Ππ System Research Articles

Analysis of hadronic transitions in Upsilon (3S) decays.

Using the CLEO II detector, we have measured the branching fractions for ϒ(3S)→ππϒ(1S), ϒ(3S)→ππϒ(2S), and the cascade ϒ(3S)→ϒ(2S)+X, ϒ(2S)→π+π−ϒ(1S), analyzing the exclusive mode where the daughter ϒ state decays to a e+e− or μ+μ− pair, as well as the inclusive π+π− transitions where the final ϒ state decays into hadrons. Properties of the ππ system are analyzed. Searches for the cascade decay ϒ(3S)→π+π−hb, hb→γηb and ϒ(3S)→π0hb were also performed.Received 8 July 1993DOI:https://doi.org/10.1103/PhysRevD.49.40©1994 American Physical Society

Observation of a 2+ + resonance at 1515 MeV in proton-antiproton annihilations into 3π0

Antiproton-proton annihilations at rest into 3 π 0 are analyzed in terms of π 0 π 0 final state interactions. A J PC = 2 + + resonance at a mass of 1515 ± 10 MeV with a width of 120 ± 10 MeV is required to explain the data. This result is supported by previous observations made on the ππ system in p p and p n annihilations which show that the resonance has I = 0. This resonance is practically degenerate in mass with the f′ 2 (1525) but the two resonances have very different production and decay characteristics.

Measurement of pi 0 pi 0 production in two-photon collisions.

The reaction e+e−→e+e−π0π0 has been analyzed using 97 pb−1 of data taken with the Crystal Ball detector at the DESY e−e+ storage ring DORIS II at beam energies around 5.3 GeV. For the first time we have measured the cross section for γγ→π0π0 for π0π0 mvariant masses ranging from threshold to about 2 GeV. We measure an approximately flat cross section of about 10 nb for W=mπ0π0<0.8 GeV, which is below 0.6 GeV, in good agreement with a theoretical prediction based on an unitarized Born-term model. At higher invariant masses we observe formation of the f2(1270) resonance and a hint of the f0(975). We deduce the following two-photon widths: Γγγ(f2(1270))=3.19±0.16±0.280.29 keV and Γγγ(f0(975))<0.53 keV at 90% C.L. The decay-angular distributions show the π0π0 system to be dominantly spin 0 for W<0.7 GeV and spin 2, helicity 2 in the f2(1270) region, with helicity 0 contributing at most 22% (90% C.L.).Received 9 February 1990DOI:https://doi.org/10.1103/PhysRevD.41.3324©1990 American Physical Society

A partial-wave analysis of the K ππ system produced in the reaction [formula omitted

A partial-wave analysis of the K 0π +π − system from the reaction K − p → K 0π + π − n has been carried out using data obtained at 8.25 GeV/ c in a high-statistics experiment. A strong signal appears in the 1 +SO + (K ∗π) wave at the Q 2 mass (≈ 1.4 GeV). The 1 +S0 + (ϱK) wave behaves rather like a background and does not exhibit the characteristics of a resonance. A prominent signal also appears in the 2 +D wave (via K ∗π and ϱK); it is interpreted as the K ∗(1430). In the L region (1.6–2.1) GeV, there is evidence for the 3 −K ∗(1780) while the 2 − wave also gives some contribution. A strong signal shows up in the 1 − P0−(K ∗π) wave around 1.47 GeV; it does not seem to be due to a resonance. The 0 − S0 + (κπ) wave gives a large contribution in the 1–2 GeV region, but it cannot be identified with the resonating 0 − S0 + (ε K) or 0 − P0 + ( K ∗π) waves observed in diffractive channels.

Rare decay modes of K ∗(1420) and K ∗(892)

A 2.5 standard deviation signal is observed for the decay K ∗−(1420) → K ∗−(892)π +π − . From this signal a partial width of 13.0 ± 5.0 MeV for the decay mode K ∗(1420)→ K ∗(892)ππ is deduced under the assumption that the ππ system is in an I = 1 state. For the rare decay mode K ∗(892)→ Kππ no signal is observed. The following upper limits can be given for the decay widths: Γ K ∗−(892) → K −π +π − < 40 keV , Γ K ∗−(892) → K 0π −π 0 < 35 keV , Γ K ∗0(892) → K 0π +π − < 35 keV .

The πNN vertex function

The πNN vertex function is calculated with a dispersion theoretic approach which reduces to standard field theoretic results in the limit of zero-width σ- and ρ-mesons. Results are shown to be sensitive to the choice of various on-shell-equivalent couplings of the J = 1 ππ system to the NN̄ system.

New data on coherent K −d interactions at 12 GeV/ c

The coherent deuteron reactions K −d → K − π + π −d and K −d → K 0 π − π 0d have been studied at 12 GeV/ c in the SLAC 82 inch deuterium bubble chamber. The cross section for the first reaction is 312 ± 30 μb and for the second 270 ± 50 μb. Resonance signals or enhancements observed are K 890 ∗, K 1420 ∗, d ∗, and Q; no clear L-meson signal is seen. In the K ππ system, components with spin ⩾ 2 only occur in events with d ∗ production. Helicities in the s-and t-channels are not conserved in K ππ production.

A study of the slope-mass correlation in the low mass K ππ system

The shape of the four-momentum transfer t′ distribution in the reactions K ±p→K ± π + π −p at 13 GeV/ c is studied for various partial waves as a function of the Kππ mass. Strong variation of the slope with effective mass is observed for the dominant J P =1 + waves. For natural parity exchange it is found that, for Kππ masses up to 1.6 GeV, the slope of the t′ distribution is decreasing with increasing spin of the excited system, B 0−> B 1+> B 2 +.

Spin-parity structure of the [formula omitted] system produced in the charge-exchange reaction [formula omitted] at 4.2 GeV c

A partial wave analysis of the K 0π +π − system produced in the charge exchange reaction K − p → ( K 0π +π −) n at 4.2 GeV/ c has been performed both as a function of Kππ mass and of t′. The 1 +S wave forms the largest contribution to the K ππ system and peaks at roughly the same mass as the Q in diffractive K ππ production. The polarization properties of the 1 + S ( K ∗π) and 1 +S (Kϱ) waves differ fromt those of the diffractive 1 + wave. There is some evidence for a resonance contribution to 1 + S( K ∗π) . The strong 2 + wave the K ∗(1420) and the Kϱ/ K ∗ϱ decay branching ratio determined to be 0.36±0.10. An enhancement with spin-parity 1 − is observed under K ∗ (1420) .

Features of the π0π0 system produced in 5π final states by 3.5 GeV/ cπ+

Angular correlations between two pions in the 5-pion final states are studied. A comparison between neutral, like-charged and unlike-charged pion-pion system is done. Evidence is shown for a strong π 0 π 0 correlation in the asymmetry parameters as well as in the mass distribution. This effect is interpreted in terms of the scattering length approximation.

Partial-wave analysis of the Q region in the reactions K −p → K −π+π−p and [formula omitted] at 14.3 GeV/ c

We have studied the reactions K −p → K − π + π −p and K − p → K 0π −π 0 p at 14.3 GeV/ c using respectively 15 992 and 3723 events. Partial-wave analysis of the region 1.0 < m(K ππ) < 1.7 GeV have been made using a modified version of the method developed at the University of Illinois. We have measured the cross section as a function of mass for each spin-parity state of the K ππ system. Results from the two reactions are in excellent agreement and indicate an overall isospin of 1 2 . We find that the Q enhancement is predominantly J P = 1 + but that the partial-wave substates have very different branching ratios into K ϱ and K ∗π , the K ∗π component of the 1 + state being similar to the 1 + state of the 3 π system produced in the reaction πp → (3 π)p.

Spin-parity analysis of ππ systems produced in pp → pp π+π− and the question of double pomeron exchange

Data on the reaction pp π + π − in the momentum range 2–25 GeV/ c are analysed for features indicating double pomeron exchange (DPE). The moments of the ππ angular distributions are examined in the quasi three-body frame of reference which is analogous to the s-channel helicity frame for quasi two-body processes. Inequality constraints on the moments are shown to suggest the presence of both S and P waves of the ππ system in the relevant pp(ππ) kinematic region, arguing against the dominance of DPE. The energy dependence of cross sections for selected events in different ranges of ππ mass is shown to be in accord with the predictions of a double Regge exchange model incorporating pomeron-reggeon exchange and not DPE. Both the cross sections and the ππ angular distributions are also examined in the light of a reggeized one-pion exchange model.

Analysis of the reaction K −p → K −π−π+p at 40 GeV/ c

The reaction K −p → K − π − π +p has been measured at 25 and 40 GeV/ c at the Serpukhov Proton Accelerator. The production cross section at 25 and 40 GeV/ c as a function of momentum transfer and K ππ mass is presented, and results of the partial-wave analysis of the K ππ system yielding information about Q(1300), K ∗(1400) and L(1770) mesons are discussed.

Study of the [formula omitted] system produced in [formula omitted] reactions at 14.3 GeV/ c

We have studied the K ππ system in the 14.3 GeV/ c reactions K −p → K − π + π −p, K − p → K 0π −π 0 and K − p → K 0π +π − n . The data have been obtained from a 500 000 picture exposure of the CERN 2m HBC. The first two final states are dominated by Q-production in the Kππ system; there is also an L-signal at M (K ππ) ∼ 1.75 GeV. The reaction cross sections are compared to K −p data at other energies. We discuss the K ππ mass dependence of the diffractive production slope. Evidence is presented for a Q −p versus Q +p differential cross section cross-over around | t| = 0.17 GeV 2. A t-channel isospin analysis for the KN → K∗(890)π N channels in the Q-region shows that the I = 1 exchange amplitude is ⋍ 10% of the dominant I = 0 exchange amplitude. The K ππ decay distributions indicate a predominant J P = 1 + state in the Q-region, and an important J P = 2 − contribution in the L-region. We find neither s-channel nor t-channel helicity conservation at the meson vertex in the Q- or L-regions. The K π angular correlation moments within the K ππ diffractive system are characteristic of K π elastic scattering, suggesting a π-exchange Deck-type production mechanism. There is evidence for a Kf 0 and κπ contribution (where κ is the J P (K π) = 0 + state) to the diffractive K ππ system. A fit to the K −π +π − and K 0π −π 0 Dalitz-plot distributions for the Q-re gion indicates that the ratio of Kϱ to K ∗π decay amplitudes decreases with increasing K ππ mass.

An analysis of the reaction K +p → Q +p from 2.5 to 12.7 GeV/c

The Q-region of Kππ mass (i.e. 1.10 ⩽ M(K ππ) < 1.5 GeV) is studied for the reactions K +p → K + π + π −p (≈ 77 000 events) and K +p → K seen 0 π + π 0p (≈ 19 000 events). The data samples correspond to K +p interactions at fourteen beam momenta in the range 2.5 – 12.7 GeV/ c. The production and the decay of the of the K ππ system are analysed as a function of the incident energy and the K ππ mass. It is shown that the differential cross section d σ/d t pp shrinks when the incident energy increases. The spin-parity is found to be predominantly 1 + for the whole Q-region and the density-matrix elements do not vary with the energy or the K ππ mass. The ratio of the number of Kϱ to the number of K ∗π events is obtained from a fit of the decay Dalitz plot and found to be maximum for 1.25 ⩽ M (K ππ) < 1.30 GeV.

Study of the 1500 and 1700 MeV enhancements in the pπ +π − mass in the reaction pp→pp π+π− at 19 GeV/ c

The 1500 MeV and 1700 MeV enhancements observed in the p ππ system are analyzed. It is found that the 1500 MeV effect is not a pure angular momentum state. Neither t- nor s-channel helicity is conserved. The 1700 MeV effect gives good fits to J= 3 2 and 5 2 decaying to p ππ. In the decay mode Δ ++π −J= 5 2 is preferred over 3 2 . Moment analysis shows consistency with t- rather than s-channel helicity conservation.

Duality and final-state interaction in the description of $$\bar p$$ n → π+π−π− at rest

A pole expansion of Veneziano four-point amplitudes which is convergent in the annihilation channel is useda) to analyse the resonance content of the previously proposed dual descriptions of the reaction\(\bar p\)n → π+π−π− at rest, andb) to construct a dual model for this reaction which exhibits a clear connection to the final-state interaction model. The important predictions of duality and crossing symmetry are the existence of the daughter resonances in the ππ system (particularly ɛ and ɛ′), and the presence of a nonresonant background in the annihilation amplitude. A reasonable fit to the data is found with this model.

Search for exotic mesons produced by baryon exchange and [formula omitted]B coupling to exotic channels

Recent evidence on exotic meson exchange reactions suggests that such mesons, if they exist, may couple strongly to baryon-decuplet vertices. Therefore, decuplet baryon-exchange reactions provide the “natural” searching grounds for these states. We have searched for possible J = 0, 2, I = 2 states in the mass range of 0.9–1.3 GeV/ c using the reaction π −p → p + x −. We show recent backward data from the reaction π +n → p + π + + π − for comparison. We tentatively conclude that the baryon-decuplet annihilation vertex shows strong coupling to the I = 2 ππ system.

Moving CDD poles and complex regge trajectories

A mechanism for generating dynamical fixed cuts in the angular-momentum plane, based on moving CDD poles, is proposed. In particular, the splitting of Regge poles coupled to the ππ system is studied. Implications on low-energy scattering and on high-energy phenomenology are discussed.

A study of K ππ states produced in 10 GeV/ c K +p interactions

We present an analysis of the K ππ system produced in 10 GeV/ c K +p interactions. We show that the low-mass enchancement between 1.2 and 1.4 GeV/ c 2 on the K ππ mass spectrum is predominantly 1 + throughout, give the relative amplitudes for the decay of this system into K ∗ (890) π and K ρ, and offer new evidence for the presence of two 1 + resonances in this mass region.