Bose-einstein correlations and color reconnection in W-pair production

We propose a systematic study of Bose-Einstein correlations between identical hadrons coming from different W decays. Experimentally accessible signatures of these correlations as well as of possible color reconnection effects are discussed on the basis of two-particle inclusive densities. Received: 7 July 1998 / / Published online: 19 November 1998     

Colour interference and confinement effects in W-pair production

In the reactione ⁺ e ^– W ⁺ W ^– ( ) ( ) we expect that normally the colour singlet systems ( ) and ( ) hadronize independently into two hadron chains or strings. However also the pairs ( ) and ( ) form colour singlets with probability 1/N _c ² =1/9. This probability could be further enhanced by gluon exchange. We therefore expect that the hadronization can give recoupled colour strings between these quark-antiquark pairs with some small probability, and a study of this effect will give interesting information about the vacuum structure. In this paper we discuss a possible experimental signal for recoupled events and show that such events can be identified also when the effect of gluon emission before the recoupling is taken into account.     

Coherent final state interaction in jet production in nucleus-nucleus collisions

The coherent final state interaction of an energetic parton produced in AA collisions is studied. This interaction is due to the change in the cutoff scale and in the running coupling constant when the parton passes from a vacuum to a quark-gluon plasma. It is demonstrated that the contribution of this new mechanism to the energy loss may be of the same order of magnitude as the induced gluon radiation. However, an accurate evaluation of this medium effect is a difficult task, because there is a strong cancellation between the cutoff and running coupling constant effects. The uncertainties in the contribution of the coherent final state interaction restrict strongly the accuracy of jet tomographic analyses of the matter density produced in AA reactions.     

Evaluation of the LEP centre-of-mass energy above the W-pair production threshold

Knowledge of the centre-of-mass energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55 GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. This test results in the largest contribution to the systematic uncertainty. Several further corrections are applied to derive the centre-of-mass energies at each interaction point. In addition, the centre-of-mass energy spread is evaluated. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of . This is small in comparison to the present uncertainty on the W mass measurement at LEP. However, the ultimate statistical precision on the W mass with the full LEP2 data sample should be around 25 MeV, and a smaller uncertainty on the beam energy is desirable. Prospects for improvements are outlined. Received: 14 December 1998 / Published online: 3 November 1999     

Collective properties of the final state in processes involving the production of hadron jets

Experimental results obtained by studying soft hadron jets in pion-proton and pion-nucleus reactions at intermediate energies with the aid of traditional collective variables are presented. Analytic approximations that describe, at a qualitative level, the dependences of collective parameters on the energy and multiplicity are proposed. Estimates obtained for strong coupling constant by studying collective variables are in reasonable agreement with its world-average value and with the results extracted by using different methods. The behavior of traditional collective variables as functions of the multiplicity in various interactions makes it possible to obtain a universal estimate for the lower boundary of the region of experimental manifestations of jets in multiparticle-production processes.     

Emission in the final state in processes with the production of charged pions and light leptons

The Drell-Yan form is proposed for the decay widths of heavy quarks and cross sections for high-energy scattering with the production of charged pions and light leptons including radiative corrections in the final state in the leading logarithmic approximation. The corresponding spectral distributions are given in terms of the convolution of the cross section (decay width) in the Born approximation with the structure functions of pions and leptons. The pion structure function is given in the explicit form.     

Bose-Einstein and colour interference in W-pair decays

We study effects on the W mass measurements at LEP2 from non-perturbative interference effects in the fully hadronic decay channel. Based on a model for Bose-Einstein interference, which is in agreement with LEP1 data, we argue that there are no Bose-Einstein correlations between bosons coming from the different W's. For small reconnection probabilities we rule out the possible experimental signal of colour interference at LEP2, suggested in [1]. The conclusions from this paper are that the theoretical uncertainties in the W mass determination should be smaller than the experimental statistical error. Received: 3 November 1997 / Published online: 10 March 1998     

The influence of final state interaction on two-particle correlations in multiple production of particles and resonances

The structure of pair correlations of interacting particle moving with nearby velocities is analysed. A general formalism of the two-particle space-time density matrix, taking into account the space-time coherence of the production process, is developed. The influence of strong final state interaction on two-particle correlations in the case of the production of a systemresonance+particle is investigated in detail. It is shown that in the limit of small distances between the resonance and particle production points the effect of final state interaction is enhanced due to logarithmic singularity of the triangle diagram. Numerical estimates indicate that, in this limit, the effect of strong final state interaction becomes important even for two-pion correlations.     

Decay width and final state in α-decay

If the theory for the decay of a prepared state is formulated in a model-independent way it leads to an equation for the final state. This continuum state is extremely non-resonant and orthogonal to the bound initial state which is used as a prepared state. The decay width is given by the matrix element of the exact Hamiltonian between these two states. Under certain conditions it can be reduced to a matrix element of the residual interaction. From the properties of the final state which are numerically tested with an example in potential scattering, follows the justification for the cluster ansatz in α-decay.     

Q-PYTHIA: a medium-modified implementation of final state radiation

We present a Monte Carlo implementation of medium-induced gluon radiation in the final-state branching process. Medium effects are introduced through an additive term in the splitting functions. We have implemented such modification within PYTHIA. We show the medium effects on the hump-backed plateau, and the transverse-momentum and angular distributions with respect to the parent parton. As expected, with increasing medium densities there is an increase (decrease) of partons with small (large) momentum fraction, and angular broadening is observed. The effects on the transverse-momentum distributions are more involved, with an enhancement of low- and intermediate-p _T partons and a decrease at large p _T, which is related to energy conservation, and to the lack of momentum exchange with the medium in our approach.     

Search for associated production of charginos and neutralinos in the trilepton final state using 2.3 fb of data

We report the results of a search for associated production of charginos and neutralinos using a data set corresponding to an integrated luminosity of 2.3 fb⁻¹ collected with the DØ experiment during Run II of the Tevatron proton–antiproton collider. Final states containing three charged leptons and missing transverse energy are probed for a signal from supersymmetry with four dedicated trilepton event selections. No evidence for a signal is observed, and we set limits on the product of production cross section and leptonic branching fraction. Within minimal supergravity, these limits translate into bounds on m₀ and m_1/2 that are well beyond existing limits.     

Photon radiation in final state parton showering

A method is described for the inclusion ofq→qγ branchings within a QCD parton showering event generator. Special attention is paid to the ‘back-to-back’ region ine ⁺ e ^? events, where a very hard photon recoils from a nearly collinear quark-antiquark pair. The resulting algorithm is also applied to back-to-backq \(\bar q\) g configurations. Results for theq \(\bar q\) γ cross-section are compared against various theoretical predictions, models and experimental data.     

Multiple pomeron exchange and final state rescattering

The influence of final state rescattering corrections on multiple pomeron exchange is studied. It is shown in a certain approximation that the multiple exchange of a simple pole pomeron violates the Froissart bound even after the inclusion of these corrections. A self consistent unitarity equation is set up and it is shown that its solution is an expanding black disk. The multiparticle properties of this solution are discussed.     

Femtoscopic correlations and final state resonance formation

The correlation femtoscopy formalism including resonance formation due to the final state interaction and its applicability conditions are discussed. The example calculations of π⁺Ξ⁻ and K ⁺ K ⁻ correlation functions are done with the account of Ξ*(1530) and ϕ(1020) resonances, respectively. It is shown that in these calculations the usual form of the smoothness approximation should be substituted by a more general one. A strong sensitivity of resonance formation in the final state to the position-momentum correlations at particle freeze-out is demonstrated.     

Polarized beams and final state heavy fermion polarization

The final state τ longitudinal polarization is discussed in e⁺e⁻ collisions with polarized electron beams. Explicit formulae are given in the context of the standard model, taking into account mass effects and energy dependence. In particular dips in the cross section and zeros in the longitudinal polarization are examined.     

Coulomb final state interaction in identical Boson interferometry

Recently, the muonic three-body systemdtμ, has aroused considerable attention in relation to the realization of a useful muon-catalyzed fusion [1, 2]. This bound system must be solved up to 6 digits in energy to establish the muon catalyzed fusion process. We applied the ATMS method [3] and the coupled-channel method [4] to the Coulomb three-body system and obtained the detailed information on the energy levels and the wave functions of the bound states. Further we investigated the effect of the nuclear interaction on the fusion rate in the bound states [5] and developed a formalism to calculate the muon sticking to⁴He [6]. In this paper we report the results obtained in our collaborations: Figure 2 shows up our main results.     

Four-quark and hybrid mixing in the light-quark vector sector

The data one ⁺ e ^– annihilation are shown to be incompatible with an interpretation of the observed vector mesons as excitations. It is necessary to consider mixing with some system. Two specific models are considered: either four-quark states or hybrid states mixing with the conventional quark-antiquark excitations. Both models are compatible with the presente ⁺ e ^– annihilation data. It is possible to incorporate the 1.3 GeVJ ^PC =1^–– state seen by LASS in the channel in the four-quark model, but this is not possible with the hybrid model.