A
bstract
Measurements of anisotropic flow coefficients with two- and multi-particle cumulants for inclusive charged particles in Pb-Pb collisions at
s
N
N
=
5.02
and 2.76 TeV are reported in the ...pseudorapidity range |
η
| < 0.8 and transverse momentum 0.2 <
p
T
< 50 GeV/
c
. The full data sample collected by the ALICE detector in 2015 (2010), corresponding to an integrated luminosity of 12.7 (2.0)
μ
b
−1
in the centrality range 0-80%, is analysed. Flow coefficients up to the sixth flow harmonic (
v
6
) are reported and a detailed comparison among results at the two energies is carried out. The
p
T
dependence of anisotropic flow coefficients and its evolution with respect to centrality and harmonic number
n
are investigated. An approximate power-law scaling of the form
v
n
(
p
T
) ∼
p
T
n
/3
is observed for all flow harmonics at low
p
T
(0.2 <
p
T
< 3 GeV/
c
). At the same time, the ratios
v
n
/
v
m
n
/
m
are observed to be essentially independent of
p
T
for most centralities up to about
p
T
= 10 GeV/
c
. Analysing the differences among higher-order cumulants of elliptic flow (
v
2
), which have different sensitivities to flow fluctuations, a measurement of the standardised skewness of the event-by-event
v
2
distribution
P
(
v
2
) is reported and constraints on its higher moments are provided. The Elliptic Power distribution is used to parametrise
P
(
v
2
), extracting its parameters from fits to cumulants. The measurements are compared to different model predictions in order to discriminate among initial-state models and to constrain the temperature dependence of the shear viscosity to entropy-density ratio.
We present a study of the inclusive charged-particle transverse momentum (
p
T
) spectra as a function of charged-particle multiplicity density at mid-pseudorapidity,
d
N
ch
/
d
η
, in pp collisions ...at
s
=
5.02
and 13 TeV covering the kinematic range
|
η
|
<
0.8
and
0.15
<
p
T
<
20
GeV/
c
. The results are presented for events with at least one charged particle in
|
η
|
<
1
(INEL
>
0
). The
p
T
spectra are reported for two multiplicity estimators covering different pseudorapidity regions. The
p
T
spectra normalized to that for INEL
>
0
show little energy dependence. Moreover, the high-
p
T
yields of charged particles increase faster than the charged-particle multiplicity density. The average
p
T
as a function of multiplicity and transverse spherocity is reported for pp collisions at
s
=
13
TeV. For low- (high-) spherocity events, corresponding to jet-like (isotropic) events, the average
p
T
is higher (smaller) than that measured in INEL
>
0
pp collisions. Within uncertainties, the functional form of
⟨
p
T
⟩
(
N
ch
)
is not affected by the spherocity selection. While EPOS LHC gives a good description of many features of data, PYTHIA overestimates the average
p
T
in jet-like events.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
The
p
T
-differential production cross section of prompt
Λ
c
+
charmed baryons was measured with the ALICE detector at the Large Hadron Collider (LHC) in pp collisions at
s
=
7
TeV and in ...p-Pb collisions at
s
N
N
=
5.02
TeV at midrapidity. The
Λ
c
+
and
Λ
¯
c
¯
were reconstructed in the hadronic decay modes
Λ
c
+
→ pK
−
π
+
,
Λ
c
+
→ pK
S
0
and in the semileptonic channel
Λ
c
+
→ e
+
ν
e
Λ
(and charge conjugates). The measured values of the
Λ
c
+
/D
0
ratio, which is sensitive to the c-quark hadronisation mechanism, and in particular to the production of baryons, are presented and are larger than those measured previously in different colliding systems, centre-of-mass energies, rapidity and
p
T
intervals, where the
Λ
c
+
production process may differ. The results are compared with the expectations obtained from perturbative Quantum Chromodynamics calculations and Monte Carlo event generators. Neither perturbative QCD calculations nor Monte Carlo models reproduce the data, indicating that the fragmentation of heavy-flavour baryons is not well understood. The first measurement at the LHC of the
Λ
c
+
nuclear modification factor,
R
pPb
, is also presented. The
R
pPb
is found to be consistent with unity and with that of D mesons within the uncertainties, and consistent with a theoretical calculation that includes cold nuclear matter effects and a calculation that includes charm quark interactions with a deconfined medium.
A measurement of the production of prompt Λ$_c^+$ baryons in Pb–Pb collisions at $\sqrt{s_{NN}}$ =5.02 TeV with the ALICE detector at the LHC is reported. The Λ$_c^+$ and $\bar{Λ}_c^-$ were ...reconstructed at midrapidity (|y|<0.5) via the hadronic decay channel Λ$_c^+$ →pK$_S^0$ (and charge conjugate) in the transverse momentum and centrality intervals 6 < pT < 12 GeV/c and 0–80%. The Λ$_c^+$/D0 ratio, which is sensitive to the charm quark hadronisation mechanisms in the medium, is measured and found to be larger than the ratio measured in minimum-bias pp collisions at $\sqrt{s}$ = 7 TeV and in p–Pb collisions at $\sqrt{s}$ = 5.02 TeV. In particular, the values in p–Pb and Pb–Pb collisions differ by about two standard deviations of the combined statistical and systematic uncertainties in the common pT interval covered by the measurements in the two collision systems. The Λ$_c^+$/D0 ratio is also compared with model calculations including different implementations of charm quark hadronisation. The measured ratio is reproduced by models implementing a pure coalescence scenario, while adding a fragmentation contribution leads to an underestimation. The Λ$_c^+$ nuclear modification factor, RAA, is also presented. The measured values of the RAA of Λ$_c^+$/D$_s^+$ , and non-strange D mesons are compatible within the combined statistical and systematic uncertainties. They show, however, a hint of a hierarchy (R$^{D^0}_{AA}$ < R$^{D^+_s}_{AA}$ < R$^{Λ_c^+}_{AA}$), conceivable with a contribution from coalescence mechanisms to charm hadron formation in the medium.
A
bstract
The elliptic (
v
2
), triangular (
v
3
), and quadrangular (
v
4
) flow coefficients of π
±
, K
±
,
p
+
p
¯
,
Λ
+
Λ
¯
,
K
S
0
, and the
ϕ
-meson are measured in Pb-Pb collisions at
s
N
N
=
...5.02
TeV. Results obtained with the scalar product method are reported for the rapidity range |
y
|
<
0.5 as a function of transverse momentum,
p
T
, at different collision centrality intervals between 0–70%, including ultra-central (0–1%) collisions for
π
±
, K
±
, and
p
+
p
¯
. For
p
T
<
3 GeV
/c
, the flow coefficients exhibit a particle mass dependence. At intermediate transverse momenta (3 <
p
T
< 8–10 GeV/
c
), particles show an approximate grouping according to their type (i.e., mesons and baryons). The
ϕ
-meson
v
2
, which tests both particle mass dependence and type scaling, follows
p
+
p
¯
v
2
at low
p
T
and π
±
v
2
at intermediate
p
T
. The evolution of the shape of
v
n
(
p
T
) as a function of centrality and harmonic number
n
is studied for the various particle species. Flow coefficients of
π
±
, K
±
, and
p
+
p
¯
for
p
T
< 3 GeV/
c
are compared to iEBE-VISHNU and MUSIC hydrodynamical calculations coupled to a hadronic cascade model (UrQMD). The iEBE-VISHNU calculations describe the results fairly well for
p
T
< 2.5 GeV/
c
, while MUSIC calculations reproduce the measurements for
p
T
< 1 GeV/
c
. A comparison to
v
n
coefficients measured in Pb-Pb collisions at
s
N
N
=
2.76
TeV is also provided.
The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of
J
/
ψ
vector mesons off proton targets in ultra–peripheral p–Pb collisions at a centre-of-mass energy per ...nucleon pair
s
NN
=
5.02
TeV. The e
+
e
-
and
μ
+
μ
-
decay channels are used to measure the cross section as a function of the rapidity of the
J
/
ψ
in the range
-
2.5
<
y
<
2.7
, corresponding to an energy in the
γ
p centre-of-mass in the interval
40
<
W
γ
p
<
550
GeV. The measurements, which are consistent with a power law dependence of the exclusive
J
/
ψ
photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We report a precise measurement of the J/ψ elliptic flow in Pb-Pb collisions at sNN=5.02 TeV with the ALICE detector at the LHC. The J/ψ mesons are reconstructed at midrapidity (|y|
The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high-transverse momentum trigger hadron in p–Pb collisions at sNN=5.02 TeV. ...Jets are reconstructed from charged-particle tracks using the anti-kT algorithm with resolution parameter R=0.2 and 0.4. A data-driven statistical approach is used to correct the uncorrelated background jet yield. Recoil jet distributions are reported for jet transverse momentum 15<pT,jetch<50GeV/c and are compared in various intervals of p–Pb event activity, based on charged-particle multiplicity and zero-degree neutral energy in the forward (Pb-going) direction. The semi-inclusive observable is self-normalized and such comparisons do not require the interpretation of p–Pb event activity in terms of collision geometry, in contrast to inclusive jet observables. These measurements provide new constraints on the magnitude of jet quenching in small systems at the LHC. In p–Pb collisions with high event activity, the average medium-induced out-of-cone energy transport for jets with R=0.4 and 15<pT,jetch<50GeV/c is measured to be less than 0.4 GeV/c at 90% confidence, which is over an order of magnitude smaller than a similar measurement for central Pb–Pb collisions at sNN=2.76TeV. Comparison is made to theoretical calculations of jet quenching in small systems, and to inclusive jet measurements in p–Pb collisions selected by event activity at the LHC and in d–Au collisions at RHIC.
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