Strange Sector Research Articles

Fluctuations and Correlations of Conserved Charges Serving as Signals for QGP Production: An Overview from Polyakov Loop Enhanced Nambu–Jona-Lasinio Model

Quark–Gluon plasma driven by the strong force is subject to the conservativeness of the baryon number, net electric charge, strangeness, etc. However, the fluctuations around their mean values at specific temperatures and chemical potentials can provide viable signals for the production of Quark–Gluon plasma. These fluctuations can be captured theoretically as moments of different orders in the expansion of pressure or the thermodynamic potential of the system under concern. Here, we look for possible explanations in the methodologies used for capturing them by using the framework of the Polyakov–Nambu–Jona-Lasinio (PNJL) model under the 2 + 1 flavor consideration with mean-field approximation. The various quantities thus explored can act to signify meaningfully near the phase transitions. Justifications are also made for some of the quantities capable of serving necessarily under experimental scenarios. Additionally, variations in certain quantities are also made for the different collision energies explored in the high-energy experiments. Rectification of the quantitative accuracy, especially in the low-temperature hadronic sector, is of prime concern, and it is also addressed. It was found that most of the observables stay in close proximity with the existing lattice QCD results at the continuum limit, with some artifacts still remaining, especially in the strange sector, which needs further attention.

Hyperons in neutron stars: studies of hyperon spectroscopy and the hyperon-nucleon interaction with the K-long Facility.

Elucidating the role of strange baryons (hyperons) in neutron stars requires detailed knowledge of hyperon-nucleon interactions in the light (u,d,s) quark sector. The structure of the hyperons and their excitation spectra also directly impact, and are an input to, models of big-bang nucleosynthesis. The upcoming K-long Facility will provide a much-needed intense and clean neutral strange meson beam, from which hyperons can be produced at rates where hyperon structure, hyperon-nucleon interactions and higher-order interactions can be studied with a new level of accuracy and for hitherto unreachable measurements. The new facility has the potential to address long-standing questions surrounding the strange sector of the strong force and its relevance to the structure of atomic nuclei, neutron stars and the cosmos at large. This article is part of the theme issue 'The liminal position of Nuclear Physics: from hadrons to neutron stars'.

Molecular pentaquarks with hidden charm and double strangeness

We analyze theoretically the coupled-channel meson-baryon interaction with global flavor c¯cssn and c¯csss, where mesons are pseudoscalars or vectors, and baryons have JP=1/2+ or 3/2+. The aim is to explore whether the nonlinear dynamics inherent in the unitarization process within coupled channels can dynamically generate double- and triple-strange pentaquark-type states (Pcss and Pcsss, respectively), for which there is no experimental evidence to date. We evaluate the s-wave scattering matrix by implementing unitarity in coupled channels, using potential kernels obtained from t-channel vector meson exchange. The required PPV and VVV vertices are obtained from Lagrangians derived through appropriate extensions of the local hidden gauge symmetry approach to the charm sector, while capitalizing on the symmetry of the spin and flavor wave function to evaluate the BBV vertex. We find four different poles in the double strange sector, some of them degenerate in spin. For the triple-strange channel, we find the meson-baryon interaction insufficient to generate a bound or resonance state through the unitary coupled-channel dynamics. Published by the American Physical Society 2024

K + N Elastic Scatterings for Estimation of the In-Medium Quark Condensate with Strange Quarks

Abstract We revisit the low-energy K+N elastic scatterings in the context of the in-medium quark condensate with strange quarks. The chiral ward identity connects the in-medium quark condensate to the soft limit value of the pseudoscalar correlation function evaluated in nuclear matter. The in-medium correlation function of the pseudoscalar fields with strangeness describes in-medium kaon propagation and is obtained by kaon–nucleon scattering amplitudes in the low-density approximation. We construct the kaon–nucleon scattering amplitudes in chiral perturbation theory up to the next-to-leading order and add some terms of the next-to-next-to-leading order with the strange quark mass to improve expansion of the strange quark sector. We also consider the effect of a possible broad resonance state around Plab = 600 MeV/c for I = 0 reported in the previous study. The low-energy constants are determined by existing K+N scattering data. We obtain good reproduction of the K+p scattering amplitude by chiral perturbation theory, while the description of the KN amplitude with I = 0 is not so satisfactory due to the lack of low-energy data. Performing analytic continuation of the scattering amplitudes obtained by chiral perturbation theory to the soft limit, we estimate the in-medium strange quark condensate.

Spectrum of the molecular tetraquarks: Unraveling the Tcs0(2900) and Tcs¯0a(2900)

We relate the interactions of the D¯(*)K* and D(*)K* systems to those of D(*)D(*) and D(*)D¯(*) respectively, considering the residual strong interactions at the near-threshold energy is too weak to excite the strange quarks inside the hadrons. We propose an effective model to describe the low-energy S-wave interactions that are undertaken by the light u, d quarks between two separated heavy hadrons. We find that the existence of molecules in the heavy-(anti)heavy sectors will naturally lead to the emergence of molecular states in D¯(*)K* and D(*)K* systems. The recently observed Tcs0(2900) and Tcs¯0a(2900) can be well identified as the 0(0+) and 1(0+) partners of Tcc(3875) and Zc(3900) in the charmed strange sector, respectively. We also predict their members under the (c and s) quark symmetry and SU(2) flavor symmetry. Most of them are very good molecule candidates, for example, (i) the 0(1+) states in D*D*, D¯K*, D¯*K*; (ii) the 0(+)(2+(+)) states in D*D¯*, D¯*K*, D*K*; (iii) the 1−(0++) state in D*D¯* and 1(1+) state in D*K*. The 0+(0++) state in DD¯ and the 0(1+) state in DK* might also exist as virtual states, and the 0(1+) DK* can serve as a key to infer the existence of 0+(0++) DD¯. The Dsπ invariant mass spectrum of Tcs¯0a(2900) is also studied within the coupled-channel approach, and the molecular interpretation of Tcs¯0a(2900) is consistent with the experimental data. Searching for the predicted states in experiments is crucial to discriminate the different pictures for interpreting these near-threshold exotica. Published by the American Physical Society 2024

Kaonic atoms with SIDDHARTA-2 at the DAΦNE collider

The most important information still missing in the field of the low-energy antikaon-nucleon inter actions studies is the experimental determination of the hadronic energy shift and width of kaonic deuterium. This measurement will be performed by the SIDDHARTA-2 experiment, installed at the DAΦNE collider and presently in data taking campaign. The precise measurement of the shift and width of the 1s level with respect to the purely electromagnetic calculated values, generated by the presence of the strong interaction, through the measurement of the X-ray transitions to this level, in kaonic hydrogen, was performed by the SIDDHARTA collaboration, the kaonic deuterium is underway by SIDDHARTA-2. These measurement will allow the first precise experimental extraction of the isospin dependent antikaon-nucleon scattering lengths, funda mental quantities for understanding low-energy QCD in the strangeness sector. The experimental challenge of the kaonic deuterium measurement is the very small X-rays yield, the even larger width (compared to kaonic hy drogen), and the difficulty to perform X-rays spectroscopy with weak signals in the high radiation environment of DAΦNE. It was, therefore, crucial to develop a new apparatus involving large-area X-rays detector system, to optimize the signal and to control and by improve the signal-to-background ratio by gaining in solid angle, increasing the timing capability, and as well implementing additional charge particle tracking veto systems.

CdZnTe detectors tested at the [formula omitted] collider for future kaonic atoms measurements

The SIDDHARTA-2 collaboration at the INFN Laboratories of Frascati (LNF) aims to perform important measurements on kaonic atoms. In parallel to the groundbreaking kaonic deuterium measurement, presently running on the DAΦNE collider at LNF, we plan to install additional detectors. The aim is to perform further kaonic atoms’ studies, taking advantage of the unique low energy and low momentum spread K− beam delivered by the at-rest decay of the ϕ meson.CdZnTe devices are ideal for detecting transitions toward both the upper and lower levels of intermediate-mass kaonic atoms, like kaonic carbon and aluminium. Measuring these transitions can have an important impact on the strangeness sector of nuclear physics.We present the results obtained in two preliminary tests conducted at DAΦNE in view of measurements foreseen in 2024, with the twofold aim to tune the timing window required to reject the extremely high electromagnetic background, and to quantify the readout saturation effect due to the high rate when placed close to the Interaction Region (IR). In the first test we used a RITEC device and electronics, while for the second one commercial REDLEN detectors were coupled to a frontend electronics customized at the University of Palermo.The results confirmed the possibility of finding and matching a proper timing window to identify the signal events and proved the better performance in terms of energy resolution of the REDLEN technology with a custom frontend electronics. In both cases, strong saturation effects were confirmed, accounting for a loss of almost 90% of the events, which will be overcome by a dedicated shielding structure foreseen for the final experimental setup.

Surface and curvature tensions of cold quark matter in the SU(3)fNJL model with vector interactions

AbstractWe investigate the surface and curvature tensions of three‐flavor cold quark matter in a state of electric charge neutrality and chemical equilibrium under weak interactions, employing the Nambu–Jona–Lasinio model with the inclusion of vector interactions. To account for finite‐size effects we adopt the multiple‐reflection expansion framework and present our results for various input parameters. Our results indicate that both surface and curvature tensions increase with heightened vector interactions and density. Furthermore, we note that the restoration of chiral symmetry in the strange quark sector leads to a modest reduction in curvature tension and a substantial increase in surface tension.

Cumulants of net-strangeness multiplicity distributions at energies available at the BNL Relativistic Heavy Ion Collider

The higher-order cumulants of net-proton number, net-charge, and net-strangeness multiplicity distributions are widely studied to search for the quantum-chromodynamics critical point and extract the chemical freeze-out parameters in heavy-ion collisions. In this context, the event-by-event fluctuations of the net-strangeness multiplicity distributions play important roles in extracting the chemical freeze-out parameter in the strangeness sector. Due to having difficulties in detecting all strange hadrons event by event, the kaon ($K$) and lambda ($\Lambda$) particles serve as a proxy for the strangeness-related observables in heavy-ion collisions. We have studied the net-$K$, net-$\Lambda$, and net-($K$ + $\Lambda$) multiplicity distributions and calculated their different order of cumulants using the ultrarelativistic quantum molecular dynamics model and hadron resonance gas calculation. To adequately account for the net-strangeness cumulants, it has been found that the inclusion of resonance decay contributions in $K$ and $\Lambda$ is necessary.

Baryons and antibaryons in compressed nuclear matter

In this work we discuss the in-medium properties of baryons and antibaryons in nuclear matter at densities beyond saturation. We focus the attention on the in-medium optical potentials of heavy baryons with strangeness content. That is, the hyperons Λ, Σ with strangeness S=-1 and the cascade particle Ξ with S=-2 . The particular treatment of the hyperons inside compressed matter is crucial in order to understand the in-medium modifications of baryons in matter conditions that may occur in the interior of neutron stars. We dicsuss further the properties of their antiparticles, since there are experimental data for the nucleonic sector (S=0) while for the strangeness sector (S=-1,-2) experimental information will be available in the close future at the new facilities, for instance, at FAIR and J-PARC.

Constraint of the Nuclear Dissipation Coefficient in Fission of Hypernuclei.

Experimental studies of nuclear fission induced by fusion, transfer, spallation, fragmentation, and electromagnetic reactions in combination with state-of-the-art calculations are successful to investigate the nuclear dissipation mechanism in normal nuclear matter, containing only nucleons. The dissipation mechanism has been widely studied by the use of many different fission observables and nowadays the dissipation coefficients involved in transport theories are well constrained. However, the existence of hypernuclei and the possible presence of hyperons in neutron stars make it necessary to extend the investigation of the nuclear dissipation coefficient to the strangeness sector. In this Letter, we use fission reactions of hypernuclei to constrain for the first time the dissipation coefficient in hypernuclear matter, observing that this coefficient increases a factor of 6 in the presence of a single Λ hyperon with respect to normal nuclear matter.

Constraining hadronization mechanisms with [formula omitted] production ratios in Pb–Pb collisions at [formula omitted] TeV

The production of prompt Λc+ baryons at midrapidity (|y|<0.5) was measured in central (0–10%) and mid-central (30–50%) Pb–Pb collisions at the center-of-mass energy per nucleon–nucleon pair sNN=5.02 TeV with the ALICE detector. The results are more precise, more differential in centrality, and reach much lower transverse momentum (pT=1 GeV/c) with respect to previous measurements performed by the ALICE, STAR, and CMS Collaborations in nucleus–nucleus collisions, allowing for an extrapolation down to pT=0. The pT-differential Λc+/D0 ratio is enhanced with respect to the pp measurement for 4<pT<8 GeV/c by 3.7 standard deviations (σ), while the pT-integrated ratios are compatible within 1σ. The observed trend is similar to that observed in the strange sector for the Λ/KS0 ratio. Model calculations including coalescence or statistical hadronization for charm-hadron formation are compared with the data.

Cross-Channel Constraints on Resonant Antikaon-Nucleon Scattering.

Chiral perturbation theory and its unitarized versions have played an important role in our understanding of the low-energy strong interaction. Yet, so far, such studies typically deal exclusively with perturbative or nonperturbative channels. In this Letter, we report on the first global study of meson-baryon scattering up to one-loop order. It is shown that covariant baryon chiral perturbation theory, including its unitarization for the negative strangeness sector, can describe meson-baryon scattering data remarkably well. This provides a highly nontrivial check on the validity of this important low-energy effective field theory of QCD. We show that the K[over ¯]N related quantities can be better described in comparison with those of lower-order studies, and with reduced uncertainties due to the stringent constraints from the πN and KN phase shifts. In particular, we find that the two-pole structure of Λ(1405) persists up to one-loop order reinforcing the existence of two-pole structures in dynamically generated states.

SIDDHARTA-2 veto system design and performance for kaonic atoms studies at DAΦNE

Light kaonic atoms spectroscopy provides a unique approach to study the low-energy strong interaction in the strangeness sector. Precise measurements of X-ray emission from light kaonic atoms provide valuable information on kaon-nucleus interaction at threshold without the need for extrapolation as required in scattering experiments. The SIDDHARTA-2 experiment at the DAΦNE collider of INFN-LNF is now poised to perform the challenging measurements of the K−- d 2p → 1s transition to extract the isospin-dependent antikaonnucleon scattering lengths. To achieve this goal, the background reduction is a crucial factor. This paper provides an overview of the SIDDHARTA-2 Veto-1 system, which uses scintillators outside the vacuum chamber to detect charged particles produced by K− absorption by the nucleus. The arrival time of these particles is correlated with the position where the kaonic atom has been created inside the setup, allowing for the rejection of kaons stopped outside the target cell, which is a critical component for reducing the background and improve the accuracy of the measurement.

Hadron research with AMBER at CERN

The recently approved NA66/AMBER experiment (Apparatus for Meson and Baryon Experimental Research) at the CERN Super Proton Synchrotron pursues a broad research program in quantum chromodynamics. It ranges in its first phase from a precision measurement of the proton radius with a 100 GeV muon beam to investigating the quark-gluon structure of mesons in Drell-Yan processes. In a second phase, radio-frequency separated kaon beams will allow to extend such investigations to the strangeness sector.

Kaonic atoms at the DAΦNE collider with the SIDDHARTA-2 experiment

Kaonic atoms are a unique tool to explore quantum chromodynamics in the strangeness sector at low energy, with implications reaching neutron stars and dark matter. Precision x-ray spectroscopy can fully unlock the at-threshold isospin dependent antikaon-nucleon scattering lengths, via the atomic transitions to the fundamental level. While the SIDDHARTA experiment at the INFN-LNF DAΦNE collider successfully measured kaonic hydrogen, its successor SIDDHARTA-2 is starting now its data taking campaign aiming to finally fully disentangle the isoscalar and isovector scattering lengths via the measurement of kaonic deuterium. An overview of the first experimental results from a preparatory run for the SIDDAHARTA-2 experiment is presented.

High precision Kaonic Deuterium measurement at the DAΦNE collider: the SIDDHARTA-2 experiment and the SIDDHARTINO run

The kaonic deuterium 2p→1s transition X-ray measurement, a fundamental information needed for a deeper understanding of the Quantum ChromoDynamics (QCD) in the strangeness sector, is still missing. The SIDDHARTA-2 collaboration is now ready to achieve this unprecedented result thanks to the dedicated experimental apparatus that will allow to obtain the values of the kaonic deuterium K-transitions with a precision comparable to the most precise kaonic hydrogen measurement to-date performed by SIDDHARTA in 2009. Both the kaonic hydrogen and kaonic deuterium X-ray spectroscopy measurements of the de-excitation towards the fundamental level are a direct probe on KN interaction at threshold, as opposed to the scattering experiments which need an extrapolation to zero energy. Combining these results through the Deser-Truemann like formula, the isospin-dependent kaon-nucleon scattering lengths can be obtained in a model-independent way. The SIDDHARTA-2 setup is presently installed at the DAΦNE (Double Annular Φ Factory for Nice Experiments) collider of Istituto Nazionale di Fisica Nucleare – Laboratori Nazionali di Frascati and it is ready to perform the challening kaonic deuterium measurement. Thispaper provides an overview on the SIDDHARTA-2 experimental apparatus and a preliminary result of the kaonic helium run, preparatory for the SIDDHARTA-2 data taking campaign, is also presented.

News from the light and strange meson sector

Recent high-quality data sets from various experiments grant new insights into the excitation spectrum of light mesons, which are composed of up, down, and strange quarks. In the non-strange light-meson sector, many recent experimental and theory efforts are focused on the search for so-called exotic states, which are states beyond the na¨ıve constituent quark-model. To this end, various potential decay modes of exotic states are studied at experiments such as GlueX or COMPASS. In addition, there is recent progress in the understanding of so-called non-resonant processes, which act as background in searches for light mesons, as well as in the prediction of light mesons from lattice QCD. While the non-strange light-meson spectrum is already mapped out rather well, many predicted strange mesons have not yet been observed experimentally or need further confirmation. Recent high-precision data from experiments such as LHCb and BESIII allow us to study strange mesons in heavy-meson decays. Alternatively, strange mesons are studied in diffractive scattering of high-energy kaon beams. The so far world’s largest data set on the diffractively produced K−π −π + final state was measured by the COMPASS experiment. A recent analysis of this data covers a large variety of strange mesons over a wide mass range in a single, self-consistent analysis

Kaon and nucleon states with hidden charm

In this talk we discuss the formation of exotic hadrons with hidden charm arising from three-body interactions. To be more specific, in the strangeness sector, we predict the existence of a mesonic state, $K^*(4307)$, which is dynamically generated from the three-body interactions of the $KD\bar D^*$ system, has a mass around 4307 MeV and quantum numbers $I(J^P) = 1/2 (1^-)$. In the baryonic sector, we predict the existence of $N^*$ states, which are generated from the three-body interactions of the $ND\bar D^*$ system, with masses around 4400∼4600 MeV, widths of 2 ∼20 MeV and positive parity.

Combined analysis of the Zc(3900) and the Zcs(3985) exotic states

We have performed a combined analysis of the BESIII data for both the $Z_c(3900)$ and $Z_{cs}(3985)$ structures, assuming that the latter is an SU(3) flavor partner of the former one. We have improved on the previous analysis of Albaladejo $et$ $al.$ [Phys. Lett. B 755, 337 (2016)] by computing the amplitude for the $D_1\bar{D}D^*$ triangle diagram considering both $D$ and $S$-wave $D_1D^*\pi$ couplings. We have also investigated effects from SU(3) light-flavor violations, which are found to be moderate and of the order of 20%. The successful reproduction of the BESIII spectra, in both the hidden-charm and hidden-charm strange sectors, strongly supports that the $Z_{cs}(3985)$ and $Z_c(3900)$ are SU(3) flavor partners placed in the same octet multiplet. The best results are obtained when an energy-dependent term in the diagonal $D^{(*)}\bar D_{(s)}^{(*)}$ interaction is included, leading to resonances (poles above the thresholds) to describe these exotic states. We have also made predictions for the isovector $Z_{c}^*$ and isodoublet $Z_{cs}^*$, $D^*\bar{D}^*$ and $D^*\bar{D}_{s}^*$ molecules, with $J^{PC}=1^{+-}$ and $J^{P}=1^{+}$, respectively. These states would be heavy-quark spin symmetry partners of the $Z_{c}$ and $Z_{cs}$. Besides the determination of the masses and widths of the $Z_c(3900)$ and $Z_{cs}(3985)$, we also predict those of the $Z_c^*$ and $Z_{cs}^*$ resonances.