The Higgs particle is the missing link in today’s particle physics. Is the Higgs just round the corner, or do we have to be patient? Likewise, supersymmetry could soon make a dramatic stage entry? A recent meeting in Florida assessed the chances.
Some of the most compelling questions in particle physics today are to do with the Higgs boson and supersymmetry (SUSY). Is the Higgs mechanism responsible for electroweak symmetry breaking and the origin of mass? Can SUSY (the symmetry under which bosons and fermions are equivalent in nature which thus predicts that for every particle there is a partner “sparticle”) point the way to the ultimate unification of forces? Where is the Higgs? Where is SUSY? How can they be found? What if we don’t find them?
To address these questions in depth, more than one hundred physicists gathered at the University of Florida, Gainesville, for the international conference entitled Higgs and Supersymmetry: Search and Discovery, earlier this year.
The goal of the conference was to provide a forum for physicists involved in Higgs and supersymmetry searches in which the present status of the research could be summarized and new directions for searches and the potential for discovery established.
The conference agenda focused on Higgs and SUSY presentations (see “http://www.phys.ufl.edu/~rfield/higgs_susy.html”). One exception was M Spiro’s (CEA/Saclay) talk on dark matter. Even then SUSY was in the limelight: the lightest supersymmetric particle is one of the best candidates for dark matter in the universe.
The elusive Standard Model
G Altarelli (CERN/Rome 3) pointed out in his presentation on “The Standard Electroweak Theory and beyond” that, in spite of the great experimental work done in the 1990s, which included high-precision electroweak measurements at LEP (CERN) and SLC (SLAC, Stanford) and the discovery of the top quark at the Tevatron (Fermilab), a clear view beyond the Standard Model (SM) continues to elude researchers. Even so, there are good reasons for optimism in the coming decade as CERN’s LEP2 electronpositron collider continues towards its highest achievable beam energy (around 100 GeV), and surprises may be just around the corner.
At the same time, physicists at Fermilab’s Tevatron collider, the scene of the CDF and D0 experiments, get ready to finalize their upgrades for Run 2. The increased collision energy (2 TeV) and the large data samples expected in the first two years of running (from integrated luminosities of 2 fb-1 or more: 20 times those gathered previously) may be the key to finding the Higgs and SUSY.
Just over the horizon the ATLAS and CMS experiments at CERN’s LHC collider await their turn to push back the high energy frontier. However the Higgs could be nearby. The most recent fit results of precise electroweak measurements, discussed by F Richard (LALOrsay), favour a relatively light Standard Model (SM) Higgs
(91 +71 41 GeV). The newest results on the SM Higgs search from LEP2, with electronpositron annihilation producing a Higgs with a Z particle (the former decaying into a beauty [“b”] quark and antiquark and the Z to any quark and antiquark being the primary search channel), give a Higgs mass greater than 95.5 GeV (95% c.l.). LEP’s ultimate sensitivity to the SM Higgs is expected to be between 105 and 110 GeV. The window for discovery remains open.
The current Tevatron Higgs searches, described by E Barberis (Berkeley), focus on Higgs production associated with W or Z bosons. The levels for these processes are still an order of magnitude away from the SM expectation. The Tevatron’s reach during Run 2 and beyond, presented by M Carena (Fermilab), has been studied extensively at the Run 2 SUSY/Higgs workshop.
For a Higgs mass less than about 130 GeV the Higgs decays predominantly into b-quarks. The key to the analysis is to have excellent b-tagging capabilities and good energy resolution for b-quark jets (to reconstruct a narrow Higgs mass peak over the quarkgluon jet background). For the SM Higgs, updated results show that, with an integrated luminosity of 2 fb-1 per experiment, the 95% exclusion limit can reach about 115 GeV. However, in 10 fb1 this sensitivity can reach about 185 GeV and, if the Higgs is in that mass range, a signal could be detected for a mass of less than 125 GeV or of 150 to 175 GeV (when the Higgs decay into W pairs dominates the b-quarkantiquark decay favoured at the lower Higgs masses).
An integrated luminosity of some 50 fb-1, although difficult to envisage, would be able to exclude a Higgs below 180 GeV. Beyond the SM, where new particles enter the game, the coupling strengths of the Higgs particle(s) can change and affect the outcome. In the popular Minimal Supersymmetric extension to the SM (MSSM), two Higgs doublets result in five physical states, the masses of which can be determined by two parameters. Radiative corrections owing to the sixth “top” quark and “stop” (supersymmetric top) can be large.
The model predicts the lightest SUSY Higgs at 130 GeV. However, the reaction rates at the Tevatron for the MSSM channels can, at best, be only slightly larger than the SM in some parts of the kinematically allowed region but mostly they are expected to be somewhat smaller. Still, with enough collisions, the lightest SUSY Higgs seems to be within the Tevatron’s reach in Run 2.
Limits from LEP on MSSM Higgs masses currently reach about 85 GeV and will increase. The Higgs searches at LEP and the
upcoming Tevatron run are definitely worth keeping a close track on. A surprise could be in store before the onset of the LHC, which has the goal of ultimately reaching 300 fb-1 in 10 or so years of running, at a collision energy of 14 TeV, and finding all there is to be found.
The prospects for Higgs and SUSY at the LHC were covered by K Jakobs (Mainz/ATLAS) and D Denegri (Saclay/CMS). Extensive detector simulations of all relevant Higgs channels have been performed to understand the various detector efficiencies and resolutions needed to optimize the physics yield. Excellent lepton and photon detection and b-quark tagging are paramount. With 30 fb-1 of integrated luminosity expected after three years of running, and by combining all signatures, both detectors show the capability of detecting the SM Higgs unequivocally up to a mass of 1 TeV.
The SM Higgs mass can be measured with a precision of 0.1% up to masses of some 400 GeV.
If the decays of MSSM Higgs bosons to SUSY particles are not allowed kinematically, then at the LHC the full parameter space can readily be covered. Open decay channels producing SUSY particles complicate the Higgs searches, but still a good fraction of the available parameter space can be probed.
If SUSY exists at the electroweak scale, the LHC will discover it easily. Gluinos and squarks (the supersymmetric partners of gluons and quarks), up to masses of about 2 TeV, will be copiously produced and their decays will give signatures that differ significantly from the SM. Sleptons (the leptons’ SUSY partners) can be detected directly up to about 400 GeV.
The status of the LEP SUSY searches was reviewed by M Schmitt (Harvard). A slew of searches for gauginos, squarks and sleptons have yielded limits on SUSY masses in the 80 to 95 GeV range. In addition to the MSSM model, other models have been addressed but no signals have been observed.
SUSY searches at the Tevatron, covered in part by D Stuart (Fermilab), are also a busy industry, and in many cases the kinematic region, excluded by the LEP experiments, has already been extended significantly: the sbottom (stop) mass limits have reached 148 (119) GeV and the gluino limits almost 270 GeV.
At DESY’s HERA electronproton collider, “looking for non-Standard Model effects is alive and well,” affirmed F Sciulli (Columbia), who showed that promising indications in the large-x (large mass in the electronproton collision) region persist in data from both the ZEUS and the H1 experiments.
Higgs and SUSY prospects at the proposed Next Linear Collider (NLC), Muon Collider and Very Large Hadron Collider were covered by D Burke (SLAC), J Lykken (Fermilab) and D Denisov (Fermilab) respectively.
H Baer (Florida State) discussed the interface between theorists and experimentalists in the context of simulations beyond SM physics, and G Kane (Michigan) predicted that, unless we are missing some basic ideas, in the next six years or so SUSY particles and the light SUSY Higgs will be discovered either at LEP or at Fermilab.
In the final presentation of the conference, J Bagger (Johns Hopkins) argued that perhaps not all sparticles may be light, given that several rare processes, such as lepton-flavour violation and proton decay, prefer unnaturally heavy scalar particles (in the 510 TeV range). Supernaturally superheavy supersymmetry provides a scenario in which the superparticles mass spectrum is the reverse of what is encountered with ordinary particles, but still has enough particles below the searchable 1 TeV scale.
As conference participants headed for other Florida attractions, the feeling was that the near future in particle physics was as bright as the sunshine.