Tevatron collides protons with antiprotons, and the production of top-antitop pairs at the parton level is dominated by quark-antiquark collisions. Thus, one can define the forward direction along the proton beam (which is also the direction of the incoming quark) and the backward direction along the antiproton beam. One can then count the number of top quarks produced in the forward and backward directions, and similarly for antitop quarks. Both CDF and D0 studied this asymmetry in the Tevatron data, and found that top quarks prefer to go forward and the antitop quarks prefer to go backward. In the first approximation, there should not be any asymmetry in the standard model. However, loop corrections and jet radiation do in fact induce a small asymmetry of order a few percent. CDF and D0 were finding the asymmetry of the correct sign but a bit larger magnitude compared to the standard model prediction, with the discrepancy at the 2-sigma level. Intriguing, but not overly exciting.
Until today.
The new CDF paper updates their earlier analysis using 5.3 inverse femtobarns of the Tevatron data. It studies the semileptonic top events when one of the top/antitop quarks decays to b-quark + electron/muon + neutrino, and the other decays into 3 quarks. Measuring the momenta of all decay products one can reconstruct the original momenta of both tops, so that we know exactly (up to experimental uncertainties) in which directions they were produced. Furthermore, measuring the charge of the lepton identifies which one was the top and which one the antitop (the former decays to l+, the latter to l-). It is then trivial to count the difference of top quarks produced in the forward versus backward direction. Actually, the asymmetry depends a bit on the reference frame in which it is measured. CDF give the asymmetries measured in the lab frame and in the t-tbar rest frame, and they also unfold background contamination and resolution effects to obtain microwave-ready parton-level results. The numbers quoted in the following are parton level in the t-tbar frame, while the plot relates to the same frame before the unfolding.
The new key element in the CDF paper is that, thanks to improved statistics, they can study various kinematical distributions related to the asymmetry. In particular, they study how the
asymmetry depends on the invariant mass of the t-tbar system (which tells you at what center of mass energy the pair was produced). While the inclusive asymmetry is small, of order few percent, the asymmetry at the high mass end of the spectrum appears to be huge, almost 50 percent. More precisely, CDF divides the t-tbar events into two groups, depending whether their invariant mass is smaller or larger than 450 GeV. In the former group the measured asymmetry is actually slightly negative, -12±15 %, perfectly consistent with the standard model prediction of 4 %. But for events with the invariant mass above 450 GeV the asymmetry is 48±11 %, as compared to 8% predicted by the standard model! The anomaly has a statistical significance of 3.4 standard deviations.
The fact that the asymmetry sharply grows with the invariant mass of the t-tbar system smells like a new heavy particle meddling into the top production process. The CDF paper tries out a heavy gluon with axial couplings to the light and the top quarks. The positive sign of the asymmetry (as observed) can be obtained assuming that the couplings to the light quarks has the opposite sign than that to the top quark. The model predicts roughly the right value and shape of the asymmetry for the gluon mass around 2 TeV (see the left plot). Of course, heavy gluon is not the unique possibility; other models have been proposed in the literature (Zprimes, color sextets, ...), all of them at least as ugly. The full spectrum of possibilities will be worked out in 123 papers due to appear on hep.ph by the end of the month.
As is the case with any anomaly, it is always more likely that the explanation is trivial. 3 sigma could well be a fluke. Also, some important physical contribution to the asymmetry may have been missed by theorists, so that the standard model prediction has been grossly underestimated. Another possibility is that CDF observed a fundon (an elementary particle produced in high-energy colliders near the end of the budgetary cycle); forward-backward asymmetry is one type of measurement where Tevatron is superior to the LHC (whose initial state is p-p, rather than p-pbar, which obscures this kind of analysis). On the other hand, if a colored particle with a TeV mass is responsible for the asymmetry, finding the particle should be a piece of cake for the LHC. Theory error, fundon, or KK gluon...time will tell. Soon.
30 comments:
In the last paragraph, I think you mean the LHC's initial state IS symmetric (i.e. protons on protons).
Depends which symmetry :-). But, sure, it was unclear, corrected.
Ooooo, pretty. Thank you!
Top quark pair production in p p collisions is a sensitive probe of QCD at high energies. SM predicts that quark pair production is symmetric under charge conjugation, at least to the lowest order.
If the result holds or it is enhanced at LHC, it may signal the breakdown of SM and the onset of a strongly asymmetric regime in charge conjugation.
This model makes quite some sense. So I gradually start to believe that it may be real and it's the fourth mirror generation (with reverted R-parity) squarks by the SUSY 1-Higgs-doublet model by Rajaraman et al. that are responsible - see my blog for the Yukawa coupling. ;-)
I was thinking on some superpartner of the gauge bosons, amazingly surviving almost unbroken at EW energies, but probably it is already discarded by other limits. Anyway, does it need to be a integer spin particle?
I'm a little confused why the asymmetry is non-zero in QCD. If QCD respects C and P then why would the tops go preferentially in the direction of the proton?
At the Tevatron, the initial state is not C or P symmetric, which introduces the preferred direction.
Arvind R. has pointed out to me that the total cross section looks really OK and constrained - while a new particle would enhance it.
So that makes any model with a non-octet intermediate state problematic. The KK gluon is really natural from this viewpoint. It doesn't look too natural from other theoretical angles but it's the experimenters that will ultimately tell us what is natural in Nature. ;-)
I don't think the KK gluon is the only possibility. See for example 0911.3237 where they show that a colored scalar triplet or sextet in the t-channel can also give a large asymmetry without screwing up the top production cross section. So it can even be an R-parity and flavor violating squark.
Alejandro, light particles interfering with the top production are strongly constrained by the top cross section measurements, especially by dσ/dm_tt.
Not sure how you want to mix fermions into that.
Jester, I was thinking mainly in the octet of gluinos. But the limits for them should be already about 400 GeV, even if you can imagine a way for them to enter play. Yep, the KK gluon is an interesting answer too.
I'm a bit puzzled by how a KK gluon can have axial couplings to the top and light quarks, which moreover have opposite sign for the top and light quarks, since QCD preserves parity - so I would have thought KK gluons would only have parity conserving couplings.
What channel would the KK gluon be in?
Is there a simple way to understand how the KK gluon can produce an asymmetry that increases with the t t bar mass, without affecting the production cross section?
In the Randall-Sundrum model KK gluon can have different couplings to left and right quarks, because in general the quarks can be differently localized in the extra dimension. So that's ok. In the CDF paper an off-shell heavy gluon enters in the s-channel, q qbar > G > t tbar. The top cross section does get modified, but experimental and theoretical uncertainties are presumably still large enough to accommodate the shift.
But don't get too attached to the idea, nobody says it's a KK gluon.
Uncle Al, that's brilliant! Thanks.
On other hand, the most stressed measure in the LEP, at 2.3 sigmas, was the FB asymmetry for bottom production (here page 27 and details here). Given that the production here is not QCD but EW decay, it is not easy to think of a common physics, but perhaps a common systematic problem, either in detectors or in calculation?? Or is there a possible common physics, at all?
Thanks, Jester, that's absolutely fascinating. I think now it might also be able to occur in models like my arXiv:0704.1476, and the requirement that it be possible is quite constraining - look for my contribution to the 123 within the next few days, with a bit of luck.
Alejandro, thanks for pointing this out. If the left-handed q, u, and d multiplets can be located at different regions in the extra dimensions, so that a KK gluon, whose wavefunction depends on position in the extra dimensions, can couple to them with different strengths, then so can the left-handed l and e, and KK Z and W, whose masses and wavefunctions will be the same as the KK gluon's before electroweak breaking, can also couple to the SM chiral fermions with different strengths.
I am somewhat surprised how people go around looking for exotic explanations. I am convinced that here there is a more prosaic reason for this and a proper NJL model could fit the bill.
Cheers,
Marco
NJL model? If you refer to the toy models treating QCD, that sounds extremely unlikely. These processes are very high energy, the realm of quarks and gluons.
TGD predicts that all gauge bosons should be accompanied by scalars and pseudo-scalars with same quantum numbers: also gluons. Scalars should be eaten to give the third polarization to gauge bosons.
Could the coupling to a pseudo-scalar variant of color octet Higgs give rise to a contribution interfering with the contribution of spin zero virtual gluons and in this manner give rise to the asymmetry? See this. Maybe there is simple objection but I am not able to invent it now.
Dear Anon,
NJL model are not toy models. Rather, they apply to the low-energy limit of QCD. But in this case we are in a fully asymptotic freedom regime where one should expect gluons to be massless. This is the more tricky aspect of this result. Observing a "massive gluon" of 450 GeV seems to be in the higher end of the spectrum of Yang-Mills theory for a massive gluon (mass gap plus excited states without no bound from above). KK gluons seem a rather interesting possibility but they imply a higher mass being O(TeV) and some other problems. This makes this finding at Tevatron really striking.
Cheers,
Marco
Marco Frasca:
NJL models are toy models, which describe certain aspects of low energy QCD. They are toy models because they do so in terms of both mesons and constituent quarks, and there is experimental evidence that there is no energy regime in which an effective theory containing both describes the physics at anything more than the qualitative level.
This different than perturbative QCD at high energies or chiral perturbation theory at low energies, both which describe the physics very well.
But the basic point is that NJL models do not even pretend to describe physics beyond 1 GeV. They simply have nothing at all to say about the energies at which top quarks are produced.
Matti Pitkanen:
Scalars with a variety of color representations (singlet, octet, sextet, triplet) have been tried; see 0911.3237 , which Jester already alluded to. The conclusions of those authors are that singlets and octets do not produce the right asymmetry, but sextets or triplets could for some couplings.
The KK gluon-like explanations have been claimed to be excluded as a possible explanation in 1007.0260
Is the Tau Lepton similarly asymmetric ?
Tau has chiral coupling to Z, so there is an asymmetry. It was measured at LEP and it agrees with the Standard Model.
Oh, come on, people. Obviously we are seeing the color carried by the Z boson under the twisted Fourier transform of the Bilson-Thompson fermion braid spectrum.
Anon:
Of course, we have in mind different NJL models as it can be proved that, at low energies below 1 GeV as you correctly states, a NJL model is the correct low-energy limit of QCD using just quarks and a proper propagator for the gluon field. So, while I can realize that my initial comment was not so clear, I should say that your knowledge of NJL models is just old-fashioned.
What I am saying is something different. My take is that the spectrum of Yang-Mills theory (this is a black box for physics so far) can explain the t-tbar asymmetry.
Cheers,
Marco
Marco Frasca:
You can of course use the term NJL however you like. But you will run into misunderstandings right and left. As commonly used, NJL refers to the work of Nambu and Jona-Lasinio and very much applies to low energy QCD.
There are applications of NJL-type dynamics to physics beyond the standard model, such as in top condensation and its descendants such as top-color. It would be interesting to see if they have anything to see here, but it is not at all obvious a viable scenario which is in agreement with other data is permitted.
However, I think it is very sloppy notation to call the low energy dynamics of a "random" Yang-Mills theory a NJL model; rather NJL captures some features of certain kinds of Yang-Mills models.
Anon:
When I say that the low energy limit of QCD is a NJL model I refer to this pair of papers:
K. I. Kondo, Phys. Rev. D 82, 065024 (2010) [ http://arxiv.org/abs/1005.0314 ]
M. Frasca, Int. J. Mod. Phys. E 18, 693 (2009) [ http://arxiv.org/abs/0803.0319 ]
My take is that Yang-Mills theory has a mass gap and a set of excited states not bounded from above. This means that one could in principle get massive glue state excitations also at very high energies like in this case producing a possible asymmetry.
Cheers,
Marco
Actually, I was the one who said low energy QCD is an NJL model. I am glad you now agree. And for the record, that body of work goes back to the original work by Nambu and Jona_Lasinio. Those papers were referenced in Nambu's Nobel prize award text.
You should explore Yang Mills theories which you think could produce this effect. I think building realistic models will be challenging, but it is an interesting idea.
All,
I am not sure there is an immediately obvious explanation of this FB asymmetry.
The analysis selects t-tbar events in the lepton plus jets channel where one top decays semi-leptonically (t->l-(nu)-b)and the other hadronically (t->q-qbar-b). So there are multiple possibilities on what causes this anomaly:
1) is it a pure high-energy QCD effect from the q-qbar->t-tbar channel?
2) is it an electroweak effect from the top decay t-> W-b?
3) is it a combination of 1) and 2)?
4) is it a "beyond the Standard Model" effect that can be attributed, for example, to the dynamics of Yang-Mills fields sliding outside equilibrium?
Cheers,
Ervin
Post a Comment