Resonant Higgs pair production as a probe of stop at the LHC

Journal of High Energy Physics, Sep 2017

Searching for top squark (stop) is a crucial task of the LHC. When the flavor conserving two body decays of the stop are kinematically forbidden, the stops produced near the threshold will live long enough to form bound states which subsequently decay through annihilation into the Standard Model (SM) final states. In the region of stop mixing angle \( {\theta}_{\tilde{t}}\to 0 \) or π/2, we note that the LHC-13 TeV diphoton resonance data can give a strong bound on the spin-0 stoponium (\( {\eta}_{\tilde{t}} \)) and exclude the constituent stop mass \( {m}_{\tilde{t}} \) up to about 290 GeV. While in the large stop mixing region, the stoponium will dominantly decay to the Higgs pair. By analyzing the process \( pp\to {\eta}_{\tilde{t}}\to h\left(\to b\overline{b}\right)h\left(\to {\tau}^{+}{\tau}^{-}\right) \), we find that a large portion of the parameter space on the \( {m}_{{\tilde{t}}_1}-{\theta}_{\tilde{t}} \) plane can be probed at 2σ- significance level at the LHC with the luminosity ℒ = 3000 fb−1.

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Resonant Higgs pair production as a probe of stop at the LHC

Received: June Resonant Higgs pair production as a probe of stop at Guang Hua Duan 0 1 2 3 6 8 9 10 Lei Wu 0 1 3 5 7 8 9 10 Rui Zheng 0 1 3 4 8 9 10 0 Davis , CA, 95616 U.S.A 1 Beijing , 100049 China 2 School of Physical Sciences, University of Chinese Academy of Sciences 3 Beijing , 100190 China 4 Department of Physics, University of California , USA 5 ARC Centre of Excellence for Particle Physics at the Terascale, School of Physics 6 Institute of Theoretical Physics, Chinese Academy of Sciences 7 Department of Physics and Institute of Theoretical Physics, Nanjing Normal University 8 nal states. In the region of stop 9 The University of Sydney , New South Wales, 2006 Australia 10 Nanjing , Jiangsu, 210023 China Searching for top squark (stop) is a crucial task of the LHC. When the avor conserving two body decays of the stop are kinematically forbidden, the stops produced near the threshold will live long enough to form bound states which subsequently decay through annihilation into the Standard Model (SM) mixing angle t~ ! 0 or =2, we note that the LHC-13 TeV diphoton resonance data can give a strong bound on the spin-0 stoponium ( t~) and exclude the constituent stop mass m~ t up to about 290 GeV. While in the large stop mixing region, the stoponium will dominantly signi cance level at the LHC with the luminosity L = 3000 fb 1. decay to the Higgs pair. By analyzing the process pp ! t~ ! h(! bb)h(! that a large portion of the parameter space on the mt~1 { t~ plane can be probed at 2 Supersymmetry Phenomenology - ArXiv ePrint: 1706.07562 1 Introduction 2 Diphoton resonance constraint on the stoponium 3 Di-Higgs decay of stoponium with bb + nal states at the LHC 4 Conclusions 1 Introduction mass splitting which leads to di erent decay modes. For instance, when mt~1 > mt + m~01 and t~1 mainly decays to t ~01, the top quark from stop decay can be quite energetic and a stop mass up to 940 GeV for a massless lightest neutralino has been excluded by the very recent LHC run-2 data [17]. When the avor-conserving two body decays channels the light stop would be the three-body decay t~1 ! W +b ~01, the two-body like t~1 ! t ~01 and t~1 ! b ~1+ are kinematically forbidden, the primary decay channels of avor-changing decay t~1 ! c ~01 or the four-body decay t~1 ! bf 0f ~01 [18{25]. The current null results of LHC searches for these decay channels have correspondingly excluded the stop mass up to 500 GeV, 310 GeV and 370 GeV for certain mass splitting between the stop and the LSP [17]. It should be mentioned that such a light stop usually has very small decay width [26] compared to the typical binding energy of t~1t~1 bound state (stoponium). In this case, two stops produced near-threshold could live long enough to form a stoponium due to the Coulomb-like attraction via the QCD interaction. In contrast to the existing direct stop pair searches, stoponium if formed, will resonantly decay to a pair of the SM particles and can be independent of the assumptions of the LSP mass and the branching ratios of the { 1 { stop. Therefore, it is expected that the search of stoponium can provide a complementary probe to the direct stop pair production at the LHC. The phenomenologies of the stoponium have been studied at colliders [26{34]. In particular, the diphoton channel was studied and found to be a promising way to observe stoponium at the LHC in refs. [26{28]. The diboson decay of stoponium with W W and ZZ nal states were also examined in [32, 33]. In [35], the authors investigated the di-Higgs decay of stoponium with bb nal states and found it to be a viable channel at the LHC. But the loop induced diphoton decay of the Higgs boson can be sizably a ected by other sparticles, such as the light stau in the MSSM [36]. high mass resonances at 13 TeV LHC. Then we explore the potential of probing the stop in Higgs pair production with bb + nal states at high-luminosity LHC (HL-LHC). As a comparison with bb channel, although the bb + channel su ers from relatively complicated backgrounds, it has a larger branching ratio. Besides, it is expected that the reconstruction e ciency of can reach 80% with the likelihood taggers in the future LHC experiment [ 37, 38 ]. This will make bb + channel become another promising way of discovering, or con rming the stoponium at the LHC. The paper is organized as follows. In section 2, we introduce productions and decays of the stoponium and display the limits on stoponium mass from the LHC-13 TeV data. In section 3, we investigate the observability of the di-Higgs decay of the stoponium with bb nal states at the LHC. Finally, we draw our conclusions in section 4. In this paper, we rst confront the stoponium with the recent data of searching for mtXty 1 A mt2~L = m2Q~3L + mt2 + m2Z sin2 W cos 2 ; mt2~R = m2U~3R Xt = At cot ; 2 3 + mt2 + m2Z sin2 W cos 2 ; 1 2 + tR 2 3 (2.1) (2.2) (2.3) (2.4) (2.5) where mQ~3L and mU~3R denote the soft-breaking mass parameters of the third generation left-handed squark doublet Q~3L and the right-handed stop U~3R, respectively. At is the soft-breaking trilinear parameter. We neglect the generation mixing in our study. The hermitian matrix eq. (2.1) can be diagonalized by a unitary transformation: t~1 ! ~ t2 = cos t~ sin t~ sin t~ cos t~ ! tL ~ ! ~ tR ; { 2 { where t~ 2 [0; ) is the mixing angle between left-handed (t~L) and right-handed (t~R) stops. A very narrow decay width of stop1 can naturally appear in the compressed region, in which the decay width of stop is suppressed either by phase space or loop factor. If the t~1 is much smaller than binding energy, stop pair produced near the threshold could form a bound state due to the strong attractive force mediated by gluons. Then, these bound states will proceed annihilation decay rather than the prompt decay of the constituent stop. The production of stoponium is mainly from the gluon fusion at the LHC. In narrowwidth approximation, the leading order (LO) cross section of stoponium is be given by [26] (gg ! t~) = 2 8m3~ t s^ Z 1 d x t~!gg s s^ x s fg(x)fg s^ xs (2.6) where s^ is squared center-of-mass energy at the parton level and is taken as s^ = m2~ in t our calculation. ~t~!gg is the width of stoponium decay to di-gluon. The next-to-leading order QCD radiative corrections to stoponium production have been calculated in [40]. We include these e ects by using the values of K-factor given in [41]. It should be noted that there are two main uncertainties in the computation of stoponium production rate. One of them lies in the parametrization of the wavefunction, which depends on the choice of QCD scale parameter [42]. Larger value of leads to greater coupling and hence stronger binding between the constituent stops. We adopt = 300 MeV by following [41]. The other uncertainty comes from the contributions of excited bound states, such as nS(n 2) and 1P states. In particular, the e ects of higher S-wave states are compared in [41]. The excited states can contribute by either rst decaying into the lowest stoponium state (1S) or decaying directly into SM nal states. For instance, the non-annihilation decay of the 2S state could go entirely to the 1S state and the signal could be merged with that of the ground state due to the detector energy resolution [26]. In general, states with di erent angular momentum could have very distinct decay modes. Without thorough knowledge of the decay modes, we will take a conservative approach and focus on the 1S state. The main decay channels of the stoponium include t~ ! The LO partial decay widths into transverse gauge bosons are [26] ; Z; ZZ; W W; gg; hh; tt. ( t~ ! gg) ' 3 S 4 2 jR(0)j2 ; m2~ t ( t~ ! ) ' 27 32 2 jR(0)j2 m2~ t where R(0) = p 4 (0) is the radial wavefunction at the origin. In the nonrelativistic limit (v ! 0), only four-point interaction contributes to the stoponium decays t~ ! gg; other decay widths can be found in [27, 33]. Radiative corrections to stoponium annihilation decays to hadrons, photons, and Higgs bosons were calculated in ref. [43]. In gure 1, we display the decay branching ratios of the stoponium with respect to the mixing angle t~, where we assume tan = 10, mt~1 = 0:2 TeV and mt~2 = 2 TeV. It can be seen that the stoponium dominantly decays to di-gluon when the mixing angle t~ approaches 0 or =2. While if t~L and t~R have a sizable mixing, the stoponium will dominantly 1If the stop has a large decay width, it could in general produce a wide resonance signal and will be hardly observed on top of the continuum background [39]. { 3 { (2.7) . All 0.100 ∼ t gg γγ Zγ WW ZZ hh tt HJEP09(217)3 0.0 0.1 0.4 0.5 0.2 0.3 =2, we plot only the region t~ 2 [0; =2] here and also in gure 5. decay to a pair of Higgs bosons because of the enhancement induced by the Higgs-stop coupling ht~1t~1 .2 We also checked and found that branching ratios of the stoponium have a weak dependence of tan . So we will assume tan = 10 in our following calculations. Due to the distinctive signature of two photon nal states, the stoponium decay to diphoton o ers a very sensitive way to observing stoponium at hadron colliders. The bound on stoponium from 8 TeV run at the LHC is given in [44]. In gure 2, we update the result with the LHC-13 TeV diphoton resonance data [45]. We can see that the stoponium mass can be excluded up to about 580 GeV for the mixing angles t~ = =2, which is stronger than that from LHC-13 TeV direct searches for the four-body decay ~ t1 ! bf 0f ~01 with pure bino LSP in the region of mt~1 m ~01 < 15 GeV [17]. However, due to the branching ratio suppression e ect, there is still no constraint on the stoponium from the diphoton data for the mixing angles t~ = =8; =4. We also checked the bounds on the stoponium from current null results of LHC searches for Z and diboson resonances and found that they can not give stronger limits than the diphoton data. 3 Di-Higgs decay of stoponium with bb + nal states at the LHC Given that the stoponium can have a large branching fraction into the two Higgs bosons, we will investigate its observability through the resonant Higgs pair production with bb + nal states at the 14 TeV LHC, pp ! t~ ! hh ! bb + ; (3.1) mv2t2 + m2Zc2 ct2 12 v2 23 s2W + st2 23 s2W + st2ct2 m2~ t1 v2 mt2~2 . 2The trilinear coupling between the SM Higgs and stop quark t~1 takes the form [44]: ht~1t~1 = p2v { 4 { 0.01 ATLAS, s =13 TeV, 36.9 fb-1 θ∼t=0 mη∼t [GeV] 2 experimental upper limit (yellow band) is taken from [45]. Here we also assumed tan where one tau lepton decays hadronically ( had) and the other decays leptonically. had is reconstructed using clusters in the electromagnetic and hadronic calorimeters with medium We generate parton-level events of the stoponium production and subsequent decay into Higgs pair using the code for resonant Higgs pair production [47] within lepton decays are modeled by TAUOLA [49]. Then we perform parton shower and hadronization with PYTHIA [50]. The fast detector simulation is implemented with Delphes [51]. We use the b-jet tagging e ciency parametrization as 80% [52] and set the misidenti cation 10% and 1% for c-jets and light jets, respectively. We also assume the tagging e ciency is 40%. We set the renormalization scale R and factorization scale F as the default event-by-event value. We cluster the jets by choosing the anti-kt algorithm with a cone radius R = 0:4 [53]. The major backgrounds come from events with a jet misidenti ed as had, including tt, Z(! + )bb and Z(! + )jj processes. In gure 3, we present distributions of the di-tau invariant mass m , two b-jets invaristruct m from the observed lepton, had and Emiss. One can see that m T ant mass mbb, the transverse mass of the lepton plus missing energy system m`T and the di-tau transverse momentum pT . The simple transverse mass method is used to recondistribution shows a relatively broad peak around the Higgs boson mass with a long tail,3 as a comparison with mbb distribution. Another variable m`T can e ectively reduce tt background since the lepton in signal is not from W boson decay. The variable pT is used to select the events with the boosted Higgs boson candidate on the transverse plane. For such events, m resolution is improved and a better separation between the signal t~ ! and the 3This can be improved by using the advanced experimental MMC reconstruction technique [54]. { 5 { HJEP09(217)3 transverse mass of the lepton plus missing energy system m`T and the di-tau transverse momentum pT . The stoponium mass is taken as m t~ = 500 GeV. background Z ! QCD multijet background. is achieved. This selection also has the advantage of reducing the In our analysis, we select events that satisfy the following criteria: We require exactly one lepton (e or ) with pT (`) > 26 GeV, j ej < 2:47 or j j < 2:5. We further require the presence of a hadronically decayed tau h carrying opposite electric charge with pT ( h) > 20 GeV and j h j < 2:5. We require at least two jets with pT (j) > 30 GeV and j j j < 2:5 and two of them are b tagged. We require 80 GeV < mbb < 150 GeV, 80 GeV < m p T > 120 GeV and jmbb m t~j < 0:08m t~. < 150 GeV, m`T < 50 GeV, m`T can suppress Z(! In table 1, we present a cut ow of cross sections for the signal and backgrounds at 14 TeV LHC. After the di-b jets and di-tau invariant mass cuts, we nd that the cut < 50 GeV can reduce the tt background by about half. The cut p > 120 GeV T )bb backgrounds by an extra factor of six. The total { 6 { 2 [80; 150] GeV tt Z( )bb Z( )jj signal(m t~ = 500 GeV) mbb < 0:08m t~ 0.29 s=14TeV, ∫Ldt=3000 fb-1, S/ B=5 400 475 550 700 775 850 625 t m ~(GeV) signal signi cance S=p m t~j < 0:08m t~ can further hurt tt background by about O(102) )bb by about O(10). nal states needed for the signal signi cance S=pB = 5 In gure 4, we plot the cross sections of the process pp ! t~ ! hh with bb + at the HL-LHC. It can be =bb seen that the cross section of the process pp ! about 800 fb/100 fb to reach 5 t~ ! hh ! bb + =bb should be signi cance at m t~ = 400 GeV. When the stoponium is heavier than about 700 GeV, the required cross section of bb + channel for a tau tagging e ciency = 40% can be comparable with that of bb channel studied in [35]. If tagging e ciency can be improved to 80% estimated in [ 37, 38 ], the sensitivity of bb + channel is expected to become better than that of bb channel for m t~ & 570 GeV. hh ! bb + In gure 5, we show the 2 exclusion limits from the di-Higgs decay channel t~ ! and the di-photon decay channel t~ ! for mt~2 = 1 TeV and 2 TeV on the plane of mt~1 versus stop mixing angle t~ at the HL-LHC. We can see that the stop mass mt~1 can be excluded up to 380 (450) GeV in the large stop mixing region =7 . t~ . =3 { 7 { m m t∼ t∼ 2 2 = = 2 1 θ∼t /π 0.0 for mt~2 = 1 TeV and 2 TeV on the plane of mt~1 versus stop mixing angle t~ at the HL-LHC. The result of di-photon decay channel is taken from ref. [44]. by the di-Higgs decay channel t~ ! hh ! bb + depends on the Higgs-stop coupling , since the branching ratio of t~ ! hh ht~1t~1 . For a given mixing angle t~, a larger mt~2 sets a mixing region, such as t~ . =7 or t~ & di erence m2~ t1 m2~ . The di-photon decay channel t~ ! t2 stronger bound on mt~1 because the Higgs-stop coupling ht~1t~1 is proportional to the mass =3, which is complementary to the di-Higgs decay mainly excludes small stop channel. 4 Conclusions In this paper, we confront the stoponium with the recent data of searching for high mass resonances at 13 TeV LHC, and explore the potential of probing the stoponium in resonant Higgs pair production with bb + nal states at the LHC. We note that the LHC-13 TeV diphoton resonance data can give a strong bound on the spin-0 stoponium ( t~) and exclude the constituent stop mass mt~1 up to about 290 GeV in the small stop mixing region. While in the large stop mixing region, the stoponium will dominantly decay to Higgs pair. 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Guang Hua Duan, Lei Wu, Rui Zheng. Resonant Higgs pair production as a probe of stop at the LHC, Journal of High Energy Physics, 2017, 37, DOI: 10.1007/JHEP09(2017)037