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The LHC is on turning on again! What does that mean?

Deep underground, on the border between Switzerland and France, the Large Hadron Collider (LHC) is starting back up again after a 4 year hiatus. Today, July 5th, the LHC had its first full energy collisions since 2018.  Whenever the LHC is running is exciting enough on its own, but this new run of data taking will also feature several upgrades to the LHC itself as well as the several different experiments that make use of its collisions. The physics world will be watching to see if the data from this new run confirms any of the interesting anomalies seen in previous datasets or reveals any other unexpected discoveries. 

New and Improved

During the multi-year shutdown the LHC itself has been upgraded. Noticably the energy of the colliding beams has been increased, from 13 TeV to 13.6 TeV. Besides breaking its own record for the highest energy collisions every produced, this 5% increase to the LHC’s energy will give a boost to searches looking for very rare high energy phenomena. The rate of collisions the LHC produces is also expected to be roughly 50% higher  previous maximum achieved in previous runs. At the end of this three year run it is expected that the experiments will have collected twice as much data as the previous two runs combined. 

The experiments have also been busy upgrading their detectors to take full advantage of this new round of collisions.

The ALICE experiment had the most substantial upgrade. It features a new silicon inner tracker, an upgraded time projection chamber, a new forward muon detector, a new triggering system and an improved data processing system. These upgrades will help in its study of exotic phase of matter called the quark gluon plasma, a hot dense soup of nuclear material present in the early universe. 

 

A diagram showing the various upgrades to the ALICE detector (source)

ATLAS and CMS, the two ‘general purpose’ experiments at the LHC, had a few upgrades as well. ATLAS replaced their ‘small wheel’ detector used to measure the momentum of muons. CMS replaced the inner most part its inner tracker, and installed a new GEM detector to measure muons close to the beamline. Both experiments also upgraded their software and data collection systems (triggers) in order to be more sensitive to the signatures of potential exotic particles that may have been missed in previous runs. 

The new ATLAS ‘small wheel’ being lowered into place. (source)

The LHCb experiment, which specializes in studying the properties of the bottom quark, also had major upgrades during the shutdown. LHCb installed a new Vertex Locator closer to the beam line and upgraded their tracking and particle identification system. It also fully revamped its trigger system to run entirely on GPU’s. These upgrades should allow them to collect 5 times the amount of data over the next two runs as they did over the first two. 

Run 3 will also feature a new smaller scale experiment, FASER, which will study neutrinos produced in the LHC and search for long-lived new particles. 

What will we learn?

One of the main goals in particle physics now is direct experimental evidence of a phenomena unexplained by the Standard Model. While very successful in many respects, the Standard Model leaves several mysteries unexplained such as the nature of dark matter, the imbalance of matter over anti-matter, and the origin of neutrino’s mass. All of these are questions many hope that the LHC can help answer.

Much of the excitement for Run-3 of the LHC will be on whether the additional data can confirm some of the deviations from the Standard Model which have been seen in previous runs.

One very hot topic in particle physics right now are a series of ‘flavor anomalies‘ seen by the LHCb experiment in previous LHC runs. These anomalies are deviations from the Standard Model predictions of how often certain rare decays of the b quarks should occur. With their dataset so far, LHCb has not yet had enough data to pass the high statistical threshold required in particle physics to claim a discovery. But if these anomalies are real, Run-3 should provide enough data to claim a discovery.

A summary of the various measurements making up the ‘flavor anomalies’. The blue lines and error bars indicate the measurements and their uncertainties. The yellow line and error bars indicates the standard model predictions and their uncertainties. Source

There are also a decent number ‘excesses’, potential signals of new particles being produced in LHC collisions, that have been seen by the ATLAS and CMS collaborations. The statistical significance of these excesses are all still quite low, and many such excesses have gone away with more data. But if one or more of these excesses was confirmed in the Run-3 dataset it would be a massive discovery.

While all of these anomalies are gamble, this new dataset will also certainly be used to measure various known entities with better precision, improving our understanding of nature no matter what. Our understanding of the Higgs boson, the top quark, rare decays of the bottom quark, rare standard model processes, the dynamics of the quark gluon plasma and many other areas will no doubt improve from this additional data.

In addition to these ‘known’ anomalies and measurements, whenever an experiment starts up again there is also the possibility of something entirely unexpected showing up. Perhaps one of the upgrades performed will allow the detection of something entirely new, unseen in previous runs. Perhaps FASER will see signals of long-lived particles missed by the other experiments. Or perhaps the data from the main experiments will be analyzed in a new way, revealing evidence of a new particle which had been missed up until now.

No matter what happens, the world of particle physics is a more exciting place when the LHC is running. So lets all cheers to that!

Read More:

CERN Run-3 Press Event / Livestream Recording “Join us for the first collisions for physics at 13.6 TeV!“

Symmetry Magazine “What’s new for LHC Run 3?”

CERN Courier “New data strengthens RK flavour anomaly“

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Discovery of a New Particle or a Sick and Twisted Santa?

Good day particle nibblers,

The last time I was here I wrote about the potentially exciting “bump” which was observed by both the ATLAS and CMS experiments at the LHC.  As you’ll recall, the “bump” I’m referring to here is the excess of events seen at around 750 GeV in data containing pairs of high energy photons, what you may have heard referred to as “the diphoton excess”. The announcement was made by the experimental collaborations just before Christmas last year, ensuring that theorists around the world would not enjoy a Christmas break as instead we plunged head first into model building and speculation of what this “bump” could be. Combined with too much holiday wine, this lead to an explosion of papers in the following weeks and months (see here for a Game of Thrones themed accounting of the papers written).

The excitement was further fueled in March at the Moriond conference when both ATLAS and CMS announced results from re-analyzed data taken at 13 TeV during 2015 (and some 8 TeV data taken in 2012). They found, after optimizing their analysis for both a spin-0 and spin-2 particle, that the statistical significance for the excess increased slightly in both experiments (see Figure 1 for ATLAS results and here for a more in depth discussion).

ATLAS 13 TeV diphoton spectrum with cuts optimized for a spin-0 heavy resonance (left) and for a spin-2 resonance (right).
Figure 1: ATLAS 13 TeV diphoton spectrum with cuts optimized for a spin-0 heavy resonance (left) and for a spin-2 resonance (right).

In the end both experiments reported a (local) statistical significance (see Footnote 1) of more than 3 standard deviations (or 3σ for short). Normally 3σ’s don’t cause such a frenzy, but the fact that two separate experiments observed this made the probability that it was just a statistical fluctuation much lower (something on the order of 1 in a few thousand chance). If this excess really is just a statistical fluctuation it is a pretty nasty one indeed and may suggest a sick and twisted Santa has been messing with the fragile emotional state of particle theorists ever since Christmas (see Figure 2).

Figure 2: Last known photo of the sick and twisted Santa suspected of perpetuating the false hope of a 750 GeV diphoton excess.
Figure 2: Last known photo of the sick and twisted Santa suspected of perpetuating the false hope of a 750 GeV diphoton excess.

Since the update at the Moriand conference in March (based primarily on 2015 data), particle physicists have been eagerly awaiting the first results based on data taken at the LHC in 2016. With the rate at which the LHC has been accumulating data this year, already there is more than enough collected by ATLAS and CMS to definitively pin down whether the excess is real or if we are indeed dealing with a demented Santa. The first official results will be presented later this summer at ICHEP, but we particle physicists are impatient so the rumor chasing is already in full swing.

Sadly, the latest rumors circulating in the twitter/blogosphere (see also here, here, and here for further rumor mongering) seem to indicate that the excess has disappeared with the new data collected in 2016. While we have to wait for the experimental collaborations to make an official public announcement before shedding tears, judging by the sudden slow down of ‘diphoton excess’ papers appearing on the arXiv, it seems much of the theory community is already accepting this pessimistic scenario.

If the diphoton excess is indeed dead it will be a sad day for the particle physics community. The possibilities for what it could have been were vast and mind-boggling. Even more exciting however was the fact that if the diphoton excess were real and associated with a new resonance, the discovery of additional new particles would almost certainly have been just around the corner, thus setting off a new era of experimental particle physics. While a dead diphoton excess would indeed be sad, I urge you young nibblers to not be discouraged. One thing this whole ordeal has taught us is that the LHC is an amazing machine and working fantastically. Second, there are still many interesting theoretical ideas out there to be explored, some of which came to light in attempting to explain the excess. And remember it only takes one discovery to set off a revolution of physics beyond the Standard Model so don’t give up hope yet!

I also urge you to not pay much attention to the inevitable negative backlash that will occur (and already beginning in the blogosphere) both within the particle physics community and the popular media. There was a legitimate excess in the 2015 diphoton data and that got theorists excited (reasonably so IMO), including yours truly. If the excitement of the excess brought in a few more particle nibblers then even better still! So while we mourn the (potential) loss of this excess let us not give up just yet on the amazing machine that is the LHC possibly discovering new physics. And then we can tell that sick and twisted Santa to go back to the north pole for good!

OK nibblers, thats all the thoughts I wanted to share on the social phenomenon that is (was?) the diphoton excess. While we wait for official announcements, let us in the meantime hope the rumors are wrong and that Santa really is warm and fuzzy and cares about us like they told us as children.

Footnote 1: The global significance was between 1 and 2σ, but I wont get into these details here.

Disclaimer 1: I promise next post I will get back to discussing actual physics instead of just social commentary =).

Disclaimer 2: Since I am way too low on the physics totem pole to have any official information, please take anything written here about rumors of the diphoton excess with a grain of salt. Stay tuned here for more credible sources.

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How to Turn On a Supercollider

Figure 1: CERN Control Centre excitement on June 5. Image from home.web.cern.ch.

After two years of slumber, the world’s biggest particle accelerator has come back to life. This marks the official beginning of Run 2 of the LHC, which will collide protons at nearly twice the energies achieve in Run 1. Results from this data were already presented at the recently concluded European Physical Society (EPS) Conference on High Energy Physics. And after achieving fame in 2012 through observation of the Higgs boson, it’s no surprise that the scientific community is waiting with bated breath to see what the LHC will do next.

The first official 13 TeV stable beam physics data arrived on June 5th. One of the first events recorded by the CMS detector is shown in Figure 2. But as it turns out, you can’t just walk up to the LHC, plug it back into the wall, and press the on switch (crazy, I know.) It takes an immense amount of work, planning, and coordination to even get the thing running.

Event display from one of the first Run 2 collisions.
Figure 2: Event display from one of the first Run 2 collisions.

The machine testing begins with the magnets. Since the LHC dipole magnets are superconducting, they need to be cooled to about 1.9K in order to function, which can take weeks. Each dipole circuit then must be tested to ensure functionality of the quench protection circuit, which will dump the beam in the event of sudden superconductivity loss. This process occurred between July and December of 2014.

Once the magnets are set, it’s time to start actually making beam. Immediately before entering the LHC, protons are circling around the Super Proton Synchroton, which acts as a pre-accelerator. Getting beam from the SPS to the LHC requires synchronization, a functional injection system, beam dump procedure, and a whole lot of other processes that are re-awoken and carefully tested. By April, beam commissioning was officially underway, meaning that protons were injected and circulating, and a mere 8 weeks later there were successful collisions at the safe energy of 6.5 TeV. As of right now, the CMS detector is reporting 84 pb-1 total integrated luminosity; a day-by-day breakdown can be seen in Figure 3.

CMS total integrated luminosity per day, from Ref 5.
Figure 3: CMS total integrated luminosity per day, from Ref 4.

But just having collisions does not mean that the LHC is up and fully functional. Sometimes things go wrong right when you least expect it. For example, the CMS magnet has been off to a bit of a rough start—there was an issue with its cooling system that kept the magnetic field off, meaning that charged particles would not bend. The LHC has also been taking the occasional week off for “scrubbing”, in which lots of protons are circulated to burn off electron clouds in the beam pipes.

This is all leading up to the next technical stop, when the CERN engineers get to go fix things that have broken and improve things that don’t work perfectly. So it’s a slow process, sure. But all the caution and extra steps and procedures are what make the LHC a one-of-a-kind experiment that has big sights set for the rest of Run 2. More posts to follow when more physics results arrive!

 

References:

  1. LHC Commissioning site
  2. Cyrogenics & Magnets at the LHC
  3. CERN collisions announcement
  4. CMS Public Luminosity results
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Welcome to ParticleBites

Welcome to ParticleBites! This is a new blog reviewing recent papers in theoretical and experimental particle physics. Our bloggers are graduate students and postdocs working in high energy physics.

ParticleBites grew out of the Communicating Science 2013 workshop, hosted by our friends at AstroBites. Another recent “Bites” blog growing out of that workshop is OceanBites.

As with the other “Science Bites” sites, our goal is to condense current research papers into one-page posts that are accessible to undergraduates and the science-minded general public.