ON March 18, the Fermi National Accelerator Laboratory (Fermilab) in Illinois, United States, announced that it had found evidence of an unexpected particle whose curious characteristics may reveal new ways that quarks can combine to form matter. Fermilab houses the Tevatron, the most powerful particle accelerator until the Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) now scheduled to start operations in July after repairs following the mishap of September 19, 2008 (Frontline, January 30, 2009) overtakes it.
The new particle has been temporarily designated as Y(4140), which signifies that it has a mass equivalent to 4,140 million electron volts (MeV) of energy in the sense of Einsteins mass-energy relation and the symbol Y is indicative of its as yet unconfirmed and ill-understood status. Once its other characteristics besides mass, such as the intrinsic quantum properties and its internal structure, get established it may get a permanent name.
At the Tevatron, counterrotating beams of protons and antiprotons (the antimatter equivalents of protons), each with an energy of 980 billion, or giga, eV (GeV), are brought to collide head on. As protons and antiprotons annihilate at the collision points, a total of 1.96 trillion eV (TeV) of energy becomes available for particle production. Two major international experiments, called D-Zero (which has significant Indian participation) and CDF, are currently running at Fermilab. The experiments involve looking at trillions of collisions and analysing the vast amounts of data generated for signals of something new among the particles produced.
The Y(4140) discovery came from the data gathered at the CDF detector. Scientists found signatures that could not be explained with known particle processes and the resulting products. For appropriate signatures, scientists look at the spectrum of energy distribution of all, as well as subsets of, relatively long-lived final particles from the decays of short-lived particles (whose typical lifetimes are in the order of 10-22 to 10-23 seconds) that are produced by the proton-antiproton collisions. A bump in the energy distribution, or a clustering of events around a particular energy, would point to something significant (Figure 1).
At present, it is not clear what exactly Y(4140) is made of. It does not seem to fit into the known scheme for making up particles from the fundamental building blocks of matter called quarks (and their antimatter equivalents, the antiquarks). It must be trying to tell us something, CDF spokesperson Jacobo Konigsberg said.
So far, we are not sure what it is, but rest assured well keep on listening, he was quoted as saying. Significantly, Y(4140) is not the first mystery particle to be discovered in recent times. In the last five years, physicists, particularly at the electron-positron collider experiments called Belle at KEK in Tsukuba, Japan, and BaBar at the Stanford Linear Accelerator Centre (SLAC), U.S., have seen signals of several other exotic particles variously designated as X-, Y- and Z-particles with different masses that cannot be explained in terms of conventional particle structure.
Clearly, all these discoveries are trying to say something that physicists are trying to decipher. Is it new physics? Or is it just new insights into the known physics of elementary particles? At this point, it seems to be the latter but physicists are yet to grasp the new insights and there are several ideas floating around. Tommaso Dorigo of Padova University and a member of the CDF collaboration wrote in his blog after the recent discovery: While all eyes are pointed at the searches for the Higgs boson and supersymmetric particles [Frontline, October 10, 2008] if not even more exotic high-mass objects, and careers are made and unmade on those uneventful searches, it is elsewhere that action develops. Such discoveries tell the tale of a very prolific research field [of low-energy particle physics] where there is really a lot to understand.
The standard picture is based on what is known as the Standard Model (Figure 2), which describes all known matter and forces (except gravity). According to this model, the fundamental constituents of all matter are six quarks and six leptons and their antiparticles. The six quarks are called (somewhat whimsically): up (u), down (d), charm (c), strange (s), top (t) and bottom (b). These quarks and their antiquarks combine in different ways to make the particles that are produced in accelerator experiments. For example, three quarks make up a proton (two u quarks and one d quark) and a neutron (two d quarks and a u quark). In general, three quarks can bind to form other proton-like particles whose generic name is baryon. Similarly, a quark-antiquark pair can also form a bound state whose generic name is meson.
For example, the positively charged pi-meson, or pion, is made of a u quark and an anti-d (d-bar) quark. The J/psi-meson is made of a c quark and a c-bar quark. Mesons and baryons are collectively referred to as hadrons.
But quarks themselves are not seen in isolation. This is because of the nature of the strong force, called the colour force, that mediates between quarks. The word colour has nothing to do with its usual meaning and is merely a name given to a new type of charge that quarks carry. This colour charge comes in three varieties, often labelled as red, blue and green. The theory that describes this force is called quantum chromodynamics (QCD), analogous to the force of electrodynamics that mediates between electric charges. QCD posits that colour charge is confined within hadrons and can never emerge to be detected in an experiment unlike electric charge, which can exist in isolation. That is, quarks must always combine so that only colourless bound states are formed, which can be detected. That is, protons, pions and other particles that are created in collision experiments in accelerators, for example, should have quarks combined in such a fashion that they are colour-neutral (Figure 3).
It is the colour force that gives rise to a residual strong force that extends beyond a colour-neutral proton or neutron to bind them into a nucleus, much like the van der Waals force that allows electrically neutral atoms to bind into the molecules of chemistry.
The electromagnetic force between charged particles is described by the exchange of photons (light particles) between them, that is, a photon is the carrier of the electromagnetic force. Analogously, particles called gluons are the carriers of the colour force between quarks. However, there are crucial differences. Photons are massless and chargeless. But gluons, though massless, have a colour charge. That is, gluons, unlike photons, can interact with each other. Also, unlike a single carrier for electrodynamics, QCD requires eight gluons. These peculiar properties of gluons make the colour force drastically different from the coulomb force between electric charges. The latter, as we know, falls off as the inverse square of the distance between the charges.
The colour force, on the other hand, does not decrease with distance as the colour-carrying entities quarks and gluons are moved apart. This means that an immense amount of energy will be required to separate two coloured objects. This is why quarks are confined forever within hadrons (Figure 4). (One could, of course, imagine imparting a huge amount of force to separate quarks for a distance, say, about the diameter of a proton. But that would only result in pumping enough energy to create a real quark-antiquark pair and the process generating two colour-neutral particles from the initial one.)
The above simple picture of hadrons as a mere collection of two or three quarks, together with a set of approximations, provides a theoretical framework for calculations called the constituent quark model (CQM), and the model seems to describe hadrons at low energies pretty well. But this picture is overly naive because quarks are actually dressed in a force field of the mediating gluons, and all of them packaged together is the particle that one sees.
In fact, QCD allows for the number of quarks and gluons that constitute a hadron to fluctuate with time. The fluctuations are in the form of additional virtual gluons or quark-antiquark pairs. This is because of the underlying vacuum in the theory. At the quantum level, the vacuum is never empty. It is actually seething with virtual particle-antiparticle pairs ever so momentarily created and annihilated without violating any conservation laws (because of the quantum uncertainty principle). But the CQM ignores the constituent gluons, neglects these vacuum fluctuations and treats quark motion as non-relativistic.
This picture of a particle as quarks plus a sea of virtual gluons and quark-antiquark pairs actually solves another problem with the naive quark model: quarks are much too light to account for the mass of the particles that they form.
Physicists now believe that these virtual particles constitute the missing mass about 98 per cent in a proton, for example that makes up a typical hadron. But can gluons, which account for the mass of hadrons, exist as colour-neutral objects devoid of quarks on their own? Since gluons carry colour charge and mutually interact, could gluons combine to form purely gluonic matter? Such objects are called glueballs. Similarly, gluons could also combine with quarks and antiquarks to form hybrid particles. Physicists have even conceived of four-quark (tetraquark) or five-quark (pentaquark) systems to form colour-neutral particles. One could also think of bound states of mesons, a kind of mesonic molecule, just as protons and neutrons bind to form nuclei. Nothing in QCD forbids such mesonic bound states as well or even baryonic bound states. These hybrids, glueballs and higher multiquark states are together called QCD exotics.
But are we now beginning to see evidence of these exotic objects? Eric Swanson of the University of Pittsburg believes that the CQM, which is an approximation that works well at lower energies, must fail somewhere. As he pointed out in a 2006 review article, quark models are not QCD. But models do not seek to be QCD, rather they attempt to capture the dominant physics relevant to the problem at hand. In this regard quark models suffer from deficiencies which will become apparent as states higher in the spectrum are discovered. Are these discoveries evidence of deeper manifestations of the strong colour force beyond the CQM?
These new discoveries have been seen in the analyses of the production of what are called charmonium states, particles that are bound states of the c quark and its antiquark such as the prototype J/psi-meson and its higher-mass avatars, which are expected in the conventional CQM framework. Physicists look for these different charmonium particles in the decays of B-mesons (mesons containing a b-quark), which are copiously produced in the experiments at Belle (KEK), BaBar (SLAC), the Tevatron (Fermilab) and CLEO (Cornell). Being heavier particles, their decay modes favour production of various charmonium states.
According to Swanson, to date 16 new charmonium states have been discovered, of which, Y(4140) is the latest. A great number of interpretations, Swanson said in an e-mail response, have been proposed for these states. Of course, since there are very many interpretations, most of them must be wrong!
The manner of discovery of Y(4140) by CDF scientists was quite similar to that of other new particles. The signal for a particle at 4,140 MeV mass was seen in the decay of a much more commonly produced B+ meson. From trillions of collision events, they found a sample of about 80 events the largest sample in the discoveries of such unusual decays so far, according to Tammaso decaying in an unexpected pattern (Figure 5). Further analysis showed that B+ mesons were decaying into Y(4140), which was, in turn, found to decay into a pair of particles, J/psi-meson and a phi-meson. (The latter is made of s and s-bar quarks.) Though this decay mode suggests that it could be a charmonium state, its characteristics did not fit what is expected of a charmonium state.
The properties of charmonium states are well predicted by theory, said Estia Eichten, a theorist at Fermilab involved with charmonium physics, in an e-mail response. Y(4140) adds an important piece to the puzzle of new states discovered recently. However, the theory of these new states is not yet clear. There is not yet a consensus on Y(4140)s composition. This, as well as the others, must have other active degrees of freedom, like gluons, to form a hybrid or an additional light quark pair to form a molecule or a tetraquark state. Theorists have not yet made reliable predictions for the spectrum and properties of such [exotic] states, Eichten added.
Swanson, however, believes that Y(4140) may not be a real particle at all, the signal seen being just a statistical fluctuation or an experimental artefact. If it is real, he said in his e-mail, one must consider the possibility that it is a threshold enhancement. This happens quite often when two particles are produced, and because of their interactions, cause a bump in the signaland eager experimentalists call them particles....Except for one thing. Threshold enhancements tend to be quite broad [but] Y(4140) is very narrow and so is quite unusual. My bottom line is that, if the state is confirmed, then it is an unusual thing indeed. But my bet is that this signal will go away with more statistics.
But Masanori Yamaguchi of the KEK believes that the state may be related to the Y(3940) discovered at Belle in 2004 and has been hypothesised to be a hybrid state. We will try to confirm this state in our own Belle data, he said. However, Swanson believes that Y(3940) is also a threshold effect. For him, in his list of 16, only two X(3872), one of the first of the new particles, discovered at Belle in 2003, and Y(4260), discovered at BaBar in 2005 make the grade as real exotic particles.
The X(3872) is the poster boy of the new heavy hadrons, Swanson wrote. It has been observed by four experiments in three decay and two production channels and continues to refuse to fit into our expectations for charmonium, he said. Though it now stands established as a distinct new particle, the exact nature of its composition is still being hotly debated by physicists. One of the widely accepted interpretations for X(3872) is that it is a bound state a molecule of two other mesons, called D0 and anti-D0*. (D0 is a combination of c and u-bar quarks; anti-D0* is an excited state of its antiparticle anti-D0 made of c-bar and u quarks.) If true, it opens up an entirely new field of chemistry of elementary particles, just like the chemistry of atoms.
If mesons can form such molecules, one could ask why we do not see molecules of the lighter mesons, such as the pion. This is a question of dynamics, says Swanson. In principle, a pi-pi bound state could exist, but when you solve the equations you find no bound states. It is because, one, there is no long range van der Waals force between pions, and two, pions are very light and therefore are difficult to bind together, he explains.
Y(4260), on the other hand, is a very good candidate for a hybrid, which is a bound state of c, anti-c and gluons, says Swanson. If confirmed, this would be the first sighting of a dramatically new form of matter, which incorporates gluons, he adds. Like these higher states of charmonium, higher state analogues of bottomonium, composed of heavier b and b-bar quarks, are expected. Searches are on for these and some candidates like Yb have been found at Belle. But top quarks are so heavy that they decay before they can form bound states. So the story is unlikely to repeat with toponium.
The discovery of glueballs would, of course, be yet another intriguing story. Though candidates for this have been seen, the f0 states discovered in 2006, they are still unconfirmed. But, as Swanson points out, a theoretical framework to handle glueballs could be particularly problematic as they are states of pure gluons, with which physicists have no experience, and existing models are untested and numerical predictions less reliable.
Though there is a lot of evidence now that nature is presenting us with a tantalising hint of unusual happenings at energy scales of 10s of GeV, something not expected in the standard picture that had worked so well until now, it appears that there is as yet no need to invoke new physics beyond the Standard Model and the extended application of QCD in detailed quark-gluon spectroscopy.
These discoveries have brought surprise and excitement only because, as Swanson put it, nature is more complex than it had indicated to date. The discoveries also teach us important things about how QCD actually works. In the end, we are learning how the Standard Model and QCD build up the universe.