13,332 research outputs found
Almost-Commutative Geometries Beyond the Standard Model III: Vector Doublets
We will present a new extension of the standard model of particle physics in
its almostcommutative formulation. This extension has as its basis the algebra
of the standard model with four summands [11], and enlarges only the particle
content by an arbitrary number of generations of left-right symmetric doublets
which couple vectorially to the U(1)_YxSU(2)_w subgroup of the standard model.
As in the model presented in [8], which introduced particles with a new colour,
grand unification is no longer required by the spectral action. The new model
may also possess a candidate for dark matter in the hundred TeV mass range with
neutrino-like cross section
Almost-Commutative Geometries Beyond the Standard Model
In [7-9] and [10] the conjecture is presented that almost-commutative
geometries, with respect to sensible physical constraints, allow only the
standard model of particle physics and electro-strong models as
Yang-Mills-Higgs theories. In this publication a counter example will be given.
The corresponding almost-commutative geometry leads to a Yang-Mills-Higgs
model which consists of the standard model of particle physics and two new
fermions of opposite electro-magnetic charge. This is the second
Yang-Mills-Higgs model within noncommutative geometry, after the standard
model, which could be compatible with experiments. Combined to a hydrogen-like
composite particle these new particles provide a novel dark matter candidate
Almost-Commutative Geometries Beyond the Standard Model II: New Colours
We will present an extension of the standard model of particle physics in its
almost-commutative formulation. This extension is guided by the minimal
approach to almost-commutative geometries employed in [13], although the model
presented here is not minimal itself.
The corresponding almost-commutative geometry leads to a Yang-Mills-Higgs
model which consists of the standard model and two new fermions of opposite
electro-magnetic charge which may possess a new colour like gauge group. As a
new phenomenon, grand unification is no longer required by the spectral action.Comment: Revised version for publication in J.Phys.A with corrected Higgs
masse
Low-Energy Properties of Antiferromagnetic Spin-1/2 Heisenberg Ladders with an Odd Number of Legs
An effective low-energy description for multi-leg spin-1/2 Heisenberg ladders
with an odd number of legs is proposed. Using a newly developed Monte Carlo
loop algorithm and exact diagonalization techniques, the uniform and staggered
magnetic susceptibility and the entropy are calculated for ladders with 1, 3,
and 5 legs. These systems show a low-temperature scaling behavior similar to
spin-1/2 chains with longer ranged unfrustrated exchange interactions. The
spinon velocity does not change as the number of legs increases, but the energy
scale parameter decreases markedly.Comment: 4 pages and 5 figure
General Relation between Entanglement and Fluctuations in One Dimension
In one dimension very general results from conformal field theory and exact
calculations for certain quantum spin systems have established universal
scaling properties of the entanglement entropy between two parts of a critical
system. Using both analytical and numerical methods, we show that if particle
number or spin is conserved, fluctuations in a subsystem obey identical scaling
as a function of subsystem size, suggesting that fluctuations are a useful
quantity for determining the scaling of entanglement, especially in higher
dimensions. We investigate the effects of boundaries and subleading corrections
for critical spin and bosonic chains.Comment: 4 pages, 2 figures. Minor changes, references added
Numerical Approach to Multi Dimensional Phase Transitions
We present an algorithm to analyze numerically the bounce solution of
first-order phase transitions. Our approach is well suited to treat phase
transitions with several fields. The algorithm consists of two parts. In the
first part the bounce solution without damping is determined, in which case
energy is conserved. In the second part the continuation to the physically
relevant case with damping is performed. The presented approach is numerically
stable and easily implemented.Comment: 18 pages, 8 figures; some comments, a reference and a table adde
Strange quark mass from e+e- revisited and present status of light quark masses
We reconsider the determinations of the strange quark mass m_s from e+e- into
hadrons data using a new combination of FESR and revisiting the existing
tau-like sum rules by including non-resonant contributions to the spectral
functions. To order alpha_s^3 and including the tachyonic gluon mass lambda^2
contribution, which phenomenologically parametrizes the UV renormalon effect
into the PT series, we obtain the invariant mass m_s=(119 +- 17)MeV leading to:
m_s(2 GeV)=(104+- 15)MeV. Combining this value with the recent and independent
phenomenological determinations from some other channels, to order alpha_s^3
and including lambda^2, we deduce the weighted average: m_s (2 GeV)=(96.1 +-
4.8)MeV . The positivity of the spectral functions in the (pseudo)scalar [resp.
vector] channels leads to the lower [resp. upper] bounds of m_s(2 GeV): (71 +-
4) MeV < m_s(2 GeV) < (151 +- 14) MeV, to order alpha_s^3. Using the ChPT mass
ratio r_3 = 2m_s/(m_u+m_d)=24.2 +- 1.5, and the average value of m_s, we
deduce:
(m_u+m_d)(2 GeV)=(7.9 +- 0.6) MeV, consistent with the pion sum rule result,
which, combined with the ChPT value for m_u/m_d, gives: m_d(2 GeV)=(5.1 +-
0.4)MeV and m_u(2 GeV)=(2.8 +- 0.2)MeV. Finally, using (m_u+m_d) from the pion
sum rule and the average value of m_s (without the pion sum rule), the method
gives: r_3= 23.5 +- 5.8 in perfect agreement with the ChPT ratio, indicating
the self-consistency of the sum rule results. Using the value: m_b(m_b)=(4.23
+- 0.06) GeV, we also obtain the model-building useful scale-independent mass
ratio: m_b/m_s=50 +- 3.Comment: Updated and improved average values. Version to appear in Phys. Rev.
Coulomb correlations and coherent charge tunneling in mesoscopic coupled rings
We study the effect of a strong electron-electron (e-e) interaction in a
system of two concentric one-dimensional rings with incommensurate areas A_1
and A_2, coupled by a tunnel amplitude. For noninteracting particles the
magnetic moment (persistent current) m of the many-body ground state and first
excited states is an irregular function of the external magnetic field. In
contrast, we show that when strong e-e interactions are present the magnetic
field dependence of m becomes periodic. In such a strongly correlated system
disorder can only be caused by inter-ring charge fluctuations, controllable by
a gate voltage. The oscillation period of m is proportional to 1/(A_1 + A_2) if
fluctuations are suppressed. Coherent inter-ring tunneling doubles the period
when charge fluctuations are allowed.Comment: 4 pages, 4 eps figure
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