Papers by Gabriela Agostinelli
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
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 ALICE Collaboration has measured inclusive J/ψ production in pp collisions at a center of mas... more The ALICE Collaboration has measured inclusive J/ψ production in pp collisions at a center of mass energy √ s = 2.76 TeV at the LHC. The results presented in this Letter refer to the rapidity ranges |y| < 0.9 and 2.5 < y < 4 and have been obtained by measuring the electron and muon pair decay channels, respectively. The integrated luminosities for the two channels are L e int = 1.1 nb −1 and L µ int = 19.9 nb −1 , and the corresponding signal statistics are N e + e − J/ψ = 59 ± 14 and N µ + µ − J/ψ = 1364 ± 53. We present dσ J/ψ /dy for the two rapidity regions under study and, for the forward-y range, d 2 σ J/ψ /dydp t in the transverse momentum domain 0 < p t < 8 GeV/c. The results are compared with previously published results at √ s = 7 TeV and with theoretical calculations.
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 Collaboration has studied J/ψ production in pp collisions at √ s = 7 TeV at the LHC thr... more The ALICE Collaboration has studied J/ψ production in pp collisions at √ s = 7 TeV at the LHC through its muon pair decay. The polar and azimuthal angle distributions of the decay muons were measured, and results on the J/ψ polarization parameters λ θ and λ φ were obtained. The study was performed in the kinematic region 2.5 < y < 4, 2 < p t < 8 GeV/c, in the helicity and Collins-Soper reference frames. In both frames, the polarization parameters are compatible with zero, within uncertainties. * See Appendix A for the list of collaboration members J/ψ polarization in pp collisions at √ s=7 TeV 3
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 ALICE experiment has measured low-mass dimuon production in pp collisions at √ s = 7 TeV in t... more The ALICE experiment has measured low-mass dimuon production in pp collisions at √ s = 7 TeV in the dimuon rapidity region 2.5 < y < 4. The observed dimuon mass spectrum is described as a superposition of resonance decays (η, ρ, ω, η ′ , φ ) into muons and semi-leptonic decays of charmed mesons. The measured production cross sections for ω and φ are σ ω (1 < p t < 5 GeV/c, 2.5 < y < 4) = 5.28 ± 0.54(stat) ± 0.50(syst) mb and σ φ (1 < p t < 5 GeV/c, 2.5 < y < 4) = 0.940 ± 0.084(stat) ± 0.078(syst) mb. The differential cross sections d 2 σ /dydp t are extracted as a function of p t for ω and φ . The ratio between the ρ and ω cross section is obtained. Results for the φ are compared with other measurements at the same energy and with predictions by models.
Angular correlations between unidentified charged trigger (t) and associated (a) particles are me... more Angular correlations between unidentified charged trigger (t) and associated (a) particles are measured by the ALICE experiment in Pb-Pb collisions at √ s NN = 2.76 TeV for transverse momenta 0.25 < p t, a T < 15 GeV/c, where p t T > p a T . The shapes of the pair correlation distributions are studied in a variety of collision centrality classes between 0 and 50% of the total hadronic cross section for particles in the pseudorapidity interval |η| < 1.0. Distributions in relative azimuth ∆φ ≡ φ t − φ a are analyzed for |∆η| ≡ |η t − η a | > 0.8, and are referred to as "long-range correlations". Fourier components V n∆ ≡ cos(n∆φ ) are extracted from the long-range azimuthal correlation functions.If particle pairs are correlated to one another through their individual correlation to a common symmetry plane, then the pair anisotropy V n∆
The p T -differential inclusive production cross section of the prompt charm-strange meson D + s ... more The p T -differential inclusive production cross section of the prompt charm-strange meson D + s in the rapidity range |y| < 0.5 was measured in proton-proton collisions at √ s = 7 TeV at the LHC using the ALICE detector. The analysis was performed on a data sample of 2.98×10 8 events collected with a minimum-bias trigger. The corresponding integrated luminosity is L int = 4.8 nb −1 . Reconstructing the decay D + s → φ π + , with φ → K − K + , and its charge conjugate, about 480 D ± s mesons were counted, after selection cuts, in the transverse momentum range 2 < p T < 12 GeV/c. The results are compared with the prediction of a model based on perturbative QCD. The ratios of the cross sections of four D meson species (namely D 0 , D + , D * + and D + s ) were determined both as a function of p T and integrated over p T after extrapolating to full p T range, together with the strangeness suppression factor in charm fragmentation. The obtained values are found to be compatible within uncertainties with those measured by other experiments in e + e − , ep and pp interactions at various centre-of-mass energies.
The ALICE experiment at the LHC has studied inclusive J/ψ production at central and forward rapid... more The ALICE experiment at the LHC has studied inclusive J/ψ production at central and forward rapidities in pp collisions at √ s = 7 TeV. In this Letter, we report on the first results obtained detecting the J/ψ through the dilepton decay into e + e − and µ + µ − pairs in the rapidity ranges |y| < 0.9 and 2.5 < y < 4, respectively, and with acceptance down to zero p T . In the dielectron channel the analysis was carried out on a data sample corresponding to an integrated luminosity L int = 5.6 nb −1 and the number of signal events is N J/ψ = 352 ± 32 (stat.) ± 28 (syst.); the corresponding figures in the dimuon channel are L int = 15.6 nb −1 and N J/ψ = 1924 ± 77 (stat.) ± 144 (syst.). The measured production cross sections are σ J/ψ (|y| < 0.9) = 12.4 ± 1.1 (stat.) ± 1.8 (syst.) +1.8 −2.7 (syst.pol.) µb and σ J/ψ (2.5 < y < 4) = 6.31 ± 0.25 (stat.) ± 0.76 (syst.) +0.95 −1.96 (syst.pol.) µb. The differential cross sections, in transverse momentum and rapidity, of the J/ψ were also measured.
Physical Review Letters, 2011
The production of K * (892) 0 and φ (1020) in pp collisions at √ s=7 TeV was measured by the ALIC... more The production of K * (892) 0 and φ (1020) in pp collisions at √ s=7 TeV was measured by the ALICE experiment at the LHC. The yields and the transverse momentum spectra d 2 N/dydp T at midrapidity |y|< 0.5 in the range 0<p T <6 GeV/c for K * (892) 0 and 0.4<p T <6 GeV/c for φ (1020) are reported and compared to model predictions. Using the yield of pions, kaons, and Ω baryons measured previously by ALICE at √ s=7 TeV, the ratios K ratios are similar to those found at lower centre-of-mass energies. In contrast, the φ /π − ratio, which has been observed to increase with energy, seems to saturate above 200 GeV. The (Ω + Ω)/φ ratio in the p T range 1-5 GeV/c is found to be in good agreement with the prediction of the HIJING/BB v2.0 model with a strong colour field.
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.
We present measurements of Underlying Event observables in pp collisions at √ s = 0.9 and 7 TeV. ... more We present measurements of Underlying Event observables in pp collisions at √ s = 0.9 and 7 TeV. The analysis is performed as a function of the highest charged-particle transverse momentum p T,LT in the event. Different regions are defined with respect to the azimuthal direction of the leading (highest transverse momentum) track: Toward, Transverse and Away. The Toward and Away regions collect the fragmentation products of the hardest partonic interaction. The Transverse region is expected to be most sensitive to the Underlying Event activity. The study is performed with charged particles above three different p T thresholds: 0.15, 0.5 and 1.0 GeV/c. In the Transverse region we observe an increase in the multiplicity of a factor 2-3 between the lower and higher collision energies, depending on the track p T threshold considered. Data are compared to PYTHIA 6.4, PYTHIA 8.1 and PHOJET. On average, all models considered underestimate the multiplicity and summed p T in the Transverse region by about 10-30%.
Physics Letters B, 2012
The ALICE Collaboration reports the measurement of the relative J/ψ yield as a function of charge... more The ALICE Collaboration reports the measurement of the relative J/ψ yield as a function of charged particle pseudorapidity density dN ch /dη in pp collisions at √ s = 7 TeV at the LHC. J/ψ particles are detected for p t > 0, in the rapidity interval |y| < 0.9 via decay into e + e − , and in the interval 2.5 < y < 4.0 via decay into μ + μ − pairs. An approximately linear increase of the J/ψ yields normalized to their event average (dN J/ψ /dy)/ dN J/ψ /dy with (dN ch /dη)/ dN ch /dη is observed in both rapidity ranges, where dN ch /dη is measured within |η| < 1 and p t > 0. In the highest multiplicity interval with dN ch /dη(bin) = 24.1, corresponding to four times the minimum bias multiplicity density, an enhancement relative to the minimum bias J/ψ yield by a factor of about 5 at 2.5 < y < 4 (8 at |y| < 0.9) is observed.
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 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.
In this Letter we report the first results on π ± , K ± , p and p production at mid-rapidity (|y|... more In this Letter we report the first results on π ± , K ± , p and p production at mid-rapidity (|y| < 0.5) in central Pb-Pb collisions at √ s NN = 2.76 TeV, measured by the ALICE experiment at the LHC. The p T distributions and yields are compared to previous results at √ s NN = 200 GeV and expectations from hydrodynamic and thermal models. The spectral shapes indicate a strong increase of the radial flow velocity with √ s NN , which in hydrodynamic models is expected as a consequence of the increasing particle density. While the K/π ratio is in line with predictions from the thermal model, the p/π ratio is found to be lower by a factor of about 1.5. This deviation from thermal model expectations is still to be understood. π, K, and p production in central Pb-Pb collisions at √ s NN = 2.76 TeV 1
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Papers by Gabriela Agostinelli