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.
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.
A
bstract
Two-particle angular correlations are measured in high-multiplicity proton-proton collisions at
s
= 13 TeV by the ALICE Collaboration. The yields of particle pairs at short-(∆
η
∼ 0) and ...long-range (1
.
6
<
|∆
η
|
<
1
.
8) in pseudorapidity are extracted on the near-side (∆
φ
∼ 0). They are reported as a function of transverse momentum (
p
T
) in the range 1
< p
T
<
4 GeV/
c
. Furthermore, the event-scale dependence is studied for the first time by requiring the presence of high-
p
T
leading particles or jets for varying
p
T
thresholds. The results demonstrate that the long-range “ridge” yield, possibly related to the collective behavior of the system, is present in events with high-
p
T
processes as well. The magnitudes of the short- and long-range yields are found to grow with the event scale. The results are compared to EPOS LHC and PYTHIA 8 calculations, with and without string-shoving interactions. It is found that while both models describe the qualitative trends in the data, calculations from EPOS LHC show a better quantitative agreement for the
p
T
dependency, while overestimating the event-scale dependency.
Differential cross sections of charged particles in inelastic pp collisions as a function of
p
T
have been measured at
at the LHC. The
p
T
spectra are compared to NLO-pQCD calculations. Though the ...differential cross section for an individual
cannot be described by NLO-pQCD, the relative increase of cross section with
is in agreement with NLO-pQCD. Based on these measurements and observations, procedures are discussed to construct pp reference spectra at
up to
p
T
=50 GeV/
c
as required for the calculation of the nuclear modification factor in nucleus–nucleus and proton–nucleus collisions.
The measurement of the azimuthal-correlation function of prompt D mesons with charged particles in pp collisions at
s
=
5.02
TeV
and p–Pb collisions at
s
NN
=
5.02
TeV
with the ALICE detector at the ...LHC is reported. The
D
0
,
D
+
, and
D
∗
+
mesons, together with their charge conjugates, were reconstructed at midrapidity in the transverse momentum interval
3
<
p
T
<
24
GeV
/
c
and correlated with charged particles having
p
T
>
0.3
GeV
/
c
and pseudorapidity
|
η
|
<
0.8
. The properties of the correlation peaks appearing in the near- and away-side regions (for
Δ
φ
≈
0
and
Δ
φ
≈
π
, respectively) were extracted via a fit to the azimuthal correlation functions. The shape of the correlation functions and the near- and away-side peak features are found to be consistent in pp and p–Pb collisions, showing no modifications due to nuclear effects within uncertainties. The results are compared with predictions from Monte Carlo simulations performed with the PYTHIA, POWHEG+PYTHIA, HERWIG, and EPOS 3 event generators.
A
bstract
The first measurement of the e
+
e
−
pair production at low lepton pair transverse momentum (
p
T
,
ee
) and low invariant mass (
m
ee
) in non-central Pb–Pb collisions at
s
NN
= 5
.
02 TeV ...at the LHC is presented. The dielectron production is studied with the ALICE detector at midrapidity (
|η
e
| <
0
.
8) as a function of invariant mass (0.4
≤ m
ee
<
2
.
7 GeV/
c
2
) in the 50–70% and 70–90% centrality classes for
p
T
,
ee
<
0.1 GeV/
c
, and as a function of
p
T
,
ee
in three
m
ee
intervals in the most peripheral Pb–Pb collisions. Below a
p
T
,
ee
of 0.1 GeV/
c
, a clear excess of e
+
e
−
pairs is found compared to the expectations from known hadronic sources and predictions of thermal radiation from the medium. The
m
ee
excess spectra are reproduced, within uncertainties, by different predictions of the photon–photon production of dielectrons, where the photons originate from the extremely strong electromagnetic fields generated by the highly Lorentz-contracted Pb nuclei. Lowest-order quantum electrodynamic (QED) calculations, as well as a model that takes into account the impact-parameter dependence of the average transverse momentum of the photons, also provide a good description of the
p
T
,
ee
spectra. The measured
p
T
,
ee
2
of the excess
p
T
,
ee
spectrum in peripheral Pb–Pb collisions is found to be comparable to the values observed previously at RHIC in a similar phase-space region.
Three-body nuclear forces play an important role in the structure of nuclei and hypernuclei and are also incorporated in models to describe the dynamics of dense baryonic matter, such as in neutron ...stars. So far, only indirect measurements anchored to the binding energies of nuclei can be used to constrain the three-nucleon force, and if hyperons are considered, the scarce data on hypernuclei impose only weak constraints on the three-body forces. In this work, we present the first direct measurement of the p–p–p and p–p–
Λ
systems in terms of three-particle correlation functions carried out for pp collisions at
s
=
13
TeV. Three-particle cumulants are extracted from the correlation functions by applying the Kubo formalism, where the three-particle interaction contribution to these correlations can be isolated after subtracting the known two-body interaction terms. A negative cumulant is found for the p–p–p system, hinting to the presence of a residual three-body effect while for p–p–
Λ
the cumulant is consistent with zero. This measurement demonstrates the accessibility of three-baryon correlations at the LHC.
A
bstract
The production of J/
ψ
is measured as a function of charged-particle multiplicity at forward rapidity in proton-proton (pp) collisions at center-of-mass energies
s
= 5.02 and 13 TeV. The J/
...ψ
mesons are reconstructed via their decay into dimuons in the rapidity interval (2.5
< y <
4.0), whereas the charged-particle multiplicity density (d
N
ch
/
d
η
) is measured at midrapidity (
|η| <
1). The production rate as a function of multiplicity is reported as the ratio of the yield in a given multiplicity interval to the multiplicity-integrated one. This observable shows a linear increase with charged-particle multiplicity normalized to the corresponding average value for inelastic events (d
N
ch
/d
η
/〈d
N
ch
/d
η
〉), at both the colliding energies. Measurements are compared with available ALICE results at midrapidity and theoretical model calculations. First measurement of the mean transverse momentum (〈
p
T
〉) of J/
ψ
in pp collisions exhibits an increasing trend as a function of d
N
ch
/d
η
/〈d
N
ch
/d
η
〉 showing a saturation towards high charged-particle multiplicities.
A
bstract
Measurements of elliptic (
v
2
) and triangular (
v
3
) flow coefficients of
π
±
, K
±
, p+
p
¯
,
K
S
0
, and Λ+
Λ
¯
obtained with the scalar product method in Xe-Xe collisions at
s
NN
= 5
....
44 TeV are presented. The results are obtained in the rapidity range |
y
|
<
0
.
5 and reported as a function of transverse momentum,
p
T
, for several collision centrality classes. The flow coefficients exhibit a particle mass dependence for
p
T
<
3 GeV/
c
, while a grouping according to particle type (i.e., meson and baryon) is found at intermediate transverse momenta (3
< p
T
<
8 GeV/
c
). The magnitude of the baryon
v
2
is larger than that of mesons up to
p
T
= 6 GeV/
c
. The centrality dependence of the shape evolution of the
p
T
-differential
v
2
is studied for the various hadron species. The
v
2
coefficients of
π
±
, K
±
, and p+
p
¯
are reproduced by MUSIC hydrodynamic calculations coupled to a hadronic cascade model (UrQMD) for
p
T
<
1 GeV/
c
. A comparison with
v
n
measurements in the corresponding centrality intervals in Pb-Pb collisions at
s
NN
= 5
.
02 TeV yields an enhanced
v
2
in central collisions and diminished value in semicentral collisions.
A
bstract
Inclusive
ψ
(2S) production is measured in p-Pb collisions at the centre-of-mass energy per nucleon-nucleon pair
s
NN
= 8
.
16 TeV, using the ALICE detector at the CERN LHC. The production ...of
ψ
(2S) is studied at forward (2
.
03
< y
cms
<
3
.
53) and backward (
−
4
.
46
< y
cms
< −
2
.
96) centre-of-mass rapidity and for transverse momentum
p
T
<
12 GeV/
c
via the decay to muon pairs. In this paper, we report the integrated as well as the
y
cms
- and
p
T
-differential inclusive production cross sections. Nuclear effects on
ψ
(2S) production are studied via the determination of the nuclear modification factor that shows a strong suppression at both forward and backward centre-of-mass rapidities. Comparisons with corresponding results for inclusive J/
ψ
show a similar suppression for the two states at forward rapidity (p-going direction), but a stronger suppression for
ψ
(2S) at backward rapidity (Pb-going direction). As a function of
p
T
, no clear dependence of the nuclear modification factor is found. The relative size of nuclear effects on
ψ
(2S) production compared to J/
ψ
is also studied via the double ratio of production cross sections
σ
ψ
(2S)
/σ
J
/ψ
pPb
/
σ
ψ
(2S)
/σ
J
/ψ
pp
between p-Pb and pp collisions. The results are compared with theoretical models that include various effects related to the initial and final state of the collision system and also with previous measurements at
s
NN
= 5
.
02 TeV.
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