DPNC - ATLAS

Particle Physics

(Français/English)

The first proton-proton physics run of the Large Hadron Collider (LHC) is expected to take place in late 2009. The LHC is expected to run first at a centre of mass energy √s=10 TeV during 2009-2010, and then at √s=14 TeV following a shutdown. The first heavy ion collisions (Pb+Pb) may be expected at the LHC in 2010 just before the shutdown. The DPNC group at the University of Geneva is involved in several early physics studies of Standard Model processes as well as in searches for new physics, such as supersymmetry, at the ATLAS experiment. Physicists in the DPNC group are working on the following topics:

Minimum Bias studies

Charged particle density for non-single diffractive events as a function of the centre of mass energy
Charged particle density for non-single diffractive events as a function of the centre of mass energy
The properties of inelastic proton-proton and proton anti-proton interactions have previously been studied over a wide range of energies. Previous analyses have selected events with minimal bias and illustrated their behaviour through dNCh/dη, dNCh/dpT, KNO scaling, and <pT> vs NCh distributions. These distributions are typically produced from a selection of non-single-diffractive events, defined as a sample of inelastic events, where the trigger acceptance for single diffractive events is very low. Previous results from CERN and Fermilab experiments have been used to tune the PYTHIA event generator such that the generator properly describes previous measurements. In particular the associated figure shows the measured and predicted charged particle density for non-single diffractive events as a function of the centre of mass energy. There are clear differences in the predicted multiplicities of PYTHIA (ATLAS and CDF tune-A) and PHOJET at Large Hadron Collider (LHC) centre of mass energies of √s = 10TeV and √s = 14TeV.

Data collected from the first physics run at the LHC will allow models of soft quantum chromodynamics (QCD) processes to be constrained. These studies are vital to understand QCD within the LHC energy regime and to model additional proton-proton interactions, which will be abundant at higher instantaneous luminosities.

Study of J/Ψ → ee production

Accepted differential cross-section as a function the di-electron mass
Accepted differential cross-section as a function the di-electron mass
The LHC will produce quarkonium states such as J/Ψ and Υ in abundance already in the initial low luminosity period. Their importance for ATLAS is threefold:

Measurement of the direct photon production cross-section

Single direct photon cross-section as a function of the photon transverse momentum
Single direct photon cross-section as a function of the photon transverse momentum
Direct photon production is defined as the production of one or more photons in the hard scattering process. Studies of single and double direct photon production provide an arena for testing perturbative QCD predictions and to constrain the gluon parton density function. These prompt photon processes represent also an important contribution in estimating the background to other physics processes in the SM (such as H → γγ) and beyond. The current feasibility studies estimate the accuracy with which ATLAS will be able to measure the direct photon cross section and distinguish it from the dominant background process π0 → γγ. The analysis includes studies of acceptance and efficiencies of trigger, photon identification and reconstruction in the kinematical region where the transverse momentum of the photon is pT > 20 GeV, and a Monte Carlo based estimation of the signal and background cross section for direct photon production.

Study of (W → e ν) + jets production

Direct production of electroweak gauge bosons in association with jets are important hard processes at LHC.

Search for light stop

Expected cross section for stop pair production at 10 TeV and 14 TeV as a function of the stop mass
Expected cross section for stop pair production at 10 TeV and 14 TeV as a function of the stop mass
Despite the spectacular agreement with current experimental results, the Standard Model is only a low-energy effective description of a more fundamental theory as it leaves many questions unanswered. Two such unanswered questions drive our SUSY studies on the ATLAS experiment: what is the nature of dark matter and why does the universe have an overwhelming abundance of matter over antimatter.

Assuming that inflation washes out any initial baryon asymmetry after the Big Bang, there should be a dynamic mechanism to generate the asymmetry after inflation. The three Sakharov requirements for baryogenesis are satisfied in both the SM and the Minimal Supersymmetric Standard Model (MSSM) during the electroweak phase transition. However, SM processes do not allow for a sufficient departure from thermal equilibrium. The MSSM allows for electroweak baryogenesis if the supersymmetric partner of the top, the stop, is lighter than the top.The lightest SUSY particle in this scenario, the neutralino, also provides an ideal candidate for dark matter. If the light stop is found it may be evidence of electroweak baryogenesis. Despite the search efforts at the Tevatron yielding no sign of the stop to date, its existence cannot be ruled out. The allowed region still lies predominantly within reach of the LHC. The analysis is a prime candidate for early SUSY searches at the LHC, due to its high cross section and lepton based event topologies.

The decay topologies of the light stop are similar to that of the top. The Geneva group is preparing analyses to search for the light stop, as deviations from the expected top signatures. Direct searches will also be performed with a focus on establishing clear experimental signatures, with respect to SM processes. Data driven background subtraction techniques and pure background samples can be used to ascertain the shape and normalisation of the SM backgrounds.

Data-based background determination for SUSY searches

In R-parity conserving supersymmetry (SUSY), the hard-scattering interaction of two protons can only produce an even number of supersymmetric particles, which will then decay incoherently in cascades to ever lighter particles, conserving the initial negative R parity in each decay. Consequently, the lightest SUSY particle is stable and escapes detection. The resulting characteristic missing transverse energy signature drives the search strategy for such events. In this environment the masses of the involved SUSY particles can not be reconstructed on an event-by-event basis. One needs therefore to search for a statistical excess in events with large missing transverse energy, rather than mass peaks over broader background. While background under a peak can be empirically determined from the observed data without relying on Monte Carlo simulation, the use of simulation is required when comparing event abundances only, without extracting shape information from the data. However, for experiments operating at the high energy frontier it is not possible to rely solely on MC simulation. These are inaccurate due to various uncertainties, including parton density distributions for protons at LHC energies, cross sections of the involved Standard Model processes, and the details of the detector response. It is thus mandatory to develop strategies to determine the expected Standard Model background (and, simultaneously, the observed signal abundance) in an as model-independent manner as possible.

Search for excited electrons

The hierarchical structure observed among the masses of quark and lepton SU(2) doublets can be considered as a hint for an underlying substructure. The existence of excited states is a general prediction of composite models of elementary fermions. Their discovery would provide an unequivocal evidence for an underlying substructure for leptons and quarks. We search for singly produced excited electrons in the process pp→ee*→eeγ which extends the search sensitivity close to the LHC center of mass energy. Such an intecation would lead to an excess, over the Standard Model expectation, of high transverse energy di-electron plus photon production with a resonance in the electron-photon mass.

Heavy Ion Studies

In a very near future, in addition to proton beams, Pb beams will be run in the LHC at √s= 5.5 TeV per colliding nucleon pair (1.2 PeV if all nucleons participate to the collision) and will provide a unique opportunity to create and study the quark gluon plasma at the highest temperatures and densities ever created in the laboratory. The results obtained by the SPS and the RHIC experiments, in particular a discovery of a new form of matter with the properties resembling those of an ideal fluid, emphasize the importance of the study of nucleus-nucleus collisions at LHC, where the center-of-mass energy is 30 times larger than the one reached at RHIC, and where the incursion in the quark gluon plasma phase lasts longer. ATLAS has demonstrated that most sub-detectors retain nearly their full capability despite the additional soft background from nucleus-nucleus collisions.

Global measurements (particle multiplicities, transverse energy, collective flow, etc.) reflect the time integral over all physics processes happening during the collision, and give access into thermodynamic and hydrodynamic basic event properties.

Measuring complete jets out to 100's of GeV allows detailed studies of energy loss and jet quenching. Jet quenching is due to the energy loss by gluon radiation of the fast partons at the origin of jets, which is expected to be larger in a dense partonic medium than in ordinary nuclear matter. This induced radiation should result in a rearrangement of energy inside the jets, and, consequently, in the modification of jet properties like a suppression of high-z hadrons from the jet fragmentation, correlated with an increase in the number of low-momentum hadrons inside the jet, and even with a conical structure with a dip in the center as observed at RHIC, in contrast with the typical Gaussian-like jet shape seen in pp collisions. In particular, direct photons in coincidence with jets (γ-jet events) are a useful tool to study the plasma properties because the medium is transparent to photons. Therefore the photons can be used to measure the original energy and direction of the away-side jets, which should be strongly modified by the medium.

Quarkonia suppression provides a handle on deconfinement mechanisms: color screening effect in a quark gluon plasma prevents heavy quarkonia to be formed when the color screening length becomes shorter than the size of the resonances, allowing to probe the long-range confinement of QCD. As each quark-antiquark bound state is characterized by a different dissociation temperature, the systematic measurement of the suppression of these resonances should provide some sort of thermometer for the early stage of the system evolution. Thus, in addition to Υ, Υ', J/Ψ, Ψ'decaying into muon or electron pairs, the possibility of measuring χcdecaying into J/Ψ is investigated. Due to the mass of the J/Ψ, a low pT muon trigger with a threshold of 1.5 GeV is needed to measure the J/Ψ from pT=0, and is under study.

Last modified: 2009/06/17