Soft James Monk UK LHC HEP Forum 07 Sept. 2011, Cosener s House

Transcription

1 Soft James Monk UK LHC HEP Forum 7 Sept., Cosener s House

2 Outline Soft QCD program at the LHC is extremely large. Not possible to cover anywhere near all of it in 5 minutes. A few areas Minimum Bias Results Min Bias with particle ID Correlations and Event Shape Results Underlying Event Results

3 Minimum Bias Minimum bias is experimentally defined (by the trigger or some other phase-space cuts). It is a measure of what happens on average when you collide two protons It is important because it forms a background to the higher p physics. Multiple independent collisions (a.k.a pile-up) are min bias, which provides a background plus noise contamination of signal Fluctuations in the average event can look like signal herefore we need min bias to be well modelled by Monte Carlo = compare measurements to different MC tunes.

4 Charged particle multiplicity distributions dn ev /dn ch Ratio /N ev n ch, p > 5 MeV, <.5 ALAS s = 7 ev PYHIA ALAS AMB PYHIA ALAS MC9 PYHIA DW PYHIA 8 PHOJE Uncertainties MC / 6 8 n ch dn ev /dn ch Ratio /N ev n ch, p > 5 MeV, <.5 ALAS s = 7 ev PYHIA ALAS AMB PYHIA ALAS MC9 PYHIA DW PYHIA 8 PHOJE Uncertainties MC / 6 8 n ch dn ev /dn ch Ratio /N ev n ch, p > 5 MeV, <.5 ALAS s = 7 ev PYHIA ALAS AMB PYHIA ALAS MC9 PYHIA DW PYHIA 8 PHOJE Uncertainties MC / 6 8 n ch P n (b) CMS NSD CMS 7 ev (x ) PYHIA D6 PYHIA 8 PHOJE ev (x ).9 ev (x) η <. p >.5 GeV/c 6 8 n Note slight differences between phase space and event definition. η <.5 (ALAS),. (CMS) or. (ALICE). ALAS and ALICE use events with > charged particle inside their η acceptance, whereas CMS define non-single diffractive sample

5 / N ev u dn ch / dd Pseudo-Rapidity distributions.8 nch *, p > 5 MeV, d <.5 ALAS s = 7 ev Ratio. More similar event PYHIA ALAS AMB PYHIA ALAS MC9 PYHIA DW PYHIA 8 PHOJE Uncertainties MC / definitions this time d

6 Common plots ALICE, ALAS and CMS defined a common set of cuts for easy comparison of MB results: η <.8, p > 5 MeV or GeV. All events with Nch > 6

7 Min Bias with Particle ID

8 !/ K S. LHCb LHCb LHCb MC Perugia s =.9 ev. Are the p spectra of individual particle species well modelled?.5.5 Rapidity Ratio of e.g. / Ks shows baryon production rate relative to meson pbar/p ratio gives an indication of baryon production (two baryons in initial beams) Rapidity. LHCb LHCb MC Perugia LHCb LHCb MC Perugia... LHCb s =.9 ev.5 s = 7 ev. "/ K S LHCb LHCb LHCb MC Perugia. Particle ID can provide useful input to hadronisation models "/ K S!/ K S Min Bias with Particle ID LHCb s = 7 ev!.5.5 ransverse Momentum [GeV/c].5 Generators not doing a great job of this 8!.5.5 ransverse Momentum [GeV/c]

9 Min Bias with Particle ID Page of Fig. 6 (Color online) Ratios (K+ + K )/(π + + π ) and K /π as a function of s. (full symbols) are from pp collisions, (at s = 7.9 GeV by NA9 [, 5], at s = GeV by SAR [] ALICE, present work) and (open symbols) from pp and at s = 9 interaction (at s = 56 GeV by UA5 [7] and at the EVARON by E75 [6, 8]) Eur. Phys. J. C () 7:655 Page of Eur. Phys. J. C () 7:655 Alice have measured mesons were measured and K /π is used instead. he pt -integrated (K+ + K )/(π + + π ) ratio shows a slight increase from s = GeV (K/π =. ±.8) to s = 9 GeV (K/π =. ±. ±.) [], yet consistent within the error bars. he results at 7 ev will show whether the K/π ratio keeps rising slowly as a func tion of s or saturates. Protons and antiprotons in able have been corrected Page of for feed down (mainly from Λ), while the results from the SAR Collaboration are not. he proton spectra measured by PHENIX, on the other hand, have a lower pt -cut of.6 GeV/c. his makes a direct comparison with RHIC data difficult. Figure 7 shows a comparison of the measured pion, kaon and proton spectra with several tunes of the PYHIA event generator [5] and with PHOJE [8]. he PYHIA CSC 6 [9] tune provides a very poor description of the particle spectra for all species. Similar deviations were already seen for the unidentified charged hadron spectra []. he other PYHIA tunes, Perugia [] and D6 [6], and PHOJE give a reasonable description of the charged pion spectra, but show large deviations in the kaon and proton spectra. he measured kaon pt -spectrum falls more slowly Fig. (Color online) Specific loss generators de/dx vs. predict. momentum with increasing pt thanenergy the event A simfor tracks measured with the ALICE PC. he solid lines are a ilar trend is seen for the proton spectra, except for PYHIA parametrization of the Bethe Bloch curve [5] tune D6, which describes the proton spectra reasonably well. Efficiency correction heof raw hadron spectra corrected of he upper panel Fig. 8 shows the are pt -dependence for the the reconstruction shown inbyfig. 6, deterk/π and alsoefficiency, the measurements the E75 [6] and mined by doing the same analysis on Monte-Carlo events. SAR Collaborations []. It can be seen that the observed he efficiency the number ofon re-colliincreaseisofcalculated K/π withby pt comparing does not depend strongly constructed particles to the number of charged primary parsion energy. π, k and p p spectra at 9 GeV collision energy Fig. 6 (Color online) Ratios (K+ + K )/(π + + π ) and K /π as a function of s. (full symbols) are from pp collisions, (at s = 7.9 GeV by NA9 [, 5], at s = GeV by SAR [] ALICE, present work) and (open symbols) from pp and at s = 9 interaction (at s = 56 GeV by UA5 [7] and at the EVARON by E75 [6, 8]) Eur. Phys. J. C () 7:655 mesons were measured and K /π is used instead. he pt -integrated (K+ + K )/(π + + π ) ratio shows a slight + + K )/(π + + π ) and K /π Fig. 6 (Color online) Ratios (K increase from s= GeV (K/π =. ±.8) to as a function of s. (full symbols) are±from pp ± collisions, (at yet s = 9 GeV (K/π =...) [], s = 7.9 GeV by NA9 [, 5], at s = GeV by SAR [] consistent within the error bars. he results at 7 ev will present work) and (open symbols) from pp and at s = 9 ALICE, show whether the K/π ratio keeps rising slowly as a funcinteraction (at s =56 GeV by UA5 [7] and at the EVARON by tion of s or saturates. E75 [6, 8]) Protons and antiprotons in able have been corrected for feed down (mainly from Λ), while the results from the SARmeasured Collaboration not. he protoninstead. spectra measured mesons were and are K /π is used he + + by PHENIX, on the other hand, have a lower pt -cut of pt -integrated (K + K )/(π + π ) ratio shows a slight.6 GeV/c. his makes a direct comparison with RHIC increase from s = GeV (K/π =. ±.8) to data s = 9difficult. GeV (K/π =. ±. ±.) [], yet Figure 7 shows a comparison of the measured pion, consistent within the error bars. he results at 7 ev will kaon and proton spectra with several tunes of the PYHIA show whether the K/π ratio keeps rising slowly ashe a funcgenerator [5] and with PHOJE [8]. PYHIA event () 7:655 Page of tion of scsc or saturates. 6 [9] tune provides a very poor description of the Protonsparticle and antiprotons Similar have been corrected spectra for in all able species. deviations were alfor feed down Λ), whilecharged the results from the []. ready(mainly seen for from the unidentified hadron spectra he other PYHIA [] andmeasured D6 [6], and SAR Collaboration are not.tunes, he Perugia proton spectra PHOJE giveother a reasonable description of the by PHENIX, on the hand, have a lower ptcharged -cut of pion spectra, but show large deviations in the kaon proton.6 GeV/c. his makes a direct comparison with RHICand data spectra. he measured kaon p -spectrum falls more slowly t difficult. with increasing pt than the event generators predict. A simfigure 7 shows a comparison of the measured pion, Fig. 7 (Color measured pion, kaon and trend iscomparison seen for theofproton spectra, except forproton PYHIA ilaronline) kaon and spectra with several tunes of the PYHIA stune = 9 GeV (both charges combined) with various tunes spectra at proton D6, which describes the proton spectra reasonably event and with [8]. PYHIA of eventgenerator generators. Statistical errorsphoje only. See text forhe details well. [5] ticles from PYHIA in the chosen rapidity range. For transcsc 6 [9] tune provides very8poor description of the of he upper panel ofa Fig. shows the pt -dependence verse momenta above 8 MeV/c the efficiency saturates particle spectra for all species. Similar deviations were al- and the K/π and also the measurements by the E75 [6] at roughly 8%. For kaons, the decay reduces the efficiency ready seensar for the unidentified[]. charged Collaborations It canhadron be seenspectra that the[]. observed by about % at 5 MeV/c and % at.5 GeV/c. he increase of K/π with p does not depend strongly on collihe other PYHIA tunes, Perugia [] and D6 [6], and t Fig. 7 (Color online) β of tracks of particles measured by sion OF energy. vs. range with a reconstruction efficiency lower than 6% (for PHOJE give a reasonable description of the charged pion their momentum Particle Id by de/dx and time of flight (ß=length travelled/ time) Comparison to MC shows different discrepancies for different species 9 Fig. 7 (Color online) Comparison of measured pion, kaon and proton spectra at s = 9 GeV (both charges combined) with various tunes of event generators. Statistical errors only. See text for details

10 similar measurement for neutral 9 GeV.5 < y <. LHCb. < y <.5 LHCb 9 data Perugia LHCb MC LHCb MC + PYHIA 6 diff. MC/data LHCb performed a.5 < y <. dp dy d σ (pp K S X) [mb/(gev/c)] Min Bias with Particle ID p_ [GeV/c] p [GeV/c].6 p [GeV/c]. p [GeV/c] s =.9 ev as a function of transverse momentum p and rapidity y. he points represent LHCb data, with total uncertainties shown as vertical error bars and statistical uncertainties as tick marks on the bars. he histograms are predictions from different settings of the PYHIA generator (see text). he lower plots show the MC/data ratios, with the shaded band representing the uncertainty for one of these ratios, dominated by the uncertainty on the measurements (the relative uncertainties for the other ratios are similar). Figure 5: Double-differential prompt KS production cross-section in pp collisions at ranges in rapidity or pseudo-rapidity. he ability of LHCb to contribute measurements that extend the kinematic range towards high rapidities and very low p is apparent.

11 s =.9 ev. ratio of p/pbar production gives an indication of baryon production the contribution to the systematic (beam baryon e different sources. he total systemtical for both energies and amounts number = )! n corrected p=p ratio R integrated pt acceptance rises from Rjyj<:5 ¼ pffiffi :ðsystþ at s ¼ :9 pffiffi ev to 5ðstatÞ " :ðsystþ at s ¼ 7 ev.! p=p ratio, : " :8ðstatÞ, is e systematic errors at both energies Lambda/ ALICE.5 LHCb s = 7 ev..9 PYHIA 6.: ALAS-CSC PYHIA 6.: Perugia-SOF.8.8 HIJING/B.6.8 pt [GeV/c]! he pt dependence of p the ratio ffiffi p=p integrated over pffiffi jyj < :5 for pp collisions at s ¼ :9 ev (top) and s ¼ 7 ev (bottom). Only statistical errors are shown for the data; the width of the Monte Carlo bands indicates the statistical uncertainty of the simulation results. LHCb LHCb MC Perugia Perugia NOCR.5!.5.5 ransverse Momentum [GeV/c] s = 7 ev Lambda-bar. production ors, the measured ratio R shows no erse momentum (Fig. ) or rapidity shows similar. he ratio is also independent of FIG. (color online). ty for all generators in our acception of HIJING/B, which predicts a ing transverse momentum for the LHCb LHCb MC Perugia Perugia NOCR!/! uncertainty to the accuracy of the d analysis corrections. ulting from the subtraction of secom the feed-down corrections was by using different functional forms btraction and for the contribution of ducts [9]. f diffractive reactions to our final ied with different event generators ess than %, resulting into a negli:%) to the systematic uncertainty. e analysis was repeated using only without using any of the IS detecfference was negligible at both en- "/ " week ending AUGUS PHYSICAL REVIEW LEERS p/p ratio ) Min Bias with Particle ID LHCb s = 7 ev.5.5 Rapidity

12 Correlations & Event Shapes

13 wo-particle Correlations he existence of correlations between final state particles is an indication that there is a common origin for their production. Simple example: decays of clusters could give rise to particles close together in η and ϕ. Another example: if radiation is emitted at a given angle, ϕ, then there will tend to also be emission close to π-ϕ because of momentum conservation. In general, the pattern of correlations can be quite complicated. Models of soft QCD dynamics (as encapsulated in Monte Carlo generators) need to be able to describe this.

14 wo-particle Correlations wo particle correlations consist of a foreground and a background. Foreground = take Δη and Δϕ between each pair of particles in an event. Fill a D histogram with those values Falls with Δη because of phase space, but there is also structure (e.g. peak at,) F ( η, φ) = Foreground is normalised by dividing by total number of events N ch (N ch ) { Means that each track has the same weight in the distribution, regardless of the track multiplicity of the event i j=i Foreground Background δ ηi η j ηδ φi φ j φ N ch = number of (charged) particles in the event

15 5 wo-particle Correlations For the background take the Δη and Δϕ between particle pairs in independent events. Accounts for the phase space effect plus some other detector effects Divide the foreground by the background to give the observable Foreground Background B ( η) = dη dη δ (η η η) dn ch dη η=η dn ch dη η=η Note the different normalisation: the background is normalised by dividing by the number of entries (= the no. of tracks) to give unit integral

16 6 wo-particle Correlations R( ) 5 7 ev < < ALAS Preliminary MC9 une DW une Phojet une Perugia une Pythia8 une R( ) 5 9 GeV < < ALAS Preliminary MC9 une R( ) MC.5.5 MC9 une DW une Phojet une Perugia une Pythia8 une - R( ) MC.5.5 MC9 une R( ) -.5 R( ) Easier to compare to MC by integrating over Δϕ Δη distribution (by integrating the foreground and background separately over {, π} at 7 ev and 9 GeV)

17 wo-particle Correlations R( ) - R( ) R( ) MC MC9 une DW une Phojet une Perugia une Pythia8 une 7 ev < < ALAS Preliminary MC9 une DW une Perugia une Phojet une Pythia8 une - R( ) - R( ) R( ) MC GeV < < ALAS Preliminary MC9 une MC9 une - Δϕ distribution (by integrating the foreground and background separately Δη over {, } at 7 ev and 9 GeV) Double peak due to back-toback recoil. Similar to some underlying event distributions N ch /d d d p >.5 GeV and <.5, lead p ALAS >,,, 5 GeV, bottom to top PYHIA ALAS MC9 s = 7 ev Away ransverse oward ransverse Away [rad] wrt lead Δϕ distribution (by integrating the foreground and background separately Δη over {, 5} at 7 ev and 9 GeV) Integrating over Δϕ region that does not include main peak - we see the away side recoil, but a dip on the near side R( ) - R( ) R( ) MC ev < < 5 ALAS Preliminary MC9 une DW une Phojet une Perugia une Pythia8 une MC9 une DW une Perugia une Phojet une Pythia8 une - R( ) - R( ) R( ) MC GeV < < 5 ALAS Preliminary MC9 une MC9 une - 7

18 8 wo-particle Correlations at higher multiplicity (a) CMS MinBias, p >.GeV/c (b) CMS MinBias,.GeV/c<p <.GeV/c In a specific region of phase space, Nch(p > MeV), and for particles with GeV < p < GeV, CMS observe an interesting ridge showing long range correlations between tracks Interpretation is open for debate... R( η, φ) - φ (c) CMS N, p >.GeV/c R( η, φ) - - φ η η R( η, φ) R( η, φ) φ φ η (d) CMS N,.GeV/c<p <.GeV/c η

19 9 wo-particle Correlations at higher multiplicity In a specific region of phase space, Nch(p > MeV), and for particles with GeV < p < GeV, CMS observe an interesting ridge showing long range correlations between tracks Interpretation is open for debate... R( φ) R( φ) R( φ) R( φ) GeV/c<p <.GeV/c N<5 CMS pp PYHIA8 5 N<9 9 N< N - φ.gev/c<p <.GeV/c -.GeV/c<p <.GeV/c φ φ φ.gev/c<p <.GeV/c.< η <.8 - Same D plot in profile

20 Soft Diffraction Results from the exchange of (composite) colourless objects - a large and important contribution to the total cross section Exchange of colour singlet is typically accompanied by a rapidity gap devoid of radiation in the detector. dm/d6df [mb] Diffraction is the low t (momentum exchange) limit of scattering processes. ALAS has measured the cross section as a function of that gap size L = 7. µb- Systematic unc. MC Pythia6 MC Pythia8 MC Phojet s = 7 ev p > MeV Gap defined as a region with no track of p > MeV and no calorimeter cell with an energy deposit above a noise pedestal. he noise pedestal is defined such that the probability for a noisy cell to exceed the threshold is. ALAS Preliminary MC/ df

21 Bose Einstein Correlations Determine dn/dq (Q = (p-p)) for all pairs of like-sign charged particles in each event. Do same for un-correlated particles (mixed events) ake the ratio Final state pions are bosons, therefore they may originate from the same quantum state (hence require like-signed particles) his would show up as an increase as Q->

22 Underlying Event Mearurements

23 SHERPA: An event generator for the LHC Underlying Event. Gleisberg, S. Höche, F. Krauss, A. Schälicke, S. S. and J. Winter, JHEP :56, In its current version SHERPA includes: Underlying event is a feature of the ME generator AMEGIC++ (Monte Carlo) models that (providing the ME s for hard processes and describes what happens themodel) decays in the SM, MSSM and theto ADD part the proton does not theof parton shower that module APACIC++ participate the ordered hard scatter (containing a in virtuality initial Hadronisation Parton shower and final state parton shower) combination of ME s and PS s á la CKKW soft interactions Secondary between the to proton remnants an interface the Pythia string Underlying event fragmentation and hadron decays next release will contain a simple because it can add hard UE model Important (see talk to by your S. Höche) radiation final state, fake your signal, your jetsresponsible areinitialization a set of observables here Sherpamess is theup framework for the of to the which thegeneration underlying event the different phases and for steering event Related to, but not the same as models can be compared and min bias their parameters tuned... Steffen Schumann HERA/LHC Workshop, CERN,.-. October p.

24 Underlying Event Identify leading particle or track or calo-cluster in each event Define regions relative to this: oward: Away: ransverse: 6q leading track toward 6q <6 o Δϕ < 6 transverse o Δϕ > 6 < Δϕ < Determine p sum, multiplicity, av. p of tracks and other observables in each region 6< 6q < transverse o o 6< 6q < away 6q > o o

25 Underlying event from leading track Spike at Δϕ = ->radiation d-p/dd d6 q Shows Δϕ distribution of p correlated to the leading track corresponds to recoil radiation s = 7 ev ALAS PYHIA ALAS MC9 PYHIA DW PYHIA Perugia PHOJE HERWIG+JIMMY ALAS MC9.5 p >.5 GeV and d <.5 plead >. GeV Away ransverse plead >. GeV.5 d-p/dd d6q Increase as Δϕ-> π.5 oward ransverse Away Away oward ransverse Away oward ransverse Away plead > 5. GeV.5 ransverse plead >. GeV Away ransverse.5 his structure gets more obvious as lead p increases (emergence of jets) oward ransverse 6q Away transverse region) 5 - [rad] wrt lead Note the dip in between (the Away - ransverse - 6q [rad] wrt lead

26 ransverse Region ALAS ransverse Region s = 7 ev.5 Av. p sum <d-p /dddq> [GeV] p>. GeV and d <.5.5 ALAS ransverse Region.5 s = 7 ev p>. GeV and d <.5 <dnch/dddq> Av. Multiplicity.5 HERWIG+JIMMY ALAS MC9 PHOJE.5 MC/ PYHIA ALAS MC9 PYHIA DW PYHIA Perugia.. PYHIA ALAS MC9 HERWIG+JIMMY ALAS MC9 Lead rack PYHIA DW PYHIA Perugia PHOJE plead [GeV] 6 8 plead [GeV].5 CMS s = 7 ev. ch..8 PYHIA-6 Z PYHIA-8 C PYHIA-6 D6 PYHIA-6 Z Herwig++ UE-EE-.6.. CMS / #$ #(#") <!p > [GeV/c].6 / "# "("!) <N > MC/.5 charged particles (p >.5 GeV/c, # <, 6 < "! < ) Lead rack Jet.5 PYHIA-6 Z PYHIA-8 C PYHIA-6 D6 PYHIA-6 Z Herwig++ UE-EE-.5 charged particles (p >.5 GeV/c, $ <, 6 < #" < ) 6 8 s = 7 ev Leading track-jet p [GeV/c] 6 8 Leading track-jet p [GeV/c] 6 (difference to lead track shows in different x axis scale)

27 ransverse Region.5... ALAS ransverse Region s = 7 ev <!p >7 ev / <!p >.9 ev p >.5 GeV and d <.5. <p > [GeV] Av. p per particle.9 MC/.8 PYHIA DW PYHIA ALAS MC9 PYHIA Perugia.6 HERWIG+JIMMY ALAS MC9 PHOJE CMS PYHIA-6 Z PYHIA-8 C PYHIA-6 D6 PYHIA-6 Z Herwig++ UE-EE-..5. charged particles (p >.5 GeV/c, $ <, 6 < #" < ) Leading track-jet p [GeV/c] Nch Ratio of p sum at 9GeV: Pythia overshoots <p>, 7 ev. Shows energy evolution of the underlying event Herwig undershoots (a bit) 7

28 <d-p /dddq> [GeV]. ransverse region ALAS s= 7 ev.8 ransverse region ALAS.8 s= 7 ev PYHIA ALAS MC9 PYHIA Perugia PYHIA DW HERWIG+JIMMY PHOJE.6.. PYHIA ALAS MC9 PYHIA Perugia PYHIA DW HERWIG+JIMMY PHOJE.6.. charged+neutral particles p>.5 GeV d <.5 MC/ ALAS has performed the same measurement using neutral particles in the calorimeter together with tracks for charged particles MC/ <dn/dddq> Neutral particles charged+neutral particles p>.5 GeV d < lead p [GeV] plead [GeV]

29 7 CMS Preliminary s=7 ev Pythia-6 DW Pythia-6 Z 6 Pythia-8 C 5 Here the direction of the di-µ system is the towards region and is most sensitive to the underlying event (because an electroweak interaction is responsible for the µ)..8.6 Pythia-6 DW CMS Preliminary s=7 ev Pythia-6 Z Pythia-8 C charged particles o "! >, p >.5 GeV/c, # <. /Nev " $p/" # " ("!) [GeV/c] CMS have a preliminary measurement of UE in Drell-Yan µµ production. 8 /Nev " $p /" # " ("!) [GeV/c] Drell Yan 6 8 µµ p [GeV/c] Away 9.. charged particles o "! < 6, p >.5 GeV/c, # <. 6 8 p [GeV/c] µµ owards

30 Conclusion he first year of data from the LHC has lead to a panoply of new soft physics results hese valuable results have already fed into Monte Carlo tuning efforts, and will continue to do so Could only show a small selection here More coming in the future...