ANCORA ...SULLA PRESUNTA "RILEVAZIONE" DEL BOSONE DI HIGGS

CON PIACERE PUBBLICHIAMO IL POST DELL'AMICO E COLLABORATORE DOTT. MIGUEL LUNETTA (FISICO), SULLA "CONTROVERSA" QUESTIONE DEL BOSONE DI HIGGS.

I FREQUENTATORI DEL BLOG SONO AL CORRENTE DEL MIO PENSIERO SULL'ARGOMENTO, QUINDI SENZA ULTERIORI COMMENTI, VI INVITO A LEGGERE CON ATTENZIONE LA DETTAGLIATA RELAZIONE DI MIGUEL SUL TORTUOSO PERCORSO CHE, INFINE, HA PORTATO IL NOBEL PER LA FISICA A PETER HIGGS.

BUONA LETTURA

MLR

On Higgs boson detection

di: Dott. M. Lunetta

Abstract: Initially we talk on Higgs’ prediction, standard model, Higgs field properties and describe particle accelerators. Successively we explain why the Higgs boson gained the nickname of God particle, why the Standard Model is refuted, and we described the Tevatron and LHC experiments. Then, after mentioning the Higgs boson production, the collision of two proton beams and the evaluation of standard model mass, we conclude reporting how the Tevatron found Higgs boson.

Résumé: Nous parlons initialement sur la prévision de Higgs, le modèle standard, les accélérations de particules. Ensuite, nous expliquons pourquoi le boson de Higgs gagna le nom de particule de Dieu, pourqoui le modèle standard fut réfuté, et nou décrions les expériences du Tevatron et du LHC. Ainsi, après mentionner la production du boson de Higgs, le choc de deux rayons de protons et le calcule de la mass du modèle standard, nous concluions expliquant comment le Tevatron détecta le boson de Higgs.

Key words : Higgs boson, standard model, particle accelerator, God particle, Tevatron, Large Hadron Collider (LHC), Big Bang, Large Electron-Positron Collider (LEP), Giant particle detectors ATLAS and CMS, Nuclear Physics Laboratory of High Energies (LPNHE).

I.             INTRODUCTION

A.           Higgs prediction

The Higgs particle is postulated as a part of the mechanism by which all other fundamental particles acquire mass. The theory behind it was

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published by six physicists - including Edimburg University physicist Peter Higgs - within a few months of each other in 1964(1). Higgs idea was that the universe contains an as yet undiscovered field. Interaction with what is now called the Higgs field gives particles their mass and would explain why some particles have high mass (that is are heavy) and some particles have almost no mass. “Higgs bosons would not naturally exist in the universe today”, says David Krofcheck, a physicist from the University of Auckland(2).

Physicists therefore, have the challenge of searching for a theoretical particle that may have briefly existed immediately after the Big Bang that created the universe. The Higgs boson is a hypothetical elementary particle which belongs to a class of subatomic particles known as bosons, characterized by an integer value of their spin quantum number. The Higgs boson is an excitation of the Higgs field above its ground state(3).

B.            Standard model

The existence of the Higgs boson is predicted by the Standard Model to explain how spontaneous breaking of electroweak symmetry (the Higgs mechanism) takes place in nature, which in turn explain why other elementary particles have mass. Only 1% of the mass of composite particles, such as the proton and neutron, is due to the Higgs mechanism. The other 99% is due to the strong interaction (3). “If you discover there is some particle at 120 GeV, to be absolutely sure it is the Standard Model Higgs which we think it is, one would have to measure precisely how it is produced and how it decays to determine its properties,” said Söldner-Rembold. “That’s quite a way down the road because, for that, you need much more data. There still remains quite a bit of work to be done to be absolutely sure that it is actually the Higgs boson as the Standard Model predicts it.”(1)

The Higgs boson is the key piece of the Standard Model, an ambitious suite of equations that has ruled the universe of high-energy physics for the last few decades, explaining how three of the four fundamental forces of nature work. But the boson itself has never been observed. The theory describes how it should work and behave but does not predict one of its key attributes, namely its mass.(4)

C.            Higgs field properties

The theory hypothesizes that a sort of lattice, referred to as the Higgs field, fills the universe. This is something like an electromagnetic field, in that it affects the particles that move throughout, but it is also related to the physics of solid materials. Scientists know that when an electron passes through a positively charged crystal lattice of atoms (a solid), the electron’s mass can increase as much as 40 times. The same might be true in the Higgs field: a parting moving through it creates a little bit of distortion – like the croud around the star at the party – and that lends mass to the particle.

The Standard Model describes three of nature’s  four forces: electromagnetic and the strong weak nuclear forces. Electromagnetism has been fairly well understood for many decades. Recently, physicists have learned much more about the strong force, which binds the elements of atomic nuclei together, and the weak force, which governs radioactivity and hydrogen fusion (which generates the sun’s energy).

Electromagnetism describes how particles interact with photons, tiny packets of electromagnetic radiation. In a similar way, the weak force describes how or three entities, the W and Z particles, interact with electrons, quarks, neutrinos and others. There is one very important difference between these two interactions: photons have no mass, while the masses as W and Z are huge. In fact, they are some of must massive particles known.

The first inclination is to assume that W and Z simply exist and interact with other elemental particles. But, for mathematical reasons, the giant masses of W and Z raise inconsistencies in the Standard Model. To address this, physicists postulate that there must be at least one other particle – the Higgs boson. The simplest theories predict only one boson, but others say there might be several. In fact, the search for the Higgs particle(s) is some of the most exciting research happening, because it could lead to completely new discoveries in particle physics. Some theorists say it could bring to light entirely new types of strong interactions, and others believe research will reveal a new fundamental physical symmetry called “supersymmetry.”

First, Though, scientists want to determine whether the Higgs boson exists. The search has been on for over ten years both at CERN’s Large Electron Positron Collider (LEP) in Geneva and at Fermilab in Illinois. To look for the particle, researchers must smash other particles together at very high speeds. If the energy from that collision is high enough, it is converted into smaller bits of matter - particles – one of which could be a Higgs boson. The Higgs will only last for a small fraction of a second, and then decay into other particles. So in order to tell whether the Higgs appeared in the collision, researchers look for evidence of what it would have decayed into.(5)

D.           Particle accelerators

The particle accelerators performing experiments to find out whether the Higgs boson exists are the following:

1.            Large Hadron Collider (LHC) at CERN, in Geneva, Switzerland.

2.            Large Electron – Positron Collider (LEP) at CERN, by detector ALEPH.

3.            Tevatron particle collider in the US.

4.            Fermi National Laboratory in Batavia, Illinois.

5.            Giant particle detectors ATLAS and CMS from the CERN collider.

6.            Nuclear Physics Laboratory of the High Energies (LPNHE) in Paris.

II.            DISCUSSION

A.           Why the God particle?

In the popular media, the particle Higgs boson is sometimes referred to as the God particle, a title generally disliked by the scientific community as media hyperbole that misleads readers.(6)The Higgs boson is often referred to as ‘the God particle’ by the media(7) after the title of Leon Lederman’s popular science book on particle physics, The God Particle. If the Universe Is the Answer, what is the Question?(8, 9) While the use of this term may have contributed to increased medi9a interest, many scientists dislike it, since it overstates the particle’s importance, not least since its discovery would still leave unanswered questions about the unification of QCD, the electroweak interaction and gravity, and the ultimate origin of the universe.

Lederman said he gave it the nickname “The God Particle” because the particle is “so central to the state of physics today, so crucial to our understanding of the structure of matter, yet so elusive,(10) but jokingly added that a second reason was because “the publisher wouldn’t let us call it the Goddamn Particle, through that might be a more appropriate title, given its villainous nature and the expense it is causing.”

A renaming competition conducted by the science correspondent for the British Guardian newspaper chose the name “the champagne bottle boson” as the best from among their submissions: “The bottom of a champagne bottle is in the shape of the Higgs potential and is often used as an illustration in physics lectures. So it’s not are embarrassingly grandiose name, it is memorable, and [it] has some physics connection too.”(11)

B.            Refutation of Standard Model

Physicists claim evidence of a new particle only if the probability that the data could be due to a statistical fluctuation is less than 1 in 740, or three sigmas. A discovery is claimed only if that probability is less than 1 in 3.5 million, or five sigmas. This result sits well within the stringent constraints established by earlier direct and indirect measurements made by CERN’s Large Hadron Collider, the Tevatron, and other accelerators, which place the mass of the Higgs boson within the range of 115 to 127 GeV. These findings are also consistent with the December 2011 announcement of excesses seen in that range by LHC experiments, which searched for the Higgs in different decay patterns. None of the hints announced so far from the Tevatron or LHC experiments, however, are strongly enough to claim evidence for the Higgs boson.(12)

Theories that go beyond the “standard model” of particle physics (of which the Higgs is the keystone – the one missing piece needed to explain how the universe we know come to be) may be necessary. Steven Weinberg, who in this landmark 1967 paper on the unification of the electromagnetic and weak interactions had made key use of the Higgs for “breaking the symmetry” and separating the electromagnetic from weak forces, has since gone beyond the standard model in his research.

Weinberg has proposed a theory called Technicolor, within which the primeval symmetry o0f our universe can be broken through a different mechanism than the action of the elusive Higgs. But to prove the validity of the Technicolor theory may require an energy level that would  dwarf that available to the LHC – at an equal astronomical cost.(13)

From recent data of Fermilab in Batavia, Ill. It results that Higgs boson has not been discovered yet, but its mass is 125 billion electron volts. If the Higgs does not exist, they will have to come up with a new model of how the universe works. If they do find the Higgs, studying it might give then clues to deeper mysteries the Standard Model does not solve.

The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and 180 GeV/c2, then the Standard Model can be valid at energy scales all the way up to the Planck scale (1015TeV). Many theorists expect new physics beyond the Standard Model to emerge at the TeV scale, based on unsatisfactory properties of the Standard Model. The higher possible mass scale allowed for the Higgs boson (or some other electroweak symmetry breaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such a mechanism, because unitarity is violated in certain scattering processes.

On 7 March 2012, the DØ and CDF Collaborations announced that, after analyzing the full data set from the Tevatron accelerator, they found excesses in their data might be interpreted as coming from a Higgs boson with a mass in the region of 1/5 to 135 GeV/c2. It is expected that the LHC provide sufficient data to either exclude or confirm the existence of the Standard Model Higgs boson by the end of 2012.(14)

C. Description of Tevatron and LHC experiments.

One reason the Tevatron results are important is that there look for the Higgs in a different way to the detectors at the LHC. In the LHC’s announcement in December,, scientists had largely been looking for a signal in which the Higgs boson decayed into a pair of photons, whereas the Tevatron located its signal in the decay of the boson into a pair of heavy “bottom quarks”.

The Higgs particle can decay into different particles and the Tevatron looks for different decays of the Higgs than the LHC. The Large Hadron Collider (LHC), in Geneva, smashes protons together at high energy, simulating in the laboratory the conditions that occurred in the early universe, a trillionth of a second after the Big Bang. This is the epoch when the Higgs boson is theorized to have been abundant and to have seeded the structured universe within which we now exist. More than four decades after the ideas were conceived, the experiments began in earnest last year. On December 13 came the news that something exciting was beginning to show – perhaps a Higgs Boson. However, no one seemed prepared to go the full distance and claim that it has been discovered, at least not yet.

The LHC is currently operating at only half its planned power. So if this is indeed the long sought Higgs, this offers the prospect that a wealth of discoveries may be within the LHC’s reach when operating at full capacity. The hints are also consistent with the theories that predict the Higgs to be but one number of new families of particles, including the seeds of the mysterious “dark matter” throughout the cosmos.(15)

Colliding protons remain intact but still generate new particles, according to results from Fermilab. A similarly clean process could produce the elusive Higgs particle at CERN’s Large Hadron Collider (LHC). Particle physicists at LHC hope to discover the Higgs boson amid the froth of particles born from proton – proton collisions. Results in the 19 June Physical Review Letters show that there may be a way to cut through some of that froth. An experiment at Fermilab’s proton – antiproton collider in Illinois has identified a rare process that produces matter from the intense field of the strong nuclear force but leaves the proton and antiproton intact. There’s a chance the same basic interaction could give LHC physicists a clean look at the Higgs.

In the new experiment, researchers were looking for signs that the interaction of virtual gluons had generated short – lived particles including the Xc(Chi-c) and J/ψ mesons, which are charm – anticharm quark pairs that decay into muons and antimuons. Now the team has sifted through nearly 500 muons – antimuons pairs, identifying 65 that must have come from the decay of the Xc – very close to the rate predicted in 2005 by a team at Durham University in England.(16)

Because the Xc has similar particle properties to the much heavier Higgs boson, the same basic reaction should produce the Higgs at the higher collision energies provided by the LHC, says Albrow. “It’s the strongest evidence that the Higgs boson must be produced this way, if it does exist.” Based on the rate of Xc production, Albrow estimates LHC collisions could produce 100 to 1000 Higgs bosons per year in each of the accelerator’s two largest particle detectors, ATLAS and CMS.

“It looks hard, but one should never say never,” says Joseph Incandela of the University of California, Santa Barbara, deputy physics coordinator for CMS. Incandela points out that once the LHC is operating at full capacity, every crossing of its twin proton beams is expected to yeld about 20 collisions, throwing up other particles that may obscure exclusive reactions. If a Higgs boson is created in a high – energy particle collision, it immediately decays into lighter more stable particles before even the world’s best detectors and fastest computers can snap a picture of it. To find the Higgs boson, physicists retraced the path of these secondary particles and ruled out processes that mimic its signal.

III. METHODOLOGY

A.  Production of Higgs boson

Unknown Lamer on Wednesday March 07, 10:36 AM, 2012 writes with exciting news from the world of the particle physics: “A hint of the Higgs boson, the missing piece in the standard model pf particle physics, has been found in data collected by the Tevatron, the now – shuttered U.S. particle collider at Fermilab in Batavia, Illinois. While not statistically significant enough in themselves to count as a ‘discovery’, the indications announced on 7 March at the Moriond conference in La Thuile, Italy, are consistent with 2011 reports of a possible standard model Higgs particle with a mass of around 125 GeV from experiments at the Large Hadron Collider at CERN near Geneva, Switzerland. The data is more direct evidence of the Higgs than the constraints in its mass offered by the precise W boson mass measurement reported on Monday. On a sad note, the find vindicates Tevatron scientists who campaigned unsuccessfully to extend the collider’s run. The request was turned down by the Department of Energy but this last hurrah suggests that Tevatron might indeed have found the Higgs ahead of CERN’s Large Hadron Collider if they’d recued the funding required. The Tevatron is currently being raided for parts.”

New measurements announced today’(March 7, 2012) by scientists from CDF and DZero collaborations at the U.S. Department of Energy’s Fermi National Accelerator Laboratory indicate that the elusive Higgs boson may nearly be cornered. After analyzing the full data set from the Tevatron accelerator, which completed its last run in September 2011, the two independent experiments see hints of a Higgs boson.(17)

“The end game is approaching in the hunt for Higgs boson,” said Jim Siegrist, DOE Associate Director of Science for High Energy Physics. “This is an important milestone fo9r the Tevatron experiments, and demonstrates the continuing importance of independent measurements in the quest to understand the building blocks of nature.” Fermilab scientist Don Lincoln describes the concept of how the search for the Higgs boson is accomplished.

B. Collision of proton beams

The Large Hadron Collider (LHD), with a circumference of 27 Km, boosts protons to high speeds, smashes them together, and uses sophisticated detectors to see what results from the high – speed, High – temperature collisions that mimic collisions one – thousand of a billionth of a second after the Big Bang.

The search for Higgs boson has been on for over ten years, both at CERN’s Large Electron Position Collider (LEP) in Geneva and at Fermilab in Illinois. To look for the particle, researchers must smash other particles together at very high speeds. If the energy from that collision is high enough, it is converted into smaller bits of matter – particle – one of which could be a Higgs boson. The Higgs will only last for a small fraction of a second, and then decay into other particles. In 2011, the LHC collided two proton beams at 7 tera-electronvolts (TeV). On average more than 100 million collisions per second took place, but most of those collisions were “conventional”, in that they involved only the most common elementary particles. If the standard model is correct, then on rare occasions, a Higgs boson should have been produced.

Assuming the Higgs mass lies in the expected mass range, about 75,000 Higgs bosons were produced in the ATLAS and CMS collider detections in 2011.

C.Evaluation of standard model mass

There may be a 95% chance that the Higgs does not exist between 146 – 446 GeV, but that it exists between 140 – 145 GeV.

Three papers now appearing in Physical Review Letters, two from ATLAS Collaboration(18) and one from CMS Collaboration(19) report searches for the Higgs boson in the debris of high – energy  proton – proton collisions at LHC. The results of these searches, and several others being published elsewhere(20), were announced in December, 2011. Collectively, they have shown that the Higgs boson of the standard model, if it exists, must be lie in a narrow range of masses around 126 giga – electron – volts (GeV). Moreover, an excess of events around this mass value provides a tantalizing hit that experimentalist could be only the verge of discovering the long – sought particle.

From experiments at the CERN  LEP colliders(21) which shut down in 2000, it is known that the standard model Higgs boson mass cannot be below 114GeV, while subsequent data from the Fermilab Tevatron(22), which shut down  in 2011, excludes a Higgs boson mass between 156 and 177 GeV.

Based on a statistical analysis of LEP and Tevatron colliders, the Higgs mass cannot be larger than 169 GeV((23). Combining the direct Higgs research limits quoted above with the indirect constraints implies that the standard model is viable if, and only if, the standard model Higgs boson mass lies between 114 and 156 GeV(24).

The results of two recent experiments at the Large Hadron Collider (LHC) near Geneva suggest physicists are close to discovering the Higgs boson, the so – called God particle. Combined, the two experiments have narrowed the possible band of possible Higgs boson masses to between 115 and 130 GeV (gigaelectron volt). Rather than look directly for this fleeting would – be particle itself, physicists look for the various combination of particles  into which Higgs bosons are though to decay. Independent analyses have verified excesses of these particles from the low mass region 124 to 126 GeV.(25)

IV.CONCLUSIONS

A.Tevatron might found Higgs boson

Last December, two groups, which run giant particle detectors named ATLAS  and C.M.S. from the CERN collider, reported that they have found promising bumps in their data at masses of 124 billion electron volts and 126 billion electron volts, respectively, those being the units of mass or energy preferred by particle physicists.

The Fermilab physicists have found a broad hump in their data in the same region, between 115 billion and 135 billion electron volts. Those results came from combining the data from two detectors operated on the Tevatron: the Collider Detector at Fermilab, and DZero. The chances of this signal being the result of a random fluctuation in the data where only about 1 in 100, the group said.

This is the first time in the long search for the particle that  different groups, indeed different colliders, are in vague agreement. It has led to a joke in physics circles now: the Higgs boson has not been discovered yet, but its mass is 125 billion electron volts.

Physicists base in the US have presented evidence of the Higgs boson particle that correlates closely with European researchers’ work at the  Large Hadron Collider.(26)

The data, from the Tevatron particle collider, was presented at a physics conference in Italy, and indicate  that the particle could exist at a mass of between 115 gigaelectronvolts and 135 gigaelectronvolts. This result is consistent  with last December’s finding from CERN’s Large Hadron Collider in Switzerland, wich narrowed down the Higgs Boson’s mass to about 125 gigaelectronvolts.

While not statistically significant enough in themselves to count as a ‘discovery’, the indication announced on 7 March at the Moriond conference in La Thuile, Italy, are consistent with 2011 reports of a possible standard model Higgs particle with a mass of around 125 GeV from experiments at the Large Hadron Collider at CERN near Geneva, Switzerland.(27)

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14CERN press release # 25.11.13 December 2011, “The statistical significance is not large enough to say anything conclusive. As of today what we see is consistent either with a background fluctuation or with the presence of the boson. Refined analyses and additional data delivered in 2012 by this magnificent machine will definitely give an answer”.

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24(http://teonphwg.fnal.gov/results/SM_Higgs_Winter_12/).

25James Holloway, LHC physicists sniff Higgs boson discovery, 04: 59 December 14, 2011.

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27Unknown Lamer, Final Analysis Suggests Tevatron Saw Hint of the Higgs Boson, Slashdot, Wednesday March 07, 10: 36 AM, 2012, from the America-hates-science dept.