HIG-12-002

"Search for the fermiophobic model Higgs boson decaying into two photons"

This is a condensed description with plots for the analysis HIG-12-002.

Table of contents

Abstract

A search for a Higgs boson decaying into two photons is described. The analysis is performed using a dataset recorded in 2011 by the CMS experiment at the LHC from pp collisions at a centre-of-mass energy of 7 TeV, and corresponds to an integrated luminosity of 4.8 /fb Limits are set on the cross section of the fermiophobic model Higgs boson decaying to two photons. The expected exclusion limit at 95\% confidence level increases monotonically between 0.1 and 3.0 times the fermiophobic model cross section in the mass range between 110 and 150~GeV, while the expected exclusion range is below 136 GeV. The observed limit excludes at 95\% confidence level the fermiophobic model Higgs boson decaying into two photons in the mass range 110 to 124~GeV and 128-136 GeV. The largest excess of events over the expected background is observed around 126 GeV. The excess has a local significance of 2.7. When taking in consideration the look-elsewhere effect in the search range 100-150 GeV, the global significance is 1.2 sigma. More data are required to ascertain the origin of this excess.

Further details

This search for the FP Higgs is performed using 3 channels (dijet-tag, lepton-tag, inclusive channel. Techiniques used in this analysis are fundamentally different from the FP search reported previously, HIG-11-021. Due to 2D method in the inclusive channel and the use of the exclusive channels, the limit is improved by significantly in terms of cross section.

To improve the sensitivity of the search, selected diphoton events are subdivided into mutually exclusive classes tagged in three different channels

• dijet-tag channel: one event class
• lepton-tag channel: electron and muon-class
• inclusive channel: four classes according to photon eta and R9 variables: EBEB highR9, EBEB lowR9, NotEBEB highR9, NotEBEB lowR9. Events already tagged by dijet- or lepton-tag are removed from this class.

The background model is obtained by fitting polynomial to the observed diphoton mass distributions in the dijet-tag channel. In the lepton-tag channel the model is obtained by fitting the exponential which is obtained from MC due to extremely low statistic in data. In the inclusive channle we use two physical observables, diphoton mass and piT (piT is defined as the ratio of the diphoton pT and mass), and build a 2D maximum likelihood model. We explicitely account for the correlations between the observables in the background model. We fit the the observed diphoton mass by a power law function with a linear correlation term in the power. The higher order correlation terms can be neglected. The observed diphoton piT is modeled with a sum of an exponential and a gaussian function centered at zero. 2D compared to 1D treatment is 30% more sensitive in terms of cross section measurement.

Figures

Image	File links	Description
	pdf png	Figure 1: Background model fit to the mgg distribution for the three event classes of the exclusive channels. We also show a simulated signal (mH=120 GeV). The magnitude of the simulated signal is what would be expected if its cross section were equal to the FP expectation. (top) dijet-tag: observed diphoton mass is fit with a 2nd order polynomial, (midle) muon-tag: observed diphoton mass is fit with an exponential with shape derived from irreducible background MC , (bottom) electron-tag: observed diphoton mass is fit with an exponential with shape derived from a combination of ireducible background MC and a control sample defined by requiring one of the photons to be matched with a track. For the top plot the statistical uncertainty bands shown are computed from the data fit uncertainty on the background yield. For the two bottom plots the dominant uncertainty comming from the nor- malization to the data is not shown. Signal of FP Higgs at m=120 GeV (red line) is overlaid for reference. Individual plots are given for each event class of this inclusive channel.
	pdf png	a) Dijet-tagged diphoton event class. Fit with a 2nd order polynomial
	pdf png	b) Muon-tagged diphoton event class. Fit from MC, normalized to data.
	pdf png	c) Electron-tagged diphoton event class. Fit from MC and CS, normalized to data.

	pdf png	Observed piT distribution in inclusive channel after dijet-tagged and lepton-tagged events have been removed. We also show a simulated signal (mH=120 GeV). The magnitude of the simulated signal is normalized to data. Discriminating power of this observable's shape between background and signal is evident and motivates the use of this observable together with the diphoton mass to build a 2D likelihood model. Not shown on this plot, but the function used to model the signal piT distributions is a sum of a Gaussian and Bifurcated Gaussian.
	pdf png	Bias observed if correaltion not included in the 2D background model of the inclusive channel. Bias is defined the mean of the pull distribution of signal strength relative to statistical error of pseudo-experiment.

	pdf png	Figure 2: Background model fit projected to the mgg distribution for the four event classes of the inclusive channel after the dijet-tagged and lepton-tagged events have been removed. In the power we use a term linear with piT to account for the correlation between the two observables. We also show a simulated signal (mH=120 GeV). The magnitude of the simulated signal is what would be expected if its cross section were equal to the FP expectation. Individual plots are given for each event class of this inclusive channel.
	pdf png	a) The best inclusive channel event class defined by diphoton EBEB HighR9.
	pdf png	b) The inclusive channel event class defined by diphoton EBEB LowR9.
	pdf png	c) The inclusive channel event class defined by diphoton NotEBEB HighR9.
	pdf png	d) The inclusive channel event class defined by diphoton NotEBEB LowR9.

pdf png

Fig 3. Background model fit to the piT distribution for the four event classes of the inclusive channel after the dijet-tagged and lepton-tagged events have been removed. We also show a simulated signal (mH=120 GeV). The magnitude of the simulated signal is normailzed to data to better ilustrated the difference in shape between the signal and backround.

	pdf png	Figure 4: Exclusion limit on the cross section of a FG Higgs boson decaying into two photons as a function of the boson mass relative to the FP cross section, where the theoretical uncertainties on the cross section have been included in the limit setting. The limit is calculated in the asymptotic CLs approximation. The expected limit is provided for the combined channels and the indiviudal channels (dijet-tag, lepton-tag and inclusive (with events removed) is also shown for reference, with the following paticipation in the combined limit (dijet 65%, lepton 17%, inclusive 17%).
	pdf png	Figure 5: Observed local p-values for the three channels combined is shown in black line. P-value is also shown for individual channels: dijet-tagged class, lepton-tagged classes, and inclusive 2D classes.
	pdf png	Figure 6: The best fit signal strength, in terms of the standard model Higgs boson cross section, for the combined fit to the five classes (vertical line) and for the individual contributing classes (points) for the hypothesis of a FP Higgs boson mass of 126 GeV. The band corresponds to +- 1sigma uncertainties on the overall value. The horizontal bars indicate +-sigma uncertainties on the values for individual classes.