Evidence for the production of thermal muon pairs with masses above 1 GeV/c 2 in 158 A GeV indium-indium collisions

The yield of muon pairs in the invariant mass region 1<M<2.5 GeV/c ² produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of prompt muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are independent of mass and significantly lower than those found at masses below 1 GeV/c ², rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/c ² to be of largely partonic origin, when radial flow has not yet built up.     

The diagrammatic structure of deep inelastic scattering in asymptotically free theories

We investigate the structure of the leading diagrams in QCD. The diagrams of a simple scalar gluon model form the basis of a generalisation to the more complicated vector gluon theory. It is found that a certain set of a generalised ladder diagrams gives the standard QCD result for the non-singlet structure function in the limit x → 1. A simple brems-strahlung model is presented which forms a useful link between QCD field theory and more intuitive parton formulations. The value of a diagrammatic treatment is that it provides an accessible approach to the discussion of other processes such as the Drell-Yan mechanism for heavy muon pair production.     

Large perturbative corrections to the Drell-Yan process in QCD

The total cross section $\text{d}\text{σ}\text{d}\text{Q}{}^2$ for the production of a muon pair of invariant mass Q²via the Drell-Yan mechanism and the Feynman x_F differential cross section $\text{d}{}^2\text{σ}\text{d}\text{Q}{}^2\text{d}\text{x}{}_{\mathrm{<mtext>F</mtext>}}$ are calculated in QCD retaining all terms up to order α_s(Q². The calculations are performed using dimensional regularisation of the intermediary infrared and collinear singularities, but we present our results in a form independent of such details. The corrections to both these cross sections coming from radiative corrections to the lowest-order $\text{q}\text{q}$ annihilation diagram are found to be large at present values of Q² and S when the cross section is expressed in terms of parton densities derived from leptonproduction, for all Drell-Yan processes of practical interest. Numerical calculations are presented which show, for any reasonable parametrisation of the parton densities, that the neglect of higher-order terms in α_s(Q²) is not justifiable. The quark-gluon diagrams on the other hand give small corrections in this order and are only important for PP scattering.     

Chiral Invariant Phase Space model

The CHIPS (CHiral Invariant Phase Space) model is used for approximation of the Drell-Yan process in hadron-nucleon interactions. It is proved that the Drell-Yan cross-section at low p _T ² is a function of only x₁ and x₂ and does not evolve as a function of the squared mass of the lepton pair M². The CHIPS model predicts that the Drell-Yan structure functions evolve as a function of p _T ² . This p _T ² evolution can be measured in experiments at RHIC and LHC.     

μSR Investigation of magnetic phases in R1−xSrxCoO3 oxides (R=Pr, Nd)

We report results of our muon spin relaxation measurements in the series of polycrystalline compounds Pr_1−xSr_xCoO₃ (x=0.3, 0.4 and 0.5) and Nd_1−xSr_xCoO₃ (x=0.3 and 0.5). For the Pr-based samples our data clearly indicate the existence of two magnetic transitions, as also inferred from macroscopic measurements. While the high temperature transition is typical for cobaltites (∼200 K), the low temperature one is unusual. In our experiments it occurs below about 120 K and it manifests itself as a change in the slope of the temperature variation of the muon spin depolarization rate λ(T). For the Nd-based samples we found an increase of the muon spin depolarization rate below 45 K, temperature at which the sample is ferrimagnetic, when the Nd sublattice coupled antiparallel to the Co lattice. No phase separation could be evidenced in our samples by μSR experiments.     

Drell-Yan production of massive muon pairs

Cross-sections for Drell-Yan production of massive muon pairs by incident π^±, K^±, p and $\text{p}$ on an isoscalar, nuclear target are calculated.     

Production of γ by 194 GeV/c negative pions on tungsten

On the basis of 2000 ?, ?′ and ?″ events obtained in π^?-W interactions at 194 GeV/c we extract a value for the cross section times the branching ratio of B_μμσ = (0.96±0.04±0.17) pb nucleon^?1. The $\text{(?′ + ?″)}\text{?}$ ratio is found to be 0.53 ± 0.19. The observed differential x_F and P_T distributions are compared with those of the Drell-Yan continuum, and the cross section and the x_F distribution are compared with theoretical predictions.     

Approach to scaling in deep inelastic scattering and lepton pair production

The covariant parton model is used to investigate the approach to scalling in deep inelastic lepton scattering and in lepton pair production by hadron beams. The subasymptotic effects in these two reactions are controlled by rather different features. Of particular importance for lepton pair production is how far the partons are off shell before they annihilate, while for deep inelastic scattering what matters is rather the mass of the parton after it has absorbed the virtual photon. There are uncertainties because of problems with gauge invariance, but it seems that subasymptotic effects in lepton pair production may be large even at SPS/Fermilab energies.Transverse momentum and x distributions of partons in hadrons are discussed in a very simple model. In particular it is found that, while the model is constructed such that F₂(x) ～ (1 ? x)³ as x → 1, for values of x up to 0.85 F₂(x) is better approximated by (1 ? x)⁴.     

Electron spin polarization in consecutive spin-correlated radical Pairs: Application to short-lived and long-lived precursors in type 1 photosynthetic reaction centres

An analytical treatment of the spin dynamics in sequential photoinduced correlated coupled radical pairs is presented and applied to the spectra of the states P⁺A ₁ ^? and P⁺F _x ^? in type 1 photo-synthetic reaction centres. Expressions for the spin polarized spectra are derived for the specific limiting cases of a very short-lived and very long-lived primary radical pair which correspond to the situation found in heliobacteria and photosystem I (PSI), respectively. The inhomogeneous line-broadening due to the unresolved hyperfine couplings is taken explicitly into account. It is shown that the density matrix of the secondary pair ρ₂ can be written as the sum of two terms corresponding to (i) the part which is independent of the spin dynamics in the precursor, (ii) the additional spin polarization which is generated during the lifetime of the precursor and transferred to the secondary pair. The latter term contains two contributions which arise from the difference of the Zeeman interactions of the radicals in the primary pair and from the inhomogeneous line broadening. The predicted polarization patterns are compared to those established for chemically induced dynamic electron polarization (CIDEP) when uncoupled radicals are generated from a radical pair precursor. The expressions are then used to simulate the experimental spectra of the consecutive pairs P⁺A ₁ ^? and P⁺F _x ^? in PSI using parameters derived entirely from independent experimental data. Excellent agreement with the experimental results is obtained. The spectra of P⁺F _x ^? in heliobacteria at X- and K-band are also simulated and it is shown that the observed polarization patterns can be reproduced assuming direct electron transfer from A₀ to F_x with a time constant ofτ = 600 ps.     

What can Drell-Yan data tell us about the EMC effect and nucleon parton distributions?

We show that knowledge of thex ₁ andx ₂ dependence of the ratio of Drell-Yan cross-sections measured on heavy nuclei and deuterium can give us information about the origin of the EMC effect. Conversely, we show that an understanding of the EMC effect, together with (non-ratio) Drell-Yan data, can provide us with a way of discriminating between candidates for nucleon parton distributions.     

A possible determination of the quark radiation length in cold nuclear matter

We calculate the differential Drell-Yan production cross section in proton-nucleus collisions by including both next-to-leading order perturbative effects and effects of the nuclear medium. We demonstrate that dilepton production in fixed target experiments is an excellent tool to study initial-state parton energy loss in large nuclei and to accurately determine the stopping power of cold nuclear matter. We provide theoretical predictions for the attenuation of the Drell-Yan cross section at large values of Feynman xF and show that for low proton beam energies experimental measurements at Fermilab?s E906 can clearly distinguish between nuclear shadowing and energy loss effects. If confirmed by data, our results may help determine the quark radiation length in cold nuclear matter X₀∼¹⁰−13 m.     

Low-mass Drell-Yan pairs at RHIC and LHC

We summarize the calculation of Drell-Yan transverse-momentum distributions using QCD perturbation theory. In particular, the transversemomentum spectrum of low-mass Drell-Yan pairs is calculated with all-order resummation. We demonstrate that the transverse-momentum distribution of low-mass Drell-Yan pairs is an advantageous source of constraints on the gluon distribution and its nuclear dependence. We argue that low-mass Drell-Yan pairs in the forward region provide a good and clean probe of small-x gluons at RHIC and LHC.     

Search for massive photon pair production at the CERN intersecting storage rings

A search for massive photon pair production at \(\sqrt s = 63 GeV\) has been carried out on the data sample previously employed for the electron pair production study. Positive evidence is reported form _γγ>6GeV, with a production cross-section similar to Drell-Yan electron pairs. The ratio γγ/π⁰π⁰ was measured to be ～10^?3 for ap _T of each γ or γ or π⁰ above 3 GeV/c.     

Thermal conductivity of single crystals of the Ca1 − x Y x F2 + x solid solution

The thermal conductivity of single crystals of the solid solution of yttrium fluoride in calcium fluoride Ca_{1 ? x} Y _x F_{2 + x} with the fluorite structure (x ≤ 0.20) and the Ca_0.27Y_0.73F_2.73 phase with the tisonite structure has been studied by the absolute steady-state longitudinal heat flow method in the temperature range 50–300 K. It has been established that the thermal conductivity drops sharply with increasing yttrium trifluoride concentration, especially in the low-temperature region.     

Search for the stannous ion in a chloride/fluoride matrix: cases of Ba1−x Sn x Cl1+y F1−y and of Ba2SnCl6

With the exception of anhydrous SnCl₂, in divalent tin fluorides and chlorides, tin(II) is always covalent bonded, i.e. its valence orbitals are hybridized and the tin lone pair is located in one of the hybrid orbitals. This lone pair is highly stereoactive and generates a large efg, resulting in a large quadrupole splitting. A doubly disordered Ba_1?x Sn _x Cl_1+y F_1?y solid solution has been prepared and found to contain either ionic tin(II) (Sn²⁺ ions) or a mixture of ionic and covalent tin(II), depending on x, y and the method of preparation. The ionic tin(II) spectrum in Ba_1?x Sn _x Cl_1+y F_1?y gives a Mössbauer single line that is broadened by the lattice efg, like in SnCl₂. Now, Sn²⁺ has been found to be present in an undistorted octahedral coordination in a newly isolated compound, Ba₂SnCl₆. It should be the first example of Sn²⁺ that is fully ionic and has a perfectly spherical lone pair.     

Angular distributions of muon pairs produced by negative pions on deuterium and tungsten

We present the angular distributions of high-mass muon pairs produced in a high-statistics experiment by 140 and 194 GeV/c π^? beams impinging on a tungsten target, and by 286 GeV/c π^? beam on deuterium and tungsten targets. We find no evidence for a center-of-mass energy dependence or a nuclear dependence of the angular distribution parameters. The two parameters λ and μ are found to be essentially independent of any kinematical variable. In contrast, the parameterv increase with the dimuon transverse momentumP _T , at variance with recent perturbative QCD predictions. Our statistics at largex ₁ are insufficient to substantiate the highertwist prediction.     

Production of high-energy muons in gamma showers

We have calculated the number of high-energy muons in gamma showers generated by photoproduction and by muon pair creation. The prompt muons have flatter energy spectrum than the muons, which come from photoproduction and contribute significant fraction of the total muon rates for E_μ ? 1 TeV. The total rate of high-energy muons in gamma showers is, however, very low.     

Thermal dependence of impurity induced fluorescence in the system RbMgxMn1-xF3

The absorption spectrum of RbMnF₃ and the excitation spectra of the system RbMg_xMn_1-xF₃ at 10 K as well as the fluorescence spectra and lifetimes of Mn²⁺ in the systems RbMg_xMn_1-xF₃ and KMg_xMn_1-xF₃ in the region 10–300 K were measured. The lifetime and fluorescence temperature dependence suggest that the origin of the fluorescence occurs at Mn²⁺ sites slightly perturbed by impurity ions and that a non-radiative energy transfer mechanism is responsible for the observed thermal quenching. By using different Mn²⁺ concentrations in the above systems the dependence of the energy transfer on the Mn²⁺ concentration is shown. Finally, a preliminary observation on laser stimulated Mn²⁺ luminescence in the system RbMg_xMn_1-xF₃ is reported.     

Muon-spin-rotation study of muon polarization losses in plastic scintillators and quartz

Muon polarization losses in plastic scintillators of two types and in fused quartz have been studied by the μSR method. The muon and muonium spin precession spectra have been measured on the μSR setup placed at the output of the muon channel of the Gatchina synchrocyclotron. It has been shown that a significant fraction of stopped muons participate in the formation of the muonium. As a result, these muons lose their polarization completely. The magnitude of muon depolarization depends considerably on the type of plastic. It has been found that the muon spin precession frequency in fused quartz in an external magnetic field (F _{Q, μ} = 0.116 ± 0.002 MHz) is shifted with respect to that in plastic scintillators (F _{1, μ} = 0.101 ± 0.005 MHz and F _{2, μ} = 0.101 ± 0.002 MHz).     

Lepton pair production in two-dimensional QCD

Large q² lepton pair production is calculated in leading order in 1/N_c in two-dimensional quantum chromodynamics (QCD). From this confinement model the (Drell-Yan) parton model result is recovered.