Midrapidity production of π±, K±, and (¯p)p measured by the ALICE experiment at the CERN Large Hadron Collider, in Pb-Pb and inelastic pp collisions at √sNN=5.02 TeV, is presented. The invariant ...yields are measured over a wide transverse momentum (pT) range from hundreds of MeV/c up to 20 GeV/c. The results in Pb-Pb collisions are presented as a function of the collision centrality, in the range 0–90%. The comparison of the pT-integrated particle ratios, i.e., proton-to-pion (p/π) and kaon-to-pion (K/π) ratios, with similar measurements in Pb-Pb collisions at √sNN=2.76 TeV show no significant energy dependence. Blast-wave fits of the pT spectra indicate that in the most central collisions radial flow is slightly larger at 5.02 TeV with respect to 2.76 TeV. Particle ratios (p/π, K/π) as a function of pT show pronounced maxima at pT≈3GeV/c in central Pb-Pb collisions. At high pT, particle ratios at 5.02 TeV are similar to those measured in pp collisions at the same energy and in Pb-Pb collisions at √sNN=2.76 TeV. Using the pp reference spectra measured at the same collision energy of 5.02 TeV, the nuclear modification factors for the different particle species are derived. Within uncertainties, the nuclear modification factor is particle species independent for high pT and compatible with measurements at √sNN=2.76 TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily.
Comprehensive results on the production of unidentified charged particles, π±, K±, K$_S^0$, K*(892)0, $p, \bar{p}, ϕ$(1020), Λ, Λ, Ξ-, Ξ+, Ω-, and $\bar{Ω}^+$ hadrons in proton-proton ...(pp) collisions at $ \sqrt{s}$=7 TeV at midrapidity (|y|<0.5) as a function of charged-particle multiplicity density are presented. In order to avoid autocorrelation biases, the actual transverse momentum (pT) spectra of the particles under study and the event activity are measured in different rapidity windows. In the highest multiplicity class, the charged-particle density reaches about 3.5 times the value measured in inelastic collisions. While the yield of protons normalized to pions remains approximately constant as a function of multiplicity, the corresponding ratios of strange hadrons to pions show a significant enhancement that increases with increasing strangeness content. Furthermore, all identified particle-to-pion ratios are shown to depend solely on charged-particle multiplicity density, regardless of system type and collision energy. The evolution of the spectral shapes with multiplicity and hadron mass shows patterns that are similar to those observed in p-Pb and Pb-Pb collisions at Large Hadron Collider energies. The obtained pT distributions and yields are compared to expectations from QCD-based pp event generators as well as to predictions from thermal and hydrodynamic models. These comparisons indicate that traces of a collective, equilibrated system are already present in high-multiplicity pp collisions.
The pseudorapidity density of charged particles, dNch / d η , at midrapidity in Pb-Pb collisions has been measured at a center-of-mass energy per nucleon pair of √ s N N = 5.02 TeV . For the 5% ...most central collisions, we measure a value of 1943 ± 54 . The rise in dNch / d η as a function of √ s N N is steeper than that observed in proton-proton collisions and follows the trend established by measurements at lower energy. The increase of dNch / d η as a function of the average number of participant nucleons, ⟨ N part ⟩ , calculated in a Glauber model, is compared with the previous measurement at √ s N N = 2.76 TeV . A constant factor of about 1.2 describes the increase in dNch / d η from √ s N N = 2.76 to 5.02 TeV for all centrality classes, within the measured range of 0%–80% centrality. The results are also compared to models based on different mechanisms for particle production in nuclear collisions.
We report the measurements of correlations between event-by-event fluctuations of amplitudes of anisotropic flow harmonics in nucleus-nucleus collisions, obtained for the first time using a new ...analysis method based on multiparticle cumulants in mixed harmonics. This novel method is robust against systematic biases originating from nonflow effects and by construction any dependence on symmetry planes is eliminated. We demonstrate that correlations of flow harmonics exhibit a better sensitivity to medium properties than the individual flow harmonics. The new measurements are performed in Pb-Pb collisions at the center-of-mass energy per nucleon pair of √ s N N = 2.76 TeV by the ALICE experiment at the Large Hadron Collider. The centrality dependence of correlation between event-by-event fluctuations of the elliptic v2 and quadrangular v4 flow harmonics, as well as of anticorrelation between v2 and triangular v3 flow harmonics are presented. The results cover two different regimes of the initial state configurations: geometry dominated (in midcentral collisions) and fluctuation dominated (in the most central collisions). Comparisons are made to predictions from Monte Carlo Glauber, viscous hydrodynamics, ampt, and hijing models. Together with the existing measurements of the individual flow harmonics the presented results provide further constraints on the initial conditions and the transport properties of the system produced in heavy-ion collisions.
We report the first results of elliptic (v2), triangular (v3), and quadrangular (v4) flow of charged particles in Pb-Pb collisions at a center-of-mass energy per nucleon pair of √sNN=5.02 TeV with ...the ALICE detector at the CERN Large Hadron Collider. The measurements are performed in the central pseudorapidity region |η|<0.8 and for the transverse momentum range 0.2<pT<5 GeV/c. The anisotropic flow is measured using two-particle correlations with a pseudorapidity gap greater than one unit and with the multiparticle cumulant method. Compared to results from Pb-Pb collisions at √sNN=2.76 TeV, the anisotropic flow coefficients v2, v3, and v4 are found to increase by (3.0±0.6)%, (4.3±1.4)%, and (10.2±3.8)%, respectively, in the centrality range 0%–50%. This increase can be attributed mostly to an increase of the average transverse momentum between the two energies. The measurements are found to be compatible with hydrodynamic model calculations. This comparison provides a unique opportunity to test the validity of the hydrodynamic picture and the power to further discriminate between various possibilities for the temperature dependence of shear viscosity to entropy density ratio of the produced matter in heavy-ion collisions at the highest energies.
In this Letter, comprehensive results on π±, K±, KS0, p(p¯) and Λ(Λ¯) production at mid-rapidity (0<yCMS<0.5) in p–Pb collisions at sNN=5.02 TeV, measured by the ALICE detector at the LHC, are ...reported. The transverse momentum distributions exhibit a hardening as a function of event multiplicity, which is stronger for heavier particles. This behavior is similar to what has been observed in pp and Pb–Pb collisions at the LHC. The measured pT distributions are compared to d–Au, Au–Au and Pb–Pb results at lower energy and with predictions based on QCD-inspired and hydrodynamic models.
We present a study of the inclusive charged-particle transverse momentum (
p
T
) spectra as a function of charged-particle multiplicity density at mid-pseudorapidity,
d
N
ch
/
d
η
, in pp collisions ...at
s
=
5.02
and 13 TeV covering the kinematic range
|
η
|
<
0.8
and
0.15
<
p
T
<
20
GeV/
c
. The results are presented for events with at least one charged particle in
|
η
|
<
1
(INEL
>
0
). The
p
T
spectra are reported for two multiplicity estimators covering different pseudorapidity regions. The
p
T
spectra normalized to that for INEL
>
0
show little energy dependence. Moreover, the high-
p
T
yields of charged particles increase faster than the charged-particle multiplicity density. The average
p
T
as a function of multiplicity and transverse spherocity is reported for pp collisions at
s
=
13
TeV. For low- (high-) spherocity events, corresponding to jet-like (isotropic) events, the average
p
T
is higher (smaller) than that measured in INEL
>
0
pp collisions. Within uncertainties, the functional form of
⟨
p
T
⟩
(
N
ch
)
is not affected by the spherocity selection. While EPOS LHC gives a good description of many features of data, PYTHIA overestimates the average
p
T
in jet-like events.
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 detailed study of pseudorapidity densities and multiplicity distributions of primary charged particles produced in proton–proton collisions, at
s
=
0.9, 2.36, 2.76, 7 and 8 TeV, in the ...pseudorapidity range
|
η
|
<
2
, was carried out using the ALICE detector. Measurements were obtained for three event classes: inelastic, non-single diffractive and events with at least one charged particle in the pseudorapidity interval
|
η
|
<
1
. The use of an improved track-counting algorithm combined with ALICE’s measurements of diffractive processes allows a higher precision compared to our previous publications. A KNO scaling study was performed in the pseudorapidity intervals
|
η
|
<
0.5, 1.0 and 1.5. The data are compared to other experimental results and to models as implemented in Monte Carlo event generators PHOJET and recent tunes of PYTHIA6, PYTHIA8 and EPOS.
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.
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