

 1 Sep 1995

MSUHEP-50831

August 1995

Top Quark Physics at the Tevatrona

C.-P. Yuan Department of Physics and Astronomy

East Lansing, MI 48824 , USA

We discuss physics of the top quark at the Fermilab Tevatron. By the year 2000, many properties of the top quark can be measured at the Tevatron.

1 Discovery of the Top Quark

The discovery of the top quark is one of the most important discoveries at the Fermilab Tevatron which is currently the only collider that produces top quark on its mass-shell. The mass of the top quark has been measured to be mt = 176 \Sigma 8 (stat:) \Sigma 10(sys:) GeV by CDF group and mt = 199+29\Gamma 21 (stat:) \Sigma 22(sys:) GeV by DO/ group, by detecting the t_t events. The standard t_t event selection is based on the expected Standard Model (SM) decay chain t_t ! (W +b)(W \Gamma _b) and the subsequent decays of the W 's into fermion pairs. At least one W is tagged in the mode W ! ` + * by requiring an isolated high pT (transverse momentum) lepton (` = e or _) and large 6ET (missing transverse energy). In the "dilepton" analysis the leptonic decay of the other W is identified with a loose lepton selection; this mode has small backgrounds but small branching ratio of just 4=81 ' 5%. In the case of the "lepton + jets" mode, the second W decays to quark pairs, giving larger branching ratio of 24=81 ' 30%. The final state of (`*b)(jj_b) is separated from the primary background, W + jets, by requiring a large multiplicity of high pT jets and also evidence of a b-jet, using either secondary vertex (silicon detector) or soft lepton (b ! c`*X) identification. For more detailed discussions on the event selection and the detector configuration which determines the acceptance and the detection efficiency of the events, we refer the readers to Ref. [1].

aTalk given at the International Workshop on Elementary Particle Physics: Present and Future, Univ. of Valencia, Valencia, Spain, June 5 to 9, 1995; and at the Fermilab Users Annual Meeting, Fermilab, Illinois, July 13 & 14, 1995.

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Assuming the SM decay mode of the top quark, the cross section oet_t for the QCD production processes q _q; gg ! t_t was measured to be 7:6+2:4\Gamma 2:0 pb (by CDF) and 6:4 \Sigma 2:2 pb (by DO/ ). For comparison, the SM result for mt = 175 GeV at ps = 1:8 TeV is oet_t = 5:52+0:07\Gamma 0:45 pb quoted from Ref. [2] in which the effects of multiple soft-gluon emissions have been properly resummed. Since the measurement of the cross section (obtained from the "counting" experiments, i.e. counting the observed total t_t event numbers in various decay modes) and the measurement of the mass of the top quark (obtained from reconstructing the invariant mass of the top quark) are two independent measurements, one can combine these results to find the best fitted values for mt and oet_t [3]. We find that mt = 174 \Sigma 4 GeV and oet_t = 5:9 \Sigma 0:7 pb [4]. If the t_t event rate is indeed given by the SM prediction for oet_t, then the results of the fit described above conclude the branching ratio (BR) of t ! X for X 6= bW has to be less than , 10% [4].

2 Is Top Quark just another heavy quark?

mt (' v=p2 = 174 GeV) is about the scale (v) of the electroweak symmetry breaking (EWSB). Studying top quark might provide clues to generation of the fermion masses which could be closely related to the EWSB. Furthermore, effects of new physics originating from the EWSB would be more apparent in the top quark sector than any other light sector of the electroweak theory. Hence, the top quark system not only serves as a stage for testing the SM but also provides a window to new physics beyond the SM.

A few examples are discussed in Ref. [5] to illustrate that different models of EWSB mechanism will induce different interactions among the top quark and the W - and Z-bosons. These interactions may strongly modify the production and/or the decay of the top quark. In some models (e.g., the TopColor model [6]) observable flavor-changing neutral current (FCNC) processes (e.g., t ! cZ; cg; cfl : : :) can be mediated by new underlying dynamics, and some new resonances can strongly couple to t_t (e.g., a degenerate, massive color octet of "colorons" and a singlet heavy Z0) or t_b (e.g., a triplet of "top-pions") system. With all these new effects possibly appearing in the top quark system, we conclude that top quark is likely not just another heavy quark.

Is it a SM top quark? What do we know about the interaction of the top quark? Can we learn about that from the radiative effects to the precision LEP/SLC data (physics at the Z-pole)? A few analyses for studying the top quark couplings to the gauge bosons show that current low energy data still allow rooms for new physics [5, 7], and that only the direct measurements (with on-shell top quark produced) of these couplings can be conclusive.

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3 Top Quark Physics for the Tevatron at Run-II and Beyond

To determine how well an observable can be measured at the Tevatron in the Main Injector Era (Run-II and beyond), we need to set up a reference for top quark event rates. For this purpose, we consider a _pp collider with pS = 2 TeV and an integrated luminosity of 2 fb

\Gamma 1 (or, 1 fb\Gamma 1 for each experimental

group). In the SM, for a 175 GeV top quark, there will be about 1:4 \Theta 104 t_t pairs and 5\Theta 103 single-t or single-_t events produced. After taking into account the b-tagging efficiency and the detection efficiency [1], there are about 1000 single-b-tagged t_t pairs in the `+ jets sample (among those 600 are also doubleb-tagged), 100 in the dilepton sample, and 250 single-t or single-_t events (in the `+ jets sample) available for testing various properties of the top quark. In the following, we discuss the relevant observables and show that with a 2 fb

\Gamma 1

luminosity, many first measurements can already be done to a good accuracy at the Tevatron. With a (10\Gamma 100) fb

\Gamma 1 integrated luminosity (beyond Run-II),

many further improvements are expected.

3.1 t_t production rate oet_t

At the Tevatron, the dominant t_t pair production mechanism is q _q ! t_t not gg ! t_t, the former contributes about 90% of the rate because the quark luminosities are larger than the gluon luminosities for large x (i.e. for producing heavy top quarks). To test QCD, we need an accurate measurement for oet_t which is experimentally limited by the systematic uncertainties. It is concluded in Ref. [1] that the experimental error in oet_t is ffioet_t ' 10% which is about the same as the theoretical error for calculating oet_t [2]. (Note that counting the dilepton event rate is a direct measurement of oet_t, and the statistical error for the dilepton sample of t_t events is about 1=p100 = 10%.)

3.2 Mass of the top quark mt

With enough t_t pair events, the accuracy in measuring mt will be determined by the systematic uncertainty which is dominated by the error in measuring the jet energy scale due to the imperfections in the calorimetry and the effects of initial and final state gluon radiations. The determination on the jet energy scale can be greatly improved by studying the Z + 1 jet and fl + 1 jet events [1]. It is expected that mt can be measured to within a couple percent. So, the uncertainty in measuring mt is ffimt ' (2 \Gamma 4) GeV.

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3.3 Distributions of invariant mass Mt_t and transverse momentum pT (t)

If a heavy new resonance (V ) can be produced in _pp collision and can strongly couple to t_t [8], then the observed distributions of Mt_t and pT (t) will be different from the SM predictions. (The event rates can either increase or decrease.) By carefully comparing these distributions with the predictions of a given theory, one can then either approve or exclude that theory. Since Mt_t can be reconstructed on an event-by-event basis by requiring that there are two top quarks observed in the event, the shape and the magnitude of this distribution can therefore set a model-independent limit on possible new physics coupled to t_t pairs. Demanding a resonance to be observed at the 5oe level (i.e. S=pB ?, 5) over the t_t continuum in the Mt_t spectrum, one can set the minimum bound on oe(_pp ! V ) \Lambda (V ! t_t) to about (0:4 \Gamma 0:8) pb and (0:1 \Gamma 0:2) pb for MV equal to 500 GeV and 800 GeV, respectively [1].

3.4 Top quark decays and FCNC decay modes of top

Because top quark is heavy, it will decay via weak interaction before it feels non-perturbative strong interaction. This is the first opportunity we have for studying the properties of a bare quark. In the SM, the total decay width of a SM top quark is \Gamma t ' 1:6 GeV(mt=180 GeV)3, and the branching ratio of the weak two body decay t ! bW + is about one hundred percent. In this decay mode the top quark will analyze its own polarization [9]. If t is found dominantly decaying to bW +, the second top (_t) in each t_t event should be carefully studied as a window for small non-SM decay modes of top quarks. We call this t-tagging.

In the SM, the branching ratios for the FCNC decay modes were found to be too small to be detected, but they can be observable in some other models. Consider the MSSM with light chargino and top-squark. One of the t in the t_t event decays to ~t1 ~O/01, and ~t1 subsequently decays to c ~O/01. The signature of this event is W + 2 jet+ 6ET which is not included in the counting experiments that only count events with W + * 3 jets. A careful study on this signature can approve the MSSM or set limit on the MSSM parameters [4]. Other studies [1] show that a 2 fb

\Gamma 1 luminosity can be sensitive to BR(t ! cZ) , 2% (from

3` + 2 jets or 2` + 4 jets sample) and BR(t ! cfl) , 0:3% (from ` + fl + 2 jets or fl + 4 jets sample).

3.5 Ratios of branching ratios: R` and Rb

Define R` to be the ratio of the t_t cross sections measured using dilepton events to that measured using ` + jets events. If R` differs from the SM

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prediction, then it implies new physics that would allow t decay without a W boson in the final state, such as charged Higgs (t ! bH+ [10]) and top-squark (t ! ~t1 ~O/01). Hence, R` measures BR(t ! bW ). With 2 fb

\Gamma 1, the error on

BR(t ! bW ) is about 10% [1]. Another useful ratio Rb is the ratio of the t_t cross sections measured using double-b-tagged events to that measured using single-b-tagged events. This determines the upper limit on the branching ratio of t ! X where X does not contain any b-quark. (This can be applied to both ` + jets and dilepton samples, from a known b-tagging efficiency.) With 2 fb

\Gamma 1,

this upper limit can be set to about (3\Gamma 5)% [1]. This result can be interpreted as the error on measuring BR(t ! bW ) if a W boson is confirmed in t decay. (Generally a small BR(t ! X) is better measured from direct search of the rare decay mode than from the measurements of R` and Rb.)

3.6 Partial decay width \Gamma (t ! bW ) and the lifetime of top

The total decay width \Gamma t of a SM top quark cannot be measured from the invariant mass of t reconstructed in the t_t events [11]. An elegant way to determine the lifetime (the inverse of the total decay width) of the top quark is to measure the partial decay width \Gamma (t ! bW ) and the branching ratio BR(t ! bW ), since \Gamma t = \Gamma (t ! bW )=BR(t ! bW ). As shown in Ref. [11], \Gamma (t ! bW ) can be model-independently measured by counting the production rate of single-top events produced from the W -gluon fusion process (which is equivalent to the W -b fusion process), because the production rate of W b ! t is directly proportional to the decay rate of t ! bW . The cross section for the W -gluon fusion process is known to about (15 \Gamma 20)% [11], the lifetime of the top quark can therefore be determined to about (20 \Gamma 30)%.

Before concluding this section, we note that measuring the single-top event rate from the W -gluon fusion process is an inclusive method for detecting effects of new physics which might produce large modifications to the interactions of the top quark [11]. This provides the first hint of possible large new physics effects in the production of the single-top events at the Tevatron, and a further detailed test of the t-b-W couplings can be achieved by studying the decay of top quarks in both the t_t pairs and the single-top events.

3.7 Form factors of t-b-W , Vtb and mt

To describe the decay of t ! bW (! `*), the most general form factors contain fL;R1 and fL;R2 [9]. Using the invariant mass (mb`) of b and `, one can determine the polarization of the W boson which depends on the values of these form factors. A study [12] shows that the errors ffif L1 , (2 \Gamma 3)% and ffifR1 , 0:2, if f L1 ss 1 and fR1 and fL;R2 are almost zero (SM values).

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The measurement of fL1 can be interpreted as the measurement of the CKM element Vtb which is therefore known to within 3%. (We note that this result is more accurate than that obtained from measuring the qq0 ! W \Lambda ! t_b production rate which yields a 10% error in measuring Vtb [1].) Furthermore,

the fraction (FL) of longitudinal W 's from top decays is equal to 12 m

2 t

M2W , and

is independent of fL;R1 , it is therefore a good tool for measuring mt [9]. (The quadratic dependence of FL on mt helps in ffimt by a factor of 2.) A 2% accuracy in determining FL yields a , 2 GeV error in measuring mt.

3.8 Exotic production mechanism and testing CP violation in top

If new physics strongly modifies the coupling of t-c-g , then the production rate of t-c pair from q _q ! g ! t_c can be largely enhanced. By measuring its production rate in high Mtc, one can set the minimum energy scale \Lambda tcg at which new physics must set in. It is found that \Lambda tcg ?, 2 TeV [13].

Besides all the potential physics discussed above, the Tevatron, as a _pp collider, is unique for being able to test CP violation by measuring the production rates of single-top events. A nonvanishing asymmetry (At) in the inclusive production rates of the single-t events and the single-_t events signals CP violation [14]. With 2 fb

\Gamma 1, it is possible to observe this asymmetry for At ?, 20%

[14].

This work is supported in part by the NSF under grant no. . We thank our colleagues who contribute to the studies in Ref. [1].

References

1. Dan Amidei and Chip Brock, "Report of the T eV 2000 Study Group

on Future ElectroWeak Physics at the Tevatron", 1995; and references therein. 2. E.L. Berger and H. Contopanagos, ANL-HEP-PR-95-31, May 1995. 3. D. Soper, talk at the QCD session of the XXX Rencontre de Moriond,

Les Arcs, France, March 1995. 4. S. Mrenna and C.-P. Yuan, in preparation. 5. Ehab Malkawi and C.-P. Yuan, Phys. Rev. D50, 4462 (1994). 6. C.T. Hill, Phys. Lett. B345, 483 (1995). 7. J. Feliciano, F. Larios, R. Martinez and M.A. Perez, CINVESTAV-FIS09-95, 1995; T. Han, R.D. Peccei and X. Zhang, Fermilab-pub-95/160-T, 1995.

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8. C.T. Hill and S. Parke, Phys. Rev. D49, 4454 (1994);

E. Eichten and K. Lane, Phys. Lett. B327, 129 (1994). 9. G.L. Kane, G.A. Ladinsky and C.-P. Yuan, Phys. Rev. D45, 124 (1992). 10. J. Guasch, R. Jimenez and J. Sola, UAB-FT-370, July 1995. 11. D.O. Carlson and C.-P. Yuan, MSUHEP-50823, August 1995. 12. D.O. Carlson, Ph.D. thesis, Michigan State University, MSUHEP050727, August 1995. 13. E. Malkawi and T. Tait, private communication. 14. C.-P. Yuan, Mod. Phys. Lett. A10, 627 (1995).

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