K\rightarrow \pi\pi e^+e^- decays and chiral low-energy constants

The branching ratios of the measured decay and of the still unmeasured decay are calculated to next-to-leading order in chiral perturbation theory (CHPT). Recent experimental results are used to determine two possible values of the combination of weak low-energy couplings (LECs) from the chiral Lagrangian. The values obtained are compared to the predictions of theoretical approaches to weak counterterm couplings to distinguish between the two values. Using the favoured value of the combination and taking into account additional assumptions suggested by the considered models, one obtains the branching ratio of the second decay as a function of the unknown combination of weak low-energy couplings. Finally, using values of the individual LECs derived from a particular model, one predicts the branching ratio of the decay. Received: 27 February 2001 / Published online: 11 May 2001     

4\pi-decays of scalar and vector mesons

Two different annihilation modes, and , are used to study the 4–decays of scalar and vector mesons. The data are dominated by 4 scalar isoscalar interactions. At least two states are needed, the and the . The 4–decay width of the is more than 6 times larger than the sum of all observed partial decay widths to two pseudoscalar mesons. The state has important couplings to and to . The 4-decays of the (1500) represent about half of its total width. The and the are observed in several 4 decay modes. The ratio of the relative to the decay of the is in contradiction to its proposed interpretation as a pure hybrid state but it suggests that it is not a pure –state either. Our results favour the assignment of the as state, its interpretation as -state is less plausible. Received: 7 December 2000 / Published online: 19 July 2001     

The processes \gamma\gamma\to Z H in SM and MSSM

The process first arises at the one loop level, and as such it provides us with remarkable tests of the structure of the electroweak Higgs sector. These tests are complementary to those in the gauge sector involving . We show that in the standard model (SM) where , as well as in the supersymmetric case where or , observables exist (like e.g. the energy dependence, angular distribution, photon polarization dependence or final Z polarization) which present rather spectacular properties. Such properties involve strong threshold effects with steps, bumps or peaks, reflecting the type of Higgs and heavy quarks and chargino masses and couplings predicted by the SM and supersymmetric models. Received: 13 March 2001 / Published online: 13 June 2001     

Search for single leptoquark and squark production in electron-photon scattering at $\sqrt{s_{\rm ee}}=189$ GeV at LEP

A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks () in R-parity violating SUSY models with the direct decay of the into Standard Model particles has been performed using ee collisions collected with the OPAL detector at LEP at an ee centre-of-mass energy of 189 GeV. The data correspond to an integrated luminosity of about 160 pb. The dominant process for this search is , where a photon, which has been radiated by one of the beam electrons, serves as a source of quarks. The numbers of selected events found in the two decay channels are in agreement with the expectations from Standard Model processes. This result allows to set lower limits at the 95% confidence level on the mass of first generation scalar and vector leptoquarks, and of squarks in R-parity violating SUSY models. For Yukawa couplings to fermions larger than , the mass limits range from 121 GeV to GeV ( GeV to GeV) depending on the branching ratio of the scalar (vector) leptoquark state. Furthermore, limits are set on the Yukawa couplings for leptoquarks and for squarks, and on as a function of the scalar leptoquark/squark mass. Received: 22 May 2001 / Published online: 25 January 2002     

Radiation zeros in W^+W^-\gamma production at high-energy colliders

The vanishing of the cross section for particular points in phase space – radiation zeros – is examined for the process at high energy. Unlike the process , actual zeros only occur in the soft-photon limit. However, for photon energies that are not too large, the cross section does exhibit deep dips in regions of phase space corresponding to the position of the actual zeros. We show that in these regions the sensitivity to possible anomalous quartic couplings is very large. Received: 9 June 1999 / Revised version: 8 July 1999 / Published online: 21 December 1999     

Effect of top quark spin on the unparticle couplings in $\gamma\gamma \to t\bar{t}$

We investigate the potential of γ γ collisions to probe scalar unparticle couplings via top–antitop quark pair production. We find 95% confidence level limits on the unparticle couplings with an integrated luminosity of 500 fb⁻¹ and an energy of  TeV. We investigate the effect of the top quark spin polarization on the unparticle couplings. It is shown that spin polarization of the top quark leads to a significant improvement in the sensitivity limits.     

Process $$\gamma \gamma \to \nu \bar \nu$$ in a strong magnetic field

The three-vertex loop amplitude in a strong magnetic field are analyzed in a general form by using the asymptotic behavior of the electron propagator in an external field. The process \(\gamma \gamma \to \nu \bar \nu\) is studied in terms of the scalar-vector-vector (SVV), pseudoscalar-vector-vector (PVV), vector-vector-vector (VVV), and axial-vector-vector-vector (AVV) combinations of couplings. It is shown that only in the case of the SVV combination does the amplitude grow linearly with increasing magnetic-field strength, the amplitudes evaluated with the other combinations of couplings (PVV, VVV, and AVV) featuring no linearly increasing terms. The process \(\gamma \gamma \to \nu \bar \nu\) is also studied within the left-right model, which is an extension of the Standard Model of electroweak interactions and which may involve an effective scalar ννee coupling. Possible astrophysical manifestations of this process are discussed.     

Scanning of the ee\rightarrow \pi\pi cross-section below 1 GeV by radiative events with untagged photon

We discuss an inclusive approach to the measurement of the cross-section by the radiative return method without photon tagging. The essential part of this approach is the choice of rules for event selection which provide rejection of events with 3 (or more) pions and decrease the final-state radiation background. The radiative corrections to the initial-state radiation process are computed for DANE conditions, using the quasi-real electron approximation for both, the cross-section and the underlying kinematics. The two cases of restricted and unrestricted pion phase space are considered. Some numerical calculations illustrate our analytical results. Received: 4 February 2002 / Published online: 5 July 2002     

Four-fermion production at $\gamma\gamma$ colliders: 1. Lowest-order predictions and anomalous couplings

We have constructed a Monte Carlo generator (the corresponding FORTRAN code can be obtained from the authors upon request) for lowest-order predictions for the processes and in the standard model and extensions thereof by an effective coupling as well as anomalous triple and quartic gauge-boson couplings. Polarization is fully supported, and a realistic photon beam spectrum can be taken into account. For the processes all helicity amplitudes are explicitly given in a compact form. The presented numerical results contain, in particular, a survey of cross sections for representative final states and their comparison to results obtained with the program package Whizard/Madgraph. The impact of a realistic beam spectrum on cross sections and distributions is illustrated. Moreover, the size of various contributions to cross sections, such as from weak charged- or neutral-current, or from strong interactions, is analyzed. Particular attention is paid to W-pair production channels where we investigate the impact of background diagrams, possible definitions of the W-pair signal, and the issue of gauge-invariance violation caused by finite gauge-boson widths. Finally, the effects of triple and quartic anomalous gauge-boson couplings on cross sections as well as the possibility to constrain these anomalous couplings at future colliders are discussed.Received: 19 May 2004, Published online: 16 July 2004     

Gauge invariance, infrared/collinear singularitiesand tree level matrix element for e^ + e^- \to \nu_e \bar{\nu}_e \gamma \gamma

One of the necessary steps in constructing a high-precision option of KKMC, a Monte Carlo program for the high-precision simulation of fermion pair production at LEP and Linear Collider energies, was to make a careful study of the appropriate matrix elements calculated from QED and the complete standard model. In particular, the installation of the double bremsstrahlung matrix element for the process into the scheme of coherent exclusive exponentiation (CEEX) was necessary. In the CEEX scheme one has to define an extrapolation and/or reduction procedure to enable the use of the matrix elements for kinematical configurations with a large number of outgoing particles. The process under study is particularly interesting because of the gauge cancellation of contributions for photon emission from incoming fermion lines and t-channel W. The QED U(1) gauge properties require terms of the triple and quartic gauge couplings to be taken into consideration as well. A natural separation of the complete amplitude into gauge invariant parts was found and is among the main results of the paper. Each part has a well defined physical interpretation, which after partial integration over phase space provides infrared singular, leading-log, next-to-leading-log, and other terms. Contributions related to the triple and quartic gauge coupling of W (extracted with the help of an expansion around the contact W-interaction) have been ordered as well. The separation can be of broader interest; it originates from the rigorous calculation of matrix elements; it visualizes, in the language of spin amplitudes, the properties of factorization necessary for the common multi-process picture. For example, the multiple photon algorithm of PHOTOS, based on the parton shower-like approach, profits from similar considerations. These somewhat speculative aspects of the calculation will be mentioned in the paper as well. Received: 24 April 2005, Revised: 22 July 2005, Published online: 11 October 2005 Supported in part by the EU grant MTKD-CT-2004-510126, in patnership with the CERN Physics department, and Polish State Committee for Scientific Research (KBN) grant 2 P03B 091 27 for years 2004-2006.     

Calculation ofn^3 S_1 (Q\bar Q) \to \gamma + (pseudo)scalar

The decays of³ S ₁ quarkonia into a photon and a scalar or pseudoscalar Higgs particle are examined, taking into account the bound-state dynamics in the framework of a nonrelativistic potential model. We find that for realistic quark potentials the naive calculation [1] overestimates the scalar rate. Numerical results are obtained for the \(\bar bb\) and \(\bar tt\) quark systems.     

Search for pair-produced leptoquarks in e + e- interactions at $\sqrt{s} \simeq 189\text{--}209$ GeV

A search for pair-produced leptoquarks is performed using collision events collected by the OPAL detector at LEP at centre-of-mass energies between 189 and 209 GeV. The data sample corresponds to a total integrated luminosity of 596 pb^-1. The leptoquarks are assumed to be produced via couplings to the photon and the Z⁰. For a given search channel only leptoquark decays involving a single lepton generation are considered. No evidence for leptoquark pair production is observed. Lower limits on masses for scalar and vector leptoquarks are calculated. The results improve most of the LEP limits derived from previous searches for the pair production process by 10-25 GeV, depending on the leptoquark quantum numbers.Received: 6 May 2003, Published online: 24 October 2003     

W\gamma and Z\gamma production at hadron colliders

We present a general purpose Monte Carlo program for the calculation of any infrared safe observable in and production at hadron colliders at next-to-leading order in . We treat the leptonic decays of the W and Z-boson in the narrow-width approximation, but retain all spin information via decay angle correlations. The effect of anomalous triple gauge boson couplings is investigated and we give the analytical expressions for the corresponding amplitudes. Furthermore, we propose a way to study the effect of anomalous couplings without introducing the ambiguity of form factors. Received: 14 February 2000 / Published online: 18 May 2000     

Analysis and interpretation of new low-energy \pi\pi scattering data

The recently published E865 data on charged decays and phases are reanalyzed to extract values of the two S-wave scattering lengths, of the subthreshold parameters and , of the low-energy constants and as well as of the main two-flavour order parameters: and in the limit taken at the physical value of the strange quark mass. Our analysis is exclusively based on direct experimental information on phases below 800 MeV and on the new solutions of the Roy equations by Ananthanarayan et al. The result is compared with the theoretical prediction relating 2a ₀ ⁰-5a ₀ ² and the scalar radius of the pion, which was obtained in two-loop Chiral Perturbation Theory. A discrepancy at the 1- level is found and commented upon. Received: 11 December 2001 / Revised version: 28 March 2002 / Published online: 7 June 2002     

\pi\pi correlations in \gamma + A reactions

Preliminary differential cross-sections of the reactions and with A = ¹ H, ¹² C, and ^nat Pb are presented. A significant nuclear-mass dependence of the invariant-mass distribution is found in the channel. The dependence is not observed in the channel. The in-medium observation in the channel is consistent with an in-medium modification of the interaction in the I = J = 0 channel, changing width and pole position of a resonant state.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Le Meson production - 21.65. + f Nuclear matterJ.G. Messchendorp: For the TAPS and A2 Collaborations     

Observation of a tensor glueball in the reactions p\bar p \to \pi \pi ,\eta \eta ,\eta \eta '

Partial wave analysis of the reactions $p\bar p \to \pi \pi ,\eta \eta ,\eta \eta '$ in the region of invariant masses 1900–2400 MeV indicates the existence of four relatively narrow tensor-isoscalar resonances ? ₂(1920), ? ₂(2020), ? ₂(2240), ? ₂(2300), as well as the broad state ? ₂(2000). The determined decay couplings of the broad resonance ? ₂(2000) → π⁰π⁰, ηη, ηη′ satisfy the relations appropriate to those of a tensor glueball, while the couplings of other tensor states do not, thus verifying the glueball nature of ? ₂(2000).     

Study of anomalous couplings of gauge bosons in the reaction e^ - \gamma \to W^ - \nu \to \ell \bar \nu \nu

The possibility of observing an anomaly in triple interactions of a W boson and a photon in the process eγ → Wν is considered in the leptonic mode of W-boson decay. It is shown that, in some cases, the use of a bounded phase-space domain instead of the total phase space improves upper limits on the anomalous couplings. The estimates obtained in the above mode under conditions of the TESLA project are poorer than analogous current estimates for all modes of the e ⁺ e ^? channel.     

Analyticity of \eta \pi isospin-violating form factors and the \tau \rightarrow \eta \pi \nu second-class decay

We consider the evaluation of the \(\eta \pi \) isospin-violating vector and scalar form factors relying on a systematic application of analyticity and unitarity, combined with chiral expansion results. It is argued that the usual analyticity properties do hold (i.e. no anomalous thresholds are present) in spite of the instability of the \(\eta \) meson in QCD. Unitarity relates the vector form factor to the \(\eta \pi \rightarrow \pi \pi \) amplitude: we exploit progress in formulating and solving the Khuri–Treiman equations for \(\eta \rightarrow 3\pi \) and in experimental measurements of the Dalitz plot parameters to evaluate the shape of the \(\rho \) -meson peak. Observing this peak in the energy distribution of the \(\tau \rightarrow \eta \pi \nu \) decay would be a background-free signature of a second-class amplitude. The scalar form factor is also estimated from a phase dispersive representation using a plausible model for the \(\eta \pi \) elastic scattering \(S\) -wave phase shift and a sum rule constraint in the inelastic region. We indicate how a possibly exotic nature of the \(a_0(980)\) scalar meson manifests itself in a dispersive approach. A remark is finally made on a second-class amplitude in the \(\tau \rightarrow \pi \pi \nu \) decay.     

QCD anatomy of B\to X_s\gamma decays

We present an updated next-to-leading order analysis of the branching ratio and photon spectrum, including consistently the effects of Fermi motion in the heavy-quark expansion. For the Standard Model, we obtain for the integral over the high-energy part of the photon spectrum with GeV, where the first error reflects the uncertainty in the input parameters, and the second one the uncertainty in the calculation of Fermi motion. This prediction agrees with the CLEO measurement of the same quantity within one standard deviation. From a reanalysis of the CLEO data, we obtain for the total branching ratio using the measured rate above 2.2 GeV, and using a fit to the photon energy spectrum. Both values are consistent with the Standard Model prediction of . Our analysis contains an improved discussion of renormalization-scale dependence and QED corrections. We also study the sensitivity of the branching ratio and photon spectrum to hadronic parameters such as the b-quark mass, and to possible contributions from New Physics beyond the Standard Model. Received: 13 May 1998 / Published online: 3 September 1998