Near-threshold Cross Sections Research Articles

Three-loop soft function for energetic electroweak boson production at hadron colliders

We calculate the three-loop soft function for the production of an electroweak boson (Higgs, γ, W±, Z) with large transverse momentum at a hadron collider. It is the first time a soft function for a three-parton process is computed at next-to-next-to-next-to-leading order (N3LO). As a technical novelty, we perform the calculation in terms of forward-scattering-type loop diagrams rather than evaluating phase space integrals. Our three-loop result contains color-tripole contributions and explicitly confirms predictions on the universal infrared structure of QCD scattering amplitudes with three massless parton legs. The soft function is a central ingredient in the factorized cross section for electroweak boson production near the kinematic endpoint (threshold), where the invariant mass of the recoiling hadronic radiation is small compared to its transverse momentum. Our result is required for predictions of the near-threshold cross sections at N3LO and for the resummation of threshold logarithms at primed next-to-next-to-next-to-leading logarithmic (N3LL′) accuracy.

Near-threshold π− photoproduction on the deuteron

The first experimental investigation of the near-threshold cross section for incoherent $\pi^-$ photoproduction on the deuteron $\gamma$ d -> $\pi^-$ pp is presented. The experimental technique involved detection of the ~131 MeV gamma ray resulting from the radiative capture of photoproduced $\pi^-$ in the target. The total cross section has been measured using an unpolarized tagged-photon beam, a liquid-deuterium target, and three very large NaI(Tl) spectrometers. The data are compared to theoretical models that give insight into the elementary reaction $\gamma$ n -> $\pi^-$ p and pion-nucleon and nucleon-nucleon final-state interactions.

Electron-impact single ionisation of W14+ ions: subconfiguration-average and level-to-level distorted wave calculations

The cross section for electron-impact single ionisation of ions has been calculated by using two different approaches, i.e. the subconfiguration averaged distorted-wave (SCADW) method and the more involved level-to-level distorted-wave (LLDW) method. Both methods are found to yield very similar results except for the excitation-autoionisation (EA) channels that straddles the ionisation threshold. Accordingly, a hybrid theoretical cross section where the EA SCADW cross section is replaced by its LLDW counterpart is in good agreement with the experimental result from an electron-ion crossed-beams experiment. This is in contrast to pure SCADW calculations for and neighbouring charge states which exhibit significant deviations from the experimental near-threshold cross sections of Schury et al (2020 J. Phys. B: At. Mol. Opt. Phys. 53 015201).

Exclusive open-charm near-threshold cross sections in a coupled-channel approach

Data on 7 open-charm channels collected by the Belle Collaboration are analyzed simultaneously using a unitary approach based on a coupled channel model in a wide energy range $ \sqrt s $ = 3.7 − 4.7 GeV. The resulting fit provides a remarkably good overall description of the line shapes in all studied channels. Parameters of 5 vector charmonium resonances are extracted from the fit. It is demonstrated, that this approach could be used account for all exclusive channels and thus solve the long-term problem of the charmonium spectra near threshold.

Near-Threshold Cross Sections for Electron and Positron Impact Ionization of Atomic Hydrogen.

Using classical arguments Wannier [Phys. Rev. 90, 817 (1953)PHRVAO0031-899X10.1103/PhysRev.90.817] proposed an electron-impact ionization cross section for neutral atoms to behave as E^{1.127}, where E is the excess energy above threshold. Using similar arguments Klar [J. Phys. B 14, 4165 (1981)JPAMA40022-370010.1088/0022-3700/14/21/027] obtained E^{2.65} to be the corresponding threshold law for positron impact. Recently, Babij etal. [Phys. Rev. Lett. 120, 113401 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.113401] measured near-threshold positron-impact breakup behavior to be similar to that expected for electrons. Using the convergent close-coupling method for the atomic hydrogen target, we examine cross sections at near-threshold energies for electron and positron impact. Contrary to the experiment, the calculated cross sections are found to behave differently for the two projectiles and consistently with the aforementioned threshold laws, despite the entirely quantum nature of these problems. For electron impact, the threshold behavior holds while the total electron spin asymmetry remains constant, whereas for positron scattering the threshold law holds for breakup while the positronium-formation component of the ionization cross section remains constant.

Exclusive open-charm near-threshold cross sections in a coupled-channel approach

Data on open-charm channels collected by the Belle Collaboration are analysed simultaneously using a unitary approach based on a coupled-channel model in a wide energy range $\sqrt{s}=3.7\div 4.7$ GeV. The resulting fit provides a remarkably good overall description of the line shapes in all studied channels. Parameters of 5 vector charmonium resonances are extracted from the fit.

Cold magnetically trapped2Dgscandium atoms. II. Scattering dynamics

The binary collision dynamics of ${}^{2}{D}_{g,3/2}$ ground state scandium atoms is studied from first principles. We employ 30 coupled diabatic ab initio potentials in a coupled-channels study of the scattering dynamics of cold and ultracold scandium atoms in external magnetic fields. Due to the long-ranged magnetic dipolar interaction, the field dependence of the cross section does not follow the threshold laws derived by Volpi and Bohn [Phys. Rev. A 65, 052712 (2002)]. In the field-free case, the near-threshold cross section is independent of the collision energy, and hence the cross section does not follow the well-established Wigner threshold laws. The observed threshold behavior is explained in the Born approximation. For energies above $1\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}$, inelastic collisions are driven by the anisotropic nonrelativistic electronic interaction. For energies below $100\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}$, the ratio of elastic-to-inelastic collisions is likely to be favorable for evaporative cooling. Both anisotropy in the long-range interaction and in the short-range potential contribute to large cross sections for inelastic collisions at higher energies and lead to a small ratio of elastic-to-inelastic collisions. This is in agreement with the large rates for Zeeman relaxation of submerged-shell atoms observed experimentally. The effect of the uncertainty in the ab initio potential is sampled by scaling the reduced mass and is found to have little influence on the conclusions drawn from this work.

Quantum dynamics of Renner–Teller and isotopic effects in NH(a1Δ) + D(2S) reactions

A quantum dynamics study for the NH(a(1)Δ) + D((2)S) reactions using coupled channel time dependent real wavepacket formalism is presented. Moreover, the Renner-Teller (RT) interactions between two electronic states of NHD (X[combining tilde](2)A'' and Ã(2)A') have been taken into account by means of semiempirical RT matrix elements. The introduction of RT effects opens the possibility of studying not only the adiabatic reactions [depletion (d) to N((2)D) + HD(X(1)Σ(+)) and exchange (e) to ND(a(1)Δ) + H((2)S)] but also nonadiabatic ones [quenching (q) to NH(X(3)Σ(-)) + D((2)S) and exchange-quenching (eq) to ND(X(3)Σ(-)) + H((2)S)]. Reaction probabilities, cross sections, isotopic effects, and rate constants are presented for all the before mentioned reactions. RT results are compared with Born-Oppenheimer, quasiclassical, and experimental data. Contrasting with previous NH + H results, we point out interesting RT and isotopic effects, which depend on the D and H masses and on the tunneling of the H atom. In fact, RT effects, near-threshold cross sections, and rate constants are smaller in NH + D than in NH + H, as expected from the masses of the attacking atoms. Our rate constants and quenching branching ratio agree well with previous quasiclassical and experimental data, validating the semiempirical RT coupling we employ. Some small differences between calculated and measured rate constants might be due to the theoretical approximations and to the large experimental error bars.

Φ PRODUCTION IN pp COLLISIONS

The data for near-threshold cross section for the reaction pp → ppɸ published by the DISTO and ANKE Collaboration are analyzed in order to evaluate the OZI ratio at low energies by taking into account corrections from the kinematics and the final-state interaction. Combining these new data with the few measurements available at higher energies the limit for the OZI rule violation is evaluated and the possible contribution from a five-quark baryonic resonance coupled to the ɸp system is estimated.

Near-threshold absoluteM-shell x-ray production cross sections of Au and Bi by electron impact

Near-threshold absolute<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>M</mml:mi></mml:math>-shell x-ray production cross sections of Au and Bi by electron impact

Strangeness Production Process pp → nK+Σ+ within Resonance Model

We explore production mechanism and final state interaction in the pp → nK+Σ+ channel based on the inconsistent experimental data published respectively by COSY-11 and COSY-ANKE. The scattering parameter a > 0 for nΣ+ interaction is favoured by large near-threshold cross section within a nonrelativistic parametrization investigation, and a strong nΣ+ interaction comparable to pp interaction is also indicated. Based on this analysis we calculate the contribution from resonance Δ* (1920) through π+ exchange within resonance model, and the numerical result suggests a rather small near-threshold total cross section, which is consistent with the COSY-ANKE data. With an additional sub-threshold resonance Δ* (1620), the model gives a much better description to the rather large near-threshold total cross section published by COSY-11.

The role of [formula omitted] resonances in [formula omitted] reaction and its important implications

The pp→nK+Σ+ reaction is a very good isospin 3/2 filter for studying Δ++∗ decaying to K+Σ+. With an effective Lagrangian approach, contributions from a previous ignored sub-K+Σ+-threshold resonance Δ++∗(1620)1/2− are fully included in addition to those already considered in previous calculations. It is found that the Δ++∗(1620)1/2− resonance gives an overwhelmingly dominant contribution for energies very close to threshold, with a very important contribution from the t-channel ρ exchange. This solves the problem that all previous calculations seriously underestimate the near-threshold cross section by order(s) of magnitude. Many important implications of the results are discussed.

Associated charmonium production in low energypp¯annihilation

The QCD mechanisms underlying the exclusive strong decays and hadronic production amplitudes of charmonium remain poorly understood, despite decades of study and an increasingly detailed body of experimental information. One set of hadronic channels of special interest are those that include baryon-antibaryon states. These are being investigated experimentally at BES and CLEO-c in terms of their baryon resonance content, and are also of interest for the future PANDA experiment, in which charmonium and charmonium hybrids will be produced in $p\overline{p}$ annihilation in association with light mesons. In this paper we develop a simple initial-state light meson emission model of the near-threshold associated charmonium production processes $p\overline{p}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\Psi}$, and evaluate the differential and total cross sections for these reactions in this model. (Here we consider the states $\ensuremath{\Psi}={\ensuremath{\eta}}_{c}$, $J/\ensuremath{\psi}$, ${\ensuremath{\psi}}^{\ensuremath{'}}$, ${\ensuremath{\chi}}_{0}$ and ${\ensuremath{\chi}}_{1}$.) The predicted near-threshold cross section for $p\overline{p}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}J/\ensuremath{\psi}$ is found to be numerically similar to two previous theoretical estimates, and is roughly comparable to the (sparse) existing data for this process. The theoretical charmonium angular distributions predicted by this model are far from isotropic, which may be of interest for PANDA detector design studies.

Aspects of φ-meson production in proton-proton collisions

We analyze near-threshold cross section data for the reaction pp->pp phi published by the DISTO collaboration and recent, still preliminary results presented by the ANKE Collaboration. We formulate a procedure to evaluate the OZI ratio at low energies by taking into account corrections from the kinematics and the final-state interaction. Combining the new data with the few measurements available at higher energies we give a limit for the OZI rule violation and estimate the possible contribution from a five-quark baryonic resonance coupled to the phi-p system.

Systematic comparison of positron- and electron-impact excitation of theν3vibrational mode ofCF4

Absolute measurements are presented for the excitation of the ${\ensuremath{\nu}}_{3}$ vibrational mode in ${\mathrm{CF}}_{4}$ by positron and electron impact from 0.1 to 2 eV. To minimize systematic differences, these measurements were made using the same trap-based electron or positron beam, associated experimental apparatus, and procedures. Unlike other vibrational excitation cross sections studied to date, the near-threshold cross section for the ${\ensuremath{\nu}}_{3}$ vibrational mode in ${\mathrm{CF}}_{4}$ is similar, both in magnitude and shape, for positrons and electrons. Comparison of the cross sections with an analytic Born dipole model yields good agreement, while comparison of this model with other measured positron-impact vibrational cross sections indicates that the contribution of this long-range dipole coupling varies widely. The maximum value of the cross section in ${\mathrm{CF}}_{4}$ is the largest of any positron-impact vibrational excitation cross section measured to date. This provides a likely explanation of the observation that ${\mathrm{CF}}_{4}$ is very effective when used as a buffer gas to cool positron gases and plasmas.

Calculations of near-threshold cross sections for photodissociation of CH+ using the Lanczos algorithm

We combine the Lanczos algorithm with the absorbing-potential method, implemented in a discrete variable representation to calculate the near-threshold photodissociation cross sections of CH+. The method is iterative, based on a continued fraction representation of the Green function and avoids any explicit matrix diagonalization. A very good agreement is found with experiment and close-coupling calculations.

Determination of closed-flavour πJ/ψ and πϒ cross sections at low energies from dipion decays

The scale of low energy and cross sections on light hadrons is of great importance to searches for the quark–gluon plasma using the heavy-quarkonium suppression signature. Unfortunately, little is known about these near-threshold cross sections at present, and recent theoretical estimates span many orders of magnitude. Here we use experimental data on the four observed closed-flavour heavy quarkonium hadronic decays ψ′ → ππJ/ψ, ϒ′ → ππϒ, ϒ″ → ππϒ and ϒ″ → ππϒ′, combined with simple models of the transition amplitudes, to estimate the pion scattering cross sections of and mesons near threshold. Specifically we consider the closed-flavour reactions πJ/ψ → πψ′, πϒ → πϒ′, πϒ → πϒ″ and πϒ′ → πϒ″ and their time-reversed analogues. Our results may be useful in constraining theoretical models of the strong interactions of heavy quarkonia, and can be systematically improved through future detailed studies of dipion decays, notably ψ′ → ππJ/ψ and ϒ″ → ππϒ.

Production of η mesons in nucleon-nucleon collisions

A microscopic calculation of near-threshold \ensuremath{\eta}-meson production in the reaction $N\stackrel{\ensuremath{\rightarrow}}{N}\mathrm{NN}\ensuremath{\eta}$ is presented. It is assumed that the \ensuremath{\eta} meson is produced via direct emission and via elementary rescattering processes $M\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\eta}N$ of various mesons $M=\ensuremath{\pi},\ensuremath{\rho},$ etc. As a novel feature the amplitudes for the latter production mechanism are taken from a multichannel meson-exchange model of the $\ensuremath{\pi}N$ system developed by the J\ulich group which contains explicitly the channels $\ensuremath{\pi}N,\ensuremath{\rho}N,\ensuremath{\eta}N,\ensuremath{\sigma}N,$ and \ensuremath{\pi}\ensuremath{\Delta}. Furthermore, effects of the $\mathrm{NN}$ interaction in the final as well as in the initial state are taken into account microscopically. Our results are compared with recent data from the COSY and CELSIUS accelerator facilities. Reasonable agreement with available near-threshold cross section data for the reactions $p\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{pp}\ensuremath{\eta},p\stackrel{\ensuremath{\rightarrow}}{n}\mathrm{pn}\ensuremath{\eta},$ and $p\stackrel{\ensuremath{\rightarrow}}{n}d\ensuremath{\eta}$ is found.

Near-threshold photodetachment of heavy alkali-metal anions

We calculate near-threshold photodetachment cross sections for Rb{sup -}, Cs{sup -}, and Fr{sup -} using the Pauli equation method with a model potential describing the effective electron-atom interaction. Parameters of the model potential are fitted to reproduce ab initio scattering phase shifts obtained from Dirac R-matrix calculations. Special care is taken to formulate the boundary conditions near the atomic nucleus for solving the Pauli equation, based on the analytic solution of the Dirac equation for a Coulomb potential. We find a {sup 3}P{sub 1}{sup o} resonance contribution to the photodetachment cross section of Rb{sup -}, Cs{sup -}, and Fr{sup -} ions. Our calculated total photodetachment cross sections for Cs agree with experiments after tuning the resonance position by 2.4 meV. For Rb{sup -} and Fr{sup -} the resonance contribution is much smaller than for Cs. We therefore also provide angle-differential cross sections and asymmetry parameters which are much more sensitive to the resonant contribution than total cross sections.

Control of near-threshold detachment cross sections via laser polarization.

The behavior of near-threshold cross sections for dissociation of a target into a pair of particles, as described by Wigner's threshold law, can depend sensitively on the angular momentum of the particles. In this Letter, we investigate the near-threshold nonresonant two-photon detachment process in the negative ion of gold. The expected s-wave threshold behavior is observed with linearly polarized light. Closure of the s-wave channel is realized by using circular polarization, allowing the first observation of a d-wave threshold. Practical applications are discussed, including extensions which could prove valuable for investigations of negative ions with near-threshold structure.