# Sebastian Neubert$${}^{a}$$, Martin Hoferichter$${}^{b}$$, Miriam Fritsch$${}^{c}$$, Simon Eidelman$${}^{d,e}$$, Michael Pennington$${}^{f}$$, Bernhard Ketzer$${}^{g}$$, Jonas Rademacker$${}^{h}$$, Wolfgang Gradl$${}^{j}$$, Christoph Hanhart$${}^{k}$$, Bastian Kubis$${}^{l}$$, José R. Peláez$${}^{m}$$, Claudia Patrignani$${}^{n}$$, Stephan Paul$${}^{o}$$ $${}^{a}$$Physikalisches Institut Heidelberg, $${}^{b}$$INT Seattle, $${}^{c}$$Ruhr-University Bochum, $${}^{d}$$Budker Institute of Nuclear Physics SB RAS, $${}^{e}$$Novosibirsk State University, Novosibirsk, $${}^{f}$$JLab, $${}^{g}$$University of Bonn, $${}^{h}$$University of Bristol, $${}^{j}$$Johannes Gutenberg University of Mainz, $${}^{k}$$Forschungszentrum Jülich (Juelich Research Centre), $${}^{l}$$University of Bonn, $${}^{m}$$Complutense University of Madrid, $${}^{n}$$University of Bologna and INFN Bologna, $${}^{o}$$TU Munich

We propose to form a joint panel, composed of delegates from experimental collaborations and members of the theory community, who are engaged in the investigation of the structure of hadronic amplitudes. Seeking a deeper understanding of strong interaction dynamics, the panel will work towards making data analysis techniques consistent across experiments and enabling joint analyses of multiple data sets. Such global analyses will only become possible through cooperation between experiments and between experimentalists and theorists and will significantly reduce systematic uncertainties on the extracted physics across a wide range of energies and processes. PHASE aims to become the central hub for amplitude analysis expertise in Europe and will publish regular reviews on the state of the art of the field.

This letter of intent is an invitation to all researchers in hadron physics to contribute to the project. It lays out the planned structure and activities of PHASE and is intended to bring together the efforts across the community to form a common forum and start taking joint action.

## Vision and Goals

The vision of PHASE is to enable joint analyses of hadronic processes across multiple experiments and multiple data sets. It provides a forum and infrastructure for joint developments within the hadron physics community and defines best practices.

To make progress in the understanding of strong interaction phenomena, it is necessary to combine the information gathered by several experiments across a wide range of energies in a coherent fashion. Due to the complexity of hadron dynamics, this is a non-trivial problem. Current state-of-the-art techniques for extracting hadronic amplitudes from data exhibit model dependencies, which need to be quantified. Amplitude parameterisations have to be made consitent across different experiments. Only then will the envisioned global analyses become possible. One of the main reasons for the difficulties currently experienced is, that in hadronic decays often many processes are coupled together. For a comprehensive treatment, all of them need to be taken into account, while each analysis typically only investigates a single final state. It is at this point that combining data from different experiments will have the biggest impact, as now each channel can be constrained from the experimental data, which is most sensitive to that particular hadronic system. PHASE will bring together expertise and data from both the baryon and meson sectors.

To advance further, methods are being developed which will enable the extraction of universal features from the data. In addition, procedures for data handling across experiments are needed, which will allow the transfer of those universal features between different production processes and final states. Theorists have already developed the basic concepts how such universal features can be defined. However, considerable technical challenges have so far prohibited the full implementation of these ideas into the data analysis frameworks of the current generation of experiments. The outstanding challenges and possible angles of attack have been discussed and documented in a series of workshops during the past few years (Battaglieri 2015, Briceño 2016, Lutz 2016). It is now a consensus in the community that to overcome said challenges, a closer cooperation between phenomenologists and experimentalists is needed.

The first and foremost goal of the PHASE network is to facilitate this cooperation.

The activities carried out in the project proposed here aim at providing researchers with useful tools for data analysis. They will produce actual software implementations of amplitude models. The focus will be on advanced models that cannot be produced by a single collaboration but need combined expertise from several actors. PHASE puts the software implementation of the physics models into the center of its activities. Its goal is to advance complex analysis projects from the conceptual to the actual.

## PHASE Structure

\label{sec:structure}

PHASE consists of two representatives (liaisons) from each experiment and a number of representatives (liaisons) from the theory community. Experimental and theory liaisons make up the PHASE panel as illustrated in figure \ref{fig:structure}. The panel elects a speaker to represent PHASE to the public and to funding agencies. It holds regular virtual meetings to coordinate the activities outlined in section \ref{sec:activities}.

The panel is supported by a core team of volunteers.