Context. Hickson Compact Group (HCG) 16 is a prototypical compact group of galaxies in an intermediate stage of the previously proposed evolutionary sequence, where its galaxies are losing gas to the ...intra-group medium (IGrM). The group hosts galaxies that are H I-normal, H I-poor, and centrally active with both AGNs and starbursts, in addition to a likely new member and a H I tidal feature of ∼160 kpc in length. Despite being a well-studied group at all wavelengths, no previous study of HCG 16 has focused on its extraordinary H I component. Aims. The characteristics of HCG 16 make it an ideal case study for exploring which processes are likely to dominate the late stages of evolution in compact groups, and ultimately determine their end states. In order to build a coherent picture of the evolution of this group we make use of the multi-wavelength data available, but focus particularly on H I as a tracer of interactions and evolutionary phase. Methods. We reprocess archival VLA L-band observations of HCG 16 using the multi-scale CLEAN algorithm to accurately recover diffuse features. Tidal features and galaxies are separated in three dimensions using the SlicerAstro package. The H I deficiency of the separated galaxies is assessed against the benchmark of recent scaling relations of isolated galaxies. This work has been performed with particular attention to reproducibility and is accompanied by a complete workflow to reproduce all the final data products, figures, and results. Results. Despite the clear disruption of the H I component of HCG 16 we find that it is not globally H I deficient, even though HCG 16a and b have lost the majority of their H I and almost 50% of the group’s H I is in the IGrM. The H I content of HCG 16d shows highly disturbed kinematics, with only a marginal velocity gradient that is almost perpendicular to its optical major axis. The tail of ∼160 kpc in length extending towards the southeast appears to be part of an even larger structure which spatially and kinematically connects NGC 848 to the northwest corner of the group. Conclusions. This study indicates that in the recent past (∼1 Gyr) galaxies HCG 16a and b likely underwent major interactions that unbound gas without triggering significant star formation. This gas was then swept away by a close, high-speed encounter with NGC 848. The starburst events HCG 16c and d, likely initiated by their mutual interaction, triggered galactic winds which, in the case of HCG 16d, appear to have disrupted its H I reservoir. The tidal features still connected to all these galaxies indicate that more H I will soon be lost to the IGrM, while that which remains in the discs will likely be consumed by star-formation episodes triggered by their ongoing interaction. This is expected to result in a collection of gas-poor galaxies embedded in a diffuse H I structure, which will gradually (over several Gyr) be evaporated by the UV background, resembling the final stage of the evolutionary model of compact groups.
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.
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.
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.
The study of (anti-)deuteron production in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this ...paper the production of
(anti-)deuterons
is studied as a function of the charged particle multiplicity in inelastic pp collisions at
s
=
13
TeV using the ALICE experiment. Thanks to the large number of accumulated minimum bias events, it has been possible to measure (anti-)deuteron production in pp collisions up to the same charged particle multiplicity (
d
N
ch
/
d
η
∼
26
) as measured in p–Pb collisions at similar centre-of-mass energies. Within the uncertainties, the deuteron yield in pp collisions resembles the one in p–Pb interactions, suggesting a common formation mechanism behind the production of light nuclei in hadronic interactions. In this context the measurements are compared with the expectations of coalescence and statistical hadronisation models (SHM).
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.
A
bstract
The inclusive J
/ψ
production in Pb–Pb collisions at the center-of-mass energy per nucleon pair
s
NN
= 5
.
02 TeV, measured with the ALICE detector at the CERN LHC, is reported. The J
/ψ
...meson is reconstructed via the dimuon decay channel at forward rapidity (2
.
5
< y <
4) down to zero transverse momentum. The suppression of the J
/ψ
yield in Pb–Pb collisions with respect to binary-scaled pp collisions is quantified by the nuclear modification factor (
R
AA
). The
R
AA
at
s
NN
= 5
.
02 TeV is presented and compared with previous measurements at
s
NN
= 2
.
76 TeV as a function of the centrality of the collision, and of the J
/ψ
transverse momentum and rapidity. The inclusive J
/ψ R
AA shows a suppression increasing toward higher transverse momentum, with a steeper dependence for central collisions. The modification of the J
/ψ
average transverse momentum and average squared transverse momentum is also studied. Comparisons with the results of models based on a transport equation and on statistical hadronization are carried out.
A
bstract
The transverse momentum (
p
T
) differential cross section of the charm-strange baryon
Ξ
c
0
is measured at midrapidity (|
y
|
<
0.5) via its semileptonic decay into e
+
Ξ
−
ν
e
in pp ...collisions at
s
= 5
.
02 TeV with the ALICE detector at the LHC. The ratio of the
p
T
-differential
Ξ
c
0
-baryon and D
0
-meson production cross sections is also reported. The measurements are compared with simulations with different tunes of the PYTHIA 8 event generator, with predictions from a statistical hadronisation model (SHM) with a largely augmented set of charm-baryon states beyond the current lists of the Particle Data Group, and with models including hadronisation via quark coalescence. The
p
T
-integrated cross section of prompt
Ξ
c
0
-baryon production at midrapidity is also reported, which is used to calculate the baryon-to-meson ratio
Ξ
c
0
/
D
0
= 0
.
20 ± 0
.
04
stat
.
−
0.07
+
0.08
(syst
.
). These results provide an additional indication of a modification of the charm fragmentation from e
+
e
−
and e
−
p collisions to pp collisions.
A
bstract
Anisotropic flow coefficients,
v
n
, non-linear flow mode coefficients,
χ
n,mk
, and correlations among different symmetry planes,
ρ
n,mk
are measured in Pb-Pb collisions at
s
NN
= 5
.
02 ...TeV. Results obtained with multi-particle correlations are reported for the transverse momentum interval 0
.
2
< p
T
<
5
.
0 GeV/
c
within the pseudorapidity interval 0
.
4
<
|
η
|
<
0
.
8 as a function of collision centrality. The
v
n
coefficients and
χ
n,mk
and
ρ
n,mk
are presented up to the ninth and seventh harmonic order, respectively. Calculations suggest that the correlations measured in different symmetry planes and the non-linear flow mode coefficients are dependent on the shear and bulk viscosity to entropy ratios of the medium created in heavy-ion collisions. The comparison between these measurements and those at lower energies and calculations from hydrodynamic models places strong constraints on the initial conditions and transport properties of the system.