recent qcd-related results from atlas pierre-hugues beauchemin tufts university hep seminar, tata...

Recent QCD-related results from ATLAS Pierre-Hugues Beauchemin Tufts University HEP seminar, Tata Institute, December 5 th 2014, Mumbai, India

Outline Any discussions about progresses on a theory should start with the hard facts: experimental results A lot of improvements have recently been made from the theory side Experimental results tell us where more improvements are needed Introduction: Studying the strong interaction at the LHC QCD studies in ATLAS Parton distribution function (PDF) QCD bremsstrahlung QCD hard parton emission Heavy flavors Conclusions 2 Soft QCD studies will not be covered at all Very rich QCD studies program in ATLAS

Studying the strong interaction at the LHC 3

The strong interaction at the LHC (I) Events with jets in final state are typically copiously produced via the strong interaction at hadron machines How many new physics signatures contain jets??? 4

The strong interaction at the LHC (II) New Physics or Standard Model processes with no jets in the final state also involve the strong interaction in their production A good understanding of soft and perturbative QCD is crucial for the LHC physics program both for searches and measurements 5 ss PDF

The strong interaction at the LHC (III) The strong interaction intervenes in various ways and at various scales in an event Hard scatter QCD bremsstrahlung Parton density function Fragmentation and hadronization Multiple interactions Allows for calculable predictions for a many observables 6 Factorization theorem: Predictions can be obtained from the convolution of short distance physics and a universal non-perturbative regime:

The strong interaction at the LHC (IV) An accurate predictions from QCD are crucial for a successful LHC physics program: There are many regions of phase space of interest for new physics searches in which current approximations used to make predictions are questionable Multiple different scales in the same event topology Test predictions in unprobed phase space and kinematic regimes 7 Higher order QCD effects can be very large and mimic new physics Need good modeling of QCD in order to extend our knowledge beyond SM

The strong interaction at the LHC (V) What can we learn at the LHC that we havent learned at the Tevatron? Higher center of mass energy Higher jet P T reach Larger parton radiation Higher jet multiplicity Different PDF regime Involve different type of partons Different mixtures of final state quark and gluons Different colliding partons (pp vs ppbar) Processes with heavy flavors in initial state New experimental context Larger acceptance (eg: forward jets) More statistics Better precision to test higher order effects From J. Campbell ~30% @ LHC vs ~10% @ Tevatron 8 Also true for coming 14 TeV LHC runs Good complementarity

Huge theoretical progresses on QCD over the last decade NLO revolution o Up to 5 partons in the final state o Approximate NNLO for V+1 NNLL resummation NLO ME+PS matching Despite these improvements, there are still sources of uncertainty related to the modeling of various QCD effects : Complete NNLO predictions PDF Parton shower and matching Mass of heavy flavor quarks Underlying event Electroweak corrections 9 Use LHC data to constrain these effects and show where theory needs improvements The strong interaction at the LHC (VI)

Various level of predictions (I) MC generators are the test bench of the various progress made on the modeling of the different QCD effects over the past years 10 LO ME +PSPythia (6, 8), Herwig Leading Order Matrix Element calculations Soft and collinear emission corrections integrated in a parton shower model Ordering: angular, P T, virtuality Fragmentation and hadronization Model: string, clusters These generators incorporate different modeling of QCD corrections

Various level of predictions (II) MC generators are the test bench of the various progress made on the modeling of the different QCD effects over the past years 11 Tree-level multiple parton ME +PS Alpgen, Sherpa 1.x, MadGraph These generators incorporate different modeling of QCD corrections A matching procedure is needed to avoid the double counting of ME and PS partons MLM cone CKKW Alpgen interfaced to Pythia or Herwig Sherpa: own PS+Had

Various level of predictions (III) MC generators are the test bench of the various progress made on the modeling of the different QCD effects over the past years 12 NLO multi-parton MEMCFM, BlackHat An NLO cross section for N=1,5 partons Different calculation techniques Feynman graphs: MCFM (up to 2 partons) Unitarity: BlackHat (up to 5 partons) No PS, had, or UE corrections Need some kind of cross section sum rules for inclusive final states NNLO: Drell-Yan = FEWZ Approximate V+j: Loopsim

Various level of predictions (IV) MC generators are the test bench of the various progress made on the modeling of the different QCD effects over the past years 13 NLO ME for lowest multiplicity +PS POWHEG, MC@NLO These generators incorporate different modeling of QCD corrections Different matching/merging procedures Interfaced to Pythia or Herwig for PS, had and UE

Various level of predictions (V) MC generators are the test bench of the various progress made on the modeling of the different QCD effects over the past years 14 NLO ME for low multiplicity, LO for others + PS Sherpa 2.0 (MEPS@NLO), MEnloPS: NLO inclusive cross section Normalization at NLO accuracy MEPS@NLO: NLO for 1 and 2 partons LO + PS for higher multiplicity State of the art for V+jets

Various level of predictions (VI) MC generators are the test bench of the various progress made on the modeling of the different QCD effects over the past years 15 ResummationResbos, HEJ Resum large logs at all orders in s Extend validity of perturbation theory Improve normalization and shape Analytical functions for Leading Logs, etc Low energy regime: Resbos (NNLL) Logs from IR and collinear parton emission High Energy Regime: HEJ Large invariant mass between jets Approx which captures hard wide angle emission BFKL-inspired

Generators used and tested * For comparison to data, correction for hadronization and underlying events applied to parton-level MC using ratio of Pythia events 16 GeneratorInterfacesComments ALPGENHERWIG+ JIMMY, PHOTOS, CTEQ6L1, AUET2 tune mP- ME up to 5 parton, MLM matching SHERPACTEQ6L1, Default UE tunemP-ME up to 5 parton, CKKW matching; also NLO PYTHIAPHOTOS, MRST 2007 LOLO ME+PS+Hadronization MCFM*CTEQ6.6/CTEQ6L1NLO up to 2-jets (Feynman graph) BLACKHAT*+SHERPACTEQ6.6MNLO up to 5-jets (unitarity) POWHEGPYTHIANLO+PS (1-jet) MC@NLOHERWIGNLO+PS FEWZ*NNLO Drell-Yan LOOPSIM*nNLO W/Z+jets Non exhaustive list

Effects to be measured Different LHC measurements made to address specific aspects of QCD PDF: o W+c/D, inclusive jet, dijet, trijet and prompt photon Resummation in soft regime: o ZPT, * Resummation at high PT (HEJ generator) o W+jets, rapidity gap and angular decorrelation in dijet events Hard QCD radiation and matching to ME o V+jets (W/R+jets), (pseudo-) top-quark kinematics Heavy flavor jets o W+b, Z+b, Z+bb 17

ATLAS at the LHC Most of the results to be presented here have been obtained with the 2011 ATLAS data 4.57 fb -1 18

QCD studies in ATLAS: PDF 19

PDF: W/Z inclusive (I) Physics motivation: Light quark PDF fitting Reference cross section Lepton momentum calibration and reconstruction efficiency Measurement results: Reached a precision of 1.25% ( (W,Z)) o Expect better with full 2011 dataset o Measured d /dp T d Phys. Rev. D85 (2012) 072004 20

PDF: W/Z inclusive (II) Highly precise measurements of the ratio of W to Z cross sections allow for a test of Lepton Universality Results already comparable to PDG value for the case of W! 21

PDF: W charge asymmetry Different W+ and W- production provides more information about proton structure W produced mostly via valence quarks interaction |y|-dependence sensitive to differences between u and d Parton Distribution Functions Difficult to reconstruct W-boson rapidity: use decay lepton pseudorapidity Measurement compares well with predictions No clear best PDF Additional experimental input in PDF fit Common phase space agreed between ATLAS, CMS and LHCb in LHC EWK working group Phys. Lett. B701 (2011) 31-49 22

Impact on the PDF W/Z results included in NNPDF since version 2.2 Impact the precision of quark density at low x Also introduce a novel sensitivity to s-quark density at x~

Inclusive prompt photon cross section Measurement of all photons that are not secondaries Include both direct from hard processes and fragmentation from parton Sensitive to gluon-PDF through dominant qg->q process Comparison to JETPHOX: NLO calculation of both sources of Discrimination between PDF at low E T ; constrain PDF uncert. at high E T Tensions between data and NLO predictions in low P T regime where photon fragmentation effects are important 27 Phys. Rev. D 89, 052004 (2014)

QCD studies in ATLAS: resummation 28

QCD bremsstrahlung: Vector Boson P T Test multiple aspect of QCD predictions Intrinsic-K T Low-PT (W,Z): logarithmic resummations High-PT (W,Z): (N)NLO perturbative QCD Important test of parton shower tuning o No color flow between initial and final state Phys. Lett. B705 (2011) 415-434 Resummation: Good description at low and average P T Fixed order (NNLO): Poor description at average P T PS more important than NLO ME JHEP 09 (2014) 145 29

QCD bremsstrahlung: Vector Boson * Complementary physics can be obtained by measuring Z * Finer resolution (bin) at low P T Z Smaller systematic o From 0.1% to 0.6% o 2% to 6% for theory Best agreement: NNLL at low values PS matched to multiple parton ME at high values Sensitivity to PS tuning Depend solely on angle of two leptons Highly correlated to P T Z /M ll 0< * < 1 P T Z < 100 GeV Phys. Lett. B720 (2013) 32-51 JHEP 09 (2014) 145 30

Need for tuning Need Resummation or PS to describe inclusive Z-system distributions Huge advantage of tuning over more accurate ME predictions for low and average P T region, less critical for high Z P T value for NLO o True for both Z P T and * o Tuning effects larger for LO than NLO, but end point comparable 31 JHEP 09 (2014) 145

Jet veto in dijet events Events with large rapidity gap are in phase space region where DGLAP reaches its limit of validity BFKL resummation in terms of ln(1/x) is an alternative Implemented in HEJ Expect and observe an exponential suppression in function of ln(P T /Q 0 ) Good agreement with Powheg+Pythia and HEJ+Ariadne PS has the bigger effect Probe BFKL vs DGLAP??? 32 EPJ C74 (2014) 3117

Azimuthal decorrelations in dijet events Better agreement for HEJ+Ariadne But: PS has a bigger effect than BFKL Powheg = HEJ for rapidity None reproduces all data 33

QCD studies in ATLAS: hard emission 34

Hard radiation: W+jets (I) 35 Multiplicities in good agreement with NLO PS lacks of large angle hard emission Normalization issue with MEPS@NLO Large difference between Alpgen and Sherpa in PS regime Test pQCD in more exclusive final states Probe hard QCD radiation from matrix element Less sensitive to fragmentation and UE Direct study of SM in regions where new physics is expected arXiv:1409.8639

Lack of PS is detrimental to jet momentum modeling MEPS@NLO features large disagreement at low PT -> tuning? Exclusive sum and Loopsim not sufficient to restore agreement in HT Again: need for PS High Energy resummation is totally off despite huge uncertainties Hard radiation: W+jets (II) 36

Hard radiation: Z+jets (I) Similar conclusions as in W+jets measurements NLO and mP-ME+PS describe data up to 4/5 jets PS jets are too soft or at low angle o Not an effect of missing NLO corrections on ME Some observables need higher multiplicity calculation Phys. Rev. D85 (2012) 032009 37 JHEP 07 (2013) 032

Hard radiation: scaling patterns in Z+jets Study the probability that an extra hard jet get radiated from a given hard process: R (n+1)/n = P n+1 /P n Two patterns provide limiting cases for most LHC processes Staircase scaling R (n+1)/n = e -b : non-abelian QCD effects o Radiation depend on the hard process due to PDF Poisson scaling : gluon radiation off hard quark o Log enhanced radiation ME due to large hierarchy of scales 38 Theoretically Understood! 1/02/13/24/3 5/4 6/5 2/13/24/35/4 6/5 P T jet > 30 GeV P T lead > 150 GeV

Systematic uncertainties 39 Systematic uncertainty at same level as the NLO theory uncertainty for W/Z+jets measurements Even below for some observables o E.g.: leading jet P T in Z+jets Dominated by Jet energy scale uncertainty o More data allow for in-situ calibration studies Statistical uncertainty important in a large part of the spectrum probed R jets W+jets

Hard Radiation: Rjets The ratio of W+jets to Z+jets retains sensitivity to parton emission with very high precision, but little dependence on other QCD effects Sensitive to difference between Wjets and Zjets (not flat) PS Good way to test tuning with high precision o Significant discrepancy at high rapidity between Alpgen and Sherpa Mismodeling due to missing n-parton contribution cancels in this ratio 40 arXiv:1408.6510

QCD radiation in top events Top quarks do not fragment before decaying, but can test ISR and decay products FSR (for had top decay) Large sensitivity to ISR for fiducial top defined from observable particles Effect stabilize for NLO matched/merged to PS Also less sensitivity for kinematic of full ttbar system 41 ATLAS-CONF-2014-059 41

QCD studies in ATLAS: heavy flavor 42

Heavy Flavor: W+b-jets Further theoretical uncertainties for heavy flavor jets: Heavy flavor content of PDF Gluon splitting pushed in the limit of invalidity when m q is large Mass effects in ME calculation JHEP 06 (2013) 084 4FNS vs 5FNS: b-quark from gluon splitting vs DGLAP Tension between predictions and data Systematically increasing with P T 43 Not yet significant Need more precision t0 conclude

Heavy flavor: Z+b-jets Good agreement with NLO MCFM and aMC@NLO Seems to favor scheme where b-quark is taken from PDF (5 FNS) LO+PS generators are underestimating the cross section Cant constrain PDF yet due to too large uncertainty Good description of b-jet p T shape Normalization is off JHEP 10 (2014) 141 44

Heavy flavor: Z+bb It is the 4FNS that tends to be favored when there are 2 b-jets Gluon splitting seems not to be very well understood 45

Conclusion 46

Conclusion Sensitivity to new physics and parameter values require precise QCD predictions Higher cross section, smaller x and larger phase space for parton radiation might inflate QCD uncertainties in Run-2 An extensive set of ATLAS measurements reached sufficient precision for testing state-of-the-art QCD calculations Inclusive W/Z cross section measurement used in PDF constraints Differential cross section for various W/Z and W/Z+jets observable Multijet and top-quark observables W/Z+heavy flavor cross section measurements Measurements are challenging NLO predictions and other sophisticated calculation techniques (e.g. NNLL resummation) Theory community is also very active, and improved predictions are regularly made available, in response to our measurements In process of developing a broad ATLAS QCD physics program for Run-2 47

Back-up slides 48

The observables Measured d /d for various observables O: Rapidity (Y l), and masses (M ll ) are particularly sensitive to PDF P T Z/W, *, R(jj), y(jj) study soft vs hard radiation P T j, y j, N j probe higher order ME or multiple partons combination H T is the (robust) scale used in NLO calculations Test new NLO calculations up to 5-jets Measured ratios with precision in function of jet observables Cancel some experimental and theoretical uncertainties Explore transition region of EWK scale in perturbative jet production The differential cross section for a given jet observable (O): Results in electron and muon channels combined using BLUE method 49 Where U(O) unfold detector effetcs

Well defined measurements The objective of such SM measurements is precision: Measurement designed to minimize experimental errors Minimal dependence of measurement results on theory input Well defined quantities and physically meaningful final states Fiducial measurement: Unfold to phase space as close as possible to observable phase space o Lepton p T >20 GeV, lepton | | 25 GeV o M T (W) > 40 GeV, 66 (71) < M ll < 116 (106) for Z+jet (Z+b) QED treatment: Unfolded lepton definition includes sum of all photons in a 0.1 cone Particle level b-jets defined as jets containing a B hadron 50

W/Z selection efficiencies Lepton triggers are used to collect data P t ele > 15 GeV, 100% efficient P T > 13 GeV, 85% efficient (L1-trigger acceptance limitation) To limit background contamination: identification criteria are applied to leptons Track quality (muons and electrons) Isolation (mu. and ele.) Track-cluster (ele.); inner detector-muon spectrometer track matching (mu.) EM-like cluster shape (ele.) Lepton E or P T scale and resolution corrections are applied, Based on data to MC comparison of Z lineshape and E/p measurements E.g.: Scale correction on P T < 1% Data-driven estimate of the efficiency with tag & probe method Electrons: ~70%, with data-MC scale factors of ~3% Muons: ~80%, with data-MC scale factors < 1% 51

Backgrounds MC are used for Electroweak backgrounds (top, W, Z, dibosons) ~3.5% for W (in both the muon and the electron channels) ~0.5% for Z (electrons and muons) Essentially the same level of electroweak background, regardless of the jet activity recoiling the vector bosons Special care brought to top in high jet multiplicity events 52 Dominant background: QCD multijets Fake electrons from single hadrons or conversions (dominant in electron channel) Heavy flavor decays (dominant in muon channel) Data-driven approach adopted for QCD < 2% for Z (ee, ) with or without jets ~2.5% W (+jets); ~4.5% for We but ~15% when jets are required

Data to MC comparisons Transverse mass (M T ) in We events Jet multiplicity (N jet ) in W +jets events 53

Jets in the measurements Jets are reconstructed using the anti-kT algorithm with R = 0.4 Infrared and collinear safe simple cone-like geometrical shape Used both on predictions and data Calibration: numerical inversion met hod Dominant systematic in W/Z+jets Use in-situ studies to keep systematics low o single hadron, +jets and Z+jets events o 2010 analyses used MC-based corrections SelectionW+jetsZ+jetsRjetsW/Z+b-jets Jet p T 2030 25 Jet |y 4.4 2.82.1 R jet-lep 0.5 Jet ignored Event rejected if [0.2,0.5] As W+jets JVF > 0.75Applied to all to reject fake pile-up jets JVF = p T (tracks) in a jet pointing towards the primary vertex / p T (tracks) in a jet 54

Unfolding Measurement-theory comparison done at particle level: Raw observation corrected for detector effects Theory predictions corrected for hadronization, underlying events, etc. Allow for direct comparison to calculations, and tune the theory Correct for flavor effects in calibration Compare two different methods: Iterative Bayesian unfolding method o Lower MC dependence and better stat. treatment o Different number of iterations in different bins Bin-by-bin unfolding o Simpler and better understood for ratios Dependence on priors is tested by comparing unfolding obtained from different generators (ALPGEN vs SHERPA). 55

W/Z+Heavy flavor (I) b-tagging require a displaced secondary vertex in a jet with a decay length significance of > 5.85 o ~50% b-jet efficiency in a ttbar sample o Efficiency from muon p T relative to the jet axis B-tagging affects sample composition Challenging: Small cross section but large background (especially W+b) b-jets are identified by exploiting the long lifetime (1.5 ps) and large mass of B-hadrons 56

W/Z+Heavy flavor (II) Fraction of W+b/c/l jets from a fit to the mass distribution of the secondary vertex Estimate template shapes from MC Systematic uncertainties on shapes by comparing data and MC in control regions Cross section at event level First measurement in exclusive jet bins Vetoed on number of jets (

recent qcd-related results from atlas pierre-hugues beauchemin tufts university hep seminar, tata...

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