A
bstract
We report about the properties of the underlying event measured with ALICE at the LHC in pp and p–Pb collisions at
s
NN
= 5
.
02 TeV. The event activity, quantified by charged-particle ...number and summed-
p
T
densities, is measured as a function of the leading-particle transverse momentum
p
T
trig
. These quantities are studied in three azimuthal-angle regions relative to the leading particle in the event: toward, away, and transverse. Results are presented for three different
p
T
thresholds (0.15, 0.5 and 1 GeV/
c
) at mid-pseudorapidity (|
η
|
<
0
.
8). The event activity in the transverse region, which is the most sensitive to the underlying event, exhibits similar behaviour in both pp and p–Pb collisions, namely, a steep increase with
p
T
trig
for low
p
T
trig
, followed by a saturation at
p
T
trig
≈
5
GeV/
c
. The results from pp collisions are compared with existing measurements at other centre-of-mass energies. The quantities in the toward and away regions are also analyzed after the subtraction of the contribution measured in the transverse region. The remaining jet-like particle densities are consistent in pp and p–Pb collisions for
p
T
trig
>
10
GeV/
c
, whereas for lower
p
T
trig
values the event activity is slightly higher in p–Pb than in pp collisions. The measurements are compared with predictions from the PYTHIA 8 and EPOS LHC Monte Carlo event generators.
This article presents new measurements of the fragmentation properties of jets in both proton–proton (pp) and heavy-ion collisions with the ALICE experiment at the Large Hadron Collider (LHC). We ...report distributions of the fraction
z
r
of transverse momentum carried by subjets of radius
r
within jets of radius
R
. Charged-particle jets are reconstructed at midrapidity using the anti-
k
T
algorithm with jet radius
R
= 0
.
4, and subjets are reconstructed by reclustering the jet constituents using the anti-
k
T
algorithm with radii
r
= 0
.
1 and
r
= 0
.
2. In proton–proton collisions, we measure both the inclusive and leading subjet distributions. We compare these measurements to perturbative calculations at next-to-leading logarithmic accuracy, which suggest a large impact of threshold resummation and hadronization effects on the
z
r
distribution. In heavy-ion collisions, we measure the leading subjet distributions, which allow access to a region of harder jet frag- mentation than has been probed by previous measurements of jet quenching via hadron fragmentation distributions. The
z
r
distributions enable extraction of the parton-to-subjet fragmentation function and allow for tests of the universality of jet fragmentation functions in the quark–gluon plasma (QGP). We find no significant modification of
z
r
distributions in Pb–Pb compared to pp collisions. However, the distributions are also consistent with a hardening trend for
z
r
<
0
.
95, as predicted by several jet quenching models. As
z
r
→ 1 our results indicate that any such hardening effects cease, exposing qualitatively new possibilities to disentangle competing jet quenching mechanisms. By comparing our results to theoretical calculations based on an independent extraction of the parton-to-jet fragmentation function, we find consistency with the universality of jet fragmentation and no indication of factorization breaking in the QGP.
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
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.
The multiplicity dependence of jet production in pp collisions at the centre-of-mass energy of
s
=
13
TeV
is studied for the first time. Jets are reconstructed from charged particles using the anti-
...k
T
algorithm with resolution parameters
R
varying from 0.2 to 0.7. The jets are measured in the pseudorapidity range
|
η
jet
|
<
0.9
-
R
and in the transverse momentum range
5
<
p
T
,
jet
ch
<
140
GeV
/
c
. The multiplicity intervals are categorised by the ALICE forward detector V0. The
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
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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
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
Σ
(
1385
)
±
resonance production at midrapidity in Pb–Pb collisions at
s
NN
=
5.02
TeV with the ALICE detector is presented in this Letter. The resonances are reconstructed via their hadronic decay channel,
Λ
π
, as a function of the transverse momentum (
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
Σ
(
1385
)
±
, a similar behaviour as
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
A
bstract
The production of the W
±
bosons measured in p–Pb collisions at a centre-of-mass energy per nucleon–nucleon collision
s
NN
= 8
.
16 TeV and Pb–Pb collisions at
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
<
η
lab
μ
< −
2
.
5 with transverse momentum
p
T
μ
>
10 GeV
/c
. While in Pb–Pb collisions the measurements are performed in the forward (2
.
5
<
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
cms
μ
< −
2
.
96) and forward (2
.
03
<
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 measurement of the production of charm jets, identified by the presence of a D
0
meson in the jet constituents, is presented in proton–proton collisions at centre-of-mass energies of
s
...= 5.02 and 13 TeV with the ALICE detector at the CERN LHC. The D
0
mesons were reconstructed from their hadronic decay D
0
→ K
−
π
+
and the respective charge conjugate. Jets were reconstructed from D
0
-meson candidates and charged particles using the anti-
k
T
algorithm, in the jet transverse momentum range 5
< p
T
,
chjet
<
50 GeV/
c
, pseudorapidity |
η
jet
|
<
0
.
9 −
R
, and with the jet resolution parameters
R
= 0
.
2
,
0
.
4
,
0
.
6. The distribution of the jet momentum fraction carried by a D
0
meson along the jet axis
z
‖
ch
was measured in the range 0
.
4
<
z
‖
ch
<
1
.
0 in four ranges of the jet transverse momentum. Comparisons of results for different collision energies and jet resolution parameters are also presented. The measurements are compared to predictions from Monte Carlo event generators based on leading-order and next-to-leading-order perturbative quantum chromodynamics calculations. A generally good description of the main features of the data is obtained in spite of a few discrepancies at low
p
T
,
chjet
. Measurements were also done for
R
= 0
.
3 at
s
= 5
.
02 and are shown along with their comparisons to theoretical predictions in an appendix to this paper.
The transverse-momentum
(
p
T
)
spectra of K
∗
(
892
)
0
and
ϕ
(
1020
)
measured with the ALICE detector up to
p
T
= 16 GeV/
c
in the rapidity range
-
1.2
<
y
<
0.3
,
in p–Pb collisions at the ...center-of-mass energy per nucleon–nucleon collision
s
NN
=
5.02
TeV are presented as a function of charged particle multiplicity and rapidity. The measured
p
T
distributions show a dependence on both multiplicity and rapidity at low
p
T
whereas no significant dependence is observed at high
p
T
. A rapidity dependence is observed in the
p
T
-integrated yield (d
N
/d
y
), whereas the mean transverse momentum
⟨
p
T
⟩
shows a flat behavior as a function of rapidity. The rapidity asymmetry (
Y
asym
) at low
p
T
(< 5 GeV/
c
) is more significant for higher multiplicity classes. At high
p
T
, no significant rapidity asymmetry is observed in any of the multiplicity classes. Both K
∗
(
892
)
0
and
ϕ
(
1020
)
show similar
Y
asym
. The nuclear modification factor
(
Q
CP
)
as a function of
p
T
shows a Cronin-like enhancement at intermediate
p
T
, which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high
p
T
(> 5 GeV/
c
), the
Q
CP
values are greater than unity and no significant rapidity dependence is observed.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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