Scientific Highlights 2014 and earlier

2014

12 December 2014

A measurement of the neutron to 199Hg magnetic moment ratio

The neutron gyromagnetic ratio has been measured relative to that of the 199Hg atom with an uncertainty of 0.8 ppm. We employed an apparatus where ultracold neutrons and mercury atoms are stored in the same volume and report the result γnHg = 3.8424574(30).
Facility: UCN

Reference: S. Afach et al, Physics Letters B 739, 128 (2014)

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2 October 2014

The μ → eγ decay in a systematic effective field

We implement a systematic effective field theory approach to the benchmark process μ → eγ, performing automated one-loop computations including dimension 6 operators and studying their anomalous dimensions. We obtain limits on Wilson coefficients of a relevant subset of lepton-flavour violating operators that contribute to the branching ratio μ → eγ at one-loop. In addition, we illustrate a method to extract further constraints induced by the mixing of operators under renormalisation-group evolution. This results in limits on the corresponding Wilson coefficients directly at the high scale. The procedure can be applied to other processes as well and, as an example, we consider also lepton-flavour violating decays of the τ.
Reference: G.M. Pruna et al, Journal of High Energy Physics 10, 014 (2014)

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2013

Measurement of the B0s → μ+μ- Branching Fraction and Search for B0 → μ+μ- with the CMS Experiment

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Results are presented from a search for the rare decays B0s → μ+μ- and B0 → μ+μ- in pp collisions at √s = 7 and 8 TeV, with data samples corresponding to integrated luminosities of 5 and 20 fb-1, respectively, collected by the CMS experiment at the LHC. An unbinned maximum-likelihood fit to the dimuon invariant mass distribution gives a branching fraction B(B0s → μ+μ-)=(3.0+1.0-0.9) x 10-9, where the uncertainty includes both statistical and systematic contributions. An excess of B0s → μ+μ- events with respect to background is observed with a significance of 4.3 standard deviations. For the decay B0 → μ+μ- an upper limit of B(B0 → μ+μ-) < 1.1 x 10-9 at the 95% confidence level is determined. Both results are in agreement with the expectations from the standard model.
Reference: S. Chatrchyan et al. (CMS collaboration), Physical Review Letters 111, 101804 (2013)

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New Constraint on the Existence of the μ+→e+γ Decay

The analysis of a combined data set, totaling 3.6⋅1014 stopped muons on target, in the search for the lepton flavor violating decay μ+→e+γ is presented. The data collected by the MEG experiment at the Paul Scherrer Institut show no excess of events compared to background expectations and yield a new upper limit on the branching ratio of this decay of 5.7⋅1013 (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.
Reference: J. Adam et al (MEG collaboration), Physical Review Letters 110, 201801 (2013)

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Proton Structure from the Measurement of 2S-2P Transition Frequencies of Muonic Hydrogen

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Accurate knowledge of the charge and Zemach radii of the proton is essential, not only for understanding its structure but also as input for tests of bound-state quantum electrodynamics and its predictions for the energy levels of hydrogen. These radii may be extracted from the laser spectroscopy of muonic hydrogen (μp, that is, a proton orbited by a muon). We measured the 2S1/2F=0 - 2P3/2F=1 transition frequency in μp to be 54611.16(1.05) gigahertz (numbers in parentheses indicate one standard deviation of uncertainty) and reevaluated the 2S1/2F=1 - 2P3/2F=2 transition frequency, yielding 49881.35(65) gigahertz. From the measurements, we determined the Zemach radius, rZ = 1.082(37) femtometers, and the magnetic radius, rM = 0.87(6) femtometer, of the proton. We also extracted the charge radius, rE = 0.84087(39) femtometer, with an order of magnitude more precision than the 2010-CODATA value and at 7σ variance with respect to it, thus reinforcing the proton radius puzzle.
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Reference: A. Antognini et al., Science 339, 417 (2013)

Die schwache Seite des Protons

Error: (1) can't find mucap.png at LIVE_WWW/LTP.ScientificHighlights2014EarlierEN Ein internationales Forschungsteam hat mit grosser Genauigkeit bestimmt, wie das Proton an der schwachen Wechselwirkung – einer der vier fundamentalen Kräfte der Natur – teilhat. Die Ergebnisse bestätigen die theoretischen Voraussagen des Standardmodells der Teilchenphysik. In dem Experiment wurde beobachtet, mit welcher Wahrscheinlichkeit Myonen von Protonen einfangen werden – ein Prozess, der von der schwachen Wechselwirkung bestimmt wird. Das Experiment wurde am Paul Scherrer Institut PSI durchgeführt, dem einzigen Ort weltweit, an dem genügend Myonen erzeugt werden, damit es in einer realistischen Zeit durchgeführt werden kann. Die Forscher haben darauf geachtet, nicht nur die zahlreichen technischen Fehlerquellen auszuschliessen, sondern auch die „psychologischen“. Durch ein trickreiches Verfahren konnten sie ausschliessen, dass sie sich bei der Auswertung der Messergebnisse unbewusst von den bekannten theoretischen Voraussagen beeinflussen liessen. Die moderne Beschreibung des untersuchten Prozesses beruht auf Ideen, die vor 50 Jahren vom amerikanischen Physiker Y. Nambu entwickelt wurden, der 2008 dafür den Physiknobelpreis erhalten hat. Erst jetzt war es möglich, die theoretischen Vorhersagen mit der notwendigen Genauigkeit zu überprüfen. Das Projekt zeigt, dass die Teilchenphysik neben Experimenten an den grossen Beschleunigeranlagen im Hochenergiebereich, auch Untersuchungen mit hohen Teilchenzahlen braucht, für die das PSI die besten Voraussetzungen bietet. Das Ergebnis ist im Fachjournal Physical Review Letters erschienen. Das American Institute of Physics hat das Ergebnis mit einer Zusammenfassung auf der Webseite gewürdigt.

2011

Search for Bs → μ+μ- decays in pp collisions at √s = 7 TeV

Bs mesons, composed of a 'beauty' and a 'strange' quark, are produced copiously at the LHC. The fraction that subsequently decays (known as the 'branching fraction') to a pair of muons is highly suppressed in the Standard Model - only about three such decays are expected per billion Bs meson produced. Several extensions of the SM, for instance supersymmetric models, predict (significant) enhancements of this branching fraction. CMS has searched for the decays of Bs to muon pairs using proton-proton collision data collected up to June 2011. The CMS (barrel) pixel detector, developed and constructed at PSI, has a central role in reconstructing the signal with high efficiency while strongly reducing the background. The number of candidate decays observed in the available data sample is consistent with the Standard Model expectations for signal and background. CMS has excluded (at 95% confidence level) branching fractions larger than 1.9*10-8 for the decay Bs → μ+μ-. This result is among the most stringent exclusion limits achieved until now and implies significant constraints on extensions of the Standard Model. The CMS result has been accepted for publication in PRL and has been combined with the corresponding result of the LHCb collaboration.
Reference: CMS collaboration (author), Physical Review Letters 107, accepted for publication (2011)

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The MEG experiment at PSI reaches an unprecedented level of precision in the search for the μ → e γ decay

The international collaboration forming the MEG-experiment, one of PSI’s “flagship” particle physics experiments at the proton accelerator facility in Switzerland, is publishing its latest results in the search for “New Physics” beyond the Standard Model (SM), in the journal Physical Review Letters. The goal of the experiment is the search for the existence of the decay μ → e γ, a quest that started more than sixty years ago using cosmic-ray muons and now uses one of the world’s most intense surface muon beams. Data collected by MEG in 2009-2010 shows no evidence, as yet, of an excess of events above the expected background. However, even while collecting thirty-million decays a second in the detector, the experiment will require about two more years of data-taking to reach the expected sensitivity. This new MEG result does however exclude, with 90% confidence, that more than one muon in approximately five-hundred billion, does decay into a positron (positive electron) and photon. This therefore translates into the most stringent constraint on the existence of the μ → e γ decay to date (Branching Ratio B ≤ 2.4⋅10-12, 90% C.L.) and improves the previous best limit by a factor of five.
Reference: J. Adam et al., MEG collaboration, Physical Review Letters 107, accepted for publication (October 2011)

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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 2×1012 decays. Two different stopping target configurations were employed in independent data-taking periods. The combined results give τμ+(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: GF(MuLan)=1.166378(7)×10-5GeV-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 gP.
Reference: D.M. Webber et al, Physical Review Letters 106, 041803 (2011)

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2011

New constraints on Lorentz invariance violation from the neutron electric dipole moment

We propose an original test of Lorentz invariance in the interaction between a particle spin and an electromagnetic field and report on a first measurement using ultracold neutrons. We used a high-sensitivity neutron electric dipole moment (nEDM) spectrometer and searched for a direction dependence of an nEDM signal leading to a modulation of its magnitude at periods of 12 and 24 hours. We constrain such a modulation to d12 < 10 × 10−25 e cm and d24 < 14 × 10−25 e cm at 95% C.L. The result translates into a limit on the energy scale for this type of Lorentz violation effect at the level of εLV > 1010 GeV.
Reference: I. Altarev et al, Europhysics Letters 92, 51001 (2010)

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Transverse-Momentum and Pseudorapidity Distributions of Charged Hadrons in pp Collisions at √s=7  TeV

Charged-hadron transverse-momentum and pseudorapidity distributions in proton-proton collisions at √s=7  TeV are measured with the inner tracking system of the CMS detector at the LHC. The charged-hadron yield is obtained by counting the number of reconstructed hits, hit pairs, and fully reconstructed charged-particle tracks. The combination of the three methods gives a charged-particle multiplicity per unit of pseudorapidity dNch/dη||η|<0.5=5.78±0.01(stat)±0.23(syst) for non-single-diffractive events, higher than predicted by commonly used models. The relative increase in charged-particle multiplicity from √s=0.9 to 7 TeV is [66.1±1.0(stat)±4.2(syst)]%. The mean transverse momentum is measured to be 0.545±0.005(stat)±0.015(syst)  GeV/c. The results are compared with similar measurements at lower energies.
Reference: V. Khachatryan et al, Physical Review Letters 105, 022002 (2010)

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The size of the proton

Here, a technically challenging spectroscopic experiment is described: the measurement of the muonic Lamb shift. The results lead to a new determination of the charge radius of the proton. The new value is 5.0 standard deviations smaller than the previous world average, a large discrepancy that remains unexplained. Possible implications of the new finding are that the value of the Rydberg constant will need to be revised, or that the validity of quantum electrodynamics theory is called into question.
Reference: R. Pohl et al, Nature 466, 213 (2010)

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2009

Test of Lorentz Invariance with Spin Precession of Ultracold Neutrons

A clock comparison experiment, analyzing the ratio of spin precession frequencies of stored ultracold neutrons and 199Hg atoms, is reported. No daily variation of this ratio could be found, from which is set an upper limit on the Lorentz invariance violating cosmic anisotropy field bperpendicular<2×10-20 eV (95% C.L.). This is the first limit for the free neutron. This result is also interpreted as a direct limit on the gravitational dipole moment of the neutron |gn|<0.3 eV/c2 m from a spin-dependent interaction with the Sun. Analyzing the gravitational interaction with the Earth, based on previous data, yields a more stringent limit |gn|<3×10-4 eV/c2m.
Reference: I. Altarev et al, Physical Review Letters 103, 081602 (2009)

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Next-To-Leading Order QCD Corrections to pp -> tt'bb'+X at the LHC

We report on the calculation of the full next-to-leading-order QCD corrections to the production of tt'bb' final states at the LHC, which deliver a serious background contribution to the production of a Higgs boson (decaying into a bb' pair) in association with a tt' pair. While the corrections significantly reduce the unphysical scale dependence of the leading-order cross section, our results predict an enhancement of the tt'bb' production cross section by a K factor of about 1.8.
Reference: A. Bredenstein et al, Physical Review Letters 103, 012002 (2009)

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Measurement of the Transverse Polarization of Electrons Emitted in Free-Neutron Decay

Both components of the transverse polarization of electrons (sigma_T1, sigma_T2) emitted in the beta-decay of polarized, free neutrons have been measured. The T-odd, P-odd correlation coefficient quantifying sigma_T2, perpendicular to the neutron polarization and electron momentum, was found to be R=0.008±0.015±0.005. This value is consistent with time reversal invariance and significantly improves limits on the relative strength of imaginary scalar couplings in the weak interaction. The value obtained for the correlation coefficient associated with sigma_T1, N=0.056±0.011±0.005, agrees with the Standard Model expectation, providing an important sensitivity test of the experimental setup.
Reference: A. Kozela et al, Physical Review Letters 102 , 172301 (2009)

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