Résumé

Transition en douceur à la tête d'ATLAS

Pour la première fois depuis sa naissance par fusion de deux proto-collaborations, en 1992, ATLAS a changé de porte-parole. Peter Jenni a assumé cette fonction jusqu'en mars 2009 et Fabiola Gianotti a maintenant pris sa succession. La collaboration est passée de 800 à près de 3000 membres. Si Peter Jenni a suivi la conception et la construction du détecteur ATLAS jusqu'au démarrage du LHC, Fabiola Gianotti conduira la collaboration à ses premières acquisitions de données et analyses de physique. Ils s'entretiennent ici avec Antonella Del Rosso de leur rôle à la tête de la plus grande collaboration de physique des particules du monde.

If you think that it might be time to retire after more than 15 years of leading a constantly growing international collaboration and of constructing the world's largest-volume particle detector, then Peter Jenni would disagree. Nicknamed the "father of ATLAS" by his colleagues, Jenni was there in 1992 when the ATLAS collaboration was born out of two early proto-collaborations (CERN Courier October 2008 p42). Initially co-spokesperson, he was spokesperson from 1995 until March 2009, when he handed over to Fabiola Gianotti. Now he looks forward to getting back to the main purpose of ATLAS: the physics.

"I am very proud to have helped the collaboration to construct ATLAS. Twenty years ago we could only imagine the experiment in our dreams and now it exists," says Jenni. "I could lead the collaboration for so long because I was supported by very good ATLAS management teams where the right people, such as Fabiola Gianotti, Steinar Stapnes, Marzio Nessi and Markus Nordberg over the past five years, were in the right places."

As with most particle-physics experiments, the management of one of the two largest detectors at the LHC is a challenge that changes during the lifetime of the collaboration: it starts with the design phase, continues with the R&D and the construction and ends up with the data-taking and analysis. "Over the years I tried to balance the emphasis given by the collaboration to the different aspects, that is, the hardware part (initially very strong), the data preparation, computing and software," confirms Jenni.

Originally "only" about 800-strong, the ATLAS collaboration today has almost 3000 members from all over the world. "Keeping the groups united, inviting new groups to join the collaboration, negotiating to find the funds necessary for the construction… these have been among my key tasks during the past 15 years," he explains. "My efforts also went into keeping groups whose technologies were not retained in the collaboration. Most of the time we managed to have everyone accept the best arguments, but unfortunately there were a few exceptions."

With such a vast amount of experience, what does Jenni regard as the key element for managing a successful collaboration? "Talking with as many people as possible is a key factor," he says. "ATLAS members, even the youngest ones, knew that I was available to discuss all problems or issues at any time. With the exception of the Christmas period, I have tried to reply to all e-mails within 24 hours. By the way, that is why my son thinks physics is crazy and decided to study microtechnologies instead!"

While Jenni's functions have changed, his engagement with ATLAS definitely has not. "A significant part of my work remains the same, particularly in the relationships of ATLAS with the outside world. My main duty is to help obtain a smooth transition, which is facilitated by the fact that Fabiola was one of my two deputies – and I have enjoyed working with her before." Indeed, having more freedom now, he can think of doing more than just sharing some management duties. "In the medium term I have the ambition to study physics with ATLAS," he says. "I am already 'selling' LHC physics in many public talks but I would like to contribute some real physics myself."

The ATLAS collaboration is clearly appreciative of its father's dedication over the years. At the party organized in Jenni's honour on 19 February, the Collaboration Board (CB) chairs directed by Katie McAlpine – the author and singer of the LHC rap (CERN Courier December 2008 p25) – sang: "We've been CB chairs/and we're here to affirm /Peter's time was more an era/ than just a few terms/ leading ATLAS to completion/ like no one else can/ Of course he did it/ Jenni is the man."

The changeover

Now with the construction complete, it's Gianotti's turn to fill the spokesperson's many shoes, after Jenni passed her the leadership baton in March. At the very beginning she joined LHC R&D activities and then the proto-ATLAS collaboration in 1990. "Heading such an ambitious scientific project, and a large and geographically distributed collaboration, is certainly a big honour, responsibility and challenge," she says. "However, I have inherited a very healthy situation from Peter: the experiment has already shown that it performs well, the collaboration is united and strong, and we can continue to prepare for the first collisions without any major worry."

Indeed, activity on ATLAS hasn't stopped since the LHC incident on 19 September 2008 (CERN Courier November 2008 p5). "The first single beams that circulated in the machine before the incident were very useful for studying several aspects of the experiment, such as the timing of the trigger system. After the LHC stopped, we decided to focus on some repairs to the detector and on the optimization of the software and computing infrastructure, of the data distribution chain, and of the event simulation and reconstruction," confirms Gianotti.

An effective distribution of data to the worldwide community is a key point for the new ATLAS spokesperson because she thinks that this is the prime requisite for a motivated and successful collaboration. "The crucial challenge for me is to make sure that each single member of ATLAS can participate effectively and successfully in the adventure that this experiment represents. ATLAS has a very exciting future ahead, with many possible discoveries that will change the landscape of high-energy physics. I consider it very important that each individual in this experiment can actively participate in the data analysis, regardless of whether he or she can physically be at CERN or not. In particular, we have to make sure the younger generations are nurtured in a stimulating environment, share the excitement for the wonderful physics opportunities and are given visibility and recognition," she explains.

While the sharing of data relies mostly on the performance of the Grid and the software and computing infrastructure put in place by the collaboration, it cannot occur without the other side of the coin – effective and open communication in real-time with all members of the collaboration. "The solution we have envisaged is a web space where ATLAS people will be able to find updated 'on-line' news about the machine, the experiment, the physics results, anything that is relevant to ATLAS' life," explains Gianotti.

Asked about the potential "competition" among many people working on the same analysis, she says: "I think it is healthy that people from different groups work on the same topic with a collaborative and constructive spirit. This will allow us to produce solid, verified and fully understood results." Regarding the relationship with CMS, the other general-purpose LHC experiment, she says, "There is a healthy competition, but also collaboration. For instance, ATLAS and CMS have set up a common group that works on statistics tools and how to combine the information coming from both experiments."

The excitement about the restart of the LHC is growing again at CERN and around the world, and the experiments all have their own plans and strategies. "Before undertaking the path towards discoveries, we will need to understand the performance of our detector in all details and 'rediscover' the Standard Model," says Gianotti. "I believe that we will be ready to start investigating new territories when we have observed top-quark production. Indeed, final states arising from the production of top quark–antiquark pairs contain most of the interesting physics objects, from leptons to missing energy and light- and heavy-flavour jets. In addition, this process is the main background to many searches for new physics. Being able to reconstruct these events successfully, and perform our first measurements of the top production cross-section and mass, will give us a clear indication that we are ready for discoveries."

When does Gianotti expect ATLAS to release the first results? "It all depends on the performance of the machine – and its luminosity and energy profile. If everything goes well we expect to have first results, mainly addressing the detector performance, for the winter physics conferences early in 2010; then we hope to present the first interesting physics results at the summer conferences of the same year."