The production yield of the Λ (1520) baryon resonance is measured at midrapidity in Pb-Pb collisions at s NN = 2.76 TeV with the ALICE detector at the Large Hadron Collider (LHC). The measurement is ...performed in the Λ (1520)→ p K - (and charge conjugate) hadronic decay channel as a function of the transverse momentum ( p T ) and collision centrality. The ratio of the p T -integrated production of Λ (1520) baryons relative to Λ baryons in central collisions is suppressed by about a factor of 2 with respect to peripheral collisions. This is the first observation of the suppression of a baryonic resonance at the LHC and the first 3σ evidence of Λ (1520) suppression within a single collision system. The measured Λ (1520)/ Λ ratio in central collisions is smaller than the value predicted by the statistical hadronization model calculations. The shape of the measured p T distribution and the centrality dependence of the suppression are reproduced by the EPOS3 Monte Carlo event generator. The measurement adds further support to the formation of a dense hadronic phase in the final stages of the evolution of the fireball created in heavy-ion collisions, lasting long enough to cause a significant reduction in the observable yield of short-lived resonances.
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.
A
bstract
Inclusive J
/ψ
production is studied in minimum-bias proton-proton collisions at a centre-of-mass energy of
s
= 5
.
02 TeV by ALICE at the CERN LHC. The measurement is performed at ...mid-rapidity (
|y| <
0
.
9) in the dielectron decay channel down to zero transverse momentum
p
T
, using a data sample corresponding to an integrated luminosity of
L
int
= 19
.
4
±
0
.
4 nb
−
1
. The measured
p
T
-integrated inclusive J
/ψ
production cross sec- tion is d
σ/
d
y
= 5
.
64
±
0
.
22(stat.)
±
0
.
33(syst.)
±
0
.
12(lumi.)
μ
b. The
p
T
-differential cross section d
2
σ/
d
p
T
d
y
is measured in the
p
T
range 0–10 GeV
/c
and compared with state-of- the-art QCD calculations. The J
/ψ
〈
p
T
〉 and
p
T
2
are extracted and compared with results obtained at other collision energies.
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.
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.
ϒ production in p–Pb interactions is studied at the centre-of-mass energy per nucleon–nucleon collision sNN=8.16 TeV with the ALICE detector at the CERN LHC. The measurement is performed ...reconstructing bottomonium resonances via their dimuon decay channel, in the centre-of-mass rapidity intervals 2.03<ycms<3.53 and −4.46<ycms<−2.96, down to zero transverse momentum. In this work, results on the ϒ(1S) production cross section as a function of rapidity and transverse momentum are presented. The corresponding nuclear modification factor shows a suppression of the ϒ(1S) yields with respect to pp collisions, both at forward and backward rapidity. This suppression is stronger in the low transverse momentum region and shows no significant dependence on the centrality of the interactions. Furthermore, the ϒ(2S) nuclear modification factor is evaluated, suggesting a suppression similar to that of the ϒ(1S). A first measurement of the ϒ(3S) has also been performed. Finally, results are compared with previous ALICE measurements in p–Pb collisions at sNN=5.02 TeV and with theoretical calculations.
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.
An excess of J/ψ yield at very low transverse momentum (pT<0.3 GeV/c), originating from coherent photoproduction, is observed in peripheral and semicentral hadronic Pb–Pb collisions at a ...center-of-mass energy per nucleon pair of sNN=5.02 TeV. The measurement is performed with the ALICE detector via the dimuon decay channel at forward rapidity (2.5<y<4). The nuclear modification factor at very low pT and the coherent photoproduction cross section are measured as a function of centrality down to the 10% most central collisions. These results extend the previous study at sNN=2.76 TeV, confirming the clear excess over hadronic production in the pT range 0−0.3 GeV/c and the centrality range 70–90%, and establishing an excess with a significance greater than 5σ also in the 50–70% and 30–50% centrality ranges. The results are compared with earlier measurements at sNN=2.76 TeV and with different theoretical predictions aiming at describing how coherent photoproduction occurs in hadronic interactions with nuclear overlap.
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.
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