This article reports on the inclusive production cross section of several quarkonium states,
J
/
ψ
,
ψ
(
2
S
)
,
Υ
(
1
S
)
,
Υ
(
2
S
)
, and
Υ
(
3
S
)
, measured with the ALICE detector at the LHC, ...in pp collisions at
s
=
5.02
TeV. The analysis is performed in the dimuon decay channel at forward rapidity (
2.5
<
y
<
4
). The integrated cross sections and transverse-momentum (
p
T
) and rapidity (
y
) differential cross sections for
J
/
ψ
,
ψ
(
2
S
)
,
Υ
(
1
S
)
, and the
ψ
(
2
S
)
-to-
J
/
ψ
cross section ratios are presented. The integrated cross sections, assuming unpolarized quarkonia, are:
σ
J
/
ψ
(
p
T
<
20
GeV/c) = 5.88 ± 0.03 ± 0.34
μ
b,
σ
ψ
(
2
S
)
(
p
T
<
12
GeV/c) = 0.87 ± 0.06 ± 0.10
μ
b,
σ
Υ
(
1
S
)
(
p
T
<
15
GeV/c) = 45.5 ± 3.9 ± 3.5 nb,
σ
Υ
(
2
S
)
(
p
T
<
15
GeV/c) = 22.4 ± 3.2 ± 2.7 nb, and
σ
Υ
(
3
S
)
(
p
T
<
15
GeV/c) = 4.9 ± 2.2 ± 1.0 nb, where the first (second) uncertainty is the statistical (systematic) one. For the first time, the cross sections of the three
Υ
states, as well as the
ψ
(
2
S
)
one as a function of
p
T
and
y
, are measured at
s
=
5.02
TeV at forward rapidity. These measurements also significantly extend the
J
/
ψ
p
T
reach and supersede previously published results. A comparison with ALICE measurements in pp collisions at
s
=
2.76
, 7, 8, and 13 TeV is presented and the energy dependence of quarkonium production cross sections is discussed. Finally, the results are compared with the predictions from several production models.
A
bstract
The production of J/
ψ
is measured as a function of charged-particle multiplicity at forward rapidity in proton-proton (pp) collisions at center-of-mass energies
s
= 5.02 and 13 TeV. The J/
...ψ
mesons are reconstructed via their decay into dimuons in the rapidity interval (2.5
< y <
4.0), whereas the charged-particle multiplicity density (d
N
ch
/
d
η
) is measured at midrapidity (
|η| <
1). The production rate as a function of multiplicity is reported as the ratio of the yield in a given multiplicity interval to the multiplicity-integrated one. This observable shows a linear increase with charged-particle multiplicity normalized to the corresponding average value for inelastic events (d
N
ch
/d
η
/〈d
N
ch
/d
η
〉), at both the colliding energies. Measurements are compared with available ALICE results at midrapidity and theoretical model calculations. First measurement of the mean transverse momentum (〈
p
T
〉) of J/
ψ
in pp collisions exhibits an increasing trend as a function of d
N
ch
/d
η
/〈d
N
ch
/d
η
〉 showing a saturation towards high charged-particle multiplicities.
Partons traversing the strongly interacting medium produced in heavy-ion collisions are expected to lose energy depending on their color charge and mass. We measure the nuclear modification factors ...for charm- and bottom-decay electrons, defined as the ratio of yields, divided by the number of binary nucleon–nucleon collisions, in
s
NN
=
200
GeV Au+Au collisions to
p
+
p
collisions (
R
AA
), or in central to peripheral Au+Au collisions (
R
CP
). We find the bottom-decay electron
R
AA
and
R
CP
to be significantly higher than those of charm-decay electrons. Model calculations including mass-dependent parton energy loss in a strongly coupled medium are consistent with the measured data. These observations provide evidence of mass ordering of charm and bottom quark energy loss when traversing through the strongly coupled medium created in heavy-ion collisions.
The longitudinal and transverse spin transfers to Λ ($\overline{Λ}$) hyperons in polarized proton-proton collisions are expected to be sensitive to the helicity and transversity distributions, ...respectively, of (anti)strange quarks in the proton, and to the corresponding polarized fragmentation functions. We report improved measurements of the longitudinal spin transfer coefficient, DLL, and the transverse spin transfer coefficient, DTT, to Λ and $\overline{Λ}$ in polarized proton-proton collisions at √s = 200 GeV by the STAR experiment at RHIC. The dataset includes longitudinally polarized proton-proton collisions with an integrated luminosity of 52 pb–1, and transversely polarized proton-proton collisions with a similar integrated luminosity. Both datasets have about twice the statistics of previous results and cover a kinematic range of |ηΛ($\overline{Λ}$)| < 1.2 and transverse momentum pT,Λ($\overline{Λ}$) up to 8GeV/c. We also report the first measurements of the hyperon spin transfer coefficients DLL and DTT as a function of the fractional jet momentum z carried by the hyperon, which can provide more direct constraints on the polarized fragmentation functions
Particle production sensitive to nonfactorizable and nonperturbative processes that contribute to the underlying event associated with a high transverse momentum ( pT) jet in proton + proton ...collisions at √s = 200 GeV is studied with the STAR detector. Each event is divided into three regions based on the azimuthal angle with respect to the highest- p T jet direction: in the leading jet direction ("Toward"), opposite to the leading jet ("Away"), and perpendicular to the leading jet ("Transverse"). In the Transverse region, the average charged particle density is found to be between 0.4 and 0.6 and the mean transverse momentum, ⟨pT⟩, between 0.5 and 0.7 GeV/c for particles with pT > 0.2 GeV/c at mid-pseudorapidity (|η| < 1) and jet pT > 15 GeV / c . Both average particle density and ⟨pT⟩depend weakly on the leading jet pT. Closer inspection of the Transverse region hints that contributions to the underlying event from initial- and final-state radiation are significantly smaller in these collisions than at the higher energies, up to 13 TeV, recorded at the LHC. Underlying event measurements associated with a high-pT jet will contribute to our understanding of QCD processes at hard and soft scales at RHIC energies, as well as provide constraints to modeling of underlying event dynamics.
A
bstract
The first measurements of elliptic flow of
π
±
, K
±
,
p
+
p
¯
,
K
S
0
,
Λ
+
Λ
¯
,
ϕ
,
Ξ
−
+
Ξ
¯
+
, and
Ω
−
+
Ω
¯
+
using multiparticle cumulants in Pb–Pb collisions at
s
NN
= 5
.
02 TeV ...are resented. Results obtained with two- (
v
2
{2}) and four-particle cumulants (
v
2
{4}) are shown as a function of transverse momentum,
p
T
, for various collision centrality intervals. Combining the data for both
v
2
{2} and
v
2
{4} also allows us to report the first measurements of the mean elliptic flow, elliptic flow fluctuations, and relative elliptic flow fluctuations for various hadron species. These observables probe the event-by-event eccentricity fluctuations in the initial state and the contributions from the dynamic evolution of the expanding quark–gluon plasma. The characteristic features observed in previous
p
T
-differential anisotropic flow measurements for identified hadrons with two-particle correlations, namely the mass ordering at low
p
T
and the approximate scaling with the number of constituent quarks at intermediate
p
T
, are similarly present in the four-particle correlations and the combinations of
v
2
{2} and
v
2
{4}. In addition, a particle species dependence of flow fluctuations is observed that could indicate a significant contribution from final state hadronic interactions. The comparison between experimental measurements and CoLBT model calculations, which combine the various physics processes of hydrodynamics, quark coalescence, and jet fragmentation, illustrates their importance over a wide
p
T
range.
We present measurements of the differential cross sections of inclusive J/ψ meson production as a function of transverse momentum (pTJ/ψ) using the μ+μ− and e+e− decay channels in proton+proton ...collisions at center-of-mass energies of 510 and 500 GeV, respectively, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The measurement from the μ+μ− channel is for 0<pTJ/ψ<9 GeV/c and rapidity range |yJ/ψ|<0.4, and that from the e+e− channel is for 4<pTJ/ψ<20 GeV/c and |yJ/ψ|<1.0. The ψ(2S) to J/ψ ratio is also measured for 4<pTmeson<12 GeV/c through the e+e− decay channel. Model calculations, which incorporate different approaches toward the J/ψ production mechanism, are compared with experimental results and show reasonable agreement within uncertainties.
A
bstract
The first measurement of the e
+
e
−
pair production at low lepton pair transverse momentum (
p
T
,
ee
) and low invariant mass (
m
ee
) in non-central Pb–Pb collisions at
s
NN
= 5
.
02 TeV ...at the LHC is presented. The dielectron production is studied with the ALICE detector at midrapidity (
|η
e
| <
0
.
8) as a function of invariant mass (0.4
≤ m
ee
<
2
.
7 GeV/
c
2
) in the 50–70% and 70–90% centrality classes for
p
T
,
ee
<
0.1 GeV/
c
, and as a function of
p
T
,
ee
in three
m
ee
intervals in the most peripheral Pb–Pb collisions. Below a
p
T
,
ee
of 0.1 GeV/
c
, a clear excess of e
+
e
−
pairs is found compared to the expectations from known hadronic sources and predictions of thermal radiation from the medium. The
m
ee
excess spectra are reproduced, within uncertainties, by different predictions of the photon–photon production of dielectrons, where the photons originate from the extremely strong electromagnetic fields generated by the highly Lorentz-contracted Pb nuclei. Lowest-order quantum electrodynamic (QED) calculations, as well as a model that takes into account the impact-parameter dependence of the average transverse momentum of the photons, also provide a good description of the
p
T
,
ee
spectra. The measured
p
T
,
ee
2
of the excess
p
T
,
ee
spectrum in peripheral Pb–Pb collisions is found to be comparable to the values observed previously at RHIC in a similar phase-space region.
A
bstract
Production of inclusive charmonia in pp collisions at center-of-mass energy of
s
= 13 TeV and p–Pb collisions at center-of-mass energy per nucleon pair of
s
NN
= 8
.
16 TeV is studied as a ...function of charged-particle pseudorapidity density with ALICE. Ground and excited charmonium states (
J/ψ
,
ψ
(2S)) are measured from their dimuon decays in the interval of rapidity in the center-of-mass frame 2
.
5
< y
cms
<
4
.
0 for pp collisions, and 2
.
03
< y
cms
<
3
.
53 and −4
.
46
< y
cms
<
−2
.
96 for p–Pb collisions. The charged-particle pseudorapidity density is measured around midrapidity (|
η
|
<
1
.
0). In pp collisions, the measured charged-particle multiplicity extends to about six times the average value, while in p-Pb collisions at forward (backward) rapidity a multiplicity corresponding to about three (four) times the average is reached. The
ψ
(2S) yield increases with the charged-particle pseudorapidity density. The ratio of
ψ
(2S) over
J/ψ
yield does not show a significant multiplicity dependence in either colliding system, suggesting a similar behavior of
J/ψ
and
ψ
(2S) yields with respect to charged-particle pseudorapidity density. Results for the
ψ
(2S) yield and its ratio with respect to
J/ψ
agree with available model calculations.
PDF
