Sheldon Lee Glashow, premio Nobel de Física, considera que los indicios de la nueva partículas son insuficientes para decir que es un descubrimiento
Autor: Alicia Rivera Madrid 16 DIC 2011 - 12:46 CET13
El físico teórico Sheldon Lee Glashow ha estado esta semana muy pendiente de los resultados sobre la búsqueda de la partícula Higgs presentados en el CERN (Laboratorio Europeo de Física de Partículas). “La verdad es que me he quedado un poco decepcionado”, dice, “porque los indicios que tienen son más débiles de lo que yo creía”. Enseguida apunta que los científicos del CERN han sido “escrupulosamente honestos, incluso conservadores, sin exageración alguna” al exponer los resultados que tienen hasta ahora, pero se había creado una expectación enorme.
Glashow, Premio Nobel de Física en 1979 (junto con Steven Weinberg y Abdus Salam), ahora profesor de la Universidad de Boston, con 79 años recién cumplidos, un auténtico coloso de la física teórica reconocido internacionalmente, no acaba de explicarse el enorme interés social que esta búsqueda del bosón de Higgs ha suscitado en todo el mundo. “Tal vez es porque la gente comparte la sensación que yo tengo de que tenemos la obligación de conocer el universo en que vivimos”, apunta.
Explica que los indicios de Higgs presentados tienen aún una probabilidad de error que es inaceptablemente alta en esta disciplina para que algo sea considerado un descubrimiento. “Yo he visto muchas veces en mi vida, muchas veces, desaparecer señales con 3 sigma [medida del error estadístico del experimento]. Un descubrimiento tiene que tener 5 sigma”, afirma categórico. “Uno de los grupos del CERN asegura que tiene indicios de la partícula de Higgs en un rango, el otro grupo no lo excluye, así que no me impresionó mucho. A algunos de mis colegas les ha impresionado más que a mí, a otros menos. Pero lo interesante es que ahora sabemos que el año que viene, cuando se ponga de nuevo en operación el LHC, se tomarán muchísimos más datos y será posible determinar si efectivamente está ahí el Higgs o no”.
Glashow estaba el martes pasado en la Universidad de Santiago de Compostela, invitado en el programa Conciencia, y siguió la transmisión en directo desde el CERN de la presentación de los resultados de los detectores Atlas y CMS del acelerador LHC. “Había mucha gente y mucha emoción”, dice ya en Madrid, donde asiste esta semana a la conferencia inaugural del Instituto de Física Teórica (CSIC-Universidad Autónoma de Madrid). Además, mañana impartirá una conferencia en la Fundación BBVA en la sesion El acelerador LHC: resultados y perspectivas, junto con Albert de Roeck, investigador del CERN.
A la vista del interés mundial que ha suscitado esta investigación uno puede pensar que la partícula de Higgs es la definitiva, la última que se descubrirá. “Por supuesto que no.., eso espero porque en realidad no lo sabemos”, responde Glashow tajante (pero afable y sonriente en todo momento). “Los físicos que trabajan en supersimetrías esperan encontrar todo un nuevo grupo de partículas que podrían ser descubiertas en el LHC, especialmente cuando funcione a mayor energía aún, tal y como está planeado. Otros han anticipado que tal vez sea posible hallar la partícula de la materia oscura del universo….. También se piensa en nuevos niveles complejos de estructuras, porque la partícula de Higgs per se no soluciona todos los problemas de la física. Así que necesitamos más, es clave pero no suficiente”.
Glashow no descarta que la famosa partícula pueda incluso no existir, aunque él da más de un 50% de probabilidad a que sí, pero reconoce que casi preferiría que no apareciese “porque entonces sabríamos que tiene que haber cosas nuevas, otras estructuras que podrían aparecer en el LHC”.
Lo que tiene claro es que el gran acelerador del CERN está funcionando “maravillosamente” y está previsto aumentar su energía en los próximos años. “Estamos muy orgullosos”, manifiesta. No se trata de una máquina científica exclusivamente europea, ya que participan en ella, además de los países miembros, instituciones de otros muchos, incluido, con una importante contribución, Estados Unidos. “Igual que los europeos contribuyen en el telescopio Hubble, nosotros participamos en el LHC, son relaciones de reciprocidad”, dice Glashow. Señala que centenares de científicos estadounidenses de numerosas instituciones y universidades (incluida la suya) forman parte de los experimentos del LHC. “Pero hubiera preferido haber podido operar a la vez un acelerador equivalente en EE UU”, indica, aunque sin nombrar el ambicioso SSC, que se comenzó a construir en Estados Unidos y que el Congreso canceló en la década de los noventa.
Si se encuentra la partícula de Higgs, ¿seguirá siendo interesante el LHC? “Sí, incluso más", asevera Glashow, "porque sabríamos que está ahí, pero no entenderíamos, por ejemplo, por qué esa partícula no puede tener una masa enorme…. Hay algo más allá de nuestra comprensión actual de la física y el Higgs sería solo un primer escalón para avanzar; sería el último escalón del Modelo Estándar de física de partículas y el primer escalón de la física de más allá del Modelo Estándar”. Desde luego, dice este gran científico, si se descubre todo el mundo estará emocionadísimo. “Llevamos buscando esta partícula 40 años, en la historia de la física no ha habido nunca una búsqueda tan larga de una partícula”. No duda de que si aparece, Peter Higgs, el físico británico que propuso su existencia, “estará muy arriba en la lista de candidatos al Premio Nobel”.
“Si me pregunta si el descubrimiento de estas partículas tiene aplicación directa, le diré que no”, señala Glashow. “La última que ha tenido utilidad es el positrón, descubierta en 1932 y aplicada en los escáneres por PET. Se han descubierto muchas después, algunas hace 60 años, y no tienen utilidad todavía”. Pero sobre las tecnologías que se desarrollan en esta disciplina científica, en los aceleradores y detectores, no tiene ninguna duda: “No sé las cifras concretas, pero el CERN tiene calculado el gran efecto económico que tienen las tecnologías que desarrollan, que suponen unos beneficios económicos enormes para los países miembros”.
miércoles, 21 de diciembre de 2011
viernes, 25 de noviembre de 2011
Quantum Theory's 'Wavefunction' Found to Be Real Physical Entity
Quantum Theory's 'Wavefunction' Found to Be Real Physical Entity
The wavefunction is a real physical object after all, say researchers.
November 17, 2011 , Nature reproduction by Scientific American
--------------------------------------------------------------------
By Eugenie Samuel Reich of Nature magazine
At the heart of the weirdness for which the field of quantum mechanics is famous is the wavefunction, a powerful but mysterious entity that is used to determine the probabilities that quantum particles will have certain properties. Now, a preprint posted online on November 14 reopens the question of what the wavefunction represents--with an answer that could rock quantum theory to its core. Whereas many physicists have generally interpreted the wavefunction as a statistical tool that reflects our ignorance of the particles being measured, the authors of the latest paper argue that, instead, it is physically real.
"I don't like to sound hyperbolic, but I think the word 'seismic' is likely to apply to this paper," says Antony Valentini, a theoretical physicist specializing in quantum foundations at Clemson University in South Carolina.
Valentini believes that this result may be the most important general theorem relating to the foundations of quantum mechanics since Bell's theorem, the 1964 result in which Northern Irish physicist John Stewart Bell proved that if quantum mechanics describes real entities, it has to include mysterious "action at a distance".
Action at a distance occurs when pairs of quantum particles interact in such a way that they become entangled. But the new paper, by a trio of physicists led by Matthew Pusey at Imperial College London, presents a theorem showing that if a quantum wavefunction were purely a statistical tool, then even quantum states that are unconnected across space and time would be able to communicate with each other. As that seems very unlikely to be true, the researchers conclude that the wavefunction must be physically real after all.
David Wallace, a philosopher of physics at the University of Oxford, UK, says that the theorem is the most important result in the foundations of quantum mechanics that he has seen in his 15-year professional career. "This strips away obscurity and shows you can't have an interpretation of a quantum state as probabilistic," he says.
Historical debate
The debate over how to understand the wavefunction goes back to the 1920s. In the `Copenhagen interpretation' pioneered by Danish physicist Niels Bohr, the wavefunction was considered a computational tool: it gave correct results when used to calculate the probability of particles having various properties, but physicists were encouraged not to look for a deeper explanation of what the wavefunction is.
Albert Einstein also favoured a statistical interpretation of the wavefunction, although he thought that there had to be some other as-yet-unknown underlying reality. But others, such as Austrian physicist Erwin Schrödinger, considered the wavefunction, at least initially, to be a real physical object.
The Copenhagen interpretation later fell out of popularity, but the idea that the wavefunction reflects what we can know about the world, rather than physical reality, has come back into vogue in the past 15 years with the rise of quantum information theory, Valentini says.
Rudolph and his colleagues may put a stop to that trend. Their theorem effectively says that individual quantum systems must "know" exactly what state they have been prepared in, or the results of measurements on them would lead to results at odds with quantum mechanics. They declined to comment while their preprint is undergoing the journal-submission process, but say in their paper that their finding is similar to the notion that an individual coin being flipped in a biased way--for example, so that it comes up 'heads' six out of ten times--has the intrinsic, physical property of being biased, in contrast to the idea that the bias is simply a statistical property of many coin-flip outcomes.
Quantum information
Robert Spekkens, a physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, who has favoured a statistical interpretation of the wavefunction, says that Pusey's theorem is correct and a "fantastic" result, but that he disagrees about what conclusion should be drawn from it. He favours an interpretation in which all quantum states, including non-entangled ones, are related after all.
Spekkens adds that he does expect the theorem to have broader consequences for physics, as have Bell's and other fundamental theorems. No one foresaw in 1964 that Bell's theorem would sow the seeds for quantum information theory and quantum cryptography--both of which rely on phenomena that aren't possible in classical physics. Spekkens thinks this theorem may ultimately have a similar impact. "It's very important and beautiful in its simplicity," he says.
This article is reproduced with permission from the magazine Nature. The article was first published on November 17, 2011.
The wavefunction is a real physical object after all, say researchers.
November 17, 2011 , Nature reproduction by Scientific American
--------------------------------------------------------------------
By Eugenie Samuel Reich of Nature magazine
At the heart of the weirdness for which the field of quantum mechanics is famous is the wavefunction, a powerful but mysterious entity that is used to determine the probabilities that quantum particles will have certain properties. Now, a preprint posted online on November 14 reopens the question of what the wavefunction represents--with an answer that could rock quantum theory to its core. Whereas many physicists have generally interpreted the wavefunction as a statistical tool that reflects our ignorance of the particles being measured, the authors of the latest paper argue that, instead, it is physically real.
"I don't like to sound hyperbolic, but I think the word 'seismic' is likely to apply to this paper," says Antony Valentini, a theoretical physicist specializing in quantum foundations at Clemson University in South Carolina.
Valentini believes that this result may be the most important general theorem relating to the foundations of quantum mechanics since Bell's theorem, the 1964 result in which Northern Irish physicist John Stewart Bell proved that if quantum mechanics describes real entities, it has to include mysterious "action at a distance".
Action at a distance occurs when pairs of quantum particles interact in such a way that they become entangled. But the new paper, by a trio of physicists led by Matthew Pusey at Imperial College London, presents a theorem showing that if a quantum wavefunction were purely a statistical tool, then even quantum states that are unconnected across space and time would be able to communicate with each other. As that seems very unlikely to be true, the researchers conclude that the wavefunction must be physically real after all.
David Wallace, a philosopher of physics at the University of Oxford, UK, says that the theorem is the most important result in the foundations of quantum mechanics that he has seen in his 15-year professional career. "This strips away obscurity and shows you can't have an interpretation of a quantum state as probabilistic," he says.
Historical debate
The debate over how to understand the wavefunction goes back to the 1920s. In the `Copenhagen interpretation' pioneered by Danish physicist Niels Bohr, the wavefunction was considered a computational tool: it gave correct results when used to calculate the probability of particles having various properties, but physicists were encouraged not to look for a deeper explanation of what the wavefunction is.
Albert Einstein also favoured a statistical interpretation of the wavefunction, although he thought that there had to be some other as-yet-unknown underlying reality. But others, such as Austrian physicist Erwin Schrödinger, considered the wavefunction, at least initially, to be a real physical object.
The Copenhagen interpretation later fell out of popularity, but the idea that the wavefunction reflects what we can know about the world, rather than physical reality, has come back into vogue in the past 15 years with the rise of quantum information theory, Valentini says.
Rudolph and his colleagues may put a stop to that trend. Their theorem effectively says that individual quantum systems must "know" exactly what state they have been prepared in, or the results of measurements on them would lead to results at odds with quantum mechanics. They declined to comment while their preprint is undergoing the journal-submission process, but say in their paper that their finding is similar to the notion that an individual coin being flipped in a biased way--for example, so that it comes up 'heads' six out of ten times--has the intrinsic, physical property of being biased, in contrast to the idea that the bias is simply a statistical property of many coin-flip outcomes.
Quantum information
Robert Spekkens, a physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, who has favoured a statistical interpretation of the wavefunction, says that Pusey's theorem is correct and a "fantastic" result, but that he disagrees about what conclusion should be drawn from it. He favours an interpretation in which all quantum states, including non-entangled ones, are related after all.
Spekkens adds that he does expect the theorem to have broader consequences for physics, as have Bell's and other fundamental theorems. No one foresaw in 1964 that Bell's theorem would sow the seeds for quantum information theory and quantum cryptography--both of which rely on phenomena that aren't possible in classical physics. Spekkens thinks this theorem may ultimately have a similar impact. "It's very important and beautiful in its simplicity," he says.
This article is reproduced with permission from the magazine Nature. The article was first published on November 17, 2011.
Universe Expands While Minds Contract
The proof is in the pudding only if you concede the fact of the pudding
By Steve Mirsky | Wednesday, November 23, 2011 | 30
The leaves are turning as I write in early October. Also turning is my stomach, from the accounts coming out of something called the Values Voter Summit in Washington, D.C. According to Sarah Posner writing online in Religion Dispatches, talk-radio host Bryan Fischer went out of his way to attack me. And probably you. Anybody, really, who accepts science as an arbiter of reality. Fischer told the assembled that America needs a president who will “reject the morally and scientifically bankrupt theory of evolution.”
Evolution is a strange process indeed, to cobble together organisms who so completely and emotionally reject it. Well, evolution concerns itself only with differential survival, and brainpower may not be a crucial factor. Fischer may as well have gotten out of a car at the convention center and proclaimed that the car had not brought him there and did not in fact exist. To thunderous applause. One’s only reasonable response to this whole scene is to bring forefinger to mouth and rapidly toggle the lips while humming, so as to produce a sound roughly in accord with a spelling of “Blblblblblblblblblb.”
A few days before the summit, over in the rational world, Saul Perlmutter won a share of the 2011 Nobel Prize in Physics. He and his fellow laureates, Adam Riess and Brian Schmidt, showed that the universe is not only expanding, the expansion is accelerating. (On hearing this news, my brother asked me if there was a limit. I told him yes, no more than three people can share any one Nobel Prize.)
Perlmutter’s Nobel led to an additional, highly coveted prize. His University of California, Berkeley—home to 22 Nobelists over the years—gives newly minted laureates a campus-wide parking permit. And, if asked, every time Perlmutter exits his car he will no doubt respond that he arrived in it and that it exists.
Perlmutter the driver also surely has the good sense to know that alcohol impairs judgment and neuromuscular skills. Contrast that mind-set with Miami Herald reporter Jose Cassola—well, former Miami Herald reporter now—who ran a stop sign shortly before Perlmutter was getting news of his Nobel and then told the cop who pulled him over, “You can’t get drunk off of vodka.”
As Cassola explained to the arresting officer: “I’m fat, I won’t be able to get drunk from only seven shots.” He later expounded on his unique theories about alcohol and its effects to media-watch reporter Gus Garcia-Roberts of the Miami New Times: “Dude, I go to Chili’s all the time and have two-for-one margaritas, and then I get in my car. Am I drunk? No!”
The disoriented mind pronouncing itself whole is always a wonder to behold. Which brings us back to the Values Voter Summit. Oddly, Fischer’s enraptured audience may have been morphologically identifiable. That notion appears in an article in the June 25, 1885, issue of the journal Nature by Charles Darwin’s half cousin Francis Galton. (It’s probably a good example of our information inundation that less than an hour after I discovered this 126-year-old article, I cannot re-create the steps by which I wound up reading it. E-mail? Twitter? Link within a link? It’s all part of the mystery.)
Galton found himself at a boring lecture and decided to study the sea of heads in front of him. He noted that “when the audience is intent each person ... holds himself rigidly in the best position for seeing and hearing.” In other words, they sit up straight. When the talk got tedious, “the intervals between their faces, which lie at the free end of the radius formed by their bodies, with their seat as the centre of rotation varies greatly.” In other words, they lean.
By all accounts, the audience at the Values Voter Summit was sitting ramrod straight, indicating great engagement with the material being presented. Although a scientific mind-set requires a consideration of another possibility: that x-rays would reveal in each attendee a stick responsible for the vertical attitude and in desperate need of removal.
Source...Scientific American
Permanent Address: http://www.scientificamerican.com/article.cfm?id=respect-for-evidence
By Steve Mirsky | Wednesday, November 23, 2011 | 30
The leaves are turning as I write in early October. Also turning is my stomach, from the accounts coming out of something called the Values Voter Summit in Washington, D.C. According to Sarah Posner writing online in Religion Dispatches, talk-radio host Bryan Fischer went out of his way to attack me. And probably you. Anybody, really, who accepts science as an arbiter of reality. Fischer told the assembled that America needs a president who will “reject the morally and scientifically bankrupt theory of evolution.”
Evolution is a strange process indeed, to cobble together organisms who so completely and emotionally reject it. Well, evolution concerns itself only with differential survival, and brainpower may not be a crucial factor. Fischer may as well have gotten out of a car at the convention center and proclaimed that the car had not brought him there and did not in fact exist. To thunderous applause. One’s only reasonable response to this whole scene is to bring forefinger to mouth and rapidly toggle the lips while humming, so as to produce a sound roughly in accord with a spelling of “Blblblblblblblblblb.”
A few days before the summit, over in the rational world, Saul Perlmutter won a share of the 2011 Nobel Prize in Physics. He and his fellow laureates, Adam Riess and Brian Schmidt, showed that the universe is not only expanding, the expansion is accelerating. (On hearing this news, my brother asked me if there was a limit. I told him yes, no more than three people can share any one Nobel Prize.)
Perlmutter’s Nobel led to an additional, highly coveted prize. His University of California, Berkeley—home to 22 Nobelists over the years—gives newly minted laureates a campus-wide parking permit. And, if asked, every time Perlmutter exits his car he will no doubt respond that he arrived in it and that it exists.
Perlmutter the driver also surely has the good sense to know that alcohol impairs judgment and neuromuscular skills. Contrast that mind-set with Miami Herald reporter Jose Cassola—well, former Miami Herald reporter now—who ran a stop sign shortly before Perlmutter was getting news of his Nobel and then told the cop who pulled him over, “You can’t get drunk off of vodka.”
As Cassola explained to the arresting officer: “I’m fat, I won’t be able to get drunk from only seven shots.” He later expounded on his unique theories about alcohol and its effects to media-watch reporter Gus Garcia-Roberts of the Miami New Times: “Dude, I go to Chili’s all the time and have two-for-one margaritas, and then I get in my car. Am I drunk? No!”
The disoriented mind pronouncing itself whole is always a wonder to behold. Which brings us back to the Values Voter Summit. Oddly, Fischer’s enraptured audience may have been morphologically identifiable. That notion appears in an article in the June 25, 1885, issue of the journal Nature by Charles Darwin’s half cousin Francis Galton. (It’s probably a good example of our information inundation that less than an hour after I discovered this 126-year-old article, I cannot re-create the steps by which I wound up reading it. E-mail? Twitter? Link within a link? It’s all part of the mystery.)
Galton found himself at a boring lecture and decided to study the sea of heads in front of him. He noted that “when the audience is intent each person ... holds himself rigidly in the best position for seeing and hearing.” In other words, they sit up straight. When the talk got tedious, “the intervals between their faces, which lie at the free end of the radius formed by their bodies, with their seat as the centre of rotation varies greatly.” In other words, they lean.
By all accounts, the audience at the Values Voter Summit was sitting ramrod straight, indicating great engagement with the material being presented. Although a scientific mind-set requires a consideration of another possibility: that x-rays would reveal in each attendee a stick responsible for the vertical attitude and in desperate need of removal.
Source...Scientific American
Permanent Address: http://www.scientificamerican.com/article.cfm?id=respect-for-evidence
Hunt for Higgs Particle Enters Endgame
Large Hadron Collider could soon deliver a clear verdict on missing boson.
November 18, 2011 | 12 Nature and reproduced by Scientific America
----------------------------------------------------------------------------------
By Geoff Brumfiel of Nature magazine
Bill Murray is a man with secrets. Along with a handful of other scientists based at CERN, Europe's particle-physics facility near Geneva, Switzerland, Murray is one of the few researchers with access to the latest data on the Higgs boson -- the most sought-after particle in physics.
Looking at his laptop, he traces a thin black line that wiggles across a shaded area at the centre of a graph. This is the fruit of his summer's labours. "It's interesting, actually, looking at this again," he muses. A tantalizing pause. "But no, I can't say..."
Despite Murray's coyness, there are few places left for the Higgs to hide. Billed as the particle that helps to confer mass on other matter, and the final missing piece in the `standard model' of particle physics, the Higgs would be a huge prize for CERN's Large Hadron Collider (LHC), the world's most powerful particle accelerator. But so far, the two massive detectors there--ATLAS, where Murray works, and the Compact Muon Solenoid (CMS) -- have not seen any convincing signals of the elusive particle.
At a conference in Paris on November 18, teams from ATLAS and the CMS experiments presented a combined analysis that wipes out a wide swathe of potential masses for the Higgs particle. Gone is the entire mass range from 141 to 476 gigaelectronvolts (GeV; energy and mass are interchangeable in particle physics). Together with earlier results from the 1990s, the analysis leaves a relatively narrow window of just 114-141 GeV in which the Higgs could lurk (see `Cornering the Higgs').
Analysis of the very latest data from this autumn--which Murray isn't yet ready to share -- will scour the range that remains. If it turns out to be empty, physicists may have to accept that the particle simply isn't there. Working around the clock, the detector teams hope to have this larger data set analysed before the end of December. "We'll know the outcome within weeks," says Guido Tonelli, spokesman for the CMS detector.
Waiting for God
The quest for the Higgs boson, often called the `God particle' after the title of a 1993 book by Nobel prizewinner Leon Lederman, is the public face of science at the LHC. Most high-energy physicists wince at the deistic designation, but they hold a near-religious devotion to the boson. Contrary to the popular view, their belief has less to do with mass than with fundamental forces.
Four fundamental forces are at work in nature: gravity, the strong nuclear force, the weak nuclear force and electromagnetism. Since the mid-1960s, physicists have strongly suspected that the weak and electromagnetic forces are actually different aspects of a single `electroweak' force. This is partly because the photon, the force-carrying particle of electromagnetism, is highly similar to the force-carrying particles of the weak force -- the W and Z bosons. Moreover, a single electroweak theory successfully predicts the interactions of fundamental particles.
There is one problem, however: the W and Z bosons are extremely heavy, nearly 100 GeV, whereas the photon is massless. To explain the difference, a number of physicists (including Peter Higgs in 1964) proposed a new field and particle. The eponymous Higgs mechanism would interact with the W and Z bosons, giving them mass, but would ignore the photon, allowing it to remain massless. Relatively straightforward tweaks to the Higgs machinery allow it to endow other particles, such as quarks, with their observed masses as well.
"The Higgs now sought at CERN is expected on the basis of the simplest picture" for the electroweak theory, says Steven Weinberg, a theorist who won a Nobel prize in 1979 for his work unifying electromagnetism and the weak force. "But there are other possibilities," he adds, reckoning the odds that the LHC's detectors will find the Higgs at 50/50 (see `Do you believe?').
If there is no Higgs, then what? Gian Giudice, a theorist at CERN, recently published work suggesting that giant clusters of W bosons might serve the same purpose, but even he admits that "it would be a great surprise if it were true". Other models without the Higgs boson invoke extra dimensions of space, but they are not yet sufficiently developed to guide experiments.
Perhaps the most likely alternative is that the Higgs is not a single particle, but rather a class of particles, which together do the job of unifying the two forces. Such a concept might appeal theoretically if a single Higgs is not found, but it would be a major headache for experimentalists to check. Theorists believe that the conventional Higgs boson would leave only a subtle mark on the detectors as it decays into W and Z bosons, high-energy photons and other particles. If there were two Higgs-like particles instead of one, the signal of each would be weaker still, says Murray. "It starts to get quite messy to do the analysis," he says.
The answer to the Higgs question lies in the data now being crunched at CERN and other academic-computing centres around the world. The first 70 trillion or so collisions turned up intriguing Higgs-like decays in the ATLAS and CMS experiments, hinting at a particle of around 140 GeV (see Nature 475, 434; 2011). But the second batch of collisions showed nothing. If the collisions now being analysed show further evidence of Higgs decays, then the teams on the two experiments are likely to announce that they have found a tentative signal, to be firmed up in 2012. If not, the search will probably continue until the LHC is shut down for an upgrade at the end of next year.
Even if that continued search shows no evidence for a Higgs or anything else, the LHC will push on. Without a unified electroweak force, the standard model is unable to predict how certain particles and forces interact inside the collider, says Matthew Strassler, a theorist at Rutgers University in Piscataway, New Jersey. The LHC will gather data on exactly those processes, and that information could potentially be used to find a way in which electromagnetism and the weak force fit together. That process, Strassler adds, is likely to take many years.
This article is reproduced with permission from the magazine Nature. The article was first published on November 18, 2011. Source Scientific American
November 18, 2011 | 12 Nature and reproduced by Scientific America
----------------------------------------------------------------------------------
By Geoff Brumfiel of Nature magazine
Bill Murray is a man with secrets. Along with a handful of other scientists based at CERN, Europe's particle-physics facility near Geneva, Switzerland, Murray is one of the few researchers with access to the latest data on the Higgs boson -- the most sought-after particle in physics.
Looking at his laptop, he traces a thin black line that wiggles across a shaded area at the centre of a graph. This is the fruit of his summer's labours. "It's interesting, actually, looking at this again," he muses. A tantalizing pause. "But no, I can't say..."
Despite Murray's coyness, there are few places left for the Higgs to hide. Billed as the particle that helps to confer mass on other matter, and the final missing piece in the `standard model' of particle physics, the Higgs would be a huge prize for CERN's Large Hadron Collider (LHC), the world's most powerful particle accelerator. But so far, the two massive detectors there--ATLAS, where Murray works, and the Compact Muon Solenoid (CMS) -- have not seen any convincing signals of the elusive particle.
At a conference in Paris on November 18, teams from ATLAS and the CMS experiments presented a combined analysis that wipes out a wide swathe of potential masses for the Higgs particle. Gone is the entire mass range from 141 to 476 gigaelectronvolts (GeV; energy and mass are interchangeable in particle physics). Together with earlier results from the 1990s, the analysis leaves a relatively narrow window of just 114-141 GeV in which the Higgs could lurk (see `Cornering the Higgs').
Analysis of the very latest data from this autumn--which Murray isn't yet ready to share -- will scour the range that remains. If it turns out to be empty, physicists may have to accept that the particle simply isn't there. Working around the clock, the detector teams hope to have this larger data set analysed before the end of December. "We'll know the outcome within weeks," says Guido Tonelli, spokesman for the CMS detector.
Waiting for God
The quest for the Higgs boson, often called the `God particle' after the title of a 1993 book by Nobel prizewinner Leon Lederman, is the public face of science at the LHC. Most high-energy physicists wince at the deistic designation, but they hold a near-religious devotion to the boson. Contrary to the popular view, their belief has less to do with mass than with fundamental forces.
Four fundamental forces are at work in nature: gravity, the strong nuclear force, the weak nuclear force and electromagnetism. Since the mid-1960s, physicists have strongly suspected that the weak and electromagnetic forces are actually different aspects of a single `electroweak' force. This is partly because the photon, the force-carrying particle of electromagnetism, is highly similar to the force-carrying particles of the weak force -- the W and Z bosons. Moreover, a single electroweak theory successfully predicts the interactions of fundamental particles.
There is one problem, however: the W and Z bosons are extremely heavy, nearly 100 GeV, whereas the photon is massless. To explain the difference, a number of physicists (including Peter Higgs in 1964) proposed a new field and particle. The eponymous Higgs mechanism would interact with the W and Z bosons, giving them mass, but would ignore the photon, allowing it to remain massless. Relatively straightforward tweaks to the Higgs machinery allow it to endow other particles, such as quarks, with their observed masses as well.
"The Higgs now sought at CERN is expected on the basis of the simplest picture" for the electroweak theory, says Steven Weinberg, a theorist who won a Nobel prize in 1979 for his work unifying electromagnetism and the weak force. "But there are other possibilities," he adds, reckoning the odds that the LHC's detectors will find the Higgs at 50/50 (see `Do you believe?').
If there is no Higgs, then what? Gian Giudice, a theorist at CERN, recently published work suggesting that giant clusters of W bosons might serve the same purpose, but even he admits that "it would be a great surprise if it were true". Other models without the Higgs boson invoke extra dimensions of space, but they are not yet sufficiently developed to guide experiments.
Perhaps the most likely alternative is that the Higgs is not a single particle, but rather a class of particles, which together do the job of unifying the two forces. Such a concept might appeal theoretically if a single Higgs is not found, but it would be a major headache for experimentalists to check. Theorists believe that the conventional Higgs boson would leave only a subtle mark on the detectors as it decays into W and Z bosons, high-energy photons and other particles. If there were two Higgs-like particles instead of one, the signal of each would be weaker still, says Murray. "It starts to get quite messy to do the analysis," he says.
The answer to the Higgs question lies in the data now being crunched at CERN and other academic-computing centres around the world. The first 70 trillion or so collisions turned up intriguing Higgs-like decays in the ATLAS and CMS experiments, hinting at a particle of around 140 GeV (see Nature 475, 434; 2011). But the second batch of collisions showed nothing. If the collisions now being analysed show further evidence of Higgs decays, then the teams on the two experiments are likely to announce that they have found a tentative signal, to be firmed up in 2012. If not, the search will probably continue until the LHC is shut down for an upgrade at the end of next year.
Even if that continued search shows no evidence for a Higgs or anything else, the LHC will push on. Without a unified electroweak force, the standard model is unable to predict how certain particles and forces interact inside the collider, says Matthew Strassler, a theorist at Rutgers University in Piscataway, New Jersey. The LHC will gather data on exactly those processes, and that information could potentially be used to find a way in which electromagnetism and the weak force fit together. That process, Strassler adds, is likely to take many years.
This article is reproduced with permission from the magazine Nature. The article was first published on November 18, 2011. Source Scientific American
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