Surprising high-energetic event at the LHC

[mfb]

Probably just a weird fluctuation, but still something to watch.
Event Display of a Candidate Electron-Positron Pair with an Invariant Mass of 2.9 TeV

Edit: Follow-up after data-taking in 2015 ended: no further events with that high energy, now one event is reasonable. Was just a weird fluctuation to have this event so early on.
CMS result with the full 2015 dataset
ATLAS result with the full 2015 dataset

The CMS detector found an event with a very high-energetic electron and a very high-energetic positron. If they are from the decay of a particle, this particle would have a mass of around 2.9 TeV (the so-called "invariant mass" of the electron/positron pair). The most likely candidate would be the hypothetical Z', a heavier version of the known Z boson.
The event is significantly more high-energic than everything seen in run 1 (2010-2012). Sure, the proton energy increased, but the analyzed data from this year represents just 0.3% of the collisions from run 1.

How surprising is it? There are well-known processes that can produce those pairs, but they rarely do that at very high energy. One could calculate the probability to find an event with an invariant mass of at least 2.9 TeV, but that is unfair - the choice of 2.9 comes from the one observed event. CMS calculated the expected number of events above 1.0, 2.0 and 2.5 TeV for the size of their analyzed dataset:
Above 1.0 TeV: 0.21
Above 2.0 TeV: 0.007
Above 2.5 TeV: 0.002

Those numbers don't have proper systematic uncertainties yet, so they might change a bit in the future, but it is clear that an event with 2.9 TeV was not expected that early in run 2.

What comes next? CMS studied just 1/3 of their dataset, and the full ATLAS dataset is even a bit larger. I'm quite sure both collaborations look at that now. If they find a second event, things get really interesting. If they find nothing, it is probably just a statistical fluctuation (with then ~0.04 expected events above 2 TeV it is not that surprising any more. Keep in mind that there are many possible particle combinations where one can look for events).

If it is a Z', it should also decay to muons with the same probability. Looking for high-energetic events with a muon and an antimuon will be interesting as well.

 

[lightarrow]

How does it work? They can only analyze the other data they already have, or wait for another event like that to happen, or they can fine tune the collisions energy to 2.9 TeV to increase the probability of it?
Thanks.

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lightarrow

 

[mfb]

Processing and analyzing the collected data takes some time, so there are collisions recorded that still need analysis.
Besides that, more collisions will come soon.

There is no way to improve the collisions (a higher energy would help, but preparing the machine for that takes months so it won't happen this year) - all you can do is try to get as many collisions as possible for better statistics.

 

[Vanadium 50]

They wait. Reducing the energy will not help. The rate will go way down.

The odds of this being anything are small. Every couple of days the experiments get an oddball event that is extreme in some way or another. More importantly, 65 pb-1 at 13 TeV corresponds to 1.3 fb-1 at 8 TeV (at 2.9 TeV). The experiments have analyzed the equivalent of 15x as much data already and not seen anything.

 

[mfb]

On the other hand, the Z' exclusion limit did not reach 3 TeV with run 1 data. Sure, you probably don't expect 1 event every 65/pb, but even combined with the whole run 1 data the observed event rate is above the expectation (but the significance goes away).

 

[Vanadium 50]

The Z-prime exclusion limit is for a SSM Z-prime benchmark model. We know that's not the case (precision electroweak tells us) so it's just a benchmark: the exact number for the mass doesn't tell us much. I can always come up with a number where the limit is less (or for that matter, more) stringent.

The best apples-to-apples comparison is the partonic luminosity.

 

[Buzz Bloom]

More importantly, 65 pb-1 at 13 TeV corresponds to 1.3 fb-1 at 8 TeV (at 2.9 TeV).

I would much appreciate it if someone would define for me "pb-1". I can guess that pb might mean "part per billion", and that "-1" might mean inverse, but "inverse parts per billion" makes no sense to me, and in the conexs of the discussion about Z', I cannot imagine what "pb-1" means.

Ah, I found the following in a list of possible abbreviations for pb: petabyte. Is this what is intended? If so, then it might be useful to mention what the petabyte accumulation rate is for the LHC test runs.

 

[lightarrow]

I would much appreciate it if someone would define for me "pb-1". I can guess that pb might mean "part per billion", and that "-1" might mean inverse, but "inverse parts per billion" makes no sense to me, and in the conexs of the discussion about Z', I cannot imagine what "pb-1" means.

It means (pico barn)^-1. So "65 pb-1" means 65 events for every pico barn of cross section. Multiplying that value for the cross section of a specific reaction you find the total number of events for that reaction.
Or at least this is how I've understood it

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lightarrow

 

[Buzz Bloom]

Hi lightarrow:

It means (pico barn)-1

Thaks very much for the definition.

Regards,
Buzz

 

[mfb]

I wrote about cross-sections and luminosity in this insights article.

The Z-prime exclusion limit is for a SSM Z-prime benchmark model. We know that's not the case (precision electroweak tells us) so it's just a benchmark: the exact number for the mass doesn't tell us much. I can always come up with a number where the limit is less (or for that matter, more) stringent.

Okay.
Well, we still have more events than expected, but it is probably just a weird fluctuation, as I wrote in post 1.

 

[Buzz Bloom]

Hi @mfb:

Although the paper had a September 2015 date, the CERN cite gives August 30 as the date of the "discovery". How do you interpret that there has been no further reports about this since then?

Regards,
Buzz

 

[mfb]

See post 1:

Edit: Follow-up after data-taking in 2015 ended: no further events with that high energy, now one event is reasonable. Was just a weird fluctuation to have this event so early on.

CMS result with the full 2015 dataset. The 2.9 TeV event stayed by far the most high-energetic one.

ATLAS result with the full 2015 dataset. Nothing above 2 TeV.

 

[Buzz Bloom]

Hi mfb:

I interpret your post that the original finding was a false positive. Is that right?

Regards,
Buzz

 

[Vanadium 50]

Nothing false about it. It's a real event. We expect to see one event or so.

 

[Buzz Bloom]

Nothing false about it. It's a real event. We expect to see one event or so.

Hi Vanadium:

Thnaks for your prompt answer.

My mistake. I thought that to get an acceptable confidence level, a second detection was required.

Regards,
Buzz

 

[mfb]

Confidence level of what?
You can always construct confidence levels. If no event is observed, they typically include zero of whatever parameter would lead to more events, so they are called "upper limits". If one event is observed, the confidence interval depends on your expected background and on what you want to measure.

Some event had to be the most high-energetic in 2015, just by the ordering of energies. This event is at 2.9 TeV. So what? It is not surprising to have an event at 2.9 TeV in the full 2015 datasets.

 

[Buzz Bloom]

Confidence level of what?

Hi mfb:∈

I mean a calculated value (confidence level) X∈[0,1] WRT which a measured value M with an error measurement range +/-Y is such that the probability that a random measurement R is in the range M+/-Y is < 1-X.

Regards,
Buzz

 

[mfb]

I know what confidence levels are, but what is the measured value you want to consider?

 

[Buzz Bloom]

Event Display of a Candidate Electron-Positron Pair with an Invariant Mass of 2.9 TeV

Hi mfb:

WRT quote: the measured value is 2.9 TEV as the energy at which data from the experimental run indicates that a "candidate" particle was created from the annihilation energy of the electron/positron pair.

BTW, as I think I understand it, the analysis of the data was not specific about the specific nature of the particle. Is this correct?

Regards,
Buzz

 

[mfb]

The 2.9 TeV are the measured value of a single event. That is not a physical parameter you could constrain. You can ask what the true invariant mass of the pair was, then you get a very narrow window around the measured value, but I don't think that is what you mean.

The LHC is a proton-proton collider. The dominant production process is quark+antiquark -> photon/Z -> electron+positron. No electron/positron annihilation.

BTW, as I think I understand it, the analysis of the data was not specific about the specific nature of the particle.

I don't understand that question.
Which particle? And no, that is not the main part that I don't understand.

 

[Buzz Bloom]

Hi mfb:

I apologize for being so dense. I confess do not understand particle physics theory or experimentation in general, nor specifically the significance regarding

The CMS detector found an event with a very high-energetic electron and a very high-energetic positron. If they are from the decay of a particle, this particle would have a mass of around 2.9 TeV (the so-called "invariant mass" of the electron/positron pair).

Here is my likely misunderstanding of the quote.
A proton crashed into a proton with a total energy of 2.9 TeV. A very short time afterwards, detectors detected a electron and a positron. This is interpreted as some particle being created by the proton-proton collision that subsequently decayed into the electron and positron, and possibly some other stuff that was not detected, e.g. a photon.

The most likely candidate would be the hypothetical Z', a heavier version of the known Z boson.

This quote makes a guess about the identity of the created particle.

Is it possible for a proton-proton collision to directly create a electron-positron pair?
Is it possible for the detector to see a false positive electron-positron pair?
Are there any plausible possibilities that a 2.9 TeV proton-proton collision could have created one or more non-hypothetical particles that could subsequently create other particles which collectively might look like an electron-positron pair (maybe e.g., muons or taus)?

If all of the above are extremely unlikely, then I will then understand that it becomes very likely that some hypothetical particle was created.

Thanks for your patience.

Regards,
Buzz

 

[Buzz Bloom]

If they find nothing, it is probably just a statistical fluctuation (with then ~0.04 expected events above 2 TeV it is not that surprising any more. Keep in mind that there are many possible particle combinations where one can look for events).

Hi mfb:

This quote is the one that prompted me to think in terms of "a false alarm", although I did not have a clear idea about how that might be defined in this context.

Regards,
Buzz

 

[ChrisVer]

A proton crashed into a proton with a total energy of 2.9 TeV. A very short time afterwards, detectors detected a electron and a positron.

The protons have approximately energy ~13TeV (well it depends on which energy the experiment runs)... What is colliding are the partons (quarks or gluons) of the proton, which can carry a fraction of the total proton energy... as a result the partons should be at at least 2.9TeV to give a particle of 2.9TeV mass. Then this particle yes decays...

and possibly some other stuff that was not detected, e.g. a photon.

photons can in general be detected.

Is it possible for a proton-proton collision to directly create a electron-positron pair?

I guess that such a scenario would require a coupling of 4 fermions (as for example the Fermi's 4-point interaction). having the same problems as Fermi's theory; and solved by the introduction of the heavy bosons... However you can have a proton-proton collision, that would (in the inner mechanics) produce a Z (or gamma) and give an electron-positron pair...This would be a background process which [together with all the rest] allows you to predict the expected events at that energy ("with then ~0.04 expected events above 2 TeV it")

Is it possible for the detector to see a false positive electron-positron pair?

yes I think otherwise it would consist a discovery... it can still be an error of statistics

Are there any plausible possibilities that a 2.9 TeV proton-proton collision could have created one or more non-hypothetical particles that could subsequently create other particles which collectively might look like an electron-positron pair (maybe e.g., muons or taus)?

Again yes, all those things make up the background...but not muons or taus (neither of them give a pair of electron positron originating from the same point). However you can still have % ocf muons or taus falsely identified as electrons by the ID algorithms. One result of this is that in the Z background it doesn't only count the $Z\rightarrow e^-e^+$ but also the $Z\rightarrow \mu^-\mu^+$ and $Z\rightarrow \tau^-\tau^+$ (in the last page of the slides, it states the background processes that they considered, and SM Drell Yan was one of them, so Z->ll ).. the thing is that even if you have several sources fo background, getting a signal so high in energies is quiet rare because at high energies the cross sections become very small.
The problem is that this "presentation" of that event lacks information over the background and nothing is said about the systematic uncertainties.

 

[mfb]

A proton crashed into a proton with a total energy of 2.9 TeV.

No, the total energy was 13 TeV.

A very short time afterwards, detectors detected a electron and a positron.

Plus various other particles, right.

This is interpreted as some particle being created by the proton-proton collision that subsequently decayed into the electron and positron, and possibly some other stuff that was not detected, e.g. a photon.

Well, "particle" - it was probably a virtual photon, not a new heavy particle. If it would have been a new particle, this new particle would probably have a mass of about 2.9 TeV.

Is it possible for a proton-proton collision to directly create a electron-positron pair?

What is "directly"? Quarks and gluons do not couple directly to electrons and positrons, there has to be some particle in between - like a (virtual) photon or a Z boson.

Is it possible for the detector to see a false positive electron-positron pair?

It is possible to mis-identify other particles as electrons or positrons, but the probability is small.

If all of the above are extremely unlikely, then I will then understand that it becomes very likely that some hypothetical particle was created.

No. We know that the existing particles can produce high-energetic electron/positron pairs, and they did so with a rate that we expected.

(in the last page of the slides, it states the background processes that they considered, and SM Drell Yan was one of them, so Z->ll )

This is purely Z->ee. The probability to misidentify tau or muon as electron is negligible.

 

[ChrisVer]

The probability to misidentify tau or muon as electron is negligible.

Is there really any table (if that makes any sense) that contains all those miss-ID probabilities?
I don't know, but I work the other way around (look taus and so take into account backgrounds with electrons and muons as well)...of course there is no need for things to be symmetric : prob[tau_IDas_e] != prob[e_IDas_tau]... judging by how taus or muons are IDed, not only there is no need, but there is no reason to even consider equal probabilities (only that they exist).

 

[ChrisVer]

I am not understanding however, why they used so many backgrounds that contain information missing energy (like Ws, tops) for searches such as Z' (where you don't expect a detected event to correspond to some missing energy)...

 

[mfb]

Is there really any table (if that makes any sense) that contains all those miss-ID probabilities?

They depend on the detector, selection criteria, eta, pt, pileup and probably a few other variables. Hard to make a universal table.
In doubt, you always have to consider the probability that a jet fakes your object, just because jets are so frequent. In the other direction, while other objects can fake jets, you rarely have to care about them.
Electrons and photons look quite similar.
Short-living tau can produce muons or electrons without a notable displacement to identify them - but then those particles usually have lower energy, a pair of taus faking an electron/positron event at 2.9 TeV would be really remarkable.

I am not understanding however, why they used so many backgrounds that contain information missing energy (like Ws, tops) for searches such as Z' (where you don't expect a detected event to correspond to some missing energy)...

I don't know the electron energy resolution, but all the calibration methods work at much lower energy - if the uncertainty is 5%, you can get O(100 GeV) missing ET just from measurement uncertainty in the electron/positron pair.