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 ${\rm d} N_{\rm ch}/{\rm d} η$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $η/π^0$ 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.
The first measurement of dielectron ($\mathrm{e}^{+}\mathrm{e}^{-}$) production in central (0$-$10$\%$) Pb-Pb collisions at $\mathbf{\sqrt{\textit{s}_{\mathrm{NN}}}}$ = 2.76 TeV at the LHC is presented. The dielectron invariant-mass spectrum is compared to the expected contributions from hadron decays in the invariant-mass range $0 < m_{\mathrm{ee}}<3.5\ \mathrm{GeV}/\textit{c}^{2}$. The ratio of data and the cocktail of hadronic contributions without vacuum $ρ^{0}$ is measured in the invariant-mass range $0.15 < m_{\mathrm{ee}}<0.7\ \mathrm{GeV}/\textit{c}^{2}$, where an excess of dielectrons…
Dielectrons are an excellent probe for the QCD matter created in created in ultra-relativistic heavy-ion collisions, since they are emitted during the whole evolution of the collision and do not interact strongly with the medium. To isolate the QGP signals, measurement of the dielectron production in vacuum and its modifications due to the presence of cold nuclear matter is necessary. We present and discuss results from a low magnetic field detector setup in proton-proton collisions at $\sqrt{s} = 13$ TeV, as well as the measurement of dielectron production in pp, p-Pb, and Pb-Pb collisions at…
Spectra of unbound electron-positron pairs (dielectrons, in brief) and photons from decays of parapositronia produced in ultraperipheral collisions of electrically charged objects are calculated. Their shapes at energies of the NICA collider are demonstrated. Soft dielectrons and photons are abundantly produced. The relevance of these processes to the astrophysical problem of cooling electron-positron pairs and the intense emission of 511 keV photons from the Galactic center is discussed.
Dielectrons probe a wide range of phenomena in heavy-ion collisions. These include light- and heavy-flavour meson production, thermal radiation and coherent photo-production. The latter process is distinguished by dielectron production at low transverse-pair momentum ($ \it{p}_{\textrm{T,ee}}$). Transverse momentum spectra of dielectrons in central and peripheral Pb-Pb collisions are extracted and compared to the corresponding expectations. In central collisions the data fit the expected spectrum, which is dominated by semi-leptonic decays of correlated heavy-flavour hadrons. In peripheral col…
The ALICE Collaboration measured dielectron production as a function of the invariant mass ($m_{\rm ee}$), the pair transverse momentum ($p_{\rm T,ee}$) and the pair distance of closest approach ($\rm DCA_{ee}$) in pp collisions at $\sqrt{s} = 7$ TeV. Prompt and non-prompt dielectron sources can be separated with the $\rm DCA_{ee}$, which will give the opportunity in heavy-ion collisions to identify thermal radiation from the medium in the intermediate-mass range dominated by contributions from open-charm and beauty hadron decays. The charm and beauty total cross sections are extracted from th…
The measurement of dielectron production is presented as a function of invariant mass and transverse momentum ($p_{\rm T}$) at midrapidity ($|y_{\rm e}|<0.8$) in proton-proton (pp) collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV. The contributions from light-hadron decays are calculated from their measured cross sections in pp collisions at $\sqrt{s}=7$ TeV or 13 TeV. The remaining continuum stems from correlated semileptonic decays of heavy-flavour hadrons. Fitting the data with templates from two different MC event generators, PYTHIA and POWHEG, the charm and beauty cross sections…
The first measurement of e$^+$e$^-$ pair production at mid-rapidity ($|η_{\rm e}|$ $<$ 0.8) in pp collisions at $\sqrt{s} = 7$ TeV with ALICE at the LHC is presented. The dielectron production is studied as a function of the invariant mass ($m_{\rm ee}$ $<$ 3.3 GeV/$c^{2}$), the pair transverse momentum ($p_{\rm T,ee}$ $<$ 8 GeV/$c$), and the pair transverse impact parameter (DCA$_{\rm ee}$), i.e., the average distance of closest approach of the reconstructed electron and positron tracks to the collision vertex, normalised to its resolution. The results are compared with the expectations from …
Heavy quarks are useful probes to investigate the properties of the Quark-Gluon Plasma (QGP) produced in heavy-ion collisions at the LHC, since they are produced in initial hard scattering processes. To single out the signals that are characteristic of the QGP, it is nevertheless crucial to understand the primordial heavy-quark production in vacuum, and to disentangle hot from cold nuclear matter effects. Moreover, observations of collective effects in high-multiplicity pp and p-Pb collisions show surprising similarities with those in heavy-ion collisions. Heavy-flavour production in such coll…
Low-mass dielectron measurements play an essential role in the study of the Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions. They are produced in all stages of the collision and are not affected by final-state interactions. Thus, they provide a penetrating probe of the created medium. In the dielectron intermediate-mass region a measurement of the thermal radiation from the QGP gives information on the medium temperature. However, in this region the main component of the dielectron continuum stems from correlated semi-leptonic decays of charm and beauty hadrons. The…
Measurements of the inclusive J/$ψ$ yield as a function of charged-particle pseudorapidity density ${\rm d}N_{\rm ch}/\rm{d}η$ in pp collisions at $\sqrt{s}$ = 13 TeV with ALICE at the LHC are reported. The J/$ψ$ meson yield is measured at midrapidity ($|y|<0.9$) in the dielectron channel, for events selected based on the charged-particle multiplicity at midrapidity ($|η|<1$) and at forward rapidity ($-3.7<η<-1.7$ and $2.8<η<5.1$); both observables are normalized to their corresponding averages in minimum bias events. The increase of the normalized J/$ψ$ yield with normalized ${\rm d}N_{\rm ch…
Inclusive J/$ψ$ production is studied in minimum-bias proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 5.02 TeV by ALICE at the CERN LHC. The measurement is performed at mid-rapidity ($|y| < 0.9$) in the dielectron decay channel down to zero transverse momentum $p_{\rm T}$, using a data sample corresponding to an integrated luminosity of $L_{\rm int} = 19.4 \pm$ 0.4 nb$^{-1}$. The measured $p_{\rm T}$-integrated inclusive J/$ψ$ production cross section is d$σ$/d$y$ = 5.64 $\pm$ 0.22 (stat.) $\pm 0.33$ (syst.) $\pm 0.12$ (lumi.) $μ$b. The $p_{\rm T}$-differential cross sectio…
The study of the J/$ψ$ production in pp collisions provides important information on perturbative and non-perturbative quantum chromodynamics. Using high multiplicity pp events, we can study how charmonium production depends on the event activity. These measurements are used to investigate the possible influence of multiple partonic interactions to the J/$ψ$ production and the interplay between soft and hard processes.In this work we report on studies of J/$ψ$ production as a function of event multiplicity in pp collisions at $\sqrt{s}$ = 13 TeV at mid-rapidity with ALICE. The J/$ψ$ are recons…
We report a precise measurement of the J/$ψ$ elliptic flow in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with the ALICE detector at the LHC. The J/$ψ$ mesons are reconstructed at mid-rapidity ($|y| < 0.9$) in the dielectron decay channel and at forward rapidity ($2.5<y<4.0$) in the dimuon channel, both down to zero transverse momentum. At forward rapidity, the elliptic flow $v_2$ of the J/$ψ$ is studied as a function of transverse momentum and centrality. A positive $v_2$ is observed in the transverse momentum range $2 < p_{\rm T} < 8$ GeV/$c$ in the three centrality classes studied an…
A measurement of beauty hadron production at mid-rapidity in proton-lead collisions at a nucleon-nucleon centre-of-mass energy $\sqrt{s_{\rm NN}}=5.02$ 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 ${\rm {b\overline{b}}}$ production cross section at mid-rapidity, ${\rm d}σ_{\rm {b\overline{b}}}/{\rm d} y$, and the total cross section extrapolated over full phase space, $σ_{\rm {b\overline{b}}}$, are obtained. Th…
The inclusive J/$ψ$ meson production in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon collision of $\sqrt{s_{\rm NN}}$ = 5.02 TeV at midrapidity ($|y|$ < 0.9) is reported by the ALICE Collaboration. The measurements are performed in the dielectron decay channel, as a function of event centrality and J/$ψ$ transverse momentum $p_{\rm T}$, down to $p_{\rm T}$ = 0 GeV/$c$. The J/$ψ$ mean transverse momentum $\langle p_{\rm T} \rangle$ and $r_{\rm AA}$ ratio, defined as $\langle p^{\rm 2}_{\rm T} \rangle_{\rm PbPb}/\langle p^{\rm 2}_{\rm T} \rangle_{\rm pp}$, are evaluated. Both …
We report systematic measurements of dielectron ($e^{\pm}e^{\pm}$) invariant-mass $M_{ee}$ spectra at mid-rapidity in Au+Au collisions at $\sqrt{s_{NN}}$ = 27, 39, and 62.4 GeV taken with the STAR detector at the Relativistic Heavy Ion Collider. For all energies studied, a significant excess yield of dielectrons is observed in the low-mass region (0.40$ < M_{ee} < 0.75$ MeV/$c^2$) compared to hadronic cocktail simulations at freeze-out. Models that include an in-medium broadening of the $ρ$-meson spectral function consistently describe the observed excess. In addition, we report acceptance-cor…
