arXiv subjects

R. Biondi

Publications and source records attributed to R. Biondi.

62 records · Page 4Linked to original sources

arXiv2021

Improved search for neutron to mirror-neutron oscillations in the presence of mirror magnetic fields with a dedicated apparatus at the PSI UCN source

While the international nEDM collaboration at the Paul Scherrer Institut (PSI) took data in 2017 that covered a considerable fraction of the parameter space of claimed potential signals of hypothetical neutron ($n$) to mirror-neutron ($n'$) transitions, it could not test all claimed signal regions at various mirror magnetic fields. Therefore, a new study of $n-n'$ oscillations using stored ultracold neutrons (UCNs)is underway at PSI, considerably expanding the reach in parameter space of mirror magnetic fields ($B'$) and oscillation time constants ($τ_{nn'}$). The new apparatus is designed to test for the anomalous loss of stored ultracold neutrons as a function of an applied magnetic field. The experiment is distinguished from its predecessors by its very large storage vessel (1.47\,m$^3$), enhancing its statistical sensitivity. In a test experiment in 2020 we have demonstrated the capabilities of our apparatus. However, the full analysis of our recent data is still pending. Based on already demonstrated performance, we will reach a sensitivity to oscillation times $τ_{nn'}/\sqrt{\cos(β)}$ well above hundred seconds, with $β$ being the angle between $B'$ and the applied magnetic field $B$. The scan of $B$ will allow the finding or the comprehensive exclusion of potential signals reported in the analysis of previous experiments and suggested to be consistent with neutron to mirror-neutron oscillations.

physics.ins-det↗

arXiv2021

Identification of the cosmogenic $^{11}$C background in large volumes of liquid scintillators with Borexino

Cosmogenic radio-nuclei are an important source of background for low-energy neutrino experiments. In Borexino, cosmogenic $^{11}$C decays outnumber solar $pep$ and CNO neutrino events by about ten to one. Highly efficient identification of this background is mandatory for these neutrino analyses. We present here the details of the most consolidated strategy, used throughout Borexino solar neutrino measurements. It hinges upon finding the space-time correlations between $^{11}$C decays, the preceding parent muons and the accompanying neutrons. This article describes the working principles and evaluates the performance of this Three-Fold Coincidence (TFC) technique in its two current implementations: a hard-cut and a likelihood-based approach. Both show stable performances throughout Borexino Phases II (2012-2016) and III (2016-2020) data sets, with a $^{11}$C tagging efficiency of $\sim$90 % and $\sim$63-66 % of the exposure surviving the tagging. We present also a novel technique that targets specifically $^{11}$C produced in high-multiplicity during major spallation events. Such $^{11}$C appear as a burst of events, whose space-time correlation can be exploited. Burst identification can be combined with the TFC to obtain about the same tagging efficiency of $\sim$90 % but with a higher fraction of the exposure surviving, in the range of $\sim$66-68 %.

physics.ins-det↗

arXiv2021

Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun

For most of their existence stars are fueled by the fusion of hydrogen into helium proceeding via two theoretically well understood processes, namely the $pp$ chain and the CNO cycle. Neutrinos emitted along such fusion processes in the solar core are the only direct probe of the deep interior of the star. A complete spectroscopy of neutrinos from the {\it pp} chain, producing about 99\% of the solar energy, has already been performed \cite{bib:Nature-2018}. Here, we report the direct observation, with a high statistical significance, of neutrinos produced in the CNO cycle in the Sun. This is the first experimental evidence of this process obtained with the unprecedentedly radio-pure large-volume liquid-scintillator Borexino detector located at the underground Laboratori Nazionali del Gran Sasso in Italy. The main difficulty of this experimental effort is to identify the excess of the few counts per day per 100 tonnes of target due to CNO neutrino interactions above the backgrounds. A novel method to constrain the rate of \bi contaminating the scintillator relies on the thermal stabilisation of the detector achieved over the past 5 years. In the CNO cycle, the hydrogen fusion is catalyzed by the carbon (C) - nitrogen (N) - oxygen (O) and thus its rate, as well as the flux of emitted CNO neutrinos, directly depends on the abundance of these elements in solar core. Therefore, this result paves the way to a direct measurement of the solar metallicity by CNO neutrinos. While this result quantifies the relative contribution of the CNO fusion in the Sun to be of the order of 1\%, this process is dominant in the energy production of massive stars. The occurrence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe has been proven.

hep-ex↗

arXiv2021

The Low Polonium Field of Borexino and its significance for the CNO neutrino detection

Borexino is a liquid scintillator detector located at the Laboratori Nazionale del Gran Sasso, Italy with the main goal to measure solar neutrinos. The experiment recently provided the first direct experimental evidence of CNO-cycle neutrinos in the Sun, rejecting the no-CNO signal hypothesis with a significance greater than 5$σ$ at 99\%C.L. The intrinsic $^{210}$Bi is an important background for this analysis due to its similar spectral shape to that of CNO neutrinos. $^{210}$Bi can be measured through its daughter $^{210}$Po which can be distinguished through an event-by-event basis via pulse shape discrimination. However, this required reducing the convective motions in the scintillator that brought additional $^{210}$Po from peripheral sources. This was made possible through the thermal insulation and stabilization campaign performed between 2015 and 2016. This article will explain the strategy and the different methods performed to extract the $^{210}$Bi upper limit in Phase-III (Jul 2016- Feb 2020) of the experiment through the analysis of $^{210}$Po in the cleanest region of the detector called the Low Polonium Field.

physics.ins-det↗

arXiv2021

First detection of CNO neutrinos with Borexino

Neutrinos are elementary particles which are known since many years as fundamental messengers from the interior of the Sun. The Standard Solar Model, which gives a theoretical description of all nuclear processes which happen in our star, predicts that roughly 99% of the energy produced is coming from a series of processes known as the "pp chain". Such processes have been studied in detail over the last years by means of neutrinos, thanks also to the important measurements provided by the Borexino experiment. The remaining 1% is instead predicted to come from a separate loop-process, known as the "CNO cycle". This sub-dominant process is theoretically well understood, but has so far escaped any direct observation. Another fundamental aspect is that the CNO cycle is indeed the main nuclear engine in stars more massive than the Sun. In 2020, thanks to the unprecedented radio-purity and temperature control achieved by the Borexino detector over recent years, the first ever detection of neutrinos from the CNO cycle has been finally announced. The milestone result confirms the existence of this nuclear fusion process in our Universe. Here, the details of the detector stabilization and the analysis techniques adopted are reported.

hep-ex↗

arXiv2020

Sensitivity to neutrinos from the solar CNO cycle in Borexino

Neutrinos emitted in the carbon, nitrogen, oxygen (CNO) fusion cycle in the Sun are a sub-dominant, yet crucial component of solar neutrinos whose flux has not been measured yet. The Borexino experiment at the Laboratori Nazionali del Gran Sasso (Italy) has a unique opportunity to detect them directly thanks to the detector's radiopurity and the precise understanding of the detector backgrounds. We discuss the sensitivity of Borexino to CNO neutrinos, which is based on the strategies we adopted to constrain the rates of the two most relevant background sources, pep neutrinos from the solar pp-chain and Bi-210 beta decays originating in the intrinsic contamination of the liquid scintillator with Pb-210. Assuming the CNO flux predicted by the high-metallicity Standard Solar Model and an exposure of 1000 daysx71.3 t, Borexino has a median sensitivity to CNO neutrino higher than 3 sigma. With the same hypothesis the expected experimental uncertainty on the CNO neutrino flux is 23%, provided the uncertainty on the independent estimate of the Bi-210 interaction rate is 1.5 cpd/100t. Finally, we evaluated the expected uncertainty of the C and N abundances and the expected discrimination significance between the high and low metallicity Standard Solar Models (HZ and LZ) with future more precise measurement of the CNO solar neutrino flux.

hep-ex↗

arXiv2020

New high-sensitivity searches for neutrons converting into antineutrons and/or sterile neutrons at the European Spallation Source

The violation of Baryon Number, $\mathcal{B}$, is an essential ingredient for the preferential creation of matter over antimatter needed to account for the observed baryon asymmetry in the universe. However, such a process has yet to be experimentally observed. The HIBEAM/NNBAR %experiment program is a proposed two-stage experiment at the European Spallation Source (ESS) to search for baryon number violation. The program will include high-sensitivity searches for processes that violate baryon number by one or two units: free neutron-antineutron oscillation ($n\rightarrow \bar{n}$) via mixing, neutron-antineutron oscillation via regeneration from a sterile neutron state ($n\rightarrow [n',\bar{n}'] \rightarrow \bar{n}$), and neutron disappearance ($n\rightarrow n'$); the effective $Δ\mathcal{B}=0$ process of neutron regeneration ($n\rightarrow [n',\bar{n}'] \rightarrow n$) is also possible. The program can be used to discover and characterise mixing in the neutron, antineutron, and sterile neutron sectors. The experiment addresses topical open questions such as the origins of baryogenesis, the nature of dark matter, and is sensitive to scales of new physics substantially in excess of those available at colliders. A goal of the program is to open a discovery window to neutron conversion probabilities (sensitivities) by up to three orders of magnitude compared with previous searches. The opportunity to make such a leap in sensitivity tests should not be squandered. The experiment pulls together a diverse international team of physicists from the particle (collider and low energy) and nuclear physics communities, while also including specialists in neutronics and magnetics.

physics.ins-det↗

arXiv2017

New experimental limits on neutron - mirror neutron oscillations in the presence of mirror magnetic field

Present probes do not exclude that the neutron ($n$) oscillation into mirror neutron ($n'$), a sterile state exactly degenerate in mass with the neutron, can be a very fast process, in fact faster than the neutron decay itself. This process is sensitive to the magnetic field. Namely, if the mirror magnetic field $\vec{B}'$ exists at the Earth, $n-n'$ oscillation probability can be suppressed or resonantly amplified by the applied magnetic field $\vec{B}$, depending on its strength and on the angle $β$ between $\vec{B}$ and $\vec{B}'$. We present the results of ultra-cold neutron storage measurements aiming to check the anomalies observed in previous experiments which could be a signal for $n-n'$ oscillation in the presence of mirror magnetic field $B'\sim 0.1$~G. Analyzing the experimental data on neutron loses, we obtain a new lower limit on $n-n'$ oscillation time $τ_{nn'} > 17$ s (95 % C.L.) for any $B'$ between 0.08 and 0.17 G, and $τ_{nn'}/\sqrt{\cosβ} > 27 $s (95 % C.L.) for any $B'$ in the interval ($0.06\div0.25$) G.

hep-ex↗