Subito dopo il Big Bang, lo spazio era caratterizzato da una sorta di “zuppa primordiale” composta di quark e gluoni, ossia particelle di materia e di interazioni fondamentali. Questo plasma super denso si raffreddò quasi istantaneamente e la sua, seppure breve, esistenza contribuì sostanzialmente a creare le condizioni iniziali da cui si è successivamente evoluto il nostro Universo. Ma per capire meglio queste fasi iniziali della storia cosmica, gli scienziati devono ricreare nei grandi acceleratori di particelle quel plasma primordiale: è il caso del Relativistic Heavy Ion Collider (RHIC) presso il Brookhaven National Laboratory (BNL) dove si stanno analizzando i dati degli ultimi anni grazie ad un esperimento noto come STAR (Solenoidal Tracker at RHIC). A tale complesso è stato aggiunto di recente un nuovo rivelatore, denominato Heavy Flavor Tracker, il più avanzato nel suo genere e che servirà per studiare i processi di decadimento degli adroni costituiti da quark charm e bottom.
Scientists and engineers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab), have played a major role in the development of the STAR Heavy Flavor Tracker. The STAR HFT is actually the collective name for three separate silicon-based detector systems that make it possible for the first time to directly track the decay products of hadrons comprised of flavors (types) of quarks, “charm” and “bottom,” with heavy mass. Heavy quarks are considered ideal probes for quark-gluon plasma studies; however, their low production yield and short life-span (a fraction of a microsecond) make them difficult to study in heavy ion collisions that also produce huge quantities of light flavor particles. The HFT was first conceived nearly 15 years ago by Berkeley Lab’s Howard Wieman, a physicist with the Lab’s Nuclear Sciences Division who also played a prominent role in the creation of STAR. The HFT construction project, which began a few years later, was initially led at Berkeley Lab by Hans Georg Ritter, a physicist who served as head of the Nuclear Science Division’s Relativistic Nuclear Collisions program (RNC) for many years. “The HFT enables precision tracking measurements of heavy quarks at low momentum where the particle production is most sensitive to the bulk medium created in heavy ion collisions”, says Nu Xu, a physicist also with Berkeley Lab’s Nuclear Science Division who is the current spokesperson for the STAR experiment. “This allows us to distinguish the decay vertices of heavy flavor particles from primary vertices and significantly reduces combinational background, which yields cleaner measurements with a higher level of significance”. The importance of the HFT’s precision measurements at low momentum to quark-gluon plasma studies is explained by Peter Jacobs, a Berkeley Lab physicist who now heads the Nuclear Science Division’s RNC program. “Theorists claim they can calculate the dynamical behavior of heavy quarks in matter more accurately than that of light quarks or gluons. Some even think they can calculate the dynamical behavior of heavy quarks in the quark-gluon plasma using models inspired by string theory”, Jacobs says. “One of the things we will be testing with the HFT is the different predictions of the behavior of heavy flavors in the quark-gluon plasma made by string-inspired models versus more conventional physics”.
Berkeley Lab scientists and engineers are now developing a new, larger version of the HFT which they propose to be fabricated for the ALICE detector at CERN’s Large Hadron Collider. “If approved, this will be an upgrade to the Inner Tracking System of the ALICE experiment at the LHC that is a direct follow-on to the STAR HFT, utilizing a number of HFT developments”, says Jacobs. “It is proposed to be installed during the next long LHC shutdown in 2018 and will essentially be a 25 giga-pixel camera made up of 11 square meters of silicon, about 30 times larger than the HFT at STAR”.