D0 : Graphical representation of a top/ antitop quark pair production event.
Physics of the elementary
This is a simple question: what is matter made of, and what is its origin? A long occidental tradition answers this through division, claiming that the variety of nature is obtained from the assembly of elementary bricks available in a small number of types.
The idea is strong and simple, and even today, the discovery of a new “particle” is a big event for the public as well as for scientists. However, with the advent of modern physics, the notion of constituent and interaction is now much wider: since quantum mechanics, a particle is also a wave; since general relativity, matter and even space-time are also fields.
In the most recent theories, constituents and interactions are strings and space-time is a membrane…
Standard Model : Top quark mass (abscissa) vs W mass.
Red ellipse: LEP 1 results,
green ellipse : LEP 2 and TeVatron results. The yellow band shows the Standard Model predictions for the whole range of possible masses for the Higgs boson. The measurements are coherent with the Model for lower Higgs masses.
Structure and coherence
Behind the multiple phenomena, physicists search for structures and the coherence of the model. In the last 50 years, this capability of explaining the world around us has been extended in a spectacular way to the whole universe: we can now link the structure and
evolution of the universe to the laws which describe matter on a microscopic scale.
The keystone is the Standard Model, which describes accurately the constituents of matter and their interactions. Any new measurement, coming from a particle accelerator or from a satellite, is immediately checked against the Standard Model.
Today, very precise measurements performed on various objects and in various energy ranges, all confirm the Standard Model.
More questions
Despite its successes, the Standard Model is full of imperfections: a large number of free parameters (27), some of its structures totally unexplained, and more importantly an ontological deficit. While we know how to describe particles and their behaviour, we know
much less about what it takes to be a particle. Why is there, at location X and time t, a concentration of energy which appears to us as a massive particle?
These questions have been formulated in a detailed way, and we have some hints how to answer them.
Edelweiss : 320g detector (Germanium crystal).
Experimentalists
SPP physicists are experimentalists. While designing experiments, they evaluate theoretical ideas, sometimes contribute directly to them, and confront them to the technical possibilities for testing. Then they participate to the detailed design of the detectors, their construction, and operation. Finally, they analyze the experimental data and compare it to the theoretical models considered.
The strong link between physicists and technical support in DAPNIA is an essential asset. In most cases, the experiments are performed by international collaborations, each with several tens of participating teams. However, the impact of SPP physicists is often clearly apparent in the global design of an experiment, its mode of operation, or the results of its analysis.
The main research programs The search for the elementary is presently pursued through four programs, described in this report :
• Tests and extensions of the Standard Model ;
• CP violation ;
• Neutrinos ;
• Dark matter and cosmology.
This fi gure shows the number of physicists by program in 2003.
SPP status and scientific production
End 2003 , SPP has 81 CEA positions, 5 CNRS, 2 University, 9 postdocs and 16 graduate students, forming 15 groups of 3 to 18 physicists. Support is provided by the division secretariat (management assistant, employment, conferences…) and two secretariats for the physicists (travel,…).
The quality of theses work is given a high priority. Each student is followed by a tutor, whom (s) he chooses outside her/his group.
About 20 physicists have teaching activities outside DAPNIA, with various amounts and audiences. Several physicists are members of the NEPAL network which offers physics presentations for high-schools.
The Scientific and Technical Committee of SPP meets about twice a year to examine proposals
or monitor approved experiments. Specific to SPP are a weekly seminar and preprint club.
This table summarizes the scientifi c output for years 2001, 2002, 2003.
Scientific prizes:
2001 : Emmanuelle Perez : CNRS Bronze Medal.
2002 : Vanina Ruhlmann-Kleider: CNRS Silver Medal.
maj : 25-04-2013 (537)
