We report on two-particle charge-dependent correlations in pp, p–Pb, and Pb–Pb collisions as a function of the pseudorapidity and azimuthal angle difference,
Δ
η
and
Δ
φ
respectively. These ...correlations are studied using the balance function that probes the charge creation time and the development of collectivity in the produced system. The dependence of the balance function on the event multiplicity as well as on the trigger and associated particle transverse momentum (
p
T
) in pp, p–Pb, and Pb–Pb collisions at
s
NN
=
7, 5.02, and 2.76 TeV, respectively, are presented. In the low transverse momentum region, for
0.2
<
p
T
<
2.0
GeV/
c
, the balance function becomes narrower in both
Δ
η
and
Δ
φ
directions in all three systems for events with higher multiplicity. The experimental findings favor models that either incorporate some collective behavior (e.g. AMPT) or different mechanisms that lead to effects that resemble collective behavior (e.g. PYTHIA8 with color reconnection). For higher values of transverse momenta the balance function becomes even narrower but exhibits no multiplicity dependence, indicating that the observed narrowing with increasing multiplicity at low
p
T
is a feature of bulk particle production.
An invariant differential cross section measurement of inclusive
π
0
and
η
meson production at mid-rapidity in pp collisions at
s
=
8
TeV was carried out by the ALICE experiment at the LHC. The ...spectra of
π
0
and
η
mesons were measured in transverse momentum ranges of
0.3
<
p
T
<
35
GeV/c
and
0.5
<
p
T
<
35
GeV/c
, respectively. Next-to-leading order perturbative QCD calculations using fragmentation functions DSS14 for the
π
0
and AESSS for the
η
overestimate the cross sections of both neutral mesons, although such calculations agree with the measured
η
/
π
0
ratio within uncertainties. The results were also compared with PYTHIA 8.2 predictions for which the Monash 2013 tune yields the best agreement with the measured neutral meson spectra. The measurements confirm a universal behavior of the
η
/
π
0
ratio seen for NA27, PHENIX and ALICE data for pp collisions from
s
=
27.5
GeV to
s
=
8
TeV within experimental uncertainties. A relation between the
π
0
and
η
production cross sections for pp collisions at
s
=
8
TeV is given by
m
T
scaling for
p
T
>
3.5
GeV/c
. However, a deviation from this empirical scaling rule is observed for transverse momenta below
p
T
<
3.5
GeV/c
in the
η
/
π
0
ratio with a significance of
6.2
σ
.
A
bstract
The production of charm jets in proton-proton collisions at a center-of-mass energy of
s
=
7
TeV was measured with the ALICE detector at the CERN Large Hadron Collider. The measurement is ...based on a data sample corresponding to a total integrated luminosity of 6
.
23 nb
−1
, collected using a minimum-bias trigger. Charm jets are identified by the presence of a D
0
meson among their constituents. The D
0
mesons are reconstructed from their hadronic decay D
0
→K
−
π
+
. The D
0
-meson tagged jets are reconstructed using tracks of charged particles (track-based jets) with the anti-
k
T
algorithm in the jet transverse momentum range
5
<
p
T
,
jet
ch
<
30 GeV
/c
and pseudorapidity |
η
jet
|
<
0
.
5. The fraction of charged jets containing a D
0
-meson increases with
p
T
,
jet
ch
from 0
.
042 ± 0
.
004 (stat) ± 0
.
006 (syst) to 0
.
080 ± 0
.
009 (stat) ± 0
.
008 (syst). The distribution of D
0
-meson tagged jets as a function of the jet momentum fraction carried by the D
0
meson in the direction of the jet axis
z
∥
ch
is reported for two ranges of jet transverse momenta,
5
<
p
T
,
jet
ch
<
15 GeV
/c
and
15
<
p
T
,
jet
ch
<
30 GeV
/c
in the intervals
0.2
<
z
‖
ch
<
1.0
and
0.4
<
z
‖
ch
<
1.0
, respectively. The data are compared with results from Monte Carlo event generators (PYTHIA 6, PYTHIA 8 and Herwig 7) and with a Next-to-Leading-Order perturbative Quantum Chromodynamics calculation, obtained with the POWHEG method and interfaced with PYTHIA 6 for the generation of the parton shower, fragmentation, hadronisation and underlying event.
Direct photons have been measured in √sNN=200 GeV d+Au collisions at midrapidity. A wide pT range is covered by measurements of nearly real virtual photons (1<pT<6 GeV/c) and real photons (5<pT<16 ...GeV/c). The invariant yield of the direct photons in d+Au collisions over the scaled p+p cross section is consistent with unity. Theoretical calculations assuming standard cold-nuclear-matter effects describe the data well for the entire pT range. This indicates that the large enhancement of direct photons observed in Au+Au collisions for 1.0<pT<2.5 GeV/c is attributable to a source other than the initial-state nuclear effects.
A
bstract
The energy deposited at very forward rapidities (very forward energy) is a powerful tool for characterising proton fragmentation in pp and p-Pb collisions. The correlation of very forward ...energy with particle production at midrapidity provides direct insights into the initial stages and the subsequent evolution of the collision. Furthermore, the correlation with the production of particles with large transverse momenta at midrapidity provides information complementary to the measurements of the underlying event, which are usually interpreted in the framework of models implementing centrality-dependent multiple parton interactions.
Results about very forward energy, measured by the ALICE zero degree calorimeters (ZDCs), and its dependence on the activity measured at midrapidity in pp collisions at
s
= 13 TeV and in p-Pb collisions at
s
NN
= 8
.
16 TeV are discussed. The measurements performed in pp collisions are compared with the expectations of three hadronic interaction event generators: PYTHIA 6 (Perugia 2011 tune), PYTHIA 8 (Monash tune), and EPOS LHC. These results provide new constraints on the validity of models in describing the beam remnants at very forward rapidities, where perturbative QCD cannot be used.
First measurements of balance functions (BFs) of all combinations of identified charged hadron (π, Κ, ρ) pairs in Pb–Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV recorded by the ALICE detector are ...presented. The BF measurements are carried out as two-dimensional differential correlators versus the relative rapidity (Δy) and azimuthal angle (Δφ) of hadron pairs, and studied as a function of collision centrality. The Δφ dependence of BFs is expected to be sensitive to the light quark diffusivity in the quark–gluon plasma. While the BF azimuthal widths of all pairs substantially decrease from peripheral to central collisions, the longitudinal widths exhibit mixed behaviors: BFs of ππ and cross-species pairs narrow significantly in more central collisions, whereas those of KK and pp are found to be independent of collision centrality. This dichotomy is qualitatively consistent with the presence of strong radial flow effects and the existence of two stages of quark production in relativistic heavy-ion collisions. Finally, the first measurements of the collision centrality evolution of BF integrals are presented, with the observation that charge balancing fractions are nearly independent of collision centrality in Pb–Pb collisions. Overall, the results presented provide new and challenging constraints for theoretical models of hadron production and transport in relativistic heavy-ion collisions.
Abstract
Angular correlations of heavy-flavour and charged particles in high-energy proton–proton collisions are sensitive to the production mechanisms of heavy quarks and to their fragmentation as ...well as hadronisation processes. The measurement of the azimuthal-correlation function of prompt D mesons with charged particles in proton–proton collisions at a centre-of-mass energy of
$$\sqrt{s} = 13$$
s
=
13
TeV with the ALICE detector is reported, considering
$$\mathrm D^{0} $$
D
0
,
$$\mathrm D^{+} $$
D
+
, and
$$\mathrm D^{*+} $$
D
∗
+
mesons in the transverse-momentum interval
$$3< p_{\mathrm{T}} < 36$$
3
<
p
T
<
36
GeV/
$$c$$
c
at midrapidity (
$$|y| < 0.5$$
|
y
|
<
0.5
), and charged particles with
$$p_{\mathrm{T}} > 0.3$$
p
T
>
0.3
GeV/
$$c$$
c
and pseudorapidity
$$|\eta | < 0.8$$
|
η
|
<
0.8
. This measurement has an improved precision and provides an extended transverse-momentum coverage compared to previous ALICE measurements at lower energies. The study is also performed as a function of the charged-particle multiplicity, showing no modifications of the correlation function with multiplicity within uncertainties. The properties and the transverse-momentum evolution of the near- and away-side correlation peaks are studied and compared with predictions from various Monte Carlo event generators. Among those considered, PYTHIA8 and POWHEG+PYTHIA8 provide the best description of the measured observables. The obtained results can provide guidance on tuning the generators.
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