Archivi tag: higgs boson

Rencontres de Blois on Particle Physics and Cosmology

Particle Physics and Cosmology will emphasize the increasing interplay between high energy accelerator based physics and cosmology. The conference will consist of plenary sessions for invited in depth oral presentations (review talks and talks on specific specialised topics), and contributed papers, in the form of relatively short talks. Continua a leggere Rencontres de Blois on Particle Physics and Cosmology


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

Possible evidence for dark matter particles

Dark matter, the mysterious substance estimated to make up approximately more than one-quarter of the mass of the Universe, is crucial to the formation of galaxies, stars and even life but has so far eluded direct observation. At a recent UCLA symposium attended by 190 scientists from around the world, physicists presented several analyses that participants interpreted to imply the existence of a dark matter particle.
The likely mass would be approximately 30 billion electron-volts“, said the symposium’s organizer, David Cline, a professor of physics in the UCLA College of Letters and Science and one of the world’s experts on dark matter. The physicists at the Feb. 26–28 event were in agreement that “there seems to be an excess in the available data that could be due to dark matter“, Cline said. “At this symposium, it was obvious that excitement is building in the fields of dark matter theory and, especially, detection“, said Cline, who noted that there are several ways dark matter can be observed and that all were discussed at the UCLA meeting. “Because dark matter makes up the bulk of the mass of galaxies and is fundamental in the formation of galaxies and stars, it is essential to the origin of life in the Universe and on Earth“, Cline said. The first evidence for dark matter was discovered in 1933 using the Mt. Wilson telescope outside of Los Angeles. More recently, various theoretical models and detector improvements have made it possible to search for dark matter particles at extremely sensitive levels, some of the most sensitive measurements made by any scientists in the world. One search technique involves using the vast amount of dark matter in our galaxy. The NASA Fermi Satellite Telescope, an international collaboration involving NASA, the Goddard Space Flight Center and the SLAC National Accelerator Laboratory, searches for gamma rays, very high-energy light particles, from this dark matter. There are models of dark matter that would allow a signal in the galactic dark matter consistent with the claims at the meeting and provide a small interaction consistent with the “null results” in the direct dark matter searches all over the world. Much larger direct dark matter detectors are being planned in the U.S., Italy, Canada and China (including Xenon 3 Ton, LUX-ZEPLIN 7 Ton and DarkSide, which will weigh five tons). “These larger detectors potentially could see a dark matter signal in the next few years“, Cline said.
Dark matter is widely thought to be a kind of massive elementary particle that interacts weakly with ordinary matter. Physicists refer to these particles as WIMPS, for weakly interacting massive particles, and think they originated from the Big Bang. WIMPs are thought to be streaming constantly through the solar system and the Earth.
Another search method is to look for an interaction of a WIMP with xenon or argon nuclei and others (like germanium) in very low-background laboratories deep underground in Italy, the U.S., Canada, China and other countries. While these experiments have seen no signal of a WIMP above 30 billion electron volts, “there is no incompatibility with the interesting excess in the FERMI data“, Cline said. “The discovery of the Higgs boson, which won the 2013 Nobel Prize in physics, plays a role in the search for dark matter“, Cline said, adding that this topic was discussed in detail at the meeting. “Dark matter“, he said, “could consist of axions, WIMPs or sterile neutrinos, all of which were discussed at the symposium” (post). The UCLA dark matter symposium is convened every two years; this was the 11th such meeting. Cline said he and his colleagues hope to clarify the dark matter puzzle at the 2016 symposium.
It was at this same dark matter symposium in 1998 that two groups of scientists reported that the Universe is accelerating, as well as expanding, a finding Cline described as “one of the greatest discoveries in the history of science”.
See more on last week’s conference.
UCLA: Possible evidence for dark matter particle presented at UCLA physics symposium

Evidence for single top quark production through the weak nuclear force

Dopo una serie di tentativi che durano ormai da circa 20 anni, finalmente gli scienziati che lavorano agli esperimenti CDF e DZero presso il Fermi National Accelerator Laboratory hanno annunciato di aver trovato il modo di produrre un quark top. I due gruppi hanno affermato di aver osservato uno dei tanti metodi decisamente rari di produrre questa particella attraverso la forza nucleare debole, nel cosiddetto “canale-s”. Per arrivare a questo risultato, i ricercatori hanno dovuto analizzare più di 500 trilioni di collisioni protoni-antiprotoni che sono state realizzate con l’acceleratore Tevatron tra il 2001 e il 2011. I risultati indicano che in circa 40 collisioni, dove è stata prodotta la forza nucleare debole, sono stati identificati singolarmente quark top assieme a quark bottom.

Top quarks are the heaviest and among the most puzzling elementary particles. They weigh even more than the Higgs boson, as much as an atom of gold, and only two machines have ever produced them: Fermilab’s Tevatron and the Large Hadron Collider at CERN. There are several ways to produce them, as predicted by the theoretical framework known as the Standard Model, and the most common one was the first one discovered: a collision in which the strong nuclear force creates a pair consisting of a top quark and its antimatter cousin, the anti-top quark. Collisions that produce a single top quark through the weak nuclear force are rarer, and the process scientists on the Tevatron experiments have just announced is the most challenging of these to detect.

This method of producing single top quarks is among the rarest interactions allowed by the laws of physics.

The detection of this process was one of the ultimate goals of the Tevatron, which for 25 years was the most powerful particle collider in the world. “This is an important discovery that provides a valuable addition to the picture of the Standard Model Universe”, said James Siegrist, DOE associate director of science for high energy physics. “It completes a portrait of one of the fundamental particles of our universe by showing us one of the rarest ways to create them”. Searching for single top quarks is like looking for a needle in billions of haystacks. Only one in every 50 billion Tevatron collisions produced a single s-channel top quark, and the CDF and DZero collaborations only selected a small fraction of those to separate them from background, which is why the number of observed occurrences of this particular channel is so small. However, the statistical significance of the CDF and DZero data exceeds that required to claim a discovery. “Kudos to the CDF and DZero collaborations for their work in discovering this process”, said Saul Gonzalez, program director for the National Science Foundation. “Researchers from around the world, including dozens of universities in the United States, contributed to this important find”. The CDF and DZero experiments first observed particle collisions that created single top quarks through a different process of the weak nuclear force in 2009. This observation was later confirmed by scientists using the Large Hadron Collider. Scientists from 27 countries collaborated on the Tevatron CDF and DZero experiments and continue to study the reams of data produced during the collider’s run, using ever more sophisticated techniques and computing methods. “I’m pleased that the CDF and DZero collaborations have brought their study of the top quark full circle”, said Fermilab Director Nigel Lockyer. “The legacy of the Tevatron is indelible, and this discovery makes the breadth of that research even more remarkable”.

Fermilab: Scientists complete the top quark puzzle

Fermilab: Observation of s-channel production of single top quarks at the Tevatron
arXiv: Evidence for s-channel Single-Top-Quark Production in Events with one Charged Lepton and two Jets at CDF

arXiv: Search for s-channel Single Top Quark Production in the Missing Energy Plus Jets Sample using the Full CDF II Data Set

arXiv: Evidence for s-channel single top quark production in pp¯ collisions at s√ = 1.96 TeV

The need for a new, giant particle collider

Al CERN di Ginevra sono pronti per lanciare una nuova sfida, ancora più grande. E’ quanto hanno affermato in questi giorni i fisici delle particelle che oggi come non mai ritengono necessaria la costruzione di un nuovo, gigantesco acceleratore, almeno sette volte più potente dell’attuale Large Hadron Collider (LHC) grazie al quale è stato possibile rivelare una particella che tanto assomiglia al bosone di Higgs (post).

Particle physics takes the long-term view. Originally conceived in the 1980s, the LHC took another 25 years to come into being. This accelerator, which is unlike any other, is just at the start of a programme which is expected to run for another 20 years. Even now, as consolidation work aimed at a restart in 2015 continues, detailed plans are being hatched for a large-scale upgrade to increase luminosity and thereby exploit the LHC to its full potential.

The HL (High Luminosity) LHC is CERN1’s number-one priority and will increase the number of collisions accumulated in the experiments by a factor of ten from 2024 onwards.

Even though the LHC programme is already well defined for the next two decades, the time has come to look even further ahead, so CERN is now initiating an exploratory study for a future long-term project centred on a new-generation circular collider with a circumference of 80 to 100 kilometres. A worthy successor to the LHC, whose collision energies will reach 14 TeV, such an accelerator would allow particle physicists to push back the boundaries of knowledge even further. The Future Circular Colliders (FCC) programme will focus especially on studies for a hadron collider, similar to the LHC, capable of reaching unprecedented energies in the region of 100 TeV. The FCC study will be a global venture for particle physics and stems from the recommendation in the European Strategy for Particle Physics, published in May 2013, that a feasibility study be conducted on future fundamental research projects at CERN. It will be conducted over the coming five years and starts with an international kick-off meeting at the University of Geneva from 12 to 15 February. The FCC will thus run in parallel with another study that has already been under way for a number of years, the Compact Linear Collider, or “CLIC”, another option for a future accelerator at CERN.  The aim of the CLIC study is to investigate the potential of a linear collider based on a novel accelerating technology. “We still know very little about the Higgs boson, and our search for dark matter and supersymmetry continues. The forthcoming results from the LHC will be crucial in showing us which research paths to follow in the future and what will be the most suitable type of accelerator to answer the new questions that will soon be asked,” says Sergio Bertolucci, Director for Research and Computing at CERN“We need to sow the seeds of tomorrow’s technologies today, so that we are ready to take decisions in a few years’ time,” adds CERN’s Director for Accelerators and Technology, Frédérick Bordry. The goal of the two studies is to examine the feasibility of the various possible machines, to evaluate their costs and to produce a conceptual design report for the FCC and elaborate on the one already produced for CLIC in time for the next European Strategy update around 2018/2019.

For more information on:

CERN: CERN prepares its long-term future

Lisa Randall on the importance of the Higgs discovery

Oggi mi piace segnalare l’e-book “Higgs Discovery: The Power of Empty Space” di Lisa Randall che descrive l’importanza ed il significato della recente scoperta del bosone di Higgs, o di una particella che gli assomiglia tanto (post). L’e-book ha avuto talmente successo al punto che l’editore ha deciso di pubblicare una versione cartacea. Quella che segue è l’intervista della Harvard Gazette all’autrice che spiega le motivazioni che l’hanno portata a scrivere il libro.

GAZETTE: Why did you write this book, and for whom is it written?

RANDALL: The discovery of the Higgs boson was a remarkable event (post). The particle was predicted based on the need for a consistent theory to describe what was known about nature half a century ago. The Large Hadron Collider (LHC), the 27-kilometer-circumference machine near Geneva, was built to look for new particles and forces and the underlying nature of matter. One of the goals was to find the Higgs boson, a particle that helps us understand how elementary particles acquire their masses. Yet no one knew for certain when, and even if, it would be found. When the discovery was announced on July 4, 2012, I was on vacation on the island of Patmos, Greece, where I was attending a wedding. I literally heard about the discovery on the balcony of the one café with Internet nearby on the island. I listened to a live feed (supplemented with Twitter for when the signal was too low). I was thrilled, but also frustrated to be so far from my work and my colleagues at the time. And many people immediately wrote to me to express excitement but also to ask many questions. Even though they were fascinated, they didn’t quite know what it was that had been found. Having just written a book, “Knocking on Heaven’s Door,” which among other things explained the LHC and the search for the Higgs boson (see video), I wanted to be able to answer those questions and complete the story (at least this part of it). But I also wanted to return to full-time research and not be burdened with another book so soon after finishing the previous one. So I decided to write a short e-book, but only if I could do it in a week. I happened to be in London en route to a European science conference immediately after Greece and spoke to a partner of my book agent who advised against an e-book, suggesting (correctly, in my opinion) that if writers agree to the terms, they will lose out in the end. But she saw that I really was excited and read on my laptop what I had managed to write the evening before and quickly changed her mind, agreeing that it could be a good thing. My publisher, Ecco, signed on and agreed to a quick turnover (amazing in the publishing world) and added two chapters I had written on the Higgs boson for my previous two books.

GAZETTE: In the simplest, easiest, most rudimentary terms, what is the Higgs boson?

RANDALL: The Higgs boson is a particle associated with the masses of elementary particles.  Notice the careful phrasing. There are two common misconceptions about the Higgs boson that are important to know if you want to truly understand it. First of all, the Higgs boson is associated only with elementary particle masses such as that of the electrons or particles called quarks inside protons and neutrons. Most of the mass of common matter is a result of the strong binding force in those protons and neutrons. It would exist even without the Higgs boson. But the mass of the most basic particles we know about, those building blocks of matter of which all ordinary stuff is made, can only be explained by something called the Higgs mechanism. The Higgs mechanism is a result of something called a field that extends throughout space, even where no particles are present. This notion is probably most familiar to you from a magnetic field. You feel a force between a magnet and your refrigerator even when “nothing” is there. A field can fill “empty” space. The Higgs field extends throughout space. Elementary particles acquire their masses by interacting with this field. It is kind of like space is charged and particles get mass through their interactions with this charge. Now, back to the Higgs boson. The Higgs boson is not directly responsible for mass. The Higgs field is. The boson is a particle that tells us our understanding of this mechanism is correct. It also is a big clue as to where that field came from in the first place. Its discovery tells us that what we expected to be true was indeed correct, and it gives us clues as to what else might underlie the Standard Model.

GAZETTE: What are the implications of its discovery? What’s next?

RANDALL: First of all, it means our understanding of what is called the Standard Model of particle physics is correct. We understand the basic building blocks of matter and the forces through which they interact. We now also know how they get their masses. But there are still many properties we have yet to understand. Chief among the questions we’d like to answer is why masses are what they are. Not only are all the elementary particle masses different from each other, they are one ten-thousand-trillion times smaller than we would expect if we tried to estimate them based on the equations given by quantum mechanics and special relativity. It turns out this is an extraordinarily challenging problem. The answer could give us deep insights, not only into particle physics but into the nature of space itself. The LHC will search for answers to these questions, both by looking for new particles and by better measurements of the Higgs boson’s properties. Any new information helps us move forward.

GAZETTE: Why should the layperson care about the Higgs boson?

RANDALL: No one necessarily “should.” They can live their lives without this knowledge. But I do think those who care should have access to understanding. And I also have seen that many do care. I think the reason has to do with being human and curious and wanting to understand the world and Universe in which we live. I also think the discovery is rewarding in that it truly represents progress, both technical and scientific. In a world with many problems where progress isn’t always clear, it is wonderful to see science so clearly advance and for us to be able to answer such basic questions that help us better understand our Universe.

GAZETTE: What are you at work on now?

RANDALL: I’m currently thinking about a couple of different questions. One major research focus is the matter that isn’t part of the Standard Model, namely dark matter. That is matter that doesn’t interact via the forces like electromagnetism under which ordinary matter interacts. We know about it because of its gravitational influence. We would like to know more about what it is. Along with others, I’m thinking about the possibility that some part of the dark matter interacts under its own forces, dark light if you like. This could have dramatic effects on our galaxy and structure formation, all of which makes it rather rich and interesting.

In a world with many problems where progress isn’t always clear, it is wonderful to see science so clearly advance and for us to be able to answer such basic questions that help us better understand our Universe.

Courtesy Harvard Gazette: Explaining the Higgs

Testing the string theory with astronomy

Il “Sacro Graal” della fisica potrebbe venire alla luce. E’ oggi quello che un gruppo di fisici del Department of Physics, Astronomy and Geosciences presso la Towson University (TU) sperano di aver trovato dopo quasi mezzo secolo di ricerche: verificare sperimentalmente una delle teorie più elusive e più complicate da capire, la teoria delle stringhe, osservando il moto dei pianeti, della Luna e degli asteroidi in una sorta di reminiscenza di uno dei più famosi test realizzati da Galileo sulla caduta dei gravi dalla Torre di Pisa.

Scientists have joked about how string theory is promising…and always will be promising, for lack of being able to test it”, says James Overduin, professor in Towson’s Department of Physics, Astronomy and Geosciences and lead author on a paper about the test TU scientists are developing. The paper was presented at the 223rd Meeting of the American Astronomical Society in Washington.

String theory posits an explanation for the connection between all the forces in the Universe. If it sounds overly broad, it is; string theory is nicknamed “the theory of everything.”

Scientific theories need tests in order to be truly valid, and string theory hasn’t been testable because the energy level and size to see its effects are just too big. “What we have identified is a straightforward method to detect cracks in general relativity that could be explained by string theory, with almost no strings attached”, Overduin explains. For most people, the understanding of string theory goes about as far as CBS’s “The Big Bang Theory” can convey it. The very basic explanation of the complex concept is that all matter and energy in the Universe is made of one-dimensional strings, a quintillion times smaller than the extremely tiny hydrogen atom. That means the strings are too small to detect indirectly, and finding signs of them in an instrument like a particle accelerator would require millions of times more energy than what was used to uncover, for example, the Higgs boson, a particle pivotal to the explanation and further proof of particle theory. The Higgs boson was posited in the 1960s, around the same time as string theory’s introduction; the boson’s identification was announced in 2012. The TU team’s string theory test borrows from Galilean and Newtonian laws of gravity. History holds that Galileo tested rates of acceleration by simultaneously dropping balls of two different weights off the Tower of Pisa to demonstrate that, despite the weight difference, they would hit the ground at the same time. Newton later found that Jupiter and its moons, in their orbits, “fall” at the same rate of acceleration toward the Sun. Much later, Einstein developed the theory of relativity when he recognized that gravity pulls all masses with precisely the same amount of strength, regardless of size.

Overduin and his team use those understandings for their test because string theory posits violations of Einstein’s relativity. It asserts that there are other fields that couple with objects differently, depending on the objects’ composition. That makes them accelerate differently, even within the same gravitational field.

But why does it matter? According to Overduin, the answer is nothing short of revolution. “Every time physicists have succeeded in unifying two different branches of physics, society has been transformed”, Overduin says. The Scientific Revolution was born of Newton’s unification of physics and astronomy. The Industrial Revolution, steam engines leading to train and boat transportation, began after physicists unified mechanics and heat. Electrification came when James Clerk Maxwell unified electricity and magnetism. Einstein’s relativity ushered in the Atomic Age, and then the Information Age, when relativity entered the quantum mechanics. That leaves two parts of physics still unconnected: gravitation and everything else. Physicists believe unifying them, as a test of string theory could do, would spark yet another revolution. But for all this time, they couldn’t do it. Towson University scientists might.

TU: TU scientists may spark revolution with string theory test

arXiv: Expanded solar-system limits on violations of the equivalence principle

Will the Universe collapse in a Big Crunch?

Nel 1922, il cosmologo russo Alexander Friedmann propose delle soluzioni alle equazioni di campo di Einstein nel caso di un Universo omogeneo, isotropo e non statico. Sotto queste ipotesi, è possibile definire una densità media dell’Universo, ossia una densità di massa-energia e, come lo stesso Friedmann dimostrò a suo tempo, descrivere lo spazio in ogni istante con un solo numero, la curvatura scalare. Da qui si ricavano tre modelli di Universo, detti modelli di Friedmann, in funzione del parametro di curvatura k per cui l’Universo può assumere o una geometria iperbolica (k=-1, aperto), o una geometria euclidea (k=0, piatto), oppure una geometria ipersferica (k=1, chiuso). Dunque, una previsione di questi modelli implica che l’Universo potrebbe un giorno collassare a causa della mutua interazione gravitazionale dovuta alla materia presente nell’Universo, una ipotesi che sembra essere confermata oggi da alcuni calcoli eseguiti da un gruppo di fisici dell’University of Southern Denmark. Il risultato è che il rischio di una possibile contrazione gravitazionale sembra essere molto maggiore di quanto sia stato finora ipotizzato.

Sooner or later a radical shift in the forces of the Universe will cause every little particle in it to become extremely heavy. Everything, every grain of sand on Earth, every planet in the Solar System and every galaxy, will become millions of billions times heavier than it is now, and this will have disastrous consequences: the new weight will squeeze all material into a small, super hot and super heavy ball, and the Universe as we know it will cease to exist. This violent process is called a phase transition and is very similar to what happens when, for example water turns to steam or a magnet heats up and loses its magnetization. The phase transition in the Universe will happen if a bubble is created where the Higgs-field associated with the Higgs-particle reaches a different value than the rest of the Universe. If this new value results in lower energy and if the bubble is large enough, the bubble will expand at the speed of light in all directions. All elementary particles inside the bubble will reach a mass, that is much heavier than if they were outside the bubble, and thus they will be pulled together and form supermassive centers. “Many theories and calculations predict such a phase transition, but there have been some uncertainties in the previous calculations. Now we have performed more precise calculations, and we see two things: 1) yes, the Universe will probably collapse, and 2) a collapse is even more likely than the old calculations predicted“, says Jens Frederik Colding Krog of the Center for Cosmology and Particle Physics Phenomenology (CP ³ – Origins) at University of Southern Denmark and co-author of an article on the subject in the Journal of High Energy Physics. “The phase transition will start somewhere in the Universe and spread from there. Maybe the collapse has already started somewhere in the Universe and right now it is eating its way into the rest of the Universe. Maybe a collapsed is starting right now right here here. Or maybe it will start far away from here in a billion years. We do not know”, says Jens Frederik Colding Krog. More specifically he and his colleagues looked at three of the main equations that underlie the prediction of a phase change. These are about the so-called beta functions, which determine the strength of interactions between for example light particles and electrons as well as Higgs bosons and quarks. So far physicists have worked with one equation at a time, but now the physicists from CP3 show that the three equations actually can be worked with together and that they interact with each other. When applying all three equations together the physicists predict that the probability of a collapse as a result of a phase change is even greater than when applying only one of the equations.

The theory of phase transition is not the only theory predicting a collapse of the Universe. Also the so-called Big Crunch theory is in play.

This theory is based on the Big Bang, the formation of the Universe. After the Big Bang all material was ejected into the Universe from one small area, and this expansion is still happening. At some point, however, the expansion will stop and all the material will again begin to attract each other and eventually merge into a small area again. This is called the Big Crunch. “The latest research shows that the Universe’s expansion is accelerating,  so there is no reason to expect a collapse from cosmological observations. Thus it will probably not be Big Crunch that causes the Universe to collapse“,  says Jens Frederik Colding Krog.

Although the new calculations predict that a collapse is now more likely than ever before, it is actually also possible, that it will not happen at all.

It is a prerequisite for the phase change that the Universe consists of the elementary particles that we know today, including the Higgs particle. If the Universe contains undiscovered particles, the whole basis for the prediction of phase change disappears. “Then the collapse will be canceled”, says Jens Frederik Colding Krog. In these years the hunt for new particles is intense. Only a few years ago the Higgs-particle was discovered, and a whole field of research known as high-energy physics  is engaged in looking for more new particles. At CP3 several physicists are convinced that the Higgs particle is not an elementary particle, but that it is made up of even smaller particles called techni-quarks. Also the theory of supersymmetry predicts the existence of yet undiscovered particles, existing somewhere in the Universe as partners for all existing particles (superparticles). According to this theory there will be a selectron for the electron, a fotino for the photon, etc. While the physical results discussed in the article were partially established earlier in the literature, the work of the Danish based researchers deals with the mathematical foundations of the technique used among other things also to determine the stability of the Universe. In their work the researchers assumed valid the current knowledge of the Standard Model interactions augmented by the discovery of the Higgs and the latest mathematical constraints.

University of Southern Denmark: Collapse of the universe is closer than ever before

arXiv: Standard Model Vacuum Stability and Weyl Consistency Conditions

First evidence of Higgs boson decay to fermions

I fisici del CERN che lavorano all’esperimento ATLAS hanno pubblicato in un comunicato stampa i dati preliminari relativi alla prima evidenza del decadimento del bosone di Higgs in due particelle tau che appartengono ad un gruppo di particelle subatomiche, chiamate fermioni. Questo risultato è stato ottenuto con un livello di confidenza pari a 4,1 sigma su una scala di 5 punti, che di solito viene utilizzata per determinare il grado di certezza di un esperimento, e rappresenta la prima evidenza sperimentale del decadimento del bosone di Higgs in fermioni.

On 4 July 2012, the ATLAS and CMS experiments at CERN announced the discovery of a new particle, which was later confirmed to be a Higgs boson. For physicists, the discovery meant the beginning of a quest to find out what the new particle was, if it fit in the Standard Model, our current model of nature in particle physics, or if its properties could point to new physics beyond that model.

An important property of the Higgs boson that ATLAS physicists are trying to measure is how it decays.

The Higgs boson lives only for a short time and disintegrates into other particles. The various possibilities of the final states are called decay modes. So far, ATLAS physicists had found evidence that the Higgs boson decays into different types of gauge bosons, the kind of elementary particles that carry forces. The other family of fundamental particles, the fermions, make up matter. The tau is a fermion and behaves like a very massive electron. The Brout-Englert-Higgs mechanism was first proposed to describe how gauge bosons acquire mass. But the Standard Model predicts that fermions also acquire mass in this manner, so the Higgs boson could decay directly to either bosons or fermions.

The new preliminary result from ATLAS shows clear evidence that the Higgs boson indeed does decay to fermions, consistent with the rate predicted by the Standard Model.

This important finding was made possible through careful analysis of data produced by the LHC during its first run. Only with new data will physicists be able to determine if the compatibility remains or if other new models become viable. Fortunately, the next LHC run, which begins in 2015, is expected to produce several times the existing data sample. In addition, the proton collisions will be at higher energies, producing Higgs bosons at higher rates. ATLAS’ broad physics programme, which includes precision measurements of the Higgs boson, will continue to test the Standard Model. The years ahead will be exciting for particle physics as the LHC experiments have found new territory that they have only just begun to explore.

CERN: ATLAS sees Higgs boson decay to fermions