417 research outputs found
A consistent picture for large penguins in D -> pi+ pi-, K+ K-
A long-standing puzzle in charm physics is the large difference between the
D0 -> K+ K- and D0 -> pi+ pi- decay rates. Recently, the LHCb and CDF
collaborations reported a surprisingly large difference between the direct CP
asymmetries, Delta A_CP, in these two modes. We show that the two puzzles are
naturally related in the Standard Model via s- and d-quark "penguin
contractions". Their sum gives rise to Delta A_CP, while their difference
contributes to the two branching ratios with opposite sign. Assuming nominal
SU(3) breaking, a U-spin fit to the D0 -> K+ pi-, pi+ K-, pi+ pi-, K+ K- decay
rates yields large penguin contractions that naturally explain Delta A_CP.
Expectations for the individual CP asymmetries are also discussed.Comment: 24 pages, 8 figure
Long-Lived Neutralino NLSPs
We investigate the collider signatures of heavy, long-lived, neutral
particles that decay to charged particles plus missing energy. Specifically, we
focus on the case of a neutralino NLSP decaying to Z and gravitino within the
context of General Gauge Mediation. We show that a combination of searches
using the inner detector and the muon spectrometer yields a wide range of
potential early LHC discoveries for NLSP lifetimes ranging from 10^(-1)-10^5
mm. We further show that events from Z(l+l-) can be used for detailed kinematic
reconstruction, leading to accurate determinations of the neutralino mass and
lifetime. In particular, we examine the prospects for detailed event study at
ATLAS using the ECAL (making use of its timing and pointing capabilities)
together with the TRT, or using the muon spectrometer alone. Finally, we also
demonstrate that there is a region in parameter space where the Tevatron could
potentially discover new physics in the delayed Z(l+l-)+MET channel. While our
discussion centers on gauge mediation, many of the results apply to any
scenario with a long-lived neutral particle decaying to charged particles.Comment: 31 pages, 12 figure
Z' signals in polarised top-antitop final states
We study the sensitivity of top-antitop samples produced at all energy stages
of the Large Hadron Collider (LHC) to the nature of an underlying Z' boson, in
presence of full tree level standard model (SM) background effects and relative
interferences. We concentrate on differential mass spectra as well as both
spatial and spin asymmetries thereby demonstrating that exploiting combinations
of these observables will enable one to distinguish between sequential Z's and
those pertaining to Left-Right symmetric models as well as E6 inspired ones,
assuming realistic final state reconstruction efficiencies and error estimates.Comment: 21 pages, 6 colour figures, 10 table
Composite GUTs: models and expectations at the LHC
We investigate grand unified theories (GUTs) in scenarios where electroweak
(EW) symmetry breaking is triggered by a light composite Higgs, arising as a
Nambu-Goldstone boson from a strongly interacting sector. The evolution of the
standard model (SM) gauge couplings can be predicted at leading order, if the
global symmetry of the composite sector is a simple group G that contains the
SM gauge group. It was noticed that, if the right-handed top quark is also
composite, precision gauge unification can be achieved. We build minimal
consistent models for a composite sector with these properties, thus
demonstrating how composite GUTs may represent an alternative to supersymmetric
GUTs. Taking into account the new contributions to the EW precision parameters,
we compute the Higgs effective potential and prove that it realizes
consistently EW symmetry breaking with little fine-tuning. The G group
structure and the requirement of proton stability determine the nature of the
light composite states accompanying the Higgs and the top quark: a coloured
triplet scalar and several vector-like fermions with exotic quantum numbers. We
analyse the signatures of these composite partners at hadron colliders:
distinctive final states contain multiple top and bottom quarks, either alone
or accompanied by a heavy stable charged particle, or by missing transverse
energy.Comment: 55 pages, 13 figures, final version to be published in JHE
Vectorlike Confinement at the LHC
We argue for the plausibility of a broad class of vectorlike confining gauge
theories at the TeV scale which interact with the Standard Model predominantly
via gauge interactions. These theories have a rich phenomenology at the LHC if
confinement occurs at the TeV scale, while ensuring negligible impact on
precision electroweak and flavor observables. Spin-1 bound states can be
resonantly produced via their mixing with Standard Model gauge bosons. The
resonances promptly decay to pseudo-Goldstone bosons, some of which promptly
decay to a pair of Standard Model gauge bosons, while others are charged and
stable on collider time scales. The diverse set of final states with little
background include multiple photons and leptons, missing energy, massive stable
charged particles and the possibility of highly displaced vertices in dilepton,
leptoquark or diquark decays. Among others, a novel experimental signature of
resonance reconstruction out of massive stable charged particles is
highlighted. Some of the long-lived states also constitute Dark Matter
candidates.Comment: 33 pages, 6 figures. v4: expanded discussion of Z_2 symmetry for
stability, one reference adde
Comparative analysis of the semileptonic transition in SM and different SUSY scenarios using form factors from full QCD
We work out the semileptonic
transition in standard as well as different supersymmetric models. In
particular, considering the parametrization of the matrix elements entered the
low energy effective Hamiltonian in terms of form factors in full QCD, we
calculate the amplitude and differential decay rate responsible for this decay
channel in supersymmetric models. We then use the form factors calculated via
light cone QCD sum rules in full theory to analyze the differential branching
ratio and lepton forward-backward asymmetry of this decay channel in different
supersymmetric models and compare the obtained results with those of the
standard model. We also discuss how the results of different supersymmetric
models deviate from the standard model predictions and which SUSY scenarios are
favored.Comment: 23 Pages, 8 Figures and 11 Table
Flavourful Production at Hadron Colliders
We ask what new states may lie at or below the TeV scale, with sizable
flavour-dependent couplings to light quarks, putting them within reach of
hadron colliders via resonant production, or in association with Standard Model
states. In particular, we focus on the compatibility of such states with
stringent flavour-changing neutral current and electric-dipole moment
constraints. We argue that the broadest and most theoretically plausible
flavour structure of the new couplings is that they are hierarchical, as are
Standard Model Yukawa couplings, although the hierarchical pattern may well be
different. We point out that, without the need for any more elaborate or
restrictive structure, new scalars with "diquark" couplings to standard quarks
are particularly immune to existing constraints, and that such scalars may
arise within a variety of theoretical paradigms. In particular, there can be
substantial couplings to a pair of light quarks or to one light and one heavy
quark. For example, the latter possibility may provide a flavour-safe
interpretation of the asymmetry in top quark production observed at the
Tevatron. We thereby motivate searches for diquark scalars at the Tevatron and
LHC, and argue that their discovery represents one of our best chances for new
insight into the Flavour Puzzle of the Standard Model.Comment: 18 pp., 8 figures, references adde
Pure-glue hidden valleys through the Higgs portal
We consider the possibility that the Higgs boson can act as a link to a
hidden sector in the context of pure-glue hidden valley models. In these models
the standard model is weakly coupled, through loops of heavy messengers fields,
to a hidden sector whose low energy dynamics is described by a pure-Yang-Mills
theory. Such a hidden sector contains several metastable hidden glueballs. In
this work we shall extend earlier results on hidden valleys to include
couplings of the messengers to the standard model Higgs sector. The effective
interactions at one-loop couple the hidden gluons to the standard model
particles through the Higgs sector. These couplings in turn induce hidden
glueball decays to fermion pairs, or cascade decays with multiple Higgs
emission. The presence of effective operators of different mass dimensions,
often competing with each other, together with a great diversity of states,
leads to a great variability in the lifetimes and decay modes of the hidden
glueballs. We find that most of the operators considered in this paper are not
heavily constrained by precision electroweak physics, therefore leaving plenty
of room in the parameter space to be explored by the future experiments at the
LHC.Comment: 44 pages, 16 figures. Major revision for JHEP, corrected an error in
Eq. 5.1, comments adde
Heavy-to-light baryonic form factors at large recoil
We analyze heavy-to-light baryonic form factors at large recoil and derive
the scaling behavior of these form factors in the heavy quark limit. It is
shown that only one universal form factor is needed to parameterize Lambda_b to
p and Lambda_b to Lambda matrix elements in the large recoil limit of light
baryons, while hadronic matrix elements of Lambda_b to Sigma transition vanish
in the large energy limit of Sigma baryon due to the space-time parity
symmetry. The scaling law of the soft form factor eta(P^{\prime} \cdot v),
P^{\prime} and v being the momentum of nucleon and the velocity of Lambda_b
baryon, responsible for Lambda_b to p transitions is also derived using the
nucleon distribution amplitudes in leading conformal spin. In particular, we
verify that this scaling behavior is in full agreement with that from
light-cone sum rule approach in the heavy-quark limit. With these form factors,
we further investigate the Lambda baryon polarization asymmetry alpha in
Lambda_b to Lambda gamma and the forward-backward asymmetry A_{FB} in Lambda_b
to Lambda l^{+} l^{-}. Both two observables (alpha and A_{FB}) are independent
of hadronic form factors in leading power of 1/m_b and in leading order of
alpha_s. We also extend the analysis of hadronic matrix elements for Omega_b to
Omega transitions to rare Omega_b to Omega gamma and Omega_b to Omega l^{+}
l^{-} decays and find that radiative Omega_b to Omega gamma decay is probably
the most promising FCNC b to s radiative baryonic decay channel. In addition,
it is interesting to notice that the zero-point of forward-backward asymmetry
of Omega_b to Omega l^{+} l^{-} is the same as the one for Lambda_b to Lambda
l^{+} l^{-} to leading order accuracy provided that the form factors
\bar{\zeta}_i (i=3, 4, 5) are numerically as small as indicated from the quark
model.Comment: 19 page
Probing Colored Particles with Photons, Leptons, and Jets
If pairs of new colored particles are produced at the Large Hadron Collider,
determining their quantum numbers, and even discovering them, can be
non-trivial. We suggest that valuable information can be obtained by measuring
the resonant signals of their near-threshold QCD bound states. If the particles
are charged, the resulting signatures include photons and leptons and are
sufficiently rich for unambiguously determining their various quantum numbers,
including the charge, color representation and spin, and obtaining a precise
mass measurement. These signals provide well-motivated benchmark models for
resonance searches in the dijet, photon+jet, diphoton and dilepton channels.
While these measurements require that the lifetime of the new particles be not
too short, the resulting limits, unlike those from direct searches for pair
production above threshold, do not depend on the particles' decay modes. These
limits may be competitive with more direct searches if the particles decay in
an obscure way.Comment: 39 pages, 9 figures; v2: more recent searches include
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