Studies of dielectron pair emission from bayronic resonances in vacuum and dense nuclear matter with HADES


Research Grant Harmonia 2013/10/M/ST2/00042 awarded by National Centrum of Science - Poland


Research Project Objectives

The basic aim of the hadron physics is to understand the structure of hadrons and their interactions in elementary interactions and in complex heavy ion collisions, in the energy regime where quark and hadronic degrees of freedom interleave. The underlying theory is quantum chromodynamics (QCD), which is well understood on the short distance scales below 1 fm. Such scales are probed in reactions with high four momentum transfer (Q2 > a few GeV2), where the coupling constant s is small and perturbation theory is applicable. This energy regime is experimentally explored by means of high energy e+e annihilation, lepton–nucleon scattering and reveals partonic degrees of freedom.

Going to lower energies, non-perturbative processes like confinement and chiral symmetry breaking, come into play. The spectacular effect of the latter one is the mass generation of light quarks (u, d), which are the components of the majority of the visible matter in universe, and the appearance of bound states: baryons and mesons. At this energy region it is safe to regard these physically observed particles as the relevant degrees of freedom and use them in the description of hadronic interactions. Here, the application of effective field theories, accounting for the symmetries of QCD and calculations on lattice are the relevant theoretical methods.

A beautiful example of the aforementioned interplay of various degrees of freedoms is the structure of baryons. In the high energy, lepton-nucleon scattering partonic content of a nucleon is probed by a virtual space-like photon with the four momentum Q2Î(2-104) GeV2, penetrating the nucleon on distances much smaller than 1 fm. The results, expressed in form of the nucleon structure functions, reveal a complex structure with contribution from valence, see quarks and gluons. At lower energy, a wealth of new precise data obtained from photon induced reactions at the Jefferson Lab (USA), MAMI in Mainz (Germany) or ELSA in Bonn (Germany) allows for an insight into electromagnetic structure of nucleon and its excited states (baryon resonances) at four momentum transfer Q2Î(0-7) GeV2 . The quantities describing baryon structure are electromagnetic form factors of nucleon (eFF) and electromagnetic transition form factors (eTFF) for the transitions gN->N*, where N* (D) can be any isospin I=1/2 (3/2) resonance i.e N(1440), N(1520)… (or (1232) and higher mass  states). The form factors depend in general on q2 (= -Q2) and reveal two important contributions originating from a quark core and a surrounding pion “cloud”, where the effects of the latter one are especially important at small Q2 (for recent review see [burk2012]). On the other hand, dependence of the form factors in the complementary time-like region (Q2<0) is not well known. Only data for the proton eFF are available from the proton-antiproton annihilation experiments at q2 (= -Q2) > 4mn2. In the intermediate, so called “unphysical 0 < q2 < 4mn2” region, no data on eFF nor eTFF exist. The latter one, however, can be accessed by studies of the electromagnetic Dalitz decay N*(D) -> Ng* -> N e+e, a very rare process (BR are of order 10-5) where intermediate massive virtual photon (with q2 > 0) converts into e+e pair with mass me+e- =Öq2. Indeed, according to recent model calculations [pena2012], the respective eTFF are very sensitive to both contributions: the quark core and the pion cloud. Moreover, according to these calculations, a q2 dependence of eTFF exhibits a rich structure with the strongest enhancement in the dielectron invariant mass spectrum near q2 = (me+e-)2  0.6 GeV2, reflecting the important role played by the r/w vector mesons in the N*(D) -> Ng* transition [kriv2002]. The special role of the vector mesons in such transitions, acting as the interpolating field between the hadron and the photon, was suggested already in 60’es by Sakurai [sakurai] in the framework of the Vector Meson Dominance model (VMD). Moreover, it recently turned out that the baryons decaying into real and virtual massive photons, are also important for the understanding of the radiation emitted from the dense nuclear matter created in heavy ion collisions, as discussed below.

The first aim of this project is to reconstruct electromagnetic Dalitz decays of baryon resonances in the exclusive processes by means of the HADES (High Acceptance Dielectron Spectrometer) detector, currently operating at GSI in Darmstadt [hades, salabura2012].

The strong interactions experienced by hadrons in a compressed nuclear medium (e.g. created in course of heavy ion collisions) can modify basic hadron properties like masses or life times. Such changes have been predicted by various theoretical models due to the effect of chiral symmtery restoration in dense and hot nuclear matter. Chiral symmetry is the basic symmetry of QCD valid in the limit of mass-less quarks and is spontaneously broken at the vacuum level. A commonly used order parameter of the chiral symmetry breaking is a non vanishing expectation value of two-quark condensate 〈¯q q〉 . Various studies with the lattice QCD and effective field theories predict significant changes of the two-quark condensate as a function of nuclear matter density and/or temperature. Also studies on the lattice QCD for vanishing baryon chemical potential show that the expectation value of the quark condensate vanishes at temperatures close to phase transition hadron gas – quark qluon plasma . A strong reduction of the quark condensate is also predicted at lower temperatures but the high baryon densities, even below the phase transition boundary (for recent review see [CBM2012]).

The expectation value of 〈¯q q〉 cannot be directly accessible by experiments. However, there are inspiring suggestions for the related observables which concern the measurement of in-medium mass hadron masses by means of a dilepton radiation.

1) The most famous ones stem from Brown and Rho in 90’ies [BR1996] and QCD sum rules of Hatsuda [Hat1996]. Both relate the expectation value of the quark condensate in medium to the mass of the light vector mesons (r, w). Either the meson mass dropping (Brown-Rho) or/and the meson mass broadening (QCD sum rules) were predicted. Many experiments were performed, aiming at a direct measurement of the vector meson mass inside the medium, by means of their dilepton decays. A big advantage of such decays is no final state interaction of outgoing leptons with the surrounding medium what allows for a direct reconstruction of the meson in-medium mass distribution (for the recent reviews see [Leu2010], [salabura2011]). The most precise and spectacular data were obtained at CERN SPS by the NA60 experiment [na602006] showing the significant broadening of the r meson spectral function, equivalent to almost complete disappearance, or “melting” of the resonance, inside the dense and hot nuclear matter. A successful explanation of this effect is given by the model of R. Rapp et al. [Rapp2008] which explains the broadening of the meson by many body interactions with the surrounding meson gas and baryon resonances. In particular, the effects related to baryons are the dominant source of the observed melting due to a strong r->(R N-1) (baryon resonance - nucleon hole) coupling. This process can be directly related by cutting the RN-1 loop to the discussed process of the baryon resonance R->Nr -> Ne+e Dalitz decay, provided that virtual photon indeed couples to the resonance by the intermediate vector meson state. The suggested process explains also dielectron data obtained in the Beam Energy Scan at RHIC by the STAR [qm2012] and seems to be the universal mechanism for the  meson mass modification in a broad energy range 17 < Ös < 200 GeV. One should note, however, that a theoretical challenge to prove that such process is the way in which chiral symmetry restoration is realized still remains.

2) The second relation of the dielepton invariant mass spectrum to the properties of dense nuclear matter can be defined in a more general concept of the “emissivity”, which expresses the way the matter radiates photons. The emissivity is the radiation originating from hot and compressed zone of the collisions, where individual hadrons starts to overlap. It is also called “excess radiation” and can be obtained from the total measured dilepton yield by the subtraction of the dilepton rate from the freeze-out stage of HI collisions, where all produced hadrons cease to interact. There are experimental techniques which allow for the model independent extraction of such an “excess radiation”, as shown for example by the NA60 experiment [na602006]. The measurement of the mass spectrum of such radiation and its possible changes as a function of the beam energy is one of the most important goals of heavy ion experiments. In this context a low energy domain, where nuclear matter is composed mainly from baryons, plays a special role. The question we are after is the following: how does the radiation from such composite objects as baryon resonances changes, if the matter is compressed and respective wave functions start to overlap?

The research task related to the dielectron radiation from the dense nuclear matter are the following:

a) Investigate the “emissivity” of compressed baryonic matter at the low energy heavy ion collisions (1 < Ös < 2 GeV) with the present HADES at SIS18.

b) Perform development studies for the future HADES research at FAIR at the higher energy (3 < Ös < 7 GeV)

Significance of the Project

Measurements of the electromagnetic Dalitz decays of baryon resonances in the exclusive processes, probing the resonance eTFF in a time like region, have never been done before. The results will provide very interesting complementary information to the one obtained from the ongoing measurements of the eTFF in a space like region by means of an electron scattering. In particular, the pion cloud and the quark core effects established by the measurement of eTFF in a space like region, will be investigated in a time like region. It is expected that the q2 dependence of the eTFF in a time like region is directly reflected in the dielectron invariant mass spectrum, resulting from the R-> Ne+e decay [pena2012]. The investigation of eTFF of baryons is also complementary to the on-going research for mesons and is a part of the European network Meson-Net [mesonnet].

Furthermore, as argued above, the process of Baryon resonance Dalitz decays is directly connected to the in-medium r->(RN-1) couplings, explaining the observed dramatic increase of the in-medium r meson width [rapp2008]. It is therefore of major importance to confirm the predicted strong r meson–resonance coupling by the independent measurement. Strong in-medium effects observed at CERN-SPS and at RHIC are driven by baryons and therefore call for investigations at lower beam energies. Measurement of the dielectron radiation from the compressed and hot nuclear matter (“emissivity of nuclear matter”) as a function of the beam energy is also an important observable for our understanding of nuclear matter phase diagramme [cbm2012]. A combination of the region of the phase diagramme covered by the HADES and the selection of highly penetrating dielectron probes (and other rare hadronic states as hyperons, f mesons measured by the HADES) makes this research unique in heavy ion physics and cannot be done elsewhere. The nuclear matter created in a few AGeV heavy ion collisions is composed of nucleons and of baryon resonances compressed to 3-4 nuclear nuclear matter densities. The effects of meson gas (mainly pions) plays a minor role, in contrary to the higher energies where pion densities are much larger. Therefore, the research in this energy regime is complementary to studies carried out at the higher energies with relativistic heavy ion collisions at CERN SPS (experimenta Na61-Shine) and at RHIC with the Beam Energy Scan programme. Obtained results will present an important reference and a natural link to measurements for the future programme of the Compressed Baryonic Matter experiments at FAIR, which will start in 2017 [cbm2012]. The HADES at FAIR will open new possibilities to study both aforementioned aspects at the higher beam energies (4-10 GeV) .

The results will be published in the word leading journals and presented during the most important conferences. Last but not least, the results of the work conduced within this project will be an essential part of four PhD theses (G. Korcyl, S. Harabasz, J. Biernat, P. Strzempek) and one habilitation thesis (W. Przygoda).