37 OCR Output

F. Pietropaolo. L. Rolandi. R. Santacesaria. S. Schlenstedt, F. Vanucci and K. Winter.

A. Eredidato, E. Fernandez, _I. Gomez, A.B. Kaidalov, P.F. Lovere, V. Palladino,Working group on neutrino physics: L. Camilleri, A. Capone, A. De Rujula, LI. Dore,

have given negative results. A search in a beam dump experiment with vu and we at

UN ·=* T_X

reactions which produce a tau-lepton

identity with we or uu can indeed be excluded. Searches for charged current neutrino

guishes it from the electron- and the muon-neutrino? The possibility of a complete

Xmay be a neutrino. Is it a sequential neutrino, with a flavour property which distin

analysing semi-leptonic tau—deca)s with multipion final states [5]. Hence, the particle

ticle X is a spin l /2 particle. Its mass has been constrained to less than 35 MeV by

The muon energy spectrum has the familiar [2] Michel shape suggesting that the par

1:- Q u_v,,X.

decays [4]. e.g.

Further indirect evidence about the tau—neutrino can be obtained from leptonic tau

same lepton flavour. the tau-neutrino.

natural assignment of the "up" state (I3 = +1/2) would be to a neutrino with the

leptons as the "down" state (I3 = -1 /2) of a weak isospin doublet. Hence, the most

was deduced in agreement with ga = -1 /2 for the assignment of lel’t—handed tau

a value of the axial-vector neutral current coupling constant of ga = — 0.—l5 1 0.05

e+e* +* ft

ward angular asymmetry of tau produced in e+e` annihilations [3], [4]

lepton belonging to a third lepton Family [2]. From measurements of the forward-bacl<

rators [1 ], all evidence today favours the hypothesis that it is a sequential heavy

Following the discovery of the tau-lepton in e+e` collisions by M. Perl and collabo

I. Introduction

CERN. Geneva. Switzerland

Klaus Winter

AT me LI·IC*

DETECTION OF THE TAIPNEIITRINO

38 OCR Output

decay path of ~i cm. The majority of the tau decay modes (86%) produce a single

them. With a mean tau—neutrino event energy of 300 GeV at LHC we expect a mean

ground. The short lifetime itseIF is an unambiguous method to detect and to identify

ance due to undetected neutrinos is not possible in the presence of a large back

has shown that their identificaton by kinematics and by transverse momentum unbal

Previous experience with the detection of short-lived particles, e.g. of charm-mesons

2. The Concept of Tau-Neutrino Detection

bilities have arisen which will be discussed in the Following sections.

in the LISSR. With the advent of high energy pp colliders (LHC and SSC) new possi

proton accelerator UNK which is presently under construction at Protvino (Serpukhov)

A new experiment [9] has been designed to detect the tau-neutrino at the 3TeV

[8] have been indefinitely deferred.

GeV Tevatron the situation would be slightly more favourable. However, the experiments

trinos is too low at the CERN proton energy of 450 GeV to be detectable. At the 800

meson (cs) into tvt. The contribution of the latter channel which produces tau-neu

there are semiieptonic decays of the D meson (cd) into ve and vu and of the Ds

mesons (lifetime ~3‘i0`s) are not suppressed. Among the known decay channelsl3

are suppressed by a Factor of ~l0‘. Neutrinos From decays of short—lived charm

Because of their strong absorption in copper, decays of long-lived mesons (rc and K)

this type of experiment the proton beam is absorbed by a long block of copper [6].

so—called beam dump experiments have been performed at the 450 GeV CERN—SPS. ln

All attempts to directly observe the reaction have failed up to now. Searches in

vTN - t" X.

current reaction

answered by yes or no. Its existence can be proved by direct observation of the charged

Does the tau-neutrino exist as a particle? Surprisingly, this question cannot be

tau—neutrin0 is distinct from ve and v

time and leptonic branching ratio measurements [3] [4], these results show that the

0.0021 and on the coupling constant oF vp and t of 0.073. Together with the life

muon-neutrino beam at FNAL [7] gave limits on the coupling constant of vn and t of

CERN [6] excluded the identity ve E vt by six standard deviations. A search in a

the tau lepton in DQ decay. and is there dominating the expected event rate. OCR Output

has higher energy. on average 300 GeV at LHC energy, because of the Lorentz boost of

which is expected to occur with a branching ratio of ~3%. The secondary vt from t decay

D; -» vt t, 1*» vt + X++

(cc. bb) in proton-proton collisions. The main source of vT is the leptonic decay

Tau neutrinos can be obtained from the production and decay of heavy quark flavors

3. The Tau-Neutrino Beam

and in Chapter 5 the expected performance.

briefly describing the vt beam and the expected vt event rate in Chapter 4 the detector

liquid argon drift chambers [10] for detecting the tau- neutrino. ln Chapter 3, I am

either Silicon micro-strip detectors or scintillating fibres [9], others have developed

bubble chamber as detectors. The CHARM lI·—Zeuthen Collaboration has been considering

to have good time resolution (E, l0 us). This condition eliminates the emulsion and the

so been investigated [IO]. Because of the high muon flux (see Chapter 3) it is essential

detectors. Other detection techniques using e.g. a liquid argon drift chamber have al

exist proven methods: emulsions, holographic bubble chambers or Silicon micro-strip

Detection of short lived particles requires good space resolution (~20 um). There

thods is shown in figure 3 [9]. lt illustrates the basic concept.

is large; a spectacular event with a t + uvnvr decay, simulated by Monte Carlo me

riant with respect to the spread of tau—neutrino energies. The transverse decay angle

tion with mean values of XT = 243 um and X. = 65 um. These values are nearly inva

Figure 2 shows the expected transverse decay length and impact parameter distribu

respect to the event vertex (figure I).

decay length (kink) or a visible transverse impact parameter of a particle track with

For the identification of a tau decay vertex we require instead a visible transverse

and. hence. its lifetime in the rest system cannot be determined.

energy and momentum carried away in tau—decay by the vT the energy of each tau-lepton

a broad distribution because of the wide tau—neutrino energy spectrum. Because of the

small. about 1 O on average and therefore difficult to detect. The decay length hascharged particle. The decay angle (kink) between the tau and the charged particle is

39

accelerator (LINK) with 10protons/s. OCR Output13

may be as high as 4-1034. For comparison we also show the rate at a fixed target

mode we have assumed a mean luminosity of 103’1cm`2s`1 whereas the peak luminosity

1010 protons/s by slow ejection operate in the fixed target mode; for the beam—beam

operation: a gas jet with a density of p~ 4-1014 nucleons/cm3 and a beam dump receiving

detector with a mass of 6 kg/cm2 is given in Table 1 for three different modes of

the rate of v,, and vn induced reactions (from heavy quark decays) in a conventional

detector with a mass of 2 kg/cm2 subtending an angular acceptance of :2.5mrad, and

to the proton direction (see figure 5). The luminosity, the vt event rate for 10’s for athe angular distribution of the vt flux is strongly peaked at small angles with respect

Rujula, Fernandez and Gomez. Because of the energy imparted to the parent DS mesons

Figure #1- shows the energy spectrum of vt at —/s = 16TeV as calculated by De

tests.

fixed target mode at 1/s ~ 120GeV. These predictions must be subjected to experimental

predicted. with o(D+) ~ o(Dd)~o(DS)~ 1 mb at 16 TeV, a factor ~20 higher than in thexp dependence at x ~0, like 1/x and a strong cross section increase with energy is

dalov et al. [12] and by the PYTHIA QCD Monte Carlo program. A very steeply rising

been estimated by a scaling law approach [11], by the quark-gluon string model of Kai

The cross section dependence on w/s and on the Feynman scaling variable xl: has

lisions.

of A. De Rujula and R. Riickl [11], a ti., beam can be produced in —/s z 16 TeV pp col

ISR and at the pp colliders [9]. Llsing the collider mode of LHC, following a suggestion

the cross section for DQ production can be safely estimated from results obtained at the

At the energy of fixed target accelerators (LINK, LHC, SSC) of »/s #-80- 120GeV

41 OCR Output

is smaller than in the longitudinal fibre version. However, the effects of nuclear

and it is expected that the efficiency of tau-lepton detection of this detector version

order to eliminate all prompt events a minimum impact parameter has to be required

characteristic transverse impact parameter distribution with a mean value of 65 jim. In

tional impact parameter technique (see figure 1) the tau-lepton can be identified by its

beam (figure 7). The (fibre) strip dimensions would again be 20 um. Using the conven

and micro strip detectors (Silicon or scintillating fibres) oriented perpendicular to the

In the other version the detector would consist of alternating layers of live targets

the LIA2 scintillating fibre detector at CERN [14].

will be made using image intensifiers and CCD's following the development made for

CERN [13]. The read-out of the transverse image at the end face of the fibre target

coherent bundles of glass capillaries filled with liquid scintillator is under way at

of 20pm diameter oriented in the direction of the neutrino beam. The development of

technique. ln one version (see figure 6) the detector will consist of scintillating fibres

We have considered two versions of a tau neutrino vertex detector using the fibre

-l. The Detector

tX may be possible. except in the gas jet mode.

and a detection efficiency (see Chapter -l-) of E~30% observation of the reaction vTN

Detecting t decays in the leptonic decay it -¢ uvHvT with a branching ratio of 17.8%

S) target mass~6kg/cm2, A@~:2.5 mrad, l07s

4) target mass ~2kg/cm2, A(E)~12.5mrad, 107s

3) slow ejection 1013 protons/s

2) slow ejection 10protons/s10

1) p (jet) ~ L10nucleons/cm14

20'0001038LINK fixed target3

500002500Beam dump2'(fixed iargi-;t>| 2-103

500Gas—_]et” (fixed target) I 2·103

80'0O03800Beam-Beam 1034

{i€Mode L(cm_2s`1) I N(v,)4) I N(v)S)= N(v)

operation modes at the LHC and at LINK.

Comparison of event rates in different

Table 1

42 OCR Output

the possibility to reduce it by passive or magnetic shielding.

concerned with the detector. with its location and with the background of muons and

Further work is required before an experiment can be planned. This work must be

6. Conclusions

possible to detect the tau neutrino.

The efficiency for detection of a decay kink is ~30% [9]. It should therefore be

A full simulation gives a background due to vu and ve of ~S%.

+u + neutrals.(JUN *» MDI? }i

SHN —* p+(D_ + ii" + neutrals)X

production

Background produced by conventional neutrinos (vu, vp) comes mainly from charm

to background ratio of IO to 1 must therefore be achieved.

for a situation in which the reaction may not occur at all or with reduced rate. A signal

Aiming at the detection of the reaction vTN + tX the experiment must be designed

5. Background

shielding remain, however, to be calculated.

after magnetic deflection in the LHC magnet lattice and the possibility of passive

and we experiments at a distance of SOO m. The background due to hadrons and muons

section region 1 there is a possibility to install a large and massive detector for v

by 18cm. A detector of rectangular shape (15cm x 30cm) may find space here. In inter

proceedings). At a distance of ~13O m from the interaction point the beams are separated

The possible location of a detector at LHC has been studied by L. Camilleri (these

muon flux.

the two—track resolution. The time resolution of ~lOO{.is may cause problems with the

(figure 8). However. there are problems with the resolution in the second view and with

Nedelec and Vannucci [I0]. They obtained a resolution of 0~60u in the drift direction

Detection with a liquid argon drift chamber has been investigated by Du Marchez,

the two detector versions have to be determined by a test experiment.

detector version more than the transverse (fibre) strip detector. The relative merits of

fragments and evaporation nucleons are expected to affect the longitudinal fibre

R.E. Ansorge et al.. LlA2 Collaboration. Nucl. Instr. Methods A265(l988)33. OCR Output[1+]

A. Artamonov et al., Nucl. Instr. Methods (in the press) and CERN—EP/90-58.U3]

A.B. Kaidalov and O. Piskunova. Z. Phys. C30(1986)1—i5 and preprint Moscow (1990).[12]

A. De Rujula, R. Riickl, ECFA 8-1-85/CERN 84-10. Vol. II, p.571 (Geneva 198-i).[U]

P. Astier et al. LPN HEP/8907(1990).[10]

K. Winter. CERN-EP/89-182. Acta Physica Hungarica (in the press).

Munich - Naples - Rome) and [HEP (Berlin, Zeuthen), and

ll Collaboration (Brussels — CERN — Hamburg — Louvain — Moscow

[*9] Letter of intent "Detection of the vT at LINK", 6 january 1989, CHARM

ITEP preprint N1 33(1984) using emulsions.

chamber (1980), and FNAL proposal E—646, A.E. Asratyan et al.

[8] FNAL proposal E—636, E.S. Hafen et al. using a holographic bubble

[7] N. Llshida et al., Phys. Rev. Lett. 57(1986)2897.

[0] M. Talebzadeh et al., Nucl. Phys. B291(1987)503.

H. Albrecht et al., ARGLIS, Phys. Lett. 202B(1988)149.[5]

[-L] Particle Data Group. Phys. Lett. B20—l(1988)1 .

Springer (R. Kotthaus, j.H. Kuhn. editors) 1988, p. 156.

T. Kamae. Proc. 24th Int. Conf. on High Energy Physics (Munich).[3]

B.C. Barish. R. Stroynowski, Phys. Rep. 157(1988)1[2]

[1] M. Perl et al., Phys. Rev. Letters 35(1975)1489.

References

43

Illustration of the impact parameter technique.

Figure 1

Hadron OCR Output

Hadron

.... - - Ja --——————~~— - —·—

impact parameter

Transverse

Hadron

44

45 OCR Output

I'l"1€Y,€l` Of I decays.

Distribution of transverse decay length and 0F transverse‘ impact para

Figure 2

1L (pm)

200 L00 600

(ip: 65 pm

XL (mm)

()_6 1.2 1.8

<)`_L>= 2L+5 pm

46 OCR Output

t" + if decay kink is very spectacular in this event,

region of the longitudinal scintillating fiber (20 um) detector [9]. The

Simulated event of the type vIN —> t`(» [1_Y}|lVt)X in a 2 mm by 2 mm

Figure 3

-5 + I {H: - ` all + H n 'i H H

-1. I r' F.-, `ln I

· '¤I - _ I 1 ‘T;-`¤. .

I I - lu . ,_ _, —-: L arI · `"·‘ I. n I-I

_ _

® N 1 ·"1¤L · ' `;j·· 11+¤¤ ul ¤ I v.rI n

_ I l lI I I gIIi l - -I.*- 1 --

.IE 'l- 1..1 I- : I{f. n I Ll x . il

' ' .1*¤ ' if II I l` ' "} J

.}.1 I- J I'" P '..1l I I I I 11

.r - ¤ g?®FI.H

Schematic view of the transverse Fibre detector [9]. OCR Output

Figure 7

FIBREUBUNDLEBEAMBUNDLE

1000mm "‘ **/!;¢" " ',x¤ /};"'| r_/JS ‘ ""’“' -" ’'

.;L "§I oi :

a, { \"·4 Ir'tts z"’I"

IIQAGE PLANE"

system [9].

Schematic view of the longitudinal Fibre target detector and read—0ut

Figure 6

. - · Fibre bundleBundle block S

OCR Output1000mm '‘‘‘ ll »’......¥7 I J!}

I

: L{_l: , 1 »j¢ fz- E10mm

EE ? -." Ir

`\ ," II "'

i ' ‘>"`z’ i ,`»"}t; . ·Tr

plane _Transverse image

48

49

Ncdelec, Vannucci).

Resolution in drift direction in a liquid argon drift chamber (Du Marchez,

Figure 8

RESIDU POSITION

i-soo -200 -100 0 IO0 200 300

20

80

{gg O': }..LfT\

120