I What are the challenges faced by LHC in the initial data-taking of 2017?

[Drakkith]

Then we need more scrubbing or simply more data-taking at 2300 bunches - that acts as scrubbing as well.

What do you mean? What does data-taking have to do with scrubbing?

 

[mfb]

Scrubbing = have as many protons as possible circling in the machine.
Data-taking = have as many protons as possible at high energy circling in the machine
The second approach has fewer protons, as higher energies means the magnets get more heat (that is the problem reduced by scrubbing).

Scrubbing runs have 2820 bunches, data-taking might be limited to 2300. The latter is not so bad - especially as it means more data keeps coming in. And that is what counts.2.3/fb, 10 hours in stable beams already. We might get 2300 bunches as early as Wednesday evening.

Edit 13:00 CERN time: 21 hours in stable beams, 0.5/fb in less than a day, the beam will get dumped soon. New records for this year. And enough time to go up another step, to 2173 bunches.

Edit on Thursday 12:00: 2173 bunches, initial luminosity was about 140% the design value. At the level of the 2016 record. 2.8/fb in total. We'll get 2317 bunches later today, probably with a new luminosity record.

 

[mfb]

We have a new all-time luminosity record! Over the night, a fill with 2317 bunches had more than 140% the design luminosity. Close to 150%.

Unfortunately, the LHC encountered multiple issues in the last day, so the overall number of collisions collected was very low (just 1 hour of stable beams since yesterday afternoon). One of these issues lead to a lower number of protons in the ring than usual - we can get new luminosity records once that is fixed.

The heat in the magnets is now close to its limit, I expect that we get data-taking at 2300 bunches for a while before the beam pipe is "clean" enough to put even more protons in.Edit: They decided that the margin is large enough. 2460 bunches! And a bit more than 150% the design luminosity.

 

[websterling]

The current fill has 2556 bunches. Is this a record? I looked but didn't find the max from 2016.

Also, earlier in the fill there was a notice about a failed 'Hobbit scan'. What's a Hobbit scan?

 

[mfb]

The current fill has 2556 bunches. Is this a record? I looked but didn't find the max from 2016.

It is a record at 13 TeV. I don't know if we had more at the end of 2012 where they did some first tests with 25 ns bunch spacing (most of 2012 had 50 ns, where you are limited to ~1400 bunches).

No idea about the Hobbit scan. They are not new, but apart from the LHC status page I only find amused Twitter users.The LHC started its first phase of machine development, followed by a week of technical stop. Data-taking will probably resume July 10th. https://beams.web.cern.ch/sites/beams.web.cern.ch/files/schedules/LHC_Schedule_2017.pdf.

In the last days we had a couple of runs starting at ~150% design luminosity with a record of 158%. The initial phase of rapid luminosity increase is over. While the machine operators will try to increase the luminosity a bit more, this is basically how most of the year will look like now.

ATLAS and CMS got 6.3/fb so far. As comparison: Last year they collected about 40/fb.
LHCb collected 0.24/fb. Last year it was 1.9/fb.
https://lpc.web.cern.ch/lumiplots_2017_pp.htm
In both cases, the final 2017 dataset will probably be similar to the 2016 dataset. In 2018 we will get a bit more than that, 2019 and 2020 are reserved for machine and detector upgrades. Long-term schedule. With the 2017 dataset you can improve some limits a bit, you can improve the precision of some measurements a bit, but many studies will aim for an update after 2018.

LHC report: full house for the LHC

 

[Amrator]

So mfb, what can you tell us about the recent discovery? https://press.cern/press-releases/2...nce-observation-new-particle-two-heavy-quarks

 

[mfb]

A nice start for EPS.

The first baryon with two heavy quarks. It needs the production of two charm/anticharm pairs in the collision, with the charm quarks with similar momenta, that makes the production very rare.

Now that $ccu$ (=quark content) is found (with a very clear signal peak), $ccd$ should be possible to find as well - the mass should be extremely similar, but it has a shorter lifetime and a larger background. $ccs$ will be much more challenging - it needs an additional strange quark, that makes it even rarer. In addition, its lifetime should be even shorter.

Baryons with bottom and charm together: A naive estimate would suggest one such baryon per 20 double-charm baryons. That is probably too optimistic. The lifetime could be a bit longer. Maybe with data from 2016-2018?
Two bottom: Another factor 20, maybe more. Good luck.

--------------------

ATLAS showed updated results for Higgs decays to bottom/antibottom. Consistent with the Standard Model expectation, the significance went up a bit, now at 3.6 standard deviations. If CMS also shows new results, we can probably get 5 standard deviations in a combination. It is not surprising, but it would still be nice to have a good measurement how often this decay happens.I'll have to look through the EPS talks for more, but I didn't find the time for it today.
The technical stop is nearly done, the machine will continue operation tomorrow.

 

[Vanadium 50]

In addition, its lifetime should be even shorter.

Why? The strange lifetime is much, much longer than the charm. I'd expect that, up to final state effects, the lifetimes would be about the same.

 

[ChrisVer]

When they find those mesons or hadrons, and claim a discovery, I am really shocked that it took so long to discover them... it's only 3.something GeV (~half+ the mass of the B mesons) and not so "extraordinary" (just a ccu)...

 

[mfb]

I don't know how large the effect would be, but it should have a larger overall mass, although its decay products could have a higher mass as well. I didn't draw Feynman diagrams and I certainly didn't calculate it.

When they find those mesons or hadrons, and claim a discovery, I am really shocked that it took so long to discover them... it's only 3.something GeV (~half+ the mass of the B mesons) and not so "extraordinary" (just a ccu)...

They found 300 in the whole Run 2 dataset. Double charm production is rare, and both charm in the same hadron is rare even for this rare process.

 

[Vanadium 50]

I am really shocked that it took so long to discover them... it's only 3.something GeV (~half+ the mass of the B mesons) and not so "extraordinary" (just a ccu)...

Do you always denigrate the accomplishments of others?

 

[ChrisVer]

Do you always denigrate the accomplishments of others?

That'd be an offensive behavior from my side. Nope, I don't denigrate their or any discovery...
I am just wondering what factors made it take so long. We have undeniably found heavier particles, so the machines that produced those heavy particles could also produce the ccu ...

 

[mfb]

The mass is not everything that matters. See the top discovery long before the Higgs discovery.
The cross section, the decays, the backgrounds - all these things matter.

Could LHCb have seen a hint of this particle in Run 1? Probably. But manpower is limited, they probably didn't look into this particular channel at that time.
Could other experiments have seen it before? Every other experiment has a much smaller dataset for heavy baryons. Probably not, at least not with a high significance.Edit: Beam is back in the machine. Some issues with the accelerating cavities delay the operation. We'll probably get collisions on Sunday, with rapidly increasing number of bunches, and get back to the full intensity on Monday.

 

[dlgoff]

I don't think I've ever seen anything here on PF about the LHC's beam tube vacuum. Seeing the problems/leaks I'm having with my little rough vacuum system @ around 2 or 3 mTorr, how do you guys maintain, I'm assuming an ultrahigh vacuum, on such a huge system? @mfb and @Vanadium 50.

Thanks

Edit: Thanks again guys. You've given me information about the beam pipe vacuum I would have never known about.

 

[Vanadium 50]

Lots of pumps, lots of getters, and the fact that it's at cryogenic temperatures helps - residual gas tends to freeze.

 

[mfb]

~800 getter pumps, plus various others.
The pressure in the beam pipe is 1 to 10 nPa. A weaker vacuum would mean too many protons get lost. That would be bad for the magnets (heat load) and for the luminosity (the runs are several hours long, most protons should survive that long).Some more issues with the machine delayed the recovery from the technical stop. We had 600 bunches overnight, now the cryogenics system has another issue. Once that is fixed a few hours with 1300 bunches will be needed, and a few more hours of tests, then the machine goes back to the previous record of 2556 bunches and the experiments can resume regular data-taking at the full luminosity.

 

[dlgoff]

The pressure in the beam pipe is 1 to 10 nPa.

I'm blown away by this. Numbers do tell.

 

[Vanadium 50]

To put that in perspective, this is comparable to the lunar atmosphere. (It's better than 1 nPa at the IP, 10 nPa in the arcs, and the moon is about 0.3 nPa if I remember right. The moon's atmosphere is mostly argon, the LHC is atomic hydrogen, molecular hydrogen, helium and possibly CO)

 

[mfb]

The LHC beam pipe should be the largest vacuum of that quality (150 m³).
The LIGO vacuum is much larger (10,000 m³), but it has ~100 nPa.
The LHC magnets are in an insulation vacuum (to limit heat transfer) - 9000 m³, but at a "high" pressure of about 100 µPa (1µtorr).

For smaller volumes, it is possible to make the vacuum orders of magnitude better. Pump out the system, close all exits, and then cool everything down until all remaining atoms freeze out at the walls. BASE (also at CERN) has a vacuum so good that they can store antiprotons for more than a year without annihilations. The expected number of remaining gas atoms is zero in their 1.2 liter vacuum chamber, and there is no tool that can detect any remaining gas. They didn't observe annihilations, based on that they set an upper limit of ~1 fPa on the remaining pressure, or 3 atoms per cubic centimeter. Here is an article about it.

 

[Vanadium 50]

And not only does the pressure of the residual gas vary around the ring, but so does its composition.

 

[dlgoff]

@mfb and @Vanadium 50

Thanks for your replies. I don't mean to hijack this thread, but these things are what I live for.

 

[dlgoff]

... but so does its composition.

Speaking of composition (materials), don't these very low pressures evaporate some components? or degrade them?

 

[mfb]

Steel and copper (outside the experiments) and beryllium (at the experiments) don't evaporate notably, especially at cryogenic temperatures (some parts of the beam pipe are at room temperature, however). The LHCb VELO detector uses an AlMg₃ foil, no idea about that but it has a small surface anyway. I don't see how vacuum would degrade these materials.

 

[mfb]

Recovery from technical stop is still ongoing. The RF system (radio frequency cavities to accelerate the beam) cannot handle the 2556 bunches we had before, the problem is under investigation. With 2317 bunches it works, for now the LHC is running with this lower number of bunches. Still enough to collect a lot of collisions. ATLAS and CMS reached 7/fb, LHCb collected 0.24/fb.I made a thread about results from EPS.

Edit on Wednesday: Finally back at 2556 bunches.

 

[mfb]

Both ATLAS and CMS reached 10/fb, about 1/4 of the 2016 dataset. 16 more weeks for data-taking are planned. At pessimistic 2/(fb*week), we get the same number of collisions as last year, at optimistic 3.5/(fb*week) we get 65% more.
If everything in the LHC would work perfectly 100% of the time, more than 5/(fb*week) would be possible, but that is unrealistic with such a complex machine.We had a short break for machine development and van-der-Meer scans:
Cross section measurements are an important part of the physics program, and they require an accurate luminosity estimate. What the machine can deliver from normal operation has an uncertainty of a few percent. That is good for the machine operators, but for physics you want to get the uncertainty to be smaller - 2% is nice, 1% is better. The luminosity depends on a couple of machine parameters:$$\mathcal{L} = \frac{N_1 N_2 f N_b S}{4 \pi \sigma_x \sigma_y}$$
$f$ is the revolution frequency - fixed and known to many decimal places.
$N_b$ is the number of bunches per beam - known exactly.
$N_1$ and $N_2$ are the numbers of protons in the bunches, they can be measured via the electromagnetic fields they induce when moving around the ring.
$S \leq 1$ is a factor that takes the crossing angle into account, it can be calculated precisely. See also post 58.
$\sigma_x$ and $\sigma_y$ are the widths of the bunches in x/y direction. There is no good direct way to measure that accurately.

To estimate the width of the bunches, the machine operators shift the relative positions of the beams around at the collision points while the experiments monitor the collision rate as function of the shift. A fit to the observed rates leads to the widths. This procedure was named after Simon van der Meer.

 

[mfb]

A few updates: The LHC experiments got collisions at a high rate, and the machine operators found some methods to improve the rate further.

ATLAS and CMS reached 15.5/fb. 11 days since they had 10/fb, this means 0.5/(fb*day) or 3.5/fb per week.
Wednesday 6:46 to Thursday 6:46 this week we had a record of 0.83/fb in 24 hours. As comparison: In these 24 hours, the LHC experiments had 4 times the number of Higgs boson and 8 times the number of top quarks the Tevatron experiments had - in their 20 years of operational history.

LHCb surpassed 0.5/fb, nearly 1/3 of the 2016 dataset.

The stepwise reduction of the crossing angle, discussed earlier was studied in more detail. Previously it was reduced in steps of 10 millirad (150 -> 140 -> 130 -> ...). That increases the collected data by about 3.5%. The process now works so smoothly that it became possible to reduce it in steps of 1 millirad, always following the optimal angle. This increases the number of collisions by additional 1.5%. That doesn't sound much, but all these small improvements add up.

The number of protons per bunch went up a bit. We reached a record of 3.1*10¹⁴ protons per beam at high energy, or 320 MJ per beam. Correspondingly, the initial luminosity reached a new record, 174% the design value.
The machine operators tried to get even more, but that lead to problems, so they added a day of scrubbing.

Another thing discussed is the focusing of the beams at the collision points. Based on the analysis of the machine development block, it can be improved a bit more. That could increase the luminosity by ~20%. 1.74*1.2=2.09. There is still hope for the absolute luminosity record!

 

[mfb]

ATLAS and CMS reached 20/fb. We have gained 4.5/fb since the previous post 21 days ago, or 1.5/fb per week, even below the pessimistic estimate from above. You can see this clearly in https://lpc.web.cern.ch/lumiplots_2017_pp.htm as well.

A problem appeared in a region called 16L2, which lead to the dump of many fills, often before collisions started. Although the cause is not well understood, the process is always the same: Some beam particles are lost in this region, and a few milliseconds (tens of revolutions) later many more particles are lost roughly at the opposite side of the ring - more than acceptable, this triggers a beam dump. This can happen from either beam 1 or beam 2, although they fly in separate beam pipes in 16L2.
The problem appeared early in the year already, but until August, the dump rate could be managed by adjusting the control magnets in this region a bit. With increasing beam currents, it got more problematic and the machine operators wanted to get rid of the problem. The losses look gas-induced. The gas can stick to parts called "beam screen", and get released during the run, the collision of the beam with gas particles leads to the observed losses. The usual approach is to heat this beam screen, then all the gas evaporates, and gets pumped out or sticks to even colder parts of the beam pipe where it stays.
That was done on August 10 - and then everything got worse. Now more than half of the fills were dumped due to 16L2, even at lower numbers of bunches. The smaller fraction of time in stable beams plus the reduced number of bunches lead to the slower accumulation of collision data in the last three weeks. The leading hypothesis is gas in other components of 16L2 that redistributed when heating the beam screen and other components, leading to even more gas there.

What to do?

• The problem could be solved by heating up the whole sector and pumping it out properly. That would probably take 2-3 months, doing it now would mean most of the time planned for data-taking this year is gone. Unless data-taking becomes completely impossible this won't be done before the winter shutdown.
• The machine operators see if there is a stable running condition that works for now. The last few runs with 1550 bunches were promising, at this rate the LHC would be limited to ~2/fb per week, but that is still a reasonable rate that would double the 2016 dataset by the end of the year.
• Gaps between bunches can reduce losses, e. g. "8 bunches in a row, then 4 slots free, then 8 bunches in a row, then 4 slots free, ...". This might be tested. It would also mean the number of bunches has to be reduced compared to the initial plan, but if it reduces the number of dumps sufficiently it can be worth it.
• There are some special runs planned/proposed for 2018, some at lower energies and some with a very low collision rate, for something like 1 week in total. They might be shifted to 2017 as they won't be affected by the 16L2 issue as much as the regular operation at high energy and collision rate.
• The machine operators discuss what else can be done.

LHC report: Something in the nothing

 

[mfb]

ATLAS and CMS reached about 24/fb.
The mitigation approaches, especially the "8 bunches, then 4 slots free, repeat" pattern worked, in the last days ~2/3 of the time could be spent with data-taking. The luminosity is lower, but still at the design value. There are still some dumps due to 16L2 but they don't break everything any more.

A https://beams.web.cern.ch/sites/beams.web.cern.ch/files/schedules/LHC_Schedule_2017.pdf machine development block started, followed by a few days of technical stop. About 9 weeks for data-taking left in 2017. Unless there are some new ideas how to solve the 16L2 issue, I guess they will just keep the current configuration, it should lead to about 2-2.5/fb per week, so we will still get more than the 40/fb of last year.

LHC Report: operation with holes

 

[mfb]

Back to data-taking. Currently 1900 bunches, with the "8 bunches, 4 free, repeat" pattern. Initial luminosity was 120% the design value, quite nice for the relatively low number of bunches.

The machine operators work around the 16L2 issue:

• Combine the bunch/empty pattern with BCMS, a different way to prepare the beam in the preaccelerators. This will reduce the number of bunches to 1800, but give more collisions per bunch crossing. This will be tested in the next few days.
• Focus the beam better at the collision point. This was tested during the machine development block, and the operators are confident they can do this (technically: reduce $\beta^*$ from 40 cm to 30 cm).
• Move some special runs from 2018 to 2017:
  • collisions of xenon ions, probably for a day in November
  • proton-proton collisions at lower energy and lower collision rate to cross-check heavy ion results (as they have a lower energy per nucleon) and for some studies that don't care much about energy (or even work better at low energy) but suffer from many simultaneous collisions. About two weeks in December.

Other news: The machine development block included a run where some bunch crossings lead to 100 simultaneous collisions in ATLAS and CMS, compared to 40-50 during normal operation. This is an interesting test for future running conditions (~150-200 expected for the HL-LHC upgrade). These are averages, individual bunch crossings vary in the number of collisions, of course. An average of 100 means you have events with more than 130 collisions.

 

[sandy stone]

I assume it must be very challenging to track the collision products back to the specific collision that produced them.