Nuclear Modification Factor Of Electrons Research Articles

Inclusive and multiplicity dependent production of electrons from heavy-flavour hadron decays in pp and p-Pb collisions

Measurements of the production of electrons from heavy-flavour hadron decays in pp collisions at sqrt{s} = 13 TeV at midrapidity with the ALICE detector are presented down to a transverse momentum (pT) of 0.2 GeV/c and up to pT = 35 GeV/c, which is the largest momentum range probed for inclusive electron measurements in ALICE. In p-Pb collisions, the production cross section and the nuclear modification factor of electrons from heavy-flavour hadron decays are measured in the pT range 0.5 < pT< 26 GeV/c at sqrt{s_{textrm{NN}}} = 8.16 TeV. The nuclear modification factor is found to be consistent with unity within the statistical and systematic uncertainties. In both collision systems, first measurements of the yields of electrons from heavy-flavour hadron decays in different multiplicity intervals normalised to the multiplicity-integrated yield (self-normalised yield) at midrapidity are reported as a function of the self-normalised charged-particle multiplicity estimated at midrapidity. The self-normalised yields in pp and p-Pb collisions grow faster than linear with the self-normalised multiplicity. A strong pT dependence is observed in pp collisions, where the yield of high-pT electrons increases faster as a function of multiplicity than the one of low-pT electrons. The measurement in p-Pb collisions shows no pT dependence within uncertainties. The self-normalised yields in pp and p-Pb collisions are compared with measurements of other heavy-flavour, light-flavour, and strange particles, and with Monte Carlo simulations.

PHENIX measurements of bottom and charm quark production

Heavy quarks are produced primarily in the early stage of high-energy collisions, before the formation of any quark-gluon plasma (QGP). Therefore heavy quarks provide valuable insight to the properties of the QGP, especially by changing the mass of the heavy quark. PHENIX is able to measure the nuclear modification factor of electrons from bottom and charm composed hadrons. PHENIX has also made measurements of the momentum integrated nuclear modification factor of B → J/ψ and prompt J/ψ production, observing no nuclear modification for B mesons and a significant suppression of prompt J/ψ due to its breaking in the medium. Additionally, the cross-section was measured in p+p collisions where it shows a factor of two larger than the center value of the Fixed-Order-Next-to-Leading-Log (FONLL) prediction.

Measurement of beauty-hadron decay electrons in Pb–Pb collisions at = 2.76 TeV with ALICE

The ALICE Collaboration at the LHC studies heavy-ion collisions to investigate the properties of the Quark-Gluon Plasma (QGP). Heavy quarks (charm and beauty) are effective probes for this purpose. Both their energy loss in the medium as well as their possible thermalization yield information about the medium properties. In ALICE, the reconstruction of exclusive decays is currently accessible for charm, but not for beauty hadrons. For hadrons with beauty valence quarks a promising strategy is the measurement of their decay electrons. To separate these from the background electrons (mainly from charm hadron decays, photon conversions or light-meson decays) the large decay length of beauty hadrons can be utilized. It leads to a relatively large typical impact parameter of the decay electrons. By comparing the impact parameter distribution of the signal electrons with those from the background sources, the signal can be statistically separated from the background. For this purpose a maximum likelihood fit is employed using impact parameter distribution templates from simulations. The resulting nuclear modification factor for electrons from beauty-hadron decays shows a sizeable suppression for pT > 3 GeV/c, albeit still with large uncertainties.

Measurement of electrons from heavy-flavour hadron decays in pp and p-Pb collisions with ALICE

ALICE measured electrons from inclusive heavy-flavour hadron decays and beauty-hadron decays at mid-rapidity in pp collisions at = 7 TeV and 2.76 TeV and p-Pb collisions at = 5.02 TeV. For both pp collision energies, pT-differential electron production cross sections are presented and compared to pQCD predictions. For p-Pb collisions, the pT-dependent nuclear modification factor of electrons from inclusive heavy-flavour hadron decays is presented.

Measurement of electrons from heavy-flavour decays in pp and Pb–Pb collisions with ALICE at the LHC

The ALICE experiment has measured at mid-rapidity electrons from heavy-flavour decays in pp collisions at $\sqrt{s}$ = 2.76 and 7 TeV, and in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV. In pp collisions, electrons from charm-hadron and from beauty-hadron decays are identified by applying cuts on displaced vertices. The relative yield of electrons from beauty-hadron decays to those from heavy-flavour decays is extracted using electron-hadron correlations. Results are compared to pQCD-based calculations. In Pb-Pb collisions, the $p_{\rm T}$ dependence of the nuclear modification factor of electrons from heavy-flavour decays is presented in two centrality classes. The status on the analysis of electrons from beauty-hadron decays is reported in Pb-Pb collisions, in view of the measurement of the corresponding nuclear modification factor.

Nuclear modification factor for heavy flavors: An energy loss effect or more baryons than mesons?

The properties of the nuclear modification factor for heavy flavors are usually attributed to the energy loss suffered by the heavy quark propagating in a QCD plasma. Nevertheless it is a bit surprising that the suppression of this factor is as strong as the one suffered by light flavors. In this work we show that when accounting for the momentum shift associated to the opening of the recombination channel to produce hadrons in the QCD plasma, it is not necessary to invoke such a strong energy loss. We show that when the heavy baryon to meson ratio is larger in nuclear than in proton collisions, data from RHIC and LHC for the nuclear modification factor of electrons coming from heavy flavor decays as well as for charmed masons can be accounted for.

Di-electron production from vector mesons with medium modifications in heavy ion collisions

We reproduce the di-electron spectra in the low and intermediate mass regions in most central Au+Au collisions by the STAR Collaboration incorporation of the STAR detector acceptance. We also compare our results with PHENIX data constrained by the PHENIX acceptance. We include the medium modifications of vector mesons from scatterings of vector mesons by mesons and baryons in the thermal medium. The freezeout contributions from vector mesons are also taken into account. The space-time evolution is described by a 2+1 dimensional ideal hydrodynamic model. The backgrounds from semi-leptonic decays of charm hadrons are simulated by the PYTHIA event generator and corrected by the nuclear modification factor of electrons from charm decays. It is difficult to extract the thermal contributions from those from charm decays in the invariant mass spectra alone and in the current detector acceptances. Other observables such as transverse momenta and collective flows may provide additional tools to tag these sources.

Measurement of the nuclear modification factor of electrons from heavy-flavour decays at mid-rapidity in Pb–Pb collisions at = 2.76 TeV with ALICE

We present results on inclusive electrons for 1.5 <pT < 6 GeV/c in Pb–Pb collisions at = 2.76 TeV measured with ALICE at the LHC and compare these to a cocktail of background electron sources. The excess of electrons beyond the cocktail at high momenta (pT > 3.5 GeV/c) is attributed to electrons from heavy-flavour decays. The corresponding nuclear modification factor indicates heavy-flavour suppression by a factor of 1.5–4.

Recent heavy flavour and quarkonia measurements by the PHENIX experiment

The discovery at the RHIC of large high-pT suppression and flow of electrons from heavy quarks has altered our view of the hot and dense matter formed in central Au+Au collisions at 200 GeV. Meanwhile, the nuclear modification of the J/ψ yield has shown weak energy density dependence when comparing measurements at CERN-SPS and at RHIC energies and measurements in different rapidities. PHENIX has carried out a comprehensive study of heavy quarks including a baseline measurement of heavy flavour, J/ψ and ϒ in p+p collisions, and the fractional contribution to J/ψ from ψ′ and χc decays. The initial state and other cold nuclear matter effects were studied using a large data set collected in d+Au collisions. A modest nuclear modification factor of electrons from heavy flavour decays at mid-rapidity was observed in d+Au and in Cu+Cu collisions. We found a significant suppression of J/ψ in d+Au for all rapidity ranges covered by PHENIX. This suppression is stronger at forward rapidity than at mid-rapidity and the dependence with the nuclear thickness is not linear. A similar suppression trend was also observed for ϒ(1S+2S+3S) but with large statistical uncertainties. A more precise measurement of the J/ψ yield in Au+Au collisions in the forward rapidity is consistent with our previous observation. The centrality dependence of the J/ψ suppression also shows similar behaviour when looking at the data collected in Au+Au collisions at = 62 GeV and GeV, supporting previous comparisons with CERN-SPS data.

Measurement of the Bottom Quark Contribution to Nonphotonic Electron Production inp+pCollisions ats=200 GeV

The contribution of B meson decays to nonphotonic electrons, which are mainly produced by the semileptonic decays of heavy-flavor mesons, in p + p collisions at √s=200 GeV has been measured using azimuthal correlations between nonphotonic electrons and hadrons. The extracted B decay contribution is approximately 50% at a transverse momentum of pT≥5 GeV/c. These measurements constrain the nuclear modification factor for electrons from B and D meson decays. The result indicates that B meson production in heavy ion collisions is also suppressed at high pT.

Heavy quark three-body collisional energy loss in QGP

Heavy quark drag coefficients due to three-body elastic scattering in a quark–gluon plasma have been evaluated in the lowest order in QCD. They are found to have similar values as those due to two-body elastic scattering, and both are larger than those due to two-body radiative scattering if the momenta of charm and bottom quarks are below about 4 and 9 GeV/c, respectively. Including three-body elastic scattering increases the momentum degradation of heavy quarks in QGP, bringing the nuclear modification factor for electrons from the decays of produced heavy mesons in Au+Au collisions at closer to the measured one.

Suppression of Heavy Quarks in Heavy-ion Collisions

Abstract Heavy quark drag coefficients in a quark-gluon plasma due to three-body elastic scattering have recently been approximately evaluated in the lowest order in QCD. For both charm and bottom quarks, their values are found to be comparable to those from two-body elastic scattering. In the Fokker-Planck approach based on a schematic expanding fireball model, its effect on the transverse momentum spectra of heavy quarks produced from Au+Au collisions at center of mass energy s NN = 200 has been studied, and the results show that adding contributions from heavy quark three-body scattering to those due to two-body elastic and radiative scattering brings the nuclear modification factor for electrons from heavy meson decays closer to available experimental data.

Testing radiative energy loss at RHIC and at the LHC

The ratio of nuclear modification factors of high − p T heavy-flavored mesons to light-flavored hadrons (heavy-to-light ratio) is shown to be a sensitive tool to test medium-induced energy loss at RHIC and LHC energies. Heavy-to-light ratios of D mesons at the Relativistic Heavy Ion Collider RHIC in the region 7 p T 12 GeV , and of D and B mesons at the Large Hadron Collider LHC in the region 10 p T 20 GeV , are proposed for such a test. Finally, the perturbative uncertainties in the nuclear modification factor for electrons at RHIC are analyzed. Data from the PHENIX and STAR Collaborations are compatible with radiative energy loss provided the contribution of electrons from beauty decays is small compared to that from charm. The results stress the need to control the p-p baseline form charm and beauty production.

Heavy-to-light ratios as a test of medium-induced energy loss at RHIC and the LHC

The ratio of nuclear modification factors of high- p T heavy-flavored mesons to light-flavored hadrons (heavy-to-light ratio) is shown to be a sensitive tool to test medium-induced energy loss at RHIC and LHC energies. Heavy-to-light ratios of D mesons at RHIC in the region 7 < p T < 12 GeV , and of D and B mesons at the LHC in the region 10 < p T < 20 GeV , are proposed for such a test. Finally, the different contributions to the nuclear modification factor for electrons at RHIC are analyzed. Preliminary PHENIX and STAR data are compatible with radiative energy loss provided the contribution of electrons from beauty decays is small compared to that from charm.