSpinner ver.2.0.0 has been presented which includes a provision for introducing non-standard states and couplings and study their effects in the vector-boson-fusion processes by exploiting the spin correlations of τ-lepton pair decay products in processes where final states include also two hard jets. In the present paper we document how this can be achieved taking as an example the non-standard spin-2 state that couples to Standard Model particles and tree-level matrix elements with complete helicity information included for the parton–parton scattering amplitudes into a τ-lepton pair and two outgoing partons. This implementation is prepared as the external (user-provided) routine for the TauSpinner algorithm. It exploits amplitudes generated by MadGraph5 and adapted to the TauSpinner algorithm format. Consistency tests of the implemented matrix elements, re-weighting algorithm and numerical results for observables sensitive to τ polarisation are presented.
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