Beyond the Standard Model: Results with the 7 TeV LHC Collision Data September Search for Dijet Resonances at ATLAS

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1 6- Beyond the Standard Model: Results with the 7 TeV LHC Collision Data 9 - September 0 Search for Dijet Resonances at ATLAS Georgios Choudalakis University of Chicago U.S.A.

2 Dijet searches in ATLAS Georgios Choudalakis on behalf of the ATLAS Collaboration University of Chicago Trieste September 9, 0

3 Introduction Overview Two complementary ways of analyzing dijet events: Dijet resonance search in m jj. Dijet angular distribution analysis. In this presentation: Flashback of 0 results Dijet resonance search in m jj with.0 fb. Model-independent search Limits to benchmark models Model-independent limits Total Integrated Luminosity [fb ].5 s = 7 TeV ATLAS Online Luminosity LHC Delivered ATLAS Recorded Total Delivered:.75 fb Total Recorded:.6 fb 0 5/0 5/0 /04 0/05 7/06 5/07 /08 09/09 Day in 0 Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches

4 Flashback of 0 results, with 6 pb. New J. Phys. (0) 05044

5 Dijet angular analysis The observables of the angular analysis /dχ ev )dn ev (/N Ldt = 6 pb QBH(M D, s=7 TeV = TeV) (+0.8) 50<m jj <800 GeV 800<m jj <00 GeV (+0.04) 00<m jj <600 GeV (+0.08) 600<m jj <000 GeV (+0.) m jj >000 GeV (+0.8) QCD Prediction Theoretical Uncertainties Total Systematics ATLAS y -y χ = e F χ Ldt = 6 pb, s=7 TeV QCD Prediction Theoretical uncertainties Total Systematics Λ = 5 TeV data 0.05 ATLAS m jj [GeV] The signal is expected in y y <., namely χ<.. F χ = fraction of events in y y <.. Background: Pythia QCD k-factor from NLOJET++. Systematics: Jet Energy Scale, μ R and μ F,PDF. Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 4

6 Dijet angular analysis Flashback Summary of results with 6 pb Analysis / observable 95% C.L. Limits (TeV) Expected Observed Excited Quark q Resonance in m jj [Bayesian].07.5 F χ (m jj ) [Frequentist]..64 Randall-Meade Quantum Black Hole for n =6 Resonance in m jj [Bayes.] F χ (m jj ) [Freq.] F χ for m jj > TeV [Freq.].7.69 dn dχ for m jj > TeV [Freq.] Axigluon Resonance in m jj [Bayes.].0. Contact Interaction Λ F χ (m jj ) [Freq.] F χ (m jj ) [Bayes.] F χ for m jj > TeV [Freq.] dn dχ for m jj > TeV [Freq.] Three observables used in the angular analysis: Whole F χ (m jj ) spectrum F χ for m jj > TeV in χ bins, for m jj > TeV dn dχ Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 5

7 Dijet angular analysis Dijet m jj limits with 6 pb Flashback A [pb] σ 4 q* MC09 q* Perugia0 q* MC09 A (MRST) Observed 95% CL upper limit Expected 95% CL upper limit Expected limit 68% and 95% bands L dt = 6 pb s = 7 TeV A [pb] σ QBH n= QBH n=4 QBH n=6 Observed 95% CL upper limit Expected 95% CL upper limit 68% and 95% bands ATLAS L dt = 6 pb s = 7 TeV ATLAS Resonance Mass [GeV] M D [GeV] Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 6

8 New results, from the dijet resonance search in m jj.

9 Dijet resonance search Resonance search in dijet mass Background determined by (smart) fitting ( x) f (x) =p p 0, x m jj x p +p ln x s. Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 8

10 Dijet resonance search About the function f (x) =p 0 ( x) p x p +p ln x, x m jj s Able to fit wonderfully Pythia QCD, Alpgen, Herwig, NLOJET++. No inflection points. big bumps.] f (m = s)=0. [Unable to fit [A physical property.] It is a parabola in log-log scale, except for the numerator which is addedtomakeitgoto0at s. Known, and used several times before. [e.g., CDF, Phys.Rev.D 79 (009) 00]. Events/(bin width) [Events/GeV] - - s=7 TeV Pythia QCD Fit ATLAS Preliminary jj Reconstructed m [GeV] Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 9

11 Dijet resonance search Trigger and event selection Event selection The obvious: Stable beams, good detector conditions, good vertex, etc. m jj in trigger plateau. Two leading jets be of good quality. No other jet of poor quality that has p T > 0.p j T. Δy <. Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches

12 Dijet resonance search Suppression of QCD background Trigger and event selection QCD events in pb b s / 5 4 η, η <.5 j j η, η η j j, η <. j <0.8 j η j 0 - ATLAS Preliminary - 0 η j ATLAS Preliminary Great sensitivity boost, by selecting central events. η -η j j max [This coheres with dijet angular analysis, where F χ is, practically, the fraction of events passing this cut.]. Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches

13 The data Dijet resonance search The data Events 5 4 ATLAS Data Fit s = 7 TeV L dt =.0 fb significance Reconstructed m jj [GeV] The fit, overall, is fantastic: The p-value of the ( log L) statistic is about 8%. Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches

14 Event at m jj =4TeV Dijet resonance search The data Both jets at p T.8 TeV. Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches

15 Dijet resonance search Search phase The BumpHunter hypertest is used to look for excesses Hypertests, and the BH in specific, are explained in arxiv:.090 [physics.data-an]. What does it do? It counts events in all intervals, and computes Poisson p-values. It keeps the smallest Poisson p-value it finds. BH s test statistic. It generates several 0-signal pseudo-experiments and scans them too. It counts how often a pseudo-spectrum would have any interval that is more significantly discrepant than the one observed in the data. BH s p-value. What is the point of this? Hypertests account for the trials factor. Sensitive to excesses, without presuming their shape or location. Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 4

16 A demo of sensitivity Toy MC used; no ATLAS data involved. Dijet resonance search Search phase Gassian signal is injected, as shown here [At 00±50 GeV]: Events 4 Prob p-val <.6σ effect.5.0 -ln(l(s=0)/l(s)) BumpHunter BumpHunter, no sidebands -ln(l(s=0)) χ TailHunter KS Jeffreys jj Reconstructed m [GeV] We find the probability of making a claim, as a function of expected signal Number of Signal Events The BumpHunter is the most sensitive model-independent test. The absolute winner (black) knows a-priori the signal shape and location. Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 5

17 Dijet resonance search The search phase of the analysis Search phase Events 5 4 ATLAS Data Fit s = 7 TeV L dt =.0 fb significance Reconstructed m jj [GeV] The p-value of the BumpHunter statistic is 8%. Totally insignificant. Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 6

18 Dijet resonance search Limit-setting Systematic uncertainties Jet Energy Scale: From to 4% at p T above 0 GeV. Jet Energy Resolution: Negligible. Fit uncertainty: Ranges from < % at TeV, to 0% at 4 TeV. Luminosity uncertainty:.7% Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 7

19 Dijet resonance search Limits to specific models Limit-setting σ xa [pb] q* A Observed 95% CL upper limit Expected 95% CL upper limit 68% and 95% bands σ xa [pb] s8 Observed 95% CL upper limit Expected 95% CL upper limit 68% and 95% bands - ATLAS L dt =.0 fb s = 7 TeV Mass [GeV] At 95% credibility level, with constant priors: m q >.99 TeV. [expected.8] m A >. TeV. [expected.07] m s8 >.9 TeV. [expected.77] - ATLAS L dt =.0 fb s = 7 TeV Mass [GeV] Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 8

20 Dijet resonance search Model-independent limits Limits to Gaussian signal templates 95% CL Limit on σ xa [pb] ATLAS s = 7 TeV L dt =.0 fb σ Gaussian / m Gaussian signals, of various means and widths. Approximation to virtually any resonance. Systematic uncertainties included, just like for specific models Mass [GeV] Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 9

21 Summary Summary Reviewed the results from 6 pb of 0. Updated the dijet resonance search in m jj with.0 fb of data. Model-independent search (BumpHunter) found no significant discrepancy. Updated limits to benchmark models, like q. (.99 TeV) Added scalar octet to the repertoire. Updated model-independent limits. A [pb] σ 4 ATLAS q* MC09 q* Perugia0 q* MC09 A (MRST) Observed 95% CL upper limit Expected 95% CL upper limit Expected limit 68% and 95% bands L dt = 6 pb s = 7 TeV Resonance Mass [GeV] σ xa [pb] - Our reach extended by about TeV. q* A Observed 95% CL upper limit Expected 95% CL upper limit 68% and 95% bands ATLAS L dt =.0 fb s = 7 TeV Mass [GeV] Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches 0

22 Backup

23 Signal examples Events 5 4 s=7 TeV, Data Fit q*(00) q*(700) q*(750) L dt = 0.8 fb significance 0 - ATLAS Preliminary Reconstructed m jj [GeV] Georgios Choudalakis (ATLAS, U.Chicago) BSMLHC 0 ATLAS Dijet searches