Cosmic-ray Antiprotons Research Articles

The Cosmic‐Ray Antiproton Flux between 3 and 49 GeV

We report on a new measurement of the cosmic ray antiproton spectrum. The data were collected by the balloon-borne experiment CAPRICE98 which was flown on 28-29 May 1998 from Fort Sumner, New Mexico, USA. The experiment used the NMSU-WIZARD/CAPRICE98 balloon-borne magnet spectrometer equipped with a gas Ring Imaging Cherenkov (RICH) detector, a time-of-flight system, a tracking device consisting of drift chambers and a superconducting magnet and a silicon-tungsten calorimeter. The RICH detector was the first ever flown capable of mass-resolving charge-one particles at energies above 5 GeV. A total of 31 antiprotons with rigidities between 4 and 50 GV at the spectrometer were identified with small backgrounds from other particles. The absolute antiproton energy spectrum was determined in the kinetic energy region at the top of the atmosphere between 3.2 and 49.1 GeV. We found that the observed antiproton spectrum and the antiproton-to-proton ratio are consistent with a pure secondary origin. However, a primary component may not be excluded.

Successive measurements of cosmic-ray antiproton spectrum in a positive phase of the solar cycle

The energy spectrum of cosmic-ray antiprotons (p̄'s) has been measured by BESS successively in 1993, 1995, 1997 and 1998. In total, 848 p̄'s were clearly identified in energy range 0.18–4.20 GeV. From these successive measurements of the p̄ spectrum at various solar activity, we discuss about the effect of the solar modulation and the origin of cosmic-ray p̄'s. The p̄/p ratios showed no distinctive year-to-year variation during the positive Sun's polarity phase.

Performance of the CAPRICE98 balloon-borne gas-RICH detector

A RICH counter using a gas radiator of C 4F 10 and a photosensitive MWPC with pad readout has been developed, tested in particle beam at CERN and used in the CAPRICE98 balloon-borne experiment. The MWPC was operated with a TMAE and ethane mixture at atmospheric pressure and used a cathode pad plane to give an unambiguous image of the Cherenkov light. The induced signals in the pad plane were read out using the AMPLEX chip and CRAMS. The good efficiency of the Cherenkov light collection, the efficient detection of the weak signal from single UV photons together with a low noise level in the electronics of the RICH detector, resulted in a large number of detected photoelectrons per event. For β≃1 charge one particles, an average of 12 photoelectrons per event were detected. The reconstructed Cherenkov angle of 50 mrad for a β≃1 particle had a resolution of 1.2 mrad (rms). The RICH was flown with the CAPRICE98 magnetic spectrometer and was the first RICH counter ever used in a balloon-borne experiment capable of identifying charge one particles at energies above 5 GeV . The RICH provided an identification of cosmic ray antiprotons up to the highest energies ever studied (about 50 GeV of total energy). The spectrometer was flown on 28–29 May 1998 from Fort Sumner, New Mexico, USA.

Cosmic-ray antiprotons and antinuclei

Recent progress in experimental observations and theoretical investigations of cosmic-ray antiprotons ( p 's) have brought new light on their origin. The p energy spectrum measured by the recent BESS experiment shows the characteristic peak around 2 GeV, which was expected from “secondary” p 's, i.e., produced in cosmic-ray interactions with the interstellar gas. The MASS and CAPRICE experiments have detected p 's in the higher energy range and show that the flux decreases beyond the peak. These new data are well reproduced by theoretical calculations recently developed. Further sensitive observations with still higher statistics are required to investigate the propagation and solar modulation models in detail and to search for the primary p component from novel sources, e.g. evaporating black holes or annihilating neutralino dark matter. In the case of antinuclei searches, an upper limit of a level of 10 −6 is obtained for the antihelium/helium ratio both from the BESS balloon flight and the AMS space mission. In the future, BESS-polar and AMS on the ISS (International Space Station) may be expected to reach upper limits of 10 −7 and 10 −8 , respectively.

Precise measurements of cosmic-ray antiproton spectrum in a positive phase of the solar cycle

The BESS spectrometer has measured the energy spectrum of cosmic-ray antiprotons successively in 1993, 1995, 1997 and 1998. In total, 848 antiprotons were clearly identified in energy range 0.18 to 4.20 GeV based on magnetic-rigidity, time-of-flight, energy loss, and Cherenkov radiation measurements. The measured antiproton spectra and antiproton/proton ratios were nearly identical at the upper end of the instrumental energy range, which is consistent with theoretical calculations. In the low energy region, our data show variations that are somewhat larger than expected. This might be due to statistical fluctuation, or might suggest a contribution of primary antiprotons from novel sources.

Supersymmetric dark matter

When combined with the wealth of observational evidence for a nonrelativistic matter density Ω 0≳0.3 , the big-bang-nucleosynthesis constraint on the baryon density, Ω b ≲0.1 indicates the existence of a significant amount of nonbaryonic dark matter. Several lines of reasoning suggest that the dark matter consists of some new, as yet undiscovered, weakly interacting massive particle (WIMP). Of the WIMP candidates that have been considered, perhaps the best-motivated and certainly the most theoretically developed is the neutralino, the lightest supersymmetric particle (LSP) in many supersymmetric theories. There is now a vast experimental effort being mounted to detect these particles in the Galactic halo. Techniques include direct detection in low-background laboratory detectors, indirect detection through observation of energetic neutrinos from annihilation of WIMPs that have accumulated in the Sun and/or the Earth, and observation of anomalous cosmic-ray antiprotons, positrons, and gamma rays.

First Mass-resolved Measurement of High-Energy Cosmic-Ray Antiprotons

We report new results for the cosmic-ray antiproton-to-proton ratio from 3 to 50 GeV at the top of the atmosphere. These results represent the first measurements, on an event-by-event basis, of mass-resolved antiprotons above 18 GeV. The results were obtained with the NMSU-WIZARD/CAPRICE98 balloon-borne magnet spectrometer equipped with a gas-RICH (Ring-Imaging Cerenkov) counter and a silicon-tungsten imaging calorimeter. The RICH detector was the first ever flown that is capable of identifying charge-one particles at energies above 5 GeV. The spectrometer was flown on 1998 May 28-29 from Fort Sumner, New Mexico. The measured p&d1;/p ratio is in agreement with a pure secondary interstellar production.

Precision Measurement of Cosmic-Ray Antiproton Spectrum

The energy spectrum of cosmic-ray antiprotons ( &pmacr;'s) has been measured in the range 0.18-3.56 GeV, based on 458 &pmacr;'s collected by BESS in a recent solar-minimum period. We have detected for the first time a characteristic peak at 2 GeV of &pmacr;'s originating from cosmic-ray interactions with the interstellar gas. The peak spectrum is reproduced by theoretical calculations, implying that the propagation models are basically correct and that different cosmic-ray species undergo a universal propagation. Future BESS data with still higher statistics will allow us to study the solar modulation and the propagation in detail and to search for primary &pmacr; components.

The Japanese balloon program

Abstract The Japanese scientific ballooning program has been organized by ISAS since the institute was founded in mid 1960s. Since then, the balloon group of ISAS has been engaged in the development of the balloon technologies and scientific observations in collaboration with scientists and engineers in other universities and organizations. Here, I describe several subjects of recent activities, the details of some items will also be reported in the separate papers in this meeting. • • Preparation of a new mobile receiving station. • • Balloons of made of the EVAL (Ethylene-Vinyl-Alcohol) films. EVAL film has specific Infra-red absorption bands, and is expected to be useful for saving the ballast for a long duration flight. • • A high altitude balloon with thin polyethylene films achieving at an altitude of above 50km. Further improvement of this type of balloons is continued by inventing how to extrude thin films less than 5 microns of thickness. • • Recent achievement of Antarctica Flights under the collaboration of ISAS and National Polar Institute. • • Other new efforts to long duration flights such as satellite link boomerang balloon systems and others. • • New balloon borne scientific instrumentation for observations of high energy electrons and Anti-protons in cosmic-rays.

Precision measurement of antiproton spectrum with BESS

The absolute fluxes of the cosmic-ray antiproton are measured at solar minimum in the energy range 0.2 to 3.2 GeV, based on 460 antiprotons unambiguously detected by BESS spectrometer during its '95 and '97 balloon flights from Lynn Lake, Canada. In the resultant antiproton spectrum we have detected a clear peak around 2 GeV and measured its flux to 10 % accuracy. The position and the absolute flux of the peak agree with the prediction of the Standard Leaky Box model for the “secondary” antiprotons. At low energies below 1 GeV, we observe an excess antiproton flux over the simple Standard Leaky Box prediction. This might indicate that the propagation mechanism needs to be modified, or might suggest a contribution of low-energy antiproton component from novel sources such as evaporating primordial black holes or the annihilating neutralino dark matter. Data from '98 and future flights are expected to help us to clarify the situation.

Cosmic Antiprotons as a Probe for Supersymmetric Dark Matter?

The flux of cosmic ray antiprotons from neutralino annihilations in the galactic halo is computed for a large sample of models in the MSSM (the Minimal Supersymmetric extension of the Standard Model). We also revisit the problem of estimating the background of low-energy cosmic ray induced secondary antiprotons, taking into account their subsequent interactions (and energy loss) and the presence of nuclei in the interstellar matter. We consider a two-zone diffusion model, with and without a galactic wind. We find that, given the uncertainties in the background predictions, there is no need for a primary (exotic) component to explain present data. However, allowing for a signal by playing with the uncertainties in the background estimate, we discuss the characteristic features of the supersymmetric models which give a satisfactory description of the data. We point out that in some cases the optimal kinetic energy to search for a signal from supersymmetric dark matter is above several GeV, rather than the traditional sub-GeV region. The large astrophysical uncertainties involved do not, one the other hand, allow the exclusion of any of the MSSM models we consider, on the basis of data. We present besides numerical results also convenient parameterizations of the antiproton yields of all `basic' two-body final states. We also give examples of the yield and differential energy spectrum for a set of supersymmetric models with high rates. We also remark that it is difficult to put a limit on the antiproton lifetime from present measurements, since the injection of antiprotons from neutralino annihilation can compensate the loss from decay.

Combining the data of annual modulation effect in WIMP direct detection with measurements of WIMP indirect searches

In previous papers we showed that the data of the DAMA/NaI experiment for WIMP direct detection, which indicate a possible annual modulation effect, are widely compatible with an interpretation in terms of a relic neutralino as the major component of dark matter in the Universe. In the present note we discuss the detectability of the relevant supersymmetric neutralino configurations by two of the most promising methods of indirect search for relic particles: measurement of cosmic-ray antiprotons and measurement of neutrino fluxes from Earth and Sun.

Which fraction of the measured cosmic-ray antiprotons might be due to neutralino annihilation in the galactic halo?

We analyze the data of low-energy cosmic-ray antiproton spectrum, recently published by the BESS Collaboration, in terms of newly calculated fluxes for secondary antiprotons and for a possible contribution of an exotic signal due to neutralino annihilation in the galactic halo. We single out the relevant supersymmetric configurations and discuss their explorability with experiments of direct search for particle dark matter and at accelerators. We discuss how future measurements with the Alpha Magnetic Spectrometer (AMS) on the Shuttle flight may disentangle the possible neutralino-induced contribution from the secondary one.

Measurement of Low-Energy Cosmic-Ray Antiprotons at Solar Minimum

The absolute fluxes of the cosmic-ray antiprotons at solar minimum are measured in the energy range 0.18 to 1.4 GeV, based on 43 events unambiguously detected in BESS '95 data. The resultant energy spectrum appears to be flat below 1 GeV, compatible with a possible admixture of primary antiproton component with a soft energy spectrum, while the possibility of secondary antiprotons alone explaining the data cannot be excluded with the present accuracy. Further improvement of statistical accuracy and extension of the energy range are planned in future BESS flights.

Development of a large-area aerogel Čherenkov counter onboard BESS

This paper describes the development of a threshold-type aerogel Čherenkov counter with a large sensitive area of 0.6 m 2 to be carried onboard the BESS rigidity spectrometer to detect cosmic-ray antiprotons. The design incorporates a large diffusion box containing 46 finemesh photomultipliers, with special attention being paid to achieving good performance under a magnetic field and providing sufficient endurance while minimizing material usage. The refractive index of the aerogel was chosen to be 1.03. By utilizing the muons and protons accumulated during the cosmic-ray measurements at sea level, a rejection factor of 10 4 was obtained against muons with β≈1, while keeping 97% efficiency for protons below the threshold.

Effects of Solar Modulation on the Low‐Energy Cosmic‐Ray Antiproton/Proton Ratio

We examine the transport of cosmic-ray protons and anitprotons from local interstellar space through the interplanetary medium to Earth and discuss the resulting effects on the low-energy antiproton/proton ratio at 1 AU. We find that the antiproton/proton ratio at energies above ~3 GeV is a useful diagnostic of cosmic-ray transport in the Galaxy. However, at energies below ~1 GeV the expected ratio is much more uncertain because of differences in the energy spectra and the resulting relative modulation of protons and antiprotons over the solar cycle, as well as uncertainties in the interstellar spectra. Using calculated interstellar spectra as references, we find that the antiproton/proton ratio at low energies varies by as much as an order of magnitude over the solar cycle. As a result, we recommend that attention be given instead to interpretation of the measured cosmic-ray antiproton energy spectrum rather than to the antiproton/proton ratio.

Cosmic ray antiproton spectrum measured by BESS in 1993–1995

Balloon-borne experiments using a superconducting spectrometer were carried out successfully in northern Canada, in summer 1993 to 1995. In '93 flight, seven antiprotons were detected in the kinetic range between 200 and 600 MeV. The corresponding cosmic ray antiproton flux at the top of the atmosphere is found to be 6.4 −3.5 +5.5×10 −3(m 2 sr s GeV) −1. In '95 flight, improvement on the TOF resolution resulted to the observation of the forty antiprotons in the kinetic energy region 175 MeV to 1.3 GeV. The analysis on '95 flight is under way.

Cosmic‐Ray Antiproton Flux in the Energy Range from 200 to 600 MeV

We have studied the low-energy antiprotons in the cosmic rays by utilizing data obtained by the Balloon-borne Experiment with a Superconducting magnetic rigidity Spectrometer (BESS) flown in 1993 July from Lynn Lake, Manitoba, Canada. A detailed description of the event selection criteria and background corrections is given. Seven antiprotons are found that give an antiproton flux of 6.4 -->+ 5.5?3.5 ? 10-3(m2 sr s GeV)-1 and an antiproton/proton ratio of 5.2 -->+ 4.4?2.8 ? 10-6 in the 200-600 MeV energy range. These results are consistent with a secondary origin of low-energy cosmic-ray antiprotons within our uncertainties, but they still require the precise measurement of the spectrum shape below 500 MeV to clarify exactly the model of particle propagation and possible contributions from exotic sources.

Expected enhancement of the primary antiproton flux at the solar minimum

We calculate the solar-modulated energy spectra of cosmic-ray antiprotons ( p 's) from two candidate primary sources, i.e., evaporating primordial black holes and the annihilation of neutralino dark matter, as well as for the secondary p 's produced by cosmic-ray interactions with interstellar gas. A large enhancement toward the solar minimum phase emerges in the low-energy flux of p 's from the primary sources, whereas the flux of the secondary p 's, falling steeply below 2 GeV, does not significantly vary. This enables us to conduct a very sensitive search for primary p components by precisely measuring the p spectrum, especially at low energies, throughout the forthcoming solar minimum phase.

Cosmic ray antiproton/electron discrimination capability of the CAPRICE silicon-tungsten calorimeter using neural networks

A data analysis based on an artificial neural network classifier is proposed to identify cosmic ray antiprotons detected with the CAPRICE silicon-tungsten imaging calorimeter against electron background in the energy range 1.2–4.0 GeV. A set of new physical variables, describing the events inside the calorimeter on the base of their different patterns, are introduced in order to discriminate between hadronic and electromagnetic showers. The ability of the artificial neural network classifier to perform a careful multidimensional analysis gives the possibility to identify antiprotons with an electron rejection 408±85 (stat) at 95.0±0.2 (stat)% of signal detection efficiency. The high accuracy achieved by this method improves substantially the efficiency in the evaluation of the cosmic ray antiproton spectrum.