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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The pT-differential cross sections of prompt charm-strange baryons Ξc0 and Ξc+ were measured at midrapidity (|y|<0.5) in proton-proton (pp) collisions at a center-of-mass energy s=13 TeV with the ...ALICE detector at the LHC. The Ξc0 baryon was reconstructed via both the semileptonic decay (Ξ-e+νe) and the hadronic decay (Ξ-π+) channels. The Ξc+ baryon was reconstructed via the hadronic decay (Ξ-π+π+) channel. The branching-fraction ratio BR(Ξc0→Ξ-e+νe)/BR(Ξc0→Ξ-π+)=1.38±0.14(stat)±0.22(syst) was measured with a total uncertainty reduced by a factor of about 3 with respect to the current world average reported by the Particle Data Group. The transverse momentum (pT) dependence of the Ξc0- and Ξc+-baryon production relative to the D0 meson and to the ςc0,+,++- and Λc+-baryon production are reported. The baryon-to-meson ratio increases toward low pT up to a value of approximately 0.3. The measurements are compared with various models that take different hadronization mechanisms into consideration. The results provide stringent constraints to these theoretical calculations and additional evidence that different processes are involved in charm hadronization in electron-positron (e+e-) and hadronic collisions.
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 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.
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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.
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.
The study of the production of nuclei and antinuclei 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 protons, deuterons and
3
He
and their charge conjugates at midrapidity is studied as a function of the charged-particle multiplicity in inelastic pp collisions at
s
=
5.02
TeV using the ALICE detector. Within the uncertainties, the yields of nuclei in pp collisions at
s
=
5.02
TeV are compatible with those in pp collisions at different energies and to those in p–Pb collisions when compared at similar multiplicities. The measurements are compared with the expectations of coalescence and Statistical Hadronisation Models. The results suggest a common formation mechanism behind the production of light nuclei in hadronic interactions and confirm that they do not depend on the collision energy but on the number of produced particles.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This article reports on the inclusive production cross section of several quarkonium states,
J
/
ψ
,
ψ
(
2
S
)
,
Υ
(
1
S
)
,
Υ
(
2
S
)
, and
Υ
(
3
S
)
, measured with the ALICE detector at the LHC, ...in pp collisions at
s
=
5.02
TeV. The analysis is performed in the dimuon decay channel at forward rapidity (
2.5
<
y
<
4
). The integrated cross sections and transverse-momentum (
p
T
) and rapidity (
y
) differential cross sections for
J
/
ψ
,
ψ
(
2
S
)
,
Υ
(
1
S
)
, and the
ψ
(
2
S
)
-to-
J
/
ψ
cross section ratios are presented. The integrated cross sections, assuming unpolarized quarkonia, are:
σ
J
/
ψ
(
p
T
<
20
GeV/c) = 5.88 ± 0.03 ± 0.34
μ
b,
σ
ψ
(
2
S
)
(
p
T
<
12
GeV/c) = 0.87 ± 0.06 ± 0.10
μ
b,
σ
Υ
(
1
S
)
(
p
T
<
15
GeV/c) = 45.5 ± 3.9 ± 3.5 nb,
σ
Υ
(
2
S
)
(
p
T
<
15
GeV/c) = 22.4 ± 3.2 ± 2.7 nb, and
σ
Υ
(
3
S
)
(
p
T
<
15
GeV/c) = 4.9 ± 2.2 ± 1.0 nb, where the first (second) uncertainty is the statistical (systematic) one. For the first time, the cross sections of the three
Υ
states, as well as the
ψ
(
2
S
)
one as a function of
p
T
and
y
, are measured at
s
=
5.02
TeV at forward rapidity. These measurements also significantly extend the
J
/
ψ
p
T
reach and supersede previously published results. A comparison with ALICE measurements in pp collisions at
s
=
2.76
, 7, 8, and 13 TeV is presented and the energy dependence of quarkonium production cross sections is discussed. Finally, the results are compared with the predictions from several production models.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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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