Data Collection Begins: Monitoring the Diphoton Excess

[mfb]

Machine development is over, but all they got in the last 2.5 days were two short runs. It cannot work always as nicely as we had it in the last weeks.

The initial problem was from communication in the cryogenic system, afterwards one magnet did not work properly. This morning the preaccelerators needed some intervention - now fixed, but the magnet still has problems.

Once the magnet is running again, they try to quickly go to a larger number of bunches (see the post by dukwon).

Another thing that will be tested is luminosity leveling - LHCb uses it already, ATLAS and CMS want to use it later: The beams are deliberately collided with some position offset to reduce the interaction rate to something the detectors can reasonably process. Currently ATLAS and CMS are interested in as many collisions as the machine can give them (up to ~40 per bunch crossing), but with the high-luminosity upgrade they will need this leveling procedure to limit the collisions per bunch crossing to about 150 while the machine could achieve something like 250. LHCb is designed for a lower luminosity, they have been running with luminosity leveling since 2011.

 

[dukwon]

Next fill will be the first to take advantage of the new Abort Gap Keeper position. 2173b @ 96bpi

 

[dukwon]

From the most recent LPC meeting...

 

[mfb]

I saw that ;).
21.6 or 23.7/fb delivered to ATLAS and CMS, depending on which number you trust. Data-taking after MD was slower than before due to various issues, but it is still better than the 2016 projection.

The LHC is now running with 2220 bunches per beam, but at a lower number of protons per bunch. One magnet seems to have an electrical problem inside and could get damaged if there is a quench, so they are very careful about that magnet now. If it gets damaged, a replacement could easily take two months, which basically means the end of data-taking for this year.

 

[cube137]

Just a question.. if the LHC has a proton collision energy of 13 TeV. What is the maximum energy of particles it can produce. Half or 6.5 TeV only? or smaller.

 

[ChrisVer]

Just a question.. if the LHC has a proton collision energy of 13 TeV. What is the maximum energy of particles it can produce. Half or 6.5 TeV only? or smaller.

the actual collision energy is less than 13TeV (what collides are the quarks or gluons, and they carry a fraction $x$ of the proton's energy/momentum).
It depends on what is produced... in general the collision will produce several particles, and so the energies are not "fixed" by the energy-momentum conservation, as they are in the case you produce just two particles... some can be higher some lower.

 

[cube137]

the actual collision energy is less than 13TeV (what collides are the quarks or gluons, and they carry a fraction $x$ of the proton's energy/momentum).
It depends on what is produced... in general the collision will produce several particles, and so the energies are not "fixed" by the energy-momentum conservation, as they are in the case you produce just two particles... some can be higher some lower.

Are they high enough to detect KK particles which may weight up to 2 GeV or do we have to wait another 20 years and $10 billion dollars for the China collider?

 

[mfb]

A reaction like gluon+gluon -> particle with 10 TeV mass is not impossible (if such a particle exists), but incredibly unlikely as (a) the gluons would have to carry a large fraction of the total energy of both protons and (b) single particle production is always problematic in terms of phase space and conserved quantities.

As far as I know, the searches for black holes (which are not actual particles, but share many of their properties) are the only searches with exclusion limits above 6.5 TeV.
Here is a summary of CMS run 1 exclusion limits, the corresponding ATLAS plots look similar: no exclusion limit beyond 4 TeV as the production rate would be too tiny to see it. Only black holes have a huge production rate at high masses if they are possible at the LHC energies.Currently there are some problems with the magnets of ALICE and ATLAS.

 

[cube137]

A reaction like gluon+gluon -> particle with 10 TeV mass is not impossible (if such a particle exists), but incredibly unlikely as (a) the gluons would have to carry a large fraction of the total energy of both protons and (b) single particle production is always problematic in terms of phase space and conserved quantities.

As far as I know, the searches for black holes (which are not actual particles, but share many of their properties) are the only searches with exclusion limits above 6.5 TeV.
Here is a summary of CMS run 1 exclusion limits, the corresponding ATLAS plots look similar: no exclusion limit beyond 4 TeV as the production rate would be too tiny to see it. Only black holes have a huge production rate at high masses if they are possible at the LHC energies.

I've been googling or researching about the searches for KK particles for the past two hours.. what are the mass or TeV range already investigated? Are KK particles still possible to be found by the LHC?

 

[mfb]

Randall-Sundrum Gravitons would form such a structure. The diphoton peak appeared in a search for those particles. There are also searches for heavier versions of a Z or W, those could be new particles or KK-like heavier states of the Z/W. I guess searches for excited quark states are also similar to this.

 

[ChrisVer]

I am not sure if the W' has been used for such a search (because they have to come from models that have an extra SU(2) )... Z' though have (I guess because they can be connected to just a U(1) that comes from string theories and so on).
I may be wrong though...

 

[mfb]

I would expect the W to have heavier partners as well if the other particles have them, but I'm not sure.

 

[ChrisVer]

A E6 gauge group for example (that can appear from string-theory low-energy phenomenology http://www.sciencedirect.com/science/article/pii/0370157389900719 ) can break down into an SU(5) group + two additional U(1) groups http://journals.aps.org/prd/pdf/10.1103/PhysRevD.34.1530 ... (the reason I said that I cannot be sure is because all these can be very model-dependent, for example you can have the SU(4)xSU(2)xSU(2) which can bring a W').
Those U(1) can predict heavier Z' gauge bosons, but not W'.
I guess people then adopt the last paper's notation when they search for them and denote them with $Z_\psi$ and $Z_\chi$.

 

[vanhees71]

The appealing thing with additional U(1) gauge fields is that they can be massive gauge fields without additional Higgs particles in the physical spectrum. In the Abelian case you can just have a naive mass term without violating gauge invariance by introduing an additional scalar field, the Stueckelberg ghost. The point is that in the Abelian case this Stueckelberg ghost decouples completely from the dynamics (as also the Faddeev-Popov ghosts in the Abelian case). At finite temperature, the ghosts are important since they give the correct counting for the bosonic degrees of freedom: You have four gauge-field degrees of freedom and one Stueckelberg ghost (which is quantized as a true boson, i.e., as a c-number field in the path integral) and two Faddeev-Popov ghosts (which are quantized as pseudo-fermionic fields to provide the determinant of the gauge transformation in the Faddeev-Popov formalism). So together you have 4+1-2=3 physical and true bosonic degrees of freedom, corresponding to the three physical spacelike degrees of freedom of a massive vector field.

 

[cube137]

Randall-Sundrum Gravitons would form such a structure. The diphoton peak appeared in a search for those particles. There are also searches for heavier versions of a Z or W, those could be new particles or KK-like heavier states of the Z/W. I guess searches for excited quark states are also similar to this.

Randall-Sundrum RS1 and RS2 warped and extra dimensions including different sizes have different unique KK particles signatures.. I've been looking for the KK particles already excluded by current and past LHC searches.. what websites summarize the KK particles already excluded (as well as the corresponding dimensional stuff)? Thanks.

 

[ChrisVer]

Randall-Sundrum RS1 and RS2 warped and extra dimensions including different sizes have different unique KK particles signatures.. I've been looking for the KK particles already excluded by current and past LHC searches.. what websites summarize the KK particles already excluded (as well as the corresponding dimensional stuff)? Thanks.

Well, if there exist any such public result from either ATLAS or CMS, you can find it in their "Exotics" results:
https://twiki.cern.ch/twiki/bin/view/AtlasPublic/ExoticsPublicResults
https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsEXO
most of the times the papers refer to the "signatures" they search for (for example charged lepton + MET , or dilepton*, or jet+stuff and so on) and maybe in keywords like "extra dimensions".
Also the pdg review on the topic you are interested in (eg http://pdg.lbl.gov/2015/reviews/rpp2015-rev-extra-dimensions.pdf) contains information on the searches for those particles, so you could check it out (and the references therein)

*Here I use "leptons" to refer to all SM leptons: eμτ... most of the times, leptons in the paper titles refer to light leptons (e and μ) like here http://arxiv.org/abs/1407.2410 and if the search is τ-specific, the taus are used in the title...(mainly due to the differences between e/μ and τ signals)

 

[cube137]

A reaction like gluon+gluon -> particle with 10 TeV mass is not impossible (if such a particle exists), but incredibly unlikely as (a) the gluons would have to carry a large fraction of the total energy of both protons and (b) single particle production is always problematic in terms of phase space and conserved quantities.

As far as I know, the searches for black holes (which are not actual particles, but share many of their properties) are the only searches with exclusion limits above 6.5 TeV.
Here is a summary of CMS run 1 exclusion limits, the corresponding ATLAS plots look similar: no exclusion limit beyond 4 TeV as the production rate would be too tiny to see it. Only black holes have a huge production rate at high masses if they are possible at the LHC energies.Currently there are some problems with the magnets of ALICE and ATLAS.

Would like to verify something. At the bottom of the paper you shared above is the description: "Summary of CMS limits on new physics particle-masses/scales in different BSM searches". What does it mean? Are the bars those already tested or the capability of the machine.? For example. In the "RS Gravitons" in the second line RS1(ee,uu), k=0.1 with bar reaching 2.75 TeV. Is it 2.75TeV the capability of the machine or energy already tested??

 

[ChrisVer]

Are the bars those already tested or the capability of the machine.?

tested and excluded, so if they exist they have mass above the written in the bar.

 

[cube137]

tested and excluded

But in Compositeness, there is a bar in the dielectrons, A+ LUM which has bar reaching 18.3 TeV. But the LHC has only 14 TeV hadron collision energy.. can the components of the debris have more energy than 14 TeV?

 

[ChrisVer]

But in Compositeness, there is a bar in the dielectrons, A+ LUM which has bar reaching 18.3 TeV. But the LHC has only 14 TeV hadron collision energy.. can the components of the debris have more energy than 14 TeV?

Well , we don't have 14TeV yet... and I don't know about those high masses... I think it can be possible depending on the actual particle/model... for example if those particles existed with masses below 18TeV they might affect some observable we have at the accessible energies.
what I am sure about is that the "Heavy gauge bosons" show the actual limits observed... well some don't make sense at all (eg in the W'->τν CMS got 3.3TeV in their latest release, but in the W'->(e/μ)ν they got 4.4TeV , so I don't understand why their bar for SSM W' is at 3.3TeV)

 

[ChrisVer]

An example is here:
https://arxiv.org/pdf/1202.5535v2.pdf
with pp collisions @ sqrt(s)=7TeV they ruled out those models for up to 14.5 TeV.

 

[cube137]

Well, if there exist any such public result from either ATLAS or CMS, you can find it in their "Exotics" results:
https://twiki.cern.ch/twiki/bin/view/AtlasPublic/ExoticsPublicResults
https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsEXO
most of the times the papers refer to the "signatures" they search for (for example charged lepton + MET , or dilepton*, or jet+stuff and so on) and maybe in keywords like "extra dimensions".
Also the pdg review on the topic you are interested in (eg http://pdg.lbl.gov/2015/reviews/rpp2015-rev-extra-dimensions.pdf) contains information on the searches for those particles, so you could check it out (and the references therein)

*Here I use "leptons" to refer to all SM leptons: eμτ... most of the times, leptons in the paper titles refer to light leptons (e and μ) like here http://arxiv.org/abs/1407.2410 and if the search is τ-specific, the taus are used in the title...(mainly due to the differences between e/μ and τ signals)

Do you or does anyone have any idea what's the maximum TeV before the RS1 and ADD models were totally excluded? or is there no limit even reaching up to 50 TeV in future colliders?

 

[mfb]

A direct production of real particles is not the only way you can find new physics.

For all searches, more data allows to set better exclusion limits, and the increased energy of run2 helps massively in nearly all searches.

Do you or does anyone have any idea what's the maximum TeV before the RS1 and ADD models were totally excluded? or is there no limit even reaching up to 50 TeV in future colliders?

They could appear anywhere, including millions of TeV. But the nice features of the theory go away if they are not reasonably close to the scale of electroweak symmetry breaking.

 

[cube137]

A direct production of real particles is not the only way you can find new physics.

For all searches, more data allows to set better exclusion limits, and the increased energy of run2 helps massively in nearly all searches.
They could appear anywhere, including millions of TeV. But the nice features of the theory go away if they are not reasonably close to the scale of electroweak symmetry breaking.

What range for you is this "reasonably close" to the scale of EWSB.. maybe from 1 TeV to 20 TeV or 1 TeV to 70 TeV? or 1 TeV to 3Tev?

 

[mfb]

There is no fixed limit, higher masses just make the theories less and less plausible. If the LHC doesn't find anything with its full dataset (~2035), then I would expect many theorists to look for new approaches.

 

[cube137]

There is no fixed limit, higher masses just make the theories less and less plausible. If the LHC doesn't find anything with its full dataset (~2035), then I would expect many theorists to look for new approaches.

You mean up to year 2035? That's very long! It's only 2016 now.. that's still 19 years to go.. or did you mean up to 2035 TeV?

 

[ChrisVer]

year 2035

yup
Well LHC was not built to work for 3-4 years.

 

[cube137]

yup
Well LHC was not built to work for 3-4 years.

But just within 1 year of run2.. LHC has already excluded say up to 2.8 TeV for RS1 warped dimension model.. why would it need 19 more years when it's limit is only up to 13 TeV hadron collision energy. Or were you saying that they need to look at the data for the next 19 yrs and all those supersymmetric particles can suddenly become visible say 7 years from now? Please clarify. Thank you.

 

[mfb]

Here is the current schedule (page 2)

We collected about 4/fb for ATLAS and CMS each in 2015.
This year we should get between 30 and 40, 2017 and 2018 probably another 40 to 50 each, for a combined dataset of ~100-150/fb.
Then two years of shutdown for improvements to LHCb and ALICE and various machine components. If it doesn't happen earlier, we can probably go to 14 TeV afterwards.
2021-2023 the experiments hope for more than 50/fb per year, for a total of ~300/fb.
2024-2026 the machine will upgraded to the High-Luminosity (HL) LHC, pushing the collision rate to about 7 times the current value from 2027 on (with a shorter break in 2031), ATLAS and CMS get major upgrades as well. That should allow to collect about 300/fb per year to have about 3000/fb by 2035.Larger datasets allow to increase the exclusion limits, but also to make them harder: you can often tune the signal strength in a model (the exclusion limits are then given for a fixed signal strength), and to find weaker signals you simply need more data.

 

[ChrisVer]

why would it need 19 more years when it's limit is only up to 13 TeV hadron collision energy.

it's not only the energies that matter, but the amount of data...
With the 2015 dataset at sqrt(s)=13TeV with 3.2fb^- luminosity the limit of an exotic particle was at ~4.0TeV
With the 2016 dataset at sqrt(s)=13TeV (same energy) with ~13.3fb^- the limit went at ~4.7TeV

Also read about top's discovery.

 

[cube137]

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.

 

[ChrisVer]

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 referring to?

 

[cube137]

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 referring 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..."

 

[mfb]

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.

 

[ChrisVer]

I still don't see why those quotes made you set an "interesting" (or important) energy window between 1 and 4 GeV.