Bill Murray gave a nice summary of the recent results of the ATLAS collaboration at the ICNFP meeting this morning. I excerpted some graphs from his presentation to show that the Large Hadron Collider (LHC) is one of his two major microscopes that his amazing ATLAS detector lens As you can see, we are offering it recently.
LHC is getting data. what again?
Since Run 3 of the LHC has recently begun, it may be necessary to first and foremost describe the business of particle collisions at CERN. When the LHC was first launched 12 years ago, the world’s media were busy explaining the scale of the apparatus, the challenges, and even the dangers of reaching hitherto unattainable impact energies. In 2012, the discovery of the Higgs boson brought the experiment back into the spotlight. Aren’t they already done?
In fact it is not – the concept of the experiment as very well defined in time is for tabletop devices. When we talk about experiments on the scale of the LHC collider, we have to convey the concept that once the device is built, we start collecting data and continue until it dies. must be collected and the experiment will continue to run forever.
In my public lectures, I often use the analogy of deep underground caverns that speleologists have access to for the first time. It’s dark and her flashlight can only see her within her crevice some distance, and taking long-exposure photos allows her to see deeper. If she can expose her photos for even longer, she’ll be able to discern more details in the cave, farther afield. This means that very little light shining on a distant wall leaves an imprint on the photo. On the same line of thinking, accelerator experiments should run for a long time. The longer you collect data, the deeper you can look. It gives you the opportunity to see small details that would otherwise be lost.
The details are important to better test the current theory of the standard model of particle physics. We know that our theory, though surprisingly accurate, is imperfect. There must be a physical phenomenon that breaks it. We have not seen such a phenomenon in previous experiments, so we need to increase the sensitivity to explore the abyss.
So while many proton-proton collisions were collected in LHC Run 1 (2010-2012) and Run 2 (2015-2018), now Run 3 collected twice as much data as before and reduced collision energies. I would like to increase it slightly. Collision energy can be likened to the power of the light that illuminates it in a cave. Illuminated by a more powerful lamp, it doesn’t take long to see the faintest details. It is difficult to increase the collision energy beyond the already impressive value of 13 TeV. Run 3 pushes this to 13.6 TeV, a modest increase of 5% and equivalent to up to a doubling of the probability of very rare events with very large particle generation. increase.
ATLAS Result Potpourri
So, without further ado, here are some of the most interesting results from Bill Murray’s talk. This is a choice being made, and of course it is not representative of the vast amount of great science that experimentation has produced. It’s just what I found interesting from a biased and narrow perspective.
The first result is just a very clean bump in the mass distribution – the Higgs boson. In this case, if accompanied by the signature of the forward jet, it is seen to decay into photon pairs. This is because Higgs generation is called “vector boson fusion”. In other words, the quarks colliding within the proton both emitted a W boson or a Z boson, and their (virtual) particles annihilated into the Higgs boson. What we see in the experiment is the decay of Higgs, along with the signals of the two quarks that first emitted vector bosons.
I am showing the graph because it shows very distinct peaks. This proves that this channel is the one with the highest signal-to-noise ratio. What can be seen in the distribution is a Gaussian line at 125 GeV (the Higgs boson mass) above a smoothly descending background at all mass values. The reason for this is that we are asking the following questions for data displaying two energy photons. That question is answered by taking the energies and directions of the photons and calculating their ‘constant mass’.
In the graph, the mass distribution is in the upper right inset. The peak data are used by ATLAS to extract information about some detailed properties of the Higgs boson through related observables (“OO”, best-fit observables). Shown in the main graph. The blue contribution is from the actual Higgs decay and the red is from the background in the 118–132 GeV mass region.
The second result I want to flash below is the search for the generation of top quark pairs associated with energetic photons. What interest is there in searching for this process? In fact, we know that the LHC produces pairs of top quarks, and that the top quarks carry an electric charge, so we know that these top quarks It goes without saying that you must expect some to emit photons from time to time. – This is what *all* charged particles do.
The interesting thing lies in the fact that when we study very rare processes, we may encounter other physical reactions that were previously invisible and could not be predicted by theory. Furthermore, the presence of top quarks in the final state may identify new natural forces that preferentially bind to these particles. can be a way to become sensitive to these possible new processes.
Below is a picture of Murray’s slide on search. At the top left you can see Feynman’s diagram showing how photons are emitted from the quark leg of Top before Top collapses into the observed final state body. The right side shows the distribution of neural network outputs when the network is trained to separate the signal (orange) from all competing backgrounds as much as possible. ATLAS can then measure 580 femtoburn cross-sections of the process. In other words, such reactions occur about 6 times in every trillion collisions. This is a larger percentage than predicted (406 femtoburns), but the uncertainty puts this impact at the compatibility level of a few percent. In other words, there is room for new physical processes in the data, but no real evidence. As with particle physics, we need a tougher level of evidence to believe we have discovered something new.
The final result shown below is for the search for rare Higgs decays. This is a topic that interests me. This is because we have previously used CDF data to search for the generation of related vector bosons and upsilon mesons. The Higgs boson had not yet been discovered, and there were some new physics ideas that the final state combination of particles could be the result of the collapse of the supersymmetric Higgs boson. In my searches, I didn’t observe the process, just set a cap.
Here, ATLAS searches for related processes (photons and J/Psi mesons or photons and Upsilon mesons). Now that we know the Higgs mass, we can go directly to the mass peak of the total mass of the observed final-state particles. This is shown in the bottom left graph, where the color indicates the expected peak if signal is present. However, no traffic lights are displayed, so ATLAS can only set an upper bound for the processes found (see table below left).
To access all the results presented at the presentation, please visit the conference website.