Europhysics News (2004) Vol. 35 No. 3

Particle physics from the Earth and from the sky

Daniel Treille, CERN, Geneva, Switzerland

Recent results in particle physics offer a good balance between the news "coming from the Earth", namely results from the various colliders, and news "coming from the sky", concerning solar and atmospheric neutrinos, astroparticle programmes, searches for dark matter, cosmic microwave background (CMB), cosmology, etc.

   In the light of this information, gathered in particular from the 2003 Summer Conferences (EPS in Aachen, Lepton-Photon in Fermilab), an account of the status of our field is given. It will appear in two parts, corresponding approximatively to the division between the Earth and the sky. The first one covers the Electroweak Theory, ideas beyond the Standard Model, Quantum Chromodynamics (QCD), Beauty and heavy ion physics.

Electroweak Theory
The Electroweak Theory (EWT), together with Quantum Chromodynamics (QCD), modern version of the strong interaction, builds the Standard Model (SM) of Particle Physics [1]. The EWT is a fully computable theory. All EW measurable quantities, called "observables", as for instance the properties of the various Z⁰ decay modes, can be predicted with great accuracy and compared to measurements. Each of them allows in particular to determine the Weak Mixing Angle, i.e. the parameter of the 2X2 unitary matrix which transforms the two abstract neutral bosons of the EWT into the two neutral physical states, photon and Z⁰. The internal consistency of the EWT implies that all values of the Weak Mixing Angle obtained should coincide. In terms of the standard Big Bang model, the breaking of the EW symmetry, namely the time at which known elementary particles got their mass, presumably through the Higgs mechanism*, occured at about 10^-11s after the Big Bang.

   The e⁺e^- high-energy colliders LEP at CERN and SLC (SLAC Linear Collider) have delivered their quasi-final results. Their contribution to the validation of the EWT has been invaluable. However, besides celebrating this great success, it is worth considering the few areas of obscurity left and discussing how one can hope to improve the precision measurements in the future.

   It is amusing to remember what was expected from LEP, for instance at the time of the meeting held in Aachen, in the same place as the 2003 EPS Conference, in 1986. In nearly all domains the quality and accuracy of the final results of Z⁰ and W^± physics* have been much better than foreseen, in particular due to the progress made during the last decade on detectors (microvertex devices allowing a clean tag of beauty particles, by revealing their long lifetime (flight path) of about 1 picosecond (few mm), luminometers providing a very accurate absolute normalization of the various processes, etc), on methods (such as how to determine the number of neutrinos from the Z⁰ properties, ...) and on the mastering of theoretical calculations.

   Figure 1 and its legend recall what is the scenery of e⁺e^- collisions. Sitting on the huge Z⁰ resonance, LEP recorded about 18 millions Z⁰ events and SLC about half a million only, but with the strong bonus of a large polarization of the incident electrons and better conditions for beauty tagging. From this large amount of data, many observables were measured, often with an accuracy of one per mil or better. Later, LEP200 measured e⁺e^- interactions at higher center-of-mass energies, up to 206 GeV: it recorded about 40K W pair events and set quite strong lower mass limits on the Higgs boson and Supersymmetric Particles.

   If one summarizes the whole set of available EW measurements (LEP/SLC and others) by performing a global fit [2], one finds that the SM accounts for the data in a satisfactory but nevertheless imperfect way: the probability of the fit is only 4.5%.

   The measurement lying furthest from the average is that of the weak mixing angle by the NuTeV experiment in Fermilab [3], which scatters neutrinos and antineutrinos on target nuclei. Before invoking new physics, the possible "standard" causes of such a disagreement were carefully investigated: unexpected features of the quark distribution inside nucleons are the most likely culprits. If this measurement is excluded from the fit, the probability becomes 27.5%, a reassuring number.

   The other noticeable disagreement concerns the two most precise electroweak measurements, namely the spin asymmetry A_LR at SLC, i.e. the relative change of rate of Z⁰ production in e⁺e^- collisions when one flips the electron helicity (i.e. the component of its spin along the direction of motion), and the forward-backward asymmetry of beauty production on the Z⁰ at LEP, A_FB^b, i.e. the manifestation of the violation of particle-antiparticle conjugation C (and of parity P) in e⁺e^-Z⁰beauty-antibeauty, which give values of the weak mixing angle differing by 2.7 standard deviation with no hint of an explanation, neither instrumental nor theoretical.

Fig 1 The scenery of e+e- collisions as a function of energy. LEP1 was "sitting" on the huge Z0 resonance. Beauty factories exploit the Y(4S) resonance located in the family of Y beauty-antibeauty resonances near 10 GeV. The J/y is the lowest charm quark-antiquark bound state near 3 GeV. (Fig. courtesy U. Almaldi.)

  
   An ambiguity which is not yet removed concerns the theoretical interpretation of the muon g-2 measurement [4] obtained in Brookhaven with an experimental accuracy of ~ 5 10^-7. The slight departure of the muon g factor, relating the magnetic moment to the spin, from its canonical value of 2 (i.e. the value given by the Dirac equation describing pointlike relativistic fermions) is due to the fact that the electromagnetic interaction of a muon and a photon is perturbed by the exchange of one (or more) additional photon(s) (figure 2a). After correction of a small error, the theoretical frame is sound. However, the tiny hadronic contribution (figure 2b) to this quantity expected in the SM, which reflects the probability that the additional photon fluctuates into a light hadronic system, differs, depending on the way it is estimated. To obtain its value one has to resort to subsidiary experimental data. Using for this purpose the hadronic decays of the tau* (plus a set of assumptions) leads to a relatively fair agreement between theory and experiment (the latter larger than the former by 1.4 s. However using hadronic production in low energy e⁺e^- collisions leads to an excess of experiment over expectation which according to the most recent analyses [5] amounts to 2.7 s. The situation may still change with the advent of data from the B Factories in SLAC and in KEK (Japan) and from KLOE in Frascati. This residual discrepancy is all the more unfortunate given that the g-2 observable is potentially a powerful telltale sign of new physics, in particular Supersymmetry [1], since new particles can contribute to the perturbation as virtual states (figure 2c). While a significant excess of the measured value over theory could point to an appetizing window for the masses of some supersymmetric particles, good agreement could on the contrary eventually turn into a noticeable constraint on the minimal value of their masses.

   A low energy measurement which "returned to the ranks" is that of atomic parity violation (APV). APV [6] occurs because in an atom the electrons and the nucleus interact not only by photon exchange but also by Z⁰ (and its possible recurrences at higher mass Z⁰') exchange. Alkali atoms, having a single outer electron, are the only ones that lead to tractable atomic calculations. Due to recent refinements of some theoretical estimates, there is presently a good agreement between the expectation and the 0.6% accurate measurement on cesium made in Boulder in 1997. The APV measurement does not weight much in the EW fit. However, a remarkable result for such a small sized experiment is that the lower mass limit it sets on a potential Z' (600-800 GeV) is quite competitive with those of LEP or Tevatron. However, to stay so in the face of future LHC data, the APV measurement should reach ~ 1‰ or so. The possibility of a programme using francium, the next alkali atom, much more sensitive but radioactive, is sometimes mentioned.

   It is worth underlining here the promises of another set of low energy measurements concerning Electric Dipole Moments (EDM), in particular of the neutron. For particles to have a permanent EDM the forces concerned must violate the invariance under time reversal T (and therefore under CP*), and the SM expectations are out of reach, far below existing and foreseeable limits. But various scenarios beyond the SM may lead to strong enhancements. Very sophisticated methods involving ultra cold neutrons are under study and may bring an improvement of two orders of magnitude on the present neutron EDM upper limit. Limits on the muon EDM, as a by-product of the g-2 measurement, and on the electron EDM, through measurements made on various atoms, in particular Hg, are likely to improve as well. If no positive evidence is found, these limits will in particular become a major constraint for Supersymmetry.

   Let us finally quote a potential problem concerning the unitarity of the Cabbibo-Kobayashi-Maskawa (CKM) matrix⁷, and more precisely its first row. The CKM matrix gives the relationship between the quarks seen as mass and as flavour eigenstates. Unitarity just means that when one "rotates" from one base to the other the probability has to be conserved. The CKM matrix is a 3X3 unitary matrix, entirely defined in terms of four real parameters. It gives a concise description of all that we know at present about the weak interactions of quarks. The first row of the matrix concerns essentially the ud and the us (Cabbibo angle) transitions and the fact that their moduli squared do not add exactly to unity could indicate that the value of the Cabibbo angle is slightly underestimated. Actually, after including recent results, like the data of E865, at Brookhaven Alternate Gradient Synchrotron, on the decay Kpen, the remaining deficit relative to unity amounts only to ~1.8 s and is not a big worry

Fig 2 Examples of loop diagrams (a) The lowest order diagram contributing to g-2. (b) The hadronic contribution to g-2. (c) SUSY particles in the loops as a possible contribution to g-2. (d) Penguin diagrams (SM and SUSY) contributing to one B decay mode. The ressemblance to a penguin is a matter of taste. (e) The loop diagrams responsible for beauty-antibeauty oscillation.

  
The message from LEP
In spite of the few open questions quoted above, the first message of LEP/SLC is therefore the quality of the agreement of the SM, or more exactly of its neutral current (i.e. Z⁰) sector with data. Any theory attempting to go beyond the SM (see below) must therefore mimic it closely and offer very similar predictions of the various EW observables. Most interestingly, because of the extreme accuracy of the measurements, the agreement has been demonstrated at the quantum loop-level. Before expanding this last point, let us remark that the situation is less precise for the charged current sector of the SM. As for its scalar (Higgs or equivalent) sector, still largely untested, it will need the CERN Large Hadron Collider (LHC) to be explored.

   In a given process, particles, even if they are too heavy to be produced as "real" particles, can nevertheless intervene as "virtual" states and slightly influence the process. Figure 2 presents a variety of such loop diagrams. Accurate measurements on a process can thus yield information on these virtual particles. At LEP the "missing pieces" of the SM were the top quark, too heavy to be pair produced but whose existence was never in doubt, and the Higgs boson, not yet observed directly at present. As G.Altarelli put it, LEP physicists were in the situation of a bush hunter, his ear to the ground, trying to hear the pace of a tiger (the Higgs) while an elephant (the top) was rampaging around.

It is well known that Z⁰ physics at LEP gave a rather accurate "indirect" estimate of the top quark mass (presently 171.5 ^+11.9 _-9.4 GeV), in very good agreement with the value that later the Tevatron measured "directly" by producing the top, presently 174.3 ± 5.1 GeV (figure 3a). Once the "large" effect of the elephant-top on the relevant electroweak observables was well under control, one could search for the tiny effect expected from the tiger-Higgs boson, which in the SM is assumed to be the only missing piece. Ignoring the disagreements quoted above, essentially that existing between A_LR and A_FB^b, and considering only the mean values, one can thus deduce, in the strict frame of the SM, the preferred mass region for the Higgs boson (remembering that the information concerns the logarithm of its mass):

   M_h = 91⁺⁵⁸ _-37 GeV, and m_h <219 GeV at 95% CL (figure 3b).

   Taken alone, the A_LR observable would give for the boson mass a range between about 15 and 80 GeV, while the observable A_FB^b would give it between about 200 and 700 GeV. The W mass value (the world average is 80.426 ± 0.034 GeV) indicates also a Higgs mass region on the low side.

   Let us remark that the SLC measurement seems to contradict the lower limit of 114.2 GeV set on the Higgs mass by the direct Higgs search* at LEP200, as well as the indication for an effect near 115 GeV which is presently at the 1.7 s level. However the problem would be less acute if the top mass was a few GeV, say one standard deviation, higher than one states presently, a possibility that a reanalysis by the Tevatron [8] experiment D0 of its Run I data might suggest. If it were so the limit on m_h would be raised from 219 to ~280 GeV. For this reason, and many other good ones, a precise determination of the top mass is "devoutly to be wished". The Tevatron will reduce the uncertainty to ~2.5-3 GeV, per experiment and with an integrated luminosity of 2fb^-1 (i.e. providing 2 events for a process having a cross-section of a femtobarn, i.e. 10^-39 cm²). The LHC should reach an uncertainty of ~1-2 GeV, while a Linear Collider will do about ten times better.

Fig 3 Left: The top mass from indirect LEP measurements (open circles) and from the direct Tevatron measurements (colour triangles). Right: The preferred region for the SM Higgs mass (near the bottom of the c2 curve) deduced from electroweak measurements.

  
   The other key message of LEP/SLC is thus the indication of a light Higgs boson. Is this the truth, or could it be an illusion? Clearly if one quits the frame of the SM by introducing new physics, it is quite possible to invent "conspiracies", i.e. interference of amplitudes of different processes, by which a heavy Higgs boson has its effect on electroweak observables compensated by something else, like new particles or extradimensions of space. However, these solutions are more or less artificial: it is thus reasonable to focus on the simplest scenario and to test in priority the assumption of a light boson by obtaining direct evidence for it.

Beyond the standard model
Exhaustive reviews of the direct searches for new physics at colliders, updating the existing limits, have been given. Unfortunately, besides the D_sJ particles* found by the Beauty Factories [9] and the Pentaquarks [10]*, no discovery has showed up at the high-energy frontier.

   Nevertheless the motivations pushing to go beyond the SM are still present and more compelling than ever. The main one is the Hierarchy Problem that can be stated as follows. Gravity exists and defines a very high energy scale, the Planck scale* (~10¹⁹ GeV) at which the gravitational force becomes strong. In the SM all other masses, in particular the Higgs mass, should be irredeemably pulled towards this high scale by the radiative effects already quoted. Something more is needed to guarantee the stability of low-mass scales. Traditionally the routes leading beyond the SM either call for new levels of structure and/or new forces, as Technicolour (TC) [11] does, or involve more symmetry among the players of the theory, as in the case of Supersymmetry (SUSY) [1,12], in which SM particles and their "superpartners", i.e. the new particles of opposite spin-statistics (a boson as partner of a SM fermion and vice-versa) that SUSY introduces, conspire to solve the Hierarchy Problem.

   TC breaks the EW symmetry in an appealing way, very reminiscent of the way the electromagnetic one is broken by supraconductivity (which, crudely speaking, gives a mass to the photon). However TC meets serious problems in passing the tests of electroweak measurements, because it harms too much the predictions. On the other hand SUSY, which has a more discrete effect in this respect, keeps its eminent merits and remains the most frequented and even crowded route. In this context another important result [13] derived from the LEP data is the quasi-perfect convergence near 10¹⁶ GeV of the electromagnetic, weak and strong coupling "constants" in the frame of SUSY, the so-called Supersymmetric Grand Unification (SGU) (figure 4b). This "running" of coupling constants with the energy scale is another consequence of the quantum nature of the theory: it is due to the effect of virtual particles appearing in the loop diagrams. The presence of superpartners explains why the "running speed" is different in SUSY and in the SM.

Fig 4 Left: The evolution of the strong coupling constant with the energy scale. Right: The convergence of the SM coupling constants, approximate in the SM (upper figure), exact in SUSY (lower figure). One should distinguish this smooth running of couplings from the evolution of the intensity of the interaction with the energy scale, depending on the mass of the exchanged boson.

  
   SUSY is certainly a broken symmetry as no partner of known particles with opposite spin-parity exists with the same mass. These partners are assumed to be heavy, but not too much (few hundred GeV to few TeV) as otherwise SUSY would no longer cure the hierarchy problem. Furthermore the convergence of couplings quoted above requires that the superpartners appear at relatively low mass, say 1 to 10 TeV.

   With the diversity of the possible SUSY breaking mechanisms*, this theory presents a complex phenomenology with many different possible mass spectra for the supersymmetric particles. Its minimal version however offers a golden test: it predicts a very light Higgs boson, i.e. <130 GeV in full generality (for m_top=175 GeV), and <126 GeV once SUSY is broken, as it has to be, and in particular in all versions of Supergravity¹⁴ presently considered as the reference points for future searches. This is a mass window that LEP, with 80 additional (i.e. 30% more) superconducting accelerating cavities and the magnificent performances of the accelerating field finally reached, could have explored and which stays as the first objective of future programmes. If SUSY represents the truth, the LHC, or maybe, with much luck and considerable improvements, the Tevatron, will discover it by observing, besides the light Higgs boson, some supersymmetric particles. But a Linear Collider will be needed to complete its metrology in the mass domain it will give access to.

   However, quite interesting new roads have appeared in recent years.

   One, the Little Higgs scenario, leaving aside the Big Hierarchy problem (the one we introduced above) for the time being, tackles first the Small Hierarchy one, namely the fact that LEP announces a light Higgs boson while it pushes beyond several TeV the scale of any new physics (except SUSY which can still be "behind the door"): again the Higgs mass should be pulled to this high scale and the fact that it is not calls for efficient cancellation mechanisms to be at work. Keen to do without SUSY, this model, by an algebraic tour de force, manages to realize the compensations needed by inventing new particles, a Z', a W', a new quark, etc., at the mass scale of few TeV. The existing EW measurements put however the model under a severe tension. True or not, this theory has the merit to reinvigorate the LHC phenomenology by introducing new particles into the game and in particular insisting on quantitative tests concerning their decay modes.

   The other new route postulates the existence, so far uncontradicted, of extra dimensions of space (ED), large enough to generate visible effects at future experiments. The general idea of an ED, due to Kaluza and Klein, is rather old (around 1919). The Superstring Theory requires EDs since it is consistent only in 9 or 10 spatial dimensions. For long, however, these EDs were thought to be "curled up" (compactified) at the Planck scale, until it was realized that things could be different. Several versions are presently put forward [15]. With substantial differences between them, they all predict Kaluza-Klein recurrences of the graviton or some of the SM particles, i.e. new states which can be produced if their mass is at the TeV scale or below, or that may change the rate of SM processes through their effect as virtual particles.

   Such an eventuality, which has to be fully explored, would be an extraordinary chance for LHC and its prospective study also contributes an agreeable diversification of its phenomenology. However, before dreaming too much, it is important to appreciate correctly the existing limits, drawn either from accelerators or from astrophysics. For the ADD scenario, one should also consider the impact of dedicated tests of Newtonian gravity at small scale [16], which, besides micro-mechanical experiments, use sophisticated methods involving Ultra Cold Neutrons and maybe in the future Bose-Einstein Condensates, which build interesting bridges between particle physics and other sectors of physics.

   Moreover, it is still a rather natural attitude to assume that extra dimensions, if they play a role, would do so at much higher energy scales, for instance the one of Grand Unification (GU). Many studies follow that path and analyse what one or more extra dimensions bring to the already very successful theories of Supersymmetric GU. This complements the class of studies which, to the symmetry group of GU, add other ones (a new U(1), a new SU(3), etc.) whose role is to deal in particular with the mystery of the triplication of families (i.e. the existence of the electron, muon and tau families).

   The hope is that these attempts, performed from "bottom to top", i.e. from low towards high energies, and those, from "top to bottom", of Superstrings [17] will meet one day and guide each other.

Fig 5 The "d-b" Unitary Triangle. Adding to zero three complex numbers like VudVub*, etc. naturally lead to draw a triangle. We indicate which B decay modes give access to its angles and sides. Vij is the element of the CKM matrix connecting the flavour eigenstate quark i to the mass eigenstate quark j.

  
Quantum Chromodynamics (QCD)
QCD is the modern version of the strong interaction. The numerous experimental successes of QCD [18], besides its natural simplicity (a single parameter, the strong coupling "constant", if one forgets the quark masses), make it an exemplary theory. Most of the sectors of particle physics feed QCD and need it. QCD is both turned towards the domain of very high energies, where it exhibits "asymptotic freedom"*, and towards the low energy hadronic world, where its strong coupling at large distance leads to confinement of quarks and gluons. This evolution with the energy scale has been very clearly demonstrated in the past decade (figure 4a). All QCD aspects, from perturbative to non-perturbative, are actively studied and essential to extract correctly the physics of other sectors, EW measurements, beauty physics, heavy ions, etc. Nevertheless it is clear that all of them still need much progress, in particular to meet the requirements of future experimental programmes.

   The QCD lattice simulations* built upon the basic principles of the theory, have become fundamental tools, well established and vital to many fields. The progress achieved is greatly due to that of the computing means, but also to the improvements of the algorithms and methods.

   One of the few dark points concerning QCD seemed to be an excess of the production of beauty quark-antiquark pairs in various types of collisions compared to the predictions of the theory. However, new data and refined theoretical expectations show that the problem seems to be fading away.

  
   The values of the single parameter of QCD, its coupling constant, extrapolated to the energy scale of the Z⁰ mass, aS(M_Z), obtained from very different sectors, are now well coherent. The uncertainty on this quantity, after having considerably decreased, is now stabilizing. Its absolute value, 0.118 ± 0.003, is in very good agreement with what is required by the most elaborate versions of Supersymmetric Grand Unification, including the effects appearing at the Grand Unification scale.

   The nucleon structure and the distributions of the quarks and gluons inside it are better and better known and understood, especially thanks to HERA and in particular at the very small values of the fractional momentum x carried by the constituents, a crucial region since it will govern the production cross-sections at LHC.

   However, when the spin intervenes, our understanding of nucleons is still poor. It is not yet clear how the spin of a nucleon is shared between its constituents. One is thus expecting from future polarization programs [19], in particular polarized proton-proton collisions in the RHIC collider at Brookhaven, a number of clarifications, in particular concerning the gluon helicity distribution, by measuring processes at transverse momenta large enough for the perturbative and computable version of QCD to apply.

   It is important to underline that much remains to be done in matters of QCD if one wants to enter the CERN Large Hadron Collider (LHC) [20] era in optimal conditions, namely with a good mastery of the SM prediction for the many different topologies that searches will explore. Indeed before claiming for a discovery one should be confident with the estimate of the SM background. This remark is particularly true for the indispensable Monte Carlo programs needed to simulate the expected observations.

Fig 6 The tip (yellow) of the Unitary Triangle as determined from LEP, K physics, etc. The blue cones give the value of its angle b directly measured at B Factories.

  
Heavy flavours
We recall that, in the jargon of particle physics, this corresponds to the physics of heavy quarks of the second, strangeness and charm, and especially third (beauty and top) generations. The heaviness of these quarks offers a simplified approach to the spectroscopy of hadrons containing them. Concerning the few symbols used below, B denotes a beauty meson (a beauty quark and a light antiquark, or vice-versa), while B_S is a meson containing beauty and strangeness. We recall that the f is a narrow strange-antistrange bound state, while the J/y is the equivalent for charm. K_S (K_L) is the short (long)-lived neutral strange meson.

   The progress of heavy flavour physics, especially of beauty physics is impressive. One must first underline the remarkable performances of the e⁺e^- Beauty Factories, PEPB (detector BaBaR) at SLAC, and in particular KEKB (detector BELLE) in Japan, the first machine to deliver a luminosity of 10³⁴ cm^-2s^-1 (the luminosity, multiplied by the cross-section of a given process, gives the rate of corresponding events). These colliders sit on the Upsilon(4S) resonance shown in figure 1, decaying into beauty meson-antimeson pairs.

   In the study of the CKM matrix* defined above, the highlight is the determination of the so-called Unitarity Triangle [7]. In the SM, the unitarity of the CKM matrix, expressed in a graphical way, leads to the figure of a triangle, one for each of its rows and columns. This is because one of its four parameters is a non-zero phase, so that the CKM matrix is complex. Among the 6 unitarity relations, the "d-b" one, V_udV_ub*+V_cdV_cb*+V_tdV_tb*=0, is particularly interesting because the three sides of the corresponding triangle (figure 5) have similar sizes. The length of its sides and its angles can be extracted from various measurements in the field of heavy flavour physics, and in particular of beauty physics. With enough of these measurements one can build this triangle in different ways: thus one can check that the result is unique, and, first of all, that one is indeed dealing with a triangle and not a more complicated situation that theories beyond the SM may announce.

   It is clear that a very successful first round of experiments has been accomplished [21,7]. The direct measurement at Beauty Factories of one of the angles (called b or f₁, depending on the continent…) via the theoretically very clean mode BJ/y K_S is in excellent agreement with the determination of the tip of the Triangle through the measurement of its sides made during the past decade at LEP and elsewhere, from B and K physics results (figure 6). This is another major success of the SM. However, revealing new phenomena through B physics calls for a still much better accuracy.

   The roadmap, concerning the second round of measurements, defines an ambitious programme, involving many different decay modes of beauty and extremely demanding from the experimental (luminosity needed, control of systematics, etc.) as well as from the theory side: the hadronic uncertainties must be controlled, since the b quark under study is irredeemably confined inside beauty mesons, and one must obtain a reliable estimate of the contribution from loop diagrams complicating the process, the famous "penguins" (figure 2d), which represent both an embarrassing "pollution" and a promise, since it is in their loops that new physics, like Supersymmetry, could appear.

   The kaon rare modes [22] also allow one to build another Unitarity Triangle through K⁺p⁺nn, K_Lµµ, K_Lp⁰nn, p⁰e⁺e^-, etc. Several recent results and the promise offered by future experiments like CKM in Fermilab go in the right direction.

   Finally, the muon rare modes are equally promising and the expected performances very impressive indeed: µeg with a sensitivity per event of 10^-14 at PSI, conversion µe in nuclei at 2 10^-17 in MECO at BNL.

Heavy ions
In the Big Bang model the transition from free quarks to hadrons occurred at a few microseconds. High energy heavy ions collisions are under study, to find evidence for the reverse step, i.e. the fusion of nucleons into a quark-gluon plasma (quagma) [23].

   Fresh results are coming from the RHIC collider in Brookhaven, concerning Au-Au collisions up to 200 A GeV and have brought a few "surprises" concerning the properties of the hot and dense medium thus produced. The quote expresses the fact that some of them were actually predicted long ago.

   The chemical freeze-out (at which the identity of the particles is fixed) occurs at 175 MeV (the Hagedorn temperature [24]), as at the CERN SPS, but the medium is now nearly baryon-free. The kinetic freeze-out (at which their kinematics is fixed) happens near 100 MeV. The medium undergoes an explosive expansion at a speed of 0.6 c, and shows a strong anisotropy of transverse flux, suggesting a hydrodynamic expansion due to very strong pressure gradients developing early in the history of the collision. Remarkably, the collision zone is opaque to fast quarks and gluons and this has a strong impact on hard phenomena: suppression of hadrons produced at large p_T, jet "quenching", i.e. the decrease of their rate of production, phenomena which are not observed in control collisions D-Au . Several questions concerning the Hanbury-Brown-Twiss (HBT) correlations (a concept borrowed from astronomy), e.g. the size of the collision zone, or the fate of charm in this opaque medium, etc. have still to be clarified.

   However the most prominent signatures which could reveal a quark-gluon plasma are not yet available from RHIC and it is from the CERN Super Proton Synchrotron that results are still coming. In particular, the experiment NA45 confirms that the excess of low mass e⁺e^- pairs, m_ee>0.2 GeV, implies a modification of the r resonance in the dense medium, probably linked to its baryonic density. The suppression of the production of the J/y, the lowest bound state of charm and anticharm, which could signal its fusion in the quagma [25], is confirmed by the analyses of NA50 and keeps all its interest. Unfortunately no unique prediction of this effect exists for RHIC and LHC. Data are needed: the next ones should come from PHENIX at RHIC and from NA60 at the CERN SPS.

Acknowledgements
I would like to thank several colleagues for their reading and suggestions, in particular L. Pape, P. Janot, R. Barate and I. Timmermans.

About the author
Daniel Treille is a senior physicist at CERN and a former spokesman of the DELPHI experiment.

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COMPASS: CERN Courrier, Sept. 99, article 19,
RHIC: CERN Courrier, April 02, p8, Jan-Feb 03

[20] CERN Courrier, Jan-Feb 03, article 1, July-Aug. 03, article 2

[21] CERN Courrier, April 01, article 15, may 2002, article 2, see also reference 7

[22] KTeV: CERN Courrier, April 99, article 2, July-Aug. 2001, article1,
E787: CERN Courrier, March 2002, article 2, p4
KLOE: CERN Courrier, Sept. 99, article 23
NA48: CERN Courrier, Sept. 99, article 2, July-Aug. 2001, article 1

[23] Europhysics News, (2000) Vol. 31, No. 3,
CERN Courrier, Oct. 2002, article 13

[24] CERN Courrier, Sept. 03, article 18

[25] CERN Courrier, May 1999, article 7

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