A. Belyaev

HADES is a versatile magnetic spectrometer aimed at studying dielectron production in pion, proton and heavy-ion induced collisions. Its main features include a ring imaging gas Cherenkov detector for electron-hadron discrimination, a tracking system consisting of a set of 6 superconducting coils producing a toroidal field and drift chambers and a multiplicity and electron trigger array for additional electron-hadron discrimination and event characterization. A two-stage trigger system enhances events containing electrons. The physics program is focused on the investigation of hadron properties in nuclei and in the hot and dense hadronic matter. The detector system is characterized by an 85% azimuthal coverage over a polar angle interval from 18 to 85 degree, a single electron efficiency of 50% and a vector meson mass resolution of 2.5%. Identification of pions, kaons and protons is achieved combining time-of-flight and energy loss measurements over a large momentum range. This paper describes the main features and the performance of the detector system.

We present phase space distributions and multiplicities of K + , K -, and φ mesons produced in Ar + KCl reactions at a kinetic beam energy of 1.756 GeV/nucleon and measured with the HADES spectrometer.The inverse slope parameters and yields of kaons supplement the systematics of previous measurements.The percentage of K -mesons coming from φ decay is found to be 18 ± 7%.

We present transverse momentum spectra, rapidity distribution and multiplicity of $ \Lambda$ -hyperons measured with the HADES spectrometer in the reaction Ar(1.76AGeV) + KCl . The yield of $ \Xi^{{-}}_{}$ is calculated from our previously reported $ \Xi^{{-}}_{}$ /( $ \Lambda$ + $ \Sigma^{{0}}_{}$ ) ratio and compared to other strange particle multiplicities. Employing a strangeness balance equation the multiplicities of the yet unmeasured $ \Sigma^{{\pm}}_{}$ -hyperons can be estimated. Finally a statistical hadronization model is used to fit the yields of $ \pi^{-}_{}$ , K+, K 0 s , K-, $ \phi$ , $ \Lambda$ and $ \Xi^{-}_{}$ . The resulting chemical freeze-out temperature of T = (76±2) MeV is compared to the measured slope parameters obtained from fits to the transverse mass distributions of the different particles.

We present a search for the e+e− decay of a hypothetical dark photon, also named U vector boson, in inclusive dielectron spectra measured by HADES in the p(3.5 GeV) + p, Nb reactions, as well as the Ar (1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared ϵ2 at 90% CL has been obtained for the mass range MU=0.02–0.55 GeV/c2 and is compared with the present world data set. For masses 0.03–0.1 GeV/c2, the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favored by the muon g−2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3×10−6 has been set on the helicity-suppressed direct decay of the eta meson, η→e+e−, at 90% CL.

Employing the Bonn–Gatchina partial wave analysis framework (PWA), we have analyzed HADES data of the reaction p(3.5GeV)+p→pK+Λ. This reaction might contain information about the kaonic cluster "ppK−" (with quantum numbers JP=0− and total isospin I=1/2) via its decay into pΛ. Due to interference effects in our coherent description of the data, a hypothetical K‾NN (or, specifically "ppK−") cluster signal need not necessarily show up as a pronounced feature (e.g. a peak) in an invariant mass spectrum like pΛ. Our PWA analysis includes a variety of resonant and non-resonant intermediate states and delivers a good description of our data (various angular distributions and two-hadron invariant mass spectra) without a contribution of a K‾NN cluster. At a confidence level of CLs=95% such a cluster cannot contribute more than 2–12% to the total cross section with a pK+Λ final state, which translates into a production cross-section between 0.7 μb and 4.2 μb, respectively. The range of the upper limit depends on the assumed cluster mass, width and production process.

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. $C_n \propto <N>^n$, 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 $\sqrt{s_{NN}}$.

The Nuclotron-based Ion Collider fAcility (NICA) is under construction at the Joint Institute for Nuclear Research (JINR), with commissioning of the facility expected in late 2022. The Multi-Purpose Detector (MPD) has been designed to operate at NICA and its components are currently in production. The detector is expected to be ready for data taking with the first beams from NICA. This document provides an overview of the landscape of the investigation of the QCD phase diagram in the region of maximum baryonic density, where NICA and MPD will be able to provide significant and unique input. It also provides a detailed description of the MPD set-up, including its various subsystems as well as its support and computing infrastructures. Selected performance studies for particular physics measurements at MPD are presented and discussed in the context of existing data and theoretical expectations.

We present the transverse momentum spectra and rapidity distributions of ${\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\mathrm{K}}_{S}^{0}$ in Ar $+$ KCl reactions at a beam kinetic energy of $1.756$ $A$ GeV measured with the High Acceptance Di-Electron Spectrometer (HADES). The reconstructed ${\mathrm{K}}_{S}^{0}$ sample is characterized by good event statistics for a wide range in momentum and rapidity. We compare the experimental ${\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\mathrm{K}}_{S}^{0}$ distributions to predictions by the Isospin Quantum Molecular Dynamics (IQMD) model. The model calculations show that ${\mathrm{K}}_{S}^{0}$ at low transverse momenta constitute a particularly well-suited tool to investigate the kaon in-medium potential. Our ${\mathrm{K}}_{S}^{0}$ data suggest a strong repulsive in-medium ${\mathrm{K}}^{0}$ potential of about 40 MeV strength.

The emission of e+e− pairs from C+C collisions at an incident energy of 1 GeV per nucleon has been investigated. The measured production probabilities, spanning from the π0-Dalitz to the ρ/ω invariant-mass region, display a strong excess above the cocktail of standard hadronic sources. The bombarding-energy dependence of this excess is found to scale like pion production, rather than like eta production. The data are in good agreement with results obtained in the former DLS experiment.

We report measurements of electron pair production in elementary p+p and d+p reactions at 1.25 GeV/u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n+p reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass Me+e−>0.15 GeV/c2 is about an order of magnitude larger in n+p reactions as compared to p+p. A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C+C reactions are compatible with a superposition of elementary n+p and p+p collisions, leaving little room for additional electron pair sources in such light collision systems.

We present results on dielectron production in 40 Ar+KCl collisions at 1.76A GeV.For the first time ω mesons could be reconstructed in a heavy-ion reaction at a bombarding energy which is well below the production threshold in free nucleon-nucleon collisions.The ω multiplicity has been extracted and compared to the yields of other particles, in particular of the φ meson.At intermediate e + e -invariant masses, we find a strong enhancement of the pair yield over a reference spectrum from elementary nucleon-nucleon reactions suggesting the onset of non-trivial effects of the nuclear medium.Transverse-mass spectra and angular distributions have been reconstructed in three invariant mass bins.In the former unexpectedly large slopes are found for high-mass pairs.The latter, in particular the helicity-angle distributions, are largely consistent with expectations for a pair cocktail dominated at intermediate masses by ∆ Dalitz decays.

We present an analysis of the Lambda(1405) resonance produced in the reaction p+p->Sigma^{pm}+pi^{mp}+K+p at 3.5 GeV kinetic beam energy measured with HADES at GSI. The two charged decay channels Lambda(1405) -> Sigma^{\pm}+pi^{\mp} have been reconstructed for the first time in p+p collisions. The efficiency and acceptance-corrected spectral shapes show a peak position clearly below 1400 MeV/c^2. We find a total production cross section of sigma_{Lambda(1405)}=9.2 +- 0.9 +- 0.7 +3.3-1.0 mub. The analysis of its polar angle distribution suggests that the Lambda(1405) is produced isotropically in the p-p center of mass system.

Flow coefficients $v_{n}$ of the orders $n = 1 - 6$ are measured with the High-Acceptance DiElectron Spectrometer (HADES) at GSI for protons, deuterons and tritons as a function of centrality, transverse momentum and rapidity in Au+Au collisions at $\sqrt{s_{NN}} = 2.4$ GeV. Combining the information from the flow coefficients of all orders allows to construct for the first time, at collision energies of a few GeV, a multi-differential picture of the angular emission pattern of these particles. It reflects the complicated interplay between the effect of the central fireball pressure on the emission of particles and their subsequent interaction with spectator matter. The high precision information on higher order flow coefficients is a major step forward in constraining the equation-of-state of dense baryonic matter.

We present the inclusive invariant-mass, transverse momentum and rapidity distributions of dielectrons (e$^{+}$e$^{-}$ pairs) in p+p interactions at 3.5 GeV beam kinetic energy. In the vector-meson mass region, a distinct peak corresponding to direct $\omega$ decays is reconstructed with 2% mass resolution. The data is compared to predictions from three model calculations. Due to the large acceptance of the HADES apparatus for $e^{+}e^{-}$ invariant masses above 0.2 GeV/$c^{2}$ and for transverse pair momenta p$_{t}$ $ <$ 1 GeV/$c$, acceptance corrections are to a large extent model independent. This allows us to extract from dielectron data for the first time at this energy the inclusive production cross sections for light vector mesons. Inclusive production cross sections for $\pi^o$ and $\eta$ mesons are also reported. The obtained results will serve as an important reference for the study of vector meson production in proton-nucleus and heavy-ion collisions. Furthermore, using this data, an improved value for the upper bound of the branching ratio for direct $\eta$ decays into the electron-positron channel is obtained.

The centrality determination for Au + Au collisions at 1.23A GeV, as measured with HADES at the GSI-SIS18, is described. In order to extract collision geometry related quantities, such as the average impact parameter or number of participating nucleons, a Glauber Monte Carlo approach is employed. For the application of this model to collisions at this relatively low centre-of-mass energy of $ \sqrt{s_{{\rm NN}}} = 2.42$ GeV special investigations were performed. As a result a well defined procedure to determine centrality classes for ongoing analyses of heavy-ion data is established.

We present an updated assessment of the viability of t-b-tau Yukawa coupling unification in supersymmetric models. For the superpotential Higgs mass parameter mu>0, we find unification to less than 1% is possible, but only for GUT scale scalar mass parameter m_{16}~8-20 TeV, and small values of gaugino mass m_{1/2}<400 GeV. Such models require that a GUT scale mass splitting exists amongst Higgs scalars with m_{H_u}^2<m_{H_d}^2. Viable solutions lead to a radiatively generated inverted scalar mass hierarchy, with third generation and Higgs scalars being lighter than other sfermions. These models have very heavy sfermions, so that unwanted flavor changing and CP violating SUSY processes are suppressed, but may suffer from some fine-tuning requirements. While the generated spectra satisfy b->s gamma and (g-2)_mu constraints, there exists tension with the dark matter relic density unless m_{16}<3 TeV. These models offer prospects for a SUSY discovery at the Fermilab Tevatron collider via the search for chargino_1 neutralino_2 -> 3 leptons events, or via gluino pair production. If mu<0, Yukawa coupling unification to less than 5% can occur for m_{16} and m_{1/2}>1-2 TeV. Consistency of negative mu Yukawa unified models with b->s gamma, (g-2)_mu, and relic density all imply very large values of m_{1/2} typically greater than about 2.5 TeV, in which case direct detection of sparticles may be a challenge even at the LHC.

The first LHC data on high transverse momentum hadron and dijet spectra in PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed in the frameworks of PYQUEN jet quenching model. The presented studies for the nuclear modification factor of high-p T hadrons and the imbalance in dijet transverse energy support the supposition that the intensive wide-angular (“out-of-cone”) medium-induced partonic energy loss is seen in central PbPb collisions at the LHC.

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.

The reaction p(@3.5 GeV)+p→p+Λ+K+ can be studied to search for the existence of kaonic bound states like ppK− leading to this final state. This effort has been motivated by the assumption that in p+p collisions the Λ(1405) resonance can act as a doorway to the formation of the kaonic bound states. The status of this analysis within the HADES Collaboration, with particular emphasis on the comparison to simulations, is shown in this work and the deviation method utilized by the DISTO Collaboration in a similar analysis is discussed. The outcome suggests the employment of a partial wave analysis do disentangle the different contributions to the measured pK+Λ final state.

We present results of an exclusive measurement of the first-excited state of the $\ensuremath{\Sigma}$ hyperon, $\ensuremath{\Sigma}{(1385)}^{+}$, produced in $p+p\ensuremath{\rightarrow}{\ensuremath{\Sigma}}^{+}+{K}^{+}+n$ at $3.5\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$ beam energy. The extracted data allow the detailed study of the invariant mass distribution of the $\ensuremath{\Sigma}{(1385)}^{+}$. The mass distribution is well described by a relativistic Breit-Wigner function with a maximum at ${m}_{0}=1383.2\ifmmode\pm\else\textpm\fi{}0.9\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/{c}^{2}$ and a width of $40.2\ifmmode\pm\else\textpm\fi{}2.1\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/{c}^{2}$. The exclusive production cross section comes out to be $22.27\ifmmode\pm\else\textpm\fi{}0.89\ifmmode\pm\else\textpm\fi{}1.{56}_{\ensuremath{-}2.10}^{+3.07}\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}b$. Angular distributions of the $\ensuremath{\Sigma}{(1385)}^{+}$ in different reference frames are found to be compatible with the hypothesis that $33%$ of $\ensuremath{\Sigma}{(1385)}^{+}$ result from the decay of an intermediate broad ${\ensuremath{\Delta}}^{++}$ excitation at about $2000\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/{c}^{2}$.

We report on baryon resonance production and decay in proton-proton collisions at a kinetic energy of 3.5 GeV based on data measured with HADES. The exclusive channels $ pp \rightarrow np\pi^{+}$ and $ pp\rightarrow pp\pi^{0}$ as well as $ pp\rightarrow ppe^{+}e^{-}$ are studied simultaneously for the first time. The invariant masses and angular distributions of the pion-nucleon systems were studied and compared to simulations based on a resonance model ansatz assuming saturation of the pion production by an incoherent sum of baryonic resonances (R with masses < 2 GeV/c2. A very good description of the one-pion production is achieved allowing for an estimate of individual baryon resonance production cross sections which are used as input to calculate the dielectron yields from $ R\rightarrow pe^{+}e^{-}$ decays. Two models of the resonance decays into dielectrons are examined assuming a point-like $ RN\gamma^{*}$ coupling and the dominance of the $ \rho$ meson. The results of model calculations are compared to data from the exclusive $ ppe^{+}e^{-}$ channel by means of the dielectron and $ pe^{+}e^{-}$ invariant mass distributions.

Results on $ \Lambda$ hyperon production are reported for collisions of p(3.5 GeV) + Nb , studied with the High-Acceptance Di-Electron Spectrometer (HADES) at SIS18 at GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt. The transverse mass distributions in rapidity bins are well described by Boltzmann shapes with a maximum inverse slope parameter of about 90 MeV at a rapidity of y = 1.0, i.e. slightly below the center-of-mass rapidity for nucleon-nucleon collisions, $ y_{cm}=1.12$ . The rapidity density decreases monotonically with increasing rapidity within a rapidity window ranging from 0.3 to 1.3. The $ \Lambda$ phase-space distribution is compared with results of other experiments and with predictions of two transport approaches which are available publicly. None of the present versions of the employed models is able to fully reproduce the experimental distributions, i.e. in absolute yield and in shape. Presumably, this finding results from an insufficient modelling in the transport models of the elementary processes being relevant for $ \Lambda$ production, rescattering and absorption. The present high-statistics data allow for a genuine two-dimensional investigation as a function of phase space of the self-analyzing $ \Lambda$ polarization in the weak decay $ \Lambda\rightarrow p\pi^{-}$ . Finite negative values of the polarization in the order of 5-20% are observed over the entire phase space studied. The absolute value of the polarization increases almost linearly with increasing transverse momentum for p t > 300 MeV/c and increases with decreasing rapidity for y < 0.8 .

The tagged quasi-free np→npπ+π− reaction has been studied experimentally with the High Acceptance Di-Electron Spectrometer (HADES) at GSI at a deuteron incident beam energy of 1.25 GeV/nucleon (s∼2.42 GeV/c for the quasi-free collision). For the first time, differential distributions of solid statistics for π+π− production in np collisions have been collected in the region corresponding to the large transverse momenta of the secondary particles. The invariant mass and angular distributions for the np→npπ+π− reaction are compared with different models. This comparison confirms the dominance of the t-channel with ΔΔ contribution. It also validates the changes previously introduced in the Valencia model to describe two-pion production data in other isospin channels, although some deviations are observed, especially for the π+π− invariant mass spectrum. The extracted total cross section is also in much better agreement with this model. Our new measurement puts useful constraints for the existence of the conjectured dibaryon resonance at mass M∼2.38 GeV and with width Γ∼70 MeV.

We present first data on sub-threshold production of Ks0 mesons and Λ hyperons in Au+Au collisions at sNN=2.4 GeV. We observe an universal 〈Apart〉 scaling of hadrons containing strangeness, independent of their corresponding production thresholds. Comparing the yields, their 〈Apart〉 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 them can simultaneously describe these observables with reasonable χ2 values.

The started LHC heavy ion program makes it possible to probe new frontiers of the high temperature Quantum Chromodynamics. It is expected that the role of hard and semi-hard particle production processes may be significant at ultra-high energies even for bulk properties of the created matter. In this paper, the LHC data on multiplicity, hadron spectra, elliptic flow and femtoscopic correlations from PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed in the framework of the HYDJET++ model. The influence of the jet production mechanism on these observables is discussed.

We present the first observation of K -and φ absorption 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 ± 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±0.03(stat)+0.06 -0.07 (syst) and to 0.63±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 φ meson in nuclear medium is strongly coupled to the K -dynamics.The large difference in the φ production off C and W nuclei is discussed in terms of a strong φN in-medium coupling.

A feasibility study has been performed in order to investigate the performance of the HADES detector to measure the electromagnetic decays of the hyperon resonances $\Sigma(1385)^0$, $\Lambda(1405)$ and $\Lambda{\Lambda}(1520)$ as well as the production of double strange baryon systems $\Xi^-$ and $\Lambda\Lambda$ in p+p reactions at a beam kinetic energy of 4.5 GeV. The existing HADES detector will be upgraded by a new Forward Detector, which extends the detector acceptance into a range of polar angles that plays a crucial role for these investigations. The analysis of each channel is preceded by a consideration of the production cross-sections. Afterwards the expected signal count rates using a target consisting of either liquid hydrogen or polyethylene are summarized.

We present the analysis of the inclusive ${K}^{0}$ production in $p+p$ and $p+\mathrm{Nb}$ collisions measured with the HADES detector (GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt) at a beam kinetic energy of 3.5 GeV. Data are compared to the Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU) transport model. The data suggest the presence of a repulsive momentum-dependent kaon potential as predicted by the chiral perturbation theory (ChPT). For the kaon at rest and at normal nuclear density, the ChPT potential amounts to $\ensuremath{\approx}35$ MeV. A detailed tuning of the kaon production cross sections implemented in the model has been carried out to reproduce the experimental data measured in $p+p$ collisions. The uncertainties in the parameters of the model were examined with respect to the sensitivity of the experimental results from $p+\mathrm{Nb}$ collisions to the in-medium kaon potential.

Results on the production of the double strange cascade hyperon Ξ^{-} are reported for collisions of p(3.5 GeV)+Nb, studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18 at GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt. For the first time, subthreshold Ξ^{-} production is observed in proton-nucleus interactions. Assuming a Ξ^{-} phase-space distribution similar to that of Λ hyperons, the production probability amounts to P_{Ξ^{-}}=[2.0±0.4(stat)±0.3(norm)±0.6(syst)]×10^{-4} resulting in a Ξ^{-}/(Λ+Σ^{0}) ratio of P_{Ξ^{-}}/P_{Λ+Σ^{0}}=[1.2±0.3(stat)±0.4(syst)]×10^{-2}. Available model predictions are significantly lower than the measured Ξ^{-} yield.

We report on the investigation of $\Delta$(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 $ppe^{+}e^{-}\gamma$ have been studied and compared with the partial wave analysis of the hadronic $pp\pi^{0}$ channel. They allow to access both $\Delta^+ \to p\pi^0(e^+e^-\gamma)$ and $\Delta^+ \to pe^+e^-$ Dalitz decay channels. The perfect reconstruction of the well known $\pi^0$ Dalitz decay serves as a proof of the consistency of the analysis. The $\Delta$ Dalitz decay is identified for the first time and the sensitivity to N-$\Delta$ transition form factors is tested. The $\Delta$(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.

Baryon resonance production in proton-proton collisions at a kinetic beam energy of 1.25 GeV is investigated. The multi-differential data were measured by the HADES collaboration. Exclusive channels with one pion in the final state ($np\pi^{+}$ and $pp\pi^{0}$) were put to extended studies based on various observables in the framework of a one-pion exchange model and with solutions obtained within the framework of a partial wave analysis (PWA) of the Bonn-Gatchina group. The results of the PWA confirm the dominant contribution of the $\Delta$(1232), yet with a sizable impact of the $N$(1440) and non-resonant partial waves.

The recent results reported by the CMS collaboration, indicating "bumps" in the $\gamma\gamma$ and $\tau\tau$ channels at $m_\phi\approx 95$ GeV, provide interesting hints for new physics. We find that the lightest Higgs state of the general 2HDM (2HDM Type-III) can perfectly and simultaneously accommodate the two excesses alongside with the LEP long-standing anomaly observed in the $b\bar{b}$ channel while meeting all theoretical and experimental requirements. Furthermore, the study predicts an enhanced production process for the SM-like Higgs in $pp\to t\bar t H_{\rm SM}$, offering a testable hypothesis for future experiments.

We present the results of a study of charged-pion production in 12C + 12C collisions at incident beam energies of 1A GeV and 2A GeV using the HADES spectrometer at GSI. The main emphasis of the HADES program is on the dielectron signal from the early phase of the collision. Here, however, we discuss the data with respect to the emission of charged hadrons, specifically the production of $ \pi^{{\pm}}_{}$ mesons, which are related to neutral pions representing a dominant contribution to the dielectron yield. We have performed the first large-angular-range measurement of the distribution of $ \pi^{{\pm}}_{}$ mesons for the 12C + 12C collision system covering a fairly large rapidity interval. The pion yields, transverse-mass and angular distributions are compared with calculations done within a transport model, as well as with existing data from other experiments. The anisotropy of pion production is systematically analyzed.

The combination of a production target for secondary beams, an optimized ion optical beam line setting, in-beam detectors for minimum ionizing particles with high rate capability, and an efficient large acceptance spectrometer around the reaction target constitutes an experimental opportunity to study in detail hadronic interactions utilizing pion beams impinging on nucleons and nuclei. For the 0.4-2.0GeV/c pion momentum regime such a facility is located at the heavy ion synchrotron accelerator SIS18 in Darmstadt (Germany). The layout of the apparatus, performance of its components and encouraging results from a first commissioning run are presented.

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 $2.5 \sigma$ 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.

We present a new class of dark matter (DM) models wherein the Standard Model (SM) is extended with a new $SU(2{)}_{\mathrm{D}}$ dark gauge sector. In this framework, the stability of DM is provided by the conservation of a $U(1)$ global symmetry which, upon appropriate charge assignments for the $SU(2{)}_{\mathrm{D}}$ multiplets, effectively leads to a ${\mathbb{Z}}_{2}$ symmetry subgroup. The origin of the global $U(1)$ symmetry which ensures the stability of DM can be justified in the form of a dark electroweak (EW) sector or through an underlying composite structure. The key ingredient of the model is a vectorlike (VL) fermion doublet of $SU(2{)}_{\mathrm{D}}$, the members of which are singlets of the SM EW gauge group, which mediate the interactions between the dark sector and the SM, via new Yukawa interactions. This class of models, labeled as fermion portal vector DM (FPVDM), allows multiple realizations, depending on the properties of the VL partner and the scalar potential. After spontaneous breaking of the $SU(2{)}_{\mathrm{D}}$ symmetry via a new scalar doublet, the ensuing massive vector bosons with nonzero dark isospin are DM candidates. The new class of FPVDM models suggested here has numerous phenomenological implications for collider and noncollider studies. As a practical example, we discuss here in detail a realization involving a VL top partner assuming no mixing between the two physical scalars of the theory, the SM Higgs boson and its counterpart in the dark sector. We, thus, provide bounds on this setup from both collider and astroparticle observables.

An exclusive analysis of the four-body final states $\ensuremath{\Lambda}+p+{\ensuremath{\pi}}^{+}+{K}^{0}$ and ${\ensuremath{\Sigma}}^{0}+p+{\ensuremath{\pi}}^{+}+{K}^{0}$ measured with HADES for $p+p$ collisions at a beam kinetic energy of 3.5 GeV is presented. The analysis uses various phase space variables, such as missing mass and invariant mass distributions, in the four-particle event selection $(p$,${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\ensuremath{-}})$ to find cross sections of the different production channels, contributions of the intermediate resonances ${\ensuremath{\Delta}}^{++}$ and $\ensuremath{\Sigma}{(1385)}^{+}$, and corresponding angular distributions. A dominant resonant production is seen, where the reaction $\ensuremath{\Lambda}+{\ensuremath{\Delta}}^{++}+{K}^{0}$ has a cross section about ten times higher $(29.45\ifmmode\pm\else\textpm\fi{}0.{08}_{\ensuremath{-}1.46}^{+1.67}\ifmmode\pm\else\textpm\fi{}2.06\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\text{b})$ than the analogous nonresonant reaction $(2.57\ifmmode\pm\else\textpm\fi{}0.{02}_{\ensuremath{-}1.98}^{+0.21}\ifmmode\pm\else\textpm\fi{}0.18\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\text{b})$. A similar result is obtained in the corresponding ${\ensuremath{\Sigma}}^{0}$ channels with $9.26\ifmmode\pm\else\textpm\fi{}0.{05}_{\ensuremath{-}0.31}^{+1.41}\ifmmode\pm\else\textpm\fi{}0.65\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\text{b}$ in the resonant and $1.35\ifmmode\pm\else\textpm\fi{}0.{02}_{\ensuremath{-}1.35}^{+0.10}\ifmmode\pm\else\textpm\fi{}0.09\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\text{b}$ in the nonresonant reactions.

We present the activities of the "New Physics" working group for the "Physics at TeV Colliders" workshop (Les Houches, France, 3--21 June, 2013). Our report includes new computational tool developments, studies of the implications of the Higgs boson discovery on new physics, important signatures for searches for natural new physics at the LHC, new studies of flavour aspects of new physics, and assessments of the interplay between direct dark matter searches and the LHC.

Abstract High-statistics $$\pi ^-\pi ^-$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi>π</mml:mi><mml:mo>-</mml:mo></mml:msup><mml:msup><mml:mi>π</mml:mi><mml:mo>-</mml:mo></mml:msup></mml:mrow></mml:math> and $$\pi ^+\pi ^+$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi>π</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:msup><mml:mi>π</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math> femtoscopy data are presented for Au + Au collisions at $$\sqrt{s_\mathrm{NN}} = 2.4~\hbox {GeV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:msub><mml:mi>s</mml:mi><mml:mi>NN</mml:mi></mml:msub></mml:msqrt><mml:mo>=</mml:mo><mml:mn>2.4</mml:mn><mml:mspace /><mml:mtext>GeV</mml:mtext></mml:mrow></mml:math> , 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 cube root of the number of participants. 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.

Data on inclusive pion and eta production measured with the dielectron spectrometer HADES in the reaction p+93Nb at a kinetic beam energy of 3.5 GeV are presented. Our results, obtained with the photon conversion method, supplement the rather sparse information on neutral meson production in proton-nucleus reactions existing for this bombarding energy regime. The reconstructed e+e-e+e- transverse-momentum and rapidity distributions are confronted with transport model calculations, which account fairly well for both pi0 and eta production.

Theories which provide a dynamical explanation for the large top quark mass often include TeV-scale vectorlike top quark and bottom quark partner states which can be potentially discovered at the LHC. These states are currently probed through model-independent searches for pair production via gluon fusion, as well as through model-dependent complementary electroweak single production. In this paper we study the potential to extend those searches for the partners of the third-generation Standard Model quarks on the basis of their expected chromomagnetic interactions. We discuss how current searches for ``excited'' bottom quarks produced via $b$-gluon fusion through chromomagnetic interactions are relevant, and provide significant constraints. We then explore the region of the parameter space in which the bottom quark partner is heavier than the top quark partner, in which case the top partner can be primarily produced via the decay of the bottom partner. Next, we probe the potential of the production of a single top quark partner in association with an ordinary top quark by gluon fusion. Kinematically these two new processes are similar, and they yield the production of a heavy top partner and a lighter Standard Model state, a pattern which allows for the rejection of the associated dominant Standard Model backgrounds. We examine the sensitivity of these modes in the case where the top partner subsequently decays to a Higgs boson and an ordinary top quark, and we demonstrate that these new channels have the potential of extending and complementing the conventional strategies at the LHC run III and at the high-luminosity phase of the LHC. In this last case, we find that partner masses that range up to about 3 TeV can be reached. This substantially expands the expected mass reach for these new states, including regions of parameter space that are inaccessible by traditional searches.

We study the production of Σ±π∓pK+ particle quartets in p + p reactions at 3.5 GeV kinetic beam energy. The data were taken with the HADES experiment at GSI. This report evaluates the contribution of resonances like Λ(1405), Σ(1385)0, Λ(1520), Δ(1232), N⁎ and K⁎0 to the Σ±π∓pK+ final state. The resulting simulation model is compared to the experimental data in several angular distributions and it shows itself as suitable to evaluate the acceptance corrections properly.

Results on the deep sub-threshold production of the short-lived hadronic resonance K*(892)0 are reported for collisions of Ar + KCl at 1.76A GeV beam energy, studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. The K*(892)0 production probability per central collision of $ P_{K^{*0}}=(4.4\pm 1.1 \pm 0.5)\times 10^{-4}$ and the K*(892)0/K0 ratio of $ P_{K^{*0}}/P_{K^0}=(1.9\pm 0.5\pm 0.3)\times 10^{-2}$ are determined at the lowest energy so far (i.e. deep below the threshold for the corresponding production in nucleon-nucleon collisions, $ \sqrt{s_{NN}}-\sqrt{s_{thr}}=-340$ MeV). The K*0/K0 ratio is compared with results of other experiments and with the predictions of the UrQMD transport approach and of the statistical hadronization model. The experimental K*0 yield and the K*0/K0 ratio are overestimated by the transport model by factors of about five and two, respectively. In a chemically equilibrated medium the ratio corresponds to a temperature of the thermalized system being systematically lower than the value determined by the yields of the stable and long-lived hadrons produced in Ar + KCl collisions. From the present measurement, we conclude that sub-threshold K* production either cannot be considered to proceed in a system being in thermal equilibrium or these short-lived resonances appear undersaturated, for example as a result of the rescattering of the decay particles in the ambient hadronic medium.

We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25GeV/u impinging on a liquid hydrogen target. Our measurements with HADES confirm a significant excess of $e^{+}e^{-}$ pairs above the $\pi^{0}$ mass in the exclusive channel $dp \rightarrow npe^{+}e^{-}(p_{spect})$ as compared to the exclusive channel $ ppe^{+}e^{-}$ measured in proton-proton collisions at the same energy. That excess points to different bremsstrahlung production mechanisms. Two models were evaluated for the role of the charged pion exchange between nucleons and double- $\Delta$ excitation combined with intermediate $\rho$ -meson production. Differential cross sections as a function of the $e^{+}e^{-}$ invariant mass and of the angles of the virtual photon, proton and electrons provide valuable constraints and encourage further investigations on both experimental and theoretical sides.

We investigate identical pion HBT intensity interferometry for central Au+Au collisions at 1.23A GeV. High-statistics $\pi^-\pi^-$ and $\pi^+\pi^+$ 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.

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) 3 2 -resonance in the intermediate ρ production.Results on exclusive

The search for Higgs bosons in both the standard model and its extensions is well under way at the Tevatron. As the integrated luminosity collected increases into the multiple inverse femptobarn range, these searches are becoming very interesting indeed. Meanwhile, the construction of the Large Hadron Collider (LHC) and its associated experiments at CERN are nearing completion. In this TeV4LHC workshop, it was realized that any experience at the Tevatron with respect to backgrounds, experimental techniques and theoretical calculations that can be verified at the Tevatron which have relevance for future measurements at the LHC were important. Studies and contributions to these efforts were made in three broad categories: theoretical calculations of Higgs production and decay mechanisms; theoretical calculations and discussions pertaining to non-standard model Higgs bosons; and experimental reviews, analyses and developments at both the Tevatron and the upcoming LHC experiments. All of these contributions represent real progress towards the elucidation of the mechanism of electroweak symmetry breaking.

The HADES collaboration uses the e + e − production as a probe of the resonance matter produced in collisions at incident energies of 1-3.5 GeV/nucleon at GSI. Elementary reactions provide useful references for these studies and give information on resonance Dalitz decays (R→Ne + e − ). Such processes are sensitive to the structure of time-like electromagnetic baryon transitions in a kinematic range where (off-shell) vector mesons play a crucial role. Results obtained in proton-proton reactions and in a commissioning pion-beam experiment are reported and prospects for future pion beam experiments and for first hyperon Dalitz decay measurements are described. The connection with the investigations of medium effects to be continued with HADES in the next years at SIS18 and SIS100 is also discussed.

Results on $\Lambda$p femtoscopy are reported at the lowest energy so far. At a beam energy of 1.76A~GeV, the reaction Ar+KCl was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $\Lambda$ sample was collected, allowing for the investigation of $\Lambda$ p correlations at small relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the $\Lambda$p emission source. The $\Lambda$p source radius is found slightly smaller than the pp correlation radius for a similar collis ion system. The present $\Lambda$p radius is significantly smaller than that found for Au+Au/Pb+Pb collisio ns in the AGS, SPS and RHIC energy domains, but larger than that observed for electroproduction from He. Taking into account all available data, we find the $\Lambda$p source radius to increase almost linearly with the number of participants to the power of one-third.

Results on pp, π+π+, and π−π− intensity interferometry are reported for collisions of Ar+KCl at 1.76A GeV beam energy, studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. The experimental correlation functions as a function of the relative momentum are compared to model calculations allowing the determination of the space-time extent of the corresponding emission sources. The ππ source radii are found significantly larger than the pp emission radius. The present radii do well complement the beam-energy dependences of Gaussian source radii of the collision system of size A + A ≃ 40 + 40 . The pp source radius at fixed beam energy is found to increase linearly with the cube root of the number of participants. From this trend, a lower limit of the pp correlation radius is deduced.

We present a search for the e+e- decay of a hypothetical dark photon, also names U vector boson, in inclusive dielectron spectra measured by HADES in the p (3.5 GeV) + p, Nb reactions, as well as the Ar (1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared epsilon^{2} at 90% CL has been obtained for the mass range M(U) = 0.02 - 0.55 GeV/c2 and is compared with the present world data set. For masses 0.03 - 0.1 GeV/c^2, the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favoured by the muon g-2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3 * 10^{-6} has been set on the helicity-suppressed direct decay of the eta meson, eta-> e+e-, at 90% CL.

The aim of the present analysis is to determine the relative production cross sections of the $\Lambda$(1405) and $\Sigma(1385)^{0}$ resonances in p+p collisions at E$_{kin}$=3.5 GeV measured with HADES. Upper and lower limits have been determined for the ratio $\sigma_{(\Sigma(1385)^{0}+p+K^{+})}/\sigma_{(\Lambda(1405)+p+K^{+})}=0.76_{-0.26}^{+0.54}$. The knowledge of this ratio is an essential input for the analysis of the decay $\Lambda(1405)\rightarrow\Sigma^{\pm}\pi^{\mp}$, where an unambiguous separation of the $\Lambda$(1405) and $\Sigma(1385)^{0}$ signals is not possible.

We present results on the ${K}^{*}{(892)}^{+}$ production in proton-proton collisions at a beam energy of $E=3.5$ GeV, which is hitherto the lowest energy at which this mesonic resonance has been observed in nucleon-nucleon reactions. The data are interpreted within a two-channel model that includes the three-body production of ${K}^{*}{(892)}^{+}$ associated with the $\mathrm{\ensuremath{\Lambda}}$ or $\mathrm{\ensuremath{\Sigma}}$ hyperon. The relative contributions of both channels are estimated. Besides the total cross section $\ensuremath{\sigma}(p+p\ensuremath{\rightarrow}{K}^{*}{(892)}^{+}+X)=9.5\ifmmode\pm\else\textpm\fi{}0.{9}_{\ensuremath{-}0.9}^{+1.1}\ifmmode\pm\else\textpm\fi{}0.7\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{b}$, which adds a new data point to the excitation function of the ${K}^{*}{(892)}^{+}$ production in the region of low excess energy, transverse momenta and angular spectra are extracted and compared with the predictions of the two-channel model. The spin characteristics of ${K}^{*}{(892)}^{+}$ are discussed as well in terms of the spin-alignment.

This work presents the analysis of the performance of the RPC ToF wall of the HADES, located at GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt. The behavior of the detector is studied in Au+Au collisions at 1.23 AGeV. A main characteristic of the detector is that all the active areas were designed to be electrically shielded in order to operate in high occupancies of the chambers. Here we show the achieved performance regarding efficiency and timing capabilities at different occupancies of this special design after the applied offline corrections to the data. Also the stability of the intrinsic time resolution over time of data taking is presented.

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 has been gathered for the light vector mesons ρ ω and ø. HADES is a second generation experiment operating at GSI 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 allows also for measurements of strangeness production. These abilities combined with the variety of beams provided by the SIS18 allow for 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 main emphasis on hadron properties in nuclear matter.

In this letter we report the first multi-differential measurement of correlated pion-proton pairs from 2 billion Au+Au collisions at \sqrt{s_{NN}} = 2.42 GeV collected with HADES. In this energy regime the population of \Delta(1232) resonances plays an important role in the way energy is distributed between intrinsic excitation energy and kinetic energy of the hadrons in the fireball. The triple differential d3N/dM{\pi}pdpTdy distributions of correlated {\pi}p pairs have been determined by subtracting the {\pi}p combinatorial background using an iterative method. The invariant-mass distributions in the \Delta(1232) mass region show strong deviations from a Breit-Wigner function with vacuum width and mass. The yield of correlated pion-proton pairs exhibits a complex isospin, rapidity and transverse-momentum dependence. In the invariant mass range 1.1 < Minv (GeV/c2) < 1.4, the yield is found to be similar for {\pi}+p and {\pi}-p pairs, and to follow a power law <Apart>^{\alpha}, where <Apart> is the mean number of participating nucleons. The exponent {\alpha} depends strongly on the pair transverse momentum (pT) while its pT-integrated and charge-averaged value is {\alpha} = 1.5 \pm 0.08stat \pm 0.2sys.

e + e - production was studied using the High Acceptance DiElectron Spectrometer (HADES). In pp collisions at 2.2GeV kinetic beam energy, the exclusive η production and the Dalitz decay η→γe + e - have been reconstructed. The electromagnetic form factor of the latter decay was found to be in good agreement with the existing theoretical predictions. In addition, an inclusive e + e - invariant-mass spectrum from the 12C + 12C reaction at 2AGeV is presented and compared with a simplified thermal model.

The HADES experiment, installed at GSI, Darmstadt, measures di-electron production in A + A, p/π + N and p/π + A collisions. Here, the π 0 and η Dalitz decays have been reconstruced in the exclusive p + p reaction at 2.2 GeV to form a reference cocktail for long-lived di-electron sources. In the C + C reaction at 1 and 2 GeV/u, these long-lived sources have been subtracted from the measured inclusive e + e - yield to exhibit the signal from the early phase of the collision. The results suggest that resonances play an important role in dense nuclear matter.

We report on recent data of e+e− pair emission in proton nucleus collisions at energies above the light vector meson production thresholds. Invariant mass distributions for the p+Nb system at Ekin = 3.5 GeV are compared to data from elementary p+p reactions at the same beam energy. We observe a constant π0/ω yield ratio for both systems but an excess in the mass region above the π0 mass. Furthermore we present here the normalization procedure that was applied to p+Nb collisions by measuring the production of negative pions in the HADES acceptance.

This is the TeV4LHC report of the Physics Landscapes Working Group, focused on facilitating the start-up of physics explorations at the LHC by using the experience gained at the Tevatron. We present experimental and theoretical results that can be employed to probe various scenarios for physics beyond the Standard Model.

Radiative transition of an excited baryon to a nucleon with emission of a virtual massive photon converting to dielectron pair (Dalitz decays) provides important information about baryon-photon coupling at low q 2 in timelike region. A prominent enhancement in the respective electromagnetic transition Form Factors (etFF) at q 2 near vector mesons ρ/ω poles has been predicted by various calculations reflecting strong baryon-vector meson couplings. The understanding of these couplings is also of primary importance for the interpretation of the emissivity of QCD matter studied in heavy ion collisions via dilepton emission. Dedicated measurements of baryon Dalitz decays in proton-proton and pion-proton scattering with HADES detector at GSI/FAIR are presented and discussed. The relevance of these studies for the interpretation of results obtained from heavy ion reactions is elucidated on the example of the HADES results.

With the HADES spectrometer at GSI we have studied dilepton production in various collision systems from elementary N + N , over p + A , up to the medium-heavy Ar + KCl system. We have confirmed the puzzling results of the former DLS collaboration at the Bevalac. While we have traced the origin of the excess pair yield in C + C collisions to elementary p + p and n + p processes, we find a significant contribution from the dense phase of the collision in larger Ar + KCl system. From recently obtained e + e - pair spectra in p + p and p + Nb interactions at 3.5 GeV kinetic beam energy the inclusive production cross sections for neutral pions, η , ω and ρ mesons are extracted for the first time at this beam energy. Furthermore, the production mechanisms of the vector mesons, which are not known at these energies, are investigated. The direct comparison of p + p and p + Nb data allows us to investigate in-medium mass modifications of vector mesons at nuclear ground state density. On the other hand, exclusive production of ω and η mesons in p + p reactions at 3.5 GeV were studied via their π + π - π 0 decay channel. Production cross sections and angular distributions for both states were determined.

The study of 3π and 4π meson production in np interactions was carried out at the momenta of incident neutrons P0 = 3.83, 4.42 and 5.20 GeV/c. The characteristics of the reactions were satisfactorily described by OPER model. For the better description of the reaction np → ppπ+ π− π− π0 it was necessary to take into account the production of η0 and ω0 mesons.

HADES is a secondary generation experiment operated at GSI Darmstadt with the main goal to study dielectron production in proton, pion and heavy ion induced reactions. The first part of the HADES mission is to reinvestigate the puzzling pair excess measured by the DLS collaboration in C + C and Ca + Ca collisions at 1 A GeV. For this purpose dedicated measurements with the C + C system at 1 and 2 A GeV were performed. The pair excess above a cocktail of free hadronic decays has been extracted and compared to the one measured by DLS. Furthermore, the excess is confronted with predictions of various model calculations.

Currently, the HADES spectrometer undergoes un upgrade program to be prepared for measurements at the upcoming SIS-100 synchrotron at FAIR. We describe the current status of the HADES di-electron measurements at the SIS-18 and our future plans for SIS-100.

Systematic investigations of dilepton production are performed at the SIS accelerator of GSI with the HADES spectrometer. The goal of this program is a detailed understanding of di-electron emission from hadronic systems at moderate temperatures and densities. New results obtained in HADES experiments focussing on electron pair production in elementary collisions are reported here. They pave the way to a better understanding of the origin of the so-called excess pairs earlier on observed in heavy-ion collisions by the DLS collaboration and lately confirmed in two measurements of the HADES collaboration using C+C and Ar+KCl collisions. Results of these studies are discussed.

The search for charged particle tracks in detectors with linear sensing elements, such as multiwire proportional drift chambers, silicon strip detectors etc., often begins with the identification of a straight segment of track. This can be achieved by constructing all reasonable combinations of fired detector channels. In high multiplicity environment, with many fired detector channels, this approach causes large combinatorics and significantly reduces the performance of the track finding algorithms. We provide a method which allows to find initial approximations of straight segments of track in space using four linear sensing elements. This method is especially helpful if there are no parallel elements in the setup. The procedure can also be applied as a first approximation to the case of finding curved tracks. In general, it can be used to find a line in space which crosses the other four lines.

Abstract It has been dozen years since HYDJET++ Monte-Carlo event generator for the simulation of relativistic heavy ion collisions was developed. Now the generator is widely used for the simulation of nucleus-nucleus interactions from NICA to LHC energies. The model calculations on soft and hard probes of quark-gluon plasma (including collective flow, different kinds of particle correlations, jets, D and B mesons etc.) agree well with the experimental data. In this paper the selected main results and the very new ones are presented.

The High Acceptance Di-Electron Spectrometer HADES 1 has been constructed at the SIS accelerator (GSI Darmstadt) to investigate electron-positron pairs produced in proton, pion and heavy ion induced reactions. The main goal of these studies is to explore properties of hadrons in nuclear matter. The apparatus and the experimental results from C + C at 2.0 AGeV and 1.0 AGeV and p + p at 2.2 GeV compared with Monte-Carlo events from a generator based on known cross-sections and branching ratios are presented.

The reaction $np \rightarrow np \pi^+ \pi^-$ was studied at the various momenta of incident neutrons. It was shown that the characteristics of the reaction at the momenta above 3 GeV/c could be described by the model of reggeized $\pi$ exchange (OPER). At the momenta below 3 GeV/c, it was necessary to use additionally the mechanism of one baryon exchange (OBE).

Following our recent letter [1] on inclusive e+e- pair production in proton induced reactions at Ekin 3.5 GeV on the nucleus Nb, we present here in addition the transverse and rapidity distributions for various e+e- invariant mass bins and compare them to reference data measured in p+p reactions.

We present the results of two-pion production in tagged quasi-free np collisions at a deutron incident beam energy of 1.25 GeV/c measured with the High-Acceptance Di-Electron Spectrometer (HADES) installed at GSI. The specific acceptance of HADES allowed for the first time to obtain high-precision data on π+π− and π−π0 production in np collisions in a region corresponding to large transverse momenta of the secondary particles. The obtained differential cross section data provide strong constraints on the production mechanisms and on the various baryon resonance contributions (∆∆, N(1440), N(1520), ∆(1600)). The invariant mass and angular distributions from the np → npπ+π −and np → ppπ−π0 reactions are compared with different theoretical model predictions.

Many QCD based and phenomenological models predict changes of hadron properties in a strongly interacting environment. The results of these models differ significantly and the experimental determination of hadron properties in nuclear matter is essential. In this paper we present a review of selected physics results obtained at GSI Helmholtzzentrum für Schwerionenforschung GmbH by HADES (High-Acceptance Di-Electron Spectrometer). The e+e− pair emission measured for proton and heavy-ion induced collisions is reported together with results on strangeness production. The future HADES activities at the planned FAIR facility are also discussed.

The event topology in relativistic heavy ion collisions is determined by various multi-particle production mechanisms. The simultaneous model treatment of different collective nuclear effects at high energies (such as a hard multi-parton fragmentation in hot QCD-matter, thermal resonance production, hydrodynamical flows, etc.) is actual but rather complicated task. We discuss the simulation of the above effects by means of Monte-Carlo model HYDJET++.

Abstract In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged-pion transverse-mass and rapidity spectra, as well as on pion–pion momentum correlations is investigated in Au + Au collisions at $$\sqrt{s_\text {NN}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:msub> <mml:mi>s</mml:mi> <mml:mtext>NN</mml:mtext> </mml:msub> </mml:msqrt> </mml:math> = 2.4 GeV. We show that the low- $$m_t$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>t</mml:mi> </mml:msub> </mml:math> region ( $$m_t &lt; 0.2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>t</mml:mi> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>0.2</mml:mn> </mml:mrow> </mml:math> GeV/ $$c^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>c</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:math> ) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential energy deviates strongly from a $$A_{\text {part}}^{2/3}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>A</mml:mi> <mml:mrow> <mml:mtext>part</mml:mtext> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>/</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields.

We present the transverse momentum spectra and rapidity distributions of $\pi^{-}$ and K$^0_S$ in Ar+KCl reactions at a beam kinetic energy of 1.756 A GeV measured with the spectrometer HADES. The reconstructed K$^0_S$ sample is characterized by good event statistics for a wide range in momentum and rapidity. We compare the experimental $\pi^{-}$ and K$^0_S$ distributions to predictions by the IQMD model. The model calculations show that K$^0_S$ at low tranverse momenta constitute a particularly well suited tool to investigate the kaon in-medium potential. Our K$^0_S$ data suggest a strong repulsive in-medium K$^0$ potential of about 40 MeV strength.

The HADES Collaboration G. Agakishiev, C. Agodi, A. Balanda, G. Bellia, D. Belver, A. Belyaev, J. Bielcik, A. Blanco, A. Bortolotti, J.L. Boyard, P. Braun-Munzinger, P. Cabanelas, S. Chernenko, T. Christ, R. Coniglione, M. Destefanis, J. Diaz, F. Dohrmann, I. Duran, A. Dybczak, T. Eberl, L. Fabbietti, O. Fateev, R. Ferreira-Marques, P. Finocchiaro, P. Fonte, J. Friese, I. Frohlich, T. Galatyuk, J.A. Garzon, R. Gernhauser, A. Gil, C. Gilardi, M. Golubeva, D. Gonzalez-Diaz, E. Grosse, F. Guber, M. Heilmann, T. Heinz, T. Hennino, R. Holzmann, A. Ierusalimov, I. Iori, A. Ivashkin, M. Jurkovic, B. Kampfer, K. Kanaki, T. Karavicheva, D. Kirschner, I. Koenig, W. Koenig, B.W. Kolb, R. Kotte, A. Kozuch, A. Krasa, F. Křižek, R. Krucken, W. Kuhn, A. Kugler, A. Kurepin, J. Lamas-Valverde, S. Lang, J.S. Lange, K. Lapidus, L. Lopes, M. Lorenz, L. Maier, C. Maiolino, A. Mangiarotti, J. Maŕin, J. Markert, V. Metag, B. Michalska, J. Michel, E. Moriniere, J. Mousa , M. Munch, C. Muntz, L. Naumann, R. Novotny, J. Otwinowski, Y.C. Pachmayer, M. Palka, Y. Parpottas, V. Pechenov, O. Pechenova, T. Perez Cavalcanti, P. Piattelli, J. Pietraszko, V. Posṕisil, W. Przygoda, B. Ramstein, A. Reshetin, M. Roy-Stephan, A. Rustamov, A. Sadovsky, B. Sailer, P. Salabura, P. Sapienza, A. Schmah, C. Schroeder, E. Schwab, R.S. Simon, Yu.G. Sobolev, S. Spataro, B. Spruck, H. Strobele, J. Stroth, C. Sturm, M. Sudol, A. Tarantola, K. Teilab, P. Tlustý, M. Traxler, R. Trebacz, H. Tsertos, V. Wagner, M. Weber, M. Wisniowski, T. Wojcik, J. Wustenfeld, S. Yurevich, Y. Zanevsky, P. Zhou, and P. Zumbruch 1 Istituto Nazionale di Fisica Nucleare Laboratori Nazionali del Sud, 95125 Catania, Italy 2 LIP-Laboratorio de Instrumentacao e Fisica Experimental de Particulas, 3004-516 Coimbra, Portugal 3 Smoluchowski Institute of Physics, Jagiellonian University of Cracow, 30-059 Krakow, Poland 4 GSI Helmholtzzentrum fur Schwerionenforschung, 64291 Darmstadt, Germany 5 Institut fur Strahlenphysik, Forschungszentrum Dresden-Rossendorf, 01314 Dresden, Germany 6 Joint Institute of Nuclear Research, 141980 Dubna, Russia 7 Institut fur Kernphysik, Johann Wolfgang Goethe-Universitat, 60438 Frankfurt, Germany 8 II. Physikalisches Institut, Justus Liebig Universitat Giessen, 35392 Giessen, Germany 9 Istituto Nazionale di Fisica Nucleare, Sezione di Milano, 20133 Milano, Italy 10 Institute for Nuclear Research, Russian Academy of Science, 117312 Moscow, Russia 11 Physik Department E12, Technische Universitat Munchen, 85748 Munchen, Germany 12 Department of Physics, University of Cyprus, 1678 Nicosia, Cyprus 13 Institut de Physique Nucleaire (UMR 8608), CNRS/IN2P3 Universite Paris Sud, F-91406 Orsay Cedex, France 14 Nuclear Physics Institute, Academy of Sciences of Czech Republic, 25068 Rez, Czech Republic 15 Departamento de Fisica de Particulas, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain 16 Instituto de Fisica Corpuscular, Universidad de Valencia-CSIC, 46971 Valencia, Spain

We report on a measurement of electron pair production in 12 C+ 12 C collisions at an incident energy of 2 GeV per nucleon with the HADES spectrometer.The measured pair production probabilities span over five orders of magnitude from the π 0 -Dalitz to the ρ/ω invariant-mass region.Dalitz decays of π 0 and η account for all the yield up to 0.15 GeV/c 2 , but for only about 50% above this mass.The excess yield is in agreement with the former DLS result if one assumes that it scales with beam energy like pion production.A preliminary analysis of 12 C+ 12 C collisions at an incident energy of 1 A GeV, measured with the HADES detector, supports this scenario.

The High Acceptance DiElectron Spectrometer HADES [1] is installed at the Helmholtzzentrum für Schwerionenforschung (GSI) accelerator facility in Darmstadt. It investigates dielectron emission and strangeness production in the 1-3 AGeV regime. A recent experiment series focusses on medium-modifications of light vector mesons in cold nuclear matter. In two runs, p+p and p+Nb reactions were investigated at 3.5 GeV beam energy; about 9·109 events have been registered. In contrast to other experiments the high acceptance of the HADES allows for a detailed analysis of electron pairs with low momenta relative to nuclear matter, where modifications of the spectral functions of vector mesons are predicted to be most prominent. Comparing these low momentum electron pairs to the reference measurement in the elementary p+p reaction, we find in fact a strong modification of the spectral distribution in the whole vector meson region.

Heavy-flavour quarks are predominantly produced in hard scatterings on a short time-scale and traverse the medium interacting with its constituents, thus they are one of the effective probes of the transport properties of the medium formed in relativistic heavy ion collisions. On the other hand, the thermal production of heavy-flavour quarks in quark-gluon plasma (QGP) is itself of interest. In this report, the production and elliptic flow of the prompt charmed mesons $D^0$, $D^+$, $D^{*+}$ and $J/\psi$ in PbPb collisions at the center-of-mass energy 2.76 TeV per nucleon pair are described in the framework of two-component HYDJET++ model. The model combines thermal and pQCD production mechanisms. The spectra and elliptic flow of charmed mesons are presented, the results are compared with LHC data.

The phenomenological analysis of various characteristics of $J/\psi$ and D mesons in PbPb collisions at the center-of-mass energy 2.76 TeV per nucleon pair is presented. The data on charmed meson momentum spectra and elliptic flow are reproduced by two-component model HYDJET++ including thermal and non-thermal production mechanisms. The significant part of D-mesons is found to be in a kinetic equilibrium with the created medium, while $J/\psi$-mesons are characterized by earlier (as compared to light hadrons) freeze-out.

The HADES data from p+Nb collisions at center of mass energy of $\sqrt{s_{NN}}$= 3.2 GeV are analyzed by employing a statistical model. Accounting for the identified hadrons $π^0$, $η$, $Λ$, $K^{0}_{s}$, $ω$ allows a surprisingly good description of their abundances with parameters $T_{chem}=(99\pm11)$ MeV and $μ_{b}=(619\pm34)$ MeV, which fits well in the chemical freeze-out systematics found in heavy-ion collisions. In supplement we reanalyze our previous HADES data from Ar+KCl collisions at $\sqrt{s_{NN}}$= 2.6 GeV with an updated version of the statistical model. We address equilibration in heavy-ion collisions by testing two aspects: the description of yields and the regularity of freeze-out parameters from a statistical model fit. Special emphasis is put on feed-down contributions from higher-lying resonance states which have been proposed to explain the experimentally observed $Ξ^-$ excess present in both data samples.

We discuss a field theoretical framework to describe the interactions of non-thermal quantum black holes (QBHs) with particles of the Standard Model. We propose a non-local Lagrangian to describe the production of these QBHs which is designed to reproduce the geometrical cross section for black hole production. This model is implemented into CalcHEP package and is publicly available at the High Energy Model Database (HEPMDB) for simulation of QBH events at the LHC and future colliders. We present the first phenomenological application of the QBH@HEPMDB model with spin-0 neutral QBH giving rise the $e^+e^-$ and $e\mu$ signatures at the LHC@8TeV and LHC@13TeV and produce the respective projections for the LHC in terms of limits on the reduced Planck mass and the number of the extra-dimensions.

The phenomenological analysis of the LHC data on transverse momentum spectrum and elliptic flow of $J/ψ$ and D mesons in PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair is presented. The charmed meson production pattern in PbPb collisions may be reproduced by two-component model HYDJET++ including thermal and non-thermal components. The significant part of D-mesons is found to be in a kinetic equilibrium with the created medium, while $J/ψ$-mesons are not.

This discovery of the Higgs boson last year has created new possibilities for testing candidate theories for explaining physics beyond the Standard Model. Here we explain the ways in which new physics can leave its marks in the experimental Higgs data, and how we can use the data to constrain and compare different models. In this proceedings paper we use two models, Minimal Universal Extra Dimensions and the 4D Composite Higgs model, as examples to demonstrate the technique.

First results of the differential cross section in dp elastic scattering at 1.25 GeV/u measured with the HADES over a large angular range are reported. The obtained data corresponds to large transverse momenta, where a high sensitivity to the two-nucleon and three-nucleon short-range correlations is expected.

The knowledge of baryonic resonance properties and production cross sections plays an important role for the extraction and understanding of medium modifications of mesons in hot and/or dense nuclear matter. We present and discuss systematics on dielectron and strangeness production obtained with HADES on p+p, p+A and A+A collisions in the few GeV energy regime with respect to these resonances.

The study of the elastic np-scattering was carried out at the momenta of incident neutrons P 0 =1.43, 1.73 and 5.20 GeV/c.The obtained results are compared with the available world data.The simple pole model of π-meson, ρ-meson and pomeron exchanges is satisfactory describes the main characteristics of the reaction.

We present an analysis of both the Neutral Current (NC) and Charged Current (CC) Drell-Yan processes at the LHC within a 4 Dimensional realization of a Composite Higgs model studying the cross sections and taking into account the possible impact of the extra fermions present in the spectrum.