Dissociation Of Hadrons Research Articles

Production of Exotic Particles in Electron-Positron Collisions

In this work, the results for cross sections of central exclusive exotic particles production in future linear electron-positron colliders are presented. These results show that the central production of charginos, dilatons, radions, axions and technipions can be measurable for the center-of-mass energies for these colliders. The results presented here, when compared with the production in hadronic colliders, have a smaller cross section, but with the advantage that the electron-positron collision does not have remnants coming from hadronic dissociation.

Quark Cluster Expansion Model for Interpreting Finite-T Lattice QCD Thermodynamics

In this work, we present a unified approach to the thermodynamics of hadron–quark–gluon matter at finite temperatures on the basis of a quark cluster expansion in the form of a generalized Beth–Uhlenbeck approach with a generic ansatz for the hadronic phase shifts that fulfills the Levinson theorem. The change in the composition of the system from a hadron resonance gas to a quark–gluon plasma takes place in the narrow temperature interval of 150–190 MeV, where the Mott dissociation of hadrons is triggered by the dropping quark mass as a result of the restoration of chiral symmetry. The deconfinement of quark and gluon degrees of freedom is regulated by the Polyakov loop variable that signals the breaking of the Z(3) center symmetry of the color SU(3) group of QCD. We suggest a Polyakov-loop quark–gluon plasma model with O(αs) virial correction and solve the stationarity condition of the thermodynamic potential (gap equation) for the Polyakov loop. The resulting pressure is in excellent agreement with lattice QCD simulations up to high temperatures.

Unified Quark–Hadron EoS and Critical Endpoint in the QCD Phase Diagram

We present a recent development towards a unified description of quark-hadron matter in the QCD phase diagram that is based on a cluster decomposition of the generalized Beth-Uhlenbeck approach to quark matter, selfconsistently coupled to Polyakov-loop and mesonic background fields. The Mott dissociation of hadrons under extreme conditions of temperature and density is triggered by chiral symmetry restoration and confining aspects are modeled by the coupling to the background mean fields. First results for the QCD phase diagram with the capability to describe critical endpoints as well as crossover-all-over are presented and an excellent agreement with lattice QCD on the temperature axis is obtained.

Generalized Beth--Uhlenbeck Approach to the Equation of State for Quark--Hadron Matter

A unified equation of state for quark-hadron matter is presented in the generalized Beth-Uhlenbeck form. It follows from a $\Phi-$derivable approach to the thermodynamic potential where the ansatz for the $\Phi$ functional contains all 2PI diagrams at two-loop order formed with quark cluster Green's functions for quark, diquark, meson and baryon propagators. We present numerical results using an effective model for the generic behaviour of hadron masses and phase shifts at finite temperature which shares basic features with recent developments within the PNJL model for correlations in quark matter. We obtain the transition between a hadron resonance gas phase and the quark gluon plasma where the Mott dissociation of hadrons is encoded in the hadronic phase shifts. The resulting thermodynamics is in very good agreement with recent lattice QCD simulations.

Sequential regeneration of charmonia in heavy-ion collisions

We investigate the production of ψ(2S) in nuclear collisions at RHIC and LHC energies. We first address charmonium production in 200 GeV d–Au collisions at RHIC; the strong suppression of ψ′ mesons observed in these reactions indicates mechanisms beyond initial cold nuclear matter effects. We find that a more complete treatment of hadronic dissociation reactions leads to appreciable ψ′ suppression in the thermal medium of an expanding fireball background for d–Au collisions. When implementing updated hadronic reaction rates into a fireball for 2.76 TeV Pb–Pb collisions at LHC, the regeneration of ψ′ mesons occurs significantly later than for J/ψ's. Despite a smaller total number of regenerated ψ′, the stronger radial flow at their time of production induces a marked enhancement of their RAA relative to J/ψ's in a transverse-momentum range of pt≃3–6 GeV. We explore the consequences and uncertainties of this “sequential regeneration” mechanism on the RAA double ratio and find that it can reproduce the trends observed in recent CMS data.

Mott-hadron resonance gas and lattice QCD thermodynamics

We present an effective model for the generic behaviour of hadron masses and phase shifts at finite temperature which shares basic features with recent developments within the PNJL model for correlations in quark matter. On this basis we obtain the transition between a hadron resonance gas phase and the quark gluon plasma in the spirit of the generalized Beth-Uhlenbeck approach where the Mott dissociation of hadrons is encoded in the hadronic phase shifts. Here we restrict ourselves to low-lying hadronic channels and perform a discussion of recent lattice QCD thermodynamics results from this perspective. We find agreement in the asymptotic regions while for the description of the transition itself the inclusion of further hadronic channels as well as a selfconsistent determination of the continuum thresholds is required.

Proton–proton, pion–proton and pion–pion diffractive collisions at ultrahigh energies

The LHC energies are those at which the asymptotic regime in hadron–hadron diffractive collisions (pp, πp, ππ) might be switched on. Based on results of the Dakhno–Nikonov eikonal model which is a generalization of the Good–Walker eikonal approach for a continuous set of channels, we present a picture for transformation of the constituent quark mode to the black disk one. In the black disk mode [Formula: see text], we have a growth of the logarithm squared type for total and elastic cross-sections, σ tot ~ ln 2 s and σ el ~ ln 2 s and [Formula: see text]-scaling for diffractive scattering and diffractive dissociation of hadrons. The diffractive dissociation cross-section grows as σD ~ ln s, σDD ~ ln s, and their relative contribution tends to zero: σD/σ tot → 0, σDD/σ tot → 0. Asymptotic characteristics of diffractive and total cross-sections are universal, and this results in the asymptotical equality of cross-sections for all types of hadrons (the Gribov universality). The energy scale for switching on the asymptotic mode is estimated for different processes.

Effective degrees of freedom in QCD thermodynamics

An effective model reproducing the equation of state of hadronic matter as obtained in recent lattice QCD simulations and from hadron resonance gas data is presented. The hadronic phase is described by means of an extended Mott-Hagedorn resonance gas while the QGP phase is described by the extended PNJL model. The dissociation of hadrons is obtained by including the state dependent hadron resonance width. The model gives a quantitative estimate for partial fractions of hadronic and partonic degrees of freedom above $T_c$.

Asymptotic regime for hadron-hadron diffractive collisions at ultrahigh energies

Using the pre-LHC and LHC data for $\ensuremath{\pi}p$ and $pp$ diffractive collisions, we study the ultrahigh-energy asymptotic regime in the framework of the black disk picture. The black disk picture, being constrained by the $s$-channel unitarity condition and the $t$-channel analyticity, gives rather definite predictions for diffractive processes increasing with the energy. To deal with the data, we consider the Dakhno-Nikonov eikonal model, which predicts a growth of the ${ln}^{2}s$ type for total and elastic cross sections and ($\ensuremath{\tau}={\mathbf{q}}_{\ensuremath{\perp}}^{2}{ln}^{2}s$) scaling for diffractive scattering and diffractive dissociation of hadrons. According to the calculations, ultrahigh-energy asymptotic characteristics of diffractive and total cross sections are universal, and this results in the asymptotic equality of cross sections for all types of hadrons. We estimate the energy scale of the asymptotics in different processes. The manifestation of the asymptotic regime in hadron fragmentation reactions is discussed.

An effective model of QCD thermodynamics

A combined effective model reproducing the equation of state of hadronic matter as obtained in recent lattice QCD simulations is presented. The model reproduces basic physical characteristics encountered in dense hadronic matter in the quark-gluon plasma (QGP) phase and the lower temperature hadron resonance gas phase. The hadronic phase is described by means of an extended Mott-Hagedorn resonance gas while the QGP phase is described by the extended PNJL model. The dissociation of hadrons is obtained by including the state dependent hadron resonance width.

Charmonium suppression and regeneration from SPS to RHIC

The production of charmonia is investigated for heavy-ion collisions from SPS to RHIC energies. Our approach incorporates two sources of J/Ψ yield: (i) a direct contribution arising from early (hard) parton-parton collisions, subject to subsequent nuclear absorption, quark-gluon plasma and hadronic dissociation, and (ii) statistical production at the hadronization transition by coalescence of c and c̄ quarks. Within an expanding thermal fireball framework, the model reproduces J/Ψ centrality dependencies observed at the SPS in Pb–Pb and S–U collisions reasonably well. The study of the Ψ′/Ψ ratio at SPS points at the importance of the hadronic phase for Ψ′ interactions, possibly related to effects of chiral symmetry restoration. Predictions are given for the centrality dependence of the NJ/Ψ/Ncc̄ ratio at full RHIC energy. We also calculate the excitation function of this ratio. The latter exhibits a characteristic minimum structure signalling the transition from the standard J/Ψ suppression scenario prevailing at SPS to dominantly thermal regeneration at collider energies.

From hard to soft diffraction and return

The long standing mystery of smallness of diffractive dissociation of hadrons to large effective masses (the Pomeron-proton cross section is only 2 mb) witnesses that the gluonic clouds of valence quarks are so small ( r 0 = 0.3 fm) that soft interaction hardly resolves those gluons (diffraction is ∝ r 4 0). A color-dipole light-cone (LC) approach is developed which incorporates a strong nonperturbative interaction of the LC gluons. The energy dependent part of the total hadronic cross section is calculated in a parameter-free way employing the nonperturbative LC wave functions of the quark-gluon Fock states. It rises with energy as s Δ, and we predict Δ = 0.17 ± 0.01, as well as the normalization. However, the energy independent part of the cross section related to inelastic collisions with no gluon radiated (gluons are not resolved) cannot be calculated reliably and we treat it as an adjustable parameter which is fixed fitting just one experimental point for total cross section. Then the energy dependence of the total cross section (the Pomeron part) and the elastic slope are fully predicted, as well as the effective Pomeron trajectory in impact parameter space, in a good agreement with data. These results naturally explain the x-dependence of the diffractive DIS observed at HERA. Although diffraction is expected to be dominated by soft interactions the observed effective Δ is about twice as large as one (0.08) known for total cross sections. Diffractive excitations of large effective mass correspond to diffractive gluon radiation and should be associated with our calculated Δ.

Nonperturbative effects in gluon radiation and photoproduction of quark pairs

We introduce a nonperturbative interaction for light-cone fluctuations containing quarks and gluons. The $\overline{q}q$ interaction squeezes the transverse size of these fluctuations in the photon and one does not need to simulate this effect via effective quark masses. The strength of this interaction is fixed by data. Data on diffractive dissociation of hadrons and photons show that the nonperturbative interaction of gluons is much stronger. We fix the parameters for the nonperturbative quark-gluon interaction by data for diffractive dissociation to large masses (triple-Pomeron regime). This allows us to predict nuclear shadowing for gluons which turns out to be not as strong as perturbative QCD predicts. We expect a delayed onset of gluon shadowing at $x<~{10}^{\ensuremath{-}2}$ shadowing of quarks. Gluon shadowing turns out to be nearly scale invariant up to virtualities ${Q}^{2}\ensuremath{\sim}4$ ${\mathrm{GeV}}^{2}$ due to the presence of a semihard scale characterizing the strong nonperturbative interaction of gluons. We use the same concept to improve our description of gluon bremsstrahlung which is related to the distribution function for a quark-gluon fluctuation and the interaction cross section of a $\overline{q}\mathrm{qG}$ fluctuation with a nucleon. We expect the nonperturbative interaction to suppress dramatically the gluon radiation at small transverse momenta compared to perturbative calculations.

Calculations of hadronic dissociation of 28Si projectiles at 14.6A GeV by nucleon emission.

An optical potential abrasion-ablation collision model is used to calculate hadronic dissociation cross sections for one, two, and three nucleon removal for the first time for a 14.6A GeV 28Si beam fragmenting in aluminum, tin, and lead targets. These estimates are compared with recent semi-inclusive measurements. Significant differences between some calculated and measured semi-inclusive cross sections exist which cannot be resolved without measurements of the exclusive channel hadronic cross sections. Calculations for each exclusive reaction channel contributing to the semi-inclusive cross sections are presented and discussed.

Large-t, triple-pomeron diffractive processes in QCD.

In terms of perturbative QCD we calculate the basic features of high-mass (${M}_{X}$) diffractive dissociation of hadrons at large momentum transfer t. We find that the distribution over 1-x=${M}_{X}^{2}$/s should change from 1/(1-x) at small t to / ${\mathrm{ln}}^{3}$[1/(1-x)] at large t [where ${\ensuremath{\omega}}_{0}$=${\ensuremath{\alpha}}_{S}$(t)12 ln(2)/\ensuremath{\pi}], and that the gluon distribution in the Pomeron should flatten with increasing \ensuremath{\Vert}t\ensuremath{\Vert}.

Rescattering effects in the Deck model

The authors study resonant rescattering corrections to the Deck amplitude B for diffractive dissociation of hadrons. The results of numerical calculations for a variety of physically-reasonable forms of B show that provided B falls off faster than a simple pole the rescattered amplitude is well approximated by the form Bei delta cos delta , rather than Bei delta sin delta /k. They conclude that the former expression is generally relevant to situations in which the Born amplitude B is spatially diffuse, while the latter applies if B is spatially localised.

Inelastic diffraction in the quark-glue model

Diffractive dissociation of hadrons is related to the quark-glue picture of their structure. It is shown that the quark-glue model can produce sufficiently large variations of opacities for diffractive eigenstates to account for the experimental value of diffractive cross sections if unitarity is saturated in central glue-glue collisions. This requirement simplifies the collision picture and provides a natural explanation for the observed energy independence of proton opacity in central pp collisions.

Nucleon diffractive dissociation. I. Peripheral model with absorption

A model for diffractive dissociation of hadrons into low-mass states is proposed. It is based on a peripheral mechanism with absorption. The absorption effects lead to an important modification of the amplitude by introducing an extra dependence upon momentum transfer and strong slope-mass correlation. A diffractive minimum is predicted for small values of the mass of the produced system. The connection with the crossover effects in diffractive dissociation is discussed. The nucleon dissociation is considered in detail.

Mass interference patterns in diffractive dissociation of hadrons

Recently, in order to explain the experimentally observed (1) increase in the transparency of the nuclear matter to the propagation of a diffractively dissociated hadronic system D, VAN Hov~ has proposed a continuum model (~.s). In the latter i t is assumed that the system D consists of an infinite number of mutual ly interacting systems [~> having different masses but the same quantum numbers as D. This model predicts not only the increase in transparency but also a new phenomenon, namely that the mass distribution of [~}'s should exhibit diffractionlike interference patterns. However, the theoretical foundation for this prediction is rather restricted. For example, it is claimed that a) only a continuum model can predict such an effect, whereas a discrete model with a finite number of ]~}'s cannot, b) the DD interaction potential must have a smoothness property (2). I n this paper we show that the restriction a) is not necessary and indicate how b) can be relaxed to some extent. We also identify the dynamical causes underlying the increased transparency and the predicted diffraction patterns. Consider a high-energy collision between a hadron h and a nucleus. After the collision, h gets dissociated into a system D which we assume to consist of n states [~> having different masses but the same quantum numbers as D. For concreteness, we assume h to be a pion. Then, at high energies, the multiehanncl eikonal equations of propagation of = and D can be writ ten in matrix form as

A modification of Glauber theory for diffraction dissociation of hadrons from nuclei

The formulation of the Glauber theory for diffraction dissociation of hadrons from heavy nuclei is modified to take into account the fact that the dissociated system D can oscillate between many internal states as it propagates through nuclear matter. The set of internal states of D is approximated by a continuum and the successive D-nucleon collisions are represented by a scattering amplitude which is taken to be a smooth function of the internal parameters of D. This smoothness property is argued to result from the very short time between successive D-nucleon collisions; it is therefore an off-shell effect. The physical implications of the modified scheme are an increased transparency of nuclear matter and a A-dependent modification in the distribution of internal D states, mainly a narrowing of the mass distribution. These effects are demonstrated by the numerical treatment of a highly simplified example.