arXiv subjects

W. Bertl

Publications and source records attributed to W. Bertl.

17 recordsLinked to original sources

arXiv2018

PEN experiment: a precise test of lepton universality

With few open channels and uncomplicated theoretical description, charged pion decays are uniquely sensitive to certain standard model (SM) symmetries, the universality of weak fermion couplings, and to aspects of pion structure and chiral dynamics. We review the current knowledge of the pion electronic decay $π^+ \to e^+ ν_e(γ)$, or $π_{e2(γ)}$, and the resulting limits on non-SM processes. Focusing on the PEN experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the prospects for further improvement in the near term.

hep-ex↗

arXiv2017

PEN: a low energy test of lepton universality

Allowed charged $π$ meson decays are characterized by simple dynamics, few available decay channels, mainly into leptons, and extremely well controlled radiative and loop corrections. In that sense, pion decays represent a veritable triumph of the standard model (SM) of elementary particles and interactions. This relative theoretical simplicity makes charged pion decays a sensitive means for testing the underlying symmetries and the universality of weak fermion couplings, as well as for studying pion structure and chiral dynamics. Even after considerable recent improvements, experimental precision is lagging far behind that of the theoretical description for pion decays. We review the current state of experimental study of the pion electronic decay $π^+ \to e^+ν_e(γ)$, or $π_{e2(γ)}$, where the $(γ)$ indicates inclusion and explicit treatment of radiative decay events. We briefly review the limits on non-SM processes arising from the present level of experimental precision in $π_{e2(γ)}$ decays. Focusing on the PEN experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the prospects for further improvement in the near term.

hep-ex↗

arXiv2014

New results in rare allowed muon and pion decays

Simple dynamics, few available decay channels, and highly controlled radiative and loop corrections, make pion and muon decays a sensitive means of exploring details of the underlying symmetries. We review the current status of the rare decays: pi+ -> e+ nu, pi+ -> e+ nu gamma, pi+ -> pi0 e+ nu, and mu+ -> e+ nu nu-bar gamma. For the latter we report new preliminary values for the branching ratio B(E_gamma >10 MeV, theta_(e-gamma) > 30deg) = 4.365 (9)_stat (42)_syst x 10^{-3}, and the decay parameter eta-bar = 0.006 (17)_stat (18)_syst, both in excellent agreement with standard model predictions. We review recent measurements, particularly by the PIBETA and PEN experiments, and near-term prospects for improvement. These and other similar precise low energy studies complement modern collider results materially.

nucl-ex↗

arXiv2012

New studies of allowed pion and muon decays

Building on the rare pion and muon decay results of the PIBETA experiment, the PEN collaboration has undertaken a precise measurement of B_{πe2} = R^π_{e/μ}, the π^+ -> e^+ν(γ) decay branching ratio, at the Paul Scherrer Institute, to reduce the present 40\times experimental precision lag behind theory to ~ 6-7\times. Because of large helicity suppression, R^π_{e/μ} is uniquely sensitive to contributions from non-(V-A) physics, making this decay a particularly suitable subject of study. Even at current precision, the experimental value of B_{πe2} provides the most accurate test of lepton universality available. During runs in 2008-10, PEN has accumulated over 2\times 10^7 π_{e2} events; a comprehensive maximum-likelihood analysis is currently under way. The new data will also lead to improved precision of the earlier PIBETA results on radiative πand μdecays.

hep-ex↗

arXiv2009

PEN: a sensitive search for non-(V-A) weak processes

A new measurement of $B_{πe2}$, the $π^+ \to e^+ν(γ)$ decay branching ratio, is currently under way at the Paul Scherrer Institute. The present experimental result on $B_{πe2}$ constitutes the most accurate test of lepton universality available. The accuracy, however, still lags behind the theoretical precision by over an order of magnitude. Thanks to the large helicity suppression of $π_{e2}$ decay, the branching ratio is susceptible to significant contributions from new physics, making this decay a particularly suitable subject of study.

hep-ex↗

arXiv2009

PEN experiment: a precise measurement of the pi+ -> e+ nu decay branching fraction

A new measurement of $B_{πe2}$, the $π^+ \to e^+ν(γ)$ decay branching ratio, is currently under way at the Paul Scherrer Institute. The present experimental result on $B_{πe2}$ constitutes the most accurate test of lepton universality available. The accuracy, however, still lags behind the theoretical precision by over an order of magnitude. Because of the large helicity suppression of the $π_{e2}$ decay, its branching ratio is susceptible to significant contributions from new physics, making this decay a particularly suitable subject of study.

hep-ex↗

arXiv2009

New Precise Measurement of the Pion Weak Form Factors in the Pi+ -> e+ nu gamma Decay

We have measured the $π^+\to {\rm e}^+νγ$ branching ratio over a wide region of phase space, based on a total of 65,460 events acquired using the PIBETA detector. Minimum-$χ^2$ fits to the measured $(E_{e^+},E_γ)$ energy distributions result in the weak form factor value of $F_A=0.0119(1)$ with a fixed value of $F_V=0.0259$. An unconstrained fit yields $F_V=0.0258(17)$ and $F_A=0.0117(17)$. In addition, we have measured $a=0.10(6)$ for the dependence of $F_V$ on $q^2$, the ${\rm e}^{+}ν$ pair invariant mass squared, parametrized as $F_V(q^2)=F_V(0)(1+a\cdot q^2)$. The branching ratio for the kinematic region $E_γ> 10 $MeV and $θ_{{\rm e^+}γ} > 40^\circ $ is measured to be $B^{\rm exp}=73.86(54) \times 10^{-8}$. Earlier deviations we reported in the high-$E_γ$/low-$E_{{\rm e}^+}$ kinematic region are resolved, and we find full compatibility with CVC and standard $V$$-$$A$ calculations without a tensor term. We also derive new values for the pion polarizability, $α_E = \rm 2.78(10) \times 10^{-4} fm^3$, and neutral pion lifetime, $τ_{π0} = (8.5 \pm 1.1) \times 10^{-17} $s.

hep-ex↗

arXiv2008

Precise Measurement of pi+ -> e+ nu Branching Ratio

The PEN Collaboration is conducting a new measurement of the pi+ -> e+ nu branching ratio at the Paul Scherrer Institute, with the goal uncertainty of delta B/B(pie2)=5E-4 or lower. At present, the combined accuracy of all published pie2 decay measurements lags behind the theoretical calculation by a factor of 40. In this contribution we report on the PEN detector configuration and its performance during two development runs done in 2007 and 2008.

hep-ex↗

arXiv2007

CMS Barrel Pixel Detector Overview

The pixel detector is the innermost tracking device of the CMS experiment at the LHC. It is built from two independent sub devices, the pixel barrel and the end disks. The barrel consists of three concentric layers around the beam pipe with mean radii of 4.4, 7.3 and 10.2 cm. There are two end disks on each side of the interaction point at 34.5 cm and 46.5 cm. This article gives an overview of the pixel barrel detector, its mechanical support structure, electronics components, services and its expected performance.

physics.ins-det↗

arXiv2004

Precise Measurement of the Pi+ -> Pi0 e+ nu Branching Ratio

Using a large acceptance calorimeter and a stopped pion beam we have made a precise measurement of the rare Pi+ -> Pi0 e+ Nu,(pi_beta) decay branching ratio. We have evaluated the branching ratio by normalizing the number of observed pi_beta decays to the number of observed Pi+ -> e+ Nu, (pi_{e2}) decays. We find the value of Gamma(Pi+ -> Pi0 e+ Nu)/Gamma(total) = [1.036 +/- 0.004(stat.) +/- 0.004(syst.) +/- 0.003(pi_{e2})] x 10^{-8}$, where the first uncertainty is statistical, the second systematic, and the third is the pi_{e2} branching ratio uncertainty. Our result agrees well with the Standard Model prediction.

hep-ex↗

arXiv2003

Precise Measurement of the Pion Axial Form Factor in the Pi+ -> e+ nu gamma Decay

We have studied radiative pion decays Pi+ -> e+ nu gamma in three broad kinematic regions using the PIBETA detector and a stopped pion beam. Based on Dalitz distributions of 42,209 events we have evaluated absolute Pi -> e nu gamma branching ratios in the three regions. Minimum chi^2 fits to the integral and differential (E(e+),E(gamma)) distributions result in the axial-to-vector weak form factor ratio of gamma = F_A/F_V = 0.443(15),or F_A = 0.0115(4) with F_V = 0.0259. However, deviations from Standard Model predictions in the high-E(gamma)/low-E(e+) kinematic region indicate the need for further theoretical and experimental work.

hep-ex↗

arXiv2003

Design, Commissioning and Performance of the PIBETA Detector at PSI

We describe the design, construction and performance of the PIBETA detector built for the precise measurement of the branching ratio of pion beta decay, pi+ -> pi0 e+ nu, at the Paul Scherrer Institute. The central part of the detector is a 240-module spherical pure CsI calorimeter covering 3*pi sr solid angle. The calorimeter is supplemented with an active collimator/beam degrader system, an active segmented plastic target, a pair of low-mass cylindrical wire chambers and a 20-element cylindrical plastic scintillator hodoscope. The whole detector system is housed inside a temperature-controlled lead brick enclosure which in turn is lined with cosmic muon plastic veto counters. Commissioning and calibration data were taken during two three-month beam periods in 1999/2000 with pi+ stopping rates between 1.3*E3 pi+/s and 1.3*E6 pi+/s. We examine the timing, energy and angular detector resolution for photons, positrons and protons in the energy range of 5-150 MeV, as well as the response of the detector to cosmic muons. We illustrate the detector signatures for the assorted rare pion and muon decays and their associated backgrounds.

hep-ex↗

arXiv1998

New Bounds from Searching for Muonium to Antimuonium Conversion

A new upper limit for the probability of spontaneous muonium to antimuonium conversion was established at ${\rm P_{M\bar{M}}} \leq 8.2 \cdot 10^{-11}$ (90%C.L.) in 0.1~T magnetic field, which implies consequences for speculative extensions to the standard model. Coupling parameters in R-parity violating supersymmetry and the mass of a flavour diagonal bileptonic gauge boson can be significantly restricted. A Z$_8$ model with radiative mass generation through heavy lepton seed and the minimal version of 331-GUT models are ruled out.

hep-ex↗

arXiv1998

Improved upper limit on Muonium to Antimuonium Conversion

A new experiment has been set up at the Paul Scherrer Institut to search for muonium to antimuonium conversion. No event was found to fulfil the requested signature which consists of the coincident detection of both constituents of the antiatom in its decay. Assuming an effective (V-A)$\times$(V-A) type interaction an improved upper limit is established for the conversion probability of ${\rm P_{M\bar{M}}} \leq 8 \cdot 10^{-9}$ (90%C.L.), which is almost two orders of magnitude lower compared to previous results and provides a sensitive test for theoretical extensions of the standard model.

nucl-ex↗

arXiv1998

Increased Sensitivity to Possible Muonium to Antimuonium Conversion

A new experimental search for muonium-antimuonium conversion was conducted at the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysis yielded one event fulfilling all required criteria at an expected background of 1.7(2) events due to accidental coincidences. An upper limit for the conversion probability in 0.1 T magnetic field is extracted as $8 \cdot 10^{-11}$ (90% CL).

hep-ex↗

arXiv1998

Results from Searching for Muonium to Antimuonium Conversion

A new result from searching for muonium to antimuonium conversion is reported which sets an upper limit on the coupling constant in an assumed $(V-A) \times (V-A)$ type interaction of $G_{M\bar{M}} \leq 3\cdot 10^{-3} G_F$ ~ (90% C.L.). A particular Z_8 and a minimal 331 GUT model can be ruled out. Further new and stringent limits can be set for masses of bileptonic gauge bosons and $λ$ parameters in R-parity breaking supersymmetric models.

nucl-ex↗

arXiv1996

Radiative Muon Capture on Hydrogen and the Induced Pseudoscalar Coupling

The first measurement of the elementary process $μ^- p \rightarrow ν_μ n γ$ is reported. A photon pair spectrometer was used to measure the partial branching ratio ($2.10 \pm 0.22) \times 10^{-8}$ for photons of k > 60 MeV. The value of the weak pseudoscalar coupling constant determined from the partial branching ratio is $g_p(q^{2}=-0.88m_μ^2) = (9.8 \pm 0.7 \pm 0.3) \cdot g_a(0)$, where the first error is the quadrature sum of statistical and systematic uncertainties and the second error is due to the uncertainty in $λ_{op}$, the decay rate of the ortho to para $p μp$ molecule. This value of g_p is $\sim$1.5 times the prediction of PCAC and pion-pole dominance.

nucl-ex↗