The ALICE Collaboration reports three measurements in ultraperipheral proton-lead collisions at forward rapidity. The exclusive two-photon process γγ→μ+μ− and the exclusive photoproduction of J/ψ are ...studied. J/ψ photoproduction with proton dissociation is measured for the first time at a hadron collider. The cross section for the two-photon process of dimuons in the invariant mass range from 1 to 2.5 GeV/c2 agrees with leading-order quantum electrodynamics calculations. The exclusive and dissociative cross sections for J/ψ photoproductions are measured for photon-proton center-of-mass energies from 27 to 57 GeV. They are in good agreement with HERA results.
We present measurements of two-particle correlations with neutral pion trigger particles of transverse momenta 8<pTtrig<16 GeV/c and associated charged particles of 0.5<pTassoc<10 GeV/c versus the ...azimuthal angle difference Δφ at midrapidity in pp and central Pb–Pb collisions at sNN=2.76 TeV with ALICE. The new measurements exploit associated charged hadrons down to 0.5 GeV/c, which significantly extends our previous measurement that only used charged hadrons above 3 GeV/c. After subtracting the contributions of the flow background, v2 to v5, the per-trigger yields are extracted for |Δφ|<0.7 on the near and for |Δφ−π|<1.1 on the away side. The ratio of per-trigger yields in Pb–Pb to those in pp collisions, IAA, is measured on the near and away side for the 0–10% most central Pb–Pb collisions. On the away side, the per-trigger yields in Pb–Pb are strongly suppressed to the level of IAA≈0.6 for pTassoc>3 GeV/c, while with decreasing momenta an enhancement develops reaching about 5 at low pTassoc. On the near side, an enhancement of IAA between 1.2 at the highest to 1.8 at the lowest pTassoc is observed. The data are compared to parton-energy-loss predictions of the JEWEL and AMPT event generators, as well as to a perturbative QCD calculation with medium-modified fragmentation functions. All calculations qualitatively describe the away-side suppression at high pTassoc. Only AMPT captures the enhancement at low pTassoc, both on the near and away side. However, it also underpredicts IAA above 5 GeV/c, in particular on the near-side.
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
$$ \sqrt{s} $$
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.
A
bstract
A measurement of inclusive, prompt, and non-prompt J/
ψ
production in p-Pb collisions at a nucleon-nucleon centre-of-mass energy
$$ \sqrt{s_{\mathrm{NN}}} $$
s
NN
= 5
.
02 TeV is presented. ...The inclusive J/
ψ
mesons are reconstructed in the dielectron decay channel at midrapidity down to a transverse momentum
p
T
= 0. The inclusive J/
ψ
nuclear modification factor
R
pPb
is calculated by comparing the new results in p-Pb collisions to a recently measured proton-proton reference at the same centre-of-mass energy. Non-prompt J/
ψ
mesons, which originate from the decay of beauty hadrons, are separated from promptly produced J/
ψ
on a statistical basis for
p
T
larger than 1.0 GeV/
c
. These results are based on the data sample collected by the ALICE detector during the 2016 LHC p-Pb run, corresponding to an integrated luminosity
$$ \mathcal{L} $$
L
int
= 292
±
11
μ
b
−
1
, which is six times larger than the previous publications. The total uncertainty on the
p
T
-integrated inclusive J/
ψ
and non-prompt J/
ψ
cross section are reduced by a factor 1.7 and 2.2, respectively. The measured cross sections and
R
pPb
are compared with theoretical models that include various combinations of cold nuclear matter effects. From the non-prompt J/
ψ
production cross section, the
$$ \mathrm{b}\overline{\mathrm{b}} $$
b
b
¯
production cross section at midrapidity,
$$ {\mathrm{d}\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$
d
σ
b
b
¯
/
d
y
, and the total cross section extrapolated over full phase space,
$$ {\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$
σ
b
b
¯
, are derived.
The production cross section of inclusive isolated photons has been measured by the ALICE experiment at the CERN LHC in pp collisions at a centre-of-momentum energy of
s
=
7 TeV. The measurement is ...performed with the electromagnetic calorimeter EMCal and the central tracking detectors, covering a range of
|
η
|
<
0.27
in pseudorapidity and a transverse momentum range of
10
<
p
T
γ
<
60
GeV
/
c
. The result extends the
p
T
coverage of previously published results of the ATLAS and CMS experiments at the same collision energy to smaller
p
T
. The measurement is compared to next-to-leading order perturbative QCD calculations and to the results from the ATLAS and CMS experiments. All measurements and theory predictions are in agreement with each other.
A study of multiplicity and pseudorapidity distributions of inclusive photons measured in pp and p–Pb collisions at a center-of-mass energy per nucleon–nucleon collision of
s
NN
=
5.02
TeV using the ...ALICE detector in the forward pseudorapidity region 2.3
<
η
lab
<
3.9 is presented. Measurements in p–Pb collisions are reported for two beam configurations in which the directions of the proton and lead ion beam were reversed. The pseudorapidity distributions in p–Pb collisions are obtained for seven centrality classes which are defined based on different event activity estimators, i.e., the charged-particle multiplicity measured at midrapidity as well as the energy deposited in a calorimeter at beam rapidity. The inclusive photon multiplicity distributions for both pp and p–Pb collisions are described by double negative binomial distributions. The pseudorapidity distributions of inclusive photons are compared to those of charged particles at midrapidity in pp collisions and for different centrality classes in p–Pb collisions. The results are compared to predictions from various Monte Carlo event generators. None of the generators considered in this paper reproduces the inclusive photon multiplicity distributions in the reported multiplicity range. The pseudorapidity distributions are, however, better described by the same generators.
Abstract
A study of multiplicity and pseudorapidity distributions of inclusive photons measured in pp and p–Pb collisions at a center-of-mass energy per nucleon–nucleon collision of
...$$\sqrt{s_{\textrm{NN}}}~=~5.02$$
s
NN
=
5.02
TeV using the ALICE detector in the forward pseudorapidity region 2.3
$$<~\eta _\textrm{lab} ~<$$
<
η
lab
<
3.9 is presented. Measurements in p–Pb collisions are reported for two beam configurations in which the directions of the proton and lead ion beam were reversed. The pseudorapidity distributions in p–Pb collisions are obtained for seven centrality classes which are defined based on different event activity estimators, i.e., the charged-particle multiplicity measured at midrapidity as well as the energy deposited in a calorimeter at beam rapidity. The inclusive photon multiplicity distributions for both pp and p–Pb collisions are described by double negative binomial distributions. The pseudorapidity distributions of inclusive photons are compared to those of charged particles at midrapidity in pp collisions and for different centrality classes in p–Pb collisions. The results are compared to predictions from various Monte Carlo event generators. None of the generators considered in this paper reproduces the inclusive photon multiplicity distributions in the reported multiplicity range. The pseudorapidity distributions are, however, better described by the same generators.
Hadronic resonances are unique tools to investigate the interplay of re-scattering and regeneration effects during the hadronization phase in heavy-ion collisions. Measurements in small collision ...systems provide a necessary baseline for heavy-ion data, help to tune pQCD inspired event generators and give insight into the search for the onset of collective effects. As the ϕ meson has a longer lifetime compared to other resonances, it is expected that its production would be much less affected by regeneration and re-scattering processes. We report on measurements of ϕ meson production in minimum bias pp collisions at different beam energies and as a function of charged particle multiplicity with the ALICE detector at the LHC. The results include the transverse momentum (pT) distributions of ϕ as well as the particle yield ratios. Finally, we have also studied the ϕ effective strangeness content by comparing our results to theoretical calculations.
A
bstract
The measurement of prompt D-meson production as a function of multiplicity in p-Pb collisions at
s
N
N
=
5.02
TeV with the ALICE detector at the LHC is reported. D
0
, D
+
and D
∗+
mesons ...are reconstructed via their hadronic decay channels in the centre-of-mass rapidity range −0
.
96
< y
cms
<
0
.
04 and transverse momentum interval 1
<p
T
<
24 GeV
/c
. The multiplicity dependence of D-meson production is examined by either comparing yields in p-Pb collisions in different event classes, selected based on the multiplicity of produced particles or zero-degree energy, with those in pp collisions, scaled by the number of binary nucleon-nucleon collisions (nuclear modification factor); as well as by evaluating the per-event yields in p-Pb collisions in different multiplicity intervals normalised to the multiplicity-integrated ones (relative yields). The nuclear modification factors for D
0
, D
+
and D
∗+
are consistent with one another. The D-meson nuclear modification factors as a function of the zero-degree energy are consistent with unity within uncertainties in the measured
p
T
regions and event classes. The relative D-meson yields, calculated in various
p
T
intervals, increase as a function of the charged-particle multiplicity. The results are compared with the equivalent pp measurements at
s
=
7
TeV as well as with EPOS 3 calculations.
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