141 research outputs found
Two-loop QCD corrections to gluon-gluon scattering
We present the virtual QCD corrections to gluon–gluon scattering due to the interference of tree- and two-loop amplitudes. We work in conventional dimensional regularisation and give analytic expressions renormalised in the scheme. The structure of the infrared divergences agrees with that predicted by Catani while formulae for the finite remainder are given in terms of logarithms and polylogarithms that are real in the physical region. These results, together with those previously obtained for quark–quark and quark–gluon scattering, complete the two-loop matrix elements needed for the next-to-next-to-leading order contribution to inclusive jet production at hadron colliders
Two-loop QCD corrections to the scattering of massless distinct quarks.
We present the two-loop virtual QCD corrections to the scattering of distinct massless quarks, , in conventional dimensional regularisation. The structure of the infrared divergences agrees with that predicted by Catani while expressions for the finite remainder are given for each of the s-, t- and u-channels in terms of polylogarithms. The results presented here form a vital part of the next-to-next-to-leading order contribution to inclusive jet production in hadron colliders and will play a crucial role in improving the theoretical prediction for jet cross sections in hadron-hadron collisions
Two-Loop g -> gg Splitting Amplitudes in QCD
Splitting amplitudes are universal functions governing the collinear behavior
of scattering amplitudes for massless particles. We compute the two-loop g ->
gg splitting amplitudes in QCD, N=1, and N=4 super-Yang-Mills theories, which
describe the limits of two-loop n-point amplitudes where two gluon momenta
become parallel. They also represent an ingredient in a direct x-space
computation of DGLAP evolution kernels at next-to-next-to-leading order. To
obtain the splitting amplitudes, we use the unitarity sewing method. In
contrast to the usual light-cone gauge treatment, our calculation does not rely
on the principal-value or Mandelstam-Leibbrandt prescriptions, even though the
loop integrals contain some of the denominators typically encountered in
light-cone gauge. We reduce the integrals to a set of 13 master integrals using
integration-by-parts and Lorentz invariance identities. The master integrals
are computed with the aid of differential equations in the splitting momentum
fraction z. The epsilon-poles of the splitting amplitudes are consistent with a
formula due to Catani for the infrared singularities of two-loop scattering
amplitudes. This consistency essentially provides an inductive proof of
Catani's formula, as well as an ansatz for previously-unknown 1/epsilon pole
terms having non-trivial color structure. Finite terms in the splitting
amplitudes determine the collinear behavior of finite remainders in this
formula.Comment: 100 pages, 33 figures. Added remarks about leading-transcendentality
argument of hep-th/0404092, and additional explanation of cut-reconstruction
uniquenes
Evidence for orbital and North Atlantic climate forcing in alpine Southern California between 125 and 10 ka from multi-proxy analyses of Baldwin Lake
We employed a new, multi-proxy record from Baldwin Lake (∼125–10 ka) to examine drivers of terrestrial Southern California climate over long timescales. Correlated bulk organic and biogenic silica proxy data demonstrated high-amplitude changes from 125 to 71 ka, suggesting that summer insolation directly influenced lake productivity during MIS 5. From 60 to 57 ka, hydrologic state changes and events occurred in California and the U.S. Southwest, though the pattern of response varied geographically. Intermediate, less variable levels of winter and summer insolation followed during MIS 3 (57–29 ka), which likely maintained moist conditions in Southern California that were punctuated with smaller-order, millennial-scale events. These Dansgaard-Oeschger events brought enhanced surface temperatures (SSTs) to the eastern Pacific margin, and aridity to sensitive terrestrial sites in the Southwest and Southern California. Low temperatures and reduced evaporation are widespread during MIS 2, though there is increasing evidence for moisture extremes in Southern California from 29 to 20 ka. Our record shows that both orbital-scale radiative forcing and rapid North Atlantic temperature perturbations were likely influences on Southern California climate prior to the last glacial. However, these forcings produced a hydroclimatic response throughout California and the U.S. Southwest that was geographically complex. This work highlights that it is especially urgent to improve our understanding of the response to rapid climatic change in these regions. Enhanced temperature and aridity are projected for the rest of the 21st century, which will place stress on water resources
Talk the talk, walk the walk: Defining Critical Race Theory in research
Over the last decade there has been a noticeable growth in published works citing Critical Race Theory (CRT). This has led to a growth in interest in the UK of practical research projects utilising CRT as their framework. It is clear that research on 'race' is an emerging topic of study. What is less visible is a debate on how CRT is positioned in relation to methodic practice, substantive theory and epistemological underpinnings. The efficacy of categories of data gathering tools, both traditional and non-traditional is a discussion point here to explore the complexities underpinning decisions to advocate a CRT framework. Notwithstanding intersectional issues, a CRT methodology is recognisable by how philosophical, political and ethical questions are established and maintained in relation to racialised problematics. This paper examines these tensions in establishing CRT methodologies and explores some of the essential criteria for researchers to consider in utilising a CRT framework. © 2012 Copyright Taylor and Francis Group, LLC
LHC sensitivity to the resonance spectrum of a minimal strongly interacting electroweak symmetry breaking sector
We present a unified analysis of the two main production processes of vector
boson pairs at the LHC, VV-fusion and qqbar annihilation, in a minimal strongly
interacting electroweak symmetry breaking sector. Using a unitarized
electroweak chiral Lagrangian formalism and modeling the final V_L V_L strong
rescattering effects by a form factor, we describe qqbar annihilation processes
in terms of the two chiral parameters that govern elastic V_L V_L scattering.
Depending on the values of these two chiral parameters, the unitarized
amplitudes may present resonant enhancements in different angular
momentum-isospin channels. Scanning this two parameter space, we generate the
general resonance spectrum of a minimal strongly interacting electroweak
symmetry breaking sector and determine the regions that can be probed at the
LHC.Comment: Final version to appear in Phys. Rev. D, including a more detailed
exposition and a few more references. Conclusions and results unchanged. 14
pages, 5 figure
Observing the First Stars and Black Holes
The high sensitivity of JWST will open a new window on the end of the
cosmological dark ages. Small stellar clusters, with a stellar mass of several
10^6 M_sun, and low-mass black holes (BHs), with a mass of several 10^5 M_sun
should be directly detectable out to redshift z=10, and individual supernovae
(SNe) and gamma ray burst (GRB) afterglows are bright enough to be visible
beyond this redshift. Dense primordial gas, in the process of collapsing from
large scales to form protogalaxies, may also be possible to image through
diffuse recombination line emission, possibly even before stars or BHs are
formed. In this article, I discuss the key physical processes that are expected
to have determined the sizes of the first star-clusters and black holes, and
the prospect of studying these objects by direct detections with JWST and with
other instruments. The direct light emitted by the very first stellar clusters
and intermediate-mass black holes at z>10 will likely fall below JWST's
detection threshold. However, JWST could reveal a decline at the faint-end of
the high-redshift luminosity function, and thereby shed light on radiative and
other feedback effects that operate at these early epochs. JWST will also have
the sensitivity to detect individual SNe from beyond z=10. In a dedicated
survey lasting for several weeks, thousands of SNe could be detected at z>6,
with a redshift distribution extending to the formation of the very first stars
at z>15. Using these SNe as tracers may be the only method to map out the
earliest stages of the cosmic star-formation history. Finally, we point out
that studying the earliest objects at high redshift will also offer a new
window on the primordial power spectrum, on 100 times smaller scales than
probed by current large-scale structure data.Comment: Invited contribution to "Astrophysics in the Next Decade: JWST and
Concurrent Facilities", Astrophysics & Space Science Library, Eds. H.
Thronson, A. Tielens, M. Stiavelli, Springer: Dordrecht (2008
Acute Brain MRI Findings in 120 Malawian Children with Cerebral Malaria: New Insights into an Ancient Disease
Association between Features of Spontaneous Late Preterm Labor and Late Preterm Birth
Objective This study aimed to evaluate the association between clinical and examination features at admission and late preterm birth. Study Design The present study is a secondary analysis of a randomized trial of singleton pregnancies at 34 0/7 to 36 5/7 weeks' gestation. We included women in spontaneous preterm labor with intact membranes and compared them by gestational age at delivery (preterm vs. term). We calculated a statistical cut-point optimizing the sensitivity and specificity of initial cervical dilation and effacement at predicting preterm birth and used multivariable regression to identify factors associated with late preterm delivery. Results A total of 431 out of 732 (59%) women delivered preterm. Cervical dilation ≥ 4 cm was 60% sensitive and 68% specific for late preterm birth. Cervical effacement ≥ 75% was 59% sensitive and 65% specific for late preterm birth. Earlier gestational age at randomization, nulliparity, and fetal malpresentation were associated with late preterm birth. The final regression model including clinical and examination features significantly improved late preterm birth prediction (81% sensitivity, 48% specificity, area under the curve = 0.72, 95% confidence interval [CI]: 0.68-0.75, and p -value < 0.01). Conclusion Four in 10 women in late-preterm labor subsequently delivered at term. Combination of examination and clinical features (including parity and gestational age) improved late-preterm birth prediction
Physical Processes in Star Formation
© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00693-8.Star formation is a complex multi-scale phenomenon that is of significant importance for astrophysics in general. Stars and star formation are key pillars in observational astronomy from local star forming regions in the Milky Way up to high-redshift galaxies. From a theoretical perspective, star formation and feedback processes (radiation, winds, and supernovae) play a pivotal role in advancing our understanding of the physical processes at work, both individually and of their interactions. In this review we will give an overview of the main processes that are important for the understanding of star formation. We start with an observationally motivated view on star formation from a global perspective and outline the general paradigm of the life-cycle of molecular clouds, in which star formation is the key process to close the cycle. After that we focus on the thermal and chemical aspects in star forming regions, discuss turbulence and magnetic fields as well as gravitational forces. Finally, we review the most important stellar feedback mechanisms.Peer reviewedFinal Accepted Versio
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