arXiv subjects

J. Biernat

Publications and source records attributed to J. Biernat.

At least 19 recordsLinked to original sources

arXiv2021

Weak phases and CP-symmetry tests in sequential decays of entangled double-strange baryons

Using a sample of $1.31\times10^9$ $J/ψ$ events collected with the BESIII detector at the electron-positron collider BEPCII, we analyse the full $J/ψ\to$ $Ξ^-\overlineΞ^+$, $Ξ^-\to Λπ^-$, $Λ\to pπ^-$, $\overlineΞ^+\to\overlineΛπ^+$, $\overlineΛ\to\overline{p}π^+$ decay chain. A new method, exploiting the fact that the $Ξ^-\overlineΞ^+$ pair is entangled and sequentially decaying, and where the complete decay chains are reconstructed, is applied for the first time. This enables precision measurements of the decay parameters for the $Ξ^-\toΛπ^-$ decay ($α_Ξ$, $ϕ_Ξ$) as well as the $\overlineΞ^+\to\overlineΛπ^+$ decay ($\overlineα_Ξ$, $\overlineϕ_Ξ$). From the decay parameters, two independent CP tests were performed, quantified by the observables $A_{\rm CP}^Ξ$ and $Δϕ_Ξ$. Our results, $A_{\rm CP}^Ξ$ = $(6.0\pm13.4\pm5.6)\times10^{-3}$ and $Δϕ_Ξ= (-4.8\pm13.7\pm2.9)\times10^{-3}~{\rm rad}$, are consistent with CP symmetry. Furthermore, our method enables a separation of strong and weak $Ξ\toΛπ$ decay amplitudes. This results in the first direct measurement of the weak phase difference for any baryon decay. The result is found to be $(ξ_{P} - ξ_{S}) = (1.2\pm3.4\pm0.8)\times10^{-2}$ rad and is one of the most precise tests of CP symmetry for strange baryons. The strong phase difference is measured to be $(δ_P - δ_S) = (-4.0\pm3.3\pm1.7)\times10^{-2}$ rad. In addition, we provide an independent measurement of the recently debated $Λ$ decay parameter, $α_Λ = 0.757 \pm 0.011 \pm 0.008 $. The $Λ\overlineΛ$ asymmetry is measured to be $A_{\rm CP}^Λ = (-3.7\pm11.7\pm9.0)\times10^{-3}$.

hep-ex↗

arXiv2020

Proton number fluctuations in $\sqrt{s_{NN}}$ = 2.4 GeV Au+Au collisions studied with HADES

We present an analysis of proton number fluctuations in $\sqrt{s_{NN}}$ = 2.4 GeV Au+Au collisions measured with the High-Acceptance DiElectron Spectrometer (HADES) at GSI. With the help of extensive detector simulations done with IQMD transport model events including nuclear clusters, various nuisance effects influencing the observed proton cumulants have been investigated. Acceptance and efficiency corrections have been applied as a function of fine grained rapidity and transverse momentum bins, as well as considering local track density dependencies. Next, the effects of volume changes within particular centrality selections have been considered and beyond-leading-order corrections have been applied to the data. The efficiency and volume corrected proton number moments and cumulants Kn of orders n = 1, . . . , 4 have been obtained as a function of centrality and phase-space bin, as well as the corresponding correlators C_n . We find that the observed correlators show a power-law scaling with the mean number of protons, i.e. $C_n \propto ^n$, indicative of mostly long-range multi-particle correlations in momentum space. We also present a comparison of our results with Au+Au collision data obtained at RHIC at similar centralities, but higher $\sqrt{s_{NN}}$.

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arXiv2020

Analysis of the decay $D^0\rightarrow K_{S}^{0} K^{+} K^{-}$

Using a data sample of $2.93~fb^{-1}$ of $e^+e^-$ collisions collected at $\sqrt{s}=3.773 GeV$ in the BESIII experiment, we perform an analysis of the decay $D^0\rightarrow K_{S}^{0} K^{+} K^{-}$. The Dalitz plot is analyzed using $1856\pm 45$ flavor-tagged signal decays. We find that the Dalitz plot is well described by a set of six resonances: $a_0(980)^0$, $a_0(980)^+$, $ϕ(1020)$, $a_2(1320)^+$, $a_2(1320)^-$ and $a_0(1450)^-$. Their magnitudes, phases and fit fractions are determined as well as the coupling of $a_0(980)$ to $K\bar{K}$, $g_{K\bar{K}}=3.77\pm 0.24\text{(stat.)}\pm0.35\text{(sys.)} GeV$. The branching fraction of the decay $D^0\rightarrow K_{S}^{0} K^{+} K^{-}$ is measured using $11660\pm 118$ untagged signal decays to be $(4.51\pm 0.05\text{(stat.)}\pm 0.16\text{(sys.)})10^{-3}$. Both measurements are limited by their systematic uncertainties.

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arXiv2020

Identical pion intensity interferometry at sqrt(s_{NN})=2.4 GeV

High-statistics $π^-π^-$ and $π^+π^+$ femtoscopy data are presented for Au+Au collisions at $\sqrt{s_\mathrm{NN}}=2.4$ GeV, measured with HADES at SIS18/GSI. The experimental correlation functions allow the determination of the space-time extent of the corresponding emission sources via a comparison to models. The emission source, parametrized as three-dimensional Gaussian distribution, is studied in dependence on pair transverse momentum, azimuthal emission angle with respect to the reaction plane, collision centrality and beam energy. For all centralities and transverse momenta, a geometrical distribution of ellipsoidal shape is found in the plane perpendicular to the beam direction with the larger extension perpendicular to the reaction plane. For large transverse momenta, the corresponding eccentricity approaches the initial eccentricity. The eccentricity is smallest for most central collisions, where the shape is almost circular. The magnitude of the tilt angle of the emission ellipsoid in the reaction plane decreases with increasing centrality and increasing transverse momentum. All source radii increase with centrality, largely exhibiting a linear rise with the number of participants, irrespective of transverse momentum. A substantial charge-sign difference of the source radii is found, appearing most pronounced at low transverse momentum. The extracted source parameters are consistent with the extrapolation of their energy dependence down from higher energies.

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arXiv2020

Charged pion production in $\mathbf{Au+Au}$ collisions at $\mathbf{\sqrt{s_{NN}}}$ = 2.4$\mathbf{GeV}$

We present high-statistic data on charged pion emission from Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 2.4 GeV (corresponding to $E_{beam}$ = 1.23 A GeV) in four centrality classes in the range 0 - 40$\%$ of the most central collisions. The data are analyzed as a function of transverse momentum, transverse mass, rapidity, and polar angle. Pion multiplicity per participating nucleon decreases moderately with increasing centrality. The polar angular distributions are found to be non-isotropic even for the most central event class. Our results on pion multiplicity fit well into the general trend of the world data, but undershoot by $2.5 σ$ data from the FOPI experiment measured at slightly lower beam energy. We compare our data to state-of-the-art transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find substantial differences between the measurement and the results of these calculations.

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arXiv2020

Two-Pion production in the second resonance region in $π^- p$ collisions with HADES

Pion induced reactions provide unique opportunities for an unambiguous description of baryonic resonances and their coupling channels by means of a partial wave analysis. Using the secondary pion beam at SIS18, the two pion production in the second resonance region has been investigated to unravel the role of the $N(1520) \frac{3}{2}^-$ resonance in the intermediate $ρ$ production. Results on exclusive channels with one pion ($π^{-}p$) and two pions ($π^{+}π^{-}n$, $π^{0}π^{-}p$) in the final state measured in the $π^{-}-p$ reaction at four different pion beam momenta (0.650, 0.685, 0.733, and 0.786 GeV/c) are presented. The excitation function of the different partial waves and $Δπ$, $Nσ$ and $Nρ$ isobar configurations is obtained, using the Bonn-Gatchina partial wave analysis. The $N(1520) \frac{3}{2}^-$ resonance is found to dominate the $Nρ$ final state with the branching ratio $BR=12.2 \pm 1.9\%$.

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arXiv2020

Observation of a resonant structure in $e^{+}e^{-} \to K^{+}K^{-}π^{0}π^{0}$

Using $e^{+}e^{-}$ collision data samples with center-of-mass energies ranging from 2.000 to 2.644 GeV, collected by the BESIII detector at the BEPCII collider, and with a total integrated luminosity of 300 pb^{-1}, a partial-wave analysis is performed for the process $e^{+}e^{-} \to K^{+}K^{-}π^{0}π^{0}$. The total Born cross sections for the process $e^{+}e^{-} \to K^{+}K^{-}π^{0}π^{0}$, as well as the Born cross sections f or the subprocesses $e^{+}e^{-} \to ϕπ^{0}π^{0}$, $K^{+}(1460)K^{-}$, $K^{+}_{1}(1400)K^{-}$, $K^{+}_{1}(1270)K^{-}$ and $K^{*+}(892)K^{*-}(892)$, are measured versus the center-of-mass energy. The corresponding results for $e^{+}e^{-} \to K^{+}K^{-}π^{0}π^{0}$ and $ϕπ^{0}π^{0}$ are consistent with those of BaBar and have much improved precision.By analyzing the cross sections for the four subprocesses, $K^{+}(1460)K^{-}$, $K^{+}_{1}(1400)K^{-}$, $K^{+ }_{1}(1270)K^{-}$ and $K^{*+}K^{*-}$, a structure with mass $M$ = (2126.5 $\pm$ 16.8 $\pm$ 12.4)~MeV/c^{2} and width $Γ$ = (106.9 $\pm$ 32.1 $\pm$ 28.1)~MeV is observed with an overall statistical significance of 6.3 $σ$, although with very limited significance in the subprocesses $e^{+}e^{-} \to K^{+}_{1}(1270)K^{-}$ and $K^{*+}(892)K^{*-}(892)$. The resonant parameters of the observed structure suggest it can be identified with the $ϕ(2170)$, thus the results provide valuable input to the internal nature of the $ϕ(2170)$.

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arXiv2019

Technical Design Report for the PANDA Endcap Disc DIRC

PANDA (anti-Proton ANnihiliation at DArmstadt) is planned to be one of the four main experiments at the future international accelerator complex FAIR (Facility for Antiproton and Ion Research) in Darmstadt, Germany. It is going to address fundamental questions of hadron physics and quantum chromodynamics using cooled antiproton beams with a high intensity and and momenta between 1.5 and 15 GeV/c. PANDA is designed to reach a maximum luminosity of 2x10^32 cm^2 s. Most of the physics programs require an excellent particle identification (PID). The PID of hadronic states at the forward endcap of the target spectrometer will be done by a fast and compact Cherenkov detector that uses the detection of internally reflected Cherenkov light (DIRC) principle. It is designed to cover the polar angle range from 5° to 22° and to provide a separation power for the separation of charged pions and kaons up to 3 standard deviations (s.d.) for particle momenta up to 4 GeV/c in order to cover the important particle phase space. This document describes the technical design and the expected performance of the novel PANDA Disc DIRC detector that has not been used in any other high energy physics experiment (HEP) before. The performance has been studied with Monte-Carlo simulations and various beam tests at DESY and CERN. The final design meets all PANDA requirements and guarantees suffcient safety margins.

physics.ins-det↗

arXiv2019

Identical pion intensity interferometry in central Au+Au collisions at 1.23A GeV

We investigate identical pion HBT intensity interferometry for central Au+Au collisions at 1.23A GeV. High-statistics $π^-π^-$ and $π^+π^+$ data are measured with HADES at SIS18/GSI. The radius parameters, derived from the correlation function depending on relative momenta in the longitudinal-comoving system and parametrized as three-dimensional Gaussian distribution, are studied as function of transverse momentum. A substantial charge-sign difference of the source radii is found, particularly pronounced at low transverse momentum. The extracted Coulomb-corrected source parameters agree well with a smooth extrapolation of the center-of-mass energy dependence established at higher energies, extending the corresponding excitation functions down towards a very low energy. Our data would thus rather disfavour any strong energy dependence of the radius parameters in the low energy region.

nucl-ex↗

arXiv2019

Measurements of Weak Decay Asymmetries of $Λ_c^+\to pK_S^0$, $Λπ^+$, $Σ^+π^0$, and $Σ^0π^+$

Using $e^+e^-\toΛ_c^+\barΛ_c^-$ production from a 567 pb$^{-1}$ data sample collected by BESIII at 4.6 GeV, a full angular analysis is carried out simultaneously on the four decay modes of $Λ_c^+\to pK_S^0$, $Λπ^+$, $Σ^+π^0$, and $Σ^0π^+$. For the first time, the $Λ_c^+$ transverse polarization is studied in unpolarized $e^+e^-$ collisions, where a non-zero effect is observed with a statistical significance of 2.1$σ$. The decay asymmetry parameters of the $Λ_c^+$ weak hadronic decays into $pK_S^0$, $Λπ^+$, $Σ^+π^0$ and $Σ^0π^+$ are measured to be $0.18\pm0.43(\rm{stat})\pm0.14(\rm{syst})$, $-0.80\pm0.11(\rm{stat})\pm0.02(\rm{syst})$, $-0.57\pm0.10(\rm{stat})\pm0.07(\rm{syst})$, and $-0.73\pm0.17(\rm{stat})\pm0.07(\rm{syst})$, respectively. In comparison with previous results, the measurements for the $Λπ^+$ and $Σ^+π^0$ modes are consistent but with improved precision, while the parameters for the $pK_S^0$ and $Σ^0π^+$ modes are measured for the first time.

hep-ex↗

arXiv2019

Complete measurement of $Λ$ electromagnetic form factors

The exclusive process $e^+e^-\rightarrowΛ\barΛ$, with $Λ\to pπ^-$ and $\barΛ \to \bar{p}π^+$, has been studied at $\sqrt{s} =$ 2.396 GeV for measurement of the $Λ$ electric and magnetic form factors, $G_E$ and $G_M$. A data sample, corresponding to an integrated luminosity of 66.9 pb$^{-1}$, was collected with the BESIII detector for this purpose. A multi-dimensional analysis with a complete decomposition of the spin structure of the reaction enables a determination of the modulus of the ratio $R=|G_E/G_M|$ and, for the first time for any baryon, the relative phase $ΔΦ=Φ_E-Φ_M$. The resulting values are obtained using the recent and most precise measured value of the asymmetry parameter $α_Λ$ = $0.750~\pm~0.010$ to be $R~=~0.96\pm0.14~(\rm stat.)\pm~0.02~(\rm sys.)$ and $ΔΦ=37^{\mathrm{o}}\pm~12^{\mathrm{o}}~(\rm stat.)\pm~6^{\mathrm{o}}~(\rm sys.)$, respectively. In addition, the cross section is measured with unprecedented precision to be $σ= 118.7~\pm~5.3~(\rm stat.)\pm~5.1~(\rm sys.)$ pb, which corresponds to an effective form factor of $|G|=0.123~\pm~0.003~(\rm stat.)\pm~0.003~(\rm sys.)$. The contribution from two-photon exchange is found to be negligible. Our result enables the first complete determination of baryon time-like electromagnetic form factors.

hep-ex↗

arXiv2019

Observation of $ψ(3686)\to p\bar{p}ϕ$

Using a data sample of $4.48\times10^{8}$ $\psip$ events collected with the BESIII detector, we present a first observation of $ψ(3686)\to p\bar{p}ϕ$, and we measure its branching fraction to be $[6.06\pm0.38 ($stat.$) \pm 0.48 ($syst.$)]\times10^{-6}$. In contrast to the earlier discovery of a threshold enhancement in the $p\bar{p}$-mass spectrum of the channel $J/ψ\toγp\bar p$, denoted as $X(p\bar{p})$, we do not find a similar enhancement in $ψ(3686)\to p\bar{p}ϕ$. An upper limit of $1.82\times10^{-7}$ at the $90\%$ confidence level on the branching fraction of $ψ(3686)\to X(p\bar{p})ϕ\to p\bar{p}ϕ$ is obtained.

hep-ex↗

arXiv2018

Sub-threshold production of K^{0}_{s} mesons and Λ hyperons in Au(1.23A GeV)+Au

We present first data on sub-threshold production of K0 s mesons and Λ hyperons in Au+Au collisions at $\sqrt{s_{NN}}$ = 2.4 GeV. We observe an universal scaling of hadrons containing strangeness, independent of their corresponding production thresholds. Comparing the yields, their scaling, and the shapes of the rapidity and the pt spectra to state-of-the-art transport model (UrQMD, HSD, IQMD) predictions, we find that none of the latter can simultaneously describe all observables with reasonable \c{hi}2 values.

nucl-ex↗

arXiv2018

Strong absorption of hadrons with hidden and open strangeness in nuclear matter

We present the first observation of K- and ϕabsorption within nuclear matter by means of π- -induced reactions on C and W targets at an incident beam momentum of 1.7 GeV/c studied with HADES at SIS18/GSI. The double ratio (K-/K+)W / (K-/K+)C is found to be 0.319 \pm 0.009(stat)+0.014-0.012 (syst) indicating a larger absorption of K- in heavier targets as compared to lighter ones. The measured ϕ/K- ratios in π-+C and π^- +W reactions within the HADES acceptance are found to be equal to $0.55 \pm 0.03(stat)+0.06-0.07$ (syst) and to $0.63 \pm 0.05(stat)-0.11+0.11$ (syst), respectively. The similar ratios measured in the two different reactions demonstrate for the first time experimentally that the dynamics of the ϕmeson in nuclear medium is strongly coupled to the K- dynamics. The large difference in the ϕproduction off C and W nuclei is discussed in terms of a strong \phiN in-medium coupling.

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arXiv2018

Improved limit on neutrinoless double beta decay of $^{76}$Ge from GERDA Phase II

The GERDA experiment searches for the lepton number violating neutrinoless double beta decay of $^{76}$Ge ($^{76}$Ge $\rightarrow$ $^{76}$Se + 2e$^-$) operating bare Ge diodes with an enriched $^{76}$Ge fraction in liquid argon. The exposure for BEGe-type detectors is increased threefold with respect to our previous data release. The BEGe detectors feature an excellent background suppression from the analysis of the time profile of the detector signals. In the analysis window a background level of $1.0_{-0.4}^{+0.6}\cdot10^{-3}$ cts/(keV$\cdot$kg$\cdot$yr) has been achieved; if normalized to the energy resolution this is the lowest ever achieved in any 0$νββ$ experiment. No signal is observed and a new 90 \% C.L. lower limit for the half-life of $8.0\cdot10^{25}$ yr is placed when combining with our previous data. The median expected sensitivity assuming no signal is $5.8\cdot10^{25}$ yr.

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arXiv2017

Deep sub-threshold ϕ production and implications for the K+/K- freeze-out in Au+Au collisions

We present data on charged kaons (K+-) and ϕ mesons in Au(1.23A GeV)+Au collisions. It is the first simultaneous measurement of K and ϕ mesons in central heavy-ion collisions below a kinetic beam energy of 10A GeV. The ϕ/K- multiplicity ratio is found to be surprisingly high with a value of 0.52 +- 0.16 and shows no dependence on the centrality of the collision. Consequently, the different slopes of the K+ and K- transverse-mass spectra can be explained solely by feed- down, which substantially softens the spectra of K- mesons. Hence, in contrast to the commonly adapted argumentation in literature, the different slopes do not necessarily imply diverging freeze- out temperatures of K+ and K- mesons caused by different couplings to baryons.

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arXiv2017

$Σ^0$ production in proton nucleus collisions near threshold

The production of $Σ^{0}$ baryons in the nuclear reaction p (3.5 GeV) + Nb (corresponding to $\sqrt{s_{NN}}=3.18$ GeV) is studied with the detector set-up HADES at GSI, Darmstadt. $Σ^{0}$s were identified via the decay $Σ^{0} \rightarrow Λγ$ with subsequent decays $Λ\rightarrow p π^{-}$ in coincidence with a $e^{+}e^{-}$ pair from either external ($γ\rightarrow e^{+} e^{-}$) or internal (Dalitz decay $γ^{*}\rightarrow e^{+} e^{-}$) gamma conversions. The differential $Σ^0$ cross section integrated over the detector acceptance, i.e. the rapidity interval $0.5 < y < 1.1$, has been extracted as $Δσ_{Σ^{0}} = 2.3 \pm (0.2)^{stat} \pm \left(^{+0.6}_{-0.6}\right)^{sys} \pm (0.2)^{norm}$ mb, yielding the inclusive production cross section in full phase space $σ^{total}_{Σ^{0}} = 5.8 \pm (0.5)^{stat} \pm \left(^{+1.4}_{-1.4}\right)^{sys} \pm (0.6)^{norm} \pm (1.7)^{extrapol}$ mb by averaging over different extrapolation methods. The $Λ_{all}$/$Σ^{0}$ ratio within the HADES acceptance is equal to 2.3 $\pm$ $(0.2)^{stat}$ $\pm$ $(^{+0.6}_{-0.6})^{sys}$. The obtained rapidity and momentum distributions are compared to transport model calculations. The $Σ^{0}$ yield agrees with the statistical model of particle production in nuclear reactions.

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arXiv2017

$Δ$(1232) Dalitz decay in proton-proton collisions at T=1.25 GeV measured with HADES

We report on the investigation of $Δ$(1232) production and decay in proton-proton collisions at a kinetic energy of 1.25 GeV measured with HADES. Exclusive dilepton decay channels $ppe^{+}e^{-}$ and $ppe^{+}e^{-}γ$ have been studied and compared with the partial wave analysis of the hadronic $ppπ^{0}$ channel. They allow to access both $Δ^+ \to pπ^0(e^+e^-γ)$ and $Δ^+ \to pe^+e^-$ Dalitz decay channels. The perfect reconstruction of the well known $π^0$ Dalitz decay serves as a proof of the consistency of the analysis. The $Δ$ Dalitz decay is identified for the first time and the sensitivity to N-$Δ$ transition form factors is tested. The $Δ$(1232) Dalitz decay branching ratio is also determined for the first time; our result is (4.19 $\pm$ 0.62 syst. $\pm$ 0.34 stat.) $\times $ 10$^{-5}$, albeit with some model dependence.

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