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The OBELIX Collaboration,
V. G. Ableev, A. Adamo,
M. Agnello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G. C. Bonazzola, [......],
G. Usai,
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: The branching ratios of Φπ− and ωπ− final states were measured for the antiproton annihilation in gaseous deuterium at rest. A significant violation of the OZI
rule was found.
Il Nuovo Cimento A 10/1994; 107(11):2315-2320.
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The OBELIX Collaboration,
V. G. Ableev, A. Adamo,
A. Andrighetto,
M. Angello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio, [......],
G. Usai,
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: We describe some results obtained by the OBELIX experiment at LEAR by exploiting the whole spectrometer, including the electromagnetic
calorimeter, to study the antiproton-proton annihilation at rest in an NTP hydrogen target. In particular, we give some information
on the calibration of the calorimeter, as obtained from the analysis of two-prong events, and we describe the status of the
analysis of the first sample of «all neutrals» events collected in gaseous hydrogen. A preliminary result on the branching
ratio of the annihilation reaction
[`(p)]p ® p0 p0\bar pp \to \pi ^0 \pi ^0
is reported.
Il Nuovo Cimento A 10/1994; 107(11):2287-2303.
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The OBELIX Collaboration,
V. Ableev, A. Adamo,
M. Agnello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G. C. Bonazzola, [......],
G. Usai,
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: A study of
[`(n)]p\bar np
in-flight annihilations is presented. The 4π invariant mass spectrum resulting from the analysis of the five-prong exclusive
events, following the annihilation of ∼ (100–297) MeV/c
[`(n)]\bar n
, reveals, besides a large structure centred at ∼ 1500 MeV/c2, some evidences for a new state around 1660 MeV/c2.
Il Nuovo Cimento A 10/1994; 107(11):2243-2252.
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V. G. Ableev, A. Adamo,
M. Agnello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G. C. Bonazzola,
T. Bressani, [......],
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
E. Zavattini,
A. Zenoni,
A. Zoccoli,
G. Zosi,
The OBELIX Collaboration
[show abstract]
[hide abstract]
ABSTRACT: The effect of contaminants (H2, N2, Ne, Ar, Xe) on the time distribution of delayed annihilations (average delay time about 3 μs) of antiprotons stopped in
helium gas, at 3 atm and room temperature, has been experimentally investigated. The annihilation rate is expressed in terms
of an empirical «quenching cross-section»,
[`(s)]\bar \sigma
, that is different for the different contaminants. For hydrogen and xenon at low concentrations,
[`(s)]\bar \sigma
is around 3.5·10−16 cm2, and is more than ten times larger than for the other contaminants. It has been observed that the addition of a contaminant,
with the exception of neon, gives also rise to a delayed annihilation’s component that decays with an average time of some
hundreds of nanoseconds. It seems that this fast component increases and its average time decreases as the contaminant concentration
increases.
Il Nuovo Cimento A 07/1994; 107(8):1325-1337.
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V. G. Ableev, A. Adamo,
M. Agnello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G. C. Bonazzola,
E. Botta, [......],
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi,
OBELIX Collaboration
[show abstract]
[hide abstract]
ABSTRACT: Annihilation cross-sections σann for antineutrons on some nuclei (C, Al, Cu, Sn and Pb) at three antineutron momenta (180, 240 and 280 MeV/c) were measured
at LEAR (CERN) with the OBELIX spectrometer. A behaviour σann=σ0
A
ν has been found withν≈2/3. The data are discussed following some models for antineutron-nucleus interaction.
Il Nuovo Cimento A 05/1994; 107(6):943-953.
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The OBELIX Collaboration, A. Adamo,
M. Agnello,
F. Balestra,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G. C. Bonazzola,
T. Bressani,
M. Bruschi, [......],
G. Usai,
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: A preliminary study of
[`(n)]p\bar np
three-prong annihilations into the channels K− K+ π+ and π− π+ π+ π0 is presented for ∼ 100–297 MeV/c
[`(n)]\bar n
momenta. The extracted φπ+/ωπ+ production ratio shows that also in this channel, never measured before using an
[`(n)]p\bar np
beam, the OZI rule is strongly violated.
Il Nuovo Cimento A 01/1994; 107(11):2271-2278.
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The OBELIX Collaboration,
V. G. Ableev, A. Adamo,
M. Agnello,
A. Andrighetto,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio, [......],
G. Usai,
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: The possibility of selecting the initial angular-momentum state from which antiproton-proton annihilation at rest can occur
can be a fundamental tool in meson spectroscopy. With the OBELIX detector different target systems have been employed, ranging
fromS-wave toP-wave dominance. The preliminary results both on meson spectroscopy and branching ratio measurements are discussed.
Il Nuovo Cimento A 01/1994; 107(11):2279-2286.
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A. Ableev, A. Adamo,
A. Andrighetto,
M. Agnello,
F. Balestra,
G. Bell,
G. Bendiscioli,
A. Benin,
P. Boccaccio,
G. Bonazzola, [......],
G. Usai,
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: Recent experimental results in antinucleon-nucleus interaction obtained by the OBELIX collaboration at LEAR are presented. Preliminary results concerning the total antineutron-nucleus annihilation cross section at low energy are reported. They represent the first complete set of measurements on medium and heavy nuclei available in the momentum region below 300 MeV/c. A new measurement of the exotic one-meson annihilation at rest (Pontecorvo reaction) with the better statistics and precision up to now, is reported. Finally, the measurement
Nuclear Physics A.
-
A. Adamo,
M. Agnello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G.C. Bonazzola,
T. Bressani,
M. Bruschi, [......],
V.I. Tretyak,
G. Usai,
L. Vannucci,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: We have measured the K+K-/π+π- ratio R from annihilations in gaseous D2 and H2 at atmospheric pressure. The measurement was performed with the OBELIX spectrometer. From the measured value in gaseous D2 (0.27±0.02) we infer a P wave contribution to −p annihilation in D2 of (18±7)%.
Physics Letters B.
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A. Adamo,
M. Agnello,
F. Balestra,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G.C. Bonazzola,
T. Bressani,
M. Bruschi,
M.P. Bussa, [......],
G.L. Usai,
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: First results concerning -p and -p annihilation cross section and -p total cross sections at low energies () obtained by the OBELIX spectrometer are reported. The agreement with the few existing data in this energy range is good; the comparison with some models gives interesting indications on the S and P wave contribution at very low energy and on the range of the elastic interaction.
Nuclear Physics A.
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A. Adamo,
M. Agnello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G.C. Bonazzola,
T. Bressani,
M. Bruschi, [......],
G.L. Usai,
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: This paper reports new experimental results on the annihilation of antiprotons stopped in a4He gas target. The measurement was carried out with the Obelix spectrometer installed at the LEAR machine of CERN. Main results can be summarized as follows, (i) The invariant mass distribution of 2π+2π− produced at rest is similar to those from hydrogen and deuterium targets: the peak seen in recent experiments around 1150 MeV/c2 is absent, (ii) 2π angular correlations are very close to those found for hydrogen. The nuclear structure does not modify remarkably the pion production and emission, (iii) πp and pp angular correlations are strongly peaked close to 180°, in qualitative agreement with INC model predictions, (iv) The pp invariant mass distribution reveals events which may be due to η(ω) absorption; the estimated upper limit for probability of π and ω absorption is about 3 × 10−3. (v) Events having features compatible with two-nucleon annihilations with production of heavy mesonic systems are detected both in the 2π+ 2π− and the 2π−π+ channels. The production rate of the 2π+ 2π− channel is of the order of 5 × 10−3.
Nuclear Physics A.
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A. Adamo,
M. Agnello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G.C. Bonazzola,
T. Bressani,
M. Bruschi, [......],
G.L. Usai,
L. Vannucci,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
G. Zavattini,
A. Zenomi,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: The use of low-density hydrogen targets is demonstrated experimentally to be a new way of selecting protonium (p atom) annihilations with initial angular momentum L = 1. The mean value and the width of the time distributions which occur between protonium formation and annihilation have been measured at room temperature at 8.2, 4.1, 2.1 mbar. They are (110±10; 45±5), (145±15; 82±9) and (210±25;125±9) ns, respectively.
Physics Letters B. 285:15-20.
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G.C. Bonazzola,
E. Botta,
T. Bressani,
D. Calvo,
S. Costa,
F. D'Isep,
A. Feliciello,
A. Filippi,
S. Marcello,
N. Mirfakhrai, [......],
A. Lai,
P. Macciotta,
A. Masoni,
L. Musa,
G. Puddu,
S. Serci,
P. Temnikov,
G. Usai,
P. Gianotti,
V. Lucherini
[show abstract]
[hide abstract]
ABSTRACT: The characteristics and performances of a large time of flight detector which is composed by two scintillator barrels, installed between the poles of an axial field magnet as part of the OBELIX spectrometer, are described. The associated electronics, data acquisition system and analysis technique used in the system are discussed in details. The importance of the detector for trigger purposes and for the comprehension of the full event is shown.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment.
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A. Adamo,
M. Agnello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G.C. Bonazzola,
T. Bressani,
M. Bruschi, [......],
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
E. Zavattini,
A. Zenom,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: The stopping power of H, for was measured in the energy interval 1 keV + 120 keV using a low density gas target ( being the Hz density at STP) with the OBELIX spectrometer at the LEAR of CEBN.Furthermore, the formation of metastable states in the interaction at rest, having a mean life of the order of 3 μs, was investigated at six target gas pressures (3 atm and 400,250,200,150,50 mbar). The influence of contaminants (like Ha, Ne, Ar, N2 and Xe) on the life of such metastable states was examined at the target gas pressure of 3 atm.
Nuclear Physics A.
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V.G. Ableev, A. Adamo,
M. Agnello,
A. Andrighetto,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G.C. Bonazzola, [......],
V.I. Tretyak,
G. Usai,
L. Vannucci,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: We report the first measurement of the K+K−, π+π− frequencies for annihilations at rest in a gaseous H2 target at low pressure (5 mbar). We have obtained: fππ = (4.26±0.11)×10−3, fKK = (0.46±0.03)×10−3. The ratio R = fKK/fππ has also been measured at a pressure of 2 mbar. Comparison with previous measurements shows that fKK decreases monotonically with the target density but its value at 5 mbar is still higher than the value with L X-ray in coincidence.
Physics Letters B.
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A. Adamo,
M. Agnello,
F. Balestra,
G. Belli,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G. C. Bonazzola,
T. Bressani,
M. Bruschi, [......],
G. L. Usai,
L. Vannucci,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
G. Zavattini,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: The simultaneous measurement of the spatial coordinates and times of p¯s annihilating at rest in a H2 target at very low density ρ (ρ/ρ0<10-2, ρ0 being the STP density) gives the possibility of evaluating the behavior of the p¯ stopping power in H2 at low energies (below 120 keV). It is different from that of protons (the Barkas effect). Moreover, it is shown that a rise at low-energy values (≲1 keV) is needed to agree with experimental data.
Phys. Rev. A. 47(5).
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V.G. Ableev, A. Adamo,
M. Agnello,
F. Balestra,
G. Bendiscioli,
A. Bertin,
P. Boccaccio,
G. Bonazzola,
T. Bressani,
M. Bruschi, [......],
G. Usai,
L. Vannucci,
S. Vecchi,
G. Vedovato,
L. Venturelli,
M. Villa,
A. Vitale,
A. Zenoni,
A. Zoccoli,
G. Zosi
[show abstract]
[hide abstract]
ABSTRACT: Antinucleon-nucleus annihilations into two-body final states containing only one or no meson are unusual annihilations (Pontecorvo reactions), practically unexplored experimentally, with the exception of the channel , for which only two low-statistics measurements exist. Their physical interest lies in the possibility of exploring small-distance nuclear dynamics, in which an important role can be played by non-nucleonic degrees of freedom. A new measurement of the reaction rate at rest, performed with the OBELIX spectrometer at LEAR, with the best statistics up to now and a careful evaluation of systematic effects is reported, together with a critical analysis of the existing theoretical models. The measured branching ratio, which confirms the previous results, can represent a reference point for the studies in the field.
Nuclear Physics A.
