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Top Physics at the LHC P Heavy Quarks and Leptons, Melbourne, 2008 1 Top Physics at the LHC P. de Jong Nikhef, P.O. Box 41882, NL-1009 DB Amsterdam, the Netherlands The LHC will be a top quark factory. In this note, the central role of the top quark for LHC physics will be discussed, and an overview will be given of the studies of top quark properties in preparation, with emphasis on the systematic uncertainties that will dominate most measurements. 1. Introduction (to e, µ or τ plus corresponding neutrino), or to two jets. This leads to an overall branching fraction of The Large Hadron Collider at CERN is scheduled top-quark pairs to an all-hadronic bbjjjj final state of to deliver its first proton- proton collisions in the au- 46.2%, to a semi-leptonic bbℓνjj final state of 43.5% tumn of this year (2008). The ATLAS and CMS ex- (29% for ℓ = e, µ only), and to a di-leptonic bbℓνℓν periments are in the final phase of installation and final state of 10.3% (4.6% for ℓ = e, µ only). commissioning of their detectors, and are preparing Recently three new calculations of the top-pair pro- for measurements of the first collisions. duction cross-section have been published [6–8], each The LHC will be a top quark factory, both for top- making significant steps towards a future full NNLO quark pair production and single top-quark produc- calculation. The central values of these calculations tion. In this note, the central role of top quark pro- at the LHC are in good agreement, even though there duction for LHC physics will be explained, and an is some difference in the treatment of the scale un- overview will be given of the studies of top quark prop- certainties. Taking the conservative value of Ref [7], erties in preparation. Being the only fermion with a the top-quark pair cross-section at √s = 14 TeV, us- Yukawa coupling to the Higgs boson of (1), the top ing the CTEQ6.5 pdf’s, is calculated to be: σ(tt¯) = O quark plays a central role in almost all models of new 908 83 (scales) 30 (pdf) pb, for a top mass mt of 171 physics. Reviews of top quark physics at hadron col- GeV.± The difference± between the predictions using the liders can be found in Refs. [1–3]. MRSTW-06 and CTEQ6.5 pdf’s (53 pb), however, is Lacking data, all results discussed in this note are larger than the pdf uncertainties as evaluated within expectations based upon Monte Carlo simulations CTEQ (30 pb) and MRSTW (12 pb) individually. At with realistic detector response and backgrounds, as √s = 10 TeV, the startup centre-of-mass energy of documented by CMS [4] and ATLAS [5]. the LHC in 2008, σ(tt¯) 400 pb. Single top-quark production≈ is an electroweak pro- cess, usually divided into three mechanisms: the t- 2. Top production channel mechanism with the largest expected cross- section, associated tW production, and the small s- Figure 1 shows the leading order diagrams for top- channel (or W*) production, as shown in Fig. 2. pair production in QCD. At the LHC, top-quark pair production is dominated by gluon-gluon fusion ( 90%). The cross-section increase with respect to the Teva- tron is very large, a factor of almost 150. The back- grounds increase as well, but with a smaller factor (about 10 for W/Z + jets), making the signal/ back- ground ratio at the LHC better. On the other hand, there is now considerable phase space for radiation of extra jets. Figure 2: Single top production diagrams. 3. Role of the top at the LHC At the LHC, the top quark has many hats. It is Figure 1: Top-pair production diagrams. possible to select clean samples of top quarks, which can be used to check and calibrate the detectors, as In the Standard Model (SM), each top quark de- described in the next subsection. A precise measure- cay would produce a W and a lighter quark (usually ment of the top production cross-section is a test of ad- a b-quark), and the W can decay either leptonically vanced QCD resummation techniques that are also ap- 2 Heavy Quarks and Leptons, Melbourne, 2008 plicable in other calculations. Precise measurements cuts on one lepton, at least four jets and some miss- of top properties can give insight into new physics be- ing ET , even without the use of b-tagging, which may yond the SM affecting top couplings and decay modes. be compromised in early data. The main background New physics can also affect top production, for ex- is W+jets, but the top signal is obvious. The cross- ample via top production from decay of heavy reso- section can be extracted from a fit to the invariant nances. Finally, SM top quark production is often a mass shape, or by background subtraction. ATLAS major background in searches for new phenomena like expects in 100 pb−1 a measurement of the tt¯ cross- supersymmetry, and it is important to understand all section with 3-7% statistical uncertainty and 15% sys- aspects of top production from data. tematic uncertainty, not including the pdf error (3%) and the luminosity error (5%). The di-leptonic channel has a smaller branching 3.1. Top as a tool fraction, but less background, and also here a signal −1 Pure samples of top-pair events can be used to ver- in 10 pb seems feasible. ify trigger and lepton identification, measure the light When well-understood b-tagging is available, the and b-jet energy scale, check the missing ET recon- top signal becomes very clean, and accurate cross- struction, and measure the b-tagging efficiency. Fur- section measurements may be made. In Tab. I the thermore, with these events it is possible to study effi- CMS estimates of uncertainties in the tt¯ cross-section ciencies and fake rates in a “busy” environment. The in the semi-leptonic decay channel (muons only) are top at the LHC is thus a prime multifunctional tool listed. A major uncertainty is due to the error on the (yesterday’s sensation, today’s calibration!). b-tagging; 5% is considered conservative and may well be considerably smaller with 10 fb−1 of luminosity. 4. Observation and cross-section Table I CMS estimates of uncertainties in the measure- ment of the tt¯ cross-section in the semi-leptonic decay − Due to this special role of the top quark in detec- channel (muons only) for 1, 5 and 10 fb 1. tor commissioning, CMS and ATLAS aim for a rapid rediscovery of the top quark in first data. A measure- ∆σ/σ −1 −1 −1 ment of the cross-section in < 1 fb−1 would provide 1 fb 5 fb 10 fb an interesting early physics result, and is important Simulation samples 0.6% for searches for new physics. Pile-up (30% on-off) 3.2% Underlying event 0.8% 4.1. tt¯production Jet energy scale (light q, 2%) 1.6% Jet energy scale (heavy q, 2%) 1.6% ¯ 2 Early observation of tt production is possible both Radiation (ΛQCD,Q0) 2.6% in the semi-leptonic and in the di-leptonic channel. As Fragmentation (Lund b, σq) 1.0% an example in the semi-leptonic channel, Fig. 3 (left) b-tagging (5%) 7.0% shows the expected distribution of the number of jets Parton density functions 3.4% after a few basic cuts in 10 pb−1 of CMS data [9]; the Background level 0.9% presence of a top signal is clear. Integrated luminosity 10% 5% 3% 250 Statistical uncertainty 1.2% 0.6% 0.4% TTbar (muon) Systematic uncertainty 13.6% 10.5% 9.7% 200 TTbar (muon comb.) Total uncertainty 13.7% 10.5% 9.7% 150 Background 100 50 Number of events / 10.0 GeV 0 50 100 150 200 250 300 350 400 450 500 M [GeV] jjj 4.2. Single top production Figure 3: Expectations for early top-pair observation. Left: CMS distribution of the number of jets after simple As mentioned, single top production takes place via − cuts in 10 pb 1. Right ATLAS distribution of three-jet three mechanisms, each having its own dedicated anal- −1 invariant mass after simple cuts in 100 pb . ysis. The major backgrounds for all three are top- quark pair production, multi-jet QCD and W+jets Figure 3 (right) shows for 100 pb−1 of ATLAS data events. In particular the QCD background can be the expected invariant mass of three hadronic jets (the suppressed by only looking at leptonic decays of the three with the highest pT vector sum) after simple W from the top. Heavy Quarks and Leptons, Melbourne, 2008 3 A. t-channel C. s-channel The s-channel is interesting since other particles like CMS performs a cut-based analysis of this channel, ± H can appear in the propagator, but it is a difficult where cuts are optimized with a genetic algorithm. In − channel due to the low cross-section and large back- 10 fb 1, the statistical significance of the t-channel grounds. signal is 37, and the cross-section is measured with CMS has studied this channel with a fast detector a 2.7% statistical error and 8% systematic error (ex- simulation, and expects to reach a measurement of the cluding the luminosity uncertainty). − cross-section in 10 fb 1 with a 18% statistical error, ATLAS uses a cut-based analysis as baseline for and a 31% systematic error.
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