Charming ALPs Carmona, Adrian; Scherb, Christiane; Schwaller, Pedro
The journal of high energy physics,
08/2021, Volume:
2021, Issue:
8
Journal Article
Peer reviewed
Open access
A
bstract
Axion-like particles (ALPs) are ubiquitous in models of new physics explaining some of the most pressing puzzles of the Standard Model. However, until relatively recently, little attention ...has been paid to its interplay with flavour. In this work, we study in detail the phenomenology of ALPs that exclusively interact with up-type quarks at the tree-level, which arise in some well-motivated ultra-violet completions such as QCD-like dark sectors or Froggatt-Nielsen type models of flavour. Our study is performed in the low-energy effective theory to highlight the key features of these scenarios in a model independent way. We derive all the existing constraints on these models and demonstrate how upcoming experiments at fixed-target facilities and the LHC can probe regions of the parameter space which are currently not excluded by cosmological and astrophysical bounds. We also emphasize how a future measurement of the currently unavailable meson decay
D
→
π
+ invisible could complement these upcoming searches.
Collider constraints on dark mediators Mies, Hanna; Scherb, Christiane; Schwaller, Pedro
The journal of high energy physics,
04/2021, Volume:
2021, Issue:
4
Journal Article
Peer reviewed
Open access
A
bstract
We explore the constraints current collider searches place on a QCD-like dark sector. A combination of multi-jet, multi-jet plus missing energy and emerging jets searches is used to derive ...constraints on the mediator mass across the full range of the dark meson lifetimes for the first time.
The dark sector inherits a flavour structure from the coupling between the dark quarks and the SM quarks through the mediator. When this is taken into account, the differently flavoured dark pions become distinguishable through their lifetime. We show that also in these cases the above mentioned searches remain sensitive, and we obtain limits on the mediator mass also for the flavoured scenario.
We then contrast the constraints from collider searches with direct detection bounds on the dark matter candidate itself in both the flavoured and unflavoured scenario. Using a simple prescription it becomes possible to display all constraints in the dark matter and mediator mass plane. Constraints from direct detection tend to be stronger than the collider constraints, unless the coupling to the first generation quarks is suppressed, in which case the collider searches place the most stringent limits on the parameter space.
A
bstract
The type II seesaw mechanism is an attractive way to generate the observed light neutrino masses. It postulates a SU(2)
L
-triplet scalar field, which develops an induced vacuum expectation ...value after electroweak symmetry breaking, giving masses to the neutrinos via its couplings to the lepton SU(2)
L
-doublets. When the components of the triplet field have masses around the electroweak scale, the model features a rich phenomenology. We discuss the currently allowed parameter space of the minimal low scale type II seesaw model, taking into account all relevant constraints, including charged lepton flavour violation as well as collider searches. We point out that the symmetry protected low scale type II seesaw scenario, where an approximate “lepton number”-like symmetry suppresses the Yukawa couplings of the triplet to the lepton doublets, is still largely untested by the current LHC results. In part of this parameter space the triplet components can be long-lived, potentially leading to a characteristic displaced vertex signature where the doubly-charged component decays into same-sign charged leptons. By performing a detailed analysis at the reconstructed level we find that already at the current run of the LHC a discovery would be possible for the considered parameter point, via dedicated searches for displaced vertex signatures. The discovery prospects are further improved at the HL-LHC and the FCC-hh/SppC.
A
bstract
We propose a search for long lived axion-like particles (ALPs) in exotic top decays. Flavour-violating ALPs appear as low energy effective theories for various new physics scenarios such as ...t-channel dark sectors or Froggatt-Nielsen models. In this case the top quark may decay to an ALP and an up- or charm-quark. For masses in the few GeV range, the ALP is long lived across most of the viable parameter space, suggesting a dedicated search. We propose to search for these long lived ALPs in
t
t
¯
events, using one top quark as a trigger. We focus on ALPs decaying in the hadronic calorimeter, and show that the ratio of energy deposits in the electromagnetic and hadronic calorimeters as well as track vetoes can efficiently suppress Standard Model backgrounds. Our proposed search can probe exotic top branching ratios smaller than 10
−
4
with a conservative strategy at the upcoming LHC run, and potentially below the 10
−
7
level with more advanced methods. Finally we also show that measurements of single top production probe these branching ratios in the very short and very long lifetime limit at the 10
−
3
level.
High‐temperature corrosion was studied under multiple chemical loads on ferritic‐austenitic model alloys (Fe–13Cr, Fe–18Cr–12Ni, and Fe‐25Cr–20Ni) with KCl deposit under 0.5% SO2 ...${\text{SO}}_{2}$/99.5% Ar gas atmosphere at 560°C $560^\circ {\rm{C}}$. Postexposure characterization was done by X‐ray diffraction and scanning electron microscopy. In a pure SO2 ${\text{SO}}_{2}$/Ar environment a protective Cr2O3 ${\text{Cr}}_{2}{{\rm{O}}}_{3}$ scale was formed by all samples. The introduction of KCl deposits causes the scale to be nonprotective and multilayered, consisting of CrS, FeS, Cr2O3,Fe3O4 ${\text{Cr}}_{2}{{\rm{O}}}_{3},{\text{Fe}}_{3}{{\rm{O}}}_{4}$, and Fe2O3 ${\text{Fe}}_{2}{{\rm{O}}}_{3}$. The impact of the microstructure and alloying elements is discussed.
The effect of KCl in high temperature corrosion of highly alloyed steels in SO2 was studied, simulating biofuel environments. The effect of the alloying elements and the microstructure was determined using XRD, SEM and EDX analysis. A corrosion mechanism of highly alloyed FeCrNi steels in SO2 + KCl environments was proposed.
A
bstract
Extended scalar sectors with additional degrees of freedom appear in many scenarios beyond the Standard Model. Heavy scalar resonances that interact with the neutral current could be ...discovered via broad resonances in the tails of the four-lepton invariant mass spectrum, where the Standard Model background is small and well understood. In this article we consider a recent ATLAS measurement of four-lepton final states, where the data is in excess over the background for invariant masses above 500 GeV. We discuss the possibility that this excess could be interpreted as a “double peak” from the two extra heavy neutral scalars of a CP violating Two Higgs Doublet Model, both coupling to the
Z
boson. We apply an iterative fitting procedure to find viable model parameters that can match the excess, resulting in a benchmark point where the observed four-lepton invariant mass spectrum can be explained by two scalar particles
H
2
and
H
3
, with masses of 544 GeV and 629 GeV, respectively, being admixtures of the CP eigenstates. Our explanation predicts additional production processes for
t
t
¯
,
W
+
W
−
, 4
b
and
γγ
, some of which have cross sections close to the current experimental limits. Our results further imply that the electric dipole moment of the electron should be close to the present bounds.
The prediction of structural parameters and optoelectronic properties of compound semiconductors is very important. However, calculations often neglect chemical variability and structural defects. In ...chalcopyrite type semiconductors one of the major defects are copper vacancies (VCu). The four cation neighbors of the anion determine its position in the chalcopyrite type structure expressed by the Wyckoff position 8d (x, 1/4, 1/8). Intrinsic point defects like VCu and anti‐sites may cause variations of the anion position in the middle of the cation tetrahedron, especially in the anion position parameter x. For stoichiometric chalcopyrite type compounds a formalism according to the principle of conservation of tetrahedral bonds (CTB) can be applied to calculate the anion position parameter, but it fails in the case of off‐stoichiometric chalcopyrites. This case study of chalcopyrite type CuGaS2 and Mn‐substituted GuGaS2 shows that the experimentally determined anion position parameter x deviate from values calculated by CTB approach. The systematic investigation of off‐stoichiometric CuGaS2 and Mn‐substituted GuGaS2 demonstrates the effect of copper vacancies on the average radii of the cation sites (Wyckoff positions 4a and 4b) as well as on the anion position parameter x. By applying an elaborated CTB approach implementing copper vacancies an agreement between experimental and calculated anion position parameter x can be obtained.
In the chalcopyrite‐type semiconductor CuGaS2 a copper vacancy causes four degenerated coordination tetrahedra where the anion is dragged towards the gallium atoms. As a result, the Ga‐S bond length decreases and the Cu‐S bond length increases. A newly developed enhanced formalism for the conservation of tetrahedral bonds principle is verified by applying it to off‐stoichiometric chalcopyrite‐type phases in CuGaS2 – MnS series.
In this work, we consider the case of a strongly coupled dark/hidden sector, which extends the Standard Model (SM) by adding an additional non-Abelian gauge group. These extensions generally contain ...matter fields, much like the SM quarks, and gauge fields similar to the SM gluons. We focus on the exploration of such sectors where the dark particles are produced at the LHC through a portal and undergo rapid hadronization within the dark sector before decaying back, at least in part and potentially with sizeable lifetimes, to SM particles, giving a range of possibly spectacular signatures such as emerging or semi-visible jets. Other, non-QCD-like scenarios leading to soft unclustered energy patterns or glueballs are also discussed. After a review of the theory, existing benchmarks and constraints, this work addresses how to build consistent benchmarks from the underlying physical parameters and present new developments for the
pythia
Hidden Valley module, along with jet substructure studies. Finally, a series of improved search strategies is presented in order to pave the way for a better exploration of the dark showers at the LHC.
Insights into early damage mechanisms during high‐temperature gas corrosion provide important aspects for the prediction of the long‐term stability of high‐temperature materials exposed to a hot and ...corrosive environment. The current work presents a real time study concerning the combined oxidation and sulfidation of ferritic model alloys in a hot SO2 containing atmosphere using energy dispersive X‐ray diffraction. The applied high temperature reactor allows the transmittance of high energetic X‐rays in order to collect X‐ray diffraction pattern of the sample surface in situ during the reaction with the reactive environment. The results revealed that the first phases crystallizing on iron with 2 wt% chromium and with 9 wt% chromium are oxides. The aging experiments at T = 650° C with 0.5% SO2 and 99.5% Ar were followed in situ and caused an external mixed oxide‐sulfide layer on top of iron with 2 wt% chromium. On the higher alloyed material the external scale consists of iron oxides and the inner scale of mixed (Fe,Cr)‐oxides and chromium‐sulfides. The oxide content continuously increases parallel to an increase of the sulfide amount. Thus, the initially formed (Fe,Cr)‐oxides do not have a protective character and support the transport of sulfur through the growing oxide scale.
In situ studies of material interactions with hot and strongly corrosive gases render information of fundamental mechanisms influencing the lifetime of a material. A technique is introduced to study the early stage of high temperature corrosion in real time. First results of the aging mechanism of two model alloys and the potential for the real time analysis of high temperature corrosion by EDXRD are presented.
Mixed ionic electronic conducting ceramics Nd6−yWO12−δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO2, CO, H2, H2O or ...H2S. Due to this chemical stability, they are promising and cost‐efficient candidate materials for gas separation, catalytic membrane reactors and protonic ceramic fuel cell technologies. As in La6−yWO12−δ, the ionic/electronic transport mechanism in Nd6−yWO12−δ is expected to be largely controlled by the crystal structure, the conclusive determination of which is still lacking. This work presents a crystallographic study of Nd5.8WO12−δ and molybdenum‐substituted Nd5.7W0.75Mo0.25O12−δ prepared by the citrate complexation route. High‐resolution synchrotron and neutron powder diffraction data were used in combined Rietveld refinements to unravel the crystal structure of Nd5.8WO12−δ and Nd5.7W0.75Mo0.25O12−δ. Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm3m and doubled unit‐cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4a and 48h and is evenly distributed, in contrast with La6−yWO12−δ. X‐ray absorption spectroscopy as a function of partial pressure pO2 in the near‐edge regions excludes oxidation state changes of Nd (Nd3+) and W (W6+) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo‐substituted Nd6−yWO12−δ is therefore explained by the higher Mo reducibility and the creation of additional – disordered – oxygen vacancies.
The crystal structures of non‐substituted and Mo‐substituted neodymium tungstates are described in detail through neutron diffraction and high‐resolution X‐ray diffraction. Combined X‐ray and neutron diffraction refinements and electron probe micro‐analysis were employed to locate Mo atoms in the crystal structure of Nd6−yW1−zMozO12−δ (z = 0, 0.25), while X‐ray absorption spectroscopy in the near‐edge regions confirmed no changes in the oxidation states of Nd and W.
PDF
