# What is meant by the breakdown of unitarity at TeV-scale

1. Sep 17, 2008

### guimauve

I read a statement that there is a breakdown of unitarity in the Standard Model at 1 TeV. What exactly is meant by that?

2. Sep 17, 2008

http://en.wikipedia.org/wiki/Unitarity_(physics [Broken])
I couldn't get my head round this, might try later. Some one else can probaly explain it to you if you don't understand it either.

*Edit*
Just realised that even if you know what Unitarity is, you might not under stand what it means when it breaks down.

Last edited by a moderator: May 3, 2017
3. Sep 17, 2008

### JustinLevy

If the time evolution isn't described with unitary operators, then probability isn't conserved and quantum mechanics breaks down.

As for breakdown at TeV-scale ... I don't see why this would be. Can you site the paper? Is it in the context of what it would mean if no Higgs-like mechanism was found?

4. Sep 17, 2008

### guimauve

https://www.physicsforums.com/archive/index.php/t-187870.html

"If one looks at WW scattering, it is pretty easy to see that the process scales as energy squared (sometimes called s). This means that as energy increases, the probability for WW to scatter increases. At some point, this probability is greater than one, which doesn not make sense. ALL of quantum mechanics is based on probabilities being less than or equal to one.

The energy scale where WW scattering breaks unitarity is at 1 TeV. This means that we HAVE to see something happening at this energy scale, or our theory is not unitary, which would be an absolute disaster. SOME new physics (maybe just a single higgs) MUST come in to save unitarity."

So this would be the context of no Higgs below 1 TeV which means no SM Higgs.

5. Sep 17, 2008

### humanino

No. You need the Higgs mass to be less than approximately 1 TeV in the SM. Otherwise, its width becomes too large compared to its mass. But that's definitely the SM Higgs.

6. Sep 18, 2008

### guimauve

@humanino

Why does the width become too large compared to it's mass?

7. Sep 18, 2008

### humanino

Take a look at Fig4. It's log-log but the scales are different : the width blows up.