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T Society on29 July,Society physicist whenparticle meetingof theEuropeanvirtual Physical of four andfive , respectively. calledhadrons, , sofar.62 non-elementary includetetraquarks These made andpentaquarks — particles The Large Colliderdiscovered ,theHiggsboson,in2012. anelementary Butithasalsodiscovered PARTICLE DISCOVERIES 330 building blocks of and the fundamental called thestandard model,describes thebasic thathostslaboratory theLHC. based atCERN,theEuropean particle- are allworld firsts,” says Koppenburg, whois Subatomic Physics inAmsterdam. “These with Nikhef, theDutch NationalInstitute for Patrick Koppenburg, aparticle physicist discoveries’), according to a tally kept by LHC’s hadron bounty up to 62 (see ‘Particle hadron made of four quarks. This brought the York unveiled apreviously unknown exotic Ivan Polyakov atSyracuse University inNew and , are madeof quarks. cles calledhadrons —those that,like netted dozens parti- of thenon-elementary ButtheLHChasalso particles. elementary keystonefinal of thecurrent classification of 2012, adiscovery thatslotted into place the By Davide Castelvecchi test theoriesaboutstrong nuclearforce. Rare could helpphysicists to News in focus LARGE HADRON PARTICLE SPOTTED AT EXOTIC FOUR- Number of discovered The established pantheon of particles, The latest hadron made its debut at the 60 40 30 50 20 70 10 0 | |Vol 596 |19 August 2021 2011 the existence of theHiggs in land, isfamousfordemonstrating smashernearGeneva, Switzer- also a bighadron discoverer. The he (LHC)is Tetraquark 2012 2013 2014 2015 2016 Vanya Belyaev at the Institute for Theoretical collisions, Polyakov andhiscollaborator Sifting through dataonthedebrisfrom announced by Polyakov was no exception. tunnel that holds the LHC, and the particle detectors inthe27-kilometre-long circular been spotted by LHCb, oneof thefourgiant Most of the LHC’s new hadron types have Quark quartet collisionshead-on between protons. new kindsof hadron by causinghigh-, in afraction of asecond. theLHCcreates So the collision of otherparticles, anddecay All otherhadrons formonlyfleetingly, from when they are incorporated into atomic nuclei. stable in isolation — neutrons are stable only protons (two upsandonedown). include neutrons (one up and two downs) and ‘up’ and‘down’; theirpossible combinations two most common quarksinnature are called , oneof thefourfundamentalforces. The quarks are heldtogether by thestrong nuclear form composite particles called hadrons. The ard modelalsoincludesrulesforhow quarks including andphotons. Thestand- and several particles, otherelementary of quark,theirsixantimatter counterparts on them. Itincludes sixflavours thatact Protons are the only hadrons known to be © 2017

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A l l r i g h t 2021 s r e s e r v e d 2022 . discoveries shouldbetreated asdiscrete stunned. reaction “Myfirst was: it’s my properties soprecisely thatBelyaev was Phys. Rev. Lett . about how interact particles through the anew helpsphysicists to test theirtheories tight , says Belyaev. Butcreating them all matter was compressed inanextremely instants ofduring thefirst theUniverse, when ties as T existence of a particle with the same proper University inIsrael andhelpedto predict the offirst its kind,” says Karliner, who is at Tel Aviv genuine, tightly bound quadruplet. “It’s the Karliner thinksthatthelatest onecould bea a ,buttheoretical physicist Marek doublets attachedto eachotherlike in were likely to bepairs of quark ordinary the new one is an oddity. Previous 2003, andLHCbhasseen several more. But Organization (KEK) inTsukuba, , in ted Accelerator attheHighEnergy Research three tetraquark quarks.Thefirst was spot - known hadrons are madeupof eithertwo or hadronfour-quark —calledT the expected signature of a‘tetraquark’ —a and in Moscow found . Belyaev addsthat T 3,000 better thaninthediscovery of the was nailedwithamargin of error nearly , whichisaround 4 times that of aproton, mistake,” hesays. For example, theparticle’s strong nuclearforce. particle is,” hesays. that we needabetter of whata definition “Itendparticles. to beincreasingly convinced is amemberof theLHCbcollaboration. Koppenburg, who, like Polyakov andBelyaev, plain sight,sittinginthedataona laptop,” says how many otherstates are there just hiddenin catalogue are excited states. knows “Who the majorityof inKoppenburg’s particles have been found experimentally, and in fact is classified as aseparate particle. Many for example, how fast they —andeach heavier ‘excited states’ — distinguished by, there isanalmost limitless numberof possible nearly 50 have beenseen. (and asmany tripletsof antiquarks),of which been observed, and75 possible quarktriplets ble 2-quark hadrons, allbutoneof whichhave hadrons. Karliner says that there are 50 possi of combinations of quarkscangive rise to The search for new hadrons will go on. Dozens Limitless possibilities other particlesto lookfor. ituntilnowfind because they hadalonglist of the LHC, but he and his LHCb colleagues didn’t been discovered in data from the early years of In nature, tetraquarks probably existed only Tetraquarks are extremely unusual: most But healsowonders whetherallthese Moreover, foreachcombination of quarks, The data revealed the new particle’s cc + in 2017 (M. Karliner and J. L. Rosner 119 , 202001; 2017). 202001; , cc + . cc + could have - -

SOURCE: PATRICK KOPPENBURG