Prospects for NUMI Offaxis Initiative

1
Prospects for NUMI Off-axis Initiative

• Kwong Lau
• University of Houston
• November 28, 2003

2
Outline

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

3
Introductory Comments
The current generation of long and medium
baseline terrestial n oscillation experiments is
designed to

• Confirm SuperK results with accelerator ns (K2K)
• Demonstrate oscillatory behavior of nms (MINOS)
• Make precise measurement of oscillation
  parameters (MINOS)
• Improve limits on nm?ne subdominant oscillation
• Demonstrate explicitly nm?nt oscillation mode by
• detecting nts (OPERA, ICARUS) mode, or
  detect it (MINOS, ICARUS)
• Resolve the LSND puzzle (MiniBooNE)
• 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 nm?ne
• Measurement of q13
• Determination of mass hierarchy (sign of Dm23)
• Search for CP violation in neutrino sector
• Measurement of CP violation parameters
• Testing CPT with high precision

5
Kinematics of p Decay
Compare En spectra from 10,15, and 20 GeV ps

• Lab energy given by length of vector from origin
  to contour
• Lab angle by angle wrt vertical
• Energy of n is relatively independent of p energy
• Both higher and lower p energies give ns of
  somewhat lower energy
• There will be a sharp edge at the high end of the
  resultant n spectrum
• Energy varies linearly with angle
• Main energy spread is due to beam divergence

EnLAB
qLAB
6
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
7
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

8
Optimization of off-axis beam

• Choose optimum En (from L and Dm232)
• This will determine mean Ep and qLAB from the 90o
  CM decay condition
• Tune the optical system (target position, horns)
  so as to accept maximum p meson flux around the
  desired mean Ep

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nm ? ne transition equation
P (nm ? ne) P1 P2 P3 P4

A. Cervera et al., Nuclear Physics B 579 (2000)
17 55, expansion to second order in
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Several Observations

• First 2 terms are independent of the CP violating
  parameter d
• The last term changes sign between n and n
• If q13 is very small ( 1o) the second term
  (subdominant oscillation) competes with 1st
• For small q13, the CP terms are proportional to
  q13 the first (non-CP term) to q132
• The CP violating terms grow with decreasing En
  (for a given L)
• There is a strong correlation between different
  parameters
• CP violation is observable only if all angles ? 0

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q13 Issue

• The measurement of q13 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 q13
• Especially for low values of q13, sensitivity of
  an experiment to seeing nm?ne depends very much
  on d
• Several experiments with different conditions and
  with both n and n will be necessary to
  disentangle these effects
• The focus of next generation oscillation
  experiments is to observe nm?ne transition
• q13 needs to be sufficiently large if one is to
  have a chance to investigate CP violation in n
  sector

12
Matter Effects

• The experiments looking at nm disappearance
  measure Dm232
• Thus they cannot measure sign of that quantity
  ie determine mass hierarchy
• The sign can be measured by looking at the rate
  for nm?ne for both nm and nm.
• The rates will be different by virtue of
  different ne-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 2v2 GF ne En /
  Dm232 0.15

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Source of Matter Effects
14
CP and Matter Effects
15
Experimental Challenge
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ne 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 ne 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 ms
• Cosmic-induced neutrons
• But
• Duty cycle 0.5x10-5
• Known direction
• Observed energy gt 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 ne 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, Dm20.025-0.035 eV2
• Below the tau threshold! (BR(t-gte)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

20
The off-axis detector Stacks
28.8 m
21
The absorbers
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The active detectors scintillators
23
The active detectors WLS fibers
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The DAQ system
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CC ne 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 ne background
All
ne/nm 0.5
K decays

• At low energies the dominant background is from
  m?enenm decay, hence
• K production spectrum is not a major source of
  systematics
• ne background directly related to the nm spectrum
  at the near detector

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Sensitivity dependence on neefficiency
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
30
Sensitivity dependence on ne 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 Ue32 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
  R500 and an efficiency of order 30.
• There will be order 3 x NCC300 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
  P0.001

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Letter of Intent (LOI)

• A Letter of Intent has been submitted to Fermilab
  in June expressing interest in a new n 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

34
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 q13 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
  RD (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 q13 is small, eg sin22q13 lt 0.02, then CP
  violation effects obscure matter effects
• Hence, performing the experiment at 2nd maximum
  (n3) might be a best way of resolving the
  ambiguity
• Good knowledge of Dm232 becomes then critical
• Several locations (and energies) are required to
  determine all the parameters

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Important Reminder

• Oscillation Probability (or sin22qme) is not
  unambigously related to fundamental parameters,
  q13 or Ue32
• At low values of sin22q13 (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 n and n

39
Sensitivity for Phases I and II (for
different run scenarios)
We take the Phase II to have 25 times higher
POT x Detector mass Neutrino energy and
detector distance remain the same
40
An example of a possible detector
Low Z tracking calorimeter
NuMI off-axis detector workshop January 2003

• Issues
• absorber material (plastic? Water? Particle
  board?)
• longitudinal sampling (DX0)?
• What is the detector technology (RPC?
  Scintillator? Drift tubes?)
• Transverse segmentation (e/p0)
• Surface detector cosmic ray background? time
  resolution?

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Background rejection beam detector issue
n spectrum
NC (visible energy), no rejection
Spectrum mismatch These neutrinos contribute to
background, but no signal

• ne background

ne (Ue32 0.01)
NuMI low energy beam
NuMI off-axis beam
These neutrinos contribute to background, but not
to the signal
42
Fighting NC backgroundthe Energy Resolution
Cut around the expected signal region to improve
signal/background ratio

• M. Messier, Harvard U.

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Scaling Laws (CP and Matter)

• Both matter and CP violation effects can be best
  investigated if the dominant oscillation phase f
  is maximum, ie f np/2, n odd (1,3,)
• Thus En a L / n
• For practical reasons (flux, cross section)
  relevant values of n are 1 and 3
• Matter effects scale as q132En or q132 L/n
• CP violation effects scale as q13 Dm122 n

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CP/mass hierarchy/q13
ambiguity
Neutrinos only, L712 km, En1.6 GeV, Dm232 2.5
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Kinematics Quantitatively
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Antineutrinos help greatly

• Antineutrinos are crucial to understanding
• Mass hierarchy
• CP violation
• CPT violation
• High energy experience antineutrinos
  are expensive.

Ingredients s(p)3s(p-) (large x)
For the same number of POT
NuMI ME beam energies s(p)1.15s(p-) (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
A. Para, M. Szleper, hep- ex/0110032
More flux than low energy on-axis (broader
spectrum of pions contributing)
Neutrinos from K decays

• Neutrino event spectra at putative detectors
  located at different transverse locations

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How antineutrinos can help resolve the CP/mass
hierarchy/q13 ambiguity
Antineutrino range
Neutrino range
L712 km, En1.6 GeV, Dm232 2.5
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NuMI Beam on and off-axis
Det. 2
Det. 1

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