MiniBooNE Neutrino Result – Guest Blog from Heather Ray

By John Conway | April 11, 2007 4:15 pm
Today at Fermilab, the MiniBooNE experiment announced to a packed auditorium their long-awaited results looking for neutrino oscillations. Below is a guest post from Dr. Heather Ray, a scientist at Los Alamos National Lab, who has been working on the experiment for several years. I have known Heather since she was a graduate student on the CDF experiment at Fermilab, when she was at the University of Michigan. To the right is a photo of her with her significant other, Ivan Furic.

MiniBooNE Neutrino Experiment Results

by Dr. Heather Ray, Los Alamos National Lab

Neutrinos, a fundamental particle of nature, are believed to oscillate, or change from one type to another. In the long list of experiments which have claimed an observation of neutrino oscillations, one stands apart : LSND. The LSND result doesn’t fit in with our picture of oscillations from other experiments, and as such is highly controversial. The MiniBooNE experiment was designed to explore the LSND result, to conclusively prove or disprove the claimed oscillations. MiniBooNE announced it’s first results today (April 11th, 2007). The illustrious rulers of the Cosmic Variance blog have asked me to write a bit about this result. So, let the amazing neutrino story begin!

  • Neutrinos and Oscillations : A Quick Introduction
  • In the Standard Model of physics there are three main categories of fundamental particles: quarks, leptons, and gauge bosons, or force carriers. The leptons are the electron, muon, and tau, as well as their partner neutrinos : &#957e, &#957&#956, and &#957&#964. Neutrinos in the Standard Model have no charge and are massless. Imagine for a minute that neutrinos do have mass. If they have mass then they are able to oscillate, or change type. Neutrinos have definitively been observed changing from one type into another, yet the Standard Model of physics says that neutrinos do not have mass. The simplest solution to this conundrum is to allow the neutrinos to have mass.

    In more technical terms we say that the weak eigenstates ( &#957e, &#957&#956, and &#957&#964) are made up of a combination of mass eigenstates. For example, in a two-neutrino scenario, at the time of creation the muon neutrino is a combination of the two mass eigenstates :

    |&#957&#956(0)> = -sin &#952 |&#9571> + cos &#952 |&#9572>

    where the probability for two-neutrino oscillations is given by :

    Posc = sin2(2&#952) * sin2[ (1.27 * &#916m2 * L) / E ]

    The probability has two terms which are constrained by the design of the experiment (L, the distance from the neutrino source to the detector, and E, the energy of the neutrino beam), and two terms which are fit for when performing a two-neutrino oscillation analysis (&#916m2 and sin2(2&#952), where &#952 is the mixing angle between the two neutrino states and &#916m2ab = m2a – m2b).

    Neutrino physicists illustrate the current status of neutrino oscillations using a two dimensional plot that is the function of the two fit parameters. Oscillation results from the solar and atmospheric sectors (the tiny red and blue dots) have been observed and confirmed by several experiments. The set of several independent measurements allows us to constrain the range of fit parameters for those oscillations. The LSND result, which spans the upper third of this plot, has a large allowed region in parameter space.

    In the Standard Model there are only three neutrinos, all of which interact with matter. The &#916m2 is the mass squared difference between the two neutrino states. These three results represent three differences, or splittings, between the mass states. If the Standard Model of physics is correct and there are 3 and only 3 neutrinos, a summation law should exist : &#916m213 = &#916m212 + &#916m223. You can see that even at LSND’s lowest allowed &#916m2 point the summation law does not hold.

    If LSND’s observation is found to be a true fact of nature, the Standard Model of physics cannot fully accommodate/explain neutrino interactions! This “breaking” of the Standard Model is very exciting to physicists, and indicates there is new physics that we haven’t previously thought possible. Many things could be true – there could be new allowed interactions for neutrinos (Lorentz Violation, CP/CPT violation, the list goes on!), or there could be additional particles – sterile neutrinos, which don’t interact with other matter but only can been seen through mixing with other neutrinos.

    To properly explore the LSND signal we need an experiment that has the same experimental constraints (L/E, from the oscillation probability formula), so that the entire allowed region of LSND can be explored. The follow-up experiment also should have more events (smaller statistical errors), and a different signal signature, backgrounds, and sources of systematic errors. This experiment is MiniBooNE.

  • MiniBooNE
  • MiniBooNE is located at Fermi National Accelerator Laboratory, in Batavia, IL. To produce our neutrino beam we start with an 8 GeV beam of protons from the Booster. The proton beam enters a magnetic focusing horn where it strikes a beryllium target. The interactions of the protons+Be produce positively and negatively charged mesons (pions and kaons). The positively charged mesons will decay to produce a neutrino beam, while the negatively charged mesons will decay to produce an anti-neutrino beam.

    There are still a lot of mysteries surrounding the interactions of neutrinos. We don’t yet know if neutrinos mix with the same probability as anti-neutrinos. Therefore, the LSND result, which claims observation of anti-&#957&#956 &#8594 anti-&#957e oscillations, needs to be explored using both neutrinos and anti-neutrinos. For the first check of LSND we chose to focus the positively charged mesons, which means we’re looking for &#957&#956 &#8594 &#957e oscillations. This choice was solely dictated by physics : the proton + Be interactions produce far more positively charged mesons. This means the rate of collection for our neutrino sample is much higher than our rate of collection for an anti-neutrino sample. We chose to collect the quick data set first, and then proceed with analyzing that data while collecting the anti-neutrino data set.

    The mesons decay in flight into the neutrino beam seen by the detector : K+ / &#960+ &#8594&#956+ + &#957&#956, where the &#957&#956 comprise the neutrino beam seen at MiniBooNE. These mesons decay in flight in our vacuum decay region. Following the decay region is an absorber, put in place to stop any muons and undecayed mesons. The neutrino beam then travels through approximately 450 meters of earth before entering the MiniBooNE detector.

    MiniBooNE is a 12.2 meter diameter sphere. The detector is filled with pure mineral oil and lined with photomultiplier tubes (PMTs). PMTs work like a reverse light bulb – instead of putting in electricity to produce light the PMTs collect light from neutrino interactions in our detector and output an electrical pulse. There are two regions of the MiniBooNE detector : an inner light-tight region and an optically isolated outer region known as the veto region, which aids in vetoing cosmic backgrounds.

  • Detecting Neutrino Interactions
  • Neutrinos interact with material in the detector. It’s the outcome of these interactions that we look for. These neutrino interactions in the MiniBooNE detector leave a distinct mark in the form of Cerenkov and scintillation light. Cerenkov light is produced when a charged particle moves through the detection medium with a velocity greater than the speed of light in the medium (v > c/n). This produces an electro-magnetic shock wave, similar to a sonic boom. The shock wave is conical and produces a ring of light which is detected by the PMTs. We can use Cerenkov light to measure the particle’s direction and to reconstruct the interaction vertex. This effect occurs immediately with the particle’s creation and is known as a prompt light signature.

    Charged particles moving through the detector also may deposit energy in the medium, exciting the surrounding molecules. The de-excitation of these molecules produces scintillation light. This is an isotropic, delayed light source, and provides no information about the track direction. We can however use the PMT timing information to locate the point, or vertex, where the neutrino interaction occurred.

    We can use the patterns of light seen in our PMTs to determine what type of neutrino interacted in our detector. In the charged-current quasi-elastic events, a neutrino interaction in the detector will produce the lepton partner of the neutrino. For example, an electron neutrino interaction will produce an electron, and a muon neutrino interaction will produce a muon. Electrons travel for only a very short time before their velocity falls below the Cerenkov threshold. They multiple scatter along the way, as well. This leaves a fuzzy Cerenkov ring in the detector. Muons tend to travel for a much longer distance. As they travel through the detector they lose energy, and the angle at which the Cerenkov light is being emitted shrinks. Muons also emit scintillation light. The signature of a muon in the detector isn’t one of a ring, as in the case of an electron. It is instead a filled in circle of light. Neutral pions decay into two photons, which then pair produce. The electrons from this pair production each create a ring in the detector.

  • Components of the Oscillation Analysis
  • MiniBooNE is performing a blind analysis. This means that we can either :

    • see some of the information in all of the data : we can check the charge per PMT as a function of time, to verify our detector isn’t failing,
    • see all of the information in some of the data : we are able to select data sets which will have no oscillation events present, if we assume maximal allowed oscillations from LSND. We can use these data sets to then tune and verify our Monte Carlo simulation.

    but we can’t see all of the information in all of the data. Having access to all of the information in all of the data is unblinding. Prior to unblinding we had to have all components of the analysis completely fixed. We aren’t allowed to go back and change event selection cuts or error estimates once we unblind.

    Our oscillation analysis can be boiled down to this simple algorithm : determine a set of event selection cuts which will isolate the electron neutrino events but remove the majority of all other events. There are a certain amount of electron neutrino events inherent in the beam, which come from kaon decays. There are also a small amount of other types of events (delta decays, pi0 events) which will pass the electron neutrino cuts, but which are not from true electron neutrino events. The sum of the estimated intrinsic electron neutrino events plus the fake events is the total number of events we expect to see, if no oscillations are present. We compare the number of events observed in data to the number expected, as a function of the reconstructed neutrino energy in these events. If we observe oscillations we should see an excess of data events over the expectation, whose shape will change as a function of the oscillation parameters.

  • Awesome-o Results!
  • This plot shows the MiniBooNE final sensitivity, compared to the prediction from our 2003 Run Plan. Curves are shown overlaid on the allowed LSND region.

    This plot shows the final result from the likelihood analysis. Data points are the black dots. The expected event spectrum is shown in red, and is broken down into the intrinsic electron neutrino and fake event shapes in the green and blue.

    We have two separate analyses that are used in the oscillation search : one which depends on likelihood variables, and one which depends on a boosted decision tree. These two analyses have a small overlap in event composition, and provide a good check of each other. We have complete confidence in our analysis if these two analyses find the same result. Before we unblinded our data we had to decide which of the two analyses we would call our primary analysis, for the purpose of quoting numbers in publications. We made this decision based on the expected sensitivity found using Monte Carlo. The sensitivity is the amount of parameter space allowed by the LSND result that we expect to be able to probe. Our sensitivity studies showed that with the likelihood analysis we were able to achieve a sensitivity which agrees quite well with the sensitivity MiniBooNE was designed for. This is something to be quite proud of! We also agreed that the final result quoted for the two neutrino oscillation search would be from 475 MeV to 3 GeV, based on the LSND best fit region.

    MiniBooNE unblinded on Monday, March 26th, 2007. When we opened the box we found no evidence for an excess of events over the background prediction. The MiniBooNE neutrino data set agrees with the no neutrino oscillation hypothesis, in the range of reconstructed neutrino energy from 475 MeV to 3 GeV. The probability that MiniBooNE and LSND both are due to two-neutrino oscillations is only 2%. The likelihood and the boosting analyses also agree quite well in measured excess events.

    I’m sure by now you’ve noticed that MiniBooNE observes an excess of events in the low energy region. We first saw this excess two weeks ago, when we unblinded. At that point we began working like madmen to determine what those events could possibly be. We’re rechecking our detailed understanding of various low energy background events: interactions in the dirt surrounding the detector, radiative delta decays, low energy neutral current events, you name it, we’re exploring it. Obviously, we don’t want to make any additional comments about this excess until we’re certain that we’ve performed all possible checks on our event predictions in that region. We’re hoping to have this excess mystery resolved in the next few months.

  • Now What?
  • MiniBooNE’s neutrino oscillation analysis was the first of two analyses needed to conclusively explore the LSND result. An anti-neutrino oscillation analysis will also need to be performed. At MiniBooNE it would take many more years to accumulate the data set needed to perform this anti-neutrino analysis. Instead, I’m hoping that we can continue this exploration at the Spallation Neutron Source, at Oak Ridge Laboratory in TN. The SNS is designed to be a world-class neutron facility. One of the side-effects of the process which produces the neutrons is that you get an amazing neutrino beam for free! Funding permitting I’m hoping we can begin taking data at the SNS within 3 to 4 years. At the SNS we could perform neutrino and anti-neutrino measurements simultaneously (no need to switch the horn polarity!), look for oscillations, sterile neutrinos, and look for CP/CPT violation in the neutrino sector.


    • S. Ahmed et al. [SNO Collaboration], Phys. Rev. Lett. 92, 181301 (2004),
    • G. Fogli et al., Phys. Rev. D 67, 093006 (2003),
    • A. Aguilar et al. [LSND Collaboration], Phys. Rev. D 64, 112007 (2001),
    • D. Smith, “Calculating the Probability for Neutrino Oscillations”,
    • The HARP Collaboration, CERN-SPSC/2003-027, SPSC-P-325
    • S. Kopp, “The NuMI Neutrino Beam at Fermilab”,
    CATEGORIZED UNDER: Guest Post, Science
    • J


      everything from the 4th paragraph down is stricken out with lines, as are the blogroll and several other entries. It looks like all the letters were strung onto the lines like beads on a string.

    • Benabik

      I do not see the strikethrough lines mentioned in comment 1. Mark WORKSFORME on Safari 2.0.4. ;-)

    • http://http// NathanL

      HRay! You rock! I wondered if you’d be involved in the press release for this.

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    • Hiranya

      Awesome post!

    • Sean

      Fantastic, thanks so much for doing this. Meaning both the post and the experiment!

    • Kea

      Cool! Thanks a lot!

    • Julianne

      Terrific explanation! And go Steelers!

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    • Heinrich

      Congratulations – nice job! I’d like to add a comment though, before the general perception of this result accepts this as a water-proof evidence that LSND is wrong: True, miniBooNE ruled out standard neutrino oscillations as a solution for the LSND anomaly, but that possibility was already at the verge of being ruled out by combinations of results from solar, atmospheric, reactor and collider neutrino experiments. On the other hand we proposed a spectacular solution to the LSND anomaly involving neutrino shortcuts in extra dimensions, published in Phys.Rev.D72:095017,2005 [hep-ph/0504096]. If you look at Fig. 5 in this paper, you will see that for a choice of the resonant energy in the region 200-300 MeV we not only predicted the small counting rate for electron neutrino events above 475 MeV, but also the large rates in the 300-475 MeV region. While the anomalous effect seen by miniBooNE might have a conventional explanation, it might well be the first hint for extra dimensions of spacetime!

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    • Brian

      Thank you, Heather (for posting) and Sean (for hosting). Intriguing comment by Heinrich.

    • Paul Valletta

      This is very dramatic, and a long time coming!

      Well, it seems that the teams results, will spurn a swarm of hornets out of their nests, if the results are to be taken exact.

      The probable outcome will be that there is a finite limit to the experimental limits of dimensional observations. Neutrino’s, are the most finite particles of 3-D space?..any experiment that can be created in 3-Dimensions, cannot give results external from “other” dimensions? The Neutrino is not fleeting in and out of other dimensions.

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    • http:/// Matti Pitkanen

      The neutrino energies of LSND and MiniBoone are 60-200 MeV and 300-1500 MeV and oscillations in LSND mass range are found to be absent above 500 MeV. Evidence for oscillations is found below 500 MeV.

      Hence LSND and MiniBooNE are consistent if one accepts that neutrino mass scale depends on its energy as TGD strongly suggests. For details see the posting at my blog page.

      Matti Pitkanen

    • Kris Krogh

      We’ll also hear the preliminary results from another major experiment this week. Those from Gravity Probe B will be announced Saturday at the meeting of the American Physical Society in Florida, preceded by a NASA press/media event. (My prediction: general relativity fails.)

    • Duncan

      Crikey. My physics degree was too long ago – I’m going to have to have a lie down in a dark room and wait for the palpitations to subside :-)

    • Plato

      I think it is important that we are fabricating the experimental basis so we see this as a “natural process.” Following the work of John Bahcall and the evolution of research in this area makes this article posted here, along with all the data very interesting story about what the sun does for us. Cosmic particle in space and Fly’s eye.

      Bringing the Heavens down to Earth

      If mini black holes can be produced in high-energy particle interactions, they may first be observed in high-energy cosmic-ray neutrino interactions in the atmosphere. Jonathan Feng of the University of California at Irvine and MIT, and Alfred Shapere of the University of Kentucky have calculated that the Auger cosmic-ray observatory, which will combine a 6000 km2 extended air-shower array backed up by fluorescence detectors trained on the sky, could record tens to hundreds of showers from black holes before the LHC turns on in 2007. See here

      As well, Clifford has a post called “Quark Soup Al Dente” along the same line in regards too cosmic particle colllsions that made me think that this information supplied here should be seen in context of the collision process. I am not sure if you feel this is appropriate?

    • Plato

      One last thing, and then I’m done. Thanks


      Figure 3 shows arrival directions of cosmic rays with energies above 4 x 10^19eV. Red squares and green circles represent cosmic rays with energies of > 10^20eV , and (4 – 10) x 10^19eV , respectively.

      Agasa Results See here

      Shaded circles indicate event clustering within 2.5o. At (11h 20m, 57o), three 4 x 10^19eV cosmic rays are observed against expected 0.06 events . The chance probability of observing such triplet under an isotropic distribution is only 0.9%

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    • Arun M Thalapillil

      I missed both the talk at Fermilab and the live stream yesterday, so it was very nice to see this post. The low energy excess is surprising… it would be interesting to see what conclusion comes out of it finally.

    • Arun

      Great post!

      Will someone be kind, though, and mark on the very first chart with the green blur and the blue and red spots, what part of parameter space MiniBooNE allows? Or is that left as a homework exercise?

    • Neil B.

      Interesting. One of the thoughts I’ve (and others) had about neutrinos: if they really aren’t going at the speed of light, but even a bit less, then their spin can’t be inherently left-handed. Why not? Because, a sufficiently fast-moving observer could be moving past them, and would see a neutrino’s spin going the wrong way relative to its velocity. Their spins would be an artifact of the creation process. Any thoughts on this?

    • Diogenes

      -”Interesting. One of the thoughts I’ve (and others) had about neutrinos:
      -if they really aren’t going at the speed of light, but even a bit less, then
      -their spin can’t be inherently left-handed. Why not? Because, a sufficiently -fast-moving observer could be moving past them, and would see a
      -neutrino’s spin going the wrong way relative to its velocity. Their spins
      -would be an artifact of the creation process. Any thoughts on this?
      -Neil B. on Apr 12th, 2007 at 8:55 pm”

      You are completely correct, and this effect is included in the theory of neutrino masses as appears in standard texts. The projection of the neutrino spin along its direction of motion is it’s “helicity”. There is another measure of the “handedness” of a relativistic spin one half-particle called “chirality” that is easily defined in terms of the matrices appearing in the Dirac equation, and this measure does NOT depend on the reference frame in which you observe the particle. For massless particles these measures of “handedness” agree, but for massive particles they can disagree for exactly the reason you describe, ie. by moving past the particle you can reverse it’s direction of motion, hence it’s apparent helicity. When a neutrino is created by the weak interaction it is always created in a state that is left-handed as measured by *chirality*. For a neutrino that is not exactly massless, this means that there is a small amplitude (proportional to it’s mass divided by its energy), and hence a small probability, that it actually was created in a state that is right-handed as measured by *helicity*. If you run along in the direction of the neutrino you will see a greater probability for it to be in the “wrong helicity” state, which agrees with what I wrote above because you see it with less energy in your reference frame, so the mass divided by the energy (which gives the “wrong helicity” amplitude, hence probability) is larger. I hope that this helps!

    • archgoon

      Okay, let me see if I have this sorted out (probably not):

      1. According to the Standard Model, neutrinos are massless.

      2. According to a number of different experiments, neutrinos appear to be oscillate their flavor, which only works if they have mass.

      3. This should only be happening at a single energy range (why?), and we have 3 different ranges provided by different experiments. The MiniBooNE experiment has ruled out one of the ranges, which was suggested by the results of the LSND experiment. Does this mean we don’t know how to currently explain the LSND data?

      Is this somewhat correct?

    • not too shabby

      The masslessness of neutrinos is hardly required by the standard model. Neutrino masses can be generated through yukawa-type couplings just like all the other fermions. Of course this is not very satisfying and it’s still a puzzle why these particular yukawa couplings are so small etc etc. But I’m just saying.

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    • B

      question: could somebody comment on how reliable this cross-section

      is in the observed energy range? It is not really clear to me how much of the total cross-section is a fit and what is a calculation. thanks,


    • Count Iblis


      Isn’t that explained in the article by

    • Piecefu

      Neil B. –
      I’ve thought about that too, but it turns out not to matter too much. Wikipedia explains it better than I could:

      “The existence of nonzero neutrino masses somewhat complicates the situation. Neutrinos are produced in weak interactions as chirality eigenstates. However, chirality of a massive particle is not a constant of motion; helicity is, but the chirality operator does not share eigenstates with the helicity operator. Free neutrinos propagate as mixtures of left- and right-handed helicity states, with mixing amplitudes on the order of mν / E. This does not significantly affect the experiments, because neutrinos involved are nearly always ultrarelativistic, and thus mixing amplitudes are vanishingly small (for example, most solar neutrinos have energies on the order of 100 keV — 1 MeV, so the fraction of neutrinos with “wrong” helicity among them can’t exceed 10 − 10).”

      See complete article here.

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    • Ellipsis

      How many more years of booster running would an antineutrino analysis of comparable sensitivity to the present MiniBooNE neutrino results take?

      I.e. could you say a little about the cost/benefits of continuing at FNAL, vs. a new facility at SNS?

    • Gavin Polhemus

      Niel B.

      [If neutrinos] really aren’t going at the speed of light, but even a bit less, then their spin can’t be inherently left-handed. Why not? Because, a sufficiently fast-moving observer could be moving past them, and would see a neutrino’s spin going the wrong way relative to its velocity.

      This is correct, so there must be a right handed version of the neutrino. There is: the anti-neutrino. When you reverse the direction of a neutrino it changes helicity, but it also changes to an anti-neutrino, so no new particle needs to be added to the standard model to account for neutrino masses.

      Although somewhat academic, we should probably think of the neutrinos as their own antiparticles (i.e. they don’t get arrows in Feynman diagrams) and allow them to come in both right and left handed versions. (In technical language, we should think of them as real four component spinors rather than complex two component spinors). This doesn’t require a dramatic change in thinking. When you hear “neutrino” just think “left-handed neutrino” and when you hear anti-neutrino think “right-handed neutrino.” We all know that mass terms mix right-handed and left-handed, so there’s nothing to worry about. (Except for lepton number, which is no longer well defined. No matter. Lepton number isn’t conserved anyway once you add neutrino masses).


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    • Mustafa Mond, FCD

      Hence LSND and MiniBooNE are consistent if one accepts that neutrino mass scale depends on its energy as TGD strongly suggests. For details see the posting at my blog page.
      Matti Pitkanen

      Did I miss something? What does MiniBoonE have to do with Richard Dawkins’ The God Delusion?

    • B

      Hi Count,

      to me it’s not clear – that’s why I was asking. There is a lot of talking about method one and method two and so on in that paper. I am not an expert on this, so I find it kind of confusing. I am just puzzled that if there are different methods at all, shouldn’t there be an errorbar on that cross-section? Esp. if there are so little data points in the relevant energy region? If that cross-section was somewhat different wouldn’t it affect the number of expected events (not sure if it would do the same to the background since I am not sure exactly what contributes to that background). Best,


    • Yvette

      How exciting! Go Stillers!

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    • Susan, Heather’s Momma

      I was beyond proud when I read this, however, I was curious as to how many scientists would catch the relevance of the black and gold attire in the photo…I am even more proud to see many of you did!! The picture was taken 2/06 in Detriot at the Super Bowl.. Heather and I had built our first new house and actually spent the porch money to watch our beloved Steelers get one for the thumb!! Here we go Steelers, Here we go!!

    • HelloWorld

      Peace people

      We love you

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