Total Annihilation Cross Section Research Articles

Upper Bound on the Dark Matter Total Annihilation Cross Section

We consider dark matter annihilation into standard model particles and show that the least detectable final states, namely, neutrinos, define an upper bound on the total cross section. Calculating the cosmic diffuse neutrino signal, and comparing it to the measured terrestrial atmospheric neutrino background, we derive a strong and general bound. This can be evaded if the annihilation products are dominantly new and truly invisible particles. Our bound is much stronger than the unitarity bound at the most interesting masses, shows that dark matter halos cannot be significantly modified by annihilations, and can be improved by a factor of 10-100 with existing neutrino experiments.

Model-independent dark matter annihilation bound from the diffuse gamma ray flux

An upper limit on the total annihilation cross section of dark matter (DM) has recently been derived from the observed atmospheric neutrino background. We show that comparable bounds are obtained for DM masses around the TeV scale by observations of the diffuse gamma-ray flux by EGRET, because electroweak bremsstrahlung leads to non-negligible electromagnetic branching ratios, even if DM particles only couple to neutrinos at tree level. A better mapping and the partial resolution of the diffuse gamma-ray background into astrophysical sources by the GLAST satellite will improve this bound in the near future.

Measurement of [formula omitted] and [formula omitted]-nucleus annihilation cross sections at very low energy at LEAR

An experimental procedure for the measurement of the p ̄ p and p ̄ -nucleus total annihilation cross sections, at very low p ̄ incident momenta (between 174 and 37 MeV/c ), has been developed by the Obelix experiment at LEAR. The procedure is based on the ability in using gaseous targets at different pressures (from 10 −4 to 3 bar absolute pressure) and in recording both the coordinates of the vertex and the time of the annihilation. In this work, a detailed description of this experimental method is presented.

Antiproton annihilation on light nuclei at very low energies

The recent experimental data obtained by the OBELIX group on $\bar{p}$D and $\bar{p}^4$He total annihilation cross sections are analyzed. The combined analysis of these data with existing antiprotonic atom data allows, for the first time, the imaginary parts of the S-wave scattering lengths for the two nuclei to be extracted. The obtained values are: $Im a^{sc}_0 = [- 0.62 \pm 0.02 ({stat}) \pm 0.04 ({sys})] fm$ for $\bar{p}$D and $Im a^{sc}_0 = [- 0.36\pm 0.03({stat})^{+0.19}_{-0.11}({sys})] fm$ for $\bar{p}^4$He. This analysis indicates an unexpected behaviour of the imaginary part of the $\bar{p}$-nucleus S-wave scattering length as a function of the atomic weight A: $|Im a^{sc}_0|$ ($\bar{p}$p) > $|Im a^{sc}_0|$ ($\bar{p}$D) > $|Im a^{sc}_0|$ ($\bar{p}^4$He).

Formula omitted] and [formula omitted] annihilation cross sections at very low energy

The p ̄ D and p ̄ 4 He total annihilation cross sections have been measured with the Obelix apparatus at LEAR at, respectively, three and two values of the p ̄ incident momentum between 70 MeV/c and 36 MeV/c. The values of the p ̄ D annihilation cross section, at such low energies, are in agreement with a surprising result obtained in a recent low statistics measurement of the antiprotonic deuterium K α X-rays [M. Augsburger et al., Phys. Lett. B 461 (1999) 417] (see following article), concerning the unexpected narrow width of the antiprotonic deuterium 1s level.

New measurements of the [formula omitted] annihilation cross section at very low energy

The pp total annihilation cross section has been measured at four values of the p ̄ incident momentum, between 70 MeV/c and 38 MeV/c, with the Obelix apparatus at LEAR. The new measurements are in agreement with the trend of previous measurements of the pp total annihilation cross section at low energy, performed by the Obelix experiment [A. Bertin et al., Phys. Lett. B 369 (1996) 77; A. Benedettini et al., Nucl. Phys. B (Proc. Suppl.) 56A (1997) 58], as well as with a fit of the latter data based on a low energy expansion of the scattering amplitude [J. Carbonell, K.V. Protasov, A. Zenoni, Phys. Lett. B 397 (1997) 345]. The departure of the annihilation cross section from a smooth behaviour, suggested by a previous measurement of the cross section around 44 MeV/c [A. Bertin et al., Phys. Lett. B 369 (1996) 77], is not confirmed by the new data.

P̄P partial cross sections at low energy

New measurements of the pp total annihilation cross-section at very low momenta (around 50 MeV/c) confirm the previous results obtained by the OBELIX collaboration. The measured values are in general agreement with a scattering length approximation of the annihilation cross-section where the pp scattering lengths are obtained from measurement of the protonium shifts and widths and which accounts for the Coulomb interaction. Partial annihilation cross-sections and annihilation frequencies for some channels have been measured too. The values of the total and partial cross sections at the lowest p momentum show an unexpected behaviour

Formula omitted]p low energy parameters from annihilation cross section data

The recent experimental data obtained by the OBELIX group on total pp annihilation cross section are analysed; the low energy spin averaged parameters of the pp scattering amplitude (the imaginary parts of the S-wave scattering length and P-wave scattering volume) are extracted from the data. Their values are found to be equal to Im a sc = −0.69 ± 0.01 (stat) ± 0.03 (sys) fm, Im A sc = −0.76 ± 0.05 (stat) ± 0.04 (sys) fm 3. The results are in very good agreement with existing atomic data.

P̄p annihilation cross section at very low energy

The p̄p total annihilation cross section has been measured, with the Obelix apparatus at LEAR, at ten values of the antiproton incident momentum between 43 and 175 MeV/ c. The values of the cross section show that the well known 1 p behaviour of the annihilation cross section is drastically modified at very low momenta, which demonstrates the important role of the Coulomb force in low energy p̄p interaction. Moreover, they do not present any explicit resonant behaviour. Finally, when compared to potential model calculations, the data suggest that the percentage of P-wave in p̄p interaction around 50 MeV/ c antiproton incident momentum is less than 5%.

Nuclear physics with OBELIX

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 p d → π − p (Pontecorvo reaction) with the better statistics and precision up to now, is reported. Finally, the measurement

New limits on parameters of the supersymmetric standard model from cosmology

We show that there are large regions of parameter space for the supersymmetric standard model in which the lightest supersymmetric particle (LSP) is a nearly pure bino (partner of the hypercharge gauge boson) or particular higgsino (symmetric or antisymmetric linear combination of the two neutral higgsinos with definite hypercharges). In particular, a very massive LSP is almost always one of these possibilities. We calculate the total annihilation cross section for these LSPs, including gauge and Higgs bosons as final states. By computing the relic abundance of LSPs left over from the Big Bang, we can rule out some portions of the parameter space. In other portions there are not enough LSPs to comprise a significant fraction of the dark matter.

Formula omitted] annihilation and scattering in a quark model

p p annihilation into mesons in flight and at rest are analysed in the constituent quark model with the 3P 0 and the 3S 1 model for the q q pair creation/annihilation vertex. In the 3P 0 model, one assumes that the created/annihilated q q pair has the quantum number of the vacuum, while in the 3S 1 model, it has the quantum number of a gluon. As the meson channels we consider combinations of s-wave mesons and p-wave mesons, and in the annihilation into two mesons we compare the annihilation model (A2), the rearrangement model with a 3P 0 vertex (R2) and the rearrangement with a 3S 1 vertex (S2). The initial state interaction is taken into account by solving the coupled Lippmann-Schwinger equation including virtual N Δ ± NΔ and Δ Δ channels. The combination of R3 and A2 in the 3P 0 model roughly reproduces the branching ratios of p p annihilation into mesons, while the combination of R3 and R2 and the combination of R3 and S2 show worse agreement with the experimental branching ratios. The total p p annihilation cross section turned out to be about 60% of the experimental value for R3 + A2 and R3 + R2 and about 70% for R3 + S2, which means that we need other mechanisms of p p annihilation into mesons. The channel of virtual Δ's modifies the branching ratios of certain partial waves, but in the total annihilation cross section, the effect of Δ's is not large.

Studies of $$\bar pp \to \bar nn$$ and $$\bar np$$ annihilation at incident momenta of 700 and 760 MeV/c

Results are presented on the charge exchange reaction\(\bar pp \to \bar nn\) and\(\bar np\) annihilations from bubble chamber exposures to antiproton beam of momenta 700 and 760 MeV/c. The differential cross section of\(\bar pp \to \bar nn\) shows a forward spike followed by a clear dip bump structure. Total annihilation cross section of\(\bar np\) for average\(\bar n\) momentum of 700 MeV/c has been evaluated to be 55.4±2.2 mb. The multiplicity, Feynmanx andpT2 distributions for inclusive charged pions in\(\bar pp\) and\(\bar np\) annihilations are found to be similar. The emission of charged pions from\(\bar np\) annihilations are found to be consistent with thermodynamic models with temperature ∼110 MeV.

Electron-Positron Annihilation Cross Section inSU(2)L⊗U(1)Supergravity

For a wide range of masses of the $W$ superpartners and the supersymmetric scalars it is shown that a measurement of the total electron-positron annihilation cross section is sufficient to demonstrate the existence or nonexistence of supersymmetry.

Measurement of [formula omitted] annihilation cross sections into charged particles in the momentum range 374–680 MeV/ c

pp annihilation cross sections into 2-, 4- and 6-prong topologies, and the exclusive annihilation cross sections for the π + π −, K +K −, π + π − π 0, π + π + π − π −, π + π + π − π − π 0, π + π + π + π − π − π −, and π + π + π + π − π − π − π 0 channels in the momentum range 374–680 MeV/ c were measured. No prominent structure was observed in the momentum dependence of any of the above cross sections, but a small enhancement in the cross section was observed at 490 MeV/ c corresponding to the S meson mass in the topological, and π + π − π 0 and π + π + π − π − π 0 cross sections. If the extensive of the S meson with a mass and width of 1935.5 and 2.8 MeV/ c 2 is assumed, our measurement gives a total resonant annihilation cross section into charged particles of 10.0±3.0 mb.

Formula omitted] annihilation and theoretical models of particle-nuclei interactions

We study p d annihilation as a suitable place to test several theoretical models. The coherent tube model and models of the Glauber-Gribov type give rise to different behaviour for the momentum distributions of isolated double scattering annihilation events. Also the total distribution and total annihilation cross section are very different in both kinds of models.

Annihilation of the antideuteron-deuteron system

The interactions¯dd at intermediate energies are studied from the point of view of the annihilation mechanism. Using the Glauber approximation and wave function of the deuteron the total annihilation cross section is estimated to be 0·9±0·3 mb at 12 GeV/c primary momentum of the antideuteron. The multiplicity distribution in¯dd annihilation and non-annihilation channels is also derived. Clustering of secondary particles in single- and multiparticle distributions is studied by a sample of Monte Carlo events. The results of the study lead to a proposal for an experiment in the 2 m hydrogen bubble chamber in Serpuchov. The estimated minimal size of the experiment is to be 1 μb/event at 12 GeV/c.

Distributions for electron-positron annihilation into two and three photons

In this paper a procedure is outlined to simulate events for the reactions e +e − → γγ and γγγ. Various distributions thus obtained are presented. The standard radiative corrections are also calculated in this way. Moreover, an analytical expression for the total annihilation cross section is given up to order α 3.

Confinement, form factors, and deep-inelastic scattering in two-dimensional quantum chromodynamics

We clarify the gauge invariance, infrared structure, and completeness of 't Hooft's solution for the meson sector of two-dimensional quantum chromodynamics. Electromagnetic form factors of mesons are then shown to obey an asymptotic power law, whose power is dynamically determined and is not related to the shortdistance behavior of the theory. Following a review of the total annihilation cross section for producing hadrons, we discuss deep-inelastic lepton scattering. As expected, Bjorken scaling is obtained, but we show how the sum over hadronic final states reproduces the parton model precisely. The Drell-Yan-West relation and Bloom-Gilman duality are fulfilled for the relation between the scaling function and form factors. We conclude by speculating on the applicability of our picture of form factors to the real, four-dimensional world. We argue that this is a viable alternative to dimensional scaling and, phenomenologically, the differences between our predictions and the dimensional counting rules are slight for light quarks. Finally, we attempt to abstract those features of the model which may guide us toward a solution to the four-dimensional problem.

A new unitarity bound on e +e − annihilation

Unitarity relates σ h the electron-positron annihilation cross-section into hadrons and σ μ the cross-section into muon pairs. This relation provides much more restrictive bounds on σ h than the simple unitarity bound, for the case in which the hadronic channels dominate the total annihilation cross-section.