;-JASR 106Ii9- , ~ 5 Auguat ~11 'u > Avairable online at www.sciencedireclcom .... -" ADVANCES IN ... ; .. ScienceDirect SPACE REsEARCH (a COSPARpuhlicationj ELSEVIER Advances in Space Researrh xxx (2011 ) xxx-xxx www.elsevier.comllocatelasr

2 Search for ~osmic-ray origins and for

3 cosmological with BESS

4 Ql

5 6 7 • 9 10

II • High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan 12 b National Aeronautics and Space Administratkm, Goddard Space Flight Center (NASAIGSFC), Greenbelt, MD 20771, USA

13 C Kobe Unioersity. Kobe, Byago 657-8501. Japan :4 d Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISASIJAXA), Sagamihara, Kanagawa 229-8510, Japan

15 C [PST, University of Maryland, College Park, MD 20742, USA 16 fThe University o/Tokyo, Blmkyo, Tokyo jJ3-0033, Japan 17 I University of Denver, Denver, CO 80208, USA 18

19 Abstract

20 The balloon-borne experiment with a superconducting spectrometer (BESS) has performed cosmic-ray observations as a US-Japan 21 coopeni.tive space science program, and has provided fundamental data on cosmic rays to study elementary particle phenomena in the 22 early . The BESS experiment has measured the energy spectra of cosmic-ray to investigate signatures of possible 23 exotic origins sucb as dark candidates or primordial black boles. and searched for heavier antinuclei that might reach Earth from 24 antimatter domains formed in the early Universe. The apex of the BESS program was reached with the Antarctic flight of BESS-Polar II, 25 during the 2007- 2008 Austral Summer, that obtained over 4.7 billion cosmic-ray events from 24.5 days of observation. The flight took 26 place at the expected solar minimum, when the sensitivity of the low-energy antiproton measurements to a primary source is greatest. 27 Here, we report the scientific restilts, focusing on the long-duration flights of BESS-Polar I (2004) and BESS-Polar II (2007-2008). 28 Ii:> 2011 Published by Elsevier Ltd. on behalf of CaSPAR.

29 Ql Keywords: Cosmic rays; Antiproton; Antimatter; Primordial black hole; ];)ark matter; t;;'ssreis Far pr9J3agatisB; 5&111£ "u,ulwJ.atiaR; Salar miAimYm; 30 "Iltflr;ti;ai Scientific balloon; BESS; IU!SS Pelar " 31

• Co1Te.!lponding author. Tel.: +81 29 864 5459; fax: +81 29 864 3209. E-mail addre~~:[email protected] (A. Yamamoto), I Present address: ICRR, The University of Tokyo, Kamioka, Qifu 506-1205, Japan. 2 Present address: Instituto Nazionale di Fisica Nucleare (INFN), Perugia, ItdY. ) Also University of Maryland Baltimore County, Ba1timore. MD, USA . .. Present address: Physics Department, Technion - Israel L.,stitule of Technology, Technion City, Haifa, Israel. S Present address: Tokushima University. Tokushima, Japan. 6 AJso University of Maryland, College Park, MD, US-A. 7 Present address: Japan Atomic Energy Agency (JA'EA), Tom, lbaralri, Japan. S Present address: Okayama University, Okayama, ..Japan. ...

0273-1177/536.00 @ 2011 Published by Elsevier Ltd. on behalf of COSPAR, doi: IO.IOI6lj ,asr.2011.07 .012

Please cite this artIcle m press as, ',a,n;, !ot~), A., et aI, Search for cosmicwray antiplOton ongms and for c08lflological .. ntimatter with BESS. J. Adv. Space Rea (2011) doi:1O.1016/j.asr.2011.07.012 r ~ JASR ~10669 ~ No. of Pages S, Model 5+ i i 5 August 2011 Disk Used

2 A. Yamamoto et all Advances in Space Research xxx (2011) xxx-xxx

32 1. Introduction 2008, nine approximately one-day northern-latitude flights 71 and two long-duration Antarctic flights, as summarized in 72 33 Progress in modern observational cosmology and astro- Table 1. These have collectively recorded more than 13,000 73 34 physics has shown that the material Universe is dominated cosmic-ray low-energy antiprotons and set the most strin- 74 35 by dark matter responsible for the formation of structnre gent upper limits to the existence of antihelium and anti- 75 36 and for the dynamics of galaxies. The nature of the dark deuterium. BESS has also provided the reference 76 37 components, however, is unknown. Similarly, it is observed standard for elemental and isotopic spectra of H and He 77 38 that cosmological antimatter is apparently absent in the over more than a full solar cycle. Together with the anti- 78 39 present era, but the reason for this absence remains as a proton measurements, these provide strong constraints on 79 40 major problem for cosmology and . It has models of cosmic-ray transport in the Galaxy and Solar 80 41 been suggested that one constituent of the dark matter System. 81 42 may be primordial black holes (Hawking, 1975; Barrau 43 et al., 2002)~ formed in the early Universe due to the col- 2. Progress of the BESS and BESS-Polar experiments 82 44 lapse of dense regions formed by density fluctuations. 45 The detection of PBH through arising from The . BESS program began as an outgrowth of work 83 46 the Hawking radiation emitted as they evaporate would toward the Astromag superconducting magnet facility that 84 47 probe the early Universe at very small scales (Maki et al., was planned for the International Space Station, ~SS 85 48 1996). PBH evaporation might be detected by its effect (Ormes, 1986). From the early 1980s, there was tremendous 86 49 on the measured antiproton spectrum. Addressing these excitement over results from seminal balloon-borne exper- 87 50 issues are central scientific goals of the BESS program iments that reported detecting substantial excesses of anti- 88 51 (Yoshimura, 2001; Yamamoto et al., 2008; Mitchell protons at both high and low energies using magnetic 89 52 et al., 2009). The precise measureinents of the low-energy spectrometers or annihilation signatures. By the mid- 90 53 cosmic-ray antiproton flux and the sensitive search for hea- 1980s, the cosmic-ray community was fully engaged in an 91 54 vier antinuclei made by the BESS experiment are vital to effort to measure matter and antimatter to 92 55 constraining candidate models for dark matter, evaluating unprecedented precision. During the Astromag study, a 93 56 the possible density of primordial black holes, and seeking number of magnet configurations were proposed. BESS 94 57 for the limits of cosmological antimatter. BESS also pro- stemmed from a proposal to use a solenoidal superconduc- 95 58 vides important fundamental data on the spectra of light ting magnet with a coil thin enough for particles to pass 96 59 cosmic-ray elements and isotopes and for studies of the through with minimal interaction probability (Yamamoto 97 60 effect of the out-flowing solar wind on the Galactic cosmic et aI., 1988). This conflguration maximizes the opening 98 61 rays (Mitchell et aI., 2009). The exceptionally large collect- angle of the instrwnent, and hence the geometric factor, 99 62 ing power and precise particle identification capability of making it ideal for rare-particle measurements. BESS 100 63 the BESS instruments enable a broad scientiflc reach. began as a balloon-borne instrument to validate this con- 101 64 BESS uses a superconducting magnetic-rigidity spec- cept, and rapidly evolved into an immensely capable scien- 102 65 trometer with a time-of-flight (TOF) system and an aerogel tific program in its own right (Orito, 1987). 103 66 Cherenkov counter (ACC) to fully identify incident parti- The BESS instruments consist of thin superconducting 104 67 cles by charge, charge sign, rigidity, and velocity (Ajima solenoidal magnets and high-resolution detector systems. 105 68 et al., 2000; Yoshida et aI., 2004). The joint US-Japan For energies between about 0.1 GeV and 4 GeV, referenced 106 69 BESS program, supported by NASA and lSAS-JAXA, to the top of the atmosphere (TOA), the BESS instruments 107 70 carried out eleven successful balloon flights from 1993 to accurately identify incident particles by directly measuring 108

Table I Progress of the BESS and BESS-Polar balloon flights and observations. 1993 1994 1995 1997 1998 1999 2000 2001 2002 2004 2007 Locatio::J. Canada » » ::;:. » » » us c. Ant. Ant. Float time (h) 17.5 17 19.5 20.5 22.0 34.5 44.5 1.0 16.5 205 730 Obsenation time, float (h) 14 15 17.5 18.3 20.0 31.3 32.5 1 11.3 180 588 Observation time, asc./des. (h) 2.8 2.5 12.8 2.3 3.3 3.5 Recorded events (x 10, 4.0 4.2 4.5 16.2 19.0 19.1 17.0 N/A 13.7 900 4700 Data volume (GB) 4.5 6.5 8.0 31 38 41 38 N/A 56 2.140 13,500 Event filtering Yes Ye, Ye, Ye, Ye, Yes Yes y" Yes No No Magnetic field (T) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.8 0.8 MDR(GV) 200 200 200 200 200 200 200 1,400 1,400 240 270 TOF resolution (ps) 300 300 100 75 75 75 75 75 75 160 120 ACC index 1.03 1.02 1.02 1.02 1.02 1.02 1.02 1.03 Antiproton events observed 6 2 43 415 384 668 558 N/A 147 1520 ~8000 Antiproton's energy (GeV) :0.5 <0.5 <3.6 "3.6 "3.6 :3.6 <4.2 N/A <4.2 <4.2 <3.5 6 Anti-He/He upper limit (x 10- ) 22 4.3 2.4 1.4 1.0 0.8 0.68 N/A 0.65 0.27 0.07

Please cite this article in press as: Y;-_';:I:~lil;'tf;, A., et al. Search for cosmic-ray annpwton origins and for .:o8:.r.Iloillgical amimatter Wlth BESS. J. Adv Space Res. (2011). doi 10.1 016/; .•".2011.07.011 JASR 10669 No. of Pages 8, Model 5+ 5 August 2011 Disk Used

A. Yamamoto et aI.1 Advances in Space Research xxx (2011) xxx-xxx 3

109 their charge, charge-sign, magnetic rigidity, and velocity. penetrate the spectrometer with munmum interactions 147 110 This infonnation is subsequently used to derive their mass (Yamamoto et al., 1988; Makida et al., 2005). Since the 148 III and kinetic energy. Elemental spectra can be measured to magnetic field is very uniform inside the solenoid, the 149 112 >100 GeV. All BESS instruments, improved during the deflection measurement is very accurate for all trajectories 150 113 course of the program, use similar instrwnent configura­ within the instrument geometric acceptance. A maximum 151 114 tiom with detail changes reflecting the evolution of the detectable rigidity (MDR) of 200 GV was achieved in the 152 IW instruments and flight-specific requirements (Yoshida original BESS instrument and 280 GV in BESS-Polar. 153 116 et al.,;2004; Yamamoto et al., 2008; Mitchell et aI., 2009). For the BESS-TeV flights in 2001 and 2002, outer drift 154 117 Fig. I shows a schematic cross-sectional view of the chambers were added to raise the MDR to 1400 GV (Hai- 155 118 BESS-Polar II instrument as an example. A central no et aI., 2004). . 156 119 JET-type drift chamber tracking system and inner drift Versions of the original BESS instrument were used for 157 120 chambers (IDC), giving 52 trajectory points in the bending the initial 9 northern-latitude flights. In order to take 158 121 direction, are located inside the wann bore of the solenoid advantage of the long flight durations and low geomagnetic 159 122 to measure the trajectories of charged particles as they pass cutoff in Antarctic flights, a completely new version of the 160 123 through the magnetic field. The magnetic rigidity, R.= pel instrument, BESS-Polar, was developed (Yamamoto et al., 161 124 Ze (where p is momentum, e is light velocity, and Z is the 20020.; Yoshida et al., 2004; Mitchell et aI., 2004; 162 125 particle electric charge) is determined by fitting the curva­ Yoshimura et aI., 2008). The BESS-Polar magnet has half 163 126 ture of the track through the field. The charge-sign of an the material (radiation) thickness in the coil wall, achieved 164 127 incident particle is determined by the direction of its curved by use of improved superconducting wire with A1 stabilizer 165 128 track with respect to the local vector magnetic field. Arrays strengthened by alloying with Ni and by cold-working 166 129 of time-of-flight (TOF) scintillation connters (Shikaze (Yamamoto et aI., 2002b; Makida et al., 2005). Reduced 167 130 et aI., 2000) are located at the top (UTOF) and bottom heat transmission to the low-temperature components gives 168 13l (LTOF) of the instrument. In BESS-Polar, a middle TOF a much improved cryogen lifetime. In addition, the outer 169 132 scin:illator array (MTOF) is located inside the magnet bore pressure vessel was eliminated, the ACC was moved to 170 133 below the lower IDC. The TOF scintillators trigger readout the bottom, and the MTOF was added. The result was a 171 134 of events and measure Z, and velocity,!!, of incident parti­ spectrometer with ~4.5 glcm2 encountered by incident trig­ 172 135 cles. Particle momentum, p, is determined from Rand Z gering particles compared to ~18 glcm2 in the previous 173 136 and, in tum, particle mass, m, is determined from p and BESS instrument, lowering the effective energy threshold 174 137 p. BESS separates antiprotons from negative charge back­ to well below 100 MeV at TOA. The BESS-Polar data 175 138 ground particles, mainly muons and electrons, by mass up acquisition system has the required throughput and storage 176 139 to an energy of about 1.5 GeV. Above this energy, an capacity to record all triggered events, and so no longer 177 140 aerogel Cherenkov counter (ACC) identifies low mass, high requires down-sampling of proton data. Greatly reduced 178 141 p, backgronnd particles. Additional background rejection power consumption and a new solar-cell array power sys­ 179 142 is supplied by multiple measurements of ionization energy tem enable long-duration flights. In BESS-Polar I, the mag­ 180 143 loss (dE/dx) from the JET. The horizontal cylindrical net cryogen lifetime was 11 days. BESS-Polar I was flown 181 144 configuration of the BESS instrument allows a full opening in 2004, acquiring data for 8.5 days and recording ~2 tera­ 182 2 8 145 angle of ~90° with a resulting acceptance of 0.3 m sr. The bytes of data on 9 x 10 cosmic ray events. High-voltage 183 '146 thin soirutoid magnet allows the incoming cosmic rays to breakdown in some of the TOF photomultiplier units 184 reduced the geometric acceptance to about 0.2 m2 sr and 185 impacted TOF resolution. BESS-Polar I measured 186 Incoming particle 432 antiprotons at energies below 1.3 GeV, nearly a 4-fold 187 increase in statistics over BESS measurements duririg the 188 previous solar minimum, and 1512 antiprotons over the 189 ._--- TOF 0.1--4.2 GeV energy range. Technical improvements for 190 BESS-Polar II, see Table 1, addressed cryogen lifetime, 191 _---- IDC detector performance and stability, power system perfor­ 192 Magnet -. mance, data storage, and the efficiency of the final pre­ 193 launch assembly process. For BESS-Polar II, cryogen life­ 194 JET .---- time was increased to >25 days, the TOP resolution was 195 --.---- IDC effectively improved to ~ 120 ps, the rejection power of 196 MTOF ------.-~~~ the ACC was increased to ~6000, and the full geometric 197 2 acceptance of 0.3 m sr was maintained throughout the 198 Ace ----- "" ---.. TOF ;---" flight. BESS-Polar II operated at float altitude for 199 24.5 days with the magnet energized, recording 13.5 tera­ 200 9 o 1m bytes of data on over 4.7 X 10 cosmic ray events. This 201 I I more than doubles the combined data from all previous 202 Fig. 1. Cross section of the BESS-Polar II spectrometer. BESS flights, including BESS-Polar I, and is several times 203

Please cite this article in pres.~ as: '\"::;-iHi.;I."to, A., et aI. Search for cosmic-ray antiproton origins and for cOMnologica1 antimatter with BESS J. Adv. Space Res. (2011), dodO 1016/j.asr.2011.07.012 ;JASR,iieg- No. of Pages 8, _ 5+ i·5 A~gu~~~~ .. Disk Used 4 ..4.. Yamamoto et al.1 Advances in Space Research xxx (lOll) xxx-xxx

Mitchell et al., ~004, 2005; Yoshida et al., 2004; Yoshimura 222 et aI. , 2008; Yamamoto et al., ~008; Mitchell et al.. 2009). 223 In this report, we focus on progress in the searches for cos­ 224 mic-ray antiproton origins and for cosmological antimatter 225 from the BESS-polar program. 226

3.1. Precise measurement of the antiproton spectrwn 227

Most cosmic-ray antiprotons are produced by interac­ 228 tions of high-energy Galactic cosmic rays with the interstel­ 229 lar ·medium. Due to production kinematics and to the 230 energy spectra of the primary cosmic rays, the energy spec­ 231 Yea, trum of these secondary antiprotons has a characteristic 232 peak at around 2 GeY and decreases sharply below and 233 Fig. 2. Solar acti.vity during the last BESS balloon flights. above the peak. This feature is clearly shown by the BESS 234 204 the data expected from PAMELA in the BESS-Polar data (Oritoet al., 2000; Abe etal., 2008). Their mainly sec­ 235 205 energy range. Most important, the BESS-Polar II llight ondary origin makes antiprotons important tools to probe 236 206 took place very near solar minimum, as shown in Fig. 2, cosmic-ray transport as discussed in a recent comprehen­ 237 ')Jr/ when sensitivity to a low-energy primary antiproton source sive review (Strong et aI., 2007). Deviations from the 238 208 is greatest. The long BESS-Polar II flight gave a ~20-fold . expected antiproton spectrum may signify the contribution 239 209 increase in the number of antiprotons detected below of a primary source such as evaporation of primordial 240 210 I GeY compared to the BESS-97 data at the previous solar black holes (PBH) or annihilation of neutralino dark 241 211 minimum and a ...... 14-fold increase over the combined matter. PBH evaporation is expected to yield an antiproton 242 212 BESS-(95+97) data. After about one and two-thirds orbits spectrum with a peak well below I GeY. Superimposed on 243 213 of Antarctica, the BESS-Polar II flight was terminated over the steeply decreasing secondary antiproton spectrum, this 244 214 the West Antarctic Ice Sheet, as shown in Fig. 3, because of could cause a flattening of the observed spectrum (Mitsui 245 215 concerns over the flight trajectory. Logistics considerations et al., 1996). Although the BESS (95+97) antiproton flux 246 216 prevented immediate recovery. Recovery of the BESS­ measurements at the last solar minimum hint at an excess 247 217 Polar II instrument was successfully carried out two years at low energy (Orito et aI., 2000), successive measurements, 248 218 later in 2009-2010. .

--- Moskalenko et al. 219 3. Scientific progress from BESS-Polar oboervation - Bieber et aI. -Fuk 220 The general BESS and BESS-Polar scientific progress ~ 221 has heen reviewed in the references (Yamamoto, 2003; <> -;, to< '-"-' ..... ------.. -... ~.... " .. 1. ,'-- . ....' ...... ~ • BESS-Polar ~ " BESS (2000) i n BESS (95+97) <..

10. _.- PBR (R ·'."''''''''~ · 'I 10.1 Kinetic eoerlY (GeV)

Fig..4. Antiproton flux measured in BESS· Polar I and in previous BESS dights compared to secondary antiproton caJculation with three models (Abe et at, 20(8): the Standard Leaky Box (SLB) model modulated with a steady state drift model (solid curves: Bieber et al. (1999» and the Diffusion plus Convection (DC) n:;odel modulated with a HeliOspheric drift model (dashed curves: Moskalenko et aI., 20(2). and the DC model modulated with a spherical symmetric model (dotted curves: Fisk, 1971). .... The dash·dot curves are calculations of antiproton spectra from evapo­ ration of primordial hlack holes with an explosion rate of 2 Fig. 3. Balloon flight trajectories in BESS-Polar 11. 0.4 x 10- pc-3 yr -I modulated by 550 MV (top: in 1995-1997) and Rim MV (hi • ')fV\A\ Please cite tIm article m press as: Y.'nla.·'w;o, A., et aI. Search for cosmic-ra} dtltiproton I)ngms and for .:osmological antimatter with BESS. J. Adv Space Res (2011), dodO.l016/j.asr.201L07.012 JASlhb669 No. of Pogn 8, Model 5+ : f 5 August 2011 DIsk Used

A. Yamamoto et aL I Advances in Space Research xxx (201J ) xxx- xxx 5

249 taken after the solar minimwn period, are more consistent 3,------,,--.-~------_, 250 with. a pure secondary nature. x 1/300 251 Fig. 4 shows the antiproton spectrum measured by (wIUt onl,. 114l1tatlstlcs) 252 BESS-Polar I (Abe et al., 2(08) compared with results from 2.5 II p 253 pre-,ious BESS flights around solar minimum, 25+97 254 (Matsunaga et al., 1998; Orito et a!. , 2000) and maximum 255 (Asaoka et al., 2(02), and compared with theoretical calcu- ! 2 256 lations. The solid curves are calculations of the interstellar ...:::> 257 secondary antiproton spectra from a Standard Leaky Box 258 (SUI) model modulated with a steady state drift model Ie-~ 1.5 259 (Bieber et a!., 1999) in which the modulation is character- 260 ized by a tilt angle of the heliospheric current sheet and - 261 the Sun's magnetic polarity of (from top to bottom, and 262 the first two are very close) 10"(+), 10"(-), and]OO(-). 263 The dashed curves are calculations with the Diffusion plus 264 Cor,vection (DC) model of the secondary antiproton D.5-.4CW~-3-c:-L~~-2~J....L..I. I_1 " , '0 ' '~ t J..~r'...I 4 265 spectrum modulated with a Heliospheric drift model 266 (Moskalenko et a!., 2002; Moskalenko, 2006). The tilt RIgidity (GV) 267 angles, 10°(+),70°(- ), and 30°(-) roughly correspond to Fig. .5. The rr 1 versus rigidity plot, and antiproton selection band. For 268 the :ne.Tsurements with BEfs (95+97), BESS (2000), and the negativc rigidity, all the events with R ·: -0.8 GY/e after Cherenkov 269 BESS-Polar I (2004), respectively (Zhao and Hoeksema, veto cuts and JET dE/dx cut are shown. For the positive rigidity. O.l %or 270 1995; Hoeksema, 1995). The dotted-curves are calculations the events after Cherenkov veto cuts arc shown. 271 with the DC model (Moskalenko et a!., 2(02) modulated 272 with a standard spherically symmetric approach (Fisk, Preliminary 273 1971), in which the modulation is characterized by a single 274 pamneter ( was obtained by fitting the corresponding 276 proton spectrum measured by BESS, assuming the inter- • . ·.,·· 1 277 stellar spectrum in (Orito et al., 2000). The values of , 278 550 :\IIV, 1400 MV, and 850 MY correspond to the mea- 279 surements with BESS (95+97), BESS (2000) and BESS- 280 Polar I (2004), respectively. The dash-dot curves are 281 ca1c:.J.lations of antiproton spectra from evaporation of 282 PBH at a rate of 0.4 x 10-2 pc-3 yr- ' (Maki et al., 1996; 283 Yoshimura, 200 I) modulated by a 'spherically symmetric .. 284 approach . (Fisk, 1971) with modulation parameter ~ BESS 15+17 285 independent of solar polarity. The expected s;gnal from a IEtS-Polar 1 • 8ES8-Polar 2 286 PBH evaporation is alIected by solar modulation rr:ore .. PAMELA ~hJl"'DKJ -Sl8+Dflft1r(+J:~"al""'-l 287 thalO the secondary antiproton spectrum because of its • - SLB+Dr1ftW(-t : ENIhr ..... __1I 288 low energy spectral peak. As might "" expected, BESS- -- $L.B+F1sk ... 550t/N ~ I SlJI;+Ask.-SSOrIN -.-q 289 Polar I antiproton meast:re!Ilents, !aken dur:ng a trar:sient , 290 period in advance of solar rninim1!I:1, show 1:.0 app~. rent 291 excess, but provide a baseline secondary spectrum to be 10" 1 Kln.tJc _nergy (GeV) 292 compared with the spectrum obse::ved at solar :minimum Fig. 6. Antiproton flux measured by BESS (95+97), BESS-Polar I (2004), 293 by BESS-Polar II. aod a preliminary result by BESS-Polar n (2007-2008) which was 294 The BESS-Polar II data analysis is still in progress. The obtained rrom the data analysis using a quarter observed events. The solid 295 full BESS-Polar II dataset is expected to yield ~8000 mea- curves are secondary antiproton calculation with the SLB model modu­ 296 sured antiprotons. Fig. 5 shows particle identification plot lated with the steady state drin model (Bieber et at). The dotted curves are 297 with fJ' versus rigidity using a quarter of the data from the secondary antiproton calculations with thc'SLB model modulated. with the spherically symmetric model (Fisk). 298 BESS-Polar II. Fig. 6 shows a very preliminary antiproton 299 energy spectrum from analysis, compared with the results 300 from BESS-Polar I (2004) and BESS (95+97). The solid II flight would be about I CO( -). The dashed curves are 306 301 curves are calculations with the SLB model modulated with calculations with the SLB model modulated with the spher­ 307 302 a st,.dy state drift model (Bieber et al., 1999). The tilt ically symmetric approach (Fisk, 1971). The modulation 308 0 303 angbs of I 0°(+) and 30 ( -) approximately correspond to parameters of = 550 MV and 850 MY correspond to 309 304 the measurements with BESS (95+97) and BESS-Polar I the measurements with BESS (95+97) and BESS-Polar 310 305 (2004), respectively. The tilt angle during the BESS-Polar (2004), respectively. The modulation parameter for 311

Please cite trul article in press as: "r.·;,rn;:u,nh), A" et aI. Search for (;osuUc..·ray 811tiprotO!1 origins and for cosmo'ogtca1 antim lltt~ 1 with BESS. J. Adv Space Rca. (2011 ), doi,IO.1016/j .•".2011 07.012 GiiiSR-10669 . No. of Pages 8, Model 5+ .

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312 BESS· Polar II should be comparable to BESS (95+97). M 1tr" 313 a preliminary result, the BESS-Polar II observation shows .R""elHe Umit (95% C.L.) 314 good consistency with the secondary antiproton ! 315 calculations. aNIMe, et: ... (l''l'') aUSoTeV .2 Gold,n at el. Cll!!) 316 3,2, Search for antihelium e 1tr' >C 317 A fundamental question in cosmology is whether matter Elumnglon atel. (1911) 0;::'" 318 and f.ntimatter arc asymmetric or symmetric in the E 319 Universe. The Sakharov conditions of direct violation of .2 1tr" IBESS '95]". F. arm.. lit e •. (ltt1) 320 baryon number conservation, CP & C symmetry breaking, Gi !BESS '93 'N '9!ip. $eeId at ~. (1'" 321 and a period out of equilibrium in the very early Universe 322 indicted a way to explain the apparent baryon domination l .2 IAMSI". Alcannt e'. (U") 323 observed (Sakharov, 1967). However, direct violation of 1tr' Gi !BESS 't3 - '00] M. !!I ..e~11It eL (2002) 324 baryon number conservation has never been demonstrated, .c 2 "·:·-.. ·::'f;_.~ .~_ > .. :;:I 325 and the strength of CP violations currently measured at C 326 accelerators are insufficient to explain strong matter/anti~ c( 327 matter asymmetry. Detection of antihelium would provide 10" 328 direct evidence of antimatter domains in the Universe. ALL BESS Results 329 Although antihelium might, in principle, be produced as 330 seconcaries in cosmic-ray interactions, the resulting antihc- 12 331 lium/helium ratio should be much less than 10- (Brown 1tr" ='---'-L..Lw.il,"--,--,-,~.,... "",1 ,..-'-'-.L' .L"W" 332 and Sleeker, 1979). " 10 10' 6 333 The BESS-Polar-I experiment observed 8 x 10 helium Rigidity (GV) 334 events and no antihelium candidate was detected in the Fig. 7. Antihelium flux upper limitll progressed in BESS and BESS-Polar 335 rigidity range 1- 20 GV with an effective geometrical accep- experiments, compared with previous experiment! (Sasaki et al., 2010). 2 336 tance of 0.2 m sr, The resultant upper limit for the ratio of 337 antihelium/helium was 4.4 x 10-'. By accumulating all 338 result, from BESS thro~gh BESS-Polar I, an upper limit statistics increased a factor of ::;"' 10 compared to BESS mea- 365 339 of 2.7 x 10-' was set in the rigidity range 1- 14 GV (Sasaki surements at the previous solar minimum, BESS-Polar II 366 340 et 8.1., 2008), data shows good consistency with the secondary antjpro- 367 341 The BESS-Polar II experiment observed 4 .x 10' helium ton calculation. With further analysis, this data will place 368 342 events in a rigidity range of 1-14 GV with an effective severe limits on any possible PBH evaporation contribu- 369 2 343 geometrical acceptance of 0.3 m sr, and no antihelium tion to the low-energy antiproton spectrum. and hence to 370 344 candidate was detected, The resultant upper limit was limits on any possible density of primordial black holes. 371 345 9,4 x 10-8, By accumulating all results from BESS through No antihelium candidate was observed in BESS through 372 346 · BESS·P~lar II, the 95% confidence level upper limit for BESS-Polar II flight, and the 95"10 confidence level upper 373 347 antihelium/helium in the rigidity range 1-14 GV has been limit for antihelium/helium in the 1-14 GV rigidity range 374 348 reduced to be 6,9 x 10"" (Sasaki et aL, 2010), Fig, 7 shows has been reduced to be 6.9, X 10-'. 375 ~ 349 the BESS upper limits compared with other experiments, 350 The upper limit for antihelium/helium has been reduced 351 by two orders of magnitude compared to the first BESS 352 limit (Ormes et al., 1997; Sasaki et al., t002, 2008, 2010), A lsa.t=G it a1. (1999). ~aell''I'ar et a1. (19+8), Rlif.BSgtBS st w a!. (1981), Saaki-d ai (1998) aRe ¥am.afRBtB st at, (1994). QJ 378 353 4. Summary Acknowledgment 379 354 The BESS program has performed eleven scientific 355 balloon flights successfully in northern Canada and The authors thank NASA Headquarters, ISAS/JAXA, 380 356 Antarctica. It has aimed to search for cosmic-ray antipro- GSFC, and KEK for continuous support and encourage- 381 357 ton origins and for cosmological antimatter. The Antarctic ment in the US·Japan cooperative BESS program, The 382 358 flights of BESS-Polar I (2004) and BESS-Polar" (2007- authors also ~thank the NASA Balloon Program Office 383 359 2008) have yielded measurements of cosmic-ray antipr6- and the NASA Columbia Scientific Balloon Facility for 384 360 tons \\'ith unprecedented statistical accuracy and greatly their highly professional support of BESS conventional 385 361 increased the sensitivity of the antihelium search. The and long-

Please citl5 this article in press as' Y!'n\i>'1,'Yo, A., et al. Search fOl cosml(:-ray antJ:proton origl:18 and for cosmulogical anbmattC:l with BESS. J. Adv S?ace Res. (2011), doi lO.1016/j.asr 2011.07.012 : JASR 10669 No. of Pages 8, Model 5+

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Please cite this article in pre"s as: Y.~f.';;;l·()k, A., et aI. Search for w&mic-ul-Y antiprotoll origius and for cosmologiea.l antimatter with BESS. I. Adv. Space Res. (2011), doi:1O.1016il.asr.2011 07.012