I Data Collection Begins: Monitoring the Diphoton Excess

[mfb]

Performance in the last two weeks has been amazing. Monday->Monday set a new record with 3.1/fb collected in one week (that is nearly the same amount of data as the whole last year) while the LHC experiments could take data 80% of the time. About 12 weeks of data-taking remain for this year, 13.5/fb has been collected already (more than 3 times the 2015 dataset).

The LHC is now reliably reaching the design luminosity at the start of a run, and most runs are so long that they get dumped by the operators (instead of a technical issue) to get a new run - the number of protons goes down over time, after about 24 hours it gets more efficient to dump the rest and accelerate new protons.

The SPS vacuum issue won't get resolved this year, but there are still some clever ideas how to increase the collision rate a bit more.

 

[dukwon]

The SPS vacuum issue won't get resolved this year, but there are still some clever ideas how to increase the collision rate a bit more.

Slide 14 of the first set of slides from Monday's LPC meeting (http://lpc.web.cern.ch/lpc-minutes/2016-07-11.htm) has a table of the max. possible bunches (total and colliding at each point) given a train length, with and without moving the Abort Gap Keeper.

If the SPS will handle 144 bpi, we could see up to 2532b/beam, which translates to 24% increase in number of colliding bunches at ATLAS & CMS and 38% increase at ALICE and LHCb.

 

[mfb]

18.3/fb (or 19.0, 19.2 or 19.4 according to other sources - I'll keep the previous one to have consistent comparisons) collected for ATLAS and CMS (compare this to ~4/fb the whole last year), the first machine development block of this year started today, stable beams will probably resume on Monday.

In the last days, the LHC implemented a "BCMS" scheme ("Batch Compression, Merging and Splitting") - a different way to prepare the bunches in the preaccelerators. As result, the beams are focused better, leading to a higher luminosity. We had several runs starting at 120% the design luminosity. The availability of the machine was great as well, so the LHC experiments could collect a huge amount of data.

I updated the luminosity plot including a paint-based extrapolation, taking into account planned downtimes. The light green line was a very early official estimate, the red line was an earlier extrapolation from me.
If the LHC can continue to run as good as in the last two weeks, it will beat even the optimistic red line extrapolation significantly.The ICHEP conference starts Wednesday next week, on Friday there are talks about the diphoton mass spectrum where we can learn more about the 750 GeV bump seen last year.

 

[Lord Crc]

Are any of the parallel sessions recorded and published by any chance?

 

[Lord Crc]

I was also thinking about the Higgs boson, any hints that there might be something interesting or unexpected there?

 

[websterling]

Are any of the parallel sessions recorded and published by any chance?

I didn't see anything on the conference website 38th International Conference on High Energy Physics about the sessions being recorded or streamed. But it did say that the Proceedings will be publicly available. Also, presented papers tend to show up on the arXiv.

 

[mfb]

Typically the slides are made available quickly after the sessions (sometimes before, but then people go through the slides instead of listening to the talks even more), with the corresponding arXiv uploads a bit before or after that.

But it did say that the Proceedings will be publicly available.

Proceedings appear several months later - at a point where all those analyses updated to the full 2016 dataset already, and no one cares about proceedings any more.

 

[Hepth]

Yeah, I think they were recording only the Plenary speakers at the last ICHEP, our parallel sessions were not. But usually even the video takes time to be put on the website. A lot of the slides, especially those with preliminary experimental results, might not be made public either.

 

[Lord Crc]

Thanks for the info, guess I've been a bit spoiled by pirsa :)

 

[dukwon]

Since the Abort Gap Keeper was moved and verified before the MD period, the filling scheme should change at some point this week to one with 2200 bunches per beam (up from 2076).

This will mean a 9% increase in number of colliding bunches at ATLAS and CMS, 16% at ALICE and 18% at LHCb

 

[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)