742 research outputs found
The extraction of nuclear sea quark distribution and energy loss effect in Drell-Yan experiment
The next-to-leading order and leading order analysis are performed on the
differential cross section ratio from Drell-Yan process. It is found that the
effect of next-to-leading order corrections can be negligible on the
differential cross section ratios as a function of the quark momentum fraction
in the beam proton and the target nuclei for the current Fermilab and future
lower beam proton energy. The nuclear Drell-Yan reaction is an ideal tool to
study the energy loss of the fast quark moving through cold nuclei. In the
leading order analysis, the theoretical results with quark energy loss are in
good agreement with the Fermilab E866 experimental data on the Drell-Yan
differential cross section ratios as a function of the momentum fraction of the
target parton. It is shown that the quark energy loss effect has significant
impact on the Drell-Yan differential cross section ratios. The nuclear
Drell-Yan experiment at current Fermilab and future lower energy proton beam
can not provide us with more information on the nuclear sea quark distribution.Comment: 17 pages, 4 figure
CKM Favored Semileptonic Decays of Heavy Hadrons at Zero Recoil
We study the properties of Cabibbo-Kobayashi-Maskawa (CKM) favored
semileptonic decays of mesons and baryons containing a heavy quark at the point
of no recoil. We first use a diagrammatic analysis to rederive the result
observed by earlier authors that at this kinematic point the meson decays
via transitions can only produce a or meson. The result is
generalized to include photon emissions which violate heavy quark flavor
symmetry. We show that photons emitted by the heavy quarks and the charged
lepton are the only light particles that can decorate the decays at zero recoil, and the similar processes of heavy baryons.
Implications for the determinations of the CKM parameter are
discussed. Also studied in this paper is the connection between our
diagrammatic analysis of suppression of particle emission and the formal
observation based on weak currents at zero recoil being generators of heavy
quark symmetry. We show that the two approaches can be unified by considering
the Isgur-Wise function in the presence of an external source.Comment: 27 pages, including 11 figures using macros FEYNMAN.te
Ultra-sharp asymmetric Fano-like resonance spectrum on Si photonic platform
In this paper, we report the generation of an ultra-sharp asymmetric resonance spectrum through Fano-like interference. This generation is accomplished by weakly coupling a high-quality factor (Q factor) Fabry–Pérot (FP) cavity and a low-Q factor FP cavity through evanescent waves. The high-Q FP cavity is formed by Sagnac loop mirrors, whilst the low-Q one is built by partially transmitting Sagnac loop reflectors. The working principle has been analytically established and numerically modelled by using temporal coupled-mode-theory (CMT), and verified using a prototype device fabricated on the 340 nm silicon-on-insulator (SOI) platform, patterned by deep ultraviolet (DUV) lithography. Pronounced asymmetric resonances with slopes up to 0.77 dB/pm have been successfully measured, which, to the best of our knowledge, is higher than the results reported in state-of-the-art devices in on-chip integrated Si photonic studies. The established theoretical analysis method can provide excellent design guidelines for devices with Fano-like resonances. The design principle can be applied to ultra-sensitive sensing, ultra-high extinction ratio switching, and more applications
Semileptonic B and Lambda_b Decays and Local Duality in QCD
The inclusive and exclusive semileptonic decay distributions for b -> c decay
are computed in the Shifman-Voloshin limit. The inclusive decay distributions
(computed using an operator product expansion) depend on quark masses, and the
exclusive decay distributions depend on hadron masses. Nevertheless, we show
explicitly how the first two terms in the 1/m expansion match between the
inclusive and exclusive decays. Agreement between the inclusive and exclusive
decay rates requires a minimum smearing region of size Lambda_QCD before local
duality holds in QCD. The alpha_s corrections to the inclusive and exclusive
decay rates are also shown to agree to order (log m)/m^2. The alpha_s/m^2
corrections are used to obtain the alpha_s correction to Bjorken's inequality
on the slope of the Isgur-Wise function.Comment: 22 pages, 3 eps figures, uses revtex (Revision: a discussion of
radiative corrections to the bound K>0 of Section 7.B has been added; some
typos, including labels in fig 2
Low-light-level nonlinear optics with slow light
Electromagnetically induced transparency in an optically thick, cold medium
creates a unique system where pulse-propagation velocities may be orders of
magnitude less than and optical nonlinearities become exceedingly large. As
a result, nonlinear processes may be efficient at low-light levels. Using an
atomic system with three, independent channels, we demonstrate a quantum
interference switch where a laser pulse with an energy density of
photons per causes a 1/e absorption of a second pulse.Comment: to be published in PR
Late Holocene isotope hydrology of Lake Qinghai, NE Tibetan Plateau: effective moisture variability and atmospheric circulation changes
A sub-centennial-resolution record of lacustrine carbonate oxygen isotopes (δ<sup>18</sup>O<sub>C</sub>) from the closed-basin Lake Qinghai on the NE Tibetan Plateau shows pronounced variability over the past 1500 years. Changes in δ<sup>18</sup>O<sub>C</sub> in hydrologically closed lakes are often interpreted in terms of changing effective moisture. Under this interpretation our record would imply increasing effective moisture during the Little Ice Age (LIA) compared to the Medieval Warm Period (MWP). However, independent evidence from other archives strongly suggests the Asian summer monsoon was stronger during the MWP and weakened during the LIA. Controls other than effective moisture (the balance of water inputs over evaporative loss) must therefore have contributed to the δ<sup>18</sup>O<sub>C</sub> values. We propose the LIA signal in Lake Qinghai resulted from a reduction in evaporation caused by colder air temperatures, coupled with a decrease in oxygen isotope composition of input waters as a result of an increase in the relative importance of westerly-derived precipitation. Our results caution against simplistic interpretations of carbonate oxygen isotope records from hydrologically closed lakes and suggest all possible controlling factors must be taken into account in order to avoid misleading palaeoclimatic reconstructions
Faraday rotation in graphene
We study magneto--optical properties of monolayer graphene by means of
quantum field theory methods in the framework of the Dirac model. We reveal a
good agreement between the Dirac model and a recent experiment on giant Faraday
rotation in cyclotron resonance. We also predict other regimes when the effects
are well pronounced. The general dependence of the Faraday rotation and
absorption on various parameters of samples is revealed both for suspended and
epitaxial graphene.Comment: 10 pp; v2: typos corrected and references added, v3, v4: small
changes and more reference
Light-Front Approach for Heavy Pentaquark Transitions
Assuming the two diquark structure for the pentaquark state as advocated in
the Jaffe-Wilczek model, there exist exotic parity-even anti-sextet and
parity-odd triplet heavy pentaquark baryons. The theoretical estimate of
charmed and bottom pentaquark masses is quite controversial and it is not clear
whether the ground-state heavy pentaquark lies above or below the strong-decay
threshold. We study the weak transitions of heavy pentaquark states using the
light-front quark model. In the heavy quark limit, heavy-to-heavy pentaquark
transition form factors can be expressed in terms of three Isgur-Wise
functions: two of them are found to be normalized to unity at zero recoil,
while the third one is equal to 1/2 at the maximum momentum transfer, in
accordance with the prediction of the large-Nc approach or the quark model.
Therefore, the light-front model calculations are consistent with the
requirement of heavy quark symmetry. Numerical results for form factors and
Isgur-Wise functions are presented. Decay rates of the weak decays Theta_b+ to
Theta_c0 pi+ (rho+), Theta_c0 to Theta+ pi- (rho-), Sigma'_{5b}+ to
Sigma'_{5c}0 pi+ (rho+) and Sigma'_{5c}0 to N_8+ pi- (rho-) with Theta_Q,
Sigma'_{5Q} and N_8 being the heavy anti-sextet, heavy triplet and light
octet pentaquarks, respectively, are obtained. For weakly decaying Theta_b+ and
Theta_c0, the branching ratios of Theta_b+ to Theta_c0 pi+, Theta_c0 to Theta+
pi- are estimated to be at the level of 10^{-3} and a few percents,
respectively.Comment: 33 pages, 3 figures, version to be published in Phys. Rev.
New Upper Limit of Terrestrial Equivalence Principle Test for Rotating Extended Bodies
Improved terrestrial experiment to test the equivalence principle for
rotating extended bodies is presented, and a new upper limit for the violation
of the equivalence principle is obtained at the level of 1.6, which is limited by the friction of the rotating gyroscope. It
means the spin-gravity interaction between the extended bodies has not been
observed at this level.Comment: 4 page
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