Higgs: stronger and more exciting

Today the CMS and ATLAS collaborations dumped into public pages a dozen publications describing the Higgs searches in the 2011 LHC data. In the first approximation, these are the same results that were presented on December 13. But there is one surprise...

The CMS collaboration had every reason to think that life was unfair. For the last round of Higgs searches they made significantly more effort and analyzed more possible signatures than ATLAS. The latter updated only 2 channels to the full dataset, and in principle had worse sensitivity in the H→ZZ*→4l channel (due to slightly higher pT thresholds in the analysis). In spite of that, the significance of the Higgs-like excess near 125 GeV was much weaker in CMS than in ATLAS. Naturally, the CMS researchers have spent the last 2 months scouring their drawers for strayed Higgs events. And they found.

New interesting events are reported in the H → γγ channel. Compared to the December 13 presentation, CMS added a new category of events which, apart from 2 photons, contain 2 energetic jets in the forward (closer to the beam pipe) region of the detector. Such events could arise in the so-called vector boson fusion (VBF) process, where each of the 2 colliding quarks emits a W or Z boson which coalesce to create a Higgs boson (right graph). The 2 original quark get deflected and may be seen in a detector as two forward jets. On the other hand in gluon fusion (left graph), which is the dominant Higgs production process at the LHC, the 2 colliding gluons "vanish" and the final state rarely contains 2 forward jets. Also background processes are less likely to produce 2 photons in association with 2 such jets. Hence, by selecting diphoton events with 2 forward jets we can probe a distinct Higgs production process, with less signal (the VBF cross section is 10 times smaller than the gluon fusion one), but also with less background.

Now, in the VBF class CMS finds 7 diphoton events in a 1-GeV bin at the invariant mass 124 GeV, where only about 2 events would be expected from non-Higgs background. By itself it would be nothing, but together with the rest of events in the diphoton and 4-lepton channels it provides another support for the existence of the Higgs boson in the mass range 124-126 GeV. All in all, the local significance of the excess near 125 GeV in the combined CMS analysis is now over 3 sigma, very similar to that of ATLAS. While many small improvements have been made, my feeling is that significance was pumped up mostly by these additional VBF events.













But there is something intriguing here. Now in both ATLAS and CMS the best fit of the Higgs rate in the H → γγ channel alone is about twice the Standard Model rate, with the standard rate being over 1 sigma away in both cases. Actually, with the present amount of data CMS would not expect to see any VBF events, as rate predicted by the Standard Model is too small (see below that in this channel their fit is almost 4 times the standard rate, although with a large error). Could it be that we're seeing a non-Standard-Model Higgs boson with an enhanced decay rate and/or enhanced VBF production cross section? Of course, it is far too early for jumping into conclusions: the errors are still large and we may easily be observing an upward fluctuation. Besides,the combination of all channels doesn't show any dramatic enhancement of the Higgs rate. In any case we're free to speculate while waiting for more data (and a word from ATLAS on the VBF events).















See also Matt for more details and more caution.