Monday, 6 June 2011

More details about the CDF bump

These days the next round of LHC results is beginning to emerge but for at least another week all eyes will be on the Tevatron. Recall that the CDF collaboration is observing an unexplained bumpy feature in the invariant mass spectrum of jet pairs in events with a W boson. Last Thursday CDF released an online note describing the latest update based on 7.3 fb-1 of data. The significance of the excess has increased to 4.1 sigma. The note describes a number of additional checks that the authors of the analysis have made to exclude background mismodeling as the origin of the bump. In particular, it seems that neither the standard model top quark, nor a difference of jet energy scales between quark and gluon jets can be responsible for the excess. Moreover, the excess persists (albeit a bit smaller) when a different Monte Carlo program is used to simulate the background. All in all, currently none of the known sources can explain the peak in a way consistent with all data. It must be a more subtle detector effect, or new physics.

Furthermore, the note presents a number of kinematic distributions of the events in the
window 115 < M_JJ < 175 GeV where the excess is the largest (thanks guys!). One plot makes you jump in your chair: It shows the invariant mass of the sum of the 4-vectors of the 2 jets, the lepton, and the neutrino, the latter reconstructed from the missing energy. If they all originate from one mother resonance, as suggested by a class of models, the invariant mass should reproduce that resonance mass. Indeed, the plot above shows a clear excess just below 300 GeV. This hints at another heavy particle being produced at the Tevatron, which then decays to a W boson and a 150 GeV particle who is directly responsible for the dijet bump. However, the plot on the right, which shows the same distribution but without subtracting the background, tells you that the standard model also peaks around 300 GeV (as a results of the imposed cuts) which makes the peak less trustworthy. If not for that, we would already be dancing on the streets and indulging in wild orgies.

It's also worth looking at another plot showing the transverse mass of the lepton + neutrino system. If the two come from a decay of a W boson, as tacitly assumed in the analysis, that distribution should have an endpoint at m_W = 80 GeV. Since most of the excess is below 80 GeV we can conclude that most of the times the 150 GeV resonance is accompanied by a genuine W boson. This excludes a more exotic class of models I mentioned where the lepton and the neutrino originate from the same particle that is responsible for the bump.

What's next? We are of course dying to hear the story from D0 and from the LHC experiments. The update from D0 is imminent. We expect it to be announced on June 10 at the Wine&Cheese seminar in Fermilab (as a general fact, wine facilitates communication between experiment and theory). As for the LHC, a blog post on Quantum Diaries points to an ATLAS note based on 33 pb-1 of data which roughly repeats the CDF analysis and finds no excess. However this means nothing: ATLAS simply had no right to see the hypothetical CDF bump in their 2010 data. First of all, the larger production cross section at the LHC (5 to 40 times, depending on the production mode) combined with the better efficiency does not make up for the 200 times smaller luminosity. Moreover, the W+jets background at the LHC is about 40 times larger. For these reasons you need a larger data set to make any conclusive statement. Suppose the CDF excess indeed originates from a 300 GeV mother resonance. If that resonance is produced by gluon-gluon collisions then the LHC should be able to see the excess already in 200 pb-1, which is the amount of data used in the most recent analyses. I'm sure hundreds of people are looking into this as we speak and as soon as the peak appears it will be pasted into Peter Woit's blog ;-) If, on the other hand, that resonance is produced in quark-antiquark collisions then we need to wait for 1 inverse femtobarn to see a significant excess at the LHC. In any case, the floor should be swept by the end of this summer, one way or another.

57 comments:

Anonymous said...

I can even see two more peaks in the invariant mass plot, at around 500 and 600 GeV, where the error bars are small.

Could the resonaance be a composite, with those smaller bumps corresponding to excited states?

Jester said...

Could be, but by eye those other bumps are less than 2 sigma.

Peter Woit said...

While I don't usually add graphics to my posts, I just wanted to make it clear that if anyone from ATLAS/CMS/D0/CDF has interesting plots they want to anonymously share with the world, I'll be glad to make an exception...

Manmohan Dash said...

very nice developments, in par with my remarks so far, that the bump might have come from a kinematics inherent with the current Standard Model. So this hypothesis of this blog's author that the bump may have originated from a W and a bump at 300 GeV must be tested seriously. BTW I am not much with the arguments to the contrary that with backgrounds included this does not seem to be the cae. HOW???

Anonymous said...

hey guys, I really really really have one question. If the bump/s is at such low energy, why we have just seen them? (if they are real)

Jester said...

There is a huge background from the Standard Model W+jets production, so in spite of the low mass you need a large data set to see a significant excess over the background.

Anonymous said...

my question is actually:

have the CDF people investigated W+jets production before?

--if yes, they should have a large amount of data long long time ago, why they just found the bump?

--if no, why they just started to look into this?

thanks!

Jester said...

Yes, both D0 and CDF have studied W+jets, but only now the amount of data they've acquired is sufficient to see the excess. Another thing is that it wasn't the first place people thought new physics might show up.

Daniel de França MTd2 said...

Could that be a 4th generation down quark?

Andrew Oh-Willeke said...

"If not for that, we would already be dancing on the streets and indulging in wild orgies."

So that's why physicists toil away so hard trying to discover new particles.

Jester said...

No, it's not b'.

Daniel de França MTd2 said...

Why not it's not b'?

Kea said...

Hmm, that peak is awfully close to Dave's top mass at a Koide phase of pi/2 (ie. at 274 GeV).

Walter said...

The plot which should make you excited (and maybe even dancing in the street) is the Pt(w) (or equivalently the Pt(jj)) distribution of the events in the bump region.
If you look at it you'll see it has an edge/cut-off at 100 GeV. Assuming the underlying kinematics is a heavy resonance decaying at rest (because it is heavy) into a 80 GeV W-boson and a 150 GeV resonance you can easily determine the mass of the resonance:
m~sqrt(ptmax^2+80^2)+sqrt(ptmax^2+150^2)
Put in ptmax=100 GeV and voila, there is your 300 GeV resonance without the background problems you have with the total invariant mass distribution.

Seems the kinematics is pretty consistent... Makes a subtle detector effect explanation or software glitch less likely as they usually give inconsistent kinematics...

Kea said...

Well, it seems that CMS are looking at a Higgs triplet model for the 275 GeV peak.

Anonymous said...

Is it clear that in the absence of a mother resonance a Z' and a W would almost be produced at rest at Tevatron energies? How different and inconsistent with data should we expect the pt(jj) distribution to be in such a scenario?

Jester said...

Right Walter, but I'm confused by their statement: "The two models are similarly discrepant with respect to the leading-jet ET and the pT of the dijet system" where one of "the two" is the technirho mother resonance. I haven't simulated it (yet) so i'm not sure what they mean.

Jester said...

Daniel, you mean b' decaying to t W ? CDF says it's not a top because they see too few b-jets in the excess region.

Walter said...

@Jester: The quote is from the CDF paper? Or the web site? I cannot locate it...

Walter said...

@anonymous: Now you're getting into kinematic details and you need to actually simulate these models. It might well be the Z' mode produces a pt(jj) distribution similar to the techni-rho model and the CDF data is not precise enough to distinguish the two models.

Daniel de França MTd2 said...

Jester, no, I mean b' decaying to cW, or any combination with quarks with lower mass than the top.

Jester said...

Walter, the quote is from http://www-cdf.fnal.gov/physics/ewk/2011/wjj/7_3.html, towards the bottom, in the bullet "Model dependence of jet system"

Walter said...

@Jester: Found it, thanks. Yes, I am equally confused by that statement. The authors of the website are very responsive about clarifying what is on there. You probably get a quick reply when you email them....

Jester said...

Daniel, and the bump?

Daniel de França MTd2 said...

And the bump is caused by that.

Anonymous said...

Daniel, the 150 GeV bump is in the invariant mass of the jets (it doesn't include the W). If you're saying the b' is the "mother resonance" (before creation of the W), what's the source of the 150 GeV resonance?

Daniel de França MTd2 said...

W also generate jets So, you mean jets excluding the ones from Ws, is that it?

Anonymous said...

@Daniel, in this case the actual signal was 2 jets + lepton + missing energy. Lepton + missing energy can be a W decay (where the missing energy corresponds to a neutrino). So I assumed you were saying the b' decays to a W (which decays leptonically), and some other hadronic stuff. But that doesn't explain the 150 GeV resonance.

Daniel de França MTd2 said...

I understand now. So, why couldn't a 150GeV particle be the b' and its parent, around 300GeV, be a t'?

Anonymous said...

To follow along the same line of thought as Daniel, why couldn't the 300 GeV mother particle be a tau' and the 150 GeV daughter be a nu'?

Jester said...

Yeah, t' -> b' W, followed by b' -> W c might work provided b' is somewhat heavier than 150 GeV. Then when you pick c and one jet from a hadronic W decay the dijet mass distribution will have an edge a bit below the mass of b'. One would have to simulate it to see if everything fits, and then there are constraints from other searches and experiments one would have to address, but it's not obviously impossible. I'm currently thinking about a related but a bit simpler idea. Nu' doesn't work, it does not decay to jets.

Anonymous said...

The MET plot looks intriguing as well. also has a cutoff/edge/bump which could come from an explanation along the line of what Walter said.
why would detector effects give you these features?

Mitchell said...

Regarding the t' -> b' -> W idea, 1101.4976 is the most advanced analysis I can find of constraints on t' and b' masses, for a wide variety of mixing angles.

Chris said...

With respect to a possible 300 GeV t' there seems to be a possible hint from D0 here. But how could a b' not much heavier than 150 GeV have escaped discovery via QCD production of b' \bar{b'} at the Tevatron?

With reference to 1101.4976, it is not directly clear to me whether the lower limits on m_{b'} and m_{t'} of 290 GeV that they quote in their conclusions apply to both b' and t' in all cases, or just to the heavier of the two, because they also consider the case m_{b'} > m_{t'}.

If the 300 GeV particle was a t' produced by quark - gluon collisions, how much data would be needed to see a significant excess at the LHC?

Kea said...

At the 274 GeV t' scale, Dave's t' triplet Koide phase is pi/6 (from the trisection of pi/4 for the charged leptons), which is the phase that sets the overall top triplet scale (for both t and t' sets) by a u' mass at 23 GeV. An analogous b' mass comes out around 135 GeV, but there are many ways to define this ... hmmm.

Kea said...

Oops: pi/2 not pi/4.

Kea said...

Oh, wait, a better idea is to take the mirror top quark phase of 8/27 at the parameter r=2.5412 (set by the lepton' phase of 8/9 in Dave's applet). This gives a t' mass of 271 GeV.

Anonymous said...

I wonder if there are people actually reading what Kea writes.

ignorabimus said...

If it weren't for Kea, I would never have thought of combining hep-ph/0206021 and 1106.0971. Thanks Kea!

Ervin Goldfain said...

Let us recall that the existence of new fermion generations is constrained by anomaly cancellation in SM. In one particular scenario, it implies a massive neutrino whose mass is larger than half of the Z boson mass. This heavy neutrino has not been detected so far. New fermion generations need also comply with precision measurements of EW parameters and requirements of Higgs physics.

Daniel de França MTd2 said...

Kea, how would you tell apart a 4th generation from mirror particles of the 3rd generation?

Chris said...

I appreciate that in Fig 7, about halfway down the note, they have tried the effect of a variation in the jet energy scale, and the fit to the WW, WZ peak looks good here, with a fairly convincing excess around 150 GeV in the left-hand graph before background subtraction. But in the left-hand graphs of Figs 2, 4, and 5 of the note, i.e. before background subtraction, the fit to the WW, WZ peak would be improved if the data were uniformly shifted to the left by about 5 GeV, and that would largely wipe out the excess around 150 GeV. Is an additive error in M_{jj} of this type, as opposed to a multiplicative error, a plausible systematic error in the measurement?

Matti Pitkanen said...

For possible explanation of the Wjj mystery in terms scale up version of hadron physics and related predictions see this.

Chris said...

Re my question above about a possible 5 GeV additive error in M_{jj}, Viviana Cavaliere has sent the following response to an email enquiry:

you can't simply shift the bin by 5 GeV because that is not how Jet energy scale (JES) works. In particular JES changes also the acceptance (meaning that changes also the number of events that get into the histogram since you are changing the cut on jet Et too). For this reason it's not physical to simply move bins.

Moreover Fig 7 shows already an unreasonable jet energy scale shift (7%) while at CDF we constrain it at 3%. An additional 5 GeV shift would mean varying the JES of 10% and as we said we don't believe, given what we see from JES measurement, that this physical.

Walter said...

For those interested in the kinematic distributions:
CDF added one more to the website. There is now graph K11, which is the properly defined transverse mass distribution of the jj+ lepton and missing energy. (The previous plot, now K10, was the transverse mass as defined in ROOT.)
The significance of the transverse mass distribution is that it should have an kinematic edge at the mass of the internal resonance (+ a radiative tail).
This should, with enough statistics, distinguish e.g. a technicolor from Z' model....

Kea said...

Daniel, since our non local scheme labels particles differently to all the standard (and most non standard) attempts based on local physics, there is no reason to expect it to behave similarly. Moreover, the braid list gives us precisely SM + mirror neutrinos, and no more, in agreement with the fact that the LHC appears to be seeing little new stuff. Mirror particles are closely associated with the dark sector, which means it is best to view these bumps as a result of non perturbative QCD (and hence QG under a Zvi Bern type correspondence) rather as new particles as such. A mirror top quark is just a simple possibility for modeling the new QCD effects.

Kea said...

Yes, ignorabimus, cool combination. It would be interesting to discuss Koide's ideas with Nima et al.

Anonymous said...

Much ado about nothing?:

http://www.math.columbia.edu/~woit/wordpress/

One thing is for sure, there is plenty of hype going around these days and many cannot resist the temptation to jump the gun...

Kea said...

Will we hear soon. Rumours are just that. Rumours.

Naveen Gaur said...

I heard that D0 too will be coming about their results later on this week. So this would be interesting !!

Anonymous said...

Is this paper from D0 related to that : http://arxiv.org/abs/1106.1457 ?
("Measurements of inclusive W+jets production rates as a function of jet transverse momentum in ppbar collisions at sqrt{s}=1.96 TeV")

Anonymous said...

@Kea

Actually, people like yourself have a vested interest in selling baroque theories with no connection to physical reality. Your ideas add confusion to an existing body of hyped up speculations...

Anonymous said...

It is suspicious to have yet one more paper today:

http://arxiv.org/abs/1106.1682

Perhaps they know something we do not?

Anonymous said...

D0 paper is up now:

http://www-d0.fnal.gov/Run2Physics/WWW/results/final/HIGGS/H11B/H11B.pdf

Anonymous said...

The peak is gone.
http://www.fnal.gov/pub/today/

Anonymous said...

D0 sees nothing:

http://www-d0.fnal.gov/Run2Physics/WWW/results/final/HIGGS/H11B/

Tim Tait said...

D0 sees nothing there:

http://www-d0.fnal.gov/Run2Physics/WWW/results/final/HIGGS/H11B/H11B.pdf