Tests of quantum chromodynamics in the decay of the τ-lepton and in electron positron annihilation into hadrons

We present a method to derive upper and lower bounds for a large class of quantities measurable ine ⁺ e ^? annihilation into hadrons and in τ-decay. The theoretical input are the asymptotic expansions for the vacuum polarization functions of vector and axial vector currents in the euclidean region as given by QCD. Since our method allows straightforward inclusion of (as yet uncalculated) higher order terms in α _s , there is the potential of making high precision tests of QCD even with experiments at rather moderate energies. We argue, therefore, that it may be very worthwhile to pursue experiments ate ⁺ e ^? storage rings below theJ/Ψ resonance and to measure the branching ratios for the decays of the τ-lepton with high accuracy. In the latter case we can also get information on the behaviour of the weak axial vector current which should be very interesting in connection with chiral symmetry breaking.     

A possible scheme to obtain e-e- and e+e- collisions at energies of hundreds of GeV

As a contribution to the discussion on very long term developments in the field of high energy physics, it is pointed out that it is possible to devise e^-e^- and e⁺e^- colliding beam machines which are not affected by the large synchrotron losses typical of conventional storage rings. The scheme proposed here makes use of two collinear superconducting linacs which at the same time accelerate and recover the energy fed to the electron and positron beams.     

Charged majorons ine + e − ande − e − collisions

The experimental limits on charged majorons, i.e. charged scalars which carry lepton number, are discussed in a model independent way. Possible signals ine ⁺ e ^? ande ^? e ^? collisions are then estimated. It is stressed that the limits on majorons which conserve the different lepton flavors separately are presently very poor. These limits can however be improved by studyinge ^? e ^? collisions.     

Supersymmetry and polarization in e+e− collisions (II). Fermionic partners of the gauge bosons

We study the pair production of fermionic partners of the gauge bosons in e⁺e^? collisions with polarized beams. We calculate the cross sections and the spin asymmetries for various processes using different models for the basic properties of gauginos. If they exist and if they are rather light, they should be discovered in future e⁺e^? colliders. We show that polarization provides very special signatures for their experimental search.     

Prospects for e+e- physics at Frascati between the φ and the ψ

We present a detailed study, done in the framework of the INFN 2006 Roadmap, of the prospects for e⁺e^- physics at the Frascati National Laboratories. The physics case for an e⁺e^- collider running at high luminosity at the φ resonance energy and also reaching a maximum center of mass energy of 2.5 GeV is discussed, together with the specific aspects of a very high luminosity τ-charm factory. Subjects connected to kaon decay physics are not discussed here, being part of another INFN Roadmap working group. The significance of the project and the impact on INFN are also discussed. All the documentation related to the activities of the working group can be found in http://www.roma1.infn.it/people/bini/roadmap.html.     

Dipole formulation of QCD cascades

The formulation of a QCD parton cascade in terms of colour dipoles or colour antennae is applied to e⁺e⁻-annihilation. In this formulation the destructive interference for soft-gluon emission has a very natural interpretation, and it is easy to construct a Monte Carlo simulation program. At PEP and PETRA energies the results agree very well with experimental data, and thus also with results from conventional parton cascades. However, at higher energies observable differences emerge, which can discriminate between the models.     

Hard bremsstrahlung amplitudes fore + e − collisions with polarized beams at LEP/SLC energies

Cross sections are presented for the processese ⁺ e ^?→e ⁺ e ^?γ ande ⁺ e ^?→f \(\bar f\) γ (wheref is any light fermion) in the form of helicity amplitudes in which all complex phase information is kept. Owing to a spcial choice of the bremsstrahlung polarization and the use of spinor techniques the resultant expressions are extremely compact and lend themselves to fast numerical evaluation. An algorithm is given to obtain from the helicity amplitudes the cross section for arbitrary polarization of the incominge ⁺ e ^? beams. Special attention is given to the case of bremsstrahlung at very small angles to the beams, where a complicated dependence on the electron mass has to be taken into account.     

Scaling predictions for multiplicity distributions at LEP

This paper presents a study of the charged particle multiplicity distributions for thee ⁺ e ^? annihilation. It is shown that the KNO-G scaling, the concept which turned out to be very successful in thepp collisions, is also valid for thee ⁺ e ^?, data. Using this concept the predictions for the multiplicity distributions at LEP are given. The possible explanation of the scaling origin is postulated. It seems that the proposed mechanism, namely the Polyakov scale invariant branching, is responsible for the scaling observed in the data. It is worth to emphasize that the same mechanism was proposed to explain the intermittent behaviour of a multiparticle final state.     

Selection of jets in multihadron final states produced bye + e −-annihilation

Fore ⁺ e ^?-annihilations a procedure is described to select events of a given multiplicity of jets. The procedure is demonstrated by applying it to a Monte-Carlo sample ofe ⁺ e ^? multihadron final states. Selection of three jet events is shown to be somewhat better than more conventional techniques.     

Reconstruction and selection of J/ψ → e + e − decays registered by CBM setup in 25 AGeV AuAu collisions

The problem of reconstructing and selecting J/? → e ⁺ e ^? decays registered by the CBM setup in AuAu collisions at 25 AGeV beam energy is considered. The key task in this problem is fast and reliable electron-positron identification using the energy losses of charged particles in the Transition Radiation Detector (TRD). We consider two methods for solving this problem: an artificial neuron network (ANN) and a modified non-parametric goodness-of-fit ω _n ^k criterion. Our analysis shows that both approaches give similar results for the J/? → e ⁺ e ^? yield and the signal-to-background ratio. Compared with the ω _n ^k criterion, the method based on ANN has a number of disadvantages which are discussed in detail. Taking into consideration the very simple software implementation of the ω _n ^k algorithm, it can be used for J/? → e ⁺ e ^? decays selection in a real-time experiment.     

Parametrization of Bose–Einstein correlations and reconstruction of the space–time evolution of pion production in ee annihilation

A parametrization of the Bose–Einstein correlation function of pairs of identical pions produced in hadronic e⁺e⁻ annihilation is proposed within the framework of a model (the τ-model) in which space–time and momentum space are very strongly correlated. Using information from the Bose–Einstein correlations as well as from single-pion spectra, it is then possible to reconstruct the space–time evolution of pion production.     

Doubly excited states of positronium negative ions

A physical interpretation of the recently calculated doubly excited states of Ps^– is given. The available resonance parameters obtained by using the method of complex-coordinate rotation indicate that the doubly excited (e^–e⁺e^–) system would behave very much like an XYX triatomic molecule.     

Can a fourth lepton family exist without its quark counterpart?

Solely within the minimal standard model, we show that it is possible to have a fourth lepton familywithout its quark counterpart provided the Higgs sector is strongly coupled and can accommodate Skyrmion solutions. The triangle and Witten anomalies of the fourth lepton family are cancelled by those of the “weak” Skyrmions if the latter are quantized as fermions. Thus a fourth quark family is not needed to cancel the anomalies of the new leptons. If indeed such a family is discovered in the near future bye ⁺ e ^? machines and if no Higgs boson nor new quarks of masses less than 1TeV are found, this intriguing scenario can be a very viable one. Arguments are presented to show that if such a scheme is realized, it also implies that the Higgs fields are composite.     

Opposite side quantum number correlations ine + e −-annihilation

We claim that double moments of twoparticle distributions are very sensitive quantities for testing the opposite side quantum number correlations ine ⁺ e ^?-annihilation predicted by the quark parton model. In particular it is found that kaon correlations are extremely transparent in these quantities. This feature survives when the QCD predictedQ ²-dependence is taken into account. We also stress the importance of establishing the higher meson resonance content in quark jets in order to make a sensible QCD-analyses of the fragmentation functions.     

Breakdown of normal threshold dispersion relation for the vacuum polarization

Stating a concrete example, it is shown that the vacuum polarization develops anomalous singularities. The analytic structure of the vacuum polarization in these circumstances is discussed in some detail. It appears that its large q² behaviour is not linked anymore to the asymptotic form of the e⁺e^? total cross section. Since it is the large q² behaviour of the vacuum polarization which solely can be calculated in asymptotically free gauge theories (giving in leading order the naive quark model result), this is a warning to using the naive quark model result for the e⁺e^? total cross section as well in fitting the data.     

Observation of possible production and decay of short-living heavy neutral particles in the SKAT neutrino experiment

In the SKAT bubble chamber neutrino experiment production of a short-living neutral particle with mass (1.4?m?2.5) GeV/c² and lifetime τ≈6×10^?12 s has been detected. The event may be interpreted as production and decay of the heavy lepton M⁰→μ^?+e⁺+ν_e with mass (1.4?m_M⁰?2.1) GeV/c². It might also be the production of a charmed particle D⁰→e⁺+τ^?+ν_e (D⁰→e⁺+π^?+ν_e) with mass 1.4?m_D⁰?2.5) GeV/c² in a non-diagonal neutral current. The probability to interpret the event as other possible processes is very low.     

Hunting new vector bosons in high precision measurements without polarization

We illustrate a strategy to identify the effect of a possible extra neutral vector boson of several different theoretical origins (extended gauge groups, composite models, models with a strongly interacting Higgs sector), assuming that only precision experiments without polarized e⁺e⁻ beams were available. We show that it might be relatively easy to discover evidence of genuine New Physics (i.e., effects not depending on the value of the top mass, or on the existence of new families...) if a certain set of observables, including the final τ polarization in unpolarized e⁺e⁻ annihilation, were measured. The region of possible confusion between different theoretical models would be, though, sensibly larger than in the case in which e⁺e⁻ polarization were available.     

Confirmation of the new 3700 MeV narrow resonance in e+e−-collisions

The two new, very narrow resonant states discovered at Brookhaven and SLAC were confirmed at the storage ring DORIS of DESY in e⁺e^?-colliding beam experiments.     

Calculations of Coulomb systems with optimization of nonlinear parameters

Precise variational calculations of the energy and diverse physical characteristics are performed for the ground state of a fully nonadiabatic four-particle system—the positronium molecule e⁺e^?e⁺e^?. The nonlinear exponential parameters of correlated Gaussian basis functions dependent on all interparticle separations are systematically optimized. The dependence of the calculated energy values and the expectation values of interparticle separations (as well as their degrees and Dirac delta functions of interparticle separations) on the basis size N is examined. For N=200, the ground-state energy of the positronium molecule is found to be E=?0.5160028 au. For an extended basis set with N=4700 containing 200 basis functions with the optimized parameters and 4500 additional functions with random values of nonlinear parameters, E=?.5160036 au. At present, this result is the most accurate variational energy value of the positronium molecule, and it demonstrates the high efficiency of optimization of the nonlinear parameters in calculations of atomic-molecular systems. The lifetime of the positronium molecule e⁺e^?e⁺e^? with respect to electron-positron annihilation is calculated to be τ=2.26×10^?10 s.     

Fluctuations and anomalous dimensions in QCD cascades

A recently proposed multiplicity measure is used to study the properties of QCD cascades. We show that it is possible to define anomalous dimensions locally in rapidity space for the QCD cascade ine ⁺ e ^–-annihilation events and in deep inelastic lepton scattering. In this way it will be possible to differentiate between different suggested models for multiple gluon emission. We also show that the properties of an event are to a surprisingly large degree deterimined by the first one or two gluons. Thus e.g. the multiplicity fluctuations ine ⁺ e ^–-annihilation at theZ ⁰-pole are to about 90% determined by the hardest gluon. This implies that it may be principally difficult to make a distinct separation between the hard perturbative phase and the soft hadronization phase.