This paper is a write-up of the ideas that were presented, developed and discussed at the fourth International Workshop on QCD Challenges from pp to AA, which took place in February 2023 in Padua, Italy. The goal of the workshop was to focus on some of the open questions in the field of high-energy heavy-ion physics and to stimulate the formulation of concrete suggestions for making progresses on both the experimental and theoretical sides. The paper gives a brief introduction to each topic and then summarizes the primary results.

Australia

Javira Altmann

France

Carlota Andres

Germany

Anton Andronic

Italy

Federico Antinori

Pietro Antonioli

Andrea Beraudo

Eugenio Berti

Livio Bianchi

United States

Thomas Boettcher

Lorenzo Capriotti

Sweden

Peter Christiansen

Czechia

Jesus Contreras Nuño

Leticia Cunqueiro Mendez

Cesar Silva

Andrea Dainese

Hans Dembinski

Brazil

Austria

David Dobrigkeit Chinellato

Andrea Dubla

Mattia Faggin

Chris Flett

Vincenzo Greco

Ilia Grishmanovskii

Jack Holguin

Finland

Yuuka Kanakubo

Dong Kim

Ramona Lea

South Korea

Su Lee

CERN

Saverio Mariani

Poland

Adam Matyja

Aleksas Mazeliauskas

Vincenzo Minissale

Andreas Morsch

Lucia Oliva

Luca Orusa

Petja Paakkinen

Spain

Daniel Pablos

Mexico

Guy Paić

Tanguy Pierog

Salvatore Plumari

Francesco Prino

Andrea Rossi

Lorenzo Sestini

Peter Skands

Olga Soloveva

Francesca Soramel

Alba Soto Ontoso

Martin Spousta

Andre Stahl Leiton

Jiayin Sun

Norway

Adam Takacs

Stefano Trogolo

Rosario Turrisi

Netherlands

Marta Verweij

Vytautas Vislavicius

Jing Wang

Klaus Werner

Valentina Zaccolo

China

Mingyu Zhang

Jianhui Zhu

Davide Zuliani

QCD challenges from pp to AA collisions: 4th edition

The cosmic-ray database, CRDB, has been gathering cosmic-ray data for the community since 2013. We present a new release, CRDB v4.1, providing many new quantities and data sets, with several improvements made on the code and web interface, and with new visualisation tools. CRDB relies on the MySQL database management system, jquery and table-sorter libraries for queries and sorting, and PHP web pages and AJAX protocol for displays. A REST interface enables user queries from command line or scripts. A new (pip-installable) CRDB python library is developed and extensive jupyter notebook examples are provided. This release contains cosmic-ray dipole anisotropy data, high-energy  $$\bar{p}/p$$  <math> <mrow> <mover> <mrow> <mi>p</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> <mo>/</mo> <mi>p</mi> </mrow> </math>   upper limits, some unpublished LEE and AESOP lepton time series, many more ultra-high energy data, and a few missing old data sets. It also includes high-precision data from the last three years, in particular the hundreds of thousands AMS-02 and PAMELA data time series (time-dependent plots are now enabled). All these data are shown in a gallery of plots, which can be easily reproduced from the public notebook examples. CRDB contains 316,126 data points from 504 publications, in 4111 sub-experiments from 131 experiments.

David Maurin

Denmark

Markus Ahlers

Andreas Haungs

Pierre-Simon Mangeard

Frédéric Melot

Philipp Mertsch

Doris Wochele

Jürgen Wochele

A cosmic-ray database update: CRDB v4.1

It is well known that the fraction of energy in a hadron collision going into electromagnetic particles (electrons and photons, including those from decays) has a large impact on the number of muons produced in air shower cascades. Recent measurements at the LHC confirm features that can be linked to a mixture of different underlying particle production mechanisms such as a collective statistical hadronization (core) in addition to the expected string fragmentation (corona). Since the two mechanisms have a different electromagnetic energy fraction, we present a possible connection between statistical hadronization in hadron collisions and muon production in air showers. Using a novel approach, we demonstrate that the core-corona effect as observed at the LHC can have a significant impact and should be properly taken into account before trying to find a more exotic solution for the lack of muon production in simulations of high energy cosmic rays.

Sebastian Baur

Argentina

Matias Perlin

Ralf Ulrich

Core-corona effect in hadron collisions and muon production in air showers

A common statistical problem in particle physics is to extract the number of samples which originate from a statistical process in an ensemble containing a mix of several contributing processes. The probability density function of each process is usually not exactly known. Barlow and Beeston found an exact likelihood for the problem of fitting binned templates obtained from Monte-Carlo simulation to binned data, which propagates the uncertainty of the templates into the result. Solving the exact likelihood is technically challenging, however. The original paper also did not provide a way to use weighted simulation samples with varying weights. Other papers have introduced alternative likelihoods to address these points. In this paper, a new approximate likelihood is derived from the exact Barlow–Beeston one. The new likelihood is generalized to fits of weighted templates to weighted data. The performance of the new likelihood is evaluated based on toy examples. The performance is excellent – point estimates have small bias and confidence intervals have good coverage – and is comparable to the exact Barlow–Beeston likelihood when the templates are not weighted. The new likelihood evaluates faster than the Barlow–Beeston one when the number of bins is large.

United Kingdom

Ahmed Abdelmotteleb

A new maximum-likelihood method for template fits