Tuesday 26 July 2011

D0: top forward-backward asymmetry continues to intrigue

While the LHC has been depressingly confirming the predictions of the Standard Model, the good old Tevatron remains the only light in the tunnel. The only 2 lights actually: almost 4 sigma anomaly of the dimuon charge symmetry measured by D0, and over 3 sigma anomaly of the top quark forward-backward asymmetry at high t-tbar invariant mass (yikes) measured by CDF. Future will tell whether these lights should be interpreted as the way out or as the oncoming train. As for the present, the D0 collaboration just provided an important update concerning the forward-backward asymmetry that puts the CDF result in a slightly different... light.

For the latest update D0 used 5.4fb−1 and focused on semi-leptonic top decays. The idea of the measurement is simple: one picks the top decay products, reconstructs the original top and anti-top momenta, and check for an excess of top over antitop quarks moving forward (that is along the proton beam) in the t-tbar rest frame. The Standard Model predicts such an excess should be very small, of order 5%. Instead, after unfolding detector effects from the measured asymmetry, D0 finds the "unfolded" or "production" asymmetry to be 19.6 ± 6.5 %. This kicks in very nicely with the analogous CDF result of 15.8 ± 7.4 % (or 20% when combined with the asymmetry in the dilepton channel). Both results are about 2 sigma away from the Standard Model and both point in the same direction, which is intriguing and almost exciting.

However, not everything agrees perfectly between D0 and CDF. Most importantly, D0 does not see any significant dependence of the asymmetry on the t-tbar invariant mass. On the other hand, CDF sees a dramatic dependence: it was actually the abnormally high asymmetry in the mtt > 450 GeV bin that allowed them to claim a 3-sigma anomaly at the beginning of this year. The situation is thus a bit volatile: the good matching of the inclusive asymmetry between the two experiments is obtained after integrating over discrepant results in the low and high mtt bins.

D0 shares one more important result which I find more exciting. D0 measured the leptonic asymmetry of the leptons originating from top quark decays. The observable is defined as an excess of positive charge leptons moving forward + negative charge leptons moving backward over negative (positive) leptons moving forward (backward). For experimentalists it is more user-friendly than the top asymmetry : it is defined in the laboratory frame and one can avoid tedious and uncertain reconstruction of the momenta of the top quarks. From the theoretical point of view, the lepton asymmetry is tightly related to the top forward-backward asymmetry but not identical. It is related, because the direction of the lepton is clearly correlated with the direction of the mother top or antitop. It is not identical, because that direction is also correlated with the polarization of the mother top. In fact, if the top quark is polarized along some axis, for example in the direction of its motion, the lepton prefers to fly along that direction. See for example this paper for more details. All in all, D0 finds this leptonic asymmetry to be 15.2 ± 4.0%, compared to the Standard Model prediction of 2%. This is more than 3 sigma discrepancy! Not only we get a novel 3 sigma anomaly to cherish, but we also get a hint of anomalous top polarization.

To wrap up , D0 has brought some exciting news and some worrying news too (see the paper for more worries concerning modeling additional QCD radiation that I didn't mention here). The new results will somewhat shake the hierarchy of new physics models that address Tevatron's anomalies, but we have to wait for the next load of theory papers for quantitative details. On the experimental front, next year Tevatron will update all these measurements with twice as much data. About the same time, the LHC will be seriously joining in the game too. Although the LHC cannot measure the top asymmetry directly, due to the symmetric p-p initial state at the LHC, they can access the same physics by constructing more fancy observables. For example, a recent CMS note investigates whether top quarks move closer to the beam axis than anti-top quarks more often than the other way around. Such an effect would be a consequence of a positive forward-backward asymmetry of the t-tbar pair production in quark-antiquark collisions. No effect has been observed in CMS or ATLAS who studied a similar observable. However this doesn't mean much yet: most models addressing Tevatron's top anomaly predict the CMS observable to be the edge of their current sensitivity. It is more worrying that LHC observes no anomalous effects in other top quark observables, like the production cross section or the invariant mass distribution. Yeah, the obstinate lack of new physics at the LHC is utterly worrying. Guys, you better find something fast, otherwise there'll be nothing but darkness.

The D0 paper is available on arXiv. See also Tommaso's comments on the CMS note.

Friday 22 July 2011

Higgs won't come out of the closet

Today we had a true fireworks display in Grenoble: at the EPS conference the LHC experiments presented their Higgs search results based on 1 inverse femtobarn of analyzed data. The cream of the cream is these 2 plots:

Here is just a few fleeting remarks.
  • There was a significant chance to catch him yet the bastard escaped once again. However it's becoming increasingly clear that this year we will learn whether the Standard Model Higgs exists or not.
  • CMS excludes the Standard Model Higgs in the 149-206 GeV and 300-440 GeV windows (plus a few peeping holes here and there), while ATLAS excludes the 155-190 GeV and 295-450 GeV windows. The low mass exclusion is dominated by the search of the H→WW→2l2ν final state, while the high mass one is dominated by H→ZZ after combining different Z decay channels.
  • The exclusion range in the low mass region is smaller than expected. Indeed, there are hints of Higgs-like events in the mass range 130-140 GeV. This is nicely visualized in a plot from the ATLAS talk. The excess in the combined plot is driven by a broad excess WW → 2l+MET events. In certain mass regions the excess is amplified by γγ and ZZ→4lepton excesses, and reaches almost 3 sigma significance. CMS also has a 3-sigmish excess in that same region. This could be a fluke, a mismodeled background, or a first glimpse of the real thing. If the latter is true, we may learn it very soon!
  • We're looking forward to the ATLAS/CMS combination which should be ready for the next big conference: Lepton-Photon in Mumbai. Most of the high-mass region, up to almost 500 GeV, should be excluded by the combination, and it's not impossible that the low-mass Higgs signal will pop above the 3 sigma surface...
  • Both ATLAS and CMS presented their searches in the ZZ→4l channel. Yesterday CDF tried to launch its own firework - a statistically large excess of 4 events with two Z bosons decaying to 2 leptons each near the ZZ invariant mass of around 330 GeV. However that firework fizzled out, as none of the LHC experiments sees any ZZ → 4l excess in that mass region. Given that, there is no way the CDF result can be due to a Higgs or any other new particle; it's either a bad fluke or mismodeled background.
  • We can officially announce that Tevatron is out of the Higgs business. Both ATLAS and CMS on its own have much more powerful exclusion limits than the combined Tevatron exclusion from last summer. LHC should collect 3-5 times more luminosity by the end of the year, which will allow them to beat Tevatron's sensitivity also in the mass region near 115 GeV. Higgs hunting has moved to Geneva, for good...
On a different front, LHCb and CMS presented important limits on Bs → μμ branching fraction. Recall that CDF recently saw a 2-sigmish excess corresponding to Bs → μμ branching fraction of (1.8 ± 1) × 10−8, which, in spite of low statistical significance, prompted some excitement among theorists. However, that central value is now excluded by CMS at almost 95% and by LHCb at more than 95% confidence level. So CDF result seems to be just another fluke... bad luck.

On viXra log Phil is doing a great job of keeping us updated in real time on what is going on at EPS; see this post for a royal collection of Higgs plots. Matt Strassler is blogging live from Grenoble (Et tu, Brute?). See also Tommaso's comments on CMS searches. Tomorrow more excitement guaranteed :-)

Wednesday 13 July 2011

Another intriguing result about B-mesons

Today's update on the measurement of the Bs → μμ branching fraction from CDF makes your heart beat a little faster. The Tevatron collider produces huge numbers of neutral Bd and Bs mesons and they're being looking at from every angle in desperate attempts to spot any departures from the Standard Model predictions. One interesting process to look for is when the 2 quarks making a B-meson annihilate, inducing a decay of that meson to a μ+ μ- pair. This process is mediated by flavor changing neutral currents and therefore within the Standard Model it occurs only via loop processes (see the diagrams), as opposed to much more frequent tree-level charged current decays of the b-quarks. As a consequence, the Bx → μμ decays are suppressed by small loop and CKM factors and the branching fraction ends up being tiny, 3×10^-9 for Bs mesons and 10^-10 for Bd mesons, which is below the current sensitivity. At the same time, these decays has been searched for vigorously because it's fairly easy for new physics to mess them up. For example, additional Higgses in 2-Higgs-doublet models, Z-prime gauge bosons, or SUSY particles in R-parity violating models could mediate these decays pump up the branching fraction.

CDF just posted the latest update on that search based on 7fb-1 of data. They pick up pairs of opposite sign muons originating from the same displaced vertex and measure the dimuon invariant mass. If that mass falls into the window of the Bs or Bd meson mass then we have a Bx → μμ candidate. On top of that, several other properties of these events are cooked into a magic potion (called the neural network discriminant by those in the know) to better distinguish the signal from background. See the plot of the number of events in various bins of the NN discriminant as a function of the dimuon invariant mass. A tantalizing excess can be seen in the upper right window of the plot, with 4 observed vs. 0.9 expected dimuon events having a large likelihood of coming from Bs decays. You should not look in the 2nd left window in that row showing a large excess (16 observed, 8 expected) in the bin where they don't expect any signal ;-) Based on the 3 highest bins, CDF estimates the branching fraction of Bs → μμ is (1.8 ± 1) × 10−8, which is about 2 sigma above the expected Standard Model value. The middle row corresponds to events where one of the muons is detected in the forward region, in which case less signal is expected and no excess is seen. The lower row tells you there is no excess of Bd → μμ events.

So is it interesting or not? First of all, it's merely a 2 sigma excess. Secondly, the data do not trace very well the expected background outside the signal window which casts doubts whether CDF has everything under control. Nevertheless, the new CDF result is very exciting in the context of the D0 observation of the anomalous dimuon charge asymmetry. That anomaly is related to a different decay process where two B-mesons decay to *one* muon each. It is however plausible that both anomalies have a common origin, see for example this paper for quantitative estimates of the Bs → μμ branching fraction in concrete models addressing the D0 anomaly.

The best thing is that we'll learn more very soon. The LHCb experiment is well equipped to make the same measurement. At the moment they have over 400 pb-1 of data on tape. Their own estimates suggest they should be able to see a 3 sigma excess if the Bs → μμ branching fraction is equal to the CDF central value. Moreover, ATLAS and CMS may also try to stick a foot in the door. Hold your breath for just a bit longer; in case anyone sees something the rumor will soon be out on blogs ;-)

See also Tommaso's post. The Wine&Cheese seminar will take place this Friday 9pm Europe time.

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!