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Posts Tagged ‘Higgs boson’

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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The upcoming Higgs seminar could be the biggest announcement in particle physics for nearly 30 years. There have been several excellent blog posts and videos explaining what the Higgs is and what it does, so I’ll link to those at the bottom of the page. What I want to do here is give you the overview of what you really need to know to get the best from the talk.

Of course you should follow along with the liveblog as well!

What’s happening with the webcast?

CERN have put in a lot of resources for the webcast. General users can get to the webcast at http://cern.ch/webcast. If you have a CERN login you can use a second webcast at http://cern.ch/webcast/cern_users.

The webcast will start around 09:00 CST (that’s 00:00 US West Coast, 03:00 US East Coast, 08:00 UK, and 17:00 Melbourne.

What is the Higgs boson? What does it do?

The Higgs boson is part of the Standard Model of particle physics. The Standard Model includes the quarks and leptons (which make up all the matter see around us) and the photon, gluons, and \(W\) and \(Z\) boson (which carry all the forces in nature, except for gravity.) Three of these particles, the \(W^+\), \(W^-\) and \(Z\) bosons, have mass, but according to our framework of physics, they should not have mass, unless the Higgs boson exists. The Standard Model of physics predicts that the \(W\), \(Z\), photon and Higgs all come as a package and they are all related to each other. If we don’t see a Higgs boson, we don’t understand the world around us.

People say that the Higgs boson gives particles mass, but this isn’t quite what happens. The Higgs boson allows some particles to have mass. The Higgs boson does not explain the mass that comes from binding energies (for example, most of the mass of the proton) and it does not explain the mass associated with dark matter. If the Higgs boson is discovered it will complete the Standard Model of physics, but it will not complete our picture of the universe. There will still be many unanswered questions.

What would a discovery look like?

In order to claim a discovery an experiment would need to see a 5 sigma excess over the expected background. A sigma is a measure of uncertainty, and the chance of seeing a 5 sigma excess due to statistical fluctuations is about 1 in 3 million. If both experiments see an excess of 5 sigma in the same region the chances that this is due to a fluctuation is 1 in 9 million million!

The experiments produce “Brazil plots”, which show what they expect to see if there is no Higgs, and compare it to what they actually see. The green band shows 1 sigma deviations, the yellow bands show 2 sigma deviations, and then you have to use your imagination to see the remaining bands, and colors. When the green and yellow bands pass below the SM=1 line, and the central black line does too, then the Higgs is excluded in that region to 95% confidence. If the black line stays above the SM=1 line then we haven’t excluded the Higgs boson in that region yet. So when the green and yellow bands fall far below the SM=1 line, but the black line stays above or at the SM=1 line then we accumulate evidence for a Higgs boson.

How do we search for the Higgs boson?

The search for the Higgs boson depends on its mass. At high mass it can decay to heavy particles with clean signatures, so the high mass region was the first region to see an exclusion. At very high mass the width of the Higgs boson is large, so the events get spread out over a large range, so the searches take a little longer. At low mass the decays get very messy, so we have to pick our decay modes carefully. The cleanest modes are the two photon mode (often called gamma gamma), the ZZ* mode and the WW* mode. Of these three, the gamma gamma and ZZ* modes are the most sensitive, so we can expect to see these presented tomorrow.

The data are collected that the detectors and stored to disk, and the physicists spend their time analyzing the data. This is a slow process, full of potential pitfalls, so the internal review process is long and stringent. This is one of the reasons why we need two experiments, so that they can check each other’s findings. The experiments at Tevatron have already presented their results and they see an excess in the same region. This is vital because they are sensitive to different final states, so between the Tevatron and the LHC we have all the analyses covered.

For each analysis there are two kinds of background, the “reducible” backgrounds where particles fake the particles we are looking for (for example, a high energy electron can look just like a high energy photon) and the “irreducible” backgrounds where particles are the same kind as the ones we are looking for. So when you see plots showing the gamma gamma searches, you can expect to see four categories: gamma gamma (irreducible Standard Model background), jet gamma, jet jet, and “other”. As we make more and more stringent requirements to eliminate these backgrounds we also lose signal events, so we have trade off background rejection against signal acceptance.

On top of all these problems we also have to take reconstruction and acceptance into account. We cannot record every event, so we pick and choose events based on how interesting they look. Does an event have two high energy photon candidates? If so, record it. Does an event have four leptons in the signal state? If so, record it. These trigger decisions are affected by definitions of “high energy”, by the algorithms we use, and by the coverage of the detectors. We have to take all of these biases into account with systematic uncertainties, and these can dominate for some of the searches.

When we put all this together we end up asking some simple questions: “How many background events do we expect?” “How many events do we see in data?” “What is the total uncertainty on the background and signal?” “How many signal events do we think we see?” “How much larger is this than the uncertainty?” This then gives us the “n sigma” for that mode across the mass range. We combine these sigmas within a single experiment, taking correlated uncertainties into account, and that’s how we get our Brazil plots.

How likely is a discovery?

In 2011 we had about \(5fb^{-1}\) of luminosity and we saw about 3 sigma for each experiment. In 2012 we had about \(6.5fb^{-1}\) of luminosity at slightly higher energy (giving a factor of 1.25). So we can work out what to expect for 2012 sensitivity- just take the 3 sigma and add it in quadrature to \((\sqrt{1.25\times 6.5/5})\times 3\) sigma and that comes out at 4.9 sigma. If we’re lucky one or more experiments might see more than 5 sigma, meaning we could have a discovery!

What next for the Higgs?

If we make a discovery, either now or in the coming weeks, then we need to measure the properties of the new particle. We can’t claim to have discovered the Standard Model Higgs boson until we’ve measured its branching fractions and spin. Fortunately, if the Higgs boson is at 125GeV then we have a rich variety of decay modes, and this could give us insights into all kinds of interesting measurements, such as the quark masses.

Now go and enjoy the seminar!

Learn more about the Higgs

What comes next? (Richard Ruiz)

How difficult is it find the Higgs? (Richard Ruiz)

Why do we expect to find a Higgs boson? Part I Electroweak Symmetry Breaking (Flip Tanedo)

Why do we expect a Higgs boson? Part II Unitarization of Vector Boson Scattering (Flip Tanedo)

(Video) What is a Higgs boson? (Dom Lincoln)

(Video) Higgs boson – Latest update (Dom Lincoln)

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Higgsdependence Day

Monday, July 2nd, 2012

On July 4th CERN will hold a seminar where ATLAS and CMS will present their latest findings on the search for the Higgs boson. There’s a reasonable chance that either or both experiments will see a 5 sigma excess, and this would be enough to claim a “discovery”. One of my US friends at CERN called this day Higgsdependence Day, and all over the USA people will be celebrating with fireworks and barbecues. (Okay, perhaps they will be celebrating something else. My boss tells me he might tar and feather me as the token British member of the group…)

CERN is not the only lab to be holding a seminar. Today at 09:00 CDT Fermilab will be announcing the latest results from CDF and D0. Rumors suggest a 3 sigma excess (technically an “observation”) in the interesting region. So if you can spare the time I’d recommend you listen in on the announcement. You can see the webcast information here.

In anticipation of the CERN seminar, when I came to my office this morning I found a bottle of champagne with a label hastily pasted to the back. It seems these might be placed alongside fire extinguishers in every office at CERN! (You can get your own label here.)

No Higgs seminar is complete without a bottle of Champagne, just in case!

No Higgs seminar is complete without a bottle of Champagne, just in case!

For those of us who can’t get enough of the Higgs boson and want to brush up on the basics I would recommend the following show, put out by the BBC. This contains the latest results from the 2011 searches and it goes into quite a bit of depth about why we think the Higgs boson exists and what to expect from the 2012 searches.

Finally for those keeping score I still have $20 riding on a non-discovery. If a 5 sigma excess is seen on Wednesday there is a bit more work that needs to be done to show that it is the Standard Model Higgs, and that would probably take until the end of 2012 running. So my $20 is safe… for now.

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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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There’s an interesting New York Times article out today, titled “New Data on Elusive Particle Shrouded in Secrecy”. The headline is misleading. There’s nothing to keep secret about our Higgs boson search, because we’re simply not done.

It’s true that we looked at some of our preliminary results last Friday. Every part of the search has more to do, and some don’t have their 2012 updates in their final form at all. And we’ve allowed ourselves only two or three weeks to go from first-pass results to the final product!

The article itself actually gets this more or less right:

Right now, most of the physicists doing the work do not even know what they have. In order to avoid bias, the physicists involved avoided looking at most of the crucial data until last week, when they “unblinded” it. About 500 physicists on each team are analyzing eight different ways a Higgs boson, once produced in the collider, might decay and leave its signature.

And, as it quotes Joe Incandela, the spokesperson for my experiment:

Our final [ICHEP] results will not be even seen by the collaboration before the last day of June and then will require the usual final cosmetics for presentation.

So you’ll have to forgive us if we keep quiet for a few more weeks about our results. They’ll be shown at ICHEP in Melbourne, Australia starting on July 4. Here at CERN, we’ll be dealing with the suspense by working on the final answer almost up to the very last minute.

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Finding the Higgs boson will have no epistemic value whatsoever.  A provocative statement. However, if you believe that science is defined by falsification, it is a true one.  Can it really be true, or is the flaw in the idea of falsification?  Should we thumb our noses at Karl Popper (1902 – 1994), the philosopher who introduced the idea of falsification?

The Higgs boson, the last remaining piece of the standard model, is the object of an enormous search involving scientists from around the world.  The ATLAS collaboration alone has 3000 participants from 174 institutions in 38 different countries. Can only the failure of this search be significant? Should we send out condolence letters if the Higgs boson is found? Were the Nobel prizes for the W and Z bosons a mistake?

Imre Lakatos (1922 – 1974), a neo-falsificationist and follower of Popper, states it very cleanly and emphatically:

But, as many skeptics pointed out, rival theories are always indefinitely many and therefore the proving power of experiment vanishes.  One cannot learn from experience about the truth of any scientific theory, only at best about it falsehood: confirming instances have no epistemic value whatsoever (emphasis in the original).

Yipes! What is going on? Can this actually be true? No! To see the flaw in Lakatos’s argument, let’s consider an avian metaphor—this time Cygnus not Corvus. Consider the statement: All swans are white. (Here we go again.) Before 1492, Europeans would have considered this a valid statement. All the swans they had seen were white. Then Europeans started exploring North America. Again, the swans were white. Then they went on to South America and found swans with black necks (Cygnus melancoryphus) and finally to Australia where the swans are black (Cygnus atratus). By the standards of the falsificationist, nothing was learned when white swans were found, but only when the black swans or partially black swans were found.  With all due respect, or lack of same, that is nonsense. It is the same old problem: you ask a stupid question you get a stupid answer. Did we learn anything when white swans were found in North America? Yes. We learned that there were swans in North America and that they were white. Based on having white swans in Europe, we could not deduce the colour of swans in North America or even that they existed. In Australia, we learned that swans existed there and were black. Thus, we learned a similar amount of information in both cases—really nothing more or nothing less.  The useful question is not, ‘Are all swans white?’ Rather, ‘On which continents do swans exist and what color are they on each continent?’

Moving on from birds to model cars (after all, the standard model of particle physics is a model). What can we learn about a model car? Certainly, not if it is correct. Models are never an exact reproduction of reality. But, we can ask, ‘Which part the car is correctly described by the model? Is it the color? Is it the shape of the head lights or bumper?’ The same type of question applies to models in science. The question is not, ‘Is the standard model of particle physics correct?’ We knew from its inception that it is not the answer to the ultimate question about life, the universe and everything. The answer to that is 42 (Deep Thought, from The Hitchhiker’s Guide to the Galaxy by Douglas Adams). We also know that the standard model is incomplete because it does not include gravity. Thus, the question never was, ‘Is this model correct?’ Rather, ‘What range of phenomena does it usefully describe?’ It has long history of successful predictions and collates a lot of data. So, like the model car, it captures some aspect of reality, but not all.

Finding the Higgs boson helps define what part of reality the standard model describes. It tells us that the standard model still describes reality at the energy scale corresponding to the mass of the Higgs boson. But, it also tells us more: It tells us that the mechanism for electroweak symmetry break –a fundamental part of the model—is adequately described by the mechanism that Peter Higgs (and others) proposed and not some more complex and exotic mechanism.

The quote from Lakatos, given above, misses a very important aspect of science–parsimony. The ambiguity noted there is eliminated by the appeal to simplicity. The standard model of particle physics describes a wide range of experimental observations. Philosophers call this phenomenological adequacy. But a lot of other models are phenomenologically adequate. The literature is filled with extensions to the standard model that agree with the standard model where the standard model has been experimentally tested. They disagree elsewhere, usually at higher energy. Why do we prefer the standard model to these pretenders? Simplicity and only simplicity. And the standard model will reign supreme until one of the more complicated pretenders is demonstrated to be more phenomenolgically adequate. In the meantime, I will be a heretic and proclaim that finding the Higgs boson would indeed confirm the standard model. Popper, Lakatos, and the falsificationists be damned.

Additional posts in this series will appear most Friday afternoons at 3:30 pm Vancouver time. To receive a reminder follow me on Twitter: @musquod.

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A Grumpy Note on Statistics

Tuesday, March 13th, 2012

Last week’s press release Fermilab about the latest Higgs search results, describing the statistical significance of the excess events, said:

Physicists claim evidence of a new particle only if the probability that the data could be due to a statistical fluctuation is less than 1 in 740, or three sigmas. A discovery is claimed only if that probability is less than 1 in 3.5 million, or five sigmas.

This actually contains a rather common error — not in how we present scientific results, but in how we explain them to the public. Here’s the issue:

Wrong: “the probability that the data could be due to a statistical fluctuation”
Right: “the probability that, were there no Higgs at all, a statistical fluctuation that could explain our data would occur”

Obviously the first sentence fragment is easier to read — sorry![1] — but, really, what’s the difference? Well, if the only goal is to give a qualitative idea of the statistical power of the measurement, it likely doesn’t matter at all. But technically it’s not the same, and in unusual cases things could be quite different. My edited (“right”) sentence fragment is only a statement about what could happen in a particular model of reality (in this case, the Standard Model without the Higgs boson). The mistaken fragment implies that we know the likelihood of different possible models actually being true, based on our measurement. But there’s no way to make such a statement based on only one measurement; we’d need to include some of our prior knowledge of which models are likely to be right.[2]

Why is that? Well, consider the difference between two measurements, one of which observed the top quark with 5 sigma significance and the other of which found that neutrinos go faster than light with 5 sigma significance. If “5 sigma significance” really meant “the probability that the data could be due to a statistical fluctuation,” then we would logically find both analyses equally believable if they were done equally carefully. But that’s not how those two measurements were received, because the real interpretation of “5 sigma” is as the likelihood that we would get a measurement like this if the conclusion were false. We were expecting the top quark, so it’s a lot more believable that the excess is associated with the top quark than with an incredibly unlikely fluctuation. But we have many reasons to believe neutrinos can’t go faster than light, so we would sooner believe that an incredibly unlikely fluctuation had happened than that the measurement was correct.[3]

Isn’t it bad that we’d let our prior beliefs bias whether we think measurements are right or not? No, not as long as we don’t let them bias the results we present. It’s perfectly fair to say, as OPERA did, that they were compelled to publish their results but thought they were likely wrong. Ultimately, the scientific community does reach conclusions about which “reality” is more correct on a particular question — but one measurement usually can’t do it alone.

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[1] For what it’s worth, I actually spent a while thinking and chatting about how to make the second sentence fragment simpler, while preserving the essential difference between the two. In this quest for simplicity, I’ve left off any mention of gaussian distributions, the fact that we really give the chance of a statistical fluctuation as large or larger than our excess, the phrase “null hypothesis,” and doubtless other things as well. I can only hope I’ve hit that sweet spot where experts think I’ve oversimplified to the point of incorrectness, while non-expert readers still think it’s completely unreadable. 😉

[2] The consensus among experimental particle physicists is that it’s not wise to include prior knowledge explicitly in the statistical conclusions of our papers. Not everyone agrees; the debate is between Frequentist and Bayesian statistics, and a detailed discussion is beyond the scope of both this blog entry and my own knowledge. A wider discussion of the issues in this entry, from a Bayesian perspective, can be found in this preprint by G. D’Agostini. I certainly don’t agree with all of the preprint, but I do owe it a certain amount of thanks for help in clarifying my thinking.

[3] A systematic mistake in the result, or in the calculation of uncertainties, would be an even likelier suspect.

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Each time news comes out about the Higgs boson I get questions from media, friends and family trying to grasp why this particle is so important. The following questions come up again and again. So with experimenters from using Fermilab’s  Tevatron announcing new Higgs results Wednesday at a conference in Italy, I thought it was time to share answers to the questions that might pop into your mind.

Why should the average person care if the Higgs is found?

Understanding more about the building blocks of matter and the forces that control their interactions helps scientists to learn how to manipulate those forces to humankind’s benefit. For example, the study of the electron led to the development of electricity, the study of quantum mechanics made possible the creation of GPS systems and the study of the weak force led to an understanding of radioactive decay and nuclear power.

Now what?

The Tevatron experiments will continue to further analyze the Higgs boson data to wring out more information. In addition, the Tevatron and LHC experiments are working to combine their data for a release at an unspecified date.

Even if both teams find evidence of a Higgs boson in the same location, physicists will need to do more analysis to make sure the Higgs boson isn’t a non-Standard Model Higgs masquerading as a resident of the Standard Model. That will require physicists to measure several properties in addition to mass.

What would finding the Higgs boson mean for the field of physics?

Finding evidence of the Higgs boson would expand the following three areas of study:

• Pin-pointing the mass range of the Higgs would help physicists condense the number of theories about the existence of undiscovered particles and the forces that interact on them. For example, a Standard Model Higgs boson would rule out classic QCD-like versions of technicolor theory. A Higgs boson with a mass larger than 125 GeV would rule out the simplest versions of supersymmetry, or SUSY, which predict that every known particle has an unknown sibling particle with a different mass. Other theories would gain more support. One such SUSY theory predicts that a Standard Model Higgs boson would appear as the lightest of a group of five or more Higgs bosons. Whether the Higgs boson exists or not does not affect theories about the existence of extra dimensions.

• Knowing the mass of the Higgs boson would give physicists more data to plug into other equations about how our universe formed and about some of the least understood particle interactions, such as magnetic muon anomaly.

• Finding evidence of a heavy mass Higgs boson (larger than 150 GeV) would require the existence of undiscovered particles and/or forces. Finding a light mass Higgs boson (less than 125 GeV) would not require the existence of new physics but doesn’t rule it out either.

What is the difference between the Higgs boson and the Higgs field?

The Higgs field exists like a giant vat of molasses spread throughout the universe. Particles that travel through it end up with globs of molasses sticking to them, slowing them down and making them heavier. You can think of the Higgs boson as the molasses globs, or a particle manifestation of this energy field akin to a ball of energy.

Physicists have different theories about how many Higgs bosons exist, akin to predicting whether the molasses would stick in one giant glob or several globlets.

How long have physicists been looking for the Higgs?

More than a decade. It started with the LEP experiment at CERN in the 1990s, continued with the Tevatron and now with the LHC.

How do physicists create a Higgs boson?

A high-energy particle accelerator such as the Tevatron or LHC can recreate the energy levels that permeated the universe shortly after the Big Bang. Colliding particles at this energy level can set free the right amount of energy to produce particles, including a Higgs boson. The collision energy is localized in a small space and transforms from energy into the mass of the Higgs boson.

How is the Higgs boson related to the Big Bang theory?

The Big Bang occurred 13.7 billion years ago sending massless particles and radiation energy zooming through the universe like cars at rush hour. Shortly afterward, the Higgs field appeared, as if a truck carrying molasses overturned and leaked all over the highway. Particles such as light, which went through the puddle super fast, avoided having any molasses stick to them, similar to the way hydroplaning cars skim the surface of water. Particles that went through the molasses puddle more slowly had molasses goblets cling to them, creating a drag that slowed them even more and made them more massive. How fast a particle made it through the puddle determined how much molasses clung to it, and thus how massive it became. When the universe began to cool, slow particles with mass began to bunch up like mini-traffic jams and form composite particles and then atoms.

How do we know this is where the Higgs is located?

Just as firemen sweep building floors to rule out the existence of trapped homeowners, physicists have used direct and indirect observations from experiments to rule out the existence of the Higgs boson in most energy ranges where the Standard Model predicts it could reside.

Does the mass of the Higgs compare to its weight?

Sort of. Non-physicists think of mass as how much something weighs. But scientists consider mass to take into account weight and other factors. Weight changes with gravity, so you would weigh less on the moon than on Earth. Mass remains constant throughout the universe. However, when talking about things on Earth, mass and weight are fairly interchangeable.

How did the Higgs boson get the nickname “the God particle”?

Nobel laureate Leon Lederman, a Fermilab physicist, wrote a book in the early 1990s about particle physics and the search for the Higgs boson. His publisher coined the name as a marketable title for the book. Scientists dislike the nickname.

What countries are involved in the CDF and DZero experiments?

• CDF: US, Canada, France, Germany, Greece, Italy, Japan, Korea, UK, Russia, Slovakia, Spain and Taiwan

• DZero; Brazil, China, Columbia, Czechoslovakia, Ecuador, France, Germany, India, Ireland, Korea, Mexico, Netherlands, UK, Ukraine, US, Russia, Spain and Sweden.

What is the competitive relationship between the Tevatron and LHC experiments?

It is closer to sibling rivalry than the traditional business competition you would find in something such as the auto industry.

Fermilab supports about 1,000 US CMS scientists and engineers by providing computing facilities, office and meeting space as well as the LHC Remote Operation Center. Fermilab helped design and build the CMS detector as well as equipment for the LHC accelerator, and Fermilab scientists are working on upgrades for both and analyzing data. About one third of the members of each of the Tevatron’s experiments, CDF and DZero, are also members of the LHC experiments.

— Tona Kunz

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Christmas time brings not only presents and pretty cookies but an outpouring of media lists proffering the best science stories of the year and predicting those that will top the list in 2012.

While the lists varied wildly everyone seemed excited by a few of the same things: upsetting Einstein’s theory of special relativity, a hint of the ‘god particle’ and finding planets like our own.

Several of the stories that made nearly every media outlet’s list, though in various rankings, have a connection, directly or indirectly, to Fermilab. Here’s a sampling with the rankings from the publications.

Discover magazine had the largest list, picking the top 100 science stories.

1: A claim by researchers at the OPERA experiment at CERN that they had measured neutrinos traveling faster than the speed of light, something disallowed by Einstein’s Theory of Special Relativity. Now the scientific community is looking for another experiment to cross-check OPERA’s claim.

That brought renewed interest to a 2007 measurement by the MINOS experiment based at Fermilab that found neutrinos skirting the cosmic speed limit, but only slightly. The MINOS collaboration always planned to study this further when it upgrades its detector in early 2012 but the OPERA result added new urgency.

Look in 2012 for MINOS to update the time of flight of neutrinos debate in three stages. First, MINOS is analyzing the data collected since its 2007 result to look for this phenomena. Results should be ready in early 2012. This likely will improve the MINOS  precision in this area by a factor of three from its 2007 result. Second, MINOS is in the process of upgrading its timing system within the next few months using a system of atomic clocks to detect when the neutrinos arrive at the detector. The atomic clock system will progressively improve resolution, which is needed to make the MINOS analysis comparable to the OPERA result and improve precision from the 2007 MINOS result by as much as a factor of 10. That will tell us if OPERA was on the right track or not, but may not be the definitive answer. That answer will come with the upgrades to the MINOS experiment  and a more powerful neutrino beam, producing a larger quantity of neutrino events to study. The upgraded MINOS experiment will be in many ways a more precise system than OPERA’s and could produce a result comparable with OPERA’s precision likely by January 2014.

4: Kepler’s search for Earth-like planets that could sustain life produces a bounty of cosmic surprises, fueled, in part, by the computing skills of a Fermilab astrophysicist.
32: The on-again, off-again rumor of finding the Higgs boson particle.  Physicists working with experiments at Fermilab’s Tevatron experiments and CERN’s Large Hadron Collider expect to answer the question of whether a Standard Model version of the Higgs exists in 2012.
65: The shutdown of the Tevatron at Fermilab after 28 years and numerous scientific and technological achievements.
82: Fermilab physicist Jason Steffen’s frustration with slow airplane boarding drives him to figure out a formula to speed up the aisle crawl.

Nature’s year in review didn’t rank stories but started off by mentioning the Tevatron’s shutdown after 28 years and following up shortly with the puzzling particle news of potentially FTL neutrinos and a Higgs sighting.

For science — as for politics and economics — 2011 was a year of upheaval, the effects of which will reverberate for decades. The United States lost three venerable symbols of its scientific might: the space-shuttle programme, the Tevatron particle collider and blockbuster profits from the world’s best-selling drug all came to an end.

Cosmos magazine rankings:

The MINOS far detector in the Soudan Mine in Minnesota. Credit: Fermilab

1: Kepler’s exoplanet findings
2: FTL neutrinos
3: Higgs

Scientific American‘s choices:

3: FTL neutrinos
5: Higgs

ABC News asked science radio and TV host physicist Michio Kaku for his top 10 picks. They include:

3: Hint of Higgs
5: Kepler’s exoplanet findings
10: Nobel Prize for the discovery that the expansion of the universe is accelerating, which laid the groundwork for the today’s search for dark energy. Fermilab has several connections to to this work. The latest tool in dark energy survey experiments, the Dark Energy Camera,  was constructed at Fermilab in 2011. One of the three prize winners, Saul Perlmutter, is a member of the group that will use the camera, the Dark Energy Survey collaboration. Adam Riess, another of the winners, is a member of the SDSS-II experiment, a predecessor to DES that Fermilab was key in building and later operating its computing system.

Live Science

5: FTL neutrinos
4: Kepler’s exoplanet findings
2: Higgs

If the Higgs boson’s mass is high, it is expected to decay predominantly into two W bosons. Plushies images from the Particle Zoo.

To make the Ars Technica list stories had to be awe inspiring in 2011 AND have a chance of making the 2012 list as well.

1: FTL neutrinos
2: Kepler’s exoplanet findings
6: Higgs hunt

Science magazine chose the best scientific breakthrough of the year. Kepler’s exoplanet hunt made it into the runner up list.

Tell us who you agree with or, better, yet give us your own top 10 science stories of the year.

— Tona Kunz

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Real CMS proton-proton collision events in which 4 high energy electrons (green lines and red towers) are observed. The event shows characteristics expected from the decay of a Higgs boson but is also consistent with background Standard Model physics processes. Courtesy: CMS

Today physicists at CERN on the CMS and ATLAS experiments at the Large Hadron Collider announced an update on their search for the Higgs boson. That may make you wonder ( I hope) what is Fermilab’s role in this. Well, glad you asked.

Fermilab supports the 1,000 US LHC scientists and engineers by providing office and meeting space as well as the Remote Operation Center. Fermilab helped design the CMS detector, a portion of the LHC accelerator and is working on upgrades for both. About one-third of the members of each of the Tevatron’s experiments, CDF and DZero, are also members of the LHC experiments.

That means that a good portion of the LHC researchers are also looking for the Higgs boson with the Tevatron.  Because the Tevatron and LHC accelerators collide different pairs of particles, the dominant way in which the experiments search for the Higgs at the two accelerators is different. Thus the two machines offer a complimentary search strategy.

If the Higgs exists and acts the way theorists expect, it is crucial to observe it in both types of decay patterns. Watch this video to learn how physicists search for the Higgs boson. These types of investigations might lead to the identification of new and unexpected physics.

Scientists from the CDF and DZero collaborations at Fermilab continue to analyze data collected before the September shutdown of the Tevatron in the search for the Higgs boson.

The two collaborations will announce their latest results for the Higgs boson search at an international particle physics conference in March 2012. This new updated analysis will have 20 to 40 percent more data than the July 2011 results as well as further improvements in analysis methods.

The Higgs particle is the last not-yet-observed piece of the theoretical framework known as the Standard Model of particles and forces. Watch this video to learn The nature of the Higgs boson and how it works. According to the Standard Model, the Higgs boson explains why some particles have mass and others do not. Higgs most likely has a mass between 114-137 GeV/c2, about 100 times the mass of a proton. This predicted mass range is based on stringent constraints established by earlier measurements made by Tevatron and other accelerators around the world, and confirmed by the searches of LHC experiments presented so far in 2011. This mass range is well within reach of the Tevatron Collider.

The Tevatron experiments already have demonstrated that they have the ability to ferret out the Higgs-decay pattern by applying well-established techniques used to search for the Higgs boson to observing extremely rare but firmly expected physics signature. This signature consists of pairs of heavy bosons (WW or WZ) that decay into a pair of b quarks, a process that closely mimics the main signature that the Tevatron experiments use to search for the Higgs particle, i.e. Higgs decaying to a pair of b quarks, which has by far the largest probability to happen in this mass range. Thus, if a Standard Model Higgs exists, the Tevatron experiments will see it.

If the Standard Model Higgs particle does not exist, Fermilab’s Tevatron experiments are on track to rule it out this winter. CDF and DZero experiments have excluded the existence of a Higgs particle in the 100-108 and the 156-177 GeV/c2 mass ranges and will have sufficient analysis sensitivity to rule out this winter the mass region between.

While today’s announcement shows the progress that the LHC experiments have made in the last few months, all eyes will be on the Tevatron and on the LHC in March 2012 to see what they have to say about the elusive Higgs Boson.

— Tona Kunz

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