Femtoscopy is an effective instrument to study the size and dynamics of the system created in heavy-ion collisions. In particular, a decreasing dependence of system size on increasing pair momentum ...and particle mass could be interpreted as an evidence of the strong collective flow. Such phenomena are naturally modeled by hydrodynamics. The large data sample collected in Pb-Pb collisions at sNN=2.76TeV by the ALICE collaboration enables femtoscopic measurements for pions and kaons. In this paper, the ALICE data are compared with calculations within the EPOS 3 hadronic interaction model, which is based on a (3+1)D viscous hydrodynamical evolution and employs the UrQMD cascade to describe the hadronic phase. The femtoscopic radii for pions and kaons are considered as a function of pair transverse momentum/mass and collision centrality. The obtained results show the importance of the hadronic rescattering phase at LHC energies.
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.
Measurements of anisotropic flow coefficients (vn) and their cross-correlations using two- and multiparticle cumulant methods are reported in collisions of pp at s=13 TeV, p−Pb at a center-of-mass ...energy per nucleon pair sNN=5.02 TeV, Xe-Xe at sNN=5.44 TeV, and Pb-Pb at sNN=5.02 TeV recorded with the ALICE detector. The multiplicity dependence of vn is studied in a very wide range from 20 to 3000 particles produced in the midrapidity region |η|<0.8 for the transverse momentum range 0.2<pT<3.0 GeV/c. An ordering of the coefficients v2>v3>v4 is found in pp and p−Pb collisions, similar to that seen in large collision systems, while a weak v2 multiplicity dependence is observed relative to nucleus-nucleus collisions in the same multiplicity range. Using a novel subevent method, v2 measured with four-particle cumulants is found to be compatible with that from six-particle cumulants in pp and p−Pb collisions. The magnitude of the correlation between vn2 and vm2, evaluated with the symmetric cumulants SC(m,n) is observed to be positive at all multiplicities for v2 and v4, while for v2 and v3 it is negative and changes sign for multiplicities below 100, which may indicate a different vn fluctuation pattern in this multiplicity range. The observed long-range multiparticle azimuthal correlations in high multiplicity pp and p−Pb collisions can neither be described by pythia 8 nor by impact-parameter-Glasma, music, and ultrarelativistic quantum molecular dynamics model calculations, and hence, provide new insights into the understanding of collective effects in small collision systems.
The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity (
y
<
0.5
) are measured in proton-proton collisions at
s
= 13 TeV as a function of the charged ...particle multiplicity, using the ALICE detector at the LHC. The production rates of
K
S
0
,
Λ
,
Ξ
, and
Ω
increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV.
A
bstract
The
p
T
-differential production cross sections of prompt and non-prompt (produced in beauty-hadron decays) D mesons were measured by the ALICE experiment at midrapidity (
|
y
|
<
0
.
5) in ...proton-proton collisions at
s
= 5
.
02 TeV. The data sample used in the analysis corresponds to an integrated luminosity of (19
.
3
±
0
.
4) nb
−
1
. D mesons were reconstructed from their decays D
0
→
K
−
π
+
, D
+
→
K
−
π
+
π
+
, and
D
s
+
→
ϕ
π
+
→
K
−
K
+
π
+
and their charge conjugates. Compared to previous measurements in the same rapidity region, the cross sections of prompt D
+
and
D
s
+
mesons have an extended
p
T
coverage and total uncertainties reduced by a factor ranging from 1.05 to 1.6, depending on
p
T
, allowing for a more precise determination of their
p
T
-integrated cross sections. The results are well described by perturbative QCD calculations. The fragmentation fraction of heavy quarks to strange mesons divided by the one to non-strange mesons,
f
s
/
(
f
u
+
f
d
), is compatible for charm and beauty quarks and with previous measurements at different centre-of-mass energies and collision systems. The
b
b
¯
production cross section per rapidity unit at midrapidity, estimated from non-prompt D-meson measurements, is
d
σ
b
b
¯
/
d
y
y
<
0.5
=
34.5
±
2.4
stat
−
2.9
+
4.7
tot
.
syst
μb. It is compatible with previous measurements at the same centre-of-mass energy and with the cross section pre- dicted by perturbative QCD calculations.
This paper presents the measurements of
π
±
,
K
±
,
p
and
p
¯
transverse momentum (
p
T
) spectra as a function of charged-particle multiplicity density in proton–proton (pp) collisions at
s
=
13
TeV
...with the ALICE detector at the LHC. Such study allows us to isolate the center-of-mass energy dependence of light-flavour particle production. The measurements reported here cover a
p
T
range from 0.1 to 20
GeV
/
c
and are done in the rapidity interval
|
y
|
<
0.5
. The
p
T
-differential particle ratios exhibit an evolution with multiplicity, similar to that observed in pp collisions at
s
=
7
TeV
, which is qualitatively described by some of the hydrodynamical and pQCD-inspired models discussed in this paper. Furthermore, the
p
T
-integrated hadron-to-pion yield ratios measured in pp collisions at two different center-of-mass energies are consistent when compared at similar multiplicities. This also extends to strange and multi-strange hadrons, suggesting that, at LHC energies, particle hadrochemistry scales with particle multiplicity the same way under different collision energies and colliding systems.
The coherent photoproduction of
J
/
ψ
and
ψ
′
mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy
s
NN
=
5.02
TeV with the ALICE detector. Charmonia are detected in ...the central rapidity region for events where the hadronic interactions are strongly suppressed. The
J
/
ψ
is reconstructed using the dilepton (
l
+
l
-
) and proton–antiproton decay channels, while for the
ψ
′
the dilepton and the
l
+
l
-
π
+
π
-
decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233
μ
b
-
1
. The results are compared with theoretical models for coherent
J
/
ψ
and
ψ
′
photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-
x
of around
6
×
10
-
4
, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent
J
/
ψ
cross section including ALICE measurements at forward rapidity. The ratio of
ψ
′
to
J
/
ψ
coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.
The production cross sections for prompt charmed mesons
D
0
,
D
+
,
D
∗
+
and
D
s
+
were measured at mid-rapidity in proton–proton collisions at a centre-of-mass energy
s
=
7
TeV
with the ALICE ...detector at the Large Hadron Collider (LHC). D mesons were reconstructed from their decays
D
0
→
K
-
π
+
,
D
+
→
K
-
π
+
π
+
,
D
∗
+
→
D
0
π
+
,
D
s
+
→
ϕ
π
+
→
K
-
K
+
π
+
, and their charge conjugates.With respect to previous measurements in the same rapidity region, the coverage in transverse momentum (
p
T
) is extended and the uncertainties are reduced by a factor of about two. The accuracy on the estimated total
c
c
¯
production cross section is likewise improved. The measured
p
T
-differential cross sections are compared with the results of three perturbative QCD calculations.
The differential invariant yield as a function of transverse momentum (pT) of electrons from semileptonic heavy-flavour hadron decays was measured at midrapidity in central (0–10%), semi-central ...(30–50%) and peripheral (60–80%) lead–lead (Pb–Pb) collisions at sNN=5.02 TeV in the pT intervals 0.5–26 GeV/c (0–10% and 30–50%) and 0.5–10 GeV/c (60–80%). The production cross section in proton–proton (pp) collisions at s=5.02 TeV was measured as well in 0.5<pT<10 GeV/c and it lies close to the upper band of perturbative QCD calculation uncertainties up to pT=5 GeV/c and close to the mean value for larger pT. The modification of the electron yield with respect to what is expected for an incoherent superposition of nucleon–nucleon collisions is evaluated by measuring the nuclear modification factor RAA. The measurement of the RAA in different centrality classes allows in-medium energy loss of charm and beauty quarks to be investigated. The RAA shows a suppression with respect to unity at intermediate pT, which increases while moving towards more central collisions. Moreover, the measured RAA is sensitive to the modification of the parton distribution functions (PDF) in nuclei, like nuclear shadowing, which causes a suppression of the heavy-quark production at low pT in heavy-ion collisions at LHC.
A
bstract
The inclusive J/
ψ
elliptic (
v
2
) and triangular (
v
3
) flow coefficients measured at forward rapidity (2
.
5
< y <
4) and the
v
2
measured at midrapidity (|
y
|
<
0
.
9) in Pb-Pb ...collisions at
s
NN
= 5
.
02 TeV using the ALICE detector at the LHC are reported. The entire Pb-Pb data sample collected during Run 2 is employed, amounting to an integrated luminosity of 750
μ
b
−
1
at forward rapidity and 93
μ
b
−
1
at midrapidity. The results are obtained using the scalar product method and are reported as a function of transverse momentum
p
T
and collision centrality. At midrapidity, the J/
ψ v
2
is in agreement with the forward rapidity measurement. The centrality averaged results indicate a positive J/
ψ v
3
with a significance of more than 5
σ
at forward rapidity in the
p
T
range 2
< p
T
<
5 GeV/
c
. The forward rapidity
v
2
,
v
3
, and
v
3
/v
2
results at low and intermediate
p
T
(
p
T
≲ 8 GeV/
c
) exhibit a mass hierarchy when compared to pions and D mesons, while converging into a species-independent curve at higher
p
T
. At low and intermediate
p
T
, the results could be interpreted in terms of a later thermalization of charm quarks compared to light quarks, while at high
p
T
, path-length dependent effects seem to dominate. The J/
ψ v
2
measurements are further compared to a microscopic transport model calculation. Using a simplified extension of the quark scaling approach involving both light and charm quark flow components, it is shown that the D-meson
v
n
measurements can be described based on those for charged pions and J/
ψ
flow.
PDF
