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the measurement of electron - positron pairs ( dielectrons ) in the low invariant mass region allows studying the vacuum and in - medium properties of light vector mesons . additionally , low - mass dielectrons are produced by internal conversion of virtual direct photons . they are excellent probes to study all collision stages , since they pass through the created medium almost unaffected . to quantify modifications of the dielectron production in heavy - ion collisions , measurements in pp collisions serve as a reference , while the analysis of p - a collisions allows disentangling cold from hot nuclear matter effects . in alice @xcite at the lhc , dielectron measurements are performed using the central barrel detectors around mid - rapidity . electrons can be identified via their specific energy loss in the inner tracking system ( its ) and the time projection chamber ( tpc ) , combined with time - of - flight information from the tof detector @xcite . in this proceedings we present a virtual direct photon measurement in pp collisions and the invariant mass analyses in p  pb and pb  pb collisions . prospects of a future measurement in pb  pb after the alice upgrade for lhc run 3 are also discussed . the analysis presented here is based on @xmath1 minimum bias pp collisions at @xmath0 , recorded in 2010 . fiducial cuts on transverse momentum ( @xmath2 ) and pseudorapidity ( @xmath3 ) are applied to electron candidates . a clean electron sample is achieved by cuts on the time - of - flight and on the @xmath4 in the tpc . dielectron spectra are created for unlike - sign and like - sign combinations of these particles . the unlike - sign distribution contains a superposition of signal and combinatorial background , while the like - sign is used to provide an estimate of such a background . the acceptance may be different for unlike- and like - sign pairs due to detector effects . therefore , the signal yield is obtained as @xmath5 , where @xmath6 is built via a mixed - event technique as @xmath7 . + figure [ pp - fit ] shows the differential dielectron cross - section in a kinematic region @xmath8 which is useful to study direct photon production . also shown in fig . [ pp - fit ] are a hadronic cocktail and its components as well as the expected mass distribution of dielectrons coming from virtual direct photons , after having applied the single - electron fiducial cuts on @xmath9 and @xmath10 . a fit to the data is performed to determine the virtual photon fraction @xmath11 by using the function @xmath12 in the range @xmath13 . this fit is done in four pair-@xmath9 regions and the extracted virtual photon fraction is then multiplied by the inclusive photon cross section , also measured by alice via the photon conversion method ( pcm ) @xcite . in figure [ pp - photons ] the resulting direct photon cross section is compared to nlo pqcd calculations . reasonable agreement is found , within uncertainties , between data and model . details on this analysis can be found in @xcite . direct photon cross section compared to nlo pqcd calculations , and the inclusive photon cross section extracted from pcm . ] direct photon cross section compared to nlo pqcd calculations , and the inclusive photon cross section extracted from pcm . ] the analyzed data sample consists of @xmath15 minimum bias p  pb collisions . the analysis is carried out in @xmath3 and two different track selections @xmath2 or @xmath16 are performed , to enable comparisons with both the pp and pb  pb data . the pid scheme was modified to increase the electron efficiency : a tof signal is not required , yet used if available to purify the electron sample . kaons and protons are instead rejected by an electron inclusion cut on the @xmath4 measured by the silicon drift and silicon pixel detectors of the its . in addition , electrons from photon conversions are identified and rejected either by their displaced vertex or by the orientation of the pair plane relative to the magnetic field . more in detail , since a conversion pair does not have an intrinsic opening angle , an apparent opening after reconstruction may only appear perpendicular to the magnetic field vector , while true finite - mass signal pairs have a uniform azimuthal opening angle . therefore , an exclusion cut on the pair plane normal vector @xmath17 is applied for @xmath18 . + the signal extraction is performed as described in section [ sect - pp ] . the signal - to - background ratio is smaller than in pp data by a factor two to four , and range from @xmath19 in the @xmath20 mass region to @xmath21 at @xmath22 and to @xmath23 in the @xmath24 region . the corrected dielectron yield for @xmath2 and integrated over pair-@xmath9 , is shown in fig . [ p - pb - mee ] . the data points and systematic uncertainties are extracted from the mean values and the spread of results obtained with @xmath25 different combinations of analysis cut settings ( inspired by @xcite ) . the data are compared to the hadronic cocktail , which uses the charged pion measurement @xcite as @xmath20 input and @xmath26-scaling for the other light mesons . open heavy - flavour and @xmath24 contributions are calculated from pythia , tuned to independent alice measurements in pp and p  pb . uncertainties on the input of the cocktail sources are propagated to the shapes shown in fig . [ p - pb - mee ] . data and cocktail are in reasonable agreement within their uncertainties over the full mass range .    dielectron raw subtracted yield in central and semi - central pb pb collisions after background subtraction . ] dielectron raw subtracted yield in central and semi - central pb pb collisions after background subtraction . ] from the 2011 pb  pb run , @xmath28 and @xmath29 events in central ( 0 - 10% ) and semi - central ( 20 - 50% ) pb  pb collisions , respectively , are selected for this analysis . the pid scheme is as described for the p  pb analysis . a track selection cut @xmath30 ensures exclusion of charged pions and reduces the contribution from soft conversion and @xmath20-dalitz electrons to the combinatorial background . a residual hadron contamination at the @xmath31 level also contributes to the background pairs but monte - carlo studies show that it does not distort the signal . a conversion rejection cut @xmath32 is applied for @xmath33 . the signal - to - background ratio for @xmath34 decreases from @xmath35 for 20 - 50% to @xmath36 for 0 - 10% centrality . the combinatorial - background - subtracted uncorrected yields ( _ raw subtracted yield _ ) are shown in fig . [ pb - pb - mee ] in the low - mass region for @xmath37 for both centralities . efficiency correction and cocktail comparison are in progress . a fine - binning differential low - mass dielectron measurement in pb pb collisions is a major physics case for the alice upgrade for lhc run 3 @xcite . we evaluated to which precision a possible thermal excess yield @xcite could be reconstructed in central and peripheral pb pb collisions at full lhc energy ( @xmath38 5.5 tev ) . electron tracking and pid efficiencies from a monte - carlo simulation using the current alice setup , operated at reduced solenoid magnetic field , are used and modified according to the expected performance after the upgrade . a statistical significance is calculated from realistically constructed signal and background spectra , and is then used for poisson sampling of data points around the model input . figures [ excess - curr ] and [ excess - upg ] show the expected spectra after subtraction of hadronic sources , for @xmath39 and @xmath40 events using the current and upgraded setup , respectively . while the measurement is dominated by uncertainties in the first case , the upgrade is expected to allow for a temperature measurement in the intermediate mass region ( imr ) within @xmath41 statistical and @xmath42 systematic uncertainty @xcite . same as fig . [ excess - curr ] , but with upgraded its and tpc . reduced statistical and systematic uncertainties make a temperature measurement in the imr feasible . ] same as fig . [ excess - curr ] , but with upgraded its and tpc . reduced statistical and systematic uncertainties make a temperature measurement in the imr feasible . ] virtual direct photons are observed in pp collisions at @xmath8 . the extracted direct photon cross section agrees with nlo pqcd calculations within uncertainties . the pair-@xmath9-integrated invariant mass spectrum measured in p pb collisions is in agreement with the expected hadronic sources . from pb pb collisions , raw subtracted yields have been extracted in two centrality classes and a further analysis is ongoing . a feasibility study indicates that the alice upgrade for lhc run 3 will facilitate a measurement of the early effective temperature . 9    k. aamodt et al . ( alice collaboration ) , _ jinst _ * 3 * ( 2008 ) s08002 . b. abelev et al . ( alice collaboration ) , _ int . phys . _ a * 29 * ( 2014 ) 1430044 . m. wilde ( for the alice collaboration ) , _ arxiv:_1210.5958 ( 2012 ) . m. k. khler , doctoral thesis , tu darmstadt ( 2014 ) . r. barlow , _ arxiv:_hep - ex/0207026 ( 2002 ) . b. abelev et al . ( alice collaboration ) , _ phys . _ b * 728 * ( 2014 ) 25 . b. abelev et al . ( alice collaboration ) , cern - lhcc-2012 - 012 / lhcc - i-022 ( 2012 ) . r. rapp and j. wambach , _ eur . j. _ a * 6 * ( 1999 ) 415 and private communication . b. abelev et al . ( alice collaboration ) , _ j. phys . _ g * 41 * ( 2014 ) 087002 .
the alice collaboration measures the production of low - mass dielectrons in pp , p  pb and pb  pb collisions at the lhc . 
 the main detectors used in the analyses are the inner tracking system , time projection chamber and time - of - flight detector , all located around mid - rapidity . 
 the production of virtual photons relative to the inclusive yield in pp collisions is determined by analyzing the dielectron excess with respect to the expected hadronic sources . 
 the direct photon cross section is then calculated and found to be in agreement with nlo pqcd calculations . 
 results from the invariant mass analysis in p 
 pb collisions show an overall agreement between data and hadronic cocktail . in pb 
 pb collisions , uncorrected background - subtracted yields have been extracted in two centrality classes . a feasibility study for lhc 
 run 3 after the alice upgrade indicates the possibility for a future measurement of the early effective temperature .