Tuesday, July 22, 2014

CMS: a \(2.1\TeV\) right-handed \(W_R^\pm\)-boson

Since the beginning of this month, the ATLAS and CMS collaborations have reported several intriguing excesses such as the apparent enhancement of the \(W^+W^-\) cross section (which may be due to some large logarithms neglected by theorists, as a recent paper indicated), a flavor-violating Higgs decay, leptoquarks, and a higgsino excess, among others.

Bizarrely enough, all of us missed another, 2.8-sigma excess exactly one week ago:
CMS: Search for heavy neutrinos and \(W^\pm_R\) bosons with right-handed couplings in proton-proton collisions at \(\sqrt{s} = 8 \TeV\) (arXiv)
The ordinary \(W^\pm\)-bosons only interact with the left-handed component of the electron, muon, and tau, because only those transform nontrivially (as a doublet) under the relevant \(SU(2)_W\) part of the electroweak gauge group.

However, there exist models of new physics where this left-right asymmetry is fundamentally "repaired" at higher energies – and its apparent breakdown at accessible energies is due to some spontaneous symmetry breaking.

The CMS search assumed a new spontaneously broken non-Abelian gauge group with a gauge boson \(W^\pm_R\). Under this gauge group, the right-handed electron and muon may transform nontrivially and it doesn't create too much havoc at accessible energies as long as the gauge boson \(W^\pm_R\) is very heavy.

In the search, one assumes that the \(W^\pm_R\) boson is created by the proton-proton collisions and decays\[

pp\to W^\pm_R \to \ell_1^\pm N_\ell \to \dots

\] to a charged lepton and a (new) right-handed neutrino. The latter hypothetical particle is also in the multi-\({\rm TeV}\) range and it decays to another charged lepton along with a new but virtual (therefore the asterisk) \(W^\pm_R\) bosons, so the chain above continues as\[

\dots \to \ell_1 \ell_2 W_R^* \to \ell_1\ell_2 q\bar q

\] where the final step indicates the decay of the virtual \(W_R^*\) boson to a quark-antiquark pair. Great. They have to look for events with two charged leptons and two jets.

So the CMS folks have made a search and wrote that there is nothing interesting to be seen over there. They may obliterate the proposals of new physics (of right-handed couplings of new gauge bosons) more lethally than anyone before them, they boast, and the exclusion zone for the \(W_R^\pm\) goes as high as \(3\TeV\).

However, under this boasting about exclusion, there is a "detail" that isn't advertised too much. Look at the exclusion graph above. You must have seen many graphs of this kind. On the \(x\)-axis, you see a parameter labeling the hypothesis about new physics – in this case, it's the mass of the \(W^\pm_R\)-boson. The right-handed neutrino is assumed to have mass \(m_N=m_{W(R)}/2\).

On the \(y\)-axis, you see the number of \(\ell\ell q\bar q\) events that look like if they originated from the new particle decaying as indicated above. If there is no new physics, the expected or predicted number of events (the "background", i.e. boring events without new physics that imitate new physics) is captured by the dotted line plus minus the green and yellow (1-sigma and 2-sigma) band. The actual number of measured events is depicted by the full black line.

If there is no new physics, the wiggly black line is expected to fluctuate with the Brazil band 95% of the time. The red strip shows the prediction assuming that there is new physics – in this case, new \(W^\pm_R\)-bosons that are coupled as strongly as the known \(W_L^\pm\)-bosons.

The wiggly black curve (observation) never gets close to the red strip. However, you may see that the wiggly black curve violates the Brazil band. If the wiggly curve were black-red-yellow (German), it would tear the Brazil band apart by 7.1 sigma. (That was a stupid soccer joke.) But even the black wiggly curve deviates by 2.8 sigma, something like 99.5% confidence level.

This may be interpreted as a "near-certainty" that there are new \(W^\pm_R\)-bosons whose mass is about \(2.1\TeV\) or perhaps between \(1.9\TeV\) and \(2.4\TeV\). Well, I am of course joking about the "near-certainty" but still, this "near-certainty" is 86 times stronger than the strongest available "proofs" that global warming exists.

The CMS collaboration dismisses the excess because it is nowhere near the red curve. So it must be a fluke. Well, it may also be a sign of new physics – but a different kind of physics than what the search was assuming. It's actually easy to adjust the theory so that it does predict a signal of this sort. Somewhat lower (\(g_R=0.6 g_L\)) couplings of the right-handed bosons are enough to weaken the predicted signal.

In a new hep-ph paper today,
A Signal of Right-Handed Charged Gauge Bosons at the LHC?,
Frank Deppisch and four co-authors argue that such new gauge bosons coupled to right-handed fermions may be predicted by \(SO(10)\) grand unified theories. The minimal \(SU(5)\) group is no good. Needless to say, I indeed love the \(SO(10)\) grand unification more than I love the \(SU(5)\) grand unification – especially because it's more (heterotic and) stringy and the fermions are produced in a single multiplet, not two.

The asymmetry in the left-handed and right-handed coupling (note that they need a suppression \(0.6\) when going from the left to the right) may be achieved in "LRSM scenarios": the scalars charged under \(SU(2)_L\) have a different mass than those under \(SU(2)_R\), and the implied modifications of the RG running are enough to make the left-handed and right-handed couplings significantly different at low energies.

All these possibilities sound rather natural and Deppisch et al. are clearly excited about their proposal and think that it's the most promising potential signal of new physics at the LHC yet. I think that the probability is above 95% that this particular "signal" will go away but people who are interested in HEP experiments and phenomenology simply cannot and shouldn't ignore such news.


  1. And of course we electrons [1] herald BSM physics better than muons [2], right?

    [1] https://twiki.cern.ch/twiki/pub/CMSPublic/PhysicsResultsEXO13008/Elec_Lim2D.png
    [2] https://twiki.cern.ch/twiki/pub/CMSPublic/PhysicsResultsEXO13008/Mu_Lim2D.png

  2. LOL, nice to speak to an electron after years of waiting.

    I surely agree that it's bizarre for almost any new theory of physics to be felt by electrons only.

    Discrimination of electrons and muons is something I have spent several hours today since the morning. ;-)

  3. It is obviously good to have curious phenomenologists who, apart from the experimenters, look at the data too ...

    Nice finding :-)!

  4. The model does not say how the observed light neutrino masses (~ 240 GeV^2/ 10^15 GeV), that hint at B-L breaking seesaw scale of ~10^15 GeV, can be reconciled in the context of Left-Right model, if B-L breaks at 2-3 TeV.

  5. correction -- "observed light neutrino masses (~240^2 GeV^2/10^15 GeV)"....there should be the square on 240 that I missed before.

  6. I observe exact correspondence between theory and experiment at 3.1 - 3.2 TeV. Why do you insist upon probabilistic excuses 1.9 - 2.4 TeV? If they were so, both Ansätze would be wrong.

    The vacuum is trace left-handed toward matter, testable to at least 5×10^(-14) relative as a geometric Eötvös experiment, geometric enthalpies of fusion, geometric microwave rotation temperature. One microwave rotation spectrum of vacuum supersonic expanded racemic oxa-D_3-trishomocubane can falsify both gravitation and particle theory consistent with prior observation. " the right-handed electron and muon may transform nontrivially and it doesn't create too much havoc at accessible energies" is elegant and In need of more studies.

  7. About the possible difference between the electron and muon, perhaps you remember this poster?

  8. Hi, I was on the fence to look this documentary, but before I tried to find if it was reliable and this page and other on the same line prove to not be.
    So I'm doing this post to ask: can you suggest a good documentary on quantum physics please?

  9. Nice article! It remembers me that nowadays Brazil is doing better in particle physics than in soccer ... the world is changing in a so curious way that maybe (in the near future) even Czech republic would get a Nobel prize in Physics ;-)

  10. Dear Luboš,

    I'm a little over the age of 30, and then some. :)

    Al beeb has radically changed in my lifetime. It was never that great but there were plenty of things it did that were worthwhile. It certainly used to feel British. But all that is gone now.

    Today it's a completely alien institution that only pretends to be British (that pretence is wholly cosmetic and glaringly apparent to all but the brain dead) and encapsulates almost everything I despise.

    It's not just that I want its existence ended, I want it exterminated in the most brutal manner possible. Hate doesn't come close to describing my animus toward it.

    I used to watch its science programs, and Horizon in particular since it started in the 1960s. That programme was pretty much always worthwhile, at least up until sometime in the 1980s.

    But somewhere around the 1990s it started to decline noticeably and I found it pissed me off far more than it amused me. It got dumber and dumber. It took 50 minutes or so to say, and repeat and labour and bore on about, what could be said in about 5 minutes, if that. It was all about weird arty-farty camera work (shots up interviewees nostrils or close-ups of their earholes or some other fuckwit 'creative' shit) and 'dramatisation' to make it appealing Miss Cretinette Dumbfucker and her nose-picking, boy-racer 'partner'. On top of that increasingly it went out of its way to shove wimmin and racial hideoids to the fore.

    You are correct. The BBC is nothing if it is not anti-American — in fact IT FUCKING HATES THE YANKS.

    Now to your question:
    "Is the new pro-American bias based on the assumption that the U.S. has already become a socialist country?"

    I rarely ever listen to or watch al beeb. It has been that way for some years now, as part of my own personal rage-management programme in fact. So I need to say first that I have not seen the programme, nor do I intend to.

    OK. So, strictly speaking I don't know what I'm talking about here, specifically that is, but in general I do. Bearing that 'caveat' in mind...

    I wish to politely disagree with your premiss: there is no "new pro-American bias". At least it is not bias as one would normally understand the term, i.e. as somehow favouring Americans or America. No way. At best, such favour would be an incidental side effect.

    No, in my view, whether flippancy was intended or not, TheJollyGreenMan has put his finger on the pulse with his "The purpose of the programme was to give the science department of the BBC some nice trips to the USA. It succeeded very well and there are some happy BBC staff that can show their holiday snaps to those they love."

    That's all there is to it. You just have to remember what utter fucking shallow shits, hypocrites and parasites these traitorous arty-farty peecee champagne-socialist marxoid cunts really are.

    I hope one day they are all made destitute and forced to die horrible painful deaths, and that in the meantime they come to fully appreciate what pleasurable 'release' so many Britons, and no doubt many Americans, will acquire in contemplating that eventuality. Glee time indeed.

    Incidentally, what's true of al beeb is true of our political class. Both need ending.

    We also need to ship all those hideous asians and other third-world drossoids back to their cuntries of origin. The rest of the West should do likewise, if it has any sense.

    And since the religion of piss has never ceased war against us and never will while it exists, we should finally end it by ending them. That is the moral imperative for today.

  11. Why would that be strange, the most famous physicists like Schrödinger and Wolfgang Joseph Pauli came from the old Austro-Hungarian Empire which included Czech Rep. Pauli's parents are from Prague.


  12. laboussoleestmonpaysJul 24, 2014, 1:04:00 AM

    It is only half a tantalizing news for a non SUSY SO(10) enthousiast ;-) because the last report from ATLAS on this field is two year old (http://arxiv.org/abs/1203.5420) if I am correct! When could one expect the next one?

  13. laboussoleestmonpaysJul 24, 2014, 2:28:00 AM

    "we have embedded TeV scale Left-Right model within the framework of SO(10) model where the predicted mass for light neutrinos matches with the neutrino oscillation data" from http://arxiv.org/abs/1405.6836 (with a B-L breaking scale at (3-5)TeV)?

  14. So far, (ethnic) Czechs have one chemistry Nobel prize - Jaroslav Heyrovsky for polarography. The invention helps chemists but it is really an achiemevent of physics.

  15. It doesn't count for Czech Rep. so far, it is scored for Austria, sorry ;-)

  16. And the more assumptions are made, the easier Occam's razor cuts the hypothesis away...

  17. Just embedding is not sufficient to show this, as in SO(10) generally light neutrino masses come out ok because the B-L breaking scale is order 10^15 GeV. So embedding in SO(10) and breaking B-L at TeV scale doesn't automatically get the light neutrino masses right. It may be impossible to get it right in the minimal models considered (without fine tuning Yukawas).

  18. laboussoleestmonpaysJul 25, 2014, 11:23:00 PM

    Well if one is a susy agnostic and has some other physical motivation or mathematical guidance to get a testable and minimal or simple scalar spectrum and dynamics for example, why not fine-tuning the Yukawa parameters to find out the structure chosen by Nature !