Archivi tag: elementary particles

Tests of Fundamental Symmetries

“Tests of Fundamental Symmetries” is a one-day workshop organized jointly by the APS Topical Group on Precision Measurements and Fundamental Constants and the Few-body Topical Group. It will take place in Baltimore, MD on April 10, 2015–the day before the APS April meeting (APR15) begins. It will be held at the Baltimore Convention Center, the site of APR15. The goal of the workshop is to survey forefront efforts in searches for time-reversal and parity violation.  Continua a leggere Tests of Fundamental Symmetries

Top at Twenty

The “Top at Twenty” workshop is dedicated to the celebration of 20 years since the top quark discovery at Fermilab in 1995. Speakers from all experiments capable of studying top quark, ATLAS, CDF, CMS and DZero, will present the most recent results of the top quark studies based on Run II of the Tevatron and Run I of the LHC. Continua a leggere Top at Twenty

Iberian Cosmology Meeting 2015

This series of meetings is aimed at encouraging the interaction and collaboration between researchers working in Cosmology and related areas (Gravitation, Particle Physics and Astronomy) in Portugal and Spain.  Continua a leggere Iberian Cosmology Meeting 2015


In almost a century of progress, particle Dark Matter (DM) remains the single, most powerful physics concept able to explain otherwise anomalous observations on all cosmological scales, from motions of stars in the solar neighbourhood to its imprint on the horizon of the observable Universe.
Continua a leggere DARK MALT

Hunting for dark matter with HADES

Nonostante il 96% dell’Universo sia costituito principalmente da materia scura ed energia scura, non siamo ancora in grado di capire qual è la loro origine e natura. Alcuni astrofisici che ricercano le particelle come potenziali candidati della materia scura hanno escluso dalla lista il cosiddetto “fotone scuro” o “bosone U” grazie ad una serie di esperimenti condotti da HADES del Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in collaborazione con altri 17 istituti europei. Questi risultati negativi potrebbero portare a cambiamenti radicali nel modello standard della fisica delle particelle.

Continua a leggere Hunting for dark matter with HADES

Max Tegmark ‘welcomes’ the multiverse idea

What is the Universe made of? The ancient Greeks conceived of the “atom”, the indivisible unit of matter. Today’s physicists talk of smaller particles, quarks and electrons, neutrinos, Higgs Bosons and photons. Understand them, and the forces that hold everything together, and we may finally get a handle on what makes it all tick. The trouble is, the more we drill down into the subatomic world, the more complexity we find. The bedrock of reality seems as elusive as ever. Perhaps, says a leading theoretical physicist Max Tegmark, we do not live in a world of particles and forces at all, but of pure mathematics.

The Telegraph: It’s goodbye to the universe – hello to the multiverse

Neutrons method to test dark energy theories by sensitive measurements of gravity at small scales

Schema dello spettrometro di risonanza per lo studio degli effetti della gravità su scale molto piccole. Credit: TU
Non sempre occorre un acceleratore di particelle per fare esperimenti di fisica fondamentale. I primi risultati di un esperimento a bassa energia sulla gravità newtoniana, spinto fino ad un limite più piccolo di cinque ordini di grandezza, stringono il cerchio sulle proprietà potenziali che forze e particelle potrebbero assumere al di là di questo limite di sensibilità pari ad almeno qualche centinaia di migliaia di volte. Infatti, un nuovo metodo sviluppato da alcuni fisici della Vienna University of Technlogy, denominato spettroscopia di risonanza gravitazionale, si è rivelato così sensibile che ora potrà essere applicato per studiare le due componenti più enigmatiche dell’Universo, la materia scura e l’energia scura.

All the particles we know to exist make up only about five per cent of the mass and energy of the Universe. The rest, dark matter and dark Energy, remains mysterious. A European collaboration led by researchers from the Vienna University of Technology has now carried out extremely sensitive measurements of gravitational effects at very small distances at the Institut Laue-Langevin (ILL) in Grenoble. These experiments provide limits for possible new particles or fundamental forces, which are a hundred thousand times more restrictive than previous estimations.

Undiscovered Particles?
Dark matter is invisible, but it acts on matter by its gravitational pull, influencing the rotation of galaxies. Dark energy, on the other hand, is responsible for the accelerated expansion of the Universe. It can be described by introducing a new physical quantity, Albert Einstein’s cosmological constant. Alternatively, so-called quintessence theories have been put forward: “Perhaps empty space is not completely empty after all, but permeated by an unknown field, similar to the Higgs-field”, says Professor Hartmut Abele (TU Vienna), director of the Atominstitut and group leader of the research group. These theories are named after Aristotle’s “quintessence”, a hypothetical fifth element, in addition to the four classical elements of ancient Greek philosophy. 

If new kinds of particles or additional forces of nature exist, it should be possible to observe them here on Earth.

Tobias Jenke and Hartmut Abele from the Vienna University of Technology developed an extremely sensitive instrument, which they used together with their colleagues to study gravitational forces. Neutrons are perfectly suited for this kind of research. They do not carry electric charge and they are hardly polarizable. They are only influenced by gravity, and possibly by additional, yet unknown forces. 

Forces at Small Distances
The technique they developed takes very slow neutrons from the strongest continuous ultracold neutron source in the world, at the ILL in Grenoble and funnels them between two parallel plates. According to quantum theory, the neutrons can only occupy discrete quantum states with energies which depend on the force that gravity exerts on the particle. By mechanically oscillating the two plates, the quantum state of the neutron can be switched. That way, the difference between the energy levels can be measured. This work is an important step towards modelling gravitational interactions at very short distances. The ultracold neutrons produced at ILL together with the measurement devices from Vienna are the best tool in the world for studying the predicted tiny deviations from pure Newtonian gravity”, says Peter Geltenbort (ILL Grenoble)Different parameters determine the level of precision required to find such tiny deviations, for instance the coupling strength between hypothetical new fields and the matter we know. Certain parameter ranges for the coupling strength of quintessence particles or forces have already been excluded following other high-precision measurements. But all previous experiments still left a large parameter space in which new physical non-Newtonian phenomena could be hidden.

A Hundred Thousand Times Better than Other Methods
The new neutron method can test theories in this parameter range: “We have not yet detected any deviations from the well-established Newtonian law of gravity”, says Hartmut Abele. “Therefore, we can exclude a broad range of parameters”. The measurements determine a new limit for the coupling strength, which is lower than the limits established by other methods by a factor of a hundred thousand. Even if the existence of certain hypothetical quintessence particles is disproved by these measurements, the search will continue as it is possible that new physics can still be found below this improved level of accuracy. Therefore, Gravity Resonance Spectroscopy will need to be improved further, and increasing the accuracy by another few orders of magnitude seems feasible to the Abele’s team.

However, if even that does not yield any evidence of deviations from known forces, Albert Einstein would win yet another victory: his cosmological constant would then appear more and more plausible.

TU: Searching for Dark Energy with Neutrons
arXiv: Gravity Resonance Spectroscopy Constrains Dark Energy and Dark Matter Scenarios

The potential of string theory as an elegant unified description of physics

Nell’Agosto del 1984 due fisici arrivarono ad elaborare una formula che aprì una nuova finestra verso la comprensione della teoria delle stringhe. Lo scorso mese di Dicembre, Michael Green dell’Università di Cambridge e John Schwarz del California Institute of Technology sono stati insigniti del Fundamental Physics Prize 2014, uno dei premi della serie “Breakthrough Prizes” che riguarda le scienze fisiche e biologiche. La citazione del premio, che ammonta a 3 milioni di dollari, dice “per aver introdotto nuove prospettive sulla gravità quantistica e l’unificazione delle forze“.

Green and Schwarz are known for their pioneering work in string theory, postulated as a way of explaining the fundamental constituents of the Universe as tiny vibrating strings. Different types of elementary particles arise in this theory as different vibrational harmonics (or ‘notes’). The scope of string theory has broadened over the past few years and is currently being applied to a far wider field than that for which it was first devised, which has taken those who research into it in unexpected directions. Although the term ‘string theory’ was not coined till 1971, it had its genesis in a paper by the Italian physicist Gabriele Veneziano in 1968, published when Green was a research student in Cambridge. Green was rapidly impressed by its potential and began working seriously on it in the early 1970s. As he explains in the accompanying film, he stuck with string theory during a period when it was overshadowed by other developments in elementary particle physics. As a result of a chance meeting at the CERN accelerator laboratory in Switzerland in the summer of 1979, Green (then a researcher at Queen Mary, London) began to work on string theory with Schwarz. Green says that the relative absence of interest in string theory during the 1970s and early 1980s was actually helpful: it allowed him and a small number of colleagues to focus on their research well away from the limelight. “Initially we were not sure that the theory would be consistent, but as we understood it better we became more and more convinced that the theory had something valuable to say about the fundamental particles and their forces”, he says. In August 1984 the two researchers, while working at the Aspen Center for Physics in Colorado, famously understood how string theory avoids certain inconsistencies (known as ‘anomalies’) that plague more conventional theories in which the fundamental particles are points rather than strings. This convinced other researchers of the potential of string theory as an elegant unified description of fundamental physics. “Suddenly our world changed – and we were called on to give lectures and attend meetings and workshops”, remembers Green. String theory was back on track as a construct that offered a compelling explanation for the fundamental building blocks of the Universe: many researchers shifted the focus of their work into this newly-promising field and, as a result of this upturn in interest, developments in string theory began to take new and unexpected directions. Ideas formulated in the past few years, indicate that string theory has an overarching mathematical structure that may be useful for understanding a much wider variety of problems in theoretical physics that the theory was originally supposed to explain, this includes problems in condensed matter, superconductivity, plasma physics and the physics of fluids. Green is a passionate believer in the exchange of ideas and he values immensely his interaction with the latest generation of researchers to be tackling some of the knottiest problems in particle physics and associated fields. “The best ideas come from the young people entering the field and we need to make sure we continue to attract them into research. It is particularly evident that at present we fail to encourage sufficient numbers of young women to think about careers in physics”, he says. “Scientific research is by its nature competitive and there are, of course, professional jealousies – but there’s also a strong tradition of collaboration in theoretical physics and advances in the subject feel like a communal activity.” In 2009 Green was appointed Lucasian Professor of Mathematics at Cambridge. It comes with a legacy that Green describes as daunting: his immediate predecessor was Professor Stephen Hawking and in its 350-year history the chair has been held by a series of formidable names in the history of mathematical sciences.

The challenges of pushing forward the boundaries in a field that demands thinking in not three dimensions but as many as 11 are tremendous. The explanation of the basic building blocks of nature as different harmonics of a string is only a small part of string theory, and is the feature that is easiest to put across to the general public as it is relatively straightforward to visualise.

Far harder to articulate in words are concepts to do with explaining how time and space might emerge from the theory”, says Green. “Sometimes you hit a problem that you just can’t get out of your head and carry round with you wherever you are. It’s almost a cliché that it’s often when you’re relaxing that a solution will spontaneously present itself”. Like his colleagues Green is motivated by wonderment at the world and the excitement of being part of a close community grappling with fundamental questions. He is often asked to justify the cost of research that can seem so remote from everyday life, and that cannot be tested in any conventional sense. In response he gives the example of the way in which quantum mechanics has revolutionised the way in which many of us live. In terms of developments that may come from advances in string theory, he says: “We can’t predict what the eventual outcomes of our research will be. But, if we are successful, they will certainly be huge and in the meantime, string theory provides a constant stream of unexpected surprises.”

Michael Green will be giving a lecture, ‘The pointless Universe’, as part of Cambridge Science Festival on Thursday 13 March, 5pm-6pm, at Lady Mitchell Hall, Sidgwick Site, Cambridge. The event is free but requires pre-booking.

University of Cambridge: Strings that surprise: how a theory scaled up

Using the Universe as ‘tool’ to detect gravitons

La gravità è l’unica tra le quattro forze fondamentali per la quale gli scienziati non hanno ancora rivelato la sua unità fondamentale. Infatti, secondo il modello standard delle particelle elementari si ritiene che l’interazione gravitazionale venga trasmessa attraverso il gravitone, allo stesso modo con cui l’interazione elettromagnetica viene trasmessa dai fotoni. Oggi, nonostante esistano delle basi teoriche a favore dell’esistenza dei gravitoni rimane, però, il problema di rivelarli, almeno sulla Terra dove le possibilità sono estremamente basse se non quasi nulle.

For example, the conventional way of measuring gravitational forces, by bouncing light off a set of mirrors to measure tiny shifts in their separation, would be impossible in the case of gravitons. According to physicist Freeman Dyson, the sensitivity required to detect such a miniscule distance change caused by a graviton requires the mirrors to be so massive and heavy that they’d collapse and form a black hole. Because of this, some have claimed that measuring a single graviton is hopeless. But what if you used the largest entity you know of, in this case the Universe, to search for the telltale effects of gravitons. That is what two physicists are proposing. In the paper, “Using cosmology to establish the quantization of gravity”, published in Physical Review D (Feb. 20, 2014), Lawrence Krauss, a cosmologist at Arizona State University, and Frank Wilczek, a Nobel-prize winning physicist with MIT and ASU, have proposed that measuring minute changes in the cosmic background radiation of the Universe could be a pathway of detecting the telltale effects of gravitons.

Krauss and Wilczek suggest that the existence of gravitons, and the quantum nature of gravity, could be proved through some yet-to-be-detected feature of the early Universe.

This may provide, if Freeman Dyson is correct about the fact that terrestrial detectors cannot detect gravitons, the only direct empirical verification of the existence of gravitons”, Krauss said. “Moreover, what we find most remarkable is that the Universe acts like a detector that is precisely the type that is impossible or impractical to build on Earth”. It is generally believed that in the first fraction of a second after the Big Bang, the Universe underwent rapid and dramatic growth during a period called “inflation.” If gravitons exist, they would be generated as “quantum fluctuations” during inflation. Ultimately, these would evolve, as the Universe expanded, into classically observable gravitational waves, which stretch space-time along one direction while contracting it along the other direction. This would affect how electromagnetic radiation in the cosmic microwave background (CMB) radiation left behind by the Big Bang is produced, causing it to become polarized. Researchers analyzing results from the European Space Agency’s Planck satellite are searching for this “imprint” of inflation in the polarization of the CMB.

Krauss said his and Wilczek’s paper combines what already is known with some new wrinkles.

While the realization that gravitational waves are produced by inflation is not new, and the fact that we can calculate their intensity and that this background might be measured in future polarization measurements of the microwave background is not new, an explicit argument that such a measurement will provide, in principle, an unambiguous and direct confirmation that the gravitational field is quantized is new”, he said. “Indeed, it is perhaps the only empirical verification of this very important assumption that we might get in the foreseeable future”. Using a standard analytical tool called dimensional analysis, Wilczek and Krauss show how the generation of gravitational waves during inflation is proportional to the square of Planck’s constant, a numerical factor that only arises in quantum theory. That means that the gravitational process that results in the production of these waves is an inherently quantum-mechanical phenomenon. This implies that finding the fingerprint of gravitational waves in the polarization of CMB will provide evidence that gravitons exist, and it is just a matter of time (and instrument sensitivity) to finding their imprint. “I’m delighted that dimensional analysis, a simple but profound technique whose virtues I preach to students, supplies clear, clean insight into a subject notorious for its difficulty and obscurity”, said Wilczek. “It is quite possible that the next generation of experiments, in the coming decade or maybe even the Planck satellite, may see this background”, Krauss added.

ASU: Researchers propose a new way to detect the elusive graviton

arXiv: Using Cosmology to Establish the Quantization of Gravity

The decay of sterile neutrinos as signal of dark matter presence

Un gruppo di astrofisici dell’Università di Leiden, guidati da Alexey Boyarsky, potrebbero aver identificato alcune tracce della presenza di materia scura attraverso la rivelazione di una nuova particella, il neutrino sterile, un ipotetico tipo di neutrino che non interagisce con nessuna delle interazioni fondamentali. Intanto, qualche giorno fa, un altro gruppo di ricercatori di Harvard hanno riportato risultati simili.

The two groups this week reported that they have found an indirect signal from dark matter in the spectra of galaxies and clusters of galaxies. They made this discovery independent of one another, but came to the same conclusion: a tiny spike is hidden in the X-ray spectra of the Perseus galaxy cluster, at a frequency that cannot be explained by any known atomic transition. The Harvard group see the same spike in many other galaxy clusters, while Boyarsky also finds it in the nearby Andromeda galaxy. The researchers put it down to the decay of a new kind of neutrino, called ‘sterile’ because it has no interaction with other known neutrinos.

A sterile neutrino does have mass, and so could be responsible for the missing dark matter.

The first indications for the existence of dark matter in space were found more than eighty years ago, but there are still many questions surrounding this invisible matter. Sterile neutrinos are a highly attractive candidate for the dark matter particle, because they only call for a minor extension of the already known and extensively tested standard model for elementary particles. Boyarsky and his colleagues have already had this extension of the standard model ready for some time, but were waiting for the first observation of the mysterious particle. Measurements at higher resolution will shed light on the matter, and there is reason to hope that the spectral line just discovered will finally eliminate the problem of the missing mass.

University of Leiden: Glimmer of light in the search for dark matter

arXiv: An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster 

arXiv: Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters