The production of Ξ− and Ω− baryons and their anti-particles in Pb–Pb collisions at sNN=2.76 TeV has been measured using the ALICE detector. The transverse momentum spectra at mid-rapidity (|y|<0.5) ...for charged Ξ and Ω hyperons have been studied in the range 0.6<pT<8.0 GeV/c and 1.2<pT<7.0 GeV/c, respectively, and in several centrality intervals (from the most central 0–10% to the most peripheral 60–80% collisions). These spectra have been compared with the predictions of recent hydrodynamic models. In particular, the Kraków and EPOS models give a satisfactory description of the data, with the latter covering a wider pT range. Mid-rapidity yields, integrated over pT, have been determined. The hyperon-to-pion ratios are similar to those at RHIC: they rise smoothly with centrality up to 〈Npart〉∼150 and saturate thereafter. The enhancements (yields per participant nucleon relative to those in pp collisions) increase both with the strangeness content of the baryon and with centrality, but are less pronounced than at lower energies.
The inclusive transverse momentum (pT) distributions of primary charged particles are measured in the pseudo-rapidity range |η|<0.8 as a function of event centrality in Pb–Pb collisions at ...sNN=2.76 TeV with ALICE at the LHC. The data are presented in the pT range 0.15<pT<50 GeV/c for nine centrality intervals from 70–80% to 0–5%. The results in Pb–Pb are presented in terms of the nuclear modification factor RAA using a pp reference spectrum measured at the same collision energy. We observe that the suppression of high-pT particles strongly depends on event centrality. The yield is most suppressed in central collisions (0–5%) with RAA≈0.13 at pT=6–7 GeV/c. Above pT=7 GeV/c, there is a significant rise in the nuclear modification factor, which reaches RAA≈0.4 for pT>30 GeV/c. In peripheral collisions (70–80%), only moderate suppression (RAA=0.6–0.7) and a weak pT dependence is observed. The measured nuclear modification factors are compared to other measurements and model calculations.
The pT-differential production cross sections of the prompt charmed mesons DO, D+, D*+, and Ds+ and their charge conjugate in the rapidity interval -0.96 0.04 were measured in p-Pb collisions at a ...center-of-mass energy sNN=5.02 TeV with the ALICE detector at the LHC. The nuclear modification factor RpPb, quantifying the D-meson yield in p-Pb collisions relative to the yield in pp collisions scaled by the number of binary nucleon-nucleon collisions, is compatible within the 15%-20% uncertainties with unity in the transverse momentum interval 1<pT<24 GeV/c. No significant difference among the RpPb of the four D-meson species is observed. The results are described within uncertainties by theoretical calculations that include initial-state effects. The measurement adds experimental evidence that the modification of the momentum spectrum of D mesons observed in Pb-Pb collisions with respect to pp collisions is due to strong final-state effects induced by hot partonic matter.
Angular correlations between unidentified charged trigger particles and various species of charged associated particles (unidentified particles, pions, kaons, protons and antiprotons) are measured by ...the ALICE detector in p–Pb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV in the transverse-momentum range 0.3<pT<4 GeV/c. The correlations expressed as associated yield per trigger particle are obtained in the pseudorapidity range |ηlab|<0.8. Fourier coefficients are extracted from the long-range correlations projected onto the azimuthal angle difference and studied as a function of pT and in intervals of event multiplicity. In high-multiplicity events, the second-order coefficient for protons, v2p, is observed to be smaller than that for pions, v2π, up to about pT=2 GeV/c. To reduce correlations due to jets, the per-trigger yield measured in low-multiplicity events is subtracted from that in high-multiplicity events. A two-ridge structure is obtained for all particle species. The Fourier decomposition of this structure shows that the second-order coefficients for pions and kaons are similar. The v2p is found to be smaller at low pT and larger at higher pT than v2π, with a crossing occurring at about 2 GeV/c. This is qualitatively similar to the elliptic-flow pattern observed in heavy-ion collisions. A mass ordering effect at low transverse momenta is consistent with expectations from hydrodynamic model calculations assuming a collectively expanding system.
A measurement of the production of prompt Λ$_c^+$ baryons in Pb–Pb collisions at $\sqrt{s_{NN}}$ =5.02 TeV with the ALICE detector at the LHC is reported. The Λ$_c^+$ and $\bar{Λ}_c^-$ were ...reconstructed at midrapidity (|y|<0.5) via the hadronic decay channel Λ$_c^+$ →pK$_S^0$ (and charge conjugate) in the transverse momentum and centrality intervals 6 < pT < 12 GeV/c and 0–80%. The Λ$_c^+$/D0 ratio, which is sensitive to the charm quark hadronisation mechanisms in the medium, is measured and found to be larger than the ratio measured in minimum-bias pp collisions at $\sqrt{s}$ = 7 TeV and in p–Pb collisions at $\sqrt{s}$ = 5.02 TeV. In particular, the values in p–Pb and Pb–Pb collisions differ by about two standard deviations of the combined statistical and systematic uncertainties in the common pT interval covered by the measurements in the two collision systems. The Λ$_c^+$/D0 ratio is also compared with model calculations including different implementations of charm quark hadronisation. The measured ratio is reproduced by models implementing a pure coalescence scenario, while adding a fragmentation contribution leads to an underestimation. The Λ$_c^+$ nuclear modification factor, RAA, is also presented. The measured values of the RAA of Λ$_c^+$/D$_s^+$ , and non-strange D mesons are compatible within the combined statistical and systematic uncertainties. They show, however, a hint of a hierarchy (R$^{D^0}_{AA}$ < R$^{D^+_s}_{AA}$ < R$^{Λ_c^+}_{AA}$), conceivable with a contribution from coalescence mechanisms to charm hadron formation in the medium.
The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of
J
/
ψ
vector mesons off proton targets in ultra–peripheral p–Pb collisions at a centre-of-mass energy per ...nucleon pair
s
NN
=
5.02
TeV. The e
+
e
-
and
μ
+
μ
-
decay channels are used to measure the cross section as a function of the rapidity of the
J
/
ψ
in the range
-
2.5
<
y
<
2.7
, corresponding to an energy in the
γ
p centre-of-mass in the interval
40
<
W
γ
p
<
550
GeV. The measurements, which are consistent with a power law dependence of the exclusive
J
/
ψ
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.
A
bstract
The
p
T
-differential production cross section of prompt
Λ
c
+
charmed baryons was measured with the ALICE detector at the Large Hadron Collider (LHC) in pp collisions at
s
=
7
TeV and in ...p-Pb collisions at
s
N
N
=
5.02
TeV at midrapidity. The
Λ
c
+
and
Λ
¯
c
¯
were reconstructed in the hadronic decay modes
Λ
c
+
→ pK
−
π
+
,
Λ
c
+
→ pK
S
0
and in the semileptonic channel
Λ
c
+
→ e
+
ν
e
Λ
(and charge conjugates). The measured values of the
Λ
c
+
/D
0
ratio, which is sensitive to the c-quark hadronisation mechanism, and in particular to the production of baryons, are presented and are larger than those measured previously in different colliding systems, centre-of-mass energies, rapidity and
p
T
intervals, where the
Λ
c
+
production process may differ. The results are compared with the expectations obtained from perturbative Quantum Chromodynamics calculations and Monte Carlo event generators. Neither perturbative QCD calculations nor Monte Carlo models reproduce the data, indicating that the fragmentation of heavy-flavour baryons is not well understood. The first measurement at the LHC of the
Λ
c
+
nuclear modification factor,
R
pPb
, is also presented. The
R
pPb
is found to be consistent with unity and with that of D mesons within the uncertainties, and consistent with a theoretical calculation that includes cold nuclear matter effects and a calculation that includes charm quark interactions with a deconfined medium.
A
bstract
The elliptic (
v
2
), triangular (
v
3
), and quadrangular (
v
4
) flow coefficients of π
±
, K
±
,
p
+
p
¯
,
Λ
+
Λ
¯
,
K
S
0
, and the
ϕ
-meson are measured in Pb-Pb collisions at
s
N
N
=
...5.02
TeV. Results obtained with the scalar product method are reported for the rapidity range |
y
|
<
0.5 as a function of transverse momentum,
p
T
, at different collision centrality intervals between 0–70%, including ultra-central (0–1%) collisions for
π
±
, K
±
, and
p
+
p
¯
. For
p
T
<
3 GeV
/c
, the flow coefficients exhibit a particle mass dependence. At intermediate transverse momenta (3 <
p
T
< 8–10 GeV/
c
), particles show an approximate grouping according to their type (i.e., mesons and baryons). The
ϕ
-meson
v
2
, which tests both particle mass dependence and type scaling, follows
p
+
p
¯
v
2
at low
p
T
and π
±
v
2
at intermediate
p
T
. The evolution of the shape of
v
n
(
p
T
) as a function of centrality and harmonic number
n
is studied for the various particle species. Flow coefficients of
π
±
, K
±
, and
p
+
p
¯
for
p
T
< 3 GeV/
c
are compared to iEBE-VISHNU and MUSIC hydrodynamical calculations coupled to a hadronic cascade model (UrQMD). The iEBE-VISHNU calculations describe the results fairly well for
p
T
< 2.5 GeV/
c
, while MUSIC calculations reproduce the measurements for
p
T
< 1 GeV/
c
. A comparison to
v
n
coefficients measured in Pb-Pb collisions at
s
N
N
=
2.76
TeV is also provided.
We report a precise measurement of the J/ψ elliptic flow in Pb-Pb collisions at sNN=5.02 TeV with the ALICE detector at the LHC. The J/ψ mesons are reconstructed at midrapidity (|y|
We report on the first measurement of the triangular v3, quadrangular v4, and pentagonal v5 charged particle flow in Pb-Pb collisions at √SNN = 2.76 TeV measured with the ALICE detector at the CERN ...Large Hadron Collider. We show that the triangular flow can be described in terms of the initial spatial anisotropy and its fluctuations, which provides strong constraints on its origin. In the most central events, where the elliptic flow v2 and v3 have similar magnitude, a double peaked structure in the two-particle azimuthal correlations is observed, which is often interpreted as a Mach cone response to fast partons. Here, we show that this structure can be naturally explained from the measured anisotropic flow Fourier coefficients.
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