71 research outputs found
A new scalar resonance at 750 GeV: Towards a proof of concept in favor of strongly interacting theories
We interpret the recently observed excess in diphoton invariant mass as a new
spin-0 resonant particle. On the theoretical ground, an interesting question is
whether this new scalar resonance belongs to a strongly coupled sector or a
well-defined weakly coupled theory. A possible UV-completion that has been
widely considered in literature is based on the existence of new vector-like
fermions whose loop contributions---Yukawa-coupled to the new
resonance---explain the observed signal rate. The large total width
preliminarily suggested by data seems to favor a large Yukawa coupling, at the
border of a healthy perturbative definition. This potential problem can be
fixed by introducing multiple vector-like fermions or large electric charges,
bringing back the theory to a weakly coupled regime. However, this solution
risks to be only a low-energy mirage: Large multiplicity or electric charge can
dangerously reintroduce the strong regime by modifying the renormalization
group running of the dimensionless couplings. This issue is also tightly
related to the (in)stability of the scalar potential. First, we study---in the
theoretical setup described above---the parametric behavior of the diphoton
signal rate, total width, and one-loop functions. Then, we numerically
solve the renormalization group equations, taking into account the observed
diphoton signal rate and total width, to investigate the fate of the weakly
coupled theory. We find that---with the only exception of few fine-tuned
directions---weakly coupled interpretations of the excess are brought back to a
strongly coupled regime if the running is taken into account.Comment: 32 pages, 38 figures, version appeared in JHEP, Fig.1 and 4 revised,
references added, new section V.C adde
Anomaly-Mediation and Sequestering from a Higher-Dimensional viewpoint
We study a five-dimensional supergravity model with boundary-localized
visible sector exhibiting anomaly-mediated supersymmetry breaking, in which the
central requirements of sequestering and radius stabilization are achieved
perturbatively. This makes it possible to understand these various mechanisms
in a more integrated and transparent fashion, mostly from the
higher-dimensional viewpoint. Local supersymmetry, in the presence of visible
sector quantum effects, is enforced by the formalism of the five-dimensional
superconformal tensor calculus. The construction results in only mild warping,
which allows a natural supersymmetry-breaking mediation mechanism of (finite)
boundary-to-boundary gravity loops to co-dominate with anomaly-mediation,
thereby solving the latter's tachyonic slepton problem. We make the non-trivial
check that this can occur while dangerous loops of stabilizing fields remain
highly suppressed. Our discussion is a well-controlled starting point for
considering other generalizations of anomaly-mediation, or for string theory
realizations.Comment: 33 pages, typos corrected, added references, version appearing in
JHE
Precision Drell-Yan Measurements at the LHC and Implications for the Diphoton Excess
Precision measurements of the Drell-Yan (DY) cross section at the LHC
constrain new physics scenarios that involve new states with electroweak (EW)
charges. We analyze these constraints and apply them to models that can address
the LHC diphoton excess at 750 GeV. We confront these findings with LEP EW
precision tests and show that DY provides stronger constraints than the LEP
data. While 8 TeV data can already probe some parts of the interesting region
of parameter space, LHC14 results are expected to cover a substantial part of
the relevant terrain. We derive the bounds from the existing data, estimate
LHC14 reach and compare them to the bounds one gets from LEP and future FCC-ee
precision measurements.Comment: 21 pages, 7 figures; v2: minor change
Bottom-Flavored Mono-Tau Tails at the LHC
We study the effective field theory sensitivity of an LHC analysis for the
final state with an associated b-jet. To illustrate the improvement
due to the b-tagging, we first recast the recent CMS analysis in the
channel, using an integrated luminosity of 35.9 fb at TeV,
and provide limits on all the dimension-six effective operators which
contribute to the process. The expected limits from the b-tagged analysis are
then derived and compared. We find an improvement of approximately
in the bounds for operators with a b quark. We also discuss in detail possible
angular observables to be used as a discriminator between dimension-six
operators with different Lorentz structure. Finally, we study the impact of
these limits on some simplified scenarios aimed at addressing the observed
deviations from the Standard Model in lepton flavor universality ratios of
semileptonic B-meson decays. In particular, we compare the collider limits on
those scenarios set by our analysis either with or without the b-tagging,
assuming an integrated luminosity of 300 fb, with relevant low-energy
flavor measurements.Comment: 41 pages, 13 figures. The complete chi-square function for our CMS
recast is shared in ancillary files. v2: references added, matches the
version to be published in JHE
Group Theoretic Approach to Fermion Production
We propose a universal group theoretic description of the fermion production
through any type of interaction to scalar or pseudo-scalar. Our group theoretic
approach relies on the group , corresponding to the freedom
in choosing representations of the gamma matrices in Clifford algebra, under
which a part of the Dirac spinor function transforms like a fundamental
representation. In terms of a new () vector constructed out
of spinor functions, we show that fermion production mechanism can be analogous
to the classical dynamics of a vector precessing with the angular velocity. In
our group theoretic approach, the equation of motion takes a universal form for
any system, and choosing a different type of interaction or a different basis
amounts to selecting the corresponding angular velocity. The expression of the
particle number density is greatly simplified, compared to the traditional
approach, and it provides us with a simple geometric interpretation of the
fermion production dynamics. For the purpose of the demonstration, we focus on
the fermion production through the derivative coupling to the pseudo-scalar.Comment: 25 pages, 4 figures, v3: version accepted to JHEP. New Section V
adde
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