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Register a new userSearch for Axion-Like Particles produced in $e^+e^-$ collisions at Belle II
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We present a search for the direct production of a light pseudoscalar $a$ decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process ${e^+e^-togamma a, a togammagamma}$ in the mass range ${0.2} ,< m_a < {9.7},{text{GeV/$c$}^2}$ using data corresponding to an integrated luminosity of $(445pm 3),text{pb}^{-1}$. Light pseudoscalars interacting predominantly with standard model gauge bosons (so-called axion-like particles or ALPs) are frequently postulated in extensions of the standard model. We find no evidence for ALPs and set 95% confidence level upper limits on the coupling strength $g_{agammagamma}$ of ALPs to photons at the level of $10^{-3},{text{GeV}^{-1}}$. The limits are the most restrictive to date for $0.2,<,m_a,<,1,{text{GeV/$c$}^2}$.
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The enhancement of charged-particle pairs with large pseudorapidity difference and small azimuthal angle difference, often referred to as the ``ridge signal, is a phenomenon widely observed in high multiplicity proton-proton, proton-ion and deutron-ion collisions, which is not yet fully understood. In heavy-ion collisions, the hydrodynamic expansion of the Quark-Gluon Plasma is one of the possible explanations of the origin of the ridge signal. Measurements in the $e^+e^-$ collision system, without the complexities introduced by hadron structure in the initial state, can serve as a complementary probe to examine the formation of a ridge signal. The first measurement of two-particle angular correlation functions in high multiplicity $e^+e^-$ collisions at $sqrt{s}=10.52$ GeV is reported. The hadronic $e^+e^-$ annihilation data collected by the Belle detector at KEKB are used in this study. Two-particle angular correlation functions are measured over the full azimuth and large pseudorapidity intervals which are defined by either the electron beam axis or the event thrust as a function of charged particle multiplicity. The measurement in the event thrust analysis, with mostly outgoing quark pairs determining the reference axis, is sensitive to the region of additional soft gluon emissions. No significant ridge signal is observed with either coordinates analyses. Near side jet correlations appear to be absent in the thrust axis analysis. The measurements are compared to predictions from various event generators and expected to provide new constraints to the phenomenological models in the low energy regime.
We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), $X$, at the $B$-factory experiment Belle II. We focus on production of the ALP in the tau decays $tau to X l$ with $l=e,mu$, followed by its decay via $Xto l^- l^+$. The ALP can be either promptly decaying or long-lived. We perform Monte-Carlo simulations, recasting a prompt search at Belle for lepton-flavor-violating $tau$ decays, and propose a displaced-vertex (DV) search. For both types of searches, we derive the Belle~II sensitivity reaches in both the product of branching fractions and the ALP coupling constants, as functions of the ALP mass and lifetime. The results show that the DV search exceeds the sensitivity reach of the prompt search to the relevant branching fractions by up to about a factor of 40 in the long decay length regime.
The Heavy Photon Search experiment took its first data in a 2015 engineering run using a 1.056 GeV, 50 nA electron beam provided by CEBAF at the Thomas Jefferson National Accelerator Facility, searching for an electro-produced dark photon. Using 1.7 days (1170 nb$^{-1}$) of data, a search for a resonance in the $e^{+}e^{-}$ invariant mass distribution between 19 and 81 MeV/c$^2$ showed no evidence of dark photon decays above the large QED background, confirming earlier searches and demonstrating the full functionality of the experiment. Upper limits on the square of the coupling of the dark photon to the Standard Model photon are set at the level of 6$times$10$^{-6}$. In addition, a search for displaced dark photon decays did not rule out any territory but resulted in a reliable analysis procedure that will probe hitherto unexplored parameter space with future, higher luminosity runs.
The Heavy Photon Search experiment took its first data in a 2015 engineering run at the Thomas Jefferson National Accelerator Facility, searching for a prompt, electro-produced dark photon with a mass between 19 and 81 MeV/$c^2$. A search for a resonance in the $e^{+}e^{-}$ invariant mass distribution, using 1.7 days (1170 nb$^{-1}$) of data, showed no evidence of dark photon decays above the large QED background, confirming earlier searches and demonstrating the full functionality of the experiment. Upper limits on the square of the coupling of the dark photon to the Standard Model photon are set at the level of 6$times$10$^{-6}$. Future runs with higher luminosity will explore new territory.
Theories beyond the standard model often predict the existence of an additional neutral boson, the $Z^{prime}$. Using data collected by the Belle II experiment during 2018 at the SuperKEKB collider, we perform the first searches for the invisible decay of a $Z^{prime}$ in the process $e^+ e^- to mu^+ mu^- Z^{prime}$ and of a lepton-flavor-violating $Z^{prime}$ in $e^+ e^- to e^{pm} mu^{mp} Z^{prime}$. We do not find any excess of events and set 90% credibility level upper limits on the cross sections of these processes. We translate the former, in the framework of an $L_{mu}-L_{tau}$ theory, into upper limits on the $Z^{prime}$ coupling constant at the level of $5 times 10^{-2}$ -- $1$ $M_{Z^prime}leq 6$ GeV/$c^2$.
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