What Could Be Causing the 2 TeV Bump in Symmetry Magazine?

[edguy99]

An article in Symmetry magazine today suggests the possibility of a new particle at 2 TeV.

http://www.symmetrymagazine.org/article/july-2015/something-goes-bump-in-the-data

It also contains the quote: "An extended Standard Model predicts comparable particles at higher energies, heavier versions known as W prime and Z prime (or W’ and Z’). Several theorists suggest the bump at 2 TeV could be a type of W prime."

The article does not give any other suggestions of what it could be. Is there something in the standard model that prevents a heavier quark or lepton from being discovered? ie. Is there an existing theory or theorist predicting a 4th quark?

 

[fzero]

I don't have any personal insight, but would suggest that you read Jester's take on the situation: http://resonaances.blogspot.com/2015/06/on-lhc-diboson-excess.html Tl,dr: It is too early to tell, wait for early run 2 data.

 

[ChrisVer]

There are searches for a 4th generation of quarks... and theories that suggest them.

The 2TeV bump at the moment can be a new signal or it might be just statistical fluctuations, since the statistics are pretty small. It is however interesting because it appears in both ATLAS and CMS at ~ the same region (that's why they have to look deeper into that).

I am not sure why they'd suggest the W' or the Z' , except for if the looked channel suggests it's those particles that decay into those products.

 

[Vanadium 50]

The ATLAS paper was very careful not to state that this was a new particle, or even that it is a hint of one. The ATLAS and CMS data are also not completely consistent in mass and rate. Also, if this is a real particle, it decays weakly - to W's and Z's - but is produced with a cross-section 30-50x larger than weak production. The story that this is a new particle doesn't really hang together.

 

[ChrisVer]

Well the mass ranges seem pretty consistent (judging the lack of more data)... I don't understand how you saw the rate difference?

Even though this bump is far too small to signify a discovery and presents no predictable pattern, its presence across multiple different analyses from both CMS and ATLAS is intriguing and suspicious.

I'd stick with it...

 

[RGevo]

If it decays to SM W and Z bosons, and is only seen hadronically and not into leptons. CIt can be categorically ruled out as evidence for new physics. These searches use jet substructure techniques which I do not believe are fully understood.

 

[mfb]

We know how SM Z and W decay. There is no way to have a decay chain via those particles that does not lead to leptonic decays (more than observed). You would need at least two new exotic particles, one at 2 TeV and one close to the SM W and Z. That sounds very unlikely, and then there are also all the other issues mentioned.

For 2 TeV, I guess one or two inverse femtobarn 13 TeV data should be sufficient to get the same sample size. At the end of this year the dataset should be sufficient to completely rule out that excess.

 

[Vanadium 50]

Well the mass ranges seem pretty consistent (judging the lack of more data).

The ATLAS bump is at 2 TeV, and for CMS, it's more like 1.8. The ATLAS bump is much larger than what CMS sees, and it is not very consistent with their leptonic decay data (as mfb points out). The quoted significance is 2.5 sigma, or about 1/160. ATLAS has 400 papers. So you expect a handful of fluctuations about this size.

 

[ChrisVer]

I haven't looked the leptonic data.
The (stat) significance of the figure is ~3sigma. Small but I never said a particle was discovered.
Isn't the CMS bump bigger?

My problem is that ATLAS gives the plot in events/100GeV ... while CMS gives the cross section? If I'd use the luminosity quoted, $\Delta N_{peak-vs-bckg} = (\sigma_{peak}-\sigma_{bckg}) \times L$ I found a larger peak for CMS (~400events) than ATLAS (which was ~10 events)

 

[mfb]

You have to take the trigger and selection efficiency and the G, W and Z branching fractions into account for ATLAS to compare it with a production cross-section.
Also, the CMS plot has upper limits instead of measured cross-sections.

 

[ChrisVer]

I also think I did a mistake... the ATLAS plot is events/100GeV... if we are at 2TeV, then the ATLAS one is ~10*(2000/100)=200 events...
Using the branching fraction would mean that I'd turn ATLAS plot into a similar to the CMS? (from the #events counted to the cross section)

I also didn't know the CMS plot was showing upper limits. Maybe I misinterpreted the C.L. obs vs bckg with the actual cross section? It's giving the cross section at 95% Confidence Level?

Final note: up to now I don't think these are published yet? Because I looked for the July publications of ATLAS and I didn't find anything.

 

[mfb]

No, the bins have a width of 100 GeV, so events/(100GeV) is the same as events per bin. And there is only one bin with an excess.

Publications don't matter much in experimental high-energy physics. Something that appears on arXiv or gets shown on conferences got a much better (internal) peer-review than journals do.