636 research outputs found
Study of Baryon and Antibaryon Spectra in Lead Lead Interactions at 160 GeV/c per Nucleon
No full text% WA97 \\ \\ Hyperons are expected to be a useful probe for the dynamics of hadronic matter under extreme conditions. In particular the onset of a Quark-Gluon Plasma phase in a heavy ion collision is expected to enhance the hyperon yield with respect to normal hadronic interactions. \\ \\WA97 aims to measure the spectra of strange particles and in particular of hyperons and antihyperons produced in ultrarelativistic lead-lead interactions and to compare them with those from proton initiated reactions. The experiment covers central rapidity down to transverse momenta of a few hundred MeV/c. The experimental setup consists of: an array of multiplicity counters, a silicon based decay detector made of pixels, located in the CERN-OMEGA Spectrometer, an array of pad cathode MWPCs used as lever arm detectors and a zero degree hadron calorimeter. \\ \
Beyond the question of fixed-parameter tractability
No full textMultivariate complexity is a prominent field that over the last decades has developed a rich toolbox, not only to tackle seemingly intractable problems, but also to describe the boundaries of tractability in a richer and more fine-grained way. In this thesis we survey the research directions emerging after the question of fixed-parameter tractability has been settled. That is, we define and exemplify structural parameters, polynomial kernelizations, branching techniques, subexponential time algorithms and parameterized approximation algorithms. In addition, we display techniques for proving lower bounds for all of the above mentioned directions. After this, we give new results within this parameterized framework for several classic graph problems. The problems studied in this thesis can naturally be divided into two groups; graph modification problems and structural graph problems. With respect to graph modification problems, we study problems where one is to remove a small set of vertices in order to break the graph into small connected components. We also study problems where, instead of deleting vertices, we are to add and/or remove a small number of edges in order to obtain a graph that adheres to a specific set of properties. We resolve several questions in the literature with respect to modifying a graph to a threshold graph or to a chain graph. We prove that editing to such graphs is NP-hard and, under the widely believed Exponential Time Hypothesis (ETH), not solvable in 2o(√k) · nO(1) time. We also provide polynomial kernels and subexponential time parameterized algorithms running in time 2O(√k log k) + nO(1) for all three edge modification variants into both graph classes. We also consider edge modifications into H-free graphs, where H is any finite set of forbidden induced subgraphs, on bounded degree input graphs. We prove that for a fixed maximum degree Δ, both edge editing and edge deletion to H-free graphs in at most k operations, admit polynomial kernels with kO(Δ log Δ) vertices. Then, via the framework of cross-compositions we prove that there is a finite set H, such that completion to H-free graphs does not admit a polynomial kernelization algorithm on bounded degree graphs, when parameterized by the bound on the number operations k, unless NP ⊆ coNP/poly. With respect to structural graph problems, we first provide several results for bandwidth. We prove that, assuming ETH, there is no significant improvement over the classic dynamic programming algorithm by Saxe [SIAM’80]. In particular, we prove that, assuming ETH, there is no f(b)no(b) time algorithm for deciding whether the bandwidth of a graph is at most b. This result remains true when restricted to trees of pathwidth at most 2. By the same reduction, we prove that deciding whether the bandwidth of a graph is at most b, when parameterized by b, is W[1]-hard when restricted to the same set of trees. Furthermore, we provide the first approximation algorithm for computing the bandwidth of trees, where the approximation factor depends solely on b. We then extend this result to graphs of bounded treelength, a rich graph class containing among others chordal graphs and graphs of bounded hyperbolicity. We also provide a characterization of graphs of small bandwidth for the same graph classes. In particular, the most general of these results states that a graph of bounded treelength can only have high bandwidth if it has high local density or high pathwidth, or if it contains a slight modification of a bandwidth obstruction introduced by Chung and Seymour [Discrete Mathematics’89]. Finally, we provide a constant factor approximation algorithm for computing the treewidth of a graph that runs in O(ckn) time. The algorithm either provides a tree decomposition of width 5k + 4 or concludes that the treewidth of the input graph is larger than k. This algorithm improves several known results, like the one by Robertson and Seymour [JCTB’95] and Reed [STOC’92] and Amir [Algorithmica’10]. We point out that there are many important problems in the literature, for example computing a vertex cover, a dominating set or a steiner tree of a graph, that can be solved in O(ckn) time if provided a tree decomposition of width at most O(k). The algorithm presented, is the first that can provide such algorithms with a tree decomposition of sufficiently small width, without being the bottleneck of the composed algorithm: That is, an algorithm that first computes a decomposition of width O(k) and then solves the problem in O(ckn) time
Constraining the K ¯ N coupled channel dynamics using femtoscopic correlations at the LHC
No full textAbstract The interaction of K - with protons is characterised by the presence of several coupled channels, systems like K ¯ 0 n and \uppi \Sigma π Σ with a similar mass and the same quantum numbers as the K - p state. The strengths of these couplings to the K - p system are of crucial importance for the understanding of the nature of the Λ ( 1405 ) resonance and of the attractive K - p strong interaction. In this article, we present measurements of the K - p correlation functions in relative momentum space obtained in pp collisions at s = 13 Te, in p–Pb collisions at s NN = 5.02 Te, and (semi)peripheral Pb–Pb collisions at s NN = 5.02 Te. The emitting source size, composed of a core radius anchored to the K + p correlation and of a resonance halo specific to each particle pair, varies between 1 and 2 fm in these collision systems. The strength and the effects of the K ¯ 0 n and \uppi \Sigma π Σ inelastic channels on the measured K - p correlation function are investigated in the different colliding systems by comparing the data with state-of-the-art models of chiral potentials. A novel approach to determine the conversion weights ω , necessary to quantify the amount of produced inelastic channels in the correlation function, is presented. In this method, particle yields are estimated from thermal model predictions, and their kinematic distribution from blast-wave fits to measured data. The comparison of chiral potentials to the measured K - p interaction indicates that, while the \uppi \Sigma π Σ – K - p dynamics is well reproduced by the model, the coupling to the K ¯ 0 n channel in the model is currently underestimated
Study of the p–p–K and p–p–K dynamics using the femtoscopy technique
No full textThe interactions of kaons (K) and antikaons (K)
with few nucleons (N) were studied so far using kaonic
atom data and measurements of kaon production and interaction yields in nuclei. Some details of the three-body KNN
and KNN dynamics are still not well understood, mainly
due to the overlap with multi-nucleon interactions in nuclei.
An alternative method to probe the dynamics of three-body
systems with kaons is to study the final state interaction
within triplet of particles emitted in pp collisions at the Large
Hadron Collider, which are free from effects due to the presence of bound nucleons. This Letter reports the first femtoscopic study of p–p–K+ and p–p–K− correlations measured in high-multiplicity pp collisions at √s = 13 TeV
by the ALICE Collaboration. The analysis shows that the
measured p–p–K+ and p–p–K− correlation functions can be
interpreted in terms of pairwise interactions in the triplets,
indicating that the dynamics of such systems is dominated
by the two-body interactions without significant contributions from three-body effects or bound states
Photoproduction of KK Pairs in Ultraperipheral Collisions
No full textKK pairs may be produced in photonuclear collisions, either from the decays of photoproduced ϕ(1020) mesons or directly as nonresonant KK pairs. Measurements of KK photoproduction probe the couplings between the ϕ(1020) and charged kaons with photons and nuclear targets. The kaon-proton scattering occurs at energies far above those available elsewhere. We present the first measurement of coherent photoproduction of KK pairs on lead ions in ultraperipheral collisions using the ALICE detector, including the first investigation of direct KK production. There is significant KK production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range 1.1 < M < 1.4 GeV/c above the ϕ(1020) resonance, for rapidity |y| < 0.8 and p < 0.1 GeV/c, the measured coherent photoproduction cross section is dσ/dy = 3.37±0.61(stat)±0.15(syst) mb. The center-of-mass energy per nucleon of the photon-nucleus (Pb) system W ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for ϕ(1020) photoproduction alone. The mass spectrum is fit to a cocktail consisting of ϕ(1020) decays, direct KK photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct KK photoproduction are presented
Charged particle production in S-S collisions at 200 GeV/c per nucleon
No full textCharged particle production in central S-S collisions at 200 GeV/c per nucleon has been studied by the WA94 experiment at the CERN-SPS. Particle identification has been provided by the Omega RICH, while a silicon telescope in the Omega spectrometer and an array of MultiWire Proportional Chambers have been used to trace particles through the RICH detector. Production ratios and transverse mass spectra for pi(+/-), K+/- and p((p) over bar) at central rapidity and p(T) > 1.3 GeV/c are presented. (C) 1997 Elsevier Science B.V
Photoproduction of K+K− pairs in ultra-peripheral collisions
No full textK+K− pairs may be produced in photonuclear collisions, either from the decays of photoproduced ϕ(1020) mesons, or directly as non-resonant K+K− pairs. Measurements of K+K− photoproduction probe the couplings between the ϕ(1020) and charged kaons with photons and nuclear targets. We present the first measurement of coherent photoproduction of K+K− pairs on lead ions in ultra-peripheral collisions using the ALICE detector, including the first investigation of direct K+K− production. There is significant K+K− production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range 1.1<MKK<1.4 GeV/c2 above the ϕ(1020) resonance, for rapidity |yKK|<0.8 and pT,KK<0.1 GeV/c, the measured coherent photoproduction cross section is dσ/dy = 3.37 ± 0.61 (stat.) ± 0.15 (syst.) mb. The centre-of-mass energy per nucleon of the photon-nucleus (Pb) system WγPb,n ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for ϕ(1020) photoproduction alone. The mass spectrum is fit to a cocktail consisting of ϕ(1020) decays, direct K+K− photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct K+K− photoproduction are presented
Photoproduction of KK pairs in ultra-peripheral collisions
No full textInternational audienceKK pairs may be produced in photonuclear collisions, either from the decays of photoproduced mesons, or directly as non-resonant KK pairs. Measurements of KK photoproduction probe the couplings between the and charged kaons with photons and nuclear targets. We present the first measurement of coherent photoproduction of KK pairs on lead ions in ultra-peripheral collisions using the ALICE detector, including the first investigation of direct KK production. There is significant KK production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range GeV/ above the resonance, for rapidity and GeV/, the measured coherent photoproduction cross section is = 3.37 (stat.) (syst.) mb. The centre-of-mass energy per nucleon of the photon-nucleus (Pb) system ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for photoproduction alone. The mass spectrum is fit to a cocktail consisting of decays, direct KK photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct KK photoproduction are presented
Strange particle production p-Be, p-Pb, Pb-Pb at 158 A GeV/c (WA97 experiment)
No full textThe CERN experiment WA97, which studies strange particle production at central rapidity in Pb-Pb, p-Pb, p-Be collisions at 158 A GeV/c, has already reported a pronounced enhancement of hyperon production. This is considered as a sensitive signature for a phase transition to a new state of matter-the quark gluon plasma (QGP). A comprehensive study, including the most recent results, of yields and transverse mass spectra of K/sub S//sup 0/, Lambda , Xi /sup -/, Omega /sup -/ (and antiparticles) and negative particles as a function of the number of nucleons participating in the collisions is presented. (11 refs)
KK and KK femtoscopy in pp collisions at 5.02 and 13 TeV
No full textFemtoscopic correlations with the particle pair combinations KS0KS0 and KS0K± are studied in pp collisions at s=5.02 and 13 TeV by the ALICE experiment. At both energies, boson source parameters are extracted for both pair combinations, by fitting models based on Gaussian size distributions of the sources, to the measured two-particle correlation functions. The interaction model used for the KS0KS0 analysis includes quantum statistics and strong final-state interactions through the f0(980) and a0(980) resonances. The model used for the KS0K± analysis includes only the final-state interaction through the a0 resonance. Source parameters extracted in the present work are compared with published values from pp collisions at s=7 TeV and the different pair combinations are found to be consistent. From the observation that the strength of the KS0KS0 correlations is significantly greater than the strength of the KS0K± correlations, the new results are compatible with the a0 resonance being a tetraquark state of the form (q1,q2‾,s,s‾), where q1 and q2 are u or d quarks.Femtoscopic correlations with the particle pair combinations KK and KK are studied in pp collisions at and TeV by the ALICE experiment. At both energies, boson source parameters are extracted for both pair combinations, by fitting models based on Gaussian size distributions of the sources, to the measured two-particle correlation functions. The interaction model used for the KK analysis includes quantum statistics and strong final-state interactions through the and resonances. The model used for the KK analysis includes only the final-state interaction through the resonance. Source parameters extracted in the present work are compared with published values from pp collisions at 7 TeV and the different pair combinations are found to be consistent. From the observation that the strength of the KK correlations is significantly greater than the strength of the KK correlations, the new results are compatible with the resonance being a tetraquark state of the form , where and are or quarks
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