LHC ended 2016 proton collisions - exceeded all records; now: proton-lead collisions

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  • #101
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I know fb has to do with fbytes.. what is f (what is the complete word)? how many terabytes is one f?

So we will still have 19 years of data in the LHC. But why are some physicists already discouraged. I think the most important energy is between 1 GeV to 4 GeV because higher and you will so many new parameters that it would make the theory even have triple the constants of nature and unlikely already especially for Supersymmetry and the Hierarchy Problem Naturalness. In fact the physicist Sabine wrote:

"The idea of naturalness that has been preached for so long is plainly not compatible with the LHC data, regardless of what else will be found in the data yet to come. And now that naturalness is in the way of moving predictions for so-far undiscovered particles – yet again! – to higher energies, particle physicists, opportunistic as always, are suddenly more than willing to discard of naturalness to justify the next larger collider.

Now that the diphoton bump is gone, we’ve entered what has become known as the “nightmare scenario” for the LHC: The Higgs and nothing else. Many particle physicists thought of this as the worst possible outcome. It has left them without guidance, lost in a thicket of rapidly multiplying models. Without some new physics, they have nothing to work with that they haven’t already had for 50 years, no new input that can tell them in which direction to look for the ultimate goal of unification and/or quantum gravity."

If we will have a poll.. how many percentage of physicists here in Physicsforums agree with the above and how many agree that Supersymmetry and other major findings can still be found at 100 TeV like Lubos who is a string theorist forever.
 
  • #102
ChrisVer
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I know fb has to do with fbytes.. what is f (what is the complete word)? how many terabytes is one f?
that is really breaking the whole disussion. fb^-1 stands for reciprocal femtobarn, and it is a unit in which the integrated luminosity is measured (@mfb has written an Insight here https://www.physicsforums.com/insights/lhc-part-3-protons-large-barn/).

If we will have a poll.. how many percentage of physicists here in Physicsforums agree with the above and how many agree that Supersymmetry and other major findings can still be found at 100 TeV like Lubos who is a string theorist forever.

Check the discussion after post #14 here:
https://www.physicsforums.com/threa...data-atlas-nothing-in-spin-0-analysis.881050/
The fact that some people put so much hope over a so-called insignificant signature is their personal problem... it's OK to have something to work with (and that's the job of theoreticians who follow the experiment!!), but people who do that should always have in the back of their head that the next day it may be gone...

But why are some physicists already discouraged
based on my statement above: it's their problem...
People have been dreaming of discovering SUSY for way more years than just 19.

I think the most important energy is between 1 GeV to 4 GeV because higher and you will so many new parameters that it would make the theory even have triple the constants of nature and unlikely already especially for Supersymmetry and the Hierarchy Problem Naturalness.
I don't get what's the point there... what kind of new parameters are you refering to?
 
  • #103
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that is really breaking the whole disussion. fb^-1 stands for reciprocal femtobarn, and it is a unit in which the integrated luminosity is measured (@mfb has written an Insight here https://www.physicsforums.com/insights/lhc-part-3-protons-large-barn/).



Check the discussion after post #14 here:
https://www.physicsforums.com/threa...data-atlas-nothing-in-spin-0-analysis.881050/
The fact that some people put so much hope over a so-called insignificant signature is their personal problem... it's OK to have something to work with (and that's the job of theoreticians who follow the experiment!!), but people who do that should always have in the back of their head that the next day it may be gone...


based on my statement above: it's their problem...
People have been dreaming of discovering SUSY for way more years than just 19.


I don't get what's the point there... what kind of new parameters are you refering to?

I have Peter Woit book "Not even Wrong" he wrote in page 173 about the 105 extra parameters:

"One can come up with ways of spontaneously breaking the supersymmetry, but these all involve conjecturing a vast array of new particles and new forces, on top of the new ones that come from supersymmetry itself..."

"to define the MSSM one must include not only an unobserved superpartner for each known particle, but also all possible breaking terms that could arise from any kind of supersymmetry breaking. The end result is that the MSSM has at least 105 extra undetermined parameters that were not in the standard model. Instead of helping to understand some of the eighteen experimentally known but theoretically unexplained numbers of the standard model, the use of supersymmetry has added in 105 more. As a result, the MSSM is virtually incapable of making any predictions. In principle, the 105 extra numbers could take on any values whatsoever and, in particular, there is no way to predict what the masses of any of the unobserved superpartners will be..."
 
  • #104
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The number of new parameters is independent of the particle masses. The minimal supersymmetric model doesn't have all the >100 parameters (that's why it is called "minimal" - it has the smallest number of parameters), but all SUSY models introduce more parameters.
 
  • #105
ChrisVer
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I still don't see why those quotes made you set an "interesting" (or important) energy window between 1 and 4 GeV.
 
  • #106
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If we will have a poll.. how many percentage of physicists here in Physicsforums agree with the above and how many agree that Supersymmetry and other major findings can still be found at 100 TeV like Lubos who is a string theorist forever.

Sure it can be found. But it will become harder to convince funding agencies that "the next bump in energy will show experimental results for sure".
I've only worked close with my advisor and the way I read him was as a pragmatic physicist. In fact quite a bit of his work is to exclude possible avenues including the celebrated KKLT result.

Rule number one: confirming your theory is nice, something absolutely unexpected is even more fun!

Re Lubos; I think he gives some nice insights for some articles but he's too harsh, same as Woit (IMHO).
Discussing articles on a blog without too much technical details calls for a lot of nuance.
 
  • #107
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We are in another block of machine development / technical stop / special runs at low luminosity now. ATLAS and CMS collected nearly 30/fb, and four weeks of data-taking are left. The schedule got moved a bit, data-taking now ends a few days earlier (October 25th instead of November 1st), but with the recent performance we can still expect about 40/fb in total, much more than the original expectation of 25/fb. More than three times the amount of data analyzed for ICHEP in August. I expect that we get some results of the full 2016 dataset around the end of the year.

https://espace.cern.ch/be-dep/BEDepartmentalDocuments/BE/LHC_Schedule_2016.pdf [Broken]

Luminosity evolution, green actual data, dotted green the earlier expectation, red an early extrapolation:

lumievolution.png
 
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  • #108
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Some improvements in the procedure allowed even higher luminosities in the past weeks, up to 150% the design luminosity as peak, and reliably above than 130% at the start of runs. The values between ATLAS and CMS diverge again, this time CMS shows notably higher values, it is unclear if they actually have more collisions. Various planned fixes and upgrades should allow to get even higher luminosities next year.


Last week included a "high pile-up test": As many proton-proton collisions per bunch crossing as possible. They reached 90-95, while the design value is about 25 and the current regular runs have about 35-40 as maximum. The high values per bunch crossing came at the price of just a few bunches with this performance - not suitable for the current operation, 2200 bunches with 40 collisions each are much better than 50 with 90 collisions. The test gives the experiments a better idea how the next years might look like. The HL-LHC upgrade in ~2023-2026 will then lead to 140-200 collisions per bunch crossing.


A bit more than one week left for proton-proton collisions, then some machine development, mid November proton-lead collisions will start (3 weeks). Those collisions are an important control sample to understand the lead-lead collisions better: do they look like a collection of 208 separate nucleon-lead collisions, or which new things do they show? We had a similar run in 2013 already, but at lower energies.

The option to study those collisions is a lucky side-product of the design: Both beams have the same magnetic field strength in the bending magnets. This means the different particle types have the same momentum per charge. Protons have a mass of 1 u per electric charge, while lead has 208 u and 82 charges, a ratio of ~2.53. More mass per momentum means the lead ions are slower: If a bunch of protons collides with a bunch of lead ions at a collision point, and the bunch of protons goes around the ring once, the bunch of lead ions is not there yet, and the collision position would shift all the time. Oops.
Two features make the collisions possible: The LHC has much more energy than any previous collider. More energy means all speeds are extremely close to the speed of light, and speed differences are smaller. The second feature is the decision to have proton-proton collisions (instead of proton-antiproton): the two beams need their magnetic fields in opposite directions, which means they need separate beam pipes. This allows to steer the beams separately better - the lead ions can get the "inside curve", an orbit just a millimeter shorter over the circumference of 27 km - sufficient to keep them synchronized with the protons. At the injection energy, the necessary difference would be 40 cm - way too large to fix this. The LHC has to accelerate proton and lead to let them collide.


Luminosity evolution. The red line is my extrapolation from July 6th. CMS quotes 37.5/fb, ATLAS 34.6/fb, I plotted both. A huge dataset - I'm looking forward to first results at the end of the year!

lumievolution.png
 
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  • #109
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The last run got dumped half an hour ago. No more proton-proton collisions this year. CMS quotes 41.5/fb, ATLAS 38.5/fb, LHCb 1.9/fb. For nearly every analysis, and for all three experiments*, the data collected since ICHEP exceeds the dataset collected over all the years before.

We'll probably see results during Moriond in March, maybe earlier if something really exciting comes up.

* for ALICE, the proton-proton collisions are mainly a control sample, they care more about the lead collisions.


Next: Two weeks of technical work, then three weeks proton-lead collisions, followed by the end-of-year shutdown, with proton-proton collisions to resume in May 2017.
 
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  • #111
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Probably next year, but I don't think it's been scheduled.
 
  • #113
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The LHC is running luminosity scans now. This doesn't add to the dataset, but it does improve the precision of some measurements.
 
  • #114
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Why no lead-lead in 2016 and 2017? Not expecting much exciting results from it beyond the 2015 data?
 
  • #115
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Proton-lead collisions are important as well, both at the same time doesn't work and switching between different modes takes some days. It is more efficient to make just proton-lead (pPb) or lead-lead (PbPb) but not both within a year. I don't know why there is no lead-lead run planned for 2017. One argument could be the longer than usual shutdown this winter, proton-proton collisions would have a shorter time if there would be an additional heavy ion run at the end.
 
  • #116
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I guess there is some truth in the idea of #114. The previous pPb results are much more puzzling than the Pb Pb results obtained so far. Particularly, why in p Pb so many "collective phenomena", quite well describable by hydrodynamics, despite the large gradients involved, is quite exciting. So indeed, the pA runs are at least as exciting as new AA runs might be. Also for some Pb Pb results you don't only need the pp baseline but also the p Pb baseline (i.e., the "cold-nuclear matter" initial-state effects vs. the full hot-medium, including QGP, effects of the Pb Pb collision). E.g., to learn about heavy-quarkonium suppression vs. regeneration you need to know the initial-state effects like Cronin, shadowing etc. from the p Pb.
 
  • #117
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The current schedule has more PbPb than pPb in run 2. We had PbPb last year, now we have pPb - nothing unexpected so far. They could collide PbPb again in 2017 and pPb in 2018, but the current schedule does not have more pPb collisions in run 2.
 
  • #118
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I don't know why there is no lead-lead run planned for 2017. One argument could be the longer than usual shutdown this winter, proton-proton collisions would have a shorter time if there would be an additional heavy ion run at the end.

There's that, and I'm pretty sure Linac3 is switching to Xenon for that year
 
  • #119
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And not switching back? That would be another reason. On the other hand, collisions with other types of ions were considered for the LHC as well.
 
  • #120
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Potentially silly question: with the p-Pb run under way I'm looking at Vistars and wondering why the instantaneous luminosity of ALICE has such great fluctuations compared to the other detectors.

Is it just due to each collision having a much wider range of results depending on "how well" each proton hits the nucleus? I'm thinking bowling here.
 

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  • #121
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The luminosity just depends on beam parameters, not on details of the collisions (which happen in the kHz range anyway, not on the timescale of those fluctuations). I don't know where the fluctuations come from - could be some calibration issue with the measurement, or very frequent changes of the beam overlap by the machine operators.



The LHC registered the earthquake in New Zealand. It lead to a small deformation of the ring which changes the beam energy a tiny bit. This is the result. The long-term sine modulation are the tides. They are quite strong because we are close to a full moon.
 
  • #123
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Potentially silly question: with the p-Pb run under way I'm looking at Vistars and wondering why the instantaneous luminosity of ALICE has such great fluctuations compared to the other detectors.

Is it just due to each collision having a much wider range of results depending on "how well" each proton hits the nucleus? I'm thinking bowling here.

This is what luminosity levelling looks like when you zoom in on the y-axis scale.

Here's an example when they tried levelling ATLAS and CMS

fqiFbOZ.png
 
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  • #124
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The luminosity just depends on beam parameters, not on details of the collisions (which happen in the kHz range anyway, not on the timescale of those fluctuations). I don't know where the fluctuations come from - could be some calibration issue with the measurement, or very frequent changes of the beam overlap by the machine operators.

As the song goes, I should have known better... :)

The LHC registered the earthquake in New Zealand. It lead to a small deformation of the ring which changes the beam energy a tiny bit. This is the result. The long-term sine modulation are the tides. They are quite strong because we are close to a full moon.

Really interesting, thanks for sharing.
 

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