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.
Recent pT-integrated cross-section measurements of the ground-state charm mesons and baryons, D0, D+, D+s, Λ+c, and Ξ0c, are used to evaluate the charm fragmentation fractions and production cross ...section per unit of rapidity at midrapidity (|y|<0.5), in pp collisions at √s=5.02 TeV at the LHC. The latter is dσc¯c/dy||y|<0.5=1165±44(stat)+134−101(syst) μb. These measurements were obtained for the first time in hadronic collisions at the LHC, including the charm baryon states, recently measured by ALICE at midrapidity. The charm fragmentation fractions differ significantly from the values measured in e+e− and ep collisions, providing evidence of the dependence of the parton-to-hadron fragmentation fractions on the collision system, indicating that the assumption of their universality is not supported by the measured cross sections. An increase of a factor of about 3.3 for the fragmentation fraction for the Λ+c with a significance of 5σ between the values obtained in pp collisions and those obtained in e+e− (ep) collisions is reported. The fragmentation fraction for the Ξ0c was obtained for the first time in any collision system. The measured fragmentation fractions were used to update the c¯c cross sections per unit of rapidity at |y|<0.5 at √s=2.76 and 7 TeV, which are about 40% higher than the previously published results. The data were compared with perturbative-QCD calculations and lie at the upper edge of the theoretical bands.
The first measurement at the LHC of charge-dependent directed flow (v1) relative to the spectator plane is presented for Pb-Pb collisions at √ s NN = 5.02 TeV . Results are reported for charged ...hadrons and D 0 mesons for the transverse momentum intervals p T > 0.2 GeV / c and 3 < p T < 6 GeV / c in the 5%–40% and 10%–40% centrality classes, respectively. The difference between the positively and negatively charged hadron v 1 has a positive slope as a function of pseudorapidity η , dΔv1/dη = 1.68 ± 0.49 ( stat ) ± 0.41 ( syst ) × 10−4 . The same measurement for D0 and ¯ D0 mesons yields a positive value d Δv1/dη = 4.9 ± 1.7 ( stat ) ± 0.6 ( syst ) × 10−1, which is about 3 orders of magnitude larger than the one of the charged hadrons. These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in noncentral heavy ion collisions on the dynamics of light (u, d, and s) and heavy (c) quarks. The large difference between the observed Δv1 of charged hadrons and D0 mesons may reflect different sensitivity of the charm and light quarks to the early time dynamics of a heavy ion collision. These observations challenge some recent theoretical calculations, which predicted a negative and an order of magnitude smaller value of d Δv1/dη for both light flavor and charmed hadrons.
A
bstract
The
p
T
-differential production cross sections of prompt and non-prompt (produced in beauty-hadron decays) D mesons were measured by the ALICE experiment at midrapidity (
|
y
|
<
0
.
5) in ...proton-proton collisions at
s
= 5
.
02 TeV. The data sample used in the analysis corresponds to an integrated luminosity of (19
.
3
±
0
.
4) nb
−
1
. D mesons were reconstructed from their decays D
0
→
K
−
π
+
, D
+
→
K
−
π
+
π
+
, and
D
s
+
→
ϕ
π
+
→
K
−
K
+
π
+
and their charge conjugates. Compared to previous measurements in the same rapidity region, the cross sections of prompt D
+
and
D
s
+
mesons have an extended
p
T
coverage and total uncertainties reduced by a factor ranging from 1.05 to 1.6, depending on
p
T
, allowing for a more precise determination of their
p
T
-integrated cross sections. The results are well described by perturbative QCD calculations. The fragmentation fraction of heavy quarks to strange mesons divided by the one to non-strange mesons,
f
s
/
(
f
u
+
f
d
), is compatible for charm and beauty quarks and with previous measurements at different centre-of-mass energies and collision systems. The
b
b
¯
production cross section per rapidity unit at midrapidity, estimated from non-prompt D-meson measurements, is
d
σ
b
b
¯
/
d
y
y
<
0.5
=
34.5
±
2.4
stat
−
2.9
+
4.7
tot
.
syst
μb. It is compatible with previous measurements at the same centre-of-mass energy and with the cross section pre- dicted by perturbative QCD calculations.
Experimental results are presented on event-by-event net-proton fluctuation measurements in Pb–Pb collisions at sNN=2.76 TeV, recorded by the ALICE detector at the CERN LHC. These measurements have ...as their ultimate goal an experimental test of Lattice QCD (LQCD) predictions on second and higher order cumulants of net-baryon distributions to search for critical behavior near the QCD phase boundary. Before confronting them with LQCD predictions, account has to be taken of correlations stemming from baryon number conservation as well as fluctuations of participating nucleons. Both effects influence the experimental measurements and are usually not considered in theoretical calculations. For the first time, it is shown that event-by-event baryon number conservation leads to subtle long-range correlations arising from very early interactions in the collisions.
The coherent photoproduction of
J
/
ψ
and
ψ
′
mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy
s
NN
=
5.02
TeV with the ALICE detector. Charmonia are detected in ...the central rapidity region for events where the hadronic interactions are strongly suppressed. The
J
/
ψ
is reconstructed using the dilepton (
l
+
l
-
) and proton–antiproton decay channels, while for the
ψ
′
the dilepton and the
l
+
l
-
π
+
π
-
decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233
μ
b
-
1
. The results are compared with theoretical models for coherent
J
/
ψ
and
ψ
′
photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-
x
of around
6
×
10
-
4
, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent
J
/
ψ
cross section including ALICE measurements at forward rapidity. The ratio of
ψ
′
to
J
/
ψ
coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.
This paper presents the measurements of
π
±
,
K
±
,
p
and
p
¯
transverse momentum (
p
T
) spectra as a function of charged-particle multiplicity density in proton–proton (pp) collisions at
s
=
13
TeV
...with the ALICE detector at the LHC. Such study allows us to isolate the center-of-mass energy dependence of light-flavour particle production. The measurements reported here cover a
p
T
range from 0.1 to 20
GeV
/
c
and are done in the rapidity interval
|
y
|
<
0.5
. The
p
T
-differential particle ratios exhibit an evolution with multiplicity, similar to that observed in pp collisions at
s
=
7
TeV
, which is qualitatively described by some of the hydrodynamical and pQCD-inspired models discussed in this paper. Furthermore, the
p
T
-integrated hadron-to-pion yield ratios measured in pp collisions at two different center-of-mass energies are consistent when compared at similar multiplicities. This also extends to strange and multi-strange hadrons, suggesting that, at LHC energies, particle hadrochemistry scales with particle multiplicity the same way under different collision energies and colliding systems.
This Letter presents the first experimental evidence of the attractive strong interaction between a proton and a ϕ meson. The result is obtained from two-particle correlations of combined p − ϕ ⊕ ¯ ...p − ϕ pairs measured in high-multiplicity p p collisions at √ s = 13 TeV by the ALICE Collaboration. The spin-averaged scattering length and effective range of the p − ϕ interaction are extracted from the fully corrected correlation function employing the Lednický-Lyuboshits approach. In particular, the imaginary part of the scattering length vanishes within uncertainties, indicating that inelastic processes do not play a prominent role for the p − ϕ interaction. These data demonstrate that the interaction is dominated by elastic p − ϕ scattering. Furthermore, an analysis employing phenomenological Gaussian- and Yukawa-type potentials is conducted. Under the assumption of the latter, the N − ϕ coupling constant is found to be gN−ϕ = 0.14 ± 0.03 ( stat ) ± 0.02 ( syst ) . This work provides valuable experimental input to accomplish a self-consistent description of the N − ϕ interaction, which is particularly relevant for the more fundamental studies on partial restoration of chiral symmetry in nuclear medium.
The pT-differential cross sections of prompt charm-strange baryons Ξc0 and Ξc+ were measured at midrapidity (|y|<0.5) in proton-proton (pp) collisions at a center-of-mass energy s=13 TeV with the ...ALICE detector at the LHC. The Ξc0 baryon was reconstructed via both the semileptonic decay (Ξ-e+νe) and the hadronic decay (Ξ-π+) channels. The Ξc+ baryon was reconstructed via the hadronic decay (Ξ-π+π+) channel. The branching-fraction ratio BR(Ξc0→Ξ-e+νe)/BR(Ξc0→Ξ-π+)=1.38±0.14(stat)±0.22(syst) was measured with a total uncertainty reduced by a factor of about 3 with respect to the current world average reported by the Particle Data Group. The transverse momentum (pT) dependence of the Ξc0- and Ξc+-baryon production relative to the D0 meson and to the ςc0,+,++- and Λc+-baryon production are reported. The baryon-to-meson ratio increases toward low pT up to a value of approximately 0.3. The measurements are compared with various models that take different hadronization mechanisms into consideration. The results provide stringent constraints to these theoretical calculations and additional evidence that different processes are involved in charm hadronization in electron-positron (e+e-) and hadronic collisions.
A
bstract
The inclusive J/
ψ
elliptic (
v
2
) and triangular (
v
3
) flow coefficients measured at forward rapidity (2
.
5
< y <
4) and the
v
2
measured at midrapidity (|
y
|
<
0
.
9) in Pb-Pb ...collisions at
s
NN
= 5
.
02 TeV using the ALICE detector at the LHC are reported. The entire Pb-Pb data sample collected during Run 2 is employed, amounting to an integrated luminosity of 750
μ
b
−
1
at forward rapidity and 93
μ
b
−
1
at midrapidity. The results are obtained using the scalar product method and are reported as a function of transverse momentum
p
T
and collision centrality. At midrapidity, the J/
ψ v
2
is in agreement with the forward rapidity measurement. The centrality averaged results indicate a positive J/
ψ v
3
with a significance of more than 5
σ
at forward rapidity in the
p
T
range 2
< p
T
<
5 GeV/
c
. The forward rapidity
v
2
,
v
3
, and
v
3
/v
2
results at low and intermediate
p
T
(
p
T
≲ 8 GeV/
c
) exhibit a mass hierarchy when compared to pions and D mesons, while converging into a species-independent curve at higher
p
T
. At low and intermediate
p
T
, the results could be interpreted in terms of a later thermalization of charm quarks compared to light quarks, while at high
p
T
, path-length dependent effects seem to dominate. The J/
ψ v
2
measurements are further compared to a microscopic transport model calculation. Using a simplified extension of the quark scaling approach involving both light and charm quark flow components, it is shown that the D-meson
v
n
measurements can be described based on those for charged pions and J/
ψ
flow.
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