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New OPERA results: the atmospheric muon charge ratio

New OPERA results: the atmospheric muon charge ratio. Nicoletta Mauri Università di Bologna and INFN on behalf of the OPERA Collaboration. UZH Seminar on Particle and Astrophysics Universität Zürich , April 30 th , 2014. Outline. The OPERA experiment detector and physics case

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New OPERA results: the atmospheric muon charge ratio

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  1. New OPERA results:the atmospheric muon charge ratio Nicoletta Mauri Università di Bologna and INFN on behalf of the OPERA Collaboration UZH Seminar on Particle and Astrophysics Universität Zürich, April 30th, 2014

  2. Outline • The OPERA experiment • detector and physics case • neutrino oscillation results • New results on non-oscillation physics • atmospheric muon charge ratio (submitted to EPJC) • OPERA as a cosmic ray detector • Data Analysis and Combination • Inference on primary composition and forward physics • Conclusions N. Mauri, UZH Seminar

  3. Introduction • In the last decades several experiments provided evidence for neutrino oscillations: conversion in-flight of lepton flavor • Quantum-mechanical interference phenomenon • Solar, atmospheric, artificial beams: all disappearance experiments (or indirect appearance) • The OPERA experiment was mainly designed to unambiguously prove the oscillation phenomenon through directnt appearance • Definitely close of the discovery phase of neutrino oscillations • The detector - although optimized for beam neutrino detection - can also be exploited for non-oscillation studies • Cosmic ray physics at the Gran Sasso Lab N. Mauri, UZH Seminar

  4. The OPERA experiment Main physics goal: prove nm nt oscillations in appearance mode Full coverageof the parameterspacefor the atmospheric neutrino sector • Long baseline neutrino oscillation experiment located in the CNGS (CERN • Neutrinos to Gran Sasso) nm beam • Direct search for nm ntoscillations detecting the t lepton produced in nt • CC interactions (appearance mode) 30 April 2014 N. Mauri, UZH Seminar

  5. Appearance detection Direct observation of nm ntoscillation • Large mass ~O(kton) • signal selection and background rejection High granularity~1mm resolution Emulsion Cloud Chamber N. Mauri, UZH Seminar

  6. Neutrino interaction detector (ECC) • Target basic unit: brick of 57 nuclear emulsions interleaved by lead plates • + 2 interface emulsions (CS) • high resolution and large mass in a modular way • unambiguous measurement of the kink • “stand-alone” detector N. Mauri, UZH Seminar

  7. SM1 SM2 10m 10m 20m OPERA general structure Brick: ECC target basic unit (57 nuclear emulsion films + 56 lead plates) Target section: 27 brickwalls (75000 bricks) 31 Target Trackerwalls (TT) n Neutrino interaction trigger Brick selection Calorimetry Magnetic spectrometer: 22 RPC planes 6drift tubelayers(PT stations) B = 1.53 T m ID, charge, momentum Total target mass = 1.25 ktons N. Mauri, UZH Seminar

  8. OPERA: an hybrid detector N. Mauri, UZH Seminar

  9. nm ntsearch: signalcandidates Firstntcandidate (Phys. Lett. B 691 (2010) 138) Secondntcandidate (JHEP 11 (2013) 036) tr ( p0p-) candidate Thirdntcandidate (Phys. Rev. D 89 (2014) 051102) t3 hcandidate N. Mauri, UZH Seminar tmcandidate

  10. Neutrino oscillation results in progress Fourthntcandidate very recently presented by the OPERA Spokesperson at a Seminar at LNGS (25 March 2014) thcandidate • With a simple counting method and conservative background evaluation: 4.2s significance N. Mauri, UZH Seminar

  11. 1400 m OPERA as a cosmic ray detector CNGS beam events are identified through a timing coincidence with CERN  possibility to collect cosmic events during the physics run Gran Sasso underground lab: 1400 m of rock (3800 m.w.e) shielding, cosmic ray flux reduced by a factor 106w.r.t. surface, very reduced environmental radioactivity • OPERA vsprevious and current underground experiments:a deep underground detector with charge and momentum reconstruction and excellent timing capabilities (~10 ns) • Atmospheric muon charge ratio N. Mauri, UZH Seminar

  12. The atmospheric muon charge ratio • The atmospheric muon charge ratio Rm≡ Nm+/Nm-is being studied and measured since many decades • Depends on the chemical composition and energy spectrum of the primary cosmic rays • Depends on the hadronic interaction features • At high energy, depends on the prompt component • It provides the possibility to check HE hadronic interaction models (E>1TeV) in the fragmentation region, where no data exists • Since atmospheric muons are kinematically related to atmospheric neutrinos (same sources), Rm provides a benchmark for atmospheric n flux computations (e.g. background for neutrino telescopes) N. Mauri, UZH Seminar

  13. The physics of cosmic ray TeVmuons hadronic interaction: multiparticle productions(A,E), dN/dx(A,E)  extensive air shower Primary C.R. proton/nucleus:A, E, isotropic K (ordinary) meson decay:dNm/d cosq~ 1/ cosq p m short-lifetime meson production and prompt decay (e.g. charmed mesons) Isotropic angular distribution transverse size of bundle  PT(A,E) m m TeV muon propagation in the rock:radiative processes and fluctuations detection:Nm(A,E), dNm/dr N. Mauri, UZH Seminar

  14. Analytic predictions Naïve prediction • Assume only primary protons with a spectrum dN/dE = N0E-(1+g) • Assume only pions and neglect muon decays (HE limit) • Consider the inclusive cross-section for pions • The pion spectrum is then p (primary) air nucleus Assuming Feynman scaling p Feynman scaling N. Mauri, UZH Seminar

  15. Analytic predictions The pion spectum becomes: Spectrum weighted moments Finally, the muon charge ratio prediction: • Interpretation of the prominent features: • The result is valid only in the fragmentation region, since in the central region Feynman scaling is strongly violated • But the steeply falling primary spectrum (g ~ 1.7) in the SWM suppresses thecontribution of the central region in the secondary production  scaling holds (at least for E < 1 TeV);In other words: each pion is likely to have an energy close to the one of the projectile (primary CR proton) and comes from its fragmentation (valence quarks) positive charge • Rmdoes not depend onEp (or Em) nor on the target nature • Rmdepends on the primary spectrumg N. Mauri, UZH Seminar

  16. Neutrons in the primary flux • Elaborating the minimal model: • Introducing the neutroncomponent in the primaryflux (in heavy nuclei) and considering the isospinsymmetries: oneobtains: p0, n0 = proton and neutrons in the primaryflux N. Mauri, UZH Seminar

  17. Kaon contribution • At higher energy (>100 GeV) the contribution of K becomes important • In general, the contribution of eachcomponent to the muon flux Npar = (p, K, charmed, etc.) depends on the relative contributionof decays and interaction probabilities: where q = 0o q = 60o ep eK ei = ei(q) is the “critical energy”, i.e. the energy above which interactions dominate over decays. Along the vertical (q = 0o)ei(0)= mich/ti (h = 6.5 km) ep = 115 GeV eK = 850 GeV eX > 107GeV N. Mauri, UZH Seminar

  18. Kaon contribution to Rm • For kaons: because the reaction pp  K+ L N + anything is favoured. Conservation of strangeness requires production of K+ and K0 in association with a Λor S (associate production). On the other hand, the production of K−,K0 requires the creation of an associated baryon and an additional strange meson. This leads to a larger Rm ratio at high energy N. Mauri, UZH Seminar

  19. Parameterization of the charge ratio • Let us consider again the general form for the muon flux where we have explicited the ei(q) dependence on q where q* is the zenith angle at the production point • The correct variable to describe the evolution of Rm istherefore Emcosq* • The Rm evolution as a function of Emcosq* spans over the different sources Rm = wpRmp+ wKRmK + wcharmRmcharm +… q* q Earth POWERFUL HANDLE TO DISCRIMINATE MODELS The (magnetized) experiment with the largest Emcosq* OPERA: Emcosq* ≈ 2000GeV N. Mauri, UZH Seminar

  20. Rm measurements with Emcosq*> 1 TeV Experiments with magnetic field: • Utah: G. K. Ashley et al., Phys. Rev. D12 (1975) 20 • Underground at Utah University, flat surface above ~1400 m.w.e., magnetic spectrometer (1.63 T) + spark chambers, six bins with 46 < q < 78 • MINOS:P. Adamson et al., Phys. Rev. D76 (2007) 052003 +Phys. Rev. D 83 , 032011 (2011) • Underground at Soudan, magnetized steel, flat surface above ~2000 m.w.e., 0 < q < 90 • OPERA:N. Agafonova et al., Eur. Phys. J. C67 (2010) 25 + arXiv:1403.0244 (submittedto EPJC) • Underground magneticspectrometer (1.53 T) at LNGS, average overburden ~3800 m.w.e.,drift tubes + RPC + scintillators, 0 < q < 90 Experiments without magnetic field: • Kamiokande-IIM. Yamada et al., Phys. Rev. D44 (1991) 617 • Underground Cherenkov detector at Kamioka ~2700 m.w.e., delayed events on stopping muons, one bin with 0 < q < 90 • LVD:N. Agafonova et al., Proc. 31th ICRC, ŁÓDZ 2009 + arXiv:1311.6995 • Underground at LNGS, average overburden ~3800 m.w.e., scintillators, delayed events on stopping muons, one bin with q < 15 N. Mauri, UZH Seminar

  21. Cosmic event reconstruction in OPERA • Cosmic-event tagging: • Outside the CNGS spill window • Dedicated pattern recognition and track finding/fitting: • Cosmic events are passing-through • Cosmic events comes from all directions • Cosmic events may have multiple parallel tracks (in OPERA 5% of the events are muon bundles) • Different reconstruction philosophy N. Mauri, UZH Seminar

  22. Event reconstruction: pattern recognition • Dedicated pattern recognition for cosmic events (outside CNGS spill window): • takes into account event topology and multiple events (mbundles): • hybrid strategy: • global method (Hough Transform) + local method (pivot points) Realdoublemuonevent r y N. Mauri, UZH Seminar

  23. PT track reconstruction • 3D tracks are used to “guide”track finding and fitting in the PTsystem: • Find the linetangentto thedriftcircleswith the best c2 • 250 mm position resolution • 0.15 (1) mradangularresolutionfordoublets (singlets) forf = 0 • (improveforf > 0) Residuals ~250 mm Track reconstructed in 1 PT station (in a pair) singlet Track reconstructed in 2 PT station (in a pair) doublet N. Mauri, UZH Seminar

  24. Event reconstruction performance • Multiple muon events well reconstructed • High angular resolution in the PT system f q • Good overall angular resolution • “resolutions” < 1 deg both for zenith and azimuth direction reconstruction N. Mauri, UZH Seminar

  25. Charge and momentum reconstruction • In each side of the magnet arm we can reconstruct an independent angle fj, j=1,...,6. • Each fjcan be reconstructed with one station (singlets) or two stations (doublets) • We compute Dfk= fi – fj , k=1,...,4, angle differences between adjacent station-pairs Dfbending angle Top view of the OPERA detector B ≡Bd/l= effective magnetic field • Charge is reconstructed according to the Df sign • If each track contributes to multiple Df angles, a weighted average is computed • [weights = experimental errors on Df] N. Mauri, UZH Seminar

  26. Momentum reconstruction performance Momentum resolution N. Mauri, UZH Seminar

  27. Monte Carlo simulations • Two Monte Carlo simulations for different purposes: • 1) a fast tool for the cross-check with experimental data, for the validation of the analysis software and for unfolding • a parameterized generator (MC1) with a primary • chemical composition based on the MACRO fit model • [Phys. Rev. D56 (1997) 1418] • 2) a reliable tool for surface muon energy estimation and a link between underground variables and primary cosmic ray parameters • a FLUKA-based full Monte Carlo simulation (MC2) • withprimarycompositionmodel • fromAstropart. Phys. 19 (2003) 193 Muonflux g~2.7÷3 Ecut~3000 TeV Ecut E N. Mauri, UZH Seminar

  28. Charge misidentification • h≡ fraction of tracks reconstructed with wrong charge sign • Used to correct data (unfold) • In the LE limit (only MCS) the estimate is ~10-3 • Real h is larger due to spurious effects: • Secondary particles • Timing errors N. Mauri, UZH Seminar

  29. Data analysis • Data taken during the CNGS Physics Runs • inStandard Magnet Polarity (SP): • 2008 (from Jun 18th until Nov 3rd) • 2009 (from Jun 1st until Nov 23rd) • 2010 (from Apr 29th until Nov 22nd) • 2011 (from Mar 18th until Nov 17th) • inInverted Magnet Polarity (IP): • 2012 (from Mar 22nd until Dec 3rd) • Data Pre-Selection:on the basis of data quality and global variable distributions (based on TT, RPC, PT digits) • Live time Standard Polarity (SP): 625.0 days • Live time Inverted Polarity (IP): 234.8 days • Total number of events: ~2.22 M (20082011) + ~823 k (2012) N. Mauri, UZH Seminar

  30. Data reduction A set of progressive cutsapplied in orderto isolate a clean data sample: • at leastonereconstructedDf angle (acceptance cut) • Removeeventswithlargenumberof PT hits (clean PT cut) • Removeeventswithbendingssmallerthan the experimentalresolution (deflection cut) c’) Removeeventswithverylargebendings(effectivefor pm<5 GeV/c) N. Mauri, UZH Seminar

  31. cut at 3s Quality cuts I Clean PT Cut: number of digits allowed from geometrical considerations: Ndata(re-scaled) = Ndata – NMC(geometrical)(f) Remove events with a large number of PT tubes fired (d-rays, secondary interactions, electronic noise etc)  can induce wrong matching Re-scaled distribution of the experimental number of PT digits Geometrical MC dependence on thefangle M = M(f) N’ = N - M(f) N. Mauri, UZH Seminar

  32. Exp. data MC data Quality cuts II Deflection Cut: Cut on bending angle information below the PT resolution • Df/sDf > 3 • Rmsaturates above 3s; as expected, for lower number of s, Rm 1 (charge misidentification ~50%, total randomization of charge reconstruction) • We also requireDf< 100 mrad, to reject isolated secondary particles produced at large angles by high energy muons before Deflection cut after Deflection cut w/o deflection cut MC distributions are split in the 2 regions m+trueandm-true w/ deflection cut m- m+ m- m+ misidentification probability N. Mauri, UZH Seminar

  33. Alignment of the PT system The comparison between the two data sets with opposite magnet polarities allowed checking the systematics related to the alignment of each magnet arm Comparing the Dfdistributions for the two magnet polarities: (compatible with the systematic uncertainty quoted in the previous paper, EPJC 67(2010) 25) |dfsyst|≈ 0.1 mrad 4thmagnetarm IP SP N. Mauri, UZH Seminar

  34. Combination of data setswith opposite magnet polarities Using the two data sets with opposite magnet polarities allows disposing of misalignment systematics. Assuming the symmetric behaviour of the bin-to-bin migration matrix in the opposite polarities, the proper combination of the two data sets is given by the ratio of the sums (m+SP+ m+IP) and (m-SP+ m-IP), where the numbers are normalized by their polarity live time wherelSPandlIPare therespectivepolarity live time Finally unfolding the result with the charge misidentification: N. Mauri, UZH Seminar

  35. New systematic uncertainty on Rm Two main sources of systematic uncertainties: misalignment and charge misidentification • Misalignment: combination procedure • in principle all the systematic contributions due to misalignment cancel with the SP and IP data combination • Estimate of the residual systematic uncertainty related to the combination procedure: difference between the charge ratio Rm for muons coming from opposite directions: dRm = |Rm(up)-Rm(down)| Reversing the muon orientation, the misalignment bias flips the sign m(up) m(down) For single m: dRmunf(syst) = ±0.001 For multiple m:dRmunf(syst) = ±0.013 N. Mauri, UZH Seminar

  36. Systematic uncertainty on Rm • Charge misidentification hfrom experimental data • estimate dh = hdata – hMCfor a subsample of eventscrossing both arms of a spectrometer: computation of the probability p of reconstructing opposite charges [sign(Df1) = -sign(Df2)] • the probability that Df1 and Df2 have opposite sign is related to hthrough a function h = h(p), computed via MC dh = 0.007  dRm = 0.007 (one-sided: hreal≥ hMC) Quadratic sum of the two contributions (misalignment and misidentification): Total systematic uncertainty for single m: dRmunf(syst) = +0.007, -0.001 Total systematic uncertainty for multiple m:dRmunf(syst) = +0.015, -0.013 N. Mauri, UZH Seminar

  37. Results: underground muon charge ratio Rmcomputedseparatelyfor single and multiple muonevents • Multiple muons: computeRmwhen the 3D multiplicityis > 1, independently on the numberofmeasuredcharges in the event New OPERA data (full statistics) OK, nm=3 primary features extracted from MC2 “dilution” ofRmwhenproton-Air and neutron-Airinteractionschangetheir relative contributions N. Mauri, UZH Seminar

  38. Charge ratio of multiple muon events • The smaller value of the charge ratio of multiple muons is due to the convolution of twoeffects:larger n/p ratio in the all-nucleonspectrumdifferentxFregion Multiple muon sample: higher E/nucleon, higher average A Multiple muon sample: smaller xF, towards the central region Feynmanx: xF≅Esecondary/Eprimary n/p ratio in primary cosmic rays N. Mauri, UZH Seminar

  39. Rm as a function of pm • Rm (single muons) wascomputed in binsof the underground momentum pm and unfolded • Evolutionwith pm iscompatiblebothwith a constant and with a logarithmicenergyincrease, with a 2.4s preferencefor the latter Rm = a0 + a1 log10 pm a0 = 1.322 ± 0.023 a1 = 0.030 ± 0.012 (c2/dof = 14.99/16) Rm = c0  c0 = 1.377 ± 0.006 (c2/dof = 20.86/17) Dc2/dof = 5.87/1 (~2.4 sigma) N. Mauri, UZH Seminar

  40. Rm as a function of Emcosq* • The resolution on Emisdominatedbyfluctuations in the stochastictermbof the energy loss • Best performance is achieved using the “crude” MC2 • Build a table EmMPV = f(h,pm): • take the MPV of the Landau distribution • better resolution and residuals well centered • = a(E) • b = b(E) h (bin 2) pm (bin 3) Emsurface MPV N. Mauri, UZH Seminar

  41. Rm as a function of Em cos q* only single muons • Compute and unfold the charge ratio in each bin • Fit with the function Fixing RKp = 0.127 (weighted average of experimental values, Ref. Grashorn et al.): fp+ = 0.5512 ± 0.0014fK+ = 0.705 ± 0.014 N. Mauri, UZH Seminar

  42. Rm as a function of Emcosq* and d0 Taking into account an explicit dependence on d0= (p - n)/(p + n): (Gaisser, Astropart. Phys. 35 (2012) 801) d0 depends on Eprimary/nucleon ≈ 10 Em (not on Emcosq*!) • Different dependencies: • fit in 2-dimensions (Em, cosq*) • 20 bins: 5 energy bins × 4 angular bins • Fixed parameters (see table) • Inferred parameters:ZpK+ and d0 N. Mauri, UZH Seminar

  43. Rm as a function of Em d0 (EN ≈ 20 TeV/n)= 0.61 ± 0.02ZpK+ = 0.0086 ± 0.0004 Fit result: Projecting the fit result on the average OPERA zenith <cosq*> ≅0.7: Rm as a function of the surface muon energy only single muons N. Mauri, UZH Seminar

  44. Conclusions • The OPERA detector, designed to prove nmnt oscillations in appearance mode,was exploited for the measurement of the atmospheric muon charge ratio Rm • New data with the full OPERA statistics (2008-2012) were presented: the atmospheric muon charge ratio was measured in the highest energy region • The combination of the two data sets with opposite magnet polarities allowed minimizing systematic uncertainties • Rm was measured separately for single and for multiple muon events • We found a strong reduction of the charge ratio for multiple muon events • The integral value and the energy dependence of the charge ratio for single muons are compatible with the expectation from a simple p-K model • No significant contribution of the prompt component up to Emcosq*~ 10 TeV • We extracted relevant parameters on the primary composition (d0) and the associated kaon production in the forward fragmentation region (ZpK+moment) • The Rmbehaviour as a function of Emsupports the validity of Feynman scaling in the fragmentation region up to Em~ 20 TeV, corresponding to primary energy/nucleon EN ~ 200 TeV N. Mauri, UZH Seminar

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