196 research outputs found
Light colored scalars from grand unification and the forward-backward asymmetry in top quark pair production
The experimental results on the t bar t production cross section at the
Tevatron are well described by the QCD contributions within the standard model,
while the recent measurement of the forward-backward asymmetry cannot be
accounted for within this framework. We consider light colored scalars
appearing in a particular SU(5) GUT model within the 45-dimensional Higgs
representation. A virtue of the model is that it connects the presence of a
light colored SU(2) singlet (Delta_6) and a color octet weak doublet (Delta_1)
with bounds on the proton lifetime, which constrain the parameter space of both
scalars. We find that both the t bar t production cross section and the
forward-backward asymmetry can be accommodated simultaneously within this
model. The experimental results prefer a region for the mass of Delta_6 around
400 GeV, while Delta_1 is then constrained to have a mass around the TeV scale
as well. We analyze possible experimental signatures and find that Delta_6
associated top production could be probed in the t bar t + jets final states at
Tevatron and the LHC.Comment: 12 pages, 13 figures, version as publishe
Light Colored Scalar as Messenger of Up-Quark Flavor Dynamics in Grand Unified Theories
The measured forward-backward asymmetry in the t tbar production at the
Tevatron might be explained by the additional exchange of a colored weak
singlet scalar. Such state appears in some of the grand unified theories and
its interactions with the up-quarks are purely antisymmetric in flavor space.
We systematically investigate the resulting impact on charm and top quark
physics. The constraints on the relevant Yukawa couplings come from the
experimentally measured observables related to D0--D0bar oscillations, as well
as di-jet and single top production measurements at the Tevatron. After fully
constraining the relevant Yukawa couplings, we predict possible signatures of
this model in rare top quark decays. In a class of grand unified models we
demonstrate how the obtained information enables to constrain the Yukawa
couplings of the up-quarks at very high energy scale.Comment: 13 pages, 11 figures, version as published in PR
Minimal lepton flavor violating realizations of minimal seesaw models
We study the implications of the global U(1)R symmetry present in minimal
lepton flavor violating implementations of the seesaw mechanism for neutrino
masses. In the context of minimal type I seesaw scenarios with a slightly
broken U(1)R, we show that, depending on the R-charge assignments, two classes
of generic models can be identified. Models where the right-handed neutrino
masses and the lepton number breaking scale are decoupled, and models where the
parameters that slightly break the U(1)R induce a suppression in the light
neutrino mass matrix. We show that within the first class of models,
contributions of right-handed neutrinos to charged lepton flavor violating
processes are severely suppressed. Within the second class of models we study
the charged lepton flavor violating phenomenology in detail, focusing on mu to
e gamma, mu to 3e and mu to e conversion in nuclei. We show that sizable
contributions to these processes are naturally obtained for right-handed
neutrino masses at the TeV scale. We then discuss the interplay with the
effects of the right-handed neutrino interactions on primordial B - L
asymmetries, finding that sizable right-handed neutrino contributions to
charged lepton flavor violating processes are incompatible with the requirement
of generating (or even preserving preexisting) B - L asymmetries consistent
with the observed baryon asymmetry of the Universe.Comment: 21 pages, 4 figures; version 2: Discussion on possible generic models
extended, typos corrected, references added. Version matches publication in
JHE
The Machine Learning Landscape of Top Taggers
Based on the established task of identifying boosted, hadronically decaying
top quarks, we compare a wide range of modern machine learning approaches.
Unlike most established methods they rely on low-level input, for instance
calorimeter output. While their network architectures are vastly different,
their performance is comparatively similar. In general, we find that these new
approaches are extremely powerful and great fun.Comment: Yet another tagger included
Implications of Flavor Dynamics for Fermion Triplet Leptogenesis
We analyze the importance of flavor effects in models in which leptogenesis
proceeds via the decay of Majorana electroweak triplets. We find that depending
on the relative strengths of gauge and Yukawa reactions the asymmetry can
be sizably enhanced, exceeding in some cases an order of magnitude level. We
also discuss the impact that such effects can have for TeV-scale triplets
showing that as long as the asymmetry is produced by the dynamics of the
lightest such triplet they are negligible, but open the possibility for
scenarios in which the asymmetry is generated above the TeV scale by heavier
states, possibly surviving the TeV triplet related washouts. We investigate
these cases and show how they can be disentangled at the LHC by using Majorana
triplet collider observables and, in the case of minimal type III see-saw
models even through lepton flavor violation observables.Comment: 22 pages, 9 figures, extended discussion on collider phenomenology,
references added. Version matches publication in JHE
Nuclear physics for cultural heritage
We report about the expert review, published by the Nuclear Physics Division of the European Physical Society (NPD EPS), which aims to provide the public with a popular and accessible account of the latest developments in the field of nuclear physics application for cultural heritage. The contributions from a range of leading specialists explain how applied atomic and nuclear techniques can be used to obtain information that can help us to understand the way of life in ancient times and how they can be used to conserve cultural heritage treasures. This topical review draws heavily on European work and is extensively illustrated with important discoveries and examples from archaeology, pre-history, history, geography, culture, religion and curation. It outlines key advances in a wide range of cross-disciplinary techniques and has been written with the minimum of technical detail so as to be accessible by as wide as possible audience. The large number of groups and laboratories working in the study and preservation of cultural heritage using mainly nuclear physics methods across Europe indicates the enormous effort and importance paid by society to this activity
Characterizing the Diversity of the CDR-H3 Loop Conformational Ensembles in Relationship to Antibody Binding Properties
We present an approach to assess antibody CDR-H3 loops according to their dynamic properties using molecular dynamics simulations. We selected six antibodies in three pairs differing substantially in their individual promiscuity respectively specificity. For two pairs of antibodies crystal structures are available in different states of maturation and used as starting structures for the analyses. For a third pair we chose two antibody CDR sequences obtained from a synthetic library and predicted the respective structures. For all three pairs of antibodies we performed metadynamics simulations to overcome the limitations in conformational sampling imposed by high energy barriers. Additionally, we used classic molecular dynamics simulations to describe nano- to microsecond flexibility and to estimate up to millisecond kinetics of captured conformational transitions. The methodology represents the antibodies as conformational ensembles and allows comprehensive analysis of structural diversity, thermodynamics of conformations and kinetics of structural transitions. Referring to the concept of conformational selection we investigated the link between promiscuity and flexibility of the antibodies' binding interfaces. The obtained detailed characterization of the binding interface clearly indicates a link between structural flexibility and binding promiscuity for this set of antibodies
An Improved Standard Model Prediction Of BR(B -> tau nu) And Its Implications For New Physics
The recently measured B -> tau nu branching ratio allows to test the Standard
Model by probing virtual effects of new heavy particles, such as a charged
Higgs boson. The accuracy of the test is currently limited by the experimental
error on BR(B -> tau nu) and by the uncertainty on the parameters fB and |Vub|.
The redundancy of the Unitarity Triangle fit allows to reduce the error on
these parameters and thus to perform a more precise test of the Standard Model.
Using the current experimental inputs, we obtain BR(B -> tau nu)_SM = (0.84 +-
0.11)x10^{-4}, to be compared with BR(B -> tau nu)_exp = (1.73 +-
0.34)x10^{-4}. The Standard Model prediction can be modified by New Physics
effects in the decay amplitude as well as in the Unitarity Triangle fit. We
discuss how to disentangle the two possible contributions in the case of
minimal flavour violation at large tan beta and generic loop-mediated New
Physics. We also consider two specific models with minimal flavour violation:
the Type-II Two Higgs Doublet Model and the Minimal Supersymmetric Standard
Model.Comment: 7 pages, 13 figures, 1 table. v2: added references and discussion of
B -> D tau nu in the 2HDM. v3: added Bs->mumu in the 2HDM. Final version to
appear in PL
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