High Energy Frontier Research Articles

Searches for Heavy Quarks at the ATLAS experiment

During 2011 the ATLAS experiment at CERN collected around 5 fb−1 of proton-proton collision data from the LHC at the center-of-mass energy of = 7 TeV. At such high energy frontier it is possible to explore possible new physics scenarios. Amongst the models of physics beyond the Standard Model (SM) some predict the existence of exotic heavy quarks, which would help to explain matter-antimatter asymmetry and to solve the hierarchy problem. Several analyses at ATLAS were performed to search for a chiral 4th generation of quarks with charges +2/3 and −1/3 (t' and b' respectively) and for vector-like quarks with charges of +2/3, −1/3, +5/3, −4/3, coming in singlets or doublets depending on the model. The recent discovery of a Higgs-like, SM-like boson with mass of ~ 125 GeV makes the vector-like quark scenario the most interesting one. The status of these searches and the limits set at 95% Confidence Level (CL) are presented.

Low energy probes of physics beyond the standard model

Low-energy tests of fundamental symmetries and studies of neutrino properties provide a powerful window on physics beyond the Standard Model (BSM). In this article, we provide a basic theoretical framework for a subsequent set of articles that review the progress and opportunities in various aspects of the low-energy program. We illustrate the physics reach of different low-energy probes in terms of an effective BSM mass scale and illustrate how this reach matches and, in some cases, even exceeds that accessible at the high energy frontier.

The “Golden” cLFV channels μ → eγ and μ → eee — the high-intensity frontier

The muon as a laboratory for studying charged lepton-flavour violation (cLFV) has proven to be one of the most sensitive areas to probe for “New Physics”, due to the muon’s copious production rate and relatively long lifetime. The search at the intensity frontier with precision-type experiments is complementary to the search for new particles at the high-energy frontier of TeV colliders. Of the three “golden” muon channels: μ → eγ, μ → 3e and μ → e conversion, an overview of the status of the coincidence experiments MEG, together with the latest results, which constitute the most stringent limit to date on this decay and the recently initiated Mu3e experiment, will be given.

Search for pair-produced vector-like quarks with the ATLAS detector

The high energy frontier opened by the LHC is allowing us to explore physics scenarios where new physics might lay. The need to go beyond the Standard Model (SM) comes from various unanswered questions such as where does the matter-antimatter asymmetry comes from? What is the nature of Dark Matter? How can the hierarchy problem be solved? The recent discovery of an Higgs-like boson tends to disfavour the existence of a heavy 4th generation of quarks which would change the Higgs SM cross section and branching ratio in a way that is not experimentally observed. At the same time, vector-like quarks become a more compelling possibility due to their important role stabilizing the Higgs boson mass against r adiative corrections. The purpose of this poster is to review the latest results in the searches for pair production of vector-like quarks at the ATLAS experiment.

Electroweak baryogenesis

Electroweak baryogenesis (EWBG) remains a theoretically attractive and experimentally testable scenario for explaining the cosmic baryon asymmetry. We review recent progress in computations of the baryon asymmetry within this framework and discuss their phenomenological consequences. We pay particular attention to methods for analyzing the electroweak phase transition and calculating CP-violating asymmetries, the development of Standard Model extensions that may provide the necessary ingredients for EWBG, and searches for corresponding signatures at the high energy, intensity and cosmological frontiers.

Betatron radiation damping in laser plasma acceleration

AbstractWe explore the feasibility of accelerating electron beams up to energies much beyond 1 TeV in a realistic scale and evolution of the beam qualities such as emittance and energy spread at the final beam energy on the order of 100 TeV, using the newly formulated coupled equations describing the beam dynamics and radiative damping of electrons. As an example, we present a design for a 100 TeV laser-plasma accelerator in the operating plasma density np = 1015 cm−3 and numerical solutions for evolution of the normalized emittance as well as their analytical solutions. We show that the betatron radiative damping causes very small normalized emittance that promises future applications for the high-energy frontier physics.

Physics at the high-energy frontier: the Large Hadron Collider project

This issue of Philosophical Transactions A is devoted to the proceedings of the Royal Society Discussion Meeting on ‘Physics at the High Energy Frontier—The Large Hadron Collider Project’ held on 16th and 17th May 2011. The purpose of this meeting was to draw together the many aspects of a

Early physics results

For the past year, experiments at the Large Hadron Collider (LHC) have started exploring physics at the high-energy frontier. Thanks to the superb turn-on of the LHC, a rich harvest of initial physics results have already been obtained by the two general-purpose experiments A Toroidal LHC Apparatus (ATLAS) and the Compact Muon Solenoid (CMS), which are the subject of this report. The initial data have allowed a test, at the highest collision energies ever reached in a laboratory, of the Standard Model (SM) of elementary particles, and to make early searches Beyond the Standard Model (BSM). Significant results have already been obtained in the search for the Higgs boson, which would establish the postulated electro-weak symmetry breaking mechanism in the SM, as well as for BSM physics such as Supersymmetry (SUSY), heavy new particles, quark compositeness and others. The important, and successful, SM physics measurements are giving confidence that the experiments are in good shape for their journey into the uncharted territory of new physics anticipated at the LHC.

The high energy frontier of ultrarelativistic nucleus–nucleus collisions

An outlook on physics at the high energy frontier of nucleus–nucleus collisions is presented, on the basis of the new results presented at Quark Matter 2011 by the LHC and RHIC experiments.

Operating plasma density issues on large-scale laser-plasma accelerators toward high-energy frontier

Consideration of laser-driven plasma-based electron/positron accelerators with a 2 TeV center-of-mass energy is presented, employing a multistaging scheme consisting of successive multi-GeV laser wakefield accelerators operated at the plasma density range of ${10}^{15}--{10}^{18}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ in the quasilinear regime. A total accelerator length is determined by an operating plasma density and a coupling distance allowed for both laser and beam focusing systems. We investigate beam dynamics and synchrotron radiation due to the betatron oscillation of the beam in laser-plasma acceleration, characterizing the beam qualities such as energy spread and transverse emittance. According to the criteria on the beam qualities for applications and available laser sources, the operating plasma density will be optimized. We note that in the low density operation the required wall-plug power for the laser driver will be much reduced in comparison with the high-density options.

Inclusive electron spectrum from heavy-flavour decays in proton-proton collisions at [formula omitted] measured with ALICE at LHC

We present the status of the analysis on heavy-flavour production in proton-proton collisions at s = 7 TeV via a measurement of their semileptonic decay spectrum at mid-rapidity. An important goal of QCD studies at the LHC is a precise understanding of the heavy-quark production cross section at the high energy frontier in terms of the pQCD framework. Furthermore, heavy quarks are an excellent probe of the hot matter produced in high energy nucleus-nucleus collisions. Establishing precision measurements in proton-proton collisions is a very important baseline for the interpretation of their behaviour in the Quark Gluon Plasma. We present first measurements of the inclusive spectrum of single electrons, using the excellent particle identification capabilities of the ALICE experiment. The subtraction of a cocktail describing photonic and other background sources, as well as an analysis of impact parameter distributions relative to the interaction vertex, are ongoing and will allow the isolation of electrons from heavy-flavour hadron decays.

Charmless two-body B-meson decays in QCD

The coming years will witness tremendous progress at the ''high energy frontier'' accomplished primarily at the LHC. Important advances are also expected at the ''high precision frontier'' through the improved B_s-physics experiments at the Tevatr on and in particular LHCb at CERN. After summarizing the status of various tensions in the present Unitarity Triangle (UT) analyses, we consider a number of Supersymmetric models showing their capability to solve these tensions. Moreover, we show how the characteristic patterns of correlations among flavour observables allow to distinguish between these different SUSY scenarios. We propose a ''DNA-Flavour Test'' of NP models, with the aim of showing a tool to distinguish between these NP scenarios, once a dditional data on flavour changing processes become available.

Flavour Physics in SUSY extensions of the SM in the LHC era

The coming years will witness tremendous progress at the “high energy frontier” accomplished primarily at the LHC. Important advances are also expected at the “high precision frontier” through the improved B s -physics experiments at the Tevatron and in particular LHCb at CERN. After summarizing the status of various tensions in the present Unitarity Triangle (UT) analyses, we consider a number of Supersymmetric models showing their capability to solve these tensions. Moreover, we show how the characteristic patterns of correlations among flavour observables allow to distinguish between these different SUSY scenarios. We propose a “DNA-Flavour Test” of NP models, with the aim of showing a tool to distinguish between these NP scenarios, once additional data on flavour changing processes become available.

Development and online operation of minimum bias triggers in ATLAS

Since the restart of the LHC in November 2009, ATLAS has recorded pp collisions to investigate new phenomena at the high energy frontier. One early measurement is of charged particle multiplicities in inelastic interactions, which was performed on data sets of about 360,000 and 10,000,000 minimum bias event at (s)1/2 = 900 GeV and 7 TeV respectively. This measurements will help to constrain various phenomenological models describing inelastic interactions. Understanding trigger efficiencies for different event types is crucial to minimize any possible bias in the event selection. ATLAS has a three level trigger system and utilises two main minimum bias triggers. A primary scintillator based trigger situated in the forward regions of 2.1 < |η| < 3.8 was proven to select pp collisions with high efficiency. A complementary control trigger runs on unbiased filled proton bunches and utilizes space points reconstructed in the silicon tracking volume to trigger on inelastic interactions. Both triggers were essential for the analysis of charged particle multiplicities. Their performance and trigger efficiency measurements as well as studies on possible bias sources are presented. The triggers are shown to form an effective and inclusive triggering solution for early LHC physics.

Slepton mass-splittings as a signal of LFV at the LHC

Precise measurements of slepton mass-splittings might represent a powerful tool to probe supersymmetric (SUSY) lepton flavour violation (LFV) at the LHC. We point out that mass-splittings of the first two generations of sleptons are especially sensitive to LFV effects involving $\tau-\mu$ transitions. If these mass-splittings are LFV induced, high-energy LFV processes like the neutralino decay ${\nt}_2\to\nt_1\tau^{\pm}\mu^{\mp}$ as well as low-energy LFV processes like $\tau\to\mu\gamma$ are unavoidable. We show that precise slepton mass-splitting measurements and LFV processes both at the high- and low-energy scales are highly complementary in the attempt to (partially) reconstruct the flavour sector of the SUSY model at work. The present study represents another proof of the synergy and interplay existing between the LHC, i.e. the {\em high-energy frontier}, and high-precision low-energy experiments, i.e. the {\em high-intensity frontier}.

Searches for supersymmetry at high-energy colliders

This review summarizes the state of the art in searches for supersymmetry at colliders on the eve of the LHC era. Supersymmetry is unique among extensions of the standard model in being motivated by naturalness, dark matter, and force unification, both with and without gravity. At the same time, weak-scale supersymmetry encompasses a wide range of experimental signals that are also found in many other frameworks. We recall the motivations for supersymmetry and review the various models and their distinctive features. We then comprehensively summarize searches for neutral and charged Higgs bosons and standard model superpartners at the high energy frontier, considering both canonical and non-canonical supersymmetric models, and including results from LEP, HERA, and the Tevatron.

From LHC to SLHC, ILC and Beyond

A review and possible road map for future collider projects at the high energy frontier are presented. Opportunities for photon physics will be pointed out.

Quasi-Static Particle-In-Cell Simulation of the Plasma Wakefield Afterburner Concept

High-energy physics frontier is approaching the teraelectron-volt energy range for the study of the Higgs particle. To reach teraelectron-volt energy for charged particles, a high-gradient acceleration structure is desirable. Recent plasma wakefleld acceleration (PWFA) experiment has shown sustained acceleration gradient of 50 GeV/m in a meter-long plasma [1]. To study the relevant physics in an "afterburner" concept [2], i.e., an energy booster based on PWFA for the ILC-type parameters, we conduct 3-D particle-in-cell (PIC) simulation of the nonlinear beam-plasma interactions. A multiscale PIC model is developed using the quasi-static description of the interaction. We present images from the simulation, which illustrate the physical picture of an afterburner and the evolution of the accelerated beam.

Science on the grid with CMS at the LHC

In late 2008 a new experimental facility, the Large Hadron Collider is expected to start data taking. Two large experiments, Atlas and CMS, are getting ready to explore the high-energy frontier of elementary particle physics at this facility. We briefly explain why this new facility is a once-in-a-lifetime opportunity for the present generation of particle physicists. Using the example of the CMS experiment, we explain the computing challenge of these large experiments, and how this challenge is addressed. In this context, we explain why Grid computing is an essential enabling technology to fully exploit the science potential of these experiments. We use data transfer and data analysis as examples to give a flavor of the present state of readiness of the globally distributed computing system for data taking, and what we had to do to get there.

ATLAS and Astroparticle Physics

The construction of the ATLAS detector at the LHC is nearing completion. This report will briefly summarize the progress, the technical challenges of the detector construction, and the status in the summer of 2006. The project is on track and the ATLAS Collaboration is poised to exploit the discovery potential of the LHC physics arena. Research into the fundamental nature of matter at the high energy frontier takes place in three main areas: accelerator based particle physics; high energy astrophysics; and, the cosmology of the early universe. The LHC project provides the laboratory to perform measurements of great importance for cosmic ray astrophysics and cosmology. Also, the study of astroparticle physics can have significant implications for collider physics at the LHC. This paper reviews some of the important synergistic links between astroparticle and LHC physics with the ATLAS detector.