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Brian Dorney | USLHC | USA

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New State Discovered by the ATLAS Collaboration!

Over the Christmas Holiday the ATLAS Collaboration submitted an article to Physical Review Letters, a peer-review journal.  The article titled, “Observation of a New χb State in Radiative Transitions to Υ(1S) and Υ(2S) at ATLAS,” can be found on arXiv.

The processes under study in this paper are the following:

χb(nP)→ Υ(1S) γ → μ+μ γ

χb(nP)→ Υ(2S) γ → μ+μ γ

Where n = 1,2 or 3.

The focus of this paper was on finding a meson known as the χb(nP). Mesons are a class of particles formed by a bound state of a quark and an anti-quark; the χb(nP) happens to be a bound state of a b-quark (termed b, for beauty) and an anti-b-quark (termed b).  The (nP) part means that quark/anti-quark are bound together in a P-orbital of energy level n. As a consequence of the relativistic energy-momentum relation, different energy levels correspond to bound states with different rest masses.  So basically for each value of n you have a unique particle! The n = 3 particle has only ever been theoretically predicted, so in this paper a new particle was discovered!

Now the χb(nP) particles are very short lived and usually can’t be observed directly.  So to find them the ATLAS Collaboration has to infer their presence by summing up the energy of their decay products.  In the above two equations, the χb(nP) is decaying into another meson known as the Upsilon, Υ(kS), and in the process a photon is also emitted (hence the “radiative transition” in the title). Now the Upsilon is also made up by a bb pair.  The (kS) part means that the quark/anti-quark pair are bound together in an S-Orbital of energy level k = 1 or 2.

The Υ(kS) is also a very short lived particle (mean lifetime of approximately 10-20 seconds).  To identify the Upsilons needed for this study the ATLAS Collaboration had to look for two oppositely charged muons, called a di-muon or μ+μ pair, having a summed rest mass (termed “invariant mass”) near the published mass values for the Υ(kS).  A plot of the di-muon invariant mass can be see at right [1].  From left to right the peaks in the graph represents di-muons originating from decays of the Υ(1S),  Υ(2S), and the Υ(3S), respectively. The muons in the shaded regions from the Υ(1S) and Υ(2S) decays were used in the search for the χb(nP) particles.

Then to find the χb(nP) particles, ATLAS researchers looked for a point in the detector from which a di-muon and a photon originated from.  This point is known as a vertex.

Charged particles, such as muons, leave tracks in the ATLAS Detector’s inner tracking detector (which consists of a silicon pixel detector, a silicon microstrip detector, and a transition radiation tracker).  The inner tracking detector is like a giant CCD camera, and is based on the same technology.

However, neutral particles, like photons, do not leave a track in the tracker.  Photons are detected by energy depositions in the ATLAS Detector’s electro-magnetic calorimeter.  To see if an energy deposition marked as a photon comes from this di-muon vertex, you take every di-muon vertex, and you try and match it with one of your photon energy depositions.  If the match is “good enough” you call this di-muon plus photon a χb(nP) candidate.

Before we show you these χb(nP) candidates I want to talk about the di-muon invariant mass plot one more time.  Notice how the peaks in this plot have some width to them.  This has to do with the resolution of the ATLAS Detector.  The narrower the peaks are the better the resolution.  However, there is a limit to how thin these peaks can be.  For example, the Υ(1S) has its own natural width of  about 54 keV or 0.000054 GeV.  So suppose you had the perfect particle detector and made the measurement shown in the di-muon invariant mass plot.  Even using your perfect detector your Υ(1S) peak would still have a width of exactly 0.000054 GeV.  As you can see the peaks are no where near this, and as I said this is due to the finite resolution of the ATLAS Detector.  To account for this resolution, researchers at ATLAS worked with a variable defined as:

Δm = m(μ+μγ) – m(μ+μ)

This takes the invariant mass (e.g. rest mass) of the di-muon and the photon, the χb(nP) candidates, and subtracts the di-muon mass.  Then the ATLAS researchers add the world average values of the Upsilon masses back to Δm.

m k = Δm + mΥ(kS) = m(μ+μγ) – m(μ+μ) + mΥ(kS)

Note, for your perfect detector measuring the Υ(1S) this quantity: m(μ+μ) – mΥ(1S) is approximately zero, but has a maximum value of 0.000027 GeV, e.g. this would be half the width of the Υ(1S)!  This is how the use of Δm and the world average value of the Upsilon minimizes the affect of the ATLAS detector’s resolution.

A little side note about world average values in particle physics.  They are a single value for some experimental observation, produced by the Particle Data Group [2], and take into account every experimental result that has ever been published.

A plot of mk is shown at left [1] for the χb(nP) candidates in which the photon was measured directly (as opposed to an indirect measurement from the photon splitting into an e+e pair).  The first two peaks are the previously observed χb(1P) and χb(2P) particles.  The third peak is the first ever observation of the χb(3P)!

In case you all remember the golden rule of particle physics, the ATLAS Collaboration reports that:

“the significance of the χb(3P) signal is found to be in excess of six standard deviations in each of the unconverted and converted photon selections independently” [1]

Or put plainly, the probability that this third peak could have happened by coincidence is about 2 in one billion.  You literally have a higher probability of winning the lottery at 1 in approximately 16 million [3] or being struck by lightning this year at 1 in 775,000 [4].


So how about that? Not everyday a new particle is discovered!



Until Next Time,




[1] The ATLAS Collaboration, “Observation of a New State in Radiative Transitions to and at ATLAS,” arXiv:1112.5154v1 [hep-ex].

[2] K. Nakamura et al. (Particle Data Group), J. Phys. G 37, 075021 (2010). Note this may be found online here: http://pdg.lbl.gov/2011/tables/contents_tables.html

[3] wikiHow, “How to Calculate Lotto Odds,” http://www.wikihow.com/Calculate-Lotto-Odds, Jan 10th 2012.

[4] NOAA, “Medical Aspects of Lightning,” http://www.lightningsafety.noaa.gov/medical.htm, Jan 10th 2012.