High-energy Cosmic-ray Collisions Research Articles

Constraining (anti)nuclei measurements relevant for astrophysics with ALICE

Antinuclei can be produced in space either by collisions of highenergy cosmic rays with the interstellar medium or from the annihilation of dark matter particles stemming into standard model particles. High-energy hadronic collisions at accelerators create a suitable environment for producing light (anti)nuclei. Hence, studying the production of (anti)nuclei in pp collisions at the LHC can provide crucial insights into the production mechanisms of nuclear states in our Universe. Recent measurements of the production of (anti)nuclei in and out of jets, and as a function of rapidity in pp collisions at √S = 13 TeV have been carried out with ALICE. The latter allow for the extrapolation of the nuclear production models at forward rapidity, region of interest for indirect searches of dark matter. Recent results on the annihilation crosssection of antinuclei are also discussed in the context of astrophysical measurements of cosmic ray flux. Such information is essential to study the different sources of antinuclei in our Universe and to interpret any future measurement of antinuclei in space.

Measurement of anti-3He nuclei absorption in matter and impact on their propagation in the Galaxy

In our Galaxy, light antinuclei composed of antiprotons and antineutrons can be produced through high-energy cosmic-ray collisions with the interstellar medium or could also originate from the annihilation of dark-matter particles that have not yet been discovered. On Earth, the only way to produce and study antinuclei with high precision is to create them at high-energy particle accelerators. Although the properties of elementary antiparticles have been studied in detail, the knowledge of the interaction of light antinuclei with matter is limited. We determine the disappearance probability of {}^{3}overline{{{{rm{He}}}}} when it encounters matter particles and annihilates or disintegrates within the ALICE detector at the Large Hadron Collider. We extract the inelastic interaction cross section, which is then used as an input to the calculations of the transparency of our Galaxy to the propagation of {}^{3}overline{{{{rm{He}}}}} stemming from dark-matter annihilation and cosmic-ray interactions within the interstellar medium. For a specific dark-matter profile, we estimate a transparency of about 50%, whereas it varies with increasing {}^{3}overline{{{{rm{He}}}}} momentum from 25% to 90% for cosmic-ray sources. The results indicate that {}^{3}overline{{{{rm{He}}}}} nuclei can travel long distances in the Galaxy, and can be used to study cosmic-ray interactions and dark-matter annihilation.

Higgs-particle production through vacuum excitations.

We consider here a nonperturbative mechanism for the production of Higgs particles through vacuum excitations. It appears that it could already have been seen in ``chiron'' events, ``halo'' events, and some other signals in high-energy cosmic-ray collisions.

Can the electroweak vacuum be unstable?

The electroweak vacuum need not be absolutely stable. For certain top-quark and Higgs-boson masses in the minimal standard model, our vacuum is instead metastable with a lifetime exceeding the present age of the Universe. It has been suggested that a metastable vacuum is generally ruled out because high-energy cosmic-ray collisions would have long ago induced its decay. I argue that the reasoning for this conclusion is erroneous. As a consequence, upper bounds on the top-quark mass derived from stability arguments are relaxed. Also presented is an analytic method for accurately approximating the lifetime of the vacuum from the effective potential without solving for the 0(4) bounce solution numerically.

Cosmic-ray induced vacuum decay in the standard model

In the standard model, if the Higgs boson mass, mH, and the top quark mass, mt, satisfy the relationship mt≳95GeV + 0.60 mH, then the vacuum is unstable. However, if the top quark mass is less than 190 GeV, then the lifetime is greater than the age of the universe. There is thus a large region of parameter space in which the vacuum is unstable, but sufficiently long-lived. We examine the possibility that high energy cosmic ray collisions could induce the decay of the vacuum, and show that this region of parameter space can be excluded.

A measurement of the cosmic-ray antiproton flux and a search for antihelium

A balloon-borne instrument has measured the cosmic-ray antiproton flux between 130 and 320 MeV and searched for antihelium between 130 and 370 MeV per nuclear. These particles were selected from the background of normal-matter cosmic rays by combining a selective trigger with a detailed spark chamber visualization of each recorded event. Antiprotons are identified by their characteristic annihilation radiation. Residue from background processes meeting the selection criteria is small. The observed 14 antiprotons yield a measured differential flux of 1.7±0.5X 10^(-4) antiprotons m^(-2) sr(-1) s^(-1)i Mev^(-1) at the top of the atmosphere. The corresponding antiproton/pro-ton ratio is 2.2±0.6X10^(-4), only slightly smaller than the ratio observed by other experiments at higher energies. Thus the antiprotons have a spectral shape similar to the protons, at least down to about 100 MeV. The expected flux of these particles can be calculated under the assumption that they were created by collisions of high-energy cosmic rays with the interstellar gas. Calculations using the standard leaky box model for propagation in the Galaxy predict a flux two orders of magnitude smaller than that observed. A small low-energy flux is predicted due to a kinematic suppression of the production of low-energy antiprotons. The discrepancy between calculations and experiment may be evidence that cosmic-ray protons have passed through substantially more than 5 g cm^(-2) of material during their lifetime. In addition, the combined results from this experiment and previous ones may be evidence for stochastic, energy-changing processes in interstellar space which act upon the secondary antiprotons after their creation. The search for cosmic-ray antihelium sets a 95% confidence level upper limit on the He /He ratio of 2.2 X 10^(-5).

Energy estimates of cosmic-ray events

We propose new methods for estimating the energy of the incident particles in high-energy cosmic-ray collisions. We demonstrate their validity in accelerator experiments.

On the hydrodynamic expansion of a relativistic gas

An exact solution to the one-dimensional relativistic Euler equations is presented which is not of a self-similar nature and which satisfies certain initial conditions found to be appropriate in describing high-energy cosmic-ray collisions and models of radio galaxies. A full numerical analysis is performed to demonstrate the temporal behavior of all relevant thermodynamic quantities at any given point in the expansion. The results indicate that the motion is self-similar at the very beginning of free expansion, that this description ceases to be valid when the entire bulk of fluid is in motion, and that with the passage of time, the nonsimilar solution prevails over more and more of the fluid except for a small region near the leading edge where the fluid borders on the vacuum. The very bulk of the expansion is shown to become nonsimilar because the leading edge of the self-similar solution moves into the vacuum at the speed of light while that of the nonsimilar solution approaches c only asymptotically. The relativistic expansion of a gaseous disk is analyzed as an example.

Abundances of kaons and η-mesons in high-energy cosmic ray collisions

It is shown that a relatively small η-production significantly affects the ratio between γ-rays and charged pions in high-energy cosmic ray collisions. As a consequence, the K ±/ π ± ratio is increased to about 0.44.

Production spectrum of high-energy electrons from high-energy cosmic-ray collisions

Production spectrum of high energy electrons from high energy cosmic ray pion decay and proton- proton collisions

Pion production in high-energy cosmic-ray collisions

The Landau-Milekhin hydrodynamic model of very-high energy nucleon interactions is extended to calculate the pion energy and angle distributions in the center-of-mass system (c.m.s.) and laboratory frames. Consideration of the physical limitations on the average energy of the pions produced in the decomposition of the nucleonic fluids leads to truncation of the pion distribution and a rather slow energy dependence of the average c.m.s. pion energy on the incident proton energy. When a power law dependence is chosen for incident primary cosmic-ray protons, the pion production spectrum found in the laboratory frame has a steeper energy spectrum than those found using simpler models of nucleon interactions.

Production of Low-Energy Cosmic Rays by Collisions of High-Energy Cosmic Rays with Interstellar Gas.

The production rate of protons of energy less than 1 BeV in interstellar space is calculated from empirical cross sections for proton-proton elastic and inelastic collisions in the kinetic energy range 1 to 30 BeV. Representing the cosmic ray intensity by a simple fit to the observed spectrum in the absence of solar activity the production spectrum is found to decrease steadily from zero energy to a value one- tenth as great at 500 MeV. The integrated production rate of 0-500 MeV protons is 10-26 cm-3 sec-1 from elastic collisions, and the contribution from inelastic collisions is estimated to be of lower order. The production spectrum is combined with the slowing lifetimes of protons in the interstellar gas (Meskys and Milford, Bull. Am. Phys. Soc. 8, 305, 1963) to give the density of these 0-500 MeV protons as 10-li cm-3 with an intensity of 10-i cm-2 sec-1. Recently Hayakawa, Nishimura, and Takayanagi (Publ. Astron. Soc. Japan 13, 184, 1961) have suggested that low-energy cosmic rays contribute appreciably to the heating of interstellar gas clouds, with an assumed low-energy cosmic ray intensity of order 100 cm-2 sec-1. From the above calculations it may be seen that high-energy collisions can not supply any significant fraction of such a hypothesized high intensity. Future measurements of low-energy interstellar cosmic rays will indicate what fraction of these can be attributed to the collision mechanism discussed above, and what fraction must be due to other mechanisms, such as stellar injection and interstellar magnetic field acceleration.

Observations on Some High Energy Cosmic-Ray Collisions in Photographic Emulsions

A brief account is given of some high energy disintegrations initiated in photographic emulsions by primary cosmic-ray particles at about 90,000 feet above sea level. In particular, six events which show only relativistic or near relativistic fragments and a typical forward cone of shower particles are described in detail. The angular distribution of the shower particles is, in some of these cases, consistent with them, being due to the multiple production of mesons in a single interaction between an incoming nucleon and a hydrogen nucleus, or a nucleon on the edge of a heavier nucleus, according to the mechanism of Fermi's recent theory. One of these events has thus been interpreted as a collision between an incoming lithium nucleus with an energy of about 2\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ev per nucleon and a hydrogen nucleus in the emulsion.