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Aidan Randle-Conde | Université Libre de Bruxelles | Belgium

View Blog | Read Bio

Higgs Seminar liveblog from CERN

CERN are holding a seminar for the latest results for the ATLAS and CMS Higgs searches. This is the first such update since December 2011, and there is a reasonable chance that at least one of the experiments could show a 5 sigma excess. This is my liveblog, follow along for live updates!

“Observation of a new particle consistent with a Higgs Boson (but which one…?)”

Thank you to all who joined me on this liveblog and on twitter!

The seminar is webcast live so that you can watch from anywhere in the world. The link is http://cern.ch/webcast. The seminar will begin at 09:00 CERN time (00:00 US West Coast, 03:00 US East Coast, 08:00 UK, 17:00 Melbourne.)

This is my liveblog and I will be providing updates as the seminar proceeds. Most recent updates at the top of the page. Also follow me on twitter (@aidanatcern) and Seth Zenz (@sethzenz). Ken Bloom is also liveblogging from ICHEP, and my boss, @drsekula is liveblogging for SMU.

The liveblog

10:59: Rolf: We can all be proud of this day. Enjoy it! (Applause)

Questions, answers, and comments

10:55: Any comments from the theorists? (Applause) Many congratulations!

10:50: Many thanks offered from the front row.

10:48: Any questions from Melbourne? Any applause from Melbourne?! (Applause from Melbourne.) Any remarks? A: Grateful to take part in this historic event and wish you the best.

Overview (Rolf Heuer)

10:44: “Speaking as a layman: I think we have it.” We have a discovery consistent with a Higgs boson (but which one?) This is the beginning. “Global implications for future. Standing applause!

ATLAS talk (Fabiola Gianotti)

10:42: Local excess of 5.0 sigma, dominated by gamma gamma and ZZ* final states.

10:41: Only recorded on third of 2012 data. More data to come. The LHC is working beyond expectation. Theorists: please be patient!

10:40: Next steps: publish paper, then gather more data.

10:38: Evolution of excess with time. December saw 3.5 sigma peak. Seeing a nice 5 sigma peak today!

10:37: Excess compatible with Standard Model Higgs boson.

10:34: Excluded all points in the Higgs mass spectrum now, except around 125GeV and at very high mass.

10:33: Observe 3.4 local (2.5 global) sigma excess at 125GeV.

10:30: Slight excess above background + Standard Model signal at 125Gev. (Expect 10.4 +- 1.1 total, observe 13)

10:29: Z->4 leptons seen in the spectrum.

10:28: 1.3 times more ZZ events in data at higher masses.

10:26: Total reconstruction efficiency for electrons 98% flat in eta, pt and pileup. Required for low transverse momentum objects. 60% gain in acceptance times efficiency electrons. 45% gain for muons.

10:24: H->ZZ*->4 leptons final state. Backgrounds suppressed using isolation requirements. High efficiency needed, down to low transverse momentum objects. Gain in sensitivity of 20-30% since 2011.

10:21: 4.5 local (3.6 global) sigma excess in gamma gamma. Signal strength is 1.9 +/- 0.5. Cross section seems a little high, but consistent with Standard Model within 2 sigma.

10:19: Background model taken from data, using sidebands. Both 2011 and 2012 exclusions show compatible shapes.

10:18: Isolation of photon used to reject jets. Subtraction algorithm used to remove some pileup dependent effects.

10:17: Rejection of jets is 1 part in 10^4, at 90% signal efficiency.

10:15: Need to know the position of the vertex to get the angle of the photons and the mass. Do not use tracking information, in order to be insensitive to pileup. Use longitudinal and lateral segmentation of the electromagnetic calorimeter to point the photons.

10:14: Important to have powerful gamma identification to reject jet backgrounds. Energy scale known to 0.3% at the mass of the Z. Linearity known to better than 1% up to a few 100 GeV. Mass resolution not seriously affected by pileup.

10:11: Gamma gamma final state. Large backgrounds, split signal into 10 categories, depending on the kinematics and conversion variables. Expect gain in sensitivity by 15%. Signal to background ratio is very small. (170 signal events for 6340 background events.)

10:09: Use experience with the detector from 2011 to inform analyses in 2012. Improved reconstruction and identification of physics objects.

10:07: Previous results show exclusions except near 116GeV and 125GeV.

10:06: As center of mass energy changes from 7TeV to 8TeV, cross section increases by a factor of 1.3. Irreducible background cross sections increase by a factor of 1.2-1.25, whereas reducible backgrounds increase by a factor of 1.4-1.5. This gives an increase of sensitivity of 10%.

10:05: Many electroweak results , with cross sections of rare and rarer processes. Small amounts of tension in measurements.

10:04: Analysis not possible without dedicated computing resources. Usually 100,000 jobs in parallel at a time.

10:02: Trigger thresholds rise and luminosity rises. This keeps the good physics events for lower mass objects. Efficiency of electron trigger is flat and 94%. Stable performance required with respect to changes in pileup. Pileup changes as the run progresses.

10:00: Pileup showing big challenges for the continued analysis of data. Missing transverse energy resolution rises linearly with pileup, but is fine and flat after pileup suppression using information from the detector.

09:58: Pileup is increasing quickly. Average of 30 collisions per bunch crossing (with 50ns bunch spacing, rather than 25ns which is design performance.)

09:56: Integrated luminosity of 6.3fb^-1. 94% efficiency. 90% of delivered luminosityy is recorder to disk, in spite of very fresh data and harsher conditions.

09:55: Results are preliminary, data taking stopped two weeks ago. Pileup increased, harsher conditions. Present the highest sensitivity and best resolution modes (gamma gamma and ZZ*.) Other channels contains missing energy, poorer mass resolution and sensitive to pileup.

CMS talk (Joe Incandela)

09:51: Following lots of applause, acknowledgements. Lots of people to thank.

09:49: Event yields are self consistent across the topologies. Ratio of WW* and ZZ* states consistent. Couplings consistent with Standard Model at 95% confidence, we need more data. “We have observed a new boson with a mass of 125.3 +/- 0.6 GeV at 4.9sigma significance.”

09:48: Combined mass is 125.3 +/- 0.6 GeV. Now we need to see if it is compatible with Standard Model Higgs boson. Signal strength is 0.8+/-0.2.

09:46: Observed limit 1.06 x Standard Model cross section. Low statistics may cause some slight bias. Needs investigation. “Very interesting channel.” (Nice to hear open and candid discussion about results. Responsible science.)

09:44: tau tau channel. Challenging, lots of sub modes. 2 times improvement in sensitivity since 2011. “Use a very fancy fit that I won’t explain in detail…”

09:42: Current limits are compatible with signal or background.

09:42: Now bb, large branching fraction but huge background. Look for associated production mode. (W+H, Z+H; H->bb)

09:41: Still working on combination.

09:39: WW* analysis. Very difficult channel at low mass. DeltaPhi between leptons and invariant mass of two leptons used as discriminators.

09:37: Combined result for gamma gamma and ZZ* is 5.0 sigma. That’s a discovery!

09:35: Broader distribution for mass of Z bosons. Needs to be watched in the future…

09:34: Z->4l peak seen in the final mass spectrum! Also a bump at 126GeV.

09:32: Moving to ZZ* search. 20% improvement since 2011. Using all four (light) lepton final states. Backgrounds estimated from data. Angular analysis of leptons performed. 8 degrees of freedom in this angular analysis.

09:30: 4.2 sigma local significance, 3.2 sigma global. 1.56 +/- 0.43 x Standard Model cross section.

09:28: Peak clearly visible at 125GeV at the 2.3 sigma leve.

09:28: Classes combined weighted by signal to background ratio. Impressive bump appears!

09:27: Background model comes from data. Bias must be less than 20% of statistical error in the data.

09:25: Multivariate analysis used with kinematic variables, identification and per event mass resolution and vertex probability. Classes arranged in decreasing order of purity.

09:24: Photons selected using kinematic variables (transverse energy and mass of diphoton system.) Mass reconstruction depends on the vertex position. Aim to be within 1cm of the correct vertex. Correct to 83%(80%) in 2011 (2012).

09:23: Different algorithms for electron reconstruction, including brem recovery. Slightly better performance in Monte Carlo compared to data, so smear the data.

09:22: Analysis performed blind in 2012. Most studies are data driven.

09:21: Multivariate analysis used, using boosted decision trees. Classify different kinds of events, end up with four event classes. Crosschecked using an alternate background model, using sideband subtraction. Also a cut based crosscheck.

09:20: Standard Model cross sections well measured, including ttbar.

09:19: Jets a challenging but performing well. Shape differences are evident for pileup jets. Jet resolution good to within 15% up to the TeV scale.

09:18: Muon efficiency appears flat a function of pileup, as does isolation. 2012 has lower fake rates for electrons than 2011 for the same efficiency. Tau identification is ~70% with very low fake rates.

09:16: Particle flow used to great effect at CMS. Sophisticated electron reconstructed. Electron and photon calibrations show excellent performance. Gaining in sensitivity with identification algorithms.

09:15: Data recording and Monte Carlo production shown impressive performance and improvements.

09:14: Laser monitored correction for light loss in ECAL crystals. Resolution good to 1% using Z lineshape for calibration.

09:13: CMS detector, silicon tracker with 200m2 and 10M channels. Huge 3.8T solenoid (which is what CMS is named after.) Very fine granularity. Electromagnetic calorimeter a first for hadron experiment, using PbW04 75,000 crystals. Close to 100% up time for subsystems.

09:11: Luminosity increasing appreciably in 2012. 5.2fb^-1 collected so far in 2012.

09:10: Discovery potential: expect 5 to 6 sigma sensitivity for a Standard Model Higgs around 125GeV.

09:07: Constraints come from masses of top quark and W boson. Great exclusions coming from Tevatron.

09:08: In 2012 LHC moved from 7TeV to 8TeV. Dominant production mechanism is gluon gluon fusion. (Others include vector boson fusions, top radiation and associated produciton.

09:09: Main decay modes: WW, ZZ, bb, tautau, gammgamma.

09:05: “A lot of effort to combine all the work of thousands of people… it’s very tricky.”

09:06: Big challenge from pileup, about 50 interactions per event. Very rare particle, lots of sleepless nights.

Before the talks

09:02: Rolf Heuer: “Good morning everybody at CERN. Good afternoon everybody at Melbourne.” The seminar is about to begin. “Today is a special day.”

08:59: It is time. May the announcements begin.

08:56: Peter Higgs just arrived! Applause.

08:48: Why the Higgs boson is the “God particle”: It gives us mass. Mass is the fundamental unit of Catholicism.

08:46: Less than 15 minutes to go. I hope my typing is good enough and fast enough! Apologies for any typos.

08:40: We can see our colleagues in Melbourne and they can see us. Jon Ellis just arrived. There are many cameras here. I’m waiting for Peter Higgs to show up…

08:29: ATLAS Spokesperson, Fabiola Gianotti has arrived. As far as I know CMS will present first, and ATLAS will present second. (Last time ATLAS presented first.)

8:13: Famous faces arriving. Rolf Heuer, Director General of CERN. Guido Tonelli, the former CMS Spokesperson. Eilam Gross, the ATLAS Higgs Convener and Bill Murray (not the actor, the former ATLAS Higgs Convener).

08:02: I waited in the lobby since 11pm last night, with food and blankets and books. There was a very communal atmosphere and people tweeted their experience (search for the #occupyCERN tag!) Now we reap the benefits of the wait.

08:01: A short while ago me and my mother were interviewed by an Israeli TV station!

07:45: I waited 8 hours to get a seat, and I have a wonderful view! I should be able to hear the speakers well, all questions being asked, and the answers. I’m sitting here with my mother to my right (she flew all the way from the UK to attend!) and my boss to my left.


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