onsdag 17 november 2010

Antimatter - antihydrogen trapped at Cern.

Seconds after the Big Bang, the Universe was a soup of matter and antimatter. But today, everything around us is matter. So what and where is the antimatter? Could antimatter be the missing dark matter?

Physicists at CERN in Geneva are the first to capture and store atoms of antimatter for long enough to study its properties in detail. Working at the lab's ALPHA experiment, the team managed to trap 38 anti-hydrogen atoms for about 170 ms. Setting the antihydrogen free to annihilate with surrounding matter created several charged particles including pions. Tremendous? A door is opened to explore the symmetry question and models of neutron stars (->supernovas) and black hole collapse. The antiprotons could be used to study how antimatter interacts with gravity and to make the best ever measurement of the magnetic moment of the antiproton.
During those early moments, matter was an ultrahot, superdense brew of particles called quarks and gluons rushing hither and thither and crashing willy-nilly into one another. A sprinkling of electrons, photons and other light elementary particles seasoned the soup. This mixture had a temperature in the trillions of degrees, more than 100,000 times hotter than the sun's core. Video here.
Antimatter was first predicted in 1931, by Dirac, detected 1995. But an anti-atom is not easy to create. There are many degrees of freedom. Look at the proton only.
The new antinucleus . . . is a negatively charged state of antimatter containing an antiproton, an antineutron, and an anti-Lambda particle. It is also the first antinucleus containing an anti-strange quark. "This experimental discovery may have unprecedented consequences for our view of the world," commented theoretical physicist Horst Stoecker. "This antimatter pushes open the door to new dimensions in the nuclear chart - an idea that just a few years ago, would have been viewed as impossible."
Anti-electrons are used regularly in the medical technique of positron emission tomography scanning. Antihydrogen has been produced at Cern since 2002, and is of interest for use in a precision test of nature’s fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Any deviations from this could help physicists identify new physics – and explain why there is much more matter than antimatter in the universe. Hints at the asymmetry has come from neutrino-antineutrino scattering. Can it be seen also here?

At Nature 17.11. 2010:
Given the current experimental precision of measurements on the hydrogen atom (about two parts in 1014 for the frequency of the 1s-to-2s transition), subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter. However, so far experiments have produced antihydrogen that is not confined, precluding detailed study of its structure. Here we demonstrate trapping of antihydrogen atoms. From the interaction of about 107 antiprotons and 7×108 positrons, we observed 38 annihilation events consistent with the controlled release of trapped antihydrogen from our magnetic trap; the measured background is 1.4±1.4 events. This result opens the door to precision measurements on anti-atoms, which can soon be subjected to the same techniques as developed for hydrogen.

Fig. a, Measured tz distribution for annihilations obtained with no bias (green circles), left bias (blue triangles), right bias (red triangles) and heated positrons (violet star). The grey dots are from a numerical simulation of antihydrogen atoms released from the trap during the quench. The simulated atoms were initially in the ground state, with a maximum kinetic energy of 0.1meV. The typical kinetic energy is larger than the depth of the neutral trap, ensuring that all trappable atoms are considered. The 30-ms observation window includes 99% of the 20,000 simulated points. b, Experimental tz distribution, as above, shown along with results of a numerical simulation of mirror-trapped antiprotons being released from the trap. The colour codes are as above and there are 3,000 points in each of the three simulation plots. In both a and b, the simulated z distributions were convolved with the detector spatial resolution, of ~5mm.

Antimatter unstable.
Jun 23,2005: Particles interacting with each other through electrical or Coulomb forces. Starting with a proton, an antiproton, an electron and a positron, for instance, it is possible to form two stable atoms: hydrogen, which contains a proton and an electron, and antihydrogen, which contains an antiproton and a positron. However, Gridnev and Greiner show that these two atoms cannot form a molecule because there is no molecular state with an energy that is less than the combined energy of the individual atoms. Phys. Rev. Lett. 94 223402
Molecules can only form in such systems if a certain function of the four masses is greater than a particular value. Antihydrogen molecule is not stable, and that replacing the hydrogen atom with heavier isotopes (deuterium and tritium) does not make it stable either. Moreover, other exotic systems, such as muonium-antimuonium, are also unstable. "The hydrogen-antihydrogen molecule is unstable because the proton and antiproton get too close together and are therefore seen as a neutral combination by the other particles," Gridnev told. When hydrogen and antihydrogen meet the result is protonium (a bound state of a proton and an antiproton) and positronium (an electron-positron bound state).

March 4, 2010: Today the most heavy detected antimatter is He. Scientists at RHIC analyzed about a hundred million collisions to spot the new antinuclei, identified via their characteristic decay into a light isotope of antihelium and a positive pi-meson. Altogether, 70 examples of the new antinucleus were found. It should be possible to discover even heavier antinuclei.

The elements that make up basic matter contain protons and neutrons (up and down quarks). Now if we were to organize these elements by the amount of protons they contain, we would end up with the standard two-dimensional Periodic Table of Elements. Physicists take it a step further and organize the table by not only the number of protons, but the number of neutrons while adding an entirely new third dimension that measures a nuclei's "strangeness." (S on the table below, Z represents number of protons and N, neutrons). Nuclei containing one or more strange quarks are called hypernuclei.
For all ordinary matter, with no strange quarks, the strangeness value is zero and the chart is flat. Hypernuclei appear above the plane of the chart. The new discovery of strange antimatter with an antistrange quark (an antihypernucleus) marks the first entry below the plane.

The diagram above is known as the 3-D chart of the nuclides. The familiar Periodic Table arranges the elements according to their atomic number, Z, which determines the chemical properties of each element. Physicists are also concerned with the N axis, which gives the number of neutrons in the nucleus. The third axis represents strangeness, S, which is zero for all naturally occurring matter, but could be non-zero in the core of collapsed stars. Antinuclei lie at negative Z and N in the above chart, and the newly discovered antinucleus (magenta) now extends the 3-D chart into the new region of strange antimatter.

“The strangeness value could be non-zero in the core of collapsed stars,” said Jinhui Chen. In both nucleus-nucleus collisions at RHIC and in the Big Bang, quarks and antiquarks emerge with equal abundance. At RHIC, among the collision fragments that survive to the final state, matter and antimatter are still close to equally abundant, even in the case of the relatively complex antinucleus and its normal-matter partner featured in the present study. In contrast, antimatter appears to be largely absent from the present-day universe.

Bubbles of broken symmetry.
February 15, 2010: Data suggest symmetry may ‘melt’ along with protons and neutrons.
hints of profound symmetry transformations in the hot soup of quarks, antiquarks, and gluons produced in RHIC’s most energetic collisions. In particular, the new results, reported in the journal Physical Review Letters, suggest that “bubbles” formed within this hot soup may internally disobey the so-called “mirror symmetry” that normally characterizes the interactions of quarks and gluons.
Some crucial features of symmetry-altering bubbles speculated to have played important roles in the evolution of the infant universe. Bubbles, or local regions of “broken” symmetry may occur at extreme temperatures near transitions from one phase of matter to another, that is transformations of space, time, and particle types may be different (oppositely oriented charge separation in bubbles at different locations?). Analogous symmetry-altering bubbles created at an even earlier time in the universe helped maybe to establish the preference for matter over antimatter in our world.
RHIC exp. is 250,000* times hotter than the center of the Sun, and a transition to a new phase of nuclear matter known as quark-gluon plasma. Furthermore, as the colliding nuclei pass near each other, they produce an ultra-strong magnetic field that facilitates detecting effects of the altered symmetry.
It hint at a violation in what is known as mirror symmetry, or parity. This rule of symmetry suggests that events should occur in exactly the same way whether seen directly or in a mirror, with no directional dependence. But STAR has observed an asymmetric charge separation in particles emerging from all but the most head-on collisions at RHIC: The observations suggest that positively charged quarks may prefer to emerge parallel to the magnetic field in a given collision event, while negatively charged quarks prefer to emerge in the opposite direction. Because this preference would appear reversed if the situation were reflected through a mirror, it appears to violate mirror symmetry.
The theory suggests that particles with the same sign of electric charge should tend to be emitted from such local parity-violating regions in the same direction, either both parallel, or both anti-parallel, to the magnetic field arising in the collision, whereas unlike-sign particles should be emitted in opposite directions.

Data also suggest the local breaking of another form of symmetry, known as charge-parity, or CP invariance. According to this fundamental physics principle, when energy is converted to mass or vice-versa according to Einstein’s famous E=mc2 equation, equal numbers of particles and oppositely charged antiparticles must be created or annihilated.

Matter-antimatter oscillations.
From odd behavior in a particle called the BS meson, which flips back and forth between its matter and antimatter forms three trillions times per second (mixing). As the exotic, short-lived particles produced in the collisions progressively decayed to more stable particles such as electrons, a collision product known as a neutral B meson appeared to decay more often into muons—unstable particles that exist for roughly two millionths of a second before decaying further—than into antimuons. "We observe an asymmetry that is close to 1 percent."

"We find that the phase of the Bs mixing amplitude deviates more than 3 sigma from the Standard Model prediction. While no single measurement has a 3 sigma significance yet, all the constraints show a remarkable agreement with the combined result."

They are known as neutral mesons because they carry no net electric charge. In their brief lifetimes, they can oscillate between two forms, each the antiparticle of the other, Denisov explains. The difference is that Bs mesons oscillate much faster, giving them more flexibility to change from a matter progenitor to an antimatter progenitor, or vice versa. "Neutral B mesons are really interesting because they can basically go back and forth between matter and antimatter, to simplify things a bit, and we would have thought that they would spend an equal time as each," Denisov says. "What we're measuring now, it looks like they prefer matter."

Researchers believe that such a breakdown, known as CP violation, is required to explain why matter is so abundant. But the generally observed CP breaking is far too small to explain the difference, when about 2 - 4 % is matter. DZero and its detector, CDF, focus on the BS, which consists of a bottom quark and a strange antiquark. Data make it 99.7 percent likely that the discrepancy is real, but that is not enough yet for result to be real. Newer result is well compati-
ble with these results.

Researchers strongly suspect that the key to this riddle lies in the weak nuclear force, which governs radioactive decay, along with more exotic reactions. CP symmetry states that a particle ought to behave identically to the mirror image of its antiparticle, but not when acted on by the weak nuclear force.

See also blogs Physics Buzz, symmetry breaking, and uncertain principles for discussion.

4 kommentarer:

  1. Kea has a blogpost 21.11.10

  2. Picture at http://www.nature.com/nature/journal/v468/n7324/carousel/nature09610-f3.2.jpg

    Articles about anthydrogen research at http://asacusa.web.cern.ch/ASACUSA/

  3. http://physicsworld.com/cws/article/news/44561
    CERN moves closer to antihydrogen spectroscopy.
    Dec 9, 2010
    A big step towards making the first spectroscopic measurements on a beam of antihydrogen atoms,the Standard Model of particle physics says they should be identical to those of hydrogen.
    The ASACUSA group created their antiatom beams by combining antiprotons with positrons in a "cusp trap".

    The trap comprises 17 successive ring-shaped electrodes and two magnetic coils, which are wired to create magnetic fields in opposite directions (see figure). A cloud of antielectrons (also called positrons) from a radioactive source is first sent into the trap, where it is held as a plasma. A cloud of antiprotons – created in a nearby accelerator – is then fired into the plasma to create the antihydrogen atoms.

    Charged particles remain stuck in the trap, while neutral antihydrogen atoms are able to move further along the apparatus to a "field ionization trap". At this point, antihydrogen atoms in highly excited Rydberg states, in which the positron lies very far from the antiproton, are ionized and their antiprotons are trapped.
    Each annihilation event creates pions, which are easily spotted by a bank of detectors surrounding the trap. By comparing the number of antiprotons injected into the trap with the number of annihilations detected, the team estimated that about 7% of antiprotons combine to form antihydrogen.
    passing it through a microwave cavity in which hyperfine transitions between atomic energy states should occur. Making precise measurements of these transitions, which have not yet been carried out, could be used to study a fundamental quantum transformation known as the charge-parity-time (CPT) operation.

    When applied to a physical system, a CPT transformation converts every particle to its antiparticle, reflects each spatial co-ordinate, and reverses time. Although is currently no experimental evidence that the CPT symmetry is violated, it could show up as a slight difference in the frequency of hyperfine transitions in hydrogen and antihydrogen atoms. Phys. Rev. Lett. 105 243401.

  4. Physics World reveals its top 10 breakthroughs for 2010

    Dec 20, 2010, http://physicsworld.com/cws/article/news/44618

    It was a tough decision, given all the fantastic physics done in 2010. But we have decided to award the Physics World 2010 Breakthrough of the Year to two international teams of physicists at CERN, who have created new ways of controlling antiatoms of hydrogen.

    The ALPHA collaboration announced its findings in late November, which involved trapping 38 antihydrogen atoms (an antielectron orbiting an antiproton) for about 170 ms. This is long enough to measure their spectroscopic properties in detail, which the team hopes to do in 2011.

    Just weeks later, the ASACUSA group at CERN announced that it had made a major breakthrough towards creating a beam of antihydrogen that is suitable for spectroscopic studies. Our congratulations to both teams.