Studying extremely hot and dense nuclear matter is a very important problem of high energy physics.Quantum chromodynamics (QCD) predicts the existence of a new state of matter - the quark-gluon plasma (QGP), which may be formed in relativistic nuclear collisions . The study of the properties of the QGP is associated a number of fundamental problems, such as phase transitions in nuclear matter, the state of the QCD vacuum, the evolution of the universe and the mechanisms of neutron starsformation.
When matter is heated or compressed it can undergo a phase transition to a new state. Well known examples are the transition from ice to water and from water to stream. If you compress or heat nuclear matter by an enormous amount the transition from hadronic phase to quark-gluon plasma can occur. In this state the quarks and gluons are no longer confined to hadrons forming a quark bag in which they can move about freely (fig.1). A quark-gluon plasma can be created onlyin extreme conditions. Such conditions have existed after one microsecond after the Big Bang (high temperature T > 1012 K) and nowadays they are expected to exist in the cores of neutron stars (high density, ten times more than the density of nuclear matter).
Fig.1. Phase statesof water andnuclear matter.
The only way in wich we could create such a state of matter for study in the laboratory is by means of relativistic nucleus-nucleus collisions. Theoretical estimates show that in heavy nuclei collisions at 100 GeV per nucleon the particles are compressed to 2-3 times the normal nuclear density. A system of strongly interacting hadrons in the overlap region of the colliding nuclei is called a fireball and can reach thermodynamic equilibrium at a temperature ~200 MeV exceeding the temperature of the phase transition to QGP. This hot and dense fireball is only created from nucleons of the target and the projectile, who experienced at least one interaction (fig.2). These nucleons are called participants and the remaining nucleons are the spectators. The size of the fireball is propotional to the total number of participants. This number depends on the size of the nuclei and of the impact parameter and can serve as a measure of the centrality of the nucleus-nucleus collisions. Depending on the impact parameter nuclear collisions are consided to be peripheral or central. Fireball that is formed in central collisions with close to zero impact parameter has a maximum energy density. Studying the properties of such a fireball is one of the main problems of modern high-energy physics.
Fig.2. Nucleus-nucleus collision with impact parameter b.
Phase diagram ofnuclear matter.
First QGP signals were obtained in the Super Proton Synchrotron SPS at CERN experiments NA49, NA50 and NA57. The most convincing signatures predicted by theorists are the J/Ψ suppression and the enhanced production of strange particles in nucleus-nucleus collisions. Both effects were found in collisions of heavy nuclei at 40-160 GeV per nucleon .
Fig.3. Au+Au collision at 158 GeV per nucleon,modeled on the base of event generator (left) and registered in the experiment NA49 (right).
Despite the fact that recent experiments STAR and PHENIX in a relativistic heavy ion collider RHIC [3-4] at 60-200A GeV and the ongoing experiment ALICE  at the Large Hadron Collider LHC at 1000-6000A GeV confirmed the formation of QGP in Au+Au and Pb+Pb collisions, theoretical description of this new state of nuclear matter is far from complete. Additional experimental data on nucleus-nucleus collisions at 2-10A GeV are required to define the boundary of the phase transition of nuclear matter to a quark-gluon plasma. This fact has stimulated the new SPS experiments to search for the critical point in the phase diagram (fig. 4) and measure the excitation function of nuclear matter (fig. 5). This will be achieved by the collaboration NA61 . In addition, new experiments are designed on the base of the accelerator complex FAIR  in Darmstadt (GSI) and NICA  in Dubna (JINR). Detailed studying of the phase diagram ofnuclear matter is the main goal of the russian megaproject NICA-MPD (Nuclotron-based Ion Collider fAcility with Multi-Purpose Detector) that will be implemented in 2016.
Fig.4. Phase diagram of water and nuclear matter.
Fig.5. Heating curvesof water andnuclear matter.
Experimentalsetupsfor the studyof QGP.
A large number of secondary particles are produced in central collisions of relativistic ions (fig.3). In connection with this an experimental setup, recording such a multiple production event should comprise tracking systems having large acceptance and high resolution.
Fig.6. Detector complex ALICE (left) and the structure of its inner tracker (right).
Fig.7. Detector complex NICA-MPD (left) and the structure of its internal tracker (right).
The main tracking detectors in ALICE (fig.6) and NICA-MPD (fig.7) are the time-projection chamber (TPC) and the inner tracker (IT) based on silicon pixel and microstrip detectors having the best spatial resolution at high counting rate of events. NA61 collaboration also plans to create a silicon vertex tracker in order to increase efficiency for short-lived products of nucleus-nucleus interactions. Working group under the leadership of V.P. Kondratyev participates in the design, modeling and modifying the vertex tracking systems of ALICE, NICA-MPD and NA61 collaborations. Accurate assessment of the tracking detectors efficiency plays a key role in the reconstruction of events, acquiring particular importance in the planning stage for future experiments (fig. 8).
Fig.8. Invariant mass spectrum for Λ (mΛ=1.116 ГэВ) in Au+Au at 7 GeV detected by means of their charged decay products Λ0 → p+ π-(left) and charged particle efficiency of MPD inner tracker as a function of particle momentum (right).
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