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Posts Tagged ‘liveblog’

April 2013 AMS Liveblog

Wednesday, April 3rd, 2013

General information

Today, the Alpha Magnetic Spectrometer (AMS) experiment is going to announce its findings for the first time. The AMS experiment uses a space-based detector, mounted on the International Space Station (ISS), and was delivered on NASA’s shuttle Endeavour, on NASA’s penultimate shuttle mission. To date AMS has observed 25 billion events over the course of the last 18 months. There has been a lot of news coverage and gossip about how this might change our understanding of the universe, and how it might impact on the search for dark matter and dark energy. However until today the results have been a guarded secret for AMS. Sam Ting, who leads the AMS Experiment, will make the presentation in the CERN Main Auditorium at 17:00 CERN time.

AMS-02 on the ISS (Wikipedia)

AMS-02 on the ISS (Wikipedia)

I’ll be live blogging the event, so stay tuned for updates and commentary! This is slightly outside my comfort zone when it comes to the science, so I may not be able to deliver the same level of detail as I did for the Higgs liveblogs. All times are CERN times.

See the indico page of the Seminar for details, and for a live video feed check out the CERN Webcast.

18:25:Congratulations and applause. The seminar is over! Thanks for reading.

Questions

Q (Pauline Gagnon): How many events above 350GeV?
A: We should wait for more statistics and better understanding. Note we do not put “Preliminary” on any results.

Q: Is there a step in the spectrum?
A: Good question! Experiments in space are different to those on the ground. This was studied over Christmas, but it’s just fluctuations. “If you don’t have fluctuations something is wrong.”

Q (Bill Murray): What is the efficiency of the final layer of the Silicon tracker?
A: Close to 100%

Q: Some bins not included. Why not?
A: Less sensitive at low energy. We want a simple model for the spectrum.

Q: Are you going to provide absolute flux measurements?
A: Yes, we will provide those. We calibrated the detector very carefully for precise measurements.

Q (John Ellis): Dark matter interpretation constrained by other experiments, eg ground based experiments.
A: Good point, we have a large number of spectra to analyze very carefully.

Q: Why not use a superconducting magnet?
A: NASA could not deliver more Helium, so superconducting is not an option for a long lived experiment.

Q: You have high statistics in the final bin, so why not rebin?
A: That’s an important question! “I’ve been working at CERN for many years and never made a mistake… We will publish this when we are absolutely sure.” (To my mind this sounds like a fine tuning problem- we should not pick which binning gives us the results we want.) “You will have to wait a little bit.”

Q (Pauline Gagnon): How can you tell the difference between the sources of positrons and models?
A: The fraction will fall off very sharply at high energy as a function of the energy.
Q: How much more time do you need to explore that region?
A: It will happen slowly.

The liveblog

18:11: Ting concludes, to applause. Time for questions.
18:10: The excess of positons has been observed for about 20 years and aroused much interest. AMS has probed this spectrum in detail. The source of the excess will be understood soon.
18:09: Conclusion time. More statistics needed for the high energy region. No fine structure is observed. No anisotropy is observed. (anisotropy of less than 0.036 at 95% confidence.)
18:07: Diffuse spectrum fitted and consistent with a single power law source.
18:00: The positron fraction spectrum is shown (Twitpic) Results should be isotropic if it’s a physics effect. The most interesting part is at high energy. No significant anisotropy is observed.
17:57: Time for some very dense tables of numbers and tiny uncertainties. Is this homeopathic physics? Dilute the important numbers with lots of other numbers!
17:53: A detailed discussion of uncertainties. There seems to be no correlation between the number of positrons and the positron fraction. Energy resolution affects resolution and hence bin to bin migration as a function of energy. There are long but small tails in the TRD estimator spectra for electrons and positrons, which must be taken into account. For charge confusion the MC models are used to get the uncertainties, which are varied by 1 sigma.
17:51: Charge confusion must be take into account. The rate is a few percent with a subpercent uncertainty. Sources of uncertainty come from large angle scattering and secondary tracks. Monte Carlo (MC) simulations are used to estimate these contributions and they seem to be well modeled.
17:48: A typical positron event, showing how the various components make the measurements. (Twitpic)
17:46: Ting shows the cover of the upcoming Physical Review Letters, a very prestigious journal, with an AMS event display. Expect a paper on April 5th!
17:45: The positron fraction. Measurements of the number of positrons compared positrons+electrons can be used to constrain physics beyond the Standard Model. In particular it can be sensitive to neutralinos, particles which are present in the Supersymmetric (SUSY) models of particle physics. The models are extensions of the Standard Model. The positron fraction is sensitive to the mass of the neutralino, if it exists.
17:42: Onto the data! There have been 25 billion events, with 6.8 million electron or positron events in the past 18 months. Two independent groups (Group A and Group alpha for fairness) analyze the data. Each group has many subgroups.
17:41: AMS is constantly monitored and reports/meetings take place every day. NASA keep AMS updated with the latest technology. There’s even an AMS flight simulator, which NASA requires AMS to use.
17:40: A less obvious point: AMS have no control over the ISS orientation or position- the position and orientation must be monitored, tolerated and taken into account.
17:38: “Operating a particle physics experiment on the ISS is fundamentally different from operating an experiment in the LHC”. Obvious Ting is obvious! :)
17:34: The tracking system must be kept at constant temperature, while the thermal conditions vary by tens of degrees. It has a dedicated cooling system.
17:30: Sophisticated data readout and trigger system with 2 or 4 times redundancy. (You can’t just take a screwdriver out to it if it goes wrong.)
17:27: In addition to all the other constraints, there are also extreme thermal conditions to contend with. The sun is a significant source of thermal radiation. ECAL temperatures vary from -10 to 30 degrees Celcius.
17:25 : Data can be stored for up to two months in case of a communication problem. Working space brings all kinds of constraints, especially for computing.
17:23 : NASA was in close contact to make sure it all went to plan, with tests on the ground. NASA used 2008t of mass to transport 7.5t of AMS mass (plus other deliveries) into space! AMS was installed on May 19th 2011. (I was lucky enough to hear the same story from the point of view of the NASA team, and it was an epic story they told. Apparently AMS was “plug and play”.)
17:21: Calibration is very important, because once AMS is up in space you can’t send a student to go and fix it. (Murmurs of laughter from the audience)
17:19: The detector was tested and calibrated at CERN. (I remember seeing it in the Test Beam Area long before it was launched.)
17:18: Ting shows a slide of the AMS detector, which is smaller than the LHC physicists are used to. “By CERN standards, it’s nothing”. (Twitpic)
17:16: Lots of challenges for electronic when in space. Electronics must be radiation sensitive, and AMS needs electronics that perform better than most commercial space electronics.
17:15: The TRD system measures energy loss (dE/dx) to separate electrons and positrons. A tried and true method in particle physics! The Silicon tracker has nine layers and 200,000 channels, all aligned to within 3 microns. Now that’s precision engineering. The RICH has over 10,000 photosensors to identify nuclei and measuring their energy. This sounds like a state of the art particle detector, but In Space! The ECAL system, with its 50,000 fibers and 600kg of lead can measure up to 1TeV of energy, comparable to the LHC scale.
17:11: Permanent magnet shows <1% deviation in the field since 1997. Impressive. Cosmic rays vetoed with efficiency of 0.99999.
17:10 Studies require rejection of protons versus positrons of 1 million, a huge task! TRD and TOF provides a factor of 10^2, whereas the RICH and ECAL provide the rest of the discrimination.
17:08: AMS consists of a transition radiation detector (TRD), nine layers of silicon tracker, two layers of time of flight (TOF) systems, a magnet (for measuring the charge of the particles), and a ring imaging Cherenkov detector (RICH) and electromagnetic calorimetry system (ECAL). Charges and momenta of particles are measured independently.
17:06: Ting summarizes the contributions from groups in Italy, Germany, Spain, China, Taiwan, Switzerland, France. Nice to see the groups get recognition for their long, hard work. The individual groups are often mentioned only in passing.
17:03: “AMS is the only particle physics experiment on the ISS” which is the size of a football field. The ISS cost “about 10 LHC” units of money! It’s a DOE sponsored international collaboration. Ting is doing a good job acknowledging the support of collaborators and the awesomeness of having a space based particle physics experiment.
17:00: “Take your seats please.” The crowd goes quiet, as the introduction starts. Sam Ting was awarded the 1976 Nobel Prize for Physics, for the discovery of the J/psi particle.
16:54: Rolf Heuer has arrived. The room is nearly full now!
16:47: Sam Ting is here. He arrived about 10 minutes ago, and spoke to Sau Lan Wu, an old colleague of his. (Twitpic)
16:31: There are a few early bird arrivals. (Twitpic)

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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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Higgs Seminar 2012

Saturday, June 30th, 2012

This is the link to the liveblog

This year sees the International Conference on High Energy Physics, or ICHEP. Hundreds of physicists will flock to Melbourne, Australia, to get the latest news on physics results from around the world. This includes the latest searches for the Higgs boson, the final piece of the Standard Model. CERN will hold a seminar where ATLAS and CMS will present their results. I’ll be liveblogging the event, so join me on the day!

Information about the webcast

The webcast for the CERN seminar is available at http://cern.ch/webcast. If you have a CERN login you can also use http://cern.ch/webcast/cern_users/

Wednesday 4th July 2012 09:00.
(Other timezones: 00:00 PDT / 03:00 EDT / 07:00 GMT / 08:00 BST /09:00 CET / 17:00 VIC)

Meeting link: https://indico.cern.ch/conferenceDisplay.py?confId=197461
Webcast link: http://webcast.cern.ch/
Follow on twitter: @aidanatcern @sethzenz

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Higgs Liveblog

Tuesday, December 13th, 2011

Seminar / Webcast / My Twitter

Slides available

Go to the Seminar page for the slides that there presented today.

The liveblog

(For details about the seminar, see below. Some links to photos on the twitter feed.)

The most recent updates are at the top of the page. All times are CERN times.

More updates will be posted as they arrive. Thanks for reading!

Questions

15:49: Rolf says “No more questions, so final remarks”. Great to have first results, remember they are preliminary and with small numbers. “Keep in mind we’re also running next year.” “The window is getting smaller and smaller, but it’s still alive!” “Stayed tuned for next year”.

(G) means that Guido answered, (F) means that Fabiola answered. Thanks to Rozmin Daya for providing more detailed transcripts of the questions and answers. (Questions are ordered so that the most recent question is at the top of the post.)

Q: Regarding the crystal calorimeter, by how much has this improved your endcap H->gamma gamma mass resolution? Did I understand that you understand the scale to 0.1%?
A(G): No. for the scale, if you take a look at the plot on slide 33, this rms is what you’re able to acheive. In the endcap there’s a lot of progress, the scale does not evolve in big jumps. We have lots of room to improve. Up to now we were limited by this phenomena related to transparencies. Our tracker is fantastic but introduces extra material. Have to understand material and conversions more. We did this for barrel but must do more aggresively for barrel.

Q: For ATLAS, you have exclusion at 115.5 GeV. Is there a way to have some kind of Look Elsewhere effect for negative fluctuations? For CMS, you have excess everywhere between 115-135 GeV. Can it be that you’re simply misunderstimating background?
A(G): it could be, but we should be really precise. I cannot exclude but I’ll give small probability to this. Atlas: as far as I know we don’t have a LEE for this exclusion, but it’s an exclusion at the 95% confidence level.

Q: In the 4l final state analysis, how much would you lose if we count only 2/3 events (to ATLAS)
A(F): It would go down to 1.5sigma to 1.6sigma
Note: This question was motivated by fact that now there is discrepancy between ATLAS and CMS that wasn’t there at HCP time.

Q: What is the ultimate scale energy energy scale precision in the gamma gamma, because you were showing 0.5GeV from the Z.
A(F): Uncertainty on photon energy scale is more. It’s a few parts per million on the Z peak, but it’s below 1%. When we transfer to the photon, we have to take into account that we use the Monte Carlo simulation. We vary the material in the simulation, and we end up with a few parts per million to 1%.

Q: it’s interesting to understand what are the signal resolution for the few events in the 4 lepton final state.
A(F): The mass resolution is typically about 1.9 GeV for muons, and 1.7GeV for electrons.

Q: Question for CMS. Did you try to extract the 90% exclusion limit for gamma gamma? It’s a bit close to the ATLAS excess. Don’t understand the strategy for W->lnulnu.
A(G): We use continuous approach: cut based, and then the invariant mass of two leptons in a boosted decision tree.

Q: I’d like to understand looking at gam gam fit and the use of exponenetials to describe the background. Choice in CMS was not to do that. How confident are you to do that, knowing that QCD bg not well modeled by exponential?
A(F): You’ll find the slide in the spares. We tried several functions. We tried using background coming from Monte Carlo simulation, and also adopted a conservative estimation on background by taking as background uncertainty in a bin of 4GeV the difference between exponential and the expectation from Monte Carlo generator. We get consistent results using other functions.

Talks

15:36: Finish and applause. Rolf gives overview and opens up the floor to questions.

15:34: Two excesses at 119.5GeV and 124GeV. Both excesses seem compatible with a Standard Model Higgs. 95% confident limits include 127-600GeV. Some excess is present in all 5 channels. Statistical significance of 2.6sigma locally and 1.9sigma with Look Elsewhere Effect taken into account.

15:30: Modes split by resolution. Low and high resolution channels agree that something is happening around 125GeV. Maximum local significance is 2.6sigma. With look elsewhere effect it’s 1.9sigma in low mass region. Expect 2-3 sigma effect in region 115-127GeV. Look for best fit of Higgs cross section, shows best agreement at 124GeV.

15:29: CMS more sensitive than Tevatron experiments combined! Expected exclusion is 117-543GeV, observed is 127-600GeV. What is stopping CMS getting lower limit? There’s some kind of bump there in the low mass region…

15:26: First glance at invariant mass plot. Exclusion plot looks like it shows excess at 125GeV, but deficiency at around 128GeV. Interesting, given what ATLAS saw!

15:25: H->gamma gamma analysis. Improvements in the vertex identification, energy resolution. Vertex finding efficiency gives roughly 80% or better in all data periods. Resolution measured using Z, W decays and pi0 decays. Laser signals used to correct for transparency measurements. A lot of work has gone into understanding these issues!

15:20: Putting limits using H->ZZ* mode, one of the most powerful modes. Expected to exclude the ranges 130-160GeV, 182-420GeV, observed exclusion in 134-156GeV, 180-395GeV and 340 460GeV.

15:19: H->ZZ*(llll) and H->gammagamma have excellent resolution. H->ZZ* is the “golden channel”. Expect 67 events, observer 72 events in full mass range. In the low mass region (mH<160GeV) CMS has observed 13 events, expected 9.5 events.

15:13: H->bb mode. Very challenging! Huge background from QCD processes. Look for associated production of a boson with the Higgs boson. Better sensitivity, but lower efficiency. Require a very boosted W or Z produced in association. (pT of 100-160GeV) 5 sub channels of H->bb with associated production.

15:12: H->tautau mode. Slight hints of excess. Limit plot shows gentle excess across the low mass region (110-150GeV) compared to expectation.

15:08: Using multivariate analyses for the H->WW* state. Cut and count analysis shows most backgrounds removed by a handful of cuts. (Standard Model WW production dominates to the end. Expected exclusion is 129-236GeV, observed is 132-238GeV. Then using a boosted decision tree, split samples into different topologies. Look for discrepancies in the BDT spectrum Expected exclusion is 127-270GeV, observed is 129,-270GeV. Looks like a small excess just below 127GeV!

15:06: Now onto H->WW* analysis. Large non-resonant background from Standard Model WW processes. Angle between leptons can be used as a discriminant. Leptons emitted in small angle, so invariant mass of leptons not very large (it's all about spins of boson!) MET can be used to discriminate against background.

15:03: Monte Carlo simulation plots shown for events. Topological constraints useful for removing background. H->ZZ(ll,qq) mode extended to low mass region. Study at high mass includes H->ZZ(ll,tautau). 10.2 expected background events, 10 observed, so not sensitive in this mode yet.

14:58: 8 independent decay channels modes shown in a big table, with their sensitivities. 4.6-4.7fb^-1 of luminosity used for each of the 5 main modes (H->gamma gamma, ZZ*, WW*, tautau, bb) Resolution is 1-3% for gamma gamma and ZZ* final states. All 8 analyses made it to preliminary results to be shown today.

14:56: More than 90% data taking efficiency in each mode, and 91% overall. Impressive! Analysis requires good understanding of backgrounds and object reconstruction. Good agreement with data for identification efficiencies up to hundreds of GeV. Standard Model cross section plot shown. CMS agrees with data across all the processes, with a slight deficiency in ZZ production.

14:50: Guide starts, outlining the CMS collaboration and the detector. Overview of the Standard Model Higgs boson. Showing results up to 600GeV. Different production modes give different sensitivity.

14:52: Flashback to slides from a year ago, showing expectations. Expected sensitivity down to Standard Model across the whole range when combining channels. Projected significance decreases sharply in the low mass range. Sensitivity will come from combining channels.

14:48: Finish and applause! Guido takes the microphone. And goes through Fabiola's slides by mistake!

14:45 With current data set ATLAS has excluded 112.7-115.5GeV and 131-453GeV (except for 237-251GeV) ATLAS is now competing with LEP's low mass results! There is a large deviation in p0-values at 126GeV. 1.9e-4, or an excess of 3.6sigma (gamma gamma 2.8 has sigma, ZZ* has 2.1, WW* has 1.4sigma)

Updating all other analyses for full data set. We need more data in 2012 in order to confirm if this is the Higgs. 126GeV is a nice mass for the Higgs- it can be probed with lots of modes (gamma gamma, ZZ*, WW*, bb, tautau).

14:40: Apologies, connectivity issues.

Now discussing H->ZZ* analysis. Statistics limited background studies for SM ZZ processes. Electron identification efficiency comes from J/psi, W and Z decays. Covers wide range of transvere energy (up to 50GeV). Monte Carlo simulation tracks particle identification well as pileup increases- we understand the detector very well. Isolated muons selected, isolation performance looks impressive, even as pileup increases.

Simultation gives mass resolution of about 2GeV, 85% of signal falls within two standard deviations of mass point. 71 events seen in the full range, expected background is 62 events. In the low mass region (gamma gamma at 126GeV is 2.8sigma! If it's due to background only, it's a very large fluctuation. There are nine categories of photon, with the background modeled with an exponential function, and Crystal ball+Gaussian for signal. Excess shown at 126GeV

14:27: Discussion of the angle measurement. Need to know position performance in the calorimeter. Resolution of position of primary vertex is ~1.5cm. Potentially large background from jets and hadrons. The faking is rare, but the rate of production of jets is orders of magnitude larger than the rate of Higgs boson production.

14:26: Sensitive at lower energies. Different from previous channel, need good resolution of photon measurements. Irreducible background from Standard Model gamma gamma, also some fake gammas from jets. Mass resolution and positive robust against pileup. About 5GeV width in the invariant gamma gamma mass (in simulation, based on knowledge of detector.) Energy scale known to 0.5%, about 1% for linearity and uniformity. Z->ee mass shown, good performance there. Knowledge of how electrons interact inform energy scale for photons.

14:19: Discussion of Standard Model backgrounds for WW* channel. Turn on of ttbar background for this mode at Missing Energy (MET)>50GeV. MET strongly affected by pileup.

Expected background: 76 events, Data seen: 94 events, Expected signal :19 events. Cannot improve limit with this mode alone.

14:17: There are lots of backgrounds to consider! Concentrating on the gamma gamma, WW* and ZZ* modes. Backgrounds are jets, photons and W/Z bosons.

14:16: Huge efforts go into understanding the detector. As the regions of the Higgs search change, the requirements of the analyses change.

14:12: Outline of Higgs search motivation. The two photon sample is most sensitive at low mass ranges. Massive vector bosons sensitive at higher masses. Theorists have been working hard to update their expectations. The allowed region is small. We'll make it even more smaller today, and maybe see something very interesting in there as we do!

14:10: "The Standard Model works at 7TeV. Very Good." Good performance of Standard Model processes. We must understand these to understand the backgrounds, and also to calbirate measurements.

14:08: Discussion of pileup, the price we pay for high luminosity. We increase the number of events we record at once by having several interactions per beam crossing. A big challenge at working at the LHC, and a challenge we meet. Triggers are closely monitored to pick out the most interesting results.

14:06: Understanding of the search is "well advanced". Fabiola expresses thanks to the LHC team. Data taking efficiency is 93.5%. Good quality data fraction is greater than 90% for all analyses.

14:05: First two slides. Fabiola explains the importance and difficult of Higgs searches. The first slide shows plots from several different analyses with data and Monte Carlo simulation.

14:01: Rolf introducing Fabiola and Guido. Huge round of applause for all the experts and LHC team. Building up the moment with a great sense of community. In spite of the competition between ATLAS and CMS, we're here together to present and see the results together.

Before the talks

13:59: One minute to go. Both talks and both speakers ready.

13:54: Just spotted Guido Tonelli, the Spokesperson for CMS and the second speaker today! Both he and Fabiola are looking smart, and ready to give us the facts.

13:45: Fabiola's talk has been copied and it is ready for her. She taking a sip of water and chatting with Rolf and technical support.

13:41: Experts from the LHC are here too. They have worked very hard to make sure the machine works for us, and we've had fantastic running this year. We must not forget their role in this work!

13:40: Fabiola Gianotti, the ATLAS Spokesperson is here. She will give the first talk in about 20 minute's time. She's chatting to the Director General of CERN, Rolf Heuer. She's smiling, but if I was in her position I'd be quite nervous right now! She's taking off her coat, looking at the microphone and so on. She's given many talks before, so she knows what's she's doing. Still, it must be nerve-wracking for her!

13:32: The webcast is now available! http://webcast.web.cern.ch/webcast/

13:30: A “delegation” of very smartly dressed people are arriving in the front rows! They’re more smartly dressed than most physicists, and they have reserved seats, so they are probably management, dignitaries etc.

13:27: Of course big names from CMS are here as well, including Albert de Roeck, Jim Virdee, and Gigi Rolandi! (Being an ATLAS member, it’s easier to recognize other ATLAS members!)

13:22: ATLAS Higgs Group Conveners, including Bill Murray and Eilam Gross, arriving now. These are the people in charge of the various Higgs searches at ATLAS. Some of the analysts are here as well. Lots of big names arriving.

13:14: Some thoughts about the media, science, and what we can expect to see today. The physicist sitting next to me asked about my blog and twitter feed, and we started discussing how pressure from the media can affect what scientists do. While it’s true that we love the media to be informed, we don’t change our results or interpretation based on public opinion. The results we see today are going to be exciting, but we need to be careful and do a proper job. If it’s not 5σ yet, it’s not a discovery yet. We’ve been searching for the Higgs boson for decades, so we want to get it right and we don’t want to sacrifice our standards for the sake of getting in a few months early. If we see the Higgs in the summer (and we probably will, if the rumors are to be believed) it will be the most important discovery in high energy physics since the W was discovered in 1983!

(If we don’t see the Higgs, that will also be an important observation, as it will tell us there is something else out there . Convincing bumps have been known to disappear when we add more data.)

12:53: The projector is being tested. As soon as the webcast is available I’ll update to let you know.

12:51: Katie snapped a photo of me and Pauline! It’s on yfrog.

12:28: I’ve just heard that the security staff at the door are no longer allowing more people to enter.

12:27: This seminar is one that should not be missed. Looking through the audience I’m glad to see most of my closest colleagues have found somewhere to sit, including @marktibbetts, Matt, Tina, Rozmin, Catrin and Sudan. We are normal physicists, fighting for whatever seats we can get. The first three rows have been reserved for special guests, representatives etc. There are people sitting on all the stairs. We’ve all heard the rumors. None of us has heard the official results form both experiments yet.

11:57: There are lot of people from the blogging community here, including Pauline Gagnon (to my right), Anna Phan (to my left) and Seth Zenz (in front of me)! You can follow Seth’s tweets, @sethzenz

11:42: Seating is at a premium, I just someone bring their own chair in!

11:31: I’ve been here for an hour now, and the auditorium is nearly full. There are roughly 10 seats left (none of them have desks, or power supplies.) Nearly everyone here has their laptop with them, it’s like a commercial for Apple! Staff are checking IDs at the door. There is a lot of chatter here.

Details about the seminar

Today sees CERN’s seminar on the update of the Higgs search. I’ll be updating this page as the information comes in. Refresh this page to get the updates! The most important points will be also be tweeted.

Time

The seminar will begin at 14:00 CERN time. (08:00 East Coast, 05:00 Pacific)

There will be a talk from the ATLAS spokesperson 14:00-14:40 giving updates on the ATLAS search, and then a talk from the CMS spokesperson 14:40-15:20. There will then be questions and answers to both speakers until 16:00.

Links

Here are the links to:
Seminar page

Webcast video

Follow the updates on twitter using the hashtag #higgsliveblog (my account there is @aidanatcern.)

The seminar page also has a chat room!

Shortlink for this page: http://bit.ly/u0wALv

Shortlink for the seminar page: http://bit.ly/s5X4Zm

Shortlink for the webcast: http://bit.ly/q2QB

Connectivity

We’re expecting a lot of internet traffic at CERN today, so there is a small possibility the network may get jammed for a few seconds from time to time. Thank you to Kevin for allowing caching of this page so that it can still be accessible in case of any problems.

Please report any errors in the transcript of this blog post in the comments.

Aidan Randle-Conde

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Live blog: neutrinos!

Friday, September 23rd, 2011

This is a live blog for the CERN EP Seminar “New results from OPERA on neutrino properties“, presented by Dario Autiero. Live webcast is available. The paper is available on the arXiv.

The crowd in the auditorium (Thanks to Kathryn Grim)

The crowd in the auditorium (Thanks to Kathryn Grim)

15:39: So here I am at CERN, impatiently waiting for the Colloquium to start on the OPERA result. The room is already filling up and the chatter is quite loud. I’m here with my flatmate Sudan, and we have a copy of the paper on the desk in front of us. I just bumped into a friend, Brian, and wished him look finding a chair! (He just ran to get me a coffee. Cheers Brian!)

15:53: Wow, the room is really crowded now! People are sitting on the steps, in the aisles, and more are coming in. The title slide is already up on the projector, and some AV equipment is being brought in. I was just chatting to Sudan and Brian, and we commenting that this is probably the biggest presentation that the world’s biggest physics lab has seen in a long time! As Sudan says, “The whole world is going to be watching this man.”

15:55: Burton and Pauline are here too, getting some photos before the talk begins. Expect to see more (less hastily written) blog posts about this talk!

15:59: We’re not allowed to take photos of the talk itself, but there will be a video feed that you can watch. See this link for details about the live webcast.

16:03: The talk begins. A fairly straightforward start so far. As usual, the speaker introduces the OPERA Collaboration, and gives a bit of background. Nothing ground breaking so far!

16:06: The analysis was performed blind, which means that the physicists checked and double checked their systematic uncertainties before looking at the data. This is a common best practice in these kinds of experiments and it is a good way to eliminate a lot of experimenter bias. The speaker is now discussing past results, some of which show no faster than light speed, and one of which (from MINOS) that shows a small effect which is less than 2σ.

16:16: Autiero is currently discussing the hardware of the experiment. It looks like a standard neutrino observatory setup- large amounts of dense matter (Pb), scintillation plates and tracking hardware for the muons which get produced when the neutrinos interact. By the time the beam reaches Gran Sasso it is about 2km wide! At CERN the neutrinos are produced by accelerating protons at a target, producing pions and kaons, which are then allowed to decay to muons and muon neutrinos. The hadrons are stopped with large amounts of Carbon and Iron, so that only the neutrinos and some muons survive. By the time the neutrino beam reaches Gran Sasso the muons have long since interacted and are no longer present in the beam. The neutrinos have 17GeV of energy when they leave CERN, so they are very energetic!

16:29: The discussion has moved onto the timing system, probably the most controversial aspect of the experiment. The timing challenge is probably the most difficult part of the whole analysis, and the part that particle physicists are least familiar with. Autiero points out that the same methods of timing are commonly used in metrology experiments. For OPERA, the location of each end of the experiment in space and time is determined using GPS satellites in the normal way, and then a “common view” is defined, leading to 1ns accuracy in synchronization. It looks like variations in the local clocks are corrected using the common view method. The time difference between CERN and Gran Sasso was found to be 2.3 ± 0.9 ns, consistent with the corrections.

16:36: Things are made trickier by identifying where in the “spill” of protons a neutrino came from. For a given neutrino it’s pretty much impossible to get ns precision timing, so probability density functions are used and the time interval for a given proton spill is folded into the distribution. We also don’t know where each neutrino is produced within the decay tube. The average uncertainty in this time is about 1.4ns. Autiero is now talking about the time of flight measurement in more detail, showing the proton spills and neutrino measurements overlaid.

16:39: Geodesy is important to this analysis. OPERA need to know the distance between CERN and Gran Sasso to good precision (they need to know the distances underground, which makes things more complicated.) They get a precision of 20cm in 730km. Not bad! Autiero is now showing the position information, showing evidence of continental drift and even an earthquake. This is very cool!

16:47: Two techniques are used to verify timing, using Caesium clocks and optical fibers. These agree to ns precision. The overall timing system is rather complicated, and I’m having trouble following it all!

16:48: I just got a message from a friend who saw this blog via Twitter. Hello Angela! Welcome to all the readers from Twitter!

16:52: Currently discussing event selection at Gran Sasso. Events must have a highly relativistic muon associated with them. (The speed of the muon and slight difference in direction of flight can only increase the measured time of flight.)

16:54: Autiero is telling us about how the analysis is blinded. They used very old calibrations, intentionally giving meaningless results. A novel approach to blinding!

16:56: No evidence of variation with respect to time of day or time of year. So that’s the “Earth moved!” theory sunk.

17:01: Unblinding: Δt = -987.8ns correction to time of flight after applying corrections (ie using up to date calibration.) Total systematic uncertainty is 7.4ns. Time of flight obtained using maximum likelihood. Measured difference in time of flight between speed of light and speed of neutrinos is

\[
\delta t (c-\nu) = (60.7 \pm 6.9(stat) \pm 7.40 (syst)) ns
\]

\[
\frac{c-v_{\nu}}{c} = -(2.4 \pm 0.28 \pm 0.30)\times 10^{-5}
\]

17:03: ~16,000 events observed. OPERA has spent six months checking and rechecking systematic uncertainties. Cannot account for discrepancy in terms of systematic uncertainties.

17:04: “Thank you”. Huge ripple of applause fills the auditorium.

Questions

(These questions and answers are happening fast. I probably make an error or omission here and there. Apologies. Consult the webcast for a more accurate account or for any clarifications.)

17:05: Questions are to be organized. Questions about the distance interval, then the time interval, then the experiment itself. There will be plenty of questions!

17:08: Question: How can you be sure that the timing calibrations were not subject to the same systematic uncertainties whenever they were made? Answer: Several checks made. One suggestion is to drill a direct hole. This was considered, but has an uncertainty associated of the order of 5%, too large for this experiment.

17:12: Question: Geodesy measurements were taken at one time. There are tidal effects (for example, measured at LEP.) How can you be sure that there are no further deviations in the geodesy? Answer: Many checks made and many measurements checked.

17:14: Question: Looking for an effect of 1 part in 105. Two measurements not sufficient. Movement of the Moon could affect measurements, for example. Answer: Several measurements made. Data taken over three years, tidal forces should average out.

17:15: Question: Is the 20cm uncertainty in 730km common? Answer: Similar measurements performed elsewhere. Close to state of the art. Even had to stop traffic on half the highway to get the measurement of geodesy!

17:16: Question: Do you take into account the rotation of the Earth? Answer: Yes, it’s a sub ns effect.

17:23: Question: Uncertainty at CERN is of the order of 10μs. How do you get uncertainty of 60ns at Gran Sasso? Answer: We perform a maximum likelihood analysis averaging over the (known shape) of the proton spill and use probability density functions.

(Long discussion about beam timings and maximum likelihood measurement etc.)

17:31: Large uncertainty from internal timers at each site (antenna gives large uncertainty.) Measurements of timing don’t all agree. How can you be sure of the calibration? Answer: There are advanced ways to calibrate measurements. Perform inclusive measurement using optic fibers. Comment from timing friends in the audience? Audience member: Your answer is fine. Good to get opportunity to work on timing at CERN.

17:33 Question: What about variation with respect to time of day/year? Answer: Results show no variation in day/night or Summer vs Spring+Fall.

17:35: Question: How can you be sure of geodesy measurements if they do not agree? Answer: The measurements shown are for four different points, not the same point measured four times. Clocks are also continually resynchronized.

17:37: Question: Do temperature variations affect GPS signals? Answer: Local temperature does not affect GPS measurements. Two frequencies are used to get the position in ionosphere. 1ps precision possible, but not needed for OPERA.

17:41: Question: Can you show the tails of the timing distributions with and without the correction? Is selection biasing the shapes of the fitted distributions? Answer: Not much dependence on spatial position from BCT at CERN. (Colleague from audience): The fit is performed globally. More variation present than is shown in the slides, with more features to which the fit is sensitive.

17:43: Question: Two factors in the fit: delay and normalization. Do you take normalization into account? Answer: Normalization is fixed to number of events observed. (Not normalized to the cross section.)

17:45: Question: Do you take beam stretching/squeezing into account? Answer: Timing is measured on BCT. No correlation between position in Gran Sasso and at CERN.

17:47: Question: Don’t know where muons were generated (could be in rock.) How is that taken in to account? Answer: We look at events with and without selections on muons.

17:49: Question: Do you get a better fit if you fit to the whole range and different regions? What is the χ2/n for the fits? Answer: We perform the fit on the whole range and have the values of χ2/n, but I can’t remember what they are, and they are not on the slides.

17:50: Question: What about any energy dependence of the result? Answer: We don’t claim energy dependence or rule it out with our level of precision and accuracy.

17:52: Question: Is a near experiment possible? Answer: This is a side analysis. The main aim is to search for τ appearance. (Laughter and applause from audience.) We cannot compromise our main physics focus. E-mail questions welcome!

17:53: End, and lots of applause. Time for discussion over coffee! Thanks for reading!

The start of the neutrinos journey, taken from the OPERA paper.  (http://arxiv.org/abs/1109.4897)

The start of the neutrinos journey, taken from the OPERA paper. (http://arxiv.org/abs/1109.4897)

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