Low Pion Research Articles

Particle Identification at Belle II

We report on the charged particle identification (PID) systems for the upcoming Belle II experiment. The time of propagation counter in the central region and the proximity focusing ring imaging Cherenkov counters with aerogel radiator in the forward region will be used as the PID devices. They are expected to provide a kaon identification efficiency of more than 94% at a low pion misidentification probability of 4%. The motivation for the upgrade, method and status of both systems are discussed.

Monte Carlo study of a Belle II proximity focusing RICH with aerogel as a radiator

A proximity focusing RICH will be installed in the forward direction of the Belle II spectrometer, inside a super conducting solenoid coil with the magnetic field of 1.5T. Photons emitted in the sequence of two different aerogel radiator layers will be registered by the Hybrid Avalanche Photo Diodes. By detecting more than 11 photons per incident 4GeV/c pion, with 15mrad single photon Cherenkov angle resolution, the designed detector should enable an efficient separation of kaons from pions in the wide range of particle momenta from 0.5GeV/c up to 4GeV/c.The particle separation will be based on a two-dimensional extended maximum-likelihood analysis. The position dependence of the efficiencies for identification of pions and kaons shows a very homogeneous response over the detector area. The performance on the edge of the detector is improved by employing planar mirrors to reflect the Cherenkov photons on the detector plane. The results show that by using a designed detector configuration the kaon pion separation efficiency of more than 95% can be achieved at a very low pion misidentification probability of 1%. In the low momentum region (0.5GeV/c–0.8GeV/c) the variation of detected number of photons due to matching of aerogel refractive index and the gaps between the sensitive areas on the detector plane results in a slightly decreased separation efficiency.

Nucleon structure from Lattice QCD using a nearly physical pion mass

We report the first Lattice QCD calculation using the almost physical pion mass mπ=149 MeV that agrees with experiment for four fundamental isovector observables characterizing the gross structure of the nucleon: the Dirac and Pauli radii, the magnetic moment, and the quark momentum fraction. The key to this success is the combination of using a nearly physical pion mass and excluding the contributions of excited states. An analogous calculation of the nucleon axial charge governing beta decay has inconsistencies indicating a source of bias at low pion masses not present for the other observables and yields a result that disagrees with experiment.

Forward Neutral Pion Production inp+pandd+AuCollisions atsNN=200 GeV

Measurements of the production of forward pi0 mesons from p + p and d + Au collisions at square root sNN=200 GeV are reported. The p + p yield generally agrees with next-to-leading order perturbative QCD calculations. The d + Au yield per binary collision is suppressed as eta increases, decreasing to approximately 30% of the p + p yield at eta =4.00, well below shadowing expectations. Exploratory measurements of azimuthal correlations of the forward pi0 with charged hadrons at eta approximately 0 show a recoil peak in p + p that is suppressed in d + Au at low pion energy. These observations are qualitatively consistent with a saturation picture of the low-x gluon structure of heavy nuclei.

Evidence for different freeze-out radii of high- and low-energy pions emitted in Au+Au collisions at 1 A·GeV

Abstract Double differential production cross sections of π − and π + mesons and the number of participating protons have been measured in central Au+Au collisions at 1 A·GeV. At low pion energies the π − yield is strongly enhanced over the π + yield. The energy dependence of the π − / π + ratio is assigned to the Coulomb interaction of the charged pions with the protons in the reaction zone. The deduced Coulomb potential increases with increasing pion c.m. energy. This behavior indicates different freeze-out radii for different pion energies in the c.m. frame.

Remarks on the hadronic effect in muon g-2: Low energy behavior of V0- pi + scattering.

The behavior of the ${V}^{0}\ensuremath{-}{\ensuremath{\pi}}^{+}$ scattering amplitude (where ${V}^{0}={\ensuremath{\rho}}^{0}, \ensuremath{\omega}, or \ensuremath{\varphi}$) in the low pion momentum limit is studied, motivated by its relevance to the theory of muon $g\ensuremath{-}2$. Current algebra analysis shows that its $S$-wave component must vanish in the chiral limit under general physical assumptions. We confirm this result with various low energy models of the vector meson. Our result justifies the calculation of the charged pion loop part of the hadronic light-by-light scattering contribution to muon $g\ensuremath{-}2$ within the framework of the hidden local symmetry model of low energy hadron dynamics.

Low-energy pionic double charge exchange on the beta beta -instable nuclei 128,130Te.

Data of the (${\ensuremath{\pi}}^{+}, {\ensuremath{\pi}}^{\ensuremath{-}}$) reaction on $^{128}\mathrm{Te}$ and $^{130}\mathrm{Te}$ at ${T}_{\ensuremath{\pi}}=48$ MeV are presented, which constitute also the first observation of pionic double charge exchange (DCX) on heavy nuclei at energies below the (3,3) resonance. For the ground state transitions in these isotopes we find very small cross sections of about 15 nb/sr only at 30\ifmmode^\circ\else\textdegree\fi{} and a ratio of $\frac{\ensuremath{\sigma}(^{130}\mathrm{Te})}{\ensuremath{\sigma}(^{128}\mathrm{Te})}={1.5}_{\ensuremath{-}0.8}^{+1.8}$. The experimental results and their impact on the understanding of the $\ensuremath{\beta}\ensuremath{\beta}$ decay of the Te isotopes are discussed within the framework of recent proton-neutron quasiparticle random phase approximation calculations. For the transitions to the double isobaric analog states in $^{128}\mathrm{Xe}$ and $^{130}\mathrm{Xe}$ we obtain cross sections in the range 1 \ensuremath{\mu}b/sr which fit very well into a systematics, that is in accordance with a dominance of the short-range part of the DCX operator in analog transitions at low pion energies.

Two-body correlations in pionic systems

In close analogy to fermionic many-body theory the truncation of the bosonic BBGKY density matrix hierarchy on the two-body level leads to a coupled set of nonlinear equations of motion for the one-body density matrix and the two-body correlation function. These equations provide a nonperturbative description of the nonequilibrium time evolution of particle number conserving bosonic many-body systems including the dynamical resummation of parquet-like diagrams. Within this framework we study the properties of a pionic system as a function of temperature and density with focus on two-body quantities. For each temperature we find a related pion density for which the relative strength of the two-body correlation function assumes a maximum and the pionic system is far from the mean-field limit. Since these correlated phases up to $T$=200 MeV only appear at rather low pion density, the hot and dense pion gas as generated in ultrarelativistic nucleus-nucleus collisions should be well described within mean-field theory; i.e. the HBT analysis of pion sources from $\pi-\pi$ correlations should remain valid even in the case of strongly interacting pions.

Revisit of Coulomb effects on [formula omitted] ratio in heavy ion collisions

We show that the large π − π + ratio at low pion energies recently observed in the reaction of Au+Au at E beam A = 1.0 GeV at SIS/GSI is due to the strong Coulomb field of the reaction system.

Nuclear structure effects in ( π+, π0) and (p,n) reactions on 13C

The role of 2p-shell admixture in the ground-state wave function of 13C is studied in the case of reactions such as 13C(e,e) 13C, 13 C(π +, π 0) 13 N g.s. , and 13C(p,n) 13N g.s.. A comparison is made between microscopic ground-state wave functions elaborated in the (0+2) h ̷ ω basis and the phenomenological 1p-shell model. The best agreement with the data for electromagnetic form factors, cross sections, and asymmetries is achieved using a combination of the microscopic and the phenomenological description. At low pion energies a large suppression of differential cross section is observed for the 13 C(π +, π 0) 13 N g.s. reaction when 2ħω configurations are taken into account.

Estimate ofd?- production in protonproton collisions

An estimate of the cross section for the production of a narrow isosinglet 0−-dibaryon (d′) resonance inpp-collisions is given. This cross section is shown to be in the range of a fewμb at incident energies close to threshold. The resonance parameters used are those obtained from a recent analysis of pionic double charge exchange data which at low pion energies clearly exhibit features in accordance with the existence ofd′.

Positive-pion absorption on carbon at low pion energies

The total cross section of positive-pion absorption on carbon with a two-proton final state at pion energies of 26–39 MeV has been measured. At a pion energy of 28.5 MeV some narrow structure exists in the energy dependence of the cross section.

Low-energy pion production in the pp → pp π0 reaction

We consider pion production in the pp → pp π 0 reaction at low pion energies. The total and differential cross sections and the analyzing power of this reaction are calculated. Different assumptions about the dynamics of the πN interaction are analyzed. The obtained results are compared with the experimental data in the energy region under consideration. The pp → pp π 0 cross section turns out to be more sensitive to the off-shell behaviour of the isosymmetric πN amplitude than the pp → d π + cross section.

Pion therapy at TRIUMF. Treatment results for astrocytoma grades 3 and 4: a pilot study

At TRIUMF, (located on the University of B.C. Campus), 53 patients with supratentorial astrocytomata grades 3 and 4 were treated with pions between 1982 and 1985. A 3-dimensional spot-scanning treatment technique has proven to be practical. The accuracy of the beam alignment system used for treatment was reproducible daily within 2 mm. Low pion flux has hindered optimal beam shaping but this will soon be remedied as flux improves. The overall median survival observed (53 patients) is 262 days from date of first radiation treatment. Younger (< 49 years) patients have significantly better survival than older (> 50 years) patients ( p = 0.001). From a base line dose of 40 Gy photons whole brain and 17.5 Gy pion boost, doses were escalated to 33 Gy pions localised to the primary tumour and the median survival improved from 198 to 436 days. Survival curves for patients treated with localised pion techniques to doses above 30 Gy are significantly better than for those treated with schedules of pions mixed with photons ( p = 0.04). It appears that optimal pion dose for brain tumours is 33 Gy minimum with a possible maximum of 36 Gy and doses delivered in 15 fractions in 3 weeks. Requirements for future trials are discussed.

The nuclear equation of state in effective relativistic field theories and pion yields in heavy-ion collisions

Within the framework of relativistic field theory for nucleons, deltas, scalar and vector mesons, a systematic study of the nuclear equation of state and its relation to pion yields in heavy-ion collisions is presented. It is not the compressibility, but the effective nucleon mass at normal nuclear density that turns out to be the most sensitive parameter. Effects from vacuum fluctuations are well modelled within the mean-field no-sea approximation by self-interaction terms for the scalar meson field. Incomplete thermalization in the fireball may be the reason for the low pion yields observed in heavy-ion collisions.

Dynamic pion irradiation of unresectable soft tissue sarcomas

Since November 1981, when pion irradiation was introduced for deep seated tumors at the Swiss Institute for Nuclear Research (SIN, now Paul Scherrer Institute, PSI) a dynamic, 3-dimensional spot scan treatment technique has been in use. To exploit this technique a special optimization treatment planning system has been designed. Of a total of 331 patients treated with pions from November 1981–December 1987, 35 were irradiated for unresectable soft tissue sarcomas. In 32 35 patients, tumor sites were retroperitoneal, pelvic or in the groin or thigh. Twentynine tumors had a maximum diameter of >10 cm, 18 tumors of >15 cm; 30 tumors had grade 2 3 and 32 Stage III B/IV A/IV B. Eight of 35 patients received a low pion total dose, 7–27 Gy. Twenty-seven patients received a total dose of 30–36 Gy, fraction size 150–170 cGy (90%-isodose), 20 fractions, 4 times per week. Of these 27 patients, severe late reactions appeared in five: 2 8 patients with extremity/groin sarcomas ( 1 2 caused by biopsy) and 3/19 patients with retroperitoneal/pelvic sarcomas (one a skin reaction after Actinomycin-D, one a small bowel reaction after 36 Gy, a dose no longer used). Seven of 27 patients had metastases at the beginning of irradiation. Three of 27 were treated with excisional biopsy, 9 with incisional biopsy or partial resection and in 15 patients biopsies were performed for histology only. The median follow-up of these 27 patients was 17 months (5–66). There was no progression in eight extremity/groin tumors but in 4 of 19 retroperitoneal/pelvic tumors. Three of these were marginal progressions. The actuarial 5-year rate of local tumor control is 64%; the actuarial 5-year survival rate of patients without metastases at the beginning of treatment is 58%. Dynamic spot scan pion irradiation proves to be a successful treatment technique for unresectable sarcomas with a high rate of tumor control and a very low rate of severe late reactions.

Pion and Proton "Temperatures" in Relativistic Heavy-Ion Reactions

Pion and proton production are measured to investigate thermal equilibrium in central collisions of $^{40}\mathrm{Ar}$+KCl at 1.8 GeV/nucleon. The bulk of the pion yield is isotropic in the c.m. system, with an apparent temperature of 58\ifmmode\pm\else\textpm\fi{}3 MeV, much lower than the 118\ifmmode\pm\else\textpm\fi{}2 MeV of the protons. It is shown that the low pion "temperature" can be explained by the decay kinematics of delta resonances in thermal equilibrium. A (5\ifmmode\pm\else\textpm\fi{}1)% component in the pion spectrum is, however, found to have a temperature of 110\ifmmode\pm\else\textpm\fi{}10 MeV. The effect on the spectra of possible contributions from collective radial flow is discussed.

Relevance of final-state nucleon-nucleon charge exchange in inclusive (π,πN) reactions

The Sternheim-Silbar linear transport model for calculating the effect of final-state charge exchange of the struck nucleon in (..pi..,..pi..N) reactions has been reexamined. It is demonstrated that for /sup 12/C, the effect ranges from only 10% at low pion energies to 4% at the (3,3) resonance and plays a minor role in determining the magnitude of the inclusive cross sections. A semiclassical linear transport model is used to calculate the ..pi../sup -//..pi../sup +/ ratio for a clean, one-step nucleon knockout. Unlike the Sternheim-Silbar calculation these results are in very good agreement with intranuclear cascade model predictions.

The reaction cross section for low-energy pion-nucleus scattering

It is shown that for low pion energies it is possible to calculate the pion absorption part of the pion-nucleus reaction cross section from a detailed knowledge of the imaginary part of the pion-nucleus optical potential. Such calculations provide tests of various theories of low-energy pion-nucleus dynamics.

Pion asymmetry in the three-body radiative decays of polarized hyperons

We have calculated the energy dependence of pion asymmetry relative to the direction of hyperon polarization in the three-body radiative decays of polarized hyperons, using the inner bremsstrahlung model. We find that in the low pion energy region, the pion asymmetry in the radiative decay relative to the non-radiative decay is kinematically enhanced. For certain decays, this effect could be exploited experimentally to obtain additional information about the properties of the hyperons.