# How do we know that the 2012 LHC discovery is SM Higgs Boson and not a pseudo-dilaton?

To be more clear, are there any measurements or analysis done on the new particle to verify if it is SM Higgs boson and not a pseudo-dilaton.

• Yes, hundreds: a large fraction of CMS and Atlas papers do just that. How did you get the impression they did not and are not doing it as we speak? – Cosmas Zachos Oct 16 '17 at 16:46
• @CosmasZachos Your comment is a bit mean: as pointed out in my answer, John Ellis himself deemed it worth his time to address this question. The style of the question, especially the "or something completely new", is not great, agreed, but it is healthy to ask such questions and the guy is for sure not a pro, so he can't know. – user154997 Oct 20 '17 at 10:03
• @Luc Perhaps lacking in deliberative indulgence. For half a decade since the discovery of the Higgs, sustainable BSM facts have failed to surface. Has this basic fact languished unreported in the press, sending questions about it to this site? – Cosmas Zachos Oct 20 '17 at 13:32
• @Cosmas You can't expect a recent graduate to keep upto date with half a decades research work of a recent particle discovery. Also, I find it extremely hard to find resources addressing this issue without being surrounded by a sea of Jargon. Also, stack exchange provides a platform for amateurs like me to learn from experts like you guys, in order to alleviate the steep learning curve I face right now. – Invariance Oct 20 '17 at 14:28
• Well, the canonical PDG summary is not that technical: it is designed for the broadest audience. There is no hard evidence for pseudodilatons. – Cosmas Zachos Oct 20 '17 at 14:53

Interpreting pseudo-dilaton with its usual meaning in this context, I can add some information to what John Rennie rightly stated. John Ellis wrote a very nice summary [1] which dates a bit (2013) but already back then, the pseudo-dilaton model was strongly disfavoured, as detailed in section 2.2.3, based on a previous work of his [2]. His conclusion is that the pseudo-dilaton is compatible with experimental data if and only if it is fine-tuned to mimic the Standard Model exactly, i.e. the dilaton v.e.v $V$ which breaks scale invariance has to be nearly equal to the v.e.v. $v$ of the Higgs: see the two plots on Fig. 19, showing that $v/V$ has to be peaked near 1. He concludes with humour that

Clearly, there is no evidence for any significant deviation from the Standard Model, and Peter Higgs may continue to smile!

[1] Ellis, John, Higgs Physics, arXiv:1312.5672

[2] J. Ellis and T. You, JHEP 1306 (2013) 103 arXiv:1303.3879.

With any new particle we look at the things we can measure, e.g. spin, charge and decay channels, and compare them to the properties predicted for the particle from the Standard Model.

In this case we find the measurements all match the properties the Standard Model predicts for the Higgs boson. That doesn't prove it's really the Higgs boson since there could be another particle with identical values for the properties we have measured but different values for properties we can't measure yet. But we already have enough data that would make this one hell of a coincidence, so it seems pretty unlikely.