249 research outputs found
Instanton bundles on Fano threefolds
We introduce the notion of an instanton bundle on a Fano threefold of index
2. For such bundles we give an analogue of a monadic description and discuss
the curve of jumping lines. The cases of threefolds of degree 5 and 4 are
considered in a greater detail.Comment: 31 page, to appear in CEJ
Critical thermodynamics of two-dimensional N-vector cubic model in the five-loop approximation
The critical behavior of the two-dimensional N-vector cubic model is studied
within the field-theoretical renormalization-group (RG) approach. The
β functions and critical exponents are calculated in the five-loop approximation,
RG series obtained are resummed using Pade-Borel-Leroy and ´
conformal mapping techniques. It is found that for N = 2 the continuous
line of fixed points is well reproduced by the resummed RG series and an
account for the five-loop terms makes the lines of zeros of both β functions
closer to each other. For N > 3 the five-loop contributions are shown to
shift the cubic fixed point, given by the four-loop approximation, towards
the Ising fixed point. This confirms the idea that the existence of the cubic
fixed point in two dimensions under N >2 is an artifact of the perturbative
analysis. In the case N = 0 the results obtained are compatible with the
conclusion that the impure critical behavior is controlled by the Ising fixed
point.В рамках теоретико-польового підходу ренормалізаційної групи (РГ)
вивчається критична поведінка двовимірної N-векторної кубічної
моделі. β функції і критичні показники обчислюються в п’ятипетлевому наближенні, отримані РГ ряди пересумовуються з використанням техніки Паде-Бореля-Лєруа і конформного перетворення.
Знайдено, що для N = 2 неперервна лінія нерухомих точок добре
відтворюється пересумованими РГ рядами і врахування п’ятипетлевих членів робить лінії нулів обох β функцій ближчими один
до одного. Показано, що для N > 3 п’яти-петлеві внески зсувають
кубічну нерухому точку, отриману в чотири-петлевому наближенні,
до нерухомої точки Ізинґа. Це підтверджує ідею, що існування
кубічної нерухомої точки в двох вимірах під N > 2 є результатом
пертурбативного аналізу. У випадку N = 0 отримані результати є
сумісні з висновком, що критична поведінка, пов’язана з домішками,
контролюється нерухомою точкою Ізинґа
Universal sextic effective interaction at criticality
The renormalization group approach in three dimensions is used to estimate
the universal critical value g_6^* of the dimensionless sextic effective
coupling constant for the Ising model. The four-loop RG expansion for g_6 is
calculated and resummed by means of the Pade-Borel and Pade-Borel-Leroy
procedures resulting in g_6^* = 1.596, while the most accurate estimate for
g_6^* is argued to be equal to 1.61.Comment: 6 pages, TeX, no figure
Period- and mirror-maps for the quartic K3
We study in detail mirror symmetry for the quartic K3 surface in P3 and the
mirror family obtained by the orbifold construction. As explained by Aspinwall
and Morrison, mirror symmetry for K3 surfaces can be entirely described in
terms of Hodge structures. (1) We give an explicit computation of the Hodge
structures and period maps for these families of K3 surfaces. (2) We identify a
mirror map, i.e. an isomorphism between the complex and symplectic deformation
parameters, and explicit isomorphisms between the Hodge structures at these
points. (3) We show compatibility of our mirror map with the one defined by
Morrison near the point of maximal unipotent monodromy. Our results rely on
earlier work by Narumiyah-Shiga, Dolgachev and Nagura-Sugiyama.Comment: 29 pages, 3 figure
New precise determination of the \tau lepton mass at KEDR detector
The status of the experiment on the precise lepton mass measurement
running at the VEPP-4M collider with the KEDR detector is reported. The mass
value is evaluated from the cross section behaviour around the
production threshold. The preliminary result based on 6.7 pb of data is
MeV. Using 0.8 pb of data
collected at the peak the preliminary result is also obtained:
eV.Comment: 6 pages, 8 figures; The 9th International Workshop on Tau-Lepton
Physics, Tau0
Measurement of \Gamma_{ee}(J/\psi)*Br(J/\psi->e^+e^-) and \Gamma_{ee}(J/\psi)*Br(J/\psi->\mu^+\mu^-)
The products of the electron width of the J/\psi meson and the branching
fraction of its decays to the lepton pairs were measured using data from the
KEDR experiment at the VEPP-4M electron-positron collider. The results are
\Gamma_{ee}(J/\psi)*Br(J/\psi->e^+e^-)=(0.3323\pm0.0064\pm0.0048) keV,
\Gamma_{ee}(J/\psi)*Br(J/\psi->\mu^+\mu^-)=(0.3318\pm0.0052\pm0.0063) keV.
Their combinations
\Gamma_{ee}\times(\Gamma_{ee}+\Gamma_{\mu\mu})/\Gamma=(0.6641\pm0.0082\pm0.0100)
keV,
\Gamma_{ee}/\Gamma_{\mu\mu}=1.002\pm0.021\pm0.013 can be used to improve
theaccuracy of the leptonic and full widths and test leptonic universality.
Assuming e\mu universality and using the world average value of the lepton
branching fraction, we also determine the leptonic \Gamma_{ll}=5.59\pm0.12 keV
and total \Gamma=94.1\pm2.7 keV widths of the J/\psi meson.Comment: 7 pages, 6 figure
Search for narrow resonances in e+ e- annihilation between 1.85 and 3.1 GeV with the KEDR Detector
We report results of a search for narrow resonances in e+ e- annihilation at
center-of-mass energies between 1.85 and 3.1 GeV performed with the KEDR
detector at the VEPP-4M e+ e- collider. The upper limit on the leptonic width
of a narrow resonance Gamma(R -> ee) Br(R -> hadr) < 120 eV has been obtained
(at 90 % C.L.)
Measurement of main parameters of the \psi(2S) resonance
A high-precision determination of the main parameters of the \psi(2S)
resonance has been performed with the KEDR detector at the VEPP-4M e^{+}e^{-}
collider in three scans of the \psi(2S) -- \psi(3770) energy range. Fitting the
energy dependence of the multihadron cross section in the vicinity of the
\psi(2S) we obtained the mass value
M = 3686.114 +- 0.007 +- 0.011 ^{+0.002}_{-0.012} MeV and the product of the
electron partial width by the branching fraction into hadrons \Gamma_{ee}*B_{h}
= 2.233 +- 0.015 +- 0.037 +- 0.020 keV.
The third error quoted is an estimate of the model dependence of the result
due to assumptions on the interference effects in the cross section of the
single-photon e^{+}e^{-} annihilation to hadrons explicitly considered in this
work.
Implicitly, the same assumptions were employed to obtain the charmonium
leptonic width and the absolute branching fractions in many experiments.
Using the result presented and the world average values of the electron and
hadron branching fractions, one obtains the electron partial width and the
total width of the \psi(2S):
\Gamma_{ee} =2.282 +- 0.015 +- 0.038 +- 0.021 keV,
\Gamma = 296 +- 2 +- 8 +- 3 keV.
These results are consistent with and more than two times more precise than
any of the previous experiments
Critical behavior of the two-dimensional N-component Landau-Ginzburg Hamiltonian with cubic anisotropy
We study the two-dimensional N-component Landau-Ginzburg Hamiltonian with
cubic anisotropy. We compute and analyze the fixed-dimension perturbative
expansion of the renormalization-group functions to four loops. The relations
of these models with N-color Ashkin-Teller models, discrete cubic models,
planar model with fourth order anisotropy, and structural phase transition in
adsorbed monolayers are discussed. Our results for N=2 (XY model with cubic
anisotropy) are compatible with the existence of a line of fixed points joining
the Ising and the O(2) fixed points. Along this line the exponent has
the constant value 1/4, while the exponent runs in a continuous and
monotonic way from 1 to (from Ising to O(2)). For N\geq 3 we find a
cubic fixed point in the region , which is marginally stable or
unstable according to the sign of the perturbation. For the physical relevant
case of N=3 we find the exponents and at the cubic
transition.Comment: 14 pages, 9 figure
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
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