Precision measurements of the properties of cosmic rays at the highest energies

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg at a luminosity distance of 40 8 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at 40 Mpc) less than 11 hours after the merger by the OneMeter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position 9 and 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

⇥ 10 17 eV, and a zenith angle of 25 as measured in individual AERA radio detectors (circles filled with color corresponding to the measured value) and fitted with the azimuthally asymmetric, two-dimensional signal distribution function (background color). Both, radio detectors with a detected signal (data) and below detection threshold (subthreshold ) participate in the fit. The fit is performed in the plane perpendicular to the shower axis, with the x-axis oriented along the direction of the Lorentz force for charged particles propagating along the shower axisṽ in the geomagnetic ~ surface detector fluorescence detector radio detector X max , E cal TA 0.8*10 4 km 2 sr yr exposure to use events with only five und the one with the largest ore relaxed condition, the efincreased by 18.5%, and the vents increases correspond-113,888. The reconstruction dditional events is sufficient ee supplementary materials s in right ascension h for studying the large-scale arrival directions of cosmic a harmonic analysis in right rst-harmonic Fourier compo- he dashed line shows a coordinates, using a Hammer projection, showing the cosmic-ray flux above 8 EeV smoothed with a 45°top-hat function. The galactic center is marked with an asterisk; the galactic plane is shown by a dashed line. 3*10 4 CRs E>8*10 18 eV Anisotropy detected at >5.2 sigma dipole amplitude 6.5%

COSMIC RAYS
Observation of a large-scale anisotropy in the arrival directions of cosmic rays above 8 × 10 18 eV The Pierre Auger Collaboration* † Cosmic rays are atomic nuclei arriving from outer space that reach the highest energies observed in nature. Clues to their origin come from studying the distribution of their arrival directions. Using 3 × 10 4 cosmic rays with energies above 8 × 10 18 electron volts, recorded with the Pierre Auger Observatory from a total exposure of 76,800 km 2 sr year, we determined the existence of anisotropy in arrival directions. The anisotropy, detected at more than a 5.2s level of significance, can be described by a dipole with an amplitude of 6:5 þ1:3 À0:9 percent toward right ascension a d = 100 ± 10 degrees and declination d d = À24 þ12 À13 degrees. That direction indicates an extragalactic origin for these ultrahighenergy particles. P articles with energies ranging from below 10 9 eV up to beyond 10 20 eV, known as cosmic rays, constantly hit Earth's atmosphere. The flux of these particles steeply decreases as their energy increases; for energies above 10 EeV (1 EeV ≡ 10 18 eV), the flux is about one particle per km 2 per year. The existence of cosmic rays with such ultrahigh energies has been known for more than 50 years (1, 2), but the sites and mechanisms of their production remain a mystery. Information about their origin can be obtained from the study of the energy spectrum and the mass composition of cosmic rays. However, the most direct evidence of the location of the progenitors is expected to come from studies of the distribution of their arrival directions. Indications of possible hot spots in arrival directions for cosmic rays with energies above 50 EeV have been reported by the Pierre Auger and Telescope Array Collaborations (3, 4), but the statistical significance of these results is low. We report cosmic rays above 0.1 EeV. It is a hybrid system, a combination of an array of particle detectors and a set of telescopes used to detect the fluorescence light. Our analysis is based on data gathered from 1600 water-Cherenkov detectors deployed over an area of 3000 km 2 on a hexagonal grid with 1500-m spacing. Each detector contains 12 metric tons of ultrapure water in a cylindrical container, 1.2 m deep and 10 m 2 in area, viewed by three 9-inch photomultipliers. A full description of the observatory, together with details of the methods used to reconstruct the arrival directions and energies of events, has been published (5).
It is difficult to locate the sources of cosmic rays, as they are charged particles and thus interact with the magnetic fields in our Galaxy and the intergalactic medium that lies between the sources and Earth. They undergo angular deflections with amplitude proportional to their atomic number Z, to the integral along the trajectory of the magnetic field (orthogonal to the direction of Searches for large-scale anisotropies are conventionally made by looking for nonuniformities in the distribution of events in right ascension (15,16) because, for arrays of detectors that operate with close to 100% efficiency, the total exposure as a function of this angle is almost constant. The nonuniformity of the detected cosmic-ray flux in declination (fig. S1) imprints a characteristic nonuniformity in the distribution of azimuth angles in the local coordinate system of the array. From this distribution it becomes possible to obtain information on the three components of a dipolar model.

Event observations, selection, and calibration
We analyzed data recorded at the Pierre Auger Observatory between 1 January 2004 and 31 August 2016, from a total exposure of about 76,800 km 2 sr year. The 1.2-m depth of the water-Cherenkov detectors enabled us to record events at a useful rate out to large values of the zenith angle, q. We selected events with q < 80°enabling the declination range −90°< d < 45°to be explored, thus covering 85% of the sky. We adopted 4 EeV as the threshold for selection; above that energy, showers falling anywhere on the array are detected with 100% efficiency (17). The arrival directions of cosmic rays were determined from the relative arrival times of the shower front at each of the triggered detectors; the angular resolution was better than 1°at the energies considered here (5).
Two methods of reconstruction have been used for showers with zenith angles above and below 60°(17, 18). These have to account for the effects of the geomagnetic field (17,19) and, in the case of showers with q < 60°, also for atmospheric effects (20) because systematic modulations to the rates could otherwise be induced (see supplementary materials). The energy estimators for both data sets were calibrated using events detected simultaneously by the water-Cherenkov detectors and the fluorescence telescopes, with  ove shall be implemented through 5 sub projects.
D station, consisting of the water Cherenkov detector, the scintillator mounted on A radio antenna (this proposal -red), mounted to the mechanical structure of the scintillator.
design, pre-amplifier, mechanical mounting -PI, PD 1, engineer. ennas at SD positions in the 1500 m array and the 750 m dense sub-array. The antop of the WCD. Mechanically, we will attach the antennas to the mounting of the grade. These mountings are a contribution of RU Nijmegen/Nikhef and the relee aim to use Short Aperiodic Loaded Loop (SALLA) antennas 50 as a dipole loop radio signals between 30 and 80 MHz. The SALLA has been developed to promatches the need for both, ultra-wideband sensitivity, and low costs for produc-e/ µ Upgrade of the Pierre Auger Observatory (astro-)physics of the highest-energy particles in nature cle near 100%, the fluorescence telescopes operate only during dark nights and under favourable logical conditions, leading to a reduced duty cycle of about 12%. enhancements of the PAO include a sub-array of surface-detector stations with a spacing of 750 m e additional fluorescence telescopes with a field of view from 30° to 60°, co-located at the Coihueco ence detector site, in Fig. 3, left on the left side of the array. Co-located with these enhancements is ger Engineering Radio Array (AERA). 19,20,21 It comprises 153 autonomously operated antenna , covering an area of 17 km 2 . It records the radio emission from extensive air showers in the cy range from 10 -80 MHz at nearly 100% duty cycle. Two antenna types are employed: logarithmic dipole antennas and butterfly antennas. An AERA station, equipped with a butterfly antenna is n Fig. 3, right. ent, the Auger Collaboration is preparing a major upgrade of the observatory 10 in order to elucidate ental composition and the origin of the flux suppression at the highest energies, to search for a flux tion of protons up to the highest energies, and to study air showers and hadronic multi-particle pro-. The upgrade comprises of a plastic scintillator plane above the existing water Cherenkov detectors le the shower particles with two detectors, having different responses to muons and electromagnetic s; an upgrade of the electronics of the surface detector stations, with a faster sampling rate and an d dynamic range; an underground muon detector to provide a direct measurement of muons in air , covering an area of 24 km 2 , co-located with the enhancements (described above) and AERA; and a of the operation mode for the fluorescence telescopes, increasing their duty cycle to 20%. design, pre-amplifier, mechanical mounting -PI, PD 1, engineer. ennas at SD positions in the 1500 m array and the 750 m dense sub-array. The antop of the WCD. Mechanically, we will attach the antennas to the mounting of the grade. These mountings are a contribution of RU Nijmegen/Nikhef and the relee aim to use Short Aperiodic Loaded Loop (SALLA) antennas 50 as a dipole loop radio signals between 30 and 80 MHz. The SALLA has been developed to promatches the need for both, ultra-wideband sensitivity, and low costs for produc-e/ µ cle near 100%, the fluorescence telescopes operate only during dark nights and under favourable logical conditions, leading to a reduced duty cycle of about 12%. enhancements of the PAO include a sub-array of surface-detector stations with a spacing of 750 m e additional fluorescence telescopes with a field of view from 30° to 60°, co-located at the Coihueco ence detector site, in Fig. 3, left on the left side of the array. Co-located with these enhancements is ger Engineering Radio Array (AERA). 19,20,21 It comprises 153 autonomously operated antenna , covering an area of 17 km 2 . It records the radio emission from extensive air showers in the cy range from 10 -80 MHz at nearly 100% duty cycle. Two antenna types are employed: logarithmic dipole antennas and butterfly antennas. An AERA station, equipped with a butterfly antenna is n Fig. 3, right. ent, the Auger Collaboration is preparing a major upgrade of the observatory 10 in order to elucidate ental composition and the origin of the flux suppression at the highest energies, to search for a flux tion of protons up to the highest energies, and to study air showers and hadronic multi-particle pro-. The upgrade comprises of a plastic scintillator plane above the existing water Cherenkov detectors le the shower particles with two detectors, having different responses to muons and electromagnetic s; an upgrade of the electronics of the surface detector stations, with a faster sampling rate and an d dynamic range; an underground muon detector to provide a direct measurement of muons in air , covering an area of 24 km 2 , co-located with the enhancements (described above) and AERA; and a of the operation mode for the fluorescence telescopes, increasing their duty cycle to 20%. design, pre-amplifier, mechanical mounting -PI, PD 1, engineer. ennas at SD positions in the 1500 m array and the 750 m dense sub-array. The antop of the WCD. Mechanically, we will attach the antennas to the mounting of the grade. These mountings are a contribution of RU Nijmegen/Nikhef and the relee aim to use Short Aperiodic Loaded Loop (SALLA) antennas 50 as a dipole loop radio signals between 30 and 80 MHz. The SALLA has been developed to promatches the need for both, ultra-wideband sensitivity, and low costs for produc-e/ µ cle near 100%, the fluorescence telescopes operate only during dark nights and under favourable logical conditions, leading to a reduced duty cycle of about 12%. enhancements of the PAO include a sub-array of surface-detector stations with a spacing of 750 m e additional fluorescence telescopes with a field of view from 30° to 60°, co-located at the Coihueco ence detector site, in Fig. 3, left on the left side of the array. Co-located with these enhancements is ger Engineering Radio Array (AERA). 19,20,21 It comprises 153 autonomously operated antenna , covering an area of 17 km 2 . It records the radio emission from extensive air showers in the cy range from 10 -80 MHz at nearly 100% duty cycle. Two antenna types are employed: logarithmic dipole antennas and butterfly antennas. An AERA station, equipped with a butterfly antenna is n Fig. 3, right. ent, the Auger Collaboration is preparing a major upgrade of the observatory 10 in order to elucidate ental composition and the origin of the flux suppression at the highest energies, to search for a flux tion of protons up to the highest energies, and to study air showers and hadronic multi-particle pro-. The upgrade comprises of a plastic scintillator plane above the existing water Cherenkov detectors le the shower particles with two detectors, having different responses to muons and electromagnetic s; an upgrade of the electronics of the surface detector stations, with a faster sampling rate and an d dynamic range; an underground muon detector to provide a direct measurement of muons in air , covering an area of 24 km 2 , co-located with the enhancements (described above) and AERA; and a of the operation mode for the fluorescence telescopes, increasing their duty cycle to 20%. : Farthest axis distance at which a radio signal above noise background has been detected as a function of the air-shower zenith angle. Black dots represent the 50 events that pass the quality cuts for energy reconstruction, grey diamonds denote the remaining 511 events. The red bars show the profile of the distribution, i.e., the mean and standard deviation in each 2 ¶ bin. Please note that, as the array is significantly smaller than the radio-emission footprints, the mean values might significantly underestimate the average footprint size.  Mass separation using ratio S rad /N 19 (eletron/muon ratio) Precision shower physics (for vertical showers) in dense region of Pierre Auger observatory direct verification of deconvolution matrices (SSD/WCD) with measured showers study hadronic interactions 9 ultaneously with AERA and the SD at the PAO. 49 shall be implemented through 5 sub projects.

OPEN QUESTIONS AND GOALS OF UPGRADING THE OBSERVATORY
ation, consisting of the water Cherenkov detector, the scintillator mounted on adio antenna (this proposal -red), mounted to the mechanical structure of the scintillator.
gn, pre-amplifier, mechanical mounting -PI, PD 1, engineer. s at SD positions in the 1500 m array and the 750 m dense sub-array. The anof the WCD. Mechanically, we will attach the antennas to the mounting of the e. These mountings are a contribution of RU Nijmegen/Nikhef and the releim to use Short Aperiodic Loaded Loop (SALLA) antennas 50 as a dipole loop io signals between 30 and 80 MHz. The SALLA has been developed to protches the need for both, ultra-wideband sensitivity, and low costs for producntenna in a large-scale radio detector. The compact structure of the SALLA asy to manufacture. The response of these antennas has been measured as part their characteristics is well known and suitable for our purpose. In particular, ve to the ground conditions, i.e. ideal to be placed on top of an existing SD  than 10 18 to pion-photoproduction.

Neutrinos and Photons
The new RDs at each SD will also help to increase our sensitivity to neutrinos and photons.
Photon identification and measurement with SD and RD The new RDs at each SD will also help to increase our sensitivity to neutrinos and photons. Fig. 2. Neutrinos can initiate atmospheric showers through charged (CC) or neutral (NC) current interactions. In ν e CC interactions all the energy of the primary neutrino is transferred to the shower. This is not the case of the NC channel where the primary neutrino energy is only partially transferred to the shower while a significant fraction is carried away by the scattered neutrino. Similar behaviour is seen in the ν µ CC induced showers where the emerging high energy muon usually decays under the ground and doesn't produce a shower. Note that ν τ CC initiated showers may have a "double bang" structure due to the fact that the out-coming high energy τ may travel a long distance before decay producing a second displaced shower vertex.
responding to ∼ 1.2 years of the full SD array -was used as "training" data. From the showers that trigger the SD array [3], those arriving during periods in which instabilities in data acquisition occur are excluded. After that the FADC traces are cleaned to remove segments that are due to accidental muons not belonging to the shower but arriving close in time with the shower front. Moreover, if 2 or more segments of comparable area appear in a trace the station is classified as ambiguous and it is not used. Then a selection of the stations actually belonging to the event is done based on spacetime compatibility among them. Events with less than 4 tanks passing the level 2 trigger algorithm [3] are rejected. This sample is then searched for inclined events requiring that the triggered tanks have elongated patterns on the ground along the azimuthal arrival direction. A length L and a width W are assigned to the pattern [5], [8], and a cut on their ratio is applied (L/W >3). Then we calculate the apparent speed of the signal in the event moving across the ground along L, using the arrival times of the signals at ground and the distances between tanks projected onto L [13]. The average speed ⟨V ⟩ is measured between pairs of triggered stations, and is required to be compatible with that expected in a simple planar model of the shower front in an inclined event with θ ≥ 75 • , allowing for some spread due to fluctuations (⟨V ⟩ ≤ 0.313 m ns −1 ). Furthermore, since in inclined events the speed measured between pairs of tanks is concentrated around ⟨V ⟩ [5] we require than 0.08 · ⟨V ⟩. The zenith angle θ of the shower is also reconstructed, and those events with θ ≥ 75 • are selected. Exactly the same set of conditions is applied to the simulated neutrinos.
The sample of inclined events is searched for "young" showers using observables characterising the time duration of the FADC traces in the early region of the event.
To optimize their discrimination power we applied the Fisher discriminant method [7] to the training dataoverwhelmingly, if not totally constituted of nucleonic showers -and to the Monte Carlo (MC) simulations -exclusively composed of neutrino-induced showers. Given two populations of events -nucleonic inclined showers and ν-induced showers in our case -characterised by a set of observables, the Fisher method produces a linear combination of the various observables f the Fisher discriminant -so that the separation between the means of f in the two samples is maximised, while the quadratic sum of the r.m.s. of f in each of them is minimised. Since events with a large number of tanks N (large multiplicity) are different from events with small multiplicity the sample of training data is divided into 3 sub-samples corresponding to events with number of tanks 4 ≤ N ≤ 6, 7 ≤ N ≤ 11 and N ≥ 12, and a Fisher discriminant is obtained using each of the sub-samples as training data. We use the Area-over-Peak (AoP) [8] and its square of the first 4 tanks in each event, their product, and a global early-late asymmetry parameter of the event as the discriminant variables of s. In ν e CC interactions all the primary neutrino energy is only ilar behaviour is seen in the ν µ uce a shower. Note that ν τ CC vel a long distance before decay ngle θ of the shower is events with θ ≥ 75 • are t of conditions is applied ts is searched for "young" racterising the time duraearly region of the event. on power we applied the 7] to the training dataconstituted of nucleonic reaching to the highest energies with radio technique enhancements of the PAO include a sub-array of surface-detector stations with a spacing of 750 m ee additional fluorescence telescopes with a field of view from 30° to 60°, co-located at the Coihueco cence detector site, in Fig. 3, left on the left side of the array. Co-located with these enhancements is ger Engineering Radio Array (AERA). 19,20,21 It comprises 153 autonomously operated antenna s, covering an area of 17 km 2 . It records the radio emission from extensive air showers in the cy range from 10 -80 MHz at nearly 100% duty cycle. Two antenna types are employed: logarithmic c dipole antennas and butterfly antennas. An AERA station, equipped with a butterfly antenna is in Fig. 3, right. ent, the Auger Collaboration is preparing a major upgrade of the observatory 10 in order to elucidate mental composition and the origin of the flux suppression at the highest energies, to search for a flux ution of protons up to the highest energies, and to study air showers and hadronic multi-particle pro-. The upgrade comprises of a plastic scintillator plane above the existing water Cherenkov detectors ple the shower particles with two detectors, having different responses to muons and electromagnetic s; an upgrade of the electronics of the surface detector stations, with a faster sampling rate and an ed dynamic range; an underground muon detector to provide a direct measurement of muons in air s, covering an area of 24 km 2 , co-located with the enhancements (described above) and AERA; and a of the operation mode for the fluorescence telescopes, increasing their duty cycle to 20%. for <5 W power required for the additional electronics. If the capacity of the existing SD systems is ficient, we will add a small solar panel and a battery buffer to the system. They will be mounted as the mechanical frame of the scintillator module.

Section b. Methodology
The work plan described above shall be implemented through 5 sub projects. p r e l i m i n a r y ! o n g o i n g w o r k