arXiv subjects

A. Passeri

Publications and source records attributed to A. Passeri.

At least 19 recordsLinked to original sources

arXiv2025

Measurement of $\eta\to\pi^{0}\gamma\gamma$ branching fraction with the KLOE detector

We present a measurement of the radiative decay $\eta\to\pi^0\gamma\gamma$ using 82 million $\eta$ mesons produced in $e^+e^-\to\phi\to\eta\gamma$ process at the Frascati $\phi$-factory DA$\Phi$NE. From the data analysis $1246\pm133$ signal events are observed. By normalising the signal to the well-known $\eta\to3\pi^0$ decay the branching fraction ${\cal B}(\eta\to\pi^0\gamma\gamma)$ is measured to be $(0.98\pm 0.11_\text{stat}\pm 0.14_\text{syst})\times10^{-4}$. This result agrees with a preliminary KLOE measurement, but is twice smaller than the present world average. Results for $d\Gamma(\eta\to\pi^0\gamma\gamma)/dM^2(\gamma\gamma)$ are also presented and compared with latest theory predictions.

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arXiv2025

Design and Commissioning of Readout Electronics for a $K_L^0$ and $\mu$ Detector at the Belle II Experiment

The K-long and muon detector (KLM) constitutes the outer-most volume of the Belle II spectrometer at the interaction region of the SuperKEKB collider in Tsukuba, Japan. The KLM detector was partially upgraded since the Belle experiment by replacing many of its resistive-plate chambers with scintillators containing wavelength-shifting fibers and instrumenting it with silicon photomultipliers. We describe the readout electronics, firmware, and software created to control and acquire data from the scintillators and resistive-plate chambers.

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arXiv2024

Production cross sections of light and charmed mesons in $e^+e^-$ annihilation near 10.58 GeV

We report measurements of production cross sections for $\rho^+$, $\rho^0$, $\omega$, $K^{*+}$, $K^{*0}$, $\phi$, $\eta$, $K_S^0$, $f_0(980)$, $D^+$, $D^0$, $D_s^+$, $D^{*+}$, $D^{*0}$, and $D^{*+}_s$ in $e^+e^-$ collisions at a center-of-mass energy near 10.58 GeV. The data were recorded by the Belle experiment, consisting of 571 fb$^{-1}$ at 10.58 GeV and 74 fb$^{-1}$ at 10.52 GeV. Production cross sections are extracted as a function of the fractional hadron momentum $x_p$ . The measurements are compared to {\sc pythia} Monte Carlo generator predictions with various fragmentation settings, including those that have increased fragmentation into vector mesons over pseudo-scalar mesons. The cross sections measured for light hadrons are consistent with no additional increase of vector over pseudo-scalar mesons. The charmed-meson cross sections are compared to earlier measurements -- when available -- including older Belle results, which they supersede. They are in agreement before application of an improved initial-state radiation correction procedure that causes slight changes in their \xp shapes.

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arXiv2024

The new $K_S \rightarrow \pi e \nu$ branching fraction measurement at KLOE

A new measurement of the branching fraction for the decay $K_S \rightarrow \pi e \nu$ is presented, based on a sample of 300 million $K_S$ mesons recorded by the KLOE experiment at the DA$\Phi$NE $e^+e^-$ collider. A two-step signal selection strategy is used, exploiting first kinematic variables and then time-of-flight measurements. Data control samples of $K_S \rightarrow \pi e \nu$ decays are used to evaluate signal selection efficiencies. Normalizing the selected sample to the number of $K_S \rightarrow \pi^+ \pi^-$ decay events the result for the branching fraction is $B(K_S \rightarrow \pi e \nu)=(7.211 \pm 0.046 \pm 0.052)\times 10^-4$. The combination with our previous measurement gives $B(K_S \rightarrow \pi e \nu)=(7.153 \pm 0.037 \pm 0.043)\times 10^-4 =(7.153 \pm 0.057)\times 10^-4$. From this value we derive $f^+(0)|Vus|=0.2170 \pm 0.0009$.

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arXiv2022

Direct tests of T, CP, CPT symmetries in transitions of neutral K mesons with the KLOE experiment

Tests of the T, CP and CPT symmetries in the neutral kaon system are performed by the direct comparison of the probabilities of a kaon transition process to its symmetry-conjugate. The exchange of in and out states required for a genuine test involving an anti-unitary transformation implied by time-reversal is implemented exploiting the entanglement of $K^0\bar{K}{}^0$ pairs produced at a $\phi$-factory. A data sample collected by the KLOE experiment at DA$\Phi$NE corresponding to an integrated luminosity of about 1.7 fb$^{-1}$ is analysed to study the $\Delta$t distributions of the $\phi\to K_{S}K_{L}\to \pi^+\pi^- \: \pi^{\pm}e^{\mp}\nu$ and $\phi\to K_{S}K_{L}\to \pi^{\pm}e^{\mp}\nu \: 3\pi^0$ processes, with $\Delta$t the difference of the kaon decay times. A comparison of the measured $\Delta$t distributions in the asymptotic region $\Delta t \gg \tau_{S}$ allows to test for the first time T and CPT symmetries in kaon transitions with a precision of few percent, and to observe CP violation with this novel method.

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arXiv2022

Measurement of the $K_S \to \pi e \nu$ branching fraction with the KLOE experiment

The branching fraction for the decay $K_S \to \pi e \nu$ has been measured with a sample of 300 million $K_S$ mesons produced in $\phi \to K_L K_S$ decays recorded by the KLOE experiment at the DA$\Phi$NE $e^+e^-$ collider. Signal decays are selected by a boosted decision tree built with kinematic variables and time-of-flight measurements. Data control samples of $K_L \to \pi e \nu$ decays are used to evaluate signal selection efficiencies. A fit to the reconstructed electron mass distribution finds 49647$\pm$316 signal events. Normalising to the $K_S \to \pi^+\pi^-$ decay events the result for the branching fraction is $\mathcal{B}(K_S \to \pi e \nu) = (7.211 \pm 0.046_{\rm stat} \pm 0.052_{\rm syst}) \times10^{-4}$. The combination with our previous measurement gives $\mathcal{B}(K_S \to \pi e \nu) = (7.153 \pm 0.037_{\rm stat} \pm 0.043_{\rm syst}) \times10^{-4}$. From this value we derive $f_+(0)|V_{us}| = 0.2170 \pm 0.009$.

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arXiv2021

Precision tests of Quantum Mechanics and CPT symmetry with entangled neutral kaons at KLOE

The quantum interference between the decays of entangled neutral kaons is studied in the process $\phi\rightarrow K_S K_L \rightarrow\pi^+\pi^-\pi^+\pi^-$, which exhibits the characteristic Einstein--Podolsky--Rosen correlations that prevent both kaons to decay into $\pi^+\pi^-$ at the same time. This constitutes a very powerful tool for testing at the utmost precision the quantum coherence of the entangled kaon pair state, and to search for tiny decoherence and CPT violation effects, which may be justified in a quantum gravity framework. The analysed data sample was collected with the KLOE detector at DA$\Phi$NE, the Frascati $\phi$-factory, and corresponds to an integrated luminosity of about 1.7 fb$^{-1}$, i.e. to about $1.7 \times 10^9$ $\phi\rightarrow K_S K_L$ decays produced. From the fit of the observed $\Delta t$ distribution, being $\Delta t$ the difference of the kaon decay times, the decoherence and CPT violation parameters of various phenomenological models are measured with a largely improved accuracy with respect to previous analyses. The results are consistent with no deviation from quantum mechanics and CPT symmetry, while for some parameters the precision reaches the interesting level at which -- in the most optimistic scenarios -- quantum gravity effects might show up. They provide the most stringent limits up to date on the considered models.

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arXiv2021

$B$-flavor tagging at Belle II

We report on new flavor tagging algorithms developed to determine the quark-flavor content of bottom ($B$) mesons at Belle II. The algorithms provide essential inputs for measurements of quark-flavor mixing and charge-parity violation. We validate and evaluate the performance of the algorithms using hadronic $B$ decays with flavor-specific final states reconstructed in a data set corresponding to an integrated luminosity of $62.8$ fb$^{-1}$, collected at the $\Upsilon$(4$S$) resonance with the Belle II detector at the SuperKEKB collider. We measure the total effective tagging efficiency to be $\varepsilon_{\rm eff} = \big(30.0 \pm 1.2(\text{stat}) \pm 0.4(\text{syst})\big)\%$ for a category-based algorithm and $\varepsilon_{\rm eff} = \big(28.8 \pm 1.2(\text{stat}) \pm 0.4(\text{syst})\big)\%$ for a deep-learning-based algorithm.

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arXiv2021

Large-Area SiPM Pixels (LASiPs): a cost-effective solution towards compact large SPECT cameras

Single Photon Emission Computed Tomography (SPECT) scanners based on photomultiplier tubes (PMTs) are still largely employed in the clinical environment. A standard camera for full-body SPECT employs $\sim50$-100 PMTs of 4-8~cm diameter and is shielded by a thick layer of lead, becoming a heavy and bulky system that can weight a few hundred kilograms. The volume, weight and cost of a camera can be significantly reduced if the PMTs are replaced by silicon photomultipliers (SiPMs). The main obstacle to use SiPMs in full-body SPECT is the limited size of their sensitive area. A few thousand channels would be needed to fill a camera if using the largest commercially-available SiPMs of 6$\times$6~mm$^2$. As a solution, we propose to use Large-Area SiPM Pixels (LASiPs), built by summing individual currents of several SiPMs into a single output. We developed a LASiP prototype that has a sensitive area 8 times larger than a 6$\times$6~mm$^2$ SiPM. We built a proof-of-concept micro-camera consisting of a 40$\times$40$\times$8~mm$^3$ NaI(Tl) crystal coupled to 4 LASiPs. We evaluated its performance in a central region of $15\times15$~mm$^2$, where we were able to reconstruct images of a $^{99m}$Tc capillary with an intrinsic spatial resolution of $\sim2$~mm and an energy resolution of $\sim11.6$\% at 140 keV. We used these measurements to validate Geant4 simulations of the system. This can be extended to simulate a larger camera with more and larger pixels, which could be used to optimize the implementation of LASiPs in large SPECT cameras. We provide some guidelines towards this implementation.

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arXiv2020

Upper limit on the $\eta\to\pi^{+}\pi^{-}$ branching fraction with the KLOE experiment

Based on an integrated luminosity of 1.61 fb$^{-1}$ $e^+e^-$ collision data collected with the KLOE detector at DA$\Phi$NE, the Frascati $\phi$-factory, a search for the $P$- and $CP$-violating decay $\eta\to\pi^{+}\pi^{-}$ has been performed. Radiative $\phi\to\eta\gamma$ decay is exploited to access the $\eta$ mesons. No signal is observed in the $\pi^{+}\pi^{-}$ invariant mass spectrum, and the upper limit on the branching fraction at 90\% confidence level is determined to be ${\mathcal B}(\eta\to\pi^{+}\pi^{-})<4.9\times10^{-6}$, which is approximately three times smaller than the previous KLOE result. From the combination of these two measurements we get ${\mathcal B}(\eta\to\pi^{+}\pi^{-}) < 4.4\times10^{-6}$ at 90\% confidence level.

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arXiv2020

Measurement of the branching fraction for the decay $K_S \to πμν$ with the KLOE detector

Based on a sample of 300 million $K_S$ mesons produced in $ϕ\to K_L K_S$ decays recorded by the KLOE experiment at the DA$Φ$NE $e^+e^-$ collider we have measured the branching fraction for the decay $K_S \to πμν$. The $K_S$ mesons are identified by the interaction of $K_L$ mesons in the detector. The $K_S \to πμν$ decays are selected by a boosted decision tree built with kinematic variables and by a time-of-flight measurement. Signal efficiencies are evaluated with data control samples of $K_L \to πμν$ decays. A fit to the reconstructed muon mass distribution finds $7223 \pm 180$ signal events. Normalising to the $K_S \to π^+ π^-$ decay events the result for the branching fraction is $\mathcal{B}(K_S \to πμν) = (4.56 \pm 0.11_{\rm stat} \pm 0.17_{\rm syst})\times10^{-4}$.

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arXiv2018

Measurement of the charge asymmetry for the $K_S \rightarrow πe ν$ decay and test of CPT symmetry with the KLOE detector

Using 1.63 fb$^{-1}$ of integrated luminosity collected by the KLOE experiment about $7\times 10^4$ $K_S \rightarrow π^{\pm}e^{\mp}ν$ decays have been reconstructed. The measured value of the charge asymmetry for this decay is $A_S = (-4.9 \pm 5.7_{stat} \pm 2.6_{syst}) \times 10^{-3}$, which is almost twice more precise than the previous KLOE result. The combination of these two measurements gives $A_S = (-3.8 \pm 5.0_{stat} \pm 2.6_{syst}) \times 10^{-3}$ and, together with the asymmetry of the $K_L$ semileptonic decay, provides significant tests of the CPT symmetry. The obtained results are in agreement with CPT invariance.

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arXiv2018

Combined limit on the production of a light gauge boson decaying into $μ^+μ^-$ and $π^+π^-$

We searched for the $μ^+μ^-$ decay of a light vector gauge boson, also known as dark photon, in the $e^+ e^- \to μ^+ μ^- γ_{\rm ISR}$ 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$Φ$NE $ϕ$-factory. No structures have been observed over the irreducible $μ^+ μ^-$ background. A 90\% CL limit on the ratio $\varepsilon^2=α^{\prime}/α$ 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 $e^+ e^- \to π^+ π^- γ_{\rm ISR}$ events. The combined 90\% CL limit increases the sensitivity especially in the $ρ-ω$ 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.

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arXiv2018

Combination of KLOE $σ\big(e^+e^-\rightarrowπ^+π^-γ(γ)\big)$ measurements and determination of $a_μ^{π^+π^-}$ in the energy range $0.10 < s < 0.95$ GeV$^2$

The three precision measurements of the cross section $σ\big(e^+e^-\rightarrowπ^+π^-γ(γ)\big)$ using initial state radiation by the KLOE collaboration provide an important input for the prediction of the hadronic contribution to the anomalous magnetic moment of the muon. These measurements are correlated for both statistical and systematic uncertainties and, therefore, the simultaneous use of these measurements requires covariance matrices that fully describe the correlations. We present the construction of these covariance matrices and use them to determine a combined KLOE measurement for $σ\big(e^+e^-\rightarrowπ^+π^-γ(γ)\big)$. We find, from this combination, a two-pion contribution to the muon magnetic anomaly in the energy range $0.10 < s < 0.95$ GeV$^2$ of $a_μ^{π^+π^-} = (489.8 \pm 1.7_{\rm stat} \pm 4.8_{\rm sys} ) \times 10^{-10}$.

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arXiv2018

First Measurements of Beam Backgrounds at SuperKEKB

The high design luminosity of the SuperKEKB electron-positron collider is expected to result in challenging levels of beam-induced backgrounds in the interaction region. Properly simulating and mitigating these backgrounds is critical to the success of the Belle~II experiment. We report on measurements performed with a suite of dedicated beam background detectors, collectively known as BEAST II, during the so-called Phase 1 commissioning run of SuperKEKB in 2016, which involved operation of both the high energy ring (HER) of 7 GeV electrons as well as the low energy ring (LER) of 4 GeV positrons. We describe the BEAST II detector systems, the simulation of beam backgrounds, and the measurements performed. The measurements include standard ones of dose rates versus accelerator conditions, and more novel investigations, such as bunch-by-bunch measurements of injection backgrounds and measurements sensitive to the energy spectrum and angular distribution of fast neutrons. We observe beam-gas, Touschek, beam-dust, and injection backgrounds. We do not observe significant synchrotron radiation, as expected. Measured LER beam-gas backgrounds and Touschek backgrounds in both rings are slightly elevated, on average three times larger than the levels predicted by simulation. HER beam-gas backgrounds are on on average two orders of magnitude larger than predicted. Systematic uncertainties and channel-to-channel variations are large, so that these excesses constitute only 1-2 sigma level effects. Neutron background rates are higher than predicted and should be studied further. We will measure the remaining beam background processes, due to colliding beams, in the imminent commissioning Phase 2. These backgrounds are expected to be the most critical for Belle II, to the point of necessitating replacement of detector components during the Phase 3 (full-luminosity) operation of SuperKEB.

physics.ins-det↗

arXiv2017

Measurement of the running of the fine structure constant below 1 GeV with the KLOE Detector

We have measured the running of the effective QED coupling constant $α(s)$ in the time-like region $0.6<\sqrt s< 0.975$ GeV with the KLOE detector at DA$Φ$NE using the Initial State Radiation process $e^+e^-\toμ^+ μ^-γ$. It represents the first measurement of the running of $α(s)$ in this energy region. Our results show a more than 5$σ$ significance of the hadronic contribution to the running of $α(s)$, which is the strongest direct evidence both in time- and space-like regions achieved in a single measurement. By using the $e^+e^-\toπ^+π^-$ cross section measured by KLOE, the real and imaginary part of the shift $Δα(s)$ has been extracted. By a fit of the real part of $Δα(s)$ and assuming the lepton universality the branching ratio $BR(ω\toμ^+μ^-) = (6.6\pm1.4_{stat}\pm1.7_{syst})\cdot 10^{-5} $ has been determined.

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arXiv2016

Measurement of the $ϕ\to π^0 e^+e^-$ transition form factor with the KLOE detector

A measurement of the vector to pseudoscalar conversion decay $ϕ\to π^0 e^+e^-$ with the KLOE experiment is presented. A sample of $\sim 9500$ signal events was selected from a data set of 1.7 fb$^{-1}$ of $e^+e^-$ collisions at $\sqrt{s} \sim m_ϕ$ collected at the DA$Φ$NE $e^+e^-$ collider. These events were used to obtain the first measurement of the transition form factor $| F_{ϕπ^0}(q^2) |$ and a new measurement of the branching ratio of the decay: $\rm{BR}\,(ϕ\to π^0 e^+e^-) = (\,1.35 \pm 0.05^{\,\,+0.05}_{\,\,-0.10}\,) \times 10 ^{-5}$. The result improves significantly on previous measurements and is in agreement with theoretical predictions.

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arXiv2016

Limit on the production of a new vector boson in $\mathrm{e^+ e^-}\rightarrow {\rm U}γ$, U$\rightarrow π^+π^-$ with the KLOE experiment

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 ${\rm e^+ e^-}\rightarrow {\rm U}γ$, U$\rightarrow π^+π^-$ based on 28 million $\mathrm{e^+ e^-} \rightarrow π^+ π^-γ$ events collected at DA$Φ$NE by the KLOE experiment. The $π^+ π^-$ production by initial-state radiation compensates for a loss of sensitivity of previous KLOE ${\rm U} \rightarrow \mathrm{e^+ e^-}$, $μ^+μ^-$ 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, $\varepsilon^2$, in the U mass range between $527$ and $987$~MeV. Above 700 MeV this new limit is more stringent than previous ones.

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