Prospects for NUMI Off-axis Initiative

Kwong Lau

University of Houston

November 28, 2003

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Outline

Introductory Comments Advantages of an Off-axis Beam Important Physics Issues NuMI Capabilities Detector issues Present schedule

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Introductory Comments

The current generation of long and medium baseline terrestial oscillation experiments is designed to:

• Confirm SuperK results with accelerator ’s (K2K)• Demonstrate oscillatory behavior of ’s (MINOS)• Make precise measurement of oscillation parameters (MINOS)• Improve limits on e subdominant oscillation• Demonstrate explicitly oscillation mode by detecting ’s (OPERA, ICARUS) mode, or detect it (MINOS, ICARUS)6. Resolve the LSND puzzle (MiniBooNE)7. Confirm indications of LMA solution (KamLAND)

Many issues in neutrino physics will then still remain unresolved. Next generation experiments will try to address them.

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The Physics Goals

Observation of the transition e

Measurement of 13

Determination of mass hierarchy (sign of m23)

Search for CP violation in neutrino sector Measurement of CP violation parameters Testing CPT with high precision

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Kinematics of Decay

Lab energy given by length of vector from origin to contour

Lab angle by angle wrt vertical Energy of is relatively

independent of energy Both higher and lower energies

give ’s of somewhat lower energy There will be a sharp edge at the

high end of the resultant spectrum

Energy varies linearly with angle Main energy spread is due to beam

divergence

Compare E spectra from

10,15, and 20 GeV ’s

LAB

ELAB

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Off-axis ‘magic’ ( D.Beavis at al. BNL Proposal E-889)

NuMI beam can produce 1-3 GeV intense beams with well defined energy in a cone around the nominal beam direction

2

2

22 412

zA

Fluxγ

γ⎟⎠

⎞⎜⎝

⎛+

=

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The Off-axis Advantage

The dominant oscillation parameters are known reasonably well

One wants to maximize flux at the desired energy (near oscillation maximum)

One wants to minimize flux at other energies One wants to have narrow energy spectrum

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Optimization of off-axis beam

Choose optimum E (from L and m232)

This will determine mean E and LAB from the 90o CM decay condition

Tune the optical system (target position, horns) so as to accept maximum meson flux around the desired mean E

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e transition equation

A. Cervera et al., Nuclear Physics B 579 (2000) 17 – 55,

expansion to second order in 1212131223,,,LA

P ( e) = P1 + P2 + P3 + P4

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Several Observations First 2 terms are independent of the CP violating

parameter The last term changes sign between and If 13 is very small (≤ 1o) the second term (subdominant

oscillation) competes with 1st For small 13, the CP terms are proportional to 13; the first

(non-CP term) to 132

The CP violating terms grow with decreasing Efor a given L)

There is a strong correlation between different parameters CP violation is observable only if all angles ≠ 0

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13 Issue

The measurement of 13 is made complicated by the fact that

oscillation probability is affected by matter effects and possible CP violation

Because of this, there is not a unique mathematical relationship between oscillation probability and 13

Especially for low values of 13, sensitivity of an experiment to seeing

e depends very much on

Several experiments with different conditions and with both and will be necessary to disentangle these effects

The focus of next generation oscillation experiments is to observe e transition

13 needs to be sufficiently large if one is to have a chance to

investigate CP violation in sector

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Matter Effects The experiments looking at disappearance measure

m232

Thus they cannot measure sign of that quantity ie determine mass hierarchy

The sign can be measured by looking at the rate for e for both and .

The rates will be different by virtue of different e-e- CC interaction in matter, independent of whether CP is violated or not

At L = 750km and oscillation maximum, the size of the effect is given by A = 2√2 GF ne E / m23

2 ~ 0.15

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Source of Matter Effects

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CP and Matter Effects

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Experimental Challenge

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e Appearance: Experimental challenges

•Need to know the expected flux•Need to know the beam contamination•Need to know the NC background* rejection power (Note: need to beat it down to the level of e component of the beam only)•Need to know the electron ID efficiency

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Detector(s) Challenge

Surface (or light overburden) High rate of cosmic ’s Cosmic-induced neutrons

But: Duty cycle 0.5x10-5

Known direction Observed energy > 1 GeV

Principal focus: electron neutrinos identification

• Good sampling (in terms of radiation/Moliere length)

Large mass:

• maximize mass/radiation length

• cheap

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Receipe for a Better e Appearance Experiment

More neutrinos in a signal region Less background Better detector (improved efficiency, improved

rejection against background) Bigger detector

Lucky coincidences:

• distance to Soudan = 735 km, m2=0.025-0.035 eV2

• Below the tau threshold! (BR(->e)=17%)

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NuMI Off-axis Detector

The goal is an eventual 50 kt fiducial volume detector

Liquid scintillator strips readout by APDs with particle board absorber is the baseline design

Backup design is glass RPCs Location is 810 km baseline, 12 km off-axis (Ash

River, MN) Present cost is about 150 M$

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The off-axis detector: Stacks

Liquid scintillator strips

1.2 m × 3 cm × 14.4 m

30-cell extrusions

797 planes

570 k channels

28.8 m

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The absorbers

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The active detectors: scintillators

0

200

400

600

800

1000

0 100 200 300 400

Channel number

Performance of BC517L with

Compton edge of 137Cs gammas

Far right: MINOS

Middle: fresh BC517L

Far left: 5-year old BC517L

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The active detectors: WLS fibers

0

20

40

60

80

100

120

140

0 20 40 60 80

Relative pulse height

0 pe

1 pe

2 pe

3 pe

4 pe

Pulse height spectra

of cosmic ray muons

7.5 m from end

1.0 mm WLS fibers

HPD photodetector

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The DAQ system

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CC e vs NC events in a tracking calorimeter: analysis example

Electron candidate: Long track ‘showering’ I.e. multiple

hits in a road around the track

Large fraction of the event energy

‘Small’ angle w.r.t. beam NC background sample

reduced to 0.3% of the final electron neutrino sample (for 100% oscillation probability)

35% efficiency for detection/identification of electron neutrinos

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A ‘typical’ signal event

Fuzzy track = electron

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A ‘typical’ background event

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Sources of the e background

At low energies the dominant background is from +e+

+e+ decay, hence

K production spectrum is not a major source of systematics

e background directly related to the spectrum at the near detector

All

K decays

e/ ~0.5%

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Sensitivity dependence on eefficiency and NC rejection

Major improvement of sensitivity by improving ID efficiency up to ~50%

Factor of ~ 100 rejection (attainable) power against NC sufficient

NC background not a major source of the error, but a near detector probably desirable to measure it

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Sensitivity dependence on e efficiency and NC rejection

Major improvement of sensitivity by improving ID efficiency up to ~50%

Factor of ~ 100 rejection power against NC sufficient

NC background not a major source of the error, but a near detector probably desirable to measure it

Sensitivity to ‘nominal’ |Ue3|2 at the level 0.001 (phase I) and 0.0001 (phase II)

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Off-axis potential

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Numerology: My perspective

A 20-kton detector 712 km from Fermilab, 9 km off axis will have order NCC =10,000 muon-type charged-current interactions in 5 years of running

There will be order NNC =4,000 neutral current interactions in the same exposure.

Software will suppress neutral current interactions with a rejection factor of order R=500 and an efficiency of order 30%.

There will be order 3% x NCC=300 genuine electron-type CC interactions. These events will be reduced to same order as NC fakes due to its broad energy spectrum.

Cosmic background is negligible. The signal to noise (background fluctuation) for P=0.001

SB

≈0.001×10,000×0.3

≈2(4,000/ 500)≈

316

≈11

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Letter of Intent (LOI)

A Letter of Intent has been submitted to Fermilab in June expressing interest in a new effort using off-axis detector in the NuMI beam

This would most likely be a ~15 year long, 2 phase effort, culminating in study of CP violation

The LOI was considered by the Fermilab PAC at its Aspen July, 2002, meeting

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Fermilab Official Reaction

Given the exciting recent results, the eagerly anticipated results from the present and near future program, and the worldwide interest in future experiments, it is clear that the field of neutrino physics is rapidly evolving. Fermilab is already well positioned to contribute through its investment in MiniBooNE and NuMI/MINOS. Beyond this, the significant investment made by the Laboratory and NuMI could be further exploited to play an important role in the elucidation of 13 and the exciting possibility of observing CP violation in the neutrino sector. ( June 2002, PAC Recommendation)

We will encourage a series of workshops and discussions, designed to help convergence on strong proposals within the next few years. These should involve as broad a community as possible so that we can accurately guage the interest and chart our course. Understanding the demands on the accelerator complex and the need for possible modest improvements is also a goal. Potentially, an extension of the neutrino program could be a strong addition to the Fermilab program in the medium term. We hope to get started on this early in 2003. Michael Witherell

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The Next Steps/Schedule

Workshop on detector technology issues planned for January, 2003 (done)

Proposal to DOE/NSF in early 2003 for support of R&D (done) and subsequent construction of a Near Detector in NuMI beam to be taking data by early 2005

Proposal for construction of a 25 kt detector in late 2004

Site selection, experiment approval, and start of construction in late 2005

Start of data taking in the Far Detector in late 2007 Formation of an international collaboration to construct

a 50 kton detector

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Concluding Remarks

Neutrino Physics appears to be an exciting field for many years to come

Most likely several experiments with different running conditions will be required

Off-axis detectors offer a promising avenue to pursue this physics

NuMI beam is excellently matched to this physics in terms of beam intensity, flexibility, beam energy, and potential source-to-detector distances that could be available

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Scaling Laws (2) If 13 is small, eg sin2213 < 0.02, then CP violation effects obscure matter

effects Hence, performing the experiment at 2nd maximum (n=3) might be a best way

of resolving the ambiguity Good knowledge of m23

2 becomes then critical Several locations (and energies) are required to determine all the parameters

Detector L(km) E(GeV) Relative

matter effect

Relative

CP effect

JHF 295 0.6 1.0 1.0

NuMI Phase I 712 1.4 2.9 1.0

NuMI Phase II 985 0.7 1.1 3.0

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Important Reminder

Oscillation Probability (or sin22e) is not unambigously

related to fundamental parameters, 13 or Ue32

At low values of sin2213 (~0.01), the uncertainty could

be as much as a factor of 4 due to matter and CP effects Measurement precision of fundamental parameters can

be optimized by a judicious choice of running time between and

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Sensitivity for Phases I and II (for different run scenarios)

We take the Phase II to have25 times higher POT x Detectormass

Neutrino energyand detector distance remainthe same

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An example of a possible detector

Low Z tracking calorimeter

Issues: absorber material (plastic? Water? Particle

board?) longitudinal sampling (X0)? What is the detector technology (RPC?

Scintillator? Drift tubes?) Transverse segmentation (e/0) Surface detector: cosmic ray background?

time resolution?

NuMI off-axis detector workshop: January 2003

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Background rejection: beam + detector issue

e background

NC (visible energy), no rejection

spectrumSpectrum mismatch: These neutrinos contribute to background, but no signal

e (|Ue3|2 = 0.01)

NuMI low energy beam

NuMI off-axis beam

These neutrinos contribute to background, but not to the signal

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Fighting NC background:the Energy Resolution

M. Messier, Harvard U.

Cut around the expected signal region to improve signal/background ratio

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Scaling Laws (CP and Matter)

Both matter and CP violation effects can be best investigated if the dominant oscillation phase is maximum, ie = n/2, n odd (1,3,…)

Thus E L / n For practical reasons (flux, cross section)

relevant values of n are 1 and 3 Matter effects scale as 13

2E or 132 L/n

CP violation effects scale as 13 m122 n

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CP/mass hierarchy/13

ambiguity

Neutrinos only, L=712 km, E=1.6 GeV, m232 = 2.5

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Kinematics Quantitatively

11/28/03 Kwong Lau p.46Antineutrinos help greatly

Antineutrinos are crucial to understanding: Mass hierarchy CP violation CPT violation

High energy experience: antineutrinos are ‘expensive’.

For the same number of POT

Ingredients: (+)~3(-) (large x)

NuMI ME beam energies:

(+)~1.15(-) (charge conservation!)

Neutrino/antineutrino events/proton ~ 3

(no Pauli exclusion)

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2 Mass Hierarchy Possibilities

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Two Most Attractive Sites

Closer site, in Minnesota About 711 km from Fermilab Close to Soudan Laboratory Unused former mine Utilities available Flexible regarding exact location

Further site, in Canada, along Trans-Canada highway About 985 km from Fermilab There are two possibilities:

About 3 km to the west, south of Stewart Lodge About 2 km to the east, at the gravel pit site, near compressor

station

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Location of Canadian Sites

Stewart Lodge Beam Gravel Pit

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Medium Energy Beam

More flux than low energy on-axis (broader spectrum of pions contributing)

Neutrino event spectra at putative detectors located at different transverse locations

Neutrinos from K decays

A. Para, M. Szleper, hep- ex/0110032

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How antineutrinos can help resolve the CP/mass hierarchy/13 ambiguity

L=712 km, E=1.6 GeV, m232 = 2.5

Neutrino rangeAntineutrino range

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Det. 2

NuMI Beam: on and off-axis

Det. 1

•Selection of sites, baselines, beam energies•Physics/results driven experiment optimization