Particle physics at accelerators
Particle physics experiments using particle accelerators study fundamental interactions of matter. At present, the best theory to describe our knowledge of subnuclear physics is the Standard Model. The aim of current research is to gain a deeper understanding of certain aspects, such as the origin of the mass of these particles. In that context, the recent discovery of the Higgs boson represents a great step forward and determining its characteristics is now a priority. Scientists are hopeful that ongoing experiments will also enable them to discover new phenomena and fill some of the gaps in the Standard Model. One such example would be the discovery of supersymmetric particles, some of which are candidates for the constituents of dark matter (we know this pervades the universe, but have so far been unable to detect or describe its nature). Other examples would be the discovery of new signals that explain the asymmetry between matter and antimatter in our universe, or proof of the existence of further space-time dimensions.
In working to broaden the scope of our knowledge, experiments in subnuclear physics explore two different and complementary frontiers of our experimental limits: the energy frontier and the high intensity frontier. On the one hand, we use ever-more powerful particle accelerators to achieve ever higher collision energies and the formation of new particles at the Large Hadron Collider at CERN. Alternatively, we search for new physics phenomena in the flavor physics sector through precision measurements, inferring the presence of high mass new particles from their effect on lower energy processes.
These kinds of search require the accumulation of large samples of data that are available at high luminosity colliders, and in some cases can exceed the values of the new particle masses that can be observed directly at the LHC. Both of these lines of research are pursued at Milano University: the ATLAS experiment at the high energy frontier and the flavor experiment BaBar, as well as the SuperB project, at the high intensity frontier.
Experiments in subnuclear physics involve the use of large, highly complex equipment based on the latest technology in the field of detectors, electronics, data acquisition and computing systems. Collaborations to build this equipment involve hundreds of physicists from institutes and laboratories around the world (thousands in the case of the LHC). These projects are significant examples of effective international cooperation, bringing together the world’s best physicists and providing an opportunity for young scientists to gain experience and learn fundamental skills.
ATLAS is a particle physics experiment currently taking data at the Large Hadron Collider at CERN. The ATLAS detector is searching for new discoveries in the head-on collisions of protons of extraordinarily high energy. ATLAS will learn about the basic forces that have shaped our Universe since the beginning of time and that will determine its fate. Among the possible unknowns are the origin of mass, extra dimensions of space, unification of fundamental forces, and evidence for dark matter candidates in the Universe.
The Milano group had and is having an important role in the construction, commissioning and operation of the calorimeter and of the pixel detector of the ATLAS experiment. The group is presently involved in the exciting search for the Higgs boson, as well as for supersymmetric particles, a possible solution to Dark Matter enigma. The group and the INFN Computing center have contributed in an important way to the development of GRID computing and middleware software, and Milano host a TIER2 facility for the analysis of data from LHC.
Using data collected up to June 2012 ATLAS has observed an excess of events at a mass of approximately 126 GeV with a statistical significance of more than five standard deviations above background expectations. The probability of the background alone fluctuating up by this amount or more is about one in three million. The evidence is strongest in the two final states with the best mass resolution: first the two-photon final state and second the final state with two pairs of charged leptons (electrons or muons).
ATLAS has interpreted this to be due to the production of a previously unobserved particle with a mass of around 126 GeV. The new particle observed at about 126 GeV is compatible, within the limited statistical accuracy, with being the Standard Model Higgs boson. However, more data are required to measure its properties such as decay rates in the various channels (γγ, ZZ, WW, bb and ττ) and ultimately its spin and parity, and hence ascertain whether it is indeed the Standard Model Higgs boson or the result of new physics.
More information on the ATLAS group in Milano can be found here.
BABAR is a particle physics experiment designed to study some of the most fundamental questions about the universe by exploring its basic constituents – elementary particles. The BABAR Collaboration’s research topics include the nature of antimatter, the properties and interactions of the particles known as quarks and leptons, and searches for new physics.
Research topics of interest of the Milano group are the searches for new physics in charm e B meson decays. By analyzing the large data sample available in BABAR, we are looking for signatures that cannot be explained in the Standard Model of particle physics, and would require an alternative explanation.