GAPS: A balloon-borne cosmic-ray antimatter experiment

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GAPS: A balloon-borne cosmic-ray antimatter experiment G. Osteria, for the GAPS Collaboration 1 Istituto Nazionale di Fisica Nucleare - Sezione di Napoli, Naples, Italy

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ABSTRACT

Keywords: Dark matter Cosmic rays Antideuteron Antiproton Si(Li) tracker Time of flight system Balloon experiment

Theories beyond the Standard Model predict dark matter candidates that could provide a significant enhancement of the antideuteron and antiproton flux, in particular at low energies. The General Antiparticle Spectrometer (GAPS) experiment is the first antimatter search experiment designed specifically for low-energy cosmic ray antideuterons and antiprotons. GAPS identifies antideuterons and antiprotons using a technique based on exotic atoms. This novel detection technique allows GAPS to have unprecedented sensitivity in the low energy range (< 0.25 GeV/n) for antiprotons and antideuterons. The apparatus consists of 10 planes of lithium-drifted Si (Si(Li)) detectors, surrounded on all sides by a plastic scintillator time-of-flight. GAPS is designed to carry out its science program using long-duration balloon flights in Antarctica and is currently scheduled by NASA for its first flight in late 2020. This presentation will describe the design, status, and discovery potential of the GAPS scientific program.

1. Introduction

exceeds the background level by more than two orders of magnitude. Fig. 2 shows the predicted 𝑑̄ fluxes by secondary production and by some DM particle models. The General Antiparticle Spectrometer (GAPS) balloon-based experiment is designed to carry out a sensitive DM search by measuring low-energy cosmic-ray baryonic antiparticles, focusing on 𝑑̄ search. The red area in Fig. 2 represents the sensitivity of GAPS after three 35-days flights. A first GAPS science (Long Duration Balloon) flight is proposed from Antarctica in the austral summer of 2020.

A well motivated explanation of the Dark Matter (DM) phenomenon is that the DM is made up of weakly interacting massive particles (WIMPs) with a mass ranging from GeV to a few TeV. In a variety of WIMP scenarios, WIMPs may self-annihilate or decay in the galactic halo with standard model final states. The available observational channels depend on the mass of the DM particles, which annihilate almost at rest, and on the astrophysical background in these channels. A DM signal could be detected looking at the rare CR antimatter components. In recent years several indirect dark matter experiments have provided interesting hints measuring daughter standard model particles such as 𝛾 [1] or e± [2–5] potentially resulting from the decay or annihilation of a given dark matter species. Also the 𝑝̄ abundance has been extensively measured by magnetic-spectrometer experiments [6,7] from 200 MeV up to 450 GeV, where it has been found overall consistent, within the uncertainties, with the expected astrophysical background. By extending the 𝑝̄ measurement at lower energies, where the background is kinematically suppressed, a dark matter signature could be seen as an excess in the antiproton flux. Since the secondary antiproton flux steeply decreases at low energy (see Fig. 1), the spectrum shape would become flat if a primary component existed [8]. Among the possible indirect DM search channels a favourable signal-to-background ratio ̄ [9]. The is expected for low energy (< 0.25 GeV/n) antideuterons (𝑑) production of low energy 𝑑̄ by collisions with the interstellar medium (ISM) is more strongly suppressed than by annihilation or decay of massive DM particles, due to the kinematics of the reactions [10]. According to several calculations, below 1 GeV/n the 𝑑̄ flux from DM

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2. Anti-nuclei identification method of GAPS The operating principle of GAPS is illustrated by the cartoon of Fig. 3. An incident antinucleus slows down and stops in the silicon target forming an exotic atom in a highly excited state. The exotic atom de-excites emitting Auger electrons as well as atomic X-rays with energies depending on the captured anti-nucleus species. The process ends with the nuclear annihilation of the anti-nucleus producing a star of pions and protons. The number of particles generated in the annihilation scales with the number of antinucleons and provides an additional identification for the captured antiparticle. So the identification of the incident anti-nucleus relies on the capability of measuring with great accuracy the atomic X-rays and the pion/proton tracks. As shown in Fig. 3 antiprotons are the dominant background for the antideuteron ̄ 𝑝̄ identification requires measurement (same event topology). A good 𝑑∕ measuring simultaneously:

E-mail address: [email protected]. http://gaps1.astro.ucla.edu/gaps/authors/alist.html.

https://doi.org/10.1016/j.nima.2019.05.042 Received 31 March 2019; Received in revised form 9 May 2019; Accepted 13 May 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: G. Osteria, GAPS: A balloon-borne cosmic-ray antimatter experiment, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.05.042.

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Fig. 2. Antideuteron fluxes from three representative WIMP models, as well as the astrophysical background component. The measured limit from BESS is shown, along with the sensitivities of AMS-02, (the arrow shows the AMS geomagnetic cutoff correction size) and GAPS, after three 35-day flights [10]. The shaded bands represent the envelope of model predictions due to the uncertainties on the propagation parameters.

Fig. 1. Measured antiproton flux by BESS [2,3] and PAMELA [4] compared with theoretical calculations for secondary antiproton production and for some DM models predicting a sizeable antiproton contribution at low energy [8]. In particular, the secondary (background) antiprotons augmented by primary antiprotons from a 8 GeV neutralino is shown with the expected flux precision for GAPS from one 35-day flight.

3. The GAPS instrument • the stopping depth, the velocity as well as the 𝑑𝐸∕𝑑𝑥 energy loss

The GAPS instrument is optimized for the detection and identification of low-energy antideuterons and antiprotons using the described exotic atom technique. It is a balloon-born detector consisting of a Time-Of-Flight (TOF) system that surrounds a silicon detector array (Fig. 4). The TOF system provides 𝑑𝐸∕𝑑𝑥 and velocity measurement of incoming ionizing particles as well as high-speed trigger and veto. The

of the incident antiparticle; • the characteristic X-rays energies of the exotic atom; • the pion and proton multiplicity generated in the nuclear annihilation.

Fig. 3. A low-energy antinucleus stopped in the silicon detector generates an excited exotic atom with the silicon atom. Then, characteristic X-rays, pions and protons are radiated in the decay and in nuclear annihilation of the exotic atom.

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Fig. 4. GAPS payload design. The TOF counters surround a 1 m3 volume, filled with a silicon detector array nested into expanded polystyrene blocks. The inset shows an expanded view of the detector module.

detector module, which provides the interface with cooling, power and read-out systems (see the inset in Fig. 4). The detector modules are nested into expanded polystyrene blocks, which provide insulation and protection during parachute shock and landing, while minimizing the interference with particle detection. The Si(Li) sensors are kept at the operational temperature of −43 ◦ C by means of an Oscillating Heat Pipe (OHP) passive cooling system, developed by JAXA/ISAS for the GAPS experiment [11]. A total of 1440 detectors is required to fill the large sensitive volume of the GAPS silicon detector array. The mass production process of the GAPS Si(Li) detectors has been developed based on previous works on prototype detectors [12,13]. Fig. 5 displays the energy spectrum at a temperature of −35 ◦ C measured with one strip of a sample Si(Li) detector and 59.5 keV and 88.0 keV X-rays from 241 Am and 109 Cd respectively [14]. The result confirms the required X-ray energy resolution, < 4 keV (FWHM), has been achieved by detectors. The measured leakage current and capacitance as a function of bias is similar between all strips, indicating that the detector volume is uniformly compensated by the Li-drift. The mass production of GAPS Si(Li) detectors started in late 2018 and will extend through early 2020.

silicon detector array acts both as target material for antiparticles annihilation and as tracking device for incoming antinucleus and outgoing exotic atom products. 3.1. Lithium Drifted Silicon Tracker The GAPS silicon detector array has to measure 20 to 100 keV X-rays with FWHM < 4 keV energy resolution (necessary to distinguish X-rays from different incident antinuclei). In order to provide sufficient depth to stop incoming anti-nuclei (with kinetic energy up to 0.25 GeV/n) the array thickness must be greater then 25 mm having over 90% of the silicon thickness as sensitive layer. To achieve sensitivity to the very low antideuteron flux, more than 10 m2 of active Si area is required. Lastly, due to the limited power available to the cooling system on the balloon flight, the required operation temperature is significantly higher (−35 to −45 ◦ C) than that of typical silicon X-ray detectors. The silicon detector array designed for GAPS is based on Lithiumdrifted Silicon (Si(Li)) detectors. Si(Li) detectors, with active regions that can be made both large in area and deep in thickness at a low fabrication cost, are indeed ideal for GAPS. The silicon detector array has dimensions of ∼ 1 m3 and consists of 10 layers of Si(Li) detectors. Each tracking layer is composed of 12 × 12 Si(Li) wafers, of 10 cm diameter and 2.5 mm thickness, segmented into 8 strips. Four Si(Li) detectors, arranged in 2 × 2 matrix, are mounted into an aluminum module, the

3.1.1. Si(Li) Tracker electronics The Si(Li) Tracker electronics consists of three main sub-systems: Front-end electronics, Back-end electronics and Power electronics (High and Low Voltage). 3

Please cite this article as: G. Osteria, GAPS: A balloon-borne cosmic-ray antimatter experiment, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.05.042.

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distribution scheme adopted, one LV channel has to provide voltage to a ‘‘chain’’ of six detector modules for an estimated power of 15 W. A custom power system able to manage the 360 HV channels (one per Si(Li) detector module) and the 60 LV channels (one per chain of six Si(Li) detector modules) has been designed. The system has a MicroController-based architecture and is interfaced with the GAPS DAQ system via Ethernet protocol. 3.2. Time of flight system The TOF system has to measure the velocity (𝛽) for ≥ 95% of particles from above within 60◦ to the vertical and ≥ 30% of particles originating in the tracker. The requested resolution in measuring the charge (z) is 0.20 e (𝜎𝑞 ≤ 0.20 e). The incident angle of single stopping tracks should be reconstructed with a precision of ±5◦ respect to the vertical axis. Eventually, the TOF system has to provide a faster trigger for the readout of TOF and tracker. The GAPS TOF system shown in Fig. 4 meets all these requirements. It surrounds the tracking volume measuring the particle velocities, the energy depositions as well as the coordinates of the impact point on the outer and inner layers. The TOF consists of two main components: an outer ‘‘umbrella’’ section, and inner ‘‘cube’’ section. The top side of the ‘‘umbrella’’ has dimensions 3.6 × 3.6 m2 . The dimension of each side of the ‘‘Cortina’’ are 3.6 × 2.5 m2 . The flight distance between the outer and the inner layers is ∼ 1m. The TOF detectors are 0.6355 cm thick ‘‘paddles’’ of EJ-200 (Eljen) plastic scintillator. The paddles of the outer TOF layers have dimensions of 180 × 16 cm2 . The cube will use 156(106.5) cm × 16(10) cm × 0.635 cm side (corner) paddles. The total number of scintillation counters required for the whole TOF system is 196. Each paddle is instrumented on both ends with a preamplifier board loaded with six Hamamatsu S13360-6050VE silicon photomultipliers (SiPMs) for maximum light collection. The summed SiPM signal from 8 paddles (for 16 total channels) will be routed to a TOF readout board, which digitizes the trace using a DRS4 switched capacitor array ASIC. In addition to the readout board, the summed signals are also split and connected to a local trigger board which implements various discrimination levels and combinatorial logic to search for events. The local trigger boards are connected to a master trigger system which makes a system-wide trigger decision based on local patterns. The master trigger will initiate waveform readout on the TOF and Si(Li) system. Recent measurements on a 180 cm long paddle have demonstrated a ≤ 500 ps end-to-end timing resolution in the laboratory.

Fig. 5. The energy spectrum at a temperature of −35 C measured with one strip of a sample Si(Li) detector and 59.5 keV and 88.0 keV X-rays from 241 Am and 109 Cd respectively.

Front-end electronics. The readout electronics must handle a large range of signal amplitudes (20 keV–100 MeV) and feature a low noise performance in order to achieve the required 4 keV resolution at low energies. A custom 32-channel ASIC ‘‘Silicon LIthium DEtectors Readout’’ (SLIDER) has been developed to read out the 11520 Si(Li) strips of the GAPS Silicon Tracker. The ASIC is hosted in a Front-End board placed in the center of the Si(Li) detector module (inset of Fig. 4). The board is used for the connection of the ASIC to the detectors. The board is also used to bring the bias voltage to the sensors, to provide the bias voltage and the control signals to the ASIC and to propagate them throughout the whole tracker. Fig. 6 shows the block diagram of one channel of the ASIC. The analog conditioning scheme is based on a low-noise charge-sensitive amplifier (CSA) featuring dynamic signal compression. The solution takes advantage of the non-linear features of a MOS capacitor in the feedback loop of the charge-sensitive preamplifier itself. In the CSA, a continuous reset is provided by a Krummenacher feedback network. The amplifier is followed by a unipolar second order semi-Gaussian shaper. The signal peaking time at the shaper output is selectable among eight values (from 250 ns up to 2 μs). After the filtering stage the analog conditioning scheme is split into three different paths. On one side a comparator is used to discriminate the amplified pulse. Another branch includes a single-ended to differential Sample&Hold which provides a signal proportional to the shaper output peak to the subsequent differential SAR (Successive Approximation Register) ADC for analog information detection. On the third branch, the signal at the shaper output is differentiated. The identification of the zero-crossing of the resulting bipolar signal provides a trigger, synchronous with the shaper peaking time, for the single-ended to differential Sample&Hold. The first prototype channel, in 180 nm CMOS technology, was submitted for fabrication in August 2018 and delivered at the end of 2018. The characterization activity is now in progress and the preliminary results are very encouraging. The submission of the first prototype of the final ASIC, which will host 32 channels and a 11-bit ADC, is foreseen for the spring of 2019.

4. Conclusion The GAPS experiment will carry out a sensitive dark matter indirect search by measuring low-energy (E ≤ 0.25 GeV/nucleon) cosmic-ray antinuclei. The primary target are low-energy antideuterons produced in the annihilation or decay of dark matter. GAPS will also conduct lowenergy antiproton and antihelium searches. The instrument consists of a central tracker with a surrounding time-of-flight system. The key elements of the two detectors have been developed and successfully tested. The construction of the flight model is in progress, the integration and test of the payload is scheduled for the end of 2019. The earliest date for the first of a series a long-duration Antarctic balloon flights is the austral summer of 2020. The first GAPS flight will improve the current antideuteron limit by 1.5 orders of magnitude and will deliver the first precision measurement of antiproton flux below 0.25 GeV/n.

Back-end electronics. The back-end electronics will configure, control and acquire the data from the tracker front-end boards. A ‘‘chain’’ of 6 Si(Li) detectors modules (six ASICs) is controlled by one back-end channel through SPI protocol. One 6-channel FPGA-based DAQ box controls a tracker plane. The whole system comprises 10 DAQ boxes.

Acknowledgment This work is supported in the U.S. by NASA APRA grants (NNX17AB44G, NNX17AB45G, NNX17AB46G, and NNX17AB47G), in Japan by JAXA/ISAS Small Science Program and JSPS KAKENHI grants (JP26707015 and JP17H01136), in Italy by INFN and the Italian Space Agency through the GAPS ASI INFN agreement n. 2018-28-HH.0.

Power electronics. The power system has to provide the bias voltage (HV) to the Si(Li) detector modules and the voltages (LV) to the frontend ASIC boards. The typical bias voltage of the Si(Li) detectors varies in the range 150–300 V, the requested accuracy is 1 V. The frontend ASIC board needs four different voltages. According to the power 4

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Fig. 6. Front-end electronics block diagram.

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