28 research outputs found
Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at TeV
The elliptic, , triangular, , and quadrangular, , azimuthal
anisotropic flow coefficients are measured for unidentified charged particles,
pions and (anti-)protons in Pb-Pb collisions at 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 at different collision centralities and as a
function of transverse momentum, , out to GeV/.
The observed non-zero elliptic and triangular flow depends only weakly on
transverse momentum for GeV/. The small dependence
of the difference between elliptic flow results obtained from the event plane
and four-particle cumulant methods suggests a common origin of flow
fluctuations up to GeV/. The magnitude of the (anti-)proton
elliptic and triangular flow is larger than that of pions out to at least
GeV/ indicating that the particle type dependence persists out
to high .Comment: 16 pages, 5 captioned figures, authors from page 11, published
version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/186
Transverse sphericity of primary charged particles in minimum bias proton-proton collisions at , 2.76 and 7 TeV
Measurements of the sphericity of primary charged particles in minimum bias
proton--proton collisions at , 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 GeV/c in . The
mean sphericity as a function of the charged particle multiplicity at
mid-rapidity () is reported for events with different
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 , whereas the event generators show the
opposite tendency. The combined study of the sphericity and the mean 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.Comment: 21 pages, 9 captioned figures, 3 tables, authors from page 16,
published version, figures from
http://aliceinfo.cern.ch/ArtSubmission/node/308
Measurement of jet suppression in central Pb-Pb collisions at root s(NN)=2.76 TeV
The transverse momentum(p(T)) spectrum and nuclear modification factor (R-AA) of reconstructed jets in 0-10% and 10-30% central Pb-Pb collisions at root s(NN) = 2.76 TeV were measured. Jets were reconstructed using the anti-k(T) jet algorithm with a resolution parameter of R = 0.2 from charged and neutral particles, utilizing the ALICE tracking detectors and Electromagnetic Calorimeter (EMCal). The jet p(T) spectra are reported in the pseudorapidity interval of \eta(jet)\ 5 GeV/c to suppress jets constructed from the combinatorial background in Pb-Pb collisions. The leading charged particle requirement applied to jet spectra both in pp and Pb-Pb collisions had a negligible effect on the R-AA. The nuclear modification factor R-AA was found to be 0.28 +/- 0.04 in 0-10% and 0.35 +/- 0.04 in 10-30% collisions, independent of p(T), jet within the uncertainties of the measurement. The observed suppression is in fair agreement with expectations from two model calculations with different approaches to jet quenching. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe
J/psi Production as a Function of Charged Particle Multiplicity in pp Collisions at sqrt{s} = 7 TeV
The ALICE collaboration reports the measurement of the inclusive J/psi yield
as a function of charged particle pseudorapidity density dN_{ch}/deta in pp
collisions at sqrt{s} = 7 TeV at the LHC. J/psi 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 mu+mu- pairs. An approximately linear
increase of the J/psi yields normalized to their event average
(dN_{J/psi}/dy)/ with (dN_{ch}/deta)/ is observed
in both rapidity ranges, where dN_{ch}/deta is measured within |eta| < 1 and
p_t > 0. In the highest multiplicity interval with = 24.1,
corresponding to four times the minimum bias multiplicity density, an
enhancement relative to the minimum bias J/psi yield by a factor of about 5 at
2.5 < y < 4 (8 at |y| < 0.9) is observed.Comment: Submitted to Phys. Lett.
Measurement of event background fluctuations for charged particle jet reconstruction in Pb-Pb collisions at TeV
The effect of event background fluctuations on charged particle jet
reconstruction in Pb-Pb collisions at TeV has been
measured with the ALICE experiment. The main sources of non-statistical
fluctuations are characterized based purely on experimental data with an
unbiased method, as well as by using single high particles and
simulated jets embedded into real Pb-Pb events and reconstructed with the
anti- 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 between 0.15 GeV/ and 2 GeV/. For embedded jets
reconstructed from charged particles with GeV/, the
uncertainty in the reconstructed jet transverse momentum due to the heavy-ion
background is measured to be 11.3 GeV/ (standard deviation) for the 10% most
central Pb-Pb collisions, slightly larger than the value of 11.0 GeV/
measured using the unbiased method. For a higher particle transverse momentum
threshold of 2 GeV/, 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/ 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.Comment: 21 pages, 5 captioned figures, 3 tables, authors from page 16,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/350
Transverse momentum spectra of charged particles in proton-proton collisions at GeV with ALICE at the LHC
The inclusive charged particle transverse momentum distribution is measured
in proton-proton collisions at GeV at the LHC using the ALICE
detector. The measurement is performed in the central pseudorapidity region
over the transverse momentum range GeV/.
The correlation between transverse momentum and particle multiplicity is also
studied. Results are presented for inelastic (INEL) and non-single-diffractive
(NSD) events. The average transverse momentum for is (stat.) (syst.) GeV/ and
\left_{\rm NSD}=0.489\pm0.001 (stat.) (syst.)
GeV/, respectively. The data exhibit a slightly larger than measurements in wider pseudorapidity intervals. The results are
compared to simulations with the Monte Carlo event generators PYTHIA and
PHOJET.Comment: 20 pages, 8 figures, 2 tables, published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/390
Inclusive J/psi production in pp collisions at sqrt(s) = 2.76 TeV
The ALICE Collaboration has measured inclusive J/psi production in pp
collisions at a center of mass energy sqrt(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^mu_int=19.9 nb^-1, and the corresponding signal statistics are
N_J/psi^e+e-=59 +/- 14 and N_J/psi^mu+mu-=1364 +/- 53. We present
dsigma_J/psi/dy for the two rapidity regions under study and, for the forward-y
range, d^2sigma_J/psi/dydp_t in the transverse momentum domain 0<p_t<8 GeV/c.
The results are compared with previously published results at sqrt(s)=7 TeV and
with theoretical calculations.Comment: 7 figures, 3 tables, accepted for publication in Phys. Lett.
Measurement of the production of high-pT electrons from heavy-flavour hadron decays in Pb–Pb collisions at s
Electrons from heavy-flavour hadron decays (charm and beauty) were measured with the ALICE detector in Pb–Pb collisions at a centre-of-mass of energy sNN=2.76 TeV. The transverse momentum (pT) differential production yields at mid-rapidity were used to calculate the nuclear modification factor RAA in the interval 3<pT<18 GeV/c. The RAA shows a strong suppression compared to binary scaling of pp collisions at the same energy (up to a factor of 4) in the 10% most central Pb–Pb collisions. There is a centrality trend of suppression, and a weaker suppression (down to a factor of 2) in semi-peripheral (50–80%) collisions is observed. The suppression of electrons in this broad pT interval indicates that both charm and beauty quarks lose energy when they traverse the hot medium formed in Pb–Pb collisions at LHC