1,042 research outputs found
A momentum subtraction scheme for two--nucleon effective field theory
We introduce a momentum subtraction scheme which obeys the power counting of
Kaplan, Savage, and Wise (KSW), developed for systems with large scattering
lengths, . Unlike the power divergence subtraction scheme, coupling
constants in this scheme obey the KSW scaling for all . We comment
on the low-energy theorems derived by Cohen and Hansen. We conclude that there
is no obstruction to using perturbative pions for momenta .Comment: 12 pages, 3 fig
Jet Vetoes Interfering with H->WW
Far off-shell Higgs production in , is a particularly
powerful probe of Higgs properties, allowing one to disentangle Higgs width and
coupling information unavailable in on-shell rate measurements. These
measurements require an understanding of the cross section in the far off-shell
region in the presence of realistic experimental cuts. We analytically study
the effect of a jet veto on far off-shell cross sections, including
signal-background interference, by utilizing hard functions in the soft
collinear effective theory that are differential in the decay products of the
. Summing large logarithms of , we find that the
jet veto induces a strong dependence on the partonic centre of mass energy,
, and modifies distributions in or . The
example of is used to demonstrate these
effects at next to leading log order. We also discuss the importance of jet
vetoes and jet binning for the recent program to extract Higgs couplings and
widths from far off-shell cross sections.Comment: 31 pages, 8 figures. v2: Journal Versio
Matching the Quasi Parton Distribution in a Momentum Subtraction Scheme
The quasi parton distribution is a spatial correlation of quarks or gluons
along the direction in a moving nucleon which enables direct lattice
calculations of parton distribution functions. It can be defined with a
nonperturbative renormalization in a regularization independent momentum
subtraction scheme (RI/MOM), which can then be perturbatively related to the
collinear parton distribution in the scheme. Here we
carry out a direct matching from the RI/MOM scheme for the quasi-PDF to the
PDF, determining the non-singlet quark matching
coefficient at next-to-leading order in perturbation theory. We find that the
RI/MOM matching coefficient is insensitive to the ultraviolet region of
convolution integral, exhibits improved perturbative convergence when
converting between the quasi-PDF and PDF, and is consistent with a quasi-PDF
that vanishes in the unphysical region as the proton momentum ,
unlike other schemes. This direct approach therefore has the potential to
improve the accuracy for converting quasi-distribution lattice calculations to
collinear distributions.Comment: 18 pages, 6 figure
Quark Fragmentation within an Identified Jet
We derive a factorization theorem that describes an energetic hadron h
fragmenting from a jet produced by a parton i, where the jet invariant mass is
measured. The analysis yields a "fragmenting jet function" G_i^h(s,z) that
depends on the jet invariant mass s, and on the energy fraction z of the
fragmentation hadron. We show that G^h_i can be computed in terms of
perturbatively calculable coefficients, J_{ij}(s,z/x), integrated against
standard non-perturbative fragmentation functions, D_j^{h}(x). We also show
that the sum over h of the integral over z of z G_i^h(s,z) is given by the
standard inclusive jet function J_i(s) which is perturbatively calculable in
QCD. We use Soft-Collinear Effective Theory and for simplicity carry out our
derivation for a process with a single jet, B -> X h l nu, with invariant mass
m_{X h}^2 >> Lambda_QCD^2. Our analysis yields a simple replacement rule that
allows any factorization theorem depending on an inclusive jet function J_i to
be converted to a semi-inclusive process with a fragmenting hadron h. We apply
this rule to derive factorization theorems for B -> X K gamma which is the
fragmentation to a Kaon in b -> s gamma, and for e^+e^- -> (dijets)+h with
measured hemisphere dijet invariant masses.Comment: 26 pages, 2 figures; v3: small correction to eq.(72
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