,fe-a very successful theory for physics phe­ nomena, valid for many human applica­ Testingdiscrete symmetries tions. What about the symmetry proper­ ties of the other theories? There is an im­ portanttheorem,known as the CPT theo­ in Kdecays rem, which states that, under very general assumptions, any theory of microscopic P.DEBU- (EA Saclay- France interactions must respect the CPT sym­ February7,2000 metry. This means that in the present day the­ oretical framework ofparticle physics, it is Discrete symmetries, P and C Weak interactions and violation of P and believed that the laws of physics are in­ hen physicists try to lay down the Csymmetries variant under CPT. Wlaws that govern the processes they There are four known fundamental As a consequence, CP symmetry im­ are studying, they use as a first guidance interactions between elementary plies T symmetry, and vice-versa, because the most general properties ofthe system. . Gravitation is felt by all any CP violation should be compensated There are fundamental and universal particles. Charged particles undergo by some T violation to follow the CPT the­ examples. The laws ofNature are believed electromagnetic interactions. The orem. to be independent of the position of the and the neutron are bound in The 1957 observation that CP symme­ observer, or of his orientation. One says nuclei by the strong interaction. Weak try still holds could be seen as a necessary that they are invariant by translation and interactions transform into one consequence that microscopic phenome­ rotation. another and cause the f3decays of na obey the T symmetry. Consider now a simple experiment,like unstable nuclei. playing billiards. Imagine you are looking It was assumed for a long time that the CPviolation at the image ofthe game in a large mirror fundamental interactions respect both P CP violation was discovered in 1964 in instead of looking directly at the billiard and C symmetries, but experimental ob­ an experiment dedicated to the study of table. Would you be able to tell from the servations led two theoreticians, T.D. Lee KOand KOmesons. It was performed by moves ofthe balls that you are looking at a and C.N. Yang, to suggest in 1956 that P J.H. Christenson, J.W. Cronin, Y.L. Fitch virtual image? The answer is no. Laws of and C symmetries might be violated by and R. Turlay. classical mechanics are obeyed bythe mir­ the weak interactions. One year later, an The KOis a made of an anti s ror experiment. They are invariant under experiment led by C.S. Wu, a detailed (8) and a d quark. Because weak in­ the transformation called Parity (P), study ofthe f3decay of C0 60 in a magnetic teractions can change a in­ which consists in changing the sign ofspa­ field, brought indeed the proofthat P sym­ to a non strange quark, both KOand KO tial coordinates. metry was violated by weak interactions. mesons can decay into the same final state We say that classical mechanics respect Additional theoretical work showed that of 2 , and moreover, by a succession the P symmetry. at the same time, the C symmetry was vi­ of two weak processes, the KOcan become Imagine that instead of looking at the olated. However, it was also realized that a KOand vice-versa! billiards game in a mirror, you replace the observed processes do respect the CP During the propagation of the K every piece ofmatterbyantimatter. Could transformation, the one obtained by ap­ mesons, a continuous KOH KOoscillation you tell the difference from the moves? plying both the C and the P transforma­ occurs. Were CP conserved, the propagat­ The answer is again no. Laws of classical tions. In other words, the virtual experi­ ing states would be the symmetric and an­ mechanics apply to . The oper­ ment obtained by looking at the original tisymmetric states superpositions ation which consists in replacing a particle process in a mirror AND by replacing all K 1 = (KO+ [(0)1"'/2and K 2 = (KO- KO)1"'I2. by its is called Charge Conju­ particles by their antipartners would obey The 2 state issued of a K decay being gation (C). the known properties of weak interac­ a CP even state, this decay would be al­ Classical mechanics respect the C sym­ tions. lowed for the K 1 which is also a CP even metry. This was a very important observation. state, andforbidden for the K 2,which is CP Every particle has its corresponding an­ odd. tiparticle, which has the same mass butthe Time reversal and CPTsymmetry Indeed, physicists observed both a opposite charge. This is the origin of the There is a fundamental reason why CP short lived K meson, called Ks, decaying name Charge Conjugation. The antiparti­ symmetry plays a crucial role. It is indeed into 2 pions, and a 500 times longer lived cle ofthe e- is the positron e+.The linked to the Time Reversal transforma­ K L which could only undergo three body antiparticle ofthe p is the antipro­ tion (T). decays. The whole picture seemed consis­ ton p.The proton is made of 3 quarks, 2 u This transformation consists in "look­ tent with the previous description. quarks and 1 d quark. The an!iproton is ing" at an experiment backwards in time, The 1964 experimental demonstration made of3 antiquarks,2 ii and 1d The neu­ like playing a movie backwards. Although, ofCP violation was the detection ofKL de­ tron is made of3 quarks, 2 d quarks and! at the macroscopic level, one can immedi­ cays into 2 charged pions. Although this u quark. The antineutron is made of 2 d ately distinguish which is the real experi­ happened only rarely, about twice every and 1 ii. One can imagine building an an­ ment and which is the reversed movie, this thousand decays, it was a clear sign of CP tiworld from these elementary pieces. If is not a priori the case at the microscopic violation in microscopic processes. the laws of Nature would be C symmetric, level. Indeed, the laws ofclassical mechan­ This came as a big surprise, but it was this antiworld could not be distinguished ics remain valid after Time Reversal. also soon realized, in 1967, byA. Sakharov, from the world we know. But they are not. Classical mechanics is an example of a that CP violation is one of the necessary europhysicsnews MAY/JUNE 2000 s Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn:2000301 FEATURES

conditions to explain why our is It has been established that the K L state Although it is in principle possible to com­ almost exclusively made of , al­ is not the pure antisymmetric stateK 2, but pute the value of E'IE in the standard though it starts from a symmetricstate af­ rather an admixture ofK 1 and K 2: model, it is in practice avery difficult task. ter the Big-Bang model. Most theorists give estimates in the range 4 This made CP violation even more fas­ KL=K 2 + E K 1 of a few 10- for the ratio E'/E. cinating. In 1988, a first evidence for the exis­ At that time, it was necessary to intro­ This small impurity (E '" 2 10-3) allows the tence of direct CP violation was published duce new types of interactions to imple­ K L to decay into 2 pions. This is referred to by the NA31 collaboration at CERN [1] : ment CP violation in the theoretical as CP violation in the mixing. While the framework. One very popular model was introduction of this E parameter allows a E'IE = (3.3 ± 1.1) 10-3 proposedby1.Wolfenstein in 1964. He in­ proper phenomenological description of troduced a so called superweak interac­ the observations, itbrings no understand­ Later, their american competitors, E731 at tion, the strength and properties ofwhich ing ofthe origin of CP violation. FNAL, published [2]: were such that CP violationwas a small ef­ However, in the standard model of fect which could only be seen with the K weak interactions with 3 families, CP vio­ E'IE = (7.4 ± 5.9) 10-4 mesons, and came solely through the lation can also occur directly in the decay KOH KOoscillation. ofthe K 2 itself. This is called CP violation The NA31 completed their experiment In 1973, M. Kobayashi and T. Maskawa in the decay, or direct CP violation. Con­ and obtained in 1993 [3] : showed that CP violation could also occur trarily, the superweak model of1.Wolfen­ in weak interactions under the condition stein does not allow such a decay. E'IE = (2.3 ± .65) 10-3 that at least 3 families of2 quarks exist. To­ Many experiments have been launched day, the existence of6 quarks is established already in the late 60's to search for direct This experimental situation, two results (the heaviest one, the t quark, has been dis­ CP violation. It can be detected through a hardly compatible, did not allow to con­ covered in 1995),but at that time, only the tiny difference between decays in 2 clude. This led the two groups to launch existence of 3 quarks was postulated. This charged pions and 2 neutral pions. This more precise experiments, E832 at FNAL is a noticeable theoretical insight. difference is parametrizedby a very small and NA48 at CERN. The presence ofthree families ofquarks quantity E', which can be measured byus­ allows the introduction of one single pa­ ing the very simple relation: rameter in the theorywhich causes CP vi­ olation. The test of such a model requires several independent observations of CP violation. decay rate ofKL -7 1t + 1t -/ decay rate ofKL ~ 1t 0 1t 0 '" 1- 6 E'IE decay rate of Ks -7 1t + 1t - decay rate of K -7 1t 0 1t 0 Direct CPviolation s CP violation seen with the K mesons can occur in 2 different ways.

Muon Muon Analysis Magnet Filt~rVeto Ve~.?.?etectors • 20cmI CsI

" 'I' Vacuum ~~yReLl··1··········1 !1 I I 1 b~~s Vacuum Wi~~ow\

'-.1 R_e_g_e_n_e_ra_t_o_r-'-_---',.'--_----e __ UJ.,',,' ITrig~e,lll.1 Drift Hodoscopes ' Chambers Hadron Veto with Lead Wall ) 120 140 160 180 Distance from Target (m) fuLl The E832 detector

6 europhysicsnews MAYIJUNE2000 FEATURES

NEWS FROM EUROPHYS An experimental challenge consists in about 26000 CuBe ribbons very The smallness of the quantity to be mea­ precisely positioned, which define 13000 Appearing soon sured implies verystringent constraints on readout cells (fig. 4). the experiments. In excess of ten millions The detection and reconstruction of K Papers accepted up of K to 2 pion decays must be recorded to decays into charged particles is made with to 8 February to ap­ EUROPHYSICS reach an accuracy ofless than 10-4on E' / E, a spectrometer consisting of4 drift cham­ pear in forthcoming LETTERS­ which is necessaryto match the predicted bers and a dipole magnet. Additional issues range ofvalues. counters and detector elements are used Intrinsic non-expo­ Intense beams ofneutralparticles need for fast detection purposes and back­ nential relaxation to be produced. Both types of mesons, K L ground rejection. near the glass tran­ and Ks, must be produced and identified, Even after the online filtering of K de­ sition ofan orienta­ and both types ofdecays, 1t + 1t - and1t °1t 0, cays by a farm ofprocessors, both experi­ tionally disordered reconstructed. Detectors must stand a flux ments have to record about 100 TByte per crystal - M. Winter­ of several hundreds kHz of particles dur­ year at a sustained rate of a few Mbyte/s lich and R.Bohmer ing several months. Avery powerful selec­ during several months! Ripple topography tion of the few percent of interesting de­ Both teams have announced their first of ion beam eroded cays must be applied to reduce the amount result obtained with a partial data set [4, graphite: A key to ofdata to be recorded. 5]: ion-beam-induced damage tracks - S. The major difference between the Habenicht, W.Bolse,H.Feldermann,U.Geyer,H. 4 american and the european experiment is E832: E'/E = (28.0 ± 4.1) 10- Hofsiiss,K.P.Lieb and F.Roccaforte the production and the identification of NA48 : E'/E = (14.0 ± 74.3) 10-4 The Mott-Hubbard transition and the para­ the K mesons. In the former (fig. 1), high magnetic insulating state in the two-dimen­ energyprotons hit a single target, and col­ The american result is surprisinglyhigh in sional Hubbard model - F.Mancini limators are used to define 2 beams ofneu­ view of the previous results. Taken all to­ Distinct roles of the triplicated Zn CI4tetrahe­ tral particles. One of the beams is inter­ gether, the NA31, E731, E832 and NA48 dra in phase soliton formation and residual cepted by a block of matter which allows measurements lead to the average: commensurations in K2 Zn Cl4- Dong KeunOh, 4 the production of short lived K mesons E'/E = (19.2 ± 2.5) 10- • YoungMan Kwon, Cheol EuiLeeand S.Y.leong near the detector, while long lived mesons While this set of results demonstrates Single-particle mean kinetic in low­ remain in the matter free beam. In the lat­ the existence of direct CP violation, this density supercritical 4He - D.Colognesi,C.An­ ter (fig. 2), two targets are used to produce spread ofvalues is uncomfortable. dreani and R.Senesi the Kmesons. One is situated about 200m The additional data of E832 and NA48 Polycontinuous morphologies and interwoven upstream of the detector, so that only K L should bring extremelyvaluable informa­ helical networks -5. T.Hydeand S.Ramsden decays can reach the detector. The second tion. Moreover, a new experiment, KLOE Pseudo-contact angle due to superfluid vor­ target is lOOm downstream, and from at Frascati in Italy, has just started. It tices in 4He - R.Luusalo, A.Husmann, l. Kopu which accepted Ks decays are dominant. should produce another measurement of and P.l. Hakonen The most difficult decay to detect, re­ E'/E using a completely different tech­ Collapse of cavitation buobles in blood ­ construct and disentangle from back­ nique. E.A.Brujan ground events is the KL ~ 2 nOdecay. Both Ifa high value of E' /E is confirmed, this Comment on uPropagator ofthe Fokker-Planck nOdecay immediately into 2 gamma rays, might be an indication for sources of CP equationwith a linearforce-lie-algebraic ap­ so that the final state only consists in 4 violation outside the standard model. proach" by C. F.Lo - P.Demo,A.M. Sveshnikov neutral particles. The master piece ofboth andZ.Kozisek the american and the european detectors T violation and CPT tests with Kmesons Nucleation and growth in order·to-order tran­ is the electromagnetic calorimeter, dedi­ As was mentioned in the introduction, K sitionsofa block copolymer - G.Floudas,R.Uf­ cated to the detection and measurement of mesons are a superposition of KOand J(O rich, U.Wiesnerand B.Chu the energy and position of the gamma which transform into each other during Statistical mechanics of point particles with a rays. propagation. gravitational interaction - M.-L. Chabanol, F. The American calorimeter (fig. 3) is an The CPLEAR experiment at CERN was Corsonand Y.Pomeau assembly of about 3100 pure CsI crystals. able to measure a tiny difference between It-shifted magnetoconductance oscillations in Showers ofparticles are created bythe in­ the probability ofa KOto become a KOat a mesoscopic superconducting-normal-het­ teractions initiatedbythe incident gamma time t and that ofthe reversed process. erostructures - V.N.Antonov,H.Takayanagi,F.K. rays, and these particles produce In this experiment, KOand KOare pro­ Wilhelm and A.D.Zaikin Cerenkovlight in the crystals. The amount duced through the reactions Transmission and reflection ofwaves by an an­ of detected is a measure of the de­ pP~1t-K+ KOandpp~1t-K- KO.The an­ harmonic interface - Z.Odziana and P.Zielinski posited energy. tiprotons are extracted from the Low En­ Plasmon and charge quantization effects in a The NA48 collaboration built a liquid ergy Ring at CERN, and double-barrier quantum wire - A.Braggio,M. 3 krypton calorimeter, a tankofabout 10m stopped in a target at the center Grifoni, M.Sassetti and F.Napoli embedded in a cryostat to keep the kryp­ ofthe detector. Detecting the K+(resp. K-) Phonon-Iike and sing1o-particle dynamics in ton liquid. In this case, this is the ionisa­ identifies the KO (resp. KO) production. liquid lithium - T.Scopigno,U.Balucani,A.Cun­ tion of krypton atoms induced by the The decays KO~ 1t- e+Ve and J(O~ 1t+ e- Ye solo, C.Masciovecchic r" F -co,F.Setteand R. showers of particles which is detected by allow the identification of the decaying K Verbeni applying a high voltage between im­ meson with the sign of the emitted elec­ Note on irreducible approach to reducible sec­ mersed electrodes. The electrode structure tron. It is observed that the probability ond-class constraints - C. Bizdadea, A. Con­ has a useful aperture of about 2Am and that a KOis produced initially and oscil- stantin and S.O.Saliu europhysics news MAY/JUNE 2000 7 FEATURES

~ Fiducial Region

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~\~

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~~ 11

11 11 ~11 I JOJ2ppp

Vacuum :><: 3 r- ~ -l P- O • (") " [ i ""2" " "5" "":><: "" r- ;.: t 21 :><: en (") r- r- ~ (") "E. ~ "'"a §: g, ;,. (j x :><: .g. 3" ~. r- :::; 3" " (j .." Rc ~ g ~ ~ ~ ~ ;; " .. ~ [IOCm "~ 5cm ,{ / I N ;:;; Gcm I 0 11 0 w 0 0 v. 0 3 IV 0 ~ ~ 3 :; :; " o o o 3 :; :;

Fig 2 Schematic layout of the NA48 experiment

lates into a KOdecaying by emitting an decays to allow very stringent tests of CP observation of B H Boscillation and de­ electron is different from that of the violation and the standard model frame­ cay is quite hard. B's must be produced process where the KO and KO are ex­ work. Sensitivities to decay probabilities with sufficient energyso that the relativis­ changed (a positron being emitted in that well below IQ-ID are considered. tic expansion of their life time let them case). travel for several hundred of micrometer The relative difference is small, CPviolation and Bmesons before decaying. (6.6 ± 1.6) 10-3 [6], but significant. This is B mesons are similar to K mesons, but the The BABAR and BELLE detectors are a direct evidence for a microscopic viola­ b quarkplays the role ofthe s quark. In the attached to asymmetric e+e- coHiders. The tion of the Time Reversal symmetry. Standard Model, there are three families of positron and electron beams have differ­ Another outstanding result of CPLEAR 2 quarks: (u,d), (c,s), (t,b). Weak interac­ ent . A resonant bb state is pro­ is a comparison between the masses of the tions allow transitions between up type duced and decays into a pair of B mesons. KOand the KO,Am/m = (-3 ± 4) IQ-18 [7]. quarks (u,c,t) and down type quarks CP violation in the B system is expect­ This is the most precise test of the CPT (d,s,b). There are veryprecise relations be­ ed to be observed in many different ways, symmetry, which implies that particles tween these transitions if indeed only 3 either directlyorvia the B H Boscillation. and should have the same families ofquarks exist. As a consequence, Although all decay modes are-rare, the mass. and because in the StandardModel CP vi­ expected CP violation effects can be large. These recent results show that the K olation arises from these transitions, ep A popular example is the decay meson system provides an invaluable op­ violation processes in the K system trans­ B ~ J/IJIKs,for which the be­ portunity to test the fundamental proper­ late into CP violation processes in the B tween Band Boscillation and decay rate ties of physics laws. system. This is a very strongprediction of could be of order 60%. These opportunities are far from being the theory, which must be tested. B meson production is much higher exhausted. The previously mentioned Two dedicated experiments have just with hadronic colliders, but the selection KLOE experiment will pursue very de­ started: BABAR atSLAC (Stanford Linear and reconstruction ofinteresting decays is tailed tests of quantum mechanics and Accelerator Center, USA), and BELLE at much more efficient at an e+e- collider. discrete syrnmetries. In a longer term, ex­ KEK. About thirty millions bb could be pro­ periments are proposed at BNL There is one big difference between B duced per year at SLAC or KEK for (Brookhaven National Laboratory), FNAL mesons and K mesons, which makes their BABAR and BELLE, to be compared with (Fermi National Laboratory near Chica­ study more difficult. The B mesons are two hundreds billions at the ppFNAL col­ go), KEK (High EnergyPhysics Laborato­ about 10 times heavier, and thus can de­ lider for each of the 2 experiments called ry in Japan) and CERN (European Labo­ cayinto many different channels and have CdF (Collider Detector at Fermilab) and ratory for Particle Physics) to study rare K a very short life time (about 10-12 s !). The DO. However, about one thousand

8 europhysicsnews MAY/JUNE2000 FEATURES

Fig 3 The E832 Cslcalorimeter front face Fig 4 The electrode structure of the NA48 liquid krypton calorimeter

B ~ ]/I.JIKs decays can be used for analysis el with the Bsystem. First results will come tal (a)symmetries of Nature. in the former case, and only fifteen thou­ very soon andwith increasing precision in sands in the latter, which is relatively much th following years. less. In a longer term, extremely K decays References In spite of the very difficult extraction could provide additional theoretically [1] H. Burhardt et aI., Phys. Lett. B 206 of interesting events, CdF has already at­ very clear tests ofthe standardmodel par­ (1988) 169 tempted a measurement of one CP viola­ adigm for CP violation and one hopes to tion parameter in the B system. However, reveal surprising effects. [2] L.K. Gibbons et al., Phys. Rev. Lett. 70 the uncertainty is too large to draw any In parallel with this program of refin­ (1993) 1199 conclusion yet. An experiment at DESY ing the Standard Model confrontation (high energy physics laboratory near with experiment, another path must be [3] G.D. Barr et al., Phys. Lett. B317 (1993) Hamburg), HERA-b, is about to start, and followed. Indeed, itis widely accepted that 233 is one of the competitors in this opening the standard explanation for the CP syrn­ field. metries seen with K mesons is not at all [~]A. Alavi-Harati etal. Phys. Rev.Lett. 83 In the long term, extensive studies of B sufficient to allow for the observed matter­ (1999) 22 physics will be performed byexperiments antimatter asymmetry in the universe. As in preparation for the Large Hadron Col­ a consequence, one should also look for [5] This is a combination ofthe result ob­ lider in construction at CERN, and at the CP violation in places where it is not ex­ tained with data taken in 1997, V.Fanti et BTEV dedicated program at FNAL, ifit is pected or expected to be very small. Ex­ aI., Phys. Lett. B 465 (1999) 335, and the accepted. amples are the study of charmed mesons preliminaryresult from 1998 data, A.Cec­ (mesons similar to K's and B's butwith a c cucci, CERN seminar February 29th 2000 Conclusion quark instead of the s or b quark) and A A very important experimental step has hyperons decays, the measurement of the [6] A. Angelopoulos et al., Phys. Lett. B444 been recently reached with the demon­ electric dipole moment ofthe neutron, the (1998) 43 stration ofdirect CP violation in K decays, spectroscopy ofthe antihydrogen atom. as expected in the Standard Model ofpar­ Each of these fascinating experimental [7] A. Angelopoulos et aI., CERN-EP/99­ ticle physics. challenges offers strong perspectives to 150, submitted to Phys. Lett. B The next step is to challenge this mod- deepen our understanding of fundamen- europhysicsnews MAY/JUNE2000 9