- Email: [email protected]
Muon astrophysics with the MACRO detector
I ~ lil[qll I f-,] m tl -"k'~"][Gkl [t
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 35 (1994) 229-234 North-Holland
Muon astrophysics with the MACRO Detector The MACRO Collaboration*, presented by P. Bernaxdini a ~Dipaxtimento di Fisica dell'Uuiversit~ and INFN, Lecce, Italy Muon events collected with the streamer tube system of MACRO have been used to study the vertical muon intensity and to search for astrophysical point sources. New upper limits on the muon fluxes coming from source candidates have been obtained. The/~ pair distance distribution and multimuon rates are presented and compared with Monte Carlo predictions for di~erent primary cosmic rays composition models.
1. I n t r o d u c t i o n
b) vertical p intensity and p flux at the surface,
The MACRO detector is located under the Gran Sasso mountain in central Italy. The apparatus [1] consists of liquid scintillation counters (energy loss and time of flight measurements), streamer tubes (tracking system) and track-etch plastics (highly ionizing paxticles detection). The minimum depth of the overburden rock is 3200 hg cm -2. The minimum energy at the surface is ,,~ 1.4 TeV for a muon axriving in MACRO. The primaxy cosmic ray energy has to be > 3 TeV to produce a single muon and > 20 TeV to produce multiple muons at the detector. Furthermore the detector siJes allow to study events with high muon multiplicities and large muon separations. The lower part of the apparatus, composed of six supermodules, is operating in its full sizes (75.6 m × 12 m × 4.8 m) since June 1991 and in stable acquisition since December 1992. Its acceptance is ,~ 8000 m2sr for an isotropic flux and its intrinsic angular resolution is ,,, 0.2 ° for tracks crossing 10 streamer tube planes. Taking into account multiple scattering in the rock, the overall angular resolution is estimated ,~ 1°. The upper part of the detector is presently under test. The acceptance of the full apparatus will be ~, 10000 m2sr for isotropic flux. Here we present various analyses of muon events, studied by means of the tracking system of the lower part of the detector:
c) muon pair distance distribution,
a) search for astrophysical muon "point sources",
d) composition of ultra-high energy primary cosmlc rays. The diferent samples used in these analyses have been extracted from the data taken in the period from June 1991 through June 1993. Selections were made mainly according to the criteria of stability in data acquisition and quality of single events. 2. S e a r c h f o r a s t r o n o m i e e d p o i n t s o u r c e s Analysing a total of 5.6× 106 muons, we have studied the distribution of the normafised deviations (n - b)/v~ between the observed numbers of muons (n) and the simulated background (b). In order to simulate the background, 25 events have been generated for each real event with the same declination and different time randomly selected from real events time set. In this way the simulated samples have llve-time, efficiency and acceptance equal to the real sample. The Right Ascension (c~) distributions for real d a t a and for this isotropically simulated flux axe found to be in good agreement. We searched for excesses above the expected background in each sky bin (A~ = 3 o, A sin 6 = 0.04)[2]. Bins with less than 10 events were removed. The best fit to the curve is a Gaussian as expected and there are just three bins in which the deviation is more than 3.5 ~. Using different bidimensioned bins, in order to
0920-5632/94/$07.00 © 1994 - Elsevier Science B.V. All fights reserved.
SSDI 0920-5632(94)00460-D
230
MACRO Collaboration/Muon astrophysics with the MACRO detector
Table 1 Upper limits Source Cyg X-3 Her X-I Geminga Crab
for muon flux (95% c.l.) Steady Muon Flux 5.3 × 10-1;~crrt-2s - I 5.2 × 10-13crn-2s - I 7.6 x lO-lacm-2s-1 6.2 X l O - 1 3 c m - a s - 1
Modulated Muon Flux 3.2 × 10-x%m-zs -x 3.6 x lO-Xacm-2s -x 3.2 x lO-lacm-2s -x
avoid that a source was missed becanse it was located near the edge of a bin, similar results have been obtained. We conclude that there is no evidence in our d a t a for a point source originating /z (the upper limit is 8.2 × 10-1Scm-Zs -1 at 95% c.1.). We then searched for point sources looking at U H E gamma-sources candid&tea (Cyg X-3, Her X-l, Crab and Geminga). No steady flux excess has been found analysing the data contained in a narrow cone (1.5 ° half angle) around the source position. A search was also made for modulated signals coming from sources that in the past showed such behaviour. The obtained upper limits are reported in table 1 (see Fig. 1 for the limit to the modulated muon flux from Cyg X-3). ''''1''''1''''1''''1''''1''''[''"1'"'1''''.
1040
ModulatedMuaaFlux
~CygX3
~ 10It 0 .~ 10l~
3. T h e m e a s u r e m e n t
~m
o
10"13 .,,I .... l,,,,I,,,,ll,.,I
. . . . ] . . . . I,, ~ I , , .
1000 1500 ~00 2500 3000 3500 4000 4500 ~ 0
i500
l ~ (m.w.e.)
Figure 1. The MACRO limit to the modulated muon flux from Cyg X-3 compared with the resuits from other underground detectors
of the muon
vertical
intensity The muon intensity - depth relation has been investigated rejecting events reaching MACRO from directions where the mountain map is not reliab]y known. For the selected sample (,,, 2.2×10 6 events) the slant depth h crossed by each muon is known with an uncertainty of -~ 14 hg cm -2. In order to measure the vertical muon intensity Iv(h), the slant depths have been converted to standard rock and the following formula has been used:
(1) where AT is the live-time, N~ is the number of observed events of muon multiplicity m~ in the bin of slant depth h, Aj is the acceptance of the detector, ej the trigger and reconstruction efficiency and 8j is the muon seulth angle. The resulting vertical muon intensity distribution relative to a senith range 00-600 is shown in Fig. 2. In the depth interval with larger statistics (3200 - 7000 h 8 cm-2), the intensity distribution has been fitted by the relation
I(h) = A MA(RO~1"
Period 4.79 hr (orbital) 1.70 days (orbital) 273 ms (pulsar)
e- ~ ,
(2)
with the following results: A = (1.93 -40.01)x10 -e c m - 2 s - l s r - l , a : 0.98 :t: 0.03, h0 : (936 4- 1) hg cm -2 and x2/DoF = 2.2. The large value of X z/DoF indicates the presence of systematic uncertainties which are currently under investigations. Using the function [3]
I(h):C-
(3)
we obtain a fit with a lazger value of chi-squared
(x2/DoF
=
4.6).
MACRO Collaboration/Muon astrophysics with the MACRO detector
As a function of h and 0 the intensity is given
by:
z.(h,o)
=
~0°°
dE.,
(4)
where @(E#,0) is the surface muon flux and P(E#,h) is the probability that a muon of energy E~, survives after a depth h. P(E~,,h) was calculated for the energy range I-I00 TeV, using a
,-do ,.,I,.,ll .... I .... I . . . . . . . . . . . . . . . . . . . •1000 .Y~O 4000 4500 5000 ,%~0 6000 6500 70QO 7 ~ 0
8000
~evdo~l Rock
Figure 2. Vertical muon intensity (cm-Zs-Zsr-1) versus Standard Rock depth (hg cm -2)
simulation code conta;nlng a detailed description of muon propagation in the Gran Sasso rock. We have assumed for the surface muon flux the cos O dependence given in [4] : o) = B
cos 0"
(u)
Using this formula, a fit to the experimental intensity (in the ranges h = 3200-6700 hg cm -~ and 0 = 00-60 °) gives the following results: B = (1.44-t-0.03)×10-rcm-2s-Zsr-ZTeV 7-z, ~f = 3.744-0.03, with E~, in TeV, and x 2 / D o F = 2.2. Only statistical errors have been taken into account.
231
4. T h e m e a s u r e m e n t o f t h e ~ p a i r separation The distance distribution of the p pairs underground, as measured by a very large area detector, is sensitive to the hadronic interaction mechanlgms, allowing the rejection of some simplified treatments of the hadronic cascades [5]. Here we present the analysis of a data sample of 5.8×106 muons and 7600 hr of live-time. Using the following data selection criteria we get a sample with 190,000/~ pairs: 1. reconstruction of each track in two projectire views to allow the assignment of a 3dimensional distance to each # pair, 2. the p pairs must be "parallel" (relative angle _~ 3°) to avoid contamination by pions from hadronic cascades in the rock overburden. In order to get a detector-independent distribution, two different methods, discussed in a previons paper [5], have been used. The resulting decoherence function is directly comparable with Monte Carlo simttlation of cosmic ray cascades down to the underground laboratory. It is possible to achieve a better agreement with data by taking into account the correlation between muon multiplicity and lateral distribution in the same shower. Using the published parametrisation [6] we gain CPU time, but we lose this correlation. Therefore we used the full development of the hadronic shower with HEMAS. Moreover the simulation code has been modified by inserting the effect of the geomagnetic field and a more realistic model [7] of the nucleus-nucleus interaction, by which the showers present larger fluctuations than expected in superposition model. The distribution, for the whole pair sample, is shown in Fig. 3, where the data are compared with the Monte Carlo predictions for two primary composition models (light and heavy composition models) [8]. In the previous measurement [5] a good agreement with Monte Carlo expectations was achieved up to the mAYimum attainable separation (.~ 20 m). With the full length MACRO detector the decoherence distribution for large separations is higher than the simulations. The average distance is 10.9 m for the real data, 10.5
232
MACRO Collaboration/Muon astrophysics with the MACRO detector
5. Composition of the Ultra-High Energy P r i m a r y Cosmic R a y s 3
b -I o ~o
• reol doto
e'
a slmulotion with light composition o simulo|;on with heavy cornposit~on "b'4..
"~41-
% ~+÷ .d~" ~
,
i
i
)
10
i
t
i
i
[
20
i
i
i
i
I
30
i
,
i
l
I
,
,
i
J
[~
40 50 seporotlon Ira)
Figure 3. Multimuon distance distribution
m for the heavy model and 9.4 m for the light model. The hadron interaction model presently used for the analysis probably needs further improvement. A possible line of investigation is the use of models including the onset of hard processes, such as mlnl-jet production [9]. Muons detected underground axe mainly due to the decay of mesons produced in the f~agmentation region; however our measurement is paxtially sensitive to the features of the interaction in the central region [5]. An increase of multiplicity in this region, such as the one induced by minijet production, would eventually produce a tail in the sepaxation distribution corresponding to the high transverse momentum t~il of the secondaxy mesons. Since the experimental lateral distribution is only a few percent wider than the Monte Carlo predictions, we axe confident that the muon-multiplicity simulation used below is not strongly biased by the chosen hadronic interaction model.
The present MACRO detector is capable of collecting ~ 6.6× 106 events per year of any multiplicity; this sample includes ~ 400,000 events/yr with N~ >2 and ~ 1600 events/yr with N~ >I0. Indeed our data sample contains many high multiplicity events with laxge sepaxations. Therefore we have a good statistical accuracy over a wide range of the primaxy energy spectrum; we can investigate the spectrum above the "knee" region (~ 3000 TeV), where the knowledge of the primary composition is still rather poor. For this analysis our data sample corresponds to ~ 3300 hz of total live-time and ..- 2.5×106 muon events of which ~ t50,000 axe multiple muon events. Using an improved version of the detector simulation program, based on the GEANT [10] code, the experimental data are reproduced at a satisfactozy level of accuracy, t~dng into account various physics and detector effects (electromagnetic showering down to 500 KeV, charge induction of the streamer signal in the strips, electronic noise, inefficiencies, failuzes of the tracking algorithm and track shadowing at small sepax&tious). The accurate simulation allows to calculate the correction factors that transform the reconstructed multiplicities on the two projected views to an actual multiplicity. In this way we reduce considerably the erzors, mainly connected to visual scanning used in a previous analysis [11]. The simulation is the same used for the distance distribution analysis, with a light (i.e., proton-rich) and a heavy (i.e., Fe-rich) pdmaxy compositions [8] adjusted to fit the "all-paxticle" spectrum. Fig. 4 shows the calculated range of pfimaxy energy that corresponds to the detection of 90% of events as a function of detected multiplicity, for the two composition models. This figure shows that events with detected multiplicity N~ > 10 ori~nate fxom primaries in an energy region entirely above the "knee". The expected rates f~om our Monte Caxlo simulation at each multiplicity axe shown in Fig. 5, compaxed with the experimental data. No nozmaliJation has been applied between data and Monte Carlo
MACRO Collaboration/Muon astrophysics with the MACRO detector
233
this disagreement. The cosmic ray primary composition has been also studied analysing coincident deep underground muons and extensive aft showers as measured by MACRO and EAS-TOP experiments. The results have been presented elsewere in this conference [12].
10!
1o4
'c
o. 10~
~
10z
c e
10:
w 10~
• 6 SM data o MC heavycomposiUon [3 MC 59ht composition
4~
10
10
.e-
.oI
0
10
20 30 Muonmult~x:Ry
0
10
8
ZU ,,IU Mum mumpfic~ 1();
Figure 4. Calculated range of primary energy for the detection of 90% of events versus multiplicity. The bold line gives the mean primary energy as a function of the detected multiplicity. 4
predictions. Error bars represent statistical errors, inclusive of uncertainties from the correction procedure, and also include systematic uncertainties in the Monte Carlo predictions. Our previous results [11] are confirmed by new data. The experimental data lie in between the two models at lower multiplicities and favour the light model at higher multiplicities (strongly for
N~ _> 15). A controversial feature arising from Fig. 5 is that the measured rates at low multiplicities are considerably higher than both Monte Carlo predictions. The number of events for N~, _< 4 is 25% (31%) higher than the prediction of the light (heavy) model. The events with multiplicities N~, <4 come from regions of low primary energy (Fig. 4), where light and heavy models are very similar, being tailored to fit direct measurements. We are currently investigating possible sources of
8
12
16
20
24
28 32 36 Muon multiplicity
Figure 5. Comparison of multimuon rates between data and M.C. predictions
REFERENCES
1. MACRO Collaboration (S. Alden et al.), Nud. Instr. & Meth. A 824 (1993) 337. 2. MACRO Collaboration (S. Alden et al.), Ap.J. 412(1993) 301. 3. Ch. Berger et al., Phys. Rev. D 40(1989) 2163. 4. T.K. Gaisser, "Cosmic Rays and Particle Physics", Cambridge University Press, Cambridge, 1990.
234
MACRO Collaboration/Muon astrophysics with the MACRO detector
5. MACRO Collaboration (S. Ahlen et al.), Phys. Rev. D 46(1992) 4836. 6. C. Forti et al., Phys. Rev. D 42(1990) 3668. 7. J. Engel et al., Proceedings of the XXII ICRC vol.4 (1991) 1. 8. G. Auriemma et al., Proceedings of the XXI ICRC vol.9 (1990) 362. 9. T.K. Gaisser and T. Stamev, Phys. Lett. B 29(1989) 375. 10. R. B r u n e t al., "GEANT3 manual", CERN DD/EE/84-1 (1984). 11. MACRO Collaboration (S. Alden et al.), Phys. Rev. D 48(1992) 895. 12. MACRO and EAS-TOP Collaborations, proceedings of this conference. *The M A C R O C o l l a b o r a t i o n S. Alden s, M. Ambrosio 12, R. Antollni7, G. Auriemma 14,19, R. Baker 11, A. Baldin113, G.C. Bazbazino 1~, B.C. Bazish4, G. Battistoni 6,2d, R. Bellotti 1, C. Bemporad is, P. Bernardini 1°, H. Bilokone, V. Blsi 10, C. Bloisee, C. Bowers, S. Bussino 14, F. Cafagna 1, M. Caliccldo 1, D. Campana 12, M. Caxboni o, S. Cecchini 2'2°, F. Cei 13, V. Chiszella o, A. Corona 14, S. Coutu 11, G. De Cataldo 1, H. Dekhissi2,7,2e, C. De Mazso 1, M. De Vincenzi 14'21, A. Di Credico 9, E. Dieh111, O. Erriquez 1 , C. Favuzsi 1, C. Fortie, P. Fusco x, G. Giacomelli2, G. Giamninils'22, N. Giglietto I, M. Grassi x3, P. GreenlS~ A. Gnllo e, F. Guazino 12, P. Guaznsccia 1 C. Gustavino 7, A. Habig 7,s, K. Hanson 11, R. Heins s, J.T. Hong3, E. Iazocci6'23, E. Katsavounidis 4, E. Keazns 3, S. Kyriaxopoulou 4, E. Lamanna 14, C. Lamed , D. Levin 11, P. Lipari x4, G. Liu4, R. Liu4, M.J. Longo 11, Y.Lu ld, G. Ludiam s, G. Mamcarella 1°, G. Mandrioll 2, A. MazgiottsNeri 2, A. Mazin s, A. Mazinio, D. Maztello 1°, A. Mazsazi Chiesa 1~, M.N. Massiotta 1, D.G. Michael4, S. Mikheyev7'24, L. Millers, M. Mittelbrun d,7, P. Monacelli7,9, M. Monteno le, S. Mufson s, J. Musser s, D. Nicol6 ls'27, R. Nolty 4'7, C. Okada 3, G. Osteria 12, O. Palamaza 1°, S. Pazlati T, V. Patera o, L. Patrizii 2, B. Pavesi2, R. Passi 13, C.W. Peck4, J. Petrakis 17, S. Petrera 1°, N.D. Pignatemo4, P. Pistillil0, A. Rain61, J. Reynoldson ~, F. Ronga e, A. Sansgizi7'1d, C.
Satriano 14'19, L. Satta s,2s, E. Scapparone 2, K. Scholberg4, A. Sciubba e,2s, P. Serra Lugazesi2, M. Sevezi 14, M. Sirra xe, P. SpinelliI , M. Spinetti 8 , M. Spurio 2, J. Steele d, R. Steinberg d , J.L. Stone s, L.R. Sulak3, A. Surdo 1°, G. Tazl~ 11, V. Togo 2, V. Valente6, E. Vilela 2, C.W. Walter 4, R. Webb ld, W. Worstell 3, 1. Dip. di Fis. dell'Univ, di Bail amd INFN, Bari, Italy; 2. Dip. di Fis. dell'Univ, di Bologna and INFN, Bologna, Italy; 3. Phys. Dept., Boston Univ., Boston, MA, USA; 4. Cal. Inst. of Tech., Pasadena, CA, USA; 5. Dept. of Phys., Drexel Univ., Philadelphia, PA, USA; 6. Lab. Nas. di Frascati dell'INFN, Frascati (Roma), Italy; 7. Lab. Nas. del Gram Sasso dell'INFN~ Assergi (L'Aqnila), Italy; 8. Depts. of Phys. and of Astr., Indiana Univ., Bloomington, IN, USA; 9. Dip. di Fis. dell'Univ, dell'Aqnila and INFN, L'Aquila, Italy; 10. Dip. di Fis. dell'Univ, di Lecce and INFN, Lecce, Italy; 11. Dept. of Phys., Univ. of Michigan, Ann Arbor, MI, USA; 12. Dip. di Fis. dell'Univ, di Napoll and INFN, Napoli, Italy; 13. Dip. di Fis. dell'Univ, di Plsa and INFN, Piss, Italy; 14. Dip. di Fis. dell'Univ, di Roma and INFN, Roma, Italy; 15. Phys. Dept., Texas A&M Univ., College Station, TX, USA; 16. Dip. di Fls. dell'Univ, di Torino and INFN, Torino, Italy; 17. Baztol Res. Inst., Univ. of Delaware, Newark, DE, USA; 18. Sandis Nat. Lab., Albuquerque, NM, USA; 19. Also Univ. della Basilicata, PotenJa, Italy; 20. Also Ist. TESRE/CNR, Bologna, Italy; 21. Also Univ. di Camerino, Camerino, Italy; 22. Also Univ. di Trieste and INFN, Trieste, Italy; 23. Also Dip. di Energetica, Univ. di Roma, Roma, Italy; 24. Also Inst. for Nucl. Res., Russian Academy of Sciences, Moscow; 25. Also INFN, Mllamo, Italy; 26. Also Univ. Mohammed 1st, Oujda, Morocco; 27. Also Scuola Normale Superiore, Pisa, Italy.
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 35 (1994) 229-234 North-Holland
Muon astrophysics with the MACRO Detector The MACRO Collaboration*, presented by P. Bernaxdini a ~Dipaxtimento di Fisica dell'Uuiversit~ and INFN, Lecce, Italy Muon events collected with the streamer tube system of MACRO have been used to study the vertical muon intensity and to search for astrophysical point sources. New upper limits on the muon fluxes coming from source candidates have been obtained. The/~ pair distance distribution and multimuon rates are presented and compared with Monte Carlo predictions for di~erent primary cosmic rays composition models.
1. I n t r o d u c t i o n
b) vertical p intensity and p flux at the surface,
The MACRO detector is located under the Gran Sasso mountain in central Italy. The apparatus [1] consists of liquid scintillation counters (energy loss and time of flight measurements), streamer tubes (tracking system) and track-etch plastics (highly ionizing paxticles detection). The minimum depth of the overburden rock is 3200 hg cm -2. The minimum energy at the surface is ,,~ 1.4 TeV for a muon axriving in MACRO. The primaxy cosmic ray energy has to be > 3 TeV to produce a single muon and > 20 TeV to produce multiple muons at the detector. Furthermore the detector siJes allow to study events with high muon multiplicities and large muon separations. The lower part of the apparatus, composed of six supermodules, is operating in its full sizes (75.6 m × 12 m × 4.8 m) since June 1991 and in stable acquisition since December 1992. Its acceptance is ,~ 8000 m2sr for an isotropic flux and its intrinsic angular resolution is ,,, 0.2 ° for tracks crossing 10 streamer tube planes. Taking into account multiple scattering in the rock, the overall angular resolution is estimated ,~ 1°. The upper part of the detector is presently under test. The acceptance of the full apparatus will be ~, 10000 m2sr for isotropic flux. Here we present various analyses of muon events, studied by means of the tracking system of the lower part of the detector:
c) muon pair distance distribution,
a) search for astrophysical muon "point sources",
d) composition of ultra-high energy primary cosmlc rays. The diferent samples used in these analyses have been extracted from the data taken in the period from June 1991 through June 1993. Selections were made mainly according to the criteria of stability in data acquisition and quality of single events. 2. S e a r c h f o r a s t r o n o m i e e d p o i n t s o u r c e s Analysing a total of 5.6× 106 muons, we have studied the distribution of the normafised deviations (n - b)/v~ between the observed numbers of muons (n) and the simulated background (b). In order to simulate the background, 25 events have been generated for each real event with the same declination and different time randomly selected from real events time set. In this way the simulated samples have llve-time, efficiency and acceptance equal to the real sample. The Right Ascension (c~) distributions for real d a t a and for this isotropically simulated flux axe found to be in good agreement. We searched for excesses above the expected background in each sky bin (A~ = 3 o, A sin 6 = 0.04)[2]. Bins with less than 10 events were removed. The best fit to the curve is a Gaussian as expected and there are just three bins in which the deviation is more than 3.5 ~. Using different bidimensioned bins, in order to
0920-5632/94/$07.00 © 1994 - Elsevier Science B.V. All fights reserved.
SSDI 0920-5632(94)00460-D
230
MACRO Collaboration/Muon astrophysics with the MACRO detector
Table 1 Upper limits Source Cyg X-3 Her X-I Geminga Crab
for muon flux (95% c.l.) Steady Muon Flux 5.3 × 10-1;~crrt-2s - I 5.2 × 10-13crn-2s - I 7.6 x lO-lacm-2s-1 6.2 X l O - 1 3 c m - a s - 1
Modulated Muon Flux 3.2 × 10-x%m-zs -x 3.6 x lO-Xacm-2s -x 3.2 x lO-lacm-2s -x
avoid that a source was missed becanse it was located near the edge of a bin, similar results have been obtained. We conclude that there is no evidence in our d a t a for a point source originating /z (the upper limit is 8.2 × 10-1Scm-Zs -1 at 95% c.1.). We then searched for point sources looking at U H E gamma-sources candid&tea (Cyg X-3, Her X-l, Crab and Geminga). No steady flux excess has been found analysing the data contained in a narrow cone (1.5 ° half angle) around the source position. A search was also made for modulated signals coming from sources that in the past showed such behaviour. The obtained upper limits are reported in table 1 (see Fig. 1 for the limit to the modulated muon flux from Cyg X-3). ''''1''''1''''1''''1''''1''''[''"1'"'1''''.
1040
ModulatedMuaaFlux
~CygX3
~ 10It 0 .~ 10l~
3. T h e m e a s u r e m e n t
~m
o
10"13 .,,I .... l,,,,I,,,,ll,.,I
. . . . ] . . . . I,, ~ I , , .
1000 1500 ~00 2500 3000 3500 4000 4500 ~ 0
i500
l ~ (m.w.e.)
Figure 1. The MACRO limit to the modulated muon flux from Cyg X-3 compared with the resuits from other underground detectors
of the muon
vertical
intensity The muon intensity - depth relation has been investigated rejecting events reaching MACRO from directions where the mountain map is not reliab]y known. For the selected sample (,,, 2.2×10 6 events) the slant depth h crossed by each muon is known with an uncertainty of -~ 14 hg cm -2. In order to measure the vertical muon intensity Iv(h), the slant depths have been converted to standard rock and the following formula has been used:
(1) where AT is the live-time, N~ is the number of observed events of muon multiplicity m~ in the bin of slant depth h, Aj is the acceptance of the detector, ej the trigger and reconstruction efficiency and 8j is the muon seulth angle. The resulting vertical muon intensity distribution relative to a senith range 00-600 is shown in Fig. 2. In the depth interval with larger statistics (3200 - 7000 h 8 cm-2), the intensity distribution has been fitted by the relation
I(h) = A MA(RO~1"
Period 4.79 hr (orbital) 1.70 days (orbital) 273 ms (pulsar)
e- ~ ,
(2)
with the following results: A = (1.93 -40.01)x10 -e c m - 2 s - l s r - l , a : 0.98 :t: 0.03, h0 : (936 4- 1) hg cm -2 and x2/DoF = 2.2. The large value of X z/DoF indicates the presence of systematic uncertainties which are currently under investigations. Using the function [3]
I(h):C-
(3)
we obtain a fit with a lazger value of chi-squared
(x2/DoF
=
4.6).
MACRO Collaboration/Muon astrophysics with the MACRO detector
As a function of h and 0 the intensity is given
by:
z.(h,o)
=
~0°°
dE.,
(4)
where @(E#,0) is the surface muon flux and P(E#,h) is the probability that a muon of energy E~, survives after a depth h. P(E~,,h) was calculated for the energy range I-I00 TeV, using a
,-do ,.,I,.,ll .... I .... I . . . . . . . . . . . . . . . . . . . •1000 .Y~O 4000 4500 5000 ,%~0 6000 6500 70QO 7 ~ 0
8000
~evdo~l Rock
Figure 2. Vertical muon intensity (cm-Zs-Zsr-1) versus Standard Rock depth (hg cm -2)
simulation code conta;nlng a detailed description of muon propagation in the Gran Sasso rock. We have assumed for the surface muon flux the cos O dependence given in [4] : o) = B
cos 0"
(u)
Using this formula, a fit to the experimental intensity (in the ranges h = 3200-6700 hg cm -~ and 0 = 00-60 °) gives the following results: B = (1.44-t-0.03)×10-rcm-2s-Zsr-ZTeV 7-z, ~f = 3.744-0.03, with E~, in TeV, and x 2 / D o F = 2.2. Only statistical errors have been taken into account.
231
4. T h e m e a s u r e m e n t o f t h e ~ p a i r separation The distance distribution of the p pairs underground, as measured by a very large area detector, is sensitive to the hadronic interaction mechanlgms, allowing the rejection of some simplified treatments of the hadronic cascades [5]. Here we present the analysis of a data sample of 5.8×106 muons and 7600 hr of live-time. Using the following data selection criteria we get a sample with 190,000/~ pairs: 1. reconstruction of each track in two projectire views to allow the assignment of a 3dimensional distance to each # pair, 2. the p pairs must be "parallel" (relative angle _~ 3°) to avoid contamination by pions from hadronic cascades in the rock overburden. In order to get a detector-independent distribution, two different methods, discussed in a previons paper [5], have been used. The resulting decoherence function is directly comparable with Monte Carlo simttlation of cosmic ray cascades down to the underground laboratory. It is possible to achieve a better agreement with data by taking into account the correlation between muon multiplicity and lateral distribution in the same shower. Using the published parametrisation [6] we gain CPU time, but we lose this correlation. Therefore we used the full development of the hadronic shower with HEMAS. Moreover the simulation code has been modified by inserting the effect of the geomagnetic field and a more realistic model [7] of the nucleus-nucleus interaction, by which the showers present larger fluctuations than expected in superposition model. The distribution, for the whole pair sample, is shown in Fig. 3, where the data are compared with the Monte Carlo predictions for two primary composition models (light and heavy composition models) [8]. In the previous measurement [5] a good agreement with Monte Carlo expectations was achieved up to the mAYimum attainable separation (.~ 20 m). With the full length MACRO detector the decoherence distribution for large separations is higher than the simulations. The average distance is 10.9 m for the real data, 10.5
232
MACRO Collaboration/Muon astrophysics with the MACRO detector
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m for the heavy model and 9.4 m for the light model. The hadron interaction model presently used for the analysis probably needs further improvement. A possible line of investigation is the use of models including the onset of hard processes, such as mlnl-jet production [9]. Muons detected underground axe mainly due to the decay of mesons produced in the f~agmentation region; however our measurement is paxtially sensitive to the features of the interaction in the central region [5]. An increase of multiplicity in this region, such as the one induced by minijet production, would eventually produce a tail in the sepaxation distribution corresponding to the high transverse momentum t~il of the secondaxy mesons. Since the experimental lateral distribution is only a few percent wider than the Monte Carlo predictions, we axe confident that the muon-multiplicity simulation used below is not strongly biased by the chosen hadronic interaction model.
The present MACRO detector is capable of collecting ~ 6.6× 106 events per year of any multiplicity; this sample includes ~ 400,000 events/yr with N~ >2 and ~ 1600 events/yr with N~ >I0. Indeed our data sample contains many high multiplicity events with laxge sepaxations. Therefore we have a good statistical accuracy over a wide range of the primaxy energy spectrum; we can investigate the spectrum above the "knee" region (~ 3000 TeV), where the knowledge of the primary composition is still rather poor. For this analysis our data sample corresponds to ~ 3300 hz of total live-time and ..- 2.5×106 muon events of which ~ t50,000 axe multiple muon events. Using an improved version of the detector simulation program, based on the GEANT [10] code, the experimental data are reproduced at a satisfactozy level of accuracy, t~dng into account various physics and detector effects (electromagnetic showering down to 500 KeV, charge induction of the streamer signal in the strips, electronic noise, inefficiencies, failuzes of the tracking algorithm and track shadowing at small sepax&tious). The accurate simulation allows to calculate the correction factors that transform the reconstructed multiplicities on the two projected views to an actual multiplicity. In this way we reduce considerably the erzors, mainly connected to visual scanning used in a previous analysis [11]. The simulation is the same used for the distance distribution analysis, with a light (i.e., proton-rich) and a heavy (i.e., Fe-rich) pdmaxy compositions [8] adjusted to fit the "all-paxticle" spectrum. Fig. 4 shows the calculated range of pfimaxy energy that corresponds to the detection of 90% of events as a function of detected multiplicity, for the two composition models. This figure shows that events with detected multiplicity N~ > 10 ori~nate fxom primaries in an energy region entirely above the "knee". The expected rates f~om our Monte Caxlo simulation at each multiplicity axe shown in Fig. 5, compaxed with the experimental data. No nozmaliJation has been applied between data and Monte Carlo
MACRO Collaboration/Muon astrophysics with the MACRO detector
233
this disagreement. The cosmic ray primary composition has been also studied analysing coincident deep underground muons and extensive aft showers as measured by MACRO and EAS-TOP experiments. The results have been presented elsewere in this conference [12].
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predictions. Error bars represent statistical errors, inclusive of uncertainties from the correction procedure, and also include systematic uncertainties in the Monte Carlo predictions. Our previous results [11] are confirmed by new data. The experimental data lie in between the two models at lower multiplicities and favour the light model at higher multiplicities (strongly for
N~ _> 15). A controversial feature arising from Fig. 5 is that the measured rates at low multiplicities are considerably higher than both Monte Carlo predictions. The number of events for N~, _< 4 is 25% (31%) higher than the prediction of the light (heavy) model. The events with multiplicities N~, <4 come from regions of low primary energy (Fig. 4), where light and heavy models are very similar, being tailored to fit direct measurements. We are currently investigating possible sources of
8
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Figure 5. Comparison of multimuon rates between data and M.C. predictions
REFERENCES
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Satriano 14'19, L. Satta s,2s, E. Scapparone 2, K. Scholberg4, A. Sciubba e,2s, P. Serra Lugazesi2, M. Sevezi 14, M. Sirra xe, P. SpinelliI , M. Spinetti 8 , M. Spurio 2, J. Steele d, R. Steinberg d , J.L. Stone s, L.R. Sulak3, A. Surdo 1°, G. Tazl~ 11, V. Togo 2, V. Valente6, E. Vilela 2, C.W. Walter 4, R. Webb ld, W. Worstell 3, 1. Dip. di Fis. dell'Univ, di Bail amd INFN, Bari, Italy; 2. Dip. di Fis. dell'Univ, di Bologna and INFN, Bologna, Italy; 3. Phys. Dept., Boston Univ., Boston, MA, USA; 4. Cal. Inst. of Tech., Pasadena, CA, USA; 5. Dept. of Phys., Drexel Univ., Philadelphia, PA, USA; 6. Lab. Nas. di Frascati dell'INFN, Frascati (Roma), Italy; 7. Lab. Nas. del Gram Sasso dell'INFN~ Assergi (L'Aqnila), Italy; 8. Depts. of Phys. and of Astr., Indiana Univ., Bloomington, IN, USA; 9. Dip. di Fis. dell'Univ, dell'Aqnila and INFN, L'Aquila, Italy; 10. Dip. di Fis. dell'Univ, di Lecce and INFN, Lecce, Italy; 11. Dept. of Phys., Univ. of Michigan, Ann Arbor, MI, USA; 12. Dip. di Fis. dell'Univ, di Napoll and INFN, Napoli, Italy; 13. Dip. di Fis. dell'Univ, di Plsa and INFN, Piss, Italy; 14. Dip. di Fis. dell'Univ, di Roma and INFN, Roma, Italy; 15. Phys. Dept., Texas A&M Univ., College Station, TX, USA; 16. Dip. di Fls. dell'Univ, di Torino and INFN, Torino, Italy; 17. Baztol Res. Inst., Univ. of Delaware, Newark, DE, USA; 18. Sandis Nat. Lab., Albuquerque, NM, USA; 19. Also Univ. della Basilicata, PotenJa, Italy; 20. Also Ist. TESRE/CNR, Bologna, Italy; 21. Also Univ. di Camerino, Camerino, Italy; 22. Also Univ. di Trieste and INFN, Trieste, Italy; 23. Also Dip. di Energetica, Univ. di Roma, Roma, Italy; 24. Also Inst. for Nucl. Res., Russian Academy of Sciences, Moscow; 25. Also INFN, Mllamo, Italy; 26. Also Univ. Mohammed 1st, Oujda, Morocco; 27. Also Scuola Normale Superiore, Pisa, Italy.