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Findings uphold “Standard Model,” for now

April 13, 2007
Special to World Science  

Long-awaited in­i­tial re­sults from a dec­ade-long ex­pe­ri­ment seem to have res­cued—for now—the “stan­dard mod­el” of phys­ics, used by sci­ent­ists as a work­ing blue­print of real­ity.

The Mini­BooNE ex­per­i­ment re­lies on a 250,000-gallon tank filled with min­er­al oil, which is clear­er than wa­ter from a fau­cet. Light-sensitive de­vices mount­ed in­side the tank de­tect col­li­sions be­tween neu­tri­nos and car­bon nu­clei of oil mo­le­cules. (Cour­te­sy Fer­mi­lab)
“Sci­en­tists eve­ry­where have been ea­ger­ly wait­ing for our re­sults,” said Ja­net Con­rad of Co­lum­bia Uni­ver­si­ty, spokes­wom­an for the pro­ject, dubbed Mi­ni­BooNE. The ex­pe­ri­ment  in­ves­t­i­gated ghost­ly fun­da­men­tal par­t­i­cles called neu­tri­nos.

The Mini­BooNE team, a group of 77 re­search­ers from in­sti­tu­tions in the Unit­ed States and the U.K., an­nounced pre­lim­i­nar­y find­ings this week.

The so-called stand­ard mod­el—the con­sen­sus view of phys­i­cists on the struc­ture of mat­ter—holds that the uni­verse con­tains just 16 fun­da­men­tal par­t­i­cles, along with a crew of “antipar­t­i­cles” that are sorts of evil twins of the bas­ic par­t­i­cles. 

Whle the mod­el serves as a re­li­a­ble guide for ex­pe­ri­men­ta­tion, phys­i­cists ful­ly ex­pect it to be su­per­sed­ed by a more com­plete the­o­ry some­day, since it fails to ex­plain some key phe­nom­e­na in­clud­ing grav­i­ty.

None­the­less, simp­ly find­ing the mod­el wrong would­n’t au­to­mat­i­cally pro­vide the way for­ward. A set of find­ings in the 1990s would, if con­firmed, have spelled a rath­er messy end for the mod­el. At that time, ex­pe­ri­ments at the the U.S. De­part­ment of En­er­gy’s Los Alamos Na­tion­al Lab­o­r­a­to­ry turned up sur­pris­ing ob­ser­va­tions about neu­tri­nos, which are al­most un­de­tect­a­ble, near­ly weight­less par­t­i­cles.

Three types or “fla­vors” of neu­tri­nos are known: elec­tron, mu­on and tau neu­tri­nos. In the last dec­ade, ex­pe­ri­ments have found that neu­tri­nos can os­cil­late be­tween dif­fer­ent fla­vors. The Los Alamos find­ings, based on a de­vice called the Liq­uid Scin­til­la­tor Neu­tri­no De­tec­tor, were among those that sug­gested the pres­ence of such os­cil­la­tions. But it in­volved neu­tri­nos in a range of mass­es vast­ly dif­fer­ent from oth­er ex­pe­ri­ments.

Rec­on­cil­ing those find­ings with the oth­er os­cil­la­tion re­sults would have re­quired the pres­ence of a fourth, or “ster­ile” type of neu­tri­no, with un­u­su­al prop­er­ties. The ex­ist­ence of ster­ile neu­tri­nos would have thrown se­ri­ous doubt on the Stand­ard Mod­el.

The Mini­BooNE ex­pe­ri­ment at the En­er­gy De­part­ment’s Fer­mi­lab in Ba­ta­via, Ill., has shown there is more to the sto­ry, which is good news for the Stand­ard Mod­el, in­ves­ti­ga­tors in the work say. 

The re­search­ers mim­icked the Los Alamos ex­pe­ri­ments by look­ing for signs of neu­tri­no os­cil­la­tions in the mass range stud­ied ear­li­er. They found no ev­i­dence of os­cil­la­tions be­tween mu­on and elec­tron neu­tri­nos, as a sim­ple in­ter­pre­ta­tion of the Los Alamos data in­di­cat­ed, they said.

“It was very im­por­tant to ver­i­fy or re­fute” that ear­li­er re­sult, said Rob­in Staf­fin, as­so­ci­ate di­rec­tor of sci­ence for high en­er­gy phys­ics at the De­part­ment of En­er­gy. How­ev­er, Mini­BooNE in­ves­ti­ga­tors found some oth­er phe­nom­e­na at low en­er­gy ranges that de­fied ex­pectations, and which will re­quire some ex­plain­ing, they said.

“It clears one mys­tery but it leaves us with a puz­zle,” said Fer­mi­lab Di­rec­tor Pier Odd­one. The Mini­BooNE col­la­bo­ra­tion will fur­ther an­a­lyze its da­ta, he added.

Re­search­ers said that fur­ther Mini­BooNE re­sults still have the po­ten­tial to up­set the Stand­ard Mod­el, but this looks more un­like­ly than be­fore. “The pic­ture we have of neu­tri­no os­cil­la­tions looks se­cure,” Da­vid Wark of the U.K.’s Ruth­er­ford Ap­ple­ton Lab­o­r­a­to­ry in Did­cot, U.K. told the In­sti­tute of Phys­ics Pub­lish­ing’s Phys­ics Web web­site at physicsweb.org. Thus “we can pro­ceed with­out a trou­bled consci­ence to the next genera­tion of par­t­i­cle phys­ics ex­pe­ri­ments.”

Sci­en­tists con­sid­er neu­tri­nos to be among the more fas­ci­nat­ing par­t­i­cles of na­ture.

“You can’t see them, hear them, or tou­ch them, but neu­tri­nos are eve­ry­where,” Con­rad said. “They pass right by us and right through us. They can trav­el the dis­tance of 200 Earths lined up be­fore they hit an­y­thing, and if you put your hand on the desk­top and count to three, tril­lions will pass through it. And they are pro­duced in many ways—by the sun, or when stars ex­plode, or by us us­ing par­t­i­cle ac­cel­er­a­tors. So, it is im­por­tant for us to un­der­stand their na­ture and how they be­have.”

Mini­BooNE utilizes a de­tec­tor made of a 250,000-gallon tank filled with ul­tra­pure min­er­al oil, clear­er than wa­ter from a fau­cet. Light-sensitive de­tec­tors rec­ord ti­ny bursts of light from oc­ca­sion­al col­li­sions be­tween neu­tri­nos and car­bon atoms in the oil.

* * *

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Long-awaited initial results from a decade-long experiment have rescued—for now—the “standard model” of physics that scientists use as their working blueprint of reality. “Scientists everywhere have been eagerly waiting for our results,” said Janet Conrad of Columbia University, spokeswoman for the experi ment, dubbed MiniBooNE, which invest igated ghostly funda mental particles called neutrinos. The MiniBooNE team, a group of 77 researchers from institutions in the United States and the U.K., announced preliminary findings this week. The so-called standard model—the consensus view of physicists on the structure of matter—holds that the universe contains just 16 funda mental particles, along with a crew of “antiparticles” that are sorts of evil twins of the basic particles. Whle the model serves as a reliable guide for experi mentation, physicists fully expect it to be superseded by a more complete theory someday, since it fails to explain some key phenomena including gravity. Nonetheless, simply finding the model wrong wouldn’t automatically provide the way forward. A set of findings in the 1990s would, if confirmed, have spelled a rather messy end for the model. At that time, experi ments at the the U.S. Department of Energy’s Los Alamos National Labora tory turned up surprising observations about almost undetectable, nearly weightless particles called neutrinos. Currently, three types or “flavors” of neutrinos are known: electron neutrinos, muon neutrinos and tau neutrinos. In the last decade, experi ments have found that neutrinos can oscillate between different flavors. The Los Alamos findings, based on a device called the Liquid Scintillator Neutrino Detector, was one of those that suggested the presence of such oscillations, but it involved a range of neutrino masses vastly different from other experi ments. Reconciling those findings with the other oscillation results would have required the presence of a fourth, or “sterile” type of neutrino, with unusual properties. The existence of sterile neutrinos would have thrown serious doubt on the Standard Model. The MiniBooNE experi ment at the Energy Department’s Fermilab in Batavia, Ill., has shown there is more to the story, which is good news for the Standard Model, invest igators in the work say. The researchers mimicked the Los Alamos experi ments by looking for signs of neutrino oscillations in the mass range studied earlier. They found no evidence of oscillations between muon and electron neutrinos, as a simple interpretation of the Los Alamos results indicated, they said. “It was very important to verify or refute” that earlier result, said Robin Staffin, associate director of science for high energy physics at the Department of Energy. However, MiniBooNE invest igators found some other phenomena at low energy ranges that defied expectations, and which will require some further explaining, they said. “It clears one mystery but it leaves us with a puzzle that is important to understand,” said Fermilab Director Pier Oddone. The MiniBooNE collabo ration will further analyze its data, he added. Researchers added that further MiniBooNE results still have the potential to upset the Standard Model, but this looks more unlikely than before. “It means that the picture we have of neutrino oscillations looks secure,” David Wark of the U.K.’s Rutherford Appleton Labora tory in Didcot, U.K. told the Institute of Physics’ Physics Web website. Thus “we can proceed without a troubled conscience to the next generation of particle physics experi ments.” Scientists consider neutrinos to be among the more fascinating particles of nature. “You can’t see them, hear them, or touch them, but neutrinos are everywhere,” Conrad said. “They pass right by us and right through us. They can travel the distance of 200 Earths lined up before they hit anything, and if you put your hand on the desktop and count to three, trillions will pass through it. And they are produced in many ways—by the sun, or when stars explode, or by us using particle accel erators. So, it is important for us to understand their nature and how they behave.” For its observations, MiniBooNE relies on a detector made of a 250,000-gallon tank filled with ultrapure mineral oil, clearer than water from a faucet. Light-sensitive detectors record tiny bursts of light from occasional collisions between neutrinos and carbon atoms in the tank.