Signal Of New Physics Research Articles

Constraints from Heavy Higgs Boson Masses in the two Higgs Doublet Model

Upon the absence of signals of new physics at the LHC, a reasonable strategy is to assume that new particles are very heavy and the other model parameters are unknown yet. In the aligned two Higgs doublet model, however, heavy Higgs boson masses above 500 GeV enhance some couplings in the scalar potential, which causes a breakdown of the perturbative unitariry in general. Some tuning among model parameters is required. We find that one information on the heavy Higgs boson mass, say $M_H$, has significant theoretical implications: (i) the other heavy Higgs bosons should have similar masses to $M_H$ within $\pm \mathcal{O}(10)\%$; (ii) the inequalities from the theoretical constraints are practically reduced to an equation such that $m_{12}^2 \tan\beta$ is constant, where $m_{12}^2$ is the soft $Z_2$ breaking parameter and $\tan\beta$ is the ratio of two vacuum expectation values; (iii) the triple Higgs coupling $\lambda_{HHh}$ is constant over $\tan\beta$ while $\lambda_{HHH}$ and $\lambda_{AAH}$ are linearly proportional to $\tan\beta$. The double Higgs-strahlung process of $e^+ e^- \to ZHH$ is also studied, of which the total cross section is almost constant with the given $M_H$.

Measuring the CP state of tau lepton pairs from Higgs decay at the ILC

In the Standard Model, the Higgs boson is a CP even state with CP conserving couplings; any deviations from this would be a sign of new physics. These CP properties can be probed by measuring Higgs decays to tau lepton pairs: the transverse correlation between the tau spins depends on CP. This paper develops such an analysis, using full simulation of signal and background events in the International Large Detector concept for the International Linear Collider. We consider Higgs-strahlung events e+ e- -> Z H) in which the Z boson decays to electrons, muons, or hadrons, and the Higgs boson decays to $\tau$ leptons, which then decay either to (pi+ nu) or (pi+ pi0 nu). Assuming 2 ab^{-1} of integrated luminosity at a center-of-mass energy of 250~GeV, the mixing angle psi_CP between even and odd CP components of the tau pair from Higgs boson decays can be measured to a precision of 75~mrad (4.3 deg).

Single top-quark production at the Tevatron and the LHC

This paper provides a review of the experimental studies of processes with a single top quark at the Tevatron proton-antiproton collider and the LHC proton-proton collider. Single top-quark production in the t-channel process has been measured at both colliders. The s-channel process has been observed at the Tevatron, and its rate has been also measured at the center-of-mass energy of 8 TeV at the LHC in spite of the comparatively harsher background contamination. LHC data also brought the observation of the associated production of a single top quark with a W boson as well as with a Z boson. The Cabibbo-Kobayashi-Maskawa matrix element |Vtb| is extracted from the single-top-quark production cross sections, and t-channel events are used to measure several properties of the top quark and set constraints on models of physics beyond the Standard Model. Rare final states with a single top quark are searched for, as enhancements in their production rates, if observed, would be clear signs of new physics.

Rareb-hadron decays as probe of new physics

The unexpected absence of unambiguous signals of New Physics (NP) at the TeV scale at the Large Hadron Collider (LHC) puts today flavor physics at the forefront. In particular, rare decays of b-hadrons represent a unique probe to challenge the Standard Model (SM) paradigm and test models of NP at a scale much higher than that accessible by direct searches. This article reviews the status of the field.

First Extraction of Transversity from a Global Analysis of Electron-Proton and Proton-Proton Data.

We present the first extraction of the transversity distribution in the framework of collinear factorization based on the global analysis of pion-pair production in deep-inelastic scattering and in proton-proton collisions with a transversely polarized proton. The extraction relies on the knowledge of dihadron fragmentation functions, which are taken from the analysis of electron-positron annihilation data. For the first time, the transversity is extracted from a global analysis similar to what is usually done for the spin-averaged and helicity distributions. The knowledge of transversity is important for, among other things, detecting possible signals of new physics in high-precision low-energy experiments.

New physics with the lepton flavor violating decay τ→3μ

Lepton flavor violating (LFV) processes are a smoking gun signal of new physics (NP). If the semileptonic $B$ decay anomalies are indeed due to some NP, such operators can potentially lead to LFV decays involving the second and the third generation leptons, like $\ensuremath{\tau}\ensuremath{\rightarrow}3\ensuremath{\mu}$. In this paper, we explore how far the nature of NP can be unraveled at the next generation $B$-factories like Belle-II, provided the decay $\ensuremath{\tau}\ensuremath{\rightarrow}3\ensuremath{\mu}$ has been observed. We use four observables with which the differentiation among NP operators may be achieved to a high confidence level. Possible presence of multiple NP operators are also analyzed with the optimal observable technique. While the analysis can be improved even further if the final state muon polarizations are measured, we present this work as a motivational tool for the experimentalists, as well as a template for the analysis of similar processes.

Lepton-flavour-violating gluonic operators: constraints from the LHC and low energy experiments

Effective operators provide a model-independent description of physics beyond the standard model that is particularly useful given the absence of any signs of new physics at the Large Hadron Collider (LHC). We recast previous LHC analyses to set limits on lepton-flavour-violating gluonic effective operators of dimension 8 and compare our results to existing limits from low-energy precision experiments. Current LHC data constrains the scale Λ of the effective operators to be larger than Λ ≳ 0.5 − 1.6 TeV depending on the flavour and thus provides the most stringent limit for all operators apart from parity-conserving operators of the form GGoverline{mu}{P}_{L,R}e , where μ-e conversion in nuclei poses the most stringent constraint.

Searching for new physics with three-particle correlations in pp collisions at the LHC

New phenomena involving pseudorapidity and azimuthal correlations among final-state particles in pp collisions at the LHC can hint at the existence of hidden sectors beyond the Standard Model. In this paper we rely on a correlated-cluster picture of multiparticle production, which was shown to account for the ridge effect, to assess the effect of a hidden sector on three-particle correlations concluding that there is a potential signature of new physics that can be directly tested by experiments using well-known techniques.

New formalism of quantum field theory

In recent years, theoretical calculations of scattering amplitudes face a huge challenge because experiment data coming from colliders, such as Large Hadron Collider (LHC), become more and more accurate. The traditional Feynman diagram technique can not deal with the problem because the difficulties involving gauge interaction. To solve the problem, theorists have made many studies. Through investigations of last fourteen years, we have achieved many important progresses, such as on-shell recursion relation, the Cachazo-He-Yuan framework for scattering amplitudes of massless particles, the geometric Grassmannian and Amplituhedron frameworks for N =4 SYM planar amplitudes, etc. These new methods can calculate tree-level and loop scattering amplitudes much faster than the traditional method, so have improved our ability to analyze background contributions from Standard Model and find possible signals of new physics by comparing with experiment data. These breakthroughs have some common features. These features, such as the focus of on-shell properties, the right choice of prescription variables and the emphasize of the analytic properties of scattering amplitudes, hint a possible new formalism of quantum field theory, which is very different from traditional Lagrangian formalism. In this article, we give a brief discussion of this topic.

The Three-dimensional Spatial Distribution of Interstellar Gas in the Milky Way: Implications for Cosmic Rays and High-energy Gamma-ray Emissions.

Direct measurements of cosmic ray (CR) species combined with observations of their associated γ-ray emissions can be used to constrain models of CR propagation, trace the structure of the Galaxy, and search for signatures of new physics. The spatial density distribution of interstellar gas is a vital element for all these studies. So far, models have employed the 2D cylindrically symmetric geometry, but their accuracy is well behind that of the available data. In this paper, 3D spatial density models for neutral and molecular hydrogen are constructed based on empirical model fitting to gas line-survey data. The developed density models incorporate spiral arms and account for the warping of the disk, and the increasing gas scale height with radial distance from the Galactic center. They are employed together with the GALPROP CR propagation code to investigate how the new 3D gas models affect calculations of CR propagation and high-energy γ-ray intensity maps. The calculations reveal non-trivial features that are directly related to the new gas models. The best-fit values for propagation model parameters employing 3D gas models are presented and they differ significantly from those derived with the 2D gas density models that have been widely used. The combination of 3D CR and gas density models provide a more realistic basis for the interpretation of non-thermal emissions from the Galaxy.

The Belle II experiment: status and physics program

The Belle II experiment at the SuperKEKB asymmetric-energy e+e- collider in Japan aims to search for new physics in the flavour transitions in the quark and lepton sectors. The SuperKEKB accelerator will operate at the target instantaneous luminosity of 8 × 1035 s-1cm2. It requires a substantial upgrade of the detector subsystems which are expected to record 50 ab-1 of data. Such a huge data sample in clean background environment allows for probing signatures of new physics through suppressed flavour physics reactions and cross checks for deviations from the Standard Model measured at the LHCb experiment. Physics data taking at the Belle II experiment successfully started in April 2018.

Quark flavour-violating Higgs decays at the ILC

Flavour-violating Higgs interactions are suppressed in the Standard Model such that their observation would be a clear sign of new physics. We investigate the prospects for detecting quark flavour-violating Higgs decays in the clean ILC environment. Concentrating on the decay to a bottom and a light quark j, we identify the dominant Standard Model background channels as coming from hadronic Standard Model Higgs decays with mis-identified jets. Therefore, good flavour tagging capabilities are essential to keep the background rate under control. Through a simple cut-based analysis, we find that the most promising search channel is the two-jet plus missing energy signature . At 500 GeV, the expected 95% CL upper limit on mathrm{mathcal{B}}left(hto bjright) is of order 10−3. Correspondingly, a 5σ discovery is expected to be possible for branching ratios as low as a few 10−3.

Cabibbo\u2013Kobayashi\u2013Maskawa-favored B decays to a scalar meson and a D meson

In this work, we attempt to study the Cabibbo–Kobayashi–Maskawa-favored B rightarrow overline{D} S (“S” denoting the scalar meson) decays within the perturbative QCD approach at the leading order and the leading power. Although the light scalar mesons are widely perceived as primarily the four-quark bound states, in practice it is hard for us to make quantitative predictions based on the four-quark picture for light scalars. Hence, we calculate the decays with light scalars in the two-quark model. For the decays with scalar mesons above 1 GeV, we have explored two possible scenarios, depending on whether the light scalars are treated as the lowest lying qbar{q} states or four-quark particles. In total, we calculated the branching fractions of 72 decay modes, and most of them are in the range 10^{-4}–10^{-7}, which are measurable in the on-going LHCb experiment and the forthcoming Belle-II experiment. Moreover, since in the standard model these decays occur only through tree operators and have no CP asymmetries, any deviation will be a signal of new physics beyond the standard model. Despite large uncertainties induced by nonperturbative parameters and corrections of high order and high power, our results and discussions will be useful for the on-going LHCb and the forthcoming Belle-II experiments.

Double-Cascade Events from New Physics in Icecube.

A variety of new physics models allows for neutrinos to up-scatter into heavier states. If the incident neutrino is energetic enough, the heavy neutrino may travel some distance before decaying. In this work, we consider the atmospheric neutrino flux as a source of such events. At IceCube, this would lead to a "double-bang" (DB) event topology, similar to what is predicted to occur for tau neutrinos at ultrahigh energies. The DB event topology has an extremely low background rate from coincident atmospheric cascades, making this a distinctive signature of new physics. Our results indicate that IceCube should already be able to derive new competitive constraints on models with GeV-scale sterile neutrinos using existing data.

ATLAS fast physics monitoring: TADA

The ATLAS experiment at the LHC has been recording data from proton-proton collisions with 13 TeV center-of-mass energy since spring 2015. The collaboration is using a fast physics monitoring framework (TADA) to automatically perform a broad range of fast searches for early signs of new physics and to monitor the data quality across the year with the full analysis level calibrations applied to the rapidly growing data. TADA is designed to provide fast feedback directly after the collected data has been fully calibrated and processed at the Tier-0. The system can monitor a large range of physics channels, offline data quality and physics performance quantities. TADA output is available on a website accessible by the whole collaboration. It gets updated twice a day with the data from newly processed runs. Hints of potentially interesting physics signals or performance issues identified in this way are reported to be followed up by physics or combined performance groups. The note reports as well about the technical aspects of TADA: the software structure to obtain the input TAG files, the framework workflow and structure, the webpage and its implementation.

Is there a sign of new physics in beryllium transitions?

We discuss the current status of the anomaly in beryllium-8 nuclear transitions recently reported in the angular distribution of internal conversion electron–positron pairs. We present a phenomenological analysis of the signal and review the models proposed to explain it, focusing on those involving a new light protophobic vector gauge boson. We also elaborate on the prospects of verifying the anomaly in present and future experiments.

Interpreting hints for lepton flavor universality violation

We interpret the recent hints for lepton flavor universality violation in rare $B$ meson decays. Based on a model-independent effective Hamiltonian approach, we determine regions of new physics parameter space that give a good description of the experimental data on $R_K$ and $R_{K^*}$, which is in tension with Standard Model predictions. We suggest further measurements that can help narrowing down viable new physics explanations. We stress that the measured values of $R_K$ and $R_{K^*}$ are fully compatible with new physics explanations of other anomalies in rare $B$ meson decays based on the $b \to s \mu\mu$ transition. If the hints for lepton flavor universality violation are first signs of new physics, perturbative unitarity implies new phenomena below a scale of $\sim 100$ TeV.

Theoretical aspects of charged Lepton Flavour Violation

If observed, charged lepton flavour violation is a clear sign of new physics - beyond the Standard Model minimally extended to accommodate neutrino oscillation data. We briefly review several extensions of the Standard Model which could potentially give rise to observable signals, also emphasising the rôle of charged lepton flavour violation in probing such new physics models.1

Branching ratios, CP asymmetries and polarizations of Brightarrow psi (2S) V decays

We analyze the non-leptonic decays B/B_srightarrow psi (2S) V with V=(rho , omega , K^{*}, phi ) by employing the perturbative QCD (pQCD) factorization approach. Here the branching ratios, the CP asymmetries and the complete set of polarization observables are investigated systematically. Besides the traditional contributions from the factorizable and non-factorizable diagrams at the leading order, the next-to-leading order (NLO) vertex corrections could also provide considerable contributions. The pQCD predictions for the branching ratios of the B_{(s)}rightarrow psi (2S)K^{*}, psi (2S)phi decays are consistent with the measured values within errors. As for Brightarrow psi (2S) rho , psi (2S) omega decays, the branching ratios can reach the order of 10^{-5} and could be measured in the LHCb and Belle-II experiments. The numerical results show that the direct CP asymmetries of the considered decays are very small. Thus the observation of any large direct CP asymmetry for these decays will be a signal for new physics. The mixing-induced CP asymmetries in the neutral modes are very close to sin 2beta _{(s)}, which suggests that these channels can give a cross-check on the measurement of the Cabbibo–Kobayashi–Maskawa (CKM) angle beta and beta _s. We find that the longitudinal polarization fractions f_0 are suppressed to sim 50% due to the large non-factorizable contributions. The magnitudes and phases of the two transverse amplitudes mathcal {A}_{parallel } and mathcal {A}_{perp } are roughly equal, which is an indication for the approximate light-quark helicity conservation in these decays. The overall polarization observables of Brightarrow psi (2S) K^{*0} and B_srightarrow psi (2S) phi channels are also in good agreement with the experimental measurements as reported by LHCb and BaBar. Other results can also be tested by the LHCb and Belle-II experiments.

Probing new physics in B rightarrow D ell ^+ ell ^- decays by using angular asymmetries

We present a fully general, model-independent study of a few rare semileptonic B decays that get dominant contributions from W-annihilation and W-exchange diagrams, in particular B^0 rightarrow overline{D}{} ^0 ell ^+ell ^-, where ell = e,mu . We consider the most general Lagrangian for the decay, and define three angular asymmetries in the Gottfried–Jackson frame, which are sensitive to new physics. We show how these angular asymmetries can easily be extracted from the distribution of events in the Dalitz plot for B rightarrow D ell ^+ ell ^- decays. Especially a non-zero forward–backward asymmetry within the frame would give the very first hint of possible new physics. These observations are also true for related decay modes, such as B^+ rightarrow D^+ ell ^+ ell ^- and B^0 rightarrow D^0 ell ^+ ell ^-. Moreover, these asymmetry signatures are not affected by either B^0 –overline{B}^0 or D^0 –overline{D}^0 mixings. Then this implies that both B^0 rightarrow overline{D}{} ^0 ell ^+ ell ^- and B^0 rightarrow D^0 ell ^+ ell ^- as well as their CP conjugate modes can all be considered together in our search for signature of new physics. Hence, it would be of great importance to look for and study these decays in the laboratory, LHCb and Belle II in particular.