ATLAS: The making of a giant

19 September 2008


ATLAS is the well deserved name for the largest-volume detector ever constructed at a particle collider. It sits about 100 m underground in a cavern that could accommodate the Arc de Triomphe in Paris. A multipurpose detector, its physics goals range from the search for the Higgs boson and supersymmetric particles to the exploration of extra dimensions and other alternative scenarios.

The ATLAS collaboration was born in the autumn of 1992 from the merging of two existing groups, ASCOT and EAGLE, that had presented different expressions of interest at the meeting in Evian the previous March. By the end of 1994, the ATLAS collaboration had taken shape and submitted the technical proposal. “In summer 1995 the detector was pretty much the same as it is today with the exception of the inner detector, whose technical design report was presented later, in 1997,” says Peter Jenni, (co-)spokesperson of the ATLAS collaboration since the beginning. “When, we submitted the technical proposal in December 1994, all the big decisions, such as which type of calorimeter or magnetic field to use, had already been taken.”

So, after about 15 years in the making, not much has changed from the original design for ATLAS. There were only ever two main turning points. “Until 1997, the design of the precision chambers in the inner detector was not established,” explains Jenni. “The collaboration was hesitating between using microstrip gas chambers and silicon strips in the outer layer. It finally decided to adopt the silicon solution. In 2002, the ATLAS detector underwent an internal financial audit and the resources review board accepted a completion plan with a reduced budget. As a result, the development of some parts of the detector had to be postponed. The impact of such financial cuts was particularly significant on the high-level trigger and data acquisition, but some features of the inner detector, the muon system, the electronics of the calorimeter and the shielding system had to be reviewed as well.” Since then, not all these projects have been completed, and some of them never will be. “However,” says Jenni, “this does not affect the main design or performance of the detector.”

The detector was designed from the beginning to study a range of phenomena. “The initial design requirements of ATLAS were optimized for the search for the Higgs boson and supersymmetric particles,” confirms Jenni. “The Higgs boson always featured strongly because, depending on the mass, the decays to deal with experimentally are very different. Therefore it is an excellent benchmark for making sure you have built a detector with many capabilities.”

If the ATLAS detector has not changed much since 1995, the physics panorama has. New particles have come onto the scene, as well as new scenarios that attempt to describe the first moments of the universe. “ATLAS will be able to study the signature of still-to-be discovered heavy objects decaying into electron pairs or muon pairs, such as the Z’,” explains Jenni. “The superconducting toroid system allows us to measure muons with great precision, even with the highest luminosity, independently from the inner detector.” Jenni also expects an excellent performance for studying signatures from particles coming from possible supersymmetry (SUSY). “Our detector has a particularly good hadronic calorimeter, which will allow us to measure accurately the missing energy associated with the possible existence of SUSY or extra dimensions. Moreover, if there is a graviton-like resonance from extra-dimension scenarios we will have to measure the angular distribution. In this case, toroids have the advantage that the field is optimal also in the forward direction.” In Jenni’s opinion: “The performance of detectors with high luminosity will make the difference in the race for discovery in the long run.”

However, according to the most recent schedules, such high luminosity will not be available at the LHC until 2011 or 2012. In particular, the first protons will collide in the LHC at 5 TeV, rather than at 7 TeV. Instead of being disappointed, Jenni is pragmatic. “We will use the first two-month run to get to know and test the detector with known signatures, such as the W boson and the top quark – 10 TeV at low luminosity will already give us a lot of data to calibrate, as well as understand all the subdetectors and the chain of data preparation and analysis. Before any discovery can be claimed we first have to show that the known physics is reproduced and that the detector performs well.”

After this first learning phase, the collaboration will be ready for 2009, when the accelerator will run at full energy and increasing luminosity. If the expected Higgs boson really exists, ATLAS will start to record its signatures. “An estimate for finding the Higgs is not before 2010, but this seems rather optimistic,” says Jenni. “For SUSY or extra dimensions, the time needed to study the signatures depends on the different theoretical models. We could cast light on some of them before the Higgs can be confirmed”.

When it comes to discoveries, an important aspect for the collaboration and for CERN will be how they will be disseminated. “The first thing we will take care of is to publish our results in a scientific review, not in the New York Times,” declares Jenni. “Then will come the sharing of the excitement of the results with the public and this is a very important aspect. For an experiment like ATLAS, outreach is an important activity. I think that it is crucial to involve active scientists, although scientists do not necessarily know how to deal with it. We will all have to learn how to do it together with CERN.”

ATLAS has been a pioneer in this field, with an attractive website that features video material, interactive games, press kits, regular news etc. “Inside ATLAS we have some communication plans to deal with the publication of the first results. There is already quite a lot of preparation of educational resources to be used to explain how things work. An EU co-funded project has recently received a first approval from the Commission,” continues Jenni. “All this, however, seems rather theoretical for the moment. I feel that we will have to learn how to do things for real.”

In the race for discovery at the LHC, ATLAS is not alone. The collaborations are competitors, but they are also allied because what one detector sees will have to be confirmed by the others. “Different detectors have made different choices, giving priority to different features (calorimetry, particle identification systems etc). Physics will tell us who made the right choice,” confirms Jenni. “Having invested so much in this powerful multipurpose detector, it is clear that the ambition and duty of ATLAS is to exploit the LHC potential to the maximum”.

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