1,118 research outputs found
An example of non-attainability of expected quantum information
Introduction Braunstein and Caves [1] have clarified the relation between classical expected information i(`), in the sense of Fisher, and the analogous concept of expected quantum information I(`), by showing that I(`) is an upper bound of i(`; M) with respect to all (dominated) generalized measurements M of the state ae = ae(`) where ` is an unknown parameter and i(`; M) is the Fisher expected information for ` in the distribution of the outcome of the measurement of M . They indicate moreover that a measurement exists achieving the bound. In the present paper we show by an example, for an elementary spin- 1 2 situation, that in general there does not exist
QCD Sum Rules for the production of the X(3872) as a mixed molecule-charmonium state in B meson decay
We use QCD sum rules to calculate the branching ratio for the production of
the meson X(3872) in the decay , assumed to be a mixture between
charmonium and exotic molecular states with
. We find that in a small range for the values of the mixing
angle, , we get the branching ratio
, which is in agreement with
the experimental upper limit. This result is compatible with the analysis of
the mass and decay width of the mode and the radiative decay
mode performed in the same approach.Comment: 6 pages, 3 figures; revised versions to appear on Phys. Lett.
A Comparative Study of Pentaquark Interpolating Currents
In a diquark-diquark-antiquark picture of pentaquarks, we use two
interpolating currents to calculate the mass of the recently measured
state in the framework of QCD sum rules. We show that, even though
yielding similar values for (and close to the experimental
value), these currents differ from each other in what concerns the strength of
the pole, convergence of the OPE and sensitivity to the continuum threshold
parameter.Comment: 19 pages, 8 figures, replaced version accepted for publication in
Phys. Lett.
Diquark-Antidiquark with open charm in QCD sum rules
Using the QCD sum rule approach we investigate the possible four-quark
structure of the recently observed charmed scalar mesons
(BELLE) and (FOCUS) and also of the very narrow
, firstly observed by BABAR. We use diquak-antidiquark
currents and work to the order of in full QCD, without relying on
expansion. Our results indicate that a four-quark structure is acceptable for
the resonances observed by BELLE and BABAR: and
respectively, but not for the resonances observed by FOCUS:
.Comment: 6 pages, 5 eps figures; Contribution to the 'Workshop on Light-Cone
QCD and Nonperturbative Hadron Physics 2005 (LC2005)', Cairns-Australi
The Vertex in QCD Sum Rules
The form factor is evaluated in a QCD sum rule calculation for
both and off-shell mesons. We study the double Borel sum rule for
the three point function of two pseudoscalar and one vector meson current. We
find that the momentum dependence of the form factors is different if the
or the meson is off-shell, but they lead to the same coupling constant
in the vertex.Comment: 11 pages, Latex, 4 eps figure
Are and the Roper resonance diquark-diquark-antiquark states?
We consider a current in the QCD sum rule framework to study
the mass of the recently observed pentaquark state , obtaining
good agreement with the experimental value. We also study the mass of the
pentaquark . Our results are compatible with the interpretation
of the state as being the Roper resonance N(1440), as suggested
by Jaffe and Wilczek.Comment: 9 pages RevTex4 and 3 eps figures. Revised version accepted for
publication in Phys. Lett.
A model for string-breaking in QCD
We present a model for string breaking based on the existence of
chromoelectric flux tubes. We predict the form of the long-range potential, and
obtain an estimate of the string breaking length. A prediction is also obtained
for the behaviour with temperature of the string breaking length near the
deconfinement phase transition. We plan to use this model as a guide for a
program of study of string breaking on the lattice.Comment: 7 pages, minor improvements of the text and of the reference lis
Optimal estimation of qubit states with continuous time measurements
We propose an adaptive, two steps strategy, for the estimation of mixed qubit
states. We show that the strategy is optimal in a local minimax sense for the
trace norm distance as well as other locally quadratic figures of merit. Local
minimax optimality means that given identical qubits, there exists no
estimator which can perform better than the proposed estimator on a
neighborhood of size of an arbitrary state. In particular, it is
asymptotically Bayesian optimal for a large class of prior distributions.
We present a physical implementation of the optimal estimation strategy based
on continuous time measurements in a field that couples with the qubits.
The crucial ingredient of the result is the concept of local asymptotic
normality (or LAN) for qubits. This means that, for large , the statistical
model described by identically prepared qubits is locally equivalent to a
model with only a classical Gaussian distribution and a Gaussian state of a
quantum harmonic oscillator.
The term `local' refers to a shrinking neighborhood around a fixed state
. An essential result is that the neighborhood radius can be chosen
arbitrarily close to . This allows us to use a two steps procedure by
which we first localize the state within a smaller neighborhood of radius
, and then use LAN to perform optimal estimation.Comment: 32 pages, 3 figures, to appear in Commun. Math. Phy
On the complexity of strongly connected components in directed hypergraphs
We study the complexity of some algorithmic problems on directed hypergraphs
and their strongly connected components (SCCs). The main contribution is an
almost linear time algorithm computing the terminal strongly connected
components (i.e. SCCs which do not reach any components but themselves).
"Almost linear" here means that the complexity of the algorithm is linear in
the size of the hypergraph up to a factor alpha(n), where alpha is the inverse
of Ackermann function, and n is the number of vertices. Our motivation to study
this problem arises from a recent application of directed hypergraphs to
computational tropical geometry.
We also discuss the problem of computing all SCCs. We establish a superlinear
lower bound on the size of the transitive reduction of the reachability
relation in directed hypergraphs, showing that it is combinatorially more
complex than in directed graphs. Besides, we prove a linear time reduction from
the well-studied problem of finding all minimal sets among a given family to
the problem of computing the SCCs. Only subquadratic time algorithms are known
for the former problem. These results strongly suggest that the problem of
computing the SCCs is harder in directed hypergraphs than in directed graphs.Comment: v1: 32 pages, 7 figures; v2: revised version, 34 pages, 7 figure
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