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3-D view inside proton may be coming into focus

April 4, 2013
Courtesy of DOE Pulse Magazine 
and World Science staff

Phys­i­cists are measuring how the most basic known build­ing blocks of na­ture—quarks—are ar­ranged to make up pro­tons, key pieces of the atom­ic nu­cle­us.

The find­ings could be a step to­ward even­tu­ally un­der­stand­ing the three-di­men­sion­al in­ter­nal struc­ture of pro­tons and neu­trons, the par­t­i­cles form­ing the heart of an at­om. Pro­tons in par­tic­u­lar de­ter­mine the ident­ity of each chem­i­cal el­e­ment; this de­pends on the num­ber of pro­tons in each at­om.

New find­ings in­di­cate that n a pro­ton, quarks with spin in the "up" di­rec­tion (red and blue) tend to gath­er in the left half of the pro­ton as seen by an in­com­ing elec­tron. "Down"-spinning quarks (green) tend to gath­er in the right half. (Cour­te­sy U.S. Dept. of En­er­gy)


Phys­i­cists had al­ready de­ter­mined that the pro­ton con­tains three fun­da­men­tal par­t­i­cles called quarks. It now turns out that the quarks take up po­si­tions that de­pend largely on the di­rec­tion of their “spin,” sci­en­tists said: those with op­po­site spin tend to find them­selves in op­po­site sides of the pro­ton. 

The new work was car­ried out at the U.S. De­part­ment of En­er­gy’s Jef­fer­son Lab in New­port News, Va.. 

Suba­tom­ic par­t­i­cles are said to have spin be­cause a spin­ning mo­tion can ac­count for their mag­net­ic fields. But the par­t­i­cles aren’t con­sid­ered to be ac­tu­ally spin­ning be­cause they could still have this spin with­out hav­ing any size at all. Al­so, suba­tom­ic par­t­i­cles have re­stric­tions on the ways they can “spin,” un­char­ac­ter­is­tic of or­di­nary ob­jects.

The pro­ton lies at the heart of eve­ry at­om that builds our vis­i­ble uni­verse. Yet phys­i­cists are still try­ing to fig­ure out how this par­t­i­cle, too small to see with or­di­nary mi­cro­scopes, is com­posed of its chief build­ing blocks, quarks and glu­ons. At Jef­fer­son Lab, a par­ticle ac­cel­er­ator called CE­BAF is used to ex­tract such in­forma­t­ion by smash­ing a stream of elec­trons in­to pro­tons, and ob­serv­ing how they in­ter­act.

This work has re­vealed im­por­tant bas­ics about pro­ton struc­ture, said Harut Avakian, a Jef­fer­son Lab staff sci­ent­ist. The in­sight comes from ex­pe­ri­ments that de­tect wheth­er a quark was hit, re­veal­ing in­forma­t­ion such as the num­ber of quarks and the dis­tri­bu­tion of their mo­men­tum. Mo­men­tum is a quantity that takes in­to ac­count the speed, di­rec­tion and weight of an ob­ject.

Adding up the mo­men­tums of the com­po­nent quarks leads to a use­ful com­par­i­son with the mo­men­tum of the whole pro­ton, Ava­kian ex­plained. 

“What sci­en­tists were do­ing these last 40 years [is that] they were in­ves­ti­gat­ing the mo­men­tum dis­tri­bu­tion of quarks along the di­rec­tion in which the elec­tron looks at it—a one-dimensional pic­ture of the pro­ton,” he added. He and his col­leagues in­stead used a new meth­od meant to pro­vide three-di­men­sion­al in­forma­t­ion. This meth­od meas­ures par­t­i­cles called pi­o­ns—made of a quark and a type of mirror-im­age par­t­i­cle called an an­ti­quark—as they are formed in elec­tron-pro­ton col­li­sions.

This meth­od al­so al­lows an in­fer­ence of just where the quark was when it was hit, he added: how far the quarks are away from the pro­ton’s cen­ter, and the di­rec­tion of their spin. The tech­nique pro­jects an im­age of the quark dis­tri­bu­tion in­to a plane, or flat ar­ea of space, that cuts across the elec­tron beam.

“The one-dimensional pic­ture is ex­tend­ed to a three-di­men­sion­al im­age that al­lows us to un­der­stand how those lit­tle quarks are dis­trib­ut­ed in the space,” Ava­kian said. To do this, the re­search­ers needed to thwhack quarks with elec­trons just hard enough for the quarks to ab­sorb en­er­gy from the elec­trons and then give it away again, with­out break­ing up the pro­tons.

“You just tou­ch a sin­gle quark,” said Jef­fer­son Lab The­o­rist Chris­tian Weiss. “The elec­tron hits the quark, and this quark shakes off a pi­on. The quark re­turns to the pro­ton, and the pro­ton re­mains in­tact and re­coils. You meas­ure the pi­on and the re­coil­ing pro­ton in ad­di­tion to the scat­tered elec­tron.”

The ex­pe­ri­men­tal da­ta alone is­n’t enough, though. To get de­tailed in­forma­t­ion, the ex­pe­ri­menters plug their da­ta in­to a com­pli­cat­ed the­o­ry ex­pressed as a set of equa­tions. These pro­vide de­tailed in­forma­t­ion on how quarks and glu­ons be­have in the pro­ton. It’s thought that these ex­pres­sions, called gen­er­al­ized par­ton dis­tri­bu­tions, along with oth­er in­forma­t­ion, will pro­vide the first three-di­men­sion­al view of the pro­ton’s guts.

“It’s like you have some mo­sa­ic. These are parts of your mo­sa­ic. To get the pic­ture, you need all these pieces to put to­geth­er,” Ava­kian said.

The first re­sult from the new meth­od was pub­lished last fall in the jour­nal Phys­i­cal Re­view Let­ters by a sci­en­tif­ic team at Jef­fer­son lab called the CLAS col­la­bora­t­ion. 

It turns out “the po­si­tion of the quark de­pends on how its spin is point­ing or on its mo­men­tum. The spin of the quark af­fects the prob­a­bil­ity to find the quark in a cer­tain point in space,” Ava­kian ex­plained.

They team found that quarks tend to gath­er in op­po­site sides of the pro­ton de­pending on which way they spin. From the in­com­ing elec­tron’s point of view, the quarks in the left half of the pro­ton are those that in the jar­gon of phys­ics have “up” spin. This is akin to a di­rec­tion in which—if the quark were a really spin­ning bal­l—a dot on its sur­face would be mov­ing right. Quarks with the op­po­site spin tend to be in the right side of the pro­ton.

Avakian said the re­sult con­firms that pro­tons are com­plex sys­tems, with a rich in­ter­nal struc­ture and soph­is­t­icated dy­nam­ics, re­ferred to as Quan­tum Chromody­nam­ics. “The quarks are not just dis­trib­ut­ed in mo­men­tum in one di­rec­tion. They have mo­menta, po­si­tions, and eve­rything is mov­ing around. As of now, we don’t un­der­stand very well the dy­nam­ics, such as how this spin is cor­re­lat­ed with the po­si­tion and the mo­men­tum. That’s what we are try­ing to study—the in­ter­play of the quark’s in­ter­nal mo­tion and their spin with their spa­tial po­si­tion in the sys­tem.”


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Physicists have measured how the most fundamental particles of nature—quarks—are arranged to make up protons, key components of the atomic nucleus. The findings could be a step toward eventually understanding the three-dimensional internal structure of protons and neutrons, the particles forming the heart of an atom. Protons in particular determine the identity of each chemical element; this depends on the number of protons in an atom. Physicists had already determined that in the proton are three fundamental particles called quarks. It now turns out that the quarks take up positions that depend largely on the direction of their “spin,” scientists said: those with opposite spin tend to find themselves in opposite sides of the proton. The new work was carried out at the U.S. Department of Energy’s Jefferson Lab in Newport News, Va.. Subatomic particles are said to have spin because a spinning motion can account for certain magnetic fields. But the particles aren’t considered to be actually spinning because they could still have this spin without even having any size at all. Also, subatomic particles have restrictions on the ways they can “spin,” uncharacteristic of ordinary objects. The proton lies at the heart of every atom that builds our visible universe. Yet physicists are still trying to figure out how this particle, too small to see with ordinary microscopes, is composed of its chief building blocks, quarks and gluons. At Jefferson Lab, a machine called CEBAF is built to extract such information by smashing a stream of electrons into protons, and observing how they interact. These observations have revealed important basic facts on proton structure, said Harut Avakian, a Jefferson Lab staff scientist. The insight comes from experiments that detect whether a quark was hit, revealing information such as the number of quarks and the distribution of their momentum. Momentum is a that which takes into account the speed, direction and weight of an object. Adding up the momentums of the component quarks leads to a useful comparison with the momentum of the whole proton, Avakian explained. “What scientists were doing these last 40 years [is that] they were investigating the momentum distribution of quarks along the direction in which the electron looks at it – a one-dimensional picture of the proton,” he added. He and his colleagues instead used a new method meant to provide three-dimensional information. This method measures particles called pions—made of a quark and a type of mirror-image particle called an antiquark—as they are form in electron-proton collisions. This method also allows an inference of just where the quark was when it was hit, he added: how far the quarks are away from the proton’s center, and the direction of their spin. The technique projects an image of the quark distribution into a plane, or flat area of space, that cuts across the electron beam. “The one-dimensional picture is extended to a three-dimensional image that allows us to understand how those little quarks are distributed in the space,” Avakian said. To do this, the researchers needed to thwhack quarks with electrons just hard enough for the quarks to absorb energy from the electrons and then give it away again, without breaking up the protons. “You don’t destroy the proton, you just touch a single quark,” said Jefferson Lab Theorist Christian Weiss. “The electron hits the quark, and this quark shakes off a pion. The quark returns to the proton, and the proton remains intact and recoils. You measure the pion and the recoiling proton in addition to the scattered electron.” The experimental data alone isn’t enough, though. To get detailed information, the experimenters plug their data into a complicated theory expressed as a set of mathematical expressions. These provide detailed information on how quarks and gluons behave in the proton. It’s thought that these expressions, called generalized parton distributions, along with other information, will provide the first three-dimensional view of the proton’s structure. “It’s like you have some mosaic. These are parts of your mosaic. To get the picture, you need all these pieces to put together,” Avakian said. The first result from the new method was published last fall in the journal Physical Review Letters by a scientific team at Jefferson lab called the CLAS collaboration. It turns out “the position of the quark depends on how its spin is pointing or on its momentum. The spin of the quark affects the probability to find the quark in a certain point in space,” Avakian explained. They team found that quarks tend to gather in opposite sides of the proton depending on which way they spin. From the incoming electron’s point of view, the quarks in the left half of the proton are those that in the jargon of physics have “up” spin. This is akin to a direction in which—if the quark were a really spinning ball—a dot on its surface would be moving right. Quarks with the opposite spin tend to be in the opposite side of the proton. Avakian said the result confirms that protons are complex systems, with a rich internal structure and sophisticated dynamics, referred to as Quantum Chromodynamics. “The quarks are not just distributed in momentum in one direction. They have momenta, positions, and everything is moving around. As of now, we don’t understand very well the dynamics, such as how this spin is correlated with the position and the momentum. That’s what we are trying to study—the interplay of the quark’s internal motion and their spin with their spatial position in the system.”