Experimental test of the PCAC hypothesis in the reactionsv μ p→ μ− pπ+ and \bar v_\mu p \to \mu ^ + p\pi ^ - in theΔ(1232) region

Data on the reactionsv _μ p→ μ^? pπ⁺ and \( \bar v_\mu p \to \mu ^ + p\pi ^ - \) in the Δ(1232) region are presented and a test of the PCAC hypothesis, using a modified version of the Adler model, is performed. The analysis is based on 1081 events in the neutrino and on 180 events in the antineutrino reaction, obtained in a bubble chamber experiment with BEBC at CERN. The experimental cross-sections for an invariant hadronic massW<1.4 GeV and an (anti-)neutrino energyE _v ^L >10 GeV are determined to be (0.628±0.059)·10^?38 cm² for the neutrino and (0.168±0.023)·10^?38 cm² for the antineutrino reaction. TheQ ² andW distributions, the density matrix elements of the Δ resonance, and moments of the pion angular distribution are discussed. The data are found to be in good agreement with the Adler model in theQ ² region below 1 GeV². A maximum likelihood fit for the axial massm _A in the axial-vector form factor yields a value ofm _A =1.31±0.12 GeV. At lowQ ² the data confirm the PCAC hypothesis and the discrepancy, formerly observed between the experimental and theoretical cross-sections forv _μ p→ μ^? pπ⁺ at low momentum transfers (Q ²?0.2 GeV²), is understood as being due to inadequate pion ‘off-mass-shell’ corrections.     

Thepp —p \bar p difference and the behaviour of phases at high energies

Predictions based on axiomatic field theory are obtained from the assumption that the difference between thepp and thep \(\bar p\) elastic scattering observed at the CERN ISR is caused by an “odderon-like” term in the scattering amplitude. A wide class of amplitudes violating the Pomeranchuk relation is considered. Contrary to common opinion, severe restrictions on the high-energy behaviour of the phase of the crossing-odd amplitude are shown to follow for the whole class. Less pronounced but still visible consequences follow for the phases of thepp andp \(\bar p\) amplitudes separately. We derive the corresponding high-energy bounds and correlations using the general frame of analyticity, crossing symmetry and unitarity. As a special case, the maximal odderon amplitude is discussed.     

Rescattering mechanisms for\bar p p→φX and the OZI rule re-examined

The OZI suppressed reactions¯pp and¯pp are reanalyzed, evaluating the unitarity contributions from two meson intermediate states with full spin, partially correcting earlier results. An estimate for¯pp without spin is also given. Together with earlier VDM results for¯pp the conservative estimates reported here easily explain the observed rates for all the meson channels considered.It is a pleasure to thank the following colleagues for numerous discussions: C. Amsler, D. Buzatu, O. Gortchakov, F. Lev, J.-M. Richard and M.G. Sapozhnikov.     

A study of theKπ system in the reactionK^ - p \to \bar K^0 \pi ^ - p at 8.25 GeV/c

The production ofK ^* resonances has been studied in the reaction \(K^ - p \to \bar K^0 \pi ^ - p\) at 8.25 GeV/c. The data comes from a high statistics bubble chamber experiment (180 events/μb). Masses, widths and production cross-sections have been determined for the first threeK ^*'s. The contributions from natural and unnatural parity exchange have been obtained for theK ^*(890) and theK ^*(1420). A partial wave analysis of theK π system from threshold to 1.9 GeV provides evidence for a 0⁺ enhancement near 1.4 GeV which could be interpreted as the κ(1350).     

Scattering length expansion of the coupled channels\bar pp \to \bar \Lambda \Lambda and\bar \Lambda \Lambda \to \bar \Lambda \Lambda near theΛ production threshold

We fit the scattering lengths in the triplets-,p- andd- waves for the two channels \(\bar pp \to \bar \Lambda \Lambda\) and \(\bar \Lambda \Lambda \to \bar \Lambda \Lambda\) near theΛ production threshold to the differential cross section \(\frac{{d\sigma }}{{d\Omega }}(\bar pp \to \bar \Lambda \Lambda )\) and to the polarization P.     

No Sommerfeld resummation factor in e^ + e^ - \to p\bar p?

The Sommerfeld rescattering formula is compared to the $e^ + e^ - \to p\bar p$ BABAR data at threshold and above. While there is the expected Coulomb enhancement at threshold, two unexpected outcomes have been found: the modulus of the proton form factor is normalized to one at threshold, i.e. |G ^p (4M _p ² )| = 1, as a pointlike fermion, and, moreover, data show that the resummation factor in the Sommerfeld formula is not needed. Other e ⁺ e ^? ?? baryon-antibaryon cross-sections show similar behavior near threshold.     

Spin density matrix of the $$\omega $$ in the reaction $${\bar{p}p}\,\rightarrow \,\omega \pi ^0$$p¯p→π0

The spin density matrix of the \(\omega \) has been determined for the reaction \({\bar{p}p}\,\rightarrow \,\omega \pi ^0\) with unpolarized in-flight data measured by the Crystal Barrel LEAR experiment at CERN. The two main decay modes of the \(\omega \) into \(\pi ^0 \gamma \) and \(\pi ^+ \pi ^- \pi ^0\) have been separately analyzed for various \({\bar{p}}\)momenta between 600 and 1940 MeV/c. The results obtained with the usual method by extracting the matrix elements via the \(\omega \) decay angular distributions and with the more sophisticated method via a full partial wave analysis are in good agreement. A strong spin alignment of the \(\omega \) is clearly visible in this energy regime and all individual spin density matrix elements exhibit an oscillatory dependence on the production angle. In addition, the largest contributing orbital angular momentum of the \({\bar{p}p~}\)system has been identified for the different beam momenta. It increases from \(L^{max}_{{\bar{p}p~}}\) \(=\) 2 at 600 MeV/c to \(L^{max}_{{\bar{p}p~}}\) \(=\) 5 at 1940 MeV/c.     

Experimental determination of the branching ratiosp\bar p \to 2\pi ^0 ,\pi ^0 \gamma,and 2γ at rest

The branching ratios of \(p\bar p\) annihilations into the neutral final states 2π⁰, π⁰γ, and 2γ are measured by stopping antiprotons in liquid hydrogen. They are \(B_{2\pi ^0 } = \left( {2.06 \pm 0.14} \right) \times 10^{ - 4} \) , \(B_{\pi ^0 \gamma } = \left( {1.74 \pm 0.22} \right) \times 10^{ - 5} \) , andB _γγ<1.7×10^?6 (95% c.l.).     

Leading-particle effects in the spectra of Λc and \bar \Lambda _c produced in Σ− p, pp, and π − p interactions

The spectra of leading and nonleading charmed baryons (Λ_c and $\bar \Lambda _c$ ) and the asymmetries between these spectra measured in Σ^? A, π ^? A, and pA collisions at p _L =600 GeV/c in the E781 experiment are simultaneously described within the framework of the Quark-Gluon String Model (QGSM). It is shown that the charmed baryon spectra can be fitted by QGSM curves calculated with the parameter of diquark fragmentation, $a_f^{\Lambda _c } = 0.006$ . It was found in this experiment that the asymmetry between the spectra of Λ_c and $\bar \Lambda _c$ in π ^? A collisions is of nonzero value. It might be described in our model only assuming that the string junction is transferred from target proton into the kinematical region of pion projectile fragmentation.     

\bar K K and\bar p p off-shell annihilations fromK − +p → Λ + pions at 4.2 GeV/c

We have investigated the properties of off-shell \(\bar K\) K and \(\bar p\) p annihilations into pions by using the interactionsK ^? p → Λ + pions at 4.2 GeV/c. The properties considered are multiplicities, hadronic temperatures and angular distributions. These properties appear more akin to those of \(\bar p\) p annihilations in flight than to that of \(\bar p\) p annihilation at rest despite the fact that the annihilation energy is small.     

\bar \Lambda Production inK − p interactions at 32 GeV/c

Antilambda production is studied inK ^? p interactions at 32 GeV/c. Both total and differential cross sections are presented. The inclusive \(\bar \Lambda \) production cross section amounts to 109±7 μb. A remarkable energy dependence is observed, σ( \(\bar \Lambda \) ) increasing by a factor of four between 14.3 and 32 GeV/c. Thep _⊥ ² distribution exhibits an exponential fall-off with a slope of 3.3±0.2 (GeV/c)^?2. Most of the \(\bar \Lambda \) 's are emitted in the forward hemisphere. The invariantx distribution increases between 14.3 and 32 GeV/c. Data are presented for \(\bar \Lambda \) production inK ^- p→Λ \(\bar \Lambda \) +X K ^- p→ \(\bar \Lambda \) K ⁿ +X, andK ^- p→ \(\bar \Lambda \) p+X.     

A partial wave analysis of thef′ region in the reactionK^ - p \to K\bar K\Lambda ^0 at 8.25 GeV/c

A partial wave analysis of theK \(\bar K\) system produced by 8.25 GeV/cK ^? mesons in the reaction \(K^ - p \to K\bar K\Lambda ^{ 0} \) has been performed, taking into account the information provided by the Λ⁰ decay. Thef′ region is dominated byD ₀ ^(?) andD ₁ ⁽⁺⁾ waves. We see no evidence for the production of new 0⁺⁺ states in the mass region 1.05 to 1.75 GeV.     

Some comments on the branching ratios for \bar n p annihilation into ππ, K \bar K, and πη channels

We present some remarks on the $\bar n$ p partial branching ratios in flight at low momenta of antineutrons measured by the OBELIX Collaboration. A comparison is made to the known branching ratios from the p $\bar p$ -atomic states. The branching ratio for the reaction $\bar n$ p → π⁺π⁰ is found to be suppressed in comparison to what follows from the p $\bar p$ data. It is also shown that there is no so-called dynamical I=0 amplitude suppression for the process N $\bar N$ → K $\bar K$ .     

φ meson production fromp\bar p collisions at\sqrt s = 1.8 TeV

Fermilab experiment E735 located at the CO intersection region of the $\sqrt s = 1.8$ TeV $p\bar p$ collider analysed over 900 Φ→K ⁺ K ^? events. Measured were the transverse momentum spectrum, the correlation between the average transverse momentum <pt> and the charged particle multiphcityN _c , as well as the probability of Φ production per charged track,N _Φ /N _c , versusN _c . We have also made an estinate of the total inclusive cross section for Φ mesons, $\sigma (p\bar p \to \phi X) = 7.3 \pm 2.2 mb$ .     

InclusiveK s 0 , Λ and\bar \Lambda-production in\bar p p-interactions at 22.4 GeV/c

InclusiveK _s ⁰ , Λ and \(\bar \Lambda\) -production has been investigated in \(\bar p\) p-interactions at 22.4 GeV/c. Total and topological cross sections and multiplicity characteristics of neutral strange particles have been determined. DifferentialK _s ⁰ and Λ-cross sections and also characteristics of annihilation processes with neutral kaon production have been studied. It is shown that the difference of the shapes of the invariantx-distributions forK-mesons in the fragmentation region for annihilation and non-annihilation processes can be described in the framework of the “dual” valon model, but not by considering the current (undressed) quark recombination mechanisms only. The polarization of Λ and \(\bar \Lambda\) -hyperons has been measured.     

Charged particle and π0 multiplicity distributions and the annihilation properties of\bar p p interactions at 70 GeV/c

The elastic and inelastic \(\bar p\) p cross sections at 70 GeV/c have been determined in an experiment performed at CERN using BEBC equipped with a TST. The topological cross sections were measured and the moments of the inelastic multiplicity distribution are 〈n _c 〉=6.16±0.09, 〈n _c 〉/D=2.04±0.05 andf ₂ ^cc =2.97±0.03. The average number of Dalitz pairs per inelastic event is (3.12±0.09)×10^?2. Assuming that these all arise from π⁰ decay the average π⁰ multiplicity is \(\langle n_{\pi ^0 } \rangle = 2.71 \pm 0.14\) . The \(\bar p\) p?pp cross section differences lead to an annihilation cross section σ _A = 4.42±0.41 mb and the moments of the annihilation multiplicty distribution are 〈n _A 〉=8.0±0.3, 〈n _A 〉/D=2.5±0.2 andf ₂ ^A?? =?1.4±0.3. An independent check of σ _A was made by investigating fast forward charged and neutral secondary interactions in the TST and in the surrounding neon-hydrogen mixture, and gives a value σ _A = 5.0±1.6 mb. The ratio of fast \(\bar n\) to \(\bar p\) production in non-annihilation interactions at 70 GeV/c is found to be 0.45±0.11.     

Two photon exchange contributions to elastic $\vec{e}+p\rightarrow e+\vec{p}$ process in the non-local field formalism

We compute the two photon exchange contributions to elastic scattering of polarized electrons from target protons. We use a non-local field theory formalism for this calculation. The formalism maintains gauge invariance and provides a systematic procedure for making this calculation. The results depend on one unknown parameter, . We compute the two photon exchange correction to the ratio of electric to magnetic form factors extracted using polarization transfer experiments. The correction is found to be small if . However, for larger values of , the correction can be quite significant. The correction to the polarization transfer results goes in the right direction to explain their difference with the ratio measured by the Rosenbluth separation method. We find that the difference between the two experimental results can be explained for a wide range of values of the parameter . We also find that the corrections due to two photon exchange depend on the photon longitudinal polarization ε. Hence, we predict an ε dependence of the form factor ratio extracted using the polarization transfer technique. Finally, we obtain a limit on by requiring that the non-linearity in ε dependence of the unpolarized reduced cross section is within experimental errors.     

Nonlinear coupling between the mean polarizability \bar \gamma of molecules and the local-field anisotropy in a liquid crystal and change of the “ \bar \gamma = const” paradigm

The nonlinear relation between the mean effective polarizability $ \bar \gamma $ of molecules and the local-field anisotropy in uniaxial liquid crystals is demonstrated. Stringent constraints on $ \bar \gamma $ from below, indicative of the dependence of $ \bar \gamma $ on the liquid-crystal state and the change of the “ $ \bar \gamma $ = const” paradigm, are established. The theoretical results are confirmed by experiments with nematic liquid crystals having high and low birefringences.     

Experimental study of the inclusiveη-spectrum fromp \bar p annihilations at rest in liquid hydrogen

The inclusive η-momentum spectrum from \(\bar p\) annihilations at rest in liquid hydrogen was measured at LEAR. Branching ratios were obtained for $$\begin{gathered} p\bar p \to \eta \omega \left( {1.04_{ - 0.10}^{ + 0.09} } \right)\% ,\eta \rho ^0 \left( {0.53_{ - 0.08}^{ + 0.20} } \right)\% , \hfill \\ \pi a_2 \left( {8.49_{ - 1.10}^{ + 1.05} } \right)\% ,\eta \pi ^0 \left( {1.33 \pm 0.27} \right) \times 10^{ - 4} , \hfill \\ \end{gathered} $$ , and ηη(8.1±3.1)×10^?5. An upper limit for \(p\bar p \to \eta \eta '\) of 1.8×10^?4 at 95% CL was found. The ratio of the branching ratios is BR(η?)/BR(ηω)=0.51 _?0.06 ^+0.20 . For the ratio of branching ratios into two pseudoscalar mesons, we have BR(ηπ⁰)/BR(π⁰π⁰)=0.65±0.14, BR(ηη)/BR(π⁰π⁰), BR(η η ^′ )/BR(π⁰π⁰) at 95% CL, and BR(ηη)/BR(ηπ⁰).