Induced photon emission from quark jets in ultrarelativistic heavy-ion collisions

We study the induced photon bremsstrahlung from a fast quark produced in AA collisions due to multiple scattering in quark-gluon plasma. For RHIC and LHC conditions, the induced photon spectrum is sharply peaked at a photon energy close to the initial quark energy. In this region, the contribution of the induced radiation to the photon fragmentation function exceeds the ordinary vacuum radiation. Contrary to previous analyses [4–7], our results show that, at RHIC and LHC energies, the final-state interaction effects in quark-gluon plasma do not suppress the direct photon production and may even enhance it at p _T ～5–15 GeV.     

Two-proton correlations in <Superscript>4</Superscript>He<Emphasis Type="Italic">p</Emphasis> interactions at a primary momentum of 5 GeV/<Emphasis Type="Italic">c</Emphasis>

Correlations of secondary protons in ⁴Hep interactions are investigated in an exclusive experiment with the aid of a 2-m liquid-hydrogen bubble chamber exposed to an alpha-particle beam of momentum 5 GeV/c the kinetic energy of primary protons in the rest frame of the nucleus is T_p=620 MeV). By using data obtained in 4π geometry for six basic channels of ⁴Hep interaction that lead to the production of two protons, the total correlation function for the pp system is determined, along with two-proton correlation functions for individual channels. Experimental results are compared with the predictions of the modified Lednicky-Lyuboshitz model. The value of R=1.6±0.3 fm is obtained for the root-meansquare spacetime radius of pp emission in ⁴Hep interactions. The dependence of the correlation function on the total momentum of two emitted protons and on the momentum-transfer direction is investigated.     

Production of light vector mesons in heavy-ion nuclear collisions at RHIC,measured by the PHENIX spectrometer

The production cross sections of ω and ? mesons in p + p, d + Au, and Au + Au collisions at energies √S _NN = 63 and 200 GeV have been measured in the PHENIX experiment at RHIC. The results of the measurements in different hadronic and dielectron decay channels are in agreement within the measurement error. The nuclear modification factors R _AA measured for both mesons are consistent with the results previously obtained for light neutral mesons. The position and width of the meson mass peaks reconstructed in hadronic decay channels are in agreement with the results of measurements in vacuum.     

Influence of coalescence parameters on the production of protons and Helium-3 fragments

The time evolution of protons and ³He fragments from Au+Au/Pb+Pb reactions at 0.25, 2, and 20 GeV/nucleon is investigated with the potential version of the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) model combined with the traditional coalescence afterburner. In the coalescence process, the relative distance R₀ and relative momentum P₀ are surveyed in the range of 3-4 fm and 0.25-0.35 GeV/c, respectively. For both clusters, a strong reversed correlation between R₀ and P₀ is seen and it is time-dependent as well. For protons, the accepted (R₀, P₀) bands lie in the time interval 30-60 fm/c, while for 3He, a longer time evolution (at about 60-90 fm/c) is needed. Otherwise, much smaller R₀ and P₀ values should be chosen. If we further look at the rapidity distributions from both central and semi-central collisions, it is found that the accepted [t_cut, (R₀, P₀)] assemble can provide consistent results for proton yield and collective flows especially at mid-rapdities, while for ³He, the consistency is destroyed at both middle and projectile-target rapidities.     

The decoupling problem at RHIC

We investigate whether it is possible to dynamically generate from classical transport theory the observed surprising R_out ≈ R_side in Au+Au at \(\sqrt s = 130A\) GeV at RHIC [1,2]. We obtained covariant solutions to the Boltzmann transport equation via the MPC technique [3], for a wide range of partonic initial conditions and opacities. We demonstrate that there exist transport solutions that yield a freezeout distribution with R _out < R _side for K _⊥ ? 1.5 GeV. These solutions correspond to continuous evaporation-like freezeout, where the emission duration is comparable to the source size. Naively this would mean R _out > R _side. Nevertheless, our sources exhibit R _out < R _side because they are narrower in the‘out’ than in the‘side’ direction and, in addition, a positive x _out-t correlation develops reducing R _out further.     

Top-quark <Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript>-spectra at CMS and flavor independence of <Emphasis Type="Italic">z</Emphasis>-scaling

We present new results of analysis of top-quark differential cross sections obtained by the CMS Collaboration in pp collisions in the framework of the z-scaling approach. The spectra are measured over a wide range of collision energy \(\sqrt s = 7,8,13TeV\) and transverse momentum p _T = 30?500 GeV/c of top-quark using leptonic and jet decay modes. Flavor independence of the scaling function ψ(z) is verified in the new kinematic range. The results of analysis of the top-quark spectra obtained at the LHC are compared with similar spectra measured in \(\overline p p\) collisions at the Tevatron energy \(\sqrt s = 1.96TeV\). A tendency to saturation of ψ(z) for the process at low z and a power-law behavior of ψ(z) at high z is observed. The measurements of high-p _T is observed. The measurements of highspectra of the top-quark production at highest LHC energy is of interest for verification of self-similarity of particle production, understanding flavor origin and search for new physics symmetries with top-quark probe.     

Investigation of <Emphasis Type="Italic">pn</Emphasis> correlations in <Superscript>4</Superscript>He<Emphasis Type="Italic">p</Emphasis> interactions at a momentum of 5 GeV/<Emphasis Type="Italic">c</Emphasis>

Proton-neutron correlations in ⁴Hep interactions are studied in an exclusive experiment by using a 2-m bubble chamber exposed to a 5-GeV/c beam of α particles (the kinetic energy of the protons in the nucleus rest frame is T _p = 620 MeV). Data on the production of pn pairs in 4π geometry for three channels, where it is possible to reconstruct the neutron momentum unambiguously, are used to determine the pn correlation function in ⁴Hep interactions. The experimental results are compared with the predictions of a modified Lednicky-Lyuboshitz model. The value obtained for the root-mean-square radius of the pn-emission region is R _pn = 2.1 ± 0.3 fm. The dependence of the correlation function on the modulus of the total momentum of the emitted nucleon pair and on the direction of the momentum transfer is studied. An indication that the emission of a pn pair proceeds predominantly through the production of a virtual deuteron is obtained.     

Limits on the scalar-leptoquark and scalar-gluon masses from the parameters <Emphasis Type="Italic">S</Emphasis>, <Emphasis Type="Italic">T</Emphasis>, and <Emphasis Type="Italic">U</Emphasis> in the minimal model based on four-color symmetry

The contributions to the parameters S, T, and U of radiative corrections from the doublets of scalar leptoquarks and scalar gluons are analyzed within the minimal model based on four-color symmetry of the Pati-Salam type. It is shown that current experimental data on the parameters S, T, and U admit the existence of relatively light scalar leptoquarks and scalar gluons (of mass lower than 1 TeV), the best fit to experimental data being attained at mass values not greater than 400 GeV. In particular, the existence of scalar leptoquarks of mass below 300 GeV is found to be compatible with data on the parameters S, T, and U at χ² < 3.1 (3.2) for m_H = 115 (300) GeV as against χ _SM ² = 3.5 (5.0) in the Standard Model. The mass of the lightest scalar gluon is then predicted to be less than 850 (720) GeV. It is emphasized that the aforementioned doublets of scalar leptoquarks and scalar gluons can play a significant role in processes involving a t quark at LHC.     

High transverse momentum identified particle spectra in 200 GeV collisions from STAR

Significant baryon over meson enhancement was measured at RHIC in the intermediate transverse momentum range of p _T =2?4 GeV/c (“baryon-meson puzzle”). With STAR detector we were able to extend particle identification towards higher transverse momentum offering further insights into the particle production mechanisms at intermediate to high p _T . In this paper we present results on charged pion, proton and anti-proton spectra and ratios at intermediate to high p _T exploiting the relativistic rise of the specific ionization energy loss measured in the STAR Time Projection Chamber. These measurements provide valuable information about the production mechanisms of particles at intermediate p _T in relativistic heavy ion collisions, e.g. coalescence/recombination versus jet fragmentation.     

Collective effects in the interactions of small systems on the RHIC

It has long been believed that small colliding systems (p+Au, d+Au, ³He + Au) are can only be used to study the collective effects of cold nuclear matter. However, recent studies on the RHIC and LHC accelerators indicate there are flowlike collective effects characterized by the high multiplicity of charged particles produced in these collisions. Whether these effects result from the hydrodynamic expansion of a dense and hot thermalized medium or are caused by the initial state remains an open question. This work reports the results from measuring flow characteristics in d + Au and ³He + Au collisions at an energy of 200 GeV in the PHENIX experiment on the RHIC collider. Attempts to describe the results theoretically are discussed.     

Successes and failures with hard probes

The two major pillars of searches for the Quark Gluon Plasma have been: J/Ψ suppression, proposed in 1986, and observed at both SPS fixed target energies and at RHIC; and, more recently, the suppression of π ⁰ with p _T ≥3 GeV/c by a factor ～5 in Au+Au central collisions, observed at RHIC in 2001, which had been predicted in advance as a consequence of Landau-Pomeranchuck-Migdal coherent (gluon) bremsstrahlung by the outgoing hard-scattered partons traversing the medium. However, new effects were discovered and the quality of the measurements greatly improved so that the clarity of the original explanations has become obscured. For instance: J/Ψ suppression is the same at SpS and RHIC. Is it the QGP, comovers, something else? QCD provides beautiful explanations of π ⁰ and direct γ measurements in p–p collisions but precision fits of the best theories of π ⁰ suppression barely agree with the Au+Au data. Better data are needed for 10<p _T <20 GeV/c, systematic errors are needed in theory calculations, the values of parameters of the medium such as \(\langle\hat{q}\rangle\) derived from precision fits are the subject of controversy. Baryons are much less suppressed than mesons, leading to an anomalous \(\bar{p}/\pi\) ratio for 2≤p _T ≤4.5 GeV/c, but beautiful theoretical explanations of the effect such as recombination do not work in detail. Heavy quarks seem to be suppressed the same as the light quarks, naively arguing against the bremsstrahlung explanation and suggesting exotic, possible transformational explanations. Di-hadron correlations reveal a trigger side ridge, possible Mach cones on the away side, vanishing and reappearance of away jets, both wide and normal jet correlations with and without apparent loss of energy. Can this all be explained consistently? Preliminary results of direct γ production in Au+Au appear to indicate a suppression approaching that of π ⁰ for p _T ≈20 GeV/c and a possibly thermal component for 1≤p _T ≤ 3 GeV/c. What are the implications? Are fragmentation photons a problem? Regeneration of direct γ by outgoing partons is predicted, leading to negative v ₂—is there evidence for or against it? STAR and PHENIX have different observations relevant to the existence of monojets in d+Au collisions. Will new data clarify the situation? When? etc. These and other issues will be discussed with a view to identify which conclusions are firm and where further progress towards real understanding is required.     

Quark see-saw,Higgs mass and vacuum stability

The issue of vacuum stability of standard model (SM) is discussed by embedding it within the TeV scale left–right quark see-saw model. The Higgs potential in this case has only two coupling parameters (λ₁, λ₂) and two mass parameters. There are only two physical neutral Higgs bosons (h,H), the lighter one being identified with the 126 GeV Higgs boson. We explore the range of values for (λ₁, λ₂) for which the vacuum is stable for all values of the Higgs fields till 10¹⁶ GeV. Combining with the further requirement that the scalar self-couplings remain perturbative till 10¹⁶ GeV, we find (i) an upper and lower limit on the second Higgs (H) mass to be within the range: 0.4 ≤ (M_H/v_R) ≤ 0.7, where v_R is the parity breaking scale and (ii) the masses of heavy vector-like top, bottom and τ partner fermions (P₃, N₃,E₃) have an upper bound ≤ v_R. These predictions can be tested at LHC and future higher energy colliders.     

Charmonium-state production in heavy-ion collisions

An overview of existing experimental data on the production of charmonium states (J/ψ and ψ′) at the superproton synchrotron (SPS, CERN) and the relativistic heavy ion collider (RHIC, Brookhaven National Laboratory, United States) is given. The production of J/ψ mesons shows an anomalous suppression discovered by the NA50 Collaboration (CERN) in collisions of lead nuclei at an energy of 158 GeV per nucleon and confirmed in the NA60 experiment (CERN) in collisions of indium nuclei at the same energy. The suppression of J/ψ production depends on interaction centrality and becomes anomalous at about 122 participant nucleons in PbPb collisions and at about 86 participant nucleons in InIn collisions. The experimental data in question are compared with the predictions of existing theoretical models. None of the models is able to simultaneously describe data on PbPb and InIn collisions. Data obtained in the PHENIX experiment at RHIC from measurements of J/ψ production in AuAu and CuCu collisions at an energy of 200 GeV (in the nucleon-nucleon c.m. frame) indicate that the suppression of J/ψ production at such energies approximately corresponds to the suppression of J/ψ production in PbPb collisions at SPS. Theoretical models that describe SPS data on PbPb collisions predict a stronger suppression of J/ψ production. Models that take into account J/ψ-meson regeneration better describe experimental data obtained at RHIC. Measurement of cross sections for charmonium and bottomonium production at the Large Hadron Collider (LHC, CERN) would make it possible to study the properties of matter and to explore the mechanism of quarkonium production at ultrahigh energy densities and temperature and high transverse momenta, as well as to investigate the effect of the regeneration and suppression of quarkonium production as the energy increases.     

Breit-Wigner parameters of the <Emphasis Type="Italic">S</Emphasis><Subscript>11</Subscript>(1535) nucleon resonance

The results of a partial-wave analysis of the angular distributions for the process γp → ηp over the energy range up to 2 GeV are presented. Reliable estimates of the Breit-Wigner parameters of the S₁₁(1535) resonance, as well as the energy dependence of the real and imaginary parts of the electric dipole amplitude E₀₊ and its phase, are derived from the energy dependence of the regression coefficient a₀(W).     

Elastische Elektronenstreuung an Titan bei 33 bis 58 MeV

Cross sections for elastic electron scattering were measured for titanium relative to carbon and beryllium at the Darmstadt electron linear accelerator. The energy and angle were varied from 33 to 58 MeV and from 33° to 165°. Phase shift calculations of the scattering cross section show that the rms-radiusR _m determined from the measurements is almost independent of specific assumptions about the charge distribution. For the natural isotopic mixture of titanium the measurements yieldR _m=(3.60±0.04) fm. This result agrees well with measurements of the (2p-1s)-transition energy in the muon-titanium-atom.     

Radiative parton energy loss and jet quenching in high-energy heavy-ion collisions

We study within the light-cone path integral approach [3] the effect of the induced gluon radiation on high-p _T hadrons in high-energy heavy-ion collisions. The induced gluon spectrum is represented in a new form which is convenient for numerical simulations. For the first time, computations are performed with a realistic parameterization of the dipole cross section. The results are in reasonable agreement with the suppression of high-p _T hadrons in Au + Au collisions at \(\sqrt s = 200\)GeV observed at RHIC.     

Structure of the spatial periphery of the <Superscript>11</Superscript>Li and <Superscript>11</Superscript>Be isobars

On the basis of the shell model with an extended basis, the structure of ⁹Li-⁹Be to ¹¹Li-¹¹Be nuclei is examined with allowance for the competition of ^jj coupling and Majorana exchange forces via considering the sequential addition of neutrons, and the respective wave functions are determined. A formalism for calculating the spectroscopic factor for a dineutron and for individual neutrons in nuclei whose wave functions incorporate the mixing of shell configurations is developed. The reactions ⁹Li(t, p)¹¹Li and ⁹Be(t, p)¹¹Be treated with allowance for the mechanisms of dineutron stripping and a sequential transfer of two neutrons are considered as an indicator of the proposed structure of lithium and berylliumisotopes. The parameters of the optical potentials, the wave functions for the bound states of transferred particles, and the interaction potentials corresponding to them are determined from a comparison of the theoretical angular distribution of protons from the reaction ⁹Be(t, p)¹¹Be with its experimental counterpart. It is shown that a dineutron periphery of size about 6.4 fm is present in the ¹¹Li nucleus and that a single-neutron periphery of size about 8 fm is present in the ¹¹Be nucleus.