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PSI Muon-Neutrino and Pion Masses

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PSI Muon-Neutrino and Pion Masses Around the Laboratories More penguins. The CLEO detector at Cornell's CESR electron-positron collider reveals a clear excess signal of photons from rare B particle decays, once the parent upsilon 4S resonance has been taken into account. The convincing curve is the theoretically predicted spectrum. could be at the root of the symmetry breaking mechanism which drives the Standard Model. PSI Muon-neutrino and pion masses wo experiments at the Swiss T Paul Scherrer Institute (PSI) in Villigen have recently led to more precise values for two important physics quantities - the masses of the muon-type neutrino and of the charged pion. The first of these experiments - B. Jeckelmann et al. (Fribourg Univer­ sity, ETH and Eidgenoessisches Amt fuer Messwesen) was originally done about a decade ago. To fix the negative-pion mass, a high-precision crystal spectrometer measured the energy of the X-rays emitted in the 4f-3d transition of pionic magnesium atoms. The result (JECKELMANN 86 in the graph, page 16), with smaller seen in neutral kaon decay. can emerge. Drawing physics conclu­ errors than previous charged-pion CP violation - the disregard at the sions from the K* example was mass measurements, dominated the few per mil level of a invariance of therefore difficult. world average. physics with respect to a simultane­ Now the CLEO group has collected In view of more recent measure­ ous left-right reversal and particle- all such events producing a photon ments on pionic magnesium, a re- antiparticle switch - has been known with energy between 2.2 and 2.7 analysis shows that the measured X- for thirty years. While the basic GeV. This sample implicitly includes ray line leads to two possible equations of physics could otherwise the whole range of produced strange- negative-pion masses - run equally well forwards or back­ quark states, and is therefore more JECKELMANN 94(A) or (B) - sepa­ wards, CP violation imposes a accessible to theoretical analysis. rated by 15 parts per million. The two definite direction to the arrow of time Separate analyses (neural net and solutions use different electron- and could have played a major role in B reconstruction) clearly display the screening corrections, the strongest shaping the Universe as it emerged excess signal of photons from the component of the X-ray line being from the Big Bang. penguin processes. attributed to the presence of one or The penguin process seen at The branching ratio of some two two K-electrons, respectively. The Cornell last year was the decay of a parts in ten thousand is in excellent 1986 measurement of the pion mass B meson into a K* meson (mass 892 agreement with the Standard Model. was based on solution A, favoured MeV) and a photon. The K* is only When combined with other precision at that time by the available pionic- one example of a penguin beauty- Standard Model results, it already magnesium data. strange quark transition - in principle rules out a range of low masses for Additional information on the a whole range of strange particles possible charged higgs particles that charged-pion mass was recently 14 CERN Courier, October 1994 Around the Laboratories Charged-pion mass (vertical axis) results obtained at PSI. The values 1 and 3 are from pionic-magnesium X-ray data (A,B: different electron-screening corrections); the results 2 and 4 are based on the muon mass and the momentum of muons from pion decay at rest. Taken together, the 1994 results imply a pion mass close to 139.570 MeV, larger than the previous world average. (90% confidence level). This is interesting as certain non-standard models allow unstable muon-neutri- nos with masses above about 100 keV. BERKELEY Light Source anniversary he staff of the Advanced Light T Source (ALS) at the Lawrence Berkeley Laboratory has been too busy to celebrate the first anniversary of the facility's transition from a US Department of Energy construction project to operating third-generation synchrotron radiation source. Based on a 1.5-GeV, low-emittance electron contributed by another precision pion-mass result ASSAMAGAN 94 is storage ring that accommodates up experiment at PSI - K. Assamagan et consistent with JECKELMANN 94(B) to ten insertion-device radiation al. (PSI, Zurich University, ETH, but excludes JECKELMANN 94(A) sources optimized primarily for the University of Virginia), in which the by 6.2 standard deviations, thus soft X-ray and vacuum ultra-violet momentum of muons from the decay confirming the conclusions of the regions of the spectrum, the ALS has of positive pions at rest into a muon earlier muon-momentum measure­ completed. and a muon-neutrino was measured ment DAUM 91 with improved A year ago, we reported the blister­ by a new technique. Muons in a significance. ing pace set by accelerator physi­ surface muon beam, originating from The 1994 results of the two experi­ cists, operators, and engineers as stopped pion decays at the surface of ments together yield a charged-pion the storage ring surpassed many of a pion production target at the 590 mass close to 139.570 MeV, signifi­ its performance goals in the first six MeV proton accelerator, are momen­ cantly higher than the previous world weeks of operation (June 1993, tum analysed in a magnetic average. The neutral-pion mass is pages 8-9). In particular, the maxi­ spectrometer. The resulting muon also affected, as this is obtained by mum observed storage-ring current momentum of 29.79207(12) MeV/c is subtracting the pionic mass differ­ of 457 mA - reached at the end of consistent with the 1991 PSI meas­ ence of 4.5936(5) MeV from the April 1993 - was well beyond the urement of M. Daum et al., using a charged-pion mass. design goal of 400 mA for the normal positive pion beam. The muon-neutrino mass is derived multibunch mode of operation. The new muon momentum value from the charged-pion mass Although there is no readily recogniz­ and the muon mass lead to the JECKELMANN 94(B) (which in able barrier to reaching significantly charged-pion mass ASSAMAGAN contrast to JECKELMANN 94(A) is higher current, the policy has been to 94. In the graph, the pion mass value consistent with DAUM 91 and operate at the design value. indicated by a filled circle and the ASSAMAGAN 94), together with the After this initial round of successes, uncertainty given by the lower error muon momentum mentioned above the ALS went into a lengthy shut­ bar are obtained if the muon-neutrino and the muon mass. A muon neu­ down from May through August mass is assumed to vanish, while trino mass squared of -0.022(23) 1993. This saw the installation of the larger pion mass values are obtained MeV2 yields an upper limit for the first two insertion devices and for non-zero neutrino masses. The muon neutrino mass of 0.16 MeV undulators for beamlines 7.0 and 8.0, 16 CERN Courier, October 1994 .
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