Showing papers in "European Physical Journal C in 1994"

From the deuteron to deusons, an analysis of deuteronlike meson-meson bound states

Abstract: A systematic study of possible deuteronlike twomeson bound states,deusons, is presented. Previous arguments that many such bound states may exist are elaborated with detailed arguments and numerical calculations including, in particular, the tensor potential. This tensor potential which is crucial for the deuteron binding is shown to be very important also in the mesonic case. Especially, in the pseudoscalar3P0 pseudoscalar-vector\((P\bar V)\) and vector-vector\((V\bar V)\) channels the important observation is made that the centrifugal barrier from theP-wave can be overcome by the 1/r2 and 1/r3 terms of the tensor potential. In the heavy meson sector one-pion exchange alone is strong enough to form at least deuteronlike\(B\bar B*\) and\(B*\bar B*\) composites bound by approximately 50 MeV. Composites of\(D\bar D*\) and\(D*\bar D*\) states bound by pion exchange alone are expected near the thresholds, while in the light meson sector one generally needs some additional short range attraction to form bound states. The quantum numbers of these states areI=0, andJPC=0−+, 1++ for the\(P\bar V\) states andI=0,JPC=0++, O−+, 1+− and 2++ for the\(V\bar V\) composites. In\(B\bar B*\) the states:ηb(≈10545),χb1(≈10562) are predicted and in\(B*\bar B*\), one finds the states:ηb(≈10590),χbQ(≈10582),hb(≈10608),χb2(≈10602). Near the\(D\bar D*\) threshold the states:ηc(≈3870),χc0(3870) are predicted, and near the\(D*\bar D*\) threshold one finds the states:χb0(≈4015),ηc(≈4015),hc(≈4015),χc2(≈4015). Within the light meson sector pion exchange gives strong attraction for\(P\bar V\) and\(V\bar V\) systems with quantum numbers where the best non-\(q\bar q\) candidates exist, although pion exchange alone is not strong enough to support such bound states. Thus, although one cannot conclude with certainty it to be the case, this fact does favour the picture that the η (1440) and thef1 (1420) are mainly\(K\bar K*\) composites and thef0(1710) mainly a\(K*\bar K*\) bound state, while thef0(1515) andf2(1520) could be predominantly\((\rho \rho \pm \omega \omega )/\sqrt 2 \) composites. If the predicted\(D\bar D*\) and\(D*\bar D*\) states are found, these would support this interpretation of the light states. In channels with exotic flavour orCP quantum numbers pion exchange is generally repulsive or quite weak. Therefore one does not expect that such deuteronlike bound states exist, althoughB*B* may be an exception.

Model independent constraints on leptoquarks from rare processes

Abstract: We present model independent constraints on the masses and couplings to fermions ofB andL conserving leptoquarks Such vector or scalar particles could have masses below 100 GeV and be produced at HERA; we list the generation dependent bounds that can be calculated from rare lepton and meson decays, meson-anti-meson mixing and various electroweak tests

A new experimental limit on neutron-antineutron oscillations

Abstract: The experimental search for neutron-antineutron oscillations has been completed at the ILL high flux reactor at Grenoble. A neutron beam of intensity 1011n s−1 was propagated for a timet≅0.1 s in vacuum in a region shielded against the external magnetic field. No antineutron was detected in 2.4·107 s running time. The lower limit\(\tau _{n\bar n} \mathbin{\lower.3ex\hbox{$\buildrel>\over{\smash{\scriptstyle=}\vphantom{_x}}$}} 0.86 \cdot 10^8 \) s for\(n\bar n\) transitions was established at 90% C.L.

FLow equations for N point functions and bound states

Abstract: We discuss the exact renormalization group or flow equation for the effective action and its decomposition into one particle irreducibleN point functions. With the help of a truncated flow equation for the four point function we study the bound state problem for scalar fields. A combination of analytic and numerical methods is proposed, which is applied to the Wick-Cutkosky model and a QCD-motivated interaction. We present results for the bound state masses and the Bethe-Salpeter wave function.

Measurement of total and partial photon proton cross sections at 180 GeV center of mass energy

Abstract: Photon proton cross sections for elastic light vector meson production, σelνp, inelastic diffractive production, σndνp, non-diffractive production, σdνp, as well as the total cross section, σtotνp, have been measured at an average υp center of mass energy of 180 GeV with the ZEUS detector at HERA. The resulting values are σelνp = 18 ± 7 μb, σdνp = 33 ± 8 μb, σndνp = 91 ± 11 μb, and σtotνp 143 ± 17 μb, where the errors include statistical and systematic errors added in quadrature.

Optimal observables for the measurement of three gauge boson couplings in e+ e- ---> W+ W-

Abstract: We investigate the prospects of measuring anomalous couplings between gauge bosons at electron-positron-colliders with optimal observables Such observables are shown to contain all information on the coupling parameters that can be extracted in a given reaction Their sensitivity to the form factors in the general expressions of the triple gauge verticesWWZ andWWγ, includingCP violating terms and absorptive parts, is calculated in view of LEP2 and the NLC

A novel approach to confront electroweak data and theory

Abstract: A novel approach to study electroweak physics at one-loop level in generic SU(2)L×U(1)Y theories is introduced. It separates the 1-loop corrections into two pieces: process specific ones from vertex and box contributions, and universal ones from contributions to the gauge boson propagators. The latter are parametrized in terms of four effective form factors $$\bar e^2 (q^2 ), \bar s^2 (q^2 ), \bar g_Z^2 (q^2 )$$ and $$\bar g_W^2 (q^2 )$$ corresponding to the γγ, γZ,ZZ andWW propagators. Under the assumption that only the Standard Model contributes to the process specific corrections, the magnitudes of the four form factors are determined atq 2=0 and atq 2=m 2 by fitting to all available precision experiments. These values are then compared systematically with predictions of SU(2)L×U(1)Y theories. In all fits α s (m Z ) and $$\bar \alpha (m_Z^2 )$$ are treated as external parameters in order to keep the interpretation as flexible as possible. The treatment of the electroweak data is presented in detail together with the relevant theoretical formulae used to interpret the data. No deviation from the Standard Model has been identified. Ranges of the top quark and Higgs boson masses are derived as functions of α s (m Z ) and $$\bar \alpha (m_Z^2 )$$ . Also discussed are consequences of the recent precision measurement of the left-right asymmetry at SLC as well as the impact of a top quark mass and an improvedW mass measurement.

Diquonia and potential models

Abstract: We present four new quark-quark potentials, developed in the framework of the nonrelativistic quark model (NRQM), which can reproduce quite well the spectra of mesons and baryons. They contain a central part which is of type “Coulomb+linear” or “Coulomb +2/3-power”, and a strong but smooth hyperfine term. With these four potentials and the one proposed by Bhaduri et al., we have calculated masses and decay properties of some interesting diquonia (q2q−2 mesons). We have not found appreciable difference between the results of the five potentials considered, indicating that the properties of diquonia are, to a large extent, potential-independent in the NRQM.

Exclusive Radiative B-Decays in the Light-Cone QCD Sum Rule Approach

Abstract: We carry out a detailed study of exclusive radiative rareB-decays in the framework of the QCD sum rules on the light cone, which combines the traditional QCD sum rule technique with the description of final state vector mesons in terms of the light-cone wave functions of increasing twist. Our calculation is restricted to the leading twist-two operators. The decays considered are:Bu,d→K* +γ,Bu,d→p+γ,Bd→ω+γ and the corresponding decays of theBs mesons,Bs→ϕ+γ andBs→K*+γ. Based on our estimate of the transition form factorF1B→K* (0)=0.32±0.05, we find for the branching ratio BR(B→K*+γ)=(4.8±1.5)×10−5, which is in agreement with the observed value of (4.5±1.5±0.9)×10−5 measured by the CLEO collaboration. We present detailed estimates for the ratios of the radiative decay form factors, which are then used to predict the rates for the exclusive radiativeB-decays listed above. This in principle allows the extraction of the CKM matrix element |Vtd| from the penguin-dominated CKM-suppressed radiative decays when they are measured. We give a detailed discussion of the dependence of the radiative transition form factors on theb-quark mass and on the momentum transfer, as well as their interrelation with the CKM-suppressed semileptonic decay form factors in B→ρ+l+vl, which we also calculate in our approach.

On colour rearrangement in hadronicW+W− evemts

Abstract: We discuss the possibility of colour rearrangement in $$e^ + e^ - \to W^ + W^ - \to q_1 \bar q_2 q_3 \bar q_4 $$ events, i.e that the original colour singlets $$q_1 \bar q_2 $$ and $$q_3 \bar q_4 $$ may be transmuted, for instance, into new singlets $$q_1 \bar q_4 $$ and $$q_3 \bar q_2 $$ . The effects on event properties could be quite large if such a rearrangement would occur instantaneously, so that gluon emission would be restricted to each of the new singlets separately. We argue that such a scenario is unlikely for two reasons. Firstly, theW + andW − usually decay at separately. We argue that such a scenario is unlikely for two reasons. Firstly, theW + W − production, which leads to large relative phases for energetic radiation off the two constituents of a rearranged system, and a corresponding dampening of the QCD cascades. Secondly, within the perturbative scenario the colour transmutation appears only in order α s 2 and is colour-suppressed. Colour reconnection at longer time scales is quite feasible, however, and may affect the fragmentation phase. If so, the nature of non-perturbative QCD can be probed in a new way. We formulate several alternative toy models and use these to estimate the colour reconnection probability as a function of the event kinematics. Possible consequences for LEP 2 events are illustrated, with special attention to systematic errors inW mass determinations.

Closed expressions for specific massive multiloop self-energy integrals

Abstract: In this paper the class ofN loop massive scalar self-energy diagrams withN+1 propagators is studied in an arbitrary number of dimensions. As it is known these integrals cannot be expressed in terms of polylogarithms. Here it is shown, however, that they can be described by generalized hypergeometric functions of several variables, namely Laricella functions. These results represent previous small and large momentum expansions in closed form. Numerical comparisons for the finite part in four dimensions with a two-dimensional integral representation show good agreement.

Prospects of measuring the parity of Higgs particles

Abstract: We analyze the prospects of measuring the parity of Higgs particles in the Standard Model and its supersymmetric extensions. Higgs decays are discussed in this context as well as production processes including, in particular, the fusion of Higgs particles in linearly polarized photon-photon collisions.

Proton structure function at small q**2

Abstract: A fit is made to the data for the proton structure function up toQ 2=10 GeV2, including the real γp total cross-section. It is economical and simple, and its form is motivated by physical principles. It is extrapolated down to very small values ofx. Data for the ratio νW 2 /νW 2 are also fitted.

Measuring the photon fragmentation function at LEP

Abstract: Using an algorithm that treats photons and hadrons democratically, we discuss how the quark to photon fragmentation function,D q →γ, might be measured in ‘photon’ + jet events at LEP. Simple analytic results are given at lowest order. The possibility of determining the gluon to photon fragmentation function,D g →γ, in ‘photon’ + 2 jet events is also discussed, however, the prospects for doing so seem bleak.

Hadrons with charm and beauty

Abstract: By combining potential models and QCD spectral sum rules (QSSR), we discuss the spectroscopy of the\((b\bar c)\) mesons and of the (bcq), (ccq) and (bbq) baryons (q≡d ors), the decay constant and the (semi)leptonic decay modes of theBc meson. For the masses, the best predictions come from potential models and read:\(M_{B_c } = (6255 \pm 20)\) MeV,\(M_{B_c^* } = (6330 \pm 20)\) MeV,MΛ(bcu)=(6.93±0.05) GeV,MΩ(bcs)=(7.00±0.05) GeV,MΣ*(ccu)=(3.63±0.05) GeV andMΣ*(bbu)=(10.21±0.05) GeV. The decay constant\(f_{B_c } = (2.94 \pm 0.21)f_\pi \) is well determined from QSSR and leads to:Γ(Bc→vττ=(3.0±0.4)(Vcb/0.037)2×1010 s−1. The uses of the vertex sum rules for the semileptonic decays of theBc show that thet-dependence of the form factors is much stronger than predicted by vector meson dominance. It also predicts the almost equal strength of about 0.30×1010 s−1 for the semileptonic ratesBc intoBs,Bs*,ηc andJ/ψ. Besides these phenomenological results, we also show explicitly how the Wilson coefficients of the 〈αsG2〉 and 〈G3〉 gluon condensates already contain the full heavy quark\((\left\langle {\bar QQ} \right\rangle )\) and mixed-\((\left\langle {\bar QGQ} \right\rangle )\) condensate contributions in the OPE.

Parton distributions in the photon

Abstract: We discuss in detail the photon structure function beyond the leading logarithm approximation. Of special concern is the factorization scheme and the hadronic input; we show how to naturally absorb large terms due to the\(\overline {MS} \) factorization scheme in a modified hadronic component. The effect of the charm quark mass threshold is also discussed in relation to the phenomenology. A comparison with data shows that the modified hadronic component can be reasonably described by a VDM-type input.

Measurement of the production rates of charged hadrons in e+ e- annihilation at the Z0

Abstract: The inclusive production rates of π±,K± andp\(\bar p\) inZ0 decays have been measured with the OPAL detector at LEP. Using the energy loss measurement in the jet chamber, the momentum range up to the beam energy (45.6 GeV/c) has been covered. Differential cross sections and total particle yields are given. Comparisons of the inclusive momentum spectra and the total rates with predictions of the JETSET and the HERWIG Monte Carlo model are presented. The total single rates are found to be 17.05±0.43 π±, 2.42±0.13K± and 0.92±0.11p\(\bar p\) per hadronic event. Predictions of JETSET for cross sections and total rates agree very well for π±; however, for momenta greater than 4 GeV/c,K± rates are underestimated and\(\bar p\) rates are overestimated. Combined with data of other particle species there is evidence that the peak positions in the ξ=ln(1/x p ) distributions show a different mass dependence for mesons and baryons. However, both JETSET and HERWIG Monte Carlo predictions agree with the observed data.

Calculation of Feynman diagrams from their small momentum expansion

Abstract: A new powerful method to calculate Feynman diagrams is proposed. It consists in setting up a Taylor series expansion in the external momenta squared (in general multivariable). The Taylor coefficients are obtained from the original diagram by differentiation and putting the external momenta equal to zero, which means a great simplification. It is demonstrated that it is possible to obtain by analytic continuation of the original series high precision numerical values of the Feynman integrals in the whole cut plane. For this purpose conformal mapping and subsequent resummation by means of Pade approximants or Levin transformation are applied.

Strange particle production in nuclear collisions at 200 GeV per nucleon

Abstract: Multiplicities and spectra of strange particles (\(\Lambda ,\bar \Lambda ,K_S^0 ,K^ +\) andK− produced in central32S+S,32S+Ag and32S+Au collisions at 200 GeV per nucleon are presented and compared with data on strange particle production in protonnucleus and nucleon-nucleon interactions. It is shown that strangeness production in32S+Ag collisions is enhanced by a factor of two, similar to that found previously in central32S+S collisions.

Squark and gluino decays for large tan β

Abstract: We have studied stop, sbottom and gluino decays in a scenario with large tan β, where both $$\tilde t_L - \tilde t_R $$ and $$\tilde b_L - \tilde b_R $$ mixing as well as the Yukawa couplings have to be taken into account. In particular we have worked out the case that $$\tilde b_1 $$ is the lightest squark. The decay patterns of sbottom and stop are very different from those of the squarks of the first and second generation.

Chemical freeze-out conditions in central S-S collisions at 200 A GeV

Abstract: We determine the chemical freeze-out parameters of hadronic matter formed in central S-S collisions at 200 A GeV, analyzing data from the NA35 collaboration at CERN. In particular we study the quark (baryon number) and strange quark fugacities, as well as the strange quark phase-space occupancy and the freeze-out temperature. The strange quark chemical potential is found to be consistent with zero, and the strange quark abundance is found to be saturated as would be expected for the deconfined phase. We study the stability of our results against variations in the analysis procedure and with respect to a restriction of the data to the central rapidity region. The same analysis applied to nucleonnucleon data yields significantly different freeze-out parameters, in particular regarding the degree of strangeness saturation.

Representations of a q-deformed Heisenberg algebra

Abstract: We extend the symmetric operators of theq-deformed Heisenberg algebra to essentially self-adjoint operators. On the extended domains the product of the operators is not defined. To represent the algebra we had to enlarge the representation and we find a Hilbert space representation of the deformed Heisenberg algebra in terms of essentially self-adjoint operators. The respective diagonalization can be achieved by aq-deformed Fourier transformation.

Probing the scalar sector in $$e^ + e^ - \to f\bar fH$$

Abstract: We study possible deviations from the Standard Model in the reactione + e −→Zϕ, where ϕ denotes a spinless neutral boson. We show how theZ decay angular correlation can be used to extract detailed information on the ϕ couplings, such as the parity of ϕ, radiatively-induced form factor effects and possible CP violation in the scalar sector. Consequences of gauge invariant dimension six operators are discussed as an example.

Electroweak penguin effects beyond leading logarithms in theB-meson decaysB−→K− ϕ and\(B^ - \to \pi ^ - \bar K^{0*} \)

Abstract: Using the low energy effective Hamiltonian for |ΔB|=1, ΔC=ΔU=0 transitions, which has been calculated recently by Buras et al. beyond the leading logarithmic approximation, we analyze the penguin-inducedB-meson decaysB−→K− ϕ and\(B^ - \to \pi ^ - \bar K^{0*} \) within the framework of the Bauer-Stech-Wirbel model and find, in contradiction to naive expectations, that the decay modeB−→K− ϕ is affected strongly by electroweak penguin operators. Thes contributions depend on the value of the top-quark mass and reduce the branching ratio BR(B−→K− ϕ) by factors of 0.8 ... 0.6 form t =(130... 250) GeV, respectively, relative to the results obtained by taking into account only QCD penguin operator contributions. On the other hand, we find that the effects of the electroweak penguins are very small for the transition\(B^ - \to \pi ^ - \bar K^{0*} \).

Equivalence of dimensional reduction and dimensional regularisation

Abstract: For some years there has been uncertainty over whether regularisation by dimensional reduction (DRED) is viable for non-supersymmetric theories. We resolve this issue by showing that DRED is entirely equivalent to standard dimensional regularisation (DREG), to all orders in perturbation theory and for a general renormalisable theory. The two regularisation schemes are related by an analytic redefinition of the couplings, under which the β-functions calculated using DRED transform into those computed in DREG. TheS-matrix calculated using DRED is numerically equal to the DREG version, ensuring that both schemes give the same physics.

Mass and width of unstable gauge bosons

Abstract: It has been proposed that the mass of theZ0 boson should be defined as the real part, and the width of theZ0 as the imaginary part of the complex poleMc, for the reason thatMc is gauge invariant (g.i.). Clearly, this definition is only useful if we can calculate theS-matrix up to some accuracy in a g.i. way. This is not at all obvious, since the use of theZ0 dressed propagator mixes up the order of the diagrams, and thus order by order gauge invariance of theS-matrix, in this formulation, becomes an ill-defined concept.

Massive spin-1 field chiral Lagrangian from an extended Nambu-Jona-Lasinio model of QCD

Abstract: We present a calculation of the coupling constants in the massive spin-1 field chiral Lagrangian to chiral O(p3) in the context of the extended Nambu-Jona-Lasinio model described in [1]. Phenomenological applications of this Lagrangian to anomalous and non-anomalous low-energy hadronic transitions involving spin-1 particles are also discussed.

Equations of motion for effective Lagrangians and Penguins in rare B-decays

Abstract: We study the application of the classical equations of motion (EOM) within the framework of an effective low-energy Lagrangian treated at the loop level. Gauge-fixing and ghost terms, which enter naturally in the EOM, are found to lead to no physical effects—neither through operator mixing nor in matrix elements. Beyond first order in the effective interactions, contact terms have to be included when reducing the effective Lagrangian and we present an explicit procedure to construct them. Applied to (hadronic) rareB-decays, the EOM drastically simplify the effective Lagrangian and its matching to the underlying theory, and certain cancellations of large (logarithmic) contributions become more transparent. Finally, we discuss details of the ‘matching’ of the effective Lagrangian, which may be helpful in incorporating short distance QCD corrections in further phenomenological studies.

QCD studies using a cone-based jet finding algorithm for e + e − collisons at LEP

Abstract: We describe a cone-based jet finding algorithm (similar to that used in $$\bar p$$ p experiments), which we have applied to hadronic events recorded using the OPAL detector at LEP. Comparisons are made between jets defined with the cone algorithm and jets found by the “JADE” and “Durham” jet finders usually used ine + e − experiments. Measured jet rates, as a function of the cone size and as a function of the minimum jet energy, have been compared with O(α 2 ) calculations, from which two complementary measurements $$\alpha _s \left( {M_{Z^0 } } \right)$$ have been made. The results are $$\alpha _s \left( {M_{Z^0 } } \right)$$ =0.116±0.008 and $$\alpha _s \left( {M_{Z^0 } } \right)$$ =0.119±0.008 respectively, where the errors include both experimental and theoretical uncertainties. Measurements are presented of the energy flow inside jets defined using the cone algorithm, and compared with equivalent data from $$\bar p$$ p interactions, reported by the CDF collaboration. We find that the jets ine + e − are significantly narrower than those observed in $$\bar p$$ p. The main contribution to this effect appears to arise from differences between quark- and gluon-induced jets.

Dimensional reduction in non-supersymmetric theories

Abstract: It is shown that regularistation by dimensional reduction is a viable alternative to dimensional regularisation in non-supersymmetric theories.