I LHC Restarts in 2022 - Live Updates

[mfb]

TL;DR Summary

Beams are again circulating in the LHC, in preparation for collisions

Large Hadron Collider restarts
Live status

We last had collisions in December 2018, followed by the Long Shutdown 2 for accelerator and experiment upgrades.
At the moment it is a very low intensity beam to make sure everything is still working and behaving as expected. Over the next month the beam currents will increase and we'll see beams being accelerated (currently they stay at the energy of the preaccelerators). We will likely get initial collisions in mid/late May and first high energy collisions (at 13.6 TeV, up from 13 TeV before the shutdown) in June. Afterwards the focus will be on an increase in the collision rate.

The slightly higher energy means slightly higher production cross sections for almost everything, especially very heavy particles. The luminosity will increase as well, further increasing the rates of everything.

LHCb moved to a triggerless readout: Every event is read out fully, which means trigger algorithms are not limited to specific subdetectors for their decision. That gives a better separation between interesting events and everything else. The trigger still has to discard most events - 32 TB/s is too much to store permanently - but it can do a better selection now.

 

[Algr]

Previous results were disappointing. I hope it will eat the Earth this time.

 

[vanhees71]

Disappointing? What do you mean?

 

[Algr]

It could have stopped all the fighting and brought the whole world together.

 

[Vanadium 50]

It could have stopped all the fighting and brought the whole world together.

I see. We made the mistake of taking you seriously. Don't worry. Won't happen again.

 

[Lord Crc]

Besides finding the Higgs, what's the most prominent results from Atlas and CMS till now?

LHCb folks seem to be good at spreading their news around so heard plenty about their work.

 

[Vanadium 50]

Jet quenching, pp ridge, best top quark mass, observation of Bs decay to dimuons,

 

[pinball1970]

Summary:: Beams are again circulating in the LHC, in preparation for collisions

Large Hadron Collider restarts
Live status

We last had collisions in December 2018, followed by the Long Shutdown 2 for accelerator and experiment upgrades.
At the moment it is a very low intensity beam to make sure everything is still working and behaving as expected. Over the next month the beam currents will increase and we'll see beams being accelerated (currently they stay at the energy of the preaccelerators). We will likely get initial collisions in mid/late May and first high energy collisions (at 13.6 TeV, up from 13 TeV before the shutdown) in June. Afterwards the focus will be on an increase in the collision rate.

The slightly higher energy means slightly higher production cross sections for almost everything, especially very heavy particles. The luminosity will increase as well, further increasing the rates of everything.

LHCb moved to a triggerless readout: Every event is read out fully, which means trigger algorithms are not limited to specific subdetectors for their decision. That gives a better separation between interesting events and everything else. The trigger still has to discard most events - 32 TB/s is too much to store permanently - but it can do a better selection now.

Thanks. In terms of goals? I remember you guys discussing this I detail last year.
https://indico.cern.ch/event/976688/

 

[dukwon]

New beam-energy world record this morning with the first ramp of pilot beams to 6.8 TeV

 

[mfb]

Thanks. In terms of goals? I remember you guys discussing this I detail last year.
https://indico.cern.ch/event/976688/

These are precision measurements that take time. It will take a while until we get Run 3 results. We'll likely see a few more Run 2 results.

LHCb has collected an integrated luminosity of 3/fb at 7-8 TeV and 6/fb at 13 TeV, it should now collect ~6fb/year with a higher trigger efficiency and slightly higher production cross section, so each year now is better than all of Run 2. Run 3 results will have much smaller statistical uncertainties.
LHC long-term schedule

 

[ohwilleke]

Jet quenching, pp ridge, best top quark mass, observation of Bs decay to dimuons,

I'd say that the last two are much more notable than the first two (at least from the perspective of a more general audience).

The apparent lack of lepton universality in B meson decays is the only really live result from the LHC that still seems to be in strong tension with the Standard Model, although the data haven't yet reached "discovery level" for this apparent anomaly, in the most reasonable interpretations of the data and the strength of the anomaly to date. Everything else either refines the measurement of physical constants in, or vindicates more generally, the Standard Model. My money is on the lepton universality violation going away for technical analysis reasons, rather than proving the existence of New Physics, but it could definitely go either way at this point.

LHCb has also given us a new and independent precise W boson mass measurement (albeit contradicted strongly by a reworking of old Tevatron data by CDF this month).

I'd also add that the Higgs discovery at the LHC has really had two parts. Part I was definitely finding a roughly 125 GeV resonance in 2012. Part II, since then, has been precisely characterizing that resonance to show how its measured properties correspond to a SM Higgs boson of that mass including its width and the branching fractions of its decays (although its charge, spin and parity were determined with very little uncertainty almost immediately so those discoveries are almost more part of Part I), and to pin down the Higgs boson's mass more precisely. So far, there is a pretty good match, but there is still work to be done on that front.

Part II is important because Higgs boson branching fractions, relative to well calculated theoretical expectations, are a powerful global test of the Standard Model. Any reasonably massive non-SM particle that gets its mass via the Higgs mechanism could throw off the branching fraction percentages for the Higgs boson decay by a lot, in much the same way that W and Z boson decay fractions are powerful global measures of the completeness of the SM with respect to weakly interacting particles up to about 45.59 GeV (half the Z boson mass) (i.e. all of them except photons and gluons and the top quark), and much like muon g-2 is a powerful global measure of the completeness of the SM.

Numerically, the decays of a 125 GeV Higgs boson in the Standard Model are approximately as follows:

b-quark pairs, 58%
W boson pairs, 21.3%
gluon pairs, 8%
tau-lepton pairs, 6.3%
c-quark pairs, 3%
Z boson pairs, 2.8%
photon pairs, 0.2%
muon pairs, 0.02%

(The total does not add to 100% due to rounding errors and due to omitted low probability decays such as electron-positron pairs, strange quark pairs, down quark pairs, up quark pairs, and some asymmetric boson pairs.)

Several the decay channels of the Higgs boson have been observed and are tolerably close to the expected values in the chart above (specifically decays to ZZ, two photons, WW, bb, two tau leptons, and two muons). A new fundamental particle in the 1 GeV range or more that gets it mass via the Higgs mechanism, for example (at least up to half the Higgs boson mass of about 62.5 GeV) would really throw off these percentages greatly. Higgs boson interactions with the top quark have also been confirmed to be order of magnitude correct relative to the SM prediction.

And, while I'm not sure how much of this is LHCb v. ATLAS/CMS, the overall LHC project has played a key part in pinning down that tetraquarks and pentaquarks exist, and if I recall correctly, the LHC was also the first man made experiment to actually create a quark-gluon plasma. All of these were Standard Model predictions, that were untested and might not have panned out as expected, but in the end showed up just as they were supposed to have.

What the LHC didn't find also matters. Pretty much every group of theorists favoring some kind of beyond the Standard Model argued strenuously before the LHC was built that their theories would be vindicated and that their discovery was "right around the corner." So far, the answer in each case has been "no dice". These theories are somewhat malleable and elastic, so it is hard to say that any class of theories has been ruled out entirely, but the parameter space not ruled out for each of them has been greatly diminished. Technicolor and electroweak scale supersymmetry are two of the most notable, and the LHC has greatly disfavored a fourth generation Standard Model extension with a t', b', tau' and tau-neutrino' generation in addition to the three of the Standard Model. Several other false alarms for potential new fundamental particles have also failed to pan out in the end as more data was collected.

It also didn't eat the Earth, which is very salient to almost everyone, even if that result was widely expected.

 

[mfb]

First collisions with stable beams (conditions for data-taking) this morning:
https://op-webtools.web.cern.ch/vistar/vistars.php?usr=LHC3
Still at 450 GeV, but you can see experimental ramp-up sequences of the magnets over night (at very low beam energies).

First collisions overall happened on Tuesday.

We will likely get initial collisions in mid/late May and first high energy collisions (at 13.6 TeV, up from 13 TeV before the shutdown) in June.

Perfectly on track so far.

 

[dukwon]

First collisions with 6.8 TeV per beam (for van der Meer scans). First "stable beams" at this energy are still 5 weeks away

 

[Vanadium 50]

Here is a picture of a 900 GeV collision. Nice (low energy) muon.

 

[mfb]

First stable collisions at 13.6 TeV are now expected for July 5.

10 years and one day after the announcement of the Higgs boson discovery (4 July 2012).

 

[ohwilleke]

First stable collisions at 13.6 TeV are now expected for July 5.

10 years and one day after the announcement of the Higgs boson discovery (4 July 2012).

Fireworks in Switzerland for my son's birthday!

 

[dukwon]

It's the day of the media event!

Here's the YouTube live stream link (starts 16:00 CERN time)

... and we start the day off with 7 magnet quenches (6 dipoles and a quadrupole)

First estimate for recovery of the cryogenic conditions is 15:00. The plan was to have beams in "adjust" by 13:00. We might be waiting around a bit.

 

[mfb]

Livestream is live, magnets are fine again, currently ramping up as protons are accelerated.



https://op-webtools.web.cern.ch/vistar/vistars.php

Just three bunches.

The "English" stream is mixing German and English commentary for some reason.

 

[mfb]

Stable beams have been achieved. The luminosity is very low, there are several new detector components that need to be calibrated, and the machine is using the time to test some new procedures. It's likely these early runs will never be used for physics analyses. Just too much effort for such a small dataset.

After 9 hours of running they dumped the beam and refilled with more protons to achieve a higher collision rate (this second run is still ongoing). It will take weeks to get back to the rates we had before the long shutdown.

 

[Vanadium 50]

The experiments have been posting event displays of 13.6 GeV collisions. They look a lot like 13.0 GeV collisions.

 

[mfb]

A special run for LHCf had a single 57-hour fill, exceeding the previous record of 37 hours and 46 minutes by a bit under a day.
The run had relatively wide beams in the experiments and a low collision rate, which helps a lot.

LHCf is studying neutral particles produced in the collisions to better understand how cosmic rays interact with the atmosphere.