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CERN | Geneva | Switzerland

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New composite particle discovered by ATLAS

On December 21, a few hours after CERN was officially closed for the end-of-the-year holiday, the ATLAS experiment ended a fruitful 2011 on a happy note by releasing a paper announcing the discovery of a new quarkonium state, identified as the χb(3P), which had been predicted by theorists.

Quarkonia particles are composed by a quark and its anti-quark, such as charm plus anti-charm for the charmonium family and bottom plus anti-bottom for the bottomonium family.  Patterns of different mass and spin quantum number appear, corresponding to different configurations of how the state is bound together.  States of higher mass decay frequently to configurations of lower mass, such as the J/ψ and Y for the two quarkonium families.

Several such states have been both predicted and observed in the past. This discovery adds one more piece of information to a very complex problem. Quantum Chromodynamics (QCD) is the theory that describes the strong nuclear force that acts between quarks. It is part of the Standard Model, the theoretical model of particle physics. In principle, QCD can directly predict the properties of all particles made from quarks, including the protons and neutrons, which make up ordinary matter, and these more exotic quarkonium states. The problem is that it is mathematically very difficult to perform these calculations, and in many cases it is not currently possible.  Theorists have therefore developed simplified models that enable them to make useful predictions. Data on the newly found χb(3P) state of the bottomonium family  help to refine the models.

The newest state is the third of a series, with χb(1P) and χb(2P) being two lighter states, found in the 80’s and in the 90’s in electron-positron colliders. The χb(3P) had been predicted,  but had never been seen before, because it is difficult to observe it in transitions from states of higher mass.

The Large Hadron Collider (or LHC) at CERN is a 27-km machine that accelerates beams of protons at near the speed of light. Two separate beams circulate in opposite directions and are brought into collisions in the center of large detectors such as the ATLAS detector. Heavy but unstable particles are created from these highly energetic collisions. The detectors are used to take a snapshot of all their debris, each “picture” being called one event. Physicists then attempt to reconstruct what happened in each event and see what particles were created and how they decayed. The overall goal is to gain a clearer understanding of what matter is made of and how it interacts at the smallest level.

The χb states are found by looking for events where these states decayed into lighter bound states of b quark and anti-b quark called ϒ(1S) or ϒ(2S), plus a photon. In turn, the ϒ decays into two muons. The muons and photons are seen by the detector, and from these, the original χb states can be reconstructed.

The photon is detected in two ways: either by a system called the electromagnetic calorimeter, or, when the photon converts into a positron and an electron as it interacts with material in the detector, the latter can be seen by the tracking detector. For these decays, the tracking detector has a better resolution than the calorimeter, which means it measures the photon momentum more precisely.

The results are on the two plots below. The vertical axes show how many events containing a muon pair and a photon ATLAS detects in the collision fragments that are consistent with coming from a χb(1P), χb(2P) or χb(3P) decay. The scale on the horizontal axis is the mass; as the mass increases, the number of selected events rises, then falls, three times, leading to three peaks in the data. Each peak corresponds to one of the three states, and the position of the peak on the mass scale indicates the most likely mass of the state. The upper plot uses photons seen by the calorimeter and the lower one uses photons spotted by the tracking detector; the peaks are narrower in the latter because of the better resolution of the tracking detector.  The red curve shows candidates where the χb decayed into a ϒ(1S) state and the purple one, ϒ(2S).

The mass of the new state is equal to 10.54 GeV, about ten times that of a proton. This is somewhat higher than predicted, indicating that the quarks are less tightly bound than suggested by the models.

Each of the peaks shown in the plot is expected to be formed by separate, closely spaced sub-peaks, due to contributions from χb states of different spin. With the current quantity of data, it is not possible to see them since the limited resolution of the detector smears them into a single peak. More data from LHC will allow ATLAS, and also CMS and LHCb, to perform additional and more accurate measurements.

Theorists will then have more landmarks to refine their models, leading to a better description of QCD. Improving the understanding of the strong nuclear force is one of the major challenges facing particle physicists, and this new state provides important new information.

Pauline Gagnon

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