8 Search Results

The production of $π^+$, $π^–$, $$\hbox{K}^+$$, $$\hbox{K}^–$$, $$p$$, and $$\overline p$$ at mid-rapidity has been measured in proton-proton collisions at $$\sqrt{s}$$ = 900 GeV with the ALICE detector. Particle identification is performed using the specific energy loss in the inner tracking silicon detector and the time projection chamber. In addition, time-of-flight information is used to identify hadrons at higher momenta. Finally, the distinctive kink topology of the weak decay of charged kaons is used for an alternative measurement of the kaon transverse momentum ($$p_t$$) spectra. Since these various particle identification tools give the best separation capabilities over different momentum ranges,more » the results are combined to extract spectra from $$p_t$$=100 MeV/c to 2.5 GeV/c. The measured spectra are further compared with QCD-inspired models which yield a poor description. The total yields and the mean $$p_t$$ are compared with previous measurements, and the trends as a function of collision energy are discussed.« less

The production of mesons containing strange quarks ($$K^0_S$$, Φ) and both singly and doubly strange baryons ($$Λ, \overline Λ$$, and $$\Xi^– + \overline {\Xi}^+$$) are measured at mid-rapidity in pp collisions at $$\sqrt{s}$$ = 0.9 TeV with the $$\hbox{ALICE}$$ experiment at the $$\hbox{LHC}$$. The results are obtained from the analysis of about 250 k minimum bias events recorded in 2009. Measurements of yields (d$$N$$ / d$$y$$) and transverse momentum spectra at mid-rapidity for inelastic pp collisions are presented. For mesons, we report yields ($$\langle$$ d$$N$$ / d$$y\rangle$$) of $0.184 ± 0.002(stat.) ± 0.006(syst.)$ for $$K^0_S$$ and $0.021 ± 0.004(stat.) ±more » 0.003(syst.)$ for Φ. For baryons, we find ($$\langle$$ d$$N$$ / d$$y\rangle$$) = $0.048 ± 0.001(stat.)± 0.004(syst.)$ for $$Λ$$, $0.047 ± 0.002(stat.) ± 0.005(syst.)$ for $$\overline Λ$$ and $0.0101 ± 0.0020(stat.) ± 0.0009(syst.)$ for $$\Xi^– + \overline {\Xi}^+$$. The results are also compared with predictions for identified particle spectra from $$\hbox{QCD}$$-inspired models and provide a baseline for comparisons with both future pp measurements at higher energies and heavy-ion collisions.« less

The pseudorapidity density and multiplicity distribution of charged particles produced in proton-proton collisions at the LHC, at a centre-of-mass energy $$\sqrt{s}=7$$ TeV, were measured in the central pseudorapidity region |η| < 1. Comparisons are made with previous measurements at $$\sqrt{s}=0.9$$ TeV and 2.36 TeV. At $$\sqrt{s}=7$$ TeV, for events with at least one charged particle in |η| < 1, we obtain dN_ch/dη = 6.01 ± 0.01(stat.)$$\mathbb +0.20\atop{-0.12} $$(syst.). This corresponds to an increase of 57.6% ± 0.4%(stat.)$$\mathbb +3.6\atop{-1.8} $$(syst.) relative to collisions at 0.9 TeV, significantly higher than calculations from commonly used models. The multiplicity distribution at 7 TeV ismore » described fairly well by the negative binomial distribution.« less

Charged-particle production was studied in proton-proton collisions collected at the LHC with the ALICE detector at centre-of-mass energies 0.9 TeV and 2.36 TeV in the pseudorapidity range |η|<1.4. In the central region (|η|<0.5), at 0.9 TeV, we measure charged-particle pseudorapidity density dNch/dη= 3.02 ± 0.01 (stat.)$$ +0.08\atop{-0.05} $$(syst.) for inelastic interactions, and dN_ch/dη = 3.58 ± 0.01 (stat.)$$ +0.12\atop{-0.12} $$(syst.) for non-single-diffractive interactions. At 2.36 TeV, we find dN_ch/dη = 3.77 ± 0.01 (stat.)$$ +0.25\atop{-0.12} $$(syst.) for inelastic, and dN_ch/dη = 4.43 ± 0.01(stat.)$$ +0.17\atop{-0.12} $$(syst.) for non-single-diffractive collisions. The relative increase in charged-particle multiplicity from the lower to highermore » energy is 24.7% ± 0.5%(stat.)$$ +5.7\atop{-2.8} $$%(syst.) for inelastic and 23.7% ± 0.5%(stat.)$$ +4.6\atop{-1.1} $$% 1.1%(syst.) for non-single-diffractive interactions. This increase is consistent with that reported by the CMS collaboration for non-single-diffractive events and larger than that found by a number of commonly used models. The multiplicity distribution was measured in different pseudorapidity intervals and studied in terms of KNO variables at both energies. The results are compared to proton-antiproton data and to model predictions.« less

On 23rd November 2009, during the early commissioning of the CERN Large Hadron Collider (LHC), two counter-rotating proton bunches were circulated for the first time concurrently in the machine, at the LHC injection energy of 450 GeV per beam. Although the proton intensity was very low, with only one pilot bunch per beam, and no systematic attempt was made to optimize the collision optics, all LHC experiments reported a number of collision candidates. In the ALICE experiment, the collision region was centred very well in both the longitudinal and transverse directions and 284 events were recorded in coincidence with themore » two passing proton bunches. The events were immediately reconstructed and analyzed both online and offline. We have used these events to measure the pseudorapidity density of charged primary particles in the central region. In the range |η|<0.5, we obtain dN_ch/dη=3. 10±0. 13(stat.)±0.22(syst.) for all inelastic interactions, and dN_ch/dη=3.51±0. 15(stat.)±0. 25(syst.) for non-single diffractive interactions. These results are consistent with previous measurements in proton-antiproton interactions at the same centre-of-mass energy at the CERN Sp$$\bar{p}$$S collider. They also illustrate the excellent functioning and rapid progress of the LHC accelerator, and of both the hardware and software of the ALICE experiment, in this early start-up phase.« less