Antihydrogen and Fundamental Physics

101 Pages·2020·1.447 MB·English

by  Michael Charlton, Stefan Eriksson, Graham M. Shore

SPRINGER BRIEFS IN PHYSICS
Michael Charlton
Stefan Eriksson
Graham M. Shore
Antihydrogen
and Fundamental
Physics
SpringerBriefs in Physics
Series Editors
BalasubramanianAnanthanarayan,CentreforHighEnergyPhysics,IndianInstitute
of Science, Bangalore, India
EgorBabaev,PhysicsDepartment,UniversityofMassachusettsAmherst,Amherst,
MA, USA
MalcolmBremer,HHWillsPhysicsLaboratory,UniversityofBristol,Bristol,UK
Xavier Calmet, Department of Physics and Astronomy, University of Sussex,
Brighton, UK
FrancescaDiLodovico,DepartmentofPhysics,QueenMaryUniversityofLondon,
London, UK
Pablo D. Esquinazi, Institute for Experimental Physics II, University of Leipzig,
Leipzig, Germany
Maarten Hoogerland, University of Auckland, Auckland, New Zealand
Eric Le Ru, School of Chemical and Physical Sciences, Victoria University
of Wellington, Kelburn, Wellington, New Zealand
Dario Narducci, University of Milano-Bicocca, Milan, Italy
James Overduin, Towson University, Towson, MD, USA
Vesselin Petkov, Montreal, QC, Canada
Stefan Theisen, Max-Planck-Institut für Gravitationsphysik, Golm, Germany
Charles H.-T. Wang, Department of Physics, The University of Aberdeen,
Aberdeen, UK
James D. Wells, Physics Department, University of Michigan, Ann Arbor, MI,
USA
Andrew Whitaker, Department of Physics and Astronomy, Queen’s University
Belfast, Belfast, UK
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Michael Charlton Stefan Eriksson
(cid:129) (cid:129)
Graham M. Shore
Antihydrogen
and Fundamental Physics
123
Michael Charlton StefanEriksson
Department ofPhysics Department ofPhysics
Collegeof Science Collegeof Science
SwanseaUniversity SwanseaUniversity
Swansea, UK Swansea, UK
Graham M.Shore
Department ofPhysics
Collegeof Science
SwanseaUniversity
Swansea, UK
ISSN 2191-5423 ISSN 2191-5431 (electronic)
SpringerBriefs inPhysics
ISBN978-3-030-51712-0 ISBN978-3-030-51713-7 (eBook)
https://doi.org/10.1007/978-3-030-51713-7
©TheAuthor(s),underexclusivelicensetoSpringerNatureSwitzerlandAG2020
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Acknowledgements
MCandSEaremembersoftheALPHAcollaboration.Theirworkissupportedby
EPSRC grant EP/P024734/1. The work of GMS is supported in part by the STFC
theoretical particle physics grant ST/P00055X/1. We would like to thank
M. Fujiwara, N. Madsen, F. Robicheaux, S. Jonsell and D. P. van der Werf for
helpfuldiscussionsandC.Ø.Rasmussenforvaluablecommentsonthemanuscript.
v
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Fundamental Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 Antimatter and Causality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Lorentz Symmetry and CPT. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Breaking Lorentz Invariance and CPT. . . . . . . . . . . . . . . . . . . . . 13
2.4 General Relativity and Equivalence Principles . . . . . . . . . . . . . . . 18
2.5 Breaking General Relativity and the Equivalence
Principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.6 ‘Fifth’ Forces, B(cid:1)L and Supergravity. . . . . . . . . . . . . . . . . . . . . 30
2.6.1 Gauged B(cid:1)L Theories . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.6.2 N (cid:3) 2 supergravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.6.3 S, V, T Background Fields. . . . . . . . . . . . . . . . . . . . . . . . 37
2.7 Matter-Antimatter Asymmetry and CPT . . . . . . . . . . . . . . . . . . . 39
3 Antihydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.1 Charge Neutrality of Antihydrogen . . . . . . . . . . . . . . . . . . . . . . . 45
3.1.1 Antihydrogen Charge Measurement . . . . . . . . . . . . . . . . . 46
3.1.2 Theoretical Principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.2 Antihydrogen 1S–2S, 1S–2P and 1S Hyperfine
Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.1 Antihydrogen Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.2 Lorentz and CPT Violation . . . . . . . . . . . . . . . . . . . . . . . 50
3.2.3 New Background Fields. . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.3 Antihydrogen and Gravity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.3.1 Antihydrogen Free Fall . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.3.2 Antihydrogen Spectrum and Gravitational
Redshift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
vii
viii Contents
4 Other Antimatter Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
References.... .... .... .... ..... .... .... .... .... .... ..... .... 91
Chapter 1
Introduction
TherecentmeasurementbytheALPHAcollaborationofthe1S–2Sspectrallinein
antihydrogenwithaprecisionofacoupleofpartsin1012[1,2]marksthebeginning
ofaneweraofprecisionanti-atomicphysics.Futureexperimentsonantihydrogen
andotherantimatterspecieswillenableexceptionallyhigh-precisiontestsofmanyof
thefundamentaltenetsofrelativisticquantumfieldtheoryandgeneralrelativity,such
asCPTinvariance,LorentzsymmetryandtheEquivalencePrinciple.Itistherefore
timelytoexaminecriticallywhateachoftheseexperimentsmaybesaidtotestand
whatanyviolationfromstandardexpectationswouldmeanforfundamentalphysics.
Experimentsonpureantimattersystems,whetherelementaryparticlesorbound
statessuchasantihydrogen,areespeciallyinterestingfromthispointofviewsince
theyareconstrainedsodirectlybythefundamentalprinciplesunderlyingthestandard
model.Forexample,thediscoveryofanewZ(cid:2)boson,right-handedneutrinos,asuper-
symmetricdarkmattercandidateetc.wouldbeofimmenseinterestbutcouldreadily
be assimilated into an extension of the standard model. In contrast, an anomalous
result on the charge neutrality of antihydrogen, or a difference in the 1S–2S tran-
sitionsofhydrogenandantihydrogen,wouldimpactdirectlyonthefoundationsof
localrelativisticQFT.Inthesetheories,theexistenceofantiparticleswithprecisely
the mass and spin, and opposite charge, of the corresponding particles is required
byLorentzinvarianceandcausality.Moreover,foralocalQFT,Lorentzinvariance
impliesinvarianceunderCPT,accordingtothecelebratedtheorem[3–6].Antimatter
experimentsthereforedirectlytesttheseprinciples.
The situation is not so clear when we consider gravity, where such experi-
ments are often presented as tests of “the equivalence principle”. The difficulty is
thatthereareseveralversionsoftheequivalenceprincipleintheliterature—weak,
strong,Einstein—withdefinitionswhicharenotalwayseitheruniqueorwell-defined.
Indeed,asemphasisedbyDamour[7],itshouldnotreallybeconsideredasa‘prin-
ciple’ of GR in the more rigorous sense that Lorentz symmetry and causality are
principlesofQFT.Amoresatisfactoryapproachistorecognisethatwehaveawell-
©TheAuthor(s),underexclusivelicensetoSpringerNatureSwitzerlandAG2020 1
M.Charltonetal.,AntihydrogenandFundamentalPhysics,
SpringerBriefsinPhysics,https://doi.org/10.1007/978-3-030-51713-7_1
2 1 Introduction
defined,andextraordinarilysuccessful,theoryofgravityinGR,whichmakesclear
andprecisepredictionsforthegravitationalinteractionsofallformsofmatter.Like
otherexperiments,thoseonantimattershouldsimplybeviewedastestsofthistheory.
General Relativity is based on the idea that gravitational interactions may be
describedintermsofacurvedspacetime.AsdescribedmorepreciselyinSect.2.4,
this spacetime is taken to be a Riemannian manifold, since this has the property
that at each point it locally resembles Minkowski spacetime. The global Lorentz
symmetryofnon-gravitationalphysicsisreducedtolocalLorentzsymmetryincurved
spacetime. This is the mathematical realisation of the physical requirement of the
existenceoflocalinertialframes(i.e.freely-fallingframes)eveninthepresenceof
gravity.Furthertothis,thestandardformulationofGRmakesasimplifying,though
well-motivated,choiceofdynamicsfortheinteractionofmatterandgravity,which
isencapsulatedinthefollowingstatementoftheStrongEquivalencePrinciple:
• In a local inertial frame, the laws of physics take their special relativistic form
(SEP).
Wewillalsodiscussfrequentlytwofurtherexpressionsoftheuniversalityatthe
heartofGR.ThesearebestviewedasexperimentalpredictionsofGR,thoughwe
refertothemhereasversionsoftheWeakEquivalencePrinciple:
• Universalityoffree-fall—allparticles(orantiparticles)fallwiththesameaccel-
erationinagravitationalfield(WEPff).
• Universality of clocks—all dynamical systems which can be viewed as clocks,
e.g.atomicoranti-atomictransitionfrequencies,measurethesamegravitational
timedilationindependentlyoftheircomposition(WEPc).
Takentogether,thesethreepropertiesofGRareusuallyreferredtoastheEinstein
EquivalencePrinciple.1
Apparentviolationsofthesepredictions,especiallyWEPff,canalsoarisenotfrom
theactualviolationofanyfundamentalprincipleofQFTorGRbutfromtheexis-
tenceofnewinteractionsnotpresentinthestandardmodel,so-called‘fifthforces’.
Low-energyprecisionexperimentsonantimatter,whetherinvolvingspectroscopyor
free-fallequivalence principletests,maybesensitivetosuchnewinteractionsand
can place limits on their range and coupling strength. Here, we consider two such
possibilities,bothwell-motivatedbyfundamentaltheory.Thefirstisanextensionof
thestandardmodelgaugegrouptoincludeanewU(1)B−Lfactor,withacorrespond-
inggaugeboson Z(cid:2) couplingto B−L (baryonminusleptonnumber)charge.The
second involves the spin 1 ‘gravivector’ boson which arises in some supergravity
1TheEinsteinEquivalencePrinciplemaybestatedinvariousessentiallyequivalentways.In[8],
thethreeprinciplesarereferredtoasLocalLorentzInvariance(LLI),whichwehavecalledSEP;
theWeakEquivalencePrinciple(WEP),whichissimplyourWEPff;andLocalPositionInvariance
(LPI),whichstatesthat‘theoutcomeofanylocalnon-gravitationalexperimentisindependentof
whereandwhenintheuniverseitisperformed’[8].LPIimpliestheuniversalityofgravitational
redshift,orWEPc,andcanalsobetestedthroughthespaceandtime-independenceoffundamental
constants.NotethatwhileGRimpliesWEPc,thelatterisamoregeneralpropertyofanymetric
theoryofspacetime.Alsonotethat,asdescribedabove,ametrictheorylikeGRonaRiemannian
spacetimemanifoldexhibitsLLI,butthedynamicsneednotbethesameasspecialrelativity,orbe
independentofthelocalcurvature,ifSEPisviolated.