Production of hard photons in electron-positron annihilation into hadrons

Frequency of emission of prompt, hard photons radiated by the fast-moving quarks (antiquarks) ine ^– e ⁺ annihilation into hadrons is estimated for the string model. The differential probability of emission of such photons is obtained from a sum rule which takes for the initial state of the emitter a classical trajectory. It is found that the string models are quite effective in production of hard photons (i.e., photons whose energies are substantial fractions of the energies of the initiale ^– e ⁺ pair) associated with production of hadrons. For 100 GeVe ⁺ e ^– initial energy the frequency of emission,v, of 10 GeV<><40gev photons=" is=" estimated=" to=" be=">^–4v<>^–3.Supported in part by the Polish Ministry of Education, Grant DNS-P-04/215/90-2 and KBN Grant No 2-00054-91-01     

CP-sensitive observables in e^ + e^-\to\tilde\chi^0_i \tilde\chi^0_j and neutralino decay into the Z boson

We study CP-sensitive observables in the neutralino production and the subsequent two-body decays of the neutralino and of the Z boson . We identify the CP-odd elements of the Z boson density matrix and propose CP-sensitive triple-product asymmetries. We calculate these observables and the cross sections in the minimal supersymmetric standard model with complex parameters and M ₁ for an e ⁺ e ^- linear collider with GeV and longitudinally polarized beams. We show that the asymmetries can reach for and for and discuss the feasibility of measuring these asymmetries.Received: 4 February 2004, Revised: 17 May 2004, Published online: 2 July 2004     

\eta_b decay into two photons

We discuss the theoretical predictions for the two photon decay width of the pseudoscalar meson. Predictions from potential models are examined. It is found that various models are in good agreement with each other. Results for are also compared with those from data through the NRQCD procedure. Received: 28 July 2002 / Revised version: 16 September 2002 / Published online: 20 November 2002     

Polarization effects in the reactione \to e' + v_e + \bar v_e in a magnetic field,and μ-e decay

The reaction \(e \to e' + v_e + \bar v_e \) is analyzed for relativistic electrons in weak and strong magnetic fields with an account of the polarization states. There is shown to be a higher probability for this reaction to occur with left-polarized electrons; in very strong fields this effect is due essentially completely to left-polarized electrons. It is shown that this reaction, like synchrotron radiation, leads to electron polarization in the direction opposite the field. The effect of a magnetic field on μ-e decay is examined.     

Electroweak asymmetries in the processe^ - + e^ + \to \tilde f + \tilde f in various gauge models

The creation of a pair of scalar fermion partners accompanying the annihilation of arbitrarily polarized electrons and positrons is studied in the gauge models SU(2)× U(l), SU(2)_L× SU(2)_R× U(1), and SU(3)× U(1). Different electroweak asymmetries are found and analyzed in detail (right-left asymmetry a_rl, P-even transversal spin asymmetry a₁ and P-odd asymmetry A₂) for the e^–e⁺-beam energies s=40–360 GeV and the Weinberg parameter values 0.21 sin²_W0.27. The computations show that the presence of weak neutral currents also leads to noticeable electroweak asymmetries, which are sensible to the choice of the gauge model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 78–82, May, 1989.     

Resonance-enhanced multiphoton ionization (2+1) of the\tilde B and\tilde C' states of ammonia

Resonance-enhanced multiphoton ionization (REMPI) of the \(\tilde B\) and \(\tilde C'\) states of NH₃ in a pulsed supersonic molecular beam was observed at 304–340 nm, using the multiphoton-ionization mass-spectrometer system described herein. The observed features are the result of a 2+1 MPI process and correspond quite well with the reported 3+1 MPI of NH₃ in the literature from the ground state of opposite parity.     

Effects of gauge boson compositeness ine\bar v_e \to W^ - Z^0

Here, we treat the high-energy production process , allowing for gauge boson compositeness, using two approaches: (a) through an anomalousW boson weak magnetic moment parameterk _z , and (b) through a modification of theW propagator andWWZ vertex by a form factor depending on a compositeness scale parameter . We give the angular differential and total cross-section for different values ofk _z (approach (a)), and different values of (approach (b)). A clear deviation from the standard model result is observed in both cases.     

Interference of Higgs boson resonances in $\mu^ + \mu^- \to\tilde{\chi}^0_i\tilde{\chi}^0_j$ with longitudinal beam polarization

We study the interference of resonant Higgs boson exchange in neutralino production in m⁺ m^-\mu^ + \mu^- annihilation with longitudinally polarized beams. We use the energy distribution of the decay lepton in the process [(c)\tilde]⁰_j ? l^± [(l)\tilde]^-±\tilde{\chi}^0_j \to \ell^{\pm} \tilde{\ell}^\mp to determine the polarization of the neutralinos. In the CP-conserving minimal supersymmetric standard model a non-vanishing asymmetry in the lepton energy spectrum is caused by the interference of Higgs boson exchange channels with different CP-eigenvalues. The contribution of this interference is large if the heavy neutral bosons H and A are nearly degenerate. We show that the asymmetry can be used to determine the couplings of the neutral Higgs bosons to the neutralinos. In particular, the asymmetry allows one to determine the relative phase of the couplings. We find large asymmetries and cross sections for a set of reference scenarios with nearly degenerate neutral Higgs bosons.     

Reconstruction of $$ \tilde \tau _1 $$ mass at the LHC

The cascade mass reconstruction approach was used for mass reconstruction of the lightest $ \tilde \tau $ \tilde \tau produced at the LHC in the cascade decay $ \tilde g \to \tilde bb \to \tilde \chi _2^0 bb \to \tilde \tau _1 \tau bb \to \tilde \chi _1^0 \tau \tau bb $ \tilde g \to \tilde bb \to \tilde \chi _2^0 bb \to \tilde \tau _1 \tau bb \to \tilde \chi _1^0 \tau \tau bb . The $ \tilde \tau _1 $ \tilde \tau _1 mass was reconstructed assuming that masses of gluino, bottom squark, and two lightest neutralinos were reconstructed in advance. SUSY data sample sets for the SU(3) model point containing 160k events each were generated which corresponded to an integrated luminosity of about 8 fb⁻¹ at 14 TeV. These events were passed through the AcerDET detector simulator, which parametrized the response of a generic LHC detector. The mass of the $ \tilde \tau _1 $ \tilde \tau _1 was reconstructed with a precision of about 20% on average.     

Direct photons in ATLAS@LHC

The ATLAS detector at the LHC is capable of efficiently separating photons and neutral hadrons based on their shower shapes over a wide range in η, ϕ and E _T , either in addition to or instead of isolation cuts. This provides ATLAS with a unique strength for direct photon and γ-jet physics (“tomography”) as well as access to the unique capability to measure non-isolated photons from fragmentation or from the medium. We present a first look at the ATLAS direct photon measurement capabilities in p+p and Pb+Pb collisions at LHC energies over the region |η| < 2.4.     

Comparison of antiproton diffraction dissociation with inelastic\bar pp interactions ande + e − annihilation into hadrons

Experimental multiplicity distributions of antiproton diffraction dissociation and inelastic antiproton-proton interactions at 22.4 GeV/c with leading particles removed are compared withe ⁺ e ^? annihilation into hadrons. We find agreement with the predictions of the dual parton model.     

Search for resonant $\tilde\nu$ production at $\sqrt{s} = $ 183 to 208 GeV

Searches for resonant production in e ⁺ e ^- collisions under the assumption that R-parity is not conserved and that the dominant R-parity violating coupling is or used data recorded by DELPHI in 1997 to 2000 at centre-of-mass energies of 183 to 208 GeV. No deviation from the Standard Model was observed. Upper limits are given for the and couplings as a function of the sneutrino mass and total width. The limits are especially stringent for sneutrino masses equal to the centre-of-mass energies with the highest integrated luminosities recorded. Received: 14 January 2003 / Published online: 14 March 2003     

Double vector-charmonium production in electron-positron annihilation into two photons at $$\sqrt s = 10.6$$ GeV

In experimentally studying the exclusive production of two \(c\bar c\) mesons in electron-positron annihilation, the BELLE Collaboration obtained results that differ substantially from theoretical predictions. Recently, it was assumed that a significant part of this discrepancy can be explained by the process e⁺e^? → 2γ* → 2J/ψ, and its cross section was published. It is shown here that these results are incorrect (the cross sections are overestimated). The correct values of the cross sections for the double production of various vector charmonia are given.     

Direct probes of linearly polarized gluons inside unpolarized hadrons

We show that linearly polarized gluons inside unpolarized hadrons can be directly probed in jet or heavy quark pair production in electron-hadron collisions. We discuss the simplest cos2? asymmetries and estimate their maximal value, concluding that measurements of the unknown linearly polarized gluon distribution in the proton should be feasible in future Electron-Ion Collider or Large Hadron electron Collider experiments. Analogous asymmetries in hadron-hadron collisions suffer from factorization breaking contributions and would allow us to quantify the importance of initial- and final-state interactions.     

Hydrodynamical model of electron-positron annihilation into hadrons

Landau's hydrodynamical model of multiparticle production is formulated for e⁺e^? → hadrons at center-of-momentum energies 10 GeV and higher. An analytical solution to the problem of spherical expansion of a relativistic fluid is presented. When hadrons condense out at the last stage of expansion, fluid thermal motion is taken into account. We derive predictions for multiplicities, average secondary energies and inclusive single particle distributions for various secondary hadrons; these results are then compared to other models. Finally, we investigate the internal consistency of the model.     

Direct photons in nuclear collisions at fair energies

Using the extrapolation of existing data, estimations of prompt-photon production at FAIR energies have been made. At y = y _c.m. the rapidity density of prompt photons with p _t > 1.5 GeV/c per central Au + Au event at 25 A GeV is estimated as ∼10⁻⁴. With the planned beam intensity 10⁹ per second and 1% interaction probability, for 10% of most central events one can expect the prompt-photon rate ∼10² photons per second. Direct photons from the hadron scenario of ion collisions generated by the Hadron-String-Dynamics (HSD) transport approach with implemented meson scatterings πρ → πγ, ππ → ργ have been analyzed. Photons from short-living resonances (e.g., ω → π ⁰ γ) decaying during the dense phase of the collision should be considered as direct photons. They contribute significantly in the directphoton spectrum at p _t = 0.5–1 GeV/c. At the FAIR energy 25 A GeV in Au + Au central collisions the HSD generator predicts, as a lower estimate, γ _direct/ ≃ 0.5% in the region p _t = 0.5–1 GeV/c. At p _t = 1.5–2 GeV/c γ _prompt/ ≃ 2%. Thermal direct photons have been evaluated with the Bjorken Hydro-Dynamics (BHD) model. The BHD spectra differ strongly from the HSD predictions. The direct-photon spectrumis very sensitive to the initial temperature parameter T ₀ of the model. The 10-MeV increase in the T ₀ value leads to ∼2 times higher photon yield. The text was submitted by the authors in English.     

Parametric generation of twin photons in vertical triple microcavities

We report the realization of a monolithic vertical-cavity, surface emitting micro-optical parametric conversion nanostructure, triply resonant with the parametric frequencies, allowing parametric oscillation with ultra-low pump power threshold. The photonic phase-space naturally provides triple resonance for the parametric frequencies, together with built-in cavity phase-matching for the pump wave at normal incidence. Parametric oscillation is observed in both the strong and weak exciton–photon coupling regime, allowing a high operating temperature. Signal and idler beams can be collected at 0° or at finite angles. The OPO threshold is low enough to envisage the realization of an all-semiconductor electrically-pumped micro-parametric oscillator. To cite this article: C. Diederichs et al., C. R. Physique 8 (2007).