104,773 research outputs found
Solutions to the Cosmic Initial Entropy Problem without Equilibrium Initial Conditions
The entropy of the observable universe is increasing. Thus, at earlier times
the entropy was lower. However, the cosmic microwave background radiation
reveals an apparently high entropy universe close to thermal and chemical
equilibrium. A two-part solution to this cosmic initial entropy problem is
proposed. Following Penrose, we argue that the evenly distributed matter of the
early universe is equivalent to low gravitational entropy. There are two
competing explanations for how this initial low gravitational entropy comes
about. (1) Inflation and baryogenesis produce a virtually homogeneous
distribution of matter with a low gravitational entropy. (2) Dissatisfied with
explaining a low gravitational entropy as the product of a 'special' scalar
field, some theorists argue (following Boltzmann) for a 'more natural' initial
condition in which the entire universe is in an initial equilibrium state of
maximum entropy. In this equilibrium model, our observable universe is an
unusual low entropy fluctuation embedded in a high entropy universe. The
anthropic principle and the fluctuation theorem suggest that this low entropy
region should be as small as possible and have as large an entropy as possible,
consistent with our existence. However, our low entropy universe is much larger
than needed to produce observers, and we see no evidence for an embedding in a
higher entropy background. The initial conditions of inflationary models are as
natural as the equilibrium background favored by many theorists.Comment: 10 pages, 5 figure
On Second-Order Monadic Monoidal and Groupoidal Quantifiers
We study logics defined in terms of second-order monadic monoidal and
groupoidal quantifiers. These are generalized quantifiers defined by monoid and
groupoid word-problems, equivalently, by regular and context-free languages. We
give a computational classification of the expressive power of these logics
over strings with varying built-in predicates. In particular, we show that
ATIME(n) can be logically characterized in terms of second-order monadic
monoidal quantifiers
Measurement of Resonance Parameters of Orbitally Excited Narrow B^0 Mesons
We report a measurement of resonance parameters of the orbitally excited
(L=1) narrow B^0 mesons in decays to B^{(*)+}\pi^- using 1.7/fb of data
collected by the CDF II detector at the Fermilab Tevatron. The mass and width
of the B^{*0}_2 state are measured to be m(B^{*0}_2) =
5740.2^{+1.7}_{-1.8}(stat.) ^{+0.9}_{-0.8}(syst.) MeV/c^2 and \Gamma(B^{*0}_2)
= 22.7^{+3.8}_{-3.2}(stat.) ^{+3.2}_{-10.2}(syst.) MeV/c^2. The mass difference
between the B^{*0}_2 and B^0_1 states is measured to be
14.9^{+2.2}_{-2.5}(stat.) ^{+1.2}_{-1.4}(syst.) MeV/c^2, resulting in a B^0_1
mass of 5725.3^{+1.6}_{-2.2}(stat.) ^{+1.4}_{-1.5}(syst.) MeV/c^2. This is
currently the most precise measurement of the masses of these states and the
first measurement of the B^{*0}_2 width.Comment: 7 pages, 1 figure, 1 table. Submitted to Phys.Rev.Let
Search for lepton flavor violating decays of a heavy neutral particle in p-pbar collisions at root(s)=1.8 TeV
We report on a search for a high mass, narrow width particle that decays
directly to e+mu, e+tau, or mu+tau. We use approximately 110 pb^-1 of data
collected with the Collider Detector at Fermilab from 1992 to 1995. No evidence
of lepton flavor violating decays is found. Limits are set on the production
and decay of sneutrinos with R-parity violating interactions.Comment: Figure 2 fixed. Reference 4 fixed. Minor changes to tex
Measurement of the fraction of t-tbar production via gluon-gluon fusion in p-pbar collisions at sqrt(s)=1.96 TeV
We present a measurement of the ratio of t-tbar production cross section via
gluon-gluon fusion to the total t-tbar production cross section in p-pbar
collisions at sqrt{s}=1.96 TeV at the Tevatron. Using a data sample with an
integrated luminosity of 955/pb recorded by the CDF II detector at Fermilab, we
select events based on the t-tbar decay to lepton+jets. Using an artificial
neural network technique we discriminate between t-tbar events produced via
q-qbar annihilation and gluon-gluon fusion, and find
Cf=(gg->ttbar)/(pp->ttbar)<0.33 at the 68% confidence level. This result is
combined with a previous measurement to obtain the most precise measurement of
this quantity, Cf=0.07+0.15-0.07.Comment: submitted to Phys. Rev.
Pseudo-finite hard instances for a student-teacher game with a Nisan-Wigderson generator
For an NP intersect coNP function g of the Nisan-Wigderson type and a string
b outside its range we consider a two player game on a common input a to the
function. One player, a computationally limited Student, tries to find a bit of
g(a) that differs from the corresponding bit of b. He can query a
computationally unlimited Teacher for the witnesses of the values of constantly
many bits of g(a). The Student computes the queries from a and from Teacher's
answers to his previous queries. It was proved by Krajicek (2011) that if g is
based on a hard bit of a one-way permutation then no Student computed by a
polynomial size circuit can succeed on all a. In this paper we give a lower
bound on the number of inputs a any such Student must fail on. Using that we
show that there is a pseudo-finite set of hard instances on which all uniform
students must fail. The hard-core set is defined in a non-standard model of
true arithmetic and has applications in a forcing construction relevant to
proof complexity
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