Stephanie Wissel
With the the discovery of a di use flux of astrophysical neutrinos [1,2] and the identification of the first candidate extra-galactic source of neutrinos [3,4], neutrinos have been established a powerful messenger in the exploration of the high-energy universe. The Radio Neutrino Observatory in Greenland (RNO-G) will extend the reach of multi-messenger neutrino experiments to energies above 100 PeV (ultra-high energies UHE, see Fig. 1), with a unique view of the Northern sky 3, and the largest footprint of a radio neutrino experiment (see Fig. 4), while also providing several critical technological tests that will inform the design of the radio component of IceCube-Gen2 [5,6].
Neutrinos are excellent probes of high energy particle acceleration in the universe. Because they only interact through the weak force, neutrinos point back to their sources, can reach Earth from the most distant corners of the universe, and provide a clear indication of hadronic, cosmic ray acceleration. Above 100 PeV, astrophysical neutrinos can be generated through cosmic ray interactions with gas or radiation at (or near) some of the most energetic objects in the universe [7][8][9][10][11][12][13]. Cosmogenic neutrinos [14] expected due to interactions of cosmic rays with photon backgrounds [15,16] and from observations of the cosmic ray spectrum [17,18] -are also expected at UHE [19][20][21]. The neutrino flux encodes information about the evolution and acceleration mechanisms at cosmological length scales.
The first observation of a neutrino from a blazar, TXS 0506+056, coincident with a flare in Wrays opened a new window into the non-thermal universe through multi-messenger observations [3,4]. However, the neutrino sky must be more complex; neutrinos from blazars cannot comprise the bulk of the di use neutrino spectrum [22] and many sources may extend to higher energies [7][8][9][10][11][12][13]. Bursts of neutrinos from the most explosive sources are expected to peak in flux at the EeV scale. A full understanding of the neutrino sky over cosmological distances requires a large detector and observations expanded up to EeV energies.
RNO-G is an intermediate-scale discovery instrument for astrophysical and cosmogenic neutrinos at the highest energies that builds on prior ice radio experiments [23][24][25][26][27][28]. The design combines high single-station e ective volume achieved with the deep array with a cosmic ray veto and enhanced the neutrino e ective volume from a shallow array. Its scalable, low power, and autonomous design is a novel instrument design that provides important field testing for the larger radio array planned for IceCube-Gen2 [6]. In this proceeding, we describe the science prospects of RNO-G, the design, and the first deployment season.
RNO-G's sensitivity to the di use flux of neutrinos in the UHE range is shown in the left panel of Fig. 1 as the 90% confidence level upper limit to the all-flavor di use flux of neutrinos. This is compared to several models of the expected cosmogenic flux of neutrinos, assuming a range of cosmic ray compositions that are still consistent with current observations from TA [32,33] and Auger [19,34]. These are representative samples of a large range of models [36,37]. Within 5 years of observation, RNO-G is expected to constrain the di use flux.
Additionally, several models of astrophysical neutrinos are expected to contribute to the di use neutrino background in the UHE band. These models tend to extend to lower energies than the [19,[32][33][34]. From Ref. [35]. cosmogenic models, but remain within the reach of RNO-G. Potential candidates range from Active Galactic Nuclei (AGN) [7] to various types of gamma-ray bursts (GRBs) [8,9], pulsars [10], galaxy clusters [11], Flat Spectrum Radio Quasars (FSRQs) [12], and blazars [13].
RNO-G will also be capable of searching for transient phenomena that generate UHE neutrinos, extending multi-messenger observations of these explosive objects into a new energy range. RNO-G's neutrino fluence sensitivity in a range of zenith angle bands is shown in the right panel of Fig. 2. Flares of blazars [12,38] may generate UHE neutrinos along with neutrinos at lower energies observable with IceCube. Gamma-ray bursts (GRBs) can produce UHE neutrinos at several stages in their evolution, including during the prompt phase of short GRBs [39] and in the afterglow phase [9]. Cataclysmic events like tidally disrupted stars [40] and neutron star mergers [41,42], some of which can result in long-lived magnetars [43], can produce gravitational waves and neutrinos simultaneously. A planned design feature of RNO-G is the capability to send and respond to real-time alerts, made possible by a local LTE network and continuous satellite network connection between Summit Station and RNO-G institutions.
RNO-G will provide a unique view of the UHE Northern sky that is broad in declination and overlaps with the region of the sky where IceCube is most sensitive to lower-energy neutrinos, as shown in Fig. 3. The Earth is opaque to neutrinos at PeV to EeV energies, such that UHE neutrino observatories are most sensitive to Earth-skimming neutrinos. Therefore, follow-up of TeV-scale IceCube events at higher energies requires a Northern detector. In addition to the targets defined by IceCube, other sources visible with RNO-G include several well-known TeV gamma ray blazars 1 [38], stable magnetars [43], short GRBs [39], GRB afterglows [9] are shown for comparison at several luminosity distances. From Ref. [35].
and the TA hospot [44]. The broad sky coverage will enables studies of studies of point sources of high-energy neutrinos in the UHE band that can be combined with neutrino observations at lower energies.
RNO-G will also provide a testbed for fundamental physics at energies in a new energy range (see Ref. [45,46] and references therein). Nucleon and nuclear structure and potential new physics can be probed through cross-section measurements of the interactions of neutrinos with nucleons. Some models, such as those that include sphaelerons, supersymmetry, rolled-up dimensions, or leptoquarks, can lead to drastic increases in the cross-section at UHE energies. They can also be used to measure probe parton distributions at low Bjorken G and high & 2 , well beyond the reach of the Large Hadron Collider. Fundamental properties of the neutrino can be explored at a new energy scale and over long length scales including flavor oscillations and neutrino decay. Several opportunities are available for exploring new physics including dark matter, neutrino selfinteractions, fundamental symmetries, and interactions of neutrinos with dark backgrounds, among others. All fundamental physics measurements require the discovery of UHE neutrinos and good energy and angular resolution.
The radio detection technique relies on coherent, impulsive radio signals emitted when a neutrino interacts in a dense material [48], due to a negative charge excess in the resulting cascade. For wavelengths larger than the transverse size of the shower, the emission is coherent, leading to a large power boost for high-energy showers at frequencies . 1 GHz. This so-called Askaryan e ect [49] has been demonstrated in beam-test experiments [50][51][52][53]. The radio technique has advanced over the last two decades through several experiments [27,54], especially recently through
RNO-G [47]. The orange sources include strong gamma ray or radio emitters as well as the TA hotspot [44]. From Ref. [35].
the balloon-borne experiment ANITA [27], the ARIANNA shallow array [55] and ARA, a deep-ice detector at the South Pole [23,56]. These and other radio instruments have further validated the radio technique through cosmic ray observations [57][58][59] and radio propagation studies [60,61].
RNO-G is located at Summit Station, a site managed by the US National Science Foundation (NSF). Summit Station (72 35 0 46 00 N, 38 25 0 19 00 W) is located in near the apex of the ice sheet in Greenland over 3.2 km of ice. Early field campaigns established that the site is suitable for a large scale neutrino detector. In 2013, the depth averaged attenuation length was first measured at Summit Station [62] to be 823 +189 209 m when extrapolated from 75 MHz to 300 MHz. In 2015, the field teams deployed an analog prototype of the trigger design used in RNO-G. They also monitored both the radio frequency noise environment [62] and radio wave propagation in the upper layer of snow known as the firn [63]. Further ice studies relevant for radio neutrino detection are underway at Summit Station now.
As shown in Fig. 4, RNO-G will be a large scale observatory consisting of 35 independentlytriggered stations separated by 1.25 km. The array will cover a 50 km 2 area over the surface of glacial ice. While the first stations were already deployed in the summer of 2021, the array is planned to be completed and fully operational by 2024.
The RNO-G design combines a deep array for high-sensitivity triggering and a surface array for a cosmic-ray veto and improved event reconstruction. Most of the deep antennas, both vertically polarized (Vpol) and horizontally polarized (Hpol) antennas, are deployed on a single string of instrumentation in one hole. The other two instrumented strings provide azimuthal reconstruction and also have impulsive calibration sources for in situ characterization. The hardware for RNO-G is described in detail in these proceedings [64]. The RNO-G stations, shown in Fig. 5, each have 15 deep channels and 9 shallow channels. The power string, shown on the left, has a compact, closely packed array of antennas used in the primary trigger as well as antennas deployed at di erent depths along the same string. The helper strings provide additional antennas for reconstruction and calibration pulsers. The vertically polarized antennas consist of 5-inch diameter fat dipoles based on the RICE and GNO dipole designs. The horizontally polarized antennas use an 8-inch diameter aluminum quad-slot design. Commercially available LPDAs are used for the for the shallow channels. Three of the LPDAs point upward, while the remaining six point downward.
Because the density and index of refraction of ice increases with depth, radio signals from neutrino interactions deep in the ice bend as they travel towards the surface. Antennas positioned deeper in the ice therefore access a larger volume of ice and fractional field of view than those at the surface, which motivates using the deep channels as the primary trigger. RNO-G stations are primarily triggered with an interferometric phased array on the deep vertically polarized channels, based on a similar trigger used in the ARA experiment [65] and tested in early field campaigns in Greenland [62]. The phased array trigger coherently sums the envelopes of the waveforms with time delays corresponding to a range of angles of incident plane waves, increasing the voltage signal-to-noise ratio (SNR) for triggering by the square root of the number of antennas in the array [65]. Based on a full Askaryan trigger simulation, validated by the performance of the existing ARA system, we expect to achieve an elevation-averaged 50% trigger e ciency point at a 2f threshold in voltage. The trigger for RNO-G is tuned to the lower portion of the frequency band (80-236 MHz), which optimizes for events further from the Cherenkov cone and achieves a comparable trigger e ciency as the ARA phased array even with fewer channels [35,66]. Each station uses a custom designed digitizer board using LAB4d digitizers [67], which yield more than a GHz of bandwidth, using two bu ers of 2048 samples each.
Cosmic ray air showers provide a unique opportunity for instrument calibration and validation. Air showers from downgoing cosmic rays generate coherent radio emission from the combined e ects of geomagnetic emission and Askaryan radiation, with geomagnetic emission dominating. Geomagnetic emission occurs due to the currents induced by the Lorentz force acting on charged particles in the shower. The expected radio footprint for a range of zenith angles is shown in Fig. 6. Cosmic ray and neutrino radio signals can be distinguished by their polarization, signal shape [57,68,69], but because both signals are broadband and impulsive, the more prevalent cosmic ray events can be used to calibrate the instrument timing and energy scale [58,59]. Using an independent trigger with only the shallow channels, we expect O(10 100) cosmic ray events per day. Because the radio signal refracts into a narrow range of angles near the nulls of the deep antennas, we expect the cosmic ray rate using the primary trigger formed with the deep channels to be substantially lower. However, the instrument sensitivity can be calibrated with the surface channels.
The same characteristics that make cosmic rays a good calibration source also make them potential backgrounds. Thus the shallow channels provide an additional veto for cosmic ray events. This is especially important for more insidious cosmic-ray induced backgrounds like cosmic-ray muons that generate showers in the ice similar to neutrinos [70] and transition radiation from air showers impacting the ice [71]. The shallow channels can also veto non-thermal backgrounds originating from above the surface. These potential cosmic ray backgrounds can be O(0.1-1) events
RNO-G per array per year, which may be comparable to rate of neutrinos.
The stations are autonomous, using solar power to provide an expected 60-70% uptime and an LTE cellular network for the high-throughput communications required to transmit the full waveforms back to the Big House. A backup lower throughput LoRaWAN that can be used for station commanding and monitoring is also available. Summit Station has a continuous, 24-hour satellite connection that allows for 1 Gb/day of data to be transmitted to the RNO-G collaborating institutions. The continuous networking capabilites can be used to respond to and generate multimessenger alerts.
The first three stations, Amaroq (shown in the left panel of the stations over long distances. The remaining stations are expected to be deployed in by 2024.
The deployment schedule shown in Fig. 8 includes delays due to the COVID-19 pandemic. Three 100-m holes are drilled per station using the custom designed 11-inch drill (the British Antarctic Survey's BigRAID drill [73]), shown in Fig. 7. Over the 2021 deployment season most holes were drilled in two shifts of two people per hole, but we anticipate improvements to the drilling procedure that can complete a 100-m hole in one shift. Aspects of the hardware and station deployment procedure are illustrated in Fig. 9.
Fig. 10 shows some of the first events recorded on the vertically polarized channels in Amaroq. These events were generated by a field team member driving a snowmobile near the station. The events demonstrate the impulsive character of the signals searched for by RNO-G.
RNO-G is a mid-scale UHE neutrino detector currently under construction and operating at Summit Station in Greenland. It is the first science-level instrument to target the UHE cosmic neutrino sky in the North. The novel hybrid design combines deep component for high e ective volume from deep, low threshold trigger with shallow component for improved angular reconstruction, additional neutrino sensitivity, and background vetos. The instrument is designed to be scalable, both allowing a large number of stations to be installed in over four years and using a low power design.
We are thankful to the sta at Summit Station for supporting our deployment work in every way possible. Also to our colleagues from the British Antarctic Service for getting excited about building and operating the BigRAID drill for our project. We would like to acknowledge our home institutions and funding agencies for supporting the RNO-G work; in particular the Belgian Funds for Scientific Research (FRS-FNRS and FWO) and the FWO programme for International Research Infrastructure
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