All about a Higgs boson and more at EPS-HEP 2013.
When the Swedish warship Vasa capsized in Stockholm harbour on her maiden voyage in 1628, many hearts must have also sunk metaphorically, as they did at CERN in September 2008 when the LHC’s start-up came to an abrupt end. Now, the raised and preserved Vasa is the pride of Stockholm and the LHC – following a successful restart in 2009 – is leading research in particle physics at the high-energy frontier. This year the two icons crossed paths when the International Europhysics Conference on High-Energy Physics, EPS-HEP 2013, took place in Stockholm on 18–24 July, hosted by the KTH (Royal Institute of Technology) and Stockholm University. Latest results from the LHC experiments featured in many of the parallel, plenary and poster sessions – and the 750 or so participants had the opportunity to see the Vasa for themselves at the conference dinner. There was, of course, much more and this report can only touch on some of the highlights.
Coming a year after the first announcement of the discovery of a “Higgs-like” boson on 4 July 2012, the conference was the perfect occasion for a birthday celebration for the new particle. Not only has its identity been more firmly established in the intervening time – it almost certainly is a Higgs boson – but many of its attributes have been measured by the ATLAS and CMS experiments at the LHC, as well as by the CDF and DØ collaborations using data collected at Fermilab’s Tevatron. At 125.5 GeV/c2, its mass is known to within 0.5% precision – better than for any quark – and several tests by ATLAS and CMS show that its spin-parity, JP, is compatible with the 0+ expected for a Standard Model Higgs boson. These results exclude other models to greater than 95% confidence level (CL), while a new result from DØ rejects a graviton-like 2+ at >99.2% CL.
The mass of the top quark is in fact so large – 173 GeV/c2 – that it decays before forming particles
The new boson’s couplings provide a crucial test of whether it is the particle responsible for electroweak-symmetry breaking in the Standard Model. A useful parameterization for this test is the ratio of the observed signal strength to the Standard Model prediction, μ = (σ × BR)/(σ × BR)SM, where σ is the cross-section and BR the branching fraction. The results for the five major decay channels measured so far (γγ, WW*, ZZ*, bb and ττ) are consistent with the expectations for a Standard Model Higgs boson, i.e. μ = 1, to 15% accuracy. Although it is too light to decay to the heaviest quark – top, t – and its antiquark, the new boson can in principle be produced together with a tt pair, so yielding a sixth coupling. While this is a challenging channel, new results from CMS and ATLAS are starting to approach the level of sensitivity for the Standard Model Higgs boson, which bodes well for its future use.
The mass of the top quark is in fact so large – 173 GeV/c2 – that it decays before forming particles, making it possible to study the “bare” quark. At the conference, the CMS collaboration announced the first observation, at 6.0σ, of the associated production of a top quark and a W boson, in line with the Standard Model’s prediction. Both ATLAS and CMS had previously found evidence for this process but not to this significance. The DØ collaboration presented latest results on the lepton-based forward–backward lepton asymmetry in tt- production, which had previously indicated some deviation from theory. The new measurement, based on the full data set of 9.7 fb–1 of proton–antiproton collisions at the Tevatron, gives an asymmetry of (4.7±2.3 stat.+1.1–1.4 syst.)%, which is consistent with predictions from the Standard Model to next-to-leading order.
The study of B hadrons, which contain the next heaviest quark, b, is one of the aspects of flavour physics that could yield hints of new physics. One of the highlights of the conference was the announcement of the observation of the rare decay mode B0s → μμ by both the LHCb and CMS collaborations, at 4.0 and 4.3σ, respectively. While there had been hopes that this decay channel might open a window on new physics, the long-awaited results align with the predictions of the Standard Model. The BaBar and Belle collaborations also reported on their precise measurements of the decay B → D(*)τντ at SLAC and KEK, respectively, which together disagree with the Standard Model at the 4.3σ level. The results rule out one model that adds a second Higgs doublet to the Standard Model (2HDM type II) but are consistent with a different variant, 2HDM type III – a reminder that the highest energies are not the only place where new physics could emerge.
Precise measurements require precise predictions for comparison and here theoretical physics has seen a revolution in calculating next-to-leading order (NLO) effects, involving a single loop in the related Feynman diagrams. Rapid progress during the past few years has meant that the experimentalists’ wish-list for QCD calculations at NLO relevant to the LHC is now fulfilled, including such high-multiplicity final states as W + 4 jets and even W + 5 jets. Techniques for calculating loops automatically should in future provide a “do-it-yourself” approach for experimentalists. The new frontier for the theorists, meanwhile, is at next-to-NLO (NNLO), where some measurements – such as pp → tt – are already at an accuracy of a few per cent and some processes – such as pp → γγ – could have large corrections, up to 40–50%. So a new wish-list is forming, which will keep theorists busy while the automatic code takes over at NLO.
With a measurement of the mass for the Higgs boson, small corrections to the theoretical predictions for many measurable quantities – such as the ratio between the masses of the W and the top quark – can now be calculated more precisely. The goal is to see if the Standard Model gives a consistent and coherent picture when everything is put together. The GFitter collaboration of theorists and experimentalists presented its latest global Standard Model fit to electroweak measurements, which includes the legacy both from the experiments at CERN’s Large Electron–Positron Collider and from the SLAC Large Detector, together with the most recent theoretical calculations. The results for 21 parameters show little tension between experiment and the Standard Model, with no discrepancy exceeding 2.5σ, the largest being in the forward–backward asymmetry for bottom quarks.
There is more to research at the LHC than the deep and persistent probing of the Standard Model. The ALICE, LHCb, CMS and ATLAS collaborations presented new results from high-energy lead–lead and proton–lead collisions at the LHC. The most intriguing results come from the analysis of proton–lead collisions and reveal features that previously were seen only in lead–lead collisions, where the hot dense matter that was created appears to behave like a perfect liquid. The new results could indicate that similar effects occur in proton–lead collisions, even though far fewer protons and neutrons are involved. Other results from ALICE included the observation of higher yields of J/ψ particles in heavy-ion collisions at the LHC than at Brookhaven’s Relativistic Heavy-Ion Collider, although the densities are much higher at the LHC. The measurements in proton–lead collisions should cast light on this finding by allowing initial-state effects to be disentangled from those for cold nuclear matter.
Supersymmetry and dark matter
The energy frontier of the LHC has long promised the prospect of physics beyond the Standard Model, in particular through evidence for a new symmetry – supersymmetry. The ATLAS and CMS collaborations presented their extensive searches for supersymmetric particles in which they have explored a vast range of masses and other parameters but found nothing. However, assumptions involved in the work so far mean that there are regions of parameter space that remain unexplored. So while supersymmetry may be “under siege”, its survival is certainly still possible. At the same time, creative searches for evidence of extra dimensions and many kinds of “exotics” – such as excited quarks and leptons – have likewise produced no signs of anything new.
However, evidence that there must almost certainly be some kind of new particle comes from the existence of dark, non-hadronic matter in the universe. Latest results from the Planck mission show that this should make up some 26.8% of the universe – about 4% more than previously thought. This drives the search for weakly interacting particles (WIMPs) that could constitute dark matter, which is becoming a worldwide effort. Indeed, although the Higgs boson may have been top of the bill for hadron-collider physics, more generally, the number of papers with dark matter in the title is growing faster than those on the Higgs boson.
While experiments at the LHC look for the production of new kinds of particles with the correct properties to make dark matter, “direct” searches seek evidence of interactions of dark-matter particles in the local galaxy as they pass through highly sensitive detectors on Earth. Such experiments are showing an impressive evolution with time, increasing in sensitivity by about a factor of 10 every two years and now reaching cross-sections down to 10–8 pb. Among the many results presented, an analysis of 140.2-kg days of data in the silicon detectors of the CDMS II experiment revealed three WIMP-candidate events with an expected background of 0.7. A likelihood analysis gives a 0.19% probability for the known background-only hypothesis.
Neutrinos are the one type of known particle that provide a view outside the Standard Mode
“Indirect” searches, by contrast, involve in particular the search from signals from dark-matter annihilation in the cosmos. In 2012, an analysis of publically available data from 43 months of the Fermi Large Area Telescope (LAT) indicated a puzzling signal at 130 GeV, with the interesting possibility that these γ rays could originate from the annihilation of dark-matter particles. A new analysis by the Fermi LAT team of four years’ worth of data gives preliminary indications of an effect with a local significance of 3.35σ but the global significance is less than 2σ. The HESS II experiment is currently accumulating data and might soon be able to cross-check these results.
With their small but nonzero mass and consequent oscillations from one flavour to another, neutrinos are the one type of known particle that provide a view outside the Standard Model. At the conference, the T2K collaboration announced the first definitive observation at 7.5σ of the transition νμ → νe in the high-energy νμ beam that travels 295 km from the Japan Proton Accelerator Complex to the Super-Kamiokande detector. Meanwhile, the Double CHOOZ experiment, which studies νe produced in a nuclear reactor, has refined its measurement of θ13, one of the parameters characterizing neutrino oscillations, by using two independent methods that allow much better control of the backgrounds. The GERDA collaboration uses yet another means to investigate if neutrinos are their own antiparticles, by searching for the neutrinoless double-beta decay of the isotope 76Ge in a detector in the INFN Gran Sasso National Laboratory. The experiment has completed its first phase and finds no sign of this process but now provides the world’s best lower limit for the half-life at 2.1 × 1023 years.
On the other side of the world, deep in the ice beneath the South Pole, the IceCube collaboration has recently observed oscillations of neutrinos produced in the atmosphere. More exciting, arguably, is the detection of 28 extremely energetic neutrinos – including two with energies above 1 PeV – but the evidence is not yet sufficient to claim observation of neutrinos of extraterrestrial origin.
Towards the future
In addition to the sessions on the latest results, others looked to the continuing health of the field with presentations of studies on novel ideas for future particle accelerators and detection techniques. These topics also featured in the special session for the European Committee for Future Accelerators, which looked at future developments in the context of the update of the European Strategy for Particle Physics. A range of experiments at particle accelerators currently takes place on two frontiers – high energy and high intensity. Progress in probing physics that lies at the limit of these experiments will come both from upgrades of existing machines and at future facilities. These will rely on new ideas being investigated in current accelerator R&D and will also require novel particle detectors that can exploit the higher energies and intensities.
For example, two proposals for new neutrino facilities would allow deeper studies of neutrinos – including the possibility of CP violation, which could cast light on the dominance of matter over antimatter in the universe. The Long-Baseline Neutrino Experiment (LBNE) would create a beam of high-energy νμ at Fermilab and detect the appearance of νe with a massive detector that is located 1300 km away at the Sanford Underground Research Facility. A test set-up, LBNE10, has received funding approval. A complementary approach providing low-energy neutrinos is proposed for the European Spallation Source, which is currently under construction in Lund. This will be a powerful source of neutrons that could also be used to generate the world’s most intense neutrino beam.
The LHC was first discussed in the 1980s, more than 25 years before the machine produced its first collisions. Looking to the long-term future, other accelerators are now on the drawing board. One possible option is the International Linear Collider, currently being evaluated for construction in Japan. Another option is to create a large circular electron–positron collider, 80–100 km in circumference, to produce Higgs bosons for precision studies.
The main physics highlights of the conference were reflected in the 2013 EPS-HEP prizes, awarded in the traditional manner at the start of the plenary sessions. The EPS-HEP prize honoured both ATLAS and CMS – for the discovery of the new boson – and three of their pioneering leaders (Michel Della Negra, Peter Jenni and Tejinder Virdee). François Englert and Peter Higgs were there to present this major prize and took part later in a press conference together with the prize winners. Following the ceremony, Higgs gave a talk, “Ancestry of a New Boson”, in which he recounted what led to his paper of 1963 and also cast light on why his name became attached to the now-famous particle. Other prizes acknowledged the measurement of the all-flavour neutrino flux from the Sun, as well as the observation of the rare decay B0s → μμ, work in 4D field theories and outstanding contributions to outreach. In a later session, a prize sponsored by Elsevier was awarded for the best four posters out of the 130 that were presented by young researchers in the dedicated poster sessions.
To close the conference, Nobel Laureate Gerard ‘t Hooft presented his outlook for the field. This followed the conference summary by Sergio Bertolucci, CERN’s director for research and computing, in which he also thanked the organizers for the “beautiful venue, the fantastic weather and the perfect organization” and acknowledged the excellent presentations from the younger members of the community. The baton now passes to the organizing committees of the next EPS-HEP conference, which will take place in Vienna on 22–29 July 2015.
• This article has touched on only some of the physics highlights of the conference. For all of the talks, see http://eps-hep2013.eu/.