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    Fundamental Glossary For The Higgs Broadcast
    By Tommaso Dorigo | December 9th 2011 02:54 AM | 44 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    As everybody knows, next Tuesday we will be treated with a CERN webcast of the analysis results on the Higgs boson searches by ATLAS and CMS. I imagine many of you will want to tune in, but fear you will not grasp much given the typically technical jargon that physicists use to communicate the details of their analyses.

    So I thought I would provide here a very short glossary of terms you are likely to hear, and which you might have a hard time understanding correctly. Let me see if I can do a decent job.


    - gamma: a gamma-ray is a photon, i.e. a quantum of light. A very energetic one, to be sure: a gamma ray is such only if it carries significantly more energy than a x-ray, so above a Mega-electron-Volt or so. The gammas we will be hearing about are those directly coming from a Higgs boson decay, and these have an energy of 62.3 GeV, equivalent to the kinetic energy of a mosquito traveling at 9 centimeters per second.

    - VBF, or "vector-boson fusion": this is a process whereby the two protons (in the case of LHC collisions, but this is not specific to the VBF reaction) interact by both emitting a W, or a Z boson. The two bosons "fuse" - they annihilate - and a Higgs particle is thus created. The peculiarity of the VBF production of Higgs bosons is that from the protons, which both emitted the W or Z originating the process, emerge two energetic hadronic jets. The latter arise from the quarks which emitted the W or Z in the first place: they need to balance the momentum, so they are ejected from the parent proton (which then dissociates, since a proton does not remain stable if you pull a quark out of it); these VBF jets are typically emitted at small angle from the proton direction, but they are energetic enough that they constitute a nice "tag" of the VBF production process. VBF production of Higgs bosons is not the highest-rate production mechanism, but it is significant because of the distinctive feature of the two forward jets.

    - gluon fusion: this is the most common way of producing Higgs bosons. Both protons emit a gluon, and the two exchange a virtual pair of top quarks; the latter in turn annihilate creating the Higgs boson. Note that the top quark is the one doing this the most, because it is the heaviest fermion, and the Higgs boson "couples" preferentially to heavy stuff. Also note that unlike the VBF process discussed above, gluon-gluon fusion does not give rise to energetic, forward jets. That is because gluons are massless, and the quarks that emitted them in the protons do not typically recoil in the plane transversal to the beam axis.

    - Associated production: Higgs bosons can be produced together with a W or Z boson, when the latter "radiate out" a Higgs. The resulting signatures are richer than in the gluon-fusion process, but the rate of these events is smaller. One typically looks for this process for low Higgs masses, when the rate is not too small; in this case, one is bound to look for the Higgs decaying into two b-quark jets, because the gamma-gamma decay rate is several hundred times smaller.

    - background model: we'll hear that discussed a lot in the ATLAS and CMS talks. Indeed, we are after a very small signal (particularly in the H->gamma-gamma decay mode) on top of a large background, which unfortunately is not easy to predict with simulations. So what the experiments do is to parameterize it with a simple function: a low-degree polynomial, or a sum of two exponentials, etcetera. This functional form of the "background model" is used together with a signal model (derived from simulation with much more confidence) to fit the data. Since the signal is small, the different background models will in general produce different results. Experimentalists thus need to be very careful in estimating how their a priori assumptions bias the result. We will probably also hear a lot about how the background models have been tested in pseudoexperiments, so see the next item too.

    - pseudoexperiment: this is not how CMS members mockingly call the ATLAS detector, or vice-versa. Rather, a pseudoexperiment is a set of data derived from the random generation of events following a pre-defined model. To clarify, imagine you want to test whether your fitting procedure is sensitive to detect a Higgs decay to two gammas in the diphoton mass spectrum, when you assume that the background shape is a falling exponential distribution and the signal has the strength expected from Standard Model production. What you do is to randomly generate mass values distributed according to the background model, add the correct amount of mass values distributed according to the signal model for a specific Higgs mass hypothesis, and fit the resulting mass distribution. You obtain a signal strength estimate, an uncertainty, etcetera, as in a real experiment. This is a pseudoexperiment: you can iterate as much as you want the procedure, creating distributions of the expected relevant quantities for each mass hypothesis. This allows you to draw those fancy "brazil bands" in green and yellow which describe what is the expected result of your experiment.

    - p-value: beware! this is a very tricky thing. You will see plots of p-values and will need some insight to really understand what you are looking at. A p-value is the probability of obtaining data at least as "extreme" as the one you observe, if your null hypothesis is true. What is your null hypothesis ? Well, it can be anything, but in our case it will be the absence of a Higgs boson of a pre-defined mass. "Extreme" is quite tricky instead: it depends on what is your "alternate hypothesis" of reference, and what kind of departure it would produce on the studied statistic derived from the data. In searching for a Higgs boson we will combine channels where event excesses are sought with various techniques: the combination is performed by using a global test statistic called "CLs". You need not know what that is (it is very, very technical), but its distribution will be different for signal+background and for background-only hypotheses (the alternate and the null). So "extreme" will mean "departing from the typical values expected for the null hypothesis, toward the values expected from the alternate hypothesis".

    - Connected with the p-value is the issue of probability inversion, which is the typical pitfall that 90% of outsiders and 40% of insiders fall in (and maybe I am being a bit too optimistic with my colleagues' understanding of statistics). Again, the p-value is the probability of obtaining data at least as extreme as the ones observed, if the null hypothesis is true. This is a world apart from saying that it is the probability of the null hypothesis being true, given that you observed that extreme data! Beware! If your ability on the long jump puts you in the 99.99% percentile, that does not mean that you are a kangaroo, and neither can one infer that the probability that you belong to the human race is 0.01%.

    - But what is a "sigma" anyway ? A couple of colleagues suggested that I be more explicit on explaining this - I often give it for granted. A "sigma" is the unit of measurement of how discrepant is a result with respect to its expected value. The jargon comes from the parameter sigma of the Gaussian distribution. If we say that a result is "three sigma" away from expectations, we are saying that it is quite far from what we would get if the "null hypothesis" is correct. This does not allow one to say that the null hypothesis is false, of course - see the explanation above about probability inversion. However, the chance to obtain a result as discrepant as "one-sigma", "two-sigma", etc, can be read off from the following table (courtesy Andre David):
    z=1 sigma, p(by chance)~16% or once every 6 times.
    z=2 sigma, p(by chance)~2.3% or once every 44 times.
    z=3 sigma, p(by chance)~0.13% or once every 741 times. <= What we call "evidence" level.
    z=4 sigma, p(by chance)~0.0032% or once every 31'574 times.
    z=5 sigma, p(by chance)~0.000028% or once every 3'486'914 times. <= What we call "discovery" level.

    Comments

    bla bla bla bla just do the damn experiment already feels like been waiting forever

    "The gammas we will be hearing about are those directly coming from a Higgs boson decay, and these have an energy of 62.3 GeV"

    Did you just give away the mass of the Higgs here? (2x62.3 GeV)

    vongehr
    Moreover, look at the wording:

    "those directly coming from a Higgs boson decay"
    "this is the most common way of producing Higgs bosons."
    "Higgs bosons can be produced together with a W or Z boson
    "

    A stark contrast to just reprimanding me for writing that a Higgs discovery will be announced.
    dorigo
    I did not reprimanded you Sascha, I just alerted that what you wrote is wrong: no higgs discovery will be announced on December 13th, as the director general has clarified in his press release.

    As for my wording, it is the same I have used for years in my course of subnuclear physics and in this blog, and it is always conditional to the theory being true. I suspect you know this well and just want to argue.

    Best,
    T.
    vongehr
    no higgs discovery will be announced on December 13th, as the director general has clarified
    Tommaso, please, ... . Or are you kidding yourself? But go on, it is amusing nonetheless, and why not. Lets have fun - life is too sad otherwise. ;-)
    Are you going to post back here saying that your comments were meaningless and stupid speculation against all indications, if you are proven wrong on Tuesday?
    God, sometimes you make me miss Lubos' comments. (Well, not really.)

    PS: I'm not taking Tommaso's or anyone's side with this, I'm just bored of this brainless situation and the potential harm it holds for science and its funding when it'll repeat itself for the n-th time.

    Sh*t,, Lubos had already commented when I wrote this, and his comment was way witier than yours. Oh well.

    vongehr
    "Tulpoeid" - head, I just answered somebody this:
    "A welcome break from people who comment that I am an idiot after they only read half the article because the important bit went over their head."
    Maybe you can read his comment and see whether perhaps you are missing something? Reading levels are various. You will improve, maybe, keep trying.
    Is your answer related to what I've written? How is quoting a comment from a third party which adds no information relevant?

    I've read your posts for a while and used to find the bickering entertaining, but it's really just juvenile and taxing, ultimately. Yes, Tommaso's command of English is frightening for a scientist, but no more so than your petty pointedness, regardless of how clever you think you're being. You've a responsibility as a scientist to correct misinformation; not tirelessly contribute to and reference some cosmic scoreboard of he said/she said.

    Stop being fucking assholes, both of you.

    vongehr
    You are welcome to point out any misinformation on my articles that you think to have identified. Up to now, I never had any, and those typos or other trifles that slipped in, were promptly corrected. The nature of Alpha Meme renders it unlikely that anything wrong can slip in, because the arguments are 'on the next level' and not concerned with petty stuff like whether something is 4.2 sigma or perhaps 4.9. Alpha Meme is consciously aimed at future relevance, to be recognized as of increasing importance when discussions about sigmas will be cold coffee from last year.
    dorigo
    Well, depends on the frame of reference you're sitting on ;-)
    Cheers,
    T.
    Thanks alot Tommaso, your post is extremely illuminating.

    Also, rightly or wrongly, it does sound like you are giving away insider information. Perhaps you could have been more "hypothetical" than usually, in a situation with so much expectation.

    dorigo
    Hi Plm,

    of course I am just slightly teasing my readers... I of course have no clue of what the Higgs mass is, as proven by the fact that until a couple of weeks ago I was willing to bet it was 119 GeV and now I am more inclined to believe it's around 125.

    Cheers,
    T.
    Thanks Tammaso,

    Appreciate the help in understanding the the terminology. Getting the Jet production has not been easy on one's own so I appreciate the clarifications. I think experimentally how would you see developmental aspects of the same work as you are describing, in relation too? See: Article #1 on Standard Model Higgs: How It Is Produced at the LHC- http://profmattstrassler.com/2011/12/09/article-1-on-standard-model-higg...

    Best,

    Tommaso Dorigo announces observed Higgs mass: 124.6 GeV

    If this figure 62.3 GeV was really meant to be multiplied by two, let me mention that many people in this thread are very good at multiplication by two while Peter Shmoit or one of his sources is very bad at rounding numbers such as 124.6 to the closest integer. ;-)

    What the hell does "glossary" mean?

    dorigo
    Oh well, what solar system are you from anyway ?
    http://en.wikipedia.org/wiki/Glossary

    Cheers,
    T.
    Are there any other possibilities for particles, other than the Higgs, that could decay to two 62.3 GeV photons?

    I hope you're happy, Tommaso, that you just became famous:

    http://motls.blogspot.com/2011/12/higgs-mass-1246-gev-cms-126-gev-atlas....

    And I've quoted not one but two articles of yours, including one about the doubly measured sticks!

    Hank
    That "Hello Kitty" background over there makes it tough to read. But when you talk about physics, you are always fun.
    Dear Hank, the text near the "hello kitty" pictures says "pray for Japan" and all the other text is written at places where kittys are strictly forbidden.

    You may also read a kitty-free version of the blog:
    http://motls.blogspot.com/?m=1

    Why are you still praying for Japan? I understand that the Fukushima crisis ended on March 21... ;-)
    http://motls.blogspot.com/2011/03/fukushima-crisis-is-over.html

    I was never praying because of Fukushima that has killed 0 people and only stupid brainwashed people believe that has ever been a serious crisis. I was praying because of the earthquake and tsunami that killed tens of thousands of people and decimated a big portion of the country.

    As you have just showed, what it really depends upon is what particular source for mass brainwashing of the stupid sheep you choose before you scream nonsense about crises.

    So what would be YOUR favorite source of brainwashing? I bet it's the same as this guy's... ;-)
    www.youtube.com/watch?v=sVTRRccoTcY

    It has to sound remarkable if not blasphemous to you but my favorite source of brainwashing is no source of brainwashing.

    The official in the video you posted just drank ordinary decontaminated water from the Fukushima reactor, to show imbeciles like you that there's nothing dangerous going on beyond the contaminated areas (or volumes). Clearly, for some of the most hardcore imbeciles, even this proof isn't enough.

    right, you've not been brainwashed - you were BORN that way ;-)

    I read in Shasha's blog what we had to say about the light Higgs and the instability of the Universe vacuum and so on. When he talks of things I don't know anything about he sounds pretty self-confident. What he says now in his blog I can understand better. Most of it is simply rubbish.

    Tommaso,

    Just a stupid question from a chemist (me): Lets assume we find the Higgs boson. Then:

    Why is this so great?
    What's next?

    ss

    dorigo
    Hi Shelton,

    not stupid at all. Interesting questions to answer in one-liners. Let me have a shot:

    - It's great since it finally proves correct, after 40 years of investigations, our understanding of fundamental physics.
    - Next (short-midterm) is proving it is behaving as we expect for a SM particle (as opposed to a SUSY Higgs).

    Cheers,
    T.
    This if fascinating. I could swear i am reading a script from The Big Bang Theory.

    Thankyou for the definitions, but you miss the definition of sigma. Everybody talks about it, and it took some time to find an understandable methaphor.

    Scientists measure their experiments by a term called sigma.

    There is no scientific discovery about a sigma 2.5 or 3.50 as probably will be the case on tuesday. Maybe even lower.

    If Newton were sitting under his appletree and made 100 observations, and in one instance the apple didn't hit him in the head, it is a sigma 2.5. A sigma 5.o is EVERY time that the apple falls down, and that includes doing it a million times and more.

    THAT is a scientific discovery.

    At sigma 3 the apple fals wrong one out of every 370 times you do the experiment. That is not a scientific discovery either.

    My prediction is that the Higgs particle never will reach close to a sigma 5.

    Behind the prediction there is a theory, if you are interested.

    Google crestroyer theory and find it or visit directly at

    http://crestroyertheory.com/the-theory/

    You. Are. So. Full. Of. Words.

    You missed the definition of sigma

    Scientists measure their experiments by a term called sigma.

    There is no scientific discovery about a sigma 2.5 or 3.00 as probably will be the case on tuesday. Maybe even lower.

    If Newton were sitting under his appletree and made 100 observations, and in one instance the apple didn't hit him in the head, it is a sigma 2.5. A sigma 5.o is EVERY time that the apple falls down, and that includes doing it a million times and more.

    THAT is a scientific discovery.

    At sigma 3 the apple fals wrong one out of every 370 times you do the experiment. That is not a scientific discovery either.

    My prediction is that the Higgs particle never will reach close to a sigma 5.

    Behind the prediction there is a theory, if you are interested.

    Google crestroyer theory and find it or visit directly at

    Hello Tomaso.
    You did it once again.Write,this time indirectly,results before they are anounced.All people are working in the experiments seems to be stupid waiting for the official publications/announcements and you are clever by announcing them first by screwing them!Well,i don't find your behaviour ethical,it's not the correct way to make a name for yourself!cheers!

    dorigo
    Dear Yia,

    please tell me where I wrote results in the post above. And also tell me where I did it elsewhere, since I never actually did anything like that. And then please go fishing.

    Cheers,
    T.
    Hi again!In this blog entry you write
    "The gammas we will be hearing about are those directly coming from a Higgs boson decay, and these have an energy of 62.3 GeV"..
    I also remember taking the blog entry down about the OPERA experiment recently...

    dorigo
    So ? That was a teaser, and one must be a bit gullible to think it is half the best-fit mass of the Higgs by CMS, or anything like that which you might be thinking it is. I just took a value that had been circulated and divided by two, just to make an example of the typical photon energy we will be discussing in a few days. Of course we will be talking of photons of about that energy, but even for the Higgs the value is not fixed in the lab frame.
    In any case you will be deluded if you think the experiments will come up with a sub-GeV estimate of the Higgs mass. This will come, if the Higgs is found, only after a few additional years of running.

    As for the Opera result, yes, I discussed it before it was announced, albeit vaguely. But that is not an experiment I am part of. Sorry for not paying attention to the fact that you are concerned with the general issue, rather than the relationship with my own colleagues. If you claim I cannot discuss anything I get to hear about other experiments because it is unethical to do so, well -this is simple journalism and I cannot be blamed for what everybody else also does just because I am a scientist. If a collaboration cannot keep silent and leaks information, it's their responsibility, not mine. Of course if it's my own collaboration the matter is different; and I concede that I owe some additional care when discussing the competitors around the ring, i.e. ATLAS.

    Cheers
    T.
    A table of z scores and p values

    Hi Tommaso,
    I think it would be useful to add a table like:
    z=1 sigma, p(by chance)~16% or once every 6 times.
    z=2 sigma, p(by chance)~2.3% or once every 44 times.
    z=3 sigma, p(by chance)~0.13% or once every 741 times. <= What we call "evidence" level.
    z=4 sigma, p(by chance)~0.0032% or once every 31'574 times.
    z=5 sigma, p(by chance)~0.000028% or once every 3'486'914 times. <= What we call "discovery" level.

    Perhaps you have some meaningful numbers to illustrate these rates? Like one hour in 75 years is 1 in ~660'000.

    dorigo
    Thanks Andre, this is indeed useful information for laymen. I will add it to the text.

    Cheers,
    T.
    Oops. Can you please remove the duplicates. I did submit a couple of times because the comment would not appear after being submitted.