The European Physical Journal C

al., C. Adloff et

doi: 10.1007/s100529901072pmid: N/A

Abstract. Deep-inelastic scattering events with a leading baryon have been detected by the H1 experiment at HERA using a forward proton spectrometer and a forward neutron calorimeter. Semi-inclusive cross sections have been measured in the kinematic region \(2 \le Q^2 \le 50\,{\mathrm GeV}^2\), \(6\times 10^{-5}\le x\le 6\times10^{-3}\) and baryon \(p_{\rm T} \le 200\) MeV, for events with a final-state proton with energy \(580 \le E^{\prime} \le 740\) GeV, or a neutron with energy \(E^{\prime} \ge 160\) GeV. The measurements are used to test production models and factorization hypotheses. A Regge model of leading baryon production which consists of pion, pomeron and secondary reggeon exchanges gives an acceptable description of both semi-inclusive cross sections in the region \(0.7 \le E^{\prime} / E_{\rm p} \le 0.9\), where \(E_{\rm p}\) is the proton beam energy. The leading neutron data are used to estimate for the first time the structure function of the pion at small Bjorken-x.

al., J. Breitweg et

doi: 10.1007/s100529901051pmid: N/A

Abstract. Exclusive production of \(\rho^0\) and \(J/\psi\) mesons in e\(^+\)p collisions has been studied with the ZEUS detector in the kinematic range \(0.25 < Q^2 < 50\) GeV\(^2\), \(20 < W < 167\) GeV for the \(\rho^0\) data and \(2 < Q^2 < 40\) GeV\(^2\), \(50 < W < 150\) GeV for the \(J/\psi\) data. Cross sections for exclusive \(\rho^0\) and \(J/\psi\) production have been measured as a function of \(Q^2\), W and t. The spin-density matrix elements \(r^{04}_{00}\), \(r^1_{1-1}\) and Re\(\:r^{5}_{10}\) have been determined for exclusive \(\rho^0\) production as well as \(r^{04}_{00}\) and \(r^{04}_{1-1}\) for exclusive \(J/\psi\) production. The results are discussed in the context of theoretical models invoking soft and hard phenomena.

Mureşan, Raluca;Smirnova, Oxana;Lörstad, Bengt

doi: 10.1007/s100529901055pmid: N/A

Abstract. This work studies the algorithm which implements the Bose-Einstein correlation effect in the JETSET 7.4 event generator. This algorithm attempts to reproduce an expected correlation function with a given correlation radius and amplitude. The two-particle correlation function is studied in the generated \(Z^0\) hadronic decays for different values of the built-in radius parameter. Samples consisting of only charged particles are used, as well as subsamples of pions, pions coming from the string decay and pions from the resonance decays. The Bose-Einstein correlation function, extracted from the generated events, is parameterized analogously to the built-in JETSET correlation function and its parameters are compared with the input ones. We found that the measured correlation function reproduces the built-in one, if the input radius parameter is larger than 1 fm. For lower input radii an artificial new length scale appears due to the way the Bose-Einstein correlation is implemented.

Spagnolo, S.

doi: 10.1007/s100529800953pmid: N/A

Abstract. A relevant reduction of the theoretical uncertainty on the muon anomalous magnetic moment, dominated by the error on the leading hadronic contribution \(a^{\rm h}_\mu\), would come from a new precise measurement of the cross section of \(e^+e^-\) annihilation into hadrons (\(\sigma_h\)) below 1 GeV. An experimental approach to the evaluation of the dispersion integral defining \(a^{\rm h}_\mu\) is proposed here as an alternative to the conventional method based on the interpolation of different measurements of \(\sigma_h\) performed with a center of mass energy scan. The wide occurrence of hadron production, at variable \(q^2\), in the initial state radiation (ISR) events at the high luminosity \(\phi\)-factory DAΦNE suggests to redefine the dispersion integral in terms of the differential cross section for \(e^+e^-\rightarrow hadrons + \gamma_{\rm ISR}\). The feasibility of such measurement, which can be performed without the need of dedicated run conditions and with the benefit of the full luminosity of the machine, is discussed. The required precision is shown to be easily achieved thanks to the resolutions and performances of the KLOE detector.

Dighe, A.S.;Dunietz, I.;Fleischer, R.

doi: 10.1007/s100529800954pmid: N/A

Abstract. Suggestions for efficiently determining the lifetimes and mass difference of the light and heavy \(B_s\) mesons (\(B_s^L, B_s^H\)) from \(B_s \to J/ \psi \phi, D_s^{*+}D_s^{*-}\) decays are given. Using appropriate weighting functions for the angular distributions of the decay products (moment analysis), one can extract \((\Gamma_H, \Gamma_L, \Delta m)_{B_s}\). Such a moment analysis allows the determination of the relative magnitudes and phases of the CP-odd and CP-even decay amplitudes. Efficient determinations of CP-violating effects occuring in \(B_s \to J/\psi \phi, D_s^{*+} D_s^{*-}\) are discussed in the light of a possible width difference \((\Delta \Gamma)_{B_s}\), and the utility of this method for \(B \to J/\psi K^*, D^{\ast +}_s \overline D^{\ast}\) decays is noted. Since our approach is very general, it can in principle be applied to all kinds of angular distributions and allows the determination of all relevant observables, including fundamental CKM (Cabibbo–Kobayashi–Maskawa) parameters, as well as tests of various aspects of the factorization hypothesis. Explicit angular distributions and weighting functions are given, and the general method that can be used for any angular distribution is indicated.

Hwang, Dae Sung;Lee, Bum-Hoon

doi: 10.1007/s100529800950pmid: N/A

Abstract. We calculate the form factors \(F_0\) and \(F_1\) of \(B\rightarrow\pi\) and V, \(A_0\), \(A_1\) and \(A_2\) of \(B\rightarrow\rho\) transition matrix elements by using the factorization formalism of perturbative QCD in the region \(0\le q^2 \le M_B^2/2\). In the limit of \(m_\pi /M_B=0\), \(m_\rho /M_B=0\), \({M_b/M_B}=1\) and \((1-x)\ll 1\), the results show that \(F_0\) and \(A_1\) are of monopole type, V, \(A_0\) and \(A_2\) of dipole type, and \(F_1\) of a combination of monopole and dipole types with dipole type dominating.

Kwiecinski, J.;Lang, S.C.;Martin, A.D.

doi: 10.1007/s100529800945pmid: N/A

Abstract. We study the transverse momentum (\(p_T\)) spectrum of charged particles produced in deep inelastic scattering (DIS) at small Bjorken x in the central region between the current jet and the proton remnants. We calculate the spectrum at large \(p_T\) with the BFKL ln (1/x) resummation included and then repeat the calculation with it omitted. We find that data favour the former. We normalize our BFKL predictions by comparing with HERA data for DIS containing a forward jet. The shape of the x distribution of DIS + jet data is also well described by BFKL dynamics.

Moussallam, B.

doi: 10.1007/s100529800955pmid: N/A

Abstract. A sum rule due to Das et al. is reanalyzed using a euclidian space approach and a Padé resummation procedure. It is shown that the result is essentially determined by the matrix elements of dimension six and dimension eight operators which have recently been measured by the ALEPH collaboration. The result is further improved by using the vector spectral function which must be extrapolated to the chiral limit. This extrapolation is shown to be reliably performed under the constraint of a set of sum rules. The sum rule is employed not as an approximation to \(M_{\pi^+}-M_{\pi^0}\) but as an exact result for a chiral low-energy parameter. A sufficiently precise evaluation provides also an estimate for a combination of subleading electromagnetic low-energy parameters.