L3+C: Selected Results

Nuclear Physics B (Proc. Suppl.) 151 (2006) 314–321 www.elsevierphysics.com

L3+C: Selected Results P. Le Coultre (on behalf of the L3+C collaboration) a

a

CERN, CH-1211 Geneva, Switzerland

Final results from the L3+C experiment are presented on different topics: The atmospheric muon momentum spectrum between 20 GeV and 3 TeV, it’s angular dependence and the charge ratio, a limit on the primary anti-proton to proton ratio at 1 TeV, muon flux limits from a sky survey for different threshold energies and time windows, a measurement of the Solar anisotropy for 200 GeV protons, an analysis searching for high energy signals from GRBs, and a flux limit for protons above 40 GeV from a Solar flare.

1. Introduction The L3 detector [1] installed at LEP, CERN, allowed to measure precisely reaction products of e+ e− collisions and to determine fundamental quantities of the Standard Model of particle interactions. The excellent muon momentum resolution in particular, obtained with the large drift chamber system and the huge magnet (1000 m3 , 0.5 Tesla), initiated the idea to use this spectrometer also as a tool for cosmic ray and astroparticle physics studies. Typically the momentum resolution at 100 GeV amounts to 7.6 %, the angular resolution is better than 0.22◦ and the pointing precision better than 0.2◦ above 100 GeV. These values were obtained with cosmic ray muons after implementing the L3 detector with 202 m2 of timing plastic scintillators on top of the magnet [2] (see Figure 1) and an L3 independent trigger and data acquisition system. 30 m of molass overburden limit the minimal detectable muon energy to 15 GeV. A surface scintillator array (Figure 1) was mounted on the roof of the L3 surface hall to detect air showers related to measured muons in the cave. The coordinates of the L3+C site are 6.02◦ E, 46.25◦ N. The data (1.2 · 1010 triggers) used for the present analysis were recorded from July to October 1999 and from April to November 2000, totalizing an effective live-time of 312 days.

Figure 1. The air shower scintillator array on the roof of the surface hall (54 × 30 m2 ) and the L3 detector covered with timing scintillators in the LEP cave.

2. The vertical muon momentum spectrum The first motivation to measure the atmospheric muon momentum spectrum was the

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flux value of 2.3% is found at 150 GeV. Statistics becomes important above 0.5 TeV. Discussion of the result: In Figure 2 the vertical differential spectrum is shown together with results from other experiments providing an absolute normalization. The Kiel data e.g. [5] agree in shape, but have a higher normalization. MARS data [6] disagree in shape and normalization, whereas the most recent BESS data [7] agree above 50 GeV with L3+C. Another comparison can be made with theoretical calculations: In Figure 3 it is seen, that the L3+C spectrum cannot be reproduced correctly by present interaction models. The uncertainties of the primary flux and composition play also a role in this discussion. See the figure caption for details.

Φ·p3 [GeV2cm-2s-1sr-1]

unique experimental opportunity to use a particle physics precision spectrometer of large size. It could provide a major improvement over previous cosmic ray experiments, which gave absolute flux values with uncertainties ranging up to 25% in the energy range 20 GeV to 3 TeV. The additional possibility to determine also well the angular dependence and the charge ratio permits to constrain the calculated muon neutrino and antineutrino spectra, important for oscillation measurements and for background estimations in neutrino astronomy. In addition these results also constrain the parameters in models for hadronic interactions at very high energies, the major topic of this conference. The L3+C results [3] are summarized below. Muon tracks crossing the whole L3 detector were fitted in 6 drift chamber planes, and the spectrum was obtained with the least squares method. The momentum being also measured twice in two opposite sets of 3 drift chambers allowed for a precise estimation of the momentum resolution as a function of momentum for tracks crossing the 6 chambers (important for the understanding of the migration of events into neighbour momentum bins). The energy losses were calculated with the GEANT Monte Carlo [4] and the acceptance with the L3 GEANT Monte Carlo. The detection efficiencies were measured thanks to redundant informations and the effective live-time was determined with a special trigger counter and an independent clock. Experimental uncertainties: The momentum bias is dominated by the energy loss uncertainty at low energies (2% at 20 GeV) and by the chamber misalignement at high energies (15% at 1 TeV). The magnetic field uncertainty contributes to less than 0.4% below 3 TeV. The normalization error depends on several items: The uncertainty on the acceptance was estimated by comparing independent data samples (from different detector domains, from 1999 and 2000 data, events from different azimuth directions), checking the obtained flux values by modifying the selection criteria, and observing atmospheric effects. The influence of the momentum resolution on the minimization result was also analyzed. E.g. a total uncertainty on the absolute vertical

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Figure 2. The vertical differential muon spectrum at sea level. The L3+C measurement is compared to previous results providing an absolute flux normalization (for references see text).

Figure 4 and Figure 5 show the angular dependence of the spectrum and the charge ratio. The inner error bars denote the statistical uncertainty, and the full bar the total uncertainy (systematics included).

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Figure 3. L3+C spectrum compared to theoretical predictions. Best agreements is obtained by the HKKM95 (Fritijov) [8] and Bartol96 [9] with pre-AMS/BESS primary spectrum. Using the AMS/BESS primary flux [10,11] the following models fail to reproduce the L3+C data: CORT01 [12], HKKM04 (DPMJET) [13]. Also the Target2.1 [14], Sibyll2.1 [15] and QGSJET01 calculations (with the ”upper” Gaisser-Honda primary flux [16]) show discrepancy (worst in this case is QGSJET01, best Target 2.1).

3. The anti-proton to proton ratio at 1 TeV Besides of the excellent momentum resolution another particular property of the L3+C detector is the very good angular resolution and the pointing precision. These features allow the study of the ”Moon shadow” and through it a determination of an upper limit on the anti-proton to proton ratio of primary cosmic rays. Indeed, nuclei entering the Earth magnetic field are deflected towards the East and some secondaries produced in interactions with air nuclei may decay into muons,

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ranging from 0◦ to 58◦ at 450 m above sea level. Inner error bars denote statistical uncertainty, full bars total uncertainty.

which in turn point back to the incoming nucleus direction. If the primary is absorbed on its way by the Moon, a lack of muons from this direction is observed. In other words a shadow is produced, which is situated on the West side of the Moon. The non-observation of a shadow on the East side of the Moon indicates the absence of primary anti-nuclei [17]. Compared to most other experiments (air shower arrays, or Cherenkov detectors), L3+C is sensitive to relatively low energy primaries (good statistics!) with large deflection in the Earth field. The signature for a missing muon shadow on the East side of the Moon is therefore enhanced. The angular resolution could be estimated by

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(CORSIKA, [18]), delivered a shadow significance of 9.4 σ and a ratio r = p+He p+He = −0.07 ± 0.09. This result being unphysical the unified approach method has been applayed to get finally an upper limit on r of 0.08 with a 90 CL. With an assumed flux composition around 1 TeV of 75% protons and 25% heavier nuclei responsible for the observed deficit [19], this corresponds to a pp ratio of 0.11. This result is presented in Figure 6 together with direct measurements obtained with balloon and satellite experiments, all sensitive to anti-protons below 50 GeV and in agreement with the secondary production model [20,21]. Only Dark Matter neutralino annihilations are expected to produce larger anti-proton fluxes at high energies [22]. With the L3+C limit a solid upper limit has been obtained. Extrapolations from the measured muon charge ratio [3] are too unsecure, due to the uncertain primary flux and the high energy hadronic interaction cross-sections (see lower panel in Figure 3). 4. Search for flaring point sources

the measured angular difference of two-muon events, where both high energy muons are practically parallel to each other, since they are produced high up in the atmosphere within the same shower. The perfect agreement with the Monte Carlo prediction gave confidence in the estimated resolution for single muon tracks. The observed shadow has been analyzed with the 2-dimensional likelihood method, delivering an angular resolution of (0.22 ± 0.04)◦ for single muons with an energy larger than 100 GeV, in agreement with the previous estimate. The shadow itself has been detected with a significance of 5.5 standard deviations (σ) for muons between 65 GeV and 100 GeV (LE sample), and 8.3 σ for muons above 100 GeV (HE sample). A clear dependence on the assumed Earth magnetic field has also been observed. Out of the two tested Earth fields (simple dipole and IGRF) the IGRF field describes the situation best. The maximum likelihood procedure applied to the combined HE and LE samples, corresponding to primaries of about 1 TeV energy

Sources of cosmic rays and the acceleration mechanisms are still hot topics. L3+C has many advantages in performing a search for signals from point sources: The sky can be surveyed 24 hours a day, the muon threshold is lower than for most other underground detectors (which in turn means larger rates), the threshold can be selected off-line in order to optimize eventually the signal to background noise, the background is continuously monitored, sources could be followed accross the sky, and the angular resolution and pointing precision are excellent. A serious difficulty is that the production of muons in gamma induced showers is rather low compared to proton showers [23]. Another negative point is that during the last two decades observed known sources show rather low measured fluxes at the highest observed energies (below 60 TeV). This means that L3+C has no chance to detect steady signals and that only strong burst like events may be found. L3+C has a steady flux sensitivity of apporximately 10−10 cm−2 s−1 for muons above 20 GeV.

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entry for each sky cell and each day of data taking. The conclusion can be made, that no signal occuring and lasting one day has been observed from any direction in the northen sky. A similar conclusion can be extracted from the analysis of events originating from the direction of 10 well known sources, namely Mkn 421, Mkn 501, 3C 273, 1ES 1426+428, 1ES 2344+514 (all AGNs); Cyg X-1 (BH); Cyg X-3 (B/NS); Her X-1 (E/NS); Geminga (NS); Crab nebula (SNR/NS).

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flux ratios versus the primary energy compared to model calculations. The dotted lines show the range of the theoretical expectations [20]. The result of the present work is shown in the upper right of the figure, as an upper limit. The down pointing arrow indicates the average energy of the involved nucleons, assuming the same power index for anti-protons as for protons, and the horizontal line limits the energy band of the primaries containing 68 % of all nucleons.

Search procedure [24]: The muon flux was measured in the local equatorial coordinate system and transformed to the equatorial coordinate system (taking the event distribution as a function of the sideral time into account), in order to get the expected background muon rate to a possible signal in a given sky cell (delimited by a right ascension α and a declination δ interval). The cell sizes (defined according to the angular resolution for the selected muon momentum threshold) range from (0.9◦ )2 at 100 GeV to (3.0◦ )2 at 20 GeV. To find a rate excess in a particular sky cell, the Poisson probability is calculated to find the number of events lager than or equal to the observed number, when the number of background events is known. Cumulative distributions of log(P) are plotted. Figure 7 shows a sky survey: For four different muon energy thresholds the cumulative distributions are plotted with one

Figure 7. Cumulative distributions of -log P for different threshold muon energies. There is one entry for each direction and for each analyzed one day period.

The same analysis has been repeated for time windows of one or several months and for all sky cells without success. The 90% CL upper flux limits Iµ for steady signals (depending on the source position and the cell size) have also been determined and are listed below (312 days of data taking and assuming the source at zenith): For Eµ > 20 GeV : Iµ < 1.0 · 10−9 to 20 · 10−9 cm−2 s−1 , for Eµ > 30 GeV : Iµ < 0.2 · 10−9 to 5.0 · 10−9 cm−2 s−1 , for Eµ > 50 GeV : Iµ < 1.0 · 10−10 to 20. · 10−10 cm−2 s−1 , for Eµ > 100 GeV :Iµ < 0.2 · 10−10 to 5.0 · 10−10 cm−2 s−1 .

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window, nor in a 24 hour window. 6. Solar anisotropy L3+C’s sensitivity to the anisotropy of the arrival direction of primaries is 10−4 . No deviation from isotropy has been observed at the sideral frequency for any of the first 3 harmonics. For muons above 20 or 30 GeV (corresponding to primary protons around 250 GeV) a significant departure from isotropy has been found for the 2nd harmonics at Solar frequency (see Figure 9). The stucture found is similar in shape to the result of the GRAND experiment [25] at 0.1 GeV threshold, but with smaller amplitude.

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A particular study for a steady signal from Cyg X-3 during the full data taking period also revealed no particular signal. The obtained upper flux limits for the four muon energy thresholds is compared to other underground experimental results in Figure 8.

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More stringent limites are obtained by L3+C than given by other experiments at higher energies. The L3+C steady flux limits for the 10 sources mentioned above are comparable to the ones given here for Cyg X-3. 5. High energy signals from GRB ? The power emitted in GRBs is supposed to be mainly from the high energy part of the spectrum. Out of the BATSE catalogue 8 GRBs have been selected to search for high energy signals: GRB 990903, 990917, 991025, 991103, 991106, 000403, 000415, 000424. No signal with muons above 20 GeV have been found, neither within 10 sec following the GRB time, neither within a 1 hour

Figure 9. Anisotropy distribution in ”pseudo-right ascension” α ˜ (= [φˆ − h], with φˆ = phase of the concerned frequency and h = hour angle; the squared bracket means ”mod 2 π”) for muons with energy larger than 20 GeV. The distribution is fitted with the sum of the first two harmonics. The vertical bars represent the statistical errors.

7. Search for a signal from a Solar flare The particle acceleration mechanisms at or near the surface of the Sun are still under de-

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bate. Usual proton energies range up to 10 GeV, but according to an observation by the Baksan group [26], protons may be accelerated up to 500 GeV in solar flares (e.g. on the 29th of September 1989. - Since 1946 only 60 important flares have been observed). L3+C had the opportunity to verify this claim by analysing it’s muon data collected during the solar flare of the 14th of July 2000, when the Sun was just above Geneva at the flare time (around 10h30 UT). In one particular sky region - displaced to the North with respect to the solar track accross the sky - L3+C found an excess of 65 muons above a background of 235 with energies between 15 and 25 GeV. The probability for this excess to be a background fluctuation is 1% when taking into account the 41 sky cells analyzed. In Figure 10 the calculated upper possible flux emitted during this flare for protons with energies above 40 GeV is compared to neutron monitor data, other experiments and a theoretical limit for different proton energies and flares. The L3+C limit amounts to 2.8 · 10−3 cm−2 s−1 , according to an assumed proton spectrum proportional to Ep−6 .

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Figure 10. The upper limit of the solar flare induced proton flux obtained by this work compared with other measurements and theoretical upper limits [27].

8. Primary composition in the knee region L3+C has new tools to investigate the primary composition in the knee region. The rate of multimuon events as a function of multiplicity, the muon multiplicity as a function of the muon momenta, the spectra of muon momenta as a function of the shower size, estimated with the air shower scintillator array. - This analysis is not yet closed. Figure 11 shows a high multiplicity event reconstructed across several drift chamber octants.

Figure 11.

Example of a high multiplicity event crossing the L3+C detector.

9. Exotic events Motivated by the findings of exotic events by some underground experiments, L3+C has undertaken to search for large angle 2 and 3 prong events, apparently originating from a common vertex in- or outside of the detector. This analysis is also still on-going. A typical candidate is shown in Figure 12, which differs from an e+ e− → W + W − → μ+ μ− νµ νµ LEP event by

a shift of the vertex. The detection of the latter events allow for an estimate of the detection efficiency of the exotic events. 10. Meteorological effects Stimulated by the observed rate increase in the EAS scintillator detectors due to air radioactivity during rain fall, and the possibility to have

P. Le Coultre / Nuclear Physics B (Proc. Suppl.) 151 (2006) 314–321

5. 6. 7. 8. 9. 10. 11. 12. Figure 12. A candidate event ? 13. 14. 15. detected a rate increase during the Solar flare of the 14th July 2000, a detailed study on environmental and meteorological effects is in progress. 11. Conclusions: Different topics in ”astroparticle physics” could successfully be studied with the L3+C detector, a large ”particle physics” experiment. Acknowledgements The L3+C group would like to express their thanks to CERN, to Edgar Bugaev, John Ellis, Andreas Engel, Anatoly Erlykin, Paolo Lipari, Thomas Gaisser, Leonidas Resvanis, and Todor Stanev for helpful discussions and their continuous support. REFERENCES 1. B.Adeva et al., Nucl.Instr. Meth. A289 (1990) 35 2. O. Adriani et al., Nucl.Instr.and Meth. A488 (2002) 209 3. P.Achard et al., Phys.Lett. B598 (2004) 15; M. Unger, Ph.D. thesis, Humboldt Universit¨at Berlin, 2004, DESY-THESIS-2004-008 4. R. Brun et al.,“GEANT 3,”CERN Report DD/EE/84-1 (1987); S. Bottai and L. Per-

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