1,652 research outputs found
New Light Species and the CMB
We consider the effects of new light species on the Cosmic Microwave
Background. In the massless limit, these effects can be parameterized in terms
of a single number, the relativistic degrees of freedom. We perform a thorough
survey of natural, minimal models containing new light species and numerically
calculate the precise contribution of each of these models to this number in
the framework of effective field theory. After reviewing the relevant details
of early universe thermodynamics, we provide a map between the parameters of
any particular theory and the predicted effective number of degrees of freedom.
We then use this map to interpret the recent results from the Cosmic Microwave
Background survey done by the Planck satellite. Using this data, we present new
constraints on the parameter space of several models containing new light
species. Future measurements of the Cosmic Microwave Background can be used
with this map to further constrain the parameter space of all such models.Comment: 38 pages plus appendices and references; 10 figures and 1 table;
references added, discussion of anapole moments added; supernovae cooling
bounds added, discussion of models condense
Constraining Light Colored Particles with Event Shapes
Using recently developed techniques for computing event shapes with
Soft-Collinear Effective Theory, LEP event shape data is used to derive strong
model-independent bounds on new colored particles. In the effective field
theory computation, colored particles contribute in loops not only to the
running of alpha_s but also to the running of hard, jet and soft functions.
Moreover, the differential distribution in the effective theory explicitly
probes many energy scales, so event shapes have strong sensitivity to new
particle thresholds. Using thrust data from ALEPH and OPAL, colored adjoint
fermions (such as a gluino) below 51.0 GeV are ruled out to 95% confidence
level. This is nearly an order-of-magnitude improvement over the previous
model-independent bound of 6.3 GeV.Comment: 4 pages, 2 figure
Searching for Higgs decays to four bottom quarks at LHCb
We discuss the feasibility of seeing a Higgs boson which decays to four
bottom quarks through a pair of (pseudo-)scalars at the LHCb experiment to
argue that the use of b-physics triggers and off-line vertex reconstruction, as
opposed to jet triggers with b tagging, may be more effective for this signal.
Focusing on inclusive production for the Higgs, we find that for light scalar
masses below 20 GeV, signal reconstruction efficiencies of order a few percent
may be enough for LHCb to find evidence for a Higgs with a dominant 4b decay
channel.Comment: 6 pages, 2 figure
Top-tagging: A Method for Identifying Boosted Hadronic Tops
A method is introduced for distinguishing top jets (boosted, hadronically
decaying top quarks) from light quark and gluon jets using jet substructure.
The procedure involves parsing the jet cluster to resolve its subjets, and then
imposing kinematic constraints. With this method, light quark or gluon jets
with pT ~ 1 TeV can be rejected with an efficiency of around 99% while
retaining up to 40% of top jets. This reduces the dijet background to heavy
t-tbar resonances by a factor of ~10,000, thereby allowing resonance searches
in t-tbar to be extended into the all-hadronic channel. In addition,
top-tagging can be used in t-tbar events when one of the tops decays
semi-leptonically, in events with missing energy, and in studies of b-tagging
efficiency at high pT.Comment: 4 pages, 4 figures; v2: separate quark and gluon efficiencies
included, figure on helicity angle added, and physics discussion extende
Twisted Split Fermions
The observed flavor structure of the standard model arises naturally in
"split fermion" models which localize fermions at different places in an extra
dimension. It has, until now, been assumed that the bulk masses for such
fermions can be chosen to be flavor diagonal simultaneously at every point in
the extra dimension, with all the flavor violation coming from the Yukawa
couplings to the Higgs. We consider the more natural possibility in which the
bulk masses cannot be simultaneously diagonalized, that is, that they are
twisted in flavor space. We show that, in general, this does not disturb the
natural generation of hierarchies in the flavor parameters. Moreover, it is
conceivable that all the flavor mixing and CP-violation in the standard model
may come only from twisting, with the five-dimensional Yukawa couplings taken
to be universal.Comment: 15 pages, 1 figur
Dynamics of First Order Transitions with Gravity Duals
A first order phase transition usually proceeds by nucleating bubbles of the
new phase which then rapidly expand. In confining gauge theories with a gravity
dual, the deconfined phase is often described by a black hole. If one starts in
this phase and lowers the temperature, the usual description of how the phase
transition proceeds violates the area theorem. We study the dynamics of this
phase transition using the insights from the dual gravitational description,
and resolve this apparent contradiction.Comment: 11 pages, 1 figure. v2: minor clarifications, reference adde
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