RARE PROCESSES Looking at physics in a rarefied atmosphere

Last year the annual DESYTheory Workshop focused on rare physics - the physics that's hardest to do and sometimes difficult to understand. AndrzejBuras reports.

Rare processes and the vio­ 5xl0~4and 30xl04. lation of CP symmetry pro­ As discussed by Guido vided the focus for the Martinelli (Rome), Laurent recent Theory Workshop at Leilouch (Marseille) and the DESY Laboratory in Amarjit Soni (Brookhaven), Hamburg.As emphasized by advanced numerical lattice Chris Quigg (Fermilab) in calculations could consider­ the opening lecture, discrete ably improve the estimates symmetries (box 1) and in the coming years. How­ their violation, in particular ever, as stressed by Jamin, CP symmetry, play an impor­ it is important to develop tant role in a deeper under­ further the existing analyti­ standing of nature at both cal tools in order to confront very small and large dis­ the lattice results. tances. Studying these vio­ lations in rare processes Difficult measurements may give hints to what lies The experimental situation beyond the Standard Model for e/e, described by Martin of particle physics. Holder (Siegen), improved In the Standard Model, Relaxed atmosphere during the break. Left to right: Luca Silvestrini considerably in the past two CP violation is attributed to (Rome), chairman of the organizing comittee Andrzej Bums (Munich), years due to measurements quark transitions, which are Yosef Nir (Weizmann) and Gino Isidori (Frascati). by the KTeV collaboration at described by the three- Fermilab (28 ± 4) xlO"4 and dimensional (Cabbibo-Kobayashi-Maskawa; CKM) matrix.The clas­ the NA48 collaboration at CERN (14±4)xl0~4. These meas­ sic effect in the decays of neutral kaons into two pions, which was urements confirm the previous result of the NA31 collaboration at first seen in 1964, is attributed to "indirect" CP violation through the CERN (23 ± 7) x 10~4 that E/E is not zero, confidently ruling out cer­ mixing of the neutral kaon and its antiparticle (box 2). This type of tain hypotheses. violation is usually characterized by a small parameter, which is Also taking into account the older inconclusive measurement of measured to be roughly 2.3 x 10-3. However, the Standard Model the E731 collaboration (7 ± 5) x 10"4, one arrives at the world aver­ also allows "direct" CP violation, governed by quark mechanisms age of (19 ±3) x 10"4. In view of the spread in values obtained by involving the exchange of the sixth "top" quark. various experimental groups, the resulting small error should be The theoretical status of these effects was summarized by treated with caution. Matthias Jamin (Heidelberg). Refined gluon corrections and a more Within the next few years the experimental situation should accurate top quark mass (174 ± 5 GeV) from the CDF and DO col­ improve considerably through the new data from KTeV and NA48, laborations at Fermilab have both considerably improved the evalu­ and in particular from the KLOE experiment at DAFNE in Frascati. ation of CP violation parameters. While indirect CP violation in the Standard Model is consistent with experimental data, large uncer­ New channels tainties currently preclude a precise comparison of direct and indi­ While the theoretical estimates of E/E in the Standard Model are rect CP violation. compatible with the experimental data within the theoretical and Despite intensive efforts by theorists, estimates of this ratio (which experimental uncertainties, there is still a lot of room for new is known in the trade as e/e) by various groups range between physics. As discussed by Luca Silvestrini (Rome), important 0

CERN Courier March 2001 29 RARE PROCESSES

new contributions are still corrections and new physics possible within supersym­ contributions, in particular metric (box 3) extensions of in supersymmetric models, the Standard Model. during the last years will The present bounds on allow for stringent tests of CP violation in various the theory once the experi­ processes already give very mental branching ratios important limits for the mas­ become precise. ses and weak couplings of The channel with the final supersymmetric particles. In photon was observed in particular, the pattern of the 1993 by the CLEO collabor­ masses of squarks - the ation at Cornell, and these supersymmetric partners of data have been improved quarks - is severely restric­ considerably since by CLEO ted. However, in spite of and by the ALEPH collabor­ these constraints, large ation at CERN. Recently the supersymmetric effects in efforts to measure this chan­ CP-violating processes are nel precisely have been possible. joined by the new B-fac- It is important to study CP tories at SLAC (Stanford) violating decays and CP and KEK (Japan), so that in conserving rare decays, a few years a rather precise which are theoretically far DESY Theory Workshop 2000 - Peter Zerwas (DESY) (right) tells Martin branching ratio should bec­ cleaner than those tradition­ Holder (Siegen) to measure CP violation more precisely. ome available. ally studied. As stressed by However, as emphasized Gino Isidori (Frascati), a "gold-plated" decay in this respect is that of by Greub, the available data, while being consistent with current the long-lived kaon into a neutral pion, neutrino and antineutrino, expectations, already put powerful constraints on supersymmetric proceeding almost exclusively through direct CP violation. The pre­ extensions.The second channel, with a muon-antimuon pair in the dicted branching ratio within the Standard Model (3 x 10"11) and final state, should be observed this year at the B-factories and at the presence of neutrinos and the neutral pion make the measure­ Fermilab. For new physics, this is even more interesting than the ment of this decay formidable. photon-yielding decay. On the other hand, in certain supersymmetric models the branch­ ing ratio could be one order of magnitude higher. Most important is Larger effects that the newly approved KOPIO experiment at Brookhaven should be While CP violation has been observed so far only in kaon decays, able to measure this decay in the first half of this decade even if where the effects are rather small, much larger effects are predicted the branching ratio is at the predicted level.There are also plans to for B-mesons. As stressed by Roy Aleksan (Saclay), Yosef Nir measure this decay at Fermilab and at KEK in Japan.The optimism (Weizmann) and Robert Fleischer (DESY), there are several decays in measuring this branching ratio is strengthened by the observation where measurements should fix CP violation without almost any of one event in the CP-conserving decay of the charged kaon into a hadronic uncertainties. Here the central role is played by the "gold-

charged pion, neutrino and antineutrino by the E787 at Brookhaven, plated" decay of the Bd-mesons (bound states of an b-antiquark dating from 1997. and a down quark) into a short-lived kaon and a W (charm The branching ratio for this rare decay as of the end of 2000 is quark-antiquark bound state). around 1.5 x 10"10 - slightly higher but fully compatible with expec­ The present bounds The corresponding CP violating tations. As this decay is also theoretically very clean, the improved on CP violation in asymmetry is parametrized by an measurements of its branching ratio expected in the coming years at angle, (3, in the so-called unitarity Brookhaven, and later at Fermilab, will provide powerful constraints various processes triangle, which is related to the on the elements of the CKM quark transition matrix and the param­ already give very CKM matrix. eters of new physics. The most recent data - which important limits for were reviewed by Aleksan - from Beauty quark the masses and the BaBar and Belle experiments In the coming years, some of the most promising tests of the at SLAC and KEK respectively give Standard Model and its extensions will come from studying the weak couplings of values of sin2(3 somewhat lower decays of B-mesons (containing the fifth - "beauty" - quark) into supersymmetric than expected and lower than strange hadronsand either a photon or a lepton-antilepton pair. As earlier measurements by the CDF reviewed by Christoph Greub (Bern), refined calculations of gluon particles. collaboration at Fermilab. The

30 CERN Courier March 2001 RARE PROCESSES

three experiments taken together this mixing will provide a valuable give sin2(3 = 0.42 ±0.24, com­ 1. When symmetries fall additional constraint, as stressed pared with the Standard Model- In CP (charge/parity) symmetry, the physics of left-handed by Ali and Silvestrini. expected sin2(3 = 0.7 ± 0.15, and particles is the same as that of right-handed antiparticles. According to the Standard discussed by Nir and Ali (DESY). Changing right to left (a parity, P, transformation), or particle Model, CP violation outside the Clearly, within the experimental to antiparticle (a charge conjugation, C, transformation), are kaon and B-meson systems is uncertainties, the measured examples of what physicists call "discrete symmetries" - expected to be invisible. However, value is consistent with expecta­ operations that either you do or you don't - there is no halfway as reviewed by Werner Bern- tions, which, using among other point between left and right. reuther (Aachen), new sources of inputs the observed CP violation The combined CP symmetry became popular after physicists CP violation, present in exotic in kaon decays, is subject to the­ had been shocked in 1956 to discover that nuclear beta decay multi-Higgs models or models oretical uncertainties. - a fundamental weak interaction - is spectacularly left-right with supersymmetry, could give On the other hand, as stressed asymmetric (P-violating). rise to measurable effects. by Ali, Nir and Silvestrini, imp­ The confusion grew in 1964 when new experiments found roved measurements of sin2(3 that the new CP criterion was not 100% reliable either. Ever Tracing interactions significantly below 0.5 would sig­ since, physicists have sought to understand how and why this Extensions of the Standard Model nal the presence of new physics symmetry is flawed. predict new CP-violating interac­ contributions - in particular new tions for quarks and for leptons. CP violating phases. Attempts to trace such inter­ The large variety of CP-violating 2. Indirect and direct routes to CP violation actions include charm meson (D) asymmetries in B-decays should decays and the search for neutral allow for decisive tests of the The neutral kaon provides the classic examples of CP D mixing; a high statistics hyp- Standard Model and its exten­ violation. However, understanding the physics of the neutral eron experiment at Fermilab sions. Other CP-violation parame­ kaon is not easy. With no electric charge, the neutral kaon (E756); and the "classic" low- ters could be measured, initially particle and antiparticle can only be distinguished by their energy searches for electron and by BaBar and Belle in the coming strangeness quantum label. Unfortunately, strangeness is only neutron electric dipole moments years, as reviewed by Aleksan.The conserved in strong interactions, not in weak decays. Because and of certain atoms. Top quarks

decays of the heavier Bs meson, of this, the neutral kaon and its antiparticle get mixed up. may also be used to probe CP containing a strange quark, will Such mixing can produce a special kind of CP violation, violation once they are produced open up more possibilities.These called "indirect" because it is due not to the explicit breaking in large numbers at theTevatron measurements can only be done of the symmetry in the decay itself, but instead through the and CERN's LHC. by the dedicated experiments particle-antiparticle mixing before the decay. More CP The most interesting at present LHCB at CERN and BeTeV at violation (the "direct" kind) occurs via the decays of the are the results from CLEO Fermilab around the year 2005. strange quarks inside the neutral kaons. (Cornell) and FOCUS (Fermilab), Several strategies were reviewed which possibly indicate neutral D by Robert Fleischer (DESY). How­ mixing at a much higher level ever, he also emphasized that the 3. Supersymmetry than expected and in the area of two-body decays of Bd mesons some supersymmetric scenarios. into pions and kaons, measured The known elementary particles come in two kinds - fermions, CPT invariance - the combina­ first by CLEO at Cornell and now such as quarks, electrons and muons (matter particles), and tion of CP symmetry with time- studied by BaBar and Belle, bosons, such as photons, gluons, Ws and Zs (force carriers). reversal symmetry - is the despite some hadronic uncer­ The key feature of supersymmetry is that every matter particle bedrock theorem of quantum tainties, are likely to provide very (quark, electron,...) has a boson counterpart (squark, field theory, enforcing, for valuable constraints. selectron,...), and every force carrier (photon, gluon, W, Z) has instance, equal masses for parti­ a fermion counterpart (photino, gluino, chargino, cles and antiparticles. Ludwig Other goals neutralino,...). Supersymmetry is intellectually appealing and Tauscher (Basle) reviewed various While CP violation in B-decays is could solve several outstanding physics problems, but no tests of CPT and time-reversal the main aim of B physics in the direct evidence for these additional superparticles has yet symmetry in the CPLEAR experi­ near future, there are other impor­ been seen. ment at CERN. tant goals. These were reviewed Time-reversal violation has by Henning Schroeder (Rostock). been seen for the first time in this In addition to the rare B-meson decays already mentioned, a central experiment (March 1999 p21). While no violation of CPT symmetry

role is played by Bs mixing. While electron-positron collider experi­ has been found, CPLEAR has provided very precise values of various ments have put a stringent lower bound on this mixing, its first actual CPT-parameters and unprecedented CPT tests of mass differences measurement is expected this year at Fermilab. The knowledge of of the order of 10"19 GeV. >

CERN Courier March 2001 31 RARE PROCESSES

Graham Ross (Oxford) reviewed current progress in understanding The matter-antimatter asymmetry of the universe, in which CP vio­ quark and lepton masses and mixings. New ideas will soon be lation is believed to play an important role, was highlighted in the tested in B-factories, and in particular in neutrino experiments, from final talk of the workshop by Wilfried Buchmueller (DESY). Different which an improved knowledge of neutrino masses and mixings scenarios for baryogenesis (nuclear matter production) differ in the should be achieved. way in which Sakharov's basic conditions for matter-antimatter The status of neutrino masses and mixings was reviewed by Walter asymmetry (baryon number violation, C- and CP-violation, devia­ Grimus (Vienna). He showed that the latest results, especially from tion from thermal equilibrium) are satisfied. SuperKamiokande in Japan, give very strong indications for neu­ An attractive scenario is electroweak baryogenesis during the ini­ trino oscillations and imply that the pattern of neutrino masses and tial electroweak phase transition of the universe. Here the baryon mixings differs substantially from the quark sector. This could sug­ asymmetry can be calculated in terms of low-energy CP-violation. gest large CP-violating effects in neutrino oscillations, reviewed by Unfortunately, this situation does not work within the Standard Manfred Lindner (Munich). He described various possibilities to Model. However, even if the parameter space is already rather con­ search for leptonic CP-violation in future long-baseline experiments strained, it may work within its Minimal Supersymmetric extension. and stressed that several CP-violating effects have to be taken into A very interesting alternative - so-called "leptogenesis" - is to pro­ account in future analyses of oscillation data. duce an initial lepton asymmetry thrtjugh the decay of a special heavy neutrino that can then be reprocessed into a baryon asym­ Flavour violation metry by non-perburtative "sphaleron" electroweak effects. Given convincing data from SuperKamiokande on atmospheric neu­ Introductory lectures by Thomas Mannel (Karlsruhe), LalitSehgal trino oscillation, there is more than ever reason to search for anom­ (Aachen), Roland Waldi (Rostock) and Gerhard Buchalla (CERN) alous lepton effects (flavour violation). Especially with were appreciated.The DESY workshop certainly showed that CP vio­ supersymmetry, there is a good chance that lepton flavour-violating lation, rare processes and related issues remain key questions of transmutations between electrons, muons and taus will be seen in particle physics. the next round of experiments. A review was presented by Hitoshi Murayama (Berkeley). From Andrzej Buras, Munich Technical University.

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