286 research outputs found

### MT2-reconstructed invisible momenta as spin analizers, and an application to top polarization

Full event reconstruction is known to be challenging in cases with more than
one undetected final-state particle, such as pair production of two states each
decaying semi-invisibly. On the other hand, full event reconstruction would
allow to access angular distributions sensitive to the spin fractions of the
decaying particles, thereby dissecting their production mechanism. We explore
this possibility in the case of Standard-Model t-tbar production followed by a
leptonic decay of both W bosons, implying two undetected final-state neutrinos.
We estimate the t and tbar momentum vectors event by event using information
extracted from the kinematic variable MT2. The faithfulness of the estimated
momenta to the true momenta is then tested in observables sensitive to top
polarization and t-tbar spin correlations. Our method thereby provides a novel
approach towards the evaluation of these observables, and towards testing
t-tbar production beyond the level of the total cross section. While our
discussion is confined to t-tbar production as a benchmark, the method is
applicable to any process whose decay topology allows to construct MT2.Comment: 28 pages, 14 figures, 3 tables. Changes in v2: new paragraph on ISR
(end of sec. 2.2); new sec. 3.3 on the method's comparison with existing
ones; typos fixed; reference added. Matches journal version (JHEP

### Supersymmetric Higgs-portal and X-ray lines

We consider a Dirac singlet fermion as thermal dark matter for explaining the
X-ray line in the context of a supersymmetric Higgs-portal model or a
generalized Dirac NMSSM. The Dirac singlet fermion gets a mass splitting due to
their Yukawa couplings to two Higgs doublets and their superpartners,
Higgsinos, after electroweak symmetry breaking. We show that a correct relic
density can be obtained from thermal freeze-out, due to the co-annihilation
with Higgsinos for the same Yukawa couplings. We discuss the phenomenology of
the Higgsinos in this model such as displaced vertices at the LHC.Comment: 15 pages, 4 figures, references adde

### YAM2: Yet another library for the $M_2$ variables using sequential quadratic programming

The $M_2$ variables are devised to extend $M_{T2}$ by promoting transverse
masses to Lorentz-invariant ones and making explicit use of on-shell mass
relations. Unlike simple kinematic variables such as the invariant mass of
visible particles, where the variable definitions directly provide how to
calculate them, the calculation of the $M_2$ variables is undertaken by
employing numerical algorithms. Essentially, the calculation of $M_2$
corresponds to solving a constrained minimization problem in mathematical
optimization, and various numerical methods exist for the task. We find that
the sequential quadratic programming method performs very well for the
calculation of $M_2$, and its numerical performance is even better than the
method implemented in the existing software package for $M_2$. As a consequence
of our study, we have developed and released yet another software library,
YAM2, for calculating the $M_2$ variables using several numerical algorithms.Comment: 1+22 pages, 5 figures; matches published version; fixed title page
for inspire record; The library is distributed via
https://github.com/cbpark/YAM

### Collider probes of singlet fermionic dark matter scenarios for the Fermi gamma-ray excess

We investigate the collider signatures of the three benchmark points in the
singlet fermionic dark matter model. The benchmark points, which were
introduced previously to explain the Fermi gamma-ray excess by dark matter (DM)
pair annihilation at the Galactic center, have definite predictions for future
collider experiments such as the International Linear Collider and the
High-Luminosity LHC. We consider four collider observables: (1) Higgs signal
strength (essentially $hZZ$ coupling), (2) triple Higgs coupling, (3) exotic
Higgs decay, and (4) direct production of a new scalar particle. The benchmark
points are classified by the final states of the DM annihilation process: a
pair of $b$ quarks, SM-like Higgs bosons, and new scalar particles. Each
benchmark scenario has detectable new physics signals for the above collider
observables that can be well tested in the future lepton and hadron colliders.Comment: 14 pages, 1 figure, 1 tabl

### $B$-meson anomalies and Higgs physics in flavored $U(1)'$ model

We consider a simple extension of the Standard Model with flavor-dependent
$U(1)'$, that has been proposed to explain some of $B$-meson anomalies recently
reported at LHCb. The $U(1)'$ charge is chosen as a linear combination of
anomaly-free $B_3-L_3$ and $L_\mu-L_\tau$. In this model, the flavor structure
in the SM is restricted due to flavor-dependent $U(1)'$ charges, in particular,
quark mixings are induced by a small vacuum expectation value of the extra
Higgs doublet. As a result, it is natural to get sizable flavor-violating
Yukawa couplings of heavy Higgs bosons involving the bottom quark. In this
article, we focus on the phenomenology of the Higgs sector of the model
including extra Higgs doublet and singlet scalars. We impose various bounds on
the extended Higgs sector from Higgs and electroweak precision data, $B$-meson
mixings and decays as well as unitarity and stability bounds, then discuss the
productions and decays of heavy Higgs bosons at the LHC.Comment: 40 pages, 11 figures, 1 table; v2: references added; v3: accepted
version for publication in EPJ

### Resonant Higgs boson pair production in the $hh\rightarrow b\bar{b} \; WW \rightarrow b\bar{b} \ell^+ \nu \ell^- \bar\nu$ decay channel

Adding a scalar singlet provides one of the simplest extensions of the
Standard Model. In this work we briefly review the latest constraints on the
mass and mixing of the new Higgs boson and study its production and decay at
the LHC. We mainly focus on double Higgs production in the $hh \rightarrow b
\bar{b} WW \rightarrow b \bar{b} \ell^+ \nu \ell^- \bar{\nu}$ decay channel.
This decay is found to be efficient in a region of masses of the heavy Higgs
boson of 260 - 500 GeV, so it is complementary to the 4$b$ channel, more
efficient for Higgs bosons having masses greater than 500 GeV. We analyse this
di-leptonic decay channel in detail using kinematic variables such as $M_{\rm
T2}$ and the $M_{\rm T2}$-assisted on-shell reconstruction of invisible
momenta. Using proper cuts, a significance of $\sim$ 3$\sigma$ for 3000
fb$^{-1}$ can be achieved at the 14 TeV LHC for $m_H$ = 260 - 400 GeV if the
mixing is close to its present limit and ${\rm BR}(H \rightarrow hh) \approx
1$. Smaller mixing values would require combining various decay channels in
order to reach a similar significance. The complementarity among $H
\rightarrow hh$, $H \rightarrow ZZ$ and $H \rightarrow WW$ channels is
studied for arbitrary ${\rm BR}(H \rightarrow hh)$ values.Comment: 36 pages, 13 figure

### Minimal Flavor Violation with Axion-like Particles

We revisit the flavor-changing processes involving an axion-like particle
(ALP) in the context of generic ALP effective lagrangian with a discussion of
possible UV completions providing the origin of the relevant bare ALP
couplings. We focus on the minimal scenario that ALP has flavor-conserving
couplings at tree level, and the leading flavor-changing couplings arise from
the loops involving the Yukawa couplings of the Standard Model fermions. We
note that such radiatively generated flavor-changing ALP couplings can be
easily suppressed in field theoretic ALP models with sensible UV completion. We
discuss also the implication of our result for string theoretic ALP originating
from higher-dimensional $p$-form gauge fields, for instance for ALP in large
volume string compactification scenario.Comment: 41 pages, 3 figures; v3: a discussion on general extended Higgs
sector added in sec. 2, version published in JHE

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