Eur. Phys. J. C

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 $^{210}$Bi beta decays originating in the intrinsic contamination of the liquid scintillator with $^{210}$Pb. Assuming the CNO flux predicted by the high-metallicity Standard Solar Model and an exposure of 1000 days $\times$ 71.3 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 $^{210}\text {Bi}$ interaction rate is 1.5 $\hbox {cpd}/100~\hbox {ton}$ . 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.

Phys. Rev. Lett.

A search for Higgs boson decays into a Z boson and a light resonance in two-lepton plus jet events is performed, using a pp collision dataset with an integrated luminosity of 139  fb-1 collected at s=13  TeV by the ATLAS experiment at the CERN LHC. The resonance considered is a light boson with a mass below 4 GeV from a possible extended scalar sector or a charmonium state. Multivariate discriminants are used for the event selection and for evaluating the mass of the light resonance. No excess of events above the expected background is found. Observed (expected) 95% confidence-level upper limits are set on the Higgs boson production cross section times branching fraction to a Z boson and the signal resonance, with values in the range 17–340 pb (16-5+6–320-90+130  pb) for the different light spin-0 boson mass and branching fraction hypotheses, and with values of 110 and 100 pb (100-30+40 and 100-30+40  pb) for the ηc and J/ψ hypotheses, respectively.

Nature

For most of their existence, stars are fuelled by the fusion of hydrogen into helium. Fusion proceeds via two processes that are well understood theoretically: the proton–proton (pp) chain and the carbon–nitrogen–oxygen (CNO) cycle. Neutrinos that are emitted along such fusion processes in the solar core are the only direct probe of the deep interior of the Sun. A complete spectroscopic study of neutrinos from the pp chain, which produces about 99 per cent of the solar energy, has been performed previously; however, there has been no reported experimental evidence of the CNO cycle. Here we report the direct observation, with a high statistical significance, of neutrinos produced in the CNO cycle in the Sun. This experimental evidence was obtained using the highly radiopure, large-volume, liquid-scintillator detector of Borexino, an experiment located at the underground Laboratori Nazionali del Gran Sasso in Italy. The main experimental challenge was to identify the excess signal—only a few counts per day above the background per 100 tonnes of target—that is attributed to interactions of the CNO neutrinos. Advances in the thermal stabilization of the detector over the last five years enabled us to develop a method to constrain the rate of bismuth-210 contaminating the scintillator. In the CNO cycle, the fusion of hydrogen is catalysed by carbon, nitrogen and oxygen, and so its rate—as well as the flux of emitted CNO neutrinos—depends directly on the abundance of these elements in the solar core. This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent; however, in massive stars, this is the dominant process of energy production. This work provides experimental evidence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe.

Phys. Rev. Lett.

Ultrarelativistic heavy ion collisions recreate in the laboratory the thermodynamical conditions prevailing in the early universe up to 10-6  sec, thereby allowing the study of the quark-gluon plasma (QGP), a state of quantum chromodynamics (QCD) matter with deconfined partons. The top quark, the heaviest elementary particle known, is accessible in nucleus-nucleus collisions at the CERN LHC, and constitutes a novel probe of the QGP. Here, we report the first evidence for the production of top quarks in nucleus-nucleus collisions, using lead-lead collision data at a nucleon-nucleon center-of-mass energy of 5.02 TeV recorded by the CMS experiment. Two methods are used to measure the cross section for top quark pair production (σtt¯) via the selection of charged leptons (electrons or muons) and bottom quarks. One method relies on the leptonic information alone, and the second one exploits, in addition, the presence of bottom quarks. The measured cross sections, σtt¯=2.54-0.74+0.84 and 2.03-0.64+0.71  μb, respectively, are compatible with expectations from scaled proton-proton data and QCD predictions.

Phys.Part.Nucl.Lett.

In this paper we conduct a phenomenological study for the search of evidence for the intrinsic charm mechanism in double ${J \mathord{\left/ {\vphantom {J \psi }} \right. \kern-0em} \psi }$ production at the COMPASS experiment using the CERN ${{\pi }^{ - }}$ beam at 190 GeV/c. Estimations on ${J \mathord{\left/ {\vphantom {J \psi }} \right. \kern-0em} \psi }$ pair cross-sections and kinematic distributions are given for different production mechanisms. We also re-review the double ${J \mathord{\left/ {\vphantom {J \psi }} \right. \kern-0em} \psi }$ production data provided by the NA3 experiment using the CERN ${{\pi }^{ - }}$ beam at 150 and 280 GeV/c with incident on a platinum target.

Pisma Fiz.Elem.Chast.Atom.Yadra

The description of the muon capture system units with different targets (gas and solid-state) is presented. Using the muon-event selection system and high-purity germanium (HPGe) detectors, precise measurements of the time-energy distributions following ordinary muon capture in the isotopically-enriched $^{130}$Xe, $^{82}$Kr, and $^{24}$Mg nuclei were performed. The optimal parameters of the muon capture measuring system to ensure maximum muon stops in the targets under study and to collect effective statistics during measurements are reported. The system makes it possible to separate muonic X-ray events from gamma radiation with high accuracy, which is critical in such studies, and it is also suitable for future measurements with other targets.

Phys. Part. Nucl. Lett.

Lead converter was irradiated with electron beams with energies of 60, 80 and 100 MeV. And holmium samples were irradiated with a flux of bremsstrahlung, which was formed in a lead converter. The rates of about 20 photonuclear reactions in $^{165}$Ho, which are induced by bremsstrahlung, were measured. Simulations to determine the fluence of electrons, photons, neutrons for holmium samples and the reaction rates for the obtained isotopes in the samples were performed using the Geant4 code. For those radionuclides that are determined in experimental measurements, the results were compared.

Phys. Rev. Lett.

A study of prompt charm-hadron pair production in proton-lead collisions at sNN=8.16  TeV is performed using data corresponding to an integrated luminosity of about 30  nb-1, collected with the LHCb experiment. Production cross sections for different pairs of charm hadrons are measured and kinematic correlations between the two charm hadrons are investigated. This is the first measurement of associated production of two charm hadrons in proton-lead collisions. The results confirm the predicted enhancement of double parton scattering production in proton-lead collisions compared to the single parton scattering production.

Phys.Rev.D

The ratios of the Bc(2S)+ to Bc+, Bc*(2S)+ to Bc+, and Bc*(2S)+ to Bc(2S)+ production cross sections are measured in proton-proton collisions at s=13  TeV, using a data sample collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 143  fb-1. The three measurements are made in the Bc+ meson phase space region defined by the transverse momentum pT>15  GeV and absolute rapidity |y|<2.4, with the excited Bc(*)(2S)+ states reconstructed through the Bc(*)+π+π−, followed by the Bc+→J/ψπ+ and J/ψ→μ+μ− decays. The Bc(2S)+ to Bc+, Bc*(2S)+ to Bc+, and Bc*(2S)+ to Bc(2S)+ cross section ratios, including the unknown Bc(*)(2S)+→Bc(*)+π+π− branching fractions, are (3.47±0.63(stat)±0.33(syst))%, (4.69±0.71(stat)±0.56(syst))%, and 1.35±0.32(stat)±0.09(syst), respectively. None of these ratios shows a significant dependence on the pT or |y| of the Bc+ meson. The normalized dipion invariant mass distributions from the decays Bc(*)(2S)+→Bc(*)+π+π− are also reported.

Phys.Rev.D

The decays χcJ→Σ0p¯K++c.c. (J=0,1,2) are studied via the radiative transition ψ(3686)→γχcJ based on a data sample of (448.1±2.9)×106 ψ(3686) events collected with the BESIII detector. The branching fractions of χcJ→Σ0p¯K++c.c. (J=0,1,2) are measured to be (3.03±0.12±0.15)×10-4, (1.46±0.07±0.07)×10-4, and (0.91±0.06±0.05)×10-4, respectively, where the first uncertainties are statistical and the second are systematic. In addition, no evident structure is found for excited baryon resonances on the two-body subsystems with the limited statistics.

Phys.Rev.D

A study of the lineshape of the $\chi_{c1}(3872)$ state is made using a data sample corresponding to an integrated luminosity of $3\,$fb$^{-1}$ collected in $pp$ collisions at centre-of-mass energies of 7 and 8\,TeV with the LHCb detector. Candidate $\chi_{c1}(3872)$ and $\psi(2S)$ mesons from b-hadron decays are selected in the $J/\psi \pi^+ \pi^-$ decay mode. Describing the {\mbox{lineshape}} with a Breit--Wigner function, the mass splitting between the $\chi_{c1}(3872)$ and $\psi(2S)$ states, $\Delta m$, and the width of the $\chi_{c1}(3872)$ state, $\Gamma_{\mathrm{BW}}$, are determined to be \begin{eqnarray*} \Delta m & = & 185.598 \pm 0.067 \pm 0.068\, \mathrm{MeV} \,, \\ \Gamma_{\mathrm{BW}} & = & \phantom{00}1.39\phantom{0} \pm 0.24\phantom{0} \pm 0.10\phantom{0} \mathrm{MeV} \,, \end{eqnarray*} where the first uncertainty is statistical and the second systematic. Using a Flatt\'e-inspired model, the mode and full width at half maximum of the lineshape are determined to be \begin{eqnarray*} \mathrm{mode} & = 3871.69^{\,+\,0.00\,+\,0.05}_{\,-\,0.04\,-\,0.13} &\mathrm{MeV} \\ \mathrm{FWHM} & = 0.22^{\,+\,0.07\,+\,0.11}_{\,-\,0.06\,-\,0.13}& \mathrm{MeV} . \end{eqnarray*} An investigation of the analytic structure of the Flatt\'e amplitude reveals a pole structure, which is compatible with a quasi-bound $D^0\bar{D}^{*0}$ state but a quasi-virtual state is still allowed at the level of $2$ standard deviations.

Nucl. Instrum. Meth. A

The High-Granularity Timing Detector is a detector proposed for the ATLAS Phase II upgrade. The detector, based on the Low-Gain Avalanche Detector(LGAD) technology, will cover the pseudo-rapidity region of  2.4<|η|<4.0 with two end caps on each side and a total area of 6.4 m 2 . The timing performance can be improved by implanting an internal gain layer that can produce signals with a fast rising edge. It significantly improves the signal-to-noise ratio. The required average timing resolution per track for a minimum ionizing particle is 30 ps at the start and 50 ps at the end of the HL-LHC operation. This is achieved with several layers of LGAD. The innermost region of the detector would accumulate a 1MeV neutron-equivalent fluence up to 2.5 × 10 15 n eq /cm 2 including a safety factor of 1.5 before being replaced during the scheduled shutdowns. The addition of this new detector is expected to play an important role in the mitigation of high pile-ups at the HL-LHC. The layout and performance of the various versions of LGAD prototypes produced by Hamamatsu(HPK) have been studied by the ATLAS Collaboration. The breakdown voltages, depletion voltages, inter-pad gaps, collected charge as well as the time resolution have been measured and the production yield of large size sensors has been evaluated.

Eur.Phys.J.C

We consider the prompt double $J/\psi$ production in pp collisions at the LHC in the framework of $k_T$-factorization QCD approach. Using the fragmentation mechanism, we evaluate the color octet contributions to the production cross sections taking into account the combinatorial effects of multiple gluon radiation in the initial state driven by the Ciafaloni–Catani–Fiorani–Marchesini evolution equation. We demonstrate the importance of these contributions in a certain kinematical region covered by the CMS and ATLAS measurements. On the other hand, the experimental data taken by the LHCb Collaboration at forward rapidities and moderate transverse momenta can be described well by ${{{\mathcal {O}}}}(\alpha _s^4)$ color singlet terms and contributions from the double parton scattering mechanism. The extracted value of the effective cross section is compatible with many other estimations based on different final states.

IEEE Trans.Nucl.Sci.

In this article, development results from a new p-type high-purity germanium (HPGe) detector with a segmented n ⁺ -region are presented. The intended detector is used in spectrometry; however, its sixfold segmentation allows for the determination of the direction of individual photons emitted in the nuclear reactions from a source placed inside the detector. The capacitance and current voltage characteristics of the detector are presented. At full depletion of -1500 V, the segments' capacitance was measured to be 17 pF and leakage currents ranged between 7 and 10 pA. The energy resolution of the segments was measured with a variety of gamma sources and determined to be less than 1000 eV at 122 keV and less than 1900 eV at 1332 keV. The incident photon angle response from a radiation source placed inside the detector was also studied.

Phys. Lett. B

The top anti-top quark production cross-section is measured in the lepton+jets channel using proton–proton collision data at a centre-of-mass energy of s=13 TeV collected with the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 139 fb −1 . Events with exactly one charged lepton and four or more jets in the final state, with at least one jet containing b -hadrons, are used to determine the tt¯ production cross-section through a profile-likelihood fit. The inclusive cross-section is measured to be σinc=830±0.4(stat.)±36(syst.)±14(lumi.)pb with a relative uncertainty of 4.6%. The result is consistent with theoretical calculations at next-to-next-to-leading order in perturbative QCD. The fiducial tt¯ cross-section within the experimental acceptance is also measured.

JINST

The PreProcessor of the ATLAS Level-1 Calorimeter Trigger prepares the analogue trigger signals sent from the ATLAS calorimeters by digitising, synchronising, and calibrating them to reconstruct transverse energy deposits, which are then used in further processing to identify event features. During the first long shutdown of the LHC from 2013 to 2014, the central components of the PreProcessor, the Multichip Modules, were replaced by upgraded versions that feature modern ADC and FPGA technology to ensure optimal performance in the high pile-up environment of LHC Run 2. This paper describes the features of the new Multichip Modules along with the improvements to the signal processing achieved.

Phys. Rev. D

A measurement of the W+W- boson pair production cross section in proton-proton collisions at s=13  TeV is presented. The data used in this study are collected with the CMS detector at the CERN LHC and correspond to an integrated luminosity of 35.9  fb-1. The W+W- candidate events are selected by requiring two oppositely charged leptons (electrons or muons). Two methods for reducing background contributions are employed. In the first one, a sequence of requirements on kinematic quantities is applied allowing a measurement of the total production cross section, 117.6±6.8  pb, which agrees well with the theoretical prediction. Fiducial cross sections are also reported for events with zero or one jet, and the change in the zero-jet fiducial cross section with the jet transverse momentum threshold is measured. Normalized differential cross sections are reported within the fiducial region. A second method for suppressing background contributions employs two random forest classifiers. The analysis based on this method includes a measurement of the total production cross section and also a measurement of the normalized jet multiplicity distribution in W+W- events. Finally, a dilepton invariant mass distribution is used to probe for physics beyond the standard model in the context of an effective field theory, and constraints on the presence of dimension-6 operators are derived.

JHEP

Decays of the 125 GeV Higgs boson into a Z boson and a ρ$^{0}$(770) or ϕ(1020) meson are searched for using proton-proton collision data collected by the CMS experiment at the LHC at $\sqrt{s}$ = 13 TeV. The analysed data set corresponds to an integrated luminosity of 137 fb$^{−1}$. Events are selected in which the Z boson decays into a pair of electrons or a pair of muons, and the ρ and ϕ mesons decay into pairs of pions and kaons, respectively. No significant excess above the background model is observed. As different polarization states are possible for the decay products of the Z boson and ρ or ϕ mesons, affecting the signal acceptance, scenarios in which the decays are longitudinally or transversely polarized are considered. Upper limits at the 95% confidence level on the Higgs boson branching fractions into Zρ and Zϕ are determined to be 1.04–1.31% and 0.31–0.40%, respectively, where the ranges reflect the considered polarization scenarios; these values are 740–940 and 730–950 times larger than the respective standard model expectations. These results constitute the first experimental limits on the two decay channels.[graphic not available: see fulltext]

Phys. Rev. D

The Deep Underground Neutrino Experiment is a next-generation neutrino oscillation experiment that aims to measure CP-violation in the neutrino sector as part of a wider physics program. A deep learning approach based on a convolutional neural network has been developed to provide highly efficient and pure selections of electron neutrino and muon neutrino charged-current interactions. The electron neutrino (antineutrino) selection efficiency peaks at 90% (94\) and exceeds 85% (90\) for reconstructed neutrino energies between 2–5 GeV. The muon neutrino (antineutrino) event selection is found to have a maximum efficiency of 96% (97\) and exceeds 90% (95\) efficiency for reconstructed neutrino energies above 2 GeV. When considering all electron neutrino and antineutrino interactions as signal, a selection purity of 90% is achieved. These event selections are critical to maximize the sensitivity of the experiment to CP-violating effects.

JHEP

A search for new physics with non-resonant signals in dielectron and dimuon final states in the mass range above 2 TeV is presented. This is the first search for non-resonant signals in dilepton final states at the LHC to use a background estimate from the data. The data, corresponding to an integrated luminosity of 139 fb$^{−1}$, were recorded by the ATLAS experiment in proton-proton collisions at a center-of-mass energy of $\sqrt{s}$ = 13 TeV during Run 2 of the Large Hadron Collider. The benchmark signal signature is a two-quark and two-lepton contact interaction, which would enhance the dilepton event rate at the TeV mass scale. To model the contribution from background processes a functional form is fit to the dilepton invariant-mass spectra in data in a mass region below the region of interest. It is then extrapolated to a high-mass signal region to obtain the expected background there. No significant deviation from the expected background is observed in the data. Upper limits at 95% CL on the number of events and the visible cross-section times branching fraction for processes involving new physics are provided. Observed (expected) 95% CL lower limits on the contact interaction energy scale reach 35.8 (37.6) TeV.[graphic not available: see fulltext]

Eur.Phys.J.C

We analyze joint LHC data on the production of $\chi _{c1}$ and $\chi _{c2}$ mesons together with the polarization data obtained very recently by the CMS Collaboration at $\sqrt{s} = 8$ TeV. Our consideration is based on the $k_T$-factorization approach and nonrelativistic QCD formalism for the formation of bound states. The observed polar anisotropy of $\chi _{c1}$ and $\chi _{c2}$ decays can be described as a combined effect of the color-singlet and color-octet contributions. We extract the corresponding long-distance matrix elements from the fits. Our fits point to unequal color singlet wave functions for $\chi _{c1}$ and $\chi _{c2}$ states.

IEEE Trans.Nucl.Sci.

Characterization of the vacuum ultraviolet (VUV) reflectance of silicon photomultipliers (SiPMs) is important for large-scale SiPM-based photodetector systems. We report the angular dependence of the specular reflectance in vacuum of SiPMs manufactured by Fondazionc Bruno Kessler (FBK) and Hamamatsu Photonics K.K. (HPK) over wavelengths ranging from 120 to 280 nm. Refractive index and extinction coefficient of the thin silicon-dioxide film deposited on the surface of the FBK SiPMs are derived from reflectance data of an FBK silicon wafer with the same deposited oxide film as SiPMs. The diffuse reflectance of SiPMs is also measured at 193 nm. We use the VUV spectral dependence of the optical constants to predict the reflectance of the FBK silicon wafer and FBK SiPMs in liquid xenon.

JHEP

The ratios of the production cross sections between the excited ϒ(2S) and ϒ(3S) mesons and the ϒ(1S) ground state, detected via their decay into two muons, are studied as a function of the number of charged particles in the event. The data are from proton-proton collisions at $\sqrt{s}$ = 7 TeV, corresponding to an integrated luminosity of 4.8 fb$^{−1}$, collected with the CMS detector at the LHC. Evidence of a decrease in these ratios as a function of the particle multiplicity is observed, more pronounced at low transverse momentum ParseError: KaTeX parse error: Undefined control sequence: \upmu at position 20: …_{\mathrm{T}}^{\̲u̲p̲m̲u̲ ̲\upmu} . For ϒ(nS) mesons with ParseError: KaTeX parse error: Undefined control sequence: \upmu at position 20: …_{\mathrm{T}}^{\̲u̲p̲m̲u̲ ̲\upmu} > 7 GeV, where most of the data were collected, the correlation with multiplicity is studied as a function of the underlying event transverse sphericity and the number of particles in a cone around the ϒ(nS) direction. The ratios are found to be multiplicity independent for jet-like events. The mean ParseError: KaTeX parse error: Undefined control sequence: \upmu at position 20: …_{\mathrm{T}}^{\̲u̲p̲m̲u̲ ̲\upmu} values for the ϒ(nS) states as a function of particle multiplicity are also measured and found to grow more steeply as their mass increases.[graphic not available: see fulltext]

Nucl. Instrum. Meth. A

We report on the results of a radiation campaign with neutrons and protons of Low Gain Avalanche Detectors (LGAD) produced by Hamamatsu (HPK) as prototypes for the High-Granularity Timing Detector (HGTD) in ATLAS. Sensors with an active thickness of 50μm were irradiated in steps of roughly 2 × up to a fluence of 3 ×1015neqcm−2 . As a function of the fluence, the collected charge and time resolution of the irradiated sensors will be reported for operation at −30 °C.

Comput.Phys.Commun.

In this paper we present a new Monte Carlo event generator ReneSANCe for simulation of processes at electron–positron colliders. In the current release of the generator the Bhabha scattering ( e+e−→e−e+ ) and Higgs-strahlung ( e+e−→ZH ) process are implemented. Based on the SANC (Support for Analytic and Numeric Calculations for experiments at colliders) modules, the new generator takes into account complete one-loop and some higher-order electroweak radiative corrections with finite particle masses and polarizations. The new generator effectively operates in the collinear region and at the ZH production threshold. It is constructed in such a way that new processes can be easily added. The paper contains a theoretical description of the SANC approach, numerical validations and manual.