Measurement of cosmic-ray low-energy antiproton spectrum with the first BESS-Polar Antarctic flight

TL;DR

The paper reports a precise measurement of the low-energy cosmic-ray antiproton spectrum in the range $0.1$--$4.2$ GeV using the first BESS-Polar Antarctic balloon flight, with $N_{ar{p}}=1520$ antiprotons detected over 8.5 days. The authors present a detailed instrument description, data-analysis pipeline, and flux determination at the top of atmosphere, correcting for energy loss, survival probability, and atmospheric secondary production. The results show consistency with secondary production across most energies and place constraints on primary sources such as primordial black holes, while providing insight into solar modulation through the $ar{p}/p$ ratio and its time evolution under different drift models. Overall, the work establishes a baseline for solar-modulation studies and primaries, and offers improved low-energy antiproton data to test propagation and modulation theories ahead of future Antarctic flights.

Abstract

The BESS-Polar spectrometer had its first successful balloon flight over Antarctica in December 2004. During the 8.5-day long-duration flight, almost 0.9 billion events were recorded and 1,520 antiprotons were detected in the energy range 0.1-4.2 GeV. In this paper, we report the antiproton spectrum obtained, discuss the origin of cosmic-ray antiprotons, and use antiprotons to probe the effect of charge sign dependent drift in the solar modulation.

Figures (5)

• Figure 1: Cross-sectional view of the BESS-Polar spectrometer.
• Figure 2: The $\beta^{-1}$ versus rigidity plot, and antiproton selection band. The same band but opposite rigidity sign is applied to select protons. For the negative rigidity, all the events after Čerenkov veto cuts and JET d$E$/d$x$ cut are shown. For the positive rigidity, 0.5 % of the events after Čerenkov veto cuts are shown. Antiproton candidates are shown with dots of larger size.
• Figure 3: Antiproton flux at the top of the atmosphere obtained with the first BESS-Polar flight together with results from previous BESS flights around solar minimum (95+97) Matsunaga98Orito00 and maximum (2000) Asaoka02. The solid curves are calculations of secondary antiproton spectra with the Standard Leaky Box (SLB) model modulated with a steady state drift model Bieber99 by solar tilt angles and magnetic polarities of (from top to bottom, the first two are very close) 10$^\circ(+)$, 10$^\circ(-)$, and 70$^\circ(-)$. The dashed curves are calculations with the Diffusion plus Convection (DC) model Moskalenko02 modulated by (from top to bottom, the first two are very close) 10$^\circ(+)$, 30$^\circ(-)$, and 70$^\circ(-)$. The dotted curves are calculations with the DC model Moskalenko02 modulated with a spherically symmetric model Fisk71 by (from top to bottom) 550 MV, 850 MV, and 1400 MV. The dash-dot curves are calculations of antiproton spectra from evaporation of primordial black holes with an explosion rate of $0.4\times10^{-2}$pc$^{-3}$yr$^{-1}$ modulated by 550 MV(top) and 850 MV(bottom) Maki96Yoshimura01.
• Figure 4: p̄/p ratio obtained with the first BESS-Polar flight together with results from previous BESS flights around solar minimum (95+97) Matsunaga98Orito00 and maximum (2000) Asaoka02. The solid curves are calculations with the Standard Leaky Box (SLB) model modulated with a steady state drift model Bieber99 by solar tilt angles and magnetic polarities of (from bottom to top, the first two are very close) 10$^\circ(+)$, 10$^\circ(-)$, and 70$^\circ(-)$. The dashed curves are calculations with the Diffusion plus Convection (DC) model Moskalenko02 modulated by (from bottom to top, the last two are very close) 10$^\circ(+)$, 30$^\circ(-)$, and 70$^\circ(-)$.
• Figure 5: The top panel shows time variations of the tilt angle Zhao95WSO shown by a solid curve and modulation parameter for the spherically symmetric model Fisk71 shown by a dotted curve. The modulation parameter values continuous over time were estimated by using its linear relation with Climax neutron monitor data Climax. The linear relation was established using the modulation parameter for each BESS flight obtained by fitting the BESS proton spectrum. The other three panels show time variations of the p̄/p ratio at 0.3 GeV (2nd), 1.0 GeV (3rd), and 1.9 GeV (bottom). The data of p̄/p ratio are compared with time variations predicted by two drift models shown by solid curves from Bieber et al. Bieber99 and dashed curves from Moskalenko et al. Moskalenko02, and with the spherically symmetric moduation by Fisk Fisk71 shown by dotted curves.