1,733 research outputs found
Measurement of an excess in the yield of J/ at very low in Pb-Pb collisions at = 2.76 TeV
We report on the first measurement of an excess in the yield of J/ at
very low transverse momentum ( GeV/) in peripheral hadronic
Pb-Pb collisions at = 2.76 TeV, performed by ALICE at the
CERN LHC. Remarkably, the measured nuclear modification factor of J/ in
the rapidity range reaches about 7 (2) in the range 0-0.3
GeV/ in the 70-90% (50-70%) centrality class. The J/ production cross
section associated with the observed excess is obtained under the hypothesis
that coherent photoproduction of J/ is the underlying physics mechanism.
If confirmed, the observation of J/ coherent photoproduction in Pb-Pb
collisions at impact parameters smaller than twice the nuclear radius opens new
theoretical and experimental challenges and opportunities. In particular,
coherent photoproduction accompanying hadronic collisions may provide insight
into the dynamics of photoproduction and nuclear reactions, as well as become a
novel probe of the Quark-Gluon Plasma.Comment: 18 pages, 3 captioned figures, 1 table, authors from page 13,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/191
First measurement of production in pp collisions at = 7 TeV
The production of the charm-strange baryon is measured for
the first time at the LHC via its semileptonic decay into e
in pp collisions at TeV with the ALICE detector. The transverse
momentum () differential cross section multiplied by the branching
ratio is presented in the interval 1 8 GeV/ at
mid-rapidity, 0.5. The transverse momentum dependence of the
baryon production relative to the D meson production is
compared to predictions of event generators with various tunes of the
hadronisation mechanism, which are found to underestimate the measured
cross-section ratio.Comment: 22 pages, 6 captioned figures, 1 table, authors from page 17,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/412
Constraining the magnitude of the Chiral Magnetic Effect with Event Shape Engineering in Pb-Pb collisions at = 2.76$ TeV
In ultrarelativistic heavy-ion collisions, the event-by-event variation of
the elliptic flow reflects fluctuations in the shape of the initial state
of the system. This allows to select events with the same centrality but
different initial geometry. This selection technique, Event Shape Engineering,
has been used in the analysis of charge-dependent two- and three-particle
correlations in Pb-Pb collisions at TeV. The
two-particle correlator ,
calculated for different combinations of charges and , is
almost independent of (for a given centrality), while the three-particle
correlator
scales almost linearly both with the event and charged-particle
pseudorapidity density. The charge dependence of the three-particle correlator
is often interpreted as evidence for the Chiral Magnetic Effect (CME), a parity
violating effect of the strong interaction. However, its measured dependence on
points to a large non-CME contribution to the correlator. Comparing the
results with Monte Carlo calculations including a magnetic field due to the
spectators, the upper limit of the CME signal contribution to the
three-particle correlator in the 10-50% centrality interval is found to be
26-33% at 95% confidence level.Comment: 20 pages, 6 captioned figures, 1 tables, authors from page 15,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/382
INFN What Next: Ultra-relativistic Heavy-Ion Collisions
This document was prepared by the community that is active in Italy, within
INFN (Istituto Nazionale di Fisica Nucleare), in the field of
ultra-relativistic heavy-ion collisions. The experimental study of the phase
diagram of strongly-interacting matter and of the Quark-Gluon Plasma (QGP)
deconfined state will proceed, in the next 10-15 years, along two directions:
the high-energy regime at RHIC and at the LHC, and the low-energy regime at
FAIR, NICA, SPS and RHIC. The Italian community is strongly involved in the
present and future programme of the ALICE experiment, the upgrade of which will
open, in the 2020s, a new phase of high-precision characterisation of the QGP
properties at the LHC. As a complement of this main activity, there is a
growing interest in a possible future experiment at the SPS, which would target
the search for the onset of deconfinement using dimuon measurements. On a
longer timescale, the community looks with interest at the ongoing studies and
discussions on a possible fixed-target programme using the LHC ion beams and on
the Future Circular Collider.Comment: 99 pages, 56 figure
Energy dependence of exclusive photoproduction off protons in ultra-peripheral p-Pb collisions at = 5.02 TeV
The ALICE Collaboration has measured the energy dependence of exclusive
photoproduction of vector mesons off proton targets in
ultra-peripheral p-Pb collisions at a centre-of-mass energy per nucleon pair
TeV. The ee and decay channels
are used to measure the cross section as a function of the rapidity of the
in the range , corresponding to an energy in the
p centre-of-mass in the interval GeV.
The measurements, which are consistent with a power law dependence of the
exclusive photoproduction cross section, are compared to previous
results from HERA and the LHC and to several theoretical models. They are found
to be compatible with previous measurements.Comment: 25 pages, 3 captioned figures, 3 tables, authors from page 19,
published version, figures at http://alice-publications.web.cern.ch/node/455
Measurement of the production of charm jets tagged with D mesons in pp collisions at = 7 TeV
The production of charm jets in proton-proton collisions at a center-of-mass
energy of TeV was measured with the ALICE detector at the CERN
Large Hadron Collider. The measurement is based on a data sample corresponding
to a total integrated luminosity of , collected using a
minimum-bias trigger. Charm jets are identified by the presence of a D
meson among their constituents. The D mesons are reconstructed from their
hadronic decay DK. The D-meson tagged jets are
reconstructed using tracks of charged particles (track-based jets) with the
anti- algorithm in the jet transverse momentum range
and pseudorapidity
. The fraction of charged jets containing a D-meson
increases with from to . The distribution of D-meson tagged jets as a
function of the jet momentum fraction carried by the D meson in the
direction of the jet axis () is reported for two ranges
of jet transverse momenta, and
in the intervals
and , respectively. The
data are compared with results from Monte Carlo event generators (PYTHIA 6,
PYTHIA 8 and Herwig 7) and with a Next-to-Leading-Order perturbative Quantum
Chromodynamics calculation, obtained with the POWHEG method and interfaced with
PYTHIA 6 for the generation of the parton shower, fragmentation, hadronisation
and underlying event.Comment: 29 pages, 8 captioned figures, 3 tables, authors from page 24,
published version, figures at http://alice-publications.web.cern.ch/node/525
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Inclusive J/ψ production at mid-rapidity in pp collisions at √s = 5.02 TeV
Inclusive J/ψ production is studied in minimum-bias proton-proton collisions at a centre-of-mass energy of s = 5.02 TeV by ALICE at the CERN LHC. The measurement is performed at mid-rapidity (|y| < 0.9) in the dielectron decay channel down to zero transverse momentum pT, using a data sample corresponding to an integrated luminosity of Lint = 19.4 ± 0.4 nb−1. The measured pT-integrated inclusive J/ψ production cross sec- tion is dσ/dy = 5.64 ± 0.22(stat.) ± 0.33(syst.) ± 0.12(lumi.) μb. The pT-differential cross section d2σ/dpTdy is measured in the pT range 0–10 GeV/c and compared with state-of- the-art QCD calculations. The J/ψ 〈pT〉 and 〈pT2〉 are extracted and compared with results obtained at other collision energies. [Figure not available: see fulltext.]
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Measurement of Λ (1520) production in pp collisions at √s=7TeV and p–Pb collisions at √sNN=5.02TeV
The production of the Λ (1520) baryonic resonance has been measured at midrapidity in inelastic pp collisions at s=7TeV and in p–Pb collisions at sNN=5.02TeV for non-single diffractive events and in multiplicity classes. The resonance is reconstructed through its hadronic decay channel Λ (1520) → pK - and the charge conjugate with the ALICE detector. The integrated yields and mean transverse momenta are calculated from the measured transverse momentum distributions in pp and p–Pb collisions. The mean transverse momenta follow mass ordering as previously observed for other hyperons in the same collision systems. A Blast-Wave function constrained by other light hadrons (π, K, KS0, p, Λ) describes the shape of the Λ (1520) transverse momentum distribution up to 3.5GeV/c in p–Pb collisions. In the framework of this model, this observation suggests that the Λ (1520) resonance participates in the same collective radial flow as other light hadrons. The ratio of the yield of Λ (1520) to the yield of the ground state particle Λ remains constant as a function of charged-particle multiplicity, suggesting that there is no net effect of the hadronic phase in p–Pb collisions on the Λ (1520) yield
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Measurement of charged jet cross section in pp collisions at s =5.02 TeV
The cross section of jets reconstructed from charged particles is measured in the transverse momentum range of
- …