Meson Pair Production Research Articles

Charmed meson pair production in high‐energy pp$$ pp $$ collisions within the QCD color dipole approach

AbstractA study of double‐charmed meson production in high‐energy proton–proton collisions at the Large Hadron Collider is presented. Considering the color dipole formalism developed in the transverse momentum representation along with the double parton scattering mechanism, predictions are made for the transverse momentum differential cross section for different pairs of ‐mesons. The theoretical results cover the center‐of‐mass energy and forward rapidities available by the LHCb experiment. The proton–proton results considering different unintegrated gluon distributions are compared to the respective data collected at the LHC.

Gamma-gamma physics and meson transition form factors

I summarize recent experimental results for two photon production of mesons. These processes include the neutral pion, η and η′ transition form factors and two photon production of pseudoscalar meson pairs. In addition I discuss the related hadronic and radiative processes. All these processes are attracting attention of experiment and theory due their relevance for the hadronic light-by-light contribution to the muon anomalous magnetic moment.

Diffractive Physics at the LHC

Diffractive processes possible to be measured at the LHC are listed and briefly discussed. This includes soft (elastic scattering, exclusive meson pair production, diffractive bremsstrahlung) and hard (single and double Pomeron exchange jets, y +jet, W/Z, jet-gap-jet, exclusive jets) processes as well as Beyond Standard Model phenomena (anomalous gauge couplings, magnetic monopoles).

Tensor pomeron, vector odderon and diffractive production of meson and baryon pairs in proton-proton collisions

We review some selected results of the tensor-pomeron and vectorodderon model of soft high-energy proton-proton scattering and central exclusive production of meson and baryon pairs in proton-proton collisions. We discuss the theoretical aspects of this approach and consider the phenomenological implications in a variety of processes at high energies, comparing to existing experimental data. We consider the diffractive dipion and dikaon production including the continuum and the dominant scalar and tensor resonance contributions as well as the photoproduction processes. The theoretical results are compared with existing CDF experimental data and predictions for planned or current LHC experiments, ALICE, ATLAS, CMS, LHCb are presented.

Central exclusive diffractive production of pp¯ pairs in proton-proton collisions at high energies

We consider the central exclusive production of the $p\bar{p}$ in the continuum and via resonances in proton-proton collisions at high energies. We discuss the diffractive mechanism calculated within the tensor-pomeron approach including pomeron, odderon, and reggeon exchanges. The theoretical results are discussed in the context of existing WA102 and ISR experimental data and predictions for planned or current experiments at the RHIC and the LHC are presented. The distribution in ${\rm y}_{diff}$, the rapidity distance between proton and antiproton, is particularly interesting. We find a dip at ${\rm y}_{diff} = 0$ for the $p \bar{p}$ production, in contrast to the $\pi^{+}\pi^{-}$ and $K^{+}K^{-}$ production. We predict also the $p \bar{p}$ invariant mass distribution to be less steep than for the pairs of pseudoscalar mesons. We argue that these specific differences for the $p \bar{p}$ production with respect to the pseudoscalar meson pair production can be attributed to the proper treatment of the spin of produced particles. We discuss asymmetries that are due to the interference of $C=+1$ and $C=-1$ amplitudes of $p \bar{p}$ production. We have also calculated the cross section for the $pp \to pp \Lambda \overline{\Lambda}$ reaction. Here, the cross section is smaller but the characteristic feature for $d\sigma/d{\rm y}_{diff}$ is predicted to be similar to $p \bar{p}$ production. The presence of resonances in the $p \bar p$ channel may destroy the dip at ${\rm y}_{diff} = 0$. This opens the possibility to study diffractively produced resonances. We discuss the observables suited for this purpose.

GenEx: A Simple Generator Structure for Exclusive Processes in High Energy Collisions

A simple C++ class structure for construction of a Monte Carlo event generators which can produce unweighted events within relativistic phase space is presented. The generator is self-adapting to the provided matrix element and acceptance cuts. The program is designed specially for exclusive processes and includes, as an example of such an application, implementation of the model for exclusive production of meson pairs $pp \rightarrow p M^+M^- p $ in high energy proton-proton collisions.

Partial waves in Bs*B¯s* production in e+e− annihilation near threshold

The production of the vector meson pairs $B_s^* \bar B_s^*$ in the $e^+e^-$ annihilation is generally contributed by two $P$ wave amplitudes with different total spin $S$ of the mesons, $S=0$ and $S=2$, and also by an $F$ wave amplitude. Belle has recently reported a study of the available data at the energy of the $\Upsilon(5S)$ peak in terms of the two $P$ wave contributions only. It is argued here that although at this energy the $F$ wave should be quite suppressed, especially for the strange-bottom vector mesons, the particular studied angular distribution is very sensitive to a presence of even small $F$ wave amplitude due to a significant interference with the dominant $S=2$ $P$ wave. Thus the available data are not conclusive with regards to the partial wave structure of the production amplitude. Additional angular correlations are discussed that can be measured for a full amplitude analysis of the production process.

Production of dimeson atoms in high-energy collisions

The production of two-meson electromagnetic bound states and free meson pairs \( \pi^+\pi^-\) , \( K^+K^-\) , \( \pi^+K^{\mp}\) in relativistic collisions has been considered. It is shown that using of exact Coulomb wave functions for dimeson atom (DMA) allows one to calculate the yield of discrete states with the desired accuracy. The relative probabilities of production of DMA and meson pairs in the free state are estimated. The amplitude of DMA transition from 1S to 2P state, which is essential for the pionium Lamb shift measurements, has been obtained.

Pion polarizabilities from a γγ→ππ analysis

We present results for pion polarizabilities predicted using dispersion relations from our earlier Amplitude Analysis of world data on two photon production of meson pairs. The helicity-zero polarizabilities are rather stable and insensitive to uncertainties in cross-channel exchanges. The need is first to confirm the recent result on $(\alpha_1-\beta_1)$ for the charged pion by COMPASS at CERN to an accuracy of 10\% by measuring the $\gamma\gamma\to\pi^+\pi^-$ cross-section to an uncertainty of ~1\%. Then the same polarizability, but for the $\pi^0$, is fixed to be $(\alpha_1-\beta_1)_{\pi^0}=(-0.9\pm0.2)\times 10^{-4}$ fm$^{3}$. By analyzing the correlation between uncertainties in the meson polarizability and those in $\gamma\gamma$ cross-sections, we suggest experiments need to measure these cross-sections between $\sqrt{s}\simeq 350$ and 600 MeV. The $\pi^0\pi^0$ cross-section then makes the $(\alpha_2-\beta_2)_{\pi^0}$ the easiest helicity-two polarizability to determine.

D¯D meson pair production in antiproton-nucleus collisions

We study the $\bar D D$ (${\bar D}^0 D^0$ and $D^-D^+$) charm meson pair production in antiproton (${\bar p}$) induced reactions on nuclei at beam energies ranging from threshold to several GeV. Our model is based on an effective Lagrangian approach that has only the baryon-meson degrees of freedom and involves the physical hadron masses. The reaction proceeds via the $t$-channel exchanges of $\Lambda_c^+$, $\Sigma_c^+$, and $\Sigma_c^{++}$ baryons in the initial collision of the antiproton with one of the protons of the target nucleus. The medium effects on the exchanged baryons are included by incorporating in the corresponding propagators, the effective charm baryon masses calculated within a quark-meson coupling (QMC) model. The wave functions of the bound proton have been determined within the QMC model as well as in a phenomenological model where they are obtained by solving the Dirac equation with appropriate scalar and vector potentials. The initial- and final-state distortion effects have been approximated by using an eikonal approximation-based procedure. Detailed numerical results are presented for total and double differential cross sections for the ${\bar D}^0 D^0$ and $D^-D^+$ production reactions on $^{16}$O and $^{90}$Zr targets. It is noticed that at ${\bar p}$ beam momenta of interest to the ${\bar P}ANDA$ experiment, medium effects lead to noticeable enhancements in the charm meson production cross sections.

ϒ(6S)and triangle singularity ine+e−→B1(5721)B¯→Zb(10610)π

We discuss the possibility that production of final states with bottomonium and light mesons at the peak $\Upsilon(6S)$ in the $e^+e^-$ annihilation at approximately 11.00 GeV is in fact due to a triangular singularity at the threshold of the heavy meson pair production $B_1(5721) \bar B + c.c.$ through the process $e^+e^- \to B_1(5721) \bar B \to Z_b(10610) \, \pi$. The presence of the hidden-bottom resonance $Z_b(10610)$ then explains the observed enhanced production of the final channels with both ortho- and para- bottomonium states, $\Upsilon(nS) \pi \pi$ and $h_b(kP) \pi \pi$. The discussed mechanism also predicts a distinct pattern for production of hidden-bottom states at the $\Upsilon(6S)$ energy that can be tested by experiment.

P¯pannihilation intoD¯Dmeson pair within an effective Lagrangian model

We study the charmed meson pair (${\bar D}^0 D^0$ and $D^- D^+$) production in ${\bar p}p$ annihilation within an effective Lagrangian model that has only the baryon-meson degrees of freedom and involves the physical hadron masses. The reaction amplitudes include terms corresponding to the t-channel $\Lambda_c^+$, $\Sigma_c^+$ and $\Sigma_c^{++}$ baryon exchanges and the s-channel excitation, propagation and decay of the $\Psi$(3770) resonance into the charmed mesons. The initial- and final-state distortion effects have been accounted for by using a simple eikonal approximation-based procedure in the same way as was done in our previous study of the ${\bar p}p \to {\bar \Lambda}_c^-\Lambda_c^+$ reaction within a similar model. The ${\bar D}^0 D^0$ production reaction is dominated by the $\Lambda_c^+$ baryon exchange process, and the corresponding total cross sections are predicted to be in the range of 0.18 - 0.7 $\mu$b for antiproton beam momenta varying between threshold and 20 $GeV/c$. The $\Psi$(3770) resonance contributions have a large influence on the differential cross sections of the $D^- D^+$ production reaction.

Production of heavy meson pairs in pp̄ collisions within a double handbag approach

We study the pair-production of heavy mesons in proton-antiproton annihilations within a perturbative QCD-motivated framework. In particular we investigate pp̄ → D0 within a double handbag approach, where a hard subprocess factorizes from soft hadronic matrix elements. The soft matrix elements can be parametrized by transition distribution amplitudes, which are off-diagonal in flavor space. The transition distribution amplitudes are modelled as overlaps of light-cone wave functions. We obtain rather robust model results for pp̄ → D0 cross sections, which are expected to be measured at the future detector at GSI-FAIR.

Modelling exclusive meson pair production at hadron colliders

We present a study of the central exclusive production of light meson pairs, concentrating on the region of lower invariant masses of the central system and/or meson transverse momentum, where perturbative QCD cannot be reliably applied. We describe in detail a phenomenological model, using the tools of Regge theory, that may be applied with some success in this regime, and we present the new, publicly available, Dime Monte Carlo (MC) implementation of this for \(\pi \pi \), \(KK\) and \(\rho \rho \) production. The MC implementation includes a fully differential treatment of the survival factor, which in general depends on all kinematic variables, as well as allows for the so far reasonably unconstrained model parameters to be set by the user. We present predictions for the Tevatron and LHC, discuss and estimate the size of the proton-dissociative background, and show how future measurements may further test this Regge-based approach, as well as the soft hadronic model required to calculate the survival factor, in particular in the presence of tagged protons.

Production of Charged Heavy Quarkonium-Like States at the LHC and Tevatron

We study prompt hadroproduction of the charged bottomonium-like states Zb± (10610) and Zb± (10650), and the charged charmonium-like states Zc±(3900) and Zc±(4020), at the Tevatron and the LHC, provided that these states are S-wave hadronic molecules. Using two Monte Carlo event generators, Herwig and Pythia, to simulate the production of heavy meson pairs, we derive an order-of-magnitude estimate of the production rates for these four particles. Our estimates yield a cross section at the nb level for the Zb (10610) and Zb (10650). The results for the Zc (3900) and Zc (4020) are larger by a factor of 20–30. These cross sections are large enough to be observed, and measurements at hadron colliders in the future will supplement the study using electron-positron collisions, and therefore allow to explore the mysterious nature of these exotic states.

Latest results from meson photoproduction at ELSA and MAMI

Photoproduction of mesons plays a key role for the investigation of the excitation spectrum of the nucleon and thus for our understanding of the strong interaction in the non-perturbative regime. In this contribution we discuss recent results from the experiments at the tagged photon beams of the electron accelerators ELSA in Bonn and MAMI in Mainz. They include the measurement of cross sections and (double) polarization observables for single meson production and production of meson pairs off free protons as well as of quasi-free nucleons bound in light nuclei (in particular the deuteron).

Baryon Spectroscopy at ELSA and at MAMI - selected results

Spectroscopy of baryons and their excited states plays a key role for our under- standing of the strong interaction in the non-perturbative regime. Both, in theory and in experiment, large progress has been made during the last few years. The rapid develop- ments in lattice gauge calculations and the application of the Dyson-Schwinger equation to QCD have opened new perspectives for the interpretation of the excitation spectrum of the nucleon. In parallel, large efforts have been undertaken world-wide, and are still running, to investigate excited nucleon states experimentally, in particular with photon- induced production of mesons. In the present contribution we discuss such experimental programs conducted at the tagged photon beams of the electron accelerators ELSA in Bonn and MAMI in Mainz. These programs are diverse. They include the measurement of cross sections, single- and double polarization observables for single meson production and production of meson pairs off free protons as well as of quasi-free nucleons bound in the deuteron (and sometimes other light nuclei).

GAMMA GAMMA PHYSICS AT BELLE

We report a high-statistics measurement of the differential cross section of the process [Formula: see text] with the Belle detector. We study spectroscopy of light-quark resonances and charmonia and test QCD models using the data. We summarize our systematic measurements of different processes of γγ → meson-pair production based on the parameterization of the cross sections and comparison with QCD models. We also report on the measurement of the two-photon π0 transition form factor at Belle.

Probing the perturbative dynamics of exclusive meson pair production

We present the results of a recent novel application of the ‘hard exclusive’ perturbative formalism to the process gg→MM¯ at large angles, where M (M¯) is a light meson (anti-meson). As well as discussing the important theoretical features of the relevant leading-order gg→qq¯(gg)qq¯(gg) 6-parton amplitudes, we also comment on their phenomenological implications. In particular, we consider the central exclusive production of meson pairs at comparatively large transverse momentum k⊥, which is expected to proceed via this mechanism. We show that this leads to various non-trivial predictions for a range of exclusive processes, and that the cross sections for the η′ and η mesons display significant sensitivity to any valence gg component of the meson wavefunctions.

Υ-meson pair production at the LHC

Theoretical predictions for $pp\to 2 \Upsilon+X$ cross section at $\sqrt{s}=8$ TeV for the LHCb and ATLAS kinematical conditions are obtained. A possibility to observe the new hypothetical particles containing two valence $b$-quarks and two valence $\bar{b}$-quarks is discussed.