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:
- 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.
- 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.
- 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.
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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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%.
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?
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….
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.
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.
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?
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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