...we studied the γγ invariant mass distribution over the range of 80 to 150 GeV/c2. With 37.5 pb−1 data from 2010 and 26.0 pb−1 from 2011, we observe a γγ resonance around 115 GeV with a significance of 4σ. The event rate for this resonance is about thirty times larger than the expectation from Higgs to γγ in the standard model...

The H to γγ rate 30 times larger than the standard model one appears to be in tension with the existing Tevatron bounds. Both CDF and D0 in this mass range place a limit of order 20 times the standard model cross section. Actually, D0's limits shown at Moriond display an intriguing 2-sigma excess around 115 GeV...

ATLAS also showed their first H to γγ results at Moriond. At that time, their limits were about 40 times the standard model rate. Actually, a bumpy feature near 115 GeV can already be discerned, especially if one assumes that the background rate is overestimated...



Enhancement of the H to γγ rate by 20-30 times sounds humongous. This is certainly not a feature in any mainstream model, and I'm not aware of a single theory paper that predicts it. A recent paper argues that in the MSSM the rate is usually suppressed ; in the NMSSM an enhancement is possible, however by a factor of 2 rather than 20. One ancient paper studies this issue in a more general manner. Large enhancement of the branching fraction to 2 photons is possible if the Higgs couples only to up-type quark (h0_u), or if it does not couple to fermions at all, the so-called fermiophobic Higgs (h0_bh). But this is still not enough; one would need in addition new charged particles with a large coupling to the Higgs (4th generation?, composite fermions?) in order to pump up the production rate.

; in the NMSSM an enhancement is possible, however by a factor of 2 rather than 20. One ancient paper studies this issue in a more general manner. Large enhancement of the branching fraction to 2 photons is possible if the Higgs couples only to up-type quark (h0_u), or if it does not couple to fermions at all, the so-called fermiophobic Higgs (h0_bh). But this is still not enough; one would need in addition new charged particles with a large coupling to the Higgs (4th generation?, composite fermions?) in order to pump up the production rate. Strictly speaking, ATLAS observes a particle decaying to 2 photons, likely a scalar, and likely produced in gluon fusion. A coupling to photons or gluons is not a defining property of the Higgs boson. To demonstrate the Higgsesness of the new particle one would have to pinpoint its coupling to electroweak gauge bosons, for example by measuring its associated production with W and Z bosons. Until that is done, alternative options are on the table.

γγ

Peter's blog breaks a story that will be all over the news tomorrow. The Easter Bunny dropped in his comment section the abstract of a sensational ATLAS internal note, which saysSo, the note claims no less than a firm evidence for a 115 GeV Higgs boson decaying into 2 photons. This decay occurs in the standard model with the amplitude dominated by a W-boson loop. However, this decay is very rare - only about 0.2 percent of Higgses decay this way. It's absolutely impossible that ATLAS is seeing the standard model Higgs - the rate is way too small. However, one can imagine theories beyond the standard model where the production cross section of the Higgs and/or its branching fraction into 2 photons is enhanced.Well, it's past 4am, I really need to sleep. So just a few hasty remarksWhatever it is, it means busy days ahead, for theorists and experimentalists alike. One thing is certain: it's not a hoax, the note and the analysis really exist. But one should keep in mind that the result has not been internally reviewed yet, thus, at this point, it is not backed by the entire ATLAS collaboration. It may well turn out to be a false alarm... or it could be the discovery of the century... stay tuned.Update: Tommaso points to newer and more stringent CDF limits than I linked to above. For a 115 GeV Higgs therate must be less than 15 times the standard model rate, which further disfavors the ATLAS signal. On the other hand, CDF has some excess near 120 GeV...