This video was pretty cool until the end. “Harness the power of the Sun”? This torch emits 2,300 lumens from a battery that lasts 30 minutes. The Sun? About 98,000 lumens, equivalent to 100 billion megatonnes of TNT blowing up per second for 9 billion years. This is also a chance to marvel at the hypernova, a.k.a. the superluminous supernova: a few billion billion megatonnes of TNT per second, a.k.a. 10 million Suns blowing up per second. It’s the hypernova that actually harnesses the power of the Sun.
Tag: luminosity
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Restarting the LHC: A timeline
CERN has announced the restart schedule of its flagship science “project”, the Large Hadron Collider, that will see the giant machine return online in early 2015. I’d written about the upgrades that could be expected shortly before it shut down in 2012. They range from new pixel sensors and safety systems to facilities that will double the collider’s energy and the detectors’ eyes for tracking collisions. Here’s a little timeline I made with Timeline.js, check it out.
(It’s at times like this that I really wish WP.com would let bloggers embed iframes in posts.)
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The hunt for supersymmetry: Reviewing the first run – 2
I’d linked to a preprint paper [PDF] on arXiv a couple days ago that had summarized the search for Supersymmetry (Susy) from the first run of the Large Hadron Collider (LHC). I’d written to one of the paper’s authors, Pascal Pralavorio at CERN, seeking some insights into his summary, but unfortunately he couldn’t reply by the time I’d published the post. He replied this morning and I’ve summed them up.
Pascal says physicists trained their detectors for “the simplest extension of the Standard Model” using supersymmetric principles called the Minimal Supersymmetric Standard Model (MSSM), formulated in the early 1980s. This meant they were looking for a total of 35 particles. In the first run, the LHC operated at two different energies: first at 7 TeV (at a luminosity of 5 fb-1), then at 8 TeV (at 20 fb-1; explainer here). The data was garnered from both the ATLAS and CMS detectors.
In all, they found nothing. As a result, as Pascal says, “When you find nothing, you don’t know if you are close or far from it!”
His paper has an interesting chart that summarized the results for the search for Susy from Run 1. It is actually a superimposition of two charts. One shows the different Standard Model processes (particle productions, particle decays, etc.) at different energies (200-1,600 GeV). The second shows the Susy processes that are thought to occur at these energies.
Cross sections of several SUSY production channels, superimposed with Standard Model process at s = 8 TeV. The right-handed axis indicates the number of events for 20/fb. The cross-section of the chart is the probability of an event-type to appear during a proton-proton collision. What you can see from this plot is the ratio of probabilities. For example, stop-stop* (the top quark’s Susy partner particle and anti-particle, respectively) production with a mass of 400 GeV is 1010 (10 billion) less probable than inclusive di-jet events (a Standard Model process). “In other words,” Pascal says, it is “very hard to find” a Susy process while Standard Model processes are on, but it is “possible for highly trained particle physics” to get there.
Of course, none of this means physicists aren’t open to the possibility of there being a theory (and corresponding particles out there) that even Susy mightn’t be able to explain. The most popular among such theories is “the presence of a “possible extra special dimension” on top of the three that we already know. “We will of course continue to look for it and for supersymmetry in the second run.”
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LHC to re-awaken in 2015 with doubled energy, luminosity
This article, as written by me, appeared in The Hindu on January 10, 2012.
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After a successful three-year run that saw the discovery of a Higgs-boson-like particle in early 2012, the Large Hadron Collider (LHC) at CERN, near Geneva, Switzerland, will shut down for 18 months for maintenance and upgrades.
This is the first of three long shutdowns, scheduled for 2013, 2017, and 2022. Physicists and engineers will use these breaks to ramp up one of the most sophisticated experiments in history even further.
According to Mirko Pojer, Engineer In-charge, LHC-operations, most of these changes were planned in 2011. They will largely concern fixing known glitches on the ATLAS and CMS particle-detectors. The collider will receive upgrades to increase its collision energy and frequency.
Presently, the LHC smashes two beams, each composed of precisely spaced bunches of protons, at 3.5-4 tera-electron-volts (TeV) per beam.
By 2015, the beam energy will be pushed up to 6.5-7 TeV per beam. Moreover, the bunches which were smashed at intervals of 50 nanoseconds will do so at 25 nanoseconds.
After upgrades, “in terms of performance, the LHC will deliver twice the luminosity,” Dr. Pojer noted in an email to this Correspondent, with reference to the integrated luminosity. Precisely, it is the number of collisions that the LHC can deliver per unit area which the detectors can track.
The instantaneous luminosity, which is the luminosity per second, will be increased to 1×1034 per centimetre-squared per second, ten-times greater than before, and well on its way to peaking at 7.73×1034 per centimetre-squared per second by 2022.
As Steve Myers, CERN’s Director for Accelerators and Technology, announced in December 2012, “More intense beams mean more collisions and a better chance of observing rare phenomena.” One such phenomenon is the appearance of a Higgs-boson-like particle.
The CMS experiment, one of the detectors on the LHC-ring, will receive some new pixel sensors, a technology responsible for tracking the paths of colliding particles. To make use of the impending new luminosity-regime, an extra layer of these advanced sensors will be inserted around a smaller beam pipe.
If results from it are successful, CMS will receive the full unit in late-2016.
In the ATLAS experiment, unlike with CMS which was built with greater luminosities in mind, pixel sensors are foreseen to wear out within one year after upgrades. As an intermediate solution, a new layer of sensors called the B-layer will be inserted within the detector for until 2018.
Because of the risk of radiation damage due to more numerous collisions, specific neutron shields will be fit, according to Phil Allport, ATLAS Upgrade Coordinator.
Both ATLAS and CMS will also receive evaporative cooling systems and new superconducting cables to accommodate the higher performance that will be expected of them in 2015. The other experiments, LHCb and ALICE, will also undergo inspections and upgrades to cope with higher luminosity.
An improved failsafe system will be installed and the existing one upgraded to prevent accidents such as the one in 2008.
Then, an electrical failure damaged 29 magnets and leaked six tonnes of liquid helium into the tunnel, precipitating an eight-month shutdown.
Generally, as Martin Gastal, CMS Experimental Area Manager, explained via email, “All sub-systems will take the opportunity of this shutdown to replace failing parts and increase performance when possible.”
All these changes have been optimised to fulfil the LHC’s future agenda. This includes studying the properties of the newly discovered particle, and looking for signs of new theories of physics like supersymmetry and higher dimensions.
(Special thanks to Achintya Rao, CMS Experiment.)
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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.
