This supercomputer-generated image of a galaxy suggests that general relativity might not be the only way to explain how gravity works. Theorists at Durham University in the UK simulated the universe using hydrodynamical simulations based on “f(R) gravity” – in which a scalar field enhances gravitational forces in low-density regions (such as the outer parts of a galaxy) but is screened by the so-called chameleon mechanism in high-density environments such as our solar system (see C Arnold et al. Nature Astronomy; arXiv:1907.02977).
The left-hand side of the image shows the scalar field of the theory: bright-yellow regions correspond to large scalar-field values, while dark-blue regions correspond to to a very small scalar fields, i.e. regions where screening is active and the theory behaves like general relativity. The right-hand side of the image shows the gas density with stars overplotted. The simulation, which was based on a total of 12 simulations for different model parameters and resolutions, and which required a total runtime of about 2.5 million core-hours, shows that spiral galaxies like our Milky Way could still form even with different laws of gravity.
“Our research definitely does not mean that general relativity is wrong, but it does show that it does not have to be the only way to explain gravity’s role in the evolution of the universe,” says lead author Christian Arnold of Durham University’s Institute for Computational Cosmology.
Following the discovery of gravitational waves by the LIGO and Virgo collaborations, there is great interest in observing other parts of the gravitational-wave spectrum and seeing what they can tell us about astrophysics, particle physics and cosmology. The European Space Agency (ESA) has approved the LISA space experiment that is designed to observe gravitational waves in a lower frequency band than LIGO and Virgo, while the KAGRA experiment in Japan, the INDIGO experiment in India and the proposed Einstein Telescope (ET) will reinforce LIGO and Virgo. However, there is a gap in observational capability in the intermediate-frequency band where there may be signals from the mergers of massive black holes weighing between 100 and 100,000 solar masses, and from a first-order phase transition or cosmic strings in the early universe.
This was the motivation for a workshop held at CERN on 22 and 23 July that brought experts from the cold-atom community together with particle physicists and representatives of the gravitational-wave community. Experiments using cold atoms as clocks and in interferometers offer interesting prospects for detecting some candidates for ultralight dark matter as well as gravitational waves in the mid-frequency gap. In particular, a possible space experiment called AEDGE could complement the observations by LIGO, Virgo, LISA and other approved experiments.
The workshop shared information about long-baseline terrestrial cold-atom experiments that are already funded and under construction, such as MAGIS in the US, MIGA in France and ZAIGA in China, as well as ideas for future terrestrial experiments such as MAGIA-advanced in Italy, AION in the UK and ELGAR in France. Delegates also heard about space – CACES (China) and CAL (NASA) – and sounding-rocket experiments – MAIUS (Germany) – using cold atoms in space and microgravity.
A suggestion for an atom interferometer using a pair of satellites is being put forward by the AEDGE team
ESA has recently issued a call for white papers for its Voyage 2050 long-term science programme, and a suggestion for an atom interferometer using a pair of satellites is being put forward by the AEDGE team (in parallel with a related suggestion called STE-QUEST) to build upon the experience with prior experiments. AEDGE was the focus of the CERN workshop, and would have unique capabilities to probe the assembly of the supermassive black holes known to power active galactic nuclei, physics beyond the Standard Model in the early universe and ultralight dark matter. AEDGE would be a uniquely interdisciplinary space mission, harnessing cold-atom technologies to address key issues in fundamental physics, astrophysics and cosmology.
The 10th Higgs Hunting workshop took place in Orsay and Paris from 29–31 July, attracting 110 physicists for lively discussions about recent results in the Higgs sector. The ATLAS and CMS collaborations presented Run 2 analyses with up to 140 fb–1 of data collected at a centre-of-mass energy of 13 TeV. The statistical uncertainty on some Higgs properties, such as the production cross-section, has now been reduced by a factor three compared to Run 1. This puts some Higgs studies on the verge of being dominated by systematic uncertainties. By the end of the LHC’s programme, measurements of the Higgs couplings to the photon, W, Z, gluon, tau lepton and top and bottom quarks are all expected to be dominated by theoretical rather than statistical or experimental uncertainties.
Several searches for additional Higgs bosons were presented. The general recipe here is to postulate a new field in addition to the Standard Model (SM) Higgs doublet, which in the minimal case yields a lone physical Higgs universally associated with the particle discovered at the LHC with a mass of 125 GeV in 2012. Adding a hypothetical additional Higgs doublet, however, as in the two Higgs doublet model, would yield five physical states: CP-even neutral Higgs bosons h and H, the CP-odd pseudoscalar A, and two charged Higgs bosons H±; the model would also bequeath three additional free parameters. Other models discussed at Higgs Hunting 2019 include the minimal and next-to-minimal supersymmetric SMs and extra Higgs states with doubly charged Higgs bosons. Anna Kaczmarska from ATLAS and Suzanne Gascon-Shotkin from CMS described direct searches for such additional Higgs bosons decaying to SM particles or Higgs bosons. Loan Truong from ATLAS and Yuri Gershtein from CMS described studies of rare – and potentially beyond-SM – decays of the 125 GeV Higgs boson. No significant excesses were reported, but hope remains for Run 3, which will begin in 2021.
Nobel laureate Gerard ’t Hooft gave a historical talk on the role of the Higgs in the renormalisation of electroweak theory, recalling the debt his Utrecht group, where the work was done almost 50 years ago, owed to pioneers like Faddeev and Popov. Seven years after the particle’s discovery, we now know it to be spin-0 with mainly CP-even interactions with bosons, remarked Fabio Cerutti of Berkeley in the experimental summary. With precision on the Higgs mass now better than two parts per mille, all of the SM’s free parameters are known with high precision, he continued, and all but three of them are linked to Higgs-boson interactions.
Give me six hours to chop down a tree and I will spend the first four sharpening the axe.
Abraham Lincoln
Hunting season may now be over, Cerutti concluded, but the time to study Higgs anatomy and exploit the 95% of LHC data still to come is close at hand. Giulia Zanderighi’s theory summary had a similar message: Higgs studies are still in their infancy and the discovery of what seems to be a very SM-like Higgs at 125 GeV allows us to explore a new sector with a broad experimental programme that will extend over decades. She concluded with a quote from Abraham Lincoln: “Give me six hours to chop down a tree and I will spend the first four sharpening the axe.”
The next Higgs Hunting workshop will be held in Orsay and/or Paris from 7–9 September 2020.
The Institute of Physics of Jagiellonian University, Forschungszentrum Jülich, INFN-LNF Frascati and Institute of Nuclear Physics PAS Cracow are organizing a biennial workshop to establish closer contacts between experimentalists and theorists involved in the studies of meson production, properties and interaction. The workshop will cover lectures on both experimental and theoretical aspects, in particular the presentation of new results.
The main topics of the workshop are:
hadronic and electromagnetic meson production,
meson interaction with mesons, baryons, ground state nuclei as well as
hot and dense nuclear matter,
structure of hadrons,
precision measurements as tests of fundamental symmetries,
exotic systems in QCD,
novel approaches in theory and experiment.
The intention is to provide an overview of the present status in these fields, as well as of new developments, and a preview of the forthcoming investigations. This workshop – the sixteenth of the series – will maintain the tradition of the workshops organized since 1991 at Cracow.
This conference will bring together experts in cosmology, particle physics, and fundamental theory to address how and when the universe thermalizes following inflation, and the associated particle physics and dark matter phenomenology. Important topics that will be covered include hidden sector model building in the LHC era, thermalization of the universe following inflation, possibilities of post-inflation cosmic history prior to nucleosynthesis, and associated experimental signatures. The conference aims to attract researchers in different areas to develop new directions in model building and establish new experimental paths for probing early universe cosmology and dark matter phenomenology.
The workshop brings together the nuclear and particle physics communities to discuss the latest developments (experimental and theoretical) in exotic and conventional hadron spectroscopy. It is the third in a series of workshops on this topic, following previous events at the University of Edinburgh. The spectrum of hadrons has become increasingly rich in recent years, owing to ongoing experimental discoveries at both particle and nuclear physics experiments. The properties of known states have been measured with improved precision, and many new states (both conventional and exotic) have been discovered. Often similar states and search methods are discussed separately in the particle and nuclear physics communities, with little interaction between them. This workshop facilitates communication between these communities, with the ultimate aim of improving our understanding of the spectrum of hadrons. A particular focus is new analysis methods and search strategies for current and future experimental facilities (CERN, JLab, Mainz, BESIII, BelleII, PANDA).
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