Putting particle physics research to work

In the whole gamut of comments regarding the Higgs boson, there is a depressingly large number decrying the efforts of the ATLAS and CMS collaborations. Why? Because a lot of people think the Large Hadron Collider (LHC) is a yawning waste of time and money, an investment that serves mankind no practical purpose.

Well, here and here are some cases in point that demonstrate the practical good that the LHC has made possible in the material sciences. Another big area of application is in medical diagnostics: making the point is one article about hunting for the origin of Alzheimer's, and another about the very similar technology used in particle accelerators and medical imaging devices, meteorology, VLSI, large-scale networking, cryogenics, and X-ray spectroscopy.

Moving on to more germane applications: arXiv has reams of papers that discuss the deployment of

• Advanced pixel-planar sensors (today used commonly in retina displays, DSLRs, high-speed motion sensors, and video camera tubes) for the ATLAS detector for during the HL-LHC years,


• Geometric scaling techniques applicable in construction and modeling,


• Magnetic spectrometers that are used by NASA to investigate the nature of dark matter,


• Resistive-plate chambers that are used in rapid data-tracking and analysis,


• High-luminosity electron-proton colliders that further our understanding of the proton's structure and open gateways to advanced materials engineering,


• Gamma-ray tracking/measurement devices whose results could be used to find out our real place in the universe, whether there are other inhabitable planets or new sources of energy/revenue,

... amongst others.



The LHC, above all else, is the brainchild of the European Centre for Nuclear Research, popularly known as CERN. These guys invented the notion of the internet, developed the first touch-screen devices, and pioneered the earliest high-energy medical imaging techniques.

With experiments like those being conducted at the LHC, it's easy to forget every other development in such laboratories apart from the discovery of much-celebrated particles. All the applications I've linked to in this post were conceived by scientists working with the LHC, if only to argue that everyone, the man whose tax money pays for these giant labs to the man who uses the money to work in the labs, is mindful of practical concerns.

Gunning for the goddamned: ATLAS results explained

Here are some of the photos from the CERN webcast yesterday (July 4, Wednesday), with an adjoining explanation of the data presented in each one and what it signifies.



This first image shows the data accumulated post-analysis of the diphoton decay mode of the Higgs boson. In simpler terms, physicists first put together all the data they had that resulted from previously known processes. This constituted what's called the background. Then, they looked for signs of any particle that seemed to decay into two energetic photons, or gamma rays, in a specific energy window; in this case, 100-160 GeV.

Finally, knowing how the number of events would vary in a scenario without the Higgs boson, a curve was plotted that fit the data perfectly: the number of events at each energy level v. the energy level at which it was tracked. This way, a bump in the curve during measurement would mean there was a particle previously unaccounted for that was causing an excess of diphoton decay events at a particular energy.



This is the plot of the mass of the particle being looked for (x-axis) versus the confidence level with which it has (or has not, depending n how you look at it) been excluded as an event to focus on. The dotted horizontal line, corresponding to 1μ, marks off a 95% exclusion limit: any events registered above the line can be claimed as having been observed with "more than 95% confidence" (colloquial usage).

Toward the top-right corner of the image are some numbers. 7 TeV and 8 TeV are the values of the total energy going into each collision before and after March, 2012, respectively. The beam energy was driven up to increase the incidence of decay events corresponding to Higgs-boson-like particles, which, given the extremely high energy at which they exist, are viciously short-lived. In experiments that were run between March and July, physicists at CERN reported an increase of almost 25-30% of such events.

The two other numbers indicate the particle accelerator's integrated luminosity. In particle physics, luminosity is measured as the number of particles that can pass detected through a unit of area per second. The integrated luminosity is the same value but measured over a period of time. In the case of the LHC, after the collision energy was vamped up, the luminosity, too, had to be increased: from about 4.7 fb-1 to 5.8 fb-1. You'll want to Wiki the unit of area called barn. Some lighthearted physics talk there.



In this plot, the y-axis on the left shows the chances of error, and the corresponding statistical significance on the right. When the chances of an error stand at 1, the results are not statistically significant at all because every observation is an error! But wait a minute, does that make sense? How can all results be errors? Well, when looking for one particular type of event, any event that is not this event is an error.

Thus, as we move toward the ~125 GeV mark, the number of statistically significant results shoot up drastically. Looking closer, we see two results registered just beyond the 5-sigma mark, where the chances of error are 1 in 3.5 million. This means that if the physicists created just those conditions that resulted in this >5σ (five-sigma) observation 3.5 million times, only once will a random fluctuation play impostor.

Also, notice how the differences between each level of statistical significance increases with increasing significance? For chances of errors: 5σ - 4σ > 4σ - 3σ > ... > 1σ - 0σ. This means that the closer physicists get to a discovery, the exponentially more precise they must be!



OK, this is a graph showing the mass-distribution for the four-lepton decay mode, referred to as a channel by those working on the ATLAS and CMS collaborations (because there are separate channels of data-taking for each decay-mode). The plotting parameters are the same as in the first plot in this post except for the scale of the x-axis, which goes all the way from 0 to 250 GeV. Now, between 120 GeV and 130 GeV, there is an excess of events (light blue). Physicists know it is an excess and not at par with expectations because theoretical calculations made after discounting a Higgs-boson-like decay event show that, in that 10 GeV, only around 5.3 events are to be expected, as opposed to the 13 that turned up.

Good news from the Tevatron

Level: Jedi Master


There's some glad news that's come in today from the Tevatron at Chicago, IL. The analysis of the data it collected last year before shutting down in September shows an excess of events in the mass range of 115-135 GeV/c², with a precision of 2.8 sigma (97.4% CL). This result coincides with the ATLAS and CMS results declared on December 13 last year, providing the broader scientific community and the world the first glimpse of the Higgs boson.

The results were announced at the ongoing Moriond Conference - spanning seven days from March 3 to March 10 - in La Thuile, Italy, which opened with an address by Prof. Francois Englert, one of the contributors who shaped the Higgs mechanism. Ever since the ATLAS/CMS results came out, one important thing as far as the hunt for the Higgs boson is concerned that scientists have learned is the different ways in which the elusive particle can decay. They have used this information to add more readout channels to existing ones at the ATLAS/CMS (two W-boson channels for the former) detectors as well as at the CDF and D-Zero detectors at the Tevatron. Each of these channels will track and monitor one decay channel, or one mode of decay.

Because the Tevatron has shut down, the data it's yielded is more or less final; the only improvisations that can arise will be from refinement of the data. At the same time, apart from the addition of channels, the LHC will also run at a beam intensity of 4 TeV/beam instead of the 3.5 TeV/beam it's been running with in 2010 and 2011. This can be attributed to the encouraging results that have been returned by the experiments hunting for the Higgs boson. The bunch-spacing will remain at 50 nanoseconds.

χb (3P) discovered at the ATLAS

The ATLAS detector at the CERN confirmed the existence of another particle predicted by the Standard Model, this one much less prolific than its apotheosized cousin, the Higgs boson, on December 22. It's the Upsilon meson, which is a coupled state of the bottom quark and the bottom antiquark. Such a coupling makes the meson a quarkonium, and so it also assumes the name of bottomonium.

Such a flavourless meson, also called a χ (chi) particle, exists at various excited states, or excitation modes, just like a hydrogen atom, and it decays to each state by the radiation of a photon (γ). The Upsilon meson was first observed in 1977 in an experiment headed by Leon Lederman at the Fermilab, Chicago, with a mass of 9.46 GeV/c². The ATLAS discovery corresponds to a higher excitation state, χ_b (3P), wherein the particle's mass is 10.539 ± 0.004 GeV/c².

[caption id="attachment_21088" align="aligncenter" width="600" caption="The spike in activity corresponds to a detection. The Upsilon meson was observed in three independent channels."][/caption]

Like the electronic orbitals in an atom, each decay state is designated as belonging to an S, P or D energy-state, and the discovery is that of a χ_b (3P): a flavourless meson at the 3P excitation mode. In the second diagram, the particle shows up as the right-most peak on the purple (lowermost) curve, corresponding to ~10.5 GeV on the x-axis. It hasn't shown up in earlier experiments because only the Large Hadron Collider has been able to supply the high rate of collisions required to record the particle's existence before it decays in 1.21×10⁻²⁰ seconds (at 9.46 GeV/c²).

The corresponding pre-print paper submitted by the ATLAS Collaboration is available here.

With the quarry cornered, the real hunt for the Higgs boson begins!

The CERN has announced that the Higgs boson, a.k.a. the God particle, has been glimpsed with a mass in the vicinity of 126 GeV and with a general restriction between 115.5 GeV and 131 GeV (95% confidence level, or CL) with a 3.6-sigma local precision and a 2.3-sigma global precision at a luminosity of 4.9 inverse-femtobarn (fb^-1) by the ATLAS experiment.

In other words, there does exist a good chance of a quarry, and the quarry has been cornered.

[caption id="attachment_20965" align="aligncenter" width="530" caption="From the presentation by ATLAS's Fabiola Gianotti (unfortunate that she had to use Comic Sans)"][/caption]

This means physicists know that the mass of the Higgs boson is not in the high-mass region (above 200 GeV) but in the low-mass region (<200 GeV). Consequently, the channels that are tracking events in this region will be watched specifically and more carefully in the checks that will be run in 2012. The checks are necessary because the events will now have to be studied with greater luminosity (measured by the inverse-femtobarn) and because the standard deviation of the errors will have to be brought down from 0.54011 GeV/c² (2.3-sigma) precision to 0.00023 GeV/c² (5-sigma).

[caption id="attachment_20968" align="aligncenter" width="530" caption="The yellow box shows what will change during the 2012 checks"][/caption]

In order to detect the Higgs boson in the vicinity of 125 GeV to 126 GeV at the 5-sigma tolerance level, the luminosity will have to be increased from what is now 4.9 fb^-1 to 20 fb^-1. Also, the low-mass observation channels will each have to have a precision high enough to keep deviations below 0.400138 GeV/c². The Compact Muon Solenoid (CMS) experiment has also done a good job of excluding energy levels in between 127 GeV and 600 GeV with a 95% CL, and from between 117 GeV to 543 GeV with a 99% CL.

[caption id="attachment_20970" align="aligncenter" width="530" caption="The exclusion limits arrived at by the CMS experiment"][/caption]

[caption id="attachment_20969" align="aligncenter" width="530" caption="The center chart shows the broad excess just above the mean-mark, a flat plateau that points at an event "somewhere there", and the chart on the right, corresponding to a high-sensitivity detection channel, shows a sharp peak (toward the left) - a finer image of the plateau."][/caption]

The final results and papers will be published by the end of January, together with an update on the installation of new channels and refined analyses. The high-points of the entire experiment were the 95% CL exclusion of the 127-600 GeV range and the observation of a small excess of events in the 115-127 GeV mass range with a (high) deviation of 0.62009 GeV/c². Because of the low precision, the observation could also have been a result of background fluctuations in the experiments, necessitating a verification in 2012 - which will yield a definite answer.