Measurement of charm production at central rapidity in proton-proton collisions at sqrt(s) = 7 TeV

The pt-differential inclusive production cross sections of the prompt charmed mesons D0, 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 D0 → K−π+, D+ → K−π+π+, D∗+ → D0π+, and their charge conjugates, about 8, 400 D0, 2, 900 D+, and 2, 600 D∗+ mesons with 1 < pt < 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 Lint = 5 nb−1). The results are described within uncertainties by predictions based on perturbative QCD

Charged-particle multiplicity density at midrapidity in central Pb-Pb collisions at √sNN=2.76 TeV

The first measurement of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair √sNN=2.76 TeV is presented. For an event sample corresponding to the most central 5% of the hadronic cross section, the pseudorapidity density of primary charged particles at midrapidity is 1584±4(stat)±76(syst), which corresponds to 8.3±0.4(syst) per participating nucleon pair. This represents an increase of about a factor 1.9 relative to pp collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at √sNN=0.2  TeV. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies

Light vector meson production in pp collisions at √s = 7 TeV

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 σω(1 < pt <5 GeV/c, 2.5 < y < 4) = 5.28±0.54(stat)±0.49(syst) mb and σφ(1 < pt <5 GeV/c, 2.5 < y < 4) = 0.940 ± 0.084(stat) ± 0.076(syst) mb. The differential cross sections d2σ/dy dpt are extracted as a function of pt 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

Midrapidity Antiproton-to-Proton Ratio in pp Collisons at √s=0.9 and 7 TeV Measured by the ALICE Experiment

The ratio of the yields of antiprotons to protons in pp collisions has been measured by the ALICE experiment at √s=0.9 and 7 TeV during the initial running periods of the Large Hadron Collider. The measurement covers the transverse momentum interval 0.45&lt;pt&lt;1.05  GeV/c and rapidity |y|&lt;0.5. The ratio is measured to be R|y|&lt;0.5=0.957±0.006(stat)±0.014(syst) at 0.9 TeV and R|y|&lt;0.5=0.991±0.005(stat)±0.014(syst) at 7 TeV and it is independent of both rapidity and transverse momentum. The results are consistent with the conventional model of baryon-number transport and set stringent limits on any additional contributions to baryon-number transfer over very large rapidity intervals in pp collisions

Neutral pion and η meson production in proton– proton collisions at √s = 0.9 TeV and √s = 7 TeV

The first measurements of the invariant differential cross sections of inclusive π0 and η meson production at mid-rapidity in proton–proton collisions at √s = 0.9 TeV and √s = 7 TeV are reported. The π0 measurement covers the ranges 0.4 &lt; pT &lt; 7 GeV/c and 0.3 &lt; pT &lt; 25 GeV/c for these two energies, respectively. The production of η mesons was measured at √s = 7 TeV in the range 0.4 &lt; pT &lt; 15 GeV/c. Next-to-Leading Order perturbative QCD calculations, which are consistent with the π0 spectrum at √s = 0.9 TeV, overestimate those of π0 and η mesons at √s = 7 TeV, but agree with the measured η/π0 ratio at √s =7 TeV

Suppression of high transverse momentum D mesons in central Pb-Pb collisions at psNN = 2.76TeV

The production of the prompt charm mesons D0, D+, D∗+, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at the LHC, at a centre-of-mass energy √sNN = 2.76 TeV per nucleon- nucleon collision. The pt-differential production yields in the range 2 &lt; pt &lt; 16 GeV/c at central rapidity, |y| &lt; 0.5, were used to calculate the nuclear modification factor RAA 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, RAA 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

Femtoscopy of pp collisions at √s=0.9 and 7 TeV at the LHC with two-pion Bose-Einstein correlations

We report on the high statistics two-pion correlation functions from pp collisions at √s=0.9 TeV and √s=7 TeV, measured by the ALICE experiment at the Large Hadron Collider. The correlation functions as well as the extracted source radii scale with event multiplicity and pair momentum. When analyzed in the same multiplicity and pair transverse momentum range, the correlation is similar at the two collision energies. A three-dimensional femtoscopic analysis shows an increase of the emission zone with increasing event multiplicity as well as decreasing homogeneity lengths with increasing transverse momentum. The latter trend gets more pronounced as multiplicity increases. This suggests the development of space-momentum correlations, at least for collisions producing a high multiplicity of particles. We consider these trends in the context of previous femtoscopic studies in high-energy hadron and heavy-ion collisions and discuss possible underlying physics mechanisms. Detailed analysis of the correlation reveals an exponential shape in the outward and longitudinal directions, while the sideward remains a Gaussian. This is interpreted as a result of a significant contribution of strongly decaying resonances to the emission region shape. Significant nonfemtoscopic correlations are observed, and are argued to be the consequence of “mini-jet”-like structures extending to low pt. They are well reproduced by the Monte-Carlo generators and seen also in π+π- correlations

Measurement of the cross section for electromagnetic dissociation with neutron emission in Pb-Pb collisions at √sNN=2.76 TeV

The first measurement of neutron emission in electromagnetic dissociation of 208Pb 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 √sNN=2.76  TeV with neutron emission are σsingleEMD=187.4±0.2(stat)-11.2+13.2(syst)  b and σmutualEMD=5.7±0.1(stat)±0.4(syst)  b, respectively. The experimental results are compared to the predictions from a relativistic electromagnetic dissociation model

Underlying Event measurements in pp collisions at sqrt(s) = 0.9 and 7 TeV with the ALICE experiment at the LHC

We present measurements of Underlying Event observables in pp collisions at sqrt(s)=0.9 and 7 TeV. The analysis is performed as a function of the highest charged-particle transverse momentum pT, 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 pT 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 pT threshold considered. Data are compared to Pythia 6.4, Pythia 8.1 and Phojet. On average, all models considered underestimate the multiplicity and summed pT in the Transverse region by about 10–30%

Erratum to “Rapidity and transverse momentum dependence of inclusive J/ψJ/ψ production in pp collisions at root s = 7 TeV

We have identified a bias in the calculation of the cross section of J/ψJ/ψ production measured in the e+e−e+e− channel at central rapidity (|y|<0.9|y|<0.9) [1]. The acceptance and efficiency corrections were evaluated using a Monte Carlo simulation, based on PYTHIA [2] and [3], which did not include the radiative decays of J/ψJ/ψ (J/ψ→e+e−γJ/ψ→e+e−γ). In these decays the reconstructed dielectron invariant mass, me+e−me+e−, is biased towards smaller values than the nominal J/ψJ/ψ mass, since the photon contribution is neglected. As a consequence, the fraction of signal events in the me+e−me+e− range 2.92–3.16 GeV/c2 was overestimated by about 10%. Moreover, the requirement of the transverse momentum of the daughter electrons being larger than 1 GeV/c is more selective for radiative than non-radiative J/ψJ/ψ decays. Therefore, the J/ψJ/ψ acceptance was also overestimated by about 5%