Internal Pair Production Associated with the Emission of High-Energy Gamma Rays

Kroll, N.; Wada, Walter W.

doi:10.1103/physrev.98.1355

Cited by 343 publications

(192 citation statements)

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“…Electrons from direct γ decays, γ-conversions and Dalitz-decays of π 0 and η mesons are always produced in electron-positron pairs with a small invariant mass (m e + e − ) following a Kroll-Wada distribution [72] peaked at zero. Such correlation does not hold for heavyflavour decay electrons.…”

Section: Invariant Mass Methodsmentioning

confidence: 99%

Elliptic flow of electrons from heavy-flavour hadron decays at mid-rapidity in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}}=2.76 $$ TeV

Adam¹,

Adamová²,

Aggarwal³

et al. 2016

J. High Energ. Phys.

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Abstract:The elliptic flow of electrons from heavy-flavour hadron decays at mid-rapidity (|y| < 0.7) is measured in Pb-Pb collisions at √ s NN = 2.76 TeV with ALICE at the LHC. The particle azimuthal distribution with respect to the reaction plane can be parametrized with a Fourier expansion, where the second coefficient (v 2 ) represents the elliptic flow. The v 2 coefficient of inclusive electrons is measured in three centrality classes (0-10%, 10-20% and 20-40%) with the event plane and the scalar product methods in the transverse momentum (p T ) intervals 0.5-13 GeV/c and 0.5-8 GeV/c, respectively. After subtracting the background, mainly from photon conversions and Dalitz decays of neutral mesons, a positive v 2 of electrons from heavy-flavour hadron decays is observed in all centrality classes, with a maximum significance of 5.9σ in the interval 2 < p T < 2.5 GeV/c in semicentral collisions (20-40%). The value of v 2 decreases towards more central collisions at low and intermediate p T (0.5 < p T < 3 GeV/c). The v 2 of electrons from heavy-flavour hadron decays at mid-rapidity is found to be similar to the one of muons from heavy-flavour hadron decays at forward rapidity (2.5 < y < 4). The results are described within uncertainties by model calculations including substantial elastic interactions of heavy quarks with an expanding strongly-interacting medium. The ALICE collaboration 34 Keywords: Heavy Ion Experiments IntroductionThe main goal of the ALICE [1] experiment is the study of strongly-interacting matter at the high energy density and temperature reached in ultra-relativistic heavy-ion collisions at the Large Hadron Collider (LHC). In these collisions the formation of a deconfined state of quarks and gluons, the Quark-Gluon Plasma (QGP), is predicted by Quantum ChromoDynamic (QCD) calculations on the lattice [2][3][4][5][6]. Because of their large masses, heavy quarks, i.e. charm (c) and beauty (b) quarks, are produced at the initial stage of the collision, almost exclusively in hard partonic scattering processes. Therefore, they interact with the medium in all phases of the system evolution, propagating through the hot and dense medium and losing energy via radiative [7,8] and collisional scattering [9][10][11] processes. Heavy-flavour hadrons and their decay products are thus effective probes to study the properties of the medium created in heavy-ion collisions. Heavy-quark energy loss in strongly-interacting matter can be studied via the modification of the transverse momentum (p T ) spectra of heavy-flavour hadrons and their decay products in heavy-ion collisions with respect to the proton-proton yield scaled by the number of binary nucleon-nucleon collisions, quantified by the nuclear modification factor (R AA ). A strong suppression of open charm hadrons and heavy-flavour decay leptons is observed for p T > 3 GeV/c in central collisions, both at RHIC ( √ s NN = 200 GeV) [12][13][14][15][16] -1 - JHEP09(2016)028and LHC ( √ s NN = 2.76 TeV) [17][18][19][20] energies. The PHENIX and STAR Collabor...

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Section: Invariant Mass Methodsmentioning

confidence: 99%

Elliptic flow of electrons from heavy-flavour hadron decays at mid-rapidity in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}}=2.76 $$ TeV

Adam¹,

Adamová²,

Aggarwal³

et al. 2016

J. High Energ. Phys.

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“…1 and 5. PHENIX at RHIC has recently [23] analysed dielectron data in the mass window 0.2<M<0.3 GeV for p T >1 GeV under the extreme assumption, that the total yield for that selection would solely be due to internal conversion of real photons; the transformation back to real photons was done with the prescription in [24]. The analogous situation for NA60 is illustrated in Fig.…”

Section: Real Photons and Low-mass Dileptonsmentioning

confidence: 99%

Thermal dileptons in high-energy nuclear collisions

Damjanović

2009

Progress in Particle and Nuclear Physics

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Clear signs of excess dileptons above the known sources were found at the SPS since long. However, a real clarification of these observations was only recently achieved by NA60, measuring dimuons with unprecedented precision in 158A GeV In-In collisions. The excess mass spectrum in the region M <1 GeV is consistent with a dominant contribution fromThe associated ρ spectral function shows a strong broadening, but essentially no shift in mass. In the region M >1 GeV, the excess is found to be prompt, not due to enhanced charm production. The inverse slope parameter T eff associated with the transverse momentum spectra rises with mass up to the ρ, followed by a sudden decline above. While the initial rise, coupled to a hierarchy in hadron freeze-out, points to radial flow of a hadronic decay source, the decline above signals a transition to a low-flow source, presumably of partonic origin. The mass spectra show the steep rise towards low masses characteristic for Planck-like radiation. The polarization of the excess referred to the Collins Soper frame is found to be isotropic. All observations are consistent with a global interpretation of the excess as thermal radiation. We conclude with a short discussion of a possible link to direct photons.

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“…In terms of the "transverse masses above 500 MeV the acceptance at low pg-is good. This is illustrated in Figure 8 [18], (K-W) expression3) multiplied by a form-factor. Branching ratios well the advent of much new data.…”

Section: Magcalmentioning

confidence: 99%

Low-mass lepton-pair production in p-Be collisions at 450 GeV/c

Åkesson

Almehed

Angelis

et al. 1995

Z. Phys. C - Particles and Fields

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(to be submitted to Zeit. Phys. C} for muons, and ~ 40% for electrons.level, on any new source of lepton pairs is ~ 20% of the hadronic decay contribution of the major hadronic sources is set by the data. The upper limit, at 90% confidence decays, and there is no need to invoke any "unconventional" source. The normalisation the low-mass spectrum can be explained satisfactorily by lepton pairs from hadronic p-Be collisions at 450 GeV/ c at the CERN SPS. For both electron and muon pairsWe report on the production of low-mass electron pairs and muon pa.irs in Area. An overview of the apparatus is shown in Figure la. The main components are OCR OutputThe HELIOS spectrometer is situated in the H8 beam line of the CERN SPS North 2.2 Apparatus reliably and stably throughout the experiment.The intensity was ~ 106 per burst. The targeting of the beam on to the wire worked 0.1%), a transverse diameter less than 50 pm and divergence ~ 0.2 mrad at the target.eriment to match the wire target. This beam has excellent momentum resolution (6p/ p A special 450 GeV/ c proton f'micro"-bea.m was developed for the HELIOS exp from the decay of hadrons produced in the interaction.of only 125 pm diameter, in order to minimize the radiation length traversed by photons the design. Accordingly, we have used a 4 cm long (10% interaction length) Be wire target experiment, and so the suppression of e'*'c" pairs from conversions was a key feature ofThe study of low-mass lepton pairs was one of the prime motivations of the HELIOS analysis is presented in section 4, and in section 5 results are summarised and conclusions ing, and data-taking, followed by the event reconstruction and selection in section 3. TheThe plan of this paper is as follows. In section 2 we describe the apparatus, trigger any "unconventional" source. Upper limits on any new source are presented.can be accounted for by lepton pairs from the decay of hadrons, and there is no need forThe main result is that low-mass lepton pairs, produced centrally at `/E 2 29 GeV, a measurement of the total charged multiplicity of the event.electron identincation by both transition radiation and calorimetry; a double measurement of the momentum (or energy) of both muons and electrons;Other noteworthy points are:of certain Dalitz decay modes;the measurement of photons as well as charged leptons, affording direct measurement the detector, producing two essentially independent measurements of lepton pairs; the analysis of both electron pairs and muon pairs, emphasising different aspects of from conventional sources. The most important features of the experimental approach are:Be collisions (\/Z cx 29 GeV) in the central rapidity region, is compared to the expectationIn this paper the production of electron and muon pairs, produced in 450 GeV/ c p their production level in ordinary hadronic collisions.plasma formation in relativistic heavy-ion collisions [3]: it is essential then to understand process Furthermore, lepton pairs have been suggested as a signature for quark-gluon implied by conven...

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Internal Pair Production Associated with the Emission of High-Energy Gamma Rays

Cited by 343 publications

References 10 publications

Elliptic flow of electrons from heavy-flavour hadron decays at mid-rapidity in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}}=2.76 $$ TeV

Elliptic flow of electrons from heavy-flavour hadron decays at mid-rapidity in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}}=2.76 $$ TeV

Thermal dileptons in high-energy nuclear collisions

Low-mass lepton-pair production in p-Be collisions at 450 GeV/c

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