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Charge asymmetry of top quark-antiquark pairs Antoine Chapelain CEA-Saclay, IRFU/SPP email: [email protected] Abstract charge asymmetry at the Tevatron and LHC is there- fore complementary. Inthisnotewepresentthechargeasymmetrymeasure- mentsoftopquark-antiquarkpairsathadroncolliders. 1 Introduction 4 1 0 Among the known twelve fermions, which are the fun- 2 damental bricks of matter, the top quark is the latest to have been discovered at the Tevatron Fermilab Col- n a lider near Chicago by the CDF and D0 experiments in J 1995 [1]. The top quark was found to be the heaviest 7 particle ever observed. Its mass is about the mass of 2 the gold atom, which is extremely heavy for a point- like particle. Due to its large mass, studying the top ] x quarkcouldbeawindowtowardsso-callednewphysics, e i.e., physics that lies beyond the Standard Model of - p particle physics. The top quark can be scrutinized at e hadron colliders since the Tevatron and LHC colliders h produced numerous top quark-antiquark 1 pairs. The Figure 1: Rapidity distributions of the top quark and [ chargeasymmetryisoneofthepropertiesoftopquark- antiquark at the Tevatron (top) and LHC (bottom). 1 antiquark pairs. Indeed the strong interaction predicts v thatwhenproducedthroughquark-antiquarkcollisions Thechargeasymmetrycanalsobemeasureddirectly 6 the top quark and antiquark are not produced isotrop- using the leptons coming from the decay of the top 3 ically. The top quark is preferentially produced in the quark and antiquark since the flight direction of these 8 6 directionoftheincomingquarkwhilethetopantiquark leptonsiscorrelatedwiththeflightdirectionofthetop . is preferentially produced in the direction of the in- quark/antiquark. Thismeasurementissimplerbecause 1 0 comingantiquarkintheincomingquark-antiquarkrest the flight direction of the leptons is directly measured 4 frame. As the top quark and antiquark have oppo- inthedetectorwhilethetopquarkflightdirectionneed 1 site electric charge, it will result in a charge asymme- tobereconstructedfromthedecayproductsofthetop v: try (excess of positive/negative charge in the incoming quark. i quark/antiquark direction). In 2011 the CDF and D0 collaborations reported X To quantify this effect we use the rapidity y measurements higher than the predictions as summa- r a (or pseudorapidity η). It is defined approximatively rized in Figure 2. The Tevatron stopped data taking as y (cid:39)−ln(tan(θ/2)) where θ is the angle between inSeptember2011. Updatingtheasymmetrymeasure- the flight direction of the top quark (antiquark) and ment with the full CDF and D0 recorded dataset is the beam direction. Figure 1 shows how the charge underway. asymmetry appears at the Tevatron and LHC. At the In Sec. 2 we will focus on the measurement per- Tevatron, which is a proton-antiproton collider, there formed at D0 in the dilepton channel [2] and in Sec. 3 is a forward-backward (or right-left) asymmetry since onthemeasurementperformedatATLASinthedilep- the incoming quark-antiquark collision frame is almost ton channel at 8 TeV. Section 4 summarizes the inclu- equaltothequark-antiquarkrestframe. SincetheLHC sivechargeasymmetrymeasurementsathadroncollid- is a proton-proton collider the quarks carry on average ers. a higher momentum than the antiquarks which come from the sea of the proton. The top quark will be 2 Dilepton measurement at D0 thus emitted more forward or backward and the top antiquark will be emitted more central. Measuring the Thett¯dileptonfinalstate(seeFig.3),ordileptonchan- 1Each fermion has a corresponding antifermion, which have nel, is characterized by two leptons with opposite elec- thesamepropertiesbutoppositeelectriccharges. tric charge, at least two jets coming from the two b- 1 2 300 Top Quark Asymmetry DØ, L=9.7 fb 1 tt Z CDF L+jet (5.3 fb-1) 250 Instrum. 15.8 – 7.5 % Diboson Data D0 L+jet (5.4 fb-1) 19.6 – 6.5 % 0.4200 Lepton Asymmetry AFlB ents/150 D0 Dilepton (5.4 fb-1) 5.8 – 5.3 % Ev100 D0 L+jet (5.4 fb-1) 15.2 – 4.0 % 50 D0 combination (5.4 fb-1) 11.8 – 3.2 % 0 3 2 1 0 1 2 3 Lepton Asymmetry All q × η D0 Dilepton (5.4 fb-1) 5.3 – 8.4 % DØ, L=9.7 fb 1 tt Bernreuther & Si, Phys.Rev., D86 (2012) 034026 100 Z Instrum. 0 5 10 15 20 Diboson Asymmetry (%) 80 Data 4 0. s/ 60 nt e v Figure 2: Summary of the asymmetry measurements E 40 at the Tevatron in 2011. 20 0 4 3 2 1 0 1 2 3 4 quarks,andmissingenergyduetothetwoneutrinoses- ∆η capingthedetector(theneutrinosandthechargedlep- tonsarebothcomingfromthedecayoftheW bosons). Figure 4: Rapidity distributions used to compute the asymmetry in the dilepton channel at D0 [2]. erage of the detector. Once these corrections are made we can compare the measurements to the theoretical predictions. Table1showsthemeasuredandpredicted values. Both are agreement within the uncertainties. It is interesting to look at the ratio A(cid:96) /A(cid:96)(cid:96) since the FB Figure 3: Topology of a dilepton tt¯ event produced two asymmetries are strongly correlated and because through quark-antiquark annihilation at the Tevatron. the systematic uncertainty is reduced due to cancella- tions. Figure 5 shows the measured value in black to- gether with different predictions. The measured value Thedileptonchannelsuffersfromsmallstatisticsbe- of 0.36±0.20 is in agreement with the prediction of cause of a small branching ratio but on the other hand 0.79±0.10attheleveloftwostandarddeviations. Fig- haveasmallamountofbackground. Themeasurement ure 6 summarizes all the current measurements at the of the forward-backward asymmetries through leptons Tevatron. We can see that the tensions between mea- is performed using the two distributions in Fig. 4. The surementsandpredictionsobservedin2011(seeFig.2) single-lepton A(cid:96) asymmetry is defined with the q×η FB vanished. Two measurements from D0 have still to be distributionlookingateachleptonindependentlyifthe releasedin2014. ThefocusisnowontheCDF-D0com- leptongoesintheforward(η >0)orbackward(η <0) bination of these different results to achieve the best direction. The∆ηdistributionbuiltasthedifferenceof possible precision. lepton pseudorapidities is used to measure the lepton- pairA(cid:96)(cid:96) asymmetry. A(cid:96) andA(cid:96)(cid:96) arecomputedasthe FB relative difference between the forward and backward regionoftherelevantdistributionsusingthedatafrom Measured Predicted which we subtracted the expected background. A(cid:96) 4.4±3.7±1.1 3.8±0.3 FB In Fig. 4 the black dots represent the data, the col- A(cid:96)(cid:96) 12.3±5.3±1.5 4.8±0.4 oredhistogramsrepresentthepredictions: thett¯signal in red, and the different backgrounds in grey, yellow and blue. At this level we performed the asymmetry Table1: Measuredandpredictedvaluesofthetwolep- measurements in the detector, i.e., distorted by detec- tonic asymmetries in the D0 dilepton channel. The toreffectsthatneedtobecorrectedfor. Wefirstcorrect first uncertainty on the measured values is statistical for the selection efficiency, i.e., for the fact that we do and the second is systematic. not observe all the produced dilepton tt¯events in the detector. We then correct for the limited spatial cov- 4. CURRENT STATUS AND CONCLUSION 3 %) DØ, L=9.7 fb 1 Top Quark Asymmetry l (AFB 15 DMaCt@a NLO 3 σ CDD0 FL +Lj+ejte (t5 (.94. 4fb f-b1)-1) 19.6 – 6.5 % Model 1 16.4 – 4.7 % Model 2 2 σ Single-lepton Asymmetry AFlB SM NLO D0 Dilepton (9.7 fb-1) 10 1 σ D0 L+jet (9.7 fb-1) |y|<1.5 4.4 – 3.9 % l 4.7 – 2.6 % CDF Dilepton (9.1 fb-1) 7.2 – 6.0 % CDF L+jet (9.4 fb-1) 9.4 – 3.2 % 5 Lepton-pair Asymmetry All D0 Dilepton (9.7 fb-1) 12.3 – 5.7 % CDF Dilepton (9.1 fb-1) 7.6 – 8.1 % Bernreuther & Si, Phys.Rev., D86 (2012) 034026 0 5 10 15 20 0 5 10 15 20 All (%) Asymmetry (%) Figure 5: A(cid:96) versus A(cid:96)(cid:96) in the dilepton channel at Figure 6: Summary of the asymmetry measurement at FB D0[2]. Theblackdotrepresentsthemeasurementwith the Tevatron in 2013. the uncertainty ellipses corresponding to 1, 2 and 3 standard deviation. “SM NLO” represents the most recent theoretical prediction, “MC@NLO” is the event and detector effects. We see that the reconstructed generatorusedtosimulatedthett¯signaland“Model1” distribution reproduces the behavior of the truth dis- and “Model 2” are two new physics model that could tribution well. explain the 2011 observed tension at the Tevatron between measurements and predictions. 3 Dilepton measurement at AT- LAS As explained earlier, measuring the charge asymmetry at the LHC and the Tevatron is complementary. This section is focusing on the charge asymmetry measurement in the dilepton channel at ATLAS. Both the asymmetry of the lepton coming from the top quark/antiquarkandofthett¯pairsaremeasured. The top quark is not directly observed in the detector due to its very short lifetime (10−23 s). Thus we need to Figure 7: Rapidity distributions at truth (red) and re- reconstructitskinematicfromitsobserveddecayprod- constructed (blue) level. ucts. To do so we use the energy and momentum con- versationateachdecayverticesofthedecaychain. We Theasymmetryiscomputedusingtheobservablede- obtain then a system of 16 equations and 22 unknowns fined as ∆|y| = |y |−|y |. In the example of top antitop which cannot be solved. Making several assumptions Fig. 7 we are able to reconstruct the correct sign of and fixing the masses of the W bosons and the top ∆|y| in 70 % of the cases. This performance is rather quarks to their measured values we finally end up with satisfyingandverysimilartoperformancesofotherre- 18 equations and 18 unknowns. For a given event we constructionmethod. Thismeasurementat8TeVwith obtain several solutions. We define a weight for each theATLASdetectorisstillongoingandwillbereleased solution according to its probability to be a tt¯event. soon. This probability is computed using the matrix element of the gg → tt¯process. The solution with the highest weight is selected. This method is called the “Matrix 4 Current status and conclusion Elementmethod” [3]. Weusethesimulationtotestthe performances of this reconstruction method. The vari- The Tevatron and LHC are both the most powerful able we are interested in to compute the asymmetry is proton-antiproton and proton-proton colliders, respec- therapidityy ofthetopquarkandantiquark. Figure7 tively. They allow to conduct complementary studies shows the y distribution at the so-called “truth” level on the charge asymmetry of the top quark-antiquark andafterreconstruction. Thetruthleveliswhatisgen- pairs. In 2011 the Tevatron measurements showed ten- erated with the simulation and the reconstructed level sionbetweenmeasurementsandpredictions. Thelatest is what we reconstruct after the simulation of physics resultswiththefullstatisticsrecordedbytheCDFand 4 REFERENCES D0experimentstendtoindicateabetteragreementbe- tween predictions and measurements. At the LHC, so far all the measurements are in good agreement with the predictions. Some physics model beyond the Stan- dard Model could explain the deviations observed at the Tevatron in 2011 while still in agreement with the observation at the LHC (see Fig. 8). We can see on Fig.8thatasmallregionofphasespaceisstillallowed forthesenewphysicsmodel. ThenewresultsfromD0, as well as new results from ATLAS and CMS are expected to be able to make a conclusive statement. The year 2014 is thus very promising to understand deeper thechargeasymmetryofthetopquark-antiquarkpairs. 0.08 ATLAS G Models from: µ PRD 84 115013, 0.06 arXiv:1107.0841 W′ Ω4 ω4 0.04 φ C A 0.02 SM ATLAS 0 CMS -0.02 DF 0 C D 0 0.1 0.2 0.3 0.4 0.5 A FB Figure 8: Summary of the measurements at the Teva- tron and LHC and predictions from different physics models [4]. References [1] The CDF Collaboration, Phys.Rev.Lett. 74.2626 (1995); The D0 Collaboration, Phys.Rev.Lett. 74.2632 (1995). [2] The D0 Collaboration, Phys.Rev. D88 (2013) 112002. [3] F. Fiedler, A. Grohsjean, P. Haefner, P. Schiefer- decker, Nucl.Instrum.Meth. A624 (2010), 203-218. [4] TheATLASCollaboration,arXiv:1311.6724(2013).