The proton is the primary building block of the visible Universe, but many of its properties-such as its charge radius and its anomalous magnetic moment-are not well understood. The root-mean-square charge radius, r(p), has been determined with an accuracy of 2 per cent (at best) by electron-proton scattering experiments. The present most accurate value of r(p) (with an uncertainty of 1 per cent) is given by the CODATA compilation of physical constants. This value is based mainly on precision spectroscopy of atomic hydrogen and calculations of bound-state quantum electrodynamics (QED; refs 8, 9). The accuracy of r(p) as deduced from electron-proton scattering limits the testing of bound-state QED in atomic hydrogen as well as the determination of the Rydberg constant (currently the most accurately measured fundamental physical constant). An attractive means to improve the accuracy in the measurement of r(p) is provided by muonic hydrogen (a proton orbited by a negative muon); its much smaller Bohr radius compared to ordinary atomic hydrogen causes enhancement of effects related to the finite size of the proton. In particular, the Lamb shift (the energy difference between the 2S(1/2) and 2P(1/2) states) is affected by as much as 2 per cent. Here we use pulsed laser spectroscopy to measure a muonic Lamb shift of 49,881.88(76) GHz. On the basis of present calculations of fine and hyperfine splittings and QED terms, we find r(p) = 0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value of 0.8768(69) fm. Our result implies that either the Rydberg constant has to be shifted by -110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.

Proton Still Too Small Despite a proton's tiny size, it is possible to measure its radius based on its charge or magnetization distributions. Traditional measurements of proton radius were based on the scattering between protons and electrons. Recently, a precision measurement of a line in the spectrum of muonium—an atom consisting of a proton and a muon, instead of an electron—revealed a radius inconsistent with that deduced from scattering studies. Antognini et al. (p. 417 ; see the Perspective by Margolis ) examined a different spectral line of muonium, with results less dependent on theoretical analyses, yet still inconsistent with the scattering result; in fact, the discrepancy increased.

The deuteron is the simplest compound nucleus, composed of one proton and one neutron. Deuteron properties such as the root-mean-square charge radius rd and the polarizability serve as important benchmarks for understanding the nuclear forces and structure. Muonic deuterium μd is the exotic atom formed by a deuteron and a negative muon μ(-). We measured three 2S-2P transitions in μd and obtain r(d) = 2.12562(78) fm, which is 2.7 times more accurate but 7.5σ smaller than the CODATA-2010 value r(d) = 2.1424(21) fm. The μd value is also 3.5σ smaller than the r(d) value from electronic deuterium spectroscopy. The smaller r(d), when combined with the electronic isotope shift, yields a "small" proton radius r(p), similar to the one from muonic hydrogen, amplifying the proton radius puzzle.

The DEAR (DANE 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 and the width ÿ of the 1s state due to the K ÿ p strong interaction are 1s ÿ193 37 (stat) 6 (syst) eV and ÿ 1s 249 111 (stat) 30 (syst) eV, the most precise values yet obtained. The pattern of the kaonic hydrogen K-series lines, K , K , and K , was disentangled for the first time. DOI: 10.1103/PhysRevLett.94.212302 PACS numbers: 13.75.Jz, 25.80.Nv, 36.10.Gv Over 40 years, chiral symmetry breaking has been recognized as the essential aspect of nuclear low-energy phenomena. The outline of how the breaking plays a vital role is well known, yet its detailed dynamics is uncertain. The existence of the eight pseudoscalar mesons (; K; ) is believed to arise from spontaneous symmetry breaking of the flavor global symmetry represented by the group SU3 L SU3 R , which generates the mesons as Nambu-Goldstone bosons, leaving the vacuum only SU(3) symmetric [1]. Furthermore, the mass spectrum of these mesons reflects the explicit breaking of this symmetry [2]. In the quark model, the squares of the meson masses are proportional to the small current quark masses with the multiplicative factors of the chiral quark condensate in vacuum. The large mass difference between the mesons and the current quarks then suggests that the condensate is playing a significant role in the structure of the mesons [3].A similar situation is expected to occur in the structure of baryons and to be manifested in the baryon-pseudoscalar meson interaction [4]. In this case, the corresponding relation is that the baryon sigma terms are proportional to the current quark masses with the factors of the chiral quark condensate for the baryons [3]. The sigma terms thus serve as the measure of the significance of the condensate in the structure of the baryons. Especially of interest here is how the SU(3) flavor symmetry is realized in this aspect of the nucleon structure, but more specifically, how high is the strangeness content of the nucleon. The resolution of these issues depends quite sensitively on the value of the kaonnucleon (KN) sigma terms [5]. As the basic symmetry of QCD is SU(3), the KN sigma terms play the central role in various nuclear phenomena, such as strangeness production in heavy-ion collision and chiral restoration in nucleon matter, a topic of astrophysical interest [6].The KN sigma terms are closely related to the lowenergy KN and antikaon-nucleon (KN) scattering amplitudes [7], but the value of the KN sigma terms continues to remain with a large uncertainty [6,7] in spite of the recent efforts in lattice [8] and chiral perturbation [9] calculations, where the information on the KN and KN scattering lengths is vital. In this work, we report an accurate measurement of the ground-state x-ray transitions in kaonic hydrogen atoms. The shift and width of the atomic ground state is known to provide the most accurate information of the K ÿ -proton scattering l...

We present a detailed report of the method, setup, analysis and results of a precision measurement of the positive muon lifetime. The experiment was conducted at the Paul Scherrer Institute using a time-structured, nearly 100%-polarized, surface muon beam and a segmented, fast-timing, plastic scintillator array. The measurement employed two target arrangements; a magnetized ferromagnetic target with a ∼4 kG internal magnetic field and a crystal quartz target in a 130 G external magnetic field. Approximately 1.6 × 10 12 positrons were accumulated and together the data yield a muon lifetime of τµ(MuLan) = 2 196 980.3(2.2) ps (1.0 ppm), thirty times more precise than previous generations of lifetime experiments. The lifetime measurement yields the most accurate value of the Fermi constant GF (MuLan) = 1.166 378 7(6) × 10 −5 GeV −2 (0.5 ppm). It also enables new precision studies of weak interactions via lifetime measurements of muonic atoms.

The MuCap experiment at the Paul Scherrer Institute has measured the rate ΛS 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. ΛS 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 ± 5.4stat ± 5.1syst) s −1 and derive the proton's pseudoscalar coupling g P (q 2 0 = −0.88 m 2 µ ) = 8.06 ± 0.55.

International audienc

We report a measurement of the positive muon lifetime to a precision of 1.0 parts per million (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) = 2196980.3(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: GF (MuLan) = 1.1663788(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 .A measurement of the positive muon lifetime, τ µ + , to high precision determines the Fermi constant, G F , according to the relationHere, ∆q represents well-known phase space and both QED and hadronic radiative corrections [1], and we assume that G F is universal for weak interactions. Strictly speaking, τ µ + determines a muon-decay-specific coupling, denoted G µ , which could be compared to other G F determinations as a test of the standard model [2]. Prior to 1999, the limitation on the precision of G F was dominated by the uncertainty on ∆q. Van Ritbergen and Stuart were the first to compute the secondorder QED radiative corrections in the massless electron limit, reducing the theoretical uncertainty to below 0.3 ppm [3], and well below the then-current experimental uncertainty. This development motivated a new generation of precision muon lifetime measurements, MuLan [4] and FAST [5]. More recently, Pak and Czarnecki extended the result in [3] to finite electron mass, which shifts the predicted decay rate 1/τ µ by -0.43 ppm; alternatively, it decreases G F by 0.21 ppm [6].In Ref.[4], we reported an 11 ppm measurement of τ µ + based on a relatively short commissioning run. This Letter reports the results from a 100 times larger data set, accumulated using the final setup of the experiment.The experiment is designed to stop muons in a target during a beam-on accumulation interval and measure the decay positrons-primarily from the µ + → e + ν eνµ decay mode-during a beam-off measurement period. The two running periods, R06 and R07, used different targets. More than 10 12 decays were recorded in each period.The experiment used the πE3 beamline at the Paul Scherrer Institute (PSI). During the run, positive muons from at-rest pion decay near the surface of the production target are directed to the experiment through two opposing vertical dipole magnets and a series of 15 magnetic quadrupole lenses. A velocity-selecting E × B separator is tuned to pass muons and reject positrons. A special feature of the beamline is a custom, 60-ns switching, 25-kV kicker [8]. When energized, the electric field across the 120-mm vertical gap and 1500-mm length displaces the muon beam by 46 mm at the exit and deflects it by 45 mrad onto a downstream collimator. The muon flux of ∼ 1...