<p>Axion dark matter thermalizes by gravitational self-interactions and forms a Bose-Einstein condensate. We show that the rethermalization of the axion fluid during the initial collapse of large scale overdensities near cosmic dawn transports angular momentum outward sufficiently fast that black holes form with masses ranging from approximately <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mn>10</mn><mn>5</mn></msup></math> to a few times <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mn>10</mn><mn>10</mn></msup><msub><mi>M</mi><mo stretchy="false">⊙</mo></msub></math>. This conclusion holds for QCD axions and for axionlike particles of mass larger than <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mn>10</mn><mrow><mo>−</mo><mn>16</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi><mo>/</mo><msup><mi>c</mi><mn>2</mn></msup></math>.</p>

United States

Pierre Sikivie

Yuxin Zhao

Supermassive black hole formation in the initial collapse of axion dark matter

The Pierre Auger Collaboration has embraced the concept of open access to their research data since its foundation, with the aim of giving access to the widest possible community. A gradual process of release began as early as 2007 when 1% of the cosmic-ray data was made public, along with 100% of the space-weather information. In February 2021, a portal was released containing 10% of cosmic-ray data collected by the Pierre Auger Observatory from 2004 to 2018, during the first phase of operation of the Observatory. The Open Data Portal includes detailed documentation about the detection and reconstruction procedures, analysis codes that can be easily used and modified and, additionally, visualization tools. Since then, the Portal has been updated and extended. In 2023, a catalog of the highest-energy cosmic-ray events examined in depth has been included. A specific section dedicated to educational use has been developed with the expectation that these data will be explored by a wide and diverse community, including professional and citizen scientists, and used for educational and outreach initiatives. This paper describes the context, the spirit, and the technical implementation of the release of data by the largest cosmic-ray detector ever built and anticipates its future developments.

Australia

A. Abdul Halim

Portugal

P. Abreu

Italy

M. Aglietta

Argentina

I. Allekotte

Netherlands

K. Almeida Cheminant

A. Almela

R. Aloisio

Spain

J. Alvarez-Muñiz

A. Ambrosone

J. Ammerman Yebra

G. Anastasi

L. Anchordoqui

B. Andrada

L. Andrade Dourado

S. Andringa

L. Apollonio

C. Aramo

Germany

P. Araújo Ferreira

E. Arnone

Mexico

J. Arteaga Velázquez

P. Assis

G. Avila

E. Avocone

Czechia

A. Bakalova

F. Barbato

A. Bartz Mocellin

J. Bellido

France

C. Berat

M. Bertaina

X. Bertou

M. Bianciotto

P. Biermann

V. Binet

K. Bismark

T. Bister

J. Biteau

J. Blazek

C. Bleve

J. Blümer

M. Boháčová

D. Boncioli

C. Bonifazi

Brazil

L. Bonneau Arbeletche

Poland

N. Borodai

J. Brack

P. Brichetto Orchera

F. Briechle

A. Bueno

Belgium

S. Buitink

M. Buscemi

M. Büsken

A. Bwembya

K. Caballero-Mora

S. Cabana-Freire

L. Caccianiga

F. Campuzano

R. Caruso

A. Castellina

F. Catalani

G. Cataldi

L. Cazon

M. Cerda

B. Čermáková

A. Cermenati

J. Chinellato

J. Chudoba

L. Chytka

R. Clay

A. Cobos Cerutti

R. Colalillo

R. Conceição

A. Condorelli

G. Consolati

M. Conte

F. Convenga

D. Correia dos Santos

P. Costa

C. Covault

M. Cristinziani

C. Cruz Sanchez

S. Dasso

K. Daumiller

B. Dawson

R. Almeida

B. Errico

J. Jesús

S. Jong

J. Mello Neto

I. Mitri

J. Oliveira

D. Oliveira Franco

F. Palma

V. Souza

E. Vito

A. Popolo

O. Deligny

N. Denner

L. Deval

A. Matteo

C. Dobrigkeit

The Pierre Auger Observatory open data

It is assumed that superheavy dark matter particles with O(EeV) mass may decay to relativistic milli-charged particles (MCPs). The upward-going MCPs passing through the Earth can be measured by the fluorescence detectors (FD) of the Pierre Auger observatory (Auger). The massless hidden photon model is taken for MCPs to interact with nuclei, so that the numbers and fluxes of expected MCPs may be evaluated at the FD of Auger. Based on the assumption that no events are observed at the FD of Auger in 14 years, the corresponding upper limits on MCP fluxes are calculated at 90% C. L. These results indicate that MCPs can be directly detected in the secondaries' energy range O(1 EeV)-O(10 EeV) at the FD of Auger, when <math><mn>3</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>13</mn></mrow></msup><mo>≲</mo><msup><mrow><mi>ϵ</mi></mrow><mrow><mn>2</mn></mrow></msup><mo>≲</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></math>. And a new region of 0.7 GeV &lt; <math><msub><mrow><mi>m</mi></mrow><mrow><mi>M</mi><mi>C</mi><mi>P</mi></mrow></msub></math> &lt; 10 GeV and <math><mn>1.2</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>7</mn></mrow></msup></math> &lt; ϵ &lt; <math><mn>4.5</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></math> is ruled out in the <math><msub><mrow><mi>m</mi></mrow><mrow><mi>M</mi><mi>C</mi><mi>P</mi></mrow></msub></math>-ϵ plane with 14 years of Auger data.

China

Ye Xu

Jieqin Lan

Weiwei Gao

Measurement of upward-going milli-charged particles at the Pierre Auger Observatory

Axions have a double motivation: they solve the ‘strong CP problem’ of the Standard Model of elementary particles and they are a candidate for the dark matter of the Universe. Dark matter axions can be detected on Earth by converting them to photons in an electromagnetic cavity permeated by a strong magnetic field. A signal in such a detector, called an axion haloscope, would immediately reveal the velocity spectrum of dark matter axions on Earth in great detail. This prospect motivated the study of the special properties that axions have in large scale structure formation. It is shown that cold dark matter axions thermalize through their gravitational self-interactions, and form a Bose-Einstein condensate. As a result, axion dark matter behaves differently from the other proposed forms of dark matter. The differences are observable.

Axion dark matter

Dark matter particles could be superheavy, provided their lifetime is much longer than the age of the Universe. Using the sensitivity of the Pierre Auger Observatory to ultrahigh energy neutrinos and photons, we constrain a specific extension of the Standard Model of particle physics that meets the lifetime requirement for a superheavy particle by coupling it to a sector of ultralight sterile neutrinos. Our results show that, for a typical dark coupling constant of 0.1, the mixing angle <math><msub><mi>θ</mi><mi>m</mi></msub></math> between active and sterile neutrinos must satisfy, roughly, <math><msub><mi>θ</mi><mi>m</mi></msub><mo>≲</mo><mn>1.5</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>6</mn></mrow></msup><mo>(</mo><msub><mi>M</mi><mi>X</mi></msub><mo>/</mo><msup><mn>10</mn><mn>9</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi><msup><mo>)</mo><mrow><mo>−</mo><mn>2</mn></mrow></msup></math> for a mass <math><msub><mi>M</mi><mi>X</mi></msub></math> of the dark-matter particle between <math><msup><mn>10</mn><mn>8</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> and <math><msup><mn>10</mn><mn>11</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>.

G. A. Anastasi

P. R. Araújo Ferreira

J. C. Arteaga Velázquez

Peru

J. A. Bellido

M. E. Bertaina

G. Bhatta

P. L. Biermann

P. G. Brichetto Orchera

F. L. Briechle

K. S. Caballero-Mora

J. A. Chinellato

R. W. Clay

A. C. Cobos Cerutti

M. R. Coluccia

P. J. Costa

C. E. Covault

C. S. Cruz Sanchez

B. R. Dawson

R. M. de Almeida

J. de Jesús

S. J. de Jong

J. R. T. de Mello Neto

I. De Mitri

J. de Oliveira

D. de Oliveira Franco

F. de Palma

V. de Souza

B. P. de Souza de Errico

E. De Vito

A. Del Popolo

A. di Matteo

Romania

M. Dobre

J. C. D’Olivo

Constraints on metastable superheavy dark matter coupled to sterile neutrinos with the Pierre Auger Observatory

We find that the adiabatic fluctuations produced in the primordial plasma by cosmological inflation resonantly excite the axion field during the QCD phase transition by pumping axions from low momentum modes to modes with momentum up to of order <math><msqrt><mn>3</mn></msqrt><mi>m</mi></math> where <math><mrow><mi>m</mi></mrow></math> is the axion mass. We derive the momentum distribution of the excited axions. The fraction of cold axions that get excited is of order one if the axion mass is larger than a few <math><mi>μ</mi><mi>eV</mi></math>. The effect occurs whether inflation happens before or after the Peccei-Quinn phase transition.

Wei Xue

Resonant excitation of the axion field during the QCD phase transition

We present a measurement of the cosmic-ray spectrum above 100 PeV using the part of the surface detector of the Pierre Auger Observatory that has a spacing of 750 m. An inflection of the spectrum is observed, confirming the presence of the so-called second-knee feature. The spectrum is then combined with that of the 1500 m array to produce a single measurement of the flux, linking this spectral feature with the three additional breaks at the highest energies. The combined spectrum, with an energy scale set calorimetrically via fluorescence telescopes and using a single detector type, results in the most statistically and systematically precise measurement of spectral breaks yet obtained. These measurements are critical for furthering our understanding of the highest energy cosmic rays.

J. Albury

R. Alves Batista

H. Asorey

A. Badescu

A. Balaceanu

R. Barreira Luz

K. Becker

P. Billoir

A. Botti

T. Bretz

P. Buchholz

F. Canfora

I. Caracas

J. Carceller

A. Coleman

M. Coluccia

F. Contreras

J. Day

G. Mauro

M. del Río

A. Di Matteo

J. D’Olivo

L. Domingues Mendes

R. dos Anjos

D. dos Santos

M. Dova

The energy spectrum of cosmic rays beyond the turn-down around  $$\varvec{10^{17}}$$  <math> <mrow> <msup> <mn>10</mn> <mn>17</mn> </msup> </mrow> </math>   eV as measured with the surface detector of the Pierre Auger Observatory

We present the first measurement of the fluctuations in the number of muons in extensive air showers produced by ultrahigh energy cosmic rays. We find that the measured fluctuations are in good agreement with predictions from air shower simulations. This observation provides new insights into the origin of the previously reported deficit of muons in air shower simulations and constrains models of hadronic interactions at ultrahigh energies. Our measurement is compatible with the muon deficit originating from small deviations in the predictions from hadronic interaction models of particle production that accumulate as the showers develop.

A. Aab

J. M. Albury

A. M. Badescu

R. J. Barreira Luz

Measurement of the Fluctuations in the Number of Muons in Extensive Air Showers with the Pierre Auger Observatory