The Production of single top quarks at LHC provides an ideal framework to
investigate the properties of electroweak interaction, in particular of the
{\it tWb} coupling. Moreover, single top is a powerful mean to identify physics
beyond the standard model. All three different production mechanisms of single
top are expected to be observed at LHC. Recent studies from ATLAS and CMS are
presented.Comment: 5 pages, Presented at the XLIrst Rencontres de Moriond - QCD and High
  Energy Hadronic Interaction

Najafabadi, M. Mohammadi

English

arXiv

The Production of single top quarks at LHC provides an ideal framework to investigate the properties of electroweak interaction, in particular of the {\it tWb} coupling. Moreover, single top is a powerful mean to identify physics beyond the standard model. All three different production mechanisms of single top are expected to be observed at LHC. Recent studies from ATLAS and CMS are presented

Mohammadi Najafabadi, M.

CERN Document Server

Single Top production at LHC M. Mohammadi Najafabadi on behalf of CMS and ATLAS collaborations Institute for studies in theoretical physics and mathematics (IP M), School of Physics, P. O. Box 19395-5531, Tehran, Iran The production of single top quarks at LHC provides an ideal framework to investigate the properties of the electroweak interaction, in particular of the tWb coupling. Moreover, single top is a powerful mean to identify physics beyond the standard model. All three different production mechanisms of single top are expected to be observed at LHC. Recent studies from ATLAS and CMS are presented. 1 Introduction Top quarks are mostly produced in pairs via strong interaction at LHC. However, there are a significant number of top quarks which are produced singly, via the weak interaction. Single top production proceeds through three separate sub-processes at LHC (Fig. 1 ) :  • t-channel: the dominant process involves the exchange of a space-like W boson . This process is also called W-gluon fusion, because the b--quark ultimately arises from a gluon splitting to bb. • s-channel: involves the production of a time-like W boson, which then decays to a top and a bottom quark. • tW-channel: involves the production of a real W boson. There are several reasons for studying single top quarks at LHC 1 • 2 :  • The cross sections of single top quark processes are proportional to lvtb J2 •  They provide the only way to measure V'tb directly. 1 73 qTw: q' q-,::' b__L t g,J__ b q t ,'r:.<. t-channel s-channel 9).-< t tW-channel b b w+ Figure 1: Feynman diagrams for single top production. • Single top quarks are produced highly polarized due to V-A structure of weak interaction, so single top provides a good laboratory for studying the spin of the top quark. • Single top events are backgrounds to new physics signals. • New physics could be appeared in single top events. It provides the possibility to in­vestigate the structure of tWb coupling, FCNC, fourth generation of the quarks and the existence of W'. Table 1: The total cross sections for sin le top production processes. process t-channel s-channel t W-channel cross section (pb) 242.6(NLO) 10.62(NLO) 60(LO) The total cross sections of single top production processes are listed in Table 1. The t-channel process has the largest cross section, almost one third of tl cross section. The s-channel process has the smallest cross section, one order of magnitude less than t-channel 3•4•5•6• The recent selection startegies for separation of signal from backgrounds for the three production processes in ATLAS and CMS are described in the following sections. 2 t-channel In the t-channel process when the W boson decays leptonically the final objects at parton level are one b-quark from top, one charged lepton, neutrino, a light quark and an additional b-quark. The kinematical distribution of the final partons are shown in Fig. 2. It is obvious from Fig. 2 that all final cbjects have relatively large transverse momenta except for the additional b-quark which is very soft. The identification (even the observation) of the additional b-quark is difficult due to its softness. The light quark tends to fly in the forward/backward direction which will be useful for separation of signal from backgrounds. Since detector can measure the transverse missing energy only, one needs to calculate the longitudinal component of the neutrino. The longitudinal component of the neutrino could be obtained by using the mass of the W boson and solving a quadratic equation: This equation has two solutions: 1 74 pl ,2 _ AP,,1±,/t;: z,v - p2 T,I ( 1 )  (2) "' 600 500 000 400 500 300 000 500 200 000 additional b-quark 100 500 \._ 0 o "--5�,-1�oo�iso 0 oL.::"'5�0 -10�0---1150 Pr GeV PT' GeV 700 . 400 600 300 500 400 200 JOO 1 0 1)  2 0 0  100 char�ed kpton soo� i:: o o L-----'>=-� 0 50 100 150 0 PT, GcY 500 400 . 225 200 115 150 300 200 :�: I 100 so 25 ,, I o o w..����· -4 -2 0 2 4. 11  - 4  - 2  0 2 4 11 JOo�·q,filk 500l)j]' fil ,  '''" 250 400 200 300 150 100 200 50 100 0 ' 0 ' -4 -2 0 2 ' 11  - 4  - 2  0 2 ' 11 Figure 2: The transverse momenta and pseudo rapidity distributions of the final partons in t-channeL M2 - -A =  T + Pr,1 -ET,miss ' 6. = Ef (A2 - Ef,missPf) Two difficulties are present here. First, in some events 6. < 0, as a result no real solution exists for the lonitudinal component of neutrino. Second, there is an ambiguity in choosing the Pz,v between the two roots. Regarding the above properties of t-channel the selection strategies of ATLAS and CMS are presented below. Common selections are: • One isolated high PT lepton in the central region. • Cut on missing transverse energy (useful for suppression backgrounds with W boson in the final state) . • Exactly two high PT jets, one of them should be b-tagged in centeral region and another one should be in the forward region ( 111 1 > 2.5). In the ATLAS analysis, the events with no real solution for Pz,v are discared and between two real solutions of Pz,v , the solution which gives closest invariant mass of lvb to the nominal top mass is chosen. A window cut is applied on Hr where Hr = L:jetE}et + Er,1 + Er,miss and on the top mass. After following the above strategy and taking into account tl and W +jets backgrounds the number of survived signal events are 7000 after 30 fb-1 of integrated luminosity and � and statistical sensitivity to V'tb are: S y'S + B B � 3 , --B- = 0.014 (3) The main dominant systematics are due to background estimation, ISR/FSR, JES, b-tagging and luminosity. ISR and FSR have a significant influence on the jet multiplicity, the uncertainty on JES affects the selection efficiencies directly(via the thresholds of the PT of the jets), the uncertainty on b-tagging has an important effect on the background rejection( especially on rejection of W +njets). In CMS analysis, the events with no real solution for Pz,v are kept (in this case Mw is increased up to the value in which 6. = 0 ) and between two real solutions of Pz,v, the solution with minimal absolute value is chosen. A cut is applied on lfr l where :Er = Pr,w + Pr,bjet + Pr,forw.jet (Fig. 3). A window cut is also applied on transverse mass of the W boson where M,fr = J2(Pr,1ET,miss - Pr,1 · Er,miss ) ·  Finally, a window cut is applied on the top mass. At 10 fb-1 and after applying the above strategy, the efficiency is around 1.43 and number of survived events is around 2350 events. 1 75 3 s-channel ,. ' ...... � - signal ........ ttbar "'i':,� .. '•· ,,, '"' ··�·:.t•: ... s .. � .. -'':_ .. � W W  100 1M 1 � 1 W 1 W 200 1!';1 (GeV/c) Figure 3: The distribution of IETI in t-channel (CMS). The final objects of the s-channel mode when W-boson decays leptonically are two high PT b-jets, one charged lepton and neutrino. In principle, searching for s-channel is more difficult than t-channel because of much smaller cross section, the presence of two high PT b-jets, lack of any extra forward jet. However, the PDF's of the initial state of s-channel(u(u) and d(d)) are very well known while the PDF's of the initial state of t-channel(b(b) and gluon) are less known, so the uncertainty due to PDF in s-channel is smaller than t-channel. In the following the selection strategies for CMS and ATLAS are presented. Common selections are: • One isolated energetic charged lepton in the centeral region. • A cut is applied on transverse missing energy. • Exactly two high PT b-tagged jet. In the CMS analysis, the events with no real solution for Pz,v are kept (in this case the real part is kept) and between two real solutions of P,,v, the solution with minimal abstract value is chosen. For top reconstruction, the b-jet which has the opposite charge sign (Qjet) to lepton is used. Jet charge is defined as the sum of the charges of the tracks inside the jet cone. A cut is applied on !Er l  where Er = Pr,w + Pr,bjetl + Pr,bjet2 · A window cut is also applied on the top mass 7• In the ATLAS analysis, the selection is splitted in two parts: tb and lb. The events containing a secondary lepton with opposite sign are rejected. Among the four combinations of two b-tagged jets and two solutions for Pz,v, the combination which gives the closest invariant mass of lvb to the nominal top mass is chosen. A window cut is applied on Hr where Hr = EjetE?et+Er,t+Er,miss and also on the top mass (Fig. 4) . The mentioned strategy for ATLAS leads to the following results at 30 fb-1 : S .JS + B = 0 06 B � 0.15 ' B . (4) After application of all cuts the efficiency is 1 % and number of survived events is 2050. The dominant background comes from the top pair production in the dilepton and lepton+jets channels, followed by WQQ(Q = c, b) contamination. The remaining W +jets contamination is due to the high cross section for such events. The dominant systematics are systematics due to background estimation, ISR/FSR, JES, b-tagging and luminosity. 1 76 Energy H, (L=.30 fb-1) I tbfinal slate J � 9-i;hann .. 1 • t-ehann11I • \t --+ -rT+-rjc\ D It-+ lvb lub M u --+ 1vb jjb � WOO (O-b,c) · ·: Wjj -+ lvjj (':� W Z - + lv bb Energy H, (GeV) Men M(lvb) (L=30 fb-') ( tb !incl stale I 1 Iii ' - 00000•' � • tt-+ -r-r+-rjeti Ill ti -+ lvb lvb � a tt --+ lvb jjb J 'X WOO (O .. b,c) j Wjj --+ lvjj � •,; wz �'" " l moss in GeV/c' Figure 4: The distributions of HT and the top mass in the s-channel (ATLAS). 4 t W-channel In ATLAS the study for tW-channel has been performed for the case of leptonic decay of the W-boson from top decay and hadronic decay of another W-boson. One high PT lepton and exactly three jets are required, one of jets should be b-jet in the central region. W-boson is reconstructed by the two non b-tagged jets. Window cuts are applied on Mw, the invariant mass of the lvb system and on Hr. After applying the cuts the signal to background ratio is � and vs;B = 0.04 (at 30 fb-1) .  The dominant backgrounds are tt and t-channel. For an integrated luminosity of 30 fb-1 ATLAS expects 4700 events in tW-channel. A large systematic uncertainty is expected to come from the JES and from ISR/FSR. 5 Conclusions Single top quark is expected to be observed in Tevatron soon. Since the cross sections of single top quark production processes at LHC is much higher than Tevatron, LHC provides the possibility for precise measurements of all three processes. The electroweak model in the top quark sector could be tested and the l vtbl CKM matrix element is determined with high precision. Single top production processes could be used as a powerful mean for probing new physics. Acknowledgments I would like to thank to Arnaud Lucotte who provided the most recent updated results from ATLAS analyses. Many thanks to the CMS single top group and Joachim Mnich. References L M. Beneke et al., Proceedings of the Workshop on Standard model physics at the LHC,  CERN-2000-004, Top Quark Physics. 2. T.M.P.Tait, C.-P.Yuan, Phys. Rev. D 63, 014018 (2000) . 3. Z. Sullivan, Phys. Rev. D 70, 1 14012 (2004) .  4. T. Stelzer, Z. Sullivan and S .  Willenbrock, Phys. Rev. D 56,  5919 ( 1997). 5. M.C. Smith and S .  Willenbrock, Phys. Rev. D 54, 6696 (1996). 6. J. Campbell, R.K. Ellis and F. Tramontano, Phys. Rev. D 70, 09401 2  (2004) . 7. A. Giammanco, CMS Conference Report 2006/011. 1 77 

Single Top production at LHC

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