Systematic studies of charge-dependent two- and three-particle correlations in Pb-Pb collisions at $\sqrt{\it{s}_\mathrm{{NN}}} = $ 2.76 and 5.02 TeV used to probe the Chiral Magnetic Effect (CME) are presented. These measurements are performed for charged particles in the pseudorapidity ($\eta$) and transverse momentum ($p_{\rm{T}}$) ranges $\left|\eta \right| < 0.8$ and $0.2 < p_{\mathrm{T}} < 5$ GeV/$c$. A significant charge-dependent signal that becomes more pronounced for peripheral collisions is reported for the CME-sensitive correlators $\gamma_{1,1} = \langle \cos (\varphi_{\alpha} +\varphi_{\beta} - 2\Psi_{2}) \rangle$ and $\gamma_{1,-3} = \langle \cos (\varphi_{\alpha} -3\varphi_{\beta} + 2\Psi_{2}) \rangle$. The results are used to estimate the contribution of background effects, associated with local charge conservation coupled to anisotropic flow modulations, to measurements of the CME. A blast-wave parametrisation that incorporates local charge conservation tuned to reproduce the centrality dependent background effects is not able to fully describe the measured $\gamma_{1,1}$. Finally, the charge and centrality dependence of mixed-harmonics three-particle correlations, of the form $\gamma_{1,2} = \langle \cos (\varphi_{\alpha} +2\varphi_{\beta} - 3\Psi_{3}) \rangle$, which are insensitive to the CME signal, verify again that background contributions dominate the measurement of $\gamma_{1,1}$.
2-particle integrated correlators
2-particle differential correlator versus pT difference
2-particle differential correlator versus average pT
This paper presents isolated photon-hadron correlations using pp and p-Pb data collected by the ALICE detector at the LHC. For photons with |$\eta$| < 0.67 and 12 < $p_{\rm{T}}$ < 40 GeV/$c$, the associated yield of charged particles in the range |$\eta$| < 0.80 and 0.5 < $p_{\rm{T}}$ < 10 GeV/$c$ is presented. These momenta are much lower than previous measurements at the LHC. No significant difference between pp and p-Pb is observed, with PYTHIA 8.2 describing both data sets within uncertainties. This measurement constrains nuclear effects on the parton fragmentation in p-Pb collisions, and provides a benchmark for future studies of Pb-Pb collisions.
$\gamma^\mathrm{iso}$-hadron correlation functions for pp (red) and p$-$Pb (blue) data at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV as measured by the ALICE detector. The different panels represent three different $z_\mathrm{T}$ bins. The correlation functions are projected over the range $|\Delta\eta| < 1.2$. The darker bands at zero represents the uncertainty from the underlying event estimation in pp and p$-$Pb. The underlying event was estimated over the range $|0.4 <\Delta\varphi < 1.6|$. The vertical bars represent statistical uncertainties only. The boxes indicate the systematic uncertainties. The dashed green line represents the $\gamma^\mathrm{iso}$-hadron correlation function obtained with PYTHIA 8.2 Monash Tune. '$p$' is the p-value for the hypothesis that the pp and p$-$Pb data follow the same true correlation function.
$\gamma^\mathrm{iso}$-hadron correlation functions for pp (red) and p$-$Pb (blue) data at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV as measured by the ALICE detector. The different panels represent three different $z_\mathrm{T}$ bins. The correlation functions are projected over the range $|\Delta\eta| < 1.2$. The darker bands at zero represents the uncertainty from the underlying event estimation in pp and p$-$Pb. The underlying event was estimated over the range $|0.4 <\Delta\varphi < 1.6|$. The vertical bars represent statistical uncertainties only. The boxes indicate the systematic uncertainties. The dashed green line represents the $\gamma^\mathrm{iso}$-hadron correlation function obtained with PYTHIA 8.2 Monash Tune. '$p$' is the p-value for the hypothesis that the pp and p$-$Pb data follow the same true correlation function.
$\gamma^\mathrm{iso}$-hadron correlation functions for pp (red) and p$-$Pb (blue) data at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV as measured by the ALICE detector. The different panels represent three different $z_\mathrm{T}$ bins. The correlation functions are projected over the range $|\Delta\eta| < 1.2$. The darker bands at zero represents the uncertainty from the underlying event estimation in pp and p$-$Pb. The underlying event was estimated over the range $|0.4 <\Delta\varphi < 1.6|$. The vertical bars represent statistical uncertainties only. The boxes indicate the systematic uncertainties. The dashed green line represents the $\gamma^\mathrm{iso}$-hadron correlation function obtained with PYTHIA 8.2 Monash Tune. '$p$' is the p-value for the hypothesis that the pp and p$-$Pb data follow the same true correlation function.
A measurement of dielectron production in proton-proton (pp) collisions at $\sqrt{s} = 13$ TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron invariant mass $m_{\rm ee}$ and pair transverse momentum $p_{\rm T,ee}$ that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of $m_{\rm ee}$, $p_{\rm T,ee}$, and event multiplicity ${\rm d} N_{\rm ch}/{\rm d} \eta$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $\eta/\pi^0$ ratio in pp and proton-nucleus (p-A) collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at $0.15<m_{\rm ee}<0.6$ GeV/$c^2$ and for $p_{\rm T,ee} < 0.4$ GeV/$c$ indicates an enhancement of soft dielectrons, reminiscent of the 'anomalous' soft-photon and -dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to $1.61\pm 0.13\,(\rm{stat.})\pm 0.17\,(\rm{syst., data}) \pm 0.34\,(\rm{syst., cocktail})$ in the ALICE acceptance. Acceptance-corrected excess spectra in $m_{\rm ee}$ and $p_{\rm T,ee}$ are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.
Differential dielectron cross section in pp collisions at $\sqrt{s}$ = 13 TeV as a function of $m_{\rm ee}$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.075$ GeV/$c$.
Differential dielectron cross section in pp collisions at $\sqrt{s}$ = 13 TeV in the invariant mass interval 0.15<$m_{\rm ee}$<0.6 as a function of $p_{\rm T,ee}$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.075$ GeV/$c$.
Dielectron yield per event in the excess region(0.15<$m_{\rm ee}$<0.6, $p_{\rm T,ee}$<0.4) as a function of the event multiplicity. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.075$ GeV/$c$.
The measurements of the (anti)deuterons elliptic flow ($v_2$) and the first measurements of triangular flow ($v_3$) in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon collisions $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV are presented. A mass ordering at low transverse momentum ($p_{\rm T}$) is observed when comparing these measurements with those of other identified hadrons, as expected from relativistic hydrodynamics. The measured (anti)deuterons $v_2$ lies between the predictions from the simple coalescence and blast-wave models, which provide a good description of the data only for more peripheral and for more central collisions, respectively. The mass number scaling, which is violated for $v_2$, is approximately valid for the (anti)deuterons $v_3$. The measured $v_2$ and $v_3$ are also compared with the predictions from a coalescence approach with phase-space distributions of nucleons generated by iEBE-VISHNU with AMPT initial conditions coupled with UrQMD, and from a dynamical model based on relativistic hydrodynamics coupled to the hadronic afterburner SMASH. The model predictions are consistent with the data within the uncertainties in mid-central collisions, while a deviation is observed in central centrality intervals.
v2 as a function of pT for Pb-Pb collisions at \sqrt{s_NN} = 5.02 TeV and centrality 0-5%.
v2 as a function of pT for Pb-Pb collisions at \sqrt{s_NN} = 5.02 TeV and centrality 5-10%.
v2 as a function of pT for Pb-Pb collisions at \sqrt{s_NN} = 5.02 TeV and centrality 10-20%.
The inclusive J/$\psi$ 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 $\sqrt{s_{\rm NN}}$ = 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 $\mu$b$^{-1}$ at forward rapidity and 93 $\mu$b$^{-1}$ at midrapidity. The results are obtained using the scalar product method and are reported as a function of transverse momentum $p_{\rm T}$ and collision centrality. At midrapidity, the J/$\psi$ $v_2$ is in agreement with the forward rapidity measurement. The centrality averaged results indicate a positive J/$\psi$ $v_3$ with a significance of more than 5$\sigma$ at forward rapidity in the $p_{\rm T}$ range $2<p_{\rm T}<5$ GeV/$c$. The forward rapidity $v_2$, $v_3$, and $v_3$/$v_2$ results at low and intermediate $p_{\rm T}$ ($p_{\rm T} \lesssim 8$ GeV/$c$) exhibit a mass hierarchy when compared to pions and D mesons, while converging into a species-independent curve at higher $p_{\rm T}$. At low and intermediate $p_{\rm T}$, the results could be interpreted in terms of a later thermalization of charm quarks compared to light quarks, while at high $p_{\rm T}$, path-length dependent effects seem to dominate. The J/$\psi$ $v_2$ 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 $v_{\rm n}$ measurements can be described based on those for charged pions and J/$\psi$ flow.
Inclusive J/$\psi$ $v_2$ as a function of $p_{T}$ in the centrality interval 0$-$10.0 %
Inclusive J/$\psi$ $v_2$ as a function of $p_{T}$ in the centrality interval 10$-$30 %
Inclusive J/$\psi$ $v_2$ as a function of $p_{T}$ in the centrality interval 30$-$50 %
The first measurements of dielectron production at midrapidity ($|\eta_{c}|<0.8$) in proton-proton and proton-lead collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV at the LHC are presented. The dielectron cross section is measured with the ALICE detector as a function of the invariant mass $m_{\rm{ee}}$ and the pair transverse momentum $p_{\rm{T,ee}}$ in the ranges $m_{\rm{ee}}$ < 3.5 GeV/$c^{2}$ and $p_{\rm{T,ee}}$ < 8.0 GeV/$c^{2}$, in both collision systems. In proton-proton collisions, the charm and beauty cross sections are determined at midrapidity from a fit to the data with two different event generators. This complements the existing dielectron measurements performed at $\sqrt{s}$ = 7 and 13 TeV. The slope of the $\sqrt{s}$ dependence of the three measurements is described by FONLL calculations. The dielectron cross section measured in proton-lead collisions is in agreement, within the current precision, with the expected dielectron production without any nuclear matter effects for $\rm{e}^{+}\rm{e}^{-}$ pairs from open heavy-flavor hadron decays. For the first time at LHC energies, the dielectron production in proton-lead and proton-proton collisions are directly compared at the same $\sqrt{s_{\rm{NN}}}$ via the dielectron nuclear modification factor $R_{\rm{pPb}}$. The measurements are compared to model calculations including cold nuclear matter effects, or additional sources of dielectrons from thermal radiation.
Inclusive $e^+e^-$ cross section in pp collisions at $\sqrt{s}$ = 5.02 TeV as a function of $m_{\rm ee}$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.2$ GeV/$c$.
Inclusive $e^+e^-$ cross section in p-Pb collisions at $\sqrt{s}$ = 5.02 TeV as a function of $m_{\rm ee}$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.2$ GeV/$c$.
Inclusive $e^+e^-$ cross section in pp collisions at $\sqrt{s}$ = 5.02 TeV as a function of $p_{\rm T,ee}$ for $0.5 < m_{\rm ee} < 1.1$ GeV/$c$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.2$ GeV/$c$.
One of the key challenges for nuclear physics today is to understand from first principles the effective interaction between hadrons with different quark content. First successes have been achieved using techniques that solve the dynamics of quarks and gluons on discrete space-time lattices. Experimentally, the dynamics of the strong interaction have been studied by scattering hadrons off each other. Such scattering experiments are difficult or impossible for unstable hadrons and so high-quality measurements exist only for hadrons containing up and down quarks. Here we demonstrate that measuring correlations in the momentum space between hadron pairs produced in ultrarelativistic proton-proton collisions at the CERN Large Hadron Collider (LHC) provides a precise method with which to obtain the missing information on the interaction dynamics between any pair of unstable hadrons. Specifically, we discuss the case of the interaction of baryons containing strange quarks (hyperons). We demonstrate how, using precision measurements of p-omega baryon correlations, the effect of the strong interaction for this hadron-hadron pair can be studied with precision similar to, and compared with, predictions from lattice calculations. The large number of hyperons identified in proton-proton collisions at the LHC, together with an accurate modelling of the small (approximately one femtometre) inter-particle distance and exact predictions for the correlation functions, enables a detailed determination of the short-range part of the nucleon-hyperon interaction.
The p--$\Xi^{-}$ $\oplus$ $\overline{\mathrm{p}}$--$\overline{\Xi}^{+}$ correlation function.
The p--$\Xi^{-}$ $\oplus$ $\overline{\mathrm{p}}$--$\overline{\Xi}^{+}$ correlation function.
The p--$\Omega^{-}$ $\oplus$ $\overline{\mathrm{p}}$--$\overline{\Omega}^{+}$ correlation function.
Measurement of Z-boson production in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=8.16$ TeV and Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is reported. It is performed in the dimuon decay channel, through the detection of muons with pseudorapidity $-4 < \eta_{\mu} < -2.5$ and transverse momentum $p_{\rm T}^{\mu} > 20$ GeV/$c$ in the laboratory frame. The invariant yield and nuclear modification factor are measured for opposite-sign dimuons with invariant mass $60 < m^{\mu\mu} < 120$ GeV$c^2$ and rapidity $2.5 < y_{cms}^{\mu\mu} < 4$. They are presented as a function of rapidity and, for the Pb-Pb collisions, of centrality as well. The results are compared with theoretical calculations, both with and without nuclear modifications to the Parton Distribution Functions (PDFs). In p-Pb collisions the center-of-mass frame is boosted with respect to the laboratory frame, and the measurements cover the backward ($-4.46< y_{cms}^{\mu\mu}<-2.96$) and forward ($2.03< y_{cms}^{\mu\mu}<3.53$) rapidity regions. For the p-Pb collisions, the results are consistent within experimental and theoretical uncertainties with calculations that include both free-nucleon and nuclear-modified PDFs. For the Pb-Pb collisions, a $3.4\sigma$ deviation is seen in the integrated yield between the data and calculations based on the free-nucleon PDFs, while good agreement is found once nuclear modifications are considered.
Differential fiducial cross section in p-Pb
Integrated fiducial invariant yield in Pb-Pb
Rapidity differential fiducial invariant yield in Pb-Pb
The first measurements of the scattering parameters of $\Lambda$K pairs in all three charge combinations ($\Lambda$K$^{+}$, $\Lambda$K$^{-}$, and $\Lambda\mathrm{K^{0}_{S}}$) are presented. The results are achieved through a femtoscopic analysis of $\Lambda$K correlations in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV recorded by ALICE at the LHC. The femtoscopic correlations result from strong final-state interactions, and are fit with a parametrization allowing for both the characterization of the pair emission source and the measurement of the scattering parameters for the particle pairs. Extensive studies with the THERMINATOR 2 event generator provide a good description of the non-femtoscopic background, which results mainly from collective effects, with unprecedented precision. Furthermore, together with HIJING simulations, this model is used to account for contributions from residual correlations induced by feed-down from particle decays. The extracted scattering parameters indicate that the strong force is repulsive in the $\Lambda\rm{K}^{+}$ interaction and attractive in the $\Lambda\rm{K}^{-}$ interaction. The data hint that the and $\Lambda\rm{K}^{0}_{S}$ interaction is attractive, however the uncertainty of the result does not permit such a decisive conclusion. The results suggest an effect arising either from different quark-antiquark interactions between the pairs ($\rm s\overline{s}$ in $\Lambda$K$^{+}$ and $\rm u\overline{u}$ in $\Lambda$K$^{-}$) or from different net strangeness for each system (S = 0 for $\Lambda$K$^{+}$, and S = $-2$ for $\Lambda$K$^{-}$). Finally, the $\Lambda$K systems exhibit source radii larger than expected from extrapolation from identical particle femtoscopic studies. This effect is interpreted as resulting from the separation in space-time of the single-particle $\Lambda$ and K source distributions.
Invariant mass distributions in the 0--10\% centrality interval of (a) p$\uppi^{-}$ pairs showing the $\Lambda$ peak for V$^{0}$ candidates.
Invariant mass distributions in the 0--10\% centrality interval of $\uppi^{+}\uppi^{-}$ pairs showing the $\mathrm{K^{0}_{S}}$ peak for V$^{0}$ candidates.
Measured correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 0--10\% centrality interval.
The polarization of inclusive J/$\psi$ and $\Upsilon(1{\rm S})$ produced in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$ TeV at the LHC is measured with the ALICE detector. The study is carried out by reconstructing the quarkonium through its decay to muon pairs in the rapidity region $2.5<y<4$ and measuring the polar and azimuthal angular distributions of the muons. The polarization parameters $\lambda_{\theta}$, $\lambda_{\phi}$ and $\lambda_{\theta\phi}$ are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval $2<p_{\rm T}<10$ GeV/$c$ and $p_{\rm T}<15$ GeV/$c$ for the J/$\psi$ and $\Upsilon(1{\rm S})$, respectively. The polarization parameters for the J/$\psi$ are found to be compatible with zero, within a maximum of about two standard deviations at low $p_{\rm T}$, for both reference frames and over the whole $p_{\rm T}$ range. The values are compared with the corresponding results obtained for pp collisions at $\sqrt{s}=7$ and 8 TeV in a similar kinematic region by the ALICE and LHCb experiments. Although with much larger uncertainties, the polarization parameters for $\Upsilon(1{\rm S})$ production in Pb-Pb collisions are also consistent with zero.
J/$\psi$ polarization parameters, measured for Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, in the helicity and Collins-Soper reference frames in the rapidity interval $2.5<y<4$.The first uncertainty is statistical and the second systematic.
J/$\psi$ polarization parameters, measured for Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, in the helicity and Collins-Soper reference frames in the rapidity interval $2.5<y<4$.The first uncertainty is statistical and the second systematic.
J/$\psi$ polarization parameters, measured for Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, in the helicity and Collins-Soper reference frames in the rapidity interval $2.5<y<4$.The first uncertainty is statistical and the second systematic.
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