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In the Steps of the Antiproton CERN Courier September 2015 Antiprotons INNOVATION FOR A BRIGHT FUTURE MODULES SERVICES VALVES In the steps of the FROM CONCEPT TO PRODUCT 24/7 GLOBAL SUPPORT LEADING TECHNOLOGY COMPACT SPARE PARTS HIGH PURITY antiproton CLEAN LOCAL REPAIR ENGINEERED SOLUTIONS EFFICIENT UPGRADES AUTOMATED PRODUCTION Sixty years after the discovery of the antiproton MEET US AT at Berkeley, a look at some of the ways that VACUUM EXPO pion 2015 studies with antiprotons at CERN have cast BOOTH # W03 light on basic physics and, in particular, on antiproton proton fundamental symmetries. pion www.vatvalve.com On 21 September 1955, Owen Chamberlain, Emilio Segrè, Clyde 100 μm Wiegand and Tom Ypsilantis found their fi rst evidence of the anti- proton, gathered through measurements of its momentum and its velocity. Working at what was known as the “Rad Lab” at Berkeley, Fig. 1. One of the fi rst annihilations of an antiproton observed at they had set up their experiment at a new accelerator, the Bevatron the Bevatron with a photographic emulsion. The antiproton – a proton synchrotron designed to reach an energy of 6.5 GeV, enters from the left. The fat tracks are from slow protons or INDUSTRIAL suffi cient to produce an antiproton in a fi xed-target experiment nuclear fragments, the faint tracks from fast pions. (Image ELECTRONIC (CERN Courier November 2005 p27). Soon after, a related experi- credit: O Chamberlain et al. 1956 Nuo. Cim. 3 447.) WIRING LTD ment led by Gerson Goldhaber and Edoardo Amaldi found the SWINDON WILTSHIRE expected annihilation “stars”, recorded in stacks of nuclear emul- Sam Ting (CERN Courier May 2009 p15 and October 2009 p22). sions (fi gure 1). Forty years later, by combing antiprotons and posi- A decade later, evidence emerged for antihelium-3 and antitritium YOUR PARTNER IN CONTRACT trons, an experiment at the Low Energy Antiproton Ring (LEAR) in the WA33 experiment at CERN’s Super Proton Synchrotron, YOURELECTRONIC PARTNER MANUFACTURE IN CONTRACT at CERN gathered evidence in September 1995 for the production following the sighting of a few candidates at the 70 GeV proton syn- ELECTRONIC MANUFACTURE of the fi rst few atoms of antihydrogen. chroton at the Institute for High Energy Physics near Serpukhov. Over the decades, antiprotons have become a standard tool for More recently, the availability of colliding beams of heavy ions studies in particle physics; the word “antimatter” has entered into has led to the observation of antihelium-4 by the STAR experi- mainstream language; and antihydrogen is fast becoming a labo- ment at Brookhaven’s Relativistic Heavy-Ion Collider (CERN ratory for investigations in fundamental physics. At CERN, the Courier June 2011 p8). At CERN, the ALICE experiment at the Antiproton Decelerator (AD) is now an important facility for stud- LHC observes the production of light nuclei and antinuclei with ies in fundamental physics at low energies, which complement the comparable masses and therefore compatible binding energies investigations at the LHC’s high-energy frontier. This article looks (fi gure 2, p22). back at some of the highlights in the studies of the antiworld at CERN, and takes a glimpse at what lies in store at the AD. Exit baryonium, enter new mesons Mileon™- the longest hollow Back at the Bevatron, the discovery of the antineutron through Back in 1949, before the discovery of the antiproton, Enrico Fermi neutral particle annihilation followed in 1956, setting the scene and Chen-Ning Yang predicted the existence of bound nucleon– conductor in the world Developed in response to the customer for studies of real antimatter. Initially, everyone expected perfect antinucleon states (baryonium), when they noted that certain repul- demand for seamless magnetic windings. symmetry between matter and antimatter through the combination sive forces between two nucleons could become attractive in the of the operations of charge conjugation (C), parity (P) and time nucleon–antinucleon system. Later, quark models based on dual- reversal (T). However, following the observation of CP violation ity predicted the existence of states made of two quarks and two in 1964, it was not obvious that nuclear forces were CPT invari- antiquarks, which should be observed when a proton annihilates ant and that antinucleons should bind to build antinuclei. These with an antiproton. In the 1970s, nuclear-potential models went doubts were laid to rest with the discovery of the antideuteron at on to predict a plethora of bound states and resonance excitations Tel +44 (0) 1793 694033 [email protected] CERN by a team led by Antonino Zichichi, and at Brookhaven by around the two-nucleon mass. There were indeed reports of such iew.co.uk ▲ UNIT 10, BIRCH, KEMBREY PARK, SWINDON, WILTSHIRE, SN2 8UU a team from Columbia University, including Leon Lederman and states, among them narrow states observed in antiproton–proton 21 Luvata_Mileon_CERN_advert_10_ 09_13_.indd 2 12/09/2013 10:22 CERNCOURIER www. V OLUME 5 5 N UMBER 7 S EPTEMBER 2 0 1 5 CERN Courier September 2015 CERN Courier September 2015 Antiprotons Antiprotons ICE, the AA and LEAR 3 The construction of LEAR took advantage of the antiproton facility that was typical intensity of 106/s. LEAR started up in 1982 and saw as many as f2(1565) f (1500) built at CERN in 1980 to search for the W and Z bosons at the Super Proton 16 experiments before being decommissioned in 1996. The LEAR magnet 0 – ) 2 Synchrotron (SPS) operating as a pp collider (CERN Courier December 1999 ring lives on in the Low Energy Ion Ring, which forms part of the injection 2 f (1270) p15). The antiprotons originated when 26 GeV protons from the PS struck chain for heavy ions into the LHC. 2 ) (GeV 0 a target. Emerging with an average momentum of 3.5 GeV/c, they were LEAR also benefi tted from the Initial Cooling Experiment (ICE), a π 0 π collected in the Antiproton Accumulator (AA), and a pure antiproton beam storage ring designed in the late 1970s to test Simon van der Meer’s idea ( 2 1 with small transverse dimensions was generated by stochastic cooling. Up to of stochastic cooling on antiprotons, and later to investigate electron m f0(980) 1012 antiprotons a day could be generated and stored. The antiprotons were cooling. After essential modifi cations, the electron cooler from ICE went then extracted and injected into the PS. After acceleration to 26 GeV, they were on to assist in cooling antiprotons at LEAR, and is now serving at CERN’s transferred to the SPS where they circulated in the same beam pipe as the current antiproton facility, the AD (CERN Courier September 2009 p13). ICE protons, but in the opposite direction. After a fi nal acceleration to 270 GeV, the also contributed to measurements on antiprotons, when in August 1978, it 0 0 1 2 3 antiprotons and protons were brought into collision. successfully stored antiprotons at 2.1 GeV/c – a world fi rst – keeping them m2 (π0π0) (GeV2) For injection into LEAR, the 3.5 GeV/c antiprotons from the AA were circulating for 32 hours. The previous best experimental measurement decelerated in the PS, down to 600 MeV/c. Once stored in LEAR, they of the antiproton lifetime, from bubble-chamber experiments, was about Fig. 3. Left: The Crystal Barrel experiment installed at LEAR. Right: The Dalitz plot measured by Crystal Barrel for p̅p annihilation were further decelerated to 60 MeV/c and then slowly extracted with a 10 –4 s; now, it is known to be more than 8 × 105 years. into π0π0π0. The bright (dark) zones correspond to high (low) event density. For symmetry reasons, there are six entries per event. (Image credits: CERN and Crystal Barrel Collaboration.) of the pions had decayed. The high intensity of antiprotons (106/s small length scales, and could lead to slight differences between 0.04 Pb–Pb compared with about 102/s in extracted beams at the PS) and a high the properties of particles and antiparticles, such as lifetime, –3 –4 3He 3He momentum resolution of 10 –10 was crucial at low energies for inertial mass and magnetic moment. antiprotons stopping with very small range-straggling. At LEAR, the TRAP collaboration (PS196) performed a series 0.02 t t d The spectroscopy of mesons produced in p̅p annihilation at rest of pioneering experiments to compare precisely the charge-to- d T p p in several experiments at LEAR proved to be much more fruit- mass ratios of the proton and antiproton, using antiprotons stored A 0 K– K+ ful. This continued a tradition that had begun in the 1960s with in a cold electromagnetic (Penning) trap. The signal from a single 2 10 – e e+ antiprotons annihilating in the 81 cm Hydrogen Bubble Chamber stored antiproton could be observed, and antiprotons were stored at the PS, leading to the discovery of the E meson (E for Europe, in the trap for up to two months. By measuring the cyclotron fre- –0.02 now the η(1440)) and the D meson (now the f1(1285)) in p̅p → (E, quency of the orbiting antiprotons with an oscillator and compar- dE/dx in TPC (arb.units) – + – 2 6 10 14 18 π π D → KKπ)ππ. The former led to the long-standing controversy ing it with the cyclotron frequency of H– ions in the same trap, the t (τs) about the existence in this mass region of a glueball candidate – a team fi nally achieved a result at the level of 9× 10 –11. The experi- state made only of gluons – which was observed in radiative J/ψ ment used H– ions instead of protons to avoid biases when reversing Fig.
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