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 cross section for incoherent photonuclear production of J/ψ vector mesons as a function of the Mandelstam |t| variable is presented. The measurement was carried out with ...the ALICE detector at midrapidity, |y|<0.8, using ultraperipheral collisions of Pb nuclei at a center-of-mass energy per nucleon pair of sqrts_{NN}=5.02 TeV. This rapidity interval corresponds to a Bjorken-x range (0.3-1.4)×10^{-3}. Cross sections are given in five |t| intervals in the range 0.04<|t|<1 GeV^{2} and compared to the predictions by different models. Models that ignore quantum fluctuations of the gluon density in the colliding hadron predict a |t| dependence of the cross section much steeper than in data. The inclusion of such fluctuations in the same models provides a better description of the data.
This Letter presents the measurement of near-side associated per-trigger yields, denoted ridge yields, from the analysis of angular correlations of charged hadrons in proton-proton collisions at ...sqrts=13 TeV. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of 1.4<|Δη|<1.8 and a transverse momentum of 1<p_{T}<2 GeV/c, as a function of the charged-particle multiplicity measured at midrapidity. This Letter extends the measurements of the ridge yield to the low multiplicity region, where in hadronic collisions it is typically conjectured that a strongly interacting medium is unlikely to be formed. The precision of the new low multiplicity results allows for the first direct quantitative comparison with the results obtained in e^{+}e^{-} collisions at sqrts=91 GeV and sqrts=183-209 GeV, where initial-state effects such as preequilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range 8≲⟨N_{ch}⟩≲24 where the e^{+}e^{-} results have good precision, the measured ridge yields in pp collisions are substantially larger than the limits set in e^{+}e^{-} annihilations. Consequently, the findings presented in this Letter suggest that the processes involved in e^{+}e^{-} annihilations do not contribute significantly to the emergence of long-range correlations in pp collisions.
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.
A
bstract
The transverse momentum (
p
T
) and centrality dependence of the nuclear modification factor
R
AA
of prompt and non-prompt J/
ψ
, the latter originating from the weak decays of beauty ...hadrons, have been measured by the ALICE collaboration in Pb–Pb collisions at
s
NN
= 5.02 TeV. The measurements are carried out through the e
+
e
−
decay channel at midrapidity (|
y
| < 0.9) in the transverse momentum region 1.5 <
p
T
< 10 GeV/
c
. Both prompt and non-prompt J/
ψ
measurements indicate a significant suppression for
p
T
> 5 GeV/
c
, which becomes stronger with increasing collision centrality. The results are consistent with similar LHC measurements in the overlapping
p
T
intervals, and cover the kinematic region down to
p
T
= 1.5 GeV/
c
at midrapidity, not accessible by other LHC experiments. The suppression of prompt J/
ψ
in central and semicentral collisions exhibits a decreasing trend towards lower transverse momentum, described within uncertainties by models implementing J/
ψ
production from recombination of c and
c
¯
quarks produced independently in different partonic scatterings. At high transverse momentum, transport models including quarkonium dissociation are able to describe the suppression for prompt J/
ψ
. For non-prompt J/
ψ
, the suppression predicted by models including both collisional and radiative processes for the computation of the beauty-quark energy loss inside the quark-gluon plasma is consistent with measurements within uncertainties.
A
bstract
Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the “ridge” phenomenon, were discovered in heavy-ion collisions, and later ...found in pp and p–Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small collision systems. In this Letter, measurements of the long-range correlations in p–Pb collisions at
s
NN
= 5
.
02 TeV are extended to a pseudorapidity gap of ∆
η
~ 8 between particles using the ALICE forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of ∆
η
~ 8 for the first time in p–Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small collision systems such as p–Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient,
v
2
(
η
), is extracted from the long-range correlations. The
v
2
(
η
) results are presented for a wide pseudorapidity range of –3
.
1
< η <
4
.
8 in various centrality classes in p–Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small collision systems, the
v
2
(
η
) measurements are compared with hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small collision systems.
A
bstract
Measurements of the production cross sections of prompt D
0
, D
+
, D
*+
,
D
s
+
,
Λ
c
+
, and
Ξ
c
+
charm hadrons at midrapidity in proton-proton collisions at
s
= 13 TeV with the ALICE ...detector are presented. The D-meson cross sections as a function of transverse momentum (
p
T
) are provided with improved precision and granularity. The ratios of
p
T
-differential meson production cross sections based on this publication and on measurements at different rapidity and collision energy provide a constraint on gluon parton distribution functions at low values of Bjorken-
x
(10
−
5
–10
−
4
). The measurements of
Λ
c
+
(
Ξ
c
+
) baryon production extend the measured
p
T
intervals down to
p
T
= 0(3) GeV
/c
. These measurements are used to determine the charm-quark fragmentation fractions and the
c
c
¯
production cross section at midrapidity (|
y
|
<
0
.
5) based on the sum of the cross sections of the weakly-decaying ground-state charm hadrons D
0
, D
+
,
D
s
+
,
Λ
c
+
,
Ξ
c
0
and, for the first time,
Ξ
c
+
, and of the strongly-decaying J
/ψ
mesons. The first measurements of
Ξ
c
+
and
Σ
c
0
,
+
+
fragmentation fractions at midrapidity are also reported. A significantly larger fraction of charm quarks hadronising to baryons is found compared to e
+
e
−
and ep collisions. The
c
c
¯
production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations.
The elliptic flow
(
v
2
)
of
D
0
mesons from beauty-hadron decays (non-prompt
D
0
)
was measured in midcentral (30–50%) Pb–Pb collisions at a centre-of-mass energy per nucleon pair
s
NN
=
5.02
TeV ...with the ALICE detector at the LHC. The
D
0
mesons were reconstructed at midrapidity
(
|
y
|
<
0.8
)
from their hadronic decay
D
0
→
K
-
π
+
, in the transverse momentum interval
2
<
p
T
<
12
GeV/
c
. The result indicates a positive
v
2
for non-prompt
D
0
mesons with a significance of 2.7
σ
. The non-prompt
D
0
-meson
v
2
is lower than that of prompt non-strange D mesons with 3.2
σ
significance in
2
<
p
T
<
8
GeV
/
c
, and compatible with the
v
2
of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties.
A
bstract
The ALICE Collaboration reports a search for jet quenching effects in high-multiplicity (HM) proton-proton collisions at
s
= 13 TeV, using the semi-inclusive azimuthal-difference ...distribution ∆
φ
of charged-particle jets recoiling from a high transverse momentum (high-
p
T
,
trig
) trigger hadron. Jet quenching may broaden the ∆
φ
distribution measured in HM events compared to that in minimum bias (MB) events. The measurement employs a
p
T
,
trig
-differential observable for data-driven suppression of the contribution of multiple partonic interactions, which is the dominant background. While azimuthal broadening is indeed observed in HM compared to MB events, similar broadening for HM events is observed for simulations based on the PYTHIA 8 Monte Carlo generator, which does not incorporate jet quenching. Detailed analysis of these data and simulations show that the azimuthal broadening is due to bias of the HM selection towards events with multiple jets in the final state. The identification of this bias has implications for all jet quenching searches where selection is made on the event activity.
