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
s
= 7 TeV and
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
s
= 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at
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.
Measurements of event-by-event fluctuations of charged-particle multiplicities in Pb–Pb collisions at
s
NN
=
2.76 TeV using the ALICE detector at the CERN Large Hadron Collider (LHC) are presented ...in the pseudorapidity range
|
η
|
<
0.8
and transverse momentum
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
η
and
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
A
bstract
Jet fragmentation transverse momentum (
j
T
) distributions are measured in proton-proton (pp) and proton-lead (p-Pb) collisions at
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.
Production of ω mesons in pp collisions at s=7TeV Al-Turany, M.; Alkin, A.; Aphecetche, L. ...
The European physical journal. C, Particles and fields,
12/2020, Letnik:
80, Številka:
12
Journal Article
Recenzirano
Odprti dostop
The invariant differential cross section of inclusive
ω
(
782
)
meson production at midrapidity (
|
y
|
<
0.5
) in pp collisions at
s
=
7
TeV
was measured with the ALICE detector at the LHC over a ...transverse momentum range of
2
<
p
T
<
17
GeV
/
c
. The
ω
meson was reconstructed via its
ω
→
π
+
π
-
π
0
decay channel. The measured
ω
production cross section is compared to various calculations: PYTHIA 8.2 Monash 2013 describes the data, while PYTHIA 8.2 Tune 4C overestimates the data by about 50%. A recent NLO calculation, which includes a model describing the fragmentation of the whole vector-meson nonet, describes the data within uncertainties below
6
GeV
/
c
, while it overestimates the data by up to 50% for higher
p
T
. The
ω
/
π
0
ratio is in agreement with previous measurements at lower collision energies and the PYTHIA calculations. In addition, the measurement is compatible with transverse mass scaling within the measured
p
T
range and the ratio is constant with
C
ω
/
π
0
=
0.67
±
0.03
(stat)
±
0.04
(sys)
above a transverse momentum of
2.5
GeV
/
c
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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
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.
Two-particle Azimuthal correlations are measured with the ALICE apparatus in pp collisions at
s
=
13
TeV to explore strangeness- and multiplicity-related effects in the fragmentation of jets and the ...transition regime between bulk and hard production, probed with the condition that a strange meson (
K
S
0
) or baryon (
Λ
) with transverse momentum
p
T
>
3
GeV/
c
is produced. Azimuthal correlations between kaons or
Λ
hyperons with other hadrons are presented at midrapidity for a broad range of the trigger (
3
<
p
T
trigg
<
20
GeV/
c
) and associated particle
p
T
(1 GeV/
c
<
p
T
assoc
<
p
T
trigg
), for minimum-bias events and as a function of the event multiplicity. The near- and away-side peak yields are compared for the case of either
K
S
0
or
Λ
(
Λ
¯
) being the trigger particle with that of inclusive hadrons (a sample dominated by pions). In addition, the measurements are compared with predictions from PYTHIA 8 and EPOS LHC event generators.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The study of nuclei and antinuclei production has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. The first measurement ...of the production of H Λ 3 in p -Pb collisions at sNN=5.02 TeV is presented in this Letter. Its production yield measured in the rapidity interval -1<y<0 for the 40% highest-multiplicity p -Pb collisions is d N/ d y= 6.3±1.8 ( stat ) ±1.2 ( syst ) ×10−7 . The measurement is compared with the expectations of statistical hadronization and coalescence models, which describe the nucleosynthesis in hadronic collisions. These two models predict very different yields of the hypertriton in charged particle multiplicity environments relevant to small collision systems such as p -Pb, and therefore the measurement of d N/ d y is crucial to distinguish between them. The precision of this measurement leads to the exclusion with a significance larger than 6.9σ of some configurations of the statistical hadronization model, thus constraining the theory behind the production of loosely bound states at hadron colliders.
This article presents groomed jet substructure measurements in pp and Pb-Pb collisions at sqrts_{NN}=5.02 TeV with the ALICE detector. The soft drop grooming algorithm provides access to the hard ...parton splittings inside a jet by removing soft wide-angle radiation. We report the groomed jet momentum splitting fraction, z_{g}, and the (scaled) groomed jet radius, θ_{g}. Charged-particle jets are reconstructed at midrapidity using the anti-k_{T} algorithm with resolution parameters R=0.2 and R=0.4. In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed parton splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time. A narrowing of the θ_{g} distribution in Pb-Pb collisions compared to pp collisions is seen, which provides direct evidence of the modification of the angular structure of jets in the quark-gluon plasma. No significant modification of the z_{g} distribution in Pb-Pb collisions compared to pp collisions is observed. These results are compared with a variety of theoretical models of jet quenching, and provide constraints on jet energy-loss mechanisms and coherence effects in the quark-gluon plasma.
The elliptic flow of electrons from beauty hadron decays at midrapidity (|y|<0.8) is measured in Pb-Pb collisions at sqrts_{NN}=5.02 TeV with the ALICE detector at the LHC. The azimuthal ...distribution of the particles produced in the collisions can be parametrized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, v_{2}. The v_{2} coefficient of electrons from beauty hadron decays is measured for the first time in the transverse momentum (p_{T}) range 1.3-6 GeV/c in the centrality class 30%-50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length cτ≈500 μm compared to that of charm hadrons and most of the other background sources. The v_{2} of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75 σ. The results provide insights into the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavored by the measurement at high p_{T}, but is in agreement with the results at low p_{T}. Transport models including substantial interactions of beauty quarks with an expanding strongly interacting medium describe the measurement within uncertainties.
Luminosity determination within the ALICE experiment is based on the measurement, in van der Meer scans, of the cross sections for visible processes involving one or more detectors (visible cross ...sections). In 2015 and 2018, the Large Hadron Collider provided Pb–Pb collisions at a centre-of-mass energy per nucleon pair of √sNN = 5.02 TeV. Two visible cross sections, associated with particle detection in the Zero Degree Calorimeter (ZDC) and in the V0 detector, were measured in a van der Meer scan.This article describes the experimental set-up and the analysis procedure, and presents the measurement results. The analysis involves a comprehensive study of beam-related effects and an improved fitting procedure, compared to previous ALICE studies, for the extraction of the visible cross section. The resulting uncertainty of both the ZDC-based and the V0-based luminosity measurement for the full sample is 2.5%. The inelastic cross section for hadronic interactions in Pb–Pb collisions at √sNN = 5.02 TeV, obtained by efficiency correction of the V0-based visible cross section, was measured to be 7.67 ± 0.25 b, in agreement with predictions using the Glauber model.
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