Jet coherence in QCD media: the antenna radiation spectrum

Journal of High Energy Physics, Jan 2013

Abstract We study the radiation of a highly energetic partonic antenna in a colored state traversing a dense QCD medium. Resumming multiple scatterings of all involved constituents with the medium we derive the general gluon spectrum which encompasses both longitudinal color coherence between scattering centers in the medium, responsible for the well known Landau-Pomeranchuk-Migdal (LPM) effect, and transverse color coherence between partons inside a jet, leading, in vacuum, to angular ordering of the parton shower. We discuss shortly the onset of transverse decoherence which is reached in opaque media. In this regime, the spectrum consists of independent radiation off the antenna constituents.

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Jet coherence in QCD media: the antenna radiation spectrum

Yacine Mehtar-Tani 1 Konrad Tywoniuk 0 0 Department of Astronomy and Theoretical Physics, Lund University , Solvegatan 14A, SE-22 362 Lund, Sweden 1 Institut de Physique Tehorique , CEA Saclay, F-91191 Gif-sur-Yvette, France We study the radiation of a highly energetic partonic antenna in a colored state traversing a dense QCD medium. Resumming multiple scatterings of all involved constituents with the medium we derive the general gluon spectrum which encompasses both longitudinal color coherence between scattering centers in the medium, responsible for the well known Landau-Pomeranchuk-Migdal (LPM) eect, and transverse color coherence between partons inside a jet, leading, in vacuum, to angular ordering of the parton shower. We discuss shortly the onset of transverse decoherence which is reached in opaque media. In this regime, the spectrum consists of independent radiation o the antenna constituents. Contents 1 Introduction 2 3 4 Decoherence in opaque media A The reduction formula eq. (3.7) Color coherence phenomena in a few words Emission amplitude from classical Yang-Mills equations The antenna spectrum in the presence of a medium Introduction One of the key objectives of the heavy-ion program at the LHC is to investigate properties of the quark-gluon plasma (QGP) using hard probes. In particular, one addresses in-medium modications of the fragmentation properties of nal state energetic particles that depart from the well-known fragmentation pattern in vacuum, for instance in e+e annihilation, proton-proton collisions etc., where no dense medium is formed. These modications are assumed to be sensitive to local medium properties, such as the density, as well as their spatiotemporal evolution. Indeed, strong medium eects are observed in heavy-ion collisions for both single-inclusive leading particle spectra [1{3] and two-particle correlations [4, 5]. While such measurements have reached a high level of sophistication, shedding light on qualitative aspects of the quark-gluon plasma, studies of intrajet distributions in heavy ion collisions have recently been initiated both at RHIC [6{8] and LHC [9{11] with many promising results and prospects for the future. The increased experimental capabilities at these high energies have also triggered several eorts to improve the theoretical understanding of gluon radiation in the presence of a colored medium. Until recently, only the leading order one-gluon medium-induced emission spectrum o a highly energetic quark or gluon, which will be denoted BDMPSZ throughout, was known [12{15]. Equivalent formulations were also derived in [16{24]. This spectrum measures radiative parton energy loss in the QGP and accounts for momentum broadening of the radiated gluon which undergoes multiple scattering in the medium. The characteristic broadening of the transverse momenta of such gluons, arising due to coherence eects between medium rescatterings, sets an upper bound on the energy of the induced radiation which, nevertheless, can be quite sizable in relatively opaque media. Regrettably, since the process under consideration does not deal with interference effects between emitters, see section 2, the extension to multi-gluon emissions is bound to rely on ad hoc conjectures. In order to study the importance of these radiative interferences, lately the gluon emission spectrum o a time-like quark-antiquark ( qq) antenna was calculated. In [25, 26] we considered an antenna traversing a relatively thin medium, i.e., assuming only one scattering in the medium background potential. In [27], on the other hand, we resummed multiple scatterings in the limit of soft gluon emission. The aim of the present work is to generalize the latter results to arbitrary number of rescatterings of the quark, antiquark and gluon, thus extending the validity of our previous ndings to arbitrarily opaque media and up to large gluon energies. We also briey discuss the main dierence between the direct and interference contributions which relates to the physics of decoherence of QCD radiation, thus making contact with our previous work [25, 27]. The key result of this work is the derivation of the interference spectrum, cf. eq. (4.16), while a complete and detailed discussion of the emerging physical picture is presented in [26, 28], see also [29] for a complementary discussion. The paper is structured as follows. At the outset, in section 2, we discuss briey the known coherence phenomena relevant for high-energy physics and heavy-ion collisions. Then, in section 3, we present and solve the classical Yang-Mills equations for the qq antenna, thus obtaining a compact expression for the medium-induced gluon eld. Medium averages for the total spectrum are described in section 4 where we also present the novel interference spectrum J , given in eq. (4.16), which encodes the new ingredients of transverse coherence in medium. The general properties of this spectrum are also outlined in brief. In particul (...truncated)


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Yacine Mehtar-Tani, Konrad Tywoniuk. Jet coherence in QCD media: the antenna radiation spectrum, Journal of High Energy Physics, 2013, pp. 31, Volume 2013, Issue 1, DOI: 10.1007/JHEP01(2013)031