The NA60 experiment has studied low-mass muon pair production in proton–nucleus collisions with a system of Be, Cu, In, W, Pb and U targets, using a 400 GeV proton beam at the CERN SPS. The transverse momentum spectra of the $$\rho /\omega $$ and $$\phi $$ mesons are measured in the full $$p_{\mathrm {T}}$$ range accessible, from $$p_{\mathrm {T}}= 0$$ up to $$2 \, {\hbox {GeV/c}}$$ . The nuclear dependence of the production cross sections of the $$\eta $$ , $$\omega $$ and $$\phi $$ mesons has been found to be consistent with the power law $$\sigma _{\mathrm {pA}} \propto {\mathrm {A}}^\alpha $$ , with the $$\alpha $$ parameter increasing as a function of $$p_{\mathrm {T}}$$ for all the particles, and an approximate hierarchy $$\alpha _\eta \approx \alpha _\phi > \alpha _\omega $$ . The cross section ratios $$\sigma _\eta /\sigma _\omega $$ , $$\sigma _\rho /\sigma _\omega $$ and $$\sigma _\phi /\sigma _\omega $$ have been studied as a function of the size A of the production target, and an increase of the $$\eta $$ and $$\phi $$ yields relative to the $$\omega $$ is observed from p–Be to p–U collisions.

presented by Albert Romana for the NASO Collaboration M.C. Abreu6•0, B . Alessandro1 1 , C. Alexa3, R. Amaldi11 , J. Astruc8 , M. Atayan13, C. Baglin1 , A . Baldit2, M. Bedjidian12, F . Bellaiche12, S . Beoie11 , V . Boldea3, P. Bordalo8·b, A . Bussiilre1, V. Capony1 , L. Casagrande8, J. Castor2, T. Chambon2, B. Chaurand9, I. Chevrot2, B. Cheynis12, E. Chiavassa1 1 , C. Cicalo4 , M.P. Comets8, S. Constantinescu3, J. Cruz8 , A. De Falco\ N. De Marco11, G. Dellacasa11·<, A. Devaux2, S. Dita3, 0. Drapier12, B. Espagnon2, J.Fargeix2, S.N. Filippov7, F. Fleuret9 , P. Force2, M. Gallio11 , Y.K. Gavrilov7, C. Gerschel8, P. Giubellino11, :M.B. Golubeva7, M. Gonin9 , A.A. Grigorian13, J.Y. Grossiord12, F.F. Guber7, A. Guichard12, H. Gulkanian13, R. Hakobyan13, R. Haroutunian12, M. ldzik11•d, D. Jouan8, T.L. Karavitcheva7, L. Kluberg9, A.B. Kurepin7, Y. Le Bornec8, C. Louren�o5, M. :Mac Cormick8, P. ::vlacciotta4 , A. Marzari-Chiesa11 , M . Masera1 1 , A . :Masoni4 , S . Mehrabian13 , S. Mourgues2, A . :Musso11, F . Ohlsson-:Malek12·•, P. Petiau9, A. Piccotti1 1 , J.R. Pizzi12, W.L. Prado Da Silva11·f, G. Puddu4, C. Quintans8, C. Racca10, L. Ramello11 ·c , S. Ramos8•b, P. Rato-Mendes11 , L. Riccati1 1 , A. Romana9, S. Sartori1 1 , P. Satumini2 , E. Scomparin5·9, S. Serci4 , R. Shahoyan8·h, S. Silva8, C. Soave11 , P. Sonderegger5·b, X. Tarrago8, P. Temnikov4 , N.S. Topilskaya7, G.L. Usai4, C. Vale8, E. Vercellin11 and N. Willis8 • 1 LAPP, CNRS-IN2P3, Annecy-LeVieux, France; 2 LPG, Uni11ersite Blaise Pascal and CNRS-IN2P3, Aubiere, France; 3 !FA, Bucharest, Romania; 4 Unillersitd di Cagliari and INFN, Cagliari, Italy; 5 CERN, Gene11a, Switzerland; 8 LIP, Lisbon, Portugal; 7 INR, Moscow, Russia; 8 JPN, Uni11ersite Paris-Sud and CNRS-IN2P3 , Orsay, France; 9 LPNHE, Ecole Polytechnique and CNRS-IN2P3 , Palaiseau, France; 10 IReS, Universite Louis Pasteur and CNRS-IN2P3, Strasbourg, France; 1 14 Uni11ersita di Torino and INFN, Torino, Italy; 12 JPN, Uni11ersite Claude Bernard and CNRS-IN2P3 , Villeurbanne, France; 13 YerPhi, Yerevan, Armenia. • also at FCUL, Unillersidade de Lisboa, Lisbon,Portugal; b also at !ST, Universidade Tecnica de Lisboa, Lisbon,Portugal; c Dipartimento di Scienze e Tecnologie Avanzate, II Facolta di Scienze, Alessandria, Italy; d now at Faculty of Physics and Nuclear Techniques, University of Mining and Metallurgy, Cracow, Poland; • now at !SN, Uni11ersite Joseph Fourier and CNRS-IN2P3 , Grenoble, France; I now at UERJ, Rio de Janeiro, Brazil; 9 on leave of absence from INFN Torino, Italy; h on lea11e of absence from YerPhi, Yerevan, Armenia.

ψ production is studied in Pb-Pb collisions at 158 GeV/c per nucleon incident momentum. Absolute cross-sections are measured and production rates are investigated as a function of the centrality of the collision. The results are compared with those obtained for lighter colliding systems and also for the J/ψ meson produced under identical conditions. Accepted for publication in European Physics Journal C 1 Universitá del Piemonte Orientale, Alessandria and INFN-Torino, Italy 2 LPC, Univ. Blaise Pascal and CNRS-IN2P3, Aubière, France 3 IFA, Bucharest, Romania 4 Università di Cagliari/INFN, Cagliari, Italy 5 CERN, Geneva, Switzerland 6 LIP, Lisbon, Portugal 7 INR, Moscow, Russia 8 IPN, Univ. de Paris-Sud and CNRS-IN2P3, Orsay, France 9 LLR, Ecole Polytechnique and CNRS-IN2P3, Palaiseau, France 10 Università di Torino/INFN, Torino, Italy 11 IPN, Univ. Claude Bernard Lyon-I and CNRS-IN2P3, Villeurbanne, France 12 YerPhI, Yerevan, Armenia a) also at IST, Universidade Técnica de Lisboa, Lisbon, Portugal b) also at Faculty of Physics and Nuclear Techniques, AGH University of Science and Technology, Cracow, Poland c) on leave of absence of YerPhI, Yerevan, Armenia d) Deceased Dedicated to the memory of our colleagues Albert Romana and Jean René Pizzi, whose untimely death occurred while finalizing this article.

The NA60 experiment has studied J/ψ production in p–A collisions at 158 and 400 GeV, at the CERN SPS. Nuclear effects on the J/ψ yield have been estimated from the A-dependence of the production cross section ratios σJ/ψA/σJ/ψBe (A = Al, Cu, In, W, Pb, U). We observe a significant nuclear suppression of the J/ψ yield per nucleon–nucleon collision, with a larger effect at lower incident energy, and we compare this result with previous observations by other fixed-target experiments. An attempt to disentangle the different contributions to the observed suppression has been carried out by studying the dependence of nuclear effects on x2, the fraction of the nucleon momentum carried by the interacting parton in the target nucleus.

The NA60 experiment at the CERN SPS has studied phi meson production in In-In collisions at 158 AGeV via both the K+K- and the mu(+)mu(-) decay channels. The yields and inverse slope parameters of the m(T) spectra observed in the two channels are compatible within errors, different from the large discrepancies seen in Pb-Pb collisions between the hadronic (NA49) and dimuon (NA50) decay channels. Possible physics implications are discussed. (C) 2011 Elsevier B.V. All rights reserved.

The inclusive charged particle transverse momentum distribution is measured in proton-proton collisions at root s = 900 GeV at the LHC using the ALICE detector. The measurement is performed in the central pseudorapidity region (vertical bar eta vertical bar < 0.8) over the transverse momentum range 0.15 < p(T) < 10 GeV/c. The correlation between transverse momentum and particle multiplicity is also studied. Results are presented for inelastic (INEL) and non-single-diffractive (NSD) events. The average transverse momentum for vertical bar eta vertical bar < 0.8 is < p(T)>(INEL) = 0.483 +/- 0.001 (stat.) +/- 0.007 (syst.) GeV/c and < p(T)>(NSD) = 0.489 +/- 0.001 (stat.) +/- 0.007 (syst.) GeV/c, respectively. The data exhibit a slightly larger < p(T)> than measurements in wider pseudorapidity intervals. The results are compared to simulations with the Monte Carlo event generators PYTHIA and PHOJET. (C) 2010 Published by Elsevier B.V.

We report on the measurement of two-pion correlation functions from pp collisions at {radical}(s)=900 GeV performed by the ALICE experiment at the Large Hadron Collider. Our analysis shows an increase of the Hanbury Brown-Twiss radius with increasing event multiplicity, in line with other measurements done in particle- and nuclear collisions. Conversely, the strong decrease of the radius with increasing transverse momentum, as observed at the Relativistic Heavy Ion Collider and at Tevatron, is not manifest in our data.

The NA60 experiment at the CERN SPS studied phi production in In–In collisions at 158 A GeV via muon and kaon decay channels. The yields and transverse mass spectra observed in the two channels are compatible within errors. The results are compared to the previous measurements in Pb–Pb collisions, where large discrepancies were observed between NA50 (muon pairs) and NA49 (kaon pairs).

The J/ψ azimuthal distribution relative to the reaction plane has been measured by the NA50 experiment in Pb-Pb collisions at 158 GeV/nucleon. Various physical mechanisms related to charmonium dissociation in the medium created in the heavy ion collision are expected to introduce an anisotropy in the azimuthal distribution of the observed J/ψ mesons at SPS energies. Hence, the measurement of J/ψ elliptic anisotropy, quantified by the Fourier coefficient v_2 of the J/ψ azimuthal distribution relative to the reaction plane, is an important tool to constrain theoretical models aimed at explaining the anomalous J/ψ suppression observed in Pb-Pb collisions. We present the measured J/ψ yields in different bins of azimuthal angle relative to the reaction plane, as well as the resulting values of the Fourier coefficient v_2 as a function of the collision centrality and of the J/ψ transverse momentum. The reaction plane has been estimated from the azimuthal distribution of the neutral transverse energy detected in an electromagnetic calorimeter. The analysis has been performed on a data sample of about 100 000 events, distributed in five centrality or p_ T sub-samples. The extracted v_2 values are significantly larger than zero for non-central collisions and are seen to increase with p_ T.

The yield of muon pairs in the invariant mass region 1 < M < 2.5 GeV/c (2) produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of prompt muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are independent of mass and significantly lower than those found at masses below 1 GeV/c (2), rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/c (2) to be of largely partonic origin, when radial flow has not yet built up.

The NA60 experiment at the CERN Super Proton Synchrotron has studied dimuon production in 158A GeV In-In collisions. The strong excess of pairs above the known sources found in the complete mass region 0.2<M<2.6 GeV has previously been interpreted as thermal radiation. We now present first results on the associated angular distributions. Using the Collins-Soper reference frame, the structure function parameters lambda, mu, and nu are measured to be zero, and the projected distributions in polar and azimuth angles are found to be uniform. The absence of any polarization is consistent with the interpretation of the excess dimuons as thermal radiation from a randomized system.

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158A GeV In-in collisions. The mass and p(T) spectra associated with peripheral collisions can quantitatively be described by the known neutral meson decays. The high data quality has allowed to remeasure the electromagnetic transition form factors of the Dalitz decays eta -> mu(+)mu(-)gamma and omega -> mu(+)mu(-)pi(0). Using the usual pole approximation F = (1 - M-2/Lambda(2))(-1) for the form factors, we find Lambda(-2) (in GeV-2) to be 1.95 +/- 0.17(stat.) +/- 0.05(syst.) for the eta and 2.24 +/- 0.06(stat.) +/- 0.02(syst.) for the omega. While the values agree with previous results from the Lepton-G experiment, the errors are greatly improved, confirming now on the level of 10 sigma the strong enhancement of the omega form factor beyond the expectation from vector meson dominance. An improved value of the branching ratio BR(omega -> mu(+)mu(-)pi(0)) = [1.73 +/- 0.25(stat.) +/- 0.14(syst.) 1 x 10(-4) has been obtained as a byproduct. (C) 2009 Elsevier B.V. All rights reserved.