Hades experiments : investigation of hadron in-medium properties

Hadron modifications in nuclear matter are discussed in connection to chiral symmetry restoration and/or hadronic many body effects. Experiments with photon, proton and heavy ion beams are used to probe properties of hadrons embedded in nuclear matter at different temperatures and densities. Most of the information was gathered for the light vector mesons $\rho$, $\omega$ and $\phi$. HADES is a second generation experiment operating at GSI (Darmstadt) with the main aim to study in-medium modifications by means of dielectron production at the SIS18/Bevelac energy range. Large acceptance and excellent particle identification capabilities allow also for measurements of strangeness production. These abilities combined with the variety of beams provided by the SIS18 lead to a characterization of properties of the dense baryonic matter properties created in heavy ion collisions at these energies. A review of recent experimental results obtained by HADES is presented, with the main emphasis on hadron properties in nuclear matter

Resonance production and decays in nucleon-nucleon collisions with HADES

The High Acceptance Di-Electron Spectrometer (HADES), installed at GSI, allows to study both hadron and rare dilepton production in a fixed target experiments in the beam energy range of a few GeV per nucleon, reaching up to 4.5 GeV for a proton beam and 1.5 GeV/nucleon for a heavy-ion beam. The present interpretation of dilepton spectra measured in heavy-ion reactions at various energies is based on hadronic models. They predict in-medium modifications of the $\rho$−meson spectral function due to its coupling to resonance-hole states. In the energy range of the HADES experiments, the $\rho$−meson can be produced in primary NN or secondary $\pi$ N collisions. The measurement of a dielectron emission in elementary reactions opens the possibility to constrain the interpretation of medium effects and better understand the relation between the couplings of the baryonic resonances to the $\rho$−meson. The Dalitz decay of baryonic resonances $(R \rightarrow Ne^{+}e^{-})$ gives access to the electromagnetic structure of baryonic transitions in a timelike region at small positive values of the squared fourmomentum transferred, where couplings to light vector $\rho /\omega$ mesons are expected. In this monograph the systematic studies of resonance excitation and decays, measured in NN collisions with the increasing energy, are presented. First, the pp collisions at 1.25 GeV allowed to measure exclusive channels with one-pion in the final state. They were put to extended studies based on various observables in one-pion exchange models and with solutions obtained within the framework of a partial wave analysis (PWA) of the Bonn-Gatchina group. The obtained $\Delta$ (1232) production cross section was further used for the first experimental extraction of the $\Delta$ Dalitz decay and compared to the models of the electromagnetic transition form factors. The branching ratio of this decay, achieved in the exclusive $pp \rightarrow ppe^{+}e^{-}$ channel, has been included in the 2018 Review of Particle Physics. In quasi-free np collisions at 1.25 GeV, two-pion production was studied to conclude on double−$\Delta$, N(1440), and $\Delta$ (1600) excitation with the aim of studying the production of the $\rho$−meson in the isospin I = 1 channel. In addition, this analysis provided an independent verification of the existence of the $d^{\ast }$(2370) resonance observed by the WASA Collaboration. Furthermore, a strong excess of the dielectron yield, in comparison to the pp reaction at 1.25 GeV, was observed in the $e^{+}e^{-}$ inclusive channel. This reaction is well suited for studies of the np bremsstrahlung with strong isospin effects effects predicted by one-boson-exchange models. The excess was confirmed also in the $e^{+}e^{-}$ exclusive channel and the data were compared with the models explaining the enhancement at large invariant masses due to the off-shell $\rho$−meson production. The exclusive hadronic channels with one-pion and rho−meson were identified in pp collisions at 2.2 GeV. The $\pi ^{0}$ and eta exclusive production cross sections were extracted and compared to the predictions of a resonance model. Further, the analysis of the $ppe^{+}e^{-}$ exclusive channel allowed for the reconstruction of $\pi ^{0}$ and $\eta$ Dalitz decays. The angular distributions of $e^{+} or e^{−}$ in the $\gamma^{\ast }$ rest frame were determined to be in agreement with the QED prediction for pseudoscalar mesons. In the pp collisions measured at energy T = 3.5 GeV, a peak corresponding to direct $\omega$ decays, with 2% mass resolution, was reconstructed. The inclusive production cross sections for $\omega$ and $\rho$ mesons were determined from dielectron experimental data for the first time at this energy. In addition, the upper bound for the direct $\eta \rightarrow e^{+}e^{-}$ decay was defined. The exclusive one pion production $(pp \rightarrow pp\pi ^{0}$ and $pp \rightarrow np\pi ^{+})$ and dielectron $(pp \rightarrow ppe^{+}e^{-})$ channels were used to extract the $N^{\ast }$ and $\Delta$ resonance production cross sections. The $e^{+}e^{-}$ emission from baryonic decays was identified with the dielectron invariant mass distribution strongly modified by the intermediate rho−meson. This observation has been interpreted within the Vector Dominance Model of the resonance electromagnetic transition form factor. Comparison with various transport models (PYTHIA, GiBUU/SMASH, HSD, UrQMD) unraveled the ambiguities of the model descriptions and the important role of the intermediate rho production. To tackle this problem a systematic investigation focused on the role of N(1520) production and decay in pion-induced reactions was started. Data at four different pion beam momenta (656, 690, 748 and 800 MeV/c) were collected in $\pi ^{−}p$ collisions. In the outlook, exclusive channels with one pion $(\pi ^{−} p)$, two pions $(n\pi ^{+} \pi ^{−})$ and dileptons $(ne^{+} e^{−})$ in the final state were discussed, with a special interest for the rhoN channel, since it has a direct impact on the in-medium distortions of the $\rho$−meson spectral function. Pion beam experiments will be continued in GSI in the next years, then the HADES experimental program will be pursued using the proton and ion beams at FAIR. This monograph has been extended by Appendices covering details on dielectron production channels as well as the models of the electromagnetic transition form factors for baryon resonance Dalitz decays. The HADES spectrometer and the analysis strategy were described, too. The author hopes that this book will serve not only as a report on the scientific achievements but also as a source of information on resonance production and decays in nucleon-nucleon collisions at energies of a few GeV

Search for an axion-like particle with forward proton scattering in association with photon pairs at ATLAS

A search for forward proton scattering in association with light-by-light scattering mediated by an axion-like particle is presented, using the ATLAS Forward Proton spectrometer to detect scattered protons and the central ATLAS detector to detect pairs of outgoing photons. Proton-proton collision data recorded in 2017 at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV were analysed, corresponding to an integrated luminosity of 14.6 fb$^{−1}$. A total of 441 candidate events were selected. A search was made for a narrow resonance in the diphoton mass distribution, corresponding to an axion-like particle (ALP) with mass in the range 150–1600 GeV. No excess is observed above a smooth background. Upper limits on the production cross section of a narrow resonance are set as a function of the mass, and are interpreted as upper limits on the ALP production coupling constant, assuming 100% decay branching ratio into a photon pair. The inferred upper limit on the coupling constant is in the range 0.04–0.09 TeV$^{−1}$ at 95% confidence level

Observation of four-top-quark production in the multilepton final state with the ATLAS detector

This paper presents the observation of four-top-quark ($t\bar{t}t\bar{t}$) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 fb$^{-1}$ at a centre-of-mass energy of 13 TeV collected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of the measured $t\bar{t}t\bar{t}$ signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The $t\bar{t}t\bar{t}$ production cross section is measured to be 22.5$_{-5.5}^{+6.6}$ fb, consistent with the SM prediction of 12.0 $\pm$ 2.4 fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect $t\bar{t}t\bar{t}$ production

Search for single production of vector-like T quarks decaying into Ht or Zt in pp collisions at s\sqrt{s} = 13 TeV with the ATLAS detector

This paper describes a search for the single production of an up-type vector-like quark (T) decaying as T $\to$ Ht or T $\to$ Zt. The search utilises a dataset of pp collisions at $\sqrt{s}$ = 13 TeV collected with the ATLAS detector during the 2015–2018 data-taking period of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb$^{−1}$. Data are analysed in final states containing a single lepton with multiple jets and b-jets. The presence of boosted heavy resonances in the event is exploited to discriminate the signal from the Standard Model background. No significant excess above the Standard Model expectation is observed, and 95% CL upper limits are set on the production cross section of T quarks in different decay channels. The results are interpreted in several benchmark scenarios to set limits on the mass and universal coupling strength ($\kappa$) of the vector-like quark. For singlet T quarks, $\kappa$ values above 0.53 are excluded for all masses below 2.3 TeV. At a mass of 1.6 TeV, $\kappa$ values as low as 0.35 are excluded. For T quarks in the doublet scenario, where the production cross section is much lower, $\kappa$ values above 0.72 are excluded for all masses below 1.7 TeV, and this exclusion is extended to $\kappa$ above 0.55 for low masses around 1.0 TeV

Comparison of inclusive and photon-tagged jet suppression in 5.02 TeV Pb+Pb collisions with ATLAS

Parton energy loss in the quark–gluon plasma (QGP) is studied with a measurement of photon-tagged jet production in 1.7 nb$^{-1}$ of Pb+Pb data and 260 pb$^{−1}$ of pp data, both at $\sqrt{^{S}NN}$ = 5.02 TeV, with the ATLAS detector. The process pp $\to$ γ + jet + X and its analogue in Pb+Pb collisions is measured in events containing an isolated photon with transverse momentum (p$_{T}$) above 50 GeV and reported as a function of jet p$_{T}$. This selection results in a sample of jets with a steeply falling p$_{T}$ distribution that are mostly initiated by the showering of quarks. The pp and Pb+Pb measurements are used to report the nuclear modification factor, R$_{AA}$, and the fractional energy loss, S$_{loss}$, for photon-tagged jets. In addition, the results are compared with the analogous ones for inclusive jets, which have a significantly smaller quark-initiated fraction. The R$_{AA}$ and S$_{loss}$ values are found to be significantly different between those for photon-tagged jets and inclusive jets, demonstrating that energy loss in the QGP is sensitive to the colour-charge of the initiating parton. The results are also compared with a variety of theoretical models of colour-charge-dependent energy loss

Search for Majorana neutrinos in same-sign WW scattering events from pp collisions at s\sqrt{s} = 13 TeV

A search for Majorana neutrinos in same-sign WW scattering events is presented. The analysis uses $\sqrt{s}$ = 13 TeV proton–proton collision data with an integrated luminosity of 140 fb$^{-1}$ recorded during 2015–2018 by the ATLAS detector at the Large Hadron Collider. The analysis targets final states including exactly two same-sign muons and at least two hadronic jets well separated in rapidity. The modelling of the main backgrounds, from Standard Model same-sign WW scattering and WZ production, is constrained with data in dedicated signal-depleted control regions. The distribution of the transverse momentum of the second-hardest muon is used to search for signals originating from a heavy Majorana neutrino with a mass between 50 GeV and 20 TeV. No significant excess is observed over the background expectation. The results are interpreted in a benchmark scenario of the Phenomenological Type-I Seesaw model. In addition, the sensitivity to the Weinberg operator is investigated. Upper limits at the 95% confidence level are placed on the squared muon-neutrino–heavy-neutrino mass-mixing matrix element $\left|V_{\mu N}\right|^{2}$ as a function of the heavy Majorana neutrino’s mass $m_{N}$, and on the effective $\mu\mu$ Majorana neutrino mass $\left|m_{\mu\mu}\right|$

Evidence of off-shell Higgs boson production from ZZ leptonic decay channels and constraints on its total width with the ATLAS detector

This Letter reports on a search for off-shell production of the Higgs boson using 139 fb$^{-1}$ of pp collision data at $\sqrt{s}$ = 13 TeV collected by the ATLAS detector at the Large Hadron Collider. The signature is a pair of Z bosons, with contributions from both the production and subsequent decay of a virtual Higgs boson and the interference of that process with other processes. The two observable final states are ZZ $\to 4\ell$ and ZZ $\to 2\ell2v$ with $\ell$ = e or $\mu$. In the ZZ $\to 4\ell$ final state, a dense Neural Network is used to enhance analysis sensitivity with respect to matrix element-based discrimination. The background-only hypothesis is rejected with an observed (expected) significance of 3.3 (2.2) standard deviations, representing experimental evidence for off-shell Higgs boson production. Assuming that no new particles enter the production of the virtual Higgs boson, its total width can be deduced from the measurement of its off-shell production cross-section. The measured total width of the Higgs boson is 4.5$^{+3.3}_{-2.5}$ MeV, and the observed (expected) upper limit on the total width is found to be 10.5 (10.9) MeV at 95% confidence level

Search for dark matter produced in association with a Higgs boson decaying to tau leptons at s\sqrt{s} = 13 TeV with the ATLAS detector

A search for dark matter produced in association with a Higgs boson in final states with two hadronically decaying $\tau$-leptons and missing transverse momentum is presented. The analysis uses 139 fb$^{−1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. No evidence of physics beyond the Standard Model is found. The results are interpreted in terms of a 2HDM+a model featuring two scalar Higgs doublets and a pseudoscalar singlet field. Exclusion limits on the parameters of the model in selected benchmark scenarios are derived at 95% confidence level. Model-independent limits are also set on the visible cross-section for processes beyond the Standard Model producing missing transverse momentum in association with a Higgs boson decaying into $\tau$-leptons

Search for a new heavy scalar particle decaying into a Higgs boson and a new scalar singlet in final states with one or two light leptons and a pair of τ\tau-leptons with the ATLAS detector

A search for a new heavy scalar particle X decaying into a Standard Model (SM) Higgs boson and a new singlet scalar particle S is presented. The search uses a proton-proton (pp) collision data sample with an integrated luminosity of 140 fb$^{−1}$ recorded at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV with the ATLAS detector at the Large Hadron Collider. The most sensitive mass parameter space is explored in X mass ranging from 500 to 1500 GeV, with the corresponding S mass in the range 200–500 GeV. The search selects events with two hadronically decaying τ-lepton candidates from H $\to \tau^{+}\tau^{-}$ decays and one or two light leptons ($\ell$ = e, μ) from S $\to$ VV (V = W, Z) decays while the remaining V boson decays hadronically or to neutrinos. A multivariate discriminant based on event kinematics is used to separate the signal from the background. No excess is observed beyond the expected SM background and 95% confidence level upper limits between 72 fb and 542 fb are derived on the cross-section σ(pp $\to$ X $\to$ SH) assuming the same SM-Higgs boson-like decay branching ratios for the S $\to$ VV decay. Upper limits on the visible cross-sections σ(pp $\to$ X $\to$ SH $\to$ WW$_{\tau \tau}$) and σ(pp $\to$ X $\to$ SH $\to$ ZZ$_{\tau \tau}$) are also set in the ranges 3–26 fb and 6–33 fb, respectively