# Search for the Rare Decay $K_L \to \pi^0 \pi^0 \mu^+ \mu^-$

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### Description

This thesis describes the search for K<sub>L</sub> → π<sup>0</sup>π<sup>0</sup>μ<sup>+</sup>μ<sup>-</sup>, which was conducted using data collected by the KTeV Experiment at Fermi National Accelerator Laboratory in Batavia, Illinois. Although this decay mode is possible within the Standard Model, it is limited to a very narrow band of phase space. The HyperCP Experiment has recently observed three Σ<sup>+</sup> → pμ<sup>+</sup>μ<sup>-</sup> events within a narrow dimuon mass range of 213.8 MeV/c<sup>2</sup> to 214.8 MeV/c<sup>2</sup>. This suggests that the process could occur via a neutral intermediary particle, Σ<sup>+</sup> → pX<sup>0</sup> → pμ<sup>+</sup>μ<sup>-</sup>, with an X<sup>0</sup> mass of 214.3 MeV/c<sup>2</sup> ± 0.5 MeV/c<sup>2</sup> . Since ... continued below

204 p.

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Phillips, David Graham May 1, 2009.

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### Description

This thesis describes the search for K<sub>L</sub> → π<sup>0</sup>π<sup>0</sup>μ<sup>+</sup>μ<sup>-</sup>, which was conducted using data collected by the KTeV Experiment at Fermi National Accelerator Laboratory in Batavia, Illinois. Although this decay mode is possible within the Standard Model, it is limited to a very narrow band of phase space. The HyperCP Experiment has recently observed three Σ<sup>+</sup> → pμ<sup>+</sup>μ<sup>-</sup> events within a narrow dimuon mass range of 213.8 MeV/c<sup>2</sup> to 214.8 MeV/c<sup>2</sup>. This suggests that the process could occur via a neutral intermediary particle, Σ<sup>+</sup> → pX<sup>0</sup> → pμ<sup>+</sup>μ<sup>-</sup>, with an X<sup>0</sup> mass of 214.3 MeV/c<sup>2</sup> ± 0.5 MeV/c<sup>2</sup> . Since the X<sup>0</sup> has a light mass and a low interaction probability, then it is most likely a new neutral boson that exists beyond the Standard Model. Recent theoretical predictions suggest that the decay mode K<sub>L</sub> → π<sup>0</sup>π<sup>0</sup>μ<sup>+</sup>μ<sup>-</sup> can also occur via the aforementioned neutral boson: K<sub>L</sub> → π<sup>0</sup>π<sup>0</sup>X<sup>0</sup>→ π<sup>0</sup>π<sup>0</sup>μ<sup>+</sup>μ<sup>-</sup>. Therefore, in addition to a Standard Model measurement, the search for K<sub>L</sub> → π<sup>0</sup>π<sup>0</sup>μ<sup>+</sup>μ<sup>-</sup> is also carried out in an effort to address the viability of X<sup>0</sup> in explaining the HyperCP phenomena. Measurement of the upper limits presented in this thesis were performed using blind analysis techniques. The upper limit of the decay K<sub>L</sub> → π<sup>0</sup>π<sup>0</sup>μ<sup>+</sup>μ<sup>-</sup> has been measured in this thesis to be BR(K<sub>L</sub> → π<sup>0</sup>π<sup>0</sup>μ<sup>+</sup>μ<sup>-</sup>) &lt; 8.63 x 10<sup>-11</sup> at 90% confidence level. In addition, this thesis has measured the upper limit of the decay K<sub>L</sub> → π<sup>0</sup>π<sup>0</sup>X<sup>0</sup> → π<sup>0</sup>π<sup>0</sup>μ<sup>+</sup>μ<sup>-</sup> to be Br(K<sub>L</sub> → π<sup>0</sup>π<sup>0</sup>X<sup>0</sup> → π<sup>0</sup>π<sup>0</sup>μ<sup>+</sup>μ<sup>-</sup>) &lt; 9.44 x 10<sup>-11</sup> at the 90% confidence level.

204 p.

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• Report No.: FERMILAB-THESIS--2009-61
• Grant Number: AC02-07CH11359
• DOI: 10.2172/984004 | External Link
• Office of Scientific & Technical Information Report Number: 984004
• Archival Resource Key: ark:/67531/metadc1013213

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• May 1, 2009

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• Oct. 14, 2017, 8:36 a.m.

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Phillips, David Graham. Search for the Rare Decay $K_L \to \pi^0 \pi^0 \mu^+ \mu^-$, thesis or dissertation, May 1, 2009; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc1013213/: accessed October 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.