Dead Cone Effect Research Articles

THE INTERNAL STRUCTURE OF JETS AT COLLIDERS: LIGHT AND HEAVY QUARK INCLUSIVE HADRONIC DISTRIBUTIONS

In this paper, we report our results on charged hadron multiplicities of heavy quark initiated jets produced in high energy collisions. After implementing the so-called dead cone effect in QCD evolution equations, we find that the average multiplicity decreases significantly as compared to the massless case. Finally, we discuss the transverse momentum distribution of light quark initiated jets and emphasize the comparison between our predictions and CDF data.

Gluon radiation outside a finite cone

The medium-induced gluon radiation angular distributions of light quarks and heavy quarks outside a finite jet cone are studied. We find the effect of destructive interference between the vacuum and medium-induced radiation plays an important role in gluon radiation for very small value of path length L, which leads to the negative value of medium-induced energy loss. The medium-induced radiative energy loss outside an angle for heavy quarks is found to have a minimum and a maximum at small angle for small path length, which are caused by dead cone effect and Brownian k⊥-broadening effect, respectively. However for large path length the minimum and maximum disappear.

Why heavy and light quarks radiate energy with similar rates

The dead-cone effect has been predicted to reduce the magnitude of energy loss and jet quenching for heavy flavors produced with large ${p}_{T}$ in heavy-ion collisions. On the contrary, data from the Relativistic Heavy Ion Collider demonstrate a strong suppression of high-${p}_{T}$ electrons from charm and bottom decays. We show that vacuum radiation of a highly virtual quark produced at high ${p}_{T}$ with a stripped-off color field develops a much wider dead cone, which screens the one related to the quark mass. Lacking the field, gluons cannot be radiated within this cone until the color field is regenerated and the quark virtuality cools down to the scale of the order of the quark mass. However, this takes longer than is essential for the observed jet quenching, leading to similar nuclear effects for the light and charm quark jets. Open beauty is expected to radiate much less within the ${p}_{T}$ range studied so far in heavy-ion collisions.

Heavy quark flavour dependence of multiparticle production in QCD jets

After inserting the heavy quark mass dependence into QCD partonic evolution equations, we determine the mean charged hadron multiplicity and second multiplicity correlators of jets produced in high energy collisions. We thereby extend the so-called dead cone effect to the phenomenology of multiparticle production in QCD jets and find that the average multiplicity of heavy-quark initiated jets decreases significantly as compared to the massless case, even taking into account the weak decay products of the leading primary quark. We emphasize the relevance of our study as a complementary check of $b$-tagging techniques at hadron colliders like the Tevatron and the LHC.

Consequences of a [formula omitted] enhancement effect on the non-photonic electron nuclear modification factor in central heavy ion collisions at RHIC energy

The RHIC experiments have measured the nuclear modification factor RAA of non-photonic electrons in Au+Au collisions at sNN=200 GeV. This RAA exhibits a large suppression for pt>2 GeV/c which is commonly attributed to heavy-quark energy loss. It is expected that the heavy-quark radiative energy loss is smaller than the light quark one because of the so-called dead-cone effect. An enhancement of the charm baryon yield with respect to the charm meson yield, as it is observed for light and strange hadrons, can explain part of the suppression. This phenomenon has been put forward in a previous work. We present in this Letter a more complete study based on a detailed simulation which includes electrons from charm and bottom decay, charm and bottom quark realistic energy loss as well as a more realistic modeling of the Λc/D enhancement. We show that a Λc/D ratio close to unity, as observed for light and strange quarks, could explain 20–25% of the suppression of non-photonic electrons in central Au+Au collisions. This effect remains significant at relatively high non-photonic electron transverse momenta of 8–9 GeV/c.

Consequences of a Λc/D enhancement effect on the non-photonic electron nuclear modification factor in central heavy ion collisions at RHIC energy

The RHIC experiments have measured the nuclear modification factor RAA of non-photonic electrons in Au+Au collisions at . This RAA exhibits a large suppression for pt > 2GeV/c which is commonly attributed to heavy-quark energy loss. In order to reproduce satisfactorily the data, energy-loss models assume a heavy-quark energy loss similar to that of light quarks. However, it is expected that the heavy-quark radiative energy loss is smaller than the light quark one because of the so-called dead-cone effect. In this paper, we show that a charm baryon/meson enhancement, as is observed for the light and strange quarks, can explain a part of the suppression. This phenomenon has been put forward in a previous work. At variance, we show here that a Λc/D ratio close to unity, as observed for the light and strange quarks, could explain 20–25% of the suppression of non-photonic electrons in central Au+Au collisions. This effect remains significant at relatively high non-photonic electron transverse momenta of 8–9 GeV/c.

HEAVY QUARK ENERGY LOSS THROUGH SOFT QCD SCATTERING IN THE QGP

A strong suppression of non-photonic electrons in Au + Au collisions is observed at Rhic. This is in disagreement with the expected dominance of the energy loss via gluon radiation, which predicts a much weaker suppression of heavy flavours due to the dead cone effect. However, collisional energy loss is also important, as demonstrated recently by the Soft Colour Interaction Jet Quenching model. Based on this model we show that collisional energy loss of charm quarks gives a suppression of electrons of the observed magnitude, but the contribution from beauty decays, which dominates at intermediate and large p⊥, results in a somewhat weaker overall suppression of electrons than observed.

Medium modification of heavy flavour production measured by PHENIX in Au + Au collisions at GeV

Charm quarks serve as a probe of matter produced in heavy-ion collisions. Due to the large quark mass, the charm cross section is calculable as a point-like QCD process. Since initial-state effects should be small at RHIC energies, the charm yield should follow binary scaling at mid-rapidity. Heavy quarks are expected to lose less energy in the medium than lighter quarks due to the ‘dead-cone effect’. Heavy flavour production can be investigated in PHENIX via the pT spectra of single electrons. While the total charm yield measured from the Au + Au collisions taken by PHENIX in 2002 scales with the number of binary collisions, single-electron pT spectra suggest a strong suppression of the heavy flavour yield at high pT. We present an analysis of the centrality dependence of the electron spectra from heavy quark decays obtained from the high statistics data sample of Au + Au collisions at GeV taken in 2004.

Suppression of high-pT non-photonic electrons in Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV

The strong suppression of high pT hadrons observed at RHIC has led to the interpretation that energetic partons lose their energy via induced gluon radiation in the hot and dense matter before fragmenting into hadrons. The study of heavy quark production can extend our understanding of this scenario. Due to the dead cone effect, the suppression of heavy quark mesons at high pT is expected to be smaller than that observed for charged hadrons at the same energy. The measurement of non-photonic single electrons up to high pT provides information on charm and beauty production. The semi-leptonic decays of D and B mesons are the dominant contribution to the non-photonic electron spectra. The preliminary spectra from p+p, d+Au and Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV have been extracted for mid-rapidity non-photonic electrons in the range 1.5<pT (GeV/c)<10. The corresponding nuclear modification factors (RAA) are presented and show a large suppression in central Au+Au collisions, indicating an unexpectedly large energy loss for heavy quarks in the hot and dense matter created at RHIC. This observed suppression is compared to recent theoretical models.

Radiative Energy Loss of Heavy Quark and Dead Cone Effect inUltra-relativistic Heavy Ion Collisions

The lowest-order heavy quark radiative energy losshas been analysed to quantify the dead cone effect. Themedium-induced gluon radiation is found to fill the dead cone, itis reduced at large gluon energies compared to the radiation oflight quarks. We calculate the radiative energy loss of heavyquarks in the condition of dead cone effect. It is found that the radiativeenergy loss with dead cone effect is smaller than that without the deadcone effect.

Medium-induced gluon radiation off massive quarks fills the dead cone

We calculate the transverse momentum dependence of the medium-induced gluon energy distribution radiated off massive quarks in spatially extended QCD matter. In the absence of a medium, the distribution shows a characteristic mass-dependent depletion of the gluon radiation for angles $\ensuremath{\theta}lm/E,$ the so-called dead cone effect. Medium modifications of this spectrum are calculated as a function of quark mass m, initial quark energy E, in-medium path length and density. Generically, medium-induced gluon radiation is found to fill the dead cone, but it is reduced at large gluon energies compared to the radiation off light quarks. We quantify the resulting mass dependence for momentum-averaged quantities (gluon energy distribution and average parton energy loss), compare it to simple approximation schemes and discuss its observable consequences for nucleus-nucleus collisions at the BNL RHIC and CERN LHC. In particular, our analysis does not favor the complete disappearance of energy loss effects from leading open charm spectra at the RHIC.

Gluon Emission of Heavy Quarks: Dead Cone Effect

The lowest-order induced soft gluon radiation processes of heavy quarks have been analyzed to quantify the dead cone effect. This effect is most likely expected to suppress significantly the energy loss of charm quarks passing an amorphous color-neutral deconfined medium, as has been concluded from recent experiments at RHIC.

Perspectives for the study of charm in-medium quenchingat the LHC with ALICE

Charm mesons produced in nucleus-nucleus collisions are expected to be less attenuated (quenched) by the medium than hadrons containing only light quarks, since radiative energy loss of heavy quarks should be reduced by the `dead-cone' effect. We start from a published energy-loss model to derive the quenching for D mesons at the LHC, introducing an approximation of the dead-cone effect and employing a Glauber-based description of the geometry of central Pb-Pb collisions to estimate the in-medium path lengths of c quarks. We show that the exclusive reconstruction of D^0 \to K\pi decays in ALICE allows to measure the nuclear modification factor of the D mesons transverse momentum distribution and the D/(charged hadrons) ratio and, thus, to investigate the energy loss of c quarks.