Archivi tag: standard model

LHCb observes an exotic particle outside the quark model

The black points at the left image above show the ψ’π- invariant mass squared distribution of the data. The blue histogram shows the Z(4430) contribution. Credit. LHCb

I fisici che lavorano all’esperimento LHCb hanno pubblicato un articolo in merito ad una serie di misure di una particella esotica denominata Z(4430). Secondo il modello standard che descrive i quark, le particelle che sono soggette all’interazione forte, cioè gli adroni, sono formate sia da coppie quark-antiquark (mesoni) o da tre quark (barioni). Da quasi 50 anni, gli scienziati stanno cercando di identificare queste particelle, chiamate adroni esotici, che potrebbero non essere classificate secondo gli schemi tradizionali. Sono stati proposti numerosi candidati ma fino ad oggi non c’è stata alcuna evidenza sperimentale che confermasse con certezza la loro esistenza.

The first evidence for the Z(4430) particle has been presented in 2008 by the Belle Collaboration as narrow peak in the ψπ mass distribution in the B → ψ decays. In the latest Belle publication the observation of the Z(4430) particle is confirmed with a significance of 5.2σ and a 3.4σ evidence is presented that the quantum numbers JP = 1+ are favored over the other spin assignments. There are many so called charmonium cc* neutral states in this mass region. The fact that the Z(4430) is a charged particle does not allow to classify it as a charmonium state making this particle an excellent exotic candidate. The BaBar collaboration could explain the Z(4430) enhancement in their data by a possible feature of experimental analysis (so called reflections, for experts), not contradicting in the same time the Belle evidence. The LHCb Collaboration has reported today an analysis of about 25 200 B0 → ψ, ψ → μ+μ decays observed in 3 fb−1 of pp-collision data collected at √s = 7 and 8 TeV. The LHCb data sample exceeds by an order of magnitude that of Belle and BaBar together.

The significance of the Z(4430) signal is overwhelming, at least 13.9σ, confirming the existence of this state.

The Z(4430) quantum numbers are determined to be JP = 1+ by ruling out 0, 1, 2+ and 2 assignments at more than 9.7σ, confirming the evidence from Belle. The LHCb analysis establishes the, so called, resonant nature of the observed structure in the data, and in this way proving unambiguously that the Z(4430) is really a particle. The minimal quark content of the Z(4430) state is cc*du*. It is therefore a four quark state or a two-quark plus two-antiquark state.

LHCb: Unambiguous observation of an exotic particle which cannot be classified within the traditional quark model
The Conversation: Quirky quark combination creates exotic new particle

Quantum Diaries: Major harvest of four-leaf clover
arXiv:  Observation of the resonant character of the Z(4430)^- state

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

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

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

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


Exploring the nature of dark matter through the A’ particle

Per definizione, la materia scura è invisibile anche se la sua presenza viene “osservata” grazie ai suoi effetti gravitazionali. Nonostante ciò, secondo alcune teorie moderne che tentano di descrivere questa materia elusiva le sue proprietà potrebbero essere in definitiva rivelate. Infatti, alcuni scienziati del MIT hanno sviluppato un metodo che potrebbe permettere di verificare la validità di una teoria al fine di dare credito, o meno, alle sue previsioni.

The work is described in a paper in the journal Physical Review Letters co-authored by MIT physics professors Richard Milner and Peter Fisher and 19 other researchers. “We’re looking for a massive photon“, Milner explains. That may seem like a contradiction in terms: photons, or particles of light, are known to be massless. That’s why they travel at the speed of light, something that, according to Einstein’s theory of relativity, is impossible for anything that possesses mass. However, an exotic particle that resembles a photon, but with mass, has been proposed by some theorists to explain dark matter, whose nature is unknown but whose existence can be inferred from the gravitational attraction it exerts on ordinary matter, such as in the way galaxies rotate and clump together. “Now, an experiment known as DarkLight, developed by Fisher and Milner in collaboration with researchers at the Jefferson National Accelerator Laboratory in Virginia and others, will look for a massive photon with a specific energy postulated in one particular theory about dark matter“, Milner says. The idea is more than just a theoretical prediction, he adds: “There are hints of such a particle from other experiments, making it worthwhile to pursue a definitive answer“. But the previous hints, consisting of what Milner calls “anomalous moments of the muon”, do not rise to statistical significance.

The DarkLight experiment is designed to provide solid confirmation of the massive photon’s existence. If it does exist, that would represent a major discovery.

It’s totally beyond anything we understand about the physical world“, Milner says. “A massive photon would be totally different from anything allowed by the Standard Model, the bedrock of modern , he says. To prove the existence of the theorized particle, dubbed A’ (“A prime”), the new experiment will use a particle accelerator at the Jefferson Lab that has been tuned to produce a very narrow beam of electrons with a megawatt of power. That’s a lot of power, Milner says: “You could not put any material in that path without having it obliterated by the beam”. For comparison, he explains that a hot oven represents a kilowatt of power. “This is a thousand times that“, he says, “concentrated into mere millionths of a meter”.

The new paper confirms that the new facility’s beam meets the characteristics needed to definitively detect the hypothetical particle, or rather, to detect the two particles that it decays into, in precise proportions that would reveal its existence.

Doing so, however, will require up to two years of further preparations and testing of the equipment, followed by another two years to collect data on millions of electron collisions in the search for a tiny statistical anomaly. “It’s a tiny effect“, Milner says, but “it can have enormous consequences for our theories and our understanding. It would be absolutely groundbreaking in physics“. While DarkLight’s main purpose is to search for the A’ particle, it also happens to be well suited to addressing other major puzzles in physics. It can probe the nature of a reaction, inside stars, in which carbon and helium fuse to form oxygen, a process that accounts for all of the oxygen that now exists in the Universe. “This is the stuff we’re all made of“, Milner says, “and the rate of this reaction determines how much oxygen exists”. While that reaction rate is very hard to measure, the DarkLight experiment could illuminate the process in a novel way: “The idea is to do the inverse. Instead of fusing atoms to form oxygen, the experiment would direct the powerful beam at an oxygen target, causing it to split into carbon and helium. That would provide an indirect way of determining the stellar production rateRoy Holt, a distinguished fellow in the physics division at Argonne National Laboratory in Illinois, says “This work is a novel and significant technical development that not only opens a new window to search for a new [particle], but also for new studies in nuclear physics“. “If the planned experiment detects the A’ particle”, he says, “it would signal that  could actually be studied in a laboratory setting“.

MIT: Seeing the dark: New MIT-led experiment could finally shed light on the mysteries of dark matter
arXiv: Transmission of Megawatt Relativistic Electron Beams Through Millimeter Apertures

Higgs boson as trigger of primordial matter asymmetry?

La recente scoperta di una particella che “assomiglia” al bosone di Higgs rappresenta un traguardo importante della ricerca scientifica perchè ci permette di capire come ha avuto origine la massa delle particelle (post). Tuttavia, alcuni fisici si stanno ora chiedendo se il bosone di Higgs abbia avuto un ruolo significativo nel generare la materia scura e la materia barionica durante le fasi primordiali della storia dell’Universo, causando di conseguenza l’asimmetria osservata tra materia e antimateria.

In a paper published in Physical Review Letters, physicists Géraldine Servant at CERN and Sean Tulin at the University of Michigan in Ann Arbor, call this theoretical scenario “Higgsogenesis.” “With the Higgs discovery, the final piece of the Standard Model of particle physics has been put into place” Servant says. “Now, it is a natural question to ask: could the Higgs boson have been important in the early Universe to help explain two observational puzzles that the Standard Model cannot: the origin of dark matter and the matter-antimatter asymmetry? In the very early Universe, the Higgs particle was distinct from its antiparticle. We show that an asymmetry between Higgs and anti-Higgs might have been the missing link connecting the densities of visible and dark matter, which observationally are quite similar“. This Higgs could have provided the missing link in one of two ways. One possibility is that, if there were a dark matter asymmetry in the early Universe, then this asymmetry could have transferred to an asymmetry between the Higgs and the anti-Higgs, which then could have transferred to a baryon asymmetry between matter and antimatter. Another possibility is that this sequence could have happened in reverse, where a baryon asymmetry first transferred to a Higgs asymmetry, which then transferred to a dark matter asymmetry.

In both cases, the Higgs provides a “portal” through which asymmetries can flow from the dark sector to the visible sector or vice versa. In these scenarios, dark matter would have an asymmetry just like baryonic matter.

The physicists proposed two new fermions that couple to the Higgs boson that could have mediated the asymmetry transfers. “Our mechanism relies on the existence of an interaction between the Higgs field and the dark sector, which is a natural assumption in many extensions of the Standard Model of particle physics“, Tulin said.

“The novelty of our work is to investigate the role of the Higgs in transferring matter asymmetries between the dark and visible sectors. It offers new opportunities for baryogenesis and dark matter generation”.

In fact, previous research has shown that the Higgs boson may play a role in electroweak baryogenesis and leptogenesis, both of which describe asymmetries in the early Universe. Future experiments may be able to test these proposals. For instance, physicists could investigate Higgs decays at the Large Hadron Collider (LHC). In these decays, the proposed fermions may escape as missing energy that could be detected. “For Higgsogenesis to work, there must be new particles that interact through the weak force“, Servant said. “Actually, new weakly interacting particles are not unique to Higgsogenesis, but are part of many different new physics models, and the LHC is actively searching for them. A second prediction is that the Higgs boson can decay invisibly to  particles, and again the LHC is looking for this signature as well“.

arXiv: Higgsogenesis

Looking for ‘new physics’ in the Universe

Con il termine “nuova fisica” si intende un nuovo campo di ricerca che tenta di spiegare quei fenomeni della natura che i fisici non sono ancora in grado di descrivere. Oggi, sta prendendo piede sempre più l’idea in base alla quale l’Universo può essere caratterizzato da una struttura diversa rispetto a quanto previsto dagli attuali modelli o teorie. In tal senso, un gruppo di fisici hanno avviato uno studio che avrà lo scopo di aiutare gli scienziati a rendere più facile, almeno in parte, la comprensione di alcuni fenomeni della fisica fondamentale.

New physics is about searching for unknown physical phenomena not known from the current perception of the Universe. Such phenomena are inherently very difficult to detect“, explains Matin Mojaza from CP3-Origins. Together with colleagues Stanley J. Brodsky from Stanford University in the U.S. and Xing-Gang Wu from Chongqing University in China, Mojaza has now succeeding in creating a new method that can make it easier to search for new physics in the Universe (post).

The method is a so called scalesetting procedure, and it fills out some empty, but very important, holes in the theories, models and simulations, which form the basis for all particle physics today.

With this method we can eliminate much of the uncertainty in theories and models of today“, says Matin Mojaza. Many theories and models in particle physics today has the problem that they, together with their predictions, provide some parameters that scientists do not know how to set. “Physicists do not know what values they should give these parameters. For example, when we study the Standard Model and see these unknown parameters, we cannot know whether they should be interpreted as conditions that support or oppose to the Standard Model, this makes it quite difficult to study the Standard Model accurately enough to investigate its value”, explains Matin Mojaza. With the new approach researchers can now completely clean their models for the unknown parameters and thus become better at assessing whether a theory or a model holds water.

The Standard Model has for the last 50 years been the prevailing theory of how the Universe is constructed. According to this theory, 16 (17 if we include the Higgs particle) subatomic particles form the basis for everything in the Universe.

But the Standard Model is starting to fall short, so it is now necessary to look for new physics in the Universe. One of the Standard Model’s major problems is that it cannot explain gravity, and another is that it cannot explain the existence of dark matter, believed to make up 25 percent of all matter in the Universe. In addition, the properties of the newly discovered Higgs particle, as described in the Standard Model, is incompatible with a stable Universe. “A part of the Standard Model is the theory of quantum chromodynamics, and this is one of the first things, we want to review with our new method, so that we can clean it from the uncertainties“, explains Matin Mojaza. The theory of quantum chromodynamics predicts how quarks (such as protons and neutrons) and gluons (particles that keeps quarks in place inside the protons and neutrons) interact. Matin and his Chinese and American colleagues now estimate that there may be a basis for reviewing many scientific calculations to clean the results from uncertainties and thus obtain a more reliable picture of whether the results support or contradict current models and theories. “Maybe we find new indications of new physics, which we would not have exposed if we had not had this new method”, says Matin Mojaza.

He believes that the Standard Model needs to be extended so that it can explain the Higgs particle, dark matter and gravity.

One possibility in this regard is to examine the so-called technicolor theory, and another is the theory of supersymmetry. According to the supersymmetry theory, each particle has a partner somewhere in the Universe (these have not yet been found though). According to the technicolor theory there is a special techni-force that binds so-called techni-quarks, which can form other particles, perhaps this is how the Higgs particle is formed. This could explain the problems with the current model of the Higgs particle. Also Rolf-Dieter Heuer, director of CERN in Switzerland, where the famous 27 km long particle accelerator, the LHC, is situated, believes that the search for new physics is important. According to him, the Standard Model cannot be the ultimate theory, and it is only capable of describing about 35 percent of the Universe. Like CP3-Origins, also CERN has put focus on weeding out old theories and search for new physics, this happening in 2015, when the accelerator starts up again (post).

University of Southern Denmark: New groundbreaking research may expose new aspects of the Universe

arXiv: A Systematic All-Orders Method to Eliminate Renormalization-Scale and Scheme Ambiguities in PQCD