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Manuscript Title: NUNUGPV, a Monte Carlo event generator for e+e- -> nu nubar gamma(gamma) events at LEP.
Authors: G. Montagna, O. Nicrosini, F. Piccinini
Program title: NUNUGPV
Catalogue identifier: ADDX_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 98(1996)206
Programming language: Fortran.
Computer: DEC VAX.
Operating system: VMS, UNIX.
RAM: 136K words
Word size: 32
Keywords: Particle physics, Elementary, Qcd, High energy electron Positron collisions, Single-photon events, Radiaive neutrino Counting, Visible photons, Z0 and w bosons, Lep1, Lep2, Corrections qed, Electron structure Functions, Photon transverse Degrees of freedom, Experimental cuts, Monte carlo integration, Event generation, Importance sampling, Multichannel approach.
Classification: 11.5.

Nature of problem:
The radiative neutrino counting and, more in general, the reactions with a single-photon signature, provide a very useful tool for measuring the number of light neutrinos and eventually detecting new-physics signals at the LEP collider [2]. Within the Standard Model (SM), the bulk of the contribution to single-photon events at LEP comes from the production of a neutrino-antineutrino pair accompanied by a large-angle energetic photon, i.e. from the reaction e+e- -> nu nubar gamma. This process has already been studied in the LEP1 energy range using semi- analytical and Monte Carlo codes [3,4,5], which contain assumptions or approximations valid around the Z0 pole. However, in the light of the present data taking at LEP 1.5 (where square root(s) =~ 140 GeV) and in view of the forthcoming experiments at LEP2 (where square root(s) =~ 200 GeV), the approximations employed for the Z0 resonance have to be rediscussed (see, for instance, ref. [6]), in order to provide reliable predictions with a theoretical error of the order of 1 per cent.

Solution method:
A Monte Carlo integration technique for weighted events is employed to perform the high-dimensional numerical integration in presence of realistic cuts on the observed photons. For experimental simulation, the program can also be used as a true event generator that provides a sample of unweighted events, defined as the components of the three final-state photons, sotred into proper n-tuples. To cure the peaking behaviour of the integrand, the importance-sampling technique [7] is used, both in the integration and in the generation modes.

Processes originating the same single-photon final state as the signal e+e- -> nu nubar gamma (such as the radiative Bhabha scattering background with the two final-state electrons undetected) are not taken into account in the present version of the program.

Unusual features:
None The program uses the random number generator RANLUX [1] which is included in the program.

Running time:
As integrator, the code needs about 1 min of ALPHA 3000/700 for generating 10**5 wieghted events; the corresponding relative error on the total corss section is about 5*10**-3. The generation of a sample of 10**4 unweighted events requires about 30 min on the same system.

[1] F. James, Comput. Phys. Commun. 79 (1994) 111.
[2] F. Boudjema, B. Mele et al., Standard Model Processes, in Physics at LEP2, G. Altarelli, T. Sjostrand and F. Zwirner eds., CERN Report 96-01, 1996 (to appear); L. Trentadue et al., Neutrino Counting, in Z Physics at LEP1, G. Altarelli, R. Kleiss and C. Verzegnassi eds., CERN Report 89-08, Vol. 1 (1989) p. 129.
[3] O. Nicrosini and L. Trentadue, Nucl. Phys. B318 (1989) 1.
[4] R. Miquel, C. Mana and M. Martinez, Z. Phys. C48 (1990) 309; also in QED Structure Functions, G. Bonvicini, ed., AIP Conf. Proc. No. 201 (AIP, New York, 1990), p. 395.
[5] S. Jadach, B.F.L. Ward and Z. Was. Comput. Phys. Commun. 79 (1994) 503.
[6] G. Montagna, O. Nicrosini, F. Piccinini and L. Trentadue, Nucl. Phys. B452 (1995) 161.
[7] F. James, Rep. Prog. Phys. 34 (1980) 1145.