Friday 1 July 2011

D0: 4 sigma like-sign dimuon anomaly!

About a year ago the D0 collaboration announced a surprising result. They compared the number of events with two positive muons and those with two negative muons. Once the contribution from kaons and pions decaying to muons within the detector is subtracted and some instrumental effects are taken into account, the number of positive and negative muon pairs is expected to be the same. Instead, D0 saw a 1% excess of events with 2 negative muons which represented a 3.2 sigma deviation from the Standard Model prediction. Yesterday D0 presented an update of that measurement based on 9fb-1, that is almost the full data set they have on tape. They obtain the asymmetry of −0.787% with an error of about 0.2%. The anomaly has grown to 3.9 sigma!.

The observed dimuon charge asymmetry is most likely due to asymmetric decays of B-mesons. Bottom quarks inside these mesons can decay as b → c μ- ν, and analogously an anti-bottom quark can decay to a positive muon. Most of the time the Tevatron produces pairs of bottom and anti-bottom quarks, each of them dressing into its own B-meson. However, neutral B-mesons can oscillate into its own antiparticles. If this happens, both original b-quarks may end up decaying to same-sign muons. Furthermore, if the oscillation probability violates CP, that is oscillating Bbar → B is more likely than the other way round, then the excess of negative muon pairs may show up. In fact, such an effect occurs within the Standard Model, but the predicted asymmetry is tiny, of order 0.01%. On the other hand, the asymmetry of the size observed by D0 requires new sources of CP violation beyond the Standard Model. Like what? Like Z', W' charged Higgs, KK gluons, or whatever; we would need more clues to guess the right answer.

An important new element in the latest D0 analysis is the study how the asymmetry depends on muon's impact parameter with respect to the primary vertex of the collision. Muons from B-meson decays often have large impact parameters because decay happens picoseconds after production. On the other hand, muons from kaon decays have typically small impact parameters because the mother kaon usually comes straight from the collision point. Thus, selecting events with large impact parameters enriches the sample with dimuons from B-mesons decays. D0 concludes that the dependence of the asymmetry on the muon impact parameter is consistent with the hypothesis that it indeed originates from B-meson decays, and not from some mundane background. Moreveor, the cut on the impact parameter also affects the relative fractions of Bd and Bs meson decays in the dimuon sample (these fractions are about 50-50 without the IP cut, but due to different oscillation parameters more Bd mesons spit muons with large IP). Thus one can put better constraints on separate contributions of Bs and Bd mesons to the asymmetry. The result is this plot:
The axes are the semileptonic decay asymmetries of the Bd and Bs mesons. The pink band is the fit to the observed dimuon asymmetry without the IP cut, while the ellipse takes into account the input from the IP measurements. Unfortunately, we still cannot tell whether the asymmetry is due to Bs mesons, or Bd mesons, or both, which is of primary importance for theoretical interpretations of the anomaly.


So have we discovered new physics yet? Alas, recent history teaches us not to celebrate before the signal is confirmed by an independent experimental group. The CDF collaboration had an anomaly even larger than 4 sigma which did not stop D0 from ruthlessly shooting it down. The rules of the wild west suggest that CDF may attempt the same with the D0 pet anomaly, after which they all meet at the O.K. Corral. But maybe this time it'll be different? Maybe this time it's for real? We may learn more later this year, either from CDF or from the LHC. Actually, the LHCb experiment promised to deliver a complementary evaluation of the B-meson decay asymmetries by measuring the B →D μ ν decay rates. Because of systematic effects they find it easier to determine the difference of the Bd and Bs meson semileptonic decay asymmetries (while the D0 dimuon asymmetry depends roughly on the sum thereof). With 1fb-1 of data their expected sensitivity corresponds to the thin gray band in the plot on the right. One more reason to bite our nails while waiting for the next LHC results!

21 comments:

Kea said...

Hurrah, new physics! Triality rules!

Anonymous said...

This is BS! CDF demands satisfaction!

Mitchell said...

Is there any interaction between these results, and the recent constraints on CP violation coming from measurements of the electron dipole moment?

Jester said...

Not directly. One is related to the quark sector, the other to the lepton sector. In generic models CP violation in these two sectors depends on different parameters, unless you make some specific assumptions about unification.

Anonymous said...

Since this has been around at the 3 sigma level for a year, why didn't CDF bother to look into this before?

Jester said...

CDF has been looking into it for at least a year now but I don't know how close to finishing they are.

Anonymous said...

The SM prediction depends crucially on the B semileptonic form factors, which, if one talks purely theoretical predictions, are known only to an accuracy of few percent (that's being optimistic).

Of course one can extract them from experimental data assuming pure SM in their analysis... Still, this kind of tiny effects in heavy meson physics require a larger than usual cum granum salis.

Jester said...

D0 also gives the residual charge asymmetry that does not depend on the interpretation in terms semileptonic B decays, see Eq 40. That one is also 4 sigma away. Not that I disagree about salis...

Hannah said...

Hi Jester,

Apologies for contacting you this way. I can't find a way to contact you either through here or your website. It says you are on the CERN directory but I can't find you there.

I would like to contact you regarding a documentary my company are developing. I would be really grateful if you could pass on your email adress so I could tell you a bit more about the project.

My email is HannahB AT filmsofrecord DOT com. Please feel free to delete this comment when you've read it!

Thanks,

Hannah Boyd
Films of Record
6 Anglers Lane
London
NW5 3DG

Anonymous said...

I remember seeing the D0 presentation last year. The difficulty with this measurement is that it is a very small asymmetry in a large signal that one wants to measure. So it is important to suppress backgrounds as much as possible, in particular also backgrounds that might be induced by an asymmetry of the D0 detector.

One key ingredient of the D0 analysis was that they flip the D0 magnetic field every other day. Apparently this reduces machine background. Now as far as I understood, CDF either can't or didn't do such a flip. In that case they might not have enough sensitivity for this measurement, although I bet they are trying to do it.

I'm not sure if LHCb is able to look for this.

Anonymous said...

The ignorance of this blog and its respondents is breath-taking. (e.g, childish crap like this: "The CDF collaboration had an anomaly even larger than 4 sigma which did not stop D0 from ruthlessly shooting it down. The rules of the wild west suggest that CDF may attempt the same with the D0 pet anomaly, after which they all meet at the O.K. Corral.") In the case of the same-sign dimuons, CDF has looked for the excess and not been able to make a "definite" statement. Part of the problem (at least) is that D0's effect depends on a delicate background subtraction (sound familiar?) that CDF cannot so easily do. D0 can change the direction of its solenoidal magnetic field, and this is useful for reducing systematic uncertainties. CDF cannot do this. So...we may have to wait for LHCb to settle this. Be patient, children. And be responsible in what you say. Thanks.

Jester said...

Some time ago LHCb promised to measure the difference of the a_fs^d and a_fs^s with 1fb-1, which obviously could clarify a lot. I added their PR plot at the bottom of the post. Not sure when will they deliver.

Luboš Motl said...

Dear Anonymous,

I didn't understand your criticism of Jester's sentence. More precisely, it doesn't make any sense.

Jester just wrote that D0 has ruled out the model with an extra 150 GeV Wjj bump suggested by the CDF analyses - despite the CDF claim that the significance was above 4 sigma. Do you dispute that these different results on the Wjj channel occurred?

Jester just formulated it wittily because he's Jester, after all. He also suggests that the CDF and D0 roles may get exchanged because CDF may ultimately also say something about this asymmetry. There's no fundamental reason why it shouldn't be able to say anything.

Your decision to discuss LHCb is nice and LHCb could also be relevant but it doesn't have to be. At any rate, none of your comments have shown any evidence that Jester's description was breathtakingly childish.

I will omit universal comments on the respondents because they're a mixed bag, of course.

Cheers, LM

VF said...

You guys really have a lot of time.

Anonymous said...

@Lubos. Actually D0 ruled out the existence of a 4pb resonance with WH kinematics. They can't rule out anything else and they don't want to update the result with new data...
And people should really look and ask for the details especially to D0 that ha not released any at all.

Luboš Motl said...

Hi, I agree that D0 has excluded the theory with 4 pb production of the 150 GeV bump. I guess that Jester agrees, too. It was kind of his point which was criticized by you - or another anonymous.

Not sure what you mean by "CDF cannot rule out anything else". They have ruled out many things and they may still rule out additional ones before they're permanently abolished.

One of the other bogus claims by CDF that D0 has excluded was the claim about the top-antitop mass difference. CDF had claimed that it was huge, something like 3.3 GeV, implying a giant CPT violation.

D0 has returned physics back to Earth, claiming that it is at most 0.8 GeV with the same error. CPT is totally fine and CDF got another strike.

Mitchell said...

Ethan Siegel just suggested this might be responsible for cosmological baryon asymmetry!

Mitchell said...

OK, I see this aspect was mentioned last year, in the original paper from D0. But still, it makes the observation all the more interesting.

Anonymous said...

CDF see an excess in Bs -> mu mu :
http://arxiv.org/abs/1107.2304

Anonymous said...

"Like what? Like Z', W' charged Higgs, KK gluons, or whatever; we would need more clues to guess the right answer."
Is http://arxiv.org/abs/1107.2304 'clueish' enough ?

Anonymous said...

LHCb has decided NOT to hold a press conference regarding their result. The reason is that they are in complete agreement with the Standard Model and disagree with D0. Source is a friend on the experiment....this was learned last night.