The first measurement of the production of pions, kaons, (anti-)protons and
ϕ
mesons at midrapidity in Xe–Xe collisions at
s
NN
=
5.44
TeV
is presented. Transverse momentum (
p
T
) spectra and
p
T
...-integrated yields are extracted in several centrality intervals bridging from p–Pb to mid-central Pb–Pb collisions in terms of final-state multiplicity. The study of Xe–Xe and Pb–Pb collisions allows systems at similar charged-particle multiplicities but with different initial geometrical eccentricities to be investigated. A detailed comparison of the spectral shapes in the two systems reveals an opposite behaviour for radial and elliptic flow. In particular, this study shows that the radial flow does not depend on the colliding system when compared at similar charged-particle multiplicity. In terms of hadron chemistry, the previously observed smooth evolution of particle ratios with multiplicity from small to large collision systems is also found to hold in Xe–Xe. In addition, our results confirm that two remarkable features of particle production at LHC energies are also valid in the collision of medium-sized nuclei: the lower proton-to-pion ratio with respect to the thermal model expectations and the increase of the
ϕ
-to-pion ratio with increasing final-state multiplicity.
The multiplicity dependence of the pseudorapidity density of charged particles in proton–proton (pp) collisions at centre-of-mass energies
s
=
5.02
, 7 and 13 TeV measured by ALICE is reported. The ...analysis relies on track segments measured in the midrapidity range (
|
η
|
<
1.5
). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval
|
η
|
<
1
. The multiplicity dependence of the pseudorapidity density of charged particles is measured with mid- and forward rapidity multiplicity estimators, the latter being less affected by autocorrelations. A detailed comparison with predictions from the PYTHIA 8 and EPOS LHC event generators is also presented. The results can be used to constrain models for particle production as a function of multiplicity in pp collisions.
A
bstract
Understanding the production mechanism of light (anti)nuclei is one of the key challenges of nuclear physics and has important consequences for astrophysics, since it provides an input for ...indirect dark-matter searches in space. In this paper, the latest results about the production of light (anti)nuclei in pp collisions at
s
= 13 TeV are presented, focusing on the comparison with the predictions of coalescence and thermal models. For the first time, the coalescence parameters
B
2
for deuterons and
B
3
for helions are compared with parameter-free theoretical predictions that are directly constrained by the femtoscopic measurement of the source radius in the same event class. A fair description of the data with a Gaussian wave function is observed for both deuteron and helion, supporting the coalescence mechanism for the production of light (anti)nuclei in pp collisions. This method paves the way for future investigations of the internal structure of more complex nuclear clusters, including the hypertriton.
A
bstract
The production of J
/ψ
is measured at midrapidity (
|y| <
0
.
9) in proton-proton collisions at
s
= 5
.
02 and 13 TeV, through the dielectron decay channel, using the ALICE detector at the ...Large Hadron Collider. The data sets used for the analyses correspond to integrated luminosities of
L
int
= 19.4
±
0.4 nb
−
1
and
L
int
= 32.2
±
0.5 nb
−
1
at
s
= 5
.
02 and 13 TeV, respectively. The fraction of non-prompt J
/ψ
mesons, i.e. those originating from the decay of beauty hadrons, is measured down to a transverse momentum
p
T
= 2 GeV
/c
(1 GeV
/c
) at
s
= 5
.
02 TeV (13 TeV). The
p
T
and rapidity (
y
) differential cross sections, as well as the corresponding values integrated over
p
T
and
y
, are carried out separately for prompt and non-prompt J
/ψ
mesons. The results are compared with measurements from other experiments and theoretical calculations based on quantum chromodynamics (QCD). The shapes of the
p
T
and
y
distributions of beauty quarks predicted by state-of-the-art perturbative QCD models are used to extrapolate an estimate of the
b
b
¯
pair cross section at midrapidity and in the total phase space. The total
b
b
¯
cross sections are found to be
σ
b
b
¯
= 541
±
45 (stat
.
)
±
69
syst
.
−
12
+
10
(extr
.
)
μ
b and
σ
b
b
¯
= 218
±
37 (stat
.
)
±
31
syst
.
−
9.1
+
8.2
(extr
.
)
μ
b at
s
= 13 and 5.02 TeV, respectively. The value obtained from the combination of ALICE and LHCb measurements in pp collisions at
s
= 13 TeV is also provided.
The elliptic flow of electrons from beauty hadron decays at midrapidity (|y| < 0.8) is measured in Pb-Pb collisions at √sNN = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution ...of the particles produced in the collisions can be parametrized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, v2. The v2 coefficient of electrons from beauty hadron decays is measured for the first time in the transverse momentum (pT) range 1.3–6 GeV /c in the centrality class 30%–50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length c τ ≈ 500 μm compared to that of charm hadrons and most of the other background sources. The v2 of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75 σ. The results provide insights into the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavored by the measurement at high pT, but is in agreement with the results at low pT. Transport models including substantial interactions of beauty quarks with an expanding strongly interacting medium describe the measurement within uncertainties.
We present the charged-particle multiplicity distributions over a wide pseudorapidity range (
-
3.4
<
η
<
5.0
) for pp collisions at
s
=
0.9
,
7
, and 8 TeV at the LHC. Results are based on ...information from the Silicon Pixel Detector and the Forward Multiplicity Detector of ALICE, extending the pseudorapidity coverage of the earlier publications and the high-multiplicity reach. The measurements are compared to results from the CMS experiment and to PYTHIA, PHOJET and EPOS LHC event generators, as well as IP-Glasma calculations.
The measurement of the production of deuterons, tritons and 3He and their antiparticles in Pb-Pb collisions at $\sqrt{s_{NN}}$= 5.02 TeV is presented in this article. The measurements are carried out ...at midrapidity (|y| < 0.5) as a function of collision centrality using the ALICE detector. The pT-integrated yields, the coalescence parameters and the ratios to protons and antiprotons are reported and compared with nucleosynthesis models. The comparison of these results in different collision systems at different center-of-mass collision energies reveals a suppression of nucleus production in small systems. In the Statistical Hadronisation Model framework, this can be explained by a small correlation volume where the baryon number is conserved, as already shown in previous fluctuation analyses. However, a different size of the correlation volume is required to describe the proton yields in the same data sets. The coalescence model can describe this suppression by the fact that the wave functions of the nuclei are large and the fireball size starts to become comparable and even much smaller than the actual nucleus at low multiplicities.
A
bstract
A measurement of the inclusive b-jet production cross section is presented in pp and p-Pb collisions at
s
NN
= 5
.
02 TeV, using data collected with the ALICE detector at the LHC. The jets ...were reconstructed in the central rapidity region
|η| <
0
.
5 from charged particles using the anti-
k
T
algorithm with resolution parameter
R
= 0
.
4. Identification of b jets exploits the long lifetime of b hadrons, using the properties of secondary vertices and impact parameter distributions. The
p
T
-differential inclusive production cross section of b jets, as well as the corresponding inclusive b-jet fraction, are reported for pp and p-Pb collisions in the jet transverse momentum range 10
≤ p
T
,
ch jet
≤
100 GeV/
c
, together with the nuclear modification factor,
R
pPb
b
−
jet
. The analysis thus extends the lower
p
T
limit of b-jet measurements at the LHC. The nuclear modification factor is found to be consistent with unity, indicating that the production of b jets in p-Pb at
s
NN
= 5
.
02 TeV is not affected by cold nuclear matter effects within the current precision. The measurements are well reproduced by POWHEG NLO pQCD calculations with PYTHIA fragmentation.
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