Proof of Principle for CMS High-Granularity Calorimeter

Particle physics is the study of the fundamental building blocks of nature and the forces that govern their interactions. Over the last 40 years we have developed the so-called 'standard model' of particle physics, encapsulating our understanding of this sub-atomic world. Whilst amazingly successful, and indeed a crowning-glory of 20th century fundamental physics, we know it cannot be the final answer. For example it doesn't incorporate gravity or explain dark matter. More immediately it predicts the existence of a particle, the Higgs boson, associated with mass generation. Until the discovery by the ATLAS and CMS Collaborations at the Large Hadron Collider (LHC) at CERN in 2012 there was no experimental evidence for this corner-stone of the standard model. The discovery of a Higgs particle has opened a window onto the new physics beyond the standard model. Understanding the implications of this discovery through detailed study of the new particle and searches for other Higgs bosons is one of the highest priorities within particle physics worldwide.

To study the Higgs boson in sufficient detail, and indeed for many other important physics topics, we need much larger datasets than we have so far. The LHC will undergo extensive upgrades to generate these larger data sets by colliding the protons accelerated in the LHC at even greater rates; this upgrade is referred to as the 'High-Luminosity LHC' (HL-LHC). In parallel, the experiments, such as CMS that record these collisions, must also be upgraded to cope with the increased radiation levels, and increased rates associated with the LHC upgrades.

CMS will need to replace its so-called 'calorimeters' in the forward regions as this is where the radiation doses are highest. A novel concept is proposed for the calorimeter upgrade, and the work proposed here would form a key part of this. Traditionally calorimeters have only measured the energy of the particles produced in the collisions, but advances in technology and their application to particle physics, in part pioneered by the UK, now make it possible to consider high granularity calorimeters. Such devices can effectively also measure the path, or track, that a particle follows within the calorimeter. This brings large advantages in distinguishing close-by particles, which would be seen as one deposit in a traditional calorimeter. A high granularity calorimeter would consist of dense absorbing material that causes incident particles to lose energy, interspersed with silicon detectors able to track the passage of individual particles. This additional information can be very powerful in overcoming the challenging environment of the Hl-LHC.

This proposal will deliver on two key aspects of this project. First we will clearly demonstrate that the potential gains of individual particle tracking that such a detector offers can be fully realised in an environment such as the LHC. As part of this we will deliver an optimised detector layout i.e. determine the optimum ratio of absorber and silicon sensors, along with optimising the size of the silicon sensors themselves. The second goal is demonstrate that the additional information available from such a device can be used, even in the harsh LHC environment, to effectively trigger, i.e. select in real time, possible events of interest. As part of this, coding these trigger algorithms into a state-of-the art 'FPGA' is foreseen and this will provide a proof-of-principle that such an approach is possible with current technology.

Achieving the goals of the project will significantly help the CMS Collaboration to build a suitable detector to deliver the rich physics programme that the HL-LHC enables. As such we will be able to answer further questions about fundamental physics and the universe in which we live. Obtaining and sharing this knowledge enriches society - as seen by the world-wide interest in the Higgs discovery - and that was just the beginning.

A key aspect of the proposal is the demonstration of the flexibility and power of UK developed technology, and as such successful completion enhances the UK's credibility and leadership in these areas. Demonstrations such as that proposed of the versatility of these boards go a long way in helping raise awareness of their usefulness in a far wider context, well beyond HEP, including national security and counter terrorism applications, as well as internet security.