{"id":4718,"date":"2016-11-15T13:39:09","date_gmt":"2016-11-15T13:39:09","guid":{"rendered":"https:\/\/particlebites.com\/?p=4718"},"modified":"2017-02-27T13:41:26","modified_gmt":"2017-02-27T13:41:26","slug":"studying-the-higgs-via-top-quark-couplings","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=4718","title":{"rendered":"Studying the Higgs via Top Quark Couplings"},"content":{"rendered":"

Article: \u201cImplications of CP-violating Top-Higgs Couplings at LHC and Higgs Factories\u201d<\/strong><\/p>\n

Authors: Archil Kobakhidze, Ning Liu, Lei Wu, and Jason Yue<\/strong><\/p>\n

Reference: arXiv hep-ph 1610.06676<\/a><\/strong><\/p>\n

 <\/p>\n

It has been nearly five years since scientists at the LHC first observed a new particle that looked a whole lot like the highly sought after Higgs boson. In those five years, they have poked and prodded at every possible feature of that particle, trying to determine its identity once and for all. The conclusions? If this thing is an imposter, it\u2019s doing an incredible job.<\/p>\n

This new particle of ours really does seem to be the classic Standard Model Higgs. It is a neutral scalar, with a mass of about 125 GeV. All of its couplings<\/strong> with other SM particles are lying within uncertainty of their expected values, which is very important. You\u2019ve maybe heard people say that the Higgs gives particles mass. This qualitative statement translates into an expectation that the Higgs coupling to a given particle is proportional to that particle\u2019s mass. So probing the values of these couplings is a crucial task.<\/p>\n

\"\"<\/a>
Figure 1: Best-fit results for the production signal strengths for the combination of ATLAS and CMS. Also shown for completeness are the results for each experiment. The error bars indicate the 1\u03c3 intervals.<\/figcaption><\/figure>\n

Figure 1 shows the combined experimental measurements between ATLAS and CMS of Higgs decay signal strengths as a ratio of measurement to SM expectation. Values close to 1 means that experiment is matching theory. Looking at this plot, you might notice that a few of these values have significant deviations from 1, where our perfect Standard Model world is living. Specifically, the ttH signal strength is running a bit high. ttH is the production of a top pair and a Higgs from a single proton collision. There are many ways to do this, starting from the primary Higgs production mechanism of gluon-gluon fusion. Figure 2 shows some example diagrams that can produce this interesting ttH signature. While the deviations are a sign to physicists that maybe we don\u2019t understand the whole picture.<\/p>\n

\"\"<\/a>
Figure 2: Parton level Feynman diagrams of ttH at leading order.<\/figcaption><\/figure>\n

Putting this in context with everything else we know about the Higgs, that top coupling is actually a key player in the Standard Model game. There is a popular unsolved mystery in the SM called the hierarchy problem<\/strong><\/a>. The way we understand the top quark contribution to the Higgs mass, we shouldn\u2019t be able to get such a light Higgs, or a stable vacuum. Additionally, electroweak baryogenesis reveals that there are things about the top quark that we don\u2019t know about.<\/p>\n

Now that we know we want to study top-Higgs couplings, we need a way to characterize them. In the Standard Model, the coupling is purely scalar. However, in beyond the SM models, there can also be a pseudoscalar component, which violates charge-parity (<\/strong>CP<\/strong><\/a>) symmetry<\/strong>. Figure 3 shows a generic form for the term, where Cst is the scalar and Cpt is the pseudoscalar contribution. What we don\u2019t know right away are the relative magnitudes of these two components. In the Standard Model, Cst = 1 and Cpt = 0. But theory suggests that there may be some non-zero value for Cpt, and that\u2019s what we want to figure out.<\/p>\n

\"\"<\/a>
Figure 3<\/figcaption><\/figure>\n

Using simulations along with the datasets from Run 1 and Run 2 of the LHC, the authors of this paper investigated the possible values of Cst and Cpt. Figure 4 shows the updated bound. You can see from the yellow 2\u03c3 contour that the new limits on the values are |Cpt| < 0.37 and 0.85 < Cst < 1.20, extending the exclusions from Run 1 data alone. Additionally, the authors claim that the cross section of ttH can be enhanced up to 1.41 times the SM prediction. This enhancement could either come from a scenario where Cpt = 0 and Cst > 1, or the existence of a non-zero Cpt component.<\/p>\n

\"\"<\/a>
Figure 4: The signal strength \u00b5tth at 13 TeV LHC on the plane of Cst and Cpt. The yellow contour corresponds to a 2\u03c3 limit.<\/figcaption><\/figure>\n

Further probing of these couplings could come from the HL-LHC, through further studies like this one. However, examining the tH coupling in a future lepton collider would also provide valuable insights. The process e+e- \u00e0 hZ contains a top quark loop. Thus one could make a precision measurement of this rate, simultaneously providing a handle on the tH coupling.<\/p>\n

 <\/p>\n

References and Further Reading:<\/strong><\/p>\n

    \n
  1. \u201cEnhanced Higgs associated production with a top quark pair in the NMSSM with light singlets\u201d. arXiv hep-ph 02353<\/a><\/li>\n
  2. \u201cMeasurements of the Higgs boson production and decay rates and constraints on its couplings from a combined ATLAS and CMS analysis of the LHC pp collision data at \u221as = 7 and 8 TeV.\u201d ATLAS-CONF-2015-044<\/a><\/li>\n<\/ol>\n

     <\/p>\n

     <\/p>\n","protected":false},"excerpt":{"rendered":"

    Article: \u201cImplications of CP-violating Top-Higgs Couplings at LHC and Higgs Factories\u201d Authors: Archil Kobakhidze, Ning Liu, Lei Wu, and Jason Yue Reference: arXiv hep-ph 1610.06676   It has been nearly five years since scientists at the LHC first observed a new particle that looked a whole lot like the highly sought after Higgs boson. In … <\/p>\n

    Continue reading “Studying the Higgs via Top Quark Couplings”<\/span><\/a><\/p>\n","protected":false},"author":8,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[23,24],"tags":[],"class_list":["post-4718","post","type-post","status-publish","format-standard","hentry","category-higgs-boson","category-higgs-boson-decays"],"_links":{"self":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/4718","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/users\/8"}],"replies":[{"embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4718"}],"version-history":[{"count":5,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/4718\/revisions"}],"predecessor-version":[{"id":4727,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/4718\/revisions\/4727"}],"wp:attachment":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4718"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4718"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4718"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}