Cronin Effect Research Articles

Cronin effect and high-pTsuppression inpAcollisions

We review the predictions of the theory of Color Glass Condensate for gluon production cross section in p(d)A collisions. We demonstrate that at moderate energies, when the gluon production cross section can be calculated in the framework of McLerran-Venugopalan model, it has only partonic level Cronin effect in it. At higher energies/rapidities corresponding to smaller values of Bjorken x quantum evolution becomes important. The effect of quantum evolution at higher energies/rapidities is to introduce suppression of high-p_T gluons slightly decreasing the Cronin enhancement. At still higher energies/rapidities quantum evolution leads to suppression of produced gluons at all values of p_T.

Quark fragmentation to π±, π0, K±, p and [formula omitted] in the nuclear environment

The influence of the nuclear medium on lepto-production of hadrons was studied in the HERMES experiment at DESY in semi-inclusive deep-inelastic scattering of 27.6 GeV positrons off deuterium, nitrogen and krypton targets. The differential multiplicity for krypton relative to that of deuterium has been measured for the first time for various identified hadrons (π+, π−, π0, K+, K−, p and p̄) as a function of the virtual photon energy ν, the fraction z of this energy transferred to the hadron, and the hadron transverse momentum squared pt2. The multiplicity ratio is strongly reduced in the nuclear medium at low ν and high z, with significant differences among the various hadrons. The distribution of the hadron transverse momentum is broadened towards high pt2 in the nuclear medium, in a manner resembling the Cronin effect previously observed in collisions of heavy ions and protons with nuclei.

The Cronin effect, quantum evolution and the color glass condensate

We show that the numerical solution of the classical SU(3) Yang–Mills equations of motion in the McLerran–Venugopalan model for gluon production in central heavy ion collisions leads to a suppression at low pt and an enhancement at the intermediate pt region as compared to peripheral heavy ion and pp collisions at the same energy. Our results are compared to previous, color glass condensate inspired calculations of gluon production in heavy ion collisions. We revisit the predictions of the color glass condensate model for pA (dA) collisions in leading order and show that quantum evolution—in particular, the phenomenon of geometric scaling and change of anomalous dimensions—preserves the Cronin enhancement of pA cross section (when normalized to the leading twist term) in the leading order approximation even though the pt spectrum can change. We comment on the case when gluon radiation is included.

Saturation and parton level Cronin effect: Enhancement versus suppression of gluon production inp−AandA−Acollisions

We note that the phenomenon of perturbative saturation leads to transverse momentum broadening in the spectrum of partons produced in hadronic collisions. This broadening has a simple interpretation as parton level Cronin effect for systems in which saturation is generated by the "tree level" Glauber-Mueller mechanism. For systems where the broadening results form the nonlinear QCD evolution to high energy, the presence or absence of Cronin effect depends crucially on the quantitative behavior of the gluon distribution functions at transverse momenta kt outside the so called scaling window. We discuss the relation of this phenomenon to the recent analysis by Kharzeev-Levin-McLerran of the momentum and centrality dependence of particle production in nucleus-nucleus collisions at RHIC.

Initial state parton broadening and energy loss probed in d+Au at RHIC

The impact parameter and rapidity dependence of the Cronin effect for massless pions in d+Au reactions at sNN=200 GeV at RHIC is computed in the framework of pQCD multiple elastic scattering on a nuclear target. We introduce a formalism to incorporate initial state energy loss in perturbative calculations and take into account the elastic energy loss in addition to the transverse momentum broadening of partons. We argue that the centrality dependence of the Cronin effect can distinguish between different hadron production scenarios at RHIC. Its magnitude and rapidity dependence are shown to carry important experimental information about the properties of cold nuclear matter up to the moderate- and large-x antishadowing/EMC regions.

From deep inelastic scattering to proton-nucleus collisions in the color glass condensate model

We show that particle production in proton-nucleus (pA) collisions in the Color Glass Condensate model can be related to Deep Inelastic Scattering of leptons on protons/nuclei (DIS). The common building block is the quark antiquark (or gluon-gluon) dipole cross section which is present in both DIS and pA processes. This correspondence in a sense generalizes the standard leading twist approach to pA collisions based on collinear factorization and perturbative QCD, and allows one to express the pA cross sections in terms of a universal quantity (dipole cross section) which, in principle, can be measured in DIS or other processes. Therefore, using the parameterization of dipole cross section at HERA, one can calculate particle production cross sections in proton-nucleus collisions at high energies. Alternatively, one could use proton-nucleus experiments to further constrain models of the dipole cross-section. We show that the McLerran-Venugopalan model predicts enhancement of cross sections at large p_t (Cronin effect) and suppression of cross sections at low p_t. The cross over depends on rapidity and moves to higher p_t as one goes to more forward rapidities.

Large pT Hadron Spectra in pp, pA and AA Collisions

Large pT particle spectra in p+p, p+A and A+B collisions at different energies are calculated in a QCD parton model. Intrinsic transverse momentum, its broadening due to initial multiple parton scattering are included. These calculations are meant to estimate the normal nuclear effects (parton shadowing, pT broadening or Cronin effect) in high pT hadron spectra in heavy-ion collisions. Comparisons of these calculations with experimental data will help to determine the effect of jet quenching.

Hadron formation in a nuclear environment at HERMES

The influence of the nuclear medium on the production of charged hadrons has been studied by the HERMES experiment at DESY in semi-inclusive deep inelastic scattering of 27.5 GeV positrons off deuterium, nitrogen and krypton targets. The differential multiplicity from the nucleus relative to that from the deuterium has been measured for charged hadrons and, for the first time, for identified pions, kaons, protons and antiprotons as a function of the relevant kinematic variables: the virtual photon energy v, the fraction z of this energy transferred to the hadron and the hadron transverse momentum squared p t 2 . The multiplicity ratio R M h is strongly reduced at low v and high z with significant differences for different hadrons. The distribution of the hadron transverse momentum is broadened at high p t 2 in the nuclear medium showing the evidence of the Cronin effect in the deep inelastic scattering process.

High PT π 0 production and angular correlations in 158 AGeV p+A and Pb+Pb collisions

Recent results of the WA98 experiment with p and Pb induced reactions at 158 AGeV are presented. Azimuthal γ-γ correlations at high PT were studied in search for a signal of jet-like structures. A clear indication for back-to-back correlations can be seen in p+A with a strong dependence on the PT of the photons and the size of the system. In Pb+Pb collisions in plane elliptic flow has been observed. Results on transverse mass spectra of neutral pions measured at central rapidity are presented for impact parameter selected Pb+Pb collisions. In going from peripheral to medium central collisions there is a nuclear enhancement increasing with transverse mass similar to the Cronin effect, while for very central collisions this enhancement appears to be weaker than expected.

High-p(T) Tomography of d+Au and Au+Au at SPS, RHIC, and LHC.

The interplay of nuclear effects on the p(T)>2 GeV inclusive hadron spectra in d+Au and Au+Au reactions at sqrt[s(NN)]=17, 200, and 5500 GeV is compared to leading order perturbative QCD calculations for elementary p+p (p+p) collisions. The competition between nuclear shadowing, Cronin effect, and jet energy loss due to medium-induced gluon radiation is predicted to lead to a striking energy dependence of the nuclear suppression/enhancement pattern in A+A reactions. We show that future d+Au data can be used to disentangle the initial and final state effects.

Cronin effect in hadron production off nuclei.

Recent data from RHIC for high- pT hadrons raised again the long-standing problem of quantitatively understanding the Cronin effect, i.e., nuclear enhancement of high- pT hadrons. All existing models for the Cronin effect rely on a fit to the data to be explained. We develop a phenomenological description based on the light-cone QCD-dipole approach which allows one to explain data without fitting to them and to provide predictions for pA collisions at RHIC and LHC. We point out that the underlying mechanism drastically changes with energy, from incoherent production of high- pT hadrons on different nucleons at low energies, to an entirely coherent process at very high energies.

Transverse mass distributions of neutral pions from $^{208}$ Pb-induced reactions at 158 $\cdot A$ GeV

Results on transverse mass spectra of neutral pions measured at central rapidity are presented for impact parameter selected 158$\cdot A$ GeV Pb + Pb, and Pb + Nb collisions. The distributions cover the range $0.5 \mathrm{GeV}/c^{2} \le m_{T} - m_{0} \le 4 \mathrm{GeV}/c^{2}$. The change of the spectral shape with centrality is studied in detail. In going from peripheral to medium central collisions there is a nuclear enhancement increasing with transverse mass similar to the well known Cronin effect, while for very central collisions this enhancement appears to be weaker than expected.

High-pTpion and kaon production in relativistic nuclear collisions

High-p_T pion and kaon production is studied in relativistic proton-proton, proton-nucleus, and nucleus-nucleus collisions in a wide energy range. Cross sections are calculated based on perturbative QCD, augmented by a phenomenological transverse momentum distribution of partons (``intrinsic k_T''). An energy dependent width of the transverse momentum distribution is extracted from pion and charged hadron production data in proton-proton/proton-antiproton collisions. Effects of multiscattering and shadowing in the strongly interacting medium are taken into account. Enhancement of the transverse momentum width is introduced and parameterized to explain the Cronin effect. In collisions between heavy nuclei, the model over-predicts central pion production cross sections (more significantly at higher energies), hinting at the presence of jet quenching. Predictions are made for proton-nucleus and nucleus-nucleus collisions at RHIC energies.

Study of jet production in p– N interactions at [formula omitted] GeV in EAS multicore events

The cross section for large p t jet production for transverse momentum 10≤ p t ≤20 GeV/c and rapidity 1.6≤ η≤2.6 in p– N ( p–`air') interactions is studied from the analysis of multicore Extensive Air Showers recorded in the EAS–TOP calorimeter. The projectiles are the leading particles interacting at atmospheric depths between 250 and 480 cm −2, the CMS energy of interaction being s ≈500 GeV. The slope of the p t distribution agrees with the expected one from the p– p ̄ collider data at the same CMS energy. The measured jet production cross section in p– N interactions with respect to p– p ̄ interactions is (dσ/dp t) pN jet =(dσ/dp t) p p ̄ jet ·A α with α=1.56±0.07 for A=14.7 (average mass number of `air' nuclei). Such value is compatible within the experimental uncertainties with the one obtained in p–nucleus accelerator measurements at s ≃30 GeV in the same range of transverse momentum and rapidity. No indication of increase of α with energy, i.e. of the entity of the `Cronin effect', is found.

Hard $\pi^0$ and $\eta$ production in S + Au and Pb + Pb nuclear collisions at SPS energies and the possible signature of quark–gluon plasma

Hard $\pi^0$ and $\eta$ production is investigated in Landau and Bjorken hydrodynamical models, taking a great number of hadronic resonances (16 and 42) in the hadronic phase into account. Central and peripheral S + Au and Pb + Pb collisions are considered. We consider two different scenarios: one with quark–gluon plasma (QGP) formation and one with only hadronic gas without QGP. The Cronin effect and hard direct meson emission are taken into account. It is shown that these two scenarios give similar $p_{\perp}$ pion spectra, which approximately agree with experimental data for S + Au collisions obtained by the WA80 Collaboration. Therefore we conclude that it is difficult to extract proof of QGP formation from hadronic spectra. However, we also calculate the $\eta/\pi^0$ ratio. We show that this value agrees with experimental data only for the scenario with plasma and a phase transition for hadrons. The scenario of hadronic gas without plasma disagrees with experimental data.

Jet quenching and Cronin enhancement in A+ A at [formula omitted] versus 200 AGeV

The sensitivity of semi-hard ( p ⊥<10 GeV) hadron production to parton energy loss in high energy nuclear collisions is studied via the HIJING1.35 model. We test the model on recent WA98 data on 160 AGeV Pb+ Pb→ π 0 up to 4 GeV/c and while these data are reproduced, the results depends sensitively on the model of the Cronin effect. At (RHIC) collider energies ( s >200 AGeV), on the other hand, semi-hard hadron production becomes insensitive to the above model and is thus expected to be a cleaner probe of jet quenching signature associated with non-Abelian radiative energy loss.

Hard π0 and η production in S+ Au nuclear collisions at SPS energies and possible signature of quark-gluon plasma

Hard π 0 and η production is investigated in Landau and Bjorken hydrodynamical models taking into account great number of hadronic resonances (16 and 42) in hadronic phase. We consider two different scenario: with quark-gluon plasma (QGP) formation and with only hadronic gas one without QGP. The Cronin effect and hard direct pions emission are taken into account. It is shown that these two scenario give similar p ⊥ spectra which agrees with experimental data obtained by WA80 collaboration. Therefore we conclude that from hadronic spectra it is difficult to extract the proof of QGP formation. However we calculate also the η/ π 0 ratio. We show that this value agrees with experiment only for scenario with QGP and phase transition to hadrons. The hadronic gas scenario without QGP disagrees with experimental data.

The Firecracker Model, a possible collective effect in high energy heavy-ion collisions

We present a possible collective gluon emission effect based upon a string-like scenario (available if the QCD vacuum works like a color-superconductor type II, i.e. when the color force fields work like vortex-lines) and provide some detailed predictions on the Cronin effect, as well as on an increased (mini)jet production in the center of phase-space in high energy heavy ion collisions.