78 research outputs found
Fragmentation inside an identified jet
Using Soft-Collinear Effective Theory we derive factorization formulae for
semi-inclusive processes where a light hadron h fragments from a jet whose
invariant mass is measured. Our analysis yields a novel "fragmenting jet
function" G_i^h(s,z) that depends on the jet invariant mass \sqrt{s}, and on
the fraction z of the large light-cone momentum components of the hadron and
the parent parton i. We show that G_i^h(s,z) 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). Our analysis
yields a simple replacement rule that allows any factorization theorem
depending on a jet function J_i to be converted to a semi-inclusive process
with a fragmenting hadron h.Comment: 3 pages; presented at "Quark Confinement and the Hadron Spectrum IX -
QCHS IX" (30 August - 3 September 2010, Madrid, Spain), to appear in the
proceeding
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
Improved predictions for conversion in nuclei and Higgs-induced lepton flavor violation
Compared to and , the process
conversion in nuclei receives enhanced contributions from Higgs-induced lepton
flavor violation. Upcoming conversion experiments with drastically
increased sensitivity will be able to put extremely stringent bounds on
Higgs-mediated transitions. We point out that the theoretical
uncertainties associated with these Higgs effects, encoded in the couplings of
quark scalar operators to the nucleon, can be accurately assessed using our
recently developed approach based on Chiral Perturbation Theory that
cleanly separates two- and three-flavor observables. We emphasize that with
input from lattice QCD for the coupling to strangeness , hadronic
uncertainties are appreciably reduced compared to the traditional approach
where is determined from the pion--nucleon -term by means of an
relation. We illustrate this point by considering Higgs-mediated lepton
flavor violation in the Standard Model supplemented with higher-dimensional
operators, the two-Higgs-doublet model with generic Yukawa couplings, and the
Minimal Supersymmetric Standard Model. Furthermore, we compare bounds from
present and future conversion and experiments.Comment: 9 pages, 5 figures, journal versio
Accurate evaluation of hadronic uncertainties in spin-independent WIMP-nucleon scattering: Disentangling two- and three-flavor effects
We show how to avoid unnecessary and uncontrolled assumptions usually made in
the literature about soft SU(3) flavor symmetry breaking in determining the
two-flavor nucleon matrix elements relevant for direct detection of WIMPs.
Based on SU(2) Chiral Perturbation Theory, we provide expressions for the
proton and neutron scalar couplings and with the
pion-nucleon sigma-term as the only free parameter, which should be used in the
analysis of direct detection experiments. This approach for the first time
allows for an accurate assessment of hadronic uncertainties in spin-independent
WIMP-nucleon scattering and for a reliable calculation of isospin-violating
effects. We find that the traditional determinations of and
are off by a factor of 2.Comment: 6 pages, 2 figures; improved numerical analysis, journal versio
New Constraints on Dark Matter Effective Theories from Standard Model Loops
We consider an effective field theory for a gauge singlet Dirac dark matter
(DM) particle interacting with the Standard Model (SM) fields via effective
operators suppressed by the scale TeV. We perform a
systematic analysis of the leading loop contributions to spin-independent (SI)
DM--nucleon scattering using renormalization group evolution between
and the low-energy scale probed by direct detection experiments. We find that
electroweak interactions induce operator mixings such that operators that are
naively velocity-suppressed and spin-dependent can actually contribute to SI
scattering. This allows us to put novel constraints on Wilson coefficients that
were so far poorly bounded by direct detection. Constraints from current
searches are comparable to LHC bounds, and will significantly improve in the
near future. Interestingly, the loop contribution we find is maximally isospin
violating even if the underlying theory is isospin conserving.Comment: 6 pages, 3 figures. v2: revised manuscript, updated formulas and
plots, improved bounds, references added, journal versio
Resummation of Double-Differential Cross Sections and Fully-Unintegrated Parton Distribution Functions
LHC measurements involve cuts on several observables, but resummed
calculations are mostly restricted to single variables. We show how the
resummation of a class of double-differential measurements can be achieved
through an extension of Soft-Collinear Effective Theory (SCET). A prototypical
application is jets, where the jet veto is imposed through the
beam thrust event shape , and the transverse momentum of
the boson is measured. A standard SCET analysis suffices for and , but additional
collinear-soft modes are needed in the intermediate regime. We show how to
match the factorization theorems that describe these three different regions of
phase space, and discuss the corresponding relations between fully-unintegrated
parton distribution functions, soft functions and the newly defined
collinear-soft functions. The missing ingredients needed at NNLL/NLO accuracy
are calculated, providing a check of our formalism. We also revisit the
calculation of the measurement of two angularities on a single jet in JHEP 1409
(2014) 046, finding a correction to their conjecture for the NLL cross section
at .Comment: 43 pages, 3 figures. v2: JHEP version, discussion on non-global
logarithms adde
Connecting Dark Matter UV Complete Models to Direct Detection Rates via Effective Field Theory
Direct searches for WIMPs are sensitive to physics well below the weak scale.
In the absence of light mediators, it is fruitful to apply an Effective Field
Theory (EFT) approach accounting only for dark matter (DM) interactions with
Standard Model (SM) fields. We consider a singlet fermion WIMP and effective
operators up to dimension 6 which are generated at the mass scale of particles
mediating DM interactions with the SM. We perform a one-loop Renormalization
Group Evolution (RGE) analysis, evolving these effective operators from the
mediators mass scale to the nuclear scales probed by direct searches. We apply
our results to models with DM velocity-suppressed interactions, DM couplings
only to heavy quarks, leptophilic DM and Higgs portal, which without our
analysis would not get constrained from direct detection bounds. Remarkably, a
large parameter space region for these models is found to be excluded as a
consequence of spin-independent couplings induced by SM loops. In addition to
these examples, we stress that more general renormalizable models for singlet
fermion WIMP can be matched onto our EFT framework, and the subsequent
model-independent RGE can be used to compute direct detection rates. Our
results allow us to properly connect the different energy scales involved in
constraining WIMP models, and to combine information from direct detection with
other complementary searches, such as collider and indirect detection.Comment: 22 pages + appendices, 11 figures; v2, improved numerics, references
added, typos corrected; v3, minor changes, journal versio
Chiral extrapolations of nucleon properties from lattice QCD
We report on recent work about the study of quark mass dependence of nucleon
magnetic moments and axial-vector coupling constant. We examine the feasibility
of chiral effective field theory methods for the extrapolation of lattice QCD
data obtained at relative large pion masses down to the physical values.Comment: 5pages, LaTeX, 3 figures, uses World Scientific style file; presented
at PANIC 02, Osak
Dispersion relation for hadronic light-by-light scattering: theoretical foundations
In this paper we make a further step towards a dispersive description of the
hadronic light-by-light (HLbL) tensor, which should ultimately lead to a
data-driven evaluation of its contribution to . We first provide a
Lorentz decomposition of the HLbL tensor performed according to the general
recipe by Bardeen, Tung, and Tarrach, generalizing and extending our previous
approach, which was constructed in terms of a basis of helicity amplitudes.
Such a tensor decomposition has several advantages: the role of gauge
invariance and crossing symmetry becomes fully transparent; the scalar
coefficient functions are free of kinematic singularities and zeros, and thus
fulfill a Mandelstam double-dispersive representation; and the explicit
relation for the HLbL contribution to in terms of the coefficient
functions simplifies substantially. We demonstrate explicitly that the
dispersive approach defines both the pion-pole and the pion-loop contribution
unambiguously and in a model-independent way. The pion loop, dispersively
defined as pion-box topology, is proven to coincide exactly with the one-loop
scalar QED amplitude, multiplied by the appropriate pion vector form factors.Comment: 59 pages, 11 figures. Draws on and substantially extends
arXiv:1412.5171 [hep-ph] and arXiv:1402.7081 [hep-ph]. Version accepted for
publication in JHE
Hadronic light-by-light contribution to : a dispersive approach
After a brief introduction on ongoing experimental and theoretical activities
on , we report on recent progress in approaching the calculation of
the hadronic light-by-light contribution with dispersive methods. General
properties of the four-point function of the electromagnetic current in QCD,
its Lorentz decomposition and dispersive representation are discussed. On this
basis a numerical estimate for the pion box contribution and its rescattering
corrections is obtained. We conclude with an outlook for this approach to the
calculation of hadronic light-by-light.Comment: 18 pages, 5 figures. Proceedings of the 35th International Symposium
on Lattice Field Theory, 18-24 June 2017, Granada, Spain. Plenary tal
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