DOI 10.1393/ncc/i2015-11864-y Colloquia: LaThuile14
Search for a Higgs to Fermions with the CMS detector at the LHC
J. Swansonon behalf of the CMS Collaboration
Brown University - Providence, RI, USA ricevuto il 31 Luglio 2014
Summary. — The discovery of a new boson at the LHC in 2012 has lead to an exciting time in high-energy particle physics. The first observations were in decays to γγ, WW, and ZZ and came with tests of spin parity and measurements of the new bosons mass. An important remaining question is the coupling of this boson to fermions. In this paper the current CMS results for Higgs boson decays to τ τ , μμ and b¯b will be presented. These searches show an observed significance of 3.2 σ to τ τ , 2.1 σ to b¯b and no significant excess to μμ. The combination of the τ τ and b¯b results in a combined observed significance of 3.8 σ which gives strong evidence for the direct coupling of the 125 GeV Higgs boson to down-type fermions.
PACS 14.80.Bn – Standard-model Higgs bosons.
1. – Introduction
One of the defining goals of the LHC physics program was to discover the mechanism of electroweak symmetry breaking, which explains the mass of the W and Z bosons when compared to the massless photon. In the standard model (SM) this is done via the Higgs Mechanism, which predicts an additional boson. In 2012 a new boson was discovered at the LHC [1, 2] in decays to γγ, WW, and ZZ with a mass near 125 GeV that had properties like those predicted from a SM Higgs boson.
An important remaining question is whether this new boson also couples to fermions, in particular to down-type fermions. Determination of couplings to down-type fermions requires measuring their direct decays from the Higgs boson. In this paper searches for Higgs decays to μμ, b¯b and τ τ will be discussed followed by a combination of the b¯b and
τ τ searches. c
CERN on behalf the CMS Collaboration
2 Events/1.0 GeV/c 0 100 200 300 400 500 600 700 800 900 Data Background Model 20 × SM Higgs
CMS Preliminary Analysis A: 0,1-Jet Tight BB
-1 L = 19.7 fbs = 8TeV ] 2 ) [GeV/c μ μ M( 110 120 130 140 150 160 Fit Data-Fit -2 -1 0 1 2 /NDF: 1.02 2 χ ] 2 [GeV/c H m 120 130 140 150 )μ μ → (H SM σ/ σ 95% CL Limit on 0 5 10 15 20 25 30 35 40 CMS Preliminary -1 = 7 TeV L = 5.0 fb s -1 = 8 TeV L = 19.7 fb s Combination
Fig. 1. – The dimuon invariant mass at√s = 8 TeV along with the fit used to estimate the background for the 0,1 Jet category with tight selection and both muons in the barrel region (left). Expected and observed 95% CL Limit on σ/σSMas a function of mH(right).
2. – Higgs to μμ
In the SM decays of the Higgs boson to μμ are a very rare process with branching fractions on the order of 2.2× 10−4 at mH= 125 GeV. The CMS μμ analysis [3] is done with a total luminosity of 5.0 fb−1 at 7 TeV and 19.7 fb−1 at 8 TeV. The analysis is split into categories based on the number of reconstructed jets in the event, the transverse momentum of the μμ system and the psuedorapidity of the reconstructed muons.
The search is done looking for a resonant peak in the μμ mass spectrum coming from the combination of the Drell-Yan, t¯t and di-boson pair production backgrounds. A fit
is done to the Mμμ distribution, fig. 1, from the data. The signal is modeled with a
double Gaussian function and the background is parameterized with a polynomial and an exponential term. The analysis shows no significant excess so an observed (expected) limit, fig. 1, is set at 7.4(5.1+2.3−1.5) times the SM expectation at 95% confidence for mH= 125 GeV.
3. – Higgs to b¯b
A SM Higgs boson of mH = 125 GeV has a large branching fraction to b¯b. Along
with this large production rate also comes a large background from multi-jet events,
therefore CMS searches [4] for H→ b¯b focus on Higgs boson production in association
with a W±/Z boson. The CMS b¯b analysis is done with a total luminosity of 5.1 fb−1 at 7 TeV and 18.9 fb−1 at 8 TeV. The associated production analysis is done in a total of six topologies with a Z decaying to an electron pair, muon pair, a pair of neutrinos
or a W decaying to an electron, muon or tau. A search for H→ b¯b with Vector Boson
[GeV] H m 110 115 120 125 130 135 Local p-value -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 1 Observed
Expected from SM Higgs boson
CMS -1 = 7 TeV, L = 5.0 fb s -1 = 8 TeV, L = 18.9 fb s b b → VH; H → pp σ 1 σ 2 σ 3 μ Best fit -2 0 2 4 0.9 ± = 1.1 μ )H(bb) ν τ, ν W(l 1.0 ± = 0.8 μ )H(bb) + l -Z(l 0.8 ± = 1.0 μ )H(bb) ν ν Z( -1 = 7 TeV, L = 5.0 fb s s = 8 TeV, L = 18.9 fb-1 0.5 ± = 1.0 μ Combined CMS b b → VH; H → pp = 125 GeV H m
Fig. 2. – Local p-values and corresponding significance (measured in standard deviations) for the background-only hypothesis to account for the observed excess of events in the data for the b¯b VH analysis (left). The best-fit value of the production cross section for a 125 GeV Higgs boson relative to the standard model cross section, i.e., signal strength μ, for partial combinations of channels and for all channels combined (band) (right).
The associated production analysis utilizes a Boosted Decision Tree (BDT) to discrim-inate the Higgs boson signal from the backgrounds. The BDT includes event kinematics, b-jet tagging discriminators, and various system angles. The events are then categorized based on the transverse momentum of the vector boson. The analysis has a broad excess that is consistent with a Higgs boson of mH= 125 GeV. The excess has a significance of 2.1 standard deviations at mH= 125 GeV with a best fit μ = σ/σSM = 1.0± 0.5 (fig. 2). 4. – Higgs to τ τ
Searches for a Higgs boson decaying to a pair of taus are very important for prob-ing the properties of a SM Higgs boson. The SM predicts a branchprob-ing ratio to taus of approximately 6% for a 125 GeV Higgs boson. Due to smaller backgrounds and better resolution than b¯b searches, τ τ searches offer the highest potential sensitivity for mea-suring direct coupling to fermions. The CMS search [6] is sensitive to gluon gluon fusion (ggF), vector boson fusion (VBF) along with associated Higgs (VH) production modes. Also the search is done in all six possible τ τ decay channels: τμτh, τeτh, τhτh, τeτμ, τμτμ, and τeτe along with dedicated search channels for the VH modes. The CMS τ τ analysis is done with a total luminosity of 4.9 fb−1 at 7 TeV and 19.7 fb−1 at 8 TeV.
The main τ τ search is done in categories, fig. 3, split by the number of jets, transverse
momentum of the τ legs and the transverse momentum of the τ τ system (pτ τ
T), fig. 4. Of particular interest are high-sensitivity categories such as 1-jet categories with pτ τ
T > 100 GeV focusing on boosted ggF events and 2-jet categories requiring high Mjj, Δηjj and pτ τ
T that targets VBF production, fig. 5. The categories are then combined in a
maximum-likelihood fit of the τ τ system mass to search for the presence of a SM Higgs signal.
high-pTτh high-pTτh boosted low-pTτh low-pTτh tight VBF tag (2012 only) high-pTτh loose VBF tag pTτh > 45 GeV low-pTμ low-pTμ tight VBF tag (2012 only) high-pTμ high-pTμ loose VBF tag pTμ > 35 GeV low-pTl low-pTl high-pTl high-pTl 2-jet pTl > 35 GeV
boosted boosted highly VBF tag
pTττ > 170 GeV pT ττ > 100 GeV mjj > 500 GeV |Δηjj| > 3.5 eτh eμ ee, μμ τhτh (8 TeV only) μτh low-pTτh tight VBF tag (2012 only) high-pTτh high-pTτh high-pTτh boosted loose VBF tag pTτh > 45 GeV pTττ > 100 GeV pTττ > 100 GeV mjj > 700 GeV |Δηjj| > 4.0 mjj > 500 GeV |Δηjj| > 3.5 low-pTτh
0-jet 1-jet 2-jet
baseline baseline baseline baseline baseline > 30 GeV pTττ > 100 GeV Emiss T
Fig. 3. – Event categories for the non-VH τ τ analysis.
Important backgrounds in the τ τ search include irreducible Drell Yan to τ τ and reducible backgrounds W plus jets, QCD multi jet, t¯t. Drell Yan to τ τ is estimated using a data embedding technique which starts with real μμ events and replaces the muons with Monte Carlo taus. This technique gives jets, missing energy, pileup and underlying event from the original data which reduces systematic uncertainties. The rest of the important reducible backgrounds are measured using data driven techniques including fake rates and sidebands.
The analysis observes an excess of events over the background-only hypothesis with a local significance, fig. 6, in excess of 3 standard deviations for Higgs boson mass
hy-potheses between mH = 115 and 130 GeV, and equal to 3.2 (3.7) standard deviations
Events 2 10 3 10 4 10 at 8 TeV -1 CMS, 19.7 fb h τ μ Observed Bkg. uncertainty τ τ → Z μ μ → Z Electroweak t t QCD [GeV] h τ T p 30 35 40 45 50 55 60 65 70 75 80 Obs/Bkg 0.8 1.0 1.2 Events 10 2 10 3 10 4 10 5 10 at 8 TeV -1 CMS, 19.7 fb h τ μ Observed Bkg. uncertainty τ τ → Z μ μ → Z Electroweak t t QCD [GeV] τ τ T p 0 50 100 150 200 250 300 Obs/Bkg 0.8 1.0 1.2
Fig. 4. – Observed and predicted distributions for the τμτh channel after event selection for
the transverse momentum of the τh leg (left) and the transverse momentum of the τ τ system
(right). The yields predicted for the various background contributions correspond to the result of the final fit. The “bkg. uncertainty” band represents the combined statistical and systematic uncertainty in the background yield in each bin.
cross section for mH = 125 GeV is 0.78± 0.27 times the standard model expectation.
This constitutes evidence for a Higgs boson coupling to a pair of taus consistent with the boson observed in the ZZ and γγ searches.
5. – τ τ /b¯b combination
In addition to the individual searches for the b¯b and τ τ final states CMS has per-formed a combination [7] of the two searches. This combination results in a combined significance of 3.8 (4.4) standard deviations observed (expected) above the background
only hypothesis for mH = 125 GeV. The summary of this result along with the results
from the individual channels is available in table I. This result gives strong evidence for coupling of the 125 GeV Higgs boson to down-type fermions.
6. – Summary
This paper presents a short review of the existing searches for Higgs to fermions
done by the CMS Collaboration. Searches for a Higgs boson decaying to μμ, b¯b and
τ τ were included. No significant excess in the μμ channel was observed so a limit was
set at 7.4(5.1+2.3−1.5) times the SM expectation at 95% confidence for mH. In the b¯b search an excess of 2.1 standard deviations was observed with a best-fit signal strength of μ = 1.0±0.5. Also in the ττ search an excess is observed with an observed significance of 3.2 standard deviations and a best-fit of μ = 0.78± 0.27 for mH = 125 GeV. Lastly
a combination was performed of the b¯b and the τ τ searches that resulted in a total
observed significance of 3.8 standard deviations. This result gives strong evidence for coupling of the 125 GeV Higgs boson to down-type fermions.
[GeV] τ τ m 0 100 200 300 [1/GeV] ττ dN/dm 0.0 0.1 0.2 0.3 0.4 0.5 at 8 TeV -1 CMS, 19.7 fb h τ μ Tight VBF tag τ τ → SM H(125 GeV) Observed τ τ → Z t t Electroweak QCD Bkg. uncertainty [GeV] τ τ m 0 100 200 300 [1/GeV] ττ dN/dm 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 at 8 TeV -1 CMS, 19.7 fb h τ e Tight VBF tag τ τ → SM H(125 GeV) Observed τ τ → Z ee → Z W+jets t t QCD Bkg. uncertainty [GeV] τ τ m 0 100 200 300 [1/GeV]ττ dN/dm 0.0 0.1 0.2 0.3 0.4 0.5 at 8 TeV -1 CMS, 19.7 fb μ e Tight VBF tag τ τ → SM H(125 GeV) Observed WW → SM H(125 GeV) τ τ → Z t t Electroweak μ Misidentified e/ Bkg. uncertainty [GeV] τ τ m 0 100 200 300 [1/GeV]ττ dN/dm 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 at 8 TeV -1 CMS, 19.7 fb h τ h τ VBF tag τ τ → SM H(125 GeV) Observed τ τ → Z t t Electroweak QCD Bkg. uncertainty
Fig. 5. – Mass distribution for the tight VBF categories in the H→ ττ search after the maximum-likelihood fit. τμτh (top left), τeτh(top right), τeτμ(bottom left), τhτh(bottom right).
[GeV] H m 100 120 140 Local p-value -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 1 σ 5 σ 4 σ 3 σ 2 σ 1 Observed p-value Expected for SM H(125 GeV)
σ
1
±
Expected for SM H(125 GeV)
σ 2 ± at 8 TeV -1 at 7 TeV, 19.7 fb -1 , 4.9 fb τ τ → CMS (unpublished) H SM σ / σ
Best fit for
0 2 4 0-jet 1.09 ± 0.34 1-jet 0.46 ± 1.07 2-jet (VBF tag) 0.41 ± 0.94 h τ L + l ' + LL + ll 1.02 ± -0.33 τ τ → H 0.27 ± 0.78 = 125 GeV H m at 8 TeV -1 at 7 TeV, 19.7 fb -1 CMS, 4.9 fb [GeV] τ τ m 0 100 200 300 [1/GeV] ττ S / (S+B) W eighted dN/dm 0 500 1000 1500 2000 2500 τ τ → SM H(125 GeV) Observed τ τ → Z t t Electroweak QCD [GeV] τ τ m 0 100 200 300 -40 -20 0 20 40 SM H(125 GeV)→ττ Data - background Bkg. uncertainty at 8 TeV -1 at 7 TeV, 19.7 fb -1 CMS, 4.9 fb μ , e h τ h τ , h τ , e h τ μ
Fig. 6. – Top left: Local p-value and significance in number of standard deviations as a function of the SM Higgs boson mass for the combination of all decay channels. The observation (solid line) is compared to the red bands which correspond to 1 and 2 standard deviations from the expectation. Top right: Best-fit signal strength values, for independent categories for mH =
125 GeV. The combined value for the τ τ analysis in both plots corresponds to μ = 0.78± 0.27. The dashed line corresponds to the best-fit μ value. Bottom: The combined mass distribution for the τμτh, τeτh, τhτh, τeτμchannels. The distributions obtained in each category of each channel
are weighted by the ratio between the expected signal and signal-plus-background yields in each category. The inset shows the corresponding difference between the observed data and expected background distributions, together with the signal distribution for a SM Higgs boson at mH= 125 GeV.
TableI. – Significance and best-fit μ value for the b¯b and τ τ channels and their combination.
Channel Significance (σ) Best-fit
mH= 125 GeV Expected Observed μ
VH→ b¯b 2.3 2.1 1.0± 0.5
H→ ττ 3.7 3.2 0.78± 0.27
Combined 4.4 3.8 0.83± 0.24
REFERENCES
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