Dimuon and charm production in nucleus-nucleus collisions at the CERN-SPS

Muon pair production in p-A, S-U and Pb-Pb collisions has been studied by the NA38 and NA50 collaborations at the CERN SPS. In this paper we present an analysis of the dimuon invariant mass region between the and the J/. In p-A collisions we find that, after the subtraction of the combinatorial background due to and K decays, the superposition of dimuons from semi-leptonic decays of D mesons and from the Drell-Yan process reproduces well the measured mass and transverse momentum spectra. The yield of open charm dimuons required to fit our data leads to a charm production cross section in good agreement with previous measurements. A linear extrapolation of the p-A sources with the product of the mass numbers of the projectile and target nuclei, , underestimates the dimuon yield measured in S-U and Pb-Pb collisions. The excess increases with the number of participant nucleons, and the ratio between the observed dimuon yield and the expected sources reaches a factor 2 for central Pb-Pb interactions. The kinematical distributions of the measured dimuon excess are compatible with those expected from the open charm contribution. Received: 20 December 1999 / Revised version: 4 February 2000 / Published online: 14 April 2000     

A first look at open charm production in Indium-Indium collisions at SPS energies

NA60 is an experiment at the CERN-SPS devoted to the study of dimuon production in heavy-ion and proton-nucleus collisions. The main topics under study are low mass vector meson production, J/ production and suppression, and the sources of the dimuon continuum in the mass range 1.2-2.7 GeV/c ². In 2003, NA60 collected million dimuon events from Indium-Indium collisions. We present preliminary results of the analysis of this data sample in view of measuring the open charm contribution to the dimuon spectrum. Although we are still working on the final background subtraction procedure, we can already demonstrate that the detector performance is good enough to allow the separation of prompt dimuons from muon pairs originating in distant D decays. Arrival of the final proofs: 1 July 2005 PACS: 25.75.Dw, 25.75.Nq     

φ meson production in NA60

NA60 is a fixed-target experiment at the CERN SPS which measured dimuon production in nucleus–nucleus and proton–nucleus collisions. The experiment collected muon pair samples of unprecedented quality in heavy-ion experiments. This paper presents a high quality measurement of the p_T distribution of the φ meson, covering a broad p_T window. The data were collected in 2003 in In-In collisions at 158 GeV per nucleon. The results, presented as a function of centrality, were studied against several possible sources of systematic effects and proved to be fairly stable. We show that the inverse m_T slope measured in In-In collisions, in the φ→μμ decay channel, depends significantly on the range used to perform the fit. When the fit is performed at low transverse momentum, the effective inverse slope increases from peripheral to central collisions, as measured by other experiments. We finally show that our measurement for In-In is compatible with the overall systematics of T slope versus mass, measured in different collision systems by the NA49 experiment PACS 25.75.Nq; 25.75.-q; 25.75.Dw; 14.40.Cs; 12.38.Mh     

Leading-order QCD analysis of neutrino-induced dimuon events

The results of a leading-order QCD analysis of neutrino-induced charm production are presented. They are based on a sample of 4111 - and 871 -induced opposite-sign dimuon events with , and , observed in the CHARM II detector exposed to the CERN wide band neutrino and antineutrino beams. The analysis yields the value of the charm quark mass and the Cabibbo–Kobayashi–Maskawa matrix element . The strange quark content of the nucleon is found to be suppressed with respect to non-strange sea quarks by a factor . Received: 1 February 1999 / Published online: 14 October 1999     

Wedgebox analysis of four-lepton events from neutralino pair production at the LHC

A ‘wedgebox’ plot is a two-dimensional scatter-plot of two invariant mass quantities. Here pp→e⁺e^-μ⁺μ^-+≠E signature LHC events are analyzed by plotting the di-electron invariant mass versus the di-muon invariant mass. Data sets of such events are obtained across the MSSM input parameter space in realistic event-generator simulations, including cuts designed to remove SM backgrounds. Their study reveals several general features. Firstly, regions in the MSSM input parameter space where a sufficient number of events are expected so as to be able to construct a clear wedgebox plot are delineated. Secondly, the presence of box shapes on a wedgebox plot either indicates the presence of heavy Higgs bosons’ decays or restricts the location to a quite small region of low μ and M₂ values, ≲ 200 GeV, a region denoted as the ‘lower island’. In this region, wedgebox plots can be quite complicated and change in pattern rather quickly as one moves around in the (μ,M₂) plane. Thirdly, direct neutralino pair production from an intermediate Z^0* may only produce a wedge shape since only decays can contribute significantly. And fourthly, a double-wedge or wedge-protruding-from-a-box pattern on a wedgebox plot, which results from combining a variety of MSSM production processes, yields three distinct observed endpoints, almost always attributable to decays, which can be utilized to determine a great deal of information about the neutralino and slepton mass spectra and related MSSM input parameters. Wedge and double-wedge patterns are seen in wedgebox plots in another region of higher μ and M₂ values, denoted as the ‘upper island’. Here the pattern is simpler and more stable as one moves across the (μ,M₂) input parameter space.     

Soft-particle spectra as a probe of interconnection effects in hadronic W^{+}W^{-} events

Cross-talk between the and sources of hadron production at LEP2 offers a hope to learn about basic properties of QCD, but at the same time threatens high-precision measurements of the W boson mass. Directly visible effects are not expected to be large, however. It is, therefore, important to develop methods to measure the level of interconnection in the data –‘connectometers’. In this article we study one particular set of observables, namely the reduced rate of low-momentum particle production caused by reconnection. In realistic scenarios the expected signals are at the border of visibility, but not quite beyond reach. Special studies on kaons and protons, or with other subsets of the data, could provide supporting evidence. We also show how LEP1 events could be used to define an alternative reference sample. Received: 1 April 1998 / Published online: 12 August 1998     

Charm dimuons and determination of the strange sea

We re-examine the standard charm model for opposite-sign dimuon events in neutrino-nucleon scattering. Published dimuon rates are shown to be consistent with a simple SU(3) symmetric sea. We discuss the types of correlated studies of different dimuon observables needed to extract unambiguous information on the nucleon's strange sea from upcoming, high statistics data. Our calculations demonstrate the sizeable effects of finite energy resolution and missing final-state energy on predictions of observed dimuon rates, particularly for data obtained from wide-band incident neutrino spectra. We emphasize the crucial role of the non-zero charm mass, not only for the analysis of dimuon events, but also for the interpretation of large-y inclusive $\text{v}\text{N}\text{→}\text{μ}\text{X}$ data.     

Charm dimuon production in neutrino-nucleon interactions in the NOMAD experiment

We present our new measurement of charm dimuon production in neutrino-iron interactions based upon the full statistics collected by the NOMAD experiment. After background subtraction we observe 15,340 charm dimuon events, providing the largest sample currently available. The analysis exploits the large inclusive charged current sample (about 9 million events after all analysis cuts) to constrain the total systematic uncertainty to ∼2%. The extraction of strange sea and charm production parameters is also discussed.     

Irreducible QCD backgrounds to top searches in semi-leptonic final states at the Next Linear Collider

At future electron-positron colliders, one of the largest irreducible backgrounds to top searches in the channel ‘4 jets + lepton + missing energy’ comes from QCD events of order . We compute here such processes exactly at the parton level by resorting to 2 6 matrix elements exploiting helicity amplitude techniques. We adopt a typical selection procedure based on the tagging of a high momentum and separated lepton. We finally outline kinematic differences between signal and background events that can be exploited to further reduce such a QCD noise. Received: 12 January 1999 / Revised version: 24 March 1999 / Published online: 20 May 1999     

Facet degradation of high-power diode laser arrays

Micro-Raman facet temperatures of high-power diode lasers with different waveguide architectures are compared. For regular operation conditions, the thermal behavior of ‘unaged’ arrays emitting in the 808-nm wavelength region with different architectures is similar, however, with an increased load thermal behaviors differ significantly and exhibit failure events at facet temperatures typically between 150 and 450 °C. From various experiments, among them facet temperature measurements for ultrahigh-power operation as well as by preparative failure analytics, we provide evidence that in arrays the front facets are significantly affected by device operation and influence the failure behavior of the whole high-power diode laser also in cases when the device failure is accompanied by dislocation creation inside the device. Received: 3 October 1999 / Accepted: 9 November 1999 / Published online: 8 March 2000     

Recent results on heavy-ion physics from the SPS

The CERN SPS heavy-ion physics program was recently given an important and fresh impetus with the running of the NA60 dimuon experiment, which probed indium-indium collisions at 158 GeV per incident nucleon (in 2003), as well as proton-nucleus collisions at 158 and 400 GeV (essentially in 2004). Several interesting physics results have been obtained and were recently presented by NA60. They address such varied physics topics as the search for in-medium modifications on the ρ short-lived vector meson (which could be related to the restoration of chiral symmetry, spontaneously broken in the hadronic world), the understanding of the “anomalous” J/ψ suppression (expected to be a signature of quark-gluon deconfinement), the search for thermal dimuons (presumably radiated from a thermal system, maybe composed of deconfined quarks and gluons — the “quark-gluon plasma”), the understanding of the enhancement of θ production in heavy-ion collisions, etc. These topics were previously studied by other SPS experiments, and very interesting observations were made, but serious doubts remained concerning the interpretation of those earlier results. It is remarkable that one single experiment, NA60, is able to provide high-quality information on each of these many topics, potentially triggering a very significant step forward in our understanding of “quark-matter physics”.In this paper, after a general introduction, I describe the NA60 apparatus, the data taking conditions, and the main steps in the data reconstruction procedure. I then give some information on the muon track matching and background subtraction procedures. In the remaining sections I review some of the results presently available from the on-going physics analyses, in what concerns the studies of low mass and intermediate mass dimuon production, and J/ψ suppression, in proton-nucleus and indium-indium collisions. These new (and still preliminary) results are placed in perspective, by recalling the findings of previous experiments.     

Investigation of the ALICE muon forward spectrometer performances for upsilon measurement

ALICE (A Large Ion Collider Experiment) is the LHC detector dedicated to the study of nucleus–nucleus collisions, in which the formation of the quark–gluon plasma (QGP) is expected. Heavy quarkonia, especially the Upsilon states, are relevant for studying the QGP since they provide an essential probe of the earliest and hottest stages of heavy ion collisions. They will be measured via their dimuon decay channel in ALICE in the muon spectrometer. The muon spectrometer performance has been studied in simulations, the results will be presented with emphasis on the trigger efficiency and rate in Pb–Pb collisions. The expected yields of Upsilon states will be extracted from a simulation based on a global fit of the dimuon mass spectra for different collision centralities.     

Strong pionic intermittency in ‘cold’ events in12C-AgBr interactions at 4.5 A GeV

In this paper intermittent behaviour of the pions from ‘cold’ and ‘hot’ classes of events from¹²C-AgBr interactions at 4.5 A GeV has been studied, separately. The results reveal strong intermittent pattern in case of ‘cold’ class of events.     

What is Erased in the Quantum Erasure?

In this paper, we re-examine a series of gedanken welcher Weg (WW) experiments introduced by Scully, Englert and Walther that contain the essential ideas underlying the quantum eraser. For this purpose we use the Bohm model which gives a sharp picture of the behaviour of the atoms involved in these experiments. This model supports the thesis that interference disappears in such WW experiments, even though the centre of mass wave function remains coherent throughout the experiment. It also shows exactly what it means to say ‘that the interference can be restored by manipulating the WW detectors long after the atoms have passed’. It does not support Wheeler’s notion that ‘the past is undefined and undefinable without the observation (in the present)’.     

Nanocrystalline silicon thin-film transistors with 50-nm-thick deposited channel layer, 10 cm2V-1s-1 electron mobility and 108 on/off current ratio

Thin-film transistors were made using 50-nm-thick directly deposited nanocrystalline silicon channel layers. The transistors have a coplanar top gate structure. The nanocrystalline silicon was deposited from discharges in silane, hydrogen and silicon tetrafluoride. The transistors combine a high electron field effect mobility of ∼10 cm² V^-1s^-1 with a low ‘off’ current of ∼10^-14 A per μm of channel length and an ‘on’/‘off’ current ratio of ∼10⁸. This result shows that transistors made from directly deposited silicon can combine high mobility with low ‘off’ currents. Received: 28 May 2001 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Against ‘Realism’

We examine the prevalent use of the phrase “local realism” in the context of Bell’s Theorem and associated experiments, with a focus on the question: what exactly is the ‘realism’ in ‘local realism’ supposed to mean? Carefully surveying several possible meanings, we argue that all of them are flawed in one way or another as attempts to point out a second premise (in addition to locality) on which the Bell inequalities rest, and (hence) which might be rejected in the face of empirical data violating the inequalities. We thus suggest that the phrase ‘local realism’ should be banned from future discussions of these issues, and urge physicists to revisit the foundational questions behind Bell’s Theorem.     

Low-coherence interferometry by intensity correlation

‘Low-coherence interferometry’ is an old technique which has had a wide development recently, and is based on the fact that interference with a path difference much longer that the coherence length gives rise to a ‘channeled spectrum’, which can be detected either by a dispersive spectroscope or by a second interferometer with a variable delay. We have tested an alternative way to detect path differences in this kind of interferometry, by analyzing the output intensity fluctuation correlations by a radiofrequency spectrum analyzer, and Fourier transforming the data. This method is suitable for very long path differences. The experiments have been performed with different lengths of single-mode fibre, in Mach–Zehnder and Fabry–Pérot configurations. Received: 30 March 2000 / Revised version: 17 Juli 2000 / Published online: 20 September 2000     

Contributions from SUSY-FCNC couplings to the interpretation of the HyperCP events for the decay Σ<Superscript>+</Superscript>→pμ<Superscript>+</Superscript>μ<Superscript>-</Superscript>

The observation of three events for the decay Σ⁺→pμ⁺μ^- with a dimuon invariant mass of 214.3 ± 0.5 MeV by the HyperCP Collaboration implies that a new particle X may be needed to explain the observed dimuon invariant mass distribution. We show that there are regions in the SUSY-FCNC parameter space where the A⁰ ₁ in the NMSSM can be used to explain the HyperCP events without contradicting all the existing constraints from the measurements of the kaon decays, and the constraints from K⁰–K̄⁰ mixing are automatically satisfied once the constraints from kaon decays are satisfied. PACS  14.80.Cp; 12.60.Jv; 14.20.Jn     

The dynamics of desorption induced by atomic hydrogen: HD/Cu(111)

Recent observations of a direct reaction between adsorbates and hydrogen atoms incident from the gas phase are interpreted in terms of an Eley-Rideal reaction. A detailed comparison of the experimental data for the HD/Cu(111) system with quantum mechanical model calculations corroborates such an interpretation. The peculiar isotope effect observed can be understood from the different dynamical implications of appropriately rescaled potential energy surfaces. The width of the measured time-of-flight spectrum is explained from the overlapping contributions of the populated vibrational levels. The angular distributions are rationalized by contributions both from ‘indirect’ events, where the incident atoms make several bounces in the surface well prior to reaction, and ‘direct’ reactive events.