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VOLUME

75,

NUMBER 21

PHYS ICAL REVIEW LETTERS

Observation of Cosmic-Ray Antiprotons

20 NovEMBER 1995

at Energies below 500 MeV

K. Yoshimura, ' S. Orito, ' I. Ueda, ' K. Anraku, R. Golden, M. Imori, ' S. Inaba, B. Kimbell, N. Kimura, Y. Makida, H. Matsumoto, H. Matsunaga, ' J. Mitchell, M. Motoki, J. Nishimura, M. Nozaki, J. Ormes, T. Saeki, ' R. Streitmatter s J Suzuki, K. Tanaka, N. Yajima, T. Yamagami, A. Yamamoto, and T. Yoshida' 'University of Tokyo, Bunkyo ku, T-okyo, 113, Japan 2National Laboratory for High Energy Physics (KEK), Tsukuba, Ibaraki, Japan New Mexico State University, I as Cruces, New Mexico 88003 Kobe University, Kobe, Hyogo, Japan ~National Aeronautics and Space Administration, Goddard Space Flight Center (NASA/GSFC), Greenbelt, Maryland 2077I 6The Institute of Space and Astronautical Science (ISAS), Sagamihara, Kanagavva, 229, Japan (Received 20 March 1995)

A search for cosmic. -ray antiprotons was performed using a balloon-borne experiment with a magnetic rigidity spectrometer (BESS). We have detected four antiprotons in the superconducting kinetic energy range between 300 and 500 MeV, the first antiprotons to be identified by a direct The corresponding P/p fiux ratio at the top of the atmosphere is found to be mass measurement. 1.2 o'65 && 10 . No antiprotons were observed in the range between 175 and 300 MeV, resulting in a 90% C.L. upper limit on the flux ratio of 2.9 X 10 ~. PACS numbers: 96.40.De

The observation of antiprotons (p's) in the galactic cosmic rays in 1979 [1] and the subsequent experiment that implied a large fiux at a few hundred MeV [2] stimulated a number of theoretical speculations about their origin. Because p s below 1 GeV are kinematically difficult to produce in collisions of energetic cosmic-ray nuclei with ambient interstellar material, the low energy regime is an ideal place to look for the p's from novel sources such as the evaporation of primordial black holes (PBHs) [3,4] or the annihilation of dark matter in the galactic halo [5,6]. Now, more than a dozen years later, a new generation of experiments [7] including this one is reporting results which identify galactic cosmic-ray p s by mass measurement. The first flight of the balloon-borne experiment with a superconducting magnetic rigidity spectrometer (BESS) carried sophisticated new experimental apparatus focused on identifying p s. Using very strict selection criteria to prevent false identifications, we have identified 4 p s. The results are consistent with two separate upper limits [8,9] and with the hypothesis that most of the cosmic-ray p's observed at Earth are produced by the collisions of cosmicray nuclei with ambient interstellar matter [10—12]. Figure 1 shows a cross-sectional view of the cylindrical detector system. From inside to outside, it includes a jettype drift (JET) chamber, inner drift chambers (IDCs), a solenoid, outer drift chambers (ODCs), superconducting The solenoid and a time of fiight (TOF) hodoscope. produces a magnetic field of 1 T in the axial direction with an uniformity of ~15%. The JET chamber, as a central tracking detector, measures hit positions three dimensionally with an accuracy of 200 p, m in the r-P plane, resulting in a typical rigidity resolution of 0.5%%uo at 1 GV [cylindrical coordinates (rPz) are used with the

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magnetic field direction as z axis]. Continuous 24 r and 16 z measurements along each particle trajectory make it possible to monitor even complicated events having interactions or multiple tracks. Each of the IDCs and ODCs consists of two 12-mm-thick drift layers divided into 50-mm-wide cells. Using vernier pads with a cycle of 100 mm (120 mm for ODCs), both chambers can measure the z position within the cycle with 300 p, m resolution and the r @position w-ith 200 p, m resolution. By combining the z information from the JET chamber (resolution of 2 cm/point), the z position of the track can be determined with a precision of 300 p, m. The combination pattern of the IDC and ODC hit cells is utilized in the trigger selection. The TOF hodoscope consists of four upper and six lower plastic scintillation

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FIG. 1. Cross-sectional view of the BESS detector and one of the antiproton

events.

1995 The American Physical Society

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PHYSICAL REVIEW LETTERS

counter paddies each (110 cm X 20 cm X 2 cm). The light signals of the scintillation paddies are guided to the photomultiplier tubes (PMTs) through acrylic light guides at both ends. The timing and amplitude of all PMT signals are measured to determine the time of flight and energy loss of incident particles with resolution of 280 ps and 15%, respectively. These components along with electronics and microcomputers are enclosed by a 2-mm-thick aluminum pressure vessel. The solenoid is located between the IDC and ODC layers; including both the wall and coil, it is 5 g/cm (0.04 proton interaction length) thick. The cylindrical configuration provides a wide tracking region and a large geometrical acceptance of 0.4 m sr. The trigger system is designed to detect negatively charged particles (p s, antiheliums) with high efficiency while sampling the higher flux proton and helium nuclei. The first level trigger is the so-called "TO trigger" where a simple coincidence between the top and bottom scintillator layers initiate the data gathering. The second level is "track trigger" where the event selection is performed using IDC and ODC hit patterns. At this level, first a hit-pattern selection is performed to reject events with multiparticles or without a track. This is followed by a lookup table track-rigidity selection to reject a large part of the positively curved events. In addition, one of every 140 TO triggers is selected, irrespective of the track trigger condition, to build a set of unbiased triggers from The events that which the efficiency can be determined. pass through the above selections are processed by the microcomputers and recorded onto two 8-mm tapes with total capacity of 10 Gbytes. The data were collected in a fiight launched from Lynn Lake, Manitoba, Canada on July 26, 1993, for 13 h at a fioating altitude of 36.5 km (residual atmospheric depth of 5 g/cm ). The cutoff rigidity varied between 0.34 and 0.43 GV. The experiment was triggered at the T0 level 10s times and total 3635139 (including 517148 "unbiased trigger sample" ) events were recorded. About half of the recorded events exhibit either multiple tracks or no track in the JET chamber. The following cuts are applied to reject these events and to select clean high quality single track events which passed through the fiducial region: (1) Only one counter hit in each layer of the TOF hodoscope. (2) Only one track found, which should have spanned at least 16 readout wires in the JET chamber. This cut eliminates the tracks which clip or partially traverse the sensitive volume. A total of 2160082 events, including 221898 unbiased trigger events, pass through these cuts. The following cuts are then applied to ensure the quality of the track along with correct timing information (the track used here is defined as the combined fit using hit points of the IDC and the JET): (1) At least one hit in each of upper two and lower two layers of IDCs should be used in the r Pand z fitting. (2) The n-umber of JET hits used for the combined fittings in r-@ and

20 NovEMBER 1995

respectively, has to be at least 13 and 5. (3) There have to be less than 20 extra r P-hits in the JET chamber other than those used for the track fitting. (4) The reduced X~ of the combined fitting in the r /pl-ane and along the z axis has to be less than 4 and 3.5, respectively. (5) The extrapolated track should cross the fiducial (~z~ ~ 500 mm) region of the TOF scintillators. (6) The z position determined by the leftright time difference (zTQI:) matches the z-impact point of the extrapolated track within 80 mm, i.e. , twice the gToI; resolution. (7) The ratio of the signal amplitude of the z planes,

left and right PMTs must be consistent with the g-impact position of the track. A total of 1167350 events, including 115562 unbiased trigger events, pass through these cuts. Using the surviving events, we can check the quality of the timing = v/c), is measurement. The particle velocity, i.e. , p (— determined from the time of flight and the path length. The p ' resolution is 0.065 and relativistic up- and downward-moving particles are completely separated by more than 30cr away. We reject all up-going (albedo) particles at this stage, and limit further analysis to the down-going particles. For the present analysis, we restrict ourselves to the absolute rigidity range between 0.45 and 1.05 GV. The ' peak of the upper bound is determined such that the p proton is more than 5o. away from the peak of e/p, /vr distribution while the lower bound is set to reject the events that stop in the bottom scintillator. Figures 2(a) and 2(b) show dE/dX vs rigidity plots of the top and bottom scintillators, respectively. We require

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FIG. 2.

dE/dX band.

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YSICAL REVIEW LETTERS

that p's as well as protons must have the dE/dX inside the "dE/dX" band shown in Fig. 2. In the low rigidity region of the present analysis, this dE/dX cut rejects most of e/p, /~/deuteron and all of the particles with charge greater than 1, while keeping 90% of protons. The mass of the observed particles can then be calculated from p and rigidity: mToF = R (1/p — 1). Figure 3(a) shows the histograms of mToF before and after the dE/dX cut at various positive rigidity bins. The sharp peak of the proton is visible in each rigidity bin, clearly separated from the peak of e/p, /~. It is also clear that the dE/dX cut extracts the proton signal efficiently, while rejecting most of the e/p, /~. Figure 3(b) shows the mToF histograms for the negative rigidity events after the dE/dX cut. We observe four p candidates clustered right at m„and well separated from the cluster of e//L/~. The dE/dX of each of the four candidates is consistent with that of the proton, and is far from the most probable dE/dX value of e/p, /~. Each of the four p candidates has been closely investigated using event display software. As seen in Fig. 1 for one of the p events, the four events all exhibit excellent track qualities and show no evidence of interactions in the detector. The relative uncertainties in the (rigidity) ' determination are obtained in the fitting process to be 0.005, 0.007, 0.006, 0.007, all more than 130o away from protons with infinity rigidity. Therefore the probability of a positively curved track (proton) faking any of the four candidates is negligible. Since the measured p ' of the candidates (1.55, 1.45, 1.43, and 1.45) are all more than 37o away from the up-going particles, the chance of an albedo R =0.45 -0.60

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FIG. 3. Histograms

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particle faking any of the candidates is also extremely small. We also traced back [13] the trajectory of all candidates by integrating the bendings by the Earth' s magnetic field and found that they all came from the outer space. Therefore these events are probably not due to the reentrant albedo. 2 These events all have mTpF more than 6o away from the e/p, /~ peak. However the possibility remains that they are misidentified e//L/~ that have moved far from their response curve by some interactions or accidentals. To evaluate this likelihood, we utilize Monte Carlo (MC) simulations based on GEANT/GHEISHA taking into account the detailed material distribution and the detector response [14]. We generated e, p, and ~ entering the detector, respectively, each with 5 times as many events as the total observed negative rigidity events and found no events that mimic p by passing through the same selection criterion as used for the flight data analysis. We consider the four candidates as the p's and obtain the Ilux ratio to proton in the same ~R~ intervals. We first obtain the "total number of good proton" events (N„) which is defined as the number of the proton that passed through the hit-pattern selection and would have survived all off-line selections, if they were not rejected by the track-rigidity selection. N~ is calculated to be 1 178800 (including 772940 in the energy between 300 and 500 MeV) by counting the events in the unbiased trigger sample that survive all of the trigger and the selection criteria except for the track-rigidity condition in the track trigger, and by correcting for the sampling factor

of 140. To calculate the total number of p events (N~) in the energy range 300 —500 MeV based on the four observed events, we correct for the small inefficiency of up to 2% in the track-rigidity selection, which is estimated by using the negative e/p, /7r in the unbiased trigger sample. We then conect for the loss of the p's due to the annihilation in the 5 g/cm of the air and in the detector. The MC simulations show that the p/p ratio should be upwards corrected to a factor of 1.9 for the energy range 300— 500 MeV. A correction factor for loss of p and proton in the atmosphere is also estimated as 1.2. We also have to consider the production of p and proton by collision of primary cosmic rays with the atmosphere. Following [15] and [16], we estimate that 0.3 and 0.4 event, respectively, of atmospheric p should be observed. On the other hand, secondary protons calculated by Papini, Grimani, and Stephens [17] contribute about 10% of the observed protons [18]. This raises the p/p ratio by a factor of 1.12. The resultant p/p ratio is 1.2+o'6z X 10 ~ in the energy range 300 —500 MeV. The systematic error, due mainly to uncertainties in the atmospheric p contribution and the annihilation cross section, is estimated to be about 15% [18], much smaller than the present statistical. No p events are observed in the range 175 —300 MeV, corresponding to a 90% C.L. upper limit on the p/p ratio of 2.9 X 10 after corrections. If the four observed p's

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LETTERS

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on RS/6000 workstations supplied for the program between ICEPP and IBM Japan, Ltd. K. Y. and T. S. acknowledge fellowships from the Japan Society for the Promotion of Science for Japanese Junior

performed partnership

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with previous data. The region between the solid curves is for the secondary p Aux calculated from Gaisser and Schaefer [12], taking into account the solar modulation at the BESS '93 Aight. Also shown are theoretical curves from the annihilaton of 30 GeV neutralino [6], and from the evaporation of PBHs with two arbitrary evaporation rates [4].

are considered representative of the whole energy range from 175 to 500 MeV, the corresponding flux ratio is 8.9 47 )& 10 6 The new results on the p/p ratio from BESS '93 Aight are shown in Fig. 4. The data are consistent with recent upper limits [8,9]. This experiment is the first to low energy P's at indentify (by a mass measurement) energies below 500 MeV and determine their abundance relative to protons. As can be seen from the comparison to the calculation by Gaisser and Schaefer [12] shown in the figure, the results are consistent with their origin as being secondary produced by collisions of galactic cosmic rays with interstellar gas. Our data can also be used to place limits on novel processes such as the evaporation of PBHs which would result to a Aux of low energy 3 3 ' p s. The resulting limit [4] is 8 X 10 pc yr for the evaporation rate of PBH averaged over the region within a few kpc of the solar system. Sincere thanks are given to NASA for providing the balloon and to the NSBF for the launch. We thank Y. Ajima, Y. Higashi, and D. Righter for their help. This experiment was supported by Grant-in-Aid for Scientific Research and for International Scientific Research from the Ministry of Education, by the Kurata Research Grant, and by Sumitomo Research Grant. The analysis was

R. L. Golden et al. , Phys. Rev. Lett. 43, 1196 (1979). [2] A. Buffington, S. M. Schindler, and C. R. Pennypacker, Astrophys. J. 24S, 1179 (1981). [3] P. Kiraly, J. Wdowczyk, and A. W. Wolfendale, Nature (London) 293, 120 (1981); M. S. Turner, Nature (London) 297, 379 (1982); J. H. MacGibbons and B. J. Carr, Astrophys. J. 371, 447 (1991). [4] K. Maki, T. Mitsui, and S. Orito, Report No. ICEPP-9403, 1994. [5] J. Silk and M. Srednicki, Phys. Rev. Lett. 53, 624 (1984); J. S. Hagelin and G. L. Kane, Nucl. Phys. B263, 399 (1986); F. W. Stecker, S. Rudaz, and T. F. Walsh, Phys. Rev. Lett. 55, 2622 (1985); F. W. Stecker and A. W. Wolfendale, Nature (London) 309, 37 (1984); S. Rudaz and F. W. Stecker, Astrophys. J. 325, 16 (1988). [6] G. Jungman and M. Kamionkowski, Phys. Rev. D 49, 2316 (1994). [7] See J. W. Mitchell et al. , in Proceedings of the 24th International Cosmic Ray Conference, Rome, 1995 (to be published) and A. W. Labrador et al. , in Proceedings of the 24th International Cosmic Ray Conference, Rome, 1995 (to be published). [8] S. P. Ahlen et al. , Phys. Rev. Lett. 61, 145 (1988); M. H. Salamon et al. , Astrophys. J. 349, 78 (1990). R. Streitmatter et al. , Adv. Space Res. 9(12), 65 (1989); S. Stochaj, Ph. D. thesis, University of Maryland, 1990; A. Moats et al. , in Proceedings of the 21st International Cosmic Ray Conference, Adelaide, Australia 3, 284

(1990). [10] T. K. Gaisser and R. H. Maurer, Phys. Rev. Lett. 30, 1264 (1973). [11] R. J. Protheroe, Astrophys. J. 251, 387 (1981). T. K. Gaisser and R. K. Schaefer, Astrophys. J. 394, 174 (1992). [13] A. Inoue et al. , "Report of the Institute of Space and Astronautical Science, " 1981, p. 79. [14] K. Yoshimura, Ph. D. thesis, University of Tokyo, 1995. [15] S. A. Stephens, in Proceedings of the 22nd International Cosmic Ray Conference, Calgary 2, 144 (1993). [16] T. Mitsui and S. Orito (to be published). [17] P. Papini, C. Grimani, and S. A. Stephens, in Proceedings of the 22nd International Cosmic Ray Conference, Cal gary 3, 761 (1993). [18] BESS Collaboration (to be published). E. A. Bogomolov et aL, in Proceedings of the 20th

st

International Cosmic Ray Conference, Moscow 2, 72 (1987); E. A. Bogomolov et al. , in Proceedings of the 2I International Cosmic Ray Conference, Adelaide, Australia 3, 288 (1990).

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