New OPERA results: the atmospheric muon charge ratio

New OPERA results:the atmospheric muon charge

ratioNicoletta Mauri

Università di Bologna and INFN

on behalf of the OPERA Collaboration

UZH Seminar on Particle and AstrophysicsUniversität Zürich, April 30th, 2014

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 Seminar30 April 2014 2

Introduction

3N. Mauri, UZH Seminar30 April 2014

• 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 direct nt 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

The OPERA experiment

Full coverage of the parameter space for the atmospheric neutrino sector

Main physics goal: prove nm nt oscillations in appearance mode

• Long baseline neutrino oscillation experiment located in the CNGS (CERN Neutrinos to Gran Sasso) nm beam• Direct search for nm nt oscillations detecting the t lepton produced in n t

CC interactions (appearance mode)4N. Mauri, UZH Seminar30 April 2014

Appearance detection

5N. Mauri, UZH Seminar30 April 2014

Direct observation of nm nt oscillation

Large mass ~O(kton) signal selection and background rejection High granularity ~1mm resolution

dEEEmEPEMNN CCDA )()(),()( 2

Emulsion Cloud Chamber

30 April 2014 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


20m

10m

10m

SM1SM2

Total target mass = 1.25 ktons

Target section:27 brick walls (75000 bricks)31 Target Tracker walls (TT)

Magnetic spectrometer:22 RPC planes 6 drift tube layers (PT stations)

Brick selection

m ID, charge, momentum

Calorimetry

Neutrino interaction trigger

n

B = 1.53 T

Brick: ECC target basic unit (57 nuclear emulsion films + 56 lead plates)

OPERA general structure


OPERA: an hybrid detector

N. Mauri, UZH Seminar 930 April 2014

n m n t search: signal candidates

t (r p0 p-) candidate

First n t candidate (Phys. Lett. B 691

(2010) 138)

t 3 h candidate

Second n t candidate (JHEP 11

(2013) 036)

t m candidate

Third n t candidate (Phys. Rev. D 89

(2014) 051102)

10

Neutrino oscillation results in progress

With a simple counting method and

conservative background

evaluation: 4.2s significance

30 April 2014 N. Mauri, UZH Seminar

Fourth n t candidate very recently presented by the OPERA Spokesperson at a Seminar at LNGS (25 March 2014)

t h candidate


OPERA as a cosmic ray detector

1400

m

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 106 w.r.t. surface, very reduced environmental radioactivity

OPERA vs previous and current underground experiments:a deep underground detector with charge and momentum reconstruction and excellent timing capabilities (~10 ns)

Atmospheric muon charge ratio


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)

13

m

m

(ordinary) meson decay: dNm/d cosq ~ 1/ cosq

p

K

hadronic interaction: multiparticle production s(A,E), dN/dx(A,E) extensive air shower

m

short-lifetimemeson production and prompt decay

(e.g. charmed mesons)

Isotropic angulardistribution

detection: Nm(A,E), dNm/dr

transverse size of bundle PT(A,E)

Primary C.R. proton/nucleus: A, E, isotropic

TeV muon propagation in the rock: radiative processes andfluctuations

The physics of cosmic ray TeV muons

30 April 2014 N. Mauri, UZH Seminar


Analytic predictions

p

p (primary) airnucleus

• 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

Ed

dEEEf p

inelpp

pp

),(

),()(

)( )1(pp

E

EEfdEEE

constE

Naïve prediction

Assuming Feynman scaling

)(),(~

xfEd

dEEEf pE

p

inelpp

pp

Feynmanscaling


Analytic predictions

R (E )

(E ) (E )

(E )Z

p

Zp

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 ( ~ 1.7)g in the SWM suppresses

the contribution 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

• R does not depend on E (or E ) nor on the target nature• R depends on the primary spectrum g

Zp f p

~

(x)x 1dx0

1Spectrum weighted moments

pZEconstE )1()()(

The pion spectum becomes:


Neutrons in the primary flux

• Elaborating the minimal model:– Introducing the neutron component in the primary flux

(in heavy nuclei) and considering the isospin symmetries:

one obtains:

p0, n0 = proton and neutrons in the primary flux

npnp

ZZZZ ,

82.0)np/()np( 00000

R 101 0

1.22

where

(Zp Z p ) /(Z

p Z p )

(1 Z pp Z pn ) /(Z pp Z pn )


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

parN

i ii

Nii

NN

N

b

Za

Z 1 )(/11

)(

E

E

)1)(1(

)()1()(

1

i

iii r

iBrraa

)/log)(1)(1(

))(1)(2()(

2

1

Niii

Niiii r

rbb

2)/( ii mmr

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 GeVeX > 107 GeV

q = 0o

q = 60o

ep eK


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 Λ or 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

pnpZZZ


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 is

therefore Emcosq*

• The Rm evolution as a function of Emcosq* spans over the different sources

Rm = wpRm p + wKRm

K + wcharmRmcharm +…

par

Ni

N

i ii

i

NN

N

b

Za

Z 1* )0(/cos11

)(

E

E

*cos/)0()( ii Earth

q*q

POWERFUL HANDLE TODISCRIMINATE MODELS

OPERA: Emcosq* ≈ 2000 GeV The (magnetized) experiment withthe largest Emcosq*


Rm measurements with Emcosq* > 1 TeVExperiments 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 (submitted to EPJC)– Underground magnetic spectrometer (1.53 T) at LNGS, average overburden

~3800 m.w.e., drift tubes + RPC + scintillators, 0 < q < 90

Experiments without magnetic field:• Kamiokande-II

M. 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 < < 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 < 15


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


Event reconstruction: pattern recognition

Real double muon event

Dedicated pattern recognition for cosmic events ( outside CNGS spill window): takes into account event topology and multiple events (m bundles):

hybrid strategy: global method (Hough Transform) + local method (pivot points)

r

y


PT track reconstruction3D tracks are used to “guide”track finding and fitting in the PTsystem: Find the line tangent to the drift circles with the best c2

250 mm position resolution 0.15 (1) mrad angular resolution for doublets (singlets) for f = 0 (improve for f > 0)

Residuals ~250 mm

Track reconstructed in 1 PT station (in a pair) singletTrack reconstructed in 2 PT station (in a pair) doublet


Good overall angular resolution“resolutions” < 1 deg both for zenith and azimuth direction reconstruction

Event reconstruction performance

q f

Multiple muon events well reconstructed

High angular resolution in the PT system


Charge and momentum reconstruction

• In each side of the magnet arm we can reconstruct an independent angle fj, j=1,...,6.

• Each fj can be reconstructed with one station (singlets) or two stations (doublets)

• We compute Dfk = fi – fj , k=1,...,4, angle differences between adjacent station-pairs

2

2

11

] / ) / ( exp[ 1

) / (

xz

yzk

s

s

Be dx dE

dx dE lp

B ≡ Bd/l = effective magnetic field

Top view of the OPERA detector

• 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

Df bending angle


Momentum reconstruction performance

Momentum resolution


Monte Carlo simulationsTwo 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) with primary composition model from Astropart. Phys. 19 (2003) 193

Muon flux

Aknee

A2

Aknee

A1

EE,EKdEdN

EE,EKdEdN

A2

A1

EEcut

g~2.7÷3Ecut~3000 TeV


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

)1(

)1(

meas

measunf

R

RR

2

2222

)]1([

)()1()()21(

meas

measmeas

unf

R

RRR


Data analysis• Data taken during the CNGS Physics Runs

in Standard 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)

in Inverted 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 (20082011) + ~823 k (2012)


Data reduction

A set of progressive cuts applied in order to isolate a clean data sample:a) at least one reconstructed Df angle

(acceptance cut)b) Remove events with large number of PT hits

(clean PT cut)c) Remove events with bendings smaller than

the experimental resolution (deflection cut)c’) Remove events with very large bendings

(effective for pm<5 GeV/c)


Quality cuts IClean PT Cut: number of digits allowed from geometrical considerations:

Ndata(re-scaled) = Ndata – NMC(geometrical)(f)

Geometrical MC dependence on the fangle

cut at 3s

Remove events with a large number of PT tubes fired (d-rays, secondary interactions, electronic noise etc) can induce wrong matching

M = M(f)N’ = N - M(f)

Re-scaled distribution of the experimental number of PT digits


Quality cuts II

misidentification probability

Deflection Cut: Cut on bending angle information below the PT resolution

before Deflection cut

after Deflection cut

• /Df sDf > 3

R saturates above 3s; as expected, for lower number of s, R 1 (charge misidentification ~50%, total randomization of charge reconstruction)

We also require Df < 100 mrad, to reject isolated secondary particles produced at large angles by high energy muons

MC distributions are split in the 2 regions m+

true and m-

true

Exp. data

MC data

m+ m+m- m-

03.0total

wrong

N

N

w/o deflection cut

w/ deflection cut


Alignment of the PT systemThe comparison between the two data sets with opposite magnet polarities allowed checking the systematics related to the alignment of each magnet arm

Comparing the Df distributions for the two magnet polarities:

(compatible with the systematic uncertainty quoted in the previous paper, EPJC 67(2010) 25)

|dfsyst|≈ 0.1 mrad

IP SP

4th magnet arm


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

Rmeas

(iSP / lSP

i

iIP / lIP )

(iSP / lSP

i

iIP / lIP )

where lSP and lIP

are the respective polarity live time

Finally unfolding the result with the charge misidentification:

)1(

)1(

meas

measunf

R

RR


New systematic uncertainty on Rm

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)|

Two main sources of systematic uncertainties: misalignment and charge

misidentification

m

(up)m

(down)

Reversing the muon orientation, the misalignment bias flips the sign

For single m: dRmunf(syst) = ±0.001

For multiple m: dRmunf(syst) = ±0.013


Systematic uncertainty on Rm

Charge misidentification h from experimental data• estimate dh = hdata – hMC for a subsample of events

crossing 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)Total systematic uncertainty for single m: dRm

unf(syst) = +0.007, -0.001

Total systematic uncertainty for multiple m: dRmunf(syst) = +0.015, -0.013

Quadratic sum of the two contributions (misalignment and misidentification):


Results: underground muon charge ratio

OK,nm=3

Rm computed separately for single and multiple muon events– Multiple muons: compute R m when the 3D

multiplicity is > 1, independently on the number of measured charges in the event

Nm ‹A› ‹E/A›primary

[TeV]H fraction Np/Nn Rm

unf

= 1 3.35 ± 0.09

19.4 ± 0.1 0.667 ± 0.007

4.99 ± 0.05

1.377 ± 0.006

> 1 8.5 ± 0.3 77 ± 1 0.352 ± 0.012

2.09 ± 0.07

1.098 ± 0.023

“dilution” of Rm when proton-Air and neutron-Air interactions change their relative contributions

primary features extracted from MC2

New OPERA data (full statistics)


Charge ratio of multiple muon events

• The smaller value of the charge ratio of multiple muons is due to the convolution of two effects:larger n/p ratio in the all-nucleon spectrum different xF region

Feynman x: xF ≅ Esecondary/Eprimary

n/p ratio in primary cosmic rays

Multiple muon sample: higher E/nucleon, higher average A

Multiple muon sample: smaller xF, towards the central region


Rm as a function of pm

• Rm (single muons) was computed in bins of the underground momentum pm and unfolded

• Evolution with p m is compatible both with a constant and with a logarithmic energy increase, with a 2.4 spreference for the latter

Rm = a0 + a1 log10 p m

a0 = 1.322 ± 0.023a1 = 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)


The resolution on E m is dominated by fluctuations in the stochastic term b of the energy loss

Best performance is achieved using the “crude” MC2 Build a table EmMPV = f(h,pm): take the MPV of the Landau distribution better resolution and residuals well centered

h (bin 2)

pm (bin 3)

Emsurface

MPV

/)/( heEE

a= ( )a E = ( )b b E

EEEdh

dE)()(

Rm as a function of E m cos q*


Fixing RKp = 0.127 (weighted average of experimental values, Ref. Grashorn et al.):fp+ = 0.5512 ± 0.0014fK+ = 0.705 ± 0.014

only single muons

Rm as a function of E m cos q*

Compute and unfold the charge ratio in each bin

Fit with the function


Rm as a function of E m cos q* 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 E m cos q*!)

Different dependencies: fit in 2-dimensions (Em, cos q*)

20 bins: 5 energy bins × 4 angular bins

Fixed parameters (see table) Inferred parameters: ZpK+ and d0


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 <cos q*> ≅ 0.7:

Rm as a function of the surface muon energyonly single muons


Conclusions• The OPERA detector, designed to prove n n oscillations in

appearance mode, was exploited for the measurement of the atmospheric muon charge ratio R

– 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 E m cos q*

~ 10 TeV

We extracted relevant parameters on the primary composition (d0) and the associated kaon production in the forward fragmentation region (ZpK+ moment)

The Rm behaviour as a function of E m supports the validity of Feynman scaling in the fragmentation region up to E m ~ 20 TeV, corresponding to primary energy/nucleon E N ~ 200 TeV

New OPERA results: the atmospheric muon charge ratio

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