Ππ Invariant Mass Research Articles

Evidence of a dibaryon spectrum in coherent π0π0d photoproduction at forward deuteron angles

The coherent reaction, γd→π0π0d was studied with the BGOOD experiment at ELSA from threshold to a centre-of-mass energy of 2850MeV. A full kinematic reconstruction was made, with final state deuterons identified in the forward spectrometer and π0 decays in the central BGO Rugby Ball. The strength of the differential cross section exceeds what can be described by models of coherent photoproduction and instead supports the three isoscalar dibaryon candidates reported by the ELPH collaboration at 2.38, 2.47 and 2.63GeV/c2. A low mass enhancement in the π0π0 invariant mass is also observed at the d⁎(2380) centre-of-mass energy which is consistent with the ABC effect. At higher centre-of-mass energies, a narrow peak in the π0d invariant mass at 2114MeV/c2 with a width of 20MeV/c2 supports a sequential two-dibaryon decay mechanism.

Search for the tensor glueball

The tensor glueball is searched for in BESIII data on radiative J/ψ decays into π0π0 and KsKs. The ππ invariant mass distribution exhibits an enhancement that can be described by a pole at (2210±60)−i(180±60)MeV or by three of ϕϕ resonances suggested a long time ago to be the trace of one, two or three glueballs. We speculate that several high-mass tensor mesons might have qq¯ and glueball components.

The role of charged exotic states in e+e− → ψ(2S) π+π−

In this work, we use the dispersion theory to provide a physical description of recent BESIII data on the reaction e+e−→ψ(2S)π+π−. Taking into account explicitly the effects of charged exotic intermediate states in the t- and u-channels as well as the two-pion final state interaction, we describe the invariant mass distribution for four different e+e− center-of-mass energies. The effects of the ππ rescattering are accounted for in a single channel Omnès approach which is found to explain the ππ-invariant mass distributions at all e+e− center-of-mass energies. For q=4.226 GeV and q=4.258 GeV the already established charged exotic state Zc(3900) is considered as the intermediate state, whereas for q=4.358 GeV the rescattering of pions dominates the fits. For the highest energy, q=4.416 GeV, a heavier charged exotic state with mass mZc=4.016(4) GeV and width ΓZc=52(10) MeV is essential to describe the experimental data. Although the mass of this state is consistent with the established Zc(4020), its width is significantly larger.

Dispersive analysis of the γγ⁎ → ππ process

We present a theoretical study of the γγ⁎→π+π−,π0π0 processes from the threshold through the f2(1270) region in the ππ invariant mass. We adopt the Omnès representation in order to account for rescattering effects in both s- and d-partial waves. For the description of the f0(980) resonance, we implement a coupled-channel unitarity. The constructed amplitudes serve as an essential framework to interpret the current experimental two-photon fusion program at BESIII. They also provide an important input for the dispersive analyses of the hadronic light-by-light scattering contribution to the muon's anomalous magnetic moment.

Examination of the nature of the ABC effect

Recently it has been shown by exclusive and kinematically complete experiments that the appearance of a narrow resonance structure in double-pionic fusion reactions is strictly correlated with the appearance of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the ππ-invariant mass spectrum. Whereas the resonance structure got its explanation by the d⁎(2380) dibaryonic resonance, a satisfactory explanation for the ABC effect is still pending. In this paper we discuss possible explanations of the ABC effect and their consequences for the internal structure of the d⁎ dibaryon. To this end we examine and review a variety of proposed explanations for the ABC effect, add a new hypothesis and confront all of them with the experimental results for the np→dπ0π0 and np→npπ0π0 reactions, which are the most challenging ones for this topic.

From CELSIUS to COSY: on the observation of a dibaryon resonance

Using a high-quality beam of storage rings in combination with a pellet target and a hermetic WASA detector covering practically the full solid angle, two-pion production in nucleon–nucleon collisions has been systematically studied by exclusive and kinematically complete measurements—first at CELSIUS and subsequently at COSY. These measurements resulted in a detailed understanding of the two-pion production mechanism by t-channel meson exchange. The investigation of the ABC effect, which denotes an unusual low-mass enhancement in the ππ-invariant mass spectrum, in double-pionic fusion reactions led the trace to the observation of a narrow dibaryon resonance with about 80 MeV below the nominal mass of the conventional system. New neutron–proton scattering data, taken with a polarized beam at COSY, produced a pole in the coupled partial waves at () MeV, establishing thus the first observation of a genuine s-channel dibaryon resonance.

Measurement of the np → npπ0π0 reaction in search for the recently observed d⁎(2380) resonance

Exclusive measurements of the quasi-free np→npπ0π0 reaction have been performed by means of dp collisions at Td=2.27 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region s=(2.35–2.46) GeV, which includes the region of the ABC effect and its associated d⁎(2380) resonance. Adding the d⁎ resonance amplitude to that for the conventional processes leads to a reasonable description of the data. The observed resonance effect in the total cross section is in agreement with the predictions of Fäldt and Wilkin as well with those of Albadajedo and Oset. The ABC effect, i.e. the low-mass enhancement in the π0π0-invariant mass spectrum, is found to be very modest – if present at all, which might pose a problem to some of its interpretations.

Detailed study of the Ke4 decay mode properties

The NA48/2 collaboration at CERN has accumulated an unprecedented statistics of the semileptonic Ke4 charged kaon decay in the “charged pions” (Ke4(+-)) mode K±→π+π−eν and in the “neutral pions” (Ke4(00)) mode K±→π0π0eν with respectively <1% and ∼1% background contamination, allowing a detailed study of the decay properties.More than one million Ke4(+-) decays have been analyzed leading to the improved determination of the branching ratio and detailed studies of the form factors. The resulting BR(K±e4(+−))=(4.257±0.004stat±0.016syst±0.031ext)×10−5 is 3 times more precise than the corresponding PDG value and the error is dominated by the external uncertainty from the branching ratio of the normalization decay mode. It has been possible to obtain the absolute value of the form factor fs=5.705±0.017exp±±0.017syst±0.031ext and we can now translate into absolute form all the relative form factors previously published by NA48/2. This result complements the study of the S and P wave hadronic form factors obtained by NA48/2 in a simultaneous analysis of the ππ scattering length obtained from the same data sample.Concurrently, 65,210 K± decays into π0π0e±ν have been collected, increasing the world available statistics by several orders of magnitude. A study of the differential rate provides the first measurement of the form factors. The achieved precision makes possible also the observation of small effects such as the evidence for a cusp-like structure in the distribution of the π0π0 invariant mass squared around the threshold 4mπ+2. Using a model independent description of the form factors we have obtained the final result, inclusive of radiative decays, BR(K±e4(00))=(2.552±0.010stat±0.010syst±0.032ext)×10−5, also dominated by the external uncertainty from the branching ratio of the normalization decay mode, and improving the world average precision by an order of magnitude.The detailed study of form factors is sensitive to small isospin symmetry breaking effects and brings new inputs to low energy QCD description and crucial tests of predictions from Chiral Perturbation Theory and lattice QCD calculations.

Detailed study of the K ± → π 0 π 0 e ± ν (K e4 00 ) decay properties

A sample of 65210 K ± → π 0 π 0 e ± ν (K e4 00 ) decay candidates with 1% background contamination has been collected in 2003-2004 by the NA48/2 collaboration at the CERN SPS. A study of the differential rate provides the first measurement of the hadronic form factor variation in the plane (M 2 , M 2 ) and brings evidence for a cusp-like structure in the distribution of the squared π 0 π 0 invariant mass around $$ 4{m}_{\pi^{+}}^2 $$ . Exploiting a model independent description of this form factor, the branching ratio, inclusive of radiative decays, is obtained using the K ± → π 0 π 0 π ± decay mode as normalization. It is measured to be BR(K e4 00 ) = (2.552 ± 0.010stat ± 0.010syst ± 0.032ext) × 10−5, which improves the current world average precision by an order of magnitude while the 1.4% relative precision is dominated by the external uncertainty from the normalization mode. A comparison with the properties of the corresponding mode involving a π + π − pair (K e4 + − ) is also presented.

γγ-PHYSICS AT KLOE/KLOE-2

Photon-photon interactions have been studied at the ϕ-factory DAΦNE with the detector KLOE operating the machine at [Formula: see text] GeV, without tagging of the final leptons. The results about the γγ → η process and the evidence for γγ → π0π0 production at low π0π0 invariant mass are reviewed. The process γγ → π0 will be studied at KLOE-2 running the machine at ϕ-peak thanks to new lepton tagger detectors. In particular, the possibility to measure the two-photon width of π0 and the π0γ*γ transition form factor in the low (space-like) Q2 region is considered.

Isospin decomposition of the basic double-pionic fusion in the region of the ABC effect

Exclusive and kinematically complete high-statistics measurements of the basic double-pionic fusion reactions pn→dπ0π0, pn→dπ+π− and pp→dπ+π0 have been carried out simultaneously over the energy region of the ABC effect using the WASA detector setup at COSY. Whereas the isoscalar reaction part given by the dπ0π0 channel exhibits the ABC effect, i.e. a low-mass enhancement in the ππ-invariant mass distribution, as well as the associated resonance structure in the total cross section, the isovector part given by the dπ+π0 channel shows a smooth behavior consistent with the conventional t-channel ΔΔ process. The dπ+π− data are very well reproduced by combining the data for isovector and isoscalar contributions, if the kinematical consequences of the isospin violation due to different masses for charged and neutral pions are taken into account.

γγ physics at KLOE/KLOE-2

Two-photon physics from e+e− collision is studied at DAΦNE. The processes e+e− → e+e− X, with X being either the η meson or π0π0 have been studied with beams colliding at GeV, below the ϕ resonance peak and without tagging of the outgoing e+e−. Preliminary results are presented on the observation of the γγ → η process, with both η → π+π−π0 and η → π0π0π0 channels, and the evidence for γγ → π0π0 production at low π0π0 invariant mass. The process e+e− → e+e−π0 will be studied at KLOE-2 running the machine at ϕ peak thanks to new lepton tagger detectors. In particular, the possibility to measure the width Γγγ → π0 and the π0γγ* form factor, F(Q2), at low invariant masses of the virtual photon (in the space-like region) is considered. The feasibility of these measurements is estimated on the basis of a Monte Carlo simulation, whose results are presented here.

Two-pion production in nucleon–nucleon collisions—Back to dibaryons?

The two-pion production in nucleon–nucleon collisions has been investigated systematically by exclusive and kinematically complete measurements at CELSIUS/WASA and more recently at COSY-TOF and particularly WASA@COSY. The data for all pp-induced isovector production channels in the energy range from threshold up to Tp = 1.4 GeV can be well understood by t-channel Roper, ΔΔ and possibly Δ(1600) excitation. In contrast to that the purely isoscalar reaction channel pn→dπ0π0 exhibits a narrow resonance-like structure with m=2.37GeV and Γ=70MeV in the total cross section, which is correlated with an intriguing low-mass enhancement in the ππ invariant mass spectrum (ABC effect) as well as with the formation of a ΔΔ system in the intermediate state. From the angular distributions the spin and parity of this structure have been derived resulting in I(JP)=0(3+). The consequences of the resonance hypothesis for other reaction channels are discussed. The fact that also the double-pionic fusion reactions to 3He and 4He exhibit a corresponding resonance-like structure in the total cross section correlated with the ABC effect suggests that the underlying conjectured pn resonance is obviously robust enough to survive in nuclei.

WHAT IS THE NATURE OF THE ABC EFFECT?

The ABC effect — an intriguing low-mass enhancement in the ππ invariant mass spectrum — is known from inclusive measurements of two-pion production in nuclear fusion reactions. Exclusive measurements have been carried out at CELSIUS/WASA and WASA-at-COSY for the fusion reactions leading to d, 3 He and 4 He . They all reveal this effect to be of isoscalar nature and associated with a narrow Lorentzian-shaped structure in the total cross section. The latter could point to the formation of an isoscalar resonance, which couples to the pn channel as well as to an intermediate ΔΔ state.

THE pp → ppπ0π0 REACTION AND ITS LIMITING CASE, FUSION TO QUASI-BOUND 2He, IN SEARCH OF THE ABC EFFECT

The π0π0 production in pp -collisions has been investigated in exclusive and kinematically complete measurements from threshold up to Tp = 1.4 GeV . For incident energies Tp &gt; 1 GeV , i.e. in the region beyond the Roper excitation, the ΔΔ excitation process takes over. The data are well explained by the t-channel ΔΔ process dominated by pion exchange. There is no low-mass enhancement (ABC effect) in the π0π0-invariant mass distribution beyond that given by the conventional t-channel ΔΔ process. This is also true for the limiting case, where the protons are in the quasi-bound 2 He state.

THE ABC EFFECT AND ITS ENERGY DEPENDENCE IN THE DOUBLE-PIONIC FUSION TO 4He

The double-pionic fusion reactions dd → 4 He π0π0 and dd → 4 He π+π- were measured exclusively and kinematically complete with the WASA detector at COSY. The measurements were performed in the energy range Tp = 0.8 - 1.4 GeV covering thus the full energy region, where the so-called ABC effect – a peculiar low-mass enhancement in the ππ-invariant mass spectrum – was observed previously in inclusive measurements. Our findings for the double-pionic fusion to 4 He are in accordance with the observations for the basic pn → dπ0π0 reaction pointing to the same fundamental mechanism. In both cases we observe that the ABC effect is associated with a peak in the energy dependence of the total cross section. In the 4 He case the peak structure in the total cross section is broadened in comparison to the deuteron case. This is consistent with both Fermi motion of the basic pn system within 4 He and with collision broadening as it is known, e.g., from excitations of the Δ resonance in nuclei.

Empirical parameterization of the [formula omitted] decay Dalitz plot

As first observed by the NA48/2 experiment at the CERN SPS, the π0π0 invariant mass (M00) distribution from K±→π±π0π0 decay shows a cusp-like anomaly at M00=2m+, where m+ is the charged pion mass. An analysis to extract the ππ scattering lengths in the isospin I=0 and I=2 states, a0 and a2, respectively, has been recently reported. In the present work the Dalitz plot of this decay is fitted to a new empirical parameterization suitable for practical purposes, such as Monte Carlo simulations of K±→π±π0π0 decays.

Exclusive measurement of two-pion production in the [formula omitted] reaction

The results from the first kinematically complete measurement of the d d → He 4 π π reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculiar ππ-invariant mass distributions appearing in double pion production in light ion collisions, the so-called ABC effect. The measurements were performed at the incident deuteron energies of 712 MeV and 1029 MeV, with the WASA detector assembly at CELSIUS in Uppsala, Sweden. We report the observation of a characteristic enhancement at low ππ-invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in addition to confirming previous experimental observations, our results reveal a strong angular dependence of the pions in the overall centre of mass system. The results are qualitatively reproduced by a theoretical model, according to which the ABC effect is described as resulting from a kinematical enhancement in the production of the pion pairs from two parallel and independent N N → d π sub-processes.

RECENT NA48/2 RESULTS ON Ke4 AND K3π DECAYS AND DETERMINATION OF THE ππ SCATTERING LENGTHS

Large samples of K± → 3π and K± → π+π-e±ν (Ke4) decays have been collected by the NA48/2 experiment at CERN SPS. At the π+π- threshold, the π0π0 invariant mass spectrum of the decay K± → π±π0π0 xhibits a Wigner cusp, from which the S-wave ππ scattering lengths are extracted with high precision. The same scattering lengths are also independently determined from the accurate measurement of the form factors in the Ke4 decay K± → π+π-e±ν.

DOUBLE-PION PRODUCTION IN NUCLEON COLLISIONS ON FEW-BODY SYSTEMS — THE ABC EFFECT AND ITS POSSIBLE ORIGIN

The ABC effect – an intriguing low-mass enhancement in the ππ invariant mass spectrum – is known from inclusive measurements of two-pion production in nuclear fusion reactions. Exclusive measurements have been carried out at CELSIUS/WASA and WASA@COSY for the fusion reactions leading to d, 3 He and 4 He . They all reveal this effect to be a σ channel phenomenon combined with a resonance-like behavior in the total cross section. The latter points to the formation of an isoscalar dibaryonic resonance, which couples to the pn channel as well as to a ΔΔ intermediate state.