Search for New Physics in Dijet Resonant Signatures and Recent Results from Run 2 with the CMS Experiment

138

Proceedings of the LHCP2015 Conference, St. Petersburg, Russia, August 31 - September 5, 2015

Editors: V.T. Kim and D.E. Sosnov

Search for new physics in dijet resonant signatures and recent results from Run 2 with the CMS experiment

GIULIA D’IMPERIO

Universit`aLa Sapienza and INFN Roma

On behalf of the CMS Collaboration

Abstract. A search for narrow resonances in proton-proton collisions at a center-of-mass energy of √s = 13 TeV is presented. The dijet invariant mass distribution of the two leading jets is measured with the CMS detector using early data from Run 2 of the Large Hadron Collider. The dataset presented here was collected in July 2015 and corresponds to an integrated luminosity of 42 1 pb− . The highest observed dijet mass is 5.4 TeV. The spectrum is well described by a smooth parameterization and no evidence for new particle production is observed. Upper limits at a 95% conﬁdence level are set on the cross section of narrow resonances with masses above 1.3 TeV. When interpreted in the context of speciﬁc models the limits exclude: string resonances with masses below 5.1 TeV; scalar diquarks below 2.7 TeV; axigluons and colorons below 2.7 TeV; excited quarks below 2.7 TeV; and color octet scalars below 2.3 TeV.

Introduction

Deep inelastic proton-proton (pp) collisions often produce two or more energetic jets when the constituent partons are scattered with large transverse momenta (pT ). The invariant mass of the two jets with the largest pT in the event (the dijet) has a spectrum that is predicted by quantum chromodynamics (QCD) to fall steeply and smoothly with increasing dijet mass (m jj) [1]. Many extensions of the standard model predict the existence of resonances at the TeV scale that couple to partons (quarks and/or gluons), and therefore accessible at the proton-proton collisions of the Large Hadron Collider (LHC). The object of this search are narrow resonances, with small natural width compared to the experimental resolution, decaying to a pair of partons. The two partons will result in the detector as a pair of back-to-back hadronic jets in the transverse plane. If they come from the decay of a resonance, a bump corresponding to the mass of the resonance over the smoothly falling dijet mass distribution of the QCD processes will appear. The search is extremely powerful and promising at the beginning of LHC run 2, since the new center-of-mass energy of √s = 13 TeV is almost twice with respect to the past. The increase in energy corresponds to much larger cross sections, especially at high mass, as can be seen in Fig. 1 left, where the ratio of the parton luminosity factor between 13 and 8 TeV is shown as a function of the resonance mass 1 MX. The results presented here make use of a 42 pb− dataset collected by the CMS experiment [2] in the first three weeks of data taking in July 2015. These data already exceed the sensitivity of the 2012 dijet search for resonance masses greater than about 5 TeV, as it is shown in Fig. 1 right. Figure 1 right [3] and represents an estimate of the system mass that can be probed in BSM searches at one collider setup (“collider 2”, e.g. LHC 8 TeV with 20 fb−1) given an established system mass reach of some other 1 collider setup (“collider 1”, e.g. LHC 13 TeV with 0.04 fb− .). The mass where the grey diagonal line crosses the green (different green lines for different final states), indicates≈ the point where the two setups have the same sensitivity. Where the green lines go below the grey, the 13 TeV datasets start to be more statistically powerful than the 8 TeV full dataset. This happens around 5 TeV if we consider the average of all partonic channels. G. D’Imperio, Search for new physics in dijet resonant signatures and recent results from Run 2 with ... 139

Proceedings of the LHCP2015 Conference, St. Petersburg, Russia, August 31 - September 5, 2015

WJS2013 100 7 http://cern.ch/collide Editors: V.T. Kim and D.E. Sosnov q q ratios of LHC parton luminosities: 13 TeV / 8 TeV -1 Σ q q- 6 q ig i i g g r-reach by G.P. Salam gg_ 5 Σqq qg 4 10 and A.and Weiler or 8.00 TeV, 20.00 fb or 8.00 TeV,

luminosity ratio 2

MSTW2008NLO f system[TeV] mass 1 100 1000 0 M (GeV) 0 1 2 3 4 5 6 7 X system mass [TeV] for 13.00 TeV, 0.04 fb-1

FIGURE 1. Left: the parton luminosity ratio between √s = 13 TeV and 8 TeV as a function of the resonance mass MX for qq, qg and gg final states. The ratio increases quickly at high dijet mass. Right: estimate of the system mass that can be probed in 1 BSM searches at LHC run 1 setup (on y-axis √s = 8 TeV with 20 fb− ) given the system mass reach of run 2 setup (on x-axis 1 √s = 13 TeV with 0.04 fb− ). The different green lines indicates the different final states (qq, qg, gg). ≈ Jet reconstruction

2 2 The dijet analysis uses Particle Flow [4] jets with anti-kT algorithm [5] and width parameter ∆R = ∆η +∆φ = 0.4 (PF ak4 jets). This definition satisfies the requirements of infrared and collinear safety, and the jet energy is corrected using MC and data-based techniques in order to take into account the pile-up extra energy, the non-uniformity of the response across the detector and the residual difference in the absolute scale of the energy between data and MC. CMS has developed jet quality criteria (“Jet ID”) for PF jets which are found to retain the vast majority of real jets in the simulation while rejecting most fake jets arising from calorimeter and/or readout electronics noise. In addition to the Jet ID, all the PF ak4 jest are required to have a minimum pT > 30 GeV and to be in the tracker coverage region η < 2.5. For the leading jet the pT cut is pT > 60 GeV. | | The dijet analysis choice, as in the past, is to recluster in a larger cone the corrected PF ak4 jets that pass the selection described above, and use wide jets to reconstruct the invariant mass of the dijet system. This allows to contain better the energy of the hadrons in presence of final state radiation (FSR), and thus improves the dijet mass resolution with the resonance peak resulting both closer to the nominal mass and narrower. In the phase of the analysis preparation the optimization of the cone width has been studied in order to minimize the expected upper limits on the cross sections and the value ∆R = 1.1 is found to be optimal.

Event selection and trigger studies

The most relevant selection criteria are: Wide i The dijet mass calculated using wide jets m jj > 1.2TeV. During run 1 this cut was set to 890 GeV. ii The angular separation between the two wide jets ∆ηWide < 1.3. During run 1 this cut was the same. Opti- | jj | mization studies on 13 TeV Monte Carlo indicate that this value is still optimal. The reason of (i) is that the trigger turn-on curve is complete around 1.2 TeV (see below in this section). The requirement (ii) is a cut on the ∆ηWide between the jets. This quantity is related to the emission angle of the ﬁnal | jj | partons with respect to the beam line in the center-of-mass reference frame (the scattering angle θ∗): ∆η cos θ∗ = tanh( ) (1) 2 and the cut ∆ηWide < 1.3 corresponds to cos θ < 0.57. This criterion is introduced to improve the signal over | jj | ∗ background ratio, excluding the region close to cos θ∗ = 1 where most of the QCD processes concentrates. The analysis, with this choice, remains inclusive with respect to diﬀerent new physics hypotheses. 140 LHCP2015 Conference, St. Petersburg, Russia, August 31 - September 5, 2015

The event with the highest dijet mass passing the full selection is shown in Fig. 2 left. The dijet mass of this clean dijet event is 5.4 TeV, greater then the highest mass event observed in 2012 (of 5.2 TeV), confirming that this analysis exceeds the run 1 sensitivity above 5 TeV. The PFHT800 is the main unprescaled≈ trigger that is used for this analysis. The trigger selection is based on the scalar sum of the transverse momenta of all the jets in the event (HT ) with a threshold around 800 GeV and carries a large part of the fully hadronic physics at CMS. The PFHT475 is a prescaled path, based on the same HT selection of the main unprescaled one, with a lower threshold (around 450 GeV). It used as reference to study the relative trigger efficiency, since its turn-on region is far enough from the one of PFHT800, and it has a relatively small prescale ( (100)), that allows to collect a sufficient statistics to study the turn on region of the main trigger. The turn on curve O Wide as a function of m jj is shown in Fig. 2 right.

2015, 13 TeV

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FIGURE 2. Left: display of the highest dijet mass event in the ρ-φ view of the CMS detector. Right: Trigger relative eﬃciency as Wide function of m jj .

The study of the trigger eﬃciency curve is important to decide where the ﬁt to the dijet mass distribution in data can start, without having distortions in the low mass region due to the trigger turn on. The turn on curve of the Wide PFHT800 trigger is complete for m jj greater than about 1.2 TeV.

Data quality checks

The most relevant analysis variables are compared to a leading-order (LO) QCD Monte Carlo (MC) prediction from Pythia8 (v205) [6] with the CUETP8M1 tune [7, 8], including a GEANT 4 based [9] simulation of the CMS detector. To check the quality and stability of data some jet and event-related quantities are also monitored as a function of time. Figure 3 shows:

the data-MC comparison for the kinematic variables of the two leading jets: transverse momentum (pT ), pseu- • dorapidity (η) and azimuthal angle (φ); the data-MC comparison for the main dijet event variables: the separation in pseudorapidity (∆η) and in the • azimuthal angle (∆φ) between the two leading jets; the observed dijet cross section as a function of time (run index). • The Monte Carlo simulation is scaled to the integral of data. The shapes of pT , η and φ of the two leading jets and the angular distance between them result in good agreement with simulation. The measured dijet cross section is ﬂat versus time, conﬁrming that the data are stable and we do not observe unexpected features.

Analysis strategy

We search for narrow resonances in the dijet mass spectrum. For narrow we mean that the natural resonance width is small compared to the CMS dijet mass resolution. G. D’Imperio, Search for new physics in dijet resonant signatures and recent results from Run 2 with ... 141

CMS Preliminary s = 13 TeV L = 41.8 pb-1 CMS Preliminary s = 13 TeV L = 41.8 pb-1 CMS Preliminary s = 13 TeV L = 41.8 pb-1 700 4 |η|<2.5 |η|<2.5 Data 10 Data |∆ η|<1.3 Data |∆ η|<1.3 MC

Entries MC 1200Entries Mjj > 1.1 TeV MC Entries 600 Mjj > 1.1 TeV |η|<2.5 103 |∆ η|<1.3 1000 500 Mjj > 1.1 TeV 800 400 102 600 300

10 400 200

200 100 1 0 0 0 500 1000 1500 2000 2500 3000 3500 −3 −2 −1 0 1 2 3 −3 −2 −1 0 1 2 3 p leading jets (GeV) T η leading jets φ leading jets

-1 -1 CMS Preliminary s = 13 TeV L = 41.8 pb CMS Preliminary s = 13 TeV L = 41.8 pb CMS Preliminary s = 13 TeV L = 41.8 pb-1

|η|<2.5 Data |η|<2.5 Data 800 |η|<2.5

Entries |∆ η|<1.3 MC Entries 103 |∆ η|<1.3 MC |∆ η|<1.3 Mjj > 1.1 TeV Mjj > 1.1 TeV 700 Mjj > 1.1 TeV 600 2 10 500 Cross Section (pb)

400

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200 1 102 100 0 0 0.2 0.4 0.6 0.8 1 1.2 0 1 2 3 4 5 6 0 1 2 3 4 5 6 7 8 Dijet ∆ η Dijet ∆ φ Run index

FIGURE 3. Top: kinematic distributions of the two leading jets (pT , η, φ) in data and QCD simulation. Bottom: the dijet angular separation ∆η (left) and ∆φ (middle) in data and QCD simulation, and the observed dijet cross section as a function of the run index (right).

Figure 4 shows the dijet mass distributions for these signals using Pythia 8 and the CMS detector simulation. The predicted mass distributions have a Gaussian core coming from the jet energy resolution and a tail towards lower mass values coming primarily from QCD radiation. The contribution of this low-mass tail to the line-shape depends on the parton content of the resonance (qq, qg, or gg). Resonances containing gluons, which are more susceptible to QCD radiation than quarks, have a more pronounced tail. For high-mass resonances, there is also another significant contribution depending on both PDFs and the natural width of the Breit-Wigner resonance shape. For resonances produced by interactions of non-valence partons in the proton, the low-mass component of the Breit-Wigner resonance shape is amplified by the rise of the parton probability distribution at low fractional momentum. This effect causes a large tail at low mass values. Figure 5 shows the measured differential cross section as a function of dijet mass in predefined bins corresponding to the dijet mass resolution [10]. The data are compared to a LO QCD Monte Carlo prediction (the same used for Fig. 3). To test the smoothness of our measured cross section as a function of dijet mass, we fit the data with the parameterization dσ P (1 x)P1 = 0 , −P (2) dm jj x 2 with x = m jj/ √s and three free parameters P0, P1, P2. This functional form is a modified version of the 4-parameter function used in previous searches [10–20] to describe both data and QCD predictions. With a Fisher-test [21] it has th 1 been proved that the 4 parameter is not necessary to describe the dataset od 42 pb− presented here. In Fig. 5 we show the result of the binned maximum likelihood fit, which has a chi-squared (χ2) of 24 for 27 degrees of freedom when excluding the empty bins. The difference between the data and the fit is also shown at the bottom of Fig. 5, and that difference is normalized to the statistical uncertainty of the data in each bin. The data are well described by the fit.

Results

Figure 6 shows the model-independent observed and expected upper limits at a 95% conﬁdence level (CL) on σ B A, i.e. the product of the cross section (σ), the branching fraction (B), and the acceptance (A), for the kinematic× × 142 LHCP2015 Conference, St. Petersburg, Russia, August 31 - September 5, 2015

(13 TeV) 0.0045 CMS 0.004 Simulation Preliminary

0.0035

0.003 WideJets |η|<2.5 & |∆η |<1.3 0.0025 jj Quark-Quark Quark-Gluon 0.002 Normalized Yield / GeV Gluon-Gluon

0.0015

0.001

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0 0 1000 2000 3000 4000 5000 6000 7000 8000 Dijet Mass [GeV]

FIGURE 4. The reconstructed resonance mass spectrum generated with the Pythia MC simulation for quark-quark resonances modeled by the process qq RS graviton qq (solid), for quark-gluon resonances modeled by qg excited quark qg → → → → (dotted), and for gluon-gluon resonances modeled by gg RS graviton gg (dashed) for resonance masses of 1, 3, 5 and 7 TeV. → →

42 pb-1(13 TeV)

CMS 1 Preliminary

data 10-1 background ﬁt to data [pb / GeV]

jj QCD MC

q* (4.5 TeV) 10-2 / dm σ d 10-3 |η| < 2.5, | ηΔ | < 1.3

Mjj> 1.1 TeV -4 10 Wide Jets

10-5

3 σ 2 1 1500 2000 2500 3000 3500 4000 4500 5000 5500 0 -1 Dijet Mass (GeV) -2 -3 (Data-Fit)/ 1500 2000 2500 3000 3500 4000 4500 5000 5500 Dijet Mass [GeV]

FIGURE 5. Dijet mass spectrum using wide jets (points) compared to a smooth fit (solid line) and to predictions [6] including detector simulation of QCD (dashed line) and expectations of one narrow resonance signal at the value of resonance mass excluded by this analysis with 95% CL. The QCD prediction has been normalized to the data. The error bars are statistical only. The bin-by- bin fit residuals divided by the uncertainty of the data, (data fit)/σ , are shown at the bottom. − data G. D’Imperio, Search for new physics in dijet resonant signatures and recent results from Run 2 with ... 143

requirements ∆ηWide < 1.3 and η < 2.5, for narrow qq, qg, and gg resonances. | jj | | | Resonances with mass less than 1.3 TeV, are too close to the lower edge of our dijet mass spectrum to produce a peak distinguishable from the background, and are therefore not considered.

42 pb-1 (13 TeV) 42 pb-1 (13 TeV)

2 CMS 2 CMS 10 Axigluon/coloron 10 [pb] Preliminary [pb] Preliminary String Scalar diquark A A Excited quark W’ SSM × × Z’ SSM B B 95% CL upper limits 95% CL upper limits × × 10 Observed 10 Observed σ σ Expected Expected ± 1σ ± 1σ ± 2σ ± 2σ

1 1

10-1 10-1 1000 2000 3000 4000 5000 6000 7000 1000 2000 3000 4000 5000 6000 7000 qq resonance mass [GeV] qg resonance mass [GeV]

42 pb-1 (13 TeV) 42 pb-1 (13 TeV)

CMS 2 CMS 10 String [pb] 2 Preliminary [pb] Preliminary 10 S8 Excited quark A A Axigluon/coloron × × Scalar diquark S8 B B 95% CL upper limits W’ SSM × × Observed 10 Z’ SSM 10 RS graviton (k/M=0.1) σ σ Expected ± 1σ ± 2σ 95% CL upper limits gluon-gluon quark-gluon 1 1 quark-quark

10-1 10-1 1000 2000 3000 4000 5000 6000 7000 1000 2000 3000 4000 5000 6000 7000 gg resonance mass [GeV] Resonance mass [GeV]

FIGURE 6. The observed and expected 95% upper limits on σ B A for dijet resonances of the type (from top left) quark- × × quark, quark-gluon, gluon-gluon, summary of observed limits for the 3 ﬁnal states. The observed limits are shown as points connected by solid curves. The expected limits are shown as dotted curves and their variation at the 1σ and 2σ levels with shaded bands. Limits are compared to predicted cross sections of string resonances [22, 23], scalar diquarks [24], excited quarks [25, 26], axigluons [27, 28], colorons [28, 29], color octet scalars S8 [30], new gauge bosons and [31], and RS gravitons [32].

The expected limits are estimated with pseudo-experiments generated using background-only hypotheses. The generated mass spectra are further fitted with a background-plus-signal model to extract expected upper limits. The difference in the limits for qq, qg, and gg resonances at the same resonance mass originate from the difference in their shapes. All upper limits presented can be compared to the parton-level predictions of σ B A, without any detector simulation, in order to determine mass limits on new particles. Acceptance can be evaluated× × at the parton level for the resonance decay to two partons. The model predictions shown in Fig. 6 are calculated in narrow-width approximation [33] using CTEQ6L1 [34] PDFs at leading order and a next to leading order k-factor is included for the W, Z, axigluon, and coloron models [28]. New particles are excluded at a 95% in mass regions for which the theory curve lies at or above the observed upper limit for the appropriate final state in Fig. 6. For string resonances the observed mass limit of 5.1 TeV already extends the previous CMS limit of 5.0 TeV, confirming again that this search is more sensitive then run 1 for masses greater than 5 TeV. For the other models, the previous run 1 limits are still more stringent [15]: for scalar diquarks the observed mass limit is 2.7 TeV, compared to 4.7 TeV set in run 1; for axigluons and colorons the observed mass limit is 2.7 TeV, compared to 3.7 TeV set in run 1; for excited quarks we exclude up to 2.7 TeV, compared to 3.5 TeV set in run 1; for a color octet scalar the observed mass limit of 2.3 TeV, compared to 2.7 TeV limit set in run 1. With the current dataset we cannot set mass limits on W, Z bosons or RS Gravitons. The most stringent limits on the cross section of dijet resonance models are set from the recent results of CMS [35] and ATLAS [36], based on the full 144 LHCP2015 Conference, St. Petersburg, Russia, August 31 - September 5, 2015

1 1 dataset of 2015 at √s = 13 TeV (respectively 2.4 fb− and 3.6 fb− ).

REFERENCES

[1] Serguei Chatrchyan et al. Measurement of the differential dijet production cross section in proton-proton collisions at √s = 7 TeV. Phys. Lett. B, 700:187, 2011. [2] S. Chatrchyan et al. The CMS experiment at the CERN LHC. JINST, 3:S08004, 2008. [3] Gavin Salam and Andreas Weiler. Collider reach. http://collider-reach.web.cern.ch/ collider-reach/. [4] CMS Collaboration. Commissioning of the particle-flow event reconstruction with the first LHC collisions recorded in the CMS detector. CMS Physics Analysis Summary CMS-PAS-PFT-10-001, 2010. [5] Matteo Cacciari, Gavin P. Salam, and Gregory Soyez. The anti-kt jet clustering algorithm. JHEP, 04:063, 2008. [6] Torbjorn¨ Sjostrand,¨ Stephen Mrenna, and Peter Skands. A brief introduction to PYTHIA 8.1. Comp. Phys. Comm., 178:852, 2008. [7] CMS Collaboration. Underlying Event Tunes and Double Parton Scattering. CMS Physics Analysis Sum- mary CMS-PAS-GEN-14-001, 2014. [8] Peter Skands, Stefano Carrazza, and Juan Rojo. Tuning PYTHIA 8.1: the Monash 2013 tune. Eur. Phys. J. C, 74:3024, 2014. [9] S. Agostinelli et al. Geant4: A simulation toolkit. Nucl. Instrum. Meth. A, 506:250, 2003. [10] Vardan Khachatryan et al. Search for Dijet Resonances in 7 TeV pp Collisions at CMS. Phys. Rev. Lett., 105:211801, 2010. [Erratum 10.1103/PhysRevLett.106.029902]. [11] T. Aaltonen et al. Search for new particles decaying into dijets in proton-antiproton collisions at √s = 1.96 TeV. Phys. Rev. D, 79:112002, 2009. [12] Serguei Chatrchyan et al. Search for resonances in the dijet mass spectrum from 7 TeV pp collisions at CMS. Phys. Lett. B, 704:123, 2011. [13] Serguei Chatrchyan et al. Search for narrow resonances and quantum black holes in inclusive and b-tagged dijet mass spectra from pp collisions at √s = 7 TeV. JHEP, 01:013, 2013. [14] Serguei Chatrchyan et al. Search for narrow resonances using the dijet mass spectrum in pp collisions at √s = 8 TeV. Phys. Rev. D, 87:114015, 2013. [15] V. Khachatryan et al. Search for resonances and quantum black holes using dijet mass spectra in proton- proton collisions at √s = 8 TeV. Phys. Rev. D, 91:052009, 2015. [16] G. Aad et al. Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions with the ATLAS Detector at the LHC. Phys. Rev. Lett., 105:161801, 2010. [17] Georges Aad et al. Search for new physics in dijet mass and angular distributions in pp collisions at √s = 7 TeV measured with the ATLAS detector. New J. Phys., 13:053044, 2011. 1 [18] Georges Aad et al. Search for new physics in the dijet mass distribution using 1 fb− of pp collision data at √s = 7 TeV collected by the ATLAS detector. Phys. Lett. B, 708:37, 2012. [19] Georges Aad et al. ATLAS search for new phenomena in dijet mass and angular distributions using pp collisions at √s = 7 TeV. JHEP, 01:029, 2013. [20] G. Aad et al. Search for new phenomena in the dijet mass distribution using pp collision data at √s = 8 TeV with the ATLAS detector. Phys. Rev. D, 91:052007, 2015. [21] Richard G. Lomax and Debbie L. Hahs-Vaughn. Statistical concepts: A second course. Routledge Academic, 2007. [22] Luis A. Anchordoqui, Haim Goldberg, Dieter Lust,¨ Satoshi Nawata, Stephan Stieberger, and Tomasz R. Taylor. Dijet signals for low mass strings at the LHC. Phys. Rev. Lett., 101:241803, 2008. [23] Schuyler Cullen, Maxim Perelstein, and Michael E. Peskin. TeV strings and collider probes of large extra dimensions. Phys. Rev. D, 62:055012, 2000. [24] JoAnne L. Hewett and Thomas G. Rizzo. Low-energy phenomenology of superstring-inspired E(6) models. Phys. Rept., 183:193, 1989. [25] U. Baur, I. Hinchliffe, and D. Zeppenfeld. Excited quark production at hadron colliders. Int. J. Mod. Phys. A, 2:1285, 1987. [26] U. Baur, M. Spira, and P. M. Zerwas. Excited quark and lepton production at hadron colliders. Phys. Rev. D, 42:815, 1990. G. D’Imperio, Search for new physics in dijet resonant signatures and recent results from Run 2 with ... 145

[27] Paul H. Frampton and Sheldon L. Glashow. Chiral color: An alternative to the standard model. Phys. Lett. B, 190:157, 1987. [28] R. Sekhar Chivukula, Arsham Farzinnia, Jing Ren, and Elizabeth H. Simmons. Hadron collider production of massive color-octet vector bosons at next-to-leading order. Phys. Rev. D, 87:094011, 2013. [29] Elizabeth H. Simmons. Coloron phenomenology. Phys. Rev. D, 55:1678, 1997. [30] Tao Han, Ian Lewis, and Zhen Liu. Colored resonant signals at the LHC: largest rate and simplest topology. JHEP, 12:085, 2010. [31] E. Eichten, I. Hinchliﬀe, Kenneth D. Lane, and C. Quigg. Supercollider physics. Rev. Mod. Phys., 56:579, 1984. [32] Lisa Randall and Raman Sundrum. An alternative to compactiﬁcation. Phys. Rev. Lett., 83:4690, 1999. [33] Robert M. Harris and Konstantinos Kousouris. Searches for dijet resonances at hadron colliders. Int. J. Mod. Phys. A, 26:5005, 2011. [34] Jonathan Pumplin, Daniel Robert Stump, Joey Huston, Hung-Liang Lai, Pavel Nadolsky, and Wu-Ki Tung. New generation of parton distributions with uncertainties from global QCD analysis. JHEP, 07:012, 2002. [35] Vardan Khachatryan et al. Search for narrow resonances decaying to dijets in proton-proton collisions at sqrt(s) = 13 TeV. 2015. [36] Search for New Phenomena in Dijet Mass and Angular Distributions with the ATLAS Detector at √s = 13 TeV. 2015. [37] CMS Collaboration. CMS luminosity based on pixel cluster counting - summer 2013 update. CMS Physics Analysis Summary CMS-PAS-LUM-13-001, (2013). [38] F. Abe et al. Search for new particles decaying to dijets in pp¯ collisions at √s = 1.8 TeV. Phys. Rev. Lett., 74:3538, 1995. [39] R. Sekhar Chivukula, Arsham Farzinnia, Elizabeth H. Simmons, and Roshan Foadi. Production of massive color-octet vector bosons at next-to-leading order. Phys. Rev. D, 85:054005, 2012. [40] Tomasz Skwarnicki. A study of the radiative cascade transitions between the upsilon-prime and upsilon resonances. DESY-F31-86-02, 1986. [41] CMS Collaboration. Cms luminosity based on pixel cluster counting - summer 2012 update. CMS Physics Analysis Summary, CMS-PAS-LUM-12-001, 2012. [42] Particle Data Group and K. Nakamura. Review of Particle Physics. J. Phys. G, 37:075021, 2010. [See Ch. 33, Statistics]. [43] Zoltan Nagy. Phys. Rev. D, 68:094002, 2003. [44] F. Abe et al. Search for new particles decaying to dijets at CDF. Phys. Rev. D, 55:5263, 1997. [45] CMS Collaboration. Search for dijet resonances in the dijet mass distribution in 7 TeV pp collisions at CMS. CMS Physics Analysis Summary, http://cdsweb.cern.ch/record/1287571CMS-PAS-EXO-10-010, 2010. [46] K. Nakamura et al. Review of particle physics. J. Phys. G, 37:075021, 2010. [47] The pythia6 Z2 tune is identical to the Z1 tune described in [? ] except that Z2 uses the CTEQ6L PDF while Z1 uses CTEQ5L.