40,047 research outputs found
Soft gluons and gauge-invariant subtractions in NLO parton-shower Monte Carlo event generators
We address the problem of decomposing graphs in perturbative QCD into terms
associated with particular regions. Motivated by asking how to incorporate
next-to-leading order (NLO) QCD corrections in parton-shower algorithms, we
require that: (a) The integrand for the hard part is to be integrable even if
the corrections are applied to a process that is not infrared and collinear
safe. (b) The splitting between the terms should be defined gauge-invariantly.
(c) The dependence on cut-offs should obey homogeneous evolution equations. In
the context of one-gluon-emission graphs for deep inelastic scattering, we
explain a subtractive technique that is based on gauge-invariant Wilson-line
operators. Appropriate organization of subtractions involving the soft region
allows a connection to previous work where evolution equations with respect to
the directions of the Wilson lines have been derived.Comment: 11 pages, 2 figures; v2: comments and references added, results
unchange
Calculation of TMD Evolution for Transverse Single Spin Asymmetry Measurements
The Sivers transverse single spin asymmetry (TSSA) is calculated and compared
at different scales using the TMD evolution equations applied to previously
existing extractions. We apply the Collins-Soper-Sterman (CSS) formalism, using
the version recently developed by Collins. Our calculations rely on the
universality properties of TMD-functions that follow from the TMD-factorization
theorem. Accordingly, the non-perturbative input is fixed by earlier
experimental measurements, including both polarized semi-inclusive deep
inelastic scattering (SIDIS) and unpolarized Drell-Yan (DY) scattering. It is
shown that recent COMPASS measurements are consistent with the suppression
prescribed by TMD evolution.Comment: 4 pages, 2 figures. Version published in Physical Review Letter
Relative distributions of W's and Z's at low transverse momenta
Despite large uncertainties in the and transverse momentum
() distributions for q_T\lsim 10 GeV, the ratio of the distributions
varys little. The uncertainty in the ratio of to distributions is
on the order of a few percent, independent of the details of the
nonperturbative parameterization.Comment: 13 pages in revtex, 5 postscript figures available upon request,
UIOWA-94-0
Renormalized masses of heavy Kaluza-Klein states
Several ways of computing the radiative corrections to the heavy boson masses
in Kaluza-Klein theory are discussed. It is argued that only an intrinsically
higher dimensional approach embodies all the desired physical properties.Comment: LaTeX, 22 pages. Fully rewritten and streamlined. Five and six
dimensional cases treated separatelly. References adde
Post-Impact Thermal Evolution of Porous Planetesimals
Impacts between planetesimals have largely been ruled out as a heat source in
the early Solar System, by calculations that show them to be an inefficient
heat source and unlikely to cause global heating. However, the long-term,
localized thermal effects of impacts on planetesimals have never been fully
quantified. Here, we simulate a range of impact scenarios between planetesimals
to determine the post-impact thermal histories of the parent bodies, and hence
the importance of impact heating in the thermal evolution of planetesimals. We
find on a local scale that heating material to petrologic type 6 is achievable
for a range of impact velocities and initial porosities, and impact melting is
possible in porous material at a velocity of > 4 km/s. Burial of heated
impactor material beneath the impact crater is common, insulating that material
and allowing the parent body to retain the heat for extended periods (~
millions of years). Cooling rates at 773 K are typically 1 - 1000 K/Ma,
matching a wide range of measurements of metallographic cooling rates from
chondritic materials. While the heating presented here is localized to the
impact site, multiple impacts over the lifetime of a parent body are likely to
have occurred. Moreover, as most meteorite samples are on the centimeter to
meter scale, the localized effects of impact heating cannot be ignored.Comment: 38 pages, 9 figures, Revised for Geochimica et Cosmochimica Acta
(Sorry, they do not accept LaTeX
Single spin asymmetries in hadron-hadron collisions
We study weighted azimuthal single spin asymmetries in hadron-hadron
scattering using the diagrammatic approach at leading order and assuming
factorization. The effects of the intrinsic transverse momenta of the partons
are taken into account. We show that the way in which -odd functions, such
as the Sivers function, appear in these processes does not merely involve a
sign flip when compared with semi-inclusive deep inelastic scattering, such as
in the case of the Drell-Yan process. Expressions for the weighted scattering
cross sections in terms of distribution and fragmentation functions folded with
hard cross sections are obtained by introducing modified hard cross sections,
referred to as gluonic pole cross sections.Comment: 22 pages, 4 figures; minor text modifications and some additional
reference
Wiring Viterbi decoders (splitting deBruijn graphs)
A new Viterbi decoder, capable of decoding convolutional codes with constraint lengths up to 15, is under development for the Deep Space Network (DSN). A key feature of this decoder is a two-level partitioning of the Viterbi state diagram into identical subgraphs. The larger subgraphs correspond to circuit boards, while the smaller subgraphs correspond to Very Large Scale Integration (VLSI) chips. The full decoder is built from identical boards, which in turn are built from identical chips. The resulting system is modular and hierarchical. The decoder is easy to implement, test, and repair because it uses a single VLSI chip design and a single board design. The partitioning is completely general in the sense that an appropriate number of boards or chips may be wired together to implement a Viterbi decoder of any size greater than or equal to the size of the module
Hard-scattering factorization with heavy quarks: A general treatment
A detailed proof of hard scattering factorization is given with the inclusion
of heavy quark masses. Although the proof is explicitly given for
deep-inelastic scattering, the methods apply more generally The
power-suppressed corrections to the factorization formula are uniformly
suppressed by a power of \Lambda/Q, independently of the size of heavy quark
masses, M, relative to Q.Comment: 52 pages. Version as published plus correction of misprint in Eq.
(45
Designing Primary Prevention for People Living with HIV
Today, there are new reasons for a sharper focus on prevention for people living with HIV. Growing numbers of people with the disease are living more healthy, sexual lives. Recent evidence suggests that risk taking among both HIV-positive and negative people is increasing. After nearly two decades of life in the shadow of AIDS, communities are growing weary of traditional prevention messages and many people are openly grappling with difficult questions of intimacy and sex. Increasingly, people living with HIV also face multiple complex economic and substance abuse challenges that complicate prevention efforts.There is an urgent need -- and sufficient expertise -- to move forward with prevention campaigns focused on helping people living with HIV and AIDS avoid passing their infection along to others. Numerous innovative interventions for people with HIV show promise, including:a social marketing campaign for gay men and a five-session group intervention for women living with HIV in Massachusetts,a chat line for positives and a group session program for Latinas/Latinos in Los Angeles,Internet chat room interventions in Atlanta,a group session for gay Asian American-Pacific Islander Americans living with HIV in San Francisco, andPrevention Case Management programs newly funded by the Centers for Disease Control
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