The most promising source of SUGRA signal at the Tevatron collider is the
pair-production of electroweak gauginos, followed by their leptonic decay. In
the parameter range corresponding to dominant leptonic decay of these gauginos
one or more of the leptons are expected to be $\tau$ with $P_\tau \simeq +1$.
This polarization can be effectively used to distinguish the signal from the
background in the 1-prong hadronic decay channel of $\tau$ by looking at the
fractional $\tau$-jet momentum carried by the charged prong.Comment: 11 pages, LateX, 2 figures, discussion on polarization effect added,
  final version to be published in PL

Baer

Barger

Bullock

D.P. Roy

Dicus

Dutta

Groom

Kuhn

Lykken

Matchev

Monoranjan Guchait

Nojiri

Raychaudhuri

Stelzer

Physics Letters B

English

arXiv

The most promising source of SUGRA signal at the Tevatron collider is the pair-production of electroweak gauginos, followed by their leptonic decay. In the parameter range corresponding to dominant leptonic decay of these gauginos one or more of the leptons are expected to be $\tau$ with $P_\tau \simeq +1$. This polarization can be effectively used to distinguish the signal from the background in the 1-prong hadronic decay channel of $\tau$ by looking at the fractional $\tau$-jet momentum carried by the charged prong.The most promising source of SUGRA signal at the Tevatron collider is the pair-production of electroweak gauginos, followed by their leptonic decay. In the parameter range corresponding to dominant leptonic decay of these gauginos one or more of the leptons are expected to be $\tau$ with $P_\tau \simeq +1$. This polarization can be effectively used to distinguish the signal from the background in the 1-prong hadronic decay channel of $\tau$ by looking at the fractional $\tau$-jet momentum carried by the charged prong.The most promising source of a SUGRA signal at the Tevatron collider is the pair-production of electroweak gauginos, followed by their leptonic decay. In the parameter range corresponding to dominant leptonic decay of these gauginos one or more of the leptons are expected to be τ with P τ ≃+1. This polarization can be effectively used to distinguish the signal from the background in the 1-prong hadronic decay channel of τ by looking at the fractional τ -jet momentum carried by the charged prong

Guchait, Monoranjan

Roy, D.P.

CERN Document Server

TIFR/TH/01-36CERN/TH/01-246Using Tau Polarization to Sharpen up theSUGRA Signal at TevatronMonoranjan Guchaita and D.P. Roya,ba Theoretical Physics Division, CERNCH-1211, Geneva 23, Switzerlandb Tata Institute of Fundamental ResearchHomi Bhabha Road, Mumbai 400 005, INDIAAbstractThe most promising source of SUGRA signal at the Tevatron collider isthe pair-production of electroweak gauginos, followed by their leptonic decay.In the parameter range corresponding to dominant leptonic decay of thesegauginos one or more of the leptons are expected to be  with Pτ ’ +1.This polarization can be eectively used to distinguish the signal from thebackground in the 1-prong hadronic decay channel of  by looking at thefractional  -jet momentum carried by the charged prong.1The LEP limit of chargino mass > 90 GeV corresponds to a gluino mass> 300 GeV in the minimal SUGRA model [1], which puts them beyond thediscovery reach of the Tevatron collider. Thus the most promising source ofSUGRA signal at Tevatron seems to be the pair production of electroweakgauginos, ~W+1 ~W−1 and ~W1~Z2. The leptonic decays of ~W1 and ~Z2 into theLSP ( ~Z1) result in clean dilepton and trilepton nal states with a signicantmissing-ET (E=T ) and very little hadronic jet activity. Recently there hasbeen a good deal of interest in these processes as the main signatures ofthe SUGRA model at Tevatron [2]-[5]. The parameter space of particularinterest to this signature is one where the lighter (right-handed) sleptonslye below the ~W1 and ~Z2 masses, resulting in very large leptonic branchingfractions of these gauginos. This corresponds to two regions of the SUGRAparameter space { i.e. I) m0 signicantly less than m1/2 (m0  12m1/2),implying m˜`R,τ˜R<mW˜1,Z˜2 at any value of tan , and II) m0  m1/2 at largetan, implying mτ˜R<mW˜1,Z˜2, where ‘ denotes electron and muon. In the1st case one expects a ‘+‘− signature from ~Z2 ~W1 decay, since ~W1 ! τ ~Z1via the larger L-R mixing in the ~ sector due to the larger  mass. In the2nd case one expects  and  signatures from ~W1 ~W1 and ~Z2 ~W1 decaysrespectively. The presence of one or more  leptons in the nal state meansthat the  channel is expected to play a very important role in superparticlesearch at Tevatron, particularly in the minimal SUGRA model [2]-[5].The minimal SUGRA model predicts the polarization of  resulting fromthe above ~ decay to be = +1 to a good approximation, as we shall see below.The purpose of this note is to use this  polarization (Pτ = +1) to sharpenthe distinction between the SUSY signal and the SM background. It hasbeen shown in the context of charged Higgs boson search in the H ! channel that in the 1-prong hadronic  -jet the Pτ = +1 signal from Hdecay can be eectively distinguished from the Pτ = −1 background fromW via the sharing of the jet energy between the charged pion and theaccompanying neutrals [6]-[7]. This has been conrmed now by detailedsimulation studies for both Tevatron and LHC. We shall use a similar strategyhere to distinguish the SUSY signal from the SM background in the 1-pronghadronic  -jet channels. In particular we shall see that the Pτ = +1 signalcan be eectively separated from the Pτ = −1 background as well as the fake background from QCD jets by requiring the charged track to carry > 80%of the jet energy-momentum.2We shall concentrate on the 1-prong hadronic decay channel of  , which isbest suited for  identication. It accounts for 80% of hadronic  decay and50% of its total decay width. The main contributors to the 1-prong hadronicdecay are ! (12:5%); (26%); a1 (7:5%); (1)where the branching fractions for  and  include the small K and K? contri-butions respectively [1], which have identical polarization eects. Togetherthey account for 90% of the 1-prong hadronic decay. The CM angular dis-tribution of  decay into  or a vector meson v (= ; a1) is simply given interms of its polarization as1ΓpidΓpid cos =12(1 + Pτ cos ); (2)1ΓvdΓvL,Td cos =12m2τ ; m2vm2τ + 2m2v(1 Pτ cos ); (3)where L; T denote the longitudinal and transverse polarization states of thevector meson. The fraction x of the  lab. momentum carried by its decaymeson (i.e. the visible momentum of the  -jet) is related to the angle  viax =12(1 + cos ) +m2pi,v2m2τ(1− cos ); (4)where we have neglected the  mass relative to its lab. momentum (collinearapproximation). It is clear from eqs. (2) - (4) that the hard part of the  -jet,which is responsible for  identication, is dominated by ; L; a1L for thePτ = +1 signal, while it is dominated by T ; a1T for the Pτ = −1 background.The two can be distinguished by exploiting the fact that the transverse  anda1 decays favour even sharing of momentum among the decay pions, whilethe longitudinal  and a1 decays favour uneven sharing, where the chargedpion carries either very little or most of the momentum. It is easy to derivethis quantitatively for  decay. But one has to assume a dynamical modelfor a1 decay to get a quantitative result. We shall assume the model of ref.[8], based on conserved axial vector current approximation, which providesa good description to the a1 ! 3 data. A detailed account of the  anda1 decay formalisms including nite width eects can be found in [6],[9]. Asimple FORTRAN code for 1-prong hadronic decay of Polarized  based onthese formalisms can be obtained from one of the authors (D.P. Roy).3As specic examples of the two regions of interest in the SUGRA parame-ter space mentioned above, we have chosen two points representing the casesI and II, and evaluated the corresponding SUSY spectra using the ISAS-UGRA code { version 7.48 [10]. The resulting ~W1; ~Z2; ~Z1 and the sleptonmasses are shown in the two rows of Table 1 along with the ~ mixing angle,where~1 = ~R sin τ + ~L cos τ : (5)It may be noted here that the Polarization of the  resulting from the ~1 ! ~Z1 decay isPτ ’ 4 sin2 τ − cos2 τ4 sin2 τ + cos2 τ; (6)since ~Z1 ’ ~B in the minimal SUGRA model and R has twice as large ahypercharge as L [11]. For the mixing angles of Table 1, cos τ = 0:19(0:53),we get Pτ = 0:98(0:85). Hence the  polarization is ’ +1 to a good ap-proximation over a wide range of the relevant SUGRA parameters, notablytan.Table 1. The light sparticle masses of the minimal SUGRA model for(I) m1/2 = 175, m0 = 70, tan = 5 (top row) and (II) m1/2 = 160,m0 = 150, tan  = 40 (bottom row), where all the masses are in GeVunit. For both cases A0 = 0 and sign() = +ve.~W1 ~Z2 ~Z1 ~` ~‘L ~‘R ~τ ~2 ~1 cos τ116 120 66 130 151 106 130 153 104 0.19110 112 63 178 195 168 164 207 97 0.53We have estimated the signal and background cross-sections for the abovetwo cases using a parton level Monte Carlo programme. To simulate detectorresolution we have applied Gaussian smearing on the jet and lepton pT with((pT )=pT )2 = (0:6=ppT )2 + (0:04)2 and (0:15=ppT )2 + (0:01)2 respectively.The E=T is evaluated from the vector sum of the lepton and jet pT afterresolution smearing. The main results for the two cases are presented below.4I. m0  12m1/2 (‘+‘− signal):The sparticle spectrum of the top row of Table 1 imply that the dominantdecay modes of ~Z2 and ~W1 are~Z2 ! ‘~‘R ! ‘+‘− ~Z1; (7)~W1 ! τ ~1 ! τ ~Z1; (8)with branching fractions ’ 2=3 and 1 respectively. Thus one expects a dis-tinctive ‘+‘− signal accompanied by a signicant E=T from ~W1 ~Z2 production.Moreover this signal is expected to hold over a wide range of tan , since theproduction cross-section as well as the above decay branching fractions areinsensitive to this parameter. Note that the right-handed slepton masses ofTable 1 are fairly close to the ~W1; ~Z2 masses due to the LEP limit on m˜`,τ˜[1]. Hence the lepton from the ~Z2 ! ‘~‘R decay is expected to be relativelysoft. We have therefore imposed a modest but realistic pT cut on the softerlepton. The cuts arep`1T > 15 GeV; p`2T > 10 GeV; pτ−jetT > 15 GeV; E=T > 20 GeV;j`1,`2,τ−jetj < 2:5; `1`2 < 150; M`1`2 > 10 GeV and 6= MZ  20 GeV: (9)Table 2 summarises the signal and background cross-sections after these cuts,where we have included a  identication eciency of 50% along with a 0.5%probability of mistagging a normal hadron jet as  [12]. The latter is aconservative assumption, since the probability of a normal hadron jet fakinga 1-prong  -jet with pT  20 GeV has been estimated to be about 0.3% forthe CDF experiment in Run-1, going up to 0.8% for the (1+3)-prongs  -jet[13].Table 2. The signal and background cross-sections (in fb) in the ‘‘channel after the cuts of eq. (9). It includes a 50% eciency factor for identication along with a 0.5% probability of mistagging a normalhadron jet as  .Signal Background~W1 ~Z2 (Z?=γ?)W (Z?=γ?)j27 0.1 0.045Thanks to the E=T and the dilepton mass and opening angle cuts, the po-tentially large (Z?=γ?)j background is reduced to  0:1% of the signal. Wehave estimated this background using a simple analytic formula for the ma-trix element neglecting the vector coupling of Z to ‘‘. The matrix elementfor (Z?=γ?)W has been evaluated using MADGRAPH [14].Fig. 1 shows the Pτ = +1 signal as a function of the fractional  -jetmomentum (R) carried by the charged-prong. For comparison it also showsthe corresponding distribution assuming the signal to have Pτ = −1. Thiscould be the case e.g. in some alternative SUSY model with a higgsinoLSP. The complimentary shape of the two distributions, as discussed earlier,is clearly visible in this gure. The Pτ = +1 signal shows the peaks at thetwo ends from the L; a1L along with the pion contribution (added to the lastbin), while the Pτ = −1 distribution shows the central peak due to the T ; a1Talong with a reduced pion contribution [6],[9]. The expected luminosity of 2fb−1 per experiment in Run-2 corresponds to  54 signal events in the ‘+‘−channel for each experiment without any serious SM background. Thus onecan use this distribution in this case as a conrmatory test of the minimalSUGRA model.= -1 (dashed line)P = +1 (solid line)RNormalisedcrosssection10.90.80.70.60.50.40.30.20.1043.532.521.510.50Figure 1: The normalised ‘‘ signal cross-section in the 1-prong hadronic  -jet channel shown as a function of the  -jet momentum fraction (R) carriedby the charged prong for Pτ = +1(−1).6II. m0  m1/2 and large tan ( signal):The sparticle spectrum of the bottom row of Table 1 imply that in this casethe dominant decay modes of ~Z2 and ~W1 are~Z2 !  ~1 ! +− ~Z1; (10)~W1 ! τ ~1 ! τ ~Z1: (11)Thus one expects a  signal from ~W1 ~Z2, ~W1 ~W1 and ~~ production withPτ ’ 1 each. The 1st process contains a 3rd  from ~Z2 !  ~1, whosepolarization depends on the model parameters. The contribution from thedominant ( ~W ) component of ~Z2 coupling to the subdominant (~L) componentof ~1 has Pτ = −1, while that from the subdominant ( ~B) component of ~Z2coupling to the dominant (~R) component of ~1 has Pτ = +1. And it is theother way around for the higgsino component of ~Z2. But in any case the resulting from this decay is relatively soft for the reason mentioned aboveand rarely survives the  -identication cut of pτ−jetT > 15 GeV. Therefore weshall require the identication of two  jets with Pτ = +1, while there maybe occasionally a 3rd  jet with any polarization (inclusive  channel). Weshall neglect the contribution from this 3rd  to the signal cross-section forsimplicity, which means a marginal underestimation of the signal. The rawcross-sections for ~W1 ~Z2, ~W1 ~W1 and ~ ~ production processes are 770, 850 and40 fb respectively. We impose the following cuts:pτ−jetT > 15 GeV; E=T > 20 GeV; jτ−jetj < 2:5;30 < ττ < 150; E/T τ > 20; Mττ > 30 GeV and 6= MZ  30 GeV; (12)where we have reconstructed the invariant mass of the  -pair for the signaland background events after resolving the E=T into their respective directions.The resulting signal and background cross-sections are listed in Table 3.We see from the 1st row of Table 3 that the potentially large backgroundfrom (Z?=γ?)j can be eectively suppressed by the combination of E=T ;  and invariant mass cuts in eq. (12). However the Wj background, with thejet faking as a  , remains about 5 times larger than the  signal. In view ofthe importance of this background we have estimated it via the on-shell Wjas well as the 3-body production processes q0q(g) ! g(q) using the matrixelements from [15]. The two estimates agree to within 5%.7Table 3. The signal and background cross-sections (in fb) in the channel after the cuts of eq. (12), including a 50% eciency factor foreach  along with a 0.5% probability for mistagging a normal hadron jetas  . The last row shows the total signal and background cross-sectionsafter the R > 0:8 cut on the two  -jets.Signal Background~W1 ~Z2 ~W1 ~W1 ~~ Wj (Z?=γ?)j WW tt8 8.7 0.5 80 2 1.7 1.23.5 3.6Fig. 2 compares the Pτ = +1 signal and this Pτ = −1 background asfunctions of the  -jet momentum fraction R carried by the charged prong. Itclearly shows the complimentary shapes of the two distributions, similar tothose of Fig. 1. It means that the dierence comes mainly from the oppositepolarizations of  rather than kinematic dierence between the signal andthe background. Requiring the charged track to carry > 80% of the  -jetenergy-momentum (R > 0:8) retains 45% of the signal as against only 20%of the background. Moreover the R > 0:8 cut is also known to reduce thefake background from normal hadron jets by at least a factor of 5 [16]. Thusdemanding both the  -jets to contain hard charged tracks, carrying > 80%of their momenta, would reduce the signal by a factor of 5 while reducing thedominant background by at least a factor of 25. The  background fromWW and tt are also reduced by a factor of 25 each. On the other hand thebackground from Z?=γ? !  has Pτ = 0, and the corresponding distributionlies midway between those of Pτ = 1. The resulting suppression factor is’ (1=3)2.As we see from the bottom row of Table 3 the R > 0:8 cut on both the -jets reduces the total background to the signal level, i.e. about 3.5 fb each.With the expected Run-2 luminosity of 2 fb−1 per experiment, one expectsa combined yield of  14 signal events against a similar background fromCDF and DO=. Note that the corresponding signicance level is S=pB ’ 4with or without the R > 0:8 cut. Nonetheless it is no mean gain that thiscut can enhance the signal to background ratio from 1/5 to at least 1. Thismeans that the  channel can oer a viable SUGRA signature along withthe ‘+‘− channel at the Tevatron upgrades, starting with Run-2. It maybe noted from Table 3 that requiring the  pair to have opposite sign (same8= -1 (dashed line)P = +1 (solid line)RNormalisedcrosssection10.90.80.70.60.50.40.30.20.1043.532.521.510.50Figure 2: The normalised SUSY signal (Pτ = +1) and Wj background (Pτ =−1) cross-sections in the 1-prong hadronic  -jet channel shown as functionsof the  -jet momentum fraction (R) carried by the charged prong.sign) will retain a little over 3=4 (under 1/4) of the signal while retaining1/2 of the dominant background. Thus with sucient luminosity it may bepossible to improve the signal to background ratio by requiring the  pair tohave opposite sign. Finally it should be noted that while we have focussedthe current analysis on the SUGRA model the same polarization strategycan be used to distinguish the SUSY signal from the SM background in thegauge mediated SUSY breaking model [17], where one expects a Pτ = +1from the ~R !  ~G decay.We are grateful to Dhiman Chakraborty, Daniel Denegri and especiallyManuel Drees for many helpful discussions. DPR acknowledges partial nan-cial support from IFCPAR.References[1] Review of Particle Properties, Euro. Phys. J. C15, 1 (2000).[2] H. Baer, M. Drees, F. Paige, P. Quintana and X. Tata, Phys. Rev. D61,095007 (2001).9[3] H. Baer, C. Chen, M. Drees, F. Paige and X. Tata, Phys. Rev. Lett. 79,986 (1997); Phys. Rev. D58, 075008 (1998); Phys. Rev. D59, 055014(1999).[4] K.T. Matchev and D.M. Pierce, Phys. Rev. D60, 075004 (1999); J.D.Lykken and K.T. Matchev, Phys. Rev. D61, 015001 (2000).[5] V. Barger, C. Kao and T. Li, Phys. Lett. B433, 328 (1998); V. Bargerand C. Kao, Phys. Rev. D60, 115015 (1999).[6] S. Raychaudhuri and D.P. Roy, Phys. Rev. D52, 1556 (1995); D53,4902 (1996).[7] D.P. Roy, Phys. Lett. B459, 607 (1999).[8] J.H. Kuhn and A. Santamaria, Z. Phys. C48, 445 (1990).[9] B.K. Bullock, K. Hagiwara and A.D. Martin, Phys. Rev. Lett. 67, 3055(1991); Nucl. Phys. B395, 499 (1993).[10] H. Baer, F. Paige, S.D. Protopopescu and X. Tata, hep-ph/0001086.[11] M. Nojiri, Phys. Rev. D51, 6281 (1995).[12] CDF Collaboration: F. Abe et. al., Phys. Rev. Lett. 79, 3585 (1997);M. Hohlmann, preprint Fermilab-conf-98-376-E.[13] M. Hohlmann, Univ. of Chicago, Ph.D. Thesis (1997).[14] MADGRAPH, T. Stelzer and W.F. Long, Comp. Phys. Commun. 81,357 (1994); HELAS by H. Murayama, I. Watanabe and K. Hagiwara,KEK-91-11 (1992).[15] V. Barger and R.J.N. Phillips, Collider Physics, Addison-Wesley(1987).[16] R. Kinnunen and D. Denegri, CMS Note 1999/037; and private com-munication from D. Chakraborty and D. Denegri.[17] D.A. Dicus, B. Dutta and S. Nandi, Phys. Rev. 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Using Tau Polarization to Sharpen up the SUGRA Signal at Tevatron

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Using tau polarization to sharpen up the SUGRA signal at Tevatron

Using Tau Polarization to Sharpen up the SUGRA Signal at Tevatron

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