Calcolata (non misurata) la massa del protone

[lowenz]

Ce l'hanno fatta

http://sciencenow.sciencemag.org/cgi...8/1121/2?rss=1

It's one thing to know a fact, but it's another to explain it, as a curious advance in particle physics shows. Ever since the proton was discovered 89 years ago, physicists have been able to measure the mass of the particle--which, along with another called the neutron, makes up the atomic nucleus. But even with the best computers, theorists had not been able to start with a description of the proton's constituent parts and calculate its mass from scratch. Now, a team of theorists has reached that goal, marking the arrival of precision calculations of the ultracomplex "strong force" that binds nuclear matter.

"It's a really big deal," says John Negele, a theorist at the Massachusetts Institute of Technology in Cambridge. "It's the first time that we've really had this kind of confidence that everything is being done right."

Like a troubled teenager, the proton is a mess inside and just about impossible to figure out. In the 1970s, experimenters discovered that the proton and the neutron, known collectively as nucleons, consist of more-fundamental particles called quarks and gluons, which are the basic elements of a theory called quantum chromodynamics (QCD). In the simplest terms, a proton contains two "up" type quarks and one "down" type quark, with gluons zipping among them to bind them with the strong nuclear force. (The neutron contains two downs and an up.) In reality, a nucleon is much more complicated.

Thanks to the uncertainties of quantum mechanics, myriad gluons and quark-antiquark pairs flit in and out of existence within a nucleon. All of these "virtual" particles interact in a frenzy of pushing and pulling that's nearly impossible to analyze quantitatively. "Everything interacts with everything," says Laurent Lellouch, a theorist with the French National Center for Scientific Research at the Center for Theoretical Physics in Marseille and one of 12 physicists from France, Germany, and Hungary who performed the new calculations. Ninety-five percent of the mass of a nucleon originates from these virtual particles.

To simplify matters, the team took a tack pioneered in the late 1970s called lattice QCD. Within their computer programs, the researchers modeled space not as continuous but as a three-dimensional array of points. They also modeled time as passing in discrete ticks, as opposed to flowing smoothly. This turns space and time into a lattice of points. The researchers then confined the quarks to the points in the lattice and the gluons to the links between the points. The lattice sets a shortest distance and time for the interactions, greatly simplifying the problem.

Still, the computation involves millions of variables and requires supercomputers. Only since about 2000 have researchers attempted to include not just all of the gluons but the fleeting quark-antiquark pairs as well. The latest work, reported today in Science, incorporates a variety of conceptual improvements to obtain estimates of the mass of the nucleon and nine other particles made of up, down, and slightly heavier "strange" quarks accurate to within a couple of percent.

This isn't the first computational tour de force for particle physicists. Five years ago, others made equally precise calculations of more esoteric quantities--somewhat easier to calculate--such as those that govern the decay of a particle called a D+ meson, which contains a down antiquark and a heavy "charm" quark, notes Christine Davies, a theorist at the University of Glasgow in the U.K. Still, she says, the calculation of the well-known masses highlights the ability of lattice QCD to make accurate predictions for the strong force. "This is all good news for lattice QCD," Davies says, "because there are lots of things that we want to calculate that experimenters haven't already measured." For example, Negele says, physicists still don't know distribution of the virtual particles inside the proton or the origin of its spin.

[CioKKoBaMBuZzo]

ma, se ho capito bene, l'hanno calcolata all'interno del modello standard giusto?

edit: no aspetta, ma nel modello standard le masse non sono parametri liberi?

[Doraneko]

Tutto sto bordello e non ci dicono neanche quanto e' sta massa?

"It's a relly big deal"

[guyver]

Originariamente inviato da CioKKoBaMBuZzo

ma, se ho capito bene, l'hanno calcolata all'interno del modello standard giusto?

edit: no aspetta, ma nel modello standard le masse non sono parametri liberi?

le masse dei quark ed elettroni sono parametri liberi a quanto mi ricordo..
per le masse dei protoni il grosso dovrebbe essere energia di legame...
tutto ihmo

[lowenz]

Originariamente inviato da guyver

le masse dei quark ed elettroni sono parametri liberi a quanto mi ricordo..
per le masse dei protoni il grosso dovrebbe essere energia di legame...
tutto ihmo

Sì, il grosso è l'energia di legame ed è riportato anche sopra

Ninety-five percent of the mass of a nucleon originates from these virtual particles.

[Banus]

Questo tipo di risultati sono estremamente difficili da ottenere, anche con adeguati strumenti matematici e supercomputer.
Per dare un'idea, nonostante il calcolo della massa del protone sia il più accurato e preciso mai eseguito, l'errore è ancora così grande, 20 MeV/c^2, da non permettere di distinguere fra protone e neutrone, che hanno una differenza di massa dieci volte minore (vedi qui, serve registrazione). Le masse calcolate sono 936±22 MeV/c^2 per il neutrone e 936±25 MeV/c^2 per il protone, da confrontare con i 940 e 938 rispettivamente misurati.
Insomma, c'è ancora margine di miglioramento