Hyperfine InteractDOI 10.1007/s10751-014-1037-4

Experimental tests of fundamental symmetriesAn overview

Klaus P. Jungmann

© Springer International Publishing Switzerland 2014

Abstract Ongoing experiments and projects to test our understanding of fundamental inter-actions and symmetries in nature have progressed significantly in the past few years. At highenergies the long searched for Higgs boson has been found; tests of gravity for antimatterhave come closer to reality; Lorentz invariance - up to now very well tested in electromag-netic interactions - is now being challenged in dedicated experiments in weak interactionsas well; discrete symmetries are scrutinized in experiments on C, P, T, CP, and CPT throughsearches for permanent electric dipole moments and precise measurements of atomic parityviolation and in precision measurements involving antiprotons; a new generation of pre-cise flavor symmetry tests is coming up; and last but not least precision measurements ofStandard Model values of important fundamental constants and quantities such as the muonmagnetic anomaly are underway. All the ongoing projects in this most active field have arobust potential for discovery of New Physics, i.e. physics not yet covered by the well estab-lished Standard Model of particle physics. Antiprotons have a central position in this livelyfield with many new and novel upcoming experiments and facilities.

Keywords Fundamental symmetries and interactions · Exotic atoms · Precision tests ofstandard model · Discrete symmetries · Global symmetries

1 Introduction

As an empirical science physics depends on the constructive interplay between experi-mentalists and theorists. Any theoretical model of nature is only as good as it has beenexperimentally verified. Any theory that cannot be tested in experiment has no standing inscience at all. In particle physics the Standard Model is an enormously successful theoreticalframework that can describe all properly established experimental observations. However,

Proceedings of the 11th International Conference on Low Energy Antiproton Physics (LEAP 2013)held in Uppsala, Sweden, 10–15 June, 2013

K. P. Jungmann (�)Faculty FWN, University of Groningen, Zernikelaan 25, NL 9747 AA Groningen, Netherlandse-mail: k.h.k.j.jungmann@rug.nl

K. P. Jungmann

it lacks in many cases the ability to provide deeper explanations for observed facts. Exam-ples of this are the observed number of three particle generations or the origin of the matteranti-matter asymmetry in the universe.

The area of fundamental symmetries and interactions touches the basics of our under-standing of the physical world; a major driving force in the field arises from the potentialto steer model building decisively though testing models decicively that try to extend ourpresent established theory framework towards including explanations of observed factswhere no underlying deeper theory has been found so far. Experiments at high energies,such as searches for new particles, as well as precision experiments mostly at lower ener-gies, which test the results of accurate calculations within the already established theoryframework, are complementary. This provides powerful means to test new theories that tryto expand the standard theory in order to provide deeper explanations.

In the present decade a considerable number of such experiments has booked significantprogress already; only a small selection of them can be covered here.

2 The higgs particle at LHC

It has been a major highlight in physics when in the summer of 2012 at CERN the undoubteddiscovery of a new particle has been announced, which appears to have the major featuresof the long searched for Higgs boson (H ), the last yet missing massive object within theStandard Model. In various decay channels, such as H → bb, H → bb, H → ττ , H →γ γ , H → WW and H → ZZ, the particle was found simultaneously by both the ATLASand the CMS collaborations. The signals appeared above background with more than 5 σ

significance and at a consistent mass of some 126 GeV/c2 [1, 2]. The particle completessurprisingly well our picture of the physical nature. This observation is a major triumph ofphysics and can certainly not be overestimated as such. The Standard Model as a completeconcept for the description of fundamental particle physics can by now be considered fullyconfirmed; although, there are plenty of intriguing questions still open. They relate to the yetunknown deeper underlying concepts that would enable us to provide explanations ratherthan a description for may observations and plain facts.

It may be considered also a surprise that so far no unexpected or fundamentally new par-ticles have appeared otherwise at LHC. Unless the upcoming upgrade of LHC in energyby some factor of two will yield such a discovery, high energy physics faces the fact thatthe hunt for direct discoveries of new particles is not necessarily the only and most promis-ing road towards new fundamental insights. Precision studies of the behavior of nature inthe accessible energy regime are therefore an important independent complementary roadtowards potential discoveries and hints that would enable us to come closer to a solutionof the puzzle. The LHC data, e.g., is therefore also analyzed for various possible new phe-nomena and more exotic decays. Among those are searches for supersymmetry, Lorentzinvariance violation and many others.

3 Gravity

Among the four fundamental forces we know gravity is the one that is least understood. Itis not an integral part of the yet successfully confirmed Standard Model. There is still notheory with standing in physics that combines gravity and quantum mechanics. Although

Experimental tests of fundamental symmetries

there are many approaches pursued, none of them has yet led to a theory with experimentallyverifiable consequences. Therefore, experimental input can provide hints on how to proceed.

It is an unsolved question, yet, whether the gravitational mass of particles and anti-particles are identical. There are motivated opinions published on why there might be nodifference [3].1 The absence of the decay of the long lived kaon into two pions has beeninterpreted already in 1960 as an equality of the gravitational mass at the level of a fewparts in 10−10; it had been well recognized already then that the gravitational mass of kaonshas never been measured at all. Therefore any conclusions on gravity from kaon experi-ments can be considered to miss an essential prerequisite, independent of the sometimespronounced opinions [3]. Furthermore, the arguments are based on a quantum mechanicalinterpretation of gravitational effects. This appears to be somewhat premature in view of theabsence of a confirmed theory that is able to combine quantum mechanics and gravity, andin particular to assign eigenstates to gravity. For short lived particles also the concept of astationary state concerning gravitational interaction is an issue.

Among the various experiments underway which aim to measure the anti-gravity of anti-protonic atoms, the ALPHA collaboration has made a first step by setting limits on anti-hydrogen gravity. They have investigated the time dependence of the decay pattern of anti-hydrogen. The results pose a limit at 95% C.L. on the ratio of the gravitational mass tothe inertial mass to be < 110. It can be excluded that anti-hydrogen falls upwards witha gravitational mass > 65 times its inertial mass [4]. With this success the race towardspinning down the nature of the gravitational mass of anti-matter has started. In view of thefact that the mass of the proton and the anti-proton are not only due to the constituent quarksand anti-quarks and in view of the fact that gluons may be the same in both particles as faras gravitation is concerned, the question of anti-matter gravity can be considered to be morecomplex than just a difference in sign for the particle and anti-particle masses. One canrather motivate the expectation of small differences in the masses. At CERN-AD severalpromising experiments are underway to scrutinize anti-matter gravity.

4 Lorentz invariance

Precision tests of Lorentz invariance have been in the center of precision experiments withenormous activity in the past decade [5]. Since a general framework has been providedthrough the Standard Model Extension [5] experiments with mindboggling precision couldbe performed, testing for new physics well beyond the Planck scale. Despite the big successof these experiments, they have one common drawback: they all have tested with highestaccuracy only effects in the electromagnetic interaction. Since we know that discrete sym-metry violations are only present in weak interactions, we might wonder, whether Lorentzinvariance violation might also not be present in electromagnetic interactions. This stronglymotivates new experiments on, e.g., weak processes. Those can have a discovery potentialalready for experiments at much lower levels of precision, since they are exploring a com-pletely new territory. Such dedicated experiments exploiting weak interactions have justbeen started [6].

Among the tests of Lorentz invariance in electromagnetic interactions are spin precessionexperiments where the free spin precession of polarized 3He and 129Xe in mixed samples

1Similarly less founded argumentation had prevented searches for, e.g., parity violation before its discoveryin 1957, because one just was convinced to know that discrete symmetries would always hold.

K. P. Jungmann

(with spin relaxation times of 4-6 hours and more than 60 hours, respectively) has beenrecorded as a function of siderial time. No effect was found. From this, various Lorentzviolating parameters could be limited to the region of 10−27 GeV for protons, 3.72 · 10−32

GeV for neutrons and 10−31 for electrons [7]. Even better limits are about to be published[8].

At the TRIμP facility at the AGOR cyclotron of KVI in Groningen, NL, Lorentz viola-tion in weak interactions is being tested in the β-decay 20Na → 20Ne+β++γ . The β-decayasymmetry has been monitored as a function of the siderial time. To suppress systemat-ics the polarization was flipped every 4 seconds and the spatially symmetric γ -radiationwas used as an independent sample monitor. A novel limit on Lorentz violation could beestablished [6, 9] already in an early measurement campaign. More precise data is beinganalyzed. The experiments are complemented by theoretical work that enables comparisonof various experimentally accessible parameters in a now common framework also for weakprocesses [10, 11] .

5 Discrete symmetries

5.1 Anti-proton experiments

Among the ongoing discrete symmetry tests the ongoing work on anti-protons and atomicsystems with anti-protons plays a central role. The field has provided a plurality of preci-sion measurements that provide for comparing properties of matter and anti-matter particles.Anti-protonic helium experiments have been pioneering spectroscopic measurements insuch exotic systems [12].

Recently spin flips of single protons in the lowest quantum state in a Penning trapcould be induced [13, 14] and a measurement of the magnetic moment could already bemade in a single particle despite individual spin flips could not yet be seen. The experi-ment could also be performed on single anti-protons at CERN by the ATRAP collaboration.The experiment yielded the ratio of the magnetic moments of particle and anti-particle asμp/μp = −1.000 000(5) (5 ppm) [15]. This year also two groups at Mainz and Har-vard succeeded in observing individual spin flips from trapped single protons [16, 17].This promises a significant boost for the accuracy of the magnetic moment measurements.The further competing BASE experiment which has started recently also at CERN hasannounced their goal in precision to be below 1 ppb for the anti-proton.

Last year also the first spectroscopy experiment in an anti-atom could be performed bythe ALPHA collaboration; hyperfine transitions could be induced in anti-hydrogen atoms[18]. The measurement was conducted through flipping the positron spin in magneticallytrapped anti-hydrogen atoms by resonant microwave radiation. As a consequence the atomswere put in a non-trapping state and thereby ejected from the trap.

The hyperfine structure of anti-hydrogen arises from a contact interaction between theanti-proton and the positron. This is expected to be more sensitive to short range newforces rather than the gross structure. Several experiments are underway to measure theground state hyperfine splitting, e.g., in a beam of cold anti-hydrogen atoms in a Rabi-typeapparatus of the ASACUSA collaboration [19]. The experiment is rapidly progressing andpromises ultimately the best test of CPT, if interpreted in the framework of the StandardModel Extension [20].

Experimental tests of fundamental symmetries

5.2 Searches for permanent electric dipole moments

Among the experimental approaches to test discrete symmetries there is a plurality ofsearches for permanent electric dipole moments (EDMs, see e.g. [21]), which would violateboth parity (P) and time reversal (T). They are considered promising approaches to find newsources of CP violation and thereby might explain the matter-antimatter asymmetry in theuniverse. The search for an EDM in 199Hg provides the tightest bound of all searches foran EDM at 3.1 · 10−29 e cm (95% C.L.)[22]. It can be interpreted in terms of various limitson CP violating mechanisms The modern experiments in atoms, molecules or in ions try toexploit significant enhancement factors of elementary particle EDMs in composed systems.As such enhancement factors for elementary particle EDMs in composed systems can be oforder up to 106, new precision experiments in systems such as molecules [23, 24] promisesignificant improvements over the present limits. In this field, control of systematic effectsand possible fake signals is the most urgent issue. A particularly interesting possibility arisesfor gas mixtures of 3He and 129Xe where the long coherence times promise improvementsof up to 4 orders of magnitude over the bound established for 199Hg[8, 21].

In nuclei of Rn and Ra level of opposite parity lie close to each other (see e.g. [25]).Experiments to search for a nucleon EDM in these atoms are underway and preparationexperiments take place (see e.g. [26]).

Progress concerning EDMs in the next years can be expected from such table top exper-iments primarily, as possible EDM projects with charged systems in storage rings (see, e.g.[27–29]) are still in an orientation phase. They are primarily concerned with the develop-ment of necessary equipment and principal experimental techniques (see e.g. [30]). On thetheory side there has been progress understanding possible mechanisms that can induceEDMs in particular for lighter nuclei [31]. Although anti-protons are stable particles andstorage ring experiments for protons also would work for anti-protons, experiments capableto reach a competitive limit in the range of 10−29 e cm or beyond are not yet within sightand must be deferred to the longer term future.

5.3 Precise measurement of atomic parity violation

Atomic parity violation is best suited to test the electro-weak running at a low scale ofmomentum transfer. Atomic systems with a well calculable atomic structure and a highnuclear charge Z are favorable, because the weak effects scale stronger than Z3 and conse-quently experiments concentrate on heavy atoms and ions with one electron in the valenceshell [32–34]. Whereas the alkali-like systems Fr and Ra+ promise an improved value forthe Weinberg angle, other systems can be better suited to investigate nuclear properties suchas the anapole moment. For Ra+ a 5-fold improvement in the value of the Weinberg angleappears possible for measurements on a single Ra+ ion within a week of measurement time[35]. Also molecules such as SrF and RaF render the possibility to study weak interactioncontributions and in particular to study effects such as nuclear anapole moments [36]. Theatomic parity experiments are time consuming as next to most accurate theory values for themeasured quantities at the sub-% level highest precision experimental setups are required.For sensitive experiments with, e.g., radioactive species such as Fr and Ra, where the effectsare large [37], the effective trapping of the isotopes is essential. A profound understandingof the atomic level structure requires a constructive interplay of theorists and experimental-ists [35]. For experiments on molecules effective deceleration and trapping appears to be aprerequisite. Corresponding developments are well on their way [38].

K. P. Jungmann

6 Global symmetries

Global symmetries correspond to conservation laws. In modern physics we know of manyconserved quantities without knowing a fundamental symmetry that is associated with it.Examples are baryon and lepton number conservation or charged lepton flavor conserva-tion. That calls for precise experiments to find violations of these laws. In many modelsthat try to extend the Standard Model global symmetry violations appear naturally. Thenon-observation of such violations can in reverse rule out some of the speculative models.Therefore precision experiments that search for global symmetry violations have a robustpotential to steer model building in fundamental particle physics.

Searches for rare muon decays are in part motivated by the possibility of such decaysoccuring in supersymmetric scenarios. However, the decays are possible in many more spec-ulative frameworks. This year a new constraint has been set by the MEG collaboration atthe Paul Scherrer Institute in Villigen, Switzerland, on the decay μ+ → e+ + γ . In theCOBRA detector, which has been designed to measure precisely the energy and momen-tum of the monoenergetic γ from the decay, at 90% C.L. the branching ratio for this decayhas been found to be below 5.7 · 10−13 [39]. The collaboration has proposed an improve-ment by one more order of magnitude. At PSI also a new effort is starting to search forthe process μ → eee with an ultimate goal for a branching ration below 10−16 [40]. Plansexist for improving the limit on the process μN → eN to branching ratios of order 10−17

in the Mu2e experiment at Fermilab [41] and of order 10−18 in the COMET experiment atJ-PARC [42]. At such precision models like supersymmetry can be decisively tested.

7 Precision tests of the standard model

The muon magnetic anomaly aμ is a Standard Model quantity that has been measured to 0.5ppm in an experiment at the Brookhaven National Laboratory, Upton, New York, USA. Theanomaly of muons of both possible signs of charge agree at 0.7 ppm [43]. It appears thatthe experimental and the standard model value differ momentarily by some 4 standard devi-ations. Whereas the experimental value remains unchallenged by now for almost a decade,the theoretical value has been subject to several refinements and finally the theory valuesobtained along different routes agree now [44].

In order to settle the questions whether the difference between theory and experimentis manifest, a new experiment has been started at the Fermi National Laboratory, Chicago,USA. It uses the same experimental principle as the Brookhaven experiment. In particularthe same experimental concept, and most of all crucial pieces of equipment such as the samestorage ring magnet and the same magnetic field control concept will be used again. Thedetectors and the data acquisition will be upgraded and in particular a more intense muonbeam with much less contaminations in the particle beam will be employed. The experimentaims for half an order of magnitude improvement in aμ for positive muons, which willessentially be achieved by a higher number of muons that can be exploited at the new site.

At J-PARC a novel experimental approach to measure aμ is in its R&D phase. Theexperiment employs a small diameter storage ring at lower than magic momentum (wherethe influence of the necessary electric focussing field is cancelled). From the experimentone can expect largely different systematics as compared to the Fermilab experiment. Thisindependent experiment will therefore be very important once the Fermilab experiment willconfirm or not confirm the present difference between experiment and theory.

Experimental tests of fundamental symmetries

On the theory side the value of the g-2 result will depend on the general acceptance of thetheoretical approach towards the hadronic light-by-light scattering contribution. Its valueneeds to be taken from calculations and can unfortunately not be measured directly in anindependent way. Therefore there is room for providing a Standard Model theory value thatwill be generally undisputed by the time the experiment will produce a new value. A rigor-ous interpretation of a new muon g-2 experiment will also require better knowledge of themuon magnetic moment μμ, which can be obtained, e.g., from an improved measurement ofthe muonium ground state hyperfine structure splitting. Such an experiment is progressingat J-PARC in Jaeri, Japan [45].

Beyond aμ numerous other Standard Model quantities have been determined recently,such as, e.g., the muon lifetime τμ[46], the electron magnetic anomaly μe and the finestructure constant α [47] or details of the solar neutrino spectrum [48]. They all form arobust backbone and serve as a reference system against which all searches for new physicsmust be held in order to decide whether new physics may have been found. Any preciselymeasured value - no matter of which quantity - is an important piece in the puzzle alwaysin its own right. Preferences [49] which of the presently ongoing experiments would havehigher potential to find New Physics or to point out new directions can not be based on anyunbiased knowledge.

8 Conclusions

The progress in the recent years in the field of fundamental interactions and symmetriesis very encouraging and we can look forward to many more important results in the nearfuture. In particular, the availability of more anti-protons, such as it will be realized withthe ELENA ring at CERN [50], will enable an increasing activity in low energy anti-protonphysics towards unraveling the structures underlying the Standard Model and to limit or findsignatures of new physics. This will happen in tight competition of anti-proton experimentswith complementary experiments on particles and samples of ordinary matter.

Acknowledgments The author would like to thank LEAP 2013 for the the invitation to speak on this topicand for the support received. This work has been supported in part through the research programmes TRIμPas well as Broken Mirrors and Drifting Constants of the Foundation for Fundamental Research on Matter(FOM), which is part of the Netherlands Organization for Scientific Research (NWO).

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