Friday, March 08, 2013

ATLAS: 3-sigma excess of \(420\GeV\) type III seesaw heavy fermions

I am carefully following all the new preprints by ATLAS and CMS that are currently being presented at the Moriond 2013 conference so that you don't have to. So far, everything is compatible with the Standard Model including the \(126\GeV\) Higgs boson and the latter beast is still behaving as obediently as the Standard Model assumes. If something changes about these statements, you will probably learn about it on this blog almost instantly.

However, there's an interesting 3-sigma anomaly in an otherwise obscure search so let me tell you what it is.

It appears in the following preprint:
Search for Type III Seesaw Model Heavy Fermions in Events with Four Charged Leptons using \(5.8\,{\rm fb}^{-1}\) of \(\sqrt{s} = 8\TeV\) data with the ATLAS Detector (ATLAS-CONF-2013-019)
It is a relatively obscure search for an electroweak triplet of new fermions, \(N^\pm,N^0\), that are used in the so-called type III seesaw models. Note that all seesaw models are meant to produce the neutrino masses (equal to zero in the "truly minimal" Standard Model) – and explain why they're so small.

The type I seesaw models add at least two right-handed neutrinos \(\nu_R\) with masses near the GUT scale. The type II seesaw models add a new Higgs triplet. The type III seesaw models add the triplet of fermions \(N^\pm,N^0\) that are approximately equally heavy. It is supposed that the proton-proton collisions may produce either \(N^\pm N^\mp\) or \(N^\pm N^0\) where the latter possibilities are approximately 2 times more likely than the former possibility.

As you may expect, the ATLAS folks exclude the existence of these new fermions \(N^\pm,N^0\) up to some mass, namely \(245\GeV\). But there's an interesting 3-sigma excess near the (higher) mass \(m_N\sim 420\GeV\): its \(p\)-value (probability of a similarly strong signal according to the null hypothesis) is about \(p_0=0.20\), a statement whose origin I don't quite understand. I would understand \(0.20\%\) but maybe their figure is right and unimpressive due to some look-elsewhere reduction.

At any rate, the picture (Figure 4) says a clear story of a rather strong excess by itself:

Click to zoom in.

On the \(x\)-axis, you have the assumed mass of the new fermions, \(m_N\), in the units of \(\GeV\). The \(y\)-axis contains the relevant cross section


\frac{\sigma(pp\!\to\! N^\pm N^0)\times BF(N^\pm \!\to\! Z \ell^\pm)\times BF(N^0\!\to\! W^\pm \ell^\mp) }{\rm fb}

\] In other words, it's some cross section (in the units of one femtobarn) for the production of a pair of the new fermions (one neutral fermion and one charged fermions) using a proton pair but only the "branching fractions" in which these new fermions decay to \(W^\pm/Z^0\) gauge bosons plus leptons in the indicated way (pretty much the dominant decays expected for the new fermions) are included.

The decays of these hypothetical new fermionic triplets violate the lepton flavor if not the lepton number. They can probably achieve what they can achieve – the neutrino masses – but I haven't encountered them anywhere else. In particular, I am not aware of any top-down explanation why these things should exist. But of course, it's not impossible that these otherwise unwanted beasts are employed by Mother Nature.

It's more likely that the excess is a fluke. But even if it is due to new physics, I suspect that the details of the new physics could be a bit different (sleptons and sneutrinos of some kind?). This particular paper has only used \(5.8/{\rm fb}\) of the 2012 data. Over twenty inverse femtobarns have (already) been collected last year so when they're processed, the signals – if they're due to new physics – should grow to indisputable proportions.

TBBT and women in science

Last night, the latest episode of The Big Bang Theory made Leonard want to help young women enter science. Sheldon and Howard ultimately agreed to co-operate. They went to a high school to meet girls and the sitcom showed a very realistic picture of how hopeless disinterest most of the girls of this age have in science and how complete hypocritical waste of time similar attempts to "draft girls" are.

New Czech president

Miloš Zeman was inaugurated as the new Czech president. Lots of fun formalities at the Prague Castle and the cathedral over there. His inauguration speech was given off-hand, rather impressive. Among other things, he declared war against three main enemies of the society – mafia's godfathers, neo-Nazi guerrilla groups, and most of the journalists. ;-) The latter group (Zeman's comment about this group was the only thing that excited an applause among the audience dominated by top politicians) is composed of jealous and stupid individuals who love to criticize people for doing something they can't do at all and who love to brainwash the citizens. Fully agreed.

There were things I disagreed with, too. He rewrote the history when he presented Masaryk as the guy who wanted to eliminated all traces of monarchy and introduced pure republicanism. That's rubbish. Masaryk deliberately preserved some of the royal functions and image of the kings for the Czechoslovak presidents. At any rate, Zeman surrendered his right to declare amnesties and pardons (that's like not doing a part of his job!) and promised to be an intermediary of a political dialogue, not a judge.


  1. Lubos, a question just to be sure: ATLAS hasn't found any boson disintegrating into invisible particles right ?

  2. Dear Shannon, I would love to know! ;-)

    But if this bump is due to new physics, the new particle does decay to final products involving the invisible neutrino - because the neutrino comes from the W boson, one of the decay products of N.

    But N is a fermion. Do you have a reason to think that bosons can't disintegrate into invisible particles? Well, that can't work. The Z-bosons do often decay to neutrino pairs, in fact, in 20% of cases.

  3. I would love it to happen ! :-) But isn't it forbidden by the SM ? I've read that Atlas has recently shown that 68% of the time the boson *could* disintegrate into invisible particles... but this hasn't been observed so far...

  4. Hi Lubos

    Maybe you can answer this for me. Isn't the seesaw model supposed to have heavy neutrinos? Are these three new fermions the heavy neutrinos with two of them having a charge, are they separate fermions from the heavy neutrinos, or some kind of mixture with the heavy fermions and some other fermions? Also, is there a sterile neutrino involved and does this model fix the heirachy problem. I understand if you are too busy to answer. Thanks

  5. Hi, actually wrote the answer already but again. Only type I seesaw models have heavy neutrinos. We're talking about type III. Look for the basic descriptions above.

  6. Figure 2 in the paper suggests this excess is due to a single event. Whilst it is quite far out along the tail it doesn't look very convincing to me. As you stated, the analysis of the full dataset will clear it up either way.

  7. Lubos, it is just one event that drives that fluctuation. Hardly anything to get excited about.

  8. That's interesting that you and Dr Ancu posted this highly important comment just minutes after one another and as far as my DISQUS moderation queue may say, independently.

    Well, one event, when unlikely enough, may prove a lot. Think about one pregnant Virgin Mary if she existed or one Jesus walking on the pond, or something like that. ;-)

  9. I don't know about Dr Ancu but I am not at Moriond. Indeed the birth of 'Our Savior' was/is an unlikely event. In both cases, I'll await more data..

  10. LOL. But that's not because the "event" was just one. The Bible is full of dozens of people who lived for 1,000 years - dozens of events - but I'd await more data on this one, too. ;-)

    It's the probabilities, not the integer-valued numbers of events, that matter for the scientist's belief. So even one event may be enough - and was often enough in the history of science - and thousands of events may be too little when the background is too high etc.