Intense Muon Beams And Neutrino Factories

BNL - 67823 CAP–292–Muon-OOC ..

INTENSE MUON BEAMS AND NEUTRINO FACTORIES

Brookhaven

Zohreh Parsa National Laboratory,

New York

K?’
October 2000

Pres. at Sth International Conference on the Physics Potential Colliders, Dec. 15 – 17, 1999, San Francisco, California.

& Development

of Mu+Mu–

INTENSE MUON BEAMS AND NEUTRINO FACTORIES
Zohreh Parsa
Br-ookhaven National Laboratory (BNL)* Physics Department 510 A, Upton, NY 11973-5000, USA

Abstract. High intensity muon sources are needed in exploring neutrino factories, Iepton flavor violating muon processes, and lower energy experiments as the stepping phase towards building higher energy p + p – colliders. We present a brief overview, sketch of a neutrino source, and an example of a muon storage ring at BNL with detector(s) at Fermilab, Sudan, etc. Physics with low energy neutrino beams based on muon storage rings (pSR) and conventional Horn Facilities are described and compared. CP violation Asymmetries and a new Statistical Figure of Nlerit to be used for comparison is given. Improvements in the sensitivity of low energy experiments to study Flavor changing neutral currents are also included.

INTRODUCTION

There is a great deal of interest in high-intensity muon beams and their use in high luminosity muon colliders, neutrino factories, rare interactions and decays of muons. The theoretical interest to explore lepton flavor violating muon processes is motivated in part by supersymmetric grand unified models of particle interactions and ideas to make substantial improvements in the sensitivity of experiments to study rare muon processes.
A muon collider requires as its starting point, a very intense beam of muons with a small momentum spread. Such beams would be accelerated to collider energies and be used to search for new short distance high energy phenomena. A muon storage ring based neutrino factory is a natural path to muon collider technology, since both facilities share essentially the same subcomponents prior to the storage ring. In section 2, we briefly describe the physics and production of high-intensity muon and neutrino beams. An example of a neutrino source at BN’L is described in section 3, and a comparison of conventional Horn beam and muon storage produced

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,. ..

beam are given in section 4. In addition, a survey of low energy muon physics including muon number non-conservation, and current bounds are given in section 5, a summary in section 6 and references in section 7.

INTENSE

MUON

BEAMS - NEUTRINO PHYSICS

SOURCES -

A muon storage ring based N’eutrino Source (iXeutrino Factory) beside providing a first phase of a muon collider facility, it would generate more intense and well collimated neutrino beams than currently available. The BN-L-.AGS or some other proton driver would provide an intense proton beam that hits a target, produces pions that decay into muons. The muons must be cooled, accelerated and injected into a storage ring with a long straight section where they decay. ”The decays occurring in the straight sections of the ring would generate neutrino beams that could be directed to detectors located thousands of kilometers away. allowing studies of neutrino oscillations with precision not currently accessible.
The composition and spectra of an intense neutrino beam from a muon storage ring depends on momentum, polarization and charge of the stored muons, through the decays p- + e–uPDe or p+ + e+ fiPu,.
Table 1 illustrates (a summary of the standard model). spectrum of neutrinos with some of their basic properties. The fermions are grouped into three generations of spin 1/2 leptons and quarks which span an enormous mass range.
The idea of muon storage rings has been discussed since at least 1960 [1]. However, storage rings with enough circulating muons to provide higher intensity neutrinos than from conventional horn beams have only been considered more recently,

TABLE 1. Neutrinos and Their Properties

Symbol Ve e u d
“P P c s
VT T
t b

Spin Charge

1/2

o

1/2

-1

1/2

2/3

1/2

-1/3

1/2

o

1/2

-1

1/2

2/3

1/2

-1/3

1/2

o

1/2

-1

1/2

2/3

1/2

-1/3

Color 0 0 3 3
0 0 3 3
0 0 3 3

hlass (Gel-)
<4.5 x 10–9 0.51 x 10-3
5 x 10–3 9 x 10-3

First Generation

<.16 X 10-3 0.106 1.35 0.175

Second Generation

<2.4 x 10–3 1.777
174.3 + 5.1 4.5

Third Generation

..
in the context of muon collider technology [7]. The neutrino fluxes from the proposed muon-based beams would be higher than
ever previously achieved with a much better-understood flavor composition. In addition, since the neutrino beams from these sources would be secondary beams from high energy muon decays, they would be extremely well collimated. Distances between production and detection could, therefore span the globe. Using the precisely known flavor composition of the beam, one could envision an extensive program to measure the neutrino oscillation mixing matrix, including possible CP violating effects. The number of neutrino interactions per unit mass of a detector at distance L from a muon storage ring operating at energy Eu scales as
(1)
for the example of a proton source with 1.5 IvIJV power, in one year (107 s) of operation, there would be about 4 x 1020 muons per year decaying in the storage ring. Assuming the fraction of the ring pointing to a given detector to be about 0.25 (as in example of a bowtie-shaped muon storage) then the number of decays pointing to the given detector will be approximately 1020. It may be noted that the number of events with the 1.5 NNY neutrino factory, in a detector at the same 730 km, is approximately 100 times that in the proposed CERN - Gran Sasso experiment (N’GS) [9], and about 40 times the maximum event rate that hHiNOS [10] can expect. Upgrading the proton driver to 4 LIW, the factors become about 300 and 100 for Gran Sasso and Soudan, respectively.
At the detectors, the neutrino and the antineutrino may or may not have changed their flavor, leading to the appearance of a different flavor or the disappearance of the initial flavor, respectively. W’hen detected by a charged-current interaction, there are 6 classes of signatures in a three-neutrino model:
1) VP -+ v, + e- (appearance);
2) VP-+ VP+ p- (disappearance);
3) VV-+ v, -+ ~- (appearance);
4) D, + V, + e+ (disappearance);
5) V, -+ lJP + p+ (appearance):
6) ~. -+ ~, -+ ~+ (appearance).
For operation with positive muons, a similar list of processes may be written. The 5th process where a muon of different sign from the parent muon appears, has a very unique possibilities at a neutrino factory based on muon storage rings. Since they are the only sources of intense high energy electron (anti) neutrino beams. The ~ appearance (cases 3 and 6) are practical only for neutrino beams with 10’s of GeV energy.
Lluon Storage rings of e.g., Eu = 50,20,10, lGel’ have been considered and all else equal, higher energy is better (but the cost may be higher?). The high

..

cost estimates of a 50GeV ring has led to recent studies of lower energy rings. Experiments carried out at a neutrino factory within the next decade can add compelling new information to our understanding of neutrino oscillations, if the number of useful muon decays exceeds 1019 per year, and energy is ~ 20 Gelr.

Neutrino Source at BNL
As known, the BNL-AGS proton beam parameters are very suited for use as a source for muon storage ring based neutrino factory and muon collider. Table 2 illustrates basic BNL-AGS proton beam properties.
With a muon storage ring - neutrino source at BNL (Fig. 1), detectors at Fermilab or Soudan, Minnesota (1715 km), become very interesting possibilities. The feasibility of constructing and operating such a muon-storage-ring based NeutrinoFactory, including geotechnical questions related to building non-planar storage rings (e.g. for BNL-fermilab; at 8° angle for BNL-Soudan, and 31° angle for BNLGran Sasso) along with the design of the muon capture, cooling, acceleration, and storage ring for such a facility is being explored by our growing Neutrino Factory and hluon Collider Collaboration (NFNICC), but requires additional studies for a BNL site specific example.
Conventionally, neutrino beams employ a proton beam on a target to generate pions, which are focused and allowed to decay into neutrinos and, muons [10]. The muons are stopped in the shielding, while the muon-neutrinos are directed toward the detector. In a neutrino factory, pions are made the same way and allowed to decay, but it is the decay muons that are captured and used. The initial neutrinos from pion decay are discarded, or used in a parasitic low-energy neutrino experiment. But the muons are accelerated and allowed to decay in a storage ring with long straight sections. It is the neutrinos from the decaying muons (both muon-neutrinos and anti-electron-neutrinos) that are directed to a detector.
Figure 1 shows schematics of space angles [16] and baselines for example of a muon storage neutrino source at BNL, with detectors (placed at Fermilab; Soudan; hlinnesota (1715 km); or Gran Sasso, Italy (6527 km)) at various global locations.
In a Neutrino Factory, a proton driver of moderate energy (< 50 GeV) and high

TABLE 2. BNL- AGS Proton Beam Properties

Parameters
Proton Energy Proton/Bunch Bunch No. Proton/cycle Bunch Length Bunch spacing

[GeV]
[,us] [ns]

BNL-AGS
24 1.6 X 1013
6 1.0 x 101J
2.2 440

Lluon Collider
16-24 5 x 1013
2 1.0 x 1014
1 1000

,. ..
1’ Ring at BNL
FIGURE 1. Shows space angles and baselines for a h[uon - Storage Ring at BNL and possible detector sites (at Fermilab, Sudan, CERN, Kamioka and Gran Saso).

..
average power, (e.g., 1-4 NfJV), similar to that required for a muon collider. but with a less stringent requirements on the charge per bunch and power is needed. This is followed by a target and a pion-muons capture system. .\ longitudinal phase rotation is performed to reduce the muon energy spread at the expense of spreading it out over a longer time interval. The phase rotation system may be designed to correlate the muon polarization with time, allowing control of the relative intensity of muon and anti-electron neutrinos. Some cooling may be needed, to reduce phase space, about a factor of 50 in six dimensions. This is much smaller than the factor of 106 needed for a muon collider. Production is followed by fast muon acceleration to 50 GeV (for example), in a system of linac and two recirculating linear accelerators (RL.~’s), which maybe identical to that for a first stage of muon collider such as a Higgs Factory. .4 muon-storage ring with long straight sections could point to one or more distant neutrino detectors for oscillation studies, and to one or more near detectors for high intensity scattering studies.
.~ planar bowtie - shaped ring can be designed and oriented to send neutrino beams to any two detector sites. Since, there is no net bending, the polarization may be preserved. (.A disadvantage of the Bowtie - shaped ring is that it may need extra bending. Since there is geometry constrains on the ratio of short to long straight sections, the ring circumference may increase. ) \l-ith the ring in a tilted plane, both long straight sections would point down into the earth, such that neutrinos can be directed into two very distant detectors. Triangular-shaped storage rings also have this advantage.
Figure 2 illustrates a schematic concept of a iNeutrino Factory Facility based on a bowtie muon storage lattice. The examples described are based on some of the scenarios being explored by our XFklCC, [4].
A neutrino factory has a strong independent physics case. It would be easier to build, less expensive than a full muon collider, and could demonstrate most of the components of a collider.
Table 3, gives charged current neutrino interaction rates (per kiloton-year) as a function of baseline length L for an l?~ = 50 GeV muon storage ring in which there are 1 x 1020 unpolarized muon decays per year }~~ithin a neutrino beam-forming straight section [17]. The rates are listed for oscillations:
1) V, + Vp: Am 2 –– 3.5 X 10-3 eV2/c~ & sin220 = 0.1,
2) v. + Vp: Am 2 = 1 x 10–J e\-2/c~ & sin2 2f9 = 1,
3) v. + v,: Am2 = 3.5 x 10-3 e\”2/c4 & sin22f3 = 0.1,
4) VP + v,: Jm 2 – 3.5 X 10–3 eV2/c~ & sin2 20 = 1.
The rates for the unoscillated neutrino interactions, the corresponding statistical significance of the disappearance signal (numbers in parenthesis). and the rates for the antineutrino interactions, are also included in Table 3.
If future experiments confirm the interpretation of the LSXD data that there exist more than three light neutrinos, then use of the neutrino factory flavor-rich

,. ..

4r phase rotation No. 1
42 m rf

drift 160 m

phase rotation No.2

nT

recirculator Lirmc ?-8GeV
recirculator Lime 8.50 Gev

oi,/ ‘, )
D\

proton driver target mini-cooling
3.5 m Hydrogen cooling 80 m Linac 2GeV
storage rinoe 50 GeV 900 m circumference

neutrin~~ beam

neutrin~> k:lnl

FIGURE lattice

2. Overview of a Neutrino

Factory

Concept,

with a Bowtie shaped

Muon - Storage

.. ..

TABLE tory.

3. Neutrino

Interaction

Rates at a N-eutrino Fac-

BNL G. Sasso
6527

BNL SL.AC 4139

BNL Soudan
1715

90 1400 (2.40) 890 5 ~ ~o-2
1500 890
31 1400 (2.4?) 890
450
760 (350) 770

160 3600 (2.7U) 2200
0.86 3800 2200
60 3700 (2.7cr) 2200”
570 3100 (230) 1900

190 16000 (1.5a) 9300
1.5 16000 9400
70 1.6 X 104
(1.50) 9400
630 1.7’ x lo’~
(120) 8100

beams would be even more crucial, because the parameter space for CP/T violating effects would be considerably enlarged and could be explored in experiments with such beams [20].

LOW ENERGY

v FACTORIES AND CONVENTIONAL HORN BEAMS

An alternative source of intense muons are the conventional Horn Beams (very efficient) which seems to be not only competitive with the lower energy muon storage rings (&IR) but also at a lower cost. Low energy neutrino physics is a very competitive way to study neutrino oscillations. E.g., with VP + .ve or .v~ -+ VP. measure sin2 [email protected],CP violation, etc. [15]: CP Violation Asymmetries

A(L)~p

P(L).,+., =
P(q.=+v#

– P(L)pe+vp
+ F’(L)v,+v#

—— F’([email protected]+v, – P(L)[email protected]+ve

(2)

w)..+% + wqi7u+i7e “

Statistical Figure of Merit (F)

~ = A2(N+X)

1 – ,42

(3)

.

.. ..

which grows with E“ if all else is left equal. Where N = # of Ve + VP, ~ = # of

P.+vP,A=~,and~=&

AJm.

For example, using the BNL- AGS EP = 28GeV if one considers the BNL P889

study [14], and increase the number of proton on target (p.o. t.) e.g., from 2 x

1020 p.o.t. /yr to 6 x 1021 p.o.t. /yr one gains Vk + p–, at L = lkrn no oscillation,

a factor of 30, E~~r ”~ ~

lGe}”, with

NP- m 4.43x 10-15 /kTon/p.o.t. x6x 1021p.o.t./yr m 2.7x 107/kTon/yr, (Fiducial

Llass + 1/2).

This is Comparable to a muon storage ring with EP R 10 Gel’, 2 x 1020 p decay s/yr,

with L = 10 km; Vti + p– or De + e+; and NU- ~ 1.5 x 107/kTon/yr.-. That is, (we have about the same event rates), with the same p.o.t.

and de-

tector size, 1 GelT v~ak Horn R 10Get” p Storage Ring (statistically, if L/E is

fixed). With upgraded proton source (e.g., 6 x 10IJ p.o.t./yr ), larger detectors (e.g., 45kTon, (preferable > 500kTon)), at about 300km, with low energy VP and VP from a Horn (e.g. 300 times the BNL-P889 study) one should measure 013, 02~ (High precision), Am2~l and will have some CP violation capability (e.g., one should measure sin22f?13 >0.007 which is impressive).
If we increase L to 10 or 100 times longer, in the example of the BNL-P889 study with Detectors at 1, 3, 24, 68 km, then with 340 times the P889 event rate at L = 260 km most of VP -+ VT (Am~l N 3 x 10-3 eLT2), number of Vv + v, + e-, (5 x 104 sin2t923 sin22013 B 175 events, (sin22013 > 0.007)), and 30 measurements of Acp = 0.15 * 0.05. Upgraded VP, VP Horn Facility potentially is powerful

[15]. However, there are some questions (targetrv . R&zD issues) that needs to be addressed. E.g., how high intensity can a conventional horn tolerate? How about using a solenoid magnet as a focusing device? Solenoid may be able to handle the high intensity proton currents better?
Similar upgrades for hlINOS maybe possible. E.g., a followup efforts (hIIINOSII), could include doing muon-electron neutrino oscillations and CP violation by (upgrading the Fermilab 8 GeV Booster) intensifying their beam and doing low energy option.
stop T+ + Vp Vevp EP N 30 JIek” (LSND, KARhlEN) and Neutron Spallation Sources are also alternatives that may be considered but will not be discussed here.

LOW ENERGY MUON PHYSICS - A SURVEY
Using intense muon beams to carry forefront low energy research include the search for muon - number non - conservation, such as
p+ -+ e-y; p+ + e+e-e+; p-N + e-N.