EUSO-SPB2: A balloon experiment for UHECR and VHE neutrino observation

. The Extreme Universe Space Observatory on a Super Pressure Balloon 2 (EUSO-SPB2) experiment will make new measurements from suborbital space as a precursor for future space missions that will address the challenge of the extremely low fluxes of ultra-high energy cosmic rays (UHECR) and very high energy (VHE) neutrinos. The EUSO-SPB2 detector is comprised of two 1m diameter aperture telescopes. The Fluorescence Telescope (FT) will point in nadir and will record fluorescence light from cosmic ray EAS with energies above 1 EeV in its field of view of 36 by 12 degrees. The Cherenkov Telescope (CT) features a silicon photomultiplier focal surface with a field of view of 12 by 6 degrees. The CT will switch between two observation modes: one which points the CT above the limb to measure the Cherenkov emission of cosmic ray EAS with energies above 1 PeV and one which points the CT below the limb to record the Cherenkov emission produced by PeV scale EAS initiated by neutrino-sourced tau decay. As it is the first time such an instrument has been flown, one of the priorities of the CT will be the study of the optical background for observing neutrinos in this way. EUSO-SPB2 is


Introduction
Ultra-High Energy Cosmic Rays (UHECR) and very high energy (E>1 PeV) neutrinos provide a unique window into the most extreme astrophysical events in the universe, delivering complimentary information that may be used to identify the production and acceleration mechanisms within sources, the cosmological evolution of sources, and the propagation behavior of the resultant particles. As cosmic rays pass through Earth's atmosphere, they may interact, resulting in km-scale particle cascades, called extensive air showers (EAS). The secondary particles within the EAS produce observable emission in multiple channels, including radio, air fluorescence, and optical Cherenkov, which are frequently used in conjunction with direct secondary observation by ground-based observatories to detect cosmic rays. The flux of cosmic rays and neutrinos drops rapidly with increasing energy, and at the highest energies, requires massive instrumental volumes to reach sufficient statistics for proper analysis. For neutrinos, which have minuscule interaction probabilities * e-mail: jeser@uchicago.edu even at the highest energies, this issue is compounded further. A detailed discussion of the current status and the future of UHECR including the role of multi-messenger Astrophysics is given in [1].
One cost-effective method to increase experimental acceptance to both cosmic rays and neutrinos is to observe from high altitudes and use the entire Earth and its atmosphere as the corresponding target volume for particle interaction, allowing for gargantuan increases in geometric acceptance. Neutrinos can be observed from a high altitude detector via the Earth-skimming technique, whereby a neutrino propagates through the Earth and interacts near the Earth's surface to produce a charged lepton, which then either interacts or decays in the atmosphere to produce an upward moving EAS.
The Extreme Universe Space Observatory on a Super Pressure Balloon 2 (EUSO-SPB2) is a pathfinder mission for a space based mission, i.e. POEMMA [2], designed to identify and quantify the various backgrounds relevant for the space-based detection of cosmic rays and neutri- nos via the air fluorescence and optical Cherenkov emission channels, and to verify the overall detection strategy. It is also the third generation balloon mission undertaken by the JEM-EUSO collaboration. The first was EUSO-Balloon [3] showing the proof of concept in an overnight flight in 2014 and followed by the first attempt of a long duration flight in 2017 of EUSO-SPB1 [4] which had to be terminated early due to a leaking balloon after only 12 days at float. To address the science objectives mentioned above, EUSO-SPB2 is comprised of two telescopes: a fluorescence telescope (FT), which points directly downward and is designed for the measurement of cosmic rays, and a Cherenkov telescope (CT), which points near the Earth's limb and is designed for the measurement of Earthskimming neutrinos and direct (above-the-limb) cosmic rays. The two telescopes of EUSO-SPB2 will fly onboard a super pressure balloon, which reaches sub-orbital altitudes of ∼ 33 km and will launch from Wanaka, NZ in Spring of 2023, with the hope of achieving flight of up to 100 d. While at float, the balloon will follow a stratospheric air current that develops twice a year at these latitudes allowing for circulation around the globe. The previous trajectories are shown in Fig. 1.
The payload design is shown in the left panel of fig.  2, highlighting all the required provisions for such a long duration flight. The right panel of fig. 2 shows the fully assembled payload during hang testing at CSBF in Palestine, TX.

The Instrument
The fluorescence telescope, designed to observe ultraviolet light emitted via fluorescence of UHECR induced EAS, consists of three photo-detection modules (PDMs). Four Hamamatsu Photonics R11265-M64multianode photo-multiplier tubes (MAPMTs Hamamatsu Photonics R11265-M64) controlled by a custom application specific integrated circuit [5], are potted together in a gelatinous gel to form an elementary cell (EC). Nine of these ECs are connected to xilinx FPGA to form a PDM with 36 MAPMTs and 2,304 total pixels. Each PDM has a square field of view of roughly 12 o x12 o and can trigger internally [6]. The three PDMs (shown in top panel of fig. 3) are connected via a central data processor which issues triggers received from each PDM to the other two for a unified readout. Additionally, the data processor [7] combines the data from the PDMs and onboard differential GPS, and prioritizes data for download. The three PDMs, behind a BG3 filter 1 and field flattener (bootom panel fig.  3), are laid out in a rectangle at the focus of a Schmidt telescope consisting of a one meter diameter entrance pupil and six spherical mirror segments. An end-to-end measurement with calibrated LEDs confirm the expected overall efficiency of the detector of 19%. The telescope as assembled for field testing in August of 2022 is shown in Fig. 4 including the infrared camera on the bottom right for cloud detection.

Expected Cosmic Ray Detection Performance of the FT
The expected performance of the FT has been extensively benchmarked via simulations [8]. These simulations have been carried out utilizing the JEM-EUSO OffLine framework [9] which provides an end-to-end simulation of the response. Starting from an EAS simulated in Conex, the light production and atmospheric absorption are simulated, followed by the detectors response including the trigger algorithm. By using a Monte Carlo approach, an isotropic flux of UHECRs can be simulated. Showers are thrown over an over-sized disk on ground beneath the detector of 100 km radius. Unlike ground based experiments, there is no area on ground the EAS must land in order to be detected. As such, the over-sized sampling area allows for all possible observing geometries to be simulated. By combining the energy dependent trigger efficiency, simulated aperture, and the UHECR energy spectrum measured by the Pierre Auger Observatory [10], an expected event rate can be estimated.
Based on these simulations, the peak energy sensitivity of the FT is expected to be around 2.5 EeV. Across all

The Instrument
To record the Cherenkov emission from air showers sourced by neutrinos or cosmic rays, a Silicon Photomultipler (SiPM) camera was built using the S14521-6050AN-04 from Hamamatsu with a spectral range between 400 nm and 800 nm. The 512 pixel camera is shown in Fig. 6. A more technical description of the camera including its electronics which provide a 10 ns integration time (which matches the typical duration of a Cherenkov pulse sourced from an EAS) is given in [11]. The optical system is a modified Schmidt system with a 1 m diameter aperture providing a vertical field of view of 6.4 • and 12.8 • in azimuth. To be able to accommodate both major science objectives as outlined in section 3.2 with such a field of view, the pointing direction can be changed during the flight to any point in azimuth and from 3 deg above horizontal to ∼10 • below the Earth's limb (15 • below horizontal). The 4 segments of the mirror are aligned in a bi-focal manner, meaning parallel light from outside the instrument is focused into two distinct spots separated by 12 mm. This way, the background rate given by direct hits of charged particles can be reduced as such events produce only one spot in the camera, and coincident direct hits within the 10 ns integration time are rare. EUSO-SPB2 is the first mission equipped with such an instrument as a payload on a stratospheric balloon flight following an initial prototype flown on EUSO-SPB1 [12].

Background Measurements
In addition to the desired science goals of measuring cosmic rays and neutrinos, the CT of EUSO-SPB2 will quantify, for the first time, the night-sky airglow background from sub-orbital space. Current estimations of the airglow background are based on ground measurements, and may not be accurate at such high altitudes. Precise measurements of the airglow background are critical to the successful design of future space-based observatories, and will be provided by the CT across wide viewing ranges (both above and below the limb) and observation conditions.
The CT will also measure and quantify other sources of backgrounds relevant for future space-based missions, such as the rate of direct hits from cosmic rays (which will be well identified due to the bifocal optics) and man-made or natural phenomena. The latter category is particularly interesting, providing measurements to explore secondary science goals, such as the measurement of transient luminous events (TLEs).

Above-the-Limb Cosmic Rays
Direct Cherenkov emission is expected from above the limb due to EAS initiated by atmosphere-skimming cosmic rays. With potential near-limb arrival directions, such events represent a relevant background for neutrino searches, and need to be well understood.
EUSO-SPB2's CT sensitivity to above-the-limb cosmic rays has been investigated using a full Monte Carlo analysis where the EASCherSim 2 code has been used to generate the optical Cherenkov signal of showers, taking into account geometric and atmospheric effects [13]. The estimates provided by this study have since been updated to consider realistic assumptions of the trigger threshold of the CT obtained during field testing (see section 4). Fig. 7 shows the estimated cumulative event rate of above-the-limb cosmic rays for the CT as a function of primary energy, while fig. 8 shows the angular distribution of the accepted cosmic rays for a few primary energies. Fig. 7 shows a CT threshold of ∼ 1 PeV to EAS induced by above-the-limb cosmic rays, resulting in a total event rate of ∼ 100 events per hour of live time. Fig.  8 shows that while the majority of PeV-scale events are observed a few degrees above the limb, signals generated by more energetic primaries compete with the heavy atmospheric attenuation present for trajectories which pass closer to the Earth.
The simulated Cherenkov signals generated by abovethe-limb cosmic ray EAS show similar properties to those generated by neutrino-sourced showers, having comparable intensities, wavelength spectra, and timing and angular distributions of arriving photons. In this way, the frequently expected low energy cosmic rays from relatively high above the limb represent a reliable benchmark on which to calibrate the CT and verify the Cherenkov detection method towards the observation of potential neutrino   events. Consequently, the more rarely occurring, high energy cosmic rays from near the limb constitute a background for neutrino signals, particularly considering the expected atmospheric refraction of the generated optical emission.

Target of Opportunity
A variety of astrophysical transient events are suspected of harboring the conditions for high-energy and very-high energy neutrino production [e.g., [14][15][16][17][18]. Neutrino oscillations over large distances ensure a flux of tau neutrinos that would be detectable by EUSO-SPB2's CT through their upward-going EASs resulting from their interactions in the Earth.
The acceptance of EUSO-SPB2's CT to tau neutrinos and its transient sensitivity were studied in [19]. These calculations made use of detailed simulations of tau neu-trino propagation through the Earth, the development of upward-going EASs, and the transmission of Cherenkov signals through the atmosphere [20]. Fig. 9 and 10 show separate calculations of EUSO-SPB2's sensitivity to long-duration (lasting ≳ 1 day) and short-duration (lasting ≲ 10 3 s) events, respectively. For both scenarios, the sensitivity is presented as a band to account for variations in sensitivity that arise due to the positions of the sources behind the Earth as viewed by EUSO-SPB2 as a function of time. For long-duration events, the sensitivity presented in Fig. 9 is the average over the assumed flight duration (30 days or 100 days) in order to account for the day-to-day variation in the observation period due to the Sun and the Moon. For short-duration events, the sensitivity presented in Fig.  10 is integrated over 10 3 s and assumes the "best-case" scenario in which the source is in the optimal position for neutrino detection (i.e., when source just dips below the Earth's limb) and there is no interference from the Sun or the Moon.  [16], placing the source at various distances. Fig. 11 provides a sky plot of the time-averaged tau neutrino acceptance at 10 8.5 GeV. The plot demonstrates EUSO-SPB2's sky coverage, averaged over 100 days, assuming launch on April 1, 2023 and accounting for the reduction in the observation period due to the Sun and the Moon. For reference, select nearby astrophysics sources and reported hotspots from the Telescope Array experiment can be found in [21].

Field Test of the EUSO-SPB2 Telescopes
The full characterization of both telescopes using realistic signals prior to flight is essential to providing high quality data during the mission. In addition, the inherent risk of balloon flight makes the data taken in the field the only guaranteed data. Such full-scale end-to-end tests were performed in the field at the TA site in Delta, UT, U.S.A. for both telescopes. These tests were especially important for the CT as it was the first time such an instrument has been built. A UV laser system [22] was fired at a distance across the field of view to mimic the signals of extensive air showers in order to validate the telescopes. The same laser system has been successfully used to characterize the EUSO-SPB1 instrument in a similar setup [23], and has reliably provided test beams for several other telescopes. The CT was tested in February of 2022 for a period of 9 days. During these tests, the CT recorded 10000s of laser shots and UV LED flashes at various atmospheric and environmental conditions as well as downward going cosmic ray candidate signals. An example of a recorded signal for the CT is shown in fig. 12. These cosmic ray candidates were measured with a frequency of roughly ∼ 10 Hz, with the rate dependent on the elevation angle of the CT, which varied between 90 • and 60 • . A full monte carlo analysis using the EASCherSim code was performed to estimate the event rate of downward going cosmic rays and compare with what was measured, with the aim to confirm the validity of these candidates as cosmic ray events. The simulated event rate as a function of energy is shown in figure 13 for a CT zenith pointing angle of 30 • , and results in a total rate of 2.3 Hz. While this estimate fails to reach the measured 10 Hz observed in the field, it should be noted that the simulation results do not consider multiple pixel triggering, which can increase the event rate by allowing for triggering on less energetic events. While a full analysis is required, it is reasonable to state that the measured rate of cosmic ray candidates is a good estimate of the rate of downward going showers, therefore providing one measure of their validity.
The fluorescence telescope was tested over a period of 12 days at the end August of 2022 at the same location using the same artificial light sources as for the Figure 13: Simulated event rate of downward going cosmic rays for the CT of EUSO-SPB. In this configuration, the CT points 60 • in elevation. During field testing, the CT's orientation was varied between perfectly vertical (90 • ) and 60 • . The color bar shows the photoelectron count on the focal surface. The laser energy was set to 1.8 mJ and the laser at a distance of 24 km was pointed straight upwards while the telescope was pointed upwards by 7 • from horizontal.
CT, recording 10000s of laser shots and LED events at various monitoring conditions. Three example frames for a recorded laser track is shown in fig. 14. The tests performed included a photometric end-to-end calibration, a field of view measurement and a energy threshold study among others.
The recorded field-test data plays a special role for the FT: besides its use for an instrument characterization, the data is also the basis for the training of the neural network run on board the instrument as a download prioritizer. This method will mitigate the very limited download bandwidth of a balloon payload, making this data mission critical.
Preliminary comparisons between the recorded data and simulation show that both instruments behave as expected and validate the expected performance studies mentioned in section 3.2 and 2.2. A detailed analysis of all the collected data and a final characterization of the instruments based on these data and combined with laboratory measurements is still ongoing for both telescopes.

Summary
EUSO-SPB2 is the next major stepping stone to a space based multi-messenger detector. As such, a successful launch will raise the technological readiness for such a mission significantly and provide the proof of concept of the measurement techniques from suborbital space for the first time. The instrument is en route and launch is expected in April 2023 from Wanaka, NZ.
With an expected flight duration of at least a couple of weeks, it is expected that multiple extensive air showers with the FT will be detected, following extensive simulation studies which predict an overall event rate of 0.12 events per hour. The assumptions used in these studies were tentatively confirmed by the preliminary field test data. In addition to looking for UHECR tracks, the FT will also look for event candidates with conditions matching those of the ANITA anomalous event candidates (two classes: i) steeply upgoing, high-energy events and ii) near limb, upgoing high energy events) [24][25][26].
Studies of the novel SiPM based CT of EUSO-SPB2 show that it does not have sensitivity to the diffuse neutrino flux competitive with existing ground based experiments. However, with its ability to be pointed in zenith and azimuth on short time scales, it is possible to track potential sources across the sky following multimessenger alerts, significantly increasing the chance of neutrino observation, provided the event is within our cosmic neighbourhood. To validate and improve detection and reconstruction techniques for future missions, the CT will be pointed above the limb during flight and will record more than 100 direct cosmic rays per hour of observation. These events are guaranteed to exist, and the properties of their recorded Cherenkov emission strongly mimic those resulting from neutrino-sourced showers. Being a novel instrument, another important goal is the measurement and quantification of all relevant backgrounds for future space-based observation. The preliminary analysis of the field test data has shown that the CT behaves as expected and provides more confidence in the performed simulation studies. and 80NSSC22K1488 in the USA. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-05CH11231. We acknowledge the NASA Balloon Program Office and the Columbia Scientific Balloon Facility and staff for extensive support, the Telescope Array Collaboration for the use of their facilities in Utah. We also acknowledge the invaluable contributions of the administrative and technical staffs at our home institutions.