State Selective Scattering Angle Dependent Capture Cross Sections Measured by Cold Target Recoil Ion Momentum Spectroscopy

We have developed a new kind of recoil ion momentum spectroscopy technique, using a precooled supersonic gas jet target, to determine state selective, scattering angle dependent cross sections for swift ion-atom collisions ( 0. 25 , ..., , 1 MeV He2+ on He), by measuring the transverse and longitudinal momentum of the recoil ion. A longitudinal momentum resolution of ± 0.13 a. u. was achieved, about a factor of 30 better than ever obtained before, which enables a clear separation of K and L shell capture. In the transverse direction a resolution corresponding to a projectile scattering angle uncertainty of Δ ϑ P = ±1 μ rad was obtained

Multidifferential study of identified charged hadron distributions in ZZ-tagged jets in proton-proton collisions at s=\sqrt{s}=13 TeV

Jet fragmentation functions are measured for the first time in proton-proton collisions for charged pions, kaons, and protons within jets recoiling against a $Z$ boson. The charged-hadron distributions are studied longitudinally and transversely to the jet direction for jets with transverse momentum 20 $< p_{\textrm{T}} < 100$ GeV and in the pseudorapidity range $2.5 < \eta < 4$. The data sample was collected with the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.64 fb$^{-1}$. Triple differential distributions as a function of the hadron longitudinal momentum fraction, hadron transverse momentum, and jet transverse momentum are also measured for the first time. This helps constrain transverse-momentum-dependent fragmentation functions. Differences in the shapes and magnitudes of the measured distributions for the different hadron species provide insights into the hadronization process for jets predominantly initiated by light quarks.Comment: All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-013.html (LHCb public pages

Study of the B−→Λc+Λˉc−K−B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-} decay

The decay $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ is studied in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV using data corresponding to an integrated luminosity of 5 $\mathrm{fb}^{-1}$ collected by the LHCb experiment. In the $\Lambda_{c}^+ K^{-}$ system, the $\Xi_{c}(2930)^{0}$ state observed at the BaBar and Belle experiments is resolved into two narrower states, $\Xi_{c}(2923)^{0}$ and $\Xi_{c}(2939)^{0}$, whose masses and widths are measured to be $$m(\Xi_{c}(2923)^{0}) = 2924.5 \pm 0.4 \pm 1.1 \,\mathrm{MeV}, \\ m(\Xi_{c}(2939)^{0}) = 2938.5 \pm 0.9 \pm 2.3 \,\mathrm{MeV}, \\ \Gamma(\Xi_{c}(2923)^{0}) = \phantom{000}4.8 \pm 0.9 \pm 1.5 \,\mathrm{MeV},\\ \Gamma(\Xi_{c}(2939)^{0}) = \phantom{00}11.0 \pm 1.9 \pm 7.5 \,\mathrm{MeV},$$ where the first uncertainties are statistical and the second systematic. The results are consistent with a previous LHCb measurement using a prompt $\Lambda_{c}^{+} K^{-}$ sample. Evidence of a new $\Xi_{c}(2880)^{0}$ state is found with a local significance of $3.8\,\sigma$, whose mass and width are measured to be $2881.8 \pm 3.1 \pm 8.5\,\mathrm{MeV}$ and $12.4 \pm 5.3 \pm 5.8 \,\mathrm{MeV}$, respectively. In addition, evidence of a new decay mode $\Xi_{c}(2790)^{0} \to \Lambda_{c}^{+} K^{-}$ is found with a significance of $3.7\,\sigma$. The relative branching fraction of $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ with respect to the $B^{-} \to D^{+} D^{-} K^{-}$ decay is measured to be $2.36 \pm 0.11 \pm 0.22 \pm 0.25$, where the first uncertainty is statistical, the second systematic and the third originates from the branching fractions of charm hadron decays.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-028.html (LHCb public pages

Measurement of the ratios of branching fractions R(D∗)\mathcal{R}(D^{*}) and R(D0)\mathcal{R}(D^{0})

The ratios of branching fractions $\mathcal{R}(D^{*})\equiv\mathcal{B}(\bar{B}\to D^{*}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(\bar{B}\to D^{*}\mu^{-}\bar{\nu}_{\mu})$ and $\mathcal{R}(D^{0})\equiv\mathcal{B}(B^{-}\to D^{0}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(B^{-}\to D^{0}\mu^{-}\bar{\nu}_{\mu})$ are measured, assuming isospin symmetry, using a sample of proton-proton collision data corresponding to 3.0 fb${ }^{-1}$ of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode $\tau^{-}\to\mu^{-}\nu_{\tau}\bar{\nu}_{\mu}$. The measured values are $\mathcal{R}(D^{*})=0.281\pm0.018\pm0.024$ and $\mathcal{R}(D^{0})=0.441\pm0.060\pm0.066$, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is $\rho=-0.43$. Results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the Standard Model.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-039.html (LHCb public pages

and

The Crystal Ball/TAPS setup at MAMI is ideally suited to detect neutral and electromagnetic decays of the η and η′ mesons. In this contribution recent results on η and η′ decays from the Crystal Ball/TAPS experiment at MAMI are presented. These are the accurate measruement of η′ photoproduction cross-sections in the threshold region, the determination of the timelike electromagnetic transition form factors, and a test of low-energy QCD with the η → π0γγ decay. Finally, further perspectives are discussed

Bestimmung des Dalitz-Plot-Parameters α für den Zerfall η→3π⁰ mit dem Crystal Ball am MAMI

Der Dalitz-Plot-Parameter beschreibt in erster Ordnung die Abweichung der Ereignisdichte im Dalitz-Diagramm von der reinen Phasenraumverteilung. Dieser Unterschied wird durch die Endzustandswechselwirkung der drei Pionen verursacht. Für die neutralen Pionen aus dem Zerfall η→3π0 ist nur der Zustand mit Gesamtisospin I=1 möglich; das η-Meson aber hat I=0. Deshalb kann dieser Zerfall nur durch den isospinverletzenden Teil der Hamiltonfunktion der Quantenchromodynamik, der die Massendifferenz der beiden leichtesten Quarks u und d enthält, erzeugt werden. Da elektromagnetische Effekte hier vernachlässigbar sind, ist die Zerfallsamplitude demnach proportional zu dieser Massendifferenz. Damit stellt dieser Zerfall eine sehr gute Möglichkeit dar, Symmetrien und symmetriebrechende Verhalten in der QCD zu studieren und Berechnungen in der chiralen Störungstheorie zu testen. Um den Dalitz-Plot-Parameter α zu bestimmen, wurden Experimente, die mit dem Crystal Ball/TAPS-Aufbau am Beschleuniger MAMI (Mainzer Mikrotron) des Instituts für Kernphysik an der Johannes Gutenberg-Universität in Mainz in den Jahren 2004 und 2005 durchgeführt wurden, ausgewertet. Mit den Elektronen des Beschleunigers MAMI wurden Photonen durch den Bremsstrahlungsprozess an einem Radiator erzeugt. Die Elektronen wurden von einem magnetischen Spektrometer (Glasgow-Mainz-Tagger) von den Photonen getrennt und impulsselektierend nachgewiesen. Durch eine Koinzidenzanalyse mit den Produktdetektoren konnte somit der Photonenstrahl in seiner Energie markiert werden. Im Strahlengang der Photonen wurde ein Flüssig-Wasserstoff-Target platziert, an dessen freien Protonen η-Mesonen über die Reaktion γp→pη erzeugt wurden; über die Zerfallskette η→3π0→6γ wurden anschließend sechs Photonen emittiert. Diese Reaktionsteilchen wurden von den Photonenspektrometern Crystal Ball und TAPS, die zusammen ca. 97% des gesamten Raumwinkels abdeckten, registriert. Durch die Verwendung eines zylindrischen Szintillationsdetektors um das Target und von Veto-Plastikszintillatoren vor der TAPS-Wand wurden weitere Informationen über geladene Teilchen gewonnen. Durch Kombination aller Angaben ließen sich die Teilchen identifizieren. Neben der Auswertung der experimentellen Daten erforderte die Berechnung von α die genaue Kenntnis des Phasenraums des Zerfalls, der durch eine Simulation ermittelt wurde. Mit Hilfe von kinematischen Anpassungen wurden sowohl die gemessenen als auch die simulierten Ereignisse hinsichtlich einer Reaktionshypothese überprüft und selektiert. Zur Beurteilung der Datenanalysen wurden zusätzlich totale Wirkungsquerschnitte für die Reaktionen γp→pπ0 und γp→pη bestimmt, die in weitgehender Übereinstimmung mit anderen experimentellen Werten und theoretischen Rechnungen waren. Mit der getroffenen Auswahl von insgesamt ca. 1,8 Millionen Ereignissen wurde die radiale Ereignisdichte im Dalitz-Diagramm histogrammiert und das Verhältnis zwischen den Verteilungen der gemessenen und der simulierten Daten gebildet, um den Phasenraumanteil zu eliminieren. Dem theoretisch geforderten, in erster Ordnung linearen Verlauf dieses Verhältnisses wurde durch Anpassen einer Gerade an diese Verteilung Folge geleistet. Aus der Geradensteigung s wurde der Dalitz-Plot-Parameter durch α=s/2 bestimmt. Durch Variieren einzelner Analyseparameter wurden Studien zum systematischen Fehler angefertigt. Die erhaltenen Ergebnisse stimmen weitgehend mit früheren statistisch relevanten Experimenten überein.Determination of the Dalitz plot parameter α for the decay η→3π0 with the Crystal Ball at MAMI The Dalitz plot parameter describes to first order the deviation of the event density in the Dalitz plot from the pure phase space distribution. This difference is caused by the final state interaction of the three pions. The system of the neutral pions from the decay η→3π0 can only have a total Isospin I=1. The η-Meson has I=0. Therefore this decay can only happen through the isospin breaking part of the QCD hamiltonian, which contains the mass difference between the two lightest quarks u and d. Because electromagnetic effects are negligible, the decay amplitude is proportional to this mass difference. With this it represents a very nice possibility to study symmetries and symmetry breaking patterns in the QCD and to test calculations in the chiral perturbation theory. To determine the Dalitz plot parameter α, experiments carried out in the years 2004 and 2005 with the Crystal Ball/TAPS setup at the accelarator MAMI (Mainzer Mikrotron) of the Institut für Kernphysik at the Johannes Gutenberg-Universität in Mainz were analysed. With the electrons from the accelarator MAMI photons were produced by the Bremsstrahlung process at a radiator. The electrons were separated from the photons by a magnetic spectrometer (Glasgow-Mainz-Tagger). This device then measured the electron momenta. By demanding a coincidence with the final state detectors the photon beam could be tagged in its energy. In the photon beam line a liquid hydrogen target had been placed. On its free protons η mesons were produced by the reaction γp→pη. Through the decay chain η→3π0→6γ 6 photons were emitted afterwards. These particles were measured by the photon spectrometers Crystal Ball und TAPS, which together covered roughly 97% of the total solid angle. By using a cylindrical szintillation detector around the target und veto plastic szintillators in front of the TAPS wall further information about charged particles could be gained. Combining all these data made the identification of the measured particles possible. Besides the analysis of the experimental data the determination of α requires a precise knowledge of the phase space for the decay, which had been found out with a simulation. With the help of kinematic fitting procedures the measured as well as the simulated events were tested concerning a reaction hypothesis und selected. To judge the goodness of the data analysis in addition total cross sections for the reactions γp→pπ0 and γp→pη were calculated. These showed good agreement with other experimental results und theoretical calculations. In total almost 1.8 million η→3π0 events were found. With them the radial event density in the Dalitz plot was plotted and the ratio between the distributions from the measured and the simulated data was calculated to eliminated the phase space fraction. This ratio obeyed the theoretically expected linear behaviour to first order, which was shown by fitting a straight line to it. From the resulting slope s the Dalitz plot parameter was determined by α=s/2. Through variation of several parameters in the analysis the systematic uncertainties were studied. The obtained results agreed quite well with some former statistically relevant experiments

η

The Crystal Ball/TAPS setup at MAMI is ideally suited to detect neutral and electromagnetic decays of the η and η′ mesons. In this contribution recent results on η and η′ decays from the Crystal Ball/TAPS experiment at MAMI are presented. These are the accurate measruement of η′ photoproduction cross-sections in the threshold region, the determination of the timelike electromagnetic transition form factors, and a test of low-energy QCD with the η → π0γγ decay. Finally, further perspectives are discussed

Hadronic Contributions to the Muon Anomalous Magnetic Moment: strategies for Improvement of the Accuracy of the Theoretical Prediction

We present the mini-proceedings of the workshops Hadronic contributions to the muon anomalous magnetic moment: strategies for improvements of the accuracy of the theoretical prediction and $(g-2)_{\mu}$: Quo vadis?, both held in Mainz from April 1$^{\rm rst}$ to 5$^{\rm th}$ and from April 7$^{\rm th}$ to 10$^{\rm th}$, 2014, respectively