Analysis of Roper Resonance N(1440) in Pion Electroproduction Process $p(e,e'p)\pi^0$

Štajner, Samo (author), Širca, Simon (mentor)

Electroproduction of pseudoscalar mesons on free nucleons is a very efficient and most sensitive tool for exploration of nucleon resonances. The lowest lying excited nucleon state with positive parity is the resonance $P_{11}(1440)$, also known as the Roper resonance. Due to its unusual characteristics it has been a source of controversy since the day of its discovery. We have performed a recoil-polarization experiment, where helicity dependent proton polarizations $P_x′$ and $P_z′$ as well as helicity-independent component $P_y$ have been measured in the $p(\vec{e},e′\vec{p})\pi^0$ pion electroproduction process. We have extracted the polarization components in a single kinematic point at four-momentum transfer $Q^2 \simeq 0.1$~GeV$^2$, center-of-mass emission angle $\theta_p^\ast \simeq 90^\circ$, and invariant mass $W \simeq 1440$ MeV. This work represents the first precise measurement of double-polarization observables in the energy region of the Roper resonance. We have compared the extracted polarization components with the state-of-the-art electroproduction models MAID and DMT, as well as with the full-scale partial-wave analysis SAID. We have identified the MAID model predictions as the most adequate and performed a model-dependent extraction of the Roper’s scalar electroexcitation amplitude $S_{1/2}$ .

• Language: English
• Keywords: elementary particle physics, nuclear reactions, electron scattering, Roper resonance, electroproduction, coincidence experiments, polarization, response functions, helicity amplitudes
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2017
• PID: 20.500.12556/RUL-99560
• COBISS.SI-ID: 3168612
• Publication date in RUL: 02.02.2018

Secondary language

• Language: Slovenian
• Title: Analiza procesa $p(e,e'p)\pi^0$ v območju Roperjeve resonance N(1440)

Abstract

Elektroprodukcija psevdoskalarnih mezonov je najučinkovitejše in najobčutljivejše orodje za raziskovanje jedrskih resonanc. Najnižje ležeče vzbujeno stanje nukleona s pozitivno parnostjo P11(1440), imenovano tudi Roperjeva resonanca, že vse od odkritja postavlja raziskovalce pred številne izzive zaradi svojih nenavadnih lastnosti. Izvedli smo meritev polarizacije iz elektroprodukcijske reakcije $p(\vec{e},e′\vec{p})\pi^0$ odrinjenih protonov in določili od sučnosti odvisni polarizacijski komponenti Px′ in Pz′ ter od sučnosti neodvisno komponento Py v energijskem območju Roperjeve resonance. Komponente smo izluščili v kinematični točki s prenosom štirivektorja gibalne količine $Q^2 \simeq 0.1$~GeV$^2$, težiščnim emisijskim kotom $\theta_p^\ast \simeq 90^\circ$ in invariantno maso hadronskega sistema $W \simeq 1440$ MeV. To delo predstavlja prvo namensko meritev dvojno-polarizacijskih opazljivk v energijskem območju Roperjeve resonance. Ob primerjavi izluščenih polarizacijskih komponent z napovedmi naprednih elektroprodukcijskih modelov MAID in DMT ter napovedmi analize parcialnih valov SAID, smo prepoznali napovedi modela MAID kot najbolj zanesljive. MAID smo nato uporabili za modelsko odvisno analizo Roperjeve skalarne sučnostne amplitude $S_{1/2}$ ter njeno optimalno vrednost pri $Q^2$ našega poskusa tudi izluščili.

• Keywords: fizika osnovnih delcev, jedrske reakcije, elektronsko sipanje, Roperjeva resonanca, elektroprodukcija, koincidenčni poskusi, polarizacija, strukturne funkcije, sučnostne amplitude