Monday 31 January 2011

2 more years at 7

It's official now. Here is an excerpt from DG's email:
...The main decisions we have taken are that the LHC will run through 2012 before a long shutdown, we'll keep the energy at 3.5 TeV during 2011, and we'll work hard to increase the luminosity steadily...
Seems reasonable enough to me. With the pressure from the Tevatron gone, it is not critical to squeeze every bit out of the machine. Hence the decision to remain at the 7 TeV center-of-mass energy of collisions which increases the chances that the thing will not explode in our hands again. More importantly, the run-1 is going to continue until the end of 2012. By that time, the LHC should have acquired some 5 inverse femtobarns of data, maybe more. This will be enough to see glimpses of new physics, provided there is anything below a TeV. But the most solid advantage of extending the run-1 is that the Higgs will be discovered earlier than in the alternative scenario with the 2012 shutdown. Thus, the minimum plan should be accomplished by the early 2013, if only Higgs is where we expect him to be. The disadvantage is that for 5 more years, rather than 4, I'll have to listen to talks about supersymmetry.

For a more illuminating analysis of the Higgs prospects, see Tommaso's blog.

Monday 24 January 2011

Another Intriguing Result from Tevatron's CDF

Two weeks ago we got excited about the forward-backward asymmetry of the top quark pair production. The CDF experiment says that the asymmetry at large t-tbar invariant mass is nearly 50%, as far as 3.4 sigma away from the standard model. A last week paper of Gilad Perez & co. points out that there exists another CDF measurement related to top quarks which also shows a large discrepancy from the standard model. The funny thing is that the latter result has been available as a public note since last summer but it went relatively unnoticed. The numbers and plots quoted below are take from this recent CDF talk.

The result I'm talking about is the CDF search for boosted tops. Here, "boosted" refers to top quarks wheezing out of the collision point with pT > 400 GeV. Such large pT implies that all the decay products of the top -- one b-quark + two quarks or two leptons from the W -- merge into a single object that looks very much like an ordinary QCD jet. Therefore CDF scanned their data for dijet events with at least one jet of pT > 400 GeV. The basic handle to distinguish boosted tops in that sample is the invariant mass of the jet: when the top quark decays fully hadronically (t → b + q + q) the mass of the jet parented by the top should be in the vicinity 170 GeV, whereas the masses of ordinary QCD jets span a wider range with a preference toward a lower value. The plot below shows the distribution of the masses of the high-pT jets as measured by CDF:
Although it is not immediately clear to the eye, there is an excess in the signal box. One counts 32 events in the top mass window, while QCD predicts only a third of that. It looks like CDF has captured 10-20 boosted tops. But the standard model predicts much less than that: only about 3 top events should be in the window! The excess is 3.4 sigma.

One could easily imagine new physics producing an excess of boosted tops; the canonical example would be a heavy gluon in the Randall-Sundrum scenario. Then why is CDF not jumping around claiming an evidence for new physics? The reason is that there is a certain tension with that interpretation of the signal. The fully hadronic decays, targeted by the above plot, constitute only a subset of the top events. Almost as often, one of the top quarks should decay leptonically, that is t → b + lepton + neutrino. For boosted tops, the charged lepton (e,μ,τ) merges with the b-jet and cannot be easily discerned, but the missing energy carried by the escaping neutrino should be registered. Thus, in the case of semileptonic decays of a boosted top pair one should observe one high-pT jet in the top mass window accompanied by large missing energy (those events were excluded from the previous plot). The relevant plot is pasted on the right. There is no excess, but actually a small deficit in the signal box where QCD predicts some 31 events and CDF observes 26. Because of the unclear interpretation, CDF decided to play it safe /swipe it under the carpet, chose one. In their note they dumped together the fully hadronic and semileptonic samples, in which case the excess becomes insignificant, and quoted only this unexciting in the abstract.

There are several possibilities on the table now. By far the most probable is that the hadronic excess is a fluke or is due to an underestimation of QCD background. Conversely, the deficit in the semileptonic channel could be a downward fluke, and there is indeed an excess of boosted tops at the Tevatron. The third possibility is to take all the hints from experiment seriously. An excess in the fully hadronic channel and the lack of an excess in the semileptonic channel may suggest that Tevatron is seeing a new particle that decays exclusively to hadrons and whose mass is accidentally close to the top quark mass. For example, the paper of Gilad & co. argues that a light gluino decaying into 3 quarks (that is assuming supersymmetry with R-parity violation) can be made consistent with the data. Whatever it is, this is a kind of signal that the LHC should eat for breakfast, if only it is real.

Monday 10 January 2011

No Bosons for America

Today Facebook and blogs are abuzz with the news that the operation of the Tevatron will not be extended beyond the financial year 2011. At first sight this may appear a short-sighted decision. If Tevatron continued until 2014 and doubled the luminosity acquired so far it would have a good chance to snatch the Higgs boson, possibly the biggest prize in particle physics in this century. So why backing down now? Why slaying a goose that is about to lay a golden egg? Of course, the real reason for closing the Tevatron is not the operation costs (peanuts) or the competition from the LHC (for a light Higgs, the Tevatron could get there first). The real reason is much more profound. The real reason is the fundamental law that I pointed out some time ago, which is known as Pauli's other exclusion principle:
Fermions are discovered in the US, whereas bosons are discovered in Europe.
This law has been tested in multiple instances, and has been established beyond all doubt. Evidently, the policy makers read blogs and are aware that any attempts to discover the Higgs boson at the Tevatron would be doomed from the start. Conversely, the DOE decision to shut down the Tevatron is yet another proof that Pauli's other exclusion principle is the fundamental law of nature that can never ever be violated.

Tuesday 4 January 2011

New Physics for the New Year from CDF

The year 2011 begins with an earthquake. Just yesterday I was speculating what good old Tevatron would bring as this year. The following morning we're awaken by trumpets and angel choirs announcing the new CDF paper. Inside it, the 2-sigma blip previously seen in the forward-backward asymmetry of the top quark production is promoted to a 3-sigma blip.

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 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.

Monday 3 January 2011

The Year of Living Dangerously

The previous 4 years of Resonaances were but a constant decay. As you can read from the plot of my posting activity versus time, the end is expected in late 2012. The end of the blog, or the of the world, don't know which. Yet before this happens we have the year 2011 that, for obvious reason, is going to provide us with more excitement than the entire last decade. Here is what I'm waiting for the most (ordered according to the level of my impatience to see the results):
  • LHC
    It's the year of truth for particle physics. If there exist new colored particles below TeV (expected in basically any theory that addresses the hierarchy problem of the standard model) we should catch at least a glimpse of them this year. If nothing shows up...well, we can always dream of new physics behind the next corner, but we better start telling people that the LHC has always been about the Higgs :-)
  • Xenon100
    The most sensitive dark matter detection experiment is now sitting on almost 150 days of unpublished data, in addition to the small bone of 11 days they threw us in early 2010. The new results should be published some time soon, although Xenon100 does not commit to a specific date. The prospective limits on dark matter - nucleon cross section should not be too far from the dotted red line on the plot (which assumes 200 days)....unless the signal is there. If not this year then when?
  • Tevatron
    It's probably the final year of running for this machine. At this stage it is unlikely that new physics will jump in our faces, unless it is concealed in some truly exotic channel that nobody has thought to look at so far. Nevertheless, the Tevatron still has a potential to provide some excitement this year. First of all, there will surely be new Higgs limits this summer: another chunk of Higgs masses will be crossed out, and maybe we'll even see a 2-sigma hint of the real thing. On the new physics front, we are waiting for updates on CP violation in the B-meson system, as this is the place where the most intriguing anomalies have showed up. And, who knows, maybe some other 2-sigma blip will be promoted to a 3-sigma blip, the good candidates being the forward-backward top-quark production asymmetry, the t-prime hunt, or the Higgs partner search in the 3-b channel.
  • IceCube
    This neutrino telescope, named in honor of the gansta rapper, has just been completed last month. IceCube is a chunk of ice at the south pole where high energy muon neutrinos are invited to convert into muons, whose Cherenkov radiation is then detected by strings of photomultipliers embedded in the ice. These new eyes on the universe will surely bring a fascinating progress into conventional astrophysics. Can there be hints of new physics in cosmic ray neutrinos, or in neutrinos from the Sun? Why not.
  • MEG
    This one could be a dark horse, although it's not even clear if they will be joining the race this year. MEG is another experiment with muons at PSI, Switzerland. It is looking for the μ →e γ decay whose rate is unobservable in the Standard Model, but could be easily enhanced in many of its extensions. Last summer at ICHEP MEG presented the results of an early 2-month run that showed an intriguing clustering of events near the signal region (although none of the events passes the timing + angle cuts). They're planning to continue running until the end 2012 so as to bring the sensitivity down to 10^-13 branching fraction. But if the events keep popping up we may hear about them sooner.
  • Up in the Sky
    This year it will be more quiet up there . The AMS-II experiment is going to be launched to the ISS early this year (unless they decide to replace their magnet with a refrigerator magnet) but we'll have to wait years for any interesting results. PAMELA and Fermi are still up there and we may still learn something new about the cosmic ray spectrum. But, most likely, the sky won't rock before 2012 when Planck publishes their first CMB analysis.
If i forgot something to be impatient about, don't hesitate to point it out.