2,023 research outputs found
Operational Experience with the ALICE Pixel detector
The Silicon Pixel Detector (SPD) constitutes the two innermost layers of the
Inner Tracking System of the ALICE experiment and it is the closest detector to
the interaction point. As a vertex detector, it has the unique feature of
generating a trigger signal that contributes to the L0 trigger of the ALICE
experiment. The SPD started collecting data since the very first pp collisions
at LHC in 2009 and since then it has taken part in all pp, Pb-Pb and p-Pb data
taking campaigns. This contribution will present the main features of the SPD,
the detector performance and the operational experience, including calibration
and optimization activities from Run 1 to Run 2
The heavy-ion programme of the ALICE experiment at LHC
The ALICE experiment at LHC is mainly dedicated to heavy-ion physics. An overview of its performances, some predictions related to its first measurements and QGP observable measurements will be given.The ALICE experiment at LHC is mainly dedicated to heavy-ion physics. An overview of its performances, some predictions related to its first measurements and QGP observable measurements will be given
φ(1020)-meson identification with the HMPID detector in the ALICE experiment
The ALICE experiment at CERN is devoted to study hadronic
matter at extreme conditions of temperature, energy density and its phase transition to QGP. The φ(1020)-meson is a good probe for studying the features of the quarkgluon
plasma. The ALICE detectors will identify particles at high momenta. In particular the HMPID would identify kaons with 1 ≤ p ≤ 3 GeV/c, therefore it will be possible to identify φ-mesons through the channel φ → K+K− up to 6 GeV/c. In this paper will be shown the physical motivations for the study of φ-meson and its invariant mass spectrum
On the different flavours of Lense–Thirring precession around accreting stellar mass black holes
Type-C quasi-periodic oscillations (QPOs) in X-ray binaries have been often interpreted as a consequence of relativistic Lense-Thirring precession around a spinning black hole and they potentially offer a way to measure black hole spins and masses. The connection between relativistic precession and the resulting QPOs has been made either in terms of a simplified model involving a single test particle producing the QPO, or in terms of a global model where a geometrically thick accretion flow precesses coherently as a rigid body. In this paper, we analyse similarities and differences between these two models, sometimes considered as in opposition to each other. We demonstrate that the former is the limiting case of the latter when the radial extent of the precessing flow is very small, and that solid lower limits to the black hole spin can be obtained by considering the test particle model alone. We also show that the global precession model naturally accounts for the range of frequencies observed for type-C QPOs without the need to invoke a truncation of the inner accretion flow before it reaches the innermost stable circular orbit. Finally, we show that, in order to maintain rigid precession, the thick accretion flow should be radially narrow, and that if it extends beyond 10-10(2) gravitational radii, it aligns with the black hole spin too fast to produce a coherent QPO
Modelling correlated variability in accreting black holes:the effect of high density and variable ionization on reverberation lags
We present a new release of the RELTRANS model to fit the complex
cross-spectrum of accreting black holes as a function of energy. The model
accounts for continuum lags and reverberation lags self-consistently in order
to consider the widest possible range of X-ray variability timescales. We
introduce a more self-consistent treatment of the reverberation lags,
accounting for how the time variations of the illuminating flux change the
ionisation level of the accretion disc. This process varies the shape of the
reflection spectrum in time causing an additional source of lags besides the
light crossing delay. We also consider electron densities in the accretion disc
up to cm, which are found in most of the stellar mass black
holes and in some AGN. These high densities increase the amplitude of the
reverberation lags below keV since the reflection flux enhances in the same
energy range. In addition, we investigate the properties of hard lags produced
by variations in the power-law index of the continuum spectrum, which can be
interpreted as due to roughly variability in the corona's optical depth
and temperature. As a test case, we simultaneously fit the lag energy spectra
in a wide range of Fourier frequency for the black hole candidate MAXI
J1820+070 observed with NICER. The best fit shows how the reverberation lags
contribute even at the longer timescales where the hard lags are important.
This proves the importance of modelling these two lags together and
self-consistently in order to constrain the parameters of the system.Comment: Accepted for publication in MNRA
Contribution of the HMPID detector to the high-pT physics at LHC
The LHC will deliver unexplored energy regimes for proton-proton and
heavy-ion collisions. As shown by the RHIC experiments, particle identification
over a large momentum range is essential to disentangle physics processes,
especially in the intermediate p (1 GeV/c) region. The novel
design of the High-Momentum Particle Identification Detector (HMPID), based on
large surface CsI photocathodes, is able to identify , ,
and in the momentum region where bulk medium properties and hard
scatterings interplay. Furthermore, measurement of resonance particles such as
the could provide information on the system evolution. The
HMPID layout and segmentation are optimized to study particle correlations at
high momenta describing the early phase and the dynamical evolution of the
collision. At LHC, the increased hard cross section will significantly be
enhanced compared to RHIC. Jet reconstruction via Deterministic Annealing can
address jet quenching and detailed measurements of jet properties. In this
paper, we present these selected topics from the possible HMPID contributions
to the physics goals of LHC.Comment: 6 pages, 7 figures, Contribution to QCD @ Work 2007: International
Workshop on Quantum Chromodynamics Theory and Experiment, Martina Franca,
Italy, 16-20 June 200
High-density disc reflection spectroscopy of low-mass active galactic nuclei
The standard alpha-disc model predicts an anti-correlation between the
density of the inner accretion disc and the black hole mass times square of the
accretion rate, as seen in higher mass () active
galactic nuclei (AGNs). In this work, we test the predictions of the alpha-disc
model and study the properties of the inner accretion flow for the low-mass end
() of AGNs. We utilize a new high-density
disc reflection model where the density parameter varies from to cm and apply it to the broadband X-ray (0.3-10
keV) spectra of the low-mass AGN sample. The sources span a wide range of
Eddington fractions and are consistent with being sub-Eddington or
near-Eddington. The X-ray spectra reveal a soft X-ray excess below
keV which is well modeled by high-density reflection from an ionized accretion
disc of density cm on average. The results
suggest a radiation pressure-dominated disc with an average of 70% fraction of
the disc power transferred to the corona, consistent with that observed in
higher mass AGNs. We show that the disc density higher than cm
can result from the radiation pressure compression when the disc surface does
not hold a strong magnetic pressure gradient. We find tentative evidence for a
drop in black hole spin at low-mass regimes.Comment: 20 pages, 10 figures, 6 tables. Accepted for publication in MNRA
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