Tuesday 22 March 2011

The World after Moriond Electroweak

Rencontres de Moriond is a conference series taking place high in the Italian Alps where particle physics experiments like to present their latest analyses. This year the quality of snow was not quite satisfactory, but the quality of physics results somewhat made up for it. Some highlights have already been discussed on this and other blogs, but I think it's worth giving a short recap anyway.
  • New SUSY searches from ATLAS and CMS.
    Searches in several new channels have been presented: jets+MET+b-tags (relevant for sbottom production), trileptons (relevant for gauge mediated models), e-mu resonance (relevant for certain R-parity violating scenarios), and so on. No excess has been seen, and the parameter space of SUSY has been further constrained. Later I may write something more about the significance of these results, but actually the most striking observation here is how both experiments illustrated their analyses. CMS showed a series of cartoons that pretty accurately summarizes the evolution of SUSY from the early 90s till today:
    ATLAS, on the other hand, for the illustration picked a screen from Impossible Mission, a computer game from the 80s. Those who grew up on ZX Spectrum remember well that the game was long, frustrating, and actually impossible to complete ;-)
  • New Higgs combination from the Tevatron.
    In the last couple of years we got used to Tevatron shrinking the available range for the Higgs mass. This time the 95 percent exclusion range is actually slightly worse than in summer 2010 due to an upward background fluctuation. This may be a sign that the Tevatron Higgs searches are approaching the end of the line, and the full data set that will be analyzed next year may not bring significant improvements. See Tommaso's blog for more comments.
  • First exciting Higgs results from the LHC.
    The ATLAS and CMS searches for Higgs-like scalars in the tau-tau final state chops off a new portion the MSSM Higgs parameter space. See this post for more details.
  • LHC measurements of top quark properties.
    Top is at the moment the hottest issue in particle theory due to the anomaly in the forward-backward asymmetry of the top quark production measured by Tevatron's CDF. Most new physics explanations predict new phenomena that should affect various top quark properties measured at the LHC. CMS flashed some plots the very important distribution of the differential top pair production cross section as a function of the pair invariant mass.Nothing unusual can be seen there, except for a small glitch near 700 GeV. This measurement should severely constrain some explanations of the CDF anomaly, for example those involving the heavy gluon partner.
Two additional results are worth pointing out. Both are rather a show-off at this point, but they demonstrate the amazing potential of the LHC experiment and promise interesting physics in the coming year.
  • CMS observation of single top.
    It took 8 years of the Tevatron Run-2 to pinpoint the single top production, and even now it cannot be extracted from the background without some neural network hocus pocus. On the other hand, CMS was able to observe that process with just 35 fb-1 of data. As new physics often modifies the coupling of the top quark to W and b, which in turn affects the cross section for the single top production, single top may provide important constraints or discoveries in the near future. See also this post on Symmetry Breaking.
  • LHCb limits Bs → μμ.
    This is supposed to be the flag measurement of the LHCb experiment. The importance of this rare decay process stems from the fact that the branching fraction is very suppressed in the Standard Model whereas it can easily be enhanced in many theories beyond the Standard Model, in particular in the MSSM. The first LHCb limit is already close to that from the Tevatron, so that LHCb should take over already this summer. See Collider Blog for more details on the measurement.
This week is taking place the second part of Moriond'11 oriented more on QCD, so we are guaranteed another load of interesting results. As for me, I'm dying to see more of dijets and top quarks results.

Tuesday 15 March 2011

LHC seriously into Higgs searches

As of today the Higgs search industry is still dominated by the Tevatron; for comments on the latest results based on 8.2 inverse femtobarn see Tommaso's blog. But the LHC is catching up faster than expected. Yesterday I saw the first interesting Higgs limits from the LHC. At Moriond, both ATLAS and CMS presented the results of the MSSM Higgs searches in the ττ final state. This search is of quite some interest because of earlier reports from the Tevatron: both CDF and D0 claimed a 2 sigma excess for MSSM Higgs searches in another channel with 3 b-quarks in the final state.

In the MSSM, the Higgs sector is extended as compared to the Standard Model. Apart from the Higgs boson there are 2 additional electrically neutral scalar particles. The production cross section of Higgs and its partners can be largely enhanced for large values of the parameter tanβ. Once produced, part of the time the Higgses decay into a pair of τ-leptons. Lacking any observable excess in the ττ channel, CMS and ATLAS thus produced the limits in the tanβ-mA plane:

See that they beat the corresponding Tevatron limits. It is now clear that the Tevatron 3-b excess is unlikely to be explained within the MSSM. However, the excess can still be a hint of a more general extended Higgs sector, for example a non-susy 2-Higgs doublet model.

One more interesting result was presented by ATLAS. This time the search was for a light particle in the 10 GeV mass ballpark decaying to a pair of muons. This could be for example a pseudoscalar Higgs in the NMSSM, a dark Higgs in the hidden-valley scenario, etc. The ATLAS limits display an intriguing bump near 7 GeV. It's probably nothing but a harmless fluke, but it's worth keeping an eye on. Especially if you can keep an eye over a shoulder of an experimentalist ;-)


Contrary to what the name suggests, NEUTEL is not a telecom company but a series of conferences focused on neutrino physics. This year's edition is currently taking place in Venice. For me, the most expected talk is the one from the Xenon100 collaboration who may or may not present their new results of dark matter direct detection searches. For this and other highlights, check out the conference blog that Tommaso is running.

Update: It's confirmed that Xenon100 will not present new results at NEUTEL. We have to bite our fingernails for a few more weeks.

Saturday 5 March 2011

CDF: curiouser and curiouser

The Tevatron may be a dead man walking but it continues to kick ass. The CDF collaboration just posted a new measurement of the forward-backward asymmetry of the top pair production. Recall that earlier this year CDF made a surprising claim about that asymmetry. Restricting to the top pairs with the invariant mass larger than 450 GeV (about 30% of all t-tbar events) the asymmetry is stunning 48±11 percent, which is 3.4 sigma away from the Standard Model prediction of 9 percent. This is completely crazy: 3 times more often top quarks choose to shoot forward rather than backward (with respect to the direction of the proton beam), even though in the first approximation they should not prefer any direction. Even fancy new physics model have a hard time to predict such a huge asymmetry.

The previous CDF measurement was dealing with semileptonic top decays when one top or anti-top quark decays leptonically to an electron/muon + neutrino + b-quark, while the other decays to 3 quarks. The new measurement that I'm reviewing here is focused on dileptonic decays when both the top and the anti-top decay leptonically. This type of event is more rare; only about 5% of the top pairs decay in this manner. Nevertheless, the top stash at the Tevatron is now large enough. In 5.1 inverse femtobarn that went into the new analysis one expects over 200 dileptonic top events. This is enough to study differential distributions and asymmetries. The CDF note gives the result for the inclusive forward-backward asymmetry, that is for the entire dileptonic sample regardless of the invariant mass of the reconstructed top pair. The measured inclusive asymmetry is 14± 5% which, after unfolding the background and instrumental effects, corresponds to the parton level asymmetry of 42 ± 16 percent. The Standard Model predicts meager 5% so the discrepancy is 2.3 sigma. Much as in the semileptonic sample, the asymmetry is larger at higher t-tbar invariant masses (see the pictures), however poor statistics precludes any firm conclusions. One should also compare that result to the inclusive asymmetry of the semileptonic sample. The latter is much smaller, 16±7%, nevertheless the two results are consistent within 2 sigma.

Formally, the new CDF result is merely a 2 sigma deviation from the Standard Model. However, when combined with the previous 3 sigma anomaly, it has a much stronger psychological impact. One could worry that the CDF measurement of the asymmetry suffers from some unaccounted for systematic effects. In fact, the semileptonic sample has a quirky trait that the entire asymmetry comes from the events featuring a muon, while the events containing an electron do not show a significant asymmetry. Until very recently the following explanations of the anomaly seemed equally plausible:
  • a cat got stranded in the CDF muon chambers,
  • the QCD contribution to the asymmetry has been underestimated,
  • the asymmetry is a manifestation of new physics.
The new result makes the cat hypothesis less likely. In the dileptonic sample the asymmetry is actually the largest for the dielectron events. The systematic effects are quite different for the two measurements, yet both consistently show a large positive asymmetry, which is reassuring.

Now, it remains to make sure that higher order QCD corrections are not playing a dirty trick on us. If not, there will be one option left on the table....