74 research outputs found
Top Charge Asymmetry -- Theory Status Fall 2013
I review the current status of the charge asymmetry in hadronic top-quark
pair production from a theory perspective. The standard-model predictions for
the observables at the Tevatron and LHC are being discussed, as well as
possible explanations of the Tevatron excess in terms of new physics. I give an
outlook for future investigations of the top-quark charge asymmetry, focussing
on novel observables at the LHC.Comment: prepared for the proceedings of "Top 2013 -- 6th International
Workshop on Top Quark Physics", Sep 14-19 2013, Durbach, German
Top-Quark Charge Asymmetry with a Jet Handle
Pairs of top and antitop quarks are produced at the LHC to a large extent in
association with a hard jet. We investigate the charge asymmetry in top pair +
jet production in quantum chromodynamics (QCD) and with additional massive
color-octet vector bosons. The total charge asymmetry at the LHC is suppressed
by the large charge-symmetric background from gluon-gluon fusion. We show to
what extent the asymmetry can be enhanced by suitable phase space cuts and, in
particular, elaborate on the kinematics of the hard jet in the top pair + jet
final state. We demonstrate that in QCD, the asymmetry amounts to -1.5% for
central jets without an excessive reduction of the cross section. By applying
additional kinematical cuts, the asymmetry can be enhanced to -4%, but at the
cost of a strong reduction of the cross section. Massive color-octet states can
generate sizeable effects in top pair + jet production, both on the charge
asymmetry and on the cross section. The charge asymmetry probes both vector and
axial-vector couplings to quarks. We show that massive color octets can
generate asymmetries up to +-10% for moderate and up to +-30% for strong
kinematical cuts to be used in experimental analyses at the LHC. Jet kinematics
can be used to obtain further information about the nature of the couplings and
thereby to discriminate between different models.Comment: 29 pages, 12 figure
Integrating in the Higgs Portal to Fermion Dark Matter
Fermion dark matter (DM) interacting with the standard model through a Higgs
portal requires non-renormalizable operators, signaling the presence of new
mediator states at the electroweak scale. Collider signatures that involve the
mediators are a powerful tool to experimentally probe the Higgs portal
interactions, providing complementary information to strong constraints set by
direct DM detection searches. Indirect detection experiments are less sensitive
to this scenario. We investigate the collider reach for the mediators using
three minimal renormalizable models as examples, and requiring the fermion DM
to be a thermal relic. The Large Hadron Collider in its high-energy,
high-luminosity phase can probe most scenarios if DM is lighter than about 200
GeV. Beyond this scale, future high-energy experiments such as an
electron-positron collider or a 100-TeV proton-proton collider, combined with
future direct detection experiments, are indispensable to conclusively test
these models.Comment: 23 pages; v2: references added and correction of direct detection
limits in section VI.
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