7,286 research outputs found
Single soft gluon emission at two loops
We study the single soft-gluon current at two loops with two energetic
partons in massless perturbative QCD, which describes, for example, the soft
limit of the two-loop amplitude for . The results are presented as
Laurent expansions in in spacetime dimension. We
calculate the expansion to order analytically, which is a
necessary ingredient for Higgs production at hadron colliders at
next-to-next-to-next-to-leading order in the soft-virtual approximation. We
also give two-loop results of the single soft-gluon current in
Super-Yang-Mills theory, and find that it has uniform transcendentality. By
iteration relation of splitting amplitudes, our calculations can determine the
three-loop single soft-gluon current to order in
Super-Yang-Mills theory in the limit of large .Comment: typos corrected; journal versio
Weak Decays of Doubly-Heavy Tetraquarks
We study the weak decays of exotic tetraquark states
with two heavy quarks. Under the SU(3) symmetry for light quarks, these
tetraquarks can be classified into an octet plus a singlet: . We will concentrate on the octet tetraquarks with
, and study their weak decays, both semileptonic and nonleptonic.
Hadron-level effective Hamiltonian is constructed according to the irreducible
representations of the SU(3) group. Expanding the Hamiltonian, we obtain the
decay amplitudes parameterized in terms of a few irreducible quantities. Based
on these amplitudes, relations for decay widths are derived, which can be
tested in future. We also give a list of golden channels that can be used to
look for these states at various colliders.Comment: 14 pages,3 figure
Weak Decays of Stable Open-bottom Tetraquark by SU(3) Symmetry Analysis
The exotic state which was observed by D0 Collaboration is very
likely to be a tetraquark state with four different valence quark flavors,
though the existence was not confirmed by other collaborations. The possibility
of such state still generate lots of interests in theory. In the paper, we will
study the properties of the state under the SU(3) flavor symmetry. This four
quark state with a heavy bottom quark and three light quarks(anti-quark) can
form a or representation. The weak decays can be dominant
and should be discussed carefully while such state is stable against the strong
interaction. Therefor we will study the multi-body semileptonic and nonleptonic
weak decays systematically. With the help of SU(3) flavor symmetry, we can give
the Hamiltonian in the hadronic level, then obtain the parameterized
irreducible amplitudes and the relations of different channels. At the end of
the article, we collect some Cabibbo allowed two-body and three-body weak decay
channels which can be used to reconstruct states at the branching
fraction up to be .Comment: 53 pages, 2 figure
Mechanism of unidirectional movement of kinesin motors
Kinesin motors have been studied extensively both experimentally and
theoretically. However, the microscopic mechanism of the processive movement of
kinesin is still an open question. In this paper, we propose a hand-over-hand
model for the processivity of kinesin, which is based on chemical, mechanical,
and electrical couplings. In the model the processive movement does not need to
rely on the two heads' coordination in their ATP hydrolysis and mechanical
cycles. Rather, the ATP hydrolyses at the two heads are independent. The much
higher ATPase rate at the trailing head than the leading head makes the motor
walk processively in a natural way, with one ATP being hydrolyzed per step. The
model is consistent with the structural study of kinesin and the measured
pathway of the kinesin ATPase. Using the model the estimated driving force of ~
5.8 pN is in agreements with the experimental results (5~7.5 pN). The
prediction of the moving time in one step (~10 microseconds) is also consistent
with the measured values of 0~50 microseconds. The previous observation of
substeps within the 8-nm step is explained. The shapes of velocity-load (both
positive and negative) curves show resemblance to previous experimental
results.Comment: 22 pages, 6 figure
Model for processive movement of myosin V and myosin VI
Myosin V and myosin VI are two classes of two-headed molecular motors of the
myosin superfamily that move processively along helical actin filaments in
opposite directions. Here we present a hand-over-hand model for their
processive movements. In the model, the moving direction of a dimeric molecular
motor is automatically determined by the relative orientation between its two
heads at free state and its head's binding orientation on track filament. This
determines that myosin V moves toward the barbed end and myosin VI moves toward
the pointed end of actin. During the moving period in one step, one head
remains bound to actin for myosin V whereas two heads are detached for myosin
VI: The moving manner is determined by the length of neck domain. This
naturally explains the similar dynamic behaviors but opposite moving directions
of myosin VI and mutant myosin V (the neck of which is truncated to only
one-sixth of the native length). Because of different moving manners, myosin VI
and mutant myosin V exhibit significantly broader step-size distribution than
native myosin V. However, all three motors give the same mean step size of 36
nm (the pseudo-repeat of actin helix). Using the model we study the dynamics of
myosin V quantitatively, with theoretical results in agreement with previous
experimental ones.Comment: 18 pages, 7 figure
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