Table Of Content9 TAUOLA, TAUOLA universal interface PHOTOS and MC-TESTER:
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Status Report
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2
Z. Wa¸sa ∗
n
a aInstitute of Nuclear Physics, Polish Academy of Sciences,
J
ul. Radzikowskiego152, 31-342Cracow, Poland
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The status of Monte Carlo programs for the simulation of τ-lepton production and decay in high-energy
] accelerator experiments is reviewed. No basic changes in the organization of the programs were necessary since
h
the previous τ-lepton conference in 2006. Minor in size, but practical, extensions for the use of the programs in
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high precision mixed languages enviromens are being introduced: (i) For the TAUOLA Monte Carlo generator of
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p τ-leptondecays,automated andsimultaneoususeofmanyversionsofform-factors forthecalculation ofoptional
e weights for fits was discussed. A pilot example of its use was presented. (ii) New tests for PHOTOS Monte Carlo
h for QED bremsstrahlung in W decays were shown. (iii) Prototype version of the TAUOLA universal interface
[ based on HepMC (theC++ event record) was mentioned. Itstests with thehelp of MC-TESTER were discussed.
1 Presented results illustrate the status of the projects performed in collaboration with Vladimir Cherepanov,
v NadiaDavidson, Piotr Golonka, Gizo Nanava, Tomasz Przedzin´ski Elz˙bieta Richter-W¸as and others.
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6 Presented at International workshop on Tau Lepton Physics, TAU08 Novosibirsk, Russia September, 2008
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1 preprint IFJPAN-IV-2008-10
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0 1. Introduction ages (e.g. generating events for LHC, LC, Belle
:
v orBaBarphysicsprocesses). Theprogramsstruc-
The TAUOLA package [1,2,3,4] for the simu-
i ture, prepared for the convenience of users, was
X lation of τ-lepton decays and PHOTOS [5,6] for
presented during previous τ conferences, and we
r the simulation of radiative corrections in decays,
a will not repeat it here.
are computing projects with a rather long his-
This time, let us concentrate on physics ori-
tory. Written and maintained by well-defined
ented results and special techniques being devel-
authors, they nonetheless migrated into a wide
opped and used for fits, over the years. They
rangeofapplications where they became ingredi-
may be worth some attention now, when high
ents of complicated simulation chains. As a con-
precision, high statistic data are being analyzed.
sequence,alargenumberofdifferentversionsare
FORTRANto C++migrationis not as simple as one
presently in use. From the algorithmic point of
may think. On one hand it is generally believed
view,they oftendifferonlyinafewsmalldetails,
thatlargeprojectsareeasiertomaintaininmore
but incorporate many specific results from dis-
modern programming frameworks. On the other
tinctτ-leptonmeasurementsorphenomenological
hand, FORTRAN software can not be abandoned.
projects. Such versions were mainly maintained
It usually carries a lot of physics expertise. It
(and will remain so) by the experiments taking
is also impractical to replace the software envi-
precision data on τ leptons. On the other hand,
ronment of mature experiments with something
many new applications were developed recently,
new. Fortunately, co-existence of the two lan-
oftenrequiringaprograminterfacetootherpack-
guages is not a problem, at least not from the
∗SupportedinpartbythePolishGovernmentgrantN202 software point of view.
06434 (2008-2010) and EU-RTN Programme, Contract Our presentation is organized as follows. Sec-
No. MRTN–CT-2006-035482,“Flavianet”.
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tion 2 is devoted to the discussion of optional resolve such system in one automated step using
weights in TAUOLA and their use for fits to the themethodsuchas[7,8]forexample. Thiscanbe
data already at the level of comparison with ofcourseverydangerousfromthepointofviewof
raw data. In section 3 we present some new systematic error control. We will return to this
results for the simulation with PHOTOS of ra- point later. At this moment let us discuss nec-
diative corrections in decays. Section 4 is de- essary adaptations for TAUOLA for such methods
voted to MC-TESTER; the program which can be to be used. In fact the principle of the changes
used for semi-automatic comparisons of simula- is quite simple. The method is in use since long
tionsamples originatingfromdifferentprograms. time and in different situations, see eg. [9]. It is
The prototype, C++ based, TAUOLA universal enough,tocalculate,foreachgeneratedevent(for
interfaceis mentioned in that Section too. eachpresentinitdecayofτ+ andτ− separately)
Because of the limited space of the contribu- alternative weights; the ratios of matrix element
tion,andsizableamountofotherphysicallyinter- squared obtained with new currents provided by
estingresults,someofthemwillbeexcludedfrom the user, and the one actually used in generation
conference proceedings. Let us hope that they arethenthevectorofweights. Ifthecurrentscan
will find place in future works, possibly with col- berepresentedasalinearcombinationofelemen-
laborators mentioned in the Abstract. For these tary ones, then the weight for arbitary values of
works, the present paper may serve as an an- coefficients is a bilinear form in the relative coef-
nouncement. ficients1. In more general cases, linearization is
necessary, and the whole procedure needs to be
repeated with the currents obtained at one step
2. OptionalweightsinTAUOLAMonteCarlo
used for generation at the next one. In this way,
Physics of τ lepton decays requires sophis- effects of the new currents will migrate to spin
ticated strategies for the confrontation of phe- correlations between τ+ and τ− as well.
nomenological models with experimental data. The first example of the use of the method, is
On one side high statistics experimental samples demonstrated in fig. 1. At this step of the work
are collected, and the obtained precision is high, no detector effects were included. Experimen-
on the other hand, there is a significant cross- tal data were provided as a one-dimensional his-
contaminationbetweendistinctτ decaychannels. togram. Simulation of τ production process was
Starting from a certain precision level all chan- not performed. One can see, that the theoretical
nels need to be analyzed simultaneously. Change modelprovidesdoublepeakwhichisabsentinthe
of parametrization for one channel contributiong data. This is because of an interplay between in-
to the background may be important for the fit termediate state resonance parametrization and
of another one. This situation leads to a com- phase space jacobian. The same parametrization
plex configuatin where a multitude of parame- was used for 3-scalar and 2-scalar systems. This
ters needs to be simultaneously confronted with points to the necessity of further investigations
a multitude of observables. One has to keep in of unitarity constraints on the running width or
mind that the models used to obtain distribu- better control of the data.
tionbs in the fits may requirerefinements or even Options how to handle, in experimental soft-
substantial rebuilds. The isospin relation to dis- ware, information for weight calculation is under
tribution measured in e+e− collistions at low en- study now. In principle, information stored in
ergies offer additional control, but may be the the event record should be enough. This may
sourceofevenmorecomplexsystematicerrorcon- not be a perfect match in all cases. For exam-
siderations. Effects of QEDbremsstrahlungneed ple, effects of the boost fromlaboratoryframe to
to be taken ito account as well. It may affect rest-frame of τ lepton and back, may result in
selectioncriteriaandbackgroundcontaminations
1We have applied such method in one of our other pro-
in quite complex and unexpected ways.
grams[9]forthestudiesofanomalouscouplingsofZ and
From the statistical point of view it is best to LEPdata.
3
t - fi h p -p 0n
t
800 simpler not to include changes into spin corre-
lations. In such a case the vector of weights
700
MMeeaann 11..443322
would depend on kinematical configurations of
600 RcRc 22MM // SSnn dd ff 442244 .. 1100 ..//11 22117711 τ+, τ− and of the production process simulta-
500 PPrroobb 00 neously. The optimal choice require further in-
pp00 --33229944 –– 771100..99 teraction with collaborations. Technical side of
400 pp11 11113300 –– 228833..99 theactivitywilldependonthewaytheuserwant
pp22 44115577 –– 777733..77
300 pp33 --55220055 –– 999988..66 tocalculatethehadroniccurrent. Itcanbeeasily
pp44 88002200 –– 11553322..99 calculated as external generalized function com-
200
municating with TAUOLA at the level of linker or
100 throughthefifopipes. Theycanbeprovidedas
FORTRAN or eg. C++ code and have well defined
0
and easy to control input/output for modifica-
0.6 0.8 1 1.2 1.4 1.6 1.8 2 tions introduced into TAUOLA.
Q, GeV
Int the past, and for a good reason, signifi-
cant effort was devoted to strategies where the
Figure1. Preliminaryresult from the study of τ−→ methotds as indicated here can be avoided. For
ηπ−π0ν decay using external weight. Experimental example in [10] a system of projections was pro-
data and theoretical predictions obtained at the inter- posedfor the 3-scalarτ decays. If sucha method
mediate step of the work are shown. The distribu- canbe usedit is possible to obtainfromthe data
tion of the invariant mass of all final state hadrons just oneofthe contributiontothe current,which
is given. This plot is available thanks to courtesy of isthenparametrizedbyasinglescalarfunctionof
Vladimir Cherepanov. two invariants only. This is of course on the ex-
pense of statistical strength on the measurement
and abstraction of experimental sytematic error.
Such methods, easy to apply for final states of
rounding errors. In some cases the kinematical two or three scalars, are less practical for higher
configuration may be changed because of QED multiplicities. Nonetheless they can be of great
bremsstrahlung generated by PHOTOS. The order help to control the analysis and to verify which
of final state particles stored in the event record aspects of the models require improvements.
maynotbethesameasusedinternallyinTAUOLA
attheeventconstructionstep. Thisofcoursecan 3. PHOTOS and NLO effects in W and γ∗ →
be cross checked on the basis of inspection of a π+π− decays
smaller sample and without detector simulation
part being used. Such tests are nonetheless nec- The changesintroducedoverthe lasttwoyears
essary,but mayneedlesstime thanwouldbe the into PHOTOS Monte Carlo program itself were
case if collaboration software would be rebuild. rather small. On the other hand, the complexity
From that point of view, the easiest would be to oftheworkonitstheoreticalfoundationwaslarge
calculate optional weights from information al- andmatchesneithersizenorthemainpurposeof
ready available in experiments production tapes, thepresenttalk(someoftherelatedtopicsarere-
and only for those events where bremsstrahlung vievedin[11]). Previoustestsoftwobodydecays
effects are not generated at all. This can be cor- of the Z into a pair of charged leptons [12] and
reted at the next step of form-factor iteration. scalar B into a pair of scalars [13] was recently
Relatedslowdownofthe convergenceshouldnot supplemented[14]withthestudyofW± →l±νγ.
be large. Possibly two steps will be enough, the The study of the process for γ∗ → π+π− is on-
second one for the verification. It will be neces- going[15]. Inallofthesecasesuniversalkernelof
sary for control of spin correlations between the PHOTOSwasreplacedwiththeonematchingexact
two decaying τ-s as well. It is technically much first order matrix element. In this way terms for
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NLO/NLLlevelareimplemented. Asusual,algo- HEPEVTeventrecordisusedandC++whereHepMC
rithmcoversthefullmultiphotonphase-spaceand [17] is used instead. For the sake of automatiza-
is exactin the infraredregionof the phase space. tion of tests MC-TESTER was prepared [18]. Re-
ThisisratherunusualfortheNLLcompatibleal- cently, new functionalities were introduced into
gorithms. OneshouldnotforgetthatPHOTOSgen- the package [19]. In particular, it works now
eratesweightoneeventsanddoesnotrequireany with the HepMC event record, the standard of
factorization scales. There is full overlap of the C++ programs. The complete set-up for bench-
phase space where hard matrix element is used marking the interfaces,such as interface between
with the one for iterative photon emissions. All τ-lepton production and decay, including QED
interferenceeffectsareimplementedwiththehelp bremsstrahlung effects is prepared. The exam-
of internal weights. ple is chosen to illustrate the new options intro-
The results of all tests of PHOTOS with a NLO ducedintotheprogramandnovelwaysofitsuse.
kernel are at sub-permille level. No differences From the technical perspective, [19] documents
with benchmarks can be observed, even for sam- program updates and supplements previous doc-
ples of 109 events. For the comparisons when umentation [18].
simpler physics assumptions are used, differences As in the past, test consists of two steps. Dis-
were sub-permille too, if compared to total rates tinct Monte Carlo programs are run separately;
(or were matching precisionof programsused for events are searched for the decays of the chosen
tests). object, and information is stored by MC-TESTER.
This is very encouraging, and points to the Then, at the analysis step, information from a
possible extension of the approach outside of the pair of such runs may be compared and repre-
QED (scalar QED) only. In particular to the do- sented in the form of tables and plots.
main of QCD or if phenomenologicallagrangians One may have the impression that the modifi-
for interactions of photons need to be applied. cations introducedintothe releaseareminorand
For that work to be completed spin amplitudes consist of simple improvements in the graphical
need to be further studied. Let us point here to representationofthe outputandpurelytechnical
the paper [16]. reorganizationthanks to the use of C++ lists.
The discussed here refinements only indirectly To some degree this is true, but on the other
affect practical side of simulations for τ physics. hand the changes were introduced because of
Changes in the kernels necessary for NLO may pressurefromapplications. Letusshowhowpro-
remain as options for tests only. They are avail- gram modifications can be used for non-trivial
able from the PHOTOS web page [14] but are not practical applications.
recomended for wider use. An example of such It is quite common that information stored in
new tests is given in fig. 2b. the event record is too large. For example in-
dividual soft photons which remain undetectable
are present. Not only they do not influence the
4. MC-TESTER and C++ TAUOLA universal
detector response at all, but they exhibit tech-
interface
nical aspects of eg. infrared regulators of QED
Inthe developmentofpackagessuchasTAUOLA bremsstrahlung. In response, MC-TESTER should
or PHOTOS, question of tests and appriopriate re- ignore those photons while analyzing decays (or
lationstousersapplicationsareessentialfortheir group together with other particles). Other-
usefulness. In fact, users applications may be wise, comparisons of different Monte Carlo pro-
much larger in size and human effort than the grams would be dominated by the technical as-
programs discussed here. Good example of such pect of the implementation of the infrared regu-
‘user applications’ are complete environments to lator; MC-TESTER operation need to be adopted
fit experimentaldata and controldetector uncer- for this, see eg. [20,12].
tainties at the same time. As a consequence our Another example where the event tree may
programs need to work well with FORTRAN where needtobesimplifiedforvalidationisifspincorre-
5
CCoommppaarriissoonn ooff MMaassss((22)) ooff ppii-- ppii++ nnuu__ttaauu~~ iinn cchhaannnneell ZZ00 ==>> ppii-- ppii++ nnuu__ttaauu~~ nnuu__ttaauu SDP production processes. Let us use as an example2
0.0261 pp → Z/γ∗+X, Z → τ+τ−. It is convenient to
1.2 4500 start the test by restricting τ decays to the sim-
4000 plest decay mode, that is τ± → π±ν, and look
1 at distributions in cascade decay Z → π+π−νν¯.
3500
In this case the effects of spin correlations are
0.8 3000
largest. The distribution of the π− energy spec-
2500
0.6 trum (in the Z rest-frame), manifests the τ po-
2000 larization through its slope (see fig 2a). Fortu-
0.4 1500 nately,thisframedependentspectrumisequalto
1000 the distribution of the square of invariant mass
0.2 500 of π−π+ν¯. This distribution can be obtained in
MC-TESTERthanks to the new options.
0 0
0 0.2 0.4 0.6 0.8 1
Final state activities will lead to final states
a
of Z decays where the τ-pair is accompanied by
bremsstrahlung photons or soft hadrons. Such
CCoommppaarriissoonn ooff MMaassss((11)) ooff ggaammmmaa mmuu++ iinn cchhaannnneell WW++ ==>> ggaammmmaa nnuu__mmuu mmuu++ SDP distribution are usually strongly peaked. In
3.09e-05 fig. 2b we provide an example of such a distribu-
1 tionbutforthe comparisonofPHOTOSwithexact
106 marix element for the decay W+ →µ+νγ.
0.8 Withtheabovebriefexampleswehavedemon-
105
stratedthatMC-TESTERmaybe usefulfortestsof
0.6 104 libraries of particles decays, as well as for tests
of their interfaces. The work on HepMC based
103
0.4 TAUOLA universal interface is on-going. A
102 preliminary version is already available [21], but
from physicspoint of view it is still less complete
0.2
10 than the FORTRAN version [22].
The updated version of MC-TESTER was found
0 1
0 10 20 30 40 50 60 70 80 90 100
[20,12]to handle wellcaseswhere physicallyspu-
b rious information (eg. on soft photons) need to
be ignored.
Figure 2. Examples of interesting benchmarks:
In the decay Z → τ+τ− → π+π−νν¯, the π− en- 5. Summary and future possibilities
ergy spectrum in the Z rest-frame has identical
The status of the computer programs for the
shape as the distribution of the invariant mass
squared of π+π−ν¯, see plot a. This linear distri- decayofτ leptonsTAUOLAandassociatedprojects
bution can be used to measure decaying τ− polar- TAUOLA universal interface and MC-TESTER
was reviewed. The high-precision version of
ization. In plot b, an example of the PHOTOS test
PHOTOS for radiative corrections was presented
is given. Program version with full matrix ele-
too.
ment for W+ →µ+ν γ channel is compared with
µ New results for PHOTOS and decay of W were
the standard one. The distribution of the µ+γ
mentioned. For this channel complete next-to-
pair mass is shown.
leadingordereffects cannowbe simulated. How-
ever, for most of the applications these effects
are not necessary, leaving the standard modu-
2Spincorrelationsindecays ofW,H,H± intoτ lepton(s)
lations are appropriately introduced into various arenearlyidentical.
6
lar version of PHOTOS sufficient. This exercise is 69.
important not only for photonic bremsstrahlung. 3. S.Jadach,Z.Was,R.Decker,andJ.H.Kuhn,
It helps to understand better questions related Comput. Phys. Commun. 76 (1993)361–380.
with matching hard emission matrix elements 4. P. Golonka et al., Comput. Phys. Commun.
with parton showers without necessity to intro- 174 (2006) 818–835,hep-ph/0312240.
duce any boundaries within phase space. The 5. E. Barberio, B. van Eijk, and Z. Was, Com-
important result of the above work is that the put. Phys. Commun. 66 (1991) 115.
path to include electromagnetic form-factors of 6. E.BarberioandZ.Was,Comput.Phys.Com-
the particles participating in decay is now open mun. 79 (1994) 291–308.
for future fits to the data. These form-factor ef- 7. A. Hoecker, P. Speckmayer, J. Stelzer,
fects may be significantly larger and physically F. Tegenfeldt, H. Voss, K. Voss, et al.,
more justified than complete next-to-leading or- http://tmva.sourceforge.net/.
der effects of scalar QED as in B meson decays. 8. A. Hocker et al., PoS ACAT (2007) 040,
The presentation of the TAUOLA general- physics/0703039.
purpose interface was brief. The C++ version ex- 9. A. Jacholkowska,J. Kalinowski, and Z. Was,
ist, but is still physically less refined than the Comput. Phys. Commun. 124 (2000) 238–
FORTRAN version. Examples for its use can be 242, hep-ph/9905225.
foundonthecontinuouslymodifiedwebpage[21]. 10. J. H. Kuhn and E. Mirkes, Z. Phys. C56
ThenewversionofMC-TESTER[19]isalreadypub- (1992) 661–672.
lic now. It not only workswith HepMC[17] ofC++ 11. Z. Was, Acta Phys. Polon. B39 (2008) 1761,
but enables user defined tests. 0807.2775.
Distinct versions of the TAUOLA library for τ 12. P. Golonka and Z. Was, Eur. Phys. J. C50
lepton decay, and of PHOTOS for radiative correc- (2007) 53–62, hep-ph/0604232.
tions in decays, are now in use. The principles 13. G. Nanava and Z. Was, Eur. Phys. J. C51
howtousethedistributionpackagearepresented (2007) 569–583,hep-ph/0607019.
in refs. [4,23], onthe other hand, developmentof 14. P. Golonka, G. Nanava, and Z. Was,
new and recovery of codes for old strategies of Tests of PHOTOS Hard Bremsstrahlung,
fits is becoming available. This includes strategy http://mc-tester.web.cern.ch/MC-TESTER/PHOTOS-MCTESTER/
to use weighted events, and the projection oper- 15. Z. Was and Q. Xu, In preparation.
ators [10] for the three scalars final states. Let 16. A. van Hameren and Z. Was, 0802.2182.
us hope that the present talk will provide some 17. M. Dobbs and J. B. Hansen, Com-
stimulationforthelargerworkinthenearfuture. put. Phys. Commun. 134 (2001) 41–46,
https://savannah.cern.ch/projects/hepmc/.
Acknowledgements
18. P. Golonka, T. Pierzchala, and Z. Was,
Discussions with members of the Belle and Comput. Phys. Commun. 157 (2004) 39–62,
BaBarcollaborationsarealsoacknowledged. Ex-
hep-ph/0210252.
change of e-mails and direct discussions with S.
19. N.Davidson,P.Golonka,T.Przedzinski,and
Banerjee,S.Eidelman,H.Hayashii,K.Inami,A.
Z. Was, 0812.3215.
Korchin, J. H. Ku¨hn and O. Shekhovtsova was 20. P. Golonka and Z. Was, Eur. Phys. J. C45
a valuable input to present and future steps in
(2006) 97–107,hep-ph/0506026.
project development.
21. N.Davidson,T.Przedzinski,E.Richter-Was,
and Z. Was, Presently the code is evolving
quickly. Interested pilot users can download
REFERENCES
code from http://www.ph.unimelb.edu.au/
1. S. Jadach,J. H. Kuhn, and Z. Was, Comput. ˜ndavidson/tauola/doxygen/index.html It is
Phys. Commun. 64 (1990) 275. automatically updated daily from our source
2. M. Jezabek, Z. Was, S. Jadach, and J. H. code repository. Version information can be
Kuhn, Comput. Phys. Commun. 70 (1992) found in a text file contained in the distribu-
7
tion.
22. T. Pierzcha la, E. Richter-Wa¸s, Z. Wa¸s, and
M. Worek, Acta Phys. Polon. B32 (2001)
1277–1296,hep-ph/0101311.
23. Z. Was and P. Golonka, Nucl. Phys. Proc.
Suppl. 144 (2005) 88–94,hep-ph/0411377.
