High-mass dimuon resonances in Pb-Pb collisions at 5.5 TeV in CMS

The measurement of the charmonium (J/ψ, ψ′) and bottomonium (γ, γ′, γ″) resonances and Z ⁰ boson in nucleus-nucleus collisions provides crucial information on high density QCD matter. The observation of anomalous suppression of J/ψ at the CERN-SPS and RHIC is well established but the clarification of some important questions requires equivalent studies of the γ family, only possible at LHC energies. The Z ⁰ boson will be produced for the first time in heavy-ion collisions at the LHC and, since its dominant production channel is through q-q fusion, it is an excellent probe of the nuclear modification of quark distribution functions. This paper reports the capabilities of the CMS detector to study quarkonium and Z ⁰ production in Pb-Pb collisions at 5.5 TeV, through the dimuon decay channel.     

Resolved photon processes

We review high-energy scattering processes that are sensitive to the hadronic structure of the photon, describing theoretical predictions as well as recent experimental results. These processes include deep-inelastic electron-photon scattering ate ⁺ e ⁻ colliders; and the production of jets, heavy quarks and isolated photons in the collision of real photons ate ⁺ e ⁻ colliders, as well as in photon-photon collisions atep colliders. We also comment on minijet based calculations of totalγp andγγ cross-sections, and discuss the possibility that future lineare ⁺ e ⁻ colliders might produce very large photon fluxes due to the beamstrahlung phenomenon; in the most extreme cases, we predict more than one hadronicγγ event to occur at every bunch crossing.     

Heavy quarkonia production in p+p, d+Au and Cu+Cu collisions by PHENIX experiment at ?{sNN }\sqrt {s_{NN} } = 200 GeV

The PHENIX experiment has measured, J/ψ, ψ′ and γ productions for different collision systems in the forward rapidities 1.2 < | η | < 2.2 at ?{S_NN }\sqrt {S_{NN} } = 200 GeV. We have observed significant suppressions of J/ψ production in both Cu+Cu and Au+Au collisions relative to the yield in p+p system. The measurements of higher mass heavy quarkonia states (ψ′ and γ) will help us to constrain various quarkonium suppression models in heavy ion collisions. A first hint of ψ′ and γ productions in 200GeV p+p collisions has been observed at forward and backward rapidities at PHENIX.     

Bimodal fission in binary and ternary spontaneous fission of 252Cf

The hot bimodal fission of ²⁵²Cf is reexamined with new high-statistics data. We constructed a γ-γ-γ coincidence cube for binary fission and LCP-gated γ-γ matrix for ternary fission. By identifying the secondary fission fragments from their γ-ray transitions, we measured the yields for various fission splits. The normal neutron yield distribution is found to be Gaussian for Xe-Ru. However, the binary fission split of Ba-Mo is found to exhibit a bimodal neutron distribution with the “hot mode” corresponding to ≈3.1% of the total yield. In α ternary fission, the first measurements of yields for specific fission splits are presented. The Te-α-Ru and Xe-α-Mo neutron yields fit well with a single mode, but the Ba-α-Zr split shows evidence for an enhanced hot mode with an intensity of ≈13.8% of the normal mode. The text was submitted by the authors in English.     

Quantum chromodynamics studies at LEP2

Several studies have been made to the hadronic final states in e ⁺ e ⁻ collisions at LEP. Studies of the annihilation process at LEP2 have given rise to results on jet rate, event shape, heavy flavour production, inclusive momentum spectra, Bose-Einstein correlation and colour reconnection effects. Event shape studies have given rise to accurate determination of the strong coupling constant α _s using O (α _s ² ) with resummed leading and next-to-leading log calculation and also with power law corrections. Studies of 2-photon processes have yielded results on γγ cross-section, heavy flavour production, photon structure function and γ*γ* scattering.     

Modified fragmentation function in heavy ion collisions at RHIC via direct <Emphasis Type="Italic">γ</Emphasis>-jet measurements

The presented results are the first measurements at RHIC for direct γ-charged hadron azimuthal correlations in heavy ion collisions. We use these correlations to study the color charge density of the medium through the medium-induced modification of high-p _T parton fragmentation. Azimuthal correlations of direct photons at high transverse energy (8<p _T <16 GeV) with away-side charged hadrons of transverse momentum (3<p _T <6 GeV/c) have been measured over a broad range of centrality for Au+Au collisions and p+p collisions at  GeV in the STAR experiment. A transverse shower shape analysis in the STAR Barrel Electromagnetic Calorimeter Shower Maximum Detector is used to discriminate between the direct photons and photons from the decays of high p _T π ⁰. The per-trigger away-side yield of direct γ is smaller than from π ⁰ trigger at the same centrality class. Within the current uncertainty the I _CP of direct γ and π ⁰ are similar.     

Nuclear form factor,validity of the equivalent photon approximation and Coulomb corrections to muon pair production in photon–nucleus and nucleus–nucleus collisions

We study in detail the influence of the nuclear form factor both on the Born cross section and on the Coulomb corrections to the photo-production of muon pairs off heavy nuclei (γ Z→μ ⁺ μ ⁻ Z) and in heavy-ion collisions (ZZ→ZZ μ ⁺ μ ⁻). Our findings indicate a number of issues which have not been sufficiently described as yet in the literature: (i) the use of a realistic form factor, based on the Fermi charge distribution for the nucleus, is absolutely indispensable for reliable theoretical predictions; (ii) we checked quantitatively that the equivalent photon approximation has a very good accuracy for the discussed processes; and (iii) we present a leading logarithmic calculation of the Coulomb corrections which correspond to multi-photon exchange of the produced μ ^± with the nuclei. These corrections are found to be small (on the percent level). Our result justifies using the Born approximation for numerical simulations of the discussed process at the RHIC and LHC colliders. Finally, we calculate the total cross section for muon pair production at RHIC and LHC.     

On the high-statistics investigation of the <Emphasis Type="Italic">a</Emphasis><Subscript>0</Subscript>(980) resonance in the γγ → π<Superscript>0</Superscript>η reaction

High-statistics data on the γγ → π⁰η reaction will make it possible to conclude whether the K ⁺ K ⁻-loop rescattering mechanism, γγ → K ⁺ K ⁻ → a ₀(980) → π⁰η, is the main mechanism of the production of a ₀(980) isovector resonance. This mechanism provides a reasonable value of 20–30 nb at the maximum for the manifestation of a ₀(980) in the γγ → π⁰η cross section. It also gives rise to a noticeable narrowing of the a ₀(980) peak to its effective (observed) width ≈20–30 MeV in the γγ → π⁰η channel. The decay width averaged over the resonance mass distribution is 〈Γ_{α 0} → K ⁺ K ⁻ → γγ〉_πη ≈ 0.13 keV. The experimental confirmation of this scenario would be important evidence in favor of the q ² $ \bar q $ \bar q ² nature of light scalar mesons.     

Subthreshold and near threshold K+ meson photoproduction\newline on nuclei

The inclusive K⁺ meson production in photon–induced reactions in the near threshold and subthreshold energy regimes is analyzed with respect to the one–step (γN→K ⁺ Y, Y=Λ,Σ) incoherent production processes on the basis of an appropriate new folding model, which takes properly into account the struck target nucleon removal energy and internal momentum distribution (nucleon spectral function), extracted from recent quasielastic electron scattering experiments and from many–body calculations with realistic models of the NN interaction. Simple parametrizations for the total and differential cross sections of the K⁺ production in photon–nucleon collisions are presented. Comparison of the model calculations of the K⁺ differential cross sections for the reaction γ+C¹² in the threshold region with the existing experimental data is given, that displays the contributions to the K⁺ production at considered incident energies coming from the use of the single–particle part as well as high momentum and high removal energy part of the nucleon spectral function. Detailed predictions for the K⁺ total and differential cross sections from γH², γC¹² and γPb²⁰⁸ reactions at subthreshold and near threshold energies are provided. The influence of the uncertainties in the elementary K⁺ production cross sections on the K⁺ yield is explored. Received: 12 April 1999 / Revised version: 11 September 1999     

Constraints on the time scale of nuclear breakup from thermal hard-photon emission

Measured hard-photon multiplicities from second-chance nucleon-nucleon collisions are used in combination with a kinetic thermal model to estimate the breakup times of excited nuclear systems produced in nucleus-nucleus reactions at intermediate energies. The obtained nuclear breakup time for the ¹²⁹Xe + ^natSn reaction at 50 A MeV is Δτ ≈ 100-300 fm/c for all reaction centralities. The lifetime of the radiating sources produced in seven other different heavy-ion reactions studied by the TAPS experiment is consistent with Δτ ≈ 100 fm/c, such relatively long thermal photon emission times do not seemingly support the interpretation of nuclear breakup as due to a fast spinodal process for the heavy nuclear systems studied.     

Comparative emission characteristics of a negative hydrogen ion source with a reflex discharge with and without Cs

A theoretical and experimental investigation is made to determine how cesium in the volume and at the surfaces of an ion source influences its emission characteristics. It is shown that under the real conditions of an ion source, cesium in the volume makes an appreciable influence to the kinetic processes but barely affects the H⁻ ion current extracted from the source. However, cesium at the surface of the source increases the H⁻ ion current severalfold even when the H-H⁻ conversion efficiency is low (γ≈10⁻³). The theoretical conclusions show good agreement with the experimental data obtained in the present study. Zh. Tekh. Fiz. 69, 102–109 (April 1999)     

Production of medical radioisotopes with high specific activity in?photonuclear reactions with γ-beams of high intensity and?large brilliance

We study the production of radioisotopes for nuclear medicine in (γ,xn+yp) photonuclear reactions or (γ,γ′) photoexcitation reactions with high-flux [(10¹³–10¹⁵)γ/s], small diameter ∼(100 μm)² and small bandwidth (ΔE/E≈10⁻³–10⁻⁴) γ beams produced by Compton back-scattering of laser light from relativistic brilliant electron beams. We compare them to (ion,xn+yp) reactions with (ion = p,d,α) from particle accelerators like cyclotrons and (n,γ) or (n,f) reactions from nuclear reactors. For photonuclear reactions with a narrow γ-beam the energy deposition in the target can be managed by using a stack of thin target foils or wires, hence avoiding direct stopping of the Compton and pair electrons (positrons). However, for ions with a strong atomic stopping only a fraction of less than 10⁻² leads to nuclear reactions resulting in a target heating, which is at least 10⁵ times larger per produced radioactive ion and often limits the achievable activity. In photonuclear reactions the well defined initial excitation energy of the compound nucleus leads to a small number of reaction channels and enables new combinations of target isotope and final radioisotope. The narrow bandwidth γ excitation may make use of the fine structure of the Pygmy Dipole Resonance (PDR) or fluctuations in γ-width leading to increased cross sections. Within a rather short period compared to the isotopic half-life, a target area of the order of (100 μm)² can be highly transmuted, resulting in a very high specific activity. (γ,γ′) isomer production via specially selected γ cascades allows to produce high specific activity in multiple excitations, where no back-pumping of the isomer to the ground state occurs. We discuss in detail many specific radioisotopes for diagnostics and therapy applications. Photonuclear reactions with γ-beams allow to produce certain radioisotopes, e.g. ⁴⁷Sc, ⁴⁴Ti, ⁶⁷Cu, ¹⁰³Pd, ^117m Sn, ¹⁶⁹Er, ^195m Pt or ²²⁵Ac, with higher specific activity and/or more economically than with classical methods. This will open the door for completely new clinical applications of radioisotopes. For example ^195m Pt could be used to verify the patient’s response to chemotherapy with platinum compounds before a complete treatment is performed. Also innovative isotopes like ⁴⁷Sc, ⁶⁷Cu and ²²⁵Ac could be produced for the first time in sufficient quantities for large-scale application in targeted radionuclide therapy.     

Laser Ionization and Penning Trap Mass Spectrometry – A Fruitful Combination for Isomer Separation and High-precision Mass Measurements

We have demonstrated for the first time that element-selective laser ionization in combination with ultra-high resolution mass spectrometry can be used to prepare isomerically pure ion ensembles. Together with β–γ coincidence studies this method allowed a determination of the low-energy structure and the unambiguous identification of triple β-decaying isomerism in ⁷⁰Cu. By selective resonant ionization and measurement of the masses of these three states using ISOLTRAP at ISOLDE/CERN with a relative uncertainty of δm/m ≈ 5 ⋅ 10⁻⁸ a clear state-to-mass assignment was possible which resolved the assignment puzzle in ⁷⁰Cu.     

The equivalent photon approximation for coherent processes at colliders

We consider coherent electromagnetic processes for colliders with short bunches, in particular the coherent bremsstrahlung (CBS). CBS is the radiation of one bunch particles in the collective field of the oncoming bunch. It can be a potential tool for optimizing collisions and for measuring beam parameters. A new simple and transparent method to calculate CBS is presented based on the equivalent photon approximation for this collective field. The results are applied to theφ-factoryDAΦNE. For this collider about 5·10¹⁴ d E _γ/E _γ photons per second are expected in the photon energyE _γ range from the visible light up to 25 eV.     

Anomalies, symmetries and strangeness content of the proton

The matrix elements of the operators of strange quark fieldssГs where Г is 1 orγμγ ⁵ between a proton state is calculated. The sigma term is found to be ≈ 41 MeV and theSU(3) singlet axial matrix element is found to be ≈ 0.22, both in agreement with experiment. The sigma term is found using the trace anomaly, while the determination of the axial vector current matrix element is from QCD sum rules. These correspond to (ie.943-1)≈ 0.12 and for the axial current Δs ≈ −0.12, respectively. The role of the anomalies in maintaining flavor symmetry in the presence of substantial differences in quark masses is pointed out. This suggests that there is no need to invoke an intrinsic strange quark component in the proton wave function.     

ESR studies of sepiolites

The electron spin resonance (ESR) spectra for each beige and brown sepiolites from Polatli-Ankara, Turkey, a clay mineral of Mg₈Si₁₂O₃₀(OH)₄(OH₂)₄(H₂O)₈, at 77 K have their own single characteristic peak atg ≈ 1.99, which begins to anneal from 437 and 536°C, respectively. The sextet lines of Mn²⁺ were also observed in addition to two peaks (g = 4.4 and 5.0) associated with Fe³⁺. A high-quality sepiolite called “meerschaum” from Sepetci-Eskisehir, Turkey, has two characteristic peaks atg ≈ 1.99, which begins to anneal from 419°C, and atg ≈ 2.03. ESR intensities of the signal atg ≈ 1.99 for beige, brown sepiolites and meerschaum are enhanced by γ-ray irradiation to give equivalent dosesD _E = 11.8 ± 3.4, 4.7 ± 2.3 and 4.6 ± 1.2 kGy, respectively. The ESR ages obtained by assessing the annual dose rate from the content of²³⁸U,²³²Th and⁴⁰K determined by γ-ray spectroscopy are 1.8 ± 0.8, 0.7 ± 0.5 and 0.9 ± 0.2 Ma, respectively, falling into the Pliocene Epoch in geological time scale in agreement with the stratigraphy.     

Search for 100 TeV gamma-rays from SN 1987 A

Summary We present the results of a search for very high energy γ-rays (E _γ≈100 TeV) from SN 1987A between August 1987 and May 1988. No counting rate excess from the source is observed. The larger enhancement is seen in January 1988, but is statistically not significant (1.4 s.d.). The derived upper limits to the γ-ray and proton luminosities of the SN areL _γ≤1.9·10⁴⁰ erg/s andL _p≤1.2·10⁴² erg/s. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Investigation of the influence of incomplete fusion on complete fusion of <Superscript>12</Superscript>C-induced reactions at ≈ 4–7.2 MeV/nucleon

In this paper, we present the results of our investigation of reaction dynamics leading to incomplete fusion of heavy ions at moderate excitation energies, especially the influence of incomplete fusion on complete fusion of ¹²C -induced reactions at specific energies ≈ 4–7.2M eV/nucleon. Excitation functions of various reaction products populated via complete and/or incomplete fusions of a ¹²C projectile with ⁹³Nb, ⁵⁹Co and ⁵²Cr targets were measured at several specific energies ≈ 4–7.2 MeV/nucleon, using a recoil catcher technique, followed by off-line γ-ray spectrometry. The measured excitation functions were compared with theoretical values obtained using the PACE4 statistical model code. For representative non-α-emitting channels in the ¹²C + ⁹³Nb system, the experimentally measured excitation functions were, in general, found to be in good agreement with the theoretical predictions. However, for α-emitting channels in the ¹²C + ⁹³Nb, ¹²C + ⁵⁹Co, and ¹²C + ⁵²Cr systems, the measured excitation functions were higher than the predictions of the theoretical model code, which may be credited to incomplete fusion reactions at these energies. An attempt was made to estimate the incomplete fusion fraction for the present systems, which revealed that the fraction was sensitive to the projectile energy and mass asymmetry of the entrance channel.