Production Of Charged Pion Pairs Research Articles

Recent ATLAS results on forward physics and diffraction

The ATLAS detector at the LHC is equipped with dedicated systems designed for the detection of forward protons produced in diffractive and photon-induced processes. These detectors significantly extend the ATLAS physics reach. Recent measurements performed using these forward proton detectors: elastic proton-proton scattering, exclusive charged pion-pair production, and two-photon production of lepton pairs are presented.

Inclusive production of charged pion pairs in proton-antiproton collisions

In this study, we have considered the contribution of the higher-twist (HT) effects of the subprocesses to inclusive pion pair production cross section in the high energy proton-antiproton collisions by using various pion distribution amplitudes (DAs) within the frozen coupling constant approach and compared them with the leading-twist contributions. The feature of the HT effects may help the theoretical interpretation of the future PANDA experiment. The dependencies of the HT contribution on the transverse momentum $p_T$, the center of mass energy $\sqrt s$, and the variable $x_T$ are discussed numerically with special emphasis put on DAs. Moreover, the obtained analytical and numerical results for the differential cross section of the pion pair production are compared with the elastic backward scattering of the pion on the proton. We show that the main contribution to the inclusive cross section comes from the HT direct production process via gluon-gluon fusion. Also, it is strongly dependent on the pion DAs, momentum cut-off parameter $\triangle p$ and $<{q_{T}^2}>$ which is the mean square of the intrinsic momentum of either initial parton.

Exploring universality of transversity in proton-proton collisions

We consider the azimuthal correlations of charged hadron pairs with large total transverse momentum and small relative momentum, produced in proton-proton collisions with one transversely polarized proton. One of these correlations directly probes the chiral-odd transversity parton distribution in connection with a chiral-odd interference fragmentation function. We present predictions for this observable based on previous extractions of transversity (from charged pion pair production in semi-inclusive deep-inelastic scattering) and of the interference fragmentation function (from the production of back-to-back charged pion pairs in electron-positron annihilations). All analyses are performed in the framework of collinear factorization. We compare our predictions to the recent data on proton-proton collisions released by the STAR collaboration at RHIC, and we find them reasonably compatible. This comparison confirms for the first time the predicted role of transversity in proton-proton collisions and it allows to test its universality.

Observation of Transverse Spin-Dependent Azimuthal Correlations of Charged Pion Pairs inp↑+pats=200 GeV

We report the observation of transverse polarization-dependent azimuthal correlations in charged pion pair production with the STAR experiment in p^{↑}+p collisions at RHIC. These correlations directly probe quark transversity distributions. We measure signals in excess of 5 standard deviations at high transverse momenta, at high pseudorapidities η>0.5, and for pair masses around the mass of the ρ meson. This is the first direct transversity measurement in p+p collisions.

FRAGMENTATION FUNCTIONS AT BABAR

Inclusive hadron production cross section in e+e- annihilation shed light on fundamental questions of hadronization and fragmentation processes. We present measurements of inclusive spectra of charged pions, kaons, and protons (antiprotons) produced in e+e- collisions at the center-of-mass energy of 10.54 GeV, and tests of QCD predictions and hadronization models. We also report the results on the measurement of the azimuthal modulation induced by the Collins effect in inclusive production of charged pion pairs e+e- → ππX, where the two pions are produced in opposite hemispheres. These data allows the determination of the polarized Collins fragmentation functions.

Radiative corrections to the charged pion-pair production process π − γ → π + π − π − at low energies

We calculate the one-photon loop radiative corrections to the charged pion-pair production process $\pi^-\gamma\to\pi^+\pi^-\pi^-$. In the low-energy region this reaction is governed by the chiral pion-pion interaction. The pertinent set of 42 irreducible photon-loop diagrams is calculated by using the package FeynCalc. Electromagnetic counterterms with two independent low-energy constants $\widehat k_1$ and $\widehat k_2$ are included in order to remove the ultraviolet divergences generated by the photon-loops. Infrared finiteness of the virtual radiative corrections is achieved by including soft photon radiation below an energy cut-off $\lambda$. The purely electromagnetic interaction of the charged pions mediated by one-photon exchange is also taken into account. The radiative corrections to the total cross section (in the isospin limit) vary between $+10\%$ close to threshold and about $-1\%$ at a center-of-mass energy of $7m_\pi$. The largest contribution comes from the simple one-photon exchange. Radiative corrections to the $\pi^+\pi^-$ and $\pi^-\pi^-$ mass spectra are studied as well. The Coulomb singularity of the final-state interaction produces a kink in the dipion mass spectra. The virtual radiative corrections to elastic $\pi^-\pi^-$ scattering are derived additionally.

Phenomenological model of multiphoto-production of charged pion pairs on the proton

The production of charged pion pairs via multiphoton absorption from an intense X-ray laser wave colliding with an ultrarelativistic proton beam is studied. Our calculations include the contributions from both the electromagnetic and hadronic interactions where the latter are described approximately by a phenomenological meson-exchange potential. Order-of-magnitude estimates for π+π− production on the proton by two- and three-photon absorption from the high-frequency laser field are obtained and compared with the corresponding rates for μ+μ− pair creation.

Chiral corrections to [formula omitted] processes at low energies

We calculate in chiral perturbation theory the double-pion photoproduction processes π − γ → π − π 0 π 0 and π − γ → π + π − π − at low energies. At leading order these reactions are governed by the chiral pion-pion interaction. The next-to-leading order corrections arise from pion-loop diagrams and chiral-invariant counterterms involving the low-energy constants ℓ ¯ 1 , ℓ ¯ 2 , ℓ ¯ 3 and ℓ ¯ 4 . The pertinent production amplitudes A 1 and A 2 depending on five kinematical variables are given in closed analytical form. We find that the total cross section for neutral pion-pair production π − γ → π − π 0 π 0 gets enhanced in the region s < 7 m π by a factor 1.5 – 1.8 by the next-to-leading order corrections. In contrast to this behavior the total cross section for charged pion-pair production π − γ → π + π − π − remains almost unchanged in the region s < 6 m π in comparison to its tree-level result. Although the dynamics of the pion-pair production reactions is much richer, this observed pattern can be understood from the different influence of the chiral corrections on the pion-pion final state interaction ( π + π − → π 0 π 0 versus π − π − → π − π − ). We present also results for the complete set of two-pion invariant mass spectra. The predictions of chiral perturbation theory for the π − γ → 3 π processes can be tested by the COMPASS experiment which uses Primakoff scattering of high-energy pions in the Coulomb field of a heavy nucleus to extract cross sections for π − γ reactions with various final states.

Cross-sections for low-energy πgamma; reactions

We review the cross-sections for low-energy π-γ reactions in the framework of chiral perturbation theory. Charged-pion Compton scattering, π-γ → π-γ , is considered up to one-loop order where the pion's internal structure enters through the difference of the electric and magnetic pion polarizability, απ - βπ . The ongoing COMPASS experiment aims at measuring this important structure constant with high statistics using the Primakoff effect. In the same way, the reaction π-γ → π-π0 serves as a test of the QCD chiral anomaly (via the γ3π -coupling constant F γ3π . Furthermore, we calculate the total cross-sections for neutral and charged pion-pair production, π-γ → π-π0π0 and π-γ → π-π+π- , which are governed by the chiral ππ -interaction. Finally, we investigate the radiative (correction) process π-γ → π-γγ and calculate the corresponding two-photon mass spectrum. This information will be useful for analyzing the π- Primakoff scattering events with photons in the final state.

Erratum: Pion pair production ine+e−annihilation [Phys. Rev. D73, 094021 (2006)

We present an analysis of the process $e^+e^-\to V^*\to\pi\pi\gamma$, where $V=\gamma$ or $V=Z$ boson, in the kinematical region where $\sqrt{s}$, the c.m. energy of the $e^+e^-$ pair, is large but much below the $Z$-pole. The subprocess $V^*\to \pi\pi \gamma$ can be described by the convolution of the hard scattering coefficient $V\to q \bar{q} \gamma$ and the general distribution amplitude of two pions $q\bar{q}\to \pi \pi$. In the case of neutral pion production, $V=\gamma$ is the dominant process, which can therefore be used to access the GPAs of the pion, especially their $C$-even parts. The $\gamma Z$ interference term provides an alternative approach to extract the weak mixing angle $\sin \theta_W$ through measuring the helicity asymmetry in the process $e^+e^-\to \pi^0\pi^0\gamma$. In the case of charged pion pair production, the Bremsstrahlung process dominates and its interference with $e^+e^-\to \gamma^ \star \to \pi^+\pi^-\gamma$ can be applied to study the process $\gamma^ \star \to \pi\pi\gamma$ at the amplitude level.

Generalized distribution amplitudes at theZpole

We investigate the exclusive two-pion production at the Z-pole through the process e{sup +}e{sup -}{yields}Z{yields}{pi}{pi}{gamma}. In the kinematical region where the invariant mass of the two pions is much smaller than the mass of the Z boson, the process can be factorized into the convolution of a hard coefficient and a soft matrix element, the generalized distribution amplitude of two pions. We calculate the differential cross-section of charged pion pair production and neutral pion pair production and find the former to be much larger than the latter. We show that combining the measurements of charged pion pair production and neutral pion pair production at the Z-pole provides a convenient approach to access the C-odd part of the 2{pi} distribution amplitudes.

Cross sections of muon and charged pion pair production in electron-positron annihilation near the threshold

The processes of muon (tau) and charged pion pair production at electron-positron annihilation with $\mathcal{O}(\ensuremath{\alpha})$ radiative corrections are considered. The calculation results are presented assuming the energies of final particles (center of mass system implied) to be not far significantly from threshold production. The invariant mass distributions for the muon (tau) and pion pairs are obtained both for the initial and final state radiation. Some analytical calculations are illustrated numerically. The pions were assumed to be pointlike objects and scalar QED was applied for calculation. The QED radiative corrections related to the final state radiation, in addition to the well-known Coulomb factor, are treated near threshold region exactly.

Isospin resolved double pion production in the reaction p+d→ 3He+2 π

Neutral and charged two-pion production in p+d→ 3He+2 π reactions has been studied at CELSIUS at a proton beam energy of 477 MeV. The total cross section for double pion production is 0.22±0.03 μ b. The ratio of the cross sections for the production of charged pion pairs with isospin T=1 and T=0 was determined to be σ( π + π −; T=1)/ σ( π + π −; T=0)=1.4±0.4.

Pion polarizabilities and charged pion-pair production to two loops

We evaluate the electric and magnetic polarizabilities of charged pions in the framework of chiral perturbation theory at next-to-leading order. This requires a two-loop evaluation of the Compton amplitude near threshold. In addition we discuss the amplitude and cross section of pion-pair production for charged pions to two loops. The three new coupling constants that enter the result at this order in the low-energy expansion are estimated via resonance saturation. The predictions are compared with available experimental information.