Earlier press-briefs

Thursday, July 18, 8:30

Today kicks off the first sessions of this year's European Physical Society Conference on High Energy Physics. More than 700 physicists from all over the world are gathered in Stockholm, Sweden to hear the latest experimental results in high-energy physics.

Highlights will include more details about the enigma of dark matter, the mysterious substance making up about a quarter of the universe. We will also learn more about the precision studies done with the Higgs boson, whose discovery was announced a year ago this month. The particulars of these latest results will help build a better and clearer picture of what our universe is and how it operates.

The detailed agenda is located here: http://indico.cern.ch/conferenceTimeTable.py?confId=218030&ttLyt=room#20130718.detailed

Thursday, July 18, 18:00

The first day of the EPS HEP 2013 conference presented us with more precise measurements of several particles and particle decays - including the Higgs, W and Z bosons, as well as many others - from the Large Hadron Collider (LHC) and the Tevatron. With a year of studies behind us, the Higgs boson results indicate that though the particle behaves, at first sight, much like what is predicted by the Standard Model, the possibility of exploring new physics is still an exciting quest.

One such quest involves gravity, a force not explained by the Standard Model. Physicists in ATLAS and CMS are testing explanations as to why gravity, one of nature’s fundamental forces, is so weak. Scientists are looking for evidence of two possible causes- either gravity is propagated elsewhere and we feel just a whisper of its true strength or we need to push it to become stronger using the immense energy of the LHC. New results from ATLAS and CMS were also presented on searches for new particles that decay to top quarks - the heaviest elementary particles discovered so far.

Turning to the sky with astrophysics, the IceCube collaboration reported detecting 28 extremely energetic neutrinos. They first reported two events earlier this year, each one measuring at an energy above 1 Petaelectronvolt, much higher than what was reached with the LHC.

The Fermi Large Area Telescope collaboration reported on a puzzling gamma ray signal, first reported by Christoph Weniger and collaborators using publicly released LAT data, with an energy of 130 GeV. One interesting possibility is that these gamma rays could be generated when dark matter particles annihilate but another cause could be an instrumental effect. Interpretation is difficult though, so more analysis is necessary. The HESS 2 experiment is currently accumulating data and will soon be able to cross-check these results.

Friday, July 19, 8:30

A series of interesting results are expected today. T2K, a neutrino experiment based in Japan, will present new neutrino oscillation measurements. Neutrinos are intriguing particles coming in three different species. They can morph from one type into another, a phenomenon the T2K experiment studies using neutrinos from a particle accelerator at the J-PARC laboratory in Tokai Japan. Two detectors, placed 280 metres and 295 kilometres away from the accelerator, track the changes.

The France-based neutrino experiment, Double Chooz, will present results regarding the measurement of the θ13 angle, one parameter characterizing neutrino oscillations. The collaboration used two different independent techniques on two separate neutrino samples, which were then combined to achieve a higher precision measurement.

Three LHC experiments, LHCb, CMS and ATLAS, as well as the D0 collaboration from the Tevatron, will present their latest results on a rare B mesons decay. The Standard Model predicts Bs mesons can decay into pairs of muons but only about three times in a billion decays. Several other important new studies of B meson decays will also be presented. Particles not predicted by the Standard Model would affect the probability of these decays, making it a very sensitive place to look for new physics.

Friday, July 19, 18:30

The latest studies on the Higgs boson properties, covering the production modes, decay channels, couplings, spin and parity, were presented by the CMS and ATLAS collaborations. All properties are consistent with the Higgs boson as predicted by the Standard Model.

However, the possibility that there is more to this particle than what is predicted by the Standard Model still exists. Standard Model still exists. Theorists and experimentalists are examining alternative ways to search for new physics through exotic decay patterns to exploit the full potential of the LHC data. For example, they are looking for hints of supersymmetry in physics decays with little missing energy or long-lived particles.

The CMS and LHCb collaborations presented strong evidence for the decay of Bs particles (made of a b and s quark) into pairs of muons, with significances of 4.3 sigma and 4.0 sigma, respectively. For every billion Bs particles produced, the Standard Model predicts that around three will decay into muon pairs. Both collaborations precisely measured these decay rates to nearly the predicted values. Any deviations from the Standard Model would have indicated the presence of new physics, but the Standard Model has proven resilient once again.

The BaBar and Belle experiments both reported on very precise measurements of B meson decays. B mesons are composite particles made of a heavy b and light u or d quarks. The measurements were first reported a year ago, but continue to puzzle physicists since they disagree with the Standard Modell. The BaBar measurement is strong enough to completely rule out one version of a popular supersymmetric Higgs model known as the Two Higgs Doublet model (Type-II 2HDM). Ruling out different theoretical models guides theorists in establishing which one of the many hypothesized models may be correct.

In neutrino physics, the T2K collaboration announced a definitive observation of a muon neutrino changing to an electron neutrino. Coming in three different flavours, neutrinos can morph from one type into another. This is the first time scientists have seen this particular change so clearly.

Double CHOOZ presented a refined measurement of θ13, one parameter characterizing neutrino oscillations, using two independent methods. These separate analyses allow the scientists much better control of the background events.

On the astrophysics front, out of Tibet, the ARGO-YBJ presented remarkable results with the long-term monitoring of the active galactic nucleus MRK421 that showed a dramatic correlation of the steady and flaring emissions at TeV energies. Satellites observed the same emissions at a much lower energy (tens of keV). There is also a puzzling, unexpected anisotropy (directionally dependent property) in the cosmic-ray flux in the TeV energy range, which confirms and strengthens the previous observation made by the MILAGRO experiment.

Saturday, July 20, 8:30

The Italy-based GERDA collaboration will present the results of a test on the Standard Model using neutrinos. They want to test if neutrinos are their own antiparticle.

The LHCb collaboration will present its recent measurement of b baryons lifetime. A b baryon is a composite particle made of two light quarks (u or d) and one heavy b quark. Like B mesons, particles made of one b quark and a light quark, b baryons are unstable and decay in about a picosecond. Theorists predict that both types of particles should have similar lifetimes but several previous experiments reported systematically shorter lifetimes for b baryons. Earlier this year, both ATLAS and CMS presented values in line with the B meson lifetime measurement. With this latest precise result from the LHCb experiment, there is now enough evidence to close the case on this two-decades-old discrepancy.

The GFitter group, a collaboration of theorists and experimentalists, will present a simultaneous comparison of the latest measurements from all of the experiments to the Standard Model using the most recent theoretical calculations. With the Higgs boson mass known for the first time, small corrections to the theoretical predictions for many measurable quantities, such as the ratio between the mass of W boson and top quark, can now be calculated precisely. The goal is to see if the Standard Model gives a consistent and coherent picture when everything we know is put together. Any tension could be a sign for new physics.

And finally, theorists are looking at the very start of the universe. When nuclear matter is heated up to thousands of billions of degrees in physics experiments, it 'melts' into a fluid of elementary constituents that interact strongly with each other. By combining analytical calculations, computer simulations and insight from techniques derived from String Theory, scientists are studying how this fluid affects the motion of a particle traveling through it. Progress in this research field may shed new light on a very hot topic: the conditions of our universe a few millionths of a second after the Big Bang.

Saturday, July 20, 18:30

Among the top physics results presented by the CMS and ATLAS collaborations, CMS announced the first observation (at more than 5 sigma) of a rare process: the associated production of a single top quark and a W boson. Both ATLAS and CMS had previously seen evidence for this process but not to this significance. The observation confirms the Standard Model prediction, lending even more credence to the resiliency of the theory.

The Tevatron experiment D0 updated today their results on forward-backward lepton asymmetry in top quark pair production. Previous Tevatron results of lepton and reconstructed top quark asymmetries, from both CDF and D0, indicated that top quarks are produced in one direction more often than the other in comparison with theoretical predictions. New and improved measurements of the lepton asymmetry with the full Tevatron data set presented by D0 are now consistent with the theory within error margins. CDF and DZero experiments continue studies of the top quark production asymmetry.

The ATLAS and CMS collaborations both presented extensive, creative and comprehensive searches for dark matter, gravitons and supersymmetric particles. Nothing was found that would indicate the presence of new particles predicted by various models going beyond the Standard Model. In particular, both experiments opened a new era in the search for dark matter with new results reaching into regions never before explored.

The LHCb collaboration presented intriguing new results on a channel of B meson decays that has been of intense interest to theoreticians due to its sensitivity to contributions from non-Standard Model particles. Previous results show tension with the Standard Model prediction, and the new results appear to increase the significance of the discrepancy, although not yet to the level of being definitely established.

The ALICE, LHCb, CMS and ATLAS collaborations presented new results from the high-energy lead-lead and proton-lead collisions at the LHC. Individual protons in one beam collided with lead nuclei (each containing a total of 208 protons and neutrons) in the other beam. 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 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.

On neutrino physics, the GERDA collaboration showed its first results after about a year of taking data. They are trying to determine if neutrinos are their own antiparticles. Theoretically, the signal would come from one quark in a nucleus emitting a neutrino and an electron. The neutrino would be reabsorbed by another quark and a second electron emitted at the same energy. Researchers found a handful of events containing two electrons but nothing significantly in excess of the expected background predicted by the Standard Model.

Two proposals for new neutrino facilities could provide the means to help explain the dominance of matter over antimatter in the universe. LBNE, the Long-Baseline Neutrino Experiment, would create a beam of high-energy muon neutrinos at Fermilab in the US and detect the appearance of electron neutrinos with a massive detector 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 (ESS), currently under construction in Lund, Sweden. This will be a powerful source of neutrons, but it 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 collided protons. Looking to the long-term future of physics, other accelerators are now on the drafting table. One possible option is the International Linear Collider, currently being evaluated for construction in Japan, which would help expand the search for physics beyond the Standard Model. Another option is to create a larger circular collider at 80-100 km in circumference to collide electrons and positrons to produce Higgs bosons for precision studies. The tunnel built to house this machine could host a hadron collider capable of energies higher than the LHC after the precision studies are completed. Whatever machine comes next, it's clear physicists are expanding the limits of technology as they design new machines to dive further into the depths of our universe.

Monday Juy 22, 8:30

The morning begins with the award of the 2013 EPS High Energy and Particle Physics Prize to the ATLAS and CMS collaborations "for the discovery of a Higgs boson, as predicted by the Brout-Englert-Higgs mechanism" as well as to three of the pioneers of these experiments.The prize ceremony continues with the award of the Giuseppe and Vanna Cocconi Prize, the Gribov Medal, the Young Experimental Physicist Prize, and the Outreach Prize. Following the ceremony, Peter Higgs will talk on "The ancestry of a new boson".

Presentations on Higgs bosons in the Standard Model and beyond, and on the properties of the new boson discovered at CERN last July, will continue the theme of the EPS-HEPP prize.

The Standard Model will be further examined in overviews of electroweak studies and top physics. Both the electroweak interaction and top physics are well-established, but with the mass of a Higgs boson now known, precision measurements are allowing much more precise comparisons between theory and experiment.
String theory takes the description of particles and forces to a more fundamental level. Encompassing Standard Model physics, it could also account for things the model does not cover, such as gravity, and the possible unification of all known forces in one theory.

Finally, the latest result from the experiments with heavy ions will be presented. High energy nuclear collisions are used to address the properties of strongly interacting matter in extreme conditions of density and temperature, analogous to the primordial soup from which the universe evolved.

Tuesday Juy 23, 8:30

Tuesday's sessions begin with a focus on high-energy particle physics in the wider universe and its relationship with cosmology. While particle physics focuses mainly on the ingredients of the universe - the fundamental particles and forces - cosmology deals principally with its evolution.

One question that links both fields is the mystery of dark matter, invisible through any radiaition, which cosmological studies show should make up some 23% of the universe. Particle physics experiments are searching for possible particles that could form this dark matter. "Direct" searches look for dark matter particles in the local galaxy as they pass through highly sensitive detectors. Experiments at accelerators such as the LHC look for the production of new kinds of particles with the correct properties to make dark matter.

Cosmic rays are high-energy charged particles that are known to arrive from outer space but their precise sources remain a mystery. Specialised particle detectors on Earth and in space are continuing the century-long quest to understand these particles and their origins. High energy cosmic neutrinos could hold the answer.

These neutral particles with very small masses are interesting in their own right and may hold the key to beyond the standard model physics. They are currently studied mainly at sources on Earth, which vary from high energy particle accelerators, to nuclear reactors and radiaoctive materials emitting neutrinos.

A whole range of experiments at particle-physics accelerators currently take place at two frontiers - high energy and high intensity. Progress in probing physics that lies at the limit of current experiments will come from upgrades of exisiting machines such as the LHC and at future facilities. These will rely on new ideas being investigated in current accelerator R&D. They also require novel particle detectors that can exploit the higher energies and intensities.


Latest update: 23 July, 2013 by Sten Hellman