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Search for new long-lived elementary particles decaying to tau leptons in the CMS Experiment

Final Report Summary - LONG-LIVED PARTICLES (Search for new long-lived elementary particles decaying to tau leptons in the CMS Experiment)

The driving motivation and aims for the project, apart from training, were
- For Steve to reconnect back with the experimental particle physics field in general and with the CMS experiment in particular.
- To bring back a project to the UK for the CMS detector upgraded, that can be developed in the years after the fellowship.
In short, both aims have been achieved as will be detailed in the following. In addition the training aspect has been fulfilled continuously during the last two years, from training with the software and analysis tools in the experiment, up to training for more management and leadership type of positions.

Steve proposed to reconnect with particle physics through the searches for new physics in the CMS experiment at the Large Hadron Collider (LHC) in Geneva Swtizerland. The choice of CERN as a place of location was natural as it is the central place for the experiment and its activity. Steve had prior experience in searches for new physics at a different accelerator, quite a few years back.

The integration of Steve in the group and the CMS experiment worked wonderfully well. He decided to concentrate on the study of the search for new long-lived particles in the LHC at 7 and 8 TeV energy of the proton-proton collisions. The analyses he was driving were based on searches for displaced tracks of charged particles and displaced jets. An observation of long lived particles, produced at the LHC, would have a major impact on the field and on our understanding of the evolution of the early Universe. It would for sure drive the experimental particle physics program for the next 10-20 years.

Soon after he's started setting up these analyses and getting first results, he got invited to be the coordinator of the long lived search group, a sub-group the so called 'Exotica' physics group of CMS. This is an important token of recognition in our field. So instead of picking particular analysis on long-lived particles to complete, he became responsible for all long-lived particle searches. In the last two years we published about seven of these analyses, hence his leadership was quite successful. In fact, after a year-s time he got promoted to become the convener of the complete exotica group, at a time that we had 50 or so searches being performed by in total hundreds of people, that all needed to be guided to be completed as scientific papers. The Exotica group is the group in CMS that produces most of the papers as a single group (but unfortunately has not been lucky enough to find new physics yet).

It is clear that Steve is completely back on track in the field of experimental particle physics, thanks to CERN and the MC fellowship. To illustrate that, he gave in 2012 the review talk on exotic phenomena searches for CMS at the most important summer conference in our field, the ICHEP conference in Melbourne (4-12 July). He also gave various other talks at distinguished conferences and particle physics schools. Hence Steve is yet again recognised as an expert in the field.

For the future connection and follow-up with the the UK groups on his return, Steve has followed the path of searching for an engagement of the UK groups in the so so called pixel detector upgrade project. The pixel detector is the detector closest to the beam pipe and has about 70 million channels. For the expected higher luminosity in the future we will have to replace the present detector by a more radiation hard one with a better granularity. CMS has started an R&D program for new detectors and soon interested institutes will have to make their bid for the part they are interested in to build such an upgraded pixel detector. Steve has developed a strategy such that the UK groups could participate in a hardware part of that project, namely the construction of one of the pixel layers. Furthermore there is enough expertise that would allow UK scientists to participate in the online software for the data acquisition. These proposals have been evaluated in the UK-CMS community. At this point the exact engagement of the UK will depend on the available funding, but clearly there will be an important participation in the pixel detector upgrade program in the next years.

Hence, in all Steve met with the original aims of the the project. He is back and in a leading role in experimental particle physics, in one of the most exciting and challenging experiments presently operational: CMS at the LHC. With his work and under his direct convenorship, a large number of scientific papers on searches for exotica got published in peer reviewed journals, among which a number on papers long lived particles. No evidence for the production of long-lived particles as been found to date.