Live Blog: Webcast from Moriond Electroweak


Study of full ATLAS dataset for H->bb and H->tau tau is underway.


She shows all the channels – in none of them is a deviation observed from the background-only expectation in ATLAS data.


pp->bb production is a million times higher than production of Higgs and subsequence decay H -> bb. That’s a real challenge.


She will present new results from H->invisible decays and H->mu+ mu-. They use the full ATLAS data set in their results, compared to other results (bb, tau tau) last updated in 2012.


She reviews the motivation for looking at fermion decays of the Higgs.


The ATLAS talk is given by Dr. Victoria Martin (Univ. of Edinburgh).


Now it’s ATLAS’s turn to talk about decays of a Higgs-like particle to fermions.


Reminder: webcast here:


For the pure hadronic H -> tau tau final state, how much of the background is from real a1 -> 3pion decays?

For the background, you have an a1 or rho inside the tau decay from a Z decay. Most background we see is due to this effect. There are also combinatoric backgrounds from random triplets of tracks. However, they do try to reconstruct the a1 and rho inside the tau hadronic jet.


Bill Murray from ATLAS asks first question. There appears to be a deficit below the Z peak in tau tau and of course you are looking for an excess above the Z peak – how well is the tau energy scale understood?

She mentioned that they calibrate from the Z peak.




Results use 5+19/fb (7 + 8 TeV data). mu+hadronic tau channel is single most sensitive channel, but all channels combined to get clearer picture (with more weight given to more sensitive categories).

Data supports the existence of a signal contribution around 125 GeV, albeit with big uncertainties after background subtraction (background is huge).

Limits derived from data show observed excess over background-only expectation – observed sensitivity is 1.8 times the Standard Model. They measure a signal strength of 1.1 +/- 0.4.

Maximal local significance is 2.9 sigma at 120 GeV; at 125 GeV they have a signficance of 2.9 sigma as well.

Strong indications that new particle at 125 GeV does decay to taus.


Huge background from Z -> tau+ tau-; careful modeling is critical to success in this channel.


Now on to Higgs -> tau+ tau-!


They expected to set a limit around 5.2 times the Standard Model expectation; the observed limit is about 5.8 times the Standard Model, consistent with expectations from background-only. This also has a ways to go before it’s sensitive by itself to exactly the Standard Model prediction.


She now presents results from pp -> ttH, where the H->bb. This is an independent way of looking for the Higgs decaying to bottom quarks. This uses 5/fb each from 7 TeV and 8 TeV data.


They see a slight excess in data over pure background-only hypotheses, at around 125 GeV – but the resolution is not great in this channel (compared to ZZ or two-photon), so it’s hard to say exactly where the excess might be located. Signal strength is presently about 1.3 +/- 0.7 – still a ways to go to make this a strong channel all on its own.


The data is consistent with di-boson background (e.g. pp -> ZZ, WZ, WW, etc.), but can accommodate a Standard Model Higgs signal as well, though without much resolving power for now. They try to improve sensitivity using advanced multivariate techniques to take advantage of a suite of discriminating variables that each by themselves can separate signal from background, though perhaps not very well; when combined, the separation improves.


They take the “usual” strategy – look for Higgs produced in association with a boson, such as Z or W. This reduces the production rate of the Higgs, but cleans up a lot of the background that otherwise hides a possible signal. They are using 5/fb at 7 TeV and 12/fb of 2012 data – results are NOT UPDATED since November of last year.


She begins her results discussions with Higgs decays to a pair of bottom quarks – this is the largest branching fraction at low Higgs mass, but is challenging due to pollution by the copious quark and gluon jets produced by the LHC.


She is discussing the importance of good measurements of “missing energy” – energy that is assumed to arise from particles that cannot be detected, such as neutrinos. Good missing energy reconstruction is critical in flagship channels such as Higgs decay to a pair of tau leptons, where the tau leptons then also decay to longer-lived particles and neutrinos.


Now Valentina Dutta (MIT) is giving the status of CMS searches for Higgs decays to fermions. Fermions are particles that carry one-half-integer spin angular momentum (S=1/2, 3/2, 5/2, etc.). Examples are electrons, protons, and neutrons.


That’s it for now on experimental talks from ATLAS and CMS. Sorry I missed the CMS talk on the live stream. I’ll post notes on both the ATLAS and CMS results in the blog later, when the slides are available from the Moriond EW website:


Question: what is so different between ZZ and gamma-gamma that the masses measured from each appear to be in tension? There was concern about the energy scale of the ATLAS calorimeter.

Dr. Hubaut talked about how the calorimeter is calibrated and how to map from photons to electrons in energy measurements. These have all been extensively studied in the current data. More needs to be understood about material in front of the calorimeter and how to model it. Also, we can now use the new state itself to do calibration studies.


Mass from the two-photon channel is measured to be 126.8 +/- 0.2 (stat.) +/- 0.7 (syst.).

The signal strength for the ZZ channel (ratio of measured rate to Standard Model predicted rate) is 1.7 +/- 0.5, roughly. This is slightly larger than what was measured last year (1.4); but a lot has changed in the analysis since then, both additional data and improvements to the procedures. This is not an unexpected change.


Questions time.


The observation of the new state is at a significance greater than 6 standard deviations in both the $H^0\to\gamma\gamma$ and $H^0 \to ZZ^{(*)}$ channels separately. This is amazing. To go from combined discovery to this kind of strong individual channel discovery in just under a year is an amazing testament to the power of the LHC to deliver data and the power of ATLAS experimentalists to understand and present the data analysis.


Dr. Hubaut is now showing $H^0 \to Z \gamma$; SMU was central in making this a formal analysis within ATLAS after the discovery of the new state last summer, and with our many partner institutions in ATLAS we have completed the very first result from ATLAS on the search for this channel. Unfortunately, the channel is challenging and we are still far from being able to measure the Standard Model predicted rate for $H^0 \to Z \gamma$; but, we can at least say that there doesn’t appear to be an anomalously large coupling that produces this final state.


Dr. Hubaut has just shown the latest results from the ATLAS Experiment in the decay $H^0 \to ZZ^{(*)}$. ATLAS confirms again that the new particle indeed decays to this final state with good resolution, and reports the mass of the new particle to be measured in this channel to be still about 125 GeV.


Current, Dr. Fabrice Hubaut (CPPM) is presenting the results of Higgs decays to pairs of bosons. Bosons are particles with an integer unit of internal angular momentum, (S=0,1,2,…). Particles in nature that are boson are the familiar photon, the particle of light, and the less familiar Z and W particles that mediate the weak interaction (critical to nuclear decay, which makes the burning of the sun possible).


The webcast from the Moriond Electroweak Conference is live here:

This is the “Higgs Day” of the conference, where the latest studies of the new state at 125 GeV will be presented on behalf of the LHC and Tevatron experiments.


About Stephen Sekula

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