TOP SEARCH AT THE LIiC : SIGNAL VFR,SITS BAÇItCR.Ot1ND AND TOP-MASS MFASTJREMENT

Flavio CAVANNA

LNGS (INFN) and University of AQUiLA, Italy

Various ways of searching for the t-quark at the LHC, concentrating on the expected Standard Model mass-range100 S m~ S 250GeVlc2 are presented. ExteLsive simulations of the signal and of the expected backgrounds havebeen performed, the different techniques of reducing the backgrounds are discussed and the expected signal-to-background ratios in the main experimental channels are evaluated. Different methods to determine the topmass are presented toghether v~nth the achievable limits in precision .

1. INTRODUCTION

In the past decâde the t-quark has been system-atically searched at e+e- and hadron colliders. Nev-ertheless the t-quark still escapes discovery; its 'non-existence' would require a total overhaul of the Standard

Model and is not plausible. All this makes the searchfor the t-quark one of the most urJent and coraapelling

challenges in high energy physics. The most stringent

lower limits on mt come from experiments at pp' collider :UA1, UA2 and CDF' . The best one has been recently

set by the CDF Collaboration2 : mt > 89 GeV/c2, i.e.

above the W mass. On the other hand, an upper limitof mf ~ 200 GeV/ca is obtained by combining, for ex-ample, the measurements o~ mZ and myylmZ with theelectroweak radiative corrections to sin26yy, which de-pends on the t-quark massa.

The total integrated luminosity foreseen for the com-

ing years at Fermilab (~ 200 pb-1 ) should allow the mtrange to be explored at least up to ti 150 GeV, c2 .

The future CERN hadron Collider, LHC, with muchhigher luminosity and c.m. energy, would allow the ex-ploratian of a large t-quark domain, with nia,ss u.p ti 1TeV/c2 . In the mt range expected from the StandardModel, namely 100 ~ m~ ~ 200 GeV/c2, the produc-tion rates are very large. A precision measurement ofthe t-quark mass would be possible in this mass range,after a potential discovery in the ~ 150 GeV/ez region .Since the t-quark maybe an important source of charged

(192I~Sfiî2l92â05_0(1 Cc) 1992 - Elsevier SciencE P~ahlishers B_V

All rinhts reserved_

Nuclear Fhysics B (Pros. Suppl.) 27 (1992) 374-386North-Holland

Higgs particles expected in the SUSY extension of the

Standard Mode14, a precise measurement of the t-quark

decay branching ratios is very important too.

We summarize here the results of an extensive simu-lation oft-quark production and ofthe main background

sourcess. After a consistency check between the Monte

Carlo and the theoretical cross-sectionss,7, the main t-

quark signatures are considered, with the corresponding

backgrounds. The aim is to define selection criteria toobtain clean samples for detailed studies (mass, BR's):

a set of acceptance cuts are gïven for the 'single-lepton'

and 'dilEptons t-quark decay modes. We also discuss

the presence of additional (non-isolated) QC's tagging theb's produced in the t-quark decay as a further mean

of improving signal-to-background ratios . The t-mass

reconstruction is considered then in various ways, em-ploying different event samples, and the errors on mt

are then evaluated.

the event generators we used are EUROJETS and

ISAJET9 for the t-quark signal, andISAJET and LDWIe

for the various background sources, bb production, W~-jets production, WWproduction, etc.

2. TOP PRODUCTION AND DECAY

The t-quark production mechanisms and decay modeare reviewed in detail elsewhere?. We recall here only

the basic features . In hadronic collisions the t-quarkcan be produced either via strong interaction or via

explore,

TOPMASS (GeV)

100 â 'mt â 250 GeV/c2 ,

F. Cavanna/ Top search at the LHC

100 200 300 400 500

FIGURE 2.1Top quark production cross-sections : strong and elec-troweak processes at f = 16 TeV .

electroweak interactions . In the mass range we want to

the QCD contribution dominates with a tt pair pro-duced by gg and qq fusion. Next-to-leading order (NLO)contributions have been theoretically computeds andalso partially parametrized in the MC generators . Theweak interactions contribute significantly only for a veryheavy t-quark and become dominant above a mass ofti 300 GeV/cz, fig .2 .1 . Among the weak mechanismsthe dominant one is the W-gluon fusion, where a singlet-quark is produced in a tb pair, In fig .2 .1 the theoret-ical tt and tb production crass-sections? are given as afunction of t?~e t=quark mass. For the MC study we pro-duced large samples of tt events, from Pp collisions at

= 16 TeV, corresponding to different values of thet-quark mass (100,125,150, 200, 300 GeV/c2 ) . For ttproduction we employed EUIt,OJ?~T, and then ISAJETas a test of consistency. The EUROJET MC explicitlyincorporates matrix elements approximating the next-to-leading order contributions . For high-PT phenomena,this approach may be more reliable than the higher-o~rder contributions approximation via parton evolution

FIGURE 2.2Comparison between tt Monte Carlo cross-sections andtheoretical reference calculations in terms of mt .

schemes .In order to reproduce the expected theoretical ti

cross-section, the l~TLO contribution can be tuned inthe MC via a cut-off on the extrajet pT . Chasinga cut-off of pT = 40 GeV/c, the agreement betweenthe EUROJET simulation and theory is good (within^_" 20%), as shown in fig.2 .2 . the uncertainties in theevaluation of these cross-sections due to the choice ofthe Structure Functions (SF) set and to the Q` scaleintroduce a 20 - 30% variation of cam. We consider asa SF default choice the EHLQ1 set a ` and a Qs scale of(PT + mé)~

In the 1!~Iin:mal Standard Model (MSM) frameworkthere is only" one t-quark decay mode :

t -~

W b

375

(The W in the final state is on-shell if m= > mw -~ mb) .

If the scheme is extended to include charged Higgs,Ht, there is a possibility for a competitive t decaymechanism 12,13 of t --> H~ b . We shall consider hereonly the MSM option where BR(t -~ Wb) = 1 .

The total tt cross-section for a t-quark in the 150 -200 GeV/c2 range of mass is of the order of ti 1 nb .With an integrated luminosity of ti 104 pb-1 , i.e. one

3?b

cm-as-1 , a ver;~ large annual production of ~ 107 events

is thus expected .The t~ pair can give rise to several final-state signa-

tures, with each of the intermediate decay components

(W i~T b b) having a hadronic or a (semi-)leptonic de-

cay, producing in the latter case a (1, vt) pair with a

branching ratio of .. 1/9. Different search strategies

can thus be developed, according to the number of final

state leptons.

Although the statistics is largest in the purely hadronic

tt decay mode,

ti ---~ W+W-bb ~ > 6 jets

this channel is nonetheless overwhe'Lned by t1CD multi-jets backgroundsl~. The solution is to look for leptonicsignatures, where the leptons cän come either from Wor b decays. The W-decay leptons are harder and wellisolated ; thus they are the main topological signature.

According to whether one or both W's decay to a (d, v)

pair, we have 'single-' or 'di-'lepton decay channels . Forthe hard and isolated leptons from W decays, both elec-trons and muons can be considered . The b-decay leptonsare usually softer and embedded in the b-jet; thus only

muons are considered and we discuss this in the fo1_-lowing as b-tagging. Multilepton (> 3) final states willcle~Tly have lower backgrounds, but the loss ofstatistics

is large, even at the highest LHC luminoSYiy.

3. ISOLATED SINGLE-LEPTON t-DECAY MODFThe experimental chararcteristic of this channel is

the detection of an isolated, high pa' lepton coming froma W decay. The final-state signature is

W+W- bb

---~I v=

~- jets.

For mt = 150 GeV/c2, the tt cross-section is ~ 0.5 nb,mid the branching ratio in this channel BRA S is :

ti2x 1/9x2/3=0.14$

where the factor of 2 comes from the two W's in thea~>iaï state ~d we consider here electron ~o~r rouon finale a

vt.fâi.P~.

F. Cavanna/Topsea~h at the LHC

are:

/~y\\\\\\\\\\\\\\\~~''ây_v "

bbUA1(P P)

a (t 1). ß(b b ) . Q(WW). Q (VV + 3 Jets)o'

0`

0

0

,r (TedFIGURE 3.1

ticross-section (mt = 200 GeV/c2) and the major back-ground cross-sections as a function of the c.m . energy.

Themain backgrounds in this t-quark search channel

®

b~ (cc) ~ I v -;- jets

®

W + jets --~ d v +jets

W+W- -> I v -i- jets

1mb

1El b

1nb

1pb

In fig.3 .1 we show the ~ dependence of the signaland background cross-sections over the Tevatron-UNK-LHC energy range. The bb background has a very large

production cross-section (~~ ti 200tcb for ~ = 16TeV), with an uncertainty of almost an order of mag-nitude. The ti signal to bb background ratio (S/B) atproduction level is very unfavourable, SlB .~: 10-g . Fig-ure 3.1 also shows that the W -I- jets background ismuch smaller, but still exceeds the signal by more thanan ::rder of n~agnit~.ade, :vhits~ the WWbackground issmall and plays no significant role as long as mt ~ 250GeV/c2. The bb background signature is kinematically

10' I~

bbNA10(~P)

LDW~E ~a 20 GeV

bbWA78 EuRwET

10° (àP)/ r ~~~ o°~yQ=pi+m~aw * 3 ;w EHLOt

~'m~= 200GeV

10 ßtt"-isaEr

10 -~~-aww

LHC

~ ( ~ ~1100 5 10 15 20

O+FW

ZvÊ

FIGURE 3.2tt signal and bb background lepton kinematics : (a) Thetransverse-momentum distribution, (b) the pseudara-pidity distribution .

very differen~ in respect with the tf sign~.l because of the

large mass difference. Figure 3.2 .a compares the shapes

of the fin~1 state lepton PT spectrum coming from b-

decay with the one from a t-quark decay, wh;_le fig.3 .2 .b

compares the corresponding lepton g~~eudorapidity (n)distributions. Obviously a selection on hard and cen-

trally produced leptons allows a strong background re-

duction. The lepton isolation criterions is indeed the

most efficient way to separate the bb background . The

measurement of the energy flow (EET), around the lep-

ton (in a cone of radius OR(~J, ~)) above a given PT

threshold, leads to very different distributions for sig-

nal and bb background in the EET variable, as shown

in fig.3 .3 . Selecting high pT leptons (PTr z50 GeV/c)

centrally produced (~~t~ ti 1 .5) with EET < 10 GeVin a

cone of OR E 0.4, the ~/B66 ratio gets larger than one

(SlB ti 5 for mt = 130 GeV/c2 and is close to one for

F. Cavanna/Topsearch at the LHC

G 40 80 120 160 200 240ISOLATION: E E t (Ge1~

eR .Oa

FIGURE 3.3The lepton isolation requirement . EET distributionsfor: (a) tt signal, (b) b~ background .

mt = 200 GeV/c2). The most dangerous background

to the 'single-isolated-lepton' channel is, however, rep-

resented by W(--, dv) production accompanied by jets,

since in this ease no lepton isolation or ET cut can

separate signal from background . A detailed study of

this irreducible background has been performed, using

a large : MC event sample from the L~+VD generatorli .

The present version of this MC program slows the gen-

eration of W, from Pp collision, with up to 3 jets from

higher-order QCD processes - recently a W+4jets MC

simulation~ b was made available, the comparison of this

background with the t-quark signal in the single-lepton

mode is still under way - and at ~= 16 TeVwe obtain

a cross-section of

Q(W + 3 jets) ^-~ 10 nb

377

with ETe' % 20 GeV. This cross-section is a factor 5-10

largèr than the tt production cross section for m¢ in the

range of 150 - 200 Ge4'/c2 (fig .3 .4) and ways must be

37~

FIGUR,.E 3.4}i signal for various me values vs W+n-jets and1NH; WZ background total cross-sections .

found to improve the S;B ratio . The main handle toseparate the tt signal îrom the W -~- jets backgroundis provided by jets . Figure 3.5 .a sl=ows the normalized

pTc' spectrum for signal and background . As far as thefinal-state lepton is conczrned, same background sup-pression can still be obtained, in particular for a heavyt-quark, using a lepton pT cut, thanks to the harder sig-nal W transverse momentum distribution, fig.3.6 . Forthe tiproduction (PT; -� O(mc ) -., 100 Gel'/c, whilst

(pT 1 ~. 25 GeV/c for the W -f- jets production at theLHC. An optimization study Was cârâacû oui tv find themost suitable set of kinematical cuts on p~c', rli°c ', PT

and rtl . A further cut was introâuced to zrlsure lep-ton isolation and unambiguous jet counting, namely weasked for a mànimal angular separation of DR = U.4 be-tween any two jets, as weü as between the lepton and allthe jets . The best S/B ratio (S/B ^_- 2) correspondsto me ^_~ 150 GeV/cz where the optimization studywas performed, fig .3 .7 . In conclusion, fig .3 .8 shows thet~ signal production cross-section and the successive ef-fects of the branching ratio (BR,~) and of the lepton andjets acceptance cuts (A°pc ), as a function of the t-quarkmass . The expected level of W -;- 3jets background af-ter the same cuts is also shown in fig.3 .8 . The annual

F. Cava~tna i Tvpsearch attheLNC

Y~Z

r

FIGUIi.F 3.5ti signal and W -I_ jets background jet kinematics : (a)the inclusive 3-jets pr distribution, (b) the pseudorapid-ity ~4istribution .

~o

ioo zoo soo aooEmd

~~~°~t(L'aeV)T- t t -+WW b b

leuaa~t

FIGURE 3.6tt signal and W -~ jets background kinematics : the lep-ton PT distributions .

.2

0.8

Eß (GeV)

FIGURE 3.?4ptïmïzatïon ofthe ttsignal ta W-i-jets backgraund ra-tio : (a} variation with the inclusive 3jets gT threshold,(b} variation with the lepton pT threshold.

rate of observable tt events carrespandîng le,to the 'low' luminosity option, L~�c = 10 3 pb'1 wouldthus be :

~(rrxt = 150 t~eVjczL;nc x aci x RR,~ x A~` :~ 5 x 104 eut

Thïs rate shauid be further reduced by the lepton andjet detection efficiencies e of the apparatus (expected to

be of the order of ~ $0%) . Clther constraints can bettser_l to further gt?C!Sü'P tha background, as the MJ~ cor-relation or the azimuthal a~(f, *l,.} correlations, e~,,ch of

these havïng additir~nal rejection factors of 2 tr, 3. This

type of study would most probably allow the first obser-vation of the t-quark at the LH{J start-up with low lumi-

nosity. At ahigher luminosity of ~ lrss cm-zs-1, where

loo events per year should be produced, if we want

to exploit this 'sïngle-isolated-lepton' made far t-quarxxuass and decay branching ratio determinations, furthersubstantial background suppression can be achieved, at

the expense of efficiency, taggïng the b-quarks by ask

additional lepton (moon) in the final state. We

F. Cavanna1?~~srcrr,~r ~: : 4LHC

ACC. :3.(e~: E,~:40 GeY, (~~ <t5t le~:P~ :3QGaV i~ie~ <t5

SEPAP4T~t:eR>0.4

FIGURE 3.8tt crass-section in the single-lepton channel aad eapected W+ jets backgraund ., aftee branching racla re-ductïan and acceptance cuts .

discuss this in section 5.

4. ISOLATED DILEPTON t-DEGAV NI~?DEThe e?cperimental signature in this channel âs the

presence oftwo isolated and hïgh transverse momentum

leptons (e, t~) comïng from 1%V' decays:

ti -; W+W- bb --~ lv fv + } 2 jets

The presence of b-jets in the event fnal state can also

help in the separation of the signal from other sources

of dileptans from SM processes.

The physical backgrounds considered here are:

o bb --> l~ fu -F jets

~ I}.~ -#- jets -~ 1Î -!- jets

M m~tGeV)

l l + jets

for example, a t-quark with me = 20~;

Ge~1'/c~, a cross-section of .~ 4 x 10-s nb is expected în

ê.~T

t.lnis isolated dilepton mode, before any acceptance cut;

the ( e-p ) branching ratio BRditept is :

e~= 2 x BR{Wi --> hv) x BR(Wa --~ lav)

= 2 x 1/9 x 1/9 = 0.025

and a factor of 2 larger if we consider also e-e and p-ppairs .

The t-quark signal and the bb, WW, Z° and DYbackground processes have been studied using EURO-JET and ISAJET generators . Thz various backgroundcross-sections as a îunction of the c.m . energy are com-

pared with the t-quark signal in fig.3 .1 . ElectroweakWW pair production is lower than the t-quark signalby a factor of 10 (for mt = 200 GeV/c2); therefore itdoes not represent amajor problem and is relevant onlyafter the bb background has been sufficiently reduced.The last two processes, Z° -~ jets and DY +jets, haveto be considered for e+e- or tt+te- final states, but canobviously be avoided by asking for a mixed e-p leptonpair.

As akeady discussed in section 3, a very efficient wayto suppress the bb(g) background is to exploit the muchharder p~ spectrum of the decay lepton . The suppres-sion factor is much larger now , since the p~ thresholdcut can be applied to both leptons. A good signal tobackground separation can thus be obtained by requir-ing just kinematical cuts (PT, ~t ) . However the leptonisolation criterion, again applied here to both leptons,represents the most ef$cient way to get rid of the bbbackground . A strong background suppression is thusobtained:

if a PTr Z 50 GeV/c is required on both leptons. Thislarge background suppression allows a lower PTr to beimplemented (depending on mt), thereby gaining sub-stantially on signal statistics .

Combining thus kinematical cuts and isolation cuts,we can obtain a very clean sample of tt -; etc events,free of bb background . The observable tt signal, withtwo hard (for example PT'12 > 50 GeV/c) and isolated

F. Cayanna!Top sears& a!!he LHC

10~

10~~

ç t~2

103

10~

to~

o too 200 aoo aooM ~F (GeV)

to e

10~

10 5

to o

to i

to t

°o

0

z

FIGURE 4.1ttcross-section in the dilepton channel and the expectedbb background levels, after branching ratio reductionand acceptance cuts .

(EET < 10 GeV) leptons, when compared with the bbbackground, is at the level of

S/B

z

100 for 100 ~ mt ~ 250 Geâf/c2

The reduction of the signal, due to the dilepton branch-ing ratio (BRd;i~pt ), to the ikinematical acceptance (Akin)

to the isolation (Ai'°c) and angular (A°~) acceptances,is shown in fig.4.1 as a function of t-quark mass . Thebb background level, after the same set of cuts, is alsoshown in fig.4 .1 . The band reflects the uncertaintieson the bb cross-section and on the effects of the cuts,as obtained comparing EUR.O._TFT and IS_A_,TFT . Theannual rate of observable isolated dilepton tt events(with PT > 50 GeV/c), for an intermediate luminosityof Lint = 104 pb-1 , is :

N°-es = L~

x ~ - x BR

x Akin x A°~ x A = '°~tt snt tt di!ept

^_- 104 evts

(We can assume a large lepton recognition efficiency,et ^_" 0.9 for each lepton, for PT > 50 GeV/c.)

At the highest luminosity, L^_~ 10 34 cm'2s-1 , hardercuts on the pT of the leptons can be used to compensate

the loss of rejection power of the lepton isolation dueto the pile-up. Requiring a third lepton tagging the

b's (section 5), a further large rejection factor can beobtained. In any case this two-(or three)- lepton channelcan be exploited for t-quark physics up to the highestluminosity envisaged.

Once the bb background has been reduced to a neg-

ligible level, the residual background comes from theelectroweak W+W- pair production. As it is shown in

fig.3 .1 this background is not dangerous, since the ttproduction cross-section is larger by a factor of z 10for mt S 200 GeV/c2. Applying the same kinematicalcuts as for the bb reduction, i.e. a pT > 50 GeV/c on

both leptons in the central rapidity region ~~~ < 1.5, anadditional rejection factor of RF ti 2 is obtained. Thesignal-to-ûackground ratio is

S/B Z 20

for mt = 200 GeV/c2. This background could presum-

ably be further reduced by requiring a minimal (hardand central) jet multiplicity.

5. b-JET TAGGING WITH MUONS

The ti event yields in the single- and t.vo-isolated-

lepton channels are still large even after the appropri-

ate kinematical and acceptance cuts have been applied

to reduce the background (106 to 104 top events per

year at 1033 cm-2s-1 ). This makes it possible to further

improve the signal-to-background ratio by requiring a

(non-isolated), relatively low pT muon in the final state

coming from the b decay. In fact, b-tagging is neces-

sary if we want to improve substantially the signal to

background ratio in the 'single-isolated-lepton' channel.

Requiring an additional lepton is also an efficient way to

reduce the background in the 'isolated dilepton' chan-

nel, in particular with e+e- or ~+/e- final states ; it also

allows looser cuts on isolation and azimuthal correla-

tion . Asking for an additional muon in the event is also

an efficient way to reduce the trigger rate one would

F: Cavanna/Topsearch at theLHC

mc~

ân

m

pT (GeV)

FIGUF~E 5.1Inclusive pT spectrum for -any lepton (muon) from ttproduction and decay the muon pT spectrum from bdecay (t ~ Wb) is shown explicitly.

have at high luminosity, especially in the s~Wglz lepton

mode .

Requiring a further lepton (muon) the statistics are

resealed firstly by a leptonic branching ratio (RR

1/9), for both the t-quark signal and the bb background;

a further reduction comes from the muon pT detection

threshold and from the geometrical acceptance (rl``) .

The inclusive pT distribution for any muon (either

from Wor b decay) in a tt event is shown in fig.5 .1 . This

spectrum has two contributions, the hard one from W

decay and the soft one, strongly depending on mt, from

the b decay. In the isolated-single-lepton t-quark detec-

tion channel, the second lepton, selected without any

isolation constraint, is then a mixture of hard pT muons

from the second W decay (this is the 'isolated dilep-

ton' mode, discussed in section 4), and rather softer pT

muons from a b (or b) decay. The signal to bb back-

ground ratio, in the single-isolated-lepton channel, was

enhanced above one by requiring rather hard kinemat-

ical cuts (pTr > 50 GeV/c), quite expensive in terms

of signal statistics. Demanding kinematical cuts on the

second (non-isolated) lepton, p~ >_ 20 GeV/c, ~r?is ~

2.5, this background can be largely reduced even lower-

ing the pT threshold as the first isolated lepton {P~r >

30GeV/c} .

Further ïmpravements in the SlB ratio can always

be obtained using other tools, like the M,ta cut and

the d~r(t3, J~ ) correlation. After all these cuts the

background ïs definitely reduced to a negligible level

However, the main reason to ask for the presence

of a second softer and non-isolated lepton in this final

state is to reduce the more dangerous background from

W-~-jet, (S/13w+~tc, =' 2) to a more suitable level. Onlyprocesses like qq --> gW -; bbW can generate a prompt

second lepton in the W -F jets final state. The fractionof Wbb events in the W+ > 3jets is expected to besmall}s, less than 5%a. Thus the W+ jets backgroundto the 'tl: --} sïngle-isolated-lepton -1- non-isolated-moan'signal should be at aS/B > 201eve1, i.e . reduced below

the big) background.

The WW -~ dv -~ Iv -~- X background to this double

lepton channel is not particularly dangerous since, asakeady mentioned, the WW production crass-sectio~ris smaller {factor of ~ 10) than the tt cross-section

in the t-mass range we are cansïdering . In the ti --}WWbb --~ lvd'vbb channel with a b-tagging moon, thesignal crass-section is reduced by a BR = 2l9 and by afurther factor (depending on rot) carrespondïng to thethird moon-detection threshold (fig.5 .2) . The observ-

able cross-sectïon, after kinematicat cuts on the thirdlepton and the cuts discussed ïn section 4 on the isolateddileptons, is reduced by a factor ofti 3Q . The Pvent rate,

for m~ = 200 GeV/c2, ïs ti 0.5 x 103 events per' yearsity of 2033 cm's-~. In this '3-lepton'

channel there ïs no signïficant contamination from thebackground sources previously considered tbb(g), W -f-

W, Z-Fjets, Z?Y~-~-jets . The known mechanismsyielding 3 leptons in the final state, like W Z -} lv Z l,fig.3.5, or W 2 -} Iv TT -} lv l t and W W W --~dv lv dv, have very small cross-sections ( ~ 10"5 nb after

F. Cauanna/Topsearch at the LHC

f00 200 300 400

p~ (GeV}

FIGURE 5.2pT distributïon ofthe (non-isolated) moon from b-quark(t --> Wbti, after cuts an the isolated lepton from Wdecal

leptonic branching ratiois ). Once the acceptance cutsare applïed, the observable cross-sections for these back-

grounds become totally negligible in comparison withthe tt signal. This channel is particularly important athigh luminosity, when the isolation criteria become lessefficient and reliable.

6. TOP-MASS 1V~FASUR.EMENTThe selection criteria far top events, as previously

discussed, allav~ to identify samples relatively large instatistics and clean from background contaminations . Aprecise lop mass determination is the mast importantresult expected from the analysis of these samples.

A first estïmatïon of the top mass in principle can beed exploiting the strong dependence of the fusion

crass section on the quark mass, fig.2 .1 . The uncer-tainties ïntroduced through the NLO corrections, theparton densities and the fragmentation functions in the

top cross sectïon make this method not suitable far a~1

accurate determination of m~~ and a precision better

"n0

ûC7

taoo

:200

tboo

800

800

400

200

0

500

400

300

200

100

50 t00 t50 200 250 300 350 400

Mosse(j j) mt-200

50 100 t50 200 250 300 350 a00

MosSe(j j j) mt=200

FIGURE 6.1ti events (mt = 200GeV/c2 ) : (a) Invariant mass for anyjet pair in the emisphere opposite to the lepton, (b)three jets invariant mass.

than about 10% can not be achieved for a top massin the 130 - 200 GeV/c2 range . A model independentmeasurement of mt can be carried out by studying the3-jets invariant mass distribution of the system recoil-ing against the lepton in the isolated single lepton eventsamplel3 . This method has the advantage that the in-variant mass of the 3-jets system gives a direct indi-cation of the parent top mass . On the other hand, themain difficulties are related to the efficiency of thejet al-gorithm and to the calorimeter performance in terms ofjet resolution (i.e. to its e/a ratio) . The detector simu-lation for this study was rather simple and standard e.m .and hadror_~c rPseh~tions were assumed. tingle isolatedlepton events, with >_ 3 jets in the emisphere oppositeto the lepton, were selected according to the criteriadiscussed in section 3. The invariant mass of each jetpair (PTi,~a > 40 GeV/c) in the emisphere opposite tothe lepton, for ti events (mt~ = 200 GeV/c2 ) and forW + jets background events are shown in fig .6 .l .a and6.2.a. A clear peak around myv, corresponding to theinvariant mass of the two jets from W decay, is seen

F. Cavanna /Top search at theLf>fC

2Cevk= t0~pb~

3~i

mD ~m~ (GeVAcs)

FIGURE 6.2W +jets background events : (a) Invariant mass for anyjet pair in the emisphere opposite to the lepton, (b)three jets invariant mass .

in the signal event sample. The 3-jets invariant mass,two jets of the three being selected with the constraint~m~i,~z - miy~ < 20GeV/c2 (fig .6 .l .a, 6.2.a) and thethird one with pT > 50GeV/c in the hemisphere opgo-site to the lepton, is shown in fig.6.l .b and fig .6 .2 .b forsignal and background respectively. The large statis-tics of the single isolated lepton sample allow the re-quirement for an extra high p~ jet (p~ > 50 GeV/c),in the emisphere opposite to the lepton, from the bquark, tagged by a microvertex detector (in this casethe b-tagging can not be done by muons because in thesemi-Ieptonic decay of the b-quark the presense of the

.,

* . .~r~ .. . Le +1.'

.1' e..+ .~.. . aiI°~S.i3:rc°.aMaaeût iûa-âeintâaiiv r iv

u:ar c, n.raâs r.ir~...~ a.~i

possible) . The top mass is obtained adding the thirdjet tagged as b to the jet pair fulfiling the previous con-straint on mJ1,J2 . The large combinatorial backgroundas well as the physical W ~- jets background can bereduced tagging the b-jets, fig .6 .3 .a. A much clearerpeak around the parent tap mass (200GeV/c2 in thepresent simulation) is shown in fig.6 .3.b . A comprehen-sive studyl3 of the systematic and statistic uncertain-

F. Cavanna /Topseareh at thet:HC

FIGURE 6.3tt events (m= = 200GeV/c2 ) with b-jet tagging: (a)Invariant mass for any jet pair in the emisphere oppositeto the lepton, (b) three jets invariant mass .

ties shows that the global error on the mtop determi-nation can be reduced to about 4% (f8 GeV/e2 for atop of 200GeV/c2 ), the main contribution being comingfrom the systematic error. Other methods for indirecttop mass measurements can be exploitedl3 , leading, inprinciple, to more precise determination of mt~. Forexample from the dilepton mass distribution in the tri-lepton event sample, where an isolated lepton is pairedwith an oppositely charged lepton (muon) from a b-jet,in the emisphere opposite to an other isolated lepton .In this case the b-quark can be tagged by the presenceof a muon above a minimal pT threshold as indicated insection 5 . Since only leptons are used, the experimentalsystematics are expected to be smaller . On the otherhand the presence of missing energy (a v from W and av from b) in the emisphere where the top mass is (indi-rectly) measured, requires an adequate cut on the toptransverse momentum . Only assuming a reliable MCcontrol of pT and taking into account the uncertaintydue to the b-quark fragmentation into muons, one mayexpect the best mt~ measuerement with a total error of

ti 5 GeV/cz for a top of 200GeV/c = of mass. The high-est LHC luminosity (L = 104pb-1) is required in thiscase to reduce the statistical error below the systematicincertitude .

7 . CONCLUSIONSThe search and study of the t-quark is one of the

major issues at the LHC. The production cross-sectionfor mt ~ 150 GeV/c2 is a factor of ti 300 larger thanat the Tevatron . At the nominal LHC luminosity thet-quark production rates will be very large, of the orderof 106 to 107 events per year for mt in the expected SMmass range, 100 ~mt S 200 GeV/c2 . The basic handleto extract the t-quark signal is the observation of hardand central isolated leptons (pT ~ 30 to 50 GeV/c in

~g l ~ â 1.5 ) from the t -"Wb -~ lvb decay.The production rate is largest in the single-isolated

lepton channel, tt -; lv -~-jets . This channel may allowfirst observations of the t-quark at machine start-up andmakes possible a direct determination of the t-quarkmass, without missing particles . The bb(g) backgroundcan be reduced to a SlB ti O(10) level by a combinationof pT (~ 30 to 50 GeV/c) cuts and isolation cuts . TheW +jets QCD background is however reducible only toa SlB ti O(1) level by kinematical cuts alone . Taggingthe b-quarks with an additional muon (in jet) improvesvery much this SlB ratio and allows this channel to beexploited at high luminosity with an acceptable rate.

The decay mode with two isolated leptons, tt -;WWbb -- ; lvl'vbb, is much cleaner . The bb(g) back-ground can be reduced to a S/B ~ O(100) level by acombination of pT (~ 30 to 50 GeV/c) and isolationcuts on both leptons . A pT z 100 GeV/c cut alone suf-fices to have S/B ~ 1, thus allowing t-quark physics atthe highest LHC luminosity envisaged .

Additional muons (from b-tagging) would providevery clean samples . Electroweak WW production issmaller than tt production, provided mt ~ 250 GeV/c2 ,and thus should not represent a major difficulty. Aprecise top mass measurement, with a global error ofthe order of 2-3% for a top in the mass range of 150-

200GeVfc~ can be performed exploiting this event sam-ple at the highest luminosity envisaged at the LHC.

ACKNOWLEDGEMENTThis work was done in the framework of the ECFA-

LHt: Workshop (Aachen, Nov.1990), and contributionsfrom many people of the "Top Group" are contained . Iwould like to express my thanks to all of them and inparticular to D . Denegri far hïs faundamental help ar}dcontinous encouragement .

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