Properties of the proposed τ charged lepton

The anomalous eμ and 2-prong μx events produced in e⁺e⁻ annihilation are used to determine the properties of the proposed τ charged lepton. We find the τ mass is 1.90 ± 0.10 GeV/c²; the mass of the associated neutrino, ν_τ, is less than 0.6 GeV/c² with 95% confidence; V - A coupling is favored over V + A coupling for the τ − ν_τ current; and the leptonic branching ratios are 0.186 ± 0.010 ± 0.028 from the eμ events and 0.175 ± 0.027 ± 0.030 from the μx events where the first error is statistical and the second is systematic.     

Absorption cross sections for antiprotons on carbon, aluminum and copper at 485 and 597 MeV/c

Absorption cross sections for antiprotons on C, Al and Cu have been measured by a transmission method at 485 and 597 MeV/c. Nuclei are shown to be black to antiprotons at these momenta. Using the available data up to 280 GeV/c, the momentum dependence of the absorption cross sections is investigated. Empirically, a simple expression σ_abs = γ(p/p₀)^−β, with β and γ constants and p₀ = 1 GeV/c, gives a good account of the data in a remarkably wide momentum range, 0.5–280 GeV/c.     

Double charge exchange and one-pion production in π He collisions at 1.7 GeV/c

We present the results of a study of the exclusive reactions π^{+ 4}He → π⁻ 4p (double charge exchange, DCE) and π^{+ 4}He → π⁺ π⁻ 3pn (one-pion production). The experiment was performed with the Oxford/RHEL helium bubble chamber irradiated with a 1.7 GeV/c π⁺ beam. A general review of DCE models is presented and experimental results at other energies are discussed in the light of these models. None of the existing theoretical models is compatible with out data at 1.7 GeV/c. We propose a new mechanism for DCE, involving three nucleons, which reproduces the integrated as well as the differential cross sections. Some consequences for other models are also discussed.     

Search for narrow NNπ resonances in exclusive pp→ppππ measurements

Narrow structures in the range of a few MeV have been searched for in ppπ⁺ and ppπ⁻ invariant mass spectra (M_ppπ⁺ and M_ppπ⁻) obtained from exclusive measurements of the pp→ppπ⁺π⁻ reaction at T_p=725,750 and 775 MeV using the PROMICE/WASA detector at CELSIUS. The selected reaction is particularly well suited for the search for dibaryon resonances decoupled from NN and/or NΔ. In the mass range 2020 MeV/c²<m_dibaryon<2085 MeV/c² no narrow structures could be identified on the 3σ level of statistical significance neither in M_ppπ⁻ nor in M_ppπ⁺ giving an upper limit (95% C.L.) for dibaryon production in this reaction of σ<20 nb.     

Evidence of kaon nuclear and Coulomb potential effects on soft K production from nuclei

The ratio of forward K⁺ production on copper, silver and gold targets to that on carbon has been measured at proton beam energies between 1.5 and 2.3 GeV as a function of the kaon momentum p_K using the ANKE spectrometer at COSY-Jülich. The strong suppression in the ratios observed for p_K<200–250 MeV/c may be ascribed to a combination of Coulomb and nuclear repulsion in the K⁺A system. This opens a new way to investigate the interaction of K⁺-mesons in the nuclear medium. Our data are consistent with a K⁺A nuclear potential of V_K⁰≈20 MeV at low kaon momenta and normal nuclear density. Given the sensitivity of the data to the kaon potential, the current experimental precision might allow one to determine V_K⁰ to better than 3 MeV.     

Experimental observation of a copious yield of electrons with small transverse momenta in pp collisions at high energies

Inclusive electron and positron emission have been observed for θ_cm = 30° and S = 2800 GeV² at the CERN Intersecting Storage Rings (ISR). Over the transverse momentum interval 0.2 GeV/c < p_T < 1.5 GeV/c, electrons and positrons, which are equal in number within the experimental accuracies, appear to grow with respect to other particles (pions) approximately like 1/p_T. We are unable to explain their number and p_T-dependence in terms of “conventional” mechanisms.     

Determination of the K(1800) spin parity

A spherical harmonic moment analysis of the reactions K⁻p → K⁻π⁺n and K⁺p → K⁺π⁻Δ⁺⁺ at 13 GeV/c demonstrates the existence of a broad K^* state with mass in the vicinity of 1800 MeV and spin parity 3⁻.     

Study of impulse model in reaction Kd → pΛπ+ neutrals

In an analysis of 1114 events of the type K⁻d → pΛ⁰π⁻+ neutrals with ‘spectator’ proton momentum in excess of 0.11 GeV/c we have found that the number of protons having a momentum in the range 0.25–1.2 GeV/c exceeds that predicted on the basis of the impulse model using the Hulthén distribution by 3 : 1. Final-state interactions in which the proton does not form resonances with the other final-state particles are mainly held responsible for this effect.     

Single and double dissociation in πp collisions at 11 and 16 GeV/c

The reactions π⁻p → 2π⁻π⁺p, π⁻p → 2π⁻π⁺π^op and π⁻p → 2π⁻2π⁺n are analysed at 11 and 16 GeV/c using longitudinal phase space (LPS) plots. The weighted LPS distributions for π⁻p → 2π⁻π⁺p is dominated by two well separated structures corresponding to single diffraction dissociation of the pion, π⁻p → (2π⁻π⁺)p, and of the proton, π⁻p → π⁻(π⁻π⁺p). The former is more abundant than the latter, and both are approximately constant with energy. In contrast, processes of type π⁻p → (2π)(πp) decrease with increasing energy.

In the five-body reactions the weighted LPS distribution reveals especially at 16 GeV/c a maximum for single dissociation of the proton into 3πp, namely π⁻p → π⁻(π⁻π⁺π^op); this process is likely to be diffractive. The neutron channel has a corresponding maximum displaced toward a multiperipheral configuration π⁻p → π⁻(π⁻2π⁺)n. Another strong maximum corresponds to the pion dissociation π⁻p → (2π⁻π⁺π^o)p. This is interpreted to be an ω-exchange process because no analogous structure occurs in π⁻p → (2π⁻2π⁺)n. Finally, a broad structure reveals double dissociation of both incident particles; it occurs in the two channels π⁻p → (2π⁻π⁺)(π^op) and π⁻p → (2π⁻π⁺)(π⁺n), being stronger in the latter. Further analysis of this process in terms of isospin exchange suggests that it is partially diffractive. Factorization is also discussed.

An appendix gives general aspects of the LPS analysis for the asymptotic study of n-body collisions at very high energy.     

Self energies of the pion and the Δ isobar from the He(e,e′π)H reaction

In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the ³He(e,e′π⁺)³H reaction have been measured in the excitation region of the Δ resonance to determine the longitudinal (L), transverse (T), and the LT interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and Δ-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the π⁺ mass of MeV/c² in the neutron-rich nuclear medium at a density of fm⁻³. Our data are consistent with the Δ self energy determined from measurements of π⁰ photoproduction from ⁴He and heavier nuclei.     

Large-angle pD scattering, the form factor and the NN content of the deuteron

Single scattering and n-exchange are shown to account for all large-q² pD data (k_L 1.5 GeV/c) our to q² 8.5 (GeV/c)². From the extracted body-form factor we predict the deuteron charge form factor which agrees with the recent SLAC data. The agreement invalidates previous estimates of the NN^* content of the neuteron.     

Measurement and analysis of the infrared reflection spectrum of semiconducting SnS

The infrared reflection spectrum of p-type SnS single crystals was measured between 2 and 25μ. The observed dispersion was ascribed to the combined effects of lattice vibrations and free holes. An analysis gave for the index of refraction n₀ = 3.6 ± 0.1, the dielectric constant = 19.5 ± 2, and the effective charge on the atoms e^* = 0.7e₀.

The effective mass of holes is isotropic in the plane perpendicular to the c-axis, with m^* = 0.20m₀; the effective mass parallel to the c-axis is much larger: m^* m₀.     

Hadron and nuclear physics with inverse compton gamma-rays at SPring-8

A new facility for GeV γ-ray beams in an energy range of 1.5–2.4 GeV has been constructed for developing hadron physics at SPring-8. GeV γ-rays are produced via the inverse Compton scattering process, and are used to study the sub-nucleonic structure of nucleon via the productions of φ and K⁺ mesons. The recent experimental results to observe φ and K⁺ mesons are discussed to look for the physics in relation to strangeness quarks in baryon resonances. A new beam line for MeV γ-rays at SPring-8 to give new possibilities for the studies of the weak-strong coupling effects in nuclear mediums through the observation of parity violation is also discussed.     

Study of K*(892)0 and φ (1020) meson production in proton-proton and Pb-Pb collisions at $\sqrt{{s}_{NN}}$=2.76 TeV

In this paper, we describe a study of charged particle yield as a function of p_T for K^*(892)⁰ and φ (1020) mesons in proton-proton (pp) and Pb-Pb collisions at $\sqrt{{s}_{NN}}$=2.76 TeV in the central rapidity region of ∣y∣<0.5, in a p_T range of 0<p_T <15 GeV c⁻¹ in pp collisions and in a p_T range of 0<p_T<20 GeV c⁻¹ in Pb-Pb collisions. We also investigated a very important ratio, the nuclear modification factor, to study the effects of the medium in the most central region, i.e. 0%-5% centrality. For data simulation, we used the EPOS-LHC and EPOS-1.99 models. To check the validity of these models’ simulations, we compared the data obtained from these Monte Carlo simulation programs with ALICE experimental data for $\sqrt{{s}_{NN}}$=2.76 TeV. It was concluded that the models’ predictions for the φ-meson in pp and for the most central Pb-Pb collisions disagreed with the ALICE data, and that the difference increased with p_T. This may be connected with the essential role of collective parton behaviors which could not have been taken into account by the models. For K^*0 mesons, both programs gave almost the same predictions, and with p_T in the interval p_T>3 GeV c⁻¹, the predictions were very close to the experimental data. Both models gave higher predictions for the soft p_T interval and lower predictions for the hard interactions. The values of the R_AA distributions were lower than unity and both models were very close to the ALICE data. It is very interesting that the models were not able to describe the p_T distributions, but they gave good predictions for their ratios. This may possibly be due to parton collective behaviors. We observed some additional suppression of K^*0 at low values of p_T with respect to φ-mesons, which may be related to the role of the masses of the particles in soft interactions. The rising trend for R_AA in the region from p_T =10 GeV c⁻¹ to 20 GeV c⁻¹ observed by the ALICE experiment was absent for the φ-mesons.     

The ΔNγ electromagnetic transition form factors GM(q) and GE(q)

A parameter-free, nonperturbative calculation of the ΔNγ electromagnetic transition amplitudes GM*(q2), GE*(q2), and the resonant multipole ratio REM(q2)≡E1+3/2(q2)/M1+3/2(q2) is performed in terms of the well-known nucleon isovector Sachs form factor GMV. Our methods are fully relativistic with conservation of the electromagnetic current guaranteed. We find that GM*(q2) decreases more rapidly than the nucleon dipole form factor when −q21 GeV2/c2 and that REM(q2) remains small even for very high four-momentum transfer implying that the perturbative QCD prediction REM(q2)→1 is purely asymptotic and is valid only for extremely high |q2|.     

Hyperon-Nucleon bound states and electroproduction of strangeness on He

The A(e, e′K⁺)YX reaction has been investigated in Hall C at Jefferson Lab. Data were taken for Q² ≈ 0.35 and 0.5 GeV² at a beam energy of 3.245 GeV for ³He, ⁴He. The missing mass spectra are fitted with Monte Carlo simulations including Λ, Σ⁰, Σ⁻ hyperon production. Models for quasifree production are compared to the data, excess yields close to threshold are attributed to FSI. Evidence for Λ-hypernuclear bound states is seen for ^3,4He targets.     

Simultaneous measurements of the p+d→(A=3)+π reactions

A stack of annular detectors made of high-purity germanium was used to measure simultaneously pd → ³H π⁺ and pd → ³He π⁰ differential cross sections at beam momenta of 750 MeV/c, 800 MeV/c, and 850 MeV/c over a large angular range. The extracted total cross sections for the pd → ³He π⁰ reactions bridge a gap between near threshold data and those in the resonance region. The ratio of the cross sections for the two reaction channels taken at the same η=p^cm_π/m_π yields 2.11±0.08 indicating that a deviation from isospin symmetry is very small.     

Renormalization group invariant matrix elements of ΔS=2 and ΔI=3/2 four-fermion operators without quark masses

We introduce a new parameterization of four-fermion operator matrix elements which does not involve quark masses and thus allows a reduction of systematic uncertainties. In order to simplify the matching between lattice and continuum renormalization schemes, we express our results in terms of renormalization group invariant B-parameters which are renormalization-scheme and scale independent. As an application of our proposal, matrix elements of ΔI=3/2 and SUSY ΔS=2 operators have been computed. The calculations have been performed using the tree-level improved Clover lattice action at two different values of the strong coupling constant (β=6/g²=6.0 and 6.2), in the quenched approximation. Renormalization constants and mixing coefficients of lattice operators have been obtained non-perturbatively. Using lowest order χPT, we also obtain ππ|O₇|K^NDR_I=2=(0.11±0.02) GeV⁴ and ππ|O₈|K^NDR_I=2=(0.51±0.05) GeV⁴ at μ=2 GeV.     

Strangeness production in proton collisions close to threshold at COSY-TOF

The K-meson production in elementary proton induced reactions is studied exclusively at the external COSY beam using the time-of-flight spectrometer TOF. The complete measurement of all primary and decay particle tracks allows the extraction of total and differential cross sections as well as Dalitz plots and invariant mass spectra of the subsystems for the channels pp → K⁺ Δp, K⁰Σ⁺p, K⁺Σ⁰p and K⁺Σ⁺n. For all channels the full phase space is covered from the reaction threshold up to the COSY-limit of about 3.5 GeV/c. Especially the analysis of the Dalitz plots of the channel pp→K⁺Δp show a strong influence of N₋^* resonances.     

Trying to catch the elusive A and more. Reactions initiated by b-quarks and the Higgs sector of the MSSM

We study the cross sections for the production of neneural, intermediate mass Higgs boson in the process pp→ tq′φ, pp→tW⁻φ and pp→bZ⁰φ in the Minimal Supersymmetric Standard Model (φ = H⁰, h⁰ and A⁰) at Supercollider energies. The additional heavy particles (t, W, Z) in the final state can be used for tagging purposes, increasing the signal to background ratio. These reactions are dominated by bg and bg fusion. Their relevance for Higgs particle searches is discussed taking into account the expected efficiencies and purities for b-tagging. We find that, for tan β = 30, the cross sections for pp → bZ⁰φ are larger than 14 pb, over the whole intemediate range of M_A⁰, for A⁰ and at least one of the other two Higgses. Therefore this reaction is an excellent candidate for the discovery of one or more MSSM Higgs particles.