A Run 4 Zero Degree Calorimeter for CMS

The Run 4 Heavy Ion program of the LHC offers great physics opportunities to CMS and ATLAS. The ZDCs are increasingly important for characterizing nucleus-nucleus, proton-nucleus, photon-nucleus and photon-photon collisions, particularly the impact parameter between the colliding particles. However, the increasing LHC luminosity places a significant strain on the current ZDC design, and the space in the absorber region will be reduced by a factor of two, requiring all new detectors for LHC Run 4. Recent developments in rad-hard materials allow the construction of compact rad-hard fast ZDCs to fully exploit the potential for QED and QCD physics during Run 4. In this proposal, groups from ATLAS and CMS are collaborating to produce high-performance, cost-effective ZDCs for this new era. This document describes scientific motivation for the project and the CMS specific aspects.