Nuclear Physics Division

As part of the new CLAS spectrometer project for the 12 GeV electron energy upgrade of the Jefferson Lab (USA) IRFU has been conducting R&D for more than 10 years to design and build a new generation tracker, using thin and flexible MICROMEGAS detectors that are now operating with the new CLAS12 spectrometer. After one year of installation, this tracker is operational and meets the expected characteristics with more than 95% detection efficiency and a spatial resolution of less than 100μm. After a dedicated data collection to measure the detector response, the new CLAS12 spectrometer is now collecting data for the DVCS physics experiment, where IRFU also participates and which objective is to measure the internal structure of the proton in three dimensions.
The exceptional success of the  tracker project, that results from a close collaboration between IRFU's engineering and physics departments (DEDIP, DIS and DPHN), has been an example for other projects. Let us quote  the LHC experiments for particle hunting, the muonic imaging of the pyramids, as well as a transfer of know-how to  industry.

The first triplet of superconducting multipoles of the S3 Super Separator Spectrometer arrived at Ganil on August 29, 2018. S3 is one of the experiment rooms of the Spiral2 facility. The magnet, with a mass of 2.8 tonnes, is 1.8 m long and almost as high. This innovative type of magnet is very compact despite the number of optical functions it can generate (quadrupole, sextupole, octupole and dipole). It is the first of a series of seven to be delivered to the Ganil.

The magnetic field is generated by a niobium-titanium alloy (NbTi) conductor arranged in an epoxy/glass fibre matrix and operate at the temperature of the liquid helium (4.2 kelvins). The power supply leads are composed of two types of high-temperature superconductors and nitrogen-cooled.
It’s a unique design resulting from a collaboration between Ganil, CEA/Irfu, the American laboratory in Argonne Nat. Lab. and the two manufacturers in charge of prototyping and series (Advanced Magnet Lab. for superconducting coils, Cryomagnetics Inc. for cryostats and integration).
This element was financed by EQUIPEX n° 10-EQPX-0046, awarded to S3 by the National Research Agency in 2011.

Contacts: Antoine Drouart, Myriam Grar (Ganil) et Hervé Savajols (Ganil)

In ultra-relativistic heavy ion collisions at CERN's LHC accelerator, a new state of matter is formed: the quark-gluon plasma (QGP). It is a kind of very dense and hot "soup" containing only the most elementary constituents of matter. A few microseconds after the Big Bang, the Universe would have passed through this state. Because of the interactions between its constituents, the QGP flows like a fluid. At the LHC, interactions between constituents of the QGP are so strong that even objects as massive as the charmed quarks are carried away by this flow, as suggested by the measurement of the flow of the J/ψ (particle composed of a charm quark and its antiparticle) of ALICE during the first campaign of the LHC (see highlight 2013). This result has just been confirmed by the ALICE collaboration using data from the new LHC campaign (2015-2018). The precision obtained suggest the need of including new mechanisms in the theoretical models. The Saclay group played a key role in analyzing this data.