Dielectron measurements in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC
Electromagnetic probes are excellent messengers from the hot and dense medium created in high-energy heavy-ion collisions. Since leptons do not interact strongly, their spectra reflect the entire space-time evolution of the collision. The surrounding medium can lead to modifications of the dielectron production with respect to the vacuum rate. To quantify modifications in heavy-ion collisions, measurements in pp collisions serve as a reference, while the analysis of p–A collisions allows for the disentanglement of cold nuclear matter effects from those of the hot and dense medium.
In this proceedings, dielectron measurements with the ALICE central barrel detectors are presented. The invariant mass distributions in the range GeV/ are compared to the expected yields from hadronic sources for pp collisions at TeV, and for p–Pb collisions at TeV. The cross section of direct photons measured via virtual photons in pp collisions is compared to predictions from NLO pQCD calculations as a function of the transverse momentum. The status of the analysis of Pb–Pb collisions at TeV is presented.
keywords:Heavy-ion collisions, Electromagnetic probes, ALICE
00 \journalnameNuclear Physics A \runauthM. K. Köhler et al. \jidnupha \jnltitlelogoNuclear Physics A
Dielectrons were proposed several decades ago Shuryak1978 () to be an important source of information from the hot and dense medium, which can be created in heavy-ion collisions. Since dielectrons are emitted throughout the collision process and do not interact via the strong interaction, they are ideal probes for all stages of the collision. Moreover, the measurement of virtual photons, i.e. photons which convert internally into dileptons, allows to reduce systematic uncertainties significantly compared to the measurement of real photons, since the main sources of the background, photons and dielectrons from decays, can be rejected at finite mass.
However, to access information on the medium in heavy-ion collisions, the dielectron production in the vacuum and possible cold nuclear matter effects need to be evaluated. Therefore, it is necessary to have reference measurements from proton-proton (pp) and proton-nucleus (p–A) collisions.
LHC provided during Run 1 three different collision systems, i.e. pp, p–Pb and Pb–Pb. In this proceedings, preliminary results from pp collisions at TeV on the dielectron invariant mass continuum and direct photons measured via virtual photons are summarized. The dielectron continuum as a function of invariant mass and dielectron transverse momentum is compared to the expected hadronic sources of dielectrons in p–Pb collisions at TeV. In addition, the status of the analysis for Pb–Pb collisions at TeV will be discussed.
2 Data analysis
ALICE has capabilities for particle identification in the low transverse momentum () regime that are unique at the LHC. Electrons with the transverse momentum GeV/ are identified by combined energy loss information from the Time Projection Chamber (TPC) and, in the case of p–Pb and Pb–Pb, the outermost four layers of the Inner Tracking System (ITS). Additionally, the Time-Of-Flight detector (TOF) is used in the range GeV/ to reject kaons and protons. The remaining hadron contamination is at most % in pp collisions and up to % in Pb–Pb collisions.
When measuring unlike-sign dielectron pairs, , one of the main challenges is the estimation of the combinatorial background, which arises from random dielectron combinations and is superimposed on the physics signal. The signal-over-background ratio is in the order of for pp and p–Pb collisions and about a factor lower in central Pb–Pb collisions for GeV/. The combinatorial background is measured by the same-event like-sign method. This method holds under the assumption that the physics signal consists only of unlike-sign pairs. The like-sign spectra are normalized via , where is a correction factor for the difference between the acceptance of unlike-sign pairs and like-sign pairs and and are positive and negative like-sign dielectron pairs, respectively. The acceptance correction is calculated as , where indicates mixed event distributions. depends on the minimum single electron and is consistent with unity within its statistical uncertainties for the pp and the p–Pb analysis for GeV/. For GeV/ in Pb–Pb collisions, the deviation from unity is of the order of % for GeV/ and approaches unity for increasing mass. The raw signal is calculated as .
The data are corrected for detector and reconstruction efficiency via Monte-Carlo (MC) simulations. Single electron efficiencies are calculated as a function of . Every electron is weighted with its efficiency in a dielectron generator with realistic electron and dielectron kinematics.
The expected hadronic sources of dielectrons at the moment of freeze-out, the so-called hadronic cocktail, are calculated based on measured differential cross sections for and in pp collisions, and on the charged pion spectrum in p–Pb collisions. The mass shape of resonances is based on Gounaris () and the Dalitz pair mass distributions are following KrollWada (). To estimate the expected yield of correlated dielectrons from semi-leptonic decays of open heavy-flavor mesons, the PYTHIA event generator pythia (), tuned to NLO calculations mangano (), is used. The pair distributions are normalized to the cross sections measured in pp collisions and scaled by the number of binary collisions for p–Pb collisions. See ALICE_cocktail () for a compilation of references. Contributions from hadrons, which have not been measured, are estimated by scaling of the cross section.
The preliminary results for pp collisions at TeV are shown in Figure 1. In the upper left panel, the dielectron data are compared to the cocktail as a function of invariant mass for integrated pair . The hadronic cocktail is consistent with the dielectron data. Virtual photon production is studied in pp collisions. Virtual photons convert internally into dielectrons. The relation between the dielectron invariant mass distribution and the virtual photon yield is given for by the Kroll-Wada equation KrollWada ().
In the upper right panel of Figure 1, pp data are compared to the cocktail for GeV/. The different sources are indicated as dashed lines. The function is fitted to the data in the region GeV/, where is the cocktail contribution, is the photon input from the Kroll-Wada equation and is the only fitting parameter. reflects the ratio of direct over inclusive photons. The result for as a function of the photon is shown in the lower left panel of Figure 1. Under the assumption that the ratio of direct over inclusive photons is the same for real and virtual photons, the direct photon cross section can be calculated by , where and are the direct and inclusive photon yields. The inclusive photon cross section has been measured via photon conversions, see e.g. Wilde (). In the lower right panel of Figure 1, the direct photon cross section is shown as a function of the photon . NLO pQCD calculations Vogelsang () are consistent with the data.
In the upper left panel of Figure 2, the dielectron invariant mass spectrum is compared to the cocktail for p–Pb collisions at TeV. The cocktail is in good agreement with the data. In the upper right and lower left panels, data are compared to the cocktail distributions as a function of dielectron transverse momentum for GeV/ and GeV/, respectively. The data are well described by the cocktail. These two mass regions are of special interest. The mass region GeV/ is sensitive to hot hadronic medium effects. The region GeV/ is dominated by semi-leptonic decays of heavy-flavour mesons. In this mass region, heavy quark pair correlations can be studied.
In the lower right panel of Figure 2, the raw yield as a function of invariant mass is shown for Pb–Pb collisions in % centrality at TeV. Further analysis of this spectrum will allow the study of the virtual photon yield and for the exploration of a possible low-mass enhancement in Pb–Pb collisions.
4 Summary and outlook
The dielectron invariant mass spectrum measured in pp collisions at TeV is consistent with the expectation from hadronic sources.
The same is observed for the invariant mass and transverse momentum distributions of dielectrons in p–Pb collisions at 5.02 TeV. The direct
photon yield extracted from dielectron data in pp collisions at 7 TeV is consistent with NLO pQCD calculations. The study of dielectron
production in Pb–Pb collisions at TeV is ongoing.
At the end of Run , statistical uncertainties will be reduced significantly. After the second long shutdown at the LHC, which is expected to end in , ALICE will run with upgraded detector components ALICE_upgrade (). The upgrade of the ITS will allow high precision vertexing to measure and reject dielectrons from correlated heavy flavour decays and the continuous read-out of the TPC will allow to take full advantage of the high luminosity at the upgraded LHC. Hence, detailed studies of the dielectrons will become feasible in Pb–Pb collisions.
- A list of members of the ALICE Collaboration and acknowledgements can be found at the end of this issue.
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