jump to navigation

Gravitons are heavier than 500 GeV! December 23, 2008

Posted by dorigo in news, personal, physics, science.
Tags: , , ,

About a year ago I reported here on a search performed by CDF for events featuring two Z bosons, both decaying to electron-positron pairs: I had been an internal reviewer of that analysis, and I discussed it in some detail after we approved it for publication. While the standard model expectation for electroweak production of two Z bosons is of about 1.5 pb, and the process has indeed been put in evidence in CDF and D0 Run II data, the analysis was rather focused on a search for heavy mass resonances decaying to the ZZ final state: new physics, that is, either in the form
of a heavy Higgs boson, or of a graviton (in the Randall-Sundrum scenario), or other still fancier (and improbable) beasts.

CDF has now repeated that search by increasing the dataset size by a factor of three, and by including mixed final states which include muon pairs and even jet pairs. This makes the analysis intrinsically interesting to me, since I have started a similar analysis with the CMS experiment, together with a PhD student in Padova, Mia Tosi. Mia and I will be looking for Higgs bosons in the dilepton plus dijet final state, with particular emphasis on the Z \to b \bar b decay, which is a signal with which we have quite some familiarity.

The new CDF search for high-mass ZZ events configures itself as a “signature-based” one: despite the reference to the Randall-Sundrum graviton, the analysis cuts are kept generic, such that a signal can be found for anything that decays to two Z bosons, and in case no signal is seen, a model-independent limit on the cross section can be set. The only limitation of the search is that the four-body mass is studied only above the minimum value of 300 GeV. Such a requirement allows to steer away from phase space regions where backgrounds dominate.

Once four objects (electrons, muons, and jets, with the specification that at most two jets are present) are selected with loose cuts, a statistical estimator is built to test the hypothesis that they originate from the decay X \to ZZ \to llll (lljj). It is a simple \chi^2 function, which utilizes the expected resolution on the two two-body masses and the resulting four-body mass to estimate how much the event departs from the tentative signal interpretation. Only in the case of jet pairs, an explicit cut is set on the dijet mass to lay between 65 and 120 GeV, to avoid accepting too many random jet combinations.

While the M_x>300 GeV region is the one where the signal is sought, the complementary region of the four-body mass is used as a control sample, to verify that background estimates obtained with Monte Carlo simulations are in agreement with the observed data. The nice thing about such a spectacular signature as the production of two Z bosons is that backgrounds are exclusively of electroweak nature: by having at least one Z \to ll decay in the final state, the signal cannot be mimicked easily by purely quantum chromodynamical processes, which plague most hadron collider searches with high rates. Besides regular ZZ pairs from standard model processes, backgrounds include WZ, WW, and Z+jets production. At high four-body mass, however, all of these are really small, and even in the 3 inverse femtobarns of proton-antiproton collisions analyzed by CDF for this search, they contribute only few events; only the dilepton+dijet signature accepts a few hundred events, because of the large cross-section of Z+2 jet production processes.

In the end, no signal is seen, and a cross-section limit is extracted as a function of the X mass. The limit is shown below, compared to the expected cross section for graviton production and decay to the ZZ final state. The comparison of upper limit (the red curve) with the theory hatched line allows to exclude gravitons with masses below 491 GeV, for a particular choice of model parameters k/M_p=0.1 (k is a warp factor for the extra dimensions, and M_p is the Planck mass).

As a by-product of this analysis, a new set of excellent standard-model-like ZZ decay candidates have been selected. I am unable to show any of the new event displays here, because they have not been approved for public consumption by CDF yet… So please see the lego plot of a ZZ \to eeee candidate below, extracted last year by the same authors. The two pairs of electrons make masses very close to that of the Z boson, as evidenced by the two pink numbers.

To read this graph, you have to know that the greek letter \eta is the pseudorapidity, basically a function of the angle that particles make with the beam axis. A pseudorapidity of zero means that the particle is emitted at 90 degrees from the beam, while positive and negative values indicate the proton and antiproton directions. The other coordinate, \phi, indicates the azimuthal angle in the transverse plane. The z axis (the height of the bars) indicates how much energy is deposited in the \eta - \phi interval span by the bars. In bright pink are shown the four electron candidates, as measured by the CDF calorimeter, and each bar is labeled by the energy in GeV measured for each.

I am only left with the pleasant task of congratulating my colleagues Antonio Boveia, Ben Brau, and David Stuart for this new result, which greatly extends the scope of the analysis I have reviewed last year. During my review I had encouraged them to pursue the other decay modes of ZZ pairs, and so they did. Well done, folks!


1. Michael Schmitt - December 24, 2008

Dear Tommaso,

as much as I admire your efforts to promote an interest in, and an understanding of, collider physics, I believe you have stepped over a line with this post.

The results that you describe are not public yet – and they are not close to being public either. The paper reporting on the ZZ search in the eeee final state is in the second draft stage, and there is no draft at all for the final state you describe here. I don’t think you should pluck the results from Ben, Antonio and David and report them in your blog before they even circulate a paper within the CDF collaboration. The fact that you are starting a similar study for CMS does not release you from the normal agreements and trust of a collaboration, even a rather large one like CDF.

I admire you a lot, and I support your blog, so I am not looking for a nasty scandal here. But this particular entry is imprudent at best, and certainly unkind to the people who did the work.

respectfully yours,

2. Zhen-Ta Klos - December 24, 2008

Despite your post title, I bet you that gravitons are massless!

3. dorigo - December 24, 2008

Dear Michael,

as far as I know the result is public, but I will double-check with the CDF spokespersons.

As for the ZZ->eeee paper, it has been published to PRD last July, see here.

In any case, I have to thank you for your support. Two and a half years ago your strong encouragement was really a driving force towards making this blog a physics outreach site. I am sincerely grateful for that, and I have a deep respect for you as a scientist.


4. dorigo - December 24, 2008

Dear Zhen-Ta,

you have a point, of course. The search reported above is for a specific kind of graviton, and the exclusion limit only applies to it (and to similarly decaying particles). The search would have been quite different if we were going after lighter gravitons!


5. Michael Schmitt - December 24, 2008

Hi Tommaso,

yes you are right – the eeee paper has been published. I had relied upon the internal drafts web page, which still lists this paper in second draft form. I did not notice that the date was 2007, not 2008! (I’m embarrassed for such a silly error!!) I also rely on that same page for checking for an lljj draft, and there is none. If the paper is indeed public, and I missed it, I apologize for my earlier criticisms, and withdraw them. I also checked SPIRES and did not spot any paper there. Of course, the confidence level excluding two errors of this sort is not very high…

Merry Christmas to you and your family!


6. Ptrslv72 - December 26, 2008

Dear Tommaso,
Zhen-Ta Klos is telling you that gravitons are always massless, otherwise gravity would not be a long-range force. Even in the Randall-Sundrum model the 5-dimensional graviton is massless. However, when you look at it from the 4-dimensional point of view, the graviton manifests itself as a tower of Kaluza-Klein states, the first of which is in turn massless and acts as the 4-dimensional graviton. I presume that what you are putting bounds on is the mass of the first excited (=non massless) state of the KK tower.
Merry Christmas, Ptrslv72

dorigo - December 26, 2008

Well, yes, these are Kaluza-Klein states which the theory should manifest if there are hidden extra dimensions; they also depend on the value of k assumed…
Sometimes comments do not let me understand if they come from insiders or from non-physicists, and my answer is not always to the point in such cases!

7. Luboš Motl - December 27, 2008

I fully share Zhen-Ta Klos’ discomfort with the lightness how you redefine gravitons. It shows how counterintuitive quantum gravity is for most people (and experimenters) and how normal it is to say things that would be viewed as really stupid in the context of more familiar quantum forces.

Imagine that you would title your posting “Photons are heavier than 500 GeV!”. It’s exactly the same thing in a context of a different force. You redefine the concept of a graviton (or photon) to mean a newer, more exotic, less important particle than the crucial (massless) graviton (or photon), more concretely, you only mean the massive KK modes of gravitons, and pretend that the crucial massless particles don’t even exist – which they surely do, even in all the braneworlds you can imagine.

I think that such a summary is deeply misleading both for laymen and insiders. Laymen should first of all know that gravitons and photons are massless because they mediate infinite-range forces, and insiders won’t be sure what you have exactly excluded. Do I understand that you have only excluded KK gravitons between 300 and 500 GeV? That’s a pretty short interval, logarithmically speaking, and there can be a lot of stuff going on both above and beneath it.

dorigo - December 27, 2008

Ok, Lubos, point taken. My focus in the post was about the experimental signature, not on the theoretical implications. In fact, the limit can be easily converted into one for any new particle decaying to Z pairs (although a precise number would require a kinematical analysis).


8. Luboš Motl - December 28, 2008

Dear Tommaso,

understood and accepted. Allow me an unimportant philosophical/linguistic thought. It’s funny how the word “signature” is actually being twisted in the experimental context. Every child knows that a signature should belong to someone or something – it should be his or her or its signature.

So even though “experimental signature” sounds very experimental, it is heavily theory-laden because it is a theoretical construct “who” is supposed to sign. When the brain is allowed to be used here, it follows that the same kind of object may have different signatures in different contexts. Yes, gravitons are criminals because they change their signature depending on their mass – on how much they have eaten during the Xmas.

Still, the word “signature” is only sensible if there is a sufficiently decodable link between what you see and the identity of the person or particle whom you are supposed to be seeing. So if an experimenter sees something that can mean 20 different things, and on the other hand, each of these things only has these signatures given some additional assumptions, I feel that the experimenters should invent and use a different word than “signature”.

There should be a whole pyramid of alternative terms reflecting the degree of uniqueness of the “signature”. For example, “fingerprints” are the most unique signatures, tightly associated with the owner. Maybe, “footprint” could be a less unique. Perhaps, people could recycle many words like fingerprint, signature, footprint, trace, memory, flavor…

Happy New Year

9. dorigo - December 28, 2008

I am embarassed to say I totally agree with you. We even speak of “signature-based searches”, which should imply we look for events with a well-defined final state which could be the result of a dozen different subnuclear processes. It is more than a twist, it is a flip-over of meaning.


10. Luboš Motl - December 28, 2008

Tommaso, it’s really embarrassing to agree with you – in fact, the only thing I could disagree with is the number of letters “r” in the word “embarrassing” but as long as you don’t write Dorrigo, it’s not a signature-based search or signature-based typo, anyway. 😉

dorigo - December 30, 2008

ok ok – I bet you use a spellchecker, I don’t.

11. Luboš Motl - December 30, 2008

What about the alternative theory that I actually know grammar (if I have to choose a very testable piece of knowledge as an example) better than you do? 😉

12. Guess Who - December 30, 2008

Since grammar is not about spelling, I would rate the “altermative theory” as orthogonal to the “embarassing” issue.

On the other hand, using the wrong word in a sentence, like “grammar” instead of “spelling”, could probably be construed to be a grammatical error.

13. Luboš Motl - December 31, 2008

Dear Guess Who, this is a nice story of yours but you have given us no evidence that either of my theories – about spelling or grammar – is incorrect, so I hope you will kindly allow me to insist on every letter I have written. 😉

Concerning the spelling issues, check the letters that appear in the word “alternative”. 🙂

14. Luboš Motl - December 31, 2008

Incidentally, had I used the word “grammar” with a wrong meaning, which I have not, that would have been neither a spelling error (which I have not suggested) nor a grammatical error (which you have suggested): it would have been a semantic error. 😉

That’s also the kind of error that you have made when you referred to semantics as “grammar”. 🙂 Unfortunately, as far as I can say, you haven’t managed to squeeze a syntactic error into your short reply, unlike a spelling and a semantic error, so I can’t explain the difference efficiently. Thanks for your understanding, anyway. 🙂

15. Guess Who - December 31, 2008

Yeah, I make spelling errors all the time, but at least I know what they are. Using the phrase “could probably be construed” was a much more serious error, one of judgment. I should have know it was way over your head, Lubos. Sorry about that.

16. George Theofilos - April 7, 2009

Gravitons are massless and make photon go back and forth in the direction of travel.

17. Alfred Herman Schrader - October 10, 2009

I discovered the graviton particle, so you could ask me.
Gravitons create gravity & inertia by bumping into stuff.
Gravity pushes, it doesn’t attract.
And why hasn’t the universe all been pushed into one great big lump ?
Simple. When a lump gets big enough, the gravitational forces are so strong that the atoms grind together and decay back into photons or light. Our sun, for example is powered by gravity. The photons blast
back out into space, and re-collect back into dark matter restarting the whole process. I’ve performed this in my lab and you can repeat it by experiment.This is the math formula E = MC 2 …Alfred Herman Schrader

Sorry comments are closed for this entry

%d bloggers like this: