The content of spectator matter in relativistic
O–
O collisions was explored by means of the Abrasion–Ablation Monte Carlo for Colliders (AAMCC-MST) model with accounting for
-clustering in
O. Based ...on the Gaussian, Woods–Saxon and harmonic oscillator nuclear density distributions, three different parameterizations of the cluster density in
O were used to calculate the charge distributions of spectator nuclear fragments and multiplicity distributions of spectator
-particles. The sensitivity of the calculated distributions to the parametrization of the excitation energy of spectator matter, the density distributions of
-clusters and the total contribution of clustered states in
O were investigated. From the comparison of the calculations with experimental data the contribution of clustered states was evaluated at the level of 30
.
Fragmentation of
208
Pb nuclei at the LHC collider is studied using the Abrasion-Ablation Monte Carlo for Colliders (AAMCC) model. Various characteristics of fragmentation are investigated, such as ...the dependence of the maximum charge of a fragment in the event and the average multiplicity of fragments on the total charge of the fragments,
, which correlates with the event centrality. By comparing the simulation results for
208
Pb–
208
Pb collisions at the CERN SPS,
GeV, with those at the LHC,
TeV, the evolution of these dependences with an increase in the initial collision energy is studied. The possibility of estimating the centrality dependence of
by measuring the yields of free spectator neutrons and protons and measuring the event centrality through the number of particles produced in
NN
-collisions is discussed. The characteristics of spectator matter that are available for study in collider experiments on nucleus–nucleus collisions—the number of neutrons and the
ratio, as well as their correlation with the number of nucleon–nucleon collisions in the event—are calculated.
The Abrasion–Ablation Monte Carlo for Colliders (AAMCC) model is used to study the characteristics of spectator matter in central collisions of relativistic
238
U nuclei, which are sensitive to the ...mutual orientation and the degree of deformation of the colliding nuclei. It is shown that the multiplicity of spectator neutrons and their forward-backward asymmetry substantially depend on the initial mutual orientation of
238
U nuclei, which makes it possible to separate tip-body events with a high forward-backward asymmetry and a maximum multiplicity of spectator neutrons, as well as side-side events with a similar multiplicity, but with a minimal asymmetry. It is found that an increase in the quadrupole deformation parameter
of
238
U nuclei leads to a proportional increase in the multiplicity of spectator neutrons in tip-body events. Thus, the detection of spectator neutrons in collisions of relativistic deformed nuclei in an experiment can be used to study a degree of their deformation and to select collisions of a certain orientation.
The properties of spectator matter created in
197
Au–
197
Au and
208
Pb–
208
Pb collisions with energies typical for the NICA accelerator complex were studied with the Abrasion–Ablation Monte Carlo ...for Colliders (AAMCC) model. We review predictions made with AAMCC for several characteristics of spectator matter and compare them with available data. The sum of squares of spectator charges per spectator nucleon is proposed as an additional indicator of collision centrality in BM@N and MPD experiments at NICA. The forward-backward asymmetry of free spectator nucleons is calculated and compared to the asymmetry of the total volume of spectator matter. The sensitivity of calculated yields of spectator neutrons in central
208
Pb‒
208
Pb collisions to the presence of neutron skin in
208
Pb is demonstrated.
A
bstract
The cross section for coherent photonuclear production of J/
ψ
is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE ...detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of
s
NN
= 5.02 TeV. Cross sections are presented in five different J/
ψ
rapidity ranges within |
y
| < 4, with the J/
ψ
reconstructed via its dilepton decay channels. In some events the J/
ψ
is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17 <
W
γ
Pb
,
n
< 920 GeV, where
W
γ
Pb
,
n
is the centre-of-mass energy per nucleon of the
γ
Pb system. This range corresponds to a Bjorken-
x
interval spanning about three orders of magnitude: 1.1 × 10
−
5
<
x
< 3.3 × 10
−
2
. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.
A
bstract
Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at
s
= 13 TeV and p–Pb collisions at
s
NN
= 5
.
02 TeV. ...The correlation functions are measured as a function of relative azimuthal angle ∆
φ
and pseudorapidity separation ∆
η
for pairs of primary charged particles within the pseudorapidity interval |
η
|
<
0
.
9 and the transverse-momentum interval 1
< p
T
<
4 GeV/
c
. Flow coefficients are extracted for the long-range correlations (1
.
6
<
|∆
η
|
<
1
.
8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.
A
bstract
The transverse momentum (
p
T
) and centrality dependence of the nuclear modification factor
R
AA
of prompt and non-prompt J/
ψ
, the latter originating from the weak decays of beauty ...hadrons, have been measured by the ALICE collaboration in Pb–Pb collisions at
s
NN
= 5.02 TeV. The measurements are carried out through the e
+
e
−
decay channel at midrapidity (|
y
| < 0.9) in the transverse momentum region 1.5 <
p
T
< 10 GeV/
c
. Both prompt and non-prompt J/
ψ
measurements indicate a significant suppression for
p
T
> 5 GeV/
c
, which becomes stronger with increasing collision centrality. The results are consistent with similar LHC measurements in the overlapping
p
T
intervals, and cover the kinematic region down to
p
T
= 1.5 GeV/
c
at midrapidity, not accessible by other LHC experiments. The suppression of prompt J/
ψ
in central and semicentral collisions exhibits a decreasing trend towards lower transverse momentum, described within uncertainties by models implementing J/
ψ
production from recombination of c and
c
¯
quarks produced independently in different partonic scatterings. At high transverse momentum, transport models including quarkonium dissociation are able to describe the suppression for prompt J/
ψ
. For non-prompt J/
ψ
, the suppression predicted by models including both collisional and radiative processes for the computation of the beauty-quark energy loss inside the quark-gluon plasma is consistent with measurements within uncertainties.
A
bstract
Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the “ridge” phenomenon, were discovered in heavy-ion collisions, and later ...found in pp and p–Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small collision systems. In this Letter, measurements of the long-range correlations in p–Pb collisions at
s
NN
= 5
.
02 TeV are extended to a pseudorapidity gap of ∆
η
~ 8 between particles using the ALICE forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of ∆
η
~ 8 for the first time in p–Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small collision systems such as p–Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient,
v
2
(
η
), is extracted from the long-range correlations. The
v
2
(
η
) results are presented for a wide pseudorapidity range of –3
.
1
< η <
4
.
8 in various centrality classes in p–Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small collision systems, the
v
2
(
η
) measurements are compared with hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small collision systems.
A
bstract
Measurements of the production cross sections of prompt D
0
, D
+
, D
*+
,
D
s
+
,
Λ
c
+
, and
Ξ
c
+
charm hadrons at midrapidity in proton-proton collisions at
s
= 13 TeV with the ALICE ...detector are presented. The D-meson cross sections as a function of transverse momentum (
p
T
) are provided with improved precision and granularity. The ratios of
p
T
-differential meson production cross sections based on this publication and on measurements at different rapidity and collision energy provide a constraint on gluon parton distribution functions at low values of Bjorken-
x
(10
−
5
–10
−
4
). The measurements of
Λ
c
+
(
Ξ
c
+
) baryon production extend the measured
p
T
intervals down to
p
T
= 0(3) GeV
/c
. These measurements are used to determine the charm-quark fragmentation fractions and the
c
c
¯
production cross section at midrapidity (|
y
|
<
0
.
5) based on the sum of the cross sections of the weakly-decaying ground-state charm hadrons D
0
, D
+
,
D
s
+
,
Λ
c
+
,
Ξ
c
0
and, for the first time,
Ξ
c
+
, and of the strongly-decaying J
/ψ
mesons. The first measurements of
Ξ
c
+
and
Σ
c
0
,
+
+
fragmentation fractions at midrapidity are also reported. A significantly larger fraction of charm quarks hadronising to baryons is found compared to e
+
e
−
and ep collisions. The
c
c
¯
production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations.
