Heaviest particle detected so far

[Floyd_13]

TL;DR Summary

Upper limit for the mass of new particles at the LHC

I'm reading this article on Dark Matter and at some point the authors say '' if the LKP [Lightest Kalusa-Klein Particle] is to account for the observed quantity of dark matter, its mass [...] should lie in the range of 400 to 1200 GeV, well above any current experimental constraint.''

My questions are the following:
1. What is the heaviest particle that has been detected so far at the LCH? Is it the top quark at 172Gev?
2. What is the upper limit for the mass of a new particle that could be detected at the LHC based on the current available energy?

 

[mfb]

The heaviest known particle is the top quark. It's seen routinely at the LHC.

The collision energy is 13 TeV, so obviously you can't produce anything beyond that, but most limits are much lower. LHC collisions can be described as parton collisions: Individual quarks and gluons collide while the rest of the protons doesn't do much. The energy carried by these colliding partons varies: The larger the required energy the less common these collisions are. That means the collisions produce tons of light particles like charm and bottom quarks, while top quarks are much less common, and hypothetical 400 GeV particles would be even less common. To make things worse most hypothetical particles (just like most known particles) carry some conserved quantum number, so we expect that we need to produce them in pairs: You need at least 800 GeV to produce a 400 GeV particle and its antiparticle. Combine that with the challenge to distinguish such a process from background processes.

The LHC experiments have published hundreds of exclusion limits for all sorts of different models, you can check the publications. Supersymmetry exclusion limits tend to be in the low hundreds of GeV range while exclusion limits for some things like gravitons can be higher than 1 TeV. The highest exclusion limits (close to the collision energy) can be set on black holes because we would expect them to have a large cross section and an unmistakable decay signature.

 

[ChrisVer]

My questions are the following:
1. What is the heaviest particle that has been detected so far at the LCH? Is it the top quark at 172Gev?
2. What is the upper limit for the mass of a new particle that could be detected at the LHC based on the current available energy?

1. The heaviest particle is the top quark yes. It's followed by the Higgs boson and then the W/Z bosons...

2. As already mentioned, the energy at which protons collide is at 13TeV. Of course, it's not the protons that interact during a collision, but their quarks or gluons (partons), which carry just a fraction of that energy. So if you have a parton of proton #1 carrying $x_1$ of the proton's momentum, and a parton of proton #2 carrying $x_2$ of the proton's momentum, then the available energy they will have at the collision will be: $(x_1+ x_2) E$ , where $E= E_1 = E_2$ the energy of the protons. As $0<x_1,x_2<1$ (they fractions), then you can't really produce new particles with masses above 13TeV (and getting very close to 13TeV is impossible for many cases). The only "exclusions" you can place above that threshold are exclusions in indirect searches (such as the contact interactions, where you set limits on the new physics' scale that would potentially change the distributions that you can observe).

Please note that the article you are reading is a bit old. Of course this is up to the model parameters but for KK excitations, the limits set by LHC are pretty high, as you can see e.g. in the top figure here:
https://twiki.cern.ch/twiki/pub/CMSPublic/SummaryPlotsEXO13TeV
just from CMS, many KK particles are already excluded in the range you mention, with a small window probably for the RS GKK in gg+qq of 400-500GeV. But that is covered by the diphoton exclusion, so the model would have to be "photophobic".

 

[Floyd_13]

Thank you both for your answers.

@ChrisVer can you please point out any references for the new limits for KK particles? I don't seem to have access to the plot you mention.

 

[ChrisVer]

Can you try this?
https://twiki.cern.ch/twiki/bin/view/CMSPublic/SummaryPlotsEXO13TeV
I don't know why my other link doesn't work (and it's not private). Nevertheless, to add something new to the thread, you can also have a look at the same "plots" from ATLAS:
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PUBNOTES/ATL-PHYS-PUB-2020-011/

On the plot, depending on your "desired" final state, you can also see the arxiv id and get the relevant searches...