Caveat: I am not an experimentalist and I do not pretend to properly understand experimental nuances… but I’m doing my best to try to keep up with what I think are interesting results in particle physics. This post is primarily based on notes from the talk `Updated Oscillation Results from MiniBooNE.’

MiniBooNE photomultiplier tubes, image from image bank.

Oh no! Alien robot drones coming to enslave us! No, just kidding. They're just MiniBooNE photomultiplier tubes. Image from image bank.

The MiniBooNE experiment’s initial goal when it started taking data in 2002 was to test the hypothesis of neutrino mixing with a heavy sterile neutrino that had been proposed to explain the so-called `LSND-anomaly.’ In 2006 (07?) many watched as the collaboration revealed data that disproved this hypothesis, though their data set had an unexplained excess in low energy (below 475 MeV) electrons. Since this was in a region of large background and didn’t affect the fits used in the neutrino mixing analysis, they mentioned this in passing and promised to look into it.

A couple of months ago the collaboration came back with an improved background analysis showing that the low-energy excess still appears with over 3 sigma confidence (0812.2243). One novel model came from the paper `Anomaly-mediated neutrino-photon interactions at finite baryon density,’ (0708.1281), which was apparently a theorists’ favorite.  The model, however, predicted a similar excess for anti-neutrinos, which the latest analysis does not indicate (see the `2009 tour‘ talk of the MiniBooNE spokesperson, R. van de Water).

Some background

Neutrinos are slippery little particles that only interact via the weak force. They have also been of interest for beyond-the-standard model theorists since they are they key to several approaches to new physics, including:

  • lepton flavor physics (the PMNS matrix as the analogy for the CKM matrix in the quark sector)
  • see-saw mechanism (neutrino masses coming from mixing with GUT-scale right-handed neutrinos)
  • majorana mass terms (lepton number violating)
  • leptogenesis (transferring CP violation from the lepton sector to the hadron sector).

One of the big events of 1998 was the discovery of neutrino mixing (i.e. masses). This is actually a rather subtle topic, as recent confusion over the GSI anomaly has shown; my favorite recent pedagogical paper is 0810.4602. (See also 0706.1216 for an excellent discussion of why neutrinos oscillate rather than the charged leptons.)

The mixing probability between two neutrino mass eigenstates goes like \sin^2(\Delta m^2 L/E). I’ve dropped a numerical factor in the second sine, but this is a heuristic discussion anyway. The L and E represent the `baseline’ (distance the neutrinos travel) and the neutrino energy. A similar expression occurs for three neutrino mixing, such as between the three light neutrinos that we’ve come to know and love since 1998.

The early probes of neutrino oscillations came from `medium’ and `long’ baseline experiments where the neutrinos detected came from cosmic ray showers in the atmosphere and the sun respectively. The LSND experiment was the first to probe `short-baseline’ neutrinos, with an L/E of about 30 m / 50 MeV. What LSND found was incompatible with the standard story of three light neutrino mixing (hep-ex/0104049). They found a 3.8 sigma excess of electron anti-neutrinos over what one would expect, leading to the suggestion that this `LSND anomaly’ may have been due to mixing of the light neutrinos with a fourth, heavy `sterile’ neutrino.


MiniBooNE set out to test the sterile neutrino hypothesis by looking at the muon neutrino to electron neutrino mixing (LSND loked at anti-mu neutrinos to anti-e neutrinos). The experimental set-up had Fermilab shooting 8 GeV protons into a fixed target to produce pions and kaons. These are focused with a magnetic `horn’ so that they decay into relatively collimated neutrinos (mu and e) and charged particles. The horn can be run with opposite polarity to study the analogous anti-neutrino processes. The leptons then go through around 500 meters of dirt, which provides ample matter to stop the charged particles while leaving the neutrinos to hit an 800 ton mineral oil detector. (The energy and baseline are chosen to match the L/E of LSND.) These neutrinos may produce charged leptons, which produce Cerenkov radiation (the electromagnetic equivalent of sonic-booms) which is picked up by an array of photomultiplier tubes to read out information about the particle energy. Apparently the pattern of Cerenkov light is even enough to distinguish muons from electrons.

I don’t know why, but the MiniBooNE people measure their data in terms of protons-on-target (POT). It seems to me that the natural units would be something like luminosity or number of neutrino candidates… but perhaps these are less-meaningful in this sort of experiment?

The first results

Here’s what MiniBooNE had to say in 2007 (0704.1500):

MiniBooNE 2007 Results, from 0704.1500

MiniBooNE 2007 Results, from 0704.1500

As is standard in particle physics, did a blind analysis, i.e. analyzed the data without looking at the entire dataset, to prevent the analysts from inserting bias in their cuts. The found that their signal does not fit the LSND sterile neutrino hypothesis (pink and green solid lines on the bottom plot). Part of their blind analysis was to focus on the data above 475 MeV, since the data below this had larger backgrounds (top plot). Above this scale their data is very close to a fit to the standard 3-light-neutrino model. Below this, however, they found an odd excess of low energy electrons. Since this region has more difficult background than the 475+ MeV region and the latter region had [conclusively?] ruled out the natural LSND interpretation, they decided to publish their result look more carefully into the low energy region.

One year later

The group has since put up a further analysis of the sub 475 MeV region (0812.2243), and the result is that there is still a 3-sigma deviation. The new analysis includes several improvements to get better handles on backgrounds. I do not properly understand most of these (“theorist’s naievte”), but will mention a few to the extent that I am capable:

  • Pion neutral current distributions were reweighted to model pion kinematics properly
  • “Photonuclear absorption” was taken into account. This is the process where a pion decays into two photons and one of the photons is absorbed by a carbon atom. The remaining photon Cerenkov radiates is misidentified in the detector as an electron. (This apparently contributed 25% to the background!)
  • A new cut on the data was imposed to get rid of pion-to-photon decay backgrounds in the dirt (a generic term for earthy matter) immediately outside the detector. Signals that are pointing opposite the neutrino beam and originate near the exterior of the detector are removed since Monte Carlo simulations showed that these events are primarily background.

If I understand correctly, systematic errors are now smaller for the 200-475 MeV region (13% compared to 15% in the 475+ MeV region). The new result is:

Still an electron-like excess at low energies, from 0812.2243

Still an "electron-like" excess at low energies, from 0812.2243

The significance at each energy range is something like

  • 200 – 300 MeV: 1.7 sigma
  • 300 – 475 MeV: 3.4 sigma
  • 475 – 1250 MeV: 0.6 sigma

The 200 – 475 MeV data combine to an overall 3 sigma discrepancy. The MiniBooNE spokesperson also points out that since we now understand this low energy region better than the high-energy region (in terms of systematic errors), this is still solid indication that there is a MiniBooNE excess. It seems we’ve justed traded the `LSND anomaly’ for a `MiniBooNE anomaly’.

[For theorists: this is a familiar concept in duality called anomaly matching. For experimentalists: that was a bad joke.]

At this point, people who were model building for MiniBooNE could rest easy and keep earning their wages.


That is, until we look at the anti-neutrino data. Since the neutrino/anti-neutrino character of the beam is based on the “horn” polarity, the experiment can run in either nu or anti-nu mode, but not both simultaneously. Last December the MiniBooNE collaboration also “unblinded” their antineutrino data and found…

MiniBooNE antineutrino data fits background.

MiniBooNE antineutrino data fits background.

… what is it? A big piece of coal in their stocking. Well, ok, that was overly harsh. In fact, this is actually rather interesting. Recall that LSND was running in the antineutrino mode when it found its anomaly. [In fact, I don't properly understand why MiniBooNE didn't run in the antineutrino mode initially? I suspect it may have something to do with how well one can measure electrons vs positrons in the detector.]

The antineutrino data matches the background predictions rather well. No story here. Unfortunately this non-signal killed the favorite model for the electron excess (axial anomaly mediation), which predicted an analogous excess in the antineutrino mode. In fact, it killed most of the interesting interpretations. Apparently the betting game during the blind antineutrino analysis was to use the neutrino data to predict the excess in the antineutrino data in various scenarios. The unblinded data suggests the excess only exists in the neutrino mode.

One of the few models that survived the antineutrino data is based on “multichannel oscillations,” nucl-th/0703023.

Where to go from here

MiniBooNE continues to take data and the collaboration is planning on combining the neutrino and antineutrino data (I’m not sure what this means). They’re waiting on a request for extra running, with some proposals for exploring this anomaly at different baselines. Since background goes as 1/L^2 while oscillations go as \sin^2(L/E)/E, this L/E dependence can shed some light about the nature of the electron excess. One proposal is to actually just move the MiniBooNE detector to 200m. Since this would be closer, it would only take one year to accumulate data equivalent to the entire seven year run so far.

There was also a nice remark by the MiniBooNE spokesperson to `cover all the bases,’ so to speak: even if the low energy excess in MiniBooNE is completely background (i.e. uninteresting), this would still turn out to be very important for long baseline experiments (T2K, NoVA, DUSEL-FNAL) since they operate in this energy range.

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