Raja Baba - Academia.edu (original) (raw)

Papers by Raja Baba

The production cross section of electrons from semileptonic decays of beauty hadrons was measured... more The production cross section of electrons from semileptonic decays of beauty hadrons was measured at mid-rapidity (|y| < 0.8) in the transverse momentum range 1 < p T < 8 GeV/c with the ALICE experiment at the CERN LHC in pp collisions at a center of mass energy √ s = 7 TeV using an integrated luminosity of 2.2 nb −1 . Electrons from beauty hadron decays were selected based on the displacement of the decay vertex from the collision vertex. A perturbative QCD calculation agrees with the measurement within uncertainties. The data were extrapolated to the full phase space to determine the total cross section for the production of beauty quark-antiquark pairs. * See Appendix A for the list of collaboration members Electrons from beauty hadron decays in pp collisions at √ s = 7 TeV 1

Measurements of cross sections of inelastic and diffractive processes in proton-proton collisions... more Measurements of cross sections of inelastic and diffractive processes in proton-proton collisions at LHC energies were carried out with the ALICE detector. The fractions of diffractive processes in inelastic collisions were determined from a study of gaps in charged particle pseudorapidity distributions: for single diffraction (diffractive mass M X < 200 GeV/c 2 ) σ SD /σ INEL = 0.21 ± 0.03, 0.20 +0.07 −0.08 , and 0.20 +0.04 −0.07 , respectively at centre-of-mass energies

The elliptic, v 2 , triangular, v 3 , and quadrangular, v 4 , azimuthal anisotropic flow coeffici... more The elliptic, v 2 , triangular, v 3 , and quadrangular, v 4 , azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at √ s NN = 2.76 TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range |η| < 0.

The first measurement of neutron emission in electromagnetic dissociation of 208 Pb nuclei at the... more The first measurement of neutron emission in electromagnetic dissociation of 208 Pb nuclei at the LHC is presented. The measurement is performed using the neutron Zero Degree Calorimeters of the ALICE experiment, which detect neutral particles close to beam rapidity. The measured cross sections of single and mutual electromagnetic dissociation of Pb nuclei at √ s NN = 2.76 TeV with neutron emission are σ single EMD = 187.2 ± 0.2 (stat.) +13.8 −12.0 (syst.) b and σ mutual EMD = 6.2 ± 0.1 (stat.) ±0.4 (syst.) b respectively. The experimental results are compared to the predictions from a relativistic electromagnetic dissociation model.

The inclusive transverse momentum (p T ) distributions of primary charged particles are measured ... more The inclusive transverse momentum (p T ) distributions of primary charged particles are measured in the pseudo-rapidity range |η| < 0.8 as a function of event centrality in Pb-Pb collisions at √ s NN = 2.76 TeV with ALICE at the LHC. The data are presented in the p T range 0.15 < p T < 50 GeV/c for nine centrality intervals from 70-80% to 0-5%. The results in Pb-Pb are presented in terms of the nuclear modification factor R AA using a pp reference spectrum measured at the same collision energy. We observe that the suppression of high-p T particles strongly depends on event centrality. The yield is most suppressed in central collisions (0-5%) with R AA ≈ 0.13 at p T = 6-7 GeV/c. Above p T = 7 GeV/c, there is a significant rise in the nuclear modification factor, which reaches R AA ≈ 0.4 for p T > 30 GeV/c. In peripheral collisions (70-80%), only moderate suppression (R AA = 0.6-0.7) and a weak p T dependence is observed. The measured nuclear modification factors are compared to other measurements and model calculations.

Information Technology Journal, 2009

Journal of High Energy Physics, 2012

The effect of event background fluctuations on charged particle jet reconstruction in Pb-Pb colli... more The effect of event background fluctuations on charged particle jet reconstruction in Pb-Pb collisions at √ s NN = 2.76 TeV has been measured with the ALICE experiment. The main sources of nonstatistical fluctuations are characterized based purely on experimental data with an unbiased method, as well as by using single high p t particles and simulated jets embedded into real Pb-Pb events and reconstructed with the anti-k t jet finder. The influence of a low transverse momentum cut-off on particles used in the jet reconstruction is quantified by varying the minimum track p t between 0.15 GeV/c and 2 GeV/c. For embedded jets reconstructed from charged particles with p t > 0.15 GeV/c, the uncertainty in the reconstructed jet transverse momentum due to the heavy-ion background is measured to be 11.3 GeV/c (standard deviation) for the 10% most central Pb-Pb collisions, slightly larger than the value of 11.0 GeV/c measured using the unbiased method. For a higher particle transverse momentum threshold of 2 GeV/c, which will generate a stronger bias towards hard fragmentation in the jet finding process, the standard deviation of the fluctuations in the reconstructed jet transverse momentum is reduced to 4.8-5.0 GeV/c for the 10% most central events. A non-Gaussian tail of the momentum uncertainty is observed and its impact on the reconstructed jet spectrum is evaluated for varying particle momentum thresholds, by folding the measured fluctuations with steeply falling spectra.

The production of the prompt charm mesons D 0 , D + , D * + , and their antiparticles, was measur... more The production of the prompt charm mesons D 0 , D + , D * + , and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at the LHC, at a centre-of-mass energy √ s NN = 2.76 TeV per nucleon-nucleon collision. The p t -differential production yields in the range 2 < p t < 16 GeV/c at central rapidity, |y| < 0.5, were used to calculate the nuclear modification factor R AA with respect to a proton-proton reference obtained from the cross section measured at √ s = 7 TeV and scaled to √ s = 2.76 TeV. For the three meson species, R AA shows a suppression by a factor 3-4, for transverse momenta larger than 5 GeV/c in the 20% most central collisions. The suppression is reduced for peripheral collisions. * See Appendix A for the list of collaboration members Suppression of high p t D mesons in Pb-Pb collisions at √ s NN = 2.76 TeV 3 Suppression of high p t D mesons in Pb-Pb collisions at √ s NN = 2.76 TeV Suppression of high p t D mesons in Pb-Pb collisions at √ s NN = 2.76 TeV 7 central collisions), aimed at reducing the large combinatorial background.

Journal of High Energy Physics, 2012

The p t -differential inclusive production cross sections of the prompt charmed mesons D 0 , D + ... more The p t -differential inclusive production cross sections of the prompt charmed mesons D 0 , D + , and D * + in the rapidity range |y| < 0.5 were measured in proton-proton collisions at √ s = 7 TeV at the LHC using the ALICE detector. Reconstructing the decays D 0 → K − π + , D + → K − π + π + , D * + → D 0 π + , and their charge conjugates, about 8,400 D 0 , 2,900 D + , and 2,600 D * + mesons with 1 < p t < 24 GeV/c were counted, after selection cuts, in a data sample of 3.14 × 10 8 events collected with a minimum-bias trigger (integrated luminosity L int = 5 nb −1 ). The results are described within uncertainties by predictions based on perturbative QCD.

Physical Review Letters, 2011

Measurements of the sphericity of primary charged particles in minimum bias proton-proton collisi... more Measurements of the sphericity of primary charged particles in minimum bias proton-proton collisions at √ s = 0.9, 2.76 and 7 TeV with the ALICE detector at the LHC are presented. The observable is linearized to be collinear safe and is measured in the plane perpendicular to the beam direction using primary charged tracks with p T ≥ 0.5 GeV/c in |η| ≤ 0.8. The mean sphericity as a function of the charged particle multiplicity at mid-rapidity (N ch ) is reported for events with different p T scales ("soft" and "hard") defined by the transverse momentum of the leading particle. In addition, the mean charged particle transverse momentum versus multiplicity is presented for the different event classes, and the sphericity distributions in bins of multiplicity are presented. The data are compared with calculations of standard Monte Carlo event generators. The transverse sphericity is found to grow with multiplicity at all collision energies, with a steeper rise at low N ch , whereas the event generators show the opposite tendency. The combined study of the sphericity and the mean p T with multiplicity indicates that most of the tested event generators produce events with higher multiplicity by generating more back-to-back jets resulting in decreased sphericity (and isotropy). The PYTHIA6 generator with tune PERUGIA-2011 exhibits a noticeable improvement in describing the data, compared to the other tested generators.

The differential production cross section of electrons from semileptonic heavy-flavour hadron dec... more The differential production cross section of electrons from semileptonic heavy-flavour hadron decays has been measured at mid-rapidity (|y| < 0.5) in proton-proton collisions at √ s = 7 TeV with ALICE at the LHC. Data were collected in the transverse momentum range 0.5 < p t < 8 GeV/c. Predictions from a fixed order perturbative QCD calculation with next-to-leading-log resummation agree with the data within the theoretical and experimental uncertainties. Electrons from heavy-flavour decays in pp collisions at √ s = 7 TeV 1 Electrons from heavy-flavour decays in pp collisions at √ s = 7 TeV 25 the Ministry of Science and Technology of China (MSTC);

The ALICE experiment at the LHC has studied J/ψ production at mid-rapidity in pp collisions at √ ... more The ALICE experiment at the LHC has studied J/ψ production at mid-rapidity in pp collisions at √ s = 7 TeV through its electron pair decay on a data sample corresponding to an integrated luminosity L int = 5.6 nb −1 . The fraction of J/ψ from the decay of long-lived beauty hadrons was determined for J/ψ candidates with transverse momentum p t > 1.3 GeV/c and rapidity |y| < 0.9. The cross section for prompt J/ψ mesons, i.e. directly produced J/ψ and prompt decays of heavier charmonium states such as the ψ(2S) and χ c resonances, is

The production cross section of electrons from semileptonic decays of beauty hadrons was measured... more The production cross section of electrons from semileptonic decays of beauty hadrons was measured at mid-rapidity (|y| < 0.8) in the transverse momentum range 1 < p T < 8 GeV/c with the ALICE experiment at the CERN LHC in pp collisions at a center of mass energy √ s = 7 TeV using an integrated luminosity of 2.2 nb −1 . Electrons from beauty hadron decays were selected based on the displacement of the decay vertex from the collision vertex. A perturbative QCD calculation agrees with the measurement within uncertainties. The data were extrapolated to the full phase space to determine the total cross section for the production of beauty quark-antiquark pairs. * See Appendix A for the list of collaboration members Electrons from beauty hadron decays in pp collisions at √ s = 7 TeV 1

Measurements of cross sections of inelastic and diffractive processes in proton-proton collisions... more Measurements of cross sections of inelastic and diffractive processes in proton-proton collisions at LHC energies were carried out with the ALICE detector. The fractions of diffractive processes in inelastic collisions were determined from a study of gaps in charged particle pseudorapidity distributions: for single diffraction (diffractive mass M X < 200 GeV/c 2 ) σ SD /σ INEL = 0.21 ± 0.03, 0.20 +0.07 −0.08 , and 0.20 +0.04 −0.07 , respectively at centre-of-mass energies

The elliptic, v 2 , triangular, v 3 , and quadrangular, v 4 , azimuthal anisotropic flow coeffici... more The elliptic, v 2 , triangular, v 3 , and quadrangular, v 4 , azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at √ s NN = 2.76 TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range |η| < 0.

The first measurement of neutron emission in electromagnetic dissociation of 208 Pb nuclei at the... more The first measurement of neutron emission in electromagnetic dissociation of 208 Pb nuclei at the LHC is presented. The measurement is performed using the neutron Zero Degree Calorimeters of the ALICE experiment, which detect neutral particles close to beam rapidity. The measured cross sections of single and mutual electromagnetic dissociation of Pb nuclei at √ s NN = 2.76 TeV with neutron emission are σ single EMD = 187.2 ± 0.2 (stat.) +13.8 −12.0 (syst.) b and σ mutual EMD = 6.2 ± 0.1 (stat.) ±0.4 (syst.) b respectively. The experimental results are compared to the predictions from a relativistic electromagnetic dissociation model.

The inclusive transverse momentum (p T ) distributions of primary charged particles are measured ... more The inclusive transverse momentum (p T ) distributions of primary charged particles are measured in the pseudo-rapidity range |η| < 0.8 as a function of event centrality in Pb-Pb collisions at √ s NN = 2.76 TeV with ALICE at the LHC. The data are presented in the p T range 0.15 < p T < 50 GeV/c for nine centrality intervals from 70-80% to 0-5%. The results in Pb-Pb are presented in terms of the nuclear modification factor R AA using a pp reference spectrum measured at the same collision energy. We observe that the suppression of high-p T particles strongly depends on event centrality. The yield is most suppressed in central collisions (0-5%) with R AA ≈ 0.13 at p T = 6-7 GeV/c. Above p T = 7 GeV/c, there is a significant rise in the nuclear modification factor, which reaches R AA ≈ 0.4 for p T > 30 GeV/c. In peripheral collisions (70-80%), only moderate suppression (R AA = 0.6-0.7) and a weak p T dependence is observed. The measured nuclear modification factors are compared to other measurements and model calculations.

Information Technology Journal, 2009

Journal of High Energy Physics, 2012

The effect of event background fluctuations on charged particle jet reconstruction in Pb-Pb colli... more The effect of event background fluctuations on charged particle jet reconstruction in Pb-Pb collisions at √ s NN = 2.76 TeV has been measured with the ALICE experiment. The main sources of nonstatistical fluctuations are characterized based purely on experimental data with an unbiased method, as well as by using single high p t particles and simulated jets embedded into real Pb-Pb events and reconstructed with the anti-k t jet finder. The influence of a low transverse momentum cut-off on particles used in the jet reconstruction is quantified by varying the minimum track p t between 0.15 GeV/c and 2 GeV/c. For embedded jets reconstructed from charged particles with p t > 0.15 GeV/c, the uncertainty in the reconstructed jet transverse momentum due to the heavy-ion background is measured to be 11.3 GeV/c (standard deviation) for the 10% most central Pb-Pb collisions, slightly larger than the value of 11.0 GeV/c measured using the unbiased method. For a higher particle transverse momentum threshold of 2 GeV/c, which will generate a stronger bias towards hard fragmentation in the jet finding process, the standard deviation of the fluctuations in the reconstructed jet transverse momentum is reduced to 4.8-5.0 GeV/c for the 10% most central events. A non-Gaussian tail of the momentum uncertainty is observed and its impact on the reconstructed jet spectrum is evaluated for varying particle momentum thresholds, by folding the measured fluctuations with steeply falling spectra.

The production of the prompt charm mesons D 0 , D + , D * + , and their antiparticles, was measur... more The production of the prompt charm mesons D 0 , D + , D * + , and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at the LHC, at a centre-of-mass energy √ s NN = 2.76 TeV per nucleon-nucleon collision. The p t -differential production yields in the range 2 < p t < 16 GeV/c at central rapidity, |y| < 0.5, were used to calculate the nuclear modification factor R AA with respect to a proton-proton reference obtained from the cross section measured at √ s = 7 TeV and scaled to √ s = 2.76 TeV. For the three meson species, R AA shows a suppression by a factor 3-4, for transverse momenta larger than 5 GeV/c in the 20% most central collisions. The suppression is reduced for peripheral collisions. * See Appendix A for the list of collaboration members Suppression of high p t D mesons in Pb-Pb collisions at √ s NN = 2.76 TeV 3 Suppression of high p t D mesons in Pb-Pb collisions at √ s NN = 2.76 TeV Suppression of high p t D mesons in Pb-Pb collisions at √ s NN = 2.76 TeV 7 central collisions), aimed at reducing the large combinatorial background.

Journal of High Energy Physics, 2012

The p t -differential inclusive production cross sections of the prompt charmed mesons D 0 , D + ... more The p t -differential inclusive production cross sections of the prompt charmed mesons D 0 , D + , and D * + in the rapidity range |y| < 0.5 were measured in proton-proton collisions at √ s = 7 TeV at the LHC using the ALICE detector. Reconstructing the decays D 0 → K − π + , D + → K − π + π + , D * + → D 0 π + , and their charge conjugates, about 8,400 D 0 , 2,900 D + , and 2,600 D * + mesons with 1 < p t < 24 GeV/c were counted, after selection cuts, in a data sample of 3.14 × 10 8 events collected with a minimum-bias trigger (integrated luminosity L int = 5 nb −1 ). The results are described within uncertainties by predictions based on perturbative QCD.

Physical Review Letters, 2011

Measurements of the sphericity of primary charged particles in minimum bias proton-proton collisi... more Measurements of the sphericity of primary charged particles in minimum bias proton-proton collisions at √ s = 0.9, 2.76 and 7 TeV with the ALICE detector at the LHC are presented. The observable is linearized to be collinear safe and is measured in the plane perpendicular to the beam direction using primary charged tracks with p T ≥ 0.5 GeV/c in |η| ≤ 0.8. The mean sphericity as a function of the charged particle multiplicity at mid-rapidity (N ch ) is reported for events with different p T scales ("soft" and "hard") defined by the transverse momentum of the leading particle. In addition, the mean charged particle transverse momentum versus multiplicity is presented for the different event classes, and the sphericity distributions in bins of multiplicity are presented. The data are compared with calculations of standard Monte Carlo event generators. The transverse sphericity is found to grow with multiplicity at all collision energies, with a steeper rise at low N ch , whereas the event generators show the opposite tendency. The combined study of the sphericity and the mean p T with multiplicity indicates that most of the tested event generators produce events with higher multiplicity by generating more back-to-back jets resulting in decreased sphericity (and isotropy). The PYTHIA6 generator with tune PERUGIA-2011 exhibits a noticeable improvement in describing the data, compared to the other tested generators.

The differential production cross section of electrons from semileptonic heavy-flavour hadron dec... more The differential production cross section of electrons from semileptonic heavy-flavour hadron decays has been measured at mid-rapidity (|y| < 0.5) in proton-proton collisions at √ s = 7 TeV with ALICE at the LHC. Data were collected in the transverse momentum range 0.5 < p t < 8 GeV/c. Predictions from a fixed order perturbative QCD calculation with next-to-leading-log resummation agree with the data within the theoretical and experimental uncertainties. Electrons from heavy-flavour decays in pp collisions at √ s = 7 TeV 1 Electrons from heavy-flavour decays in pp collisions at √ s = 7 TeV 25 the Ministry of Science and Technology of China (MSTC);

The ALICE experiment at the LHC has studied J/ψ production at mid-rapidity in pp collisions at √ ... more The ALICE experiment at the LHC has studied J/ψ production at mid-rapidity in pp collisions at √ s = 7 TeV through its electron pair decay on a data sample corresponding to an integrated luminosity L int = 5.6 nb −1 . The fraction of J/ψ from the decay of long-lived beauty hadrons was determined for J/ψ candidates with transverse momentum p t > 1.3 GeV/c and rapidity |y| < 0.9. The cross section for prompt J/ψ mesons, i.e. directly produced J/ψ and prompt decays of heavier charmonium states such as the ψ(2S) and χ c resonances, is