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Name of Excellence: Investigations with the CMS Detector at LHC

Author: Assoc. Prof. Dr. Leandar Litov, Atomic Physics Department

Research domain: Particle Physics


Assoc. Prof. Dr. Leandar Litov i s a lecturer in Particle Physics at the Atomic Physics Department, Faculty of Physics. Department head in the period 2007–2012.

His scientific interests are in the fi eld of particle physics, design and development of detectors of ionizing radiation, high-performance computing, and biophysics. He conducts investigations at the CERN accelerator complex since 1992.

L. Litov is a founding member of the CMS Collaboration, member of the CB and Team Leader of Sofia University team at LHC. Author and co-author of more than 400 publications. Laureate of a number of national and international scientific awards. Chair of the Institution Board of RPC CMS Collaboration. Member of the European Commission for Future Accelerators (ECFA) and of the CERN Council.

Assoc. Prof. Dr. Leandar Litov

Research team: Assoc. Prof. Dr. Leandar Litov ( team leader), Dr. A. Dimitrov, Head Ass. Prof. V. Kozhuharov, Head Ass. Prof. B. Pavlov, Head Ass. Prof. P. Petkov, R. Hadjiiska.


A complex detector for registration of elementary particles interactions (Compact Muon Solenoid (CMS)) performing investigations at the Large Hadron Collider (LHC) at CERN was designed and constructed. The CMS detector was taking data in the period 2010–2012. The physics results obtained were published in more than 200 scientific papers.

The Standard Model (SM) of electroweak and strong interactions was tested and many of its parameters (particle masses, cross-sections and decay probabilities, interaction constants, quark distributions etc.) were measured with unprecedented accuracy.

A new particle with mass 125.4 GeV having properties compatible with the one predicted by the SM for the Higgs boson was discovered. This result is considered as historical in the development of particle physics. For the fi rst time new resonant states (particles) with heavy quark constituents were observed and their masses were determined. The prediction of many models generalizing the SM (such as supersymmetric models, models with extra dimensions etc.) were tested and strong limits on their parameters were imposed.

New unexpected physics phenomena were observed in proton–lead and lead–lead collisions. A clear evidence for existence of quark-gluon plasma was obtained. Those results triggered a strong reaction and many discussions in the scientific community (more than 8000 citations) and were met with an exceptional interest by the society.

The contributions of Sofia University team are in the following: design, prototypes construction and investigation of their characteristics, production, construction and running of the CMS Hadron Calorimeter; design, production, tests, installation, commissioning and maintenance, and running of the Resistive Plate Chambers system (2000 m2); data taking, analysis and interpretation of physics results. At Sofia University campus the necessary infrastructure (CMS center and Grid cluster) ensuring the participation of the Bulgarian scientist in the data taking and analysis was established.

The CMS Collaboration includes scientists, engineers, technicians and students from 179 universities and institutes in 41 countries.

The work of Sofi a University team was carried out in the framework of the project “Physics investigations with the CMS detector at LHC” (2008-2012), supported by the Ministry of Education and Science.


Detector CMS


The CMS detector. Installation of the Central Tracker


Detector CMS sabitie


CMS reconstructed event - candidate for Higgs boson decaying to two photons


CMS center


The CMS center at Sofia University “St. Kliment Ohridski”