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| CMS-EXO-14-008 ; CERN-PH-EP-2015-063 | ||
| Search for third-generation scalar leptoquarks in the t$\tau$ channel in proton-proton collisions at $\sqrt{s}= $ 8 TeV | ||
| CMS Collaboration | ||
| 31 March 2015 | ||
| J. High Energy Phys. 07 (2015) 042 [Erratum] | ||
| Abstract: A search for pair production of third-generation scalar leptoquarks decaying to top quark and $\tau$ lepton pairs is presented using proton-proton collision data at a center-of-mass energy of $\sqrt{s}=$ 8 TeV collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The search is performed using events that contain an electron or a muon, a hadronically decaying $\tau$ lepton, and two or more jets. The observations are found to be consistent with the standard model predictions. Assuming that all leptoquarks decay to a top quark and a $\tau$ lepton, the existence of pair produced, charge $-1/3$, third-generation leptoquarks up to a mass of 685 GeV is excluded at 95% confidence level. This result constitutes the first direct limit for leptoquarks decaying into a top quark and a $\tau$ lepton, and may also be applied directly to the pair production of bottom squarks decaying predominantly via the R-parity violating coupling $\lambda^{\prime}_{333}$. | ||
| Links: e-print arXiv:1503.09049 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; Public twiki page ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
One of the $ {\mathrm {LQ}_3} {\overline {\mathrm {LQ}}_3} $ decay chains with both same-sign and opposite-sign $\ell {\tau _\mathrm {h}} $ pairs. Labels u and d denote up and down type quarks, and $\ell $ denotes an electron or a muon. |
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Figure 2:
Comparison between data and simulation in the $| \eta | $ (left) and $ {S_\mathrm {T}} $ (right) distributions using the signal-depleted selection of events in category A with a same-sign $\mu {\tau _\mathrm {h}} $ pair. Other backgrounds refer to contributions predominantly from processes such as diboson and single top quark production, as well as QCD multijet and other rare SM processes detailed in Section {DataMCsamples}. The hatched regions in the distributions and the shaded bands in the Data/MC ratio plots represent the statistical uncertainties in the expectations. The data-simulation agreement is observed to be within 20%, and is assigned as the normalization systematic uncertainty for the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $+jets, DY+jets and diboson contributions in the signal region. |
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Figure 2-a:
Comparison between data and simulation in the $| \eta | $ (left) and $ {S_\mathrm {T}} $ (right) distributions using the signal-depleted selection of events in category A with a same-sign $\mu {\tau _\mathrm {h}} $ pair. Other backgrounds refer to contributions predominantly from processes such as diboson and single top quark production, as well as QCD multijet and other rare SM processes detailed in Section {DataMCsamples}. The hatched regions in the distributions and the shaded bands in the Data/MC ratio plots represent the statistical uncertainties in the expectations. The data-simulation agreement is observed to be within 20%, and is assigned as the normalization systematic uncertainty for the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $+jets, DY+jets and diboson contributions in the signal region. |
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Figure 2-b:
Comparison between data and simulation in the $| \eta | $ (left) and $ {S_\mathrm {T}} $ (right) distributions using the signal-depleted selection of events in category A with a same-sign $\mu {\tau _\mathrm {h}} $ pair. Other backgrounds refer to contributions predominantly from processes such as diboson and single top quark production, as well as QCD multijet and other rare SM processes detailed in Section {DataMCsamples}. The hatched regions in the distributions and the shaded bands in the Data/MC ratio plots represent the statistical uncertainties in the expectations. The data-simulation agreement is observed to be within 20%, and is assigned as the normalization systematic uncertainty for the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $+jets, DY+jets and diboson contributions in the signal region. |
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Figure 3:
Comparison between data and simulation in the leading $\tau $ lepton $ {p_{\mathrm {T}}} $ distributions using the signal-depleted selection of events in category B in the $\mu {\tau _\mathrm {h}} $ channel (left) and in the $\mathrm{ e } {\tau _\mathrm {h}} $ channel (right). Other backgrounds refer to contributions predominantly from processes such as diboson and single top quark production, but also include QCD multijet and rare SM processes detailed in the text. The hatched regions in the distributions and the shaded bands in the Data/MC ratio plots represent the statistical uncertainties in the expectations. |
png pdf |
Figure 3-a:
Comparison between data and simulation in the leading $\tau $ lepton $ {p_{\mathrm {T}}} $ distributions using the signal-depleted selection of events in category B in the $\mu {\tau _\mathrm {h}} $ channel (left) and in the $\mathrm{ e } {\tau _\mathrm {h}} $ channel (right). Other backgrounds refer to contributions predominantly from processes such as diboson and single top quark production, but also include QCD multijet and rare SM processes detailed in the text. The hatched regions in the distributions and the shaded bands in the Data/MC ratio plots represent the statistical uncertainties in the expectations. |
png pdf |
Figure 3-b:
Comparison between data and simulation in the leading $\tau $ lepton $ {p_{\mathrm {T}}} $ distributions using the signal-depleted selection of events in category B in the $\mu {\tau _\mathrm {h}} $ channel (left) and in the $\mathrm{ e } {\tau _\mathrm {h}} $ channel (right). Other backgrounds refer to contributions predominantly from processes such as diboson and single top quark production, but also include QCD multijet and rare SM processes detailed in the text. The hatched regions in the distributions and the shaded bands in the Data/MC ratio plots represent the statistical uncertainties in the expectations. |
png pdf |
Figure 4:
The $ {S_\mathrm {T}} $, $\tau $ lepton $ {p_{\mathrm {T}}} $, and jet multiplicity distributions in the signal region of category A for central (left column) and forward (right column) channels, using the optimized selection for $M_{ {\mathrm {LQ}_3} }= $ 200 GeV (all other optimized selection criteria yield events that are a subset of this selection). The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. The systematic uncertainty for each bin of these distributions is determined independently. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. |
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Figure 4-a:
The $ {S_\mathrm {T}} $, $\tau $ lepton $ {p_{\mathrm {T}}} $, and jet multiplicity distributions in the signal region of category A for central (left column) and forward (right column) channels, using the optimized selection for $M_{ {\mathrm {LQ}_3} }= $ 200 GeV (all other optimized selection criteria yield events that are a subset of this selection). The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. The systematic uncertainty for each bin of these distributions is determined independently. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. |
png pdf |
Figure 4-b:
The $ {S_\mathrm {T}} $, $\tau $ lepton $ {p_{\mathrm {T}}} $, and jet multiplicity distributions in the signal region of category A for central (left column) and forward (right column) channels, using the optimized selection for $M_{ {\mathrm {LQ}_3} }= $ 200 GeV (all other optimized selection criteria yield events that are a subset of this selection). The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. The systematic uncertainty for each bin of these distributions is determined independently. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. |
png pdf |
Figure 4-c:
The $ {S_\mathrm {T}} $, $\tau $ lepton $ {p_{\mathrm {T}}} $, and jet multiplicity distributions in the signal region of category A for central (left column) and forward (right column) channels, using the optimized selection for $M_{ {\mathrm {LQ}_3} }= $ 200 GeV (all other optimized selection criteria yield events that are a subset of this selection). The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. The systematic uncertainty for each bin of these distributions is determined independently. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. |
png pdf |
Figure 4-d:
The $ {S_\mathrm {T}} $, $\tau $ lepton $ {p_{\mathrm {T}}} $, and jet multiplicity distributions in the signal region of category A for central (left column) and forward (right column) channels, using the optimized selection for $M_{ {\mathrm {LQ}_3} }= $ 200 GeV (all other optimized selection criteria yield events that are a subset of this selection). The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. The systematic uncertainty for each bin of these distributions is determined independently. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. |
png pdf |
Figure 4-e:
The $ {S_\mathrm {T}} $, $\tau $ lepton $ {p_{\mathrm {T}}} $, and jet multiplicity distributions in the signal region of category A for central (left column) and forward (right column) channels, using the optimized selection for $M_{ {\mathrm {LQ}_3} }= $ 200 GeV (all other optimized selection criteria yield events that are a subset of this selection). The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. The systematic uncertainty for each bin of these distributions is determined independently. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. |
png pdf |
Figure 4-f:
The $ {S_\mathrm {T}} $, $\tau $ lepton $ {p_{\mathrm {T}}} $, and jet multiplicity distributions in the signal region of category A for central (left column) and forward (right column) channels, using the optimized selection for $M_{ {\mathrm {LQ}_3} }= $ 200 GeV (all other optimized selection criteria yield events that are a subset of this selection). The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. The systematic uncertainty for each bin of these distributions is determined independently. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. |
png pdf |
Figure 5:
The leading $\tau $ lepton $ {p_{\mathrm {T}}} $, $ {S_\mathrm {T}} $, and jet multiplicity distributions in the signal region of category B for $\mu {\tau _\mathrm {h}} $ (left column) and $\mathrm{ e } {\tau _\mathrm {h}} $ (right column) channels. The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. The four regions of the $\tau $ lepton $ {p_{\mathrm {T}}} $ correspond to the four search regions. |
png pdf |
Figure 5-a:
The leading $\tau $ lepton $ {p_{\mathrm {T}}} $, $ {S_\mathrm {T}} $, and jet multiplicity distributions in the signal region of category B for $\mu {\tau _\mathrm {h}} $ (left column) and $\mathrm{ e } {\tau _\mathrm {h}} $ (right column) channels. The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. The four regions of the $\tau $ lepton $ {p_{\mathrm {T}}} $ correspond to the four search regions. |
png pdf |
Figure 5-b:
The leading $\tau $ lepton $ {p_{\mathrm {T}}} $, $ {S_\mathrm {T}} $, and jet multiplicity distributions in the signal region of category B for $\mu {\tau _\mathrm {h}} $ (left column) and $\mathrm{ e } {\tau _\mathrm {h}} $ (right column) channels. The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. The four regions of the $\tau $ lepton $ {p_{\mathrm {T}}} $ correspond to the four search regions. |
png pdf |
Figure 5-c:
The leading $\tau $ lepton $ {p_{\mathrm {T}}} $, $ {S_\mathrm {T}} $, and jet multiplicity distributions in the signal region of category B for $\mu {\tau _\mathrm {h}} $ (left column) and $\mathrm{ e } {\tau _\mathrm {h}} $ (right column) channels. The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. The four regions of the $\tau $ lepton $ {p_{\mathrm {T}}} $ correspond to the four search regions. |
png pdf |
Figure 5-d:
The leading $\tau $ lepton $ {p_{\mathrm {T}}} $, $ {S_\mathrm {T}} $, and jet multiplicity distributions in the signal region of category B for $\mu {\tau _\mathrm {h}} $ (left column) and $\mathrm{ e } {\tau _\mathrm {h}} $ (right column) channels. The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. The four regions of the $\tau $ lepton $ {p_{\mathrm {T}}} $ correspond to the four search regions. |
png pdf |
Figure 5-e:
The leading $\tau $ lepton $ {p_{\mathrm {T}}} $, $ {S_\mathrm {T}} $, and jet multiplicity distributions in the signal region of category B for $\mu {\tau _\mathrm {h}} $ (left column) and $\mathrm{ e } {\tau _\mathrm {h}} $ (right column) channels. The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. The four regions of the $\tau $ lepton $ {p_{\mathrm {T}}} $ correspond to the four search regions. |
png pdf |
Figure 5-f:
The leading $\tau $ lepton $ {p_{\mathrm {T}}} $, $ {S_\mathrm {T}} $, and jet multiplicity distributions in the signal region of category B for $\mu {\tau _\mathrm {h}} $ (left column) and $\mathrm{ e } {\tau _\mathrm {h}} $ (right column) channels. The rightmost bin of each distribution includes overflow and no statistically significant excess is observed in the suppressed bins. Shaded regions in the histograms represent the total statistical and systematic uncertainty in the background expectation. The Z-score distribution is provided at the bottom of each plot. The four regions of the $\tau $ lepton $ {p_{\mathrm {T}}} $ correspond to the four search regions. |
png pdf |
Figure 6:
The expected and observed exclusion limits at 95% CL on the $ {\mathrm {LQ}_3} $ pair production cross section times $\beta ^{2}$ in category A (upper left), category B (upper right) and the combination of the two categories (lower left). The theoretical uncertainty in the LQ pair production cross section includes the PDF and renormalization/factorization scale uncertainties as prescribed in Ref. [19]. The expected and observed limits on the LQ branching fraction $\beta $ as a function of the LQ mass (lower right). The total excluded region (shaded) is obtained by including the results in Ref. [17], reinterpreted for the $ {\mathrm {LQ}_3} \to \mathrm{ b } \nu $ scenario. |
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Figure 6-a:
The expected and observed exclusion limits at 95% CL on the $ {\mathrm {LQ}_3} $ pair production cross section times $\beta ^{2}$ in category A (upper left), category B (upper right) and the combination of the two categories (lower left). The theoretical uncertainty in the LQ pair production cross section includes the PDF and renormalization/factorization scale uncertainties as prescribed in Ref. [19]. The expected and observed limits on the LQ branching fraction $\beta $ as a function of the LQ mass (lower right). The total excluded region (shaded) is obtained by including the results in Ref. [17], reinterpreted for the $ {\mathrm {LQ}_3} \to \mathrm{ b } \nu $ scenario. |
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Figure 6-b:
The expected and observed exclusion limits at 95% CL on the $ {\mathrm {LQ}_3} $ pair production cross section times $\beta ^{2}$ in category A (upper left), category B (upper right) and the combination of the two categories (lower left). The theoretical uncertainty in the LQ pair production cross section includes the PDF and renormalization/factorization scale uncertainties as prescribed in Ref. [19]. The expected and observed limits on the LQ branching fraction $\beta $ as a function of the LQ mass (lower right). The total excluded region (shaded) is obtained by including the results in Ref. [17], reinterpreted for the $ {\mathrm {LQ}_3} \to \mathrm{ b } \nu $ scenario. |
png pdf |
Figure 6-c:
The expected and observed exclusion limits at 95% CL on the $ {\mathrm {LQ}_3} $ pair production cross section times $\beta ^{2}$ in category A (upper left), category B (upper right) and the combination of the two categories (lower left). The theoretical uncertainty in the LQ pair production cross section includes the PDF and renormalization/factorization scale uncertainties as prescribed in Ref. [19]. The expected and observed limits on the LQ branching fraction $\beta $ as a function of the LQ mass (lower right). The total excluded region (shaded) is obtained by including the results in Ref. [17], reinterpreted for the $ {\mathrm {LQ}_3} \to \mathrm{ b } \nu $ scenario. |
png pdf |
Figure 6-d:
The expected and observed exclusion limits at 95% CL on the $ {\mathrm {LQ}_3} $ pair production cross section times $\beta ^{2}$ in category A (upper left), category B (upper right) and the combination of the two categories (lower left). The theoretical uncertainty in the LQ pair production cross section includes the PDF and renormalization/factorization scale uncertainties as prescribed in Ref. [19]. The expected and observed limits on the LQ branching fraction $\beta $ as a function of the LQ mass (lower right). The total excluded region (shaded) is obtained by including the results in Ref. [17], reinterpreted for the $ {\mathrm {LQ}_3} \to \mathrm{ b } \nu $ scenario. |
| Tables | |
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Table 1:
Summary of the search strategies in event categories A and B. |
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Table 2:
Systematic uncertainty sources and their effects on background ($B$) and signal ($S$) estimates. Uncertainties affecting the signal yields in both categories and the background yields in category A are calculated using the selection criteria for the $M_{ {\mathrm {LQ}_3} }= $ 550 GeV hypothesis. In category A, the uncertainties are reported for central/forward channels separately, where appropriate. In category B, all uncertainties are averaged over the four $ {p_{\mathrm {T}}} ^\tau $ search bins. All values are symmetric except for the PDF uncertainty in the signal acceptance in category A, and the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $ factorization and normalization scale uncertainty in category B. The $\tau $ misidentification rate uncertainties considered in category B are included in the matrix method uncertainty in category A. All uncertainties in the background estimates are scaled according to their relative contributions to the total expected background. |
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Table 3:
Category A search results in the signal region for several $ {\mathrm {LQ}_3} $ mass hypotheses. The $\tau $ lepton $ {p_{\mathrm {T}}} $ and $ {S_\mathrm {T}} $ columns represent the optimized thresholds defined in the text. The corresponding expected number of prompt-prompt and total background events, as well as the observed number of data events are listed as $N_{\text {Bkg}}^{\mathrm {PP}}$, total $N_{\text {Bkg}}^{\text {Exp}}$, and $N^{\text {Obs}}$. The statistical and systematic uncertainties quoted in the expected number of background events are combinations of misidentified lepton and prompt-prompt components. The $\epsilon _{ {\mathrm {LQ}_3} }$ is the expected signal efficiency at a given $ {\mathrm {LQ}_3} $ mass with respect to the total number of expected $ {\mathrm {LQ}_3} $ signal events at $\sqrt {s}=$ 8 TeV with a $\mu {\tau _\mathrm {h}} $ pair of any charge combination. No expected signal efficiency for $M_{ {\mathrm {LQ}_3} }=$ 200 GeV is reported in the forward channel since the associated yield in the signal sample was measured to be zero. |
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Table 4:
Category B search results for the four $ {p_{\mathrm {T}}} ^\tau $ search regions of the $\mu {\tau _\mathrm {h}} $ channel. All expected values for background and signal processes ($ {\mathrm {LQ}_3} $ masses indicated in parentheses) are reported with the corresponding statistical and systematic uncertainties. The expected signal efficiency $\epsilon _{ {\mathrm {LQ}_3} }$ at a given $ {\mathrm {LQ}_3} $ mass is determined with respect to the total number of expected $ {\mathrm {LQ}_3} $ signal events at $\sqrt {s}=8$ TeV with a $\mu {\tau _\mathrm {h}} $ pair of any charge combination, and $\epsilon _{ {\mathrm {LQ}_3} }$ is reported separately for opposite-sign (OS) and same-sign (SS) $\mu {\tau _\mathrm {h}} $ events. |
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Table 5:
Category B search results for the four $ {p_{\mathrm {T}}} ^\tau $ search regions of the $\mathrm{ e } {\tau _\mathrm {h}} $ channel. All expected values for background and signal processes ($ {\mathrm {LQ}_3} $ masses indicated in parentheses) are reported with the corresponding statistical and systematic uncertainties. The expected signal efficiency $\epsilon _{ {\mathrm {LQ}_3} }$ at a given $ {\mathrm {LQ}_3} $ mass is determined with respect to the total number of expected $ {\mathrm {LQ}_3} $ signal events at $\sqrt {s}=8$ TeV with an $\mathrm{ e } {\tau _\mathrm {h}} $ pair of any charge combination. |
| Summary |
| A search for pair produced, charge $-1/3$, third-generation scalar leptoquarks decaying to top quark and $\tau$ lepton pairs has been conducted in the $\ell\tau_{\mathrm{h}}$ channel with two or more jets, using a proton-proton collisions data sample collected with the CMS detector at $\sqrt{s}=8$ TeV corresponding to an integrated luminosity of 19.7 fb$^{-1}$. No statistically significant excess is observed over the SM background expectations. Assuming that all leptoquarks decay to a top quark and a $\tau$ lepton, the pair production of charge $-1/3$, third-generation scalar leptoquarks is excluded at 95% CL for masses up to 685 GeV (695 GeV expected). This constitutes the first direct result for leptoquarks decaying in this channel, and the mass limit is also directly applicable to pair produced bottom squarks decaying via the RPV coupling $\lambda_{333}^{\prime}$. |
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