Search results for "Standard Model"
showing 10 items of 1206 documents
Search for Production of Invisible Final States in Single-Photon Decays of Υ(1S)
2010
We search for single-photon decays of the Upsilon(1S) resonance, Upsilon->gamma+invisible, where the invisible state is either a particle of definite mass, such as a light Higgs boson A0, or a pair of dark matter particles, chi chi-bar. Both A0 and chi are assumed to have zero spin. We tag Upsilon(1S) decays with a dipion transition Upsilon(2S)->pi+pi-Upsilon(1S) and look for events with a single energetic photon and significant missing energy. We find no evidence for such processes in the mass range m_A0<=9.2 GeV and m_chi<=4.5 GeV in the sample of 98e6 Upsilon(2S) decays collected with the BaBar detector and set stringent limits on new physics models that contain light dark ma…
Photons, Photon Jets and Dark Photons at 750 GeV and Beyond
2016
In new physics searches involving photons at the LHC, one challenge is to distinguish scenarios with isolated photons from models leading to "photon jets". For instance, in the context of the 750 GeV diphoton excess, it was pointed out that a true diphoton resonance $S \to \gamma\gamma$ can be mimicked by a process of the form $p p \to S \to a a \to 4\gamma$, where $S$ is a new scalar with a mass of 750 GeV and $a$ is a light pseudoscalar decaying to two collinear photons. Photon jets can be distinguished from isolated photons by exploiting the fact that a large fraction of photons convert to an $e^+e^-$ pair inside the inner detector. In this note, we quantify this discrimination power, an…
Bottom-up approach within the electroweak effective theory: Constraining heavy resonances
2020
The LHC has confirmed the existence of a mass gap between the known particles and possible new states. Effective field theory is then the appropriate tool to search for low-energy signals of physics beyond the Standard Model. We adopt the general formalism of the electroweak effective theory, with a non-linear realization of the electroweak symmetry breaking, where the Higgs is a singlet with independent couplings. At higher energies we consider a generic resonance Lagrangian which follows the above-mentioned non-linear realization and couples the light particles to bosonic heavy resonances with $J^P=0^\pm$ and $J^P=1^\pm$. Integrating out the resonances and assuming a proper short-distance…
Axion search with BabyIAXO in view of IAXO
2020
Axions are a natural consequence of the Peccei-Quinn mechanism, the most compelling solution to the strong-CP problem. Similar axion-like particles (ALPs) also appear in a number of possible extensions of the Standard Model, notably in string theories. Both axions and ALPs are very well motivated candidates for Dark Matter, and in addition, they would be copiously produced at the sun's core. A relevant effort during the last decade has been the CAST experiment at CERN, the most sensitive axion helioscope to-date. The International Axion Observatory (IAXO) is a large-scale 4th generation helioscope. As its primary physics goal, IAXO will look for solar axions or ALPs with a signal to backgro…
Electroweak Higgs production with HiggsPO at NLO QCD
2017
We present the HiggsPO UFO model for Monte Carlo event generation of electroweak $VH$ and VBF Higgs production processes at NLO in QCD in the formalism of Higgs pseudo-observables (PO). We illustrate the use of this tool by studying the QCD corrections, matched to a parton shower, for several benchmark points in the Higgs PO parameter space. We find that, while being sizable and thus important to be considered in realistic experimental analyses, the QCD higher-order corrections largely factorize. As an additional finding, based on the NLO results, we advocate to consider 2D distributions of the two-jet azimuthal-angle difference and the leading jet $p_T$ for new physics searches in VBF Higg…
Leptogenesis from oscillations and dark matter
2019
An extension of the Standard Model with Majorana singlet fermions in the 1–100 GeV range can explain the light neutrino masses and give rise to a baryon asymmetry at freeze-in of the heavy states, via their CP-violating oscillations. In this paper we consider extending this scenario to also explain dark matter. We find that a very weakly coupled B−L gauge boson, an invisible QCD axion model, and the singlet majoron model can simultaneously account for dark matter and the baryon asymmetry.
Spin-dependence of gravity-mediated dark matter in warped extra-dimensions
2021
We study the spin-dependence of Dark Matter (DM) particles which interact gravitationally with the Standard Model (SM) in an extra-dimensional Randall-Sundrum scenario. We assume that both the Dark Matter and the Standard Model are confined to the TeV (Infra-red) brane and only interact via gravitational mediators, namely Kaluza-Klein gravitons and the radion. We analyze the different DM annihilation channels and find that it is possible to achieve the presently observed relic abundance of Dark Matter, $\Omega_{\rm DM}$, within the freeze-out mechanism for DM particles of spin 0, 1/2 and 1. We study the region of the model parameter space for which $\Omega_{\rm DM}$ is achieved and compare …
Determination of the muonic branching ratio of the W boson and its total width via cross-section measurements at the Tevatron and LHC
2016
The total $W$-boson decay width $\Gamma_W$ is an important observable which allows testing of the standard model. The current world average value is based on direct measurements of final state kinematic properties of $W$-boson decays, and has a relative uncertainty of 2%. The indirect determination of $\Gamma_W$ via the cross-section measurements of vector-boson production can lead to a similar accuracy. The same methodology leads also to a determination of the leptonic branching ratio. This approach has been successfully pursued by the CDF and D0 experiments at the Tevatron collider, as well as by the CMS collaboration at the LHC. In this paper we present for the first time a combination o…
Fermion pair production in e+e- collisions at 189-209 GeV and constraints on physics beyond the Standard Model
2006
Cross sections, angular distributions and forward-backward asymmetries are presented, of two-fermion events produced in e+e- collisions at centre-of-mass energies from 189 to 209 GeV at LEP, measured with the ALEPH detector. Results for e+e-, mu+mu-, tau+tau-, qq, bb and cc production are in agreement with the Standard Model predictions. Constraints are set on scenarios of new physics such as four-fermion contact interactions, leptoquarks, Z' bosons, TeV-scale quantum gravity and R-parity violating squarks and sneutrinos.
The Z boson spin observables as messengers of new physics
2017
We demonstrate that the 8 multipole parameters describing the spin state of the $Z$ boson are able to disentangle known $Z$ production mechanisms and signals from new physics at the LHC. They can be extracted from appropriate asymmetries in the angular distribution of lepton pairs from the $Z$ boson decay. The power of this analysis is illustrated by (i) the production of $Z$ boson plus jets; (ii) $Z$ boson plus missing transverse energy; (iii) $W$ and $Z$ bosons originating from the two-body decay of a heavy resonance.