Higgsino at LHC, could it be seen?

[skydivephil]

If there is super symmetric partner of the Higgs could it in principle be dectected at the lHc? if so what sort of decay products would be seen?

 

[copernicus1]

As far as I know, no one knows what the masses of the supersymmetric partner particles would be (just larger than the scales we've probed so far), so we can't really say whether they could be detected at the LHC

 

[mfb]

There are searches for Higgsinos and other exotic Higgs-like particles. It depends on the model if they are visible in current (or future) data, but with the right parameters this is certainly possible.

 

[Bill_K]

There are searches for Higgsinos and other exotic Higgs-like particles.

I'm confused! The Higgsino, if it exists, is not at all Higgs-like. For one thing, it's a fermion.

I don't even get the point of this thread. Why the Higgsino in particular? The common assumption is that the four supersymmetric partners - Higgsino, gluino and both gauginos, mix together forming a state called the neutralino.

 

[fzero]

I gave a very sketchy argument in https://www.physicsforums.com/showpost.php?p=3938801&postcount=3 that, even if the neutralino mixing is such that there are a pair of "mostly higgsinos," it would be a rather unusual to have them be amongst the lightest SUSY particles. The argument is sketchy for many reasons, primarily because SUSY has obviously not been detected, so any speculation about parameters is just that, speculation. If SUSY is detected at the LHC, it's likely that some other particle would be observed first.

 

[skydivephil]

The point of the thread is my own curiosity. So are we saying that they could only detect signs of SUSY and not specifically signs of the Higgsino? If so what would the signature look like?

 

[mfb]

I'm confused! The Higgsino, if it exists, is not at all Higgs-like. For one thing, it's a fermion.

Similar mass (if it can be detected), no electric charge, related to EWSB. Similar enough for me.

Anyway, here are two experimental searches:
ATLAS
D0

 

[Bill_K]

Similar mass (if it can be detected), no electric charge, related to EWSB. Similar enough for me.

How is the Higgsino related to electroweak symmetry breaking?

 

[mfb]

Is it not?
I don't think SUSY is realized (at least not at the TeV-scale) anyway, I don't know details about the Higgs sector in SUSY.

 

[lpetrich]

First, let's review some basic features of supersymmetry. It is an extension of space-time symmetries, and like those symmetries, it commutes with all other symmetries and operators. This commuting is from the Coleman–Mandula theorem for space-time and the Haag-Lopuszanski-Sohnius theorem for space-time with SUSY. In fact, in unbroken SUSY, all members of a supermultiplet have the same mass.

In the usual scenarios, SUSY breaking creates some mass terms and certain interaction terms, but nothing that alters the overall features of the SUSY partners' interactions.

So a higgsino will have Higgs-like interactions: electroweak ones with the photon, W, and Z, and Yukawa ones with the elementary fermions in proportion to those particles' masses. Likewise for other SUSY partners.


In the Minimal Supersymmetric Standard Model, there is not one Higgs multiplet, as in the Standard Model, but two. Here is the multiplet structure of the particles in the MSSM, as QCD, weak isospin, weak hypercharge:

Gauge: gluon (8,1,0), W: (1,3,0), B: (1,1,0)

Left-handed EF's: quark: (3,2,1/6), lepton: (1,2,-1/2)
Antiparticles: right-handed: (3*,2,-1/6), (1,2,1/2)
Right-handed EF's: up: (3,1,2/3), down: (3,1,-1/3), neutrino: (1,1,0), electron: (1,1,-1)
Antiparticles: left-handed: (3*,1,-2/3), (3*,1,1/3), (1,1,0), (1,1,1)

Higgs: up (1,1,1/2), down: (1,1,-1/2) - higgsinos left-handed
Antiparticles: (1,1,-1/2), (1,1,1/12) - right-handed

It's necessary to have 2 Higgs multiplets in the MSSM, because SUSY Yukawa interactions require the same chirality of fermion field.

Electroweak symmetry breaking makes three neutral Higgs particles and a charged one, with both +1 and -1 charges. The three neutral ones are two CP-even ones and one CP-odd one. One of the two CP-even ones is a light one that closely resembles the Standard-Model Higgs particle.

Their fermionic partners mix with the electroweak gauginos to make four neutralinos and two charginos (both charges).

In an extension called the NMSSM, there is an additional pair of neutral Higgs particles, one CP-even and one CP-odd, and one additional neutralino.

 

[Bill_K]

So a higgsino will have Higgs-like interactions: electroweak ones with the photon, W, and Z, and Yukawa ones with the elementary fermions in proportion to those particles' masses. Likewise for other SUSY partners.

Sorry, a spin-1/2 Higgsino cannot mediate a Yukawa interaction between leptons. All of these interaction terms are supersymmetric, which means for example that the Higgsino instead couples a lepton to a slepton. And likewise while a Higgs can decay into two photons, the corresponding Higgsino decay would involve instead a photino. So while from the SUSY point of view these properties are "the same", in practice they are quite different.

 

[lpetrich]

Now the Large Hadron Collider. With the energies that it accelerates protons and nuclei to, it's essentially a light-quark-and-gluon collider. So let's see how these particles interact.

Their strongest one is QCD, and so it will make mostly particles with QCD interactions. These include the SUSY partners of the quarks and gluon, the squarks and gluino. They have not been observed, and their lowest possible mass is thus about 1 TeV.

Electroweak interactions are next, with electromagnetism and the weak interaction unified at LHC energies.

Higgs-particle interactions are proportional to the masses of the particles that they interact with. They thus interact much more strongly with heavy quarks than light quarks, and they interact fairly strongly with the W and Z, and not at all with the photon and gluon.

So it'll be much harder to find evidence of neutralinos and charginos than of squarks and gluinos. Comparable with the difficulty in finding the Higgs particle.

 

[Bill_K]

So it'll be much harder to find evidence of neutralinos and charginos than of squarks and gluinos. Comparable with the difficulty in finding the Higgs particle.

Even though squarks and gluinos may be more easily produced, they'll promptly decay into the lightest supersymmetric partners, namely the neutralinos.