Study on search of a SM Higgs (120GeV) produced via

VBF and decaying in two hadronic taus

V.Cavasinni, F.Sarri, I.Vivarelli

OUTLINE

Motivation

Signal and backgrounds

Preliminary results of analysis

Trigger issues

Cross sections and branching ratios for the Higgs boson

BR(H 120) = 0.0679

(value from Hdecay)

σVBF H120= 4.35 pb(value from VV2H with CTEQ5L for PDF )

Tau channel for low Higgs mass

Previous study (hep-ph/0402254) : ll , l h .In 30 fb-1 about 32 signal events and 22 background events

Mass reconstruction in collinear approximation

For had had, if x1,2 are the visible energy fraction of the two taus :

10 ,/ 2,121 xxxmm hh

mean =128 GeV

sigma = 12 GeV

- 2 high pT forward jets - depleted jet activity in the central region

VBF signature

Rapidity distribution for the tagged forward jets for signal and for ttbar background.

HVBF hh

Main features of this event :-4 jets :

2 forward jets 2 central tau jets

-Missing transverse energy due to neutrinos

Main background: QCD

fbhhBRHBRVBFsig 125)()(

mbQCD 45.9

Tau decays 65% of times in hadronsThe BR(hh) is 42.25 %

11103.1 QCD

sig

Other backgrounds : ttbar, Z/γ+jets

Due to the high cross section, impossible to generate all the needed events for the QCD, even with ATLFAST.

After all the analysis cuts, very poor background statistics.

To “solve” the problem : when requiring tau jets, keep all the background events and weight them with the rejection factor for tau jets.

The performance of the tau identification algorithm in FULLSIM depending on pT and is parameterized and put directly into ATLFAST. (M.Heldmann work)

Treatment of QCD backgrounds

Reconstruct tau-candidate:Start from Calorimetric Clusters (default).Associate tracks to the candidate.Calibrate candidate.Build the set of pT dependent variables for tau-identification (REM, FISO, Ntrack, Strip Width, Nstrip, Charge, Impact parameter, ET/pT(1sttrack)) and then calculates Likelihood.Apply a set of the

basic cuts for tau-identification.

Tau id in full simulation - tauRec

L > 4 M.Heldmann

Typical cuts of the analysis (hep-ph/0402254)

• at least 4 jets in the event;• 2 tau tagged jets with pTs over given thresholds;• 2 candidate forward jets with pTs above given

thresholds;• negative eta product for the forwards and > 4; • taus between the 2 forwards ;• 0 < x1(2) < 1 and taus not back to back;• invariant mass for the forwards > 700 GeV; • missing transverse energy above a threshold ;• no jet with pT > 20 GeV between the 2 forward,

apart from the taus;• cut on mass window.

Two different sets of cuts used for the analysis

• at least 4 jets;• 2 tau tagged jets with pT above 35** and 30GeV;• missing transverse energy > 45 GeV**;• 2 forward candidates with pT above 50 and 30 GeV;• negative eta product for the forwards and > 4; • taus between the forwards ;• 0 < x1(2) < 1 and taus not back to back ( < 3);• cut on mass window : 104 GeV < m< 150 GeV.

** dedicated trigger menu

Soft cuts

cut Signal efficiency

Events 30 fb-

1 (300 fb-1) QCD

rejectionEvents 30 fb-

1 (300 fb-1)

4jet (2 tau), Et

miss > 45 GeV

4.4 % 3.4 10-8

Pt tau 2.5 % 1.21 10-10

Pt forward 1.9 % 4.81 10-11

forward 0.94 % 3.56 10-12

cuts 0.65 % 24 (241) 5.4 10-13 155 (1551)

Mass window 0.55 % 21 (206) 8.1 10-14 23 (231)

Soft cuts

using the trigger thresholds for low luminosity period

Soft cuts

• at least 4 jets; •2 tagged tau jets with pt over 45 and 35 GeV;•missing transverse energy over 45 GeV;•2 forward candidates with pt above 60 and 40 GeV;•negative product of forward jet eta and > 4; •taus between 2 forward ;• 0 < x1(2) < 1 and taus not back to back (< 3);•forward jet invariant mass > 700 GeV; •no jet with pt > 30 GeV between the forwards,

apart from the taus;•cut on mass window : 104 GeV < m < 150 GeV.

Hard cuts

cut Signal efficiency

Events 30 fb-1 (300 fb-1)

QCD rejection

Events 30 fb-1 (300 fb-1)

4jet (2 tau), Et

miss > 45 GeV

4.4 % 3.4 10-8

Pt tau 1.6 % 3.96 10-11

Pt forward 0.97 % 1.22 10-11

forward 0.49% 7.8 10-13

cuts 0.33 % 1.22 10-13

Mjj 0.3% 9.5 10-14

Jet veto 0.26% 10 (99) 5.1 10-14 14 (144)

Mass window 0.24% 9 (89) 5.3 10-15 2 (15)

Hard cuts

using the trigger thresholds for low luminosity period

Hard cuts

ttbar background

pbinclttbar 548)( Both Ws dacay in ~

In 30 fb-1 about 17*10^6 events.

Almost 40 *10 ^6 generated events.

4103.2 ttbar

sig

On ttbar events the rejection

from parameterization is worse by about a factor 2.

cut Signal efficiency

Events 30 fb-1 (300 fb-1)

ttbar rejection

Events 30 fb-

1 (300 fb-1)

4jet (2 tau), Et

miss > 45 GeV

4.4 % (7.3/2) 10-4

Pt tau 2.5 % (1.78/2) 10-4

Pt forward 1.9 % (1.46/2) 10-4

forward 0.94 % (5.09/2) 10-6

cuts 0.65 % 24 (241) (1.02/2) 10-6 64 (338)

Mass window 0.55 % 21 (206) (5.07/2) 10-8 4 (32)

Soft cuts

using the trigger thresholds for low luminosity period

cut Signal efficiency

Events 30 fb-

1 (300 fb-1) ttbar

rejectionEvents 30 fb-

1 (300 fb-1)

4jet (2 tau), Et

miss > 45 GeV

4.4 % (7.3/2) 10-4

Pt tau 1.6 % (1.1/2) 10-4

Pt forward 0.97 % (6.8/2) 10-5

forward 0.49% (2.4/2) 10-6

cuts 0.33 % (0.56/2) 10-

6

Mjj 0.3% (4.05/2) 10-

7

Jet veto 0.26% 10 (99) (1.77/2) 10-

7

6 (58)

Mass window 0.24% 9 (89) 0

Hard cuts

using the trigger thresholds for low luminosity period

Important background

*/Z + jets with */Z has a = 1742 pb:736 pb with two true hadronic taus.

UNDER STUDY

*/Z +jets background

Hadronic Tau Trigger (I) (ATL-COM-DAQ-2003-030)

LVL1 trigger: look at 4X4 matrix of calorimetric towers ( = 0.1 x 0.1 each).ET threshold for the central core (EM+Had) and isolation thresholds between core and 12 external towers for e.m. and had. calorimeters.

+ track multiplicity in the RoI

second layer of EM calorimeter LVL2 trigger:

look at the shower shape in the 2nd layer of e.m. calorimeter and at the track multiplicity inside the RoI defined at LVL1.Cut on the ratio between ET contained in a 3x7 cell cluster and ET in a 7x7 cell cluster and on track multiplicity

Hadronic Tau Trigger (II) (ATL-COM-DAQ-2003-030)

LVL3 (Event Filter) :look at the complete event.By now the variables of the offline algorithms are used as an approximation LVL3 trigger five variables:number of reconstructed tracks, within R = 0.3 of the candidate calorimeter cluster, between 1 and 3; cut on isolation fraction, defined as the difference between the ET contained in a cone size of R=0.2 and 0.1 normalized to the total jet ET;cut on EM jet radius, an energy weighted radius calculated only in the e.m. calorimeter ;cut on EM energy fraction, defined as the fraction of the total jet energy in the e.m. calorimeter;threshold on the pT of the highest pT track.

LVL1 Trigger RatesIllustrative menu

M.Bosman talk@ATLAS Physics Workshop June 2005

Inclusive High Level Trigger Event Selection

Selection 2x1033 cm-2s-1 Rates (Hz)

Electron e25i, 2e15i ~40

Photon g60i, 2g20i ~40

Muon m20i, 2m10 ~40

Jets j400, 3j165, 4j110 ~25

Jet & ETmiss j70 + xE70 ~20

tau & ET

miss t35 + xE45 ~5

B-physics 2m6 with mB /mJ/y ~10

Otherspre-scales, calibration, …

~20

Total ~200

Current global understanding of trigger rates

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M.Bosman talk@ATLAS Physics Workshop June 2005

Trigger efficiency

While the trigger rate depends on the QCD backgrounds, the trigger efficiency depends on the physics channel under study.

By now, only studies on Z and A/Hhh.

The trigger impact on the VBF H hh has to be studied.

ConclusionsThe channel VBF Higgs hh appears

to be promising to improve the statistical significance in the critical low Higgs mass region.

Fast simulation shows that QCD and ttbar backgrounds may be rejected at the desired level. Z+jet background still under study.

Efficiency of the trigger has to be studied for this channel.

Future work : redo the analysis with full simulated events and with ME QCD generators.

BACKUP SLIDES

id in full simulation -

tauRec• Builds set of variables for -identification – they are pT dependant

• Calculate Likelihood from: REM, FISO, Ntrack, Strip Width, Nstrip, Charge,

Impact parameter, ET/pT(1sttrack)

REM F

ISO

Ntrack

Strip Width

Nstrip

Charge

Impact parameter ET/p

T(1sttrack)

M.Heldmann

Signal A→, background QCD, 0< pT<44 full line and 134< pT< 334 dashed line

Rejection vs. pT (jet)

Rejection vs. (jet)

From M.Heldmann id in fast Simulation -

Parameterization

Parametrisation (R vs. pT)

R grows with pTR falls again with high pT

Rmax~300GeVRmax and slope depend on

20% 25% 30%

35% 40% 45%

50% 55% 60%

From M.Heldmann

d

Parametrisation (R vs. η)

20% 25% 30%

35% 40% 45%

50% 55% 60%

R falls for > 1.5(endcap, higher

granularity, lower ID resolution)

At 1.5 crack region should be

excluded

a lot of structure in R vs. , but taken to be flat and than falling

like a gauss

From M.Heldmann

• Previous studies (ATL-DAQ-98-127) used:– ETCore(em+h) > 50– fr(Core)= ETCore(em) / ETRoI(em) > 0.85– 1 ≤ Ntrk ≤ 3

• Now using infrastructure of LVL2 calo algorithm (TrigT2Calo):– Adding AlgTools for Tau calculations at LVL2.

• Variables considered:– ETCore(em), ETCore(h), ETRoI(em), ETRoI(h) in

regions set in the jobOptions.– Evaluating offline variables: em radius of the cluster,

width in energy deposition, isolation fraction.• Procedure in developing HLT code:

– Use offline athena for programming– Test the code in the multithreading environment with

athenaMT.– Testbed integration. Pilar Casado,

Martine Bosman & Carlos Osuna

Hadronic Tau Trigger