Antihydrogen is the antimatter counterpart of hydrogen. Whereas the common hydrogen atom is composed of an electron and proton, the antihydrogen atom is made up of a positron and antiproton. Antihydrogen began to be produced artificially in accelerator experiments in 1995, but the atoms produced had such "hot" velocities as to collide with matter and annihilate before they could be examined in detail.
The standard symbol for antihydrogen is H.
In November 2010, for the first time, cold antihydrogen was produced and magnetically confined for about a sixth of a second by the Antihydrogen Laser Physics Apparatus (ALPHA) team at CERN, and in 2011 antihydrogen was maintained for over 15 minutes. An important goal in antihydrogen research was published on March 7th, 2012, where researchers on ALPHA explored the internal structure of H by modifying the apparatus to use resonant microwave radiation on the magnetically trapped anti-atoms. Scientists hope studying antihydrogen may help shed light on the Baryon asymmetry problem or why there is more matter than antimatter in the universe.
According to the CPT theorem of particle physics, antihydrogen atoms should have many of the characteristics regular hydrogen atoms have; i.e., they should have the same mass, magnetic moment, and transition frequencies (see atomic spectroscopy) between their atomic quantum states. For example, excited antihydrogen atoms are expected to glow with the same color as that of regular hydrogen. Antihydrogen atoms should be attracted to other matter or antimatter gravitationally with a force of the same magnitude that ordinary hydrogen atoms experience. This would not be true if antimatter had negative gravitational mass, which is considered highly unlikely, though not yet empirically disproven (see gravitational interaction of antimatter).
When an antihydrogen atom comes into contact with ordinary matter, its constituents quickly annihilate. The positron, which is an elementary particle, annihilates with an electron, with their mass-energy being released as gamma rays. The antiproton, on the other hand, is made up of antiquarks that combine with the quarks in either neutrons or protons in normal matter; the annihilation results in high-energy pions. These pions in turn quickly decay into muons, neutrinos, positrons, and electrons. If antihydrogen atoms were to be suspended in a perfect vacuum, however, they should survive indefinitely.
As an antielement, it is expected to have exactly the same properties as hydrogen in every respect. For example, antihydrogen would be a gas under standard conditions and would combine with antioxygen to form antiwater, H2O.
Production in more detail 
In 1995, the first antihydrogen was produced by a team of researchers under the lead of Walter Oelert at the CERN laboratory in Geneva. The experiment took place in the LEAR, where antiprotons, which were produced in a particle accelerator, were shot at xenon clusters. When an antiproton gets close to a xenon nucleus, an electron-positron pair can be produced, and with some probability the positron will be captured by the antiproton to form antihydrogen. The probability for producing antihydrogen from one antiproton was only about 10−19, so this method is not well suited for the production of substantial amounts of antihydrogen, as detailed calculations had shown before.
In 1997 the CERN experiments were reproduced at Fermilab in the United States where a somewhat different cross section for the process was identified. Both experiments resulted in highly energetic, or warm, antihydrogen atoms, which were unsuitable for detailed study. Subsequently CERN built the Antiproton Decelerator in order to support efforts towards creating low-energy antihydrogen which could be used for tests of fundamental symmetries.
Low-energy antihydrogen 
In experiments carried out by the ATRAP and ATHENA collaborations at CERN, positrons from a sodium radioactive source and antiprotons were brought together in Penning traps, where synthesis took place at a typical rate of 100 antihydrogen atoms per second. Antihydrogen was first produced by ATHENA and subsequently by ATRAP in 2002, and by 2004, millions of antihydrogen atoms were produced in this way.
The low-energy antihydrogen atoms synthesized so far have had a relatively high temperature (a few thousand kelvin), thus hitting the walls of the experimental apparatus as a consequence and annihilating. A new experiment, ALPHA, a successor of the ATHENA collaboration, as well as ATRAP, is pursuing the making of antihydrogen at low enough kinetic energy to be magnetically confined.
Most precision tests of the properties of antihydrogen can be done only if the antihydrogen is trapped, meaning held in place for a long time. While antihydrogen atoms are electrically neutral, their spin produces magnetic moments. These magnetic moments will interact with an inhomogeneous magnetic field; some of the antihydrogen atoms will be attracted to a magnetic minimum. Such a minimum can be created by a combination of mirror and multipole fields. Antihydrogen can be trapped in such a magnetic minimum (minimum-B) trap; in November 2010, the ALPHA collaboration announced that they had trapped 38 antihydrogen atoms for about a sixth of a second. This was the first time that neutral antimatter had been trapped. In June 2011, ALPHA announced that they had trapped 309 antihydrogen atoms, some for times as long as 1000s.  Up to three antihydrogen atoms were trapped simultaneously.
The biggest limiting factor in the large scale production of antimatter is the availability of antiprotons. Recent data released by CERN states that, when fully operational, their facilities are capable of producing 10 million antiprotons per minute. Assuming a 100% conversion of antiprotons to antihydrogen, it would take 100 billion years to produce 1 gram or 1 mole of antihydrogen (6.02×1023 atoms of antihydrogen).
Larger antimatter atoms such as antideuterium (D), antitritium (T), and antihelium (He) are much more difficult to produce than antihydrogen. Antideuterium, antihelium-3 (3He) and antihelium-4 (4He) nuclei have been produced so far; these have such very high velocities that synthesis of their corresponding atoms poses several technical hurdles.
See also 
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