Experimental research

The experimental research is based around two main research themes:

[1] Search for physics beyond the standard model;

[2] Understanding the strong force.

 

The high energy experiments under construction provide a unique possibility to search deeper into the nature of our Universe, in search for the Higgs particle(s), supersymmetric particles or other dark matter candidates, extra dimension and unexpected new phenomena. However, a correct interpretation of signals at the LHC, depends crucially on a good understanding of the severe background mediated by the "conventional" strong force.

Although the fundamental strong force Lagrangian is well known, a good understanding of its properties is still lacking, in particular in the non-perturbative regime and the perturbative–non-perturbative transition. This includes the nature of the quark-gluon plasma and strong colour fields or high gluon densities. The strong interaction is not only important in itself. A better understanding of the mechanisms behind the formation of a condensate and symmetry breaking may also shed light on the properties of other non-linear phenomena.

Progress needs work on several frontiers, and a close interaction involving experimentalists at different types of experiments as well as theorists.

The projects listed below present complementary contributions to this overall objective. They are a selection of the large range of research choices available to incoming ESR's. It will be the individual ESR who chooses the topic to match their own personal curiosity, interest and training/career needs.

The experiments are:

PHENIX[2]: An experiment at the BNL RHIC outside New York, where collisions of Gold Nuclei are studied up to the energy of 100 GeV/n providing the best possibility today to create extended volumes of de-confined quark matter (so called Quark-gluon plasma, QGP), a state of matter, prevailing during the first microseconds after the Big Bang, just before a phase transition to normal hadronic matter took place in the rapidly expanding universe. A wide exploration of the phase diagram of matter is pursued, by measuring the properties of different final states for different initial conditions. Possible investigations for the ESR's are: (a) Measure the production of light vector mesons (w,r,f) whose mass-width and branching-rations are sensitive to QGP formation. Estimate the background, fit the mass distributions and compare with simulation models. (b) Measure the jet production and compare with proton collisions. This provides an evaluation of the extension of, and mean free path through, a QGP. (c) Measure charm-production. The environment of a QGP is predicted to produce a suppression of hidden charm (e.g. J/Y) and enhancement of open charm (D-mesons).

The experiments under preparation are:

ALICE[3]: An experiment at the CERN LHC to study colliding Pb nuclei up to an energy of 2’760 GeV/n. Collisions of Pb beams at energy about a factor 30 higher than at RHIC will offer outstanding opportunities in heavy-ion physics. The combined studies at the RHIC and the LHC would provide a synergy important for global understanding of the properties and dynamics of dense QCD matter. Two classes of questions have to be addressed: Characterization of the initial state, and characterization of the produced dense matter using hard probes. Possible investigations for the ESRs are: (a) Measure global observables. Multiplicity and transverse energy give information on the energy densities. (b) Measure jet tomography. To establish the composition of the jet and its relation to the initial conditions of the collisions. (c) Measure jet–jet and g–jet correlations. To study the propagation of the jets through the high density/high temperature matter. (d) Measure Onium production. Charm and bottom production test various theoretical scenarios on color screening in high density matter.

ATLAS[4]: A general purpose facility under construction for the CERN LHC. It will study pp collisions at Ös = 14 TeV, and be a facility at the energy frontier for many years to come. Research-topics include supersymmetry (SUSY), extra dimensions, the mass generating mechanism, CP-violation and, of course, general explorations. (a) If SUSY is an underlying theory, then hundreds of SUSY-events could be expected each day already at 1033 cm-2s-1 luminosity. A possible investigation for the ESRs is: Search for missing-ET multi-jet events, assuming R-parity is conserved. Evaluate the background from Z/W plus jets, t-tbar and QCD from data control-samples (and which selection and pre-scaling to be implemented during data-taking). Work out which parameter to construct from the jets and missing-ET, to get analysed in search for a signal. Evaluate the statistical procedure for the hypothesis testing and the statistical and the systematic errors. (b) Another possible investigation could be to search for effects from large extra dimensions: The so-called hierarchy problem, the large Planck mass compared with the electroweak scale, is speculated to be solved by the existence of large (1/TeV) extra dimensions into which gravity is diluted. This could then be manifested as a gauge-boson excitation spectrum, so-called Kaluza-Klein excitations. Investigation: Search for the influence from such excitations. Select events with leptons and missing-ET, and measure the lepton-pair mass-spectrum, and the lepton transverse-mass-spectrum at large values. Evaluate and subtract the background from t-tbar, WW, WZ and ZZ, and search for signs of excited Z/g and W. Search both for peaks and for interference effects on the continuum. Potential to reach an excitation-peak out to about 6 TeV, and measuring the tail-influence for excitations out to 10 TeV. Model the Standard Model background to allow for a Likelihood fit, and understand measurement tail effects that could fake a signal. (c) The ESRs could also investigate physics associated with the b-quark. The LHC allows high-precision measurements of B-hadrons, in particular of the B0s. The origin of CP violation can be tested with measurements related to the Unitarity Triangle and other measurements in B-hadron systems. This gives the possibility to observe deviations from the SM and to make measurements of physics beyond the SM. Lepton flavour violation can also be investigated, like t -> 3m, t -> eg etc. as the LHC with 108 t per year just from W/Z decays, is a large-pT t-factory. Predictions from models which agree with the neutrino mixing data can be compared with experimental signatures at the LHC.

 

[2] http://www.phenix.bnl.gov/

[3] http://alice.web.cern.ch/Alice/AliceNew/

[4] http://atlas.web.cern.ch/Atlas/