Forward Jets with the CAL

Transcription

1 Forward Jets with the CAL Sabine Lammers Juan Terron March 4, 004 ZEUS Collaboration Meeting Motivation Event Selection Monte Carlo Programs Forward Jet Measurements Summary and Plans

2 Parton Evolution Schemes Perturbative expansion of parton evolution equations: /x 0 5 saturation n 0 4 A mn ln Q m ln x (can't be calculated explicitly to all orders) DGLAP resummation: BFKL resummation: s ln Q s ln x n n high energy limit BFKL Evolution DLLA Evolution DGLAP Evolution Q As terms small in x contribute strongly to BFKL resummation scheme, BFKL evolution may become important at the lowest x values HERA can measure.

3 $ $ "" " " "! ## # # # %& ' * ) +, + - Gluon Ladder DGLAP and BFKL formalism based on ordering of partons emitted along the parton ladder: DGLAP: x = x n < x n- <... < x Q = k >>... >> T,n k T, forward fadeout BFKL : x = x n << x n- <<...<<x no k T ordering eta democracy ln tan ( BFKL additional hadrons from high transverse momentum forward partons, above the DGLAP prediction. k T η=. forward jet η= GeV e + HERA forward region HAC jets + X event topology HAC FCAL EMC FORWARD TRACKING BCAL EMC CENTRAL TRACKING HAC HAC η=0 SOLENOID current jet HAC.5 m 3.3 m.9 m cos h p x, p x, A requirement on the hadronic angle (current jet) allows the exploration of lower x Bj RCAL EMC hadronic angle γ h > 90º p y p y E E p z p z η=-.75 η= GeV p 3

4 Event Selection Data Set: ZEUS 96/97 (~38.6 pb - ) Monte Carlo: Detector acceptance estimated with LO Color Dipole Model (CDM) implemented with Ariadne, using CTEQ4M PDFs Trigger Chain: FLT40,4,4,43,44; SLT DIS6; TLT DIS03,04; DST,,4 Inclusive Sample: Q > 5 GeV y > 0.04 E T,jet > 6 GeV - < η jet < 3 E el > 0 GeV Detector Cuts: Z vtx < 50 y el < 0.95; y jb > < E-p z < 65 p T CAL / E T CAL < 3 Phase Space Selections QPM Suppressed Sample: Q > 5 GeV y > 0.04 E T,jet > 6 GeV 0 < η jet < 3 cos(γ h ) < 0 E el > 0 GeV Sinistra E el > 0 GeV 4x4 boxcut electron isolation 0. BFKL Forward Jets Sample: Q > 5 GeV y > 0.04 E T,jet > 6 GeV 0 < η jet < 3 cos(γ h ) < 0 E el > 0 GeV 0.5 < Q /E T,jet < effective Q >8 GeV jets are selected using k T -inclusive algorithm in the laboratory frame better reach to low x better resolution in the forward 4

5 NLO implementations of NLO calculation by DISENT Inclusive Jet (QPM) Phase Space () QPM Suppressed Phase Spaces (&3) d d LO NLO A 0 A B s d d LO NLO C C s s D s employs subtraction method µ r = µ f = Q estimated renormalisation scale uncertainty: r Q PDF : CTEQ6 corrected from partons to hadrons using Ariadne (CDM MC) Q

6 Inclusive Jet Cross Section vs. η jet dσ/dη jet (pb) data/nlo C had NLO Disent : O(α s ) ZEUS (Prel.) 96/97 CDM : Ariadne Energy Scale Uncertainty MEPS: Lepto NLO : 0.5Q < (µ r ) < Q NLO PDF Uncertainty η jet Cross section drops in forward region due to y-cut Significant discrepancy with NLO at high η, Ariadne (BFKL-like LO MC) can describe the data Lepto (DGLAP-like LO MC) gives fairly good description Cross section dominated by QPM events - should be well understood! NLO is O(α s ) BFKL? Parton shower missing from NLO? 6

7 Inclusive Jet Cross Section vs. Q, x dσ/dq (pb/gev ) NLO Disent : O(α s ) CDM : Ariadne MEPS: Lepto ZEUS (Prel.) 96/97 Energy Scale Uncertainty dσ/dx Bj (pb) NLO Disent : O(α s ) CDM : Ariadne MEPS: Lepto ZEUS (Prel.) 96/97 Energy Scale Uncertainty data/nlo C had NLO : 0.5Q < (µ r ) < Q NLO PDF Uncertainty Q (GeV ) data/nlo C had NLO : 0.5Q < (µ r ) < Q NLO PDF Uncertainty x Bj Discrepancy between data and NLO localized in lowest x Bj and Q bins, regions where BFKL may be important 7

8 Event Topology: Isolating the Signal Remember that our signal events are Boson-Gluon Fusion and QCDC events with high-e T forward going jets! η=. forward jet η= GeV e + HAC jets + X event topology HAC FORWARD TRACKING η=0 current jet HAC η=-.75 η= GeV p In QPM events, only jet => hadronic angle = jet angle FCAL EMC CENTRAL TRACKING SOLENOID BCAL EMC RCAL EMC HAC To enhance our signal-to-background ratio (reject QPM), we restrict our phase space: events must have hadronic angle > 90 o jet η must be in forward half of detector cos h HAC.5 m 3.3 m.9 m p x p x p y p y hadronic angle γ h > 90º E E p z p z 8

9 Reselection of Phase Space Inclusive Jet Phase Space Q > 5 GeV y > 0.04 E el > 0 GeV E T,jet > 6 GeV - < η jet < 3 Disent Calculations: LO = O(α s0 ) = QPM NLO = QPM + corrections to suppress QPM with hadronic angle requirement QPM = 0 for η > 0 BGF + QCDC for η > 0 QPM Suppressed Phase Space Q > 5 GeV y > 0.04 E el > 0 GeV E T,jet > 6 GeV 0 < η jet < 3 cos (γ had ) < 0 LO = O(α s ) = BGF + QCDC NLO = O(α s ) = BGF + QCDC + corrections orders in the series of α s Just order in the series of α s orders in the series of α s 9

10 Inclusive Jet Cross Section vs. η jet for QPM Suppressed Phase Space dσ/dη jet (pb) ZEUS (Prel.) 96/97 Energy Scale Uncertainty NLO Disent : O(α s ) CDM : Ariadne MEPS: Lepto For our signal events, agreement with NLO within errors. NLO now includes terms O(α s ) 500 data/nlo C had NLO : 0.5Q < (µ r ) < Q NLO PDF Uncertainty η jet Ariadne gives good description of data Lepto gives fair description of data Renormalization scale uncertainty grows in the forward region 0

11 Inclusive Jet Cross Section vs. Q,x for QPM Suppressed Phase Space dσ/dq (pb/gev ) data/nlo C had ZEUS (Prel.) 96/97 Energy Scale Uncertainty NLO Disent : O(α s ) CDM : Ariadne MEPS: Lepto NLO : 0.5Q < (µ r ) < Q NLO PDF Uncertainty Q (GeV ) dσ/dx Bj (pb) data/nlo C had ZEUS (Prel.) 96/97 Energy Scale Uncertainty NLO Disent : O(α s ) CDM : Ariadne MEPS: Lepto NLO : 0.5Q < (µ r ) < Q NLO PDF Uncertainty x Bj NLO based on DGLAP agrees with data within errrors.

12 " BFKL Phase Space Further restrictive phase space suggested by Mueller, Navalet limitation on Q /E T,jet suppresses events exhibiting DGLAP evolution E T, jet Q!!!!! BFKL Jets Sample: Q > 5 GeV y > 0.04 E el > 0 GeV Inclusive Sample: QPM Suppressed Sample: Q > 5 GeV y > 0.04 E el > 0 GeV Q > 5 GeV y > 0.04 E el > 0 GeV E T,jet > 6 GeV E T,jet > 6 GeV E T,jet > 6 GeV 0 < η jet < 3 0 < η jet < 3 - < η jet < 3 cos(γ h ) < 0 cos(γ h ) < < Q /E T,jet <

13 BFKL Phase Space Data/MC Comparison CDM (Ariadne) describes data well. Events Events Jets Jets (GeV ) Q DA log(x DA ) (GeV ) 0 E T,jet (GeV) η jet Events Events 0000 Jets y JB γ h φ jet plots are area normalized 3

14 BFKL Phase Space Efficiencies, Purities efficiency purity correction factor Efficiencies, purities reasonable. Low purity in highest eta bin. 0.8 Can improve with eta correction η jet η detector efficiency purity correction factor E T,jet η hadron 4

15 Inclusive Jet Cross Section vs η jet for BFKL Phase Space dσ/dη jet (pb) ZEUS (Prel.) 96/97 Energy Scale Uncertainty NLO Disent : O(α s ) CDM : Ariadne MEPS: Lepto Data shows excess over NLO Large renormalization scale uncertainty persists 0 data/nlo C had NLO : 0.5Q < (µ r ) < Q NLO PDF Uncertainty Ariadne (BFKL-like MC) gives excellent description of data over entire region Lepto (DGLAP-like MC) cannot describe data η jet 5

16 Inclusive Jet Cross Section vs Q,x for BFKL Phase Space dσ/dq (pb/gev ) NLO Disent : O(α s ) CDM : Ariadne MEPS: Lepto ZEUS (Prel.) 96/97 Energy Scale Uncertainty dσ/dx Bj (pb) NLO Disent : O(α s ) CDM : Ariadne MEPS: Lepto ZEUS (Prel.) 96/97 Energy Scale Uncertainty data/nlo C had NLO : 0.5Q < (µ r ) < Q NLO PDF Uncertainty Q (GeV ) data/nlo C had NLO : 0.5Q < (µ r ) < Q NLO PDF Uncertainty x Bj NLO Calculation can describe the data. 6

17 Summary Inclusive jet cross sections at Q > 5 GeV, y >0.04 have been measured over the full rapidity acceptance region in three phase space regions NLO Calculation Ariadne (BFKL-like MC) Lepto (DGLAP-like MC) Inclusive PS cannot describe data in forward good description good description QPM Suppressed PS data above NLO; agreement w/in errors good description fair description BFKL PS data above NLO excellent description data above Lepto 7

18 Conclusions and Plans Large renormalization scale uncertainty indicates higher order contributions are important for obtaining an accurate prediction from the theory. A resummed NLO calculation, perhaps using the BFKL implementation, would be interesting to compare to the data, both for its cross section predictions and as a measure of the renormalization scale uncertainty in the low-x Bj and high-η jet region Paper Publication: writing has begun SL leaves at end of April action items cross sections in most forward region < η < 3 CASCADE prediction fine tuning of systematics 8

19 End of Talk 9

20 HERA LHC HERA densities extrapolate into LHC region DGLAP parton densities, QCD knowledge from HERA LHC measurements HERA measurements crucial for understanding signal + background at LHC! 0

21 Dijet Event Looking for presence of strong forward jets accompanied by hadronic activity in central and/or rear parts of the detector

22 DGLAP Evolution Equations Quark and gluon parton distribution functions (PDF's) are predicted at a certain x and Q, given an initial distribution at x 0 and Q 0. dq i x, Q d lnq s Q x dz z q i y, Q P qq x z g y, Q P qg x z splitting functions -calculable by QCD dg x, Q d lnq s Q x dz z q i y, Q P gq x z g y, Q P g g x z In the perturbation series calculation of the evolution of the PDF's with x and Q, there are terms proportional to (α s lnq ) n, (α s ln(/x)) n and (α s lnq ln(/x)) n DGLAP Approximation: sums terms α s lnq, ignores α s ln(/x) has limited applicability ---> s ln Q DGLAP = Dokshitzer, Gribov, Lipatov, Altarelli, Parisi s ln x

23 Previous ZEUS Measurement ZEUS 995 dσ/dx [nb] ZEUS Data ARIADNE 4.08 LEPTO 6.5 (SCI) HERWIG 5.9 LDC.0 Issues: all monte carlo models understimate the data at low x LO monte carlo models are not consistent with each other 40 0 a) Improvements: x new data set: 6x more statistics new calculation: NLO higher reach in η jet finding with k T -algorithm 3

24 Monte Carlo LEPTO: k T -ordered parton shower - DGLAP Hadronization: Lund String Model ARIADNE: Parton showering with CDM (Color Dipole Model: BFKL-like) Hadronization: Lund String Model Lund String Model: Color string stretched across pairs of final state partons. Energy stored in the string gives rise to hadrons. Detector acceptance estimated with LO Color Dipole Model (CDM) implemented with Ariadne, which has the best description of data 4

25 Inclusive Jet Cross Sections vs. total Inclusive Cross Sections using DISENT Inclusive jet phase space Q > 5 GeV E T,jet > 6 GeV y > < η jet < 3 Fully inclusive DIS phase space Q > 5 GeV no jet y > 0.04 selected! dσ/dq (pb) Inclusive DIS Inclusive Jet dσ/dx Bj (pb) Inclusive DIS Inclusive Jet Incl. jet/incl. DIS Q Incl. jet/incl. DIS x Bj A hard lower cut-off in the jet E T significantly limits the phase space inclusive jet cross section does not dominate inclusive DIS cross section at low x Bj and Q 5

26 Systematic Uncertainties Systematic uncertainties arise from data measurement resolution poor description of data by MC at cut boundary model dependencies in MC Systematic Checks. Lepto instead of Ariadne.Calorimeter Energy Scale ±3% 3. Jet Et cut variation ± GeV 4. Jet η cut (forward) variation ± Electron energy cut variation ± GeV 6. Q cut variation ± GeV 7. Vtx cut variation ± 0 cm. 8. High E-pz cut variation ± 3 GeV 9. Low E-pz cut variation ± 3 GeV 0. Hadronic angle cut variation ± 0. Typical/Maximal (in a bin) Variation 6% / 5% 5% / 3% % / 3% % / 5% % / 5% % / 3% % / % % / % % / % 3%/ % 6