First J/Ψ peak: one more resonance to tick off our list

12 May 2010

In 1974 the J/Ψ meson was first discovered, heralding a new era of understanding in particle physics. Now again, in 2010, it's ‘rediscovery’ in ATLAS and the other LHC experiments’data signals an important stage towards what will hopefully also become an historic milestone in high energy physics.



Made from a charm and anti-charm quark, and bound together by the strong force of quantum chromodynamics (QCD), the J/Ψ was at the time of its discovery at Brookhaven National Laboratory and SLAC the evidence needed to prove the existence of the charm quark as suggested by Glashow, Iliopoulos and Maiani.

Two features of the J/Ψ make it a useful object to study in ATLAS -- about 6% of the time it decays to a muon anti-muon pair, and it has a very narrow ( ~ 93 keV) natural decay width meaning that in a plot of the di-muon pair invariant mass it produces a very sharp resonance as a signature of its existence, at the J/Ψ mass of 3.097 GeV. Producing a muon in a collision at ATLAS is relatively rare compared to the plethora of other particles produced in a standard collision, and is usually a sign that a significant enough interaction may have taken place that we may want to record the event for further study. The decay of a J/Ψ has a striking signature: two muons produced in one collision, coming from the same vertex, which allows us to dramatically reduce the number of candidate events that need to be studied by many orders of magnitude.

As soon as the 7 TeV data started to arrive on the grid, ATLAS physicists began to start the hunt for this early resonance using software and techniques refined from experiences with the lower energy runs. ATLAS‘ first signs of a J/Ψ signal come from the high statistics minimum bias data collected before a more sophisticated trigger was required, which also means that this first search did not rely on the muon trigger. Many weeks of work ensued, involving close collaboration between the B-Physics and Muon Combined Performance communities, improving the muon selection and understanding the events seen with respect to our expectations from simulation. As data flowed in and a signal became more apparent we began to be able to say more and more about the kinematic distributions of the events we were seeing and the effects of different selections. A flurry of activity and many things to understand meant sometimes working right through the night and getting just an hour of rest before having to present a report on the latest findings the next morning!

The first public J/Ψ performance plot shown here is the di-muon invariant mass spectrum in the vicinity of the J/Ψ mass, the result of those first studies. The plot uses 320 inverse microbarns of minimum bias data (a tiny fraction of the data that will be available by the end of the 2011 LHC run), corresponding to tens of millions of collision events. The plot shows the J/Ψ as a distinct peak above a constant background made, at this early stage, primarily from a combination of ‘fakes’ (pions/kaons in the muon spectrometer which fake a muon signal), decays of beauty and charm quarks to muons, and less commonly Drell-Yan production where a photon or off-shell Z decays to a di-muon pair leaving an irreducible background mimicking the signature of J/Ψ.

The two muons were required to have an energy above 3 GeV (the minimum energy real muons from a collision would need to reach the muon spectrometer), have a measured Inner Detector track with a certain number of silicon hits (to ensure excellent track parameter measurement) and at least one of the two muons should have been fully reconstructed in both the Inner Detector and muon spectrometer (a very stringent requirement on such low transverse momentum muons, which ensures accuracy of identification). Out of the millions of events analysed, 49 J/Ψ candidates were seen above the background (calculated with an unbinned maximum likelihood fit), with a signal to background ratio at the peak of about 5:1 and a width of 80 +/- 20 MeV, in line with expectation (as most J/Ψ in this dataset were produced in the endcaps where overall mass resolution is reduced).

As we gain more data and a better understanding we will be able to tune our selection to reduce the background under the peak. Nonetheless, that the J/Ψ is already so clearly visible with this early data is testament to the hard work by members of the collaboration in tuning the simulation and learning from the vast amounts of data for alignment and calibration of the detector in the cosmic ray, 900 GeV, 2.36 TeV data-taking phases. The J/Ψ will soon be used as a tool itself in ATLAS, helping in areas such as Inner Detector tracking performance, trigger performance, muon reconstruction efficiency and detector alignment to name but a few.

Despite the length of time since the initial discovery of the J/Ψ, its continued study, its use as a signature for other physics processes, and now its use in ATLAS as a probe of our detector performance, the process by which it is produced in high energy collisions is still not well understood. Following on the heels of these first studies are a range of measurements that will investigate the production mechanisms of J/Ψ as a probe of our understanding of QCD. Looking further ahead we can look forward to B- Physics measurements that use J/Ψ as a signature, CP violation studies and eventually rare B-decay studies that will search for new physics. The story of J/Ψ at ATLAS has only just begun.



 

 

Darren Price

Indiana University