View source for Antiprotonic helium

{{Short description|Exotic matter with an antiproton in place of an electron}}
[[File:Pbarhelium 2020.jpg|thumb|Schematic drawing of an antiprotonic helium atom.]]
{{Antimatter}}

'''Antiprotonic helium''' is a three-body atom composed of an [[antiproton]] and an [[electron]] orbiting around a [[helium]] [[Atomic nucleus|nucleus]]. It is thus made partly of matter, and partly of antimatter. The atom is electrically neutral, since an electron and an antiproton each have a charge of −1&nbsp;''e'', whereas a helium nucleus has a charge of +2&nbsp;''e''. It has the longest lifetime of any experimentally produced matter–antimatter bound state.<ref>{{Cite web|title=PROGRESS REPORT OF THE ASACUSA AD-3 COLLABORATION|url=https://cds.cern.ch/record/2799130/files/SPSC-SR-301.pdf|url-status=live|archive-url=https://web.archive.org/web/20220707213409/https://cds.cern.ch/record/2799130/files/SPSC-SR-301.pdf|archive-date=7 July 2022|access-date=30 July 2022}}</ref>

== Production ==

These [[exotic atom]]s can be produced by mixing antiprotons with ordinary helium gas; the antiproton spontaneously displaces one of the two electrons contained in a normal helium atom in a chemical reaction, and then begins to orbit the helium nucleus in the electron's place. This will happen in the case of approximately 3% of the antiprotons introduced to the helium gas.  The antiproton's orbit, which has a large [[principal quantum number]] and [[angular momentum quantum number]] of around 38, lies far away from the surface of the helium nucleus. The antiproton can thus orbit the nucleus for tens of microseconds, before finally falling to its surface and [[annihilation|annihilating]]. This contrasts with other types of exotic atoms of the form {{Subatomic particle|Antiproton}}{{px2}}''X''{{sup|+}}, which typically decay within picoseconds.<ref name="Hori, Sótér et al., 2011">
{{cite journal
 |last1=Sótér  |first1=Anna 
 |last2=Aghai-Khozani |first2=Hossein 
 |last3=Barna |first3=Dániel 
 |last4=Dax |first4=Andreas 
 |last5=Venturelli |first5=Luca 
 |last6=Hori |first6=Masaki
 |last7=Hayano |first7=Ryugo
 |last8=Friedreich |first8=Susanne
 |last9=Juhász |first9=Bertalan
 |last10=Pask |first10=Thomas
 |last11=Horváth |first11=Dezső
 |last12=Widmann |first12=Eberhard
 |last13=Venturelli |first13=Luca
 |last14=Zurlo |first14=Nicola
 |date=2011-07-27 
 |title=Two-photon laser spectroscopy of antiprotonic helium and the antiproton-to-electron mass ratio 
 |journal=Nature 
 |volume=475 
 |issue=7357 
 |language=en 
 |pages=484–488 
 |doi=10.1038/nature10260|pmid=21796208 
 |arxiv=1304.4330 
 |s2cid=4376768 
}}
</ref>

== Laser spectroscopy ==

Antiprotonic helium atoms are under study by the [[ASACUSA]] experiment at [[CERN]]. In these experiments, the atoms are first produced by stopping a beam of antiprotons in helium gas. The atoms are then irradiated by powerful laser beams, which cause the antiprotons in them to resonate and jump from one atomic orbit to another.

As in spectroscopy of other bound states, Doppler broadening and other effects present challenges to precision. Researchers use a variety of techniques to obtain accurate results. One way to exceed Doppler-limited precision is two-photon spectroscopy.<ref name="Hori, Sótér et al., 2011"/> The ASACUSA Collaboration has studied {{Subatomic particle|Antiproton}}{{px2}}{{sup|3}}He{{sup|+}} and {{Subatomic particle|Antiproton}}{{px2}}{{sup|4}}He{{sup|+}} atoms with the {{Subatomic particle|Antiproton}} occupying a high Rydberg state with large principal and orbital quantum numbers, <math>n \sim l + 1 \sim</math> 38 using 2-photon spectroscopy.<ref name="Hori, Sótér et al., 2011"/>
Counterpropagating Ti:Sapphire lasers with pulses of duration 30−100 ns excited nonlinear 2-photon transitions in the deep UV, including spectral lines of wavelengths, <math>\lambda =</math> 139.8, 193.0 and 197.0&nbsp;nm. These lines correspond to transitions between states of the form <math>(n, l) \rightarrow (n-2, l-2)</math>. Such transitions are improbable. However, the probability is increased by a factor of {{val|e=5}} when the laser frequencies sum to within 10&nbsp;GHz of an intermediate state <math>(n-1, l-1)</math>. States were selected pairwise such that Auger emission to {{Subatomic particle|Antiproton}}{{px2}}He{{sup|2+}} and rapid annihilation produced a detectable Čerenkov signal. The reduced Doppler shift resulted in narrower spectral lines accurate to between 2.3 and 5&nbsp;ppb. Comparison of the results with three-body quantum electrodynamics calculations made possible a determination of the antiproton to electron mass ratio of {{val|1836.1526736|(23)}}.

In 2022 ASACUSA [[ASACUSA experiment#ASACUSA physics|found]] unexpected narrowing of antiprotonic helium spectral lines.<ref>{{Cite journal |last1=Sótér |first1=Anna |last2=Aghai-Khozani |first2=Hossein |last3=Barna |first3=Dániel |last4=Dax |first4=Andreas |last5=Venturelli |first5=Luca |last6=Hori |first6=Masaki |date=2022-03-16 |title=High-resolution laser resonances of antiprotonic helium in superfluid <sup>4</sup>He |journal=Nature |volume=603 |issue=7901 |language=en |pages=411–415 |doi=10.1038/s41586-022-04440-7|pmid=35296843  |pmc=8930758 |bibcode=2022Natur.603..411S |issn=1476-4687}}</ref><ref>{{Cite web |title=ASACUSA sees surprising behaviour of hybrid matter–antimatter atoms in superfluid helium |url=https://home.cern/news/news/physics/asacusa-sees-surprising-behaviour-hybrid-matter-antimatter-atoms-superfluid |access-date=2022-03-17 |website=CERN |language=en}}</ref><ref>{{Cite web |date=2022-03-16 |title=Icy Antimatter Experiment Surprises Physicists |url=https://www.quantamagazine.org/icy-antimatter-experiment-surprises-physicists-20220316/ |access-date=2022-03-17 |website=Quanta Magazine |language=en}}</ref>

== Measurement of the mass ratio between the antiproton and electron ==

By measuring the particular [[frequency]] of the laser light needed to resonate the atom, the [[ASACUSA]] experiment determined the mass of the antiproton, which they measured at {{val|1836.1536734|(15)}} times more massive than an [[electron]].<ref name=Mhori>{{cite journal | journal=Science | volume=354 |issue=6312 | pages=610–4 | year=2016 |last=Hori |first=M. | title=Buffer-gas cooling of antiprotonic helium to 1.5 to 1.7&nbsp;K, and antiproton-to-electron mass ratio |doi=10.1126/science.aaf6702 | pmid=27811273 | display-authors=etal|bibcode = 2016Sci...354..610H | s2cid=37796298 }}</ref> This is the same as the mass of a "regular" proton, within the level of certainty of the experiment. This is a confirmation of a fundamental symmetry of nature called [[CPT symmetry|CPT]] (short for charge, parity, and time reversal). This law says that all physical laws would remain unchanged under simultaneous reversal of the [[electric charge|charge]] axis, [[parity (physics)|parity of the space axes]], and the orientation of the [[time]] axis. One important prediction of this theory is that particles and their [[antiparticles]] should have exactly the same mass.

== Comparison of antiproton and proton masses and charges ==

By comparing the above results on laser spectroscopy of antiprotonic helium with separate high-precision measurements of the antiproton's [[cyclotron]] frequency carried out by the [[ATRAP]] and [[BASE experiment|BASE]] collaborations at CERN, the [[mass]] and [[electric charge]] of the antiproton can be precisely compared with the proton values. The most recent such measurements show that the antiproton's mass (and the absolute value of the charge) is the same as the proton's to a precision of 0.5 parts in a billion.

== Antiprotonic helium ions ==

An '''antiprotonic helium ion''' is a two-body object composed of a helium nucleus and orbiting antiproton. It has an electric charge of +1&nbsp;''e''. Cold ions with lifetimes of up to 100&nbsp;ns were produced by the ASACUSA experiment in 2005.

== Pionic helium ==
{{See also|Kaonic hydrogen}}

In 2020 ASACUSA in collaboration with the [[Paul Scherrer Institute|Paul Scherrer Institut (PSI)]] reported the experimental verification of long lived [[pion]]ic helium by spectroscopic measurements, the first time on an exotic atom containing a meson. Its existence had been predicted in 1964 by George Condo at University of Tennessee to explain some anomalies from bubble chamber tracks but no definite proof of its existence had ever been obtained. In the experiment negatively charged pions from a ring [[cyclotron]] were magnetically focused into a tank filled with superfluid helium so that they would expel an electron from the atom and take its place. Later, to confirm the production, laser light was fired at various frequencies until they found a specific one at 1631&nbsp;nm where the pion would resonate undergoing a [[quantum jump]] from its orbit into an inner one and eventually into the nucleus which would break down into a proton, a neutron and a deuteron. The experiment proved highly technical to perform and took 8 years, including the design and construction of the experiment.<ref>{{Cite journal |last1=Hori |first1=Masaki |last2=Aghai-Khozani |first2=Hossein |last3=Sótér |first3=Anna |last4=Dax |first4=Andreas |last5=Barna |first5=Daniel |date=6 May 2020 |title=Laser spectroscopy of pionic helium atoms |url=https://www.nature.com/articles/s41586-020-2240-x |journal=Nature |language=en |volume=581 |issue=7806 |pages=37–41 |doi=10.1038/s41586-020-2240-x |pmid=32376962 |bibcode=2020Natur.581...37H |s2cid=218527999 |issn=1476-4687}}</ref><ref>{{Cite web |title=ASACUSA sees surprising behaviour of hybrid matter–antimatter atoms in superfluid helium |url=https://home.cern/news/news/physics/asacusa-sees-surprising-behaviour-hybrid-matter-antimatter-atoms-superfluid |access-date=2022-03-16 |website=CERN |language=en}}</ref><ref>{{Cite web |title=Pionic helium |url=https://www.mpq.mpg.de/4937857/pionichelium |access-date=2022-03-16 |website=www.mpq.mpg.de |language=en}}</ref>

== See also ==

* [[Positronium]]
* [[Protonium]]

== References ==

{{reflist}}

== Further reading ==

* [https://home.cern/news/news/physics/asacusa-improves-measurement-antiproton-mass ASACUSA improves measurement of antiproton mass]

[[Category:Antimatter]]
[[Category:Atomic physics]]
[[Category:Exotic atoms]]
[[Category:Helium]]