Philippe Bloch has stepped aside after five years as Head of the PH Department in order to return to CMS and continue his research work.

He achieved to combine successfully fundamental research work with an administrative position. In this interview, we discuss his first steps in particle physics, when forty years ago he arrived at CERN for the first time, as well as the challenges he had to face in the early years of the LHC and the lessons he draws as he returns to physics research.

I would like to persnally thank Philippe for the great experience we had over the last year and most of all for being a great teacher. I hope that he will continue inspiring young generations of physicists with his passion for science and his interest in discussing the latest physics results and asking the right questions!

P.C.  When did you first arrive at CERN and why did you decide to work in particle physics?

P.B.  In 1971 I was accepted to the Ecole Normale Supérieure in Paris. I did the competition in mathematics but quickly realized that I would prefer something more hands-on and decided to change to physics.

One of the reasons that I decided to focus on particle physics was due to my sister doing her thesis in this field and convincing me that it was an interesting subject! She helped me to get in touch with the Department of Physics in the Saclay laboratory where there was a strong team working in particle physics.

Particle physics were blossoming at that time and high-energy physics was a growing field. The Standard Model was emerging as more and more experimental evidences were gathered, such as the discovery of the J/ψ and the observation of neutral weak currents. It was a very promising field and for me it was a rather easy decision to start working in it. During the last year of my Masters in 1973 I joined a group in Saclay, which was performing an experiment at CERN: this year was therefore the 40^th anniversary of the first time that I came to the laboratory.

P.C.  Which was the first experiment that you joined?

P.B. I initially worked in an experiment in the South Hall of the PS where the Saclay group had been running a number of experiments since the early 70’s. We studied rare decays of charged Kaons. I continued working with the same group for my PhD and we joined a neutrino experiment that was led by Jack Steinberger, the CDHS/WA1 experiment at the SPS. It was a fantastic time as this experiment allowed a much better and more precise understanding of the structure of the proton. Of course experiments at SLAC and the Gargamelle Bubble Chamber at CERN had already taught us many things, but our group made a very significant contribution in this area of research. For example we confirmed the number of valence quarks to be 3 in the nucleon and by comparing the structure functions from neutrino experiments with those measured with electrons at SLAC we also confirmed the fractional charge of quarks. Moreover, with this experiment we got the first quantitative measurements that confirmed predictions of the recently born QCD theory.

Following a break for my military service I came back to work for the UA2 experiment. Again a very exciting time with the discovery of the W and Z bosons! In 1985, after being offered a job at CERN, the Director of Research R. Klapisch said to me that if I wanted to do something of my own then the LEAR accelerator could be a unique opportunity, so I joined CPLEAR, a medium size experiment that began at LEAR. I moved from very high energies of proton-antiproton collisions in the SppbarS to antiprotons at rest! I worked for almost 15 years in CPLEAR, firstly building a gas calorimeter, then studying CP violation and CPT symmetry. This experiment showed for the first time direct evidence for Time reversal violation and set limits on CPT, which are still competitive today. Working for this LEAR experiment was a very enriching experience. Not only was it a rather complex and challenging experiment but we also had to fight hard to keep a smooth flow of resources in order to keep it running. At that time CERN was putting a lot of effort into building the LEP and therefore most of its resources were directed towards that project.

P.C. How did you decide to join the CMS experiment?

P.B.  Whilst already working in the UA2 experiment I participated in the first workshop – that took place in Lausanne in 1984- discussing the concept of a Larger Collider in Europe. However it was in 1994 that I strongly felt that I wanted to join this new project and get more actively involved. The physics questions at stake were close to my previous research and I was also eager to work with colleagues on the flagship CERN project at the energy frontier.

The concept of the CMS detector was very appealing and when I decided to join the CMS Collaboration I met with Michel della Negra and Jim Virdee to discuss how I could help them in building the new detector. We decided that the best thing would be for me to join the electromagnetic calorimeter (ECAL) group, and I started working on the pre-shower detector and spent a couple of years further developing this subdetector. Then in 2002, Michel and Jim asked me to take over the project management of the whole ECAL, including the crystal calorimeter.

P.C. What were the main challenges of the CMS calorimeter?

P.B. The main challenge was the unprecedented number of crystals and their required radiation hardness for LHC. At this point I should add that I wasn’t at all an expert on crystals (I had never seen any before!), but the group included world experts on this subject and their experience was essential in setting up the production. An important issue was to secure the resources and to build the calorimeter in time.

The next big challenge was the electronics that we would use for the calorimeter readout. Soon after starting my mandate in 2002 we decided that the electronics used in the calorimeter should change completely as the cost of the proposed scheme was not affordable. The decision was taken to abandon the old electronics and start from scratch using the recent 0.25μm CMOS technology. It was only a couple of years before the start-up of the LHC that was scheduled for 2005-2006, and this decision seemed like madness. The credit for this goes to our late colleague Peter Sharp, who was the real initiator of this wise decision that helped us a lot both in saving resources (nearly 20 million CHF) and at the same time obtaining a better performing and more radiation hard front-end electronics.

P.C. And what about the physics questions that were most intriguing for you?

P.B. I was mainly interested in the search for the Higgs, especially in the channels of the Higgs to γγ or Higgs to 4 electrons. That was our main target and the ECAL benchmarks were based on those channels.

However, as I moved to the PH department management in 2009, I did not have the chance to get involved in the data taking and analysis. Having worked for the CMS calorimeter for almost 15 years, from 1994 to 2008, and not being part of the team that worked on the Higgs analysis has certainly been frustrating! Of course I was extremely pleased to see the results we got in both channels, which contributed in an essential way to the Higgs boson discovery in July 2012.

P.C. You started your mandate as Head of the PH department in 2009. Was it an easy decision?

P.B. Yes and no. I would say that it is my personal attitude to try to help colleagues and the feeling I had is that, from this position and using my knowledge of having worked in an LHC experiment, I could help the experiments situation within the PH Department.

P.C. 2009 was a crucial period for CERN and the PH department. What do you recall from that time?

P.B. Our mandate started a few weeks after the start-up accident in the LHC. The accident was a shock for all the people who had worked so hard over the previous years/months and particularly for young students who were looking for the first data in order to complete their theses.  However, I have to admit that most of the pressure was put on the Accelerator Departments and not on PH or the experiments. In some ways the experiments were able to benefit from the unwanted delay; for instance in CMS we installed some of the missing parts of the detector and also commissioned the whole setup with cosmic ray tests, to be completely ready once the LHC restarted. The outcome after the LHC restart has been so rewarding that we have all forgotten this difficult period.

P.C. What lessons have you learnt from your time as Head of the Department? How did it meet your expectations?

P.B. From 2007 – 2008 I was appointed as CERN Team Leader of CMS and so I already had an idea of the issues that were in front of us.

Fortunately, the new directorate valued the role of PH in the experiments and supported the need for maintaining or sometimes increasing the level of the resources that were allocated to the department. I have to say that without Rolf Heuer and Sergio Bertolucci’s strong support I would have never succeeded in the objectives that I set myself when I started as Head of the Department.

First of all it was essential to maintain our manpower. In the medium term plan of 2008 (for the period 2009-2013) it was foreseen that PH would lose 50 people in the long term, going from 440 to 390 staff. Discovering this was a great shock for me. However, both Rolf and Sergio immediately understood that this wasn’t possible and decided to maintain the number of positions and even slightly increase them after the flexibility positions we got CERN-wide in 2012. I am not claiming that we still don’t need more manpower and I am aware that there are still some issues. For example, more than 30 people are still paid by external funds and this shows that we haven’t reached an ideal situation but at least we are not in the dramatic situation that was foreseen before.

The next important task was to get funds for consolidation, R&D and upgrades. Similarly, nothing was firmly planned, and R&D was only funded through the so-called White Paper projects until 2011. Again, our directors took decisions to support these programmes. From very early on CERN ensured funding for the upgrade plans phase-1 of the four LHC experiments. This gave a clear message to the external funding agencies that later came to support the phase-1 upgrade plans. It showed the strong commitment of the organization in these projects and the actual support of the department to these plans. I hope that the same will happen with the phase-2 upgrades, which remain to be introduced in the future MTPs.

To support these plans we ensured that the number of applied fellows remains high. More recently we introduced a new category of associates, specialized in the technical fields.

Physics was also not forgotten. During the LHC construction period there was a strong reduction in the appointments for experimental research physicists at CERN.  We managed to restore it to 6 new positions per year in EP whilst keeping constant the number of positions for the Theory Unit.  Today, the CERN Physics groups play a prominent role in the LHC data analysis.

Finally, I think it is important to retain scientific diversity at CERN for the non-LHC experiments. It is an issue I am particularly sensitive to perhaps due to my previous background and the time I spent working at the PS, SPS and LEAR experiments. The NA62 experiment might be one of the most prominent examples, as we have now obtained the budget necessary for the participation of CERN to the construction of the experiment and have also injected a large technical support. The same is true for CLOUD and AEGIS. And PH continues to contribute to COMPASS, ISOLDE, CAST, DIRAC and LCD.

P.C. Are there areas where you feel that more effort is needed?

P.B. If you read the interview that I gave to the Bulletin in 2009 when starting my new role as department head, one of the points that I raised was the need to improve the “team spirit” in the department. It proved to be very difficult and we could have done much better work. People are too involved in their experiment without knowing what’s going on elsewhere and without having many occasions to be informed about other issues. Nevertheless we made a few attempts in this direction. One of these things was the PH newsletter that we launched with Kate Ross, which aimed to give broader coverage of our activities and share information among the different groups. The new web-based format of the Newsletter since March 2013 has been an additional strong step and I hope it will continue and even broaden.  Another initiative is the annual PH retreat during which all the PH group leaders are brought together for one day to present their current work to the other groups in the department. The LPCC programme is another example and of course the credit for this goes fully to Michelangelo Mangano.

P.B Why did you decide to step down and what will be your next steps?

P.C. I have enjoyed this job and the good relations that I have had with the directorate, the other department heads, my deputies, the leaders of the various PH groups and many colleagues.

However I think that rotation is important as after five years you start to feel a bit like you are doing the same routine work. There has never been a PH Department Head that stayed in the position for seven years and on average it has been around 3.5 years.

I also mentioned the frustration that I felt as I haven’t been able to work with data after spending almost fifteen years in the design and commissioning of the CMS detector. I think that the new run at the LHC offers a unique and exciting opportunity. Furthermore, I am about 4 years away from retirement and following my retirement I would like to continue with scientific activities at CERN. I believe that if I spend a few years working back with the experiments I will be in a much better position to contribute in the future. I will therefore be returning to the CERN CMS physics group. Having worked with him for many years I am sure that my successor Livio Mapelli will do a great job.

P.C. Finally, I would like to ask you about your advice to younger generations of physicists that are now starting their careers in particle physics.

P.B. When I started working at CERN, experiments were small, with only 30-40 people working on them and physicists had to get involved in most of the tasks including building the detectors, developing the software for the analysis and of course analysing the data. My personal advice to young people is to try as much as possible to participate in many aspects of the experiment. I understand that this might not be always easy within a large collaboration like those of the LHC experiments but they should look for new opportunities and try to enjoy the diversity of their work as much as possible.  Learning new things, contributing to many aspects of our experimental work was always for me a great pleasure.