A
bstract
We report the measured transverse momentum (
p
T
) spectra of primary charged particles from pp, p-Pb and Pb-Pb collisions at a center-of-mass energy
s
N
N
=
5.02
TeV in the kinematic range ...of 0
.
15
< p
T
<
50 GeV/
c
and |
η
|
<
0
.
8. A significant improvement of systematic uncertainties motivated the reanalysis of data in pp and Pb-Pb collisions at
s
N
N
=
2.76
TeV, as well as in p-Pb collisions at
s
N
N
=
5.02
TeV, which is also presented. Spectra from Pb-Pb collisions are presented in nine centrality intervals and are compared to a reference spectrum from pp collisions scaled by the number of binary nucleon-nucleon collisions. For central collisions, the
p
T
spectra are suppressed by more than a factor of 7 around 6–7 GeV/
c
with a significant reduction in suppression towards higher momenta up to 30 GeV/
c
. The nuclear modification factor
R
pPb
, constructed from the pp and p-Pb spectra measured at the same collision energy, is consistent with unity above 8 GeV/
c
. While the spectra in both pp and Pb-Pb collisions are substantially harder at
s
N
N
=
5.02
TeV compared to 2.76 TeV, the nuclear modification factors show no significant collision energy dependence. The obtained results should provide further constraints on the parton energy loss calculations to determine the transport properties of the hot and dense QCD matter.
In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as ...described by the theory of quantum chromodynamics (QCD)
. These partons subsequently emit further partons in a process that can be described as a parton shower
, which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass m
and energy E, within a cone of angular size m
/E around the emitter
. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques
to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.
The interaction of
K
-
with protons is characterised by the presence of several coupled channels, systems like
K
¯
0
n and
π
Σ
with a similar mass and the same quantum numbers as the
K
-
p state. The ...strengths of these couplings to the
K
-
p system are of crucial importance for the understanding of the nature of the
Λ
(
1405
)
resonance and of the attractive
K
-
p strong interaction. In this article, we present measurements of the
K
-
p correlation functions in relative momentum space obtained in pp collisions at
s
=
13
Te, in p–Pb collisions at
s
NN
=
5.02
Te, and (semi)peripheral Pb–Pb collisions at
s
NN
=
5.02
Te. The emitting source size, composed of a core radius anchored to the
K
+
p correlation and of a resonance halo specific to each particle pair, varies between 1 and 2 fm in these collision systems. The strength and the effects of the
K
¯
0
n and
π
Σ
inelastic channels on the measured
K
-
p correlation function are investigated in the different colliding systems by comparing the data with state-of-the-art models of chiral potentials. A novel approach to determine the conversion weights
ω
, necessary to quantify the amount of produced inelastic channels in the correlation function, is presented. In this method, particle yields are estimated from thermal model predictions, and their kinematic distribution from blast-wave fits to measured data. The comparison of chiral potentials to the measured
K
-
p interaction indicates that, while the
π
Σ
–
K
-
p dynamics is well reproduced by the model, the coupling to the
K
¯
0
n channel in the model is currently underestimated.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
Measurements of charged-particle production in pp, p–Pb, and Pb–Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined ...event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum
p
T
trig
in the range 8
<
p
T
trig
<
15 GeV
/c
. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier,
R
T
=
N
ch
T
/
N
ch
T
, is used to group events according to their UE activity, where
N
ch
T
is the charged-particle multiplicity per event in the transverse region and
N
ch
T
is the mean value over the whole analysed sample. The energy dependence of the
R
T
distributions in pp collisions at
s
= 2
.
76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle
p
T
spectra as a function of
R
T
in the three azimuthal regions in pp, p–Pb, and Pb–Pb collisions at
s
NN
= 5
.
02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p–Pb).
A
bstract
Measurements of the production of electrons from heavy-flavour hadron decays in pp collisions at
s
= 13 TeV at midrapidity with the ALICE detector are presented down to a transverse ...momentum (
p
T
) of 0.2 GeV/
c
and up to
p
T
= 35 GeV/
c
, which is the largest momentum range probed for inclusive electron measurements in ALICE. In p-Pb collisions, the production cross section and the nuclear modification factor of electrons from heavy-flavour hadron decays are measured in the
p
T
range 0
.
5
< p
T
<
26 GeV/
c
at
s
NN
= 8
.
16 TeV. The nuclear modification factor is found to be consistent with unity within the statistical and systematic uncertainties. In both collision systems, first measurements of the yields of electrons from heavy-flavour hadron decays in different multiplicity intervals normalised to the multiplicity-integrated yield (self-normalised yield) at midrapidity are reported as a function of the self-normalised charged-particle multiplicity estimated at midrapidity. The self-normalised yields in pp and p-Pb collisions grow faster than linear with the self-normalised multiplicity. A strong
p
T
dependence is observed in pp collisions, where the yield of high-
p
T
electrons increases faster as a function of multiplicity than the one of low-
p
T
electrons. The measurement in p-Pb collisions shows no
p
T
dependence within uncertainties. The self-normalised yields in pp and p-Pb collisions are compared with measurements of other heavy-flavour, light-flavour, and strange particles, and with Monte Carlo simulations.
A
bstract
The Chiral Magnetic Wave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics ...of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision
s
NN
= 5
.
02 TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic (
v
2
) and triangular (
v
3
) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope
r
3
Norm
is found to be larger than zero and to have a magnitude similar to
r
2
Norm
, thus pointing to a large background contribution for these measurements. Furthermore,
r
2
Norm
can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW (
f
CMW
) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for
f
CMW
, and in the 10–60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level.
A
bstract
The production of the W
±
bosons measured in p–Pb collisions at a centre-of-mass energy per nucleon–nucleon collision
$$ \sqrt{s_{\textrm{NN}}} $$
s
NN
= 8
.
16 TeV and Pb–Pb collisions at
...$$ \sqrt{s_{\textrm{NN}}} $$
s
NN
= 5
.
02 TeV with ALICE at the LHC is presented. The W
±
bosons are measured via their muonic decay channel, with the muon reconstructed in the pseudorapidity region
−
4
<
$$ {\eta}_{\textrm{lab}}^{\mu } $$
η
lab
μ
< −
2
.
5 with transverse momentum
$$ {p}_{\textrm{T}}^{\mu } $$
p
T
μ
>
10 GeV
/c
. While in Pb–Pb collisions the measurements are performed in the forward (2
.
5
<
$$ {y}_{\textrm{cms}}^{\mu } $$
y
cms
μ
<
4) rapidity region, in p–Pb collisions, where the centre-of-mass frame is boosted with respect to the laboratory frame, the measurements are performed in the backward (
−
4
.
46
<
$$ {y}_{\textrm{cms}}^{\mu } $$
y
cms
μ
< −
2
.
96) and forward (2
.
03
<
$$ {y}_{\textrm{cms}}^{\mu } $$
y
cms
μ
<
3
.
53) rapidity regions. The W
−
and W
+
production cross sections, lepton-charge asymmetry, and nuclear modification factors are evaluated as a function of the muon rapidity. In order to study the production as a function of the p–Pb collision centrality, the production cross sections of the W
−
and W
+
bosons are combined and normalised to the average number of binary nucleon–nucleon collision 〈
N
coll
〉. In Pb–Pb collisions, the same measurements are presented as a function of the collision centrality. Study of the binary scaling of the W
±
-boson cross sections in p–Pb and Pb–Pb collisions is also reported. The results are compared with perturbative QCD calculations, with and without nuclear modifications of the Parton Distribution Functions (PDFs), as well as with available data at the LHC. Significant deviations from the theory expectations are found in the two collision systems, indicating that the measurements can provide additional constraints for the determination of nuclear PDFs and in particular of the light-quark distributions.
A
bstract
The production of non-prompt D
0
mesons from beauty-hadron decays was measured at midrapidity (|
y
|
<
0
.
5) in Pb-Pb collisions at a nucleon-nucleon center-of-mass energy of
$$ ...\sqrt{{\textrm{s}}_{\textrm{NN}}} $$
s
NN
= 5
.
02 TeV with the ALICE experiment at the LHC. Their nuclear modification factor (
R
AA
), measured for the first time down to
p
T
= 1 GeV
/c
in the 0–10% and 30–50% centrality classes, indicates a significant suppression, up to a factor of about three, for
p
T
>
5 GeV
/c
in the 0–10% central Pb-Pb collisions. The data are described by models that include both collisional and radiative processes in the calculation of beauty-quark energy loss in the quark-gluon plasma, and quark recombination in addition to fragmentation as a hadronisation mechanism. The ratio of the non-prompt to prompt D
0
-meson
R
AA
is larger than unity for
p
T
>
4 GeV
/c
in the 0–10% central Pb-Pb collisions, as predicted by models in which beauty quarks lose less energy than charm quarks in the quark-gluon plasma because of their larger mass.
Abstract
Hadronic resonances are used to probe the hadron gas produced in the late stage of heavy-ion collisions since they decay on the same timescale, of the order of 1–10 fm/
c
, as the decoupling ...time of the system. In the hadron gas, (pseudo)elastic scatterings among the products of resonances that decayed before the kinetic freeze-out and regeneration processes counteract each other, the net effect depending on the resonance lifetime, the duration of the hadronic phase, and the hadronic cross sections at play. In this context, the
$$\Sigma (1385)^{\pm }$$
Σ
(
1385
)
±
particle is of particular interest as models predict that regeneration dominates over rescattering despite its relatively short lifetime of about 5.5 fm/
c
. The first measurement of the
$$\Sigma (1385)^{\pm }$$
Σ
(
1385
)
±
resonance production at midrapidity in Pb–Pb collisions at
$$\sqrt{s_{\textrm{NN}}}= 5.02$$
s
NN
=
5.02
TeV with the ALICE detector is presented in this Letter. The resonances are reconstructed via their hadronic decay channel,
$$\Lambda \pi $$
Λ
π
, as a function of the transverse momentum (
$$p_\textrm{T}$$
p
T
) and the collision centrality. The results are discussed in comparison with the measured yield of pions and with expectations from the statistical hadronization model as well as commonly employed event generators, including PYTHIA8/Angantyr and EPOS3 coupled to the UrQMD hadronic cascade afterburner. None of the models can describe the data. For
$$\Sigma (1385)^{\pm }$$
Σ
(
1385
)
±
, a similar behaviour as
$$\textrm{K}^{*} (892)^{0}$$
K
∗
(
892
)
0
is observed in data unlike the predictions of EPOS3 with afterburner.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The interaction of
$$\textrm{K}^{-}$$
K
-
with protons is characterised by the presence of several coupled channels, systems like
$${\overline{\textrm{K}}}^0$$
K
¯
0
n and
$$\uppi \Sigma $$
...π
Σ
with a similar mass and the same quantum numbers as the
$$\textrm{K}^{-}$$
K
-
p state. The strengths of these couplings to the
$$\textrm{K}^{-}$$
K
-
p system are of crucial importance for the understanding of the nature of the
$$\Lambda (1405)$$
Λ
(
1405
)
resonance and of the attractive
$$\textrm{K}^{-}$$
K
-
p strong interaction. In this article, we present measurements of the
$$\textrm{K}^{-}$$
K
-
p correlation functions in relative momentum space obtained in pp collisions at
$$\sqrt{s}~=~13$$
s
=
13
Te, in p–Pb collisions at
$$\sqrt{s_{\textrm{NN}}}~=~5.02$$
s
NN
=
5.02
Te, and (semi)peripheral Pb–Pb collisions at
$$\sqrt{s_{\textrm{NN}}}~=~5.02$$
s
NN
=
5.02
Te. The emitting source size, composed of a core radius anchored to the
$$\textrm{K}^{+}$$
K
+
p correlation and of a resonance halo specific to each particle pair, varies between 1 and 2 fm in these collision systems. The strength and the effects of the
$${\overline{\textrm{K}}}^0$$
K
¯
0
n and
$$\uppi \Sigma $$
π
Σ
inelastic channels on the measured
$$\textrm{K}^{-}$$
K
-
p correlation function are investigated in the different colliding systems by comparing the data with state-of-the-art models of chiral potentials. A novel approach to determine the conversion weights
$$\omega $$
ω
, necessary to quantify the amount of produced inelastic channels in the correlation function, is presented. In this method, particle yields are estimated from thermal model predictions, and their kinematic distribution from blast-wave fits to measured data. The comparison of chiral potentials to the measured
$$\textrm{K}^{-}$$
K
-
p interaction indicates that, while the
$$\uppi \Sigma $$
π
Σ
–
$$\textrm{K}^{-}$$
K
-
p dynamics is well reproduced by the model, the coupling to the
$${\overline{\textrm{K}}}^0$$
K
¯
0
n channel in the model is currently underestimated.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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