Is the 750 GeV Diphoton Excess Real? Analyzing the Moriond Results

[mfb]

You wouldn't call it particle then. PDG has resonances with Γ/M>0.3, e. g. h1(1170), or even resonances with no proper width estimate, e. g. a1(1260). Careful angular analysis of three-body decays or direct production is the only chance to see some order in the mess of extremely broad resonances.

 

[Vanadium 50]

Let me to point out also Resonaances entry "the loose-cuts analysis was not approved in time by the collaboration"

And how would Adam know what the experiments have and do not have? "Not approved in time" implies "approved too late" and not simply "not approved".

One of the sadder things about this whole affair is that the experiments have been very careful in specifying what they do and they do not see, and this has been almost completely ignored by the theoretical community. Several of the 280+ preprints state the experiments claim discovery, which is simply false. ATLAS quotes 2 sigma significance, CMS less than 1 sigma. Yes these 280+ papers go on and pretend this is real. (And why this particular low significance excess is launching so many papers while other, more significant excesses don't is mystifying. Well, it would be. You explained it - you think the experiments have something more significant but have "chickened out" and are not telling the truth.

Here are the problems you have to deal with.

• The significances are very low. One and two sigma are nowhere near five.
• ATLAS and CMS see somewhat different masses.
• ATLAS and CMS see somewhat different yields.
• ATLAS and CMS see rather different widths.
• The 8 TeV data does not confirm the 13 TeV data.
• CMS doesn't see the signal in all parts of their detector.

If you want to explain these with statistical fluctuations, why not take the next step and explain the whole shebang as a statistical fluctuation? It's no less likely.

Finally, even if it's real, you won't settle things with twice as much data. You need ten times as much data.

 

[mfb]

The significances are very low. One and two sigma are nowhere near five.

Don't take the LEE twice. This statement would be a proper description if ATLAS would have the excess at 750 GeV and CMS at 1 TeV. I don't think it would have triggered so many theory papers then. The masses are not exactly the same (how could they?), but very close and the yield comparison depends on your favorite theory model as the selection is different. CMS 8+13 TeV quotes 750 GeV, ATLAS quotes 750 GeV, CMS 13 TeV quotes 760 GeV, it looks like all values are rounded to multiples of 10.

ATLAS and CMS see rather different widths.

The narrow width gives a fine fit for ATLAS, so does a slightly larger width for CMS. That difference is way less significant than the excesses.

The 8 TeV data does not confirm the 13 TeV data.

There is a slight excess, not significant on its own but compatible with gg production.

CMS doesn't see the signal in all parts of their detector.

The interpretation depends on your favorite theory model.

Finally, even if it's real, you won't settle things with twice as much data. You need ten times as much data.

To figure out what it is (if it is real): sure. To get highly confident that something is there: I don't think so. If the ~3.5 local significance is the expected strength of an actual signal, twice that data with the same conditions will give an expected local significance of ~4.9 sigma. But you can do better. You can check the various theory papers to improve the selection, you can improve the background suppression and estimation and so on. Pileup will increase a bit, but that should be manageable.

 

[arivero]

You wouldn't call it particle then. PDG has resonances with Γ/M>0.3, e. g. h1(1170), or even resonances with no proper width estimate, e. g. a1(1260). Careful angular analysis of three-body decays or direct production is the only chance to see some order in the mess of extremely broad resonances.

I assume there is some experimental reason, but I was also thinking that if the resonance pole is at M - i Γ with Γ = M then its square is in (M^2 - Γ^2) - 2 i M Γ the pure imaginary line, and that could be theoretically relevant. Also, perhaps the indeterminacy principle in energy-time has some say here, as Γ is inverse lifetime.

 

[bbbl67]

Please note also that I started the thread in the Lounge in order to allow for wide discussion. Of course, with more that 750 papers in the arxiv, almost everybody can quote its favorite model from some arxiv paper :-) I am particularly surprised that R-D gravitons are considered as a major possibility

Can you tell me what R-D gravitons refers to?

 

[arivero]

Can you tell me what R-D gravitons refers to?

Sorry, RS, Randall-Sumdrun

 

[bbbl67]

Sorry, RS, Randall-Sumdrun

Okay, thanks, looking that up now.