Strong Final State Interactions Research Articles

Bounding the penguin effects in determinations ofαfromB0(t)→π+π−

In the absence of the QCD penguin contributions a measurement of the time-dependent asymmetry in the decay $B^0(t)\to \pi^+\pi^-$ gives directly the weak angle $\alpha$. Several bounds have been proposed in the literature on the magnitude of the penguin effects on this determination, the prototype of which is the isospin bound of Grossman and Quinn. It is pointed out that large strong final state interactions could cause these bounds to overestimate the real penguin effect. A new flavor SU(3) bound is proposed, requiring only the charge-averaged branching ratios for $B^0\to \pi^+\pi^-$ and $B_s\to K^+K^-$, which exactly takes into account all relevant amplitudes and electroweak penguin effects. This bound on the penguin-induced error on the determination of the weak phase $\alpha$ holds even without a knowledge of the direct CP asymmetry in the $\pi^+\pi^-$ channel.

Comparison ofΛandΣ0Production near Threshold in Proton-Proton Collisions

Threshold measurements of the associated strangeness production reactions pp→pK+Λ and pp→pK+Σ0 are presented. The most remarkable feature of the data is that at the same excess energy the total cross section for the Σ0 production appears to be about a factor of 28−9+6 smaller than for the Λ particle. It is concluded that strong Σ0p final state interactions, and in particular the ΣN→Λp conversion reaction, are the likely cause of the depletion in the Σ signal. This hypothesis is in line with other experimental evidence in the literature.Received 4 November 1998DOI:https://doi.org/10.1103/PhysRevLett.83.682©1999 American Physical Society

Near-Threshold Production ofωMesons in thepp→ppωReaction

The total cross section for $\ensuremath{\omega}$ production in the $\mathrm{pp}\ensuremath{\rightarrow}\mathrm{pp}\ensuremath{\omega}$ reaction has been measured at 5 c.m. excess energies from 3.8 to 30 MeV. The energy dependence is easily understood in terms of a strong proton-proton final state interaction combined with a smearing over the width of the state. The ratio of near-threshold $\ensuremath{\varphi}$ and $\ensuremath{\omega}$ production is, to within relatively large uncertainties, consistent with the predictions of a simple one-pion-exchange model. The degree of violation of the Okubo-Zweig-Iizuka rule is similar to that found in the ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}n\ensuremath{\omega}/\ensuremath{\varphi}$ reactions.

Recent results from the CELSIUS/WASA collaboration on meson production in proton-hydrogen collisions

Different π and η meson production channels in proton-proton and proton-deuteron collisions have been studied at the CELSIUS storage ring using an internal cluster-jet target setup [1]. During the last years more emphasis has been put on ππ and all but one of the possible channels in proton-proton collisions have been studied. Recently also the quasi-free p+nd+η reaction cross section has been measured at threshold using 1295 MeV protons and target deuterium. Excess energies from threshold to 10 MeV in the center of mass of the final dη system were covered. Approaching the threshold the cross section is enhanced compared to phase-space expectations. This behaviour is typical for a strong final-state interaction.

ϱ-meson production and decay in proton-nucleus collision

We analyze the production of ϱ-mesons in p + A reactions including both the production by proton-nucleon as well as pion-nucleon collisions within a coupled channel transport approach. The final state interactions of the ϱ-meson with nucleons are evaluated from a resonance model which allows to extract elastic and inelastic cross sections. We include the latter final state interactions, the ϱ-meson decay into two pions as well as the final pion-nucleon interactions within the transport approach. We find the invariant mass distribution of pion pairs to be sensitive to the ϱ-meson properties in the nuclear medium. However, due to the strong final state interactions of pions only light targets like 12C might be suited to extract a ϱ-signal from the uncorrelated two pion background which carries information about the in-medium properties of the ϱ-meson.

Energy spectrum of leptons from e+e− → W+W− in the presence of strong WL+WL− interaction

The energy spectrum of leptons, produced by the decay of one or both W's in the reaction e + e − → W + W −, is a significant probe of the helicity structure of this process. We calculate the energy spectrum dσ dE of a single decay lepton, as well as the two-dimensional energy distribution dσ dE +dE − . We then consider a possible strong final state interaction in the longitudinally polarized state of the W + W − system, parametrized by a ϱ-like resonance in the region of 1.8 TeV. We show that such a resonance produces measureble effects in the lepton spectrum at s =500 GeV. The results are compared with those obtained from a non-resonant interaction approximated by a phase factor e iδ in the l = 1 part of the W L + W L − amplitude.

Coulomb and strong interactions for Bose-Einstein correlations

We present an analytical formula for the Bose-Einstein correlations (BEC) which includes effects of both Coulomb and strong final state interactions (FSI). It was obtained by using Coulomb wave function together with the scattering partial wave amplitude of the strong interactions describing data on thes-wave phase shift. We have proved numerically that this method is equivalent to solving Schrödinger equation with Coulomb and thes-wave strong interaction potentials. As an application we have analysed, using our formula which includes the degree of coherence and the long range correlation, the data fore + e − annihilations. We have found that the degree of coherence present in our formula approaches approximately unity whereas the long range correlation parameter becomes approximately zero. These results suggest that the physical meanings of the fractional degree of coherence and the long range correlation observed in various other analyses can most probably be attributed to FSI.

Reactions pd-->3He eta and pd-->3He pi + pi - near the eta threshold.

The reaction pd${\ensuremath{\rightarrow}}^{3}$He\ensuremath{\eta} has been studied with the SPES2 spectrometer, for proton energies between 0.2 MeV and 11 MeV above threshold. The total cross section rises from 0.25 to 0.40 \ensuremath{\mu}b in this region. The observed energy dependence can be attributed to the strong final state interaction. The angular distribution is nearly isotropic, consistent with S wave production. A measurement of the pd${\ensuremath{\rightarrow}}^{3}$He${\mathrm{\ensuremath{\pi}}}^{+}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$ cross section in the threshold region is also presented. The setup, which is capable of producing ${10}^{5}$ tagged \ensuremath{\eta}/s with only a few percent background, is used to investigate rare \ensuremath{\eta} decays. \textcopyright{} 1996 The American Physical Society.

The influence of final state interaction on two-particle correlations in multiple production of particles and resonances

The structure of pair correlations of interacting particle moving with nearby velocities is analysed. A general formalism of the two-particle space-time density matrix, taking into account the space-time coherence of the production process, is developed. The influence of strong final state interaction on two-particle correlations in the case of the production of a systemresonance+particle is investigated in detail. It is shown that in the limit of small distances between the resonance and particle production points the effect of final state interaction is enhanced due to logarithmic singularity of the triangle diagram. Numerical estimates indicate that, in this limit, the effect of strong final state interaction becomes important even for two-pion correlations.

Decays of B mesons to two pseudoscalars in broken SU(3) symmetry.

The decays of $B$ mesons to two-body hadronic final states are analyzed within the context of broken flavor SU(3) symmetry, extending a previous analysis involving pairs of light pseudoscalars to decays involving one or two charmed quarks in the final state. A systematic program is described for learning information {}from decay rates regarding (i) SU(3)-violating contributions, (ii) the magnitude of exchange and annihilation diagrams (effects involving the spectator quark), and (iii) strong final-state interactions. The implication of SU(3)-breaking effects for the extraction of weak phases is also examined. The present status of data on these questions is reviewed and suggestions for further experimental study are made.

Final state interactions of B → DK decays

We study the final state strong interactions of the B → DK decay processes, using the Regge model. We conclude that the final state interaction phases are very small, typically a few degrees. Neglecting final state interactions in obtaining the weak decay amplitudes is a good approximation.

Two-pion correlation behavior in a small relative momentum region.

We argue that the standard calculation leading to the Gamov factor should be significantly corrected in the region of small relative momenta due to final-state strong interactions. We recalculate the new correction factor based on the reduced Bethe-Salpeter equation, which is relevant in the momentum region under consideration, and find out that the mentioned factor is appreciably smaller compared to the standard Gamov factor.

The reaction pd→3Heη near threshold

Abstract The cross section and deuteron tensor analysing power of the reaction pd→ 3 Heη are studied near threshold in a two-step model, where a pion beam, created in a NN→dπ reaction on one of the nucleons in the target deuteron, is converted into an η-meson on the second nucleon via a πN→ ηN process. By taking into account the strong final-state interaction of the η with the 3 He nucleus, the energy dependence of the cross section is well reproduced, though its predicted absolute value is a factor of 2.5 too low. The analysing power is much more sensitive to the assumptions in the model.

Nonfactorizable contributions to nonleptonic weak decays of heavy mesons

Nonfactorizable contributions to exclusive two-body nonleptonic weak decays of heavy mesons arising from color octet currents, characterized by the parameter r 2, are extracted from the data. It is found that r 2 is equal to ∼ −0.67, −(0.9−1.1), −(1.2−1.3), ∼ 0.36 respectively for D → Kπ, K ∗π, K ∗ϱ, B → Dπ decays. As expected, soft-gluon effects become stronger when the decay particles are less energetic, allowing more time for significant final-state strong interactions. As a consequence, the parameter a 2 is not universal and is channel or class dependent, contrary to the anticipation of the factorization approach. The leading 1 N c expansion works most successfully for D → Kπ decays as the subleading 1 N c factorizable contribution is almost compensated by the soft-gluon effect. We argue that, in contrast to what happens in B − → D (∗) π(ϱ) decays, the nonfactorizable term and the subleading 1 N c factorizable term in B → ψ K (∗) decays are opposite in signs, in accordance with a recent QCD sum rule calculation. Implications are discussed.

Investigation of polar and azimuthal distributions of subthreshold pions at intermediate energies

Abstract We present data on π 0 emission coincident with projectile-like fragments from 36 Ar + 197 Au and 36 Ar + 12 C reactions at 95 MeV/u. The pion polar and azimuthal distributions, obtained for different impact parameters, are interpreted in terms of strong final-state interactions. We propose to exploit pion shadowing as a probe of the competition between attractive and repulsive forces in nucleus-nucleus collisions.

Pion electroproduction on proton and deuteron.

The cross sections for the p(e,e'${\mathrm{\ensuremath{\pi}}}^{+}$)n and d(e,e'${\mathrm{\ensuremath{\pi}}}^{+}$)2n reactions are calculated using a simple model with pion, nucleon and \ensuremath{\Delta}-resonance degrees of freedom, for the kinematical conditions used in a recent Saclay experiment. The final-state interactions between the two outgoing neutrons are found to have a large effect on the d(e,e'${\mathrm{\ensuremath{\pi}}}^{+}$)2n reaction. The calculated quenching of the ${\mathrm{\ensuremath{\pi}}}^{+}$ production cross section for deuterium is in agreement with that observed experimentally. It is predicted that, due to the strong final-state interaction in the singlet S state of the two neutrons, the e,e'${\mathrm{\ensuremath{\pi}}}^{+}$ cross-section can have a large dependence on the orientation of deuteron spin. In principle this dependence can be observed with tensor polarized deuterium targets under suitable kinematical conditions.

Fully probing the Cabibbo-Kobayashi-Maskawa unitarity triangle at the Upsilon (4S) resonance.

We analyze the joint decays of C=\ifmmode\pm\else\textpm\fi{}1 ${\mathit{B}}_{\mathit{d}}^{0}$B\ifmmode\bar\else\textasciimacron\fi{} $_{\mathit{d}}^{0}$ pairs at a symmetric B factory. A few time-integrated measurements of decay rates and CP-violating asymmetries are proposed for ${\mathit{B}}_{\mathit{d}}^{0}$,B\ifmmode\bar\else\textasciimacron\fi{} $_{\mathit{d}}^{0}\mathrm{\ensuremath{\pi}}^{+}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$,${\mathrm{\ensuremath{\pi}}}^{0}$${\mathrm{\ensuremath{\pi}}}^{0}$,\ensuremath{\psi}${\mathit{K}}_{\mathit{S}}$ and ${\mathit{B}}_{\mathit{u}}^{\ifmmode\pm\else\textpm\fi{}}$\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{\ifmmode\pm\else\textpm\fi{}}$${\mathrm{\ensuremath{\pi}}}^{0}$ both on the \ensuremath{\Upsilon}(4S) resonance and just above it. We show that it is possible to extract three angles of the Cabibbo-Kobayashi-Maskawa unitarity triangle from such measurements unambiguously, even in the presence of significant final-state strong interactions and in the absence of exclusive ${\mathit{B}}_{\mathit{s}}^{0}$ versus B\ifmmode\bar\else\textasciimacron\fi{} $_{\mathit{s}}^{0}$ decay modes.

Photodissociation of trifluoroiodomethane at 304 nm: effects of photon energy and curve crossing on the internal excitation of trifluoromethyl

We present correlated measurements of the CF{sub 3} internal energy, the initial electronic excitation, and the final iodine state in the photodissociation of CF{sub 3}I at 304 nm by using state-selective photofragment translational spectroscopy. The simultaneous determination of the three properties is achieved by measuring the translational energy distribution of the state-selected iodine fragment as a function of its recoil angle with respect to the electric vector of the photolysis laser. We found that the excited state I*({sup 2}P{sub 1/2}) atoms are formed predominantly from the parallel {sup 3}Q{sub 0} {l_arrow} N transition while the I{sup 2}P{sub 3/2} atoms are formed from both the perpendicular {sup 3}Q{sub 1} {l_arrow} N transition (70%) and the parallel {sup 3}Q{sub 0} {l_arrow} N transition (30%) followed by curve crossing to the {sup 1}Q{sub 1} state. By comparing the average internal energies of CF{sub 3} produced in the two dissociation channels of the {sup 3}Q{sub 1} {l_arrow} N excitation at 304 nm with the recent results of Felder at 248 nm, we examine the effect of the photon energy and that of the curve crossing on the dynamics of the vibrational excitation. For the I* channel, we observe a strong photon energy dependence ofmore » the CF{sub 3} internal excitation, suggesting a strong final state interaction the dynamics of the {sup 3}Q{sub 0} potential as suggested by a number of model calculations. The best agreement obtained with the model calculation of van Veen et al. supports the assumption of the dominant involvement of the {nu}{sub 2} umbrella vibration. For the {sup 3}Q{sub 0} component of the I channel we observe a similar strong photon energy dependence in terms of its slopes, but with an enhanced CF{sub 3} internal excitation. This suggests the importance of the curve crossing dynamics as well as the dynamics of the {sup 1}Q{sub 1} potential. 34 refs., 7 figs., 2 tabs.« less

N( pi pi )S decay: A flux-tube-model picture.

We calculate the ${N(\ensuremath{\pi}\ensuremath{\pi})}_{S}$ decay width of nucleon resonances up to 2 GeV within the frame of a semirelativistic flux-tube model, using the constant amplitude flux-tube-breaking decay mechanism. To deal with the absence of a known $q\overline{q}$ intermediate state of sufficiently low mass, we use a pseudoresonating intermediate state with no assumption on its composition other than the flux-tube structure constraints. Its mass, width, and radius are varied. The agreement with data is comparable to the one obtained for other hadronic strong decay channels calculations if the intermediate state has a mass of about 600 and a typical hadronic radius, a result consistent with $\ensuremath{\pi}\ensuremath{\pi}$ strong final-state interaction.

Proton-helium correlation in 94 MeV/nucleon 16O-induced reactions on Al, Ni, and Au targets.

Azimuthal distributions of helium ions have been measured in coincidence with high-energy protons in reactions induced by {sup 16}O at 94 MeV/nucleon on {sup 27}Al, {sup 58}Ni, and {sup 197}Au. Helium ions have been detected in a large area multidetector. Protons have been observed at 90{degree}. Mean multiplicities of light charged particles (H and He) are found slightly dependent on the target mass. Strong azimuthal asymmetries whose intensity is larger for the Al target and vanishes with the increasing of the target mass are observed in the He distributions. Experimental data are discussed in the framework of the participant-spectator picture of a modified fireball model, taking into account intermediate energy corrections. In this framework the behavior of the azimuthal asymmetries, as a function of the target mass, indicates a strong final-state interaction between participant and spectator fragments. Such a result is found in agreement with interaction time predictions of a microscopical calculation based on the Boltzmann-Nordheim-Vlasov equation.