QCD, diffrac,on and forward physics at the LHC

Transcription

1 QCD, diffrac,on and forward physics at the LHC Andrew Pilkington IPPP Durham and Manchester Presented at Diffrac.on 2010, Otranto, Italy, September 2010 Overview 1) Introduc,on to LHC in ) Review of soi QCD and diffrac,on results* 3) Review of first forward physics measurements* *thanks to colleagues at ATLAS, CMS, ALICE and LHCb for providing: the measurements & data valuable input to the talk See also talks by M. Tasevsky (ATLAS), A. Vilela Pereira & B. Roland (CMS), M. Poghosyan & R. Schicker (ALICE)

2 The LHC The LHC is expected to deliver about 1P 1 of pp collisions at s=7tev in 2010/2011 to IP1 and IP5.

3 The LHC in 2010 so far CMS LHC has now delivered nearly 4pb 1 to ATLAS, CMS and LHCb [average of nearly 2 interac,ons per bunch crossing] ALICE running in low pile up configura,on. Experimental data taking efficiency is high.

4 SoI QCD measurements The inelas,c cross sec,on in pp collisions is usually divided into diffrac,ve (SD,DD) and nondiffrac,ve (ND) components: σ inel = σ nd + σ sd + σ dd The majority of these events are soi, i.e. W/Z s and high p T jets form a small frac,on of the total inelas,c rate. Many results on soi QCD from ATLAS, CMS and ALICE. Here, will focus on aspects of these measurements that have some impact on understanding diffrac,on: Describe the average inelas,c event in pp collisions at s=0.9, 2.36 and 7TeV Methods to reduce or enhance the diffrac,ve components. Descrip,on of the underlying event in pp collisions underlying event ac,vity is linked to rescahering effects in diffrac,on, i.e. the soi survival probability.

5 Charged par,cle mul,plicity measurements The star,ng point for all soi QCD studies at these experiments was the measurement of the mul,plicity, transverse momentum and pseudo rapidity of charged par,cles. ATLAS follows an average inelas,c event approach (here present latest low p T results): Take all inelas,c events, N ev, that produce two tracks in the inner detector ( η <2.5). Count tracks above reconstruc,on threshold, normalize the distribu,ons using N ev. Hence measure the inelas,c event rate for charged par,cles above a given threshold CMS measure a non single diffrac,ve (NSD) event rate: Single diffrac,ve events suppressed by requiring ac,vity in the forward region on both sides of the detector. Expected remaining single diffrac,ve component subtracted (using PYTHIA). Data corrected for tracks below reconstruc,on threshold of 100MeV using Tsallis fits to measured spectrum above 100MeV. ALICE has presented results with (i) an NSD approach similar to that adopted by CMS, (ii) an inelas,c measurement and (iii) an average event measurement similar to ATLAS (INEL>0) Diffrac,ve MC event samples normalized to previous experimental data (UA5, E710). For approaches (i) and (ii) data is corrected for SD and low p T tracks a.l.a CMS.

6 Charged par,cle mul,plicity (ATLAS) ATLAS measure mul,plicity of tracks with p T >100MeV for events that contain at least two tracks in the region η <2.5. Excess of low pt charged par,cles that is not described by the standard MC. AMBT1 is PYTHIA tuned to the first track mul,plicity / underlying event data (with p T >0.5GeV). Events were triggered by hits on one side of the Minimum Bias Trigger Scin,llators (MBTS), which cover 2.1< η <3.8. ATLAS CONF

7 Charged par,cle mul,plicity at CMS CMS measure track mul,plicity of NSD events in region η <2.4. Data corrected for charged par,cles with p T <100MeV. Events were triggered by hits in any of the Beam Shower Counters (BSC), which cover 3.23< η <4.65. NSD events selected by requiring at least one forward hadron calorimeter tower on each side of CMS with energy greater than 3 GeV. CMS PAS QCD , CMS PAS QCD

8 Charged par,cle mul,plicity at ALICE Trigger provided by Hits in the silicon pixel detector (SPD) covering η <2 or Hits in the VZERO hodoscopes ( 3.7<η< 1.7, 2.8<η<5.1) Charged mul,plicity measured for η <1.0. Data corrected for tracks to account for tracks below 50MeV reconstruc,on threshold NSD analysis requires coincidence of hits on both sides of the interac,on point in the VZERO detectors.

9 The underlying event /jet The underlying event is the collec,ve term for beam beam remnants and the mul,ple parton interac,ons. Cannot be calculated from perturba,ve QCD, we rely on phenomenological models implemented (and tuned) in MC event generators. Underlying event contribu,on is studied by iden,fying a hard scaher (leading track, or leading jet) and studying charged track proper,es in the region transverse in azimuth to the hard scaher. ATLAS: Leading track iden,fies the hard scaher, other tracks have p T >0.5GeV and η <2.5 CMS: Leading track jet iden,fies the hard scaher, other tracks have p T >0.5GeV and η <2

10 UE at ATLAS Transverse region in data has larger track density for all leading track p T ATLAS CONF

11 UE at CMS More events in data with large track mul,plicity in transverse region. D6T tune reasonable descrip,on when leading track jet has high p T CMS PAS QCD

12 What is the diffrac,ve contribu,on to the average event? The diffrac,ve contribu,ons in the event generators are not able to account for the low p T excess in the data. The charged track mul,plicity in NSD events rises faster with s than for inelas,c events (which contain SD events)

13 Observa,on of diffrac,on at 0.9 and 2.36 TeV at CMS Triggered using the BSC counters (3.23< η <4.65). Event sample required to have primary vertex with three tracks to reduce beam related backgrounds. Diffrac,ve events iden,fied in three ways: The mul,plicity of calorimeter towers in the forward hadron calorimeter (HF), which covers 2.9< η <5.2. A tradi,onal forward gap defini,on. The sum of HF tower energy. Also a forward gap defini,on. Sum over all calorimeter towers: Σ(E i + p zi ). This variable is approximately propor,onal to twice the pomeron energy for diffrac,ve events. MC CMS PAS FWD

14 Observa,on of diffrac,on at 0.9 and 2.36 TeV at CMS

15 Diffrac,on enhanced event sample at ATLAS Aim to probe diffrac,ve contribu,ons by studying events in which there is ac,vity only on one side of the MBTS. Define the inelas,c sample as those events that contain at least one track with p T >500MeV and η <2.5. Define the gap events as the subset of the inelas,c events that have ac,vity on only one side of the MBTS. Study the kinema,cs and mul,plicity of tracks in the gap events. Calculate the ra,o, R ss, of gap to inelas,c events. MBTS hits MBTS hits η

16 Diffrac,on enhanced event sample at ATLAS Gap event sample is dominated by SD and DD events. (Uncorrected) data is consistent with MC predic,ons in track mul,plicity and kinema,c distribu,ons. R ss = 4.52 ± 0.63 %, which is achieved in all MC if the ra,o of diffrac,ve to inelas,c events is about 30% this does not fit with the PHOJET default! ATLAS CONF

17 Energy flow in forward calorimeters at CMS (I) Aim to measure energy flow in forward hadronic calorimeter (HF) at 0.9,2.36 and 7TeV. Trigger using hits in each of the BSC. Primary vertex required to suppress beam induced backgrounds. Summa,on of all HF towers with energy above 4 GeV in each slice of pseudo rapidity. Analysis performed for two classes of event: Minimum Bias events, i.e. those that pass the cuts above. Dijet events; defined as the subset of events that have two jets in η <2.5 with 1.0 < Δϕ π < 1.0. Minimum jet p T is (i) 8GeV for s=0.9,2.36tev and (ii) 20GeV for s=7tev. CMS PAS FWD

18 CMS PAS FWD Energy flow in forward calorimeters at CMS (II)

19 ATLAS CONF Forward jets at ATLAS and CMS

20 Dijet produc,on with a jet veto at ATLAS Measures the frac,on of dijet events that do not contain an addi,onal jet in the rapidity region bounded by the dijet system Sensi,ve (long term) to BFKL dynamics, wide angle soi gluon radia,on, colour singlet exchange y Require at least two good an, k T jets (R=0.6) with p T >30GeV and rapidity y < 4.5. Two defini,ons of boundary jets: Selec,on A; highest p T jets in the event. Selec,on B; most forward and most backward jets in the event. Gap events defined as the subset of events that do not contain an addi,onal jet with p T above the veto scale (Q 0 =30GeV). Gap frac,on studied as a func,on of the average transverse momentum of the boundary jets and the rapidity separa,on of the boundary jets. p T Q 0

21 Dijet produc,on with a jet veto at ATLAS Sel A Sel B Sel A Sel B ATLAS CONF

22 Forward J/Psi produc,on at LHCb LHCb is essen,ally a forward spectrometer! Forward J/Psi sample studied in the di muon decay channel. Di muon sample obtained using L0/HLT muon triggers. HLT requirement of muon with p T >1.3GeV OR 2 muons with invariant mass greater than 2.5GeV. Offline selec,on: 2 opposite sign muons making a good vertex with p T >0.7GeV. Inclusive J/Psi p T spectrum is not well described by MC. Polariza,on choice systema,c:

23 Forward B hadron produc,on at LHCb (I) Two methods used at LHCb to extract events containing B hadrons in 2<η<6. Both measurements in good agreement with each other. First method: J/Psi from B hadrons (H b ). Separate from prompt J/Psi using pseudo proper,me: Observe frac,on of J/Psi from B = 11.1 ± 0.8 %. Use LEP b >J/Psi frac,ons to obtain the B hadron cross sec,on: For H b in 2<η<6, LHCb observe σ(pp >H b X) = 84.5 ± 6.3 ± 15.6 μb

24 Forward B hadron produc,on at LHCb (II) Second method H b D 0 μν μ X Inclusive D 0 sample measured through Kπ decay channel. D 0 s origina,ng from a b quark decay are separated from the prompt D 0 s by requiring: right sign muon in the final state large impact parameter of D 0 μ system. For H b in 2<η<6, LHCb observe σ(pp >H b X) = 74.9 ± 5.3 ± 12.8μb

25 Summary So0 QCD 1) Excess of low p T charged par,cles in inelas,c and non single diffrac,ve events not well described by standard event generator tunes. 2) Excess of charged par,cles in the underlying event with respect to the MC tunes. 3) Observa,on of diffrac,ve events at ATLAS and CMS PHOJET overes,mates the non diffrac,ve component (CMS) the ra,o of diffrac,ve to inelas,c events should be about 30% at s=7tev (ATLAS). Early forward physics measurements 1) Energy flow in the forward region at CMS will help to constrain future MC tunes. 2) Observa,on of forward jets at ATLAS and CMS demonstrate that forward calorimeters are reasonably well understood at both experiments. 3) First measurement of dijet produc,on with a jet veto at ATLAS. 4) Measurement of forward J/Psi produc,on at LHCb highlights data/mc discrepancy. 5) Measurement of forward B hadron produc,on at LHCb in region 2<η<6.