Live Blogging: #Higgs Boson

One day before: Update from CDF/D0

Tevatron’s experiments CDF and D0 just published their most up-to-date searches for the Standard Model Higgs boson. You can read the preprint here [arXiv:1207.0449] and take a closer look at the bonanza of new plots here.What they see is a mild excess especially in the h->bb decay channel, for which they have a better signal-to-background ration with respect to the LHC. Taking into account the look-elsewhere-effect, this excess can be estimated as a 2.9 sigma deviation from the background-only hypothesis in the same mass range pointed out by ATLAS and CMS, at about 125 GeV. However, combining this result with the other channels (mainly h->γγ, h->WW->lnln, h->ZZ->4l) the deviation is much smaller. The breakdown of the separate contributions shows an enhancement in the h->γγ channel and a deficit in the h->WW decay. Overall, the three rates are still barely consistent with the Standard Model prediction, but the Higgs mass fit as a function of the W and top masses at present seems to prefer a low-mass SUSY-like scenario.

This excess did not show up only yesterday: taking a look at the same plots as the data piled up, there is an evident positive trend in the signal region, while the background region tends just to fluctuate as expected.

In the end, after 25 years of hard work, the bottom line from Tevatron is that there is an evidence of a scalar particle with mass around 125 GeV compatible with the Higgs boson. Whether this is a Standard Model one or not is still a matter of debate (perhaps for just one day!).

#Higgsdependence day

Starting now! (9:00 Geneva time)

Directeur Genéral Rolf Heuer takes the floor: “today it’s a special day”

Joe Incadela, spokesperson of the CMS experiments, presents their new results on the Higgs boson searches!

JI: “The Standard Model has been confirmed by more than 100 analyses to better than 1%. Now only the Higgs boson remains to be discovered”.

JI: “Stellar performance of the LHC enables all experiments to produce significant physics results”.

JI: First channel: h->γγ ! Analysis is a multi-variate one, with ~15% gain w.r.t. cut-based, the latter used as a cross-check thanks to its robustness.

This decay channel has a low rate, but it’s very clean and allows a very sharp mass resolution


JI: Largest excess at 125 GeV !!! Local p-value corresponds to 4.1 σ, global in the full range 3.2 σ

Here comes the h->ZZ->4l channel, which is very clean but has a very low yield.

And here again, it’s very easy to spot the peak! Statistical significance is lower, 3,2σ at 125.5 GeV

9:38: Higgs discovered combining the γγ and ZZ channels! A shiver down the spine! 5 σ reached!!

The next channels will not show a peak, but only a broad excess (if any!)

Some excess found, but not significant in WW and ZH channels. Same story for the ττ channel.

And now the full result!

JI: “All channels combined together: 4.9 σ . Expected from the SM: 5.9 σ”

Why this deficit? Is it a statistical fluctuation or what?

JI: “Coupling to fermions and bosons consistent with the SM expectation within 95% confidence level”

 m_H = 125.3 ± 0.6 GeV at 4.9σ

JI: “Don’t forget, #Higgs is global: “These results are shared by all of mankind”

Now it’s time for ATLAS’s spokesperson Fabiola Gianotti. Go Fabiola Go! (btw: still using MS Comic Sans)

FG: “It’s not easy to speak second as all the clever things have been said”

FG: “Sizeable gain the efficiency of reconstruction of physical objects for e/γ/μ. Sensitivity improved for H->γγ and H->ZZ->4l channels”

FG: “Di-jet identification added to tag VBF events in the H->γγ decay”

Combining 2011+2012 data, the excess can be clearly seen! Including the look-elsewhere-effect it reaches 3.6 σ, local significance is 4.5 σ compared to the SM expectation of 2.4 σ! Signal strength 1.9 ±0.5 w.r.t. SM prediction!

Both experiments confirm a signal strength larger than expected!

Now showing: high-mass region in the ZZ production

FG: “1.3 times more ZZ events in data than SM prediction, but still compatible within uncertainty with the Standard Model”

Now showing: low-mass region in the ZZ production

3.4 σ excess, compared to 2.6 σ as predicted by the Standard Model at 125 GeV. Signal strength  1.2 ± 0.3
Total combination: 5.0 σ at 126.5 GeV! (expected: 4.6 σ) 
The best-fit value of the mass is still different between the two experiments. Perhaps the most interesting part is the signal strength larger than predicted. New physics lurking just around the corner?
FG: “We have only recorded 1/3 of data expected in 2012. This is just the beginning.”
Rolf Heuer says “I think we have it, do you agree?”
Peter Higgs: “I’m glad it happened in my lifetime”

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