10,964 research outputs found
Diquark and light four-quark states
Four-quark states with different internal clusters are discussed within the
constituent quark model. It is pointed out that the diquark concept is not
meaningful in the construction of a tetraquark interpolating current in the QCD
sum rule approach, and hence existing sum-rule studies of four-quark states are
incomplete. An updated QCD sum-rule determination of the properties of diquark
clusters is then used as input for the constituent quark model to obtain the
masses of light tetraquark states ({\it i.e.\} a bound state of two
diquark clusters). The results support the identification of ,
and as the light tetraquark states, and seem to
be inconsistent with the tetraquark state interpretation of the new BES
observations of the near-threshold enhancements, X(1835) and X(1812),
with the possible exception that X(1576) may be an "exotic" first orbital
excitation of or .Comment: 7 pages, 4 eps figures, RevTex, two figures and some references
added, published version in PR
Malignancy within a tail gut cyst:a case of retrorectal carcinoid tumour
Purpose. Tailgut cysts with malignant transformation are rare entities. We discuss the diagnostic strategy and treatment of a malignancy within a tailgut cyst. Methods. In this study we report on the case of a 61-year-old man with a malignant neuroendocrine tumour arising within a tailgut cyst and an overview of the literature emphasising the histopathological characteristics and differential diagnosis. Results. Our patient presented with lower back pain, rectal pain, and increased urgency of defecation. MRI scan and CT-guided biopsy on histological analysis revealed a diagnosis of carcinoid tumour of the presacral space. The patient subsequently underwent an abdominoperineal excision of the rectum. Conclusions. This case highlights the importance of tailgut cysts as a differential diagnosis of presacral masses. It is a rare congenital lesion developing from remnants of the embryonic postanal gut and is predominantly benign in nature. Approximately half of cases remain asymptomatic; therefore, diagnosis is often delayed. Magnetic resonance imaging is the investigation of choice and an awareness of the possibility of malignant potential is critical to avoiding missed diagnosis and subsequent morbidity. Complete surgical excision allows accurate diagnosis, confirmation of oncological clearance, and prevention of mortality
On the Standard Approach to Renormalization Group Improvement
Two approaches to renormalization-group improvement are examined: the
substitution of the solutions of running couplings, masses and fields into
perturbatively computed quantities is compared with the systematic sum of all
the leading log (LL), next-to-leading log (NLL) etc. contributions to
radiatively corrected processes, with n-loop expressions for the running
quantities being responsible for summing N^{n}LL contributions. A detailed
comparison of these procedures is made in the context of the effective
potential V in the 4-dimensional O(4) massless model,
showing the distinction between these procedures at two-loop order when
considering the NLL contributions to the effective potential V.Comment: 6 page
Multiwavelength observations of the Be/X-ray binary 4U1145-619
We report optical and infrared observations of the massive X-ray binary
system 4U1145-619 (V801 Cen) which show that the circumstellar disc of the Be
star component is in decline. Infrared J,H,K,L magnitudes of V801Cen have been
monitored from 1993 March to 1996 April. H alpha spectra have been obtained
throughout the same period. We find that both the infrared excess and the
Balmer emission have been in decline throughout the period of observations. A
13 year optical and X-ray history of the source has been collated, revealing a
possible correlation between the optical and X-ray activity. In addition, we
have used u,v,b,y,beta indices, corrected for both circumstellar and
interstellar effects, to calculate the physical parameters of the underlying B
star.Comment: 8 pages postscript. Accepted by MNRA
NLO Effects in QCD Sum-Rule Analyses of as a Tetraquark state
QCD sum-rule studies have been useful to understand and get an insight on the
structure of exotic states, such as tetraquark systems. Moreover, the majority
of these studies are performed only at leading-order (LO) within the light
tetraquarks systems picture, overlooking the effects of higher order
corrections, thus motivating our analysis. Our study focused on the effects of
next-to-leading order (NLO) contributions to the mass estimates of the lightest
tetraquark state (), the so-called or ,
using ratios of QCD Laplace sum-rules. A variety of different models were used,
which included multiple resonances and width effects, resulting in a final mass
prediction of . Even though the
ratios of sum-rules demonstrated some insensitivity under superficially large
NLO contributions, they added the beneficial feature of canceling the
dependence on the anomalous dimension. Our findings were in good agreement with
patterns found in Chiral Lagrangian studies regarding the four-quark structure
of the state, including the relative coupling strengths within the
multiple resonance analysis
High-Pressure Synthesis of a Pentazolate Salt
The pentazolates, the last all-nitrogen members of the azole series, have
been notoriously elusive for the last hundred years despite enormous efforts to
make these compounds in either gas or condensed phases. Here we report a
successful synthesis of a solid state compound consisting of isolated
pentazolate anions N5-, which is achieved by compressing and laser heating
cesium azide (CsN3) mixed with N2 cryogenic liquid in a diamond anvil cell. The
experiment was guided by theory, which predicted the transformation of the
mixture at high pressures to a new compound, cesium pentazolate salt (CsN5).
Electron transfer from Cs atoms to N5 rings enables both aromaticity in the
pentazolates as well as ionic bonding in the CsN5 crystal. This work provides a
critical insight into the role of extreme conditions in exploring unusual
bonding routes that ultimately lead to the formation of novel high nitrogen
content species
Probing the charge of a quantum dot with a nanomechanical resonator
We have used the mechanical motion of a carbon nanotube (CNT) as a probe of
the average charge on a quantum dot. Variations of the resonance frequency and
the quality factor are determined by the change in average charge on the
quantum dot during a mechanical oscillation. The average charge, in turn, is
influenced by the gate voltage, the bias voltage, and the tunnel rates of the
barriers to the leads. At bias voltages that exceed the broadening due to
tunnel coupling, the resonance frequency and quality factor show a double dip
as a function of gate voltage. We find that increasing the current flowing
through the CNT at the Coulomb peak does not increase the damping, but in fact
decreases damping. Using a model with energy-dependent tunnel rates, we obtain
quantitative agreement between the experimental observations and the model. We
theoretically compare different contributions to the single-electron induced
nonlinearity, and show that only one term is significant for both the Duffing
parameter and the mode coupling parameter. We also present additional
measurements which support the model we develop: Tuning the tunnel barriers of
the quantum dot to the leads gives a 200-fold decrease of the quality factor.
Single-electron tunneling through an excited state of the CNT quantum dot also
changes the average charge on the quantum dot, bringing about a decrease in the
resonance frequency. In the Fabry-P\'{e}rot regime, the absence of charge
quantization results in a spring behaviour without resonance frequency dips,
which could be used, for example, to probe the transition from quantized to
continuous charge with a nanomechanical resonator.Comment: 17 pages, 12 figure
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