166 research outputs found
Higher twist effects in parity-violating electron deuteron scattering
Parity-Violating Deep Inelastic Scattering (PVDIS) of polarized electrons from deuterium can in principle probe higher twist quark-quark correlations beyond the parton model. As first observed by Bjorken and Wolfenstein, the
dominant contribution to the electron polarization asymmetry, proportional to the axial vector electron coupling, receives corrections at twist-four from the matrix
element of a single four-quark operator. In particular, because the contribution of the relevant twist four operator satisfies the Callan-Gross relation, the ratio of
parity-violating longitudinal and transverse cross sections, RγZ, is identical to that for purely electromagnetic scattering, Rγ, up to perturbative and power suppressed contributions. This result simplifies the interpretation of the asymmetry for experiments planned at the Thomas Jefferson National Accelerator Facility (JLab).
The results of MIT Bag Model calculations are used to give benchmark estimates of the relevant twist four operator contribution to the leading term in the asymmetry as a function of Bjorken x and Q2. These are compared with corrections to the asymmetry due to violation of charge symmetry in the parton distribution functions
Factorization and Resummation of Higgs Boson Differential Distributions in Soft-Collinear Effective Theory
We derive a factorization theorem for the Higgs boson transverse momentum
(p_T) and rapidity (Y) distributions at hadron colliders, using the Soft
Collinear Effective Theory (SCET), for m_h>> p_T>> \Lambda_{QCD} where m_h
denotes the Higgs mass. In addition to the factorization of the various scales
involved, the perturbative physics at the p_T scale is further factorized into
two collinear impact-parameter Beam Functions (iBFs) and an inverse Soft
Function (iSF). These newly defined functions are of a universal nature for the
study of differential distributions at hadron colliders. The additional
factorization of the p_T-scale physics simplifies the implementation of higher
order radiative corrections in \alpha_s(p_T). We derive formulas for
factorization in both momentum and impact parameter space and discuss the
relationship between them. Large logarithms of the relevant scales in the
problem are summed using the renormalization group equations of the effective
theories. Power corrections to the factorization theorem in p_T/m_h and
\Lambda_{QCD}/p_T can be systematically derived. We perform multiple
consistency checks on our factorization theorem including a comparison with
known fixed order QCD results. We compare the SCET factorization theorem with
the Collins-Soper-Sterman approach to low-p_T resummation.Comment: 66 pages, 5 figures, discussion regarding zero-bin subtractions adde
Low Mass Dark Matter and Invisible Higgs Width In Darkon Models
The Standard Model (SM) plus a real gauge-singlet scalar field dubbed darkon
(SM+D) is the simplest model possessing a weakly interacting massive particle
(WIMP) dark-matter candidate. In this model, the parameters are constrained
from dark matter relic density and direct searches. The fact that interaction
between darkon and SM particles is only mediated by Higgs boson exchange may
lead to significant modifications to the Higgs boson properties. If the dark
matter mass is smaller than a half of the Higgs boson mass, the Higgs boson can
decay into a pair of darkons resulting in a large invisible branching ratio.
The Higgs boson will be searched for at the LHC and may well be discovered in
the near future. If a Higgs boson with a small invisible decay width will be
found, the SM+D model with small dark matter mass will be in trouble. We find
that by extending the SM+D to a two-Higgs-doublet model plus a darkon (THDM+D)
it is possible to have a Higgs boson with a small invisible branching ratio and
at the same time the dark matter can have a low mass. We also comment on other
implications of this model.Comment: RevTeX, 15 pages, 11 figures. A few typos corrected and some
references adde
Jets from Massive Unstable Particles: Top-Mass Determination
We construct jet observables for energetic top quarks that can be used to
determine a short distance top quark mass from reconstruction in e+ e-
collisions with accuracy better than Lambda_{QCD}. Using a sequence of
effective field theories we connect the production energy, mass, and top width
scales, Q>> m>> Gamma, for the top jet cross section, and derive a QCD
factorization theorem for the top invariant mass spectrum. Our analysis
accounts for: alpha_s corrections from the production and mass scales,
corrections due to constraints in defining invariant masses, non-perturbative
corrections from the cross-talk between the jets, and alpha_s corrections to
the Breit-Wigner line-shape. This paper mainly focuses on deriving the
factorization theorem for hemisphere invariant mass distributions and other
event shapes in e+e- collisions applicable at a future Linear Collider. We show
that the invariant mass distribution is not a simple Breit-Wigner involving the
top width. Even at leading order it is shifted and broadened by
non-perturbative soft QCD effects. We predict that the invariant mass peak
position increases linearly with Q/m due to these non-perturbative effects.
They are encoded in terms of a universal soft function that also describes soft
effects for massless dijet events. In a future paper we compute alpha_s
corrections to the jet invariant mass spectrum, including a summation of large
logarithms between the scales Q, m and Gamma.Comment: 54 pages, 10 figures, typos corrected, figures update
Electroweak Gauge-Boson Production at Small q_T: Infrared Safety from the Collinear Anomaly
Using methods from effective field theory, we develop a novel, systematic
framework for the calculation of the cross sections for electroweak gauge-boson
production at small and very small transverse momentum q_T, in which large
logarithms of the scale ratio M_V/q_T are resummed to all orders. These cross
sections receive logarithmically enhanced corrections from two sources: the
running of the hard matching coefficient and the collinear factorization
anomaly. The anomaly leads to the dynamical generation of a non-perturbative
scale q_* ~ M_V e^{-const/\alpha_s(M_V)}, which protects the processes from
receiving large long-distance hadronic contributions. Expanding the cross
sections in either \alpha_s or q_T generates strongly divergent series, which
must be resummed. As a by-product, we obtain an explicit non-perturbative
expression for the intercept of the cross sections at q_T=0, including the
normalization and first-order \alpha_s(q_*) correction. We perform a detailed
numerical comparison of our predictions with the available data on the
transverse-momentum distribution in Z-boson production at the Tevatron and LHC.Comment: 34 pages, 9 figure
Forward Jets and Energy Flow in Hadronic Collisions
We observe that at the Large Hadron Collider, using forward + central
detectors, it becomes possible for the first time to carry out calorimetric
measurements of the transverse energy flow due to "minijets" accompanying
production of two jets separated by a large rapidity interval. We present
parton-shower calculations of energy flow observables in a high-energy
factorized Monte Carlo framework, designed to take into account QCD logarithmic
corrections both in the large rapidity interval and in the hard transverse
momentum. Considering events with a forward and a central jet, we examine the
energy flow in the interjet region and in the region away from the jets. We
discuss the role of these observables to analyze multiple parton collision
effects.Comment: 9 pages, 5 figures. Version2: added results on azimuthal
distributions and more discussion of energy flow definition using jet
clusterin
Fully-Unintegrated Parton Distribution and Fragmentation Functions at Perturbative k_T
We define and study the properties of generalized beam functions (BFs) and
fragmenting jet functions (FJFs), which are fully-unintegrated parton
distribution functions (PDFs) and fragmentation functions (FFs) for
perturbative k_T. We calculate at one loop the coefficients for matching them
onto standard PDFs and FFs, correcting previous results for the BFs in the
literature. Technical subtleties when measuring transverse momentum in
dimensional regularization are clarified, and this enables us to renormalize in
momentum space. Generalized BFs describe the distribution in the full
four-momentum k_mu of a colliding parton taken out of an initial-state hadron,
and therefore characterize the collinear initial-state radiation. We illustrate
their importance through a factorization theorem for pp -> l^+ l^- + 0 jets,
where the transverse momentum of the lepton pair is measured. Generalized FJFs
are relevant for the analysis of semi-inclusive processes where the full
momentum of a hadron, fragmenting from a jet with constrained invariant mass,
is measured. Their significance is shown for the example of e^+ e^- -> dijet+h,
where the perpendicular momentum of the fragmenting hadron with respect to the
thrust axis is measured.Comment: Journal versio
Probing the low transverse momentum domain of Z production with novel variables
The measurement of the low transverse momentum region of vector boson
production in Drell-Yan processes has long been invaluable to testing our
knowledge of QCD dynamics both beyond fixed-order in perturbation theory as
well as in the non-perturbative region. Recently the D\O\ collaboration have
introduced novel variables which lead to improved measurements compared to the
case of the standard QT variable. To complement this improvement on the
experimental side, we develop here a complete phenomenological study dedicated
in particular to the new \phi* variable. We compare our study, which contains
the state-of-the-art next-to-next-to-leading resummation of large logarithms
and a smooth matching to the full next-to-leading order result, to the
experimental data and find excellent agreement over essentially the entire
range of \phi*, even without direct inclusion of non-perturbative effects. We
comment on our findings and on the potential for future studies to constrain
non-perturbative behaviour.Comment: 20 pages, 7 figures. Version accepted for publication in JHEP. A
figure with comparison to RESBOS has been adde
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