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Long-sought Higgs particle probably found, scientists say

July 4, 2012
Courtesy of the Weizmann 
Institute of Science, CERN 
and World Science staff

The long jour­ney to de­tect an ele­men­tary par­t­i­cle known as Higgs bos­on, which started a quar­ter of a cen­tury ago, might fi­nally have reached its goal, phys­i­cists say.

Sci­en­tists at the Eu­ro­pe­an Lab­o­r­a­to­ry for Par­t­i­cle Phys­ics, CERN, near Ge­ne­va an­nounced on Wed­nes­day that a part­i­cle fit­ting the correct des­crip­tion had been found, though there remains the un­like­ly poss­ibi­lity it is a mis­i­dent­i­fi­ca­tion.

The white circle marks where the tracks of the Large Had­ron Col­lider lie un­der­ground. (Cour­tesy CERN)


“We have reached a mile­stone in our under­stand­ing of na­ture,” CERN director general Rolf Heuer said. “The discovery of a particle consistent with the Higgs bo­son opens the way to more de­tailed studies... which will pin down the new par­ticle’s pro­per­ties, and is likely to shed light on other mys­ter­ies of our uni­verse,” he added.

The Higgs bos­on is the fi­nal build­ing block that has been mis­sing from the “Stan­dard Mod­el,” a work­ing pic­ture of na­ture that main­stream physicists rely on and that de­scribes the struc­ture of mat­ter in the uni­verse. The Higgs bos­on com­bines two forc­es of na­ture and indi­cates that they are, in fact, dif­fer­ent as­pects of a more fun­da­men­tal force. The par­t­i­cle is al­so re­spon­si­ble for the ex­ist­ence of mass in el­e­men­ta­ry par­t­i­cles—the qua­lity we feel as weight.

Phys­i­cists have been hop­ing for an overarch­ing theory of na­ture that can uni­fy the four basic forc­es known in the uni­verse: the weak force re­spon­si­ble for ra­dioac­ti­vity; the elec­tro­mag­netic force; the strong force re­spon­si­ble for the ex­ist­ence of pro­tons and neu­trons, the core of the atom; and gra­vi­tat­ion.

The first step in the jour­ney to un­ify the forc­es would be com­plet­ed with the dis­cov­ery of the Higgs par­t­i­cle: the un­ion of two el­e­men­ta­ry forc­es – the elec­tro­mag­netic and weak force, to be­come the “elec­tro­weak” force. One as­pect of the Higgs bos­on, named af­ter the Scot­tish phys­i­cist Pe­ter Higgs, man­i­fests it­self in the giv­ing of mass to the car­ri­ers of the weak force, known as “W” and “Z” par­t­i­cles.

In the ef­fort to dis­cov­er the Higgs bos­on, un­ify the fun­da­men­tal forc­es and un­der­stand the or­i­gin of mass in the uni­verse, sci­en­tists built the world’s larg­est ma­chine. It’s a par­t­i­cle ac­cel­er­a­tor nes­tled in a 27-km- (17 miles-) long cir­cu­lar tun­nel, 100 me­ters or yards be­neath the bor­der be­tween France and Switz­er­land, in the Eu­ro­pe­an par­t­i­cle phys­ics lab­o­r­a­to­ry, CERN, near Ge­ne­va.

This ac­cel­er­a­tor, the Large Had­ron Col­lider, speeds up beams of pro­tons, sub­a­tom­ic par­t­i­cles in the co­re of the at­om, up to 99.999998 per­cent the speed of light. Ac­cord­ing to Ein­stein’s the­o­ry of rel­a­ti­vity, this in­creases their mass by 7,500 times. The ac­cel­er­a­tor aims the beams straight at each oth­er, caus­ing collisi­ons that re­lease so much en­er­gy, the pro­tons them­selves ex­plode. For much less than the blink of an eye, conditi­ons si­m­i­lar to those that ex­isted in the uni­verse in its first fracti­on of a sec­ond are found in the ac­cel­er­a­tor.

As a re­sult, par­t­i­cles of mat­ter are turned in­to en­er­gy, in ac­cord­ance with Al­bert Ein­stein’s fa­mous equ­ation de­scrib­ing the conversi­on of mat­ter in­to en­er­gy: E=mc2. The sys­tem then cools back down, and en­er­gy turns back in­to par­t­i­cles.

The collisi­ons pro­duce en­er­get­ic par­t­i­cles, some of which ex­ist for ex­tremely short pe­ri­ods of time. The only way to dis­cern their ex­ist­ence is to iden­ti­fy the foot­prints they leave be­hind. For this pur­pose, a va­ri­e­ty of par­t­i­cle de­tectors were de­vel­oped, each op­ti­mized for cap­tur­ing par­tic­u­lar types of par­t­i­cles.

The sci­en­tists an­a­lyzed da­ta from a thou­sand trilli­on pro­ton collisi­ons. In these, Higgs bos­ons are ex­pect­ed to arise along with many oth­er si­m­i­lar par­t­i­cles. Ev­i­dence to sug­gest the ex­ist­ence of the Higgs arises through searches for anoma­lies in the col­lect­ed da­ta in com­par­i­son with the ex­pected da­ta if such a par­t­i­cle does not ex­ist. This search fo­cus­es on the es­ti­mat­ed mass of the par­t­i­cle: 126 trilli­on elec­tron volts (Gev), the un­its of mass used for at­omic-scale par­t­i­cles. When the sci­en­tists do man­age to find such anoma­lies, they must then rule out the pos­si­bil­ity that it is due to sta­tis­ti­cal fluc­tu­ati­on.

The cal­cul­ati­ons car­ried out by sci­en­tists in re­cent weeks have re­vealed, with a high de­gree of sta­tis­ti­cal sig­nif­i­cance, a new par­t­i­cle with a mass si­m­i­lar to the ex­pected mass of the Higgs, the re­search­ers an­nounced. Their word­ing is pur­posely cau­tious, leav­ing room for the pos­si­bil­ity that a new par­t­i­cle oth­er than the Higgs can be found with­in this mass range. 

The prob­a­bil­ity that this is, in­deed, an­other par­t­i­cle, is quite low, they added, but if that does turn out to be the case that could point to some inter­est­ing sci­en­tific poss­i­bili­ties in its own right. “We stated last year that in 2012 we would either find a new Higgs-like particle or ex­clude the exis­tence of the Stand­ard Model Higgs. With all the ne­ces­sary cau­tion, it looks to me that we are at a branch­ing point,” said CERN Re­search Dir­ec­tor Ser­gio Ber­to­luc­ci.

“This is the biggest day of my life,” said physi­cist Eilam Gross of the Weiz­mann Institute of Sci­ence in Reho­vot, Israel, a se­nior member of the in­tern­ational scien­tific team in­volved in the re­search. “I have been search­ing for the Higgs since I was a student in the 1980's. Even after 25 years, it still came as a sur­prise. No matter what you call it – we are no long­er searching for the Higgs but meas­uring its prop­erties.”

* * *

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The long journey to detect a subatomic particle known as Higgs boson, which started with one small step about 25 years ago, might finally have reached its goal, physicists say. The findings was was reported by Large Hadron Collider particle accelerator scientists today at the European Laboratory for Particle Physics, CERN, near Geneva. The Higgs boson is the final building block that has been missing from the “Standard Model,” which describes the structure of matter in the universe. The Higgs boson combines two forces of nature and shows that they are, in fact, different aspects of a more fundamental force. The particle is also responsible for the existence of mass in the elementary particles. Most of us experience the world as a diverse and complex place. But the physicists among us are not content with visible reality. They are striving to get to the bottom of that reality and to see whether it is, as they think, based on the absolute simplicity displayed by the early universe. They expect to observe a range of particles that are different “ensembles” of a handful of elementary particles. The scientists are hoping to see a unification of the four fundamental forces of nature that act on these particles (the weak force responsible for radioactivity, electromagnetic force, the strong force responsible for the existence of protons and neutrons, and gravitation). The first step in the journey to unify the forces was completed with the almost certain discovery of the Higgs particle: The union of two elementary forces – the electromagnetic and weak force, to become the electroweak force. One aspect of the Higgs boson, named after the Scottish physicist Peter Higgs, manifests itself in the giving of mass to the weak force carriers – the “W” and “Z” particles. In the effort to discover the Higgs boson, unify the fundamental forces and understand the origin of mass in the universe, scientists built the world’s largest machine: a particle accelerator nestled in a 27-km-long circular tunnel, 100 meters beneath the border between France and Switzerland, in the European particle physics laboratory, CERN, near Geneva. This accelerator, the Large Hadron Collider, accelerates beams of protons, subatomic particles in the core of the atom, up to 99.999998% the speed of light. According to Einstein’s theory of relativity, this increases their mass, or weight, by 7,500 times. The accelerator aims the beams straight at each other, causing collisions that release so much energy, the protons themselves explode. For much less than the blink of an eye, conditions similar to those that existed in the universe in the first fraction of a second after the Big Bang are present in the accelerator. As a result, particles of matter are turned into energy, in accordance with Albert Einstein’s famous equation describing the conversion of matter into energy: E=mc2. The energy then propagates through space and the system cools. Consequently, energy turns back into particles of matter and the process is repeated until particles that can exist in reality as we know it are formed. The collisions produce energetic particles, some of which exist for extremely short periods of time. The only way to discern their existence is to identify the footprints they leave behind. For this purpose, a variety of particle detectors were developed, each optimized for capturing particular types of particles. The scientists analyzed data from a thousand trillion proton collisions; in these Higgs bosons are created along with many other similar particles. Evidence to suggest the existence of the Higgs arises through searches for anomalies in the collected data in comparison with the expected data if such a particle does not exist. This search focuses on the estimated mass of the particle: 126 trillion electron volts (Gev), the units of mass used for atomic-scale particles. When the scientists do manage to find such anomalies, they must then rule out the possibility that it is due to statistical fluctuation. The calculations carried out by scientists in recent weeks have revealed, with a high degree of statistical significance, a new particle with a mass similar to the expected mass of the Higgs, the researchers announced. The wording is purposely cautious, leaving room for the possibility that a new particle other than the Higgs can be found within this mass range. The probability that this is, indeed, a new particle, is quite low, they added.