Jan Hajer

Jan Hajer
University of Lisbon | UL · Instituto Superior Técnico

PhD

About

38
Publications
2,179
Reads
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718
Citations
Additional affiliations
November 2021 - present
University of Lisbon
Position
  • Researcher
January 2021 - October 2021
University of Basel
Position
  • PostDoc Position
October 2017 - January 2021
Université Catholique de Louvain - UCLouvain
Position
  • PostDoc Position
Education
April 2010 - May 2013
University of Hamburg
Field of study
  • High Energy Physics
October 2003 - March 2010
University of Hamburg
Field of study
  • Theoretical Physics

Publications

Publications (38)
Preprint
Full-text available
We present a framework for the construction of portal effective theories (PETs) that couple effective field theories of the Standard Model (SM) to light hidden messenger fields. Using this framework we construct electroweak and strong scale PETs that couple the SM to messengers carrying spin zero, one half, or one. The electroweak scale PETs encomp...
Preprint
Full-text available
Opportunities for searches for phenomena beyond the Standard Model (BSM) using heavy-ions beams at high energies are outlined. Different BSM searches proposed in the last years in collisions of heavy ions, mostly at the Large Hadron Collider, are summarized. A few concrete selected cases are reviewed including searches for axion-like particles, ano...
Article
In this paper, we describe the potential of the LHCb experiment to detect Stealth physics. This refers to dynamics beyond the Standard Model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or tog...
Article
Full-text available
A bstract We study how oscillations of a scalar field condensate are damped due to dissipative effects in a thermal medium. Our starting point is a non-linear and non-local condensate equation of motion descending from a 2PI-resummed effective action derived in the Schwinger-Keldysh formalism appropriate for non-equilibrium quantum field theory. We...
Article
Full-text available
A bstract We present a framework for the construction of portal effective theory (PETs) that couple effective field theories of the Standard Model (SM) to light hidden messenger fields. Using this framework we construct electroweak and strong scale PETs that couple the SM to messengers carrying spin zero, one half, or one. The electroweak scale PET...
Preprint
Full-text available
We study how oscillations of a scalar field condensate are damped due to dissipative effects in a thermal medium. Our starting point is a non-linear and non-local condensate equation of motion descending from a 2PI-resummed effective action derived in the Schwinger-Keldysh formalism appropriate for non-equilibrium quantum field theory. We solve thi...
Article
Full-text available
The next generation of circular high energy collider is expected to be a lepton collider, FCC-ee at CERN or CEPC in China. However, the civil engineering concepts foresee to equip these colliders with bigger detector caverns than one would need for a lepton collider, so that they can be used for a hadron collider that may be installed in the same t...
Preprint
Full-text available
In this paper, we describe the potential of the LHCb experiment to detect Stealth physics. This refers to dynamics beyond the Standard Model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or tog...
Preprint
Full-text available
The next generation of circular high energy collider is expected to be a lepton collider, FCC-ee at CERN or CEPC in China. However, the civil engineering concepts foresee to equip these colliders with bigger detector caverns than one would need for a lepton collider, so that they can be used for a hadron collider that may be installed in the same t...
Article
Full-text available
Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton–proton collision. Such LLP signatures are di...
Article
Full-text available
This document summarises proposed searches for new physics accessible in the heavy-ion mode at the CERN Large Hadron Collider (LHC), both through hadronic and ultraperipheral γγ interactions, and that have a competitive or, even, unique discovery potential compared to standard proton–proton collision studies. Illustrative examples include searches...
Article
Full-text available
Novelty detection is the machine learning task to recognize data, which belong to an unknown pattern. Complementary to supervised learning, it allows us to analyze data model-independently. We demonstrate the potential role of novelty detection in collider physics, using autoencoder-based deep neural network. Explicitly, we develop a set of density...
Article
Full-text available
We show that heavy-ion collisions at the LHC provide a promising environment to search for signatures with displaced vertices in well-motivated new physics scenarios. Compared to proton collisions, they offer several advantages: (i) the number of parton level interactions per collision is larger, (ii) there is no pileup, (iii) the lower instantaneo...
Article
Full-text available
We show that heavy-ion collisions at the LHC provide a promising environment to search for new long-lived particles in well-motivated new physics scenarios. One advantage lies in the possibility to operate the main detectors with looser triggers, which can increase the number of observable events by orders of magnitude if the long-lived particles a...
Article
Full-text available
A bstract We study the sensitivity of displaced vertex searches for heavy neutrinos produced in W boson decays in the LHC detectors ATLAS, CMS and LHCb. We also propose a new search that uses the muon chambers to detect muons from heavy neutrino decays outside the tracker. The sensitivity estimates are based on benchmark models in which the heavy n...
Article
Full-text available
Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton–proton collision. Such LLP signatures are di...
Conference Paper
Full-text available
Preprint
Full-text available
We show that heavy ion collisions at the LHC provide a promising environment to search for new long lived particles. A main advantage lies in the possibility to operate the main detectors with lower triggers, which can increase the number of observable events by orders of magnitude if the long lived particles are produced with low transverse moment...
Preprint
We study the sensitivity of displaced vertex searches for heavy neutrinos produced in W~boson decays in the LHC detectors ATLAS, CMS and LHCb. We also propose a new search that uses the muon chambers to detect muons from heavy neutrino decays outside the tracker. The sensitivity estimates are based on benchmark models in which the heavy neutrinos m...
Preprint
Full-text available
Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton-proton collision. Such LLP signatures are di...
Preprint
Full-text available
This document summarizes proposed searches for new physics accessible in the heavy-ion mode at the LHC, both through hadronic and ultraperipheral $\gamma\gamma$ interactions, and that have a competitive or, even, unique discovery potential compared to standard proton-proton collision studies. Illustrative examples include searches of new particles...
Preprint
We show that heavy ion collisions at the LHC provide a promising environment to search for signatures with displaced vertices in well-motivated New Physics scenarios. The lower instantaneous luminosity in heavy ion collisions allows to operate the LHC experiments with very loose triggers. For scenarios in which long-lived particles are produced in...
Preprint
Full-text available
Novelty detection is the machine learning task to recognize data, which belongs to a previously unknown pattern. Complementary to supervised learning, it allows the data to be analyzed in a model-independent way. We demonstrate the potential role of novelty detection in collider analyses using an artificial neural network. Particularly, we introduc...
Preprint
Full-text available
The sensitivity of beam dump experiments to heavy neutrinos depends on the relative size of their mixings with the lepton flavours in the Standard Model. We study the impact of present neutrino oscillation data on these mixing angles in the minimal type I seesaw model. We find that current data significantly constrains the allowed heavy neutrino fl...
Article
Full-text available
A bstract The sensitivity of beam dump experiments to heavy neutral leptons depends on the relative strength of their couplings to individual lepton flavours in the Standard Model. We study the impact of present neutrino oscillation data on these couplings in the minimal type I seesaw model and find that it significantly constrains the allowed heav...
Article
Full-text available
Solutions to the electroweak hierarchy problem typically introduce a new symmetry to stabilize the quadratic ultraviolet sensitivity in the self-energy of the Higgs boson. The new symmetry is either broken softly or collectively, as for example in supersymmetric and little Higgs theories. At low energies such theories contain naturalness partners o...
Article
Full-text available
Solutions to the electroweak hierarchy problem typically introduce a new symmetry to stabilize the quadratic ultraviolet sensitivity in the self-energy of the Higgs boson. The new symmetry is either broken softly or collectively, as for example in supersymmetric and little Higgs theories. At low energies such theories contain naturalness partners o...
Article
Full-text available
We present strategies to search for heavy neutral Higgs bosons decaying to top quark pairs, as often occurs at low $\tan \beta$ in type II two Higgs doublet models such as the Higgs sector of the MSSM. The resonant production channel is unsatisfactory due to interference with the SM background. We instead propose to utilize same-sign dilepton signa...
Article
Full-text available
In this write-up, we summarize the production of non-SM Higgses in the Type II Two Higgs Doublet Model at a 100 TeV pp collider, as well as their decays. We present the reach for pp → bbH0/A → bbtt, bbττ as well as pp → tbH± → tbtb, tbτν at the 100 TeV pp collider and outline the possible search channels via Higgs exotic decays. We point out that a...
Article
Full-text available
Searching for Higgs bosons beyond the Standard Model (BSM) is one of the most important missions for hadron colliders. As a landmark of BSM physics, the MSSM Higgs sector at the LHC is expected to be tested up to the scale of the decoupling limit of \( \mathcal{O} \)(1) TeV, except for a wedge region centered around tan β ∼ 3-10, which has been kno...
Article
Full-text available
We investigate the LHC sensitivity to supersymmetric models with light higgsinos, small R-parity breaking and gravitino dark matter. The limits on decaying gravitino dark matter from gamma-ray searches with the Fermi-LAT put a lower bound on the higgsino-like neutralino NLSP decay length, giving rise to a displaced-vertex collider signature. Using...
Article
Full-text available
A recently proposed class of supersymmetric models predicts rather light and nearly mass-degenerate higgsinos, while the other superparticles are significantly heavier. In this paper we study the early LHC phenomenology of a benchmark model of this kind. If the squarks and gluinos, and in particular the lighter stop, are still light enough to be wi...
Article
Full-text available
We study supersymmetric extensions of the Standard Model with small R-parity and lepton number violating couplings which are naturally consistent with primordial nucleosynthesis, thermal leptogenesis and gravitino dark matter. We consider supergravity models where the gravitino is the lightest superparticle followed by a bino-like next-to-lightest...
Article
Full-text available
Supersymmetric extensions of the Standard Model with small R-parity and lepton number violating couplings are naturally consistent with primordial nucleosynthesis, thermal leptogenesis and gravitino dark matter. We consider supergravity models with universal boundary conditions at the grand unification scale, and scalar τ-lepton or binolike neutral...

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Projects

Projects (6)
Project
Heavy neutrinos at fixed target experiments and collider