Weight function method for precise determination of top quark mass at Large Hadron Collider

Abstract:We have recently established a new method for measuring the mass of unstable particles produced at hadron colliders based on the analysis of the energy distribution of a massless product from their two-body decays. The central ingredient of our proposal is the remarkable result that, for an unpolarized decaying particle, the location of the peak in the energy distribution of the observed decay product is identical to the (fixed) value of the energy that this particle would have in the rest-frame of the decaying particle, which, in turn, is a simple function of the involved masses. In addition, we utilized the property that this energy distribution is symmetric around the location of peak when energy is plotted on a logarithmic scale. The general strategy was demonstrated in several specific cases, including both beyond the standard model particles, as well as for the top quark. In the present work, we generalize this method to the case of a massive decay product from a two-body decay; this procedure is far from trivial because (in general) both the abovementioned properties are no longer valid. Nonetheless, we propose a suitably modified parametrization of the energy distribution that was used successfully for the massless case, which can deal with the massive case as well. We test this parametrization on concrete examples of energy spectra of Z bosons from the decay of a heavier supersymmetric partner of top quark (stop) into a Z boson and a lighter stop. After establishing the accuracy of this parametrization, we study a realistic application for the same process, but now including dominant backgrounds and using foreseeable statistics at LHC14, in order to determine the performance of this method for an actual mass measurement. The upshot of our present and previous work is that, in spite of energy being a Lorentz-variant quantity, its distribution emerges as a powerful tool for mass measurement at hadron colliders.

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“…[72][73][74] for related recent work and ref. [75] for an earlier discussion on this property in the context of cosmic-ray physics.…”

Section: Jhep04(2016)151mentioning

confidence: 99%

Energy spectra of massive two-body decay products and mass measurement

Agashe

Franceschini

Hong

et al. 2016

J. High Energ. Phys.

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“…These virtues are shared with leptonic-observable methods proposed in Refs. [59][60][61]. In order to estimate the sensitivity of the method, we perform pseudo-experiments assuming the LHC 13 TeV with 3 ab −1 data, and the FCC 100 TeV with 1 ab −1 and 10 ab −1 data.…”

mentioning

confidence: 99%

Top-quark mass from the diphoton mass spectrum

Kawabata

Yokoya²

2017

Eur. Phys. J. C

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We calculate the gg → γ γ amplitude by including the tt bound-state effects near their mass threshold. In terms of the non-relativistic expansion of the amplitude, the LO contribution is an energy-independent term in the oneloop amplitude. We include the NLO contribution described by the non-relativistic Green function and part of the NNLO contribution. Despite a missing NLO piece which can be accomplished with the two-loop-level amplitude via massive quarks, the shape of the diphoton mass spectrum is predicted with a good accuracy. Thanks to the simple and clean nature of the observable, its experimental measurement can be a direct method to determine the short-distance mass of the top quark at hadron colliders.At the LHC, a diphoton mass spectrum dσ/dm γ γ has attracted broad attention for observations of the properties of the Higgs boson in the standard model (SM) [1][2][3][4][5] and searches for new phenomena beyond the SM [6][7][8][9][10][11]. At hadron colliders, pairs of high-p T photon are produced by qq annihilation and gluon-fusion mechanisms [12][13][14], and processes which involve fragmentation-photon contributions [15]. The gg → γ γ process, which is the main focus of this letter, is described by loop diagrams with quarks in the SM. The analytic expression of the one-loop amplitude has been known for a long time for both the massless-and the massive-quark loops [16][17][18][19][20][21][22]. The two-loop amplitude has been calculated only for the massless-quark loops [23][24][25].The threshold structure of the massive-quark-loop amplitude deserves particular interests [21,26,27] where the massive quark is regarded as the top quark or a hypothetical particle beyond the SM. Beyond the one-loop level, the amplitude receives large QCD corrections due to the Coulomb-gluon exchanges between the nearly on-shell and low-velocity heavy quarks in s-channel. Thus, the description of the amplitude requires an elaborate treatment based on the non-relativistic QCD formalism. For gg → γ γ process, a e-mail: skawabata@seoultech.ac.kr b e-mail: hyokoya@kias.re.kr such a study cannot be found in the literature. The aim of this letter is to compile present knowledge of the non-relativistic QCD theory for the description of the bound-state effects in the massive-quark-loop amplitude, and to present a dedicated and quantitative study on the diphoton mass spectrum near the tt threshold. Our framework follows the preceding studies on h → γ γ [28,29], and some of our numerical results overlap with that in Ref. [26].We discuss further to utilize the predicted mass spectrum for a precise determination of the top-quark mass, which is one of the fundamental parameters in the SM. Although the top-quark mass has been measured with an error of sub-GeV level [30], its interpretation in terms of well-defined mass parameters is not settled yet in perturbative QCD. It is known that the well-defined mass parameters can be determined by using the threshold scan method at future e + e − colliders [31][32][33]. We show that the ...

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“…This should be considered, however, a less severe problem, since in principle this error could be reduced by increasing the resolution. 16 To conclude, let me say again that I am very aware that this is a controversial subject. The overall view that I have expressed should be taken as my personal one.…”

Section: Discussionmentioning

confidence: 98%

“…In ref. [16], the direct measurements are criticized on the basis that they reconstruct the top mass making use of jets, and jet reconstruction is affected by hadronization effects that are modelled by the shower Monte Carlo. In contrast, they propose using observables that only depend upon the lepton kinematics and that are insensitive to production dynamics, and thus should not (in their opinion) be affected by hadronization effects.…”

Section: The "Non Perturbative" Argumentmentioning

confidence: 99%