The first evidence of spin alignment of vector mesons ( K*0 and ϕ ) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element ρ 00 is measured at ...midrapidity ( | y | < 0.5 ) in Pb-Pb collisions at a center-of-mass energy ( √sNN ) of 2.76 TeV with the ALICE detector. ρ 00 values are found to be less than 1/3 (1/3 implies no spin alignment) at low transverse momentum ( p T < 2 GeV / c ) for K*0 and ϕ at a level of 3 σ and 2 σ , respectively. No significant spin alignment is observed for the K0S meson ( spin = 0 ) in Pb-Pb collisions and for the vector mesons in p p collisions. The measured spin alignment is unexpectedly large but qualitatively consistent with the expectation from models which attribute it to a polarization of quarks in the presence of angular momentum in heavy-ion collisions and a subsequent hadronization by the process of recombination.
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.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A bstract The azimuthal anisotropy of particles associated with jets (jet particles) at midrapidity is measured for the first time in p-Pb and Pb-Pb collisions at $$ \sqrt{{\textrm{s}}_{\textrm{NN}}} ...$$ s NN = 5 . 02 TeV down to transverse momentum ( p T ) of 0.5 GeV/ c and 2 GeV/ c , respectively, with ALICE. The results obtained in p-Pb collisions are based on a novel three-particle correlation technique. The azimuthal anisotropy coefficient v 2 in high-multiplicity p-Pb collisions is positive, with a significance reaching 6.8 σ at low p T , and its magnitude is smaller than in semicentral Pb-Pb collisions. In contrast to the measurements in Pb-Pb collisions, the v 2 coefficient is also found independent of p T within uncertainties. Comparisons with the inclusive charged-particle v 2 and with AMPT calculations are discussed. The predictions suggest that parton interactions play an important role in generating a non-zero jet-particle v 2 in p-Pb collisions, even though they overestimate the reported measurement. These observations shed new insights on the understanding of the origin of the collective behaviour of jet particles in small systems such as p-Pb collisions, and provide significant stringent new constraints to models.
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.
The ALICE experiment: a journey through QCD Alocco, G.; Antonioli, P.; Appelshäuser, H. ...
The European physical journal. C, Particles and fields,
08/2024, Letnik:
84, Številka:
8
Journal Article
Recenzirano
Odprti dostop
The ALICE experiment was proposed in 1993, to study strongly-interacting matter at extreme energy densities and temperatures. This proposal entailed a comprehensive investigation of nuclear ...collisions at the LHC. Its physics programme initially focused on the determination of the properties of the quark–gluon plasma (QGP), a deconfined state of quarks and gluons, created in such collisions. The ALICE physics programme has been extended to cover a broader ensemble of observables related to Quantum Chromodynamics (QCD), the theory of strong interactions. The experiment has studied Pb–Pb, Xe–Xe, p–Pb and pp collisions in the multi-TeV centre of mass energy range, during the Run 1–2 data-taking periods at the LHC (2009–2018). The aim of this review is to summarise the key ALICE physics results in this endeavor, and to discuss their implications on the current understanding of the macroscopic and microscopic properties of strongly-interacting matter at the highest temperatures reached in the laboratory. It will review the latest findings on the properties of the QGP created by heavy-ion collisions at LHC energies, and describe the surprising QGP-like effects in pp and p–Pb collisions. Measurements of few-body QCD interactions, and their impact in unraveling the structure of hadrons and hadronic interactions, will be discussed. ALICE results relevant for physics topics outside the realm of QCD will also be touched upon. Finally, prospects for future measurements with the ALICE detector in the context of its planned upgrades will also be briefly described.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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.
A
bstract
The production of prompt D
0
, D
+
, and D
*+
mesons was measured at midrapidity (|
y
|
<
0.5) in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair
$$ ...\sqrt{s_{\mathrm{NN}}} $$
s
NN
= 5
.
02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decay channels and their production yields were measured in central (0–10%) and semicentral (30–50%) collisions. The measurement was performed up to a transverse momentum (
p
T
) of 36 or 50 GeV/c depending on the D meson species and the centrality interval. For the first time in Pb–Pb collisions at the LHC, the yield of D
0
mesons was measured down to
p
T
= 0, which allowed a model-independent determination of the
p
T
-integrated yield per unit of rapidity (d
N/
d
y
). A maximum suppression by a factor 5 and 2.5 was observed with the nuclear modification factor (
R
AA
) of prompt D mesons at
p
T
= 6–8 GeV/c for the 0–10% and 30–50% centrality classes, respectively. The D-meson
R
AA is compared with that of charged pions, charged hadrons, and J
/ψ
mesons as well as with theoretical predictions. The analysis of the agreement between the measured
R
AA
, elliptic (
v
2
) and triangular (
v
3
) flow, and the model predictions allowed us to constrain the charm spatial diffusion coefficient
D
s
. Furthermore the comparison of
R
AA
and
v
2
with different implementations of the same models provides an important insight into the role of radiative energy loss as well as charm quark recombination in the hadronisation mechanisms.
A
bstract
Jet fragmentation transverse momentum (
j
T
) distributions are measured in proton-proton (pp) and proton-lead (p-Pb) collisions at
$$ \sqrt{s_{\mathrm{NN}}} $$
s
NN
= 5
.
02 TeV with the ...ALICE experiment at the LHC. Jets are reconstructed with the ALICE tracking detectors and electromagnetic calorimeter using the anti-
k
T
algorithm with resolution parameter
R
= 0
.
4 in the pseudorapidity range
|η| <
0
.
25. The
j
T
values are calculated for charged particles inside a fixed cone with a radius
R
= 0
.
4 around the reconstructed jet axis. The measured
j
T
distributions are compared with a variety of parton-shower models. Herwig and P
ythia
8 based models describe the data well for the higher
j
T
region, while they underestimate the lower
j
T
region. The
j
T
distributions are further characterised by fitting them with a function composed of an inverse gamma function for higher
j
T
values (called the “wide component”), related to the perturbative component of the fragmentation process, and with a Gaussian for lower
j
T
values (called the “narrow component”), predominantly connected to the hadronisation process. The width of the Gaussian has only a weak dependence on jet transverse momentum, while that of the inverse gamma function increases with increasing jet transverse momentum. For the narrow component, the measured trends are successfully described by all models except for Herwig. For the wide component, Herwig and PYTHIA 8 based models slightly underestimate the data for the higher jet transverse momentum region. These measurements set constraints on models of jet fragmentation and hadronisation.
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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