Spin asymmetry in γe ↑→e+2 jets: A laboratory for final state interactions

We isolate final state interactions not parametrized by fragmentation functions and yet present in QCDO(α _s ) corrections to two-jet production in photon-longitudinally polarized electron scattering. Our calculation focuses on dynamical imaginary (absorptive) corrections that are absent in any tree diagrams. We explicitly demonstrate cancellation of imaginary infinities in the sum over graphs. The resulting asymmetry is predicted to be small. Such a quantity should be soon measured in experiments exploiting the naturally large electron polarization occurring in standard high energy storage rings.     

The \gamma \gamma \to \gamma Z process at high energies and the search for virtual SUSY effects

We study the helicity amplitudes and the observables of the process at high energy. As in the case of the process studied before, the relevant diagrams in the standard model (SM) involve W, charged-quark, and lepton loops, while in SUSY we also have contributions from charginos and charged-sfermion or Higgs-loop diagrams. Above 250 GeV, the dominant SM amplitudes are themselves dominated by the W loop, and as for , they are helicity conserving and almost purely imaginary. We discuss the complementary information provided by for the identification of possible nonstandard effects. This process, together with , should provide very useful information on the nature of possible new physics particles, above the threshold of their direct production. Received: 26 April 1999 / Published online: 8 September 1999     

Signatures of anomalous couplings in boson pair production throughγγ collisions

We discuss possible New Physics (NP) effects on the processesγγ →W ⁺ W ⁻,ZZ, Zγ, γγ, HH which are observable inγγ collisions. Such collisions may be achieved through laser backscattering at a high energye ⁺ e ⁻ linear collider. To the extent that no new particles will be directly produced in the future colliders, it has already been emphasized that the new physics possibly hidden in the bosonic interactions, may be represented by the sevendim=6 operatorsO _W,O _BΦ,O _WΦ,O _UB,O _UW and (the last two ones being CP-violating). In this paper, we show that the above processes are sensitive to NP scales at the several TeV range, and we subsequently discuss the possibility to disentangle the effects of the various operators. Partially supported by the EC contract CHRX-CT94-0579     

Anomalous events in e+e−→μ+μ−γ. excited states and/or new interactions?

We first show that the enhancement observed by Cello can neither be explained by real or virtual μ¹ production through μ¹μγ or μ¹μZ couplings nor by μμγγ or μμγZ contact terms, when one imposes the usual constraints from $\text{g -2, e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}\text{→γγ, Z→}\text{ll}\text{γ}$. We then propose an explanation using an eeμμ¹ contact term which is less constrained. In addition to $\text{μ}{}^1\text{→μγ}$ this leads to a direct decay $\text{μ}{}^1\text{→μee}$. Generalized to other types of fermion pairs we have here an interesting source of multifermion anomalous events.     

A modified parton model for reactions with time-like photons

A time-like photon is considered as a kind of mesonic state with valence + sea multipartons configurations; consequences for e⁺e^? annihilation and leptons production in hadronic collisions are examined.     

Observability at LEP2 hadronic channels of a Z’ with small lepton couplings

We consider the effects of a number of models with one extra Z, with enhanced couplings to quarks, in the final hadronic channels at LEP2. We show that, for a number of representative cases, visible effects could be produced even for very low values of the lepton couplings, much smaller than the existing LEP1/SLC and the future LEP2 (lepton channel) bounds.     

A FORTRAN code for γγ→ZZ in SM and MSSM

Through the present paper, the code gamgamZZ is presented, which may be used to calculate all possible observables related to the process γγ→ZZ, in either the standard model, or the minimal supersymmetric standard model with real parameters.