Overview of radio experiments for UHE cosmic particles detection Simon Chiche Sorbonne Universit e CNRS UMR 7095 Institut dAstrophysique de Paris 98 bis bd Arago 75014

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Overview of radio experiments for UHE cosmic particles detection

Simon Chiche

Sorbonne Universit´e, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014

Paris, France

Valentin Decoene

Department of Physics, The Pennsylvania State University, USA, University Park 16802 PA

Center for Multimessenger Astrophysics, Institute for Gravitation and the Cosmos, The Pennsylvania

State University, USA, University Park 16802 PA

Radio-detection is a mature technique that has gained large momentum over the past decades.

Its physical detection principle is mainly driven by the electromagnetic part of the shower,

and is therefore not too sensitive to uncertainties on hadronic interactions. Furthermore its

technical detection principle allows for a 100% duty cycle, and large surface coverage thanks

to the low cost of antennas. Various detection methods of UHE particles now rely on the

radio signal as main observable. For instance, ground based experiments such as AERA on

the Pierre Auger Observatory or LOFAR detect the radio emission from air-showers induced

by high-energy particles in the atmosphere; in-ice experiment such as ARA, IceCube, or

ARIANNA beneﬁts from a detection in denser media which reduces the interaction lengths;

ﬁnally, balloon experiments such as ANITA allow for very sensitive UHE neutrino detection

with only a few antennas. Radio-detection is now focused on building increasingly large-scale

radio experiments to enhance the detector sensitivity and address the low ﬂuxes at UHE.

In this proceeding we give an overview of the past, current and future experiments for the

detection of UHE cosmic particles using the radio technique in air (AERA, Auger-Prime,

GRAND), in balloon (ANITA, PUEO) or in other media (IceCube-Gen2, BEACON, RNO-

G).

1 Motivations for Radio-detection of ultra-high energy astroparticles

Over the last decade, high energy astroparticles have been observed in coincidence with several

high energy events such as TXS0506+056 1and AT2019dsg 2. Yet, more than 50 years after

the ﬁrst observation of ultra-high energy cosmic rays (UHECRs), their origin is still unknown.

Towards the understanding of this long-standing mystery increasing experimental eﬀorts are

made to detect cosmic-rays and their secondary messengers (gamma rays and neutrinos) at the

highest energies despite the very low incoming ﬂux. This would allow to not only constrain

UHECRs sources but also to probe the most powerful sources in the universe in the advent of

the multi-messenger era.

The detection of high-energy astroparticles relies on the signal from the particle cascades they

induce while arriving on Earth as illustrated in Fig. 1. Typically the interaction of a cosmic-ray

with air atoms from the atmosphere creates an extensive air-shower that propagates over 10 to

100 of kilometers and emits Cerenkov radiation, ﬂuorescence light and electromagnetic radiation

detectable on Earth alongside with the particles reaching the ground 3. Similar showers can also

be induced by tau-neutrinos arriving with Earth-skimming trajectories going through a dense

arXiv:2210.13560v2 [astro-ph.HE] 7 Nov 2022

Figure 1 – Sketch of UHE astroparticles detection principle for (1) in-air showers, (2) tau-neutrino induced showers

and (3) in-ice showers. The typical longitudinal proﬁle is represented atop of the showers.

medium such as a mountain or Earth’s surface. In these media, the neutrino can interact to

give a tau particle, which then, can escape to decay in the atmosphere inducing an air-shower.

Finally, particle cascades can develop in denser media such as ice or water, where due to the

higher density, the showers extent over smaller distances, of the order of a few meters for the

longitudinal proﬁle and of a few centimeters for the lateral one. Similar to air-showers these, in

ice showers also emit Cerenkov radiation and electromagnetic-waves.

The radio emission from particle showers has been extensively studied over the past decades.

For in air showers, the radiation results from two main mechanisms, with an intensity peaking in

the MHz regime. (1) The geomagnetic emission: the deﬂection of the lightest charged particles

in the shower, i.e., positrons and electrons in opposite directions induce a current varying in time

as the particle content in the shower varies over time leading to a radio signal polarized along

the −v×Bdirection (with Bbeing the direction of the magnetic ﬁeld and vthe direction of the

shower). (2) The charge-excess or Askaryan emission: while the shower propagates, electrons

from air-atoms are struck by high energy shower particles and then travel along with the shower

front. This combined with positron annihilation leads to a build up of a net negative charge in

the shower front inducing a signal radially polarized in a plane perpendicular to the shower axis.

The geomagnetic emission is dominant in air where the charge-excess account for only ∼10%

of the signal but it is negligible in ice where the charge-excess is much stronger and corresponds

to the dominant emission. In addition to that, the radio signal undergoes a geometrical time

compression phenomenon called Cerenkov eﬀect. It is due to the relativistic speed of the particles

which conﬁne the emission in a cone with a typical aperture angle of 1◦for an air-shower and

of roughly 40◦to 60◦for in ice showers.

2 First generation of radio experiments

It was proved in 1965 that air-showers emit radio waves 4, yet it is only in the 2000s that

radio detection really took oﬀ, mainly due to the improvements in digital signal processing and

motivated by an expected duty cycle of 100%.

Figure 2 – Left CODALEMA butterﬂy tripole antenna from 41 and right LOPES inverted v-shape dipole antenna

from 9.

2.1 CODALEMA and LOPES

This emergence of radio-detection as a promising technique was led by 2 pioneering experiments,

CODALEMA5,6,7and LOPES10,8,9which aimed at probing that radio-detection of cosmic-ray

induced showers in the atmosphere was feasible. These experiments relied on the fact that radio

waves emitted by particle cascades travel in air with almost no absorption and could be detected

at ground level with radio-antennas. Radio-detection was however impeded by the numerous

anthropogenic radio emissions from various sources such as the FM band, airplanes or satellites.

CODALEMA, the Cosmic-ray Detection Array with Logarithmic Electro-magnetic Antennas is

an experiment initiated in the Nancay radio-astronomy station in 2003. It combined a sparse

array of 57 autonomous radio antennas (Fig. 2, left panel) detecting signals in the 20-200 MHz

band with a compact array of cabled antennas triggered by 13 scintillators over 1 km2. LOPES,

the LOFAR Prototype Station, is a radio experiment made of 30 LOFAR prototype antennas

(Fig. 2, right panel) that ran between 2003 and 2013. It used interferometry triggered by the

KASKADE-Grande experiment in the 40 −80 MHz band. Even though the layout was set in

the radio loud environment of the Karlsruhe Institute of Technology, LOPES showed that a

radio detection with energy and angular resolution comparable to particle array performances

was achievable. It also allowed the radio community to better understand the radio-emission

mechanisms, highlighting the clear dependency of the signal with the geomagnetic angle.

2.2 RICE and AURA

In parallel to the in-air pioneering experiments, two detector concepts were investigated for

the in-ice detection of neutrino-induced in-ice showers. These investigations were motivated

by the fact that the ice cap oﬀers a gigantic interaction volume for neutrinos, making possi-

ble, in principle, a gigatonne sized detector with only few antennas in a relatively radio quiet

environment. These two detector concepts called RICE, Radio Ice Cherenkov Experiment 14

(1995-2005) and AURA, Antarctic Under-ice Radio Array 15 (2003-2009), aimed at deploying

sub-surface and deep-ice antennas respectively. RICE was co-located with the AMANDA pro-

totype and was composed of 20 half-wave antennas (200 −1000 MHz), in a 200 ×200 m2layout

between 100 −300 m depth. AURA was deployed with IceCube in a layout of 6 clusters of

4 antennas (100 −450 MHz) between 300 −1300 m. Sub-surface arrays while being obviously

more convenient for the deployment faces more refraction and ray-bending eﬀects than deep-ice

ones, where the ice is more cold and more stable. One of the big advantage of the in-ice radio

detection is that the ice provides a denser interaction medium than air for high energy neutrinos.

However, the radio attenuation length in ice is of only ∼1 km on average (a factor thousand

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OverviewofradioexperimentsforUHEcosmicparticlesdetectionSimonChicheSorbonneUniversite,CNRS,UMR7095,Institutd'AstrophysiquedeParis,98bisbdArago,75014Paris,FranceValentinDecoeneDepartmentofPhysics,ThePennsylvaniaStateUniversity,USA,UniversityPark16802PACenterforMultimessengerAstrophysics,Institutefor...

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Overview of radio experiments for UHE cosmic particles detection Simon Chiche Sorbonne Universit e CNRS UMR 7095 Institut dAstrophysique de Paris 98 bis bd Arago 75014

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