16 research outputs found
Algorithms for Game Metrics
Simulation and bisimulation metrics for stochastic systems provide a
quantitative generalization of the classical simulation and bisimulation
relations. These metrics capture the similarity of states with respect to
quantitative specifications written in the quantitative {\mu}-calculus and
related probabilistic logics. We first show that the metrics provide a bound
for the difference in long-run average and discounted average behavior across
states, indicating that the metrics can be used both in system verification,
and in performance evaluation. For turn-based games and MDPs, we provide a
polynomial-time algorithm for the computation of the one-step metric distance
between states. The algorithm is based on linear programming; it improves on
the previous known exponential-time algorithm based on a reduction to the
theory of reals. We then present PSPACE algorithms for both the decision
problem and the problem of approximating the metric distance between two
states, matching the best known algorithms for Markov chains. For the
bisimulation kernel of the metric our algorithm works in time O(n^4) for both
turn-based games and MDPs; improving the previously best known O(n^9\cdot
log(n)) time algorithm for MDPs. For a concurrent game G, we show that
computing the exact distance between states is at least as hard as computing
the value of concurrent reachability games and the square-root-sum problem in
computational geometry. We show that checking whether the metric distance is
bounded by a rational r, can be done via a reduction to the theory of real
closed fields, involving a formula with three quantifier alternations, yielding
O(|G|^O(|G|^5)) time complexity, improving the previously known reduction,
which yielded O(|G|^O(|G|^7)) time complexity. These algorithms can be iterated
to approximate the metrics using binary search.Comment: 27 pages. Full version of the paper accepted at FSTTCS 200
On the equivalence of game and denotational semantics for the probabilistic mu-calculus
The probabilistic (or quantitative) modal mu-calculus is a fixed-point logic
de- signed for expressing properties of probabilistic labeled transition
systems (PLTS). Two semantics have been studied for this logic, both assigning
to every process state a value in the interval [0,1] representing the
probability that the property expressed by the formula holds at the state. One
semantics is denotational and the other is a game semantics, specified in terms
of two-player stochastic games. The two semantics have been proved to coincide
on all finite PLTS's, but the equivalence of the two semantics on arbitrary
models has been open in literature. In this paper we prove that the equivalence
indeed holds for arbitrary infinite models, and thus our result strengthens the
fruitful connection between denotational and game semantics. Our proof adapts
the unraveling or unfolding method, a general proof technique for proving
result of parity games by induction on their complexity
Higher harmonic anisotropic flow measurements of charged particles in Pb-Pb collisions at 2.76 TeV
We report on the first measurement of the triangular , quadrangular
, and pentagonal charged particle flow in Pb-Pb collisions at 2.76
TeV measured with the ALICE detector at the CERN Large Hadron Collider. We show
that the triangular flow can be described in terms of the initial spatial
anisotropy and its fluctuations, which provides strong constraints on its
origin. In the most central events, where the elliptic flow and
have similar magnitude, a double peaked structure in the two-particle azimuthal
correlations is observed, which is often interpreted as a Mach cone response to
fast partons. We show that this structure can be naturally explained from the
measured anisotropic flow Fourier coefficients.Comment: 10 pages, 4 figures, published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/387
Measurement of charm production at central rapidity in proton-proton collisions at root s=7 TeV
The p(t)-differential inclusive production cross sections of the prompt
charmed mesons D-0, D+, and D*(+) in the rapidity range vertical bar y
vertical bar < 0.5 were measured in proton-proton collisions at root s =
7 TeV at the LHC using the ALICE detector. Reconstructing the decays D-0
-> K-pi(+), D+ -> K-pi(+)pi(+), D*(+) -> D-0 pi(+), and their charge
conjugates, about 8,400 D-0, 2,900 D+, and 2,600 D*(+) mesons with 1 <
p(t) < 24 GeV/c were counted, after selection cuts, in a data sample of
3.14 x 10(8) events collected with a minimum-bias trigger (integrated
luminosity L-int = 5 nb(-1)). The results are described within
uncertainties by predictions based on perturbative QCD
Charged-Particle Multiplicity Density at Midrapidity in Central Pb-Pb Collisions at root s(NN)=2.76 TeV
The first measurement of the charged-particle multiplicity density at
midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon
pair root s(NN) = 2.76 TeV is presented. For an event sample
corresponding to the most central 5% of the hadronic cross section, the
pseudorapidity density of primary charged particles at midrapidity is
1584 +/- 4(stat) +/- 76(syst), which corresponds to 8.3 +/- 0.4(syst)
per participating nucleon pair. This represents an increase of about a
factor 1.9 relative to pp collisions at similar collision energies, and
about a factor 2.2 to central Au-Au collisions at root s(NN) = 0.2 TeV.
This measurement provides the first experimental constraint for models
of nucleus-nucleus collisions at LHC energies
Higher Harmonic Anisotropic Flow Measurements of Charged Particles in Pb-Pb Collisions at root s(NN)=2.76 TeV
We report on the first measurement of the triangular nu(3), quadrangular
nu(4), and pentagonal nu(5) charged particle flow in Pb-Pb collisions at
root s(NN) = 2.76 TeV measured with the ALICE detector at the CERN Large
Hadron Collider. We show that the triangular flow can be described in
terms of the initial spatial anisotropy and its fluctuations, which
provides strong constraints on its origin. In the most central events,
where the elliptic flow nu(2) and nu(3) have similar magnitude, a double
peaked structure in the two-particle azimuthal correlations is observed,
which is often interpreted as a Mach cone response to fast partons. We
show that this structure can be naturally explained from the measured
anisotropic flow Fourier coefficients
Measurement of event background fluctuations for charged particle jet reconstruction in Pb-Pb collisions at root s(NN)=2.76 TeV
The effect of event background fluctuations on charged particle jet
reconstruction in Pb-Pb collisions at root s(NN) = 2.76 TeV has been
measured with the ALICE experiment. The main sources of non-statistical
fluctuations are characterized based purely on experimental data with an
unbiased method, as well as by using single high p(t) particles and
simulated jets embedded into real Pb-Pb events and reconstructed with
the anti-k(t) jet finder. The influence of a low transverse momentum
cut-off on particles used in the jet reconstruction is quantified by
varying the minimum track p(t) between 0.15 GeV/c and 2 GeV/c. For
embedded jets reconstructed from charged particles with p(t) > 0.15
GeV/c, the uncertainty in the reconstructed jet transverse momentum due
to the heavy-ion background is measured to be 11.3 GeV/c (standard
deviation) for the 10% most central Pb-Pb collisions, slightly larger
than the value of 11.0 GeV/c measured using the unbiased method. For a
higher particle transverse momentum threshold of 2 GeV/c, which will
generate a stronger bias towards hard fragmentation in the jet finding
process, the standard deviation of the fluctuations in the reconstructed
jet transverse momentum is reduced to 4.8-5.0 GeV/c for the 10% most
central events. A non-Gaussian tail of the momentum uncertainty is
observed and its impact on the reconstructed jet spectrum is evaluated
for varying particle momentum thresholds, by folding the measured
fluctuations with steeply falling spectra
Centrality Dependence of the Charged-Particle Multiplicity Density at Midrapidity in Pb-Pb Collisions at root s(NN)=2.76 TeV
The centrality dependence of the charged-particle multiplicity density
at midrapidity in Pb-Pb collisions at root s(NN) = 2: 76 TeV is
presented. The charged-particle density normalized per participating
nucleon pair increases by about a factor of 2 from peripheral
(70%-80%) to central (0%-5%) collisions. The centrality dependence
is found to be similar to that observed at lower collision energies. The
data are compared with models based on different mechanisms for particle
production in nuclear collisions
Measurement of inelastic, single- and double-diffraction cross sections in proton-proton collisions at the LHC with ALICE
Measurements of cross sections of inelastic and diffractive processes in
proton-proton collisions at LHC energies were carried out with the ALICE
detector. The fractions of diffractive processes in inelastic collisions
were determined from a study of gaps in charged particle pseudorapidity
distributions: for single diffraction (diffractive mass M-X < 200
GeV/c(2)) sigma(SD)/sigma(INEL) = 0.21 +/- 0.03, 0.20(-0.08)(+0.07,) and
0.20(-0.07)(+0.04), respectively at centre-of-mass energies root s =
0.9, 2.76, and 7 TeV; for double diffraction (for a pseudorapidity gap
Delta eta > 3) sigma(DD)/sigma(INEL) = 0.11 +/- 0.03, 0.12 +/- 0.05, and
0.12(-0.04)(+0.05), respectively at root s = 0.9, 2.76, and 7 TeV. To
measure the inelastic cross section, beam properties were determined
with van der Meer scans, and, using a simulation of diffraction adjusted
to data, the following values were obtained: sigma(INEL) =
62.8(-4.0)(+2.4)(model) +/- 1.2(lumi) mb at root s = 2.76 TeV and
73.2(-4.6)(+2.0)(model) +/- 2.6(lumi) mb at root s = 7 TeV. The single-
and double-diffractive cross sections were calculated combining relative
rates of diffraction with inelastic cross sections. The results are
compared to previous measurements at proton-antiproton and proton-proton
colliders at lower energies, to measurements by other experiments at the
LHC, and to theoretical models
Transverse sphericity of primary charged particles in minimum bias proton-proton collisions at root s=0.9, 2.76 and 7 TeV
Measurements of the sphericity of primary charged particles in minimum
bias proton-proton collisions at root s = 0.9, 2.76 and 7 TeV with the
ALICE detector at the LHC are presented. The observable is measured in
the plane perpendicular to the beam direction using primary charged
tracks with p(T) > 0.5 GeV/c in vertical bar eta vertical bar < 0.8. The
mean sphericity as a function of the charged particle multiplicity at
mid-rapidity (N-ch) is reported for events with different p(T) scales
(”soft” and “hard”) defined by the transverse momentum of the
leading particle. In addition, the mean charged particle transverse
momentum versus multiplicity is presented for the different event
classes, and the sphericity distributions in bins of multiplicity are
presented. The data are compared with calculations of standard Monte
Carlo event generators. The transverse sphericity is found to grow with
multiplicity at all collision energies, with a steeper rise at low N-ch,
whereas the event generators show an opposite tendency. The combined
study of the sphericity and the mean p(T) with multiplicity indicates
that most of the tested event generators produce events with higher
multiplicity by generating more back-to-back jets resulting in decreased
sphericity (and isotropy). The PYTHIA6 generator with tune PERUGIA-2011
exhibits a noticeable improvement in describing the data, compared to
the other tested generators