Wednesday, 8 February 2012

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.

12 comments:

Kea said...

Great update, thanks. Twice the SM rate is interesting indeed.

Beijixiong said...

In the di-jet tagged gamma-gamma spectrum, bins with zero events have no data points. The bin with 7 events, which you think is "driving" the significance, is next to a bin that has apparently has no events. If that is so, the average number of events in the 2 adjacent bins is 3.5, pretty close to the SM expectation.

Jester said...

It seems to me the adjacent bins have 2 events each (note that the signal profile in blue is at 120 GeV, not 125)

Anonymous said...

is there any sensible theory that explains a possible enhanced VBF? It seems it requires a larger VVh coupling than in the SM. But I thought that no theory in the UV allows that. Perhaps an enhanced hgg vertex is more likely.
cheers

paolo said...

Though not the first one to give a report on this, but You are really spot on.. again. Thanks.

Jester said...

Anon, increased VVh coupling is impossible in popular extensions of the SM, like the MSSM or composite Higgs, but there is no no-go theorem. There is an old obscure model by Georgi and Machacek where this can be realized.

coraifeartaigh said...

Great post, many thanks

Chris Austin said...

Are you concerned at all that in the CMS combined local p-value plot, (their Fig. 6, your 3rd plot), the valley at 124 GeV is totally due to H -> 2 gamma, while H -> ZZ -> 4l produces a completely separate valley at 119 GeV: neither of these two channels shows a valley where the other does?

Jester said...

Not too much, because ATLAS has 3 H->ZZ->4l events near 125 GeV. CMS may have just been unlucky. We'll with more data...

Jester said...

Comment to my comment one before: it may not be clear that to boost sigma(VBF)xBR(h->gamma gamma) it's not necessary to pump up the hWW coupling. Another way is to increase Br(H->gamma gamma) and that can be done in popular models like composite Higgs or, up to a certain point, the MSSM.

GastroBear said...

As in December the 95% CL exclusion line is, in both experiments, between 1 and 2 sigma upwards to respect to the expected one over a huge mass range, up to 180+ GeV.
Am I the only one finding this feature "disturbing"?

Even if one considers a terrible sensitivity for the WW channel (let's say 40 GeV spread) it is difficult to have that kind of plateau.
It makes people think that they are not understanding something (background?). What do you think?

Anonymous said...

mmm.... so there were good reasons for stressing the point on hWW couplings... |-)