Dijet Azimuthal Decorrelations in $pp$ Collisions at $\sqrts = 7$ TeV

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Dijet Azimuthal Decorrelations in pp Collisions at ffiffiffi p s

¼ 7 TeV

V. Khachatryan et al.*

(CMS Collaboration)

(Received 26 January 2011; published 22 March 2011) Measurements of dijet azimuthal decorrelations in pp collisions at ffiffiffi

ps

¼ 7 TeV using the CMS detector at the CERN LHC are presented. The analysis is based on an inclusive dijet event sample corresponding to an integrated luminosity of 2:9 pb¹. The results are compared to predictions from perturbative QCD calculations and various Monte Carlo event generators. The dijet azimuthal distributions are found to be sensitive to initial-state gluon radiation.

DOI:10.1103/PhysRevLett.106.122003 PACS numbers: 13.85.Ni, 12.38.Bx, 12.38.Qk, 13.87.Ce

High-energy proton-proton collisions with high momentum transfer are described within the framework of quantum chromodynamics (QCD) as pointlike scatterings between the proton constituents, collectively referred to as partons.

The outgoing partons manifest themselves, through quark and gluon soft radiation and hadronization processes, as localized streams of particles, identified as jets. At Born level, dijets are produced with equal transverse momenta pT

with respect to the beam axis and back to back in the azimuthal angle (’dijet¼ j’jet1’jet2j ¼ ). Soft-gluon emission will decorrelate the two highest pT (leading) jets and cause small deviations from . Larger decorrelations from occur in the case of hard multijet production. Three- jet topologies dominate the region of 2=3 < ’dijet< , whereas angles smaller than 2=3 are populated by four-jet events.

Dijet azimuthal decorrelations, i.e., the deviation of

’dijetfrom for the two leading jets in hard-scattering events, can be used to study QCD radiation effects over a wide range of jet multiplicities without the need to measure all the additional jets. Such studies are important because an accurate description of multiple-parton radiation is still lacking in perturbative QCD (pQCD). Experiments there- fore rely on Monte Carlo (MC) event generators to take these higher-order processes into account in searches for new physics and for a wide variety of precision measurements. The observable chosen to study the radiation effects is the differential dijet cross section in ’dijet, normalized by the dijet cross section integrated over the entire ’_dijet phase space: ð1=dijetÞðddijet=d’dijetÞ. By normalizing the ’_dijetdistributions in this manner, many experimental and theoretical uncertainties are significantly reduced.

Measurements of dijet azimuthal decorrelations at the Tevatron have previously been reported by the D0

Collaboration [1]. In this Letter, we present the first measurements of dijet azimuthal decorrelations in pp collisions at ffiffiffi

ps

¼ 7 TeV at the CERN Large Hadron Collider (LHC).

The central feature of the Compact Muon Solenoid (CMS) apparatus is a superconducting solenoid, of 6 m internal diameter, providing an axial field of 3.8 T.

Charged particle trajectories are measured by the silicon pixel and strip tracker, covering 0 < ’ < 2 in azimuth and jj < 2:5, where pseudorapidity ¼ ln½tanð=2Þ and is the polar angle relative to the counterclockwise proton beam direction with respect to the center of the detector. A lead-tungstate crystal electromagnetic calorimeter and a brass-scintillator hadronic calorimeter surround the track- ing volume. The calorimeter cells are grouped in projective towers of granularity ’ ¼ 0:087 0:087 at central pseudorapidities. The granularity becomes coarser at for- ward pseudorapidities. A preshower detector made of silicon sensor planes and lead absorbers is installed in front of the electromagnetic calorimeter at 1:653 < jj < 2:6.

Muons are measured in gas-ionization detectors embedded in the steel magnetic field return yoke. A detailed description of the CMS detector can be found elsewhere [2].

CMS uses a two-tiered trigger system to select events on-line: level 1 and the high level trigger. In this analysis, events were selected by using two inclusive single-jet triggers that required a level-1 jet with p_T> 20 (30) GeV and a high level trigger jet with p_T> 30 (50) GeV. The jets at level 1 and the high level trigger are reconstructed by using energies measured by the electromagnetic and hadronic calorimeters and are not corrected for the jet energy response of the calorimeters.

The trigger efficiency for a given corrected pT threshold of the leading jet (p_T^max) was measured by using events selected by a lower-threshold trigger. For the event selection, p_T^maxthresholds were chosen so that this efficiency exceeded 99%. The corresponding off-line corrected p_T^max values are 80 (110) GeV for the low (high) threshold jet trigger.

Jets were reconstructed off-line by using the anti-kT

clustering algorithm with a distance parameter R ¼ 0:5 [3].

*Full author list given at the end of the article.

Published by American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

PRL106, 122003 (2011) P H Y S I C A L R E V I E W L E T T E R S 25 MARCH 2011

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The four-vectors of particles reconstructed by the CMS particle-flow algorithm were used as input to the jet- clustering algorithm. The particle-flow algorithm combines information from all CMS subdetectors to provide a com- plete list of long-lived particles in the event. Muons, elec- trons, photons, and charged and neutral hadrons are reconstructed individually. As a result, the residual corrections to the jet four-vectors, arising from the detector response, are relatively small (at the level of 5%–10% in the central region) [4]. A detailed description of the particle- flow algorithm can be found elsewhere [5,6].

Spurious jets from noise and noncollision backgrounds were eliminated by applying loose quality cuts on the jet properties [7]. Events were required to have a primary vertex reconstructed along the beam axis and within 24 cm of the detector center [8]. Further cuts were applied to reject interactions from the beam halo. Events were selected having two leading jets each with p_T> 30 GeV and rapidity jyj < 1:1, where y ¼¹₂ln½ðE þ pzÞ=ðEp_zÞ, with E being the total jet energy and p_zthe projection of the jet momentum along the beam axis. Each event is put into one of five mutually exclusive regions, which are based on the p_T^max in the event. The five regions are 80 < p_T^max< 110 GeV, 110 <

p_T^max< 140 GeV, 140 < p_T^max< 200 GeV, 200 <

p_T^max< 300 GeV, and 300 GeV < p_T^max. The data corre- spond to an integrated luminosity of 0:3 pb¹for the lowest p_T^max region and 2:9 pb¹ for the other p_T^max regions.

The uncertainty on the integrated luminosity is estimated to be 11% [9]. After the application of all selection criteria, the numbers of events remaining in each of the five p_T^max regions, starting from the lowest, are 60 837, 160 388, 69 009, 14 383, and 2284.

The ’dijetdistributions are corrected for event migration effects due to the finite jet p_Tand position resolutions of the detector. The distributions are sensitive to the jet p_T resolution because fluctuations in the jet response can cause low-energy jets to be misidentified as leading jets, and events can migrate between different p_T^max regions.

The finite resolution in azimuthal angle causes event migration between ’_dijet bins, while the resolution in rapidity can move jets in and out of the central rapidity region (jyj < 1:1). The correction factors were determined by using two independent MC samples: PYTHIA 6.422 (PYTHIA6) [10] tune D6T [11], and_HERWIGþþ 2.4.2 [12].

The p_T, rapidity, and azimuthal angle of each generated jet were smeared according to the measured resolutions [13].

The ratio of the two dijet azimuthal distributions (the generated distribution and the smeared one) determined the unfolding correction factors for each p_T^max region, for a given MC sample. The average of the correction factors for each p_T^max region from the two MC samples was used as the final unfolding correction applied to the data. The unfolding correction factors modify the measured ’dijetdistributions by less than 2% for 5=6 <

’dijet< . For ’dijet =2, the changes range from

11%, for the highest p_T^max region, to 19%, for the lowest.

The main sources of systematic uncertainty arise from uncertainties in the jet energy calibration, the jet pT resolution, and the unfolding correction. The jet energy calibration uncertainties have been tabulated for the considered phase space in the variables of jet p_T and [4]. Typical values are between 2.5% and 3.5%. The resulting uncertainties on the normalized ’_dijet distributions range from 5% at ’dijet =2 to 1% at ’dijet . The effect of the jet pT resolution uncertainty on the ’dijet

distributions was estimated by varying the jet p_T resolutions by 10% [13] and comparing the ’_dijetunfolding correction before and after the change. This yields a varia- tion on the normalized ’_dijetdistributions ranging from 5% at ’dijet =2 to 1% at ’dijet . The uncertainties on the unfolding correction factors were estimated by comparing the corrections from different event generators andPYTHIA6tunes that vary significantly in their modeling of the jet kinematic distributions and ’dijetdistributions.

The resulting uncertainty varies from 8% at ’_dijet =2 to 1.5% at ’dijet . The systematic uncertainty from using a parametrized model to simulate the finite jet pTand position resolutions of the detector to determine the unfolding correction factors was estimated to be about 2.5% in all p_T^max regions. The combined systematic uncertainty, calculated as the quadratic sum of all systematic uncertainties, varies from 11% at ’dijet =2 to 3% at

’dijet .

The corrected differential ’dijetdistributions, normalized to the integrated dijet cross section, are shown in Fig. 1 for the five p_T^max regions. The distributions are scaled by multiplicative factors for presentation purposes.

Each data point is plotted at the abscissa value for which the predicted differential ’dijetdistribution has the same value as the bin average obtained by using PYTHIA6 tune D6T, which provides a good description of the data [14].

The ’dijet distributions are strongly peaked at and become steeper with increasing p_T^max. The simulated

’dijet distributions from the PYTHIA6 (D6T and Z2 [15]

tunes), PYTHIA 8.135 (PYTHIA8) [16], HERWIGþþ, and

MADGRAPH 4.4.32 [17] event generators are presented for comparison. TheMADGRAPHgenerator is based on leading- order matrix element multiparton final-state predictions, usingPYTHIA6for parton showering and hadronization, and the Mangano method [18] to map the parton-level event into a parton shower history. The MADGRAPH predictions included tree-level processes of up to four partons. For

PYTHIA6, PYTHIA8, and MADGRAPH event generators the CTEQ6L [19] parton distribution functions (PDFs) were used; forHERWIGþþ, the MRST2001 PDFs [20].

Figure 2 shows the ratios of the measured ’dijet

distributions to the predictions of PYTHIA6, PYTHIA8,

HERWIGþþ, and ^MADGRAPH in the five p_T^max regions.

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The combined systematic uncertainty on the experimental measurements is shown by the shaded band. The predictions fromPYTHIA6and_HERWIGþþ describe the shape of the data distributions well, while MADGRAPH (PYTHIA8) predicts less (more) azimuthal decorrelation than is observed in the data.

Figure 3 displays the ratios of the measured ’dijet

distributions to the next-to-leading-order (NLO) predictions of pQCD calculations from the parton-level generator

NLOJETþþ [21] within the FASTNLOframework [22]. The predictions include 2 ! 3 processes at NLO, normalized to _dijetat NLO:

1

dijet

_NLO ddijet

d’_dijet

_NLO:

The predictions near ’_dijet¼ have been excluded because of their sensitivity to higher-order corrections not included in the present calculations.

Uncertainties due to the renormalization (_r) and factorization (_f) scales are evaluated by varying the default choice of r¼ _f¼ p_T^max between p_T^max=2

and 2p_T^maxin the following six combinations:ð_r; _fÞ ¼ ðp_T^max=2; p_T^max=2Þ, ð2p_T^max; 2p_T^maxÞ, ðp_T^max; p_T^max=2Þ, ðp_T^max; 2p_T^maxÞ, ðp_T^max=2; p_T^maxÞ, and ð2p_T^max; p_T^maxÞ.

These scale variations modify the predictions of the normalized ’dijetdistributions by less than 50%. The PDFs and the associated uncertainties were obtained from CTEQ6.6 [19]. The PDF uncertainties were derived by using the 22 CTEQ6.6 uncertainty eigenvectors and found to be 9% at ’_dijet =2 and 2% at ’_dijet< .

Following the proposal of the PDF4LHC working group [23], the impact of other global PDF fits [24–26] was investigated and found to be negligible in the context of this analysis.

Nonperturbative corrections due to hadronization and multiple-parton interactions were applied to the pQCD predictions. The correction factors were determined from the

PYTHIA6andHERWIGþþ simulations and modify the predictions from þ4% (’_dijet ) to 13% (’_dijet =2).

The uncertainty due to the nonperturbative corrections is estimated to be 6% at ’dijet =2 and 2% at

’_dijet . The effects due to the scale variations, as well as the uncertainties due to PDFs and nonperturbative corrections, are also shown in Fig.3. The NLO predictions provide a good description of the shape of the data

0.5 1 1.5 2

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PYTHIA6 D6T PYTHIA6 Z2 PYTHIA8

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= 7 TeV |y| < 1.1

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π/2 2π/3 5π/6 π

[ rad ]

dijet

ϕ

∆

dijetϕ∆d

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FIG. 2 (color online). Ratios of measured normalized ’dijet

distributions to PYTHIA6,PYTHIA8,HERWIGþþ, and^MADGRAPH predictions in several p_T^maxregions. The shaded bands indicate the total systematic uncertainty.

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dijet

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MADGRAPH L = 2.9 pb-1

= 7 TeV s

|y| < 1.1

CMS

π/2 2π/3 5π/6 π

FIG. 1 (color online). Normalized ’dijetdistributions in several p_T^maxregions, scaled by the multiplicative factors given in the figure for easier presentation. The curves represent predictions fromPYTHIA6,PYTHIA8,HERWIGþþ, and^MADGRAPH. The error bars on the data points include statistical and systematic uncertainties.

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distributions over much of the ’dijet range. Compared to the data, the reduced decorrelation in the theoretical prediction and the increased sensitivity to the r and f scale variations for ’_dijet< 2=3 shown in Fig.3are attributed to the fact that the pQCD prediction in this region is effec- tively available only at leading order, since the contribution from tree-level four-parton final states dominates.

The sensitivity of the ’_dijetdistributions to initial-state parton shower radiation (ISR) is investigated by varying the input parameter k_ISR [PARP(67)] inPYTHIA6 tune D6T.

The product of k_ISR and the square of the hard-scattering scale gives the maximum allowed parton virtuality (i.e., the maximum allowed pT) in the initial-state shower. Previous studies have shown that kISR is the only parameter in

PYTHIA6that has significant impact on the ’dijetdistributions [27]. The default value of kISRinPYTHIA6tune D6T is 2.5, determined from the D0 dijet azimuthal decorrelation results [1]. Figure4shows comparisons of the measured

’dijetdistributions toPYTHIA6 distributions with various kISR values. The effects are more pronounced for smaller

’dijet angles, where multigluon radiation dominates.

Varying kISR by 0:5 about its default value yields a change of about 30% on the PYTHIA6 prediction for

’dijet =2, suggesting that our results could be used to tune parameters in the MC event generators that control radiative effects in the initial state. In PYTHIA6tune D6T, the maximum pTallowed in the final-state radiation parton shower is controlled through the parameter PARP(71). We varied the value of this parameter from 2.5 to 8 (the default value is 4.0) and observed less than10% changes in the

’dijetdistributions in all pT regions.

In summary, we have measured dijet azimuthal decorrelations in different leading-jet p_T regions from pp collisions at ffiffiffi

ps

¼ 7 TeV. The^PYTHIA6andHERWIGþþ event generators are found to best describe the shape of the measured distributions over the entire range of ’dijet. The predictions from NLO pQCD are in reasonable agree- ment with the measured distributions, except at small

’_dijetwhere multiparton radiation effects dominate. The

’dijetdistributions are found to be sensitive to initial-state gluon radiation.

We congratulate our colleagues in the CERN accelerator departments for the excellent performance of the LHC

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NLO QCD Predictions CTEQ 6.6

max

= pT

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∆

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dijetσd dijetσ1 THEORYDATA

distributions to NLO pQCD predictions with nonperturbative corrections in several pTmaxregions. The error bars on the data points include statistical and systematic uncertainties. The effects on the NLO pQCD predictions due to r and f scale variations and PDF uncertainties, as well as the uncertainties from the nonperturbative corrections, are shown.

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3 CMS > 300 GeV

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distributions toPYTHIA6tune D6T with various values of kISRin several p_T^max regions. The shaded bands indicate the total systematic uncertainty.

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machine. We thank the technical and administrative staff at CERN and other CMS institutes and acknowledge support from FMSR (Austria); FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria);

CERN; CAS, MoST, and NSFC (China); COLCIENCIAS (Colombia); MSES (Croatia); RPF (Cyprus); Academy of Sciences and NICPB (Estonia); Academy of Finland, ME, and HIP (Finland); CEA and CNRS/IN2P3 (France);

BMBF, DFG, and HGF (Germany); GSRT (Greece);

OTKA and NKTH (Hungary); DAE and DST (India);

IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU (Korea); LAS (Lithuania); CINVESTAV, CONACYT, SEP, and UASLP-FAI (Mexico); PAEC (Pakistan); SCSR (Poland); FCT (Portugal); JINR (Armenia, Belarus, Georgia, Ukraine, Uzbekistan); MST and MAE (Russia);

MSTD (Serbia); MICINN and CPAN (Spain); Swiss Funding Agencies (Switzerland); NSC (Taipei);

TUBITAK and TAEK (Turkey); STFC (United Kingdom); DOE and NSF (USA).

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G. Sultanov,¹³V. Tcholakov,¹³R. Trayanov,¹³I. Vankov,¹³M. Dyulendarova,¹⁴R. Hadjiiska,¹⁴V. Kozhuharov,¹⁴ L. Litov,¹⁴E. Marinova,¹⁴M. Mateev,¹⁴B. Pavlov,¹⁴P. Petkov,¹⁴J. G. Bian,¹⁵G. M. Chen,¹⁵H. S. Chen,¹⁵ C. H. Jiang,¹⁵D. Liang,¹⁵S. Liang,¹⁵J. Wang,¹⁵J. Wang,¹⁵X. Wang,¹⁵Z. Wang,¹⁵M. Xu,¹⁵M. Yang,¹⁵J. Zang,¹⁵ Z. Zhang,¹⁵Y. Ban,¹⁶S. Guo,¹⁶Y. Guo,¹⁶W. Li,¹⁶Y. Mao,¹⁶S. J. Qian,¹⁶H. Teng,¹⁶L. Zhang,¹⁶B. Zhu,¹⁶W. Zou,¹⁶ A. Cabrera,¹⁷B. Gomez Moreno,¹⁷A. A. Ocampo Rios,¹⁷A. F. Osorio Oliveros,¹⁷J. C. Sanabria,¹⁷N. Godinovic,¹⁸

D. Lelas,¹⁸K. Lelas,¹⁸R. Plestina,^18,dD. Polic,¹⁸I. Puljak,¹⁸Z. Antunovic,¹⁹M. Dzelalija,¹⁹V. Brigljevic,²⁰ S. Duric,²⁰K. Kadija,²⁰S. Morovic,²⁰A. Attikis,²¹M. Galanti,²¹J. Mousa,²¹C. Nicolaou,²¹F. Ptochos,²¹ P. A. Razis,²¹H. Rykaczewski,²¹Y. Assran,^22,eM. A. Mahmoud,^22,fA. Hektor,²³M. Kadastik,²³K. Kannike,²³ M. Mu¨ntel,²³M. Raidal,²³L. Rebane,²³V. Azzolini,²⁴P. Eerola,²⁴S. Czellar,²⁵J. Ha¨rko¨nen,²⁵A. Heikkinen,²⁵ V. Karima¨ki,²⁵R. Kinnunen,²⁵J. Klem,²⁵M. J. Kortelainen,²⁵T. Lampe´n,²⁵K. Lassila-Perini,²⁵S. Lehti,²⁵

T. Lindeń,²⁵P. Luukka,²⁵T. Maënpaä¨,²⁵E. Tuominen,²⁵J. Tuominiemi,²⁵E. Tuovinen,²⁵D. Ungaro,²⁵ L. Wendland,²⁵K. Banzuzi,²⁶A. Korpela,²⁶T. Tuuva,²⁶D. Sillou,²⁷M. Besancon,²⁸S. Choudhury,²⁸M. Dejardin,²⁸

D. Denegri,²⁸B. Fabbro,²⁸J. L. Faure,²⁸F. Ferri,²⁸S. Ganjour,²⁸F. X. Gentit,²⁸A. Givernaud,²⁸P. Gras,²⁸ G. Hamel de Monchenault,²⁸P. Jarry,²⁸E. Locci,²⁸J. Malcles,²⁸M. Marionneau,²⁸L. Millischer,²⁸J. Rander,²⁸ A. Rosowsky,²⁸I. Shreyber,²⁸M. Titov,²⁸P. Verrecchia,²⁸S. Baffioni,²⁹F. Beaudette,²⁹L. Bianchini,²⁹M. Bluj,^29,g C. Broutin,²⁹P. Busson,²⁹C. Charlot,²⁹T. Dahms,²⁹L. Dobrzynski,²⁹R. Granier de Cassagnac,²⁹M. Haguenauer,²⁹

P. Mine´,²⁹C. Mironov,²⁹C. Ochando,²⁹P. Paganini,²⁹D. Sabes,²⁹R. Salerno,²⁹Y. Sirois,²⁹C. Thiebaux,²⁹ B. Wyslouch,^29,hA. Zabi,²⁹J.-L. Agram,^30,iJ. Andrea,³⁰A. Besson,³⁰D. Bloch,³⁰D. Bodin,³⁰J.-M. Brom,³⁰ M. Cardaci,³⁰E. C. Chabert,³⁰C. Collard,³⁰E. Conte,^30,iF. Drouhin,^30,iC. Ferro,³⁰J.-C. Fontaine,^30,iD. Gele´,³⁰

U. Goerlach,³⁰S. Greder,³⁰P. Juillot,³⁰M. Karim,^30,iA.-C. Le Bihan,³⁰Y. Mikami,³⁰P. Van Hove,³⁰F. Fassi,³¹ D. Mercier,³¹C. Baty,³²N. Beaupere,³²M. Bedjidian,³²O. Bondu,³²G. Boudoul,³²D. Boumediene,³²H. Brun,³² N. Chanon,³²R. Chierici,³²D. Contardo,³²P. Depasse,³²H. El Mamouni,³²A. Falkiewicz,³²J. Fay,³²S. Gascon,³² B. Ille,³²T. Kurca,³²T. Le Grand,³²M. Lethuillier,³²L. Mirabito,³²S. Perries,³²V. Sordini,³²S. Tosi,³²Y. Tschudi,³² P. Verdier,³²H. Xiao,³²L. Megrelidze,³³V. Roinishvili,³³D. Lomidze,³⁴G. Anagnostou,³⁵M. Edelhoff,³⁵L. Feld,³⁵ N. Heracleous,³⁵O. Hindrichs,³⁵R. Jussen,³⁵K. Klein,³⁵J. Merz,³⁵N. Mohr,³⁵A. Ostapchuk,³⁵A. Perieanu,³⁵

F. Raupach,³⁵J. Sammet,³⁵S. Schael,³⁵D. Sprenger,³⁵H. Weber,³⁵M. Weber,³⁵B. Wittmer,³⁵M. Ata,³⁶ W. Bender,³⁶M. Erdmann,³⁶J. Frangenheim,³⁶T. Hebbeker,³⁶A. Hinzmann,³⁶K. Hoepfner,³⁶C. Hof,³⁶ T. Klimkovich,³⁶D. Klingebiel,³⁶P. Kreuzer,³⁶D. Lanske,^36,aC. Magass,³⁶G. Masetti,³⁶M. Merschmeyer,³⁶

A. Meyer,³⁶P. Papacz,³⁶H. Pieta,³⁶H. Reithler,³⁶S. A. Schmitz,³⁶L. Sonnenschein,³⁶J. Steggemann,³⁶ D. Teyssier,³⁶M. Bontenackels,³⁷M. Davids,³⁷M. Duda,³⁷G. Flu¨gge,³⁷H. Geenen,³⁷M. Giffels,³⁷ W. Haj Ahmad,³⁷D. Heydhausen,³⁷T. Kress,³⁷Y. Kuessel,³⁷A. Linn,³⁷A. Nowack,³⁷L. Perchalla,³⁷O. Pooth,³⁷

J. Rennefeld,³⁷P. Sauerland,³⁷A. Stahl,³⁷M. Thomas,³⁷D. Tornier,³⁷M. H. Zoeller,³⁷M. Aldaya Martin,³⁸ W. Behrenhoff,³⁸U. Behrens,³⁸M. Bergholz,^38,jK. Borras,³⁸A. Cakir,³⁸A. Campbell,³⁸E. Castro,³⁸ D. Dammann,³⁸G. Eckerlin,³⁸D. Eckstein,³⁸A. Flossdorf,³⁸G. Flucke,³⁸A. Geiser,³⁸I. Glushkov,³⁸J. Hauk,³⁸

H. Jung,³⁸M. Kasemann,³⁸I. Katkov,³⁸P. Katsas,³⁸C. Kleinwort,³⁸H. Kluge,³⁸A. Knutsson,³⁸D. Kru¨cker,³⁸ E. Kuznetsova,³⁸W. Lange,³⁸W. Lohmann,^38,jR. Mankel,³⁸M. Marienfeld,³⁸I.-A. Melzer-Pellmann,³⁸ A. B. Meyer,³⁸J. Mnich,³⁸A. Mussgiller,³⁸J. Olzem,³⁸A. Parenti,³⁸A. Raspereza,³⁸A. Raval,³⁸R. Schmidt,^38,j

T. Schoerner-Sadenius,³⁸N. Sen,³⁸M. Stein,³⁸J. Tomaszewska,³⁸D. Volyanskyy,³⁸R. Walsh,³⁸C. Wissing,³⁸ C. Autermann,³⁹S. Bobrovskyi,³⁹J. Draeger,³⁹H. Enderle,³⁹U. Gebbert,³⁹K. Kaschube,³⁹G. Kaussen,³⁹ R. Klanner,³⁹J. Lange,³⁹B. Mura,³⁹S. Naumann-Emme,³⁹F. Nowak,³⁹N. Pietsch,³⁹C. Sander,³⁹H. Schettler,³⁹

P. Schleper,³⁹M. Schro¨der,³⁹T. Schum,³⁹J. Schwandt,³⁹A. K. Srivastava,³⁹H. Stadie,³⁹G. Steinbru¨ck,³⁹ J. Thomsen,³⁹R. Wolf,³⁹C. Barth,⁴⁰J. Bauer,⁴⁰V. Buege,⁴⁰T. Chwalek,⁴⁰W. De Boer,⁴⁰A. Dierlamm,⁴⁰ G. Dirkes,⁴⁰M. Feindt,⁴⁰J. Gruschke,⁴⁰C. Hackstein,⁴⁰F. Hartmann,⁴⁰S. M. Heindl,⁴⁰M. Heinrich,⁴⁰H. Held,⁴⁰

K. H. Hoffmann,⁴⁰S. Honc,⁴⁰T. Kuhr,⁴⁰D. Martschei,⁴⁰S. Mueller,⁴⁰Th. Mu¨ller,⁴⁰M. Niegel,⁴⁰O. Oberst,⁴⁰ A. Oehler,⁴⁰J. Ott,⁴⁰T. Peiffer,⁴⁰D. Piparo,⁴⁰G. Quast,⁴⁰K. Rabbertz,⁴⁰F. Ratnikov,⁴⁰M. Renz,⁴⁰C. Saout,⁴⁰ A. Scheurer,⁴⁰P. Schieferdecker,⁴⁰F.-P. Schilling,⁴⁰G. Schott,⁴⁰H. J. Simonis,⁴⁰F. M. Stober,⁴⁰D. Troendle,⁴⁰ J. Wagner-Kuhr,⁴⁰M. Zeise,⁴⁰V. Zhukov,^40,kE. B. Ziebarth,⁴⁰G. Daskalakis,⁴¹T. Geralis,⁴¹S. Kesisoglou,⁴¹

A. Kyriakis,⁴¹D. Loukas,⁴¹I. Manolakos,⁴¹A. Markou,⁴¹C. Markou,⁴¹C. Mavrommatis,⁴¹E. Ntomari,⁴¹ E. Petrakou,⁴¹L. Gouskos,⁴²T. J. Mertzimekis,⁴²A. Panagiotou,⁴²I. Evangelou,⁴³C. Foudas,⁴³P. Kokkas,⁴³

N. Manthos,⁴³I. Papadopoulos,⁴³V. Patras,⁴³F. A. Triantis,⁴³A. Aranyi,⁴⁴G. Bencze,⁴⁴L. Boldizsar,⁴⁴ G. Debreczeni,⁴⁴C. Hajdu,^44,bD. Horvath,^44,lA. Kapusi,⁴⁴K. Krajczar,^44,mA. Laszlo,⁴⁴F. Sikler,⁴⁴ PRL106, 122003 (2011) P H Y S I C A L R E V I E W L E T T E R S 25 MARCH 2011

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G. Vesztergombi,^44,mN. Beni,⁴⁵J. Molnar,⁴⁵J. Palinkas,⁴⁵Z. Szillasi,⁴⁵V. Veszpremi,⁴⁵P. Raics,⁴⁶Z. L. Trocsanyi,⁴⁶ B. Ujvari,⁴⁶S. Bansal,⁴⁷S. B. Beri,⁴⁷V. Bhatnagar,⁴⁷N. Dhingra,⁴⁷R. Gupta,⁴⁷M. Jindal,⁴⁷M. Kaur,⁴⁷J. M. Kohli,⁴⁷

M. Z. Mehta,⁴⁷N. Nishu,⁴⁷L. K. Saini,⁴⁷A. Sharma,⁴⁷A. P. Singh,⁴⁷J. B. Singh,⁴⁷S. P. Singh,⁴⁷S. Ahuja,⁴⁸ S. Bhattacharya,⁴⁸B. C. Choudhary,⁴⁸P. Gupta,⁴⁸S. Jain,⁴⁸S. Jain,⁴⁸A. Kumar,⁴⁸R. K. Shivpuri,⁴⁸ R. K. Choudhury,⁴⁹D. Dutta,⁴⁹S. Kailas,⁴⁹S. K. Kataria,⁴⁹A. K. Mohanty,^49,bL. M. Pant,⁴⁹P. Shukla,⁴⁹T. Aziz,⁵⁰

M. Guchait,^50,nA. Gurtu,⁵⁰M. Maity,^50,oD. Majumder,⁵⁰G. Majumder,⁵⁰K. Mazumdar,⁵⁰G. B. Mohanty,⁵⁰ A. Saha,⁵⁰K. Sudhakar,⁵⁰N. Wickramage,⁵⁰S. Banerjee,⁵¹S. Dugad,⁵¹N. K. Mondal,⁵¹H. Arfaei,⁵² H. Bakhshiansohi,⁵²S. M. Etesami,⁵²A. Fahim,⁵²M. Hashemi,⁵²A. Jafari,⁵²M. Khakzad,⁵²A. Mohammadi,⁵²

M. Mohammadi Najafabadi,⁵²S. Paktinat Mehdiabadi,⁵²B. Safarzadeh,⁵²M. Zeinali,⁵²M. Abbrescia,^53a,53b L. Barbone,^53a,53bC. Calabria,^53a,53bA. Colaleo,^53aD. Creanza,^53a,53cN. De Filippis,^53a,53cM. De Palma,^53a,53b

A. Dimitrov,^53aL. Fiore,^53aG. Iaselli,^53a,53cL. Lusito,^53a,53b,bG. Maggi,^53a,53cM. Maggi,^53aN. Manna,^53a,53b B. Marangelli,^53a,53bS. My,^53a,53cS. Nuzzo,^53a,53bN. Pacifico,^53a,53bG. A. Pierro,^53aA. Pompili,^53a,53b G. Pugliese,^53a,53cF. Romano,^53a,53cG. Roselli,^53a,53bG. Selvaggi,^53a,53bL. Silvestris,^53aR. Trentadue,^53a S. Tupputi,^53a,53bG. Zito,^53aG. Abbiendi,^54aA. C. Benvenuti,^54aD. Bonacorsi,^54aS. Braibant-Giacomelli,^54a,54b

L. Brigliadori,^54aP. Capiluppi,^54a,54bA. Castro,^54a,54bF. R. Cavallo,^54aM. Cuffiani,^54a,54bG. M. Dallavalle,^54a F. Fabbri,^54aA. Fanfani,^54a,54bD. Fasanella,^54aP. Giacomelli,^54aM. Giunta,^54aS. Marcellini,^54a M. Meneghelli,^54a,54bA. Montanari,^54aF. L. Navarria,^54a,54bF. Odorici,^54aA. Perrotta,^54aF. Primavera,^54a A. M. Rossi,^54a,54bT. Rovelli,^54a,54bG. Siroli,^54a,54bR. Travaglini,^54a,54bS. Albergo,^55a,55bG. Cappello,^55a,55b M. Chiorboli,^55a,55b,bS. Costa,^55a,55bA. Tricomi,^55a,55bC. Tuve,^55aG. Barbagli,^56aV. Ciulli,^56a,56bC. Civinini,^56a R. D’Alessandro,^56a,56bE. Focardi,^56a,56bS. Frosali,^56a,56bE. Gallo,^56aC. Genta,^56aS. Gonzi,^56a,56bP. Lenzi,^56a,56b M. Meschini,^56aS. Paoletti,^56aG. Sguazzoni,^56aA. Tropiano,^56a,bL. Benussi,⁵⁷S. Bianco,⁵⁷S. Colafranceschi,^57,p

F. Fabbri,⁵⁷D. Piccolo,⁵⁷P. Fabbricatore,⁵⁸R. Musenich,⁵⁸A. Benaglia,^59a,59bF. De Guio,^59a,59b,b L. Di Matteo,^59a,59bA. Ghezzi,^59a,59b,bM. Malberti,^59a,59bS. Malvezzi,^59aA. Martelli,^59a,59bA. Massironi,^59a,59b

D. Menasce,^59aL. Moroni,^59aM. Paganoni,^59a,59bD. Pedrini,^59aS. Ragazzi,^59a,59bN. Redaelli,^59aS. Sala,^59a T. Tabarelli de Fatis,^59a,59bV. Tancini,^59a,59bS. Buontempo,^60aC. A. Carrillo Montoya,^60aA. Cimmino,^60a,60b

A. De Cosa,^60a,60bM. De Gruttola,^60a,60bF. Fabozzi,^60a,qA. O. M. Iorio,^60aL. Lista,^60aM. Merola,^60a,60b P. Noli,^60a,60bP. Paolucci,^60aP. Azzi,^61aN. Bacchetta,^61aP. Bellan,^61a,61bD. Bisello,^61a,61bA. Branca,^61a R. Carlin,^61a,61bP. Checchia,^61aM. De Mattia,^61a,61bT. Dorigo,^61aF. Gasparini,^61a,61bU. Gasparini,^61a,61b P. Giubilato,^61a,61bF. Gonella,^61aA. Gresele,^61a,61cS. Lacaprara,^61a,rrI. Lazzizzera,^61a,61cM. Margoni,^61a,61b M. Mazzucato,^61aA. T. Meneguzzo,^61a,61bM. Nespolo,^61a,bL. Perrozzi,^61a,bN. Pozzobon,^61a,61bP. Ronchese,^61a,61b

F. Simonetto,^61a,61bE. Torassa,^61aM. Tosi,^61a,61bA. Triossi,^61aS. Vanini,^61a,61bP. Zotto,^61a,61bG. Zumerle,^61a,61b P. Baesso,^62a,62bU. Berzano,^62aC. Riccardi,^62a,62bP. Torre,^62a,62bP. Vitulo,^62a,62bC. Viviani,^62a,62bM. Biasini,^63a,63b

G. M. Bilei,^63aB. Caponeri,^63a,63bL. Fano`,^63a,63bP. Lariccia,^63a,63bA. Lucaroni,^63a,63b,bG. Mantovani,^63a,63b M. Menichelli,^63aA. Nappi,^63a,63bA. Santocchia,^63a,63bL. Servoli,^63aS. Taroni,^63a,63bM. Valdata,^63a,63b R. Volpe,^63a,63b,bP. Azzurri,^64a,64cG. Bagliesi,^64aJ. Bernardini,^64a,64bT. Boccali,^64a,bG. Broccolo,^64a,64c R. Castaldi,^64aR. T. D’Agnolo,^64a,64cR. Dell’Orso,^64aF. Fiori,^64a,64bL. Foa`,^64a,64cA. Giassi,^64aA. Kraan,^64a F. Ligabue,^64a,64cT. Lomtadze,^64aL. Martini,^64a,rA. Messineo,^64a,64bF. Palla,^64aF. Palmonari,^64aS. Sarkar,^64a,64c

G. Segneri,^64aA. T. Serban,^64aP. Spagnolo,^64aR. Tenchini,^64aG. Tonelli,^64a,64b,bA. Venturi,^64a,bP. G. Verdini,^64a L. Barone,^65a,65bF. Cavallari,^65aD. Del Re,^65a,65bE. Di Marco,^65a,65bM. Diemoz,^65aD. Franci,^65a,65bM. Grassi,^65a

E. Longo,^65a,65bG. Organtini,^65a,65bA. Palma,^65a,65bF. Pandolfi,^65a,65b,bR. Paramatti,^65aS. Rahatlou,^65a,65b N. Amapane,^66a,66bR. Arcidiacono,^66a,66cS. Argiro,^66a,66bM. Arneodo,^66a,66cC. Biino,^66aC. Botta,^66a,66b,b

N. Cartiglia,^66aR. Castello,^66a,66bM. Costa,^66a,66bN. Demaria,^66aA. Graziano,^66a,66b,bC. Mariotti,^66a M. Marone,^66a,66bS. Maselli,^66aE. Migliore,^66a,66bG. Mila,^66a,66bV. Monaco,^66a,66bM. Musich,^66a,66b M. M. Obertino,^66a,66cN. Pastrone,^66aM. Pelliccioni,^66a,66b,bA. Romero,^66a,66bM. Ruspa,^66a,66cR. Sacchi,^66a,66b

V. Sola,^66a,66bA. Solano,^66a,66bA. Staiano,^66aD. Trocino,^66a,66bA. Vilela Pereira,^66a,66b,bS. Belforte,^67a F. Cossutti,^67aG. Della Ricca,^67a,67bB. Gobbo,^67aD. Montanino,^67a,67bA. Penzo,^67aS. G. Heo,⁶⁸S. Chang,⁶⁹ J. Chung,⁶⁹D. H. Kim,⁶⁹G. N. Kim,⁶⁹J. E. Kim,⁶⁹D. J. Kong,⁶⁹H. Park,⁶⁹D. Son,⁶⁹D. C. Son,⁶⁹Zero Kim,⁷⁰ J. Y. Kim,⁷⁰S. Song,⁷⁰S. Choi,⁷¹B. Hong,⁷¹M. Jo,⁷¹H. Kim,⁷¹J. H. Kim,⁷¹T. J. Kim,⁷¹K. S. Lee,⁷¹D. H. Moon,⁷¹ S. K. Park,⁷¹H. B. Rhee,⁷¹E. Seo,⁷¹S. Shin,⁷¹K. S. Sim,⁷¹M. Choi,⁷²S. Kang,⁷²H. Kim,⁷²C. Park,⁷²I. C. Park,⁷²

S. Park,⁷²G. Ryu,⁷²Y. Choi,⁷³Y. K. Choi,⁷³J. Goh,⁷³J. Lee,⁷³S. Lee,⁷³H. Seo,⁷³I. Yu,⁷³M. J. Bilinskas,⁷⁴ I. Grigelionis,⁷⁴M. Janulis,⁷⁴D. Martisiute,⁷⁴P. Petrov,⁷⁴T. Sabonis,⁷⁴H. Castilla Valdez,⁷⁵E. De La Cruz Burelo,⁷⁵ PRL106, 122003 (2011) P H Y S I C A L R E V I E W L E T T E R S 25 MARCH 2011