Wednesday, 6 April 2011

Another 3 sigma from CDF

Gosh, I was damn busy all day, and by now all major blogs have already run the story. Anyway, better late than never... CDF claims a 3 sigma excess in the dijet invariant mass distribution in the lepton + neutrino + 2 jets events. This analysis was originally devised to search for the diboson WW and WZ final states, where a W boson decays leptonically and the other electroweak boson decays into 2 jets. The latter should show up as a broad peak in the dijet invariant mass spectrum, on top of a much larger background from the generic W+jets production. Indeed, both D0 and CDF could see the peak below 100 GeV, which allowed them to pinpoint the diboson production at the 5 sigma level and measure the diboson production cross section. However, CDF had also a small blip around 150 GeV which was not expected.












All in all, the excess advertised in today's CDF paper is not exactly new, and it has been widely discussed among theorists. Moreover, the analysis published today has been publicly available for some time in the form a PhD thesis. What changed with respect to the earlier CDF diboson search is that the cuts have been slightly revamped to make the bump more pronounced. The excess over the standard model prediction is estimated to be slightly above 3 sigma.

It is well known that sigmas come in varieties: there or more significant 3 sigmas, less significant 3 sigmas, and astrophysical 3 sigmas. To my taste the latest CDF claim belongs to the 2nd category. We are dealing with a small hump on top of a huge background, and a small unaccounted for systematic error could easily show up as a false positive. Furthermore, D0 does not see anything; they actually have a small deficit near 150 GeV (although with much less data and different cuts). Finally, the recent experience with papers submitted simultaneously to arXiv and to popular news outlets is not quite encouraging ;-)

In spite of these caveats the effect is definitely interesting and we cannot exclude it is real. What could it be? It is not a Higgs; anything Higgsish with 150 GeV mass would prefer decaying to a pair of W bosons or b-quarks rather than to 2 light jets. The simplest explanation, proposed in this April Fools' paper, involves a 150 GeV Z' boson. A light Zprime with a significant coupling to leptons is excluded by LEP and the Tevatron, but if the coupling to leptons is small then the limits are surprisingly weak. In particular, 150 GeV Z' with electroweak size couplings to quarks is perfectly allowed, and would have the right cross section to produce a bump observed by CDF. One should note that Z' with the mass of that order could generate a large forward backward asymmetry of the top production, as observed in another CDF analysis. But one should also note that generating the asymmetry requires a large flavor violating coupling u t Z' which in principle is not related to the coupling to the light quarks that is probed by today's CDF paper. In a month or so, when I post an update, there will surely by dozens of new models explaining the bump, and some of them may link to the top asymmetry in a more direct way.

For more comments see Flip, Tommaso, Sean, and Michael, Peter, Lubos, and again Tommaso.

16 comments:

Kea said...

As Marco Frasca says on Tommaso's post, surely it is time that this so called 'theory' community stopped jumping to the conclusion that the right answer must be expressed in terms of new localised states. We already know FOR CERTAIN that QCD has a new description in terms of non local physics, and it may therefore not behave entirely as expected. Call it a Z' Lagrangian term if you must, but expect us to laugh at you. How many other people were writing about color Z boson structure BEFORE the Cavaliere thesis appeared?

Ptrslv72 said...
This comment has been removed by the author.
Kea said...

No, Ptrslv72. He means that the QCD background is not understood because its theoretical description, as usually understood, is incorrect.

Matti Pitkanen said...

TGD suggests two explanations. The explanation based on predicted exotic variants of gauge bosons fails because quark pairs are preferred over lepton pairs. Second explanation is in terms of the decay of a charged pion of scaled up variant of hadron physics to W boson and gluon producing the quark pair see my blog posting.


Matti Pitkanen

Anonymous said...

Kea, there are no indications whatsoever that QCD (by which I mean SU(3) gauge theory) is anything but a perfect description of the strong interactions. The problem is that making predictions in QCD involves you in very difficult calculations. Perturbation theory is not always valid, and even when it is life is tough.

The fact that gauge theories like QCD might have an alternative description (you mention non-local physics) is fascinating, but it does not change anything in terms of phenomenology.

Kea said...

Um, yes there is. One of them is called twistor scattering theory.

Ptrslv72 said...

Oh, Kea you're probably right on this Frasca. I checked his record on SPIRES and - judging by his mostly unpublished work - it might well be that he was advocating some crackpottery instead of simply referring to Tommaso's hypothesis of a poorly simulated background. I deleted my comment before realizing that you had replied. My bad, Ptrslv72

Anonymous said...

Hi Jester,
If it is a resonaance of 150 GeV decaying into light jets, then these two jets should have a large average transverse momentum of about 60-70GeV or so.

Should not CMS show what happens to the bumb if they require jets of a higher pt-threshold than 25GeV that they use in their jet algorithm? If it is a real particle, then one should expect smaller backgrounds and a similar signal cross-section, ie bigger significance.

Have they done this?

Sibab

RIen said...

Kea, we know no such thing about QCD. Everything we know about nature so far can be explained in terms of localized QFT. Expect us to laugh at you, too.

Anonymous said...

@Kea: twistor scattering theory aims at reformulating the usual perturbation expansion of local QFT into some other form. That doesn't mean it stops being a local QFT: think of it as different variables. A hope is that you could do more with it, but that is just a hope at this point.

Moreover, massive particles (such as appear in nature!) do not fit nicely into a twistor framework. Maybe some extension exists, but that very much remains to be seen.

Anonymous said...

Trivial example of a perfectly local theory with a non-local formulation:

Classical or quantum electrodynamics in Coulomb gauge.

Kea said...

Gee, you people are ignorant. Best you stay anonymous.

Jester said...

@Sibab, they tried higher jet ET cuts, see Table 9.3 in Cavaliere's thesis. They say the bump remains (although it shifts to slightly to higher values). But the statistics drops as well, so you probably don't gain on significance by choosing a higher ET cut.

Anonymous said...

Ladies and Gentlemen, we are only a fraction of a sigma away from an observation of the mythical Fundon particle.

Clearly it is a T Channel process, that also displays fascinating nonlocality. In the sense that as soon as it is observed, a hep-ph article appears explaining its existence thus demonstrating entanglement between Fermilab, Europe and China!

Mitchell said...

Speaking of deformations of QCD, back in 1997 someone wrote a paper in which they said:

"It is also at least remotely conceivable that the sort of generalization of standard gauge theory considered here could show up directly in accelerator experiments (presumably with a small value of the deformation parameter and not the large value that makes possible semiclassical analysis via branes)"

Unfortunately I can't tell if it's relevant for the present situation.

Chris said...

With reference to arXiv:1104.2302 by Anchordoqui et al, which claims that the Wjj anomaly could be due to a 150 GeV Z' coupling mainly to baryon number, is there a straightforward way to understand how they avoid violating the sigma x branching ratio bound for Z' -> jj from the UA2 Collaboration? They state after their equation (9) that a previous comprehensive study of similar models to theirs concluded that gauge bosons with M_{Z'} < 700 GeV are excluded by the Z-pole data from LEP. Can they really avoid this just by arranging to suppress the Z - Z' mixing by an adjustable parameter?

The possible observation of a light Z' coupling mainly to baryon number is extremely interesting from the point of view of models with a low gravity scale, because it provides a way to suppress proton decay. But it seems extremely strange that all previous dijet-mass searches at the Tevatron are limited to M_{jj} > 200 GeV, and then this suddenly appears a few months before the Tevatron is scheduled to close. How difficult would it be for the LHC to observe a Z' such as they propose?