Small Mass Difference Research Articles

Reactor-based neutrino oscillation experiments

The status of neutrino oscillation searches employing nuclear reactors as sources is reviewed. This technique, a direct continuation of the experiments that proved the existence of neutrinos, is today an essential tool in investigating the indications of oscillations found in studying neutrinos produced in the sun and in the earth’s atmosphere. The low energy of the reactor νe makes them ideal for exploring oscillations with small mass differences and relatively large mixing angles. In the last several years the determination of the reactor antineutrino flux and spectrum has reached a high degree of accuracy. Hence measurements of these quantities at a given distance L can be readily compared with the expectation at L=0, thus testing the disappearance of νe. Two recent experiments, CHOOZ and PALO VERDE, with baselines of about 1 km and sensitive to the neutrino mass differences associated with the atmospheric neutrino anomaly have collected data and published results recently. An ambitious project with a baseline of more than 100 km, KAMLAND, has now began to take data. This last reactor experiment will have a sensitivity sufficient to explore part of the oscillation phase space relevant to solar neutrino scenarios. It is the only envisioned experiment with a terrestrial source of neutrinos capable of addressing the solar neutrino puzzle.

Layer-resolved depth profiling at single crystal surfaces

The stopping power and straggling of ions in channeling and blocking directions depend on multiple scattering and impact parameter dependent effects. Therefore, the extraction of high-resolution depth profiles of single crystals based on simulation of the energy spectra obtained from medium-energy ion scattering (MEIS) is not straightforward. An alternative is to analyze the MEIS angular (blocking) scattering yields from several angular configurations. We have applied this technique to obtain layer-resolved depth profiles in a dilute substitutional alloy, Fe-9 wt% W (1 0 0), and in an alloy with a small mass difference between the base metals, Fe-15 wt% Cr (1 0 0). As a result, layer-resolved depth profiling combined with conventional MEIS structural analysis allowed a detailed study of surface segregation phenomena in these systems.

Heat Conduction in a One-Dimensional Gas of Elastically Colliding Particles of Unequal Masses

We study the nonequilibrium state of heat conduction in a one-dimensional system of hard point particles of unequal masses interacting through elastic collisions. A BBGKY-type formulation is presented and some exact results are obtained from it. Extensive numerical simulations for the two-mass problem indicate that, even for arbitrarily small mass differences, a nontrivial steady state is obtained. This state exhibits local thermal equilibrium and has a temperature profile as predicted by kinetic theory. The temperature jumps typically seen in such studies are shown to be finite-size effects. The thermal conductivity appears to have a very slow divergence with system size, different from that seen in most other systems.

Isotope analysis of environmental substances by a new laser-spectroscopic method utilizing different pathlengths

We demonstrate a new method of absorption spectroscopy for precise isotopic composition measurements of environmental molecules such as 13 CH 4/ 12 CH 4 (∼1/100) and 15 N 14 N 16 O/ 14 N 15 N 16 O/ 14 N 14 N 16 O (∼0.4/0.4/100). We combined wavelength-stabilized near-infrared semiconductor lasers with a multipass absorption cell in which laser beams passed different times to compensate the large abundance difference and balance the signal levels. Then we could choose strong absorption lines whose absorption coefficients have almost the same temperature dependencies so as to minimize the effect of the changes in gas temperature on the ratio measurements. Normalization of the second-harmonic absorption signal by the dc signal restored a good proportionality between the signal amplitude and the absorbance. An intercomparison between absorption spectroscopy and mass spectrometry for pure natural-abundance methane samples proved that the precision of the present method is ±3×10 −4 (±0.3‰) and the relative accuracy is about the same. The demonstrated high-precision indicates that the laser-spectroscopic method can complement mass spectrometry, especially for isotopic molecular species of a small mass difference, and is potentially a useful means for precise analysis of the biogeochemical cycles of the substances.

DNA sequencing and genotyping by transcriptional synthesis of chain-terminated RNA ladders and MALDI-TOF mass spectrometry.

Sets of RNA ladders can be synthesized by transcription of a bacteriophage-encoded RNA polymerase using 3'-deoxynucleotides as chain terminators. These ladders can be used for sequencing of DNA. Using a nicked form of phage SP6 RNA polymerase in this study substantially enhanced yields of transcriptional sequencing ladders. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) of chain-terminated RNA ladders allowed DNA sequence determination of up to 56 nt. It is also demonstrated that A-->G and C-->T variations in heterozygous and homozygous samples can be unambiguously identified by the mass spectrometric analysis. As a step towards single-tube sequencing reactions, alpha-thiotriphosphate nucleotide analogs were used to overcome problems caused by chain terminator-independent, premature termination and by the small mass difference between natural pyrimidine nucleotides.

Cosmological nucleosynthesis and active-sterile neutrino oscillations with small mass differences: the resonant case

We have provided a numerical study of the influence of the resonant active-sterile neutrino oscillations between electron and sterile neutrinos, on the primordial production of helium-4. The evolution of the neutrino ensembles was followed selfconsistently with the evolution of the nucleons, using exact kinetic equations for the neutrino density matrix and the nucleon number densities in momentum space, from the time of neutrino decoupling till the freeze-out of nucleons at 0.3 MeV. The exact kinetic approach enabled us to study precisely the neutrino depletion, spectrum distortion and neutrino mixing generated asymmetry due to oscillations at each momentum mode, and to prove that their effect on nucleosynthesis is considerable. We have calculated the dependence of the primordially produced helium-4 Y_p on the oscillation parameters for the full range of mixing parameters of the model of oscillations with small mass differences less than 10^{-7} eV^2. We have obtained iso-helium contours on the oscillation parameters plane. Cosmological constraints on oscillation parameters, more precise than the existing ones, were extracted, due to the exact kinetic approach and the proper account for the neutrino spectrum distortion and the oscillations generated asymmetry.

Search for a scalar top almost degenerate with the lightest neutralino in e +e − collisions at [formula omitted] up to 202 GeV

Data collected at centre-of-mass energies from 189 GeV to 202 GeV by the ALEPH detector at LEP corresponding to an integrated luminosity of 411 pb −1, are analysed in a search for the scalar top in the decay channels t ̃ → c/uχ for small mass differences between the stop and the lightest neutralino. No evidence for deviations from the Standard Model expectation is found and a lower limit of 59 GeV/ c 2 is set for the stop mass, independent of the stop to neutralino mass difference and of the stop lifetime.

OSCILLATION EFFECTS ON NEUTRINOS FROM THE EARLY PHASE OF A NEARBY SUPERNOVA

Recent observations of atmospheric and solar neutrinos strongly support the phenomenon of neutrino oscillations — a manifestation of a nonzero and nondegenerate mass spectrum. Neutrinos emitted during stellar core collapse leading to a supernova are of the electron neutrino type at source — as for solar and reactor (anti-)neutrinos — and provide another useful tool in the search for flavor oscillations. Their propagation to an earth-bound detector involves length scales that can uniquely probe very small neutrino mass differences hitherto unobservable. Although the number of neutrinos emitted during the collapse phase is much smaller than that emitted in the post-bounce epoch (in which all flavors of neutrinos are emitted), a nearby supernova event may nevertheless register a substantial number of detections from the collapse phase at SuperKamiokande (SK) and the Sudbury Neutrino Observatory (SNO). The measurement of the fluence of these neutrinos at SNO and the distortion of the spectrum detected at SK can yield valuable information about neutrino mass difference and mixing which are illustrated here in terms of two- and three-flavor oscillation models. In particular, we find that R SNO , the ratio of the calorimetric detection of the neutrino fluence via the neutral current channel to the total energy integrated fluence observed via the charged current channel at SNO, is a sensitive probe for oscillations. We also find that αn, the ratio of the nth central moments of the distributions seen at SK and SNO (charged current), can be a useful tool (especially for n=3) to look for neutrino oscillations.

Search for charginos with a small mass difference to the lightest supersymmetric particle at [formula omitted] = 189 GeV

A search for charginos nearly mass-degenerate with the lightest supersymmetric particle is performed using the 176 pb −1 of data collected at 189 GeV in 1998 with the L3 detector. Mass differences between the chargino and the lightest supersymmetric particle below 4 GeV are considered. The presence of a high transverse momentum photon is required to single out the signal from the photon-photon interaction background. No evidence for charginos is found and upper limits on the cross section for chargino pair production are set. For the first time at LEP2, in the case of heavy scalar leptons, chargino mass limits are obtained for any χ ̃ ± 1− χ ̃ 0 1 mass difference.

Leptogenesis with heavy Majorana neutrinos reexamined

The mass term for Majorana neutrinos explicitly violates lepton number. Several authors have used this fact to create a lepton asymmetry in the universe by considering CP violating effects in the one loop self-energy correction for the decaying heavy Majorana neutrino. We compare and comment on the different approaches used to calculate the lepton asymmetry including those using an effective Hamiltonian and resummed propagators. We also recalculate the asymmetry in the small mass difference limit.

Characterization and production of amylovorin L471, a bacteriocin purified from Lactobacillus amylovorus DCE 471 by a novel three-step method.

The strongly hydrophobic bacteriocin amylovorin L471 from Lactobacillus amylovorus DCE 471 was isolated and purified to homogeneity from complex culture broth by a novel, rapid and simple three-step protocol including (i) ammonium sulphate precipitation, (ii) chloroform/methanol extraction/precipitation and (iii) reversed-phase HPLC, the only chromatographic step involved. The molecular mass of the peptide was determined to be 4876.9 Da by electrospray mass spectrometric analysis. N-terminal amino acid sequencing identified 35 amino acid residues as being identical to the N-terminal sequence of lactobin A, a bacteriocin from another L. amylovorus strain. These non-identical strains produce bacteriocins that display small differences in molecular mass and inhibitory spectrum. The amino acid sequence of amylovorin L471 shared significant homology with lactacin X, one of the two bactericidal peptides produced by Lactobacillus johnsonii VPI11088. A purified amylovorin L471 preparation permitted confirmation of the inhibitory spectrum previously established with a crude extract. It displayed a bactericidal mode of action on lactobacilli after an extremely rapid adsorption to the target cells. Two Listeria spp. were only weakly sensitive. Amylovorin L471 appears to be produced constitutively. Ethanol not only stimulated specific bacteriocin production but also prevented adsorption of the bacteriocin molecules to the producer cells upon prolonged fermentation. The latter result supports the hypothesis that the apparent inactivation of bacteriocin observed during the stationary phase of batch fermentations is due to adsorption.

Lepton-flavor violation in the left-handed slepton production at future lepton colliders

The Super-Kamiokande atmospheric neutrino data suggest existence of the large lepton-flavor violating (LFV) interaction in the higher energy scale. If the minimal supersymmetric standard model is extended to have right-handed neutrinos, the left-handed sleptons in the second and third generations are expected to have the LFV masses in the minimal supergravity scenario. In this article we study the LFV signals in the left-handed slepton production at $\mu^+\mu^-$ colliders and $\e^+\e^-$ linear colliders (LC's), $\mu^+\mu^-(\e^+\e^-)\to\tau\mu +4jets + \E$ and $\mu^+\mu^-(\e^+\e^-)\to\tau\mu l+ 2jets+ \E$. The main background comes from decay of a tau lepton into a muon in the lepton-flavor conserving slepton pair production. They are significantly reduced by the energy and the impact parameter cuts for the muon. At $\mu^+\mu^-$ colliders (LC's) it may be possible to reach the mixing angle $\sin 2\theta_{\tilde{\nu}} \gsim 0.1(0.5)$ and the mass difference $\Delta m_{\tilde{\nu}}\gsim 0.1(0.4)$ GeV for the sneutrinos in the second and third generations at the statistical significance of $3\sigma$. Such small mass difference and a mixing angle may be induced radiatively even if the Yukawa coupling constant for the tau neutrino is of the order of 0.1.

Relativistic treatment of hypernuclear decay

We compute for the first time the decay width of $\ensuremath{\Lambda}$-hypernuclei in a relativistic mean-field approximation to the Walecka model. Due to the small mass difference between the $\ensuremath{\Lambda}$-hyperon and its decay products---a nucleon and a pion---the mesonic component of the decay is strongly Pauli blocked in the nuclear medium. Thus, the in-medium decay becomes dominated by the nonmesonic, or two-body, component of the decay. For this mode, the $\ensuremath{\Lambda}$-hyperon decays into a nucleon and a spacelike nuclear excitation. In this work we concentrate exclusively on the pionlike modes. By relying on the analytic structure of the nucleon and pion propagators, we express the nonmesonic component of the decay in terms of the spin-longitudinal response function. This response has been constrained from precise quasielastic $(\stackrel{\ensuremath{\rightarrow}}{p},\stackrel{\ensuremath{\rightarrow}}{n})$ measurements done at LAMPF. We compute the spin-longitudinal response in a relativistic random-phase-approximation model that reproduces accurately the quasielastic data. By doing so, we obtain hypernuclear decay widths that are considerably smaller---by factors of 2 or 3---relative to existing nonrelativistic calculations.

Inelastic Capture Ratio at Rest and Low-Energy Cross Sections of -p Scattering in the SU6 Quark Model

The inelastic capture ratio at rest, TR, and low-energy total cross sections of E-p scat­ tering are examined in the recent SU6 quark model developed for a unified description of the N Nand Y N interactions. The effect of the small mass difference between E-p and EOn channels is treated approximately and the Coulomb attraction in the E-p channel is properly incorporated in the particle basis. The model FSS predicts TR = 0.461, which is very close to the experimental value. The low-energy cross sections for the charge exchange reaction E-p -+ EOn are improved, although those for E-p -+ An are still too large. Pre­ dictions by another model RGM-H are less favorable, even for the very scarce experimental data presently available. The effect of the Coulomb force is estimated, and the effective range approximation is examined in the low-energy S-matrix. The important roles of the one-pion exchange tensor force and the antisymmetric spin-orbit force in the EN-AN coupling are discussed.

Higgs scalar-pseudoscalar mixing in the minimal supersymmetric Standard Model

In the minimal supersymmetric Standard Model, the heaviest CP-even Higgs boson H and the CP-odd Higgs scalar A are predicted to be almost degenerate in mass at the tree level, for the wide kinematic range M A >2 M Z and tan β≥2. However, if large soft-CP-violating Yukawa interactions involving scalar quarks of the third generation are present in the theory, then the CP invariance of the Higgs potential can be maximally broken beyond the Born approximation, and the high degree of mass degeneracy between H and A may be lifted through a sizeable HA mixing. After taking the CP-odd tadpole renormalization of the A boson into account, we find that the small mass difference M H − M A , which is about 1% of the A-boson mass at the tree level, can be substantially enhanced to the 25% level at the one-loop order. We also find that the loop-induced mixing between the lightest CP-even Higgs boson h and the A boson may be of comparable size to M h . We briefly discuss the main phenomenological implications of the predicted hA and HA mixings for the general Higgs-boson mass spectrum and for CP-violating observables at collider and lower energies.

Cosmological nucleosynthesis and active-sterile neutrino oscillations with small mass differences: The nonresonant case

We study the nonresonant oscillations between left-handed electron neutrinos nu_e and nonthermalized sterile neutrinos nu_s in the early Universe plasma. The case when nu_s do not thermalize till 2 MeV and the oscillations become effective after nu_e decoupling is discussed. The evolution of neutrino ensembles was described numerically by integrating the kinetic equations for the neutrino density matrix in momentum space. We have provided a detail numerical study of the influence of the nonequilibrium nu_e - nu_s oscillations on the primordial production of He-4. The exact kinetic approach enabled us to calculate the effects of neutrino population depletion, the distortion of the neutrino energy spectrum and the generation of neutrino-antineutrino asymmetry on the kinetics of neutron-to-proton transitions during the primordial nucleosynthesis epoch and correspondingly on the cosmological He-4 production. It was shown that the neutrino population depletion and spectrum distortion play an important role. The asymmetry effect, in case the lepton asymmetry is accepted initially equal to the baryon one, is proved to be negligible for the discussed range of delta m^2. Constant helium contours in delta m^2 - theta plain were obtained for small mass differences delta m^2 < 10^{-7} eV^2. More precise cosmological constraints on mixing parameters were obtained.

Distinguishing Small Molecular Mass Differences of Proteins by Mass Spectrometry

Electrospray ionization–Fourier transform ion cyclotron resonance (ESI–FTICR) mass spectrometryallows for high-resolution, accurate mass analysisof multiply charged ions of proteins. In the workdescribed here, the ability of ESI–FTICR to distinguish small differences in molecular mass is evaluated. Ubiquitin was used as an internal mass calibration standard to measure the molecular mass of cytochromec, myoglobin, and several carbonic anhydrase isoforms. Mass calibration was based onthe tallest isotopic peak of each ubiquitin chargestate. Ubiquitin performed well as an internal standard because its charge states covered the appropriate mass range, interference was minimal, and thetallest peak was easily identified. The peak massesof cytochrome c (12.5 kDa) and myoglobin (17 kDa) were measured to an accuracy of about 0.02 Da (<2ppm). However, errors of 1.0 Da were observedfor some individual determinations because of the difficulty in identifying the tallest peak. When the technique was applied to bovine carbonic anhydrase II, even combining data from several charge statesdid not yield an unequivocal assignment of thetallest peak, resulting in a mass assignment of 29,023.7 or 29,024.7. Similarly, measurements of two isoforms with a mass difference of 1 Da, human carbonic anhydrase I, pI6.0 and 6.6, yielded overlapping values for the mass of the tallest peak. However, these two isoforms were clearly distinguished by (a) identification of the tallest peak using a measurement of average mass as a guide and (b) comparison of the isotopic peak intensity patterns.

Defects and energy accommodation in epitaxial sputter deposited Mo/W superlattices studied by molecular dynamics

We report here the results of a Molecular Dynamics—Embedded Atom Method—investigation of the pathways generating point defects in Mo/W superlattices during bombardment with energetic (50 to 200 eV) Ar and Kr neutrals. Energy accommodation coefficients are computed for the different structures and are found to be roughly independent of the incident energy, and substantially higher for structures with Mo on top. Several different types of defects are shown, and two general processes generating those are discussed. Trapping of the incoming noble gas was observed for the case of Kr impinging on structures with Mo as the top monolayer; this is interpreted as an effect of the small mass difference between the Mo and the Kr atoms. An increase in atomic mass of the gas translates into a more disparate behaviour of the studied structures. The energy exchange with the surface layer dictates the behaviour of the superlattice; this is accentuated when bombarding with the heavier gas, Kr.

Sequencing RNA by a combination of exonuclease digestion and uridine specific chemical cleavage using MALDI-TOF.

The determination of DNA sequences by partial exonuclease digestion followed by Matrix-Assisted Laser Desorption Time of Flight Mass Spectrometry (MALDI-TOF) is a well established method. When the same procedure is applied to RNA, difficulties arise due to the small (1 Da) mass difference between the nucleotides U and C, which makes unambiguous assignment difficult using a MALDI-TOF instrument. Here we report our experiences with sequence specific endonucleases and chemical methods followed by MALDI-TOF to resolve these sequence ambiguities. We have found chemical methods superior to endonucleases both in terms of correct specificity and extent of sequence coverage. This methodology can be used in combination with exonuclease digestion to rapidly assign RNA sequences.

Strange magnetic moment and isospin symmetry breaking

The small mass difference $m_n-m_p=1.3 MeV$ between the proton and neutron leads to an excess of $n=\pi^- p$ over $p=\pi^+ n$ fluctuations which can be calculated by using a light-cone meson-baryon fluctuation model of intrinsic quark-antiquark pairs of the nucleon sea. The Gottfried sum rule violation may partially be explained by isospin symmetry breaking between the proton and neutron and the same effect introduces correction terms to the anomalous magnetic moments and the anomalous weak magnetic moments of the proton and neutron. We also evaluated the strange magnetic moment of the nucleon from the lowest strangeness $K \Lambda$ fluctuation and found a non-trivial influence due to isospin symmetry breaking in the experimental measurements of the strange magnetic moment of the nucleon.