Clustered Institutions (Clusters) are typically intended for institutes in nations where the High-Energy Physics community is still developing, and where it may be difficult to form individual university groups large enough to stand alone in ATLAS. It is expected that members of a Cluster (Cluster institutes) work closely together and build up a coherent effort in ATLAS.

Each Cluster will have one vote in the ATLAS Collaboration Board.

The requirements for a new Cluster to join ATLAS are the same as for single Institutions. The obligations on joining fees, M&O and OT contributions have to be fulfilled by the Cluster as a whole.

The requirements for a new institute joining a Cluster as an additional institute are less stringent. No entrance fee has to be paid, however the financial and effort obligations (M&O and OT contributions) of the Cluster increase according to the number of additional authors. The application should benefit the Cluster being joined, as well as the ATLAS Collaboration as a whole.

Clusters appear – and are marked as such – in the list of institutes in ATLAS publications. The individual Cluster institutes are listed conse­cutively.

• Initial contact should be made with the leader of the Cluster team the new institute proposes to join, or directly with the ATLAS Spokesperson. In any case, informal contact should be made at an early stage with the Spokesperson and with the National Contact Physicist (NCP) of the nation concerned.
• The applicant group should prepare a short statement of interest, including a description of the general structure (staff, students, engineers) and background of the group, expectations for its evolution, specific interests (projects, activity areas in ATLAS) and work programme. Connections and relations to other institutes in the Cluster they wish to join should be described. Members of the applicant institute are expected to begin qualification to become ATLAS authors.
• Support will be required from the leader of the Cluster being joined and the NCP of the nation, who should also ensure that the funding agency concerned is in agreement.
• The applicant institute, and the Cluster they propose to join, need to accept the financial and effort implications of joining, i.e. increased shares of M&O and OT contributions, commensurate with the number of additional authors, qualifiers and students (see above). Payment of the additional M&O contributions will start for the calendar year after admission. The leader of the Cluster is responsible for ensuring that these commitments will be met by the engaged institutes in the Cluster.
• The addition of the institute to the Cluster is at the discretion of the Spokesperson and Collaboration Board chair. The Collaboration Board shall be informed and comments solicited, before the admission to the Cluster is completed. The admission of a new institute into a Cluster does not require a CB vote or endorsement by the RRB, as the CB composition is unchanged.