An overview of the recent results on strangeness production and spectroscopy from the electron-proton collider experiments H1 and ZEUS at HERA is presented. Production of particles with light and strange quarks is discussed and compared with both theoretical predictions and with data from e+e- experiments. Measurements in the charm sector cover studies of the radially and orbitally excited charm states. Finally, the investigation of exotic states in the strangeness sector at HERA is reviewed.
Searches for narrow baryonic states in the K0s-p, K+p, Xi-pi and bar{X}+pi decay channels are reported. The data were collected with the ZEUS detector at HERA using an integrated luminosity of 121 pb-1. The searches were performed in the central rapidity region of inclusive deep inelastic scattering at an $ep$ centre-of-mass energy of 300--318 GeV for exchanged photon virtuality, Q2, above 1 GeV2. The results support the existence of a narrow baryonic state with strangeness in K0s-p and K0s-\bar{p} decay channels, consistent with the pentaquark prediction. No pentaquark signals were found in t…
The production of neutral strange hadrons is investigated using deep-inelastic scattering events measured with the H1 detector at HERA. The measurements are made in the phase space defined by the negative four-momentum transfer squared of the photon 2 < Q^2 < 100 GeV^2 and the inelasticity 0.1 < y < 0.6. The K_s and Lambda production cross sections and their ratios are determined. K_s production is compared to the production of charged particles in the same region of phase space. The Lambda - anti-Lambda asymmetry is also measured and found to be consistent with zero. Predictions of leading or…
HERA-B is a fixed target experiment at the 920 GeV HERA proton beam at DESY which uses a variety of nuclear targets. During the last data taking period from Nov. 2002 to Feb. 2003, 200 million minimum bias events were recorded. A large sample of V^0(K_zero, Lambda_zero, Lambda_zero_bar), approximately 20000 cascade hyperons Xi^(-/+) and 1200 Omega^(-/+) over low background were reconstructed using these data. About 1 million K^*(892) --> K pi and 60,000 phi(1020) --> K^+K^- decays in central production $(-0.15 < x_F < 0.1, 0.5 < p_t^2 < 12.1 (GeV/c)^2 were analyzed. Preliminary results for dif…
Measurements are presented of $K^0$ meson and $Λ$ baryon production in deep-inelastic positron-proton scattering (DIS) in the kinematic range $10 < Q^2 < 70 $GeV$^2$ and $10^{-4} < x < 10^{-2}$. The measurements, obtained using the H1 detector at the HERA collider, are discussed in the light of possible mechanisms for increased strangeness production at low Bjorken-$x$. Comparisons of the $x_F$ spectra, where $x_F$ is the fractional longitudinal momentum in the hadronic centre-of-mass frame, with results from electron-positron annihilation are made. The $x_F$ spectra and the $K^0$ ``seagull'' …
The K0s-p invariant mass spectrum was reconstructed in several kinematic regions with the main emphasis on the studies of the production mechanism of the Theta+ candidate recently observed by ZEUS. The candidate Theta+ signal was found to be produced predominantly in the forward hemisphere in the laboratory frame. This is unlike the case for the Lambda(1520) or the Lambda(c), and indicates that the Theta+ may have an unusual production mechanism related to proton-remnant fragmentation. H1 does not observe a signal and sets an upper limit at 95% C.L. which does not exclude the ZEUS observation.
Inclusive phi-meson production in neutral current deep inelastic e+p scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 45 pb^{-1}. The phi mesons were studied in the range 10<Q2<100 GeV2, where Q2 is the virtuality of the exchanged photon, and in restricted kinematic regions in the transverse momentum, p_T, pseudorapidity, eta, and the scaled momentum in the Breit frame, x_p. Monte Carlo models with the strangeness-suppression factor as determined by analyses of e+e- annihilation events overestimate the cross sections. A smaller value of the strangen…
A new phenomenological approach is suggested for determining the strangeness content of light-flavour isoscalars. This approach is based on phenomenological laws of hadron production related to the spin, isospin, strangeness content and mass of the particles. From the total production rates per hadronic Z decay of all light-flavour hadrons, measured so far at LEP, the values of the nonstrange-strange mixing angles are found to be |ϕ_P| = 42.3^{\circ} \pm 3.5^{\circ}, |ϕ_V| = 10^{\circ} \pm 8^{\circ}, |ϕ_T| = 16^{\circ} \pm 11^{\circ} and |ϕ_S| = 13^{\circ} \pm 9^{\circ}. Our results on the η-η…
These proceedings summarise recent measurements of angular correlations between the $Ξ$ baryon and identified hadrons in pp collisions at $\sqrt{s}=13\,\mathrm{TeV}$ using the ALICE detector. The results are compared with both string-based (PYTHIA8 with extensions) and core-corona (EPOS-LHC) models, to improve our understanding of strangeness and baryon production in small systems. The results favour baryon production through string junctions over diquark breaking, but the PYTHIA models fail at describing the relatively wide $Ξ-$strangeness jet peak, indicating stronger diffusion of strange qu…
This article summarizes a talk given at the International Symposium on Multiparticle Dynamics 1999 in Providence/USA. It provides an overview on the variety of measurements of the hadronic final state in jet production for deep-inelastic scattering and photoproduction at HERA.
Recent results from H1, ZEUS, HERMES and HERA-B on searches for exotic baryons in ep collisions, eD scattering and pA scattering at HERA are reviewed. Evidence for the production of the strange pentaquark Theta^+ and of a narrow anti-charmed baryon decaying to D^{*-}p together with negative results of pentaquark searches at HERA are presented.
Recent results from H1 and ZEUS on searches for exotic baryons in $ep$ collisions at HERA are reviewed. Evidence for the production of the strange pentaquark $Θ^+$ and of a narrow anti-charmed baryon decaying to $D^{*-} p$ together with negative results of pentaquark searches at HERA are presented.
Inclusive photoproduction cross-sections of the neutral mesons eta, rho^0, f_0(980) and f_2(1270) have been measured by H1 and compared to the photoproduction of pi^+ in ep collisions at HERA. Also, inclusive KsKs production and evidence for a narrow baryonic state decaying to Ksp have been observed by ZEUS at HERA.
The universality and scaling behaviour of hadronisation have been investigated in semi-inclusive neutral current deep inelastic ep scattering; specifically the multiplicity and scaled momentum spectra of charged hadrons have been measured and analysed. The measurements are performed in the current region of the Breit frame, as well as in the current fragmentation region of the hadronic centre-of-mass frame. The data collected at the HERA ep collider by the H1 and ZEUS experiments are compared with similar measurements obtained in e+e- annihilation and with previous ep measurements as well as w…
The measured D*+/-, D0, D+/-, D_s+/- and /\_c+/- photoproduction cross sections have been used to determine charm fragmentation ratios and fractions of c quarks hadronising as a particular charm hadron, f(c -> D, /\_c). Events with a D*+/- meson produced in association with an energetic jet have been used to measure the charm fragmentation function. The results are compared with different models and with previous measurements.
A detailed study of vector meson production ($ϕ$ and $J/ψ$) in $e^\pm p$ collisions at HERA with the ZEUS and H1 detector has been performed. The cross sections are measured as a function of $Q^2$, $W$ and $t$. In this contribution, the results are summarised, compared to theoretical calculations and the dynamical picture emerging in perturbative QCD is highlighted. The measurement of the hadronic component of the photon light-cone wave function in the exclusive production of di-pions, $ep\to e π^+π^- p$ is also reported.
Recent results on spectroscopy and the measurement of hadronic final states in ep collisions from the H1 and ZEUS collaborations are presented using data sets with an integrated luminosity between 44 and 121 pb^-1 collected during the HERA I running period. Besides a search for resonant states which could be interpreted as pentaquarks, a study of charged particle momentum spectra in the Breit frame and a measurement of neutral strange hadron production is shown. Furthermore two recent measurements of prompt photons are presented and compared with pQCD calculations. The measurements are perform…
The cross sections for the charged current processes ${e^{-}p}\rightarrow{ν_e+hadrons}$ and, for the first time, ${e^{+}p}\rightarrow{\overlineν_e+hadrons}$ are measured at HERA for transverse momenta larger than 25 GeV.
The paper gives an overview of strangeness-production experiments at the Cooler Synchrotron COSY. Results on kaon-pair and $ϕ$ meson production in $pp$, $pd$ and $dd$ collisions, hyperon-production experiments and $Λp$ final-state interaction studies are presented as well as a search for a strangeness $S=-1$ resonance in the $Λp$ system.
We present the new studies which are performed with the aim of better understanding the production mechanisms for strange particles, and hence the strangeness enhancement phenomenon, in small-collision systems. In one of the recent studies, the very forward energy transported by beam remnants (spectators) and detected by the Zero Degree Calorimeters (ZDC) is used to classify events. The contribution of the effective energy and the particle multiplicity on strangeness production is studied using a multi-differential approach in order to disentangle initial and final state effects. In the second…
The production of D^0, D^*+, D^+, D_s+ and Lambda_c^+ charm hadrons and their antiparticles in ep scattering at HERA has been studied with the ZEUS detector, using a total integrated luminosity of 372 pb^-1. The fractions of charm quarks hadronising into a particular charm hadron were derived. In addition, the ratio of neutral to charged D-meson production rates, the fraction of charged D mesons produced in a vector state, and the stangeness-suppression factor have been determined. The measurements have been performed in the photoproduction regime. The charm hadrons were reconstructed in the r…
Recent results from the ep (electron-proton) collider HERA are presented. Jet production and diffraction type reactions allow tests of predictions of Quantum Chromodynamics (QCD). Jet measurements are used to improve the combined H1-ZEUS parton density fits by adding an additional constraint on the gluon distribution in the proton and, at the same time, providing information on strong coupling constant. Measurements of inclusive diffraction and diffractive dijet data are discussed in the context of factorisation in diffraction. Diffractive Upsilon(1S) production was studied to extract the t-sl…
One of the key results of the LHC Run 1 was the observation of an enhanced production of strange particles in high multiplicity pp and p--Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 7 and 5.02 TeV, respectively. The strangeness enhancement is investigated by measuring the evolution with multiplicity of single-strange and multi-strange baryon production relative to non-strange particles. A smooth increase of strange particle yields relative to the non-strange ones with event multiplicity has been observed in such systems. We report the latest results on multiplicity dependence of strange and mult…
The GlueX experiment located at Jefferson Lab studies the spectrum of hadrons using photoproduction on a LH$_2$ target in a wide variety of final states. With its detector system capable of measuring neutral and charged final state particles over almost the full solid angle, and very good particle identification capabilities, GlueX can measure many different hadrons containing strangeness. A linearly polarized photon beam allows the measurement of polarization observables, which contain information about the production mechanisms involved in generating strange particles in photoproduction. In …
New facilities for particle and nuclear physics research, which will be available due to constructing the TESLA linear electron-positron collider tangentially to the HERA proton ring, are discussed.
Probing strangeness with event topology classifiers in pp collisions at energies available at the CERN Large Hadron Collider with the rope hadronization mechanism in PYTHIA
Although strange quarks are produced in $s\bar{s}$ pairs, the ratio of $Ω^{-}$ to ${\barΩ}^{+}$ is greater than one in heavy-ion collisions at lower RHIC energies. Thus the produced $Ω$ hyperons must carry net baryon quantum numbers from the colliding nuclei. We present results of $K$-$Ω$ correlations from AMPT model simulations of Au+Au collisions at $\sqrt{s_{NN}}$ = 14.6 GeV, to probe dynamics for baryon number transport to mid-rapidities at this beam energy. We use both the default and string-melting versions to illustrate how hadronization schemes of quark coalescence and string fragmenta…
An experimental overview of the energy dependence of strangeness production is presented. The strange hadrons are considered a good probe to study the QCD matter created in relativistic nucleus-nucleus collisions. The heavy-ion experiments at SPS, RHIC, and LHC have recorded a wealth of data in proton-proton, proton-nucleus and nucleus-nucleus collisions at several beam energies. In this proceeding, I discuss the invariant yield and azimuthal anisotropy measurement of strange hadrons in nucleus-nucleus collisions at SPS, RHIC, and LHC.
The increase in strangeness production with charged particle multiplicity, as seen by the ALICE collaboration at CERN in p-p, p-Pb and Pb-Pb collisions, is investigated in the hadron resonance gas model taking into account interactions among hadrons using S-matrix corrections based on known phase shift analyses. Strangeness conservation is taken into account in the framework of the canonical strangeness ensemble. A very good description is obtained for the variation of the strangeness content in the final state as a function of the number of charged hadrons in the mid-rapidity region using the…
We present the measurements of strange and multi-strange hadrons in high energy pp and Pb-Pb collisions at the Large Hadron Collider (LHC) at CERN. The data for these measurements were collected by the ALICE detector in 2010 and 2011. By comparing strangeness data where Quark-Gluon Plasma (QGP) is not expected to be produced (pp collisions) with data where the energy-density of the collision is sufficient to produce the QGP (central and mid-central Pb-Pb collisions), we are able to investigate the properties of the QGP. We characterize the matter created in the collision by presenting strange …
The study of energy and multiplicity dependence of strange hadron production in proton-proton collisions provides a powerful tool to understand similarities and differences between small and large collision systems. In order to better understand the origin of strangeness enhancement in pp new multi-differential analyses have been performed. The first separates the contribution of soft and hard processes, such as jets, to strange hadron production through two-particle correlation techniques. The second exploits the concept of the effective energy available for particle production in the event, …
Since the discovery of strangeness almost five decades ago, interest in this degree of freedom has grown up and now its investigation spans the scales from quarks to nuclei. Measurements with identified strange hadrons can provide important information on several hot topics in hadronic physics: the strange distribution and fragmentation functions, the nucleon tomography and quark orbital momentum, accessible through the study of the {\it generalized} parton distribution and the {\it transverse momentum dependent} parton distribution functions, the quark hadronization in the nuclear medium, the…
We study the data on mean hadron yields and contrast the chemical freezeout conditions in p+p, p+Pb and Pb+Pb at the Large Hadron Collider (LHC) energies. We study several schemes for freezeout that mainly differ in the way strangeness is treated: i. strangeness freezes out along with the non-strange hadrons in complete equilibrium (1CFO), ii. strangeness freezes out along with non-strange hadrons with an additional parameter $γ_S$ accounting for non-equilibrium production of strangeness (1CFO+$γ_S$), and iii. strangeness freezes out earlier than non-strange hadrons and in thermal equilibrium …
