Sunday, 16 November 2008

Hitchhiker's Guide to Ghosts and Spooks in Particle Physics

On Halloween this year the CDF collaboration at Fermilab's Tevatron announced the presence of ghosts in their detector. And not just one meager Poltergeist rattling his chain, but a whole hundred-thousand army. As for today, the ghosts could not be exorcised by systematical effects. While waiting for theorists to incorporate the ghosts into their favorite models of new physics it is good to know that the CDF anomaly is by no means the only puzzling experimental result in our field. There are other ghosts at large: I guess most of them are due to unknown systematical errors, but some may well be due to new physics. Below I pick up a few anomalous results in subjective order of relevance. The list is not exhaustive - you are welcome to complain about any missing item.

So, off we go. In this post I restrict to collider experiments, leaving astrophysics for the subsequent post.

Muon Anomalous Magnetic Moment

This experimental result is very often presented as a hint to physics beyond the Standard Model. For less oriented: there is nothing anomalous in the anomalous magnetic moment itself - it is a well-understood quantum effect that is attributed to virtual particles. But in the muon case, theoretical predictions slightly disagree with experiment. The E821 experiment in Brookhaven measured $a_\mu = (11 659 208 \pm 6)\cdot 10^{-10}$. The Standard Model accounts for all but $28\cdot 10^{-10}$ of the above, which represents a 3.4 sigma discrepancy.

The discrepancy can be readily explained by new physics, for example by low-energy supersymmetry or by new light gauge bosons mixing with the photon. But there is one tiny little subtlety. The Standard Model prediction depends on low-energy QCD contributions to the photon propagator that cannot be calculated from first principles. Instead, one has to use some experimental input that can be related to the photon propagator using black magic and dispersion relations. Now, the discrepancy between theory and experiment depends on whether one use the low-energy e+e- annihilation or the tau decays as the experimental input. The quoted 3.4 sigma arises when the electron data are used, whereas the discrepancy practically disappears when the tau data are used. It means that some experimental data are wrong, or some theoretical methods employed are wrong, or both.

In near future, a certain measurement may help to resolve the puzzle. The troublesome QCD contribution can be extracted from a process studied in BaBar, in which a photon decays into two pions (+ initial state radiation). There are rumors that the preliminary BaBar results point to a larger QCD contribution (consistent with the tau data). This would eradicate the long-standing discrepancy of the muon anomalous magnetic moment. But, at the same time, it would imply that there is a flaw in the e+e- annihilation data, which would affect other measurements too. Most notably, the electron data are used as an input in determining the hadronic contribution to the electromagnetic coupling, which is one of the key inputs in fitting the Standard Model parameters from electroweak observables. As pointed out in this paper, if the low-energy QCD contribution where larger than implied by the electron data, the central value of the fitted Higgs boson mass would decrease. Currently, the electroweak fit determines the Higgs boson mass as $77^{+28}{}_{-22}$, which is already uncomortable with the 114 GeV direct search limit. Larger QCD contributions consisent with the tau data would increase this tension. Interesting times ahead.

Forward-Backward Asymmetry

CERN's LEP experiment has been desperately successful: it beautifully confirmed all theoretical predictions of the Standard Model. The mote in the eye is called $A_{fb}^b$: the forward-backward asymmetry in decays of the Z-boson into the b-quarks. This observable measures the asymmetry in the Z boson interactions with left-handed b-quarks and right-handed ones. The results from LEP and SLD led to a determination of $A_{FB}^b$ that deviates 3 sigma from the Standard Model prediction. On the other hand, the total decay width of the Z-boson into the b-quarks (summarized in the so-called Rb) seems to be in a good agreement with theoretical predictions.

One possible interpretation of these two facts is that the coupling of the Z-boson to the right-handed b-quarks deviates from the Standard Model, while the left-handed coupling (who dominates the measurement of Rb) agrees with the Standard Model. At first sight this smells like tasty new physics - the Zbb coupling is modified in many extensions of the Standard Model. In practice, it is not straightforward (though not impossible) to find a well-motivated model that fits the data. For example, typical Higgsless or Randall-Sundrum models predict large corrections to the left-handed b-quark couplings, and smaller corrections to the right-handed b-quark couplings, contrary to what is suggested by the electroweak observables.

Maybe this discrepancy is just a fluke, or maybe this particular measurement suffers from some systematic error that was not taken into account by experimentalists. But the funny thing is that this measurement is usually included in the fit of the Standard Model parameters to the electroweak observables because...it saves the Standard Model. If $A_{FB}^b$ was removed from the electroweak fit, the central value of the Higgs boson would go down, leading to a large tension with the 114 GeV direct search limit.

Bs Meson Mixing Phase

The results from BaBar and Belle led to one Nobel prize and zero surprises. This was disappointing, because flavor-changing processes studied in these B-factories are, in principle, very sensitive to new physics. New physics in sd transitions (kaon mixing) and bd transitions is now tightly constrained. On the other hand, bs transitions are less constrained, basically because the B-factories were not producing Bs mesons. This gap is being filled by the Tevatron who has enough energy to produce Bs mesons and study its decays to J/psi. In particular, the mass difference of the two Bs eigenstates was measured and a constraint on the phase of the mixing could be obtained. The latter measurement showed some deviation from the Standard Model prediction, but by itself it was not statistically significant.

Later in the day, the UTfit collaboration combined the Bs meson data with all other flavor data. Their claim is the Bs mixing phase deviates from the Standard Model prediction at the 3 sigma level. This could be a manifestation of new physics, though it is not straightforward to find a well-motivated model where the new physics shows up in bs transitions, but not in bd or sd transitions.

NuTeV Anomaly

Nu-TeV was an experiment at Fermilab whose goal was a precise determination of the ratio of neutral current to charged current reactions in neutrino-nucleon scattering. Within the Standard Model, this ratio depends on the Weinberg angle $\sin \theta$. It turned out that the magnitude of the Weinberg angle extracted from the NuTeV measurement deviates at the 3 sigma level from other measurements.

It is difficult to interpret this anomaly in terms of any new physics scenario. A mundane explanation, e.g. incomplete understanding of the structure of the nucleons, seems much more likely. The dominant approach is to ignore the Nu-TeV measurement.

HyperCP Anomaly

This measurement was sometimes mentioned in the context of the CDF anomaly, because the scales involved are somewhat similar. Fermilab's HyperCP experiment found evidence for decays of the hyperon (a kind of proton with one s quark) into one proton and two muons. This by itself is not inconsistent with the Standard Model. However, the signal was due to three events where the invariant mass of the muon pair was very close to 214 MeV in each case, and this clustering appears very puzzling.

The HyperCP collaboration proposed that this clustering is due the fact that the hyperon first decays into a proton and some new particle with the mass 214 MeV, and the latter particle subsequently decays into a muon pair. It is very hard (though, again, not impossible) to fit this new particle into a bigger picture. Besides, who would ever care for 3 events?

GSI Anomaly

For dessert, something completely crazy. The accelerator GSI Darmstadt can produce beams of highly ionized heavy atoms. These ions can be stored for a long time and decays of individual ions can be observed. A really weird thing was noticed in a study of hydrogen-like ions of praseodymium 140 and promethium 142. The time-dependent decay probability, on top of the usual exponential time-dependence, shows an oscillation with a 7s period.

So far the oscillation remains unexplained. There were attempts to connect it to neutrino oscillations, but this has failed. Another crazy possibility is that the ions in question have internal excitations with a small $10^{-15}$ eV mass splitting.

13 comments:

Anonymous said...

Very good post. Thanks.

Anonymous said...

good one Jester, nice to see all the ghosts happy together. hadn't heard about the GSI one, it looks strange indeed...

Kea said...

I heard a talk on the GSI at Neutrino08 and the authors' conclusion tended towards it being an as yet not understood systematic effect, but I suppose that is to be expected.

Matti Pitkanen said...

Dear Jester,

thank you for a nice summary.

A couple of anomalies might be added: the anomalous e+e- pairs in heavy ion collisions, the ortopositronium decay rate anomaly (which both can be understood as evidence for electro-pions identified as bounds states of color excited leptons). It would be interesting to know also the status of Karmen anomaly.

It would be nice to see how muon g-2 for electron and muon are affected by two-photon coupling to leptopions.

I give some references in the hope that someone might get interested.

Electro-pion anomaly

S. Barshay (1992) , Mod. Phys. Lett. A, Vol 7, No
20, p. 1843.

J.Schweppe et al.(1983), Phys. Rev. Lett. 51, 2261.

M. Clemente et al. (1984), Phys. Rev. Lett. 137B,
41.


A. Chodos (1987), Comments Nucl. Part. Phys., Vol 17, No 4, pp. 211, 223.


L. Kraus and M. Zeller (1986), Phys. Rev. D 34,
3385.

Orto-positronium decay rate anomaly

C. I. Westbrook ,D. W Kidley, R. S. Gidley, R. S
Conti and A. Rich (1987), Phys. Rev. Lett. 58 ,
1328.


Karmen anomaly

KARMEN Collaboration, B. Armbruster et al (1995) , Phys. Lett. B 348, 19.

V. Barger, R. J. N. Phillips, S. Sarkar (1995),
Phys. Lett. B 352,365-371.

Matti Pitkanen said...

Dear Jester,

as I told earlier, the leptonic color predicted by TGD promises a solution to large number of anomalies, also CDF anomaly. The predicted lifetime for charge tau-pion is same as the lifetime of the possibly existing new particle. The neutral tau-pions and their p-adically scaled up variants with masses coming as powers of two would correspond to the three states proposed by CDF collaboration: mass predictions are consistent with the proposal of CDF. The decays of these neutral pions to 3 pions almost at rest explain the jet like structure.

The remaining challenge was to estimate the production cross section. A brief article summarizing the details of the calculation of the tau-pion production cross section can be found from my home page. Here is the abstract.

The article summarizes the quantum model for tau-pion production. Various alternatives generalizing the earlier model for electro-pion production are discussed and a general formula for differential cross section is deduced. Three alternatives inspired by eikonal approximation generalizing the earlier model inspired by Born approximation to a perturbation series in the Coulombic interaction potential of the colliding charges. The requirement of manifest relativistic invariance for the formula of differential cross section leaves only two options, call them I and II. The production cross section for tau-pion is estimated and found to be consistent with the reported cross section of about 100 nb for option I under natural assumptions about the physical cutoff parameters (maximal energy of tau-pion center of mass system and the estimate for the maximal value of impact parameter in the collision which however turns out to be unimportant unless its value is very large). For option II the production cross section is by several orders of magnitude too small. Since the model involves only fundamental coupling constants, the result can be regarded as a further success of the tau-pion model of CDF anomaly. Analytic expressions for the production amplitude are deduced in the Appendix as a Fourier transform for the inner product of the non-orthogonal magnetic and electric fields of the colliding charges in various kinematical situations. This allows to reduce numerical integrations to an integral over the phase space of lepto-pion and gives a tight analytic control over the numerics.

See also the postings in my blog.

Matti Pitkanen said...

A little addition to the previous posting. The plot for differential production cross section is here.

a quantum diaries survivor said...

Great post Jester!
T.

Matti Pitkanen said...

Dear Jester,

GSI anomaly brings in mind the nuclear decay rate anomalies which I discussed some time ago in the posting Tritium beta decay anomaly and variations in the rates of radioactive processes in my blog. These variations in decay rates are in the scale of year and decay rate variation correlates with the distance from Sun. Also solar flares seem to induce decay rate variations.


The TGD based explanation relies on nuclear string model in which nuclei are connected by color flux tubes having exotic variant quark and antiquark at their ends (TGD predicts fractal hierarchy of QCD like physics). These flux tubes can be also charged: the possible charges +,-1,0. This means a rich spectrum of exotic states and a lot of new low energy nuclear physics. The energy scale corresponds to Coulomb interaction energy alpha m, where m is mass scale of exotic quark. This means energy scale of 10 keV for MeV mass scale. The well-known poorly understood X-ray bursts from Sun during solar flares in the wavelength range 1-8 A correspond to energies in the range 1.6-12.4 keV -3 octaves in good approximation- might relate to this new nuclear physics and in turn might excite nuclei from the ground state to these excited states and the small mixture of exotic nuclei with slightly different nuclear decay rates could cause the effective variation of the decay rate.

The question is whether there could be a natural varying flux of X rays in time scale of 7 seconds causing the rate fluctuation by the same mechanism also in GSI experiment. In any case, the prediction is what might be called X ray nuclear physics and artificial X ray irradiation of nuclei would be an easy manner to kill or prove the hypothesis.

See my blog and the chapter Nuclear String Hypothesis of "p-Adic length scale Hypothesis and Dark Matter Hierarchy".

Anonymous said...

Dear Matti Pitkanen,
Please stop filling this nice blog with your spam! It gets annoying to have to scroll down through your long posts. Surely if someone is interested in your theory they can just go to your blog and discuss it there.

Thomas D said...

and now particles that go bump in the ATIC ...

Matti Pitkanen said...

Dear Anonymous,

I have been always wondering where people like you having nothing constructive to say come from, and why they are not automatically moderated out of serious discussion.

Anyone interested in 7 second time scale of GSI anomaly and able to read and understand simple ten line long argument can come and see the discussion in my blog.

Jester said...

I suspect that Matti is God's wrath sent down upon me for all my sins. I guess I have to take it stoically, like the Plagues of Egypt, or the Bubonic Pest.

Matti Pitkanen said...

Dear Jester,

I suggest that you come to my blog and demonstrate that the argument deriving the oscillation period 7 seconds for the decay rate variation in GSI experiment contains a fatal mistake. We could even make a bet: 1000 euros for you if you find the mistake and vice versa. In this manner you could also prove that you are not only misusing your academic position.


The structure of argument is very simple.

a) Nuclear string model predicts existence of two states corresponding to charged and neutral color bonds.


b) The interaction potential corresponds to W Coulombic potential predicted by induced gauge field concept (classical gauge fields as projections of CP_2 spinor curvature to space-time surface: basic difference from standard model). This induces oscillating charge entanglement between nucleon and quark at the end of color bond. Nucleon and quark oscillate between their two charge states. Total charge is of course conserved.

c) Anyone worked with 2-state systems knows how to model the situation. It is simple thing to calculate the value of interaction potential energy giving oscillation frequency omega= V/hbar. If the distance of the end of quark from nucleon is fraction .61 of protons Compton radius you get 7 seconds.

Please come to blog and demonstrate me were the fatal error is and you own 1000 euros.

Matti