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This Letter presents the first experimental evidence of the attractive strong interaction between a proton and a ϕ meson. The result is obtained from two-particle correlations of combined p-ϕ⊕ p[over ¯]-ϕ pairs measured in high-multiplicity pp collisions at sqrt[s]=13 TeV by the ALICE Collaboration. The spin-averaged scattering length and effective range of the p-ϕ interaction are extracted from the fully corrected correlation function employing the Lednický-Lyuboshits approach. In particular, the imaginary part of the scattering length vanishes within uncertainties, indicating that inelastic processes do not play a prominent role for the p-ϕ interaction. These data demonstrate that the interaction is dominated by elastic p-ϕ scattering. Furthermore, an analysis employing phenomenological Gaussian- and Yukawa-type potentials is conducted. Under the assumption of the latter, the N-ϕ coupling constant is found to be g_{N-ϕ}=0.14±0.03(stat)±0.02(syst). This work provides valuable experimental input to accomplish a self-consistent description of the N-ϕ interaction, which is particularly relevant for the more fundamental studies on partial restoration of chiral symmetry in nuclear medium.

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... The scattering length |α φp | ≃ 2.37 was obtained by analyzing the QCD van der Waals potential [13]. And in 2021, the real part of the scattering length of φ-p was found to be 0.85 ± 0.34 fm by pp collision in ALICE Collaboration [16]. Note that these results go beyond the scattering length of the φ-p extracted from the vector meson photoproduction data. ...

... The olive-green triangle is |α φp | ≃ 2.37 fm by QCD van der Waals [13]. The orange diamond is the result of the ALICE Collaboration [16]. |α 2 φp | based on the differential cross section is barely larger than that obtained from the total cross section. ...

... However, two relatively large results clearly go beyond our normal understanding of the scattering length of φ-p. A result from QCD van der Waals [13] is |α φp | ≃ 2.37 fm and a real part of the scattering length of φ-p interaction calculated by ALICE Collaboration [16] from the cross section of the high-multiplicity pp collisions collision is 0.85 ± 0.34 fm. We suppose there should exist different reactions, so these two results are ignored for the time being. ...

In this work, we systematically study the the interaction between of $\phi$ meson and nucleus by analyzing and fitting the cross sections of $\gamma N$$\rightarrow \phi$$N$ ($N$ represent the nucleus) reactions near the threshold. With the help of vector meson dominant model, the distribution of $\phi$-$N$ scattering length as a function of energy is presented, and the results show that there is a slight increase in scattering length with increasing energy. Based on this, the average scattering length of $\phi$-proton is $ =0.10\pm0.01$ fm by combining experimental data and theoretical models. Moreover, the average scattering length of $\phi$-deuteron interaction is derived to be $0.014\pm0.002$ fm for the first time. Further, the effect of the momentum transfer $|t_{min}|$ on the $\phi$-$N$ scattering length at the threshold is discussed. The obtained results not only provide important theoretical information for a more comprehensive and accurate study of the $\phi$-$N$ scattering length, but also provide a basis for future experimental measurements of $\phi$ meson production.

... (14) syst. fm [7], an order of magnitude larger than that obtained from the photo-production data by the CLAS Collaboration at JLab [8] combined with the vector meson dominance [9]. ...

... Our a (3/2) 0 is substantially larger in magnitude than the previous calculations of the spin-averaged a 0 using QCD sum rules but is comparable to the spin-averaged a 0 by ALICE Collaboration within the error bar [7]. Also, our r (3/2) eff is about three times smaller than the spinaveraged r eff by ALICE Collaboration. ...

First results on the interaction between the $\phi$-meson and the nucleon ($N$) are presented based on the ($2+1$)-flavor lattice QCD simulations with nearly physical quark masses. Using the HAL QCD method, the spacetime correlation of the $N$-$\phi$ system in the spin 3/2 channel is converted into the $N$-$\phi$ scattering phase shift through the interaction potential. The $N$-$\phi$ potential appears to be a combination of a short-range attractive core and a long-range attractive tail. The latter is found to be consistent with the two-pion exchange (TPE) obtained from the interaction between a color-dipole and the nucleon. The resultant scattering length and effective range for $m_{\pi}=$ 146.4 MeV are $ a^{(3/2)}_0=-1.43(23)_{\rm stat.}\left(^{+36}_{-06}\right)_{\rm syst.} {\rm fm}$ and $ r^{(3/2)}_{\rm eff}=2.36(10)_{\rm stat.}\left(^{+02}_{-48}\right)_{\rm syst.} {\rm fm}$, respectively. The magnitude of the scattering length is shown to have non-trivial dependence of $m_{\pi}$ and is sensitive to the existence of the long-range tail from TPE.

... The situation has drastically changed in recent years, thanks to the novel employment of the femtoscopy technique [37] in pp and p-Pb collisions at the LHC applied to almost all combinations of protons and strange hadrons [38]. The ALICE Collaboration could precisely study the following interactions: pp, pK ± , pΛ, pΛ, pΣ 0 , ΛΛ, ΛΛ, pΞ − , pΩ − and pφ [38][39][40][41][42][43][44][45][46]. Since conventional scattering experiments cannot be performed with D mesons and charm nuclei [47] have not been discovered yet (searches for charm nuclear states are included in the scientific program of the Japan Proton Accelerator Research Complex [48]), the femtoscopy technique can be employed to study the ND and ND interactions. ...

... The resulting correlation function is parametrized by a third-order polynomial in k * ∈ [0, 1.5] GeV/c and is displayed by the green curve in Fig. 1. The observed behavior is determined by meson-meson and baryon-meson mini-jets and residual two-body interactions among the quadruplet, as previously observed [41,45]. ...

This Letter presents the first measurement of the interaction between charm hadrons and nucleons. The two-particle momentum correlations of $\mathrm{pD^-}$ and $\mathrm{\overline{p}D}^+$ pairs are measured by the ALICE Collaboration in high-multiplicity pp collisions at $\sqrt{s} = 13~\mathrm{TeV}$. The data are compatible with the Coulomb-only interaction hypothesis within (1.1-1.5)$\sigma$. Considering an attractive nucleon(N)$\overline{\mathrm{D}}$ strong interaction, in contrast to most model predictions which suggest an overall repulsive interaction, slightly improves the level of agreement. This measurement allows for the first time an estimation of the 68% confidence level interval for the isospin $\mathrm{I}=0$ inverse scattering length of the $\mathrm{N\overline{D}}$ state ${f_{0,~\mathrm{I}=0}^{-1} \in [-0.4,0.9]~\mathrm{fm^{-1}}}$, assuming negligible interaction for the isospin $\mathrm{I}=1$ channel.

... Recently, the method has been applied to study FSIs of hadrons produced in pp and p-Pb collisions. The large data samples allowed for the precise measurement of correlation functions for multiple hadronic pairs (p-p [25], p-K + and p-K − [26], p-Λ [25], p-Σ 0 [27], Λ-Λ [28], p-Ξ − [29], p-Ω − [30], p-φ [31] and baryon-antibaryon [32]). By using these results, several models for the two-body strong interaction could be validated (for a complete review see Ref. [33]). ...

Three-body nuclear forces play an important role in the structure of nuclei and hypernuclei and are also incorporated in models to describe the dynamics of dense baryonic matter, such as in neutron stars. So far, only indirect measurements anchored to the binding energies of nuclei can be used to constrain the three-nucleon force, and if hyperons are considered, the scarce data on hypernuclei impose only weak constraints on the three-body forces. In this work, we present the first direct measurement of the p$-$p$-$p and p$-$p$-\Lambda$ systems in terms of three-particle mixed moments carried out for pp collisions at $\sqrt{s}$ = 13 TeV. Three-particle cumulants are extracted from the normalised mixed moments by applying the Kubo formalism, where the three-particle interaction contribution to these moments can be isolated after subtracting the known two-body interaction terms. A negative cumulant is found for the p$-$p$-$p system, hinting to the presence of a residual three-body effect while for p$-$p$-\Lambda$ the cumulant is consistent with zero. This measurement demonstrates the accessibility of three-baryon correlations at the LHC.

This article presents the first measurement of the interaction between charm hadrons and nucleons. The two-particle momentum correlations of pD− and p¯D+ pairs are measured by the ALICE Collaboration in high-multiplicity pp collisions at s=13 TeV. The data are compatible with the Coulomb-only interaction hypothesis within (1.1–1.5)σ. The level of agreement slightly improves if an attractive nucleon (N)D¯ strong interaction is considered, in contrast to most model predictions which suggest an overall repulsive interaction. This measurement allows for the first time an estimation of the 68% confidence level interval for the isospin I=0 inverse scattering length of the ND¯ state f0,I=0−1∈[−0.4,0.9] fm−1, assuming negligible interaction for the isospin I=1 channel.

We investigate $$DD^*$$ D D ∗ and $$D\bar{D}^*$$ D D ¯ ∗ momentum correlations in high-energy collisions to elucidate the nature of $$T_{cc}$$ T cc and X (3872) exotic hadrons. Single range Gaussian potentials with the channel couplings to the isospin partners are constructed based on the empirical data within the pure hadronic-molecule picture. The momentum correlation functions of the $$D^0D^{*+}$$ D 0 D ∗ + , $$D^+D^{*0}$$ D + D ∗ 0 , $$D^0\bar{D}^{*0}$$ D 0 D ¯ ∗ 0 , and $$D^+D^{*-}$$ D + D ∗ - pairs are computed with including the coupled-channel effects. We discuss how the nature of the exotic states are reflected in the behaviors of the correlation results.

In recent years the femtoscopy technique has been used by the ALICE Collaboration in small colliding systems at the LHC to investigate the strong-interaction of hadron pairs in the low-energy regime. The extension of this technique to the study of many-body correlations aims to deliver in the next years the first experimental measurements of the genuine many-hadron interactions, provided that the contributions due to the lower order terms are properly accounted for. In this paper we present a method that allows to determine the residual lower order contributions to the three-body correlation functions, based on the cumulant decomposition approach and on kinematic transformations. A procedure to simulate genuine three-body correlations in three-baryon correlation functions is also developed. A qualitative study of the produced correlation signal is performed by varying the strength of the adopted three-body interaction model and comparisons with the expectations for the lower order contributions to the correlation function are shown. The method can be also applied to evaluate the combinatorial background in the two-body correlation functions, providing an improved statistical accuracy with respect to the standard techniques. The example of the contribution by the pK $$^+$$ + K $$^-$$ - channel to the recently measured p $$\upphi $$ ϕ correlation is discussed.

The strong interaction among hadrons has been measured in the past by scattering experiments. Although this technique has been extremely successful in providing information about the nucleon–nucleon and pion–nucleon interactions, when unstable hadrons are considered the experiments become more challenging. In the last few years, the analysis of correlations in the momentum space for pairs of stable and unstable hadrons measured in pp and p+Pb collisions by the ALICE Collaboration at the LHC has provided a new method to investigate the strong interaction among hadrons. In this article, we review the numerous results recently achieved for hyperon–nucleon, hyperon–hyperon, and kaon–nucleon pairs, which show that this new method opens the possibility of measuring the residual strong interaction of any hadron pair.

One of the key challenges for nuclear physics today is to understand from first principles the effective interaction between hadrons with different quark content. First successes have been achieved using techniques that solve the dynamics of quarks and gluons on discrete space-time lattices1,2. Experimentally, the dynamics of the strong interaction have been studied by scattering hadrons off each other. Such scattering experiments are difficult or impossible for unstable hadrons3–6 and so high-quality measurements exist only for hadrons containing up and down quarks7. Here we demonstrate that measuring correlations in the momentum space between hadron pairs8–12 produced in ultrarelativistic proton–proton collisions at the CERN Large Hadron Collider (LHC) provides a precise method with which to obtain the missing information on the interaction dynamics between any pair of unstable hadrons. Specifically, we discuss the case of the interaction of baryons containing strange quarks (hyperons). We demonstrate how, using precision measurements of proton–omega baryon correlations, the effect of the strong interaction for this hadron–hadron pair can be studied with precision similar to, and compared with, predictions from lattice calculations13,14. The large number of hyperons identified in proton–proton collisions at the LHC, together with accurate modelling15 of the small (approximately one femtometre) inter-particle distance and exact predictions for the correlation functions, enables a detailed determination of the short-range part of the nucleon-hyperon interaction.

We report on the measurement of the size of the particle-emitting source from two-baryon correlations with ALICE in high-multiplicity pp collisions at s=13 TeV. The source radius is studied with low relative momentum p–p, p‾–p‾, p–Λ , and p‾–Λ‾ pairs as a function of the pair transverse mass mT considering for the first time in a quantitative way the effect of strong resonance decays. After correcting for this effect, the radii extracted for pairs of different particle species agree. This indicates that protons, antiprotons, Λ s, and Λ‾ s originate from the same source. Within the measured mT range (1.1–2.2) GeV/c2the invariant radius of this common source varies between 1.3 and 0.85 fm. These results provide a precise reference for studies of the strong hadron–hadron interactions and for the investigation of collective properties in small colliding systems.

The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 3,324 new measurements from 878 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as supersymmetric particles, heavy bosons, axions, dark photons, etc. Particle properties and search limits are listed in Summary Tables. We give numerous tables, figures, formulae, and reviews of topics such as Higgs Boson Physics, Supersymmetry, Grand Unified Theories, Neutrino Mixing, Dark Energy, Dark Matter, Cosmology, Particle Detectors, Colliders, Probability and Statistics. Among the 120 reviews are many that are new or heavily revised, including a new review on High Energy Soft QCD and Diffraction and one on the Determination of CKM Angles from B Hadrons.
The Review is divided into two volumes. Volume 1 includes the Summary Tables and 98 review articles. Volume 2 consists of the Particle Listings and contains also 22 reviews that address specific aspects of the data presented in the Listings.
The complete Review (both volumes) is published online on the website of the Particle Data Group (pdg.lbl.gov) and in a journal. Volume 1 is available in print as the PDG Book. A Particle Physics Booklet with the Summary Tables and essential tables, figures, and equations from selected review articles is available in print and as a web version optimized for use on phones as well as an Android app.

The production of K*(892)0 and ϕ(1020) in pp collisions at s=8 TeV was measured by using Run 1 data collected by the ALICE collaboration at the CERN Large Hadron Collider (LHC). The pT-differential yields d2N/dydpT in the range 0<pT<20 GeV/c for K*0 and 0.4<pT<16 GeV/c for ϕ have been measured at midrapidity, |y|<0.5. Moreover, improved measurements of the K*0(892) and ϕ(1020) at s=7 TeV are presented. The collision energy dependence of pT distributions, pT-integrated yields, and particle ratios in inelastic pp collisions are examined. The results are also compared with different collision systems. The values of the particle ratios are found to be similar to those measured at other LHC energies. In pp collisions a hardening of the particle spectra is observed with increasing energy, but at the same time it is also observed that the relative particle abundances are independent of the collision energy. The pT-differential yields of K*0 and ϕ in pp collisions at s=8 TeV are compared with the expectations of different Monte Carlo event generators.

The striking similarities that have been observed between high-multiplicity proton-proton (pp) collisions and heavy-ion collisions can be explored through multiplicity-differential measurements of identified hadrons in pp collisions. With these measurements, it is possible to study mechanisms such as collective flow that determine the shapes of hadron transverse momentum (pT) spectra, to search for possible modifications of the yields of short-lived hadronic resonances due to scattering effects in an extended hadron-gas phase, and to investigate different explanations provided by phenomenological models for enhancement of strangeness production with increasing multiplicity. In this paper, these topics are addressed through measurements of the K⁎(892)0 and ϕ(1020) mesons at midrapidity in pp collisions at s= 13 TeV as a function of the charged-particle multiplicity. The results include the pT spectra, pT-integrated yields, mean transverse momenta, and the ratios of the yields of these resonances to those of longer-lived hadrons. Comparisons with results from other collision systems and energies, as well as predictions from phenomenological models, are also discussed.

The high-accuracy ϕ-meson photoproduction data from the CLAS experiment in Hall B of Jefferson Laboratory allow us to determine the near-threshold total cross section of the γp→ϕp reaction and use it for evaluating the ϕp scattering length αϕp. These data result in an absolute value of |αϕp|=(0.063±0.010) fm, which is smaller than the typical hadron size. A comparative analysis of αϕp with the previously determined scattering lengths for ωp and J/ψp from the A2 and GlueX experiments is performed.

This Letter presents the first direct investigation of the p–Σ0 interaction, using the femtoscopy technique in high-multiplicity pp collisions at s=13 TeV measured by the ALICE detector. The Σ0 is reconstructed via the decay channel to Λγ, and the subsequent decay of Λ to pπ−. The photon is detected via the conversion in material to e+e− pairs exploiting the capability of the ALICE detector to measure electrons at low transverse momenta. The measured p–Σ0 correlation indicates a shallow strong interaction. The comparison of the data to several theoretical predictions obtained employing the Correlation Analysis Tool using the Schrödinger Equation (CATS) and the Lednický–Lyuboshits approach shows that the current experimental precision does not yet allow to discriminate between different models, as it is the case for the available scattering and hypernuclei data. Nevertheless, the p–Σ0 correlation function is found to be sensitive to the strong interaction, and driven by the interplay of the different spin and isospin channels. This pioneering study demonstrates the feasibility of a femtoscopic measurement in the p–Σ0 channel and with the expected larger data samples in LHC Run 3 and Run 4, the p–Σ0 interaction will be constrained with high precision.

The study of the strength and behavior of the antikaon-nucleon (K¯N) interaction constitutes one of the key focuses of the strangeness sector in low-energy quantum chromodynamics (QCD). In this Letter a unique high-precision measurement of the strong interaction between kaons and protons, close and above the kinematic threshold, is presented. The femtoscopic measurements of the correlation function at low pair-frame relative momentum of (K+p⊕K−p¯) and (K−p⊕K+p¯) pairs measured in pp collisions at s=5, 7, and 13 TeV are reported. A structure observed around a relative momentum of 58 MeV/c in the measured correlation function of (K−p⊕K+p¯) with a significance of 4.4σ constitutes the first experimental evidence for the opening of the (K¯0n⊕K0n¯) isospin breaking channel due to the mass difference between charged and neutral kaons. The measured correlation functions have been compared to Jülich and Kyoto models in addition to the Coulomb potential. The high-precision data at low relative momenta presented in this work prove femtoscopy to be a powerful complementary tool to scattering experiments and provide new constraints above the K¯N threshold for low-energy QCD chiral models.

This document corrects two errors in Eur. Phys. J. C77 (2017) no. 8, 56: the incorrect referencing of Fig. 1 labels in three paragraphs in the results section and a missing acknowledgements section.

A bstract
Two-particle correlations in high-energy collision experiments enable the extraction of particle source radii by using the Bose-Einstein enhancement of pion production at low relative momentum q ∝ 1/ R . It was previously observed that in pp collisions at $$ \sqrt{s} $$ s = 7TeV the average pair transverse momentum k T range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena. To investigate this further, an event-shape dependent analysis of Bose-Einstein correlations for pion pairs is performed in this work. By categorizing the events by their transverse sphericity S T into spherical ( S T > 0:7) and jet-like ( S T < 0:3) events a method was developed that allows for the determination of source radii for much larger values of k T for the first time. Spherical events demonstrate little or no background correlations while jet-like events are dominated by them. This observation agrees with the hypothesis of a mini-jet origin of the non-femtoscopic background correlations and gives new insight into the physics interpretation of the k T dependence of the radii. The emission source size in spherical events shows a substantially diminished k T dependence, while jet-like events show indications of a negative trend with respect to k T in the highest multiplicity events. Regarding the emission source shape, the correlation functions for both event sphericity classes show good agreement with an exponential shape, rather than a Gaussian one.

This Letter presents the first experimental observation of the attractive strong interaction between a proton and a multistrange baryon (hyperon) Ξ−. The result is extracted from two-particle correlations of combined p−Ξ−⊕p¯−Ξ¯+ pairs measured in p−Pb collisions at sNN=5.02 TeV at the LHC with ALICE. The measured correlation function is compared with the prediction obtained assuming only an attractive Coulomb interaction and a standard deviation in the range [3.6, 5.3] is found. Since the measured p−Ξ−⊕p¯−Ξ¯+ correlation is significantly enhanced with respect to the Coulomb prediction, the presence of an additional, strong, attractive interaction is evident. The data are compatible with recent lattice calculations by the HAL-QCD Collaboration, with a standard deviation in the range [1.8, 3.7]. The lattice potential predicts a shallow repulsive Ξ− interaction within pure neutron matter and this implies stiffer equations of state for neutron-rich matter including hyperons. Implications of the strong interaction for the modeling of neutron stars are discussed.

This work presents new constraints on the existence and the binding energy of a possible Λ–Λ bound state, the H-dibaryon, derived from Λ–Λ femtoscopic measurements by the ALICE collaboration. The results are obtained from a new measurement using the femtoscopy technique in Image 1 collisions at s=13 TeV and p–Pb collisions at sNN=5.02 TeV, combined with previously published results from Image 1 collisions at s=7 TeV. The Λ–Λ scattering parameter space, spanned by the inverse scattering length f0−1 and the effective range d0, is constrained by comparing the measured Λ–Λ correlation function with calculations obtained within the Lednický model. The data are compatible with hypernuclei results and lattice computations, both predicting a shallow attractive interaction, and permit to test different theoretical approaches describing the Λ–Λ interaction. The region in the (f0−1,d0) plane which would accommodate a Λ–Λ bound state is substantially restricted compared to previous studies. The binding energy of the possible Λ–Λ bound state is estimated within an effective-range expansion approach and is found to be BΛΛ=3.2−2.4+1.6(stat)−1.0+1.8(syst) MeV.

We report on the first femtoscopic measurement of baryon pairs, such as p−p, p−Λ, and Λ−Λ, measured by ALICE at the Large Hadron Collider (LHC) in proton-proton collisions at s=7TeV. This study demonstrates the feasibility of such measurements in pp collisions at ultrarelativistic energies. The femtoscopy method is employed to constrain the hyperon-nucleon and hyperon-hyperon interactions, which are still rather poorly understood. A new method to evaluate the influence of residual correlations induced by the decays of resonances and experimental impurities is hereby presented. The p−p, p−Λ, and Λ−Λ correlation functions were fitted simultaneously with the help of a new tool developed specifically for the femtoscopy analysis in small colliding systems: Correlation Analysis Tool using the Schrödinger equation (CATS). Within the assumption that in pp collisions the three particle pairs originate from a common source, its radius is found to be equal to r0=1.125±0.018(stat)−0.035+0.058(syst) fm. The sensitivity of the measured p−Λ correlation is tested against different scattering parameters, which are defined by the interaction among the two particles, but the statistics is not sufficient yet to discriminate among different models. The measurement of the Λ−Λ correlation function constrains the phase space spanned by the effective range and scattering length of the strong interaction. Discrepancies between the measured scattering parameters and the resulting correlation functions at LHC and RHIC energies are discussed in the context of various models.

We present a new analysis framework called "Correlation Analysis Tool using the Schr\"odinger equation" (CATS) which computes the two-particle femtoscopy correlation function $C(k)$, with $k$ being the relative momentum for the particle pair. Any local interaction potential and emission source function can be used as an input and the wave function is evaluated exactly. In this paper we present a study on the sensitivity of $C(k)$ to the interaction potential for different particle pairs: p-p, p-$\mathrm{\Lambda}$, $\mathrm{K^-}$-p, $\mathrm{K^+}$-p, p-$\mathrm{\Xi}^-$ and $\mathrm{\Lambda}$-$\mathrm{\Lambda}$. For the p-p Argonne $v_{18}$ and Reid Soft-Core potentials have been tested. For the other pair systems we present results based on strong potentials obtained from effective Lagrangians such as $\chi$EFT for p-$\mathrm{\Lambda}$, J\"ulich models for $\mathrm{K(\bar{K})}$-N and Nijmegen models for $\mathrm{\Lambda}$-$\mathrm{\Lambda}$. For the p-$\mathrm{\Xi}^-$ pairs we employ the latest lattice results from the HAL QCD collaboration. Our detailed study of different interacting particle pairs as a function of the source size and different potentials shows that femtoscopic measurements can be exploited in order to constrain the final state interactions among hadrons. In particular, small collision systems of the order of 1~fm, as produced in pp collisions at the LHC, seem to provide a suitable environment for quantitative studies of this kind.

Two-particle angular correlations were measured in pp collisions at s=7 TeV for pions, kaons, protons, and lambdas, for all particle/anti-particle combinations in the pair. Data for mesons exhibit an expected peak dominated by effects associated with mini-jets and are well reproduced by general purpose Monte Carlo generators. However, for baryon–baryon and anti-baryon–anti-baryon pairs, where both particles have the same baryon number, a near-side anti-correlation structure is observed instead of a peak. This effect is interpreted in the context of baryon production mechanisms in the fragmentation process. It currently presents a challenge to Monte Carlo models and its origin remains an open question.

In the project B.4, the modification of meson properties (mass, width) in a nuclear medium has been studied in photoproduction of mesons off nuclear targets. This work has been motivated by theoretical expectations of in-medium modifications of hadrons based on the conjecture of a partial restoration of chiral symmetry in a strongly interacting medium. It has been shown that these in-medium changes can be discussed in a compact form in terms of an optical potential describing the meson-nucleus interaction. Experimental approaches to determine the real and imaginary part of the meson-nucleus potential have been developed. The experiments have been performed with the Crystal Barrel/TAPS detector at the electron accelerator ELSA (Bonn) and the Crystal Ball/TAPS detector at MAMI (Mainz). Measuring the excitation function and momentum distribution for photo production of ω and η′ mesons, the real parts of the ω and η′-nucleus potential, given by the in-medium mass shift, have been determined. For the η′ meson a lowering of the mass at normal nuclear matter density by -(39±7(stat)±15(syst)) MeV is observed, while for the ω meson a slightly smaller mass shift is found, however, with much larger uncertainties, not excluding a zero mass shift. The imaginary part of the potentials has been extracted from the measurement of the transparency ratio which compares the meson production cross section per nucleon within a nucleus to the production cross section off the free proton. For the η′ meson the imaginary part of the potential is found to be smaller than the real part. In case of the ω meson the opposite is observed. This makes the η′ meson a good candidate for the search for meson-nucleus bound states while no resolved ω mesic states can be expected. The results are compared with theoretical predictions. An outlook on future experiments is given.

We present measurements of $e^+e^-$ production at midrapidity in Au$+$Au
collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The invariant yield is studied
within the PHENIX detector acceptance over a wide range of mass ($m_{ee} <$ 5
GeV/$c^2$) and pair transverse momentum ($p_T$ $<$ 5 GeV/$c$), for minimum bias
and for five centrality classes. The \ee yield is compared to the expectations
from known sources. In the low-mass region ($m_{ee}=0.30$--0.76 GeV/$c^2$)
there is an enhancement that increases with centrality and is distributed over
the entire pair \pt range measured. It is significantly smaller than previously
reported by the PHENIX experiment and amounts to $2.3\pm0.4({\rm
stat})\pm0.4({\rm syst})\pm0.2^{\rm model}$ or to $1.7\pm0.3({\rm
stat})\pm0.3({\rm syst})\pm0.2^{\rm model}$ for minimum bias collisions when
the open-heavy-flavor contribution is calculated with {\sc pythia} or {\sc
mc@nlo}, respectively. The inclusive mass and $p_T$ distributions as well as
the centrality dependence are well reproduced by model calculations where the
enhancement mainly originates from the melting of the $\rho$ meson resonance as
the system approaches chiral symmetry restoration. In the intermediate-mass
region ($m_{ee}$ = 1.2--2.8 GeV/$c^2$), the data hint at a significant
contribution in addition to the yield from the semileptonic decays of
heavy-flavor mesons.

Measurements of the sphericity of primary charged particles in minimum bias proton-proton collisions at root s = 0.9, 2.76 and 7 TeV with the ALICE detector at the LHC are presented. The observable is measured in the plane perpendicular to the beam direction using primary charged tracks with p(T) > 0.5 GeV/c in vertical bar eta vertical bar < 0.8. The mean sphericity as a function of the charged particle multiplicity at mid-rapidity (N-ch) is reported for events with different p(T) scales ("soft" and "hard") defined by the transverse momentum of the leading particle. In addition, the mean charged particle transverse momentum versus multiplicity is presented for the different event classes, and the sphericity distributions in bins of multiplicity are presented. The data are compared with calculations of standard Monte Carlo event generators. The transverse sphericity is found to grow with multiplicity at all collision energies, with a steeper rise at low N-ch, whereas the event generators show an opposite tendency. The combined study of the sphericity and the mean p(T) with multiplicity indicates that most of the tested event generators produce events with higher multiplicity by generating more back-to-back jets resulting in decreased sphericity (and isotropy). The PYTHIA6 generator with tune PERUGIA-2011 exhibits a noticeable improvement in describing the data, compared to the other tested generators.

We report the results of the femtoscopic analysis of pairs of identical pions measured in p-Pb collisions at sNN=5.02 TeV. Femtoscopic radii are determined as a function of event multiplicity and pair momentum in three spatial dimensions. As in the pp collision system, the analysis is complicated by the presence of sizable background correlation structures in addition to the femtoscopic signal. The radii increase with event multiplicity and decrease with pair transverse momentum. When taken at comparable multiplicity, the radii measured in p-Pb collisions, at high multiplicity and low pair transverse momentum, are 10%–20% higher than those observed in pp collisions but below those observed in A−A collisions. The results are compared to hydrodynamic predictions at large event multiplicity as well as discussed in the context of calculations based on gluon saturation.

Two-particle angular correlations between unidentified charged trigger and associated particles are measured by the ALICE detector in p–Pb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV. The transverse-momentum range 0.7<pT,assoc<pT,trig<5.0 GeV/c0.7<pT,assoc<pT,trig<5.0 GeV/c is examined, to include correlations induced by jets originating from low momentum-transfer scatterings (minijets). The correlations expressed as associated yield per trigger particle are obtained in the pseudorapidity range |η|<0.9|η|<0.9. The near-side long-range pseudorapidity correlations observed in high-multiplicity p–Pb collisions are subtracted from both near-side short-range and away-side correlations in order to remove the non-jet-like components. The yields in the jet-like peaks are found to be invariant with event multiplicity with the exception of events with low multiplicity. This invariance is consistent with the particles being produced via the incoherent fragmentation of multiple parton–parton scatterings, while the yield related to the previously observed ridge structures is not jet-related. The number of uncorrelated sources of particle production is found to increase linearly with multiplicity, suggesting no saturation of the number of multi-parton interactions even in the highest multiplicity p–Pb collisions. Further, the number scales only in the intermediate multiplicity region with the number of binary nucleon–nucleon collisions estimated with a Glauber Monte-Carlo simulation.

ALICE is the heavy-ion experiment at the CERN Large Hadron Collider. The experiment continuously took data during the first physics campaign of the machine from fall 2009 until early 2013, using proton and lead-ion beams. In this paper we describe the running environment and the data handling procedures, and discuss the performance of the ALICE detectors and analysis methods for various physics observables.

We report on the high statistics two-pion correlation functions from pp collisions at root s = 0.9 TeV and root s = 7 TeV, measured by the ALICE experiment at the Large Hadron Collider. The correlation functions as well as the extracted source radii scale with event multiplicity and pair momentum. When analyzed in the same multiplicity and pair transverse momentum range, the correlation is similar at the two collision energies. A three-dimensional femtoscopic analysis shows an increase of the emission zone with increasing event multiplicity as well as decreasing homogeneity lengths with increasing transverse momentum. The latter trend gets more pronounced as multiplicity increases. This suggests the development of space-momentum correlations, at least for collisions producing a high multiplicity of particles. We consider these trends in the context of previous femtoscopic studies in high-energy hadron and heavy-ion collisions and discuss possible underlying physics mechanisms. Detailed analysis of the correlation reveals an exponential shape in the outward and longitudinal directions, while the sideward remains a Gaussian. This is interpreted as a result of a significant contribution of strongly decaying resonances to the emission region shape. Significant nonfemtoscopic correlations are observed, and are argued to be the consequence of "mini-jet"-like structures extending to low p(t). They are well reproduced by the Monte-Carlo generators and seen also in pi(+)pi(-) correlations.

The photo-production of ϕ mesons from Li, C, Al, and Cu at forward angles has been measured at Eγ=1.5–2.4GeV. The number of events for incoherent ϕ photo-production is found to have a target mass number dependence of A0.72±0.07 in the kinematical region of |t|⩽0.6GeV2/c2. The total cross section of the ϕ–nucleon interaction, σϕN, has been estimated as 35−11+17mb using the A-dependence of the ϕ photo-production yield and a Glauber-type multiple scattering theory. This value is much larger than σϕN in free space, suggesting that the ϕ properties might change in the nuclear medium.

Possible existence of $(c\bar{c})-$nucleus bound states are examined. We
adopt Gaussian potentials for the $\eta_{c}-N$ and $J/\psi-N$ interactions. The
relations between the scattering lengths $a$ of $(c\bar{c})-N$ interactions and
the binding energies of $\eta_{c}-NN$, $J/\psi-NN$ and $J/\psi-^{4}$He are
given. The results show that scattering lengths $a \le -0.95$ fm are needed to
make $\eta_{c}-NN$ and $J/\psi-NN$ bound states, while for $a \le -0.24$ fm
there may exist a $J/\psi-^{4}$He bound state.

We show that the differential cross section d/dt of the pp reaction at threshold is finite and its value is crucial to the mechanism of -meson photoproduction and for the models of N interaction.

ALICE is an LHC experiment devoted to the study of strongly interacting
matter in proton--proton, proton--nucleus and nucleus--nucleus collisions at
ultra-relativistic energies. The ALICE VZERO system, made of two scintillator
arrays at asymmetric positions, one on each side of the interaction point,
plays a central role in ALICE. In addition to its core function as a trigger,
the VZERO system is used to monitor LHC beam conditions, to reject beam-induced
backgrounds and to measure basic physics quantities such as luminosity,
particle multiplicity, centrality and event plane direction in nucleus-nucleus
collisions. After describing the VZERO system, this publication presents its
performance over more than four years of operation at the LHC.

ALICE (A Large Ion Collider Experiment) is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model. It is designed to address the physics of strongly interacting matter and the quark-gluon plasma at extreme values of energy density and temperature in nucleus-nucleus collisions. Besides running with Pb ions, the physics programme includes collisions with lighter ions, lower energy running and dedicated proton-nucleus runs. ALICE will also take data with proton beams at the top LHC energy to collect reference data for the heavy-ion programme and to address several QCD topics for which ALICE is complementary to the other LHC detectors. The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries. Its overall dimensions are 16 × 16 × 26 m3 with a total weight of approximately 10 000 t. The experiment consists of 18 different detector systems each with its own specific technology choice and design constraints, driven both by the physics requirements and the experimental conditions expected at LHC. The most stringent design constraint is to cope with the extreme particle multiplicity anticipated in central Pb-Pb collisions. The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification (PID) over a broad range in momentum, up to the highest multiplicities predicted for LHC. This will allow for comprehensive studies of hadrons, electrons, muons, and photons produced in the collision of heavy nuclei. Most detector systems are scheduled to be installed and ready for data taking by mid-2008 when the LHC is scheduled to start operation, with the exception of parts of the Photon Spectrometer (PHOS), Transition Radiation Detector (TRD) and Electro Magnetic Calorimeter (EMCal). These detectors will be completed for the high-luminosity ion run expected in 2010. This paper describes in detail the detector components as installed for the first data taking in the summer of 2008.

Differential cross sections and decay asymmetries for coherent ϕ-meson photoproduction from deuterons were measured for the first time at forward angles using linearly polarized photons at Eγ=1.5–2.4 GeV. This reaction offers a unique way to directly access natural-parity pomeron dynamics and gluon exchange at low energies. The cross sections at zero degrees increase with increasing photon energy. The decay asymmetries demonstrate a complete dominance of natural-parity exchange processes, showing that isovector unnatural-parity π-meson exchange is small. Nevertheless the deduced cross sections of ϕ-mesons from nucleons contributed by isoscalar t-channel exchange processes are not well described by the conventional pomeron model.

The two-particle momentum correlation function of a K−p pair from high-energy nuclear collisions is evaluated in the K¯N−πΣ−πΛ coupled-channel framework. The effects of all coupled channels together with the Coulomb potential and the threshold energy difference between K−p and K¯0n are treated completely for the first time. Realistic potentials based on the chiral SU(3) dynamics are used which fit the available scattering data. The recently measured correlation function is found to be well reproduced by allowing variations of the source size and the relative weight of the source function of πΣ with respect to that of K¯N. The predicted K−p correlation function from larger systems indicates that the investigation of its source size dependence is useful in providing further constraints in the study of the K¯N interaction.

We review the present status of the experimental and theoretical developments in the field of strangeness in nuclei and neutron stars. We start by discussing the K̄N interaction, that is governed by the presence of the Λ(1405). We continue by showing the two-pole nature of the Λ(1405), and the production mechanisms in photon-, pion-, kaon-induced reactions as well as proton–proton collisions, while discussing the formation of K̄NN bound states. We then move to the theoretical and experimental analysis of the properties of kaons and antikaons in dense nuclear matter, paying a special attention to kaonic atoms and the analysis of strangeness creation and propagation in nuclear collisions. Next, we examine the ϕ meson and the advances in photoproduction, proton-induced and pion-induced reactions, so as to understand its properties in dense matter. Finally, we address the dynamics of hyperons with nucleons and nuclear matter, and the connection to the phases of dense matter with strangeness in the interior of neutron stars.

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)−0.012+0.014(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±0.04(stat)−0.07+0.06(syst) and to 0.63±0.06(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. These results are relevant for the description of heavy-ion collisions and the structure of neutron stars.

Two-particle momentum correlation functions as measured in heavy ion collisions or in high-energetic proton–proton collisions are studied. Special emphasis is put on systems like ΛΛ or K⁻p where effects from the coupling to other channels could be relevant. In both cases other channels open at relatively low momenta or are already open at the reaction threshold. To have a solid basis, realistic coupled-channel interactions for ΛΛ−ΞN−ΛΣ−ΣΣ and πΛ−πΣ−K¯N are utilized in the actual calculations. It is found that the opening of the ΞN channel leaves a trace in the ΛΛ correlation function that could be detectable in experiments. Should the proposed H-dibaryon be located close to or below the ΞN it will have a very pronounced effect. The presence of open channels in systems like Ξ⁻p or K⁻p does influence the correlation functions significantly at low momenta and will certainly complicate any dedicated analysis.

We investigate the $\phi$ meson nuclear transparency using some recent theoretical developments on the $\phi$ in medium self-energy. The inclusion of direct resonant $\phi N$-scattering and the kaon decay mechanisms leads to a $\phi$ width much larger than in most previous theoretical approaches. The model has been confronted with photoproduction data from CLAS and LEPS and the recent proton induced $\phi$ production from COSY finding an overall good agreement. The results support the need of a quite large direct $\phi N$-scattering contribution to the self-energy.

The $\phi$-meson properties in cold nuclear matter are investigated by implementing resonant $\phi N$ interactions as described in effective approaches including the unitarization of scattering amplitudes. Several $N^*$-like states are dynamically generated in these models around $2$ GeV, in the vicinity of the $\phi N$ threshold. We find that both these states and the non-resonant part of the amplitude contribute sizably to the $\phi$ collisional self-energy at finite nuclear density. These contributions are of a similar strength as the widely studied medium effects from the $\bar K K$ cloud. Depending on model details (position of the resonances and strength of the coupling to $\phi N$) we report a $\phi$ broadening up to about $40$-$50$ MeV, to be added to the $\phi\to\bar K K$ in-medium decay width, and an attractive optical potential at threshold up to about $35$ MeV at normal matter density. The $\phi$ spectral function develops a double peak structure as a consequence of the mixing of resonance-hole modes with the $\phi$ quasi-particle peak. The former results point in the direction of making up for missing absorption as reported in $\phi$ nuclear production experiments.

A detailed analysis of the lowest two moments of the $\phi$ meson spectral function in vacuum and nuclear matter is performed. The consistency is examined between the constraints derived from finite energy QCD sum rules and the spectra computed within an improved vector dominance model, incorporating the coupling of kanonic degrees of freedom with the bare $\phi$ meson. In the vacuum, recent accurate measurements of the $e^+ e^- \to K^+ K^-$ cross section allow us to determine the spectral function with high precision. In nuclear matter, the modification of the spectral function can be described by the interactions of the kaons from $\phi \rightarrow K\bar{K}$ with the surrounding nuclear medium. This leads primarily to a strong broadening and an asymmetric deformation of the $\phi$ meson peak structure. We confirm that, both in vacuum and nuclear matter, the zeroth and first moments of the corresponding spectral functions satisfy the requirements of the finite energy sum rules to a remarkable degree of accuracy. Limits on the strangeness sigma term of the nucleon are examined in this context. Applying our results to the second moment of the spectrum, we furthermore discuss constraints on four-quark condensates and the validity of the commonly used ground state saturation approximation.

The PYTHIA program is a standard tool for the generation of events in
high-energy collisions, comprising a coherent set of physics models for the
evolution from a few-body hard process to a complex multiparticle final state.
It contains a library of hard processes, models for initial- and final-state
parton showers, matching and merging methods between hard processes and parton
showers, multiparton interactions, beam remnants, string fragmentation and
particle decays. It also has a set of utilities and several interfaces to
external programs. PYTHIA 8.2 is the second main release after the complete
rewrite from Fortran to C++, and now has reached such a maturity that it offers
a complete replacement for most applications, notably for LHC physics studies.
The many new features should allow an improved description of data.

We report the STAR measurements of dielectron ($e^+e^-$) production at
midrapidity ($|y_{ee}|<$1) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200
GeV. The measurements are evaluated in different invariant mass regions with a
focus on 0.30-0.76 ($\rho$-like), 0.76-0.80 ($\omega$-like), and 0.98-1.05
($\phi$-like) GeV/$c^{2}$. The spectrum in the $\omega$-like and $\phi$-like
regions can be well described by the hadronic cocktail simulation. In the
$\rho$-like region, however, the vacuum $\rho$ spectral function cannot
describe the shape of the $\rho$-like dielectron excess. In this range, an
enhancement of 1.77$\pm$0.11(stat.)$\pm$0.24(sys.)$\pm$0.34(cocktail) is
determined with respect to the hadronic cocktail simulation that excludes the
$\rho$ meson. The enhancement shows no significant dependence on the collision
centrality in 0-80% and on the dielectron transverse momentum ($p_{\rm T}$) in
$p_{\rm T}<$2 GeV/$c$. Theoretical models with in-medium broadened $\rho$
contributions can provide a consistent description of the dilepton mass spectra
for the measurement presented here and the earlier data at the Super Proton
Synchrotron energies.

A consistent treatment of quantum chromodynamical (QCD) sum rules in the nuclear medium is developed. Its close relation to the structure functions of the nucleon in the deep inelastic scattering is emphasized. The formalism is applied to the spectral changes of vector mesons (ρ, ω, and φ) in the nuclear matter. A linear decrease of the masses as a function of density is found. The four-quark condensate 〈(q¯q)2〉 and a twist-two condensate 〈q¯γμq¯μDνq〉 in medium play dominant roles for the mass-shift of light mesons. Physical implications of the result in finite nuclei and in heavy ion collisions are also discussed.

The Time-Of-Flight (TOF) detector of the ALICE experiment at the CERN LHC is based on Multi-gap Resistive Plate Chambers (MRPCs) technology. During the 2009-2013 data taking the TOF system had very stable operations with a total time resolution of 80ps. Details of the different calibration procedures and performance with data from collisions at the LHC will be described.

A new QCD sum rule analysis on the spin-isospin averaged ρ, ω and ϕ meson-nucleon scattering lengths is presented. By introducing
the constraint relation on the low energy limit of the vector-current nucleon forward scattering amplitude (low energy theorem),
we obtain aρ = −0.47±0.05 fm, aω = −0.41±0.05 fm and aϕ = −0.15±0.02 fm, which suggests that these V-N interactions are attractive. It is also proved that previous studies on the mass shift of these vector mesons in the nuclear
medium are essentially those obtained from these scattering lengths in the linear density approximation.

We report on measurements of low-mass electron pairs in 450 GeV p-Be, p-Au, and 200 GeV/nucleon S-Au collisions at central rapidities. For the proton induced interactions, the low-mass spectra are, within the systematic errors, satisfactorily explained by electron pairs from hadron decays, whereas in the S-Au system an enhancement over the hadronic contributions by a factor of 5.0±0.7(stat)±2.0(syst) in the invariant mass range 0.2<m<1.5GeV/c2 is observed. The properties of the excess suggest that it arises from two-pion annihilation ππ→e+e−.

We show that the QCD van der Waals attractive potential is strong enough to bind a {phi} meson onto a nucleon inside a nucleus to form a bound state. The direct experimental signature for such an exotic state is proposed in the case of subthreshold {phi} meson photoproduction from nuclear targets. The production rate is estimated and such an experiment is found to be feasible at the Jefferson Laboratory.

Previous studies on a systematics and phenomenology of meson family are
reviewed and extended based on the intuitive pictures. In these
discussions, the triplet configurations of hadrons are extensively used,
from which the meaning of the selection principle is made clear. This
principle is applied to the meson-baryon vertices. We further suggest an
indirect test of both the ur-baryon models are the non-relativistic
descriptions of meson nonets, where the E1-transitions, 1^+ - 9 {→}
1^- - 9 + γ are taken as the test processes.

The production of K*(892)^0 and phi(1020) in pp collisions at sqrt(s)=7 TeV
was measured by the ALICE experiment at the LHC.The yields and the transverse
momentum spectra d^2N/dydpT at midrapidity |y|<0.5 in the range 0<pT<6 GeV/c
for K*(892)^0 and 0.4<pT<6 GeV/c for phi(1020) are reported and compared to
model predictions. Using the yield of pions, kaons, and Omega baryons measured
previously by ALICE at sqrt(s)=7 TeV, the ratios K*/K^-, phi/K*, phi/K^-,
phi/\pi^-, and (Omega + anti-Omega)/phi are presented. The values of the
K*/K^-, phi/K* and phi/K^- ratios are similar to those found at lower
centre-of-mass energies. In contrast, the phi/pi^- ratio, which has been
observed to increase with energy, seems to saturate above 200 GeV. The (Omega +
anti-Omega)/phi ratio in the pT range 1-5 GeV/c is found to be in good
agreement with the prediction of the HIJING/BB v2.0 model with a strong colour
field.

QCD sum rules are evaluated at finite nucleon densities and temperatures to determine the change of mass parameters for the
lightest vector mesons ρ, ω and φ in a strongly interacting medium. For conditions relevant for the starting experiments at
HADES we find that the in-medium mass shifts of the ρ- and ω-mesons are governed, within the Borel QCD sum rule approach,
by the density and temperature dependence of the four-quark condensate. In particular, the variation of the strength of the
density dependence of the four-quark condensate reflects directly the decreasing mass of the ρ-meson and can lead to a change
of the sign of the ω-meson mass shift as a function of the density. In contrast, the in-medium mass of the φ-meson is directly
related to the chiral strange quark condensate which seems correspondingly accessible.

We investigate the density dependence of the chiral order parameter or quark condensate in nuclear matter using relativistic Brueckner-Hartree-Fock theory. While the leading behavior linear in the density is known to be determined model independently by the pion-nucleon sigma term, we demonstrate that higher order corrections are extremely sensitive to the interpretation of the scalar “σ”-meson and its substructure.

Based on an effective Lagrangian which combines chiral SU(3) dynamics with vector meson dominance, we have developed a model for the forward vector meson-nucleon scattering amplitudes. We use this as an input to calculate the low energy part of the current-current correlation function in nuclear matter. Its spectrum enters directly in the “left-hand side” of QCD sum rules. For the isovector channel we find a significant enhancement of the in-medium spectral density below the ϱ resonane while the ρ meson mass itself changes only slightly. The situation is different in the isoscalar channel, where the mass and peak position of the ω meson move downward while its width increases less drastically than in the ρ meson case. For the φ meson we find almost no mass shift; the width of the peak broadens moderately. We observe a remarkable degree of consistency with the operator product expansion of QCD sum rules in all three channels. We point out, however, that these results cannot simply be interpreted, as commonly done, in terms of a universal rescaling of vector meson masses in matter.