HERA's Intriguing Top Candidates
    By Tommaso Dorigo | April 28th 2009 04:09 PM | 30 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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    These are hard times for evil guys like me, who are always willing to speculate wildly on particle physics results -only to secretly chuckle at the ripples their extrapolations make, knowing for a fact that the Standard Model is as solid as it has ever been.

    Suggestive new results which offer themselves as the first hint of a breakdown of the Standard Model are indeed quite rare nowadays. In a famous post which originated a $1000 bet (taken up in part by Prof. Gordon Watts and in part by Prof. Jacques Distler), no less than 32 months ago I was writing in my old blog:

    What do we have to show for these 30 years+ of investigations, smart tests, cunning double ratios, global fits, and the like ? Let me try and summarize - despite I am sure I am going to do a very incomplete job:

    - Neutrinos mix and have mass. Ok, that is something. But it does not change by much the physics of electroweak interactions.

    - Rb is off, but by less than three sigma.

    - the gluon distribution function is larger than it used to be at high x.

    - low-energy QCD has made big steps forward, but it also did not change the picture much.

    - Radiative corrections imply the existence of the Higgs boson, or something else yielding the proper effect on measurable electroweak parameters. Ok, but the Higgs is part of the SM.

    What else ? In Run I, CDF did have a few inconsistencies to keep hope alive, but they died one by one under the blows of merciless re-analysis or more data. HERA had these leptoquark signals, but they died away. Aleph had fancy signals at some point, but it also bubbled down…. One could make this list much longer, but things would not change much.

    So, you can well see that during the additional 32 months that passed since September 2006 nothing much has happened in particle physics. That is, if we exclude no-news setbacks such as the continuous delay of the LHC startup. As far as hints of new physics beyond the Standard Model, I can only think at two things:

    • the suggestion of a departure from SM expectations of a parameter -the "strong-CP phase"- extracted from Bs mesons at the Tevatron; this is an anomaly staying at about 2-sigma away from the SM;

    • the intriguing Anomalous Muon signal unearthed by CDF in a controversial paper: this effect has so far not been confirmed nor (it is my opinion) disproven; it is however quite unlikely (again, my personal opinion) to constitute new physics beyond the Standard Model.

    A similar, yet more complete and deeper, list can be found e.g. at the Resonaances blog. In any case, you can well understand that under such shortages, speculative sons-of-a-bitch like me have to cling to whatever sticks its butt above a statistical significance of two standard deviations, or even less. So I cheer up at the new preprint published a few days ago by the H1 collaboration at the HERA facility in Hamburg: they searched for top quark events where none were expected, and found some!

    Now, just a moment before I start receiving ugly phone calls, let me qualify that: they do not observe any signal, and thus set upper limits on single top-quark production in electron-proton collisions. However, there is a lot of water between observing a signal and ruling it out. Indeed, what H1 finds is that there is a 2-sigmaish like upper fluctuation which does indeed look like a top signal in their data. A handful of events, to be sure. So let me discuss the result in some detail for you below.

    Producing top quarks at HERA

    HERA is a quite atypical particle accelerator: it combines some of the strenghts of both electron-positron and proton-antiproton colliders -and a few of the weaknesses- in mixed electron-proton collisions. The collision of 920 GeV protons with 30 GeV electrons (or positrons) produces the release of up to 320 GeV of energy, which can materialize into new states of matter.

    320 GeV is less than the total mass of a pair of top quarks, but it definitely is more than the mass of a single one (the most recent Tevatron measurements set that fundamental parameter of the Standard Model at M=173.1+-1.3 GeV). So it is energetically possible that a HERA collision yields a top quark. But are there reactions capable of yielding such events ?

    The question is not a vacuous one. The interaction between an electron and a quark within the proton is mediated by electroweak gauge bosons -W or Z particles, or photons- but the Standard Model dictactes that only a W boson can change the flavor of quarks. And since top quarks are not contained in the proton (well, theoretically a proton may be found to contain pairs of those for vanishingly short instants, but it is a really remote possibility), two distinct possibilities are left.

    The first: an electron emits a W boson and thus turns into an electron-neutrino; the W boson interacts with a down-type quark (a down, a strange, or a bottom quark) found in the proton, and turns it into a top quark. This process is really rare, and utterly unobservable in the present H1 dataset.

    The second: an electron emits a Z or a photon, retaining its identity; the Z boson interacts with a up-type quark (a up, or a charm quark), and magically turns the latter into a top.

    Now the most alert of you will be about to object: haven't you just said that only W bosons can change a quark's flavor ? Yes, I did. In fact, the second possibility would be a anomalous process, one which is not possible in the Standard Model until one considers complicated loop diagrams which have ridiculously small probability to occur. So here is a clean process worth searching for: if HERA finds top quarks, they must be due to a so-called "Flavor-changing neutral current" interaction (FCNC), a very striking case of Physics Beyond the Standard Model (BSM)!

    A Feynman diagram of FCNC top quark production is shown on the left. You may notice that the signature includes an outgoing electron, a W boson decay, and a b-quark-originated hadron jet. Those are the bodies one needs to measure to collect top-quark candidates at HERA. Also, the red blob has a specific function: it hides our ignorance of what might produce a change of the flavor in the quark line.

    There is another thing to note here. The most acute of you might have observed that the W boson in the diagram is positively charged: that is because producing anti-top quarks is much, much harder, because that would imply that the parton interacting with the photon or Z boson is a antiquark. Now, antiquarks do exist in the proton, but it is really difficult to find them with a large fraction of the proton's momentum -which is really called for if the collision is to release an energy of at least 173 GeV.

    The H1 Search

    H1 (see right) is endowed with a particle tracker embedded in a uranium-liquid argon calorimeter; both systems are immersed in a 1.16-Tesla solenoidal magnetic field, which allows the measurement of transverse momentum of charged particles. Electrons are well-measured in the calorimeter, and muons can be detected by streamer tubes and drift chambers.

    The data correspond to an integrated luminosity of 474 inverse picobarns. Luminosity is a measure of collisions since to get the number of collisions producing a given reaction you just have to multiply luminosity by the "cross section" of any given process: cross section is like a probability, but it is measured in units of area: squared centimeters. A square centimeter contains 10^36 picobarns: a billion of billions of billions of billions of them. Oh well: better just say that for a very rare process, one with a cross section of one picobarn, the H1 data should contain about 474 of them.

    H1 selects events containing a leptonic W decay -yielding an identified positron or a positive muon- plus missing energy due to the escaping neutrino; a hadronic jet, resulting from the b-quark hadronization, must also be present. The colliding electron which emitted the photon or Z boson is not always detected. After suitable preselection of these objects, 30 events with a positron are found in the data, with a prediction of 31.5+-4.0 from background sources; 8 more with a positive muon are found with a background expectation of 8.0+-0.9. At this level, it seems top quarks are not needed to explain the data -and in fact, the Standard Model does not predict any from the sought FCNC process.

    Three reconstructed kinematical quantities are now studied to better discriminate the possible single top signal from the concurring background processes -which have quite different characteristics:
    - the transverse momentum of the b-jet candidate;
    - the invariant mass of the top quark, reconstructed from the lepton, the neutrino, and the b-jet;
    - and an angle describing the W decay properties.

    The three observable quantities above are used to construct a multi-variate discriminant based on a Neural Network. The output of the neural network is shown for electron and muon events in the two graphs on the right (electrons on top, muons on the bottom). Data are shown as black points with error bars, while expected background is shown with a black histogram with blue shaded uncertainty; the pink hatched histogram instead shows the shape of the discriminant expected for FCNC top decays.

    Any questions ? As I saw these plots I almost fell off my chair. There is a distinct signal-like excess in both distributions!

    The paper is adamant in explaining that they do see some top-like events, but such signal has an insufficient statistical significance to be interesting:
    The top-like events observed in the data at high values of the discriminant lead to observed limits that are weaker than the expected limits. [...] Extracting a production cross section from the discriminant distributions gives for all channels combined sigma(ep -> etX) = 0.11+-0.07 pb. This value is compatible with zero within two standard deviations.
    Two standard deviations, no more. Rats! But those few outliers in the plots above do look like a nagging signal... If those were the tip of an iceberg just waiting to be digged out, we would be sitting on top of a massive new discovery.

    Unfortunately, I must get back to sobriety, and assure you that they are indeed, in all probability, no more than a unfortunate statistical fluctuation. Sure, unfortunate: because of those events, H1 extracts a limit twice higher than it expected on the rare FCNC process. That result is summarized in the graph shown below.

    The figure is a bit too technical for the level of this post, but in summary what is plotted is the value of two "anomalous couplings", numbers which parametrize the possibility that the Standard Model needs to be extended with interactions connecting a flavor-changing quark line with a photon (on the horizontal axis) or with a Z boson (on the vertical axis). The name of the game is restricting these two parameters to be as close as possible to zero -zero being the prediction for the Standard Model: no FCNC interactions.

    Several experiments have set limits on these couplings: CDF, ZEUS (the experiment competing with H1 at HERA), and even L3 -one of the experiments working at the large electron-positron collider at CERN. The H1 result is an exclusion of the yellow area, which extends down to the blue area if the top quark mass is assumed to be smaller -a lighter top quark would be produced more frequently by HERA, and the constraints are in that case stricter.
    In conclusion, H1 does slightly narrow the allowed range of FCNC coupling parameters for single top production. But the experiment also sees a small, nagging top-like signal! Enough to excite all of us HEP perverts.


    must be a leptoquark ;)

    'Two standard deviations is enough for this HEP pervert'

    I think we found the slogan for our physics t-shirts!

    P.S.  I'll find a way to work 'anomalous couplings' in there too.
    Brilliant, Hank! We should really do it.
    We'll do it, but you have to wear it!   We have shirts for every category except physics done but if we find ideas that are funny and smart, we can have one in your hands next week.
    Evil guy, son-of-a-bitch and HEP pervert. Let`s simply leave at wannabe. Some of you bloggers should get a life.

    Wanna-be's don't make Feynman diagrams.  They're too much work.   Are you Cosmic Variance pranksters at it again??

    I do like the clever substitution of 'armonyous' for anonymous though.   Even funnier because it's written.
    Wannabe what, my dear fellow ? I am a HEP researcher.

    For TGD inspired comment see this.

    Matti Pitkanen

    Rick Ryals
    Hi Tommaso,

    Where are the rest of the theorists, experimentalists, and everybody else that usually follow your blog?  I know that people are really bad about following links, but this is almost as ridiculous as loop diagrams.

    Anyway, I just wanted to take this opportunity to mention that I have made my own Long Bets prediction, which should appear this week if they get the new system up and running as promised.

    I will put a link on my profile when this happens, but I'm going to find and scold some of your former fans right now.

    Hi Rick,

    I think most of the people who followed my blog on wordpress are following it here now. What is ridiculous ? I did not get many theorists commenting my physics posts in the old site either.

    Tell me more about these Long Bets. Should I know about it ?

    Rick Ryals
    Really?  I have been reading the site since you moved, and I've only seen limited participation from a few of the people at wordpress.  Of course, things have been a little slow for you anyway due to the LHC delays, I guess.  And I was only making reference to your assertion that a Z Boson has a ridiculously small probability for turning a charm quark into a top.

    Ha!... you had a lot more theorists commenting on your blog than would ever come out of their anonymous closet!... due to your radical nature, mostly... ;)

    My prediction is a rather safe bet, since it is contingent on the LHC not finding the Higgs or new physics, and you are familiar with my assertion that Einstein was never disproved.

    So now I'm going to put it right in their face.
    Daniel de França MTd2
    Hi Tommaso,

    I must agree with our friend. The quality of the posts in general had an abysmal decrease.  I used to learn from the other people's questions, but now it seems I am reading slashdot.
    Some of it may be people have not updated their bookmarks and some of it may be google.  After a period of time, people looking for quality physics writing will have found him on google but his site here won't have the same authority yet.    Some people also just don't like group sites or resent anyone who moves to one.  Every blogger who has ever moved from their own site to a larger one has had complaints in the beginning but a few months in they had more readers than they ever imagined.  And science outreach and education  is why people write so a larger audience is good.

    People are impatient but there are really no good truly well-known physics blogs out there - Tommaso will have the first.
    I know you are HEP researcher. Wannabe refers to "evil, pervert, son-of-a-bitch". It's simply ridiculous to use that language in this context (and I'm no prude, just have an ear for logically handicapped expressions). Show more respect for the proper use of words. It is important. (Same goes for that condescending "dear fellow")

    It's interesting that he's objecting that you are not perverted, evil, etc. enough to label yourself such.   In other words, you need to step up your son-of-a-bitchness if you want more respect as a HEP pervert.

    I don't think a t-shirt alone can do that.
    Dear anon, or armon,

    you want to know the real me ? I'm more a son-of-a-bitch than you could handle. For instance,  insistence in logical flaws in a post about science pictures a sexually repressed unhappy soul, probably spending way too much time over the keyboard trying to please him (or her)self by anonymously posting sarcastic remarks in the pitiful attempt to sound in the know. It also shows that the wish to know some physics gets unfulfilled, because otherwise there would have been arguing about the many logical flaws of the content above, rather than a nit-picking on the form of a text written by a non-native.

     It is really appropriate for you to ask me to get a life: this is a transfert, and you are telling that to yourself. I do have a life, dear fellow: the post you have just mis-read was written in the spare time between a walk around the lake of Bled with my children and some sound exercise with my wife.

    My very best regards,
    Touchy, touchy, means touchè.

    or in other words,,, you're pretty fly (for a white guy).

    Just dawned on me, are you trying to look tough to imitate the great Bela Lubosi and make more money out of this? Forget it, boy, it comes across too bland.
    (Ah, and I said get a Life, not a Wife!)

    According to the traditional poem, Thursday's child has far to go.

    Limerick For The Long Road Soul

    There once was a dear fellow, Armonyous,
    Who considered the S.M. too laborious.
    He took exception to words
    That others sincerely heard,
    And kept verifying his gibberish as glorious.

    P.S. And if one is going to tell wife jokes, it's best to read the masters first.

    Henny Youngman:

    "My wife is an earth sign. I'm a water sign. Together we make mud."

    "My wife told me the car wasn't running well, there was water in the carburetor. I asked where the car was, and she told me it was in the lake."

    Rodney Dangerfield:

    "My wife and I were happy for twenty years. Then we met."

    "Last night my wife met me at the front door. She was wearing a sexy negligee. The only trouble was, she was coming home."

    Fred, I don't care about my gibberish (I don't have a blog to show off). I simply would like this guy to spare us his smart-ass bits. It would make his blog much more readable and interesting.

    You really do not get it, huh ? Maybe you do deserve an answer after all.

    Among experimental particle physicists, speculating about small fluctuations is highly frowned upon. It is something we leave to theorists, whose job is of course to pick up those effects and build a story around them. For a particle physicist, to attach meaning to a 2-sigma fluke is either a sign of not understanding how statistics works, how experimental measurements work, or of playing the devil's advocate. The latter is not in itself bad, but when one talks to the media about these effects the matter is considered very harmful. Now, having a blog visited by reporters worldwide becomes a responsibility, because those folks will usually not understand that you are joking when you put exclamation marks on no-news 2-sigma effects. They will immediately pick the bit of news up, build a story around it, and proceed to publish a very incorrect piece.

    Such was the fate of a 2-sigma MSSM Higgs signal found by CDF at the start of 2007, for instance. I discussed it in my blog, speculated tongue-in-cheek about it, and got interviewed by New Scientist about the matter as a reward. I tried to be careful and let the guy know that the effect was a fluke, but he proceeded to write about it. The NS piece prompted a piece on the Economist, and the CDF collaboration got pissed in the end, because the outcome was considered damaging to the possibilities of getting funding from the DoE and the other funding agencies in the future. If you cry wolf too much, they are soon going to not trust you anymore.

    So, I do not blame you if you do not get my tongue-in-cheekness: you probably were not around in 2007 when the matter arose -or later, in a similar case, a few months afterwards, when another signal made it to the NYT. Or in other instances. You of course know nothing of the strict policies that CDF and later other experiments have adopted toward collaborators who blog, the constant watch that people like me are exposed to, and the feeling of being "an evil guy" who only cares about his hit counter.

    In a nutshell, attaching too much meaning to flukes is a perversion for an experimental physicist, because it appears to show lack of understanding of statistics, lack of consideration for the underlying politics, and ultimately, masochism. But I not only am a "pervert" - you are really calling for these quotes -, but also a "son of a bitch", because I have lived through all the shots that have been directed at me because of my reckless blogging, and those who fired the bullets had to bite a few more. People have started to understand that what I do is not damaging to the field, but actually is raising interest in non-scientists. We are spending too little effort in getting to the laymen, letting them figure out why it is important to do science even if a new technology is not at arm's reach.

    Now spit out your relic sarcasm and let's move on, I think you got your explanation. If you do not like my style of writing please walk away, you will not be missed.

    I do understand it better now, so thanks for your time and trouble. I do not like your style, it's true, but I appreciate the contents so I'll be back.

    Glad I did not lose my time with the answer,
    Hola VA,

    You've made a wise decision and responded like a gentleman, to boot. There is so much to learn here. What I know about physics wouldn't fill the back of a postcard. Having followed Dorigo's blog since the early days of 2005 when he posted in Quantum Diaries has created a positive residual effect for me in terms of understanding and developing methods and realizing ideas within my totally unrelated occupational fields. Part of reason is the very essence and nature of physics itself. But the greater credit goes to the dedication and effort he puts forth week in and week out for the benefit of others, many of those who are out of his league. As a great added bonus, the words of his colleagues and peers that touch this site are many times the unsung heroes who unknowingly encourage me to reevaluate my thoughts and actions to raise the bar higher. But most of all, I marvel at his idiosyncratic command of the english language (It is not his first language nor his home country) in writing about a subject that very few people have the talent to convey to a single group much less the broad spectrum of readers who frequent here. I hope to hear some good thoughts and questions from you in the future.

    Buenas tardes.

    The "Events" scale is puzzling. What does it mean to have a fraction of 1 event?

    Anyway, this reminds of an infamous book on IQ (The Bell Curve). The authors find a result that based on their IQ models, American blacks are overrepresented in the medical profession. Underlying the calculation, it turns out their sample has an expected value of 0.6 black doctors, and it actually had 1.

    Seems to be here too, the expected value of events for a whole range of D is less than 1, and occasionally 1 event is seen.

    Hi Arun,

    it is not strange at all to expect a fraction of an event in a particle physics search. At the basis, there is the absolutely perfect indeterminacy of quantum processes. If you know that the cross section for producing a top quark at HERA is 1 femtobarn, and you collected half an inverse femtobarn, you expect on average 0.5 events of that kind in your data sample.

    What does an expectation value of 0.5 events mean ? It means an average, of course. It is the average of a Poisson distribution, namely P(N)=[exp(-0.5)0.5^N]/N!. The probability to observe N events has that shape. Note that no value of N has a probability equal to zero, if the expectation value is larger than zero; but it rapidly becomes vanishingly small for large N if the average is small.

    We cannot observe fractions of events, but the average, even if fractional, still has meaning: the meaning is the one outlined above. In the case of observing one black doctor when 0.6 are expected, the probability is pretty large, of the order of 33%. If the question was instead, what is the probability of observing AT LEAST ONE black doctor, the probability is 1-P(0)=45.1%.

    No need of particle physics nor of quantum indeterminacy: just throw some dirty coin.

    Yes, any coin would do -dirty or clean. The probability of heads is 0.5, while the number of heads will always be a positive integer. But this time the distribution is binomial.

    Hi Tommaso,

    Interesting post ! I personally love unconventional speculation (with conventional ones like e.g. SUSY searches we tend to forget we are speculating).

    I am a bit taken aback by this statement : "The Standard Model dictactes that only a W boson can change the flavor of quarks." I didn't know that ! While it is true that flavor-changing is only observed through W bosons, I still can imagine that is because processes involving the Z are suppressed due to its larger mass or a small coupling, or that they may be more difficult to isolate. I fail to see a reason why the Z could not also change the flavor of quarks, as straight-fowardly as the W does - just by considering a coupling, say, Z->d+sbar. Aslo, if such processes are found, is it really a big deal ? It seems to me that quark flavor changing was introduced by hand anyway in the SM ; is there anything that explicitely forbids the Z to behave like the W ?

    Then, I have another question, related to your old post about searches for fourth generations :
    There is mention of an early Tevatron search looking at the decay b'->b+Z. It seems to me that such a decay implicitely supposes flavor-changing neutral currents ! If FCNC is forbidden in the SM, it makes two hypotheses beyond the SM (fourth gen + FCNC), or perhaps theories of fourth generations usually also predict (or allow) FCNC to occur ?