A
bstract
The jet angularities are a class of jet substructure observables which characterize the angular and momentum distribution of particles within jets. These observables are sensitive to ...momentum scales ranging from perturbative hard scatterings to nonperturbative fragmentation into final-state hadrons. We report measurements of several groomed and ungroomed jet angularities in pp collisions at
s
= 5
.
02 TeV with the ALICE detector. Jets are reconstructed using charged particle tracks at midrapidity (
|η| <
0
.
9). The anti-
k
T
algorithm is used with jet resolution parameters
R
= 0
.
2 and
R
= 0
.
4 for several transverse momentum
p
T
ch
jet
intervals in the 20–100 GeV/
c
range. Using the jet grooming algorithm Soft Drop, the sensitivity to softer, wide-angle processes, as well as the underlying event, can be reduced in a way which is well-controlled in theoretical calculations. We report the ungroomed jet angularities,
λ
α
, and groomed jet angularities,
λ
α
,g
, to investigate the interplay between perturbative and nonperturbative effects at low jet momenta. Various angular exponent parameters
α
= 1, 1.5, 2, and 3 are used to systematically vary the sensitivity of the observable to collinear and soft radiation. Results are compared to analytical predictions at next-to-leading-logarithmic accuracy, which provide a generally good description of the data in the perturbative regime but exhibit discrepancies in the nonperturbative regime. Moreover, these measurements serve as a baseline for future ones in heavy-ion collisions by providing new insight into the interplay between perturbative and nonperturbative effects in the angular and momentum substructure of jets. They supply crucial guidance on the selection of jet resolution parameter, jet transverse momentum, and angular scaling variable for jet quenching studies.
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
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.
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
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
The first measurements of elliptic flow of
π
±
, K
±
,
p
+
p
¯
,
K
S
0
,
Λ
+
Λ
¯
,
ϕ
,
Ξ
−
+
Ξ
¯
+
, and
Ω
−
+
Ω
¯
+
using multiparticle cumulants in Pb–Pb collisions at
s
NN
= 5
.
02 TeV ...are resented. Results obtained with two- (
v
2
{2}) and four-particle cumulants (
v
2
{4}) are shown as a function of transverse momentum,
p
T
, for various collision centrality intervals. Combining the data for both
v
2
{2} and
v
2
{4} also allows us to report the first measurements of the mean elliptic flow, elliptic flow fluctuations, and relative elliptic flow fluctuations for various hadron species. These observables probe the event-by-event eccentricity fluctuations in the initial state and the contributions from the dynamic evolution of the expanding quark–gluon plasma. The characteristic features observed in previous
p
T
-differential anisotropic flow measurements for identified hadrons with two-particle correlations, namely the mass ordering at low
p
T
and the approximate scaling with the number of constituent quarks at intermediate
p
T
, are similarly present in the four-particle correlations and the combinations of
v
2
{2} and
v
2
{4}. In addition, a particle species dependence of flow fluctuations is observed that could indicate a significant contribution from final state hadronic interactions. The comparison between experimental measurements and CoLBT model calculations, which combine the various physics processes of hydrodynamics, quark coalescence, and jet fragmentation, illustrates their importance over a wide
p
T
range.
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.
A
bstract
A measurement of inclusive, prompt, and non-prompt J/
ψ
production in p-Pb collisions at a nucleon-nucleon centre-of-mass energy
s
NN
= 5
.
02 TeV is presented. The inclusive J/
ψ
mesons ...are reconstructed in the dielectron decay channel at midrapidity down to a transverse momentum
p
T
= 0. The inclusive J/
ψ
nuclear modification factor
R
pPb
is calculated by comparing the new results in p-Pb collisions to a recently measured proton-proton reference at the same centre-of-mass energy. Non-prompt J/
ψ
mesons, which originate from the decay of beauty hadrons, are separated from promptly produced J/
ψ
on a statistical basis for
p
T
larger than 1.0 GeV/
c
. These results are based on the data sample collected by the ALICE detector during the 2016 LHC p-Pb run, corresponding to an integrated luminosity
L
int
= 292
±
11
μ
b
−
1
, which is six times larger than the previous publications. The total uncertainty on the
p
T
-integrated inclusive J/
ψ
and non-prompt J/
ψ
cross section are reduced by a factor 1.7 and 2.2, respectively. The measured cross sections and
R
pPb
are compared with theoretical models that include various combinations of cold nuclear matter effects. From the non-prompt J/
ψ
production cross section, the
b
b
¯
production cross section at midrapidity,
d
σ
b
b
¯
/
d
y
, and the total cross section extrapolated over full phase space,
σ
b
b
¯
, are derived.
A
bstract
The production of inclusive, prompt and non-prompt J/
ψ
was studied for the first time at midrapidity (−1
.
37
< y
cms
<
0
.
43) in p-Pb collisions at
s
NN
= 8
.
16 TeV with the ALICE ...detector at the LHC. The inclusive J/
ψ
mesons were reconstructed in the dielectron decay channel in the transverse momentum (
p
T
) interval 0
< p
T
<
14 GeV/
c
and the prompt and non-prompt contributions were separated on a statistical basis for
p
T
>
2 GeV/
c
. The study of the J/
ψ
mesons in the dielectron channel used for the first time in ALICE online single-electron triggers from the Transition Radiation Detector, providing a data sample corresponding to an integrated luminosity of 689 ± 13
μ
b
−
1
. The proton-proton reference cross section for inclusive J/
ψ
was obtained based on interpolations of measured data at different centre-of-mass energies and a universal function describing the
p
T
-differential J/
ψ
production cross sections. The
p
T
-differential nuclear modification factors
R
pPb
of inclusive, prompt, and non-prompt J/
ψ
are consistent with unity and described by theoretical models implementing only nuclear shadowing.
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.