ASACUSA experiment

'''Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA)''', '''AD-3''', is an experiment at the Antiproton Decelerator (AD) at CERN. The experiment was proposed in 1997, started collecting data in 2002 by using the antiprotons beams from the AD, and will continue in future under the AD and ELENA decelerator facility.

== ASACUSA physics == ASACUSA collaboration is testing for CPT-symmetry by laser spectroscopy of antiprotonic helium and microwave spectroscopy of the hyperfine structure of antihydrogen. It compares matter and antimatter using antihydrogen and antiprotonic helium and looks into matter-antimatter collisions.<ref>{{cite web|title=ASACUSA – General|url=https://home.cern/science/experiments/asacusa|access-date=30 July 2022}}</ref><ref>{{Cite web|title=Archived copy|url=http://asacusa.web.cern.ch/ASACUSA/home/spsc/proposal.pdf|url-status=dead|archive-url=https://web.archive.org/web/20131213115540/http://asacusa.web.cern.ch/ASACUSA/home/spsc/proposal.pdf|archive-date=13 December 2013|access-date=9 February 2011}}</ref><ref>{{cite web |title=CERN experiment improves precision of antiproton mass measurement with new innovative cooling technique |url=https://phys.org/news/2016-11-cern-precision-antiproton-mass-cooling.html |website=phys.org |language=en}}</ref> It also measures atomic and nuclear cross-sections of antiprotons on various targets at extremely low energies.<ref>{{Cite web|title=Archived copy|url=http://asacusa.web.cern.ch/ASACUSA/|url-status=dead|archive-url=https://web.archive.org/web/20130415140116/http://asacusa.web.cern.ch/ASACUSA/|archive-date=15 April 2013|access-date=17 February 2010}}</ref>

In 2020 ASACUSA in collaboration with the Paul Scherrer Institut (PSI) reported spectral measurements of long lived pionic helium.<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|url-access=subscription }}</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>

In 2022 ASACUSA reported spectral measurements of antiprotonic helium suspended in gaseous and liquid (He-I and He-II) targets. An abrupt narrowing of spectral lines was discovered at temperatures near the superfluid phase transition temperature. The narrowness and symmetry of the spectral lines for antiprotonic helium contrasts with other types of atoms suspended in He-I and He-II. This is hypothesized to be related to the order of magnitude smaller orbital radius of <math>\sim</math>40 pm which is comparably unaffected during laser excitation. <ref name="Hori, Sótér et al., 2022"> {{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 4He |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>

== Experimental setup == === Antiproton Trap === ASACUSA receives antiproton beams from the AD and ELENA decelerator. These beams are decelerated to 0.01 MeV energy using a radiofrequency decelerator and the antiprotons are stored in the MUSASHI traps. The positrons to form antihydrogen atoms are obtained from <chem>Na^{22}</chem> radioactive source and stored in a positron accumulator. The mixing of antiprotons and positrons forms polarised and cold antihydrogen inside a double-Cusp trap. The polarised antihydrogen atoms from this system then enter the spectrometer where the measurements are done.<ref>{{Cite book|last1=Amsler|first1=C.|url=https://cds.cern.ch/record/2748998?ln=en|title=Status report of the ASACUSA experiment - progress in 2020 and plans for 2021|last2=Barna|first2=D.|last3=Breuker|first3=H.|last4=Chesnevskaya|first4=S.|last5=Costantini|first5=G.|last6=Ferragut|first6=R.|last7=Giammarchi|first7=M.|last8=Gligorova|first8=A.|last9=Higaki|first9=H.|date=2021|others=CERN. Geneva. SPS and PS Experiments Committee, SPSC}}</ref>

thumb|220x220px|ASACUSA team at beam setup preparation in September 2018 === Beam Spectroscopy === Hyperfine spectroscopy measurements on H beams in flight have been made using a Rabi experiment. The collaboration plans to conduct similar measurements on {{Subatomic particle|Antihydrogen}} in flight.<ref name="Malbrunot, et al., 2018"> {{Cite journal |date=2018-02-19 |title=The ASACUSA antihydrogen and hydrogen program: results and prospects |journal= Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume= 376 |issue= 2116 |language=en |pages= |doi=10.1098/rsta.2017.0273|pmid= 29459412 |pmc= 5829175 |arxiv=1710.03288 |bibcode=2018RSPTA.37670273M |last1=Malbrunot |first1=C. |last2=Amsler |first2=C. |last3=Arguedas Cuendis |first3=S. |last4=Breuker |first4=H. |last5=Dupre |first5=P. |last6=Fleck |first6=M. |last7=Higaki |first7=H. |last8=Kanai |first8=Y. |last9=Kolbinger |first9=B. |last10=Kuroda |first10=N. |last11=Leali |first11=M. |last12=Mäckel |first12=V. |last13=Mascagna |first13=V. |last14=Massiczek |first14=O. |last15=Matsuda |first15=Y. |last16=Nagata |first16=Y. |last17=Simon |first17=M. C. |last18=Spitzer |first18=H. |last19=Tajima |first19=M. |last20=Ulmer |first20=S. |last21=Venturelli |first21=L. |last22=Widmann |first22=E. |last23=Wiesinger |first23=M. |last24=Yamazaki |first24=Y. |last25=Zmeskal |first25=J. |last26=Zmeskal |first26=J. }} </ref><ref name="ASACUSAPROG2021" />

=== Cryogenic Target Spectroscopy === ==== Electrostatic Beamline ==== [[File:Asacusa1.jpg|thumb|200x200px|ASACUSA team preparing beam setup for the ELENA beams in September 2018.]] Anticipating completion of ELENA, with the aim of making spectral measurements of previously undetected atomic resonances in antiprotonic helium, a new 6 m electrostatic beamline was constructed to transport {{Subatomic particle|Antiproton}}s to a cryogenic target. <ref name="ASACUSAPROG2021"> {{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> (Previous experiments, including the antiprotonic helium spectral measurements of March 2022 used a 3 m Radio-frequency Quadrupole to decelerate {{Subatomic particle|Antiproton}}s from the Antiproton Decelerator. <ref name="Hori, 2018"> {{Cite journal |last1=Hori |first1=Masaka |date=2018-10-24 |title=Single-photon laser spectroscopy of cold antiprotonic helium |journal=Hyperfine Interactions |volume=239 |issue=1 |language=en |pages=411–415 |article-number=44 |doi=10.1007/s10751-018-1518-y|bibcode=2018HyInt.239...44H |s2cid=105937408 |doi-access=free }} </ref><ref name="Hori, Sótér et al., 2022"/><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>) 0.1 MeV ELENA {{Subatomic particle|Antiproton}}s entering the beamline are focussed to a width of <math>\le</math>1 mm and pass through an aperture (30 mm length and 8 mm diameter). The transverse horizontal and vertical dimensions of the beam are determined by beam monitors consisting of a grid of gold-coated tungsten-rhenium wires with grid spacing of 20 μm.<ref name="Hori, 2018"/> (There are 3 such monitors along the beamline, one of which is <math>\le</math> 300 mm upstream of the cryogenic chamber.<ref name="ASACUSAPROG2021"/>) Further along the beamline, there is a configuration of 3 quadrupole magnets to counteract {{Subatomic particle|Antiproton}} beam expansion and 2 more apertures of diameters 30 mm and 16 mm. A beam emerging from the apertures is focussed to 3 mm diameter and impinges on a 6 mm diameter titanium window in an OFHC copper flange mounted on the cryogenic target chamber wall.<ref name="ASACUSAPROG2021"/> Acrylic and lead fluoride Čerenkov detectors monitor the beamline for {{Subatomic particle|Antiproton}} annihilations. The beamline pressure is 0.8 mb, much higher than the ELENA beamline pressure of <math>\sim10^{-9}</math> mb. The pressure difference is maintained by three 500 L/s titanium ion and 4 turbomolecular pumps.<ref name="ASACUSAPROG2021"/>

==== Cryogenic Chamber ==== The helium targets are contained in a 35 mm diameter vessel made of titanium (gaseous or supercritical phase with 70% He-I) or OFHC copper (He-I and He-II) mounted on a liquid helium constant-flow cryostat. The vessel is enclosed within copper thermal shielding: an inner shield cooled by coolant helium vapour and an outer shield cooled by liquid nitrogen. A configuration of manometers and temperature sensors provide data used to characterize the state of the helium in the chamber. Pressures <math>\ge</math> 1 MPa can be sustained.<ref name="Hori, Sótér et al., 2022"/> The chamber is accessible to antiprotons through an annealed titanium window of diameter 75 μm or 50 μm vacuum brazed into the chamber wall.<ref name="Hori, Sótér et al., 2022"/> Opposite this, a 28-mm diameter, 5-mm thick UV-grade sapphire window transmits laser light, antilinear to an incident particle beam.<ref name="Hori, Sótér et al., 2022"/> Two 35-mm diameter Brewster windows made of fused silica ({{chem|Si|O|2}}) mounted on flanges on opposite sides of the chamber walls perpendicular to the beam axis transmit laser light.<ref name="ASACUSAPROG2021"/><ref name="Hori, Sótér et al., 2022"/> Near the cryostat, beneath the beampipe, is positioned a 300 <math>\times</math> 200 <math>\times</math> 20 mm<math>^3</math> Čerenkov detector. Particles emerging from the cryostat, such as pions from {{Subatomic particle|Proton}}-{{Subatomic particle|Antiproton}} annihilations emit Čerenkov radiation in the detector which is detected by a photomultiplier.<ref name="Hori, Sótér et al., 2022"/>

== ASACUSA collaboration == {{columns-list|colwidth=30em| *Stefan Meyer Institute for Subatomic Physics, ÖAW, Austria *Imperial College London, UK *Experimental Physics Department, CERN, Switzerland *Ulmer Fundamental Symmetries Laboratory, RIKEN, Japan *University of Brescia, Italy *Polytechnic University of Milan, Italy *Hiroshima University, Japan *Nishina Center for Accelerator-Based Science, RIKEN, Japan *University of Tokyo, Japan *University of Milan, Italy *Tokyo University of Science, Japan *Aarhus University, Denmark *Istituto Nazionale di Fisica Nucleare, Italy *Max-Planck-Institut für Quantenoptik, Germany }}

== References == {{Reflist}}

== External links == Record for [https://inspirehep.net/experiments/1110478 ASACUSA experiment] on INSPIRE-HEP Category:Particle experiments Category:CERN experiments