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.
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.
Abstract
The multiplicity dependence of jet production in pp collisions at the centre-of-mass energy of
$$\sqrt{s} = 13\ {\mathrm {TeV}}$$
s
=
13
TeV
is studied for the first time. Jets are ...reconstructed from charged particles using the anti-
$$k_\mathrm {T}$$
k
T
algorithm with resolution parameters
R
varying from 0.2 to 0.7. The jets are measured in the pseudorapidity range
$$|\eta _{\mathrm{jet}}|< 0.9-R$$
|
η
jet
|
<
0.9
-
R
and in the transverse momentum range
$$5<p_\mathrm {T,jet}^{\mathrm{ch}}<140\ {\mathrm {GeV}}/c$$
5
<
p
T
,
jet
ch
<
140
GeV
/
c
. The multiplicity intervals are categorised by the ALICE forward detector V0. The
$$p_{\mathrm T}$$
p
T
differential cross section of charged-particle jets are compared to leading order (LO) and next-to-leading order (NLO) perturbative quantum chromodynamics (pQCD) calculations. It is found that the data are better described by the NLO calculation, although the NLO prediction overestimates the jet cross section below
$$20\ {\mathrm {GeV}}/c$$
20
GeV
/
c
. The cross section ratios for different
R
are also measured and compared to model calculations. These measurements provide insights into the angular dependence of jet fragmentation. The jet yield increases with increasing self-normalised charged-particle multiplicity. This increase shows only a weak dependence on jet transverse momentum and resolution parameter at the highest multiplicity. While such behaviour is qualitatively described by the present version of PYTHIA, quantitative description may require implementing new mechanisms for multi-particle production in hadronic collisions.
A
bstract
Understanding the production mechanism of light (anti)nuclei is one of the key challenges of nuclear physics and has important consequences for astrophysics, since it provides an input for ...indirect dark-matter searches in space. In this paper, the latest results about the production of light (anti)nuclei in pp collisions at
$$ \sqrt{s} $$
s
= 13 TeV are presented, focusing on the comparison with the predictions of coalescence and thermal models. For the first time, the coalescence parameters
B
2
for deuterons and
B
3
for helions are compared with parameter-free theoretical predictions that are directly constrained by the femtoscopic measurement of the source radius in the same event class. A fair description of the data with a Gaussian wave function is observed for both deuteron and helion, supporting the coalescence mechanism for the production of light (anti)nuclei in pp collisions. This method paves the way for future investigations of the internal structure of more complex nuclear clusters, including the hypertriton.
Abstract
The study of the production of nuclei and antinuclei in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic ...collisions. In this paper, the production of protons, deuterons and
$$^{3}\mathrm {He}$$
3
He
and their charge conjugates at midrapidity is studied as a function of the charged-particle multiplicity in inelastic pp collisions at
$$\sqrt{s}=5.02$$
s
=
5.02
TeV using the ALICE detector. Within the uncertainties, the yields of nuclei in pp collisions at
$$\sqrt{s}=5.02$$
s
=
5.02
TeV are compatible with those in pp collisions at different energies and to those in p–Pb collisions when compared at similar multiplicities. The measurements are compared with the expectations of coalescence and Statistical Hadronisation Models. The results suggest a common formation mechanism behind the production of light nuclei in hadronic interactions and confirm that they do not depend on the collision energy but on the number of produced particles.
A
bstract
The transverse momentum (
p
T
) differential cross section of the charm-strange baryon
$$ {\Xi}_{\mathrm{c}}^0 $$
Ξ
c
0
is measured at midrapidity (|
y
|
<
0.5) via its semileptonic decay ...into e
+
Ξ
−
ν
e
in pp collisions at
$$ \sqrt{s} $$
s
= 5
.
02 TeV with the ALICE detector at the LHC. The ratio of the
p
T
-differential
$$ {\Xi}_{\mathrm{c}}^0 $$
Ξ
c
0
-baryon and D
0
-meson production cross sections is also reported. The measurements are compared with simulations with different tunes of the PYTHIA 8 event generator, with predictions from a statistical hadronisation model (SHM) with a largely augmented set of charm-baryon states beyond the current lists of the Particle Data Group, and with models including hadronisation via quark coalescence. The
p
T
-integrated cross section of prompt
$$ {\Xi}_{\mathrm{c}}^0 $$
Ξ
c
0
-baryon production at midrapidity is also reported, which is used to calculate the baryon-to-meson ratio
$$ {\Xi}_{\mathrm{c}}^0 $$
Ξ
c
0
/
D
0
= 0
.
20 ± 0
.
04
$$ {\left(\mathrm{stat}.\right)}_{-0.07}^{+0.08} $$
stat
.
−
0.07
+
0.08
(syst
.
). These results provide an additional indication of a modification of the charm fragmentation from e
+
e
−
and e
−
p collisions to pp collisions.
A
bstract
The ALICE Collaboration reports the first fully-corrected measurements of the
N
-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data ...recorded in pp and Pb-Pb collisions at centre-of-mass energies of
$$ \sqrt{s} $$
s
= 7 TeV and
$$ \sqrt{s_{\mathrm{NN}}} $$
s
NN
= 2
.
76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness,
τ
2
/τ
1
, which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy-ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of
R
= 0
.
4 and charged jet transverse momentum of 40 ≤
p
T
,
jet
≤ 60 GeV/
c
, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the
τ
2
/τ
1
observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of
τ
2
/τ
1
, together with the splitting aperture angle ∆
R
, are also performed in pp collisions at
$$ \sqrt{s} $$
s
= 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at
$$ \sqrt{s} $$
s
= 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for
τ
2
/τ
1
are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions.
Abstract
Measurements of event-by-event fluctuations of charged-particle multiplicities in Pb–Pb collisions at
$$\sqrt{s_{\mathrm {NN}}}$$
s
NN
$$=$$
=
2.76 TeV using the ALICE detector at the ...CERN Large Hadron Collider (LHC) are presented in the pseudorapidity range
$$|\eta |<0.8$$
|
η
|
<
0.8
and transverse momentum
$$0.2< p_{\mathrm{T}} < 2.0$$
0.2
<
p
T
<
2.0
GeV/
c
. The amplitude of the fluctuations is expressed in terms of the variance normalized by the mean of the multiplicity distribution. The
$$\eta $$
η
and
$$p_{\mathrm{T}}$$
p
T
dependences of the fluctuations and their evolution with respect to collision centrality are investigated. The multiplicity fluctuations tend to decrease from peripheral to central collisions. The results are compared to those obtained from HIJING and AMPT Monte Carlo event generators as well as to experimental data at lower collision energies. Additionally, the measured multiplicity fluctuations are discussed in the context of the isothermal compressibility of the high-density strongly-interacting system formed in central Pb–Pb collisions.
Abstract We report on the inclusive $$\text {J}/\psi $$ J / ψ production cross section measured at the CERN Large Hadron Collider in proton–proton collisions at a center-of-mass energy ...$$\sqrt{s}~=~13$$ s = 13 TeV. The $$\text {J}/\psi $$ J / ψ mesons are reconstructed in the $$\text {e}^{+}\text {e}^{-}$$ e + e - decay channel and the measurements are performed at midrapidity ( $$|y|<0.9$$ | y | < 0.9 ) in the transverse-momentum interval $$0<p_{\mathrm{T}} <40$$ 0 < p T < 40 GeV/ $$c$$ c , using a minimum-bias data sample corresponding to an integrated luminosity $$L_{\text {int}} = 32.2~\text {nb}^{-1}$$ L int = 32.2 nb - 1 and an Electromagnetic Calorimeter triggered data sample with $$L_{\text {int}} = 8.3~\mathrm {pb}^{-1}$$ L int = 8.3 pb - 1 . The $$p_{\mathrm{T}}$$ p T -integrated $$\text {J}/\psi $$ J / ψ production cross section at midrapidity, computed using the minimum-bias data sample, is $$\text {d}\sigma /\text {d}y|_{y=0} = 8.97\pm 0.24~(\text {stat})\pm 0.48~(\text {syst})\pm 0.15~(\text {lumi})~\mu \text {b}$$ d σ / d y | y = 0 = 8.97 ± 0.24 ( stat ) ± 0.48 ( syst ) ± 0.15 ( lumi ) μ b . An approximate logarithmic dependence with the collision energy is suggested by these results and available world data, in agreement with model predictions. The integrated and $$p_{\mathrm{T}}$$ p T -differential measurements are compared with measurements in pp collisions at lower energies and with several recent phenomenological calculations based on the non-relativistic QCD and Color Evaporation models.
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