Results on angular distributions of thermal dileptons in nuclear collisions

The NA60 experiment at the CERN SPS has studied dimuon production in 158 AGeV In-In collisions. The strong pair excess above the known sources found in the mass region $0.2<M<2.5$ GeV has been previously interpreted as thermal radiation. In this paper results on the associated angular distributions for $M<1$ GeV, as measured in the Collins-Soper reference frame, are presented. The structure function parameters $\lambda$, $\mu$, $\nu$ are consistent with zero and the projected polar and azimuth angle distributions are uniform. The absence of any polarization is consistent with the interpretation of the excess dimuons as thermal radiation from a randomized system.Comment: 4 pages, 4 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Accessing the strong interaction between baryons and charged kaons with the femtoscopy technique at the LHC

The interaction between Λ baryons and kaons/antikaons is a crucial ingredient for the strangeness S=0 and S=−2 sector of the meson–baryon interaction at low energies. In particular, the ΛK ̅ might help in understanding the origin of states such as the Ξ(1620), whose nature and properties are still under debate. Experimental data on Λ–K and Λ–K ̅ systems are scarce, leading to large uncertainties and tension between the available theoretical predictions constrained by such data. In this Letter we present the measurements of Λ–K⊕+Λ ̅–K− and Λ–K⊕−Λ ̅–K+ correlations obtained in the high-multiplicity triggered data sample in pp collisions at s=13 TeV recorded by ALICE at the LHC. The correlation function for both pairs is modeled using the Lednický–Lyuboshits analytical formula and the corresponding scattering parameters are extracted. The Λ–K⊕−Λ ̅–K+ correlations show the presence of several structures at relative momenta k⁎ above 200 MeV/c, compatible with the Ω baryon, the Ξ(1690), and Ξ(1820) resonances decaying into Λ–K− pairs. The low k⁎ region in the Λ–K⊕−Λ ̅–K+ also exhibits the presence of the Ξ(1620) state, expected to strongly couple to the measured pair. The presented data allow to access the ΛK+ and ΛK− strong interaction with an unprecedented precision and deliver the first experimental observation of the Ξ(1620) decaying into ΛK−

Measurement of the Λ hyperon lifetime

A new, more precise measurement of the Λ hyperon lifetime is performed using a large data sample of Pb-Pb collisions at sNN=5.02 TeV with ALICE. The Λ and Λ ̄ hyperons are reconstructed at midrapidity using their two-body weak decay channel Λ→p+π- and Λ ̄→p ̄+π+. The measured value of the Λ lifetime is τΛ=[261.07±0.37(stat.)±0.72(syst.)] ps. The relative difference between the lifetime of Λ and Λ ̄, which represents an important test of CPT invariance in the strangeness sector, is also measured. The obtained value (τΛ-τΛ ̄)/τΛ=0.0013±0.0028(stat.)±0.0021(syst.) is consistent with zero within the uncertainties. Both measurements of the Λ hyperon lifetime and of the relative difference between τΛ and τΛ ̄ are in agreement with the corresponding world averages of the Particle Data Group and about a factor of three more precise

Energy dependence of coherent photonuclear production of J/ψ mesons in ultra-peripheral Pb-Pb collisions at √sNN = 5.02 TeV

The cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of sNN = 5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y| &lt; 4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17 &lt; Wγ Pb,n &lt; 920 GeV, where Wγ Pb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1 × 10 −5 &lt; x &lt; 3.3 × 10 −2. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies

Measurements of Groomed-Jet Substructure of Charm Jets Tagged by D0 Mesons in Proton-Proton Collisions at √s=13 TeV

Understanding the role of parton mass and Casimir color factors in the quantum chromodynamics parton shower represents an important step in characterizing the emission properties of heavy quarks. Recent experimental advances in jet substructure techniques have provided the opportunity to isolate and characterize gluon emissions from heavy quarks. In this Letter, the first direct experimental constraint on the charm-quark splitting function is presented, obtained via the measurement of the groomed shared momentum fraction of the first splitting in charm jets, tagged by a reconstructed D0 meson. The measurement is made in proton-proton collisions at s=13 TeV, in the low jet transverse-momentum interval of 15≤pTjet ch&lt;30 GeV/c where the emission properties are sensitive to parton mass effects. In addition, the opening angle of the first perturbative emission of the charm quark, as well as the number of perturbative emissions it undergoes, is reported. Comparisons to measurements of an inclusive-jet sample show a steeper splitting function for charm quarks compared with gluons and light quarks. Charm quarks also undergo fewer perturbative emissions in the parton shower, with a reduced probability of large-angle emissions

Multiplicity and rapidity dependence of K∗(892)0 and φ(1020) production in p–Pb collisions at √sNN = 5.02 TeV

The transverse-momentum (pT) spectra of K ∗(892)0 and φ(1020) measured with the ALICE detector up to pT = 16 GeV/c in the rapidity range - 1.2 &lt; y&lt; 0.3 , in p–Pb collisions at the center-of-mass energy per nucleon–nucleon collision sNN=5.02 TeV are presented as a function of charged particle multiplicity and rapidity. The measured pT distributions show a dependence on both multiplicity and rapidity at low pT whereas no significant dependence is observed at high pT . A rapidity dependence is observed in the pT -integrated yield (dN/dy), whereas the mean transverse momentum (⟨ pT⟩) shows a flat behavior as a function of rapidity. The rapidity asymmetry (Yasym) at low pT (&lt; 5 GeV/c) is more significant for higher multiplicity classes. At high pT , no significant rapidity asymmetry is observed in any of the multiplicity classes. Both K ∗(892)0 and φ(1020) show similar Yasym . The nuclear modification factor (QCP) as a function of pT shows a Cronin-like enhancement at intermediate pT , which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high pT (&gt; 5 GeV/ c), the QCP values are greater than unity and no significant rapidity dependence is observed

Study of the p-p-K+ and p-p-K- dynamics using the femtoscopy technique

The 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 K ̄ NN 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

Measurement of non-prompt D0-meson elliptic flow in Pb-Pb collisions at √sNN=5.02 TeV

The elliptic flow (v2) of D mesons from beauty-hadron decays (non-prompt D) was measured in midcentral (30–50%) Pb–Pb collisions at a centre-of-mass energy per nucleon pair sNN=5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed at midrapidity (| y| &lt; 0.8) from their hadronic decay D → K -π + , in the transverse momentum interval 2 &lt; pT&lt; 12 GeV/c. The result indicates a positive v2 for non-prompt D mesons with a significance of 2.7 σ . The non-prompt D -meson v2 is lower than that of prompt non-strange D mesons with 3.2 σ significance in 2&lt;8GeV/c , and compatible with the v2 of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties

K*(892)0 and φ(1020) production in p-Pb collisions at √sNN=8.16 TeV

The production of K∗(892)0and φ(1020)resonances has been measured in p-Pb collisions at sNN = 8.16 TeV using the ALICE detector. Resonances are reconstructed via their hadronic decay channels in the rapidity interval -0.5 &lt; y &lt; 0 and the transverse momentum spectra are measured for various multiplicity classes up to pT = 20 GeV/c for K∗(892)0and pT = 16 GeV/c for φ(1020). The pT-integrated yields and mean transverse momenta are reported and compared with previous results in pp, p-Pb and Pb-Pb collisions. The xT scaling for K∗(892)0and φ(1020)resonance production is newly tested in p-Pb collisions and found to hold in the high-pT region at Large Hadron Collider energies. The nuclear modification factors (RpPb) as a function of pT for K∗0 and φ at sNN = 8.16 TeV are presented along with the new RpPb measurements of K∗0, φ, Ξ, and ω at sNN = 5.02 TeV. At intermediate pT (2-8 GeV/c), RpPb of Ξ, ω show a Cronin-like enhancement, while K∗0 and φ show no or little nuclear modification. At high pT (&gt;8 GeV/c), the RpPb values of all hadrons are consistent with unity within uncertainties. The RpPb of K∗(892)0and φ(1020)at sNN = 8.16 and 5.02 TeV show no significant energy dependence