Particle physics marches on

By Sean Carroll | March 31, 2006 1:32 pm

Physicists (like us) are, with good reason, eagerly anticipating results from the Large Hadron Collider at CERN, scheduled to turn on next year. The LHC will collide protons at much higher energies than ever before, giving us direct access to a regime that has been hidden from us up to now. But until then, a whole host of smaller experiments are interrogating particle physics from a variety of different angles, using clever techniques to get indirect information about new physics. Just a quick rundown of some recent results:

  1. Yesterday the MINOS experiment at Fermilab (Main Injector Neutrino Oscillation Search) released their first results. (More from Andrew Jaffe.) This is one of those fun experiments that shoots neutrinos from a particle accelerator onto an underground journey, to be detected in a facility hundreds of miles away — in this case, the Soudan mine in Minnesota. They confirm the existence of neutrino oscillations, with a difference in mass between the two neutrino states of Δm2 = 0.0031 eV2. The neutrinos left Fermilab as muon neutrinos, and oscillated into either electron or tau neutrinos, or something more exotic. MINOS can be thought of as a follow-up to the similar K2K experiment in Japan, with a longer baseline and more neutrinos.
  2. The previous week, the D0 experiment at Fermilab’s Tevatron (the main proton-antiproton collider) released new results on the oscillations of a different kind of particle, the Bs meson (a composite of a strange quark and a bottom antiquark), as reported in this paper. For better or for worse, the results are splendidly consistent with the predictions of the minimal Standard Model. These B-mixing experiments are very sensitive to higher-order contributions from new physics at high energies, such as supersymmetry. D0 is telling us something we have heard elsewhere: that susy could already have easily been detected if it is there at the electroweak scale, but it hasn’t been seen yet. Either it’s cleverly hiding, or there is no susy at the weak scale — which would come as a surprise (a disappointing one) to many people.
  3. Finally, a little-noticed experiment in Italy has been looking for axion-like particles — and claims to have seen evidence for them! (See also Doug Natelson and Chad Orzel.) The usual (although still hypothetical) axion is a light spin-0 particle that helps explain why CP violation is not observed in the strong interactions. (There is a free parameter governing strong CP violation, that should be of order unity, and is experimentally constrained to be less than 10-10.) The axion is a “pseudoscalar” (changes sign under parity), and couples to electromagnetism in a particular way, so that photons can convert into axions in a strong magnetic field. (Another mixing experiment!) The axion relevant to the strong CP problem has certain definite properties, but other similar spin-0 particles may exist that couple to photons in similar ways, and these are generically referred to as axion-like. Zavattini et al. have fired a laser through a magnetic field and noticed that the polarization has rotated, which can be explained by an axion-like particle with a mass around 10-3eV, and a coupling of around (4×105eV)-1. My expert friends tell me that the experimentalists are very good, and the result deserves to be taken seriously. Trouble is, the particle you need to invoke is in strong conflict with bounds from astrophysics — these particles can be produced in stars, leading to various sorts of unusual behavior that aren’t observed. Now maybe the astrophysical bound can somehow be avoided; in fact, I’m sure some clever theorists are working on it already. But it would also be nice to get independent confirmation of the experimental effect.
  • Plato

    Wolfgang Pauli lecturing in 1929. The next year, when he devised the notion of the neutrino, he allegedly said to a friend, “I have done something very bad today by proposing a particle that cannot be detected; it is something no theorist should ever do.”

    The history followed is of course a interesting one.

    John Bahcall gave reality to those things Pauli might have rejected or thought ill advised? Yet, the pursuate brought an extremely diverse experimental culture around it?

    So we look at the particle nature a little differently, and “all hell” breaks loose? :)

  • Torbjorn Larsson

    Are neutrino masses consistent with the “minimal Standard Model”?

  • Mark

    Hi Torbjorn. Consistent – yes; technically included in – no.

  • lambda T

    There is also recent experimental evidence for the existence of a pentaquark particle, using theoretical work based upon the chiral soliton model. This experimental evidence has been found in several different experiments and Thomas Jefferson National Accelerator Facility (Newport News, VA) is currently studying this. This paper examines pentaquark baryons in correlation with the dual string theory of QCD. And, for all we know, the pentaquark may, in fact, turn out to be experimental evidence supporting string theory.

  • David M

    As for the axions, the results of an experiment at CERN that is actively searching for solar axions, called the CAST Experiment, are also in direct contradiction to the Italian PVLAS results…making the axion interpretation difficult even experimentally (not only using astrophysical constraints). As a member of the CAST collaboration, I can say that some of us are quite interested in these recent findings, and several proposals for follow up experiments are being made.

    In addition, the LBNL axion experiment is under is under way and also has a good chance at seeing something, although in a completely different regime than CAST (m_a sim meV)

    This is the type of physics that makes life interesting!

  • David M

    err….m_a sim ueV….sorry 😉 (how does one make mu show up in html anyway?)

  • David M

    I am not sure if you were referring to CAST, etc….but what exactly do you mean “how much”?

    The conversion of an axion into a photon does certainly depend on the coherence length of the oscillation (which depends on the energy and mass of the incoming axion). For example, on the CAST limits you see that around a mass of 10^-2 eV your limits start to degrade. This is due to a loss of coherence in the axion-photon oscillation for higher masses in the 9.5 m, 10T field we use.

    Not sure if that answers the question….

  • David M

    well, for CAST we can’t really control how many axions we get for several reasons: they might not even exist, by definition there would be very few (weakl coupled in the production processes), we do not control the production in any case (they come from the sun).

    the case is somewhat different for the italian experiment, where the axions are production from laser photons, which they can control. however, no one can control the axion-photon coupling strength, which is what’s gonna determine how many axions acutally come out ;)…but apparently they see something pretty big!

  • Scott O.

    Hi Lambda (#4). There seems to be as many experiments that have failed to find a pentaquark as have seen it. I would not be at all surprised if in the end the whole thing goes away.

    Are neutrino masses consistent with the minimal Standard Model (#2)? This depends exactly on what you mean. Strictly speaking the ansewr is no, since the minimal SM contains no right-handed neutrino states with which to form mass terms. Now, you can introduce the right-handed fields by hand and use them to give neutrinos mass. The problem is that the right-handed fields themselves are sterile—that is, they have not even weak interactions, making them unlike the left-handed neutrinos we are familiar with. That’s a very peculiar kind of particle. Personally I think that if neutrinos require you to introduce new fields that interact differently from all other SM fields, then there’s no way I would call that consistent with a “minimal” SM. Neutrino oscillations forces one to add new fields to the SM particles, new fundamental parameters, and new phenomena—it’s very clear physics beyond the SM.

    There is another way to add masses to the SM called a “Majorana mass”. This is absolutely not consistent with the SM, and would be even more exciting than the fact that neutrinos have mass themselves.

  • Alejandro Rivero

    Well, not exactly sterile: they couple with the Higgs field :-)

  • Dumb Biologist

    I’ve read these sterile right-handed neutrinos are a dark matter candidate. Meanwhile, astronomical observations appear to indicate DM particles fly around at pretty languid speeds compared to the motion of your typical neutrino (somewhere around 10^3 m/s…lukewarm, maybe). Does this imply the sterile neutrinos, if they are a viable DM candidate, must be pretty hefty? Does this cause any problems with theory?

  • bittergradstudent

    DB #11:

    yes, and it would also requre a weird sort of behaviour for these right-handed neutrinos–it seems kind of grotesque to have a left-handed neutrino with a mass of a few eV, but have it’s right handed partner have a mass much, much larger than that, especially considering for other particles, the right and left handed versions have the same mass. But aesthetics aside, there really isn’t much reason to rule this out, though most of what I’ve read hints that the lightest supersymmetric particle is a better DM candidate than the sterile neutrino, as the non-observance of supersymmetry hints that these particles SHOULD be quite heavy, already.

  • Dumb Biologist

    Well, jeez, there’s purported axionoid particles showing up, hypothetical sterile neutrinos, and results indicating weak-scale SUSY ain’t there. My understanding is weak-scale SUSY is somehow attractive b/c it takes care of aesthetically displeasing fine-tuning of the Higgs mass. At what point does the LSP become a less-favorable candidate? Seems like, evidentially, things look kinda wide open at this point, though I be a humble dumb biologist :).

    Anyhoo, thanks very much for your answer, bgs!

  • Count Iblis

    According to a recent article the PVLAS results could be due to QED vacuum polarization or possibly axions without contradicting CAST.

  • Torbjorn Larsson

    Hi Mark, Scott and thanks for your answers! Whether it’s not yet technically included or physics beyond the SM it’s exciting that there are some observations to explain.

  • asshole

    To correct some of what’s said above about neutrino masses and the minimal standard model:

    Fermion `mass,’ as usually spoken of without further qualification in this context, is what’s more
    precisely called DIRAC mass, and is by construction or definition a coupling, mixing, or
    oscillation amplitude between left and right chiral eigenstates. The various observations of
    neutrino oscillations prove at least one such term involving at least the electron and muon
    neutrinos is non-zero. The right handed neutrinos involved, assuming the standard model, also
    have related Higgs sector interactions and are obviously not sterile, but are not excluded as a
    dark matter candidate by the standard primordial nucleosynthesis argument although they may not be
    particularly attractive to explain galactic evolution.

    It was arguably always gratutious to _define_ the minimal standard model to have no right handed
    neutrinos, and hence necessarily massless neutrinos, as some people did. It’s probably most
    physically meaningful to distinguish the minimal gauge group (viz. SU(2)_L X U(1)_Y X SU(3)_c), and
    the assignment of the well-known fermions to its smallest non-trivial representations; the right
    handed neutrino is in the trivial, identity or singlet representation. There’s also a smallest or
    minimal Higgs boson representation and sector, but no one will find it radical if it turns out to
    be more complicated (axions are a example).

    A MAJORANA mass is a coupling between a chiral fermion and its antiparticle of the same chirality,
    and thus cannot occur for fermions in the non-trivial representation, but once right handed
    neutrinos in the trivial representation are admitted cannot be a priori excluded. The observed
    neutrino oscillations cannot be explained by right handed neutrino Majorana masses, and so far
    neither prove nor disprove they vanish. Unless one is talking about a Majorana mass, it makes no
    sense to speak of left and right fermion masses.

  • Scott O.

    Thanks asshole (#16—nice name!) To add a couple of points, first I note that neutrino oscillations imply that at least TWO massive neutrino exists, since solar and atmospheric neutrino oscillations involve different Delta m^2 values. When we say that a right-handed neutrino is “sterile”, normally we exclude its Higgs interaction—I agree entirely that a right-handed Dirac neutrino couples to a Higgs, but that’s not what is usually meant by sterile. By ‘sterile’ we mean that it doesn’t couple to any of the vector bosons. One should also be very careful to distinguish between the ‘sterile’ right-handed states corresponding to the ordinary Dirac neutrinos, and the possible existence of a whole fourth flavour of neutrino that doesn’t couple to W or Z. The former is ruled out as the bulk of the dark matter by BBN (since these are just the right-handed components of the regular active neutrinos), while a fourth sterile state isn’t ruled out as best I know, and is at least hinted at by the LSND results.

    I’m not sure I agree that it was gratuitous not to include Dirac neutrino masses in the Standard Model to begin with. Given that no one had ever seen a right-handed neutrino, Ockham’s razor would say that it was simpler to assume that they don’t exist than to posit the existence of a right-handed state that was in principle undetectable, since it doesn’t couple of any vector bosons. This turns out to have been wrong, but I can’t say that it was such a ridiculous way to proceed at the time.

  • Plato

    One had to break free of “the rigidity” with which language might have been seen, while “WMAP polarization maps” might been seen in analogistical ways?

    Maybe, it’s taken further, in Dirac’s geometrical thinking?

  • Tony Smith

    Sean said (in section 2. of this blog entry) “… the D0 experiment … released new results on the oscillations of … the Bs meson … as reported in …[ hep-ex/0603029 ]… the results are splendidly consistent with the predictions of the minimal Standard Model. … susy could already have easily been detected if it is there at the electroweak scale, but it hasn’t been seen yet. Either it’s cleverly hiding, or there is no susy at the weak scale – which would come as a surprise (a disappointing one) to many people. …”.

    Various comments in another blog entry by Sean (about grad school) said:
    (Belizean) “… prospective grad students should ask themselves this question, “Do I have a burning desire to be a physics professor, or merely a deep interest in physics?” If the latter, I would recommend skipping graduate school, investing your time and energy in becoming financially independent, and studying physics as a leisure pursuit. …”.
    (A Serious Question) “… It seems much more likely to me for a bright person who goes down this path to come up with truly creative and groundbreaking physics, then it is for somebody who enters the modern “publish-or-publish” world of modern grad students and postdocs. …”.
    (Anonymous Beaver) “… You shouldn’t do it because you’ll become a crackpot and nobody will pay any attention to you. … In the unlikely event you do discover some great insight, you’ll have incredible difficulty communicating it to people besides your long-suffering colleagues. …”.
    (Ambitwistor) “… forgoing an advanced degree and going into academic seclusion without regular contact with one’s peers is a route to crankdom …”.
    (Ponderer of Things) “… for incoming graduate students NOT to pick their school based on prestige is a recipee for post-PhD career suicide …”.

    Even if Chad Orzel finds it offensive, the very existence of the buzz-phrase

    “physics is subdivided into “string theory” and “stamp collecting””

    indicates that theoretical physics in “prestige” schools is dominated by conventional “string theory” which is founded on the existence of “susy” supersymmtry. If that is the case,
    is it really very likely that anyone following the grad school career advice will be able to construct a physics model consistent with experiment, i.e.,
    a physics model either without susy or with it very “cleverly hiding” in some unconventional way ?

    I do agree that the above-stated grad school career advice is accurate advice for someone wanting a tenured professorship at a “good” school.
    it seems to me that, in times in which the dominant fashion in theory may not consistent with experimental results, the way to advance a “career” may not be the way to advance the “onward march of particle physics”.

    Tony Smith

  • Alejandro Rivero

    Other possibility is that susy is cleverly hidden, and it is not at the electroweak but at the strong/chiral scales. I told about this at hep-ph/0512065; you need to build diquarks antsymmetrising SU(3) color and symmetrizing “SU(5)” flavour, where the quotes in “SU(5)” tell us that we need consider isospin and hypercharge so that 4/3 diquarks are excluded and that it is not SU(6), ie it does not contain the top quark, whose role is different. With these rules you get the scalars you need to cancel hierarchy; you only need a clever coupling with the higss sector so that they get the same mass that single quarks.

    As for how the top quark is different, in addition to the usual argument that it is so massive that it disintegrates before pairing into a meson, let me note that the last measure of its yukawa coupling is still one sigma compatible with unity: From we have 0.991 pm 0.013

  • Dumb Biologist

    Thanks for a neat discussion!

  • Count Iblis

    So, the Italians lead the world with their two claimed observations of physics beyond the standard model so far: DAMA and PVLAS :)

  • Sean

    I think that dark matter, dark energy, and the baryon asymmetry are rather stronger pieces of evidence for physics beyond the standard model. (I will refrain from being a smartass and mentioning “gravity.”)

  • Count Iblis

    Yes :) ”direct detection” is perhaps a better description than ”observation”.

  • Plato

    The Devil is in the details, while the true cast, is here?

    :)More on name

  • Plato

    I should say, when you click on name that you go to the bottom of the article and see the images in relation to the statement above. It should make sense then.

    More details, the less humour? I hope not:)

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  • Collin

    I just thought I’d let everyone know that at today’s Wine & Cheese seminar at Fermilab, CDF announced a measurement of Delta(m_s) in the B_s system of 17.33 with an error of about 2.5%.

  • Sean

    Collin, thanks for the update — I had heard that the CDF results were imminent. So what does that mean in comparison with the D0 results, and constraints on susy?

  • JoAnne

    Ah, Collin – the importance of competition between 2 experiments! We have been waiting – for years – for this result which CDF in particular had promised very early on. Now that D0 announced it first, it seems that CDF suddenly is ready to go public too. I recall attending a talk, just last Fall, where CDF stated it would be awhile before they could measure B_s mixing, while D0 kept silent on the matter….

  • Collin

    I’m not exactly qualified to say anything about constraints on SUSY, though I’d be thrilled if someone (JoAnne?) would chime in with that. I’d also like to hear what this does for the Unitarity Triangle. Along with this measurement, CDF reported a measurement of |V_td|/|V_ts| = 0.208 +/- 0.008, a pretty significant increase in the precision of this quantity. Specifically, does this measurement improve the constraints on rho?

    As for a comparison with the D0 result, it’s certainly consistant. But, the CDF result is much nicer, if a bit later. In their amplitude scan (basically, you scan a range of possible Delta(m_s) values, fit the oscillation data and measure a mixing amplitude; this should be 1 at the true value of Delta(m_s) and 0 otherwise), D0 found a 5 ps^-1 window where the data were inconsistant with 0 at a 90% CL. In comparison, CDF actually measured Delta(m_s) to better than +/- .5 ps^-1. If you wanted to put it in a 90% CL range to compare directly with D0, it’d be less than 1 ps^-1 and contained entirely within D0’s range, though certainly on the low side.

  • JoAnne

    The Tevatron results on B_s mixing are smack dab in the middle of the Standard Model expectations, factoring in the remainer of the B-physics results. Quite disappointing, actually – us theorists were hoping for a much larger number!.

    Nonetheless, there is significant play in the overall prediction due to uncertainties in theoretical quantities such as the B meson decay constant and bag factor. Due to this, I don’t see where this result will place any significant constraints on supersymmetry. Sorry, wish it were different myself. However, there are a few wilder theories that predicted maximal B_S mixing that will now be excluded.

  • Thomas Larsson

    Does anyone know how stringent constraints on SUSY are, if one takes all types of experiments into account? Wouldn’t one expect to see several things, e.g.

    1. Sparticles
    2. Light Higgs
    3. Muon g-2
    4. Permanent electric dipole moment
    5. Proton decay
    6. WIMPS
    7. B_s mixing
    and perhaps others. AFAIU, none of these experiments can rule out SUSY by themselves, but taken together they should provide much stronger constraints than each of them taken separately, no?

  • JoAnne

    Thomas, you make a good case and indeed many people have put all the constraints together and studied the supersymmetry parameter space in detail. The result is that supersymmetry is very elusive, particularly if you consider the general theory and not the very constrained case of minimal supergravity mediated SUSY. In the former (general SUSY), then no, there are no real constraints except for the kinematic limits from sparticle searches at colliders. In constrained models, one sign of the Higgsino mixing parameter (mu) is disfavored if the sparticles are relatively light (

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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