Measurement of muon capture on the proton to 1% precision and determination of the pseudoscalar coupling gP

The MuCap experiment at the Paul Scherrer Institute has measured the rate Λ(S) of muon capture from the singlet state of the muonic hydrogen atom to a precision of 1%. A muon beam was stopped in a time projection chamber filled with 10-bar, ultrapure hydrogen gas. Cylindrical wire chambers and a segmented scintillator barrel detected electrons from muon decay. Λ(S) is determined from the difference between the μ(-) disappearance rate in hydrogen and the free muon decay rate. The result is based on the analysis of 1.2 × 10(10) μ(-) decays, from which we extract the capture rate Λ(S) = (714.9 ± 5.4(stat) ± 5.1(syst)) s(-1) and derive the proton's pseudoscalar coupling g(P)(q(0)(2) = -0.88 m(μ)(2)) = 8.06 ± 0.55.

Is the proton radius a player in the redefinition of the International System of Units?

It is now recognized that the International System of Units (SI units) will be redefined in terms of fundamental constants, even if the date when this will occur is still under debate. Actually, the best estimate of fundamental constant values is given by a least-squares adjustment, carried out under the auspices of the Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants. This adjustment provides a significant measure of the correctness and overall consistency of the basic theories and experimental methods of physics using the values of the constants obtained from widely differing experiments. The physical theories that underlie this adjustment are assumed to be valid, such as quantum electrodynamics (QED). Testing QED, one of the most precise theories is the aim of many accurate experiments. The calculations and the corresponding experiments can be carried out either on a boundless system, such as the electron magnetic moment anomaly, or on a bound system, such as atomic hydrogen. The value of fundamental constants can be deduced from the comparison of theory and experiment. For example, using QED calculations, the value of the fine structure constant given by the CODATA is mainly inferred from the measurement of the electron magnetic moment anomaly carried out by Gabrielse's group. (Hanneke et al. 2008 Phys. Rev. Lett. 100, 120801) The value of the Rydberg constant is known from two-photon spectroscopy of hydrogen combined with accurate theoretical quantities. The Rydberg constant, determined by the comparison of theory and experiment using atomic hydrogen, is known with a relative uncertainty of 6.6×10(-12). It is one of the most accurate fundamental constants to date. A careful analysis shows that knowledge of the electrical size of the proton is nowadays a limitation in this comparison. The aim of muonic hydrogen spectroscopy was to obtain an accurate value of the proton charge radius. However, the value deduced from this experiment contradicts other less accurate determinations. This problem is known as the proton radius puzzle. This new determination of the proton radius may affect the value of the Rydberg constant . This constant is related to many fundamental constants; in particular, links the two possible ways proposed for the redefinition of the kilogram, the Avogadro constant N(A) and the Planck constant h. However, the current relative uncertainty on the experimental determinations of N(A) or h is three orders of magnitude larger than the 'possible' shift of the Rydberg constant, which may be shown by the new value of the size of the proton radius determined from muonic hydrogen. The proton radius puzzle will not interfere in the redefinition of the kilogram. After a short introduction to the properties of the proton, we will describe the muonic hydrogen experiment. There is intense theoretical activity as a result of our observation. A brief summary of possible theoretical explanations at the date of writing of the paper will be given. The contribution of the proton radius puzzle to the redefinition of SI-based units will then be examined.

Measurement of the positive muon lifetime and determination of the Fermi constant to part-per-million precision

We report a measurement of the positive muon lifetime to a precision of 1.0 ppm; it is the most precise particle lifetime ever measured. The experiment used a time-structured, low-energy muon beam and a segmented plastic scintillator array to record more than 2×10(12) decays. Two different stopping target configurations were employed in independent data-taking periods. The combined results give τ(μ(+)) (MuLan)=2 196 980.3(2.2)  ps, more than 15 times as precise as any previous experiment. The muon lifetime gives the most precise value for the Fermi constant: G(F) (MuLan)=1.166 378 8(7)×10(-5)  GeV(-2) (0.6 ppm). It is also used to extract the μ(-)p singlet capture rate, which determines the proton's weak induced pseudoscalar coupling g(P).

Improved measurement of the positive-muon lifetime and determination of the Fermi constant

The mean life of the positive muon has been measured to a precision of 11 ppm using a low-energy, pulsed muon beam stopped in a ferromagnetic target, which was surrounded by a scintillator detector array. The result, tau(micro)=2.197 013(24) micros, is in excellent agreement with the previous world average. The new world average tau(micro)=2.197 019(21) micros determines the Fermi constant G(F)=1.166 371(6)x10(-5) GeV-2 (5 ppm). Additionally, the precision measurement of the positive-muon lifetime is needed to determine the nucleon pseudoscalar coupling g(P).

Measurement of the muon capture rate in hydrogen gas and determination of the proton's pseudoscalar coupling gP

The rate of nuclear muon capture by the proton has been measured using a new technique based on a time projection chamber operating in ultraclean, deuterium-depleted hydrogen gas, which is key to avoiding uncertainties from muonic molecule formation. The capture rate from the hyperfine singlet ground state of the microp atom was obtained from the difference between the micro(-) disappearance rate in hydrogen and the world average for the micro(+) decay rate, yielding Lambda(S)=725.0+/-17.4 s(-1), from which the induced pseudoscalar coupling of the nucleon, g(P)(q(2)=-0.88m(2)(micro))=7.3+/-1.1, is extracted.

Measurement of the kaonic hydrogen x-ray spectrum

The DEAR (DAPhiNE exotic atom research) experiment measured the energy of x rays emitted in the transitions to the ground state of kaonic hydrogen. The measured values for the shift epsilon and the width Gamma of the 1s state due to the K(-)p strong interaction are epsilon(1s)=-193 +/- 37 (stat) +/- 6 (syst) eV and Gamma(1s)=249 +/- 111 (stat) +/- 30 (syst) eV, the most precise values yet obtained. The pattern of the kaonic hydrogen K-series lines, K(alpha), K(beta), and K(gamma), was disentangled for the first time.

Measurement of the resonant d(mu)t molecular formation rate in solid HD

Measurements of muon-catalyzed dt fusion ( d(mu)t-->4He + n + mu(-)) in solid HD have been performed. The theory describing the energy dependent resonant molecular formation rate for the reaction (mu)t + HD-->[(d(mu)t)pee](*) is compared to experimental results in a pure solid HD target. Constraints on the rates are inferred through the use of a Monte Carlo model developed specifically for the experiment. From the time-of-flight analysis of fusion events in 16 and 37 microg x cm(-2) targets, an average formation rate consistent with 0.897+/-(0.046)(stat)+/-(0.166)(syst) times the theoretical prediction was obtained.

Resonant formation of d&mgr;t molecules in deuterium: An atomic beam measurement of muon catalyzed dt fusion

Resonant formation of d&mgr;t molecules in collisions of muonic tritium ( &mgr;t) on D2 was investigated using a beam of &mgr;t atoms, demonstrating a new direct approach in muon catalyzed fusion studies. Strong epithermal resonances in d&mgr;t formation were directly revealed for the first time. From the time-of-flight analysis of 2036+/-116 dt fusion events, a formation rate consistent with 0.73+/-(0.16)(meas)+/-(0.09)(model) times the theoretical prediction was obtained. For the largest peak at a resonance energy of 0.423+/-0.037 eV, this corresponds to a rate of (7.1+/-1.8)x10(9) s(-1), more than an order of magnitude larger than those at low energies.

Diverse modes of alternative splicing of human splicing factor SF1 deduced from the exon-intron structure of the gene

Several cDNAs encoding the essential human splicing facor (SF) 1 have been cloned. Comparison of the cDNA sequences suggested that the corresponding mRNAs are generated by alternative splicing from a common pre-mRNA. To confirm this assumption and to analyze possible modes used in the generation of these mRNAs, we have determined the structure of the gene encoding SF1. The gene extends over approximately 15kb and contains 14 exons. The exon/intron structure and sequences at the splice sites are highly conserved in the corresponding mouse gene. The human SF1 gene is located on chromosome 11 close to the gene encoding Menin, recently identified as the gene responsible for multiple endocrine neoplasia-type 1 (MEN1). The absence of a TATA box in the 5' flanking region of the SF1 transcription unit suggests that the SF1 gene represents a housekeeping gene. However, genomic sequence analysis revealed putative binding sites for regulatory transcription factors upstream of the 5' end of the cDNA. Analysis of the SF1 genomic and cDNA sequences predicts the use of duplicated 5' and 3' splice sites as well as exon skipping and intron inclusion to generate six SF1 mRNAs by alternative splicing events.

Mammalian splicing factor SF3a120 represents a new member of the SURP family of proteins and is homologous to the essential splicing factor PRP21p of Saccharomyces cerevisiae

Mammalian splicing factor SF3a consists of three subunits of 60, 66, and 120 kDa and functions early during pre-mRNA splicing by converting the U2 snRNP into its active form. A cDNA encoding the 120-kDa subunit of SF3a has been cloned. The SF3a120 gene was localized to human chromosome 22, and three mRNAs of 3.2, 3.8, and 5.7 kb are ubiquitously expressed. The N-terminal half of the deduced SF3a120 amino acid sequence contains a tandemly repeated motif (the SURP module) that has recently been identified in the essential splicing factor PRP21p of Saccharomyces cerevisiae, the Drosophila alternative splicing regulator suppressor-of-white-apricot, and four proteins from nematodes and mammals; the C-terminal half is organized into a proline-rich region and a ubiquitin-like domain. The spacing between the SURP modules and the protein's essential function in constitutive splicing identify SF3a120 as the mammalian homologue of yeast PRP21p. Binding studies with truncated derivatives of SF3a120 revealed that the SURP domains function in binding to SF3a60, whereas a region of 130 amino acids C-terminal to these domains is essential for contacts with SF3a66.

Splicing factor SF3a60 is the mammalian homologue of PRP9 of S.cerevisiae: the conserved zinc finger-like motif is functionally exchangeable in vivo

A cDNA encoding the 60 kDa subunit of mammalian splicing factor SF3a has been isolated. The deduced protein sequence reveals a 30% identity to the PRP9 splicing protein of the yeast S.cerevisiae. The highest homology is present in a zinc finger-like region in the C-terminal domain of both proteins. The PRP9 zinc finger-like motif has been replaced by the equivalent region of mammalian SF3a60. The chimeric protein rescues the temperature-sensitive phenotype of the prp9-1 mutant strain demonstrating that not only the structure but also the function of this domain has been conserved during evolution.

Site-directed mutagenesis of the P2 region of the Rous sarcoma virus gag gene: effects on Gag polyprotein processing

The Pr76Gag and Pr180Gag-Pol polyprotein precursors of Rous sarcoma virus contain a 22-amino-acid spacer peptide, called p2, located between the amino acid sequences of the mature Gag proteins MA and p10. This spacer peptide is present in stoichiometric amounts in the virion, albeit cleaved into two parts, but its function is unknown. The primary sequence of this peptide includes a region that is highly conserved among retroviruses, consisting of four prolines followed by tyrosine. We have investigated the role of p2, particularly the polyproline motif, in the virus life cycle by site-directed mutagenesis. Mutations in this region result in the intracellular accumulation of a truncated Gag precursor, due either to a block in the intracellular processing of the precursor or to the premature activation of the viral protease. Since in cells infected by Rous Sarcoma Virus there is no significant intracellular processing of the Gag polyprotein precursor, our data suggest that the p2 domain plays a role in controlling the activation of the protease. These mutations also result in a reduction in virus particle release, probably as a direct consequence of the aberrant precursor processing since a construct in with both p2 and the protease active site were mutated did not exhibit aberrant processing of the Gag polyproteins and formed particles with an efficiency similar to that of the wild type. This indicates that it is the viral protease that is responsible for the aberrant processing and suggests that the p2 region is not required for assembly. Although the virus genomic RNA packaged into virions produced by the p2 mutants is more susceptible to degradation, it appears that the p2 domain does not have a direct role in RNA packaging and protection.