The ATLAS experiment at the Large Hadron Collider(LHC) at CERN is one of the largest detectors dedicated to the elementary particle physics studies ever built. It's structure is typical for the so called general purpose detectors aiming at measurements of the wide spectrum of physics phenomena that may appear at the LHC. It contains several layers of different detectors. The outermost part is the Muon Spectrometer dedicated to measurement of tracks and momentum of muons. Going inside, the Hadron and Electromagnetic Calorimeters are placed to measure the energy of particles.
Groups from Institute of Nuclear Science Polish Academy of Science participate in building and operation of the detectors listed below.
The most internal part, the so-called Inner Detector, is laid the closest to the proton-proton interaction point and it serves for a very precise measurements of the charge particles trajectories and momentum. These measurements make the reconstruction of primary and secondary interaction vertices possible and thus are fundamental for the majority of physics studies conducted in ATLAS.
The Inner Detector is a large scale system composed of 80 millions readout channel. It has a complex structure and employs detectors built in three different technologies: the most precise Silicon Pixel Detector is placed in the immediate vicinity of the LHC beam pipe; then the silicon Microstrip Detector (SCT) follows and the external part of ID is formed by gaseous straw transition radiation detectors, the so-called Transition Radiation Tracker (TRT).
Krakow ATLAS group has been involved in the Inner Detector hardware activities since the phase of design and development, throughout tests, assembly and commissioning to the data taking. In particular we contributed to the design, production and assembly of SCT detector modules and their front-end readout electronics. The specialized, multi-channel High Voltage Power Supply system for SCT detector was designed, built and commissioned by our group. We developed a large scale, distributed control systems for the SCT and the whole ID environment. During the data taking at the LHC we are responsible for various tasks concerning maintenance and optimisation of the Inner Detector to keep it in the best possible shape for the physics measurements.
ALFA detectors are designed to measure the proton-proton elastic scattering. In such a process, the final state consists of two protons with the same energy as before the collision and moving in opposite directions at a certain angle with respect to the primary direction. Elastic scattering of protons can be caused both by the strong and the electromagnetic forces.
Since the scattering angles in elastic interactions are very small (of the order of microradians), the trajectories of the scattered protons are very close to those of the beam particles that circulate in the accelerator. In order to register the scattered particles and precisely measure the scattering angles, the detectors are placed very close to the beam and very far from the interaction point.
ALFA detectors are housed in special devices which are called the Roman pots. These devices allow insertion of the detectors into the LHC beam pipe (from the top and bottom). As a result, the detectors may take data at a distance of just a few millimeters from the beam centre. When the beams are accelerated or otherwise unstable, the ALFA detectors are retracted at a safe distance.
AFP detectors are the newest part of the ATLAS forward detector system. They allow measurements of protons that were scattered at small angles and, in addition, lost a small part of their energy. The AFP detectors are used to study the properties of processes in which such protons are produced. They can also be used to search for new particles.
Like the ALFA detectors, the AFP detectors are installed inside the Roman pots, but they approach the beam in the horizontal plane. AFP detectors use the state-of-the-art silicon pixel sensors (left side of the photo) to measure the position of the scattered proton trajectory and detectors based on Cherenkov radiation produced in quartz bars with special geometry (right side of the photo) allowing the ultra-precise measurement of proton arrival time.
Grid Computing for High Energy Physics
The goal of the WLCG Computing Grid is to federate in one system the existing distributed computing resources available in countries participating in High Energy Physics experiments running on the LHC accelerator at CERN. The ATLAS experiment group from IFJ PAN in Kraków participates in the development of the WLCG system and in the management of ATLAS computing production running both on Polish resources and in the World. We collaborate with the Academic Computer Center CYFRONET AGH in Kraków and the Poznań Supercomputing and Networking Center in order to deploy and support ATLAS grid services at these sites that constitute the Polish Tier2 Federation in the WLCG organization. We monitor the quality and volume of ATLAS computing production at Polish sites and provide support in case of problems to assure that the Polish obligations are fulfilled.
At IFJ PAN, we also have a local computing cluster. It is utilized by the ATLAS experiment group for computing tasks running small simulations and for data analysis. This cluster is built on IBM HS21 and HS22 servers, providing 72 CPU cores and a dedicated storage matrix Sun X4500 with 27 TB of storage space.