Search

Despite being a great success of the standard model (SM) of particle physics in the description of natural phenomena, there is still some information missing that may lead to a better understanding of the Universe. One of the most important issues is dark matter (DM). The only information we have about it comes from gravitational measurements; while other interactions are still unknown. In this thesis, we are planning to do some indirect searches of dark matter via coupling to the SM particles. Since these interactions are very weak, large amounts of data are required. Belle II experiment is a successor of the Belle experiment located at KEK in Tsukuba. Belle II already started its data commissioning in 2018, plans to accumulate 50 ab−1 of data in near future. We are planning to search for a Z′ boson at Belle II, a new gauge boson that couples only to the second and third lepton family, produced alongside two muons and further decaying into another pair of muons, resulting in a four muon final state (muonic dark force). As this kind of event is mainly suppressed due to large backgrounds, one can use machine learning techniques to separate signals and backgrounds. A dedicated fitting technique we are planning to implement to give our discovery potential. Also, we are planning to do possible displaced vertex signatures, in case of a long-lived Z′ boson.Being dominated by backgrounds, these kinds of searches strongly depend on particle identification algorithms for identifying leptons. Belle II is using a standard particle identification algorithm using the Likelihood of different sub-detectors, one can improve it by using a Machine Learning based particle identification algorithm. For that purpose, I am planning to give focus on the electromagnetic calorimeter and central drift chamber of the Belle II sub-detector.

We searched for the $\mu^+\mu^-$ decay of a light vector gauge boson, also known as dark photon, in the $e^+ e^- \to \mu^+ \mu^- \gamma_{\rm ISR}$ process by means of the Initial State Radiation (ISR) method. We used 1.93~fb$^{-1}$ of data collected by the KLOE experiment at the DA$\Phi$NE $\phi$-factory. No structures have been observed over the irreducible $\mu^+ \mu^-$ background. A 90\% CL limit on the ratio $\varepsilon^2=\alpha^{\prime}/\alpha$ between the dark coupling constant and the fine structure constant of $ 3\times 10^{-6}-2\times 10^{-7}$ has been set in the dark photon mass region between 519 MeV and 973 MeV. This new limit has been combined with the published result obtained investigating the hypothesis of the dark photon decaying into hadrons in $e^+ e^- \to \pi^+ \pi^- \gamma_{\rm ISR}$ events. The combined 90\% CL limit increases the sensitivity especially in the $\rho-\omega$ interference region and excludes $\varepsilon^2$ greater than $(13-2)\times 10^{-7}$. For dark photon masses greater than 600 MeV the combined limit is lower than 8~$\times\, 10^{-7}$ resulting more stringent than present constraints from other experiments., Comment: 7 pages, 9 figures, 1 table, accepted for publication in Physics Letters B

The recent interest in a light gauge boson in the framework of an extra U(1) symmetry motivates searches in the mass range below 1 GeV. We present a search for such a particle, the dark photon, in e + e − → U γ , U → π + π − based on 28 million e + e − → π + π − γ events collected at DAΦNE by the KLOE experiment. The π + π − production by initial-state radiation compensates for a loss of sensitivity of previous KLOE U → e + e − , μ + μ − searches due to the small branching ratios in the ρ – ω resonance region. We found no evidence for a signal and set a limit at 90% CL on the mixing strength between the photon and the dark photon, e 2 , in the U mass range between 527 and 987 MeV . Above 700 MeV this new limit is more stringent than previous ones.

We searched for evidence of a Higgsstrahlung process in a secluded sector, leading to a final state with a dark photon U and a dark Higgs boson h', with the KLOE detector at DAFNE. We investigated the case of h' lighter than U, with U decaying into a muon pair and h' producing a missing energy signature. We found no evidence of the process and set upper limits to its parameters in the range 2m_mu, Comment: 16 pages, 7 figures, submitted to Physics Letters B

We have searched for a light vector boson U, the possible carrier of a "dark force", with the KLOE detector at the DAΦNE e+e- collider, motivated by astrophysical evidence for the presence of dark matter in the Universe. Using e+e- collisions collected with an integrated luminosity of 239.3 pb-1, we look for a dimuon mass peak in the reaction e+e-→μ+μ-γ, corresponding to the decay U→μ+μ-. We find no evidence for a U vector boson signal. We set a 90% CL upper limit for the mixing parameter squared between the photon and the U boson of 1.6×10-5 to 8.6×10-7 for the mass region 520