Photoproduktion neutraler Pionen am Proton mit linear polarisierten Photonen im Bereich der Delta(1232)-Resonanz

Hinweise auf die innere Struktur des Nukleons, dessenbeobachtbares Quadrupolmoment verschwindet, lassen sich u.a.durch die Untersuchung des N->Delta(1232)-Übergangsgewinnen. Dieser wird von der magnetischen Dipolanregung M1- einem Spinflip-Übergang - dominiert. In der Reaktion (gamma(lin. pol.) p -> p pi0) gelingt es mittels der Photonasymmetrie Sigma, das Signal der kleinen elektrischen Quadrupolamplitude E2 in einem Interferenztermmit der M1-Amplitude zu verstärken und das VerhältnisREM=E2/M1 des betrachteten Übergangs zu bestimmen. DieE2-Amplitude des N->Delta-Übergangs läßt auf eineDeformation des Nukleons und/oder der Delta-Resonanzschließen. Das zugehörige Experiment wurde am MainzerElektronenbeschleuniger MAMI durchgeführt. Durch kohärenteBremsstrahlung der Elektronen an einem Diamantradiatorstanden im Bereich der Delta(1232)-Resonanz linear polarisierte Photonen zur Verfügung. Insgesamt wurden reellePhotonen im Bereich Egamma=(200-790) MeV von derA2-Photonenmarkierungsanlage (Glasgow--Tagger)energiemarkiert. Mit dem Photonenspektrometer TAPS wurden die pi0-Mesonen über ihre beiden Zerfallsphotonennachgewiesen. Die gewählte Anordnung der 504BaF2-Einzelkristalle um ein Flüssigwasserstofftargeterlaubte den pi0-Nachweis im vollen Polarwinkelbereich. Die Datenbasis zur pi0-Photoproduktion am Proton konntehinsichtlich der Wirkungsquerschnitte und Photonasymmetriendurch Datenpunkte über den gesamten Polarwinkelbereichhinweg nachdrücklich erweitert werden.Eine weiterführende Multipolanalyse der neuen(Proton-pi0)-Daten ermöglichte im Energiebereich derDelta-Resonanz die Bestimmung der s- und p-WellenIsospinamplituden von E0+, M1-, E1+ und M1+.

Elektromagnetische Pionproduktion in manifest Lorentz-invarianter baryonischer chiraler Störungstheorie

In dieser Arbeit wurde die elektromagnetische Pionproduktion unter der Annahme der Isospinsymmetrie der starken Wechselwirkung im Rahmen der manifest Lorentz-invarianten chiralen Störungstheorie in einer Einschleifenrechnung bis zur Ordnung vier untersucht. Dazu wurden auf der Grundlage des Mathematica-Pakets FeynCalc Algorithmen zur Berechnung der Pionproduktionsamplitude entwickelt. Bis einschließlich der Ordnung vier tragen insgesamt 105 Feynmandiagramme bei, die sich in 20 Baumdiagramme und 85 Schleifendiagramme unterteilen lassen. Von den 20 Baumdiagrammen wiederum sind 16 als Polterme und vier als Kontaktgraphen zu klassifizieren; bei den Schleifendiagrammen tragen 50 Diagramme ab der dritten Ordnung und 35 Diagramme ab der vierten Ordnung bei. In der Einphotonaustauschnäherung lässt sich die Pionproduktionsamplitude als ein Produkt des Polarisationsvektors des (virtuellen) Photons und des Übergangsstrommatrixelements parametrisieren, wobei letzteres alle Abhängigkeiten der starken Wechselwirkung beinhaltet und wo somit die chirale Störungstheorie ihren Eingang findet. Der Polarisationsvektor hingegen hängt von dem leptonischen Vertex und dem Photonpropagator ab und ist aus der QED bekannt. Weiterhin lässt sich das Übergangsstrommatrixelement in sechs eichinvariante Amplituden zerlegen, die sich im Rahmen der Isospinsymmetrie jeweils wiederum in drei Isospinamplituden zerlegen lassen. Linearkombinationen dieser Isospinamplituden erlauben letztlich die Beschreibung der physikalischen Amplituden. Die in dieser Rechnung auftretenden Einschleifenintegrale wurden numerisch mittels des Programms LoopTools berechnet. Im Fall tensorieller Integrale erfolgte zunächst eine Zerlegung gemäß der Methode von Passarino und Veltman. Da die somit erhaltenen Ergebnisse jedoch i.a. noch nicht das chirale Zählschema erfüllen, wurde die entsprechende Renormierung mittels der reformulierten Infrarotregularisierung vorgenommen. Zu diesem Zweck wurde ein Verfahren entwickelt, welches die Abzugsterme automatisiert bestimmt. Die schließlich erhaltenen Isospinamplituden wurden in das Programm MAID eingebaut. In diesem Programm wurden als Test (Ergebnisse bis Ordnung drei) die s-Wellenmultipole E_{0+} und L_{0+} in der Schwellenregion berechnet. Die Ergebnisse wurden sowohl mit Messdaten als auch mit den Resultaten des "klassischen" MAID verglichen, wobei sich i. a. gute Übereinstimmungen im Rahmen der Fehler ergaben.

Single meson production in proton diffraction at 190 GeV/c at COMPASS

The quark model successfully describes all ground state bary-ons as members of $SU(N)$ flavour multiplets. For excited baryon states the situation is totally different. There are much less states found in the experiment than predicted in most theoretical calculations. This fact has been known for a long time as the 'missing resonance problem'. In addition, many states found in experiments are only poorly measured up to now. Therefore, further experimental efforts are needed to clarify the situation.rnrnAt mbox{COMPASS}, reactions of a $190uskgigaeVperclight$ hadron beam impinging on a liquid hydrogen target are investigated.rnThe hadron beam contains different species of particles ($pi$, $K$, $p$). To distinguish these particles, two Cherenkov detectors are used. In this thesis, a new method for the identification of particles from the detector information is developed. This method is based on statistical approaches and allows a better kaon identification efficiency with a similar purity compared to the method, which was used before.rnrnThe reaction $pprightarrow ppX$ with $X=(pi^0,~eta,~omega,~phi)$ is used to study different production mechanisms. A previous analysis of $omega$ and $phi$ mesons is extended to pseudoscalar mesons. As the resonance contributions in $peta$ are smaller than in $ppi^0$ a different behaviour of these two final states is expected as a function of kinematic variables. The investigation of these differences allows to study different production mechanisms and to estimate the size of the resonant contribution in the different channels.rnrnIn addition, the channel $pprightarrow ppX$ allows to study baryon resonances in the $pX$ system.rnIn the mbox{COMPASS} energy regime, the reaction is dominated by Pomeron exchange. As a Pomeron carries vacuum quantum numbers, no isospin is transferred between the target proton and the beam proton. Therefore, the $pX$ final state has isospin $textstylefrac{1}{2}$ and all baryon resonances in this channel are $N^ast$ baryons. This offers the opportunity to do spectroscopy without taking $Delta$ resonances into account. rnrnTo disentangle the contributions of different resonances a partial wave analysis (PWA) is used. Different resonances have different spin and parity $J^parity$, which results in different angular distributions of the decay particles. These angular distributions can be calculated from models and then be fitted to the data. From the fit the contributions of the single resonances as well as resonance parameters -- namely the mass and the width -- can be extracted. In this thesis, two different approaches for a partial wave analysis of the reaction $pprightarrow pppi^0$ are developed and tested.

Extraction of the s-wave pi N scattering lengths from data on pionic atoms

For many years a combined analysis of pionic hydrogen and deuterium atoms has been known as a good tool to extract information on the isovector and especially on the isoscalar s-wave pN scattering length. However, given the smallness of the isoscalar scattering length, the analysis becomes useful only if the pion–deuteron scattering length is controlled theoretically to a high accuracy comparable to the experimental precision. To achieve the required few-percent accuracy one needs theoretical control over all isospin-conserving three-body pNN !pNN operators up to one order before the contribution of the dominant unknown (N†N)2pp contact term. This term appears at next-to-next-to-leading order in Weinberg counting. In addition, one needs to include isospin-violating effects in both two-body (pN) and three-body (pNN) operators. In this talk we discuss the results of the recent analysis where these isospin-conserving and -violating effects have been carefully taken into account. Based on this analysis, we present the up-to-date values of the s-wave pN scattering lengths.

New Constraints on Dark Matter Effective Theories from Standard Model Loops

We consider an effective field theory for a gauge singlet Dirac dark matter particle interacting with the standard model fields via effective operators suppressed by the scale Λ≳1  TeV. We perform a systematic analysis of the leading loop contributions to spin-independent Dirac dark matter–nucleon scattering using renormalization group evolution between Λ and the low-energy scale probed by direct detection experiments. We find that electroweak interactions induce operator mixings such that operators that are naively velocity suppressed and spin dependent can actually contribute to spin-independent scattering. This allows us to put novel constraints on Wilson coefficients that were so far poorly bounded by direct detection. Constraints from current searches are already significantly stronger than LHC bounds, and will improve in the near future. Interestingly, the loop contribution we find is isospin violating even if the underlying theory is isospin conserving.

Accurate evaluation of hadronic uncertainties in spin-independent WIMP-nucleon scattering: Disentangling two- and three-flavor effects

We show how to avoid unnecessary and uncontrolled assumptions usually made in the literature about soft SU(3) flavor symmetry breaking in determining the two-flavor nucleon matrix elements relevant for direct detection of weakly interacting massive particles (WIMPs). Based on SU(2) chiral perturbation theory, we provide expressions for the proton and neutron scalar couplings fp,nu and fp,nd with the pion-nucleon σ term as the only free parameter, which should be used in the analysis of direct detection experiments. This approach for the first time allows for an accurate assessment of hadronic uncertainties in spin-independent WIMP-nucleon scattering and for a reliable calculation of isospin-violating effects. We find that the traditional determinations of Vfpu−fnu and fpd−fnd are off by a factor of 2.

First πK atom lifetime and πK scattering length measurements

The results of a search for hydrogen-like atoms consisting of π∓K±π∓K± mesons are presented. Evidence for πK atom production by 24 GeV/c protons from CERN PS interacting with a nickel target has been seen in terms of characteristic πK pairs from their breakup in the same target (178±49178±49) as well as in terms of produced πK atoms (653±42653±42). Using these results, the analysis yields a first value for the πK atom lifetime of View the MathML sourceτ=(2.5−1.8+3.0) fs and a first measurement of the S-wave isospin-odd πK scattering length View the MathML source|a0−|=13|a1/2−a3/2|=(0.11−0.04+0.09)Mπ−1 (aIaI for isospin I).

Updated Next-to-Next-to-Leading-Order QCD Predictions for the Weak Radiative B -Meson Decays

Weak radiative decays of the B mesons belong to the most important flavor changing processes that provide constraints on physics at the TeV scale. In the derivation of such constraints, accurate standard model predictions for the inclusive branching ratios play a crucial role. In the current Letter we present an update of these predictions, incorporating all our results for the O(α2s) and lower-order perturbative corrections that have been calculated after 2006. New estimates of nonperturbative effects are taken into account, too. For the CP- and isospin-averaged branching ratios, we find Bsγ=(3.36±0.23)×10−4 and Bdγ=(1.73+0.12−0.22)×10−5, for Eγ>1.6  GeV. Both results remain in agreement with the current experimental averages. Normalizing their sum to the inclusive semileptonic branching ratio, we obtain Rγ≡(Bsγ+Bdγ)/Bcℓν=(3.31±0.22)×10−3. A new bound from Bsγ on the charged Higgs boson mass in the two-Higgs-doublet-model II reads MH±>480  GeV at 95% C.L.

A search for an exotic meson in the gamma + P decaying to delta++ + pi- + eta reaction

A Partial Waves Analysis (PWA) of γp → Δ ++X → pπ+ π - (η) data taken with the CLAS detector at Jefferson Lab is presented in this work. This reaction is of interest because the Δ++ restricts the isospin of the possible X states, leaving the PWA with a smaller combination of partial waves, making it ideal to look for exotic mesons. It was proposed by Isgur and Paton that photoproduction is a plausible source for the Jpc=1–+ state through flux tube excitation. The π1(1400) is such a state that has been produced with the use of hadron production but it has yet to be seen in photoproduction. A mass independent amplitude analysis of this channel was performed, followed by a mass dependent fit to extract the resonance parameters. The procedure used an event-based maximum likelihood method to maintain all correlations in the kinematics. The intensity and phase motion is mapped out for the contributing signals without requiring assumptions about the underlying processes. The strength of the PWA is in the analysis of the phase motion, which for resonance behavior is well defined. In the data presented, the ηπ– invariant mass spectrum shows contributions from the a0(980) and a2(1320) partial waves. No π1 was observed under a clear a2 signal after the angular distributions of the decay products were analyzed using an amplitude analysis. In addition, this dissertation discusses trends in the data, along with the implemented techniques.

Aspects of Chiral Symmetry Breaking in Lattice QCD

Thesis (Ph.D.)--University of Washington, 2016-08