Thursday, June 08, 2017

ALEPH at LEP strangely "discovers" new 25 GeV, 55 GeV, 80 GeV particles in four-jets

A different paper on condensed-matter and particle physics interface: Zhao and Liu argue that SUSY doesn't emerge at critical points as proposed by several others
Once upon a time, there was no LHC collider in the LHC tunnel. Instead, LEP, the large electron-positron collider, was thriving over there. It had several runs and the LEP2 run is what we will talk about.

Just like LHC has ATLAS and CMS and the Tevatron had D0 and CDF, LEP had several detectors: ALEPH, DELPHI, OPAL, and L3. We will talk about ALEPH. You must have heard me as saying that it's sometimes bizarre how much time it takes to the LHC experimenters to analyze their data. Isn't is weird that we're still getting new papers based on the 2012 data?

Well, the LEP collider was dismantled around 2001 and only historians focusing on the Holy Roman Empire remember it very well. (OK, that was an exaggeration.) In spite of that, Julian (unaffiliated) and Jennifer (CFTP Lisbon; just two names is unusual in the experimental particle physics run by big teams these days) released a fun article today:
Localized 4σ and 5σ Dijet Mass Excesses in ALEPH LEP2 Four-Jet Events
What did they discover in the dataset that is older than 16 years?

Well, they looked at the four-jet collisions – note that jets were a bit scarcer at LEP because it was leptons, and not hadrons, that were colliding. They sorted the four jets in each collision to minimize something. And with these conventions, they claim that there is a significant excess that is comparable to 5 sigma before the look-elsewhere effect is taken into account. There are actually two excesses, one that is slightly below 5 sigma and the other that is above 5 sigma.

OK, they divide the four jets to two pairs of jets (dijets) and evaluate the invariant masses \(M_1,M_2\) of these two dijets. At least I sincerely hope that they're the invariant masses because this adjective doesn't seem to be mentioned in the paper.

And it turns out that both excesses appear when\[

M_1+M_2 = 110\GeV

\] and they may be found at preferred separate values of \(M_1,M_2\), namely\[

(M_1,M_2) = (80\GeV,25\GeV)\\
(M_1,M_2) = (55\GeV,55\GeV)

\] Well, \(80+25=105\) and not \(110\) but who cares. Maybe it should be \(30\) and not \(25\).

So these excesses suggest new particles of masses \(25\GeV\) and \(55\GeV\). \(80\GeV\) could be new or the W-boson or Z-boson. OK, the \(25\GeV\) particle is suggested to be "another neutral Higgs boson" while the \(55\GeV\) particle could be a charged Higgs boson.

There are several aspects of the paper that reduce my faith. One of them is the number of authors which is two and it is a low number. Another one is the enthusiasm with which they talk about \(M_1+M_2\) which should be \(110\GeV\). As far as I an say, it makes no sense to talk about the sum of two invariant masses. You know, only the total mass/energy \(p^0_1+p^0_2\) is conserved, the sum of invariant masses isn't. The quantity \(p^0_1+p^0_2\) is something else than \(M_1+M_2\); instead, it is the invariant mass of all four jets.

So if these excesses are real, there have to exist new particles of masses \(25\GeV\) and/or \(55\GeV\). And once several new particles like that exist, there is no reason for the sums of invariant masses in this list (such as \(25+80\) and \(55+55\)) to be equal to each other. Well, I have mentioned that they're not really equal to each other – one of the sums is \(105\) and the other one is \(110\GeV\). However, what I am bothered by is the very fact that they would focus on the value of this quantity or sell it in the final paper even though the quantity \(M_1+M_2\) only has relevance for the numerologists.

If the authors were two numerologists, there would probably be lots of things that they could do incorrectly. I am worried about their "sorting of the jets". The procedure only makes sense if they carefully calculate the theoretical predictions assuming the same sorting. I don't want to accuse them of anything but I am not sure whether they have done so. If they haven't, there could be spurious deviations sitting especially near the "equal division of \(110\GeV\) to two equal parts" (yes, there is an excess there) and perhaps some other critical places.

I would predict that there would be more serious "strange" things in the paper than the enthusiasm about \(M_1+M_2\). But of course, particle physics may be lucky and I may be wrong. There are reasons to think that new particles of masses \(25\GeV\) or \(55\GeV\) could agree with some astrophysical hints of dark matter. So I am going to keep the status of these two "discoveries" as "to be decided".

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