Latest results from NA60

The NA60 experiment has measured the production of muon pairs and of charged particles in In+In collisions at a beam energy of 158 AGeV. For invariant dimuon masses below the phi the space-time averaged rho spectral function was isolated by a novel procedure. It shows a strong broadening but essentially no shift in mass. The production of J/psi was measured as a function of the collision centrality. As in previous experiments studying Pb+Pb collisions an anomalous supression is observed, setting in at approximately 90 participant nucleons. Using the charged particles the reaction plane was reconstructed. The elliptic flow of charged particles increases with pt showing a saturation for pt > 2GeV/c. For the first time azimuthal distributions for J/psi are shown.Comment: 9 pages, 11 figures, talk given at the conference "Strangeness in Quark Matter 2006 (SQM2006)", March 2006, Los Angeles, USA, accepted for publication in Journal of Physics

Evidence for radial flow of thermal dileptons in high-energy nuclear collisions

The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158 AGeV In-In collisions. An excess of pairs above the known meson decays has been reported before. We now present precision results on the associated transverse momentum spectra. The slope parameter Teff extracted from the spectra rises with dimuon mass up to the rho, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.Comment: Accepted for publication in Physical Review Letter

First Measurement of the rho Spectral Function in High-Energy Nuclear Collisions

We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360 000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi+pi-->rho-->mu+mu- annihilation. The associated space-time averaged rho spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masse

NA60 results on thermal dimuons

The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158A GeV In-In collisions. A strong excess of pairs above the known sources is observed in the whole mass region 0.2<M<2.6 GeV. The mass spectrum for M<1 GeV is consistent with a dominant contribution from pi+pi- -> rho -> mu+mu- annihilation. The associated rho spectral function shows a strong broadening, but essentially no shift in mass. For M>1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differences to Drell-Yan pairs. The slope parameter Teff associated with the transverse momentum spectra rises with mass up to the rho, followed by a sudden decline above. The rise for M<1 GeV is consistent with radial flow of a hadronic emission source. The seeming absence of significant flow for M>1 GeV and its relation to parton-hadron duality is discussed in detail, suggesting a dominantly partonic emission source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Planck-like shape of the mass spectra at low pT and the lack of polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on omega in-medium effects.Comment: 10 pages, 12 figures, submitted to Eur. Phys. J.

First results from NA60 on low mass muon pair production in In-In collisions at 158 GeV/nucleon

The NA60 experiment at the CERN SPS studies dimuon production in proton-nucleus and nucleus-nucleus collisions. The combined information from a novel vertex telescope made of radiation-tolerant silicon pixel detectors and from the muon spectrometer previously used in NA50 allows for a precise measurement of the muon vertex and a much improved dimuon mass resolution. We report on first results from the data taken for Indium-Indium collisions at 158 AGeV/nucleon in 2003, concentrating on a subsample of about 370 000 muon pairs in the mass range $<1.2$ GeV/$c^{2}$. The light vector mesons $\omega$ and $\phi$ are completely resolved, with a mass resolution of about 23 MeV/$c^{2}$ at the $\phi$. The transverse momentum spectra of the $\phi$ are measured over the continuous range $0<p_{\rm T}<2.5$ GeV/c; the inverse slope parameter of the spectra is found to increase with centrality, with an average value of $T=252\pm3$ MeV.Comment: 9 pages, 6 figures. Plenary talk, SQM2004 conference, Cape Town, South Africa 15-20 September, 2004. To be published in Journal of Physics G: Nuclear and Particle Physic

Low Mass Dimuon Production in Indium-Indium Collisions at the CERN SPS

NA60 is a fixed-target experiment at the CERN SPS which measured dimuon production in nucleus-nucleus and proton-nucleus collisions. In this paper we report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions. A significant excess of pairs is observed above the yield expected from neutral meson decays. The excess can be isolated by subtraction of expected sources, thanks to the excellent mass resolution and large sample size.Comment: 4 pages, 3 figures, Contribution at XLIst Rencontres de Moriond, "QCD and High Energy Hadronic Interactions

NA60 results on pTp_T spectra and the ρ\rho spectral function in In-In collisions

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158 AGeV In-In collisions. A strong excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of close to 400K events and the good mass resolution of about 2% have made it possible to isolate the excess by subtraction of the decay sources (keeping the $\rho$). The shape of the resulting mass spectrum exhibits considerable broadening, but essentially no shift in mass. The acceptance-corrected transverse-momentum spectra have a shape atypical for radial flow and show a significant mass dependence, pointing to different sources in different mass regions.Comment: 4 pages, 4 figures, Quark Matter 2006 conference proceeding

Study of the electromagnetic transition form-factors in \eta -> \mu^+\mu^-\gamma and \omega -> \mu^+\mu^-\pi^0 decays with NA60

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158A GeV In-In collisions. The mass and pT 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_{\mu\mu}^{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.)]*10^{-4} has been obtained as a byproduct.Comment: Submitted to Phys. Lett.

Intermediate mass dimuon production in Indium-Indium collisions at the CERN SPS

We present preliminary results on the production of intermediate mass dimuons in 158 A·GeV In-In collisions at the CERN SPS. NA38 and NA50 observed a strong excess in this region in S-U and Pb-Pb interactions with respect to the dimuon production rate expected from p-A data. Thanks to the use of a pixel vertex tracker, NA60 can separate the prompt dimuons from the pairs resulting from open charm decays and show that the excess dimuons are of prompt origin

Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV

A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7 TeV is presented. The data were collected at the LHC, with the CMS detector, and correspond to an integrated luminosity of 4.6 inverse femtobarns. No significant excess is observed above the background expectation, and upper limits are set on the Higgs boson production cross section. The presence of the standard model Higgs boson with a mass in the 270-440 GeV range is excluded at 95% confidence level.Comment: Submitted to JHE