Sofia University

About: Sofia University is a education organization based out in Sofia, Bulgaria. It is known for research contribution in the topics: Large Hadron Collider & Laser. The organization has 8533 authors who have published 15730 publications receiving 306320 citations. The organization is also known as: University of Sofia & BFUS.

Measurement of the Isolated Prompt Photon Production Cross Section in $pp$ Collisions at $\sqrt{s} = 7$~TeV

Abstract: The differential cross section for the inclusive production of isolated prompt photons has been measured as a function of the photon transverse energy E_T-gamma in pp collisions at sqrt(s)=7 TeV using data recorded by the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 2.9 inverse picobarns. Photons are required to have a pseudorapidity |eta_gamma| 21 GeV, covering the kinematic region 0.006 < x_T < 0.086. The measured cross section is found to be in agreement with next-to-leading-order perturbative QCD calculations.

Search for decays of stopped long-lived particles produced in proton-proton collisions at sqrt(s) = 8 TeV

Abstract: A search has been performed for long-lived particles that could have come to rest within the CMS detector, using the time intervals between LHC beam crossings. The existence of such particles could be deduced from observation of their decays via energy deposits in the CMS calorimeter appearing at times that are well separated from any proton-proton collisions. Using a data set corresponding to an integrated luminosity of 18.6[Formula: see text] of 8[Formula: see text] proton-proton collisions, and a search interval corresponding to 281 h of trigger livetime, 10 events are observed, with a background prediction of [Formula: see text] events. Limits are presented at 95 % confidence level on gluino and top squark production, for over 13 orders of magnitude in the mean proper lifetime of the stopped particle. Assuming a cloud model of R-hadron interactions, a gluino with mass [Formula: see text]1000[Formula: see text] and a top squark with mass [Formula: see text]525[Formula: see text] are excluded, for lifetimes between 1 [Formula: see text]s and 1000[Formula: see text]. These results are the most stringent constraints on stopped particles to date.

Measurement of the B± Meson Nuclear Modification Factor in Pb-Pb Collisions at √sNN=5.02 TeV

Abstract: The differential production cross sections of B-+/- mesons are measured via the exclusive decay channels B-+/- -> J/psi K-+/- -> mu(+)mu K--(+/-) as a function of transverse momentum in pp and Pb-Pb collisions at a center-of-mass energy root s(NN) = 5.02 TeV per nucleon pair with the CMS detector at the LHC. The pp(Pb - Pb) data set used for this analysis corresponds to an integrated luminosity of 28.0 pb(-1) (351 mu b(-1)). The measurement is performed in the B-+/- meson transverse momentum range of 7 to 50 GeV/c, in the rapidity interval vertical bar y vertical bar < 2.4. In this kinematic range, a strong suppression of the production cross section by about a factor of 2 is observed in the Pb-Pb system in comparison to the expectation from pp reference data. These results are found to be roughly compatible with theoretical calculations incorporating beauty quark diffusion and energy loss in a quark-gluon plasma.

Measurement of prompt J/ψ pair production in pp collisions at √s = 7 Tev

Abstract: Production of prompt J/psi meson pairs in proton-proton collisions at sqrt(s)=7 TeV is measured with the CMS experiment at the LHC in a data sample corresponding to an integrated luminosity of about 4.7 inverse femtobarns. The two J/psi mesons are fully reconstructed via their decays into muon pairs. This observation provides for the first time access to the high-transverse-momentum region of J/psi pair production where model predictions are not yet established. The total and differential cross sections are measured in a phase space defined by the individual J/psi transverse momentum pt[J/psi] and rapidity abs(y[J/psi]): abs(y[J/psi]) 6.5 GeV; 1.2 4.5 GeV. The total cross section, assuming unpolarized prompt J/psi pair production is 1.49 +/- 0.07 (stat.) +/- 0.13 (syst.) nb. Different assumptions about the J/psi polarization imply modifications to the cross section ranging from -31% to +27%.

Measurement of the Splitting Function in pp and Pb-Pb Collisions at sNN =5.02 TeV

Abstract: Data from heavy ion collisions suggest that the evolution of a parton shower is modified by interactions with the color charges in the dense partonic medium created in these collisions, but it is not known where in the shower evolution the modifications occur. The momentum ratio of the two leading partons, resolved as subjets, provides information about the parton shower evolution. This substructure observable, known as the splitting function, reflects the process of a parton splitting into two other partons and has been measured for jets with transverse momentum between 140 and 500 GeV, in pp and PbPb collisions at a center-of-mass energy of 5.02 TeV per nucleon pair. In central PbPb collisions, the splitting function indicates a more unbalanced momentum ratio, compared to peripheral PbPb and pp collisions.. The measurements are compared to various predictions from event generators and analytical calculations.