Friday, October 21, 2016

A bump at LEP near \(30\GeV\): weak but possibly justifiable

Update: This article was mentioned at The Wire in India. I believe it's a nice article except that, as Tommaso Dorigo pointed out, has some wrong numbers translating 3 sigma to probabilities etc.

In the morning, I was intrigued by a hep-ex paper by Arno Heister
Observation of an excess at \(30\GeV\) in the opposite sign di-muon spectra of \(Z\to b\bar b+X\) events recorded by the ALEPH experiment at LEP
To make the story short, he claims that the 1992-1995 data from the LEP (Large Electron-Positron) Collider at CERN contains a less-than-3-sigma bump at \(M_{\mu^+\mu^-}\sim 30.4\GeV\) indicating a boson of width \(1.8\GeV\).

Recall that the LHC is located in a tunnel that was the largest European infrastructure project before it was surpassed by the Channel Tunnel. But the LHC isn't the first collider that has lived or lives in the LHC tunnel. Before it was born, the LEP collider – that used to collide electrons with positrons – was happily living there.

The song above shows what LEP looked like to the horny girlfriends of (male and female) particle physicists. However, if you dared to claim that there was no LHC in the tunnel, you would be wrong. The very song was sung (in 2000) by the LHC, the Les Horribles Cernettes. ;-)

I was hesitating whether the bump deserves a blog post but I decided that the answer is Yes after I saw Matt Strassler's text
A Hidden Gem At An Old Experiment?
minutes ago. Strassler starts by saying that while he admires the intelligence of a physicist, he or she is completely wrong when she says that the nightmare scenario has come true. After all, Strassler points out, there can't be any nightmare because it's not even a nighttime yet.

When he gets to the technicalities, it becomes clear that the events pointed out by Arno Heister are events that contain a muon pair, \(\mu^+\mu^-\), or an electron-positron pair, \(e^+ e^-\), along with the bottom quark-antiquark pair, \(b\bar b\). The invariant mass of the leptons \(\ell^+\ell^-\) is measured and the bump is found near \(30\GeV\), around one-third of the Z-boson mass.

But on top of the lepton pair, the events are requested to contain the bottom quarks. So the idea is that you can't see these new hypothetical bosons decaying to the lepton pair in isolation. You may only see them along with another new particle that decays to the quark-antiquark pair.

Matt Strassler says (and said a decade ago) that this is rather natural in the "hidden valley" models. Those may predict a new spin-one (vector) boson \(V\), with the mass of \(30\GeV\) if you want to explain these events; along with a new spin-zero (scalar) boson \(S\) which decays to \(b\bar b\) and sometimes perhaps to \(\tau^+\tau^-\).

Both new particles \(V\) and \(S\) may arise from the decay of\[

Z \to V + S

\] the good old Z-boson whose mass is \(91\GeV\). So in the electron-positron collisions, the Z-boson could have been created. This Z-boson could have decayed to two (new) bosons \(V,S\) and those decayed to the lepton pair and the quark-antiquark pair, respectively.

However, I find the whole class of "hidden valley" models rather unmotivated. Just to be sure, it's not wrong or inconsistent – like loop quantum gravity or other piece of debunkable garbage that I have repeatedly written about. But I don't feel that they're needed for anything, that there really exists evidence that they should be right. There's no strong evidence that they shouldn't exist, either. But if you like to use some Occam's razor, you could think that there is a good reason – the razor – to prefer the Standard Model (or MSSM) over the "hidden valley" models. On the other hand, razors may be dangerous. You may cut your throat if you use them unwisely.

The "hidden valleys" may occur but if they do, one may ask: Who ordered them?

It's actually hard to extract a reasonable estimate from Matt Strassler of the probability that the "hidden valley" models are relevant for Nature – or even relevant for those particular bumps at ALEPH (one of the detectors at LEP, i.e. a counterpart of ATLAS and CMS at the LHC) that are being talked about here. Why?

Because Matt Strassler is, along with Kathryn Zurek and two or so less important collaborators, a co-father of the "hidden valleys". See Strassler-Zurek 2006. In fact, the first three references in Heister's paper are papers written or co-written by Matt Strassler. Of course, a blogger's proposed set of new particles may be real. But worries about the blogger's impartiality may potentially be justified, too.

OK, can't the existence of the new \(30\GeV\) boson be settled by the LHC?

The theme song from "Friends of the Green Valley", a Czechoslovak communist-era soap opera about kids (of my age, at that time) who befriend wildlife thanks to a gamekeeper. Just to be sure, the theme song in Czech is sung by a non-musician actor who is Slovak, to make things worse.

It seems to me that Strassler basically says that the number of collisions at the LHC is more than enough to do so but the 6,000 LHC experimenters haven't actually done a search for the channel equivalent to Heister's paper – or many other searches for the "hidden valleys" that should be done. I am somewhat surprised by this claim but it's plausible.

It would surely be exciting in the medium term if this new physics were found. However, in the long run, I wouldn't know what to do with that. I think that the effective field theory describing all of our experiments in Nature would get "more messy" and "more urgently waiting for answers", not less so. The "hidden valleys" could have far-reaching consequences for our guesses about the shape of compactified dimensions in string theory and related high-energy issues. But I have some worries that the anthropic hand-waving and "typicality" could be our best explanation why the hidden valleys exist at all and why they have the rough properties they have. That could amplify some features of the status quo that some people find frustrating.

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