The Large Hadron Collider, on the Franco-Swiss border near Geneva. Photo: Getty Images
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The Large Hadron Collider has made another exciting quantum discovery

Scientists working on one of the four experiments at the LHC have gathered enough evidence to confirm the existence of a four-quark particle.

Since the spectacular discovery of the Higgs boson in 2012, physicists at the Large Hadron Collider (LHC), the gigantic particle accelerator outside Geneva, have suffered a bit of a drought when it comes to finding new particles. In a welcome relief, the LHCb collaboration, who run one of four large experiments at the LHC, have announced one of the most genuinely exciting observations to come out of the 27km super-collider so far – an exotic particle that cannot be explained by current theories.

In the early 1930s physicists had a clean picture of the subatomic particles that make up our world. Every known atom has a tiny nucleus at its heart surrounded by a cloud of electrons, and each nucleus was made out of varying numbers of protons and neutrons. However, as the decades wore on a number of new, and somewhat unwelcome, particles were discovered, at first in detectors studying particles from outer space and later in particle-collider experiments.

By the 1950s, dozens of apparently elementary particles had been discovered, causing frustration among physicists who often brandish an inability to memorise a list of facts as a badge of honour. The famous physicist Enrico Fermi perhaps best expressed the mood of his colleagues in an infamous remark:

Young man, if I could remember the names of these particles, I would have been a botanist.”

Help came in the 1950s when physicists came up with a new model that explained most of these particles as being made up of a small number of truly elementary particles. Borrowing a line from James Joyce’s Finnegans Wake (a book that is even harder to understand than quantum field theory), Murray Gell-Mann dubbed these new particles “quarks”.

By the late 1960s the existence of quarks had been verified experimentally. We now know that there are six in total – the up, down, strange, charm, bottom and top quarks, along with six antiquarks (their anti-matter copies).

The quark model neatly explained all these peculiar particles. Protons, neutrons and many others besides are made of three quarks, belonging to a family known as baryons. Alternatively, a quark and an antiquark can pair up to form a meson.

Since then the quark model has been extremely successful, and is now a cornerstone of our understanding of particle physics. It was only at the turn of the millennium that some strange results started to suggest that the model might be incomplete. Until 2003 quarks had only been seen in twos or threes, but then a number of particles that looked like combinations of four quarks started to reveal themselves.

In 2008 the Belle Collaboration in Japan reported the observation of a new exotic particle – the unfortunately drably named Z(4430) (where for its negative charge). This has a mass that places it in a dense forest of charmonium states – particles that are made up of a charm quark and a charm antiquark. Crucially though, the Z is electrically charged whereas all charmonium states must be neutral, clearly marking it out as something unusual.

After a careful analysis of data from 25,000 decays of mesons resulting from more than 180 trillion collisions at the LHC in 2011 and 2012, the new announcement confirms the existence of Z(4430) with extremely high confidence. The particle was observed with an overwhelming significance of 13.9 sigma, well above the usual 5 sigma threshold required to declare a discovery. LHCb also went further than Belle by measuring the spin and parity of Z(4430), two quantum-mechanical properties that give a firm handle on the internal makeup of the particle.

The observation by LHCb is important because few physicists will take a result seriously until it has been seen by two independent experiments. This is why hundreds of millions of Euros were spent building two large detectors at the LHC. The observation of the Higgs boson by two independent teams, ATLAS and CMS, was what really convinced the scientific community that the particle was real.

This result is the clearest evidence yet of the existence of a tetraquark – a four-quark state, with the LHCb analysis suggesting that Z(4430) is most likely to be made of a charm, anti-charm, down and anti-up quark. Theorists are now able to add a whole new type of particle to the quark model and begin the hard work of trying to understand exactly how these four quarks are bound together.

Meanwhile, physicists working at the LHC experiments will continue to explore unmapped regions of the subatomic world, with the hope of turning up more members of this exotic new family. Now that we know that at least one is out there, it is very unlikely that Z(4430) is alone.The Conversation

Harry Cliff is affiliated with the University of Cambridge, CERN, the LHCb experiment.

This article was originally published on The Conversation. Read the original article.

Alan Schulz
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An Amazonian tribe is challenging scientific assumptions about our musical preferences

The Tsimane’ – a population of people in a rural village in Bolivia – are overturning scientists' understanding of why humans prefer consonant sounds over dissonant ones.

It was 29 May 1913. Hoards of Parisians packed out the newly-opened Théâtre des Champs-Élysées. Messrs Proust, Picasso and Debussy were in attendance. Billed for the evening was the premiere of Le Sacre du PrintempsThe Rite of Spring, a ballet and orchestral work debuted by Russian composer Igor Stravinsky.

The attention and conjecture focused on the theatre that day meant expectations were high. However, within moments of the piece beginning, all preconceived notions held by the audience were shattered, as what was unfolding in front of them was a musical tragedy unlike anything they had ever witnessed.

A bassoon hummed into the ether before ballet dancers stomped on stage; the music, unpredictable with its experimental edge, drove forth the onstage narrative of a young girl whose selection during a pagan ritual saw her sacrificially dance towards death. Stravinsky’s composition and the ensemble of the night caused the room to descend from laughter and disruption to chaos and uproar.

The employment of dissonance – sharp, unstable chords – largely contributed to the audience’s disturbed reaction. Dissonant chords create a tension, one which seeks to be resolved by transitioning to a consonant chord – for example an octave or perfect fifth. These musical intervals sound far calmer than the chords which riveted the audience of The Rite of Spring.

Dissonant and consonant intervals find themselves as binary opposites; the frequencies at which notes played together vibrate determine whether an interval is consonant or dissonant. Consonant intervals have simple mathematical relationships between them, but greater digression from that simplicity makes an interval increasingly dissonant.

It’s long been believed  both experimentally and anecdotally – that the preference among Westerners for consonant chords highlights a universal, perhaps biologically-rooted, leaning among all humans towards consonant sounds. If you were present at the introduction of Stravinsky’s The Rite of Spring on that night of furore in Paris, you’d find it hard to disagree.

There is, however, a growing movement against this consensus. Ethnomusicologists and composers alike argue that favouring consonance may just be a phenomenon that has evolved from Western musical culture. And following the visit of a group of researchers to a remote Amazonian society, these claims could well be grounded in scientific evidence.

Led by Josh McDermott, an MIT researcher who studies how people hear, the group travelled to a village in the Amazon rainforest called Santa Maria. It’s populated by the Tsimane’ – a group of native Amazonians whose rural abode is inaccessible by road and foot, and can be reached only by canoe. There are no televisions in Santa Maria and its inhabitants have little access to radio, meaning exposure to Western cultural influences is minimal.

The researchers were curious to see how the Tsimane’ would respond to music, in order to determine whether they too had a preference for consonant sounds over dissonant ones. To everyone’s surprise, the Tsimane’ showed no preference for consonance; the two different sounds, to the Tsimane’ at least, were equally pleasant.

Detailing their research in a paper published by Nature, the group explains how the Tsimane’ people’s indifference to dissonance is a product of their distance from Western culture and music, removing any purported notion that humans are hard-wired to praise perfect fifths and fourths.

McDermott tells me that the Western preference for consonance may just be based on familiarity. “The music we hear typically has more consonant chords than dissonant chords, and we may like what we are most exposed to,” he says. “Another possibility is that we are conditioned by all the instances in which we hear consonant and dissonant chords when something good or bad is happening, for example in films and on TV. Music is so ubiquitous in modern entertainment that I think this could be a huge effect. But it could also be mere exposure.”

To fully gauge the Tsimane’ responses to the music, 64 participants, listening via headphones, were asked to rate the pleasantness of chords composed of synthetic tones, and chords composed of recorded notes sung by a vocalist. At a later date, another 50 took part in the experiment. They had their responses compared to Bolivian residents in a town called San Borja, the capital city La Paz, and residents in the United States – locations selected based on their varying exposures to Western music.

What made the Tsimane’ particularly interesting to McDermott and his group was the absence of harmony, polyphony and group performances in their music. It was something the researchers initially thought may prevent an aesthetic response from forming, but the worry was quickly diminished given the Tsimane’ participants’ measure of pleasantness on the four-point scale they were provided.

Unsurprisingly, the US residents showed a strong preference for consonance – an expected preference given the overrunning of Western music with consonant chords. Meanwhile, the San Borja and La Paz residents demonstrated inclinations towards consonant sounds similar to the US residents. The implication of these results – that consonance preferences are absent in cultures “sufficiently isolated” from Western music – are huge. We most probably aren’t as polarised by consonance and dissonance as we assume; cultural prevalence is far more likely to have shaped the consonant-dominant sounds of Western music.

McDermott raised the question about why Western music may feature certain intervals over others to begin with:

“One possibility is that biology and physics conspire to make conventionally consonant and dissonant chords easy to distinguish, and so that distinction becomes a natural one on which to set up an aesthetic contrast even if the preference is not obligatory. We have a little evidence for this in that the Tsimane' could discriminate harmonic from inharmonic frequencies, which we believe form the basis of the Western consonance/dissonance distinction, even though they did not prefer harmonic to inharmonic frequencies.”

There has been some criticism of this. Speaking to The Atlantic, Daniel Bowling from the University of Vienna said:

“The claim that the human perception of tonal beauty is free from biological constraint on the basis of a lack of full-blown Western consonance preferences in one Amazonian tribe is misleading.”

Though the results from the Amazonian tribe demonstrate a complete refutation of previous assumptions, people's musical preferences from other cultures and places will need to be analysed to cement the idea.

With research beginning to expand beyond WEIRD people – those from a Western, Educated, Industrialised, Rich and Democratic background – the tastes in music of people the world over may continue to surprise, just as the Tsimane’ did.

The Rite of Spring, which was met with ridiculing reviews has now been canonised and is considered to be one of the most important pieces of music of the twentieth century. A Tsimane’ crowd on that tender night a century ago in Paris may have responded with instant praise and elation. With further research, the imagined Bolivian adoration of a Russian composer’s piece in the French city of love may prove music to be the universal language after all.