Matter & antimatter

The RHIC complex at the Brookhaven National Laboratory has several accelerator facilities chained together to provide beams that are collided in detectors located inside the RHIC ring.-RHIC WEBSITE/BNL

The Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) in Geneva is to start its year-and-a-half-long continuous physics runs from March end. Its two counterrotating high-intensity proton beams will be accelerated to the highest energy ever of 3.5 tera electronvolt (TeV) each and made to slam against each other to deliver a total of 7 TeV energy for particle creation.

But even as physicists eagerly await the exciting new physics that the LHC is likely to reveal, there have been exciting new developments in high-energy physics experiments in recent months in other accelerator laboratories as well.

In February, the Relativistic Heavy Ion Collider (RHIC, pronounced rick), an underground ringed atom smasher at the Brookhaven National Laboratory (BNL) at Upton, New York, established a record for the highest temperature ever created on the earth.

0Analyses of nuclear collision events observed in the collider established that collisions of gold (Au) ions travelling at nearly the speed of light created matter at a temperature of about four trillion degrees Celsius, about 250,000 times hotter than the centre of the sun. This is higher than the temperature required to melt the protons and neutrons of the nuclei into a liquid-like plasma of quarks and gluons, the fundamental constituents of matter.

RHIC is an accelerator designed to study matter as it existed fractions of a second after the birth of the universe. To achieve this, the machine accelerates heavy ions (atoms of heavy elements stripped of their electrons) to high energies and collides them to mimic the very hot and extremely dense conditions of the early universe. For Au-Au collisions, the total available energy is typically 200 GeV per nucleon (neutron or proton).

RHIC was the most powerful heavy-ion collider in the world until the LHC went online in late 2009 and collided ions at higher energies. In fact, the LHC is designed to collide lead ions with a total energy of 1,150 trillion electronvolt (TeV) in the later phase of its operations.

RHIC is actually two accelerators in one, enclosed in a tunnel with a circumference of 3.84 kilometres. In the two rings, beams comprising bunches of heavy ions are accelerated in opposite directions and held in their orbits by an array of 1,740 superconducting magnets. The particles collide at six points around the circle where the two rings intersect. Tens of thousands of collisions take place every second, each producing a spray of thousands of subatomic particles.

There are four complementary detectors located at four of the six intersection points: BRAHMS, PHENIX, PHOBOS and STAR. These detectors collect data of the collision products, which are analysed in a bid to understand the physics of the early universe.

India is one of the 13 countries participating in the STAR experiment, which is designed to obtain fundamental data about the microscopic structure of the ion interactions by tracking the thousands of particles that emerge from the collisions. The Variable Energy Cyclotron Centre (VECC), Kolkata; the Indian Institute of Technology Bombay; the University of Jammu; the University of Rajasthan, Jaipur; and Punjab University, Chandigarh are among the 54 institutions from around the world involved in the experiment. Indian scientists were responsible for the conception, design, fabrication and installation of one of STARs detectors called the Photon Multiplicity Detector (PMD), which measures the photon signatures of the quark-gluon plasma (QGP).

The temperature measurement was, however, based on the measurement by the PHENIX experiment. This temperature measurement, combined with other observations of the four experiments analysed over the nine years of RHICs operation, indicates that RHICs Au-Au collisions produce a freely flowing liquid QGP, according to a BNL press release. The QGP is believed to have filled the universe a few microseconds after it came into existence 13.7 billion years ago before cooling and condensing to form the protons and neutrons that make the matter we see around us, from atoms to stars, to planets and people. At RHIC, QGP liquid appeared and a temperature of 3 1012C was reached in less than a billionth of a second, and the QGP itself lasted for less than a billionth of a trillionth of a second, the release said.

The temperature is inferred from the distribution of the frequency, or the energy, of the light emitted from the QGP liquid just like a hot iron rod emits a red glow. Since light interacts very little with the hot liquid, it is an accurate measure of the hot conditions within.

These data, according to Steven Vigdor of BNL, who oversees the RHIC programme, provide the first measurement of the temperature of the QGP at RHIC. According to him, the temperature inferred from these new measurements is considerably higher than the long-established maximum possible temperature attainable without liberating the quarks and gluons from their normal confinement inside individual protons and neutrons.

Theoretical predictions before the start of RHIC in 2000 expected that the QGP would exist as a perfect gas of free quarks, antiquarks and gluons. But analysis of data from RHICs first three years of operation produced the astonishing result that the matter behaved like a liquid whose constituent particles interact very strongly among them. The trajectories of the thousands of particles produced indicate that the particles produced tend to move collectively, more like the flow of a liquid in motion.

However, unlike ordinary liquids, in which individual molecules move about randomly, the hot QGP seems to move in a pattern that exhibits a high degree of coordination among the particles, somewhat like a school of fish. This liquid matter has been described as a nearly perfect liquid in the sense that it flows with almost no viscosity. Such a perfect liquid, however, does not fit in with the picture of free quarks and gluons that physicists had hitherto used to characterise the QGP.

In 2005, RHIC scientists planned a set of crucial measurements to clarify the nature and constituents of the perfect liquid. Temperature measurement was one of them. Models had suggested that the initial temperature attained in RHIC collisions would be high enough to melt protons, but a more direct measurement of the temperature required detecting photons that are emitted near the beginning of the collision and travel outward practically unhindered.

This was an extraordinarily challenging measurement, explained Barbara Jacak of Stony Brook University and spokesperson for PHENIX. There are many ways that photons can be produced in these violent collisions. We were able to eliminate the contribution from these other sources by exploiting RHICs flexibility to measure them directly and to make the same measurement in collisions of protons, rather than of gold nuclei. Thus, we could pin down excess production in the gold-gold collisions and determine the temperature of the matter that radiated the excess photons. By matching theoretical models of the expanding plasma to the data, we can determine that the initial temperature of the perfect liquid had reached about four trillion degrees.

From amongst the particles emitted from the high-energy (200 GeV) gold ion collisions, physicists working in the STAR collaboration have for the first time found evidence of nuclei of certain exotic forms of antimatter. Scientists have previously created antimatter nuclei (anti-nuclei) in the form of anti-deuterium, anti-tritium and anti-helium. But the STAR experiment has created the first-ever anti-nucleus containing an anti-strange quark. The anti-nucleus discovered is a negatively charged state of antimatter consisting of three particles: an anti-proton, an anti-neutron and an anti-lambda particle. It is also the heaviest anti-nucleus discovered to date.

The finding opens up an entirely new terrain of antimatter. It could also shed light on the question of the dominance of matter over antimatter in the universe and the type of matter in the highly dense core of collapsed stars. The results were published online in the March 4 issue of Science Express.

According to the Standard Model (SM) of particle physics, the fundamental constituents of all matter are six quarks up, down, charm, strange, top and bottom and six leptons electron, muon, tau and three neutrinos linked to each respectively. But all familiar terrestrial nuclei are made of protons and neutrons (which in turn are composed of only up and down quarks) and have zero value for the quantum attribute strangeness. Like protons and neutrons, which are made of three quarks, there are also other particles composed of three quarks but which include a strange quark and thus have a non-zero strangeness value. These are called hyperons. The lambda particle, for example, is the lightest known hyperon, which is also stable, and is composed of an up quark, a down quark and a strange quark. The lightest hypernucleus is a triton, written 3H, containing a neutron, a proton and a lambda hyperon. The new anti-nucleus discovered is the antimatter counterpart, composed of the antimatter equivalent of all the three particles (anti-proton, anti-neutron and anti-lambda), and hence it is an anti-hypertriton. Hypernuclei are themselves not new. They were first discovered by Marian Danysz and Jerzy Pniewski in 1952. It was formed in a cosmic ray interaction in a balloon-flown emulsion plate. Today, there are many laboratories around the world studying the properties of hypernuclei.

The standard periodic table arranges the elements according to the atomic number, or the number of protons (Z) in the nuclei, which determines an elements chemical properties. If we wish to include isotopes, which have a different number of neutrons for a given element, in the same table, we could think of a two-dimensional chart with the two axes representing the number of protons and the number of neutrons (N) in the nuclei respectively. If the axes are extended in the negative directions as well, we can represent the antimatter counterparts of normal nuclei in the same chart. We could now think of a three-dimensional representation where the third axis represents the strangeness quantum number, thus extending the nuclear terrain to include the newly discovered strange antimatter.

As we have seen, heavy-ion collisions at RHIC fleetingly produce conditions that existed a few microseconds after the Big Bang, creating the QGP. Now in the QGP of the early universe, quarks and antiquarks, including pairs of strange and anti-strange quarks, would have formed with equal abundance. As the plasma expanded and cooled, different bound states of these quarks would have formed, including protons, neutrons and other strongly interacting particles like hyperons, as well as their corresponding antiparticles. As the plasma expanded further, light nuclei and anti-nuclei would have formed.

Likewise, if high-energy collisions really recreate the conditions of the early universe, among the collision fragments from the QGP that survive until the final state at RHIC, abundance of matter and antimatter would still be nearly equal. The mystery is that from such a symmetric state the universe as we know it seems to be devoid of antimatter. Understanding how and why there is this asymmetry in the universe remains a major unsolved problem in physics. Future antiparticle measurements from heavy-ion collisions at RHIC and in other high-energy particle experiments at the LHC, for example will be addressing this question.

The STAR experiment has found that the rate at which the new anti-hypernucleus is produced is consistent with expectations based on the Quark Coalescence Model of particle production from high-energy heavy-ion collisions. Basically, the model assumes that in the hot soup of quarks, antiquarks and gluons in the QGP, a statistical collection of antiquarks gives rise to antimatter production. Extrapolating from this result the present experiment observed 70 events with this particular anti-hypernuclei scientists expect to see even heavier anti-nuclei in future runs at RHIC. Theoretical physicist Horst Stoecker and associates have predicted that strange nuclei around double the mass of the newly discovered state should be stable. This will have implications for novel ideas relating to the structure of dense nuclear matter in stars and cosmic ray experiments searching for new physics like dark matter, according to Jinhui Chen and Declan Keane of Kent State University, the lead authors of the paper in Science Express.

About a hundred million collisions between gold nuclei were painstakingly scanned with the new anti-nucleus being identified by its decay into a light isotope on anti-helium (anti-helium3 and a positively charged pion. Corresponding to the 70 anti-hypernuclei detected, 160 hypernuclei (of normal matter) were also seen. These numbers very closely matched the number of anti-helium3 and helium3 that were seen. This was also significantly larger than that measured at lower energies, according to the authors of the paper. This implies that the hot soup of quarks and gluons must have contained similar amounts of strange quarks and up and down quarks, which is what one would expect from a real QGP. Thus, this discovery is yet another confirmation of the formation of the QGP in the heavy-ion collisions at RHIC. The antimatter studies at RHIC have restarted with greatly enhanced capabilities. The scientists expect to increase the collision rate by a factor of 10 in the next few years.

Another exciting, and related, finding from RHIC that throws new light on the nature of the QGP is the first evidence of profound symmetry transformations within the hot soup of quarks, antiquarks and gluons. This finding, which has been published in the journal Physical Review Letters, suggests that bubbles, or local domains, form in the hot strongly interacting matter of the QGP in the interiors of which parity, or mirror symmetry a fundamental symmetry that says that the physics of a system and its counterpart in a mirror-reflected world should be the same is violated.

Parity (P) symmetry and charge-parity (CP) symmetry, in which the mirror-reflected world also has all its particles replaced by their antiparticles, are known to be violated in the nuclear weak interaction that causes radioactive decay. However, there is no evidence to date of these symmetries being broken in strong interactions that bind quarks into protons and neutrons and hold the nuclei together. Although the theory of strong force, known as quantum chromodynamics (QCD), itself does not forbid P violation or CP violation in strong interactions, experiments so far have put very stringent limits on its occurrence. Theorists have, therefore, predicted an increasing probability of finding such local domains of broken symmetry at extreme temperatures near transitions from one phase of matter to another. Accordingly, physicists believe that matter inside these bubbles will exhibit different symmetries behaviour of the dynamics of such matter under simple transformations of space, time and particle types from the surrounding matter. Such symmetry violation bubbles created during the early universe may have led to the dominance of matter over antimatter, it is believed.

As has been described earlier, RHICs most energetic collisions have been able to create the extreme conditions that might be just right for producing such localised bubbles of altered symmetry: a temperature of four trillion degrees Celsius and a transition to a new phase of matter in the form of a free-flowing perfect liquid QGP. The new results suggest that RHIC may have a unique opportunity to test in the laboratory some crucial features of symmetry-altering bubbles speculated to have played a role in the evolution of the infant universe, said Steven Vigdor. What STAR has observed is an asymmetric charge separation in particles emerging from all directions except from the collisions that are most head-on.

Since there is nothing to distinguish one direction from another a priori, both positively and negatively charged particles are expected to be emitted in equal numbers above and below the plane of interaction from symmetry considerations. But colliding nuclei passing each other are known to produce coherent ultra-strong magnetic fields, stronger than the fields that can be generated under laboratory conditions, which can lead to local effects of altered symmetry. This magnetic field acts as an external field on the quarks and gluons that make up the hot matter and induce the separation of positively and negatively charged particles in the plasma.

In an off-central (not head-on) heavy-ion collision, the magnetic field is preferentially oriented perpendicular to the reaction plane. Now the separation of charges creates a localised electric dipole moment in the hot matter containing quarks, antiquarks and gluons that have been deconfined from the nuclei along the direction perpendicular to the reaction plane. This causes positively charged quarks to align themselves along the direction of the magnetic field and negatively charged quarks along the opposite direction. As a result, positively charged particles in the final state are emitted preferentially along one direction and negatively charged particles in the opposite direction. Because this appearance would appear reversed in a mirror-reflected world, mirror symmetry appears to be violated in these high-energy collisions.

There is an important difference between this observed effect and the effect observed in weak interactions. In strong interactions, nature makes a random choice in the direction of the magnetic field, or equivalently, in the manner of separation of charges, and hence the induced dipole moment.

Writing about the finding, Evan Finch and associates of the STAR experiment make the following analogy with the wind vane. In weak interactions, they wrote, the vane is always pointing in the same direction. In strong interactions, the vane is rotating rapidly while it does always point in some direction at any given moment, there is no preferred direction on the average. That is, even though there are violations locally in each event, averaged over many events there would appear to be no parity violation. This makes it necessary to study local parity violation event by event, and fluctuations of charge symmetry with respect to the collision plane would, therefore, be a signature for local parity violation. Local parity violation has been inferred from observations of such fluctuations at STAR.

The key to observing the effect is to study correlations among the particles emerging from the collision, says Nu Xu of Lawrence Berkeley National Laboratory, STAR experiment spokesperson. We have observed a correlation among emitted charged particles of the predicted type, with the degree of directional preference increasing as the collisions vary from head-on to more grazing, Xu added. STAR data also seem to suggest the local breaking of CP symmetry. If CP symmetry had not been broken at some early time in the evolution of the universe, the particles and antiparticles that were created in equal numbers during the Big Bang would have annihilated each other in pairs, leaving no matter to form the stars and planets we see today.

Some small violations of CP symmetry have been seen in previous experiments as well, but these are too weak to explain matter dominance in the universe. Likewise, STAR finding too does not provide an explanation for that but may offer some insight into how such symmetry-breaking occurs. According to Vigdor, the features observed at STAR were found to be qualitatively consistent with predictions of symmetry-breaking domains in hot quark matter. Confirmation of this effect and understanding how these domains of broken symmetry form at RHIC may help scientists understand some of the most fundamental puzzles of the universe and will be a subject of intense study in future RHIC experiments, he said.

To probe these and other questions, the BNL is planning to upgrade RHIC over the next few years to increase its collision rate and detector capabilities.