The Heavy Flavor Averaging Group HFAG has combined time-integrated and time-dependent measurements of CP asymmetries, taking account of the different decay time acceptances, to obtain wo
Trang 1Evidence for CP Violation in Time-Integrated D0! hhþDecay Rates
R Aaij et al.*
(LHCb Collaboration) (Received 6 December 2011; published 12 March 2012; publisher error corrected 12 March 2012)
A search for time-integrated CP violation in D0! hhþ (h ¼ K, ) decays is presented using
0:62 fb1of data collected by LHCb in 2011 The flavor of the charm meson is determined by the charge
of the slow pion in theDþ! D0þandD! D0decay chains The difference inCP asymmetry
between D0! KKþ and D0! þ, ACP ACPðKKþÞ ACPðþÞ, is measured to be
½0:82 0:21ðstatÞ 0:11ðsystÞ% This differs from the hypothesis of CP conservation by 3.5 standard
deviations
DOI: 10.1103/PhysRevLett.108.111602 PACS numbers: 13.25.Ft, 11.30.Er, 13.85.Ni
The charm sector is a promising place to probe for the
effects of physics beyond the standard model (SM) There
has been a resurgence of interest in the past few years since
evidence forD0mixing was first seen [1,2] Mixing is now
well established [3] at a level which is consistent with, but
at the upper end of, SM expectations [4] By contrast, no
evidence for CP violation in charm decays has yet been
found
The time-dependent CP asymmetry ACPðf; tÞ for D0
decays to aCP eigenstate f (with f ¼ f) is defined as
ACPðf; tÞ ðD0ðtÞ ! fÞ ð D0ðtÞ ! fÞ
ðD0ðtÞ ! fÞ þ ð D0ðtÞ ! fÞ; (1)
where is the decay rate for the process indicated In
generalACPðf; tÞ depends on f For f ¼ KKþ andf ¼
þ,ACPðf; tÞ can be expressed in terms of two
contri-butions: a direct component associated withCP violation
in the decay amplitudes, and an indirect component
asso-ciated withCP violation in the mixing or in the
interfer-ence between mixing and decay In the limit of U-spin
symmetry, the direct component is equal in magnitude and
opposite in sign forKKþ andþ, though the size of
U-spin breaking effects remains to be quantified precisely
[5] The magnitudes ofCP asymmetries in decays to these
final states are expected to be small in the SM [5 8], with
predictions of up toOð103Þ However, beyond the SM the
rate ofCP violation could be enhanced [5,9]
The asymmetryACPðf; tÞ may be written to first order
as [10,11]
ACPðf; tÞ ¼ adir
CPðfÞ þt
aindCP; (2)
where adir
CPðfÞ is the direct CP asymmetry, is the D0 lifetime, and aind
CP is the indirect CP asymmetry To a good approximation this latter quantity is universal [5,12] The time-integrated asymmetry measured by an experiment,ACPðfÞ, depends upon the time acceptance of that experiment It can be written as
ACPðfÞ ¼ adir
CPðfÞ þhti
aindCP; (3) where hti is the average decay time in the reconstructed sample Denoting by the differences between quantities for D0! KKþ andD0 ! þ it is then possible to write
ACP ACPðKKþÞ ACPðþÞ
¼ ½adir
CPðKKþÞ adir
CPðþÞ þhti
aindCP: (4)
In the limit that hti vanishes, ACP is equal to the difference in the direct CP asymmetry between the two decays However, if the time acceptance is different for the
KKþandþfinal states, a contribution from indirect
CP violation remains
The most precise measurements to date of the time-integrated CP asymmetries in D0 ! KKþ and D0!
þwere made by the CDF, BABAR, and Belle collab-orations [10,13,14] The Heavy Flavor Averaging Group (HFAG) has combined time-integrated and time-dependent measurements of CP asymmetries, taking account of the different decay time acceptances, to obtain world average values for the indirectCP asymmetry of aind
CP¼ ð0:03 0:23Þ% and the difference in direct CP asymmetry between the final states of adirCP¼ ð0:42 0:27Þ% [3]
In this Letter, we present a measurement of the differ-ence in time-integrated CP asymmetries between D0!
KKþ and D0! þ, performed with 0:62 fb1 of data collected at LHCb between March and June 2011 The flavor of the initial state (D0 or D0) is tagged by requiring a Dþ! D0þ
s decay, with the flavor deter-mined by the charge of the slow pion (þ
s) The inclusion
*Full author list given at the end of the article
Published by the American Physical Society under the terms of
the Creative Commons Attribution 3.0 License Further
distri-bution of this work must maintain attridistri-bution to the author(s) and
the published article’s title, journal citation, and DOI
Trang 2of charge-conjugate modes is implied throughout, except
in the definition of asymmetries
The raw asymmetry for taggedD0decays to a final state
f is given by ArawðfÞ, defined as
ArawðfÞ NðDþ! D0ðfÞþsÞ NðD! D0ðfÞsÞ
NðDþ! D0ðfÞþ
sÞ þ NðD! D0ðfÞ
sÞ; (5) whereNðXÞ refers to the number of reconstructed events of
decayX after background subtraction
To first order the raw asymmetries may be written as a
sum of four components, due to physics and detector
effects:
ArawðfÞ ¼ ACPðfÞ þ ADðfÞ þ ADðþ
sÞ þ APðDþÞ: (6) Here, ADðfÞ is the asymmetry in selecting the D0 decay
into the final statef, ADðþ
sÞ is the asymmetry in selecting the slow pion from theDþ decay chain, andAPðDþÞ is
the production asymmetry forDþmesons The
asymme-tries AD and AP are defined in the same fashion asAraw
The first-order expansion is valid since the individual
asymmetries are small
For a two-body decay of a spin-0 particle to a
self-conjugate final state there can be no D0 detection
asym-metry, i.e., ADðKKþÞ ¼ ADðþÞ ¼ 0 Moreover,
ADðþ
sÞ and APðDþÞ are independent of f and thus in
the first-order expansion of Eq (5) those terms cancel in
the differenceArawðKKþÞ ArawðþÞ, resulting in
ACP¼ ArawðKKþÞ ArawðþÞ: (7)
To minimize second-order effects that are related to the
slightly different kinematic properties of the two decay
modes and that do not cancel in ACP, the analysis is
performed in bins of the relevant kinematic variables, as
discussed later
The LHCb detector is a forward spectrometer covering
the pseudorapidity range 2 < < 5, and is described in
detail in Ref [15] The Ring Imaging Cherenkov (RICH)
detectors are of particular importance to this analysis,
providing kaon-pion discrimination for the full range of
track momenta used The nominal downstream beam
di-rection is aligned with theþz axis, and the field direction
in the LHCb dipole is such that charged particles are
deflected in the horizontal (xz) plane The field polarity
was changed several times during data taking: about 60%
of the data were taken with the down polarity and 40% with
the other
Selections are applied to provide samples of Dþ!
D0þ
s candidates, with D0 ! KKþ or þ Events
are required to pass both hardware and software trigger
levels A looseD0 selection is applied in the final state of
the software trigger, and in the offline analysis only
can-didates that are accepted by this trigger algorithm are
considered Both the trigger and offline selections impose
a variety of requirements on kinematics and decay time to
isolate the decays of interest, including requirements on the track fit quality, on theD0andDþvertex fit quality, on the transverse momentum (pT> 2 GeV=c) and decay time (pT > 100 m) of the D0 candidate, on the angle between the D0 momentum in the lab frame and its daughter mo-menta in the D0 rest frame (j cosj < 0:9), that the D0 trajectory points back to a primary vertex, and that the
D0daughter tracks do not In addition, the offline analysis exploits the capabilities of the RICH system to distinguish between pions and kaons when reconstructing the D0 meson, with no tracks appearing as both pion and kaon candidates
A fiducial region is implemented by imposing the re-quirement that the slow pion lies within the central part of the detector acceptance This is necessary because the magnetic field bends pions of one charge to the left and those of the other charge to the right For soft tracks at large angles in thexz plane this implies that one charge is much more likely to remain within the 300 mrad horizontal detector acceptance, thus making ADðþ
sÞ large Although this asymmetry is formally independent of the
D0 decay mode, it breaks the assumption that the raw asymmetries are small and it carries a risk of second-order systematic effects if the ratio of efficiencies of D0!
KKþ and D0 ! þ varies in the affected region. The fiducial requirements therefore exclude edge regions
in the slow pion (px,p) plane Similarly, a small region of phase space in which one charge of slow pion is more likely to be swept into the beampipe region in the down-stream tracking stations, and hence has reduced efficiency,
is also excluded After the implementation of the fiducial requirements about 70% of the events are retained The invariant mass spectra of selected KKþ and
þ pairs are shown in Fig. 1 The half width at half maximum of the signal line shape is 8:6 MeV=c2 forKKþand 11:2 MeV=c2 forþ, where the differ-ence is due to the kinematics of the decays and has
no relevance for the subsequent analysis The mass differ-ence (m) spectra of selected candidates, where m mðhhþþ
sÞ mðhhþÞ mðþÞ for h ¼ K, , are shown in Fig 2 Candidates are required to lie inside a widem window of 0–15 MeV=c2, and in Fig.2and for all subsequent results candidates are in addition required to lie
in a mass signal window of 1844–1884 MeV=c2 TheDþ signal yields are approximately 1:44 106 in the KKþ sample, and 0:38 106in theþsample Charm from b-hadron decays is strongly suppressed by the requirement that theD0 originate from a primary vertex, and accounts for only 3% of the total yield Of the events that contain at least one Dþ candidate, 12% contain more than one candidate; this is expected due to background soft pions from the primary vertex and all candidates are accepted The background-subtracted average decay time of D0 candidates passing the selection is measured for each final state, and the fractional difference hti= is obtained
Trang 3Systematic uncertainties on this quantity are assigned for
the uncertainty on the world averageD0lifetime (0.04%),
charm fromb-hadron decays (0.18%), and the
background-subtraction procedure (0.04%) Combining the systematic
uncertainties in quadrature, we obtain hti= ¼ ½9:83
0:22ðstatÞ 0:19ðsystÞ% The þandKKþaverage
decay time ishti ¼ ð0:8539 0:0005Þ ps, where the error
is statistical only
Fits are performed on the samples in order to determine
ArawðKKþÞ and ArawðþÞ The production and
detec-tion asymmetries can vary withpT and pseudorapidity,
and so can the detection efficiency of the two differentD0
decays, in particular, through the effects of the particle
identification requirements The analysis is performed in
54 kinematic bins defined by the pT and of the Dþ
candidates, the momentum of the slow pion, and the sign of
px of the slow pion at the Dþ vertex The events are
further partitioned in two ways First, the data are divided
between the two dipole magnet polarities Second, the first
60% of data are processed separately from the remainder,
with the division aligned with a break in data taking due to
an LHC technical stop In total, 216 statistically
indepen-dent measurements are considered for each decay mode
In each bin, one-dimensional unbinned maximum like-lihood fits to them spectra are performed The signal is described as the sum of two Gaussian functions with a common mean but different widths i, convolved with a function Bðm; sÞ ¼ ðmÞms taking account of the asymmetric shape of the measuredm distribution Here,
s ’ 0:975 is a shape parameter fixed to the value deter-mined from the global fits shown in Fig 2, is the Heaviside step function, and the convolution runs over
m The background is described by an empirical function
of the form 1 eðmm0 Þ=, wherem0 and are free parameters describing the threshold and shape of the func-tion, respectively TheDþandDsamples in a given bin are fitted simultaneously and share all shape parameters, except for a charge-dependent offset in the central value and an overall scale factor in the mass resolution The raw asymmetry in the signal yields is extracted directly from this simultaneous fit No fit parameters are shared between the 216 subsamples of data, nor between the KKþ and
þfinal states.
The fits do not distinguish between the signal and back-grounds that peak inm Such backgrounds can arise from
Dþdecays in which the correct slow pion is found but the
FIG 2 (color online) Fits to them spectra, where the D0 is reconstructed in the final states (a)KKþ and (b)þ, with
mass lying in the window of 1844–1884 MeV=c2 The dashed line corresponds to the background component in the fit
FIG 1 (color online) Fits to the (a) mðKKþÞ and
(b)mðþÞ spectra of Dþ candidates passing the selection
and satisfying 0 < m < 15 MeV=c2 The dashed line
corre-sponds to the background component in the fit, and the vertical
lines indicate the signal window of 1844–1884 MeV=c2
Trang 4D0 is partially misreconstructed These backgrounds
are suppressed by the use of tight particle identification
requirements and a narrow D0 mass window From
studies of the D0 mass sidebands (1820–1840 and
1890–1910 MeV=c2), this contamination is found to be
approximately 1% of the signal yield and to have small
raw asymmetry (consistent with zero asymmetry
differ-ence between theKKþandþfinal states) Its effect
on the measurement is estimated in an ensemble of
simu-lated experiments and found to be negligible; a systematic
uncertainty is assigned below based on the statistical
pre-cision of the estimate
A value of ACPis determined in each measurement bin
as the difference betweenArawðKKþÞ and ArawðþÞ
Testing these 216 measurements for mutual consistency,
we obtain 2=ndf ¼ 211=215 ( 2 probability of 56%) A
weighted average is performed to yield the result ACP¼ ð0:82 0:21Þ%, where the uncertainty is statistical only Numerous robustness checks are made The value of
ACP is studied as a function of the time at which the data were taken (Fig.3) and found to be consistent with a constant value ( 2 probability of 57%) The measurement
is repeated with progressively more restrictive RICH par-ticle identification requirements, finding values of ð0:88 0:26Þ% and ð1:03 0:31Þ%; both of these values are consistent with the baseline result when corre-lations are taken into account TableIlists ACPfor eight disjoint subsamples of data split according to magnet polarity, the sign of px of the slow pion, and whether the data were taken before or after the technical stop The 2 probability for consistency among the subsamples is 45% The significances of the differences between data taken before and after the technical stop, between the magnet polarities, and betweenpx> 0 and px< 0 are 0.4, 0.6, and 0.7 standard deviations, respectively Other checks include applying electron and muon vetoes to the slow pion and to the D0 daughters, use of different kinematic binnings, validation of the size of the statistical uncertainties with Monte Carlo pseudoexperiments, tightening of kinematic requirements, testing for variation of the result with the multiplicity of tracks and of primary vertices in the event, use of other signal and background parameterizations in the fit, and imposing a full set of common shape parameters between DþandDcandidates Potential biases due to the inclusive hardware trigger selection are investigated with the subsample of data in which one of the signal final-state tracks is directly responsible for the hardware trigger decision In all cases good stability is observed For several
of these checks, a reduced number of kinematic bins are used for simplicity No systematic dependence of ACPis observed with respect to the kinematic variables
Systematic uncertainties are assigned by loosening the fiducial requirement on the slow pion, assessing the effect
of potential peaking backgrounds in Monte Carlo pseu-doexperiments, repeating the analysis with the asymmetry extracted through sideband subtraction inm instead of a fit, removing all candidates but one (chosen at random) in events with multiple candidates, and comparing with the result obtained without kinematic binning In each case the
FIG 3 (color online) Time dependence of the measurement
The data are divided into 19 disjoint, contiguous, time-ordered
blocks and the value of ACP measured in each block The
horizontal red dashed line shows the result for the combined
sample The vertical dashed line indicates the technical stop
referred to in TableI
TABLE I Values of ACPmeasured in subsamples of the data,
and the 2=ndf and corresponding 2probabilities for internal
consistency among the 27 bins in each subsample The data are
divided before and after a technical stop (TS), by magnet
polar-ity (up, down), and by the sign of px for the slow pion (left,
right) The consistency among the eight subsamples is 2=ndf ¼
6:8=7 (45%)
Pre-TS, up, left 1:22 0:59 13=26ð98%Þ
Pre-TS, up, right 1:43 0:59 27=26ð39%Þ
Pre-TS, down, left 0:59 0:52 19=26ð84%Þ
Pre-TS, down, right 0:51 0:52 29=26ð30%Þ
Post-TS, up, left 0:79 0:90 26=26ð44%Þ
Post-TS, up, right þ0:42 0:93 21=26ð77%Þ
Post-TS, down, left 0:24 0:56 34=26ð15%Þ
Post-TS, down, right 1:59 0:57 35=26ð12%Þ
TABLE II Summary of absolute systematic uncertainties for
ACP.
Trang 5full value of the change in result is taken as the systematic
uncertainty These uncertainties are listed in TableII The
sum in quadrature is 0.11% Combining statistical and
systematic uncertainties in quadrature, this result is
consistent at the 1 level with the current HFAG world
average [3]
In conclusion, the time-integrated difference in CP
asymmetry between D0! KKþ and D0! þ
de-cays has been measured to be
ACP¼ ½0:82 0:21ðstatÞ 0:11ðsystÞ%
with 0:62 fb1 of 2011 data Given the dependence of
ACP on the direct and indirect CP asymmetries, shown
in Eq (4), and the measured value hti= ¼ ½9:83
0:22ðstatÞ 0:19ðsystÞ%, the contribution from indirect
CP violation is suppressed and ACPis primarily sensitive
to direct CP violation Dividing the central value by the
sum in quadrature of the statistical and systematic
uncer-tainties, the significance of the measured deviation from
zero is 3:5 This is the first evidence for CP violation in
the charm sector To establish whether this result is
con-sistent with the SM will require the analysis of more data,
as well as improved theoretical understanding
We express our gratitude to our colleagues in the CERN
accelerator departments for the excellent performance of
the LHC We thank the technical and administrative staff at
CERN and at the LHCb institutes, and acknowledge
sup-port from the National Agencies: CAPES, CNPq, FAPERJ
and FINEP (Brazil); CERN; NSFC (China); CNRS/IN2P3
(France); BMBF, DFG, HGF and MPG (Germany); SFI
(Ireland); INFN (Italy); FOM and NWO (The
Netherlands); SCSR (Poland); ANCS (Romania); MinES
of Russia and Rosatom (Russia); MICINN, XuntaGal and
GENCAT (Spain); SNSF and SER (Switzerland); NAS Ukraine (Ukraine); STFC (United Kingdom); NSF (USA) We also acknowledge the support received from the ERC under FP7 and the Region Auvergne
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M Nicol,7V Niess,5N Nikitin,31A Nomerotski,51A Novoselov,34A Oblakowska-Mucha,26V Obraztsov,34
S Oggero,23S Ogilvy,47O Okhrimenko,41R Oldeman,15,dM Orlandea,28J M Otalora Goicochea,2P Owen,49
K Pal,52J Palacios,39A Palano,13,bM Palutan,18J Panman,37A Papanestis,45M Pappagallo,47C Parkes,50,37
C J Parkinson,49G Passaleva,17G D Patel,48M Patel,49S K Paterson,49G N Patrick,45C Patrignani,19,i
C Pavel-Nicorescu,28A Pazos Alvarez,36A Pellegrino,23G Penso,22,lM Pepe Altarelli,37S Perazzini,14,c
D L Perego,20,jE Perez Trigo,36A Pe´rez-Calero Yzquierdo,35P Perret,5M Perrin-Terrin,6G Pessina,20
A Petrella,16,37A Petrolini,19,iA Phan,52E Picatoste Olloqui,35B Pie Valls,35B Pietrzyk,4T Pilarˇ,44D Pinci,22
R Plackett,47S Playfer,46M Plo Casasus,36G Polok,25A Poluektov,44,33E Polycarpo,2D Popov,10B Popovici,28
C Potterat,35A Powell,51J Prisciandaro,38V Pugatch,41A Puig Navarro,35W Qian,52J H Rademacker,42
B Rakotomiaramanana,38M S Rangel,2I Raniuk,40G Raven,24S Redford,51M M Reid,44A C dos Reis,1
S Ricciardi,45K Rinnert,48D A Roa Romero,5P Robbe,7E Rodrigues,47,50F Rodrigues,2P Rodriguez Perez,36
G J Rogers,43S Roiser,37V Romanovsky,34M Rosello,35,nJ Rouvinet,38T Ruf,37H Ruiz,35G Sabatino,21,k
J J Saborido Silva,36N Sagidova,29P Sail,47B Saitta,15,dC Salzmann,39M Sannino,19,iR Santacesaria,22
C Santamarina Rios,36R Santinelli,37E Santovetti,21,kM Sapunov,6A Sarti,18,lC Satriano,22,mA Satta,21
M Savrie,16,eD Savrina,30P Schaack,49M Schiller,24S Schleich,9M Schlupp,9M Schmelling,10B Schmidt,37
O Schneider,38A Schopper,37M.-H Schune,7R Schwemmer,37B Sciascia,18A Sciubba,18,lM Seco,36
A Semennikov,30K Senderowska,26I Sepp,49N Serra,39J Serrano,6P Seyfert,11M Shapkin,34I Shapoval,40,37
P Shatalov,30Y Shcheglov,29T Shears,48L Shekhtman,33O Shevchenko,40V Shevchenko,30A Shires,49
R Silva Coutinho,44T Skwarnicki,52A C Smith,37N A Smith,48E Smith,51,45K Sobczak,5F J P Soler,47
A Solomin,42F Soomro,18B Souza De Paula,2B Spaan,9A Sparkes,46P Spradlin,47F Stagni,37S Stahl,11
Trang 7O Steinkamp,39S Stoica,28S Stone,52,37B Storaci,23M Straticiuc,28U Straumann,39V K Subbiah,37
S Swientek,9M Szczekowski,27P Szczypka,38T Szumlak,26S T’Jampens,4E Teodorescu,28F Teubert,37
C Thomas,51E Thomas,37J van Tilburg,11V Tisserand,4M Tobin,39S Topp-Joergensen,51N Torr,51
E Tournefier,4,49M T Tran,38A Tsaregorodtsev,6N Tuning,23M Ubeda Garcia,37A Ukleja,27P Urquijo,52
U Uwer,11V Vagnoni,14G Valenti,14R Vazquez Gomez,35P Vazquez Regueiro,36S Vecchi,16J J Velthuis,42
M Veltri,17,gB Viaud,7I Videau,7X Vilasis-Cardona,35,nJ Visniakov,36A Vollhardt,39D Volyanskyy,10
D Voong,42A Vorobyev,29H Voss,10S Wandernoth,11J Wang,52D R Ward,43N K Watson,55A D Webber,50
D Websdale,49M Whitehead,44D Wiedner,11L Wiggers,23G Wilkinson,51M P Williams,44,45M Williams,49
F F Wilson,45J Wishahi,9M Witek,25W Witzeling,37S A Wotton,43K Wyllie,37Y Xie,46F Xing,51Z Xing,52
Z Yang,3R Young,46O Yushchenko,34M Zavertyaev,10,aF Zhang,3L Zhang,52W C Zhang,12Y Zhang,3
A Zhelezov,11L Zhong,3E Zverev,31and A Zvyagin37
(LHCb Collaboration)
1Centro Brasileiro de Pesquisas Fı´sicas (CBPF), Rio de Janeiro, Brazil
2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil
3Center for High Energy Physics, Tsinghua University, Beijing, China
4LAPP, Universite´ de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France
5Clermont Universite´, Universite´ Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France
6CPPM, Aix-Marseille Universite´, CNRS/IN2P3, Marseille, France
7LAL, Universite´ Paris-Sud, CNRS/IN2P3, Orsay, France
8LPNHE, Universite´ Pierre et Marie Curie, Universite´ Paris Diderot, CNRS/IN2P3, Paris, France
9Fakulta¨t Physik, Technische Universita¨t Dortmund, Dortmund, Germany
10Max-Planck-Institut fu¨r Kernphysik (MPIK), Heidelberg, Germany
11Physikalisches Institut, Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany
12
School of Physics, University College Dublin, Dublin, Ireland
13Sezione INFN di Bari, Bari, Italy
14Sezione INFN di Bologna, Bologna, Italy
15Sezione INFN di Cagliari, Cagliari, Italy
16Sezione INFN di Ferrara, Ferrara, Italy
17Sezione INFN di Firenze, Firenze, Italy
18Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy
19Sezione INFN di Genova, Genova, Italy
20Sezione INFN di Milano Bicocca, Milano, Italy
21
Sezione INFN di Roma Tor Vergata, Roma, Italy
22Sezione INFN di Roma La Sapienza, Roma, Italy
23Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands
24Nikhef National Institute for Subatomic Physics and Vrije Universiteit, Amsterdam, The Netherlands
25Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Kraco´w, Poland
26AGH University of Science and Technology, Kraco´w, Poland
27Soltan Institute for Nuclear Studies, Warsaw, Poland
28Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania
29Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia
30Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia
31Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia
32Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia
33Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia
34Institute for High Energy Physics (IHEP), Protvino, Russia
35Universitat de Barcelona, Barcelona, Spain
36Universidad de Santiago de Compostela, Santiago de Compostela, Spain
37European Organization for Nuclear Research (CERN), Geneva, Switzerland
38Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Lausanne, Switzerland
39Physik-Institut, Universita¨t Zu¨rich, Zu¨rich, Switzerland
40NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine
41Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine
42H.H Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom
43Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom
44
Department of Physics, University of Warwick, Coventry, United Kingdom
Trang 845STFC Rutherford Appleton Laboratory, Didcot, United Kingdom
46School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom
47School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom
48Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom
49Imperial College London, London, United Kingdom
50School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom
51Department of Physics, University of Oxford, Oxford, United Kingdom
52Syracuse University, Syracuse, New York, USA
53
CC-IN2P3, CNRS/IN2P3, Lyon-Villeurbanne, France
54Pontifı´cia Universidade Cato´lica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil
55University of Birmingham, Birmingham, United Kingdom
56Physikalisches Institut, Universita¨t Rostock, Rostock, Germany
aAlso at P.N Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia
bAlso at Universita` di Bari, Bari, Italy
c
Also at Universita` di Bologna, Bologna, Italy
dAlso at Universita` di Cagliari, Cagliari, Italy
eAlso at Universita` di Ferrara, Ferrara, Italy
fAlso at Universita` di Firenze, Firenze, Italy
gAlso at Universita` di Urbino, Urbino, Italy
hAlso at Universita` di Modena e Reggio Emilia, Modena, Italy
iAlso at Universita` di Genova, Genova, Italy
jAlso at Universita` di Milano Bicocca, Milano, Italy
kAlso at Universita` di Roma Tor Vergata, Roma, Italy
lAlso at Universita` di Roma La Sapienza, Roma, Italy
mAlso at Universita` della Basilicata, Potenza, Italy
nAlso at LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain
oAlso at Hanoi University of Science, Hanoi, Vietnam