Bar Pn Research Articles

Exploring the simplest purely baryonic decay processes

Though not considered in general, purely baryonic decays could shed light on the puzzle of the baryon number asymmetry in the universe by means of a better understanding of the baryonic nature of our matter world. As such, they constitute a yet unexplored class of decay processes worth investigating. We propose to search for purely baryonic decay processes at the LHCb experiment. No such type of decay has ever been observed. In particular, we concentrate on the decay $\Lambda_b^0\to p\bar pn$, which is the simplest purely baryonic decay mode, with solely spin-1/2 baryons involved. We predict its decay branching ratio to be ${\cal B}(\Lambda_b^0\to p\bar pn)=(2.0^{+0.3}_{-0.2})\times 10^{-6}$, which is sufficiently large to make the decay mode accessible to LHCb. Our study can be extended to other purely baryonic decays such as $\Lambda_b^0\to p\bar p \Lambda$ and $\Lambda_b^0\to \Lambda\bar \Lambda\Lambda$, as well as to similar decays of anti-triplet $b$-baryons such as $\Xi_b^{0,-}$.

Study of N*(1440) from J/Ψ decays

For $J/\Psi\to\bar pp \pi^0$ and $\bar pp\pi^+\pi^-$, the $\pi^0p$ and $p\pi^+\pi^-$ systems are limited to be pure isospin 1/2 due to isospin conservation. This is a big advantage in studying $N^*$ resonances from $J/\Psi$ decays, compared with $\pi N$ and $\gamma N$ experiments. The process $J/\Psi\to\bar pN^*$ or $p\bar N^*$ provides a new way to probe the internal structure of the $N^*$ resonances. Here we report a quark model calculation for $J/\Psi\to\bar pp$, $pN^*(1440)$ and $\bar N^*N^*$. The implication for the internal structure of $N^*(1440)$ is discussed.

$$N\bar N$$ annihilation at rest into five pions

\(N\bar N\) annihilation at rest into five pions was studied in the ASTERIX spectrometer by stopping antiprotrons from LEAR in a H2 or D2 gas target. In\(\bar pp\) annihilations in H2 leading to the π+π−π+π−π0 final state, the invariant mass spectra of two, three, and four pions show no evidence for any new narrow states. In\(\bar pn \to \pi ^ + \pi ^ - \pi ^ + \pi ^ - \pi ^ - \) the difference of the four pion mass spectra recoiling against a π− and those recoiling against a π+ shows a resonance-like structure (“ζ (1480)”), confirming a previous analysis with a smaller event sample. The ζ (1480) mass is shown to depend strongly on the momentum of the spectator proton. In H2, we searched for the ζ (1480) in the reaction\(\bar pp \to \pi ^0 \zeta \) (1480), ζ (1480) →π+π−π+π− to clarify whether the ζ (1480) is identical to the 4π-mode of thef2 (1520). No evidence for this decay mode was found. A prominent resonance in\(\bar pp\) innihilation into five pions is the ω meson. We find two quasi-two-body intermediate states:\(\bar pp \to \rho \omega \) and\(\bar pp \to \pi ^ \pm b_1^ \mp \) (1235). The mass and width of theb1 (1235) are determined to be: $$M_{bI} = 1225 \pm 5\user2{MeV, }\Gamma _{b1} = 113 \pm 12MeV.$$ (1)

Proton spectrum from $$\bar pd \to 5\pi p$$ at rest and the elementary $$\bar pn$$ annihilation

We calculate the shape of the proton spectator distribution in\(\bar pd \to 5\pi p\) at low proton momenta. The sensitivity to atomicS- andP-state orbits and to the size of elementarys- andp-wave annihilation is discussed. Implications for the elementary annihilation amplitude\(\bar pn \to 5\pi \) subthreshold are presented.

Multi-quark structure ofU(3.1)

We interpret the recently observedU(3.1) mesons with the\(\Lambda \bar p\) + pions decays as the bound state of\(\Lambda ,\bar p\) andX0(1480). TheX0(1480) is a mesonium with\(Q^2 \bar Q^2 \) structures observed in γγ reactions and\(\bar pn\) annihilations. With this interpretation, we can understand its decay modes. Furthermore, we predict the ratio of\(\sigma (\Lambda \bar p\pi ^ + \pi ^ - )/\sigma (\Lambda \bar p\pi ^ + \pi ^ + )\) to be ⋟3.1 for centrally produced events and that the width of\(U^ - (\Lambda \bar p\pi ^ + \pi ^ - )\) to be greater than that of\(U^ + (\Lambda \bar p\pi ^ + \pi ^ + )\). Both predictions seem to be in reasonable accord with the available data. We call for the detection of the\(\Lambda \bar p\pi ^ - \pi ^ - \) mode to verify the present interpretation.

A study of the reaction $$\bar pn \to \pi ^ - \pi ^ - \pi ^ + $$ at centre-of-mass energies between 1.9 and 2.3 GeVat centre-of-mass energies between 1.9 and 2.3 GeV

We have studied the reactions\(\bar pn \to \pi ^ - \pi ^ - \pi ^ + (\pi ^0 )\) at incident momenta between 0.42 and 1.65 GeV/c using two exposures in deuterium-filled bubble chambers. The ratio of the rates for 3π to 4π final states shows a sharp increase at a momentum of about 0.8 GeV/c. A full partial-wave analysis of the 3π final state has been made and complex structure is observed at approximately the same momentum, associated in particular with theJP=3+ final state.

The $$N\bar N$$ and ?? decay modes of baryonium

A non-relativistic quark model is used to calculate the decay widths of baryonium states to\(N\bar N,N\bar \Delta \),\(N\bar N,N\bar \Delta \) and\(\Delta \bar \Delta \). Large widths are found and used to estimate the\(\bar pp\) and\(\bar pn\) elastic cross-sections. The couplings of baryonium states to two identical mesons are also investigated.

Study of the Goldhaber effect in $$\bar pn$$ annihilations between 1.0 and 1.6 GeV/cannihilations between 1.0 and 1.6 GeV/c

Various features of the correlation between pions from the annihilation\(\bar pn \to 3\pi ^ - 2\pi ^ + \) are qualitatively explained by two Monte Carlo simulations including resonance production and Bose-Einstein correlations parametrized with the GGLP or with the Kopylov-Podgoretsky formula. The analysis of theqt2,q0 distribution alone in terms of the Kopylov-Podgoretsky theory indicates that the sources are only partially (∼25%) incoherent: we attribute our previous observation of total incoherence to the use of (+ −) pairs as background, which we criticize and conjecture to be also responsible for the high radiusR we would obtain for the reaction\(\bar pn \to 3\pi ^ - 2\pi ^ + \pi ^0 \), if we used the (+ −) pairs as background.

Charged-particle multiplicity distribution in 100-GeV/c $$\bar pd$$ and π− interactions

We present data on\(\bar pn\) and π− n collisions obtained from an exposure of the 30′' FNAL deuterium filled bubble chamber to a mixed\({{\bar p} \mathord{\left/ {\vphantom {{\bar p} {\pi ^ - }}} \right. \kern-\nulldelimiterspace} {\pi ^ - }}\) beam with a momentum of 100 GeV/c. We find that in 17±2% of the collisions with the antiproton there is an interaction on the spectator while for the collisions with π− mesons the corresponding number is 15±2%. The\(\bar pn\) and π− n multiplicity distributions have average charged multiplicities of 6.46±0.07 and 6.53±0.08 respectively. The average multiplicities for both types of interactions are slightly smaller than those for the corresponding reactions on hydrogen by an amount that is the same as observed at other energies. As an estimate of\(\bar pn\) annihilation we have calculated the difference\(\sigma _n (\bar pn) - \sigma _n (pn)\) for each prong numbern. We find an average multiplicity of 9±1, a value close to that for\(\bar pp\) annihilation at the same energy. combining our data with lower energy\(\bar pn\) annihilation data, we observe that the average negative multiplicity is systematically larger than that for\(\bar pp\) annihilation similar to the difference between neutron and proton target data with other beam projectiles.

Is theS-meson a baryonium state?

If theS-meson is assumed to be a baryonium state composed of an isospin one diquark and antidiquark, it will be produced in\(\bar pp\) reactions as a mixture ofI=0 andI=1 baryonium states. The experimentally observed large ratio of the cross sections of the reactions\(\bar pp \to S \to \bar pp\) and\(\bar pp \to S^0 \to \bar nn\) is then explained on basis of quark additivity and conservation of isospin in thes-channel. The model predicts:\(\sigma (\bar pp \to S^0 \to \bar pp):\sigma (\bar pp \to S^0 \to \bar nn):\sigma (\bar pn \to S^ - \to \bar pn) = 25:1:16\).

Phenomenological analysis of hadron total cross-sections

Total cross-sections of hadron collisions are phenomenologically analysed from several to 300 GeV/c. In the momentum range of 10 to 50 GeV/c, they are found to be represented by a fine straight line\(\sqrt 8 \sigma _T = \sqrt 8 P + R\), when\(\sqrt 8 \sigma _T \) is plotted against\(\sqrt 8 \) (i.e. Lipkin plot), and above 100 GeV/c they clearly deviate from the straight line by [log (s/s0)]∼1.5 universally for all processes of pp,\(\bar pp\), π±p, K±p, pn,\(\bar pn\) and K±n scatterings. The vector- and tensor-reggeon exchange contributions are well separated from the pomeron exchange one by the use of the difference of theirs-dependences, and the various relations about σT's due to the vector and the tensor exchange are examined. These reg geon exchange contributions are also checked by experimental quantities, which are connected with a real part of amplitudes, such as the real-to-imaginary ratio of forward elastic amplitudes and the forward differential cross-sections of two-body processes.

$$\bar pn$$ annihilation into π−π0 between 1.0 and 1.6 GeV/c

The annihilation\(\bar pn\) → π−π0 has been studied in a D2 bubble chamber at several beam momenta between 1.0 and 1.6 GeV/c. The cross-section has been measured to be (110±15) μb at an average beam momentum of 1.3 GeV/c. The study of the angular distribution shows the presence of anL = 3 wave.

Experimental study of $$\bar pn$$ annihilations between 1.0 and 1.6 GeV/cannihilations between 1.0 and 1.6 GeV/c

\(\bar pn\) annihilations into three and five charged mesons with or without neutrals have been studied in order to measure the contributions of the various final states to the 5.5 mb enhancement discovered in the totalI=1\(\bar {\mathcal{N}}\mathcal{N}\) cross-section at 2190 MeV total c.m. energy. Fits have been performed to the cross-sections as functions of the incident-\(\bar p\); momentum with background plus a resonant term. The fits show enhancements in several final states, the statistically most significant one being in the reaction\(\bar pn \to \pi ^ + 2\pi ^ - \) ((0.51 ± 0.13) mb over a background of 1.4 mb). Less significant effects are also found in all other pionic annihilation final states regardless of theirG-parity.

Cross-sections for resonance production in $$\bar pn$$ annihilation around 2190 MeV centre-of-mass energyannihilation around 2190 MeV centre-of-mass energy

Cross-sections for resonance production in $$\bar pn$$ annihilation around 2190 MeV centre-of-mass energyannihilation around 2190 MeV centre-of-mass energy

Inclusive analysis of $$\bar pn$$ annihilations between 1.0 and 1.6 GeV/cannihilations between 1.0 and 1.6 GeV/c

Inclusive analysis of $$\bar pn$$ annihilations between 1.0 and 1.6 GeV/cannihilations between 1.0 and 1.6 GeV/c

Properties of the ϕ-meson as observed in $$\bar pn$$ annihilations at restannihilations at rest

Data on\(\bar pn\) annihilation at rest into π−ϕ are presented. The production frequency, mass, width and decay angular distribution of the ϕ-meson are measured.

Experimental systematics of particle lifetimes and widths

By comparing the lifetimes of the metastable (τ>10−17 s) elementary particles with one another, we find experimentally that these lifetimes occur both as ratios of 2 and as ratios of α=e2/ħc, with supposedly dissimilar particles grouped together, and with no experimental counterexamples. When short-lived (τ ∼ 10−22 s) meson and baryon resonances are studied, it is found that the width is a key identification symbol. Grouping together resonances that have similar (narrow) widths, we obtain very accurate linear mass intervals. This mapping can be extended to include essentially all of the observed narrow-width meson and baryon resonances in a comprehensive pattern. These results suggest a weak-binding-energy approach to elementary-particle structure. This is the same conclusion that emerges from a broad overview of the successes of the quark model. The empirical level spacings point to the existence of two basic mass quanta, a spinless quantum μ ⋍ 70 MeV and a spin-1/2 quantumS ⋍ 330 MeV. Electromagnetic properties of nucleons also indicate the existence of the 330 MeV mass quantum. In reconciling a 330 MeV mass quantumS with a 939 MeV nucleon mass and a 1795 MeV\(\bar pn\) bound-state mass, we are led to the Fermi and Yang formulation of the nucleon rather than to the formulation of Gell-Mann and Zweig. The observed spectrum of narrow-width meson and baryon resonances can be reproduced by forming suitable combinations of the quanta μ andS. Broad-width resonances are interpreted as rotational excitations. Basis states 3 ≡ 3μ and 4 ≡ 4μ, initially selected to account for observed level spacings in hyperon resonances, are shown to have significance with respect to strangeness quantum numbers and with respect to basic characteristics of baryon and meson resonances. These basis states can also be used to account phenomenologically for the observed factors of 2 and α in the lifetimes of the metastable resonances. The predictive power of the present linear systematics is illustrated in a «meson excitation tower» for narrow-width resonances. First published in 1970, this excitation tower is shown with separate groupings for the resonances that were identified in 1970 and for the resonances that have subsequently been identified.

Dual, crossing-symmetric representations with finite-width resonances and Regge asymptotic behaviour

A class of dual, crossing-symmetric representations having finite-width resonances and Regge asymptotic behaviour for all physi cally attainable regions of the crossed-channel variables is obtained. One of these representations is examined in detail. A number of physical applications are also presented including an explanation of the Dalitzplot distribution for\(\bar pn \to 3\pi \), the behaviour of the most important ππ phase shifts and a determination of the ππS-wave scattering lengths.

The $$\bar pn$$ bound state and the mass of the nucleon quarkbound state and the mass of the nucleon quark

The $$\bar pn$$ bound state and the mass of the nucleon quarkbound state and the mass of the nucleon quark

Annihilations $$\bar pn$$ at rest into final states containing K-mesons 0216n at rest into final states containing K-mesons 0216

We have analysed 2.22·105\(\bar pn\) annihilations at rest into final states with two K-mesons and pions. The annihilation rates into three-body (\(K\bar K\pi \)) and four-body (\(K\bar K\pi \pi \)) final states are found to be comparable, whereas the rates for the five-body states (\(K\bar K\pi \pi \pi \)) are one order of magnitude lower, with the exception of the rate for the quasi-three-body state K−K10ω. No signal for the production of the Kππ resonance C or for the\(K\bar K\pi \) resonance E is observed. These results are compatible with the production of C and E mesons from\(\bar pp\) annihilations at rest if the 0216 isospin of C is 1/2 and that of E is 0, but not ifI(C)=3/2 andI(E)=1.