The pseudorapidity density of charged particles, Formula omitted, in p-Pb collisions has been measured at a centre-of-mass energy per nucleon-nucleon pair of Formula omitted = 8.16 TeV at ...mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, Formula omitted. The Formula omitted value is Formula omitted at Formula omitted. This quantity divided by Formula omitted / 2 is Formula omitted, where Formula omittedis the average number of participating nucleons, is 9.5% higher than the corresponding value for p-Pb collisions at Formula omitted = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agree within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the Formula omitted distribution. Saturation-based models reproduce the distributions well for Formula omitted. The Formula omitted is also measured for different centrality estimators, based both on the charged-particle multiplicity and on the energy deposited in the Zero-Degree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p-Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
First results on Formula omitted, Formula omitted and K/p fluctuations are obtained with the ALICE detector at the CERN LHC as a function of centrality in Formula omitted collisions at Formula ...omitted. The observable Formula omitted, which is defined in terms of the moments of particle multiplicity distributions, is used to quantify the magnitude of dynamical fluctuations of relative particle yields and also provides insight into the correlation between particle pairs. This study is based on a novel experimental technique, called the Identity Method, which allows one to measure the moments of multiplicity distributions in case of incomplete particle identification. The results for Formula omitted show a change of sign in Formula omitted from positive to negative towards more peripheral collisions. For central collisions, the results follow the smooth trend of the data at lower energies and Formula omitted exhibits a change in sign for Formula omitted and K/p.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
The ALICE Collaboration reports a search for jet quenching effects in high-multiplicity (HM) proton-proton collisions at
s
= 13 TeV, using the semi-inclusive azimuthal-difference ...distribution ∆
φ
of charged-particle jets recoiling from a high transverse momentum (high-
p
T
,
trig
) trigger hadron. Jet quenching may broaden the ∆
φ
distribution measured in HM events compared to that in minimum bias (MB) events. The measurement employs a
p
T
,
trig
-differential observable for data-driven suppression of the contribution of multiple partonic interactions, which is the dominant background. While azimuthal broadening is indeed observed in HM compared to MB events, similar broadening for HM events is observed for simulations based on the PYTHIA 8 Monte Carlo generator, which does not incorporate jet quenching. Detailed analysis of these data and simulations show that the azimuthal broadening is due to bias of the HM selection towards events with multiple jets in the final state. The identification of this bias has implications for all jet quenching searches where selection is made on the event activity.
A
bstract
Results on the transverse spherocity dependence of light-flavor particle production (
π
, K, p,
ϕ
, K
*0
,
K
S
0
, Λ, Ξ) at midrapidity in high-multiplicity pp collisions at
s
= 13 TeV were ...obtained with the ALICE apparatus. The transverse spherocity estimator
S
O
p
T
=
1
categorizes events by their azimuthal topology. Utilizing narrow selections on
S
O
p
T
=
1
, it is possible to contrast particle production in collisions dominated by many soft initial interactions with that observed in collisions dominated by one or more hard scatterings. Results are reported for two multiplicity estimators covering different pseudorapidity regions. The
S
O
p
T
=
1
estimator is found to effectively constrain the hardness of the events when the midrapidity (|
η
| < 0.8) estimator is used.
The production rates of strange particles are found to be slightly higher for soft isotropic topologies, and severely suppressed in hard jet-like topologies. These effects are more pronounced for hadrons with larger mass and strangeness content, and observed when the topological selection is done within a narrow multiplicity interval. This demonstrates that an important aspect of the universal scaling of strangeness enhancement with final-state multiplicity is that high-multiplicity collisions are dominated by soft, isotropic processes. On the contrary, strangeness production in events with jet-like processes is significantly reduced.
The results presented in this article are compared with several QCD-inspired Monte Carlo event generators. Models that incorporate a two-component phenomenology, either through mechanisms accounting for string density, or thermal production, are able to describe the observed strangeness enhancement as a function of
S
O
p
T
=
1
.
A
bstract
Measurements of inclusive charged-particle jet production in pp and p-Pb collisions at center-of-mass energy per nucleon-nucleon collision
s
NN
= 5
.
02 TeV and the corresponding nuclear ...modification factor
R
pPb
ch
jet
are presented, using data collected with the ALICE detector at the LHC. Jets are reconstructed in the central rapidity region |
η
jet
|
<
0
.
5 from charged particles using the anti-
k
T
algorithm with resolution parameters
R
= 0
.
2, 0
.
3, and 0
.
4. The
p
T
-differential inclusive production cross section of charged-particle jets, as well as the corresponding cross section ratios, are reported for pp and p-Pb collisions in the transverse momentum range 10
<
p
T
,
jet
ch
<
140 GeV/
c
and 10
<
p
T
,
jet
ch
<
160 GeV/
c
, respectively, together with the nuclear modification factor
R
pPb
ch
jet
in the range 10
<
p
T
,
jet
ch
<
140 GeV/
c
. The analysis extends the
p
T
range of the previously-reported charged-particle jet measurements by the ALICE Collaboration. The nuclear modification factor is found to be consistent with one and independent of the jet resolution parameter with the improved precision of this study, indicating that the possible influence of cold nuclear matter effects on the production cross section of charged-particle jets in p-Pb collisions at
s
NN
= 5
.
02 TeV is smaller than the current precision. The obtained results are in agreement with other minimum bias jet measurements available for RHIC and LHC energies, and are well reproduced by the NLO perturbative QCD P
owheg
calculations with parton shower provided by P
ythia
8 as well as by J
etscape
simulations.
Inclusive J/ψ production is studied in p-Pb interactions at a centre-of-mass energy per nucleon-nucleon collision sNN=8.16 TeV, using the ALICE detector at the CERN LHC. The J/ψ meson is ...reconstructed, via its decay to a muon pair, in the centre-of-mass rapidity intervals 2.03 < ycms < 3.53 and −4.46 < ycms < −2.96, where positive and negative ycms refer to the p-going and Pb-going direction, respectively. The transverse momentum coverage is pT < 20 GeV/c. In this paper, ycms- and pT-differential cross sections for inclusive J/ψ production are presented, and the corresponding nuclear modification factors RpPb are shown. Forward results show a suppression of the J/ψ yield with respect to pp collisions, concentrated in the region pT ≲ 5 GeV/c. At backward rapidity no significant suppression is observed. The results are compared to previous measurements by ALICE in p-Pb collisions at sNN=5.02 TeV and to theoretical calculations. Finally, the ratios RFB between forward- and backward-ycmsRpPb values are shown and discussed.
Neutral pion and Formula omitted meson invariant differential yields were measured in non-single diffractive p-Pb collisions at Formula omitted = 5.02 TeV with the ALICE experiment at the CERN LHC. ...The analysis combines results from three complementary photon measurements, utilizing the PHOS and EMCal calorimeters and the Photon Conversion Method. The invariant differential yields of Formula omitted and Formula omitted meson inclusive production are measured near mid-rapidity in a broad transverse momentum range of Formula omitted and Formula omitted, respectively. The measured Formula omitted ratio increases with Formula omitted and saturates for Formula omitted > 4 Formula omitted at Formula omitted. A deviation from Formula omitted scaling is observed for Formula omitted 2 Formula omitted. The measured Formula omitted ratio is consistent with previous measurements from proton-nucleus and pp collisions over the full Formula omitted range. The measured Formula omitted ratio at high Formula omitted also agrees within uncertainties with measurements from nucleus-nucleus collisions. The Formula omitted and Formula omitted yields in p-Pb relative to the scaled pp interpolated reference, Formula omitted, are presented for Formula omitted 20 Formula omitted and Formula omitted 20 Formula omitted, respectively. The results are compared with theoretical model calculations. The values of Formula omitted are consistent with unity for transverse momenta above 2 Formula omitted. These results support the interpretation that the suppressed yield of neutral mesons measured in Pb-Pb collisions at LHC energies is due to parton energy loss in the hot QCD medium.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We report on the measurement of the inclusive J/ Formula omitted polarization parameters in pp collisions at a center of mass energy Formula omitted TeV with the ALICE detector at the LHC. The ...analysis is based on a data sample corresponding to an integrated luminosity of 1.23 pb Formula omitted. J/ Formula omitted resonances are reconstructed in their di-muon decay channel in the rapidity interval Formula omitted and over the transverse-momentum interval Formula omitted Formula omitted. The three polarization parameters ( Formula omitted, Formula omitted, Formula omitted) are measured as a function of Formula omitted both in the helicity and Collins-Soper reference frames. The measured J/ Formula omitted polarization parameters are found to be compatible with zero within uncertainties, contrary to expectations from all available predictions. The results are compared with the measurement in pp collisions at Formula omitted TeV.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Formula omitted meson measurements provide insight into strangeness production, which is one of the key observables for the hot medium formed in high-energy heavy-ion collisions. ALICE measured ...Formula omitted production through its decay in muon pairs in Pb-Pb collisions at Formula omitted TeV in the intermediate transverse momentum range Formula omitted GeV/c and in the rapidity interval Formula omitted. The Formula omitted yield was measured as a function of the transverse momentum and collision centrality. The nuclear modification factor was obtained as a function of the average number of participating nucleons. Results were compared with the ones obtained via the kaon decay channel in the same Formula omitted range at midrapidity. The values of the nuclear modification factor in the two rapidity regions are in agreement within uncertainties.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A measurement of beauty hadron production at mid-rapidity in proton-lead collisions at a nucleon–nucleon centre-of-mass energy sNN=5.02 TeV is presented. The semi-inclusive decay channel of beauty ...hadrons into J/ψ is considered, where the J/ψ mesons are reconstructed in the dielectron decay channel at mid-rapidity down to transverse momenta of 1.3 GeV/c. The bb¯ production cross section at mid-rapidity, dσbb¯/dy, and the total cross section extrapolated over full phase space, σbb¯, are obtained. This measurement is combined with results on inclusive J/ψ production to determine the prompt J/ψ cross sections. The results in p–Pb collisions are then scaled to expectations from pp collisions at the same centre-of-mass energy to derive the nuclear modification factor RpPb, and compared to models to study possible nuclear modifications of the production induced by cold nuclear matter effects. RpPb is found to be smaller than unity at low pT for both J/ψ coming from beauty hadron decays and prompt J/ψ.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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