{"id":3012,"date":"2015-05-06T02:01:51","date_gmt":"2015-05-06T02:01:51","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=3012"},"modified":"2015-05-06T02:09:28","modified_gmt":"2015-05-06T02:09:28","slug":"uncovering-a-higgs-hiding-behind-backgrounds","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=3012","title":{"rendered":"Uncovering a Higgs Hiding Behind Backgrounds"},"content":{"rendered":"

Hello particle munchers,<\/p>\n

\"hidingHiggs\"<\/a>
Figure 1: Monsieur Higgs boson hiding behind a background.<\/figcaption><\/figure>\n

Last time I discussed the Higgs boson decay into photons, i.e. `shining light on the Higgs boson<\/a>‘.\u00a0This is a followup discussing more generally the problem of\u00a0uncovering a Higgs boson which is\u00a0hiding buried behind what can often be a large background (see Figure 1).<\/p>\n

Perhaps the first question to ask is, what the heck is a background? Well, basically a background is anything that we `already know about’. In this case, this means the well understood\u00a0Standard Model<\/a> (SM) processes which do not involve a Higgs boson (which in this case is our `signal’), but can nevertheless\u00a0mimic one of the possible decays of the Higgs. For most of these processes, we have very precise theoretical predictions in addition to previous experimental data from the\u00a0LEP<\/a> and Tevatron<\/a>\u00a0experiments (and others) which previously\u00a0searched for the Higgs boson. So it is in reference to these non-Higgs SM processes when we use the term `background’.<\/p>\n

As discussed in my previous post, the Higgs can decay to a variety of combinations of SM particles, which we call `channels’. Each of these channels has its own corresponding background which obscures the presence of a Higgs. For some channels the backgrounds are huge. For instance the background for a Higgs decaying to a pair of bottom quarks<\/a>\u00a0is so large (due to QCD<\/a>) that, despite the fact this is the dominant decay channel (about\u00a060% of Higgs’ decay to bottom quarks<\/a>\u00a0at 125 GeV), this channel has yet to be observed.<\/p>\n

This is in contrast to the Higgs decay to four charged leptons (specifically\u00a0electrons<\/a> and muons<\/a>) channel. This decay (mediated by a pair of virtual Z bosons<\/a>) was one of the first discovery channels of the Higgs at the LHC despite the fact that roughly only one in every 10,000 Higgs bosons decays to four charge leptons. This is because this channel has a small\u00a0background and is measured with very high precision. This high precision allows LHC experiments to scan over a range of\u00a0energies in very small increments or `windows’. Since the background is very small, the probability of observing a background event in any given window is tiny. Thus, if an excess of events is seen in a particular window, this is an indication that there is a non background process occurring at that particular energy.<\/p>\n

\"hto4lpeak\"<\/a>
Figure 2: The energy spectrum of a Higgs decaying to four charged leptons (red) and its associated background (blue).<\/figcaption><\/figure>\n

This is how the Higgs was discovered in the decay to four charged leptons<\/a> at around 125 GeV. This can be\u00a0seen in Figure 2 where in the window around the Higgs signal (shown in red) we see the background (shown in blue) is very small. Thus,\u00a0once\u00a0about a dozen\u00a0events were observed at around\u00a0125 GeV, this was already enough\u00a0evidence for\u00a0experiments at the LHC to be able to claim discovery of the long sought after monsieur Higgs boson.<\/p>\n

\u00a0Further Reading:<\/strong><\/p>\n

Seeking and Studying the Standard Model Higgs Particle<\/a><\/p><\/blockquote>\n