Some properties of elementary particles or a determination of the lifetime of the neutral pi meson
<p>Theoretical estimates of the lifetime of the neutral π meson have indicated a value of the order of 10<sup>−17</sup> secs. In order to determine the lifetime by flight distance measurements a method employing very fine resolution is required. Nuclear emulsions provide a medium f...
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| Format: | Thesis |
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1963
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| _version_ | 1826287436746981376 |
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| author | Evans, D |
| author_facet | Evans, D |
| author_sort | Evans, D |
| collection | OXFORD |
| description | <p>Theoretical estimates of the lifetime of the neutral π meson have indicated a value of the order of 10<sup>−17</sup> secs. In order to determine the lifetime by flight distance measurements a method employing very fine resolution is required. Nuclear emulsions provide a medium for resolution of distances of the order of a micron.</p> <p>This experiment uses the same principle as that first tried by Harris et al.(1957). The decay at rest of the K<sup>+</sup> meson in the K<sub>π2</sub> mode (π<sup>+</sup>π<sup>0</sup>) provides a π<sup>0</sup> of unique velocity and a direction directly opposite to that of the π<sup>+</sup>; the latter being charged, leaves a track in the emulsion. The decay of the π<sup>0</sup> is detected in emulsion if it takes place in the Dalitz mode:-</p> <p>π<sup>0</sup> γ+e<sup>+</sup>+e<sup>−</sup> (br. ratio 1.2%)</p> Detection is possible because of the emission of charged particles. <p>A stack of Ilford L4 emulsions was exposed to a beam of stopping K mesons. After processing, the stack was scanned for K<sub>π2</sub> decays with the emission of a Dalitz pair. Other π<sup>0</sup> producing decays of the K<sup>+</sup> (K<sub>μ3</sub>, K<sub>e3</sub>, τ′) were at first indistinguishable from the desired K<sub>π2</sub> decays.</p> <p>In order to reduce this contamination, grain density measurements were made on the charged secondary from the K<sup>+</sup> decay. By rejecting events in which g* was greater than 1.3 all τ′ and 30% of the K<sub>μ3</sub> events were rejected. A correction factor was calculated to account for the remaining decays.</p> <p>The co-ordinates of the grains composing the events were measured relative to axes fixed in the field of view. A bifilar micrometer eyepiece was used which has the motions of the cross-wires digitised to enable rapid measurement. The co-ordinates were automatically punched out on to tape ready for input to the computer.</p> <p>The tracks of the particles were obtained by fitting straight lines to the measured co-ordinates. The decay point of the π<sup>0</sup> was defined as the intersection of the electron tracks with the line of flight of the π<sup>+</sup>. The point of production was taken as the foot of the perpendicular from the measured position of the K<sup>+</sup> on to the π<sup>+</sup> trajectory.</p> <p>The effects of errors due to measurement, "grain noise" and grain size were thoroughly investigated. Use was made of co-ordinate measurements on τ decays of the K<sup>+</sup> (π<sup>+</sup>π<sup>+</sup>π<sup>−</sup>) where it is known that the three tracks converge on the point of decay of the K<sup>+</sup>.</p> <p>An estimation of the lifetime was made using a likelihood calculation. It was discovered that the presence of events giving anomalous flight distances affected the estimate considerably. A suitable cutoff criterion was applied to reject these events.</p> <p>The result after allowing for the effect of contamination events referred to earlier was:-</p> <table> <tr> <td rowspan="2">τ<sub>π<sup>0</sup></sub></td> <td rowspan="2"> = (1.56 </td> <td>+ 0.55</td> <td rowspan="2">) × 10<sup>−16</sup> secs.</td> </tr> <tr> <td>− 0.45</td> </tr> </table> |
| first_indexed | 2024-03-07T01:58:39Z |
| format | Thesis |
| id | oxford-uuid:9c9972ee-d45e-49cb-906a-0363b6d7443d |
| institution | University of Oxford |
| last_indexed | 2024-03-07T01:58:39Z |
| publishDate | 1963 |
| record_format | dspace |
| spelling | oxford-uuid:9c9972ee-d45e-49cb-906a-0363b6d7443d2022-03-27T00:37:08ZSome properties of elementary particles or a determination of the lifetime of the neutral pi mesonThesishttp://purl.org/coar/resource_type/c_db06uuid:9c9972ee-d45e-49cb-906a-0363b6d7443dPolonsky Theses Digitisation Project1963Evans, D<p>Theoretical estimates of the lifetime of the neutral π meson have indicated a value of the order of 10<sup>−17</sup> secs. In order to determine the lifetime by flight distance measurements a method employing very fine resolution is required. Nuclear emulsions provide a medium for resolution of distances of the order of a micron.</p> <p>This experiment uses the same principle as that first tried by Harris et al.(1957). The decay at rest of the K<sup>+</sup> meson in the K<sub>π2</sub> mode (π<sup>+</sup>π<sup>0</sup>) provides a π<sup>0</sup> of unique velocity and a direction directly opposite to that of the π<sup>+</sup>; the latter being charged, leaves a track in the emulsion. The decay of the π<sup>0</sup> is detected in emulsion if it takes place in the Dalitz mode:-</p> <p>π<sup>0</sup> γ+e<sup>+</sup>+e<sup>−</sup> (br. ratio 1.2%)</p> Detection is possible because of the emission of charged particles. <p>A stack of Ilford L4 emulsions was exposed to a beam of stopping K mesons. After processing, the stack was scanned for K<sub>π2</sub> decays with the emission of a Dalitz pair. Other π<sup>0</sup> producing decays of the K<sup>+</sup> (K<sub>μ3</sub>, K<sub>e3</sub>, τ′) were at first indistinguishable from the desired K<sub>π2</sub> decays.</p> <p>In order to reduce this contamination, grain density measurements were made on the charged secondary from the K<sup>+</sup> decay. By rejecting events in which g* was greater than 1.3 all τ′ and 30% of the K<sub>μ3</sub> events were rejected. A correction factor was calculated to account for the remaining decays.</p> <p>The co-ordinates of the grains composing the events were measured relative to axes fixed in the field of view. A bifilar micrometer eyepiece was used which has the motions of the cross-wires digitised to enable rapid measurement. The co-ordinates were automatically punched out on to tape ready for input to the computer.</p> <p>The tracks of the particles were obtained by fitting straight lines to the measured co-ordinates. The decay point of the π<sup>0</sup> was defined as the intersection of the electron tracks with the line of flight of the π<sup>+</sup>. The point of production was taken as the foot of the perpendicular from the measured position of the K<sup>+</sup> on to the π<sup>+</sup> trajectory.</p> <p>The effects of errors due to measurement, "grain noise" and grain size were thoroughly investigated. Use was made of co-ordinate measurements on τ decays of the K<sup>+</sup> (π<sup>+</sup>π<sup>+</sup>π<sup>−</sup>) where it is known that the three tracks converge on the point of decay of the K<sup>+</sup>.</p> <p>An estimation of the lifetime was made using a likelihood calculation. It was discovered that the presence of events giving anomalous flight distances affected the estimate considerably. A suitable cutoff criterion was applied to reject these events.</p> <p>The result after allowing for the effect of contamination events referred to earlier was:-</p> <table> <tr> <td rowspan="2">τ<sub>π<sup>0</sup></sub></td> <td rowspan="2"> = (1.56 </td> <td>+ 0.55</td> <td rowspan="2">) × 10<sup>−16</sup> secs.</td> </tr> <tr> <td>− 0.45</td> </tr> </table> |
| spellingShingle | Evans, D Some properties of elementary particles or a determination of the lifetime of the neutral pi meson |
| title | Some properties of elementary particles or a determination of the lifetime of the neutral pi meson |
| title_full | Some properties of elementary particles or a determination of the lifetime of the neutral pi meson |
| title_fullStr | Some properties of elementary particles or a determination of the lifetime of the neutral pi meson |
| title_full_unstemmed | Some properties of elementary particles or a determination of the lifetime of the neutral pi meson |
| title_short | Some properties of elementary particles or a determination of the lifetime of the neutral pi meson |
| title_sort | some properties of elementary particles or a determination of the lifetime of the neutral pi meson |
| work_keys_str_mv | AT evansd somepropertiesofelementaryparticlesoradeterminationofthelifetimeoftheneutralpimeson |