Detecting and Distinguishing Top Partners

Transcription

1 Detecting and Distinguishing Top Partners Ayres Freitas and Devin Walker After the discovery of tt+e/ T events at a hadron collider it will be crucial to determine the properties of the new particles which generate the process pp T T t t + XX. (1) Here T are the top-quark partners and X are stable neutral particles. Commonly discussed top-quark partners are supersymmetric stops and heavy top quarks (e.g, little higgs scenarios with t-parity). Given the signal in equation (1), we provide an overview of several top partner measurements at the LHC: LHC reach for top partners; Top partner mass measurement; Top partner spin measurement; Determining top partner chirality. Before moving on, we note a lepton collider with sufficiently high center-ofmass energy to produce T T pairs can measure all relevant properties of the T and X particles with high precision 1. A. LHC Potential for Searching for Top Partners: In this section, we consider identifying tt + E/ T signatures in a model independent fashion (i. e.) in contrast to the previous section fermionic and vector top partners are also considered). For simplicity, we focus on tt decays into a final state with one lepton l = e, µ in the final state. p p T T t t + XX lν b b jj + XX (2) The isolated lepton in the final state is needed in order to reduce the dominant QCD background. In the following, we assume the top partner decays to 1 The typical observables for mass, spin and coupling measurement are very similar to the case of slepton pair production, which are summarized for example in section 3.2 of Ref. [1]. 1

2 top and dark matter candidates with a 100% branching fraction. The key parameter for top partner searches is the mass difference [2], δm = m T m X, (3) between the top partner and dark matter. When δm m top, then tt + E/ T signature is almost always swamped by the dominant Standard Model (SM) tt background (see section stealth stop). In the coming paragraphs, we review existing model-independent techniques to dig out top partner signatures. Later, we sketch a novel technique that promises to help to maximize the search potential for top partners. Again, see X in this Snowmass proceedings for additional/more in depth discussion related to the search for supersymmetric stops. The following is also applicable to those searches. Basic experimental searches for spin-0 supersymmetric top partners (called scalar tops or stops) are also sensitive to top partners with higher spin. These methods rely heavily a combination of E/ T and transverse mass cuts. For the semi-leptonic top decays under consideration, the W transverse mass is often used. It is defined as m T (W ) 2 = 2E/ T p T l (1 cos φ lν ). (4) The t t background satisfies m T (W ) < m W, whereas the signal (1) can have large values of m T (W ) when δm is sizeable. Furthoremore, in this case, a large E/ T cut is also effective to separate the signal from the background SM processes. See section vanilla stop by Bai, Golling and Stolarski for more details and different choices of cuts. The current bounds exclude stops with mass below about 550 GeV for large δm [3, 4]. Note that most searches by ATLAS and CMS are presented only in terms of stops (an exception is Ref. [5], which pertains to fermionic top partners, see Fig. 1). However, the advantage of simple cut-based methods is that they do not rely on details of the stop interaction and thus are also applicable to fermionic and vector top partner searches. Due to the larger cross-section, the reach for fermionic top partners extends to masses that are roughly GeV higher than for scalar top partners at the 8-TeV LHC, while for 14 TeV the difference is GeV [2, 6]. Razor Analysis: The CMS collaboration has used the razor analysis method to search for heavy supersymmetric particles [7], which goes beyond the traditional E/ T cuts described in the previous paragraph. Nevertheless, this 2

3 Mass [GeV] A m(t) m(a ) < m(t) 0 3pb 2pb 1.5pb 1pb ATLAS 1 L dt=1.04 fb s=7 TeV 100 Expected Limit (±1σ) 50 Obs. Limit (Theory Unc.) CDF Exclusion Excl. σ BR(TT tta 0 A 0 ) T Mass [GeV] Figure 1: ATLAS exclusion plot for heavy fermionic top partner production during the 7 TeV run [5]. Here A 0 is the dark matter candidate X. technique is also based only on kinematic features and thus suitable for model-independent searches. It will be applied [8] to the signal in eq. (1) during the LHC shutdown. The razor, R, is defined as the combination of two kinematic variables where R M R T M R, (5) ( ) ( p q z q p z ) 2 1/2 M R = 2, (6) (p z q z ) 2 ( p q ) 2 MT R = 1 1/2 ( p / T ( p + q ) p / T ( p + q)). (7) 2 Here all reconstructed objects are arranged into two hemispheres with 3- momentum p and q. p/ T is the missing momentum in the event. MT R behaves similarly to M T 2, see paragraph B. M R is effectively a boost from the T T pair center of mass frame. M R is defined in this way because it has a peak 3

4 at M R m2 T m2 X 2m T (8) Similarly, MT R has an edge at the same spot. See [9] for more information on the derivation of these variables. To date, razor has put strong constraints on gluino and squark production. A Model Independent Searches for Top Partners: A new way to search for top partners that generate the signal in equation 1 to define new kinematic variables that no longer treat missing energy solely as a scalar quantity. [add section on this physics when [10] appears on the arxiv... will replace partonic level plots later as well] B. Top Partner Mass Measurement: At hadron colliders, the independent measurement of the T and X masses in processes in equation 1 is very challenging because each individual event is kinematically under-constrained. The endpoint of the distribution of the variable [11] { M T2 = min max ( M t,x 1 T, M t,x 2 ) } T (9) p T,X1 +p T,X2 =p/ T gives an accurate determination of m T, provided m X is known. Both masses can be obtained independently if M T2 is augmented by including information about initial-state radiation [12]. Alternatively, m T and m X can be determined from a likelihood fit based on the Matrix-Element Method [13]. For more information on suitable mass measurement techniques, see Ref. [14]. The analyses carried out in Refs. [12, 13] indicate that one can expect an uncertainly of roughly 20 30% for the measurement of the absolute masses, while the mass difference m Y m X can be obtained with a precision of a few percent. Note, however, that these studies were carried out for leptonic final states, rather than t t final states. Nevertheless, algorithms for top tagging reaching high levels of efficiency and purity are now available [15] so that these estimates are expected to be approximately valid. 4

5 Figure 2: Standard model backgrounds. Here X represents additional the beam remnants and/or other possible hadronic activity... [scales off] 5

6 d σ d Tanh( y/2) 1 σ 1.8 (T spin, X spin) (0, 1/2) 1.6 (1/2, 0) 1.4 (1/2, 1) (1, 1/2) Tanh( y/2) Figure 3: Unit-normalized distribution of tanh( y t t/2) for the different toppartner spin scenarios, and m T = 300 GeV, m X = 100 GeV and s = 14 TeV. From Ref. [6]. C. Top Partner Spin Measurement: The simplest method for probing the spin of the top partner T is through the T T production cross section [16, 2, 17], which increases with increasing spin of T, σ(scalar) < σ(fermion) < σ(vector) < σ(kk gravition) (10) However, this method requires sufficiently accurate knowledge of branching fractions and masses of the underlying model. Frequently, at hadron colliders like the LHC, this precision is unobtainable. Alternatively, scalar and non-scalar T particles can be distinguished based on the distribution of the production angle. In the q q sub-channel, this distribution follows a p-wave profile in the scalar case, and is dominated by the s-wave otherwise. The production angle of T / T can be measured approximately from the rapitidy of the observed t/ t [6], see Fig. 3 and also Ref. [16]. If T and X are part of a larger new physics sector, with additional particles that are produced at the LHC (or future hadron machines) and decay into T, these decay chains provide additional handles for spin determination [18]. Besides using distributions constructed from the t and t momenta, spin information can also be derived from spin correlations between the observed 6

7 top and antitop. In particular, if T is a scalar the t and t spins are completely uncorrelated, whereas the fermionic T particles will in general lead to observable correlations. This feature can be used both for discovery of a new physics signal and for model discrimination [19]. D. Top Partner Chirality: The chirality structure of the decay T tx (i. e. the relative size of left- and right-handed couplings) can be determined from the the polarization of the final-state top quark (the absolute polarization, not the polarization correlation that was used for spin determination in the prevous paragraph). The actual values for the observable polarization depend on m T and m X, but a distinction between the extreme cases of purely left- and right-handed couplings is possible independent of that [6, 20]. 7

8 Bibliography [1] J. A. Aguilar-Saavedra et al. [ECFA/DESY LC Physics Working Group Collaboration], hep-ph/ [2] T. Han, R. Mahbubani, D. G. E. Walker and L. T. E. Wang, JHEP 0905, 117 (2009) [arxiv: [hep-ph]]. [3] G. Aad et al. [ATLAS Collaboration], ATLAS-CONF (Dec 2012). [4] S. Chatrchyan et al. [CMS Collaboration], CMS-PAS-SUS (Nov 2012). [5] G. Aad et al. [ATLAS Collaboration], Phys. Rev. Lett. 108, (2012) [arxiv: [hep-ex]]. [6] C.-Y. Chen, A. Freitas, T. Han and K. S. M. Lee, JHEP 1211, 124 (2012) [arxiv: [hep-ph]]. [7] S. Chatrchyan et al. [CMS Collaboration], arxiv: [hep-ex]. [8] Private conversation with C. Rogan. [9] C. Rogan, arxiv: [hep-ph]. [10] A. Ismail, A. Schwartzman, R. Schwienhorst, J. Virzi and D. G. E. Walker, to appear. [11] C. G. Lester and D. J. Summers, Phys. Lett. B 463, 99 (1999) [arxiv:hep-ph/ ]. 8

9 [12] P. Konar, K. Kong, K. T. Matchev and M. Park, Phys. Rev. Lett. 105, (2010) [arxiv: [hep-ph]]; T. Cohen, E. Kuflik and K. M. Zurek, JHEP 1011, 008 (2010) [arxiv: [hep-ph]]. [13] J. Alwall, A. Freitas and O. Mattelaer, AIP Conf. Proc. 1200, 442 (2010) [arxiv: [hep-ph]]. [14] See sect. 4 in A. J. Barr and C. G. Lester, J. Phys. G 37, (2010) [arxiv: [hep-ph]], and refs. therein; sect. X in A. J. Barr, T. J. Khoo, P. Konar, K. Kong, C. G. Lester, K. T. Matchev and M. Park, Phys. Rev. D 84, (2011) [arxiv: [hep-ph]], and refs. therein. [15] T. Plehn and M. Spannowsky, J. Phys. G 39, (2012) [arxiv: [hep-ph]]. [16] P. Meade and M. Reece, Phys. Rev. D 74, (2006) [hepph/ ]. [17] G. L. Kane, A. A. Petrov, J. Shao and L. T. Wang, J. Phys. G 37, (2010) [arxiv: [hep-ph]]. [18] A. J. Barr, Phys. Lett. B 596, 205 (2004) [hep-ph/ ]; J. M. Smillie and B. R. Webber, JHEP 0510, 069 (2005) [hepph/ ]. [19] Z. Han, A. Katz, D. Krohn and M. Reece, JHEP 1208, 083 (2012) [arxiv: [hep-ph]]. [20] M. Perelstein and A. Weiler, JHEP 0903, 141 (2009) [arxiv: [hep-ph]]; J. Shelton, Phys. Rev. D 79, (2009) [arxiv: [hep-ph]]. 9