MINOS in 2010

Peter Litchfield

HEP Seminar March 2nd 2010

MINOS is a mature experiment with a number of published results. I will

give you a short introduction to MINOS and neutrino oscillations

show you the published results from the last ~1 year on , ,sterile neutrinos

show you something of what further will come from the current data

e

MINOS Timeline

The Soudan 2 collaboration had the first thoughts of a long baseline neutrino experiment at Soudan around 1989. The detector would be Soudan 2. But the Main Injector was still years in the future

In 1994 Fermilab requested letters of intent for neutrino experiments at the Main Injector. The MINOS collaboration formed to design and construct a new, bigger detector

Final approval was given in 1998 and construction started

The far detector was completed in 2002 and started collecting data on atmospheric neutrinos and cosmic ray muons

The near detector and the beam were completed at the end of 2004

First data March 2005

First oscillation publication, July 2006

Neutrino running ended June 2009

Antineutrino running started September 2009

MINOS running

The exposure of MINOS is measured in terms of the number of 120GeV protons delivered to the target (POT)

~7.2 x 1020 POT have been accumulated for the neutrino oscillation data

~1.5 x 1020 POT for the antineutrino data which is still running

Neutrino Phenomenology

We assume that there are three neutrinos (if there are more things are more complicated yet)

Neutrinos can be described as eigenstates of flavour (e,,) or of mass, they are not necessarily the same.

The flavour eigenstates (e,,) are a mixture of the mass eigenstates (1,2,3)

When they are produced neutrinos are eigenstates of flavour, e.g.

When neutrinos propagate they do so as the mass eigenstates This is what produces neutrino oscillations

i

iiU

Why do Oscillate?

Quantum mechanical phenomenon, not a special property of neutrinos

Initial state has pure flavor, e.g.

But is a mixture of mass statesEach mass state has the same initial energy but different mass, therefore different velocityAfter traveling some distance the particle wave packets will have changed phase Now a different mixture of mass

statesTherefore a different mixture of flavor states when it interacts

1

2

3

+e+

Time t Distance L later

Three Neutrino Phenomenology

The Matrix U can be decomposed into three submatrices with elements which are the sines and cosines of 3 angles 12, 13, 23 and a phase (responsible for CP violation)

MINOS/Super-K NOA/T2K/ Double-CHOOZ

SNO-Kamland

Formalism: 3 Flavour Oscillations

Probability of → P=1.0-sin2223 sin2(1.27m232L/E)+small terms

Probability of oscillating to e in vacuum:

P=P1+P2+P3+P4

P1=sin2θ23 sin22θ13 sin2(1.27m132L/E) “atmospheric”

P2=cos2θ23sin22θ12sin2(1.27m122L/E) “solar”

P3= Jsinδsin(1.27m122L/E)

P4=Jcosδcos(1.27m122L/E)

J=cosθ13sin2θ12sin2θ13sin2θ23sin(1.27m122L/E)sin(1.27m13

2L/E)

“atmospheric- solar interference”

At the distance of Soudan and the known oscillation parameters the blue terms are dominant for MINOS

Nova, with its increased mass and off axis location will be more sensitive to 13 and possibly , if 13 is large enough .

Matter effects

Matter is not CP invariant, it is made up of matter, not antimatter. Neutrinos passing through matter interact differently than anti-neutrinos

All flavours of neutrinos can interact with electrons via Z exchange, only e interact via W exchange

The effect depends on the sign of m2, i.e whether nature has the normal or inverted heirachy

In matter at oscillation maximum, P1P1(1 ± 2E/ER). ER11GeV for the earths crust. The + sign is for neutrinos with normal mass hierarchy and antineutrinos with inverted mass hierarchy.

Z e

e

e

eW

x x

e e

In principle we want to determine all the elements of the mixing and mass matrices for neutrinos and anti-neutrinos

MINOS was designed 15 years ago when neutrino oscillations were an unproved speculation

We went for the easiest and at that point the defining oscillation property, the disappearance of

Coarse, magnetised detectors to measure

We measure very well m232 and reasonably well sin2223

Having done that we would like to measure and sin2213

Do oscillate in the same way as ? Is CPT conserved?

Are there more than 3 neutrinos, extras have to be sterile?

What is MINOS doing?

e...

......

The 2 Detector Paradigm

The principle of MINOS is very simple. One detector at Fermilab measures the beam composition there. Another detector in Soudan measures the composition after it has travelled 735km. If it is different the neutrinos have oscillated. BUT;

The near detector is close to the end of the beam decay pipe and thus sees an extended line source of neutrinos. The far detector is far away and sees a point source. The energy distributions are different independent of oscillations

Although designed as closely similar as possible there are differences in the detectors and the beam conditions are different

Have to make corrections to the near detector spectrum to predict the far

Near-Far Beam Extrapolation

1) Far/Near:

2) Beam Matrix: The truth far energy distribution is expressed as a function of the near (previous slide) as a matrix. The near detector measured energy distribution vector is converted to a truth distribution by efficiency and background matrix corrections, multiplied by the far-near matrix, and converted back to a far detector measured distribution vector by far detector efficiency and background matrices

3) Event-by-Event: The beam MC calculates the relative probability of the neutrino from each beam decay hitting the near and far detectors. The near detector MC is corrected using the near detector data. Far detector distributions are built up event by event from the corrected near MC using the relative probabilities of hitting the two detectors

The Far/Near is the simplest and adequate for low statistics data

For the best sensitivity we need to allow for details of the beam extrapolation.

MC

pred dataF EF E N E

N E

→e

When e interact they produce electrons

Ignoring smaller components, the oscillation probability for →e is given by

We know that sin2213 is small (<~0.15) from the CHOOZ experiment, there will not be many events

MINOS is a coarse detector, not designed to identify electrons

There is a large background of electrons coming from 0→

THIS IS A REALLY TOUGH JOB IN MINOS

We need NOA (and/or T2K), but we have tried anyway

22 2 2 23

13 23

1 272e

. m LP sin sin sin

E

e N e .....

→e Analysis Strategy

Develop a selection procedure using MC and the near detector data to select e events and reject background

Measure the rate of selected events in the near detector. Assuming no oscillated events in the near detector this rate is

Intrinsic beam e (calculated from the beam MC)

Background misidentified NC and events

Extrapolate the near detector data to the far detector (need to do the NC and separately as the oscillate)

Compare the far detector prediction with the data.

Hope for an excess due to oscillations.

→e Near Detector Background

Selection of e events is done using an Artificial Neural Network

11 different properties of e events describing the shower length, width and shape are fed into the network

Selected

Area normalised output of the ANN

Signal efficiency: 41%Total CC background rejection: 99.4%Total NC background rejection: 92.3%

Predicted FD MC Background, 3.2x1020 pot

69% NC,

19% CC,

8% beam e,

4%

Far Detector Background

The Far detector background is predicted from the near detector data by the Far/Near method

The strength of a two detector experiment is that many systematics cancel (e.g. background cross-sections)

Have to extrapolate the different background components separately as the Charged Current background will oscillate, the NC will not

Separate background components using different beam configurations

Horn on, pions are focussed and we have a low energy peak

Horn off, no focussing, only the very forward (high energy) neutrinos are detected

MCpred dataFD

FD NDND

Horn on-off NC/CC decomposition

on cc nc eon

cc ncoffMC offMCoff cc nc e

offcc nconMC onMC

N N N N

N NN N N N

N N

17

e Selected Far Detector Data

We observed a total of 35 events We expect 27±5(stat)±2(sys) background events. The data is 1.5 above expected background. Published PRL 103:261802 (2009)

MINOS 90% CL in sin2213

A Feldman-Cousins method was used.

Fit simply to the number of events from 1-8 GeV, no shape or correlation information used.

Best fit and 90% CL limits are shown:as a function of CP

for both mass hierarchiesat MINOS best fit value for m23

2 and sin2223

allowed region

Limits for Normal (Inverted) hierarchy for (CP=0) sin2213<0.29 <0.42 (90% CL)

7.0 x1020 POT

Future limit if excess goes away with more

data.

Future measurement if data excess persists.

We have already doubled the data in current running!Expect new results shortly

Future 90% Contours

Is CPT conserved?

If CPT is conserved then , the oscillation parameters of must be the same as

Unfortunately are harder to study the

The production of - is lower than +

The cross sections for are lower than for

Only about 1/3 the events per POT

There is no previous data on separated and interactions.

Super-K has produced unseparated limits

A global fit produces the limits in the plot

M.C. Gonzalez-Garcia & M. Maltoni, Phys. Rept.460:1-129 (2008)

Beam Antineutrinos

Standard NuMI focuses positive mesons, parents of

are mostly produced by mesons going through the field free center of the horns

NuMI beam composition:– 91.7% peak at 3 GeV– 7.0% peak at 8 GeV

– 1.3% e and

π−

π+120 GeV p+

Target Focusing Horns

2 m

675 m

νμ

νμ

15 m 30 m

Decay Pipe

e

Most true are high energy and come from unfocussed -

Most events in the oscillation region around 2-3 GeV are background events

Neutral Current events with a fake muon

Mis-reconstructed or Mis-identified CC events

+ are selected with cuts based on three parameters;

CC/NC separation parameter

2 curvature measurements

Selecting

Far Detector SpectrumThe near detector data was projected to the far detector using the beam matrix method

64.68.0(stat)3.6(syst) events were predicted with no oscillations

58.37.6(stat)3.6(syst) with CPT conserving oscillations

42 data events were obtained, a 1.9 deficit

The deficit is at high energies, NOT where we expect to see oscillations

Checked by examining + events produced in the rock upstream, find ~1 excess. Conclude the deficit is statistical

Oscillation Fit

Allowed region contours were calculated using the Feldman Cousins prescription including systematics.

The best fit point is at high because of the deficit at highenergy.

The CPT conserving point fromthe MINOS analysis is withinthe 90% contour. A previously allowed CPTviolating region (unshaded) is excluded at 99.7% C.L.

2m

Search for transitionsIf could transform into during the flight from Fermilab we would observe an enhancement in the + energy spectrum.

Assuming that the transition probability has the same form as the the oscillation probability, we can define a transition probability

2

2 2 1 272

. m LP sin sin

E

<0.0206 at 90% conf

Reverse Horn Current Running The beam setting for the analysis just described selected negative tracks and thus By reversing the horn currents we can select positives and thusMINOS is currently running in this mode

Event rate is ~1/3 lower than in mode

But spectrum peaks in the oscillation region and thus is more sensitive to oscillations

With the full data sample we expect to collect we will reduce the error on by about a factor of 4

2m

Sterile Neutrinos

We know from LEP data that there are only three low mass neutrinos with standard model interactions

However this does not exclude neutrinos that do not couple to normal matter (sterile).

Some models beyond the standard model can include such objects

If only standard model interactions exist the rate of neutral current interactions does not change in oscillations since all neutrinos have the same neutral current cross sections

If sterile neutrinos exist and neutrinos can oscillate to them, as well as a deficit of charged current interactions there will be a deficit of neutral current interactions.

Sterile Neutrinos

Select events with no reconstructed or which are rejected by the charged current selection

Observed 388 events

Far/Near extrapolation predicts 37719.4(stat)18.5(syst)

Small correction if 130

No evidence for NC disappearance

Fraction of that disappear by converting to sterile neutrinos given by

1s

s

Pf

P

<51% at 90% confidencesf

Oscillations to sterile neutrinos will reduce the number of NC events

New Oscillation analysis CC events have the most statistics and best resolution on the neutrino energy, analysis of these events is the money analysis

Measure muon energy E by range or curvature in the field

Measure the hadronic shower energy Eshw by summing scintillator pulse height.

E=E+Eshw

The 2008 analysis used 3.2 x 1020 protons on target

Currently we have more than doubled this exposure to ~7.2 x 1020 POT

A new analysis is in process with results this summer.

The 2008 analysis used only selected (-) CC events with interaction vertices in the fiducial volume, 848 events. Only ~40% of our total events

The new analysis will have ~4,500 events of which ~1700 will be CC and will include all the extra events

Published CC AnalysisMINOS 2008 Result (3.2 1020 POT) Phys. Rev. Lett. 101, 131802 (2008)m2 = (2.43±0.13) x 10-3 eV2 sin2(2) > 0.95

Decay disfavoured at 3.9

Decoherence disfavoured at 5.7

New, improved, Oscillation Analysis The extra events that will be included are;

CC + events: combining the and analyses

NC events: contain a significant component of true CC events with low energy muons

Events with production vertices outside the fiducial volume: only ~20% of events and with worse resolution but another source of true CC events

Events with produced in the rock upstream of the detector: we only see the muon in general and it has lost energy in the rock so very poor resolution on E but the total rate is sensitive to oscillations.

The rock and anti-fiducial volume events are the subject of Matt Strait’s thesis. Seminar to come in a few weeks.

Although the CC events are only ~40% of the data they have by far the most statistical weight, adding the remainder will gain 15-20% in sensitivity

New, improved, Oscillation Aanlysis

Analysis improvements include;

A new CC selection which will improve the efficiency for low energy events

A new method of estimating the shower energy using a knn nearest neighbour method

Fitting will be made as a function of event resolution to increase sensitivity

The event-by-event extrapolation method will be used along with the beam matrix method for the extrapolation.

Fitting can be done as a function of E and Eshw instead of just E which implements the resolution fitting, improves the separation of true CC and NC events in the NC selected sample and reduces some systematics

MC plots of event energies: CC fiducial The following plots show a MC Generated Fake experiment with

7.0x1020 pot and the previous best fit oscillation parameters. The predictions for no oscillations and truth oscillations are also shown.

MC plots of Event energies: NC

Pre

dic

ted

e

ve

nts

Pre

dic

ted

e

ve

nts

True NC

True CC

2) In fiducial NC events. Note the different distributions for true CC and NC events

MC event energies: Anti-fiducial events

The out of fiducial volume CC events have a strong oscillation signal but the total of events is only ~1/7 of the fiducial events

MC plots of event energies: Rock events

All data will be fitted simultaneously to give the final oscillation parameters.

Results out this summer….

Summary

MINOS has published results on several oscillation channels based on 3.2 x 1020 pot

We have by far the best measurement of m2 and have searched for oscillations in anti-neutrinos, sterile neutrinos and in →e

Results on the full 7.2x1020 pot →e will appear shortly

Results from this data and hot off the press anti-neutrino data is being analysed and results should appear this summer (Neutrino 2010 is in Athens in June)

Don’t forget the other analyses being done on MINOS

Near detector data on neutrino interactions, by far the highest statistics in the world with the Minerva fine grain detector just coming on stream

Cosmic ray muon and neutrino data

Onward to Nova and a detector designed for →e